Theoretical evidence for fast H-divacancy rotation on H/Pd(1 1 1)

By ab initio calculations, we show that when two H vacancies occupy neighboring fcc sites on a hydrogen-covered Pd(1 1 1) surface, a very low-barrier process exists, leading to a fast effective rotation of the divacancy. As a consequence, divacancies are likely to display a three lobed shape in typical STM images, as reported in a recent experimental work [T. Mitsui, M.K. Rose, E. Fomin, D.F. Ogletree, M. Salmeron, Nature 422 (2003) 705]. Our results allow for a better understanding of the experimental data, in showing the atomic-scale path responsible for the rotation and in providing a theoretical estimate of the activation energy of the process.