Femtosecond fluorescence upconversion studies have been performed for auramine ͑a diphenylmethane dye͒, dissolved in ethanol, as a function of temperature. It is found that the ͑sub͒picosecond decay components in the fluorescence slow down as the temperature is lowered from 293 K to 173 K. From the observation of a residual fluorescence, with a viscosity-dependent lifetime of about 30 ps ͑or longer at higher viscosity͒, and transient absorption results it is concluded that the two-state sink function model ͓B. Bagchi, G. R. Fleming, and D. W. Oxtoby, J. Chem. Phys. 78, 7375 ͑1983͔͒ does not apply in the case of auramine. Comparison of the auramine fluorescence kinetics in ethanol and decanol shows that diffusional twisting and not solvation is the main cause for the ͑sub͒picosecond excited state relaxation. To explain the experimental results, adiabatic coupling between a locally excited emissive state ͑F͒ and a nonemissive excited state ͑D͒ is considered. Torsional diffusion motions of the phenyl groups in the auramine molecule are held responsible for the population relaxation along the adiabatic potential of the mixed state, S 1 ͑comprised of the F and D states͒. Simulation of the excited state dynamics is feasible assuming a barrierless-shaped potential energy for S 1 and applying the Smoluchowski diffusion equation. The temporal behavior of the auramine band emission was simulated for the temperature range 293 K ϾTϾ173 K, with the temperature, T, and the viscosity coefficient, , being the only variable parameters. The simulated temporal behavior of the emission in the investigated temperature range is compatible with that obtained experimentally. The rotational diffusion coefficient for the auramine phenyl groups as extracted from the simulations is found to follow the Einstein-Stokes relation. From the numerical calculations the effective radius of the twisting phenyl groups is determined as 1.0 Å which compares well with the actual value of 1.2 Å.

We address two questions: to what extent does geographical distance to parents, siblings and children living outside the household influence receiving support from them? And to what extent does the availability of other network members living closer play a part in receiving support? We use the Netherlands Kinship Panel Study and logistic regression models of receiving support from parents, children and siblings. The results indicate a strong impact of distance on support from family members, particularly from siblings and fathers but less so from mothers and children. Particularly siblings, sons and mothers are more likely to provide support if there are no other family members living closer. Copyright © 2009 John Wiley & Sons, Ltd.

Subpicosecond fluorescence up-conversion and transient absorption spectroscopy is applied to study the excited-state dynamics of auramine, a diphenylmethane dye, in liquid solutions. The fluorescence decays, on a time scale of a few picoseconds to a few tens of picoseconds, are found to be nonexponential and solvent viscosity dependent. They can be fitted as a sum of two exponentials in ethanol and three exponentials in decanol with a larger average lifetime in the more viscous solvent. The decays exhibit wavelength-dependent time constants, whereas the fluorescence rise time is instrument limited (150 fs) at all wavelengths. The average decay time increases with the wavelength across the steady-state emission spectrum. The spectral reconstruction indicates a few hundred wavenumbers dynamic Stokes shift accompanied by a drop in the intensity in both solvents. From transient absorption experiments, the fluorescent state population is shown to decay to an intermediate dark state and then to the ground state, with a viscosity-dependent rate. A barrierless or quasi-barrierless photoreaction involving the rotational diffusion of the phenyl rings, with a change in the radiative transition rate along the reaction path, is proposed to explain the wavelength-dependent nonexponential fluorescence decays. Both fluorescence and transient absorption data are discussed in support of an adiabatic photoreaction involving internal twisting and charge shift.

No abstract

No abstract