Lanthanide ions electronic spectra

Europium, like all other rare-earth elements, generally forms compounds in which the metal atoms lose three electrons to become ions with three positive charges. This type of europium ion can emit light in the richest red part of the visible spectrum, when suitably stimulated by an energy source. But, unlike the other lanthanides (with the exception of samarium), europium also... 

Most lanthanide ions absorb electromagnetic radiation, particularly in the visible region of the spectrum, exciting the ion from its ground state to a higher electronic state, as a consequence of the partly filled 4f subshell. The f-f transitions are excited both by magnetic dipole and electric dipole radiation. Normally the magnetic dipole transitions would not... 

Question 7.5 Why are yttrium compounds good host materials for heavier Ln + ions Answer 7.5 Because Y + has no f (or d electrons), its compounds have no absorptions in the visible region of the spectrum and do not affect the magnetic properties of any added lanthanide ions. Because its ionic radius is similar to that of Ho +, it can accommodate several of the later lanthanide ions with minimal distortion. 

Absorption and Fluorescence Spectra. The absorption spectra of actinide and lanthanide ions in aqueous solution and in crystalline form contain narrow bands in the visible, near-ultraviolet, and near-infrared regions of the spectrum (13,14,17,24). Much evidence indicates that these bands arise from electronic transitions within the 4f and 5/shells in which the 4f and bf configurations are preserved in the upper and lower states for a particular ion. 

The crystal-field Hamiltonian, Hcef, must be invariant under all operations belonging to the point-symmetry group of the lanthanide-ion site. This symmetry is usually determined by X-ray and neutron-diffraction studies of the host crystal, which yield the space group of the entire crystal, the lattice constants, and the positions of all the constituents of the crystal within a unit cell. If X-ray or neutron-diffraction experiments have not been performed, the symmetry of the lanthanide-ion sites can be determined (although not with the same level of confidence) by electron paramagnetic resonance studies or by the optical spectrum itself. This symmetry must be known or assumed before the crystal-field calculation is initiated. 

It might be wondered how N can be considered an integer at all for an ion in a crystal, where inter-penetrating electronic orbitals are a commonplace. The answer is that the exact superposition of a multitude of doublets, each one being provided by an identical lanthanide ion for which N is odd, becomes smeared into a narrow band. Its width is usually small compared to fiH, and so the effect on the optical spectrum is negligible. 

The lanthanide ion-ligand interaction undergoes changes at thermal excitations of a crystal as well. When the temperature rises, the population of the excited sublevels produces anomalies in the thermal expansion of crystals. Redistribution of electron density in a lanthanide crystal results in significant changes in the vibrational spectrum of a lattice, when variations in temperature occur. This is particularly evident for the anomalous temperature behaviour of elastic constants. 

Spectral bands of an aquated lanthanide ion arising from vibronic contributions were reported first by Haas and Stein (1971) in their study of the emission spectrum of aquated Gd. These bands are termed vibronic because they arise from a simultaneous change in the electronic state of the metal ion and the vibrational state of a coordinated ligand. Stavola et al. (1981) noted additional examples of such bands and presented a theoretical model based on the importance of electronic factors for calculating the intensities of lanthanide-ion vibronic transitions. Their theoretical model also predicts selection rules for such transitions. The intensities of observed bands assigned by these workers as being vibronic typically were at least 50 times weaker than the parent purely electronic band. Faulkner and Richardson (1979) have... 

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