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Metal L-edge spectroscopy is a spectroscopic technique used to study the electronic structures of transition metal atoms and complexes. This method measures X-ray absorption caused by the excitation of a metal 2p electron to unfilled d orbitals (e.g. 3d for first-row transition metals), which creates a characteristic absorption peak called the L-edge. According to the selection rules, the transition is formally electric-dipole allowed, which not only makes it more intense than an electric-dipole forbidden metal K pre-edge (1s → 3d) transition,〔T. E. Westre, P. Kennepohl, J. G. DeWitt, B. Hedman, K. O. Hodgson, E. I. Solomon. "A Multiplet Analysis of Fe K-Edge 1s → 3d Pre-Edge Features of Iron Complexes" J. Am. Chem. Soc. 1997, ''119'', pp. 6297-6314. http://pubs.acs.org/doi/abs/10.1021/ja964352a〕 but also makes it more feature-rich as the lower required energy (~400-1000 eV from scandium to copper) results in a higher-resolution experiment.〔S. P. Cramer, F. M. F. deGroot, Y. Ma, C. T. Chen, F. Sette, C. A. Kipke, D. M. Eichhorn, M. K. Chan, W. H. Armstrong, E. Libby, G. Christou, S. Brooker, V. Mckee, O. C. Mullins, J. C. Fuggle. "Ligand field strengths and oxidation states from manganese L-edge spectroscopy" J. Am. Chem. Soc. 1991, ''113'', pp. 7937-7940. http://pubs.acs.org/doi/abs/10.1021/ja00021a018〕 In the simplest case, that of a cupric (CuII) complex, the 2p → 3d transition produces a 2p53d10 final state. The 2p5 core hole created in the transition has an orbital angular momentum L=1 which then couples to the spin angular momentum S=1/2 to produce J=3/2 and J=1/2 final states. These states are directly observable in the L-edge spectrum as the two main peaks (Figure 1). The peak at lower energy (~930 eV) has the greatest intensity and is called the L3-edge, while the peak at higher energy (~950 eV) has less intensity and is called the L2-edge. == Spectral components == As we move left across the periodic table (e.g. from copper to iron), we create additional holes in the metal 3d orbitals. For example, a low-spin ferric (FeIII) system in an octahedral environment has a ground state of (''t2g'')5(''eg'')0 resulting in transitions to the ''t2g'' (dπ) and ''eg'' (dσ) sets. Therefore, there are two possible final states: ''t2g''6''eg''0 or ''t2g''5''eg''1(Figure 2a). Since the ground-state metal configuration has four holes in the ''eg'' orbital set and one hole in the ''t2g'' orbital set, an intensity ratio of 4:1 might be expected (Figure 2b). However, this model does not take into account covalent bonding and, indeed, an intensity ratio of 4:1 is not observed in the spectrum. In the case of iron, the d6 excited state will further split in energy due to d-d electron repulsion (Figure 2c). This splitting is given by the right-hand (high-field) side of the d6 Tanabe-Sugano diagram and can be mapped onto a theoretical simulation of a L-edge spectrum (Figure 2d). Other factors such as p-d electron repulsion and spin-orbit coupling of the 2p and 3d electrons must also be considered to fully simulate the data. For a ferric system, all of these effects result in 252 initial states and 1260 possible final states that together will comprise the final L-edge spectrum (Figure 2e). Despite all of these possible states, it has been established that in a low-spin ferric system, the lowest energy peak is due to a transition to the ''t2g'' hole and the more intense and higher energy (~3.5 eV) peak is to that of the unoccupied ''eg'' orbitals.〔E. C. Wasinger, F. M. F. de Groot, B. Hedman, K. O. Hodgson, E. I. Solomon. "L-edge X-ray Absorption Spectroscopy of Non-Heme Iron Sites: Experimental Determination of Differential Orbital Covalency" J. Am. Chem. Soc. 2003, ''125'', pp. 12894–12906. http://pubs.acs.org/doi/abs/10.1021/ja034634s〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Metal L-edge」の詳細全文を読む スポンサード リンク
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