Colloquia Abstracts

New Chemistry: Compounds That Contain Positrons

David Schrader, Marquette University

A few dozen positron-containing compounds have been shown to be stable against dissociation. These include: 

• PsCl, positronium chloride
• e⁺Li, positronic lithium
• Ps₂O, dipositronium oxide
• Ps₂, dipositronium
• PsH, positronium hydride
• e+CnH2n+2 for ethyl positride or positroethane, etc.

All but the last three are known from quantum calculations only. Annihilation lifetimes are ~500 ps, half that for the species containing two positrons. This is several orders of magnitude longer than typical vibrational periods.

I will describe the single direct experimental determination of a binding energy, that of PsH, which was performed by me and collaborators in Denmark in 1991. A modern version of that experiment using the new positron beam at the CSE Radiation Chemistry Group is proposed and will be discussed.

Diatomic positronium, Ps2, was recently prepared by Cassidy and Mills. We will discuss prospects for preparing the 2-positron compounds CPs2 and Ps2O with the ANL/CSE beam.

1) D. M. Schrader, F. M. Jacobsen, N.-P. Frandsen, and U. Mikkelsen, Phys. Rev. Lett. 69, 57 (1992).
2) D. B. Cassidy and A. P. Mills Jr., Nature 449, 195 (2007).
3) N. Jiang and D. M. Schrader, Phys. Rev. Lett. 81, 5113 (1998). [Erratum, ibid. 82, 4735 (1999)].

The Worst of Global Warming and What We Can Do to Ameliorate It

Wallace S. Broecker
Newberry Professor of Earth & Environmental Sciences
Lamont-Doherty Earth Observatory of Columbia University

Isaac Held, a prominent atmospheric physicist, predicts that as a result of global warming, precipitation will be more strongly focused on the tropics. As a result, the adjacent drylands will become even more arid. The cold glacial analogue available from paleorecords lends strong support to this prediction.

In addition to energy conservation, alternate energy, and coal gasification coupled with CO₂ capture and storage, we must develop the means to capture CO₂ directly from the atmosphere. Led by Klaus Lackner, GRT, a small company in Tucson, Arizona, has figured out how to do this in an economically acceptable manner. In its absence, I fear that we will be driven to add SO₂ to the stratosphere.

Uncovering Details of Combustion Chemistry by Using Synchrotron Photoionization Mass Spectrometry

Craig A. Taatjes
Combustion Research Facility, Sandia National Laboratories

Multiplexed mass spectrometry with photoionization by tunable synchrotron radiation is a powerful method for investigation of elementary reaction kinetics and the chemistry of low-pressure flames. In both of these applications, multiple-mass detection and the easy tunability of synchrotron radiation make it possible to acquire full sets of data as a function of mass, photon energy, and of the physical dimension of the system, e.g., distance from the burner or time after reaction initiation. This multidimensional data can be quantitatively correlated and integrated along one or more dimensions to probe different aspects of the reaction systems. The particular strength of the tunable synchrotron photoionization method is the discrimination of different isomers by their distinct photoionization efficiency curves. In this talk I will discuss recent flame chemistry and chemical kinetics measurements at the Advanced Light Source that combine tunable photoionization with multiple-mass detection, emphasizing the overall insight that can be gained from multidimensional data on these systems. In particular I will highlight recent product formation measurements of combustion and tropospheric relevance, including reactions of OH with alkenes and cycloalkenes, in which enol formation is observed, and Cl-initiated oxidation of dimethyl sulfoxide, which forms the Criegee intermediate, CH2OO.

Time for Molecular Dynamics

Stephen R. Leone
University of California and Lawrence Livermore National Laboratory

Experiments are described to study molecular dynamics over 16 orders of magnitude in time, from seconds to attoseconds. By the generation of ultrafast high order harmonics in the vacuum ultraviolet and soft x-ray region, molecular dissociation and ionization processes are explored by probing x-ray core-level spectroscopic transitions. Results reveal alignment effects, molecular dissociation pathways, and optically induced absorption changes. In extreme limits, attosecond bursts of soft x-rays are generated by selected half cycles of intense, few-cycle laser pulses and, for comparison, the statistics of long-time single quantum dot intermittencies are investigated with tunable excitation sources, exhibiting memory effects and effects of environment.

X-ray Views of the Defect Structure and Chemistry of Environmental Nanoparticles

Alain Manceau
CNRS, Grenoble (France)

Major goals of environmental science are to control mobility of toxic elements and to remediate contaminated soils, sediments, and subsurface waters. These goals cannot be met without first having in hand a fundamental understanding of the elemental composition, defect structure, and surface properties of environmental nanoparticles. Key problems are their short-range order and high density of defects, the multiplicity of reactive surface sites and bonding mechanisms of impurities, and the partitioning of elements into coexisting organic and inorganic phases. In most cases, the information sought can be obtained by application of synergistic synchrotron-based x-ray techniques, including microfluorescence (micro-SRXF), microdiffraction (micro-XRD), high-energy scattering (HEXS), and microspectroscopy (micro-EXAFS), and data modelling with meaningful structure models. In this approach, micro-SRXF is used to map trace contaminants among coexisting constituents in a natural matrix, thus determining their distribution and relative abundance with unrivalled sensitivity. Then, micro-XRD, complemented by powder XRD and HEXS, is employed to identify nanocrystalline minerals and, more importantly, to determine the nature of structural and chemical defects (stacking faults, cationic and anionic vacancies and occupancies, site occupation of impurities, stoichiometry) through modelling of their scattering properties. Finally, micro-EXAFS gives the uptake and complexation mechanism of trace contaminants by individual constituents. Since the distribution of trace elements is heterogeneous on nanometer- to micrometer-length scales and nanoparticles are generally aggregated in environmental systems, the combination of these three microscopic techniques provides just the tool needed to scrutinize the nature and fundamental properties of environmental matter. This new kind of hybridization of experimental and modelling approaches to the characterization of natural nanoparticles will be illustrated with examples that show its potential for expanding our knowledge in environmental science.