STRUCTURE AND PHASE-TRANSITIONS OF XENON MONOLAYERS ON CU(110)

The structure of xenon adsorbed on the Cu(110) surface was determined in a combined experimental and theoretical study. The experimental res ults were obtained using helium-atom diffraction. In the entire temper ature and coverage regime studied (20 K less than or equal to T-s less than or equal to 70 K and Theta < 1 monolayer) the xenon adlayer can be described in terms of (n x 2) high-order commensurate (HOC) structu res, with n greater than or equal to 7. As a result of the weak commen surability along the [1(1) over bar0$] direction, a series of uniaxial first-order phase transitions between (n x 2) structures with differe nt n is observed as a function of coverage and annealing temperature. In most cases these transitions are not completely reversible, indicat ing that the apparent stability of some of the HOC phases might be due to kinetic limitations, i.e. an effective ''pinning'' of the adlayer by the substrate. Along the highly corrugated [001] direction, the adl ayer is in perfect registry with the substrate lattice. Inside the (n x 2) unit cell, the xenon atoms form a quasi-hexagonal array. The expe rimental data were compared to the minimum free-energy configurations of the xenon adlayer calculated for surface temperatures between 0 and 75 K. These calculations are based on parameterized interaction poten tials fitted to the measured thermodynamic properties of xenon on Cu(1 10). The experimental results, in particular the stability of the vari ous HOC phases and their sequence with temperature, is well reproduced by the calculations assuming a corrugation of the holding potential a long the [1(1) over bar0$] direction of about 4 meV. The energy differ ence between the most stable HOC structures is found to be quite small , in agreement with the observed ''metastability'' of the structures. The calculations further reveal that the derails of the sequence and t he temperature range of stability of the HOC phases strongly depends o n the corrugation and the exact lattice misfit.