Scanning Microscopy


When an electron beam of less than 1000 eV interacts with a surface layer a variety of phenomena may occur. In this paper I will discuss those interactions that lead to either chemical changes on the surface and/or desorption of species or fragments from the surface. Theoretical models of electron stimulated desorption, (ESD) will be presented, specifically the Menzel, Gomer, and Redhead model, the Knotek, Feibelman model and the Ramaker, White and Murday model. Experiments that display the angular distribution of the desorbing ionic or metastable fragments, (referred to as ESDIAD for Electron Stimulated Desorption: Ion Angular Distributions) are the primary emphasis. The process of electron beam induced conversion of CO on metal surfaces (Pt (111) and Ni (110)) with the emission of O+, CO+, and CO* from the surface as seen in ESDIAD experiments shows a change of phase of the surface CO on the nickel surface above 0.75 CO/Ni and an interesting change in the bonding configuration in the coverage range of 0.50 - 0.66 CO/Pt on the platinum surface.

The ESDIAD data show that NH2 adsorbed on the silicon (100) reconstructed surface yields a very broad elliptical ESDIAD distribution that is peaked normal to the (100) surface and oriented with its major axis perpendicular to the Si surface dimers. The hydroxyl group, OH, has a four beam ESDIAD pattern that indicates off normal orientations for the H bond of OH on Si (100). Fluorine is emitted from the Si (100) surface along the direction of the Si dangling bond.

The conversion of NH3 to NH2 on Ni (110) is a beam induced effect in a surface layer as seen by ESDIAD. The electron beam dissociates the NH3 by releasing an H+ ion and leaving NH2 which produces a two lobed ESDIAD pattern. The conversion of PF3 (another surface rotor) to PF2 and PF on Ni (111) surfaces is manifest in a six lobed ESDIAD pattern that rotates 30° and acquires a strong central beam as a result of electron bombardment. These ESDIAD beams are correlated with bonding orientation and sites for PF2 and PF on the Ni (111) surface.

The surface spectroscopy of electron energy loss spectroscopy, EELS, is presented to demonstrate the electron beam induced decomposition of dimethyl and difluoromethyl ether on an alumina (Al2O3) surface. The resultant surface species from the fluorinated ether appears to contain a very stable form of an AI-F bond.

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