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In this paper we describe a lattice density functional theory for the rotator-herringbone phase transition in a liquid supported monolayer system of long chain acids or alcohols. It is assumed that both phases have long-range translational order and that the rotator phase is characterized by a uniform distribution of backbone orientation. We find that orientational interactions among the backbone planes are sufficient to drive a second order transition, and we characterize the phase of the system by a parameter associated with the width of the peaks in a distribution which describes the orientations of the backbones. We also briefly discuss generalizations of the theory to include lattice vibrations and distortions. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present the results of a self-consistent Monte Carlo simulation of the liquid–vapor interface of magnesium. The longitudinal density profile shows stratification in the liquid–vapor transition zone, as has also been found in previous simulations of the liquid–vapor interfaces of alkali metals and mercury. [ABSTRACT FROM AUTHOR]

We report calculations, based on the Cai–Rice theory [J. Chem. Phys. 96, 6229 (1992)], of the relative importance of chain–surface and chain–chain interactions for the occurrence of the tilting transition in expanded Langmuir monolayers. The interaction between amphiphile molecules, and that between an amphiphile molecule and the surface, are described by Lennard-Jones potentials. It is found that for a reasonable set of interaction parameters the structure of the expanded monolayer supports a collective tilt, and that the existence of a nonzero amphiphile chain–surface interaction plays an important role in determining the collective tilt. However, over much of the range of surface density of interest, an expanded monolayer with nonzero collective tilt is unstable with respect to one with zero collective tilt and a smaller separation of the molecules. The existence or nonexistence of a collective tilt in the monolayer is very sensitive to the relative magnitude of the amphiphile chain–amphiphile chain and amphiphile chain–surface interactions; only a modest increase in the latter is sufficient to drive the collective tilt in the monolayer toward 90°. Our results are in accord with the observed behavior of monolayers of perfluorinated amphiphiles and ordinary amphiphiles if the ratio of chain–surface to chain–chain interactions is allowed to vary. [ABSTRACT FROM AUTHOR]

We show that a reaction path Hamiltonian can be used, with the basic concepts of the Davis–Gray analysis of unimolecular reaction rate, to generate an accurate description of the dynamics of a model isomerization reaction. [ABSTRACT FROM AUTHOR]

We report the results of static and dynamic evanescent wave light scattering studies of a monolayer of a diblock copolymer, polystyrene-b-polymethylmethacrylate (PS-b-PMMA) with weight averaged molecular weights (Mw) of 880 000:290 000 supported at the air/water interface. Our studies probe the interfacial structural and dynamic properties of the monolayer on a length scale which is a fraction of the wavelength of light. The static light scattering studies were carried out as a function of polymer surface coverage and temperature; we also report some preliminary data for the dependence of the static structure function on the relative molecular weights of the PS and PMMA blocks. The complementary dynamic light scattering studies were carried out only as a function of surface coverage. Our data suggest that, upon spreading in the air/water interface, PS-b-PMMA (880:290 K) copolymers form thin disklike aggregates containing about 240 molecules. These data are consistent with a model in which each such aggregate is a ‘‘furry disk’’ with a dense core consisting of a layer of collapsed PS blocks atop a thin layer of extended PMMA blocks on the water surface and a brushlike boundary of extended PMMA blocks. The data show that the furry disks diffuse freely when the surface coverage is small, but when the surface coverage is large, they are immobile. Our data also suggest that the furry disks can aggregate to form even larger ‘‘islands’’ of disks with an extension greater than 20 μm. The static structure function of the assembly of furry disks is well described, over a wide range of surface coverage, by the structure factor of a two-dimensional hard disk fluid modulated by a two-dimensional hard disk form factor. [ABSTRACT FROM AUTHOR]

There is a steadily growing body of evidence that much of the information available concerning the properties of Langmuir monolayers concerns systems which are not in a state of equilibrium. What are now known to be unrelaxed monolayers of stearyl alcohol exhibit a high surface pressure phase transition whose signature in the isotherm changes from a ‘‘kink’’ (0–8 °C), to a small nearly flat region (8–15 °C), and finally to another kink (at higher temperatures). We have carried out external reflection infrared spectroscopic studies of relaxed monolayers of stearyl alcohol along a representative isotherm from each of the mentioned temperature regions. The results of our studies indicate that in each region the surfactant hydrocarbon chain becomes more ordered (i.e., has fewer gauche conformations) as the surface pressure is increased. We find that (i) at 5 °C, throughout the surface pressure range where the change in intramolecular chain order occurs, the collective tilt of the hydrocarbon chains remains nearly constant. For this isotherm the kink signals the point at which the hydrocarbon chains have achieved a very high degree of intramolecular order, perhaps the all-trans limit. (ii) In the 11 °C monolayer, in the phase often referred to as ‘‘superfluid’’, the intramolecular disorder in the hydrocarbon chains is measurably greater than in the equivalent phases in the higher and lower temperature regions. We also show that in the relaxed monolayer the nearly flat portion of the isotherm observed in the unrelaxed monolayer is almost totally absent, leaving only a kink very similar to those observed at higher and lower temperatures. (iii) At 25 °C, although the hydrocarbon chains in the relaxed monolayer attain a relatively high degree of intramolecular order, the high surface pressure phase transition is not observed. Instead, the surface pressure levels off at a value... [ABSTRACT FROM AUTHOR]

