Sequential counting of single molecules in a microcapillary

We studied photon bursts of individual rhodamine 6G molecules in ethylene glycol flowing through a microcapillary (diameter 1 micrometers ). Excitation was provided by an argon ion laser ((lambda) equals 514.5 nm). The laser beam was coupled into a confocal microscope by a glass plate and focussed onto the sample by an oil-immersion objective (magnification 100X). Fluorescence was collected by the same objective and detected by a photomultiplier. In order to reject out-of-focus signals a pinhole (diameter 100 micrometers ) was placed in the image plane. Several filters were used to suppress laser stray light and Raman scattered photons. With the setup described sequential counting of individual molecules is possible.