Theoretical studies of cross sections and photoelectron angular distributions in the valence photoionization of molecular oxygen

Coupled nuclear-electronic decay dynamics of O2 inner valence excited states revealed by attosecond XUV wave-mixing spectroscopy

Multiple Rydberg series converging to the O2+c4Σ-u state, accessed by 20-25 eV extreme ultraviolet (XUV) light, serve as important model systems for the competition between nuclear dissociation and electronic autoionization. The dynamics of the lowest member of these series, the 3sσg state around 21 eV, has been challenging to study owing to its ultra-short lifetime (<10 fs). Here, we apply transient wave-mixing spectroscopy with an attosecond XUV pulse to investigate the decay dynamics of this electronic state. Lifetimes of 5.8 ± 0.5 fs and 4.5 ± 0.7 fs at 95% confidence intervals are obtained for v = 0 and v = 1 vibrational levels of the 3s Rydberg state, respectively. A theoretical treatment of predissociation and electronic autoionization finds that these lifetimes are dominated by electronic autoionization. The strong dependence of the electronic autoionization rate on the internuclear distance because of two ionic decay channels that cross the 3s Rydberg state results in the different lifetimes of the two vibrational levels. The calculated lifetimes are highly sensitive to the location of the 3s potential with respect to the decay channels; by slight adjustment of the location, values of 6.2 and 5.0 fs are obtained computationally for the v = 0 and v = 1 levels, respectively, in good agreement with experiment. Overall, an intriguing picture of the coupled nuclear-electronic dynamics is revealed by attosecond XUV wave-mixing spectroscopy, indicating that the decay dynamics are not a simple competition between isolated autoionization and predissociation processes.

Resonant photoionization of O2 up to the fourth ionization threshold

We present a detailed theoretical study of valence-shell photoionization of the oxygen molecule by using the recently proposed XCHEM method. This method makes use of a hybrid Gaussian and B-spline basis in the framework of a close-coupling approach to describe electron correlation in the molecular electronic continuum at a level comparable to that provided by multi-reference configuration interaction methods in bound state calculations. The computed total and partial photoionization cross sections are presented and discussed, with emphasis on the series of autoionizing resonances that appear between the first and the fourth ionization thresholds, i.e., photon energies between 12 and 18 eV. More than fifty autoionizing states are identified, including series not previously reported in the literature, and their energy positions and widths are provided. The present results illustrate the potential of the XCHEM approach to accurately describe molecular autoionization, which is mostly due to electron correlation. This is relevant in view of current experimental efforts aimed at providing real-time (attosecond) imaging of autoionization dynamics in molecules.