Unoccupied-Band Narrowing in Na and its Effect on the X-Ray-AbsorptionKEdge

The first Na $K$-edge absorption measurements are reported. The data reveal a narrowing of the unfilled density of states analogous to that observed in a recent photoemission study of the conduction band and in very good agreement with new band-structure calculations. The narrowing is found to be essential for obtaining compatibility among the $K$-edge threshold exponent, the $L_{2,3}$-edge exponent, and the singularity index in core-level photoemission.