J. Phys. Chem. 1985,89, 1631-1636 National Laboratories, Livermore) for extremely helpful discussion and cooperation to solve the "H204"problem. This research has been supported by the National Science Foundation Grant CHE-82-17121. Registry NO. HZCO, 50-00-0; H2C202, 107-22-2; H,C"O, 3228-24-8; H2I3CO, 3228-27-1; D2C0, 1664-98-8; HDCO, 1664-99-9; 02,7782-
1631
44-7; 1802, 32767-18-3; H20, 7732-18-5; CO, 630-08-0; C"0, 4906-87-0; "CO, 1641-69-6; Hz"0, 14314-42-2; D 2 0 , 7789-20-0; D2180, 1467467-0; HDO, 14940-63-7; OD, 13587-54-7; c o 2 , 124-38-9; Co"0, 18983-82-9; 13C02,11 11-72-4; H202, 7722-84-1; H2180,, 29736-88-7; D202, 6909-54-2; D,"02, 95407-91-3; HOOD, 34322-1 1-7; H02, 3170-83-0; H1'02, 37006-04-5; DO,, 13587-55-8; DI'O,, 37006-06-7; (H02)2, 29683-94-1; H02.D02, 95407-92-4; (H1802)2,95407-93-5.
Picosecond Ultraviolet Multiphoton Laser Photolysis and Transient Absorption Spectroscopy of Liquid Benzenes Hiroshi Miyasaka, Hiroshi Masuhara; and Noboru Mataga* Department of Chemistry, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan (Received: October 5, 1984)
Ultraviolet multiphoton excitation with a picosecond laser pulse at the wavelength of no appreciable ground-state absorption has been used to excite benzene and alkylbenzeneshomogeneously in the neat liquid state and to investigate their photophysical and photochemical primary processes by picosecond time-resolved transient absorption spectral measurements. The excimer formation processes in neat liquid benzenes excited by two-photon absorption of the 355-nm picosecond pulse from a Nd3+:YAG laser have been confirmed to proceed via ionization and recombination processes, and the lifetime of the ionized (ion pair) state has been determined to be 10-20 ps in neat benzene.
Introduction Ultrashort laser pulses in the picosecond regime are providing a powerful means for the study of the most fundamental processes in chemistry such as energy relaxation, molecular structural changes, electron transfer as well as proton transfer, and other chemical changes, since they all take place on a very short time scale. In the case of transient absorption spectral measurement by means of the picosecond laser photolysis method, a rather high peak power of the exciting picosecond laser pulse is required to produce the transient species sufficient for spectral detection. This high peak power of the picosecond pulse, however, can induce rather easily nonlinear phenomena such as multiphoton excitation, ionization, and decomposition of molecules, which complicates sometimes the results of picosecond laser photolysis and transient spectral measurements. On the other hand, the method of picosecond multiphoton excitation and transient spectral studies is very suitable for the investigation of the excited state of neat liquids. If we excite the neat liquid with a laser pulse at the wavelength where the ground-state absorption is appreciable, high-density excitation in a very limited part of the sample leads to a rapid deactivation of excited states due to the efficient interactions among them. By means of multiphoton excitation with a laser pulse at the wavelength where the liquid shows no appreciable ground-state absorption, we can achieve a much more homogeneous excitation and can investigate photochemical and photophysical processes in neat liquids by picosecond time resolution. Furthermore, it is rather easy, by means of ultraviolet picosecond multiphoton absorption, to excite molecules in liquids to the energy levels higher than 7-10 eV. Since the ionization energy in the liquid phase is considerably lower than that in the gas phase, the ionization following the formation of the highly excited state by multiphoton absorption is expected to take place easily not only in simple aromatic liquids and some simple molecules containing heteroatoms but also even in liquid aliphatic hydrocarbons. We have already demonstrated that the solvation process of electrons can be investigated with picosecond transient absorption spectral measurements, by ionizing water and alcohols with pithesent address: Department of Polymer Science and Engineering, Kyoto Institute of Technology, Matsugasaki, Kyoto 606, Japan.
