7535
J . Phys. Chem. 1989, 93,1535-1536 I
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0
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8.49
8.60
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8.82
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ENERGY (eV)
Figure 1. Comparison of near threshold efficiency functions for the one-photon (points, ref 8) and two-photon (line, ref 1) photoionization of benzene dimer. The data sets have been arbitrarily normalized to one another. Error bars reflect an uncertainty of f l u due to statistics of counting.
the efficiency function in the threshold region of van der Waals Our own experience is that, of six dimers studied, four show obvious autoionizing structure in the threshold region (C6H6*HCI," C6H6-02,'6 C6F6*?2116and CH2CHCHCHyS02).I7 Furthermore, pronounced autoionization has been observed in the threshold region of benzene monomer.@ It would therefore not be at all surprising to find that autoionization is also important in the threshold ionization region of benzene dimer, and indeed, the difference in the results shown in Figure 1 could well be the result of more autoionization in the one-photon experiment than in the two-photon experiment. The two-photon results of Bornsen et al. using the SI,0-0 transitions on (C6H& and C6H6.C6D6gave threshold efficiency functions that are not greatly different than that shown in Figure 1, consistent with essentially no involvement of autoionization. (3) The dimer IP can be mistakenly assigned to the appearance potential (AP) for the production of dimer ion by dissociative photoionization of trimers or larger cl~sters.'~This problem does not occur here because the A P is larger than the IP; a simple energy diagram shows that the AP is equal to the IP plus the energy required to separate the neutral dimer from the third molecule. In some cases the dimer ion can rearrange to a new configuration of increased stability during the photoionization of the trimer, as in the production2' of C&+ from (C2H4)3, so that the AP can be smaller than the apparent IP. However, this complication is not possible with (C6H&+ from either (C6H6)3or (C6H6)2Arbecause an ion CI2Hl2+more stable than (C&)2+ does not exist. (4) Field-induced ionization from high Rydberg states of the vdW dimer will give an IP that is too low. The classical Coulomb model gives, as an upper boundIE for the magnitude of this effect, A(1P) = -6E'/* cm-', where the ~
~
(10) Dehmer, P. M. J . Chem. Phys. 1982, 76, 1263-1272. ( 1 1 ) Dehmer, P. M.; Pratt, S. T. J. Chem. Phys. 1982, 77, 4804-4817. (12) Trevor, D. J.; Pollard, J. E.; Brewer, W. D.; Southworth, S.H.; Truesdale, C. M.; Shirley, D. A.; Lee, Y. T. J . Chem. Phys. 1984, 80, 6083-6091 ~~~. ... ~. (13) Kamke, W.; Kamke, B.; Kiefl, H. U.;Hertel, I. V. Chem. Phys. Lett. 1985, 122, 356-360. (14) Sobolewski, A. L.; Domcke, W. J . Chem. Phys. 1987,86, 176-187. (15) Walters, E. A.; Grover, J. R.: White, M. G.: Hui, E. T. J . Phvs. Chem.' 1985,89, 38 14-38 18. (16) Work in progress. (17) Grover, J. R.; Walters, E. A.; Newman, J. K.; White, M. G. Sub-
mitted. (18) Duncan, M. A.; Dietz, T. G.; Smalley, R. E. J . Chem. Phys. 1981, 75, 2118-2125. (19). Mixed clusters of benzene and argon were not detectable under the
very mild jet expansion conditions employed in the work described in ref 8. The assertion in ref 1 that such species were a problem is unsupported and ignores without explanation the experimental evidence to the contrary presented and cited in ref 8. (20) Tzeng, W.-B.; Ono, Y.;Linn, S.H.; Ng, C. Y. J. Chem. Phys. 1985, 83, 2813-2817.
