bluuebunny/crossref_metadata_2025 huggingface dataset:
Estimation of free energy via single particle sampling in Monte Carlo simulations
Authors: Talbot J. , Bereolos P. , Chao K. C.
Published: July 2002

A new method for calculating the free energy in Monte Carlo simulations without the use of particle insertion is discussed. It is shown that while the method is exact, its implementation with one particle sampling requires an approximation. The validity of the approximation is examined by studying a one-dimensional Ising model.
Phase stability of dense multicomponent charged and uncharged hard-sphere fluid mixtures
Authors: Caccamo C. , Varisco M. , Floriano M. A. , Caponetti E. , Triolo R. , Lucido G.
Published: July 2002

We report calculations of the phase diagram and spinodal decomposition line of a charged hard-sphere fluid mixture constituted of two different cations, one anion and one neutral particle species. In the high-density regime the model is suitable to mimic molten silicate and metal halides mixtures, as those forming a magmatic fluid; in the low-density regime it can conveniently describe the properties of charged colloidal suspensions or micellar solutions. In this work the attention is focused mainly on the high-density regime of the mixture, and phase stability conditions are determined through the knowledge of the Gibbs free energy of mixing, Gmix at constant pressure, as calculated in the mean spherical approximation. It turns out that the solubility of the neutral component in the ionic mixture depends substantially on the potential-to-kinetic-energy ratio, that is, on the so-called coupling strength or plasma parameter, and increases when this decreases. This result is rationalized by considering the different terms contributing to Gmix and, specifically, the confi-gurational Coulombic internal energy term and entropic term. The fine balance between these two quantities is detailed and the onset of phase segregation discussed on these same bases. Such a representation also allows us to interpret why mixtures like those envisaged here are characterized by a phase diagram with an upper consolution point and why the critical temperature turns out to be an increasing function of the pressure. The implications of this last result seem interesting since, e.g., molten salt-silicate mixtures and, therefore, presumably fused rocks or magmas, could segregate not only under cooling, but also at constant temperature as the result of pressure variations.
Living poly-α-methylstyrene near the polymerization line. II. Phase diagram in methylcyclohexane
Authors: Zheng K. M. , Greer S. C. , Corrales L. René , Ruiz-Garcia J.
Published: July 2002

We present the experimental determination of the liquid–liquid coexistence curve of living poly-α-methylstyrene (initiated by n-butyllithium) in methylcyclohexane. We measured the coexistence curve by measuring the phase separation temperatures of a set of samples of different mole fractions of the initial monomer, x*m. All the samples had the same ratio, r(=0.008), of the mole fraction of the initiator to the mole fraction of the monomer. We also measured the polymerization line by measuring the temperatures at which increases in viscosity signaled the onset of polymerization. The measured upper critical solution point for this system is at a temperature of 274±1 K and at x*m = 0.18 ± 0.02. At this x*m, the polymerization temperature Tp is 285 K, well above the critical temperature. Tp decreases as x*m decreases, so that the polymerization line meets the coexistence curve at about x*m = 0.12. We compare the predictions of a lattice model which is equivalent to the mean field limit of the dilute n→0 magnet model for constant r to the measured phase diagram and find good qualitative agreement. Better agreement might result if we could solve the model without taking the mean field limit.
A new direct infrared laser absorption method for state-to-state rotational energy transfer in crossed supersonic jets: Experimental results and quantum scattering analysis for Ar+CH4
Authors: Nesbitt David J. , Nibler Joseph W. , Schiffman Aram , Chapman William B. , Hutson Jeremy M.
Published: July 2002

A new method for measuring state-to-state rotational energy transfer in crossed supersonic beams is described. The method is based on direct absorption of tunable, high-resolution infrared laser light by target molecules collisionally excited into final rotational states. The direct IR absorption approach offers high sensitivity, full quantum state resolution, a Doppler probe of final velocity components, and is applicable to any target molecule that absorbs in the near IR. Preliminary results are presented for Ar+CH4 scattering in crossed supersonic beams at a mean center-of-mass collision energy of 41 meV. Because of the high spectral resolution, the method can readily distinguish rotational fine structure states of A, F, and E symmetry in the tetrahedral group, as well as the much more energetically separated final j states. The results are compared with full quantum close-coupling calculations on two different Ar+CH4 potential energy surfaces. The state-to-state scattering results provide a sensitive measure of the potential anisotropy, and in particular probe the relative magnitudes of the different anisotropic terms in the potential (V3 and V4).
The thermalized Fokker–Planck equation
Authors: Frisch H. L. , Nowakowski Bogdan
Published: July 2002

