Relative yields of
Ca+ and ${\mathrm{Ca}}^{\mathrm{\ensuremath{-}}}$ ions from thermal-energy collisions between highly excited
Ca∗ 4snd
1D Rydberg atoms and Ca ground-state atoms are measured as a function of n. The collisions take place within an effusive Ca atomic beam. The cross section for a collision resulting in a bound ${\mathrm{Ca}}^{\mathrm{\ensuremath{-}}}$ and a ground-state
Ca+ is expected to be large when the binding energy of the excited Rydberg state nearly matches the electron affinity of Ca. We observe a large increase in positive and negative ions for n=25 only. This result strongly suggests that the binding energy of ${\mathrm{Ca}}^{\mathrm{\ensuremath{-}}}$ is 24\ifmmode\pm\else\textpm\fi{}1.4 meV.
Electron-transfer processes are studied in thermal collisions of Rydberg atoms with alkaline-earth Ca(4s 2), Sr(5s 2), and Ba(6s 2) atoms capable of forming negative ions with a weakly bound outermost p-electron. We consider the ion-pair formation and resonant quenching of highly excited atomic states caused by transitions between Rydberg covalent and ionic terms of a quasi-molecule produced in collisions of particles. The contributions of these reaction channels to the total depopulation cross section of Rydberg states of Rb(nl) and Ne(nl) atoms as functions of the principal quantum number n are compared for selectively excited nl-levels with l ≪ n and for states with large orbital quantum numbers l = n − 1, n − 2. It is shown that the contribution from resonant quenching dominates at small values of n, and the ion-pair formation process begins to dominate with increasing n. The values and positions of the maxima of cross sections for both processes strongly depend on the electron affinity of an alkaline-earth atom and on the orbital angular momentum l of a highly excited atom. It is shown that in the case of Rydberg atoms in states with large l ∼ n − 1, the rate constants of ion-pair formation and collisional quenching are considerably lower than those for nl-levels with l ≪ n.
The formation of
Ca+ ions is measured in an effusive Ca beam by exciting through either the 4s4
p1P1 or the 4s5
p1P1 state to highly excited Ca*(4
snd1D2) Rydberg states. In agreement with previous work by McLaughlin and Duquette [Phys. Rev. Lett. 72, 1176 (1994)], when the Rydberg states are accessed through the 4s5
p1P1 state, we detect a large increase in
Ca+ ions for n=25, compared to other high-n Rydberg states. It had been reported (Fabrikant [Phys. Rev. A 48, R3411 (1993)]; McLaughlin and Duquette) that ``ground-state--Rydberg-state'' collisions cause this large increase in ion production and that the result provides a precise value of the electron affinity of Ca through the binding energy of the Rydberg electron. However, a variation of the original experiment in which the same Rydberg states are accessed through the 4s4
p1P1 level instead of the 4s5
p1P1 level shows no enhancement of ions for any particular Rydberg level. With a three-laser experiment we show that the enhanced ionization effect for the 25d state is due to the 3${\mathit{d}}_{3/2}
9{\mathit{f}}_{5/2}$, ${\mathit{J}}^{\mathrm{\ensuremath{\pi}}}$=${2}^{\mathrm{\ensuremath{-}}}$ autoionizing resonance, which is coincidentally accessed from the Ca(4s25
d1D2) state by the 808-nm photon involved in the original experiment by McLaughlin and Duquette. The existence of this resonance close to the experimentally observed energy is also established by R-matrix calculations. \textcopyright{} 1996 The American Physical Society.
We calculated the electronic interaction associated with the exchange of an electron between an atom of calcium excited to a Rydberg state (n∼10−15) and another, neighbouring calcium atom in its ground state. In this range, the Rydberg states have an energy that is comparable to the electron affinity of Ca, enabling resonant or near resonant charge transfer at large internuclear separations (200–700 a0). We calculated the interaction strength while systematically and critically assessing the approximations made, and found it to be large, ranging from 10−5 Eh (70 GHz) to 10−8 Eh. Charge transfer is thus expected to be efficient and to significantly affect the molecular dynamics at a range of internuclear distances where ultra-long-range Rydberg molecules also exist.
We discuss the theory of quenching of Rydberg states by atoms with small electron affinities and perform calculations for quenching of s and d states of Ne by Ca. There are two contributions to the quenching cross section: the ion-covalent coupling contribution, which is due to the formation of an intermediate ionic state, and the impulse contribution. The first is very sensitive to the electron affinity of the projectile and may be used for studies of weakly bound anions. The ion-covalent coupling contribution is dominant for the process of quenching of Rydberg states with large quantum defects (e.g. s states) at relatively low principal quantum number n , whereas the impulse contribution dominates the process of quenching of states with small quantum defects (e.g. d states of Ne). We present calculations of quenching of s states by Yb atoms with a hypothetical electron affinity of 2 meV and show that experimental studies of quenching of Rydberg states by Yb might help in determining the electron affinity of Yb even if it is negative. In the latter case the quenching cross section as a function of n should exhibit oscillations (Borodin V M and Kazansky A K 1992 J. Phys. B: At. Mol. Opt. Phys. 25 971).
The resonant mechanism of interaction of alkaline-earth atoms having a low electron affinity to Rydberg atoms in circular (l = |m| = n–1) and near-circular states has been studied. To describe the dynamics of resonant processes accompanied by nonadiabatic transitions between ionic and Rydberg covalent terms of a quasimolecule, an approach based on the integration of coupled equations for the probability amplitudes has been developed taking into account the possibility of the decay of an anion in the Coulomb field of the positive ionic core of a highly excited atom. The approach involves the specific features of the problem associated with the structure of the wavefunction of a Rydberg electron in states with high orbital angular momenta l ~ n–1. This approach provides a much more accurate description of the dynamics of electronic transitions at collisions between atoms than that within the modified semiclassical Landau–Zener model. In addition, this approach makes it possible to effectively take into account many channels of the problem. The cross sections for resonant quenching of Rydberg states of the Li(nlm) atom with given principal n, orbital l = n–1, and magnetic m quantum numbers at thermal collisions with the Ca(4s 2) and Sr(5s 2) atoms have been calculated. The dependences of the results on n, m, and angle α between the relative velocity of the atoms and the normal to the plane of the orbit of the Rydberg electron have been obtained. The influence of orientational effects on the efficiency of the collisional destruction of circular and near-circular states has been studied. The results indicate a higher stability of such states to their perturbations by neutral particles as compared to usually studied nl states with low values of l (l ≪ n).