Energy Spectra Of Ejected Electrons Research Articles

Energy Spectrum of Ejected Electrons of H(2P) Ionization by Electrons in Coplanar Asymmetric Geometry

We have investigated the energy spectrum of ejected electrons of hydrogen atoms ionization from metastable 2P state by electrons in coplanar asymmetric geometry. A multiple scattering theory of Das and Seal is applied. The present results show very interesting binary peak features. As well as a significant qualitative agreement with hydrogenic ground state results is obtained. New theoretical results and experimental verification for such treatment in hydrogenic metastable states by electrons will be interesting.

Break-up of positronium (Ps) atom in collision with a He+ ion

The dynamics of electron loss to the continuum (ELC) from the light neutral projectile positronium (Ps) atom in collision with He+ ion is studied in the framework of post-collisional Eikonal approximation. The full three-body interaction, which is crucial for such ELC process is taken into account in the final channel. Both the triple differential cross sections (TDCS) and the double differential cross sections (DDCS) are studied at some intermediate and high incident energies, with respect to the threshold energy for this process. A distinct broad ELC peak centred around ve (velocity of the ejected electron) ≈vp (velocity of the positron) is noted in the ejected electron energy spectrum (DDCS) for forward ejection of e/e+, corroborating the experimental findings for the ELC process. The probability of emission of the e+ at higher angles is almost negligible as compared to its forward emission. Another salient feature noted in the present work is the shifting of the ELC DDCS (summed over the electron ejection angles) peak towards higher value of the ratio ve/vp with increasing incident energy, in agreement with the experimental findings of Armitage et al for Ps–He break-up process.

Coulomb-Volkov approach of ionization by extreme-ultraviolet laser pulses in the subfemtosecond regime

In conditions where the interaction betweeen an atom and a short high-frequency extreme ultraviolet laser pulse is a perturbation, we show that a simple theoretical approach, based on Coulomb-Volkov-type states, can make reliable predictions for ionization. To avoid any additional approximation, we consider here a standard case : the ionization of hydrogen atoms initially in their ground state. For any field parameter, we show that the method provides accurate energy spectra of ejected electrons, including many above threshold ionization peaks, as long as the two following conditions are simultaneously fulfilled : (i) the photon energy is greater than or equal to the ionization potential ; (ii) the ionization process is not saturated. Thus, ionization of atoms or molecules by the high order harmonic laser pulses which are generated at present may be addressed through this Coulomb-Volkov treatment.

Modelling interaction cross sections for intermediate and low energy ions.

When charged particles slow in tissue they undergo electron capture and loss processes that can have profound effects on subsequent interaction cross sections. Although a large amount of data exists for the interaction of bare charged particles with atoms and molecules, few experiments have been reported for these 'dressed' particles. Projectile electrons contribute to an impact-parameter-dependent screening of the projectile charge that precludes straightforward scaling of energy loss cross sections from those of bare charged particles. The objective of this work is to develop an analytical model for the energy-loss-dependent effects of screening on differential ionisation cross sections that can be used in track structure calculations for high LET ions. As a first step a model of differential ionisation cross sections for bare ions has been combined with a simple screening model to explore cross sections for intermediate and low energy dressed ions in collisions with atomic and molecular gas targets. The model is described briefly and preliminary results compared to measured ejected electron energy spectra.

Electron ejection by positron impact ionization

The ejected-electron energy spectrum resulting from positron impact ionization of molecular hydrogen has recently been measured at an incident energy of 50 eV. The ejected electron and the scattered positron have been measured in coincidence around 0° with a new parallel-plate analyser having time-focusing properties. A 1.6 eV shift towards lower energies has been found in the electron-spectrum relative to a theoretical prediction. The shift may indicate the occurrence of ionization simultaneous to another energy loss, e.g. vibrational excitation or molecular dissociation of the target. To test this hypothesis, studies have begun with a helium target.

