Laser-induced fluorescence study of laser sputtering of graphite

Abstract

Pulsed laser irradiation of graphite surfaces has been known for some time to lead to the ejection of C, C 2, and C 3 neutrals as well as related ions. Since most relevant thermodynamic quantities are known, graphite represents an ideal system for further study. We report on the sputtering of pyrolytic, polycrystalline, and vitreous graphite by 20 ns pulses of laser light at 351 nm. The threshold energy density for sputtering is found to be 0.5 to 0.6 J/cm 2. At this fluence, the material removal rate is of the order of a monolayer/pulse. This is consistent with pulsed evaporation provided that the surface reaches a peak temperature of ~ 4000 K. The emitted particles are probed using laser-induced fluorescence (LIF). Kinetic (i.e. translational) energies are obtained by time-of-flight and correspond, for the lowest fluences, to ~ 4600 K. Rotational and vibrational distributions are obtained by analysis of the LIF spectra for the D 1Σ u +X 1Σ g + Mulliken bands of the dimer, C 2. Detailed analysis indicates a rotational temperature of 4100 ± 300 K and a vibrational temperature of 3650 ± 350 K. Since the temperatures are all similar at the lowest it is concluded that the laser sputtering of graphite involves thermally activated vaporization, i.e. is what is normally termed “thermal sputtering”. At higher fluences, the time-of-flight information appears to be significantly perturbed by Knudsen layer formation.