Emission characteristics and dynamics of neutral, ionic and molecular species in a laser produced CaF2 plasma

The emission characteristics of a laser-ablated calcium fluoride CaF2 plasma have been investigated using time and space optical emission spectroscopy. The plasma-plume was produced by focusing 266 nm, 15 ns pulses from a Nd: YAG laser on a calcium fluoride crystal substrate placed in a vacuum chamber. The emission observed in the plasma region is due to electronic relaxation of excited neutral Ca and F, ionic fragments Ca2+, Ca+ and F+ and molecular features of CaF (A(2)Pi(-) X-2 Sigma(+), B-2 Sigma(+)- X-2 Sigma(+) and C-2 Pi(-) X-2 Sigma(+)) band systems. Plasma species were analyzed both as a function of the delay from the laser pulse incidence and location from the target surface. Spectroscopic diagnostics were employed to estimate time-resolved electron densities, electron temperatures and velocity expansion of different species. The results show a faster decay of the continuum emission, ionic F+, Ca2+, Ca+, F species than in the case of neutral Ca atoms and molecular species of CaF. Different excitation temperatures were measured by comparing Ca and F atomic/ionic Saha-local thermodynamic equilibrium (LTE) spectra with experimental ones at various delay times after the ablation event. Vibrational temperatures of CaF were assessed from the temporal analysis of the A(2)Pi X-2 Sigma(+) Delta v = 0 band sequence molecular emission. We observed a non-equilibrium plasma although the plasma is found to maintain partial LTE. (C) 2021 The Author(s). Published by Elsevier Ltd.