Energy spectrum of ejected electrons in ionization of hydrogen atoms by electrons

Energy spectrum of ejected electrons in ionization of hydrogen atoms has been calculated following a multiple scattering theory of Das and Seal [15]. The results show peaks around two to three Rydbergs of energies of the ejected electrons, for incident electron energy of 250 eV and 500 eV, considered here, and for different combinations of the angular variables of the scattered and the ejected electrons, for scattering in a plane. The peaks are very similar to those observed in relativistic K-shell ionization of Ag atoms by electrons at 500 KeV energy [6]. The physical origin of these peaks may be traced to the second order scatterings, scattering first by the atomic nucleus (or the atomic electron) and then a second time by the atomic electron. These peaks are, however, absent in the first Born results. Experimental verification of the present results and theoretical calculation by some other well-known methods will be interesting.

Theoretical studies of charge transfer in molecular ion–metal surface collisions

The collision of a molecular ion with a metal surface involves charge transfer. The effects of charge transfer can be readily seen in the increase of dissociation of ionized molecules as compared to neutral molecule dissociation, and to detectable electron emission. In the present work we present the results of a theoretical study of electron emission during the collision of hydrogen and nitrogen with an Al surface. In both cases the dominant electron emission process for slow ions is the de-excitation of the H 2 * and N 2 * molecules formed by resonance neutralization of the molecular ion in front of an Al surface. The de-excitation process is enhanced by the formation of a negative ion, which corresponds to the free-molecule shape resonances 2Σ u + and 2Π g for H 2 and N 2 , respectively. The energy spectra of the emitted electrons are much broadened, making it difficult to recognize the signature of a transient negative ion in the energy spectra of ejected electrons. Finally, the ubiquitous presence of negative ions in molecule-surface scattering is emphasized.

Ionization - excitation of helium by fast protons and antiprotons

The process of simultaneous ionization and excitation of helium by protons and antiprotons is investigated. Total and single differential cross sections for the ionization - excitation process and ionization - excitation probabilities versus the impact parameter are calculated by perturbation theory through the second order in the interaction potential. In the present calculations only the on-energy shell transitions are taken into account. The helium electron wavefunctions for the initial and final states are determined with allowance for the configuration interaction. The normalization of the helium continuous spectrum wavefunctions is examined. The different helium ionization - excitation mechanisms are analysed. The calculations predicted the exceeding of the total cross section for protons over the antiproton one for all collision energies considered. It is shown that the resonant channel coupling causes the ejected electron energy spectrum to have sharp drops at electron energies (here, n, l and L are the principal and orbital quantum numbers and the orbital angular momentum for the states of the ion lying above the one; is the energy of the state of the ion). The present cross sections are compared with those derived from other theoretical approximations.

Resonant channel coupling at electron transitions to the continuum

The process of simultaneous ionization and excitation of helium by proton impact, p + He(1s 2 1S) → p + e + He + (nl 2L) , is investigated using perturbation theory alllowing for the configuration interaction. It is shown that the resonant channel coupling causes sudden drops at electron energies E e = E n 1 l 1 2 L 1 − E nl 2 L in the ejected electron energy spectrum (here, E nl 2 L is the energy of the nl 2 L state of the He + ion; n 1, l 1 and L 1 are the principal and orbital quantum numbers and the orbital angular momentum for the states of the He + ion above the nl 2 L state). The drops in the energy spectrum of electrons ejected at the 1s 2 1S → nlκ transition are located in the range of electron energies 0 < E e ≤ − E nl 2 L .

Saddle-point shifts in ionizing collisions.

Experimental studies of electrons ejected in ionizing collisions between ${\mathrm{C}}^{+}$, ${\mathrm{C}}^{2+}$, and ${\mathrm{C}}^{3+}$ projectiles and He and Ne targets have been made for incident projectile energies of 83, 100, and 150 keV/u. The data exhibit pronounced maxima in the 10\ifmmode^\circ\else\textdegree\fi{} ejected-electron energy spectra and projectile-charge-dependent shifts of these maxima which strongly supports the hypothesis of an independent ``saddle-point'' ionization mechanism. A possible cause of disagreement between these results and existing experimental data is also discussed.

Observation of quantum beats in autoionizing electron spectra created by slow Na+-Ne collisions.

