bluuebunny/crossref_metadata_2025 huggingface dataset:
Self-diffusion across the liquid–crystal interface
Authors: Bagchi Biman
Published: July 2002

We report a theoretical study of the variation of the self-diffusion coefficient across a liquid–crystal interface at equilibrium. The study is based on the assumption that the interface is broad compared to the molecular diameter of the diffusing particle. The structure at the interface is described by a set of position-dependent order parameters which change smoothly across the interface from zero in the pure liquid to nonzero constant values in the pure solid phase. The effective potential experienced by a particle at the interface is expressed in terms of these order parameters. A Smoluchowski equation is used to describe the dynamics in the presence of this effective potential. The calculated self-diffusion coefficient changes sharply over a length of 4–5 molecular layers in the interface and reproduces the essential features observed in recent computer simulations. We find that significant topological ordering takes place on the liquid side of the interface before the self-diffusion coefficient shows any detectable change from its liquid phase value. We discuss the limitations of the present approach and propose future applications of the model.
Concentration-dependent fluorescence quenching and electron scavenging in liquids
Authors: Cukier R. I.
Published: July 2002

We present a theory for the dependence of the fluorescence quenching lifetime τ on quencher concentration and the dependence of the electron-scavenger rate constant kf on scavenger concentration. At low quencher (scavenger) concentration these quantities are obtained by solution of the Smoluchowski–Debye diffusion equation. For nondilute quenchers we find positive deviations from the linear Stern–Volmer equation for 1/τ and for nondilute scavengers we find positive deviations from linear behavior in kf. With reactants that interact by a long range potential, such as charged fluorophores and quenchers, and electrons with charged scavengers, a length ã∼100–300 Å vs the collisional encounter length a ∼1–10 Å is the significant length. Corrections to dilute behavior are found to depend on the effective volume fraction of quenchers (or scavengers) φ̃=4π3ã3c/3 which can be much larger than the material volume fraction φ=4π3a3c/3 [c is the quencher (or scavenger) number density]. We consider both the initial value and steady state situations and find that, for our exact results, no transient behavior of the rate coefficient enters the initial value expression.
Multiphoton ionization spectroscopy of ClO and BrO
Authors: Duignan Michael T. , Hudgens Jeffrey W.
Published: July 2002

We report the resonance enhanced multiphoton ionization spectra of ClO and BrO free radicals between 415 and 475 nm. The observed electronic states were prepared by simultaneous absorption of three identical photons from a dye laser. Absorption of at least one additional photon induced ionization. ClO showed spectra originating from the D, E, and F states previously reported by N. Basco and R. D. Morse. The origin of the F 2Σ←X 2Πi bands were reassigned to ν00=70 183 cm−1 and ν00=69 868 cm−1. BrO showed three new vibrational progressions originating from transitions between the X 2Π3/2 state to Rydberg states with assignments of E 2Σ (ν00=65 003 cm−1), F 2Σ (ν00=67 470 cm−1), and an apparently inverted state, designated G (ν00=70 504 cm−1), that possesses orbital angular momentum greater than zero. The G-state bands were separated by 139(±3) cm−1 which should approximate the magnitude of the spin-orbit coupling constant of this excited state if it is of 2Πi symmetry.
The properties of CO and K co-adsorbed on Ni(111), studied by thermal desorption and metastable quenching spectroscopy
Authors: Lee Jiwha , Arias Jose , Hanrahan Ciaran P. , Martin Richard M. , Metiu Horia
Published: July 2002

We use thermal desorption and metastable quenching spectroscopy to study the (CO+K)/Ni(111) system. We find that CO and K co-adsorption increases the binding energy (to the surface) of both molecules; that at coverages of more than one potassium atom per, roughly, eight nickel atoms the adsorbed CO molecules exchange atoms; that this exchange takes place without formation of a detectable (by MQS) number of adsorbed carbon and oxygen atoms; that the 2π* peak in the Penning spectrum is enhanced by the presence of the potassium; that at high potassium coverages CO and K desorb at practically the same temperature giving rise to a very narrow thermal desorption peak. We discuss the spectroscopic evidence concerning the detection of the 2π* orbital. We suggest that our experiments indicate that at low potassium coverage the surface has two local work functions: one corresponds to Ni atoms unmodified by the presence of potassium, and the other to strongly modified, low work function sites.
An improved potential energy surface for F+H2→HF+H and H+H′F→HF+H′
Authors: Brown Franklin B. , Steckler Rozeanne , Schwenke David W. , Truhlar Donald G. , Garrett Bruce C.
Published: July 2002

We present an improved analytic potential energy surface for the F+H2→FH+H and H+FH′→HF+H′ reactions. The final surface is obtained in two stages. First we create a surface, called No. 4, which is based in the F–H–H barrier region on a previous partly empirical and partly theoretical fit and is based on the F–H⋅⋅⋅H exit channel and H–F–H barrier regions on new large-basis-set configuration interaction calculations. The final surface, called No. 5 incorporates more empirical information for collinear geometries in both the F–H–H and FH⋅⋅⋅H regions but remains a good representation of the ab initio calculations for bending potentials and in the strong-interaction regions. Variational-transition-state theory rate constants and WKB adiabatic barrier heights indicate that the final surface is more accurate than previous surfaces for thermal rate constants and overall reaction thresholds for F+H2→HF+H, F+D2→DF+D, and F+HD→HF+D and for product-state thresholds for HF (n′=3) and DF(n′=4), where n′ is the final vibrational quantum number.
Theory of resonantly enhanced multiphoton processes in molecules
Authors: Dixit S. N. , McKoy V.
Published: July 2002

