H3+ as the benchmark for rigorous ab initio theory

The observed ro-vibrational spectral lines of the simplest polyatomic molecule, H3+, serve as the benchmark to test the most rigorous ab initio theory of intra-molecular dynamics. The ground state equilibrium structure of H3+ is an equilateral triangle, but near the energies of ∼ 9913 cm−1, the so called barrier to linearity, it begins to sample linear configurations for which theoretical calculations are challenging because of the singularity of the Hamiltonian. We present here a continuation of the spectroscopy of H3+ above the barrier to linearity using a Ti:sapphire laser based high sensitivity spectrometer which allowed us to observe the spectrum with near shot-noise limited sensitivity. 121 new lines have been recorded from the near infrared 10 300 cm−1-13 700 cm−1 entering the visible region bringing the total number of transitions above 10 000 cm−1 to 143. The observed spectrum is compared with theoretical predictions. The remaining discrepancies mostly due to vibrational and rotational non-adiabatic effects are discussed.