Spin polarization of Auger- and of photoelectrons from barium atoms exposed to circularly polarized radiation and their cross comparison

New results of spin polarization of both photoelectrons and Auger electrons are reported after 5p photoionization of free Ba atoms with circularly polarized light. A substantial polarization transfer from the spin polarized photons to the spin polarized photoelectrons and via the hole state orientation to the spin polarized Auger-electrons is observed. The cross comparison of the results for photoelectrons and Auger-electrons allows a quantitative test of the assumed two step model where both electron-emission processes occur in sequence.     

Intra- and inter-atomic auger processes in collisions of low energy He++, He+, He* (23,S, 21 S) and Li+ with Li covered W(110) surfaces

Electron spectra from He⁺⁺, He⁺ and Li⁺ (10 to 1500 eV) ions colliding under grazing incidence with Li covered W (110) surfaces are reported. The results are compared with those obtained from thermal collisions of (2³ S; 2¹ S) metastable He atoms. In these collisions 1s vacancies are either produced during the collision event (energetic He⁺ (Li ⁺) collisions) or are brought into the collision (slow He⁺⁺ (He⁺, He*) collisions). Population of the 2s orbitals by two electrons produces states which decay by intraatomic Auger processes: we observe autoionization of He** (2s ²) and Li** (1s 2s ²) as well as autodetachment of He^?* (1s 2s ²). Alternatively the 1s-holes in the projectile or target (Li) can be filled by Auger processes involving one or two surface electrons. The processes leading to electron emission are studied as a function of the Li coverage in the submonolayer region (0≦Θ_Li≦1Ml) and as a function of the projectile energy. It is concluded that with one or two 1s vacancies present in the projectile the double capture of two surface electrons constitutes an important process responsible for electron emission of low work function surfaces.     

CO inner-shell excitation studied by electron impact spectroscopy

The inner-shell excitation and decay of the CO molecule have been studied in electron impact experiments. The dipole-forbidden transition (1sσ_c)⁻¹(2pπ) ³Π has been characterized by angular resolved electron energy loss spectroscopy and its decay via the measurement of resonant Auger spectra. The contribution of the (1sσ_c)⁻¹(2pπ) ³Π state to the CO resonant Auger spectrum in the region of the “spectator transitions” has been isolated and the population of CO⁺ quartet final states has been observed.     

Angle- and spin-resolved photoelectron spectroscopy of thepπ outer valence shell of HBr molecules

Using circularly polarized synchrotron radiation, the photoionization of HBr molecules was studied by angle- and spin-resolved photoelectron spectroscopy in the photon energy range from 11.7 eV to 21 eV. For photoelectrons corresponding to the final ionic states HBr⁺ X ²Π_3/2(v=0) andX ²Π_1/2(v=0), the energy dependence of the dynamical photoionization parameters was measured and compared with ab initio calculations for HBr⁺ by Raseev et al. and RRPA calculations for Kr⁺ by Huang et al., This comparison indicates that, for energies above the electronic autoionization region, photoemission from the outer valence orbital exhibits distinct atomic behavior. By combining the experimental data for the cross section σ and the spin polarization parameter A, sums of partial cross section contributions to σ were determined and analyzed to obtain specific information on the outgoing partial electron waves. Furthermore, the validity of the so-called non-relativistic relationships for the dynamical photoionization parameters was tested as a function of equal photon and kinetic photoelectron energy, respectively.     

Use of spin-labelling techniques to probe the dynamics of He(23 S) deexcitation at solid surfaces

Spin labelling techniques, specifically the use of electron-spin-polarized He(2³ S) metastable atoms coupled with energy-resolved spin analysis of the ejected electrons, are used to investigate the dynamics of He(2³ S) deexcitation at solid surfaces. Data for a clean Au(100) surface are presented that show that deexcitation occurs exclusively through resonance ionization followed by Auger neutralization. The electrons involved in Auger neutralization are observed to be correlated in spin and possible reasons for this are discussed. Results obtained at Xe and NO films adsorbed on cooled Au(100) and Cu(100) substrates, respectively, show that He(2³ S) metastable atom deexcitation is analogous to gas-phase Penning ionization. Detailed differences are apparent that can be attributed to effects associated with the underlying substrate and interactions involving neighboring atoms in the film.     