Angle-resolved energy spectra of ejected electrons have been measured following the decay of autoionizing states created by slow Na/sup +/ impact on Ne. Spectral structures with two major peaks due to molecular and atomic autoioniztion have been observed at the ion impact energy of 200 eV. Remarkable substructures have been found in the lower-energy wing of the atomic autoionization peak. These substructures are associated with quantum beats due to the interference between the coherent autoionizing transitions of the doubly excited state to the doublet substates /sup 2/P/sub 1/2/ and /sup 2/P/sub 3/2/ of Ne/sup +/(2p/sup 5/).

Observation of electrons from the 1P degrees resonance of D-

We have measured the electron energy spectra near 0\ifmmode^\circ\else\textdegree\fi{} produced in collisions of ${\mathrm{D}}^{\mathrm{\ensuremath{-}}}$ with Ar. Using a 400-keV ${\mathrm{D}}^{\mathrm{\ensuremath{-}}}$ beam and with good experimental energy and angular resolution we have found structure in the ejected electron energy spectra which is due to the decay of the $^{1}\mathrm{P}$ \ifmmode^\circ\else\textdegree\fi{} shape resonance. The doubly differential cross sections (DDCS's) have been measured as a function of angle and it was found that this structure disappeared for laboratory angles greater than 1\ifmmode^\circ\else\textdegree\fi{} as expected. A resonance contribution to the DDCS's was extracted at ${\ensuremath{\theta}}_{L}$=0\ifmmode^\circ\else\textdegree\fi{}, transformed to the projectile frame, and fit with a Breit-Wigner shape. Our resonant energy is in reasonable agreement with other experiments. We also find a small asymmetry in the two resonant structures in the laboratory measurements at ${\ensuremath{\theta}}_{L}$=0\ifmmode^\circ\else\textdegree\fi{}.

Interaction of He+ and Ne+ ions with Ni(100)-K and Cu(100)-K surfaces having variable potassium coverage.

This paper presents an electron-spectroscopic study employing slow ${\mathrm{He}}^{+}$ and ${\mathrm{Ne}}^{+}$ ions incident on Ni(100) and Cu(100) surfaces with varying amounts of adsorbed potassium. Investigation of the changes in electron-energy spectra as the work function of the surface is varied probes the pattern of resonance tunneling, excitation conversion, and Auger electron-ejection processes. When the surface is clean the work function \ensuremath{\varphi} is high and the mode of electron ejection is the two-electron process of Auger neutralization of the incident ions yielding a folded two-electron spectrum. As the macroscopic \ensuremath{\varphi} is reduced by K adsorption the surface develops more and more regions near K atoms where the local \ensuremath{\varphi} is small, permitting resonance neutralization of the incident ion to form a metastable atom which on Auger deexcitation ejects electrons in a one-electron spectrum.This paper reports an investigation of this transition from the two-electron, kinetic energy spectrum to the one-electron spectrum and of the specific characteristics of each of these spectral types. Results are compared in detail with a published study by other investigators of the interaction of thermal He metastable atoms with a Cu(110)-K surface having variable K coverage. This comparative study has illuminated the differences observed in the spectra produced by incident 12-eV ions which turn to metastables in the high-K-coverage regime and thermal metastables which turn to ions at low K coverage. We have demonstrated that the local work function of ``clean'' sites decreases as K coverage increases in the low-coverage regime by interpreting small observed changes in the kinetic energy spectra of ejected electrons. This, in turn, leads to an estimate of the variation of the mean separation of K atoms with increasing K adsorption. We present new data for ion neutralization at the clean Cu(100) surface. From these we obtain the deconvolution of the kinetic energy spectrum that is the initial-state transition density for comparison with photoemission spectra. These results are also compared in detail, particularly with respect to energy broadening, with the results of an investigation by others employing ions descendent from incident thermal metastables.

