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

Abstract The very weak 3ν 1 +3ν 2 +ν 3 absorption band of the main isotopologue of nitrogen dioxide, 14 N 16 O 2 , is investigated for the first time near 7587 cm −1 . The absorption spectrum was recorded by high sensitivity Continuous Wave-Cavity Ring Down Spectroscopy with a noise equivalent absorption of α min ≈1×10 −10  cm −1 . 414 lines of the 3ν 1 +3ν 2 +ν 3 band were assigned with rotational quantum numbers N and K a 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 −3  cm −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 3ν 1 +3ν 2 +ν 3 band. With maximum line intensity on the order of 2.5×10 −27  cm/molecule at 296 K, the 3ν 1 +3ν 2 +ν 3 band is the weakest band of the NO 2 molecule rovibrationnally assigned so far.