Optical uv-visible-near-ir spectroscopy

Electromagnetic radiation in the visible and near-ir and uv regions of the spectrum, particularly with energies between approximately 33,000 and 4,000 cm (3,000-25,000 A, 4-0.5 eV) interacts with the eleetrons in solids (or liquids and gases). These interactions give rise to the processes of absorption and reflection qualitatively observed in minerals as the properties of color and luster. The quantitative measurement of such absorption and reflection processes forms the basis of eleetronic (optieal) absorption spectroscopy. The absorption and reflection phenomena arise from electronic excitation processes involving the valence electrons, excitation processes that may be of several kinds  

Crystal-field (or d-d) transitions. Splitting of the J-orbital energy levels of a transition-metal ion by the crystal (or ligand) field of the surrounding anions gives rise to the possibility of electronic transitions between these levels. Such d-d transitions are responsible for the colors of many transition-metal-bearing minerals and are best treated within the formalism of crystal-field theory. 

Charge-transfer transitions. Electrons may be transferred between filled (or partly filled) orbitals and empty orbitals on adjacent anion (or ligand) and cation (commonly a metal) or between adjacent cations (usually metals). Such ligand-metal and metal-metal charge-transfer transitions are best treated within the formalism of molecular-orbital theory. 

---