The Fokker–Planck equation for a thermalized ensemble of Brownian particles is exactly solved by a method of (nontruncated) moments, using the expansion of the velocity distribution in Hermite polynomial eigenfunctions. We explicitly studied only the cases of linear and quadratic external potentials bounded by a reflecting wall. The double Laplace transforms (with respect to position and time) of the moments of the velocity distribution were obtained from the generating function, which was found to be a functional only of the density of particles. The closed equation for the density can be derived for the thermalized system. The solution of this equation (obtained by means of a Laplace transform in time) is analyzed in the long and short time limits. The (truncated) moment solution for a nonthermalized problem is also discussed. In the long time limit, the solutions for both thermalized and nonthermalized systems approach the Smoluchowski diffusive approximation.
Calculations of magnetic properties. II. Electron-correlated nuclear shielding constants for nine small molecules
Authors: Cybulski Sl/awomir M. , Bishop David M.
Published: July 2002

Calculations of the nuclear magnetic shielding constants at the self-consistent-field (SCF) and second-order Mo/ller–Plesset (MP2) levels of theory are reported for H2, N2, F2, HF, CO, HCN, HNC, H2O, and NH3. The reliability of the calculations is verified by the high degree of satisfaction of the appropriate sum rule. The second-order correlation corrections to the shielding constants are found to be particularly important for the multiply bonded atoms in molecules such as N2, CO, HCN, and HNC. Vibrational corrections to the shielding constants for the diatomic molecules are shown to be significant.
Width of particle beams desorbed in electron stimulated desorption: O+ and metastable CO from CO/Pt(111)
Authors: Szabó András , Yates John T.
Published: July 2002

The role of adsorbate vibrations parallel to the surface on the width of particle beams escaping from the surface in electron stimulated desorption (ESD) has been studied. Chemisorbed CO on Pt(111) was employed as a model system. The angular distribution of O+ and of the metastable a 3π–CO (CO*) species desorbed in ESD was measured. A cluster calculation was used to model the CO vibration on Pt(111), from which it was possible to calculate the CO* and O+ ESDIAD (electron stimulated desorption-ion angular distribution) beam widths. For CO* we were able to calculate the width of the ESDIAD beams for θCO=0.15 ML with an accuracy better than the error bar of the measurement in the temperature range of 100–300 K. This calculation shows that the CO* angular distribution is related to both the bond direction and the parallel momentum of the surface oscillator at the time of excitation. For θCO=0.5 ML, the width of the measured and calculated values are in good agreement in the temperature range of 100–230 K; above 230 K the experimental results show a positive deviation from the calculated values, indicating that an extra broadening mechanism occurs above ∼230 K, probably because of the onset of an order–disorder transition. For the O+ ESDIAD beam widths the experimental results show a somewhat sharper distribution than the calculated values, most likely because of the angularly anisotropic neutralization rate that was not quantitatively considered in the calculation.
Determining repulsive potentials of InAr from oscillatory bound→continuum emission
Authors: Fawzy Wafaa M. , Le Roy Robert J. , Simard Benoit , Niki Hideaki , Hackett Peter A.
Published: July 2002

Oscillatory bound→continuum emission from vibrational levels v′=0–6 of the B(2Σ+) state of InAr onto the repulsive walls of the X1(2Π1/2), X2(2Π3/2), and A(2Σ+) electronic states, has been measured. In the B(2Σ+)→X1(2Π1/2) spectrum, the intensity extrema have been associated with particular extrema and nodes of the radial wave functions of the emitting levels, and the resulting phase vs energy information directly inverted to yield a pointwise potential for the X1(2Π1/2) state. Analysis of the observed peak heights then showed that on the range 2.9–3.8 Å the associated transition moment function is constant. The overlapping of the B(2Σ+)→X2(2Π3/2) and B(2Σ+)→A(2Σ+) spectra prevents application of the above inversion procedure, but reliable estimates of these two final-state potentials were obtained by matching quantum mechanical simulated spectra with experiment. The simulations also showed that the transition moment functions associated with all three transitions are of approximately equal strength.
Photodissociation and vibrational relaxation of I2− in ethanol
Authors: Kliner Dahv A. V. , Alfano Joseph C. , Barbara Paul F.
Published: July 2002

The dissociation, internal conversion, and vibrational relaxation of photoexcited I2− in ethanol have been examined using ultrafast transient-absorption spectroscopy. I2− was photoexcited at 770 nm (1.6 eV) and probed on the subpicosecond time scale at 15 wavelengths between 580 and 950 nm, permitting a determination of the temporal evolution of the absorption spectrum. The data reveal that internal conversion and vibrational relaxation at the top of the well are extremely rapid (≤0.3 ps), with loss of the final 0.3 eV of energy (v≤20) occurring on a time scale of ∼4 ps. Simple kinetic and spectral models are able to qualitatively account for the observed behavior of the transient-absorption signals.
Laser vaporization generation of Y 10B+, Y 11B+, and YAl+ for electron spin resonance studies in neon matrices at 4 K: Comparison with theoretical calculations
Authors: Knight Lon B. , Babb Robert M. , King Gina M. , McKinley Allan J. , Morse Michael D. , Arrington Caleb A.
Published: July 2002

