Electronic and structural properties of ultra-thin rare earth overlayers.

Photoelectron spectroscopy, low energy electron diffraction (LEED), Auger electron spectroscopy (AES), secondary electron emission crystal current (SEECC) and work function change measurements have been used to correlate the electronic and geometric structures of well characterised overlayers of Tm and Gd on Mo(110) and Ag(001). Tm deposited on Mo(110) is shown to form two ordered structures with (10x2) and c(8x4) symmetry before a hexagonal monolayer is formed. Mixed valence, caused by low co-ordinated adatoms, has been observed for sub-monolayer coverages of this system at room temperature. Tm alloys with Ag(001) at room temperature and above, and is found to be trivalent. At 130K the diffusion is inhibited and low co-ordinated atoms are again found to be the cause of mixed valence. Resonant photoemission measurements of thick, mixed valent Tm layers at the 4d-4f absorption threshold indicate that the 4d hole, caused by three strong absorption processes, decays in all cases by direct recombination. Strong enhancement of the trivalent 5s and 5p photoemission are noted for the lowest energy absorption peak and the second two peaks result in enhanced trivalent 4f features. There is also evidence of divalent 4f level enhancement. Gd grows on Mo(110) in a layer by layer mode, and forms six different sub-monolayer structures. The symmetry of the overlayers is similar to Gd/W(110) but with an extra (4x2) phase occurring. The initial growth is characterised by a constant ordering in the [001] direction and gradual contraction in the [110] direction, before isotropic growth of hexagonally based structures. The 3d-4f absorption process for thick Gd layers, reveals two strong peaks due to absorption from the 3d3/2 and 3d5/2 electron energy levels. The 3d hole decays by direct recombination leading to enhanced photoemission features, although a significant Auger intensity is observed on the high energy sides of the absorption peaks.