In this paper we formulate a theory for the analysis of resonant enhanced multiphoton ionization processes in molecules. Our approach consists of viewing the (n+m) photon ionization process from an isotropic initial state as m-photon ionization out of an oriented, excited state. The orientation in this resonant state, which is reached by n-photon excitation from the initial state, is nonisotropic, and is characteristic of this absorption process. The ionization simply probes this anisotropic population. The calculation of the REMPI process thus consists of determining the anisotropy created in the resonant state and then coupling this anisotropic population to ionization out of it. While the former is accomplished by the solution of appropriate density matrix equations, the latter is done by coupling these density matrix elements to angle-resolved ionization rates out of this state. An attractive feature of this approach is that the influence of saturation effects, and other interactions, such as collisions, on the photoelectron properties is easily understood and incorporated. General expressions are derived for photoelectron angular distributions. Based on these, several properties of the angular distributions that follow purely on symmetry considerations are discussed. One of the new features that emerge out of this work is the saturation induced anisotropy in REMPI. In this effect higher order contributions to the angular distributions appear since saturation influences different ionization channels differently thereby creating an additional anisotropy in the excited state.
Pressure effect on the coupling between rotational and translational motions of supercooled viscous fluids
Authors: Artaki I. , Jonas J.
Published: July 2002

Spin-lattice relaxation times, viscosities, and densities have been measured as a function of temperature and pressure for several supercooled viscous fluids in order to investigate the adequacy of the modified Debye equation to describe the microscopic behavior of these systems. By supercooling the range of experimentally accessible viscosities has been extended to 10 000 cP. It has been shown that for anisotropically reorienting molecules, the rotational-translational coupling parameter increases with increasing density, in contrast to the case of symmetric molecules which possess an axis of symmetry for relatively unhindered reorientations, for which the coupling parameter decreases with increasing density. By examining a sufficiently broad viscosity range, it has been possible to show that the rotational-translational coupling parameter is not a pressure independent quantity, as previously reported. Under isochoric conditions, the coupling parameter was found to be insensitive to kinetic energy changes. It was concluded that volume rather than kinetic energy plays the decisive role in determining the nature and the degree of the coupling.
Ultraviolet resonance Raman spectroscopy of benzene vapor with 220–184 nm excitation
Authors: Gerrity Daniel P. , Ziegler Lawrence D. , Kelly Peter B. , Desiderio Russell A. , Hudson Bruce
Published: July 2002

Raman scattering spectra of benzene vapor with excitation in the 220–184 nm region reveal strong enhancement of the binary overtones and combinations of the e2g vibrations ν8 and ν9, supporting the conclusions of a previous study of benzene with 213 nm radiation [Ziegler and Hudson, J. Chem. Phys. 74, 982 (1981)]. The fact that e2g vibrations are vibronically active with all excitation wavelengths confirms the assignment of the electronic state in this region as one of B1u symmetry. The dominance of the spectra at all excitation wavelengths by transitions involving ν8 and ν9 demonstrates that these are the particular motions with the greatest vibronic activity. Progressively higher excitation energies result in increased relative intensity in higher order combinations and overtones. The appearance of weak bands corresponding to two quanta of the a2u mode ν11 in the 213 and 204 nm spectra provides tentative evidence for Raman activity of the E1g symmetry state in this 6 eV region. The characteristic symmetric ring expansion mode ν1 at 992 cm−1 becomes very weak in the 204 and 200 nm spectra but reappears as a very strong progression in the 184 nm spectrum. This behavior is reminiscent of that observed for methyl benzenes with 213 nm excitation [Ziegler and Hudson, J. Chem. Phys. 79, 1134 (1983)] where progressive methyl substitution shifts the 1La band to lower energy. This intensity variation is shown to be due to destructive interference between the vibronic C-type contribution to the intensity and A-type contributions from higher energy allowed transitions. The 184 nm spectrum shows very strong bands due to even overtones and combinations of the b2g mode ν4 and combinations of these bands with strong ν1 progression. This demonstrates that either there is a vibronically active electronic state with E2u symmetry in this region or that the E1u state is strongly modified relative to the ground state for displacement along this b2g coordinate.
Microwave detected, microwave–optical double resonance spectra of NO2: A test of Hardwick’s ergodicity conjecture
Authors: Lehmann Kevin K. , Coy Stephen L.
Published: July 2002

