Exciton and intracenter radiative recombination in ZnMnTe and CdMnTe quantum wells with optically active manganese ions

The emission spectra of Zn_1?x Mn _x Te/Zn_0.6Mg_0.4Te and Cd_1?x Mn _x Te/Cd_0.5Mg_0.5Te quantum-well structures with different manganese concentrations and quantum-well widths are studied at excitation power densities ranging from 10⁵ to 10⁷ W cm^?2. Under strong optical pumping, intracenter luminescence of Mn²⁺ ions degrades as a result of the interaction of excited managanese ions with high-density excitons. This process is accompanied by a strong broadening of the emission band of quantum-well excitons due to the exciton-exciton interaction and saturation of the exciton ground state. Under pumping at a power density of 10⁵ W cm^?2, stimulated emission of quantum-well excitons arises in CdTe/Cd_0.5Mg_0.5Te. The luminescence kinetics of the quantum-well and barrier excitons is investigated with a high temporal resolution. The effect of the quantum-well width and the managanese concentration on the kinetics and band shape of the Mn²⁺ intracenter luminescence characterized by the contribution of the manganese interface ions is determined.     

Luminescence of CdMgTe with CdMnTe ultrathin nanolayers

The luminescence spectra of Cd_0.75Mn_0.25Te/Cd_{1 ? y} Mg _y Te superlattices with narrow-band-gap Cd_0.75Mn_0.25Te nanolayers having nominal thicknesses of 0.5, 1.5, and 3.0 monolayers (MLs) exhibit exciton emission bands of Cd_{1 ? y} Mg _y Te barriers and nanolayers, as well as intracenter luminescence of Mn²⁺ ions. For all samples, the luminescence spectra of Cd_0.75Mn_0.25Te nanolayers consist of two bands. From analyzing the dependences of the luminescence intensity on the nanolayer thickness and temperature, these bands are assigned to excitons localized at two-and zero-dimensional potentials. The intracenter emission of Mn²⁺ in 3.0-ML-thick Cd_0.75Mn_0.25Te clearly reveals excitation migration.     

Eine Modellvorstellung für das „Auroral Afterglow“ und das „Pink Afterglow“

Based on own measurements of the decay of light intensity and electron density, a simple model for the auroral afterglow and the pink afterglow is proposed. To a certain extent the model can account for the influence of pressure and tube diameter on the behaviour of the afterglow. The first decay of the electron density towards the first minimum seems not to be influenced by creation processes for electrons during 3 to 15 ms. Therefore, the first decay can be used to measure the neutralization processes of electrons and positive ions. At pressures lower than 3 torr the electrons are removed by ambipolar diffusion with a rate given byD _a ·p≈2100 torr·cm²·s^?1. From this value an electron temperature of about 7200 °K could be derived. Assuming that ambipolar diffusion will remain the dominant loss process the time dependence of the electron creation rate has been evaluated. At a pressure of 1,75 torr the maximum electron creation rate was 4·10¹² s^?1·cm^?3. At the same time the emission density rate of the first negative bands was found to be 2.7·10¹³ cm^?3 s^?1. During one decay process about five times more electrons are created than the initial number. To explain the proportionality found between the electron density and the emission of the first negative bands, it is assumed that the ions, presumable N ₂ ⁺ -ions, are repeatedly excited to theB ² Σ _u ⁺ -state by metastables.     

Concentration quenching and migration of excitations in a bulk Cd<Subscript>0.5</Subscript>Mn<Subscript>0.5</Subscript>Te crystal

The curves of intracenter luminescence decay for Mn²⁺ ions in the Cd_0.5Mn_0.5Te semiconductor solid solution, obtained in a low-temperature experiment, have been simulated by the Monte Carlo method. The features of the kinetics of the 2-eV band in the time interval where significant nonexponentiality of relaxation at different points of the emission band profile manifests itself, as well the integral kinetics and energy relaxation, have been considered. Migration of ion excitations and concentration quenching (which was previously disregarded) are considered to be the main mechanisms determining the kinetic curve formation. It was established that excitation by 2.34-eV photons leads to both selective (intracenter) and band excitation of Mn²⁺ ions. Comparison of the results of numerical simulation and experiment showed that the characteristic values of the migration and quenching rates (W _m and W _q , respectively) are close in magnitude and W _{q, m} ≈ 0.1/τ, where τ is the lifetime at the long-wavelength band wing with the exponential kinetics. The estimated quantum yield (0.56) indicates significant influence of the concentration quenching on the 2-eV luminescence quantum yield in Cd_{1 ? x} Mn _x Te and Zn_{1 ? x} Mn _x S crystals with a high concentration of Mn²⁺ ions.     

