High-resolution inner-shell coincidence spectroscopy

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High-resolution inner-shell coincidence spectroscopy

Catalin Miron�, Paul Morin

Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin, BP 48, 91192 Gif-sur-Yvette Cedex, France

a r t i c l e i n f o

Available online 25 December 2008

Keywords:

Inner-shell

Auger

Coincidence

Nuclear dynamics

Dissociation

Electron spectrometer

Time-of-flight

a b s t r a c t

The present review article gives a historical overview of high-resolution inner-shell coincidence

spectroscopies followed by selected examples showing their scientific impact in studying molecular

fragmentation dynamics. The development of such spectroscopies was intimately related to the last 20

years spectacular advances of the instrumentation. They include the construction of high-performance

soft X-ray beamlines at the newest synchrotron radiation facilities, but also the remarkable

improvement of charged particles analyzers and detectors and the extensive use of multidimensional

coincidence techniques allowing for simultaneous recording and further correlation of several physical

parameters. From the examples shown, it clearly appears the importance of using coincidence

measurements in addition to pure electron spectroscopy methods in order to fully characterize the

complex decay processes of inner-shell excited species leading for instance to site-selective

fragmentation, vibrational motion mediated dissociation or ultrafast intramolecular recombination

processes. The perspectives of the field will be briefly drawn in the end.

& 2008 Elsevier B.V. All rights reserved.

1. Introduction

Electron–ion coincidence spectroscopy has been revealed asbeing a unique tool to enlighten complex dynamics followinginner-shell excitation or ionization of molecules. Indeed, thistechnique combines electron spectroscopy, which gives anaccurate picture of the system a few fs only after the photonabsorption, with ion mass spectrometry, which allows identifyingthe resulting ionic fragments usually produced on a longer timescale (a few ms). When applied in the best possible conditions ofenergy resolution, this technique permits a real state-to-statephotochemistry to be performed and provides key information tounderstand intricate photochemical processes. Concerning morespecifically the inner-shell ionization processes, fundamentalquestions like selective photochemistry or matter degradationupon interaction with ionizing radiation have been addressedmuch more efficiently using the coincidence approach. Indeed,relaxation of core-excited molecules involves a tremendousnumber of electronic and ionic decay pathways due to the largevariety of Auger decay channels, including cascade effects. Thecoincidence techniques usually allow at least disentangling thislarge amount of decay processes. A number of physical situationswhere electron–ion coincidence measurements were used toprobe the decay dynamics of core-excited molecules can be foundin the review by Nenner and Morin [1].

Impressive progresses have been made during the last decadesin both spectroscopic instrumentation and photon sources,resulting in fine in a considerable improvement of the coin-cidental detection efficiency. The spectral brightness of the thirdgeneration synchrotron radiation (SR) facilities allows nowadays avery accurate photon energy selection which is essential, forinstance, to select a given vibrational population of molecularcore-excited states. An important issue in coincidence experi-ments is the overall detection efficiency of the ejected particlesanalyzers. Indeed, coincidence experiments present an intrinsiclimitation due to the contribution of fortuitous coincidences,i.e. originating from uncorrelated particle emission events thatoccur in the same detection time window. In the trivial case ofsingle electron–ion coincidence measurements, the rate Ni

c of thedetected coincidence events for a given ionization channel i can bewritten as follows:

Nic ¼ Ni

� we � wi þ N � we � N � wi � t (1)

where N and Ni are the total and partial ionization rates, we and wi

are the electron and ion detection efficiencies, respectively, and tis the length of the single event acquisition time window. One canclearly see from Eq. (1) that the first term of the sum, whichdescribes the contribution of ‘‘true’’ coincidence events, scales1 asN whereas the second term of the sum, which describes thecontribution ‘‘false’’ coincidence events, scales as N2. Thus,increasing the incoming photon flux (and then N) is not an

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Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/nima

Nuclear Instruments and Methods inPhysics Research A

0168-9002/$ - see front matter & 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.nima.2008.12.104

�Corresponding author. Tel.: +33169 35 96 05.

E-mail address: [email protected] (C. Miron).

1 This is because Ni¼ ðsi=sÞ � N, with si and s the partial and total

photoionization cross sections, respectively.

Nuclear Instruments and Methods in Physics Research A 601 (2009) 66–77


appropriate way of increasing the true coincidence rate withoutbecoming dominated by the fortuitous coincidences contribution.Consequently, most of the experimental efforts in the pasthave been oriented towards the improvement of the detectionefficiency of the analyzers, leading to the development of paralleldetection techniques for both electrons (position sensitivedetectors, PSDs) and ions (time-of-flight, TOF, technique). More-over, the analysis of higher coincidence levels (i.e. coincidentdetection of not only two, but three, four, or more particles) or thesimultaneous measurement of additional physical parameters likeparticle momenta, introduce an even more severe limitation forthe coincident counting rate. For this reason, high-resolutionelectron–ion coincidence experiments have been possible onlyrecently.

The first pioneering coincidence experiments on inner-shellexcited species have been performed in the early eighties byEberhardt and coworkers [2–4], and later on by Hanson andcoworkers in the United States [5,6], or by Ueda and coworkers inJapan [7], trying to show that a selective bond breaking could beachieved in a molecule by either selecting an appropriate photonenergy corresponding to the excitation of an antibondingmolecular orbital or by choosing the type of final electronic state(one hole versus one hole-one particle) created by resonant Augerdecay. These experiments have been performed with a verymoderate resolution for the Auger electron kinetic energy (a feweV), using an electrostatic electron energy analyzer, but theresolution was, however, sufficient to distinguish the variouselectronic states populated after resonant Auger decay of themolecule. In this type of experiment, a constant electric field wasapplied in the ionization region in order to extract the fragmentions towards the TOF mass spectrometer: this represented aconsiderable limitation for the electron energy analysis due to theangle dependent electron energy shift induced by the appliedelectric field. Later, Lindle and coworkers have used an ion TOFspectrometer facing a cylindrical mirror electron energy analyzer(CMA) in order to perform coincidence studies at high photonenergy. For instance, by analyzing Ar resonant excitation in the1s–4p transition region (around 3206 eV), they were able todisentangle the various multiple Auger decay processes followingthe initial core-hole creation much more accurately as comparedto a non-coincident measurement. Of special interest was theobservation, even far above threshold, of electron recaptureprocesses into bound Rydberg orbitals, as induced by the so-called postcollision interaction (PCI) effect [8]. Together withthese pioneering experiments, one could also mention the firstelectron–electron coincidence experiment performed by Lablan-quie and coworkers who investigated in detail the energy sharingbetween two departing electrons following direct double photo-ionization of Ar [9].

