Highly accurate experimentally determined energy levels of H₃

A sub-Doppler rovibrational spectroscopic survey of H₃ ⁺ has been conducted which included 36 transitions in the ν₂ ← 0 fundamental band, 15 transitions in the 2ν₂ ²←ν₂ hot band, and 7 transitions in the 2ν₂ ²←0 overtone band, improving the uncertainties of most transitions by more than an order of magnitude to ∼4 MHz. Combination differences were used to determine relative energy levels and forbidden rotational transitions up to J = 6. A fit of the ground state to an effective Hamiltonian was used to connect ortho and para states, and to determine the absolute energy levels relative to the forbidden (0, 0) state. Ultimately, 62 rovibrational energy levels in the ground, ν₂, and 2ν₂ ² states were determined with ∼10 MHz uncertainty. Comparing the experimentally determined energy levels with ab initio calculations revealed an unexpected dependence of the residuals on the quantum number G.