On the rotation-vibration interaction of diatomic molecular impurities in cubic crystals

Devonshire's model of the dumb-bell, subjected to a stationary potential of octahedral symmetry type is generalized to be applied to a vibrating rotor. In the framework of this model the influence of the interaction of vibration and hindered rotation is investigated for Ar:HCl.     

Resonant Mandelstam-Brillouin light scattering and Raman scattering in semiconductors with intermediate exciton states belonging to discrete exciton bands and the continuous spectrum

The results of a theoretical and experimental investigation of resonant Mandelstam-Brillouin light scattering by thermal acoustic phonons with k=0 near the direct absorption edge (in the case of ZnSe crystals) are analyzed. The appearance of a new type of resonant increase in the intensity of Raman scattering by optical phonons with k≠0, which corresponds to resonance with the scattered light in the output channel, near the indirect absorption edge (in the case of semi-insulating GaP:N crystals) is also reported. The resonant gain reaches ∼4×10³ at frequencies corresponding to overtone scattering assisted by LO(X) and LO(L) phonons. Exciton states belonging to both discrete exciton bands and to the continuous spectrum are considered as the intermediate states involved in the scattering processes in calculations of the resonant scattering tensors. In addition, all the intraband transitions, as well as the interband transitions between the conduction band, the valence bands, and the spin-orbit split-off band are taken into account, and good agreement with the experimental results is obtained. Fiz. Tverd. Tela (St. Petersburg) 40, 938–940 (May 1998)     

Far-infrared absorption due to impurities in CsBr crystals

The far-infrared absorption in the phonon band of CsBr due to Na⁺, K⁺, Rb⁺, In⁺, and Tl⁺ impurities has been measured and compared with the calculated density of phonon states of the host lattice.     

Light scattering spectrum of disordered molecular crystals: KCN

We have investigated the effect of the low temperature order-disorder phase transitions on the Raman spectrum of KCN. In the disordered cubic room temperature phase we only find a broad central component in the spectrum which sharpens near the transition temperature at 168K. From the doubling of the librational mode of the CN^--molecules we conclude that an antipolar orientation of the CN^--molecules is probable for the low temperature phase below 83 K.     

Angular auto-correlation functions in molecular crystals and liquids: application to incoherent neutron scattering law

The effect of the local hindered molecular motion in the incoherent neutron scattering spectra in crystals and liquids is studied theoretically on the basis of the angular auto-correlation functions symmetrized on the dynamical point groups. The extended angular jump model simulates the motion. The microscopic properties of the matter, the time scale and the point symmetry of the molecule motion as well as the site symmetry of the molecule, are taken into account with the help of the dynamical variables of the model. The incoherent neutron scattering function is anisotropic in a monocrystalline sample. The scattering function consists of elastic plus quasi-elastic components. The shape of the quasi-elastic scattering spectrum is expressed by the sum of the weighted Lorentzian curves symmetrized with respect to the non-identical irreducible representations of the molecule motion point symmetry group. The elastic part of the intensity is increased by the contribution arising from the molecule motion of the identity representation symmetry. The scattering expression related to the molecule motion symmetry of a perfect cubic group is similar to the well-known expression derived for the scattering from the spherical top molecules exhibiting rotation diffusion.     

Superliquid state of an exciton mixture with small exciton radii in molecular crystals

The energy spectrum of excitons with a small radius in a superliquid state in molecular crystals has been investigated. A more complicated structure of the exciton energy spectrum with a number of equivalent minima for k 0 has been considered. The problem was solved as that of determining the energy spectrum of a mixture of various types of non-ideal exciton gases in a superliquid state, corresponding to the individual exciton energy minima. The appropriate branches of the energy spectrum of elementary excitations (hydrons) have been determined and the conditions for their stability have been investigated.     

Theory of diffuse X-ray scattering due to orientational correlation in plastic molecular crystals

The X-ray diffuse scattering intensity of plastic molecular crystals due to orientational correlations between molecules is presented. In the case of symmetric simple molecules such as N₂, CO₂, CCl₄, and benzene, the intensity equation is much simplified by the use of symmetry adapted functions. The intensity of powdered samples is also presented. It is suggested that orientational correlations can be estimated from experimentally observed X-ray spectra and computer simulations of plastic molecular crystals are able to test the approximation employed.     

Incoherent scattering law for diffusion in crystals with random impurities

We derive the incoherent scattering law for interstitials diffusing in a crystal with a random distribution of simple traps. The average-T-matrix approximation in the lowest order of the trap concentration has been used and the thermal population of the traps has been explicitly taken into account.     

