Fluorescence microscopy and spectroscopy of an isolated micro-droplet

Abstract

Fluorescence microscopy and spectroscopy experiments are carried out on individual microdroplets (μ-drops) isolated within an electrodynamic levitator-trap (Paul type) at atmospheric pressure. Imaging experiments on spatially homogeneous donor and acceptor dyes within particles of radius a ~ 10 μm reveal a ‘global’ transfer mechanism with a yield exceeding conventional dipole-dipole transfer (Förster transfer) by ~ 1000 × between molecules at a particular radial distance 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 energy transfer through morphology dependent resonances (MDRs) of the sphere (P.T. Leung, K. Young, J. Chem. Phys. 89 (1988) 2894). To gain a better understanding for the primary step in this process, we investigate fluorescence from a particle with a dilute layer of surface active molecules. These surfactant molecules have a fixed orientation relative to the surface as revealed both qualitatively from a heuristic interpretation of polarization analyzed images (S. Arnold, L.M. Folan, Proc. SPIE 1862 (1993) 218), and quantitatively through the preferential cavity enhanced selection of various polarized modes in emission spectroscopy. The spectra are adequately described through semi-classical theory (S.D. Druger, S. Arnold, L.M. Folan, J. Chem. Phys. 87 (1987) 2649). This theory has also been applied to the description of reported ‘anomalous’ fluorescence lifetime measurements (M.D. Barnes, C.-Y. Kung, 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 describe the data at particle sizes for which the homogeneous linewidth is larger than the free spectral range (M.D. Barnes, C.-Y. Kung, W.B. Whitten, J.M. Ramsey, S. Arnold, S. Holler, Phys. Rev. Lett. 76 (1996) 3931), semi-classical theory, including all modes, is in considerably better agreement with the data