114 research outputs found
Theoretical studies of photoexcitation and ionization in H_2O
Theoretical studies are reported of the complete dipole excitation and ionization spectrum in H_2O employing FranckâCondon and staticâexchange approximations. Large Cartesian Gaussian basis sets are used to represent the required discrete and continuum electronic eigenfunctions at the groundâstate equilibrium geometry, and previously devised momentâtheory techniques are employed in constructing the continuum oscillatorâstrength 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 impactâexcitation cross sections, and dipole (e,â2e)/(e,âe+ion) and synchrotronâradiation studies of partialâchannel photoionization cross sections. The various calculated excitation series in the outerâvalence (1b(^â1)_1, 3a(^â1)_1, 1b(^â1)_2) region are found to include contributions from valenceâlike 2b_2â(Ï*) and 4a_1(Îł*) 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 âŒ7 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(^â1)_1)X(^â2)B_1, (3a_1(^â1))^2A_1, and (1b_2(^â1))(^2)B_2 partialâchannel cross sections are found to be largely atomicâlike and dominated by 2pâkd components, although the 2b_2(Ï*) orbital gives rise to resonanceâlike contributions just above threshold in the 3a_1âkb_2 and 1b_2âkb_2 channels. It is suggested that the latter transition couples with the underlying 1b_1âkb_1 channel, accounting for a prominent feature in the recent highâresolution synchrotronâradiation measurements. When this feature is taken into account, the calculations of the three outerâvalence channels are in excellent accord with recent synchrotronâradiation and dipole (e,â2e) photoionization crossâsectional measurements. The calculated innerâvalence (2a_1(^â1)) cross section is also in excellent agreement with corresponding measured values, although proper account must be taken of the appropriate finalâstate configurationâmixing 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 Kâedge electron energyâloss profile in H_2O are seen to be clarified fully by the present calculations
Symmetry-breaking in the independent particle model: nature of the singular behavior of HartreeâFock potentials
Scattering length of the ground state Mg+Mg collision
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
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
Large multiconfigurational Hartree-Fock calculations on the hyperfine-structure constants of the Li
NATO Advanced Study Institute on Problem Solving in Computational Molecular Science : Molecules in Different Environments
Understanding the spectra of a few electrons confined in a quasi-one-dimensional nanostructure
Confined quantum systems: dipole polarizability of the two-electron quantum dot, the hydrogen negative ion and the helium atom
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