Secondary-electron effects in photon-stimulated desorption

The magnitude of secondary-electron contributions to electron- or photon-stimulated desorption (ESD) or (PSD) yields is considered. In particular, we have reexamined three systems where a dominant x-ray-induced ESD (XESD) effect has been postulated. Recent ESD ion-angular-distribution data on the ${\mathrm{NH}}_3$/Ni system and a detailed determination of the mechanisms involved in ${\mathrm{H}}^{+}$ desorption indicate that all of the features previously attributed to the XESD effect may in fact arise from direct core-level processes. A reexamination of the PSD ${\mathrm{N}}^{+}$ and ${\mathrm{O}}^{+}$ yields from condensed ${\mathrm{N}}_2$-${\mathrm{O}}_2$ reveals that the indirect XESD mechanism contributes just one-third of the ${\mathrm{N}}^{+}$ yield, but dominates the ${\mathrm{O}}^{+}$ desorption. This arises because the direct Auger-stimulated desorption (ASD) process following core-hole excitation is inactive for ${\mathrm{O}}^{+}$ desorption, but remains active for ${\mathrm{N}}^{+}$. Finally, a detailed interpretation of ${\mathrm{H}}^{+}$ desorption from OH/Ti and OH/Cr, and comparison with the system OH/YbO-Sm indicates that the direct ASD process is also inactive in the latter case. This investigation concludes that in cases for which the direct ASD process is active, the indirect XESD contribution is generally on the order of 35% or less. When the ASD process is suppressed, the XESD process generally dominates. However, in chemisorbed systems, even when the direct process is relatively inactive, the XESD process does not dominate. The various reasons for this are discussed.