Dense, cold, nonequilibrium plasma states in nanosecond-scale pulsed laser microdischarges

We describe studies of the generation of dense non-equilibrium plasma states in atmospheric pressure air driven by pulsed, nanosecond laser discharges. In our most recent studies, we generate initial dense, near-Loschmidt level, plasmas using a Nd:YAG laser (15 ns, 15 mJ, 532 nm) and these are followed by second-stage electron heating to further elevate the electron/ion density using a relatively low energy picosecond laser (20 ps, 1.2 mJ, 532 nm). A third, continuous-wave HeNe laser (10 mW, 632.8 nm), is used to record time-resolved (but line-of-sight averaged) inverse Bremsstrahlung absorption from which electron number density is inferred. The analysis of the data relies weakly on estimates of electron temperature obtained from the continuous background emission in the visible range of the spectrum. Comparisons are made to electron density inferred from Stark broadening the OI 777 nm line. We find that second-stage picosecond laser heating elevates the average electron density by approximately 20 %. A simple model analysis suggests that the level of ionization in air extends into doubly and possibly triply-ionized states.