Charge compensation and the luminescence of Cr3+ in KMgF3

Studies of the photoluminescence spectra of Cr³⁺ ions in KMgF₃ crystals co-doped with Cr³⁺ and Ni²⁺ ions are reported. Several crystal field sites are identified by the different R-line spectra due to the ² E⁴ A ₂ transition and broadband luminescences associated with the ⁴ T ₂⁴ A ₂ transitions. Cr³⁺ ions substituting without local charge compensation on the octahedral Mg²⁺ site give rise to a low temperature R line in photoluminescence at =702.3 nm with a radiative decaytime of 3 ms at T=14 K. At T=300 K this isotropic centre gives rise to an unpolarized broadband ⁴ T ₂⁴ A ₂ emission, which results from the thermal occupancy of an excited ⁴ T ₂ state just above the ² E level which, at lower temperature, gives rise to emission in the R-line. Other crystal field sites are due to some Cr³⁺ ions having Mg²⁺ or K⁺ vacancies in nearest-neighbour positions, these vacancies being required to maintain charge neutrality in doped fluoride perovskites. The Cr³⁺–K⁺ vacancy complex results in the centre having trigonal symmetry, and low temperature, photoluminescence via R ₁ and R ₂ lines at 716.8 nm and 716.0 nm, respectively. Finally, Cr³⁺ ions having a nearest neighbour Mg²⁺ vacancy have tetragonal symmetry, experiencing weak crystal fields. In consequence, the ⁴ T ₂ level lies below ² E and the photoluminescence spectrum at low temperature takes the form of a polarized broad ⁴ T ₂⁴ A ₂ band with peak at 760 nm and radiative decaytime of 54 s.     

Rate of abstraction of hydrogen atoms from ethane by muonium

Thermal reaction rates for the gas-phase reaction Mu+C₂H₆MuH+C₂H₅ have been measured bySR over the temperature range 510–730 K. The usual Arrhenius expression,k=Aexp(–E _a /RT), fits the data well, giving parametersA=1.0×10^–9 cm³ molecule^–1 s^–1 andE _a =15.35 kcal/mol. The activation energyE _a is 5.5 kcal/mol higher than for the H atom variant of this reaction, indicating a marked difference in reaction dynamics. Preliminary analysis indicates a still greater difference between Mu and H for the corresponding CH₄ reaction.     

Gluon and quark jets in deep-inelastic scattering of electrons by nucleons

The paper considers the formation of gluon and quark jets in the inelastic scattering of electrons by nucleons with large momentum transfers. It is assumed that these jets are created by the processes e^–+qe^–+q+g and e^–+ g e^–+q+q, where q is a quark and g is a gluon. The angular distributions of gluon and quark jets are calculated in the lowest order in the quark-gluon coupling constant. It is shown that after integration the processes e^–+qe^–+q+g and e^–+g e^–+q+¯q lead to violation of the scaling invariance of deep-inelastic scattering of electrons by nucleons.Translated from Izvestiya Vysshkikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 68–73, September, 1981.     

Emission of unbound 8Be and 12C * (0+ 2) clusters in compound nucleus reactions

We have studied the emission of light unbound clusters, ⁸Be and ¹²C ^* (0⁺₂), in the reactions ¹⁸O + ¹³C ^{31} Si ^{23} Ne + ⁸Be and ²⁸Si + ²⁴Mg ^{52} Fe ^{40} Ca + ¹²C ^* (0⁺₂). The -ray spectra obtained in coincidence with ⁸Be and ¹²C ^* (0⁺₂) emission have been studied relative to the statistical emission of two or three -particles. The angular-momentum-to-energy balance of the cluster emission is compared with that of multiple- emission. The properties of the energy spectra of the binary process and the population of the residual nuclei by cluster emission are discussed. It is observed that cluster emission carries away less excitation energy on average than the sequential emission of the individual components.     

Processes of associated production of the Higgs boson with the Z boson in a superstrong magnetic field

We compute probabilities for the processes ee+Z+H, and e⁺+e^–Z+H in a superstrong magnetic field. It is shown that the magnetobremsstrahlung of the Higgs boson with the Z boson in the magnetic field Bs>B₀=m²/e=4.41·10¹³ G may be a relatively probable process, and that the superstrong magnetic field significantly influences the process e⁺+e^–Z+H, which is possible even in the absence of the field.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 104–108, January, 1991.The author thanks V. Ch. Zhukovskii and A. V. Borisov for discussions on the results of this work.     

