ALBERT

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.

Notes: The high-resolution infrared spectrum of 1-chloro-2-fluoroethane in a molecular beam was collected over the 2975–2994 cm−1 spectral region. The spectral region of 2975–2981 cm−1 contains a symmetric C–H stretching vibrational band of the gauche conformer containing the 35Cl isotope. The spectral region of 2985–2994 cm−1 contains three vibrational bands of the trans conformer. Two of the three bands are assigned as an antisymmetric C–H stretch of each of the two different chlorine isotopes. The third band is assigned as a symmetric C–H stretch of the 35Cl isotope. The gauche conformer of 1-chloro-2-fluoroethane showed doublet patterns similar to those previously observed in 1,2-difluoroethane. The model for 1,2-difluoroethane is further refined in the present work. These refinements suggest that the coupling dark state in 1,2-difluoroethane is composed of 1 quantum C–H bend, 1 quantum C–C stretch, and 12 quanta of torsion. For 1-chloro-2-fluoroethane the dark state could not be identified due to a small data set. The trans conformer of 1-chloro-2-fluoroethane showed no evidence of mode coupling in the three vibrational bands. Including 2-fluoroethanol in this series of molecules, the extent of vibrational mode coupling did not correlate with the density of states available for coupling. Therefore, density of states alone is insufficient to explain the observed trend. A correlation was observed between the degree of intramolecular interaction and vibrational mode coupling. © 1995 American Institute of Physics.

Notes: Optothermal detection has been used to observe nonradiative relaxation channels in aniline, p-bromoaniline, and trans-stilbene. p-Bromoaniline has no detectable fluorescence due to a heavy atom effect which increases the rate of intersystem crossing to the triplet state. An optothermal spectrum of p-bromoaniline was observed with the origin at 32 625 cm−1. For trans-stilbene, the differences between the laser excitation spectrum and the optothermal spectrum of the S1 state clearly show the onset of isomerization at ∼1250 cm−1 above the origin. Absolute quantum yields of fluorescence, Franck–Condon factors, nonradiative rates, and radiative rates have been obtained for a series of vibronic transitions. For low energy vibrational states, there is good agreement between the current study and previous work. For vibrational energies above the barrier of isomerization, predicted quantum yields do not agree with our experimental results. © 1995 American Institute of Physics.

Notes: Microwave/radio-frequency-infrared multiple resonance has been used with an electric-resonance optothermal spectrometer to characterize a weak 21.6 MHz perturbation in the infrared spectrum of the ν14 C–O stretching vibration of 2-fluoroethanol. The infrared spectrum of 2-fluoroethanol was recorded at a resolution of ∼2 MHz using a tunable microwave-sideband CO2 laser. The spectrum is fit by an asymmetric-rotor Hamiltonian to a precision of 0.6 MHz, except for the transitions to the 413 upper state which are split into doublets by an interaction between the 413 level and a rotational level of a nearby background, or dark, vibrational state. Microwave/radio-frequency-infrared double and triple resonance reveals that the 413 level of the C–O stretching vibration is interacting with the 431 level of the dark state. The rotational constants determined for the dark state allow us to assign the perturbing state to the ν18+4ν21 combination vibration of the lowest energy conformer, where ν18 is the CCO bending vibration and ν21 is the C–C torsional vibration. From the weak ΔKa=2 matrix element between ν14 and ν18+4ν21 it is possible to derive a J=0 anharmonic interaction between these states of ∼3.5 GHz.

Notes: The high resolution IR spectrum of cyclobutane in a supersonic molecular beam was obtained for the region of 2981 to 2991 cm−1. The spectrum reveals four overlapping bands suggestive of vibrational mode coupling in the C–H stretching region. Ground state combination differences demonstrate that these bands originate from two different ground states, the symmetric and asymmetric ring puckering states. Evidence of vibrational mode coupling is present in all four bands. The coupling depends on both J and the symmetry of the puckering state. A model coupling scheme involving two qualitatively different types of couplings is developed to explain the observed spectrum. Symmetry restrictions and the interaction between molecular rotation and ring puckering qualitatively accounts for the dramatically different coupling behavior between the two ring puckering states.

