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 |JM〉 components of ethylene oxide and 3,3,3-trifluoropropyne and on |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.