Tunable far-infrared spectroscopy

The first experimental observations to our knowledge of sub-Doppler linewidths in a cell made using tunable far-infrared radiation are reported. A double-resonance scheme has been used, combining CO2-laser infrared radiation with tunable far-infrared radiation to observe a sub-Doppler line shape in an excited vibrational state of CH30H. Recent progress in frequency multiplication and in nonlinear mixing of coherent radiation has made the generation of tunable far-infrared (TuFIR) radiation possible.1 In the measurements reported here, FIR difference-frequency radiation was generated by mix-ing the radiation from two C02 lasers using the point-contact metal-insulator-metal diode.2 The radiation is tuned by varying the output frequency of a wave-guide C02 laser over its pressure-broadened gain pro-file. A high-sensitivity spectrometer that uses this tech-nique has been constructed,1 and a minimum detect