Oxygen and pyridine on Ag(110) studied by second harmonic generation: Coexistence of two phases within monolayer pyridine coverage

The adsorption of oxygen and pyridine on Ag(110) in UHV is studied by second harmonic generation (SHG) from the surface. The adsorption of oxygen at room temperature (atomic) and at 110 K (molecular) causes a monotonic decrease in the second harmonic intensity with coverage which obeys simple Langmuir kinetics. Model fits to the data yield initial sticking coefficients, s0, of 0.0049 and 0.0088 for atomic and molecular oxygen adsorption, respectively. In contrast to oxygen adsorption, the changes in SHG signal induced by the exposure of Ag(110) to pyridine are more complicated and cannot be fit to a single-site Langmuir adsorption model. The SHG measurements indicate the presence of two distinct phases in the pyridine monolayer. A model, which depicts a broad transition from a flat-lying pyridine configuration at low coverages to a stand-up configuration at higher coverages, reproduced both the SHG and the work function measurements. The flat-lying phase dominates at low coverage, but the stand-up phase prevails when the coverage exceeds 0.3 monolayers. s0 for pyridine adsorption at 110 K was determined to be close to unity.