ExoMol photodissociation cross sections I: HCl and HF

TL;DR

This paper computes temperature-dependent photodissociation cross sections for HCl and HF, highlighting their variation with stellar radiation fields and providing data crucial for modeling exoplanet atmospheres.

Contribution

It introduces a new series of temperature-dependent photodissociation cross sections for key molecules, computed by solving the Schrödinger equation, and discusses their implications for exoplanet atmospheric chemistry.

Findings

Photodissociation rates increase exponentially for temperatures above 1000 K in Sun-like and cooler star fields.

In UV-rich stellar environments, photodissociation rates are nearly temperature-independent.

Rates are highly sensitive to the radiation model used for cool stars.

Abstract

Photon initiated chemistry, \textit{i.e.} the interaction of light with chemical species, is a key factor in the evolution of the atmosphere of exoplanets. For planets orbiting stars in UV-rich environments, photodissociation induced by high energy photons dominates the atmosphere composition and dynamics. The rate of photodissociation can be highly dependent on atmospheric temperature, as increased temperature leads to increased population of vibrational excited states and the consequent lowering of the photodissociation threshold. This paper inaugurates a new series of papers presenting computed temperature-dependent photodissociation cross sections with rates generated for different stellar fields. Cross sections calculations are performed by solving the time-independent Schr\"{o}dinger equation for each electronic state involved in the process. Here photodissociation cross sections for hydrogen chloride and hydrogen fluoride are computed for a grid of 34 temperatures between 0 and 10~000 K. Use of different radiation fields shows that for the Sun and cooler stars the photodissociation rate can increase exponentially for molecular temperatures above 1000 K; conversely the photodissociation rates in UV rich fields instead are almost insensitive to the temperature of the molecule. Furthermore, these rates show extreme sensitivity to the radiation model used for cool stars, suggesting that further work on these may be required. The provision of an ExoMol database of cross sections is discussed.