First-principles studies of adsorption of CO on the Na(100) surface

We have investigated the interaction of CO with the Na(100) substrate in the framework of density functional theory formulated for periodic systems. We established that there is an attractive but weak chemisorption bond between CO and Na(100). The binding energy of the CO molecule at an on-top site and adsorbed normal to the surface with the carbon end down is ∼0.25 eV for the Perdew–Burke–Enzerhof (PBE96) exchange correlation functional. The structural, electronic and vibrational properties indicate weakening of the intramolecular CO bond by adsorption on Na(100). The occupied orbitals of CO are generally shifted up in energy upon adsorption, with exception of the highest occupied orbital 5σ. The adsorbate–surface interaction is primarily characterized by: (i) partial charge transfer from the occupied 3sp band of Na to the CO antibonding 2π∗ orbital and (ii) the interaction between the 5σ orbital of CO and the metal states.