Dynamics of recombinative desorption of H2 and D2 from Cu(110), Cu(111), and sulfur-covered Cu(111)

Abstract

Rotational and vibrational state distributions have been determined for H2 and D2 recombinatively desorbing from clean Cu(110) and Cu(111) surfaces in ultrahigh vacuum, and also from sulfur-covered Cu(111). A 2+1 resonanced-enhanced multiphoton ionization technique is employed. For clean copper the (v″=1)/(v″=0) vibrational population ratio for both H2 and D2 is ∼50 times greater than the value expected for an equilibrium ensemble at the surface temperature Ts for desorption from Cu(110) and ∼100 times greater from Cu(111). In contrast, for sulfur-covered Cu(111), the H2 (v″=1)/(v″=0) ratio is 10 to 100 times less than for clean Cu(111) at the start of permeation but recovers sharply about 45 min later to a value close to that measured for the clean surface. We find that the total near-surface sulfur concentration appears to remain constant over the same time period, but that the overlayer structure is radically altered.