Angular Distribution Of Ejected Electrons Research Articles

Angular distributions of photoelectrons following photoionization of nonspherically symmetric Rydberg states in alkaline-earth atoms

Angular distributions of ejected electrons from the mpnd autoionizing states in the alkaline-earth atoms ( m = 3 to 6 for Mg, Ca, Sr and Ba atoms) have been investigated. Starting from the most general theory of photoexcitation, we have carried out specific analytic expressions in terms of the reduced dipole matrix elements for various angular distributions corresponding to different ion-core states. The properties of the symmetry parameters which characterize the angular distribution are studied in detail. Manifestations involved in both total and differential cross sections of photoionization are interpreted.

Angular distributions of electrons following photoionization of spherically symmetric Rydberg states in alkaline-earth atoms.

Two theoretical treatments have been employed to investigate various angular distributions of ejected electrons following the photoionization of the msns Rydberg states in the alkaline-earth atoms. The specific analytic expressions of angular distributions in terms of reduced dipole matrix elements have been carried out. The characteristics of angular distributions are interpreted in detail. \textcopyright{} 1996 The American Physical Society.

THEORY OF BARRIER-SUPPRESSION IONIZATION OF ATOMS

The theoretical description of the ionization of an atom (ion) by external electromagnetic radiation up to now concerned two alternative situations; multiphoton ionization and tunneling ionization. For both cases the formulas describing the ionization probability when the intensity of the laser field is not too strong are well known. However, if the field is strong, then there exists a new channel; the so-called barrier-suppression (or above-barrier) ionization of the atom. How does this process occur? It is obvious that barrier-suppression ionization and sub-barrier tunneling ionization by low-frequency laser field transform smoothly into one another as the field strength F is varied near the value of the barrier-suppression field F BSI . This paper contains a review of various theoretical approaches developed during the last few years, especially analytical considerations. Some new semi-analytical expressions for ionization rates, energy and angular distributions of ejected electrons are also derived.

Two-dimensional photoelectron imaging of state-selected iodine atom by (2+1) resonance-enhanced multiphoton ionization

The state-selective photoelectron imaging technique is applied to investigate the angular distribution of ejected electrons and the electronic state of the atomic ion produced by (2+1) resonance-enhanced multiphoton ionization of iodine atom in the photolysis of CH 3I. Strong anisotropic distribution behaviors have been depicted on two electron images observed, one via the two-photon resonance 2 D 5 2 0 state from ground state iodine and, another via 4 D 5 2 0 from spin-orbit excited iodine. The configuration of core electrons is preserved in the ionization from the 2 D 5 2 0 intermediate state, while the ionization from the 4 D 1 2 0 state shows two distinct ionization paths, a preserved and a non-preserved path of core electrons.

Energy shift and broadening of H levels in front of a metal surface

The energy position and width of H levels in front of an Al surface are studied with the nonperturbative coupled angular mode (CAM) method. The results are compared with previously published results by Nordlander and Tully [Phys. Rev. Lett. 61 (1988) 990 and Surf. Sci. 211/212 (1988) 207]. A strong hybridization is observed inside the various manifolds. The angular distributions of ejected electrons associated with the hybrids of the n = 2 and 3 manifolds are analyzed and correlated with the size of the level widths.

Angular distributions of ejected electrons from autoionizing 3pnd states of magnesium.

We have measured the angular distributions of electrons ejected from doubly excited autoionizing Rydberg 3pnd J=1 and 3 states of magnesium, where n=10--18. To excite the aligned 3snd $^{1}$${\mathit{D}}_{2}$ states of Mg, we used two dye lasers crossed with an atomic beam. A third laser, linearly polarized in the same direction as the other two, excited the atoms in the 3snd states to the 3pnd states. The angular distributions of ejected electrons are given by d\ensuremath{\sigma}/d\ensuremath{\Omega}=${\mathit{tsum}}_{\mathit{k}}$${\mathit{a}}_{\mathit{k}}$${\mathit{P}}_{\mathit{k}}$(cos\ensuremath{\theta}), where k=0,2,4,6. We measured the angular distribution parameters ${\mathit{a}}_{\mathit{k}}$ as a function of the energy of the third laser photon using an electron detector and a ${\mathrm{Mg}}^{+}$ ion detector. We compare our results with the predictions of an R-matrix multichannel quantum-defect theory. We find good agreement between experiment and theory.

Relativistic effects in the angular distribution of ejected electrons in tunneling ionization of atoms by strong electromagnetic fields

The effect of magnetic dipole interactions on the angular distribution of electrons in the tunneling ionization of atoms is considered theoretically. Linearly polarized radiation is assumed. It is shown that an asymmetry in the angular distribution of ejected electrons in the azimuthal angle appears. We conclude that the angular distribution is strongly changed by relativistic effects even at electron velocities that are small compared with the speed of the light.