Alternatively to the use of energy dispersive types of spectro-meters for the electrons detection, a double TOF configuration canbe used to analyze both electrons and ions in electron–electron orelectron–ion coincidence schemes. This arrangement presents areal advantage in terms of detection efficiency due to the widecollection angle and to the natural ‘‘parallel detection’’ of theTOF spectrometers (i.e. improvement of we in Eq. (1)). However,the electron kinetic energy resolution of such a spectrometer israther limited, especially for high energy electrons associatedwith the decay of inner-shell excited species. Indeed, the kineticenergy resolution of an electron TOF spectrometer scales, in a firstapproximation, as E1=2

kin , where Ekin is the kinetic energy of theanalyzed electron. This dependence makes this technique compe-titive as compared to the use of the energy dispersive one only upto approximately 30 eV of electron kinetic energy. Improvementshave been later made based on the use of a retarding lens into thedrift tube in order to analyze high kinetic energy electrons at

lower analysis energies, at the expense of both analysis energywindow and acceptance angle.

This multiple TOF technique has been extensively used byBecker’s group starting from the early nineties. A complete andwell illustrated review of such electron–electron or electron–ioncoincidence studies of atoms and molecules using this multipleTOF technique, including the historical aspects, was published byBecker [10]. As a nice illustration of this technique, Heiser andcoworkers were able to demonstrate a forward–backward asym-metry in the angular distribution of C1s photoelectrons from COas a function of the molecular orientation with respect to the lightpolarization vector [11]. In this experiment, a PSD was used forthe ions allowing to extract the ion ejection angle. Dowek andcoworkers have developed an electron–ion coincidence setupusing PSDs for both electron and ion TOF spectrometers. Vectorcorrelations between the photoelectrons and fragment ionsfollowing inner-valence shell ionization of spatially orientedNO molecules were obtained in such a way by Lafosse et al.[12]. Li et al. have studied, using the same technique, the inner-shell photoionization of NO using elliptically polarized light [13].Another improvement of the TOF based coincidence techniqueshas been made by using an additional magnetic field, with twopossible configurations. In the first case, a strong magnetic field isapplied in the drift region in order to parallelize the electrontrajectories towards the detector in the so-called ‘‘magneticbottle’’ type of spectrometers; in the second case, one uses asubtle combination of parallel electric and magnetic fields so thatelectrons in a given energy range reach the detector after a shortcurved trajectory (see, for instance Ref. [14]). In this later case,time (T) and position ðX;YÞ measurements allow to retrieve bothkinetic energy and emission angle of the particle.

The most extensive use of the magnetic bottle arrangement forinner-shell studies has been made by Lablanquie, Penent andcoworkers [15,16] using an experimental setup inspired by the onebuilt in Oxford [17]. For instance, they have been able to preciselydetermine breakdown curves of dissociating doubly ionized N2þ

2

species following Auger decay, with vibrational resolution [18].This work nicely showed the corresponding roles of directdouble ionization processes at threshold on the one hand and ofthe inner-shell ionization process on the other hand on the decay,and in particular on the final fragmentation pathway of the ion.This is not surprising because in one case (threshold doublephotoionization) only highly correlated electrons contribute to theprocess, whereas in the second case (core ionization followed byAuger decay) the involved orbitals should present a significantoverlap with the initial hole. Taking advantage of the very highacceptance angle (close to 2p) of the electron spectrometer, thesame group has more recently been able to detect in coincidencenot only two, but up to four electrons, producing thus a precisedescription of the energy sharing between the departing electronsduring a multi-ionization process following Xe2þ4d�2 doublephotoionization [19].

Dorner and coworkers have been the first to extend to thephotoionization field the use of electron–ion imaging coincidencesetups originally developed in collision physics, combining theuse of electric and magnetic fields and applied on cold atomsor molecules (COLTRIMS technique). More details about theCOLTRIMS technique can be found in the review by Dorner et al.[20]. In that case, by drastically reducing the so-called Dopplerbroadening related to the thermal motion of the targets at roomtemperature, it is possible to measure the ion recoil after electronejection and to use this information in order to determine the fullelectron momentum distribution. A very nice example addressingthe fundamental question of core-hole localization process indiatomic homonuclear molecules or, more generally, in highlysymmetric multicenter systems was described in Ref. [21] for N2.

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C. Miron, P. Morin / Nuclear Instruments and Methods in Physics Research A 601 (2009) 66–77 67


This study provided a remarkable experimental evidence of thequantum entanglement of the states corresponding to the emittedphotoelectron and Auger electron and showed that whether oneobserves a localized or a delocalized picture for the core hole is aquestion of choice of the detection method used to observe thephenomenon. Even in such a state-of-the-art setup, there is stillan upper electron kinetic energy limitation inherent to the TOFtechnique which makes it difficult to apply to fast Auger electronanalysis from polyatomic molecules or more complex species likeclusters or nanoparticles.

To the best of our knowledge, Auger electron–ion coincidencestudies have been performed with reasonably good electronenergy resolution only using electrostatic electron analyzers.However, even most recent studies based on the use ofcommercial high-resolution electron spectrometers suffer fromvery low coincidence counting rates leading to data of limitedstatistical quality [22]. Efforts in improving we in Eq. (1) led to anincrease of the acceptance angle, to a more optimized operation ofthe electrostatic lens of the analyzer over a broad kinetic energyrange, to the development of parallel acquisition modes using PSDor to the optimization of the geometry of the analyzer itself.Whereas hemispherical electron analyzers have shown theirefficiency in terms of ultimate energy resolution (below 1 meV),their acceptance angle is limited to a cone centered alongcylindrical lens axis. A value of 5� is currently used as a halfaperture of the collection cone; only recently wide angle lenseshave been developed (with a half aperture up to 15�) whichincrease significantly the electron counting rate. A major im-provement has been obtained using a toroidal geometry of thedispersive part: the acceptance angle is typically increased byone order of magnitude due to the 2p aperture in one direction(see Section 2 for details). Another major improvement of thecoincidence rate consisted in applying a pulsed field insteadof the continuous field to extract the ions. Indeed, a high field isbetter suited in order to efficiently extract the ions (increase of wi

in Eq. (1)) and to obtain the best mass and ion kinetic energyresolution. However, as stated before, the extraction field stronglydisturbs the electron energy analysis. To overcome this limitation,the ions are extracted only once the electron has been detected(usually very fast). Using such a pulsed extraction technique

together with an enhanced luminosity electron analyzer of doubletoroidal geometry, it has been possible to reach resolutions of lessthan 180 meV for the Auger electron analysis [23], which is closeto the vibrational energy of simple molecules.

In this review, after this brief historical recall, we aim toillustrate the remarkable scientific impact of the Auger electron–ion coincidence technique based on three selected examples wehave investigated in the past. The conditions needed to observeselective photochemistry, namely by using a selected photonenergy to induce a selective bond breaking in a complex molecule,will be emphasized using the case of the bromochloromethaneðCH2BrClÞ molecule after Br3d or Cl2p ionization. The secondexample deals with carbon K-shell resonant excitation in carbondioxide ðCO2Þ, for which we show that the excitation of bendingmotion in the core-excited state plays a determinant role in thefragmentation of the COþ2 ion into Oþ þ CO, whereas the stretchingmode excitation does not especially favor fragmentation! Finally,in the last example, we have combined both ultimate electronenergy resolution and the use of position sensitive detection of theejected ions in order to analyze the angular correlation betweenthe Auger electron ejection and the fragmentation axis of methylchloride ðCH3ClÞ following Cl2p excitation to the antibonding C–Clorbital. This is certainly one of the most sophisticated examples,where the coincidence technique appears to be a unique tool todisentangle competing processes and allowing to clarify nucleardynamics at the fs time scale in a complex system.

