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 O-2(+) 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 underestimates these rates pointing towards additional velocity-randomizing processes like collective excitations