The UV photodissociation dynamics of ClO radical using velocity map ion imaging

THE JOURNAL OF CHEMICAL PHYSICS 123, 174303 �2005�

The UV photodissociation dynamics of ClO radical using velocity mapion imaging

Hahkjoon Kim, Jiho Park, Tracy C. Niday, and Simon W. Northa�

Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, Texas 77842

�Received 3 August 2005; accepted 31 August 2005; published online 28 October 2005�

We have studied the wavelength-dependent photodissociation dynamics of jet-cooled ClO radicalfrom 235 to 291 nm using velocity map ion imaging. We find that Cl�2P3/2�+O�1D2� is thedominant channel above the O�1D2� threshold with minor contributions from the Cl�2PJ�+O�3PJ�and Cl�2P1/2�+O�1D2� channels. We have measured the photofragment angular distributions foreach dissociation channel and find that the A 2� state reached via a parallel transition carries mostof the oscillator strength above the O�1D2� threshold. The formation of O�3PJ� fragments withpositive anisotropy is evidence of curve crossing from the A 2� state to one of several dissociativestates. The curve crossing probability increases with wavelength in good agreement with previoustheoretical calculations. We have directly determined the O�1D2� threshold to be 38 050±20 cm−1

by measuring O�1D2� quantum yield in the wavelength range of 260–270 nm. We also report on thepredissociation dynamics of ClO below the O�1D2� threshold. We find that the branching ratio ofCl�2P3/2� /Cl�2P1/2� is 1.5±0.1 at both 266 and 291 nm. The rotational depolarization of theanisotropy parameters of the Cl�2P3/2� fragments provides predissociation lifetimes of 1.5±0.2 psfor the 9-0 band and 1.0±0.4 ps for the 8-0 band, in reasonable agreement with previousspectroscopic and theoretical studies. © 2005 American Institute of Physics.�DOI: 10.1063/1.2083487�

I. INTRODUCTION

The role of photodissociation of chlorine compounds inthe stratosphere is critical in understanding stratosphericozone depletion.1 Chlorine monoxide is a radical intermedi-ate that plays an important role in the stratospheric ozonedepletion via the photodissociation as well as bimolecularreactions.2,3 The UV photodissociation of ClO is estimated tobe responsible for 2%–3% of the total ozone loss in thestratosphere.4 In addition, since ClO is produced as a primaryphotoproduct of ClONO2, an understanding photodissocia-tion dynamics of ClO is critical for modeling recent ClONO2

photodissociation experiments.5,6

The ClO absorption spectrum is characterized by a con-tinuum that spans from 220 to near 263 nm and the subse-quent rovibrational-resolved structure that terminates at316 nm.7 Details of the electronic transitions responsible forthe ClO absorption spectrum can be explained consideringelectronic configurations of ClO. The ground state of ClOhas the configuration ��2��4��*�3��*�0 and the term sym-bol X 2�� which correlates to ground-state atoms. Promo-tion of an electron from a � orbital to a �* orbital results inthe ��2��3��*�4��*�0 configuration and the term symbolA 2�� and correlates to Cl�2PJ�+O�1D2� fragments based onthe Wigner-Witmer correlation rule.8 There are four elec-tronic pathways for ClO photodissociation which are ener-getically accessible in the UV region:

a�
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ClO�X 2�� → Cl�2P3/2� + O�3P2,1,0� �1�

→Cl�2P1/2� + O�3P2,1,0� �2�

→Cl�2P3/2� + O�1D2� �3�

→Cl�2P1/2� + O�1D2� . �4�

Channels �1� and �2� are accessible at wavelengths between316 and 263 nm and all four channels are accessible at wave-lengths shorter than 263 nm. The high-resolution absorptionspectrum ClO was first studied by Durie and Ramsay.9 Theauthors predicted that only Cl�2P3/2� and O�1D2� fragments�channel �3�� were produced as a result of photodissociationin the continuum region based on a comparison with IO.Analysis of the convergence of the vibronic transitions re-sulted in the onset wavelength for Cl�2P3/2�+O�1D2� of263.01±0.1 nm�38 052 cm−1�.10 McLoughlin et al. studiedthe high-resolution absorption spectrum of ClO using laserabsorption spectroscopy.11 The authors estimated the onsetwavelength for Cl�2P3/2�+O�1D2� to be 262.90±0.05 nm.

Photodissociation at wavelengths longer than the O�1D2�threshold involves predissociation of the bound A 2�� statevia several repulsive states. Predissociation lifetimes for spe-cific vibrational levels of the A 2�� state predissociationbased on spectral linewidths have been previouslyreported.11–13 A theoretical treatment of the predissociationwas first made by Bunker and Klein.14 The authors neglecteda rotational contribution to the predissociation lifetime andpredicted predissociation rates using the spin-orbit interac-

2
tion between A �� state and a single repulsive state corre-
© 2005 American Institute of Physics03-1

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http://dx.doi.org/10.1063/1.2083487

http://dx.doi.org/10.1063/1.2083487

http://dx.doi.org/10.1063/1.2083487

174303-2 Kim et al. J. Chem. Phys. 123, 174303 �2005�

lating to ground-state atoms. The authors concluded thatmore than a single repulsive state must be involved to ex-plain the observed oscillatory pattern of predissociation life-times with wavelength. Recently, Toniolo et al. and Laneet al. performed high-level ab initio calculations of the ex-cited states of ClO and identified several states involved inpredissociation of A 2�� state.15,16 Both studies concludedthat the 1 4�+, 2 4�−, and 3 2� states were the principalstates responsible for the predissociation of the A 2�� state.Lane et al. predicted predissociation lifetimes ranging from0.5 to 10 ps depending on the A 2�� vibrational level.

