227 research outputs found
Correcting the polarization effect in low frequency Dielectric Spectroscopy
We demonstrate a simple and robust methodology for measuring and analyzing
the polarization impedance appearing at interface between electrodes and ionic
solutions, in the frequency range from 1 to Hz. The method assumes no
particular behavior of the electrode polarization impedance and it only makes
use of the fact that the polarization effect dies out with frequency. The
method allows a direct and un-biased measurement of the polarization impedance,
whose behavior with the applied voltages and ionic concentration is
methodically investigated. Furthermore, based on the previous findings, we
propose a protocol for correcting the polarization effect in low frequency
Dielectric Spectroscopy measurements of colloids. This could potentially lead
to the quantitative resolution of the -dispersion regime of live cells
in suspension
Quantum Transition State Theory for proton transfer reactions in enzymes
We consider the role of quantum effects in the transfer of hyrogen-like
species in enzyme-catalysed reactions. This study is stimulated by claims that
the observed magnitude and temperature dependence of kinetic isotope effects
imply that quantum tunneling below the energy barrier associated with the
transition state significantly enhances the reaction rate in many enzymes. We
use a path integral approach which provides a general framework to understand
tunneling in a quantum system which interacts with an environment at non-zero
temperature. Here the quantum system is the active site of the enzyme and the
environment is the surrounding protein and water. Tunneling well below the
barrier only occurs for temperatures less than a temperature which is
determined by the curvature of potential energy surface near the top of the
barrier. We argue that for most enzymes this temperature is less than room
temperature. For physically reasonable parameters quantum transition state
theory gives a quantitative description of the temperature dependence and
magnitude of kinetic isotope effects for two classes of enzymes which have been
claimed to exhibit signatures of quantum tunneling. The only quantum effects
are those associated with the transition state, both reflection at the barrier
top and tunneling just below the barrier. We establish that the friction due to
the environment is weak and only slightly modifies the reaction rate.
Furthermore, at room temperature and for typical energy barriers environmental
degrees of freedom with frequencies much less than 1000 cm do not have a
significant effect on quantum corrections to the reaction rate.Comment: Aspects of the article are discussed at
condensedconcepts.blogspot.co
Electron attachment to valence-excited CO
The possibility of electron attachment to the valence state of CO
is examined using an {\it ab initio} bound-state multireference configuration
interaction approach. The resulting resonance has symmetry;
the higher vibrational levels of this resonance state coincide with, or are
nearly coincident with, levels of the parent state. Collisional
relaxation to the lowest vibrational levels in hot plasma situations might
yield the possibility of a long-lived CO state.Comment: Revtex file + postscript file for one figur
Decoherence in trapped ions due to polarization of the residual background gas
We investigate the mechanism of damping and heating of trapped ions
associated with the polarization of the residual background gas induced by the
oscillating ions themselves. Reasoning by analogy with the physics of surface
electrons in liquid helium, we demonstrate that the decay of Rabi oscillations
observed in experiments on 9Be+ can be attributed to the polarization phenomena
investigated here. The measured sensitivity of the damping of Rabi oscillations
with respect to the vibrational quantum number of a trapped ion is also
predicted in our polarization model.Comment: 26 pdf pages with 5 figures, http://www.df.ufscar.br/~quantum
The Buffer Gas Beam: An Intense, Cold, and Slow Source for Atoms and Molecules
Beams of atoms and molecules are stalwart tools for spectroscopy and studies
of collisional processes. The supersonic expansion technique can create cold
beams of many species of atoms and molecules. However, the resulting beam is
typically moving at a speed of 300-600 m/s in the lab frame, and for a large
class of species has insufficient flux (i.e. brightness) for important
applications. In contrast, buffer gas beams can be a superior method in many
cases, producing cold and relatively slow molecules in the lab frame with high
brightness and great versatility. There are basic differences between
supersonic and buffer gas cooled beams regarding particular technological
advantages and constraints. At present, it is clear that not all of the
possible variations on the buffer gas method have been studied. In this review,
we will present a survey of the current state of the art in buffer gas beams,
and explore some of the possible future directions that these new methods might
take
Experimental evidence of the ferroelectric phase transition near the point in liquid water
We studied dielectric properties of nano-sized liquid water samples confined
in polymerized silicates MCM-41 characterized by the porous sizes \sim 3-10nm.
We report the direct measurements of the dielectric constant by the dielectric
spectroscopy method at frequencies 25Hz-1MHz and demonstrate clear signatures
of the second-order phase transition of ferroelectric nature at temperatures
next to the \lambda- point in the bulk supercooled water. The presented results
support the previously developed polar liquid phenomenology and hence establish
its applicability to model actual phenomena in liquid water.Comment: 4 pages, single figur
The Library as Safe Space
Purpose: This chapter will explain how libraries define safe space through policies, procedures, and professional codes of ethics. The chapter will generate a history of the concept of libraries as safe space, will explain how libraries attempt to create safe spaces in physical and online environments, and will show how library practices both help and harm patrons in need of safe space.
Design/methodology/approach: This chapter provides a review of the literature that illustrates how libraries provide safe space—or not—for their patrons. The author will deconstruct the ALA Code of Ethics and Bill of Rights to demonstrate how libraries remain heteronormative institutions that do not recognize the existence of diverse patrons or employees, and how this phenomenon manifests in libraries.
Findings: Libraries, either through their physical construction or through policies and procedures, have become spaces for illegal activities and discrimination. Populations who would be most likely to use libraries often report barriers to access.
Practical Implications: Libraries should revisit their policies and procedures, as well as assess their physical and online spaces, to determine whether or not they truly provide safe space for their patrons. While libraries can become safer spaces, they should clearly communicate what types of safety they actually provide.
Originality/value: This chapter offers a critique of libraries as safe spaces, which will challenge popular opinions of libraries, and compel the profession to improve
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