Volume 90, number 6 CHELllCAL PHYSICS LETTERS ‘0 Auyrr 1982

ROTATIONALLY EXCITED CO FROM FORMALDEHYDE PHOTODISSOCIATION *

P~ubne HO and Arlee V. ShlITH Sandra Xarronal Lnborarories. Albuquerque. New.II~rlco 671S.5. US-I

Reccwd 18 June 1982

COU= 26-63) produced by 355 nm phorol}srs oi HzCO !\as obbwcd by VU\’ Ixcr-c\arcd tluorcsccncc The tcmporti

bchanor oiCO(.‘= 36) mdrcatcs that rotxron~lly cwrcd CO tilij lhc rcilc oi thr long-lncd mtcrmcdwc III iamsldrhbdc phorodrssonatron

1. Introduction

The measurement oiproduct quantum-state dtstri- butrons yrelds detaded “state-to-state” information about molecular reactions that can grve important in- sights mto reacuon mechamsms and dynamrcs. In thts paper, we descnbe the apphcauon of vacuum ultra- violet (WV) laser-excited fluorescence techniques to the measurement of the rotational state dlstnbuuon of CO produced by formaldehyde photodlssociauon. The results answer an unportant question regardmg the iormaldehyde phototissoclatlon mechanism: namely, the quesuon of the existence of a long-hved intermedtate state as proposed by earlier workers.

Formaldehyde has been the subject of eYtenstve eu- perimental and theoretical work aimed at elucldatmg its phot$issoclaJon mechanism [I] . After excitation of the A IA1 + X ‘A1 transition (280-355 nm), for- maldehyde can drssoctate via two channels [2] :

HzCO(SO) + Iru --c H,CO(S,. u) ,

*H+HCO,

with dissocration to radicals becoming important with decreasing wavelength between 330 and 340 run.

l T~JS work performed at Sandra National Laborstones sup- ported by the US Department of Energy under contrxr

number DE-ACW76DP00789

Appearance rates for CO fomrcd by H,CO photol- ysis have previously been measured by time-rcsolred absorpuon of indrvtdual lmes from a CO laser [3,4]. whtch lases on a llnuted number of tnnsrtions wth J

= 10 The CO appearance rates ncre much slower than the correspondmg decay rates for H,CO(St) and hnearly extrapolated to approximately zero at zero pressure, indxatmg that colhsions were required for product formation. The presence of a long-hved mter- medlatc state (I) tn the dlssoclarton process has been proposed to explam these observauons. Candrdates ior this state include the tnplet T,, htghly vibrattonally excited levels of So, and the hydrouycarbene tsomer HCOH [3-S].

In contrast to the pressure-dependent product ap- pearance rates, other evidence supports colhsionless &ssociatton of formaldehyde Extensive mcasurcments of HzCO(S t ) tluorescence decays [5.9-17-l showed that even under collisionless conditrons. decay times for single rovtbromc levels near the S, ongm were constderably shorter than the estimated radiauve life- time. Furthermore, they vaned over two orders oi mag- nitude with httle systematrc dependence on energy or rotahonaf quantum number. The interpretarton of these results, based on radrattonless transttton the;l.,: led to the concluston that collisron-free dtssociauon of HXO to H, + CO should occur. A recent molecular be&photof&gnentatton expertment [ 131 contirm- ed that such collision-free dissociation does Indeed oc- cur, in apparent contradictron wtth the CO appearance rate measurements.

407

Volomc Y@, nunibcr 6 CHEMICAL PHYSICS LLT’KRS

WC report hcrz the obsrrvatron of rotation~l~ +x- cited CO(J = 15-63) produced by rhc photodasocrs- non of formaldehyde. it JS now vicar that the above contradictton arose bccduse the eakr CO laser appear- .mcc rate measurements were only capable of dctectmg CO m low rotational levels. Obxxwttron of rotationa@ hot products indicates that rhe behavior of CO(f = IO) nngh~ not bc repr~sr7ntatnc of rhc entlre popula- uon. Indeed, the rcmporai beha~lor observed for CO(J = 36) mdaarci Chat rotationally cxcrted CO com- prlscs tht proposed Io&ned tntrmledrarc.

1. E\perimentat

The cupcrimental apparatus mctuded a L! photoi- ysis lxcr and a VW probe laser with a varrable delay ~O-IO~~u1th~~~trcrof~15ns~betwrm theruolaser pulses. Form~del~~de monomer. prepared from para- form~deh~de [ 141) was photolyzed by the third hap monk oia Nd:YAG laser (-25 mJ. 5 ns, 2 passes). The rotanonaUy hot CO product was detected by WV laser-exited tluorescence using the A I n(u= I) -‘; I S’(u = 0) transltlon VW I&t m the 150 nm re- goon was generated by four-wave mning in hlg vapor ( 13) Two dye lasers wcrc pumped fey the thrrd har- monic of a second Nd Y.% laser One dye laser was set ar 43 1 1 nm(E\srtonStrlbcne-17,0). wfuch 1s two- phoron resonant wrth the 3s * S-3d 1 D transltlon of Mg.