In contrast to the numerous spectroscopic studies, therehave been a limited number of direct experimental studies ofClO photodissociation. Davis and Lee investigated the pho-todissociation of ClO at 248 nm using photofragment trans-lational spectroscopy and concluded that channel �3� was thedominant channel �97%� with a minor contribution fromchannels �1� and �2�.17 The measured nonlimiting anisotropyparameter of 1.2±0.2 for channel �3� suggested that one ormore perpendicular transitions contribute significantly �30%�to the oscillator strength at this wavelength. Zou et al. stud-ied the photodissociation of ClO near 235 nm using 2+1resonance-enhanced multiphoton ionization �REMPI� withtime-of-flight mass spectrometry �TOFMS�.18 The authorsreported that channel �3� was the dominant channel with an-isotropy parameter of 1.2±0.2, suggesting that the photodis-sociation dynamics of ClO at 230–250 nm is not stronglywavelength dependent. Schmidt et al. investigated photodis-sociation of ClO over the wavelength range of 205–270 nmemploying 2+1 REMPI detection of O�1D2�.19 Only the sumof channels �3� and �4� in the continuum region was reportedsince the authors were unable to distinguish between the co-incident Cl�2P3/2� and Cl�2P1/2� fragments due to insufficientvelocity resolution of the mass spectrometer. They also de-termined the wavelength corresponding to the onset of thecontinuum to be 263.4±0.2 nm based on the wavelength-dependent O�1D2� yield near threshold. Recently, Fleschet al. employed single-photon ionization in the wavelengthrange of 262–265 nm and determined the O�1D2� thresholdto be 263.71±0.01 nm.20

In this paper, we report the wavelength-dependent pho-todissociation dynamics of ClO from 235 to 291 nm corre-sponding to the wavelengths above, near, and below theO�1D2� formation threshold using velocity map ion imaging.In Sec. III A, photodissociation of ClO above the O�1D2�threshold is addressed. Here we focus on the identification ofphotoproducts and angular distributions to unravel the role ofexcited states other than the A 2�� state and the role ofnonadiabatic curve crossing of ClO. Our discussion reliesheavily on the recent theoretical work of Persico. In Sec.III B, we present an accurate determination of the O�1D2�formation threshold wavelength and discuss the observedbreakdown of the axial recoil approximation. In Sec. III C,we examine the state-selected predissociation lifetimes basedon the analysis of the photofragment angular distributionsand discuss the Cl�2P3/2� /Cl�2P1/2� branching ratio below the

1
O� D2� threshold.

II. EXPERIMENT

A pulsed ClO molecular beam, collimated by a conicalskimmer, was intersected at 90° by two copropagating lin-early polarized laser beams. The dissociation beam was gen-erated using a 10 Hz Nd:yttrium aluminum garnet �YAG��Spectra Physics GCR-150-10� pumped dye laser �LAS LDL2051� operating on the dye Coumarin 500 followed by afrequency doubling crystal �beta-barium borate �BBO��. Thefundamental dye outputs were accurately calibrated using aNe-filled hollow cathode lamp. The probe beam wavelengthswere provided by mixing 1064 nm with the frequency-doubled dye output from a second dye laser �Quantel TDL-51� running on the dye mixture of Rhodamin 610 and 640. Aphotoelastic modulator �PEM-80, HINDS� was used to rotatethe polarization of the probe beam. Typical dissociation andprobe pulse energies were 30 and 50 �J, respectively. Thetime delay between the dissociation and the probe beam wasset to 10 ns in order to detect fast fragments traveling per-pendicular to the laser propagation direction. The chlorineatoms �Cl�2P3/2� /Cl�2P1/2�� were probed using 2+1 REMPItransitions at 235.336 nm �4p 2D3/2←3p 2P3/2� and235.205 nm �4p 2P1/2←3p 2P1/2�, respectively.21 The oxy-gen atoms were probed using 2+1 REMPI transitions near226 nm �3p 3P2,1,0←2p 3P2,1,0�.22 The resulting chlorineand oxygen ions were accelerated by velocity mapping ionoptics pioneered by Eppink and Parker, a variant of tradi-tional ion imaging developed by Chandler and Houston,23

before entering the 50-cm-long field-free flight tube alongthe axis defined by the molecular beam.24 The voltages of ionoptics were adjusted until the speed resolution of the frag-ment was maximized. The ions were projected on theposition-sensitive microchannel plate phosphor assemblygated to detect the mass of interest. A photomultiplier �PMT�positioned off-axis was used to record mass spectra and es-tablish proper timing. A charge-coupled device �CCD� cam-era �Mintron MS-2821E/C� acquired images of the phosphorscreen and sent raw images to a frame grabber controlled bycommercial software �CODA32� which involved centroidingand event counting.25 The final images were obtained by re-peatedly scanning Doppler profiles of Cl�2P3/2� /Cl�2P1/2� toachieve homogeneous detection efficiency. Since cylindricalsymmetry with respect to the electric field was maintainedthroughout the experiments, the two-dimensional projectionswere reconstructed to the original three-dimensional ionspheres by applying the basis-set expansion �BASEX�method developed by Drinbinski et al.26

In our previous work on ClO photodissociation the radi-cals were generated using a pulsed electric discharge27,28

similar to the approach of Cooper et al.18,29 We find that anintense molecular beam of ClO radicals can be generatedusing the flash pyrolysis of a Cl2O/He mixture. The activeregion of the pyrolysis source consists of a 3 cm aluminatube wrapped with nickel-chrome alloy wire which is resis-tively heated to approximately 800 K.30 Figure 1 demon-strates the production of ClO radicals using the pyrolyticsource. The images were collected using one color near235 nm to probe the Cl�2P3/2� fragments. At room tempera-

