High resolution studies of low-energy electron attachment to SF5Cl: Product anions and absolute cross sections

Low energy electron attachment to SF$_5$Cl was studied at high energy resolution by mass spectrometric detection of the product anions. Two variants of the laser photoelectron attachment (LPA) technique (Kaiserslautern) were used for determining the threshold behaviour of the yield for SF$_5^-$ formation at about 1 meV resolution, and to investigate the relative cross sections for Cl$^-$, FCl$^-$, and SF$_5^-$ formation towards higher energies (up to 1 eV) at about 20 meV resolution. Thermal swarm measurements (Birmingham) were used to place the relative LPA cross sections on an absolute scale. A trochoidal electron monochromator (Innsbruck) was used for survey measurements of the relative cross sections for the different product anions over the energy range of 0-14 eV with a resolution of 0.30 eV. Combined with earlier beam data (taken at Berlin, J. Chem. Phys. 88 (1988) 149), the present experimental results provide a detailed set of partial cross sections for anion formation in low-energy electron collisions with SF$_5$Cl

Evaporative attachment of slow electrons to alkali nanoclusters

The abundance spectrum of Na^-_{n~7-140} anions formed by low energy electron attachment to free nanoclusters is measured to be strongly and nontrivially restructured with respect to the neutral precursor beam. This restructuring is explained in quantitative detail by a general framework of evaporative attachment: an electron is captured by the long-range polarization potential, its energy is transferred into thermal vibrations, and dissipated by evaporative cooling. The data also affirm a formulated relation between the binding energies of cationic, neutral, and anionic clusters, and an adjustment to the prior values of dimer evaporation energies.Comment: 9 pages, 3 figures, revise

What is in a pebble shape?

We propose to characterize the shapes of flat pebbles in terms of the statistical distribution of curvatures measured along the pebble contour. This is demonstrated for the erosion of clay pebbles in a controlled laboratory apparatus. Photographs at various stages of erosion are analyzed, and compared with two models. We find that the curvature distribution complements the usual measurement of aspect ratio, and connects naturally to erosion processes that are typically faster at protruding regions of high curvature.Comment: Phys. Rev. Lett. (to appear

The shape and erosion of pebbles

The shapes of flat pebbles may be characterized in terms of the statistical distribution of curvatures measured along their contours. We illustrate this new method for clay pebbles eroded in a controlled laboratory apparatus, and also for naturally-occurring rip-up clasts formed and eroded in the Mont St.-Michel bay. We find that the curvature distribution allows finer discrimination than traditional measures of aspect ratios. Furthermore, it connects to the microscopic action of erosion processes that are typically faster at protruding regions of high curvature. We discuss in detail how the curvature may be reliable deduced from digital photographs.Comment: 10 pages, 11 figure

Negative Ions in Space

Until a decade ago, the only anion observed to play a prominent role in astrophysics was H–. The bound–free transitions in H– dominate the visible opacity in stars with photospheric temperatures less than 7000 K, including the Sun. The H– anion is also believed to have been critical to the formation of molecular hydrogen in the very early evolution of the Universe. Once H₂ formed, about 500 000 years after the Big Bang, the expanding gas was able to lose internal gravitational energy and collapse to form stellar objects and “protogalaxies”, allowing the creation of heavier elements such as C, N, and O through nucleosynthesis. Although astronomers had considered some processes through which anions might form in interstellar clouds and circumstellar envelopes, including the important role that polycyclic aromatic hydrocarbons might play in this, it was the detection in 2006 of rotational line emission from C₆H– that galvanized a systematic study of the abundance, distribution, and chemistry of anions in the interstellar medium. In 2007, the Cassini mission reported the unexpected detection of anions with mass-to-charge ratios of up to ˜10 000 in the upper atmosphere of Titan; this observation likewise instigated the study of fundamental chemical processes involving negative ions among planetary scientists. In this article, we review the observations of anions in interstellar clouds, circumstellar envelopes, Titan, and cometary comae. We then discuss a number of processes by which anions can be created and destroyed in these environments. The derivation of accurate rate coefficients for these processes is an essential input for the chemical kinetic modeling that is necessary to fully extract physics from the observational data. We discuss such models, along with their successes and failings, and finish with an outlook on the future

A Role for the Juxtamembrane Cytoplasm in the Molecular Dynamics of Focal Adhesions

Focal adhesions (FAs) are specialized membrane-associated multi-protein complexes that link the cell to the extracellular matrix and play crucial roles in cell-matrix sensing. Considerable information is available on the complex molecular composition of these sites, yet the regulation of FA dynamics is largely unknown. Based on a combination of FRAP studies in live cells, with in silico simulations and mathematical modeling, we show that the FA plaque proteins paxillin and vinculin exist in four dynamic states: an immobile FA-bound fraction, an FA-associated fraction undergoing exchange, a juxtamembrane fraction experiencing attenuated diffusion, and a fast-diffusing cytoplasmic pool. The juxtamembrane region surrounding FAs displays a gradient of FA plaque proteins with respect to both concentration and dynamics. Based on these findings, we propose a new model for the regulation of FA dynamics in which this juxtamembrane domain acts as an intermediary layer, enabling an efficient regulation of FA formation and reorganization

Rho GTPase function in flies: insights from a developmental and organismal perspective.

Morphogenesis is a key event in the development of a multicellular organism and is reliant on coordinated transcriptional and signal transduction events. To establish the segmented body plan that underlies much of metazoan development, individual cells and groups of cells must respond to exogenous signals with complex movements and shape changes. One class of proteins that plays a pivotal role in the interpretation of extracellular cues into cellular behavior is the Rho family of small GTPases. These molecular switches are essential components of a growing number of signaling pathways, many of which regulate actin cytoskeletal remodeling. Much of our understanding of Rho biology has come from work done in cell culture. More recently, the fruit fly Drosophila melanogaster has emerged as an excellent genetic system for the study of these proteins in a developmental and organismal context. Studies in flies have greatly enhanced our understanding of pathways involving Rho GTPases and their roles in development

G-protein signaling: back to the future

Heterotrimeric G-proteins are intracellular partners of G-protein-coupled receptors (GPCRs). GPCRs act on inactive Gα·GDP/Gβγ heterotrimers to promote GDP release and GTP binding, resulting in liberation of Gα from Gβγ. Gα·GTP and Gβγ target effectors including adenylyl cyclases, phospholipases and ion channels. Signaling is terminated by intrinsic GTPase activity of Gα and heterotrimer reformation — a cycle accelerated by ‘regulators of G-protein signaling’ (RGS proteins). Recent studies have identified several unconventional G-protein signaling pathways that diverge from this standard model. Whereas phospholipase C (PLC) β is activated by Gαq and Gβγ, novel PLC isoforms are regulated by both heterotrimeric and Ras-superfamily G-proteins. An Arabidopsis protein has been discovered containing both GPCR and RGS domains within the same protein. Most surprisingly, a receptor-independent Gα nucleotide cycle that regulates cell division has been delineated in both Caenorhabditis elegans and Drosophila melanogaster. Here, we revisit classical heterotrimeric G-protein signaling and explore these new, non-canonical G-protein signaling pathways