Phonons In Ionic Crystals Research Articles

PLASMON ENERGY LOSSES OF ELECTRONS IN MULTILAYER DIELECTRIC STRUCTURES

The possibilities of engineering energy losses of electrons created in the cascade process of electron-electron scattering during the interaction of multilayer dielectric structures with ionizing radiation are considered. It is shown that the contribution of surface plasmons associated with layer boundaries to electron energy losses is significant only for nanometer layer thicknesses and increases with increasing electron energy. At the same time, surface states associated with longitudinal optical phonons in ionic crystals significantly change energy losses during electron thermalization and can lead to an increase in the efficiency and growth rate of scintillation in nanostructured systems.

Optical phonons in ionic crystals with a diatomic unit cell

Optical phonons in an ionic crystal with two atoms per unit cell have been considered. The equation of relative motion of oppositely charged sublattices has been derived with the use of the concept of the self-consistent electromagnetic field. The frequency corresponding to transverse optical phonons and the expression for the frequency of the eigenmodes of the ionic crystal in terms of the physical characteristics of the crystal have been obtained from the condition of electrostatic equilibrium. The calculated values of the latter frequency agree with the experimental data.

Optical resonances in reflectivity due to crystal modes with spatial dispersion

We study the effect of spatial dispersion of crystal modes on optical properties such as the reflectivity $R$. As an example for isotropic media, we investigate the simplest model for phonons in ionic crystals and compare with previous results for highly anisotropic plasmons, which are now understood from a more general point of view. As a consequence of the wave vector dependence of the dielectric function small changes in the lineshape are predicted. Beyond that, if the frequency of minimal $R$ is near a pole of the dispersionless dielectric function, the relative amplitude of dips in $R$ with normal and anomalous dispersion differ significantly, if dissipation and disorder are low.

On the role of umklapp processes in the scattering of delocalized positronium by acoustic phonons in ionic crystals

The effect of umklapp processes in the scattering of delocalized positronium atoms by acoustic phonons in ionic crystals is analyzed. It is shown that at temperatures above which this effect becomes large, umklapp scattering renormalizes the constant of the deformation potential of positronium. This renormalization has recently been observed in experiments measuring the momentum distribution of delocalized positronium in a MgF2 crystal.

Temperature dependent dephasing of internal phonons in ionic crystals: picosecond cars measurements in K 2SO 4 and KClO 4 crystals.

The decay time constants of the totally symmetric components of the internal vibrations of K 2SO 4 and KClO 4 crystals have been measured as a function of temperature; the experimental data are interpreted in terms of three- and multi-phonon scattering processes. The role played by isotopic impurities is also discussed.

The spectroscopy of surface vibrations in ionic crystals by inelastic scattering of atoms

The recent progress in the production of highly monochromatic atomic beams is paving the way for a full determination of the surface vibrational structure. Account is given of the recent determination of Rayleigh wave dispersion curves from angular distributions exploiting the kinematical focussing effect. A short review is then presented on the direct measurement of surface phonon dispersion curves, first achieved by Brusdeylins, Doak and Toennies in alkali halides, from time-of-flight (TOF) spectra of scattered He atoms. A comparison is made with the existing theories of surface phonons in ionic crystals. The one phonon energy loss spectra of He scattering from LiF(001) calculated for a hard corrugated surface model are found to be in general good agreement with the experimental TOF spectra. From such a comparison evidence is obtained that: (i) one-phonon processes are predominant, and (ii) in addition to Rayleigh waves important contributions to the inelastic scattering come from the surface projected density of bulk phonons. Important effects due to inelastic resonances with surface bound states are put in evidence and explained by simple kinematical arguments. The possible observation of surface optical modes in NaF(001) is finally discussed.

The spectroscopy of surface vibrations by atom scattering

The recent progress in the production of highly monochromatic atomic beams is opening new perspectives in surface physics, having paved the way for a full determination of the surface vibrational structure. After a discussion on the possible determination of Rayleigh wave dispersion curves from angular distributions exploiting the kinematical focussing effect, a short review is presented on the direct measurement of surface phonon dispersion curves, first achieved by Brusdeylins, Doak and Toennies in alkali halides, from time-of-flight (TOF) spectra of scattered He atoms. A comparison is made with the existing theories of surface phonons in ionic crystals. The state of the art in the theory of inelastic processes is briefly illustrated in order to discuss the theoretical interpretation of TOF spectra. The one-phonon energy loss spectra of He scattering from LiF(001) calculated for a hard corrugated surface model are found to be in general good agreement with the experimental TOF spectra. From such a comparison evidence is obtained that: i) one-phonon processes are predominant, and ii) in addition to Rayleigh waves important contributions to the inelastic scattering come from the surface-projected density of bulk phonons. Important effects due to inelastic resonances with surface bound states are put in evidence and explained by simple kinematical arguments. The possible observation of surface optical modes in NaF(001) is finally discussed.

THE SPECTROSCOPY OF SURFACE PHONONS BY INELASTIC ATOM SCATTERING

The recent great advance in the production of highly inonochromatic atcmic beams is opening new pers2ectives in surface physics, having made way to a full determination of the surface vibrational structure. After a short review of the earlier att- empts to detect surface phonons from the angular distributions of scattered atoms, we re2ort on the direct measurement of surface phonon dispersion curves, first achieved by Brusdeylins, Doak and Toennies in alkali halides, from time-of-flight (TOF) spectra of scattered He atoms. A comparison is made with the existing theo- ries of surface phonons in ionic crystals. The state of the art in the theory of inelastic scattering processes is briefly illu- strated in order to discuss the theoretical inter2retation of TOF spectra. The one-phonon energy-loss spectra of He scattered from LiF(001) calculated for a hard corrugated surface model are found to be in good agreement with TOF spectra at all the incidence angles. Evidence is given that, in addition tl ayleigh waves, important contributions to the inelastic scatte g come from the surface- projected density of bulk phonons. The possible observation of optical surface modes in KCl(001) is finally discussed.

The effects of nonuniform electromagnetic fields on the dielectric properties of crystals

All the treatments of the interaction between electromagnetic radiation and phonons in ionic crystals assume that the electromagnetic field can be considered as uniform. With the very strong absorption present in such crystals this assumption leads to serious errors for all finite angles of incidence. Taking into account the nonuniformity of the field leads to the conclusion that significant interaction with the LO modes exists even in crystals of cubic symmetry, contrary to all other theoretical models assuming the uniformity of the electromagnetic field.

Pressure Dependence of Long-Wavelength Optical Phonons in Ionic Crystals

Pressure Dependence of Long-Wavelength Optical Phonons in Ionic Crystals