Electron-atom Interaction Research Articles

Transient thermal process after a high-energy heavy-ion irradiation of amorphous metals and semiconductors.

Following a description used to explain a phase transformation observed after pulsed femtosecond laser irradiation, a transient thermal process is used to describe latent-track formation after high electronic excitation induced by energetic (GeV) heavy ions. The transient thermal calculation is restricted to the amorphous materials a-Ge, a-Si, and a-${\mathrm{Fe}}_{85}$${\mathrm{B}}_{15}$, for which nearly all latent-track radii and/or macroscopic thermodynamic properties are known. The heat-flow equation is solved numerically in cylindrical geometry. The time-dependent heat-generation term is assumed to be due to the electron-atom interaction. The characteristic length \ensuremath{\lambda} of the energy transport by secondary electrons is taken as the only free parameter and the maximum diameter of the cylinder of liquid matter is considered as the diameter of the observed latent track. Using the single value \ensuremath{\lambda}=14 nm, we have been able to calculate these diameters in a-Si and a-Ge in reasonable agreement with experimental track diameters, taking into account the large differences between the macroscopic thermodynamic parameters of both materials. This \ensuremath{\lambda} value is less than that for the crystalline state. In the case of a-${\mathrm{Fe}}_{85}$${\mathrm{B}}_{15}$, the diameters calculated with use of \ensuremath{\lambda}=19 nm are in agreement with the ones determined recently by electrical-resistivity change.

Electric field effects on resonance structures in negative ion photodetachment

Abstract The photodetachment of negative ions in a static electric field exhibits some new characteristic features and has beer considered in various theortical approaches.1 Most of them, however, neglect the short-range interaction between the escaping electron and the atomic core, and must be modified to describe various resonant effects. Experiments2 have shown very rich resonant structure in a dc-field, which can be attributed to the mixing of different excited states in the negative ion, to competition between elastic and inelastic decay channels, and to tunneling effects induced by the field. It is known that various resonant structures in Photoprocesses can be successfully described within standard multichannel quantum defect theory (MQDT). We present a modified MQDT frame transformation approach to extend the standard method to long-range potentials with nonspherical symmetry. In our treatment both the electron-field and electron-atom interactions are treated nonperturbatively and on an equal foot...

Two-photon emission processes in electron-atom interactions

We will describe the experimental techniques and preliminary results of an on-going three-parameter coincidence experiment in which two-photon spectra are obtained by electron bombardment of target foils of selected thicknesses. The atomic number of the foils ranges from Z = 26 to 82. Incident electrons have been provided both by the β spectrum of a 147Pm source and by a 50–150 keV electron beam from an accelerator. Processes considered to date include: double bremsstrahlung, double innershell ionization, and coherent emission of both a characteristic and a continuum X-ray. The cross sections for these two-photon processes are much smaller than the processes that produce single photons in the target. This can lead typically to a large accidental rate in the accelerator experiment, or accentuate the need for a target thickness study of the real rate in the source experiment where thicker targets have been used to increase the statistics.

Hanle signal formation in the discharge negative glow

Recent experimental data of the Hanle signal obtained from a hollow-cathode discharge are analyzed. It is shown that the electron-atom interactions is the main reason for the self-alignment of the atomic levels. The shape of the Hanle signal is calculated and shown to be in good agreement with the experimental observations.

Electron-Atom interaction potential by DCS minimization

The angular and energy dependence of minima of the differential scattering cross section (DCS) for electrons scattered elastically by selected target atoms is studied using the nonrelativistic Hartree–Fock wavefunction for the static field and local energy-dependent function for the exchange and polarization potentials for the target atoms. The parameters contained in the polarization potential are varied to determine the minima of the DCS with respect to the scattering angle and incident energy with the constraint that the integral cross section agrees with the experimental value within the uncertainty of 5% at a selected energy. The resulting effective interaction potential is then used to compute the DCS over the intermediate energy range for the target atoms. The computed values are in good agreement with recently published experimental and theoretical cross section for helium and neon atoms.

The distribution function of electrons and the conductivity of a charged particle system in a weak electric field of arbitrary frequency: II. The case of a partially ionized plasma

The effective frequency of electron-atom collisions and frequency dispersion of the conductivity of the weakly ionized plasma are investigated with the account of the results obtained in part I of the present work. The structural effects in the system of scatterers and the interaction with collective excitations are also taken into account. The correct description of the frequency of collisions in the t-matrix approximation with respect to the electron-atom interaction is carried out for the adiabatic approximation. The results obtained are valid for the case of arbitrarily degenerating electrons.

Laser-induced detachment processes in an electric field.

