The Scattering of Positive Ions from a Platinum Surface

The velocity and intensity of positive ions scattered at various angles from a metal surface have been measured by allowing the ions scattered from a heated platinum target to pass through slits into the electric and magnetic analysing fields of a positive ray box. The source, oxide catalyst emitting potassium, caesium, or lithium ions, could be rotated round the target, so that the latter could be bombarded from any angle. When the target was brought to a red heat the intensity of scattering was markedly increased, presumably because of the removal of a layer of adsorbed gas.Velocity distribution. With a homogeneous initial beam the ions scattered at a given angle are nearly homogeneous in velocity. But the energy retained by the ions varies continuously with the angle of incidence, rising from 20 percent of the initial energy to over 80 percent as grazing incidence is approached. The velocity of ions scattered from a cold target on which an adsorbed layer has formed is much less, the energy retained rising to only 40 percent of the initial energy near grazing incidence.Angle distribution. A separate apparatus was used to explore the intensity of scattering in all directions, by means of a collector which could be rotated around the source. For angles of incidence near the normal the intensity of scattering is very small or even zero, but increases with increasing angle of incidence. The most intense scattering takes place in a forward direction, the number of particles which suffer large deflections being small. Thus for a fixed angle of incidence the number of particles scattered in a direction normal to the target is very small or even zero, but increases steadily with increasing angle of reflection.Critical energy. The intensity of scattering varies greatly with the energy of the incident ions, rising to a sharp maximum at about 40 volts. This critical energy for intense scattering is found to be the same for caesium, potasium, and lithium ions.