Three-Coulomb-Wave Pluvinage Model for Compton Double Ionization of Helium in the Region of the Cross-Section Maximum

Double ionization of ground-state helium by the Compton scattering of a photon is calculated using the well-known product of three Coulomb waves (3C) wave function for the final double-continuum state of the atom and the Pluvinage wave function for the initial ground state (also a product of three Coulomb waves). The theoretical model fails to predict both the shape and the overall magnitude of the observed integrated cross section vs. Incident-photon energy. However, when the monopole part of the electron-photon interaction is removed by an ad hoc procedure, qualitative agreement is obtained, although the absolute size of the cross section is still almost an order of magnitude too large

Test of the Pluvinage Wave Function for the Helium Ground State

The accuracy of the Pluvinage wave function for the ground state of helium is investigated by considering a number of different physical processes including double ionization by photoabsorption, Compton scattering, and electron impact. In the high-energy limit of these processes, the accuracy of the initial ground state can be ascertained without reference to the final double-continuum state. In this limit, we find that a Hylleraas description is superior to the Pluvinage one. For intermediate energies, final-state correlation becomes important, so we employ a 3C description of the final state (the 3C wave function is the double-continuum analog of the Pluvinage wave function). In this case, however, better agreement with experiment is obtained with the Pluvinage initial state. A possible explanation for this seemingly paradoxical result is suggested

Spin Exchange Rates in Proton-Hydrogen Collisions

The spin temperature of neutral hydrogen, which determines the optical depth and brightness of the 21 cm line, is determined by the competition between radiative and collisional processes. Here we examine the role of proton-hydrogen collisions in setting the spin temperature. We use recent fully quantum mechanical calculations of the relevant cross sections, which allow us to present accurate results over the entire physically relevant temperature range 1-10,000 K. For kinetic temperatures T_K>100 K, the proton-hydrogen rate coefficient exceeds that for hydrogen-hydrogen collisions by about a factor of two. However, at low temperatures (T_K < 5 K) H-p collisions become several thousand times more efficient than H-H and even more important than H-e^- collisions.Comment: submitted to MNRAS, 5 pages, 2 figures, typos correcte

Uses Genres and Media Ensembles : A Conceptual Roadmap for Research of Convergent Audiences

This chapter tackles one of the main methodological and conceptual challenges to current audience research: fragmentation of viewers’ practices of reception. The use of digital and networked media and the consequent multiplication of screens, distribution channels and content sources have further complicated the notion of “watching television” and, along with that, academic and applied audience research. The chapter reintroduces Maria Bakardjieva’s concept of uses genres and connects it with the concept of media ensemble, suggesting that for research on the domestic consumption of films and TV series, the application of these concepts in qualitative (ethnographic) research and in audience surveys comes with strong advantages. Firstly, the concepts help to identify distinct types of consumption practices linked with specific technological objects, with specific audiovisual content and with typical everyday situations, and they enable us to analyze consumption explicitly within the contexts of the spatiotemporal and social organization of everyday life. Secondly, in cases of small- and peripheral-market audiences, the concepts enable us to identify specifics in audiences’ practices linked with the characteristics of these markets (e.g., with localized and non-localized content, with domestic and global production, etc.). And thirdly, the concepts explicitly acknowledge power both involved in and shaping the analyzed practices by emphasizing the “generative process of technology,” i.e., the transformative role of users’ practices in shaping technological and economic systems

How quantum bound states bounce and the structure it reveals

We investigate how quantum bound states bounce from a hard surface. Our analysis has applications to ab initio calculations of nuclear structure and elastic deformation, energy levels of excitons in semiconductor quantum dots and wells, and cold atomic few-body systems on optical lattices with sharp boundaries. We develop the general theory of elastic reflection for a composite body from a hard wall. On the numerical side we present ab initio calculations for the compression of alpha particles and universal results for two-body states. On the analytical side we derive a universal effective potential that gives the reflection scattering length for shallow two-body states.Comment: final publication version, new lattice results on alpha particle compression, 5 pages, 2 figure

Threshold Laws for the Break-up of Atomic Particles into Several Charged Fragments

The processes with three or more charged particles in the final state exhibit particular threshold behavior, as inferred by the famous Wannier law for (2e + ion) system. We formulate a general solution which determines the threshold behavior of the cross section for multiple fragmentation. Applications to several systems of particular importance with three, four and five leptons (electrons and positrons) in the field of charged core; and two pairs of identical particles with opposite charges are presented. New threshold exponents for these systems are predicted, while some previously suggested threshold laws are revised.Comment: 40 pages, Revtex, scheduled for the July issue of Phys.Rev.A (1998

Differential Impacts of Willow and Mineral Fertilizer on Bacterial Communities and Biodegradation in Diesel Fuel Oil-Contaminated Soil.

Despite decades of research there is limited understanding of how vegetation impacts the ability of microbial communities to process organic contaminants in soil. Using a combination of traditional and molecular assays, we examined how phytoremediation with willow and/or fertilization affected the microbial community present and active in the transformation of diesel contaminants. In a pot study, willow had a significant role in structuring the total bacterial community and resulted in significant decreases in diesel range organics (DRO). However, stable isotope probing (SIP) indicated that fertilizer drove the differences seen in community structure and function. Finally, analysis of the total variance in both pot and SIP experiments indicated an interactive effect between willow and fertilizer on the bacterial communities. This study clearly demonstrates that a willow native to Alaska accelerates DRO degradation, and together with fertilizer, increases aromatic degradation by shifting microbial community structure and the identity of active naphthalene degraders