a) Describe the process for the emission of an LMM Auger electron from the surfa
ID: 1030402 • Letter: A
Question
a) Describe the process for the emission of an LMM Auger electron from the surface of a sample. An XPS electron was found to have a kinetic energy of 1050.5 eV when a Mg K source was employed (=0.98900 nm) in an instrument with a work function of 15.5 eV. What is the binding energy of the emitted electron? b) c) If the surface was oxidized and then re-analysed by XPS, would the kinetic and d) Would you expect the Zn(2p) electron signal to be found at higher or lower kinetic e) Explain the concept of tunneling current in STM. Include the two possible modes binding energies of this emitted electron increase or decrease? Why? energies than the Zn(3p) electron signal in an XPS spectrum? Why? of operation of STM in your answer. (3) (2) QIESTION 5
Explanation / Answer
The exact binding energy of an electron depends not only upon the level from which photoemission is occurring, but also upon :
Changes in either (1) or (2) give rise to small shifts in the peak positions in the spectrum - so-called chemical shifts .
Such shifts are readily observable and interpretable in XP spectra (unlike in Auger spectra) because the technique :
Atoms of a higher positive oxidation state exhibit a higher binding energy due to the extra coulombic interaction between the photo-emitted electron and the ion core. This ability to discriminate between different oxidation states and chemical environments is one of the major strengths of the XPS technique.
In practice, the ability to resolve between atoms exhibiting slightly different chemical shifts is limited by the peak widths which are governed by a combination of factors ; especially
In most cases, the second factor is the major contribution to the overall line width.
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