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Hyper-Raman scattering from cubic BaTi03 is studied to clarify the controversies about the low-frequency dielectric response in this material. Applying the Auctuation-dissipation theorem, we obtain the imaginary part e"(0) of the dielectric function in the wave-number range from 3 to 150 cm '. e"(0) can be adequately described by a classical singleoscillator dispersion formula. In approaching the Curie temperature, we find a continuous decrease of the mode frequency 00. The relative damping constant y/Qo exceeds 2, so that the mode may be referred to as intermediate between oscillator and relaxator. Because of the high damping e"(0) can be formally written as the sum of two overdamped oscillator contributions. This would lead to the concept of a soft-mode saturation and an extra dispersion mechanism as has been recently inferred from the far-infrared reflectivity spectrum. However, we do not find any evidence for this mode splitting and, so far, regard it as artificial.

The double electron capture process by He'+ ions in collisions with helium is studied in the high velocity regime with the measurement of total cross sections at 1.5, 4 and 6 MeV beam energy and angular differential cross sections far 1.5 MeV. The experimental results compare well with theoretical calculations which include correlation effects in the initial and final He ground states for the derivation of the capture amplitude in a continuum distorted-wave approximation.Electron capture processes in fast ion-atom collisions are of great interest both from theoretical and experimental points of view. One of the interests in studies of this process arises from the difficulties in describing the rapid transfer of the electron from a target hound state to a projectile bound state at high velocities (see e.g. Shakeshaft and Spruch 1979). In the case of non-radiative capture this requires a considerably

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We measure the relative intensities of the seven hyper-Raman (HR) lines of SrT&03 and KTa03 at room temperature as a function of the scattering geometry. From our experimental data we deduce relative values of the HR tensor elements. There are two different HR tensor elements for each TO and LO component of the three Fl"phonons and one for the silent mode of symmetry type F2".Taking the phonon eigenvectors and longitudinal effective charges from improved versions of Cowley's shell model, we obtain the derivatives BP';ii'/Bu (v) and BX';,i'/BE of the second-order susceptibility with respect to the displacements of the five sublattices and the macroscopic electric field of the LO phonons, respectively. We interpret the parameters BX'"i'/Bu (~) in terms of a nonlinear bond-polarizability model and confirm the dominant role of the Ti-0 and Ta-0 interactions in the nonlinear-optical response.