The Compton profile of Bismuth excited below the K absorption edge

A cylindrically symmetric Compton profile spectrometer stimulates resonant Raman scattering at the L₂, L₃, M and N edges in a Bi target when using a ¹⁰⁹Cd γ-ray source with energy 2.50 keV below the K-shell binding energy of Bi. Similarly, an ²⁴¹Am source (E_γ = 59.536 keV) excites a resonance response at the L₃ and M edges of Yb (B.E. = 61.332 keV) but not in Lu (B.E. = 63.314 keV).     

Determination of the chiral structure of [Formula: see text] using anomalous x-ray scattering near the Cs K absorption edge

A structural study of [Formula: see text], grown from aqueous solution, was performed using anomalous x-ray scattering near the Cs K absorption edge to determine an absolute configuration of constituent atoms. The sense of the helical structure of the [Formula: see text] chain was found to be predominantly right-handed through a comparison of observed Bragg Bijvoet ratios with calculated ones. Assuming that [Formula: see text] consists of the two domains (i.e. right- and left-handed helices), we estimate that the volume fraction for the right-handed helix is [Formula: see text].     

Emission near the absorption edge in a degenerate GaAs crystal

The stimulated and spontaneous emission functions have been calculated for the band-to-tail and tail-to-tail transitions respectively in n-type photolumineseence, and p-type photoluminescence and electrolu-minescence. The peak in the photon energy as a function of doping for degenerate GaAs at 77 K in the present theory is in good agreement with the experiments of Nathan et al.[1], and of Dapkus et al.[2]. It is also compared to the theory of Adams [3]. It is suggested that the maximum value of the stimulated emission in the n-type photolumineseence is stronger than in p-type photolumineseence for very dense impurities.     

Spontaneous emission in the near field of two-dimensional photonic crystals

We show theoretically that photonic crystal membranes cause large variations in the spontaneous emission rate of dipole emitters, not only inside but also in the near field above the membranes. Our three-dimensional finite-difference time-domain calculations reveal an inhibition of more than five times and an enhancement of more than ten times for the spontaneous emission rate of emitters with select dipole orientations and frequencies. Furthermore, we demonstrate theoretically the potential of a nanoscopic emitter attached to the end of a glass fiber tip as a local probe for mapping the large spatial variations of the photonic crystal local radiative density of states. This arrangement is promising for on-command modification of the coupling between an emitter and the photonic crystal in quantum optical experiments.     

Effect of copper impurity on the edge emission of ZnTe crystals

The results are given of studies on the edge emission of undoped and copperdoped ZnTe crystals in the temprature range 4.2–300K. The copper impurity has been found to increase the intensity of the principal edge emission band substantially. Analysis of the structure of this emission band indicated that a temprature below 160K the main role is played by the emission from excitons that are most probably bound in Cu_Zn and Li_Zn centers while at higher temperatures it is played by emission during the transition of free electrons to these centers. An appreciable role is played over the entire range of temperatures by emission from free electrons with an energy of 13 MeV. The luminescence quenching energy below 160 K has a value of 7 MeV (the binding energy of a bound exciton) and at higher temperatures it is 78 MeV.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 68–73, August, 1987.     

锗在吸收边附近的压力—折射率系数

目前半导体锗在吸收边附近(1550 nm)的压力-折射率系数在实验和理论上并未研究清楚.本文通过测量在不同压力下镀在光纤端面的高结晶度锗薄膜的反射率,来计算得到锗在吸收边附近的压力-折射率系数.本文的实验结果显示,锗在吸收边附近出现反常色散现象,即折射率随能量变化呈正相关,并且其压力-折射率系数出现反常,为正值,这是由于多晶结构中的激子吸收所引起.通过引入描述激子色散的临界点模型,得到锗在吸收边附近的反常色散范围和压力-折射率系数呈正值的范围.本文的结果将有助于基于锗薄膜的通信C波段光学器件的研究.     

Determination of optical constants near the fundamental absorption edge in semiconductors

Abstract

The photoconductivity of semiconductors was considered under the following general conditions: the surface recombination velocity was not equal to zero, the space charge near the surface is taken into account, the value of absorption coefficient was not limited. Solving both the continuity and Poisson equations for an illuminated thick specimen, we obtained a solution which suggests a new combined measurement in order to determine various optical constants. The absorption coefficient and the diffusion length of electrons in CZ grown n-type Si (N _a = 10¹²-10¹³ cm^?3) were measured at room temperature, using the new technique.     