We report the results of molecular dynamics simulations of a water supported monolayer of F(CF2)6(CH2)6OC6H4COOH. A grazing incidence x-ray diffraction study of this system showed, unexpectedly, the absence of any strongly ordered condensed phase. We find that the monolayer has a partially ordered structure with a range of order intermediate between that typically found in monolayers of, e.g., arachidic acid, and that expected for a two-dimensional liquid. The calculated width of the first order diffraction peak is about ten times larger than for arachidic acid, yet is ten times smaller than expected for a two-dimensional liquid. We argue that monolayers with intermediate order represent a different class of systems that those heretofore studied, and that these systems may have some unusual properties. [ABSTRACT FROM AUTHOR]

We report the results of molecular dynamics simulations of a monolayers of long chain carboxylic acids and alcohols supported on the 0001 face of ice Ih. The simulations are designed to provide information concerning the influence of the atomic structure of the head group and of the nature of the head group–surface interactions on the collective tilt of the molecules in the monolayer. The results of the simulations show that the structure of the head group is a more important influence on the collective tilt of the molecules than is the specific character of the head group–surface interactions. It is also shown that the constraint imposed by the potential field of the rigid ice Ih 0001 surface renders the ice supported monolayer different from the liquid water supported monolayer. Specifically, the collective tilt of the molecules in the ice supported monolayer is not accompanied by a distortion of the projection of the unit cell of the monolayer in the plane of the surface; that projection remains hexagonal for all tilt angles. In contrast, the introduction of a collective tilt of the molecules in a liquid water supported monolayer always leads to a distortion of the projection of the unit cell in the plane of the surface, typically from hexagonal at zero tilt to centered rectangular for all nonzero tilt. The tilting of the monolayer supported on ice Ih is favored by the molecular spacing induced by the field of the 0001 surface, which is slightly greater than closest packing separation of the molecules. The equilibrium tilt angle appears to be determined primarily by the chain–chain interactions since the surface pressure attained at equilibrium is sensibly independent of the precise value of the tilt angle and of the character of the head group–head group and the head group–surface interactions. [ABSTRACT FROM AUTHOR]

We report the results of an infrared external reflection spectroscopic study of phase transitions in Langmuir monolayers of heneicosanol. One of our main goals was to examine monolayers which were pure and at equilibrium. The monolayers we have studied were fully relaxed, judging from the exceptionally good long term stability demonstrated by each state on the isotherms. In addition, our fully relaxed systems displayed good reversibility, though each compression–expansion cycle showed a small hysteresis, which may possibly be a real physical property of these systems. The phase boundaries displayed in these fully relaxed systems differed in number and location from those in the partially relaxed systems usually studied. Our second goal was to advance the utility of infrared external reflection spectroscopy for the study of Langmuir monolayer structure, especially molecular conformation and orientation, by examining the hydrocarbon CH2 symmetric and antisymmetric stretching modes as a function of surface pressure, temperature, etc. In pursuit of this goal we have exploited nonimaging optics to improve the signal to noise in these difficult experiments.Our studies of the relaxed structure of Langmuir monolayers of heneicosanol have uncovered new and interesting phase behavior. In general, we find a clear trend showing that at high surface pressure and low temperature the hydrocarbon chains are rather highly ordered, perhaps all-trans; at low surface pressure and high temperature the hydrocarbon chains become somewhat disordered, containing a higher concentration of gauche defects, though that concentration is still lower than that found in the fluid phase. In all of the cases studied the region of the isotherm immediately prior to the highest density phase can be associated with a distinct ordering transition of the hydrocarbon chain. Regarding the molecular orientation, we have obtained data which are often in disagreement with the current x-ray data,... [ABSTRACT FROM AUTHOR]

Infrared external reflection spectroscopy has enjoyed enormous success in the characterization of adsorbates on metal substrates, but has been used only rarely to study adsorbates on nonmetallic substrates. In this report we present the theory of infrared external reflection spectroscopy as applied to the study of adsorbates on dielectric substrates. We begin by reviewing the well-established general macroscopic theory in which Maxwell’s equations are applied to the model system ambient-phase/adsorbate/substrate. Then we explicitly consider the case of external reflection from an arbitrary dielectric substrate and derive simple, intuitive equations for the infrared external reflection intensity as a function of macroscopic optical parameters. Our interest is in formulating a theory which will allow us to extract from experimental data information about the microscopic structure of the monolayer, such as the orientation of adsorbates on dielectric substrates, without the necessity for concomitant use of other spectroscopic techniques, such as infrared transmission or internal reflection spectroscopies, which may be impossible or inconvenient to implement; such is the case for Langmuir monolayers of long chain amphiphile molecules at the air–water interface. Unfortunately, the use of only external reflection spectroscopy to study a system results in more unknown optical constants than experimental data. To overcome this difficulty we propose for Langmuir monolayers a microscopic model which relates the optical parameters of the adsorbate monolayer to select molecular parameters, those which will allow us to extract the adsorbate orientation from experimental data. To illustrate the method we model the external reflection spectroscopy of long chain amphiphile CH2 stretching modes of typical fatty acids and alcohols. We also derive for this system a very useful dichroic ratio which facilitates the direct interpretation of experimental data. We... [ABSTRACT FROM AUTHOR]

