Theory of molecular auger spectroscopy
Creators: name: Chelkowska, Elzbieta Zofia
Publisher: name: University Of Tasmania
Year: 2023

description: Auger electron spectra of molecules are in general more complex than the spectra of the isolated constituent atoms because the increased number of valence electrons in molecules results in a significantly larger number of final double-hole states. Ab initio calculations are possible but their complexity restricts optimized studies to a limited number of relatively small molecules. Alternative approaches are sought to simplify calculations. Semi-empirical methods are very promising since the results are often at least of the same level of accuracy as minimum basis ab initio calculations, yet the answer is obtained within a small fraction of effort required for ab initio computations. In this work a semi-empirical method developed for atomic problems and adapted for molecules by Larkins is extended and appraised. All molecular Auger calculations are performed at the INDO or STO-3G level. In the present approach the intensities of molecular Auger transitions are calculated using the intra-atomic model presented by Siegbahn. The exact expressions for intensity calculations depend upon the symmetry of the orbitals involved while the complexity of the calculations increases with the size of the basis set. For calculations within this project using first- or second-row atoms the smallest basis set sp is considered sufficient. Molecular Auger energies are calculated as the difference in the experimental binding energies of the core and valence molecular orbitals involved in transitions corrected by a term which includes the hole-hole interaction energy. The correction term depends on the symmetry of the final double-hole state and is calculated as a simple function of Coulomband exchange-type integrals. Using the above approach some KVV and LVV Auger spectra of molecules containing first and second-row elements are calculated. In particular, the method is applied to interpret the KVV and LVV spectra of the first- and second-row hydrides respectively. Moreover, theoretical and experimental spectra of CH3-X (where X=F, OH, NH2, CH3) are compared when the influence of the chemical environment on the carbon KVV Auger transitions is investigated. The theoretical results are in satisfactory agreement with the experiment. In addition, the theory is tested by comparison with the experimental spectra of the first- and second-row tetrafluorides. There are some discrepancies between theory and experiment for highly symmetric molecules. On the basis of undertaken calculations C(KVV) molecular Auger spectra of XCHO (where X=F, OH, NH2, CH3) are predicted. The theory is also verified for an example of secondrow polyatomic molecule - OCS. The intensity calculations of the molecular S(LVV) Auger spectrum are the first direct evaluation based upon the molecular wavefunction for this type of molecules. The calculations can be extended to the compounds containing the third row elements. The overall agreement between theory and experiment is most encouraging nevertheless absolute energy calculation requires some minor refinement. It is concluded that this method represents an efficient approach for the interpretation of complex Auger spectra and may be extended to larger polyatomic systems with d orbitals.