2. CH2BrCl: selective photochemistry does exist

A really new start for the energy selected Auger electron–ion coincidence experiments was given by the developmentof dedicated Auger electron spectrometers of which design wasparticularly optimized in terms of a high electron collectionefficiency required by the coincidence experiments. Such instru-ments are directed to the analysis of high energy electrons, andinvolve electrostatic analyzers based on original designs allowingmuch higher geometrical collection solid angles.

Fig. 1 shows one of these optimized designs developed in ourgroup more than a decade ago especially for Auger electron–ion

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Fig. 1. Original design of a double toroidal analyzer (DTA) of optimized collection efficiency for Auger electron–ion coincidence studies [24]. (a) 2D schematic

representation of the DTA indicating its main parts: source volume/interaction region, conical lens, entrance slit, toroidal deflecting plates, position sensitive detector (PSD).

Simulated electron trajectories are also shown for several kinetic energy ranges impinging the detector at different distances from the spectrometer axis. (b) A cut in the 3D

view of the mechanical design of the DTA.

C. Miron, P. Morin / Nuclear Instruments and Methods in Physics Research A 601 (2009) 66–7768


coincidence experiments [24]. It is based on the use of a toroidalelectric field for the dispersive stage and of a four-element conicalelectrostatic lens [25] to retard the high energy electrons down tothe analysis energy (also called pass energy) and to focus them onthe entrance slit of the dispersive stage. As shown by thesimulated trajectories in panel (a) of Fig. 1 the electrons arethen spatially dispersed according to their kinetic energy andfocused on a PSD allowing the analysis of both their kinetic energyand azimuthal emission angle. The possibility of simultaneouslymeasuring the electrons kinetic energy and emission angle is areal advantage with respect to the magnetic type of spectrometerssince it gives access to electron–ion angular correlationmeasurements and surface analysis. The collection efficiency ofthis double toroidal analyzer (DTA) amounts to 5% of 4p sr whichis at least one order of magnitude more than conventional designs(see Ref. [24] and references therein for more details on theDTA and the original works which inspired its design). Panel (b)in Fig. 1 illustrates a 3D view of the mechanical design of theDTA spectrometer. The numerical simulations performed duringthe design phase predicted that the resolving power of thisspectrometer is strongly dependent on the extension of theelectron source (a gain of a factor of four is expected in resolvingpower when decreasing the linear source extension by a factor of10) and this trend was further confirmed by the experiment.

In the resulting coincidence setup, entitled EPICEA, the DTAelectron analyzer is facing a conventional TOF mass spectrometer.When an electron is detected, a strong (up to 1.8 kV) pulsedextraction field is applied into the ionization region in order toextract the ions towards the drift tube, which allows differentiationof the fragments based on their m=q ratio. This setup provided in thenineties the first true opportunity for studying state-to-statedynamics of inner-shell excited species, i.e. the possibility to directlycorrelate the quantum states of singly or multiply charged ionicspecies created after resonant or normal Auger decay and theirspecific ionic fragmentation channels. The precise quantum stateof the cation is unambiguously selected by the measurement of thekinetic energy of the Auger electron. Insofar as the vibrationalresolution is reached in the electron analysis channel, the qualitativeand quantitative role of the vibrational excitation in both core-excited state prepared with a given amount of vibrational excitationand in the final ionic state, on the further decay dynamics, can beinvestigated in detail. Due to the significantly improved collectionefficiency available in the EPICEA setup, the first high-resolutioncoincidence measurements between energy selected high energyelectrons and ions were performed using the pulsed extraction fieldtechnique described above [26–28]. In the following we are goingto concentrate on one of the nicest applications of the Augerelectron–ion coincidence technique which is the investigation of thesite-selective photofragmentation of a polyatomic molecule demon-strated in the case of bromochloromethane [26].

The idea of site-selective photofragmentation of a core-excitedmolecule using soft X-rays naturally merged-up more than twodecades ago in relation with the very localized character of thecore orbitals [2]. Indeed, by tuning the photon energy to theappropriate transition, one is able to excite a well defined site in acomplex molecule and expects to selectively break the surround-ing chemical bonds. Such studies, referred to as site-selectivephotochemistry, have not only a fundamental motivation re-lated to the general topic of the specific damages induced bymonochromatic radiation on complex systems but are also highlyinteresting in several related fields such as photodesorption,photoreactivity and photochemistry in general, which explainthe numerous review papers [1,29,30] (and references therein)and articles [5,31–36] broaching this subject.

When site-selective fragmentation is considered, the first issuewhich is of interest is the ‘‘degree of localization’’ of the primary

Auger process following the core excitation or ionization event.In the case of core ionization, the electronic decay process canbe treated sequentially: photoelectron emission followed by anormal Auger process leading to a doubly charged ion. One canexpect a site specific dissociation process of this doubly chargedion only if the location of the two positive charges in the two-holefinal state of the Auger process preserves the memory of the initialsite of photoionization. Indeed, for most of the known situations, a‘‘localized’’ Auger process takes place following core ionizationbecause the Auger decay rate depends upon the Coulomb matrixelement describing the overlap between the core-hole orbital andthe molecular orbitals involved in the Auger decay process.However, exceptions where the dicationic final states of the Augerprocess completely lose the memory of the initial ionization sitehave been demonstrated, as in the case of the silicon Augerspectrum of the highly ionic SiF4 molecule, where stronglocalization effects of the two holes take place [37,38]. On theother hand, even if the two holes are localized on or close to theinitially excited site, a fragmentation process around this site isexpected to occur only if other possible electronic relaxationchannels are not faster than dissociation, as discussed later. Theseother relaxation channels are expected to become more efficientwith an increase of the molecular ion size. In the frame of sitespecific photofragmentation, the halogenated alkanes represent aparticular class of molecules to which several studies have beendedicated [31,36,39,40]. The relevance of di-halogenated alkanescomes from the possibility of selectively excite the two halogenatoms that are sitting in identical or similar geometrical andchemical environments. For instance, after bromine 3d ionizationof ClCH2Br, Schmelz et al. observed that the molecule preferen-tially dissociates along the C–Br chemical bond whereas afterchlorine 2p ionization it was the C–Cl chemical bond which waspreferentially broken [30,36]. However, in that study the observedfragmentation was summed-up over all open electronic channelsand the origin of the selective effect could not be extracted fromthe experiment. By using the Energy Selected Auger ElectronPhotoIon Coincidence (ES-AEPICO) technique based on the EPICEAsetup we were able to obtain evidence of strong site-selectivephotofragmentation processes in the core-ionized bromochlor-omethane molecule and disentangle its origin. The main advan-tage of this technique is that the detection of only one electronsimultaneously determines the binding energy of the doublycharged ion and the hole which was initially created.

