Photon-Induced Desorption Processes in Astrophysical Ices
This chapter presents the photon-induced desorption processes in interstellar ice analogs simulated in dedicated laboratory experiments. In dense interstellar clouds, most molecules are expected to accrete on dust grains and remain frozen. The observed gas-phase abundances of certain molecules like carbon monoxide in dense clouds suggest that a non-thermal desorption mechanism is active in ice mantles. In addition to chemical desorption and direct cosmic-ray bombardment of the ice, photon-induced desorption is expected to drive the expected non-thermal desorption. Well inside the cloud, ice mantles are processed by the secondary-UV field generated by cosmic-ray excitation of molecular hydrogen. Because the photodesorption of volatiles like CO is less efficient in water-dominated ices, we introduce another type of photon-induced desorption, i.e., the photochemical desorption, which consists in the inmediate desorption of new molecules triggered by photons after their formation on the ice surface. Photochemical desorption operates in multi-component ice mixtures and can therefore play an important role in the real astrophysical scenario.