ON THE DEPENDENCE WITH BOND LENGTHS OF THE OBSERVED ENERGIES OF NEXAFS RESONANCES OF DIATOMIC-MOLECULES

An analytical theory is developed for the position of the resonances i n near-edge X-ray absorption fine structure (NEXAFS) which yields extr emely good agreement with experiment and allows a universal curve to b e calculated for the resonance energies. The analytical calculations i ndicates that, if scattering events are taken as purely atomic, the pr oduct k rho = constant, where k is the wavevector of the outgoing elec tron at resonance and rho the internuclear distance. This is in accord with a rule previously proposed by Natoli. It is found, however, that both the constant and the muffin-tin zero energy in the NEXAFS region depend on internuclear distance. Their variation as a function of bon d length is determined and reveals a more appropriate form of the depe ndence of sigma resonance energy Delta (measured relative to the ioniz ation potential) with bond length should be: Delta = A + B/rho + C/rho 2. This equation shows good agreement with the experimentally observe d variation in resonance position with bond lengths for series of mole cules with constant values of (Z(1) + Z(2)) where Z(1) and Z(2) are th e atomic numbers of the scattering nuclei. In fact, this function is r ather linear over the bond length range commonly encountered in organi c molecules. Finally, the observation that empirical rules for the var iation in resonance energy versus geometry are obeyed for molecules wi th constant (Z(1) + Z(2)) is also rationalized. (C) 1997 Elsevier Scie nce B.V.