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We have built a high-resolution and high-signal-to-noise ratio spectrometer for line shape studies of greenhouse gases in the mid infrared. The infrared radiation is generated in a AgGaS_2 nonlinear crystal by the well-known difference-frequency method. The choice of crystal is explained, and a brief literature review is presented. With two tunable dye lasers and a type I, 90° phase-matching geometry, the infrared is continuously tunable from 7 to 9 μm when Rhodamine 6G and Sulforhodamine 640 dyes are used. The total infrared power exceeds 30 nW and is limited by both the damage threshold and thermal loading of the crystal. Phase-sensitive detection allows us to reach signal-to-noise ratios in excess of 3500:1 while maintaining an instrumental linewidth of 1.5 MHz. However, we show that the spectrometer may be used to measure the positions of spectral lines within ±400 kHz.

The temperature and pressure cycles inside a pressure modulator cell (PMC) of the type used for gas-correlation radiometry aboard the Measurements of Pollution in the Troposphere (MOPITT) satellite instrument have been determined from dynamic measurements of the spectral line shapes of the R(0) and R(18) transitions in the fundamental vibrational-rotational band of carbon monoxide. The line strengths and linewidths were used to calculate the temperature and pressure, respectively, with a temporal resolution of approximately 200 μs, or 1/100 of a PMC cycle. The results are compared with a thermodynamic box model.