Abstract

The use of fluorescent probes is both widespread and varied in application. Standard fluorescence polarisation experiments whilst providing useful information on probe relaxation dynamics in isotropic media^1,2 possess two disadvantages in studies of molecular motion and order in anisotropic systems. Firstly there is inherent sensitivity to only one degree of angular freedom (θ motion) in the laboratory frame, secondly the symmetry constraints of single photon electric dipole transitions preclude the ability to precisely control of the initial order of a photoselected population. Work in our laboratory has sought to address these two issues. Use of a variable excitation polarisation in a 180° excitation-detection geometry permits the photoselection of a range of initial non-equilibrium probe orientations in an anisotropic medium which are sensitive to both θ and ϕ motions and whose evolution can be monitored by picosecond time resolved polarised fluorescence⁵'⁵ [Fig. 1(a)]. We have recently developed a novel technique which permits full alignment control of an excited state population using the net photoselection of three synchronised laser pulses [Fig. 1(b)].⁶-⁷ Here we report the first combination of these two techniques to determine the full orientational dynamics and ordering of Rhodamine B (2 × 10 ⁴M) in nematic 5CB.

© 1999 Optical Society of America