Ultrashort broadband terahertz pulses are applied to probe the electron dynamics of gaseous Ar and O2 following ionization by an intense femtosecond laser pulse. The conductivity in the plasma center is extracted by a modi¯ed WKB approach. It exhibits a nearly perfect Drude-like spectral shape and yields the temporal evolution of the free-electron density and collision rate. While the electron density in the Ar plasma remains nearly constant during the first 200 ps after generation, it decays much faster in O2 due to dissociative recombination which is only possible in molecular plasmas. Adding a small amount of the electron scavenger SF6 to Ar reduces the electron lifetimein the plasma dramatically and allows to determine the electron temperature to about 20000 K. Furthermore, anomalously high, metal-like electron collision rates of up to 25 THz are derived. Kinetic plasma theory substantially underestimates these rates pointing towards additional and more complex processes randomizing the total electronic momentum. Our results are relevant to both lightning control and generation of terahertz generation by intense laser pulses in gases .
The publication is available at link DOI: 10.1103/PhysRevE.76.066401