Diastereoselectivity in the 1,2-addition of silylazoles to chiral aldehydes. Stereocontrolled homologation of α-hydroxyaldehydes.

The addition of 2-trimethylsilylazoles (thiazole, benzothiazole, oxazole) to α-asymmetric aldehydes give the corresponding $\text{O}$-silylcarbinols in good chemical yield and high stereoselectivity; the reaction is employed for the stereoselective homologation of $\text{D}$-glyceraldehyde.     

Optical absorption and emission spectra of Cu+ : NaF single crystals

The optical absorption spectrum of monovalent copper in single-crystal sodium fluoride to 1300 Å, and its emission spectrum is reported. The temperature dependence of the observed bands shows that Cu⁺ is on-center in NaF.     

Electronic structure and luminescent properties of Cu+ and Ag+ impurity centers in NaCl

Fluorescence studies of Cu⁺ and Ag⁺ impurity centers embedded in NaCl are reported. The multiple scattering X_α method is used to describe the electronic structure of these ions in NaCl matrix. It is shown that taking into account the external lattice potential is necessary to obtain good results which can be compared to experiment. This has been done through the computation of an embedded NaCl cluster potential. Moreover, in NaCl:Ag⁺, it is also necessary to include relativistic corrections in order to obtain a reliable energy diagram. This is easily done by use of the Wood Hamiltonian, which allows the self-consistent inclusion of Darwin and mass–velocity terms in MS X_α codes. A good quantitative agreement with experiment is finally obtained for the optical excitations and the emission mechanism of the luminescent centers. The metal–chlorine distance is predicted slightly larger than the NaCl bulk value and the harmonic force constant of the a_1g vibrational breathing deformation of the cluster is calculated and found in good agreement within experiment. For the first time the Stoke shift of such systems is evaluated.     

Trapping and self-trapping in ytterbium-doped oxides with charge transfer luminescence

Temperature dependence of the charge transfer luminescence (CTL) of Yb-doped yttrium aluminum garnet Y₃Al₅O₁₂-Yb (YAG-Yb) and Yb-doped lutetium aluminum perovskite LuAlO₃-Yb (LuAP-Yb) crystals under X-ray excitation and their thermostimulated luminescence are investigated in the temperature range 30-350 K and compared to those of undoped crystals. Simulation using a set of kinetic equations describing the processes of creation of excitons, electron-hole pairs, their trapping and self-trapping, radiative relaxation and quenching is presented for the systems under investigation to analyze qualitatively two different types of experimentally observed temperature dependences: CTL yield decline with the temperature decrease below 110 K as in case of YAG-Yb and constant yield in the same temperature range as in case of LuAP-Yb.     

Relaxation processes upon photoexcitation of semiconductor CdF<Subscript>2</Subscript> by laser pulses

The microwave-cavity-based technique is used to study the processes of photoionization of electrons from donor levels to the conduction band in semiconductor CdF₂ crystals doped with Y, In, or Ga. The samples were excited by periodic pulses of Nd-laser (λ = 1.06 μm, pulse width ～10 ns) in the temperature range 6–77 K. The transient processes were detected in the absorption and dispersion modes related to variation of the imaginary and real parts of the complex permittivity ?₁ ? i?₂ induced by the light pulses. The observed signals consisted of short peak at t ～ 0, approximately 40–70 ns in length, and a long tail with a duration of ～100 ms. The short peak is likely to be related to the stay of the photoexcited carriers in the conduction band, while the long tail is associated with the processes of excitation relaxation after the electrons coming back to the donor levels of the impurity band. The weak temperature dependence of the width of the peak at t ～ 0 is explained by the tunneling mechanism of relaxation of electrons through the energy (or, probably, spatial) barrier separating the bound and free states of the carriers in the semiconductor CdF₂.     

Fluorescence line narrowing in GGG:Mn4+ in a strong magnetic field

Published in Zhurnal Prikladnoi Spektroskopii, Vol. 62, No. 5, pp. 181–188, September–October, 1995.     

Errata

For the first time, luminescence of Ag⁺ impurities in LiCl is observed and analysed by exciting in the UV vacuum range using synchrotron radiation. One of the two flourescence emissions observed is likely due to isolated silver ions, while the centers responsible of the other one are not yet clearly identified.     

High-resolution quantitative phase microscopic imaging in deep UV with phase retrieval

High-resolution three-dimensional (3D) microscopic imaging requires the use of short wavelengths. Quantitative 3D imaging techniques, such as digital holographic microscopy, require interference between the object beam and a known reference background for the extraction of phase information. At shorter wavelengths, due to short coherence lengths, it may be difficult to implement a two-beam off-axis setup. Thus, a single-beam technique, which provides complete phase information, may be better suited for short wavelengths. This Letter describes the development of a quantitative microscopy technique at 193 nm using multiple intensity samplings and phase retrieval.