H3+ in dense and diffuse clouds.

Interstellar H3+ has been detected in dense as well as diffuse clouds using three 3.7 microns infrared spectral lines of the nu 2 fundamental band. Column densities of H3+ from (1.7-5.5) x 10(14) cm-2 have been measured in dense clouds in absorption against the infrared continua of the deeply embedded young stellar objects GL2136, W33A, MonR2 IRS 3, GL961E, and GL2591. Strong and broad H3+ absorptions have been detected in dense and diffuse clouds towards GC IRS 3 and GCS3-2 in the region of the galactic center. A large column density of H3+, comparable to that of a dense cloud, has been detected towards the visible star Cygnus OB2 No. 12, which has a line of sight that crosses mostly diffuse clouds. The H3+ chemistry of dense and diffuse clouds are discussed using a very simple model. Some future projects and problems are discussed.