An analytic momentum-space wave function for an electron in both laser and static uniform electric fields is presented. It is used to obtain analytic multiphoton detachment cross sections in a static, uniform electric field which include effects of static-field-induced electron-photon interactions. These general results are not restricted to weak laser intensities or to weak static-field strengths and depend only on (a) the electric dipole approximation and (b) the approximation that final-state electron-atom interactions are ignored. Four specific predictions of our general formulas for the most interesting case of linearly polarized light polarized along the static-field direction are presented for weakly bound electrons initially in an s state. First, the effects of a weak static electric field on N-photon detachment cross sections near threshold are shown to be described by two modulation factors, one for odd N and one for even N, which depend only on a scaled energy. Second, for photodetachment in a static electric field, effects of static-field-induced electron-photon interactions are demonstrated. Third, the lifetime against field ionization is presented. Fourth, the cross section for electric-field-induced stimulated emission is presented. Numerical results for the latter three effects are presented for the ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ ion. The simpler case of circularly polarized light directed along the static-electric-field direction is treated briefly. In particular, we show for this case that the static- and laser-field effects are uncoupled and that the near-threshold, weak static-electric-field modulation factors for the N-photon detachment cross sections are dependent on N.

Multiphoton detachment in a static uniform magnetic field.

Multiphoton detachment cross sections for a negative ion in a static, uniform magnetic field are obtained in the approximation that electron-atom interactions are ignored in the final state. The final state is described by the analytic momentum-space wave function for an electron in the combined field of a laser and a static uniform magnetic field. The effects of a static-field-induced electron-photon interaction are stressed. Three consequences of our general formulas are discussed. First, the ponderomotive shift is modified by the magnetic field. It goes through a resonance at the cyclotron frequency and becomes negative for smaller frequencies. Second, for N-photon detachment of an s-shell electron by a laser of arbitrary strength, the effect of a weak magnetic field can be described near threshold by two modulation factors, one for even N and one for odd N, which depend only on a scaled energy. Third, cyclotron resonances are shown to exist under certain conditions, as demonstrated here for the specific example of multiphoton detachment of ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ by a weak laser in an arbitrarily strong magnetic field.

Electron-impact excitation of atoms in the presence of a nearly resonant laser field

We have studied the electron-impact excitation of an atom, in the presence of a laser field whose photon energy is tuned close to the energy difference between two excited final states. Both the laser-projectile and the laser-target interactions are treated nonperturbatively, while the electron-atom interaction is treated within the first Born approximation. As an application, we have analyzed the resonant laser-assisted excitation of the 21S and 21P states of helium. The agreement between the present nonperturbative results and previous perturbative ones is excellent, except for very small detunings. The present nonperturbative treatment also shows that the results given by perturbation theory on both sides of the resonance, when plotted as a function of the laser frequency, correspond to the excitation of different Floquet pseudostates. This is related to the presence of avoided crossings in the diagram of the Floquet pseudoenergies, as a function of the laser frequency. © 1990 The American Physical Society.

Electron-atom interaction mechanism: xenon states between the 2P(3/2) and 2P(1/2) limits

The interaction mechanism between low-energy electrons and atoms is examined in detail. Electron interaction within a few meV of the energy of an excited state is studied. High-resolution data showing total ionisation cross sections of low-energy electrons with Xe atoms are reported. Interactions appear as small perturbations superimposed on the ionisation continuum. The observed perturbations are compared with the theoretical predictions for negative-ion resonances, excitation of neutral states and direct ionisation. These interactions are reported in the energy range between the states 2P(3/2) and 2P(1/2) of Xe+ and their spectroscopic configurations are given. These data do not show any obvious post-collision interaction shifts that are reported in other channels.

Electron-impact ionization of atomic hydrogen in the presence of a laser field.

The influence of a laser field on the dynamics of fast (e,2e) collisions in atomic hydrogen is analyzed in the asymmetric, coplanar geometry. Particular attention is devoted to the construction of the dressed (laser-modified) target wave functions, in both the initial and final states. A detailed account is also given of the techniques we have used to evaluate the scattering amplitudes. In addition to a nonperturbative approach (valid to all orders in the laser-projectile interaction), we also discuss a second-order treatment in the combined electron-atom and laser-system (electron plus atom) interactions. The influence of the laser parameters (frequency, intensity, and direction of polarization) on the angular distribution of the ejected electron is discussed, and a number of illustrative examples are given. The structure of the triple differential cross section in the vicinity of resonances is also analyzed.

Atomic hydrogen in circularly polarized, high-intensity and high frequency laser fields

We present the application of a recently developed nonperturbative theory for electronatom interactions in intense, high-frequency laser fields to the calculation of the structure of atomic hydrogen in a monochromatic, circularly polarized plane wave. This theory predicts that the atom is stable in the high-frequency limit and that the levels are given by a time-independent Schrodinger equation containing a “dressed” Coulomb potential. The laser frequency and intensity enter only combined in a parameter α0. The energy eigenvalue equation was solved in the angular momentum representation by adopting the “decoupledl-channels approximation”. The weak field limit (α0 → 0) of the levels could be solved analytically using perturbation theory. Our numerical calculation gives the eigenvalues corresponding to principal quantum numbern≦4, over an extended range of α0:0≦α0≦100. The results are compared with those for the case of linear polarization obtained earlier by a similar approximation. The rapid decrease of the ionization potential at fixed (high) frequency and increasing intensity shows a remarkable resemblance in the two cases. This decrease is shown to be connected with a steady increase with α0 of the average of the radial coordinater, such that the atom in its ground state may attain Rydberg sizes at values of α0 presently achieved in experiment. Further, the existence of a new symmetry in the strong field limit (α0→∞) is signalled, leading to a peculiar multiplet structure. Finally, predictions are made concerning the energy spectrum of the electrons ionized at high (but finite) frequencies. In contrast to current experiments, no suppression of peaks can occur in our case, and large shifts of the peaks towards higher energies in comparison to the weak field case are expected.

