The Molecular Jet Experiment
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 has been set up, whose first light has been seen in 2018. The molecular beam, a gas generated by the mixture of different chemical samples connected to 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 of the molecular beam into the chamber is supersonic thanks to the high pressure gradient between the valve (few kTorr/ few bar) and the chamber. This supersonic expansion allows the adiabatic cooling of the molecular beam, yielding temperatures in the range of approximately 7 to 20 K, depending on the buffer gas used, significantly lower than those reachable in the CASAC spectrometer (~ 80 K). The coupling of the molecular beam to the mm- and submm-wave radiation is obtained through a roof-top mirror placed inside the chamber, which also contains the aperture through which the molecular sample is injected. The probing radiation enters the vacuum section through a teflon window on the opposite side of the roof-top mirror, interacts twice with the gas, and finally reflects back outside the chamber. The production of unstable species is achieved by attaching a high-voltage low-current DC nozzle to the front of the valve, through which the molecules pass right after the pulsed valve and prior to free expansion. It is this free expansion (Figure 5.2.4) where the molecular sample is quickly stabilised in the region dubbed the “zone of silence”; since the gas expansion moves at supersonic velocities, only a few intermolecular collisions are experienced, thus obtaining an efficient 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 CPFTS. 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.