Microwave and submillimeter molecular transitions and their dependence on fundamental constants

Abstract

Microwave and submillimeter molecular transition frequencies between nearly degenerated rotational levels, tunneling transitions, and mixed tunneling-rotational transitions show an extremely high sensitivity to the values of the finestructure constant, α, and the electron-to-proton mass ratio, μ. This review summarizes the theoretical background on quantum-mechanical calculations of the sensitivity coefficients of such transitions to tiny changes in α and μ for a number of molecules which are usually observed in Galactic and extragalactic sources, and discusses the possibility of testing the space-and time-invariance of fundamental constants through comparison between precise laboratory measurements of the molecular rest frequencies and their astronomical counterparts. In particular, diatomic radicals CH, OH, NH + , and a linear polyatomic radical C 3 H in electronic ground state, polyatomic molecules NH 3 , ND 3 , NH 2 D, NHD 2 , H 2 O 2 , H 3 O + , CH 3 OH, and CH 3 NH 2 in their tunneling and tunneling-rotational modes are considered. It is shown that sensitivity coefficients strongly depend on the quantum numbers of the corresponding transitions. This can be used for astrophysical tests of Einstein's Equivalence Principle all over the Universe at an unprecedented level of sensitivity of ∼10 −9 , which is a limit three to two orders of magnitude lower as compared to the current constraints on cosmological variations of α and μ: α/α < 10 −6 , μ/μ < 10 −7 .

The status of varying constants: a review of the physics, searches and implications

“…• Not all transitions are sufficiently sensitive to variations, and only relatively few astrophysical systems are clean enough to provide accurate measurements: more than one hundred lines of sight enable α measurements, though only a fraction of them are ideal (more on this in Section 8) and in any case atomic sensitivity coefficients are typically small; conversely molecular Hydrogen or other molecules enabling µ measurements are far less common, but several molecular transitions are highly sensitive to variations [49].…”

Perspective: Tipping the scales: Search for drifting constants from molecular spectra

“…32 Likewise, molecules that possess nuclear spin have hyperfine splittings that can be comparable to rotational splittings. 33 In polyatomic molecules, splittings due to classically forbidden large-amplitude motions, such as inversion [34][35][36] or internal rotation, 24,37 can be comparable to rotational splittings. Finally, the Renner-Teller splitting that originates from the interaction between electronic and vibrational angular momenta in linear polyatomic molecules, can be comparable to rovibrational splittings.…”

Limits on the spatial variations of the electron-to-proton mass ratio in the Galactic plane

“…Thus, any progress in improving the existing limits can be achieved from observations of narrow spectral lines involving higher spectral resolutions to resolve their profiles completely. At the moment, the resolution of radio telescopes exceeds that of optical facilities by order(s) of magnitude; an additional and very attractive property of microwave radio observations is that some molecular transitions from this frequency range are extremely sensitive to the putative variations of the fundamental physical constants (for a review see, e.g., Kozlov & Levshakov 2013). In 2007, Flambaum & Kozlov proposed the so-called ammonia method to test the variability of μ.…”