Final Electronic States Research Articles

Vibronic Sidebands of Coherent Phonon States

Recently experiments have been reported about phonon sidebands in doped crystals, which may originate from coherent phonon states. The corresponding modes are either confined phonon modes in nanocrystals or localized phonon modes in bulk materials, both showing small damping due to phonon-phonon interaction. We present a theory of the lineshape of vibronic sideband spectra due to coherent phonon states using the conventional model of linear electron-phonon coupling and displaced equilibrium positions of the oscillators in the initial and final electronic states. Unlike in the conventional theory, the initial state of the oscillator is taken as a coherent phonon state and not as a thermalized one. Under these conditions we got an exact analytical solution for the lineshape of the vibronic sideband. The lineshape is determined by two parameters, the Huang-Rhys parameter S and the coherence parameter α of the phonon state. For α = 0 the lineshape converts into the standard Pekarian form for T = 0.

Correlation in fast collisions of highly charged ions with atoms

Atomic hydrogen and helium ionization by impact of 3.6 MeV/u Au53+ is investigated within classical trajectory Monte Carlo and quantum-mechanical continuum distorted wave models. For helium ionization, a description of initial and final electron states in terms of Hartree–Fock potentials is compared with hydrogenic models. Different ionization mechanisms are characterized. A strong correlation of the electron, projectile and target nucleus is observed. Good agreement is obtained between calculations performed with the different models.

Detailed calculation of lepton flavor violating muon-electron conversion rate for various nuclei

The coherent muon-electron conversion rates in various nuclei are calculated for general lepton flavor violating interactions. We solve the Dirac equations numerically for the initial state muon and the final state electron in the Coulomb force, and perform the overlap integrals between the wave functions and the nucleon densities. The results indicate that the conversion branching ratio increases for a light nucleus up to the atomic number Z ~ 30, is largest for Z = 30 -- 60, and becomes smaller for a heavy nucleus with Z > (~ 60). We also discuss the uncertainty from the input proton and neutron densities. The atomic number dependence of the conversion ratio calculated here is useful to distinguish theoretical models with lepton flavor violation.

Negatively Charged Exciton on a Quantum Ring

We present calculations of the magnetic field dependence of the emission spectrum of charged exciton (X - ) localised on a quantum ring. They are carried out using exact diagonalisation techniques in a coordinate system localised on a valence hole. The ground state energy of charged X - complex, unlike neutral exciton, exhibits Aharonov-Bohm oscillations for any disk radius. The emission energy is found as a difference of Aharonov-Bohm oscillations in the charged exciton and the final state electron energy.

Elastic scattering of neutrinos off polarized electrons

We calculate the cross sections for elastic ν l + e −→ ν l + e − and ν ̄ l+e −→ ν ̄ l+e − scattering ( l= e, μ or τ) in the Born approximation and with exactly fixed polarization states of target and final-state electrons, discussing their sensitivity to the incident (anti)neutrino flavor. We suggest investigation of the flavor composition of a (anti)neutrino beam by a flux-independent analysis of the scattering of its constituents off polarized electrons.

Extended description for electron capture in ion-atom collisions: Application of model potentials within the framework of the continuum-distorted-wave theory

We perform an extension of the continuum-distorted-wave (CDW) approximation for single electron capture by introducing model potentials to describe the interaction of the active electron with the residual-target and projectile ions. The cross sections for electron transfer in collisions of bare and dressed projectile ions with H and He atoms are calculated and compared with experimental data and previous CDW calculations that make use of approximate analytical wave functions to represent the active electron initial and final states. For electron capture in proton-He collisions into definite final states the present calculations are in better agreement with experiments. We trace the differences in the results from both models to a different behavior at projectile scattering angles close to the Thomas peak. In the case of dressed projectiles with different charge states impinging on H and He the present version of CDW gives a better representation of the filling of the unoccupied orbitals of the dressed ion and, therefore, of the cross section as a function of the impinging ion charge state.

On the day–night effect and CC to NC event rate ratio predictions for the SNO detector

Detailed predictions for the DN asymmetry for the Super-Kamiokande experiment, as well as for the full night and core DN asymmetries in the solar neutrino induced CC event rate and the day and night ratios of the CC and NC event rates, measured in the SNO experiment, are derived in the cases of the LMA MSW and LOW solutions of the solar neutrino problem. The indicated observables for the SNO experiment are calculated for two values of the threshold (effective) kinetic energy of the final state electron in the CC reaction on deuterium: Te,th=6.75 MeV and 5.0 MeV. The possibilities to further constrain the regions of the LMA MSW and LOW solutions of the solar neutrino problem by using the forthcoming SNO data on the DN asymmetry and on the CC to NC event rate ratio are also discussed.

Dalitz plot analysis of three-body fragmentation of Na3+ excited by He impact

Three-body fragmentation of Na3+ ions to Na++Na(3s)+Na(3s) following excitation by He is studied experimentally and theoretically. The three reduced kinetic energies of the products in the center-of-mass are determined for each fragmentation event, and the results are displayed in a Dalitz plot. The fragmentation involves three adiabatic A′1 electronic states of Na3+ that become degenerate at the detector. It is possible to determine the final electronic state for each event, and here we show that each of the three product states appears in a particular sector of the Dalitz plot. Theoretical and experimental Dalitz plots for the three-body fragmentation of Na3+ are presented, and the results are related to various mechanisms for three-body fragmentation of this system.

