Das Emissionsspektrum gebundener Excitonen in Zinkoxydkristallen in Abhängigkeit von Temperatur und uniaxialer Verspannung

The recombination radiation of bound excitons in ZnO-crystals (spectral range 3,38–3,35 eV at 4.2 °K) and the adjacent phonon-assisted spectrum have been studied at temperatures between 4.2 °K and 90 °K. The phonon-assisted spectrum changes its structure at about 30 °K. With increasing temperature the exciton lines shift toward smaller quantum energies (ΔE～T ²) and their half widthH increases (H～T ²). The line shift is explained by a band gap variation due to deformation potential coupling of the holes toTA phonons. The line width is explained by broadening of an infinitely sharp line under the influence of the mean square fluctuation of thermally generated crystal deformations. The influence of uniaxial stress on the spectral positions of the lines has been studied.     

Optical investigation of ErFeO3

The absorption spectrum of single crystals of ErFeO₃ has been investigated in the red and near infrared spectral region in the temperature range between 1.2 °K and 4.2 °K and at 20 °K and 77 °K. Between 77 °K and 4.2 °K a constant splitting of the absorption lines is observed. Below the Néel-temperature of the erbium sublattice at 4.5 °K the splitting of the absorption lines increases; the saturation value extrapolated to 0 °K of the splitting of the lowest crystal field level of the⁴ I _15/2 groundterm is (6.08±0.30) cm^?1. By measuring the Zeeman effect the groundstate magnetic moment is determined asμ=(6.6±0.2)μ _B. The measured temperature dependence of the splitting of the lowest crystalfield level of the⁴I_15/2 groundterm is compared with that calculated by a Monte Carlo method.     

Native defect changes in cds single crystal platelets induced by vacuum heat treatments at temperatures up to 600°C

Physical properties of selected CdS single crystal platelets as-grown and after vacuum heat treatments at temperatures up to 600°C have been studied using u.v. excited edge emission, mass spectrometry, electrical resistivity and electron paramagnetic resonance (EPR). It was found that sulfur leaves the crystal at temperatures as low as 100°C creating a depletion layer. The native defect changes were monitored by edge emission studies at 4.2°K in combination with etch treatments. The defect structure throughout the crystal is not only dependent upon the temperature and atmosphere of the treatments, but is also strongly dependent upon the cooling rate.     

Donor-acceptor pair recombination and phonon replica in GaSxSe1−x

The photoluminescence of a series of mixed crystals GaS_xSe_1?x, (0?x?1) has been determined as a function of excitation intensity, polarization, temperature between 4.2°K and 60°K, and delay between excitation pulse and detection. All the spectra at 4.2°K contain similar series of lines whose energy varies with the composition x like the indirect gap. The highest energy line in each spectrum is attributed to transitions between shallow and distant donors and acceptors forming pairs. The width of the line, its energy and its behaviour as a function of delay are interpreted. The other lines are interpreted as phonon replica of the highest energy line.     

The photophysics of the 2,6 dialkoxy anthracenes: Evidence for excited state side-chain conformational relaxation

Three 2,6 dialkoxyanthracenes with varying ether sidechain lengths (methyl, n-propyl and n-hexyl) are synthesized and characterized. When compared to unsubstituted anthracene, the spectroscopic properties of the alkoxy-anthracenes are significantly different. The oscillator strength appears to be evenly distributed between the L_a and L_b states, rather than concentrated in the low-energy L_a state, and the transition dipole moments are rotated by ～30° in the molecular frame. More importantly, all three derivatives undergo an intramolecular conformational change in the excited state that gives rise to a reshaping of the fluorescence spectrum on the nanosecond timescale. This process has an activation energy of 8±1 kJ/mol. By ～150 K the fluorescence spectrum reflects only the high energy conformer, whose emission lineshape is similar to that of the unsubstituted anthracene. The temperature-dependent changes in the monomeric fluorescence spectra of these compounds will have to be taken into account in future studies of their solid-state spectroscopy.     

Pressure broadening and shift of argon emission lines

Resonance and van der Waals broadening coefficients, as well as width to shift ratios, have been obtained by scanning argon emission lines produced in a high-pressure (～ 1 atm) glow discharge at a temperature of 1130°K. The resonance broadening coefficients yield absolute oscillator strengths of 0·283 ± 0·024 and 0·076 ± 0·008 for the two resonance lines at 1048 Å and 1067 Å. These oscillator strengths are in good agreement with previous measurements. The importance of higher-order terms in the van der Waals potential expansion is demonstrated by the measured coefficients.     

