Question

Which method, UPS or XPS, is better for resolving chemical shifts and why? Which direction would a peak in a given spectrum move if an atom loses a valence charge? What if the atom gains a valence charge? Does the kinetic energy of the photoelectron depend on the X-ray or UV beam energy?

Answer 1

XPS give better resolution peaks than UPS because because of the high frequency of the light, and the substantial charge and energy of emitted electrons. For each and every element, there will be a characteristic binding energy associated with each core atomic orbital i.e. each element will give rise to a characteristic set of peaks in the photoelectron spectrum at kinetic energies determined by the photon energy and the respective binding energies.

The actual chemical shift value of an element depends on

i) oxidation state of the atom

ii) The chemical environment of the atom

Atoms of a positive oxidation state exhibit a higher binding energy due to the extra coulombic interaction between the photo-emitted electron and the ion core and the peak shift towards left side i.e., more binding energy region

Atoms of a negative oxidation state exhibit a lower binding energy due to the less coulombic interaction between the photo-emitted electron and the ion core and the peak shift towards right side i.e., less binding energy region

The kinetic energy of the emmited electron is given by KE = hv-BE. Therefore KE of an electron depends on the energy of the photoelectron and binding energy of electron in the atom