ALBERT

Notes: Molecular dynamics simulations have been performed on a simple fluid in the non-Newtonian regime in order to study the nature of diffusion in the presence of shear stress. The nonequilibrium molecular dynamics (NEMD) sllod algorithm is used to generate a homogeneous boundary driven Couette flow. The elements of the tensor of diffusion coefficients have been evaluated by three different routes: through Einstein relations with the corresponding elements of the tensor of molecule displacements, through Green–Kubo expressions involving tensor of momentum autocorrelation functions and cross-correlation functions, and by utilizing an NEMD color field method. All three routes to the tensor of diffusion coefficients are shown to yield consistent results. A simple spherically symmetric Lennard-Jones fluid at its triple point was used in the simulation study. We find that in a Couette strain field of sufficiently large strain rate, diffusion in all directions is enhanced substantially. We propose and verify a new asymptotic relationship between the diagonal elements of the tensor of diffusion coefficients and the strain rate.

Notes: We explore the conformations of an isolated chain molecule as it approaches an impenetrable planar surface, by exhaustive enumeration of all the three-dimensional self-avoiding flights on simple cubic lattices. We confirm the well-known results that as a chain approaches a surface, the number of accessible configurations diminishes, so there is an entropic repulsion, and the radius of gyration becomes anisotropic and the chain flattens. Our principal observation here is that proximity to a surface causes a chain molecule to develop enhanced amounts of certain forms of internal structural organization, namely, helices and antiparallel sheets, the main architectures observed in globular proteins. In addition, we compare the process of adsorption of: (i) chains that have no intrachain attraction (i.e., "open'' chains); with (ii) chains that have strong intrachain attraction (i.e., "compact'' chains). We confirm the well-known result that open chains undergo an adsorption transition onto the surface. However, we find that the adsorption of compact chains differs in one respect. Compact chains appear to undergo two types of transition depending on the strength of chain–surface attraction: (i) weak attraction leads to a "docking'' transition, in which the chain adsorbs without much deformation; (ii) strong attraction leads to a "flattening'' transition in which the chain adopts a two-dimensional ensemble of conformations on the surface.

Notes: We study the conformations of chain molecules near interfaces by exhaustive simulation. We explore all the conformations accessible to a short isolated chain (16 monomers) on a 2-dimensional square lattice as a function of the distance of the center of mass of the chain from an interface. Our principal focus is on the "internal structure'' of the chain, certain simple patterns of intrachain contacts such as helices and sheets (planar zigzags). In the process of enumeration, we confirm the well-known result that chains near interfaces have fewer conformations than chains in the bulk. We also find that the persistence length increases, and the radius of gyration and end-to-end length become anisotropic as the chain approaches the interface. The main conclusion of this work is that chain molecules are predicted to often have enhanced amounts of internal structure when they are at or near interfaces. Steric constraints imposed by the interface are selective and exclusive, eliminating open conformations but not eliminating compact conformations such as helices and sheets. Therefore when a polymer, protein, or peptide chain is weakly attracted to an interface, internal structure should be induced or, if already present, it should be enhanced. In 2 dimensions, stronger attraction in some cases flattens the chain and obliterates this structure.

Notes: The multiphoton decomposition (MPD) of 1-bromo 2-fluoroethane, CH2BrCH2F, with a pulsed CO2 infrared laser is reported. The decomposition study was done at constant fluence and at pressures up to 2 kPa. Empirical representations of the pressure dependence of the MPD are reported. A collision-dependent reaction scheme is presented to explain the pressure dependence of the up-pumping through the vibrational manifold.

Notes: Thermochemical properties of cluster ions, involving Cl−, Br− and I− solvated by ammonia, were measured by using high-pressure mass spectrometry. Standard enthalpy changes of −8.2, −7.7, and −7.4 kcal/mol and entropy changes of −15.4, −19.1, and −20.9 cal/K mol were obtained for the association of NH3 onto Cl−, Br−, and I−, respectively. Comparison with the halide affinities of SO2 and H2O along with alkali affinities of NH3, SO2, and H2O suggests the importance of quadrupole moments and polarizabilities in the qualitative agreement of electrostatics with the ordering of association affinities. Electrostatic and ab initio calculations were made which suggest a variation of halide orientation with respect to the ammonia where I− is closest to alignment with the ammonia dipole and F− is farthest removed from the dipole axis. A value of 11 kcal/mol is estimated for association energy of NH3 with F− by comparison of extrapolated and calculated values along with similar chemical systems.

Notes: The spatial distribution of neutral deuterium has been determined in a reversed field pinch by fluorescence scattering. The first series of measurements, with a resolution of 4 cm were made with the laser tuned to the Dα, n=2−3(6561 A(ring)) transition. The second set of measurements, with both 4 and 2 cm resolution were performed with the laser tuned to the Dβn=2−4(4861 A(ring)) wavelength. The improvement in signal to noise obtained was substantial, being increased by a factor of (approximately-equal-to)2.5 in general. This is believed to be the first such measurement at this wavelength. The advantages of using this transition are such that this technique can be extended to measure the electron density near the plasma edge.

