Excited 4f Research Articles

Resonance Auger and autoionization processes in solid lanthanum after 4d to 4f resonant excitation by synchrotron radiation

The decay processes of metallic lanthanum after resonant 4d to 4f excitation by synchrotron radiation have been studied by means of electron spectroscopy. Excitation to the 4d94f1(3D1) state, which is below the 4d ionization threshold, was found to decay predominantly through N4,5O2,3O2,3 and N4,5O2,3V spectator resonance processes. The energy shift relative to the corresponding normal Auger energies was 1.6+or-0.3 eV in the case of NOO resonance Auger decay, whereas no energy shift was found for the NOV resonance Auger electrons. The excitation to the main 4d to 4f giant resonance was found to decay through autoionization leading to a one-hole final state 5s-1 or 5p-1, or through normal Auger processes. Autoionization processes are strong on the low energy side of the resonance, but when the excitation energy increases, normal Auger processes become more and more dominant, indicating a change in the nature of the giant resonance.

Dipole forbidden f-f excitation in ytterbium oxide

The dipole forbidden f-f excitation in Yb2O3 has been investigated with EELS. The oxide with its large band gap provides in EEL-spectra a reliable separation of the low lying dipole forbidden 4f excitations from other, optically allowed transitions. For primary energies ranging from 1245 eV down to 10 eV we have performed EELS measurements on thin oxide films. In the loss spectra the intensity of the 1.33 eV loss increases drastically with decreasing primary electron energy. Additional EELS measurements on differently prepared films of ytterbium oxide and on Lu2O3 throw light on the nature of the 1.33 eV loss. The origin of a previously reported unexplained 2.5 eV loss is discussed.

Dipole forbidden f-f excitation in ytterbium oxide

Abstract The dipole forbidden f-f excitation in Yb2O3 has been investigated with EELS. The oxide with its large band gap provides in EEL-spectra a reliable separation of the low lying dipole forbidden 4f excitations from other, optically allowed transitions. For primary energies ranging from 1245 eV down to 10 eV we have performed EELS measurements on thin oxide films. In the loss spectra the intensity of the 1.33 eV loss increases drastically with decreasing primary electron energy. Additional EELS measurements on differently prepared films of ytterbium oxide and on Lu2O3 throw light on the nature of the 1.33 eV loss. The origin of a previously reported unexplained 2.5 eV loss is discussed.

Saturation by resonant up-conversion in Tb-doped phosphors

We have investigated the influence of second order energy loss processes in Tb-doped phosphors on the saturation behavior under cathode ray excitation. A model has been developed assuming resonant electric dipole interaction between excited activators to be the dominant loss mechanism at excitation densities per pulse below 10 −2 J/cm 2. The loss process encompasses up-conversion of 5D j -excitations ( J = 3, 4) into highly excited 4f- and 5d-states with subsequent nonradiative decay back to the 5D j -states. Experimentally, the absorption cross-sections between the excited states are accessed by a dedicated excited state absorption (ESA) technique for technique for phosphor powders. The cross-sections for transitions from the excited 5D j -states to the lower 7F j -multiplet are obtained from calibrated emission spectra. It is concluded that the different saturation behavior of the investigated Tb-doped phosphors (YAG, YAGaG, LaOBr, and Y 2SiO 5) can be explained on the basis of our model.

On the saturation of Tb phosphors under cathode-ray excitation. I. Excited-state absorption in Tb-activated phosphor powders

Optical transitions between the 5D4 excited state of the Tb3+ (4f)8 configuration and the crystal-field split components of the (4f)7(5d) configuration are observed using high-resolution laser excitation of Tb-doped powder samples at room temperature. Excited-state absorption spectra of Tb3+ in YAG, YAGaG, Y2SiO5, and LaOBr are presented. Superimposed on the broadband excitation spectrum of these transitions we find the relatively narrow 4f→4f lines. We present a theoretical model for interpreting the 4f→5d transitions, which predicts a strict proportionality between the (4f)8 5D4→(4f)7(5d) and the (4f)8 7F6→(4f)7(5d) transitions. This model is used to estimate the optical-absorption cross section for the former transitions. Although the terminating 4f levels lie at the same energy as the 5d bands, the 4f→4f transitions can be interpreted on the basis of the Judd-Ofelt theory. We derived a method for calibration of the Tb3+ 4f→5d fluorescence-excitation spectra from powder samples. For YAG:Tb, the resulting optical cross sections were confirmed by direct-absorption measurements on a Tb3+ -doped epitaxial YAG layer. To our knowledge, this is the first time that the probabilities for excited-state absorption to highly excited 5d and 4f states in Tb3+ have been obtained theoretically and experimentally from Tb-doped powder materials.