2
ture the Cl� P3/2� image is consistent with Cl2O photodisso-

174303-3 Photodissociation dynamics of ClO J. Chem. Phys. 123, 174303 �2005�

ciation at 235 nm.31 As the nozzle is heated, features associ-ated with Cl2O decrease in intensity and are replaced by asharp ring consistent with diatomic photodissociation. Athigher temperatures there is little evidence of Cl2O photodis-sociation and a bright spot in the center image from en-trained Cl atoms is observed. Although the pyrolysis of Cl2Oshould, by simple stoichiometry, provide a 1:1 yield of ClOand Cl in the expansion, we find that the relative signalsassociated with these two species vary greatly depending onthe experimental conditions. This is due, in part, to the dif-ference in the rate constants for the reaction of these specieswith Cl2O. The reaction of Cl2O with Cl is rapid �k=7.3�10−11 cm3 s−1 at 800 K�,32

Cl2O + Cl → Cl2 + ClO, �5�

and results in a net loss of Cl and net production of ClO,while the reaction of ClO with Cl2O is considerably slower.33

In addition, since the entrained Cl atoms are limited to a fewpixels in the center of the image, intense signals can result inattenuation using ion-counting techniques. We find that theformation of hot ClO radicals is significantly reduced byusing a water-cooled endcap located at the nozzle tip prior toexpansion. Based on images taken near the threshold forO�1D� production from ClO �vide infra� we estimate that theClO radicals can be characterized by Trot�100 K, Tvib

�200 K, and Telec�200 K.Cl2O was synthesized by the method of Cady.34 Cl2 was

collected on prebaked HgO �Aldrich� powder and the reac-tion was allowed to run overnight at 195 K. The product wasthen purified by vacuum distillation, and the purity�90% � was checked by UV absorption spectroscopy.

III. RESULTS AND DISCUSSION

A. Photodissociation of ClO above the O„

1D2…

threshold

The one-color photodissociation of ClO at 235 nm re-sulted in Cl�2P3/2� and Cl�2P1/2� images, as shown in Fig. 2.The speeds of the atomic fragments corresponding to eachasymptotic channel can be obtained from the total transla-

FIG. 1. Cl�2P3/2� images collected as a function of source temperature. Thecenter spot in the rightmost image is due to entrained Cl atoms from thepyrolysis of Cl2O.

tional energy of the photofragments by energy conservation


Etranstotal = h + E,J,e

ClO − D0�ClO� − EeCl − Ee

O, �6�

where h is the energy of dissociation photon, D0�ClO� is thebond energy of ClO, Ee

Cl and EeO are the electronic energy of

Cl and O, and E,J,eClO is the initial energy of the ClO prior to

dissociation which is a function of both Franck-Condon fac-tors and the internal temperature. The accurate determinationof the ClO internal temperature and bond dissociation energywill be discussed in the following section. The Cl�2P3/2� im-age in Fig. 2 shows a single ring with a speed consistent withthe Cl�2P3/2�+O�1D2� channel. No evidence for theCl�2P3/2�+O�3PJ� channel is observed, consistent with theearlier work of Zou et al. In contrast, the Cl�2P1/2� imageshows two rings with speeds consistent with the Cl�2P1/2�+O�1D2� and Cl�2P1/2�+O�3PJ� channels. The signals asso-ciated with Cl�2P1/2� are weak, indicating that the contribu-tions of the Cl�2P1/2�+O�1D2� and Cl�2P1/2�+O�3PJ� chan-nels at 235 nm are minor. We obtained a quantitativebranching ratio of Cl�2P3/2� and Cl�2P1/2� at 235 nm by in-tegrating Doppler profiles of 2+1 REMPI transitions col-lected at the identical laser power. The integrated signals arecalibrated by using the detection efficiency of Cl�2P3/2� andCl�2P1/2�.35,36 We find a Cl�2P1/2� /Cl�2P3/2� ratio of0.02±0.01 in good agreement with our previous work.

We have measured two-color images for ClO photodis-sociation at several wavelengths from 235 to 262 nm. Figure3 shows the speed distributions of Cl�2P3/2� �bottom panel�and Cl�2P1/2� �top panel� derived from images acquired at255 nm. The solid lines represent forward-convolution simu-lations to the data �circles� and the arrows in the figure indi-cate the speeds associated with the various dissociation chan-nels. We do not observe the Cl�2P1/2�+O�1D2� channel at255 nm but observe a small yield of Cl�2P3/2�+O�3PJ�. Wehave not attempted to measure the relative yields of the oxy-gen spin-orbit states. The measured Cl�2P3/2� and Cl�2P1/2�speed distributions provide accurate relative branching ratiosbetween the O�1D2� and O�3PJ� channels coincident witheither Cl�2P3/2� or Cl�2P1/2�. Determination of the relativebranching between Cl�2P3/2� and Cl�2P1/2� channels provedmore difficult since the Cl�2P1/2� channels are produced inminor yield ��1% –3% �. We do not observe a significantvariation in this yield with wavelength above the O�1D2�threshold based on a comparison of features associated withthe photodissociation at a given wavelength with featuresassociated with dissociation by the 235 nm probe in theCl�2P3/2� and Cl�2P1/2� images. We have fixed this value at0.02 based on our measurements at 235 nm for the resultspresented in Table I. A comparison to the work of Davis andLee at 248 nm, where the neutral TOF technique permits anaccurate measurement of the O�1D2� /O�3PJ� branching ratio,demonstrates that a wavelength-independentCl�2P1/2� /Cl�2P3/2� branching ratio is reasonable. There areseveral conclusions regarding the relative product yields,however, that do not depend on the absoluteCl�2P1/2� /Cl�2P3/2� branching ratio: �1� The yield of O�3PJ�is nonzero above the O�1D2� threshold, decreasing from amaximum value of 0.06 at 262 nm to 0.01 at 235 nm. �2�The Cl�2P3/2�+O�3PJ� channel is not observed at wave-
lengths shorter than 248 nm and the relative yield of this


