Resonance effects in two-photon double ionization of H 2 by femtosecond XUV laser pulses

We investigate the effect of the pulse length on the two-photon double ionization (DI) of H${}_{2}$ in the direct domain, for a femtosecond (fs) laser with a polarization vector oriented along the molecular axis. In the fixed-nuclei approximation, we find that the doubly excited ${Q}_{1}\phantom{\rule{0.28em}{0ex}}{}^{1}{\ensuremath{\Sigma}}_{u}^{+}$ states manifest themselves as resonances in the angle-integrated cross sections if the laser interaction lasts longer than about 3 fs. Decay into single-ionization channels does not significantly affect the shape of the angular distribution. A sharp rise in the probability for DI, due to virtual sequential ionization, occurs when the photon energy approaches the threshold for sequential DI.