We investigate the laser induced ionization and plasma formation in pure oxygen and nitrogen by means of optical pump-terahertz probe spectroscopy. Focused amplified 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 N-2 and 0.4 ps for O-2). We demonstrate the importance of the centrifugal barrier for the photoionization process using circularly polarized light. (c) 2008 Elsevier B. V. All rights reserved.