Size effects in photoexcitation of triplet states of ammonium tetraphenylborate

This paper reports on the results of complex investigations of photoexcited states of ammonium tetraphenylborate, which are characterized by self-sensitized luminescence. It has been established that the excitation by UV light at 77 K leads to the formation of stable triplet states due to the capture of electrons on electron traps. The EPR and luminescence excitation spectra exhibit the formation of a set of triplet states with different distances between electrons and holes. The performed investigations give grounds to affirm that, in bulk samples, cations in the structure of ammonium tetraphenylborate are electron traps. When the size of the ammonium tetraphenylborate sample is changed to 6 and 3 nm, the capture of excited electrons on sorbed oxygen molecules becomes dominant. In this case, the appearance of the spectrum of O₂⁻ anion radicals has been detected by the EPR method. The proposed interpretation of the observed effects has been confirmed by the thermoluminescence data on the recombination of electron-hole pairs, which correlate with a change in the intensity of the EPR spectra during annealing.     

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.     

Effect of polar phonons on exciton states in semiconducting crystals

By means of exact integration, the energy and quasimomentum dependence of the real part of the exciton Green's function mass operator ReM^{(2) (k,)} is derived for the first time. Using the analytical form of ReM^{(2) (k,)}, the renormalization of the exciton effective mass by polarizing lattice vibrations and the threshold value of the quasimomentum of a photon emitting exciton are derived.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 28–31, April, 1982.     

Trapped exciton states in lead halides

The absorption spectra of thin films of PbCl₂ containing 4 mol.% of NaCl, and PbI₂ containing 4 mol.% of KI measured at room temperature are presented. The study indicates the presence of effective mass states associated with substitutional sodium (or potassium) impurities. The character of these states as intermediate excitons in lead halides are discussed.     

Shallow acceptor bound exciton and free exciton states in high-purity zinc telluride

We investigated excitons bound to shallow acceptors in high-purity ZnTe and measured excitation spectra of two-hole luminescence lines at 1.6 K using a tunable dye-laser. The electron-hole coupling in the bound exciton (BE) states appears to be very different for the various acceptors even for almost identical exciton localisation energies. Three different types of BE are reported. For the Li-acceptor BE we observe three sub-components separated by 0.22 and 0.17 meV and interpreted as J = $\text{1}\text{2}$, $\text{3}\text{2}$, $\text{5}\text{2}$ states. The Ag-acceptor BE exhibits a strong ground state and a weak excited state at 1.3 meV higher energy. For the as yet unidentified k-acceptor we observe a single BE level, degenerate with the Ag-acceptor BE ground state. Dips in the excitation spectra due to absorption into free exciton 1S, 2S, and 3S states yield an exciton Rydberg R₀ = 12.8±0.3 meV and a free exciton binding energy FE(1S) = 13.2±0.3 meV.     

Two-photon transitions to exciton states in semiconductors

We consider the gauge invariance of the two-photon transition rate from the crystal ground state to exciton states in semiconductors in the framework of the Wannier model. We show that the non-locality correction in the velocity gauge is essential to preserve the gauge invariance. As a numerical example, we calculate in the length and in the velocity gauge the transition rates for the two-photon direct creation of 2p excitons in a two-band semiconductor.     

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.     

Triplet exciton diffusion and phosphorescence quenching in iridium(III)-centered dendrimers

A study of triplet-triplet exciton annihilation and nonradiative decay in films of iridium(III)-centered phosphorescent dendrimers is reported. The average separation of the chromophore was tuned by the molecular structure and also by blending with a host material. It was found that triplet exciton hopping is controlled by electron exchange interactions and can be over 600 times faster than phosphorescence quenching. Nonradiative decay occurs by weak dipole-dipole interactions and is independent of exciton diffusion, except in very thin films (<20 nm) where surface quenching dominates the decay.     

The excited states of an intersubband exciton

The energy of intersubband collective spin-and charge-density excitations is calculated for a system of quasi-two-dimensional electrons at certain values of the angular-momentum component perpendicular to the plane of the electron free motion. The calculation is carried out within the Hartree-Fock approximation. It is shown that the excited states correspond to the electron transitions in the vicinity of the Fermi momentum. The dispersion of the collective excitations is considered.     

Optical activity of CdS crystals in exciton spectral region

Natural optical activity of wurtzite CdS crystals has been studied for the first time both theoretically and experimentally in the spectral region of the B_n=1 exciton resonance. It has been shown that the observed optical activity is due to mixing the longitudinal Γ_5L exciton state with the transverse Γ₁ state by the energy terms linear in wave vector.     

Pool-Frenkel thermoelectric modulation of exciton photoluminescence in GaSe crystals

Effect of external field on the exciton photoluminescence of GaSe crystals has been investigated and it has been observed that the PL is quenched with the applied field. The changes observed in the PL spectra have been analyzed with impact exciton, Franz-Keldysh and Pool-Frenkel effects. From the analyses of the experimental data, it has been found that the intensity of direct free, indirect free and bound exciton peaks decreased exponentially with the square root of applied field as I∼exp-β√E. The energy positions of emission peaks were found to shift to longer wavelength with the applied field as ΔE∼β√E. From these findings, the Pool-Frenkel thermoelectric field effect is seen to be the dominant mechanism in the variation of exciton PL with the applied field even though the impact exciton and Franz-Keldysh effects contribute.     

Tuning of exciton states in a magnetic quantum ring

The exciton states in a CdTe quantum ring subjected to an external magnetic field containing a single magnetic impurity are investigated. We have used the multiband approximation which includes the heavy hole–light hole coupling effects. The electron–hole spin interactions and the s, p–d interactions between the electron, the hole and the magnetic impurity are also included. The exciton energy levels and optical transitions are evaluated using the exact diagonalization scheme. We show that due to the spin interactions it is possible to change the bright exciton state into the dark state and vice versa with the help of a magnetic field. We propose a new route to experimentally estimate the s, p–d spin interaction constants.     

Resonant exciton trapping at impurity states in silver bromide

Excitation spectra near the indirect exciton edge of AgBr at 1.8K are reported for several luminescence lines from weakly localized excitons. Excitation below the exciton absorption threshold reveals several excited bound exciton states the energetic positions of which are determined. For excitation above the threshold, strong energy dependent structure is observed. It is interpreted in terms of resonant trapping of free excitons in both ground and excited bound exciton states associated with emission of LO(Γ), long wavelength acoustic and intervalley TA(X) and LA(X) phonons as well as combinations and overtones of these. From measurements in doped crystals two bound exciton systems are found to be correlated with Cd²⁺ and Ca²⁺, respectively.     

有限晶体中的电子态

传统的固体物理中的电子态的理论实质上是无限晶体中的电子态的理论。但是任何真实晶体的尺寸都是有限的。文章简单介绍了作者最近提出的有限晶体中的电子态的理论的一些背景材料和主要的新结果。其中一些结果和固体物理界的传统看法很不一样。