Formation of p-xylylene from p-xylene by a two-photon process and hexamethyl Dewar benzene from hexamethylbenzene by a one-photon process at 193 nm

Article abstract of DOI:10.1016/j.jphotochem.2011.02.031

While studying a series of methyl-substituted benzenes, C₆H _6-n(CH₃)_n with n = 2, 3, 4, 6, and perfluorobenzene in the gas phase using 193-nm laser flash photolysis, we observed the formation of p-xylylene (benzoquinodimethane) due to the elimination of two hydrogen atoms as a result of a two-photon process. The results were explained in terms of an intermediate hot molecule formed by internal conversion which finally led to the ground electronic state. Quadratical dependencies on the photoproducts were observed for toluene, xylene, mesitylene, and perfluorobenzene in the presence of a foreign gas, while linear dependencies were observed for durene and hexamethylbenzene. Dewar-type benzene was detected from photolysis of hexamethylbenzene.

Full text of DOI:10.1016/j.jphotochem.2011.02.031

Journal of Photochemistry and Photobiology A: Chemistry 219 (2011) 273–277
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Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Formation of p-xylylene from p-xylene by a two-photon process and hexamethyl
Dewar benzene from hexamethylbenzene by a one-photon process at 193 nm
a
a
b,∗
Naoya Mitsubayashi , Tomoyuki Yatsuhashi , Nobuaki Nakashima
a
Department of Chemistry, Graduate School of Science, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi, Osaka 558-8585, Japan
Toyota Physical and Chemical Research Institute, Nagakute Yokomichi 41-1, Nagakute, Aichi 480-1192, Japan
b
a r t i c l e i n f o
a b s t r a c t
Article history:
6 6−n
While studying a series of methyl-substituted benzenes, C H (CH3)n with n = 2, 3, 4, 6, and perfluo-
robenzene in the gas phase using 193-nm laser flash photolysis, we observed the formation of p-xylylene
Received 21 December 2010
Received in revised form 22 February 2011
Accepted 28 February 2011
Available online 5 March 2011
(benzoquinodimethane) due to the elimination of two hydrogen atoms as a result of a two-photon pro-
cess. The results were explained in terms of an intermediate hot molecule formed by internal conversion
which finally led to the ground electronic state. Quadratical dependencies on the photoproducts were
observed for toluene, xylene, mesitylene, and perfluorobenzene in the presence of a foreign gas, while lin-
ear dependencies were observed for durene and hexamethylbenzene. Dewar-type benzene was detected
from photolysis of hexamethylbenzene.
Keywords:
Two-photon process
Benzene derivatives
ArF laser
©
2011 Elsevier B.V. All rights reserved.
Xylylene
Hexamethyl Dewar benzene
1
. Introduction
technique, multimass ion imaging, are now available. In addition,
target molecules of Scheme 1 have been extended to bio-related
molecules, including tryptophan at 193-nm nanosecond pulses [17]
and adenine in the femtosecond time domain [18].
Vacuum ultraviolet (VUV) and ultraviolet laser chemistry, in
which molecules are excited to high electronic states but at energy
levels below the ionization potential, have been extensively stud-
ied. In the VUV (193 nm) laser chemistry of benzene, a hot molecule,
a highly vibrationally excited state in the ground electronic state,
denoted S , is a key intermediate. The products of benzene can be
explained in terms of passing through S [1–6].
These photochemical mechanisms at 193-nm laser irradiation
in Scheme 1 have been shown to work for single-substituted
alkylbenzenes [10–16,19,20]. Doubly and triply substituted ben-
zenes, xylenes and mesitylene, have been photolyzed, and a major
part of the dissociation was explained in terms of Scheme 1
based on nanosecond laser photolysis and mass spectrometry
[10,21–25]. Benzocycloalkenes and cyclophanes can be regarded
as dialkyl-substituted benzenes and dissociate into the correspond-
ing radicals and molecules via Scheme 1 [26–28]. Isomerizations of
toluene [19] and m-xylene [24] have been found by multimass ion
imaging techniques to compete with direct C–C and C–H dissocia-
tions.
∗
∗
0
∗
∗
0
Products (1.1) are the result of a one-photon process at
1
84.9 nm: fulvene [1], 1-3-hexadiene-5-yne [2], and phenyl rad-
ical (C–H dissociation) [3,4]. Products (1.2) are the results of
two-photon processes through S : C –C4 fragments [5], two
hydrogen-eliminated molecules [3], 1-3-hexadiene-5-yne, and
others [6]. The hot benzene, S , at 193 nm was detected in transient
∗
∗
0
2
∗
∗
0
spectra in 1983 by one of the authors [7].
