Threshold detachment of negative ions by electron impact

The description of threshold fragmentation under long range repulsive forces is presented. The dominant energy dependence near threshold is isolated by decomposing the cross section into a product of a back ground part and a barrier penetration probability resulting from the repulsive Coulomb interaction. This tunneling probability contains the dominant energy variation and it can be calculated analytically based on the same principles as Wannier's description for threshold ionization under attractive forces. Good agreement is found with the available experimental cross sections on detachment by electron impact from $D^{-}$, $O^{-}$ and $B^{-}$.Comment: 4 pages, 4 figures (EPS), to appear in Phys.Rev.Lett, Feb. 22nd, 199

Radio-frequency discharges in Oxygen. Part 1: Modeling

In this series of three papers we present results from a combined experimental and theoretical effort to quantitatively describe capacitively coupled radio-frequency discharges in oxygen. The particle-in-cell Monte-Carlo model on which the theoretical description is based will be described in the present paper. It treats space charge fields and transport processes on an equal footing with the most important plasma-chemical reactions. For given external voltage and pressure, the model determines the electric potential within the discharge and the distribution functions for electrons, negatively charged atomic oxygen, and positively charged molecular oxygen. Previously used scattering and reaction cross section data are critically assessed and in some cases modified. To validate our model, we compare the densities in the bulk of the discharge with experimental data and find good agreement, indicating that essential aspects of an oxygen discharge are captured.Comment: 11 pages, 10 figure

Electron-impact detachment of D<SUP>-</SUP>: near-threshold behavior and the nonexistence of D<SUP>2-</SUP> resonances

D- has been stored in the heavy-ion storage ring ASTRID and merged with the electron beam from an electron cooler. The cross section for electron-impact detachment of D- was measured for relative energies from 0 to 20 eV. No evidence is found for the earlier reported resonances that were ascribed to short-lived H2- states. We present a simple classical "reaction-zone" model which essentially reproduces the data except in the region close to the threshold where the cross section is small

Electron-impact detachment from negative ions

Electron-impact detachment cross sections have been measured from threshold to about 30 eV for D<SUP>-</SUP> and O<SUP>-</SUP>. The purpose was to investigate the cross section near threshold and to reinvestigate earlier claims of resonances due to short-lived states of the doubly charged negative ion. Initial results for D- showed no resonances [L. H. Andersen, D. Mathur, H. T. Schmidt, and L. Vejby-Christensen, Phys. Rev. Lett. 74, 892 (1995)]. Here we show that there is no detectable resonance structure for O<SUP>-</SUP> either. The applied experimental technique is discussed in detail. Classical and semiclassical models for the detachment process are presented and compared with the data