J . Phys. Chem. 1990, 94, 3648-3651
3648
the loss of acetylene through reaction with CI atoms should be included in chemical kinetic models of the upper atmosphere. In the case of ethene the reaction with O H radicals is considerably more rapid, with a rate constant under the above conditions of cm3 molecule-' s-l. Consequently, loss of ethene via -6 X reaction with C1 atoms is not important in the upper atmosphere. In polluted atmospheres where C12 emissions are large, the role played by CI atoms in the destruction of hydrocarbons may be significantly more important. In a study by H o v of ~ ~the plume generated from a highly industrialized region in Telemark, Norway, it was assessed that destruction of ethene by C1 atoms was -50% more important than destruction by O H radicals. In the assessment. a value of 6.6 X lo-" cm3 molecule-' s-I was used
for k,.24 Our data show that a value closer to k , = 1 X 1O-Io cm3 molecule-' s-' would have been more appropriate, and thus the impact of CI is seen to be even larger. The C1 atom reactions in such industrial plumes are of particular importance because the ultimate products of the reactions are highly It is clear that the reactions of C1 atoms with ethene and acetylene are important in the earth's atmosphere. Further work on these reactions is required; the temperature dependence of reaction 1 has not been determined, and the fate of the Cl-organic adducts is uncertain. Registry No. CI, 22537-15-1; C2H,, 74-85-1; C2H2.74-86-2; C2H,, 74-84-0; chloroethene, 75-01 -4. (25) Laffond, M.; Foster, P.; Massot, R.; Perraud, R. Atmos. Emiron.
(24) Hov, 0. Atmos. Emiron. 1985, 19, 471.
1985, 19, 1277.
Photodtssociation of the Benzene Dimer Cation in the Gas Phase J . T. Snodgrass, R. C. Dunbar,+ and M. T. Bowers* Department of Chemistry, University of California, Santa Barbara, California 931 06 (Received: September 7 , 1989)
The photodissociation of the benzene dimer cation, (C6H6)2+, has been investigated by crossing a mass-selected ion beam with a linearly polarized laser beam and measuring the kinetic energy distribution of the charged photodissociation products with an electrostatic energy analyzer. The C6H6' + C6H6 product channel was the Only channel observed. A photodissociation onset of 2.56 0.03 eV is observed. The average relative translational energy of the C6H6'/C&6 photofragmentswas determined to be 0.070 f 0.007 eV at a photon energy of 2.71 eV. The small fraction of the available energy (EAv 2.0 eV) partitioned into the kinetic energy of the photofragments indicates that the upper state in the photoabsorption process is a bound state. Comparison of the experimental results with predictions of statistical phase space theory indicates that after the bound upper state is accessed via photon absorption, dissociation occurs by internal conversion to highly vibrationally excited levels of the ground state, followed by statistical vibrational predissociation. The observation of a photodissociation threshold allows an estimate of the binding energy of the (C,H&+ bound excited state accessed by the photon to be made. We obtain @(C6H6+ - C6H6)*= 0.50 hO.17 eV. The relevance of the results to previous condensed-phase investigations of (C6H6)2+ is discussed.
*
I. Introduction Gas-phase neutral clusters of benzene have been extensively studied both e~perimentally'-~ and theoretically" with a view toward a better understanding of the fundamental intermolecular interactions between aromatic molecules. Experimental work has included electric resonance,' infrared photodissociation,2 fluorescence e~citation,~ and s i n g l e - p h ~ t o nand ~ ~multiphotonbI0 ~ ionization studies. With the exception of some of the more recent work,I0 most of these studies have emphasized smaller clusters, especially the dimer. Charged benzene clusters have received much less attention. Some information has been provided by neutral-cluster photoionization studies.6-10 In particular, a precise measurement of the ionization potential of the neutral dimer,4 (C6H6)2, led to determination of dissociation energies for both the neutral dimer (Do[(C&,),] = 0.10 f 0.02 eV) and the dimer cation (Do[(C,H&+]. = 0.663 f 0.039 ev). The dissociation energy of the dimer cation has also been determined by thermochemical and calculated by theory.I5 Other work on gas-phase (C6H6)2+ has included kinetic measurements of formation and reaction rate constant^.'^^^^ The only spectroscopic data available on (C6H6)2+ has been Studies of obtained in condensed-phase y-irradiated benzene solutions have found a visible absorption band centered near 900 nm and another near 460 nm7.l8-l9Analysis of these dataZosuggests a symmetric, D6,,"sandwich structure" for (C6H&+. The broad absorption band near 900 nm has been 'Permanent address: Department of Chemistry, Case Western Reserve University, Cleveland, O H 44106.
