Spin-Polarized Photoelectron Spectroscopy

Electrons are spin one-half particles. In the absence of magnetic fields in the vacuum region where the photoelectrons travel from the surface to the detector, the orientations of their spins do not change, and the measurement of their spin components behind the detector plane permits to draw conclusions about the spins 

The spin polarization along the same quantization axis is defined as P He, k) - P (c]dn,k) 

12) Owing to the finite size of the detectors, they usually collect backscattered electrons over a fairly large solid angle, which 

In a spin-polarized photoemission experiment, the measured quantities are thus the spin-integrated spectrum /(eyn, t) = P f(ei3n, t) + f ( kin.fc) 3nd the spin polarization Pi(c]dn, k), from which the spin-dependent spectra can be recovered as 

As an example of the renewed interest in spin-polarized photoemission in surface science, we discuss here the recent observation of a spin sphtting in reciprocal space of the Shockley surface state on Au(lll). 

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Information on the spin resolved band structure of ferromagnetic materials can directly be obtained from spin resolving photoelectron spectroscopy. Using polarized radiation spin integrating photoemission techniques already enable to have access to magnetic properties. An enhancement of the surface sensitivity can be achieved using neutral excited spin polarized atoms which move towards the sample and are de-excited by tunneling electrons from the surface with a subsequent emission of electrons. 

DV-Xa molecular orbital calculation is demonstrated to be very efficient for theoretical analysis of the photoelectron and x-ray spectroscopies. For photoelectron spectroscopy, Slater s transition state calculation is very effective to give an accurate peak energy, taking account of the orbital relaxation effect. The more careful analysis including the spin-polarized and the relativistic effects substantially improves the theoretical results for the core level spectrum. By consideration of the photoionization cross section, better theoretical spectrum can be obtained for the valence band structure than the ordinary DOS spectrum. The realistic model cluster reproduce very well the valence state spectrum in details. 

SPUPS Spin polarized ultraviolet photoelectron spectroscopy... 

Kallury K, MacDonald P, Thompson M (1994) Effect of surface water and base catalysis on the silaniation of silica by (aminopropyl)alkoxysilanes studied by x-ray photoelectron spectroscopy and 13C cross-polarization/magic angle spinning nuclear magnetic resonance. Langmuir 10(2) 492 99... 

By combining the electron energy analyser with a polarization sensitive Mott Detector separate spin-up and spin-down valence band density of states spectra can be produced for magnetic samples. This technique is known as spin polarised ultraviolet photoelectron spectroscopy (SPUPS). 

Solid-state NMR (using cross-polarization magic-angle spinning techniques), isotopic substitution, and faster methods such as infrared, Raman, and, especially. X-ray photoelectron spectroscopy (ESCA) are particularly useful in investigating these systems. Some typical examples are depicted in Fig. 3. 

Eriksson et al. (1991b) treated the fee (100) surface of a-Ce in the paramagnetic state and included the 4f states in the conduction bands. The calculation used a film (five-layer slab) linearized muffin tin orbitals method (Fernando et al. 1986, Ma et al. 1986). From the results they concluded a magnetic instability for the surface layer. Based upon earlier experiences of such Ce instabilities, they suggested that the surface layer of a-Ce should be y-like. This prediction was subsequently verified by Weschke et al. (1991) by means of photoelectron spectroscopy experiments. This demonstrates that surface properties can be radically different from those of the bulk. The same type of surface effect has also been seen for a-like cerium compounds, Celrj, CePdj ad CeRhj (Laubschat et al. 1990), where again the Ce surface atoms show y-like behaviour. This means that some photoelectron spectroscopy data for a-like Ce systems should be reanalyzed before spectra representing the bulk can be obtained. Calculations by Eriksson et al. (unpublished), where both spin and orbital polarization are allowed for, explicitly show the y-like property of the surface layer. 

A considerable amount of research, both theoretical and experimental, has been devoted to the molecular and the electronic structure of nitroxides. From these works, it is well accepted that the spin density between the N and O atoms depends on various parameters such as the pyramidalization of the nitrogen atom, the resonance effects, or the polarity of the medium. The gain in energy from the delocalization of the unpaired electron has been calculated to be approximately 30kcalmor It was recently suggested that this value was overestimated and a lower value of 23 kcal mol" determined by Hel/Hell photoelectron spectroscopy, was proposed. 

During the coming years, one can expect the activity in this field to become even more intense, in part because of the increase in the number of synchrotron facilities and techniques for photoelectron spectroscopy. It is expected that there will be very extensive measurements of photoionization with excitation including resonance structure and double photoionization over wide energy ranges including studies of inner shells. There will be measurements of partial cross sections, angular distributions, and spin polarizations. This last topic, which has recently received considerable attention,was deferred to the paper by W. Johnson in this volume since it depends on relativistic effects. 

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