Substrate-related feature in the loss structure of contamination C 1s

It has been shown that the characteristic loss structure associated with the photoelectron peak can be separated into two components related, respectively, to extrinsic and intrinsic energy loss. The extrinsic component is related to transport of the photoelectron through the solid phase and has been well described by Tougaard. The intrinsic component is related to excitation of alternative final states in or adjacent to the photoexcited atom. Although it is difficult to discern, separately, the two contributions in the energy-loss region outside the peak, it is theoretically predicted that the extrinsic contribution has a negligible effect in the peak region. This has been confirmed by Tougaard: his background removes very little intensity under the peak and eventually leaves an asymmetric peak whose tail can be related only to 'intrinsic' processes. We have suggested that the background step 'under the peak position' as derived by the Shirley algorithm-used for background subtraction in most data systems-is related to this latter process. We have also shown that it is useful to characterize the shape of the Shirley-type background by means of a parameter κ. The intensity of the Shirley background at the peak maximum, and thus the value of K, was shown to be primarily a function of the position of the element in the Periodic Table: the transition elements have very intense backgrounds whilst elements such as carbon in organic compounds have little or no background. In compounds there is an opportunity for photoexcitation of one element to excite electrons to higher energy states in its chemical partner, and hence to show similar intrinsic losses, even when such losses are not observed in the pure element itself. We have given examples of this in the XPS spectra of oxides and intermetallic alloys. In this paper we illustrate the manner in which the 'intrinsic' background of the C 1s peak in adsorbed contamination varies with the substrate on which it is adsorbed. We believe this to arise from the fact that photoexcitation of the C 1s is able to create excited final states in the valence band of the underlying material. It is concluded that strong chemisorption of organic molecules can be recognised by reference to the shape of the intrinsic loss background.