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Comparative studies of negative ion formation in Rydberg atom collisions with attaching and polar targets

L. Suess

Published Jan 1, 2004• 0 citations
OpenAlex

Abstract

Low-energy electron transfer in collisions between a Rydberg atom and a target molecule can result in the formation of negative ions. Depending as to the properties of the target molecule the electron may be bound in a localized molecular orbital (valence-bound) or bound by its dipole potential in a diffuse orbital located at the positive end of the dipole (dipole-bound). Here we examine the properties of valence-bound and dipole-bound negative ions created through electron transfer in collision with Rydberg atoms, in particular, their lifetimes, collisional properties and electric-field-induced detachment characteristics. Intercomparison of the various data also leads to new insights into electron attachment to molecules that can support both valence-bound and dipole-bound negative ion states.

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•Jan 1, 2008•1 citation

Formation of dipole-bound negative ions in Rydberg atom collisions: A signature

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Determination of electron binding energies of ground-state dipole-bound molecular anions

C. Desfrançois

We describe two simple models for the determination of electron binding energies (${\mathit{E}}_{\mathit{b}}$) of several dipole-bound negative ions from field-detachment experiments and charge-transfer formation-rate-constant measurements in collisions between cold neutral molecules and Rydberg atoms. These models are able to correctly reproduce the experimental data, from which we deduce two compatible sets of ${\mathit{E}}_{\mathit{b}}$ values. A curve-crossing model for calculating anion formation rate constants allows us to qualitatively understand the sharply peaked shapes of the experimental curves and to derive a simple quantitative law between ${\mathit{E}}_{\mathit{b}}$ values and principal Rydberg quantum numbers at which anion formation rate constants are maximum.

Physical Review A•May 1, 1995•100 citations
Low-energy electron transfer in collisions between a Rydberg atom and a target molecule can result in the formation of negative ions. Depending as to the properties of the target molecule the electron may be bound in a localized molecular orbital (valence-bound) or bound by its dipole potential in a diffuse orbital located at the positive end of the dipole (dipole-bound). Here we examine the properties of valence-bound and dipole-bound negative ions created through electron transfer in collision with Rydberg atoms, in particular, their lifetimes, collisional properties and electric-field-induced detachment characteristics. Intercomparison of the various data also leads to new insights into electron attachment to molecules that can support both valence-bound and dipole-bound negative ion states.
Electron transfer in collisions between atoms in high Rydberg states and polar targets can lead to the formation of dipole-bound negative ions in which the excess electron is weakly bound by the dipole potential of the neutral molecule. Their lifetimes and collisional destruction rates are studied using a Penning ion trap. Ion decay in the trap is characterized by a single exponential lifetime, ∼60 to 100 mus governed by BBR-induced photodetachment. The rate constants for destruction of these ions by rotational energy transfer in collisions with residual target gas present in the trap are large, ∼ 10-7cm3s-1. The dynamics of CH3CN- ion production through electron transfer are examined using velocity selected Rydberg atoms. The CH3CN- ion formation rate shows a strong velocity dependence and is relatively small, the associated rate constants being ∼ 0.5--1.0 x 10-8cm 3s-1. A curve-crossing model, which considers the effect of crossings between the diabatic potential curves for the covalent K(np)/CH3CN system and the ionic K+/CH 3CN- system is discussed and provides a clear explanation for the observations. Electron transfer in collisions between dipole-bound negative ions (CH 3CN- and C2H3CN-) and target gas, including attaching molecules like SF6 and polar molecules like CH3NO2, is examined using the Penning trap. A free-electron capture model is used to describe electron transfer to attaching molecules. A near-resonant charge transfer model is employed to describe collisions between dipole-bound anions and polar molecules. The rate constants predicted by these models are consistent with the experimental observations.
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.
We describe two simple models for the determination of electron binding energies (Eb{\mathit{E}}_{\mathit{b}}Eb​) of several dipole-bound negative ions from field-detachment experiments and charge-transfer formation-rate-constant measurements in collisions between cold neutral molecules and Rydberg atoms. These models are able to correctly reproduce the experimental data, from which we deduce two compatible sets of Eb{\mathit{E}}_{\mathit{b}}Eb​ values. A curve-crossing model for calculating anion formation rate constants allows us to qualitatively understand the sharply peaked shapes of the experimental curves and to derive a simple quantitative law between Eb{\mathit{E}}_{\mathit{b}}Eb​ values and principal Rydberg quantum numbers at which anion formation rate constants are maximum.