Dynamical resonant electron capture in atom surface collisions: H- formation in H-Al(111) collisions.

The formation of ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ ion by grazing-angle collisions of hydrogen on an Al(111) surface is investigated with the newly developed coupled angular mode method. The capture process involves a dynamical resonant process induced by the collision velocity. All the resonance properties of the ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ level in front of an Al(111) surface are determined: position, width, and angular distribution of ejected electrons. The results are shown to account for the recent observations on ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ formation by Wyputta, Zimny, and Winter.

Angular distribution of ejected electrons from autoionizing 3pns states of magnesium.

We have measured the angular distribution of electrons ejected from doubly excited autoionizing Rydberg 3pns J=1 states of magnesium, where n=10--20. Two dye lasers cross an atomic beam and excite 3${\mathit{sns}}^{1}$${\mathit{S}}_{0}$ states of Mg. A third linearly polarized laser excites the 3pns states. The angular distribution is of the form (\ensuremath{\sigma}/4\ensuremath{\pi})[1+\ensuremath{\beta}${\mathit{P}}_{2}$(cos\ensuremath{\theta})]. We measure \ensuremath{\beta} as a function of the energy of the third laser. The \ensuremath{\beta} parameter varies over its full range of values, -1 to 2. Our data agree quite well with the predictions of the reaction-matrix--multichannel-quantum-defect theory.

Evidence for electron correlation in the two-electron continuum during double ionization in 300-keV H++He collisions.

Absolute differential ionization probabilities for small impact parameter H{sup +}+He collision were measured as a function of the electron energy, electron ejection angle, and final recoil-ion charge state. Good agreement was found between an experimental angular distribution of ejected electrons and results of a coupled-channel calculation in the case of single ionization. In the case of double ionization, significant discrepancies to theoretical predictions are found, indicating a breakdown of the independent electron model. Furthermore, evidence is provided that dynamic correlation effects in the two-electron continuum strongly influence the angular distribution of ejected electrons for double ionization.

Angular distribution of ejected electrons in atomic collisions: The three-dimensional solvable model and the adiabatic approach.

A three-dimensional model is studied in which a particle moves in the field of two colliding zero-range potentials. Exact wave functions are constructed for the case of a head-on collision. The probability distribution of the ejected particle in momentum space is investigated. The numerical calculations are compared with the results of the adiabatic approximation.

Influence of surface defects on thermal desorption spectra

The energy position and width of H levels in front of an Al surface are studied with the nonperturbative coupled angular mode (CAM) method. The results are compared with previously published results by Nordlander and Tully [Phys. Rev. Lett. 61 (1988) 990 and Surf. Sci. 211/212 (1988) 207]. A strong hybridization is observed inside the various manifolds. The angular distributions of ejected electrons associated with the hybrids of the n = 2 and 3 manifolds are analyzed and correlated with the size of the level widths.

Angular distribution of ejected electrons in resonant Auger processes of Ar, Kr, and Xe.

Angle-resolved electron spectroscopy with the help of synchrotron radiation has been used to study resonant Auger processes near the core shells of Ar 2p, Kr 3d, and Xe 4d. Results for the lowest-energy resonances have received special attention: argon (Ar 2${p}_{3/2}$\ensuremath{\rightarrow}4s) at a photon energy of 244.4 eV, krypton (Kr 3${d}_{5/2}$\ensuremath{\rightarrow}5p) at 91.2 eV, and xenon (Xe 4${d}_{5/2}$\ensuremath{\rightarrow}6p) at 65.1 eV. The angular distribution parameters \ensuremath{\beta} are evaluated for each of the resolved Auger peaks. Most striking is the occurrence of large negative \ensuremath{\beta} values for some of the higher kinetic energy peaks. The results are most apparent under high electron resolution. The theoretical basis for having \ensuremath{\beta} values near -1 is discussed. In particular, the experimental results for argon are found to be in general agreement with the prediction of -1 \ensuremath{\beta} values by using either angular momentum transfer theory or normal Auger theory. However, a better understanding of the range of \ensuremath{\beta} values will have to await explicit calculations. Experimental results on the Auger spectra are also given at lower kinetic energies, for shakeup states, and for higher-energy resonances, especially those involving vacancies in the core shells of the lower spin states. It is shown that, although transitions having the same final state of the singly charged ion frequently have similar relative intensities and \ensuremath{\beta} values, occasionally they differ quite markedly. The theoretical consequences of variant behavior for processes having the same final states are also discussed.