0022-3654/90/2094-3648$02.50/0
assigned to the "intervalence transition" between the bound (C6H6)2+ ground state and its sister repulsive excited ~ t a t e . ~ ' - ~ ~ ( I ) Janda, K. C.; Hemminger, J. C.; Winn, J. S.; Novick, S. E.; Haris, S. J.; Klemperer, W. J . Chem. Phys. 1975, 63, 1419. (2) Nishiyama, 1.; Hanazaki, I. Chem. Phys. Lett. 1985, I 17, 99. (3) Langridge-Smith, P. R. R.; Brumlaugh, D. V.; Haynam, L. A,; Levy, D. H . J . Phys. Chem. 1981, 85, 3742. (4) Grover, J . R.; Walters, E. A.; Hui, E. T. J . Phys. Chem. 1987, 91, 3233. (5) Riihl, E.; Bisling, P. G. F.; Brutschy, B.; Baumgartel, H. Chem. Phys. Lett. 1986, 126, 232. (6) Hopkins, J. B.; Powers, D. E.; Smalley, R. E. J . Phys. Chem. 1981.85, 3799. (7) Bornsen, K. 0.;Selzle, H. L.; Schlag, E. W. J . Chem. Phys. 1986,85, 1726. ( 8 ) Law, K. S.; Schauer, M.; Bernstein, E. R. J . Chem. Phys. 1984,81, 487 I , (9) Fung, K. H.; Selzie, H . L.; Schlag, E. W. J . Phys. Chem. 1983, 87, 5113. (IO) Hahn, M. Y.; Schriver, K. E.; Whetten, R. L. J . Chem. Phys. 1988, 88, 4242. ( 1 1) See references to theoretical papers in ref 4. (12) Mautner, M.; Hamlet, P.; Hunter, E. P.; Field, F. H. J . Am. Chem. SOC.1978, 100, 5466. (13) Field, F. H.; Hamlet, P.; Libby, W. F. J . Am. Chem. SOC.1969, 91, 2839. (14) Jones, E. G.; Bhattacharya, A. K.; Tiernan, T. 0. Int. J . Mass Spectrom. Ion Phys. 1975, 17, 147. ( 1 5 ) Milosevick, S. A.; Saichek, K.; Hinchey, L.; England, W. B.; Kovacic, P. J . Am. Chem. SOC.1983, 105, 1088. (16) (a) Liu, S.; Jarrold, M. F.; Bowers, M. T. J . Phys. Chem. 1985,89, 3127. (b) Anicich, V. G.; Bowers, M. T. J . Am. Chem. SOC.1974, 96, 1279. (17) Stone, J. A; Lin, M. S. Can. J . Chem. 1980, 58, 1666. (18) Shida, T.; Hamill, W. H. J . Chem. Phys. 1966, 44, 4372. (19) Badger, 8.; Broklehurst, B. Trans. Faraday SOC.1969, 65, 2582. (20) Badger, B.; Brocklehurst, B. Trans. Faraday Soc. 1970, 66, 2939. ( 21) Badger, B.: Brocklehurst, B. Nature 1968, 219, 263
0 1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. 9, 1990 3649
Photodissociation of the Benzene Dimer Cation
These states arise due to the interaction of the wave functions of two like molecular units in different electronic states, C6H6+(Xz El,) and C6H6(X1A,,), in in-phase and out-of-phase combinat i o n ~ .The ~ ~ existence of such bound-state/repulsive-statepairs is a common phenomenon and explains why nearly all homogeneous cluster ion dimers, Az+ or AT, photodissociate in the visible or near-UV. The second absorption band, centered near 460 nm, has been assigned in some to the molecular ion C6H6+ and in others19Jz*23 to the dimer cation (C&)Z+. Photodissociation of aromatic cations in the gas phase has proven to be an informative spectroscopic and structural tool,z5-z7 as well as an energy-selective method of studying unimolecular reactions.28 In general, all of the substituted benzene cations photodissociate at visible photon energies less than 3 eV,25*z6 while the benzene cation itself does not. The lowest energy dissociation pathway of C6H6+, which yields C6HS+ + H products, has a threshold energy of 3.83 eV.z8 However, C&+ does absorb in the visible via an X zEI, ZAzutran~ition.~'After absorbing a visible photon, C6H6+ appears to undergo a rapid internal conversion to vibrationally hot levels of the ground state. The vibrationally excited C6H6+ion has a radiative decay lifetime on the order of 40 ms.z9 Thus, even at moderate photon intensities, a second photon can often be absorbed via a second X zE ZAzutransition, thereby energizing the ion sufficiently to e?fect dissociati~n.