Molecular photodissociation

TL;DR

This review discusses the processes and factors affecting molecular photodissociation in various astrophysical environments, emphasizing the importance of electronic structure, dissociation dynamics, and radiation fields.

Contribution

It systematically reviews methods for determining photodissociation rates, integrating electronic structure, dynamics, and radiation field considerations with illustrative examples.

Findings

Photodissociation is key in molecular removal in UV-exposed regions.

Accurate rates require detailed electronic and dynamical analysis.

The chapter provides systematic approaches and examples for rate determination.

Abstract

Photodissociation is the dominant removal process of molecules in any region exposed to intense ultraviolet (UV) radiation. This includes diffuse and translucent interstellar clouds, dense photon-dominated regions, high velocity shocks, the surface layers of protoplanetary disks, and cometary and exoplanetary atmospheres. The rate of photodissociation depends on the cross sections for absorption into a range of excited electronic states, as well as on the intensity and shape of the radiation field at each position into the region of interest. Thus, an acccurate determination of the photodissociation rate of even a simple molecule like water or carbon monoxide involves many detailed considerations ranging from its electronic structure to its dissociation dynamics and the specifics of the radiation field that the molecule is exposed to. In this review chapter, each of these steps in determining photodissociation rates is discussed systematically and examples are provided.