Microscopic structure in a Shpol'skii system:: A single-molecule study of dibenzanthanthrene in n-tetradecane

A study is presented of the orientation of single 2.3, 8.9-dibenzanthanthrene molecules in n-tetradecane as a function of their spatial position on a length scale of 2 mm down to a few hundreds of nanometers at 1.4 K, which allows us to obtain structural information about this host/guest system. In the confocal detection volume of similar to 10 mum(3) the guest molecules show two preferential orientations. A broad distribution of orientations of about +/- 17 degrees is observed around these two mean orientations. The mean orientations exhibit long-range order even over a distance of 2 mm with gradual variations on a micrometer scale. Over distances smaller than the lateral dimension of the confocal detection volume these variations disappear almost completely whereas the widths of the distributions remain qualitatively the same. The results do not point to the presence of a polycrystalline structure which contradicts the common picture of Shpol'skii systems.