RELATION BETWEEN X-RAY-ABSORPTION NEAR-EDGE SPECTRA AND INTERATOMIC DISTANCES

We have performed first-principles calculations for the X-ray absorption near-edge structure (XANES) of PX(3) (X = F, Cl) molecules and AO(4)(n-) (A = P, S, Cl) cluster ions, using the discrete variational X alpha method. Theoretical X-ray spectra for PX(3) agree with the experimental ones. The theoretical spectra for AO(4)(n-) correspond to the major peaks in the experimental spectra which have been identified as contributions of the AO(4)(n-) clusters in the M(n)AO(4) (M = Li, Na, K, Rb, Cs) compounds. In the theoretical as well as the experimental spectra, the peak energies vary with the P-X and A-O interatomic distances. The XANES spectra and the wavefunctions are examined in terms of potential scattering. The results indicate that the wavefunction in each intra-atomic region is essential to approximate the whole quasi-standing wave of a shape resonance. In consistency with the calculated results, a model of the wavefunction is proposed for the XANES peaks of simple molecules and an equation is derived which expresses a relation among interatomic distance, peak energy and atomic number. The interatomic distances calculated with this equation including three optimized parameters have a notably high correlation with the experimental data for the free molecules involving B, C, N, O and F atoms.