heavy-Rydberg Ion-pair States Research Articles

Dissociative electron attachment studies with hyperthermal Rydberg atoms.

Earlier studies of the velocity distributions of heavy-Rydberg ion-pair states formed in collisions between potassium Rydberg atoms with low-to-intermediate values of n, 10 ≲ n ≲ 15, and targets that attach free low-energy electrons have shown that such measurements can provide a window into the dynamics of dissociative electron capture. Here we propose that the reaction dynamics can be explored in much greater detail through studies using hyperthermal Rydberg atoms. This is demonstrated using, as an example, helium Rydberg atoms and a semi-classical Monte Carlo collision code developed specifically to model the dynamics of Rydberg electron transfer in collisions between Rydberg atoms and attaching targets. The simulations show that the outcome of collisions is sensitive not only to the lifetime and decay energetics of the excited intermediate negative ion formed upon initial Rydberg electron capture but also to the radial electron probability density distribution in the Rydberg atom itself, i.e., to its ℓ value.

Probing dissociative electron attachment through heavy-Rydberg ion-pair production in Rydberg atom collisions.

Electron transfer in collisions between low-n, n = 12, Rydberg atoms and targets that attach low-energy electrons can lead to the formation of heavy-Rydberg ion-pair states comprising a weakly-bound positive-negative ion pair that orbit each other at large separations. Measurements of the velocity and angular distribution of ion-pair states produced in collisions with 1,1,1-C2Cl3F3, CBrCl3, BrCN, and Fe(CO)5 are used to show that electron transfer reactions furnish a new technique with which to examine the lifetime and decay energetics of the excited intermediates formed during dissociative electron capture. The results are analyzed with the aid of Monte Carlo simulations based on the free electron model of Rydberg atom collisions. The data further highlight the capabilities of Rydberg atoms as a microscale laboratory in which to probe the dynamics of electron attachment reactions.

Use of heavy-Rydberg ion-pair states to probe dissociative electron attachment

Electron transfer in collisions between Rydberg atoms and targets that attach free low-energy electrons can lead to the formation of heavy-Rydberg ion-pair states comprising a positive-negative ion pair that orbit each other at large separations weakly bound by their mutual electrostatic attraction. It is shown that measurements of the velocity distribution of the ion-pair states produced in such collisions provide a novel probe of the dynamics of dissociative electron attachment.

Use of heavy-Rydberg ion-pair states to probe dissociative electron attachment

SynopsisMeasurements of the velocity distributions of heavy Rydberg ion-pair states formed through electron transfer in collisions between Rydberg atoms and attaching targets are used to probe the dynamics of dissociative electron attachment.

Probing dissociative electron attachment through formation of heavy-Rydberg ion pair states in Rydberg atom collisions

A new approach to investigating the properties, i.e., lifetimes and decay energetics, of short-lived intermediates formed during dissociative electron attachment reactions is described that is based on measurements of the velocity and angular distributions of heavy-Rydberg ion pair states formed through electron transfer in collisions with Rydberg atoms. The experimental results are analyzed with the aid of a Monte Carlo collisions code and show that while electron capture by CF3I and CH2Br2 leads to creation of very-short-lived intermediates capture by CCl4 results in formation of much longer lived intermediates.

Dynamics of heavy-Rydberg ion-pair formation in K(14p,20p)-SF6, CCl4 collisions.

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.

Electric-field-induced dissociation of heavy Rydberg ion-pair states

A classical trajectory Monte Carlo approach is used to simulate the dissociation of H(+)⋅⋅⋅F(-) and K(+)⋅⋅⋅Cl(-) heavy Rydberg ion pairs induced by a ramped electric field, a technique used experimentally to detect and probe ion-pair states. Simulations that include the effects of the strong short-range repulsive interaction associated with ion-pair scattering are in good agreement with experimental results for Stark wavepackets probed by a ramped field, demonstrating that many of the characteristics of field-induced dissociation can be well described using a quasi-classical model. The data also show that states with a given value of principal quantum number (i.e., binding energy) can dissociate over a broad range of applied fields, the exact field being governed by the initial orbital angular momentum and orientation of the state.

