CAVITY RINGDOWN SPECTRUM OF THE $\nu_{8}$ BAND OF METHYLENE BROMIDE USING A QUANTUM CASCADE LASER

Abstract

Author Institution: Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, IL 61801; Departments of Chemistry and Astronomy, University of Illinois at Urbana-Champaign, Urbana, IL 61801; Department of Electrical Engineering, Princeton University, Princeton Institute for the Science and Technology of Materials, Princeton, NJ, 08544; Departments of Chemistry and Astronomy, University of Illinois at Urbana-Champaign, Urbana,; IL 61801In recent years the development of continuous wave quantum cascade lasers (QCLs) has enabled high-resolution laser spectroscopy at wavelengths between 4 $\mu$m and 12 $\mu$m. This is a major advancement in the availability of lasers in this wavelength region, as previously only lead salt diode lasers were available beyond 5 $\mu$m. Coverage in this region is necessary to allow for high-resolution spectroscopic studies of lower frequency stretching fundamentals, bending fundmentals, and overtones in a variety of molecules. In this study a sample of methylene bromide was expanded through a pinhole source and interrogated by continuous wave cavity ringdown spectroscopy (cw-CRDS) using a Fabry-Perot quantum cascade laser (FP-QCL). The primary motivation for this study was to use methylene bromide as a rotational temperature probe of the expansion in preparation for a cw-CRDS experiment of C$_{60}$ (Widicus Weaver et al., this meeting). For this reason the cw-CRDS spectrum of the $\nu_{8}$ band for the three dominant isotopomers of methylene bromide (CH$_{2}$$^{79}$Br$_{2}$,CH$_{2}$$^{81}$Br$_{2}$, CH$_{2}$$^{79}$Br$^{81}$Br) was acquired in the 8.5 $\mu$m region. This talk will discuss the current state of the assignment of the $\nu_{8}$ band of methylene bromide for all three isotopomers, and will also describe the necessary steps taken to ensure adequate thermal and mechanical stability of the FP-QCL