J . Phys. Chem. 1990, 94, 6321-6326
6321
C-C and C-H Bond Splits of Laser-Excited Aromatic Molecules. 3. UV Multiphoton Excitation Studies H. Hippler, Ch. Riehn, J. Troe,* and K.-M. Weitzel Institut fur Physikahche Chemie der Universitat Gottingen, Tammannstrasse 6, 0 - 3 4 0 0 Gottingen, West Germany (Received: August 28, 1989; In Final Form: February 14, 1990)
Parent disappearance and fragment formation yields are measured in the U V multiphoton excitation of cycloheptatriene, toluene, and ethylbenzene. The dependence on laser fluence and bath gas pressure is recorded. The results are interpreted in terms of a competition between the absorption of one or two photons, unimolecular reactions, and collisional deactivation of the parent molecules. The yields of the products CH, and benzyl decrease at high laser fluences, probably because of secondary spontaneous fragmentations of these highly excited primary fragments.
1. Introduction Multiphoton excitation in the UV often leads to ionization of gas-phase molecules. However, the transient electronic excitation can also be destroyed by rapid internal conversion processes such that ionization is "by-passed" and high vibrational excitation of the molecules in the electronic ground state arises. UV multiphoton excitation then permits a considerable extension of the energy scale accessible for studying collisional energy transfer, unimolecular isomerization, and dissociation processes of neutral molecules. This concept can be exploited in two ways: either the multiphoton process is followed in time-resolved experiments or the yields of the process are measured as a function of laser fluence and bath gas pressure. The former approach involves ultrashort and intense excitation and probing pulses. The latter method is experimentally less involved; however, the analysis of the data is possible only for simple cases and, for a quantitative interpretation, it requires known reference rates. In spite of the limited experimental work done so far, UV multiphoton excitation promises to become a powerful tool for studying the dynamics of competing processes of vibrationally very highly excited polyatomic molecules. The simplest system, which so far was studied in quantitative detail, was the UV multiphoton excitation of azulene at an excitation wavelength of 308 nm.' The yields of the isomerization reaction leading to naphthalene were measured as a function of laser fluence and bath gas pressure. The results could be modeled quantitatively. Absorption of up to three photons was identified in the parent molecule. Ratios of optical pumping, collisional energy transfer, and unimolecular isomerization rates at the various levels of excitation could be deduced from the analysis. Independent information on one (or two) of these processes was required for an absolute determination of at least one unknown rate. So far, internal consistency of the observations was obtained with thermal isomerization experiments, measurements of collisional energy transfer of one-photon-excited azulene, and studies of the hot UV absorption spectra. The second study of this type concerned the UV multiphoton dissociation of toluene, ethylbenzene, and n-butylbenzene2 using the excitation wavelength 193 nm. In this case benzyl yields were measured as a function of laser fluence under conditions where fragmentation of one-photon-excited molecules was collisionally quenched completely. The yields at first increased with the square of the laser fluence; at fluences above about 5 mJ/cm2 a smaller fluence dependence became apparent. The reduced benzyl yields here were interpreted by the absorption of a third photon by benzyl which leads to destruction of this reaction product. UV multiphoton excitation can be the primary object of interest. However, it also can be quite disturbing when the time dependence of single-photon-induced processes is of interest. It was present ( I ) Damm, H.; Hippler, H.; Troe, J. J. Chem. Phys. 1988, 88, 3564. (2) Nakashima, N.; Ikeda, N.: Yoshihara, K. J. Phys. Chem. 1988, 92, 4389. ,
