Excited 4f Research Articles

Core electron energy loss spectroscopy of cerium: Surface oxidation

Core electron excitations of clean Ce and of Ce exposed to varying amounts of oxygen and H2O have been studied by electron energy loss spectroscopy (ELS) in reflection geometry. The valence change of Ce atoms during surface oxidation with oxygen is well reflected in ELS of core levels. 4d→4f excitations provide the best spectral fingerprint of the valence state, but 3d→4f and 4p→5d excitations also show spectral modifications which can be correlated to the change in f count. The spectral profiles of core level excitations of a CeO2-like oxide phase are discussed in terms of screening effects on the core hole–excited electron final states.

Ultraviolet photoemission and 4d core reflectance spectra of SmSe(100)

Ultraviolet photoemission (UPS) and inner core reflectance spectra have been measured on SmSe(100) surface with synchrotron radiation. Besides the multiplet structures observed in UPS spectra for the Sm 4f, 5p and 5s excitations, the reflectance spectrum has revealed a multiplet structure associated with the Sm 4d→4f excitation. Each band of the UPS spectra has shown remarkably different hν dependence and the resonant behavior of the cross section. Sm 4f- and Se 4p-derived valence band features are unambiguously identified by the angle resolved UPS.

4d to 4f excitations of Ce intermetallics in CEELS

Core electron energy loss spectra (CEELS) of 4d to 4f excitations are presented for Ce intermetallic compounds. The spectral fine structures of 4d94fn+1 final states show features which can be correlated to the f-count as derived from other core level spectroscopies. Comparison with 4d CEELS of gamma Ce and CeO2 reveals, however, that the detailed spectral shapes of 4d excitations in Ce intermetallics differ from those expected from a simple superposition of f1 and f2 final state configurations.

The position of the 4f level in Ce metal

The available spectroscopic data for Ce and its compounds are evaluated and it is concluded that in the groundstate the Ce 4f level is situated probably notmore than 1 eV below the Fermi energy. This does not exclude that with photoelectron spectroscopy an excited 4f hole state can be observed 2 eV or more below the Fermi energyEF.

Direct observation of localized impurity excited states degenerate with conduction band (CdF 2: Eu)

On the basis of the coupled absorption, conductivity, thermoluminescence and ESR photoquenching experiments it is shown that all the excited 4f 6 5d states of Eu 2+ are degenerate with the conduction band of CdF 2. This is a unique property of CdF 2 among other flourites.

Lifetime- and quantumbeat measurements in the excited 4f 14 6s6d D-states in the Yb I-spectrum using stepwise laser excitation

Time resolved lifetime- and quantumbeat-measurements have been performed to determine lifetimes and gJ-values in the D-states of the 4f 14 6s6d configuration in the Yb I-spectrum using stepwise laser excitation. The results are: τ( 1D 2) = 35.2 (10) ns , τ( 3D 1) = 22.7 (7) ns , τ( 3D 2) = 24.2 (12) ns , τ( 3D 3) = 30.8 (60) ns , gJ( 1D 2 = 1.020 (7) , gJ( 3D 1) = 0.500 (4) , gJ( 3D 2) = 1.163(7) .

Study of the 4f and valence band density of states in rare-earth metals. II. Experiment and results

For pt.I see ibid., vol.11, p.113 (1980). The 4f and valence states of all metallic rare earths have been studied using X-ray photoelectron spectroscopy (XPS) for the occupied part and bremsstrahlung isochromat spectroscopy (BIS) for the unoccupied part. It is found that the bandwidth increases from Gd to Lu, and that the valence band spectra are in fair agreement with APW calculations. The intensities of the 4f final-state multiplets are well described in terms of the coefficients of fractional parentage. There is a symmetry between the fn XPS and f14-n BIS spectra. The observed energies of the 4f excitations correspond to transitions to completely screened final states. These values enable one to predict the elements which are liable to interconfiguration fluctuation when their 4f levels are shifted to EF by the chemical environment or by compression. The large Coulomb correlation energies, U, which prevent the formation of 4f bands in these elements, are directly obtained from the authors' spectra, and are found to be in good agreement with recent calculations. The linewidths and singularity indices of the XPS and BIS 4f lines are determined and discussed.

