Local structure of <i>c</i> (2×2)-Na on Al(001): Experimental evidence for the coexistence of intermixing and on-surface adsorption

Angular distributions of x-ray-excited Na 1<i>s</i> photoelectrons and Na <i>KVV</i> Auger electrons emitted from <i>c</i> (2×2)-Na/Al(001) have been measured over the full 2π solid angle above the Al(001) surface. A detailed study involving single-scattering cluster calculations and an R-factor analysis has been performed to determine the room-temperature adsorption site of Na on Al(001). The best agreement with experiment is obtained for a configuration in which domains of Na atoms adsorbed in the hollow site and in the substitutional site coexist on the surface. This unusual and unexpected behavior has not been observed for an alkali-metal-on-metal adsorption system up to now, but it can be understood on the basis of very recent results from first-principles density-functional theory [C. Stampfl, J. Neugebauer, and M. Scheffler, Surf. Rev. Lett. <b>1</b>, 222 (1994)].