Measurements of circular polarization correlations following excitation of Ar and Kr by electron impact

We report measurements of the circular polarization correlation parameterP ₃ for the excitation of the 4s′[1/2] _j=1 state in Ar by 40 eV electrons and for the excitation of the Kr 5s[3/2] _J=1 state by 30 eV electrons at electron scattering angles up to 50°. The measured Ar data are compared to predictions of a distorted wave Born approximation (DWBA). The agreement is in general not very good with theory predictingP ₃ values considerably larger than the measured values in the regime of small scattering angles. In Kr, where the measured data are compared to theoretical predictions from a DWBA, a first-order many-body theory (FOMBT) and a relativistic distorted wave theory (RDW), the level of agreement between experiment and theory is somewhat better. The measuredP ₃ parameters in conjunction with the previously measured linear coherence parametersP ₁ undP ₂ under the same scattering conditions yield the total polarizationP _tot ⁺ which is a measure for the level of coherence in the excitation process. In both cases studied here, we found values of eliminateP _tot ⁺ that were consistent with a fully coherent excitation process.     

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.     

An optically pumped,highly polarized cesium beam for the study of spin-dependent electron scattering

We have set up an atomic beam of cesium for the study of spin-dependent electron-cesium scattering. The beam is produced by an effusive oven with continuous recirculation of the condensed metal. The beam is optically pumped by circularly polarized light from two laser diodes tuned to the 6² S _1/2(F=3)→6² P _3/2(F′=4) and 6² S _1/2(F=4)→6² P _3/2(F′=5) transitions, respectively. Nearly all atoms are transferred into theF=4,m _F =+4 orm _F =?4 Zeeman level of the ground state, depending on the sense of circular polarization of the pumping light. The population distribution in the optically pumped beam is analyzed in terms of them _J =?1/2 andm _J =+1/2 components with a Stern-Gerlach magnet. We find the atomic polarization to be very close to unity at a density of 8×10⁸ atoms/cm³ in the scattering center. The polarization decreases slightly with increasing density of the cesium beam due to radiation trapping. A spin flipper serves as a means of polarization reversal, introducing no systematic errors in the spin asymmetry measurements. Lock-in technique is used to stabilize the atomic beam polarization by detecting fluorescence light signals.     

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.     

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.     

Radiation transport within the natural isotopic mixture of rubidium vapor after isotope selective excitation

The radiation transport within the natural isotopic mixture of rubidium is studied by detecting the time dependent resonance fluorescence after isotope selective excitation of⁸⁷Rb or⁸⁵Rb to the 5² P _3/2 state by the pulsed light of a laser diode. At atomic densities of about 10¹² cm^?3 the mean number of photon scatterings is 10 or 25 after excitation of⁸⁷Rb or⁸⁵Rb, respectively. Monte Carlo simulations of multiple photon scattering are used to reproduce the time dependent fluorescence signals. Satisfactory agreement between experiments and calculations can be achieved only if a radiative transfer of excitation energy between the different isotopes is considered. The energy transfer can be explained by radiative coupling of the different isotopes due to two partially overlapping hfs components of the 780 nm resonance line. As a test of our theoretical approach fluorescence signals recorded after excitation of⁸⁷Rb and⁸⁵Rb under identical experimental conditions are fitted both by using the same set of two parameters.     

Depolarization and fine structure effects in half collisions of sodium-noble gas systems

Population ratios and polarization of the Na3P _J fine structure states following far wing photon excitation of Na-noble gas collision pairs are studied with respect to the underlying interaction potentials and molecular coupling schemes. For this purpose spectral profiles of these quantities, i.e. its dependencies on excitation frequency, have been measured up to ±200 cm^?1 detuning from the NaD lines for NaAr and NaHe under nearly single collision conditions. Comparing the measured population ratios with quantummechanical coupled channels calculations the Σ potential well depths differ considerably from results of model potential calculations. Large residualJ=3/2 alignment observed in the far wings disagrees with simple models assuming incoherent excitation and/or full adiabatic reorientation of the radiating dipole along straight trajectories. By comparison with Lewis model calculations, using realistic trajectories and decoupling radius, it is found instead, that realistic trajectories are constitutive for alignment after Σ-excitation (blue wing), whereas coherence between the²Π_1/2 and²Π_3/2 states determine primarily alignment after Π-excitation (red wing).     