Notes: Long wavelength density fluctuations can be observed by scattering even with a probe beam of much shorter wavelength provided the scattering angle is small enough. This paper is concerned with experiments in which the scattering angle is comparable with the probe beam divergence so the scattered and incident radiation never achieve spatial separation. Under these circumstances, the role of diffraction is preeminent and Fourier optics methods are used to describe the propagation of the beam, which is taken to be TEM00 mode Gaussian. Interaction between the probe beam and the plasma disturbance is described by refraction and no appeal is made to explicit scattering theory. Analysis of the effect of a monochromatic wave disturbance confined to a plane perpendicular to the probe beam (a plane grating in effect) reveals oscillations at the wave frequency induced on the probe with an intensity varying over the beam profile in a regular pattern symmetric about the beam axis. Detail of the pattern depends on the wavelength of the disturbance, its direction, and its axial position relative to a local beam waist. These oscillations are readily identified as due to radiation scattered by the plasma wave into diffraction orders, beating with the unperturbed part of the beam. Indeed, it can be shown1 that Fourier optics plus refraction produce almost the same result as conventional scattering theory,2 the small discrepancy being traceable to the neglect in the latter of incident beam wavefront curvature. The results of the two approaches coincide in the Fraunhofer limit. Computations of this sort have been confirmed by experiments using transducer-driven waves in air3 and by plasma experiments where the same regular patterns are observed from spontaneous plasma waves.4,5 Calculation suggests and experiments have demonstrated6 that additional information, such as the absolute direction of wave propagation, can be deduced from phase, measured with a multichannel detector array.Asymmetric profiles are frequently encountered in far forward scattering experiments in plasma, and they are attributed either to (1) the volume effect, that is, the finite width of a plasma wave, or (2) a pair of counter-propagating waves, such as poloidal waves in a torus met twice by a probe beam traversing a minor diameter. The first explanation rests on the difference between the multiple order scattering of a two-dimensional grating (Raman–Nath) and the single-order scattering of a three-dimensional crystal (Bragg). In a regime intermediate between these extremes, both +1 and −1 orders are present, but of unequal intensity, therefore giving rise to asymmetry in the beam profile. The Fourier optics treatment can be extended to describe a wave of arbitrary interaction length L, and a controlling parameter Q=κ2L/k (κ and k being wave numbers of the plasma wave and the probe radiation, respectively) which is (very-much-less-than)1 for Raman–Nath and (very-much-greater-than)1 for Bragg, determines the precise regime that prevails.7 Calculations describing the counter-propagating waves model have been performed and verified experimentally, again using transducer-driven waves in air.8 Profiles based on this model are currently providing best fits to data recently recorded from tokamak plasmas in TOSCA. A preliminary inspection of the results of these measurements reveals, from the orientation of the beam profile pattern, predominantly poloidal waves. Their maximum intensity is near 100 kHz and they fall away towards higher frequencies as ν−2.5. Evidence for coherent gross modes at lower frequencies is also seen. Wave numbers are in the range 1 cm−1〈κ⊥〈30 cm−1, bracketing the neighborhood where κ⊥ρi∼1. The strength of the relative density fluctuation ñe/n¯e of a few per cent is consistent with diffusion coefficients D⊥∼104 cm2 s−1, and there is evidence for inverse correlation between ñe/n¯e and confinement time τE.

Notes: Photo-induced electron transfer experiments examine intrinsically nonequilibrium processes. A theoretical description of photoinduced processes should take into account the fact that the approximations and assumptions made for equilibrium electron transfer need not be appropriate. Under nonequilibrium conditions, anharmonic distortions in the potential energy surfaces of nuclear motion coupled to the electron transfer may effect the dynamics. This work is a study of the effects of anharmonicity on photo-induced electron transfer reactions for condensed phase systems where one vibrational mode is strongly coupled to the electron transfer and a stochastic bath. For this vibrational mode, both harmonic and anharmonic potential energy surfaces for the excited states are considered and the electron transfer dynamics is monitored in a range of system–bath coupling regimes. The study focuses on two effects due to anharmonic distortions of the intramolecular modes: changes to the system Hamiltonian, and differences in the dephasing processes caused by the anharmonic distortions. These calculations show that for small differences in the donor and acceptor state energies, the effects of vibrational anharmonicity is very small. However, when this energy difference is large, the dynamics for anharmonic and harmonic modes is significant. The relative role played by the competing physical processes is easily understood by examining the vibronic state populations obtained using a multistate Redfield approach. © 2001 American Institute of Physics.

Notes: The phenomenon of molecular chattering (a collision between two rigid ovaloids involving two or more impulsive hits) is investigated with emphasis on the importance of these collisions on kinetic theory calculations for dilute molecular systems. To facilitate this analysis, the pseudo-Liouville formalism, commonly employed in the kinetic theory of rigid spheres, is generalized to a form appropriate to rigid ovaloids. From this formalism, we derive a chattering expansion of the bracket integrals, the terms of which successively incorporate higher order chattering sequences. Finally, the results of scattering trajectories between a rigid sphere and a rigid ellipsoid are reported. It is demonstrated that the neglect of chattering sequences leads to large errors when calculating the bracket integrals for realistic model parameters; the errors ranging from ∼1% to as high as ∼200%.

Notes: Separation of a many-body system into a primary system plus a bath of background modes enables approximate calculation of electronic absorption spectra and zero-temperature resonance Raman scattering cross sections in cases where there is nonadiabatic coupling between two or more Born–Oppenheimer excited-state potential surfaces. In particular, the low-resolution optical line shape theory recently developed to describe curve-crossing phenomena [D. G. Evans and R. D. Coalson, J. Chem. Phys. 99, 6264 (1993)] is extended to systems where there is a primary nuclear coordinate mode that is characterized by large excited-state displacements and an ensemble of weakly displaced bath modes. The accuracy of the resulting approximation scheme is illustrated using the spectroscopic spin-boson model, in which all surfaces are harmonic, and path integral techniques can be used to obtain exact results. Application to more complicated systems is discussed.