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We present models of the chemical evolution of molecular clouds in regions of enhanced ultraviolet flux. We include the major gas phase formation pathways for carbon-, oxygen-, and nitrogen-bearing species. The model also treats the depletion and evaporation of species onto the surfaces of grains. A variety of desorption processes have been examined in order to asses their effectiveness in removing molecules or atoms from the grain mantles. A wide range of parameter space has been explored to examine the abundance of species present both on the grain mantle and in the gas phase as a function of both position in the cloud (visual extinction) and of evolutionary state. The chemical models show a complex interaction between the gas and surface phase with systematic differences in the grain mantle properties between the exterior and interior regions of the cloud. The models suggest that understanding the history of grain mantle growth is critical to unraveling the chemical evolution of molecular clouds.
The research of WDL was carried out by JPL-Caltech under contract with NASA.
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