Bound and continuum-embedded states of cyanopolyyne anions

Abstract

Cyanopolyyne anions were among the first anions discovered in the interstellar medium. The discovery has raised questions about the routes of formation of these anions in space. Some of the proposed mechanisms assumed that anionic excited electronic states, either metastable or weakly bound, play a key role in the formation process. The verification of this hypothesis requires detailed knowledge of the electronic states of the anions. Here we investigate the bound and continuum states of four cyanopolyyne anions, CN, CN, CN, and CN, by means of ab initio calculations. We employ the equation-of-motion coupled-cluster method augmented with complex absorbing potential. We predict that already in CN, the smallest anion in the family, there are several low-lying metastable states of both singlet and triplet spin symmetry. These states, identified as shape resonances, are located between 6.3-8.5 eV above the ground state of the anion (or 2.3-4.5 eV above the ground state of the parent radical) and have widths of a few tenths of eV up to 1 eV. We analyze the identified resonances in terms of leading molecular orbital contributions and Dyson orbitals. As the carbon chain length increases in the CN series, these resonances gradually become stabilized and eventually turn into stable valence bound states. The trends in the energies of the transitions leading to both resonance and bound excited states can be rationalized by means of the Hückel model. Apart from valence excited states, some of the cyanopolyynes can also support dipole bound states and dipole stabilized resonances, owing to a large dipole moment of the parent radicals in the lowest Σ state.

Complex absorbing potentials for stark resonances

“…The 1D xenon model is solved using two different forms of CAPs: SES‐CAP, which is associated with smooth exterior scaling transformations, and the quadratic CAP, which is implemented within quantum chemistry packages for calculation of autoionization resonances . Our study suggests that the SES‐CAP is more appropriate for calculation of Stark resonances, while the quadratic CAP is less accurate.…”

“…The results obtained by using the SES‐CAP are compared with the results obtained by the quadratic CAP. The quadratic CAP is implemented within the quantum chemistry packages and is commonly used for calculation of field‐free molecular resonances . To the best of our knowledge, the first ab‐initio many‐electron study using a quadratic CAP is presented in ref.…”

EOM-CC guide to Fock-space travel: the C2 edition

“…In realvalued EOM-CCSD calculations, we used the aug-cc-pVTZ basis. In the CAP-augmented CCSD and EOM-IP-CCSD calculations, we used the aug-cc-pVTZ+3s3p and aug-cc-pCVTZ+6s6p6d basis sets (the exponents of the additional diffuse sets were generated using the same protocol as in our previous studies 54,81 ). Two core orbitals, σ 1s and σ * 1s , were frozen in correlated calculations except when employing the aug-cc-pCVTZ basis.…”

The quest to uncover the nature of benzonitrile anion

“…Among the negative ions previously detected in the interstellar medium 10 , many are carbon-chain species [11][12][13][14][15] , such as C 2n−1 N − and C 2n H − . In these systems, electron capture by the neutral molecule may involve doorway dipole-bound states [16][17][18][19][20][21] or dipole-stabilized resonances 22,23 . The large dipole moment of benzonitrile is a predictor of the existence of a dipole-bound state of its anion which may be a precursor for other chemical and photochemical pathways involving negative ions in both the laboratory environments and interstellar space.…”