Electron-stimulated desorption of positive ions from methane and fluoromethane nanoclusters

Electron-stimulated positive-ion desorption has been investigated for CH4, CF4, CHF3, and CH2F2 molecules adsorbed on a solid Ar substrate. The ion yields from CH4 are enhanced markedly at the smallest coverage and decay steeply with increasing coverage. In contrast, ion yields from CF4 increase monotonically with increasing coverage. The ions from hydrofluorocarbons exhibit both behaviors. Coulomb explosion associated with the delocalization of valence holes is responsible for the enhanced ion yields from nanoclusters, whereas the monotonic increase of the ion yields with coverage is caused by the dissociative ionization of individual molecules in which valence holes are localized. The localization (delocalization) of valence holes can be ascribed to the nature of the F 2p nonbonding (C-H bonding) orbitals. (C) 2002 American Institute of Physics.