Time-of-flight investigation of the intensity dependence of laser-desorbed positive ions from SrF2

The intensity dependence of the total and specific yields of positive ions desorbed from SrF₂ under 193 nm and 308 nm excimer-laser irradiation has been investigated by the time-of-flight method. The following positive ion species have been detected: F⁺, Sr⁺, Sr⁺⁺, SrF⁺⁺ and SrF ₂ ⁺ . The Sr⁺ and SrF⁺ emission yields are found to increase as E ⁿ, where E represents the laser energy per pulse. The exponent n is related to defect-initiated neutral particle emission and gas-phase ionization. The influence of surface damage on this power dependence is investigated. The F⁺ emission yield showed a quite different behaviour compared to that of the Sr⁺ and SrF⁺ emission. At both wavelengths the total positive ion emission yields saturate at a certain laser energy. In the saturation regime the SrF⁺ emission vanishes and alternative emission of F⁺ and Sr⁺ was observed at both wavelengths, but the total emission yield in the saturation regime (F⁺ + Sr⁺) remained constant. A Scanning Electron Microscope (SEM) was used to investigate the damage spots after laser irradiation for thermal effects.