Polarization effects in infrared–infrared double resonance in methyl fluoride

Abstract

A series of infrared–infrared double-resonance experiments are described in which a low-pressure gas sample interacts with pump and probe beams that have a variety of different polarizations. It is shown that orientation or alignment of the sample, previously demonstrated for electronic spectra with visible lasers, can be performed by irradiating a sample with a polarized infrared laser that is coincident with a vibration–rotation transition. By switching the polarization of the pump beam between horizontal and vertical polarization, linear dichroism is observed. By switching the polarization of the pump beam between right and left circular polarization, either circular dichroism or circular birefringence may be seen with the appropriate experimental arrangement. Also, as shown previously for electronic transitions, a combination of dichroism and birefringence experiments can be used to determine whether pump and probe transitions in the infrared region are P, Q, or R branch transitions. It is found that induced molecular alignment or orientation can be observed in four-level double resonance, indicating that alignment or orientation survives one or more collisionally induced rotational transitions. All of the three-level double-resonance effects may be predicted by means of the Jones calculus by using a Jones matrix for the pumped sample considered as an optical device. Equations for the elements of the Jones matrix have been derived for a sample pumped by either plane-polarized or circularly polarized radiation of arbitrary intensity.