Rotational spectrum of molecular ion NH+ as a probe for alpha- and me/mp-variation
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.