SURFACE PI-BONDING AND THE NEAR-1ST-ORDER DESORPTION-KINETICS OF HYDROGEN FROM GE(100)2X1

We show by temperature-programmed desorption that hydrogen desorbs from Ge ( 100) 2 X 1 near 570 K with near-first-order kinetics, similar to the behavior of hydrogen on Si(100) 2 X 1. The near-first-order desorption kinetics are attributed to pairing on surface dimers induced by the pi bond on unoccupied dimer atoms, and a pairing energy of 5 +/- 1 kcal/mol is inferred. However, a comparison between the pairing energies for H atoms on Ge(100) and Si(100) with the electronic structure of the respective clean surfaces indicates that estimates of the pi bond strength based on the surface band structure do not correlate with the propensity for pairing.