Adiabatic theory of multiphoton ionisation. II. Resonant phenomena

For pt.I, see ibid., vol.20, no.11, p.2397 (1987). The adiabatic approach to the problem of multiphoton ionisation is extended to the case when the resonant phenomena are important. Mathematically this implies inclusion of new types of singularities (the poles) in the three-term recurrence relations defining the system quasi-energy states. The regimes of strong and weak coupling of the resonating states are discussed. It is shown that in a strong external field the quasi-energy state can be stabilised. The resonance with the weakly bound state (its role can also be played by the Rydberg series of levels) can lead to the substantial modification of the ejected electron energy spectrum. It decreases more slowly with energy and all peaks exhibit a similar dependence on the non-stationary field strength. These features are in qualitative agreement with recent experimental observations on the above-threshold ionisation.

Ionisation in atom-Rydberg atom collisions: ejected electron energy spectra and reaction rate coefficients

Using the resonant internal energy conversion mechanism for inelastic atom-Rydberg atom slow collisions, the energy spectra of ejected electrons and the rate coefficients for the ionisation process are calculated analytically. Reaction rates for H*(n)+H, Cs*(n)+Cs, Rb*(n)+Rb and K*(n)+Rb colliding systems are calculated and compared with the available experimental data.

Energy and angular distributions of electrons ejected in the ionization of SF6 and TeF6 by fast protons

Absolute cross sections, differential in ejected electron energy and emission angle, have been measured for ionization of SF6 and TeF6 molecules by protons in the energy range from 0.3 to 1.8 MeV. Evidence of potential barrier effects is observed in the structure of the low-energy portion of the ejected electron energy spectra. This structure, as well as cross sections for direct ionization processes, is discussed with respect to dependence on emission angle and incident particle energy. Emission cross sections are compared to similar measurements for molecules with constituents of only low atomic number.

Inner shell electron ejection in the α-decay of 210Po

A model for inner shell electron ejection in nuclear α-decay is proposed. The model accounts for the change of the electron Hamiltonian on nuclear decay, treats the α-motion classically as it emerges and also includes the effect on the electron due to nuclear recoil. We have used the Dirac wave function for the electron with and without shielding. We calculate both the K- and L-shell electron ejection probabilities in 210Po α-decay. The ejected electron energy spectra fit measured data reasonably well. The calculated ejection probabilities are P K S = 2.24 × 10 −6 and P L S = 2.74 × 10 −4 with shielding, and the assumption that the α-tunnels through the nuclear barrier in a time short compared to electron orbital times is made. These values compare favourably with measured values.

Experimental study of the resonances 2s21S, 2p21D and 2s2p1P of the helium atom in energy spectra of electrons ejected in H+-He collisions (below 150 keV)

The most important experimental results concerning the 2s2 1S, 2p2 1D and 2s2p1P resonances of the helium atom, as observed in energy spectra of ejected electrons, are given. Characteristic parameters are obtained numerically with Shore's formula (1969). The shape of the resonance is studied with Shore's parameter a, in the angular range 16.5 degrees to 160 degrees and for 33 to 150 keV proton energies; additional results concerning only the parameter q have been obtained at 10 degrees ejection angle, down to 26 keV proton energy. The intensity of the resonances is also measured in the same angular and energy ranges, and is characterized by Shore's parameter b. The results are compared with other experimental or theoretical works by means of the annulment diagram for the parameter a. This method permits the authors to synthesize all the experimental data actually available up to 500 keV.

Analysis of the 2s21S, 2p21D and 2s2p1P autoionizing levels of helium, in proton-helium collisions

The authors have parametrized, using Fano's formula, the three resonances associated with the 2s2 1S, 2p2 1D and 2s2p 1P autoionizing levels of helium. They were observed in energy spectra of ejected electrons produced by proton-helium collisions. For the 1S resonance, the authors determined the variations with electron emission angle of the shape parameter q and the resonant continuum cross section sigma r, in the angular range 16.5 degrees to 160 degrees , for 150 keV protons. The dependence on proton energy of these parameters, has also been obtained for 16.5 degrees angle, in the energy range 14 to 150 keV. Additional results concern the variation of the profile parameters for 1D and 1P resonances, with ejection angle, at 150 keV.