channel increases with wavelength. �3� The Cl�2P1/2�+O�1D2� channel is not observed at wavelengths longer than248 nm and the relative yield of this channel decreases withwavelength.

The angular distributions of the Cl�2P3/2� and Cl�2P1/2�fragments at 235 nm are shown in the lower panel of Fig. 2and the experimental angular distributions are fitted to thefollowing equation;37

I�� =1

4��1 + �P2�cos �� , �7�

where � is the spatial anisotropy parameter, P2�cos �� is thesecond Legendre polynomial, and � is the angle between thefragment recoil direction and laser polarization direction. Wefind best-fit anisotropy parameters of 1.9±0.1 for theCl�2P3/2�+O�1D2� channel, −1.0±0.1 for the Cl�2P1/2�+O�1D2� channel, and 1.4±0.1 for the Cl�2P1/2�+O�3P2�channel. The anisotropy parameters for the four dissociationchannels above O�1D2� threshold are provided in Table I. Wedid not observe any evidence for orbital alignment of theCl�2PJ� fragments. There was no appreciable difference inthe measured anisotropies when the polarization of the probelaser was aligned parallel or perpendicular to the photodis-sociation laser. We found, however, that the ClO rotational

FIG. 2. The upper panel shows raw ion images of Cl�2P3/2� �left� and Cl�2Pdistributions. The solid lines are the best-fit simulations obtained using Eq.

1/2� �right� acquired at 235 nm. The lower panels show the corresponding angular�7� in the text.

temperature was a sensitive function of nozzle temperature


FIG. 3. Speed distributions derived from Cl�2P3/2� �bottom panel� andCl�2P1/2� �top panel� ion images for ClO photodissociation at 255 nm. Thearrows indicate the speeds associated with the formation of specific productchannels. The dashed lines indicate the speeds associated with the 235 nm
probe laser photodissociation.


and the pulsed valve conditions and that photodissociationchannels involving the O�1D2� result in recoil velocities lowenough that rotational depolarization is expected and ob-served. This effect is especially pronounced near the O�1D�threshold �vide infra�. In order to extract intrinsic anisotropyparameters for these channels we fit only the leading edge ofthe peaks corresponding to the lowest ClO rotational states.This resulted in anisotropy parameters closer to the limitingvalues than anisotropy parameters averaged over the entirespeed distribution. The photodissociation channels corre-sponding to the O�3PJ� production result in larger recoil ve-locities and the measured anisotropy parameters are not assensitive to parent rotational state. Thus, for these channelsthe analysis involved an average over the entire velocity dis-tribution.

Davis and Lee measured a nonlimiting anisotropy pa-rameter of 1.2±0.2 for the dominant Cl�2P3/2�+O�1D2�channel at 248 nm which is lower than the present result.The authors attributed this low anisotropy parameter to the2 2�+ state which would be required to carry �30% of theoscillator strength at this wavelength. Such a large contribu-tion of the 2 2�+ state is inconsistent with theoretical calcu-lations which suggest that this state makes a minor contribu-tion ��1% � at this wavelength. We have also previouslyreported a nonlimiting anisotropy parameter of 1.2±0.2 forthis channel at 235 nm. We believe that the low value re-ported in our previous work arose, in part, from the largeentrained atomic signal which made analysis of the core-sampled TOF spectra difficult.

Although both the 2 2�+ and A 2� states diabaticallycorrelate to O�1D2� products, we find strong evidence for therole of the 2 2�+ state in the dissociation. The minorCl�2P1/2�+O�1D2� channel has an anisotropy parameter of−1.0±0.1 which is consistent with excitation to the 2 2�+

state via a perpendicular transition � �= ±1�. An alternativesource of these fragments could be curve crossing from an-other state reached via a perpendicular transition. Thewavelength-dependent yield of the Cl�2P1/2�+O�1D2� chan-nel, assuming that these products arise from direct dissocia-tion of the 2 2�+ state, is in good agreement with the previ-ous theoretical study and is therefore a more likelyexplanation. Toniolo et al. conclude that absorption to thisstate occurs from 210 to 242 nm, contributing approximately

TABLE I. Summary of relative quantum yields and anisotropy parameters

Wavelength�nm�

Cl�2P3/2�+O�3PJ� Cl�2P1/2�+O�

� � �

235 0.00 ¯ 0.01240 0.00 ¯ 0.01244 0.00 ¯ 0.02248 0.02 0.2 0.02255 0.03 0.2 0.02260 0.03 0.1 0.02262 0.04 0.1 0.02

2.5% of the oscillator strength in this wavelength range. Fig-


ure 4 summarizes the wavelength-dependent branching be-tween channels �1�–�4� above the O�1D2� threshold.