Isotope selective laser enhancement of the Li+H2 reaction

The enhancement of the Li+H₂LiH+H reaction due to laser excitation of the Li atom to the2p state has been investigated using absorption techniques. Using a rate equation model we obtain from our data a reaction cross-section [Li(2p)+H₂LiH+H]=(0.10±0.03)Ų at 515°C. By selectively exciting one Li isotope the reaction is made isotope selective, suggesting an efficient means of isotope separation.     

Effect of violating t-invariance of the electromagnetic interaction of hadrons in the processes n + p → d + γ and n + → d + e++e−

T-odd correlations that are possible in the processes n + p d + and n + p d + e⁺ + e^e- and that arise due to the capture of thermal neutrons are determined in terms of threshold amplitudes (for the process n + p d + @#@) or electromagnetic inelastic form factors (for the process n + p d+ e⁺ + e^e§-). The mechanism of these reactions is not given in concrete form, but those general properties of the electromagnetic hadronic current are used that are preserved with three-dimensional reflections.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 8–13, October, 1986.     

Energy spectrum and angular distribution of muons from the decay of heavy leptons produced in colliding electron-positron beams

On the basis of an arbitrary (V, A) structure of the neutral weak ¯ee and LL currents (L=^–, M⁰) a study is made of the processes of production in colliding electron-positron beams of pairs of heavy leptons with subsequent decays in accordance with the schemes e⁺e^––(µ^–v_µv) + ⁺( anything) and e⁺e^–M⁰(µ^–e⁺v_e) + M⁰( anything). The energy spectrum and asymmetry of the distribution of the produced muons are investigated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 66–70, February, 1981.     

Room temperature emission from the Pr3+ 1 S 0-level in PrF3

PrF₃ as well as PrF₃-LiF and PrF₃-MgF₂ mixtures were investigated by means of high-energetic excitation with synchrotron radiation. In the PrF₃ emission spectrum bands due to the intra-configurational 4f²4f² transitions originating from the ¹S₀ level of the Pr³⁺ ion have been identified. The emission from the ³P₀ multiplet is very weak due to non-radiative decay by cross-relaxation processes. Therefore, PrF₃ is not an efficient direct cascade emitter. In the PrF₃ excitation spectra in the ultraviolet/vacuum-ultraviolet spectral range, the ³H₄¹S₀ transition at 46858 cm^-1 as well as broad bands due to the inter-configurational 4f²4f5d transitions are observed. By comparison with data of Pr³⁺ doped YF₃, the branching ratios of the emission transitions for PrF₃ from the ¹S₀ were determined; these are 0.008, 0.075, 0.262, 0.023 and 0.629 for transitions ¹S₀³H₄, ³F₄, ¹G₄, ¹D₂ and ¹I₆, respectively. For the polycrystalline PrF₃-LiF and PrF₃-MgF₂ samples investigated by us, the measured emission and excitation spectra are nearly identical to those of PrF₃. For polycrystalline PrF₃-KMgF₃ the observed spectra are superpositions of the PrF₃ and Pr³⁺:KMgF₃ spectra. PACS 42.70.-a; 78.55.-m; 78.55.Hx     

The dilution of silica in a NiZnCo spinel ferrite matrix

Silica may dissolve up to about 0.05 wt% in an iron excess NiZnCo spinel matrix, affecting noticeably the magnetic properties at low and high values of the applied field. The initial permeability is reduced by about 50% with the addition of only 0.1 % SiO₂, while the coercivity almost triples, without noticeable changes in porosity or grain distribution. The vacancy and Fe²⁺ content of the sintered material also show a considerable change. The energy associated to the transformation Fe³⁺ Fe²⁺ for the samples without SiO₂ was found to be 0.41–0.42 eV, while for the samples with SiO₂ additions it decreased to 0.26 eV. The changes in magnetic properties are thought to be caused, more than because of the mentioned changes in ion distribution, by the stresses and defects created in the lattice due to the very small size of the Si⁴⁺ ion, which substitutes Fe³⁺ ions in tetrahedral sites. The transition Fe³⁺ Fe²⁺ seems to take place exclusively in the octahedral sites.     