cosecond 266-nm multiphoton excitation.' We have also demonstrated the possibility of the picosecond multiphoton ionization of liquid aliphatic hydrocarbons by the same methods2 The ionization followed by neutralization as well as solvated electron formation in neat liquid has been investigated mainly in the field of radiation chemistry. Comparison of the results obtained by the picosecond pulse radiolysis with those of picosecond ultraviolet multiphoton laser photolysis will be fruitful and lead to a more satisfactory understanding of the electronic, ionic, and excitation migration processes in liquids. Along this line, we are studying various organic liquids and soltuions by means of picosecond ultraviolet multiphoton laser photolysis. In the present paper, we report the results of picosecond multiphoton laser photolysis of liquid benzene and its derivatives. Especially, the mechanism of the excimer formation in neat benzene liquids from the excited state with 2hu energy of the 355-nm photon and the possibility of the participation of the ionic intermediates in this excimer formation process will be discussed, by comparing the present results with those obtained by means of picosecond pulse radiolysis. A preliminary result of the excimer formation in neat benzenes observed by means of picosecond 355-nm multiphoton laser photolysis was given in a previous rep~rt.~
Experimental Section A microcomputer-controlled picosecond laser photolysis system with a repetitive mode-locked Nd3+:YAG laser was used to measure time-resolved transient absorption spectra. The details of this system have been given elsewhere$v5 Samples were excited by the third harmonic (355 nm) with 22-ps fwhm or the fourth harmonic (266 nm) with 20-ps fwhm. The relative output power of the excitation pulse was measured by a handmade power meter calibrated with Molectron J3-OSDW joule meter. The excitation (1) Miyasaka, H.; Masuhara, H.; Mataga, N. Chem. Phys. Lett. 1983,98, 277. (2) Miyasaka, H.; Masuhara, H.; Mataga, N. "Abstract of the Japanese Symposium on Radiation Chemistry", 1983 Osaka, Japan; Japanese Society of Radiation Chemistry: p 36; to be submitted for publication. (3) Masuhara, H.; Miyasaka, H.; Ikeda, N.; Mataga, N. Chem. Phys. Lett.
1981, 828 59. (4) Masuhara, H.; Ikeda, N.; Miyasaka, H.; Mataga, N. J. Specrrosc.Soc. Jpn. 1982, 31, 19. (5) Miyasaka, H.; Masuhara, H.; Mataga, N. Laser Chem. 1983, 1, 357.
0022-3654/85/2089-163 1%01.50/0 0 1985 American Chemical Society
1632 The Journal of Physical Chemistry, Vola89, No. 9, 1985
Y
I
,
,
Miyasaka et al.
t
,
-0,
I
0
400
500
I
.
600 700 Wavelength I n m
Figure 1. Transient absorption spectra of neat liquids observed at 100 ps after excitation with the 355-nm picosecond laser pulse: (a) benzene, (b) toluene, (c) p-xylene, (d) m-xylene, (e) mesitylene, (f) cumene, (g)
5
pulse was focused onto a spot of 2-3-mm diameter by using a quartz lens with a focal length of 10 cm. The center of this spot was monitored by a picosecond continuum, which was generated by focusing the fundamental pulse (1064 nm) into a quartz cell of 10-cm path length containing DzO(Uvasol 99.5%). A doublebeam optical arrangement was used, and it is possible to cover the spectral range of 380 nm in one shot by using a multichannel photodiode array (512 channel). The origin for the delay time in the experimental measurement was determined as follows. The peak absorbance at 468 nm of the S, SIabsorption spectra of pyrene in cyclohexane was plotted against the optical delay. The time when the absorbance was half of the plateau value was defined as the origin of the time axis. Benzene (Wako, G R grade) was purified by fractional distillation. Toluene, m-xylene, p-xylene, mesitylene, and cumene (Wako, G R grade) were purified by distillation under reduced pressure. tert-Butylbenzene (Wako, chemically pure grade) was passed through a column of silica gel (Wako, Wakogel C-200) and distilled under reduced pressure. All the samples were deaerated by irrigating with an Nzgas stream and measured in a quartz cell of 1-cm path length. Measurements were performed at 22 f 1 O C .
