J . Phys. Chem. 1988, 92, 4389-4396
overall fit which is acceptable, and no certain conclusion regarding the rate of reaction 12 can be drawn. As mentioned previously, the establishment of a partially equilibrated HCO density via reaction 13 and subsequent reactions of the HCO radical, particularly with H atoms (reaction 15), contribute to the OH decay in this moderately rich flame. Because reaction 13 is near equilibrium, the OH decay rate responds primarily to the equilibrium constant of this reaction and not to the absolute forward and reverse rate constants. Decreasing the rates of both the forward and reverse processes of reaction 13 by a factor of 10 produces no change in the observed OH decay rate as a function of distance. Changing the equilibrium constant, however, does affect the OH profile, although the relatively small contribution of HCO reactions to the radical consumption rates in this flame means that only a large change causes a significant perturbation. As an example, early estimates of the thermochemistry of the H C O radical5 yielded an equilibrium constant for reaction 13 which is 30 times smaller than more recent estimate~!.'~ The OH decay profile calculated by use of this early HCO thermochemistry is presented as curve B in Figure 5. This simulated curve decreases much faster toward equilibrium than does the experimental data, providing additional confirmation that the earlier thermochemistry of the HCO radical is incorrect. Note that for reaction 13 the third-body efficiencies are assumed to be one for all species. This assumption was made because little data are available concerning such efficiencies for this reaction. However, as mentioned above, this reaction is nearly equilibrated, and the absolute rate of the reaction does not affect the simulation significantly. (14) Chase, M. W.; Curnutt, J. L.; Hu, A. T.; Prophet, H.; Syverud, A. N.; Walker, L. C. J . Phys. Chem. Ref. Data 1974, 3, 311. (15) Frank, P.; Just, Th. Ber. Bunsen-Ges. Phys. Chem. 1985, 89, 181. (16) Baulch, D. L.; Drysdale, D. D. Combust. Flame 1974, 23, 215. (17) Baulch, D. L.; Cox, R. A.; Hampson, Jr., R. F.; Kerr, J. A.; Troe, J.; Watson, R. T. J. Phys. Chem. Ref Data 1984, 13, 1259. (18) Nicovich, J. M.; Wine, P. H. J . Phys. Chem. 1987, 91, 5118. (19) Warnatz, J. In Combustion Chemistry; Gardiner, W. G., jr., Ed.; Springer-Verlag: New York, 1984; p 106. (20) Sridharan, U. C.; Qiu, L. X.;Kaufman, F. J . Phys. Chem. 1984.88, 1281. (21) Baulch, D. L.; Drysdale, D. D.; Horne, D. G.; Lloyd, A. C. Evaluated Kinetic Data for High Temperature Reactions; Butterworth: London, 1972; Vol. 1, p 327.
4389
Summary In this paper, computer-simulated O H profiles in the postflame gas of atmospheric-pressure propane- and methane-air flames have been compared in detail to those measured by UV absorption spectroscopy. The chemical mechanism used in the simulation, which consists of 15 reactions, predicts the O H profiles as they decay toward equilibrium from superequilibrium densities in the flame zone with errors of less than 15% when compared to the experimental data. The flames studied cover a wide range of equivalence ratios varying from 4 = 0.63 (fuel-lean) to 4 = 1.45 (fuel-rich). The results of sensitivity analyses show that the formation and subsequent reactions of the H 0 2 radical determine the shape of the OH profile for very lean flames. For a moderately rich flame (4 = 1.17), the recombination of H atoms with O H radicals is of primary importance. In a richer flame (4 = 1.46), recombination of H atoms and reactions of the HCO radical begin to be significant in addition to the (H + OH) recombination. The results of these simulations provide sensitive tests of the rates of addition of H atoms to O2and of the recombination of H atoms with O H radicals. Finally, measurements of the H1, 02,and O H density profiles in the postflame gas of a stoichiometric methane-air flame show that the OH radical is in partial equilibrium with H2 and O2as predicted by the chemical mechanism. Therefore, the reaction mechanism and rate constants shown in Table I, coupled with the diffusion coefficients summarized in Table 11, constitute a set of parameters which can simulate O H profiles in the postflame gases of atmospheric-pressure flames over a wide span of fuel-air equivalence ratios. Acknowledgment. I thank C. K. Westbrook for generously providing the HCT computer code. I also thank A. Schoene for adapting the program for use on the FPS M64/60 computer and D. Anderson for his invaluable assistance during the adaptation process. Registry No. C,H8, 74-98-6; CH,, 74-82-8; OH, 3352-57-6; HOz, 3170-83-0. (22) Temps, F.; Wagner, H. Gg. Eer. Bunsen-Ges. Phys. Chem. 1984,88, 415. (23) Timonen, R. S.;Ratajczak, E.; Gutman, D. J . Phys. Chem. 1987,91, 692.
Hot Toluene as an Intermediate of UV Multiphoton Dissociation Nobuaki Nakashima,*vt Noriaki Ikeda,t and Keitaro Yoshihara Institute for Molecular Science, Myodaiji, Okazaki 444, Japan (Received: December 17, 1987)
The primary process of excited toluene irradiated by using ArF laser light (193.2 nm) is internal conversion to the ground state in the gas phase. This process produces hot toluene. Hot toluene shows strong absorption in the UV region and hence absorbs a second photon which causes dissociation into a benzyl radical. The apparent yield is 0.24 under a laser fluence of 11.5 mJ cm-2. This is the first observation of UV multiphoton chemistry via a hot molecule. This mechanism is also operative in the ArF laser photolysis of alkylbenzenes.
