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Physical Review Letters

Role of the valence electron in reactive processes: Penning ionization by a Rydberg atom

Charles E. Burkhardt, M. Ciocca, Jacob J. Leventhal, J. D. Kelley

Published Nov 5, 1990• 4 citations
Publisher

Abstract

We report results of ${\mathrm{Ba}}^{\mathrm{*}\mathrm{*}}$-Na Penning ionization experiments in which the reactivity of pure Rydberg states is compared with that of mixed Rydberg-valence states. Pure states having energy near the ionization threshold of Na are highly reactive, \ensuremath{\sigma}\ensuremath{\approxeq}2500 \AA{} $^{2}$ for Ba(17d $^{1}$${\mathit{D}}_{2}$), but mixed states are unreactive. These results are explained using a model that includes an ion-induced dipole ${\mathrm{Ba}}^{+}$-Na interaction and a modified Coulomb interaction resulting from fractional charge accumulation around the Na.

DOI
10.1103/physrevlett.65.2351

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We report results of Ba∗∗{\mathrm{Ba}}^{\mathrm{*}\mathrm{*}}Ba∗∗-Na Penning ionization experiments in which the reactivity of pure Rydberg states is compared with that of mixed Rydberg-valence states. Pure states having energy near the ionization threshold of Na are highly reactive, \ensuremath{\sigma}\ensuremath{\approxeq}2500 \AA{} 2^{2}2 for Ba(17d ^{1}$${\mathit{D}}_{2}), but mixed states are unreactive. These results are explained using a model that includes an ion-induced dipole Ba+{\mathrm{Ba}}^{+}Ba+-Na interaction and a modified Coulomb interaction resulting from fractional charge accumulation around the Na.
The mechanism of reactive processes in binary atomic encounters is investigated by employing a Rydberg atom as one of the reactants. This permits isolation of the excited valence electron so that its role in effecting the heavy-body process can be assessed. Penning ionization by a Rydberg atom, which is studied as an example of a reactive process, is shown to have a cross section significantly higher than might be inferred solely on the basis of overlap of the atomic wave functions of the ionic Rydberg core and the collision partner. A model to describe these reactive processes in which the nearly isolated Rydberg electron polarizes, and is therefore attracted by, the neutral ground-state reactant is proposed. This attraction occurs at an internuclear separation greater than that at which the ionic core of the Rydberg atom can substantially polarize the neutral atom but polarization of the neutral atom by the Rydberg electron leads to a transitory negative ion and concomitant Coulomb attraction to the ionic core. This Coulomb attraction thus leads to cross sections that are higher than would be calculated solely on the basis of atomic overlap.
The ionization of 6sns1S06sns^{1}S_{0}6sns1S0​ Rydberg states of 138Ba^{138}\mathrm{Ba}138Ba has been investigated by high-resolution electron spectroscopy. The Rydberg states were excited by two cw dye lasers via the 6s6p1P16s6p^{1}P_{1}6s6p1P1​ level. The electron spectra show rich structure between 0.025 and 3 eV, largely attributable to collisional (Penning) ionization, photoionization, and autoionizing transitions. The first process might be interpreted as superelastic scattering of the Rydberg electron by optically pumped Ba(6s6p1P1)\mathrm{Ba}(6s6p^{1}P_{1})Ba(6s6p1P1​) or Ba(6s5d1D2)\mathrm{Ba}(6s5d^{1}D_{2})Ba(6s5d1D2​) atoms.
In rubidium Rydberg states, the collision nD_(5/2)+nD_(5/2)-->(n-2)F_(7/2)+(n+2)P_(3/2) is nearly resonant in the vicinity of n=43. As a result, over a short range of n centered around n approximately 43 the Rydberg-Rydberg interaction potential is quite large and turns from repulsive to attractive [Phys. Rev. A 75, 032712 (2007)10.1103/PhysRevA.75.032712]. We use state-selective field ionization to investigate the effect of this resonance on instantaneous excitation of mixed two-particle states, state-mixing collisions, and Penning ionization. We find that these processes depend on the magnitude and sign of the two-particle interaction potential, and thus on n near the resonance. The large magnitude of the observed state mixing provides evidence for many-body effects.
Using a classical trajectory Monte Carlo method, we have computed the ionization resulting from the interaction between two cold Rydberg atoms. We focus on the products resulting from close interaction between two highly excited atoms. We give information on the distribution of ejected electron energies, the distribution of internal atom energies and the velocity distribution of the atoms and ions after the ionization. If the potential for the atom is not purely Coulombic, the average interaction between two atoms can change from attractive to repulsive giving a Van de Graaff-like mechanism for accelerating atoms. In a small fraction of ionization cases, we find that the ionization leads to a positive molecular ion where all of the distances are larger than 1000 Bohr radii.
Abstract Features of Penning ionization in cold gaseous media of Rydberg alkali-metal atoms have been investigated. In contrast to the hydrogen atom, the corresponding autoionization widths exhibit a strong (by orders of magnitude) dependence on the orbital quantum numbers of the atoms involved in the long-range dipole–dipole interaction. An important feature of Penning ionization is the nontrivial dependence of its efficiency on the Rydberg particle size. For all types of alkali-metal atoms, the optimal highly nonsymmetric configurations of Rydberg pairs have been found that lead to the explosive (by several orders of magnitude) intensification of the free electron formation by means of the Penning ionization processes. This property makes Penning ionization an important means of forming primary charged particles during the cold Rydberg plasma formation. The numerical data for pairs of potassium atoms are reported that demonstrate a significant effect of the Förster resonance on the values of the Penning ionization rate constants.