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Physical Review A

Ionization in Rydberg-atom– SF6 collisions at highn

Byron George Zollars, C. Higgs, F. Lu, C. W. Walter, L. G. Gray, K. A. Smith, F. B. Dunning, R. F. Stebbings

Published Dec 1, 1985• 52 citations
Publisher

Abstract

Absolute rate constants for ionization of Rb(ns,nd) atoms in collisions with ${\mathrm{SF}}_{6}$ 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 ${\mathrm{SF}}_{6}$ 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 ${\mathrm{cm}}^{2}$.

DOI
10.1103/physreva.32.3330

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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 sec\ensuremath−1{\mathrm{sec}}^{\mathrm{\ensuremath{-}}1}sec\ensuremath−1, is given by \ensuremath{\sigma}(v)=(4.2\ifmmode\pm\else\textpm\fi{}1.0)\ifmmode\times\else\texttimes\fi{}10\ensuremath−7{10}^{\mathrm{\ensuremath{-}}7}10\ensuremath−7/v cm2{\mathrm{cm}}^{2}cm2.
Rate constants for Rydberg-atom destruction through electron transfer in collisions between K(np) Rydberg atoms and SF6{\mathrm{SF}}_{6}SF6​ and CCl4{\mathrm{CCl}}_{4}CCl4​ have been measured for values of principal quantum number n up to n\ensuremath{\sim}400 using an apparatus specifically developed for very-high-n collision studies. Analysis of the data using the free-electron model provides cross sections for electron attachment to SF6{\mathrm{SF}}_{6}SF6​ and CCl4{\mathrm{CCl}}_{4}CCl4​ at electron energies down to \ensuremath{\sim}10 \ensuremath{\mu}eV, which are far below those accessible using any alternate technique. The measured cross sections are inversely proportional to electron velocity and are consistent with the Wigner threshold law for s-wave attachment.
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The ionization of He atoms, prepared in a well-defined (n=14) 1^{1}1P Rydberg state, in collisions with electron-attaching molecules like SF6{\mathrm{SF}}_{6}SF6​ and NO2{\mathrm{NO}}_{2}NO2​, is studied in a crossed-beam experiment at thermal energies. The ionization cross section is observed to be large for SF6{\mathrm{SF}}_{6}SF6​, reaching 4.2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}13}$ cm2{\mathrm{cm}}^{2}cm2, and to be smaller for NO2{\mathrm{NO}}_{2}NO2​, close to ${10}^{\mathrm{\ensuremath{-}}15}$ cm2{\mathrm{cm}}^{2}cm2. The dependence of these two cross sections on the relative velocity of the reactants exhibits a characteristic behavior with a saturation at low velocities, a dependence that is completely different from that obtained with polar molecules. The observation of such a dependence for NO2{\mathrm{NO}}_{2}NO2​ indicates that, for this molecule, the reaction product is an ion pair, as for SF6{\mathrm{SF}}_{6}SF6​, and that ionization is not due to rotational energy transfer even though NO2{\mathrm{NO}}_{2}NO2​ has a dipole moment. This dependence cannot be explained by the ``free-electron model'' usually used in Rydberg-atom--molecule collisions; it clearly shows that the interaction of the molecule with the ion core cannot be neglected.
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.
Atoms in highly excited Rydberg states possess physical characteristics quite unlike those associated with atoms in the ground or low-lying excited states. In particular, they are physically very large and are only very weakly bound. In consequence, collisions can lead to a wide variety of reaction processes many of which are unique to Rydberg species and have very large collision cross sections. In collisions with neutral targets, Rydberg atoms behave not as an atom but rather as a pair of well-separated independent scatterers, namely the core ion and the excited Rydberg electron. In the present article we discuss many of the different reactions that can occur when Rydberg atoms collide with neutral targets, focusing principally on reactions that are dominated by (binary) Rydberg electron-target interactions and include collisions with molecules that attach free low-energy electrons and with polar targets. In certain situations, however, interactions involving the Rydberg core ion are important. This is illustrated using as an example the destruction of ultralong-range Rydberg molecules excited in a cold dense gas.
The ionization of low-Rydberg-state He atoms in the n 1P^{1}P1P state (n=14) in collisions with the polar molecules NH3{\mathrm{NH}}_{3}NH3​, SO2{\mathrm{SO}}_{2}SO2​, and acetone (C3{\mathrm{C}}_{3}C3​H6{\mathrm{H}}_{6}H6​O) is carried out in a crossed-beam experiment at thermal kinetic energies. The dependence of the experimental ionization cross sections ${\ensuremath{\sigma}}_{i}^{\mathrm{expt}}$ on the relative velocity of the reactants is obtained by a time-of-flight method. We observe that it varies mainly as 1/vr2{v}_{r}^{2}vr2​ in a large part of the studied velocity range, while a 1/vr{v}_{r}vr​ velocity dependence is predicted by the free-electron model for the higher Rydberg levels.