Fig. 2a shows an overview of the electron emission spectra ofClCH2Br molecule recorded with a resolution better than 1 eV athn ¼ 219 eV and represented on an electron kinetic energy scale.The excitation energy is situated about 10 eV above the chlorine2p3=2;1=2 and about 140 eV above the bromine 3d5=2;3=2 ionizationthresholds. The four main bands correspond to the chlorine 2pphotoelectrons (below 18 eV), the Br(3d)VV Auger bands (in therange 20–70 eV), the bromine 3d photoelectrons (in the range137–144 eV) and to the Cl(2p)VV Auger bands (above 151 eV),respectively. Fig. 2b is an enlargement of the Br(3d)VV andCl(2p)VV Auger bands plotted on a binding energy scale.Obviously the spectral resolution is not sufficient to resolve thefinal electronic states of the ClCH2Br2þ ion. However, a shift ofabout 3 eV between the onsets (low binding energy side) of thetwo Auger bands can be clearly observed. This is due to a differentlocalization of the two valence holes in these lowest final statespopulated in the two cases: these lowest final states correspond totwo vacancies in the bromine or in the chlorine atoms lone pairs,respectively. Indeed, using the binding energies of these lone pairsin ClCH2Brþ measured by Novak et al. (10.77 and 11.81 eV,respectively) [41], an empirical rule of thumb [42] allows toestimate an energy shift between these states of 2.9 eV, which isin very good agreement with our data.

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We have performed ES-AEPICO coincidence experiments withelectrons corresponding to binding energies of the ClCH2Br2þ ionranging from �27 to �57 eV by steps of �5 eV. Fig. 3 illustratesselected ES-AEPICO coincidence spectra for binding energies of�27 and �30 eV (a) and �57 and �55 eV (b), respectively. The topspectra correspond to the creation of the initial core hole in thebromine 3d shell whereas the bottom spectra correspond to thecreation of the initial core hole in the chlorine 2p shell. Comparingthe spectra in Fig. 3a, it is obvious that the dissociation patterns ofthe lowest electronic states created via Auger processes followingBr 3d or Cl 2p ionization are very different. In the light of theprevious discussion, a different location of the two valence holesin the doubly charged ion is expected, due to an Auger processpreserving the memory of the initial photoionization site.Moreover the observed site selectivity is very strong: Br(3d)VVAuger processes lead to a coincidence spectrum which is stronglydominated by the Brþ and CHxClþ ions (signature of a C–Br bondbreaking) whereas Cl(2p)VV Auger processes lead to one which isdominated by the Clþ and CHxBrþ ions (signature of a C–Cl bondbreaking). Let us concentrate now on the spectra in Fig. 3bcorresponding to high binding energies (55 eV, 57 eV) of thedissociating ion. The two spectra are very similar and thecomponents due to the molecular fragment ions are negligibleas compared to those associated with the atomic fragment ions. Inthis case of high binding energies of the doubly charged ion thephotoselectivity is completely lost and the fragmentation pattern

seems to be independent of the ionization path leading toatomization.

In order to quantitatively describe the selectivity and to be ableto follow it as a function of the binding energy of the dissociatingClCH2Br2þ ion, we have defined two experimental selectivitycoefficients Cs

Br ðCsClÞ derived from the branching ratio for

preservation of the C–Br (C–Cl) chemical bonds after excitationat the Br 3d (Cl 2p) edge. This dependence, not shown here (formore details, see Ref. [26]), indicates that for the creation of aninitial Br3d�1 hole or a Cl2p�1 hole the site selectivity for the firstaccessible electronic states of the ClCH2Br2þ ion is indeed verystrong. The site selectivity and the differences observed in theBr(3d)VV and Cl(2p)VV Auger spectra (Fig. 2b) reflect differencesin the topologies of the corresponding potential surfaces alongthe C–Br and C–Cl coordinates for these electronic states. Noticethat the time scales of the relaxation processes must also beconsidered at this stage, because the system behavior indicatesthat the dissociation occurs before other relaxation processes (e.g.internal conversion of the electronic into vibrational energy)which probably take place over larger characteristic time scales.According to the previous discussion concerning the ‘‘local’’character of the Auger process, we conclude that this pronouncedsite-selective fragmentation is probably induced by a very rapidand direct C–X bond breaking between the halogen atom andthe rest of the molecular ion. In order to be able to make thedistinction between the photoinduced site-selective fragmentation

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Fig. 2. (a) ClCH2Br electron emission spectra recorded at hn ¼ 219 eV. The

assignment of the four bands is also indicated on the figure. (b) Br(3d)VV and

Cl(2p)VV Auger bands plotted together on the same binding energy scale. A shift of

about 3 eV between the onsets of the Auger bands corresponding to the two inner-

shell vacancies is observed.

Fig. 3. ClCH2Br ES-AEPICO coincidence spectra for low (a) and the high (b) binding

energy states of ClCH2Br2þ . At low binding energy a strong site-selective

fragmentation of the doubly charged ion is observed (top panel), whereas at high

binding energy atomization processes are dominant (bottom panel).



and the naturally selective bond breaking solely due to a possibledifference in the thermodynamical dissociation energies ofthe C–Br and C–Cl chemical bonds, coincidence experimentsbetween energy selected C1s photoelectrons and fragment ions(ES-PEPICO) at hn ¼ 330 eV were performed. Indeed, after ioniza-tion of the central atom ðC1s�1Þ one can expect to reach theso-called basic fragmentation pattern reflecting the thermodyna-mical behavior [26]. Since ES-PEPICO type experiments contain anaverage information about the fragmentation of all final electronicstates of the doubly charged ion, they are the most appropriate togive an estimation of the basic Cs;0

Br , Cs;0Cl selectivity coefficients

(found to be 0.64 and 0.36, respectively).The pure photoselective effect can be emphasized by subtracting

these basic selectivity coefficients from the first ones CphotoX ¼

CsX � Cs;0

X ðX ¼ Br;ClÞ and representing the result as a function of theion electronic state binding energy. The analysis of their variationleads to the following conclusions: (a) their magnitude is thehighest at the lowest binding energy and (b) the photoselectivitycoefficients Cphoto

X have a similar variation with binding energy andtheir asymptotic value for high binding energy tends to zero. Thesimilarity for the lowest binding energies shows that the localizedcharacter of the lone pair orbitals is the same in Br and Cl, leading tosame type of selective fragmentation. The monotonously decreasingselectivity tends to the basic selectivity coefficients at high energy.There are two possible ways of explaining this loss of selectivity. Thefirst one involves direct formation of high binding energy electronicstates via non-local Auger processes. The second possibility is a fastinternal conversion process where the electronic energy is con-verted into vibrational energy of a lower electronic state of the ion.The rate of these energy redistribution processes is known tostrongly depend on the actual density of states, which is likely todramatically increase with binding energy. In such a statisticalenergy redistribution, the initial localization is lost and thefragmentation pattern is only governed by thermodynamicalproperties. Furthermore, as the available energy in the systemincreases, a more efficient explosion of the molecule is observed.

