Time-resolved photoemission study of the electronic structure and dynamics of chemisorbed alkali atoms on Ru(0001)

We investigate the electronic structure and photoexcitation dynamics of alkali atoms (Rb and Cs) chemisorbed on transition-metal Ru(0001) single-crystal surface by angle- and time-resolved multiphoton photoemission. Three- and four-photon photoemission (3PP and 4PP) spectroscopic features due to the $\ensuremath{\sigma}$ and $\ensuremath{\pi}$ resonances arising from the $ns$ and $np$ states of free alkali atoms are observed from $\ensuremath{\sim}2$ eV below the vacuum level in the zero-coverage limit. As the alkali coverage is increased to a maximum of 0.02 monolayers, the resonances are stabilized by formation of a surface dipole layer, but in contrast to alkali chemisorption on noble metals, both resonances form dispersive bands with nearly free-electron mass. Density functional theory calculations attribute the band formation to substrate-mediated interaction involving hybridization with the unoccupied $d$ bands of the substrate. Time-resolved measurements quantify the phase and population relaxation times in the three-photon photoemission (3PP) process via the $\ensuremath{\sigma}$ and $\ensuremath{\pi}$ resonances. Differences between alkali-atom chemisorption on noble and transition metals are discussed.