This conference has the unusual distinction that it is sponsored by two of the great international scientific unions, the International Unions of Pure and Applied Chemistry and of Pure and Applied Physics. The Australian Academy of Science, which is the Australian affiliating body to the International Council of Scientific Unions also warmly supports EXCON '94. One could expect that a conference in the field of solid state science should be sponsored by international bodies in both chemistry and physics, but joint support is not a common event. We have to thank EXCON '94 for arranging this happy and natural accommodation on the present occasion.

High resolution cw photoluminescence spectroscopy, energy transfer studies, time and spectrally resolved fluorescence decay as well as quenching of photoluminescence by strong electric fields support the notion that photon absorption in PPV-type conjugated polymers creates neutral excitations. They undergo a random walk among segments of the polymer chain thereby relaxing energetically. In the presence of an electric field they can form off- chain geminate pairs acting as precursors for free charge carriers.

In the present paper the following subjects are discussed about conjugated polymers: the first is a model of the relaxation of excitons, the second is the Raman spectrum of the excitons with 1 ps lifetime, and the third is the optical Stark effect of the Raman gain spectrum. (1) The decay kinetics of self-trapped (ST) excitons in 1D conjugated polymers were explained with a model in which potential crossing and tunneling between two potential curves of the ST exciton and the ground state are the major processes in the relaxation. The weak temperature dependence indicates that the activation process over the potential barrier between the ST exciton and the ground state is not dominant in the radiationless relaxation of the ST exciton. (2) Geometrical relaxation of main chain configuration in PDA was also investigated by time- resolved Raman gain spectroscopy was 300 fs resolution which is the highest time resolution of the vibrational spectrum ever reported. The Raman signal due to the ST exciton in PDA was observed at 1200 cm^-1. The observed Raman signals offer most direct evidence of the structure of ST exciton being butatriene-like structure after geometrical relaxation from acetylene-like structure in free exciton state. (3) The stimulated Raman gain signals of a single-crystal PDA-DFMP, with the sidegroups 2,5-bis trifluoromethyl-phenyl, were reported to exhibit an optical Stark shift of about 50 cm^-1. Using an ultrashort, near-resonant high intensity laser, the intensity dependent signal shifts to higher probe photon energies for both the CequalsC and CequalsC stretching modes. Semi- quantitative agreement between the data and a 3-level system density matrix calculation is achieved using a nonlinear optical mechanism in which a pump laser field is optically coupled to both the exciton--zero-phonon ground state and the exciton--one-phonon ground state transitions.

Results of a series of nonlinear optical studies of organic conjugated systems are reported and discussed with a view towards deriving an understanding of the origins and nature of the near infrared nonlinear optical properties of organic conjugated polymers. Measurements were made in solution by the method of self-diffraction from laser induced gratings at a wavelength of 1.064 micrometers . In a series of enyne oligomers, concentration dependences of the diffraction efficiencies show large contributions from imaginary components of the nonlinear susceptibilities. The origins of these imaginary components are linked to the nature of the response of 1D conjugated systems to a perturbation of the electronic configuration in the form of electron-lattice interaction. The role of the influence of such interactions in limiting the nonlinear optical response of polymers is discussed. In a range of polymeric systems, intensity dependences of the diffraction efficiencies reveal the strong influence of the multiphoton resonant enhancement of the nonlinearity. In three polymers, exhibiting strong similarities in linear optical properties, a striking variation of the order of the nonlinear optical susceptibilities at the wavelength studied is observed, originating in differing distributions of multiphoton states. The positioning of these states is largely determined by electron correlation interactions and the strength of these interactions are discussed in terms of monomeric structure. Finally the inclusion of electron rich species in the polymeric backbone is investigated and studies of metal containing organic species are reported.

The dynamics of excitonic transitions in semiconductors have been investigated by degenerate four-wave mixing experiments. We have studied the coherence, interference and dephasing of free, bound and localized excitons in bulk semiconductors and of quasi-2D excitons in quantum well structures. The influence of inhomogeneous broadening is investigated and compared with quantum interference in a continuum of states. The nature of four-wave mixing beats in a system of bound excitons and biexcitons is discussed.

We theoretically study one- and two-dimensional extended Peierls-Hubbard models, so as to clarify nonlinear natures of lattice relaxation processes of photo-generated excitons in charge- density wave (CDW) states. This theory is mainly based on the adiabatic approximation for phonons, and on the mean field approximation for inter-electron interactions, but is also reinforced by taking the electron-hole correlation into account within the first order perturbation, so as to obtain exciton effects. Various potential energy surfaces related to the lattice relaxation processes are calculated within this approximation, and the relaxation paths of exciton are clarified. In the 1D CDW, the exciton relaxes down to a macroscopic excited state, in which the phase of the Peierls distortion is completely inverted from that of the starting ground state. In the 2D case, on the other hand, the exciton relaxes down to form a local excited domain, wherein the spin density wave type order appears over several lattice sites. Thus, the excitons are shown to relax down to low lying collective excitations, wherein many excitons have been condensed. This is nothing but the `proliferation' of excitons during the relaxation, and this new characteristic is mainly due to the multi-stable nature of the CDW ground state.

We calculate the Hartree, exchange and correlation energies of a quasi-2D electron gas in a semiconductor quantum well as an expansion in Coulomb interaction. This allows us to get the well width dependence of these energies analytically. This also shows that such an expansion is quite valid for the Hartree term in usual experimental situations (electrons in the lowest subband only and well width smaller than the Bohr radius).

The nonlinear optics of excitons in ZnSe-based heterostructures is investigated by means of fast femtosecond and narrow-band picosecond differential transmission spectroscopy. Pulse lengths amount to 100 fs and 1 ps, respectively. Results taken at Zn_1-xCd_xSe/ZnSe quantum wells, ZnSe layers with ZnS_xSe_1-x cladding, and ZnSe epilayers are presented and discussed. Because of strain relaxation the usual bleaching of the excitonic absorption observed in quantum wells and waveguide-type structures reverses into an induced absorption in case of thick ZnSe epilayers. This phenomenon is discussed by use of a simple empirical model for the optical absorption of a set of inhomogeneously broadened Lorentz oscillators which react on the local exciton density with a nonlinear damping and a reduction of the oscillator strength. Additionally, a very fast change in transmission observed in the waveguide-type structures is discussed in terms of the resonant optical Stark effect.