Deviation from the standard model in the decay <Emphasis Type="Italic">π</Emphasis><Superscript>+</Superscript> → <Emphasis Type="Italic">e</Emphasis><Superscript>+</Superscript><Emphasis Type="Italic">νγ</Emphasis>

A precise investigation of radiative pion decay (π⁺ → e⁺νγ) in a pion beam from the meson factory of the Paul Scherrer Institute (Switzerland) was performed by the PIBETA Collaboration with the aid of the PIBETA detector. This resulted in finding 41 601 events of radiative pion decay in three kinematical regions. The absolute values of the branching ratio for radiative pion decay were determined in each of these regions. To a precision approximately four times higher than that known previously, the ratio of the axial-vector to the vector form factor was found to be γ ≡ F _A /F _V = 0.443 (15), the latter being fixed at F _V = 0.0259. The number of events found in the kinematical region specified by photon energies of E_γ > 55.6 MeV, positron energies of E _e > 20.0 MeV, and angles of θ_{γ, e} > 40° between the momenta of the corresponding particles (B region) was 5233. In region B, the measured branching ratio for radiative pion decay, R_π→evγ(expt) = 11.6(3) × 10^?8, proved to be smaller by eight standard deviations than that which follows from the Standard Model, R_π→evγ (theor) = 14.34(1) × 10^?8.     

Spectroscopic study of double sodium-yttrium fluoride crystals doped with erbium Na<Subscript>0.4</Subscript>Y<Subscript>0.6</Subscript>F<Subscript>2.2</Subscript>:Er<Superscript>3+</Superscript>: I. Intensities of luminescence spectra and kinetics of luminescence

Na_0.4Y_0.6F_2.2:Er³⁺ (NYF:Er) crystals with an erbium concentration as high as 100 at. % (Na_0.4Er_0.6F_2.2) were grown by the Bridgman-Stockbarger method. The optical spectra were investigated at low (6 K) and room temperatures. It is shown that the absorption spectrum of NYF:Er crystals contains wide bands (790–801 and 965–980 nm) corresponding to the emission range of laser diodes. The peak absorption cross section σ_a for the band peaked at λ=970.4 nm is 0.15×10^?20 cm². On the basis of the analysis of the absorption and luminescence spectra at low (6 and 12 K) temperatures, the structure of the Stark splitting of erbium levels was determined as a structure of quasi-centers for which Stark components are inhomogeneously broadened. The oscillator strengths of the transitions from the ground state of erbium to excited multiplets were calculated from the absorption spectra measured at T=300 K, and the intensity parameters Ω_t were determined by the Judd-Ofelt method: Ω₂=1.65×10^?20 cm², Ω₄=0.56× 10^?20 cm², and Ω₆=1.01×10^?20 cm². These values of the intensity parameters were used to calculate the probabilities of radiative transitions and the branching ratios. The rates of multiphonon nonradiative transitions in NYF: Er were estimated. The luminescence decay kinetics for radiative levels of erbium ions upon their selective excitation by nanosecond laser pulses was studied. The intracenter lifetimes of radiative levels of erbium ions were determined from the luminescence kinetics upon selective ion excitation by low-intensity light in a sample with a low erbium concentration (0.5%). It is demonstrated that, with an increase in temperature from 6 to 300 K, luminescence from the ⁴ G _11/2, ² G(H)_9/2, and ⁴ F _9/2 levels is quenched as a result of multiphonon nonradiative transitions. Luminescence from the ⁴ I _9/2 level is quenched only insignificantly with increasing temperature, and no quenching of luminescence from the ⁴ I _11/2 and ⁴ I _13/2 levels is observed. The spectra of steady-state luminescence of NYF:Er(0.5–15%) crystals were investigated upon broadband excitation by UV and UV-visible lamp light and selective time-resolved laser excitation. It is shown that low-lying levels of erbium ions separated by an energy gap smaller than 2500 cm^?1 are populated via cascade mechanisms. On the basis of the results obtained, it is concluded that NYF:Er ³⁺ crystals are promising candidates for active media of tunable diode-pumped lasers.     

