Step-ladder Rabi oscillations in molecules exposed to intense ultrashort vuv pulses

We theoretically investigate the production of Rabi-type oscillations in two-photon resonant ionization of H-2 by intense ultrashort vuv pulses. We show that, with such pulses, oscillations proceed through a step-ladder mechanism that successively involves higher and higher vibrational levels of the lower and upper electronic states. The consequences of this mechanism are (i) the relatively large proportion of H-2 molecules in excited electronic states not accessible by direct photon absorption, (ii) the splitting in the energy distribution of ionized electrons, and (iii) the unusually broad (non-Franck-Condon) vibrational distribution of both H-2 and H-2+.