The binding energy of an exciton in a type II polar-polar semiconductor heterojunction is calculated including both the interface image potential and the electron-phonon coupling. There are two branches of interface optical modes and two branches of the half-space longitudinal optical modes confined in the different media coupling with both the electron and hole. The contributions of the half space and the interface modes of phonons and the image potential of the binding energy are obtained by a variational calculation. It is shown that both the interface and half space phonons play more important role than image potential for the binding energy of the exciton with a large mass difference between the electron and hole. When this difference is small, their contributions to the exciton are all unimportant.

A theory of excitons in semiconductors in the Fractional Quantum Hall (FQH) Effect regime is presented. Non-conventional properties of magnetoexcitons in this regime originate from the fact that elementary excitations of the FQH phases carry fractional charges. Properties of excitons strongly depend on the separation h between electron and hole confinement planes. When this separation is not too small, h <EQ l, where l is the magnetic length, excitons look like quasiatoms consisting of a valence hole and several fractional charges. Charge fractionalization results in a multiple-branch structure of the exciton energy spectrum. Symmetric classification of the branches is proposed, and their relation to the low-energy part of the Hilbert space of the charged elementary excitations of the FQH phases is established. Spectroscopic implications of the spectra classification, including the selection rules, are discussed.

Exciton-polaron formation and self-trapping (ST) due to short-ranged coupling with phonons depend strongly on the degree of freedom of exciton motion on a lattice. When excitons can move three-dimensionally, the self-trapped (S) state appears suddenly as a strongly contracted localized state when the coupling constant (g) exceeds a certain critical value. When exciton motion is limited only in one dimension, excitons are always self-trapped and free (F) states are unstable however small g ($NEQ 0) is in the adiabatic approximation. The S state appears as a strongly extended localized state in the limit of g yields 0, and its spatial extension decreases as g increases. For excitons mobile in two dimensions, ST takes place when g exceeds a certain critical value, as in three dimensions. In two dimensions, however, ST begins with either a strongly contracted or a strongly extended S state, depending on whether the exciton-phonon interaction is caused respectively by modulation of site energies of excitons by phonons or by that of transfer integrals of excitons. Spatially extended large-radius S states characterize ST in low dimensions, being called the states of weak self-trapping in tune with weak localization in a low-dimensional-random lattice. Absorption spectra of these states including their phonon structures were investigated, by the dynamical coherent-potential approximation extended so as to incorporate spatially extended lattice distortions. It is shown that even an S state is mobile when it is a large-radius one, forming an energy band of Bloch waves of exciton polarons. Its bandwidth is, however, smaller than the phonon-energy quantum, and decrease to a value much smaller than it as the S state changes itself into a strongly contracted immobile state with increasing g.

In this paper, we shall describe a modulated photoluminescence technique which, in semiconductors quantum wells, provides results similar to the conventional electromodulated absorption measurements. The advantage of this new technique is that it can be used on as grown samples or devices without the need for elaborate sample geometries or sample preparation which are necessary in the modulated absorption measurements.

Magnetooptical study of the mixed type I-type II quantum well heterostructures (In,Ga)As/GaAs is presented, where a type I potential is realized for a heavy hole exciton and a type II quantum well takes place for a light hole exciton. The oscillatory structure of magnetoabsorption spectra allowed to restore the `fan diagrams' for HH1E1 and LH1E1 transitions taking into account the exciton binding energies calculated variationally. Thus, cyclotron masses of carriers were extracted for quantum wells with different content of Indium. An additional transition split-up from LH1E1 state is found in the spectra and interpreted as electron-third oscillatory hole state n equals 2 (LH3E1) exciton resonance. The self-consistent variational solution of the excitonic problem in a structure under study shows that in a weak type II potential LH1E1 and LH3E1 excitons remain spatially direct both having substantial oscillator strength. The splitting of 8 meV between these two states at zero magnetic field is in excellent agreement with experiment. We found that the electron Coulomb potential effect on a hole confinement results in an increase of LH3E1 exciton oscillator strength during type I-type II transitions, so that an appearance of a doublet structure in light hole exciton spectrum and substantial oscillator strength of light hole excitons is a signature of combined shallow type II quantum well and Coulomb potential.

The propagation of ballistic excitons in Cu₂O is studied using the pump-probe techniques. An excitonic packet in the shape of a soliton propagating with subsonic velocity is observed, providing a support for excitonic superfluidity.

Nonlocal formulation has been applied to a pump probe process (optical Stark effect) for an exciton-biexciton system in a slab of CuCl of thickness d equals 100 - 10000 angstroms. The evolution of the pump probe spectra shows the development of wave number selection rule with increasing d, reflecting the d-dependent spatial variation of radiation field. The spectra contain several different processes at the same time; two-photon absorption to biexciton states, the optical Stark shift and pump induced resonant broadening of exciton levels. The direction of optical Stark shift can be well interpreted by the pump induced change in the linear susceptibility. A comparison with the experiments for thicker samples shows a good agreement for d equals 1500 angstroms sample, but a marked disagreement for micrometers size samples. A comparison with a former theory has been made both analytically and numerically.

Dynamics of dark polariton solitons near exciton resonances in semiconductors has been considered. The influence of optical losses on the soliton dynamics has been examined. Results of numerical calculations for CuCl crystals are presented.

Excitation spectra for the free (FE) and bound (BE) exciton luminescence in PbI₂ have been investigated in the exciton absorption region. The excitation spectrum for FE consists of a series of sharp maxima. The structure is shown to arise from phonon scattering processes of exciton polaritons. There is a difference in the spectral profile between the excitation spectrum for FE and that for the phonon sideband of FE. The difference is explained by the change in the spatial distribution of polaritons with the incident photon energy. It is found that the excitation spectrum for BE has a spectral feature quite different from the cases of other materials such as CdS. This is due to the trapping process peculiar to PbI₂, in which bottleneck polaritons are captured with emission of an optical phonon. The difference in the trapping process between PbI₂ and CdS is discussed.

