^{1,a)}, C. H. Wang

^{1}, Y. Liu

^{1}, F. B. Dunning

^{1}and J. D. Steill

^{2}

### Abstract

Electron transfer in collisions, which leads to formation of both and anions, is investigated as a function of target temperature over the range of 300–650 K. Measurements at high show that the likelihood of production increases rapidly with temperature indicating the presence of a dissociation barrier. The data yield an activation energy of . A broad distribution of product 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 show a substantial fraction of the product and 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 atomvelocities. In the case of bound 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 , much less than the available excess energy of reaction,.

Research supported by the Robert A. Welch Foundation.

I. INTRODUCTION

II. EXPERIMENTAL METHOD

III. RESULTS AND DISCUSSION

A. Measurements at high ,

B. Measurements at low ,

IV. CONCLUSION

### Key Topics

- Rydberg states
- 38.0
- Dissociation
- 17.0
- Dissociation energies
- 8.0
- Electron capture
- 6.0
- Electron transfer
- 6.0

## Figures

Schematic diagram of the apparatus.

Schematic diagram of the apparatus.

Arrival time distribution of negatively charged particles at the upper PSD following collisions at target temperatures of (a) 300, (b) 450, and (c) 550 K for a Rydberg atom velocity of . The data sets are normalized to equal Rydberg atom production rates and target gas densities. The inset shows the temperature dependence of the size of the feature normalized to that measured at low temperature.

Arrival time distribution of negatively charged particles at the upper PSD following collisions at target temperatures of (a) 300, (b) 450, and (c) 550 K for a Rydberg atom velocity of . The data sets are normalized to equal Rydberg atom production rates and target gas densities. The inset shows the temperature dependence of the size of the feature normalized to that measured at low temperature.

Time evolution of the population in the Penning trap following collisions at temperatures of (○) 300, (▲) 355, and (▼) 400 K. The data sets are normalized to the initial number of ions injected and the time axis is measured from the time of the laser pulse.

Time evolution of the population in the Penning trap following collisions at temperatures of (○) 300, (▲) 355, and (▼) 400 K. The data sets are normalized to the initial number of ions injected and the time axis is measured from the time of the laser pulse.

Internal energy dependence of autodetachment lifetimes as predicted by QET (see text). The calculations assume a rate constant for electron capture of . The energies used for the vibrational modes of the parent neutral and anion are listed in Table I. The inset shows the change in the geometry from a planar point group to symmetry induced by electron attachment.

Internal energy dependence of autodetachment lifetimes as predicted by QET (see text). The calculations assume a rate constant for electron capture of . The energies used for the vibrational modes of the parent neutral and anion are listed in Table I. The inset shows the change in the geometry from a planar point group to symmetry induced by electron attachment.

(a) Temperature dependence of the signal produced in collisions for the values of and Rydberg atom velocities indicated. The various data sets are normalized to each other at 650 K. The data for are corrected for postattachment interactions (see text). (b) Arrhenius plot of the signal as a function of inverse temperature. The solid line is an exponential fit to the data. The inset shows the onset in the signal observed for collisions at a Rydberg atom velocity of and target temperature of 550 K. The dashed line indicates the build up of the signal predicted assuming immediate dissociation of the intermediates (see text).

(a) Temperature dependence of the signal produced in collisions for the values of and Rydberg atom velocities indicated. The various data sets are normalized to each other at 650 K. The data for are corrected for postattachment interactions (see text). (b) Arrhenius plot of the signal as a function of inverse temperature. The solid line is an exponential fit to the data. The inset shows the onset in the signal observed for collisions at a Rydberg atom velocity of and target temperature of 550 K. The dashed line indicates the build up of the signal predicted assuming immediate dissociation of the intermediates (see text).

Model calculations of the Rydberg atom velocity dependence of the escape probabilities for (a) and (b) ion pairs produced in collisions for the values of and temperatures indicated.

Model calculations of the Rydberg atom velocity dependence of the escape probabilities for (a) and (b) ion pairs produced in collisions for the values of and temperatures indicated.

Arrival time distributions for negatively charged particles at the upper PSD following collisions for Rydberg atom velocities of (a) 680 and (b) and the target temperatures indicated. The data sets are normalized to equal Rydberg atom production rates and target gas densities.

Arrival time distributions for negatively charged particles at the upper PSD following collisions for Rydberg atom velocities of (a) 680 and (b) and the target temperatures indicated. The data sets are normalized to equal Rydberg atom production rates and target gas densities.

(a) Rydberg atom velocity dependence of the signal produced in room-temperature collisions normalized to equal initial Rydberg atom production rates. The lines show the calculated ion pair escape probabilities assuming the mean translational energy releases indicated. The experimental data are normalized to the calculations for . (b) Velocity dependence of the production of unbound ions (see text). The lines show the calculated escape probabilities for ion pairs obtained using an escape fraction and the mean internal-to-translational energy conversions indicated. The experimental data are normalized to the calculations for at the highest velocity (see text). (c) Velocity dependence of the ratio of the and signals together with that predicted using the same values of (with ) as in (b), and .

(a) Rydberg atom velocity dependence of the signal produced in room-temperature collisions normalized to equal initial Rydberg atom production rates. The lines show the calculated ion pair escape probabilities assuming the mean translational energy releases indicated. The experimental data are normalized to the calculations for . (b) Velocity dependence of the production of unbound ions (see text). The lines show the calculated escape probabilities for ion pairs obtained using an escape fraction and the mean internal-to-translational energy conversions indicated. The experimental data are normalized to the calculations for at the highest velocity (see text). (c) Velocity dependence of the ratio of the and signals together with that predicted using the same values of (with ) as in (b), and .

Detail of the onset of the signal for the values of and Rydberg atom velocities indicated. The dashed line shows the build up of the signal expected assuming initial formation of unbound ion pairs.

Detail of the onset of the signal for the values of and Rydberg atom velocities indicated. The dashed line shows the build up of the signal expected assuming initial formation of unbound ion pairs.

## Tables

Calculated frequencies for the vibrational modes of and along with measured values for the neutral. All modes are given in and have a degeneracy of one. The results of the *ab initio* (MP2, CCSD) calculations are scaled by 0.95, the DFT calculations by 0.98.

Calculated frequencies for the vibrational modes of and along with measured values for the neutral. All modes are given in and have a degeneracy of one. The results of the *ab initio* (MP2, CCSD) calculations are scaled by 0.95, the DFT calculations by 0.98.

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