A fluorescence lifetime apparatus using a mode-locked argon ion laser as excitation source and single photon counting detection is described. The high repetition rate and intensity of the excitation source allows twofold discrimination in the stop channel, giving excellent time resolution. The data analysis is based on statistical concepts. The standard deviations of the Fourier transforms of the deconvolved decay curves are discussed and the advantages of performing a curve fit in the Fourier space are outlined.

band of 1-CNC as the temperature increases. ConclusionsIntramolecular hydrogen bonding between the N-H bond and the carbonyl group in 1-COOEC as well as the intermolecular double hydrogen bonding of the N-H-N=Ctype between molecules of 1-CNC in CC14 solutions have been investigated by infrared spectroscopy.A distance of about 2.1 Á between the N-H proton and the C=0 group favors the occurrence of intramolecular hydrogen bonding in 1-COOEC. Nevertheless, the rotation of the carboethoxy group around the Car-C bond causes this compound to exist in two configurations: the free N-H and the hydrogen-bonded N-H. These two configurations manifest themselves by the two well-separated bands in the N-H stretching frequency range of the infrared spectrum. From the temperature dependence of the intensities of these bands the intramolecular association constant K = 1.33 ± 0.26 at 25 °C, the enthalpy change AH0 = -7.40 ± 0.22 kJ mol"1, and the entropy change AS°= -22.6 ± 1.4 J mol"1 K"1 have been calculated for the intramolecular hydrogen bonding in 1-COOEC.1-CNC in nonpolar solvent is undergoing hydrogen-bonding self-association. The infrared spectrum of 1-CNC in carbon tetrachloride possesses two bands which have been well characterized as N-H band of the monomer and the double N-H band of the cyclic dimer. The relative intensities of these bands change with the concentration of 1-CNC. Moreover, the intensity of the N-H (dimer) band strongly depends on the variation of the temperature. The equilibrium constant of the dimer formation of 1-CNC in CC14 at 25 °C has been obtained as 92 ± 16 mol"1 dm3 4. The enthalpy change and the entropy change for the complexation of 1-CNC in CC14 are -33.3 ± 2.8 kJ mol"1 and -74 ± 10 J mol"1 K"1, respectively. These results are in good agreement with the cyclic structure of the dimer discussed.Acknowledgment. The authors thank the Natural Sciences and Engineering Research Council of Canada and the "Ministére de 1'Education du Quebec" for financial assistance. Thanks are also due to Mr. Jean-Claude Bolduc for having taken many of the infrared spectra on the FTIR apparatus.Registry No. l-(Carboethoxy)carbazole, 56995-05-2; 1-cyanocarbazole, 83415-88-7.

The fluorescence lifetime of trans‐stilbene in solution was measured in the temperature range 300–80 K by three different methods. The decay was found to be mono‐exponential throughout, but prone to falsification by impurities, which may be responsible for the bi‐exponential decays reported by some authors. At room temperature τF is 80–130 ps, increasing to 1200–1400 ps at 77 K.

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