Temperature dependence of emission intensities and lifetimes of hetero-nuclear relaxed excitons [(ClI)^-+e], in KCl:I has been studied in the range of 30 - 200 K. From population conversion between the type II and type III configurations observed below 100 K, the energy difference between the two configurations of 13 meV is obtained. An anomalous behavior observed above 100 K suggests that the singlet state above 74 meV from the triplet state of type III is thermally populated to serve as an additional decay channel effective at higher temperatures.

Temperature dependence of spectral and temporal behaviors of resonant emission in NaNO₂ has been investigated under the excitation of the isotopic ¹⁵NO₂-molecule. It has been found that time response of the exciton line includes a rise component and can be interpreted by taking account of the transfer from the isotopic level. Time-resolved spectra of the exciton line under the excitation of the isotropic molecule shows broadening with the evolution of time. Spectral and temporal behaviors of the line reflect the transfer from the isotopic level as well as the thermalization of the excitons within the exciton band.

Evolution of excitonic luminescence has been investigated in Pb_1-xCd_xI₂ solid solutions over the whole range of composition. A significant change in the luminescence characteristics is found to occur at the composition x around 0.5, which corresponds to percolation threshold of the substituted metal ions. The luminescence in the crystals with x larger than 0.5 is associated with PbI₂ clusters dispersed in CdI₂ matrix. Time- resolved measurements of the emission spectra reveal that the luminescence consists of singlet and triplet components. The energy split between the singlet and triplet states increases with the decrease in the cluster size. The origin of the triplet state is discussed.

Optical studies of excitons in mica crystals have been carried out using the synchrotron radiation as a light source. Reflection measurements reveal that the direct allowed exciton transition appears at 9.6 eV in both natural phlogopite and synthetic fluor-phlogopite. Absorption and luminescence spectra of these two micas are also investigated. An intrinsic luminescence is found in synthetic mica to occur at 5.46 eV, having two decay components. This luminescence is supposed to arise from the radiative annihilation of self-trapped excitons. The above results are discussed in comparison with those of crystalline quartz.

Optical properties of gallium iodide GaI₃ have been studied for the first time in the energy range of 2 - 8 eV. The fundamental absorption edge is located at 3.1 eV at 15 K, and it shows anomalously large red shift with increasing temperature. It is found that a logarithmic plot of the absorption coefficient gives a straight line for all temperatures investigated, indicating that the absorption edge of GaI₃ is well described by the Urbach rule. The exciton-phonon interaction and the exciton relaxation process are discussed on the basis of these results as well as the luminescence result.

Electron emission has been observed with crystals of pyrene, (alpha) -perylene and anthracene excited with photons whose energies are less than the threshold for conventional monophotonic emission. It has been established that exciton fusion is responsible for this emission. In the case of pyrene, the lifetime of excitons observed through the excitonic photoemission is the same with that obtained from the fluorescence decay, whereas in (alpha) -perylene and anthracene the lifetime obtained from photoemission experiments is shorter compared to the fluorescence lifetime. It is considered that fluorescence measures the lifetime of excitons in the bulk of a crystal, whereas electron emission of this type probes excitons at the surface.

Indirect free-exciton luminescence in AgCl single crystal accompanied by TO(L) phonon emission is found for the first time at 382.9 nm (3.238 eV) at 2 K for picosecond pulse laser excitation at 351 nm (3.53 eV). The luminescence decay curve monitored at 382.9 nm has a fast decay component, 20 ps, which is interpreted as the self-trapping time of holes generated at the L point. A luminescence band peaking at 385 nm (3.221 eV) is found, which is assigned as due to the optical transition from a point around the exit of the quantum mechanical tunneling process through the self-trapping potential barrier. The rise time 70 ps is interpreted as the time required for tunneling. A luminescence band peaking at 386.5 nm (3.208 eV) is found and assigned to be due to excitons bound by shallow impurity.

Three different kinds of condensed samples of all-trans-(beta) -carotene, i.e., a spin-coated film, a cast film, and crystals, were prepared. Excitation spectra of photoconductivity at room temperature and resonance Raman scattering at 11 K were obtained for these samples, and the influence of oxygen to these excitation spectra were examined. In the near infrared region below the lowest allowed transition to the electronically excited ¹B_u state, clear bands were observed in those two kinds of excitation spectra and were characteristic of the crystals as well as the cast film. The excitation bands are attributable to the same transition to the 2¹Ag molecular exciton state. A similar series of experiments were carried out for all-trans-(beta) -apo-8'-carotenal. The results support the above assignment for all-trans- (beta) -carotene.

Triplet absorption and phosphorescence spectra of NaNO₂ crystal have been studied in the temperature range of 1.8 approximately 40 K by means of high resolution spectroscopy. Profiles of the absorption and phosphorescence phonon sidebands of the 0 - 0 line show that the triplet excitons are not in thermal equilibrium at 2 K but approach to the equilibrium above approximately 10 K. The intensity and decay time of the phosphorescence were found to decrease drastically in the temperature range of 3 approximately 6 K. The quenching of the phosphorescence is discussed in terms of spin-lattice relaxation caused by two-phonon processes. The relation between the thermalization of excitons and quenching of the phosphorescence are also discussed.

We present a study of the excitonic luminescence contribution in Hg_1-xCd_xTe for a wide CdTe mole fraction range (0.23 < x < 0.97). This mixed crystal semiconductor system is relatively well understood and offers a unique possibility to study the contribution of excitons to the luminescence of a material with a continuously decreasing energy gap. This reexamination is based on a set of uniform (carrier concentration, degree of compensation) samples grown by the traveling heater method. Furthermore the luminescence from Hg_0.15Cd_0.85Te/Hg_0.7Cd_0.3Te superlattices is analyzed with respect to the luminescence contributions of confined 2D excitons.

We introduce a concept which allows to study the nonlinear optical response of conjugated polymers taking into account many-body effects of the excitation process. The formation of charge transfer excitons and the creation of lattice distortions are studied in short polymer chains by solving the self consistent equations of motion for the reduced single electron density matrix and the lattice displacement. The coupling of electronic motions to lattice dynamics gives rise to the formation of excitonic polarons, which can be identified in the femtosecond pump-probe signal.

