Resonant spectra of quadrupolar anions

TL;DR

This paper investigates the properties of quadrupolar anions, revealing their halo nature, rotational band formation, and phase transition behavior using advanced quantum formalism, thus providing insights into open quantum systems.

Contribution

It introduces a non-adiabatic coupled-channel approach with the Berggren ensemble to analyze quadrupolar anions, highlighting their halo characteristics and transition phenomena.

Findings

Quadrupolar anions follow halo scaling laws.

Rotational bands extend into the continuum.

Critical behavior resembles a second-order phase transition.

Abstract

In quadrupole-bound anions, an extra electron is attached at a sufficiently large quadrupole moment of a neutral molecule, which is lacking a permanent dipole moment. The nature of the bound states and low-lying resonances of such anions is of interest for understanding the threshold behavior of open quantum systems in general. In this work, we investigate the properties of quadrupolar anions as extreme halo systems, the formation of rotational bands, and the transition from a subcritical to supercritical electric quadrupole moment. We solve the electron-plus-molecule problem using a non-adiabatic coupled-channel formalism by employing the Berggren ensemble, which explicitly contains bound states, narrow resonances, and the scattering continuum. We demonstrate that binding energies and radii of quadrupolar anions strictly follow the scaling laws for two-body halo systems. Contrary to the case of dipolar anions, ground-state band of quadrupolar anions smoothly extend into the continuum, and many rotational bands could be identified above the detachment threshold. We study the evolution of a bound state of an anion as to dives into the continuum at a critical quadrupole moment and we show that the associated critical exponent is consistent with the second-order phase transition. Everything considered, quadrupolar anions represent a perfect laboratory for the studies of marginally bound open quantum systems.