The levels of atomic hydrogen in intense, high-frequency laser fields

A recently developed theory for electron-atom interactions in intense, high-frequency laser field is used as framework for the calculation of the levels of a hydrogen atom placed in a monochromatic, linearly polarized plane wave. An appropriate level-classification scheme is introduced, similar to that for homonuclear diatomic molecules. To lowest order within the theory, the levels depend on the frequency ω and on the intensity I of the plane wave only through the parameter α 0=I 1 2 ω -2 (au). We have computed the levels corresponding to principal quantum number n⩽4 over an extended range of α 0 values (0<α 0<100). A drastic decrease of the ionization potential with α 0 was found, which is of considerable consequence for the energy spectrum of the ionized electrons. A new peak pattern is predicted, differing from that currently seen in multiphoton ionization experiments: no peak suppression occurs now, and substantial shifts of the peaks with respect to the weak field case may appear.

Effective electron-atom interactions and virial coefficients in alkali plasmas

Using thermodynamic Green's functions, effective potentials and related virial coefficients for electron-atom interactions are derived within the second Born approximation. In addition to the exact results for hydrogen, improved values for the virial coefficients of electron-alkali-atom interactions are obtained.

Electron-hydrogen collisions in the presence of a laser field: one-photon excitation

The authors present a new calculation of the excitation cross sections of atomic hydrogen by the impact of fast electrons in the presence of a laser field. The computation is performed within the framework of perturbation theory to lowest non-vanishing order. In contrast with previous work, the authors do not restrict themselves to the Bethe-Born approximation for the electron-atom interaction; this opens many new channels for the scattering process. The Coulomb Green's function is represented accurately and efficiently in either of two L2 basis sets, one of Slater-Laguerre type and one Sturmian, rather than as a sum and integral over intermediate states. One result is to show that the Bethe-Born approximation does not provide reliable estimates of the cross sections for such processes. Another interesting feature of the authors' model is the prediction of deep minima in the cross sections. Their origin, as well as the magnitudes of the different terms contributing to the overall transition amplitudes, is discussed.

Polarization potentials for positron- and electron-atom systems.

Semiempirical polarization potentials with several cutoff functions are investigated for positron- and electron-atom interactions. A disposable parameter, an effective radius, in each cutoff function is adjusted so as to reproduce well-established reference values (scattering lengths and binding energies) in calculations. Striking regularities in fitted parameter values are found despite the very wide variety of cutoff functions examined. The parameter exhibits a strong dependence upon spin and angular momentum in the electronic systems, which is rationalized in terms of the Pauli exclusion principle. Simple semiempirical relations between the effective radii for positronic systems and those for the corresponding electronic systems are proposed, which enable one to find the parameter value for an electron by fitting a positron calculation to a reference value, or vice versa. In other words, the number of adjustable parameters is reduced to a single effective radius for each target regardless of the projectile.

Linear response theory for thermoelectric transport coefficients of a partially ionized plasma

Abstract Electrical conductivity, thermopower and thermal conductivity for a partially ionized plasma are expressed within an extended Zubarev approach by equilibrium correlation functions. The Green function technique is used to evaluate the correlation functions in different approximations. Improvements of the Lenard-Balescu approximation are considered, which account for dynamical screening effects and higher Born approximations for the electron-electron, electron-ion and electron-atom interaction.

Theory of angular-correlation experiments in electron scattering, including fine structure

A theory of electron-impact excitation of atoms in which fine structure is included as a perturbation is presented in detail. Hyperfine structure is included in the target atom but neglected in the electron-atom interaction. Expressions for electron-photon angular-correlation parameters and scattering cross sections are derived in terms of products of complex scattering amplitudes, with emphasis on electron-sodium excitation. Examples are given in which zero-field quantum beats may be observed in electron-photon angular-correlation and differential-cross-section measurements. Some of the interference terms are only observable in angular-correlation experiments. Examples are also given of partial cross sections for electron-impact excitation of state-to-state hyperfine levels in terms of direct- and exchange-scattering amplitudes plus additional terms due to the inclusion of fine-structure effects. The expressions given allow a comparison between the various possible scattering experiments.

Variational method on axial-channeling radiation of MeV electrons

Abstract Peak energies of axial-channeling radiations of MeV electrons in Si are calculated by the variational method with a orthonormal trial fucntion, where the Doyle-Turner potential is used as the electron-atom interaction. The calculated peak energies for a Si crystal show good agreements with measured ones.

A simple analysis on axial channeling radiation of MeV electrons

Abstract Axial channeling radiation of MeV electrons in Si have been investigated using the Doyle-Turner string potential for the electron-atom interaction potential. The energy eigenvalues are determined by the variational method with an analytic trial function, where the modified Hermite polynomial is used as a trial function. Calculated photon energies for a Si crystal show good agreements with experimental ones.