The abundance and excitation of interstellar H3+

Observations of interstellar H-3(+) test a central part of theories of ion-molecule chemistry in space. It is possible to measure the abundances of cold H-2, of H-3(+), and of CO in the same parcel of gas by means of infrared absorption spectroscopy toward obscured stars. The observed populations of the lowest rotational levels of H-2 and H-3(+) confirm that reactive collisions tend to thermalize the ortho- and para-nuclear-spin species in interstellar clouds. The rotational population distributions in CO provide diagnostic information on physical conditions (density and kinetic temperature). Taken together, observations of these three molecules make it possible to infer the rate of ionization in neutral, molecular clouds by penetrating cosmic rays or Xrays. Recent observations and their interpretation are summarized. The prospects for detecting weak, pure rotational transitions of H-3(+) are considered.