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

Experiments on the interaction of metal targets with a Nd:YAG laser beam ((Formula presented.) = 1,064 nm, intensity (Formula presented.)-(Formula presented.)) are carried out in a finite Nitrogen pressure environment. The observed (Formula presented.) 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. © 2014 Springer-Verlag Berlin Heidelberg.