Untersuchung des „Auroral Afterglow“ und seiner Präparationsstadien

The intensity decay of negative bands of N ₂ ⁺ and of first positive bands of N₂ in the auroral afterglow has been measured. An explanation, seeming possible for the decay curves of the first ones is proposed. By this the intensity decay in the beginning of the afterglow at high ionic densities is determined by the recombination of N ₂ ⁺ with electrons. Later, the ambipolar diffusion of the charge carriers to the wall causes the time dependance. A value of the ambipolar diffusion coefficient in nitrogen, derived from the measurements is given by the relationD _a ·p ≈ 124cm² sec^?1 Torr. The temperature of the charge carriers is estimated to be 313? K. The intensity decay of the pos. group in the afterglow seems mainly to be due to the diffusion of metastable particles to the wall.     

Luminescence of yttrium–aluminum garnet crystals with Eu<Superscript>2+</Superscript> impurity ions

Luminescence of yttrium-aluminum garnet Y₃Al₅O₁₂: Eu,Si samples is studied. It is found that luminescence is associated with Eu²⁺ ions, but does not correspond to intracenter 4f ⁶5d ¹–4f ⁷ transitions. Measurements of the excited state lifetime point to the luminescence mechanism involving charge-transfer states.     

Radiative decays at LHCb

Precise measurements on rare radiative B decays are performed with the LHCb experiment at LHC. The LHCb results regarding the ratio of branching fractions for two radiative decays, B ⁰ → K ^*0 γ and B _s → ?γ, the direct CP asymmetry in B ⁰ → K ^*0 γ decay channel and the observation of the photon polarization in the B ^± → K ^±π^?π^± γ decay, are included. The first two measurements were performed in 1 fb^–1 of pp collisions data and the third one in 3 fb^–1 of data, respectively.     

Thermo- and photostimulated luminescence in LiF : Mg,Ti single crystals irradiated by ions and VUV light

A coordinated study of the relaxation of optical absorption induced by vacuum ultraviolet radiation, x-rays, and α-particles, as well as of photo- and thermostimulated luminescence (TSL) of LiF : Mg, Ti crystals (TLD-100) in the 295–750-K interval, has revealed that TSL regions characterized by activation energies E _a = 2.2–2.4 eV and anomalously high frequency factors p ₀ = 10²¹–10²² s^?1 alternate with regions where E _a = 1.5 eV and p ₀ = 10¹²–10¹⁴ s^?1. The relative intensities of the TSL peaks produced by UV illumination (10–17 eV) differ strongly under the conditions of selective photon-induced generation of anion excitons, free electrons and holes, or near-impurity electronic excitations. The latter are responsible for the high efficiency of tunneling radiative (involving titanium centers) or nonradiative (involving hydroxyl ions) recombination. The analysis of TSL peaks of LiF: Mg, Ti and LiF took into account two-step processes, namely, thermal dissociation of three-fluorine F ₃ ^? molecules and recombination of the products of their decay (V _K and V _F centers, H interstices).     

Mn<Superscript>2+</Superscript> 3<Emphasis Type="Italic">d</Emphasis> luminescence kinetics in Zn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript>x</Subscript>Se

Intracenter luminescence (IL) of Mn²⁺ in Zn_1?xMn_xSe (x=0.07, 0.02) was studied under pulsed excitation by the neodymium laser second harmonic. At 4 K, the IL saturation originates from the nonlinearity of the system only at the instant of excitation, the IL decay kinetics after the exciting pulse termination depending only weakly on the pumping level. At 77 K, the decay kinetics in Zn_0.93Mn_0.07Se depends substantially on the pumping level, because the migration of intracenter excitation over the manganese ions initiates up-conversion, which is a slow nonlinear process. As shown by IL decay measurements in Zn_0.98Mn_0.02Se (x=0.02), excitation migration over the Mn²⁺ ions is insignificant even at a high temperature and under strong optical pumping.     