I. Introduction This paper demonstrates that hot molecules can be one of the intermediates in UV multiphoton chemistry. The hot molecule (denoted as so**)is formed by internal conversion to the ground
electronic state and carries a high vibrational energy, which is to the photon energy absorbed. Hot molecules have been detected by laser flash photolysis of cycloheptatriene,'P2 b e n ~ e n e alkylbenzenes? ,~ and olefins.s Hot
t Present address: Institute of Laser Engineering, Osaka University, 2-6 Yamada-oka Suita, Osaka 565, Japan. *Present address: Chemistry Department, College of General Education, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560, Japan.
(1) Srinivasan, R. J . Am. Chem. SOC.1962, 84, 3432, 4141. Thrush, B. A.; Zwolenik, J. J. Bull. SOC.Chim. Belg. 1962, 71, 642. (2) Hippler, H.; Luther, K.; Troe, J.; Wendelken, H. J. J . Chem. Phys. 1983, 79, 239.
0022-3654/88/2092-4389$01.50/0
0 1988 American Chemical Society
4390 The Journal of Physical Chemistry, Vol. 92, No. IS, 1988
molecules show a broad, strong absorption in the UV region.14g6 Therefore, a hot molecule can easily be excited by a second photon during a nanosecond laser pulse. UV multiphoton photochemistry and spectroscopy have been investigated extensively.' The excited singlet state and/or triplet states have been proposed as the most probable intermediates. The second or third photons often ionize the excited molecules. However, if the hot molecule is an intermediate in multiphoton chemistry, the electronic excitation of So** may not reach the level of photoionization. Major reactions in this case will be radical fragmentation and/or isomerization. The following four findings and related discussions are described in this paper. (i) Benzyl radical formation under high-pressure conditions. It has been shown that the benzyl radical forms via So** under low-pressure conditions with a rate constant of 2.0 X lo6 s-I (ref 4,8, and 9). This slow dissociation reaction (process 3) in schemes in section IIIA) is easily quenched upon addition of a few tens of Torr of a foreign gas. However, radical formation is not quenched under 800 Torr of nitrogen. (ii) A quadratic relation between concentration of the benzyl radical vs the laser fluence indicates that the benzyl radical forms by a two-photon process. (iii) The concentration of benzyl radical is quantitatively predictable on the basis of the q,value, which is the molar extincion coefficient of SO**a t the laser wavelength. The intermediate of the two-photon dissociation is assigned to So**. Yields of radical cations were lower than our detection limit, presumably because of the short lifetimes of the excited state at 193 nm. (iv) The multiphoton chemistry of other systems (benzenes and olefins) will be briefly discussed on the basis of the present hot molecule mechanism. 11. Experimental Section
A . Laser Flash Photolysis and Laser Fluence. Transient absorption spectra were measured by the method of laser flash photolysis, the details of which are described in our previous paper? The irradiation light source was an ArF laser (193.2 nm with a spectral fwhm of 0.6 nm, Lambda Physik excimer laser EMG 101), with a typical width of 14 ns (fwhm). Alkylbenzenes with pressures of 0.5-8 Torr were irradiated in a cell of dimensions 0.5 X 4.4 cm, with a parallel beam. The laser fluence (0.6-25 mJ cm-*) was primarily controlled by using a telescope and was adjusted by the applied voltage of the power supply. It was important to observe transient species in the region close to the surface (see Appendix A). The laser energy in the cell was calibrated by comparing transient absorbances of alkylbenzenes and of benzene. The molar extinction coefficient of the transient of benzene at 230 nm and a t t = 0 ns has been determined to be 3500 M-' ~2m-I.~ The molar extinction coefficient of methyl radical at 216.4 nm was measured by using this standard (ref 10) and was in very good agreement with that in the literature. The accuracy was estimated to be better than &5% in one u. The accuracy of quantum yield measurements of benzyl radical depended on that of its molar extinction coefficients. We determined the coefficients with the same apparatus." Therefore, the ac(3) Nakashima, N.; Yoshihara, K. J. Chem. Phys. 1983, 79, 2727. (4) Ikeda, N.; Nakashima, N.; Yoshihara, K. J . Chem. Phys. 1985, 82, 5285. Kajii, Y.; Obi, K.; Tanaka, I.; Ikeda, N.;Nakashima, N.; Yoshihara, K.J . Chem. Phys. 1987,86, 61 15. (5) Nakashima, N.; Shimo, N.; Ikeda, N.; Yoshihara, K. J . Chem. Phys. 1984, 81, 3738. Shimo, N.; Nakashima, N.; Ikeda, N.; Yoshihara, K. J . Photochem. 1986.33.279. Nakashima, N.; Ikeda, N.;Shimo, N.;Yoshihara, K. J . Chem. Phys. 1987.87, 3471. (6) Hippler, H.; Troe, J.; Wendelken, H. J. Chem. Phys. Lett. 1981.84, 257. Hippler, H.; Troe, J.; Wendelken, H. J. J . Chem. Phys. 1983, 78, 5351. (7) Lin, S.H.; Fujimura, Y.; Neusser, H. J.; Schlag, E. W. In Multiphoton Spectroscopy of Molecules; Academic: London, 1984. Schlag, E. W.; Neusser, H. J. Acc. Chem. Res. 1983, 26, 355. KUhlewind, G. H.; Kiermeier, A.; Neusser, H. J. J . Chem. Phys. 1986, 85, 4427. (8) Hippler, H.; Schubert,V.; Troe, J.; Wendelken, H. J. Chem. Phys. Lett. 1981, 84, 253. (9) Tsukiyama, K.; Bersohn, R. J. Chem. Phys. 1987, 86, 745. (10) Nakashima, N.; Yoshihara, K. Lnser Chem. 1987, 7, 177.
Nakashima et al.