~~ In this paper, we report the results of a photodissociation study of the benzene dimer cation, (c&&+, in the wavelength range of 458-514 nm. In addition to the potential complication of solvent perturbations being eliminated, one noteworthy advantage of these gas-phase measurements is that the sample is mass-spectrometrically "purified" before being irradiated. Thus, there is no ambiguity in ascertaining the species responsible for photon absorption. Dissociation can be expected to be a reliable means of monitoring photon absorption since upon absorbing a 2.7 1-eV (458-nm) photon, the (C6H6)2+ ion is energized with over 2 eV in excess of the 0.66 eV necessary for dissociation. Furthermore, in these experiments, the photofragment translational energy distribution is determined, allowing the upper state involved in photon absorption to be identified as either bound or repulsive. To our knowledge, this is the first report of the photodissociation of a benzene or substituted-benzene cluster ion. Considering the current interest in cluster-ion spectroscopy and ph~tochemistry,~~ and the considerable knowledge base that has been built up on the photoinduced behavior of benzene and substituted benzene ion^,*^-^^ the study of such cluster ions promises to be an active and rewarding area of investigation. The results of our study follow.
-
;z I nl o [ W
C
.
I
.
051
2
t
I 0.oL..
..-,...--',
3945
,
,
I
,
,
., ...:... ,.._.. I
4005
,
.. 4065
LABORATORY ION KINETIC ENERGY ( e V )
Figure 1. Laboratory frame kinetic energy distribution of C6H6+produced from the photodissociation of 8010-eV (c&,)2+ ions at a wavelength of 458 nm (2.71 eV). The apparent structure on the low kinetic energy side of the peak is noise.
-
ions were produced from pure benzene in a high-pressure temperature-variable ion source. The likely formation mechanism is direct electron impact ionization followed by three-body association to form the dimer cation. The benzene pressure in the ion source was lo-' Torr, the temperature was held at -273 K, and the electron energy was -200 eV. Ions that effused out of the source were accelerated to 8 kV, mass selected by a magnetic sector, and brought to a spatial focus where they were crossed by 458-, 476-, 488-, and 514-nm light generated by the argon ion laser. A polarization rotator was used to fix the angle between the electric vector of the laser and the ion beam direction at the "magic angle" of 54.7O in order to remove any angular dependence from the product kinetic energy distribution^.^^ C6H6+photofragment ions were mass and energy analyzed by a high-resolution electrostatic sector analyzer and detected by single-ion counting. The fwhm of the (C&)z+ parent ion beam was - 5 eV in the laboratory reference frame, while the width of the C6H6+product beam was -25 eV. Hence, it was not necessary to deconvolute the contribution of the (c&&)Z+ main beam width from the C&,+ product beam width prior to differentiation to obtain the kinetic energy di~tribution.~~ The pressure in the ion acceleration region was in the low Torr range. In the laser-interaction and energy-analysis regions, the pressure was in the low 10-9-Torr range. Background signals due to metastable dissociations or collision-induced dissociation processes were subtracted by alternately blocking and passing the laser beam and doing up/down counting.