The paper demonstrates the applicability of a special type of density matrix theory for the derivation of generalized Master equations. The density matrix theory has been formulated for the description of the dissipative electron transfer dynamics in molecular complexes. The theoretical approach is based on a representation of the density matrix in appropriately taken diabatic electron-vibrational states. Dissipative effects are taken into account by a coupling of these states to further vibrational modes of the molecular complex as well as to environmental degrees of freedom. The approach is applied to a two-center system as well as to a molecular chain. Memory kernels are derived in second order with respect to the inter-center coupling. The kernels are discussed under the assumption of a quick intra-center relaxation for a part of the vibrational modes as well as for all vibrational modes. Standard expressions for the transition rates between different sites are extended to include finite life times of the vibrational levels. Results which have been obtained in the study of the so-called spin boson model can be simply reproduced. The application of the derived generalized Master equations to the investigation of the motion of Frenkel excitons in molecular chains is also presented.

At low laser intensity, the coupling between photons and electrons in a direct gap semiconductor gives rise to a coupled mode known as the exciton polariton; in this mode, the exciton appears as a non interacting boson. Under intense laser irradiation made of below-gap photons, this coupling gives rise to a blue shift of the exciton line which increases with the laser intensity. This effect known as the exciton optical Stark effect, is due to interactions between (virtual) excitons. At small detuning, Coulomb interaction and Pauli exclusion contribute to the shift, while at large detuning the shift is controlled by Pauli exclusion only. It is then identical to the shift of a two-level atom for a semiconductor having one valence and one conduction band, while it is the shift of a (2+4) level atom when the conduction band is twofold and the valence band fourfold. The cross-over between the exciton-polariton and the exciton optical Stark effect takes place when interactions between excitons cannot be neglected, i.e. when there is more than one photon (or virtual exciton) per exciton Bohr radius. These two effects correspond in fact to the same phenomenon: an exciton dressed by photons, the photon density being either small or large.

Temperature dependence of luminescence spectra, intensity and life-time has been measured precisely in CdBr₂ to investigate whole decay kinetics of self-trapped excitons (STE's). Below 50 K, a near-ultraviolet (UV) emission band appears at 3.3 eV. The initial state for the UV-emission consists of spin singlet and triplet states. The ratio of population branching of the optically created excitons into singlet to triplet was observed as 6:94. The triplet state further split into closely lying three levels. From analysis on the decay components, the ratio among initial populations of these levels was determined as 1:6:0 in order from lower to higher levels. Above 50 K, the intensity of the UV-emission decreases rather rapidly. About half of the STE's in the UV-state are transferred into another STE state responsible for yellow (Y) emission at 2.2 eV. Another half of STE's decay directly into ground state non-radiatively. The Y-state comprise two kinds of states, each of which further split into two states. Two pairs of decay components, namely four components, were observed between 50 and 150 K. Temperature dependence of time-integrated intensity was determined for each decay component. Above 150 K, all STE's are to be de-excited non-radiatively through thermal processes.

We report here results of experiments on the transient absorption spectra of CdSe microcrystals embedded in a germanium dioxide glass matrix. The transient absorption spectra were measured by means of femtosecond pump-probe method. In the sample with the smaller microcrystal size within the category of the strong confinement regime (the individual particles confinement regime), the absorption bleachings due to the carrier dynamics in the quasi-zero- dimensional quantum confined states were observed. In the sample with the larger microcrystal size within the category of the weak confinement regime (the exciton confinement regime), the absorption bleachings due to the hot carrier in the 3D electronic states were studied at various excitation intensities.

Atomic processes induced by electronic excitation in the bulk and on surfaces of insulators are surveyed, emphasizing the difference in the atomic processes induced in different types of solids. The phenomena treated involve local lattice modification in the bulk and atomic emission from the surfaces. First the relaxation of holes and excitons in halides and oxides are compared and the way how the self-trapping of excitons leads to formation of defects is discussed. It is argued also that the self-trapping of holes and excitons is favorable in amorphous materials and the difference in the behaviors of holes and excitons in crystalline and amorphous SiO₂ is discussed. The relaxation of excitons on surfaces of ionic crystals and consequent atomic emission processes are argued. The relaxation of densely generated excitons and electron-hole pairs in the bulk is also discussed.

We describe how defect induced one phonon absorption of far infrared radiation can be utilized for monochromatic generation of high frequency acoustic phonons in doped insulating materials. By combining this monochromatic phonon generation technique with an absorption vibronic sideband spectrometer we have been able to use time resolved high resolution phonon spectroscopy to investigate a range of fundamental dynamical processes which determine the behavior of a nonequilibrium phonon population. The combination of spectral and temporal resolution has enabled us to investigate the interaction of phonons both with electronic states and with themselves. We describe the mechanisms of energy transfer in insulating materials with particular emphasis on phonon assisted processes. While phonon assisted transfer is usually studied in a regime where a steady state phonon population stimulates energy transfer we describe results where a nonequilibrium phonon population stimulates the energy transfer between disparate Pr³⁺ sites in PrF₃.

Highlights concerning the creation and the decay dynamics of excitons in rare-gas solids and in solid rare-gas clusters will be presented. The decay dynamics include exciton transport, exciton trapping and exciton-induced desorption. Pronounced quantum-size effects are observed in rare-gas clusters. Recent experimental results on electronic excitations in condensed He, obtained from liquid He-clusters in a supersonic beam will be included.

In the vicinity of the direct intrinsic absorption threshold in semiconductors nonlinearity of the optical response is dominated by manybody interaction effects in the polarization cloud accompanying the propagation of the electromagnetic field. This polarization is essentially the system of electron-hole pairs. Its evolution with increasing density (radiation intensity) is analyzed in the framework of simplified model substituting effective short range pseudopotential instead of real Coulomb potential. At moderate densities excitonic resonance is essentially modified by the formation of excitonic molecules. Multistable behavior arise. Transient processes are discussed including gradual formation of bound state-excitons and excitonic molecules. The latter manifest themselves in anomalously large four-wave mixing amplitude.

