Characterization of Astrophysical molecules: OH and OD

Abstract

Spectroscopic studies are crucial for understanding the atmospheres of planets and stars, providing insights into their components and dynamics. Astronomical spectra of different species (atomic and molecular) belonging to a given atmosphere are usually used to monitor their abundance variations at different altitudes and, as a consequence, are a crucial tool for understanding the photochemistry of the investigated medium. However, a deep knowledge of the species' emission properties and their line list is essential to understanding such spectra. The hydroxyl radical (OH) is a significant constituent of cometary gases and planetary atmospheres, including the terrestrial troposphere and stratosphere [1], [2] . This molecule exhibits a wide range of thermochemical properties and plays a vital role in several chemical processes taking place in different astronomical environments [3].

Until now, different spectrometers onboard spacecraft have identified the presence and abundance of OH in different media [4], [5] by using the radical's known spectral signatures. However, surprisingly, despite all the current experimental and theoretical work dedicated to understanding its electronic structure [6], [7], [8], [9], [10], not all of its rovibronic transitions have been theoretically identified in the literature, especially in the UV region of the spectrum.

In this work, we unravel this missing information for the first time by generating a comprehensive spectroscopic model for OH radical and its isotopologue OD using high-level CASSCF/MRCI+Q ab initio calculations. We specifically investigate the rovibronic lines among the B²Σ⁺ - X²Π and D²Σ⁻ - X²Π transitions. These are obtained using adiabatic potential energy functions (PEF) and transition dipole moment functions (TDMF), calculated in the representation ²S⁺¹Λ^(±). The computed transition's radiative lifetimes and synthetic spectra show strong agreement with previously published experimental data, demonstrating the accuracy of the calculations. These results should be of high importance to spectroscopists interested in the radical's properties and constitute an essential tool for its detection in different atmospheres.

Date of Award	14 Dec 2024
Original language	American English
Supervisor	Nayla El Kork (Supervisor)