V.N. Makhov

VUV spectroscopy of KYF4 crystals doped with Nd3+, Er3+ and Tm3+

Abstract Emission, excitation and absorption spectra as well as luminescence decay kinetics of KYF 4 crystals doped with Nd 3+ , Er 3+ and Tm 3+ were studied in the VUV spectral range. It is shown that all the crystals have intense VUV luminescence due to interconfiguration transitions originating from the 5d states of the rare earth (RE) ions and terminating on the 4f states. The UV luminescence caused by intraconfiguration 4f–4f transitions in the RE ions is also observed from the crystals doped with the Nd 3+ or the Er 3+ ions under 4f–5d excitation. The spin-allowed 5d–4f luminescence from the crystals doped with Er 3+ or Tm 3+ is extremely weak, i.e. there exists very efficient nonradiative relaxation from higher lying 5d states to the lowest 5d level responsible for spin-forbidden luminescence. This peculiarity of the studied crystals doped with Er 3+ or Tm 3+ is very attractive for developing VUV solid-state lasers based on the crystals of such a type.

VUV spectroscopy of wide band-gap crystals doped with rare earth ions

Abstract Spectral and kinetic properties of vacuum ultraviolet (VUV) luminescence from different fluoride crystals doped with Nd 3+ , Er 3+ or Tm 3+ are discussed. Concentration quenching of VUV luminescence, the impact mechanism of the energy transfer from the matrix to the emission centers as well as nonradiative relaxation processes in the rare earth ions have been detected and studied. The crystal properties are analyzed from the viewpoint of their possible applications as materials for fast VUV scintillators and VUV solid state lasers. Only some of Nd 3+ doped crystals possess simultaneously both high light output and relatively good timing properties of VUV luminescence and such crystals can be considered as promising materials for fast VUV scintillators. On the other hand, many of Er 3+ and Tm 3+ doped fluoride crystals are very attractive candidates as active media for tunable VUV solid state lasers with optical pumping.

Spectroscopic properties of Pr3+ luminescence in complex fluoride crystals

Abstract Spectroscopic properties of several complex fluoride crystals K 2 YF 5 , KYF 4 , LiKYF 5 , CsY 2 F 7 and CsGd 2 F 7 doped with Pr 3+ have been studied under synchrotron radiation and F 2 excimer laser excitation. The Pr 3+ doped K 2 YF 5 and LiKYF 5 crystals with only a single crystallographic site for Pr 3+ ions show typical properties as in the case of spectroscopic systems where the low edge of the 4f5d Pr 3+ configuration lies slightly below the 1 S 0 level of the 4f 2 Pr 3+ configuration. The Pr 3+ :KYF 4 crystal has the same properties caused with only the 5d–4f luminescence of Pr 3+ in spite of the fact that in this crystal there are six sites suitable for Pr 3+ ions. The luminescence spectrum of the Pr 3+ :CsY 2 F 7 crystal is determined by both the 5d–4f luminescence and the 4f–4f emission originating from the 1 S 0 Pr 3+ level, which is due to two groups of optical Pr 3+ centers with the energy levels of the 1 S 0 state just between the lowest 5d levels of Pr 3+ for these two kinds of centers.

Luminescence spectroscopy from the vacuum ultra-violet to the visible for Er3+ and Tm3+ in complex fluoride crystals

Abstract Spectra and decay kinetics of luminescence from several complex fluorides of alkali elements and yttrium or gadolinium doped with Er 3+ or Tm 3+ have been studied in the vacuum ultra-violet (VUV), ultra-violet (UV) and visible spectral ranges under excitation by the 157 nm radiation from a pulsed molecular F 2 -laser. It has been found that yttrium crystals have intense VUV luminescence due to interconfiguration 5d–4f transitions in the rare earth ions. On the other hand, the 5d–4f luminescence from Er 3+ or Tm 3+ in gadolinium crystals is completely absent and the nonradiative decay as a result of cross-relaxation is the dominant decay channel from the 5d level of the doping rare earth ion in these crystals. This process leads to complete conversion of absorbed VUV radiation into the lower energy part of the spectrum. These gadolinium fluorides appear to be promising materials for VUV-excited phosphors with high-quantum efficiency.

