J. P. Von der Weid

Intensity quenching of the F3+ colour centre emission in lithium fluoride

In order to evaluate the possibility of tunable F2/F3+ CW colour centre laser operation in LiF crystals at room temperature, the authors performed an investigation of the emission characteristics of gamma -irradiated LiF crystals under CW argon ion laser excitation at room temperature. They measured a strong quenching of the F3+ emission intensity that was dependent on the pumping laser power. The characteristic decay times of the F3+ luminescence bands were also investigated with different pumping powers and a model for the recombination kinetics is discussed.

Self-trapped exciton in caesium iodide. I. Theory

The electronic structure of the self-trapped exciton (STE) in caesium iodide is investigated by a semi-empirical molecular-orbital method. The STE states are constructed by considering the relaxed electronic states of a cluster composed of an anionic diatomic I2- molecule surrounded by its nearest-neighbour Cs+ ions. For the electronic states, the 6s and 5d orbitals of the Cs+ ions are taken into account. Two types of STE states are obtained; the lower level manifold of each one is composed of a split triplet state and a singlet state. Both give rise to pi -emissions with a rather long radiative lifetime. The general expressions for the magnetic circular polarisation, the magnetic linear polarisation and the lifetime of the emissions are derived and discussed.

Line-shape of the vibrational bands of the CN- UV emission spectra in alkali halide-cyanide mixed crystals

Abstract Measurements of the line-width of the vi b rational bands associated to the a3Π→X1Σ+ electronic transition of the CN ion in alkali halide-cyanide mixed crystals at 2K–250K have been performed. It is obtained that the a3Π→X1Σ+ electronic transition is coupled to a dominant mode of 110 cm-1 with strength 1.2 in KCN0.01Cl0.99. From the line-shape of the vibrational bands in all crystals at 4.2K it is suggested that this mode can be ident i fied with a breathing mode of the neighbouring alkali ions.

Self-trapped exciton in caesium iodide. III. Polarisation of the emissions

For pt.II see ibid., vol.13, no.6, p.993 (1980). The radiative recombination of (100) oriented self-trapped holes and conduction electrons has been measured in CsI:Na between 1.4 and 50K. At the lowest temperatures, both intrinsic emissions (290 nm and 338 nm) exhibit a partial plane polarisation perpendicular to the (100) axis of the parent VK centres. The 338 nm emission polarisation remains independent of temperature but the 290 nm emission polarisation decreases with temperature and eventually reverses its sign near 11K. Calculations show that both emissions come from a split triplet state located slightly below a singlet state which has an allowed transition to the ground state for the 290 nm emission but is forbidden for the 338 nm emission. This model explains reasonably well all the previously measured properties of self-trapped exciton emissions in CsI such as intensities, polarisations and decay times.

Studies of the tunnelling recombination between VK-Na° centre pairs in CsI:Na crystals

Abstract The tunnelling recombination of trapped holes (VK) and electrons (Na°) pairs in CsI:Na crystals is studied by optical detection of ESR in the 420 nm emission. A model is developed and it explains the optically detected ESR spectra. The tunnelling probabilities are determined from the transient ODESR behavior.

Self-trapped excitons in caesium iodide. II. Magneto-optical properties

For pt.I see ibid., vol.13, no.6, p.983 (1980). The magnetic circular and linear polarisation of the self-trapped exciton (STE) luminescence of pure CsI and CsBr are studied experimentally for 1.3<T<50K and 0<B<6T. By comparing the 4.2K data with numerical calculations based on the model proposed in the previous paper, the authors have determined the orbital and spin symmetries and the best set of parameters characterising the static and kinetic properties of the lowest triplet STE states. The transient behaviour of the luminescence observed under microwave pulse (ODEPR) is also studied and good agreement with the calculation is obtained.

Molecular excitons in alkali cyanides

Abstract The emission bands of X irradiated Alkali Cyanides were measured at 4.2 K. This emission is associated to the radiative recombination of molecular (Frenkel type) excitons, for the first time observed in these ionic molecular crystals. The nature of the emitting state is discussed and identified as a long lived triplet state.

Magneto-optical effects on the emission of self-trapped exciton perturbed by Na impurity in CsI:Na crystal

The tunnelling recombination of trapped holes (VK) and electron (Na0)pairs in CsI:Na crystal was studied by optical detection of ESR and magnetic CD in the 420 nm emission. A model explains the data and allows determine of the tunnelling probabilities.

The antiferroelectric phase in KCN

Studies the 83K phase transition in KCN based on a simple model Hamiltonian, in which the electric interactions between the CN- ions are of dipole-dipole nature, and the small elastic correction is introduced phenomenologically. The order parameter (sublattice polarization) of the transition as a function both of temperature and the electric dipole moment of CN- ion is obtained. The available experimental value for the electric dipole moment of CN- in KCl (p=0.07 eAA) yields a transition temperature Tc=34.1K. As the value of p is not reliable for the KCN crystals the value that fits the experimental transition temperature (Tc2=83K) was used in order to obtain the order parameter as a function of temperature, as well as the orientational energy levels both for the disordered phase and for 0K. The local electric field is obtained as a function of the temperature.

Photoluminescence measurements of the 1.55 eV band of Ge doped AℓxGa1−xAs

Abstract The photoluminescence of the 1.55 eV band of Ge doped Al x Ga 1−x As, with x=0.30–0.33, grown by liquid phase epitaxy is presented. The broad shape was found to be due to a lattice relaxation upon optical transitions. Resonant modes with ℏω q = 35±2 meV and ℏω q = 45±2 meV are found for the optical band, yielding a zero phonon transition energy = 1.73±0.02 eV and a Franck-Condon shift = 0.17–0.20 eV for the optical center. The activation energy of thermal quenching yields an associated donnor binding energy of 0.17±0.04 eV. Possible mechanisms for the radiative transitions are discussed.