HYDROGEN ADSORPTION ON FE(211) - STRUCTURAL, THERMODYNAMIC AND KINETIC-PROPERTIES

In a combined LEED, TDS and Delta phi study of hydrogen adsorption on Fe(211) a total of six commensurate phases are reported for temperatur es well below 200 K. These structures (c(2 x 6)-8H, (2 x 1)(p1g1)-2H, c(2 x 6)-16H, c(2 x 4)-12H, (1 x 3)-5H and (1 x 1)-2H) are only metast able, since for T > 200 K the hydrogen covered surface irreversibly tr ansforms into a heavily reconstructed and thermodynamically stable (1 x 2) phase. The activation energy for the transition is determined to be 0.34 +/- 0.04 eV. Both on the unreconstructed and reconstructed Fe( 211) surface hydrogen adsorbs dissociatively with an initial sticking probability near unity and the same adsorption kinetics. The latter, h owever, is considerably affected by the development of ordered superst ructures. The thermal desorption states observed can be associated wit h hydrogen binding states on non-reconstructed (alpha-states) and reco nstructed (beta-state) surface areas. The activation energy for hydrog en desorption from a 'perfectly' reconstructed Fe(211) surface is dete rmined to be 1.05 +/- 0.02 eV. Structural defects are shown to reduce this value substantially. The alpha-states, which appear at coverages theta > 1, are characteristic to the desorption from the ordered high- density hydrogen phases. Due to repulsive interactions in these phases the activation energies for desorption are as low as 0.4-0.5 eV. Both the decrease in binding energy and the observed desorption kinetics a gree well with structural proposals for the respective superstructures .