Plume emissions accompanying 248 nm laser ablation of graphite in vacuum: Effects of pulse duration

We report a comparative study of the ultraviolet laser ablation of graphite, in vacuum, using nanosecond (34 ns), picosecond (5 ps), and femtosecond (450 fs) pulses of 248 nm radiation, focusing on the plume characteristics as revealed by wavelength, time- and spatially resolved optical emission spectroscopy. Nanosecond pulsed ablation gives a distinctively different optical emission spectrum from that observed with the two shorter pulse durations. Emissions attributable to electronically excited C*, C+* and C-2* fragments are identified in the former, while the spectra obtained when using the shorter duration, higher intensity pulses contain additional lines attributable to C2+* species but none of the C* emission lines. As before [Claeyssens , J. Appl. Phys. 89, 697 (2001)], we consider that each atomic emission is a step in the radiative cascade that follows when an electron recombines with a Cn+ species (where n is one charge state higher than that of the observed emitter) formed in the original ablation process. Broadband visible radiation attributable to blackbody emission from larger particulates is also observed following ablation with any of the three laser pulse durations. Time gated imaging studies allow estimation of the velocity distributions of various of these emitting species within the plume, and their variation with incident laser fluence and/or intensity. The deduced multicomponent structure of the plume emission following excitation with short duration laser pulses is rationalized in terms of contributions from both nonthermal and thermal mechanisms for material ejection from the target. Use of longer duration (nanosecond) laser pulses offers the opportunity for additional laser-plume interactions, which we suggest are responsible for much of the observed emission in the nanosecond pulsed laser ablation of graphite. (C) 2002 American Institute of Physics.