A NEW SPECTROSCOPIC DETERMINATION OF EMPTY F-LEVELS

We use magneto-optical techniques to determine empty f levels by makin g an optical transition in a spin-polarized sample by a strong externa l magnetic field. In a La compound, e.g. LaSe in a field of 10 T at 1. 5 K, we observe the optical transition from the occupied and spin pola rized 5d states into the empty and spin-orbit split 4f(1) state. In th e magneto-optical spectrum of LaSe we find two sharp lines at 2.77 and 3.01 eV, the stronger one having +2.1 degrees Kerr rotation. The sign of the Kerr rotation is inverted between CeSe (occupied 4f(1)) and La Se (empty 4f(1)) permitting a distinction between transitions into emp ty or from occupied f states. (C) 1998 Elsevier Science B.V. All right s reserved.