The direct observation of hindered rotation of a chemisorbed molecule: PF₃ on Ni(111)

Abstract

By using the bond direction imaging capabilities of the electron stimulated desorption ion angular distribution (ESDIAD) technique, we have observed a thermally induced azimuthal disorder effect due to the thermal population of unbound hindered rotor states in chemisorbed PF3 on Ni(111). The six beam F+ ESDIAD patterns observed for PF3 on Ni(111) are interpreted as evidence for a weak barrier that hinders the rotation of isolated PF3 molecules on the Ni (111) surface. At a coverage of ∼0.04 PF3/Ni, a barrier to rotation of ∼80 ±20 cm-1 is estimated from the observed temperature dependence of the F+ ESDIAD patterns using a two-dimensional hindered rotor model. The PF3 coverage dependence of the thermally induced azimuthal disorder effect indicates that intermolecular forces are responsible for an increase in the hindering potential at high PF3 coverages. © 1987 American Institute of Physics.