We report the observation of a $\ensuremath{\pi}$-type dipole-bound state ($\ensuremath{\pi}$-DBS) in cryogenically cooled deprotonated 9-anthrol molecular anions ($9{\mathrm{AT}}^{\ensuremath{-}}$) by resonant two-photon photoelectron imaging. A DBS is observed $191\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}1}$ (0.0237 eV) below the detachment threshold, and the existence of the $\ensuremath{\pi}$-DBS is revealed by a distinct ($s+d$)-wave photoelectron angular distribution. The $\ensuremath{\pi}$-DBS is stabilized by the large anisotropic in-plane polarizability of 9AT. The population of the dipole-forbidden $\ensuremath{\pi}$-DBS is proposed to be via a nonadiabatic coupling with the dipole-allowed $\ensuremath{\sigma}$-type DBS mediated by molecular rotations.

Observation of a π-Type Dipole-Bound State in Molecular Anions.

The diffusion Monte Carlo (DMC), auxiliary field quantum Monte Carlo (AFQMC), and equation-of-motion coupled cluster (EOM-CC) methods are used to calculate the electron binding energy (EBE) of the non-valence anion state of a model (H$_2$O)$_4$ cluster. Two geometries are considered, one at which the anion is unbound and the other at which it is bound in the Hartree-Fock (HF) approximation. It is demonstrated that DMC calculations can recover from the use of a HF trial wave function that has collapsed onto a discretized continuum solution, although larger electron binding energies are obtained when using a trial wave function for the anion that provides a more realistic description of the charge distribution, and, hence, of the nodal surface. For the geometry at which the cluster has a non-valence correlation-bound anion, both the inclusion of triples in the EOM-CC method and the inclusion of supplemental diffuse d functions in the basis set are important. DMC calculations with suitable trial wave functions give EBE values in good agreement with our best estimate EOM-CC result. AFQMC using a trial wave function for the anion with a realistic electron density gives a value of the EBE nearly identical to the EOM-CC result when using the same basis set. For the geometry at which the anion is bound in the HF approximation, the inclusion of triple excitations in the EOM-CC calculations is much less important. The best estimate EOM-CC EBE value is in good agreement with the results of DMC calculations with appropriate trial wave functions.

The Role of High-Order Electron Correlation Effects in a Model System for Non-valence Correlation-bound Anions

The diffusion Monte Carlo (DMC), auxiliary field quantum Monte Carlo (AFQMC), and equation-of-motion coupled cluster (EOM-CC) methods are used to calculate the electron binding energy (EBE) of the non-valence anion state of a model (H2O)4 cluster. Two geometries are considered, one at which the anion is unbound and the other at which it is bound in the Hartree–Fock (HF) approximation. It is demonstrated that DMC calculations can recover from the use of a HF trial wave function that has collapsed onto a discretized continuum solution, although larger EBEs are obtained when using a trial wave function for the anion that provides a more realistic description of the charge distribution and, hence, of the nodal surface. For the geometry at which the cluster has a non-valence correlation-bound anion, both the inclusion of triples in the EOM-CC method and the inclusion of supplemental diffuse d functions in the basis set are important. DMC calculations with suitable trial wave functions give EBE values in good agreement with our best estimate EOM-CC result. AFQMC using a trial wave function for the anion with a realistic electron density gives a value of the EBE nearly identical to the EOM-CC result when using the same basis set. For the geometry at which the anion is bound in the HF approximation, the inclusion of triple excitations in the EOM-CC calculations is much less important. The best estimate EOM-CC EBE value is in good agreement with the results of DMC calculations with appropriate trial wave functions.

The role of high-order electron correlation effects in a model system for non-valence correlation-bound anions.

The equation-of-motion coupled cluster method is used to characterize the low-lying anion states of (NaCl)2 in its rhombic structure. This species is known to possess a non-valence bound anion of Ag symmetry. Our calculations also demonstrate that it has a non-valence temporary anion of B2u symmetry, about 14 meV above threshold. The potential energy curves of the two anion states and of the ground state of the neutral molecule are reported as a function of distortion along the symmetric stretch normal coordinate. Implications for experimental detection of the temporary anion state are discussed. The sensitivity of the results to the inclusion of high-order correlation effects and of core correlation is examined.

Josiah S Gowen

Papers

Prediction of a Non-Valence Temporary Anion State of (NaCl)2