The interaction of atoms and molecules with solid surfaces VI—The behaviour of adsorbed helium at low temperatures

Abstract

The heat of adsorption of helium atoms on solid surfaces is so small that no one has succeeded in measuring it directly. Its value could probably be inferred from a series of accurate measurements of adsorption isotherms by using the formulae given in Paper III of this series, or it could be calculated from known interatomic force fields as has sometimes been done. Such calculations indicate that the heat of adsorption on crystals of the rock salt type would probably be of the order of 100 to 200 cals./gm. atom. Another method of determining it has, however, been found in Paper V from the diffraction experiments of Frisch and Stern for these indicate that helium atoms on a lithium fluoride crystal have two quantized vibration levels perpendicular to the surface, whose energies are —57.5 cals, and —129 cals, (reckoned from a zero in which the atoms are in the gas phase at rest). The lower one gives the heat of adsorption. From these energy levels it is possible to deduce some information as to the nature of the potential field between helium atoms and lithium fluoride crystals by choosing a function of the right properties (such that it vanishes at infinity, gives attraction at large distances and repulsion at short distances, and has a minimum at a finite distance from the surface), for which the vibrational wave equation can be solved. Such a function is the Morse potential function, which contains two parameters D and k , the former giving the depth of the potential minimum and the latter the rate at which the field falls off (exponentially) at large distances. The two energy levels given above are thus sufficient to determine D and k and they are found to be —175 cals, and (1.10) 108 cm. -1 .