The other dye laser produced tunable h&t m the 5OO- 550 nm range (Euton Coum~rtn 500). wfuch was sutnmcd wtrll two 43 1 I nm pborons to gtvc tunable WVuith a band~vtdth (f~vllrn)of 1 cm-t. The phorolysrs and probe beams v.ere each focused to 22 mm diameter and crossed at r&r angles wlthm a sampk cell Aper- ture< plased before and aiter the cell aIded laser beam dzlimtron and ahgnment.

The sample cell conslstcd of a 1 P roundbottom tksk wth four optical wmdows spaced evenly on the neck. one opucal window on the mouth, and a gas in- let. Thus contigurauon was chosen to mmu7uze the laser psthlengths whale providing a large cell volume, thereby reductng the deleterious effects oi H,CO de- p&on and CO buddup. The photolysis laser entered and exited the cell through a pair of 25 mm diameter antireflecuon coated quartz windows. The VW en- trance wmdow was CaF,, while a VUV interference Tdter (1% Tat 152 nm, 25 nm bandwidth) served as an exit window. The transmitted VUV iaser mrensity

uas monitored using an EMR 541F phototube. A fused sdrca wmdow, placed on the mouth oi the flask, allowed detectton of CO Ruorescence from the mtenc- tlon repon with an EMR 547,G solar blind phototube. The xhca wmdow absorbs the incident VUV radiation and, smce the phototube is msensitivc to 355 nm, no iurther spectral fdtermg was necessary.

The phototube ngnals were processed by a PAK I65/162 boxcar sveragsr. A 15 ps gatewidth ensured detectton of the entire fluorescence puke. A microcom- puter handled the iinng of the lasers, scanning of the Iztser wavelength, and data acquisition. For eacft point in the spectrum, the computer took fhe difference of the total VW laser-excited fluorescence signal from five shots wtth the photolysis laser on and five shots wtth the photolyns 1xer off. to remove the effects of background CO. Shots for which the VW laser ener~ varied more than 1% from the average were au toms- ucslly rejected. One loused such pomts compnsc a 0.1 nm segment of the spectrum . The spectral data were subsequently smoothed vvrth a live-pomt movtng agerage.

For time dependence studies of the Q(36) line, 50 shots were averaged for each delay time, and the back. ground subtracted. To_correct for long-term dnft, these me~urements were alternated with and normahz- sd to measurements taken using reference delay times, which were 1OI?,400 and 800 ns for the 0.3.0.15 and 0 OS Torr HICO pressures. respectiveIy_

3. Results

Transient signals produced by HzCO dissociation were observed in the 15 1.5 to 154.3 nm regon, cor- responding to the J = 25-63 lines of the CO A t ll(u = 1)-X t C+(IJ = 0) transttton No signals were detect- 2dintheAt~(u=~)-Xt~+(u=i)hotbandof~O. FIN. 1 shows a portion of the observed spectrum; as- slgnments are based on hne positions calculated from the equihbrium molecular constants of Tilford and Simmons [ 161, and Goldberg and hliiller [ 171. Inter- ference from background CO prevented the study of

CO product in rotational Ievels that are thermally pop t&ted at room tempera~re, i.e. with.! S 25. S~~rl~~ the bandh~ads of the A 1 fi (u = 0)-X 1 Z+(u = 0) and e3S-@= I)-X ~x+(u=o) transitions prevented work

at wavelengths longer than 154.3 ML Dettied line posttions and a discussion of observed spectral pertma bations WI@ be published elsewhere [IS].

Volume 90. number 6 CHCMICAL PHYSICS LIX-TERS

rig. 1. Part oi thr: I~srr-cwrcd fluorcsccnsc spr‘crrum oi CO

produced b} 355 MI photolysls 010 3 Torr HlCO This see-

mcnt shotis the P(-Ul-41). Q(+I-16) and R(49-50) lmcs

of IhC x ’ Il(u = 1)-X ’ X’ (u = 0) ~rans~tmn The dell) bc-

LNSCII IIIC phorol) SIS and probe laser pulses ~3s 1-00 ns

The tune dependence of the Q(36) signal, shown in

fig. 2, was obtained by varying the delay between the photolysis and probe lasers. Relined measurements of the CO appearance rare ior J = 36 yteld a value of

I SO 2 2.5 /.s- 1 at both 0 08 and 0.3 Torr H,CO, which corresponds to a risettme of 70 +- IO ns. The CO(J = 36) sIgnal decays on a rmcrosecond timescale and e.xhibits the pressure dependence shown m fig. 3. For comparison, fig. 3 also shows the CO(J = I?) ap-

06 - Ia

co I, = 121

APPEARANCE 7 #-

-_ 06-

DECAV

001 ! I 00 01 02 03 0.3

H&O D’ lrm

rig 3 Dcppndrncc ofthc CO(/= 36) dccJ) rxc on H2C0 prcsiurc The sohd hnc IS 3 61 lo rhc c\pcnmcnral ddlJ. r@r

comparison, rhc dxhcd hnc sho\\s th\: prcssurc dcpr’ndcnsc oi thr CO(I = I?) 3ppc3rxxc r31c irom rcis 13.4 1.

pearancr rate measured m the carher CO laser dbsorp- tlon elpenmcnrs [3.4].

