Mechanism of ion desorption by electronic transitions: A density-functional study

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 ${\mathrm{F}}^{\mathrm{-}}$ and is bound by an imagelike interaction. The bonding of ${\mathrm{F}}^{+}$ and ${\mathrm{F}}^{++}$ is, however, very different. The 3s, 3p levels which were empty in ${\mathrm{F}}^{\mathrm{-}}$ 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.