H, H₂, D, HD, andphysisorbed H as function of time using the surface chemistry model with physisorbed and chemisorbed species and a unique physisorption to chemisorption activation energy of 400 K. First row, from left to right: models at 10 and 20 K; second row: models at 50 and 100 K. Bottom row: models at 300 and 500 K. The * symbol corresponds to the abundance of unoccupied chemisorption sites.