In a recent paper, Hardwick predicted that the optical spectrum of NO2 should display ‘‘quantum ergodicity’’, meaning that almost all eigenstates allowed by total symmetry should be seen in absorption. We have performed a test of this prediction by using the technique of microwave detected, microwave–optical double resonance. We have determined the spectrum over the range 16 810–17 100 cm−1 for the two ground vibrational states 91,9(F1) and 100,10(F1) of NO2. We observed 324 transitions from the 100,10 state and 364 transitions from the 91,9 state. The number of observed transitions is a factor of 8 greater than that expected if only the allowed rotational transitions to any B2 symmetry vibronic level were observed. The number of observed lines is a factor of 3 less than the number predicted if all selection rules were broken as predicted by Hardwick. Thus we see that Hardwick’s ‘‘ergodicity’’ conjecture is a useful starting point for understanding the spectrum of NO2, but the spectrum is intermediate between the regular and the ergodic limits.
The secular equation/perturbation theory method for calculating spectra of van der Waals complexes
Authors: Hutson Jeremy M. , Le Roy Robert J.
Published: July 2002

A new method for calculating vibrational wave functions and level energies for van der Waals molecules is proposed. It consists of a minimal secular equation calculation, including only strongly coupled basis functions, followed by perturbation theory corrections to take account of the effects of weakly coupled channels. The method is very efficient, and gives results accurate to ±0.001 cm−1 for Ar–H2. It is also readily applicable to predissociating states of van der Waals molecules, and gives calculated level widths more accurate than any previous approximate method. The method obviates the resonance search problem associated with direct coupled channel methods for characterizing predissociating states, and is readily extendable to more strongly anisotropic systems.
Trapping-desorption scattering of argon from Pt(111)
Authors: Hurst J. E. , Wharton L. , Janda K. C. , Auerbach D. J.
Published: July 2002

Measurements of the velocity and angular distributions for trapping-desorption scattering of argon from a clean, well characterized Pt(111) single crystal are reported. For certain experimental conditions, both the characteristic velocity and angular distributions deviate markedly from that predicted using equipartition arguments (i.e., a Maxwellian flux distribution in velocity and a cosine distribution in angle). The average kinetic energy for the flux exiting normal to the surface at 100 K is only 80% of that expected for a Maxwellian at TS. This kinetic energy deficit decreases and approaches zero as the detector is rotated away from the surface normal. The angular flux distribution is found to be broader than cosine. These results are discussed in terms of microscopic reversibility which permits estimates of the velocity dependent condensation coefficient to be obtained.
The potential of mean force between polyatomic molecules in polar molecular solvents
Authors: Pettitt B. Montgomery , Karplus Martin
Published: July 2002

Theoretical results obtained with the extended reference interaction site (XRISM) formalism are presented for site–site solute solvent correlations and solute–solute potentials of mean force for infinitely dilute polar molecular solutes in various polar solvents. The standard RISM site–site Ornstein–Zernike like equations, in a Coulomb renormalized form, with a hypernetted chain (HNC) analog closure are used to derive results for polar molecular solutes in polar molecular solvents. For a dipolar diatomic solute the difference in the solvation behavior between atomic and molecular solvents is examined. Finite concentration results are compared with the infinite dilution intermolecular site–site potentials of mean force for diatomic molecules in a simple fluid solvent.
A simple model for the intermolecular potential of water
Authors: Bratko D. , Blum L. , Luzar A.
Published: July 2002

A simple potential for the pair interaction of two water molecules is proposed. This model is intended for use in ionic solvation problems, and is a step beyond the ion dipole model proposed by Adelman, Deutch, and Blum to study molecular effects in solvation. It consists in adding to the hard spheres with a point dipole a narrow potential well with a tetrahedral octupole of the form v(θ,φ)=A(r)sin θ sin 2θ cos 2φ. We test this potential and two improvements by running a Monte Carlo simulation on a system of 70 molecules. The results show that our model reproduces surprisingly well the essential features of the structure of water, namely the rather well defined tetrahedral structure of the first coordination shell, and the very poorly structured second and third coordination shells.
High-pressure studies of rotational reorientation dynamics: The role of dielectric friction
Authors: Philips Laura A. , Webb S. P. , Clark J. H.
Published: July 2002

Picosecond, time-resolved, fluorescence depolarization spectroscopy is used to measure the rotational reorientation times of rhodamine 6G (R6G) and p-terphenyl (PTP) as a function of solvent viscosity. The viscosity is varied either by changing the solvent or by changing the pressure in a single solvent. The differences between the two molecules, PTP and R6G, provide a means of evaluating the role of solute structure and solute–solvent interactions on the dynamics of rotational reorientation. The rotational behavior of PTP is well described by simple hydrodynamic models as embodied by the Stokes–Einstein–Debye equation. In contrast, the rotational reorientation dynamics of the charged molecule R6G are not well described by these models. It is demonstrated that dielectric friction plays an important role in governing the rotational motion of charged molecules in polar solvents. When the solvent dielectric properties are varied, the dielectric friction model accurately predicts the observed experimental trends under a wide variety of experimental conditions. This model is also shown to explain anomalous effects previously attributed to the presence of solute–solvent hydrogen bonded complexes.
Classical trajectory study of vibration-rotation interaction in highly excited triatomic molecules
Authors: Frederick John H. , McClelland Gary M. , Brumer Paul
Published: July 2002

