Oxygen B-band P-branch study: simultaneous observation of Dicke narrowing and speed-dependence effects
Oxygen is one of the two most abundant molecular gases in the terrestrial atmosphere. B band is the second most intense band in the red part of the absorption spectrum of the O2 molecule. It is less commonly used for the remote sensing of the atmosphere mainly because the B-band intensity is about 15 times lower than the A-band. Until recently another limiting factor was the lack of accurate laboratory data. Accurate reference data, including full sets of line-shape parameters, is necessary for multiple atmospheric applications, in particular, accurate determination of the pressure and temperature profiles  across the atmosphere and improved parameters of clouds such as their top height .
In this study, we measured line-shape parameters of three transitions from the P branch. In contrast to the previous study , we simultaneously observe the speed-dependence of the collisional broadening and the Dicke narrowing effect , as well as the line asymmetry due to the speed-dependence of the collisional shifting for the pressures down to 1.1 kPa. It became possible due to the significantly increased spectra signal-to-noise ratio. The measurements were done with two experimental techniques: well-established cavity ring-down spectroscopy (CRDS)  and cavity mode-width spectroscopy (CMWS) [6,7] that allowed us a more reliable estimation of the determined line-shape parameters systematic uncertainties. The unperturbed line positions are determined within 40 kHz and the line intensities at 0.2% level. New results are compared with our previously published data  of the P-branch measurements and observed systematic deviations on the line intensities are explained.
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