Ion Dynamics of Plasmas Induced in Elemental Targets by Femtosecond Laser Irradiation

Abstract

The energies and number densities of ions emitted from laser-induced plasmas were studied by means of a simple time-of-flight technique. The plasmas were formed on solid elemental targets exposed to focused pulses from a Ti: Sapphire mode-locked laser and regenerative amplifier. The pulse energy and length were 1.0 mJ and 100 fs, respectively, with irradiance at the target of 2 × 1013 W/cm2. Ion kinetic energies, number densities, and mass removal rates were measured as functions of atomic mass, repetition rate, and angular orientation of the target with respect to the detector. Ion velocities varied within the 5.0–30.0 km/s range, corresponding to an average kinetic energy of 20–50 eV. Observed angular distribution of ion energy and number density obeyed the simple cosine function, which varied slightly for different elements. The total number of atoms removed per pulse was about 2 × 1013, while the ion fraction varied in the range of 3.0–5.0% for the majority of studied elements. The estimated total ion kinetic energy per pulse was ∼ 1014 eV, which corresponds to ∼ 1.5% of laser pulse energy. Mass removal rates demonstrated a direct relationship to transport properties of free electrons.