-
Results and Discussion Transient Absorption Spectra of Benzene and Its Derivatives in the Neat Liquid State. Figure 1 shows the transient absorption spectra of benzene and its derivatives in the neat liquid state excited by the 355-nm picosecond laser pulse, although they have no ground-state absorption at this wavelength. The peak positions are 505 nm for benzene, 555 nm for toluene, 615 nm for p-xylene, 590 nm for m-xylene, 610 nm for the mesitylene, 545 nm for cumene, and 540 nm for tert-butylbenzene. These spectra obtained at 100 ps after excitation can be assigned to excimers of benzenes on the basis of the results previously reported.- This assignment is supported also by the fact that the usual excitation by their S, So transitions with the fourth harmonic of a Nd3+:YAG laser also gives the same transient absorption spectra. Excitation Intensity Effect upon Transient Absorbance. We have examined the excitation intensity effect upon the transient
-
~
( 6 ) Cooper, R.; Thomas, J. K. J . Chem. Phys. 1968, 48, 5097.
10 1520
E xc i tat ion In t e nsi t y ( x 1Ol6PHOTON /cm2)
tert-butylbenzene.
-
Figure 2. Excitation intensity dependence of transient absorbance at 100 ps after excitation with the 355-nm picosecond laser pulse: (a) S, S, absorbance of pyrene in cyclohexane, (b) excimer absorbance of neat
liquid benzene.
-
absorbance as shown in Figure 2. Figure 2a shows the result for the S, SItransition of pyene in cyclohexane. Although a linear relation between the excitation intensity and transient absoraance may be expected for the pyrene system, a saturation tendency was observed. It is worth noting that pyrene in the SI state has an extinction coefficient of 21 100 M-l-cm-l at 355 nm, compared to the Sostate extinction coefficient of 270 M-l-cm-' at the same wavelength. Therefore, reabsorption of exciting light by the SI state pyrene which is called "inner filter effect", seems to be effective. In order to analyze this effect, a computer simulation method was e m p l ~ y e d .The ~ solid line in Figure 2a represents the result of the simulation, based on pulse widths of excitating and monitoring light, extinction coefficients of ground and excited states, and incident laser power. All of these parameter values were experimentally obtained. The curve of transient absorbance of pyrene in cyclohexane simulated by assuming a one-photon process has reproduced the experimental results very well. Figure 2b shows a relation between the absorbance of benzene excimer at 100-ps delay time and the exciting laser pulse intensity on a logarithmic scale. A linear relation between the transient absorbance and the excitation intensity with a slope of 1.93 has been observed up to the excitation intensity of ca. 5 X l o i 6 photons.cm-2, which indicates that benzene excimer is produced through a twephoton process. A deviation from this linear relation observed when the excitation intensity exceeds 5 X 10l6 photons.cm-z suggests that the transient species produced through a two-photon process can absorb one more photon of the same frequency. For the estimation of a two-photon absorption cross section of benzene at 355 nm, extinction coefficients of the monomer SI state and excimer, respectively, and the equilibrium constant between these two species are necessary. According to Thomas and coworkers,1° the extinction coefficient of benzene excimer at the band peak determined by means of laser photolysis with the fourth harmonic of Q-switched glass laser for excitation is 72000 M-krn-'. In the case of picosecond pulse radiolysis studies by Copper and Thomas: the extinction coefficient of benzene excimer
(7) Bonneau, R.; Joussot-Dubien, J.; Bensasson, R. Chem. Phys. Lett. 1969, 3, 353.
(8) Thomas, J. K.; Mani, I. J . Chem. Phys. 1969, 51, 1834. (9) Schomburg, H. Ph.D. Thesis, Gbttingen, 1975.
~
~~
~
(IO) Bensasson, R.; Richards, J. T.; Thomas, J. K. Chem. Phys. Lett. 1971, 9, 13
The Journal of Physical Chemistry, Vol. 89, No. 9, 1985 1633
UV Multiphoton Laser Photolysis of Liquid Benzenes
20ps
[wI 1-k 400
500 600 700
400 500 600 700
20ps
]-,I 400 500 600 700
Wavelength I nm
Figure 3. Time-resolved transient absorption spectra of neat liquids excited with the 355-nm picosecond laser pulse: (a) benzene, (b) toluene, (c) tert-butylbenzene.