11. Experimental Section The experiments were performed using a reverse geometry double-focusing mass spectrometer (V. G. Instruments, Model ZAB-2F) and an argon ion laser (Coherent, Innova Model 20). The details of the experimental method have been published previo~sly.~~ Only a brief description will be given here. (C6H6)2+
111. Results and Discussion The only dissociation pathway energetically accessible to (C6H6)2+ after absorption of 1 photon in the range 2.41-2.71 eV leads to C&+ + C6H6 as products. This is the only product channel that was investigated. No photodissociation was observed at a wavelength of 514 nm (2.41 eV) and only an extremely weak _ absorption at 488 nm (2.54 eV). A weak absorption was observed at 476 nm and a relatively strong absorption at 458 nm (2.71 eV). The likelihood of observing a two-photon process with the experimental arrangement used is vanishingly small, since the 8-keV ions pass through the -1-mm laser beam (flux -4 X IOzo photons/(cmz.s))in only s. The background signal from the metastable dissociation reaction
~ ~ ~ _ _ _ _ _
(22) Badger, B.; Brocklehurst, B.; Russell, R. D. Chem. Phys. Lett. 1967, I , 222. (23) Miller, J. H.; Andrews, L.; Lund, P. A,; Schatz, P. N . J . Chem. Phys. 1980, 73, 4932. (24) Herzberg, G. Spectra in Diatomic Molecules. Molecular Spectra and Structure; Van Nostrand: Princeton, 1950; Vol. 1 , p 321. (25) (a) Dunbar, R. C. In Gas Phase Ion Chemistry; Bowers, M. T., Ed.; Academic Press: Orlando, FL, 1984; Vol. 3, p 130. (b) Dunbar, R. C. Ibid; Vol. 2, p 181. (26) Dunbar, R. C. J . Phys. Chem. 1979, 83, 2376. (27) Freiser, B. S.; Beauchamp, J . L. Chem. Phys. Lett. 1975, 35, 35. (28) (a) Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. J . Phys. Chem. Ref.Dara 1988,17, supplement 1 and references therein. (b) Kiihlewind, H.; Kiermeir, A,; Neusser, H. J. J . Chem. Phys. 1986.85, 4427. (29) Ahmed, M. S.; So, H. Y.;Dunbar, R. C. Chem. Phys. Lett. 1988,151, 128. (30) See, for example: Ion and Cluster ion Spectroscopy and Structure; Maier, J. P., Ed.; Elsevier: Amsterdam, 1989. (31) Jarrold, M. F.; Illies, A . J.; Kirchner, N . J.; Wagner-Redeker, W.; Bowers, M. T.; Mandich, M. L.; Beauchamp, J. L. J . Phys. Chem. 1983,87, 221 3 and Appendix and references therein.
-
was large, about (1-5) X lo4 counts/s, but after h of up/down counting and signal averaging, the photodissociation component at both 476 and 458 nm substantially exceeded the noise level. Attempts to investigate the photodissociation of (32) (a) Jarrold, M. F.; Illies, A. J.; Bowers, M. T. J . Chem. Phys. 1984, 81, 214. (b) Zare, R. N. Mol. Photochem. 1972,4, 1 . Busch, G. E.; Wilson, K. R. J . Chem. Phys. 1972,56, 3638. Zare, R. N.; Herschbach, D. R. Proc. IEEE 1963, 51, 173.
The Journal of Physical Chemistry, Vol. 94, No. 9, 1990
3650
'
(C6H6):
-
t hv
OB. h v . 2 7 1 eV
C,H;
t
Snodgrass et al. \
C6H6
* Phose Space Calculation
- Experiment
>.
c
!I
0 O4 6I\ 2 56k003 eV
00 00
24i01eV
I
I
01 02 03 C M K I N E T I C ENERGY ( e V )
04
05
Figure 2. Photoproduct relative kinetic energy distribution in the center-of-massframe obtained from the data shown in Figure 1. The solid line is the experimental data, and the points are the result of a statistical phase space theory calculation. (CsH6):
-
C6H:
(C6H6):
+ C6H6
METASTABLE
('Elg)
Figure 4. Important low-lying doublet states involved in the photodissociation of the benzene dimer cation. The vertical energy axis is drawn to scale, and the horizontal axis is schematic. The origin of the numerical values shown is given in the text.
10
>
t m z w
'I
3945
-..:.......
.r
....... ......-;......,.
4005
4065
LABORATORY ION KINETIC ENERGY ( e V )
Figure 3. Laboratory frame kinetic energy distribution of C6H6' produced by the metastable decomposition of (C6H6)2+ions in the second field free region between the magnet and electric sector. Note how much narrower this distribution is than the photodissociation distribution in Figure I .
