Hello, I am new to X-ray Absorption Spectroscopy, and I have a question about XAFS, more specifically, Time-Resolved XANES. Question: I don't understand why M5 edges are usually not used for Lanthanides. Here's some background to why I came up with this question (please read if you have some time, otherwise skip because it's long): Lanthanides have inner 4f orbitals that are shielded by outer 5d, 6s, and 6p orbitals. Thus, 4f orbitals do not involve in bonding, and 4f-4f transitions (which is parity forbidden) should not change the structure relative to the ground state. Recently, I read this article (http://pubs.acs.org/doi/abs/10.1021/ja407924m), which is the first direct observation of change in electronic structure between ground state and 4f-4f excited state using Time-Resolved XANES. They used L3-edge which is mainly 2p → 5d transition of Eu(IIII) ion. Because theoretically, 4f-4f transition involves mixing of 5d orbitals that makes 4f-4f transition partially allowed, the difference of L3-edge XANES between ground and 4f-excited state would be slightly different. They also argue that pre-edge (which is 2p → 4f electric dipole forbidden) is slightly different. Now, I think understand why they used L3-edge and not L2, L1, nor K1. I interpreted this as because K1 edge and L1 edge is electron dipole-forbidden for transitions to 5d orbitals, and L3 has twice the absorption strength because of degeneracy compared to L2 edge. That made me wonder. If you want to observe electronic structural changes in 4f orbitals, then why not use M5 edge (5p J = 5/2) or at least M4 edge? 5p → 4f should be electric dipole allowed, That should allow direct observation of 4f orbital.