TRACE-LEVEL QUANTITATION VIA TIME-RESOLVED 2-PHOTON-EXCITED FLUORESCENCE

With the use of a cavity-dumped, synchronously pumped dye laser for ex citation, two-photon fluorescence cross sections are approximately eig ht orders of magnitude smaller than those for one-photon excitation. T hus, examination of dilute solutions has been achieved only with great difficulty. Any successful instrumentation will require that the blan k be essentially eliminated. To this end, time-filtered detection has been combined with two-photon excitation and spatial filtering to prod uce fluorometric detection limits of 38 pM for 9,10-diphenylanthracene and 8.6 pM for alpha-NPO. It is believed that this latter value is th e lowest concentration yet reported for two-photon spectroscopy in flu id solution. The instrumentation and data processing are described. Ad ditionally, a comparison is made with the performance of other recent alternative approaches involving spatial filtering and second harmonic detection.