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PHYSICAL REVIEW LETTERS 18 AUGUST 1975 with photoemitted electrons and, on the basis of the observed trap filling and depopulating kinetics, propose that at fields as low as 5xl0 5 V/cm some excess electrons have sufficient kinetic energy to ionize an occupied trap. Our data clearly show that the average electron at the highest fields has less than 0.15 eV energy, but of course there will be a distribution of electrons with higher kinetic energies the shape of which is not predicted by the TF theory. There is also the possibility as suggested by both DiMaria, Feigl, and Butler 10 and TF 1 that electrons which have much more than 0.15 eV upon injection will be in the unstable part of the energy-loss versus energy curve and will be accelerated by the field until some other energy-loss mechanism drops them into the stable part of the curve. Evidence for impact ionization by hot electrons in the oxide layer has been obtained in the switching studies of Shatzkes, Av-Ron, and Anderson, 11 but only for fields over 10 7 V/cm which can be maintained by the space charge of the low-mobility holes. 12 The author gratefully acknowledges technical discussions with David Emin and the technical assistance of David Evans. tWork supported by the U 0 S. Energy Research and Unusual multiplet structure in the N ls and S 2p core-electron spectra of the charge-transfer complex tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) has been reported. 1 " 3 Both discrete intrinsic energy-loss phenomena 1 and chemical shifts due to neutral and singly charged molecules coexisting in the solid on the time scale of bond vibrations 2 have been mentioned as possible causes of the structure. To date, how-Development Administration. . Our high-field data agree with Curtis, Srour, and Chiu who did not resolve the separate contributions of the electrons and holes to the total charge.

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