Stimulated Emission Depletion Studies of Molecular Probe Dynamics

Stimulated emission depletion (STED) population and polarisation dynamics are used to determine 40 α the degree of hexadecapole alignment created in ensembles of rhodamine 6G molecules in solution following two-photon excitation. Hexadecapole molecular alignment is an unavoidable consequence of two-photon excitation but is not observed in spontaneous emission. For a single element diagonal transition tensor measurements of the fluorescence anisotropy R(t) in systems undergoing small step isotropic rotational diffusion can in principle be used to determine 40 α . STED measurements of rhodamine 6G yield a value for 40 α that is larger than that predicted for a single element transition tensor (SXX). Recent work in our laboratory indicates that whilst SXX is dominant SYY, SXY and SYX are finite, measurements of 40 α appear to be a sensitive probe of the structure of the two-photon transition tensor. STED and fluorescence anisotropy measurements are extended to Rhodamine 6G in the isotropic phase of 5CB a system where small step isotropic rotational relaxation does not take place. Here the values of 40 α are considerably larger. These results are discussed in terms of the initial hexadecapole alignment and 40 α relaxation dynamics in a restricted geometry.