Knowledge Bank

Absorption spectra recorded By Fourier Transform Spectrometers consist of the transmitted signals at discrete frequencies. Spectra of the 1 - 0 bands of $H35$Cl and $H37$Cl observed at a nominal resolution of $0.1cm-1$ contain more than 8000 signal values. The individual line positions, intensities and (Lorentz) widths can be described by proper choice of 23 adjustable parameters in the expressions for the upper and lower state term values, the band intensities, Herman-wallis factors, and the J dependence of the line widths. The observed spectrum can be fitted by synthetic spectra calculated from these expressions provided the shape (assumed triangular) and width of the spectral response function of the instrument and the shape (assumed to have quadratic dependence on frequency) and position of the background curve can be determined. Values for the 27 unknown parameters of these models have been simultaneously determined by least squares fitting more than 3500 signal values within $\pm 5cm-1$ of the HCl line centers in single spectrum. The standard deviations of the values and the correlations between the parameters were also determined. The results are compared with previously estimated values. This method of band analysis requires models for all the factors which influence the spectrum and large amounts of computer time, but no experimenter manipulation of the raw data.