0
% N N /I
w"
+H
Figure 1. A schematic diagram of UV multiphoton dissociation via hot toluene. ArF laser light (193.2 nm) pumps the S, state of toluene. Internal conversion finally leading to So** takes place. The second photon excites So** during collisional relaxation by 800 Torr of added nitrogen. The excitation of So** leads to toluene dissociation into a benzyl radical. The benzyl radical is assumed to disappear on absorption of a third photon. One of the sets of good parameters are shown. The two variable parameters are the molar extinction coefficient of So** at 193.2 nm, ch, and the molar extinction coefficient of benzyl radical, cB.
curacy of quantum yield measurements was estimated to be less than f10% in one 0. The spectral resolution (AX) was 0.6 nm (fwhm). The sample gas was renewed after one or two laser shots. A few reading were averaged for each point in the time-resolved spectra. Delay times were measured from the end of the laser pulse; i.e., 0 ns was defined as the end of the pulse. One of examples of the point of t = 0 ns can be seen in Figure 8f. B. Scattered Light Correction. Transient absorption spectra were measured down to 191.5 nm. The intensity of the monitoring light was reduced in the wavelength range shorter than 210 nm, because the absorption coefficient of toluene in the ground state is large (ca.5000 M-' cm-I). Even if the scattered light intensity was a few percent of the monitoring light, transient absorbances still had to be corrected. The details are described in Appendix B. C. Molar Extinction Coefficients of Alkylbenzenes. The molar extinction coefficients were measured with a Hitachi U-3200 spectrophotometer with a spectral resolution of 0.4 nm. They are 4910 (toluene), 5450 (ethylbenzene), and 6800 M-I cm-I (butylbenzene) at a laser wavelength of 193.2 nm. Benzene yielded a value of 6600 M-'cm-l at the peak of the lBlu IAl, transition of 200.3 nm. The extinction coefficients of toluene and benzene were in good agreement with those in the Butylbenzene was largely adsorbed on the cell surface, so the pressure of butylbenzene was measured several minutes after the cell was filled. The chemicals (with stated purity and source) were distilled from trap to trap: toluene (99.7%, Merck Spectrograde), ethylbenzene (>99%, Tokyo Kasei), and n-butylbenzene (>99%, Tokyo Kasei) .
-
111. Results A . Reaction Mechanism. Figure 1 shows the mechanism of multiphoton dissociation via hot molecule to benzyl radical. The following schemes explain the mechanism:
so s 3 s3- -so** So** ~~
+
B** + H, k3 = 2.0
X ~
lo6 s-'
(3)
~~
(11) Ikeda, N.; Nakashima, N.; Yoshihara, K. J . Phys. Chem. 1984,88, 5803.
(12) Pickett, L. W.; Muntz, M.; McPherson, E. M. J . Am. Chem. SOC. 1951, 73,4862. Pantos, E.; Philis, J.; Bolovinos, A. J. Mol. Specrrosc. 1978, 72, 36. Bolovinos, A.; Philis, J.; Pantos, E.; Tsekeris, P.; Andritsopoulos, G. J . Mol. Spectrosc. 1982, 94, 55.
The Journal of Physical Chemistry, Vol. 92, No. 15, 1988 4391
Intermediates in UV Multiphoton Dissociation
so** So**
S3
IMI
so
(5)
B** + H
(6)
-kX + Y
(7)
B**
-
200
225
250
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I
I
I
I
I
I
Toluene 0.5 Torr, NZ 800 Torr - 15000
a ) t = O nr
-
I
- 10000
[MI
B**
B
(8) The ArF laser (193 nm) pumps the valley between the S2and S3 states in scheme 1. The S2state corresponds to the lBlustate of benzene and the S3state to the lEIu. We label the pumped level the S3state in this paper. Internal conversion has been shown to be the major deactivation processes in the case of S2and S3 excitation.13J4 The final products of SO**have been directly detected by us4 and are discussed is this paper also. The fluorescence quantum yield at 184.9 nm is as low as lr5 (ref 13) and the yield of the lowest triplet state has been estimated to be of the order of 0.01 (ref 14). The rate constant of process 2 is presumably higher than 10" s-l, since internal conversion to So** competes with relaxation from S3to the fluorescence state (SI). Gregory et al. suggested a constant of l O I 4 s-l (ref 13). Process 3 is a slow dissociation process from SO**.Three independent experimental values are now a ~ a i l a b l e . The ~ - ~rate ~ ~ constant, 2.0 x 106 s-1 (ref 4 , was reported by us. Slightly large values were for the obtained as (2.6-3) x 106 s - ~ (ref 8 and 9). The differences are not clear at the present stage. The collisional in detail. These processes relaxation processes have been quench process 3, i.e., the slow formation of the benzyl radical.',* Even 2o Torr Of He suppresses the formation Of the benzyl radical? The collisional deactivation efficiency Of nitrogen is 1.4 times as great as that of h e 1 i ~ m . I ~Therefore, when the benzyl radical is detected in the presence of 800 Torr of nitrogen, it can be stated with certainty that this is not a result of process 3. The origin of the radical in the presence of high pressures of foreign gases will be explained in terms of the photodissociation of So**, as shown by process 5. Occurrence of the predissociation process 6 was lower than our detection limit (0.02). The benzyl radical is assumed to decompose photochemically on absorption of a third photon as indicated by process 7. The process 8 is collisional deactivation of hot benzyl radical. The rate constant ~ when ~ hot~benzyl - radical 1 was by nitrogen was 8 104 s-i ~ produced by photolysis of benzyl chloride.lI B. Definition of Apparent Quantum Yield and Molar EXtinction Coefficient. When multiphoton absorption takes place, the number of excited molecules per unit volume is not proportional to the incident laser fluence. We assume a hypothetical co? centration, c,,where a single photon absorbed creates a single excited molecule. In other words, C, is proportional to fie number of photons absorbed and no multiphoton absorption takes place. Then, the quantum yield can be defined in terms of the concentration C,, as 4j = Cj/C, where cjis the concentration of a i. A,, observable value is absorbance, ~ ( ~ , x )at, time t and wavelength A. D(t,X) is expressed as follows by using quantum yield 4j and its molar extinction coefficient e; D(fiX)
=
('#'heti
- $-gcg
+ $Beg + 4Y4Y)Cel
2oooo
-
-kB** + H -+
2oooo
(4)
(9)
The subscript denotes a hot g is for a ground state is for a and is for products Other The negative sign On g shows than a of the initial concentration. The Observing length I was 4.4 cm. A for c~is 409 mTorr or 2*24 L-' at a laser Of mJ and a Torr Of Equation 9 can be rewritten in the form given below. Equation 10 includes a correction for ground depletion and shows the ap"*'
(13) Gregory, T. A,; Hirayama, F.; Lipsky, S. J . Chem. P h p . 1973.58, 4697. (14) Braun, C. L.; Kato, S.; Lipsky, S. J. Chem. Phys. 1963, 39, 1645. (15) Hippler, H.; Troe, J.; Wendelken, H. J. J . Chem. Phys. 1983, 78, 6709.
