ADSORBATE-INDUCED SURFACE STRESS - CO ON NI(100) AND NI(111)

We have determined the surface stress induced by the room-temperature adsorption of carbon monoxide on the (100) and (111) surfaces of nicke l by measuring the bending of a thin nickel crystal. While CO adsorpti on induces exclusively a compressive stress on Ni(111), the stress is tensile on Ni(100) at CO coverages below 0.2 ML. The tensile stress pa sses through a maximum of +0.96 N/m at theta = 0.09 to become compress ive for theta > 0.25 with a value of -0.54 N/m at theta = 0.5. The lat ter value is small compared with the stress produced by other, more st rongly chemisorbed adsorbates on the same surface. As CO molecules pre dominantly occupy terminal binding sites on Ni(100) at 300 K, we must exclude a site conversion process in order to explain the stress-cover age dependence. The sign reversal of surface stress might be attribute d to a coverage dependent variation in the net charge transfer between the metal surface and the adsorbate, involving an enhanced splitting of the bonding and antibonding 2pi CO orbitals of neighboring molecul es at higher adsorbate coverages.