Infrared multiphoton-induced chemistry of ethyl vinyl ether: Dependence of branching ratio on laser pulse duration

Vohrme57. number3

INFRARED

1 Augost 1978

cI%EMIcAL PHYSICS JLElrmRs

MULTTPKOTON-INDUCED

DEPENDENCE OF BRANCHING

CHEMTSTIXY OF ETDYLWNYL RATIO ON EASER PULSE DURATION

D.M. BRENNER Depmtmenr of chemistry, Brvok%aven N&d V@ton. New York 11973.USA.

ETEIER:

L&oratory.

Rec.&cd21 February 1978

Revisedmanuscriptreceived27 April 1978

The ‘*collkionless”. IR multiphotonindnccd decomposition of ethylvinyletherhasbeenstudiedwheretheyieldsand ratioof thetwo unimolecular reactioncbanneis havebeenusedto pr5e theinternalenergydistribution duringlaserpumping.It is foundthattheyieldsandbranchiq ratiocanbe variedby changing thelaserpulsedurationandthatbooth reaction channels areobservedat threshold whenthepnbe durationis 0.2G.

1. introduction The observation that polyatomic molecules absorb many infrared photons during a single intense laser pulse in the absence of collisions [l] affords the opportunity to examine internal energy distributions of vibrationally excited molecules prior to unimolecular decomposition. Measurementsfrom molecular beam 121and low pressurebulb experiments [3] have been consistentwith internally randomized energy prior to reaction. In the case of SF6, these experiments have also been consistentwith a model for multiphoton absorption in which the rate limiting step depends on the total energy of the laser pulse [4]_ Indirect results from product branching ratios IS-81 and product internal energy distriitions [9] also support these conclusions. Ethyl vinyl ether (EVE) undergoes two thermal reactions [lo] : a retroene reaction

and homolytic cleavage of the C4-O bond

L

CH,

== C 0 + CH$HO log t,=

+ CHz = CHH, + CH,-CH, :s-6sGOO/2.3m[e]

Recently, Brauman et al. [S] observed that radical disproportionation (kd) in reaction (2) done competes with reaction (1 j when EVE is subjected to an intense infrared radiation field at IO.6 cr(0.3 J/pulse,focused). By measuring the ratio, acetaldehyde/ketene,as a function of pressure,they found that the reaction is independent of pressure and consistentwith a thermal distribution of energy_ in this paper, we report the 9.6 j.fCO2 laser photodissociationof EVE. The effect of fluence and pulse duration on the branching ratio and yiehis of reactions (0 and 01 is examined. The con&tions used for photoinduced reaction are different than in the pretiously reported experbncnts [S] and yield different results. Evidence is presented for a peak-power dependent, collisionlessmdtiphoton-induced reaction which may 357

Volume 57, number 3 be mcompatible with a statistical

CHEMICAL PHYSICS LETrERs

distribution of en-

1 August 1978

varies from 02 D (single spike) to x2 ius(O-2- spike

ergy prior to reaction.

followed by one or more broad p9ses) (see fig. 1). Products, quantitatively trapped in 4 4 of ethanol, are analyzed by gas chromatography with flame ioni-

2. Experimental

zation detection on a Porapak Q column. All product identifications are made by gc-mass spectrometry and by gas chromatography in which authentic samples

A tunabIe muItirnode Lumonics 103 lase. was used for these experiments_ The laser beam, 5 cm in diameter, is radiahy truncated by passing it through a 1

cm diameter fxed aperture. The intensity of this beam varies by no more than 1% over its area as measured

with a calibrated disc calorimeter (Scientech) and a 500 p pinhole. The beam was attenuated with silicon or Zn&- flats and is collimated with a gal&an telescope in which the divergent lens is used as the front window of the irradiation cell (85 cm in length, 3.3 cm in diameter)_ The pulse duration is adjusted by changing the percent composition of nitrogen in the laser cavity and

were coinjected with product mixtures.

EVE (Aldrich Chemical Co.) contains an unidentified impurity (<2%) which is inert under the conditions of irradiation. This starting material is sensitive to metal and glass surfaces and undergoes a minor degree of decomposition in the Pyrex reaction vessel. Under the reaction conditions, 0.030 + 0.002% decomposition to acetaldehyde is observed reproducibly_ The data has been corrected for this “dark reaction” and for the difference in FID response factors of butyraldehyde and acetaldehyde relative to dimethoxy-

Fig. 1. Gaschromatogaphictracesof the reactionmixtureresui~ from irradiationof 0.002 torr of nezt EVE at constantfhxence (0912 J/an*) zi& different pulse&rations: (a) zp = O-2 11s;(b) rp = 2 M_ The peaksof interest are (1) ethylene, (2) acetaldehyde, (3) butane, (4) ethyl vinyl ether, (5) butyraldehyde.

