Two-photon ionization of H₂⁺ by short laser pulses

We present a theoretical study of dissociative multiphoton ionization of the H₂⁺ molecular ion in perturbative and nonperturbative regimes including both electronic and nuclear degrees of freedom. Differential (in proton and electron energy) ionization cross sections have been evaluated for various photon energies, laser intensities, and pulse lengths. We have found that the proton energy distribution is modulated by vertical Franck-Condon transitions but also by vibrational resonances associated with intermediate electronic states. We have also found that, as expected, nonperturbative results tend to the time-independent perturbative ones when both the pulse length increases and the laser intensity decreases. No divergence near intermediate-state resonances is found in the perturbative results when the nuclear motion is properly taken into account in the calculations.