Femtosecond laser induced Coulomb explosion from calcium fluoride

Abstract

Extending our previous work on the dynamics of femtosecond laser ablation from dielectrics, we concentrate in this contribution on calcium fluoride (111) surfaces. Upon irradiation with femtosecond pulses at intensities below the single-pulse ablation threshold we find, similar to BaF2: the necessity of a considerable incubation; a non-linear dependence of the electron- and positive-ion emission on deposited energy; positive-ion kinetic energies at the order of 10 eV; the detection of negative ions after considerably longer time of flight. This indicates an ablation mechanism similar to that for BaF2, i.e. Coulomb explosion of an electrostatically unstable surface after multiphoton ionization. However, the ionic cluster distribution and the role of positive fluorine ions appears to be different, indicating different binding properties in CaF2 and BaF2. An increase in the laser fluence results in the appearance of thermal contributions, distinguishable by a second, slower contribution to the ion velocity distribution.