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Volurre 57, number 3

PHYSICS LETTERS

CHEMICAL

3. Results and discussion Irradiation of EVE (0.4 torr of neat gas) at 1041.2 cm-l

(P(26)

of the OO” l-C@0 CO*, laser transition. stretching frequency) with a fluence of 091 J/cm2 produces acetaldehyde, ethylene, and butyraldehyde as tbe major products, and butane to a lesser extent. The products are stable under these reaction conditions. At higher fluence,

corresponding to the C,-0

butyraldehyde decomposes and ketene, trapped as ethyl acetate, is also observed. Similar results are obtained when the laser frequency is tuned to 974.6 cm-l_ Evidence that butyraldehyde is derived from reaction (2) comes from the observation that addition of NO quenches its formation; under these conditions the yield of acetaldehyde decreases to a minor extent, consistent with the concerted nature of (1). If butyraldehyde and acetaldehyde had come from similar pathways, the change in their respective yields due to NO deactivation and/or quenching would have been the same. The reaction of ethyl radicals, produced from photolysis of ethyl iodide in a separate experiment, with ethyl vinyl ether does not generate butyraldehyde or acetaldehyde. This observation confirms the absence of radical chain processes leading to the observed products. Formation of butyraldehyde therefore reflects directly the primary process (2), formation of free radicals, in the decomposition of EVE. A remarkable featrrre of the IR photolysis of EVE is the unexpected simplicity of the free radical pathway,

Table 1 Variation in bynching F

(J/cm*)

ratio and yieldsa) from unimolecular

lAugust1978

reaction (2). Under thermal conditions, complications arising from chain reactions and dispropor-tionation are expected. The evidence cited above shows the absence of such processes. Moreover, when EVE is irradiated to -20% reaction, >97% of the EVE decomposed is accounted for by the products [butyraldehyde] + [acetaldehyde] _Although secondary photolysis of -vibrationally excited C,Hj or CzHSO- cannot be excluded, the importance of such complications is at best minor. Fig. 1 and table 1 compare the product composition obtained with different pulse durations (rp) at constant fluence. The pulse width is varied by changing the N2 content of the laser. Increasing the percent composition of N2 causes the fraction of energy in the first 0.2 @ to decrease with the remainder distributed in one or more peaks out to ==2 W. Hence at higher fluence but longer pulse width, it has been possible to obtain similar results as those obtained at lower fluence and narrower pulse width. The results of fig. i and table 1 indicate that at constant fluence the branching ratio between (1) and (2) as measured by the ratio of 2 X (Yieldbu~~~de)iOiel$,taldehyde)

be varied by changing the pulse duration. (It has been assumed that recombination of vinyloxy and ethyl &icals to form butyraldehyde and ethyl vinyl ether is equally probable and therefore the yield of butyraidehyde has been multiplied by 2.) The formation of butane has been neglected so that kz/kl isa lower can

decompositionof EVE as a function of pulse duration (~~1

=P

Y(A)/p x 102W

ZY(B)/p x 102e)

0.101rt0.006

Ors)

056.5

OLZ

O-40

1.48 k 0.009

0.0682

0.565 0.912 0.912 0912 0.912

2 0.2 2 0.2 2

O-40 0.10 0.11 0.0023 0.0028

0 153io.04 0 1.612 0.20 0

0.0012 0.333 f 0.018 0.00659-c0.00080 0.680 " 0.111 0.169 i 0.022

* 0.0031

0.505~0.061 1.102-c 0.285 -

a) The yieIds arecalculated from tire areas of the g@c traces relative to the known pressure of dkethoxyethane dard and the total pressure of ethyl vinyl ether included in the beam volume. b) Yield of acetaldehy&Jpulse. e) Yield of butyraldehyde/pulse.