Since the A 2� state is the only state with appreciableoscillator strength that is reached via parallel transition� �=0� and this state diabatically correlates to O�1D2�products, any curve crossing �adiabatic behavior� will resultin O�3PJ� products with an anisotropy parameter of 2.0.Thus, the cumulative crossing probability from the A 2�state to the 3 2�, 1 2 , and 2 4�− states can be determined bymeasuring the parallel component to the channels resultingin O�3PJ� production relative to the total parallel contributionat each wavelength. Based on this analysis and the data pro-vided in Table I, the curve crossing probability decreasesfrom a maximum value of 2% at 262 nm to �0.2% at244 nm, as shown in Fig. 5. This decrease derives primarilyfrom the overall decrease in the O�3PJ� fragments. We notethat both the magnitudes and wavelength dependence areconsistent with the theoretical work of Persico. It is not pos-sible, however, to obtain curve crossing probabilities be-tween specific states and A 2� state to compare with theory.The perpendicular contribution to the O�1D2� channelscould, in principle, permit a determination of the curve cross-ing probability to the A 2� from other states. Although lessthan limiting values for the anisotropy parameter are ob-served for the dominant Cl�2P3/2�+O�1D2� channel, thisvariation most likely reflects uncertainties in the anisotropymeasurements rather than very large curve crossing prob-abilities from states reached via a perpendicular transition.

B. Photodissociation of ClO near the O„

1D2…

threshold

We have investigated the photodissociation dynamics ofClO at wavelengths near the O�1D2� threshold. The forma-tion of Cl atoms in coincidence with O�1D2� in this regionresults in little translational energy. As a result, the measuredspeed distributions are extremely sensitive to the availableenergy, i.e., the precise value of the Cl–O bond dissociationenergy and the initial energy content of the ClO radicals. Thelower panel of Fig. 6 shows the Cl�2P3/2� speed distributionderived from the measured image at 262 nm. This wave-length corresponds to excitation just above the O�1D2�threshold. At higher speeds, not shown in the figure, we ob-

e ClO photodissociation above the O�1D2� threshold.

Cl�2P3/2�+O�1D2� Cl�2P1/2�+O�1D2�

� � � �

0.98 1.9 0.01 −10.98 1.6 0.01 −10.98 1.9 0.00 −10.96 1.7 0.00 ¯

0.95 1.8 0.00 ¯

0.95 1.8 0.00 ¯

0.94 1.7 0.00 ¯

for th

3PJ�

�

1.51.10.70.80.91.01.3

served a small feature consistent with the formation of coin-



cident O�3PJ�. The finite width of the measured speed distri-bution is a reflection of the initial internal state distributionof the ClO radical. The speeds associated with individualClO rotational levels in both spin-orbit states are indicatedby the vertical lines in the figure. The rotational distributionis characterized by a temperature of 100 K and the spin-orbitbranching ratio by an electronic temperature of 200 K. Wehave assumed that the vibrational temperature is �200 Kbased on a lack of clear signal corresponding to v=1 ClO.The line spectrum has been convoluted with a Gaussian ��=15 m/s� to reflect instrumental resolution. The resultingsolid line provides a reasonable fit to measured speed distri-bution. We have assumed a threshold energy of38 050±20 cm−1 for fitting the speed distributions. The fitsare extremely sensitive to this value near threshold, both interms of the absolute speeds and the spacing between peaks

FIG. 4. Wavelength-dependent relative branching ratios of the ClO photo-dissociation channels. The minor channels have been multiplied by a factorof 10 for clarity.

FIG. 5. Cumulative curve crossing probability from the A 2� state to yield2 3 1
Cl� PJ�+O� PJ� products above the O� D2� threshold.

which correspond to each spin-orbit state fixed at 318 cm−1.The threshold energy was derived from the fits to speed dis-tributions acquired at multiple wavelengths, some below thethreshold where the fragments produced arise from ClO radi-cals with nonzero internal energy.

The speed-dependent anisotropy parameters for theCl�2P3/2� fragments are shown in the upper panel of Fig. 6.Excitation in this wavelength region involves a parallel tran-sition to the A 2� state above the O�1D2� threshold followedby direct dissociation. In the axial recoil approximation,where the recoil axis and molecular axis are nearly equiva-lent, the anisotropy parameter would be near its limitingvalue of 2. The measured anisotropy parameters, however,are substantially lower than this limiting value. The decreasein anisotropy parameter is due to a breakdown in the axialrecoil approximation, i.e., a reorientation of the recoil veloc-ity resulting from initial angular momentum of the parent.Evidence for this effect was reported recently in the nearthreshold photodissociation of IBr.38–40 The breakdown ofthe axial recoil approximation is expected when the radialand tangential velocities are comparable and the larger theparent rotational state the larger the magnitude of the effect.A comparison of the measured anisotropy parameters withparent rotational states demonstrates that the measurementsare consistent with such a trend. In particular, the suddenincrease in the anisotropy parameter corresponding to thelowest rotational states of the ClO spin-orbit excited stateprovides compelling evidence. A more quantitative treatmentof the speed-dependent anisotropy parameters can be ob-tained using a simple classical model. Following the discus-sion of Wrede et al.40 the classical motion of a rotating di-atomic molecule is described by the effective Hamiltonian,

FIG. 6. Speed distribution and speed-dependent anisotropy parameters forCl�2P3/2� fragments arising from ClO photodissociation at 262 nm. The bot-tom panel shows the simulation of the speed distribution using a ClO rota-tional population of 100 K. The top panel shows the speed-dependent an-isotropy parameters. The solid line represents the classical prediction for theanisotropy parameter and the filled circles represent the data.

ignoring internal angular momentum,



H = V�R� +p2

2�+

J2

2�R2 �8�

and the relation between the angle of rotation �� and inter-nuclear separation can be expressed by solving Hamilto-nian’s equation of motion

