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

Robert J. Lade and Michael N.R. Ashfold.

Surface Coatings and Technology (1999).

Pulsed ArF (l = 193 nm) laser ablation of graphite in vacuo has been investigated using wavelength, spatially and temporally resolved measurements of the plume emission. Electronically excited neutral carbon atoms, and singly and doubly charged atomic carbon ions are identified amongst the ablated material. Plume expansion velocities are estimated from time gated CCD imaging of specific emissions, by measuring the time dependence of selected emissions through localised regions of space, and via complementary Langmuir probe measurements of the charged material within the plume. The relative merits and limitations of each measurement type considered. Neutral C atoms are observed to expand with a velocity distribution that is symmetrical about the surface normal. In contrast, the emission from C⁺ ions expanding in vacuo is found to maximise in the solid angle between the laser propagation axis and the surface normal. Such behaviour is rationalised in terms of resonant excitation, and subsequent multiphoton ionisation, of C atoms in the leading edge of the expanding plasma ball whilst the 193 nm laser pulse is incident.

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