Scheme 1 has been thought to apply to alkyl-substituted ben-
zenes. A benzyl radical produced by toluene C–H dissociation
was observed in 1985 as a product (1.1) [8]. Hot molecules can
be an intermediate for reactions following the second and/or
third photon absorption. A benzyl radical formed [9] by two-
photon absorption was also found as a product (1.2) of Scheme 1.
Detailed analysis of Scheme 1 [10–12], several reviews [13–15],
and comprehensive studies [16] based on a new experimental
A series of methyl-substituted benzenes, C H
(CH )n with
6
6−n 3
n = 2, 3, 4, 6, and perfluorobenzene were studied by conventional
ArF laser flash photolysis. As expected, two-photon processes in
the transient spectra occurred for toluene, xylene, and mesitylene
in the presence of a foreign gas. One of the findings in the photolysis
was p-xylylene formation due to the elimination of two hydrogen
atoms as a result of a two-photon process, as shown in Scheme 1.
Based on the cyclophane laser chemistry [27], and the elimination
of two hydrogen atoms from benzene by two-photon absorption
[
3], xylylene formation from p-xylene is predictable and realized.
The second finding is that hexamethyl Dewar benzene (HMDB) was
produced as a final product from photolysis of hexamethylbenzene
∗ Corresponding author. Tel.: +81 80 1620 8159; fax: +81 561 63 6302.
E-mail address: riken-nakashima@mosk.tytlabs.co.jp (N. Nakashima).
1
010-6030/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2011.02.031

2
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N. Mitsubayashi et al. / Journal of Photochemistry and Photobiology A: Chemistry 219 (2011) 273–277
2
Fig. 1. (a) Transient absorption spectra of p-xylene at 1.3 kPa in the presence of 27 kPa of nitrogen at a laser fluence of 10 mJ/cm . Full circles (᭹) are at 0 ns, and open circles
(
(
ꢀ) at 800 ns. The normalized spectrum of p-xylylene (—) is formed by the photolysis of cyclophanes [27]. (b) Typical time profiles of transient signals are shown at 235 nm
b-1) and 280 nm (b-2). The initial decay component in (b-1) and the initial rise component in (b-2) are due to the collisional relaxation of hot species by nitrogen. The
remaining components with almost constant absorbances for (b-1) and (b-2) after 200 ns are attributed to photoproducts (p-xylylene).
The HMB and durene to be photolyzed were dissolved in cyclo-
−
3
hexane, and a 100 ␮L drop of 8.3 × 10 M solution was deposited
onto the bottom of the sample tube. The solvent was then pumped
carefully away until the background pressure was less than 0.7 Pa,
and 27 kPa of nitrogen gas was added. HMB and durene were sealed
in a quartz tube and kept at 423 K for over 20 min before irradiation.
The corresponding pressure of HMB was calculated to be 0.5 kPa
at 423 K. The temperature was controlled by a chromel–alumel
thermocouple and a heater combination. Other methylbenzenes
and hexafluorobenzene were photolyzed at room temperature. The
typical irradiation energy reached 40 mJ/pulse for product analysis
and less than 35 mJ/pulse for flash photolysis.
Benzene was purchased from Aldrich Chemical Co. (Milwau-
kee, WI) with a stated purity of 99.9%. Toluene and p-xylene were
from Merck Japan (Tokyo, Japan) with stated purities of 99.5%.
Mesitylene and durene were from Nacalai Tesque (Kyoto, Japan)
with stated purities of 98%. Hexamethylbenzene (TCI, Tokyo, Japan;
>99%), hexafluorobenzene (Aldrich Chemical Co.; >99%), and hex-
amethyl Dewar benzene (Aldrich Chemical Co.; >97%) were used
as received. Nitrogen gas was purchased from Osaka Sanso (Osaka,
Japan) with a stated purity of 99.999%.
Scheme 1.
(
HMB). The laser-intensity dependencies of the product formation
showed a one-photon process. The theoretical study of benzene
photoisomerization to Dewar benzene has predicted that this reac-
tion occursthrough anS toS conical intersectionleadingtoDewar
1
0
benzene [29]. The crossing is suggested to occur in a high energy
state in S . This concept can be applied to the present experimental
results; some part of hot HMDB is collisionally relaxed to HMDB, as
shown in Scheme 2, in the presence of a foreign gas M.