On the saturation of Tb phosphors under cathode-ray excitation. II. Upconversion processes in the excited-activator bath

A physical model based on resonant electric dipole interaction between excited activators is presented that may be used for gaining insight into the second-order energy-loss process in cathode-ray-irradiated Tb phosphors. It is shown that resonant up-conversion of the 5DJ excitons (J=3,4) to highly excited 4f and 5d states may be of equal importance. The derivation of the rate constants for this second-order energy-loss process is given and experimental values for the 5D4 -5D4 and 5D3 -5D4 interaction are presented for Tb-doped YAG, YAGaG, LaOBr, and Y2SiO5. The values depend on previously determined optical cross sections for excited-state absorption. Based on this model we explain well-known differences in saturation behavior between these Tb-doped phosphors under cathode-ray excitation.

EELS and low‐energy electron emission of clean and oxygen‐exposed tantalum

AbstractThe electron energy‐loss spectra (EELS) associated with 4f and 5p excitations, and the secondary electron spectra (SES) have been used to study the oxidation of a clean polycrystalline tantalum surface. For the clean and oxygen‐exposed surfaces, the EELS results are discussed in terms of bulk and surface plasmons as well as interband transitions to unoccupied states above the Fermi level (EF). The secondary electron spectra have been used as a fingerprint for the density of unoccupied states above EF. The changes observed in the EEL spectrum upon oxygen exposure are explained as being due to oxide formation, in agreement with previous results by XPS and AES. The features observed above the NVV and OVV Auger transitions, in both the clean and oxidized surfaces, are described as auto‐ionization emission due to the decay of electrons that have been excited resonantly from the 4f and 5p levels to states well above the states well above the Fermi energy. The excitation process has been identified also in the EEL spectra.

MOVPE grown InP:Yb layers using Yb(IpCp) 3 as a new doping source

For the Yb-doping of MOVPE grown InP, we have synthesized and used for the first time tris(isopropylcyclopentadienyl)-Yb, Yb(IpCp) 3. This compound with its melting point at 47°C can be used as a liquid doping source, thus improving the reproducibility of the evaporation compared to the commonly used solid precursors. The grown InP:Yb layers revealed high photoluminescence intensities of the Yb 4f lines, although the Yb concentrations measured by SIMS were only in the range of 10 17 cm -3. This indicates the high crystal quality of our samples. Additionally, we have grown InP:Yb:S layers and InP:Yb/InP:S multilayer structures with thicknesses between 10 and 100 nm for each layer to study the dependence of the excitation and decay processes on carrier and impurity concentration. The photoluminescence intensity of the 4f emission decreases for high S concentration in InP:Yb:S samples, whereas in the multilayer structures the intensity is the same as in InP:Yb samples. Based on the assumption of a homogeneous carrier concentration throughout the whole multilayer structure, we believe that a direct interaction between Yb and S atoms is responsible for the decrease in the double-doped single layers. The lifetime of the excited 4f state of Yb 3+ is 13 μs, regardless of carrier or Yb concentration. For the InP:Yb samples co-doped with S, a fast nonexponential decay was observed, a further indication of some Yb-S pair interaction.

Spectroscopic study of potential magneto-optic storage layers

We present an experimental investigation of magneto-optic (MO) thin film materials of interest in optical storage. XPS valence band spectra of ultrahigh vacuum deposited rare earth — Fe50Co50 alloys are correlated to magneto-optic and reflectivity spectra measured ex-situ in the photon energy range 0.5 ≦E ph ≦ 5.5 eV. It is found that 4f n →4f n−15d 1 optical transitions contribute to room temperature Kerr spectra in Pr-FeCo and Nd-FeCo, but only in the UV spectral region (λ≦300 nm). Eu is found to enter Eu-FeCo intermetallic alloys with a 4f 7 ground state at −2 eV binding energy. However, no substantial contribution to the Kerr effect from excitations of this electronic state has been observed n either at room temperaturen or at 100 K. We present a general argument that 4f excitations of RE ions in typical intermetallic alloys can never be expected to contribute more than some tenths of a degree toθ k in contrast to the well known huge rotations in RE-chalcogenides. For Yb-FeCo we have observed in XPS a mixed valence behavior of Yb showing a discontinuous transition from divalent Yb2+ in the pure metal to mixed valent Yb2.55+ in amorphous Yb-FeCo films. Nof→d contributions to the room temperature MO spectra have been detected. Strong optical enhancement effects are presented for metallic bilayer and multilayer structures of the type MO-layer/reflector. The total thickness of an optimized structure corresponds to about two times the penetration depth of light. Enhancement occurs predominantly at photon energies below the plasma frequency of the reflector material and is caused by low values of the optical constantsn andk of the reflector. In storage materials like Tb-FeCo the read-out performance can be improved by about a factor of 2 with a bilayer of 30 nm Tb-FeCo on Cu. Quantitative modelling of the optical and magneto-optic properties of these films is discussed.

