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.     

Spin-orbit relativistic long-range corrected time-dependent density functional theory for investigating spin-forbidden transitions in photochemical reactions

A long-range corrected (LC) time-dependent density functional theory (TDDFT) incorporating relativistic effects with spin-orbit couplings is presented. The relativistic effects are based on the two-component zeroth-order regular approximation Hamiltonian. Before calculating the electronic excitations, we calculated the ionization potentials (IPs) of alkaline metal, alkaline-earth metal, group 12 transition metal, and rare gas atoms as the minus orbital (spinor) energies on the basis of Koopmans' theorem. We found that both long-range exchange and spin-orbit coupling effects are required to obtain Koopmans' IPs, i.e., the orbital (spinor) energies, quantitatively in DFT calculations even for first-row transition metals and systems containing large short-range exchange effects. We then calculated the valence excitations of group 12 transition metal atoms and the Rydberg excitations of rare gas atoms using spin-orbit relativistic LC-TDDFT. We found that the long-range exchange and spin-orbit coupling effects significantly contribute to the electronic spectra of even light atoms if the atoms have low-lying excitations between orbital spinors of quite different electron distributions.     

Evaluation of elastic‐scattering cross sections for electrons and positrons over a wide energy range

Quantification of surface‐ and bulk‐analytical methods, e.g. Auger‐electron spectroscopy (AES), X‐ray photoelectron spectroscopy (XPS), electron‐probe microanalysis (EPMA), and analytical electron microscopy (AEM), requires knowledge of reliable elastic‐scattering cross sections for describing electron transport in solids. Cross sections for elastic scattering of electrons and positrons by atoms, ions, and molecules can be calculated with the recently developed code ELSEPA (Elastic Scattering of Electrons and Positrons by Atoms) for kinetic energies of the projectile from 10 eV to 50 eV. These calculations can be made after appropriate selection of the basic input parameters: electron‐density distribution, a model for the nuclear‐charge distribution, and a model for the electron‐exchange potential (the latter option applies only to scattering of electrons). Additionally, the correlation‐polarization potential and an imaginary absorption potential can be considered in the calculations. We report comparisons of calculated differential elastic‐scattering cross sections (DCSs) for silicon and gold at selected energies (500 eV, 5 keV, 30 keV) relevant to AES, XPS, EPMA, and AEM, and at 100 MeV as a limiting case. The DCSs for electrons and positrons differ considerably, particularly for medium‐ and high‐atomic‐number elements and for kinetic energies below about 5 keV. The DCSs for positrons are always monotonically decreasing functions of the scattering angle, while the DCSs for electrons have a diffraction‐like structure with several minima and maxima. A significant influence of the electron‐exchange correction is observed at 500 eV. The correlation‐polarization correction is significant for small scattering angles at 500 eV, while the absorption correction is important at energies below about 10 keV. Copyright © 2005 John Wiley & Sons, Ltd.     

New relativistic ANO basis sets for transition metal atoms

New basis sets of the atomic natural orbital (ANO) type have been developed for the first, second, and third row transition metal atoms. The ANOs have been obtained from the average density matrix of the ground and lowest excited states of the atom, the positive and negative ions, and the atom in an electric field. Scalar relativistic effects are included through the use of a Douglas-Kroll-Hess Hamiltonian. Multiconfigurational wave functions have been used with dynamic correlation included using second order perturbation theory (CASSCF/CASPT2). The basis sets are applied in calculations of ionization energies, electron affinities, and excitation energies for all atoms and polarizabilities for spherically symmetric atoms. These calculations include spin-orbit coupling using a variation-perturbation approach. Computed ionization energies have an accuracy better than 0.2 eV in most cases. The accuracy of computed electron affinities is the same except in cases where the experimental values are smaller than 0.5 eV. Accurate results are obtained for the polarizabilities of atoms with spherical symmetry. Multiplet levels are presented for some of the third row transition metals.     

