N-2 MONOLAYER AND BILAYER ADSORBED ON AG(110) AT 15 K - STRUCTURE ANDORIENTATIONAL ORDERING

The growth and the structure of an N-2 monolayer and bilayer physisorb ed on Ag(110) at 15 K were investigated by means of thermal desorption spectroscopy and low-energy electron diffraction (LEED) and compared with structures determined from interaction potential calculations. Th e NZ monolayer desorbs at 40 K with a desorption energy of 104 +/- 1 m eV while the bilayer desorbs at 32 K with an energy of 73 +/- 1 meV. T he LEED pattern of the monolayer corresponds to a parallelogram struct ure. For the bilayer a hexagonal structure was found. An energy minimi zation procedure based on simulated annealing algorithm gives adsorpti on energy values very consistent with the experimental data for the mo nolayer and bilayer. It shows, furthermore, that the monolayer geometr y corresponds to a;herringbone high-order commensurate phase while the second layer displays two isoenergetical structures: a relatively dil ute herringbone phase and a dense hexagonal four-sublattice pinwheel s tructure. This latter structure has the symmetry observed in the LEED pattern.