Interference Structure in the Photoelectron Spectra Obtained from Multiphoton Ionization of Na₂ with a Strong Femtosecond Laser Pulse

Kinetic energy distributions of electrons obtained from photoionization of Na₂ with a femtosecond laser pulse are calculated. Strong pulses induce Rabi oscillations between electronic states of the molecule. The temporal variation of the population in the state which is resonantly coupled to the ionization continuum is transformed into interference patterns in the electron spectra. The situation can be compared to a diffraction experiment where the spatial variation of a grating is reflected in the intensity distribution of scattered particles. It is shown that for short enough pulses the molecule behaves like a three-level system.