Electron transfer reactions for image and image-derived states in dielectric thin films.

We have studied the cross section for electron trapping that occurs at the surfaces and interfaces of a variety of thin dielectric films (n-octane, methanol, n-butanol, and difluoromethane) that are grown on Kr buffer films. When such films are bombarded with electrons of very low incident energies (E less, similar 300 meV), charging cross sections up to the order of 10(-14) cm(2) are measured for submonolayer quantities of a variety of coadsorbed molecules: CH(3)I, CH(3)Br, CH(3)Cl, and CO(2). These huge cross sections are ascribed to the formation of image states at the dielectric film interfaces, which trap incoming electrons and, via coupling to the adsorbate electron affinity levels, dramatically enhance the capture probability. We have also shown that thin film dielectric layer structures can be created which display image-derived states, such as a "quantum well" in a sandwich structure with two "electron barrier" layers surrounding a Kr and adsorbate spacer film. These phenomena are shown to be of a general nature, occurring for a wide variety of molecular thin films, and depend on the dielectric constant and electron affinity of the selected species. We also report the absolute cross section for dissociative electron attachment of submonolayer CH(3)I adsorbed on Kr thin films.