PHOTON-STIMULATED AND ELECTRON-STIMULATED DESORPTION OF EXCITED ALKALI-METAL ATOMS FROM ALKALI-HALIDE SURFACES

We report and review progress in photon- and electron-stimulated desorption studies of excited-state alkali-metal atoms from alkali halide crystal surfaces. Recent experimental results indicate the existence of two temperature regimes each having a different mechanism for the excited-atom production. In the low-temperature regime, excited-atom desorption is shown to depend on the presence of neutral metal atoms on the surface. In the high-temperature regime excited-state production involves gas-phase excitation of the ground-state alkali-metal atoms. Based on experimental results, we propose two separate and distinct mechanisms for the excited-alkali-metal-atom production. The first proposed mechanism appropriate to the low-temperature regime for both electron- and photon-stimulated desorption, assumes that surface exoergic reactions between alkali dimers and halogen atoms produce desorbed excited alkali-metal atoms. The surface reactants are formed through an irradiation-initiated and defect-mediated process. The second proposed mechanism assumes that the gas-phase excitations of ground-state alkali-metal atoms by primary electrons, and perhaps to a small extent by secondary electrons, produce the excited alkali-metal atoms. The gas-phase excitation model applies to high-temperature electron-stimulated desorption, where the desorption yield for ground-state alkali-metal atoms is high.