Oscillation of Photoionization Thresholds of Small Photolytic Silver Clusters on Silver Bromide Grain Surface

Photoionization thresholds of small photolytic silver clusters (${\mathrm{Ag}}_2$, ${\mathrm{Ag}}_3$, and ${\mathrm{Ag}}_4$) formed on silver bromide grain surfaces were evaluated from the threshold photon energy required for inducing the photobleach of the clusters. Threshold energies needed were about 1.6, 0.75, and 1.1 eV for ${\mathrm{Ag}}_2$, ${\mathrm{Ag}}_3$, and ${\mathrm{Ag}}_4$, respectively, showing an obvious even-odd oscillatory trend of the ionization threshold. The oscillation, however, seemed to be smaller than that expected for isolated metal-atom clusters, indicating the importance of interaction with the silver bromide grain surface.