Evidence for ‘‘subsurface’’ hydrogen on Pd(110): An intermediate between chemisorbed and dissolved species

We report experimental evidence for the formation of a ‘‘subsurface’’ hydrogen species at 130 K on a Pd(110) single crystal surface. This is well separated by distinct activation energy barriers from species which are chemisorbed or which are dissolved in the Pd bulk, thereby confirming a model of H absorption in Pd in which a subsurface state acts as a reaction intermediate.