The coadsorption of potassium and co on the pt(111) crystal surface: A TDS,HREELS and UPS study

The interaction of CO with a potassium covered Pt(111) surface is investigated using thermal desorption (TDS), high resolution electron energy loss (HREELS) and ultraviolet photoelectron (UPS) spectroscopies. When submonolayer amounts of potassium are preadsorbed, the adsorption energy of CO increases from 25 to 36 kcal/mole, while substantial shifts in the site occupancy from the linear to the bridged site are observed. The CO stretching vibrational frequencies are shown to decrease continuously with either increasing potassium coverage or decreasing CO coverage. A minimum CO stretching frequency of 1400 cm^?1 is observed, indicative of a CO bond order of 1.5. The work function decreases by up to 4.5 eV at submonolayer potassium coverages, but then increases by 1.5 eV upon CO co-adsorption. The results indicate that the large adsorption energy, vibrational frequency and work function changes are due to molecular CO adsorption with a substantial charge donation from potassium through the platinum substrate and into the 2π¹_CO orbital.     

Analysis of HRTEM images for carbon nanostructure quantification

Image processing algorithms have been developed to extract fringe length, tortuosity and separation from high resolution transmission electron microscopy images. To validate the separation algorithm, a comparison is made between the image-based fringe separation and that obtained by analysis of X-ray diffraction data for a progressively heat-treated carbon black. Agreement is favorable. To illustrate the utility of the analysis parameters for a range of carbon nanostructures, analysis is applied to a series of pyrolytically prepared carbon soots – qualitatively described as containing amorphous, graphitic or fullerenic nanostructure. For all processing, the intermediate image, in the form of a skeletonized binary image of the original high resolution transmission electron micrograph, is shown and found to accurately reflect the nanostructural organization within the carbon as visually observed. Statistical results for each analysis parameter, extracted from the binary images, are presented in the form of histograms and quantitatively distinguish the different carbon nanostructures.     

Temperature dependence of photoinduced electron transfer within self-associated porphyrin: guanine monophosphate complexes

In the current report, the temperature dependence of photoinduced electron transfer between tetrakis-(4-tetramethylpyridyl)porphine (T4MPyP) and guanine monophosphate (GMP) has been examined. In the presence of GMP the fluorescence lifetime analysis reveals a Lorentzian distribution of lifetimes centered at 0.7 ns with a width of 0.9 ns displaying significant temperature dependence. Fitting temperature dependent data to the Marcus equation gives a reorganizational energy (λ) for the electron transfer reaction of 0.6 eV and an electronic coupling factor (H_AB) of 3×10⁻³ eV. These results suggest conformational regulation of electron transfer within the non-covalent porphyrin:nucleotide complex.     

The millimeter-wave spectrum of acetaldehyde in its two lowest torsional states

A large number of new millimeter-wave spectral lines of gaseous acetaldehyde have been measured at frequencies up to 250 GHz. These lines arise from rotational transitions of acetaldehyde in its two lowest (v_t = 0,1) torsional states and involve angular momentum quantum numbers J ≤ 12 and K ≤ 10. A global data set consisting of 562 lines has been obtained by combining the millimeter-wave lines with previously measured lower frequency data involving the two lowest torsional states. This data set has been analyzed via an internal axis method previously used to study the spectra of CH₃OH and CH₃SH. The root-mean-square deviation of the fit is only 685 kHz. An analogous least-squares fit to 335v_t = 0 lines yields a root-mean-square deviation of 269 kHz.     

Gasuntersuchungen

Ohne Zusammenfassung     

Phase diagram of a geometrically frustrated triangular-lattice antiferromagnet in a magnetic field

The magnetic phase diagram of a geometrically frustrated triangular-lattice antiferromagnet is evaluated as a function of magnetic field and anisotropy using a trial spin state built from harmonics of a fundamental ordering wave vector. A noncollinear incommensurate state, observed to be chiral and ferroelectric in CuFeO2, appears above a collinear state with 4 sublattices (SLs). The apparent absence of multiferroic behavior for predicted chiral, noncollinear 5-SL states poses a challenge to theories of the ferroelectric coupling in CuFeO2.     

Characterization of an actively linearized ultrabroadband chirped laser with a fiber-laser optical frequency comb

The optical frequency sweep of an actively linearized, ultrabroadband, chirped laser source is characterized through optical heterodyne detection against a fiber-laser frequency comb. Frequency sweeps were measured over approximately 5 THz bandwidths from 1530 nm to 1570 nm. The dominant deviation from linearity resulted from the nonzero dispersion of the fiber delay used as a reference for the sweep linearization. Removing the low-order dispersion effects, the residual sweep nonlinearity was less than 60 kHz rms, corresponding to a constant chirp with less than 15 ppb deviation across the 5 THz sweep.     

Expectation-maximization for Bayes-adaptive POMDPs

Bayes-adaptive POMDPs (BAPOMDPs) are partially observable Markov decision problems in which uncertainty in the state-transition and observation-emission probabilities can be captured by a prior distribution over the model parameters. Existing approaches to solving BAPOMDPs rely on model and trajectory sampling to guide exploration and, because of the curse of dimensionality, do not scale well when the degree of model uncertainty is large. In this paper, we begin by presenting two expectation-maximization (EM) approaches to solving BAPOMPs via finite-state controller (FSC) optimization, which at their foundation are extensions of existing EM algorithms for BAMDPs to the more general BAPOMDP setting. The first is a sampling-based EM algorithm that optimizes over a finite number of models drawn from the BAPOMDP prior, and as such is only appropriate for smaller problems with limited model uncertainty; the second approach leverages variational Bayesian methods to ensure tractability without sampling, and is most appropriate for larger domains with greater model uncertainty. Our primary novel contribution is the derivation of the constrained VB-EM algorithm, which addresses an unfavourable preference that often arises towards a certain class of policies when applying the standard VB-EM algorithm. Through an empirical study we show that the sampling-based EM algorithm is competitive with more conventional sampling-based approaches in smaller domains, and that our novel constrained VB-EM algorithm can generate quality solutions in larger domains where sampling-based approaches are no longer viable.