I
,
probably more frequently in earlier studies than realized. It became, e.g., apparent in earlier studies of H atom formation during the dissociation of t ~ l u e n e . ~ The present work considers again the UV multiphoton excitation of toluene and ethylbenzene, as well as that of cycloheptatriene. It is based on our recent experimental and theoretical work on the excited-state spectra: collisional energy t r a n ~ f e r and , ~ unimolecular dissociation and isomerization ratesbs of these molecules after single-photon excitation. We measure benzyl and CH, yields as well as the appearance of some final reaction products. The influence of laser fluence and bath gas pressure is inspected. The results are modeled on the basis of the available knowledge about the elementary steps involved. Unambiguous interpretations, however, require laser picosecond studies of the dynamics of two-photon-excited molecules. Such work is underway in our lab~ratory.~
2. Experimental Techniques In the present work, two different techniques were applied. In one part molecules were irradiated at very low pressures, Le., under isolated molecule conditions, using laser pulses of varying energies. The neutral products of the light-induced reaction here were detected by multiphoton ionization induced by a second laser pulse. As a byproduct of this technique,* in the absence of the second laser pulse, multiphoton ionization of the parent molecule, which was induced by the first laser pulse, could also be monitored. In the second part of this work, the neutral products of the lightinduced reaction were detected by light absorption. The yields here were investigated as a function of laser fluence and bath gas pressure. These studies were conducted at much higher pressures without the detection of ions. In the following, these two techniques are characterized only briefly since the former method was described in detail in ref 8 (part 2 of this series), whereas the latter was described in ref 7 (part 1 of this series). 2.1. CH3 Product Identification by Multiphoton Ionization. In the first part of this work, toluene and ethylbenzene were irradiated with 15-ns pulses from an ArF excimer laser at 193 nm (EMG 101 MSC, Lambda Physik). The pulse energies were varied between 5 and 40 mJ They were measured with (3) Bersohn, R. Private communication, 1987. Tsukiyama, K.; Bersohn, R. J. Chem. Phys. 1987,86, 745. (4) Hippler, H.; Troe, J.; Wendelken, H. J. J . Chem. Phys. 1983, 78, 5351. Astholz, D. C.; Brouwer, L.; Troe, J. Ber. Bunsen-Ges. Phys. Chem. 1981, 85, 559. ( 5 ) 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, 6709, 6718. (6) Hippler, H.; Luther, K.; Troe, J.; Wendelken, H. J. J. Chem. Phys. 1983, 78,239. Lohmannsroben, H. G.; Luther, K. Z . Phys. Chem. (Frunk-
furl) 1986, 149, 129. (7) Brand, U.; Hippler, H.; Lindemann, L.; Troe, J. J. Phys. Chem., in this issue (part 1). (8) Luther, K.; Troe,J.; Weitzel, K.-M. J. Phys. Chem., in this issue (part
2).
(9) Brummund, U.; Luther, K.; Sander, M.; Troe, J. To be published.
0 1990 American Chemical Society
Hippler et al.
6322 The Journal of Physical Chemistry, Vol. 94, No. 16, 1990
220
210
2oops
260
280
300
320
h/nm
c--(
Time
Figure 1. Loss of cycloheptatriene by two-photon excitation at 248 nm: laser fluence 140 mJ/cm2; bath gas, SF,, P = 460 Torr; horizontal axis, 200 p / d i v : vertical axis, intensity of analysis light at 265 nm.
