Rotational spectrum of molecular ion NH^+ as a probe for alpha- and m_e/m_p-variation

TL;DR

This paper proposes NH^+ as a promising molecular ion for detecting potential variations in fundamental constants like alpha and mu, due to its unique low-lying excited states and enhanced sensitivity of certain rotational transitions.

Contribution

It identifies NH^+ as a new candidate for probing fundamental constant variations, highlighting its spectral properties and potential for astrophysical and laboratory measurements.

Findings

NH^+ has low-lying excited states close to the ground state.

Certain low-frequency transitions in NH^+ are highly sensitive to alpha and mu variations.

NH^+ may be present in interstellar space and suitable for precision spectroscopy.

Abstract

We identify the molecular ion NH^+ as a potential candidate for probing variations in the fine structure constant alpha and electron-to-proton mass ratio mu. NH^+ has an anomalously low-lying excited Sigma state, being only a few hundred cm^-1 above the ground Pi state. Being a light molecule, this proximity is such that rotational levels of the respective states are highly intermixed for low angular momenta. We find that several low-frequency transitions within the collective rotational spectrum experience enhanced sensitivity to alpha- and mu-variation. This is attributable to the close proximity of the Pi and Sigma states, as well as the ensuing strong spin-orbit coupling between them. Suggestions that NH^+ may exist in interstellar space and recent predictions that trapped-ion precision spectroscopy will be adaptable to molecular ions make NH^+ a promising system for future astrophysical and laboratory studies of alpha- and mu-variation.