Optically detected magnetic resonance (ODMR) experiments in crystalline As₂Se₃ provide detailed evidence for the existence of triplet excitonic states in some layered chalcogenide crystals. Comparisons with analogous optical absorption, luminescence, and ODMR measurements in glassy chalcogenides, such as As₂Se₃, As₂S₃, and alloys of these two glasses with copper suggest the presence of similar excitonic states in these materials.

We overview our experimental work on exciton self-trapping in simple organic molecular crystals. First we show pressure-induced change in the exciton-phonon interaction, indicating that the interaction depends strongly on the intermolecular distance, and then we demonstrate the experimental results and compare them with theoretical results on the exciton-phonon coupling constants, presence of the self-trapping barrier, intermediate self-trapped exciton states, the radiative annihilation of excitons at the exit of the self-trapping path. Most of this paper is devoted to show exciton relaxation processes visualized in time-resolved luminescence in Pyrene.

Metal precipitation has been observed at the 2D crystals of alkylammonium metal chlorides, (C_nH_2n+1NH₃)₂MCl₄ which were X- or (gamma) -irradiated at 77 K and warmed to room temperature. It has been also observed in the 1D crystals of CsMnCl₃(DOT)2H₂O which was irradiated at room temperature similar as photochromism of silver halides. The different temperature conditions of metal precipitation suggest a variety of defect formation mechanisms. In alkylammonium metal chlorides, Cl₂^- molecule ions are formed at 77 K and electron trap centers are observed at 15 K with X-ray irradiation. The electron trap centers are very unstable and are bleached at temperatures higher than 30 K.

The electronic transitions of vapor deposited (alpha) -oligothiophenes nT with n equals 3 to 8 thiophene units and film thickness of d equals 2 to 200 nm are investigated by angular-resolved polarized absorption and fluorescence spectroscopy. The absorption spectra of monolayers are strongly blue-shifted and show extreme dichroism arising from nT molecules oriented with their long molecular axes perpendicularly to the plane of the substrate. The extent of band shifts due to H-aggregation can roughly be quantified with the model of exciton dipole-dipole interaction, but the experimental oscillator strengths are much too low. In multilayers, the uniaxial molecular order is well preserved in 5T up to 10 layers. However, in 6T the order collapses quickly with growing film thickness, leading to small domains with no preferential orientation to the substrate. In the latter case the exciton splittings are reduced and the selection rules are broken up resulting in broader and more intense absorption bands. The fluorescence yields of the films are in the order of (phi) equals 10^-4 to 10^-2, increasing gradually with the film thickness, but still definitely lower than in the monomers where (phi) equals 0.07 (3T) to 0.4 (6T). At least two weak fluorescence emissions are identified in the films originating from the low energy edge of the exciton band and/or from the two-electron excited 2¹A$g)-state.

Several new spectroscopic studies relating to the coupling and dynamics in the spin-forbidden ³MLCT excited states of the chromophores [Ru(bpy)₃]²⁺ and [Os(bpy)₃]²⁺ (bpy equals 2,2'-bipyridine) in the racemic crystal lattices [Ru(bpy)₃](PF₆)₂, [Ru(bpy)₃](ClO₄)₂ and [Zn(bpy)₃](ClO₄)₂ are presented. In the first of these lattices there are three closely related chromophoric sites at low temperatures, each with trigonal (C₃) symmetry. In the two, isomorphic perchlorate salts there is a single chromophoric site, which has C₂ symmetry. Using time resolved luminescence line narrowing, we have been able to directly measure the excitation transfer rate between two equivalent metal-ligand units in the [Ru(bpy)₃]²⁺ chromophore doped in the [Zn(bpy)₃](ClO₄)₂ lattice. The rate obtained (approximately equals 1 X 10⁸ sec^-1) is in excellent accord with estimates made from the observed linewidth in Stark swept transient hole-burning experiments made on the same system and confirm the single ligand, localized nature of the lowest emitting excited states and thus the very weak intramolecular coupling between metal ligand sub-units within this chromophore. The corresponding coupling in the [Os(bpy)₃]²⁺ system is stronger and, in contrast to the ruthenium analogue, gives rise to additional features in the optical spectra in the origin region of the lowest ³MLCT excited states. The magnitude of the coupling can be probed and assessed by preparing modified chromophoric materials, in which one or two of the bpy ligands are perdeuterated (bpy-d₈). This selective deuteration breaks the (near) degeneracy of excitations involving crystallographically equivalent ligands by approximately equals 30 - 40 cm^-1 and this competes with or completely overrides the exciton coupling process. The exciton coupling is found to be approximately equals 2.4 cm^-1 for [Os(bpy)₃]²⁺ doped in [Ru(bpy)₃](PF₆)₂ and can be understood within a mini-exciton description. Stronger couplings for the same chromophore in [Zn(bpy)₃](ClO₄)₂ are indicated in our preliminary results. In inhomogeneously broadened systems such as frozen glasses or solutions, in which MLCT excitations involving different ligands on the same chromophore may differ in energy by 100 cm^-1 or more, an effectively localized description of the low lying ³MLCT states is appropriate for [Os(bpy)₃]²⁺.

Small-radius Frenkel excitons coupled with phonons can have diverse configurational structures depending on the symmetry of the electronic states involved and that of the associated modes of the inter-ionic/molecular vibrations. It is considered here that the symmetric excitons arise from the coupling with symmetry-retaining vibrational modes and vibronic excitons from the coupling to symmetry-breaking modes. Three examples are discussed: (1) formation of the barriers of exciton self trapping, as it occurs in the dimer formation at semiconductor surfaces, (2) parity breaking that occurs at a centrosymmetric site and leads to an inversion electric dipole which enhances the polarizability and binding energy for pairing vibronic excitons, and (3) a Bose condensation model of quasi-2D excitons applicable to the layered materials.

