The adsorption of hydrogen on ruthenium (001): Adsorption states, dipole moments and kinetics of adsorption and desorption

The adsorption and desorption of hydrogen on the basal Ru(001) plane has been investigated by work function changes, thermal desorption and LEED. Two states possessing positive and negative dipole moment, respectively, and different sticking behaviour, can be distinguished which are roughly correlated to the two desorption peaks found in TPD. The coverage dependences of adsorption energy, desorption preexponential and sticking coefficients have been determined; they are characterized by repulsive lateral interactions. No ordered superstructures exist, but evidence of slight expansion of the top Ru layer upon adsorption has been found. The two states discerned may be correlated with occupation of the hcp and fcc types of threefold sites; an overlayer/underlayer conversion as suggested by recent spectroscopic work appears very unlikely. Strong influences of preadsorbed CO, O, and in particular C, on H adsorption are also described which in the last case appear to be due to formation of a CH species.