FLUORESCENCE MICROSCOPY AND SPECTROSCOPY OF AN ISOLATED MICRO-DROPLET

Fluorescence microscopy and spectroscopy experiments are carried out o n individual microdroplets (mu-drops) isolated within an electrodynami c levitator-trap (Paul type) at atmospheric pressure. Imaging experime nts on spatially homogeneous donor and acceptor dyes within particles of radius a similar to 10 mu m reveal a 'global' transfer mechanism wi th a yield exceeding conventional dipole-dipole transfer (Forster tran sfer) by similar to 1000 x between molecules at a particular radial di stance r > 0.93 (S. Arnold, S. Holler, S.D. Druger, J. Chem. Phys. 104 (1996) 7741), but is consistent with the quantum theory of enhanced e nergy transfer through morphology dependent resonances (MDRs) of the s phere (P.T. Leung, K. Young, J. Chem. Phys. 89 (1988) 2894). To gain a better understanding for the primary step in this process, we investi gate fluorescence from a particle with a dilute layer of surface activ e molecules. These surfactant molecules have a fixed orientation relat ive to the surface as revealed both qualitatively from a heuristic int erpretation of polarization analyzed images (S. Arnold, L.M. Folan, Pr oc. SPIE 1862 (1993) 218), and quantitatively through the preferential cavity enhanced selection of various polarized modes in emission spec troscopy. The spectra are adequately described through semi-classical theory (S.D. Druger, S. Arnold, L.M. Folan, J. Chem. Phys. 87 (1987) 2 649). This theory has also been applied to the description of reported 'anomalous' fluorescence lifetime measurements (M.D. Barnes, C.-Y. Ku ng, W.B. Whitten, J.M. Ramsey, S. Arnold, S. Holler, Phys. Rev. Lett. 76 (1996) 3931) from a dilute surface layer. Although sum rules based on density of states of high Q whispering gallery modes fail to descri be the data at particle sizes for which the homogeneous linewidth is l arger than the free spectral range (M.D. Barnes, C.-Y. Kung, W.B. Whit ten, J.M. Ramsey, S. Arnold, S. Holler, Phys. Rev. Lett. 76 (1996) 393 1), semi-classical theory, including all modes, is in considerably bet ter agreement with the data. (C) 1997 Elsevier Science S.A.