Threshold photoelectron spectroscopy of BC13 and fragmentation of the valence electronic states of BCl, +, studied by coincidence spectroscopies

Holger Biehl," Kenneth J. Boyle," David M. Smith," Richard P. Tuckett,"9* Karl R. Yoxall," Keith Codling,b Paul A. Hatherlyb and Marek Stankiewicz' a School of Chemistry, University of Birmingham, Edgbaston, Birmingham, U K B15 2T T

Department of Physics, University of Reading, Whiteknights, Reading, U K RG6 2AF Department of Physics, Jagellonian University, 30-059 Krakow, Poland

The valence threshold photoelectron spectrum (TPES) of BCl, is reported for the first time. Threshold photoelectron-photoion coincidence (TPEPICO), photoion-fluorescence coincidence (PIFCO) and threshold photoelectron-fluorescence coincidence (TPEFCO) spectroscopies have also been used to measure for the first time, state selectively, the decay pathways of the valence states of BCl,+ in the range 11-19 eV. Vacuum UV radiation from a synchrotron source dispersed by a 5 m normal-incidence McPherson monochromator jonises the parent molecule, and electrons and ions are detected by threshold electron analysis and time-of-flight mass spectrometry, respectively. Undispersed fluorescence from the interaction region can also be detected, allowing the three different types of coincidence experiment to be performed. There are major differences in the intensity distribution of the different bands in the TPES of BCl, compared with its He I photoelectron spectrum which we attribute to autoionisation. TPEPICO spectra are recorded continuously as a function of photon energy, allowing yields of the fragment ions to be obtained. Over the range 11-19 eV, only the ions BCl,+ and BCl,+ are observed. The ground electronic state of BC13+ is stable with respect to dissociation to BC12+, whereas the five excited electronic states fragment to BC12+ + Cl. Kinetic-energy releases into BCl,+ + Cl have been measured at the Franck-Condon maxima of the c2AI; , n2E and E2Al excited states of BC13+ with good time resolution. PIFCO and TPEFCO spectra are recorded at fixed photon energies. The former experiment can yield the fate of the lower electronic state of the parent ion to which fluorescence occurs. The latter experiment can yield the lifetime of the fluorescing state. Radiative decay is confirmed to occur from the fourth excited state of BC13'(d ,E') at 15.5 eV, and the lifetime of this state is measured to be 5.5 & 0.5 ns. The absence of parent ion signal in the energy-scanning TPEPICO spectrum through the ?, state allows us to put an upper limit on the fluorescence quantum yield of this state of ca. 0.04.

Coincidence experiments offer powerful techniques to deter- mine the decay dynamics of individual vibronic states of posi- tively charged molecular ions in the gas phase.' The use of tunable vacuum UV (VUV) radiation from a synchrotron source allows the threshold photoelectron-photoion coin- cidence (TPEPICO) technique to be applied to dissociating electronic states of polyatomic ions. Here, zero kinetic energy, threshold, electrons are detected in delayed coincidence with parent or fragment ions which have been mass-selected in a time-of-flight (TOF) mass spectrometer. The fragment ion channels that are observed and the kinetic energy (Ek) released into these channels are important guides to statistical or dynamical behaviour in the parent ion. Less well known but just as informative are two coincidence techniques that can be used if the electronic state of the ion under study is metastable and, to a lesser or greater extent, decays radiatively by photon emission. These are the photo- ion-fluorescence coincidence (PIFCO) and photo-elec- tron-fluorescence coincidence (PEFCO) techniques. If a threshold photoelectron is detected, the latter technique has the acronym TPEFCO. PIFCO spectroscopy can determine whether an excited electronic state of a molecular ion decays radiatively, and by observing which ion is detected in coin- cidence with the photon, the fate of the lower electronic state to which fluorescence occurs can be determined. In theory, it is also possible to measure the fluorescence quantum yield, GF[= k,/(k, + k,,)J, and lifetime, t[= (k, + kJ '1 of the emit- ting state, k, and k,, being unimolecular radiative and non- radiative decay rates, respectively, where these quantities are averaged over the Franck-Condon populations of the vibra- tional levels of the emitting state. The TPEFCO technique can also be used to measure GF and z, but here the resolution is

determined by the photoionising beam and the threshold elec- tron analyser. In this way, vibrational state selectivity can often be obtained. Changes of (GF)" and zv with vibrational level (0) can often be attributed to a change in k,,, i.e. the onset of a competing non-radiative channel. In practice, in our experiments we use the PIFCO technique to determine whether the excited state of the ion under study decays radi- atively or not and to observe which ion is in coincidence with the photon, and the TPEFCO technique to measure state- selected lifetimes. In this paper, we apply all these three coin- cidence techniques to study the unimolecular decay of the ground and first five excited valence electronic states of BC13+. We also report the first observation of a medium- resolution (ca. 20 meV) threshold photoelectron spectrum (TPES) of BCl, .

There have now been a significant number of different spec- troscopic experiments in the VUV leading to Rydberg and ionic states of BF,. These include photoab~orption,~*~ He I and He I1 threshold photoelectron7 and photoionisation mass spectrometric' studies. Furthermore, using polarised VUV radiation from a synchrotron source, photoelectron angular distributions and partial ionisation cross-sections have been measured? and fragmentation of the valence states of BF,' which lie below 21.2 eV has been studied by He I photoelectron-photoion coincidence spectro- copy.'^ The number of comparable studies on the next member of the family of the Group 3 halides, BCl,, is scarce. This is most surprising since, unlike BF, which has few indus- trial applications, BCl, is increasingly being used as an etching gas in the plasma dry etching of integrated silicon cir- cuits." Hence a knowledge and understanding of the spec- troscopy and fragmentation of BCl, under VUV conditions

J . Chem. SOC., Faraday Trans., 1996,92(2), 185-192 185

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(corresponding to low-energy electrons of 5-25 eV which is the main excitation source in an RF plasma etching machine) is important; the results could have diagnostic applications in the monitoring of the etching of integrated circuits by BCl,. The He I photoelectron spectrum of BCl, was reported in 1971 by Bassett and Lloyd4 and a comparable spectrum obtained one year later by King et ~1 . ; " however, the data have not been improved since then, and there have been no measurements made under threshold electron conditions, where effects due to autoionisation are most apparent. Unlike BF, , partial ionisation cross-sections into the valence states of BCl,' have not been measured. Appearance energies for BCl,+, BCl,+ and BCI+ have been observed by photoionisa- tion mass spectrometry' (that for BCl,+ obviously corre- sponding to the adiabatic ionisation energy (Ei) of the ground electronic state of BCI,+), but there have been no results of state-selected coincidence experiments on BCl, + reported.

