We investigate the laser induced ionization and plasma formation in pure oxygen
and nitrogen by means of optical pump{terahertz probe spectroscopy. Focused ampli.ed
femtosecond pulses at 405 and 810 nm are used to ionize the gas molecules by nonlinear
processes. The ionized gas is probed by picosecond terahertz pulses in the frequency range
of 0.2 - 2 THz to obtain the free electron density (10^13 - 10^17 cm.3) and the electron
scattering time (0.2 ps for N2 and 0.4 ps for O2). We demonstrate the importance of the
centrifugal barrier for the photoionization process using circularly polarized light.