The first spectroscopic investigation of 89Y 10B+, 89Y 11B+, and 89Y 27Al+ is reported, revealing that both of these diatomic cation radicals have X 4Σ− electronic ground states. The ions were generated by three high energy techniques in combination with neon matrix isolation at 4 K and studied by electron spin resonance (ESR) spectroscopy. The generation methods included pulsed laser vaporization of the metal alloys, photoionization at 16.8 eV during matrix deposition, and x-irradiation at 80 keV of the matrix sample following deposition. Hyperfine interactions were resolved for all of the above nuclei and were compared with calculated isotropic and dipolar components of the A tensor. The yttrium 5s character observed in YAl+ was significantly larger than that in YB+ based upon the observed nuclear hyperfine interactions. The calculated electronic structure properties showed agreement with this trend in the observed Aiso parameters. In addition, qualitative descriptions of the bonding properties are also presented which are consistent with the magnetic parameters obtained from the ESR measurements. The magnetic parameters for Y 11B+ in neon at 4 K are g⊥=1.959(1), A⊥ (Y)=33.6(4) MHz, and A⊥(11B)=65.9(4) MHz. For YAl+, g⊥=1.942(1), A⊥=142(1) MHz, and A⊥(Al)=73(1) MHz.
Symmetry principles in the design of pulse sequences for structural measurements in magic angle spinning nuclear magnetic resonance
Authors: Tycko Robert , Smith Steven O.
Published: July 2002

We introduce general symmetry principles that are useful in the design of radio-frequency excitation sequences for magic angle spinning (MAS) NMR experiments. Sequences derived in accordance with these symmetry principles produce excitations, evolutions, or spectra of nuclear spin systems that are insensitive to the chemical shift anisotropies (CSA) and resonance offsets of the spins. Specific DRAMA (dipolar recovery at the magic angle) sequences are derived. These sequences allow nuclear magnetic dipole–dipole couplings, which are normally averaged to zero by MAS, to be measured or otherwise used in MAS experiments. In numerical simulations of dipolar line shapes and in both simulations and experimental 13C NMR measurements of double-quantum filtering, DRAMA sequences with appropriate symmetries are shown to be less strongly affected by CSA and resonance offsets than sequences used in earlier work. The practicality of the DRAMA technique is thereby enhanced. We present experimental 13C MAS NMR spectra of a 13C-labeled undecapeptide (molecular weight ≊1100) that illustrate the feasibility of applying double-quantum filtering based on the new DRAMA sequences to the problem of simplifying MAS spectra and obtaining information about the structures of complex molecules.
Probing the NO2→NO+O transition state via time resolved unimolecular decomposition
Authors: Ionov S. I. , Brucker G. A. , Jaques C. , Chen Y. , Wittig C.
Published: July 2002

Time resolved, subpicosecond resolution measurements of photoinitiated NO2 unimolecular decomposition rates are reported for expansion cooled and room temperature samples. The molecules are excited by 375–402 nm tunable subpicosecond pulses having bandwidths ≥20 cm−1 to levels which are known to be thorough admixtures of the 2B2 electronically excited state and the 2A1 ground electronic state. Subsequent decomposition is probed by a 226 nm subpicosecond pulse that excites laser-induced fluorescence (LIF) in the NO product. When increasing the amount of excitation over the dissociation threshold, an uneven, ‘‘step-like’’ increase of the decomposition rate vs energy is observed for expansion cooled samples. The steps are spaced by ∼100 cm−1 and can be assigned ad hoc to bending at the transition state. Relying on experimental estimates for the near threshold density of states, we point out that simple transition state theory predictions give rates that are consistent with these measured values. The rates are sufficiently rapid to question the assumption of rapid intramolecular vibrational redistribution, which is implicit in transition state theories. In contrast to expansion cooled samples, room temperature samples exhibit a smooth variation of the reaction rate vs photon energy. By comparing rates for rotationally cold and room temperature NO2, the ON–O bond is estimated to be ∼40% longer in the transition state than in the parent molecule.
Dynamical effects in the calculation of quantum rates for electron transfer reactions
Authors: Egger Reinhold , Mak C. H.
Published: July 2002