.i
g
6MM
- 5000
-
-
5000
-1250
-
I
I
1
I
I
I
200
225
250
275
300
325
.-1250
Wavelength/nm
Figure 2. Transient absorption spectra of toluene in the presence of 800 Torr of nitrogen. Laser fluence was 11-12 mJ cm-2. The spectra a were Observed at a time Of ns: ( O ) Observed ).( corrected values for the ground-state depletion of toluene using a yield of 6- = 0.9. (b) Observed at t = 400 11s for open circles and corrected with 6% = 0.25. The solid line in the wavelength region longer than 220 nm is the absorption spectrum of the benzyl radical. Their molar extinction coefficients are multiplied by 0.24. The absorption spectrum of toluene is depicted in the left side of (a) as a solid line.
parent molar extinction coefficient of a wavelength X and time t.
I34icj(t,A) =
D(t,A)
- 4-gegO) CJ
(10)
C. Benzyl Radical Formation under a High Pressure of Nirrogen. Figure 2 clearly ~ h o w sformation of the benzyl radical under 800 Torr of nitrogen. The solid line in Figure 2b shows the absorption spectrum of the benzyl radical, which is measured bY PhOtOlYSb Of b e y 1 chloride." The Closed circles were obtained by photolysis Of The closed are in quite good agreement with the solid line over the wavelength range longer than 225 nm* It can be that the absorbing spies is the benzyl radical. The molar extinction coefficient at the 'A2 transition) has been deterstrongest Peak (253 nm, 4'B2 l)* The s~~~~line is normalized mined to be 28000 M-'m-' to the data by a factor of 0.24 times the molar extinction coefficients. Therefore, the quantum yield of benzyl radial, $J~,was calculated to be 0.24 under a laser fluence of 11.5 mJ cm-'. The spectrum at t = 0 (the solid line in Figure 2a) is considered to be an overlapped spectrum of SO**and hot benzyl radical. These hot species relax within 80 ns. Relaxation times in the presence of 800 Torr of nitrogen are about 15 ns for hot benzyl radical" and expected to be 19 ns for S0**.l5 Therefore, the spectra observed at t = 100 ns in Figure 2b can be regarded as those of the relaxed species, The spectra (closed circles) in the wavelength region shorter than 210 nm were obtained by correcting for the depletion of the ground-state toluene molecules. The depletion was negligibly small in the wavelength region longer than 215 nm, where the molar extinction coefficient of toluene is of the order of 2oo M - ~cm-l, 4-g was assumed be o,9 and was o.25 for the spectrum at = ns and = ns in Figure 2, respectively. The depletion yield, 4%, was estimated by using one of methods to determine a triplet quantum yield.16 We can +
(16) Labhart, H. Helu. Chim. Acta 1964, 47, 2279. Heinzelmann, W.; Labhart, H. Chem. Phys. t e f f . 1969, 4, 20.
4392 The Journal of Physical Chemistry, Vol. 92, No. 15, 1988
Nakashima et al. 225
7500
sr Toluene only 10000
-I
5000
I
-
a) t = O n s
;loo00
2500
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-a
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b) t = 4 0 0 n s
2500 -
7
L
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t=Ons
1
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EB + Na 800 Torr
c
1 5000
T
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@(CHZ)~CH~(W t=Ons
I
200
250
50M) 2500
01
r
250
Wavelengthlnm
assume that structures in a transient spectrum have no coincidence with those of absorption spectrum in the ground state. The value of &g was adjusted until there was no spectral structure of ground-state toluene. The quantum yields of benzyl radical will be discussed in section VI. In the t = 0 spectrum, r # ~ - ~was less than 1.0. This finding suggests that some of incident laser photons result in multiphoton absorption. The molar extinction coefficient of hot toluene at 193 nm is about 4 times as large as that of toluene at room temperature. Hence, because hot toluene is produced in an irradiated pulse, the later part of a laser pulse will be absorbed more effectively than the earlier part. We neglected the effect of multiphoton absorption in defining c $ ~and 4, in the previous subsection C, and as a result of this the sum of 4~ and &g at t = 0 ns is 1.14; Le., it exceeds 1.0. The depletion yield, &, decreased from its initial value of 0.9 to 0.25 in the relaxed spectrum. This recovery must be caused by the return of hot toluene to toluene at room temperature. The remaining value of 4- = 0.25 is responsible for formation of the benzyl radical. D. Transient Absorption Spectra under a Low Pressure of Nitrogen. The absorption spectrum of hot toluene was observed under low collision number conditions. The largest component in the spectra at t = 0 11s in Figure 3 is assignable to the absorption of So**. The black circles in the spectrum over the wavelength region shorter than 210 nm are corrected from the observed signals (open circles) by using the value of 4, = 0.9. The sample pressure was 0.5 Torr in this wavelength range and was 2 Torr at X > 220 nm. The