21 July 2000

Ž .Chemical Physics Letters 325 2000 146–152www.elsevier.nlrlocatercplett

Velocity map imaging and REMPI study of the photodissociationof CH SCH from the first absorption band3 3

Pablo Quintana a,1, Ralph F. Delmdahl a, David H. Parker a, Bruno Martınez-Haya b,´F.J. Aoiz c,), Luis Banares c, Enrique Verdasco c˜

a Department of Molecular and Laser Physics, UniÕersity of Nijmegen, ToernooiÕeld, 6525 ED Nijmegen, The Netherlandsb Departamento de Ciencias Ambientales, Facultad de Ciencias Experimentales, UniÕersidad Pablo de OlaÕide, 41013 SeÕille, Spain

c Departamento de Quımica Fısica I and CAI de Espectroscopıa, Facultad de Quımica, UniÕersidad Complutense, 28040 Madrid, Spain´ ´ ´ ´

Received 10 May 2000

Abstract

Ž .The photodissociation of dimethyl sulfide CH SCH at 229 nm has been studied employing a combination of velocity3 3

map imaging and time-of-flight resonance-enhanced multiphoton ionization techniques to detect the CH products.3

Translational energy and recoil angle distributions as well as rotational state populations have been determined for the CH3

photofragments formed in the ground vibrational state. The electronic excitation of CH SCH to the first absorption band is3 3Ž .found to produce fast CH SqCH Õs0 recoiling products with a negative spatial anisotropy parameter of bsy0.85"3 3

Ž . XX0.05. The CH Õs0 products are rotationally cold, the rotational distribution peaking at N s3–4. q 2000 Published by3

Elsevier Science B.V.

1. Introduction

The ultraviolet photodissociation of dimethyl sul-Ž .fide DMS is known to drive the initial steps of a

main oxidation chain in the tropospheric sulfur cyclew x1–3 and has therefore an intrinsic environmentalinterest. From a more fundamental point of view, thephotodissociation of DMS via the first absorption

) Corresponding author. Fax: q34-913944135; e-mail:aoiz@legendre.quim.ucm.es

1 Permanent address: Departamento de Quımica Fısica I, Facul-´ ´tad de Quımica, Universidad Complutense, 28040 Madrid, Spain.´

band is also attractive because it constitutes a rela-tively simple prototype for photodissociation of apolyatomic molecule involving non-adiabatic dynam-

w xics 4 . The absorption spectrum of DMS at wave-lengths 214–230 nm is dominated by the first dipole

1 Ž . 1allowed transition denoted 1 B 9a §3b §X A1 1 1 1w xin C symmetry 1,5–8 . Upon electronic excitation,2 Õ

the C symmetry of the parent molecule relaxes to2 Õ

C symmetry, and the photodissociation can thens

proceed via a superposition of the first excited elec-tronic 11AXX and 21AXX states. The latter state does notcorrelate adiabatically with ground state products,but dissociation may occur via vibronic couplingwith the lower 11AXX state. The region of conicalintersections of both potential energy surfaces lies

0009-2614r00r$ - see front matter q 2000 Published by Elsevier Science B.V.Ž .PII: S0009-2614 00 00697-7

( )P. Quintana et al.rChemical Physics Letters 325 2000 146–152 147

within the Franck–Condon region of the one-photonabsorption and presumably gives rise to a rich vibra-

w xtional activity in the dissociating complex 4 .Recent experimental studies employing reso-

nance-enhanced multiphoton ionization time-of-flightŽ . w xREMPI-TOF spectroscopy 9,10 , have shown thatthe photodissociation of the CH SCH molecule and3 3

its isotopomer CD SCD on the first absorption3 3Ž .band yields fast recoiling methyl CH , CD radi-3 3

cals with a spatial anisotropic parameter bsy0.9"0.1. These experiments confirmed the perpendicu-lar nature of the 11B §X1A dipole transition and1 1

suggested a prompt dissociative mechanism preferen-tially leading to energy transfer into fragment trans-

Ž .lation about 80% of the available energy . Themeasurements also showed that the methylphotofragment receives a moderate excitation of the

w x w xn ‘umbrella’ 9 and n ‘stretching’ 10 vibrational2 1

modes. In addition, these REMPI experiments re-Ž . Žvealed that the recoiling CH Õs0 and CD Õs3 3

.0 fragments populate only low rotational states withN XX

-6 and N XX-12, respectively.

