Unoccupied surface states on W(001) and Mo(001) by inverse photoemission

The W(001) and Mo(001) surfaces are investigated by inverse photoemission. At $\overline{\Gamma }$ we observe an unoccupied surface state just above the Fermi level. The angular dependence and chemisorption behavior identify it as a $d_{z^2}$ surface state similar to one observed and calculated just below $E_F$. For W(001) a second state is seen near $\overline{M}$ at 1.4 eV above $E_F$ and assigned to a ${\overline{\Sigma }}_2$ backbonding state. The implications of our data on the driving mechanism for the surface reconstruction are discussed using all-electron self-consistent local-density calculations.