Scientific Publications 2005
2005. "Cleavage and recovery of molecular water in silica." Journal of Physical Chemistry B 109(21):10936-10945. Abstract The incorporation and reactivity of water molecules in the bulk phases of fused silica are investigated using density functional theory. A wide range of activation barriers for the reaction of water with Si-O bonds to form a pair of Si-OH (silanol) groups is found. The activation energy can be greater than that found in the bulk state of quartz and can also be very low with the possibility of having non-activated processes. The latter is indicative of the role of water in the annealing of silica glass structure and the ability of silica to retain water in the form of silanol groups.
2005. "Silica glass structure generation for ab initio calculations using small samples of amorphous silica." Physical Review. B, Condensed Matter 71(2):024208-1 - 024208-15. doi:10.1103/PhysRevB.71.024208 Abstract Calculations of processes that depend on electronic states or charge transfer in bulk glasses are challenging because of the limitations imposed by the need to use ab initio methods. Cluster-based calculations alone cannot capture the response of the network structure that occurs upon bond rupturing or other alteration of the local molecular framework. From the atomic level perspective, it is necessary that simulations of extended (bulk) amorphous systems sample a large number of possible geometrical arrangements of molecular structures as well as their frequency and relative spatial distribution. To acheive this in sample sizes amenable to ab initio methods, we create multiple controlled samples, that as an ensemble provide a good statistical sampling of the bulk structure. In this work, glass samples are first generated using classical molecular dynamics then transferred to ab initio molecular dynamics where the samples are optimized using density functional theory. The optimal structures obtained by classical molecular dynamics (MD) and ab initio MD methods are compared to each other and to the classical simulation of a larger system.
2005. "Flux Measurements of Volatile Organic Compounds from an Urban Landscape." Geophysical Research Letters 32(20):L20802, doi: 10.1029/2005GL023356. Abstract Direct measurements of volatile organic compound (VOC) emissions that include all anthropogenic and biogenic emission sources in urban areas are a missing requirement to evaluate emission inventories and constrain current photochemical modelling practices. Here we demonstrate the use of micrometeorological techniques coupled with fast-response sensors to measure urban VOC fluxes from a neighborhood of Mexico City, where the spatial variability of surface cover and roughness is high. Fluxes of olefins, methanol, acetone, toluene and C2-benzenes were measured and compared with the local gridded emission inventory. VOC fluxes exhibited a clear diurnal pattern with a strong relationship to vehicular traffic. Recent photochemical modeling results suggest that VOC emissions are significantly underestimated in Mexico City1, but the measured VOC fluxes described here indicate that the official emission inventory2 is essentially correct. Thus, other explanations are needed to explain the photochemical modelling results.
2005. "Spin-Polarized Conduction in Localized Ferromagnetic Materials: the case of Fe3O4 on MgO(100)." Journal of Applied Physics 98(8):084507(5). Abstract The surface electronic structure of 1500 °A thick Fe3O4(100) films has been investigated by spinresolved photoemission spectroscopy with vacuum ultra-violet (VUV) synchrotron radiation. The films, epitaxially grown on MgO(100) substrates using plasma assisted molecular beam deposition, are briefly exposed to air during transfer to the photoemission chamber. It is shown that clean surfaces of Fe3O4 films after exposure to air can be recovered through mild annealing in an oxygen atmosphere. The presence of the characteristic Verwey transition, as detected in the valence band photoemission spectra, provides evidence for the excellent electronic quality at the surface of these films. The top of the valence band is found to be negatively spin-polarized, with a value of ~ −50%; this result strongly points towards the localized nature of the 3d states in this system.