Rotational spectrum of the molecular ion NH+ as a probe for alpha and m(e)/m(p) variation

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 (4)Sigma(-) state, being only a few hundred cm(-1) above the ground (2)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 (2)Pi and (4)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.