Interband phonon assisted tunneling in InAs/GaSb heterostructures

The rate of LO-phonon assisted interband transitions in an InAs/GaSb double quantum well heterostructure is compared with the elastic interband tunneling rate through the heterostructure ‘leaky window’. We show that the phonon-assisted process can dominate over the elastic tunneling if the initial and final electron states anticross and the anticrossing gap is smaller than the LO-phonon energy.

Calculated gadolinium atomic electron energy levels and Auger electron emission probability as a function of atomic number Z

Auger electron interaction with matter is gaining importance in particular in medical application of radiation. The production probability and energy spectrum is therefore of great importance. A good source of Auger electrons is the 157 Gd (n,γ) 158 Gd reaction. The present article describes calculations of electron levels in Gd atoms and provides missing data of outer electron energy levels. The energy of these electron levels missing in published tables, was found to be in the 23–24 and 6–7 eV energy ranges respectively. The probability of Auger emission was calculated as an interaction of wave function of the initial and final electron states. The wave functions were calculated using the Hartree–Fock–Slater approximation with relativistic correction. The equations were solved using a spherical symmetry potential. The error for inner shell level is less than 10%, it is increased to the order of 10–15% for the outer shells. The width of the Auger process changes from 0.1 to 1.2 eV for atomic number Z from 5 to 70. The fluorescence yield width changes five orders of magnitude in this range. Auger electron emission width from the K shell changes from 10 −2 to ∼1 eV with Z changing from 10 to 64, depending on the final state. For the L shell it changes from 0 to 0.25 when it Z changes from 20 to 64.

A search for leptoquark bosons in e−p collisions at HERA

A search for scalar and vector leptoquarks coupling to first generation fermions is performed in the H1 experiment at the ep collider HERA. The analysis uses e−p data collected in 1998 and 1999 at a centre-of-mass energy of 320 GeV, corresponding to an integrated luminosity of ∼15 pb−1. No evidence for the direct production of such particles is found in a data sample with a large transverse momentum final state electron or with large missing transverse momentum, and constraints on leptoquark models are established. For a Yukawa coupling of electromagnetic strength leptoquarks are excluded for masses up to ∼290 GeV. This analysis complements the leptoquark searches performed previously using data collected whilst HERA was operating with positrons instead of electrons.

Inelastic inverse lifetimes of medium-energy electrons: photoemission analysis of s,p-band direct transitions at Cu( [formula omitted]) and Cu( [formula omitted

We investigate the inelastic lifetime of electrons in copper at excitation energies E f between 7 and 20 eV above the Fermi level. High-resolution photoemission experiments allow to determine the inelastic inverse lifetime of final state electrons precisely by measuring the line widths of s,p-band direct transitions. Energy coincidence measurements on Cu(1 0 0) and Cu(1 1 0) show dramatically different line widths which can be quantitatively explained by calculations within the one-step model of photoemission as well as by numerical line shape calculations. Our analysis shows that the distinct line widths on the differently oriented surfaces do not result from lifetime effects but from the influence of the band structure. All observations are in good agreement with a linearly increasing lifetime width Γ e= a( E f− E F), with a=(0.13±0.01) in the energy range mentioned above.

A theory of excitation–energy transfer regarded as nonadiabatic transition and its comparison to Förster's theory

AbstractWe propose a rate‐constant expression for an excitation–energy transfer between two molecules that is regarded as a nonadiabatic transition between two adiabatic electronic states. It is shown that under weak intermolecular interaction the approximately derived rate constant increases with the increase in pseudo‐Coulombic interaction \documentclass{article}\pagestyle{empty}\begin{document}$\langle \phi_{e}^{A}\phi_{g}^{B}|H|\phi_{g}^{A}\phi_{e}^{B}\rangle_{\mathbf{r}}$\end{document} and depends on the differences in the potential energies and the potential derivatives of the initial and final electronic states. In addition, the rate constant under strong interaction is shown to decrease with increasing pseudo‐Coulombic interaction. We show that our expression has the advantage that it can be applied to the excitation–energy transfer under the (not very) weak and the strong coupling limits which cannot be addressed by Förster's theory. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001

Double scattered electrons in ion-atom collisions: an impulseapproximation

A quantum-mechanical impulse approximation for target ionization ispresented, which not only treats the electron-projectile interaction inhigher orders, but also incorporates some target effects into the finalelectronic state. This approximation gives a peak in the doubledifferential electron emission cross section which is similar to the`ternary electron ridge' observed in a few experiments. The electrons inthis peak originate from two close collisions with the projectile and thetarget and have approximately the same energy as the 0°binary-encounter peak at all laboratory emission angles.