Interference effects in 1<Emphasis Type="Italic">s</Emphasis> → π* resonance excitation processes of the no molecule

A ¹Π → X ¹Σ⁺ fluorescence in the NO⁺ molecular ion observed after Auger decay of the 1s ^?1 π* resonances of the N*O molecule and NO* was studied theoretically. The energies and probabilities of the transition between the vibrational levels of the electronic states, determining the excitation and Auger decay of the resonances of the nitrogen monoxide molecule and further radiation-induced decay of the NO⁺ molecular ion were calculated by the first principles method. Multiplet splitting of the resonances of N*O and NO* and interference of the amplitudes of excitation of the molecule through various vibrational levels of the intermediate resonance explain the observed dependences of the intensity of A ¹Π(υ′) → X ¹Σ⁺(υ″) fluorescence on the excitation radiation energy. The discrepancies between the calculated and experimental integrated intensities of fluorescence point to the necessity of studying cascade processes determined by radiation transitions in NO⁺, including dipole transitions with a changed net spin.     

Polarization transfer in then 2 P-excitation of alkali atoms by longitudinally polarized electrons

In (ns — np)-excitation of alkali atoms by polarized electrons some of the spin orientation of the primary electron beam is transfered to the excited atoms because of exchange collisions. In the present work the polarization transfer to Na, K, Rb, and Cs respectively is studied by looking for the circular light polarization of (np — ns)-decay radiation emitted in forward direction following impact excitation by a beam of longitudinally polarized electrons. Maximum polarization transfers are observed at collision energies with values about 1.5 times that ofnp-thresholds. The transfer increases with increasing atomic number. In case of (6s — 6p)-excitation of cesium 45% of primary electron spin polarization is transfered to the atoms at maximum. The agreement with available close coupling data is satisfactory.     

Photoionization of oriented molecules

A general expression for the angular distribution of photoelectrons with defined spin polarization ejected from oriented molecules is derived in the electric-dipole approximation in the limit of a weak radiation field. An analysis of its geometrical part permits to draw definite conclusions without calculating matrix elements. For linear molecules it is shown that in Hund's cases (a) and (b) photoelectrons may be polarized only parallel to the molecular axis, while in Hund's case (c) they may be polarized in any direction. Appearance of a circular dichroism for nonchiral oriented molecules of relatively low symmetry is predicted. Dependence of a circular dichroism in the angular distribution of photoelectrons on the symmetry of molecules is demonstrated. The results may serve as the framework for studying molecules oriented on surfaces, in liquid crystals, or by molecular beam techniques.     

Spin-resolved off-normal photoemission from xenon adsorbates in comparison with free-atom photoionization

Circularly polarized synchrotron radiation of BESSY has been used to study the angular dependence of the spin polarization of photoelectrons emitted from Xe(√3x√3)R 30° Pd(111). The spin-polarization component along the light helicity varies from +1 for normal emission to ?0.5 for emission angles larger than 45°. The data can be fitted by use of the theory for photoionization of free xenon atoms and yield the dynamical fit parametersA, α, β, ζ for the adsorbate. Good agreement is obtained with the gas phase data.     

Final charge state distribution in the production and decay of hollow Ar

The improved model of radiative-Auger-cascade with shake-off effect is applied to calculate the final charge state distribution and the X-ray and Auger electron spectra emitted during the cascade decay of Ar, with the initial 1s hole created by the synchrotron irradiation. The discrepancy in the predicted charge distribution without the shake-off effect is removed. It was found that the discrepancy was caused mainly by the neglect of the shake-off effect during the productin stage of the 1s hole, and not by the shake-off effect during the decay of the 1s hole. The X-ray and Auger spectra predicted by the improved model are compared with that without the shake-off. The charge distributions resulting from a 2s or 2p initial hole are also presented.     