was reported to be 76000 M-l-cm-'. The monomer-excimer equilibrium constant used in these studies was 0.121 M-'." On the other hand, Nakashima et al.I2 have reported recently the value of 28000 M-'.cm-' for this extinction coefficient, using the equilibrium constant of 0.308 M-' proposed recently by Cundall and Robinson.I3 The difference between the values of Thomas et al. and of Nakashima et al. is mainly due to the difference between the equilibrium constants employed. If the equilibrium constant of 0.308 M-l is used for the analysis of the results obtained by Thomas et al., the extinction coefficient of benzene excimer is obtained to be 27700 M-'.cm-' which is in good agreement with the value by Nakashima et al. The extinction coefficient of benzene monomer in the SI state at 500 nm is about 400 M-1.cm-1,'2 and its maximum value in the wavelength region longer than 400 nm is smaller than 1000 M-'.cm-'. Therefore, the contribution from the S1-state monomer to the transient absorption spectrum of neat benzene in the visible region can be neglected. By using the above values and the value of 0.3414 for the efficiency of internal conversion from the excited state with twice the energy of a of 355-nm photon, we have obtained (7.5 f 3.5) X cm4~s/(photon~molecule) for the two-photon absorption cross section of benzene. It is difficult, however, to get precise absorption cross sections for other liquids because of the lack of values an extinction coefficient of the excited state. Nevertheless, the extinction coefficients of those alkylbenzene excimers may not be so much different from that of the benzene excimer. In that case, values of the 355-nm two-photon absorption cross sections of liquid alkylbenzenes may be the same as that of benzene in the order of magnitude. On the other hand, two-photon absorption cross sections of various molecules have been reported cm4~s/(photon~molecule)15 which is close to be in the order of to the above estimated value. This fact also indicates that our estimation described above is reasonable. Formation Times of Excimer in Neat Benzenes. Figure 3 shows time-resolved transient absorption spectra of benzene (a), toluene (b), and tert-butylbenzene (c). The taillike spectrum around 0 ps is gradually replaced by a broad band of excimer with an increase of delay time. The spectrum at 0 ps may contain some contributions from the precursor species of the excimer. However, in view of the results of our detailed examination of the nature of the picosecond continuum as a light source for transient spectral measurement^,^*^^ the taillike band shape at 0 p seems to be partly ( 1 1 ) Birks, J . B.; Braga, C. L.; Lumb, M. D. Proc. R . SOC.London, Ser. A 1965, 283, 83. (12) Nakashima, N.; Sumitani, M.; Ohmine, I.; Yoshihara, K. J . Chem. Phys. 1980, 72, 2226. (13) Cundall, R. B.; Robinson, D. A. J . Chem. SOC.,Faraday Trans. 2 1972,68, 1133. (14) Schwarz, F. P.; Mautner, M. Chem. Phys. Left. 1982, 85, 239. (1 5) Birks, J. B. 'Photophysics of Aromatic Molecules";Wiley-Interscience: New York, 1970; p 79. (16) Masuhara, H.; Miyasaka, H.; Karen, A.; Uemiya, T.; Mataga, N.; Koishi, M.; Takeshima, A.; Tsuchiya, Y . Opt. Commun. 1983, 44, 426.
T i me Ips
Figure 4. Rise curves of transient absorbance observed by exciting neat liquids with the 355-nm picosecond laser pulse: (a) ( 1 ) hydrated electron in water produced by multiphoton excitation with 266-nm picosecond laser pulse and observed at 720 nm, (2) benzene excimer observed at 505 nm, (3) toluene excimer observed at 555 nm, (4) tert-butylbenzene excimer observed at 545 nm; (b) benzene, observed at 505).( and 420 nm (0); (c) toluene, observed at 555 ( 0 )and 420 nm (0). TABLE I: Rise Times of Peak Absorbances of Excimer in Neat Liquid Observed by Exciting with the 355-nm Picosecond Laser Pulse liquid benzene toluene mesitylene in-xylene p-xylene cumene tert-butylbenzene
rise time/ps 57 f 2 65 f 5 57 f 5 67 f 5 68 f 5 63 f 5 75 f 7
hydrated electron in water
27 f 3"
"Rise time of the absorbance of hydrated electron produced by 266-nm picosecond multiphoton excitation.