calibrated energy meters (GenTec ED 100, ED 200, ED 500). The laser pulses penetrated the low-pressure photolysis cell as a parallel beam. The pressure of the parent molecules were kept near 5 X IOd mbar. The progress of the photolysis was monitored via quantitative measurements of the yield of the CH3 fragments by resonance-enhanced multiphoton ionization (2 + 1 REMPI at 333.5 nm) with a time-delayed dye laser pulse (Lambda Physik FL 3002 pumped by EMG 101 MSC, dye DCM (LC 6500),and KDP frequency doubler (FL 30)). Details of the time-of-flight mass spectrometer for ion detection were described in refs 8 and IO. Absolute ion yields were determined by actinometry via the known photolysis yields of ethylbenzene at low laser energies. The delay times between pump and probe laser pulses were kept at 200 ns for ethylbenzene and 1 ps for toluene. At these times, photolysis of the molecules after absorption of one photon is nearly complete and loss of the fragments from the detection volume is still small. REMPI spectra were only taken from CH3 fragments of the excited neutral molecules. However, since complete time-of-flight mass spectra were always recorded, ion formation was also monitored from the excitation pulse alone, Le., in the absence of a focused probe pulse. Parent ions as well as ion fragments were recorded over the energy range 5-40 mJ cm-2. The ion yields from the primary photolysis laser were estimatedi0to be always at least 1000 times smaller than the yields of the neutral photolysis fragments. 2.2. Benzyl Product and Parent Molecule Identification by UV Absorption. In the second part of this work, toluene and ethylbenzene were irradiated at 193 nm (cycloheptatriene at 248 nm) with ArF (or KrF) excimer laser pulses (Lambda Physik EMG 200). In this case, the reaction cell was a 30 cm long tube closed at both ends by quartz windows. Photolysis and analysis light beams (from a Xe high-pressure arc lamp Osram XBO 150 W/4) were conducted coaxially through this cell. Absorption signals were recorded from single-pulse experiments without further .averaging. Laser energies again were measured with GenTec energy meters (ED 200). In this part of the work, transient absorption signals were monitored, mainly at the wavelengths 253 nm (formation of benzyl) and at 265 nm (disappearance of cycloheptatriene). The concentrations of the parent molecules in the cell before each experiment were monitored by their U V absorptions as well. After each experiment, the UV spectra of the reaction products were recorded in order to identify final products. In some cases, NO was added in order to scavenge radicals from the photolysis. The experiments were executed in a variety of bath gases such as N2,SF,, and C3Hsover the pressure range 0-760 Torr. Further details of these experiments were described in refs 7 and 1 1 .
3. Results In the following, the results of different studies are described. Experiments with absorption measurements of parent disap(IO) Weitzel, K.-M. Ph.D. Thesis, Gottingen, 1989. ( I I ) Riehn. Ch. Diploma Thesis, Gdttingen, 1989.
Figure 2. U V absorption spectrum of cycloheptatriene (CHT) and of irradiated CHT (possibly superimposed biphenyl or bibenzyl spectrum).
200
220
260
2LO
280
h lnm
Figure 3. U V absorption spectrum of biphenyl at 300 K (t,,,(235 = 14300 L m01-l cm-I).
nm)
pearance and fragment formation are considered first. REMPI experiments are described afterwards. 3.1. High-pressure Experiments with Cycloheptatriene. Figure 1 shows a typical absorption-time profile at X = 265 nm recorded during the irradiation at 248 nm of a mixture of cycloheptatriene (CHT) (0.15 Torr) and SF6 (460 Torr). The laser fluence was 135 mJ/cm2. At the given pressure, the single-photon-induced isomerization of CHT to toluene is strongly quenched by collisions; see refs 5 and 6. The absorption change of Figure 1, therefore, has to be attributed mainly to multiphoton processes. Since "cooling" of the gas and radical reactions are complete at the late times recorded in Figure 1, the absorption of toluene as a product would not contribute markedly to the recorded absorption. Therefore, the absorption step of Figure 1 at first is attributed to the amount of disappearing CHT. However, the complete absorption spectrum of the mixture, shown in Figure 2 as recorded after the experiment, indicates the formation of a new compound in addition to the disappearance of CHT. The absorption coefficients relative to the wavelength 260 nm are larger at 220 and 300 nm for the new compound than for CHT. A comparison with a biphenyl spectrum given in Figure 3 suggests the intermediate formation of phenyl radicals via a two-photon dissociation involving toluene with subsequent biphenyl formation. The broadening and shift of the central absorption appears quite consistent with this product identification. Experiments with added NO resulted in a marked decrease of the absorption steps of Figure 1 by the formation of a substance with stronger absorption than C H T or biphenyl. This could well be nitrosobenzene (with t(270 nm) = 17000 L mot-' cm-I). Without explicitly accounting for the formation of a second absorbing species, first we only represent the dependence of the absorption steps on the laser fluence and bath gas pressure in Figure 4. The chosen Stern-Volmer representation shows a quenching of the photolysis by added bath gases which becomes less pronounced with increasing laser fluence. The detailed modeling of the observations given later has to account for the superposition of the spectra of the disappearing CHT and the final products which are presumably biphenyl molecules. 3.2. High-pressure Experiments with Toluene. Mixtures of toluene and SF, were irradiated at 193 nm with laser pulses of a fluence of 20 mJ/cm2. In this case the formation of benzyl was monitored at 253 nm before any disappearance of benzyl via bimolecular radical reactions occurred. The benzyl yields de-
C-C and C-H Bond Splits of Aromatic Molecules
The Journal of Physical Chemistry, Vol. 94, No. 16, I990 6323
-
IO ps -0
200
600
400
P / torr % Figure 4. Stern-Volmer representation of C H T disappearance: irradiation at 248 nm with varying laser fluences; bath gas, SF, I$ corresponds to the amount of disappeared C H T relative to that at low SF6pressure. Dashed line: Stern-Volmer plot for one-photon-excitation experiments at small laser fluence.
