Two-electron Excitation Research Articles

Two-photon-absorption spectrum of poly(di-n-hexylsilane) films.

Two-photon-absorption (TPA) spectra of poly(di-n-hexylsilane) (PDHS) films are obtained from 605 to 410 nm at 295 and 11 K, where the intensity is an order of magnitude higher. A strong TPA band is found above 5 eV and interpreted in terms of interacting \ensuremath{\sigma} electrons in a Pariser-Parr-Pople (PPP) model. PPP models for (Si${)}_{\mathit{n}}$ chains relate the excitonic (one-photon) absorption at ${\mathit{E}}_{\mathit{g}}$=3.4 in PDHS to the 4.2-eV TPA at the alternation gap and the high-energy TPA derived from two-electron excitations at ${\mathit{E}}_{\mathit{g}}$. The smaller alternation gap in \ensuremath{\pi}-conjugated polymers and their intense TPA above ${\mathit{E}}_{\mathit{g}}$ also indicate correlated states and differ qualitatively from single-particle descriptions.

Two-electron excitation in slow ion-atom collisions: interference among autoionizing states

Structures found in the Auger electron energy distribution that are due to interferences resulting from collisions of H + ions with He atoms below 5 keV are theoretically studied within a close-coupling scheme based on molecular representation. The present study identifies the dominant interference-related contributors to the structures. These contributors are due to the interferences between (i) different autoionizing electronic states, (ii) incoming and outgoing parts of collisions on a single trajectory, (iii) different locations that have the same ejected electron energy on a single trajectory, and (iv) different trajectories.

Photodetachment of the H − ion

Detachment of electrons from the H − ion is investigated with an experimental technique whereby an H − beam moving at a relativistic velocity (2.5 × 10 10 cm/s) is intersected with a fixed frequency laser. The Doppler effect allows systematic variation of the center-of-mass (CM) photon energy over a wide range (factor of 10) by simply adjusting the angle between the ion and laser beams. The focused output from a pulsed, linearly polarized, CO 2 TEA laser operating at 10.6 μm, with peak intensities on the order of 10 GW/cm 2, was used to examine the multiphoton absorption process in H −2. The fourth harmonic (266 nm) of a Nd:YAG laser was used to investigate some of the doubly-excited state resonances in H −. In the multiphoton absorption work, electron detachment was observed at photon energies where as few as 2 and as many as 8 photons are required to get above the 1-electron detachment threshold (EDT) of H − (0.754 eV). Electron yield vs photon energy plots exhibit structure that is laser intensity dependent. Electron yield vs laser pulse energy data was obtained at a few selected CM wavelengths and laser pulse energies. In the single-photon UV laser work, numerous resonances within the H − photodetachment continuum corresponding to one-photon two-electron excitation processes were observed. The doubly-excited resonances appear to be of the Feshbach type. A simple, semi-empirical recursion formula predicts the resonance energy levels. The experimental techniques described here can be used to accurately determine accelerator beam and ion source parameters such as beam energy, energy-spread, and ion density spatial distribution.

Enhancement of excited-state population of magnesium ions upon decay of autoionizing resonances.

Decay of autoionizing resonances in atoms often leads to preferential production of excited states of the resulting ion. The population of these states can exceed the population of the ground state by more than an order of magnitude. We report measurements of the branching ratios for ${\mathrm{Mg}}^{+}$ ions resulting from the decay of selected autoionizing levels. These derive either from single excitation of a 2p electron or from two-electron excitations. For excited states of the type 2${\mathit{p}}^{5}$${(}^{2}$${\mathit{P}}_{1/2,3/2}$)3${\mathit{s}}^{2}$ns, n=4 or 5, production of ${\mathrm{Mg}}^{+}$ ions having the final configuration 2${\mathit{p}}^{6}$ns is the predominant decay feature. For n>5, the predominant decay feature is the 2${\mathit{p}}^{6}$(n+1)s configuration, indicating that shakeup is very strong in the decay of the excited state. For excited states of the type 2${\mathit{p}}^{5}$${(}^{2}$${\mathit{P}}_{1/2}$)5d and 2${\mathit{p}}^{5}$${(}^{2}$${\mathit{P}}_{3/2}$)6d, the decay behaves in a manner similar to that of the ns states. However, for the 2${\mathit{p}}^{5}$${(}^{2}$${\mathit{P}}_{1/2}$)6d state, the most prominent decay channel appears to be the 2${\mathit{p}}^{6}$11d state, which leads to the emission of an electron with the same kinetic energy as the ${\mathit{L}}_{2}$-${\mathit{M}}_{1}$${\mathit{M}}_{1}$ Auger line.

