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

Abstract

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.

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

“…However, a Boltzmann-like translational energy release distribution, which peaks at =0, is unlikely as it would require that dissociation preferentially populate highly-rotationally-excited states rather than a broad, more thermal-like distribution of such states. One consequence of rotational excitation is that, as shown in earlier measurements [7], weakly-bound K + ..CN -ion pairs have finite lifetimes due to dissociation induced by conversion of rotational energy in the CN -ion into translational energy of the ion pair. In the earlier measurements no significant decay of ion pair states with binding energies greater than ~30 meV was observed.…”

“…2 thus clearly demonstrate that a dissociation model in which all the excess energy of reaction appears in translation is not appropriate and that a different reaction model is required. Earlier studies [7,8] (2) In this case, the bond dissociation energy D 0 (CCl 3 -Cl) is ~3.07 eV and the electron affinity of chlorine is EA Cl = 3.61 eV pointing to an excess energy ~0.5 eV. If immediate dissociation were to occur and all this energy appear in translation the majority would be acquired by the (light) Cl -ion, resulting in large Cl -velocities.…”

“…As a result, for thermal-energy collisions, an increasing fraction of the ion pairs possess insufficient kinetic energy of relative motion to overcome their mutual electrostatic attraction and separate. They therefore remain bound creating an ion-pair state in which the ions orbit each other at relatively large radii [4][5][6][7][8][9]. Such states are also termed heavy-Rydberg states because many of their properties parallel those of Rydberg atoms [10,11].…”

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

“…In particular, for thermal energy collisions, an increasing fraction of the product ion pairs possess insufficient kinetic energy of relative motion to overcome their mutual Coulomb attraction and separate. They therefore remain bound creating an ion-pair state in which the positive-negative ion pair orbit each other [3][4][5][6][7], also termed heavy-Rydberg states because many of their properties parallel those of Rydberg atoms [8,9]. Ion-pair states provide one example of a long-range weakly bound molecular system, others include Rydberg atom macrodimers [10][11][12] and long-range Rydberg molecules [13][14][15][16].…”

“…A variety of ion-pair states involving both atomic (K + ··Cl − ) and molecular (K + ··SF 6 − , K + ··CN − ) negative ions have been created in this manner [3][4][5][6][7]. Whereas the focus in earlier work was on the formation of bound ion pairs and their lifetimes and decay modes, we demonstrate here that measurements of their angular and velocity distributions can also provide insights into the properties (decay energetics, lifetimes) of the short-lived intermediates.…”