4. Discussion

Ths results oi these expertrnents show that H,CO(S,) dissociates promptly to H, and roratlonally euted CO, followed by cohsional relamtlon of he

InsI TIME bSl

Kg. 2 Lxer-cwted fluorescence signal of the CO Q(36) hnr PS a function of rhe delay trmc bcw.ccn the phorol}sls lxer pulse zmd

tie VUV probe pulse SipMs are nonticd to tie nfnrd obscrvcd usmg P 400 ns d&y Jnrcr rn the delay umcs 1s = 25 ns.

409

CO to lower f. -The risetune of 70 f 10 ns we obs2nT2 avdable energy is in product translation (which eorre- for tbr appctrtlnce ofCO(J = 36) mxches the sponds to the rn~~~ in the translation energy dis-

H2CO(S,) decay rates obtained in previous fiuores- tribution observed in the molecular beam photofrag

ccnce measurements. Although our srgnal-to-noise mentatlon expenment [IS]), formation of CO m the does not allow resolution of a fast and slow compo- ./ = 45 state would imply a dissociation impact param-

nent comparable to the two components of St decay eter of 1.0 d. Such an impact parameter is reasonable observed by Weisshaar et al. [ 191 for tripled YAG ex- in view of the path of zero kinetic energy for dissocla- cltauon oiH$O, a comparison of our appearance tion on the So surface found by Goddard et al. [ 7]_

curve with their original decay curves shows good Agaun assuming that 65% of the total energy 1s in prod- agreement_ Our results can atso be compared to those uc t tra~lation, ro~atjon~ excitation of CO is energe ti-

of hllller and Lee [7_0] , who observed nngie compo- caiiy restricted to J < 7 I. Thrs energy can alternattve- nent decays with rclat~vely low stgn&. For 353.2 nm ly be distributed among Hl(u = 1) and CO@ = 0. J cx~ta~on ofH$O, they observed St decay tunes of C 53), or i&(u = 2) + CO(U = 0, .K 23). Previous es- 96 and 79.5 ns at H$O pressures of 0.05 and 0.50 perunents [3$] showed that little energy goes into Torr, rcspcctlv2ly. Studvs of the ~soluted HzCO mol- CO vibration- only ~10% of the CO is vlbrationalty ecule [9,13] have shown that the fast non-radiative exIted. This is consistent with the fact that we saw no decays of St can be expkuned by coupliig of S, signals tn the CO hot band, t.hrou& So IO the product contmuum. Calculations 1111 show rhat such a decay mechamsm produces 5. Conclusion

CO on the same tlmescale as the St decay. The agree- ment between the appearance rare ofCO(f= 36) and Our observation of prompt CO Fo~a~on in H,CO

the decay of H$O(S,) under colhstonal condttlons photopre~ssocia~on ehminates the need for a separate therefore mdicstes that collisions mtroduce no gross long-lived intermediate state, thereby removing the changes m the iormaldehyde dissociation mechanism. mechanistic dichotomy between the collisionless and

The decay of CO(u= O,J= 36) occurs with approxi- cotkonal regunes. Although more detded mforma-

mately rhe same rate constant (;= 1.7 ps- t TOIT-‘) tion on productenergy dtstnbutions, particularly for and pressure dependence (fig. 3) as the appearance of H-, , would allow a more stringent comparison with CO(u = 0,J = 12) [3,-l]. The slow appearance oiCO(J theory, many of the major questlons about HzCO

= I?_) can thus be attributed to rotational relaxanon of photochssocianon have now been resolved, and a um- the promp~y formed high rotational states. Given the fied picture of the dissociation mechanism has emerged. complexly of such a relaxttion process, the agrte-

ment between the measured J = 26 decay and the J= 12 appearance rates IS surprisingly good. The ob- ~c~owl~gemen~ served timescale is reasonable ior CO rotauonal relaxa- tion. Brechtgnac I?-21 has measured a rate constant of Professor C. Bradley Moore is acknowledged for the 12.5 r 1.5ps- I TOW-1 for the transfer of population conceptlon of thrs experiment and for helpful discus- out of the CO(J = IO) level. Relaxttion from high to stons. D.J. Bamford is also thanked for helpful dtscus. low J would be slower. sions, and J.W. Clements for his technical assistance.

Some encrgetlc and geometnc conslderauons show thar II IS reasonable for H&O dissoclauon to produce CO in such high rotational states. The t~nsirion state References for H$O dissocistion found In ab imtio c~cula~ons of the S, surface is highly asymmetnc [7,7,3-Z]. Con- [ I] WM. Gelbxt, M L. Elect and D.F. HeIIer, Chem Rev. 80

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A. Horowtz and J.C. Calbert. Intern. J. Chem. Emetics 10 (1978) 805.

Volume 90. number 6 CHEMICAL PHI SKIS LETTERS 20 August 1982

Volume 90, numbrr 6 CHEMICAL PHI SICS LETI-ERS 20 August 1982

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