Characteristic energy losses of electrons scattered from incandescent solids

Abstract

The electron emission produced when solid conductors are bombarded with electrons of controlled speed has formed the subject of a great number of investigations. It is now generally recognized that this emission consists of three different parts: (1) Primary electrons, truly reflected without loss of energy; (2) electrons scattered back with reduced energy; and (3) secondary electrons proper, with very low velocities, which would seem to be produced from the atoms of the target by the same collision processes that give rise to the second group. In recent years considerable attention has been paid to the reflected electrons, the angular distribution of which reveals their wave character, if the target is a definitely orientated crystal of the substance in question. Some time ago I made some measurements on the velocity spectrum of the emission produced by electron bombardment, using a magnetic deflection apparatus of fairly high resolving power. The principal object of this investigation was to look for evidence of groups of electrons with character­istic velocities related to the soft X-ray levels of the substance. From certain theoretical considerations such electrons might be expected to be present in the emission. Targets of lithium, beryllium, boron, carbon and aluminium were tried, but in no case was there any evidence of electrons of the kind in question. These results are discussed in the paper mentioned. The distribu­tion curves obtained for different targets and bombarding voltages ranging from 40 to 900 volts were all similar in shape. The reflected electrons pro­duced a sharp and narrow peak, separated from the rest of the curve by a very deep minimum. The curve then rapidly rose to a maximum, corresponding to scattered electrons which had lost an energy equivalent to 25 volts in the collision. In addition to these, some experiments were made with targets of platinum and carbon, which could be kept at incandescence also when readings were taken. It was found that new maxima appear at high temperature, nearer to the reflected peak, and that the 25 volt maximum becomes very faint but reappears after some time on cooling. These changes were repeated several times. It was concluded that the 25 volt maximum was produced by an adsorbed layer formed on the cold target in the high vacuum, whilst the new maxima with hot targets should probably be regarded as characteristic of the target substance itself. Somewhat similar effects have been observed by Brown and Whiddington using a photographic method.