Stabilisation of alkali-adsorbate-induced states on Cu(111) surfaces

The Li and Cs alkali-adsorbate-induced states on a model Cu(111) surface are studied theoretically using the coupled angular mode and the wave packet propagation methods. Their energy position and their decay rate via one-electron transfer into the metal are calculated. An unoccupied alkali-induced state is found with a rather long lifetime, much longer than those found in the case of free-electron metal surfaces. This is attributed to the combined effect of the hybridisation of the alkali levels and of the Cu(111) projected band gap along the normal to the surface that blocks the electron transfer between the adsorbate and the metal. This qualitatively explains the experimental finding of a long-lived state by Bauer et al. [Phys. Rev. B 55 (1997) 10 040] and Ogawa et al. [Phys. Rev. Lett. 82 (1999) 1931].