MF-EXV Laboratory with Bruker IFS 125HR
Laboratory Spectroscopy of atmospheric constituents in the far, middle and near infrared
Remote sensing for determining the distribution of atmospheric trace gases relies on a line parameter spectroscopic database (e.g., HITRAN, SAO, or JPL) for line positions, line strengths, pressure broadening parameters or absorption cross sections. The database is mandatory for linking the characteristic spectral signature obtained by remote sensing to the trace gas distribution in the atmosphere. Remote sensing of the atmosphere is carried out from ground-, airplane-, balloon-, or satellite-based platforms. The spectroscopic database is usually obtained from laboratory experiments.
Precise intensity measurements
The extension of the spectroscopic database of atmospheric trace gases requires accurate measurement of spectral intensities and line shapes. Whereas the accurate determination of line positions from Fourier Transform (FT) spectra can be achieved easily, the measurement of intensities is hampered by numerous systematic error sources which have to be characterized and minimized. Unfortunately, a line intensity standard is not available for the mid-infrared (MIR) region. However, line strengths and thus line intensities can be calculated accurately in the case of pure rotational transitions, which can be observed in the far infrared (FIR) region. Thus, FT measurement of pure rotational transitions allows assessment and quantification of systematic error sources. For the first time, line strengths were experimentally determined with an accuracy better than 1%. Currently, the line model is being extended to achieve line profile accuracies down to the 0.1% limit.
The FT spectrometer at DLR is a commercial high resolution BRUKER IFS 125 HR instrument covering the spectral range from 10 to 50000 cm-1 with a maximum resolution of 0.001 cm-1 (30 MHz). The instrument can be evacuated to avoid absorption of radiation by water and carbon dioxide in the ambient air. The spectrometer is equipped with a number of highly sophisticated absorption cells covering absorption paths from 0.15 m to 0.5 m and temperatures from 200 K to 1000 K. Several sensitive detectors cover the entire electromagnetic spectrum from FIR to UV. For the MIR region the detector optics were optimized to yield sensitivities up to 10 times higher than standard commercial detectors.
Technical details of the lab infrastructure can be found in the flyer (see link at right).