La montaña rusa de las elecciones norteamericanas de 2008

El año 2008 verá las elecciones presidenciales norteamericanas más abiertas e interesantes desde hace décadas. El año 2008 verá las elecciones norteamericanas más abiertas desde hace décadas puesto que ni un presidente en ejercicio ni un vicepresidente en ejercicio se presentan a los comicios. La impopularidad de George W. Bush ha proporcionado a Barack Obama y a los demócratas un fácil eslogan de campaña: “Cambio”. Por su parte, John McCain no es en absoluto el típico republicano, razón por la cual la contienda está siendo relativamente reñida. Sin embargo, EEUU ha cambiado menos de lo que algunos podrían creer. Sus elites siguen estando divididas sobre cuestiones sociales, su economía es su principal preocupación, sus elecciones siguen siendo decididas por votantes de la clase trabajadora blanca y puede hacer surgir todavía a personajes fascinantes y maravillosos en tiempos de crisis

Effects of vibrational excitation on the F + H2O → HF + OH reaction: dissociative photodetachment of overtone-excited [F-H-OH].

The reaction F + H2O → HF + OH is a four-atom system that provides an important benchmark for reaction dynamics. Hydrogen atom transfer at the transition state for this reaction is expected to exhibit a strong dependence on reactant vibrational excitation. In the present study, the vibrational effects are examined by photodetachment of vibrationally excited F-(H2O) precursor anions using photoelectron-photofragment coincidence (PPC) spectroscopy and compared with full six-dimensional quantum dynamical calculations on ab initio potential energy surfaces. Prior to photodetachment at hνUV = 4.80 eV, the overtone of the ionic hydrogen bond mode in the precursor F-(H2O), 2νIHB at 2885 cm-1, was excited using a tunable IR laser. Experiment and theory show that vibrational energy in the anion can be effectively carried away by the photoelectron upon a Franck-Condon photodetachment, and also show evidence for an increase of branching into the F + H2O reactant channel. The experimental results suggest a greater role for product rotational excitation than theory. Improved potential energy surfaces and longer wavepacket propagation times would be helpful to further examine the nature of the discrepancy

Theoretical/experimental Comparison of Deep Tunneling Decay of Quasi-bound H(D)OCO to H(D) + CO₂

The measured H(D)OCO survival fractions of the photoelectron-photofragment coincidence experiments by the Continetti group are qualitatively reproduced by tunneling calculations to H(D) + CO2 on several recent ab initio potential energy surfaces for the HOCO system. the tunneling calculations involve effective one-dimensional barriers based on steepest descent paths computed on each potential energy surface. the resulting tunneling probabilities are converted into H(D)OCO survival fractions using a model developed by the Continetti group in which every oscillation of the H(D)-OCO stretch provides an opportunity to tunnel. Four different potential energy surfaces are examined with the best qualitative agreement with experiment occurring for the PIP-NN surface based on UCCSD(T)-F12a/AVTZ electronic structure calculations and also a partial surface constructed for this study based on CASPT2/AVDZ electronic structure calculations. These two surfaces differ in barrier height by 1.6 kcal/mol but when matched at the saddle point have an almost identical shape along their reaction paths. the PIP surface is a less accurate fit to a smaller ab initio data set than that used for PIP-NN and its computed survival fractions are somewhat inferior to PIP-NN. the LTSH potential energy surface is the oldest surface examined and is qualitatively incompatible with experiment. This surface also has a small discontinuity that is easily repaired. on each surface, four different approximate tunneling methods are compared but only the small curvature tunneling method and the improved semiclassical transition state method produce useful results on all four surfaces. the results of these two methods are generally comparable and in qualitative agreement with experiment on the PIP-NN and CASPT2 surfaces. the original semiclassical transition state theory method produces qualitatively incorrect tunneling probabilities on all surfaces except the PIP. the Eckart tunneling method uses the least amount of information about the reaction path and produces too high a tunneling probability on PIP-NN surface, leading to survival fractions that peak at half their measured values

Stabilities of nanohydrated thymine radical cations: insights from multiphoton ionization experiments and ab initio calculations

Multi-photon ionization experiments have been carried out on thymine-water clusters in the gas phase. Metastable H2O loss from T+(H2O)n was observed at n ≥ 3 only. Ab initio quantum-chemical calculations of a large range of optimized T+(H2O)n conformers have been performed up to n = 4, enabling binding energies of water to be derived. These decrease smoothly with n, consistent with the general trend of increasing metastable H2O loss in the experimental data. The lowest-energy conformers of T+(H2O)3 and T+(H2O)4 feature intermolecular bonding via charge-dipole interactions, in contrast with the purely hydrogen-bonded neutrals. We found no evidence for a closed hydration shell at n = 4, also contrasting with studies of neutral clusters

Territorio de derechos: derechos humanos desde el rol de la Municipalidad y sus trabajadores

Nuestra propuesta nace ante el interés y la necesidad de poder brindar a un municipio cercano a la Facultad de Humanidades, Artes y Ciencias Sociales de la Universidad Autónoma de Entre Ríos, Sede Concepción del Uruguay, herramientas para pensar y abordar la cuestión de los Derechos Humanos desde el rol del Estado, y a la municipalidad como institución estatal de primera línea en contacto con la sociedad y que cuenta con la responsabilidad de bregar por el bienestar y el crecimiento de su comunidad. Por este motivo, y de forma conjunta, municipio y universidad, llegaron a un acuerdo y se propuso un cursotaller que trabaje sobre estas temáticas, teniendo en cuenta el rol, tanto de la institución como la de sus trabajadores, a la hora de brindar respuestas a las rápidas y cambiantes necesidades del entorno social y, a la vez, criticar o dar postura sobre discursos que circulan y ocultan intencionalidades que van en contra de los valores democráticos y la tolerancia pacífica entre los/las ciudadanos/as.Universidad Nacional de La Plat

