High-Resolution X-ray Emission and X-ray Absorption Spectroscopy

In this review, high-resolution X-ray emission and X-ray absorption spectroscopy will be discussed. The focus is on the 3d transition-metal systems. To understand high-resolution X-ray emission and reso-nant X-ray emission, it is first necessary to spend some time discussing the X-ray absorption process. Section II discusses 1s X-ray absorption, i.e., the K edges, and section III deals with 2p X-ray absorption, the L edges. X-ray emission is discussed in, respectively, the L edges. X-ray emission is discussed in, respec-tively, and section V on 2p3s and 2p3d X-ray emission. Section VI focuses on magnetic dichroism effects, and in section VII selective X-ray absorption experiments are discussed. To limit the scope of this review paper, many related topics (for example, EELS, XPS, and resonant photoemission, phonon-oriented inelastic X-ray scat-tering, and X-ray microscopy) will not be discussed. In addition, many aspects of X-ray absorption, such as reflection experiments, diffraction absorption fine structure, and related experiments, will remain untouched. EXAFS will be discussed very briefly, and its X-ray emission analogue EXEFS 69,71 will not be discussed

Multiplet effects in Resonant X-ray Emission

After a short discussion of all conventional core level spectroscopies within the single particle model, the effects of the coupling of the core and valence wave function on the x-ray emission spectral shapes is discussed. It will be shown that these so-called multiplet effects strongly affect all x-ray emission spectra taken around the metal 2p resonances. In case of 1s resonances, valence band x-ray emission is not affected, but the spectral shapes of 1s2p and 1s3p x-ray emission can only be sensibly described with the inclusion of multiplets. A special example is the resonant excitation into the pre-edge region, which gives rise to a quadrupole resonance

3s2p inelastic x-ray scattering of CaF 2

In this paper the 3s2p inelastic x-ray scattering (IXS) cross sections of CaF 2 are analyzed using a ligand-field multiplet approach. All dipole matrix elements necessary for the IXS cross section are calculated and it is shown that interference effects are not negligible. The important possibility to map out a specific character of the 2p^5 3d^1 intermediate states is explained. The experimental results of Rubensson et al. are reproduced with the ligand field multiplet model and the Kramers-Heisenberg equation of inelastic x-ray scattering. The values found for the ligand-field splitting (20.91 eV) and the 3s3d exchange splitting (1.2 eV) differ from the results fitted to experiment

Local spin-flip spectral distribution obtained by resonant x-ray Raman scattering

We show that resonant x-ray Raman (RXR) scattering can be used to study the local spin-flip excitation spectral distribution in magnetically ordered 3d transition-metal compounds. We demonstrate with realistic calculations on the 3p x-ray absorption edge of a Cu^2+ compound and the 2p edge of a Ni^2+ compound that the scattered x-ray energy and intensity distribution contains the excitation spectrum resulting from a single local spin flip, as well as dd excitations accompanied by local spin flip satellites. We develop the theory describing this effect and discuss the polarization conditions that can be used to observe these effects

2p3s3p, 2p3p3p, and 2p3s3s resonant Auger spectroscopy from NiO

We have investigated the behavior of the 2p3s3p, 2p3p3p, and 2p3s3s Auger lines of NiO, a model compound in the class of strongly correlated 3d systems, while varying the photon energy across the Ni L3 and L2 absorption edges. The experimental data are discussed in comparison with a theoretical model based on a charge-transfer multiplet approach. When the excitation energy is below the L3 resonance, we observe the 2p3p3p and 2p3s3p peaks at a constant binding energy. This behavior is typical of nonradiative resonant Raman scattering. If the photon energy is increased further, the 2p3p3p and 2p3s3p lines rapidly transform into constant kinetic energy features, showing a normal Auger behavior. The transition from Raman- to Auger-like behavior takes place for photon energies lower than the ones corresponding to excitations of the photoelectron into ligand-hole states. This might indicate the participation of inelastic processes in the recombination of the core hole involving energies much smaller than the NiO gap, or the possible presence of nonlocal effects. On the high photon energy side of the L3 edge, the constant kinetic energy of the 2p3p3p and 2p3s3p peaks is systematically larger than the one observed for an excitation well above the L2,3 edges. We attribute this behavior to the intervention of an intermediate state of 2p^5 3d^10 character, which has very little weight but is strongly enhanced at resonance

Magnetic circular dichroism in the 2p4d x-ray emission of EuO

Magnetic circular dichroism is observed for ferromagnetic EuO (T C 569 K) in the b2,15 fluorescence at incident photon energies resonant with the 2p 3/2 excitation. The dichroism is dominated by exchange interac-tion between the localized 4 f electrons and the final state 4d core hole. The spectral shapes obtained by atomic multiplet calculations for the 2p4d emission show good agreement with the experimental data

Spin-polarized x-ray emission of 3d transition-metal ions: A comparison via K a and K ß detection

This paper demonstrates that spin-polarized x-ray-excitation spectra can be obtained using K a emission as well as K ß lines. A spin-polarized analysis of K a x-ray emission and the excitation spectra by K a detection on a Ni compound is reported. A systematic analysis of the first-row transition-metal ions using the ligand-field multiplet calculation is presented for K a and K ß emission spectra

Influence of the core hole on Kß emission following photoionization or orbital electron capture: a comparison using MnO and 55Fe2O3

The Mn K ß fluorescence emission in MnO after photoionization and in "Fe 2 O 3 after radioactive electron capture decay from the K shell have been measured using a crystal array spectrometer with an instrumental energy bandwidth of 0.7 eV (full width at half maximum). Both compounds have a 3d 5 valence electron configuration in the ionic approximation. It is found that the spectral features after K capture in 55 Fe 2 O 3 are shifted in emission energy and are sharper, compared to the spectra following photoionization in MnO, i.e., the spectra exhibit a dependence on the mode of excitation. Crystal-field multiplet calculations including ligand-to- metal charge transfer have been carried out for the 1s intermediate states as well as for the 3p to 1s (K ß) radiative transition. The populated 1s intermediate states after photoionization are found to be spread over several eV. In comparison, only the lowest-lying 1s intermediate states split by the weak (1s,3d) exchange interaction are populated after K capture. It is proposed that the differences in population of the 1s intermediate states together with a term-dependent final-state lifetime broadening can account for the changes in the spectral shapes due to the different modes of excitation

High Resolution K Capture X-ray Fluorescence Spectroscopy: A New Tool for Chemical Characterization

The ability to probe specific chemical sites in complex systems would make X-ray spectroscopy a far more versatile spectroscopic tool. In vibrational and magnetic resonance spectroscopies, isotopic substitution is commonly employed to allow characterization of particular species. Except in a few special cases, such as gas-phase spectra of light elements, isotope effects are too small to be observed in X-ray absorption spectra. An alternative approach is to examine the X-ray emission that results after electron capture by a radioactive isotope.^1,2 Controlled introduction of electron-capture isotopes could result in specific labeling of chemically distinct sites. In this paper, we show that highresolution electron capture fluorescence spectra can be obtained on a reasonable time scale. Chemical shifts in these spectra can be used to identify elemental spin states, oxidation states, and even the types of neighboring atoms. In the electron-capture process an inner shell electron reacts with a nuclear proton to yield a neutron and a neutrino