Using fluorescence excitation ~pec t ro~copy , '~~ ' a number of Rydberg series of BCl, converging on difference valence electronic states of BCl, + have recently been observed. Assignment of the Rydberg peaks and determination of quantum defects needs accurate values for the Eis of the valence states of BCl, +. We therefore decided to re-investigate the photoelectron spectrum of BCl,; in our experiment this is recorded under threshold electron conditions. Furthermore, radiative decay was observed from one excited valence state of BC13f:15*16 the b 2 E ' state at 15.5 eV. Since dissociation of these states by loss of a chlorine atom to BC12+ is energeti- cally allowed (see later), this is a surprising phenomenon in a four-atom polyatomic molecule with three heavy atoms, because non-radiative processes are expected to dominate. It is therefore desirable to investigate some of the dynamic properties of this process in more detail (e.g. to ascertain whether the radiative decay channel is a major or a minor constituent) using fluorescence coincidence techniques. We have also measured the decay routes of all the valence states of BC13+ by TPEPICO spectroscopy.

Experimental The apparatus for performing TPEPICO experiments has been described in detail el~ewhere, '~ and can be attached to any of the VUV beamlines at the UK synchrotron source at Daresbury. Although the apparatus is optimised for this par- ticular coincidence experiment, the addition of collection optics and a photomultiplier tube for undispersed fluorescence detection means that PIFCO and TPEFCO experiments can also be performed.'* In these experiments on BC13+, we use the high-resolution 5 m McPherson normal-incidence mono- chromator at Daresbury. Its range is ca. 120-40 nm (10-30 eV), it has a resolution in the scanning mode down to 0.01 nm and a quoted flux of ca. 5 x 10' ' photons s- ' per 0.1 YO band- width per 100 mA beam current. The grating is blazed at 55 nm with second-order radiation contributing ca. 20% for wavelengths greater than 80 nm. The monochromatised radi- ation is admitted to the interaction region through a capillary, and the photon flux is monitored through a Pyrex window coated with sodium salicylate by a photomultiplier tube, allowing flux normalisation of the data.

The threshold-electron'analyser incorporates a cylindrical electrostatic lens designed with large chromatic aberrations and a 127" post-analyser which rejects the energetic electrons on axis. Even with an extraction field as large as 20 V cm-', both simulations' and experiment^'^ on the photoionisation of argon into the 2P3/2 and 2P1/2 spin-orbit ionic states show that the analyser has both high collection efficiency (ca. 30% for zero-energy electrons) and high resolution (ca. 4 meV half- width at half maximum). The ion detector consists of a two- state acceleration region configured to satisfy the space

focussing condition" and a field-free region of length 186 mm. This configuration allows sufficient TOF resolution for kinetic-energy releases from a dissociative ionisation process to be measured, whilst still maintaining a high collection effi- ciency. The electron and ion signals are collected by a chan- neltron electron multiplier (Phillips X8 18BL) and a pair of microchannel plates (Galileo MCP-40/32), respectively. Fluo- rescence from the interaction region orthogonal to the elec- tron and ion flight paths is collected by anfll biconvex quartz lens of focal length 50 mm and detected by a photon-counting, red-sensitive photomultiplier tube (Mullard X P 2254 QB, range 200-750 nm) cooled to 248 K. Anfll concave mirror of focal length 75 mm behind the interaction region enhances photon collection efficiency. The raw pulses from the electron, ion and photon detectors pass to discriminator and pulse- shaping circuits (which for the electron and ion signals are based on the LeCroy HVL 100 discriminator), and any two of the signals are fed into a time-to-digital converter (TDC, LeCroy 4208) configured in the multi-hit mode. One provides the start pulses, the other the stop pulses; hence the two signals from the same ionisation event are detected in delayed coincidence. Interchange between the three different forms of coincidence experiment is straightforward. The scanning of the McPherson monochromator and the collection of the coin- cidence data are controlled by two IBM personal computers (PCs) which communicate through CAMAC-based elec- tronics. Note that all the coincidence experiments operate in the multi-bunch quasi-cw mode of the synchrotron source (160 bunches of electrons, width 0.2 ns, 2 ns between bunches in the storage ring), and pulsed extraction using the single- bunch mode is not used for either electron or ion detection. In this way we utilise the limited time we are allocated on the synchrotron source to its optimum. Furthermore, because the flight time of the photoelectron which provides the start signals is accurate to & 13 ns, this mode of operation places very few limitations on the resolution of the TOF spectra that are measured by the TPEPICO technique or on the lifetime of an emitting ionic state that can be measured by the TPEFCO technique.

We stress the versatility of the apparatus which can perform the following experiments. First, a TPES of the sample gas can be recorded by measuring the threshold photoelectron (TPE) signal as a function of photon energy. This experiment is also used to calibrate the monochromator by recording the TPES of 0, in low vibrational levels of the XzII, ionic ground state around 12 eV, in the two spin-orbit states of Ar' around 16 eV (see above), and in the BZZ,- and c4Cu- states of 0,' around 20 and 25 eV, respectively. Values of these calibration energies are taken from Merkt et u1." and Baltzer" for Ar and 0,, respectively. We estimate that peak positions of TPE spectra in BCl, can be measured to at least one-third of the monochromator resolution, since the resolution of all such spectra is limited by that of the mono- chromator, not that of the electron analyser. Secondly, all three kinds of coincidence experiment (TPEPICO, PIFCO and TPEFCO) can be recorded at a fixed photon energy. In a TPEPICO spectrum of this kind, TOF data accumulate at as high a time resolution of the TDC as the signal level will permit in order to obtain maximum resolution in the kinetic energy release distribution (KERD)23 (see later). In a PIFCO spectrum of this kind, coincidence count rates are much lower, and time resolution is often sacrificed in order to obtain a spectrum in a reasonable period of time. As described above, we use this experiment solely to determine whether an excited state of an ion decays radiatively, and if so which ion is observed in coincidence with the photon; in neither case is high time resolution required. In a TPEFCO experiment of this kind, the time resolution used is determined by the life- time of the emitting state, except that the minimum resolution (1 ns) of the TDC can impose a limitation on the quality of

