The Molecular Jet Experiment - CASJet
Spectroscopic characterisation of reactive species requires the molecules to be produced in situ while electromagnetic radiation is probing them, in order to record their spectroscopic fingerprints. These fingerprints serve as a scientific identity card which allows the astronomer to identify them in interstellar environments.
To extend the capabilities of the CAS laboratory for rotational spectroscopy, and in particular to complement the existing absorption cell (CASAC), a free-jet millimetre and sub-millimetre-wave spectrometer (CASJet) has been set up, with its first light recorded in 2018. The molecular beam, generated from a mixture of chemical samples controlled via mass flow controllers, is injected into a high-vacuum expansion chamber (~10⁻⁵ Torr / 10⁻³ bar) through a 1-mm pinhole of a pulsed valve. The expansion into the chamber is supersonic, due to the large pressure gradient between the valve (few kTorr / few bar) and the chamber. This supersonic expansion allows adiabatic cooling of the molecular beam, yielding temperatures in the range of approximately 7–20 K, depending on the buffer gas, significantly lower than those achievable in the CASAC spectrometer (~80 K). Coupling of the molecular beam to the mm- and submm-wave radiation is achieved via a roof-top mirror inside the chamber, which also contains the injection aperture. The probing radiation enters through a Teflon window on the opposite side, interacts twice with the molecular sample, and is reflected back outside the chamber.
The production of unstable species is accomplished by attaching a high-voltage, low-current DC discharge nozzle to the front of the pulsed valve, through which molecules pass immediately after the valve and before free expansion. Additionally, the system can be operated with a heated nozzle assembly, consisting of a heating reservoir, valve, and discharge nozzle, capable of reaching ~250 °C. This configuration allows the efficient vaporization of low-vapor-pressure liquids and solids prior to expansion, extending the range of molecular precursors that can be studied.
It is within the free expansion—often referred to as the "zone of silence"—that the molecular sample is rapidly stabilized. Because the gas expands at supersonic velocities, only a few intermolecular collisions occur, resulting in effective isolation of highly reactive species.
The instrument operates in the 80–1600 GHz range (4–0.2 mm), and can also be coupled with the CP-FTS. Thus it covers the entire frequency bands accessed by state-of-the-art millimetre observing facilities, such as ALMA, and the lower THz band of the SOFIA aircraft observatory. The radiation source is an active multiplier chain driven by a cm-wave frequency synthesiser (9–50 GHz). Schottky diodes and liquidhelium cooled hot-electron bolometers are used as detectors, with the latter providing higher sensitivity and a lower noise-figure at higher frequencies. The instrument has been designed with the goal of maximum flexibility to move through projects involving different class of molecular pecies, such as radicals, ions and also stable molecules (see Figure 5.2.4, right panel, for an example spectrum). In this latter case, the discharge nozzle can simply be turned off. This case is particularly suited for the so called interstellar Complex Organic Molecules (iCOMs). These molecules are key ingredients in several interstellar regions and exhibit a very dense and complex spectra. Thanks to the low rotational temperatures that the molecular beam can reach, considerably less energy levels are populated causing a simplification of the otherwise dense and complex rotational spectrum.
