2003MNRAS.343.1257P

We explore the possibility that observational differences might exist between diffuse clouds which have been formed from the dissipation of unbound molecular clouds and those which have been formed from less dense atomic gas. Using single-point chemical models, we show that molecular-rich initial conditions will significantly enrich the gas-phase chemistry of diffuse clouds in the former scenario. An injection of hydrogenated grain mantles through photoevaporation will also enrich this chemistry further. If the population of interstellar diffuse clouds contains some members that are in contraction and others that are in expansion, then wide variations in atomic and molecular abundances are to be expected in this population. We predict several signatures of this effect.