Angle-Resolved 2-D Spectra Highlight Spin-Orbit Effects
The detailed structure of atoms with more than one electron cannot be calculated exactly because of interactions between the electrons (spin-orbit effects). Therefore, approximate theoretical methods must be used to describe such systems. Light atoms, such as neon, are typically treated in a theoretical framework known as LS coupling. By using angle-resolved two-dimensional photoelectron spectroscopy, however, researchers at the ALS have found that certain LS-coupling predictions for neon are violated. In particular, they found that the spin-orbit effects are unexpectedly large and cannot be ignored when studying detailed spectra, even in atoms as light as neon.
Researchers at the ALS studied the spectra of doubly-excited, photoionized neon, in which one photon causes one electron to be emitted and another to be boosted to a higher energy level. The resulting photoionization spectra are complex. By looking at the photoelectron yield at two angles and in two dimensions, higher-order spin-orbit effects can be probed, and the accuracy of existing computational methods can be tested.
The figure above shows photoelectron yield (in color) as a function of photon energy and binding energy at 0° and at 54.7°. The upper graphs show the spectra at a photon energy of 51.3 eV, and the vertical lines at a, b, and c indicate the positions of various fine-structure levels. Looking along the vertical lines at c, it can be seen that a signal appears at both angles in some cases while in others it is absent at 0°. In this study, according to LS coupling, signals should vanish at 0° along c, so the observation of any signal at this angle is an immediate indication of the breakdown of LS coupling. These observations have since been corroborated by state-of-the-art ab-initio calculations. The important implication of the detection of such prominent spin-orbit effects is that it is not safe to assume the validity of LS coupling, even for low-lying excitations in a system as light as neon.
Research conducted by N. Berrah (principal investigator), A.A. Wills, T.W. Gorczyca, O. Nayandin, and M. Alshehri (Western Michigan University); B. Langer (Fritz-Haber-Institut); Z. Felfli (Clark Atlanta University); E. Kukk (Western Michigan University and Berkeley Lab); and J. D. Bozek (Berkeley Lab), using Beamline 9.0.1.
Funding: U.S. Department of Energy, Office of Basic Energy Sciences.
Publication about this expermiment: A.A. Wills et al., Phys. Rev. Lett. 80, 5085 (1998).
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