At least part of the results of the above mentionedexperiments on the first absorption band are in con-trast with earlier molecular beam translational spec-

w xtroscopy experiments at 193 nm 11,12 . The pho-todissociation of DMS at this shorter wavelengthwas found to produce fragments which were scat-tered isotropically in all spatial directions, carrying

Žaway a smaller fraction of translational energy f.60% in comparison with the observations at 229 nm.

w xVelocity map imaging 13 has recently gained aleading role as an experimental tool in the field ofreaction dynamics, allowing the detection of the fullangle-velocity distribution of photofragments and themeasurement of fragment alignment. In particular,the technique has been successfully applied to exten-sive studies of the photodissociation of several meth-

w xylated molecules, such as ICH 14,15 and BrCH3 3w x16 . In this letter, we report on a combined velocitymap imaging and time-of-flight REMPI investigationof the photodissociation of CH SCH at 229 nm.3 3

The measurements focus on the determination of therecoil energy and rotational state populations of the

Ž .CH Õs0 photofragments. The most relevant re-3

sults are presented and discussed in Section 3 after abrief description of the experimental and data analy-sis methodologies given in Section 2.

2. Experimental and data analysis

2.1. Velocity map imaging

The velocity map imaging experimental set-upemployed has been described in detail in previous

w xpublications 13–15 . Briefly, a mixture of ;15%Ž .DMS vapor seeded in helium total pressure 1 bar

was expanded through a pulsed nozzle directed alongthe axis of a TOF spectrometer. The molecular beamwas crossed 40 mm downstream of the skimmerŽ .1 mm diameter at right angles by two counterprop-agating laser beams. The frequency doubled outputof a Nd:YAG-pumped dye laser operating withCoumarin 460 dye delivered the photolysis wave-length of 229 nm. The REMPI detection wavelengthŽ .286.3 nm was obtained by frequency doubling theoutput of a separate Nd:YAG-dye laser system witha mixture of Rhodamine 590 and 610 dye. Bothlasers are linearly polarized and were run with ca. 0.6mJr5ns pulses at a repetition rate of 10 Hz. Follow-ing resonant ionization, CHq fragments of equal3

velocity were accelerated by means of an electro-static immersion lens to the same point of a two-di-mensional detector consistent with an array of two

Ž .microchannel plates MCPs , a phosphor screen anda CCD camera. Mass selectivity was achieved bygating the voltage of the MCP. The energy resolutionof the Nijmegen imaging apparatus is 30 meV at1 eV kinetic energy release. Calibration of the detec-tor was performed via the well-studied O photodis-2

w xsociation at a wavelength of 225 nm 17 .

2.2. REMPI-TOF

The REMPI-TOF apparatus employed in the pre-sent experiments has been described in detail else-

w xwhere 9,10 . A pulsed free jet of CH SCH was3 3

formed by expanding 170 torr of pure vapor througha pulsed valve at room temperature. A similar 10 ns-

Ž . Ž .delayed pump 229 nm rprobe 285–288 nm laserpulse scheme was used as in the velocity map imag-ing experiments described above to dissociate theparent molecule and detect the CH fragments by3

2q1 REMPI. The 229 nm laser pulses used fordissociation were produced in a Nd:YAG-dye systemby mixing in a BBO crystal the 687 nm fundamentaland its second harmonic at 343.5 nm. The pump and

( )P. Quintana et al.rChemical Physics Letters 325 2000 146–152148

probe lasers were attenuated to 0.1 and 0.3 mJrpulse,respectively. The pump laser polarization was main-tained perpendicular to the TOF axis while the probe

Ž .laser polarization was fixed at 54.78 magic anglewith respect to the pump laser polarization. A seriesof experiments with different orientations of theprobe laser polarization were carried out and nodetectable effects on the relative intensity of therotational branches of the CH REMPI spectrum3

were observed. The laser pulses were triggered so asto interact with the early edge of the gas pulseplateau in order to minimize the signal originatingfrom clusters while still achieving an effective cool-ing of the internal degrees of freedom of the DMSmolecules. A Wiley–McLaren ion source operating

Žunder space focusing conditions extraction and ac-celeration fields of 80 Vrcm and 320 Vrcm, respec-

. qtively accelerated the CH photoions at right angles3

with respect to the jet axis towards a MCP detectorthrough a 20 cm field-free time-of-flight tube.

