Pressure and intensity dependence of multiphoton energy deposition and reaction yield in vinyl chloride

Volume 58, number 2

CREMICAL PHYSICS LETTERS

15 September 1978

PRESSUfzE AND INTENSITY DEPENDENCE OF MULTIPHOTON ENERGY DEPOSITION AND REAcrr0N

YiELD IN TfwML CHLORIDE-f

FrancesM_ LUSSIER,

J-I_ STEINFELD

Deportment of CRemiby. Mmuchusetts bstitute of Technology. Gunbridge. Mwachuserts 02139. US-4

and Thomas F. DEUTSCH Lincoln Laborutory. MrssacimsettsInstitute of Technology, Chnbridge,Massachsetts 02173. USA

Received 8 May 1978 ‘I’he f=wmcY, power. ad pressure dependenceof infraredmultiphoton absorption in vinyl chloride has been investigated. The effect of collisioon~is to aflow more energy to be coupled into the haer-pressure sampIes, which causes the yieId of the laser-induced HCl elimination reaction to increase with vinyl chloride pressure.

1. Introduction A large number of chemical reactions are known to be readily induced by multiple infrared photon absorption. Unimolecular isomerizations and decompositions, in particular, require the absorption of sufficient energy from the infrared field by an individual molecule to overcome the activation barrier for the reaction. In the most widely studied example of such a process, namely, F atom elimination from SF6 , many investigators [l-4] have concluded that the energy deposition and reaction yield depend only on the integrated intensity, or fluence (a) of the laser pulse incident on the sample. AIso, the effect of gas collisions is uniformly deleterious, in that the reaction yield is degraded at higher pressuresz. Among the other multiphoton reaction systems that have been studied is the chloro-substituted ethylene family, in which the principal reaction is elimination of HCl to leave acetylene 161; the prototype of this reaction is that of vinyl chloride 171. r The &I-I-T. work was supported by the Off%e of Advamxd Isotope Separation, U.S. Department of Energy, under Contract EY-76-S-02.2793. The Lincoln Laboratory portion of this work was supported by the National Science Foundation. *A good +mary of these data Is given in fis. 9 and 10 of ref. [5].

CH2=CHCl+

HCSH

+ HCI.

(I)

In order to understand the mechanism of this process, we have investigated the dependence of infrared anergy deposition on laser intensity and absorber pressure, and also the dependence of overall reaction yield on pressure. Optoacoustic methods have proven to be extremely useful for studying energy deposition @-lo], and were used in this investigation. The results indicate that the peak intensity of the laser pulse has an important effect on the multiphoton absorption process, and that collisions between reactant molecles improve, rather than detract from, the overall dissociation efficiency.

2. Optoacoustic

spectroscopy

Experiments were carried out with two separate systems. One, at Lincoln Laboratory, used a Lurnonits model 8Oi TEA CO2 laser having a 186 ns pulse with a 1 ccs long tail. The beam was collimated by a telescope and passed through a 10 cm long stainless steel cell with an electret condenser microphone in a side port [S]. The second, at MI-T., used a Tachisto 215G CO2 Iaser having a 45 ns pulse, and the beam was brought to 2 soft focus in the sample by means of a telescope consisting of two Ar-coated Ge lenses. 277

VoIume58,number 2

mmci%L

PHYSICS LEfl-‘ER,S

15 Septekber 1978

Laser Lme Fg_ I. Multiphoton abszu-ption spectra of vinyi chloride at a pressure of 1 toz Lower cumx srnallsignaz absorption coef’ficients Upper curver normalized optoacoustk s$naIs forCOz laser lines in 10.6 JIIIIband. (a) 0.13 J/cm2 collimated beam, 0.7 MW/cm2; (b) l-3 J/cm’ collimated beam, 7 SEW/cm ; (c) O-16 3 focused beat=. 93 _MW/an2 at beam waist;-(d) OS J focused beam, 290 MW/cm’_ The zero-amplitude origins for curves
The beam profile for both configurations was measured with a scanning pinhole; the Tachisto profile was found to beTEB&o mode with a diameter =05 mm at the beam waist_ The Lumonics beam was muhimode, with a diameter of 3 mm at the half intensity points. The MJ.T. optoacoustic cell was constructed of a l-in diameter Pyrex T-joint, with a Sony ECM-18 microphone inserted at right angIes to the beam path_ Amplitude of the acoustic signal was taken to be proportional to the amount of infrared iaser energy deposited in the gas in the vicinity of the microphone. Fig. I shows the results of these e~.ximents. At moderate photon tluences(1 J/cm’, or up to 6 MWfcm2), the frequency dependence of the optoacoustic signals followed very closely that of the small-signalabsorption coefficients fl1,12] _As the power is raised, additional energy depostiion begins to appear at the more weakly absorbed lines; b&t ii is only at the ‘highestfluences, correspor&ng to peak powers in excess of 100MW/cm2, that the rotational fine structureappears to be completely “filled in”278

