The retardation long-range potentials between two 1
1S (or 2
3S) helium atoms are calculated. The complex dynamic multipole polarizabilities of 1
1S and 2
3S states of helium are obtained through a variation-perturbation approach with B-spline and Slater-type basis functions in a configuration-interaction scheme. They are used to calculate the retardation long-range potentials. The retardation coefficients are examined. Their relative magnitudes are different for different systems. The possibility of an excited state of helium dimer from the 1
1S--2
3S
He2 state is estimated. \textcopyright{} 1996 The American Physical Society.
The electric dipole, quadrupole, and octupole polarizabilities of a helium atom and the associated dispersion coefficients for the interaction of two helium atoms are calculated for the atomic
n=2 singlet and triplet states, respectively,
He(21S) and
He(23S). The calculations utilize highly accurate, variationally determined, wave functions composed of doubled basis sets. The effects of retardation due to the finite speed of light are calculated for the dispersion potentials of two
He(21S) or two
He(23S) atoms and for the atom-surface interaction potentials of a
He(21S) or a
He(23S) atom with a perfectly conducting or a dielectric wall.
We investigate the long-range, two-body interactions between rubidium and potassium atoms in highly excited
(n=70) Rydberg states. After establishing properly symmetrized asymptotic basis states, we diagonalize an interaction Hamiltonian consisting of the standard Coulombic potential expansion and atomic fine structure to calculate electronic potential energy curves. We find that when both atoms are excited to either the
70s state or the
70p state, both the $\mathrm{\ensuremath{\Omega}}={0}^{+}$ symmetry interactions and the $\mathrm{\ensuremath{\Omega}}={0}^{\ensuremath{-}}$ symmetry interactions demonstrate a deep potential well capable of supporting many bound levels; the sizes of the corresponding dimer states are of the order of 2.25 $\ensuremath{\mu}\mathrm{m}$. We establish
n-scaling relations for the equilibrium separation
Re and the dissociation energy
De and find these relations to be consistent with similar calculations involving the homonuclear interactions between rubidium and cesium. We discuss the specific effects of $\ensuremath{\ell}$ mixing and the exact composition of the calculated potential well via the expansion coefficients of the asymptotic basis states. Finally, we apply a Landau-Zener treatment to show that the dimer states are stable with respect to predissociation.
Long-lived, excited neutral particles, arising from the interaction of monochromatic synchrotron radiation with ground state He atoms, have been observed at photon energies close to the (N = 1,2,3 and 4) ionization thresholds. The measurements have been made using an unconventional experimental arrangement in which charged particles, responsible for the dominant signal in conventional photon impact studies, are prevented from reaching the detector. For , it appears that the formation, via photoexcitation, of relatively long-lived, doubly excited Rydberg states is a necessary step in the production of the observed signal. Four processes that might account for the production of the excited neutral particles are considered. The simplest, that atoms in the initial doubly excited Rydberg states are themselves directly observable, is considered unlikely. However, the lifetimes of the photoexcited double Rydberg states may be increased through the effects of electric fields present in the apparatus. Transitions from these double Rydberg states, occurring either as a result of collisions or by fluorescence, could result in metastable, singly excited neutral particles. The presence of signal related to the existence of long-lived doubly excited states could be significant in the interpretation of other photon impact measurements.
The dominant long-range interaction of an alkali-metal atom with charged particles at low energies is given for large separation R by the dispersive potential $W\ensuremath{\simeq}\ensuremath{-}{\ensuremath{\alpha}}_{d}{/R}^{4}$ in terms of the dipole polarizability ${\ensuremath{\alpha}}_{d}.$ For atoms prepared initially in Rydberg states of quantum numbers (n, l), the potential assumes a more complicated form due to the complete or near degeneracy of the n manifold. Contributions to the polarizability are treated in two parts, (a) one for the nondegenerate states and (b) the other for the degenerate or near degenerate cases. It is shown that ${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\alpha}}}_{d}^{(b)}$ for case (b) is in general R dependent, and in the limit of complete degeneracy, diverges as
R2. That is, for a small energy gap \ensuremath{\Delta} between a pair of nearly degenerate states which are dipole coupled, the dispersion potential ${W}^{(b)}\ensuremath{\simeq}{D/R}^{2}$ for
R<Rx and ${W}^{(b)}\ensuremath{\simeq}\ensuremath{-}{\ensuremath{\alpha}}_{d}^{(b)}{/R}^{4}$ for
R>Rx, where D is the dipole moment, ${R}_{x}=[2|D/\ensuremath{\Delta}|{]}^{1/2},$ and ${\ensuremath{\alpha}}_{d}^{(b)}\ensuremath{\simeq}{D}^{2}/\ensuremath{\Delta}.$ They may also compete with a
1/R3 potential for Rydberg atoms with
l>0. The total ${\ensuremath{\alpha}}_{d}$ can be very large in magnitude for small \ensuremath{\Delta} and even assume negative values, but the corresponding
Rx also increases as \ensuremath{\Delta} decreases. The validity region in R of the ${R}^{\ensuremath{-}4}$ behavior of the potential recedes to larger R as the polarizability grows. A general formula for ${\ensuremath{\alpha}}_{d}$ is given, taking into account the effects of fine-structure splitting, the Lamb shift, and quantum defects.