Enhanced sensitivity to a possible variation of the proton-to-electron mass ratio in ammonia

TL;DR

This paper investigates ammonia's energy levels with near-degeneracies that greatly enhance sensitivity to potential variations in the proton-to-electron mass ratio, providing data to aid future astrophysical tests.

Contribution

It presents the first comprehensive calculation of sensitivity coefficients for over 350 ammonia transitions, highlighting the potential for ammonia to constrain variations in fundamental constants.

Findings

Largest sensitivity coefficients in ammonia to date.

Identification of highly sensitive transitions with T=-16,738 and T=-32 to 28.

Provision of frequency data and Einstein A coefficients for future observations.

Abstract

Numerous accidental near-degeneracies exist between the $2\nu_2$ and $\nu_4$ rotation-vibration energy levels of ammonia. Transitions between these two states possess significantly enhanced sensitivity to a possible variation of the proton-to-electron mass ratio $\mu$. Using a robust variational approach to determine the mass sensitivity of the energy levels along with accurate experimental values for the energies, sensitivity coefficients have been calculated for over 350 microwave, submillimetre and far infrared transitions up to $J\!=\!15$ for $^{14}$NH$_3$. The sensitivities are the largest found in ammonia to date. One particular transition, although extremely weak, has a sensitivity of $T=-16,738$ and illustrates the huge enhancement that can occur between close-lying energy levels. More promising however are a set of previously measured transitions with $T=-32$ to $28$. Given the astrophysical importance of ammonia, the sensitivities presented here confirm that $^{14}$NH$_3$ can be used exclusively to constrain a spatial or temporal variation of $\mu$. Thus, certain systematic errors which affect the ammonia method can be eliminated. For all transitions analyzed we provide frequency data and Einstein A coefficients to guide future laboratory and astronomical observations.