Ultrafast electron relaxation dynamics in laser-ionized gases observed with time-resolved THz spectroscopy

Ultrashort broadband THz pulses are applied to probe the electron dynamics of various gases following ionization by an intense femtosecond laser pulse. The dielectric function of the plasma is found to be Drude-like and yields the temporal evolution of the density and collision rate of the free electrons. The electron decay in a plasma with molecular ions such as O₂ ⁺ is much faster than in monatomic plasmas like Ar⁺/e^- due to dissociative recombination which is only possible in molecular plasmas. However, adding a small amount of the electron scavenger SF₆ to Ar substantially accelerates the electron decay and enables one to reliably determine the electronic temperature. Furthermore, anomalously high, metal-like electron collision rates are found. Kinetic plasma theory dramatically under-estimates these rates pointing towards additional velocity-randomizing processes like collective excitations.