This is in marked contrast to SF6 [S-lo], in which a broadening and shift to lower frequencies, as compared with the normal absorption spectrum, is apparent at fluences as Iow as 0.3 Jicm2_Thus it appears that in vinyi chloride, very little absorption occurs out of vibrationally excited statesat fluences below the threshold for reaction, which is 20-40 J/cm2[6,7]. The implication is that power broadening is much more important in the initial “ladderclimbing” absorption steps than in, for example, SF6.

3. Optoacoustic measnrement of energy deposition Optoacoustic measurementsas a function of pulse energy were made for the CO2 laser 10.6pm P(22) and P(32)lines and 9_6mP(20) and P(3O)line.s for pure vinyl chloride at a pressureof 1 torr, and also with varying amounts of added He. Separate measurements were carried out for *he 180 ns pulse from the Lumonics laser and the 45 11spulse from the Tachisto laser. In each case, the acoustic response was cali-

CHEMICAL

Volume 58, number 2

1

_-_

n!

I

I

,,,1111

1

t

I III

I

II

P(30)

0001

I

I

0001

!111111I 001

I I

IlVd

I

01

I~rill~l I.0

I

‘!!‘I

u 10

(J/cm21

@

i Ol-

01

’

I

’

,lll,i

I

I tllil1

01

1

10 Q

1

I

III0

IO

(J/cm21

Fig. 2_ Results of optoacoustic measure ments of energy deposition in 1 torr vinyl chloride for CO2 laser lines in (a) 10.6 pm band and (b) 9.6 gn band. Das&ed ties CoMeCt pointsmeasured with the coIlhated beam of 180 ns pnfses from a Lumonics laser, while the solid lines are for a weakly focused beam of 45 M pukes fkom a Tadsto laser. Also shown in (a) are data of B&k et al [I] for SF& L&e&d BYBM. (c) Energy deposition from P(32) (10.6 ~1 iases line in 1 torr vinyi chloride with O-40 tosr added He.

PHYSICS

LETTERS

15 September 1978

brated by a direct transmission measurement at low incident pulse enery. The calibration was checked by measuring for SF6; the results were in reasonable agreement with those of Black et al. [ 1] _ The results of the vinyl chloride measurements are shown in fig_ 2. The most striking feature is that the collimated 200 ns and focused 45 ns pulses are quite different in depositing energy in the molecules. This is particularly evident in fig. 2a, in which we see that at the P(22) line the short pulse is less efficient in exciting the sample, while just the opposite is true for the P(32) line_ This is consistent with the spectra in fig_ 1 in which an absorption “grows in” at the P(32) line at high peak powers, while the high peak absorption coefficient for the P(22) line begins to bleach. Similar, though less dramatic behavior is found for CO2 laser lines in the 9.6m region (fig. 2b)_ For the most part, less energy is deposited by the short, focused (thus higher-intensity) pulse. This is markedly diffent from the behavior of SF6, in which energy deposition at a given fluence is found to be only “weakly” dependent on laser pulse duration and mode structure [1.4]. A comparison is also given. in fig. la. of (n) for SF6 and for vinyl chloride at the same fluence. At pulse energies where SF6 is well into the multiphoton regime, the vinyl chloride at 1 torr is barely reaching 1 photon/molecule, on the averageFrom this it is clear that the higher dissociation threshold fluence, and corresponding lower yield per pulse, of vinyl chloride as compared with SF&stems almost entirely from the greater initial difficulty of coupling the infrared energy into the former species. The effect of added He is shown in fig_ 2c_ While the addition of He could increase the acoustic signal simply by increasing the V-T relaxation rate, the direct energy deposition measurements described below show that the energy absorbed by the vinyl chloride increases with increasing helium pressure. The addition of buffer gas can increase the absorption by two mechanisms. One is pressure broadening of the rotational fine structure, which would lead to enhanced absorption of the CO2 laser line at nearly alI frequencies. In addition, if sufficient buffer gas is added so that V-T relaxation occurs during the pump pulse, saturation effects will be reduced and the absorption increased_ In this case dissociation should decrease with added buffer gas pressure, since the molecule is merely being cycled between the ground 279

