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 Franck–Condon and static‐exchange approximations. Large Cartesian Gaussian basis sets are used to represent the required discrete and continuum electronic eigenfunctions at the ground‐state equilibrium geometry, and previously devised moment‐theory techniques are employed in constructing the continuum oscillator‐strength 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–excitation cross sections, and dipole (e, 2e)/(e, e+ion) and synchrotron‐radiation studies of partial‐channel photoionization cross sections. The various calculated excitation series in the outer‐valence (1b(^−1)_1, 3a(^−1)_1, 1b(^−1)_2) region are found to include contributions from valence‐like 2b_2 (σ*) and 4a_1(γ*) 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 ∼7 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(^−1)_1)X(^ 2)B_1, (3a_1(^−1))^2A_1, and (1b_2(^−1))(^2)B_2 partial‐channel cross sections are found to be largely atomic‐like and dominated by 2p→kd components, although the 2b_2(σ*) orbital gives rise to resonance‐like contributions just above threshold in the 3a_1→kb_2 and 1b_2→kb_2 channels. It is suggested that the latter transition couples with the underlying 1b_1→kb_1 channel, accounting for a prominent feature in the recent high‐resolution synchrotron‐radiation measurements. When this feature is taken into account, the calculations of the three outer‐valence channels are in excellent accord with recent synchrotron‐radiation and dipole (e, 2e) photoionization cross‐sectional measurements. The calculated inner‐valence (2a_1(^−1)) cross section is also in excellent agreement with corresponding measured values, although proper account must be taken of the appropriate final‐state configuration‐mixing 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‐edge electron energy‐loss profile in H_2O are seen to be clarified fully by the present calculations

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

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

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

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

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