Spectroscopy of triatomic hydrogen

The visible emission spectra of H₃, D₂H, H₂D and D₃ were studied. Triatomic hydrogen molecules were produced by neutralization of fast ion beams in alkali vapors. In addition to the well-known 0-0 bands, weak vibrational bands of the electronic 3p ² A″ ₂ → 2s ² A′ ₁ transition of D₃ and D₂H were observed and analyzed. From the decay of the emission along the molecular beam lifetimes of excited states were obtained. For all observed states, 3p ² A″ ₂, 3s ² A′ ₁ and 3d, lifetimes strongly depend on the isotopic mixture. Many lifetimes measured were considerably shorter than ab initio predictions of the radiative lifetimes. Thus radiationless decay is important for all these excited states. Predissociation of the 3p ² A″ ₂ state increases with the rotational quantum number. Possible decay channels are discussed.