The CASAC (CAS Absorption Cell) Experiment

The CAS laboratories are home to a long-pathlength absorption cell spectrometer (CASAC – CAS Absorption Cell), designed for high-resolution studies of reactive, transient, and stable molecular species relevant to astrochemistry.

The core of the instrument is a 3 m-long, 5 cm-diameter glass flow cell equipped at both ends with high-density polyethylene windows that are transparent to long-wavelength radiation. Multiple vacuum flanges along the cell provide access to backing pumps (a diffusion pump and a rotary-vane pump), pressure gauges, and sample input ports. At the center of the cell lies a 2 m discharge region, bounded by large metal electrodes. The outer wall is wrapped with cooling tubing and enclosed within a copper solenoid, enabling the formation of a magnetically confined, cooled plasma from suitable gaseous precursors. The system can reach pressures below 1 mTorr, with a diffusion pump operating at approximately 1,500 L/s (VHS-6, Agilent). The discharge is powered by a 2 kW DC supply, while the solenoid can be driven up to 50 A, corresponding to a magnetic field of about 350 G. Using liquid nitrogen, the flow cell can be cooled to –190 °C, while stable plasma operation typically requires internal pressures above 15 mTorr.

Schematic diagram of the CAS@MPE CASAC experiment.

Spectroscopic measurements are performed over the frequency range of approximately 70–1600 GHz, using Schottky-based multiplier chains (AMC, Virginia Diodes Inc.) and either room-temperature Schottky diode detectors (VDI) or hot-electron bolometers, both in wet and dry configurations (QMC Instruments). A wire-grid polarizer and retroreflector can be introduced to perform double-pass spectroscopy, enhancing sensitivity and sub-Doppler (Lamb dip) measurements to increase resolution.

In addition to the main discharge configuration, CASAC includes a static absorption cell, which allows high-sensitivity spectroscopy of interstellar complex organic molecules (iCOMs) and other stable species, as well as a pyrolysis cellconnected to a high-temperature oven (up to ~1500 °C). The pyrolysis setup enables the thermal decomposition of precursors to generate non-commercial, non-synthesizable, and semi-stable species.
A large selection of high-purity gases, glass flasks, bubblers, and heating elements are available for sample preparation. A glass manifold system with multiple ports and a cryogenic trap, located in a dedicated fume hood, allows safe preparation and handling of chemical mixtures.

Photo of the experimental layout and accessory equipment.
2f spectra of all three natural Argon-isotopologues of protonated Argon, taken in a single experimental session. Note that the relative intensities reflect simply the natural abundances of Argon, with 40Ar:36Ar being approximately 1600:1.
2f spectra of protonated CO2 (top) and the CN radical (bottom).
2f Doppler and Lamb Dip spectrum of the J = 31,3 – 20,2 rotational transition of H2O taking with CASAC. The Doppler spectrum has been recorded at a pressure 15 mTorr, and frequency modulation set at a rate of 15 kHz and a depth of 350 kHz. The much narrower saturation dip has been obtained at a pressure 0.5 mTorr and the frequency modulation set to a rate of 1.5 kHz and a depth of 30 kHz. A model has also been fit to this saturation dip, to precisely determine the center frequency at 183310.08755(12) MHz. This uncertainty is based purely on the model fit, and is thus underpredicted; repeated measurements would thus give a more accurate estimate.
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