| Fluorescence from Sodium Cryptand (2. 2. 2.) (Na;When the temperature is raised above 77 K the fluorescence spectrum has structure. For example, two bands with peak energies at 1.847 eV and 1.822 eV are observed at 125 K.;Time-resolved fluorescence from Na;At high excitation densities, fluorescence bleaching and recovery processes occur.;The shape of the fluorescence spectra was found to fit a Gaussian function on the high energy side of the peak, and to obey Urbach's rule on the low energy side; that is, the spectrum on the low energy side exhibits an exponential dependence on the emission energy. When the temperature is raised, a logarithmic plot of the intensity shows a lower slope and the straight lines at different temperatures converge at approximately one point, another characteristic of the Urbach rule.;The fluorescence spectra from ten sodides were found to have common properties. For example, the main emission band is structureless, most of them have a Stoke's shift of about 0.1 eV and the lifetimes have plateau shapes as functions of energy. However, the fluorescence intensity from the other sodides is at least three orders of magnitude weaker and the decay time constants are much shorter than those of Na;Optical absorption studies of thin films of Na;The observations are explained and discussed by using the concepts of excitons, energy transfer, phonons, traps and local temperatures. |
