The strengths and half-widths of lines in the R branch of the CO fundamental band at temperatures as low as 113 K have been determined from equivalent-width measurements in the linear and square-root regions of the curve of growth. For all temperatures, the observed line strengths are in excellent agreement with line strengths calculated on the basis of the Herman-Wallis expression. However, at the lowest temperatures the observed half-widths differ considerably from the values calculated on the basis of the usual extrapolations of room-temperature data; the half-widths of lines near the band center are much larger than the calculated half-widths, and the half-widths of lines in the band wing are apparently considerably smaller than the calculated half-widths.
GARY M. HOOVER and DUDLEY WILLIAMS, "Infrared Absorptance of Carbon Monoxide at Low Temperatures," J. Opt. Soc. Am. 59, 28-33 (1969)