Doppler-free optical double resonance spectroscopy using a single-frequency laser and modulation sidebands

Principles and applications are described for a form of Doppler-free optical double resonance spectroscopy which uses amplitude modulation sidebands (v _L ±v) imposed on a single laser frequency (v _L ). The sidebands are generated by passing the carrier radiationv _L through an electro-optic modulator, driven at a radiofrequency ν, which enables the intensity and polarization characteristics of the emerging radiation to be varied for enhancement of selected double-resonance processes. The technique has been applied to infrared-infrared double-resonance studies of the Stark effects of a variety of molecules—¹³CH₃F,¹²CH₃F, PH₃,¹⁵NH₃, GeH₄, SiH₄, and CH₃D—for which physical results are presented and discussed. These results include determination of extremely small electric dipole moments (10⁻³–10⁻⁵ debye) for GeH₄ and CH₃D and, for the dipole moment of PH₃, a vibrational state dependence which is extremely small (Δμ=0.0028(5) debye for ∣Δv ₂∣=1) and a rotational state dependence which is of an unexpected sign. The spectra recorded in some cases display unusual polarization and optical saturation effects which deviate markedly from the predictions of a simple three-wave polarization theory.