spectrum in the wavelength region longer than 21 5 nm can be compared with that of Hippler et aL6 They measured the absorption spectrum of So** produced by photoisomerization of cycloheptatriene. The hot toluene has an internal energy of 623 kJ mol-’, when produced from cycloheptatriene irradiated with KrF laser light at 248 nm. This energy is close to that (627 kJ mol-’) obtained from internal conversion of toluene when irradiated in this experiment. The spectral shape is similar to that obtained by Hippler et a1.6 The present spectrum also includes absorption of the hot benzyl radical. The molar extinction coefficient at 220 nm t = 0 ns was 6000 M-’ cm-’ at the low limit of laser fluenece. This value is in fairly good agreement with that (7930 M-I cm-I) obtained by Hippler et aL6 Figure 3b shows the spectrum at 400 ns. By the time collisional relaxation has resulted in appearance of the benzyl radical spectrum. About 40 collisions are expected in 400 ns at a pressure of 2 Torr of toluene. The peaks at 260 and 310 nm are assignable
i2500
2500
Figure 3. Transient absorption spectra of toluene in the absence of a at X < 220 nm, and foreign gas. Laser fluence was 11-12 mJ at X > 220 nm. (a) t = 0 ns; open circles are for several mJ observed values and closed ones for values after correction for depletion of toluene with 4- = 0.9. (b) t = 400 ns; the spectra at X > 220 nm were replotted from our previous mea~urements.~
I
- 5000
0
0 71
BB + N2 800 Torr t = 100 ns 0
- 5000 - 2500 0
225
250 275 300 Wavelengthlnm
325
Figure 4. Transient absorption spectra of ethyl- and butylbenzene under a laser fluence of 13.5 mJ em-? (a) Ethylbenzene 0.45 Torr system, observed at t = 0 ns. (b) Ethylbenzene 0.45 Torr in the presence of 800 Torr of nitrogen, observed at t = 100 ns. The solid line is the absorption spectrum of benzyl radical. The molar extinction coefficients were multiplied by 0.36. (e) Butylbenzene 0.36 Torr system, observed at t = 0 ns. (d) Butylbenzene, 0.36 Torr, in the presence of 800 Torr of nitrogen, observed at t = 100 ns. The absorption spectrum of benzyl radical was multiplied by 0.18. TABLE I: Relative Yields of Two-Photon Dissociation and the Slow Dissociation Rate Constant alkylbenzene re1 yield’ k d / 1 O6 s-I to1u en e 1 .o 2.0‘ ethylbenzene 1.7 20 butylbenzene 0.76 2.0 “Compared at 15 mJ cm-2. bReference 4. ‘Slightly large values X lo6 s-l) were reported.*v9
((2.6-3)
to the unrelaxed benzyl radical. E. Ethyl- and Butylbenzenes. Time-resolved absorption spectra of these molecules showed formation of So** as well as the benzyl radical. The spectra at t = 0 are depicted in Figure 4, and were obtained in the absence of foreign gases, indicating that they involve the hot species. For the spectrum at t = 100 ns, 800 Torr of nitrogen was added. Under these conditions the hot species are considered to have relaxed. The components of these spectra (Figure 4) are expected to be similar to those in the photolysis of toluene. The broad absorption peaks at 260 nm and t = 0 ns were assigned to the hot benzyl radical and the back grounds to So**. The solid line in each spectrum at t = 100 ns is the absorption spectrum of the benzyl radical from the literature.” Its molar extinction coefficients are scaled to fit the observations. It is clear that the major absorbing species in this wavelength region is the benzyl radical. The relative absorption intensities for the three alkylbenzenes are listed in Table I along with their slow dissociation rate constants. The largest yield was observed for ethylbenzene, about twice the magnitude of the others. The yield for ethylbenzene may include a small contribution from the slow dissociation process (scheme 3), since the slow dissociation rate constant is 10 times that of toluene, Le., 2.0 X lo7 s-I. However, it seems reasonable that 800 Torr of nitrogen
The Journal of Physical Chemistry, Vol. 92, No. 15. 1988 4393
Intermediates in UV Multiphoton Dissociation 0.5
0
1.0
100
200
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1 -
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a) 9.8mJ (2 Torr)
0.3
1
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c
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-
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i
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I
40.01
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t
2
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0.0031
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i l l l l l l
0.5
1.0
I
I 1 1 1 1 1 1
1
1
3 10 30 Laser Fluence/mJ/cmZ
1
/
1
1
0.003 100
Figure 5. Laser fluence dependence of absorption intensity of benzyl radical. The absorption was observed at 100 ns delay time and at 253.5 nm. Tolueqe pressures were 0.5-8 Torr in the presence 800 Torr of nitrogen. Error bars show two u for 10 shots. The solid line (curve b in Figure 7 ) is one of the simulation curves.