We report calculations of the rate of isomerization of 3-phospholene based on the theory of Gray and Rice as extended by Zhao and Rice. This system is of interest for two reasons. First, the potential energy surface for motion in the isomerization coordinates is well known because of the spectroscopic studies of Harthcock and Laane. Second, the potential energy surface generates a Hamiltonian which has important off-diagonal kinetic energy coupling terms, which provides a different test of the accuracy of the approximations used in the Gray–Zhao–Rice theory than posed by previous applications. We find that the predictions of the Gary–Zhao–Rice theory are in good agreement with the results of simulations of the rate of isomerization of 3-phospholene. We also briefly discuss the relationship between the theory presented in this paper and the Reactive Island theory of isomerization. [ABSTRACT FROM AUTHOR]

The classical theory of the rate of unimolecular isomerization developed by Gray and Rice is extended in two ways. First, an improved state dependent approximation to the system separatrix is introduced. Second, a systematic approximation for the rate of intramolecular energy transfer in one isomer is introduced. These new approximations considerably improve the accuracy of the prediction of the rate of isomerization. Indeed, using the analysis described in this paper, the calculated rates of isomerization as a function of energy for the model system studied by Gray and Rice are in excellent agreement with the exact rates obtained from trajectory calculations. [ABSTRACT FROM AUTHOR]

We report the results of calculations of the rates of predissociation of HeCl2, NeCl2, HeICl, and NeICl using a version of the classical theory of unimolecular reaction rate which includes a balanced treatment of both the intramolecular energy transfer and fragmentation processes. The agreement between theory and experiment is very good. [ABSTRACT FROM AUTHOR]

We describe a classical theory of unimolecular reaction rate which is derived from the analysis of Davis and Gray by use of simplifying approximations. These approximations concern the calculation of the locations of, and the fluxes of phase points across, the bottlenecks to fragmentation and to intramolecular energy transfer. The bottleneck to fragment separation is represented as a vibration–rotation state dependent separatrix, which approximation is similar to but extends and improves the approximations for the separatrix introduced by Gray, Rice, and Davis and by Zhao and Rice. The novel feature in our analysis is the representation of the bottlenecks to intramolecular energy transfer as dividing surfaces in phase space; the locations of these dividing surfaces are determined by the same conditions as locate the remnants of robust tori with frequency ratios related to the golden mean (in a two degree of freedom system these are the cantori). The flux of phase points across each dividing surface is calculated with an analytic representation instead of a stroboscopic mapping. The rate of unimolecular reaction is identified with the net rate at which phase points escape from the region of quasiperiodic bounded motion to the region of free fragment motion by consecutively crossing the dividing surfaces for intramolecular energy exchange and the separatrix. This new theory generates predictions of the rates of predissociation of the van der Waals molecules HeI2, NeI2 and ArI2 which are in very good agreement with available experimental data. [ABSTRACT FROM AUTHOR]

We report a calculation of the free energy difference between a reference monolayer, in which the amphiphile molecules are vertical and arranged in a hexagonal lattice, and the derivative monolayer, in which the amphiphile molecules are tilted and arranged in a distorted hexagonal lattice. For the case that the interaction between amphiphile molecules, and that between an amphiphile molecule and the surface, are described by a Lennard-Jones interaction per unit length, it is found that a tilted state of the monolayer is stable only if the ratio of the amphiphile chain–surface to amphiphile chain–amphiphile chain interactions exceeds a critical value. When the area per molecule and temperature are the same, a monolayer with molecules tilting toward their nearest neighbors has a larger tilt angle and lower free energy than a monolayer with molecules tilting between the nearest neighbors. [ABSTRACT FROM AUTHOR]

We propose a modification of the approximate classical theory of unimolecular fragmentation developed by Gray, Rice, and Davis; the important difference is the use a vibrational state dependent separatrix as the bottleneck to reaction. This version of the theory is well adapted to the description of the predissociation of van der Waals molecules, the rates of fragmentation of which are poorly described by the conventional statistical theory of unimolecular reaction. A test of the modified theory against molecular dynamics simulations of the predissociation of HeI2 reveals considerable improvement relative to the predictions of the Gray–Rice–Davis treatment. For large vibrational excitation, when intramolecular energy transfer is fast enough to not influence the predissociation rate, there is also very good agreement with the experimental data of Johnson, Wharton, and Levy. For small vibrational excitation it is necessary to explicitly account for the slow intramolecular transfer of energy in order to reproduce the experimental data. [ABSTRACT FROM AUTHOR]

