Photodissociation of semiconductor positive cluster ions

Abstract

Laser photofragmentation of Si, Ge, and GaAs positive cluster ions prepared by laser vaporization and supersonic beam expansion has been investigated using tandem time-of-flight mass spectrometry. Si clusters up to size 80, Ge clusters to size 40, and GaAs clusters up to a total of 31 atoms were studied. Si+n and Ge+n for n=12–26 give daughter ions of about half their original size. For both Si and Ge, this apparent positive ion fissioning appears to go over with increasing n to neutral loss of seven and ten, but for Si+n the range of n values where this is observed is rather small. At low fluences, the larger Ge+n clusters up to the maximum size observed (50) sequentially lose Ge10 (and in some cases with lower intensity Ge7). Larger Si+n clusters (n>30) always fragment primarily to produce positive ion clusters in the 6–11 size range with a subsidiary channel of loss of a single Si atom. At high laser fluences, Ge+n also fragments to produce primarily positive ion clusters in the 6–11 size range with an intensity pattern essentially identical to Si+n at similar fluences. GaxAs+y clusters lose one or more atoms in what is probably a sequential process with positive ion clusters in which the total number of atoms, x+y, is odd being more prominent.