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the data for lifetimes less than ca. 10 ns (see later). Thirdly, a TPEPICO spectrum can be recorded continuously as a func- tion of photon energy (the energy-scanning mode). In this mode all the ions produced in the TPEPICO spectrum are usually recorded and the time resolution of the TDC is corre- spondingly degraded. Data accumulate as a three-dimensional histogram of photon energy us. ion TOF us. coincidences, the last variable being displayed as colour on the data-collection PC. In this paper we display the data by taking cross-sectional cuts through the histogram, the most useful representation being the variation of a parent- or fragment-ion yield with photon energy. In these experiments the total threshold elec- tron count is also recorded by the PC that controls the scan- ning of the monochromator, so the TPES can be displayed simultaneously with the ion yields. Although in theory it is possible, we do not run either of the fluorescence coincidence experiments in this energy-scanning mode.

BCl, gas (Matheson) was used from a lecture-bottle cylin- der without further purification, although its temperature was stabilised by immersion in a large container of water. It is well known that BCl, can cause severe poisoning of microchannel plate detectors. Great care was taken to eliminate all traces of water vapour in the gas line because BCl, hydrolyses rapidly to HC1, and it is believed that it is the HCl, rather than the BCl, per se, which causes the Contamination of the plates. The presence or absence of HCl in our apparatus is easily deter- mined by TOF mass spectrometry. The base pressure of the apparatus is typically 5 x lop8 Torr, while the background pressure during an experiment is between 5 x and 1 x lOP4Torr.

Energetics and dissociation channels of BC13 +

The valence molecular orbitals of BCl, arise from the overlap of three C1 3s and 3p atomic orbitals with the central B 2s and 2p orbitals. In D,, symmetry, B 2s and 2p transform as a; and (a: + e') and three C1 3s and 3p orbitals as (a; + e') and (a; + a; + a'; + 2e' + e"), respectively. It is useful to divide the

C1 3p orbitals into three 3pa orbitals (a; + e') pointing along the B-Cl bonds and six 3pn orbitals (a; + a; + e' + e") lying orthogonal to the bonds. The electron configuration of the six

Table 1 Energetics of dissociation channels of BC1, ' adiabatic (vertical)

neutral/ dissociation dissociation ionisation parent ion channel energy/eV energy/eV"

BCI+ + C1 + C1 G 18.37' BCl3+ ,A', 17.70 (17.70)c

BCl, + C1+ 17.56 BCl + C1,+ 17.13 (BCI,+)* + C1 1 6.75d BCI' + C1, < 15.89

f, 'E' 15.28 (15.51)e c ,A'' 14.23 ( 14.40)f €3 2 E 2 (1 2.75)

A ,EI, 12.23 (12.65) ft 'A; 11.60 (11.82)

BCI,+ + Cl < 12.308

BC1, 8 'A; 0

" This work. ' From photoionisation mass ~pectrometry.'~ Progression in v1 in the threshold photoelectron spectrum gives

v1 = 403 k 16 cm-'. A shoulder between 17.4 and 17.7 eV to low energy of the v1 = 0 peak is, as yet, unassigned. Observed threshold for fluorescence band from 290-370 nm with a vibrational spacing of 650 _+ 30 c m - l . 1 6 Spin-orbit splitting of 0.20 f 0.01 eV. ' Progression in v1 in the threshold photoelectron spectrum gives v1 = 403 k 16 cm-'. This work, and from photoionisation mass spectrometry.'

highest-occupied outer valence molecular orbitals of BCl, is (2a;)2(2e')4( 1a;)2(3e')4(le'')4( la;)2, where the numbering scheme does not include the B 1s and C1 Is, 2s, 2p core orbitals, and the ordering of the orbitals has been confirmed by ab initio calculation^.^^ All six orbitals are accessible with He I (21.22 eV) radiation, and the spectra of Bassett and Lloyd4 and King et a1." are still the only published He I photoelectron spectra of this simple molecule to date. Vibra- tional structure was observed in two bands; the fourth, corre- sponding to ionisation to BCl,+ c 2A'; (a long and extensive progression in vl), and the sixth, corresponding to ionisation to BC13+ 2A; (a much shorter progression in v1 with u1 = 0 being the strongest peak). [The vibrational distributions are slightly surprising because both semi-empirical MNDO calculations2 and the ab initio calculation^^^ predict that the la: orbital is mainly non-bonding with limited n: bonding between B 2p, and C1 3pn, whereas 2a; is a o-bonding orbital between B 2s or 2px,y with Cl 3s and 3po. Hence one might have expected the (2a;)-' photoelectron band to show a more extensive vibrational progression than that in the (la';)- band.] Spin-orbit splitting was observed in the 2, *E' band (0.18 eV), but at the resolution of the experiment (ca. 0.03 eV) could not be resolved in the B 2E' band. As expected26 no splitting was observed in the A 2E" band. Unlike a recent high-resolution He I photoelectron study of BF, ,5 there was no evidence of Jahn-Teller or pseudo Jahn-Teller effects in any of the valence states of BC13+. Bassett and Lloyd quote vertical Eis for the first six bands in the photoelectron spec- trum of 11.73 (l), 12.39 (2), 12.66 (l), 14.42 (l), 15.54 (2) and 17.74 ( 5 ) eV. Our new values are shown in the thermochemical table describing BCl, and BC13+ (Table l), and in general there is excellent agreement. Except for the A 'E" state, the discrepancy between our data and that of Bassett and Lloyd is less than 0.1 eV. For the A state, our value, 12.65 f 0.01 eV, is 0.26 eV greater than that quoted by Bassett and Lloyd, well outside the error limits of the two measurements. The reason for this discrepancy remains unclear.

