General variational approach to nuclear-quadrupole coupling in rovibrational spectra of polyatomic molecules

TL;DR

This paper introduces a versatile variational algorithm to compute nuclear quadrupole hyperfine effects in the rovibrational spectra of polyatomic molecules, demonstrated on ammonia, aligning well with experimental data.

Contribution

It extends the TROVE method by incorporating nuclear quadrupole coupling without limitations on the number of quadrupolar nuclei, enabling detailed hyperfine structure calculations.

Findings

Accurately reproduces experimental hyperfine spectra of ammonia

Successfully models rovibrational spectra across multiple vibrational states

Enhances spectral assignment and astrophysical detection capabilities

Abstract

A general algorithm for computing the quadrupole-hyperfine effects in the rovibrational spectra of polyatomic molecules is presented for the case of ammonia (NH$_3$). The method extends the general variational approach TROVE by adding the extra term in the Hamiltonian that describes the nuclear quadrupole coupling, with no inherent limitation on the number of quadrupolar nuclei in a molecule. We applied the new approach to compute the nitrogen-nuclear-quadrupole hyperfine structure in the rovibrational spectrum of NH$_3$. These results agree very well with recent experimental spectroscopic data for the pure rotational transitions in the ground vibrational and $\nu_2$ states, and the rovibrational transitions in the $\nu_1$, $\nu_3$, $2\nu_4$, and $\nu_1+\nu_3$ bands. The computed hyperfine-resolved rovibrational spectrum of ammonia will be beneficial for the assignment of experimental rovibrational spectra, further detection of ammonia in interstellar space, and studies of the proton-to-electron mass variation.