We discuss the achievability of optimal control of the evolution of an unbounded quantum system by the action of external fields. Previous work has established that the evolution of a quantum system with a nondegenerate discrete and bounded spectrum of states can, in principle, be fully controlled, i.e., that the system wave function can be guided by external fields to approach arbitrarily closely a selected target state wave function. The optimal control of the evolution of a quantum system with a discrete and bounded spectrum of states has also been studied, using a method of analysis that depends on the localized character of bound state wave functions and the fact that the spectrum of states is bounded. In this paper, we examine whether it is possible to control, partially or fully, the evolution of an unbounded quantum system. We show that optimal control of the evolution of an unbounded quantum system is possible, in the sense that it is possible to minimize the difference between the product function formed at time t0 from a localizing function and a continuum wave function and a similarly defined target function at time tf.We have not been able to establish that such optimal control is equivalent to full control, i.e., that the difference between the initial and target functions can be made arbitrarily small, which would require showing that the set of control functions is complete with respect to the function space of the system. Our analysis establishes the existence of external fields that can optimally guide the unbounded system evolution in the absence of other constraints than the Schrödinger equation of motion, but does not provide an algorithm for the construction of such fields. The relationship between optimal control of the evolution of a quantum system and the existence of chaotic dynamics in the continuum domain is briefly discussed. We argue that the optimally controlled system cannot have chaotic dynamics even if the bare system... [ABSTRACT FROM AUTHOR]

We report the results of molecular dynamics simulations of the liquid–vapor interface of water. Two different water–water interactions were studied. The density profile of the interfacial transition region between liquid and vapor is monotone with a 10%–90% width of 3.45 Å, in good agreement with the value determined from x-ray reflectance measurements, namely 3.30 Å. The water molecules in the interface tend to lie with the HOH bisector in the plane of the surface and one OH bond pointing out of the surface. As the density falls in the transition region the tetrahedral structure of the bulk liquid breaks up and there is a tendency towards dimerization. The diffusion constant in the interfacial region is 58% larger than that in the bulk region. [ABSTRACT FROM AUTHOR]

This paper describes a simple lattice model of a liquid supported monolayer of long chain amphiphile molecules. The new feature which this model possesses is incorporation of the structure of the surface of the support liquid. The liquid–monolayer interface is represented as a multilayer region, and its structure is described in terms of the associated inhomogeneous density distributions of the support liquid molecules and of the segments of the chain molecules. The properties of this diffuse surface–amphiphile monolayer model are compared with the properties of the smooth surface–amphiphile monolayer model described in our previous work [J. Chem. Phys. 92, 1427 (1990)]. The general behavior of the monolayer equation of state, including the prediction of two successive fluid–fluid phase transitions, is preserved in the present model. We find that the existence of the diffuse surface structure of the supporting liquid leads to an increase in the number of the amphiphile chain segments that lie in the surface region, particularly at low surface densities. We also find that the multilayer character of the diffuse surface region can alter the equation of state of the monolayer at high surface densities. The two layer diffuse surface–amphiphile monolayer model supports a change in the slope of the surface pressure versus area isotherm in the very high surface density region, which can be ascribed to a conformational ‘‘freezing’’ of the chain molecules. The theory illustrates the qualitative features of the system which define the close interplay between the structure and properties of the monolayer and the structure of the surface of the support liquid. [ABSTRACT FROM AUTHOR]

We report the results of self-consistent Monte Carlo simulations of the influence of a close packed monolayer of C20H41OH on the structure of the surface of liquid Cs. The monolayer enhances the stratification of the longitudinal density distribution of Cs in the interface by an amount which can be detected in an x-ray reflectivity experiment. The transverse correlation in the Cs surface is also influenced by the monolayer, but not to a great extent; the transverse ordering in the outermost layer of atoms is found to be somewhat stronger than in bulk liquid Cs. All of these effects of the monolayer on the structure of the supporting liquid are naturally explained in terms of the interatomic forces and electronic structure of the system. [ABSTRACT FROM AUTHOR]

A new method for studying the properties of adsorbed polymer molecules is proposed. If polymer molecules are adsorbed on the low refractive index side of an interface between two transparent dielectric media, they can scatter the evanescent radiation generated by total internal reflection of light incident on the interface from the high refractive index medium. Analysis of the angular distribution of the scattered light yields information about the mean extension of the polymer perpendicular to the interface and its mean extension parallel to the interface. As an example, the theory is illustrated for the case of rods, anchored at one end to the interface, and with uniform angular distribution in the half space of the low refractive index medium. Both analytical and numerical analyses are presented. [ABSTRACT FROM AUTHOR]

Studies the scattering of a point particle from hard discs fixed on a plane. Calculation of the S matrix; Multiple scattering between the discs; Free propagation from the edges of the discs; Location of the scattering resonances.

