2003ApJ...584..331G

Because H₂formation on dust-grain surfaces completely dominates gas-phase H₂formation in local molecular clouds, it is often assumed that gas-phase formation is never important. In fact, it is the dominant mechanism in a number of cases. In this paper, I briefly summarize the chemistry of gas-phase H₂ formation and show that it dominates for dust-to-gas ratios less than a critical value D_cr. I also show that D_cris simple to calculate for any given astrophysical situation and illustrate this with a number of examples, ranging from H₂formation in warm atomic gas in the Milky Way to the formation of protogalaxies at high redshift.