Spins und γ-Multipolordnungen beim Zerfall Fe59→Co59

In the decay of Fe⁵⁹ the following quantities have been measured:γ-ray intensities, conversion coefficients andβ-(circularly polarizedγ) correlations. The conversion coefficients were found to be: for the 1.10 MeVγ-transition, α_tot.=(1.36±0.10)·10^?4 and for the 1.29 MeVγ-transition, α_tot.=(1.07±0.08)·10^?4. The asymmetry parameterA of the correlationβ(0.27 MeV)?γ(1.29 MeV) were measured to beA= ?0.17±0.10, and forβ(0.46 MeV)?γ(1.10 MeV),A=?0.13±0.04. From these data and publishedγ?γ angular correlation measurements the following spins of Co⁵⁹ levels could be deduced: 1.10 MeV,I=5/2^?; 1.29 MeV,I=5/2^?; and 1.43 MeV,I=3/2^?. The two strongβ-ray groups are pure Gamow-Teller transitions (ΔI=1). The multipolarities of all fiveγ-rays are given.     

Zerfallsschema und Matrixelemente für denβ-Übergang Ag110m→Cd110

Theβ decay Ag^110m→Cd¹¹⁰ has been investigated with a double lens spectrometer and aβ-γ circular-polarization correlation setup. The shape of the 6⁺→6⁺ β spectrum withE ₀=529 keV was found to be allowed, in disagreement with earlier work. The constantA of theβ-γ circular-polarization correlation was measured to beA=0·07±0·02. This implies a ratioX of Fermi to Gamow-Teller contribution to the decay ofX ₁=?0·02±0·03 orX ₂=?10·3 _?4·1 ^+2·3 . Qualitative shell model considerations favor the valueX ₁. Additional information is given for the disintegration schemes of Ag^110m and Ag¹¹⁰.     

Luminescence of Tl<Superscript>+</Superscript> ions in a KZnF<Subscript>3</Subscript> crystal

The luminescence spectra of a KZnF₃: Tl⁺ crystal are investigated in the energy range from 4.75 to 5.9 eV at temperatures of 10–300 K upon excitation into the A absorption band (5.7–6.3 eV). At T=300 K, the luminescence spectra exhibit an intense band with a maximum at 5.45 eV, which is attributed to single Tl⁺ ions substituted for K⁺ ions. The 5.723-eV intense narrow band observed at T<20 K is assigned to the ³Γ_1u-¹Γ_1g zero-phonon transition, which is weakly allowed by the hyperfine interaction. The luminescence decay is studied as a function of temperature. The main characteristics of the luminescence spectra are adequately described in terms of the semiclassical theory based on the Franck-Condon principle and the Jahn-Teller effect for an excited sp configuration of the Tl⁺ ion with the use of the parameters obtained earlier from analyzing the absorption spectra of the system under investigation.     

Exciton luminescence of YAlO<Subscript>3</Subscript> single crystals and single-crystal films

The paper reports on a study of exciton luminescence in single crystals (SCs) and single-crystal films (SCFs) of YAlO₃, which have substantially different concentrations of vacancy-type and substitutional defects, under excitation by synchrotron radiation near the fundamental absorption edge. The radiative annihilation of excitons in SCFs was shown to occur primarily at regular perovskite lattice sites and to be accompanied by luminescence in a band peaking at λ_max = 295 nm with τ = 5.2 ns. In contrast to SCFs, the radiative exciton decay in YAlO₃ SCs takes place predominantly near vacancy-type defects (F⁺ and F centers) and is accompanied by luminescence in the bands at λ_max = 350 nm (τ = 2.5 ns) and 440 nm (τ₁ = 1.9 ns, τ₂ = 30 ms). Photoexcitation in the 175-nm band of YAlO₃ SCs revealed photoconversion of the centers F → F⁺.     

Excitons in monoclinic zinc diphosphide: The <Emphasis Type="Italic">A</Emphasis>-exciton series and Fano effect

The A-exciton series in the absorption spectra of β-ZnP₂ monoclinic zinc diphosphide samples is investigated at different directions of the wave vector and different polarization states of radiation. It is shown that the oscillator strengths determined for the observed transitions are adequately described by the relationship F _n ∝n^?3 characteristic of S-type exciton states. The assumption is made that the A-exciton series is associated with the partially allowed dipole transitions to nS states of the orthoexciton with Γ ₂ ^? (x) symmetry at m _s =0. These states are mixed, to a first approximation, with nS states of the Γ ₂ ^? (z) singlet exciton due to the spin-orbit 2 interaction and are split off by the long-range (nonanalytical) part of the exchange interaction. The Fano antiresonances arise in the absorption spectra at resonances of the A-exciton series when the radiation vector E (or the induction vector D) has a component along the crystallographic axis c. These antiresonances are induced by the configurational interaction of discrete exciton states of the A series with the continuum of the exciton-phonon spectrum due to indirect transitions to the 1S band of the singlet exciton with phonon emission.     

