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

It has been shown that the characteristic loss structure associated with th e photoelectron peak can be separated into two components related, respecti vely, to extrinsic and intrinsic energy loss. The extrinsic component is re lated to transport of the photoelectron through the solid phase and has bee n well described by Tougaard, The intrinsic component is related to excitation of alternative final state s in or adjacent to the photoexcited atom, Although it is difficult to disc ern, separately, the two contributions in the energy-loss region outside th e 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' pr ocesses. We have suggested that the background step 'under the peak positio n' as derived by the Shirley algorithm-used for background subtraction in m ost 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 m eans of a parameter kappa. The intensity of the Shirley background at the p eak maximum, and thus the value of kappa, was shown to be primarily a funct ion of the position of the element in the Periodic Table: the transition el ements have very intense backgrounds whilst elements such as carbon in orga nic compounds have little or no background. In compounds there is an opport unity for photoexcitation of one element to excite electrons to higher ener gy states in its chemical partner, and hence to show similar intrinsic loss es, even when such losses are not observed in the pure element itself, We h ave given examples of this in the XPS spectra of oxides and intermetallic a lloys. In this paper we illustrate the manner in which the 'intrinsic' back ground 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 photo excitation of the C 1s is able to create excited final states in the valenc e band of the underlying material. It is concluded that strong chemisorptio n of organic molecules can be recognised by reference to the shape of the i ntrinsic loss background, Copyright (C) 1999 John Wiley & Sons, Ltd.