Fluorescence Properties of Dansyl Groups Covalently Bonded to the Surface of Oxidatively Functionalized Low-Density Pol yet hylene Film'

acetylene dose due to the high supply of hydrogen. Moreover, methane should not be formed, since there should not be any dissociation of acetylene on the surface. Figure 11 shows the result of an experiment in which the surface was precovered with a monolayer of hydrogen. The first dose of acetylene (0.2 X 1015 molecules of C2H2 cm-2) leads to an enormous increase in the hydrogen pressure and to a formation of methane. The formation of methane and the displacement of hydrogen must be traced back to chemisorption and decomposition of acetylene on the surface. Ethane can only be observed in the gas phase, when the surface is covered with a sufficiently high concentration of acetylene or its fragments. This observation clearly points to a Langmuir-Hinshelwood mechanism. The fact that methane and ethane are formed at lower acetylene exposures than in the experiment described in Figure 9 can easily be explained by the higher hydrogen coverage on the surface. In another experiment deuterium was used to precover the surface. Figure 12 shows a series of four mass spectra taken after adsorption of different amounts of acetylene, when equilibrium had established. The mass spectrum recorded after an addition of 0.2 X 1015 molecules of C2H2 cm-2 exhibits a relatively high intensity of deuterated methanes (amu 17,18,19,20) and its fragments (amu 14, 15,16,17,18). These findings are a proof for the cracking of the C-C bond of the admitted acetylene. The ratio of the partial pressures of HB, HD and D2 indicates that the preadsorbed deuterium equilibrates with the hydrogen originally bonded in the acetylene molecules. The deuterated ethane C2H2D, (amu 34) should be expected as the main product in the case of an Eley-Rideal mechanism. However, the mass spectra show that this product can only be found in low concentration, besides other deuterated ethanes, after admission of several doses of acetylene. The formation of C2DG (amu 36) shows convincingly that acetylene is dissociatively chemisorbed at temperatures of 273 K. In a further step the fragments react to deuterated methanes and ethanes (amu 26-36) corresponding to a Langmuir-Hinshelwood mechanism. The deuterium partial pressure decreases with increasing acetylene coverage due to the formation of deuterated methanes and ethanes. Simultaneously the hydrogen partial pressure increases due to the decomposition of additionally adsorbed acetylene molecules and the desorption of the hydrogen.