Contributions from the energy level structure of the 4f 115d 1 intermediate configuration to the electronic raman scattering intensities of TmPO 4

A direct calculation of the electronic Raman scattering intensities of TmPO 4 which includes the detailed energy level structure of the intermediate excited configuration 4f 115d 1 is given. The contributions to the scattering intensities from various perturbations within the 4f 115d 1 configuration are enumerated. Most of the calculated relative scattering intensities are improved when compared to the results of the conventional Judd-Ofelt-Axe calculations. However, there are significant differences remaining between the experimental and calculated intensities.

Dye‐Laser 4f–4f‐Line Probing of Crystal‐Field Fluctuations in ZnS:Sm3+ Polycrystalline Films

AbstractUsing the dye‐laser site‐selective excitation technique, a study is made of 4G5/2 → 6H9/2, 6H7/2 inter‐Stark level transitions for a special type of Sm3+ centers in ZnS polycrystalline films. It is found that the emission lines become narrowed compared with the case of “nonselective” excitation, with the peak positions and intensities depending on the location of the dye‐laser energy within the 4f–4f excitation line profiles. The results can be quantitatively described by a simple model which involves small centre‐to‐centre fluctuations of crystal‐field parameters. A comparison with the (much sharper) lines measured for the same type of Sm centers in ZnS bulk crystals shows that these fluctuations have a Gaussian distribution with maximum corresponding to the bulk crystal situation. An explanation can be given on the basis of random strain originating from high dislocation densities. A discussion is made of whether the linewidths are affected by interactions between the Sm centres and by electric fields due to dislocation charge.

Transition probabilities for forbidden lines in the ground-state configuration of the bromine isoelectronic sequence

Calculations of transition probabilities are reported for the magnetic dipole and electric quadrupole components of the ground-state transition in the 4p5 configuration of the bromine sequence up to Xe XX. The relativistic Hartree-Fock (HFR) self-consistent-field method was used combined with parametric fitting in a configuration interaction approach. The results reported here are an extension of similar calculations recently reported in 3pN and 4pN configurations, and some comments on the effect of 3p → 4f excitations in 3p1 and 3p5 configurations are given together with a discussion of the relative merits of the HFR and HXR methods for the calculation of spin-orbit integrals.

Optical properties of Eu 2+, Eu 3+ and Mn 2+ in cubic perovskite crystals

The optical properties of the fluoroperovskite crystals KCaF 3 : Eu, RbCaF 3 : Mn : Eu, and RbCdF 3 : Mn : Eu are characterized. In these crystals the spectra for Eu 2+ ions are observed in the KCaF 3 and RbCaF 3 crystals. This optical absorption from the 8S 7/2 ground state to the excited 4f 65d(t 2g) and 4f 65d(e g) states is common for Eu 2+ ions in alkali halides and fluorite type crystals. The absorption spectrum implies that the divalent europium is in the Ca 2+ site in KCaF 3 and RbCaF 3. In KMgF 3 or RbMgF 3 samples the Eu 2+ ions substitute in K + or Rb + sites and have a different optical spectrum. No evidence of Eu 2+ absorption characteristic of Eu 2+ in K + or Rb + sites was found in KCaF 3 or RbCaF 3. Although the lattice parameters for the three lattices are essentially the same Eu 2+ was not present in the RbCdF 3 crystals. However, Eu 3+ is observed in all the crystals. The energy levels for Eu 2+, Mn 2+ and Eu 3+ in these crystals are reported.

La 4d-->4f resonance absorption in LaF3: Temperature dependence.

Temperature-modulated transmission spectra in the region of the 4d\ensuremath{\rightarrow}4f excitations in ${\mathrm{LaF}}_{3}$ show a variety of effects, none of which can be explained by the free-atom model hitherto used to describe this absorption. The sharp lines below threshold are shown to broaden with increasing temperature and to shift slightly. The broad ``autoionization'' region is shown to be composed of several structures, only one of which is included in the atomic picture. The effects of the lattice on these transitions ``within'' the ${\mathrm{La}}^{3+}$ ion are significant.