Time
Figure 6. Transient absorptions at 253 nm (top), 260 nm (middle), and 266 nm (bottom) during 193-nm irradiation of ethylbenzene. Laser fluence, 70 mJ/cm2; bath gas, N,, P = 60 Torr; horizontal axis, IO ps/div; strong absorption at 253 nm due to benzyl.
=
H
PS
Ti me
Figure 7. As Figure 6. Benzyl decay (observed at 253 nm) in the presence of added N O PNO = 0 Torr (top), PNO 2 Torr (middle), PNo = 20 Torr (bottom) (horizontal axis, 5 ps/div).
b, I lorr
,P,, /lorr
Figure 5. Stern-Volmer representation of benzyl yields from toluene photolysis at 193 nm: laser fluence = 20 mJ/cm2; (@) experimental points; I$ corresponds to benzyl yield relative to I$(P 0); (full line) plot for one-photon-excitation experiments.
-
creased with increasing pressure due to the collisional quenching of the dissociation from one-photon-excited toluene. We studied this effect up to 20 Torr of added SF,; see Figure 5. By addition of much higher pressures, the benzyl formation is exclusively due to the two-photon processes since dissociation via the one-photon process is nearly completely quenched. Since these experiments (with 800 Torr of N2) were done in ref 2, we did not repeat them here. The detailed analysis of the Stern-Volmer plots in Figure 5 indicates, in agreement with the observations of ref 2, that for the applied laser fluence there is less benzyl formation than estimated for a clean two-photon excitation of toluene leading directly to benzyl products. An interpretation of this observation will be given below. 3.3. High-pressure Experiments with Ethylbenzene. The yields of benzyl fragments under multiphoton excitation conditions were investigated in more detail for ethylbenzene. On a short time scale, one notices a broad, nearly time independent continuum which underlies the benzyl absorption. By varying the wavelength, we can easily separate the benzyl contribution from this continuum such as shown in Figure 6. By addition of NO, the strong benzyl signal could rapidly be removed (see Figure 7) and be replaced by a weaker and much broader continuum analogous to the observations with C H T described in section 3.1. The "residual absorption" could not be identified unambiguously. Stable primary dissociation products or large radicals, formed by rapid addition of fragments like H atoms to ethylbenzene and combining on a much slower time scale, could be responsible for the signals. Similar absorptions should also have been present in our toluene experiments, perturbing the measured benzyl yields. It should be mentioned that, after the experiments, fog formation was quite noticeable under the high laser fluences of the present work. This, however, did not influence the absorption measurements to a major
8
b
"
I
o
0
1
2
3 Zvj
4 5 ,p ( A E ) / lO*S'Un'
6
7
Figure 8. Benzyl yields in UV multiphoton dissociation of ethylbenzene at 193 nm: open symbols, bath gas N,; tilled symbols, bath gas propane; squares, fluence = 20 mJ/cm2; circles; fluence = 69 mJ/cm2, triangles and dashed curve, modeled yield curve, see text; scaled pressure axis, ZLJP(AE).
