Term symbols and electronic spectroscopy

Atomic and ionic energy levels are characterized by a term symbol of the general form 25+1L/. The values of S, L, and J of lanthanide ions Ln3+ in the ground state can be deduced from the arrangement of the electrons in the 4f subshell, which are determined by Hund s rules and listed in Table 18.1.4. 

Three types of electronic transition can occur for lanthanide compounds. These are f- -f transition, nf - (n + l)d transition and ligand - metal f charge-transfer transition. 

In Ln3+ ions, the 4f orbitals are radially much more contracted than the d orbitals of transition metals, to the extent that the filled 5s and 5p orbitals largely shield the 4f electrons from the ligands. The result is that vibronic coupling is much weaker in Ln3+ compounds than in transition-metal compounds, and hence the intensities of electronic transitions are much lower. As many of 

Ln3+ 4f electronic Ground state Color of Ln3+ configuration term symbol Magnetic moment, / (298 K)//tB Calculated Observed  

The Ln2+ ions are often highly colored. This arises because the 4f orbitals in Ln2+ are destabilized with respect to those in Ln3+, and hence lie closer in energy to the 5d orbitals. This change in orbital energy separation causes the f- -d transitions to shift from the ultraviolet into the visible region of the spectrum. 

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