Rate constants for Rydberg-atom destruction through electron transfer in collisions between K(np) Rydberg atoms and ${\mathrm{SF}}_{6}$ and ${\mathrm{CCl}}_{4}$ 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 ${\mathrm{SF}}_{6}$ and ${\mathrm{CCl}}_{4}$ 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.
Rate constants for Rydberg-atom destruction through electron transfer in collisions between K(np) Rydberg atoms and SF6 and 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 and 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.
Absolute rate constants for ionization of Rb(ns,nd) atoms in collisions with 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 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.
In the present work very-high-n Rydberg-atoms (nsim100−nsim400) are used to probe electron-molecule collisions at ultra-low electron energies. Based on the essentially-free-electron model, for sufficiently high n, Rydberg atom-molecule collisions can be described in terms of a binary interaction between the target molecule and the essentially-free Rydberg electron. Since the average kinetic energy of the Rydberg electron is ultra-low rm(sim85mueV−sim1.4meV), analysis of the very-high-n Rydberg-atom collision data can provide information on electron-molecule scattering at electron energies corresponding to electron temperatures of only $\\sim1\\sp\\circ$K, which are far below those accessible using any alternate approach.\nRate constants for destruction of very-high-n Rydberg atoms in collisions with various target molecules have been measured. This study focuses on Rydberg electron transfer to an electron-attaching molecule which results in negative ion formation via the reactions rmK(itnp/)+XYtoKsp++(XY)sp−∗ortoKsp++Xsp−+Yeqno(1)and on rotational energy transfer from polar molecules which leads to Rydberg atom ionization rmK(itnp/)+XY(itJ/)toKsp++itesp−+XY(itJ/−1)eqno(2)\nThe n dependence of the rate constants for Rydberg-atom destruction depends on the reactions involved. In reaction (1) the rate constant is independent of n whereas in reaction (2) it increases nearly linearly with n. When both reactions are possible, the measured n dependence can be explained in terms of contributions from each process.\nThe essentially-free-electron model suggests that the n dependence of the rate constants for Rydberg-atom destruction reflects the energy dependence of the cross sections of the corresponding free electron-molecule collision processes which are $$\\eqalignno{&e\\sp- + \\rm XY \\to (XY)\\sp{-*}\\ or\\ \\to X\\sp- + Y&(1\\sp\\prime)\\cr &e\\sp- + \\rm XY({\\it J\\/}) \\to {\\it e}\\sp- + XY({\\it J\\/}-1)&(2\\sp\\prime)\\cr}$$Analysis of the data therefore provides the behavior of cross sections for these processes at ultra-low electron energies.
Potassium atoms in selected high-lying Rydberg states (n < 1200) are used as a tool to probe threshold behavior in electron-molecule collisions. Collisions with non-polar electron attaching molecules such as CClsb4 are dominated by electron capture in a binary interaction between the Rydberg electron and target molecule allowing the study of electron attachment at electron energies of only a few mueV. Analysis of the data shows the cross section for electron capture is consistent with the Wigner threshold law for an inelastic s-wave process. Collisions with polar molecules can lead to Rydberg atom destruction through transfer of molecular rotational energy to the Rydberg electron. Rydberg atom data obtained with polar targets are not consistent with scattering from a static dipole. However, the data are consistent with a threshold law that takes into account a rotationally-averaged induced dipole interaction that can possibly support bound or virtual states.
The dynamics of formation of heavy-Rydberg ion-pair states through electron transfer in K(np)-SF6, CCl4 collisions is examined by measuring the velocity, angular, and binding energy distributions of the product ion pairs. The results are analyzed with the aid of a Monte Carlo collision code that models both the initial electron capture and the subsequent evolution of the ion pairs. The model simulations are in good agreement with the experimental data and highlight the factors such as Rydberg atom size, the kinetic energy of relative motion of the Rydberg atom and target particle, and (in the case of attaching targets that dissociate) the energetics of dissociation that can be used to control the properties of the product ion-pair states.
The ionization of He atoms, prepared in a well-defined (n=14) 1P Rydberg state, in collisions with electron-attaching molecules like SF6 and NO2, is studied in a crossed-beam experiment at thermal energies. The ionization cross section is observed to be large for SF6, reaching 4.2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}13}$ cm2, and to be smaller for NO2, close to ${10}^{\mathrm{\ensuremath{-}}15}$ 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 indicates that, for this molecule, the reaction product is an ion pair, as for SF6, and that ionization is not due to rotational energy transfer even though 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.
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.