Volume 58, number 2

CHEJffCAL

state and the first excited vibrational state. As we shah see. the latter behavior is characteris:ic of vinyl

chloride_

4. Ressure dependence of energy deposi:ion In order to be able to interpret the data on the pressure dependence of the yield of reaction (I), di_;cussed in the following section, we need to know the amount of energy deposited in the vinyl chloride sample. This was done by a direct transmission measurement, using a Gen Tee calibrated calorimeter to measure laser pulse energy. Measurements were made for the P(22) and p(32) (10_6,~u) CO;? laser lines in l-10 torr of pure vinyl chloride, and for f&e P(32) line in I torr of vinyl chloride with from 0 to 40 torr of added He. In all cases, increasing the pressure of either absorber or buffer gas caused a marked increase in the cross secticn for absorption of the infrared photons, as was also noted in the preceding section. Similar behavior has also been found for transmission of low-power cw CO2 laser radiatron through l-6 torr of vinyl chloride [I31 _ A detailed calculation of pulse transmission through a saturable absorbing medium involves extensive nuinericaTcomputation [ 141. One result of such calculations is that the rotational relaxation time of the absorbing molecule is the most ‘knportant parameter determining pulse transmission. For our purposes, a simpler analyticaJ representation incorporating a singtie relaxation time would be sufficient. Such an expression is available for the steady-state response of a saturable absorber [15,16], and we have used this to represent the data_ For an optically thick absorber, the transmission is given by (IOIIS3t)(J - r) = ln T f qtR,

(2)

where T =I-/IO and %is the small-signal absorption coefilcient. For a system in which rotational relaxation dominates, the saturation intensity will be given by Lr

= ngo,t

relaX/oabs.

where n is the density of moiecules, 8 their mean relative velocity, and the units of ISdt are photons/ cm2 s_ For a mixture of absorber A in transparent 285

(3a)

PHYSICS

I5

LE‘l-lTRS

September 1978

d&rent B, this becomes 1st

=(nA%t&t

relax

+ w&w&

x~x)l~ab~-

(3b)

From the measured transmission, T, as a function of P at constant Io we can find I,, and from the slope (I&P) we fmd, in turn, v&es for the rotational relaxation cross sections in eq. (3). These-are orot relax = 100 A2 for viny1 chloride-vinyl

chloride

and orot relax = 8 A2 for vi@

chloride-He.

These cross sections correspond to relaxation times of pi =

65 (ns torr) (vinyl chloride-vinyi chloride)

and pr =

275 (ns torr) (vinyl chloride-He)_

These cross sections are in the same ratio as the infrared line broadening coefficients measured from tunable-diode absorption spectra [1 I], but are smaller in absolute magnitude. The linearity of the Isat versus P plots and the reasonable values obtained for trot =hv indicate that this treatment is satisfactory. More important for our purposes, eqs. (2) and (3) enable us to estimate the pulse transmission, and thus the net ener,T deposited in the sampie, over the range of vinyl chloride pressures used in the reaction yield studies described in the following section.

