2018MNRAS.473.4986O

A robust variational approach is used to investigate the sensitivity of the rotation-vibration spectrum of phosphine (PH₃) to a possible cosmological variation of the proton-to-electron mass ratio, µ. Whilst the majority of computed sensitivity coefficients, T, involving the low-lying vibrational states acquire the expected values of T ≃ -1 and T ≃ -1/2 for rotational and ro-vibrational transitions, respectively, anomalous sensitivities are uncovered for the A₁ - A₂ splittings in the ν₂/ν₄, ν₁/ν₃ and 2ν_4^ℓ=0^/2ν4^ℓ=2^ manifolds of PH₃. A pronounced Coriolis interaction between these states in conjunction with accidentally degenerate A₁ and A₂ energy levels produces a series of enhanced sensitivity coefficients. Phosphine is expected to occur in a number of different astrophysical environments and has potential for investigating a drifting constant. Furthermore, the displayed behaviour hints at a wider trend in molecules of C_3v(M) symmetry, thus demonstrating that the splittings induced by higher-order ro-vibrational interactions are well suited for probing µ in other symmetric top molecules in space, since these low-frequency transitions can be straightforwardly detected by radio telescopes.