Supersonic Jet Ionization

Many of the experiments in this lab take advantage of the convenient properties of supersonic free jets.  Gas is expanded into vacuum through a small (0.03”) orifice. If the orifice is still much larger than the mean free path of the gas, many collisions will occur in the high-density region near the orifice.  These collisions accelerate the gas while increasing directionality of the flow, giving a dense jet with a relatively narrow velocity distribution that is kinetically “cold” (often below 1 K).  For molecular gases seeded in supersonic expansions, partial or complete thermal equilibrium of internal degrees of freedom of the seed molecule with the kinetic energy distribution has proven relatively efficient, allowing for the production of cold, isolated gas-phase molecules at temperatures well below their bulk freezing point.  This is especially useful in spectroscopy experiments where spectra of cold species are significantly simplified due to the reduction in the number of possible transitions.

In some conditions, particularly those characterized by a large number of many-body collisions, densities can become high enough to induce condensation of clusters in the expansion.  We make great use of this phenomenon not only to create the clusters of interest in our studies, but especially for the condensation of extra loosely-bound “tag”  species necessary for our photofragmentation experiments.  These clusters can be generated by coexpansion of a mixture of the target cluster molecules and tags, or by entrainment of the molecules of interest in the early stages of a pure tag expansion.  The ionization of clusters and the initiation of ion and radical chemistry is typically accomplished using a high-energy (1 keV) beam of electrons directed into the orifice.  If these interactions take place near the nozzle, the ensuing expansion cools the newly created species as outlined above, providing ideal targets for our experiment

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  3. Robertson, W., Kelley, J. & Johnson, M.; A pulsed supersonic entrainment reactor for the rational preparation of cold ionic complexes. Rev. Sci. Instrum. 71, 4431–4434 (2000)