For two qualitatively different model triatomic molecules, SO2 and ‘‘bent equilibrium OCS,’’ classical trajectory calculations of the rotational and vibrational motion are presented for microcanonical ensembles of initial conditions at 35% to 85% of a bond dissociation energy. At the higher energies, many of the trajectories exhibit substantial intramolecular vibrational-rotational energy transfer (IVRET), which has a significant effect on the geometry of rotational motion including in some cases, a transition between the two types of stable asymmetric top motion. IVRET is caused principally by centrifugal interactions, and in ‘‘bent OCS,’’ it is dominated by a 2:1 resonance. The rotational motion of about half of the high energy SO2 trajectories is essentially statistical, but bent OCS never undergoes statistical IVRET.
New developments in the model potential method: ScO molecule
Authors: Andzelm Jan , Radzio Elżbieta , Barandiarán Zoila , Seijo Luis
Published: July 2002

An extension of Huizinaga’s model potential method is presented and applied to the study of the equilibrium geometry of the ground electronic state of ScO. In the extended model potential method (EMP) the exchange interaction between the outer core and valence shells are handled more efficiently than in the standard MP method; furthermore, the long range behavior of the model potential is now independent on the reduction of the valence basis set. Using the new method, agreement with all electron results can be achieved in the difficult case of ScO molecule within 0.01 Å in Re, 20 cm−1 in ωe, 0.1–0.2 eV in the binding energy, and an order of 10−3 a.u. in the valence orbital energies.
Supercritical fluid phase separations: Implications for detonation properties of condensed explosives
Authors: Ree Francis H.
Published: July 2002

High explosive experiments offer the most extensive data on mixtures at high pressures (>10 GPa=100 kbar) and high temperatures (>1000 K). We have computed the detonation properties of two explosives, PBX-9404 (C1.4H2.75N2.57O2.69Cl0.03P0.01) and PETN (C5H8N4O12), using reliable statistical mechanical theories and realistic intermolecular potentials. The composition of the chemical species is determined by minimizing the Gibbs free energy. The calculation shows that the detonation products of explosives containing C, H, N, and O atoms can separate into N2-rich and N2-poor fluid phases and that this gas–gas phase separation can affect detonation properties at some pressures and temperatures. Since N2, CO2, and H2O molecules are major detonation products, we made a separate study on binary (N2–H2O, CO2–H2O, N2–CO2) and ternary (N2–CO2–H2O) mixtures. The results of this study show that the N2–H2O system may exhibit a fluid phase separation at pressures and temperatures relevant to a detonation environment. The predicted phase separation boundary is sensitive to the N2–H2O interaction and to the addition of CO2 molecules. We suggest static and dynamic experiments on N2–H2O mixtures that should reveal whether the predicted fluid phase separation occurs and that will help us refine the N2–H2O interaction potential.
Scattered light intensity cross correlation. II. Experimental
Authors: Mos H. J. , Pathmamanoharan C. , Dhont J. K. G. , de Kruif C. G.
Published: July 2002

We have studied the cross correlation of scattered light intensity by doing experiments on well-defined colloidal systems. The results are in agreement with theoretical predictions: multiple scattering does not contaminate the experimental cross-intensity correlation function, whereas the experimental auto-intensity correlation function, which contains higher order scattering effects, does not give unambiguous results.
Enhanced stability of ion–clathrate structures for magic number water clusters
Authors: Nagashima Umpei , Shinohara Hisanori , Nishi Nobuyuki , Tanaka Hideki
Published: July 2002

The near threshold vacuum-UV photoionization of water clusters has been performed by using a resonance line emission of argon at 11.83 eV. The well-known intensity anomaly at the cluster ion (H2O)21H+ is observed even in this threshold photoionization, for the first time, with very small excess energy. Structures for the water cluster ions (H2O)21H+ and (H2O)28H+ which exhibit enhanced structural stability (magic number), are presented based on Monte Carlo simulations as well as on the analogy of our previous study on the stability of the (H2O)20NH+4 ion. The Monte Carlo calculations are carried out at the temperatures of 200, 150, 100, and 50 K for the ionized water clusters (H2O)nH+ around n=21 and also around n=28, which includes the ionic hydrogen-bond interactions between an H3O+ ion and neutral H2O molecules. The clusters (H2O)21H+ and (H2O)28H+ have greater binding energies per molecule than their neighbors although the enhancement of the latter is somewhat temperature dependent. The calculations suggest that the stable structures for (H2O)21H+ and (H2O)28H+ are represented by the ion–clathrate (ion-centered cage) configurations with either an H2O+ or (H2O ⋅ H3O)+ ion trapped inside the cage, respectively. The simulations for (H2O)20 ⋅ H3O+ also suggest that the cluster is especially stable due to the strong Coulombic interaction between the encaged H3O+ ion and the surrounding 20 water molecules which form a deformed pentagonal dodecahedral cage. The stability of the (H2O)26(H2O ⋅ H3O)+ ion can also be explained consistently within the framework of the present ion–clathrate model.
Semiclassical quantization by using the method of adiabatic switching of the perturbation: Fermi resonance
Authors: Grozdanov T. P. , Saini Subhash , Taylor H. S.
Published: July 2002

