ExoMol photodissociation cross-sections - I. HCl and HF

Photon initiated chemistry, 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 Schrodinger 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 data base of cross-sections is discussed.

Automated summary

Photon initiated chemistry plays a crucial role in the evolution of exoplanetary atmospheres. This paper presents temperature-dependent photodissociation cross-sections and rates for hydrogen chloride and hydrogen fluoride, investigating the impact of atmospheric temperature and radiation fields on photodissociation. The results show that the photodissociation rate is highly dependent on temperature and the radiation model used for cool stars.