New aspects in the oxygen related satellite observed in photoemission for high T c superconducting oxides

A peak in the photoemission process is consistently observed at approximately 9 eV for the superconducting oxides with the feature being absent for CuO. A feature near 12 eV is observed for all the Cu +2 superconducting oxides. We find that the experimental spectra can be accounted for quantitatively with two-electron excitations in addition to the usual one-electron emission. In the two-electron process, a single photon causes one electron to be excited by approximately 8 eV due to a strong interband absorption while the second electron is photoemitted. The shake-up spectra at 9 and 12 eV then reflect the local densities of Cu and O states shifted by approximately 8 eV due to the second excitation. Our model is an alternative to the 5 parameters model based on the Cu-O atomic-like excitations, where it is assumed the 9 eV peak is spurious. Virtual shake-up transitions can be used as the coupling mechanism for the metallic oxide superconductors.

Total and partial generalized oscillator strengths for transitions to the continuum of helium

The generalized oscillator strengths (GOS) for transitions to the continuum of helium between the second and third thresholds of ionization of helium by fast electrons are calculated. The total and partial GOS, the GOS ratios in the ionization of helium to the n=1 and n=2 states of the He+ residual ion, the parameters of the autoionized states converging to the threshold n=3 in the total and partial GOS and the dependence of the spectroscopic characteristics of the resonances on momentum transfer are determined. The ground state of helium is described by a many-parameter wavefunction. The continuum of helium between the second and third thresholds is described with allowance for the coupling among 20 configurations, which correspond to the two-electron excitations and four configurations with the free electron above the n=1 and n=2 states of the He+ ion.

Two-electron excitation in helium-like ions by electron impact

Calculation of cross-sections for the two-electron excitation in helium-like ions by electron impact employing Coulomb-Born-Oppenheimer (CBO) approximation is presented. Analytical expressions for the differential and total scattering cross-sections without using partial wave expansion of the wavefunction reported earlier have been used. The total and differential scattering cross-sections for each of the excitations 1s21S* → 2s21Se, 2s2p1.3P0, 2p21Se,3Pe,1De in Be2+ and B3+ are computed. Results for Li+ reported earlier are also included for comparison.

Hg 5dand 6s: Multichannel quantum-defect analysis of experimental data

Experimental spin-polarization data for the Hg 6s Cooper minimum and dipole-transition amplitudes and phase-shift differences for photoionization of the Hg 5d and 6s shells (eight channels) in the photon-energy region from the $^{2}$${\mathit{D}}_{3/2}$ threshold up to 35 eV (kinetic energies from 0 to 20 eV) are presented and compared with new relativistic random-phase approximation calculations. The data were evaluated from an experimental data set, quantum mechanically ``complete,'' consisting of cross-section and photoelectron spin-polarization data. For the 5d subshells the results show strong interchannel coupling between the outgoing p and f continuum channels. The results for Hg 6s could be used for an independent determination of the photoelectron angular distribution parameter \ensuremath{\beta} in the Cooper-minimum region. This region is shown to be perturbed by numerous two-electron excitations, which might be a possible explanation for the controversy on its location.

