I being dominant and H 2 being created transiently from association on grain surfaces. In these structures, there is a thin layer of heated, but still molecular, material, emitting more strongly than the colder, still undisturbed cloud interior. A few molecular transitions can be pumped, either collisionally or by ambient ultraviolet radiation, to give astrophysical masers . For example, the VLBA is not far from being able to measure geometric distances to the Virgo cluster via proper motions of masing region in the disks of spirals. The first ISM phase to be amenable to study is often the least important by mass, yet remains wonderfully informative - ionized gas.
Electron transitions give rise to highly diagnostic sets of emission and absorption lines.
Emission is especially useful, since it can be traced spatially using narrow-band techniques, and line ratios can tell use characteristic abundances, temperatures, and densities much more directly then other approaches such as modelling stellar atmospheres. For now, I'll note that the major processes are ionization, recombination , and collisional cooling . Narrow-band imaging can quickly trace the location of ionized gas, though extinction is often a concern. For hydrogen, this has driven work into the Paschen and Brackett lines in the near-IR, although instrumental and background limitations have made this slow going. Seeing line emission does require a source of energy input, so that hydrogen is ionized only where 13.6 eV per atom is readily available.