Ionization rate and plasma dynamics at 3.9 micron femtosecond photoionization of air

The introduction of mid-IR optical parametric chirped pulse amplifiers has catalyzed interest in multimillijoule, infrared femtosecond pulse-based filamentation. As tunneling ionization is a fundamental first stage in these high-intensity laser-matter interactions, characterizing the process is critical to understand derivative topical studies on femtosecond filamentation and self-focusing. Here, we report direct nonintrusive measurements of total electron count and electron number densities generated at 3.9 μm femtosecond midinfrared tunneling ionization of atmospheric air using constructive-elastic microwave scattering. Subsequently, we determine photoionization rates to be in the range 5.0×108-6.1×109s-1 for radiation intensities of 1.3×1013-1.9×1014W/cm2, respectively. The proposed approach paves the wave to precisely tabulate photoionization rates in mid-IR for a broad range of intensities and gas types and to study plasma dynamics at mid-IR filamentation.