Since this paper will be concerned with fragmentation of the different electronic states of BCl, +, the thermochemistry of the possible fragment ions needs to be known, and Table 1 also shows the dissociation energies of the neutral and ionic fragments from BCl, and BC13+. The neutral channels are cal- culated from the 0 K heats of formation of BCl, ( n = 1-3) and Cl.27 The ion channels involving C12+ and C1+ are calculated using literature values for their Ei s.28,29 The E i s of BC1, and BCl have not been measured directly by photoelectron spec- troscopy, and channels involving these ions use thermodyna- mic onsets of these ions from BCl, as obtained from photoionisation mass spectrometry (PIMS). PIMS will always give an upper limit to the (indirectly obtained) ionis- ation energy of the neutral fragments,,' hence if these ions turn on at their thermochemical energy, we obtain values for the E i s of BC1, and BCl of 7.71 and 10.26 eV, respectively. The true adiabatic Eis for these fragments may be lower. This is especially true for BCl,, where ionisation to the ground state of BC1,' will almost certainly involve a substantial change in bond angle (Walsh's rules3' predict this to be a 17 to 16 valence electron, bent to linear transition). Under these circumstances, accurate measurement of the adiabatic E, is notoriously difficult. Perhaps the most important point to note from Table 1 is that the energy of the BC12+ + C1 channel lies above the Franck-Condon zone of the BCl,' % state, but dissociation to these products is energetically open to the five lowest excited states of BC13+. If, as expected for a polyatomic ion, internal conversion is the dominant non- radiative decay mechanism for these states, then a statistical dissociation from the ground-state potential-energy surface of BC13+ is to be expected for these valence states of the ion. Radiative decay from any of these excited states of BCl,+ would be a surprising, and non-statistical, phenomenon.

J . Chem. SOC., Faraday Trans., 1996, Vol. 92 187

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Results Threshold photoelectron spectrum of BCI, in the range 11-19 eV

The TPES of BCl, in the range 11-19 eV recorded at a resolution of 0.1 nm is shown in Fig. 1. This range encom- passes the ground and the excited valence states of BCl,’. The fourth and sixth bands (ionisation to e 2A‘; and E ’A;) show vibrational structure in the v1 (a;) B-Cl symmetric stretching mode. Adiabatic and vertical Eis are given in Table 1, and we estimate an uncertainty of better than 0.01 eV in the vertical Ei values. The resolution of these spectra is limited by the monochromator, and they do not show the full capability of the threshold electron detector. Following Bassett and Lloyd4 and the ab initio calculation^,^^ we assign the two par- tially resolved peaks at 12.65 and 12.75 eV to ionisation to the A 2E’’ and B 2E’ states, although we note that King et a l l 2 reverse the order of the second and third photoelectron bands and assign them to A ’E’ and B 2E”, respectively. An alterna- tive assignment of these two peaks to a second-order spin- orbit splitting in the A 2E” state and the broad band between 13 and 14 eV to the B 2E’ state seems unlikely for the follow- ing reasons. First, the ab initio calculation^^^ predict the separation between the A and B states to be only 0.14 eV, not ca. 1 eV, in good agreement with our preferred assignment. Secondly, the assignment of the (3e’)-’ns (n 2 3) Rydberg state of BCI, converging on the B 2E’ state of the ion observed in fluorescence excitation experiment^'^ gives reasonable values for the quantum defects of the Rydberg states if an ion- isation threshold of ca. 12.7 eV is used. Thirdly, the first-order spin-orbit splitting in a 2E” state of a D,, molecule is predict- ed to be zero,25 so if our alternative assignment is correct the observed splitting of 0.10 eV would have to be a second-order splitting. Given its magnitude, this seems unlikely. Our main reason for rejecting the assignment of the second and third photoelectron bands to A 2E’ and B 2E’’ as proposed by King et is that dipole selection rules do not allow for a (1e”)-’ns Rydberg series, and yet a series is ob~erved , ’~ con- verging on an ionisation threshold of ca. 12.7 eV with quantum defects appropriate to an s series.

The major differences of this TPES from the He I PES reported by Bassett and Lloyd4 are: (a) a reduced relative intensity of the first band (ionisation to % 2A’,) in the thresh- old spectrum; (b) an enhanced intensity of the second band (ionisation to A 2E”) compared with the third band (ionisation to B 2E’) in the threshold spectrum; (c) a substantial high- energy shoulder to the third band between 13 and 14 eV which is not present in the He I spectrum (see below); (d) a clearly resolved spin-orbit splitting (0.20 & 0.01 eV) in the fifth band (ionisation to b 2E’); and (e) a low-energy shoulder to the sixth band (ionisation to E 2A;) between 17.4 and 17.7

eV, which again is absent in the He I spectrum. Some of these effects in the threshold spectrum may be due to autoionisation which is known to produce non-Franck-Condon vibrational distributions, owing to autoionisation from high Rydberg states of the neutral molecule competing with direct Franck- Condon ionisation. It is generally assumed that this effect is negligible for He I photoelectron spectroscopy at 21.22 eV. Thus, the most probable assignment for the shoulder between 13 and 14 eV is to unresolved Rydberg states of BCl, converg- ing on a higher-lying ionic state, of which the e ’A; state seems the most likely.

The sign of the spin-orbit splitting in the a 2E’ state of BCl,’ (which splits into E5/2 and E3/2 components of equal intensity) cannot be inferred from the TPES. However, the sign is likely to be dominated by the effect of the three chlo- rine atoms which have the electron configuration ( 3 ~ ) ~ , and hence make a negative atomic contribution to the molecular splitting. Neither the sign nor the magnitude of the splitting in the B 2E’ state can be inferred from the spectrum. The vibra- tional structure in the singly degenerate 2A’; and 2A; photoelectron bands can be due only to the totally symmetric v 1 mode of a; symmetry. Its value in the neutral molecule is 471 cm-’, 27 and in both these two ionic states we obtain v1 = 0.050 & 0.002 eV or 403 f 16 cm-’. These values should be compared with values of 444 24 and 444 & 48 cm-’ quoted by Bassett and Lloyd. Using the methods developed by Eland26 and Dujardin and Leach,j2 from these values and the intensity distribution pattern, we can infer a change in the B-Cl bond distance upon ionisation to the e ’A’; and f? 2Al states. For a long vibrational progression, Eland has shown that in the semi-classical approximation

with Ar in A, Ei in eV, p in atomic mass units (u) and v’ in cm-’. With p = 3 x 35.5 = 106.5 u and v’ = 403 cm-’, we obtain I Arl = 0.073 A for ionisation to the e ’A; state of BCl,+, where Ei,ve,., - Ei,adb = 0.17 eV. For a shorter vibra- tional progression in which the transition to v’ = 0 is the most intense, Dujardin and Leach3’ have shown that the ratio of the intensities of the bands to v’ = 1 and v’ = 0 can be related to Ar,

where the constants have the same units as above. For ionis- ation to the ’A; state of BCIJ+, we measure I, . = ‘ / I v , =o to be ca. 0.5 (Fig. 2). With v’ = 403 cm-’, v” = 471 cm-’ and p = 106.5 u, we obtain I Ar I = 0.026 A. In neither case can the

