Theoretical and experimental studies of the infrared rovibrational spectrum of He ₂-N ₂O

Rovibrational spectra of the He2-N2O complex in the v1 fundamental band of N2O (2224 cm-1) have been observed using a tunable infrared laser to probe a pulsed supersonic jet expansion, and calculated using five coordinates that specify the positions of the He atoms with respect to the NNO molecule, a product basis, and a Lanczos eigensolver. Vibrational dynamics of the complex are dominated by the torsional motion of the two He atoms on a ring encircling the N2O molecule. The resulting torsional states could be readily identified, and they are relatively uncoupled to other He motions up to at least νt=7. Good agreement between experiment and theory was obtained with only one adjustable parameter, the band origin. The calculated results were crucial in assigning many weaker observed transitions because the effective rotational constants depend strongly on the torsional state. The observed spectra had effective temperatures around 0.7 K and involved transitions with J≤3, with νt=0 and 1, and (with one possible exception) with Δνt=0. Mixing of the torsion-rotation states is small but significant: some transitions with Δνt≠0 were predicted to have appreciable intensity even assuming that the dipole transition moment coincides perfectly with the NNO axis. One such transition was tentatively assigned in the observed spectra, but confirmation will require further work.