Internal conversion coefficients and penetration effect for the 279 keV transition in203Tl

The internal conversion process of theM1+E 2 mixed 279 keVγ-ray transition in the decay of²⁰³Hg has been reinvestigated. The emission rate of theK-shell internal conversion electrons was determined with an electron X-ray coincidence measurement using a magneticΒ-spectrometer and a Si(Li) detector of high energy resolution. Conversion electron ratios were obtained from electron spectra recorded as a function of momentum. The disintegration rate has been taken from measurements with a calibrated NaI(Tl)γ-ray spectrometer. Following results have been deduced: α=0.2279±0.0024, α _K =0.1653±0.0017, α _L =0.0475±0.0013,K/(L+M+?)=2.64±0.03,K/L=3.48±0.12,L/(M+N+ ?)=3.14±0.12. On the basis of the experimental results on theK-shell internal conversion coefficient,α _k , and the conversion ratioK/(L+M+?) the penetration effect on the internal conversion of theM1 part and the mixing ratio,δ ², of the 279 keVγ-ray transition in²⁰³Tl have been studied. Calculations were made using Hager and Seltzer's formalism and their theoretical internal conversion coefficients. The results are consistent with a mixing ratio ofδ ²=1.32±0.11 and a penetration parameter ofλ=6.8 ±0.8.     

Picosecond resonant two-photon ionization of cold sodium clusters

Large sodium clusters, up ton=21, generated by a low-temperature modification of the laser ablation gas jet source were ionized with two photons of visible radiation from an amplified picosecond dye-laser system.     

Energy dependence of the rotational enhancement factor in the level density

The one-shell SU(3) energies are given and the corresponding level density is calculated approximately by use of a distribution function for the SU(3) quantum numbers. The calculation is extended to include many shells by a renormalization procedure and an effective one-shell interaction. The traditional level density is then obtained from the related mean-field hamiltonian which corresponds to a deformed harmonic oscillator potential. Various rotational enhancement factors are considered. Numerical results, are obtained and comparisons between the SU(3) and the traditional level densities allow the first computation of the energy dependence of the rotational enhancement factor. A transition from axial to spherical level density is found. A simple parametrization is suggested in terms of a deformation-dependent half-value energy and a transition width.     

Singlet-Oxygen Quenching by Carotenoids: Steady-state luminescence experiments

The near-infrared luminescence of singlet oxygen (¹O₂) has been measured in order to determine the efficiency of ¹O₂ quenching by two carotenoid compounds, β-carotene and canthaxanthin. 1H-Phenalen-1-one and rose bengal have been used as photosensitizers in those steady-state luminescence experiments. Stern-Volmer analysis of the ¹O₂ luminescence in solutions of CCl₄ and CD₃OD, containing different concentrations of the carotenoids, has shown a very efficient quenching by canthaxanthin. The rate constants are about a factor of 2 below the diffusion limited values for the given solvents, confirming earlier results in benzene. In comparison, the efficiency of ¹O₂ quenching by β-carotene is slightly lower than that by canthaxanthin in non-polar solvents and is reduced by an order of magnitude in CD₃OD, due to the aggregation of this quencher.     

There is more to be learned from the Lorentz model

The classical Lorentz model for charged noninteracting point particles in a perpendicular magnetic field is reconsidered in 2D. We show that the standard Boltzmann equation is not valid for this model, even in the Grad limit. We construct a generalized Boltzmann equation which is, and solve the corresponding initial value problem exactly. By an independent calculation, we find the same solution by directly constructing the Green function from the dynamics of the model in the Grad limit. From this solution an expression for the diffusion tensor, valid for arbitrary short-range forces, is derived. For hard disks we calculate the diffusion tensor explicitly. Away from the Grad limit a percolation problem arises. We determine numerically the percolation threshold and the corresponding geometric critical exponents. The numerical evidence strongly suggests that this continuum percolation model is in the universality class of 2D lattice percolation. Although we have explicitly determined a number of limiting properties of the model, several intriguing open problems remain.It is with great pleasure we include this paper in the issue honoring Matthieu Ernst, who not only shares our love for kinetic theory, but who also contributed so much to its modern development.