A "pressure modulator" has been constructed for use in studies of modulated infrared emission and absorption spectra. The modulator provides sinusoidal variation of the volume of a gas sample at a frequency of 8 cps with compression ratios of 2:1, 3:1, 4:1, 5:1, and 6:1. The general results obtained in grating studies of the fundamental and first-overtone absorption bands of carbon monoxide can be satisfactorily interpreted on the basis of an adiabatic modulation cycle, although detailed checks of the expected behavior for transitions between high rotational states were not entirely successful. In the adiabatic modulation of a gas for which γ = 1.66, temperatures as high as 640°K and as low as 225°K are attained in the present modulator with a compression ratio of 5:1 without raising the cell temperature more than 45 K deg above room temperature.
RICHARD R. PATTY and DUDLEY WILLIAMS, "Further Studies of Pressure-Modulated Infrared Absorption," J. Opt. Soc. Am. 51, 1351-1356 (1961)