HYDROGEN ON FE(211) - COMMENSURATE AND RECONSTRUCTED PHASES

The adsorption of hydrogen on the Fe(211) surface was investigated by LEED and thermal desorption mass spectroscopy (TDS) in the temperature range 40-300 K. For T ≤ 200 K commensurate phases c(2 × 6)-8H, (2 × 1)p1g1-2H, c(2 × 6)-16H, 1 × 2-3H, 1 × 1-2H develop corresponding to coverages θH of 2 3, 1, 4 3, 3 2 and 2. However, these phases are only metastable T > 200 K transform irreversibly into a reconstructed and streaked 1 × 2 phase ("1 × 2S"), which is only removed by complete desorption of hydrogen from the surface. The formation of the 1 × 2S-phase is a thermally activated process which is also controlled by the substrate hydrogen coverage. For coverages below θH = 1 heating to T > 200 K makes the surface start to reconstruct into the 1 × 2S phase prior to any hydrogen removal. For θH > 1 the 1 × 2S phase develops under partial desorption of hydrogen. The two reaction channels are mirrored by the binding states α and β appearing in the TD spectra associated with hydrogen desorbing from unreconstructed (α) and reconstructed (β) areas, respectively. For both the commensurate and reconstructed phases we propose real space models and compare our findings with recent data obtained for the similar adsorption systems H/Co(101̄0), H/Ni(110) and H/Pd(110). © 1990.