ascribed to the wavelength-dependent distribution of the arrival time of the picosecond continuum at the sample position. Namely, since the shorter wavelength part of the picosecond continuum is more delayed, the spectrum at 0 ps is apparently stronger in the blue part than in the red part. In Figure 4a, the rise curves of excimers in neat benzenes observed at the band maximum are indicated together with that of the hydrated electron. The latter seems to represent the time response of our apparatus since the formation time of the hydrated electron is about 0.3 ps.I7 Evidently, the excimer absorbances of neat benzenes show much slower rise. In Figure 4b,c, the time dependences of the absorbance around 420 nm are also indicated for benzene and toluene. One can recognize clearly the rise and decay. Similar results have also been obtained for other liquids. Some discussions will be given in the latter part of this paper on this behavior of the absorbance at 420 nm. The rise times of peak absorbances from 10% to 90% of their plateau values were obtained as given in Table I. These rise times are considered to be given as a result of convolution of excitation pulse, monitoring pulse, and true formation time ( t o ) . In order to estimate to, we have made computer simulations of rise curves, assuming the simple reaction scheme of eq 1
M*
+M
lib
(M.M)*
(17) Wiesenfeld, J.; Ippen, E. Chem. Phys. Left. 1980, 73, 47.
(1)
1634 The Journal of Physical Chemistry, Vol, 89, No. 9, 1985
Miyasaka et al.
TABLE II: Rise Times of Absorbance Obtained by Convolution of Excitation Pulse, Monitoring Pulse, and True Formation Time ( t o )of Excimer tn/m
rise time/vs
tnlvs
rise time / ps
0
30 (27)" 33
10 20 30
38 55 73
5
"The value obtained by assuming the exciting pulse with fwhm = 20 ps (the pulse width of the fourth harmonic). Other values were obtained by assuming the exciting pulse with fwhm = 22 ps (the pulse width of the third harmonic). 50
0
100
150
Time I p s Figure 6. Rise curves of transient absorbance of benzene observed by exciting the neat liquid with the 266-nm picosecond pulse: (0),observed at 505 nm; (O),observed at 420 nm; (-), simulated curve. TABLE 111: Ionization Potentials (IP) of Benzene and Methyl-Substituted Benzenes in the Liquid State
//
compound benzene
7.1"
6.8a,b
,/'
7.0'
d,,'
A?, 0
IP/eV
I
50
100
150
Time/ps
Figure 5. Simulated and observed rise curves of the hydrated electron and benzene excimer: (---), simulated curve; (a),observed value of hydrated electron; ( O ) , observed value of benzene excimer.
where M* and M are the monomers SI and So and (M.M)* is the excimer. By using the observed values of excitation and monitoring pulse width as well as the value of extinction coefficients of monomer SI and excimer states as given before, we have estimated approximate to values as shown in Table 11. Comparison of the results in Table I1 with those in Table I indicates that the excimer formation times in these neat liquids are about 20-30 ps and that the rise time of the hydrated electron represents the time response of the apparatus. The rise curve obtained by this simulation ( t o = 20 ps) is compared with the observed one for the benzene excimer in Figure 5. It should be noted here that the observed rise curve of the excimer shows an initial slower rise followed by a steeper one compared to the simulated curve. This result appears to indicate the existence of some intermediate (other than monomer SI)produced from the two-photon excited state. The rise and decay of the absorbance around 420 nm as shown in Figure 4b,c also support this suggestion, because the extinction coefficient of the monomer SIstate is much smaller than that of excimer in this wavelength region so that simple rise kinetics should be observed if the two-photon excited state undergoes internal conversion simply to the monomer SI state without going to some longer lived intermediate state. In the following, we examine this problem in detail. Effect of Excitation Energy upon the Excimer Formation Process. The time profiles of the benzene excimer absorbance obtained by exciting the neat benzene with the 266-nm picosecond laser pulse (one-photon process) are indicated in Figure 6. In this measurement, the exciting pulse intensity was kept sufficiently low, so that the effect of the SI-S, annihilation upon the observed rise curve can be neglected. In the case of high exciting pulse intensity, we have observed the decay of excimer absorbance in the hundreds of picoseconds region, which can be ascribed to the S1-Sl annihilation because of its second-order decay kinetics.ls No such decay of the absorbance can be recognized in Figure 6, and the rise time of the excimer absorbance from 10% to 90% of the plateau value has been obtained to be 46 ps, which is a little shorter than the value for benzene in Table I. (18) Miyasaka, H.; Nakamura, W.; Masuhara, H.; Mataga, N. "Abstract of the Symposium on Photochemistry", Oct 1983; Chemical Society of Japan: Tsukuba, Japan; p 99.