A foreign gas is effective in obtaining unstable photoproducts;
otherwise they might decompose under collision-free conditions.
In fact, 1-3-hexadiene-5-yne formation from benzene was iden-
tified in the presence of foreign gases [6]. Similar one-photon
dependency has been observed for 1,2,4,5-tetramethylbenzene
1
(
durene), suggesting that isomerization is effective for multi-
substituted benzenes. Multi-substituted benzenes have been
shown to form Dewar-type benzenes since the 1960s. Dewar ben-
zene (DB) was derived from benzene itself, when the latter was
photolyzed in the liquid phase [30]. Perfluorobenzene [31], 1,2,4,5-
tert-butylbenzene [32], and hexakis(trifluoromethyl)benzene [33]
are photochemically converted to the corresponding DBs. The
observation of a one-photon process for durene and HBM indicates
that multi-substituted benzenes can be easily converted to DBs.
3
. Results and discussion
3.1. Formation of p-xylylene by a two-photon process of p-xylene
The results of the laser photolysis of p-xylene can be summa-
∗∗
rized as Scheme 3. Formation of hot p-xylene S0 by rapid internal
conversion (ic) was probably complete in less than 1 ps, as ben-
zene ic from S2 occurs in 40 fs [35]. The second photon of 193-nm
laser light pumps the S0 state to the S state, followed by inter-
nal conversion leading to S0 ; subsequently, S0 loses 2H to form
2
. Experimental
∗∗
∗∗
3
∗∗∗
∗∗∗
A conventional method of nanosecond laser photolysis was
p-xylylene. These two-photon processes are considered to proceed
in an excitation laser pulse of 20 ns. S0 is defined as hot p-xylene
applied to gaseous benzenes in a quartz cell irradiated by an ArF
excimer laser (Lambda Physik COMPex102: 193.3 nm, fwhm 14 ns,
and maximum energy 200 mJ/pulse). The method was primar-
ily the same as that previously described [6]. Time t = 0 ns was
defined as the time at the peak absorbance in the transient sig-
nals, when a spiky signal was observed, as seen in Fig. 1b. The
∗∗∗
with an internal energy of 2hꢀ. The mechanism can be regarded as
an extension of Scheme 1, which has been established for benzene,
toluene, and some alkylbenzenes.
White smoke was seen immediately after laser irradiation,
probably due to polymerization of p-xylylene. p-Xylene has been
studied by laser flash photolysis, and the product of C–H disso-
excitation wavelength of 193 nm pumps the S state corresponding
3
to the 1E1u state of benzene for the methyl-substituted benzenes
C6H
(CH )n with n = 2, 3, 4, 6 [34].
6−n
3
Scheme 2.
Scheme 3.

N. Mitsubayashi et al. / Journal of Photochemistry and Photobiology A: Chemistry 219 (2011) 273–277
275
∗∗∗
hot p-xylene S0 were faster than the collisional relaxation rate by
the added gas.
The time profiles in Fig. 1b also support Scheme 3; the profile
(
b-1) at a short wavelength of 235 nm shows collisional cooling of
∗∗
the hot species of S0 , and the profile (b-2) around the peak of p-
xylylene at 280 nm shows collisional relaxation – in other words,
collisional spectral narrowing.
The laser fluence dependencies in Fig. 2 support Scheme 3.
The transient absorption of p-xylene at 0 ns (᭹) and 235 nm is
∗∗
assignable to S0 , which is produced by one-photon absorption. In
fact, the species at 0 ns (᭹) and 235 nm can be plotted on a line
with a slope of 1.0, indicating a one-photon process. The photo-
product p-xylylene at 800 ns and 280 nm (ꢀ) can be plotted on a
line with a slope of 2.0, indicating that p-xylylene forms by a two-
photon process. This behavior is similar to that observed for the
case of benzene irradiated by 193-nm laser pulses [6]. Scheme 3
Fig. 2. The correlation between laser fluence and transient absorption of p-xylene
at 0 ns (᭹) for 1.3 kPa in the presence of 27 kPa of nitrogen observed at 235 nm
∗∗
(
S ); the dotted line (. . .) has a slope of 1.0, indicating a one-photon process. The
0
correlation between laser fluence and transient absorption of p-xylene at 800 ns (ꢀ)
and 280 nm (p-xylylene) with the dashed line (- - - -) has a slope of 2.0, indicating a
two-photon process.
