Studies of the plume emission during the femtosecond and nanosecond ablation of graphite in nitrogen

Abstract

Comparative studies of the pulsed laser ablation of graphite in 20 mTorr of N-2 using both 15 ns and 450 fs pulses at a wavelength of 248 nm are reported. Emissions from the resulting ablation plumes, and from collisions with ablated material and the background N-2 gas molecules, have been investigated by wavelength-, space-, and time-resolved optical emission spectroscopy (OES), and the observations correlated with the results of the analyses of films formed when such material is incident on a silicon substrate. Wavelength-dispersed spectra of the plume arising in nanosecond ablation reveal C I, C II, and C-2 emissions-concentrated close to the target-and, at greater distances, strong CN and weak N-2(+) emissions. N-2(+)(B-X) emission dominates in the case of femtosecond ablation. Time-gated imaging studies have allowed estimation of propagation velocities for these various emissions. Possible production routes for secondary emitters such as CN and N-2(+) are discussed, and arguments presented to show that measurements of the apparent propagation "velocities" of such emissions are unlikely to provide meaningful measures of the velocities (or energies) with which these carriers impact on a substrate surface. Laser Raman spectroscopy confirms nitrogen incorporation within the films grown by both nanosecond and femtosecond ablations; the former films are deduced to be both thicker and to have higher N content-findings that accord with the OES analyses.

Document Downloads