The method of adiabatic switching of perturbation is applied in semiclassical calculations of the eigenvalues of classically near-integrable Hamiltonian describing a two-dimensional coupled oscillator whose dynamics is dominated by an intrinsic 2:1 resonance. At low-to-intermediate excitations the semiclassical eigenvalues are generally in good agreement with quantum-variational results. Even at high excitations, close to the classical escape energy, useful information on the location of quantal levels can be obtained. Limitations of the method are discussed.
A numerical study of the multiple Gaussian representation of time dependent wave functions of a Morse oscillator
Authors: Heather Robert , Metiu Horia
Published: July 2002

We have studied the accuracy and the numerical stability of several methods for solving time dependent quantum problems in which a localized system is disturbed for a finite time. The methods assume that the wave function can be represented throughout the interaction time as a sum of Gaussian wave packets; the time evolution of the wave function is given by the evolution of the parameters in each packet. One of the methods propagates the packets independently and the other couples them by using a variational principle. Applications were made to calculate the time evolution of the ground state of a Morse oscillator and of a linear combination of the first four Morse states, and to calculate the vibrational excitation by coupling to a strong laser field. We have found that the coupled packets method provides accurate, numerically stable solutions for these problems, while the decoupled packet method fails for some of them. We have investigated the dependence of the results on the method used to fit the initial wave function to the sum of Gaussians and found that the nonlinear, least square fitting is superior to other methods. A particularly important observation is that the results obtained by the coupled Gaussians method are improved as the number of packets is increased but the propagation equations become unstable if too many packets are used to fit the wave function. Methods for monitoring and avoiding such instabilities are discussed in the paper. Our overall conclusion is that the use of the coupled, frozen Gaussians method may play an important role in the study of time dependent quantum processes in which localized quantum degrees of freedom are disturbed for a moderately long time (i.e., ten times the time constant of the system).
Model for phase equilibria in micellar solutions of nonionic surfactants
Authors: Goldstein Raymond E.
Published: July 2002

The formation of spherical micelles in aqueous solutions of nonionic surfactants and the equilibria between two such micellar phases are studied with a phenomenological model incorporating hydrophobic interactions and the configurational entropy of the amphiphiles. The distribution of micelle sizes is determined over the entire temperature-composition phase diagram, and moments of that distribution function determine the consolute point parameters. In a generalization of an analysis given by Stillinger and Ben-Naim, the mathematical properties of various thermodynamic functions in the neighborhood of the critical micelle concentration are related to the location of branch points of the osmotic pressure in the complex concentration plane. The model attributes the experimentally observed lower critical solution points in these systems to surfactant–water hydrogen bonding, whose temperature dependence is described with a mean field approximation. Calculated phase diagrams are in qualitative agreement with those from experiments, in particular, exhibiting closed solubility loops with quite distinct upper and lower critical compositions, and values for the lower critical composition on the order of several percent volume fraction. The relevance of certain aspects of the model to the understanding of microemulsions is discussed.
Improved counterpoise corrections for the a
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o calculation of hydrogen bonding interactions
Authors: Loushin Sharilyn K. , Liu Shi-Yi , Dykstra Clifford E.
Published: July 2002

Counterpoise ab initio calculations have been carried out for the hydrogen fluoride dimer using 14 different basis sets. A counterpoise calculation evaluates the stability contribution arising from just the mutual basis set enlargement of bringing two species into proximity. Using this stability contribution to correct the total interaction energy does not always improve reliability. Furthermore, counterpoise corrections have been found to be misleadingly small in certain cases, suggesting greater reliability than is warranted. The problem is that the counterpoise correction should be the stability contribution arising from the mutual basis set enlargement in the description of the perturbed rather than the isolated bonding partners. In hydrogen bonding, much of the perturbation is an electrical polarization of charge. Thus, we have found that more useful counterpoise correction values are obtained by employing a perturbing charge field. This charge field is approximately that which would arise from the species that is absent in the counterpoise evaluation.
The 2A1–2E electronic spectrum of methoxy, CH3O: Vibrational progressions, and spin-orbit and Jahn–Teller couplings
Authors: Brossard S. D. , Carrick P. G. , Chappell E. L. , Hulegaard S. C. , Engelking P. C.
Published: July 2002

The 310 nm electronic emission system of jet-cooled methoxy radical has been examined at 4 cm−1 resolution. The spectrum shows extensive progressions in the upper and lower state CO stretch frequencies, v′3 =660±2 cm−1 and v″3 =1045±3 cm−1. Two other lower state frequencies have been determined: the hydrogen stretch v″1 =2749±5 cm−1 and the hydrogen scissors v″5 =1499±6 cm−1. The Jahn-Teller effect activates v″5 with a coupling constant of k2=0.28±0.06, but does not appreciably activate either v″4 or v″6. The corresponding frequencies have been observed in deuterated methoxy. The change in the spin-orbit splitting upon deuteration, dropping from 62 to 56 cm−1, indicates that the purely electronic spin-orbit splitting is 98±11 cm−1, while the vibronic reduction factor in the hydride is 0.63±0.06.
Tunneling-induced disorder in solids
Authors: Stratt Richard M.
Published: July 2002

