Two-electron and one-photon transitions in highly charged nickel-like ions

This paper calculates the transition wavelengths and probabilities of the two-electron and one-photon (TEOP) transition from the $(3{\rm s}^{-1}_{1/2}4{\rm d}_{j})_{J=1,2}$ to $(3{\rm p}^{-1}_{3/2}4{\rm s}_{1/2})_{J=1}$ and the $(3{\rm p}^{-1}_{1/2}4{\rm s}_{1/2})_{J=1}$ to $(3{\rm d}^{-1}_{j}4{\rm d}_{j'})_{J=1,2}$ for highly charged Ni-like ions with atomic number $Z$ in the range $47\leq Z\leq92$. In the calculations, the multi-configuration Dirac--Fock method and corresponding program packages GRASP92 and REOS99 were used, and the relativistic effects, correlation effects and relaxation effects were considered systematically. It is found that the TEOP transitions are very sensitive to the correlation of electrons, and the probabilities will be enhanced sharply in some special $Z$ regions along the isoelectronic sequence. The present TEOP transition wavelengths are compared with the available data from some previous publications, good agreement is obtained.     

Two-electron transitions in germanium

Measurements of two-electron transitions in the EHD in Ge at 2 K are presented. A comparison with Si and measurements in Ge at 100 K leads to the conclusion that these transitions are forbidden and enhanced by electron- hole correlation.     

Two-electron band to band transitions in silicon

Radiative two-electron band to band transitions are investigated experimentally in silicon. The question of phonon-participation is investigated. It turns out that at low temperature the phononless transitions dominate, whereas above room-temperature the phonon-assisted transitions become more important. A theoretical discussion leads to the conclusion that at least one participating phonon is an acoustical one.     

A simple treatment of two-electron one-photon transitions

Two electron radiative transition probabilities from the L and M shells into a doubly vacant K shell are computed non-relativistically, introducing angular correlation in the simplest conceivable way. The relative cross-sections for the different possible processes are in agreement with recently reported experimental values for Fe and Ni and predict interesting changes in the spectra of heavier systems.     

Two-electron transitions in collisions of fast multiply-charged ions with helium-like atoms

Single and double excitation and ionization of a helium-like atom are considered based on the eikonal approximation. Simple analytical formulas are derived for the cross sections of inelastic collision. A comparison with the experiment is conducted. Scientific Research Institute of Applied Physics at Tashkent State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 44–48, February, 2000.     

First observation of two-electron one-photon transitions in single-photon K-shell double ionization

Experimental evidence for the correlated two-electron one-photon transitions (1s(-2)→2s(-1)2p(-1)) following single-photon K-shell double ionization is reported. The double K-shell vacancy states in solid Mg, Al, and Si were produced by means of monochromatized synchrotron radiation, and the two-electron one-photon radiative transitions were observed by using a wavelength dispersive spectrometer. The two-electron one-photon transition energies and the branching ratios of the radiative one-electron to two-electron transitions were determined and compared to available perturbation theory predictions and configuration interaction calculations.     

Structural transitions in small nickel clusters

The processes of a thermal impact on Ni nanoclusters with a radius of up to 0.8 nm have been studied by means of molecular dynamics with the use of a tight-binding potential. The simulation indicates that the structural transition from the initial fcc phase to the icosahedral modification occurs under the influence of temperature. The transition temperature is shifted towards the cluster melting temperature with an increase in the cluster size. A similar behavior has been observed for copper and gold nanoparticles. A conclusion has been drawn that 200–250 atoms is presumably the limiting size of a metallic cluster, below which the initial fcc modification cannot be kept under realistic industrial conditions. The adequacy of the results is checked in the computer experiments with Lennard-Jones nanoparticles. The results for the Lennard-Jones and metallic nanoparticles have been shown to agree with each other.     

Magnetic dipole transitions in highly ionized titanium,chromium, iron and nickel

Wavelengths of magnetic dipole transitions in the ground configurations of Ti, Cr, Fe and Ni ions in fluorine through beryllium isoelectronic sequences have been measured in tokamak plasmas.     

Two-electron atoms in strong fields

Multiphoton excitation and ionization of two-electron atoms under strong laser pulses, with emphasis on double excitation, is reviewed. After an introduction to the basic concepts of traditional single-photon spectroscopy and methodology, the main phenomena and experimental observations under multiphoton excitation in the perturbative regime are summarized. That is followed by the discussion of the main ideas underlying various non-perturbative approaches, especially as they pertain to the two-electron problem. The present state of understanding non-perturbative time-dependent behavior, in that context and in relation to issues such as correlation, double excitation, sequential and direct double ionization, is then formulated and discussed. The article ends with a summary of effects relating to autoionizing states in strong fields and an outlook for future developments.     

