Radiative and Collisional Transitions between Coupled Vibrational Modes of CO2

Each of the two energy levels of CO₂ at 1388 and 1285 cm⁻¹ is an equal mixture of doubly excited bending mode and singly excited symmetric stretch mode. The collision numbers of these levels for ${\mathrm{CO}}_2–{\mathrm{CO}}_2$ collisions are calculated by using the mixed wavefunctions in conjunction with the SSH theory that has been expanded to include two‐quantum transitions. This relaxation is shown to take place via various near resonant V‐V processes at transition rates of the order of ${10}^4\hspace{0.17em}{\sec }^{\text{−1}}\mathrm{·\hspace{0.17em}}{\mathrm{torr}}^{\text{−1}}$ . In particular, direct de‐excitation of the lower laser level via singly excited bending mode is shown to be important. An explanation is given for transitions that involve a change of the vibrational angular momentum l, by $\text{Δ l = ± 2}$ . Three independent experiments have measured rates in reasonable agreement with those calculated here Quite striking is the substantial disagreement between these three experiments on the one hand and the published interpretation of the Rhodes—Kelly—Javan (RKJ) experiment on the other. To resolve this discrepancy, the RKJ experiment is analyzed in detail, and a new interpretation is given that explains one of the observed transient effects as caused by stimulated two‐photon processes and the other by rotational relaxation of the upper laser level.