On the problem of H2 formation

In the first part of the paper the framework for theoretical studies of the H₂ formation on interstellar grains will be shortly considered. In the second part the chemisorption of a hydrogen atom at a carbon grain will be examined and results of an ab initio perturbational approach are presented.     

Stable carbon isotope discrimination in rice field soil during acetate turnover by syntrophic acetate oxidation or acetoclastic methanogenesis

Rice fields are an important source for the greenhouse gas methane. In Italian rice field soil CH₄ is produced either by hydrogenotrophic and acetoclastic methanogenesis, or by hydrogenotrophic methanogenesis and syntrophic acetate oxidation when temperatures are below and above about 40-45 °C, respectively. In order to see whether these acetate consumption pathways differently discriminate the stable carbon isotopes of acetate, we measured the δ¹³C of total acetate and acetate-methyl as well as the δ¹³C of CO₂ and CH₄ in rice field soil that had been pre-incubated at 45 °C and then shifted to different temperatures between 25 and 50 °C. Acetate transiently accumulated to about 6 mM, which is about one-third of the amount of CH₄ produced, irrespective of the incubation temperature and the CH₄ production pathway involved. However, the patterns of δ¹³C of the CH₄ and CO₂ produced were different at low (25, 30, 35 °C) versus high (40, 45, 50 °C) temperatures. These patterns were consistent with CH₄ being exclusively formed by hydrogenotrophic methanogenesis at high temperatures, and by a combination of acetoclastic and hydrogenotrophic methanogenesis at low temperatures. The patterns of δ¹³C of total acetate and acetate-methyl were also different at high versus low temperatures, indicating the involvement of different pathways of production and consumption of acetate at the two temperature regimes. Isotope fractionation during consumption of the methyl group of acetate was more pronounced at low (α = 1.010-1.025) than at high (α = 1.0-1.01) temperatures indicating that acetoclastic methanogenesis exhibits a stronger isotope effect than syntrophic acetate oxidation. Small amounts of propionate also transiently accumulated and were analyzed for δ¹³C. The δ¹³C values slightly increased (by about 10‰) during production and consumption of propionate, but were not affected by incubation temperature. Collectively, our results showed distinct isotope discrimination for different paths of acetate (and propionate) production and consumption, albeit differences were only small, and discrimination between methanogenic and syntrophic acetate consumption in nature may be difficult to detect.     

The scattering of light by dielectric particles

We consider homogeneous particles with a frequency-dependent index of refractionn =n +in = 1 +(), ()1, in an outer electromagnetic field. From general considerations we derive an expression for the total scattering cross-section of these particles. We calculate the efficiency factor for the sphere, the infinite circular cylinder, and for an ellipsoidal particle with the main axesa=1,b=2,c=1. In contrast with the ray approximation, the present method can be used to calculate the polarization of the scattered light from ellipsoids.     

Evidence for anthropogenic surface loading as trigger mechanism of the 2008 Wenchuan earthquake

Two and a half years prior to China’s M7.9 Wenchuan earthquake of May 2008, at least 300 million metric tons of water accumulated with additional seasonal water level changes in the Minjiang River Valley at the eastern margin of the Longmen Shan. This article shows that static surface loading in the Zipingpu water reservoir induced Coulomb failure stresses on the nearby Beichuan thrust fault system at <17 km depth. Triggering stresses exceeded levels of daily lunar and solar tides and perturbed a fault area measuring 416 ± 96 km². These stress perturbations, in turn, likely advanced the clock of the mainshock and directed the initial rupture propagation upward towards the reservoir on the "Coulomb-like" Beichuan fault with rate- and state-dependent frictional behavior. Static triggering perturbations produced up to 60 years (0.6%) of equivalent tectonic loading, and show strong correlations to the coseismic slip. Moreover, correlations between clock advancement and coseismic slip, observed during the mainshock beneath the reservoir, are strongest for a longer seismic cycle (10kyr) of M > 7 earthquakes. Finally, the daily event rate of the micro-seismicity (M ≥ 0.5) correlates well with the static stress perturbations, indicating destabilization.     

On the geometrical cross-section of an N-mer

We have modeled the growth of an N-mer by means of Monte-Carlo calculations. The geometrical cross-section of the N-mer is measured and compared with the one obtained if one assumes that the N-mer is of spherical form. A difference up to a factor of 2 was found forN30.Paper presented at a Workshop on The Role of Dust in Dense Regions of Interstellar Matter, held at Georgenthal, G.D.R., in March 1986.