Accurate Prediction of the Ammonia Probes of a Variable Proton-to-Electron Mass Ratio

A comprehensive study of the mass sensitivity of the vibration-rotation-inversion transitions of ${}^{14}$NH${}_3$, ${}^{15}$NH${}_3$, ${}^{14}$ND${}_3$, and ${}^{15}$ND${}_3$ is carried out variationally using the TROVE approach. Variational calculations are robust and accurate, offering a new way to compute sensitivity coefficients. Particular attention is paid to the $\mathrm{\Delta }k=\pm 3$ transitions between the accidentally coinciding rotation-inversion energy levels of the ${\nu }_2=0^{+},0^{-},1^{+}$ and $1^{-}$ states, and the inversion transitions in the ${\nu }_4=1$ state affected by the “giant” $l$-type doubling effect. These transitions exhibit highly anomalous sensitivities, thus appearing as promising probes of a possible cosmological variation of the proton-to-electron mass ratio $\mu$. Moreover, a simultaneous comparison of the calculated sensitivities reveals a sizeable isotopic dependence which could aid an exclusive ammonia detection.