In summary, we have observed a strong site-selective photo-fragmentation of the first excited states of the ClCH2Br2þ ion,

following Br3d�1 or Cl2p�1 core ionization, emphasized afterremoval of the ‘‘natural’’ selectivity. Furthermore, the evolution ofthe observed selectivity has been studied as a function of thebinding energy of the dissociating doubly charged ion using acoincidence technique between energy selected Auger electronsand fragment ions, showing a rapid disappearing of the photo-selective effect with increasing binding energy.

3. CO2: bending motion induced by core excitation as amediator of molecular fragmentation

In the previous example, the use of the electron–ion coin-cidence setup was limited to the simultaneous extraction of bothelectron energy and ionic fragment mass/charge ratio. Whereasthis kind of data is essential in order to analyze general trends likephoton induced selective bond breaking, important pieces ofinformation are missing in order to understand in detail thedissociation process mechanisms. Indeed, the full momentumdetermination of the emitted particles (energy and angle analysis)allows a much more accurate description of the nuclear dynamics.Applied to a simple triatomic system like CO2, the coincidencetechnique allows a detailed probe of the potential energy surfacesof the ion and a deep understanding of the wavepacket evolutionafter photon absorption. The last two examples, described in thepresent and in the next section, respectively, illustrate howfruitful it is combining both high-resolution resonant Augerspectroscopy and Auger electron–ion coincidence measurementsin order to extract complementary information about the decaydynamics of core-excited CO2 [28,43] and CH3Cl [23] molecules.

From the experimental point of view, this additional informa-tion has been obtained by combining time (T) and position ðX;YÞmeasurements of the particles impinging the detector. The EPICEAsetup described previously has been thus upgraded by installing adelay line PSD at the output of a modified TOF mass spectrometermaking use of inhomogeneous fields for spatial focusing of theions coming from a spatially extended source [43]. Fig. 4 shows aschematic view of the setup together with its detectors, signal

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Fig. 4. Schematic representation of the EPICEA coincidence setup including its two position sensitive detectors and the data acquisition electronics. The synchronization of

the events is insured by the measurement of all relevant quantities as time intervals on a single time-to-digital converter (TDC).



shaping electronics and data acquisition system. The use of PSDbased on time measurements for both electron and ion detectionsignificantly improved the coincidence data collection efficiency.Moreover, the addition of the ion position measurement greatlyextends the possibilities of the coincidence setup to full Augerelectron–ion momenta correlations which allow, for instance,describing the electronic decay and nuclear relaxation ofrandomly oriented molecules in the molecular frame asdeveloped in the next section on CH3Cl [23].

To give the reader an idea about the powerful method ofcombing the measurement of the ion positions on a PSD with theelectron energy and the ion mass selection, we show in Fig. 5 atypical textbook case where soft X-ray photons are used to inducethe transition N1s�!s� in molecular nitrogen. In the case of N2, theshape resonance s� is lying in the N1 s�1 ionization continuum atabout 420 eV. The core-ionized Nþ2 ð1s�1Þ will relax by normal Augerdecay leading to the dication N2þ

2 ðNþ2 ð1s�1Þ�!N2þ

2 ðV�11 V�1

2 Þ þ e�A Þwhich can be stable or further break into two Nþ ions: N2þ

2 �!NþþNþ. As we discussed earlier, conventional TOF spectrometry allowsdistinguishing different ion flight times in a field free regionaccording to the ions m=q ratio. However, the TOF also dependsupon the projection of the particle momentum on the spectrometeraxis. The particles which are ejected with no velocity componentalong the spectrometer axis will have the mean value of the TOF, theparticles with a velocity component towards the detector will havea slightly shorter TOF and, they will have a longer one if they areejected with a non-zero velocity component in the oppositedirection. This behavior is clearly described by structured TOF

profile shown in the central part of Fig. 5: a central peak with twoside bands corresponding to the ions emitted in the forward andbackward directions, respectively. Such ions (in blue) will give arelatively uniform distribution of their impact positions on the ionPSD if the molecular axis distribution is no or slightly peaked only.The TOF spectrum alone does not allow in any way to disentanglethe parent N2þ

2 ions which have a zero kinetic energy andthe energetic Nþ ions which are emitted perpendicular to thespectrometer axis (they have same m=q ratio). These two situationsare depicted in red in Fig. 5. It is easy to remark from the right ionPSD image that the two classes of ions become distinguishable ifone measures the ion TOF in coincidence with their positiondistributions on the ion PSD. The static N2þ

2 ions arrive at the centerof the detector, whereas the energetic Nþ ions will impinge thedetector on its border due to their drift in the perpendiculardirection. This simple case explains the basis of the intermolecularaxis determination allowing to study the decay phenomena in themolecular frame even for randomly oriented molecules as describedin the last example given in next section.

Carbon dioxide molecule CO2 is linear in its ground state butreaches a stable bent geometry when a carbon 1s electron isexcited to the lowest unoccupied molecular orbital 2pu (1Pu). Thisis in agreement with the so-called ‘‘Zþ 1’’ approximation2 which

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Fig. 5. The importance of correlating time-of-flight measurements with ion position determination on the ion PSD installed at the output of the ion TOF spectrometer.

Example of the N2 dissociation following N1 s�!s� when N2þ2 ions with no kinetic energy and energetic Nþ ions ejected in the perpendicular direction with respect to the

spectrometer axis are indistinguishable by conventional time-of-flight measurement while they become distinguishable when their impact position on the TOF

spectrometer’s PSD is measured in coincidence (right detector image obtained for the ions shown in red).

2 The ‘‘Zþ 1’’ approximation allows to predict the geometries of core-ionized

or core-excited states in molecules as being those of the so-called core-equivalent

molecule which is found by replacing the core-excited atom by the next one in the

periodic table (see text for an example).



predicts a bent equilibrium geometry for this core-excited statebecause the core-equivalent molecule is ONO which is bent, withan inter-bond angle evaluated to be about 134�. In the lineargeometry ðD1hÞ the 1Pu state is doubly degenerate but thisdegeneracy is lifted through a Renner–Teller effect due to thebending motion (more information about such Renner–Tellereffects can be found in the recent review by Adachi et al. [44] andreferences therein). In the bent geometry ðC2vÞ the 1Pu state issplit in two terms: 1A1 and 1B1. The first one corresponds to a bentconformation and its minimum is at lower energy as compared tothe second, which corresponds to a linear conformation. Thismeans that when the photon energy is tuned along the C1s�!p�resonance, at the low energy part of the resonance bending modeswill be predominantly populated in the bent conformation, whileat the higher energy part of the resonance both high bendingmodes in the bent conformation and symmetric stretching modesin the linear conformation will be equally excited. Theseassumptions have been tested using both high-resolution electronspectroscopy and Auger electron–ion coincidences.