Third-order Douglas-Kroll relativistic coupled-cluster theory through connected single, double, triple, and quadruple substitutions: applications to diatomic and triatomic hydrides

Coupled-cluster methods including through and up to the connected single, double, triple, and quadruple substitutions have been derived and implemented automatically for sequential and parallel executions by an algebraic and symbolic manipulation program TCE (TENSOR CONTRACTION ENGINE) for use in conjunction with a one-component third-order Douglas-Kroll approximation for relativistic corrections. A combination of the converging electron-correlation methods, the accurate relativistic reference wave functions, and the use of systematic basis sets tailored to the relativistic approximation has been shown to predict the experimental singlet-triplet separations within 0.02 eV (0.5 kcal/mol) for five triatomic hydrides (CH2, NH2+, SiH2, PH2+, and AsH2+), the experimental bond lengths (re or r0) within 0.002 angstroms, rotational constants (Be or B0) within 0.02 cm(-1), vibration-rotation constants (alphae) within 0.01 cm(-1), centrifugal distortion constants (De) within 2%, harmonic vibration frequencies (omegae) within 8 cm(-1) (0.4%), anharmonic vibrational constants (xomegae) within 2 cm(-1), and dissociation energies (D0(0)) within 0.02 eV (0.4 kcal/mol) for twenty diatomic hydrides (BH, CH, NH, OH, FH, AlH, SiH, PH, SH, ClH, GaH, GeH, AsH, SeH, BrH, InH, SnH, SbH, TeH, and IH) containing main-group elements across the second through fifth rows of the periodic table. In these calculations, spin-orbit effects on dissociation energies, which were assumed to be additive, were estimated from the measured spin-orbit coupling constants of atoms and diatomic molecules, and an electronic energy in the complete-basis-set, complete-electron-correlation limit has been extrapolated in two ways to verify the robustness of the results: One assuming Gaussian-exponential dependence of total energies on double through quadruple zeta basis sets and the other assuming n(-3) dependence of correlation energies on double through quintuple zeta basis sets.     

Differential cross section for the H+D(2)-->HD(v(')=1,j(')=2,6,10)+D reaction as a function of collision energy

We have measured differential cross sections (DCSs) for the HD (v(')=1,j(')=2,6,10) products of the H+D(2) exchange reaction at five different collision energies in the range 1.48< or =E(coll)< or =1.94 eV. The contribution from the less energetic H atoms formed upon spin-orbit excitation of Br in the photolysis of the HBr precursor is taken into account for two collision energies, E(coll)=1.84 and 1.94 eV, allowing us to disentangle the two different channels. The measured DCSs agree well with new time-dependent quantum-mechanical calculations. As the product rotational excitation increases, the DCSs shift from backward to sideward scattering, as expected. We also find that the shapes of the DCSs show only a small overall dependence on the collision energy, with a notable exception occurring for HD (v(')=1,j(')=2), which appears bimodal at high collision energies. We suggest that this feature results from both direct recoil and indirect scattering from the conical intersection.     

Electron screening in low energy scattering of muonic hydrogen on hydrogen atoms

Electron screening corrections to the cross sections for low energy scattering of muonic hydrogen on hydrogen atoms are calculated. It is shown that the presence of the electron influences considerably the elastic cross sections at collision energies below 1 eV. This influence is relatively small for the spin-flip and isotopic exchange processes.     

Circular dichroism in the angle-resolved C 1s photoemission spectra of gas-phase carvone enantiomers

The inner-shell C 1s photoionization of randomly oriented molecules of the chiral compound carvone has been investigated using circularly polarized synchrotron radiation up to 30 eV above threshold. Binding energies of the C=O and CH2= carbon 1s orbitals were determined to be 292.8+/-0.2 and 289.8+/-0.2 eV, respectively. The remaining C-H C 1s levels substantially overlap under an intense central peak centered at 290.5+/-0.2 eV. The angle-resolved photoemission from the carbonyl carbon C=O core orbital in pure carvone enantiomers shows a pronounced circular dichroism of approximately 6% at the magic angle of 54.7 degrees to the light beam propagation direction. This corresponds to an expected 0 degrees -180 degrees forward-backward electron emission asymmetry of approximately 10%. On changing between the R and S enantiomers of carvone the sense or sign of the asymmetry and associated dichroism effectively reverses. The observed circular dichroism, and its energy dependence, is well accounted for by calculations performed in the pure electric dipole approximation.     

Left-right asymmetry in superelastic collisions of polarized electrons with unpolarized laser-excited sodium atoms

Polarized electrons have been scattered superelastically from laser-excited unpolarized sodium atoms (deexcitation of the 3² P _1/2 or 3² P _3/2 states). The left-right scattering asymmetry, which results from an interplay of the atomic charge cloud orientation, atomic fine-structure interaction and exchange processes, has been measured for energies between 6 and 20 eV and scattering angles ranging from 40 to 120°. Within the experimental uncertainty the Percival-Seaton hypothesis, which is the basis of current theoretical calculations for electron-sodium collisions, has been confirmed. However, the quantitative agreement between experimental and numerical results is satisfactory only for a 2-state close-coupling calculation of Moores.     

