Dynamics of the high-intensity multiphoton ionization of N2

The intensity dependence of the multiphoton photoelectron spectrum of N₂ has been obtained with subpicosecond laser pulses up to intensities of 10¹⁵ W/cm². Both Rydberg and valence states are observed to be ac Stark shifted into intermediate resonance during the photoionization process, although the valence states are seen only at low intensities. We show that the vibrational structure of the molecular Rydberg states cancels out in the photoelectron spectrum leading to sharp atomiclike series, even at high intensities. Photoelectrons from the ionization of each of the 3σ_g, 1π_u, and 2σ_u electrons are clearly distinguished. Implications of these results for the interpretation of multiphoton dissociation experiments are discussed.