Session H8: Invited Session: Gaseous Electronics: AMO Physics

Imaging of the dissociation dynamics of polyatomic molecules following low-energy electron resonant attachment

There is a very large body of experimental work on dissociative electron attachment to molecules but the majority of that work is focused on the measurement of absolute total cross sections or energetic positions of the resonances. There is scarce information on the dynamics of electron attachment and the subsequent dissociation that often involves highly non-Born-Oppenheimer dynamics, funneling through conical intersections or intricate nuclear motion during the dissociation process. Through COLTRIMS detection techniques we investigate the electron attachment in a fixed-in-frame manner that yields direct information on the symmetries of the neutral and negatively charged resonant states. We will present a study that combines experimental data along with theoretical analysis of dissociative electron attachment to carbon dioxide, methanol and uracil. In these studies we demonstrated that an understanding of anion dissociation dynamics beyond simple one-dimensional models is crucial in interpreting the measured angular distributions.   

Insights derived from hydrodynamic interpretations of atomic-scale interactions

Many of the properties and much of the behavior of gaseous or plasma environments are governed by interactions at the atomic-scale, that is, interactions among electrons, photons, ions, atoms, and molecules. New insight into the fundamental dynamics of these interactions, such as how energy and momentum are transferred, can be gained by considering a hydrodynamic view of the evolution of the electronic probability density. In particular, the creation, evolution, interaction, dissipation, and asymptotic survival of zeroes of the probability density, and the corresponding vortices in the electronic probability current, play significant and often dominant roles in energy and momentum transfer that has not heretofore been well recognized. Recent work to elucidate the role of these phenomena in atomic collisions and photoionization will be described as well as collaboration with the Frankfurt group to experimentally demonstrate the persistence of the predicted zeroes to macroscopic scales in reaction microscope measurements.   

High-Voltage, High-Power Gaseous Electronics Switch For Electric Grid Power Conversion

We are developing a high-voltage, high-power gas switch for use in low-cost power conversion terminals on the electric power grid. Direct-current (dc) power transmission has many advantages over alternating current (ac) transmission, but at present the high cost of ac-dc power interconversion limits the use of dc. The gas switch we are developing conducts current through a magnetized cold cathode plasma in hydrogen or helium to reach practical current densities $>1$ A/cm$^2$. Thermal and sputter damage of the cathode by the incident ion flux is a major technical risk, and is being addressed through use of a ``self-healing'' liquid metal cathode (eg, gallium). Plasma conditions and cathode sputtering loss are estimated by analyzing plasma spectral emission. A particle-in-cell plasma model is used to understand various aspects of switch operation, including the conduction phase (where plasma densities can exceed 10$^{13}$ cm$^{-3}$), the switch-open phase (where the high-voltage must be held against gas breakdown on the left side of Paschen's curve), and the switching transitions (especially the opening process, which is initiated by forming an ion-matrix sheath adjacent to a control grid).   

LIF studies of Discharge Plasma Sheaths

Sheath formation in plasma discharges is a collective effect characteristic of the plasma state of matter, and in single ion species plasmas it is well understood in terms of the Bohm Criterion. However, plasmas often contain several positive ion species. This complicates the physics of sheath formation. Many theoretical studies suggest that each of the ion species reach the sheath edge at their individual Bohm speeds. We carried out the first laser-induced fluorescence (LIF) Ar+ measurements in the presheaths of weakly collisional Ar+He plasmas and showed that this was not generally the case. When the Ar and He species had comparable densities, ions were found reach the sheath edge with nearly the same speed, the ion sound speed of the bulk plasma. We broadened the scope of the measurements to include Ar+Xe, and He+Xe plasmas. Ion velocity distribution functions (IVDFs) in the Ar+Xe case were measured with LIF with separate (tunable diode) lasers. In all cases, each species reached the sheath edge at close to the system sound velocity when the ion densities are comparable, and if the ratio of the ion densities are very large or small compared to one, each ion species reaches the sheath edge at its individual Bohm speed. Our results are in good agreement with the kinetic theory of instability enhanced ion-ion friction of Baalrud et al. We report on this and on the development of the Kr+ ion flow diagnostic using a tunable diode laser undertaken in order to significantly increase the number of two ion plasma mixtures for which both ions may be diagnosed. More recently, our LIF studies have uncovered evidence of anomalous ion-ion collisionality in single ion species plasmas that play a role in single ion species sheath formation. We contributed to the development of LIF schemes suitable for diode lasers for Ar, Xe, and now Kr discharges. Issues related to atomic physics, signal to noise, and deconvolution will be discussed.