4pnp J=0e-2e autoionizing series of calcium: experimental and theoretical analysis

The even parity 4pnp J=0, 1, 2 doubly excited autoionizing states of neutral calcium in an atomic beam are investigated by a two-step isolated core excitation (ICE) method using two different combinations of polarization of the laser beams. The different excited energy levels are assigned to nine autoionizing Rydberg series 4p_{1/2, 3/2}np J=0, 1, 2 for ≤ n ≤ 22. The theoretical interpretation is achieved by a combination of the eigenchannel R-matrix theory and the multichannel quantum defect (MQDT) method. Two, five and six closed interacting channels are introduced for the J=0, J=1 and J=2 series respectively. Theoretical energy level positions, autoionization widths and excitation profiles are compared with the experimental data, confirming the identification of the observed structures and providing evidence of extended mixing between the 4p_1/2np and 4p_3/2np series.     

Amplification of electroweak left-right asymmetry in atoms by stimulated emission

This work describes quantitatively the amplification of the electroweak left-right asymmetry, a remarkable and attractive feature of the experiment in progress on the 6S _1/2 → 7S _1/2 cesium transition using detection by stimulated emission on the 7S _1/2 → 6P _3/2 transition. The process relies on the optical anisotropy of the atomic medium resulting from the 7S _1/2 alignment created by excitation with linearly polarized light. The crucial parameter (α_⊥?α_∥)/α_∥ involves the amplification coefficients for the probe field oriented in a direction either parallel or perpendicular to the alignment axis. Explicit computations are done by a semiclassical approach (classical for the field, quantum mechanical for the atomic states). The larger the optical anisotropy and the optical density, the stronger the asymmetry amplification; among all hyperfine components of the 7S _1/2 → 6P _3/2 transition, the ΔF=0 ones and more particularly the 4 → 4 offer the largest anisotropy. It is also predicted that saturation of the probe transition by the optical field should provide manifestation of the effect at lower optical densities and notwithstanding at larger fluxes of transmitted photons. Indirect production of 6P _3/2 atoms by the excitation pulse does not reduce the left-right asymmetry, neither its amplification which then appears at lower effective optical densities.     

Inelastic and superelastic scattering of electrons from sodium

Recent experimental results of the NIST, Flinders and Münster groups on superelastic scattering of intermediate-energy polarized electrons from laser-excited polarized and unpolarized sodium atoms have been analyzed theoretically together with those on inelastic scattering data. Based on the Persival-Seaton hypothesis specific model-independent relations between correlation and polarization parameters measured in superelastic scattering experiments of different types have been suggested. The differential cross section as well as the parametersL _⊥,L ^S(T) _⊥, γ,r, and \(\bar S\) _A (J) for the 3²S?3² P transition in a broad range of scattering angels have been calculated within the distorted-wave approximation. Problem of constructing the e + Na optical potential is discussed.     

Contracted polarization functions for B to Ar

Using optimal exponents for B through Ne given by Dunning and those for Al through Ar by Woon and Dunning, d-type contracted polarization functions (2d/1d), (3d/1d), and (3d/2d) are generated from natural orbitals of atomic single and double excitation configuration interaction (SDCI) calculations, where the numbers before and after the slash are those of the primitive and contracted Gaussian type functions. The resulting contracted functions are tested on N₂ and P₂ molecules by self-consistent field and SDCI calculations, which clarify characteristics of the present polarization functions. Received: 5 June 1997 / Accepted: 20 August 1997     

Complete density matrix of HeI 3p excited in fast grazing ion-surface collisions via “Hanle-effect”

He⁺-ions with energies ranging from 6 to 25 keV are grazingly scattered off a clean and flat Al(111)-surface. We investigate the excitation of HeI 3p via fluorescent light emitted in the HeI 2s ³ S?3p ^{3 3} P transition. By observation of the polarization of light in dependence on an external longitudinal magnetic field (“Hanle-effect”), we deduce the complete density matrix of HeI 3p excited by fast ion-surface scattering.     

Study of electron impact excitation of atoms through lasers

We discuss three different experiments for studying electron-excitation of atoms where lasers have been used in combination. These are stepwise electron–photon excitation, superelastic electron scattering from laser excited atoms and excitation of atoms using spin polarized electrons produced by lasers. We present distorted wave calculations and compare our results with the recently reported such experimental measurements. In particular, the results for the alignment and orientation of the excited n ²P states of K ($n=4$) and Rb ($n=5$) atoms and the spin parameters for the lowest excited ¹P₁ and ³P_0,1,2 states of argon by polarized electrons are presented and discussed.     

