Nitrogen optical emission during nanosecond laser ablation of metals: prompt electrons or photo-ionization?

Abstract

Experiments on the interaction of metal targets with a Nd:YAG laser beam ($\lambda$ = 1,064 nm, intensity $10^{10}$–$10^{11}\,\hbox {W/cm}{^2}$) are carried out in a finite Nitrogen pressure environment. The observed $\hbox {N}_2$ spectra are unambiguous evidence of the existence of an ionization and excitation source, arriving at the observation volume prior to the plume. Such a source can be either prompt electrons or VUV radiation. The analysis reveals that the prompt electron interpretation requires energies in excess of 1 keV, incompatible with any acceleration mechanisms relevant for such laser intensities. On the other hand, VUV radiation is sufficiently strong to explain the observed spectra.