Studies the semiclassical quantization of the scattering of a point particle from a chaotic repellor. Terms of the trace function; Upper bound of the semiclassical limit; Lifetimes of the scattering resonances; Distribution of the imaginary parts of the complex wave numbers of the resonances.

Studies the scattering of a point particle from circular discs in a plane. Fractal and chaotic metastable state of the system; Particle escape rate; Lyapunov exponent of the repellor; Kolmogorov-Sinai entropy per unit time.

We study the thermodynamics and structure of a model monomolecular film of pentadecanoic acid on water. In this model the water is treated as a uniform polarizable continuum and the pentadecanoic acid molecules as chains of 15 pseudoatom with internal bond constraints, angle bending and torsional intramolecular interactions, and Lennard-Jones atom–atom intermolecular interactions. The only low pressure phases exhibited by this model at temperatures between 300 and 400 K are the very low density vapor phase and a well ordered condensed phase with a reciprocal density ≤23 Å2 per molecule. This is in sharp contrast with experimental studies which show the existence of a stable liquid-expanded phase with a density of about 35 Å2 per molecule at pressures <15 dyn cm-1. [ABSTRACT FROM AUTHOR]

A time delayed, fully resonant, version of coherent anti-Stokes Raman spectroscopy (CARS) has been used to study intramolecular and intermolecular dephasing in vibrationally excited jet-cooled tetrazine and its van der Waals complexes with argon, krypton, and xenon. When there is 1200 to 2200 cm-1 of vibrational energy in the ground electronic state manifold, the dephasing of all species is dominated by the macroscopic interference associated with the heterogeneous structure of the Q-branch transitions. The observed macroscopic dephasing times are about 300 ps to 2 ns, which implies that the lower limit for the intramolecular dephasing times falls in the same range. The influence of excited state resonances on the dephasing characteristics of these molecules is qualitatively accounted for by formulas derived from the wave packet theory of Tannor, Rice, and Weber. [ABSTRACT FROM AUTHOR]

A triple resonance experiment with fluorescence detection has been carried out to investigate the spectroscopy and intramolecular dynamics of the van der Waals molecules s-tetrazine–X (X=Ar, Kr, Xe) with vibrational energies from 1250 to 2210 cm-1 in the ground electronic state (S0). Vibrational band shifts in S0 van der Waals molecules are usually on the order of 1 cm-1, and most relaxation lifetimes are longer than 15 ns. Comparison with published data for intramolecular vibrational redistribution in the S1 electronic state indicates a strong influence of the electronic structure of the aromatic ring on the dissociation dynamics, in clear contradiction to statistical models which have been advanced previously. A perturbation theory treatment supports the existence of a correlation between vibrational band shifts and dissociation dynamics, and rationalizes qualitatively all the peculiar observations for this class of van der Waals molecules. [ABSTRACT FROM AUTHOR]

We report an analysis of the bifurcation of the solution to the nonlinear equation for the inhomogeneous singlet density in a system of hard hyperspheres; the instability examined corresponds to the liquid-to-simple hypercubic lattice transition. We propose that in the limit d→∞ the continuous bifurcation which occurs is at the maximum achievable density in a simple hypercubic lattice. Extension of this result to 1

We describe a lattice model for amphiphile monolayers in which amphiphile configurational flexibility and amphiphile–amphiphile interaction in and out of the surface layer are explicitly treated. The analysis employs a mean-field approximation based on the work of Scheutjens and Fleer. The density distribution as a function of surface coverage and the equation of state of the monolayer are predicted. Monte Carlo simulations of the same model as treated in the mean-field approximation show that the predicted density profile is quite accurate except near [lowercase_phi_synonym]=0 and that the film thickness as a function of surface coverage is well represented except near [lowercase_phi_synonym]=0, but that the predicted liquid–gas phase boundary is rather inaccurate. [ABSTRACT FROM AUTHOR]

We describe a lattice model of a supported monolayer of flexible amphiphile molecules which treats their full three-dimensional structure and includes nearest neighbor interactions between segments of the amphiphile chains, an attraction between the chain segments and the underlying substrate, and the inter- and intrachain excluded volume. The thermodynamic and conformational properties of this system are determined by a Monte Carlo simulation technique using a Metropolis sampling. At low surface coverage, the thermodynamic behavior predicted by the simulations is significantly different from that predicted by the mean-field theory of Wang and Rice; notably, the liquid–vapor critical point predicted by the mean-field theory is too high. In contrast, the mean-field theory prediction of the density profile along the normal to the surface is rather good. When the surface coverage is small, the conformations of the chains are sensibly independent of density. However, as the surface coverage increases the interchain interactions compress the chain parallel to the surface and force extension of the chain along the normal to the surface. Even at infinite dilution significant portions of the chains lie in the planes above the plane in which the head group lies. [ABSTRACT FROM AUTHOR]

