Subpicosecond ion emission from transparent dielectrics

Upon irradiation of barium fluoride (111) crystals with 100- fs pulses at 800nm, explosive emission of singly charged positive ions (Ba + , F + , and larger molecules and clusters) is observed with a kinetic energy of about 100 eV and an energy distribution corresponding to a temperature of only 1 eV, independent of the ion mass. Pump-probe experiments demonstrate that ion emission is always the consequence of preceding multiphoton surface ionization, resonantly enhanced by defect states within the band gap. Yet, the ionization process appears not to be noticeably slowed down by the resonances. Electron and light microscopic investigation of the desorption crater revealed frozen surface waves with the periodicity on the order of some microns. Superimposed, we found a ripple-like periodic fine structure, with a periodicity of 100-400 nm depending on the incident laser intensity. We suppose that this should be the consequence of self-organized relaxation of the surface, rather than the consequence of an interference effect as in the classical model for ripples formation.