extent. The ionization efficiency was measured in ionization chamber experiments: It was found to be of negligible importance with respect to the overall reaction rates. Subtracting the underlying residual absorption, the initial benzyl yields in Figure 8 are plotted as a function of the bath gas pressure and the laser fluence. The benzyl yield is not linearly dependent o n laser fluence, and pressure effects apparently are only small. 3.4. Low-Pressure Experiments with Toluene and Ethylbenzene. REMPI experiments were conducted at very low pressures such that collisions during and after the photolysis did not take place before the analysis was performed. Figure 9 shows the measured CH,ion signals, obtained under identical conditions in all experiments, with the exception of varying photolysis laser fluences at the excitation wavelength 193 nm. Near to the lowest fluences of 5 mJ/cm2 a nearly linear increase of the methyl yields with fluence is observed. The smaller methyl yield in toluene
6324
Hippler et al.
The Journal of Physical Chemistry, Vol. 94, No. 16, 1990
In E 0
.-0 )
I n 0 E
I
t
u
I
t
5
io
20
so io to 50 Fluence / m l cm'2
m
20
M
Figure 11. As Figure IO, but for toluene.
CHT**
Fluence / m l tm-'
Figure 9. REMPI signals of CH3 in the UV multiphoton dissociation of ethylbenzene (A)and toluene (H) at 193 nm (collision-freeconditions with equal geometry in all experiments).
T*"
>
c
V 0
4 .
c
5 v)
n
a 2
O 0 00 n
T*
2 t I , . , . I
I
,
,
I .
. 0 . l
I
.
.
I
Fluence / m i cm-'
Figure 10. Ion signals recorded after photoexcitation of ethylbenzene at 193 nm (fluence scale shifted; experiments all done between 8 and 40
mJ/cm2). photolysis compared to ethylbenzene photolysis in part 2* was attributed to the dominance of a C-H bond split in toluene dissociation under the present excitation conditions, whereas ethylbenzene dissociation is governed by a C-C bond split. Above about 15 mJ/cm2, Figure 9 shows a maximum in the CH3 yields; for ethylbenzene even a decrease of the yield appears to set in. The measurements at the highest fluences are still too limited to unambiguously prove different behavior of the two reactions. We have also recorded the small ion signals originating from the photolysis laser in the absence of the probe laser. Figures 10 and 11 show the fluence dependences of various fragment ion signals for ethylbenzene and toluene. One recognizes the strongest fluence dependence for the smallest fragments, confirming the multiphoton character of the fragmentation. For the largest fragments 'kturation" effects occur at the highest fluences, similar to the CH, yields in Figure 9. In both cases, fragmentation of highly excited species and/or secondary optical pumping of the fragments can be discussed in relation to this observation (see section 4). 4. Modeling of Yield Curves 4.1, Modeling of Multiphoton Excitation Experiments with Cycloheptatriene. In order to model the measured pressure and fluence dependences of the yields, collisional deactivation rates, specific rate constants k ( E ) , and optical pumping rates have to be estimated. With the knowledge of these quantities, the multiphoton excitation and reaction pathways can be specified. For experiments with CHT, first, one has to ask whether the second photon is absorbed by highly excited C H T or its isom-
L
CHT Figure 12. UV multiphoton excitation scheme for cyclohcptatriene (long
vertical arrows symbolize absorption and subsequent fast internal conversion). erization product toluene (T). &E), for isomerization at the energy of one photon at 265 nm, was measured6 as 2.7 X IO7 s-I which corresponds to 7 X lo7 s-l at 248 nm. Since the LennardJones collision frequency Z, for CHT-SF, collisions is about 1.7 X IO7 Torr-I S-I, and since on the average CHT* loses ( A E ) = -400 cm-l per collisionS with SF6, CHT* molecules in the investigated pressure range (50-600 Torr of SF,) are colliding frequently such that the isomerization to toluene competes with collisional deactivation. With the knowledge4 of the absorption coefficients of cold C H T and one-photon-excited CHT* (after fast internal conversion to the electronic ground state), the pumping rate constants for absorption under our conditions follow as ko = 4.6 X IO7 s-l and k l = 2.8 X lo7 s-l at a fluence of 80 mJ/cm2 (ko, absorption of the first photon; k l , absorption of the second photon by CHT*). After the absorption of a second photon, CHT** isomerizes so fast to