VALENCE-BAND PHOTOEMISSION-STUDY OF (BA, CS) ON W(110)

We present the first valence-band photoemission study of Ba and Cs on an atomically clean W(110) surface. The cleanness of the tungsten subs trate can be justified from both high surface 4f core emission and sha rp surface states in the valence-band spectra. Upon Ba (Cs) adsorption , the work function is lowered to 2.02 (1.76) eV, corresponding to one third of a monolayer. Further, the two symmetric high-lying surface s tates move little in energy but quench rapidly in intensity and dimini sh at the work-function minimum. On the other hand, the low-lying surf ace state at 1.41 eV shifts about 215 meV toward higher binding energy and reduces its emission at thicker coverage. The bonding of the Ba d orbital with the W d-like surface states is not found, in contrast to the theoretical calculation and angle-resolved inverse photoemission study of Ba on W(100). From the coverage-dependent interfacial electro nic structure and the parallel behaviors manifest in both Ba on W and Cs on W, the polarized-covalent model for the alkali-metal-substrate i nteractions is firmly reinforced.