We show that ultrasensitive microdroplet-stream fluorescence technique
s combined with surfactant forms of Rhodamine dyes can be used to prob
e single molecules on the surfaces of spherical microcavities. Individ
ual octadecyl Rhodamine B molecules, shown previously by ensemble meas
urements to be localized and oriented at the surfaces of liquid micros
pheres, were spatially photoselected primarily along great circles lyi
ng perpendicular or parallel to the detection axis by use of polarized
laser excitation. A polarization dependence is observed in the distri
bution of single-molecule fluorescence amplitudes that can be interpre
ted qualitatively in terms of position-dependent fluorescence-collecti
on efficiencies. (C) 1998 Optical Society of America.