Direct observations of collisional effects on the internal energy distribution of propynal following collisionless infrared multiphoton pumping

1 Novcmbcr

Vokmtz 67. number I

DIRECT OBSERVATIONS ON THE WTERNAL FOLLOWING

OF COLLISIONAL

ENERGY

COLLISIONLESS

D_M_ BRENNER

EFFECTS

DISTRIBUTION INFRARED

1979

OF PROPYNAL

MUL-IIPHOTON

PUMPING

3nd K. BREZINSKY

IR-Grbk doubk resornn~z\ is used to study kibratioru1 rekutwn in propyml ttilto~ing infrared multiphoton absorption_ Under coitisionless conditions the ~6 ~ibratiorul manifold (driwn mode) does not equilibrate with tht loser vibrational Meis of nondriven modes. Collisions induoz either intuunolecutar or iatermotecuhr energy tumsfer.

1. Introduction Intramolecular relaxation at moderrtte energies is of theoretical interest [ I], especially in connection with the “quasicontirmum theory” of IR multiphoton absorption [2 j _ Rates have been atimated from mertsurements of infr3rcd linewjdths [3], and from double resomtnce experiments [4,5]_ For e_x3mpIe, using picosecond IR and visible ptdscs, Maicr et al. [4] observed that collisionless intramolecular energy tnnsfer in coumarin 6 occurs in 4 ps at 3n energy of 5950 cm-t_ Relaxation was observed by measuring a change in the initial absorption as a function of time. The absence of coliisions was guaranteed by the use of p&es of short duration for both the pump snd probe lasers_ Because the prcssurcs required in these expcrimcnts are reiatively-high, however, processes occurring on 3 slower time scale. but in the absence of collision. cannot be studied readily_ Recently we have applied the techniques of IRvisible double resonance under f3st ffow and molecular beam conditions to the study of intr3moIecuIar reraxation under colhsionless conditions. This experimental method takes advantage of both the sensitivity OFfhtorescencemeasurements [6] (detectivity of 106 moIecuIesb3 under optimum conditions), thereby aBowing the use of very low pressures. and the mode specificity ofabsorption measnrements for moIecuIes having weli defiicd spectra. Moreover, the use of 36

molecular beams permits the study of perturbations due to various collision p3rtners on the energy distribution prep3red by IR muitiphoton absorption under collisionless conditions. We report here results of doubIe-resonance experiments in the potyatomic propynsl, H-CZC-CHO. This molecule is an 3symmetric top [7,S] having .t sparse density of vibrational states (< I st3teIcni-1 by direct counting methods [9]) up to 4000 cm-t The experiment monitors the vibrational excitation of the lower levels of the ground state subsequent to multiphoton excitation by probing electronic transitions (IA” + IA’) corresponding to hot bands having a common upper state [IO] _The resulting signal is a measure of the occupation density times a Franck-Condon factor. Beuuse the spectroscopy [IO] of this molecule has been we11 studied, changes in population of driven tend nondtiven modes due to vibrational excitation at a p3rticuIar Frequency can be observed unambiguously.

2_ Experimental Fig_ 1 IIIustrates the apparatus employed in these experiments_ A molecular beam or fast flow of propynal is crossed at 90” with a CO2 laser (w = 944 + 1 cm-‘) [ 1 I] and a nitrogen-pumped dye laser (Molectron Carp) The pump Iaser is weakly Focused to a beam waist 5 cm tong (2 X 3 mm in cross section)- The probe laser

