CARS studies of bending states of CO2: evidence of collisional rotational transitions with odd ΔJ Original Research Article

The coherent anti-Stokes Raman scattering (CARS) technique was used to study the shape transformation of the hot vibrational band (1110)2←(0110) of CO2 at 1265 cm−1 in the density range of 0.3–50 Amagat at room temperature (T=295 K). The band consists of two isotropic Q-subbranches formed by the vibrotational transitions from the even- and odd-J rotational states which are characterized by different sets of rotational constants. Theoretically, the subbranches can not be coupled by rotationally inelastic collisions if the presence of the vibrational angular momentum in the combining states is disregarded. Based on the known relaxation rates, formation of two resolved, motionally narrowed bands should then be expected at medium densities (d∼10 Amagat). However, this was not observed at any of the studied densities. With the vibrational momentum incorporated into the theory, the collisional transitions with odd ΔJ become allowed, and the resulting inter-subbranch mixing prevents the doublet formation. A simple, analytically solvable model of inter-subbranch mixing is elaborated providing a reasonable agreement with the experimentally observed Q-branch shapes.