X-ray absorption near edge structure in some Mn compounds

The shapes of the manganese K-absorption edge in the compounds MnSO₄H₂O, MnF₂, MnCO₃, MnO and MnS have been studied using a Cauchois type X-ray spectrograph. Three peaks A, B and C observed in the main edges have been assigned the electronic transitions ls → 3d, ls → 4s and ls → 4p respectively. The transitions have been explained on the basis of the Z + 1 analogy. It has also been shown that the 3d → 4s and 4s → 4p orbital separations depend on the partial charge of an ion in its compound.     

Optical absorption edge in doped and undoped ZnSe crystals

The optical absorption near the fundamental edge has been investigated for different types of doped and undoped cubic ZnSe crystals. Crystals grown from the vapour phase, with and without doping, and crystals grown by the chemical transport technique are compared.Results of absorption coefficient measurements made from 77 to 313 LK are discussed and it is shown that for undoped samples, the absorption tail is dominated by electrons-phonons interactions and Urbach's rule is nearly verified, whereas for doped samples the influence of defects is preponderant.     

Direct phonon-assisted transitions near the fundamental absorption edge in CuInSe2

Summary The absorption spectrum near the fundamental absorption edge of an-type CuInSe₂ single crystal grown by the Bridgman method has been studied at 100 and 300 K. An absorption edge corresponding to the direct-band-gap transition is observed at about 1 eV. An additional absorption is observed at lower energies ((0.93÷0.95) eV) and is attributed to direct transitions assisted by long-wave-length optical phonons.

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Riassunto Lo spettro di assorbimento vicino al bordo di assorbimento fondamentale di un cristallo singolo di tipon di CuInSe₂, cresciuto col metodo di Bridgman, è stato studiato a 100 e 300 K. Si osserva un bordo di assorbimento in corrispondenza della transizione del gap di banda diretta a circa 1 eV. Un ulteriore assorbimento è osservato a energie inferiori ((0.93÷0.95) eV) ed è attribuito a transizioni dirette alla presenza di fononi ottici a grande lunghezza d'onda.

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Резюме При 100 К и 300 К исследуется спектр поглощения вблизи основного края поглощения монокристалла CuInSe₂ n-типа, выращенного по методу Ъридгмана, Вблизи 1 эВ обнаружен край поглощения, соответствующий прямому переходу. Дополнительное поглощение наблюдается при меньших энергиях ((0.93÷0.95) эВ) и приписывается прямым переходом, связанным с длинноволновыми оптическими фононами.

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Paper presented at the ?V International Conference on Ternary and Multinary Compounds?, held in Cagliari, September 14–16, 1982.     

Origin of the generally defined absorption edge of non-stoichiometric lithium niobate crystals

The ultraviolet absorption edges of LiNbO₃ crystals with different Li₂O contents and MgO doping concentrations were investigated. The generally defined absorption edges at absorption coefficient α=15 or 20 cm⁻¹ of all these crystals fit the Urbach rule perfectly. The origin of this absorption edges in non-stoichiometric LiNbO₃ crystals is attributed to the presence of Li vacancies.     

Shift of the optical absorption edge in C60 clathrate single crystals

We measured the transmission between 1.6–2.2 eV of pure C₆₀ and three clathrate single crystals containing different aliphatic guest molecules. The absorption edge in the clathrates shows a blue shift of 0.1 eV irrespective of the guest molecule, indicating that C₆₀ is electronically isolated in these compounds.     

Mechanism of light emission excited by Joule heating in ZnO crystals

Emission of light was observed in ZnO single crystals under Joule heating at high temperatures. This thermally induced emission (thermal emission) appeared at temperatures higher than 600 °C and exhibited itself as a band peaked at 500-520 nm whose intensity increased and maximum shifted towards longer wavelengths as temperature increased. Photoluminescence, thermal emission (TE) and transmission spectra were measured at different temperatures in the range of 20-1100 °C. It was concluded that the TE resulted from radiative recombination of thermally generated equilibrium carriers through some local centers.     

X-ray absorption near edge structure (XANES) for KCl

The cluster calculations of K and L_2,3 edge XANES of K and Cl in KCl within the multiple scattering theory formalism using nonlocal HF potentials with abd without consideration of core hole field were performed. For K spectra the influence of the core hole potential is rather weak and the results are similar to those obtained with Xα potentials. For L_2,3 spectra, particularly for that of K⁺, core hole field leads to a radical redistribution of oscillator strength caused mainly by the spatial rearrangement of d like states. Calculated XANES curves show good over-all agreement with the experimental spectra (the L_2,3 absorption of K⁺ in KCl was measured using the synchrotron radiation of the USSR Academy of Sciences storage ring VEPP-2M in Novosibirsk).