A study of the excited electronic states of 9-fluorenone

Some spectroscopic properties of the low-energy electronic states of 9-fluorenone have been examined. The spectra in paraffin matrices at 4.2°K show detailed vibrational spectra. Two fluorescence spectra are observed; a diffuse emission arises from 9-fluorenone crystals in the paraffin matrix, and a sharp emission is characteristic of the molecule. The sharp fluorescence is analyzed in terms of known a₁ vibrational fundamentals. The sharp absorption is a near mirror-image to the fluorescence, so Herzberg-Teller vibrations are not prominent. The polarization in the crystal spectrum allows this low-energy transition near 23 000 cm^?1 to be assigned ${}^1\text{B}{}_2\text{←}{}^1\text{A}{}_1$. Because there is no vibronic perturbation in fluorescence, and certainly no out-of-plane modes, a $\text{π}{}^1\text{← n}$ transition seen at about 26 000 cm^?1 is tentatively assigned ${}^1\text{B}{}_1\text{←}{}^1\text{A}{}_1$. Another sharp absorption system is seen at 31 000 cm^?1 in the paraffin matrices at 4.2°K (linewidth 6 cm^?1) but no fluorescence was detected. The polarized crystal spectrum indicated the assignment of this system and another very strong system at 40 000 cm^?1 to be ${}^1\text{B}{}_2\text{←}{}^1\text{A}{}_1$, while other systems at about 34 000 cm^?1 and 44 000 cm^?1 are ${}^1\text{A}{}_1\text{←}{}^1\text{A}{}_1$.The phosphorescence spectrum of pyrene-d₁₀ held in a single crystal of 9-fluorenone at 4.2°K has been recorded. No delayed fluorescence from the host crystal is observed at 4.2°K but is intense at 77°K. The energy difference between host and guest triplet levels is estimated to be about 900 cm^?1 allowing the lowest triplet state of 9-fluorenone to be placed at 17 800 cm^?1.     

Band-edge photoluminescence of PbI2

Photoluminescence spectra of the hexagonal layered semiconductor PbI₂ have been measured at 4.2°K on various single crystalline specimens. They are interpreted in relation with the anisotropic polariton model. The emitted radiation is assumed to originate from free and bound exciton recombination, with and without phonon emission. A peak polarized E ∥ c about 4 meV above the excitonic line is related to the extraordinary polariton mode.     

Untersuchung von Halbleiter-Sperrschichtzählern bis 4,2 °K

The behaviour of Semiconductor Surface Barrier Detectors has been studied in the temperature range from 300 °K to 4.2 °K. Measurements of the resolution forα- andβ-particles are given. The mean energy? required for producing an electron-hole-pair in Si is found to be 3.54±0.10 eV at 279 °K, and 3.72±0,11 eV at 78 °K and 4.2 °K respectively. Measured rise time of detector impulses is compared with theoretical values.     

Die magnetische Struktur und das magnetische Verhalten von ErFe2: Eine Neutronenbeugungsuntersuchung

The magnetic structure of ErFe₂ has been determined from neutron diffraction measurements on a powderd sample in the temperature region between 4.2°K and 700°K. ErFe₂ exhibits ferrimagnetic ordering below 600°K with the Er- and Fe-ions being antiparallel oriented. The separate sublattice magnetisations of the Er- and Fe-sublattices have been determined. The saturation moments at 4.2°K have been found to beμ(Fe)=1.97μ _B andμ(Er)=8.47μ _B. The coupling between the two sublattices is by an antiparallel spin-spin interaction of the Er and Fe ions. The temperature dependence of the sublattice magnetization of the Er-ions is explained by a separation into two parts: an induced magnetization at high temperatures and a spontaneous magnetization at lower temperatures.     

Die Szintillations-Anisotropie von Anthrazen bei tiefen Temperaturen. II

Continuing previous work the scintillation anisotropy of anthracene bombarded with α particles has been measured as a function of temperature between 1.9 and about 80° K. With all crystal specimens investigated a pronounced minimum has been found between 20 and 30° K. The anisotropy is constant below about 9° K, whereas the light yield still increases with decreasing temperature down to 1.9° K. The light yield has been found to be a nonlinear function of the temperature.     

Untersuchungen über Elektronenhaftstellen in ZnS-Phosphoren

Glow curves of different ZnS phosphors were measured and analysed in the temperature region from 4.2 °K to 500 °K. It was attempted to clarify the origin of the glowpeaks observed above 77.4 °K and to attribute them to structural defects in the ZnS lattice. In glow curves started at 4.2 °K a strong dependence of the trap distribution on the method of preparation of phosphors was observed. In particular a ZnS(Cu) phosphor containing oxygen shows a broad continuum of shallow traps between 4.2 °K and 150 °K. In a ZnS(Cu, Ga) phosphor free of oxygen the trap density of the continuum of shallow traps is very small. Moreover this phosphor shows a sharp glowpeak at about 60 °K. The afterglow at the excitation temperatures of 4.2 °K and 77.4 °K was also studied. It is concluded that the rapid amptying of shallow and, to a lesser extent, of deep traps should be due to a tunneling process leading to luminous recombination of the trapped electrons with the activator levels.     

