1154
J. Phys. Chem. 1980, 84, 1154-1155
(Figure 4) was obtained in 2,2,4,4,-tetramethylpentane and it is reasonable to assume the spectrum would be very similar in the other nonpolar solvents used. If this assumption is valid, eq 2 can be employed to calculate quantum yields. At the first absorption maximum at 4‘70 nm (2.6 eV), the absorption cross section (a,) is 1.8 X cm2. The ratio of the photodetachment cross sections at 2.6 eV to this value gives values of the electron quantum yields. These values are listed in the penultimate column of Table I and range from 0.39 to 0.68 electron/quantum. Photodetachment is even more efficient in the second absorption band. Experimental limitations precluded detachment measurements in the region of the peak at 3.1 eV but the photodetachment spectra generally show that Ud is the largest at the shortest wavelengths employed. At 296 K the quantum yields (9,)at 3.0 eV, given in the last column of Table I, are consistently higher than the values at 2.6 eV. The average value is & = 0.62, indicating detachment is a major process. A very similar behavior was found for the anthracene anion in THF;l’ the reported photoemission spectra show the same spectral structure as the absorption spectra and the yield of photoemission is greatest in the high energy band.
Acknowledgment. This research was carried out at
Brookhaven National Laboratory under contract with the US.Department of Energy and supported by its Division of Basic Energy Sciences.
References and Notes (1) For a recent review see, e.g., W. F. Schmidt, Can. J. Chem., 55, 2197 (1977). (2) L. V. Lukin and B. S. Yakovlev, Chem. Phys. Lett., 42, 307 (1976). (3) U. Sowada and R. A. Hoiroyd, J . Chem. Phys., 70, 3586 (1979). (4) M. B. Yim and D. E. Wood, J. Am. Chem. Soc., 98, 2053 (1978). (5) C. Lifschitz, T. 0.Tiernan, and B. M.Hughes, J. Chem. Phys., 59, 3182 (1973). (6) F. M. Page and G. C. Goode in “Negative Ions and the Magnetron”, Wiley-Interscience, New York, 1969. (7) L. Nyikos, C. A. M. van den Ende, J. M. Warman, and A. Hummei, article in this issue. (8) H. A. Schwarr and R. W. Dodson, to be published. (9) J. P. Dodelet and G. R. Freeman, Can. J . Chem., 50, 2667 (1972). (10) A. M. Want, T. Mukherjee, and J. P. Mittal, Radiaf.Effects Lett., 43, 13 (1979). (11) S.Geltman, Phys. Rev., 112, 176 (1958). (12) D. S. Burch, J. J. Smith, and L. M. Branscomb, Phys. Rev., 112, !71 (1958). (13) R. Holroyd, S. Tames, and A. Kennedy, J. Phys. Chem., 79, 2857 (1975); R. Holroyd, Roc. Int. Congr. Radiat. Res., 5th, 378 (1975). (14) A. Hengiein, Can. J. Chem., 55, 21 12 (1977). (15) J. R. Frarier, L. 0. Christophorou, J. G. Carter, and H. C. Schweinler, J . Chem. Phys., 69, 3807 (1978). (16) G. J. Hoytink, Chem. Phys. Lett., 29, 154 (1974). (17) P. Delahay in “Electron-Solvent and Anion-Solvent Interactions”, L. Kevan and B. C. Webster, Ed., Elsevier, New York, 1976, p 115 ff.
High Mobility Excess Electrons in the Electron-Attaching Liquid Hexafluorobenzene Lajos Nyikos,+Cornells A. M. van den Ende, John M. Warman,” and Andries Hummel Interuniversity Reactor Institute, Mekeiweg 15, Delft, The Netherlands (Received July 16, 1979) Publication costs assisted by the Interuniversity Reactor Institute, Delft
Hexafluorobenzene (C6F6)has been found to have an electron affinity between 1 and 2 eV1p2and to undergo rapid attachment of thermal electrons in the gas phases3 We have found that this compound is also a very efficient scavenger of excess electrons in nonattaching molecular in tetraliquids (rate constant 1.2 X l O I 4 dm3 mol-’ methylsilane). On the basis of this it can be concluded with considerable certainty that excess electrons formed in pure liquid C6F6 will very rapidly form a bound negative ion state with solvent molecules. Despite this, the negative charge carrier in irradiated liquid C6F6is found to have a mobility more than an order of magnitude greater than would be expected for a molecular ion in this medium, the evidence for which will be discussed below. In the present experiments, the liquid was ionized by pulsed irradiation with 3-MeV electrons from a Van de Graaff accelerator and the conductivity change was measured by using the microwave absorption methoda4The product of the yield of ions per 100 eV absorbed and the sum of the ion mobilities, G p cm2 V1s-l (100 eV)-l, was determined from the conductivity change and the known dosimetry. The transient conductivity resulting from pulse irradiation of pure C6F6is considerably greater than that which would be expected on the basis of the formation of ions, the motion of which is dependent on molecular displacements alone. This is illustrated in Figure 1by comparison of the C6F6 data with conductivity transients observed in benzene containing mol dm-3 of SF6as an electron scavenger and in the (dissociative)electron-attachingliquid +Onleave from the Central Research Institute for Physics, H-1525 Budapest, Hungary.
cc14. The decay rate of the conductivity signal in C F6 is increased by the addition of small amounts (