2.3. Rotational population analysis of the CH frag-3

ment

ŽRotational state populations of nascent CH Õs3.0 have been determined from the measurement of

the 4p 00 rotationally state-resolved 2q1 REMPIz 0

spectrum. The methodology employed for the analy-sis of the spectrum has been described in detail

w xpreviously 9,10 and only a brief description isŽ X .included here. The intensity of each N , K §

Ž XX .N , K REMPI rotational transition follows the rela-tion:

S X XX 1N N , KX XX XXI A P PP , 1Ž .XXN N , K N , K

X2 N q1 DN , K

where P XX is the relative population of eachN , KŽ XX .N , K rotational level of ground state CH , includ-3

ing the nuclear spin degeneracy, and S X XX are theN N , K

two-photon line-strength factors for each rotationalw x Xtransition 18 . Finally, D accounts for the het-N , K

erogeneous predissociation of electronically excitedCH , which for the 4p 2AXX in the ground vibrational3 z 2

state is dominated by rotation–translation Coriolisw xcoupling 19 . Predissociation induces a natural fre-

quency broadening Dn in the REMPI transitionsnw xwhich can be expressed as 19

X X 2Dn sDn qDn P N N q1 yK . 2Ž . Ž .n h i

The present best-fit values for the homogeneous andheterogeneous predissociation line broadenings, Dn h

s1.0 cmy1 and Dn s0.12 cmy1, respectively, areiw xin good agreement with earlier works 19 . Each

individual transition has been simulated with aLorentzian peak of width Dn convoluted with an

Gaussian function of width Dn s1 cmy1 to ac-exp

count for the instrumental broadening.Ž XX .We have evaluated the N , K state distribution

of the CH fragment by means of a least-squares fit3

of the measured REMPI spectrum. The populationdistribution, P XX , was expressed asN K

XX < XXXXP sr N Pr K N , 3Ž . Ž . Ž .N K

where the overall population probabilities of the N XX

Ž XX .states, r N , were taken as least-squares fit param-Ž < XX.eters. The same three trial functions r K N intro-

w xduced in a previous study 10 were used to describedifferent possible propensities of the orientation ofthe angular momentum of the CH fragments:3

< XX < < XXXXr K N sC PG P 1q2 K rNŽ . Ž .1 N K

high K-values favored , 4Ž . Ž .< XX < < XX

XXr K N sC PG P 3y2 K rNŽ . Ž .2 N K

low K-values favored , 5Ž . Ž .< XX

XXr K N sC PG no K propensity . 6Ž . Ž . Ž .3 N K

The nuclear spin degeneracy factor, G , is equal to 4K

and 0 for N XX even and odd, respectively, and Ks0,whereas it follows the progression 2, 2, 4, 2, 2, 4,

< < Ž < < XX . w x. . . , for K s1, 2, 3, 4, 5, 6, . . . K (N 20 .The constant C XX normalizes to unity the conditionalN

distribution within each N XX-manifold.Since K is a measure of the projection of the

rotational angular momentum on the C symmetry3Ž < XX.axis, the r K N function, implies a tendency of1

the CH fragment to rotate around an axis close to3Ž < XX.the C one, whereas r K N corresponds to the3 2

preference for a ‘tumbling’ rotation around an axisŽ < XX.close to the CH plane. The r K N function3 3

describes products with uniform probability for all Kvalues except for the nuclear spin degeneracy.

( )P. Quintana et al.rChemical Physics Letters 325 2000 146–152 149

3. Results and discussion

Fig. 1 depicts typical raw and Abel-transformedŽ .images of the CH Õs0 radicals formed in the3

229 nm photodissociation of CH SCH and detected3 3Ž 0via 2q1 REMPI at 286.3 nm Q branch of the 00

.4p §X transition . The arrow next to the raw im-z

age indicates the polarization direction of the 229 nmdissociation laser.

The most salient feature observed in the images isan intense anisotropic ring located close to the outeredge of the image, which is associated with CH 3

fragments with recoil velocities close to the maxi-mum allowed by energy conservation. The analysisof the velocity map images for this ring yields a

Ž . Ž .Fig. 1. Measured raw top and Abel inverted bottom velocityŽ .map images of the CH Õs0 products from the 229nm pho-3

todissociation of CH SCH , recorded on the Q-branch of the 4p3 3 z

00 REMPI band. Darker areas represent higher ion signal levels.0

The arrow indicates the polarization direction of the pump laser.

best-fit anisotropy parameter b s y0.85 " 0.05,which is close to the limiting value of y1, corrobo-rating the predominant perpendicular character of the11B §X1A electronic transition of the parent1 1

w xCH SCH accessed at 229 nm 1,5,6 . This anisotro-3 3

py parameter is in good agreement with the bsy0.9"0.1 value obtained in recent REMPI-TOFmeasurements of the photodissociation of the deuter-

w xated variant CD SCD at 229 nm 10 in a free jet.3 3Ž .Fig. 2 depicts the center-of-mass CM recoil

