FT-IR MEASUREMENTS OF CROSS SECTIONS OF COLD C$_3$H$_8$ IN THE 7 - 15 $\mu$m FOR TITAN

Abstract

To support atmospheric remote sensing of Titan, the absorption cross sections of N$_2$-broadened C$_3$H$_8$ were obtained at temperatures between 145 and 296 K. For this, 35 spectra of pure- and N$_2$-broadened propane were recorded in the 670 to 1900 cm-1 region using a Fourier transform spectrometer (Bruker IFS-125HR) at the Jet Propulsion Laboratory. A 20.38 cm path temperature-stabilized cryogenic absorption cell was used, which was developed at Connecticut College and described previously [1]. We report the absorption cross sections at the various cold temperatures for nine strong fundamental bands ($\nu_{26}$, $\nu_8$, $\nu_{21}$, $\nu_{20}$, $\nu_7$, $\nu_{19}$, $\nu_{18}$, $\nu_4$, $\nu_{24}$) as well as for many contributions from hot and combination bands. In addition, we present results from 'pseudo -line generation' (http://mark4sun.jpl.nasa.gov/data/spec/Pseudo/Readme), which includes mean intensities and effective lower state energies on a 0.005 cm$^{-1}$ frequency grid determined in the 690 - 1536 cm$^{-1}$ region from all 35 high-resolution laboratory spectra. It was observed that the pseudo lines reproduce all the observed spectral transmittances well within 3\% and the C$_3$H$_8$ amounts within 4\% on the average. The measured cross sections and synthetic spectra from the pseudoline compilation are compared to earlier work, including the C$_3$H$_8$+N$_2$ spectra recorded at PNNL [2] and line-by-line predictions available [3, 4].