Investigation of an optical potential model for post-collision interaction in ejected-electron spectra

An optical potential method is used to formulate a model for the post-collision interaction that occurs in electron-helium scattering after excitation and decay of autoionising states. The model is an extension of an earlier description given by Nienhuis and Heideman, and is used to study the effects of post-collision interaction in ejected-electron spectra. The authors present a calculation of angular distributions of ejected electrons from the He**(2s2)1S autoionising state. It is argued that study of angular momentum exchange during the post-collision interaction requires detailed knowledge of the interfering background of the direct ionisation process.

Collisional excitation and decay of the 1P shape resonance of H-

The /sup 1/P shape resonance of H/sup -/ has been populated in collisions between H/sup -/ and Xe. We report on the decay: H/sup -/( /sup 1/P)..-->..H/sup 0/(n = 2)+e/sup -/, which for the first time is measured by high-resolution electron spectroscopy. New information on the angular distribution of ejected electrons is obtained by use of a Shore parametrization to describe the cross section. The Shore parameters are related to matrix elements of the transition operator. The obtained resonance energy and width are in good agreement with recent theoretical calculations as well as other experimental results.

Energy and angular distributions of ejected electrons for hydrogen-cluster-ion (Hn+, Dn+, n=1-3) impacts on helium in the intermediate energy region

The absolute electron production cross sections for 5-30 keV Hn+ and Dn+ (n=1-3) impacts on helium, differential in angle and energy of the ejected electrons, have been measured for electron energies of 3-200 eV at ejection angles from 30 degrees to 150 degrees with respect to the incident ion beam direction. The electron production yields have been compared between isotope ions with the same velocity. No isotope effect has been found for D+ and H+ impacts at 10 keV amu-1 and D2+ impacts at 5 keV amu-1, whereas it has been found that the electron production yield for D3+ impact is considerably larger than that for H3+ at 3.33 keV amu-1 at a forward ejection angle of about 30 degrees .

On the Theory of Ion‐Induced Electron Emission

AbstractA microscopic theory of H ion‐induced electron emission is developed in close analogy to the theory of secondary electron emission and applied to polycrystalline aluminum. The formation of excited electrons by screened Coulomb interaction with H+ ions of primary energy E0 between 20 and 800 keV is considered. Both, elastic and inelastic collisions of these electrons are taken into account. As far as possible the results calculated are compared with recent experimental data. Energy and angular distributions of ejected electrons prove to be in accordance with measurements. The calculated electronic yield γ shows a maximum in the primary energy dependence at E0 ≈ 55 keV with a maximum value γmax = 1.1. With increasing primary energy there are larger deviations from the measured yield values.

Observation of a new Cd I autoionising level by (e,2e) spectroscopy

Electron-electron coincidence spectroscopy is used to detect a previously unknown Cd I autoionising level at 12.01 eV. This is done by an analysis of the angular distribution of ejected electrons observed in coincidence with electrons scattered through a chosen angle. The new level has J>1 and, for a scattering angle of 3 degrees and 150 eV impact energy, an excitation probability one fifth that of the Cd I (4d95s25p)3P1 12.06 eV autoionising level.

Photon- and electron-impact ionization and ejected-electron angular distributions from molecules including retardation effects: Nonrelativistic theory

The nonrelativistic first-Born-approximation matrix element for the photoionization and electron impact of a single electron from a molecule with no restriction on its shape, including retardation effects and with rotational energy resolution, is evaluated within the framework of the Born-Oppenheimer approximation. Angular distributions of ejected electrons are cylindrically symmetric about the propagation direction of the photon or electron source if the light is circularly polarized or if no attempt is made to simultaneously measure the scattered and ejected electrons in electron-impact ionization. If plane polarized light is used, retardation effects destroy the cylindrical symmetry about the light propagation direction. If the scattered electron in electron-impact ionization is detected in coincidence with the ejected electron then the angular distribution of ejected electrons shows cylindrical symmetry about the momentum-transfer direction. Use of an LCAO-MO (linear combination of atomic orbitals representation of molecular orbitals) basis for both bound and continuum one-electron orbitals leads to selection and propensity rules for rotational excitation.

Angular distribution of ejected electrons from 20 keV He+impact on He

The angular distributions of ejected electrons in the energy range 5-70 eV have been measured at angles from 30 to 150 degrees for 20 keV He+ impact on He. The angular dependence of excitation cross sections of autoionisation states 2s21S and 2p21D+2s2p1P are in good agreement with previous data measured by Bordenave-Montesquieu et al. (1982). The continuous parts of the electron spectra show symmetrical angular distributions around 90 degrees in the laboratory frame for low-energy electrons (<or=30 eV). These angular distributions are discussed in connection with the molecular autoionisation mechanism.