In order to make sure that the line shape is reproduced correctly by the instrument, cell measurements are made and analysed with LINEFIT. The knowledge of the instrumental line shape (ILS) is very important, otherwise wrong gas concentrations are deduced from the atmospheric measurements.
In principle, the ILS can be determined either with a single mode laser or with cell measurements. While a laser beam is tricky to use correctly, cell measurements are made quite easily. But, for the analysis of cell spectra a software is needed, for example LINEFIT. Particularly for ILS investigations a cell has been designed by Frank Hase. Please have a look on cell.pdf giving recommendations for the use of gas cells.
Fig. 1: Homemade cell
Similar to the analysis of atmospheric spectra cell spectra are analysed by fitting synthetic spectra to measured ones. In the case of cell spectra the gas volume probed is well known (p,T, composition), and ILS parameters like modulation efficiency and line assymmetry are fitted. LINEFIT includes a simple and an extended ILS modelling: While the simple model assumes a linear decrease of modulation eff. and constant phase error along the optical path difference (OPD) the extended version fits these parameters as function of OPD. For further details please see linefit.pdf or paper on LINEFIT:
Hase, F., T. Blumenstock, C. Paton-Walsh: Analysis of the instrumental line shape of high-resolution Fourier transform IR spectrometers with gas cell measurements and new retrieval software, Appl. Opt. 38, 3417-3422, 1999.
Fig. 2: ILS of a slightly misaligned FTIR
Furthermore, LINEFIT package includes ALIGN.
ALIGN calculates the ILS and the Haidinger fringes in dependance of adjustment parameters like lateral displacement of the retroreflector or of the entrance aperture. The use of ALIGN is
- to compare calculated Haidinger fringes with observed ones
- to see the effect of misalignment on the ILS
- to calculate the ILS as input for LINEFIT runs
- to calculate the ILS for a given wave number (different from those probed by the cell).
For further details please see align.pdf.
LINEFIT has been written by Frank Hase. It is available and widely used within the NDACC-IR, TCCON and COCCON community and by BRUKER Optics. If you're also interested, please contact Frank Hase.