14000 , I 1 m v , = o I 2

1 1 12 13 14 15 16 17 18 19 ’

energ y/eV

Fig. 1 Threshold photoelectron spectrum of BCI, between 11 and 19 eV recorded at a photon resolution of 0.1 nm or ca. 0.018 eV at 15 eV. The spectrum is flux normalised, and electron count rates vary between 150 s - ’ on the A ’E” ionic state maximum to 50 s - ’ on the E ’A; state maximum.

17.30 17.50 17 70 17.90 18.10 18.30 energ y/eV

Fig. 2 Threshold photoelectron spectrum (TPES) and coincidence yield of BC12+ from BCl, between 17.3 and 18.3 eV. The photon resolution is 0.1 nm. Vibrational structure in the v,(B--Cl) symmetric stretching mode is assigned, giving v1 = 0.050 _+ 0.002 eV. At 17.70 eV, count rates are: threshold electrons 10 s-’, ions 17000 s - l , and true coincidences 0.3 s- ’.

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r I sign of Ar be inferred from the Franck-Condon distributions, but given the slightly bonding nature of both the la; and 2a; orbital^,^^,^' we assume that, in both cases, Ar is positive.

Fragmentation of the valence states of BCI,' by scanning TPEPICO spectroscopy

Four TPEPICO spectra in the energy-scanning mode were recorded from 11.4 to 14.2 eV (the Franck-Condon region of the z, A and B states of BCl,'), from 14.1 to 14.9 eV (the state), from 15.0 to 16.4 eV (the i) state), and from 17.3 to 18.3 eV (the E state). A constant monochromator resolution of 0.1 nm (corresponding to ca. 10 and 25 meV at 11 and 18 eV, respectively) was used, and for each spectrum a step size between 4 and 10 meV was chosen so that there were at least three incremental steps within the optical resolution. Over the energy range 11.4-18.3 eV, BCIJ+ and BC1,' are the only ions observed, and their yields are shown in Fig. 2-5, together with the TPE signal which is recorded simultaneously in these energy-scanning coincidence spectra. The thresholds for pro- duction of BC13+ and BCl,' are measured to be 11.60 * 0.02 and 12.30 & 0.02 eV, respectively, in exact agreement with the adiabatic Ei of the state of the parent ion (Table 1) and that from PIMS,',, respectively. BCl+ is not measured over this energy range, in agreement with the observation from PIMS that its threshold is as high as 18.37 eV.I3

Over the Franck-Condon region of the ground state of BC13+, the parent ion yield matches the threshold photoelec- tron signal for photon energies up to the appearance energy of BC12+, then decreases to zero as the BC12+ signal increases in intensity. For energies between 12.3 and 14.2 eV, the BC1,'

11.4 11.8 12.2 12.6 13.0 13.4 13.8 14.2 energyIeV

Fig. 3 Threshold photoelectron spectrum (TPES) and coincidence yields of BCl,+ and BCl,' from BCI, between 11.4 and 14.2 eV. The photon resolution is 0.1 nm. At 12.75 eV, count rates are: threshold electrons 100 s-', ions 10000 s - l , and true coincidences (i.e. with the false coincidences subtracted) 2 s - '.

1 1 1 1 1 1 1 1 V , = O I 2 3 4 5 6 7

14.1 14.3 14.4 14.5 14.6 14.7 14.8 14.9 energyIeV

Fig. 4 Threshold photoelectron spectrum (TPES) and coincidence yield of 3C1,+ from BCI, between 14.1 and 14.9 eV. The photon resolution is 0.1 nm. Vibrational structure in the v1 (B-Cl) symmetric stretching mode is assigned, giving v1 = 0.050 +_ 0.002 eV. At 14.40 eV, count rates are: threshold electrons 5 s - ' , ions 13000 s-', and true coincidences 0.1 s- l.

- TPES -1 - BCI;

15.0 15.2 15.4 15.6 15.8 16.0 16.2 16.4 energyleV

Fig. 5 Threshold photoelectron spectrum (TPES) and coincidence yield of BCI,+ from BCI, between 15.0 and 16.4 eV. The photon resolution is 0.1 nm. At 15.51 eV, count rates are: threshold electrons 16 s - l , ions 16000 s-', and true coincidences 0.6 s-'. Note that BCI,+ is not observed over this photon range. If it is present, we estimate that the ratio of the BCl,+ to BCl,+ signal over the Franck- Condon region of the b 2E' state of BC13+ (15.3-16.1 eV) is less than 1 : 30 (see text).

ion yield matches the threshold photoelectron signal through the Franck-Condon regions of the a and B states of BC13+ (Fig. 3). These results confirm what is to be expected from the thermodynamics of the system, that is, the ground state of BCl,' is bound since its adiabatic and vertical Eis (11.6 and 11.8 eV, respectively) lie below the BC12+ + C1 dissociation energy (12.30 eV). However, the majority of the Franck- Condon region of the A state of BCl,' (and certainly all of the B state) lies above this dissociation energy, and probably following rapid internal conversion into high vibrational levels of the ground state, these states dissociate from the ground-state potential-energy surface in a statistical manner to BC12+ + C1. At the resolution of our threshold photoelec- tron spectrum, we see no evidence for non-adiabatic inter- actions between the x, A and B states of BC13+ (as are present in BF,' '), but any such effects, if present, will promote efi- cient non-radiative decay of the two excited states.