Evidence for resonant two-electron capture and excitation in collisions of H-like Ge with Ne

Evidence for resonant two-electron capture and excitation, i.e. the time-reversed double Auger process with radiative stabilization, was found by measuring coincidences between two K-X-rays of a heavy ion associated with its double charge exchange. Hydrogen-like Ge 3+ ions with energies of 4.5–11.5 MeV u were directed into a neon gas target. Contributions from the double non-resonant transfer and excitation process are also discussed.

Two-electron detachment and excitation processes in S- collisions with atoms and molecules.

Results of an electron-spectroscopy study of two-electron detachment (ionization) and excitation processes in ${\mathrm{S}}^{\mathrm{\ensuremath{-}}}$ collisions with He, Ne, Ar, Kr, ${\mathrm{H}}_{2}$, ${\mathrm{O}}_{2}$, and ${\mathrm{N}}_{2}$ are presented. The electron spectra indicate that singly excited autoionizing states of S ${(}^{2}$D nl and $^{2}\mathrm{P}$ nl series) are populated in these collisions, and this should lead to substantial ${\mathrm{S}}^{+}$ production. Previously unknown autoionizing states of the $^{2}(^{2}$P)nl series are identified. In the case of the ${\mathrm{O}}_{2}$ target, charge exchange to the lowest ${\mathrm{O}}_{2}^{\mathrm{\ensuremath{-}}}$ ${(}^{2}$${\ensuremath{\Pi}}_{g}$) shape resonance is observed.

New odd-parity Rydberg and autoionization levels in uranium i

We have observed new odd-parity Rydberg and autoionization levels in uranium I close to its first ionization limit. Our experimental configuration for ion detection and for pulsed electric-field ionization of Rydberg levels results in the observation of two groups of ions, the prompt photoions and delayed electric-field ions. The latter give information exclusively on Rydberg levels. The observed Rydberg progression suggests that a two-electron excitation process of the type 5f36d7s7p → 5f37s2ns or nd is involved for the transitions to Rydberg states.

CASSCF study on the FeO 2 bond in a dioxygen heme complex

Ab initio complete active space self-consistent field (CASSCF) calculations have been performed to elucidate the electronic structure of the FeO 2 bond in an oxyheme model compound. The main configuration in the CASSCF ground-state wavefunction is 1A′ (dε) 6(π g a) 2, but its weight is small (0.641). Two-electron excitations, (d π) 2→(π g b) 2 and (π g a)(d π)→(π g b)(d σ), mix into the dominant configurations.

Correlated two-electron transitions in electron-impact ionization of Li+ ions.

Electron-impact ionization cross sections for He-like ${\mathrm{Li}}^{+}$ ions have been measured with extreme precision (0.01% statistics). Thus, two new mechanisms in the ionization of positive ions both involving two-electron excitations could be quantitatively investigated. We see (1) direct excitation of both K electrons into the L shell with subsequent autoionization and (2) resonant capture of the incident electron by the ion with correlated KK-LL excitation of both core electrons leading to a triply excited Li atom which then decays by the emission of two correlated electrons. Strong interference of process (2) with direct ionization is observed.

The spectrum of two-particle excitations of the Hubbard Hamiltonian

The spectra of two-hole and two-electron excitations are calculated for a single-band Hubbard Hamiltonian. These spectra are directly connected with the intensity of CVV Auger spectra and with the appearance potential spectra of transition metals with strong electron-electron interactions. The theory is based on a local decoupling procedure applied to the retarded two-particle double-time Green functions in the site representation. It is exact in four important limiting cases: in the non-interacting limit (U to 0), in the atomic limit (band width W to 0), in the limit of low concentration of electrons (n to 0) and holes (n to 2), and it provides a physically reasonable interpolation between these limits. In contrast with previous similar theories, it has proper transformation properties under the electron-hole transformation. The theory is illustrated numerically in a simplified model.