will almost totally quench the slow progress. One of the olefins (2,3-dimethyl-2-pentene)has a dissociation rate constant of 2.8 X lo7 s-l for the hot m o l e c ~ l e . For ~ this molecule the SternVolmer plot of the nitrogen pressure vs the radical yield indicated that 600 Torr of nitrogen completely quenched the slow dissociation of the ~ l e f i n Therefore, .~ the absorption intensity of the benzyl radical in the presence of 800 Torr of nitrogen is virtually all attributable to the fast process (schemes 5 and 6) rather than the slow process (the hot molecule mechanism; scheme 3). F. Laser Power Dependence of the Concentration of Benzyl Radical. Figure 5 shows the absorbance of benzyl radical vs the laser fluence along with a simulation curve. Benzyl radical concentration was measured at t = 100 ns in the presence of 800 Torr of nitrogen. All the absorbances were normalized to those a t 8 Torr of toluene. It was found that the absorbance was proportional to the square of the laser fluence in the region lower than several mJ cm-2. We did not observe any indication of the existence of a one-photon proCess until the lowest laser fluence (0.6 mJ cmA2). In the region higher than 10 mJ a saturation effect was seen. No strong absorption was detected other than that of the benzyl radical around the peak (253 nm) at laser fluence of 11.5 mJ cm-2 (Figure 2). Therefore, the absorbance can be directly converted into the concentration of benzyl radical by using its molar extinction coefficient (e = 28 000 M-' cm-I). This allowed the apparent yield of benzyl radical to be determined. It increased from 0.02 (0.6 mJ to 0.24 (1 1.5 mJ cm-2) and finally 0.25 (25 mJ cm-2). Examples of the oscillograms a t 253 nm are shown in Figure 6. The initial small pulse or the shoulder is attributed to absorption of So**. The rise curve up until about 90 ns is explained in terms of the collisional relaxation processes by 800 Torr of nitrogen. The molar extinction coefficient of benzyl radical is increased by relaxation." The absorption intensity of So** increases proportionally to the laser fluence; however, it is clear that the absorption intensity of benzyl radical increases much more than that of So**. The slow decay a t 9.8 mJ cm-2 in Figure 6 can be explained in terms of recombination reactions. This decay was corrected in Figure 5 . It can be concluded that the benzyl radical was formed by a two-photon absorption in the low-power
f#J&
tg
= 20000 M-' cm-' = 22500 M-' cm-I
(12)
The molar extinction coefficient, q, in eq 11, was experimentally observable and found to be 1.9 X lo4 eh < 2.7 X lo4 M-' cm-' (see the next section IVB). Therefore, (f.J, is close to 1.O. When &ah is changed to 25 000, the calculated curve shows a nearly parallel shift of 1.236 times the best fit curve. When &fh = 15OOO, it goes down by a factor of 0.754 times. In Figure 7, curve a was calculated by assuming 4B= 0. The deviation from a straight line is caused by depletion of the ground-state molecules. The observed points showed saturation behavior stronger than that of curve a. To explain it, the decomposition process of the benzyl radical was introduced, and a best fit curve was obtained at tg = 22 500 M-I cm-I, when all of the deviation was assumed to be caused by the decomposition. In fact, we have found that the benzyl radical was easily decomposed by UV irradiation. Benzyl radicals were produced by photolysis of benzyl chloride with an ArF laser. The radical fortunately has a strong absorption at the KrF laser light (248.4
The Journal of Physical Chemistry, Vol. 92, No. 15, 1988
4394
20r
05
10
-7r-
Eg
"11:;
= 4900
Nakashima et al. Time/ns
30 100 m
1
-40
0.21
,p /
a) E B = 0
b) &E = 22500
33750
I
o,2
I
o + T , / f i I ;
/I
0Y
'c1.0
0.1
-
100 Torr
N2
800Torr
N2
f-
/
r
120
80
+ Tol.
M-lcm-l
I E~=20000
40
I
107
c)
0
-,0
0.1 :
0
a
-
0
-
5 c
-
51
n 4
0.03-
-01
zo.01
-
'0.005
L
5
40 003
0 003I 1 1
1 1
I
1 1 , 1 1 / 1
I
,
3 10 30 Laser Fiuence/mJ/cmz
05
10
.
100
___ -40
Figure 7. Simulation curves of the laser fluence dependence of benzyl radical absorption. The molar extinction coefficient, tg, of the hot benzyl radical in the laser pulse is a variable parameter. (a) tg = 0; saturation behavior caused by depletion of ground-state toluene. (b) t g = 22 500 M-' cm-'. The best fit curve for the experimental points shown in Figure 5 . (c) tg = 33750 M-' cm-I. Other parameters are the same as those in Figure 1.
nm), where the parent molecule shows only weak absorption. In order to avoid irradiation of the parent molecule, KrF laser light was then introduced 100 ns after the ArF laser pulse. The absorption intensity of the benzyl radical immediately demeased upon the second laser shot. Therefore, it is reasonable to assume that it is a third photon of ArF light which caused the decrease in absorption intensity of the benzyl radical in the case of toluene photolysis. Possible decomposition products are as follows:
-
C6H7
C,H2
Laser Pulse - 0 5
05-
11.L
+ C5H5or C4H, + C3H3
--0 40
0
!
80
120
Time/ns
8. Prarsuredependentdecay curves of So** observed at 195.5 nm. Nitrogen pressure was changed from (b) 100 Torr to (e) 800 Torr. The toluene pressure is 0.5 Torr. -40
'
I
0.4L
0
40
80
100
!
0.4
A
1
0.3
L
191.5nm - 0 3
0.2
(13)
These reactions are observed under thermal reaction conditions. l7 B. Estimation of 6h and during Excitation Pulse. These quantities are not easy to determine, because the molar extinctiofi coefficients strongly depend on the particular stage of collisional relaxation. Collisional relaxation actually takes place during the irradiation pulse, but we were, however, able to estimate a minimum and maximum value for q,. Collisional effects on the absorption of hot toluene are shown in Figures 8 and 9. Figure 8 shows time profiles at 195.5 nm in the presence of 0-800 Torr of nitrogen. The time to reach the peak decreased with increasing pressure of nitrogen. The time profile clearly shows that the molar extinction coefficient increases with time in the early stage. Even in the presence of 800 Torr of nitrogen (Figure 8e), the molar extinction coefficent at the peak is larger than that observed at t = 0 ns in the absence of nitrogen (Figure sa). Figure 9 shows the wavelength dependence in the presence of 200 Torr of nitrogen. The decay curve at 220.5 nm is essentially the same as that observed by Hippler et al.15 In the wavelength region shorter than 210 nm, the molar extinction coefficient intially increased from t = 0, peaked, and then decreased, the actual position of the peak being dependent on the observation wavelength. At 200 nm the peak was observed at 15 ns while at 191.5 nm it had shifted to 50 ns. These observations indicate that the molar extinction (17) Smith, R. D. J. Phys. Chem. 1979,83, 1553. Astholz, D. C.; Troe, J. J . Chem. Soc., Faraday Trans. 2 1982, 78, 1413.