Indistinguishability in one-photon and two-photon interference

Some interference experiments with the photon pairs from two parametric down-converters are described and analyzed. They are found to contribute to the interpretation of the quantum state.     

Dispersion studies and electronic transitions in nickel phthalocyanine thin films

Thin films of the organic semiconductor nickel phthalocyanine (NiPc) are structurally investigated using X-ray diffraction and infrared light absorption. The optical absorption and dispersion studies of nickel phthalocyanine were investigated using spectrophotometric measurements of transmittance and reflectance at normal incidence in the wavelength range 190–2100 nm. The absorption spectra recorded in the UV-VIS region show two well-defined absorption bands of the phthacyanine molecules, namely, the Soret and the Q-band. The Davydove splitting of the main absorption peak in the metal phthalocyanines correlates with the relative tendencies of the metal to out-of-plane bonding. The refractive index n as well as the absorption index k were calculated and showed an independent of the film thickness in the film thicknesses range 400–770 nm. The refractive index n showed an anomalous dispersion in the absorption region as well as normal dispersion in the transparent region. Some of the important optical absorption such as the molar extinction coefficient, the oscillator strength, the electric dipole strength have been evaluated. The analysis of the spectral behavior of the absorption coefficient α in the absorption region revealed two indirect allowed transitions with corresponding energies 2.77±0.03 and 1.66±0.02 eV. An energy band diagram has been proposed to account for the optical transitions of NiPc thin film. All previous parameters were as well obtained for films annealed at 523 K for 2 h. Discussion of the obtained results and their comparison with the previously published data are also given.     

Temperature dependence of interband transitions and exchange splitting in nickel

Interband transitions of Ni have been studied as a function of temperature by observing energy losses of low energy electrons reflected from a Ni(100) surface. A threshold is found at E_g ～ 0.15 eV and interpreted as the onset of X^↑₅ → X^↓₂ transitions. The threshold remains unchanged for temperatures up to 900 K. The loss continuum below E_g exhibits an anomalous increase with temperature with a discontinuity in the slope at T ～ 1.2 T_c.     

Two-electron quantum ring in short pulses

The response of a two-electron quantum ring system to the short laser pulses of different shapes in the presence of external static electric field is studied.The variation of transition probabilities of the two-electron quantum ring from ground state to excited states with a number of parameters is shown and explained.The energy levels and wavefunctions of the system in the presence of static electric field are found by solving the time-independent Schrodinger equation numerically by the finite difference method.The shape of the pulse plays a dominant role on the dynamics.     

Two-electron entanglement in elliptically deformed quantum dots

Entropic entanglement measures of a two-dimensional system of two Coulombically interacting particles confined in an anisotropic harmonic potential are discussed in dependence on the anisotropy and the interaction strength. The harmonic approximation appears exact in the strong interaction limit, allowing determination of the asymptotic expression for the linear entropy. Entanglement properties are dramatically influenced by the anisotropy of the confining potential in the strong-correlation regime.     

Change of the photon statistics by one-photon absorption and by simultaneous one-photon and two-photon absorption

With the help of the generating function we investigate the change of photon statistics due to one-photon absorption. Photon distributions with antibunching approach by this process the Poisson distribution. In a second part we consider the simultaneous action of one-photon and two-photon absorption. We present the exact solution and the asymptotic state in the limit of great initial photon numbers.     

Two-electron absorption in a biased quantum well

Linear light absorption of 2D electrons confined within a biased quantum well is studied theoretically. We demonstrate that for light polarization perpendicular to the 2D plane, in addition to conventional absorption peak at frequency ω≈Δ, where Δ is the intersubband energy distance, there exists a peak around a double frequency ω≈2Δ. This additional peak is entirely due to electron–electron interactions, and corresponds to excitation of two electrons by one photon. The magnitude of two-electron absorption is proportional to U², where U is the applied bias.     

Two-electron bond approach to chemisorption

A new approach to chemisorption is presented which considers both neutral and ionic configurations of the adsorbate on the same footing. The two-electron bond between the adsorbate and an atom of the metal surface is characterised by the Heitler-London method, the metal band structure being described by the cluster-Bethe lattice method. The only parameter of this theory is the hopping integral of the Anderson-Newns Hamiltonian. Quantities such as charge populations, one electron levels, and excitation energies which can be immediately compared with the spectroscopic data (ELS, UPS, ESD, etc.) can be easily deduced. An application is given to the H/W(100) system.