Rotational spectrum of the molecular ion NH+ as a probe for α and me/mp variation

We identify the molecular ion NH⁺ as a potential candidate for probing variations in the fine-structure constant α and electron-to-proton mass ratio μ. NH⁺ has an anomalously low-lying excited 4Σ^- state, being only a few hundred cm^-1 above the ground 2Π 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 α and μ variation. This is attributable to the close proximity of the 2Π and 4Σ^- 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 α and μ variation.