Knowledge Bank

Quantized energy losses by slow electron impact on solids are characteristic of the substance, but independent of its thickness and of the initial energy. They have been observed for aromatics, alkanes, alkenes, alcohols, acetone, ice, alkali halides and metals. Correlated luminescence has been investigated for some of these systems. The targets are supported thin films $(\sim 100$ \AA), usually at $77\circ K$, and in a $10-9$ torr enclosure. In one mode the backscattered electron current is energy-analyzed by a modulated retarding potential ramp at each incident energy. In a second mode the thermionic emitter ramp voltage V is modulated and the current I transmitted by the film is measured. In either case the spectrum is presented as dI/dV vs V. The range 0--50 eV has been used occasionally, but events lying below the first optically allowed transition are of principal interest. Such losses have been observed without exception in all systems examined.