II
-40
0
I
I
40 Timelns
80
100
Figure 9. Wavelength-dependent decay curves caused by collisional relaxation of So**.A toluene 0.5 Torr system in the presence of 200 Torr of nitrogen was irradiated with 11 mJ cm-'.
coefficients in the wavelength region shorter than 210 nm have their maximum at some internal energy smaller than the initial energy (627 kJ mol-' (ArF laser light + RT)). This conclusion is qualitatively predicted on the basis of an improved Surzer Weiland model.15 However, quantitative predictions are poor in this particular wavelength region. Assuming the reaction scheme to be accurate, the molar extinction coefficient at the low power limit in eq 10 takes its genuine 1, dB 0, and h 0). At the low-pressure limit value e,, (&,
- -
-
The Journal of Physical Chemistry, Vol. 92, No. 15, 1988 4395
Intermediates in UV Multiphoton Dissociation 2 . I A E l /107Torr-l (kJ/mol)-l 0
I
5 I
10
'f
15
I ?
I
1;
100%
+
Figure 11. Benzyl radical formation at 20 mTorr of toluene observed at 260 nm. The initial fast rise was followed by a slow formation process. Q
'
eh,max(191.5 nm, 100 ns) = 31700 M-' cm-'
(15)
for the decrease of the yield in the high-pressure region of C3H8. The dissociation yield, &, decreases and/or the molar extinction coefficient, q,, decreases due to collisions. These findings for the pressure dependence also support the assignment of the intermediate in the UV multiphoton dissociation as hot toluene. D. Very Low Yield of Two-Photon Ionization via the S3State. We tried to detect toluene cation, which will be produced by multiphoton absorption. But the yield of the cation formation was lower than our detection limit < 0.1). Toluene cation has two broad bands at 267 nm with c = 2600 M-' cm-' and at 427 nm with c = 1300 M-' cm-' (ref 18). When the cation is produced, the dimer cation will be formed in a few tens of nanosecond under 2 Torr of toluene, on the basis of estimation of a formation rate constant of benzene dimer cation.I9 Toluene dimer cation shows a broad absorption band at 900 nm with c = 10000 M-' cm-' in solution.m We expected to see the three bands in our systems, but did not detect them. Our simulation code of multiphoton absorption for the benzyl radical formation predicted a yield of ca. 0.0026 of toluene cation at a laser fluence of 11 mJ cm-2, We assumed that the intermediate S3has a lifetime of 0.2 ns with c = 20000 M-I cm-' at 193 nm and that every photon absorbed by the S3state yields a toluene cation. If the lifetime is 1 ns, the yield increases to ca. 0.016. The S3state shows very weak fluorescence (its yield is an order of (ref 13), indicating that the lifetime is much shorter than 1 ns. Therefore, it can be concluded that the yield of toluene cation is much smaller than that of benzyl radical via hot molecule at a laser fluence of 11 mJ cm-2.
th,max(195.5nm, 70 ns) = 22600 M-' cm-'
(16)
V. Conclusions and Related Compounds
a
0.0
400(N) 800(N) 1ZOO( N) 1 I
1
0.0
at t = 0 ns and 193.2 nm was estimated as an average of q, at 191.5 and 195.5 nm. th
~ ~ ( 1 9 3nm) . 2 = y2(ch(191.5 nm)
+ ~ ~ ( 1 9 5nm)) .5
= y2(1940O + 18000) = 18700 M-' cm-'
(14)
Similarly the maximum fh( 193.2 nm) was estimated to be 27 000 M-I cm-' in the presence of 100 Torr of nitrogen.
The molar extinction coefficient of hot toluene at 193.2 nm during the pulse was initially about 18 700 M-' cm-I, reached a maximum (ca. 27 000), and then decreased to some extent. The effective t h (193.2 nm during the pulse and in the presence of 800 Torr of nitrogen) can be expressed as follows. 18700 M-' cm-' Iq, (193.2 nm, during the pulse) < 27000 M-l cm-' (17) The calculated value of td& from the fitting curve was 20 000 M-' cm-', this value being consistent with the above estimate. Therefore, it can be concluded that the reaction intermediate is hot toluene. The observed value of 4BcBwas 3000 M-' cm-' as shown in Figure 2b. Since the formation yield of benzyl radical, 46, is 0.24, cB = 12 500 M-l cm-l at 193.2 nm. This value corresponds to the relaxed state, and it would appear to be 22 500 M-l cm-' for the hot benzyl radical during the laser pulse. C. Pressure Dependence of Yield of the Benzyl Radical. The relative yields were determined from the absorbance at 253.5 nm. The results did not change when the monitoring wavelength was altered by f 0 . 5 nm. The relative yield decreased by 30% when the nitrogen pressure was changed from 800 t o 400 Torr a s shown in Figure 10. It decreased by about one-third on increasing the pressure of C3H8. The yields are plotted vs the collisional parameter, ZAE, where Z is collision number and AE is transferring energy/collision determined by Hippler et al." These observations can be explained in terms of the pressure dependence of the value of & t h . In the low-pressure region, ch became small, because collisional relaxation of the hot molecule does not completely occur and so the molar extinction coefficient does not reach its maximum value. There are two possible reasons
(i) As Figure 11 shows, the rise curve of benzyl radical has fast and slow components under low-pressure conditions. It has been clearly shown in this paper that the fast one includes a two-photon process via hot toluene (scheme 5). The slow process (scheme 3) has already been studied4 and is a dissociative process via hot toluene. (ii) Detectable benzyl radical concentrations in the presence of 800 Torr of nitrogen result from the fast process and are formed by photodissociation of hot toluene (scheme 5).
