Femtosecond time-resolved observation of above-threshold ionization in Na 2

Femtosecond pump-probe techniques are applied to the measurement of above-threshold ionization (ATI) electrons to observe the influence of vibrational wave-packet dynamics on molecular above threshold ionization in real time. When ionized by a single laser pulse structural changes in the ATI electron spectra map the changes of the vibrational wave-packet evolution in the resonant intermediate electronic states as a function of laser intensity. Molecular vibrational motion in the coherently coupled electronic states dominates the electron energy distribution rather than the energy splitting induced by Rabi oscillations.