Photoionization mechanisms of oxygen probed by terahertz pulses

We show that optical pump - terahertz (THz) probe spectroscopy is a suitable experimental tool for exploring laser induced ionization and plasma formation in gases. Plasma was produced in gaseous oxygen by focused amplified femtosecond pulses. Multi-photon versus strong-field character of the photoionization are investigated using different pump wavelengths (400 and 800 nm), polarization states and intensity. A Drude model is used to evaluate the plasma dynamics.