Fluorescence lifetimes and excitation spectra of liquid alkanes measured by means of synchrotron radiation

Abstract

Fluorescence lifetimes of liquid cyclohexane, bicyclohexyl, cis-decalin, trans-decalin, n-decane, and n-dodecane have been measured at 25 °C by a delayed coincidence single photon counting technique at several excitation wavelengths of SR (synchrotron radiation) pulse between 120 and 180 nm. Excitation spectra of these liquids have been also examined at the wavelength region between 120 and 300 nm. The lifetimes show no dependence on the excitation wavelength of SR used in the experiment. The lifetimes determined are as follows: 1.1±0.1 ns (cyclohexane), 1.8±0.3 ns (bicyclohexyl), 2.4±0.1 ns (cis-decalin), 3.0±0.1 ns (trans-decalin), 3.0±0.1 ns (n-decane), and 4.2±0.2 ns (n-dodecane). Each of excitation spectra of these liquids shows a single broad peak with FWHM of (5–23)×103 cm−1 at the excitation wavelengths between 163 and 185 nm. There is no increase of fluorescence yield in the excitation spectrum with increasing the excitation energy across the ionization potential of each of the liquids. Fluorescence yield at the ionization region seems to be much less than the peak maximum at the subionization region.