compound toluene p-xylene mesitylene
IP/eV 6.gb 6.8b 6.8b
>6.3dve
>6.5"/
OReference 14. bReference 21. CReference23. dReference 25. e Benzene-benzene charge transfer state. fGeminate ion-pair state. We have simulated the rise curve by the same method as used to obtain the results in Table 11, except that the laser pulse width for the one-photon excitation is 20-ps fwhm. As shown in Figure 6, the observed result can be reproduced satisfactorily by this simulation (with to = 12 ps). The most conspicuous difference between the results in Figures 6 and 4 is the temporal characteristics of the absorbance at 420 nm. Contrary to the rapid rise and subsequent decay to the plateau region in the case of Figure 4b,c, a simple rise to the plateau region has been observed in Figure 6. These results suggest strongly the existence of an intermediate state other than the monomer SI state in the excimer formation caused by 355-nm two-photon excitation. Relaxation Pathways From the 355-nm Two-Photon Excited State. The 355-nm two-photon absorption excited benzenes up to the energy level of 7.0 eV above the ground state. In the gas phase, benzene is nonradiatively deactivated when excited to energy levels higher than 5.2 eV (the so-called third channel),I9 the mechanism of which has been elucidated recently.20 The quenching is due to the direct internal conversion to the vibrationally excited So state, producing "hot benzene".20 In the liquid phase, on the other hand, although similar fluorescence quenching with increase of excitation energy has been observed up to 6.5 eV, further increase of the excitation energy has been confirmed to bring recovery of the fluorescence yield.2'*22 This type of excitation energy dependence of fluorescence yield has been observed also in the case of liquid toluene, p-xylene, and mesitylene.21-22The recovery of the fluorescence yield has been explained as due to the ionization of the aromatic liquid and subsequent recombination to produce the S, ~ t a t e . ' ~ . ~ ~ On the basis of the fluorescence excitation spectra and the quenching efficiency of the higher excited state by electron scavengers, ionization potentials of liquid benzenes were determined.14*22On the other hand, measurements of photocurrent and two-photon fluorescence of liquid benzene have been made by using an N, laser pumped dye laser and Q-switched YAG laser as exciting light source^.*^-^^ From these measurements, the (19) (a) Noyes, Jr. W. A.; Harter, D. A. J . Chem. Phys. 1967, 46, 674. (b) Lee, S . A,; Shite, J. H.; Noyes, Jr. W. A. [bid. 1976, 65, 2805. (20) Nakashima, N.; Yoshihara, K. J . Chem. Phys. 1982, 77, 6040. (21) Braun, C. L.; Kato, S . ; Lipsky, S . J . Chem. Phys. 1963, 39, 1645. (22) Fuchs, C.; Heisel, F.; Voltz, R. J . Phys. Chem. 1972, 76, 3867. (23) Scott, T. W.; Twarowski, A. J.; Albrecht, A. C. Chem. Phys. Lett. 1979, 66, 1.
The Journal of Physical Chemistry, Vol. 89, No. 9, 1985
UV Multiphoton Laser Photolysis of Liquid Benzenes
1635
TABLE IV: Effects of Addition of Other Liquids upon the Excimer Rise Times of Liquid Benzene and Toluene
added liquid (20 vol %) cyclohexane benzene (neat) chloroform ethanol acetonitrile cyclohexane toluene (neat) acetonitrile
benzene
to1uen e
rise time, PS
45 i 4 57 i 2
62 f 4 75 f 4 89 f 5
62 i 5 65 f 5 75 f 3
dielectric constant of added liquid 2.02 (20 "C) 2.28 (25 "C) 4.81 (20 "C) 24.6 (25 "C) 37.5 (20 " C ) 2.02 (20 "C) 2.38 (25 "C) 37.5 (20 "C)
viscosity of added liquid, CP 0.97 (20 "C) 0.65 (20 "C) 0.51 (30 " C ) 1.08 (25 " C ) 0.32 (30 "C) 0.97 (20 "C) 0.58 (20 "C) 0.32 (30 "C)
1 . p liquid gas
phase phase
')I 5r 6
IP
_.
