Linear magnetic and alignment dichroism in Auger–photoelectron coincidence spectroscopy

Abstract

Auger-photoelectron coincidence measurements for polarized targets are theoretically considered. We present a general expression for the cross section of sequential double photoionization of polarized atoms within the two-step model. Linear magnetic and alignment dichroism in coincidence experiments is discussed in more detail. For illustration, we made ab initio calculations based on the multiconfigurational Dirac-Fock theory for the 4d photoionization of atomic Sn followed by N-OO Auger decay.

Evidence for the collapse of short-range magnetic order in CoO at the Néel temperature

“…To consider the first effect, geometric dichroism in AR-APECS has been ascribed to spin pairing in the two-hole final state, and also in systems with a spin-symmetric population in the ground state [15,16]. In the present case, at LT the initial state of the Co ion is spin polarized and, within a sequential two-step double photoionization model, high-spin vs. low-spin selectivity of the NN and AN experimental geometries in AR-APECS amounts to a magnetic dichroism [18]. In an atomic model [18] the observed AR-APECS cross-section is linked to the statistical tensor of the exciting radiation, and to three relevant vectors, i.e.…”

“…In the present case, at LT the initial state of the Co ion is spin polarized and, within a sequential two-step double photoionization model, high-spin vs. low-spin selectivity of the NN and AN experimental geometries in AR-APECS amounts to a magnetic dichroism [18]. In an atomic model [18] the observed AR-APECS cross-section is linked to the statistical tensor of the exciting radiation, and to three relevant vectors, i.e. the magnetic moment and the momenta of the two unbound electrons.…”

“…In particular, angular distributions of partial waves combined with selection rules give rise to an m-level sensitivity leading to a dichroic effect in angle-resolved (DEAR-) APECS measurements in which spin-symmetric (i.e., high-spin) and spin-antisymmetric (i.e., low-spin) final states are enhanced or suppressed depending upon the chosen geometry. As the Auger process probes the valence electronic structure at the site of the initial core hole, AR-APECS is sensitive to the local magnetic properties of a solid [17,18].…”

“…3(b) would not be expected to disappear above T N . On the other hand, the collapse of the local magnetic moment would eliminate one of the three vectors relevant for the AR-APECS cross-section [18], thus likely causing sharp changes of the relative cross-section. However, the disappearance of the atomic magnetic moments would be inconsistent with early magnetic susceptibility measurements on CoO single crystal showing a Curie-Weiss-like paramagnetic behavior above the Néel temperature [37].…”

Coherence and correlations in photoinduced Auger and fluorescence cascades in atoms

“…Theoretical background for treating such a process in the two-step model (photoionization + Auger decay) and particular predictions for the LMD and the LAD for the Sn 4d photoionization and subsequent N 4,5 -O 2,3 O 2,3 Auger decay have been published recently by Da Pieve et al (2007). Calculations show that the value of the dichroism as well as the complexity of the dichroic pattern can strongly vary under different experimental conditions.…”