High sensitivity cavity ring down spectroscopy of the 3ν1+3ν2+ν3 band of NO2 near 7587 cm−1

Abstract

The very weak 3ν1+3ν2+ν3 absorption band of the main isotopologue of nitrogen dioxide, 14N16O2, is investigated for the first time near 7587cm−1. The absorption spectrum was recorded by high sensitivity Continuous Wave-Cavity Ring Down Spectroscopy with a noise equivalent absorption of αmin≈1×10−10cm−1. 414 lines of the 3ν1+3ν2+ν3 band were assigned with rotational quantum numbers N and Ka as high as 32 and 6, respectively, what corresponds to 518 rotation–vibration transitions. The overall set of spin–rotation energy levels was modeled in the frame of the effective Hamiltonian approach and reproduced with an RMS of 6×10−3cm−1 for the (obs.–calc.) deviations. The effective Hamiltonian includes interactions with three nearby dark states – (350), (062) and (312) – in Coriolis interaction with the (331) bright state. Using a selected set of experimental line intensities and the fitted values of the vibration–rotation Hamiltonian parameters, the principal parameter in the dipole moment operator expansion is determined for the 3ν1+3ν2+ν3 band. With maximum line intensity on the order of 2.5×10−27cm/molecule at 296K, the 3ν1+3ν2+ν3 band is the weakest band of the NO2 molecule rovibrationnally assigned so far.