Lifetimes of heavy-Rydberg ion-pair states formed through Rydberg electron transfer

The lifetimes of K(+)..Cl(-), K(+)..CN(-), and K(+)..SF(6)(-) heavy-Rydberg ion-pair states produced through Rydberg electron transfer reactions are measured directly as a function of binding energy using electric field induced detachment and the ion-pair decay channels discussed. The data are interpreted using a Monte Carlo collision code that models the detailed kinematics of electron transfer reactions. The lifetimes of K(+)..Cl(-) ion-pair states are observed to be very long, >100 micros, and independent of binding energy. The lifetimes of strongly bound (>30 meV) K(+)..CN(-) ion pairs are found to be similarly long but begin to decrease markedly as the binding energy is reduced below this value. This behavior is attributed to conversion of rotational energy in the CN(-) ion into translational energy of the ion pair. No long-lived K(+)..SF(6)(-) ion pairs are observed, their lifetimes decreasing with increasing binding energy. This behavior suggests that ion-pair loss is associated with mutual neutralization as a result of charge transfer.

Production of heavy-Rydberg ion-pair states in Rydberg atom collisions

The formation of bound heavy-Rydberg ion-pair states through electron transfer in collisions between K(np) Rydberg atoms and attaching targets is being examined, Measurements show that low-n collisions with a wide variety of targets can lead to formation of bound ion-pair states. Under appropriate conditions a small fraction of these can subsequently dissociate as free ions through internal-to-translational energy transfer, with typical lifetimes of ∼ 1-5 μs.

Direct observation of K+⋯Cl− heavy-Rydberg ion-pair states formed in K(np)/CCl4 collisions

Abstract The formation of K + ⋯Cl − heavy-Rydberg ion-pair states through dissociative electron transfer in K( n p)/CCl 4 , n = 16–50, collisions is examined, the product ion-pair states being detected directly through electric field-induced dissociation. The results are analyzed with the aid of a Monte Carlo collision code that models both initial Rydberg electron capture and the subsequent evolution of the ion pairs. The data demonstrate the production of long-lived ( τ ≫ 10 μs) high-angular-momentum K + ⋯Cl − ion-pair states and suggest that Rydberg atom collisions can generate a wide variety of heavy-Rydberg species facilitating study of their physical and chemical properties.

Temperature dependence of reactions involving electron transfer in K(np)/C2Cl4 collisions

Electron transfer in K(np)-C(2)Cl(4) collisions, which leads to formation of both Cl(-) and C(2)Cl(4)(-) anions, is investigated as a function of target temperature over the range of 300-650 K. Measurements at high n (n approximately 30) show that the likelihood of Cl(-) production increases rapidly with temperature indicating the presence of a dissociation barrier. The data yield an activation energy of approximately 0.1 eV. A broad distribution of product C(2)Cl(4)(-) lifetimes is observed that extends from microseconds to milliseconds, this distribution moving toward shorter lifetimes as the target temperature is increased. The measured lifetimes are consistent with the predictions of quasiequilibrium theory. Studies at low n (n approximately 14) show a substantial fraction of the product K(+)-Cl(-) and K(+)-C(2)Cl(4)(-) ion pairs is electrostatically bound leading to creation of heavy-Rydberg ion-pair states. Variations in target temperature lead to changes in kinetic energy of relative motion of the reactants that can result in marked changes in the fraction of ion pairs that is bound, especially at low Rydberg atom velocities. In the case of bound K(+)-C(2)Cl(4)(-) ion pairs a few percent subsequently dissociate by the conversion of internal energy in the anion into translational energy of the ion pair. Analysis of the data points to a mean energy conversion of approximately 60-90 meV, much less than the available excess energy of reaction, approximately 0.7 eV.

Formation of heavy-Rydberg ion-pair states in collisions of K(np) Rydberg atoms with attaching targets

The formation of heavy-Rydberg ion-pair states through electron transfer in collisions between K(np) Rydberg atoms and molecules that attach low-energy electrons is investigated. The measurements show that low-n collisions with a wide variety of target species (SF(6), c-C(7)F(14), C(6)F(6), and CCl(4)) can lead to formation of bound ion-pair states and that, under appropriate conditions, a small fraction of these can subsequently dissociate as free ions through internal-to-translational energy transfer. Analysis of the data suggests that those ion pairs that do dissociate typically have lifetimes of approximately 1 micros, although some can have lifetimes of 5 micros or longer.