A variety of otherwise well ordered crystals exhibit tunneling-induced disorder at zero temperature, but there seem to be two distinct classes of such materials: hindered rotors, such as H2, D2, and CH4, which have their tunneling suppressed by external pressure, and hydrogen-bonded ferroelectrics, such as KH2PO4 (KDP), which have tunneling promoted by pressure. With the aid of a simple model which explicitly includes the effect of lattice dynamics on the barriers to tunneling, this paper is able to show that both kinds of behavior may be thought of as largely identical. In addition, the same discretized path integral methods used to study this model in mean-field theory turn out to permit the incorporation of a fluctuation correction. The correction reveals that the combination of lattice dynamics and fluctuation effects dramatically shifts the predicted critical pressure—perhaps by enough to account for some recently emphasized discrepancies between theory and experiment for H2 and D2.
Vibrational spectroscopy of van der Waals bonds: Measurement of the perpendicular bend of ArHCl by intracavity far infrared laser spectroscopy of a supersonic jet
Authors: Ray Douglas , Robinson Ruth L. , Gwo Dz-Hung , Saykally Richard J.
Published: July 2002

We report the development of a very sensitive laser technique for measuring the vibrational motions in van der Waals bonds. This new technique has been used to detect the lowest perpendicular bending vibration in ArHCl near 35 cm−1 with signal-to-noise ratios ≥104 and with resolution of nuclear hyperfine structure. A total of 80 Stark-hyperfine components of the R(0), Q(1), Q(2), rotational lines have been measured with a precision of ≤1 MHz and analyzed by least squares techniques to obtain precise values for the vibrational band origin [ν0=1018 731.20(31) MHz], the dipole moment [μ′=0.260 26(11) D], the effective rotational constant [(B′+C′)/2=1696.70(22) MHz], the l-type doubling constant [ql =−50.90(35) MHz], and the 35Cl quadrupole coupling constants [eqQ′aa=−6.5(4.1) MHz, eqQ′bb−eqQ′ cc=−74.4(2.2) MHz] in the excited perpendicular (l=±1) bending state. Interpretation of these constants indicates a strongly bent average geometry (θ>51°) for the excited state. These measurements strongly support the results obtained from empirical potential surfaces by Hutson, and Howard and Hutson.
Theory of time and frequency resolved resonance secondary radiation from a three-level system
Authors: Melinger J. S. , Albrecht A. C.
Published: July 2002

The perturbative density matrix formalism is used to study the time-dependent resonance Raman scattering (RRS) and resonance hot luminescence (RHL) components of the light-induced spontaneous emission process that has been called resonance secondary radiation (RSR). A three-level system coupled in a stochastic way to a bath is used to study the RSR emission driven by a coherent laser pulse with a variable Gaussian width in time. The spectral filter in a light detection system is explicitly included in the detection step of the problem. Observable time and frequency resolved RSR spectra are then calculated numerically. The spectral and temporal properties of the time-dependent RSR spectrum are very sensitive to the width of the driving pulse when the pulse width is of the same order of magnitude as the important system dephasing constants. In this domain the RRS contribution to the RSR spectrum weakens relative to the RHL as the light pulse is shortened. In the limit that the laser pulse becomes ‘‘delta function-like’’ the RSR spectrum consists essentially of RHL. When appropriate, the numerical results are compared to the stationary RSR problem as well as to the case where the RSR emission is driven by an exponential pulse. The analytical solutions to both of these cases are presented.
High pressure study of associated media: Raman scattering of pyridine complexes in aqueous solution
Authors: Zakin M. R. , Grubb S. G. , King H. E. , Herschbach D. R.
Published: July 2002

The ν1 and ν12 ring breathing vibrational modes of pyridine in aqueous solution were studied as functions of concentration (up to 12 M) and pressure (up to 35 kbar) in a diamond anvil cell. The pyridine isotopes -h5 and -d5 exhibited marked differences. The pressure dependence of ν1 is quite linear for pyr-h5 but has pronounced curvature for pyr-d5 and this curvature increases as the concentration decreases. This unusual isotope effect is attributed to pyridine–water complexes involving both O–H ⋅ ⋅ ⋅N and C–H ⋅ ⋅ ⋅O hydrogen bonds. For ν12, the pressure dependence does not change with concentration, but it is at least five times more steep for pyr-h5 than pyr-d5. This may occur because the mass at all six ring vertices becomes equal for the -d5 isotope, making the volume change during vibration very small. At high concentrations, ν1 appears as two distinct peaks with different pressure dependence, indicative of uncomplexed and complexed pyridine. For certain combinations of solvent and concentration, a strong pressure dependent Fermi resonance between the ν1 and ν12 modes appears. Similar experiments were done with a weakly associating solvent, dimethylformamide, and nonassociating solvents, toluene and benzene. For these, the pressure dependence did not change with concentration; this and other features indicate which aspects of the aqueous solution results can be attributed to pyridine–water and pyridine–pyridine complexes.
Approach to glassy behavior of dielectric relaxation in 3-bromopentane from 298 to 107 K
Authors: Berberian John G. , Cole Robert H.
Published: July 2002

