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Physical Review A

Laser-induced charge-transfer collisions of calcium ions with strontium atoms

M. D. Wright, D. M. O'Brien, J. F. Young, Stephen E. Harris

Published Oct 1, 1981• 6 citations
Publisher

Abstract

Experimental studies of two laser-induced charge-transfer reactions in Ca and Sr are presented. In both reactions, a Ca ion collides with an Sr neutral in the presence of a laser field to produce a Ca neutral and an excited Sr ion. The two reactions differ in the final level of the ${\mathrm{Sr}}^{+}$ species. The collision cross sections associated with these reactions are studied as functions of applied laser frequency and power density. The largest cross section induced is equal to ${10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$, obtained at a power density of 2\ifmmode\times\else\texttimes\fi{}${10}^{9}$ W/${\mathrm{cm}}^{2}$. Differences in experimental results associated with the two reactions are explained qualitatively by an examination of the potential curves of the Ca${\mathrm{Sr}}^{+}$ quasimolecule.

DOI
10.1103/physreva.24.1750

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We report the observation of a laser-induced charge-transfer collision. In the presence of an intense laser beam, ground-state calcium ions collide with ground-state strontium atoms, selectively producing excited strontium ions and calcium neutrals. The laser-induced collision cross section has a linewidth of about 50 ${\mathrm{cm}}^{\ensuremath{-}1}$ and peaks \ensuremath{\sim} 70 ${\mathrm{cm}}^{\ensuremath{-}1}$ from that wavelength which satisfies the energy defect of the separated atoms.
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We describe two new experiments which demonstrate laser-induced inelastic collisions. Energy was stored in the Sr 5p1Po5p^{1}P^{o}5p1Po state and selectively transferred both to the Ca $4{p}^{2}^{1}S$ and 5d1D5d^{1}D5d1D states. The collision cross sections maximized at the interatomic wavelengths of 4977 and 4711 \AA{}, respectively, and had a half-power width of about 14 ${\mathrm{cm}}^{\ensuremath{-}1}$.
Cold chemical reactions between laser-cooled Ca^+ ions and Rb atoms were studied in an ion-atom hybrid trap. Reaction rate constants were determined in the range of collision energies /k_B = 20 mK-20 K. The lowest energies were achieved in experiments using single localized Ca^+ ions. Product branching ratios were studied using resonant-excitation mass spectrometry. The dynamics of the reactive processes in this system (non-radiative and radiative charge transfer as well as radiative association leading to the formation of CaRb^+ molecular ions) have been analyzed using high-level quantum-chemical calculations of the potential energy curves of CaRb^+ and quantum-scattering calculations for the radiative channels. For the present low-energy scattering experiments, it is shown that the energy dependence of the reaction rate constants is governed by long-range interactions in line with the classical Langevin model, but their magnitude is determined by short-range non-adiabatic and radiative couplings which only weakly depend on the asymptotic energy. The quantum character of the collisions is predicted to manifest itself in the occurrence of narrow shape resonances at well-defined collision energies. The present results highlight both universal and system-specific phenomena in cold ion-neutral reactive collisions.
We report measurements of cross sections greater than ${10}^{\ensuremath{-}13}$ cm2{\mathrm{cm}}^{2}cm2 for laser-induced inelastic collisions between Sr and Ca. We describe an experimental system which produces such large cross sections and which greatly facilitates the study of laser-induced collisions through its ability to reduce noise from other collisional processes.