Microwave double resonance studies of rotational relaxation in polar gases

Four‐level, modulated microwave double resonance has been used to study rotational energy transfer in some dilute polar gases. A survey has been made of double resonance signals observable in four asymmetric rotors: trimethylene oxide, β‐propiolactone, cyclobutanone, and cis‐difluoroethylene. Double resonance intensities have been measured in vibrational ground and excited states. Also, angular momentum reorientation caused by intermolecular collisions has been studied by microwave double resonance. Four‐level experiments have been performed on individual $\mathrm{|JM〉}$ components of ethylene oxide and 3,3,3‐trifluoropropyne and on $\mathrm{|JF〉}$ components of ICN. These experiments provide evidence that the energy transfer and momentum reorientation occurring during collisions are determined by the long range dipole‐dipole interaction. Qualitatively, a first‐order perturbation theory description of the collision process accounts for the selection rules of the observed collision‐induced transitions.