Photon-stimulated ejection of atoms from alkali-halide nanocrystals

Interband excitation of single alkali-halide nanocrystals—in the form of mass-selected ${\mathit{M}}_{\mathit{n}\pm 1}$ ${\mathit{X}}_{\mathit{n}}^{\pm }$ beams (M=K, Cs and X=Br I)—by ultraviolet radiation leads exclusively to halogen-atom emission with very large cross section. The final state is a midgap (F-center) state. A pathway consisting of direct ejection of a photoneutralized surface atom is derived from a band picture using the computed energetics of the (3,3,3) nanocrystal ${\mathit{M}}_{14}$ ${\mathit{X}}_{13}^{+}$. This mechanism is contrasted with photon-stimulated desorption from crystal surfaces, and related to formation of surface defect states and metallized overlayers.