Aerodynamic focusing of heavy molecules in seeded supersonic jets

Abstract

The phenomenon of aerodynamic enrichment of heavy molecules seeded in supersonic free jets has been known since 1955. But its systematic exploitation in the generation of intensely focused molecular beams has been prevented by the lack of a quantitative and realistic explanation of the observed facts. Here, the aerodynamic focusing of CBr4, W(CO)6, and C2Cl6 molecules seeded in jets of He or H2 is studied experimentally, and found to be most singular under conditions similar to those known to produce sharply focused beams of microscopic spheres suspended in air jets. The gas mixture expands through thin-plate orifice into a vacuum chamber, forming a supersonic free jet. The spacial distribution of the heavy molecules in the jet is measured at varying distances L to the nozzle by scanning a thermocouple probe across a jet diameter. The probe is sufficiently small to interfere negligibly with the flow. The increment DV in the thermocouple voltage resulting from seeding the heavy gas on a given flow of He or H2 is seen to be a sensitive indicator of the local concentration of seed molecules in the jet. The following behavior is observed in terms of the same Stokes number or inertia parameter S that governs the simpler and better understood phenomenon of aerosol focusing. Below S=0.4 for H2 and S=0.2 for He, heavy molecule and aerosol beam widths are practically identical, and the boundary of the jet of heavy molecules is rather sharp. At higher values of S, aerosol beams show further reductions in cross section, down to less than 10% of a nozzle throat diameter dn. In contrast, the measured heavy species minimal beam widths or waists at a distance L∼dn from the throat are around 0.5 d n and 0.35 dn for jets of He and H2, respectively. In units of dn, these widths are several times larger than expected from elementary considerations on the defocusing effects due to Brownian motion (of the order of the square root of the molecular mass ratio between light and heavy molecules). Nonetheless, the thin-plate orifice nozzle yields considerably more concentrated jets of heavy gases than previously seen, with far-field enrichment factors for the seed species close to 50 in the case of H2 jets. This technique, thus, appears to provide a greatly improved source for intense molecular beams. Aerodynamically focused beams have a sharp distribution of kinetic energies, being ideally suited for cross beam and beam surface studies. But they are not quite so optimal for spectroscopic studies because they require moderate source Reynolds numbers (of order 100), at which the heavy gas undergoes very little translational, rotational, or vibrational cooling.