Excitons In AgBr Research Articles

Luminescence kinetics of iodine-doped silver bromide crystals: concentration and excitation intensity dependences

The decay kinetics of iodine bound excitons in AgBr:I crystals have been studied. Luminescence kinetics, quenching and changes in kinetics are observed to depend on the iodine concentration and the excitation intensity. These observation are discussed in terms of donor - donor and donor - acceptor interactions. The probabilities of energy migration via iodide ions and probability of luminescence quenching are estimated.

Quantum beat spectroscopy: a new tool to study phase relaxation of excitons

We briefly review our recent work in which picosecond quantum beat spectroscopy was employed for the study of excitons. The principle of the method and the necessary experimental technique are described. As examples, results are presented that were obtained for the indirect free exciton in AgBr, for bound excitons in CdS, and for the exciton-polariton in Cu2O.

Quantum beats in picosecond time-resolved resonant light scattering of free excitons in AgBr

Abstract Experiments are reported in which quantum beats are observed in the resonance fluorescence of a free exciton in a solid. The beats are generated in AgBr at 2 K by pulsed excitation of the Γ+6 ⊗ L-4,5 indirect singlet-triplet exciton state split in magnetic fields up to 4 T. Originating from the coherent superposition of the split states, damped oscillations occur superimposed on the decay of the 2TO(L) scattered intensity. The quantum-beat signals calculated by means of the density matrix formalism are found to be in good agreement with the experiment. From the beat frequencies the splitting of the exciton states is obtained with high accuracy. Quantum dephasing times of several hundreds of picoseconds are determined dependent on exciton energy and magnetic field strength.

Space-Resolved Luminescence Spectra of Free Excitons in Pure AgBr. II. Momentum Relaxation Mechanism

Dynamical behaviour of free excitons in AgBr has been studied by observing the space-resolved luminescence spectra. Experiments on the spatial distribution of the free excitons have been carried out, for the first time, in pure and well-annealed AgBr crystals between 1.7 and 20 K. Observed distributions are consistent with a diffusion model due to a gradient in density of free excitons. By analysing the distributions, we can well explain the momentum relaxation rate of free excitons in terms of the scattering of the free exciton by longitudinal acoustic phonons.

Luminescence Spectra of Free Excitons in Pure AgBr down to 500 mK. I. Effects of Annealing

Luminescence spectra of pure AgBr have been investigated in the vicinity of the indirect band gap at the temperature region between 500 mK and 50 K by using 3 He-coolant under various excitation conditions. Systematic studies of the effects of annealing on luminescence spectra of free excitons in AgBr have revealed that the lifetime of free excitons in a specimen after annealing can be extremely prolonged and the excitonic temperature in the specimen can be further lowered than that in a specimen before annealing. Luminescence spectra for an annealed specimen under a uniaxial stress at low temperatures have also been studied to ensure an understanding of the cause of these two phenomena.

Tightly bound exciton in AgBr:Ir 3+

Luminescence spectral and kinetic measurements lead to a model in which an exciton bound to a highly relaxed excited-state configuration of Ir 3+ in silver bromide undergoes radiative recombination. In addition to the previously reported infrared emission bands observed at ≈0.98 and ≈1.6 μm, a new highly structured band is seen at ≈2.5 μm. The luminescence decay lifetimes of these emissions are ≈0.3 ± 0.05, 1.8 ± 0.1, and 0.35 ± 0.05 msec, respectively, at <−10 K. The luminescence rise kinetics and decay kinetics have been studied as a function of temperature and excitation wavelength. An energy-level scheme for the Ir 3+ AgBr system is presented. It is believed that a bound-exciton state is a precursor to the trapped-electron species detected by EPR spectroscopy. A model for the low-temperature photochemistry of Ir 3+ in AgBr is given.

Pressure-induced phase transition of excitons in AgBr 1−xCl x

Photoluminescence spectral change of AgBr 1−xCl x (x=0.6 and 0.15) under high pressure has been investigated at 1.5K. The spectral shape of AgBr 0.4Cl 0.6 is found to change from the self-trapped exciton type to the free exciton type at about 16 kbar. The pressure coefficients of the free exciton energy and the bound exciton energy are determined for AgBr 0.85Cl 0.15. As a result, a tentative phase diagram for exciton system in AgBr 1−xCl x is given.

Resonant Raman scattering at the indirect triplet exciton in AgBr in high magnetic fields

Resonant two-phonon Raman scattering involving the indirect triplet exciton is studied in AgBr at T = 1.8K and magnetic fields up to 20 Tesla. By a quantitative analysis of the scattered 2TO(L) intensity we are able to determine the symmetry of hole states to be L-4,5, the exchange splitting Δ = 0.17 meV and effective g-values of gc = 1.46 and g″v = 2.61 for electrons and holes. The observed diamagnetic shifts lead to a value of 32 meV for the exciton binding energy.

