Potential-dependent competitive processes on platinum in acid solution in the presence of propargyl alcohol

The electroadsorption interactions between dilute propargyl alcohol and smooth platinum in 0.5 M H2SO4 at 25° C have been investigated at different potentials. When the platinum electrode is covered by H atoms the hydrogenation of propargyl alcohol takes place. Otherwise, when the platinum electrode is held at a potential located in the double-layer region, the electroadsorption of propargyl alcohol involves a disruptive electro-oxidation yielding strongly bound adsorbed residues. The latter can be electrodesorbed presumably as carbon dioxide and protons. This process occurs within the O-electroadsorption potential range. The complete residue electrodesorption requires several voltammetric cycles. Competition between the hydrogenation reaction and the disruptive electroadsorption process can be observed at potentials where the platinum surface is partially covered by H atoms.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

A refined analysis of the low-mass eclipsing binary system T-Cyg1-12664

The observational mass-radius relation of main sequence stars with masses between ~0.3 and 1.0 Msun reveals deviations between the stellar radii predicted by models and the observed radii of stars in detached binaries. We generate an accurate physical model of the low-mass eclipsing binary T-Cyg1-12664 in the Kepler mission field to measure the physical parameters of its components and to compare them with the prediction of theoretical stellar evolution models. We analyze the Kepler mission light curve of T-Cyg1-12664 to accurately measure the times and phases of the primary and secondary eclipse. In addition, we measure the rotational period of the primary component by analyzing the out-of-eclipse oscillations that are due to spots. We accurately constrain the effective temperature of the system using ground-based absolute photometry in B, V, Rc, and Ic. We also obtain and analyze V, Rc, Ic differential light curves to measure the eccentricity and the orbital inclination of the system, and a precise Teff ratio. From the joint analysis of new radial velocities and those in the literature we measure the individual masses of the stars. Finally, we use the PHOEBE code to generate a physical model of the system. T-Cyg1-12664 is a low eccentricity system, located d=360+/-22 pc away from us, with an orbital period of P=4.1287955(4) days, and an orbital inclination i=86.969+/-0.056 degrees. It is composed of two very different stars with an active G6 primary with Teff1=5560+/-160 K, M1=0.680+/-0.045 Msun, R1=0.799+/-0.017 Rsun, and a M3V secondary star with Teff2=3460+/-210 K, M2=0.376+/-0.017 Msun, and R2=0.3475+/-0.0081 Rsun. The primary star is an oversized and spotted active star, hotter than the stars in its mass range. The secondary is a cool star near the mass boundary for fully convective stars (M~0.35 Msun), whose parameters appear to be in agreement with low-mass stellar model.Comment: 18 pages, 15 figures, 15 table

A contribution to the mechanism of “reduced” CO₂ adsorbates electro-oxidation from combined spectroelectrochemical and voltammetric data

The nature of reduced CO2 adsorbates, as well as the mechanisms for their electro-oxidation on platinum, have been reviewed through the light of new experimental data obtained by cyclic voltammetry and by Fourier transform infrared reflectance spectroscopy. Three different “reduced” CO2 adsorbates are described as “‘ensembles”. It is suggested that they involve different extents of adsorbed entities, among which weakly bound and strongly bound hydrogen atoms play the most important role.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Factorizing the time evolution operator

There is a widespread belief in the quantum physical community, and in textbooks used to teach Quantum Mechanics, that it is a difficult task to apply the time evolution operator Exp{-itH/h} on an initial wave function. That is to say, because the hamiltonian operator generally is the sum of two operators, then it is a difficult task to apply the time evolution operator on an initial wave function f(x,0), for it implies to apply terms operators like (a+b)^n. A possible solution of this problem is to factorize the time evolution operator and then apply successively the individual exponential operator on the initial wave function. However, the exponential operator does not directly factorize, i. e. Exp{a+b} is not equal to Exp{a}Exp{b}. In this work we present a useful procedure for factorizing the time evolution operator when the argument of the exponential is a sum of two operators, which obey specific commutation relations. Then, we apply the exponential operator as an evolution operator for the case of elementary unidimensional potentials, like the particle subject to a constant force and the harmonic oscillator. Also, we argue about an apparent paradox concerning the time evolution operator and non-spreading wave packets addressed previously in the literature.Comment: 24 pages; added references; one figure change

