Precise fluorescence measurement for determination of photophysical properties of dyes

The authors present a double frequency modulation technique to determine th e fluorescence power with an accuracy of at least 10(-5). Quantitative anal ysis of the modulation transfer from the excitation to the fluorescence pow er is used to derive the triplet state lifetime tau (DS) and the rate const ant for intersystem crossing k(ED) of fluorescent dyes. For that purpose th e correlation between the population of transient states and the decrease o f fluorescence power is measured. The experimental results of rhodamine 6G are presented. The influence of molecular oxygen is also examined. The phot ophysical parameters can be determined with high accuracy even if absorptio n of excited states, photoproducts or other molecules is present. This sens itivity is sufficient to determine the presence of a few hundred molecules in excited states. (C) 2001 Elsevier Science BN. All rights reserved.