Photoelectron structure factor and diffraction spectroscopy
Introduction
Photoelectron spectroscopy is a powerful technique for the investigation of both electronic and atomic structure of solids and surfaces. Photoelectron energy distribution curves for valence band and core levels reflect density of states and compositions, respectively. Behavior of valence electrons and interference of photoelectrons are projected in momentum space as photoelectron angular distribution patterns. Based on the energy and momentum conservation principles in the photoemission process, all-direction-resolved photoelectron spectroscopy provides a rich variety of information on the electronic and atomic structure of solids.
The electronic properties and chemical reactivity of materials are closely related to the behavior of electrons at vicinity of Fermi level. Angle-resolved photoelectron spectroscopy for valence band dispersion mapping is a powerful technique to study such electrons. Two-dimensional photoelectron intensity angular distribution (PIAD) corresponds to the iso-energy cross section of valence band. Three-dimensional energy band dispersion (kx, ky, binding energy) [1] and Fermi surface (kx, ky, kz) [2] are obtained by measuring a series of PIAD as function of the binding energy and excitation photon energy, respectively.
In the case of linearly polarized-light excitation, the symmetry relation in the photoelectron excitation process is also displayed as “angular distribution from atomic orbital (ADAO)” [3], which is used to distinguish the atomic orbitals constituting the energy band. Another important effect in angular distribution is the “photoelectron structure factor (PSF)” [3], [4], [5], which originates from the interference among photoelectron waves from individual atoms. The bonding character of the energy band can be clarified from the intensity inequivalency between Brillouin zones determined by PSF.
On the other hand, the photoelectron from a localized core level is an excellent element-specific probe for atomic structure analysis. Photoelectron diffraction provides information on the surrounding atomic configuration, which is recorded as forward focusing peaks (FFPs) at local interatomic directions [6], [7], [8] and diffraction rings around them in PIAD. Furthermore, FFPs were found to show rotational shifts when excited by circularly polarized light. FFP shift has been shown to be inversely proportional to the distance between the emitter and scatterer atoms both experimentally [9] and theoretically [10]. This is the basis for the stereo photography of the atomic arrangements [11], [12], [13].
Moreover, we have been developing a new approach for investigating the properties of specific atomic site non-destructively. Since PIAD differs by different surrounding atomic arrangement, emitter atom sites can be specified by their characteristic diffraction patterns. By combining this diffraction technique with core level spectroscopy – we call it diffraction spectroscopy, one can get access to each atomic site structure and have their electronic property information individually [14]. We have applied this method to study various subsurface structures and electronic properties [14], [15], [16].
In this paper, the information obtained by all-direction-resolved photoelectron spectroscopy for valence band and core levels are described. It is noteworthy that the quantum phenomena in initial states (band dispersion) and final states (photoelectron diffraction) are both well observed in the valence PIAD at high kinetic energy. The circular dichroism of photoelectron FFP rotation around the incident-light axis reflects the orbital angular momentum of excited core level [11]. But, these rotations are also found in the case of the valence band photoelectrons with high kinetic energy [17]. The orbital angular momentum quantum number of valence electron for specific site can be deduced from the FFP rotation [17], [18].
Section snippets
Experimental details
One efficient approach to measure photoelectron and Auger electron patterns at specific kinetic energy are by using a display-type analyzer. For example, a two-dimensional display-type spherical mirror analyzer (DIANA) [19] shown in Fig. 1 enables 1π-steradian (±60°) PIAD direct observation without changing the angles of incident light and the sample orientation. It consists of a hemispherical main grid (MG), outer sphere with obstacle rings (OR) and guard rings (Gd). The potential V(r) of
Principles
The electronic and chemical characters of materials are determined mostly by the behavior of electrons at vicinity of Fermi level. Angle-resolved photoelectron spectroscopy using vacuum ultraviolet light as excitation source is a powerful technique to study such electrons. Based on the rule of the energy and momentum conservation in the photoemission process, the valence band structure can be analyzed directly. The two-dimensional Fermi surface mapping reveals the driving force of the
Summary
At the surface active site, where materials, information, and energy are converted to another form, local electronic structure plays an important role in realization of useful functionality. Photoelectron diffraction and spectroscopy uncover the physics behind these phenomena. In summary, the information obtained by all-direction-resolved photoelectron spectroscopy for valence band and core levels were described. By measuring two-dimensional PIAD from valence band, the iso-energy cross section
Acknowledgements
This work was performed with the approval of the Japan Synchrotron Radiation Research Institute (Proposal nos. 2006A1688, 2006B1572, 2008A1519, 2009B1769, 2011A1471). The authors deeply thank Dr. Takayuki Muro, Dr. Tetsuya Nakamura, Dr. Yukako Kato, Dr. Kentaro Goto, Dr. Naoyuki Maejima, and Dr. Hirosuke Matsui for their support in the experiments. This research was supported by the Ministry of Education, Culture, Sports, Science, and Technology, Grants-in-Aid for Scientific Research (S) (No.
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