Abstract

To understand the onset of laser-induced damage on surfaces of transparent ionic materials, we have studied the effect of tunable laser light on cleaved BaF₂ (111) surfaces in ultrahigh vacuum conditions, using intensities well below the macroscopic damage threshold. Between 415 and 435 nm strong emission bands were observed for the positive ions Ba⁺, Ba²⁺, (BaF)⁺, F⁺, and for neutral fluorine. For power densities smaller than 5 × 10⁷W/cm² the emission yield varies from site to site. All active spots bleach out after a few thousand laser shots. We also find evidence that the desorption yield is sensitive to individual layers. From measurements of the photoelectron yield for various wavelengths a two-photon process was identified, indicating the existence of an occupied surface state with binding energy of about −4.3eV. Cluster calculations support this observation and predict in addition several excitonic states in the upper half of the band gap. We suggest that the wavelength-dependent desorption is triggered by a resonantly enhanced two-photon ionization from this occupied surface state leading to a positive charge-up of the surface which, in turn, results in the emission of positive ions.

© 1986 Optical Society of America