Commonly Used Techniques for Analysis of Surfaces and Interfaces

Less Commonly Used Techniques for Analysis of Surfaces and Interfaces 

HOFLUND University of Florida Gainesville, Florida J. C, RIVIERE AEA Technology, Oxon, UK 

An early and elegant example of the type of information obtainable from UPS is given in Fig. 1 [2]. In (a) are shown the spectra from clean Ni(l 11) (full curve) and from the same surface after exposure to 2.4 L of benzene at room temperature (broken curve). The difference spectrum (i.e., clean subtracted from adsorbed ) i.s shown in (b). Another difference spectrum, this time for benzene adsorbed at 150 K, is in (c). For comparison purposes the benzene gas-phase UPS spectrum is given in (d), where the energy scale has been. shifted to align the peaks with tho.se in (c). 

During adsorption at low temperature, benzene molecules interact only weakly (van der Waals adsorption) with Ni(lll) and as a result the UPS difference spectrum (c) at 150 K looks very similar to the gas-phase spectrum. The species on the surface is basically unaltered condensed benzene. Comparison of difference spectrum (c) with that in (b), for room-temperature adsorption, reveals both that the benzene n orbitals have shifted with respect to the other orbitals, and that most of the nickel rf-electron density has been lost. Clearly there has been a dissociative and strong interaction of the benzene with the Ni(ll 1) surface since the electronic structure of the benzene molecule has been disrupted and the Ni cf-electrons have been involved in the bonding. This type of change in electronic structure is indicative of chemisorption. 

Feuerbacher. B. Fitton and R. F. Willis (eds.), Photoemission and the Electronic Properties of Surfaces, Wiley, Chichester. 1978. 

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