Quasielastic neutron scattering and correlated self-diffusion in crystals

The effect of correlations between successive jumps of atoms during self-diffusion on the quasielastic neutron-scattering line wodth is investigated. Temporal and spatial aspects of these correlations are described in terms of the so-called “encounter model”, which had originally been developed for the interpretation of nuclear-magnetic resonance studies on self-diffusion. Owing to the rather high temperatures (i.e. large diffusivities) to which quasielastic neutron-scattering experiments on self-diffusion in crystals are restricted, the effect of both mono- and divacancies is considered. It is found that the line width associated with a correlated mechanism is smaller and in single crystals shows a different orientation dependence than expected for a random-walk diffusion mechanism. Numerical results are presented for (i) the monovacancy and two divacancy mechanisms of self-diffusion in a b.c.c. lattice, a situation likely to be encountered in sodium metal, and (ii) mono- and divacancies in the f.c.c. lattice. In the random-walk limit earlier results of Gissler and Rother and Chudley and Elliot are reproduced. All of the results derived also apply to the diffusion-induced broadening of Mössbauer lines. The article therefore presents a generalization of a recent theory in which only the effect of monovacancies on the Mössbauer line broadening has been considered.     

Near-field effect in the absorption spectrum of impurities in crystals

The effect of the near-field interaction of shallow donors in GaAs on the intracenter absorption spectrum of impurities has been considered. It has been shown that the near-field interaction leads to broadening of the absorption line, which increases with the donor density. The absorption line shape is no longer Lorentzian: its low-frequency wing becomes more prolonged than the high-frequency wing.     

Inelastic neutron scattering and molecular dynamics determination of the interaction potential in liquid CD4

Anisotropic interactions of liquid CD4 are studied in detail by comparison of inelastic neutron Brillouin scattering data with molecular dynamics simulations using up to four different models of the methane site-site potential. We demonstrate that the experimental dynamic structure factor S(Q,omega) acts as a highly discriminating quantity for possible interaction schemes. In particular, the Q evolution of the spectra enables a selective probing of the short- and medium-range features of the anisotropic potentials. We show that the preferential configuration of methane dimers at liquid densities can thus be discerned by analyzing the orientation-dependent model potential curves, in light of the experimental and simulation results.     

On the calculation of surface exciton energies in molecular crystals

In this paper a general theory is established for surface exciton states in a molecular crystal. Starting from the bulk exciton formalism the pertubation caused by the surface is taken into account by introducing a perturbation matrix. Symmetry properties of the “surface exciton bands” are derived.     

Bleaching in the region of exciton resonance of layered GaSe crystals

Light absorption in the region of exciton resonance of GaSe crystal is studied experimentally at high levels of optical excitation. A picosecond YAG:Nd³⁺ laser emitting 30-ps light pulses and a dye laser with a pulse width of ～3 ns tunable within the range 594–643 nm were used as light sources. It was found that, at high levels of optical excitation, the exciton absorption line of the GaSe crystal disappeared, which was attributed to increasing exciton density with arising mechanisms of their decay: exciton-exciton interactions and screening of excitons by the free charge-carrier plasma. It is shown that these mechanisms are also responsible for the arising new emission band in the long-wavelength region of the photoluminescence spectrum.     

ππ scattering and the meson resonance spectrum

A ππ, ˉKK, and ρρ(ωω) fully coupled channel model is used to predict the lowest isospin S, P, D, F-wave phase shifts and inelasticities for elastic ππ scattering from threshold to 2.0 GeV. As input the S-matrix is required to exhibit poles corresponding to the meson resonance table of the Particle Data Group. As expected, the ππ inelasticity is very strongly related to the opening of the ˉK channel near 1 GeV, and the opening of ρρ(4π) and ωω(6π) channels in the 1.5 GeV region. The predictions of this model are compared to the various elastic ππ→ππ amplitudes, that were obtained from analyses of π⁻ p →π⁻π⁺n data. The role of the various resonances, in particular the glueball candidate f ₀(1500) and the f _J(1710) is investigated. Received: 19 November 1997     

Structure due to indirect exciton in photocurrent spectrum of homoepitaxial diamond film

Photocurrent spectrum in homoepitaxal diamond film formed by chemical vapor deposition has been measured in the photon energy range 5-6 eV of the vicinity of the indirect band gap. It has been seen that the line shape of the spectrum in the photon energy range lower than 5.6 eV agrees with the fundamental absorption edge spectrum in natural diamond. Structures in the spectrum have been explained from the optical transition due to indirect excitons assisted by TO phonons and indirect band-to-band transition.     

Density of exciton states in one-dimensional disordered molecular crystals

We investigate the effects of structural disorder on the exciton density of states of molecular crystals, utilizing an exact soluble model which involves correlated Lorentzian distributions of both the site-excitation energies and the transfer integrals. Off-diagonal disorder results in asymmetric density of states functions, the asymmetry being prominent for one-dimensional exciton band structure.     

A contribution to the theory of exciton generation by fast electron scattering in non-metallic crystals

For the process of fast electron scattering by exciton production in non-metallic crystals estimates have been made of the energy losses along the unit path length of the fast electron, the cross section of a crystal unit volume in the case of forward scattering, and the shape of the energy loss spectrum of the electrons in the energy region corresponding to the production of excitons.The author wishes to express his gratitude to Professor M. Trlifaj for proposing this interesting problem and for valuable discussions.