A. M. Ratner

Photodissociation of water in rare gas matrices : cage effect and local heating of the lattice

The H fragment produced in the photodissociation of water (H2O→H+OH) is lighter than the lattice atoms and a local heating occurs over long time scales and large spatial regions. Therefore, use of a continuum model for the lattice is justified. The local heating is promoted by a decreasing heat conductivity with increasing temperature. Solutions of a nonlinear heat conductivity equation show that the heat release inside the cage leads to a temperature increase in its nearest surroundings up to the melting point in argon. Melting enables the light fragment to overcome the cage barrier. In the case of krypton, the thermal effect is less pronounced, while in Xe, there is none. The above results are in qualitative agreement with experimental data.

New emission continua of rare-gas clusters in the VUV region

Abstract New VUV continua of argon, krypton and xenon clusters excited by an electron beam are studied experimetally and theoretically. These continua are found to be due to the radiative decay of excimer molecules R * 2 (R is the rare-gas atom) formed in ionic argon, krypton and xenon clusters.

Branched relaxation of electronic excitations in rare-gas crystals with traps of different types

Luminescence excitation spectra are used as a probe of competition between exciton traps of two types: exciton localization centres and centres of exciton trapping followed by trans- forming to another electronic state. A strong competition between localization and transforming centres takes place only under excitation above the dielectric gap Eg (with energy E>E g). For E<E g the action of transforming traps is usually suppressed by localization centres. A strong change in excitation spectra near the point Eg is explained by a nontrivial structure of the exciton band formed by the mixing of free excitons with polaronic states—excimer quasi- molecules. The nature and action mechanism of both types of trap are explored. To localize heavy exciton polarons, small lattice defects are sufficient. Transforming traps exist in the form of stable molecular ions, conditioned by large defects capturing light electrons, and impurity centres. The trapping cross sections of the centres of different types are strongly (but in different ways) dependent on the exciton energy.

Competition and transfer of excimer resonant bindings in rare-gas crystals

Abstract Transfer of two-center excitation (a quasi-molecule with a resonant binding) is considered for rare-gas crystals. The transfer is related to the formation of an additional resonant binding and its competition with an intermolecular one. The transfer probability increases exponentially with the vibrational index of the quasi-molecule. Spectroscopy data supporting the above suggestion are presented.

Effects of exciton-photon interaction and spatial dispersion in xenon crystals

Abstract New data on luminescence from xenon crystals are discussed in terms of the polariton theory, and the treatment of a fine spectrum structure is proposed based on the polariton scattering with participation of some acoustic phonons. The effect of spatial dispersion on transmission and reflection spectra of solid Xe is considered. Verification of the basic parameters of polariton spectrum in xenon is made by comparing calculated and experimental data.

Photoproduction of long-lived holes and electronic processes in intrinsic electric fields seen through photoinduced absorption and dichroism in Ca3Ga2−xMnxGe3O12 garnets

Long-lived photoinduced absorption and dichroism in the Ca3Ga2−xMnxGe3O12 garnets with x < 0.06 were examined versus temperature and pumping intensity. Unusual features of the kinetics of photoinduced phenomena are indicative of the underlying electronic processes. The comparison with the case of Ca3Mn2Ge3O12, explored earlier by the authors, permits one to finally establish the main common mechanisms of photoinduced absorption and dichroism caused by random electric fields of photoproduced charges (hole polarons). The rate of their diffusion and relaxation through recombination is strongly influenced by the same fields, whose large statistical straggling is responsible for a broad continuous set of relaxation components (observed in the relaxation time range from 1 to about 1000 min). For Ca3Ga2−xMnxGe3O12, the time and temperature dependences of photoinduced absorption and dichroism bear a strong imprint of structure imperfection increasing with x.

Photoinduced light absorption and dichroism of Ca3Mn2Ge3O12 garnet as a probe of electronic processes and intrinsic electric fields

Measurements of photoinduced light absorption in Ca3Mn2Ge3O12 revealed some unusual features: a saturation with the pumping intensity and a broad straggling of relaxation times with a predominance of very long times. These experimental facts cannot be understood in terms of photoinduced absorption centers associated with impurities or lattice defects, but are naturally explained within the notion of random electric fields of active charges. Active charges are produced by light pumping via the dissociation of coupled pairs of charges (consisting of a Mn-hole coupled with a compensating negative impurity or a negatively charged vacancy) which exist in the ground state. Such active charges create electric fields in a larger volume than coupled pairs, thus enhancing the probability for forbidden optical transitions. On the other hand, the random fields of active charges promote hopping of holes and hence the relaxation of photoinduced effects. A broad distribution of random-field magnitudes gives rise to a ve...

Influence of the size effect in the exciton energy spectrum on exciton relaxation in rare gas clusters

The influence of the size effect in the exciton energy spectrum on exciton relaxation in argon, krypton, and xenon clusters is investigated by the method of VUV cathodoluminescence spectroscopy. It is found that as the average cluster size is decreased to a certain critical value Nc∼160–190 atoms/cluster a blocking of the usual one-phonon channel of exciton energy relaxation occurs when the increasing distance between successive excitonic levels in the band reaches the phonon Debye energy. This leads to the appearance of another channel of relaxation (suppressed in the solid) that ends in the desorption of excited atoms and molecules from rare gas clusters.

Two-site self-trapping of bulk and surface wide band excitons

The self-trapping of bulk and surface excitons is considered. Both the initial free exciton state and the final state (diatomic quasi-molecule) are formed via resonant binding. Under self-trapping, the binding is redistributed to concentrate mainly inside the molecule, forming a strong intra-molecule bond. But some of the weak resonant bonds persist in the vicinity of the molecule, which results in a lowering of the self-trapping barrier. These phenomena manifest themselves differently in the three- and two-dimensional cases, being better pronounced in the latter case.

Photoinduced absorption and anomalous dichroism in NaCa2Mn2V3O12 garnet as evidence for the formation of oxygen hole dynamics

It is shown that long-lived photoinduced dichroism in garnets is caused by photoproduced charges with anisotropic structure, which retains a memory of the pumping light polarization for a long time, while photoinduced absorption is due to all photoproduced charges irrespective of their intrinsic structure. The charges with anisotropic structure are indentified as two-center oxygen holes. The formation of an oxygen hole is preceded by the excitation of a charge-transfer state with an electron partially transferred to a cation C (V5+ for NaCa2Mn2V3O12 garnet) from an adjacent oxygen anion. To turn this excited state into a free hole state requires a time τhole during which the hole axis can reorient, resulting in a diminution of dichroism. The time τhole decreases with increasing ionization potential of the cation C (very high for V5+). Such a mechanism explains qualitatively a number of unusual experimental facts, specifically, very strong dichroism observed only in NaCa2Mn2V3O12 garnet, where after the ir...