Interstellar H+3

Protonated molecular hydrogen, H-3(+), is the simplest polyatomic molecule. It is the most abundantly produced interstellar molecule, next only to H-2, although its steady state concentration is low because of its extremely high chemical reactivity. H-3(+) is a strong acid (proton donor) and initiates chains of ion-molecule reactions in interstellar space thus leading to formation of complex molecules. Here, I summarize the understandings on this fundamental species in interstellar space obtained from our infrared observations since its discovery in 1996 and discuss the recent observations and analyses of H-3(+) in the Central Molecular Zone near the Galatic center that led to a revelation of a vast amount of warm and diffuse gas existing in the region.