THIS -- next-generation mid-infrared remote sensing of planetary atmospheres using a tuneable heterodyne infrared spectrometer

The Cologne spectrometer THIS (Tuneable Heterodyne Infrared Spectrometer) opens the mid-infrared wavelength region from 8 to 17 microns to ultra-high-resolution spectroscopy. The main scientific goal of THIS is to analyze highly resolved lineshape data of molecules (e.g. O₃, NH₃, CH₄, N_2O, H_xC_y etc.) to deduce physical parameters like wind velocities or height profiles of gases in either the Earth's or other planetary atmospheres.

Also astronomical observations of non-solar-system IR-sources like IRC+10216 as well as the measurement of pure rotational transitions of H₂ in the interstellar-medium from ground based telescopes are planned in the near future. THIS is a proposed second-generation instrument for the stratospheric observatory SOFIA.

With a system noise temperature of less than three times the quantum limit THIS is the first widely tuneable and transportable infrared heterodyne receiver having a sensitivity equivalent to CO₂-laser based heterodyne systems. A quantum-cascade-laser is used as local oscillator. Its radiation is superimposed to that from the signal by use of a Fabry-Perot ring-resonator to provide optimum efficiency. The frequency mixing is done by a Mercury-Cadmium-Telluride photomixer and spectral analysis with a resolution of up to 3x10⁷ is performed by means of an Acousto-Optical spectrometer. We report on THIS' successful first observing run performed at the west auxiliary telescope at McMath-Pierce solar observatory on Kitt Peak/Arizona in 11/2002. Very weak non-LTE CO₂ emission from the atmosphere of Venus have been observed as well as trace gases in Earth's atmosphere and molecular features in sunspots.