Studies of the ablation plume arising in 193 nm laser irradiation of graphite in vacuum

Pulsed ArF (lambda=193 nm) laser ablation of graphite in vacuum has been in vestigated using wavelength, spatially and temporally resolved measurements of the plume emission. Electronically excited neutral carbon atoms, along with singly and doubly charged atomic carbon ions are identified amongst th e ablated material. Plume expansion velocities are estimated from time gate d CCD imaging of specific emissions, by measuring the time dependence of se lected emissions through localised regions of space, and via complementary Langmuir probe measurements of the charged material within the plume. The r elative merits and limitations of each measurement type are considered. Neu tral C atoms are observed to expand with a velocity distribution which is s ymmetrical about the surface normal. In contrast, the emission from C+ ions expanding in vacuum is found to maximise in the solid angle between the la ser propagation axis and the surface normal. Such behaviour is rationalised in terms of excitation, and subsequent multiphoton ionisation, of C atoms in the leading edge of the expanding plasma ball whilst the 193 nm laser pu lse is incident. (C) 1999 Elsevier Science S.A. All rights reserved.