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 H _ { 2 } and OH also depress n ( H ^ { + } ) in partly molecular gas .
Seemingly restrictive empirical limits on \zeta _ { H } are relaxed and higher values for \zeta _ { 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 \zeta _ { H } \ga 2 \times 10 ^ { -16 } s ^ { -1 } , but such models naturally explain the presence of both HD and H _ { 3 } ^ { + } 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 .