Using density-functional theory we investigate the mechanism of ion and neutral desorption via electronic transitions. F on Al is used as a prototype. We find that fluorine adsorbs as F- and is bound by an imagelike interaction. The bonding of F+ and F++ is, however, very different. The 3s, 3p levels which were empty in F- are now partially occupied by metal electrons. The resulting states are locally nearly neutral but Franck-Condon transitions to them can lead to ion desorption. © 1987 The American Physical Society.