Activation of Molecular Hydrogen by Electron Impact. Velocity Distribution of Electrons Issuing from Small Holes. Activation of Gases by Electron Impact

Abstract

I. Activation of molecular hydrogen by electron impact. In this research we have undertaken to study the activation of hydrogen molecules by electron impact under conditions in which we know the energies of the impinging electrons. The experiments of Cario and Franck show that hydrogen molecules can be activated by excited mercury atoms by collisions of the second kind and that copper oxide and tungstic oxide can then be reduced. In their experiments the mercury atoms receive energy of 4.9 volts from the light source, which is sufficient to dissociate hydrogen molecules, the heat of dissociation of hydrogen being 3 to 4 volts. Furthermore, it has been known for some time that in a discharge tube hydrogen will disappear when a discharge is passed. Hughes, in particular, has investigated the electrical clean-up of hydrogen and nitrogen, and finds a definite decrease in the hydrogen pressure at 13.3 volts and higher. He adopts Langmuir’s conclusion that hydrogen is dissociated under these conditions, and that the decrease in pressure is due to the freezing out of atomic hydrogen on surfaces cooled by liquid air. His results will be referred to later in connection with our own experiments. In Hughes’ investigations no copper oxide was present, and the minimum electron energy at which hydrogen disappears was not accurately determined. A number of experimenters also have investigated the chemical reactivity of hydrogen activated by an electric discharge, but the energies of the impinging electrons were not known. We shall discuss four possible mechanisms by which electrons may be expected to activate hydrogen molecules. First, it might be that an electron having kinetic energy of 3 to 4 volts could transfer its energy to the hydrogen molecule and cause its dissociation into atoms. These in turn could then react with other substances. However, it is known that no kink occurs in current-potential curves of hydrogen near 4 volts, and it seems, therefore, that electrons having kinetic energy equal to the dissociation energy of hydrogen molecules cannot transfer their energy to these molecules. We then should expect to find no evidence of reaction when hydrogen is bombarded with 4-volt electrons, and our experiments actually do give no indication of reaction. This is in agreement with the commonly held idea that the dissociation of molecules does not occur as the direct result of electron impact. Second, electrons may have to possess sufficient energy to resonate the molecule, which may then dissociate if its heat of dissocitation is less than its resonance potential. The hydrogen molecule has, according to the latest results of spectroscopy, a resonance potential at 11.6 volts. Electrons of this energy can raise hydrogen molecules into an upper quantum state. These activated molecules would ordinarily return to the normal state after a short time. However, they may either dissociate into atoms upon impact with other molecules and the atoms thus react, or the resonated molecules may act directly on any oxide or other substance on which they impinge. A third mechanism of causing activation by electron impact might be that where the impinging electron transfers enough energy to the hydrogen molecule to cause its dissociation and resonance of one of the atoms. This process may be expected at 13 to 14 volts, which is the sum of the heat of dissociation and the resonance potential of the hydrogen atom. This is the mechanism postulated by Hughes. A fourth possibility may be considered. It may be necessary that an electron ionize a hydrogen molecule before the latter can be made to react. It was found by Anderson and Storch and Olson that nitrogen and hydrogen reacted to form ammonia when bombarded by 17-volt electrons. This voltage is near the ionization potentials of these molecules. We may state at once the results of our experiments. Electrons of 11.4 volts’ energy can activate hydrogen molecules, for we find that there is a definite pressure decrease when the accelerating voltage applied to our tube has this value. At the same voltage we also obtain a kink in the current-potential curves, using the Franck method. This research was aided financially by a grant made to Professor A. A. Noyes by the Carnegie Institution of Washington. II. Velocity distribution of electrons issuing from small holes. The velocity distribution of a beam of 50-volt electrons issuing from a hole 0.022 cm in diameter in a copper plate 0.02 cm thick has been measured. Seventy percent of the electrons were found to retain approximately their initial velocity. By coating the sides and edges of the hole with lampblack 95% of the electrons were transmitted without appreciable energy loss. Similar results were obtained with grids of 100 mesh copper gauze. III. Activation of gases by electron impact. The purpose of this research was to study the chemical behavior of gases activated by electron impact and to measure the energy necessary for their activation. The general plan was to use an apparatus similar to that employed in the study of the reaction between copper oxide and active hydrogen by Glockler, Baxter, and Dalton. The gases were introduced at pressures of 0.05 - 0.3 mm of mercury into a four electrode tube connected with a liquid air trap and a Birani pressure gauge. A four electrode tube was used in order that the energy loss of the electrons might be studied by making current potential curves as done by Franck and others. With this arrangement it was possible to follow the course of any reaction which involved a change in the number of molecules or produced a product condensible in liquid air. It was of course not possible to use gases which would undergo reaction when exposed the the hot filament. The systems studied were: oxygen and carbon, carbon monoxide and hydrogen, nitrogen and copper. Only the first gave definite positive results and for this reason it will receive the most attention. The author wishes to express his thanks to Professor Tolman, Dr. Glockler and Mr. Baxter for their assistance in this research; also to the Carnegie Institution of Washington for financial aid received through a grant made to Professor A.A. Noyes.