Inter-molecular interaction of photochromic compound furylfulgide dispersed in a PMMA film was studied from the measurements of luminescence spectra as a function of the concentration of the fulgide and also a function of the excitation photon energy. The peak energy of luminescence decreased as the concentration increased and as the excitation photon energy decreased for the concentration lower than 0.01 mol/dm³. However, the peak energy was the lowest one among the measured samples and independent of the excitation photon energy for a sample of the concentration of 0.1 mol/dm³. These are well understood in terms of an electric dipole-dipole interaction between the fulgide molecules. The critical distance between the molecules was about 4 nm. The conversion rate for the coloration depended strongly on the temperature as well as the excitation photon energy. It was found that the band shape and peak energy of the absorption spectrum for the light-produced C- or E-form fulgides in a PMMA film depends on the excitation photon energy. This means that one can select special free volumes (cage) from the inhomogeneously distributed ones in a polymer matrix by a narrow band excitation.

Relaxation processes of exciton polaritons in monoclinic zinc diphosphide (beta) -ZnP₂ have been investigated in detail by measuring resonant secondary emission spectra under excitation into the energy range from the interband to the exciton-bands region. In addition to the 1LO line of 32.2 meV already reported, we have found more than 30 resonant Raman lines ranging from 9.4 meV to 58.9 meV by varying the excitation energy. These lines are observed distinctly only when their scattered energies fall into the vicinity of the 1s exciton energy. It is confirmed that among phonons of 72 modes the LO phonon of 32.2 meV preferentially governs the relaxation processes of exciton polaritons in (beta) -ZnP₂. We have also observed a significant intensity decrease of the exciton polariton luminescence when the excitation energy is lowered across the 1s longitudinal exciton energy E_L. A brief discussion is made in this connection.

Biexciton lifetime is studied in wide wavenumber regions, high from 0.887 to 6.2 X 10⁶ cm^-1 and low from 0 to 0.2 X 10⁶ cm^-1. In both regions its lifetime is found within about 20 approximately 40 ps, although it decays via different intermediate states, i.e. emitting different branch polaritons. In the high wavenumber region the lifetime becomes shorter with wavenumber. The decay rates responsible for the radiative emission, acoustic phonon spontaneous emission and elastic scatterings are derived.

The exciton binding energy have been calculated as function of the magnetic field and the quantum well width for the various exciton and magnetic states. The variational calculation carried out in the adiabatic approach has shown the sizeable enhancement of the exciton binding energy with increase of the external field. For rather strong magnetic fields the exciton binding energy has been found in a framework of the perturbation theory. The results were used for the analysis of the spectra of light transmission through the thin semiconductor film containing GaAs/Al_0.3Ga_0.7As multiple quantum well structures and In_xGa_1-xAs/GaAs strained multiple quantum well structures in the external magnetic field tuning from 0 to 7.5 T at T equals 2 K.

The decay law of exciton survival probability due to quasi-1D random walks and trapping on periodic lattice with randomly distributed traps is studied. Survival probability is expressed via well known analytical asymptotics of that for strictly 1D transport at low trap concentration. Presented approximation can be generalized for large trap concentrations as well. Comparison of the new results with those reported previously (for quasi-1D trapping) and Monte Carlo simulation is made.

The coherent emission process at excitonic resonance was first investigated in short-period ZnSe/ZnS_xSe_1-x (x equals 0.18) superlattices with one period of 50 A/50 A and 19 A/19 A by observing a temporal profile with subpicosecond time resolution, of reflected pulses in the Brewster-angle configuration. As a result, an almost background-free signal with nonresonant contribution reduced greatly, was observed reflecting free induction decay (FID) of coherently driven exciton polarization. In particular, a beat signal of FID was clearly observed for the first time, which arises from interference effect between a few split spectral lines of the relevant exciton. Further, it was confirmed that the decay spread of FID becomes faster as the power density of incident pulse was increased for the SL sample with a thicker well. The results may be explained qualitatively by a simple model with the knowledge of the exciton structure as well as a preliminary data of the phase relaxation time obtained by four- wave-mixing measurement. The present results have provided a basis for that the transient Brewster-angle reflection spectroscopy serves as a powerful tool for exploring coherent interaction of exciton with light.

J-aggregates of tiacarbocyanine derivative are investigated in frozen solution at low temperatures by optical spectroscopy and spectral hole burning. It is found that absorption and luminescence bands have inhomogeneous widths of 220 and 130 cm^-1 at 5 K respectively used in contrast to pseudoisocyanine (PIC) J-bands are separated by Stokes shift of 100 cm^-1. Moreover, J-aggregate fluorescence spectrum depends drastically on the temperature and initial dye concentration. Temperature broadening of spectral hole, burnt in the J-absorption band differs qualitatively from that, burnt in the monomeric band. It differs also, but quantitatively, from broadening of the spectral hole in J-band of PIC. These observations are discussed in terms of nonradiative relaxation of excitons in the system under investigation.

Commercially available vitreous silica, Suprasil P-20 and GE-124, were irradiated in JRR-2 to the total fast neutron fluences of 2.6 X 10¹⁷ nvt. Ultraviolet optical absorption, photoluminescence (PL), PL excitation and infrared absorption were measured at room temperature. Three PL peaks at 4.3 eV, 2.7 eV and 2.2 eV were clearly observed for pile irradiated specimens excited by 254 nm (4.88 eV) light, in addition to the intrinsic PL. A 1.9 eV PL was observed in the time resolved PL experiment for pile-irradiated specimens excited by fourth harmonic Nd:YAG laser light of 5 mJ/cm² pulse. Fourier transform infrared attenuated total reflection measurement showed no bond angle changes in Si-O-Si bridging angle.

Secondary emission spectra in the higher region of exciton band of ZnP₂ single crystal are measured at 6 K with a tunable Ti:sapphire laser as an exciting light source. The luminescence bands due to the radiative recombination of the 2s, 3s and 4s excitons are clearly observed separately. It is also found that the 1LO Raman line with energy of 32.2 meV shows resonance enhancement near the 2s exciton band. On the contrary, the intensity of the 1LO Raman line becomes anomalously weak when it is just superimposed on the 3s luminescence peak. The shapes of these luminescence bands change considerably when the 1LO Raman line approaches them. The changes in intensity of the 2s and 3s exciton bands with the excitation energy are briefly discussed in regard to the relaxation processes of polariton.