The high-resolution direct photoemission ðhn ¼ 280 eVÞ andresonant Auger spectra of CO2 recorded with 85 meV overallinstrumental broadening [28] for three photon energies along theC1s�!p� transition are represented in Fig. 6 using a bindingenergy scale. The final ionic states of COþ2 are also labeled onthe figure. As indicated by the spectra in Fig. 6, only the firstexcited state A2Pu is significantly populated by the resonantAuger decay, while B2Sþu and C2Sþg ionic states are almostexclusively populated by direct photoemission. This greatlysimplifies the analysis of the coincidence spectra as discussedhereafter. The spectral changes observed between Left and Rightspectra, as for instance the apparent loss of resolution in leftspectrum due to the population of the bending mode, are in fullagreement with the Renner–Teller model discussed in theprevious paragraph (see Ref. [28] for details).

The coincidence spectra between energy analyzed Augerelectrons and fragment ions for the same excitation energies asthe high-resolution electron spectra from Fig. 6 are shown in Fig. 7

as waterfall plots for an electron binding energy (or ion internalenergy) ranging from 18.7 to 20 eV. This spectral region is mainlypopulated by the high vibrational levels of the A2Pu state. Thecoincidence spectra were normalized to the most intense TOFpeak which is the parent COþ2 ion. Even if the relevant quantitiesto compare are the peak areas and not the peak intensities [43],already form the spectra shown in Fig. 7 one can draw the maintrends: (i) The formation of the Oþ ion is strongly affected by thegeometry of the core-excited state. It seems that when the lowenergy part of the resonance is excited (bending motion), theproduction of this ion is enhanced as compared to the excitationon the high energy part (stretching motion). (ii) The parent ionCOþ2 is more stable when the stretching motion is excited (rightexcitation in Fig. 7), which is an unusual kind of behavior inmolecular fragmentation. (iii) COþ ion production rate does notdepend on the excitation energy and thus on the preparation

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Fig. 6. High-resolution direct photoemission ðhn ¼ 280 eVÞ (bottom spectrum) and

resonant Auger decay spectra of CO2 recorded after exciting along the C1s�!p�resonance: Left ðhn ¼ 290:6 eVÞ, Top ðhn ¼ 291 eVÞ and Right ðhn ¼ 291:4 eVÞ. Only

A2Pu state exhibits strong resonant contribution (see text).

Fig. 7. Resonant Auger electron–ion coincidence spectra recorded for the same

photon energies as the resonant Auger spectra in Fig. 6 but with lower overall

resolution of 200 meV [43]. Each coincidence spectrum corresponds to a binding

energy range of 200 meV starting at 18.7 eV and the time-of-flight peaks are

normalized to the most intense one which is the parent ion COþ2 in all cases. The

spectra are free from fortuitous coincidences and direct photoemission contribu-

tions which were subtracted.



mode of the core-excited state. (iv) The high resolution of thecoincidence data allows to clearly identify the various dissociationthresholds and decrypt the complex mechanisms leading to thepreferential formation of Oþ ion due to a conical intersectionoccurring when a bent conformation is selected for the core-excited state [28,43]. Indeed, in linear geometry the O+(4S) +CO(1S+) dissociation pathway (the only one accessible at thisenergy) is symmetry forbidden, whereas it becomes allowed in abent geometry, if one also accounts for the spin–orbit coupling.

Not only the ion production rates but also ions kinetic energyreleased (KER) were measured in coincidence with the Augerelectrons for the three excitation energies. The results, not shownhere, indicated that Oþ ions have a low KER independent on theexcitation energy while COþ ions showed a higher KER presentinga minimum for the highest excitation energy (Right) as comparedto the lowest (Left) and medium (Top) excitation energies. Again,this observation is in full agreement with the fact that in theconsidered binding energy range Oþ ion can only be producedstarting from a bent geometry of the core-excited state and itprobably transfers a lot of ro-vibrational energy to the neutral COfragment. As for COþ at the Right excitation, the slightly lower KERcan well be corresponding to the transfer of one vibrationalquantum to COþ in a linear geometry which would be lessefficient in a bent geometry [43]. This example shows once againthat the combination between high-resolution electron spectro-scopy and coincidence measurements provides with an extremelypowerful tool to probe decay dynamics in core-excited molecules.

4. CH3Cl: complex nuclear dynamics of a polyatomic moleculedisentangled through energy and angle correlationmeasurements

The last example aims to reinforce the previous statementshowing a situation where a multiparameter electron–ion coin-cidence approach was used in combination with high-resolutionelectron spectroscopy to clearly disentangle between decayprocesses occurring at short (molecular regime) or long (atomicregime) time scales. Methyl chloride, chosen as a prototypepolyatomic molecule, has been excited along the Cl2p! s�resonance. When CH3Cl is excited around 201 eV, the Cl2p3=2 !

s� electronic transition promotes the system to an excited statecharacterized by a multidimensional potential energy surfacewhich is strongly repulsive along the C–Cl internuclear coordi-nate. The further system evolution is rather complex and severaldecay channels are open: (a) electronic decay in a molecularregime may occur through an Auger process taking place whilethe internuclear distance C–Cl increases (for more details onsuch an Auger emission influenced by the nuclear motion seeRefs. [47–49]); (b) for molecules which did not relax throughAuger decay at short internuclear C–Cl distance, neutral dissocia-tion in CH3 and Cl* can precede an atomic Auger decay process ofthe core-excited chlorine atom (such an ultrafast dissociationprocess was first observed more than 20 years ago for HBr byMorin and Nenner [45]). In the first case (a) the decay product isthe electronically and possibly vibrationally excited CH3Clþ ionwhich can further fragment and/or rearrange. In the second case(b) the main decay products are CH�3 and Clþ and the previousAuger emission process leading to Clþ can be identified throughan Auger electron energy distribution which depends upon theelectron emission angle with respect to the neutral dissociationaxis. Such an Auger-Doppler effect was first described byBjorneholm and coworkers for the decay of O1s�1�!3suðs�Þcore-excited state in the oxygen molecule [46].

