POSSIBLE "HOT" MOLECULE DESORPTION BY ELECTRON STIMULATED DECOMPOSITION OF DIHALOETHANES ON Cu(111)

The desorption of ethene from physisorbed 1, 2-dichloroethane (DCE) and 1-bromo-2-chloroethane (BCE) on Cu(111) has been observed on irradiating the surface with electrons. The techniques used were low energy electron diffraction (LEED), Auger electron spectroscopy (AES), ultraviolet photoelectron spectroscopy (UPS), and mass spectrometric detection of the desorbed species. At 110 K physisorbed DCE and BCE underwent electron capture from low energy (<1 eV ) electrons in the secondary electron yield of the surface followed by decomposition and desorption of ethene alone. The decomposition was found to be first order in the surface coverage of the physisorbed DCE/BCE. No other molecular species desorbed from the surface, a stoichiometric amount of chemisorbed halogen was deposited and no carbon was detectable at the end of the desorption. The formation of the negative ions of these molecules by electron capture of low energy electrons in the secondary electron emission from the surface and the possible dynamics by which the negative ions undergo decomposition leaving the ethene product with sufficient energy to desorb, are discussed.