Notes: The effective trapping rate k associated with diffusion-controlled reactions among random distributions of spatially correlated and uncorrelated, oriented spheroidal traps of aspect ratio ε is determined from Brownian motion simulations. Data for k are obtained for prolate cases (ε=2, 5, and 10), oblate cases (ε=0.1, 0.2, and 0.5), and spheres (ε=1) over a wide range of trap volume fractions (φ2) and satisfy recently obtained rigorous lower bounds on k for this statistically anisotropic model. The results for the trapping rate for correlated traps always bounds from above corresponding results for uncorrelated traps. Generally, the trapping rate k, for fixed φ2, increases with decreasing aspect ratio ε, showing a precipitous rise in k as the spheroids become disklike. Using a recent theorem due to Torquato [Phys. Rev. Lett. 64, 2644 (1990)], data for the trapping rate k can be employed to infer information about the fluid permeability tensor K associated with slow viscous flow through porous media composed of the same arrays of oriented spheroidal particles.

Notes: Improved rigorous bounds on the effective elastic and transport properties of a transversely isotropic fiber-reinforced material composed of oriented, infinitely long, multisized circular cylinders distributed throughout a matrix are computed. Specifically, we evaluate such bounds on the effective axial shear modulus (which includes, by mathematical analogy, the transverse conductivity), effective transverse bulk modulus, and the effective transverse shear modulus. These are generally demonstrated to provide significant improvement over the Hill–Hashin bounds which incorporate only volume-fraction information. Although the upper bounds diverge from the lower bounds when the cylinders are much stiffer than the matrix, the improved lower bounds still yield relatively accurate estimates of the effective properties. Generally, increasing the degree of polydispersivity in cylinder size increases the effective transverse conductivity (or axial shear modulus) and effective transverse bulk modulus, and decreases (slightly) the effective transverse shear modulus for cases in which the fibers are more conducting or stiffer than the matrix.

Notes: Three-point bounds on the effective conductivity σe of isotropic two-phase composites, that improve upon the well-known two-point Hashin–Shtrikman bounds [J. Appl. Phys. 23, 779 (1962)], depend upon a key microstructural parameter ζ2. A highly accurate approximation for σe developed by Torquato [J. Appl. Phys. 58, 3790 (1985)] also depends upon ζ2. This paper reports a new and accurate algorithm to compute the three-point parameter ζ2 for dispersions of hard spheres by Monte Carlo simulation. Data are reported up to values of the sphere volume fraction φ2 near random close-packing and are used to assess the accuracy of previous analytical calculations of ζ2. A major finding is that the exact expansion of ζ2 through second order in φ2 provides excellent agreement with the simulation data for the range 0≤φ2 ≤0.5, i.e., this low-volume-fraction expansion is virtually exact, even in the high-density region. For φ2 〉0.5, this simple quadratic formula is still more accurate than other more sophisticated calculations of ζ2. The linear term of the quadratic formula is the dominant one. Using our simulation data for ζ2, we compute three-point bounds on the conductivity σe and Torquato's approximation for σe .

Notes: Calculations of collisional stochastic ripple loss of alpha particles from the new 20 toroidal field (TF) coil International Thermonuclear Experimental Reactor (ITER) [Plasma Physics and Controlled Nuclear Fusion Research (International Atomic Energy Agency, Vienna, 1991), Vol. 3, p. 239] predict small alpha ripple losses, less than 0.4%, close to the loss calculated for the full current operation of the earlier 24 TF coil design. An analytic fit is obtained to the ITER ripple data field demonstrating the nonlinear height dependence of the ripple minimum for D-shaped ripple contours. In contrast to alpha loss simulations for the Tokamak Fusion Test Reactor (TFTR) [Fusion Technol. 21, 1324 (1992)], a simple Goldston, White, and Boozer stochastic loss criterion [Phys. Rev. Lett. 47, 647 (1981)] ripple loss model is found to require an increased renormalization of the stochastic threshold δs/δGWB(approximately-greater-than)1. Effects of collisions, sawtooth broadening, and reversal of the grad-B drift direction are included in the particle following simulations. © 1996 American Institute of Physics.

Notes: The high resolution infrared spectrum of 1,2-difluoroethane (DFE) in a molecular beam has been obtained over the 2978–2996 cm−1 spectral region. This region corresponds to the symmetric combination of asymmetric C–H stretches in DFE. Observed rotational fine structure indicates that this C–H stretch is undergoing vibrational mode coupling to a single dark mode. The dark mode is split by approximately 19 cm−1 due to tunneling between the two identical gauche conformers. The mechanism of the coupling is largely anharmonic with a minor component of B/C plane Coriolis coupling. Effects of centrifugal distortion along the molecular A-axis are also observed. Analysis of the fine structure identifies the dark state as being composed of C–C torsion, CCF bend, and CH2 rock. Coupling between the C–H stretches and the C–C torsion is of particular interest because DFE has been observed to undergo vibrationally induced isomerization from the gauche to trans conformer upon excitation of the C–H stretch.