Core Relaxation Effects in Molecular Photoionization

Abstract

Ionization of K-shell or, more generally, of deep inner-shell electrons in atoms and molecules is accompanied by a considerable rearrangement of the valence (outer-shell) electrons in response to the reduced shielding of the nuclear attraction.(1) This adjustment of the valence electrons, referred to as electronic relaxation, leads to a significant energy lowering of the final ionic state relative to a state where the valence electron distribution of the initial state is maintained (“frozen”). The magnitude of this relaxation energy scales with the number of valence electrons. In the case of the K-shell ionization of second-row atoms (Z = 3−10), for example, the relaxation energies (in eV) are approximately given by Ε ^R(Z) = 3.1 (Z − 2.2). In a molecular environment the corresponding relaxation energies are typically 2−3 eV larger than the values for the free atom. Relaxation not only plays a role in the ionic core but also affects the motion of the outgoing photoelectron. The relaxation of the valence electrons, being essentially a contraction of the valence charge distribution, quite effectively screens the inner-shell hole potential experienced by the photoelectron. This means that the potential of the relaxed ionic core is less attractive than its unrelaxed (frozen) counterpart. As a consequence, resonances in the photoionization cross section will appear at higher energy for a relaxed core than for an unrelaxed (frozen) core. Concomitantly with the shift to higher energy, the resonance peaks will be lowered and broadened as a result of relaxation.