The adsorption of cyclic hydrocarbons on Ru(001): II. Cyclohexane |
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Authors: | FM Hoffmann TE Felter PA Thiel WH Weinberg |
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Institution: | Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, USA |
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Abstract: | The adsorption of cyclohexane on Ru(001) at 90 K has been investigated by thermal desorption mass spectrometry, EELS, UV photoemission and LEED. Thermal desorption indicates the adsorption of the undissociated molecule first in a chemisorbed monolayer (Td = 200 K) with subsequent formation of multilayers (Td = 165 K) at higher exposures. The vibrational spectrum obtained by EELS is characterized by a frequency shift of the C-H stretching mode from 2920 cm?1 (multilayer) to 2560 cm?1 for the chemisorbed monolayer. Off-specular EELS data indicate two different electron scattering mechanisms for the C-H stretching mode. Whereas for the C-H stretching mode of the multilayer, large angle electron impact scattering is observed, the C-H soft-mode of the monolayer is largely due to small angle dipolar scattering. The He I photoelectron spectra of cyclohexane multilayers are characteristic of the undissociated molecule. A new assignment of C(2s) and the lowest C(2p) level, based on a comparison with benzene, shows that the chemisorbed monolayer is characterized by the absence of emission or broadening of the 2a1u level. This is attributed to C3v symmetry of the chemisorbed layer and to a possible interaction of the 2aIu orbital with the metal surface. |
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