Metal L-edge

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, 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.

In the simplest case, that of a cupric (Cu^II) 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.

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 (Fe^III) system in an octahedral environment has a ground state of (t_2g)⁵(e_g)⁰ resulting in transitions to the t_2g (dπ) and e_g (dσ) sets. Therefore, there are two possible final states: t_2g⁶e_g⁰ or t_2g⁵e_g¹(Figure 2a). Since the ground-state metal configuration has four holes in the e_g orbital set and one hole in the t_2g 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.

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