Observation of overtones of vibrational-rotational molecular transitions by coherent active spectroscopy

The method of active Raman spectroscopy was used in an experimental investigation of the rotational structure of a vibrational-rotational band of the carbon dioxide molecule near the first overtone of the Π_μ vibration representing the low-frequency component of the Fermi doublet 1285.5–1388.5 cm^–1. Active centers were excited by the second harmonic of a YAG:Nd³⁺ laser and by a tunable dye laser with a solution of rhodamine 6G in ethanol. The experimental anti-Stokes signal had a central strong component, corresponding to a purely vibrational second-order transition due to deformation vibrations of the CO₂ molecule (00°0–02°0), as well as the structure in the wings due to vibrational-rotational transitions. The experimental results were used to estimate the differential cross section of spontaneous Raman scattering dδ/do ≈ 5×10^–31 cm²/sr.