Photon-stimulated desorption from thin films of ${\mathrm{CaF}}_2$ exhibit distinctly different desorption channels for ions depending on the site where the initial core hole was created. In addition to ${\mathrm{H}}^{+}$, the most prominent desorbed ionic species observed in ${\mathrm{CaF}}_2$ is ${\mathrm{F}}^{+}$. Upon the creation of a F 1s core exciton, there is enhanced ${\mathrm{F}}^{+}$ desorption compared with the ion yield for complete ionization of a F 1s electron. A similar enhancement is not observed at the fluorine edge in ${\mathrm{BaF}}_2$. In ${\mathrm{BaF}}_2$, the ${\mathrm{H}}^{+}$ ion yield is, to within an order of magnitude, the same as in ${\mathrm{CaF}}_2$, but ${\mathrm{HF}}^{+}$ ions are also observed. Excitation of Ca 2p and Ba 3d core excitons does not result in an additional desorption channel that directly leads to the production of ions, but rather the desorption is caused nonspecifically by enhancing the secondary electron current through the surface.

• Received 18 May 1992

DOI:https://doi.org/10.1103/PhysRevB.46.12388