Influence of the size factor on the dynamic characteristics of cryocrystals

We present the results of an electron-diffraction determination of the rms displacements ⟨u2⟩ of atoms in island films of Ar, Kr, and Xe, as well as the degree of orientational ordering of molecules in thin films of the a phases of N2 and CO. The effective size of the specimens varied from 2–3 to several tens of angstroms while the size L of crystallites varied from 30 to 200 Å. The value of ⟨u2⟩ in the direction perpendicular to the wave vector of the primary electrons was determined from an analysis of the intensity of the diffracted beams as a function of the temperature. The initial data for calculation of the parameter of orientational ordering consisted of the ratio of the intensities of the “superlattice” lines to those of the fundamental lines. As a result of the observations carried out we found an appreciable increase in ⟨u2⟩ in small particles of solidified inert gases. Comparison of the experimental results with a theoretical calculation enabled us to establish that this effect is due not only to the growth of the contribution of the surface but also to the substantial weakening of the interatomic interaction in small van der Waals crystals. This apparently was also responsible for the substantial lowering of the degree of orientational ordering of N2 and CO molecules in specimens with a small thickness.

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