PG 1211+143: probing high frequency lags in a high mass AGN

We present the timing analysis of the four archived XMM-Newton observations of PG 1211+143. The source is well-known for its spectral complexity, comprising a strong soft-excess and different absorption systems. Soft energy band (0.3-0.7 keV) lags are detected over all the four observations, in the frequency range \nu \lsim 6 \times 10^{-4} Hz, where hard lags, similar to those observed in black hole X-ray binaries, are usually detected in smaller mass AGN. The lag magnitude is energy-dependent, showing two distinct trends apparently connectable to the two flux levels at which the source is observed. The results are discussed in the context of disk- and/or corona-reprocessing scenarios, and of disk wind models. Similarities with the high-frequency negative lag of 1H 0707-495 are highlighted, and, if confirmed, they would support the hypothesis that the lag in PG 1211+143 represents the signature of the same underlying mechanism, whose temporal characteristics scale with the mass of the central object.Comment: 6 pages, 6 figures, accepted for publication in MNRAS Letter

Multiple adsorbate interactions between reduced CO₂ adsorbates and either allyl alcohol or propargyl alcohol residues on platinum in 0.5 M sulphuric acid

The interaction of propargyl alcohol and allyl alcohol with a platinum (Pt) electrode modified by the presence of reduced CO2 adsorbates (r-CO2) is studied through voltammetry and potentiostatic current transients by using a flow cell technique. The interaction between each alcohol and r-CO2 is promoted at three constant potentials corresponding to different degrees of Pt surface coverage by H-atoms. For both alcohols the anodic stripping peaks and the charge balance are interpreted in terms of the formation of composed adsorbates. Based upon the probable r-CO2 structure and H-bonding interactions between r-CO2 and the alcohol molecules or their adsorbed residues, average alcohol residues/r-CO2 molecular ratios are estimated.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Dynamics of zonal flow-like structures in the edge of the TJ-II stellarator

The dynamics of fluctuating electric field structures in the edge of the TJ-II stellarator, that display zonal flow-like traits, is studied. These structures have been shown to be global and affect particle transport dynamically [J.A. Alonso et al., Nucl. Fus. 52 063010 (2012)]. In this article we discuss possible drive (Reynolds stress) and damping (Neoclassical viscosity, geodesic transfer) mechanisms for the associated ExB velocity. We show that: (a) while the observed turbulence-driven forces can provide the necessary perpendicular acceleration, a causal relation could not be firmly established, possibly because of the locality of the Reynolds stress measurements, (b) the calculated neoclassical viscosity and damping times are comparable to the observed zonal flow relaxation times, and (c) although an accompanying density modulation is observed to be associated to the zonal flow, it is not consistent with the excitation of pressure side-bands, like those present in geodesic acoustic oscillations, caused by the compression of the ExB flow field

Potential-dependent competitive processes on platinum in acid solution in the presence of propargyl alcohol

The electroadsorption interactions between dilute propargyl alcohol and smooth platinum in 0.5 M H2SO4 at 25° C have been investigated at different potentials. When the platinum electrode is covered by H atoms the hydrogenation of propargyl alcohol takes place. Otherwise, when the platinum electrode is held at a potential located in the double-layer region, the electroadsorption of propargyl alcohol involves a disruptive electro-oxidation yielding strongly bound adsorbed residues. The latter can be electrodesorbed presumably as carbon dioxide and protons. This process occurs within the O-electroadsorption potential range. The complete residue electrodesorption requires several voltammetric cycles. Competition between the hydrogenation reaction and the disruptive electroadsorption process can be observed at potentials where the platinum surface is partially covered by H atoms.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Molecular dynamics simulations of nanosheets of polymeric carbon nitride and comparison with experimental observations

A computational study of the properties of polymeric carbon nitride using molecular dynamics is presented. The analysis of ideal infinite-extent sheets permits to evaluate the effect of temperature on the network of hydrogen bonds responsible for the linkage of the material. The weakening of this binding mechanism at sufficiently high temperatures, together with the inter-layer interactions characteristic of this type of 2D materials, is shown to determine the conformation properties of polymeric carbon nitride at the nanoscale. The results obtained from the simulation of finite samples in the canonical ensemble at varying temperatures are consistent with those from the characterization of our experimentally synthesized samples. Hydrogen bonding between adjacent polymer ribbons leads this process and is the cause of the typical crumpled structure of this material