Laser-induced desorption from the (111) surface of BaF2

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.