Electron energy loss spectroscopy of adsorbed ethylene

Abstract

Electron energy loss spectroscopy (e.e.l.s.) provides an alternative method to infrared spectroscopy for studying the vibrational spectra of monolayers of chemisorbed molecules on single-crystal surfaces of metals. It has the advantages over infrared spectroscopy of considerably higher sensitivity, and the operation of two scattering mechanisms (dipolar and impact) that can be used to identify vibrations involving net motions parallel or perpendicular to the metal surface. It has the disadvantages with respect to infrared spectroscopy of lower attainable resolution and the necessity of the presence of only very low pressures (under 1 nbar) (0.1 m Pa) from molecules in the gas phase over the surface. Vibrational spectroscopy provides a powerful method for identifying the chemical structures of chemisorbed metal—adsorbate complexes. This is facilitated by an extensive existing vibrational spectroscopic literature. For work on metal surfaces, more recent infrared and Raman studies of ligands attached to metal clusters in com pounds of known structure have also been of substantial assistance. The scope of this type of surface analysis will be illustrated by a review of the extensive and interesting results now available by e.e.l.s. from this and a number of other laboratories for the adsorption of ethylene on a variety of metals and crystal faces, and over a range of temperatures. At low temperatures, ca. 100 K, ethylene adsorbs on different crystal faces as a π-complex or a di-σ adsorbed complex. At room tem perature, ca. 300 K , the low -tem perature species are transformed in some cases into an ethylidyne complex, CH ₃ CM ₃ (M = metal atom) or to a (C ₂ H ₂ )M ₄ complex. More complex spectra, due to the presence of 4 different species, are obtained at room temperature by infrared transmission spectroscopy on finely divided oxide-supported metal catalysts. Three of these have been identified with the help of the e.e.l.s. results on metal single-crystal faces.