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

The plume accompanying 193 nm pulsed laser ablation of graphite in vacuum h as been studied using wavelength, time and spatially resolved optical emiss ion spectroscopy and by complementary Faraday cup measurements of the posit ively charged ions. The temporal and spatial extent of the optical emission s 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 symm etric about the surface normal, while the observed C+ emission appears loca lized in the solid angle between the laser propagation axis and the surface normal. However, Faraday cup measurements of the ion yield and velocity di stributions, taken as a function of scattering angle and incident pulse ene rgy, indicate that the total ion flux distribution is peaked along the surf ace normal. The derived ion velocity distributions are used as input for a two-dimensional model which explains the observed anisotropy of the C+ emis sion in terms of preferential multiphoton excitation and ionization of C sp ecies in the leading part of the expanding plasma ball that are exposed to the greatest incident 193 nm photon flux, prior to electron-ion recombinati on and subsequent radiative decay. (C) 2001 American Institute of Physics.