Low-temperature surface phase and phase transition of physisorbed oxygen on the Ag(111) surface

Physisorption of O₂ on the Ag(111) surface at low temperature was studied by low-energy electron diffraction and thermal-desorption spectroscopy (TDS). TDS revealed that three desorption features develop at 31.5, 35, and 38 K depending on the O₂ coverage. At a low dosage below 0.8 L where a TDS peak is observed at 38 K, LEED showed a 1×1 pattern suggesting a disordered phase. While a (

1.1	0
0.23	1.55

) pattern designated as the α_s phase was observed in LEED at a dosage between 0.8 and 3 L, a (1.15×1.15) pattern designated as β_s phase appeared above 3 L. Both phases are incommensurate with the substrate. The structure of α_s-O₂ is considered to be a cross section of the α-O₂ solid phase with the O–O bond near parallel to the surface. The α_s-O₂ irreversibly changes to a disordered structure at 25 K, which might point toward a glass transition due to frustration. The structure of the high coverage β_s phase, on the other hand, is considered to be the ab plane of the β-O₂ solid phase with the O–O bond perpendicular to the surface. The β_s-O₂ phase undergoes a reversible order-disorder phase transition at 26.5 K.