We describe a generalized regular solution model of a liquid supported monolayer of long chain amphiphile molecules. The model includes the effects of chain flexibility in the surface layer and in the space outside the surface layer, but the neglect of chain–chain interactions outside the surface layer restricts the domain of applicability of the analysis to small to modest surface coverage (up to the critical density). The analysis leads to self-consistent equations for the distribution of molecular configurations and the surface density and pressure. An examination of available data for pentadecanoic acid on water (pH=2) leads to the strong inference that even at infinite dilution individual molecules take up configurations with a large part of the chain outside the surface layer. This inference, and others drawn from the theory, are supported by Monte Carlo simulations reported in the accompanying paper. An extension of the model includes the influence of aggregation of amphiphile molecules on the properties of the monolayer. [ABSTRACT FROM AUTHOR]

We report the results of calculations, at five temperatures ranging from 243 to 313 K, of the distributions of non-short-circuited rings of hydrogen-bonded molecules in TIPS-2 water. These data are used to test two hypotheses concerning the use of ring transformations as collective excitations in water. The suggestion by Speedy that it is the concentration of five-membered rings that determines the anomalous properties of water, and the suggestion by Rice, Belch, and Sceats that it is the ‘‘reactions’’ 2(6 ring)⇄(5 ring)+(7 ring) and 8(6 ring)⇄2(4 ring)+4(8 ring) that determines these properties, are both shown to be incomplete. Further verification of the random network model assumption of separation of time scales for network relaxation and local molecular motion is presented. [ABSTRACT FROM AUTHOR]

We report the results of experiments which probe the vibrational energy redistribution which accompanies fragmentation of deuterobenzene–helium van der Waals molecules. Branching ratios for vibrational relaxation to various final states were obtained for initially prepared levels 61-He1,2, 61162-He1, 111171-He1, and 172-He1 in several deuterobenzene compounds. The probabilities of transitions to final states in the relaxation from 61-He1 show general agreement with an energy gap law, and relaxation from 61162-He1, 111171-He1, and 172-He1 favors small vibrational quantum number changes. The experiments were undertaken to illuminate how molecular symmetry influences the choice of pathways for vibrational energy redistribution associated with van der Waals molecule predissociation. The data for the various C6H6-nDn·He1,2 systems, however, show very different relaxation patterns. These patterns cannot readily be explained by the usual intramolecular vibrational coupling mechanisms. [ABSTRACT FROM AUTHOR]

The x-ray reflectivity and fluorescence, as functions of angle of incidence and angle of detection, are calculated for several models of the longitudinal density distribution in the liquid–vapor interface. The models studied range from one descriptive of the monotone density distribution in a dielectric liquid–vapor interface to one descriptive of the stratified density distribution predicted for the metal liquid–vapor interface. In addition, we report calculations for the liquid–vapor interfaces of several simple binary alloys, each characterized by varying degrees of surface segregation of the components. Our results suggest that x-ray reflectivity and fluorescence intensity variations with angle of incidence and angle of detection can be sensitive probes of density and composition variations along the normal to the interface for pure metals and alloys. [ABSTRACT FROM AUTHOR]

We examine the dynamics of isomerization reactions and point out the existence of important phase space bottlenecks. A three-state statistical theory based on the behavior of trajectories and the properties of the phase space bottlenecks is proposed; this theory is better than standard two-state RRKM theory under some circumstances and offers a new way of conceptualizing isomerization. We test our ideas on the simple system, consisting of an equivalent double well coupled to a Morse oscillator, which was studied by DeLeon and Berne. Approximate methods of obtaining the required elementary rate constants in our theory are also discussed. [ABSTRACT FROM AUTHOR]

Examines the nature of the photoselective adiabatic passage approach proposed for product formation control in chemical reactions. Insight into chemical reaction control in strong fields; Prediction of sodium dimer physical realistic simulation results; Use of intense laser fields to control chemical reactivity.

Examines the changes in the distribution of an observable nature of high frequency modes of the molecule through pulse spectroscopy. Extension of the treatment to allow for two-pulse process; Application of results to two models which have a qualitative resemblance to bacteriorhodopsin.

