Directional ion emission from thin films under femtosecond laser irradiation

Thin films of nickel have been irradiated using femtosecond laser pulses in vacuum. Subsequent emission of plasma ions is diagnosed using an ion probe. Angular distributions of the emitted ions are presented for a range of target film thicknesses. Data are compared to the Anisimov model of plasma expansion [S. I. Anisimov, D. Bauerle, and B. S. Luk'yanchuk, Phys. Rev. B 48, 12076 (1993)]. The tendency of the ions to be ejected at small angles to the normal of the target surface is explained in terms of the initial conditions of the plume. Results are explained in terms of the initial shape and adiabatic index of the plasma.