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.
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.
Collisions between atoms in high Rydberg states and polar targets can lead to the formation of dipole-bound negative ions. We examine the dynamics of CH3CN- ion production through electron transfer in K(np)/CH3CN collisions using velocity selected Rydberg atoms. The CH3CN- ion formation rate decreases markedly with decreasing Rydberg atom velocity, principally as a consequence of postattachment electrostatic interactions between the product ions. Implemented using Monte Carlo techniques, a curve-crossing model, which considers the effect of crossings between the diabetic potential curves for the covalent K(np)/CH3CN system and the K+/CH3CN - system, provides a clear explanation of several characteristics seen in electron transfer in K(np)/CH3CN collisions including: the n dependence in the CH3CN- ion production rate, the velocity dependence in the CH3CN- ion production rate, and the relatively small rate constant ∼0.5 - 1.0 x 10-8cm3 s-1 for production of long-lived CH3CN - ions.
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.
Thermal collisions between potassium Rydberg atoms with intermediate values of quantum number n and target molecules that capture free low-energy electrons are investigated. The velocity, binding energy, and angular distributions of heavy-Rydberg ion-pair states formed through such collisions are measured. The experimental data are analyzed with the aid of semiclassical Monte Carlo simulations to investigate the dynamics of electron attachment to a variety of target species. Multiple reaction channels involving short- and long-lived intermediates are seen for electron capture by 1,1,1-C2Cl3F3 and CBrCl3 (where dissociation of CBrCl3 intermediates involves production of Br- and Cl- ions). Electron attachment to BrCN produces long-lived BrCN-* intermediates whose dissociation results in the creation of rotationally-hot CN- anions. Electron capture by Fe(CO)5 leads to formation of Fe(CO)5-* intermediates which can fragment producing Fe(CO)4- anions, either directly or after partial redistribution of the excess energy within the intermediate has occurred. Electron transfer in Rydberg atom collisions with C2Cl4 provide no evidence of formation of long-lived C2Cl4 ions, only Cl- ions are seen. Moreover, strong Rydberg atom scattering is observed following collisions of K(12p) Rydberg states with CH3NO2. This finding is consistent with a theory which predicts that collisions between Rydberg atoms with intermediate values of n and polar molecules can lead to strong resonant quenching of Rydberg states, i.e., n and l changing, through formation of short-lived K+...CH3NO2- ion-pair states. Finally, the improvements in sensitivity to electron capture reaction dynamics that might be achieved using different Rydberg species and a crossed molecular beam are examined. Simulations are performed to demonstrate that the use of a hyperthermal helium Rydberg atomic beam and a crossed beam of target molecules such as CCl4, BrCN and CF3I can provide new insights into dissociative electron attachment reactions. The calculations show that not only can much detailed information on translational energy release distributions and intermediate lifetimes be obtained, but also on the electron radial probability density distributions in the Rydberg atoms themselves.
We describe two simple models for the determination of electron binding energies (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 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 values and principal Rydberg quantum numbers at which anion formation rate constants are maximum.
Heavy-Rydberg ion-pair states, molecular quantum systems that are bound by their long-range pure Coulomb attraction, are formed through Rydberg atom collisions with molecules that attach low energy electrons. Collisions of potassium Rydberg atoms K(np) with the attaching species CCl4 and SF6 lead, respectively, to dissociative and non-dissociative electron attachment with subsequent formation of bound ion-pair states K+..Cl- and K+..SF6-.\nThe experimental apparatus used to produce and detect heavy-Rydberg ion-pair states consists of a reaction region and a separate analysis region. It separates a fraction of the product ion pairs from the parent Rydberg atoms, eliminating spurious background signals associated with blackbody-induced and electric-field ionization of Rydberg atoms. A position sensitive detector in the analysis region is employed to measure their velocity, angular, and binding energy distribution and lifetimes. Measurements of the spatial distribution of ion pairs provide information on their velocity and angular distributions, and their binding energy distribution is measured using electric field-induced dissociation. The lifetime of the ion-pair states is influenced by multiple processes including internal-to-translational energy transfer, autodetachment of the electron, and neutralization through charge transfer.\nThe experimental results are compared with the results of a Monte Carlo collision code that models both the initial Rydberg electron attachment and the subsequent evolution and molecular dynamics of the ion pairs. This model highlights the factors such as the kinematics of the Rydberg atom and the attaching particle and energy released in dissociation (in the case of dissociative attachment) that are important in governing ion pair formation. The model calculations are in good agreement with the experimental data.