Water-vapor line broadening and shifting by air, nitrogen, oxygen, and argon in the 720-nm wavelength region

Abstract Pressure broadening and pressure shift in air, nitrogen, oxygen, and argon have been measured for water-vapor lines in the 720-nm wavelength region using a ring dye laser in conjunction with two long path absorption cells. Deviations resulting from the Dicke-narrowing effect were observed between the measured profiles and the standard Voigt profile. The corresponding collision-narrowing coefficients were computed using two Dicke-narrowed profiles, the soft- and the hard-collision profiles. The water-vapor absorption lines broadened by oxygen or argon, where the pressure shifts become comparable to the broadenings, were found to be asymmetric. These asymmetric lines are attributed to statistical dependence or correlation between velocity- and state-changing collisions. A vibrational dependence of the broadening and shifting was observed. Widths and shifts (in magnitude) were found to be significantly smaller for lines associated with upper vibrational states ( v ′ 1 v ′ 2 v ′ 3 ) = (221) as compared to ( v ′ 1 v ′ 2 v ′ 3 ) = (301). For each buffer gas under study, a linear relationship between the widths and shifts was measured, the broader lines having the smallest pressure shifts (in magnitude). The average air-to-nitrogen broadening ratio was measured to be 0.907 ± 0.011. We show that the air-broadening coefficient could also be retrieved from the independent measurements of the nitrogen- and oxygen-broadening coefficients. The average line shift in oxygen was found to be twice as large as the shift in nitrogen, indicating that collisions with oxygen molecules contribute significantly to the shift in air. Water-vapor line broadening and shifting in air were investigated in the temperature range from 300 to 400 K. The temperature exponent for air broadening was found to be J -dependent, the broader lines (i.e., low- J lines) having the higher exponents. The average exponent value was measured to be 0.670. The temperature-dependence exponent for the line shift in air was found also to be J -dependent but it showed the opposite behavior (i.e., the high- J lines have the higher exponents). The temperature exponent for line shifting was measured to range between 0.4 and 1.2.

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