Design of a high density atomic hydrogen source and determination of Balmer cross sections

Abstract

A high density beam of atomic hydrogen was obtained from a Wood discharge and utilized in the measurement of electron-hydrogen atom collision cross sections. Balmer optical excitation cross sections were determined from observations of crossed electronic and atomic beams. In order to choose Wood discharge parameters to obtain a high degree of dissociation under conditions of a high gas throughput, the differential equations relating the production, loss, and flow of hydrogen atoms and molecules were formulated and solved. The solutions show that recombination of atoms on the metal electrodes greatly reduces the degree of dissociation in large diameter tubes. They further show that this effect can be nearly eliminated by placing short small diameter tubes, called constrictions, between the electrodes and the main tube. A Wood tube 2 cm in diameter and 2 m in length with these constrictions was constructed which produced an atomic to molecular density ratio of 11 at a throughput of 1000 mtorr · liter/sec. The atoms and molecules from the Wood tube were passed through a nozzle and formed into a beam which was crossed with a monoenergetic electron beam. In the crossed beam volume the atomic density was approximately 5.6×1012 cm−3 and the background molecular density was approximately 3.3×1013 cm−3. Optical excitation data were obtained by observation at 90° to the crossed beams with a monochromator and photomultiplier detection system. The ratio of the atomic density with the Wood discharge on to the molecular density with the Wood discharge off in the crossed beam volume was determined from molecular light intensity measurements and a derived relation. Combining this ratio with Balmer optical excitation data with the Wood discharge on and off gave the ratio of the atomic and molecular Balmer cross sections. Absolute molecular Balmer cross sections were measured in a static gas apparatus for use in determining the absolute atomic Balmer cross sections. The atomic Balmer cross sections (in units of 10−20 cm2) for Hα, Hβ, Hγ, Hδ, and Hε at 200 eV thus determined are 229.0, 72.4, 22.5, 12.9, and 6.5, respectively. All of these cross sections are within 23% of the theoretical optical cross sections determined under zero electric field conditions.