TheK S 0 →π0 l + l − andK L 0 →π0 l + l − decays in the standard model and the extended Nambu-Jona-Lasinio model

TheK _S ^{0 0} l ⁺ l ^– andK _L ^{0 0} l ⁺ l ^– decays are considered within the framework of the Standard model in the one-photon exchange approximation, i.e.K _S,L ^{0 0}+ *⁰ +l ⁺ l ^–. To evaluate the low-energy matrix elements of theK ⁰(¯K ⁰)⁰+ * transitions, the constituentquark-loop approximation, based on the extended Nambu-Jona-Lasino (NJL) model, is used. The obtained results are compared with other theoretical estimates and the experimental bounds.     

Joint creation of Z-boson and photons by colliding e+e− beams

Density matrix elements for a Z-boson created in the reaction e⁺+e^–Z+, are computed. Two reference frames are used in computations: the center-of-mass frame of the reaction e⁺+e^–Z+ with polarization states of the Z-boson specified by the polarization 4-vector with definite helicities, and the rest frame of the Z-boson. In both cases P-odd phenomena should stipulate particular combinations of the density matrix elements. The energy behavior of the total cross-section of the process e⁺+e^–Z+ is studied.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 3–6, September, 1982.     

High resolution photon stimulated desorption spectroscopy of solid nitrogen by resonant N 1s core level excitation

Photon stimulated desorption (PSD) spectra of ions (N⁺, N⁺ ₂, and N²⁺) by N 1s 1 _g and N 1s Rydberg level excitations have been recorded from solid nitrogen for different polarization and detection conditions, and compared with electron yield data. Vibrational fine structure is clearly resolved also in photon stimulated desorption (PSD) spectra; its details depend on layer preparation, but also on the type of signal. Detailed evaluation should allow conclusions on the nature and the dynamics of the process of bond breaking and on the influence of the surroundings on it. Exploiting alignment effects by properly setting polarization and detection angles, symmetries of electronic excited states can be well analysed even for samples consisting of randomly oriented molecules.     

Search for the lowest 2+ mixed-symmetry state in102Ru

TheK-conversion coefficients of some 2_i ⁺ 2_f ⁺ transitions in¹⁰²Ru have been measured and their electric monopole components have been deduced. The intensity of theE0 component in the 2₃ ⁺ 2₁ ⁺ transition supports a possible interpretation, based on gamma decay, of the 2₃ ⁺ level as the lowest mixed-symmetry state. Negative parity has been assigned to the level at 2261 keV.We would like to thank Mr. M. Ottanelli for his continuous, important technical support. One of the authors (D.C.) thanks the INFN for hospitality and financial support during this work.     

Contribution ofp-wave (3)+1 subamplitudes to4He binding energy and scattering observables below four-body breakup threshold

The four-body equations of Alt, Grassberger and Sandhas, in the form that the (2)+(2) subsystem amplitudes are treated by the convolution method, are solved for separable two-body interactions between nucleon pairs in the channels³S₁-³D₁ (t ₀₀ alone) and¹S₀. AllL=0 andL=1 (3)+1 subamplitudes are represented in a separable form using the energy dependent pole expansion. The contribution ofp-wave (3)+1 subamplitudes to the⁴He binding energy, threshold scattering observables and low energy cross sections forn ³Hn ³H,p ³Hep ³He,dddd andddp ³H reactions is studied for different two-body potentials.     

Relaxation of 5s2 2D3/2 states of Cd ions in He atom collisions in a He-Cd medium excited by fission fragments

Efforts to obtain lasing at the 325 nm CdII transition in a high pressure He-Cd medium pumped by the nuclear reactions products were unsuccessful, in contrast with the same 441.6 nm transition. Experiments carried out at the fast burst nuclear reactor in order to obtain information about the relative intensities of the 441.6 and 325 nm luminescence lines as a function of pressure have allowed to evaluate the constant of the reaction Cd⁺ (5s^{2 2}D_3/2) + He Cd⁺ (5s^{2 2}D_5/2) + He, which has been found to be about 1.5×10^–13 cm³/s. Thus the intensities of the 441.6 and 325 nm luminescence lines measured in a nuclear pumped He-Cd medium are almost equal and the hypothesis about the strong deexcitation of the Cd⁺(5s^{2 2}D_3/2) state by He has not been confirmed.     

