The interaction of atoms and molecules with solid surfaces - X-The activation of adsorbed atoms by metallic electrons

Abstract

One object of this series of papers (Lennard-Jones and others 1935-7) is to consider in detail the mechanism of condensation, migration and evaporation of atoms and molecules at solid surfaces and to try to find the processes which govern the transition from one state to another. It has been shown that under certain conditions the thermal vibrations of a solid may activate an adsorbed atom from one vibrational state to a higher one or even eject it from the surface altogether. But the theory there developed is limited in the sense that it deals only with the transfer of single quanta to or from the solid, and consequently the quantized vibrational levels of the adsorbed atom must be closer together than the largest single quantum of energy which the solid can emit. An attempt has been made (Strachan 1937) to find the probability of the simultaneous emission or absorption of several quanta by the solid, and the indication is that the probability of several such simultaneous events is small. Now when atoms are bound to solid surfaces by valency forces, the vibrational levels are widely spaced compared with those of the solid, and many thermal quanta must be transferred simultaneously to the adsorbed atom to change its state of vibration. While this process may occur in nature, it seemed desirable to look for other possible processes whereby adsorbed atoms could be activated to higher vibrational states. One such possible mechanism, in metals at any rate, is by the transfer of energy from the conduction electrons. A simple calculation by classical methods indicates that in a typical case a surface atom may suffer as many as 10 15 collisions per second with the “free” electrons of a metal, and as, according to modern views, these electrons are moving with an energy of several volts, there is here an ample reservoir of energy from which adsorbed atoms may absorb energy or to which they can re-emit it, and thus change their vibrational state, or indeed, also their electronic state.