Low-temperature time-resolved vacuum UV spectroscopy of NH<Subscript>4</Subscript>H<Subscript>2</Subscript>PO<Subscript>4</Subscript> crystals

A complex investigation of the dynamics of electronic excitations in nonlinear optical crystals of ammonium dihydrophosphate NH₄H₂PO₄ was performed using low-temperature vacuum UV luminescence spectroscopy with time resolution upon selective photoexcitation by synchrotron radiation. Data on the photoluminescence decay kinetics, time-resolved photoluminescence spectra (2–6.2 eV), and time-resolved photoluminescence excitation spectra (4–24 eV) were obtained for the first time for NH₄H₂PO₄ crystals at 8 K. It is ascertained that the photoluminescence of NH₄H₂PO₄ crystals in the vicinity of 4.7 eV has intrinsic character due to the radiative annihilation of self-trapped excitons. Possible channels of generation and decay of relaxed and unrelaxed electronic excitations in NH₄H₂PO₄ crystals are discussed.     

The synthesis of N-substituted 5-aminofurfurals

A simple method for the synthesis of N-substituted 5-aminofurfurals based on the reaction of 5-halogenofurfurals with secondary amines has been developed. The reaction takes place through the formation of quaternary salts of N-substituted 5-aminofurfurals. 5-(Diethylamino)-, 5-(N-methyl-N-phenylamino)-, and 5-piperidinofurfurals have been obtained for the first time.     

Electronic excitations and defects in nanostructural Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

A time-resolved cathodo-and photoluminescence study of nanostructural modifications of Al₂O₃ (powders and ceramics) excited by heavy-current electron beams, as well as by pulsed synchrotron radiation, is reported. It was found that Al₂O₃ nanopowders probed before and after Fe⁺ ion irradiation have the same phase composition (the γ-phase/δ-phase ratio is equal to 1), an average grain size equal to ～17 nm, and practically the same set of broad cathodoluminescence (CL) bands peaking at 2.4, 3.2, and 3.8 eV. It was established that Al₂O₃ nanopowders exhibit fast photoluminescence (PL) (a band at 3.2 eV), whose decay kinetics is described by two exponential stages (τ₁ = 0.5 ns, τ₂ = 5.5 ns). Three bands, at 5.24, 6.13, and 7.44 eV, were isolated in the excitation spectrum of the fast PL. Two alternate models of PL centers were considered, according to which the 3.2-eV luminescence either originates from radiative relaxation of the P^? centers (anion-cation vacancy pairs) or is due to the formation of surface analogs of the F⁺ center (F _S ⁺ -type centers). In addition to the fast luminescence, nano-Al₂O₃ was found to produce slow luminescence in the form of a broad band peaking at 3.5 eV. The excitation spectrum of the 3.5-eV luminescence obtained at T = 13 K exhibits two doublet bands with maxima at 7.8 and 8.3 eV. An analysis of the luminescent properties of nanostructural and single-crystal Al₂O₃ suggests that the slow luminescence of nanopowders at 3.5 eV is due to radiative annihilation of excitons localized near structural defects.     

Time-Resolved Vacuum Ultraviolet Spectroscopy of Er<Superscript>3+</Superscript> Ions in the SrF<Subscript>2</Subscript> Crystal

The photoluminescence and photoexcitation spectra as well as the luminescence decay kinetics of Er³⁺ ions in the visible ultraviolet and vacuum ultraviolet (VUV) regions have been studied by the method of low-temperature, time-resolved VUV-spectroscopy on excitation by synchrotron radiation. In the VUV spectral region of the luminescence of SrF₂:1% Er³⁺, the 146.5-nm band with a time of decay of less than 0.6 nsec was revealed together with the well-known emission band at 164.3 nm (decay constant in the microsecond range). Its possible nature is discussed. The specific features of the formation of photoexcitation spectra of the f-f and f-d transitions in the Er³⁺ ion are considered. Competition between the processes of excitation of f-f and d-f luminescence has been revealed. It manifests itself in the inverse relationship of their photoexcitation spectra in a range of energies of incident photons that are close to the position of the 4fⁿ⁻¹5d configuration levels. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 4, pp. 519–523, July–August, 2005.     

Luminescence VUV spectroscopy of cerium-and europium-doped lithium borate crystals

The paper reports on a study of the luminescence of lithium borate crystals (Li₆Gd(BO₃)₃ doped by Eu³⁺ and Ce³⁺ ions, Li_5.7Mg_0.15Gd(BO₃)₃: Eu, and Li₆Eu(BO₃)₃) initiated by selective excitation by synchrotron radiation at excitation energies of 3.7–27 eV at 10 and 290 K. Efficient energy transfer between the rare-earth ions Gd³⁺ → Ce³⁺ and Gd³⁺ → Eu³⁺ was found to proceed by the resonance mechanism, as well as by electron-hole recombination. Fast decay kinetics of luminescence of the Ce³⁺ activator centers was studied under intracenter photoexcitation and excitation in the interband transition region. The mechanisms involved in luminescence excitation and radiative relaxation of electronic states of rare-earth ions are analyzed, and the energy transfer processes operating in these crystals are discussed.     

Low-temperature time-resolved vacuum UV spectroscopy of potassium pentaborate crystals

For the first time, subnanosecond time resolution is attained in the low-temperature (at 7 K) measurements of the photoluminescence (PL) spectra (2–6 eV), the PL excitation spectra (4–32 eV), the PL kinetics, and the reflection spectra (4–21 eV) of undoped potassium pentaborate KB₅O₈·4H₂O (KB5) crystals under selective photoexcitation by synchrotron radiation. The PL peaks associated with the intrinsic defects of the KB5 lattice are detected. The PL bands resulting from radiative annihilation of the localized and self-localized electron excitations are singled out; these excitations are most efficiently photogenerated at the fundamental absorption edge in the region where the free exciton formation is expected. The difference between the PL spectra of the fast and slow components is revealed. An effective low-temperature energy transport over the KB5 hydrogen sublattice is deduced from a drop in efficiency of PL excitation in the interband-transition region as a result of nonradiative energy loss. Long-term vacuum UV irradiation of a KB5 crystal at 7 K gives rise to defects in the hydrogen sublattice, which facilitate localization of the electron excitations and reduce the effective length of their diffusion. This leads to a decrease in the nonradiative energy loss, thus enhancing the efficiency of the PL photoexcitation in the band-to-band transition region.     

Optical absorption and luminescence for radiation defects in Bi4Ge3O12 crystals

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 52, No. 3, pp. 400–405, March, 1990.