Ž . Ž .energy distribution, P E , of the CH Õs0rec 3

products extracted from the images. On energeticgrounds, given the S–C bond energy of 3.25 eVŽ y1 . w x314 kJ mol for DMS 11 , a maximum energy ofE s1.64 eV is available for translation of themax

Ž .CH Õs0 fragment. As can be seen in Fig. 2, the3Ž .P E distribution shows a gaussian-like peak cen-rec

tered at E f1.22 eV, which is associated with therec

intense outer ring of the measured velocity mapimages. The broader and much smaller peak at Erec

f0.1 eV originates from the slower CH products3

observed in the form of a diffuse ion cloud in theŽ .central part of the images see Fig. 1 . For these slow

Ž .CH Õs0 fragments, no reasonable fit indicative3

of the formation of rovibrationally excited SCH 3

partners could be found. Therefore, we consider thatonly the fast CH radicals are formed in the pho-3

todissociation of CH SCH , whereas the slower3 3

fragments originate most likely from the photodisso-ciation of DMS clusters generated in the supersonicexpansion. A similar contribution from parent clus-ters has been observed in velocity map imaging

w xexperiments on the photodissociation of ICH 14,153w xand BrCH 16 .3

Ž .For the fast CH Õs0 fragments, the present3

results yield an average fraction of the availableenergy being transferred into translational energy of² : ² :x s E rE f0.74, which is in good agree-rec max

ment with that determined in REMPI–TOF measure-ments of the 229 nm photodissociation of CD SCD3 3Ž . w xf0.8 10 and is larger than the corresponding oneobtained in molecular beam translational spec-troscopy experiments of the photodissociation of

w xCH SCH at 193 nm 11,12 without final state se-3 3Ž .lection in the methyl fragment f0.6 .

Fig. 3 shows the CH 4p 00 REMPI rotational3 z 0

state resolved spectrum. Although no recoil velocitydiscrimination was performed in the present mea-

( )P. Quintana et al.rChemical Physics Letters 325 2000 146–152150

Ž . Ž .Fig. 2. Recoil energy distribution P E of the CH Õs0 products extracted from the images of Fig. 1. The energy diagram in the upperrec 3Ž .part of the figure indicates the nominal recoil energy for CH Õs0 photoproducts formed in conjunction with vibrationally excited3

Ž2 XX .SCH E ,Õ , N s0 fragment partners.3 3r2 k

Ž Ž .surement i.e., fast and slow CH Õs0 products3.are equally detected , the contribution from cluster

dissociation is thought to be negligible, since precau-tions were taken to make the measurements at therising edge of the gas pulse. The spectrum displaysO, P, R and S transitions from product rotationalstates with N XX s0–6, that are clearly resolved fromthe intense and congested central Q-branch. Theintensity of the observed transitions decreases rapidlywith increasing N XX value. This observation cannotsolely be attributed to the loss of detection sensitivitydue to predissociation of the intermediate electronicstate, but to an actual drop in the population proba-

w xbility for the higher rotational states 19 . Fig. 3 alsoincludes the best-fits to the measured spectrum ob-

Ž < XX. Ž .tained using the functions r K N is1, 2, 3iŽ . Ž .defined in Eqs. 4 – 6 . The conditional K-state

Ž < XX.distribution function r K N , which favours pop-2

ulation of the lower K states, provides the bestŽoverall fit to the measured spectrum top simulation

.in Fig. 3 , reproducing quite satisfactorily the rela-tive intensities and widths of all the observed lines.

Ž < XX. ŽThe fits obtained by using r K N no K-propen-3. Ž < XX. Žsity and especially r K N high K-values fa-1.vored show somewhat worse agreement. The

Ž < XX. Ž .r K N model middle simulation in Fig. 3 un-1

derestimates the O rP , O rP and O branches and2 3 3 5 4

overestimates the contribution of the P and R ones.2 3Ž < XX. Ž .The r K N model bottom simulation performs3

somewhat better, but it still leads to the same qualita-tive discrepancies with the experimental spectrum.

