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AbstractAbstract

[en] We discuss the discrepancy between the Cung et al. [Phys. Lett. B 68, 474 (1977)] calculation of the three-photon-annihilation contribution to the positronium ground-state energy, performed using the binding energy to regulate the infrared divergences, and the recent calculation of Adkins, Bui, and Zhu [Phys. Rev. A 37, 4071 (1988)], which used a photon mass to regulate these divergences. By using a simpler version of the binding-energy approach advocated by these authors, we confirm the value of the discrepancy they obtained. Furthermore, it is shown that the additional term needed in the binding-energy approach to produce agreement between the two methods is precisely the one required to make the binding-energy regularized amplitudes gauge invariant to all orders in the binding

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[en] A review is given of the atomic many-body theory in the coupled-cluster approach or exponential-Ansatz formulation. Explicit equations and corresponding graphical representations are given in the pair-approximation, where the one- and two-body parts of the cluster (exponent) operator are considered. Also the effect of a small, additional perturbation is considered. The technique of evaluating diagrams by means of one- and two-particle functions, satisfying inhomogeneous differential equations, is reviewed. Illustrative numerical results are given for the electron correlation energy, electron binding energy, hyperfine separation and specific mass shift of simple atomic systems. The extension of the non-relativistic procedure to the relativistic regime is discussed by considering the effect of the exchange of one and two virtual, transverse photons between the electrons. In lowest order this leads to the ''no-virtual-pair approximation''. (orig.)

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Adriatico research conference on relativistic many-body problems; Miramare, Trieste (Italy); 30 Jun - 4 Jul 1986

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[en] In this work we present a theoretical study of the dissociative adsorption of hydrogen molecules from a series of model potential energy surfaces. The aim is to discover those particular topological features in the potential surface which are responsible for determining the vibrational state-to-state cross sections in both the dissociated and the scattered flux. The potential energy surface is two-dimensional, and is chosen to be deliberately simple; a combination of Morse potentials and a Gaussian barrier. A quantum wave packet is chosen to represent the molecule and the dynamics are solved by a spectral grid method. Results show that the location of the barrier influences the scattering cross sections markedly. Early barriers result in vibrationally excited adsorbed species while late barriers produce translationally hot atoms. The individual state distributions resulting from the two model potentials are quite different. In addition, results are given for a potential where the activation barrier is deep in the exit channel. For this case, results show that molecules can trap near the barrier for significant times without invoking substrate degrees of freedom. This is explained in terms of trapping in dynamic wells. Finally, we assess the effect on dissociation probability following vibrational excitation of the hydrogen molecule

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[en] In an active ''microcavity'' the condition of maximum ''enhancement'' of spontaneous emission corresponds to resonant coupling of atoms with a single field mode. We demonstrate that this exceptional condition results in the effective cancellation of spontaneous emission, in the onset of stimulated emission at exceedingly low excitation levels, and in an anomalously high stimulated-emission gain. In the context of phase-transition theory the active microcavity behaves as a statistical ensemble undergoing an order-disorder transition at an extremely high value of the critical temperature

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[en] The electronic states of molecules are often characterized by strong mixing of configurations. For an accurate description of molecular potential-energy surfaces (MPES), it is essential therefore to allow considerable freedom in configuration mixing. The authors present a multi-reference-state (MRS) perturbation theory (PT) which is tailored for the evaluation of MPES. The basic idea is to include the important configurations in a reference space for which an effective Hamiltonian (H/sub eff/) is defined. The matrix elements of H/sub eff/ are computed using the Rayleigh-Schrodinger perturbation series to third or fourth order. Pilot calculations on Li/sub 2/ are encouraging, with the third-order ground-state dissociation energy -- 100 cm/sup -1/ from the experimental value. Two important computational advantages of MRS PT are: Calculations are simplified upon truncation of the SD space according to numerical cutoff criteria which are easily evaluated; without altering the overall quality of the results. The computation of different matrix elements of H/sub eff/ is easily programmed for parallel processing

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Anon; p. 37; 1987; p. 37; ISOC 87 Secretarist, Room 209; London (UK); 1. international conference of the impact of supercomputers on chemistry; London (UK); 13-16 Apr 1987

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[en] The multiconfiguration Hartree-Fock theory and many-body perturbation theory are combined in a calculation of the correlation energy of the ground state of the neutral beryllium atom. For this purpose the two-component Be multiconfiguration Hartree-Fock wave function is treated as a reference state and perturbation theory is used to systematically improve upon the accuracy of this function. The correlation energy of each pair of occupied orbitals is determined by solving numerically a coupled set of two-particle inhomogeneous equations. A detailed comparison is made with other beryllium calculations

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[en] By using an approximate analytical trial density and the consideration of a Thomas-Fermi-Dirac Weizsaecker energy density functional, total ground-state energies, its different components and the expectation values < r

^{n}> with n=-1, 1 and 2 are calculated for several atoms in the Periodic Table. The results are compared with Hartree-Fock values. (orig.)Primary Subject

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Zoller, P.; Alber, G.; Henle, W.A.; Ritsch, H.

XV international conference on quantum electronics (Digest of technical papers)

XV international conference on quantum electronics (Digest of technical papers)

AbstractAbstract

[en] A theory of Rydberg states of many electron atoms in laser fields is developed. The list of problems treated includes generation of wave packets by short pulses and excitation of Rydberg series near threshold

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Anon; p. 210; ISBN 0-936659-51-3; ; 1987; p. 210; Optical Society of America; Washington, DC (USA); 15. international conference on quantum electronics; Baltimore, MD (USA); 27 Apr - 1 May 1987

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[en] Basic facts concerning the electronic structure, energy states and transitions of molecules are outlined. It is shown that chemical and a majority of physical properties of molecules are determined by the weakly bonded outer electrons. The various kinds of chemical bonds, vibrational and rotational molecular states and molecular spectra are explained. (M.D.). 10 figs

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Molekuly

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496 p; 1990; p. 189-201; Academia; Prague (Czechoslovakia)

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[en] We study the structure of the hydrogen atom when placed in a high-frequency, superintense laser field, within the framework of a nonperturbative theory recently developed for this purpose. The theory predicts that in the high-frequency limit the atom is stable against decay by multiphoton ionization, and that its structure is determined by a time-independent Schroedinger equation containing a ''dressed'' Coulomb potential. The laser frequency ω and the intensity I enter only combined in the parameter α

_{0}=I^{1/2}ω^{-2}a.u. We first analyze the symmetry of the eigenvalue problem for the case of linear polarization under consideration and adopt an appropriate classification scheme for the levels. The small-α_{0}limit of the levels is obtained analytically. In the large-α_{0}limit scaling laws are derived for the α_{0}dependence of the eigenvalues and eigenfunctions. At finite α_{0}we have carried out a very accurate numerical computation over an extended range of α_{0}values (0≤α_{0}≤200 a.u.) for a number of symmetry manifolds, by diagonalization of the Hamiltonian in a Gaussian basis. The correlation diagrams relating the small- and large-α_{0}limits exhibit several avoided crossings. The binding energies show an overall decrease with α_{0}, in some cases preceded by an increase through a maximum. For the ground state this decrease is quite steep. The extreme distortion of the atomic structure accompanying it is studied. It is shown that, with increasing α_{0}, the (oscillating) electronic cloud undergoes radiative stretching, which eventually culminates at large α_{0}in its splitting into two parts (dichotomy). The consequences of our findings for the experimental energy spectrum of the ejected electrons are consideredPrimary Subject

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