A cryogenic Herriott cell vacuum-coupled to a Bruker IFS-125HR

A new cryogenic Herriott cell and associated transfer optics have been designed and fabricated at Connecticut College under contract with NASA Langley Research Center to operate for the first time with the broad-band Bruker IFS-125HR Fourier transform spectrometer at the Jet Propulsion Laboratory (JPL). This 0.375 m base-length optical cavity produces an absorption path length, at 293 K, of 20.941 (±0.006) m. The Herriott cell, constructed from oxygen-free high conductivity copper, is placed inside its own vacuum enclosure, which is isolated from the transfer optics chamber by one CaF2 window and separately evacuated. The transfer optics chamber is in turn coupled to the sample compartment of the Bruker IFS-125HR holding another set of transfer optics. The entire spectrometer, including the transfer optics chamber can be evacuated to ~10 mTorr; the cell vacuum enclosure is cryogenically evacuated to pressures below 10-6 Torr. A closed-cycle helium refrigerator cools the Herriott cell. Initially tested at Connecticut College for temperatures between 250 and 50 K, the system has successfully been in operation for over two years at JPL. The cell has been used for recording spectra between 75 and 250 K, achieving excellent temperature uniformity (± 0.15 K) and long term stability (< 0.05 K/day). Configured with a single indium-sealed CaF2 window, it has provided Doppler-limited infrared absorption spectra of a number of molecular species above 2000 cm-1 to investigate detailed spectroscopic properties (e.g. molecular line parameters at temperatures and pressures relevant to planetary atmospheres). The design, performance and detailed characterization of the Herriott cell system are discussed.