Investigations of breakdown of rare gases by short pulses of 1.06μm laser radiation

An analysis based on a numerical integration of the equation which governs the total growth rate of the electron density due to the combined effect of multiphoton ionization and electron cascade ionization processes is presented. The calculations are carried out for argon and xenon gases in the pressure range 1.0-5x103 Torr illuminated with 10ps pulses of 1.06μm laser radiation. This analysis shows that the two ionization processes (multiphoton and electron cascade) play an important role over the whole pressure regime considered. At low pressures (< 300 Torr) the breakdown is governed solely by the multiphoton process, indicated by the weak dependence of the threshold intensity on the gas pressure. At higher pressures (< 760 Torr) however, a strong pressure dependence of the threshold intensity is observed. This is considered as evidence for the domination of the cascade ionization process at these pressures. On the pressure range 200-760 Torr a slight decrease of the threshold intensity is observed for both gases. This is attributed to the minor role played by the cascade ionization process together with the multiphoton ionization process at these pressures. The calculated thresholds for argon and xenon are consistent with those experimentally measured. (author). 15 refs, 4 figs