Knowledge Bank

A need for precise air-mass retrievals utilizing the near-infrared O$2$ A-band has motivated measurements of the water-broadening in oxygen. Experimental challenges have resulted in very little water broadened oxygen data, especially in the near-infrared where pressure broadened linewidth must compete with the relatively large thermal linewidth. Existing water broadening data for the O$2$ A-band is of insufficient precision for application to the atmospheric data. Because of the nature of scattering processes, it is believed that broadening parameters for O$2$ from one spectral region may be transferable to other spectral regions - so we investigated the O$2$ 60 GHz magnetic dipole $Q$ branch which is also used prominently in remote sensing. Atmospheric retrievals of air-mass and temperature that use the 60 GHz magnetic dipole $Q$ branch incorporate a water-broadening parameter that is scaled to self-broadened values, but there is only high temperature data that directly supports this hypothesis. We present precise O$2$-H$2$O broadening measurements for the magnetic dipole $Q$-branch and the pure-rotational band, measured at room temperature with a Zeeman-modulated absorption cell and a frequency-multiplier spectrometer. Here we will describe the apparatus and the measurement analysis. Inter-comparisons of these and other O$2$ broadening data sets confirm the expectation of only minor band-to-band scaling of pressure broadening. The measurement provides a basis for fundamental parameterization of retrieval codes for the long-wavelength atmospheric measurements. Finally, we encourage the application of these measurements for retrievals of air-mass \emph{via} remote sensing of the oxygen $A$-band. E.M. Vess \emph{et al.} \emph{J. Phys. Chem. A} 116, 4069-4073 (2012).\ G. Fanjoux \emph{et al.} \emph{J. Chem. Phys.} 101(2) 1061-1071 (1994).