∗∗
well explains that the primary species is hot p-xylene (S0 and that
the formation of p-xylylene requires the second photon).
ciation to methyl benzyl radical has been recognized [10,21–24].
Furthermore, the rate constant has been well-explained in terms
of a statistical model [10]. One-photon processes are completely
quenched in the presence of foreign gases, as observed and dis-
cussed for the two-photon process of toluene [9].
3.2. One-photon process for formation of hexamethyl Dewar
benzene
The results for mesitylene are consistent with Scheme 1, i.e., the
laser fluence dependencies showed that the transient species at
∗∗
The transient spectra, time profiles, and laser fluence depen-
dencies in Figs. 1 and 2 support Scheme 3. The transient spectrum
at 800 ns after irradiation corresponded to p-xylylene formation.
The transients denoted by circles (ꢀ) (Fig. 1a) with a peak around
t = 0 (expected to be S0 ) have a slope of 1.0 and at t = 800 ns (prod-
ucts) a slope of 2.0 (Fig. 3). Mesitylyl radical due to H elimination
∗∗
∗∗∗
from photo-excited S0 (S ) is the most probable candidate for
0
the product according to Scheme 1. In contrast, HMB and durene
showed a slope of 1.0 for both transients (t = 0 and 800 ns), indicat-
2
75 nm at a delay of 800 ns almost reproduce the spectrum (solid
line in Fig. 1a) of p-xylylene, which was previously obtained by the
photolysis of p-cyclophane [27].
ing that a different mechanism is at work for C H
(CH )n with
6
6−n 3
n = 4, 6. The one-photon process for HMB is tentatively explained
in Scheme 2. In fact, HMDB was detected by gas chromatography
and gas chromatography–mass spectrometry (GC–MS) after HMB
was irradiated with 193-nm laser light. Only one-shot irradiation
of HMB afforded the GC-MS signal of HMDB. Products from hex-
afluorobenzene have not been identified by 193-nm excitation, but
hexafluoro Dewar benzene (HFDB) may be a candidate, as HFDB
has been reported to be a product by 253.7-nm excitation [31].
Transient spectra and time profiles can be explained in terms
of Scheme 2—i.e., after rapid internal conversion through a coni-
cal intersection, hot HMDB should be included in the spectrum at
t = 0 ns in Fig. 4a and is collisionally cooled to a relaxed HMDB, which
corresponds to the spectrum at t = 400 ns in Fig. 4a. During the
course of the collisional relaxation, some of the hot HMDB would
be relaxed to the ground state HMB. HMDB has an absorption peak
close to but shorter than 200 nm, as shown in Fig. 4c, and its tail is
extended to a long wavelength region; therefore, the absorption of
hot HMDB is expected in a wavelength region longer than 200 nm.
The spectrum at t = 0 in Fig. 4a and time profiles in Fig. 4b are con-
sistent with this explanation; the hot HMDB is collisionally relaxed
in the presence of 27 kPa of nitrogen, while almost flat signals are
observed in the absence of foreign gas. The observed relaxed signals
HMDB in Fig. 4a can be plotted on the spectrum of HMDB in Fig. 4c.
The sharp decrease in the wavelength region shorter than 235 nm
in the transient spectra in Fig. 4a can be attributed to the ground-
state depletion of HMB. The open circles were plotted without data
correction and the open squares were plotted after correcting for
HMB depletion under the assumption that 55% of the excited HMB
is converted to HMDB and the remaining 45% returns to HMB. The
spectra showed some absorptions around the 250–300 nm region,
which were not identified.
The t = 0 ns spectrum in Fig. 1a in the wavelength region shorter
∗
∗
than 260 nm can be assigned to hot p-xylene (S ) formed by inter-
0
nal conversion from the pumped state of the S , as we previously
3
observed for benzene and toluene [7,9]. The monotonical spectrum
shape between 220 and 260 nm is similar to that of hot benzene
molecules and is collisionally cooled to the spectrum at 800 ns in
the presence of 27 kPa of nitrogen. The species with weak absorp-
tion in the wavelength region below 230 nm can be explained in
terms of the shifted ground state absorption due to the temper-
ature increase of the sample. The absorption spectrum to the S2
state of the ground state p-xylene starts around 225 nm and has a
peak at 210 nm. The absorbed energy is converted to vibrationally
excited states and is collisionally averaged in the sample down to
a red-shifted absorption.
