Infrared radio-frequency double-resonance spectroscopy of molecular vibrational-overtone bands using a Fabry-Perot cavity-absorption cell

Infrared (ir) radio-frequency (rf) double-resonance spectroscopy was carried out on the vibrational-overtone band of methyl iodide molecules (CH ₃I). An optical Pabry-Perot cavity was employed as an absorption cell to record saturated ir spectral lines even by a small-power extended-cavity diode laser with a high sensitivity and a wide tunability in the presence of a rf field. These features allowed investigation of molecules strongly coupled with monochromatic and bichromatic rf fields, or dressed molecules, at various rf power levels and detuning frequencies for a variety of ir and rf transitions. At appropriate energy-level schemes, quantuminterference effects were observed. All resultant spectra showed good agreement with dressed-state theoretical calculations, indicating that the present spectrometer is valid for precise investigation of dressed molecules.