Abstract
A diffuse cloud is defined as an interstellar cloud with typical H2 (or H) density of 10–100 cm−3 and typical visual extinction of a few magnitude. Physical and chemical conditions are significantly affected by interstellar ultraviolet radiation. Chemical compositions of diffuse clouds are summarized on the basis of observational results. Chemical processes in diffuse clouds are discussed for major chemical elements including C, O, and N, on the basis of timescales of chemical reactions. Since the timescale of photodissociation is very fast, chemical compositions of diffuse clouds are almost in chemical equilibrium. Observational results are compared with steady-state chemical models reported previously. Outstanding problems such as the CH+ problem, the H3 + problem, and diffuse interstellar bands are introduced.
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Yamamoto, S. (2017). Chemistry of Diffuse Clouds. In: Introduction to Astrochemistry. Astronomy and Astrophysics Library. Springer, Tokyo. https://doi.org/10.1007/978-4-431-54171-4_4
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