The classical dynamics pertinent to van der Waals molecule vibrational predissociation of a T-shaped model for HeI2(B) is examined. An interesting phase space structure involving nonlinear resonances and stochastic motion is found. For low initial vibrational excitations of the I2 partner of the vdW complex the relevant part of phase space is dominated by quasiperiodic motion indicating a purely quantal mode of decay (‘‘dynamical tunneling’’), but for higher initial vibrational excitations van der Waals molecule predissociation is a classically allowed process. Classically determined rates of decay agree to within a factor of 3 with the rates calculated from quantum mechanics. [ABSTRACT FROM AUTHOR]

We report an analysis of the vibrational spectrum of the water dimer, including Fermi resonance and the adiabatic coupling between intra- and intermolecular modes. Three recently proposed pair potentials for water, each of which includes a representation of the intra- and intermolecular force fields, are tested by comparing the predicted and observed infrared spectra. Our results show that dynamical effects in the vibrational spectrum are underestimated by these potentials. Despite the considerable uncertainty in the experimental data it is clear that currently available water–water potentials are too inaccurate to be trusted except for predictions of the qualitative properties of the vibrational spectrum. [ABSTRACT FROM AUTHOR]

We describe a representation of coherent anti-Stokes Raman spectroscopy (CARS) suitable to the description of time-dependent measurements. This representation includes the standard energy frame formulation as a special case when cw fields are involved. The traditional CARS cw field frequency matching condition ω0=ω1-ω2+ω3 must be generalized for non-cw fields; in that case it refers to the Fourier component at ω0 of the convolution of the radiation field with the wave packet recurrences. The influence of resonance, both in the ground and electronically excited states, on the decay of time delayed CARS signals is discussed. As expected, intramolecular vibrational redistribution on the ground state potential surface of a molecule causes the CARS signal to decay in time. Model calculations show that quantum beats in the CARS signal may be observed, reflecting either a small number of coupled states (strong and regular recurrences) or sequential coupling of states (weak and early recurrences). [ABSTRACT FROM AUTHOR]

A time-dependent formulation of two-photon spectroscopy is employed to show that selectivity of reactivity can be achieved via coherent two-photon processes. The problem of finding the optimum waveform (i.e., coherent pulse sequence) that will maximize the formation of a desired chemical species is formulated as a problem in the calculus of variations, and solved for two different cases. [ABSTRACT FROM AUTHOR]

We report a study of very low energy collision induced vibrational relaxation using an approximate resonant state formalism which relates the inelastic cross section to the properties of metastable states. A study of models loosely based on the He+I2 (B 3Π0+u) system reveals that the combined effect of a low collision energy resonance and high initial diatomic vibrational excitation can lead to a large enhancement of the vibrational relaxation cross section. However, even in the absence of resonances the cross section, for very low energies, increases with decreasing energy. It is found that this effect increasingly enhances the contribution of a resonance to the cross section the closer the resonance is to zero collision energy. Because the density of resonances near zero collision energy is small the collision dynamics near zero collision energy is very sensitive to the nature of the potential energy surface. [ABSTRACT FROM AUTHOR]

It is conjectured that the hard sphere system has several distinct solid phases, all but one of which are metastable. The bifurcation theory analysis of freezing is extended to the description of the transition between a supercooled liquid and a disordered solid by defining a restricted phase space for the disordered solid. This approach leads to the prediction of a first order transition between a supercooled hard sphere fluid and a disordered metastable hard sphere solid. The results of the calculation are in qualitative agreement with the results of Woodcock’s molecular dynamics computer simulations. [ABSTRACT FROM AUTHOR]

We describe a simple, systematic and physically transparent method for calculating the elastic constants of condensed matter. This approach is equally useful when applied to such diverse materials as alkali halides and nematic and smectic A liquid crystals, as we report in this paper. Our analysis involves regarding the periodic density of the ordered phase to be representable as a small perturbation to the uniform density distribution of the corresponding fluid phase. We implement this idea by making use of recent work on the density wave theory of freezing and the statistical mechanics of nonuniform systems. The theory makes the role of the structure of the medium explicit while leaving the role of the intermolecular potential implicit. We find, for example, that the elastic constants of an alkali halide crystal can be expressed in terms of the curvatures of the Fourier transforms of the charge–charge and number–number direct correlation functions of the corresponding fluid evaluated at the reciprocal lattice vectors of the Bravais lattice. We also report molecular representations for the Frank elastic constants of a nematic liquid crystal and a smectic A liquid crystal. [ABSTRACT FROM AUTHOR]

This work examines, both experimentally and theoretically, nonradiative decay processes in a series of substituted naphthalenes. We report single vibronic level fluorescence lifetimes and fluorescence excitation spectra of jet-cooled 2-chloronaphthalene, 1- and 2-fluoronaphthalene and 1- and 2-methylnaphthalene over an energy range of about 0–4000 cm-1 in S1. While the 00 nonradiative rates of these molecules vary by a factor of 30, the energy dependences of the nonradiative rates are quite similar. At low vibrational energies the nonradiative rates depend sensitively on the level excited, but in general they increase with energy. As energy increases, the nonradiative rates become less sensitive to the level excited and eventually become almost independent of vibrational energy. We can qualitatively predict this behavior using a thermodynamic formalism which treats the density of states as an intramolecular entropy and avoids the calculation of vibrational coupling terms. In addition we report the fluorescence lifetime of the S1 vibrationless level of 1-chloronaphthalene, and single vibronic level fluorescence spectra of levels up to 1396 cm-1 in 2-chloronaphthalene. Most of the fluorescence excitation and fluorescence spectra show substantial enhancement of the origin inensity and vibrational mode mixing in S1 compared to naphthalene. We briefly discuss these substituent effects and make some tentative assignments of S1 vibrational levels. We also discuss substituent effects on the 00 fluorescence lifetimes. In particular, the order of magnitude increase in the decay rate of 1-chloronaphthalene relative to 2-chloronaphthalene cannot be explained using CNDO/S calculations. [ABSTRACT FROM AUTHOR]

