Resonance photoemission

Resonance photoemission measurements have been recently made for U metal , and show indeed a resonant enhancement of the satelUte at 2.3 eV only for the threshold energy (5 A i2. hv = 94 eV) (Fig. 15). In addition the main peak at Ep shows the expected off-resonance behaviour. Further support for such an interpretation of the satellite is given by the analysis of the photon excited Auger emission. This is shown to be composed of two different bands also separated by 2.3 eV and due to the two screening channels by 5 f or 6 d states ... 

Other resonant photoemission studies for uranium compounds show, regardless of the degree of localization, that suppression (off-resonance) and enhancement (on-resonance) of 5 f emission are always found for hv = 92 and 98 eV, respectively. On the other hand, localized 5f derived structures have been identified at 0.7, 0.9, 1.0 and 1.5 eV for USb, UTe, UPds and UO2 respectively i.e. for compounds in which 5 fs differ considerably with respect to localization. 

The other possible assignment of the 2.3 eV structure to 6d excitation (line II, Ref. 56, 66) cannot be ruled out. Resonant photoemission on Ce compounds ° shows indeed that a resonant enhancement at the 4 d 4 f threshold is not only present for 4f but also for 5 d emission. Thus an easy identification of 4f emission is not possible. Theoretical calculations of the cross section due to resonant enhancement result in a 2 eV shift to lower photon energy for the maximum 5d enhancement compared with the 4f enhancement ... 

In conclusion, the partial localization effects in the valence band spectra of the light actinides, although extremely important if convincingly verified, still need much experimental and theoretical investigation. It is expected that the situation will improve considerably when resonant photoemission studies become available for actinide compounds in which the 5 f and 6 d emissions do not overlap. In addition, theoretical calculations of 5 f and 6 d cross sections near the 5 d threshold will be very helpful. 

The 5 f character of the peak at 1.4 eV in UO2 is evidenced by varying the excitation energy in the UPS/XPS technique, as well as by other techniques such as ARPES and resonant photoemission (see Table 3). 

Struc- Eb eV) ture Occupied states Xps ) BIS ARPES ") UPS/XPS this laboratory" Resonant photoemission ... 

An attempt of identification of 5 f character has been recently reported for UO2 by resonant photoemission of the valence band region with excitation energies of 92 eV (off-resonance) and 98 eV (on-resonance). The core absorption edge used was the 5d ionization threshold, corresponding to a process ... 

Further evidence for strong mixing of Cu 3d and Cl 3p orbitals is obtained from the resonant photoemission data shown in Figure 22 for (CH NH, ) CuCl (31). In addition to the Cu 3d and Cl 3p... 

Figure 22. Resonant photoemission spectra of the valence band of D,-Cu(II)C1, as the photon energy is tuned through the Cu...

Kimura et al. (1995) reported on an investigation of the electronic structure of 7 3Au3Sb4 (R = La, Ce, Pr) by reflectivity and resonant photoemission spectra. The hybridization between the Ce4f state and the Sb5p state valence band was found to be weak as deduced from the resonant photoemission spectra of Ce3Au3Sb4. This result was found to be consistent with the electronic structure derived from an analysis of the optical data about the energy gap. 

Fig. S a Valence band spectra of Gd C82 (grey) and C82 (black) measured with Al Ka x-rays, b Symbols Gd 4f photoemission after subtraction of the empty C82 C 2s/2p spectrum. The vertical lines are individual components of atomic calculations for a 4f> multiplet, and the solid curve is their broadened sum. c Gd-N4>5 x-ray absorption spectrum (Gd 4d-4f excitations) of Gd C82. The complex lineshape comes from the widely spaced multiplet components resulting from the strong Coulomb interaction between the single hole in the 4d shell and the eight electrons present in the 4f shell in the x-ray absorption final state [see Fig. lc]. The arrows represent the two photon energies used for the data shown in panel d. d Resonant photoemission data of the valence band region of Gd C82 recorded off (hv=137 eV) and on (hv=149 eV) the Gd 4d-4f giant resonance...

