The near dissociation spectrum of H3+

Dr I. R. McNab
University of Newcastle upon Tyne

H3+ has an infrared spectrum which arises from transitions between levels at, or above the dissociation limit, of its ground electronic state. The spectrum contains nearly 27,000 lines. Similar, but different spectra have also been observed for D3+, H2D+ and D2H+. The high resolution H3+ data shows no immediately recognisable pattern, but the most intense lines in the spectrum are clumped about four (equally spaced) frequencies. Classical trajectory calculations show that the clumping of frequencies is due to remnants of quasi-stable periodic orbits in an energy region which is otherwise chaotic; full quantum calculations are able to identify several stable motions in the relevant energy region which should contribute to the most intense lines in the spectrum. The current state of knowledge and understanding of the H3+ near dissociation infra red spectrum are reviewed, and recent measurements presented.