An extension of moments calculations for optical absorption bands

In this paper the developments of Henry, Schnatterly and Slichter are extended by using a more general formulation of the principle of spectroscopic stability (for a site of symmetry O_h). The linear effects for any type of stress are obtained next. The expressions indicate how one can test the theory experimentally in a critical manner. This has not been done. The theory shows that one cannot expect to get more information on KCl from stress experiments than is now available. Finally, it is indicated how one may define the irreducible stresses and strains in an unambiguous manner by using the differential strain-energy function.     

A method for resolving individual bands in absorption spectra

A new algorithm for resolving individual bands in absorption spectra is developed. It is assumed that the spectrum consists of absorption bands with halfwidths determined by different kinds of interactions—specific and nonspecific. The method has good accuracy in the case of the strong overlap of bands (the ratio of the distance between maxima of the peaks to the halfwidth is less than 0.2) and a high computational efficiency. Arkhangelsk State Technical University, 17, Naberezhnaya Severnoi Dviny, Arkhangelsk, 163007, Russia. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 3, pp. 400–404, May–June, 1997.     

Near-infrared emission bands of Er-doped YAP and LSO crystals

The ground state absorption (GSA), photoluminescence (PL) and photoluminescence excitation (PLE) spectra for Er(1.0 at%):YAP and Er(0.5 at%):LSO were measured at room temperature. Based on the GSA spectra, the radiative transition rates and luminescence branch ratios of erbium ions were determined by the Judd-Ofelt (J-O) method. In the range of 1400-1700 nm Er(1.0 at%):YAP has intense absorption at 1509 nm (0.96×10⁻²⁰ cm²), which is almost two times larger than the peak absorption of Er(0.5 at%):LSO. From the PL and PLE spectra, four intense emission bands around 850 nm (⁴S_3/2→⁴I_13/2), 980 nm (⁴I_11/2→⁴I_15/2), 1230 nm (⁴S_3/2→⁴I_11/2) and 1520 nm (⁴I_13/2→⁴I_15/2) were observed. The stimulated emission cross-sections of the four bands were calculated by the Fuchtbauer-Ladenberg (F-L) equation. The results suggest that Er(1.0 at%):YAP has potential to realize laser oscillation at 858 nm because of the relatively large simulated emission cross-section (1.76×10⁻²⁰ cm²). The temperature dependences of the PL spectra for the two crystals were also investigated in the range of 290-12 K. The ∼1520 nm emission presents continuous increase with temperature, while the emissions around 850, 1230 and 980 nm firstly increase with temperature, then reach their own largest values at the transition temperatures (about 100 K), and finally decrease with temperature. These results were well interpreted by the temperature dependence of multi-phonon process.     

Bound-exciton emission bands in ZnSe single crystals and mixed plasmon-phonon modes

The paper reports a cathodoluminescence study of ZnSe single crystals annealed in a Bi melt at 1200 K for 120 h. It is found that the distance between the phonon structure satellites in the bound-exciton series I ₁ ^s -nLO and I ₁ ^d -nLO and the relative satellite intensity are different in samples with different conduction electron concentrations. It is shown that this difference is due to the mixing of the plasmon and phonon modes. The shape of the bound-exciton emission spectrum in ZnSe crystals in the 450–470 nm region is calculated, and a satisfactory agreement with the experiment calculations is obtained.     

Energy bands and soft x-ray absorption and emission in cuprous chloride

The observed X-ray (Cl^-L₂₃) absorption and emission spectra are compared with spectral intensities calculated with the help of energy bands in CuCl. The interpretation of the absorption spectrum puts in evidence the formation of the core excitons in addition to the usual band to band transitions. In the case of emission spectrum, the effect of correlation on the core levels seems to be an important contribution.     

Thermally induced absorption bands with vibronic structure in LiF crystals

Lithium fluoride single crystals obtained from different sources show a series of narrow absorption bands in the ultraviolet when annealed above 250°C in certain vacuum conditions. The ambient pressure and temperature at which annealing rwes place affects the defect production rate, increasing temperature and decreasing total pressure giving larger absorption strengths.

The structure of the absorption was similar for all the LiF crystals studied and consists of narrow bands some of which have half widths of less than 0.05 eV at 77°K. All but one of the bands observed, fit into a periodic structure that falls into two groups, suggesting an interpretation based on the interaction between electronic transitions within a defect centre, lattice vibrations and local mode vibrations of the centre.

It is thought that an impurity ion is responsible for this optical absorption.     

On temperature dependence of impurity absorption and luminescence bands in dielectric crystals

In terms of the adiabatic approach to determining the states of an impurity dielectric crystal, the temperature dependence of the first moment (maximum) of the optical band that arises upon vibronic transitions in a local center is studied. It is shown that the frequency of the band maximum and absolute temperature are linked with each other by a relation that is analogous to the Wien’s displacement law for equilibrium radiation. This result is consistent with experimental data obtained, in particular for F absorption and luminescence bands in alkali halide crystals.     

Metastable optical absorption of relaxed electronic excitations in BeO-Zn crystals

Spectra of metastable optical absorption and its relaxation kinetics have been studied in zinc-doped BeO crystals by time-resolved pulsed absorption spectroscopy. A comparison of the observed induced optical absorption of self-trapped excitons and small-radius excitons bound to the zinc impurity suggests that their hole components have similar structures and reveals distinctive features of “forbidden” optical transitions in the electronic components. Metastable optical absorption in Zn⁺ centers has been discovered. It is shown that the small-radius excitons bound to the zinc impurity form in the hole stages of thermally stimulated tunneling recombination processes involving Zn⁺ electronic centers. It has been found that the high recombination probability of the electronic and hole centers created in BeO-Zn crystals by an electron beam may be due to the high degree of their spatial correlation. Fiz. Tverd. Tela (St. Petersburg) 41, 601–605 (April 1999)     

Nonsingular Greens function method for crystals

By use of simple mathematical transformations, an integral equation for the Greens function method leads to a homogeneous equation which contains only the nonsingular part of the Greens function. No limitations are imposed on the crystal potential, so the solutions may be found for any unit cell. The method is applicable to any crystal structure. Using bcc iron as an example we present various calculations which make it possible to predict ahead of time which atomic levels will form bands and which will not.Altaisk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 120–123, July, 1995.     

Formation and bleaching of optically-induced absorption bands in CdS:Cd,Cu crystals

Our earlier work has revealed the occurrence of three photoinduced bands in cadmium sulphide crystals appropriately doped with both cadmium and copper: one in the visible and two in the near IR region of the optical absorption spectrum. This investigation is now extended by examining the growth kinetics, their dependences on the temperature and on the excitation-light intensity, as well as the kinetics of thermal bleaching of the induced absorption. The data obtained are discussed in terms of the formation of photochemical complexes which give rise to the visible band. The IR bands are attributed to centers which are also responsible for the IR quenching of the photoconductivity.     

A representation of the green's function for an infinite perfectly conducting wedge

Power Institute, Moscow. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 34, No. 1, pp. 92–96, January, 1991.