THERMAL SHIFT IN THE BINDING-ENERGY OF BULK W4F7 2/

High-resolution photoelectron spectroscopy using synchrotron radiation is used to measure the binding energy of the W 4f(7/2) core level ass ociated with bulk atoms of a W(111) crystal in the temperature range f rom 80 to 780 K. As temperature increases the lattice expands and the binding energy of bulk W 4f(7/2) decreases. A total shift of -45 meV i s measured at 780 K with respect to the binding energy of bulk W 4f(7/ 2) at 80 K. The mechanism responsible for this shift can be understood quantitatively in terms of a Friedel model, in which the narrowing of the d band as well as the shift of the center of the d band relative to the bottom of the conduction band due to the lattice expansion play dominant roles in shifting the binding energy toward lower energies.