Plasma Chemistry

Both ions and neutral radicals are potential stepping-stones between the hydrogen-dominated chemistry of diffuse clouds and the complex organic chemistry observed in dense star-forming regions. We use plasma discharge to prepare these ions and radicals and subsequently collect their millimeter-wave rotational spectra for direct comparison to telescopic data of the interstellar medium (ISM) and other astrophysical sources.

In our laboratory, we have two plasma sources: a hollow cathode, and a supersonic expansion source. Our liquid-nitrogen cooled hollow cathode, based on the design of T. Amano [1], is an efficient discharge source to produce positive ions, especially protonated species. The copper cathode is housed in a PVC cell where appropriate precursor gases can flow continuously through. A high voltage source is then coupled to an anode, positioned halfway along the length of the cathode, and the resultant discharge produces a plasma that fills the length of the cathode tube. The species are then cooled and stabilized by the liquid-nitrogen. The supersonic expansion source, based on the design of M. C. McCarthy [2], creates a discharge that arcs across the beginning of a supersonic molecular beam which quenches spurious reactions and decompositions as well as simplifies the rotational spectrum by trapping molecules in their ground (or lower) vibrational states. This source is versatile in producing neutral molecules, radicals, ions and molecular complexes.

The liquid-nitrogen cooled hollow cathode in operation.
View of the discharge down the length of the hollow cathode.
The McCarthy-style discharge source coupled with a supersonic expansion.
A multipass optical arrangement (used to increase pathlength and therefore sensitivity of the spectrometer) is aligned onto the supersonic expansion and discharge using a HeNe laser for visualization.

[1] T. Amano, J. Opt. Soc. Am. B, 2, 790 (1985).
[2] M. C. McCarthy et al., Astrophys. J. Suppl. 129, 611 (2000).