We performed multicoincidence and high-resolution electronspectroscopy measurements on this system. Two types of

measurements were performed with overall instrumental broad-enings of 35 meV for resonant Auger spectra (VG Scienta R4000electron spectrometer) and 180 meV for the coincidence experi-ment (performed using the EPICEA setup described previously),dominated in both cases by the electron spectrometers contribu-tions. Fig. 8 presents the high-resolution direct photoemission(OFF-RES) and the resonant Auger spectra of CH3Cl followingCl2p! s� excitation and plotted on a kinetic energy scale forthree excitation energies along the resonance profile. In order tofacilitate the visual comparison of the resonant spectra with thedirect photoemission spectrum this later was shifted byþ2:5 eV todirectly estimate the contribution of the direct photoemissionprocesses in the resonant spectra. The intensities were normalizedwith respect to the photon flux and sample gas pressure and thedirect photoemission contribution has been subtracted from theresonant spectra which thus show only resonant contributions.The spectra in Fig. 8 show the typical spectral intensitydistribution for systems where ultrafast dissociation processescompete with molecular Auger decay, i.e. sharp lines staying atconstant kinetic energy superimposed on broad molecularbackground showing a dispersive behavior against photonenergy detuning. The observed sharp structures energies are infull agreement with the 1S, 1D and 3P atomic Clþ Auger linespositions which are also drawn on the same figure as vertical bars.For this reason, they can be undoubtedly assigned to an ultrafastdissociation process where the Cl� atom leaves the CH3Cl�

molecule before molecular Auger decay and further decays as anisolated atom.

Auger electron–ion coincidence spectra have been measuredwith energy and angle analysis, at the same photon energies asthe resonant Auger spectra, in order to probe the dissociationdynamics associated with this core-excitation process. By suchcoincidence measurements, it is possible for instance to obtain theresonant Auger decay spectra associated with a given fragmention production: MSPES for mass selected photoelectron spectra(see Ref. [23] for more details). Fig. 9 shows the MSPES spectraobtained for the excitation on the top of the Cl2p! s� resonance.One can easily recognize on the Clþ spectrum the three sharp lines

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Fig. 8. High-resolution direct photoemission (OFF-RES) and resonant Auger

spectra of CH3Cl recorded along Cl2p! s� resonance: Left at 200.3 eV, Top at

200.9 eV, Right at 201.4 eV. The off-resonance spectrum shown at the bottom is

shifted by 2.5 eV to account for Dhn (see text). The energy position of the 1S, 1D and3P atomic chlorine lines is indicated by vertical dashed bars. The figure is extracted

from Ref. [23].



(1S, 1D and 3P) already observed in the total Auger decay spectrumfrom Fig. 8 and associated with the atomic Auger decay processesin the Cl� atom. However, in addition to this identified channel thespectral intensity around 178 eV suggests that Clþ can also beproduced by another mechanism, for instance the dissociation ofCH3Clþ following molecular Auger decay. Another interestingfeature of the spectra in Fig. 9 is the significant resonantproduction of HClþ. Moreover, the intensity maximum of thecorresponding MSPES differs from that of Clþ whereas it is thesame as for CHþ3 , suggesting similar production mechanisms forthese two molecular fragments. The maximum KER for eachproduced fragment was also extracted from the shape of the TOFcoincidence peaks and its variation was followed as a function ofthe excitation energy as well as of the kinetic energy of the

associated Auger electron. For this purpose, the electron spectrumwas divided in four zones which are drawn on the spectra in Fig. 8.Zones II and III are centered on the 1D and 3P atomic lines whereaszones I and IV are free from any fragment decay contribution andare populated only by molecular decay events. The analysis of theKER variation as a function of the Auger electron energy is alsosuggesting [23] that the major contribution leading to Clþ

(constant KER between zones II and III) ion production is aneutral ultrafast dissociation process taking place in the core-excited state. At variance, the KER for Clþ in zones I and IV, as wellas the KER for the molecular fragments CHþ2 and CHþ3 for all zonesincreases when the Auger electron energy decreases. Such abehavior can be understood in a molecular framework where theadditional amount of internal energy stored in the dissociatingCH3Cl�þ parent ion is almost entirely converted into translationalkinetic energy of the fragments. In the case of HClþ, for which asimilar behavior is observed, the slight energy change rules out asingle scheme implying a collisional mechanism during theneutral dissociation event. Its main production channel shouldbe rather described as a stepwise process: an early Auger decayleaves the system in an excited CH3Cl�þ state far from equilibrium.The subsequent wavepacket evolution on the correspondingmultidimensional potential energy surface allows, to someextent, the formation of stable HClþ.

Finally we have also performed further investigation bymeasuring energy and angle correlations between the ejectedparticles. Fig. 10, which is extracted from Ref. [23], illustratesthe main result. It reports the energy distribution of the Augerelectrons emitted in coincidence with Clþ for two oppositepropagation directions (parallel and antiparallel) of the electronand Clþ ion. The polar electron emission angle is determined bythe fixed DTA geometry (54� with respect to the TOF spectrometeraxis) whereas the fragment emission angle is selected to be either0� (parallel) or 180� (antiparallel) with respect to the electronemission axis, by using appropriate time and positionmulticoincidence data filtering (for details see Miron et al. [23]).Assuming the axial recoil approximation valid, these selecteddirections refer to the molecular CH3-Cl axis of the ionizedmolecule. The decay spectrum corresponding to the parallelorientation of the two particles is shown with blue triangleswhereas the one corresponding to the antiparallel orientationis shown with red diamonds. The one corresponding to a

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Fig. 9. MSPES coincidence spectra of CH3Cl obtained with an overall resolution of

about 180 meV on the top of the resonance at hn ¼ 200:74 eV. The various

fragment ions in coincidence are indicated on the left part of the figure which is

extracted from Ref. [23].

Fig. 10. Auger electron energy distributions for electrons emitted in coincidence

with Clþ ion in a parallel (blue triangles), perpendicular (black solid line) or

antiparallel (red diamonds) direction with respect to the Clþ ion emission

direction. The sharp lines associated with the 1D and 3P final states present an

Auger-Doppler shift of 250 meV in good agreement with the 215 meV shift

estimated from a classical model [23].



perpendicular emission of the Auger electron with respect to Clþ

ion emission axis is shown with a black solid line. The blue andred components of the sharp atomic-like lines in Fig. 10 clearlypresent an Auger-Doppler shift which is estimated to about250 meV. This measured shift is in very good agreement with the215 meV Auger-Doppler shift that can be evaluated from aclassical model taking into account the previously determinedKER for Clþ. In contrast, no Auger-Doppler shift is observed forthe large molecular fragment CHþ3 , nor for HClþ in spite of arather complex structure of HClþ intensity distribution of theantiparallel component as discussed in Ref. [23]. This lastobservation reinforces the analysis leading to the proposedmechanism for the formation of HClþ. A remarkable feature ofthe presented approach is the fact that it can be applied torandomly oriented molecules and does not require any photonabsorption induced alignment.

We would like to stress at this point that by means of anelectron–ion multicoincidence technique combining angle andenergy measurements of all the involved particles it was possibleto fully characterize the complex competing decay mechanismstaking place in a polyatomic molecule during the natural lifetimeof the Cl2p�1 vacancy which is about 7 fs only. On a slightly longertime scale (roughly more than 7 fs), an ultrafast dissociationprocess occurs leading to a specific spectral signature: an Auger-Doppler splitting of Clþ atomic lines. Further improvement of theelectron energy resolution will allow, in the very near future, theexploration of the vibrational energy storage in the molecularfragments (the methyl group in the presented case) during theultrafast dissociation step, which has not been yet investigated.Such an improvement is expected indeed by the use of the newultra high-resolution soft X-ray beamline PLEIADES at SOLEILwhich will provide highly monochromatic beams with a focusingof a few tens of microns only, whereas one of the main limitingfactors of the electron energy resolution in EPICEA setup isthe spatial extension of the interaction region as discussed inSection 2.