Fragmentation of the z1 state of BCl,' between 14.1 and 14.9 eV also leads to exclusive production of BClz+, and the progression in v1 of the parent ion in the TPES is also observed in the BCl,+ yield (Fig. 4). Since the yield of the daughter ion has exactly the same shape and structure as the Franck-Condon envelope of the BC13+ e state, and because radiative decay of the state has not been d e t e ~ t e d ' ~ , ' ~ [we believe the report of h a t e emission in ref. 15(a) to be erroneous], we conclude that this state may also decay non- radiatively by internal conversion into lower-lying electronic states from which dissociation to BCl, + is observed. However, the value for the kinetic energy released into translational energy of products (see below) suggests that the c state shows isolated behaviour, and it may dissociate directly to products. Energetically, no other fragment ions are open to this elec- tronic state of BCI, +.

Fragmentation of the b state of BCl,+ between 15.0 and 16.4 eV also leads to exclusive production of BC12+ (Fig. 5), with the daughter-ion yield matching the form and structure of the TPE signal. There is no evidence for parent-ion pro- duction. This is a surprising result for the following reason. Radiative decay of this state of BC13+ has been observed both by undisper~ed'~.'' and by dispersed fluorescence' '* l6 spec- troscopy. Using the vertical Ei data given in Table 1, the band centres of optically allowed transitions in BCl, + are calcu- lated to be 336, 434 and 449 nm for &x, &A and D-g, respectively. Although the spectra of Lee et al. [Fig. 6 of ref. 15(a)] imply that the &B transition has the largest electronic transition moment (which would result indirectly in the pro- duction of BC12+ following photon de-excitation of the BCl,+

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b state) and that the &A transition is very weak, the inte- grated intensity of the i%g band is still ca. 30% of that of &A/& and this decay route should result in the presezce of a parent-ion signal in the TPEPICO spectrum of the D state. The fluorescence quantum yield of the state of BC13+ cannot be measured easily in any of these forms of VUV fluo- rescence excitation spectroscopy, but a lifetime as short as 5.5 ns (see later) would suggest that a fast non-radiative process is in competition with fluorescence. The absence of a parent-ion signal in the TPEPICO spectrum therefore leads us to con- clude that djF is very small, and indeed a semi-quantitative estimate (see later) gives an upper limit for @, of 0.04. That is, radiative decay is a minor channel, and the predominant decay mechanism for this state is a non-radiative process. This route is presumably internal conversion into the state, leading ultimately to dissociation from either an excited or the ground-state potential-energy surface and production of BCl,'.

Fragmentation of the E state of BCI3+ between 17.1 and 18.0 eV also leads to exclusive production of BCl,+ (Fig. 2). We note that the shoulder to low energy of the adiabatic Ei peak at 17.7 eV in the TPES is also observed in the BC12+ TPEPICO ion yield, and that the vibrational structure in v1 in the TPES is mirrored in the daughter-ion yield. The absence of parent-ion signal is now not unexpected, because the only route that could lead to production of BC13+ is radiative decay, and fluorescence e ~ c i t a t i o n ' ~ - ' ~ and fluorescence coin- cidence experiments (see @er) have shown conclusively that radiative decay from the E state does not occur. Przsumably the BCl2+ ion arises via internal conversion of the E state of BC13+ into high vibrational levels of either the excited states or the ground state, followed by dissociation to this daughter ion.

Kinetic-energy releases from fragmentation of the valence states of BCl,'

TPEPICO spectra at fixed photon energies were measured at the Franck-Condon maxima of the three highest valence excited electronic states of BCl3+(C *A';. , ?> 2E' and E 'A;) which dissociate to BC12+ + C1. In all cases, a time resolution of 8 ns per channel was used, and as an example Fig. 6 shows the broadening of the BC12+ peak at a photon energy of 15.51 eV, the Franck-Condon maximum of the BC13+ state. Detailed KERDs, and hence mean energy releases, may be derived from an analysis of such TOF peaks.,, The method used is to compute a set of TOF peaks (each with a discrete energy release EJ, and fit this basis set to the experimental peak using a linear regression technique. The discrete energies are calculated by E, =(2n - 1)2AE, where n = 1, 2, 3 etc. and AE (the minimum release when n = 1) depends primarily on the statistical quality of the data. In practice, only reliable values can be obtained for the mean E , release. The results are

200 , l

15 15.5 16 16.5 17 time of flight@

Fig. 6 Coincidence TOF spectrum of BC12+ from BCI? photoionised at 15.51 eV into the f, 'E' state of the parent ion. The time resolution is 8 ns per channel, and the accumulation time is ca. 2 h.

Table 2 of the valence states of BC13+

Mean translational E, releases, ( E L ) , , from fragmentation

daughter ion parent ion state energy/eV (E,),/eV"

BCl,+ BCl,+ c 'A" 14.40 0.73 _+ 0.17 BC12+ BCl, + f, 'E3 15.5 1 0.75 f 0.10 BCI2+ BC13+ f? 2A; 17.70 0.87 f 0.14

" The analysis assumes that each C1 atom is a single isotope with a mass of 35.5 u, and the B atom is also a single isotope with a mass of 10.8 u.

shown in Table 2, and in each case the mean release is rela- tively insensitive to the form of the distribution. The main reason for this insensitivity is that the translational tem- perature T of molecules along the axis of the TPEPICO apparatus is 300 K, there being little directionality as the mol- ecules emerge from the inlet needle.I7 With k, T as high as 0.025 eV, this thermal effect broadens the TOF peak for each value of E , . A secondary reason is that the kinematics of the dissociation are unfavourable; for the process BCl, + -+ BCl, +

+ C1, the daughter ion has over 70% of the mass of the parent ion.

