The adsorption of xenon on a ruthenium(1 0 [formula omitted] 0) surface

Abstract

The interaction of xenon with a ruthenium(1 0 1 ̄ 0) surface was investigated between 29 and 300 K by means of low-energy electron diffraction (LEED), temperature-programmed thermal desorption (TPD), and work function ( ΔΦ) measurements. TPD reveals a single desorption state at 135 K (coverage Θ=0.01 ML) which shifts to lower temperatures as Θ increases and appears at 112 K for Θ=0.78 ML. The initial desorption energy is 28.5 kJ/mol and decreases with Θ; at Θ=0.78 ML it is merely 20 kJ/mol. Beyond this coverage there appears another, relatively sharp, desorption state at 88 K which is due to a phase transformation within the Xe monolayer. For coverages 1< Θ⩽1.35 ML, Xe desorbs via zero-order kinetics, with a desorption energy of 17.5 kJ/mol. LEED reveals several phases with long-range order. At Θ=0.5 ML, a (3×1) pattern, and at Θ=0.78 ML, a (2×5) structure is observed. For 0.78< Θ<1 ML a quasihexagonally, uniaxial commensurate (UC) structure is formed which consists of chains of Xe atoms with a mutual Xe–Xe distance of 4.2 (±0.1) Å in [1 2 ̄ 1 0] direction. Work function measurements show a strong decrease even at small Xe coverages; at Θ=0.78 ML, pronounced depolarization effects become effective, leading to a minimum of the ΔΦ( Θ)-curve near the monolayer coverage. At Θ=1 ML, a ΔΦ of −1.29 eV is reached. The data are discussed and the results are compared with other noble gas adsorption systems in view of the peculiar features, namely the clear evidence of repulsive Xe–Xe interactions, the extraordinary large work function change, and the formation of a multilayer phase with square symmetry.