Bonding configurations for nickelocene on Ag(100) and steric effects in thermal desorption

The molecular adsorption and desorption of nickelocene, Ni(C5H5)(2), on Ag(100) has been studied by coverage dependent angle-resolved thermal desorption. The angle-resolved thermal desorption of nickelocene is unusual in that the molecular orientation is seen to affect the angular dependence of molecular desorption. The coexistence of two chemisorption phases for molecular nickelocene adsorption on Ag(100) near the desorption temperature of approximately 230 K is identified. One phase, with the bonding configuration of the nickelocene molecular axis along the surface normal, dominates monolayer and submonolayer coverages. Another phase, with the molecular axis not along the surface normal, appears to dominate at higher coverages. The results are discussed in the context of rehybridization and recent angle-resolved photoemission and high-resolution electron energy loss measurements.