If dissociation of these excited states of BCl, + occurs from the ground-state potential-energy surface following rapid internal conversion, these experimental results can be com- pared with the mean E k release to be expected for a statistical dissociation of BCI, + to form dissociation products BCl, +

+ Cl.33 This phase-space theory allows for conservation of energy and angular momentum, and assumes that the excess energy of the parent ion is large enough that any activation or centrifugal barrier in the exit channel to dissociation products can be neglected. Under these conditions, Klots3, has shown that there is an analytical expression relating the mean translational-energy release, (EL), , and the excess available energy above threshold, Eavai,:

Eavail is given by the photon energy minus the energy of the BC12+ + C1 dissociation channel, r is the number of rotational degrees of freedom in the products, and vi are the vibrational frequencies of the products. For this particular dissociation process in which excess energies are greater than 1 eV (Table 2), and for a long-range ion-induced dipole (Langevin) inter- action where centrifugal barriers are the approx- imations of Klots seem appropriate. Assuming that the ground state of BC12+ is linear (r = 2) and using the same vibrational frequencies for BCl, + as for the isoelectronic neutral molecule CS, (v i = 0.081, 0.049, 0.049 and 0.190 eV),,' the excess available energy is calculated to be 3.8 f 1.0, 3.9 0.5 and 4.6 f 0.8 eV for (&)f values of 0.73 f 0.17, 0.75 0.14 eV (the values measured for disso- ciation of BCl,' c, fi and E, respectively). These results for Eavail are relatively insensitive to the chosen values of the BCl, + vibrational frequencies. Assuming no activation barrier and hence a threshold for BC12+ production of 12.30 eV, the experimental values of Eavail are 2.1, 3.2 and 5.5 eV. Thus in the case of dissociation of the z' and ?> states of BC13+ to BCl,+ + C1, the mean translational-energy releases are greater than that expected for a statistical dissociation, whereas for the E state of BCl,+, (EL), is lowe: than expected. It may be possible that dissociation of the E state occurs to an excited electronic state of BCl,', thereby reducing Eavai,, and we comment that an excited singlet state of BCl,+ lying ca. 4 eV above the ground state has been observed in fluorescence experiments." We also note that the presence of an excited triplet state of BF,+ has been postu-

0.10 and 0.87

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lated as a reason why some excited states of BF3+ have smaller mean energy releases into BF, + + F than phase space calculations suggest." The higher than expected values for (&)t from dissociation of the c and b states of BCl,+ suggest a degree of non-statistical behaviour, and we note that for an impulsive dissociation with a late release of en erg^,,^.,^ (Ek)t is calculated to be 0.70 and 1.07 eV for Eavail values of 2.1 and 3.2 eV, respectively. The c state is well separated in energy from the 8, A and states of BCl,+, and shows resolv- ed vibrational structure in the v1 mode, implying that non- radiative decay uia internal co_nversion may not be as rapid as for lower excited states. The D state decays to a small extent by radiative decay14-16 (see later) which is a non-statistical phenomenon in such a polyatomic ion, although the predomi- nant decay route is non-radiative. The similarity and high value of (E,Jt from the c and b states, despite the fact that they lie over 1 eV apart, suggest that the B state decays rapidly by internal conversion into the c state from which an impulsive, non-statistical dissociation occurs. However, the quality of the raw data (limited essentially by the 300 K trans- lational temperature along the TPEPICO axis), the complex- ity of the isotope problem (the Ek analysis has to assume that B and C1 are single isotopes with mass 10.8 and 35.5 u), the lack of spectroscopic information on both the ground and any bound excited states of BCl,+, and the absence of information on how excited states of BCl,+ evolved adiabatically or dia- batically to BCI,', lead us to believe that at this stage further analysis and interpretation of these mean energy releases is not worthwhile.

TPEFCO and PIFCO coincidence experiments on excited electronic states of BC13+

A threshold photoelectron-fluorescence coincidence spectrum was obtained for BCl, excited at the energy of the Franck- Condon maximum of the b 2E' state of the parent ion, 15.51 eV. The optical resolution was 0.04 eV, no filter was used with the photomultiplier tube, and the highest time resolution (1 ns) of the TDC was used. The spectrum obtained after 42 h of data collection is shown in Fig. 7, and within the accuracy that can be obtained with the limited time resolution, we measure the decay of the fluorescence to be single exponential, with the lifetime of the emitting state as 5.5 +_ 0.5 ns.7 Because the TPEFCO technique is sensitive to fluorescence only in an ionic species and not in a neutral species, the presence of a

-10 0 10 20 30 40 50 60 timelns

Fig. 7 Threshold photoelectron-fluorescence coincidence spectrum of the fluorescence from the f, 2E' state of BCl,+ at 15.51 eV. The time resolution is 1 ns per channel, the accumulation time is ca. 42 h, and no optical filter is used. A single exponential is drawn through the data with a rate constant of 1.8 x lo8 s - l , corresponding to a lifetime of 5.5 ns. The insert shows the non-systematic residuals on the same timescale as the decay.

t Since the start signal in the TPEFCO experiment has an error of 1 3 ns due to different electron trajectories through the threshold analyser, this may affect the measured lifetime from the decay of the fluorescence. No deconvolution of this effect has been attempted.

signal in itself is very important because it represents definitive proof that a component of the fluorescence excited at 15.51 eV is due to BC13+. (We note that the dispersed emission spec- trum of BCl, excited at 15.5 eV gives rise to both BCl A-X and BC1,' B-8, A, B bands. In the TPEFCO experi- ment, the emission in the neutral molecule will contribute to the background, and it is only the emission in the positively charged species that will contribute to the signal.) We have not attempted to calibrate the TPEFCO coincidence yield37 with a known ionic emitter (such as N,' B ,Xu+) to estimate the fluorescence quantum yield of the BCl,+ b state, mainly because branching ratios into the ionic states of the parent ion are not known, but we believe that, given the weakness of the TPEFCO signal, 45, is probably small. Attempts to observe a PIFCO spectrum from BCl, excited into the ionic b state were unsuccessful, mainly owing to the deleterious effect of long exposure times of the sample gas to the microchannel plates. (This experiment was attempted in a different period of beamtime at the synchrotron source, when both the quality of BCI, gas and the purity of the gas line were inferior to that when the other experiments reported in this paper were per- formed.) However, we can predict the results of such an experiment with some confidence. Coincidences should be observed at TOFs of 16.0 and 19.1 ps, corresponding in our apparatus to ions whose masses are 82 u (i.e. BC1,+) and 117 u (i.e. BC13+). The BC12+ peak is associated with BC13+ i)-A/B fluorescence, the BCl, + peak with 6% fluorescence. With judicious use of filters, it should be possible to show that this is indeed the case; BC13+ %A/n emission leading to BCl,' should be associated with photons in the range 430- 600 nm, whereas &8 emission with photons in the range 330-400 nm should lead to coincidences with BCl,'. [A similar situation is observed with fluorescence from the state of SiCl,'; it emits both to the repulsive A state (leading to production of SiCl,+) and the bound state (leading to production of SiC14+) of the ion."] From the dispersed spec- trum of Lee et ~ 1 . l ~ ~ and allowing for the sensitivity of the quartz bialkali tube being used in their experiments, we calcu- late that the ratio of the BCI,+ to BCl,+ peaks in an unfiltered PIFCO spectrum should be ca. 6 : 1. Neither TPEFCO nor PIFCO spectra were recorded at the ,A', state of the parent ion at 17.70 eV (again owing to the long accumulation times that are to be expected and the corre- sponding degradation in performance of the apparatus after such experiments), but following our fluorescence studies,' we can predict with some confidence that no signal would be observed in either spectrum, i.e. the fluorescence quantum yields of this state is immeasurably small.

