We show that optical pump-terahertz probe spectroscopy is a direct experimental tool
for exploring laser induced ionization and plasma formation in gases. Plasma was pro-
duced in gaseous oxygen by focused ampli.ed femtosecond pulses. The ionization mech-
anisms at 400 nm and 800 nm excitation wavelength di.er signi.cantly being primarily
of a multiphoton character in the former case and a strong-.eld process in the latter
case. The generation of the plasma in the focal volume of the laser, and its expansion
on sub-nanosecond time scale scale is directly monitored through its density dependent
susceptibility. A Drude model used to evaluate the plasma densities and electron scatter-
ing rates successfully captures the observations for a wide range of pump intensities. In
addition, rotational .ngerprints of molecular and ionic species were also observed in the
spectra.