H3+ (protonated molecular hydrogen) is an important astromolecule
— so important that it has its own comprehensive website.
For example, it is a source of protons in many astrochemical reactions. However, H3+
cannot be detected via rotational or
electronic spectroscopy. Its only observable spectroscopic features are vibrational in nature and
fall in the infrared. H3+ was not identified in extraterrestrial sources until
the late 1980s. After being found in the ionospheres of three gas giants —
it was observed toward protostars GL2136 and W33A in 1996 by
Geballe & Oka and then in the diffuse ISM
toward Cynus OB2-12 by
McCall et al.
H3+ has three vibrational modes that are shown below.
The first two are degenerate
asymmetric deformation modes, and the last one is the symmetric breathing mode. Only the asymetric
motions have non-zero IR intensities.