5. Conclusions and perspectives

The present review tried to give the reader, in addition to abrief historical overview of the electron–ion coincidence experi-ments development, some detailed examples extracted from ourown research where the combination of high-resolution electronspectroscopy and of state-of-the-art multiparameter coincidencemeasurements with both energy and angle resolution for allparticles (Auger electrons and ions) allowed to characterizeextremely complex and intricate decay processes in core-excitedmolecules. With these selected examples we have tried to showthat when dealing with core excitation, reaching high resolutionboth in terms of excitation and decay analysis is a unique way toaccess ultrafast (fs) dynamics information inaccessible by meansof other tools. At the same time, we tried to underline boththe strengths and the encountered difficulties when using suchtechniques.

At this stage, we would like to draw reader’s attention on therelevant fact that while only one or two research groups across theworld were using the coincidence methods in the late eighties,basically all the groups in the field are employing at least one typeof coincidence spectroscopy for their investigations nowadays.As a consequence, the number of published studies based on theuse of this powerful technique amazingly increased in the last10 years and will surely follow the same type of growth in theincoming years. This increasing popularity is due to the highcontent in physical information of multiparticle coincidence dataand was permitted by the recent progress of data acquisition

devices, namely for accurate measurements of time delays, as wellas to the improvement of the spatial resolution and dead time forparticle PSDs. For instance, in the recent study by Schoffler et al.addressing the N1s core-hole localization question in molecularnitrogen [21] the use of an experimental setup based on high-resolution PSDs for the photoelectrons and the ions conducted toa high-enough resolution of the particle momenta allowing toextract the momentum for the undetected Auger electronsproduced after N1s core ionization only from the photoelectronand ion momenta and using the momentum conservation law.

On the one hand, in many cases the very important aspect isthe possibility of selecting with a high degree of accuracy thequantum state of the inner-shell excited system. Indeed, such anaccurate selection allows performing a vibrational state-to-stateanalysis of the decay dynamics provided the vibrational spacing ofthe core-excited state is sufficiently large as compared to the core-hole lifetime. However, in most realistic situations this highresolution in the excitation channel is not sufficient for a highquality analysis of the system dynamics. As a consequence it isimportant to try seeing what the future of these spectroscopiescan be, in the context of the continuously improving spectralresolution and flux of the X-ray sources, like from most recentthird generation SR facilities, and the wider availability of chargedparticle spectrometers equipped with high spatial resolutionPSDs. In our opinion, one of the key features of the excitingradiation which will provide in the future most significant roomfor improvement of the resolution and efficiency of the coin-cidence measurements is the spectral brightness of the futuresources. This is particularly true for the spectroscopic measure-ments based on dispersive devices for X-rays or charged particles,where spatial dispersion according to the energy occurs. Whenaiming for high resolution, the dispersion must be kept as high aspossible to the detriment of the counting rate in a given solidangle. One way of improvement of the measurement resolutionwith dispersive devices is the improvement of the spectralbrightness of the incoming radiation. Moreover, increasing theuseful number of photons in a reasonably small bandwidth forsample irradiation will simultaneously allow using more dilutedsamples as gas phase biological molecules, clusters or nanopar-ticles. Moreover, in some types of spectrometers based on bothdispersive devices or on TOF measurements there is a directrelationship between the resolution of the measurements and thesize of the interaction region, usually defined as the crossingbetween the sample gas and the X-ray beam. In such situations,the improvement of the spectral brightness will, once again, leadto the improvement of the resolution of the measurements.

New possibilities are opened nowadays by fourth generationX-ray sources for the investigation of inner-shell excited speciesby multiphoton processes. Indeed, the interaction of isolatedspecies with very high spectral brightness radiation available fromthe future XFEL or ERL machines could take place in a non-linearregime where several photons can be absorbed simultaneously topromote the system in a core-excited or a core-ionized state. Themuch higher photoionization rate as compared to the interactionwith X-rays issued from conventional SR beamlines will produce avery high amount of charged particles making impossible the useof coincidence techniques. Indeed, as discussed in Section 1, theconventional coincidence methods suffer from a strong limitationconsisting in the necessity of keeping the ionization rate understudy, as well as the overall background, low enough to avoidfortuitous coincidences. This condition is difficult to accommo-date in multiphoton studies on future X-ray sources since, forinstance, single-photon processes can produce a high backgroundwhile studying multiphoton ionization. In order to probe thedecay dynamics of inner-shell excited systems by multiphotonprocesses other methods must be used instead. One possibility to

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follow up is the covariance mapping, which was first described forthe investigation of the fragmentation dynamics of moleculesionized by intense conventional lasers [50]. Its use is notconstrained by the occurrence of one single ionization event perradiation shot and is thus well suited for this regime. This methodsimultaneously allows aiming for high-resolution state-to-statemeasurements since the optimization of the collection efficiencyis not needed so stringently and the use of high-resolutionhemispherical electron analyzers optimized for the electronenergy resolution becomes valuable.

Last but not least, a very important aspect to mention is theimprovement of the diluted samples sources. This recent progressallows nowadays studying the inner-shell decay processes of moreexotic samples like Van der Waals or metal clusters usingsupersonic beam expansion sources, isolated nanoparticlesusing aerosol sources or even liquids, including aqueous solutions,using liquid microjet sources. Such sources usually deliversamples with a rather broad size distribution and the coincidencetechniques are well adapted to be used as an a posteriori samplesize filtering. Due to these new possibilities and the effort made toimprove more and more the resolution of the inner-shell spectra,it will be possible not only to address more and moresophisticated physical situations for limited size species asmolecules, but also to efficiently investigate more and morecomplex samples like clusters, liquids, free and depositednanoparticles, which represent essential links in the chainconnecting the isolated atom to the solid.

Acknowledgments

The authors would like to warmly thank former co-workers,especially D. Ceolin, N. Leclercq and M. Simon, for theircontribution to the examples detailed along this review as wellas our close collaborators and friends from Germany, Japan,Sweden or USA for numerous exciting scientific discussions. Wewould also like to thank both our former host institute, theCommissariat a l’Energie Atomique (CEA), and former hostlaboratory Laboratoire pour l’Utilisation du Rayonnement Electro-magnetique (LURE) for giving us the opportunity to modestlycontribute to the development of this field for many years. Lastbut not least we would like to acknowledge the financial supportfrom our present funding agencies l’Agence Nationale de laRecherche (ANR) and the RTRA ‘‘Triangle de la Physique’’ whichare funding some of CM’s research programs.

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