The triplet state of magnesiumporphin · ethanol in an n-octane crystal studied by microwave induced changes in the fluorescence intensity

The zero-field splitting of the triplet state of magnesiumporphin solvated by ethanol is represented by D = 0.035 cm^?1 and |E| = 0.010 cm^?1. The decay rates of the upper two spin components both are found to be about 20 s^?1, while that of the bottom component (where the spin lies in the molecular plane) is about 2 s^?1.     

Doubly autoionizing capture resonances ine + Mg+ collisions

Fine details in the cross section for electron impact ionization of Mg⁺ ions (configuration 1s ² 2s ² 2p ⁶ 3s) have been measured with an energy resolution of 0.3 eV. Structure on top of a smooth direct-ionization “background” reflects the presence of indirect ionization mechanisms. Such contributions in a Na-like ion involvenon-resonant excitation of a 2p electron to singly autoionizing intermediate states subsequently decaying into the channel of net single ionization of the parent ion. We observe even stronger indirect contributions fromresonant excitation of a 2p electron with simultaneous dielectronic capture of the projectile electron into doubly autoionizing states which decay by successive emission of two electrons.     

ESCA: Electron Spectroscopy for Chemical Analysis

A substance on which X-rays fall emits photoelectrons and Auger electrons. The energy spectra of the electrons emitted provide information about the electronic structure in the specimen, ranging from the innermost atomic levels and their dependence on the chemical environment to the molecular orbitals of the valence electrons and the band structure in solids. Electron spectra of this nature can now be recorded with high-resolution instruments; their analysis offers new aspects for investigation of chemical composition. The method of electron spectroscopy developed for this purpose, which has now been developed to a high degree of perfection, will be referred to in the following discussion as ESCA (Electron Spectroscopy for Chemical Analysis).     

Rate constant of free electron-hole recombination in thin cadmium sulfide films

The transient decay kinetics of electrons generated in thin cadmium sulfide films by short laser pulses was studied by the microwave photoconductivity method (9 and 36 GHz) at 295 K. The films were prepared by the pulverization method from thiocarbamide coordination compounds. At the high light intensity I ₀ > 10¹⁴ photon cm^–2 per pulse, the decay kinetics of photoelectrons corresponded to a reaction of the second order. Analysis of the kinetic data made it possible to determine the rate constant of recombination of free electrons and holes: k ₃ 2(±1)·10^–13 cm³ s^–1.     

Electron emission in collisions of slow rare gas ions with partially cesiated W (110)

We report the spectra of electrons emitted after slow collisions (50 eV) of rare gas ions (He⁺, Ne⁺, Ar⁺, and Xe⁺) with partially cesiated W(110) surfaces. The electron spectra are discussed in terms of the two interatomic Auger processes, Auger Capture and Auger Deexcitation and the intra-atomic Auger process, Autodetachment of Rg^–* (He^–* (1s 2s ^{2 2} S) in the case of He). Model calculations including these processes can qualitatively reproduce the measured spectra as well as experimental values for the electron emission coefficients, . This is demonstrated for collisions of He⁺ ions and thermal metastable He atoms with clean and partially cesiated W(110).     

Magnetic Chirality Transitions of D- and L-Alanine

A chiral spin state of (N⁺H) in D- and L-alanine was established by monitoring the temperature dependence of dc-magnetic susceptibility (dc: direct current) under the external magnetic field of 1 T. An intrinsic spin chirality of electrons in the atomic magnetic dipole moment of (N⁺H) was also supported by polarized Raman spectroscopy. Magnetic chirality was associated with a strongly correlated electron system that was related to spin rigidity. Raman vibrational spectra were unrelated to structural chirality but could reflect spin chirality due to the reversal of motion breaking. The spin transition of (N⁺H) occured at 270 K without bond breaking but was assisted by an intermediate hydrogen bond elongation, splitting and reformation with NH₃⁺ torsion. The energy difference of spin chirality transitions between D- and L-alanine was around 10⁻⁴-10⁻⁵ eV·molecule⁻¹.     

