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 laser irradiation has been investigated by the time-of-flight method. The main ion species observed were the metal ions (Sr+, Ca+, Mg+), molecular ions (SrF+, CaF+ and MgO+) and F+ or O+, respectively. Nearly all observed ion species gain kinetic energies in the order of hundreds of 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 reliable and reproducible correlation with a multiphoton process, with the exception of F+ emission from SrF2 under 308 nm irradiation. Furthermore, for laser intensities well above the threshold we observed a constant total ion yield, but large fluctuations of the yields for the components (e.g. Sr+ and F+ in the case of SrF2) showing a pronounced anticorrelation. We have analyzed this behavior in terms of consecutive laser shots and found strong evidence that a laser shot does precondition the surface for the next laser shot. To obtain information about the surface composition after a single laser shot, an Ar+ ion pulse was used to analyze the top layer.