NEW APPROACHES TO DECODING ROTATIONAL SPECTRA: APPLICATIONS TO FLUOROETHYLENE MICROSOLVATION BY CO2

Abstract

Chirped-pulse Fourier-transform microwave (CP-FTMW) spectrometers such as the instrument at the University of Virginia can acquire spectra with high sensitivity in a short amount of time. This necessitates new approaches to spectroscopic analysis to ensure identification of all species in each recorded spectrum. With an intensity range covering four orders of magnitude after averaging 1 million free induction decays, a recent spectrum of a fluoroethylene (FE)/\chem{CO_2} mixture in the 2 – 8 GHz range has over 11,000 lines with signal-to-noise ratio above $\sim$2.5. These transitions may arise from a combination of monomer, dimer and larger cluster species, including low abundance isotopes and complexes with carrier gas, water or other contaminants. Our current focus is identifying spectra of FE(\chem{CO_2})$_{n}$ clusters, containing progressively larger numbers of \chem{CO_2} molecules. Several methods have been used to facilitate identification of lines for these spectra, which are expected to lose 1-2 orders of magnitude of intensity for each increase in cluster size. These approaches include subtraction of transitions that are observed in the FE-only spectrum from the spectrum of the FE/\chem{CO_2} mixture, visual identification of patterns characteristic of asymmetric molecules, and application of extended cross correlation (XCC) techniques.\footnote{N.P. Jacobson, S.L. Coy, R.W. Field, {\it{J. Chem. Phys.}}, {\bf{107}} (1997) 8349.} In the XCC approach, several spectra with systematically varied conditions (such as pressure or concentration) are compared, and a combination of graphs and computerized algorithms is used to identify transitions that behave similarly under the changing conditions. In addition to applications for fundamental spectroscopic studies, this approach has potential application to identification of the components of complex mixtures