The magnetic circular dichroic (MCD) spectra for 160- and 290-angstroms thick films of CuCl were measured around the Z₃ exciton region up to 6 tesla at 10 K. These thicknesses are in the region where the long wave approximation is valid. These MCD spectra of the quantized states of translational motion of the Z₃ excitons were explained well by Zeeman splitting. The g-values of these quantized states in the CuCl thin films were obtained by fitting these spectra to an approximation model, and the g-values obtained are identical to that of a 1700-angstroms thick sample. No change of the g-values of each quantized state was observed. These results indicate that this quantum size effect has no contribution to the relative motion of electron and hole in excitons in these film thicknesses.

Although lasing is meanwhile achieved in various ZnSe based heterostructures, the underlying mechanism is still seriously under debate. We measured the gain spectra of various MOVPE and MBE grown samples using the variable-stripe-length method. A description of the gain spectra for low particle densities is possible in terms of the model of Ding et al. The responsible gain mechanism is assumed to be an induced recombination of strongly exchange- interacting localized excitons. Predictions and limitations of the model are discussed for various temperatures, exciton densities, excitation conditions and sample designs. However, several indications are given that under certain circumstances more than one process contribute to lasing, in particular biexciton recombination seems involved in the gain mechanism. Electron-hole plasma recombination is only important for extremely high excitation densities which are not aspired for injection laser diodes based on II-VI materials. This is in strict contrast to the III-V lasers, where the electron-hole plasma recombination is the dominant lasing process.

The codoped Cr,Nd:ZnWO₄ single crystals up to (Phi) 25 X 50 mm have been grown by the Czochralski method in our laboratory. Nd³⁺ ions have improved laser characterization of codoped Cr,Nd:ZnWO₄ crystal. The spectra of crystal were presented and analyzed. A laser pulse which wavelength is 0.95 micrometers was gained in (Phi) 6 X 19 mm crystal rod, which was pumped with ruby laser. Maximum output energy of 1.62 mJ and slop coefficient of 0.79% were achieved.

We have theoretically investigated the scattering of excitons by free electrons in a semiconducting quantum well of finite width. The elastic and ionization scattering cross sections have been calculated as a function of the well width using the Born approximation for scattering of excitons by electrons. The behavior of the scattering cross section as a function of energy of relative motion of the electron and the exciton is similar to that previously obtained from 2D calculations for narrow wells. The cross section increases as the well width increases and the relative wave vector of the electron at which the total cross section is a maximum shift to lower values as the well width increases.

As a model of excitonic behavior the paper concerns itself with the one-electron states generated by a bare Coulomb potential-Ze²/r, plus an applied magnetic field H which is assumed intense. A quantity which reflects the entire one-electron level spectrum (epsilon) _i and the corresponding wave functions (formula available in paper) is the canonical density matrix (formula available in paper) whose trace is the partition function Z(beta) .The inverse Laplace transform of C/(beta) yields the Dirac density matrix (zetz) (r,r,E), its diagonal element being the integrated local density of states. For free electrons in an intense field (formula available in paper) in three dimensions and in the presence of the Coulomb potential the Thomas-Fermi approximation replaces E by (formula available in paper). One can approximate, albeit somewhat crudely, the lowest energy state by finding the energy, E₁ say, such that (integral) (formula available in paper). Such a procedure leads to a large error in E₁ for H equals O but is expected to be a better approximation in an intense field. Applying the same approximation in two dimensions, with the magnetic field perpendicular to the plane of confinement assumed for the electrons, leads again to an estimate for the lowest one-electron state in an intense field. This motivates then a further study of this 2D case, in which the pure cylindrical symmetry of the 2D zero field case is preserved. Finally, in an Appendix, the bare Coulomb limit of a recent, self-consistent field, treatment by Lieb et al, of atoms in a hyperstrong field such that, in suitable units, H Z³ is set out.

The Josephson photoresponse of granular high-T_c superconductor films to pulsed far infrared laser radiation has been investigated in magnetic fields of up to 3T. Its value is strongly influenced by fields less than 50 mT and shows hysteresis here, while no further changes occur for larger fields. It is shown that the field dependence directly yields information about the properties of intergrain weak links. These are found to have a strongly inhomogeneous critical current distribution.

Photosynthesis is an extremely efficient converter of light into chemical energy, with an observed quantum yield for primary photochemistry approximately 90%. To achieve this the photosynthetic apparatus must be highly optimized, and some of the design principles that may be involved have been suggested. The role of delocalized exciton states of light-harvesting pigments in the energy transfer process has been considered by mathematical simulation of the light-harvesting process in model systems. Namely, it has been shown that aggregation of antenna pigments (allowing to consider each aggregate as a supermolecule) is biologically expedient, as an efficient strategy for light harvesting in photosynthesis. The question of whether this design principle is realized in a natural antenna has been examined for the 3D chlorosomal superantenna of green bacteria with the hole-burning spectroscopy. Spectral hole burning studies of intact cells of green bacteria Chlorobium phaeovibriodes. Chloroflexus aurantiacus and Chlorobium limicola have proven that the Q_y- absorption system of antenna bacteriochlorophylls e or c (BChl e or BChl c) should be interpreted in terms of the delocalized exciton level structure of an aggregate. For the first time the 0-0 transition band of the lowest exciton state of BChl e and BChl c aggregates has been directly detected as the lowest energy inhomogeneously broadened band of the 1.8 K near-infrared excitation spectrum. These lowest energy bands have different spectral position of their maximums: approximately 739 nm in C.phaeovibriodes (BChl e band), approximately 752 nm in C.aurantiacus (BChl c band) and approximately 774 nm in C.limicola (BChl c band) cells. However, these bands display a number of fundamentally similar spectral features: (1) The magnitude of inhomogeneous broadening of these bands is 90 - 100 cm^-1; (2) The width of each band is 2 - 3 times less than that of the monomeric BChl c (or BChl e) in vitro at 5 K; (3) Each band, being the lowest energy exciton band, manifests itself as the longest wavelength band in the circular dichroism spectrum; (4) At the wavelength of the maximum of each band for all the three species, the amplitude of the preburnt excitation spectrum makes up 20% of the maximum amplitude of the spectrum; (5) The weak exciton-phonon coupling of optical transitions corresponding to these bands is also a common feature. So, the hole spectra measured for C.aurantiacus, C.limicola and C.phaeovibrioides cells were shown to be consistent with the BChl c (and BChl e) organization in the chlorosomes as strongly exciton- coupled BChl c (or BChl e) aggregates, i.e. the delocalized excitons are in fact involved in the energy transfer process within these antennae. Thus, aggregation of the pigment molecules in natural light-harvesting antennae should be considered as one of the optimizing factors ensuring high efficiency of excitation energy transfer from antenna to reaction center.2362