Hyperfine fields in erbium metal

The hyperfine Splitting of the 80.6-keVγ transition in Er¹⁶⁶ has been measured in erbium metal between 4.2 °K and 40 °K using the Mössbauer effect. There is evidence for a unique magnetic field and electric fieldgradient at all nuclei in erbium metal. The magnetic field decreases from (7.55±0.20)·10⁶ Oe at 4.2 °K to (6.10±0.40)·10⁶ Oe at 40 °K. Extrapolation to 0 °K yieldsH (0 °K)=(7.60±0.20)·10⁶ Oe. The quadrupole interaction energy for the 80.6-keV state iseQ V _Z′Z′/4=(0.95±0.20)·10^?6 eV at 4.2 °K. These results are discussed and compared with other measurements.     

Phonons in surface photovoltage of GaAs

Oscillatory surface photovoltage is reported in GaAs at 4.2°K, characterized by two series of minima. Dominating series is attributed to the capture of photoexcited electrons by surface states with emission of phonons. Second, weak series coincides with oscillations in photoconductivity.     

Spectroscopic and nonlinear characteristics of anthracene-containing silicon-organic polymers in solution

Spectroscopic characteristics, i.e. absorption, fluorescence, fluorescence excitation spectra, fluorescence decay time, fluorescence polarization degree of novel silicon-containing organic polymers including main chain anthracene groups were investigated. Three kinds of emission spectra were revealed and assigned to polaron–exciton, anthracene and anthracene dimer. The measured fluorescence polarization spectra gave evidence of directed excitation energy migration along the disordered polymeric chain. Strong quenching of anthracene fluorescence during the polaron–exciton lifetime was interpreted as a result of the interaction between two excitations that causes anthracene anion-radical formation. The third-order nonlinear susceptibility of the polymers in solution measured by the Z-scan technique at 1054 nm is 190×10⁻¹⁴ cm² W⁻¹.     

Spectroscope studies of YAlO_3: (Lu~(3 )Nd~(3 ))

An increase of the Nd~(3 ) doping concentration and an enhancement of the laser induced fluorescence in the single crystal of YAlO_3: (Lu~(3 ), Nd~(3 )) have been observed and are attributed to the size compensation effect. The low temperature (4.2K) fluorescence spectra indicate that the structure of the sharp lines arising from the radiative transitions of Nd~(3 ) ions is not changed by the existence of Lu~(3 ) ions, but the emission lines are inhomogeneously more broadened. Besides, it has been observed that the fluorescence due to the radiative transition from the Stark's sublevel R_2 of the metastable level ~4F_(3/2) to the sublevels Y_1 of the manifold ~4I_(11/2) decreases with decreasing temperature and the fluorescence arising from the transitions R_1→Y_1 is relatively increased. The experimental fact is qualitatively interpreted.     

Optical investigation of HoFeO3

The absorption spectra of HoFeO₃ were investigated in the near infrared spectral region at temperatures of 1.2, 4.2, 20 and 77 °K respectively. At every temperatureT≦20 °K all the absorption lines show the same splitting which is attributed to the groundstate; this splitting is (7.2±0.5) cm^?1 at 20 °K and decreases to (4.9±0.8) cm^?1 extrapolated to 0 °K. From these splittings the holmium-iron and the holmiumholmium interactions can be deduced. Measurements with an external magnetic field yield a magnetic moment ofμ=(7.6±0.7)μ _B per holmium ion. The moments are directed at angles of ?28° and ?152° with respect to theb-axis.     

Field electron emission from an aluminum oxide covered tungsten {112} plane at temperatures up to 1700°K

Field electron emission from an aluminum oxide mono-layer, face specifically adsorbed on a tungsten {112} plane, was measured between 1670°K and room temperature and at local field strength ranging from 17 MV/cm to 35 MV/cm. In a Fowler-Nordheim plot straight lines were obtained up to about 820°K. The work function at 650°K was determined to be 3.2 eV. The experimental values were compared with those determined by Christov's unified theory of field and thermionic emission. Deviations are attributed to the change in the shape of the potential barrier, which is caused by the aluminum oxide adsorption.     

Temperature dependence of the exciton and magnon-phonon sidebands in RbMnF3 in the 5500Å region

Absorption spectra of RbMnF₃ in the 5500Å region were investigated in the temperature interval 4.2–77°K. Similar temperature dependence was observed for the exciton-magnon and the magnon-phonon sidebands. The position of the exciton lines does not vary as the temperature is increased.     

Photo-induced magnetophonon resonance in n-InSb at 4.2°K

Spin-flip and two-phonon as well as normal magnetophonon resonance are observed in CO₂-laser-irradiated photoconductivity in n-InSb at 4.2°K. The effects are attributed to the resonant cooling of photo-excited electrons due to optical phonon emission.