Spin-resolved Photoelectron Spectroscopy Research Articles

Spin-polarization effects for electrons passing through thin iron and cobalt films

Spin-dependent effects of the inelastic mean free path (IMFP) are evident for low-energy electrons passing through magnetized ferromagnetic films caused by a different attenuation within the layer. Values of IMFP for both spin components were determined for ultrathin iron and cobalt layers on W(1 1 0) by means of spin-resolving photoelectron spectroscopy.

Spin-Resolved Photoemission Study of n = 2 Core Levels of Iron

ABSTRACTThe first spin-resolved x-ray photoelectron spectroscopy (SRXPS) study of the n = 2 core levels of ferromagnetic Fe are reported. The 2p3/2, 2p1/2and 2s core levels all display interesting spin-dependent structures and splittings. The spectral complexity predicted by a purely atomic picture is not observed in the data. The results indicate that theories incorporating the delocalization of the 3d valence band are more applicable to the description of core-level photoemission in iron.

Auger electron spectroscopy of molecules: Theory for spin polarization following photoabsorption

In this article we derive an expression for the angular distribution of spin-detected electrons emitted in the Auger decay of a vacancy created by photoabsorption in a molecule belonging to one of the 32 point groups. The geometrical factor in the final expression is identical to the one recently obtained by us for angle- and spin-resolved photoelectron spectroscopy of freely rotating, unpolarized polyatomics by using a modified definition of the angular momentum transferred from the ionizing radiation to the outgoing electron. The ‘‘reduced’’ amplitude in the present case contains, of course, both the Auger decay and photoionization matrix elements. The results derived herein, therefore, mean that identical geometrical analysis is applicable to spin-resolved Auger electron as well as photoelectron spectroscopies of molecules. We, in particular, show that the degree of spin-polarization of Auger electrons emitted following photoabsorption in C∞v and D∞h linear molecules can be completely characterized by four independent parameters. The procedure developed herein is used to predict those molecular vacancies whose Auger decay will give rise to anisotropic angular distribution provided spin of the ejected electron is also observed. The degree of spin polarization of Auger electrons in this case is completely determined by a single parameter, say αa, and does not depend upon the state of polarization of the absorbed photon. In the end, αa is predicted for some of the possible Auger transitions which may follow photoionization in 2a1 orbital of CCl4, SiCl4, and GeCl4.

Exchange splitting of adsorbate-induced bands on thin iron films

Adsorbate-induced bands are investigated using energy-, angle- and spin-resolving photoelectron spectroscopy. With a ferromagnetic substrate these bands can show an exchange splitting. By means of epitaxial growth on W(110) or Au(100) we produced thin bcc iron films, layer-by-layer growth was monitored by means of MEED-oscillations. Spin-resolved spectra for different coverages with chemisorbed oxygen or CO or physisorbed Xe are shown. We found values of up to 1 eV exchange splitting of the oxygen 2p-derived band, analogous to prior measurements on bulk-crystals. Even for the physisorbed species magnetic interaction is evident. Aspects of the angular dependence as well as systematic differences of the line profiles of adsorbate-induced structures in the majority- and minority-spin channels are discussed.

Magnetic circular dichroism in valence-band photo-emission from Fe(100)

We report on a new effect in photo-emission with circularly polarized light called Magnetic Circular Dichroism in the Angular Distribution of photo-electrons (MCDAD). The phenomenon was studied in valence-band photo-emission from Fe(100) using synchrotron radiation from the storage ring BESSY (Berlin). In contrast to CDAD with nonmagnetic solids and adsorbates the MCDAD signal reverses its sign when the (macroscopic) magnetization of the sample is reversed. This effect is caused by exchange splitting in connection with spin-orbit interaction and dipole selection rules. The results are compared to data of spin-resolved photo-electron spectroscopy and a similar experiment performed in the X-ray region at Fe 2 p 1 2 and 2 p 3 2 core levels.

