Angular Scattering Of Electrons In Hydrogen And Helium

564 PHYSICS: G. P. HARNWELL PROC. N. A. S. Ag II; A. G. Shenstone, Physical Review, 31, 317, 1928. Ag I, Cd II, In III, Sn IV, Sb V; J. A. Carroll, Phil. r'rans. Roy. Soc., A225, 379, 1926; R. J. Lang, Proc. Nat. Acad. Sci., 13, 341, 1927. Ru II, Rh II; H. G. Clark and E. Cohen, Trans. Roy. Soc. Canada, 20, 55, 1926. ANGULAR SCATTERING OF ELECTRONS IN HYDROGEN AND HELIUM BY GAYLORD P. HARNWsLL* CALIFORNIA INSTITUTE oF TsCHNOLOGY, PASADZNA Communicated June 9, 1928 In view of the recent work which has been done by Davisson and Germer,l G. P. Thomson,2 and others in investigating the angular scattering of electrons by crystals, and the work of E. G. Dymond3 in extending these investigations to the scattering of electrons by helium, it was.thought worthwhile to see if similar results could be obtained for this -scattering by other monatomic and diatomic gases. The phenomena observed when electrons are scattered by crystals are susceptible of explanation in a very satisfactory way on the wave mechanics hypothesis, but the theory is less fully developed for the case of gases and the results obtained by Dymond for scattering in helium have so far met with no adequate explanation. However, selective angular scattering might conceivably be expected in a monatomic gas. In this case it might be supposed that atomic hydrogen would also give rise to similar phenomena, and in view of its greater simplicity an explanation would present less serious mathematical difficulties. Hence, the following work was undertaken primarily to investigate the possibility of selective angular scattering in atomic hydrogen. Molecular hydrogen was also used though there was less reason to expect the phenomena in a diatomic gas. Finally, the experiments performed by Dymond in helium were repeated under as nearly similar conditions as possible. The apparatus used is shown in figure 1. The scattering chamber consisted of a glass bulb about ten centimeters in diameter. A large ground glass stopper mounted vertically through a mercury seal carried the electron gun, G. This consisted of a tungsten filament mounted behind two collimating slits about a quarter of a millimeter wide and a centimeter and a half apart. By means of the ground glass stopper this could be rotated through nearly a complete circle. The angular setting could be read by means of a pointer attached rigidly to the stopper carrying the gun and an angular scale attached to the bulb. The front slit of the gun Voi. 14, 1928 PHYSICS: G. P. HARNWELL KA65 was about two and a half centimeters from the center of the bulb. As an electrometer was not available the electrons could not be analyzed magnetically and a scheme similar to that used by Davisson and Germer had to be adopted. A brass tube carrying a slit SI about a tenth of a millimeter wide projected through a wax joint to within a few millimeters of the center FIGURE 1 of the bulb. About four centimeters farther in the tube was placed a collimating slit, S2, and immediately behind this an insulating plug supported a Faraday cylinder as shown. By varying the potential of the Faraday cylinder those electrons which had lost less than any given fraction of their energy could be collected. A diaphragm was mounted be- 566 PHYSICS: G. P. HARNWELL PROC. N. A. S. tween Si and 52 and a skeleton partition supported S2 so that little resistance was offered to the flow of gas in -the brass tube. A continuous flow method was used and the pressure in the brass tube could be kept lower than that in the scattering chamber by means of diffusion pumps evacuating through El. The other exit E2 was connected to a second set of diffusion pumps. A discharge tube about a meter and a quarter long was mounted with its center directly below the scattering bulb. A capilla