1,2-DICHLOROETHANE ADSORPTION ON CU(111) - MOLECULAR ADSORPTION

LEED, AES, DELTA-phi-measurements and ARUPS have been used to study the adsorption of 1,2-dichloroethane (DCE) on Cu(111). At 110 K 1,2-dichloroethane adsorbs molecularly on Cu(111) in the trans conformer, the growth mode being layer by layer for the first three layers. The first layer is orientated, but not ordered on the surface, has DELTA-phi = -0.55 +/- 0.05 eV and a coverage of approximately 0.17 DCE molecules per surface copper atom (3.01 x 10(18) m-2). The second and third layers are also disordered and have DELTA-phi of about the same value and the same intra-layer coverage as the first layer. The molecules in the three layers are probably staggered with respect to each other, rather like crossed dumb-bells. This structure, which is consistent with the bulk structure of solid DCE, is inferred from the behaviour of the photoelectron peaks from the chlorine non-bonding orbitals in UPS. During the growth of the fourth layer, tower-like crystallites of DCE begin-to form, the surface between the crystallites retaining the original three layers of DCE. The first layer of DCE is significantly more strongly adsorbed (52 +/- 4 kJ mol-1) than the second (approximately 45 kJ mol-1) and subsequent layers (DELTA-H(sub) = 35.4 kJ mol-1) and is probably differently orientated with the Cl-C-C-Cl plane perpendicular to the surface. No chemical reaction occurs between DCE and the copper surface under UHV conditions. By contrast, a very clean electron-induced decomposition of physisorbed DCE is observed when using low-energy electron diffraction (LEED) or Auger electron spectroscopy (ABS), the products being chemisorbed chlorine and gaseous ethene.