Systematics of4felectron energies relative to host bands by resonant photoemission of rare-earth ions in aluminum garnets

Abstract

The energies of trivalent rare earth ions relative to the host valence band were measured for a series of rare-earth-doped yttrium aluminum garnets, RxY3-xAl5O12 (R=Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and 0<x<3), using ultraviolet photoemission spectroscopy. The 4f photoemission spectra were acquired using synchrotron radiation, exploiting the 4d to 4f “giant resonance” in the 4f electron photoemission cross-section to separate the 4f contribution. Theoretical valence band and 4f photoemission spectra were fit to experimental results to accurately determine electron energies. The measured 4f n ground state energies of these ions range from 700 meV above the valence band maximum for Tb 3+ to 4.7 eV below the valence band maximum for Lu 3+ , and all ground state energies, except for Tb 3+ , are degenerate with valence band states. An empirical model is successful in describing the relative energies of the 4f n ground states for rare earth ions in these materials. This model is used to estimate the positions of the lighter rare earth ions, giving good agreement with published excited state absorption and photoconductivity measurements on Ce 3+ in yttrium aluminum garnet. It is shown that the energies of the 4f electrons relative to the valence band can be estimated from the photoemission spectrum of the undoped host, providing a simple method for extending these results to related host crystals. The success of this model suggests that further studies of additional host compounds will rapidly lead to a broader picture of the effect of the host lattice on the 4f electron binding energies.