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Physical Review A

Formation of O2 − ions in K(nd)- O2 collisions at intermediaten

C. W. Walter, Byron George Zollars, C. B. Johnson, K. A. Smith, F. B. Dunning

Published Nov 1, 1986• 16 citations
Publisher

Abstract

Rate constants for the formation of ${\mathrm{O}}_{2}$${\mathrm{}}^{\mathrm{\ensuremath{-}}}$ ions in collisions between K(nd) Rydberg atoms and ${\mathrm{O}}_{2}$ have been measured for values of n in the range 8--12. The measured rate constants are small and decrease from \ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}12}$ ${\mathrm{cm}}^{3}$ ${\mathrm{sec}}^{\mathrm{\ensuremath{-}}1}$ at n=8 to \ensuremath{\sim}1\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}12}$ ${\mathrm{cm}}^{3}$ ${\mathrm{sec}}^{\mathrm{\ensuremath{-}}1}$ at n=12. The data are consistent with a reaction model in which Rydberg electron attachment leads to the formation of short-lived excited ${\mathrm{O}}_{2}$${\mathrm{}}^{\mathrm{\ensuremath{-}}}$ ions, some of which are subsequently stabilized by an interaction with the core ion.

DOI
10.1103/physreva.34.4431

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Rate constants for the formation of {\mathrm{O}}_{2}$${\mathrm{}}^{\mathrm{\ensuremath{-}}} ions in collisions between K(nd) Rydberg atoms and O2{\mathrm{O}}_{2}O2​ have been measured for values of n in the range 8--12. The measured rate constants are small and decrease from \ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}10\ensuremath−12{10}^{\mathrm{\ensuremath{-}}12}10\ensuremath−12 cm3{\mathrm{cm}}^{3}cm3 sec\ensuremath−1{\mathrm{sec}}^{\mathrm{\ensuremath{-}}1}sec\ensuremath−1 at n=8 to \ensuremath{\sim}1\ifmmode\times\else\texttimes\fi{}10\ensuremath−12{10}^{\mathrm{\ensuremath{-}}12}10\ensuremath−12 cm3{\mathrm{cm}}^{3}cm3 sec\ensuremath−1{\mathrm{sec}}^{\mathrm{\ensuremath{-}}1}sec\ensuremath−1 at n=12. The data are consistent with a reaction model in which Rydberg electron attachment leads to the formation of short-lived excited {\mathrm{O}}_{2}$${\mathrm{}}^{\mathrm{\ensuremath{-}}} ions, some of which are subsequently stabilized by an interaction with the core ion.
Thermal energy collisions of K(nd) Rydberg atoms with electron attaching molecules which result in reactions of the following types K(nd) + SFsb6\\sb6sb6 to\\toto Ksp+\\sp+sp+ + SFsb6sp−\\sb6\\sp-sb6sp− K(nd) + CFsb3\\sb3sb3I to\\toto Ksp+\\sp+sp+ + Isp−\\sp-sp− + CFsb3\\sb3sb3 have been investigated at intermediate n. Using the field ionization technique, both rate constants for Rydberg atom destruction and for formation of free negative ions are directly measured for n as low as 12.\nThe data show that, at intermediate n, the rate constants for Rydberg atom destruction start to decrease with decreasing n and to deviate from those predicted on the basis of the simple free electron model. A semi-classical model is presented to explain this decrease. It reveals explicitly the relationship between the rate constant for Rydberg atom destruction and the rate constant for free electron attachment.\nThe data also show that, at intermediate n, only a fraction of the Rydberg electron attachment events leads to formation of free negative ions. Post-attachment interaction between the positive and negative ions formed is investigated. The kinetic energy distribution of the ion pairs in their center-of-mass frame and their separation probability are calculated. The angular dependence of the kinetic energy distribution and the separation probability are also studied. The present experiments, in conjunction with the theoretical calculations, advance understanding of the collisions substantially.\nInvestigation of Zeeman-effect quantum beats observed in the selective field ionization spectra of D state potassium Rydberg atoms in also presented.
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Absolute rate constants for ionization of Rb(ns,nd) atoms in collisions with SF6{\mathrm{SF}}_{6}SF6​ have been measured for values of n in the range 38--106. In addition, the kinematics of such reactions have been investigated using Xe(nf) atoms. The data confirm that collisions of Rydberg atoms with targets that attach free low-energy electrons are well described by the ``essentially-free''-electron model. Analysis of the data shows that the cross section \ensuremath{\sigma}(v) for electron attachment to SF6{\mathrm{SF}}_{6}SF6​ at very low electron energies (\ensuremath{\lesssim}10 meV) is inversely proportional to the electron velocity v and, if v is expressed in cm ${\mathrm{sec}}^{\mathrm{\ensuremath{-}}1}$, is given by \ensuremath{\sigma}(v)=(4.2\ifmmode\pm\else\textpm\fi{}1.0)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}7}$/v cm2{\mathrm{cm}}^{2}cm2.
Scattering of K(np) Rydberg atoms by electron-attaching molecules can result in the formation of positive-negative ion pairs through electron transfer reactions. These reactions can involve the dissociation of a transient intermediate negative ion into a smaller negative ion and a neutral fragment, or they can result in the parent molecule forming a metastable negative ion, with the excess energy being distributed among its internal modes. For Rydberg atoms of intermediate n, it is possible for these ion pairs to orbit each other at large separation, weakly bound by their mutual electrostatic attraction, forming what are known as heavy-Rydberg states. Measurements of the binding energy and velocity distributions of the ion pairs are used to probe the dynamics of electron transfer reactions. The data are analyzed using a semiclassical Monte Carlo collision code to explore the lifetime of the intermediate, the nature of its dissociation, the distribution of the excess energy of reaction, and the branching ratio of the various reaction channels. Results obtained using a variety of attaching targets including CF3I, CCl4, SF6, CH2Br2, 1,1,1-C2Cl3F3, CCl3Br, Fe(CO)5, and CH3NO2 have demonstrated both the wide variety of different behaviors that can accompany electron capture as well as the different behaviors of the product ion-pair states themselves. Remarkably, despite previous work showing that it was possible for CH3NO2 to form long-lived valence-bound negative ions through Rydberg electron transfer, the present work showed that such collisions do not result in the formation of long-lived bound ion-pair states. Rather, the data show that collisions lead to very strong Rydberg atom scattering through formation of transient ion-pair states. These results are consistent with theoretical work that predicts significant collisional quenching of Rydberg atoms by highly polar molecules and highlight a new mechanism for Rydberg atom scattering.