Upconversion fluorescence of Nd3+ ions in K2YF5 single crystal

Abstract Upconversion fluorescence of Nd 3+ ions in K 2 YF 5 single crystal was studied under 785-nm (near-infrared), 514.5-nm (green) and 325-nm (ultraviolet) laser excitation. With laser excitation set at 785 nm, a near-infrared upconversion emission band centered at 745 nm with a peculiar linear power dependence was observed and explained successfully by using a multiphonon assisted upconversion mechanism. With 514.5-nm laser excitation, five upconversion emission bands centered at 358, 386, 417, 451 and 496 nm were recorded. The power dependence of these emission bands was also studied and the upconversion mechanism was attributed to the excited state absorption from 4 F 3/2 state. In addition, the ultraviolet upconversion fluorescence which peaked at 287 nm under 325-nm laser excitation was also presented and analyzed.

Inter and intraconfigurational luminescence of LiYF4:Er3+ under selective VUV excitation

Electronic relaxation of selectivity excited LiYF4:Er3+ was investigated by means of time-integrated and time-resolved photoluminescence spectroscopy, using synchrotron radiation for excitation purposes. The range of excitation covered both Er3+ and host excitation up to 200 eV. Following Er3+ excitation, radiative cascades and cross relaxation were established. Under soft x-ray excitation, inelastic scattering of photoelectrons contributes to electronic relaxation.

VUV emission of rare-earth ions doped into fluoride crystals

VUV emission of LiY 1-x Er x F 4 single crystals (x = 0.05, 0.2, 1) has been studied in an extended range of excitation energy and in a wide temperature range. It was found that strong concentration quenching occurs in highly doped crystals leading to nanosecond decay for both spin-allowed and spin-forbidden 5d-4f transitions in Er 3+ . The impact mechanism of the energy transfer from the matrix to Er 3+ ions results in relatively high quantum yield of Er 3+ d-f luminescence in such crystals at high-energy excitation. It was shown that the scheme of intermediate electron-phonon coupling should be used for the description of bandwidth temperature broadening for d-f luminescence.

Emission and Excitation Spectra of Ce3+ and Pr3+ Ions in Hexafluoroelpasolite Lattices

The emission and excitation spectra of Ce(3+) and Pr(3+) doped into the cubic host Cs(2)NaYF(6) have been recorded at room temperature and ∼10 K using synchrotron radiation. The two 5d(1) T(2g) states of Ce(3+) have been located from the excitation spectra, whereas the E(g) state is placed above the host band gap. Decay measurements of the 5d(1) → 4f(1) Ce(3+) emission, and spectra collected using selective excitation, indicate the occupation of more than one type of site by Ce(3+) in this host lattice. By contrast, the location of features in the 4f(1)5d(1) → 4f(2) emission of Pr(3+) is independent of the excitation wavelength. Assignments are presented for some of the 4f(1)5d(1) levels and for the Pr(3+)-F(-) charge transfer band. The 5d emission lifetimes for Ce(3+) and Pr(3+) in the Cs(2)NaYF(6) host are 42 and 29 ± 1 ns, respectively, and are not temperature-dependent.

CROSSLUMINESCENCE AT HIGH TEMPERATURES

Abstract The temperature dependence of crossluminescence of barium fluoride and cesium chloride single crystals has been studied in the wide temperature range from 5 to 750 K using the technique of time-resolved VUV spectroscopy under synchrotron radiation excitation. It was shown that the temperature behaviour of the bandwidths in crossluminescence spectra of both crystals can be well described in the whole temperature range in the framework of the model of the local optical centre in the vibrating lattice environment. A multi-dimensional configuration co-ordinate scheme and a coupling with different modes of lattice vibration for different crossluminescence transitions were proposed as the explanation of different temperature broadening effects for different bands in the spectra of crossluminescence.

Cross-luminescence peculiarities of complex KF-based fluorides

Luminescence excitation spectra of KMgF3, KYF4 and KLuF4 crystals were investigated under pulsed synchrotron radiation (10–30 eV) excitation. Fast (τ ≈ 1 ns) cross-luminescence was found in all these crystals with excitation thresholds at 21.8 eV for KMgF3 and 20.8 eV for KYF4 and KLuF4. Electron structure peculiarities of KMgF3, KYF4 and KLuF4 as well as the possibility of cation hole self-trapping in potassium fluorides are discussed.