Photodissociation of semiconductor positive cluster ions.

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. [ABSTRACT FROM AUTHOR]