as external stan-

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Volume 67, number 3

cE-IFxICAL PHYSICS LETTERS

Iimit, With the shorter pulse, it is observed that within the limits of the detection system it is not possible to cause the lower energy channel (I) to predominate over the higher energy channel (2) by decreasing the fluence. That is, at threshold (~0-5 J/cm’), the higher energy pathway accounts for a larger fraction of the EVE decomposed than does the lower energy pathway- This is in marked contrast to the thermal reaction of EVE [IO] _ However, with the same number of photons, but spread over a Ionger duration, the Iower energy Channel Can be made to be the excfnsive pathway. These observations imply that the rate of absorption

and the number of photons absorbed during a 0.2 I.LS versus a 2 @ p&e are different_ For the shorter pulse, the rate of pumping appears to be faster than the rate of unimoIecuIar decay via reaction (1). A decrease in the photon flux obtained by increasing the pulse duration seems to decrease the pumping raie so that reaction (I) becomes faster than further absorption_ However, the facr that both reaction channels are observed at threshoId (rP = 0.2 -us)is unexpected. Although the argument can be made that attenuation of a gaussian beam only reduces its size, and therefore the reaction volume, without affecting appreciably the actuaI fIuence, the inter&y proftie of a Lumonics 103 laser is not gaussian, but more Iike a square wave. Moreover, the same yieIds and ratio are obtained whether the intensity is attenuated with ZnSe flats or by simp!y reducing the voItage of the power supply (the latter bin be shown unequivocally to have no effect on the beam diameter). It is concluded that the threshold behavior when rP = 0.2 .USis not simpIy an artifact of the method of attenuation_

2 Auyst

1978

Table 2 shows the effect of varying the EVE pressure over a range of iO3_ At low pressures (10-S to IO-1 torr), the ratio kz/kl is essentiahy invariant, whereas at higher pressure, the decrease in yield and ratio implies colhsional quenching is important. These results show that multiphoton absorption in EVE occurs without cohisional relaxation. The branching ratio at threshold and the cohisionless nature of muhiphoton absorption and subsequent unimoIecuIar decay in EVE reveal qualitative information concerning the dynamics of the pumping process. The rate of absorption appears to be more dependent on the photon flux (peak laser power) rather than on the time integrated laser intensity_ The conclusion drawn from tb& observation is that the rate limiting step in absorption is the result of an “anharmonicity bottleneck” [3,4] or siow Tr relaxation in the vibrationaI manifold_ The former, however, appears to be unJ.ikeIybased on prehminat data from opto-acoustic experiments; the acoustic signal measured for laser puIse durations of 0.2 @ and ~500 ps at constant fhrence is invariant. In SF6 changing the pulse duration at constant fluence produces a large change in amphtude of the acoustic signal, and hence in the number of absorbing molecules [ 113. This difference has been interpreted as reflecting the effect of anhannonicitybottIenecking at low energies in SF6. The ratio k2/kl = 1.5 at threshold (rp = 0.2 @) indicates that the barrier to absorption is located below the critical energy for reaction (1). If the latter were not true, it should have been possible to prepare EVE with sufficient internal energy such that the lower energy channel predominated at threshold (rP =

TaIzIe2 Brarxhing ratio and yield as a function of pressure (rp = 0.2 JJS)

uz(sp) 0.912

0565

O-0023 0.0055 0.010 0.10 1.0 0.4 is

l-4 x 59 x 3.2 x 3.2 X 3.2 X

10-5 loa 10d lo-’ lo-’

1.3 x 10-a 2.0 x 10-g

0.01 0.03 0.06 0.6 6 2 92

h/h

Y(A)/p x 102b)

ZY(B)/p x 102c)

l-61 r 0.20 1.42 -)^0.21 1.33 f 0.14 153 + 0.04 1.05 + 0.04

0.680 f 0.111

1.102= 0.2ss

0.604 f 0.027 0.333 f 0.018 0.109 i 0.024

0.812 k 0.114 050.5 + 0.061 0.114 f 0.02tl

0.0681 r 0.001 no prociwJ%

0.101 f 0.006

1.48 + 0.09

a) The collision fkquency is u!culat+d with a hard spheremodel usingan arbitrzq coikion diameter of 12 A. b) Yield of acetaIdehyde/pu&e. c) Yield of butyraldehyde/pulse. 360

