Knowledge Bank

A comparison of ATR and transmission data for molten inorganic salts shows that the ATR and transmission infrared spectra invariably agree within $2cm-1$ for bands which can conveniently be studied by both $techniques.1$ However, disparities greater than $10cm-1$ which cannot be related to sample orientation effects have been noted for certain ionic solids. Such large differences between the ATR and transmission solid state values suggest a fundamental difference between the surface and bulk structures of ionic solids. Data for the alkali metal nitrates are consistent with surface structures intermediate to the bulk solid and melt structures. These data are also consistent with low energy electron diffraction studies suggesting distinguishable surface $structures2,3$ but contrast with the general view that the surface structure has an extension of only a few $monolayers.4$ Rather, a structure which approaches the bulk structure exponentially with increasing penetration and with a ``depth'' of the order of a micron is suggested for $LiNO3$.