Angular-momentum-transfer formalism for spin-polarized atomic photoelectrons.

A recently proposed definition of the angular-momentum transfer {bold j}{sub {ital t}}=l{sub {ital r}}{minus}{bold j} (=l+{bold s}) is used to derive a formula for the distribution of spin-polarized {ital E}1 electrons ejected by photoionization in unpolarized atoms. The final expression is identical to one obtained earlier for the angular distribution of spin-resolved photoelectrons from freely rotating, unpolarized polyatomic molecules, except for the absence of an incoherent sum over the projection of {ital j}{sub {ital t}} due to the spherical nature of an atomic field and, of course, for a different set of reduced amplitudes. This, consequently, means that the same set of formulas (with these two minimal changes) can be used for the ionization of atomic as well as molecular targets. The present paper thus puts the theories for angle- and spin-resolved photoelectron spectroscopy of both atoms and molecules on the same footing. This approach has been used to analyze the currently observed spin-resolved photoionization in atomic ytterbium.

Angle- and spin-resolved photoelectron spectroscopy in the region of the 6s6p2 autoionisation of Tl

An angle-, energy- and spin-resolved photoionisation experiment was performed in the region of the 6s6p2 autoionisation resonances of thallium. Using monochromatic circularly polarised synchrotron radiation the energy dependence of the spin-polarisation parameters A, xi and alpha and the angular asymmetry parameter beta of the differential cross section were determined. In the wavelength region investigated these dynamical parameters show a pronounced variation which agrees well with the results of the random phase approximation with exchange' calculation by Cherepkov (1980). A detailed discussion of the resonance behaviour for the autoionising states is given in terms of dipole-matrix elements and phaseshift differences which are extracted from the experimental data.

Electronic and magnetic coupling between rare-earth adatoms and the Fe(001) surface.

The spin-dependent electronic structure of monolayer coverages of rare-earth metals on Fe(001) has been studied by spin-resolved photoelectron spectroscopy with synchrotron radiation. The highly spin-polarized photoemission from the localized 4{ital f} levels of Gd, Tb, and Dy on Fe(001) reveals the antiparallel coupling between these heavy rare earths and the Fe spin moment. Exchange-split final-state multiplet terms of the 4{ital f} spectra of the heavy rare earths are explicitly distinguished by direct observation of opposite polarization. For 1 monolayer of the light rare-earth Nd on Fe(001) the rare-earth magnetic moment couples parallel to the Fe magnetic moment.

Growth mode and magnetic behavior of thin Fe films on Cu3Au(001) studied by low-energy electron diffraction and spin-resolved electron spectroscopies.

The structure and magnetic behavior of ultrathin Fe films on ${\mathrm{Cu}}_{3}$Au(001) have been examined. By low-energy electron diffraction (LEED) and Auger measurements it is shown that films up to a thickness of 7 monolayers (ML) deposited at room temperature grow pseudomorphically as face-centered-cubic Fe(001) with the ${\mathrm{Cu}}_{3}$Au lattice parameter of 3.75 A\r{} parallel to the surface. The films grow layer by layer. The magnetism of the fcc Fe films has been investigated by spin-polarized secondary-electron spectroscopy as a function of film thickness. The spin-polarization component parallel to the film plane is zero for films thinner than 3.6 ML. The nonzero spin polarization for films thicker than 3.6 ML indicates the existence of remanent magnetization parallel to the surface at room temperature. Spin-resolved photoelectron spectroscopy with synchrotron radiation has been used to study the electronic and magnetic structure of the films. The high spin polarization P>50% of the photoelectrons clearly demonstrates the ferromagnetism of \ensuremath{\gamma}-Fe with a lattice constant of 3.75 A\r{}. The spin-resolved energy distribution curves (SREDC's) may be understood according to the spin-resolved band-structure calculations of Bagayoko and Callaway [Phys. Rev. B 28, 5419 (1983)]. The results indicate that in contrast to \ensuremath{\alpha}-Fe, \ensuremath{\gamma}-Fe with a lattice parameter of 3.75 A\r{} is a strong ferromagnet.