Volume 57, number 3

CHEMICAL PHYSICS LETTERS

0.2 p). Collisional deactivation prior to reaction cannot account for the threshold characteristics since acetaldehyde is the only product when TB = 2 crs. Multiphoton absorption to an energy level much higher than the critical energy for reaction (I) thus appears to be very fast in EVE and competitive with the rate of unimolecular decay via (1). Using a quantum statistical formulation of RRK theory [12], it is estimated that absorption of 27 photons corresponding to 80.5 kcaljmol is required to reproduce the ratio kz/kl for the number of active oscillators s = 17 and 35 photons (104 kcalimol) for s = 33. The lifetimes of vibtationally excited EVE at the? energies are lo-* and lo-’ s respectively. The change in yields as a function of pulse duration shows that both channels are dependent on the peak power of the laser pulse but to different degrees. Since the “long pulse” (2 11s) at threshold has a lower photon flux iu the fust 0.2 ,~rsthan does the “short pulse” (0.2 w) and leads to acetaldehyde only, but the threshold fluence for the short pulse causes the higher energy channel to predominate, it is clear that the dynamics of multiphoton absorption are not identical in the two cases. That is, if the pumping mechanisms were identical, it should have been possible to produce only acetaldehyde at threshold with the short pulse. Although there is insufficient data to determine how energy is distributed internally prior to reaction, these resuits suggest that during laser pumping a statistical energy distribution may not be realized wlren rP = 0.2 ~LS.It is expected that near the dissociation limit the rate of energy randomization is much faster than the rate of unimolecular decay, but at Icwer energies, it is unknown how Tl relaxation times vary with energy. The observations presented here seem to indicate that a competition between intramolecular relaxation and laser up-pumping can be made to exist depending upon laser conditions. We are currently investigating the consequences of the latter, the results of which will be reported with the wavelength dependence of this reaction at a later time. Technical assistance in the preliminary stages of t&is work was provided by Mr. Charles MiIler in the laboratory of Professor Richard N. Zare at Coiumbia University. Valuable discussions with G. Flynn, E. Yablonovitch and others are acknowledged_ Frelimmary experiments involving opto-acoustic measurements were

1 August 1978

carried out at Harvard University with I. Black and E. Yablonovitch This research was performed at Brookhaven National Laboratory under contract with the U.S. Department of Energy and supported by its Offke of Basic Energy Sciences. References [ 11 N.R. &nor, V. Merchant,R.S. Hallsworthand MC.

Richardson,Can. 3. Phys. 51 (1973) 1281; R-V. Ambartzumian,Yu. A. Gorokbov. VS. Letokhov and G-N. Makarov,Zh. ETF Pis. Red. 21(1975) 37.5; J.L. Lyman, R.J. Jensen,J.P. Rink, C.P. Robinson and SD. Rockwood, Appl. Phys. Letters27 (1975) 87. [2] M.J. Co&o& P-A. Schulz, Y-T- Lee and Y-R. Shen, Phy;. Rev. Letters38 (1977) 17; E.R. Grant, MJ. Coaola, Y-T. Lee, PA. Schulz, Aa. S. Sudbo and Y-R. Shen, Chem. Phys. Letters52 (‘1977) 59.5. [3] J.G. Black, E. Yablonovitch, N. Bloembergenand S. Mukarnel,Phys. Rev. Letters38 (1977) 1131; P. KoIodner,C. Wmterfeldand E. Yablonovitch,Opt. Commun. 20 (1977) 119. [4] N. Bloembergen,Opt. Commun. 15 (1975) 416; D.h%_Larsenand N. Bloembergen,Opt. Commun. 17 (1976) 254; RV- Aiibartzm&n, Yu. A. Gorokhov, VS. J_etokbov, G.N. Makarov and A-A. Puretzski,Zh. ETF Pis. Red. 23 (1976) 26; S. Mukameland J. Jort& I. Chem. Phys. 65 (1976) 5204; C-D. CantrelIand H-W. Galbraith,Opt. CO~~IXI. 18 (1976) 215; N. Bloembezge~,C-D. Cantrelland D&i_ Larsen,in: Tunablelasersand app&ations, eds. A. Mooradian, T. Jaegerand P. Stokseth(Springer,Be&n, 1976). [S] GA Hill, E. Grunwaldand P. Keehn, J. Am. Chem. Sot. 98 (1976) 6521. [61 LG. Glatt and A. Yogev, 3. Am. Chem. Sot. 98 (1976) 7087. 171 W. Braun and W. Tsang, Chem. Phys. Letters44 (1976) 354. [S] R.N. Rosenfeld,JL Brauman,J.R. Barkerand D.M. Golden, I. Am. Chem. Sot. 99 (1977) 8063. 191 D-S. King and J-C. Stephenson,C&em.Pbys. Letters 51 (1977) 48; J.M. Pxescs,R.E. Weston Jr. and G-W. Flynn, Chem. Phys. Lett~s 48 (1977) 42.5; C-R. QGck Jr. and C. Wittig,to be published. [lo] S.N. Wang and CA. W&er, Can. 1. Res. 21(1943) 97; A-T. Bladesand G.W. Murphy, J. Am. Chem. Sot. 74 (1952) 1039; MJ. Molem,J&f. Gamboa, J.A. GarciaDoxninguezand A. Conto, J. Gas Chromatography6 (1968) 594. [ 1 l] J.G. Black and E. Yablonovitch,unpublishedre&ts. [12] PJ. Robinsonand KA. Holbrook, UnimolecuIarreactions (WiIey. New York, 1972).

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