Infrared double resonance in boron trichloride

Infrared‐infrared double resonance spectroscopy has been used to monitor the absorption changes induced in both ¹¹BCl₃ and ¹⁰BCl₃ by a pulsed CO₂ laser. It is found that the relaxation of the initial excited state to a distribution of excited states proceeds by a very fast V↔ V process with a rate of at least ${\text{2× 10}}^6{\sec }^{\text{−1}}$ Torr⁻¹. This excited distribution subsequently relaxes by a slower V↔ T deactivation from the ν₄ level at 243 cm⁻¹. The collision efficiency for deactivation of this level has been measured for collision partners He, Ne, Ar, Kr, Xe, HCl, DCl, H₂, HD, D₂, N₂, CH₃F, and CHCl₃ as well as for BCl₃ itself. The results are compared with existing theories of vibrational relaxation into translational and rotational degrees of freedom.