Photodetachment and photofragmentation studies of semiconductor cluster anions

Abstract

Silicon,germanium, and gallium arsenide negative cluster ions are produced by laser vaporization followed by free supersonic expansion. Electron affinities (EA) of the corresponding neutral clusters are roughly bracketed by measuring the fluence dependence for photodetachment from anions at discrete probe laser wavelengths (above the photodetachment threshold the dependence on fluence is linear, below it is quadratic). An even/odd alternation is found in the negative ion distribution with gallium arsenide clusters with an odd number of atoms having higher EA’s than their even neighbors. This suggests that the surfaces of the even clusters are extensively restructured in a way which eliminates dangling chemical bonds. For Ga x As y with x+y constant, EA increases with increasing ratio of y to x. The EA of silicon increases smoothly with cluster size extrapolating towards the EA of bulk silicon. Photofragmentation studies show that, like the corresponding positive ions, silicon and germanium negative ions with 11 to 23 atoms fission into mainly 5 to 10 atom negative ions. Si− 10 and Ge− 10 are the favorite daughters suggesting the existence of a special structure for ten atoms.