An introduction to the spectroscopy of H3+

Abstract
The infrared spectra of H3+ and D3 were first observed almost simultaneously in 1980. Since then there has been extensive laboratory work on H3 and its D-isotopomers. In addition, the spectrum of H3+ has been observed in emission from the atmospheres of the giant planets Jupiter, Saturn and Uranus, and probably from the supernova SN1987A, and recently in absorption in interstellar clouds. In this introductory paper, the assignment of the spectra and some of the characteristic features associated with the strong anharmonicity are discussed. As the convergence of the perturbation series is poor, the large centrifugal effects are best calculated in a variational type of approach, where the rotational energy in the Hamiltonian can be treated essentially exactly. Ab initio calculations have steadily improved over the years, and recent work has given completely theoretical vibration–rotation term values for the low vibrational states with an accuracy better than 0.1 cm-1 in almost all cases, adequate for most spectroscopic assignment purposes.