Molecular beam brightening by shock-wave suppression

TL;DR

This study demonstrates that cryo-cooling surfaces in supersonic molecular beams significantly suppress shock waves, leading to nearly tenfold increases in beam density, which enhances the brightness of cold molecular sources for various scientific applications.

Contribution

The paper introduces a novel method of cryo-cooling surfaces to suppress shock waves in supersonic beams, substantially increasing beam density and brightness.

Findings

Cryo-cooling reduces shock wave reflection and structure.

Beam density increases nearly tenfold at low surface temperatures.

Shock suppression enables scaling of beam density with source pressure.

Abstract

Supersonic beams are a prevalent source of cold molecules utilized in the study of chemical reactions, atom interferometry, gas-surface interactions, precision spectroscopy, molecular cooling and more. The triumph of this method emanates from the high densities produced in relation to other methods, however beam density remains fundamentally limited by interference with shock waves reflected from collimating surfaces. Here we show experimentally that this shock interaction can be reduced or even eliminated by cryo-cooling the interacting surface. An increase in beam density of nearly an order of magnitude was measured at the lowest surface temperature, with no further fundamental limitation reached. Visualization of the shock waves by plasma discharge and reproduction with direct simulation Monte Carlo calculations both indicate that the suppression of the shock structure is partially caused by lowering the momentum flux of reflected particles, and significantly enhanced by the adsorption of particles to the surface. We observe that the scaling of beam density with source pressure is recovered, paving the way to order of magnitude brighter cold molecular beams.