Photoelectron spectroscopic studies of polyatomic molecules: Theory for spin polarization.

A general, nonrelativistic, multichannel theory for angle- and spin-resolved photoelectron spectroscopy of polyatomic molecules in the electric dipole approximation has been developed. Expressions for angular distribution of electrons ejected with defined spin polarization, both in molecule- and laboratory-fixed frames of reference from molecular targets with fixed as well as with random orientation in space, are derived. The transformation properties of the molecular point symmetry group are used to their full advantage to reduce these expressions to their simplest possible forms. The ionization amplitude is thus shown to decompose into a sum of transitions each involving the final-state wave function belonging to an irreducible representation of the point group of the target molecule. The formulas obtained herein can therefore be used to study spin-resolved photoionization in both linear and nonlinear molecules of any symmetry corresponding to one of the 32 point groups, either in their gaseous phase or oriented on liquid and solid surfaces. In order to study both the angular distribution as well as the degree of spin polarization of photoelectrons in terms of alternative contributions of the angular momentum transfer ${(\mathrm{j}}_{t}$) from photon to the ejected electron, a new set of ionization amplitudes is introduced whose contributions to the angular distribution add incoherently. This new approach not only simplifies the resulting expressions, but also eliminates the interference terms ${j}_{t}$\ensuremath{\ne}${j}_{t}^{\mathcal{'}}$ which were present in an earlier expression for angular distribution of spin-polarized atomic photoelectrons. The theory developed in this paper has been used in the accompanying article to analyze photoionization in a nonlinear molecule whose point symmetry group is ${T}_{d}$.

Spin-resolved photoemission from Ir(111): Transitions into a secondary band and energetic position of the final state bands

Spin resolved photoelectron spectroscopy with Ir(111) was performed in normal incidence of circularly polarized VUV radiation and normal electron emission. Using the spin information the spectra were separated with regard to the symmetry of the initial states. Besides the dominant transitions into the free electron like parts of the unoccupied bands, transitions into a secondary unoccupied band were unambiguously identified. Transitions into this secondary band cannot be evidenced without spin analysis. The data are in excellent agreement with a fully relativistic first-principles band structure calculation of Noffke and Fritsche, except for an overall shift of Δε = 0.8 eV ±0.3 eV for the energy of the unoccupied final bands.

Experimental characterization of the Xe 5p photoionization by angle- and spin-resolved photoelectron spectroscopy

Spin- and angle-resolved photoelectron spectroscopy with elliptically polarized radiation has been used to fully characterize the dynamics of photoelectron emission from free Xe atoms in the 5p-autoionization and continuum region. An advantageous reaction geometry and its experimental realization at the storage ring BESSY are discussed. The three independent experimental parameters which characterize the angular dependence of the photoelectron spin-polarization vector are reported for the wavelength range from 100nm to 40 nm. The results are compared with theoretical predictions based on RRPA-, RPAE- and semiempirical MQDT-calculations. The combination of existing data for the differential photoionization cross section with the spinpolarization parameters is used to completely decouple the photoionization channels: The transition matrix elements and their relative phases are determined separately for every single dissociation channel. The results are discussed in the context of the MQDT. Correlation effects and the influence of spin-orbit interaction on the continuum states most clearly show up when the Dill-Fano angular-momentum-transfer formalism is applied.

Angle- and spin-resolved photoelectron spectroscopy of the Hg 5d 10 subshell

Parameters describing electron spin polarization in Hg 5d 10 subshell photoionization have been measured at rare-gas resonance wavelengths between 73.59 nm and 30.38 nm. The spin parameters as well as asymmetry parameters β of a recent measurement are discussed in comparison with both nonrelativistic and relativistic ab initio calculations of several authors. The importance of many-electron correlations and spin-orbit coupling effects is considered.