A Large-scale Approach to Modelling Molecular Biosignatures: The Diatomics

TL;DR

This paper introduces a new first-principles approach using the Prometheus code to model synthetic rovibrational spectra of molecules relevant to astrophysics, especially biosignatures, improving accuracy over harmonic models.

Contribution

The work extends TOSH theory within Prometheus to model rotational constants, providing a more accurate spectral simulation for diatomic molecules of astrophysical interest.

Findings

Model achieves better approximation than harmonic methods

Comparison with HITRAN and ExoMol data validates approach

Accuracy decreases for transitions farther from the band origin

Abstract

This work presents the first steps to modelling synthetic rovibrational spectra for all molecules of astrophysical interest using a new approach implemented in the Prometheus code. The goal is to create a new comprehensive source of first-principles molecular spectra, thus bridging the gap for missing data to help drive future high-resolution studies. Our primary application domain is for molecules identified as signatures of life in planetary atmospheres (biosignatures), but our approach is general and can be applied to other systems. In this work we evaluate the accuracy of our method by studying four diatomic molecules H$_2$, O$_2$, N$_2$ and CO, all of which have well-known spectra. Prometheus uses the Transition-Optimised Shifted Hermite (TOSH) theory to account for anharmonicity for the fundamental $\nu=0 \rightarrow \nu=1$ band, along with thermal profile modeling for the rotational transitions. To this end, we expand TOSH theory to enable the modeling of rotational constants. We show that our simple model achieves results that are a better approximation of the real spectra than those produced through a harmonic approach. We compare our results with high-resolution HITRAN and ExoMol spectral data. We find that modelling accuracy tends to diminish for rovibrational transition away from the band origin, thus highlighting the need for the theory to be further adapted.