��

�R=

J

p�R�R2 , �9�

where the conjugate momentum p�R� is given by

p�R� = �2��E − V�R�� − J2/R2�1/2. �10�

In Eq. �10�, J is the rotational angular momentum, V�R� is aMorse potential describing the A 2� state, and E is the pho-ton energy. The molecular constants of the X 2� state andA 2� state potential curves were obtained from previousspectroscopic results.41

The final angle of rotation is given by

� = �R0

� JdR

p�R�R2 , �11�

where R0 is the turning point of the Morse potential at thecorresponding excitation energy. Although the choice of theinner turning point as the initial condition for the classicaltrajectory is an oversimplification, the repulsive wall of theA 2� state lies nearly directly above the ground-state mini-mum at excitation energies near the O�1D2� threshold.

The azimuthally averaged addition theorem42 permits theanisotropy parameter to be written as a product of two terms

� = 2P2�cos ��P2�cos �� , �12�

where � is the recoil angle relative to the molecular axis, thesecond term reflects the reorientation of the recoil axial fromthe molecular axis by angle �. The predictions using Eqs.�9�–�12� are shown as the solid lines in the upper panel ofFig. 6. Although there is qualitative agreement with the datathe model consistently underestimates the magnitude of theeffect. The classical treatment includes the rotational angularmomentum of the ClO but ignores orbital angular momen-tum of both ClO and the angular momentum of the atomicfragments. A second source of systematic error involves thetreatment of low velocities that result in small ion images.Since all the ions are projected on a limited number of pix-els, the derived anisotropy parameters will be more isotropicthan the intrinsic angular distribution although our simula-tions indicate that the effect is small in agreement with theconclusions reached by Wrede et al.

Based on the analysis of the images discussed previouslywe have established both the energy of the threshold�38 050±20 cm−1� and the internal energy distribution of theClO radicals. As additional confirmation we have measuredthe relative O�1D2� and O�3PJ� yields near the threshold. Theinsets of Fig. 7 show Cl�2P3/2� images acquired at two wave-lengths near threshold. The outer ring in the 265 nm imagecorresponds to O�3PJ� formation arising from the predisso-ciation below threshold. The inner ring in the image is due toO�1D2� formation and the intermediate ring is from the
235 nm probe laser. In the lower 262 nm image the outer

ring is very weak and the innermost ring has increased no-ticeably in radius. In order to extract relative branching ratiosfor O�1D2� and O�3PJ� formed in coincidence with Cl�2P3/2�the wavelength-dependent images were converted to speeddistributions and the integrated areas of the peaks associatedwith each channel were compared. Previous work by Wredeet al. has shown that ClO excitation in this wavelength re-gion results in the formation of ion-pair states. We find thatphotodissociation through these states results in intense Cl+

and O+ images. A full description of this process is the sub-ject of future publication. We have selected wavelengths nearthreshold which avoid these ion-pair states to facilitateanalysis. Figure 7 shows a plot of the wavelength-dependentO�1D2� quantum yield near the threshold using

�Cl�O�1D2�� =I�O�1D2��

I�O�1D2�� + I�O�3PJ��, �13�

where I�O�1D2�� and I�O�3PJ�� represent areas of the peaksin the derived speed distributions corresponding to theCl�2P3/2�+O�1D2� and Cl�2P3/2�+O�3PJ� channels, respec-tively. The data show a dramatic change in the O�1D2� yieldbetween 262.5 and 264.5 nm consistent with the energeticthreshold for O�1D2� formation. The finite width of this onsetis the result of nonzero ClO internal energy which allows forO�1D2� formation at wavelengths corresponding to energiesbelow the threshold. The solid and dashed lines in the figurerepresent simulations of the O�1D2� fixing the quantum yieldfor total energies exceeding the threshold at 0.97. It shouldbe noted that the reported O�1D2� yields are based only onthe Cl�2P3/2� images. Although above the threshold theCl�2P1/2� yield is minor, below threshold we find that the

2 2

FIG. 7. Wavelength-dependent O�1D2� quantum yield over the range of260–270 nm based on Eq. �13�. The outer ring in panel A arises fromCl�2P3/2� in coincidence with O�3PJ� at 265 nm: the inner and intermediaterings in the middle are due to Cl�2P3/2�+O�1D2� at 235 and 265 nm, respec-tively. In panel B, two rings that can be observed arise from Cl�2P3/2�+O�1D2� at 235 nm �outer ring� and 262 nm �inner ring�, respectively.

Cl� P3/2� /Cl� P1/2� ratio is approximately 1.5 and the overall



O�1D2� yields should, therefore, include this correction. Asevidence of the Cl�2P3/2� /Cl�2P1/2� branching ratio Fig. 8shows an O�3P2� image acquired at 291.71 nm along with anexpanded region of the speed distribution �bottom panel�.The O�3P2� image exhibits two distinctive rings indicatingthe evidence of O�3P2� in coincidence with Cl�2P3/2� andCl�2P1/2�. Integration of the peaks in the speed distributionresults in a branching ratio of Cl�2P3/2� /Cl�2P1/2� of 1.5±0.1at 291.71 nm. A similar result was obtained from measure-ments at 266 nm.