Possible violation of the δS=δQ selection rule in leptonic decays of σ+-hyperons

We have found a possible example of the rare decayσ ⁺ →nΜ ⁺ Ν, which violatesδS=δQ. The positive decay track of theσ ⁺ comes to rest in the hydrogen bubble chamber and decays into ane ⁺. This track has all the characteristics of a stoppingΜ ⁺. The decay neutron fortuitously scatters twice, producing two recoil protons. The only other possible interpretation of the event isσ ⁺→nγ(π ⁺ →Μ ⁺ Ν), where theπ ⁺ →Μ ⁺ Ν decay produces no deflection (θ<0.1 rad) and no significant change in curvature. Using thep-wave radiative decay predictions ofBarshay et al. we calculate that the integrated branching ratio for such “accidental” events isγ(σ ⁺ →nγ(π ⁺ →Μ _stop ⁺ Ν))/γ(σ ⁺ →nπ ⁺)=1.6×10^?6. Most of the contribution to this “accidental” branching ratio comes from radiative decays where theπ ⁺ mesons have ranges less than 1 mm (p _π<20 MeV/c). If one excludes thoseΜ's with ranges less than 1.2 cm the above “accidental” branching ratio becomes 5.5×10^?7. With this figure we estimate that we should have seen 6.5×10^?2 events of this type thusfar in our experiment. The neutron momentum does not help in deciding between the two hypotheses. We therefore assign a confidence level of 7% for the radiative hypothesis. For the leptonic hypothesis we obtain an estimate of the branching ratio,γ(σ ⁺ →nΜ ⁺ Ν)/γ(σ ⁺ →nπ ⁺)=5×10^?5. If one further accepts theσ ⁺ →nπ ⁺ Ν event reported byBarbaro-Galtieri et al. and theσ ⁺ →ne ⁺ Ν event reported byNauenberg et al., one obtains theδS=?δQ leptonic branching ratio [γ(σ ⁺ →nΜ ⁺ Ν)+γ(σ ⁺ →ne ⁺ Ν)]/γ(σ ⁺ →nπ ⁺)=(4±3)×10^?5.     

Low-temperature time-resolved VUV luminescence spectroscopy of SrF<Subscript>2</Subscript>: Er<Superscript>3+</Superscript> crystals

Time-resolved excitation and emission spectra of SrF₂: Er³⁺ upon selective excitation with synchrotron radiation in the VUV and ultrasoft x-ray ranges at T = 8 K were studied. The VUV luminescence of SrF₂: Er³⁺ derives from high-energy interconfiguration 4f¹⁰5d-4f¹¹ transitions in the Er³⁺ ion. The VUV emission spectrum revealed, in addition to the 164.5-nm band (millisecond-range kinetics), a band at 146.4 nm (with a decay time of less than 600 ps). The formation of excitation spectra for the f-f and f-d transitions in the Er³⁺ ion is discussed.     

Derg R -Faktor des 2+-Rotationsniveaus von Er168

The rotation of the angular correlation between theγ-group at about 820 keV and the 80 keV radiation in the decay of Tm¹⁶⁸ has been observed in an external magnetic field of 20300 gauss. The result:ω _R·τ=0,485±0,051 yields for theg _R -factor of the 80 keV state of Er¹⁶⁸:g _R =+0,25±0,03. The evaluation includes a paramagnetic correction factor of:β=7,26, (B _eff=β·B _ext). The angular correlation is slightly attenuated by internal fields. For a liquid source of Tm (NO₃)₃ solved in 3 n HNO₃, a measurement of the differential angular correlation as a function of the delay time gave:λ ₂=(5,8±2,9)·10⁷sec^?1, assumingA ₂(t)=A ₂(0)·e ^?λ ₂·t. The half life of the 80 keV state was found to be:T _1/2=(1,92±0,04)·10^?9sec in agreement with earlier measurements.