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 1For an element with atomic number Z, the 1s vacancy that is produced by K-capture is similar to that created in K-edge X-ray absorption, except that the nucleus now has charge Z -1. Just as with X-ray excited emission, the core hole is subsequently filled by a higher level electron, and the extra energy is released by emission of an Auger electron or X-ray fluorescence. KR X-ray fluorescence results from 2p f 1s transitions, while K X-ray fluorescence results when the 1s core hole is filled from orbitals with 3p or 4p character. K X-ray fluorescence is often split by a 3p-3d exchange interaction into a strong K 1,3 region and a weaker K ′ satellite. 3 Chemical shifts in Mn K 1,3 lines 4 have been used to record siteselective EXAFS of different Mn oxidation states in mixed valence complexes 5 and to identify the mixtures of Mn oxidation states in photosystem II. 6 The K 2,5 region has also been shown to shift with oxidation state. 7 K-capture spectra for 55 Fe metal and 55 Fe 2 O 3 are compared with X-ray excited K emission spectra for Mn metal and MnO in Figure 1. 8 The measurements 9 were done at NSLS beamline X-25 10 and SSRL beamline 10-2 11 using a crystal array spectrometer. 12,13 The Mn metal spectrum exhibits a K 1,3 peak at 6490.6 eV, and it has a broad, structureless tail extending ∼20 eV to lower energy. The MnO spectrum has a K 1,3 peak shifted 1.4 eV to higher energy, and a K ′ maximum at 6477 eV. Similar spectra have been reported for other Mn(II) complexes. 4 The large K 1,3 -K ′ splitting for Mn(II) is attributed to a strong 3p-3d exchange interaction for high-spin 3d 5 materials. 3 The 55 Fe metal K capture spectrum resembles that of the X-ray excited Mn foil, and we find that the K 1,3 peaks are located at the same energy, in contrast to a previous report that found a 0.6 eV shift. 14 The capture spectrum K 1,3 peak is measurably sharper (fwhm ≈ 3.0 vs 3.6 eV). One essential difference between the two modes of excitation involves the effect of the core ...