0022-3654/89/2093-7535$01.50/0
electric field strength E is expressed in volts per centimeter. Thus, at E = 100 V cm-I the apparent IP can be decreased by as much as 60 cm-l (0.008 eV, 0.2 kcal mol-{) below the actual IP. Of course a correction this size is important, but it is much too small to account for the effect shown in Figure 1. To verify experimentally that this correction is insignificant at our resolution of 2.3 A (i.e., 0.016 eV at 1350 A) we measured the IP of C6H6-02 with draw-out fields of 19 and 41 V cm-' for which a difference of about 0.002 eV would be expected; no difference could be detected. (5) The neutral dimer could be present as more than one isomer, or as a metastable isomer, even when produced in a jet expansion. The apparent IP would then be too low. Conversely, if only higher energy isomers of the product ion are formed, the apparent IP would be too high. Theoretical calculations21for (C6&)2 and (C6H6)?+ indeed indicate multiple minima, but separated by low barriers of only 1 kcal mol-I, for both species. However, it is most unlikely that this can explain the observed difference, because if the single-photon thresholds sample only energetic isomers, then the independent measurements of the of (c6H6)2+ must be rejected. dissociation The most likely reasons of those listed above for the difference between the two experiments are therefore (1) and (2). The one-photon work should be repeated using deuterated benzene, e.g., C,&*C6D6 or (C,$6)2, to try to confirm the contribution of near threshold autoionization, which might show up as marked differences of the efficiency functions from one system to another. The time-delay studies of the two-photon work should be extended to times much shorter than the 12-ns and longer delays Bornsen et al. used to search for relaxation effects in the Franck-Condon factors associated with the photoionization of the benzene(g. s.).benzene(excited) complexes they prepared. From the hint from Figure 1 that interesting information can be forthcoming, we believe that comparisons of one-photon with two-photon photoionization of dimers and small clusters, especially near threshold, could prove very fruitful.
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(21) See the citations to theoretical papers in ref 8.
Department of Chemistry Brookhaven National Laboratory Upton, New York 11973
J. R. Grover*
Department of Chemistry University of New Mexico Albuquerque, New Mexico 87131
E. A. Walters
Institut f u r Physikalische Chemie Freie Universitat Berlin Takustrasse 3, 0-1000 Berlin 33, West Germany
H. Baumgartel
Received: June I , 1989
Ionization Potential of the Benzene Dimer Sir: Grover et a1.I suggest that the ionization potential of the benzene dimer measured by Bornsen et aL2 is too high when compared with other data and in particular is inconsistent with the presumed dissociation energies of the ground state of the neutral dimer and the ion dimer. We had realized that there is a problem with the IP of the dimer, a fact that we feel to be generic to all IPSof van der Waals clusters. We discussed this problem in great detail in the frame of a generalized theory, in an earlier paper,3 not cited by Grover et al. We discussed tailing in the IP's of clusters to extend to ( I ) Grover, J. R.; Walters, E. A.; Baumgartel, H. J . Phys. Chem., preceding paper in this issue. (2) Bornsen, K. 0.;Selzle, H. L.; Schlag, E. W. J . Phys. Chem. 1988, 92, 5482. ( 3 ) Lin, S. H.; Selzle, H. L.; Bornsen, K. 0.;Schlag, E. W. J . Phys. Chem. 1988, 92, 1469.
0 1989 American Chemical Society
7536
The Journal of Physical Chemistry, Vol. 93, No. 21, 1989
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1 / 1 ~ 1 1 1 1 , 1 1 1 l ~ I I I I ~ I / I I ~ I I I I ~ I I I I / I l I I
TWO P H O T O N ENERGY Lev1
Figure 1. Ion-current threshold for two-color benzene dimer photoioniza tion.
