Trace-Level Quantitation via Time-Resolved Two-Photon-Excited Fluorescence

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Abstract
With the use of a cavity-dumped, synchronously pumped dye laser for excitation, two-photon fluorescence cross sections are approximately eight orders of magnitude smaller than those for one-photon excitation. Thus, examination of dilute solutions has been achieved only with great difficulty. Any successful instrumentation will require that the blank be essentially eliminated. To this end, time-filtered detection has been combined with two-photon excitation and spatial filtering to produce fluorometric detection limits of 38 pM for 9,10-diphenylanthracene and 8.6 pM for α-NPO. It is believed that this latter value is the lowest concentration yet reported for two-photon spectroscopy in fluid solution. The instrumentation and data processing are described. Additionally, a comparison is made with the performance of other recent alternative approaches involving spatial filtering and second harmonic detection.