Infrared spectra of the isotopes of H3+

H3+ is the simplest polyatomic molecule, and its existence, physical stability, and equilateral triangular structure have been known for many years. However, precise experimental information was lacking until 1980, when infrared spectra were reported for H3+ by Oka and for D3+ by Shy, Farley, Lamb, and Wing. Since that time there have been considerable advances in the spectroscopy of H3+. We have measured extensive spectra of all four isotopes (H3+, H2D+, D2H+, D3+) in the 1800–2500-cm−1 region using tunable infrared diode laser sources, a cooled hollow-cathode absorption cell, and a discharge-modulation detection scheme. These data have permitted us to make detailed analyses of the degenerate v2 bands of H3+ and D3+, as well as of the Coriolis-coupled v2 and v3 bands of the mixed isotopes H2D+ and D2H+. The results are summarized and compared, and an extension of the H3+ spectrum to higher values of J is discussed.