68 research outputs found

    Theoretical studies of photoexcitation and ionization in H_2O

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    Theoretical studies are reported of the complete dipole excitation and ionization spectrum in H_2O employing Franck‚ÄďCondon and static‚Äźexchange approximations. Large Cartesian Gaussian basis sets are used to represent the required discrete and continuum electronic eigenfunctions at the ground‚Äźstate equilibrium geometry, and previously devised moment‚Äźtheory techniques are employed in constructing the continuum oscillator‚Äźstrength densities from the calculated spectra. Detailed comparisons are made of the calculated excitation and ionization profiles with recent experimental photoabsorption studies and corresponding spectral assignments, electron impact‚Äďexcitation cross sections, and dipole (e,‚ÄČ2e)/(e,‚ÄČe+ion) and synchrotron‚Äźradiation studies of partial‚Äźchannel photoionization cross sections. The various calculated excitation series in the outer‚Äźvalence (1b(^‚ąí1)_1, 3a(^‚ąí1)_1, 1b(^‚ąí1)_2) region are found to include contributions from valence‚Äźlike 2b_2‚ÄČ(ŌÉ*) and 4a_1(ő≥*) virtual orbitals, as well as appropriate nsa_1, npa_1, nda_1, npb_1, npb_2, ndb_1, ndb_2, and nda_2 Rydberg states. Transition energies and intensities in the ‚ąľ7 to 19 eV interval obtained from the present studies are seen to be in excellent agreement with the measured photoabsorption cross section, and to provide a basis for detailed spectral assignments. The calculated (1b(^‚ąí1)_1)X(^‚ÄČ2)B_1, (3a_1(^‚ąí1))^2A_1, and (1b_2(^‚ąí1))(^2)B_2 partial‚Äźchannel cross sections are found to be largely atomic‚Äźlike and dominated by 2p‚Üíkd components, although the 2b_2(ŌÉ*) orbital gives rise to resonance‚Äźlike contributions just above threshold in the 3a_1‚Üíkb_2 and 1b_2‚Üíkb_2 channels. It is suggested that the latter transition couples with the underlying 1b_1‚Üíkb_1 channel, accounting for a prominent feature in the recent high‚Äźresolution synchrotron‚Äźradiation measurements. When this feature is taken into account, the calculations of the three outer‚Äźvalence channels are in excellent accord with recent synchrotron‚Äźradiation and dipole (e,‚ÄČ2e) photoionization cross‚Äźsectional measurements. The calculated inner‚Äźvalence (2a_1(^‚ąí1)) cross section is also in excellent agreement with corresponding measured values, although proper account must be taken of the appropriate final‚Äźstate configuration‚Äźmixing effects that give rise to a modest failure of the Koopmans approximation, and to the observed broad PES band, in this case. Finally, the origins of the various spectral features present in the measured 1a_1 oxygen K‚Äźedge electron energy‚Äźloss profile in H_2O are seen to be clarified fully by the present calculations

    Scattering length of the ground state Mg+Mg collision

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    We have constructed the X 1SIGMAg+ potential for the collision between two ground state Mg atoms and analyzed the effect of uncertainties in the shape of the potential on scattering properties at ultra-cold temperatures. This potential reproduces the experimental term values to 0.2 inverse cm and has a scattering length of +1.4(5) nm where the error is prodominantly due to the uncertainty in the dissociation energy and the C6 dispersion coefficient. A positive sign of the scattering length suggests that a Bose-Einstein condensate of ground state Mg atoms is stable.Comment: 15 pages, 3 figures, Submitted Phys. Rev.

    Unified Treatment of Asymptotic van der Waals Forces

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    In a framework for long-range density-functional theory we present a unified full-field treatment of the asymptotic van der Waals interaction for atoms, molecules, surfaces, and other objects. The only input needed consists of the electron densities of the interacting fragments and the static polarizability or the static image plane, which can be easily evaluated in a ground-state density-functional calculation for each fragment. Results for separated atoms, molecules, and for atoms/molecules outside surfaces are in agreement with those of other, more elaborate, calculations.Comment: 6 pages, 5 figure

    Energy levels and structural properties of compressed hydrogen atom under Debye screening

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    Time dependent variation perturbation calculations have been performed for estimating the transition energies, oscillator strengths and transition probability values for a few dipole allowed states of compressed hydrogen atom confined in a weakly coupled plasma. The compression is obtained by embedding the atom at the centre of an impenetrable spherical box. The dipole polarizability of the atom is evaluated at each confinement radius with respect to different plasma screening parameters. The effect of pressure due to spatial confinement on the dipole polarizability and other atomic properties is analyzed. Results obtained are useful for the diagnostic determination of astrophysical and laboratory plasmas and for the calculation of collision rate coefficients needed for computing opacity of stellar envelopes-a quantity of importance in the context of stellar structure and pulsations

    On the influence of the Debye screening on the spectra of two- electron atoms

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    The energies of the ground state and of the first three S-1 excited states of helium-like atoms with 2 less than or equal to Z less than or equal to 10 have been calculated under plasma-type confinement. The effect of the confinement has been described by the Debye screening potential. The energy levels show interesting behaviour under increased plasma strength, which is in conformity with recent experimental observations of the change in spectra of an impurity atom in liquid helium. (C) 2002 Elsevier Science B.V. All rights reserved

    Variational calculations for the energy levels of confined two- electron atomic systems

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    An accurate variational calculation has been performed for the ground-state-energy values of confined two-electron isoelectronic series from He to Ar16+. The confinement is obtained by embedding the ion in an overall charge neutral environment like that of a plasma. The confinement potential is chosen as that of a screened Coulomb potential between charges, obtained from a Debye model. The wave function is expanded in terms of product basis sets involving interparticle coordinates. The energy levels are found to be less bound with an increase of the screening parameter and ultimately become unstable. One- and two-particle moments have been calculated for the first time under such screening. The study is expected to throw new light on the behavior of the energy levels of foreign atoms embedded in an overall neutral environment which can be treated like a plasma
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