Two-photon spectroscopy of ytterbium

Two-photon laser spectra of the Yb vapor have been obtained. Transitions to highly excited 4f 14 6s ns 1S 0 and 4f 14 6s nd 1D 2 states are seen in direct two-photon excitation. Hybrid resonances involving 4f 14 6s6p 1P 0 1 and 4f 14 5d6s 3D 2 intermediate states lead to transitions to 4f 14 6s ns 1S 0, 4f 14 6s np 3P 0 2,1 and 4f 14 6s nd 1D 2 levels.

Surface-Atom Core Level Shift in Gold

The line shape of the 4f7/2 level of gold obtained by high-resolution X-ray photoemission spectroscopy has been measured and analysed. The low binding energy side of this line shows a quite exceptional shape at variance with the Doniach-Sunjic (SD) analytical expression accounting for most core level line shape in metals. The surface sensitivity was enhanced by varying the take-off angle θ of the electrons. A shoulder then appears, revealing the presence of a line shifted by 0.4 eV and corresponding to the 4f excitation in surface atoms. The decrease of the binding energy can be interpreted as a surface-atom core level shift (SCS). The bulk and surface atom components can be accurately described by two identical DS functions; their energy position is remarkably constant, while their intensity ratio varies exceptedly with θ. A narrowing and a shift to lower binding energy is also found for the surface contribution to the valence band photoemission, indicating that the origin of the SCS is an increased localization of the valence band states.

Vacuum ultraviolet magneto-optical studies of ferromagnetic metals using synchrotron radiation

Magneto-optical spectroscopy of ferromagnetic metals is extended beyond the quartz limit using synchrotron radiation. Real and imaginary parts of the off-diagonal elements of the conductivity tensor are obtained from transverse Kerr effect measurements and optical constants. The magneto-optical spectra for rare-earth metals along with recent vacuum ultraviolet optical data show evidence of 4f excitations, and predict a 4f optical threshold for Gd at 6.1 eV. The magneto-optical spectra of transition metal ferromagnets exhibit primary features resulting from the joint density of states. Structure in the magneto-optical spectra of Ni confirms corresponding peaks observed in recent vacuum ultraviolet optical studies, and assigns the structure to threshold minority spin transitions originating from near the Fermi level. This interpretation agrees with previous conclusions based on infrared magneto-optical studies, and supports the Stoner model description of the magnetic ground state of ferromagnetic Ni.

Line emission of SrBe 2Si 2O 7:Eu 2+ and BaBe 2Si 2O 7:Eu 2+

The compounds SrBe 2Si 2O 7 and BaBe 2Si 2O 7 both have the barylite structure. With 254 nm excitation, the Eu 2+-activated compounds give UV emission peaking at 360 nm (Sr) and at 375 nm (Ba). Maximum quantum efficiencies of 40% (Sr) and 65% (Ba) were measured. The emission consists of a 5d-4f band emission as well as 4f-4f line emission, in contrast to many other Eu 2+-activated oxides which generally show only 5d-4f band emission. At 77°K, both compounds show only the 4f-4f line emission peaking at 360 nm. At higher temperatures, 5d-4f band emission shows up at the cost of the line emission. A thermal equilibrium is assumed between the lowest excited 5d and 4f levels. The energy difference between these levels, calculated from the variation in the line-band intensity ratio with temperature, was computed to be 0.15 eV (Sr) and 0.09 eV (Ba). The occurrence of the line emission in the barylites is correlated with the weakness of the crystal field at the Eu 2+ ions and with the high quenching temperature of the 5d-4f band emission.