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-dibenzanthanthre ne 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 a llows us to obtain structural information about this host/guest system. In the confocal detection volume of similar to 10 mum(3) the guest molecules s how two preferential orientations. A broad distribution of orientations of about +/- 17 degrees is observed around these two mean orientations. The me an orientations exhibit long-range order even over a distance of 2 mm with gradual variations on a micrometer scale. Over distances smaller than the l ateral dimension of the confocal detection volume these variations disappea r almost completely whereas the widths of the distributions remain qualitat ively the same. The results do not point to the presence of a polycrystalli ne structure which contradicts the common picture of Shpol'skii systems.