Electron-photon polarisation correlations in 140 eV electron impact excitation of helium

Electron-polarised photon coincidence techniques are used to determine linear and circular polarisation correlations from the differential electron impact excitation of the 2¹ P state of helium at an incident electron energy of 140 eV. At 30° and 45° electron scattering angles, all the Stokes parameters are determined, whereas at 52.5° onlyP ₂ is measured. Comparisons are made with the distorted wave (DW) calculations of Madison [11] and the first order many-body theory (FOMBT) of Cartwright and Csanak [4].     

Transfer of electronic excitation: The Arh system

A theoretical analysis of the quenching of Ar* in its two metastable states (³P₂ and ³P₀) by H is presented. It is shown that a simplified treatment of the electronic structure permits a discussion of the dynamics of that type of excitation transfer in terms of the relative position of a crossing (R_C) of potential curves and of the region (R_D) of decoupling of atomic states by the molecular field. In the present case we explain why the ³P₂ state is more reactive than the ³P₀ state.     

On the scattering of a light pulse on a single atom perturbed by collisions with inert gas atoms

Scattering of a laser pulse on a single atom perturbed by collisions with inert gas atoms is described in terms of suitably defined cross-sections. The relations of the energy balance are explored to show the connection between attenuation and total cross-section for scattering. The different scattering channels, corresponding to the polarisation, spectral decomposition and coherence properties of the light are analysed in terms of the cross-sections. As an example we consider the scattering of a light pulse on an alkali metal atom with a frequency close to the atomic transition frequency from the ground stateS _1/2 to the first excited stateP _1/2.     

Photodissociation Dynamics of OCS at 217 nm

The S(¹D₂)+CO(X¹Σ⁺) product channel from photodissociation of OCS at 217 nm has been measured using the DC slice velocity map imaging (VMI) technique in combination with resonance enhanced multiphoton ionization (REMPI). Two diflerent REMPI intermediate states (¹F₃ and ¹P₁) and several pump-probe laser polarization geometries are used to detect the angular momentum polarization of the photofragmented S(¹D₂). The molecular- frame polarization parameters, as well as the laboratory-frame anisotropy parameters, for individual rotational states of co-fragment CO, are determined using two diflerent full quantum theories. The measured total kinetic energy release spectrum from photodissociation of OCS indicates two dissociation channels, corresponding to the fast and slow recoiling velocities of S(¹D₂), respectively. The slow channel is concluded to originate from an initial photoexcitation to the A(¹A') state, followed by a non-adiabatic transition to the ground state. The fast channel is found to follow a coherent excitation to A(¹A') and B(¹A') states, where contributions of the two states are almost equal at 217 nm. The determined alignment and anisotropy parameters further indicate that the slow channel follows an incoherent excitation, while the fast channel follows a coherent excitation to A(¹A') and B(¹A') states with a phase di erence of π/2.     

Polarization and collision velocity dependence of associative ionization in cold Na(3p)-Na(3p) collisions

We studied the polarization dependence of the associative ionization (AI) process Na(3p) + Na(3p) → Na ₂ ⁺ at collision velocities between 100 and 700 m/s (5 and 200 K), using linearly and circularly polarized light for the excitation. We found that the polarization dependence varies strongly in the collision velocity range under study. At the high end of our scale preparation of both collision partners in the |m _j |=1/2 substates yields the highest AI-rate, but at low velocities this ceases being so: the polarization dependence becomes less pronounced and at the low end of our scale the preparation |m _j 2|, |m _j 2|=3/2, 1/2 is most effective. All cross sections increase strongly at lower velocities. Combined with previous work this maps the detailed polarization dependence from 100–2400 m/s (5-2500 K). From these total results it is concluded that at collision velocities ≧500 m/s mainly one molecular potential curve leads to AI, but that this changes considerably at lower collision velocities. The findings are compared with recent theoretical results by Geltman, and we find large discrepancies.     

Energy distribution of charge carriers in solids in highly non-equilibrium states considering cascade transitions and inelastic scattering

A study is made of the cascade process, which describes the energy loss and multiplication of highly non-equilibrium secondary electrons and holes in crystalline platinum irradiated by low-energy electrons. The pair-creation scattering rates are evaluated in the framework of statistical model that takes into account the electron band structure of platinum. Kinetic equations for the excited electron and hole energy distributions are solved numerically in the isotropic scattering approximation for some primary (excitation) energies E_p that do not exceed the plasma energy E_F+ℏω_pl.