The absorption-edge spectra have been studied in the temperature range from 4 to 300 K for single-crystal CdTe, ZnTe, GaAs wafers and GaAs/(Al,Ga)As multiple quantum well (MQW) structures. In all cases the frequency integrated absorption coefficient K is found to increase monotonously with temperature T up to T equals T* and to keep constant above T*. The temperatures T*, depending on semiconducting materials, might be considered as critical ones corresponding to a change in polaritonic energy transport mechanism due to the lack of spatial dispersion at T > T*. It was shown, that though the measured temperature T* approximately 102 K correspond to much larger linewidths than could be explained by using the theoretical value of the exciton damping parameter, this discrepancy can be overcome if a temperature-dependent inhomogeneous broadening is taken into account consistently. A similar temperature dependence of K with T* approximately equals 20 K has been observed for the first time in GaAs/(Al,Ga)As MQW-structures.

The specular reflection spectra of CdS crystals are determined at T equals 2 K in the region of the Anequals1 exciton resonance. The great variability in the spectra cannot not be explained by the simple model of an exciton-free dead layer at the boundary of a crystal. Generalized boundary conditions are formulated for the excitons of large radius, which includes inhomogeneous additional boundary conditions for the exciton polarization and inhomogeneous boundary conditions for the tangential component of the magnetic induction vector B. An analysis of the energy balance equations at the surface of a crystal yields additional relationships between the parameters of the theory. The proposed system of boundary conditions describes the case with a sharp internal boundary at the dead layer and allows for the effects of the intrinsic and extrinsic mechanisms for formation of this layer. The reflection spectra of normally incident light are calculated numerically for various relationships between the parameters of the theory. All quantitative features of the optical exciton reflection spectra are in good agreement with the experimental data. We also give explicit expressions for the reflection and transmission coefficients of an optical wave incident normally on the plate.

Excitons in semiconductors can be spin-polarized under optical selective excitation by polarized radiation (optical orientation) or due to spin relaxation and sublevel mixing in an external magnetic field. The paper deals with optical phenomena where spin-polarized excitons generated in semiconductor nanostructures play an important role. Firstly, both optical orientation and optical alignment of excitons in type II GaAs/AlAs superlattices are considered and effects of the anisotropic electron-hole exchange interaction and external magnetic field on the photoluminescence polarization are analyzed. Secondly, magnetic-field-induced anticrossing of excitonic sublevels is discussed taking into account the axial and anisotropic exchange splittings, spin-relaxation and difference in the lifetimes of radiative and non- radiative exciton states. Next, the localized and bound excitons are shown to act as intermediate states in resonant Raman scattering by spin flips of holes bound to acceptors in GaAs/AlGaAs multiple quantum wells. The analysis of polarized Raman spectra permits to make decisive conclusions concerning microscopic mechanisms of the observed scattering processes. Finally, the doubly-resonant 2s-1s LO-assisted secondary emission observed in CdTe/CdMnTe quantum-well structures is described as a process with spin-polarized hot 1s- excitons acting as real intermediate states.

Recent experiments of our group on excitonic energy transfer and relaxation processes in doped and undoped solid rate gases are reviewed. The samples studied were vapor deposited films, mainly solid Ar and Kr doped with Ag or Au atoms. X-ray irradiation and selective VUV-excitation were used for exciton generation. Excitonic relaxation processes during and after exciton generation were monitored via the fluorescence of self-trapped and impurity- trapped excitons as well as via thermally stimulated luminescence and conductivity measurements after irradiation, respectively. Exciton diffusion lengths, diffusion constants, and trapping rates at impurities are determined by comparing the emission yield spectra of impurity and self-trapped exciton emission. By identifying the impurity states being excited during exciton trapping we could show that in all systems studied the dominating exciton capture process is the one yielding the lowest excess energy to be dissipated into the crystal. Impurity trapping of excitons partly leads to the production of free charge carriers, which can be captured in shallow traps. The glow curves of impurity luminescence and conductivity during thermal annealing clearly reveals the existence of intrinsic and extrinsic electron and hole traps. Binding energies of these traps are evaluated from the data.

Epitaxial monocrystalline CdSe layers are deposited on ZnTe substrates by sublimation in vacuum under dynamic equilibrium. The photoluminescence spectra at 4.2 degree(s) K of undoped and oxygen doped ZnTe substrates are dominated by the recombination of bound and free excitons and their phonon replica. When ZnTe is phosphorous doped the photoluminescence spectrum is determined by radiative transitions involving impurity levels, the conduction and the valence bands. The photoluminescence spectrum of undoped CdSe epitaxial layer reveals several excitonic lines near the band edge and their phonon replica. When CdSe is indium doped, the spectrum is dominated by a single luminescent band at a lower energy. The electroluminescence spectra for CdSe/ZnTe heterojunction were found consistent with a model in which carriers injection is taking place in the ZnTe substrate. Anomalous red and blue emissions were observed. The origin of these emissions has been explained in terms of recombination of carriers in an intermediate layer formed by atomic diffusion at the interface. A detailed model for the carrier injection mechanism in CdSe/ZnTe heterojunction is presented.

A red cathode-ray phosphor, Y2OS:Eu3, shows a large decrease in luminescence efficiency at high excitation density at room temperature. The luminescence intensity is approximately proportional to the square root of the electron-beam current, indicating that a bimolecular-type nonradiative process competes with a radiative process. In undoped Y2OS, broad luminescence bands with peaks at around 35and 440nm are found at 80K. The defect levels involved in this luminescence can be a candidate of the nonradiative process at room temperature, because the luminescence is thermally quenched at room temperature.