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

- First detection and analysis of the 3ν1+3ν2+ν3 band of NO2 near 7587 cm−1.
- 414 lines were assigned with rotational quantum numbers N and Ka up to 32 and 6.
- Effective Hamiltonian modeling.
- Coriolis resonance coupling with three dark states: (350), (062) and (312).
- The 3ν1+3ν2+ν3 band is the weakest NO2 band rovibrationnally assigned so far.

The very weak 3ν1+3ν2+ν3 absorption band of the main isotopologue of nitrogen dioxide, 14N16O2, 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 αmin≈1×10−10 cm−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 6×10−3 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 3ν1+3ν2+ν3 band. With maximum line intensity on the order of 2.5×10−27 cm/molecule at 296 K, the 3ν1+3ν2+ν3 band is the weakest band of the NO2 molecule rovibrationnally assigned so far.