**
so**
s3
b
-2 +*
+
H
(18)
The dissociative rate constant, kd,from So** is 2 X lo6 s-I. Internal conversion from S3**could take place to the ground state. Such a molecule will have very high internal energy and its dissociation rate constant, kd**,could well be faster than the rate of collisional relaxation by nitrogen. (iii) The predissociation yield of toluene at 193 nm by a onephoton process (scheme 6) was lower than our detection limit (C0.02). This is similar to the C-H bond dissociation of benzene which hardly predissociates on irradiation with light of 184.9*' and 193 nm.5 However, this is not commonly found for electronic excitation at 193 nm. The C-C/C-H bonds in olefins predissociate with a yield of 0.1,5 while phenol and its derivatives (18) Teng, H.H.-I.; Dunbar, R. C. J . Phys. Chem. 1978, 68, 3133. (19) Jasinski, J. M.; Rosenfeld, R. N.; Golden, D. M.; Brauman, J. I. J. Am. Chem. SOC.1979, 101, 2259. (20) Badger, B.; Brocklehurst, B. Trans. Faraday SOC.1969,65, 2582. (21) Mellows, F.; Lipsky, S.J . Phys. Chem. 1966, 70, 4076.
4396
The Journal of Physical Chemistry, Vol. 92, No. IS, 1988 50
10
l'-------ll
'
-4,4cm--
'j
dcm / //'
Toluene P Torr
/
100
/
1.0r
Appendix A . The Effect of Optical Arrangements on the Relationship between Laser Fluence and Absorbance of Product. A relation of n = 2.0 is expected in the low fluence region between the benzyl radical concentration [B] and the laser fluence I .
-1.0
40.1
t
I
00021
Nakashima et al.
, , 05
,,,I/
1
1
5
10
50
~W.lO002 100
Laser Fluence/mJcm-2
Figure 12. Calculated curves of absorbance vs laser fluence under typical experimental conditions; pressure, P, and observation depth, d . The present optical arrangement for nanosecond laser photolysis is shown. (a) Toluene pressure P = 8 Torr and depth d = 0.025 cm. (b) P = 8 Torr and d = 0.25 cm. (c) P = 0.5 Torr and d = 0.04 cm. (d) P = 0.5 Torr and d = 0.4 cm. The shapes of the curves are almost the same except for curve b. The deviation in the high-power region is caused by "an inner filter effect".
predissociate with high quantum yields.22 (iv) The UV multiphoton chemistry via the hot molecule has a different aspect from that via excited singlets or the lowest triplet state. In the case of UV multiphoton absorption via electronically excited states, the second photon often ionizes the parent molecule and the subsequent photons induce ion fragmentation reactions. This picture is called the ladder switching model.' On the other hand, in this case the second absorbed photon does not ionize the hot toluene, because the energy of the first photon has been distributed in the vibrational modes. It should be noted that neutral fragmentation takes place rather than ion fragmentation. (v) It is interesting to make a comparison with what occurs in the case of benzene. Phenyl radicals and fulvene have been detected under high laser fluence and in the presence of 400 Torr of propane.23 These products are presumably formed by UV multiphoton absorption via hot benzene. (22) Kajii, Y.; Obi, K.; Nakashima, N.; Yoshihara, K. J . Chem. Phys. 1987,87, 5059. (23) Nakashima, N., unpublished results.
However, inner filter effects may induce a significant deviation, if a relatively high pressure ( P ) and a deep depth (d) are used in the optical arrangement as shown in Figure 12. When transient species are monitored in the region close to the surface, the observed power dependence will give the correct curve. Higher signal to noise ratio is obtained at higher pressures. Appropriate experimental conditions were estimated by simulating curves. Curve a is expected when the absorbance was observed at a depth of d = 0.025 cm. Other parameters are the same as those used for the best fit curve in Figure 5 . Curve a is nearly free from the inner filter effect. However, when it is measured at d = 0.25 cm, the data, curve b will show a significant deviation from the curve a in the high-power region. In the early stage of our experiments even a slope with n = 1.1 was observed in the laser fluence region between 2 and 10 mJ cm-2. Unnoticed, this value might have led us to an incorrect conclusion. At low pressures of toluene, the calculated curve depended very little on the observation depth as shown in curves c and d in Figure 12. Therefore, toluene pressures employed were 8 Torr in the laser fluence region less than 2 mJ cm-2 and 0.5 Torr in the power region higher than 6 mJ cm-*. B. Scattered Light Correction. The scattered light cy(X) at a wavelength X is defined in this paper as follows:
OD(X) is the measured absorbance of ground-state toluene by our optical alignment for flash photolysis. A(X) is the absolute absorbance of the same sample. A(X) is assumed to be equal to the observed value by a commercial spectrophotometer. The molar extinction coefficients of toluene were essentially the same as those in the literature. The scattered light intensity cy was 0.02-0.03 in the wavelength region of 191-210 nm. The observed transient absorbances, OD(t,X), were corrected to OC,(t,X) by the equation
O D J t J ) = log V o ( t J ) / I ( t J ) ) = -log (( 1
+ S)1O-OD(t,A) - S)
(B2) where S = c ~ l O ~ ( The ~ ) . largest corrected value was located at 19 1.5 nm and was 1.15 times the observed one. Most of the data were corrected by eq B2, including the rise and decay curves in Figures 8 and 9. The photographic data in Figure 11 were not treated in such a manner. Acknowledgment. The publication cost was paid by Institute for Laser Technology, Nishi-ku, Osaka, Japan. Registry No. Toluene, 108-88-3;ethylbenzene, 100-41-4; butylbenzene, 104-51-8; benzyl radical, 2154-56-5.