Resonant Raman scattering at the indirect triplet exciton in AgBr in high magnetic fields

Resonant two-phonon Raman scattering involving the indirect triplet exciton as an intermediate state is studied in AgBr at T=2K and magnetic fields up to 20T. The symmetry of the hole state is found to be L4,5-. From a quantitative analysis of the scattered 2TO(L) intensity as function of magnetic field and at different excitation photon energies the authors were able to determine the exchange splitting Delta =0.17 meV and effective g values of gc=1.46 and gvh=2.61 for electrons and holes, respectively. The observed diamagnetic shift leads to a value of 32 meV for the indirect exciton binding energy.

Relaxation of the indirect exciton in AgBr studied by resonant Raman scattering

By studying resonant Raman scattering in AgBr excited at 1.8K in the indirect exciton absorption, various exciton relaxation processes are revealed. They are identified by analysing both the Raman shifts and intensities of the scattering lines as a function of the kinetic energy of the exciton. Intravalley scattering by acoustic phonons near the zone centre, intervalley scattering by TA(X) and LA(X) phonons and trapping by impurities are found to be important relaxation mechanisms. Their relative scattering probabilities are determined. From the analysis of intravalley scattering involving dispersive phonons the effective exciton mass is determined as M*=(1.4+or-0.2) electron masses. A model is developed on the basis of the authors results that allows reproduction of the essential features of the experimental scattering spectra demonstrating the consistency of their analysis.

Photoluminescence study of excitons in AgBr in high magnetic fields

The luminescence spectrum of AgBr at 1.6K reveals a number of sharp lines which are associated with excitons bound to residual impurities and crystalline defects. Two lines are identified as excitons bound to iodine ( epsilon b=43 meV) and cadmium ( epsilon b=8 meV). Several lines are observed in the neighbourhood of the phonon-assisted free-exciton emission, two of them corresponding to excitons bound to unidentified centres with energies of 1 meV and 0.25 meV. Electron-hole exchange coupling is found to be weak for excitons bound to the isoelectronic centres and their Zeeman spectra are described quite adequately by a simple L-S coupling scheme. The Zeeman spectra of the unidentified shallow states are found to be qualitatively similar to those of the isoelectronic traps. The g-values obtained decrease with increasing binding energy from ge=2.29 and gh=0.76 for the shallow states to ge=1.97 and gh=0.66 for the iodine trap.

The stark effect on the iodine-bound exciton in AgBr

Iodide as an isoelectronic impurity in AgBr is known to trap holes, which then bind electrons in Wannier-like states. We have examined the Stark spectrum of the emission from this iodine-bound exciton in AgBr at liquid-helium temperatures. These data yield a polarizability of 3.6 x 10 5 Å 3, an effective Rydberg constant of 34 meV, and a Bohr radius of 20 Å.

Exciton relaxation by intervalley scattering in AgBr

By resonant Raman measurements at low temperatures intervalley scattering is demonstrated to be an efficient relaxation mechanism of the free indirect exciton in AgBr. The observed strongly enhanced scattering lines are attributed to TA(X) and LA(X) phonons connecting different L-points in the Brillouin zone. Analyzing the energy dependence of the line intensities the relative contributions of different exciton relaxation processes to the total exciton lifetime are estimated.

A new shoulder in luminescence spectra due to the triplet excitons in AgBr

Magneto-optical measurements of pure AgBr have been extended to cover the luminescence study. A new and intrinsic shoulder in luminescence spectra supposed to be due to “ the indirect allowed triplet excitons” has been found, for the first time, for zone refined AgBr crystals at high magnetic fields up to 50 kOe.

Decay of the proposed Bose-Einstein condensed excitons in AgBr

A narrow emission line close to the free singlet exciton emission in AgBr has been subject to different interpretations. Bose-Einstein condensed free triplet excitons as well as a weakly bound exciton have been suggested as the origin of this emission line. The line is investigated from 1.5 to 2.1 K with respect to the temperature dependence of the lifetime and intensity. A strong decrease of both lifetime and intensity with increasing temperature is observed and suggests this line to originate from the recombination of bound, rather than of free, excitons.

Excitons in AgBr 1-æCl æ - transition of relaxed state between free and self-traffed exciton

Optical absorption and photoluminescence studies on AgBr 1-æCl æ show that the nature of relaxed exciton states changes discontinuously from free to self-trapped excition as the composition x exceeds ∼0.45. The transition can be explained by attainment of sufficiently strong exciton-phonon coupling wehn the alloy composition is varried.

Radiative Decay-Time of Bound Excitons in Silver Bromide

Luminescence due to decay of iodine-bound exciton in AgBr:I - is observed under irradiation of a Q -switched ruby laser and can be concluded as originated from interband two-photon transition process. Radiative decay-tome of the luminescence is studied as a function of temperature (2∼120°K), emission wavelength (460∼550 nm) and iodine concentration (2.1×10 18 ∼5.3×10 19 cm -3 ). The decay kinetics and its temperature dependence are found to be different between the two emission components in the emission spectrum; high energy component (∼495 nm) and low energy one (∼520 nm). The decay time at 2°K are (25±4)×10 -6 and (16±3)×10 -6 s for high and low energy components, respectively, at iodine concentration of 2.1×10 18 cm -3 . The concentration dependence of relative intensity of the two components can be understood by assuming that the high energy component corresponds to exciton bound to isolated iodide ion and the low energy component to that bound to iodine pair.