Transient x-ray absorption spectroscopy of hydrated halogen atom

Time-resolved x-ray absorption spectroscopy monitors the transient species generated by one-photon detachment of an electron from aqueous bromide. Hydrated bromine atoms with a lifetime of ca. 17 ns were observed, nearly half of which react with excess Br- to form Br2-. The K-edge spectra of the Br atom and Br2- anion exhibit distinctive resonant transitions that are absent for the Br- precursor. The absorption spectra indicate that the solvent shell around a Br0 atom is defined primarily by hydrophobic interactions, in agreement with a Monte Carlo simulation of the solvent structure.Comment: 6 pages, 4 figures + supplement, will be submitted to PR

An x-ray probe of laser-aligned molecules

We demonstrate a hard x-ray probe of laser-aligned small molecules. To align small molecules with optical lasers, high intensities at nonresonant wavelengths are necessary. We use 95 ps pulses focused to 40 mum from an 800 nm Ti:sapphire laser at a peak intensity of 10^12 W/cm^2 to create an ensemble of aligned bromotrifluoromethane (CF3Br) molecules. Linearly polarized, 120 ps x-ray pulses, focused to 10 mum, tuned to the Br 1s --> sigma* pre-edge resonance at 13.476 keV, probe the ensemble of laser-aligned molecules. The demonstrated methodology has a variety of applications and can enable ultrafast imaging of laser-controlled molecular motions with Angstrom-level resolution.Comment: 4 pages, 4 figures, RevTeX4, corrected typo

Lobster Fishing on the Maine Coast: Past and Present

An article on lobster fishing on the Maine coast that appeared in The American Neptune, Volume 14, Issue 1, p. 203-208, 1954

Learning from the early adopters: developing the digital practitioner

This paper explores how Sharpe and Beetham’s Digital Literacies Framework which was derived to model students’ digital literacies, can be applied to lecturers’ digital literacy practices. Data from a small-scale phenomenological study of higher education lecturers who used Web 2.0 in their teaching and learning practices are used to examine if this pyramid model represents their motivations for adopting technology-enhanced learning in their pedagogic practices. The paper argues that whilst Sharpe and Beetham’s model has utility in many regards, these lecturers were mainly motivated by the desire to achieve their pedagogic goals rather than by a desire to become a digital practitioner

Bodies, technologies and action possibilities: when is an affordance?

Borrowed from ecological psychology, the concept of affordances is often said to offer the social study of technology a means of re-framing the question of what is, and what is not, ‘social’ about technological artefacts. The concept, many argue, enables us to chart a safe course between the perils of technological determinism and social constructivism. This article questions the sociological adequacy of the concept as conventionally deployed. Drawing on ethnographic work on the ways technological artefacts engage, and are engaged by, disabled bodies, we propose that the ‘affordances’ of technological objects are not reducible to their material constitution but are inextricably bound up with specific, historically situated modes of engagement and ways of life

Integrated tunneling sensor for nanoelectromechanical systems

Transducers based on quantum mechanical tunneling provide an extremely sensitive sensor principle, especially for nanoelectromechanical systems. For proper operation a gap between the electrodes of below 1nm is essential, requiring the use of structures with a mobile electrode. At such small distances, attractive van der Waals and capillary forces become sizable, possibly resulting in snap-in of the electrodes. The authors present a comprehensive analysis and evaluation of the interplay between the involved forces and identify requirements for the design of tunnelingsensors. Based on this analysis, a tunnelingsensor is fabricated by Si micromachiningtechnology and its proper operation is demonstrated

Atomistic characterization of the active-site solvation dynamics of a model photocatalyst

The interactions between the reactive excited state of molecular photocatalysts and surrounding solvent dictate reaction mechanisms and pathways, but are not readily accessible to conventional optical spectroscopic techniques. Here we report an investigation of the structural and solvation dynamics following excitation of a model photocatalytic molecular system [Ir-2(dimen)(4)](2+), where dimen is para-diisocyanomenthane. The time-dependent structural changes in this model photocatalyst, as well as the changes in the solvation shell structure, have been measured with ultrafast diffuse X-ray scattering and simulated with Born-Oppenheimer Molecular Dynamics. Both methods provide direct access to the solute-solvent pair distribution function, enabling the solvation dynamics around the catalytically active iridium sites to be robustly characterized. Our results provide evidence for the coordination of the iridium atoms by the acetonitrile solvent and demonstrate the viability of using diffuse X-ray scattering at free-electron laser sources for studying the dynamics of photocatalysis.1

Comprehensive characterization of molecular interactions based on nanomechanics

Molecular interaction is a key concept in our understanding of the biological mechanisms of life. Two physical properties change when one molecular partner binds to another. Firstly, the masses combine and secondly, the structure of at least one binding partner is altered, mechanically transducing the binding into subsequent biological reactions. Here we present a nanomechanical micro-array technique for bio-medical research, which not only monitors the binding of effector molecules to their target but also the subsequent effect on a biological system in vitro. This label-free and real-time method directly and simultaneously tracks mass and nanomechanical changes at the sensor interface using micro-cantilever technology. To prove the concept we measured lipid vesicle (approximately 748*10(6) Da) adsorption on the sensor interface followed by subsequent binding of the bee venom peptide melittin (2840 Da) to the vesicles. The results show the high dynamic range of the instrument and that measuring the mass and structural changes simultaneously allow a comprehensive discussion of molecular interactions