Alkali halides adsorbed molecule interaction

A similar behavior may be found when the charge distribution in the molecules is more complex. In carbon dioxide the charge distribution is of the character of a quadrupole. Lenel (40) calculated the influence of the interaction of this quadrupole with the surface of an alkali halide crystal and reached the conclusion that a substantial contribution of roughly 3 kcal./mole is to be expected from this polar interaction. Recently Drain (41) succeeded in approaching the remarkable fact that the heat of adsorption of N2 on ionic crystals is often appreciably higher than that of 02 and A, which is not the case when these gases are adsorbed on nonionic surfaces. He shows that the quadrupole of N2 may be responsible. We shall return to this problem in Sec. VI,2. 

There is a long history of calculations of adsorption potentials for simple gases adsorbed on the exposed low index Miller planes of ionic crystals, especially alkali halides (see the review [26] and references therein). The total interaction potential energy between an adsorbed molecule and the surface of a solid is generally expressed as a sum of dispersion, repulsion, induction, and electrostatic contributions (see, e.g.. Ref. [27]) ... 

Even in the absence of the special effects of hydrogen bonding, bandwidths of 10 cm or more are common from adsorbed species on polycrystalline substrates owing to interactions with sites that differ in their detailed environments. Absorptions obtained from adsorbed species on single-crystal planes with uniform and well defined sites can, in contrast, be very sharp with bandwidths of less than 1 cm i. In the case of methane itself, and of a number of other molecules such as CO and CO2, the resolution of the spectra on alkali metal halide single-crystal surfaces are such that even the fine-structure splitting caused by the vibrational couplings of more than one molecule in the surface unit cell can readily be resolved. 

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