R. A. Popple

Dissociative electron attachment to CCl4: Lifetime of the CCl4−* intermediate

Dissociative electron attachment to CCl4 is studied by measuring the angular and velocity distribution of Cl− ions produced in collisions with velocity selected K(np) Rydberg atoms. Analysis of the data using a Monte Carlo collision code that models the detailed kinematics of the reaction indicates that the lifetime of the CCl4−* intermediate initially formed by Rydberg electron capture is 7.5±2.5 ps and that, upon dissociation, only a small fraction of the excess energy of reaction appears as translational energy of the Cl− and CCl3 fragments.

Formation of long-lived C60− ions in Rydberg atom-C60 collisions

Abstract Studies of collisions between C 60 and K( n p) Rydberg atoms with 35 ⩽ n ⩽ 125 indicate that C 60 can attach very-low-energy electrons to form long-lived C 60 − ions. The rate constant for negative ion formation is observed to be small, ≈ 10 −8 cm 3 s −1 , and to be independent of n pointing to s-wave capture. Possible mechanisms for this are discussed, including the formation of image-charge-bound C 60 − ions.

Improved low-energy, electron-spin-polarized 4He+ ion source

Recent improvements to a source of electron-spin-polarized 4He+ ions based on an optically pumped, rf-excited helium discharge are described that have resulted in ion polarizations P+ of ∼0.18 at currents of ∼1 nA and in an increase of over an order of magnitude in the quality factor P+2I.

LOW-ENERGY, ELECTRON-SPIN-POLARIZED 4HE+ ION SOURCE

A source of low-energy, electron-spin-polarized 4He+ ions based on an optically pumped, rf-excited helium discharge is described. Ion polarizations P+ of ∼0.13 are achieved at beam currents of ∼0.1 nA, decreasing to ∼0.09 at currents of ∼0.5 nA. Ion beam energies as low as 10 eV have been realized, with an energy spread of ≲3 eV full width half maximum. The ion polarization can be reversed (P+→−P+) simply by changing the sense of circular polarization of the optical pumping radiation. The source is suitable for use in a wide variety of applications including surface physics studies.

Velocity dependence of free ion production in K(np)–C2Cl4, CS2, and O2 collisions: Internal‐to‐translational energy transfer

The rates for free ion production through electron transfer in collisions between K(np) Rydberg atoms and C2Cl4, CS2, and O2 are measured as a function of Rydberg atom velocity for intermediate values of n, 13≤n≤21. The data show that postattachment interactions between the product positive and negative ions are important and can lead to conversion of internal energy from the negative ion, which is formed in an excited state, into translational energy of the product ion pair. This energy conversion occurs without ion–ion neutralization and can stabilize the negative ion against dissociation or autodetachment.

USE OF RYDBERG ATOMS AS A NANOSCALE LABORATORY TO EXAMINE DISSOCIATIVE ELECTRON CAPTURE

Dissociative electron attachment to CF3I is investigated by measuring the angular and velocity distributions of I− ions produced in collisions with velocity-selected K(np) Rydberg atoms. Analysis of the data shows that the lifetime of the excited CF3I−∗ intermediate formed by Rydberg electron capture is short (≲ 2 ps) and that essentially all the excess energy of reaction appears as translational energy of the fragments.

Ionization of Rydberg atoms by half-cycle pulses: Effect of pulse shape and rise time

The ionization of potassium Rydberg atoms with by pulsed unidirectional electric fields, termed half-cycle pulses (HCPs), with various well characterized shapes (rectangular, triangular and sawtooth) and durations comparable to the classical electron orbital period is investigated. The experimental results are compared with classical trajectory Monte Carlo (CTMC) simulations and the classically scaled results of quantum calculations undertaken at n = 5. The data show that in the intermediate regime, , the threshold field for ionization becomes sensitive to the shape and rise time of the HCP, increasing with increasing rise time. For each pulse shape, the agreement between the experimental measurements and both the CTMC and quantum calculations is very good.

Velocity dependence of associative ion production in K (np)–C6H6 collisions

The velocity dependence of the formation of stable KC6H6+ ions through associative ionization in K(np)/C6H6 collisions has been investigated for values of n in the range 13–25. The data show that the associative ionization rate increases rapidly with decreasing collision velocity. Possible reasons for this are discussed.