LASER-INDUCED DESORPTION OF IONS FROM INSULATORS NEAR THE ABLATION THRESHOLD

The intensity dependence of the yield and kinetic energies of positive ions desorbed from SrF2, CaF2 and MgO under 193 and 308 nm excimer la ser irradiation has been investigated by the time-of-flight method. Th e main ion species observed were the metal ions (Sr+, Ca+, Mg+), molec ular ions (SrF+, CaF+ and MgO+) and F+ or O+, respectively. Nearly all observed ion species gain kinetic energies in the order of hundreds o f eV for laser intensities above the emission threshold. These results indicate that dielectric breakdown leads to the formation of a plasma at the surface. Analysis of the near threshold region has shown no re liable and reproducible correlation with a multiphoton process, with t he exception of F+ emission from SrF2 under 308 nm irradiation. Furthe rmore, for laser intensities well above the threshold we observed a co nstant total ion yield, but large fluctuations of the yields for the c omponents (e.g. Sr+ and F+ in the case of SrF2) showing a pronounced a nticorrelation. We have analyzed this behavior in terms of consecutive laser shots and found strong evidence that a laser shot does precondi tion the surface for the next laser shot. To obtain information about the surface composition after a single laser shot, an Ar+ ion pulse wa s used to analyze the top layer.