CARS STUDIES OF BENDING STATES OF CO2 - EVIDENCE OF COLLISIONAL ROTATIONAL TRANSITIONS WITH ODD DELTA-J

The coherent anti-Stokes Raman scattering (CARS) technique was used to study the shape transformation of the hot vibrational band (11(1)0)(2 ) <-- (01(1)0) of CO2 at 1265 cm(-1) in the density range of 0.3-50 Am agat at room temperature (T = 295 K). The band consists of two isotrop ic e-subbranches formed by the vibrotational transitions from the even - and odd-J rotational states which are characterized by different set s of rotational constants. Theoretically, the subbranches can not be c oupled by rotationally inelastic collisions if the presence of the vib rational angular momentum in the combining states is disregarded. Base d on the known relaxation rates, formation of two resolved, motionally narrowed bands should then be expected at medium densities (d similar to 10 Amagat). However, this was not observed at any of the studied d ensities. With the vibrational momentum incorporated into the theory, the collisional transitions with odd Delta J become allowed, and the r esulting inter-subbranch mixing prevents the doublet formation. A simp le, analytically solvable model of inter-subbranch mixing is elaborate d providing a reasonable agreement with the experimentally observed e- branch shapes. (C) 1998 Elsevier Science B.V. All rights reserved.