Two-electron Excitation and Ionization in an Intense Laser Field

Abstract We present a simple model for two-electron excitation and ionization of an atom in the presence of an intense laser field. We show, in particular, how the Coulomb interaction, via configuration interaction, prevents the excitation and ionization from being collective in all but the most intense fields. By collective we mean that each electron has the same spatial wavefunction: this implies that the electron pair may be described by a restricted time-dependent Hartree-Fock wavefunction. The configuration interaction prevents collective behaviour not only through the process of auto-ionization but also through the mixing of doubly excited independent electron states into the ground state. We are able to give criteria for establishing true collective excitation and ionization of our model two-electron system in an intense field.

Structures at the Xe 5s threshold studied by photon-induced fluorescence spectroscopy

The photoionisation of Xe has been studied by photon-induced fluorescence spectroscopy (PIFS) for photon energies between 23 and 26.5 eV. The fluorescence radiation was dispersed and recorded between 103.5 and 112 nm. A prominent peak structure of the Xe 5s photoionisation cross section was observed and was attributed to two-electron excitations in neutral Xe which strongly contribute to the ionisation process.

Configuration-average energy shift owing to configuration interaction.

A general expression for the shift in the average energy of an atomic electron configuration owing to configuration interaction is calculated in the second order of Rayleigh-Schroedinger perturbation theory, for arbitrary interacting configurations in either the relativistic (nlsj) or the nonrelativistic (nls) angular momentum classification scheme. This correction to the configuration-average energy is especially important for evaluating the effects of departures from pure j-j coupling when interpreting spectra from highly ionized atoms using the spin-orbit split array model. The total shift divides naturally into contributions from a set of distinct products of radial integrals, each multiply- ing the configuration average of a reducible four-body operator. For configurations that interact by single-electron excitation, n/sub 1/l/sub 1/j/sub 1/..-->..n/sub 2/l/sub 2/j/sub 2/, the partial shift associated with each such product reduces to two terms, each proportional to a polynomial of degree 4 in the occupation numbers. The first term is proportional to delta/sub j//sub >1/j/sub =/ and is interpreted as a correction to the central potential. The second term contains the essential effects of configuration interaction, and vanishes unless the two spectator electrons (of the four-body operator) are equivalent. For two-electron excitations, only one term appears with no restrictions on the quantum numbersmore » of the active electrons. Comparisons are presented with detailed level accounting results, and with experimental measurements.« less

Transfer and excitation processes studied in H-like S and Li-like and H-like F colliding with H 2

We have studied transfer and simultaneous excitation by three different experimental techniques. Coincidences between two K X-rays were measured for S 15+, as well as coincidences between one K X-ray and the charge-exchanged projectile for Li-like F and projectile Auger electrons for H-like F, in each case colliding with H 2. For all three collision system, the measured cross sections are dominated by resonant transfer and excitation (RTE). Also, for the F projectiles, strong contributions from two-electron transfer and excitation (2eTE) were found.

Periodic orbits in atomic hydrogen exposed to circularly polarised laser light

Exact circular periodic orbits for a hydrogen atom in a strong circularly polarised laser field are derived. A stability analysis shows one orbit to be stable or unstable depending on the laser parameters, whereas the other orbit is always unstable. These orbits are expected to manifest themselves as resonances in laser assistede − −H + scattering. The binding mechanism for these resonances is provided by the trapping of the quantum mechanical wave function onto the neighbourhood of a classical periodic orbit. This mechanism is analogous to that for a Wannier ridge resonance in a doubly excited two-electron atom.

Photoionization Cross Sections of Xenon in the L Edge Region

The total photoion yields of xenon have been measured in the L-shell ionization region covering the photon energy from 4.6 to 6.1 keV with the step width of wavelength scans of about 0.0025 A. Monochromatized undulator radiation was used as a light source and crossed with an atomic xenon beam. Sharp peaks at the L 3 and L 2 edges, which had not been observed in previous work, are clearly seen in the present data. It can be suggested that these peaks are due to the photoexcitation of 2p electrons to the unfilled higher levels. Furthermore, a structure above the L 3 edge which suggests deviations from the power lawlike energy dependence predicted by an independent electron model can be seen in the present data. This structure might be due to simultaneous two-electron excitation.