Measurement of Line Strengths at Low Pressures—Application to the 2–0 Band of CO

Abstract

The use of low pressures and a high-resolution spectrometer for the measurement of the absolute line strengths of gases in the near infrared is tested on the first overtone band of carbon monoxide. Measurements are made at pressures from 100 to 500 μ Hg, where the line profile has nearly a pure Doppler contour, and at pressures 2 to 7 cm Hg, where the effects of Doppler and collision broadening are both important. It is found that both wing and base corrections must be made in higher-pressure measurements, and these corrections are given for lines with combined Doppler and collision-broadened profiles. It is found that the use of the Doppler profile offers significant advantages for the accurate determination of line strengths, and a discussion of these advantages are included. It is shown that within the accuracy of the measurements, the true line profile at pressures from 100 to 500 μ Hg cannot be distinguished from the assumed Doppler profile. The band strength of the first overtone band of CO is calculated from the measured line strengths, and the best value obtained at a temperature of 273°K is 2.06 ± 0.07 cm− 2·atm− 1.