Specific features of the exciton+xciton annihilation process for slow diffusion systems are considered in terms of excimer luminescence in pyrene crystals. I n this case luminescence decay curves allow a direct determination of the diffusion coefficient and the mean free path of the excitation during its lifetime. Analogy between excimer excitations and the well-known slow excitons is discussed. The importance of the experimental separation of excinier and monomer excitons whose kinetic parameters are greatly different is emphasized.Die spezifischen Charakteristika des Exziton-Exziton-Vernichtungsprozesses fiir Systeme mit langsamer Diffusion werden anhand der Exzimer-Lumineszenz in Pyrenkristallen betrachtet. In diesem Fall erlauben die Lumineszenzabklingkurven eine direkte Bestimmuiig des Diffusionskoeffizienten und der mittleren freien Weglange der Anregung wilhrend ihrer Lebensdauer. Die Analogie zwischen Exzimer-Anregungen und den wohlbekannten langsamen Exzitonen wird diskutiert. Die Wichtigkeit der experimentellen Trennung von exzimeren und monomeren Exzitonen, deren kinetische Parameter sehr verschieden sind, wird betont.

A number of papers is available reporting on a positive temperature coefficient of the resistivity (PTCR) in barium titanate near the Curie point Tc. However, the scattering mechanism leading to this effect in single crystals (1) is yet unknown. Below theoretical results a r e given for the mobility of conduction electrons where the interaction Hamiltonian of electrons with transverse optical vibrations of the rrsoftfr mode is the perturbation causing the electron scattering. A s it is shown, in this case the mobility is proportional to e is the static permittivity. Since near the Curie point the dependence 6 ' where 0 0 E (T) has an anomaly (the so-called 0 A -peak), the calculated mobility in this region has an anomalous temperature dependence involving also the PTCR effect. The interaction of the trexcesslf electron with the ions of the crystal lattice in the dipole approximation can be written in the canonical form -+ where a with the q wave vector of the j-th vibrational branch, m is the mass, z is the charge of the s-th ion in the unit cell of volume v, w,(s,qj) is the phonon polarization vector component, e is the electron charge, 9 is a reciprocal lattice vector. a are the phonon creation and annihilation operators of frequency o sj' q j q j S 8

In the light of new absolute measurements of triple-differential cross sections for electron-impact ionization of helium we re-examine the various forms of the Coulomb T -matrix in the plane-wave impulse approximation. At higher energies the regularized form is supported by the measurements of Pochat et al while the Ford form is supported by the older, conflicting, measurements of van Wingerden et al. A revised calculation of the second-order Faddeev-Watson approximation is presented and compared with the measurements of Pochat et al.

The decay kinetics of the Nd3+ ions luminescence in RbMnF3:Nd3+ crystals are investigated experinlentally during excitation into the Mn2+ absorption bands. It is shown that the energy transfer process is preceded by the migration of the excitation energy over the Mn*+ ions in the crystal. A theoretica1 treatment is given of the diffusion mechanism for this energy migration. The results of the calculation are in quite good agreement with the experimental data.Die Kinetik des Abklingens der Lumineszenz von Nd3+-Ionen in RbMnF,:Kd3+-Kristallen wird wahrend des Einstrahlens in die Absorptionsbanden der MnZ+-Ionen untersucht. Es wird gezeigt, daR der EnergieubertragungsprozeR durch die Wanderung der Anregung iiber MnZ+-Ionen geleitet wird. Der Diffusionsmechanismus der Energiewanderung im Kristall wird theoretisch behandelt. Die Ergebnisse der Berechnungen sind in ziemlich guter abereinstimmung mit den experimentellen Werten.Investigations of the luminescence of antiferromagnetic manganese fluoride crystals doped with rare-earth impurities have shown [l] that the excitation energy of 3d electrons of the manganese ion is effectively transferred to 4f electrons of rare-earth ions and that the most probable transfer mechanism is a n inductive-resonance one. However, it is not clear till now whether the transfer is going directly from 3d to 4f ions, or it, a t first, migrates over 3d ions of the matrix crystal and only then the 3d ion (sensibilisator) contacting a rare-earth ion (activator) transfers the excitation energy. The aim of this work is to make a conclusion about the mechanism of the transfer process of excitation energy from Mn2+ ions to Nd3+ ions on the basis of the decay kinetics experiments for RbMnF, crystals with Nd3+ impurities, and to evaluate the characteristics of this process. For an experimental investigation RbMnF, : Nd3+ crystals are chosen due to their large spectral range between the emission bands of Mn2+ and Nd3+ in Mn2+ ion spectrum that makes possible a detailed individual study of the decay kinetics of the sensibilisator (manganese) and the activator (neodymium).The experiment was carried out in such a way that the exciting light was absorbed by manganese ions only [1, 21. The spectral distribution of the luminescence light was investigated with the help of the double monochromator SP-2 having a resolution being not worse than 100 A, which was quite sufficient for separating the manganese or neodymium luminescence. The impulse duration of the exciting light did not exceed 5 xThe most important experimental data are plotted in Figs. 1 and 2 . Curve 1 in Fig. 1 illustrates in logarithmic scale the dependence of the decay intensity of the manganese ion in the RbMnF, :Nd3+ crystal. Curve 2 represents a similar s.5 ptiysica 2Zjl

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