2003A&A...398..621L

Atomic ions are mostly neutralized by small grains (or PAH molecules) in current theories of heating and cooling in cool diffuse clouds; in the main they do not recombine with free electrons. This alters the ionization balance by depressing n(H⁺) and n(He⁺) while carbon generally remains nearly fully once-ionized: charge exchange with atomic oxygen and formation of H₂ and OH also depress n(H⁺) in partly molecular gas. Seemingly restrictive empirical limits on ζ_H are relaxed and higher values for ζ_H are favored in a wide range of circumstances, when grain neutralization is recognized. Maintenance of the proton density at levels needed to reproduce observations of HD requires ζ_H>2x10^–16/s, but such models naturally explain the presence of both HD and H₃⁺ in relatively tenuous H I clouds. In dense gas, a higher ionization rate can account for high observed fractions of atomic hydrogen, and recognition of the effects of grain neutralization can resolve a major paradox in the formation of sulfur-bearing compounds.