toluene ( k ( E ) = 2 X loio s-I without collisional deactivation) with subsequent dissociation of toluene ( k ( E ) = 5 X IO9 s-I without collisional deactivation) that further absorption of a third photon could only take place in toluene dissociation fragments. Under the investigated pressures and laser fluences, the loss of C H T is governed by the competition between isomerization, collisional deactivation, and absorption of the first and second photon. Figure 12 shows the corresponding reaction scheme. After the laser pulse, the reaction continues with the competition between collisional deactivation and reaction. Detailed modeling was done with the known expressions for k(E), the given pumping and collisional deactivation rates for a stepladder ( ( AE) = -400 cm-I) deactivation scheme. Figures 13 and 14 show the derived losses of C H T as a function of SF6 pressure and laser fluence. The modeled values are in perfect agreement
The Journal of Physical Chemistry, Vol. 94, No. 16, 1990 6325
C-C and C-H Bond Splits of Aromatic Molecules
t
I
l a ,
"0
IW
200
300
400
SO0
603
F&, llorr
Figure 13. Modeled reaction yields for UV multiphoton excitation of cycloheptatriene in SF,: points, experiments of this work; solid lines, modeled curves from section 4.1.
0
I
1
I
200
400
600
P / Torr
Figure 15. As Figure 13: (F,) fluence = 60 mJ/cm2;(F,) fluence = 135 mJ/cm2; (full line) reaction from one-photon-excited molecules; (line with points) reaction from two-photon-excited molecules).
E I m J cm-l
Figure 14. As Figure 13.
with the measurements after correction of the latter for the superimposed residual absorption from biphenyl (see section 3.1). The yield of the latter was estimated by assuming that twophoton-excited CHT** via T** fragments predominantly to phenyl methyl. The discussion of the toluene dissociation in part 28 indicated dominance of this channel at the very high temperatures expected in the bath gas during the deposition of the laser energy. The good agreement between modeled and measurement C H T losses confirms the two-photon mechanism of the present experiments. It, therefore, appears of interest also to look at the modeled percentages of C H T that have reacted after absorption of one and two photons. Figure 15 shows the results. At fixed pressure the total reaction yields increase about proportional to the laser fluence. The percentage of two-photon reaction increases quadratically with the fluence below about 20 mJ/cm2; above this value, a transition to a linear increase is observed; only a small pressure dependence is observed in contrast to the percentage of one-photon reaction. 4.2. Modeling of Multiphoton Excitation Experiments with Toluene. Whereas the disappearance of the parent molecule in the C H T system under two-photon-excitation conditions (for X = 248 nm) appears well understood, the situation is less clear with toluene and ethylbenzene. After absorption of one 193-nm photon and internal conversion, the specific rate constant for dissociation of toluene is only k ( E ) = 2 X IO6 s-I, whereas it is about 1O1O s-I with the energy of two photons. Benzyl formation arises from one- and two-photon-excited toluene where the former is strongly quenched by the moderate pressures applied in the present work (see Figure 5) in contrast to the latter. The difference between the modeled Stern-Volmer plot for collisional quenching of one-photon dissociation in Figure 5, which is clearly outside the experimental error, and the measurements demonstrates the presence of unquenched two-photon dissociation. This is, however, not as pronounced as one would expect from a simple two-photon-excitation scheme (with a pumping rate constant estimated via the hot spectra from ref 4). There is, furthermore, the problem of an underlying residual absorption which is not due to benzyl. Since the situation is quite similar to ethylbenzene, we have done a more detailed modeling for ethylbenzene where more measurements (see Figure 8) were performed. The results, however, apply for toluene as well. The apparent decrease of the benzyl yield from two-photon-excited toluene which was observed here and in ref 2 could either be due to subsequent optical pumping of benzyl radicals by a third photon leading to destruction (such
+
