ORDERED HIGH COVERAGE NO OVERLAYERS ON PD(111)

High coverage NO overlayers on Pd(111) were studied by LEED and vibrational infrared spectroscopy under equilibrium adsorption conditions. Three ordered, coincident site superlattices were found to exist at NO coverages of 0.5, 0.625 and 0.75 monolayers (ML). Structural models have been proposed to give a consistent interpretation to both the LEED and vibrational infrared data. In particular, the 0.625 ML structure can be regarded as phase/antiphase domains of low density NO separated by domain walls containing NO at a higher local density. Infrared reflection-absorption data indicates that relaxation of the domain wall structure produces a tilted NO species.