New multispectrum fitting software used at DLR for analysis of laboratory Fourier-Transform molecular spectra

Abstract

A new fitting tool written in IDL for analysis of single or multiple molecular absorption spectra recorded with a Fourier-transform spectrometer will be presented. The software utilizes a microwindow-based line-by-line approach and its capabilities include the choice of numerous different line shape models, from a simple Voigt to more sophisticated models including e.g. speed-dependence, Dicke narrowing or Rosenkranz line mixing. A comfortable interactive mode as well as a fully automatic mode have been implemented including various quality assessment procedures like the monitoring of correlation coeffcients or the supply of useful information e.g. needed for file cuts (single spectrum residuals). Two application examples will be given as illustration of the software's capabilities. First, the results of a speed-dependent analysis of room temperature water vapor spectra in the 1250-1750 cm-1 range are compared to the original results obtained by an analysis based on single spectra fits applying the Voigt procedure. The results of the new approach show significantly reduced residuals and systematically larger broadening parameters. The so far published Voigt broadening parameters are effective parameters being systematically too small since they compensate narrowing. Usually effective Voigt parameters are retrieved from non-opaque lines and should only be used when modelling non-opaque lines. Second, the results of an analysis of air broadened nitrous oxide spectra in the v3 rovibrational band utilizing the pCqSDHC modela including first order line mixing will be presented. Speed dependence of the broadening parameter as well as line mixing have to be considered in order to represent the spectral features down to the 0.1% level.