The 5d← 4fexcited states of [Ce(OH2)9]3+

A DV-Xα molecular-orbital calculation was carried out on [Ce(OH2)9]3+. The charge donation of coordinated water molecules results in a net charge of +0·354 on the central tervalent metal ion and an appreciable polarization in the coordinated waters. The coordination bond of the vertex waters is stronger than that of the equatorials. The observed transition moments and energies of the 5d ← 4f excitations as well as the observed ligand-field splitting of 2 D(5d 1) multiplet in the D 3h ligand field of [Ce(OH2)9]3+ were reasonably reproduced by use of the transition-state approximation on the base of DV-Xα molecular-orbital calculation. The model also predicts a contraction of 4f orbital while an expansion of 5d orbital in the cluster. The one-electron energies of 4f, 5d and 6s orbitals as well as the spin-orbit coupling energies in Ce3+ free ion were also calculated by the relativistic DV-Xα method. The relativistic equation reproduces the observed values only for α = 1·0 rather than the traditional value α = 0·7.

Donor‐Type Tm Centres in ZnS Crystals

Abstract4f—4f excitation and emission spectra involving various intermanifold transitions are determined for two Tm3+ centres in ZnS crystals by dye‐laser site‐selective measurements at helium temperatures. A crystal‐field analysis together with polarization data indicates that in both cases Tm is located on lattice sites, probably Zn sites. One of the sites (D3) is thought to be in „unperturbed”︁ environment, whereas in the second case (D2) the four nearest‐neighbour atoms are probably replaced by impurity atoms. The excitation and deexcitation mechanisms of the blue 1G4 → 3H6 emission of these centres are studied by site‐selective broad‐band excitation spectra and kinetical measurements. These studies demonstrate that donor‐acceptor pairs involving the two Tm centres as the donors play an important role in the excitation process. Estimates are given of the relevant pair to 4f state transfer rates.

Local density total energy description of ground and excited state properties of the rare earth metals

Ground and excited state properties of rare-earth metals are investigated by means of total energy calculations in the local density approximation using the linearized muffin-tin orbital band structure method. Equilibrium lattice constants, bulk moduli and cohesive energies obtained for the rare-earth series are found to be in good agreement with experimental trends. The localization of the 4f electrons in the rare-earths is discussed and compared with that of the 5f electrons in the actinides. The relation between the structural sequence observed in the rare-earths and d-band occupancy is examined. The 4f excitation energies and intra-atomic Coulomb correlations—obtained by taking the difference in total energy of the appropriate final states—are compared with the results of photoemission and inverse photoemission experiments. A comparison of the localization of the 4f electrons in the rare-earths with that of the 5f electrons in the actinides is presented using additional results obtained for the actinide metals with the same approach.

Local-density total-energy supercell description of excited-state properties of solids: Ce photoemission and inverse photoemission spectra.

A total-energy theoretical description of peak positions in the experimental photoemission and inverse photoemission spectra of Ce is presented. The linearized muffin-tin-orbital method and local-density theory are employed to calculate the electronic structure and total energy of the excited ''impurity'' atom inside a supercell. Both 4f valence-band spectra and 3d core-hole spectra are determined. The energy positions of the 4f excitations are derived from the total energy of appropriate final-state configurations with a localized hole or localized electron picture. The 3d core-hole and 4f valence-band excitations are compared with x-ray-photoemission experiments and inverse photoemission experiments. The results show that the peak positions in these spectra can be explained by ordinary screening effects, i.e., the availability of distinct groups of (metastable) final states for the photoemission process.

Site Selective Spectroscopy of ZnS:Tm3+ Crystals

AbstractThe method of site selective excitation spectroscopy is applied to Tm3+ ions in ZnS crystals containing various codopants. By dye‐laser 4f–4f excitation at helium and neon temperatures it is possible to separate three different sites, for which 4f–4f emission, excitation, and absorption spectra are determined in the range of several free‐ion transitions. Energy spectra are deduced from these data, and some considerations are made about the nature of the sites observed.

Core level excitations in the lanthanides by electron energy loss spectroscopy. I. 4d and 3d excitations

Core electron energy loss spectroscopy in reflection geometry is applied to 4d and 3d excitations in the lanthanide series La-Lu. In 4d to 4f excitation electric dipole forbidden transitions are clearly identified in the lower half of the series, while the heavy lanthanides tend to show enhancement of already dipole-allowed transitions, to states of high total angular momentum, if the exciting primary electron energy is lowered towards excitation threshold. Even stronger non-dipole effects appear for 3d to 4f transitions, but individual multiplets are not clearly resolved due to reduced d-f exchange coupling.