Abstract
Subject of this paper is the interaction between laser radiation and surfaces of optically transparent ionic crystals at low-power densities of the order of 106–107 W/cm2. By studying the wavelength and intensity dependencies of electron and ion-emission yields, it is shown that surface states play a decisive role for photon absorption at surfaces of divalent ionic crystals. They resonantly enhance multiphoton processes which leads to a strong-wavelength dependence of both photoemission and ion desorption. Cluster calculations confirm the existence of occupied surface states in the middle of the band gap and of unoccupied states in its upper half as well as above the ionization limit. The dependence on laser intensity reveals both two- and five-photon processes for photoemission, and ten-photon processes for the desorption of positive ions. The latter is interpreted as the creation of two holes, each by five-photon absorption, with subsequent Coulomb repulsion of a positive ion.
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