  The introduction of mid-IR optical parametric chirped pulse amplifiers
(OPCPAs) has catalyzed interest in multi-millijoule, 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 constructive-elastic microwave
scattering-based measurements of total electron count, electron number
densities, and photoionization rates generated by 3.9 micron femtosecond
mid-infrared tunneling ionization of atmospheric air. Consequently, we
determine photoionization rates in the range of 5.0x10$^{8}$-6.1x10$^{9}$
s$^{-1}$ for radiation intensities 1.3x10$^{13}$-1.9x10$^{14}$ W/cm$^{2}$,
respectively. The proposed approach paves the wave to precisely tabulate
photoionization rates in mid-IR for broad range of intensities and gas types
and to study plasma dynamics at mid-IR filamentation.
