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

Comparative studies of the pulsed laser ablation of graphite in 20mTorr of N2 using both 15ns and 450fs pulses at a wavelength of 248nm are reported. Emissions from the resulting ablation plumes, and from collisions with ablated material and the background N2 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 CI, CII, and C2 emissions—concentrated close to the target—and, at greater distances, strong CN and weak N2+ emissions. N2+(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 N2+ are discussed, and arguments presented to show that measurements of...