This work examines, both experimentally and theoretically, the lifetimes of nominally degenerate benzene 1B2u levels split by differential coupling of the vibrational angular momentum components. We report fluorescence lifetimes for the 61162, 61102, and 61172 levels of the S1 electronic state in cold C6D6 in a supersonic jet. The higher energy vibrational angular momentum component has the longer lifetime. Calculations of the nonradiative decay rates for these levels are presented. Calculations based on conventional radiationless transition theory, assuming the prepared state is a single vibronic level, are not able to explain the experimental results, and we conclude that another mechanism must be responsible for the observed lifetime differences. We present a qualitative explanation of the data that takes into account perturbations of zeroth-order harmonic oscillator states by cubic and quartic vibrational anharmonicities and by Coriolis coupling. These perturbations affect the split vibrational angular momentum components differently and are capable of producing differences in lifetimes. [ABSTRACT FROM AUTHOR]

We report measurements of the in-plane (transverse) structure function, S||(k), of the liquid–vapor interface of Hg at 298 K, obtained using the method of grazing incidence x-ray diffraction. Our results show that: (i) The transverse structure function of the liquid–vapor interface of Hg is very similar to the bulk liquid structure function, but there are subtle differences between the two functions. (ii) As k→0, S||(k) appears to diverge as k-2, in agreement with theoretical predictions and computer simulation studies. (iii) The observable S||(k) for different longitudinal (parallel to the normal) density distributions in the liquid–vapor interface are very nearly the same, the largest differences being for momentum transfers less than that at the first peak of S||(k). For this reason the measurement of S||(k) is not a good monitor of the longitudinal density distribution. (iv) The interpretation of the differences in S||(k) in the small k region (but outside the region where the k-2 divergence overtakes other effects) requires consideration of the effect of the longer ranged part of the effective pair potential and assessment of the relative contributions of the ionic and electronic components of the liquid metal to the compressibility. A comparison of the shape of S||(k) in the threshold region just before the first peak and that of the same region in a bulk liquid metal leads to the weak conclusion that our data are consistent with the Monte Carlo simulations which predict that the liquid–vapor interface of Hg is stratified for about three atomic layers. [ABSTRACT FROM AUTHOR]

We present a novel approach to the control of selectivity of reaction products. The central idea is that in a two-photon or multiphoton process that is resonant with an excited electronic state, the resonant excited state potential energy surface can be used to assist chemistry on the ground state potential energy surface. By controlling the delay between a pair of ultrashort (femtosecond) laser pulses, it is possible to control the propagation time on the excited state potential energy surface. Different propagation times, in turn, can be used to generate different chemical products. There are many cases for which selectivity of product formation should be possible using this scheme. We illustrate the methodology with numerical application to a variety of model two degree of freedom systems with two inequivalent exit channels. Branching ratios obtained using a swarm of classical trajectories are in good qualitative agreement with full quantum mechanical calculations. [ABSTRACT FROM AUTHOR]

A plot of final vibrational action vs initial vibrational phase for a two degree of freedom model for He + I2 collisions is examined and shown to exhibit a chattering region. The chattering region is shown to exhibit a very intricate structure, including an infinite number of regions of regularity which we term icicles. Structure is evident on all scales of examination and a fractal dimension close to 2 is obtained for some parts of the chattering region. The survival probability associated with the complexes shows an initial fast decay, due to the icicles, but can be roughly characterized over a longer and wider time range by a more slowly decaying exponential. [ABSTRACT FROM AUTHOR]

We report the results of two studies of aspects of the consistency of truncated nonlinear integral equation based theories of freezing: (i) We show that the self-consistent solutions to these nonlinear equations are unfortunately sensitive to the level of truncation. For the hard sphere system, if the Wertheim–Thiele representation of the pair direct correlation function is used, the inclusion of part but not all of the triplet direct correlation function contribution, as has been common, worsens the predictions considerably. We also show that the convergence of the solutions found, with respect to number of reciprocal lattice vectors kept in the Fourier expansion of the crystal singlet density, is slow. These conclusions imply great sensitivity to the quality of the pair direct correlation function employed in the theory. (ii) We show the direct correlation function based and the pair correlation function based theories of freezing can be cast into a form which requires solution of isomorphous nonlinear integral equations. However, in the pair correlation function theory the usual neglect of the influence of inhomogeneity of the density distribution on the pair correlation function is shown to be inconsistent to the lowest order in the change of density on freezing, and to lead to erroneous predictions. [ABSTRACT FROM AUTHOR]