Relaxation times determined by time domain reflection (TDR), bridge and transient measurements range from 12 ps at 298 K to 590 s at 107 K. Arrhenius temperature dependence with activation energy of 2.75 kcal and Debye relaxation function begin to change to the Fulcher–Vogel rate law and Cole–Davidson (skewed arc) form of relaxation well above the melting point. Methods used to establish that the Williams–Watts (stretched exponential) relaxation function gives a less satisfactory representation are described, as are methods for correcting systematic high frequency errors in TRD measurements, and implications for theory of the results are briefly discussed.
Photoionization cross sections of rovibrational levels of the B 1Σ+u state of H2
Authors: Rudolph H. , Lynch D. L. , Dixit S. N. , McKoy V.
Published: July 2002

We report theoretical cross sections for direct photoionization of specific rovibrational levels of the B 1Σ+u electronic state of H2. The calculated cross sections differ considerably from values recently determined by resonant enhanced multiphoton ionization (REMPI) studies. In an attempt to understand the disagreement, we analyze in detail the REMPI dynamics and find that the multiphoton ionization probability is extremely sensitive to the spatial and temporal profiles of the laser pulses. Accurate characterization of laser profiles and their jitter is therefore necessary for a comparison between theory and experiment.
Close-coupling wave packet approach to numerically exact molecule–surface scattering calculations
Authors: Mowrey R. C. , Kouri D. J.
Published: July 2002

In this paper we describe the theory and application of the recently developed close-coupling wave packet (CCWP) method to the study of transition probabilities of H2 scattered from flat and corrugated surfaces. We present an improved method of analyzing the final wave function which permits S matrices and transition probabilities to be obtained over a wide range of energies from the propagation of a single wave packet. Transition probabilities obtained using the CCWP method are in excellent agreement with those obtained from time-independent close-coupling (CC) calculations. For the present three-dimensional H2-corrugated surface scattering study, the CCWP calculations require roughly one-tenth of the computation time of the CC method. Our results indicate that the CCWP method should be a very efficient method of obtaining highly accurate scattering results both for standard collision problems and for collision problems which cannot be readily treated using standard CC methods.
Infrared rotational transitions in CH2 X̃ 3B1 observed by diode laser absorption
Authors: Sears Trevor J.
Published: July 2002

Five rotational transitions involving Ka=4←3 in the ground vibronic state of the methylene radical have been detected by diode laser absorption spectroscopy in the infrared at wavelengths around 25 μm. The radical was formed in a mild glow discharge in a dilute mixture of ketene in helium that was pumped continuously through the absorption cell, and the methylene absorption lines were detected using a Zeeman modulation technique. The solenoid used to generate the required oscillating magnetic field was powered by a simpler and less expensive electrical circuit than has previously been described for this type of spectroscopy.
Dynamics and spectroscopy of near threshold nonadiabatic resonances in photodissociation to open shell atoms: CH+ a model system
Authors: Williams Carl J. , Freed Karl F.
Published: July 2002

Close-coupled theoretical calculations of the near threshold photodissociation of CH+ exhibit a wealth of structure. This structure arises because of the presence of nonadiabatic interactions which always exist and which are significant at large distances when molecules dissociate to open shell atoms. Detailed analysis of the calculated photodissociation spectrum, using an extensive set of theoretical diagnostics, displays a series of systematic trends in the spectrum which enable the assignment of resonance features in this extremely complicated spectrum. The results show how general spectroscopic principles may be used in analyzing this type of complicated structure, and our calculations provide general insight into the experimental data for the photodissociation of CH+. The theoretical calculations also indicate how additional assistance with analyzing experimental data can come from measurements of fragment kinetic energy, angular distributions, and polarizations.
Theoretical dipole moments for the first-row transition metal hydrides
Authors: Chong Delano P. , Langhoff Stephen R. , Bauschlicher Charles W. , Walch Stephen P. , Partridge Harry
Published: July 2002

Spectroscopic parameters (De, re, μ) are determined for the first-row transition metal hydrides using better than DZP basis sets at the modified coupled pair functional (MCPF) level. Extensive comparisons between MCPF and complete-active space self-consistent field (CASSCF)/MRCI calculations with natural orbital iterations, and studies with more extensive basis sets, show this level of treatment to supply an accurate and cost-effective treatment of these systems. For the transition metal hydrides, the bonding can arise from either the 3dn4s2 or 3dn+14s1 atomic asymptotes, or a mixture of both. Since the dipole moment arising from these two bonding mechanisms is very different, the dipole moment is found to be directly related to the 3d population. Thus, the magnitude of the dipole moments provide a sensitive test of the wave function, and gives insight into the nature of the bonding.
Vibrational predissociation of the nitric oxide dimer: Total energy distribution in the fragments
Authors: Casassa Michael P. , Stephenson John C. , King David S.
Published: July 2002

Rotational, spin-orbit, lambda doublet, and kinetic energy distributions were measured by laser-excited fluorescence techniques for the nitric oxide fragments formed from the vibrational predissociation of nitric oxide dimers in a free jet expansion. The NO fragments, produced following excitation in the dimer ν1 fundamental, were described by a rotational ‘‘temperature’’ of TR≂100 K, with full equilibration of lambda doublet states, and approximately equal populations in the two spin-orbit states. The velocity distributions were isotropic with an average fragment kinetic energy of 400 cm−1. Time-resolved measurements placed a 15 ns upper limit on the predissociation lifetime.
Homogeneous nucleation of binary vapors partially miscible in liquid state
Authors: Ray A. K. , Chalam M. , Peters L. K.
Published: July 2002