Formation of multi-charged Kr ions through photoionization of 2p electrons studied with a coincidence technique

Multi-charged Kr ions have been measured using monochromatized undulator radiation combined with a coincidence technique. The coincidence measurements between multi-charged ions and energy-selected Auger electrons have clarified decay processes, as follows. The Auger final states formed through L₃M₄₅M₄₅ decays turn significantly into Kr⁴⁺ and those through L₃M₂₃M₄₅ decays generate Kr⁵⁺ mainly. The Auger decays of L₃M₂₃M₂₃ types yield Kr⁶⁺ dominantly. These findings are consistent with the consideration on energy levels of Kr ions.     

Production of a spin-polarized,metastable He(23 S) beam for studies in atomic and surface physics

This beam was developed as a target for a crossed-beam electron-atom scattering experiment on the interaction of a polarized spin-1/2 electron with a polarized spin-1 atom. In the future this beam will be used in “Spin-Polarized Metastable Atom Deexcitation Spectroscopy” (SPMDS) for studying ferromagnetic surfaces without and with adsorbate layers. We use a discharge source for producing a beam of metastable helium atoms, a permanent sextupole magnet with a central stop at its exit for selecting He(2³ S) atoms in the Zeeman substatem _s =+1, a zero-field spin flipper for reversing the atomic beam polarization with respect to a magnetic guiding field, and a Stern-Gerlach magnet for analyzing the atomic polarization. At a distance of 90 cm beyond the exit of the sextupole, in the “interaction region” of an experiment, the polarized beam has a circular cross section of about 6 mm FWHM and a particle density of 1 · 10⁷ atoms/cm³. The reversible spin polarization was determined asP=0.90±0.02. A possible contamination of the beam with metastable singlet atoms is included within this value; the ground-state He atoms are not considered to be part of the polarized beam. An observed contamination with long-lived Rydberg atoms can easily be destroyed by applying a high electric field.     

Multiple valence electron detachment following Auger decay of inner-shell vacancies in gas-phase DNA

We have studied soft X-ray photoabsorption in the doubly deprotonated gas-phase oligonucleotide [dTGGGGT–2H]²⁻. The dominating decay mechanism of the X-ray induced inner shell vacancy was found to be Auger decay with detachment of at least three electrons, leading to charge reversal of the anionic precursor and the formation of positively charged photofragment ions. The same process is observed in heavy ion (12 MeV C⁴⁺) collisions with [dTGGGGT–2H]²⁻ where inner shell vacancies are generated as well, but with smaller probability. Auger decay of a single K-vacancy in DNA, followed by detachment of three or more low energy electrons instead of a single high energy electron has profound implications for DNA damage and damage modelling. The production of three low kinetic energy electrons with short mean free path instead of one high kinetic energy electron with long mean free path implies that electron-induced DNA damage will be much more localized around the initial K-shell vacancy. The fragmentation channels, triggered by triple electron detachment Auger decay are predominantly related to protonated guanine base loss and even loss of protonated guanine dimers is tentatively observed. The fragmentation is not a consequence of the initial K-shell vacancy but purely due to multiple detachment of valence electrons, as a very similar positive ion fragmentation pattern is observed in femtosecond laser-induced dissociation experiments.

A K-shell vacancy in DNA that is induced by a (therapeutically relevant) soft X-ray of MeV carbon ion, decays by Auger processes accompanied by emission of at least 3 low energy electrons.     

Alignment of atomic autoionizing states created by slow Na++He collisions

Ejected-electron spectra have been measured for collisions of He-atoms with Na⁺-ions, whose impact energy ranged from 1.7 to 7.0 keV. The ion impact-energy dependence of the angular-differential cross-section of the ejected electrons has been investigated for an aligned autoionizing state Na**(2p ⁵ 3s ^{2 2} P), which has been created by charge transfer from the He-atoms. The alignment of the autoionizing state Na**(2p ⁵ 3s ^{2 2} P) is discussed in relation to the scattering angles of the Na⁺-ions. A complete longitudinal alignment has been observed with respect to the quasimolecular axis.