Photoionization of strongly correlated many-electron atoms and moleculesA hybrid-basis close-coupling interface to quantum chemistry packages

Resumen

The theoretical description of observables in attosecond experiments requires a good representation of the system's ionization continuum. For polyelectronic molecules, however, this is still a challenge, due to the complicated short-range structure of correlated electronic wave functions. Whereas quantum chemistry packages (QCP) implementing sophisticated methods to compute bound electronic molecular states are well-established, comparable tools for the continuum are not widely available yet. To tackle this problem, we have developed a new approach, XCHEM, that by means of a hybrid Gaussian-B-spline basis (GABS), interfaces existing QCPs with close-coupling scattering methods. We illustrate the performance of the GABS hybrid basis for the hydrogen atom by solving both the time-independent and the time-dependent Schrödinger equation for a few representative cases. The results are in excellent agreement with those obtained with a purely B-spline basis, with analytical results, when available, and with recent above-threshold ionization spectra from the literature. In the latter case, we report fully differential photoelectron distributions which offer further insight into the process of above-threshold ionization at different wavelengths. To illustrate the viability of the XCHEM approach, we report results for the multichannel ionization of the helium atom and of the hydrogen molecule that are in excellent agreement with existing accurate benchmarks. We also present a theoretical study of the multichannel photoionization of Ne in the vicinity of the autoionizing states lying between the 2s²2p⁵ and 2s2p⁶ ionization thresholds. The calculated total photoionization cross sections are in very good agreement with absolute measurements and with independent benchmark calculations performed at the same level of theory. From these cross sections, we have extracted resonance positions, total autoionization widths, Fano profile parameters and correlation parameters for the lowest three autoionizing states of ¹P⁰ symmetry. The values of these parameters are in good agreement with those reported in earlier theoretical and experimental work. We have also evaluated partial photoionization cross sections, and, from them, partial autoionization widths and Starace parameters for the same resonances, not yet available in the literature. We have found that the three lowest resonances preferentially decay into the 2p⁻¹εd continuum rather than into the 2p⁻¹εs one, in agreement with previous expectations, and that in the vicinity of the resonances the partial 2p⁻¹εs cross section can be larger than the 2p⁻¹εd one, in contrast with the accepted idea that the latter should amply dominate in the whole energy range. With the matrix elements obtained within XCHEM, we used the two-photon finite-pulse model for resonant transitions to compute the side bands modulation, and reproduce a very recent RABITT experiment in Ne with a very good agreement. These findings, together with the versatility of QCPs to describe a broad range of chemical systems, indicate that this is a valid approach to study the ionization of polyelectronic systems in which correlation and exchange symmetry play a major role.