Diatomic Rovibronic Transitions as Potential Probes for Proton-to-Electron Mass Ratio Across Cosmological Time

Abstract

Astrophysical molecular spectroscopy is an important method of searching for new physics through probing the variation of the proton-to-electron mass ratio, µ, with existing constraints limiting variation to a fractional change of less than 10 −17 /year. To improve on this constraint and therefore provide better guidance to theories of new physics, new molecular probes will be useful. These probes must have spectral transitions that are observable astrophysically and have different sensitivities to variation in the proton-to-electron mass ratio. Here, we concisely detail how astrophysical observations constrain the set of potential molecular probes and promising sensitive transitions based on how the frequency and intensity of these transitions align with available telescopes and observational constraints. Our detailed investigation focuses on rovibronic transitions in astrophysical diatomic molecules, using the spectroscopic models of 11 diatomics to identify sensitive transitions and probe how they generally arise in real complex molecules with many electronic states and fine structure. While none of the 11 diatomics investigated have sensitive transitions likely to be astrophysically observable, we have found that at high temperatures (1000 K) five of these diatomics have a significant number of low intensity sensitive transitions arising from an accidental near-degeneracy between vibrational levels in the ground and excited electronic state. This insight enables screening of all astrophysical diatomics as potential probes of proton-to-electron mass variation, with CN, CP, SiN and SiC being the most promising candidates for further investigation for sensitivity in rovibronic transitions.

The 2020 release of the ExoMol database: Molecular line lists for exoplanet and other hot atmospheres

“…Detections in the atmospheres of exoplanets are discussed below. More diverse applications include modelling laser analysis of Solar System objects [40,41] , isotope abundance quantification in stars [42] , search for molecules highly sensitive to the proton-to-electron mass ratio variation [43,44] , models of molecular steering [45,46] , design of THz lasers [47] , design of dark matter detection schemes [48] and the design of novel propulsion systems [49] .…”

PhysChem 2019: RACI Australian Conference on Physical Chemistry, Perth, 11–14 February 2019

“…The next paper in the issue, 'Diatomic Rovibronic Transitions as Potential Probes for Proton-to-Electron Mass Ratio Across Cosmological Time', [10] Laura McKemmish and coworkers propose that a suite of molecular probes might be useful in astrophysics for determining proton-to-electron mass variation. They detail how the application of molecular probes is constrained by the available telescopes and, while none of the 11 molecules they studied would be observable, that due to accidental near-degeneracies at higher temperatures, five of the species show promise.…”

Molecular diatomic spectroscopy data

“…Over the decades, there have been thousands of theoretical and computational studies of the high resolution spectroscopy of hundreds of diatomic molecules 1–3 . These data are used across the sciences including monitoring pollutants like CO, NO, SO in Earth's atmosphere, 4 understanding astrophysical objects 5 like planets 6 and stars 7 using molecules like TiO, 8 assessing the accuracy of theoretical methodologies, 9,10 creating and manipulating ultracold molecules, assessing the suitability of a molecule as a probe of variation of fundamental constants, 11,12 for laser cooling 13 and for quantum computing 14 …”