Molecular beam measurement of cross sections of collision induced reorientation of molecule

The cross sections for collision induced transitions of molecules between magnetic sublevels slightly separated by a weak Stark field are directly measured with a method of infrared–infrared double resonance in a molecular beam. The ΔM=±1,±2 transitions in the J=K=2 rotational levels in the ground and the ν3 excited vibrational states of CH3F are studied as functions of the energy separation between the M sublevels. It is shown that the cross sections do not have a significant dependence on the energy separation which is less than 1 GHz, and their magnitudes are of a comparable order to the pressure broadening parameters of the infrared absorption lines. The results are compared with other ‘‘indirect’’ measurements of the cross sections.