Investigations of the plume accompanying pulsed ultraviolet laser ablation of graphite in vacuum

Abstract

The plume accompanying 193 nm pulsed laser ablation of graphite in vacuum has been studied using wavelength, time and spatially resolved optical emission spectroscopy and by complementary Faraday cup measurements of the positively charged ions. The temporal and spatial extent of the optical emissions are taken as evidence that the emitting species result from electron–ion recombination processes, and subsequent radiative cascade from the high n,l Rydberg states that result. The distribution of C neutral emission is symmetric about the surface normal, while the observed C+ emission appears localized in the solid angle between the laser propagation axis and the surface normal. However, Faraday cup measurements of the ion yield and velocity distributions, taken as a function of scattering angle and incident pulse energy, indicate that the total ion flux distribution is peaked along the surface normal. The derived ion velocity distributions are used as input for a two-dimensional model which explains the observed anisotropy of the C+ emission in terms of preferential multiphoton excitation and ionization of C species in the leading part of the expanding plasma ball that are exposed to the greatest incident 193 nm photon flux, prior to electron–ion recombination and subsequent radiative decay.