Free ion formation in K(np)/SF6 collisions at low-to-intermediate n: Velocity dependence of the number and lifetime of the product ions

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.

Temperature dependence of negative ion lifetimes

The autodetachment lifetimes of SF6-* and C6F6-* ions formed by charge transfer in K(np)/SF6, C6F6 collisions are measured as a function of target temperature over the range of approximately 300-600 K with the aid of time-of-flight techniques and a Penning ion trap. At room temperature only formation of long-lived SF6 -* ions with lifetimes tau >or similar to 1 ms is seen. As the temperature is increased the lifetime of these long-lived ions is reduced, some having lifetimes as short as approximately 0.4 ms. The appearance of a short-lived, tau <or similar to 10 micros, SF6-* signal is also observed. Rydberg electron transfer to room temperature C6F6 leads predominantly to formation of short-lived, tau <or similar to 10 micros, C6F6-* ions, although a small number of longer-lived anions with lifetimes of approximately 50-100 micros is also evident. This signal disappears as the temperature is raised and the lifetime of the short-lived ions decreases dramatically. The measured lifetimes are compared to those predicted using quasiequilibrium theory and updated values of the input parameters, including calculated vibrational frequencies for the anions. For C6F6, the calculated anion lifetimes are in good agreement with those measured experimentally. While similarly good agreement is obtained for SF6 at room temperature, the predicted temperature dependence of the lifetime is very different to that observed. This suggests that the excitation energy is not completely randomized in the anion and the measurements point to formation of at least two quasi-independent groups of anion states that have very different lifetimes. The present results are compared to those of earlier work.

Threshold Behavior in Electron-Transfer Collisions between Rubidium Atoms and C2F5Cl or C2F5I Molecules

Rubidium atoms are accelerated in a high-temperature expansion of hydrogen to produce beams with energies high enough to observe collisional ionization with a cross beam. The speed of the atoms is directly measured by time-of-flight techniques, and the positive and negative ions produced are detected in separate mass spectrometers and detected in coincidence. Chloroperfluoroethane produces C(2)F(5)(-) and Cl(-) ions, whereas iodoperfluoroethane produces I(-), C(2)F(5)(-), and C(2)F(5)I(-) ions. When the measured speed distributions are used, the signal versus energy may be deconvolved to yield thresholds and electron affinities (EAs). The EA for C(2)F(5)I is measured to be 0.96 +/- 0.1 eV. Anomalously high EA values result for C(2)F(5) apparently because C(2)F(5)(-) is produced by parts per million concentrations of Rb(2).

Lifetime of C2Cl4− ions produced by nondissociative electron attachment to C2Cl4

The lifetimes of long-lived C2Cl4(-) ions formed by Rydberg electron transfer in K(np)/C2Cl4 collisions are investigated using a Penning ion trap. Measurements at high n, n > or = 30, show that low-energy electron attachment to C2Cl4 leads to the production of C2Cl4(-) ions with a broad range of lifetimes that extends up to at least 1 ms. This is attributed to capture by molecules in different initial vibrational states. At low n, internal-to-translational energy transfer in postattachment interactions between the product K+ and C2Cl4(-) ions becomes important and leads to a substantial increase in ion lifetimes.

Rydberg electron transfer to SF6: Product ion lifetimes

The lifetimes of SF6- ions produced by Rydberg electron transfer in K(np)SF6 collisions at high n, n greater or similar to 30, are examined using a Penning ion trap. The data point to the formation of ions with a range of lifetimes that extends from approximately 1 to greater or similar to 10 ms. Sizable numbers of ions remain in the trap even 40 ms after initial injection and at least part of this signal can be attributed to radiative stabilization. Measurements of free low-energy electron attachment to SF6 in the trap show that the product ions have lifetimes similar to those of SF6- ions formed by electron transfer in high-n collisions.

Rydberg electron transfer to C6H5NO2: lifetimes and characteristics of the product C6H5NO2- ions

The nature of electron binding in C6H5NO2- ions produced by Rydberg electron transfer in K(np)C6H5NO2 collisions is investigated through measurements of the number and the lifetimes of the product ions and their dependence on Rydberg atom velocity and principal quantum number n in the range 12 <or approximately n <or approximately 30. The data are interpreted by comparison to results obtained using well-known dipole-bound and valence-bound anions. At high n direct capture into valence-bound states with a lifetime of approximately 1.6 ms is observed. At low n the data suggest that, while direct capture into valence-bound states is still possible, the majority of the observed C6H5NO2- ions result from the onset of a second reaction channel that involves the formation of a dipole-bound "doorway" state that rapidly evolves into a state with predominantly valence-bound character. These findings are discussed in the light of earlier work on electron binding to C6H5NO2.

Dynamics of Rydberg electron transfer to CH3CN: velocity dependent studies

The dynamics of free-ion production through electron transfer in K(np)/CH3CN collisions are examined through measurements using velocity-selected Rydberg atoms. The data show that Rydberg electron transfer leads to the creation of two groups of dipole-bound CH3CN- ions, one long lived (tau>85 micros), the other short lived (tau<1 micros). The velocity dependences associated with the production of both groups of ions are similar, the ion formation rate decreasing markedly with decreasing Rydberg atom velocity, principally as a consequence of postattachment electrostatic interactions between the product ions. The results are in reasonable accord with the predictions of a Monte Carlo collision model that considers the effect of crossings between the diabatic potential curves for the covalent K(np)/CH3CN system and the K+/CH3CN- ion pair. This model also accounts for the relatively small reaction rate constants, approximately 0.5-1.0 x 1.0(-8) cm(3) s(-1), associated with the formation of long-lived CH3CN- ions. No velocity dependence in the lifetime of the CH3CN- ions is observed.