12 September 2005 Stimulated emission depletion studies of molecular probe dynamics
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Stimulated emission depletion (STED) population and polarisation dynamics are used to determine <α40> 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 <α40>. STED measurements of rhodamine 6G yield a value for <α40> 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 <α40> 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 <α40> are considerably larger. These results are discussed in terms of the initial hexadecapole alignment and <α40> relaxation dynamics in a restricted geometry.
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R. J. Marsh, R. J. Marsh, N. D. Leonczek, N. D. Leonczek, D. A. Armoogum, D. A. Armoogum, E. M. Monge, E. M. Monge, A. J. Bain, A. J. Bain, } "Stimulated emission depletion studies of molecular probe dynamics", Proc. SPIE 5925, Nanophotonic Materials and Systems II, 59250C (12 September 2005); doi: 10.1117/12.634088; https://doi.org/10.1117/12.634088

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