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An analytical method is described for obtaining the precise location of an absorption line and the pressure broadening coefficients due to self-broadening or foreign gas broadening from experimental measurements using a laser operating on a single line near the absorption line. These absorption line characteristics are obtained from the pressure dependence of the transmittance of the laser radiation for the gas of interest, the analysis involving a least squares fit to a family of Lorentz curves. The method includes a computer search for the region of best fit to the Lorentz profile and provides both the values of and errors in the above coefficients. The pressure broadening coefficient obtained is the same as for the more general Voigt curve. The method is applied to the absorption line of vinyl chloride near 27.972 μm and of methane near 3.392 μm and the results compared with a graphical fit to a Voigt profile. The self-broadening coefficients obtained were α = 0.15 × 10−3 ± 0.97 × 10−4 cm−1 Torr−1 for vinyl chloride and α = 0.14 × 10−3 ± 0.46 × 10−4 cm−1 Torr−1 for methane. The separation between the helium–neon laser line at 2947.903 cm−1 and the methane absorption line at 2947.888 ± 0.015 cm−1 was found to be 0.21 × 10−2 ± 0.15 × 10−2 cm−1.
© 1973 Optical Society of America
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