Electron diffraction investigation of pulsed supersonic jets

Abstract

A pulsed nozzle source has been developed for electron diffraction studies of molecular clusters. Disturbances of the electron beam by the field actuating the pulser have been reduced to a negligible magnitude by introducing magnetic shielding and by carefully timing the electron shutter. Advantages of pulsed operation in diffraction experiments with a Laval nozzle source are considerable. In the Michigan unit stagnation pressures can be increased by an order of magnitude, and pumping requirements are greatly reduced. Moreover, cleaner diffraction patterns can be obtained because exposures can be adjusted to probe a freshly developed jet before gas scattered from the walls can reach the region of sampling. Timing works so effectively that good diffraction patterns can be recorded even when the skimmer isolating the diffraction chamber from the nozzle chamber is removed. Indeed, patterns from unskimmed jets can be particularly informative when cluster beams are composed of mixed structural forms. An unorthodox ‘‘vee’’ skimmer has proven to be useful for examining narrow regions in a jet. Moreover, the vee skimmer makes it possible to measure the diffraction geometry much more precisely than has been the case with conventional skimming configurations. Pulsed operation also facilitates an accurate characterization of the density and velocity distributions in the supersonic jet. The principal design features of a pulsed apparatus are presented together with characteristic results.