The CASAC (CAS Absorption Cell) Experiment

The CAS laboratories is home to a long-pathlength absorption cell spectrometer.

The centerpiece of this spectrometer is a long-pathlength glass tube (3m-long x 5cm-diameter), which serves as the main flow cell. At each end, it is equipped with high-density polyethylene windows transparent to long-wavelength radiation. Near each end are a number of vacuum flanges, one of which leads to two backing pumps (a diffusion pump and a rotary-vane pump), and others which enable access to pressure gauges and sample input ports. In the center is a region 2 meters in length, which has a large metal electrode at each end, is wrapped with tubing on the outside of the cell, and is centered in a copper solenoid, enabling the ability to form a cooled, confined plasma from an appropriate mixture of gaseous precursors. Spectroscopy through the cell can then be performed in the range of 80–1100 GHz.

Schematic diagram of the CAS@MPE CASAC experiment.

Standard operating pressure of the flow cell can reach below the 1 mTorr level. The diffusion pump that serves as the main workhorse functions at ca. 1,500 L/s (VHS-6, Agilent), and a large butterfly valve can serve to choke the pumping speed as one wishes.
A 2 kW DC power supply is provided for the discharge electrodes, and a separate power supply can power the solenoid up to 50 A (i.e. a 350 Gauss magnetic field). Liquid nitrogen is used to cool the cell down to temperatures of ca. -190°C. The plasma discharge requires an internal pressure above ca. 15 mTorr for stable operation.
Spectroscopy is provided by Schottky-based multiplier chain (AMC, Virginia Diodes Inc.) and either a Schottky detectors (VDI) or a hot-electron bolometer (QMC Instruments). Optionally, a wire-grid polarizer and retroreflector can be used to perform double-pass spectroscopy, which can help with saturation spectroscopy (i.e. Lamb Dip).
We have a number of high-pressure gas cylinders available for sample mixtures, as well as glass flasks and bubblers and heating tape. Found mounted in a large fumehood is a large glass manifold containing four ports and a cryogenic trap, for preparing chemical samples or gas 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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