The hump around 280–300 nm can be assigned to hot p-xylylene
at the t = 0 ns spectrum in Fig. 1a. The xylyl (p-methylbenzyl) radi-
cal has a peak at 265 nm [10], and the spectrum of p-xylylene with
a peak at 275 nm was previously obtained from the photolysis of p-
cyclophane [27]; therefore, the transient spectrum at 800 ns can be
assigned to p-xylylene. The xylyl radical by a one-photon process
would not be included in the transient spectra. Xylyl radical for-
∗
∗
mation due to the C–H bond dissociation from S by a one-photon
0
process has been observed under collision-free conditions with the
5
−1
formation rate constant of 2.7 × 10 s at 193-nm excitation [10].
This slow process can be completely quenched by addition of nitro-
∗∗
gen as a foreign gas. The survival of multiphoton processes via S0
after quenching the slow process has been thoroughly discussed
for the case of toluene [9] and has also been applied to the case
of benzene photolysis [6]. The xylyl radical by a two-photon pro-
cess was not clearly observed, but p-xylylene was detected in the
transient spectra. The activation energy of xylyl radical formation
is 381 kJ/mol, but the xylyl radical decomposes to p-xylylene with
an activation energy of 295 kJ/mol [36]. This low activation energy
was probably the reason why p-xylylene was easily detected. As a
result, the dissociation rates of the two hydrogen atoms from the
Since DB was photochemically generated in the liquid phase
[30], some substituted benzenes were successfully converted to
the corresponding DBs [31–33]. Palmer et al. have predicted that
the isomerization occurs at a conical intersection from a high-
energy region of an S /S0 crossing surface [29]. Just after crossing,
1

2
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N. Mitsubayashi et al. / Journal of Photochemistry and Photobiology A: Chemistry 219 (2011) 273–277
∗∗
Fig. 3. The correlation between laser fluence and transient absorption at 0 ns (᭹) for 0.5 kPa in the presence of 27 kPa of nitrogen observed at 235 nm (expected to be S0
)
and at 275 nm at 800 ns; the generated products are (a) mesitylene, (b) hexafluorobenzene, (c) durene, and (d) hexamethylbenzene. The products at 800 ns (ꢀ) appear to
have been formed by two-photon processes for mesitylene and hexafluorobenzene, and by one-photon processes for durene and hexamethylbenzene. The dotted lines (. . .)
have a slope of 1.0 for all four cases. The dashed lines (- - - -) have a slope of 2.0 in panels (a) and (b), and 1.0 in (c) and (d).
there is an excess of energy; therefore, the hot DB requires
rapid cooling to reach a state with a low internal energy, and
will otherwise yield a wider distribution of products, including
some that return to the S0 state. Efficient isomerization has been
observed for substituted benzenes. The following two observa-
tions are notable with regard to the isomerization. The low-energy
difference between HMB and HMDB has been estimated to be
234–251 kJ/mol, versus a difference of 331 kJ/mol between ben-
zene and Dewar benzene [37]. Another point is the effective cooling
provided by the added gases. The larger the size of the molecule,
2
Fig. 4. (a) Transient spectra at t = 0 (᭹) and t = 400 ns (ꢀ) of hexamethylbenzene in the presence of 27 kPa of nitrogen, with a laser fluence of 10 mJ/cm . (b) A typical time
profile at 235 nm for 0.5 kPa of hexamethylbenzene in the absence (upper trace) and presence (lower trace with a spike around t = 0) of 27 kPa of nitrogen. (c) Absorption
spectra of hexamethylbenzene (gray line) and hexamethyl Dewar benzene (black line). Open circles are the points of the spectrum at t = 800 ns from Fig. 4a. Open squares
(ꢀ) are plotted after correction for hexamethylbenzene depletion with a conversion efficiency of 0.55 to hexamethyl Dewar benzene.

N. Mitsubayashi et al. / Journal of Photochemistry and Photobiology A: Chemistry 219 (2011) 273–277
277
the higher the energy of collisional energy transfer. Energy-transfer
4. Conclusions
quantities from self-collisions of p-xylene are 1.7 times those of
benzene [38].