5_ Dependence of rraction yield on pressure The fraction of vinyl chloride undergoing reaction (1) was measured over the pressure range OS to 20 torr, with irradiation conditions held fured at 1500 pulses of the P(32) CO2 laser line at 0.1 J/pulse. The yield was determined by infrared spectrometry of the decrease in vinyl chloride peak absorbance and/or increase in acetylene absorbance; this was supplemented by gas-chromatographic analysis of the lowerpressure samples. As shown in fig_ 3, the fractional yield of dissociation products increases markedly over this pressure range_ This increase is attributable to the increase in energy absorbed from the beam per molecule, as the pressure is increased, which was discussed in the preceding section. If one divides the

Volume 58, number 2

CHEMICAL PHYSICS LE+PTERS

./ .

I/

/

Fig. 3. Reaction yield of &sex-induced HCl elimination from v-by1 chIoride as a function of reactant pressure. Lower

panel: percentage decomposition based on starting material. Upper panel: the yield has been normalized by the amount of infrared energy absoibed, to s%vea “‘quantum yield” which reflects more accurately the efficiency with which the absorbed photons ze utilized. fractional yield by the amount of energy absorbed, the relative “multiquantum yield” shown in the upper haIf of fig. 3 is obtained. This quantity turns out to be a rather weak function of pressure, decreasing by less than one-half its value from P = 05 torr, at which there is fewer than one coIIision per molecule during the laser pulse, up to 20 torr, which permits about 10 collisions to occur_ When helium Is added to a constant pressure of vinyl chloride (1 torr), the dissociation yield drops by 50!% for a IIe pressure of less than I torr, even though much more infrared energy i? being coupled into the system at the higher IIe pressures (cf. fig. 2c)_ These results show that, while self-collisions do not seriously degrade the dissociation efficiency, the effect of a cold burffer gas is quite different. The reaction is taking place in a smrdl, localized volume defmed by the beam condensing optics, in which all the molecules have been multiphoton activated to some extent. Near-resonant V-V collisions will redistribute this energ, but net cooling of the ensemble of molecules by V-T processes will proceed slowly, compared to the half-time of the reaction. Since this time is of the order of the laser pulse length itself 161, a very efficient V-T process

15 September 1978

wilI be required to effect such cooling. Such an efficient deactivation pathway can be provided by diluting the reactant species with a “cold” component or a simple monatomic buffer gas. The buffer gas (helium, in this case) must remove a substantial amount of vibrational enerm from the molecule in a single collision. This behavior is characteristic of highly excited vibrational leve!s of polyatomic molecules [ 17,181; ordinary V-T deactivation rates for low-lying levels, which may require IO00 or more collisions, do not apply to these highly activated molecules. Since it Is only the high-energy tail of the vibrational srate distribution of the multiphoton pumped vinyl chloride molecules which actually eliminate HCI [6], this much deactivation can be very effective in quenching the reaction. The effect of collisions is to overcome saturation of the ground-state absorption, and permit more of the incident laser energy to be coupled into the system. However, if the collisions are with an inert diluent, most of this energy will be lost from the reactive molecules by efficient V-T processes_ The practica? Implication of this result is that, while many of the reactions induced by infrared m-ultiphoton absorption could be scaled to higher pressures of pure absorbing reactant at little or no sacrifice in yield or energy efficiency, the addition of a scavenger or other diluent may indeed cause much of the absorbed energy to be wasted. especially in systems near threshold.

6. Conclusions The results presented in this paper differ in several ways from those obtained for other systems. Ethylene and its halogen derivatives, of whkh vinyl chloride b typical, are six-atomic molecules possessing mostly high-frequency vibrations; in vinyl chloride, there =e only two normal modes below 900 cm-’ [I91 _ Such molecules appear to be much more resistant to multiphoton excitation than, e.g., (a) SF6,which possesses a “ladder” of allowed one-photon transitions reaching from the ground state to the quasi-continuum [20,21]; (b)

gge molecules with many low-frequency vibrations, such as ethyl ethar [22] or ethyl 281

