A study of the alkene-ozone reactions, 2,3-dimethyl 2-butene + O3 and 2-methyl propene + O3, with photoelectron spectroscopy: measurement of product branching ratios and atmospheric implications

The reactions of ozone with the alkenes 2,3-dimethyl 2-butene (DMB) and 2-methyl propene (2MP) have been investigated using a flow-tube interfaced to a u.v. photoelectron spectrometer. These reactions were studied at low pressure at different reagent partial pressures, both with the alkene in excess and ozone in excess. In each case, photoelectron spectra recorded as a function of time have been used to estimate partial pressures of the reagents and products as a function of time using photoionization cross-sections of selected photoelectron bands of the reagents and products, which were measured separately. The yields of all the main products have been determined, some of which have been measured in previous studies. For each reaction, oxygen was observed as a product for the first time and its yield was measured. Kinetics simulations were performed using reaction schemes which were developed for these reactions, which are consistent with that used earlier for the ozone-ethene reaction, in order to determine the main reactions for production of the products. The experimental product yields have been used in a global model to estimate their global annual emissions in the atmosphere. For example, for the reaction of O(3) with 2MP the formaldehyde, formic acid and acetone global annual emissions are calculated as 0.4 Tg, 25.0 Gg and 0.16 Tg respectively, which are estimated as 0.02, 0.3 and 0.2% of the total annual emission respectively. For the reaction of O(3) with DMB, the acetone yield is higher at 0.9 Tg which is approximately 1% of the total annual estimated emission.     

Computation of the analysis error covariance in variational data assimilation problems with nonlinear dynamics

The problem of variational data assimilation for a nonlinear evolution model is formulated as an optimal control problem to find the initial condition function. The data contain errors (observation and background errors), hence there will be errors in the optimal solution. For mildly nonlinear dynamics, the covariance matrix of the optimal solution error can often be approximated by the inverse Hessian of the cost functional. Here we focus on highly nonlinear dynamics, in which case this approximation may not be valid. The equation relating the optimal solution error and the errors of the input data is used to construct an approximation of the optimal solution error covariance. Two new methods for computing this covariance are presented: the fully nonlinear ensemble method with sampling error compensation and the ‘effective inverse Hessian’ method. The second method relies on the efficient computation of the inverse Hessian by the quasi-Newton BFGS method with preconditioning. Numerical examples are presented for the model governed by Burgers equation with a nonlinear viscous term.     

The free energy of a reaction coordinate at multiple constraints: a concise formulation

The free energy as a function of the reaction coordinate (rc) is the key quantity for the computation of equilibrium and kinetic quantities. When it is considered as the potential of mean force, the problem is the calculation of the mean force for given values of the rc. We reinvestigate the PMCF (potential of mean constraint force) method which applies a constraint to the rc to compute the mean force as the mean negative constraint force and a metric tensor correction. The latter allows for the constraint imposed to the rc and possible artefacts due to multiple constraints of other variables which for practical reasons are often used in numerical simulations. Two main results are obtained that are of theoretical and practical interest. First, the correction term is given a very concise and simple shape which facilitates its interpretation and evaluation. Secondly, a theorem describes various rcs and possible combinations with constraints that can be used without introducing any correction to the constraint force. The results facilitate the computation of free energy by molecular dynamics simulations.     

The trousers problem revisited

Anderson and DeWitt considered the quantization of a massless scalar field in a spacetime whose spacelike hypersurfaces change topology and concluded that the topology change gives rise to infinite particle and energy production. We show here that their calculations are insufficient and that their propagation rule is unphysical. However, our results using a more general propagation rule support their conclusion.     

Cosmic superstrings II

Modern string theory provides a rich array of objects which could potentially appear as cosmic strings in our observable four-dimensional universe. However, there are many possible decay modes which would prevent the appearance of such strings. We investigate the conditions under which metastable strings can exist, and we find that such strings are present in many models. Hence cosmic strings give a potentially large window into string physics. To cite this article: E.J. Copeland et al., C. R. Physique 5 (2004).

Résumé

La théorie moderne des supercordes fournit un choix riche d'objets qui pourraient potentiellement apparaître en tant que cordes cosmiques dans notre univers quadridimensionnel observable. Cependant, il y a beaucoup de modes possibles de désintégration qui empêcheraient l'apparition de telles cordes. Nous étudions les conditions dans lesquelles des cordes métastables peuvent exister, et nous constatons que de telles cordes sont présentes dans beaucoup de modèles. Par conséquent les cordes cosmiques ouvrent une fenêtre potentiellement grande vers la physique des supercordes. Pour citer cet article : E.J. Copeland et al., C. R. Physique 5 (2004).     

Model-independent dark energy differentiation with the integrated Sachs-Wolfe effect

We study the integrated Sachs-Wolfe effect using a model-independent parametrization of the dark energy equation of state, w(z). Cosmic variance severely restricts the class of models distinguishable from one based on cold dark matter and a cosmological constant unless w(z) currently satisfies w(o)(Q)>-0.8, or exhibits a rapid, late-time, transition at redshifts z<3. Because of the degeneracy with other cosmological parameters, models with a slowly varying w(z) cannot be differentiated from each other or from a cosmological constant. This may place a fundamental limit on our understanding of the origin of the currently observed acceleration.     

In silico modeling of functionalized graphene oxide-metal cluster conjugates as Raman probe: Raman activity of pyridine

Functionalized graphene – metal nano-conjugates are used as Raman probes, in recent years, for trace level identification of materials having specific Raman active modes. In the present paper, model Raman probes were modeled through conjugation of Au₄ and Ag₄ clusters with functionalized graphene systems. In silico models of functionalized (5,5)-graphene sheets were designed at the density functional theory (DFT) level through attachments of epoxy, -OH and –NH(CH₃)₂SH/-CONH(CH₃)₂SH groups. Model Raman probes were designed through attachment of Au₄ and Ag₄ clusters to the functional sites. Full geometry optimizations followed by vibrational analysis were carried out to ensure that the designed Raman probes have acceptable geometric characteristics to attach Raman-active molecules to the metal site. Pyridine was used as a test system to investigate the functionality of such model Raman probes through attachment with the metal clusters. It was observed that the chemical effects due to such attachments increase the Raman intensities (RI) of specific Raman modes of pyridine (in-plane symmetric bending (1040 cm^-1) and asymmetric stretch-bend (1634 cm^-1)), which are too weak in the isolated molecule. Furthermore, the suggested in silico system could provide an important model for basic understanding of RI-enhancements of molecules through increase of the size of the metal clusters, as the observed enhancement was found to be dependent on the polarizability of the metal clusters attached to the molecule of interest.