An investigation of the effects of electron collisions with platinum and with hydrogen, to ascertain whether the production of ionisation from platinum is due to occluded hydrogen

Abstract

In the course of an investigation of the effects of electron collisions with helium atoms, it was found that positive ions were produced from a positively charged platinum gauze when this was bombarded by electrons with a minimum velocity of about 11 volts. The production of positive ions in this way does not appear to have been observed before for such small velocities of the impacting electrons, though their detection with primary rays of 30 volts speed is recorded by Campbell in the account of his experiments on ionisation by charged particles. In this paper Campbell gives an excellent summary of the work which has been done in connection with the bombardment of metal surfaces by slow cathode rays. The work of Lenard, Baeyer, Gehrts, Campbell, and others has established the facts that, when such rays fall on a negatively charged metal surface, electrons leave the surface, and that the number and the speed of these electrons depends on the velocity of impact of the primary stream. When the velocity of the incident rays is less than 11 volts, the electrons leaving the plate are those of the primary stream which have been reflected at the metal surface, the characteristic of these reflected rays being that most of them have a velocity comparable with that of the incident rays. When the velocity of impact reaches 11 volts, in addition to reflexion, an excitation of secondary rays begins and gradually increases in amount as the speed is further increased up to about 200 volts. On this account the curve showing the relation between the velocity of impact and the number of electrons leaving the plate takes an upward turn at 11 volts, but no other bend occurs until 200 volts is reached. It has therefore been concluded that the two processes mentioned above, viz., reflexion, and excitation of secondary rays beginning at 11 volts, are the only causes of electrons leaving the impacted surface. It was found by Baeyer that with a minimum velocity of impact of about 25 volts more electrons leave the plate than fall on it, and this result, combined with the fact that Campbell detected a positive current (presumably from a positively charged plate) when the bombarding electrons had a minimum velocity of 30 volts, has given rise to the view that the second process—that which is operative above 11 volts—is an ionisation at the metal surface, the material ionised being either the metal itself or gas attached to it. On this view it is concluded that the critical velocity of 11 volts is the “ionisation potential ” of the material ionised. This velocity was found to be the same for all the metal surfaces tested, and as it agreed with the usually accepted value of the “ionisation potential” for hydrogen, it has been suggested that the material ionised is hydrogen present in the surface of the metal. Some confirmation of this view has been obtained by Campbell from experiments with metal surfaces subjected to treatment designed to modify the amount of hydrogen present, but the evidence hitherto produced cannot be said to prove conclusively that the process which begins at 11 volts is a genuine ionisation by electron collisions. That ionisation occurs ultimately can hardly be doubted, for it has been observed that when the velocity of the electron stream is sufficiently increased, the number of electrons leaving a bombarded electrode is in some cases as much as twenty times as great as the number arriving at it, but the first direct proof that ionisation occurs at the metal surface when the impacting electrons have a velocity as small as 11 volts seems to be that given by the experiments made in the course of our investigation of the ionisation of helium. The present research is a fuller investigation of this effect, undertaken in order to ascertain whether the evidence of ionisation at 11 volts could be substantiated, and, if so, to determine whether it should be attributed to the metal itself or to hydrogen attached to the metal surface.