Volume 58, number 2

CHEUICAL

acetate [23], which are just about in the quasi-continuum at room temperatureFor rigid, low-symmetry moIecules, the frequency dependence of the muhiphoton absorption, as well as the resulting energy deposition, is affected by the peak intensity of the laser pulse. Such molecules are more easily saturated by high-peak power pulses_ If the initial saturation can be overcome by pressure or power broadening then the reaction wiil occur just as readily as for more easily excited molecules. The reaction threshold Cuence is higher (1040 J/cm’) for the chloroethylene reactions [6,7] than for species such as SF5 [l-a] or ethyl acetate 123) (I- 5 J/cm’), reflecting ohe greater difticulty of initial excitation_ Increased pressure may be beneficial, since it improves the coupling ofthe infrared energy to the moIecule, while for the more easily excited species increased pressure generally leads only to thermal side reactions at the expense of the directly excited multiphoton process. The effects of powtr and pressure broadening on the saturated absorption coefficient at individual laser lines depend on frequency mismatches (vo - vd), at each line. The optoacoustic technique is limited in that the response of the system can be determined only at fmed Iaser frequencies_ A better probe of the power and pressure broadening effects on the singteand mdtiplephoton absorption spectrum would be an infrared doubIe resonance experiment using tunable diode lasers, such as has been appLied recently to SF6 [24,25]_ An experiment of this type will also allow more direct measurement of the relaxation times inferred from the saturation behavior.

References [I ] J-G_ Black, E. YabIonovitch. N_ Bloembergen and S. Mukamd, Phys. Rev. Letters 38 (1977) 1131.

282

PHYSICS

LETTERS

15 SeptemSer 1978

[2] W_ Fuss and T9. Cotter, AppL Phys. 12 (1977) 265. f3] N_ Bloembergen and E. Yablonovitch, Phys. Today 31 (May, 1978) 23. [4] J_L_ Lyman, B-J. Feldman and RA_ Fisher, Opt. Commun. 25 (1978) 391. (5) CD_ CantreB, S&f_ Freund and J-L. Lyman, in: Laser handbook, Vol. 3, ed. M. Ross (North-Hohand, Amsterdam, 1978). [6] C. Reiser, FM. Lussier, C.C. Jensen and J.I. Steinfeld, to be pubhshed. [?I F&f_ Lu&r and If. Steinfeld, Chem. Phys_Letters 50 (1977) 175. [8] T.F. Deutsch, Opt. Letters l(l977) 25. [9] VN. Bagmtashvhi, 1-N. Knyazev, VS. Letokhov and V-V. Lobko, Opt. Commun. 14 (19763 426. [ 101 DM. Cox. Opt. Commun. 24 (1978) 336. (111 B.D. Green and JJ. Steinfeld. Technical Report on NASA Grant NGR-22-009-766 (1976). [12] BD_ Green and J-1. SteinfeId, Appt Opt 15 (1976) 1690.. [ 131 R_ Ale_xandrescu, DC_ DumitrDC_ Dufu. N_ Comauiciu and V. Dr@nescu, Rev. Roum. Phys. 22 (1977) 793. [ 141 D.G_ Sutton, I. Burak and J_L Steinfeld, IEEE J. Quantum Electron. QE-7 (1971) 82. f 151 M_ Hercher, AppL Opt. 6 (1967) 947. [ 163 S.R.J. BNeck, T-F. Deutsch and RM. Osgood Jr., Chem. Phys. Letters 51 (1977) 339. [ 171 D.C. Tardy and B.S. Rabinotitch, Chem. Rev. 77 (1977) 369. 118J CC. Jensen, RD_ Lzviue and J.I. Steiufeld, to be published. [ 19) G. Herrberg, Electronic spectra and electronic structure of poIyatomic molecules (Van Nostrand. princeton, 1966) p_ 632. [to] C.C. Jensen, W-B. Person, B.J. Krohn and I. Overcnd, Opt. Commun. 20 (1977) 275. [21] CD_ Cantrell and H-W.. Galbraith, Opt_ Commun. 21 (1977) 374. [22] D-M. Breruter, Chent. Phys Letters 57 (1978) 357. !23] WC_ Danen. W-D- MunsIow and D-W_ Setser, J. Am_ Chem. Sot. 99 (1977) 6961. (24] P-F. Moulton. D-M Larsen. JN_ WaIpoIe and A. Mooradian, Opt. Commun. l(l977j 51. [25] J-1. Steinfeld and CC. Jensen, in: Tunable lasers and applications. edr A_ Mooradian, T. Jaeger and P. Stokseth (Springer- Berlin, 1976) p_ 190.