Photoemission from solid neon

The photoelectric yield of solid Ne has been studied for photon energies between 15 and 30 eV. Strong photoemission from the valence band sets in at 21.4 ± 0.1 eV. The value of the electron affinity derived from this is discussed. A surface resonance for Ne atoms adsorbed on gold is observed at 17.1 eV.     

Steric effects in electron transfer from potassium to pi-bonded oriented molecules CH3CN, CH3NC, and CCl3CN

Electron transfer from K atoms to oriented CH3CN, CH3NC, and CCl3CN is studied in crossed beams at energies near the threshold for forming an ion pair. For the methyl compounds, the dominant ions are K+ and CN-; the steric asymmetry is very small and energy-independent, characteristic of sideways attack with the electron apparently entering the pi*CN antibonding orbital. Migration of the electron to the sigma*CC orbital to break the C-C bond is greatly facilitated by interaction with the atomic donor. CH2CN- is formed in collisions preferring CH3-end attack, and the steric asymmetry becomes very large near threshold. CCl3CN mostly forms Cl- in collisions slightly favoring the CCl3 end with a small energy dependence with the electron apparently entering the sigma* LUMO. CN- is formed in much smaller yield with a slight preference for the CN end. The parent negative ion CCl3CN- is observed, and a lower limit for its electron affinity is estimated to be 0.3 eV. Fragment ions CCl2CN- and CClCN- are also observed with upper limits for the quantity bond dissociation energy - electron affinity (BDE - EA) estimated to be 0.6 and 1.0 eV, respectively.     

Product spin-orbit state resolved dynamics of the H+H2O and H+D2O abstraction reactions

The product state-resolved dynamics of the reactions H+H(2)O/D(2)O-->OH/OD((2)Pi(Omega);v',N',f )+H(2)/HD have been explored at center-of-mass collision energies around 1.2, 1.4, and 2.5 eV. The experiments employ pulsed laser photolysis coupled with polarized Doppler-resolved laser induced fluorescence detection of the OH/OD radical products. The populations in the OH spin-orbit states at a collision energy of 1.2 eV have been determined for the H+H(2)O reaction, and for low rotational levels they are shown to deviate from the statistical limit. For the H+D(2)O reaction at the highest collision energy studied the OD((2)Pi(3/2),v'=0,N'=1,A') angular distributions show scattering over a wide range of angles with a preference towards the forward direction. The kinetic energy release distributions obtained at 2.5 eV also indicate that the HD coproducts are born with significantly more internal excitation than at 1.4 eV. The OD((2)Pi(3/2),v'=0,N'=1,A') angular and kinetic energy release distributions are almost identical to those of their spin-orbit excited OD((2)Pi(1/2),v'=0,N'=1,A') counterpart. The data are compared with previous experimental measurements at similar collision energies, and with the results of previously published quasiclassical trajectory and quantum mechanical calculations employing the most recently developed potential energy surface. Product OH/OD spin-orbit effects in the reaction are discussed with reference to simple models.     

Ionization interference in the acetylene-nitrous oxide flame

The influence of excess cesium upon the atomic line intensities or absorbances in the acetylene-nitrous oxide flame has been investigated for twenty elements with ionization energies up to 8 eV. Volatilization interference has been corrected for. It is shown that true degrees of ionization can be derived from the intensity increase observed upon cesium addition if corrections for incomplete atomization are made and no molecular ions are formed.     

Triplet states in 1,3-butadiene

The electron impact excitation spectrum 0f 1,3-butadiene has been studied at 20, 35 and 55 eV impact energies and scattering angles of 10° to 80°. Two low lying states are observed with maxima at 3.2 and 4.9 eV, and are identified as the ³B_u state and ³A_g state respectively     

Dissociative ionization of O2 and N2 by electron impact — N+ and O+ kinetic energies and angular distributions

An apparatus containing cross molecular and pulsed electron beams has been used to obtain distributions in kinetic energy and angle of fast (? 0.5 eV) positive ions produced through dissociative ionization of N₂ and O₂ by impact of 50 to 2000 eV electrons. Four main O⁺ ion groups are observed with peak energies of 0.8, 2.0, 3.0, and 5.0 eV. Two main N⁺ groups peaking at 2.0 and 3.0 eV are seen. Angular distributions of both N⁺ and O⁺ ions are essentially isotropic for electron-beam-ion detection angles from 30° to 110°.     

