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Physical Review Letters

Evidence for resonant formation of Ca− in collisions of Rydberg and ground-state Ca atoms

K. W. McLaughlin, D. W. Duquette

Published Feb 21, 1994• 35 citations
Publisher

Abstract

Relative yields of ${\mathrm{Ca}}^{+}$ and ${\mathrm{Ca}}^{\mathrm{\ensuremath{-}}}$ ions from thermal-energy collisions between highly excited ${\mathrm{Ca}}^{\mathrm{*}}$ 4snd $^{1}$D 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 ${\mathrm{Ca}}^{+}$ 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.

DOI
10.1103/physrevlett.72.1176

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Relative yields of Ca+{\mathrm{Ca}}^{+}Ca+ and Ca\ensuremath−{\mathrm{Ca}}^{\mathrm{\ensuremath{-}}}Ca\ensuremath− ions from thermal-energy collisions between highly excited Ca∗{\mathrm{Ca}}^{\mathrm{*}}Ca∗ 4snd 1^{1}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 Ca\ensuremath−{\mathrm{Ca}}^{\mathrm{\ensuremath{-}}}Ca\ensuremath− and a ground-state Ca+{\mathrm{Ca}}^{+}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 Ca\ensuremath−{\mathrm{Ca}}^{\mathrm{\ensuremath{-}}}Ca\ensuremath− is 24\ifmmode\pm\else\textpm\fi{}1.4 meV.
The formation of Ca+{\mathrm{Ca}}^{+}Ca+ ions is measured in an effusive Ca beam by exciting through either the 4s4p1{\mathit{p}}^{1}p1P1{\mathit{P}}_{1}P1​ or the 4s5p1{\mathit{p}}^{1}p1P1{\mathit{P}}_{1}P1​ state to highly excited Ca*(4snd1{\mathit{snd}}^{1}snd1D2{\mathit{D}}_{2}D2​) 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 4s5p1{\mathit{p}}^{1}p1P1{\mathit{P}}_{1}P1​ state, we detect a large increase in Ca+{\mathrm{Ca}}^{+}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 4s4p1{\mathit{p}}^{1}p1P1{\mathit{P}}_{1}P1​ level instead of the 4s5p1{\mathit{p}}^{1}p1P1{\mathit{P}}_{1}P1​ 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${\mathit{d}}_{3/2}999{\mathit{f}}_{5/2}$, ${\mathit{J}}^{\mathrm{\ensuremath{\pi}}}$=${2}^{\mathrm{\ensuremath{-}}}$ autoionizing resonance, which is coincidentally accessed from the Ca(4s25d1{\mathit{d}}^{1}d1D2{\mathit{D}}_{2}D2​) 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.
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.
Using crossed atomic beams and mass spectrometric ion detection we have studied the formation of negative Ca ions in slow collisions between laser-excited state-selected Ne** ( ns , J = 2) and Ne** ( nd , J = 4) Rydberg atoms and ground state Ca atoms. The rate coefficients exhibit resonant behaviour with maxima at an effective principal quantum number n * of about 11.5 and an estimated peak value of about 10 −8 cm 3 s −1 . Two different theoretical calculations, based on a curve-crossing model and on a modified adiabatic theory, respectively, yield n *-dependences of the rate coefficients in good agreement with the experimental results.
We formulate the semiclassical close-coupling method for a theoretical description of ion-pair formation in Rydberg-atom--ground-state-atom collisions at thermal energies and apply it to the process $\mathrm{Ca}{(4s}^{2})+{\mathrm{Ne}}^{*}(n)\ensuremath{\rightarrow}{\mathrm{Ca}}^{\ensuremath{-}}{(4s}^{2}4p)+{\mathrm{Ne}}^{+}.$ Our results for the formation of an isolated negative-ion state are very close to those obtained previously using the decay model, although the results for S states are somewhat higher, since the decay model underestimates the survival probability for the ion pair. After the inclusion in our close-coupling calculations of the two fine-structure components of ${\mathrm{Ca}}^{\ensuremath{-}},$ we find that the cross section increases substantially (by about a factor of 2) because of correlation between probabilities for population of two fine-structure sublevels.
The process of the electron transfer in collisions of highly excited Rydberg atoms with ground-state atoms is treated in the impulse approximation using the three-body Faddeev-equations approach. The cross section for the electron transfer in Ca∗{\mathrm{Ca}}^{\mathrm{*}}Ca∗(4snl)-Ca(4s2{\mathit{s}}^{2}s2) collisions is calculated as a function of the electron affinity of Ca. It is shown that the cross section is extremely sensitive to the electron affinity and thereby a recent experimental observation of this reaction by McLaughlin and Duquette provides a very accurate value of the electron affinity of Ca.
We develop the distorted-wave and adiabatic theories of the ion-pair formation in collisions of a Rydberg atom A∗∗{\mathit{A}}^{\mathrm{*}\mathrm{*}}A∗∗ with a ground-state atom B at thermal collision velocities. The e-B interaction is modeled by a zero-range potential. Both theories give results that are in qualitative agreement with the standard Landau-Zener model, although their prediction for m-changing transitions is different, where m is the projection of the angular momentum. To incorporate these transitions into the adiabatic theory, we take into account the rotation of the internuclear axis before and after a nonadiabatic transition. The m-averaged cross sections are substantially higher than those following from the standard Landau-Zener model, and the distorted-wave results are slightly higher than the adiabatic results with the internuclear-axis rotation taken into account. We apply our results to a description of the McLaughlin and Duquette experiment on Ca+{\mathrm{Ca}}^{+}Ca+-${\mathrm{Ca}}^{\mathrm{\ensuremath{-}}}$ pair formation. The calculated n dependence of the rate for the ion-pair formation is too flat as compared to the experimental observations and does not agree with known values of the electron affinity of Ca. We discuss other possible mechanisms responsible for the ion-pair formation in this process. \textcopyright{} 1996 The American Physical Society.