High-brilliance free-electron-laser photoionization of N{sub 2}: Ground-state depletion and radiation-field-induced modifications

Valence photoionization of N{sub 2} has been performed with high-brilliance vacuum ultraviolet radiation at h{nu}=38.5{+-}0.2 eV from the free-electron laser at Hamburg (FLASH). As a function of fluence, saturation of photoionization occurs due to the depletion of the ground-state molecular target within each femtosecond radiation pulse. With increasing fluence, the vibrational envelope of the valence photoionization spectral distribution experiences radiation-field-induced changes.