in the diffuse interstellar medium

Three forms of solely hydrogen-bearing molecules—H 2 , HD and —are observed in diffuse or optically transparent interstellar clouds. Although no comprehensive theory exists for the diffuse interstellar medium or its chemistry, the abundances of these species can generally be accommodated locally within the existing static equilibrium frameworks for heating/cooling, H 2 -formation on large grains, etc. with one modification demanded equally by observations of HD and , i.e. a pervasive low-level source of H and H 2 ionization ca 10 times faster than the usual cosmic ray ionization rate ζ H ≈10 −17 s −1 per free H-atom. We discuss this situation with reference to observation and time-dependent modelling of H 2 and formation. While not wishing to appear ungrateful for the success of what are very simplistic notions of the interstellar medium, we point out several reasons not to feel smug. The equilibrium conditions which foster high H 2 and abundances are very slow to appear and these same simple ideas of static equilibrium cannot explain any, but a few, of the simplest of the trace species, which are ubiquitously embedded in H 2 -bearing diffuse gases.