New method to establish frequency calibration of FTS with multireflection cells

Abstract

In laboratory FT spectroscopy in the infrared region, instrumental line shape (ILS) determination and the associated frequency calibration is necessary for accurate measurements. For high pressure measurements, both can be obtained from a previous measurement of a calibration gas at low pressure, preferably under otherwise similar conditions. However, especially in combination with a multireflection cell, a subsequent change in pressure causes an alteration in the adjustment of multireflection cell due to mechanical deformations and change of refractive index. The beam path and profile are thus shifted, which especially comes into effect for high absorption paths. An unavoidable correction of the adjustment then usually causes a deviation from the original ILS and thus from frequency calibration. ILS contributions are distributed over the infrared beam in the spectrometer optics and therefore the adjustment can influence the ILS. Especially the detector and detector window exhibit a strong angular dependence of reflectivity, thus influencing the ILS. The known method of using a small cell with a calibration gas in series with the multireflection cell fails when narrow band filters are applied for obtaining good signal to noise ratios and there is no calibration gas available with strong lines in the spectral region of interest. Here, a method is presented by which the re-adjustment of the instrument can be performed in a way to restore the previous ILS and frequency calibration for large absorption paths and pressure changes of about 1 bar. Explained is the detailed restoring procedure as well as exemplary results using a multireflection cell of 0.8 m base length set to an absorption path of more than 100 m, where a secondary reference cell was used to validate the method.