Dissociation dynamics and stability of cyclopentoxy and cyclopentoxide

Abstract Cyclopentoxide c-C 5 H 9 O À undergoes photodetachment to stable cyclopentoxy or the ring-opened 5-oxo-pentan-1-yl radical and dissociative photodetachment, yielding C 3 H 5 O and C 2 H 4 photofragments, at both 532 and 355 nm. The adiabatic electron affinity of c-C 5 H 9 O À is estimated from the experimental results and ab intio calculations to be 1:5 AE 0:1 eV. The results show that c-C 5 H 9 O À is stable relative to dissociation into C 3 H 5 O À and C 2 H 4 by 1:23 AE 0:07 eV, whereas c-C 5 H 9 O is unstable relative to C 3 H 5 O and C 2 H 4 by À0:12 AE 0:12 eV. These results are discussed in terms of the factors affecting the stability of cyclic alkoxides and the corresponding alkoxy radicals

An ion mobility mass spectrometer for investigating photoisomerization and photodissociation of molecular ions

An ion mobility mass spectrometry apparatus for investigating the photoisomerization and photodissociation of electrosprayed molecular ions in the gas phase is described. The device consists of a drift tube mobility spectrometer, with access for a laser beam that intercepts the drifting ion packet either coaxially or transversely, followed by a quadrupole mass filter. An ion gate halfway along the drift region allows the instrument to be used as a tandem ion mobility spectrometer, enabling mobility selection of ions prior to irradiation, with the photoisomer ions being separated over the second half of the drift tube. The utility of the device is illustrated with photoisomerization and photodissociation action spectra of carbocyanine molecular cations. The mobility resolution of the device for singly charged ions is typically 80 and it has a mass range of 100-440 Da, with the lower limit determined by the drive frequency for the ion funnels, and the upper limit by the quadrupole mass filter

Photoelectron-Photofragment Coincidence Spectroscopy With Ions Prepared in a Cryogenic Octopole Accumulation Trap: Collisional Excitation and Buffer Gas Cooling

A cryogenic octopole accumulation trap (COAT) has been coupled to a photoelectron-photofragment coincidence (PPC) spectrometer allowing for improved control over anion vibrational excitation. The anions are heated and cooled via collisions with buffer gas <17 K. Shorter trapping times (500 μs) prevent thermalization and result in anions with high internal excitation while longer trapping times (80 ms) at cryogenic temperatures thermalize the ions to the temperature of the buffer gas. The capabilities of the COAT are demonstrated using PPC spectroscopy of O3- at 388 nm (Ehν = 3.20 eV). Cooling the precursor anions with COAT resulted in the elimination of the autodetachment of vibrationally excited O2- produced by the photodissociation O3- + hν → O + O2-(v ≥ 4). Under heating conditions, a lower limit temperature for the anions was determined to be 1,500 K through Franck-Condon simulations of the photodetachment spectrum of O3-, considering a significant fraction of the ions undergo photodissociation in competition with photodetachment. The ability to cool or heat ions by varying ion injection and trapping duration in COAT provides a new flexibility for studying the spectroscopy of cold ions as well as thermally activated processes

New Journal of Physics Measuring positron-atom binding energies through laser-assisted photorecombination

Abstract. Described here is a proposed experiment to use laser-assisted photorecombination of positrons from a trap-based beam and metal atoms in the gas phase to measure positron-atom binding energies. Signal rates are estimated, based in part upon experience studying resonant annihilation spectra using a trapbased positron beam. While positrons are important in many areas of science and technology including materials science, medicine and astrophysics, there are a number of open, fundamental questions regarding positron interactions with ordinary matter. One such topic is positron binding to atoms and molecules. There are accurate theoretical calculations of positron binding to atoms The goal of this paper is the development of a method to study positron-atom bound states using laser-assisted photorecombination, which should lead to enhanced annihilation

Time-resolved predissociation of the vibrationless level of the B state of CH3I

The predissociation dynamics of the vibrationless level of the first Rydberg state 6s (B 2E) state of CH3I has been studied by femtosecond-resolved velocity map imaging of both the CH3 and I photofragments. The kinetic energy distributions of the two fragments have been recorded as a function of the pump-probe delay, and as a function of excitation within the umbrella and stretching vibrational modes of the CH3 fragment. These observations are made by using (2+1) Resonant Enhanced MultiPhoton Ionization (REMPI) via the 3pz 2A2" state of CH3 to detect specific vibrational levels of CH3. The vibrational branching fractions of the CH3 are recovered by using the individual vibrationally state-selected CH3 distributions to fit the kinetic energy distribution obtained by using nonresonant multiphoton ionization of either the I or CH3 fragment. The angular distributions and rise times of the two fragments differ significantly. These observations can be rationalized through a consideration of the alignment of the CH3 fragment and the effect of this alignment on its detection efficiency. Two extra dissociation channels are detected: one associated with Rydberg states near 9.2 eV that were observed previously in photoelectron studies, and one associated with photodissociation of the parent cation around 15 eV.Comment: submitted Physical Chemistry Chemical Physics (2011