Injection of molecules from the surfaces of grains into the gas phase chemistry of an expanding cloud

We present model results for the chemistry in an expanding cloud or clump in which molecules are injected into the gas phase from grain surfaces when the clump reaches a certain visual extinction A^sub v^ during the expansion. We consider separately injection at two different values of A^sub v^, and include a representative large hydrocarbon, C^sub 6^H, and sulphur-bearing molecule, H^sub 2^SO^sub 4^, as well as H^sub 2^O and CO. We examine the abundances of certain molecules which have been observed in diffuse and translucent clouds, and compare the results obtained for these abundances with and without an injection during expansion. We also compare our results withpublished observations, and conclude that in most clouds an injection of molecules has occurred.[PUBLICATION ABSTRACT]