as postulated in ref 2) or it could be due to the high energy content of benzyl originating from two-photon-excited toluene which leads to spontaneous fragmentation. A discussion of these alternatives will be given in the next section. 4.3. Modeling of Multiphoton Excitation Experiments with Ethylbenzene. In ethylbenzene the specific rate constant k ( E ) for dissociation of one-photon-excited molecules EtB* was measured to best9 2 X IO7 s-l. The absence of a marked pressure dependence of the benzyl yields in Figure 8 demonstrates the necessity of absorption of a second photon in order to produce benzyl. Since k(E) is equal to the collision rate constant at 1 Torr of bath pressure, our high-pressure experiments with benzyl detection probe the two-photon process. However, the benzyl yield is not linearly dependent on laser fluence. A similar observation was made in ref 2 with the toluene system. These observations could be explained by the absorption of a third photon by benzyl via a ladder-switching mechanism such as suggested in ref 2. In order to reproduce the results, absorption coefficients at 193 nm of 20 000 L mo1-I cm-' for hot toluene and of 22 500 L mol-l cm-I for hot benzyl were required. However, thermal measurements in ref 12 at 1600 K led to absorption coefficients which were about a factor of 2 smaller. Therefore, as an alternative, one may think of new reaction pathways opening up at high excitation energies. Apart from the unexplained benzyl yields, however, there would so far be no evidence for an explanation which does not involve benzyl radicals. In the following we propose a third alternative for an explanation which involves the secondary dissociation of benzyl radicals via the high internal excitation received from the primary dissociation reactions. The dissociation of two-photon-excited ethylbenzene at 193 nm leaves an energy of about 78 000 cm-l to be distributed between the dissociation fragments benzyl + CH3. Assuming statistical energy distribution, this corresponds to an apparent temperature of about 3200 K of the fragments, such as calculated with the vibrational frequencies of benzyl and CH,. The hotter part of the benzyl radical energy distribution will dissociate fast with specific rate constants of the order of 108-109s--I (similar to the toluene r e s ~ l t s ~at. ~78 ) 000 cm-' from parts 1 and 2). The cooler part of the distribution will be collisionally stabilized and appear as benzyl products. Quantitative modeling shows that both the ladder-switching mechanism and the hot benzyl fragmentation mechanism can contribute. Unfortunately, the fit to the limited experimental data is not unique. Figure 8 includes one model calculation accounting for both mechanisms which well reproduces the slight maximum of the benzyl yield at medium pressures. Since the methyl yield from the REMPI experiments shows the same drop with increasing laser fluence (see section 4.4) and since a ladder-switching mechanism here probably cannot contribute due to a small absorption coefficient of hot CH,, we favor a general dominance of the hot product fragmentation mechanism. 4.4. Methyl Yields from Toluene and Ethylbenzene. The methyl yields recorded by REMPI under isolated molecule con-
6326 The Journal of Physical Chemistry, Vol. 94, No. 16. 1990 ditions represent the sum of the results of one- and two-photon processes. In the presence of a simple two-photon-excitation process they should rise linearly with laser fluence as long as no saturation sets in. This simple picture can change for a variety of reasons. First, the relative yields of methyl in competing dissociations of the parent molecules may depend on the excitation energy. Whereas ethylbenzene is assumed to fragment dominantly via C-C bond split to benzyl and CH, under one- and two-photon-excitation conditions, there is competing C-H and C - C bond split in toluene such as reflected in the smaller CH, yields from toluene in Figure 9. However, for the essentially rotationally cold parent molecules, our SACM modeling of the [CH,]/([CH,] + [C7H7])branching ratio does not indicate a marked change between one- and two-photon-excitation energies. Second, CH, could be destroyed by further light absorption under high laser fluence conditions, similar to the ladder-switching mechanism postulated for benzyl destruction ref 2. However, the absorption coefficient of hot CH, at 193 nm most probably is much smaller than the value of 22 500 L mol-] cm-' required for interpreting the observed drop of benzyl signals2 at fluences similar to those where CH, yields level off in Figure 9. We, therefore, favor