Observation of Molecular Rotors on Surfaces by ESDIAD: Studies of PF₃ and NH₃ Chemisorption on Ni Surfaces

Abstract

We demonstrate in this paper that ESDIAD (electron stimulated desorption ion angular distribution) can give detailed information about the dynamical behavior of chemisorbed species and about the intermolecular forces which modify the dynamical motions of these species. Traditionally, spectroscopic methods have been used to study the dynamical behavior of molecules in gases, liquids, and in the adsorbed layer. In studies of the adsorbed molecular rotors H₂, HD, and D₂, EELS has been used to observe transitions between essentially free rotational states present on Cu(100) [1]. For CO hydrogen-bonded to OH groups on SiO₂ surfaces, rotational wings have been observed by IR [2]. Recently, the axial rotation of adsorbed methoxy species (CH₃O) on Al₂O₃ surfaces has also been studied in detail by IR [3]. These spectroscopic methods detect transitions between librational/rotational levels, but do not give information about the directionality of the surface forces which determine the barrier for hindered rotational motion. To provide this type of geometrical information, the ESDIAD method, by virtue of its ability to image chemical bond directions, has been employed here to observe hindered rotation in a chemisorbed layer. In addition, the freezing of the rotational motion of adsorbed molecules through the action of intermolecular forces between neighbor molecules has been directly observed using ESDIAD.