2000-3000 cm-I. Shifts of the equilibrium distance in the dimer ion of as little as 0.5 A lead to the adiabatic IP totally moving out of the field of vision. Hence the effects they discuss qualitatively have been quantitatively treated by us in a prior paper. The experimental answer to this is not, as the authors suggest, the comparison of one- and two-photon values of the ionization potentials but rather new experiments employing ionization via highly excited (preferably Rydberg) states of the neutral states which are expected to be more ionlike in structure. Only these measurements, preferably at high resolution, will give “correct” adiabatic ionization potentials. How far the previous IP of Bornsen et a1.,2 or indeed any published IP, is away from a “correct” adiabatic value can also be determined from experimental values of the dissociation energies in the neutral ground state and the ion state and the known IP of the monomer. We have measured this dissociation energy in the ground state since our first p~blication,~ in a paper4 not cited by Grover et al., and obtained a value of 70 meV. For the value of the ion, high-pressure mass spectrometry yields values from 347 to 740 meV.5 (4) Kiermeier, A.; Ernstberger, B.; Neusser, H. J.; Schlag, E. W. J . Phys. Chem. 1988, 92, 3785.
Comments Choosing the most recent value for the dissociation energy of the ion by Field and co-workers of 740 meV,5c we can now uniquely, on the basis of experiments only, predict the “correct” adiabatic ionization potential to be 8.57 eV subject to the uncertainties of these measurements. This value is outside the range of all presently measured IPS, including the value of Grover et aL6s7 Hence, we differ with their statement that their value or indeed any existing value is reasonable. Existing values even among Grover et al. and Riihl et a1.* differ widely, demonstrating the immense experimental difficulties in obtaining such ionization potentials due to long tailing of the ion current, a characteristic signature of Franck-Condon difficulties. To confirm our suggestion that there is no difference between two-photon and one-photon results, we have repeated our twophoton work in a more sensitive experiment. We indeed could see signal further down in the tail (Figure 1). In fact, our two-photon experiments yield now an IP at 8.65 eV, a value below that of Grover et al.’ In summary, we agree that indeed there is a problem with measuring IP’s of the benzene dimer; indeed there is such a problem with all van der Waals clusters. We have attributed this to Franck-Condon difficulties in a previous paper.3 We show here that this experimental problem applies to all presently known I P S of the benzene dimer, including the value published by Grover et al. (8.69 eV),7 which again differs from that of Baumgartel et al. (8.84 eV).* We predict now on the basis of known experimental data that the correct value must be near 8.57 eV and suggest experimental techniques for establishing this directly. Probably the new extremely high-resolution zero kinetic energy (ZEKE) spectra will serve to resolve this problem definitively, at high resol~tion.~ Registry No. Benzene dimer, 6842-25-7. ~
( 5 ) (a) Wexler, S.; Pobo, L. G. J . Phys. Chem. 1970, 74, 257. (b) Field, F. H.; Hamlet, P.; Libby, W. F. J. Am. Chem. 1%9,91, 2839. (c) Meot-ner
(Mautner), M.; Hamlet, P.; Hunter, E. P.; Field, F. H. J . Am. Chem. Soc. 1978,100,5466. ( d ) Jones, E. G.; Rhattacharya, A. K.; Tiernan, T. 0. Int J. Mass Spectrom. Ion Processes 1915, 17, 147. ( 6 ) Walters, E. A.; Hui, E. T.; Grover, J. R.; White, M. G. Abstracts of Papers, 187th National Meeting of the American Chemical Society, St. Louis, MO, American Chemical Society: Washington, DC, 1954; Abstract PHYS 125. (7) Grover, J. R.; Walters, E. A.; Hui, E. T. J. Phys. Chem. 1987, 91, 3233. (8) Ruhl, E.; Biding, P. G. F.; Brutschy, B.; BaumgBrtel, H. Chem. Phys. Lett. 1986, 126, 232. ( 9 ) Chewter, L. A,; Muller-Dethlefs, K.; Schlag, E. W. Chem. Phys. Leff. 1987, 135, 219.
Institut f u r Physikalische und Theoretische Chemie der Technischen Universitat Miinchen Lichtenbergstrasse 4 8046 Garching, West Germany Received: July 5, I989
H. L. Selzle H. J. Neusser B. Ernstberger H. Krause E. W. Schlag*