Energy transfer in YAlG:Nd codoped with Ce

Laser excited luminescence has been used to study an energy transfer between Ce³⁺ and Nd³⁺ ions in YAlG:Nd,Ce laser crystal with varying small Ce concentrations. Radiative Ce³⁺Nd³⁺ energy transfer dominates in the studied single crystal samples but this process can also be accompanied by multistep transfer mechanisms (for example by (Ce³⁺Ce³⁺)_n Nd³⁺ transfer mechanism). The Ce³⁺Nd³⁺ radiative energy transfer improves pumping of green and yellow excitation lines in YAlG:Nd,Ce laser rods because the contribution of the additional transfer pumping of Nd³⁺ ions from Ce³⁺ ions is more than three times greater than should be expected from concentration differences between Nd and Ce. This favourable behaviour is explained from high quantum efficiency of Ce³⁺ emission and higher Ce³⁺ absorption cross sections in comparison with the Nd³⁺ ones (more than one order of magnitude difference).This work was done in cooperation with Monokrystaly Turnov, Research Institute for Single Crystals. The author is grateful to Jos. Kvapil and J. Kubelka for supplying him with samples used in this work. He would like to acknowledge many fruitful discussions with J. Kvapil and K. Blaek of Monokrystaly Turnov.     

Luminescence sensitization in Nd-Cr-Ce doped yttrium aluminates

Energy transfer from Ce to Cr in YAG is radiative and relatively inefficient. If excited below 500 nm, the opposite transfer is dominant. YAP: Ce, Cr showed relatively efficient CeCr transfer only. The sensitization of Nd³⁺ luminescence by Ce³⁺ ions depends on the overlap of the Ce³⁺ emission band with Nd³⁺ absorption lines. Cr³⁺Nd³⁺ transfer is characterized by an expressive non-radiative portion. It is inefficient in YAG but very efficient in YAP. No UV-induced colour centres were found in YAG: Nd, Cr grown under Ar-H₂ atmosphere and doped with 10^–3 wt. % Cr, but at a higher Cr concentration anomalous absorption between UV absorption edge and 650 nm was stabilized. Ce³⁺ admixture in YAG:Nd, Cr and/or reducing treatment of the crystals facilitate the decomposition of the centres. The decomposition is accompanied with a strong Nd³⁺ luminescence. Therefore, YAG: Nd, Ce, Cr is an advisable active laser material. On the other hand the same centre in YAP: Nd, Cr seemed to be more stable even in the presence of cerium ions.     

KL,S → l+l− decays in the quark model of superconductivity type

The probabilities of K_L,S l⁺l^– decays are calculated by considering only the dominant two-lepton channel K_L,S ^{* *} l⁺l^–. The transitions K⁰ X, where X=⁰,, or (700) dominating in the K_L,S ^{* *}decay amplitudes, are described by the effective weak Lagrangian. The matrix elements of four-quark operators are calculated within the framework of the quark model of superconductivity type. It is shown that the leading contribution to matrix elements of K⁰ X transitions comes from the penguin operator matrix elements.     

Padé-approximant techniques for processing scattering data II. Energy-dependent phase-shift analysis of low-energy4He +2H scattering

The general method for Padé-parametrization of experimental scattering data proposed by the authors recently has been applied to analytic parametrizations of theS-matrix and of partialwave amplitudes with the aim of carrying out an energy-dependent phase-shift analysis. It has been shown that the high universality and flexibility of the Padé-approximants make it possible to describe the partial-wave amplitudes by a very small number of parameters (say 2–3 in each partial wave) and to create a universal scheme for parametrization of the full reaction amplitude based on the following hybrid approach: the energy dependence of the inner partial amplitudes is represented by Padé approximants whereas the peripheral part of the full amplitude is taken from the low-order iterations of the exact many-particle Faddeev —Yakubovsky equations. As an example of the technique an energy-dependent phase-shift analysis of low-energy²H +⁴He scattering has been carried out and the parameters of the 3⁺ 0,2⁺ 0 and 1⁺ 0 resonant states of⁶Li have been obtained together with the value of the vertex constant of the virtual decay⁶Li_g.s. ⁴He +²H. The values of the energy and width obtained from experimental data of the 2⁺ 0 and particularly of the 1⁺ 0 level differ significantly from the values published in the literature.Dedicated to Professor J. Kvasnica on the occasion of his sixtieth birthday.