CHEMICAL

Volume 67. number 1

1 November 1979

PHYSICS LETTERS MOLECULAR

BEAM

a_ //

/

\\\ LASER

PULSED

caa--Hx PIN

ascrLLo-

SCOPE

TO VACUUM PUMPS

STRIP CHART RECORDER

--l--PDP

11-34

Fig_ I_ Schematic of molecular becm-doubk resommce appxotus end associnted electronics- The optics (3) 40 cm nnd 1 m focnI length lenses and (b) 3 m nnd 1 m foul Iength BnFz lenses nrc used to offset Inser-benm divergence and to produce cohimated benms of well defmed dimensions in the interaction region. Suttered light is minintized by a series of cone-shaped bnffles extending 1 m Lens (c) hns n 5 cm focal Ienrgh nnd is used to maximize the fluorescence signal. A 420 mm cut-off falter is pIxed between this lens and the photomultiplierto the pump laser with (Tektronix PG508) and monitored with a Tektronix 7904 oscilloscope_ A bialkali photomultiplier tube (E.M_I_-Gencom 9635QB), placed 90” to the interaction region, is used to detect total fluorescence produced by excitation at absorption frequencies corresponding to hot band transitions. In the absence of IR pumping, the spectrum obtained is an absorption spectrum uncorrected for quantum yields of fluorescence_ The fluorescence signal is averaged on a boxcar integrator (Princeton Applied Research, models I62 mainframe and 164 plug-m) with an aperture duration of 0.5 !s, placed 100 11safter the dyeis variabIy

delayed

with respect

a delaying pulse generator

laser pulse, and a time constant of 10 ps. Subsequently, the averaged signal is normalized with respect to laser intensity and stored for further processing in a PDP 11-34 minicomputer_ Thus, by measuring the percent change in fluorescence intensity when the CO2 Iaser is on relative to when it is off, a measure of the population change due to vibrational excitation is obtained for either the driven mode or nondriven modes at varying delays relative to the excitation pulse. Propynal is prepared according to the method cf Sauer [ 12]_ The crude extract is dried over magnesium sulfate and then distilled, trap-to-trap, at reduced pressure. No measureable impurities in the purified 37

satttpfc arc detected by IK spcct:oscop> _ Absorption measurements here c.uricd ant at 1 Torr in a 60 X 2.5 cm cc11wirft a cc~ffimatrd bc.mt. Energy mezur~inents wcrc inde wit&l .t Scientccl; disc czf0ruileter (model _58-0102 of SO0 nlJ/srn~ -

\&une

absorber)

xt ZI ffuence

=\t this ilucnce an rtvcr.igc oi3--? pf~utorlslmi~fecufe is _rbsorbcd. in&cating a moderate lc\el of nluftlp!loton absorption. etltgies

Tf~is excitation wfnc!l

datributaf sity

at xl>

in ;t Siten

of states rind

nwdes

of vibrational

chcitation ofp6 at a tluence is sftown m fig_ 2 for tfir *J= f fcrcf.

fal IR

I P OFF

ON

iOO-

IO0 F

I

. . . . . . . f

1t

I

1f

1

il....-‘-.)..

II

1 I

r+..

OO

1

2’4

,..,,,,,.. 6

8

10

12

..I.’

14

i 16

: 18 20

li

40

Cd fps)

Fig_ i_ (a) Excitation speetra, uncorrected for tIuoresaXKe quantum HeId, of the 6y and 10: transitior~ when the CO.2 lyeE is OP and off_ The fluence is ~700 mJ/cm’. the pressure I mTorr. and the dehy time, 6 .u_ (b) Percent cha%e in intensity (%An of 67 as a function of defay time (rd) betWeen pump aud probe hers_ JZrrorbars are based OR at Ieast three kdependent measurements_The insert shows the intensity vfxnts time prome of the CO2 tier FUke-

38

nwfecufe

depending

of Irv,,,,

coupling

on tfle denainoil~

tiloss

= 9&l cm-l_ no EWc1

u= 1 of v6 and other

rind it is predicted

of

wilt be inttrnall)

vibr;ltiorlA

t1t.n in tile absence

of

in v6 (u= I). This is demonstrated by the escitation spectr.i of !-ig_3a. For ftigher energy fevek. II > I _ population of nondriren modes is posstbfc onfl if a mechanism whicft conser\cs energy and couples the initial and ftnaf states exists As indicated in table I ~ tfte nondriven modes V10-V4. and vg are not populated at tire u= 1 fcvef in the absence of collisions. This result is obtained for times to IO@ even though tfie popufation of “6 (u’ f) decays exponentially to 15 ps (fig_ 2b). At 7 mTorr. the time between coffisions is =z2Ojts and it CJI~be assumed that the tiffing of u6 is coffisionless and foffows the faser pulse (see insert of tig_ 2b). We attribute the exponential decay to diffusion [ 131 out of tfre pat11of the probe laser (I 5 mm diameter requiring a maximum of 4~ traversal time dt 2 mTorr) and not to a coffisiondly induced relaxation phenomenon (vida infra). These results demonstrate that during muftiphoton absorption, the fow lying states of nondriven modes do not become equiftbrated in the absence of coffisions. States at a particular energy &v,~. can equilibrate internally at that Ievel but the vibrational manifofd is not filled below that level. As a consequence. d nonBoltzmann vibrational energy distribution is established and the vibrationaf heat capacity [i4] is considerably smaffer than would be predicted for a V-V equilibrated system- For exampfe, in the case of propynai, “the vibrational temperature” for molecules which have absorbed 4i1ulaser is somewhere between the extremes 6 13 1 K (energy locafized in v6) and 1040 K (complete V-V equilibration)_ Although not surprising it should be appreciated that the “missing heat capacity” results from nonequilibration in low lying states and excludes the application of equilibrium thermodynamics to such systemsTabfe 1 also shows the effect of collisional perturbation on the populations of driven and nondriven modes. At a pressure of 20 mTorr. the collision time is z23~5; external

