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.
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.