Fig. 6 Main picture valence band photoemission spectra of Tm C82 recorded at photon energies across the Tm 4d-4f threshold. The inset shows the Tm-N4 5 x-ray absorption spectrum, indicating the choice of photon energies for the resonant photoemission experiment. The photon energies are (1) 169.4 eV, (2) 173.7 eV, (3) 177.8 eV and (4) 183.9 eV...

In the Tm case, resonant photoemission proves that there is no significant hybridisation between the rare-earth 5d levels and the carbon electronic states thus the Tm ions have an essentially ionic interaction with their fullerene host. In the Gd case, the magnitude of the resonant enhancement of emission from the 4f levels signals the presence of hybridisation between the Gd valence levels and those of the carbon cage. However, this does not alter the conclusion as regards the Gd s trivalency and the transfer of essentially three electrons to the fullerene MOs. 

Fig. 9 Resonant photoemission data from a film of Ce2 C72 recorded at photon energies crossing the Ce-N4 3 core-level excitation spectrum. The numbers refer to the energy positions as indicated in the right panel of Fig. 8 and are 1 94 eV 2 107.2 eV 3 110.2 eV 4 123 eV (at the maximum of the giant resonance, spectrum shown in black) 5 150 eV...

Now the question arises, as before, of the degree to which the rare earth s valence electrons (and in particular 5d) are populated by hybridisation with the electron-rich 7T-MO system of the C72 host molecule. Figure 9 shows resonant photoemission spectra of Ce2 C72, with photon energies selected to span the N4>5 core level excitation spectrum, as indicated in Fig. 8. 

In this case, the fact that a large resonant enhancement is seen of the states at around 1 eV is consistent with the behaviour of other dominantly trivalent Ce-based systems, as it is here that the 4f° photoemission final-state spectral weight occurs for systems with a 4f1 initial-state configuration. Thus, from the resonant photoemission data, we can confirm the trivalent nature of the Ce ions in Ce2 C72 and conclude that there is relatively weak hybridisation between the Ce 5d and the C 2s/2p states of the fullerene cage in this case. A shorthand notation for the cerium dimetallofullerene could thus be (Ce2)6+(C72)6. ... 

Resonant photoemission at the Ce 4d edge shows the Ce valence electrons to be practically fully ionised, leading to a shorthand for this molecule of... 

Sinkovic B, Tjeng LH, Brookes NB et al (1997) Local electronic and magnetic structure of Ni below and above Tc a spin-resolved circularly polarized resonant photoemission study. Phys Rev Lett 79 3510... 

Fig. 17. Resonant photoemission at 57.5 eV photon energy from WO3(001). Left hand panel surface with defect troughs, giving defect states predominantly deep in the bandgap at 2 eV. Right hand panel surface supporting large areas of the p>(lxl) reconstruction. There is now a stronger pe at the Fermi energy with a metallic density of states. Adapted from ref 275.

Finally, a subject of fundamental importance in atomic physics is the study of how electronic properties are modified by the atomic environment, as in molecules or in the solid state. New situations have been found at the frontier between atomic physics, molecular physics and the physics of condensed matter. This area has grown considerably with the discovery of giant resonances which, though atomic in origin, were first observed in the soft X-ray spectra of solids. Since then, resonant photoemission has become a well-established experimental technique in solid state physics, and valence fluctuations and intermediate valence effects in solids have been shown to involve localised orbitals which are partly atomic in character. 

The position of the 5f intensity in the valence band of UPt3 was studied by photoemission and found to be roughly the same as for a-U, which allowed conclusions to be drawn about the itinerant nature of the 5f states (Schneider and Laubschat 1981b). A narrow feature at EF (FWHM = 0.15 eV) was distinguished in high-resolution photoemission studies performed at low temperatures (Arko et al. 1984). A considerable f spectral weight at EF was observed in resonant photoemission studies which revealed that the 5f states extend down to 2 eV below EF (Parks et al. 1984, Allen et al. 1985). 

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