the third interpretation, attributing the decrease of CH, detection to spontaneous fragmentation of the products from two-photon-excited parent molecules due to their high internal excitation. As an upper limit of this effect, Figure 9 would just correspond to C H 3 produced from one-photon-excited parent molecules whereas CH, from two-photon dissociation would have completely fragmented. The high internal "temperatures" of the fragments from twophoton dissociation (near 3200 K) suggest that only a minor fraction of the products of the two-photon dissociation of the parents survive until the moment of detection. The present lowpressure experiments differ considerably from the experiments in ref 2 which were conducted with 800 Torr of N2. Under the high-pressure conditions of ref 2, absorption of the second photon competes with considerable collisional deactivation of one-photon excited molecules such that one detects benzyl with much smaller excitation than in the present isolated molecule experiments. The benzyl yield-fluence curves of ref 2, thus, could also be interpreted by the competition between collisional deactivation, optical pumping, and benzyl fragmentation instead of the ladder-switching mechanism proposed in ref 2. Although constant pressures were applied in ref 2, different pumping rates at different laser fluences resulted in strong changes of the internal energies of benzyl with changing laser fluence. There is an additional argument in favor of our interpretation of the CH, yields in Figure 9 by the products from one-photonexcited parent molecules only. Our studies of the (0,O) and the ( 1 , l ) bands of the REMPI signals in ref 8 confirm statistical energy distributions in CH, during the primary dissociation of
Hippler et al. ethylbenzene. After two-photon excitation, the CH, would be so highly excited that its detection sensitivity by REMPI decreases strongly. Therefore, in addition to the CH, lability, the low detection sensitivity would reduce the CH, signals from two-photon dissociation. It should be noted that the high power of the REMPI probe pulse has no influence on the observed methyl profiles, since no CH, signals were obtained from pure benzene or phenylproducing molecules. Assuming that Figure 9 only shows CH, from one-photon-excited parent molecules, the approach of a plateau at high fluences may be attributed either to saturation, Le., the approach of complete excitation of all parent molecules, or to the depletion of the one-photon-excited molecules by a particularly fast absorption process for the second photon, or to the combination of both effects (the decrease of the CH, yields at the highest fluences in ethylbenzene suggests contributions from fast absorption of the second photon). In the former case, a pumping rate constant of about 4.5 X lo7 s-I would be required at the laser fluence of 25 mJ cm-2 which is about 3 times the value estimated from the room temperature absorption coefficient. In the second case the absorption coefficient for absorption of the second photon would have to be about 4-5 times larger than that of the first photon (such as concluded also in ref 2). Our studies of the hot UV spectrum of toluene in refs 4 and 12, however, do not seem to support the second possibility. We, therefore, have to leave open the question of the precise origin of the CH, plateaus at high fluences. 5. Conclusions
The present work has confirmed the importance of U V multiphoton excitation even under relatively modest laser fluences. The observed product yields as a function of laser fluence and bath gas pressure can be satisfactorily modeled by simple two-photon-excitation schemes without invoking the absorption of a third photon by photolysis fragments. Instead, it suggests a marked contribution from spontaneous fragmentation of the highly excited radicals formed from two-photon-excited parent molecules. Further elucidation of the dynamics of the elementary processes involved requires time-resolved experiments with intense picosecond laser pulses. Such experiments are underway in our laborat~ry.~
Acknowledgment. Financial support of this work by the Deutsche Forschungsgemeinschaft (SFB 93 "Photochemie mit Lasern") is gratefully acknowledged. Registry No. CH,,2229-07-4; cycloheptatriene, 544-25-2; toluene, 108-88-3; ethylbenzene, 100-41-4; benzyl, 2154-56-5. (12) Brouwer, L. D.; Muller-Markgraf, W.; Troe, J. J . Phys. Chem. 1988, 92, 4995.
