[en] Photoelectron angular distributions from fixed-in-space H₂ molecules exposed to ultrashort xuv laser pulses have been evaluated. The theoretical method is based on the solution of the time-dependent Schroedinger equation in a basis of stationary states that include all electronic and vibrational degrees of freedom. Asymmetric angular distributions are observed as a consequence of the delayed ionization from the H₂ doubly excited states, which induces interferences between gerade and ungerade ionization channels. The analysis of this asymmetry as a function of pulse duration can provide an estimate of the corresponding autoionization widths.

[en] We show the relevance that molecular autoionizing states display in some recent experiments related to the symmetry-breaking in molecular-frame photoelectron angular distributions in H₂ when exposed to intense xuv femtosecond laser pulses, and others related to the electron (proton) localization when subject to attosecond pump-probe laser schemes. Our theoretical method solves the time-dependent Schroedinger equation with an spectral method that expands the wave function in terms of H₂ correlated stationary vibronic states including all electronic and vibrational degrees of motion. Time-resolved asymmetric electron angular distributions are obtained at specific proton kinetic energies due to the delayed autoionization from H₂ doubly excited states, which induces interferences between gerade (1sσ_g) and ungerade (2pσ_u) ionization channels. We also study photoionization of H₂ exposed to a xuv attosecond pump pulse plus a time-delayed IR femtosecond probe pulse. Fast alternating asymmetries in the proton ejection (electron localization) are obtained as a function of the time delay between the pump and the probe pulses. Finally, we deal with the process of (xuv) two-photon double ionization of H₂ under the assumption of having both sequential and non-sequential absorption processes.

[en] A theoretical study of two-photon ionization of H₂ by low-intensity ultrashort xuv laser pulses is reported. The method is based on the solution of the time-dependent Schroedinger equation in a basis of stationary molecular vibronic states which include all electronic and vibrational degrees of freedom. In contrast with previous work, the Q₁ doubly excited states, which also contribute to the ionization probabilities through autoionization, are explicitly included. We have found that, just below the one-photon ionization threshold, molecular autoionization leads to an enhancement of dissociative ionization, whose corresponding probability can be an order of magnitude larger than that of the nondissociative ionization process, and even larger than the corresponding dissociative probability in the one-photon absorption region. This result suggests that multicoincidence experiments, in which the orientation of the molecule with respect to the polarization axis is determined, might be easier to perform in the two-photon absorption regime than in the one-photon absorption regime. Electron angular distributions in the same range of photon energies are also reported.

[en] Ab initio calculations of H₂ photoionization induced by low-intensity ultrashort xuv laser pulses are reported in the region of two-photon absorption. Our computational approach is based on the solution of the time-dependent Schroedinger equation including all electronic and vibrational degrees of freedom. We find that, as one approaches the one-photon ionization threshold, dissociative ionization largely dominates over non dissociative ionization. We show that this behavior is mostly induced by the presence of molecular autoionizing states in this photon energy region.

[en] A simple theoretical model is used to interpret recent experimental results for two-photon double ionization (DI) of D₂ at 38 eV. We show that the measured kinetic energy distribution associated with emission of two protons can be interpreted as a sum of two processes: a sequential and an instantaneous absorption of the two incident photons. These processes lead to peaks in different regions of the spaectrum.

[en] We used a split-mirror setup attached to a reaction microscope at the free-electron laser in Hamburg (FLASH) to perform an XUV-pump-XUV-probe experiment by tracing the ultrafast nuclear wave-packet motion in the D₂⁺(1sσ_g) with <10 fs time resolution. Comparison with time-dependent calculations shows excellent agreement with the measured vibrational period of 22±4 fs in D₂⁺, points to the importance of accurately knowing the internuclear distance-dependent ionization probability, and paves the way to control sequential and nonsequential two-photon double-ionization contributions.

[en] Two-photon double ionization (TPDI) of D₂ is studied for 38-eV photons at the Free Electron Laser in Hamburg (FLASH). Based on model calculations, instantaneous and sequential absorption pathways are identified as separated peaks in the measured D⁺+D⁺ fragment kinetic energy release (KER) spectra. The instantaneous process appears at high KER, corresponding to ionization at the molecule's equilibrium distance, in contrast to sequential ionization mainly leading to low-KER contributions. Measured fragment angular distributions are in good agreement with theory.