Abstract It is shown that in the process of the exciton two‐center self‐trapping the resonance bonds generating a free exciton state are redistributed so that the total resonance contribution to the energy increases. 50% of the electronic excitation is concentrated inside a two‐center quasimolecule close to the self‐trapping threshold. The rest is distributed in neighbouring atoms, resulting in a generation of weaker extrinsic bonds along with the intermolecular resonance one. The attendant deformation of the molecular term produces a decrease in the self‐trapping barrier and an appreciable increase in its transparency.
The influence of disorder and localization on optical dephasing of excitons in the semiconductor mixed crystals ${\mathrm{CdS}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$
Sex and
Alx${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As has been investigated by means of time-resolved four-wave mixing and photon echo experiments. A dephasing time of several hundreds of picoseconds is found for resonantly excited localized excitons in ${\mathrm{CdS}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$
Sex while the dephasing time in
Alx${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As amounts to only a few picoseconds. In ${\mathrm{CdS}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$
Sex dephasing results mainly from hopping processes, i.e., exciton-phonon interaction. The contribution of disorder is negligible in terms of phase relaxation in ${\mathrm{CdS}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$
Sex. In contrast, in
Alx${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As elastic disorder scattering yields an essential contribution to the dephasing rate. We present a theoretical model, which treats dephasing of optical excitations in a disordered semiconductor, including the influence of disorder as well as exciton-phonon interaction. On the base of this model, the experimentally observed differences in the dephasing behavior of excitons in ${\mathrm{CdS}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$
Sex and
Alx${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As are related to the microscopic structure of the disorder potential and the mechanism of exciton localization.
Abstract The luminescence of Ne* centers in solid Ne is investigated. The samples are excited with synchrotron radiation in the excitonic range. By an appropriate selection of the excitation energy and by time correlated measurement of the emission spectra, overlapping luminescence bands are separated and assigned to Ne* centers in different electronic states (1P1, 3P1, 3P2) and different environments in the bulk and at the surface of the solid. Lifetime measurements and excitation spectra of spectrally selected luminescence bands support the assignments, Electronic relaxation from n ≧ 2 free excitons to the n = 1 state is found to be less effective than exciton trapping into Ne* centers. Resonance fluorescence of free Ne* atoms indicates a close connection between exciton trapping and desorption.
This paper presents the first results on a correlation between the luminescence of excitons self-trapped at the surface of solid argon and desorption. Recently, Jennison and Emin showed that localisation of electronic excitation at the surface of the crystal is an essential condition, and that especially self-trapping of excitons can lead to desorption of neutral atoms1. In the case of solid argon, self-trapping of excitons at the surface has been proved to exist2. It leads to characteristic luninescence bands which were also measured with electron excitation techniques3. Two of them exactly coincide with the 3P1 and 1P1 emission of isolated argon atoms and were not understood up to now. In the course of this work it turned out that they are emitted by desorbing neutral atoms.
Diamond samples of various origins containing NV centers were studied using optically detected magnetic resonance (ODMR) and photoluminescence (PL) methods. It is shown that various impurities and deformation-induced defects can significantly influence the crystal environment and properties of NV centers. This influence is reflected in such characteristics as the distribution of NV centers throughout the crystal volume, their charge state (the ratio of neutral to negatively charged NV centers), mechanical stresses and strains in their crystalline environment, interaction with a nitrogen donor, which affects their coherent properties, and their orientation within the diamond lattice.