Overtone Spectroscopy of N2O between 10 000 and 12 000 cm−1: A Test of the Polyad Approach

Abstract Eleven very weak overtone bands of 14 N 2 16 O have been newly detected by intracavity laser absorption spectroscopy between 10 000 and 12 000 cm −1 . The rotational analysis of three others has been significantly improved. The vibrational assignment of the upper states of the observed transitions has been readily performed on the basis of effective Hamiltonian predictions. The couplings involved in this Hamiltonian connect states of the same polyad characterized by the pseudo quantum number n r =2ν 1 +ν 2 +4ν 3 . The values of the parameters of this polyad model of effective Hamiltonian have been updated by a new global fit to all previously observed rovibrational energy levels in the ground electronic state. The newly observed transitions are used to test the predictive capabilities of this effective Hamiltonian. The deviations of the predicted line positions from the observed ones are found to be less than 0.2 cm −1 for most of the levels and reach 1.16 cm −1 for an extremely weak Σ–Σ transition centred at 11 428.068 cm −1 . A local rotational perturbation is observed around J =45 for the vibrational state at 10 079.56 cm −1 in full agreement with the effective Hamiltonian predictions. This perturbation is identified as resulting from l -resonance and anharmonic + l -resonance interactions with the vibrational dark state at 10 064.335 cm −1 .