A general analysis of nucleation in a vapor phase containing two components that are partially miscible in the liquid state is presented. The analysis has been applied to three systems: diethylene glycol+n-heptane, methanol+cyclohexane and diethylene glycol+benzene. On the basis of the classical nucleation theory, the results indicate that a range of saturation ratios, depending on the degree of miscibility, leads to the formation of two types of critical nuclei differing in size and composition. The likelihood of formation of two types of critical nuclei increases as the miscibility gap of the system increases. In some situations, both kinds of nuclei can have the same free energy of formation, and can nucleate at the same rate.
Anderson localization in topologically disordered systems: The effects of band structure
Authors: Logan David E. , Wolynes Peter G.
Published: July 2002

A self-consistent theory of localization in a tight-binding model of topologically disordered systems is developed, which explicitly incorporates the influence of irreducible many-body interactions. These interactions are responsible for the detailed band structure of the system and stem from many-body terms in the renormalized perturbation series for the self-energy. The theory employs our previously developed disordered reference system in which the structure of the medium is taken into account, and makes considerable use of statistical mechanical methods which have direct parallels in conventional applications of liquid state theory. The resultant formulation leads to a self-consistent theory for the density of states and the localization characteristics of the system. The central notion of screening is introduced, whereby the simple transfer matrix element is replaced by an energy and density dependent renormalized transfer matrix element. The effect of screening on the mobility edge trajectories is found to be pronounced, and leads to substantially enhanced (diminished) stability of localized state near the upper (lower) band edge in the density of states.
Exact enumerations of self-avoiding lattice walks with different nearest-neighbor contacts
Authors: Ishinabe T. , Chikahisa Y.
Published: July 2002

Self-avoiding walks on a lattice with m inner contacts are investigated as a model of a polymer chain in solutions for different solvents. Values of several critical exponents are estimated from the exact enumeration data for up to 22 and 20 steps on the square and tetrahedral lattices, respectively. The values of ν and γ estimated for neighbor-avoiding walks (m=0) are in good agreement with those for self-avoiding walks. However, such agreement is not found in δ, the exponent for the end-distance distribution; it suggests a possibility that δ is ruled out from the hypothesis that self- and neighbor-avoiding walks are in same class of universality in contrast to ν and γ. The limiting value of average m per step q is proposed as a parameter to describe the compactness of self-avoiding lattice walks.
Fragmentation of nitrous oxide by monochromatic soft x rays
Authors: Murakami J. , Nelson M. C. , Anderson S. L. , Hanson D. M.
Published: July 2002

Studies of the fragmentation of nitrous oxide using time-of-flight mass spectroscopy following excitation by monochromatic synchrotron radiation are reported. The consequences of core electron excitation of all three atoms to the lowest unoccupied molecular orbital are described. Clear changes in the mass spectra with the atomic site of excitation are observed. These changes can be understood qualitatively in terms of the atomic populations, which govern the Auger decay channels, and the overlap populations, which determine the bond structure, of the valence molecular orbitals. Mass spectra correlated with Auger electrons are very similar to mass spectra correlated with events producing low energy electrons. This result may indicate that the selectivity of the primary Auger decay process is not affected by additional excitation events.
Natural expansion of vibrational wave functions: RRGM with residue algebra
Authors: Moiseyev Nimrod , Friesner Richard A. , Wyatt Robert E.
Published: July 2002

The natural expansion (NE) of vibrational eigenstates is useful for identifying the optimum local coordinates for any vibrational energy and it provides a positive test for regular (nonstochastic) behavior. In previous NE analyses, both eigenvalues and eigenvectors of the Hamiltonian matrix were required. However, through use of the recursive residue generation method (RRGM), we will illustrate how to perform the NE analysis without the need to compute eigenvectors of the N×N Hamiltonian matrix. In addition, a new computational method to obtain all transition amplitudes among a set of states is developed. The method, based upon residue algebra, reduces the CPU requirement by a factor of N/2. To illustrate these procedures, the A1 symmetry eigenfunctions in the classically chaotic regime (where the modes are strongly coupled) of a 2D model Hamiltonian are analyzed with the modified RRGM.
The rotational reflection principle in the direct photodissociation of triatomic molecules. Close-coupling and classical calculations
Authors: Schinke Reinhard
Published: July 2002

We investigate rotational state distributions following the direct photodissociation of triatomic model systems. In view of several recent measurements cases of very high rotational excitation are studied. The dissociation cross sections are calculated exactly by the quantal close-coupling method. All distributions are smooth and highly inverted depending sensitively on the anisotropy of the dissociative potential energy surface. They are explained as a mapping of the bound state wave function onto the quantum number axis. This mapping is mediated by the so-called classical excitation function which is determined by running classical trajectories on the potential energy surface within the dissociative state. We call this effect rotational reflection principle. It establishes a rather direct relation between the bound state wave function of the parent molecule, the anisotropy of the dissociative potential, and the final rotational state distribution. The classical calculations agree qualitatively and in most cases even quantitatively very well with the exact quantal results. Differences are most pronounced in the region of a rotational rainbow where the classical cross section is singular. Because of the special choice of initial conditions only very few trajectories are needed to get converged results.