Experimental evidence for impulsive energy transfer in electronically adiabatic low-energy collison of Na+ with CF4

Time-of-flight measurement of the inelastic scattering of Na⁺ ions from CF₄ reveal two energy-loss mechanisms which have been observed at center-of-mass collison energies between 4 and 24 eV. At small angles θ_1ab?20° a single energy transfer peak is observed and attributed to normal mode excitation. At larger angles an additional sharp peak is observed out to θ_lab = 70° with nearly double the energy transfer. the final velocities and angular dependence agree with an impulsive interaction involving only one of the F atoms with an effective mass which approaches that of the F atoms with increasing collison energy. This is the first evidence for an impulsive mechanism in inelastic collisons at energies below the threshold for electonic excitation.     

Nitrogen-rich oligoacenes: candidates for n-channel organic semiconductors

The successive replacement of CH moieties by nitrogen atoms in oligoacenes (benzene to hexacene) has been studied computationally at the B3LYP/6-311+G(d,p)//6-31G(d) level of theory, and the effects of different heteroatomic substitution patterns on structures, electron affinities, excitation, ionization, and reorganization energies are discussed. The calculated tendencies are rationalized on the basis of molecular orbital arguments. To achieve electron affinities of 3 eV, a value required to allow for efficient electron injection from common metal electrodes, at least seven nitrogen atoms have to be incorporated into tetracenes or pentacenes. The latter require rather small reorganization energies for electron transfer (<0.20 eV) making these compounds promising candidates for n-channel semiconducting materials. Particularly interesting are heptaazapentacenes 5 and 6 in which the nitrogen atoms are arranged to form self-complementary systems with a maximum number of intermolecular CH-N contacts in planar oligomers. These interactions are expected to facilitate the formation of graphite-like sheet structures with cofacial arrangements of the pi systems and short interlayer distances due to attractive N-C(H) interlayer interactions. This should not only be ideal for charge transfer but also might contribute to improved air stability of these semiconductors. Self-complementarity is maintained in azaacenes containing two cyano groups in the terminal rings. These compounds require lower reorganization energies than the unsubstituted heterocycles (0.13-0.14 eV), show high electron affinities (3.3 eV), and are thus promising candidates for materials applications.     

Electron-impact coherence parameters for the excitation of the 51 P state of Sr

Electron-photon polarization correlation measurements have been carried out for the excitation of the 5¹P state of Sr at electron impact energies of 30.3 and 58.4 eV and electron scattering angles of 20°–130° for 30.3 eV and 20°–100° for 58.4 eV. The resulting Stokes parametersP ₁,P ₂,P ₃ are used to derive the usual complete scattering parameter sets λ, χ and γ,L _⊥,P _l. New FOMBT calculations for these parameters are reported alongside the measured data and show substantial agreement with the experiment and with recent calculations by Srivastava et al.     

Electrical transport properties of a nonstoichiometric rare earth sulfide,EuGd2S4

The electrical transport properties of nonstoichiometric EuGd₂S₄ prepared by heating under a high vacuum have been investigated. The samples heated were classified into two groups on the basis of their electrical transport behavior. One group comprised semiconducting materials heated at 1500 and 1600°C, for which the transport mechanism was found to be via electron hopping with activation energies ranging from 0.013 to 0.027 eV. Another group comprised metallic materials heated at 1700 and 1800°C. Their electrical transport was carried out through ordinary band conduction over the measured temperature range except at temperatures lower than 120°K, where hopping with a very small activation energy (～0.0035 eV) occurred predominantly.     

Spin asymmetry in near threshold (e, 2e) process for lithium and hydrogen

The singlet and triplet cross sections and the asymmetry parameter for e-H and e-Li ionization have been calculated in a distorted-wave Born approximation for incident energies in the range 0.5 to 20 eV above the ionization threshold. The two final-state continuum electrons are assumed to share equal energy and to come out 180° apart. The variation with respect to the angle of scattering is found to be qualitatively very different in the two cases. It is found that, in the case of lithium, the ¹D and ³F components in the scattering amplitude are quite significant even at very low energies.