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.

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

In one experiment, the CASAC (CAS Absorption Cell) spectrometer , we are focused on light 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.

Fine and hyperfine structure of the N = 1 - 0 rotational transition of 15NH in its ground vibrational state. The bottom plot illustrates a schematic representation of the fine structure splitting. The top plot shows the recordings of the J = 0 - 1, J = 2 - 1, and J = 1 - 1 fine-structure components. The blue traces are the experimental spectra recorded with the time constant RC = 3 ms and accumulation times of ca. 350 s. In both plots, the red bars indicate the line positions and the relative intensities computed from the best-fit parameters.
Recording (blue trace) of a portion of the J = 2–1 fine-structure transition of (15)NH showing the four strongest components F1,F= 5/2,2 The red dotted trace plots the modelled spectrum computed with the proFFit code using a modulated Voigt profile. The green trace plots the difference between the observed and the calculated spectrum.
In addition, a free-jet millimetre and sub-millimetre-wave spectrometer has been set up that uses a pulsed valve configured with a number of accessories at its front, to perform spectroscopy on unstable species within a supersonic free-jet expansion. The supersonic free-jet expansion provides 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.
Laboratory spectrum of the HSCO+ JKa,Kc = 41,4 − 30,3 rotational transition around 318 GHz, acquired with the free–unit jet experiment. The integration time is ~ 15 mins with 30 μs time constant. The red dashed line represents the best fit to a speed-dependent Voigt profile. Each rotational transition has a double–peaked line shape, the result of the Doppler shift of the supersonic molecular beam relative to the two travelling waves that compose the radiation beam.
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