The resulting least-squares N XX-state populationŽ XX.probabilities, r N , for the CH fragments result-3Ž < XX .ing from the r K N model fit are shown in2

Ž XX. Ž .Fig. 4. Notice that r N , as defined by Eq. 3 ,includes implicitly the 2 N XX q1 rotational degener-acy. The N XX population distribution peaks at N XX s3

Ž .and 4 show that the CH Õs0 photofragments are3

rotationally cold. This corresponds to an average² : Ž y1 .rotational energy of E f18 meV 1.7 kJ mol ,rot

i.e., roughly 0.8% of the maximum available energy.The energy disposal into the methylthio radical

deserves some consideration. Note that the methylkinetic energy distribution reflects the internal en-

( )P. Quintana et al.rChemical Physics Letters 325 2000 146–152 151

Fig. 3. Top: measured 2q1 REMPI spectrum of the 4p 00 bandz 0

of the CH fragments arising from the photodissociation of DMS3

at 229 nm. Rotationally state-resolved lines are observed at bothsides of the intense Q-branch and are assigned to the two-photonallowed O, P, R and S branches as indicated. Bottom: least-squaressimulations obtained as described in Section 2.3 using the K-state

Ž < XX . Ž . Ž < XX . Ž .distributions r K N top simulation , r K N middle and2 1Ž < XX . Ž . Ž . Ž .r K N bottom of Eqs. 4 – 6 .3

ergy distribution of the methylthio radicals corre-Ž .lated with the CH Õs0 products. These, how-3

ever, represent the major fraction of the methylw xvibrational distribution 9 . According to the present

experimental results, the SCH fragment stores as3

internal energy an average of 25% of the total avail-able energy, i.e. as much as f0.54 eV. This energyis enough to produce rovibrationally excited SCH 3

radicals in the ground electronic state X2 E and3r22 Žthe spin–orbit excited E state ;32 meV split-1 r 2

.ting . The energy diagram shown in the upper part ofFig. 2 indicates the nominal recoil energy for CH 3Ž .Õs0 photoproducts formed in conjunction with

Ž2 XX .SCH E ,Õ , N s0 fragment partners with Õ3 3r 2 k kŽ .ks1y6 quanta of vibrational excitation in the

w xdifferent normal modes n –n 21 . According to the1 6Ž .P E measured for the methyl fragments, the pos-rec

sible number of excitation quanta for each modereceiving significant population in the photodissocia-tion process varies from a maximum of 2 for themodes n and n to a maximum of 8–10 for n and1 4 3

n .6w xThe calculations of Manaa and Yarkony 4 sug-

gest that the excitation of DMS to the first absorp-tion band involves an initial C–S–C asymmetricstretch in the transition complex eventually leadingto an impulsive breaking of the C–S bond, and thusto a large translational excitation of the fragmentswhich is in good agreement with our observations. Inaddition, the C–S–C asymmetric stretch of the par-ent molecule couples the 11AXX and 21AXX electronicstates giving rise to conical intersections within theFranck–Condon region of the one-photon excitationof DMS. In the vicinity of the conical intersectionsof both 1AXX states, significant geometrical changesare predicted by the calculations. In particular, atorque acts on the C–S–C and S–C–H bond anglesof the excited parent molecule, which might lead toan effective excitation of the n degenerate methyl6

rock vibration or other modes of SCH , as well as to3

a substantial rotational excitation of this fragment.We conclude that the photoexcitation of dimethyl

sulfide to the first absorption band at wavelengths

XX Ž .Fig. 4. N rotational distribution of the CH Õs0 products3

originated in the photodissociation of CH SCH at 229 nm. The3 3Ž XX . XXdistribution r N includes implicitly the 2 N q1 rotational

Ž Ž ..degeneracy see Eq. 3 .

( )P. Quintana et al.rChemical Physics Letters 325 2000 146–152152

f227.5–229 nm takes place via a perpendiculardipole transition that is followed by a direct pho-todissociation process implying an inefficient inter-nal excitation of the nascent methyl fragment and apreferential channeling of the available energy intorecoil translation of the fragments and, to a lesserextent, into the internal modes of the CH S partner.3

In fact, the energy disposal into product translation islarger at 229 nm than that found in previous experi-

w xments at 193 nm 11,12 . The complex nature of theexcited states accessed at 193 nm, presumably mainly

1 w xa A state 5 , allows for a more effective coupling1

to the internal energy of the fragments compared tothe more direct dissociation process which takesplace on the first absorption band.

Acknowledgements

P.Q. gratefully acknowledges the financial sup-port from the Spanish Ministry of Education. Weacknowledge the facilities provided by the CAI deEspectroscopıa–Servicio de Espectroscopıa Multi-´ ´

Ž .fotonica REMPI of the Universidad Complutense´de Madrid. This project has been financed by the

Ž .CICYT of Spain grant no. PB98–0762–C03–01and by the EU Research Training Network ‘ReactionDynamics’ HPRN-CT-1999-00007. The imagingwork in Nijmegen is supported by the EU ResearchTraining Network ‘Imagine’ ERFBMRXCT 970110and by the research program of The Netherlands‘Stichting voor Fundamenteel Onderzoek der Ma-

Ž .terie’ FOM .

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