Atomic and molecular processes in interstellar shocks

Shocks propagate in the interstellar gas, including those regions in which molecules exist. The passage of a shock wave modifies the rates of physical and chemical processes occurring in the gas, with important consequences for its composition. The author discusses the chemistry of carbon- and oxygen-bearing species under the conditions appropriate to interstellar shocks. The collisional excitation of rotational transitions in molecules and of fine-structure transitions in atoms, which play key roles in the thermal balance of the medium, are also considered. So too are the effects of shocks on the degree of chemical fractionation of deuterium, a process which can modify considerably the ratios of deuterium- and hydrogen-bearing isotopes in interstellar clouds.