Tile effect of =800 mJ/&

exist

.I distribution

~rlwt,

the ailb~rnwnie

st.tta_ At d11 cnsrgy reson;lnces between

3. Results and dbcuuiua

produces

nluftiple

perturbations

energy

will remain

IocJfized

Volume

1 November 1979

ClIEVIC4L PHYSICS LJXTERS

67. number 1

T.ible 1 Effect of Iow Ie\eI rlbr.ttiotxl e\cnzmon on drlwn

2nd nondriven modes. The %ll(intensity) = [(ill on - 11~ off)/IlR off] X 100% n presented for it &en mode, pressure. And dsh\ rime (rd)_ The delay time between pump and probe pulses is measured from rhe Ieadin$ edge of the former. Error bars are deriwd from at ieasr three independent measurements and reflfcr bcrter than 68% contidencc (for n large number of data points at fixed delay. \cdues zre not observed to deviate from indicated r.mgcj_e) ----_ -___ -____-__-_____.~_ _- --

Pressure (mTorr)

---2.0 10 2.0 1.0 19.9 2.0 90.0 1.0 2.0 -_____

_-----

rd (W)

__-

_~____ _

0 1 2 4 4 6 6 IO 15 -~_---

‘?~\l for mode ___- Q(944 cm-‘) 5’ vA(1697 an-‘) (C-C stretch) (C=O stretch) .~_____ .- _-- __ __. 1.0 2 4-s 16.8 5 1.3 17.5 5 3 2 0.3 r 2 6 23Sz?O 15.4 r 05 19.0 f 0.9 3 0 *a.7 I:! 0 * 0.9 11.3 5 1.3 13.3 f 3.1 1.6 f 5.8 54+08 ___ _-_-___-___.

a) T&m from Brmd and Watson (81. b, _4t 0.1 mTorr, %1i(z+) is 0.1 I I.9 (cd = 2 JZS).-0.2

2 1.6 (6&,rindO

for a delay time between pump and probe pulses of 6~. collisional effects. therefore_ become important. Thts is verified by the observation that J small number of_collisions effect the redistribution of energy into v4 and “to _ However. the mechanism by which these levels are filled is unclear. Collisions cdn either perturb t!ie initiai distribution by coupling states which would otherwise be uncoupIed due to symmetry restrictions or conservttion of energy, or result in actual V-V, intermolecular energy transfer_ The former c.m be illustrated by the equations

+v6(u=o)

v4(u= I) + &I -

I80 cm-l

_

The filling of vlo(u= I), although slightly endothermic, is nearly a resonant process and should occur at gas kinetic rates- We assume from absorption measuremetItS that u = 2 of “6 is populated; an unfavorable Fran&-Condon factor precludes observation of this band at low pressures_ Alternatively, sbnilar equations can be written for processes involving intermolecular energy exchange- By varying hl, we hope to distinguish between these two cases on a single collision basis_

v,o(980 cm-‘1 (form) 1 CH \\a~) - -._-_ - -- _ ___ -

~~(206 cm-‘) (W-C bend)

2.3 + 6 6 -0-3 r 0.3 b) 1.5260 0.1 + 4.9 9.0 5 1.7 5.8 r 55

-0 6 -02 -0-7 0.1

= 0.1 b) z 0.8 f 0.8 b) f 05

8 z 1.4 (10~s).

