Multireference Approach to Normal and Resonant Auger Spectra Based on the One-Center Approximation

Abstract

A methodology to calculate the decay rates of normal and resonant Auger processes in atoms and molecules based on the One-Center Approximation (OCA), using atomic radial Auger integrals, is implemented within the restricted-active-space self-consistent-field (RASSCF) and the multistate restricted-active-space perturbation theory of second order (MS-RASPT2) frameworks, as part of the OpenMolcas project. To ensure an unbiased description of the correlation and relaxation effects on the initial core excited/ionized states and the final cationic states, their wave functions are optimized independently, whereas the Auger matrix elements are computed with a biorthonormalized set of molecular orbitals within the state-interaction (SI) approach. As a decay of an isolated resonance, the computation of Auger intensities involves matrix elements with one electron in the continuum. However, treating ionization and autoionization problems can be overwhelmingly complicated for nonexperts, because of many peculiarities, in comparison to bound-state electronic structure theory. One of the advantages of our approach is that by projecting the intensities on the atomic center bearing the core hole and using precalculated atomic radial two-electron integrals, the Auger decay rates can be easily obtained directly with OpenMolcas , avoiding the need to interface it with external programs to compute matrix elements with the photoelectron wave function. The implementation is tested on the Ne atom, for which numerous theoretical and experimental results are available for comparison, as well as on a set of prototype closed- and open-shell molecules, namely, CO, N 2 , HNCO, H 2 O, NO 2 , and C 4 N 2 H 4 (pyrimidine).

Disentangling the resonant Auger spectra of ozone: overlapping core-hole states and core-excited state dynamics

“…In the present study, we have given insights to and new interpretations of the experimental resonant Auger decay spectra of ozone measured at different regions of the XAS spectrum. 12,13 Our study is based on a newly implemented RASPT2 one-centerapproximation protocol 52 and on molecular dynamics simulations. For the 1s O T → π * (2b 1 ) resonant Auger spectrum, we have demonstrated the relevance of few femtosecond molecular dynamics, specially connected with a prominent feature observed at around 512 eV, emerging after 4 fs from the creation of the corehole state.…”

“…50 The most practical choice, which we adopt here, is to use, as MBS, the first fully contracted functions of the atomic Gaussian Type orbital (GTO) basis, which are accurate representations of the atomic orbitals. 52 For further details about our OCA-RASPT2 implementation and the automed selection of a MBS, please consult Tenorio et al. 52…”

“…52 For further details about our OCA-RASPT2 implementation and the automed selection of a MBS, please consult Tenorio et al. 52…”

“…At its ground state the ozone molecule belongs to the C 2v point group symmetry with experimentally determined structural parameters 80 Similar to what we did in previous studies, 52,66 to facilitate the application and analysis of the OCA projection on symmetry equivalent atomic sites, we have reduced the point group symmetry from C 2v to C s , and localized the core orbitals by applying a Cholesky localization procedure. 81 Moreover, we find it particularly convenient to use C s symmetry because we calculate Auger spectra along a (dissociative) molecular dynamics trajectory, where C s symmetry can always be applied.…”

“…Here we use the OCA to evaluate Auger decay rates based on a restricted-active-space perturbation theory of second order (RASPT2) description of the bound states. We recently implemented this OCA-RASPT2 approach 52 within the OpenMolcas program. 60 More specifically, we here demonstrate the capabilities of OCA-RASPT2 on the resonant and non-resonant Auger spectra of ozone, especially in a situation of overlapping core-hole states, as observed for the 1s O C → σ*(7a 1 ) and 1s O T −1 states.…”