A series of methyl-substituted benzenes and perfluorobenzenes
were irradiated in the gas phase by an ArF excimer laser. One of
the findings was the formation of xylylene or the elimination of
two hydrogen atoms as a result of a two-photon process of p-
xylene, and the other was the generation of Dewar-type benzene
from HMB as an example of a one-photon process. The isomer-
ization would occur via a conical intersection, and the hot species
would be collisionally relaxed to HMDB and in part to HMB. The
two-photon process, as shown in Scheme 1, can be enhanced
under the condition that the intermediate S0 has a reasonably
high molar extinction coefficient at the laser wavelength; oth-
erwise, a one-photon process will be dominant. The one-photon
process for HMB indicates that the plausible intermediate HMDB**
has an extinction coefficient too low to effectively absorb the
second photon of the laser pulse within the excitation pulse
duration.
3.3. Two-photon process versus one-photon process
If the molar extinction coefficient of the intermediate of a
two-photon process is high enough at the irradiation wavelength,
the two-photon process will be dominantly observed, while a
low molar extinction coefficient of the intermediate will lead to
a one-photon process. In the case of benzene, the two-photon
intermediate S is predicted to have a molar extinction coeffi-
cient of 8300 M cm at a laser wavelength of 193 nm, while the
ground state benzene has a coefficient of 4650 M cm [6]. The
molar extinction coefficients at a high temperature can be evalu-
ated using a modified Sulzer–Wieland model. The Sulzer–Wieland
model was originally developed for explaining the absorption spec-
tra of diatomic molecules at a high temperature, and a modified
version of the model has been useful for polyatomic molecules,
including benzene [7]. These predictions indicate that the ben-
zene system sometimes displays a significant increase in the molar
extinction coefficient during excitation. It is quite reasonable, then,
that a two-photon process occurs. On the other hand, a dominant
one-photon process has been found to occur in the cases of C–H
and C–C dissociations of tetramethylethylene (TME) and C7 olefins
∗∗
∗
−
∗
1
0
−1
−
1
−1
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0
−
1
−1
conversion is predicted to be 20,000 M cm ; in fact, two-
photon benzyl radical formation has been observed for the toluene
system [9]. For HFB, an increase in the molar extinction coef-
ficient at 193 nm and a two-photon process are expected from
the results of the laser photolysis of HFB [41]. The molar extinc-
[
[
22] J. Park, R. Bersohn, I. Oref, J. Chem. Phys. 93 (1990) 5700.
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[
[
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−
1
−1
tion coefficient of HMB is 60,000 M cm
at 193 nm for the
[27] Y. Hosoi, T. Yatsuhashi, K. Ohtakeyama, S. Shimizu, Y. Sakata, N. Nakashima, J.
Phys. Chem. A 106 (2002) 2014.
1
∗∗
E1u transition [34], while that of the HMB** (= S ) is predicted
0
[
[
28] T. Yatsuhashi, T. Akiho, N. Nakashima, J. Am. Chem. Soc. 123 (2001) 10137.
29] I.J. Palmer, I.N. Ragazos, F. Bernardi, M. Olivucci, M.A. Robb, J. Am. Chem. Soc.
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to be considerably lower on the basis of the Sulzer–Wieland
model. If the hot species is hot HMDB (HMDB**), according
to Scheme 2, the original V ← N transition should be lower in
the hot state at 193 nm. The absorption peak of HMDB is not
clear, but would be shorter than 201 nm, and the molar extinc-
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[
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[
−1
−1
tion coefficient is only 2800 M cm at room temperature at
this wavelength [42]. The absorption spectrum will be broad-
ened and the molar extinction coefficient of HMDB** will be
well lowered. Neither S nor HMDB** would have molar extinc-
tion coefficients substantially lower than that of 60,000 M cm
for HMB. The one-photon process of HMB at 193 nm is accept-
able. It is not easy to predict whether the one- or two-photon
process is dominant for p-xylene; the results showed the two-
photon process, and the one-photon process is a likely mechanism
for mesitylene; however, the two-photon process was seen
at a particular wavelength of 235 nm in the laser intensity
dependence.
[33] D.M. Lemal, J.V. Staros, V. Austel, J. Am. Chem. Soc. 91 (1969) 3373;
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9
∗
∗
[35] W. Radloff, T. Freudenberg, H.H. Ritze, V. Stert, F. Noack, I.V. Hertel, Chem. Phys.
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0
−
1
−1
[
[
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[
[
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17.
42] http://webbook.nist.gov/UV/Visible spectrum Dewar benzene, hexamegthyl-.
1
[