ethylbenzene dimer cation were thwarted by a metastable signal of -2 X lo5 counts/s, which overwhelmed any photodissociation signal that might have been present. The laboratory kinetic energy distribution of C6H6+ produced by the photodissociation of (C6H6)2+ at a wavelength of 458 nm is shown in Figure 1 . After transformation to the center-of-mass (CM) frame,3' the product-relative kinetic energy distribution shown in Figure 2 is obtained. The distribution peaks near zero and falls off smoothly and essentially exponentially at higher kinetic energies. The average C M product kinetic energy for the distribution shown in Figure 2 is 0.070 eV. The reproducibility of this average value was *lo% in repetitive trials. The laboratory kinetic energy distribution of C6H6+, produced by the metastable reaction ( 1 ) is shown in Figure 3. This distribution is clearly much narrower than the distribution from the photodissociation process and yields an average C M product kinetic energy of 0.014 eV. The C M product kinetic energy distribution shown in Figure 2 has fallen to essentially zero at -0.2 eV, well before the -2-eV available energy limit. The low C M product kinetic energies indicate that the upper state in the photon absorption process is almost certainly a bound state and not a repulsive state. A schematic diagram of the electronic states that are most likely involved in the photodissociation process is shown in Figure 4. The energy separation of 2.4 f 0.1 eV between the two asymptotic limits is based on the benzene photoelectron spectrum33and the C6H6+photodissociation work of Freiser and Bea~champ.~'The ground-state binding energy for the dimer is known: (0.66 f 0.04 ~~
( 3 3 ) Carlson, T. A , Anderson, C P Chem Phys Lett. 1971, I O , 561
0
!?' 2';
/f 2'5''2.6
2'7
2'8
P h o t o n Energy, eV
Figure 5. Plot of the relative photodissociation cross section for (c6H6)2+ + hv --* C6H6+ + C.5H6 VerSUS photon energy.
eV) but no information is available on the binding energy of the upper state. From our data, we have been able to estimate this binding energy, as discussed below. If photon absorption induces a transition to an upper bound state, a possible dissociation mechanism is internal conversion to vibrationally excited levels of the (C6H&+ ground state, followed by statistical vibrational excited levels of the (c&6)2+ ground state, followed by statistical vibrational predissociation. If this were the case, the experimental product translational energy distribution should match that predicted by statistical phase space theory.34 In order to test this hypothesis, we performed statistical phase space calculation^.^^ The results are shown in Figure 2. The agreement between experiment and theory is quite good, providing strong evidence in support of the proposed mechanism. A plot of the relative photodissociation cross section versus photon energy is given in Figure 5. It is clear there is a sharp onset near 2.56 f 0.03 eV. A photodissociation signal at 2.41 (34) Chesnavich, W. J.; Bowers, M.T. J. Am. Chem. Soc. 1976,98,8301. Chesnavich, W. J.; Bowers, M. T. Prog. React. Kinet. 1982, 98, 8301. Chesnavich, W. .I.Bass, ; L.; Su,T.; Bowers, M . T. J . Chem. Phys. 1981, 74, 228.
(35) The parameters required for the phase space theory calculations were taken from: Jarrold, M. F.; Wagner-Redeker, W.; Illies, A. J.; Kirchner, N. J.; Bowers, M. T. Int. J . Mass Spectrom. Ion Phys. 1984,58, 63. Reference 16a and references therein. The method used in the calculations is summarized in ref 34.
Photodissociation of the Benzene Dimer Cation eV could not be observed even after extensive signal averaging. At 2.54 eV, a very weak but reproducible signal was observed. The argon ion laser lines at 2.41 and 2.54 eV are approximately 5 times more intense than the 2.60- or 2.71-eV lines. Experiment indicates the signal for the 2.71-eV line is at least 5 times greater than the signal at 2.54 eV. Consequently, we can make a conservative estimate that the photodissociation cross section is at least 25 times as strong at 2.71-eV photon energy as at 2.54-eV photon energy. A direct comparison indicates the signal at 2.60 eV is 0.10 f 0.02 as strong as the 2.71-eV signal. The measured relative intensities are given in the plot in Figure 5. A simple energy cycle (Figure 4) indicates D!(CsH6+ - C6H6)* = IPC6H6(2A2u) - IpC6H6(x2EIg) + D!(C6H6+ - C6H6)gs - hvthreshold (2) where IPc6H6(2A2u) and IPc,H (X 2E1,) stand for the ionization potentials of the ground state ofC6H6 to the 2A2uand X 2EI, states of C6H6+, respectively. The ionization potentials are taken from the photoelectron spectrum of benzene3' as supported by the photodissociation work of Freiser and B e a ~ c h a m p , ~the ' ground-state binding energy from the photoionization analysis of Grover et al.,4 and the threshold photon energy from this work. Substituting these values into eq 2 gives the binding energy of the electronically excited benzene dimer ion as @(C6H6+ - C6H6)* = 0.50 f 0.17 eV