177nm -52266nm
't2F
Third m eI
=3
...................
3
355nm
1
01
Figure 7. Energy diagrams of benzene in the liquid and gas phase. D represents the energy level of excimer above the monomer So state.
-
ionization potential of liquid benzene as well as the energy of the benzene benzene charge-transfer state has been concluded to ~-~~ these results, be 7.0 and 6.5 eV, r e s p e ~ t i v e l y . ~Summarizing we have indicated the energy levels of benzene in Figure 7, and ionization potentials of liquid benzene and methyl-substituted benzenes are collected in Table 111. The energy level corresponding to the 355-nm two-photon excitation (7.0 eV) is close to or higher than the ionization potentials of liquid benzenes. Therefore, it is plausible that the excimer formation induced by the 355-nm two-photon excitation involves the ionization and recombination processes, which is in accordance with the results in previous sections that we should assume some precursor state other than the SImonomer for the excimer formation by 355-nm two-photon excitation. In order to confirm the above possibility of the participation of ionic transient species to the excimer formation, we have examined the effects of addition of other liquids upon the excimer rise times of benzene. As shown in Figure 8, the rise time of excimer absorbance increases with increase of the polarity of added liquid. Addition of cyclohexane into liquid benzene shortens the excimer rise time, although benzene is diluted. On the other hand, the rise time is lengthened considerably by addition of acetonitrile. Moreover, the absorbance at 420 nm also seems to be affected by addition of these liquids in benzene. When cyclohexane is added, the 420-nm absorbance becomes less pronounced compared with that of the neat benzene liquid. However, when acetonitrile is added, it is much pronounced and its decay time is considerably longer compared with the case of cyclohexane addition. In Table IV, the excimer rise times of liquid benzene and toluene under the addition of other liquids are collected. We can see clearly that the rise time increases with increase of the dielectric constant of the added liquid, but it does not show any systematic relation with the viscosity of the added liquid. These results indicate strongly that the 355-nm two-photon excitation of these liquids (24) Scott, T. W.; Albrecht, A. C. J. Chem. Phys. 1981, 74, 3807. (25) Scott, T. W.; Braun, C. L.; Albrecht, A. C. J . Chem. Phys. 1982, 76, 5195.
......
0 -50
0
50
100 150 Time I p s
200
Figure 8. Effects of addition of other liquids in benzene upon the rise curves of transient absorbance observed (at absorption maximum) by exciting the liquids with the 355-nm picosecond laser pulse: (a) (1) neat benzene, (2) 20 vol % cyclohexane in benzene, (3) 20 vol % acetonitrile in benzene; (b) 20 vol % cyclohexane in benzene, ( 0 )observed at 505 nm, (0)observed at 420 nm; (c) 20 vol % acetonitrile in benzene, ( 0 ) observed at 505 nm, (0)observed at 420 nm.
leads to a charge separation reaction and the produced ion pair plays the role of precursor of excimer formation. The recombination reaction of the ion pair seems to be slowed down by solvation with the added polar liquids. As it is pointed out in the Introduction, the charge separation and recombination reactions in liquids are the main fundamental processes not only in photochemistry but also in radiation chemistry and comparison of the present results with those of radiation chemical studies will be fruitful. According to the picosecond pulse radiolysis studies by Beck and Thomas,26the benzene excimer formation lags behind that of the hydrated electron about 10 ps. This is in agreement with our result of picosecond m@tiphoton laser photolysis. Moreover, the rise curve of benzene excimer obtained by picosecond pulse radiolysis26showed a large initial delay and could not be reproduced completely by such a simple convolution method as used in Figures 5 and 6.26 This is also quite similar to the present result and may be ascribed to the ionization followed by neutralization to give the excimer. Mechanisms of Excimer Formation in Neat Liquids. As discussed in previous sections, the ionization followed by neutralization is playing an important role in the excimer formation in neat benzenes caused by 355-nm two-photon excitation. In this section, we make a more detailed examination of the excimer formation mechanisms in neat liquids. We have determined the excimer formation rate constant of benzene (0.4 M) in cyclohexne to be k, = (2 f 1) X 1O'O M-'-S-','~ directly observing the time-resolved transient absorption spectra by exciting the solution with a 266-nm picosecond pulse. On the other hand, we have obtained the excimer formation time ( t o ) of (26) Beck, G. Thomas, J.
