Phase transitions of a two-dimensional chemisorbed system: H on Fe(110)

Dissociative adsorption of hydrogen on a Fe(110) surface below 300 K. causes the formation of ordered overlayer structures as monitored by a fast Video LEED system enabling minimization of electron beam and contamination effects. Up to a hydrogen coverage of θ = 0.5 a c(2 × 2) phase is formed. Between θ = 0.5 and θ = 0.67 this phase coexists with and transforms into a (3 × 3) phase as indicated by a splitting of the ( {1}/{2}{1}/{2}) beams in[11̄] direction until at θ = 0.67 the c(2 × 2) phase has completely converted into the (3 × 3) phase. The fractional order beam intensities as well as their angular profiles were recorded as a function of temperature at fixed coverages; they clearly show continuous order-disorder transitions for both hydrogen phases. The corresponding phase diagrams were constructed from the points of inflection of the intensity-temperature curves as well as from the onset of the broadening of the angular beam profiles. Critical temperatures of 245 (±5) K and 265 (±5) K were derived for the c(2 × 2) and the (3 × 3) phases, respectively. The results are discussed and qualitatively interpreted in terms of the interaction energies between the adsorbed hydrogen atoms.