(3) The uncertainty in D:(C6H6+ - C6H6)* was obtained by adding the maximum uncertainties of each of the values in eq 2 (see Figure 4). Implicit in this analysis is the assumption that the bound upper state accessed by the photon correlates to the ZA2ustate of C6H6+ upon dissociation. This is a good assumption for two reasons. It is widely held4-I9that the (c6H&+ ground state has a "stacked" sandwich structure of D6h symmetry. This symmetry is the same as the monomer ion and neutral C6H6. Consequently, since the ground state of C6H6 is ]Al, symmetry, the dimer states must retain the symmetries of the C6H6+ states they correlate to upon dissociation. The 2A2ustate of C6H6+ is the lowest energy excited state accessible by an electric dipole-allowed transition from the 'E,, ground state of C6H6+. Consequently, this state should generate the lowest energy bound state that is dipole accessible from the ground state of the dimer ion. (The "intravalence" repulsive state of El, symmetry that correlates to the ground state of C6H6+ is also dipole allowed.) The above reasoning assumes a D6h symmetry for the dimer ion. If this is not the case, then other excited states of benzene must be considered. From analysis of the photoelectron spectrum,''" it is clear that the only other C6H6+ state in the energy range of interest is the 2E2, state occurring 2.2 f 0.1 eV above the ground state. This is a gerade state, and optical transitions from the gerade ground state are strictly forbidden. Further, it is a u state, and T u transitions are typically weak. Other symmetry restrictions on photoaccessing the 2Ezgstate in C6H6+ also occur. If all of these restrictions are lifted upon dimerization, and a bound state correlating to the 2E2, is photoaccessed, then the resultant binding energy of the excited state would be 0.30 f 0.17 eV. While such a state could exist, it appears much less
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1*he Journal of Physical Chemistry, Vol. 94, No. 9, 1990 3651
likely than the 2A2ustate to be responsible for the photoabsorption/photodissociation we observe. The observation in this work of the onset of photodissociation near 484 nm of (c6H6)2+ in a mass-selected ion beam substantiated the assertion made in some of the condensed-phase studies22s23*29 that (c6H6)2+ absorbs in this region of the spectrum. However, the relative contributions of (c&)2+ and C6H6' to the absorption intensity observed in solution and in the solid state is difficult to assess, since both of these species absorb in this region of the spectrum, and both are likely to be present. The onset for absorption of the monomer C6H6+ ion in the gas phase occurs around 2.4 eV (514 nm), while our work indicates the dimer absorption onset is blue-shifted to 2.56 f 0.03 eV (484 f 6 nm). Our value is in good agreement with the 465-nm band assigned to (c6H6)2+ by Badger and Brockelh~rst'~ in an irradiated isopentane/n-butyl chloride glass at 77 K and with a 453-nm band observed by Miller et al.23at 21 K in a solid argon matrix, which they also assigned to the dimer cation. The product translational energy distribution we measure indicates that photodissociation is initiated by a transition to a bound excited state. Thus, absorption of photons in the range 276-258 nm does not induce the "intervalence transition", which leads to a repulsive final state, in accord with the assignments made in the condensed-phase ~ o r k . ~ Such ' - ~ ~a state is shown schematically in Figure 4. It would be informative to conduct our experiment using a photon energy near 1.38 eV (900 nm). At this wavelength, the (c6H6)2+ ions would be energized -0.5 eV above the dissociation threshold, and a large fraction of this energy should appear as product translation if a repulsive surface were accessed. Measurement of the product angular distributions would also be diagnostic of the presence of a repulsive upper state. Unfortunately, we cannot presently access this energy range but hope to be able to do so in the future. IV. Conclusions The principal conclusions of this work are as follows: (1) The (c6H6)2+ dimer ion from benzene exhibits a dissociation threshold at 256 f 0.03 eV (484 f 6 nm). This result suggests that absorptions observed in irradiated glasses near 465 nm are most likely due to (c6H6)2+ and not C&+. (2) The upper state accessed by the photon is a bound state. Arguments are presented that this state is of 2A2usymmetry and dissociates to C6H6+(2A2u)4- ~ & ( ' A I , ) . With the measured threshold for absorption and a thermodynamic cycle, it is concluded the excited state is bound by 0.50 i 0.17 eV. By comparison, the ground state has a binding energy of 0.66 f 0.04 eV. (3) Analysis of the kinetic energy release distribution using statistical phase space theory indicates the dissociation occurs by vibration predissociation of the ground state. Hence, the mechanism for the photodissociation process is (C6H6)2+tX 2
~ 1 , ~
(c~H~)~+[*A~,I
_e (C6H6)2+[X2E~gl* C6H6+(X 'A2J -I-C & w 'Ai,) --+
Acknowledgment. This work was supported by the Air Force Office of Scientific Research, Grant AFOSR 89-0102, and the National Science Foundation, Grant CHE88-17201.
