Nature of Resonant Photoemission in Gd

Abstract

The phenomenon of resonant photoemission happens when, in addition to a direct photoemission channel, a second indirect channel opens up as the absorption threshold of a core level is crossed. A massive increase in emission cross section can occur, but the nature of the process remains clouded. Using novel magnetic linear dichroism in photoelectron spectroscopy experiments and theoretical calculations, we can now clearly demonstrate that temporal matching of the processes as well as energy matching is a requirement for true "resonant photoemission." [S0031-9007(98)06819-7] PACS numbers: 75.70. -i, 75.50. -y, 79.60. -i The photoemission of 4f and 5p electrons from rareearth metals and their compounds is strongly enhanced when the photon has just enough energy to excite a 4d electron to an unoccupied 4f level, leading to a process called "resonant photoemission." (See Fig. 1.) In a generic picture, the indirect channel of the resonant photoemission is interpreted as due to a process where a 4d electron in the initial state is first excited to the unoccupied 4f level, forming a tightly coupled, bound intermediate state, 4d core hole plus 4f electrons. Then a decay via autoionization occurs, producing a final state identical to that obtained by a direct photoemission process for the ejected electron [1]. The transition rate is greatly enhanced if the excited state decay is by a (super)-Coster-Kronig [(s)CK] process [2,3]. The key question is whether these processes are coherent or incoherent: Is it truly resonant photoemission or merely the incoherent addition of a second emission channel? Should the overall intensity be treated as a squaring of the sum of the amplitudes (coherent) or summing of the squares of the amplitudes (incoherent)? A true resonant photoemission process should be coherent, involving interference terms between the direct photoemission and indirect photoemission channels. Possibly, incoherence would give rise to the loss of photoemission characteristics in the process, with a domination of Auger-like properties.To this problem we have applied the new photoelectron spectroscopy technique of magnetic linear dichroism in angular distributions (MLDAD) [4][5][6][7]. This technique is related to but distinct from the techniques of magnetic x ray circular dichroism (MXCD) in photoelectron spectroscopy and x ray absorption [8][9][10][11][12][13]. The key is that while strong MXCD effects in ferromagnets can be observed with photoemission and absorption, the large MLDAD effect in ferromagnets is solely a photoemission, not an absorption-driven, process. This is because the chirality which gives rise to magnetic sensitivity is due to the vectorial configuration in MLDAD as opposed to the FIG. 1. (a) Schematic diagram of the direct and indirect channels in Gd 4f resonant photoemission. Time estimates are based on Refs. [2,3]. (b) Same for Gd 5p emission. (c) Comparison of coherent and incoherent additions of channel contributions. A0 (AI) is the direct (indirect) amplitude. (d) The photoabsorption of Gd͞...

Orbital Specificity in the Unoccupied States ofUO2from Resonant Inverse Photoelectron Spectroscopy

“…In IPES, a very different set of Problem." Thus, because of the unique constraints placed upon RIPES owing to its quantum-mechanical nature [26,27] and the availability of a recently verified model of the complex internal structure of the unoccupied density of states (UDOS) of uranium dioxide involving both U5f and U6d states [16], it will be shown that the main and satellite peaks in the RIPES correspond to the 5f and 6d thresholds of the unoccupied DOS. This result opens the door to application of this approach to the more radioactive actinides and the isolation of their 5f and 6d components in the UDOS.…”

First-Principles Calculations of Angle-Resolved and Spin-Resolved Photoemission Spectra of Cr(110) Surfaces at the2p−3dCr Resonance

“…The huge experimental output from RPES on correlated materials [7,[11][12][13][14][15][16] and the intriguing quest for a determination of local magnetic properties put forward by pioneering experiments [14][15][16] call for advancements in the theoretical description of this spectroscopy. In experiments on CuO and Ni, it was shown that the RPES photocurrent with circular polarized light is spin polarized in antiferromagnets [14,15] and Curie paramagnets [16].…”

“…However, second order processes, like resonant inelastic X-ray scattering (RIXS) and resonant photoemission (RPES), remain a major challenge for theory. For RPES, existing approaches are semiempirical [7][8][9][10], based on a well defined two-holes final state and on small clusters, and thus do not take into account the delocalization of intermediate states, the bandstructure of the system and multiple scattering effects in the propagation of photoelectrons.The huge experimental output from RPES on correlated materials [7,[11][12][13][14][15][16] and the intriguing quest for a determination of local magnetic properties put forward by pioneering experiments [14][15][16] call for advancements in the theoretical description of this spectroscopy. In experiments on CuO and Ni, it was shown that the RPES photocurrent with circular polarized light is spin polarized in antiferromagnets [14,15] and Curie paramagnets [16].…”

“…However, second order processes, like resonant inelastic X-ray scattering (RIXS) and resonant photoemission (RPES), remain a major challenge for theory. For RPES, existing approaches are semiempirical [7][8][9][10], based on a well defined two-holes final state and on small clusters, and thus do not take into account the delocalization of intermediate states, the bandstructure of the system and multiple scattering effects in the propagation of photoelectrons.…”

Changes in the electronic structure of cerium due to variations in close packing

“…The prepeak structure remains as individual peaks but at higher photon energies, where the outgoing electron can actually escape, coupling to the continuum states generates a large, broad peak, that is referred to as the giant resonance. 24 The prepeaks are of atomic character, and thus, in Ce are rather insensitive to the phase of the metal. Only in ␣-Ce is a slight broadening of the pre-peaks observed due to d-f hybridization.…”