Molecular Spectroscopy of Reactive Species

Nearly 200 molecules have been detected in extraterrestrial environments. Due to the extreme nature of many such environments compared to those on Earth, many of these species are difficult to prepare and observe, and require only specific conditions that only a laboratory can provide: specific temperatures ranges, specific pressures, specific non-thermal excitations, specific reactants, or any combination of the aforementioned.

In one experiment, the CASAC (CAS Absorption Cell) spectrometer, we are focused on light (up to ~4 atoms) molecular ions and radicals. The centerpiece of this spectrometer is a long-pathlength glass tube, 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, via a Schottky-based multiplier chain and either a Schottky detectors or a hot-electron bolometer.

A discharge experiment using the CASAC instrument. The colored glow is from the high-voltage discharge of a mixture of argon and hydrogen.

In another experiment that is still under development, we will use a pulsed valve configured with a number of accessories at its front, to perform spectroscopy on unstable species within a supersonic free-jet expansion. In some cases, the supersonic free-jet expansion can provide an ideal place to study unstable species at the very low temperatures found in interstellar environments. The isolating nature of the supersonic expansion also provides the opportunity to perform violent processes on molecules and then immediately, in some sense, “freeze” them in time before they are able to further react (that is, destroyed or converted) to stable states.

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