Previous work on small molecules like CH3F [ 151 has shown that V-V equilibration requires IO’-IO’ collisions_ From these preliminary results it appears that energy is redistributed (and probably equilibrated) on a much faster time scaIe_ However, the rules which gorern collisional!y-induced equilibration of the various levels in larger polyatomics will be of future interestFor example, it would be expected that ug(L‘= I) should be populated rapidly since

involves only 4u = I changes and no changes in symmetry_ However, substantial filling of vg(u = I) in G 3 collisions does not occur. The advantage of using IR-visible fluorescence double resonance under crossed beam or beam-gas conditions is its sensitivity and time resolution in studying collision-induced intramolecular and intermoIecuIar energy transfer problems_ The major disadvantage is the small Frank-Condon overlap for high quantum levels and therefore the need for larger densities of molecules_ To some extent this problem can be minimized by using supersonic nozzle conditions_ The preliminary results reported here demonstrate the usefulness of this technique for studying energy transfer in appropriately chosen systems. Details of the f!uence and waveIength dependence (944 versus 3300 39

Votume 67. numbcr

cm-‘) for the

c I11-\lICAL

I

collisionless

PIIYSICS

case wiii be reported at a

LFI-lx 131 LV. faer

later time_

1979

Rcdd) . R G. Bray and \l.J_ Berry. in: Adv3nccs in chemistry.ed.

A.M. Zewzdl (Springer.

Berlin. 1978):

J-W. Perry and A.11. Zrrxail. J. Chem. Phys. 70 (1979)

Acknowledgement Research arried

1 Norember

RS

out zt Brookhven

ratory under contrrrct with the US_ Energy and supported

Srttiond

Department

Laboof

by its Oftice of Basic Energy

Sciences_

582. [4 J J-P. XI&. A. Seiimcir. A. L.mbvx.zw nnd W_ Kaiser. Cbem. Phzs. Lcttsrs36 (1977) 517. [Sj DS. rraniel Jr.. J. Ghan. Phks_ 65 (1976) 1696. 161 RX. Zxe and PJ. Dzgdi$m.Science IS5 (1974) 739. 171 CC. Costnin and J-R. Morton, J_Chcm- Phls. 31 (I9591 389. 181 J.C.D_ Bmnd and JKG. Watson. Trans. Faraday Sot_ 56 (1960) 1582[9l R-E_ Eiarriqton. BS. Rabinorith and R-W_ Dkarz. J. Chem. Phys. 32 (1960) 1145: B S. Rabinwitch and J H. Current, J. Chrm. Phls. 35

References [ 1; J. Ford. Advx!. Chem. Ph) s. 24 (1973) 155. K. Store. J. h’ordhohn and S-A_ Ritz. J. Char. (1974) 203:

(1961) [!O] Ph>s. 61

R-A. Uxcus. Ber. 65 (1976) 2150. 121 N_ Bloembrrgen. CO_ Csnrrell md D_SS. L.usen. in: Tunzbte ixers and spptications. edr A_ Mooradiar. I_ J,vuer and P_ Stokseth (Springer. Berrlin. 1976): S_ bfuktmel and J. Jortncr. J. Chem_ Ph) s. 65 (1976) 5104; R-V_ Ambartzumixn and VS_ Lctokhov. in: Chemical and bioIo$caI applications of ktscrs, VoL 3. rrd. C_B_Jfoor8? (Academic Press. New York. 1977).

7,250.

J.C.D. Brand. J.H. Callomon and JKG. Watson. Cm. J. Phys. 39 (1961) 1508: Dkcussiom I;trJday Sot. 35 (1963) 175s

[I 11 A.B. Ilorxkitz and S-R_ Leone. Rek_ Sci_ Instr_. to bc pubkhed. [IZj JS_Sauer.Or,.Syn_Colkcr.4 (1963) 813[ 131 M_ Kov;lrs, D_ Rzunachrtndm Rzo zmd X_ Jwzm. J. Chrm_ Ph)s.48 (1968) 3339; R.D. Bates. J-T_ Knudtson. G_\V_Flynn and A.M. Ronn. J_Chem_Fh)s.57(1973)4174. [ 141 1. Shamah and G. Fi) nn. J. Chrm. Ph)s- 69 (1978) 2474.

And references therein. [ 15 1 E_ Wcitz zmd G-W_ Fi) nn. Ann. Rev. Phys. Chum- 15 (1974)

275. and references therein.