ALBERT

Notes: Asymmetrically cut perfect crystals, in both the Laue and Bragg geometries, are examined as single crystal monochromators for x-ray beams that are collimated to a small fraction of the Darwin width, as is typical in experiments with coherent x rays. Both the Laue and asymmetric Bragg geometries are plagued by an inherent chromatic aberration that increases the beam divergence much beyond that of the symmetric Bragg geometry. Measurements from a recent experiment at the ESRF are presented to compare Si(220) (symmetric Bragg), diamond(111) (asymmetric Laue), and diamond(111) (symmetric Bragg inclined) geometries. © 1995 American Institute of Physics.

Notes: A scanning tunneling microscope in ultrahigh vacuum has been used to investigate the growth, morphology, and surface atomic structure of ultrathin titanium silicide films on Si(111) substrates. Microstructural considerations have been used to identify various stages of the silicide growth. Atomic resolution images of a titanium silicide crystallite facet, formed at 850 °C, have been identified as a 2×2 silicon termination of a C54-TiSi2(010) surface. Possible epitaxial silicide/silicon relationships are provided. Theoretical consideration has been given to the interatomic bonding in the C54-TiSi2 lattice and the dangling bond density of ideally terminated silicide planes has been calculated. The highly reconstructed atomically flat surface of a large crystallite, formed at 1200 °C, has been assigned as a C54-TiSi2(311) plane giving the epitaxial relation C54-TiSi2(311)(parallel)Si(111). The presence of pairs and linear chains of defects, with common orientations, is attributed to the decomposition of a diatomic gas on the facet, producing sites of preferential adsorption on the silicide surface. © 1995 American Institute of Physics.

Notes: A scanning tunneling microscope (STM) in ultrahigh vacuum has been used to investigate the growth, morphology, and surface atomic structure of ultrathin titanium silicide films on Si(100) substrates. Microstructural considerations have been used to identify various stages of the silicide growth. Methods for STM crystallography have been developed and used to identify possible epitaxial silicide/silicon relationships based on morphological considerations. Atomic resolution images of a titanium silicide crystallite have identified a 2×2 silicon termination of a C54-TiSi2(111) surface. It is shown that unambiguous identification of epitaxial relationships requires images of the atomic structure of the silicide crystallite surfaces in addition to morphological information. © 1995 American Institute of Physics.

Notes: The transformation of titanium silicide from the C49 to the C54 structure was studied using x-ray diffraction of samples containing arrays of narrow lines of preformed C49 TiSi2. Using a synchrotron x-ray source, diffraction patterns were collected at 1.5–2 °C intervals during sample heating at rates of 3 or 20 °C/s to temperatures of 1000–1100 °C. The results show a monotonic increase in the C54 transition temperature by as much as 180 °C with a decreasing linewidth from 1.0 to 0.1 μm. Also observed is a monotonic increase in (040) preferred orientation of the C54 phase with decreasing linewidth. The results demonstrate the power of in situ x-ray diffraction of narrow line arrays as a tool to study finite size effects in thin-film reactions. © 1995 American Institute of Physics.

Notes: Subpicosecond lasers measured the appearance rate of OH X(v=0) following 267 nm photolysis of the CH4⋅O3 van der Waals complex. The rise of the OH A←OH X laser-induced fluorescence with respect to the photolysis/probe delay time, tD, was LIF(tD)=1−exp(−tD/τ) with τ approximately 3 ps, indicating that the reaction CH4+O(1D2)→CH3+OH involves a CH3OH* intermediate with that lifetime. No prompt OH(v=0) from a direct or fast reaction was observed. © 1995 American Institute of Physics.

Notes: : Conditions under which a given topological space does, or does not, possess a stronger maximal feebly compact topology are given. Procedures are developed which can be used, for certain feebly compact spaces, to obtain stronger maximal feebly compact spaces. Several examples are constructed.

Notes: A finite-element method which incorporates mesh adaptation is used to calculate ground-water flow and pollutant transport. The formulation is based on the equations for conservation of mass, Darcy's law for an anisotropic medium, and the time-dependent species transport equation. Modifications have been implemented to the finite-element formulation to enhance computational speed and reduce storage; Petrov-Galerkin weighting of the advection terms provides numerical stability. An explicit time marching scheme is used to solve the transient equations. By utilizing unstructured adaptive meshing, species concentration and location of steep fronts are accurately resolved, even though one begins with a coarse mesh. The algorithm currently runs on PC and workstation class computers.

Notes: The mechanism of the reaction CH4+O(1D2)→CH3+OH was investigated by ultrafast, time-resolved and state-resolved experiments. In the ultrafast experiments, short ultraviolet pulses photolyzed ozone in the CH4⋅O3 van der Waals complex to produce O(1D2). The ensuing reaction with CH4 was monitored by measuring the appearance rate of OH(v=0,1;J,Ω,Λ) by laser-induced fluorescence, through the OH A←X transition, using short probe pulses. These spectrally broad pulses, centered between 307 and 316 nm, probe many different OH rovibrational states simultaneously. At each probe wavelength, both a fast and a slow rise time were evident in the fluorescence signal, and the ratio of the fast-to-slow signal varied with probe wavelength. The distribution of OH(v,J,Ω,Λ) states, Pobs(v,J,Ω,Λ), was determined by laser-induced fluorescence using a high-resolution, tunable dye laser. The Pobs(v,J,Ω,Λ) data and the time-resolved data were analyzed under the assumption that different formation times represent different reaction mechanisms and that each mechanism produces a characteristic rovibrational distribution. The state-resolved and the time-resolved data can be fit independently using a two-mechanism model: Pobs(v,J,Ω,Λ) can be decomposed into two components, and the appearance of OH can be fit by two exponential rise times. However, these independent analyses are not mutually consistent. The time-resolved and state-resolved data can be consistently fit using a three-mechanism model. The OH appearance signals, at all probe wavelengths, were fit with times τfast(approximate)0.2 ps, τinter(approximate)0.5 ps and τslow(approximate)5.4 ps. The slowest of these three is the rate for dissociation of a vibrationally excited methanol intermediate (CH3OH*) predicted by statistical theory after complete intramolecular energy redistribution following insertion of O(1D2) into CH4. The Pobs(v,J,Ω,Λ) was decomposed into three components, each with a linear surprisal, under the assumption that the mechanism producing OH at a statistical rate would be characterized by a statistical prior. Dissociation of a CH4O* intermediate before complete energy randomization was identified as producing OH at the intermediate rate and was associated with a population distribution with more rovibrational energy than the slow mechanism. The third mechanism produces OH promptly with a cold rovibrational distribution, indicative of a collinear abstraction mechanism. After these identifications were made, it was possible to predict the fraction of signal associated with each mechanism at different probe wavelengths in the ultrafast experiment, and the predictions proved consistent with measured appearance signals. This model also reconciles data from a variety of previous experiments. While this model is the simplest that is consistent with the data, it is not definitive for several reasons. First, the appearance signals measured in these experiments probe simultaneously many OH(v,J,Ω,Λ) states, which would tend to obfuscate differences in the appearance rate of specific rovibrational states. Second, only about half of the OH(v,J,Ω,Λ) states populated by this reaction could be probed by laser-induced fluorescence through the OH A←X band with our apparatus. Third, the cluster environment might influence the dynamics compared to the free bimolecular reaction.