Direct Transitions at Optical Frequencies in Rare-Earth Metals

Heavy rare earths in thin films depart from light rare-earth behavior by exhibiting two absorption maxima. One of those is located between 2.5 and 3.5 eV and exists in heavy rare earths only. An electronic transition between energy bands explains the presence of this maximum; plasma oscillations are not consistent with the data. The other absorption peak, located at 5.7 eV, is characteristic of all lanthanide elements. The 4f electrons are not responsible for it; a M2− → M4+ transition is more likely.