The solid line in Fig. 7 corresponds to a threshold energyof 38 050 cm−1 and the dashed lines represent ±40 cm−1, as-suming the thermal distribution of ClO is characterized byTrot=100 K, Tvib�200 K, and Telec=200 K. We have in-cluded the small shift in the onset of individual rovibrationalstates due to the centrifugal barrier by using a Morse poten-tial for the A 2�. The effect is minor, resulting in a correctionof only a few cm−1 for the largest rotational states. We findthat the best-fit value of the threshold for O�1D2� formationfor fitting the onset data is in excellent agreement with inde-pendent fits to the fragment speed distributions. Our result isin excellent agreement with the 38 052 cm−1 reported byCoxon and Ramsay based on the convergence of vibronictransitions.

To date, direct measurement of O�1D2� threshold by de-tection of O�1D2� has been performed by two groups. Fleschet al. recently employed vacuum ultraviolet �vuv� photoion-ization detection to measure the O�1D2� onset. The authors

FIG. 8. Panel A shows O�3P2� image acquired at 291.71 nm. Panel B is theenlargement of panel A. The lower panel shows an expanded region of thespeed distribution derived from the O�3P2� image. The slower feature cor-responds to the Cl�2P1/2�+O�3P2� channel and the faster feature correspondsto the Cl�2P3/2�+O�3P2� channel.

used a sigmoidal fit to the onset curve and reported a value


of 37 820±2 cm−1 corresponding to 10% of the maximumO�1D2� yield. The apparent redshift of the onset value to ourthreshold value can be attributed to the ClO thermal energy.In addition, the authors observed a steeper onset curve thananticipated, suggesting the presence of a rotational barrier.However, we fail to detect evidence for such an effect.Schmidt et al. made a similar measurement using 2+1REMPI detection of the O�1D2� fragments and reported avalue of 37 965±30 cm−1 corresponding to the cutoff of theO�1D2� signal. The value is consistent with a thermal redshiftfrom the energetic threshold. Neither Flesch et al. norSchmidt et al. addressed the nonzero yield of O�3PJ� abovethe O�1D2� threshold or observed evidence for a separateonset due to spin-orbit excited �A 2�1/2� ClO. Our approachdiffers from the previous studies in that each measurement ofthe Cl�2P3/2� speed distribution provides the yield of bothO�3PJ� and O�1D2� and given the resolution of the techniqueavoids interference from other dissociation channels.

C. Photodissociation of ClO below the O„

1D2…

threshold

In this section we report on our initial investigation ofClO photodissociation below the O�1D2� threshold. We focuson the determination of state-selected predissociation life-times based on the analysis of photofragment angular distri-butions and discuss the Cl�2P3/2� /Cl�2P3/2� branching ratio.At wavelengths below �267 nm only channels �1� and �2�,which involve O�3PJ� fragments, are observed.

The top panel of Fig. 9 shows the wavelength depen-dence of the anisotropy parameter in the region of 9-0 band�left� and 8-0 band �right� for the Cl�2P3/2�+O�3PJ� channel.The bottom panel shows spectral simulations of 9-0 band and8-0 band along with the measured photofragment excitation�PHOFEX� spectra �closed circles�, corresponding to theCl�2P3/2�+O�3PJ� image features as a function of wave-length. The spectral simulations based on the sum of P, Q,and R branches of the A 2�3/2←X 2�3/2 transition using mo-lecular constants are obtained from previous spectroscopicresults. The simulations are restricted to the 35Cl16O isoto-pomer since we detect only m /z=35 and we have used arotational temperature of 100 K. Although the PHOFEX dataqualitatively matched the band envelopes we were unable toresolve the rotational structure since the individual imagesconsisted of a limited number of laser shots, necessary toavoid drifts in the source and laser powers. It should benoted that it was necessary to analyze images, rather than thetotal ion signals, as a function of wavelength in order todistinguish between the true signal and signal due to en-trained Cl�2P3/2� atoms or ion-pair signal. The effect of finitelifetime on the anisotropy parameter can be calculated usingthe following equation:43–45

�� = ��=0�1 + �2�2

1 + 4�2�2 , �14�

where � is the predissociation lifetime, � is the rotationalangular velocity, �� is the observed anisotropy parameter,and ��=0� is the intrinsic anisotropy parameter which should

2 2
be 2.0 for a parallel transition �A �←X ��. Since the ob-


served anisotropy parameter is very sensitive to rotationalstate as well as the predissociation lifetime, we measured aseries of image intensities across the bandhead to identify therotational state of ClO.46 Lane et al. reported that the lifetimebroadening showed no dependence on the rotational states,i.e., the predissociation mechanism is dominated by spin-orbit interaction between the A 2� and the dissociative state.Assuming a J�-independent predissociation lifetime, we canfit the wavelength-dependent anisotropy parameters for the9-0 band and 8-0 band using Eq. �14�. The peaks in the lowerpanels of Fig. 9 represent overlapping P�J� and R�J+5�branch lines and the anisotropy parameters calculated usingEq. �14� represent weighted averages of the excited-state Jvalues. Based on this analysis, and illustrated in the top pan-els of Fig. 9, we have determined predissociation lifetimes of1.5±0.2 and 1.0±0.4 ps for the 9-0 and 8-0 bands, respec-tively. Our results are in reasonable agreement with the life-times reported in previous high-resolution spectroscopicstudies, as indicated in Table II.11–13 The differences betweenthe lifetimes estimated using Eq. �14� and the lifetimes re-ported from line broadening measurements are not unex-pected. The derivation of Eq. �14� relies on the coherentexcitation of different upper J states via P-, R-, and Q-branchtransitions which leads to a classical time evolution in orien-tation sampled by delayed dissociation. However, the presentcase is characterized by narrow-band excitation, essentiallycw on the time scale of predissociation. The optical selectionof individual parent J states �albeit overlapping J states dueto P- and R-branch transitions� implies predissociation slowenough to permit partially resolved rotational lines, suggest-ing that the conditions necessary for the implementation ofEq. �14� are not realized. We are currently pursuing studies
aimed at further investigation of the angular distributions

arising from the photodissociation of these resolved bandsusing imaging of the O�3P0� fragment to provide an unam-biguous measure of the spatial anisotropy. A recent high-level ab initio study employing Fermi golden rule calcula-tions on the excited states of ClO by Lane et al. identified theprincipal dissociative states that couple to A 2�3/2 state. Can-didate states for the predissociation of A 2� state were se-