The absence of a parent-ion signal in the energy-scanning TPEPICO spectrum through the fi state of BC13+ at 15.5 eV (Fig. 5) allows us to make an upper estimate of the fluores- cence quantum yield of this state in the following manner. The only ion observed is BCl,+, and it can arise from two routes; non-radiative dissociation of the BCl, + 0 state, and radiative decay of the state to lower-lying electronic states of the parent ion which fragment to BC12+ (i.e. the A and B states). Were BCl,' to be observed, it could only arise from radiative decay of the state via b-x emission. From the TPEPICO spec- trum, we estimate that the intensity of any BCl,+ signal present must be at least 30 times weaker than the main BCl, +

where k, and k,, are radiative and non-radiative dissociation rates of the 0 state of BC13+, and B is a fluorescence branch- ing ratio. Therefore we can use a value for B i j - d B ~ x , ~ of 1/6 derived from the spectrum of Lee et ~ l . , ' ' ~ and obtain k,/k,, (which for small values of k, is equivalent to djF) to be less than 0.04. Combining this value with a lifetime of 5.5 ns, we

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obtain values of k, < 7 x lo6 s-' and k,, > 2 x lo8 s-'. In a qualitative sense, an upper limit for the quantum yield of ca. 0.04 seems sensible. Both undispersed and dispersed emission has been observed from this state with good signal-to-noise ratio14-' implying that the quantum yield cannot be exces- sively small, yet TPEFCO coincidences accumulate at a very slow rate [compared with ionic emitters (e.g. N2+ B ,ZU+, CF4+ c ,T, and SiC14+ e 'T, 19) which have near-unity quantum yields38]. To quantify these statements, partial ionis- ation cross-sections must be incorporated into the calcu- lations, values for which are not known for the valence states of BCl,+. However, overall a value of the quantum yield of between 10- ' and lo-, seems intuitively sensible.

Conclusions The main results and conclusions from this paper are as follows: (1) We have recorded TPES and TPEPICO spectra in the energy-scanning mode over the ground and the outer- valence excited states of BC13+ in the energy range 11-19 eV, and have established that the BC1, + daughter ion is produced from fragmentation of each of the excited states. Since the dis- sociation energy to BC1,' + C1 lies above the energy of the majority of the Franck-Condon region of the ground elec- tronic state of BC13+, no daughter ion is produced from the ground state. In each of the excited states, the ion yield of BCl,' matches the TPE signal, even to the extent of vibra- tional structure in v1 in the z1 ,A'; and B 'A; photoelectron bands being observed in the daughter-ion yields. (2) The threshold photoelectron spectrum shows a number of differ- ences (mainly in relative intensities of the bands and the pres- ence of shoulders on the main peaks) from the He I photoelectron spectrum. We attribute these features to effects of autoionisation. Although not directly obtained from the TPES, we believe that there is now strong evidence to suggest that the second and third photoelectron bands of BCl, arise from electron removal from the le" and 3e' molecular orbitals, respectively (leading to ionic electronic states 'E" and B ,E') as suggested by Bassett and Lloyd,4 and not the reverse order as suggested by King et al." Conversely, from an analysis of the Rydberg series of BBr, converging on the ground and first two excited states of BBr, f,39 we believe that the assignment of the photoelectron bands in BBr, by King et a/." (in increasing energy of the ionic electronic states, 2 'A; < A

< B ,E) is correct, and that that suggested by Bassett and Lloyd4 for BBr, (8 2A; < 'E' < B ,En) is wrong. (3) Mean kinetic-energy releases to BCl,' + C1 have been measured from the Franck-Condon maxima of the third, fourth and fifth excited states of BCl,'. For the 21 and 2, states, (Ek), is larger than that expected for a statistical dissociation from the ground-state potential-energy surface of the parent ion in which energy and angular momentum are conserved, suggest- ing a degree of isolated-state behaviour. For the E state, (&)t

is less than that expected, suggesting that dissociation may be occurring to an excited electronic state, and not the ground state of BCl2+. (4) Radiative decay is confirmed from the fourth excited state of BCl,', fi 2E', at 15.5 eV. The lifetime of this state is 5.5 f 0.5 ns, and the absence of parent-ion signal in the energy-scanning TPEPICO spectrum through this state allows us to put an upper limit on the fluorescence quantum yield of this state of ca. 0.04.

We thank the staff at the Daresbury Laboratory for consider- able assistance, and Dr. I. Powis (Nottingham University) for the use of his kinetic energy analysis programme. The UK Engineering and Physical Sciences Research Council is thanked for an equipment grant, an Advanced Fellowship (P.A.H.), a Post-Doctoral Fellowship (D.M.S.) and Research Studentships (K.J.B. and K.R.Y.). K.J.B. thanks the Daresbury Laboratory for a CASE award. The Deutsche Forschungsge-

meinschaft is thanked for a Post-Doctoral Fellowship (H.B.). Finally, R.P.T. and M.S. thank the British Council for a bilat- eral travel grant between Birmingham and Krakow Uni- versities.

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Paper 5/05147I; Received 2nd August, 1995

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