Theoretical and experimental study of infrared spectra of He ₂-CO₂

Additional high-resolution lines in the ν₃ CO₂ fundamental band of the rovibrational spectrum of He₂-CO₂ have been observed using a tunable infrared laser to probe a pulsed supersonic jet expansion. The ro-vibrational spectrum was calculated using Euler angles and five vibrational coordinates that specify the positions of the He atoms with respect to the CO₂ molecule, a product basis, and a Lanczos eigensolver. Rotational states with J = 0, 1, 2, and 3 associated with the vibrational ground state and different states (ν_t) of the torsional motion of the two He atoms about the CO₂ axis are identified and assigned. The assignment is consistent with having different principle axis orientiations in ν_t = 0 and ν_t = 1 states. Δν_t = 1 infrared transistions are forbidden due to the symmetry of CO₂, but Δν_t = 1 microwave transistions are possible. Theoretical line positions and intensities are predicted. Good agreement between experiment and theory was obtained. The calculated energy levels and intensities were crucial in assigning some of the weaker observed transitions.