K.J . Phys. Chem. 1972, 76, 3856.
1636 The Journal of Physical Chemistry, Vol. 89, No. 9, 1985
12 ps in neat benzene by transient absorption measurements with excitation at 266 nm (Figure 6). If we formally put to = 1/ k,,[M(So)], we obtain the bimolecular rate constant of excimer formation in neat benzene as k,, = (7.8 f 1.1) X lo9 M-lns-'. Strictly speaking, the true bimolecular rate constant corresponding to the reaction M*(S,) M(So) (M.M)t may be smaller than this because the to represents the relaxation time to the equilibrium state between M*(S,) and (M*M)*. This result indicates that the bimolecular process of excimer formation in neat benzene is slower compared with that in cyclohexane solution, which might be ascribed to the more extensive rearrangements of surrounding molecules in neat benzene in the course of the excimer formation process. Nevertheless, the to value of neat benzene in the case of 266-nm one-photon excitation is considerably smaller than that in the case of 355-nm two-photon excitation, where the ionized species plays an important role in excimer formation. Moreover, the excimer rise curve in the former case can be simulated by a simple exponential function, while it is more complex in the latter case, where several processes might be contributing to the excimer formation. For example, (a) the neutralization of the ionized state will produce the monomer S1 state which subsequently forms excimer by the usual process or (b) a dimer cation may be formed in the ion-pair state and it will give excimer directly under neutralization. Although we cannot assess quantitatively the contributions of these processes, both of them seem quite plausible. Summarizing the above arguments, we suggest the scheme of eq 2 for the mechanism of excimer formation in neat benzenes
-
+
M
'2 M*
-
... M-
(M+
or (M.M)+
Illl
L ) [
M* (st)
...M - )
U($,
(2)
(M.M)*
in the case of 355-nm two-photon excitation, where (MeM)' represents the dimer cation. It should be noted here that our picosecond laser studies predict that the lifetime of the ion-pair
Miyasaka et al. state in neat benzene is 10-15 ps if path a is the dominant process since it takes 12 and 20-30 ps, respectively, for the excimer formation in the S1 state and for the whole process including ion-pair neutralization and excimer formation, and it is 20-30 ps if path b is the main one. On the other hand, the lifetime of the geminate ion pair in neat benzene has been estimated to be ca. 30 ps from the analysis of fluorescence quenching by an electron scavenger, exciting liquid benzene to the energy level higher than the ionization p0tentia1.l~ This value of the ion-pair lifetime agrees with the above values of 10-30 ps obtained in the present investigation in its order of magnitude.
Concluding Remarks In this paper, we have demonstrated the picosecond time-resolved absorption spectral studies of dynamic behaviors of neat liquid benzenes highly excited to the energy levels near to ionization, which have been made possible by means of the picosecond ultraviolet multiphoton laser photolysis and spectroscopy method. The ionization from the highly excited state and recombination prior to the excimer formation have been concluded from the quantitative time-resolved absorption spectral studies and the lifetime of the ionized (ion pair) state has been determined, which are in agreement with the results of previous picosecond radiolysis studies as well as fluorescence quenching investigations with ultraviolet excitation to the ionized state. Thus, our results have demonstrated the potentiality of the picosecond ultraviolet multiphoton laser photolysis method to elucidate the dynamic behaviors of the highly excited liquid state and especially to investigate the problems in the boundary region between photochemistry and radiation chemistry. Acknowledgment. This work was partly supported by Grant-in-Aid (No. 58430003, No. 58045097) from the Japanese Ministry of Education, Science and Culture to N.M. Registry No. Benzene, 7 1-43-2; toluene, 108-88-3; mesitylene, 10867-8; m-xylene, 108-38-3;p-xylene, 106-42-3; cumene, 98-82-8; tert-butylbenzene, 98-06-6; cyclohexane, 1 10-82-7; chloroform, 67-66-3; ethanol, 64-1 7-5; acetonitrile, 75-05-8.