Collision-induced electronic energy transfer from v=0 of the E(0g+) ion-pair state in I2: Collisions with He and Ar

The electronic energy transfer pathways that occur following collisions between I2 in the E ion-pair electronic state (v=0, J=55) and He and Ar atoms have been determined. The nearby D, D′, and β ion-pair states are populated, but with relative branching ratios that vary with the rare gas collision partner. In He/I2 collisions, the D state is preferentially populated, while Ar/I2 collisions preferentially populate the β electronic state. Bimolecular rate constants and effective hard sphere collision cross sections have been determined for each channel; the cross sections range from 7.0±1.0 Å2 for populating the β state with Ar collisions to 0.9±0.2 Å2 for populating the D′ state with He collisions. For both rare gas collision partners, and all three final electronic states, low vibrational levels are populated, in rough accord with the relevant Franck–Condon factors. There is little propensity observed for population of vibrational levels that are in near resonance with the initially prepared level in the E state.

Distribution of final electronic states following three-body fragmentation of Na 3+ excited by He impact

Three-body fragmentation of Na 3 + ions to Na ++Na(1s)+Na(1s) is studied experimentally and theoretically. A procedure is developed for distinguishing the three adiabatic electronic 1A ′ states in the product region, even though these states are degenerate at the detector. The procedure requires knowledge only of the three atom–atom and ion–atom relative velocities, and this information is obtained in our coincidence experiments. This paper reports the first experimental determination of the populations of the three-product adiabatic states in a three-body fragmentation process. The results, which are different for each state (despite of degeneracy), are compared with theoretical calculations.

Epitaxial growth and the electronic structure of MgSe on ZnSe/GaAs (001)

Abstract We have examined the growth and electronic properties of thin layers of MgSe on relaxed 0.6 μm ZnSe epilayers. This avoids potential complications when MgSe is grown directly on GaAs substrates. Using relatively low energy photons from the BESSY1 synchrotron radiation facility to enhance the photoemission cross-sections, we have monitored the evolution of the 3 d core levels of Zn, and Se together with the 2 s and 2 p levels of Mg as a function of MgSe layer thickness, using the technique of angle resolved photoemission. By using a substrate temperature of 350°C, we obtained consistently good c(2×2) LEED patterns after each evaporation cycle: the maximum MgSe thickness investigated was 3.2 nm. From an examination of the energy shift of the substrate Se 3 d level following MgSe deposition (attributed to band bending) and determination of the apparent shift in the position of the valence band maximum, we estimate the true value of the valence band offset as −0.34 eV (relative to ZnSe). Both surface and “bulk” Se contributions have been identified and an estimate of 0.44 nm for the inelastic mean-free-path of 56 eV electrons in MgSe has been derived. Normal emission valence band data utilizing a wide range of photon energies has been used to determine the band structure of MgSe (001) for the first time using the usual assumptions associated with a free electron final state model. Using an inner potential of 9.1 eV we find the binding energies of the X 5 and X 3 points to be 1.5 and 3.85 eV respectively relative to the valence band maximum.

Modeling final-state interaction effects in inelastic X-ray scattering from solids: resonant and non-resonant

Abstract We present resonant inelastic X-ray scattering calculations that realistically incorporate intermediate-state electron–core hole interactions and final-state electron–valence hole interactions. Intermediate-state interactions primarily affect the total fluorescence yield, whereas final-state interactions can change the relative strengths of spectral emission features. This is especially true in systems with strong valence–hole exciton effects. We also consider non-resonant excitation of core electrons. This work considers resonant scattering from B 1s electrons in cubic boron nitride (cBN) and hexagonal boron nitride (hBN) and non-resonant scattering from Mg and O 1s electrons in MgO, and compares theoretical results to experiment.

Magnetic and orbital dichroism in (e,2e) ionization of sodium.

We present the first measurement of (e,2e) ionization cross sections for a laser oriented atomic target by spin polarized electrons. Cross sections are presented as a function of target orientation and polarization direction of the incident electron beam. This study provides insight into mechanisms by which angular momentum is transferred from the valence electron to the two final-state continuum electrons in both singlet and triplet spin channels, by comparing measurement with distorted wave Born approximation and the dynamically screened three Coulomb wave calculations.

Phonon-assisted optical transitions in GaN with impurities and defects

We have classified all the possible types of phonon-assisted optical processes involving bound or extended initial, final, and virtual (intermediate) electron states and formulated, for the first time, in terms of site symmetry, the selection rules for corresponding transitions. We apply this theory to phonon-assisted transitions in hexagonal GaN involving substitutional impurities and vacancies with C 3v site-symmetry as well as interstitial impurities and molecular point defects (paired impurities, double vacancies, and vacancy-impurity complexes) occupying sites with C 3v, C s, and C 1 ones. We show that phonon-assisted optical recombination is allowed in any polarization for free and bound carriers and excitons whatever is the number of involved phonons. Just, the nature of virtual state(s) and phonon(s) can depend on the polarization of the emitted light. We discuss, in particular, the case of excitons bound to neutral donors or acceptors. Our predictions are in good agreement with experimental optical spectra published in the literature which exhibit numerous lines assigned to one- and multi-phonon-assisted transitions.