FIG. 9. The bottom panels show thespectral simulations of 35ClO absorp-tion �solid lines� in the region of the9-0 band �left� and 8-0 band �right� at100 K. The filled circles represent therelative Cl�2P3/2� image intensity. Thetop panels show the measured aniso-tropy parameters �filled circles� andassociated simulations �dashed line�using Eq. �14�. The best fits providepredissociation lifetimes of 1.5±0.2and 1.0±0.4 ps for the 9-0 �left� andthe 8-0 �right� bands, respectively.

TABLE II. -dependent lifetimes for the A 2�3/2 states of ClO.

Band�a

�ps��b

�ps��c

�ps��d

�ps�

0-0 4.431-0 2.002-0 1.973-0 0.974-0 1.285-0 1.06 0.836-0 1.15 0.53 1.0±0.37-0 1.08 1.06 1.028-0 0.92 1.11 1.0±0.49-0 2.27 1.03 1.5±0.2

10-0 1.45 0.94 1.0±0.311-0 0.98 1.4812-0 1.92 1.4213-0 2.5614-0 2.9215-0 5.0716-0 3.0417-0 13.3118-0 10.24

aMcLoughlin et al. �Ref. 11�.bHowie et al. �Ref. 13�.cBarton et al. �Ref. 12�.d
Present work.


lected based on electronic configuration arguments. A com-parison of the calculated predissociation lifetimes for eachvibrational level and to experimental predissociation life-times derived from spectroscopic linewidths led the authorsto assign the predissociation primarily to the 1 4�+, 2 4�−,and 3 2� states.

Figure 10 shows all the anisotropy parameters measuredfor the Cl�2P3/2�+O�3PJ� channel below the O�1D2� thresh-old �upper panel� along with ClO absorption spectrum �bot-tom panel� taken from Trolier et al. The dashed line in theupper panel shows the classical long-time limit predicted byEq. �14�. The variation of anisotropy parameter with wave-length across specific vibronic bands is observed, consistentwith Eq. �14� as previously discussed. At wavelengths ap-proaching the O�1D2� threshold the anisotropy parametersfor the Cl�2P3/2�+O�1D2� channel are far from their limitingvalue and several lie below the classical limit. The origin ofthese low anisotropy parameters below �267 nm is due toseveral factors. There is spectroscopic evidence that thesehigh vibrational states have long predissociation lifetimes�Table II�. In addition, at wavelengths shorter than 267 nmthe yield of O�3PJ� decreases as the O�1D2� channels becomeenergetically accessible and an increasing fraction of theO�3PJ� fragments arises from direct dissociation rather thanpredissociation. Since O�3PJ� formation above the thresholdhas contributions from direct dissociation of states reachedvia perpendicular transitions �Table I� anisotropy parametersbelow 0.5 are not unexpected.

IV. CONCLUSIONS

We have investigated the UV photodissociation dynam-ics of ClO above, near, and below the O�1D2� formation

FIG. 10. Wavelength-dependent anisotropy parameters associated with theCl�2P3/2�+O�3PJ� channel �top panel�. The dashed line is the classical long-time limit based on Eq. �14�. The ClO absorption spectrum from Ref. 7 isshown for comparison in the bottom panel.

threshold using velocity map ion imaging. We find that


Cl�2P3/2�+O�1D2� is the dominant dissociation pathway�0.94� above O�1D2� threshold with minor contributionsfrom the Cl�2PJ�+O�3PJ� and Cl�2P1/2�+O�1D2� channels.Based on the anisotropy parameter for the Cl�2P1/2�+O�1D2� channel we believe that this channel results fromdirect dissociation of the 2 2�+ state. We observe O�3PJ�fragments with positive anisotropy, strong evidence of curvecrossing from the A 2� to one of several dissociative states.Both the magnitude and wavelength dependence of the curvecrossing probability are in good agreement with the theoret-ical calculations of Persico. A careful analysis of the speeddistribution of Cl�2P3/2� fragments just above the O�1D2�threshold permits the accurate energetic determination of thisthreshold of 38 050±20 cm−1. Predissociation lifetimes ClOfor selected vibronic bands have been obtained using therotational depolarization of the photofragment angular distri-bution. We find values in the range of 1.0–1.5 ps dependingon the upper-state vibrational level. We also find that theCl�2P3/2� /Cl�2P1/2� branching ratio is approximately 1.5±0.1below the O�1D2� threshold based on careful measurementsat 266 and 291 nm, respectively.

ACKNOWLEDGMENTS

The authors wish to thank Maurizo Persico for discus-sions regarding ClO excited states and acknowledge GregoryHall. The technical assistance of Erin E. Greenwald, KatieAnderson, and Kristin Dooley is gratefully appreciated. Oneof the authors �S.W.N.� wishes to thank Arthur Suits for hishelp and support in getting the imaging experiments up andrunning. Support for this project was provided by the RobertA. Welch Foundation �Grant No. A-1402�.

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mutual error bars.

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The UV photodissociation dynamics of ClO radical using velocity map ion imaging

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