This paper presents results of path-integral quantum dynamics simulations of electron transfer rates in a simple class of model Hamiltonians for symmetric and asymmetric electron transfer systems. Our study is aimed at testing the practical usefulness of a centroid factorization of the electron transfer rate constant in the deep tunneling regime. To circumvent the sign problem in quantum Monte Carlo simulations of the electron flux, local filtering techniques have been employed. The simulations show that due to dynamical effects, the reactive flux far outside the transition state region also plays an important role in determining the rate. These results suggest that while the centroid formulation of the equilibrium (imaginary-time) quantum transition state theory (QTST) applied to the electron path is accurate for nonadiabatic electron transfer reactions, dynamical (real-time) effects can produce significant corrections to the QTST estimate for the rate outside this region. For model parameters characteristic for a chemical electron transfer system, we found that the rate is enhanced by a factor Ze≊4. The origin of these dynamical corrections can be understood within a perturbation theory in the number of kinks on the imaginary-time electron quantum path.
Determination of electric dipole moments and transition probabilities of low-lying singlet states of CO
Authors: Drabbels Marcel , Meerts W. Leo , ter Meulen J. J.
Published: July 2002

Transitions from the X 1Σ+(v=0) ground state of the carbon monoxide molecule to the electronically excited A 1Π(v=0), B 1Σ+(v=0), and C 1Σ+(v=0) states have been studied by 2-photon laser induced fluorescence spectroscopy. Accurate molecular constants for the B and C state have been determined. The electric dipole moments for all three electronically excited states have been deduced from the observed Stark effects. The dipole moments for the A, B, and C states are found to be 0.335±0.013, 1.95±0.03, and 4.50±0.07 D, respectively. From the observed radiative lifetimes the transition probabilities of the B–A, B–X and C–B, C–A, and C–X transitions have been determined.
Properties of a statistical model of ice at low temperatures
Authors: Barkema G. T. , Boer Jan de
Published: July 2002

A simple statistical model for ice is presented, based on a point charge model for H2O. To simulate this model, a Monte Carlo algorithm is constructed that samples proton configurations according to the Boltzmann distribution. The ground state of the model is numerically found to be an ordered nonferroelectric state with a unit cell of eight water molecules. The same structure has been previously proposed for the low-temperature phase of ice, called ice XI, on the basis of water–water potential calculations. The model is simulated at various temperatures, and the internal energy, entropy, and static dielectric constant are obtained as a function of the temperature. The model has a phase transition towards the ground state at T=36 K, and no partial ordering is observed. This transition is compared with the phase transition towards ice XI in KOH-doped ice.
Phase separation in deuterated polycarbonate/poly(methylmethacrylate) blend near glass transition temperature
Authors: Motowoka M. , Jinnai H. , Hashimoto T. , Qiu Y. , Han C. C.
Published: July 2002

Early stage of spinodal decomposition kinetics of deuterated polycarbonate/poly(methylmethacrylate) blend has been studied by the time resolved small angle neutron scattering measurements. A trapped miscible state was obtained through fast solution casting of film specimens. Time dependence of the structure factor, S(q,t), was measured after the specimen temperature was quickly increased to a final temperature which is above its glass transition temperature, Tg. Thus, the early stage of spinodal decomposition kinetics has been observed starting from the dimension (q−1) comparable to the single chain radius of gyration, Rg, for a binary polymer mixture. The results provide an unequivocal quantitative measure of the virtual structure factor, S(q,∞); the relationship of qm and qc through rate of growth, Cahn-plot analysis, and singularity in S(q,∞); the growth of fluctuation of qRg<1 and intrachain relaxation of qRg≳1; and also a clear proof of the Cahn–Hillard–Cook theory in the early stage of spinodal decomposition of a mean-field system.
Wave packet dynamics of the HgI2 photodissociation reaction in solution
Authors: Pugliano N. , Palit D. K. , Szarka A. Z. , Hochstrasser R. M.
Published: July 2002

Using femtosecond absorption spectroscopy, the photodissociation dynamics of HgI2 in solution were investigated. Upon photolysis, the HgI fragment is formed in a coherent superposition of its vibrational states. Solvent dependence of the wave packet motion is evidenced by the different results for ethanol and DMSO and between solution and gas phase.
Dissociative adsorption of H2 using the close-coupling wave packet method
Authors: Mowrey R. C.
Published: July 2002

A time-dependent quantum mechanical wave packet method was used to study the dynamics of the dissociative adsorption of H2 and D2 on a flat, static surface. A three-dimensional (3D) model was used in which the molecular rotational, vibrational, and center-of-mass translational motion normal to the surface are treated. In the close-coupling wave packet method the wave function is represented using a combination of a basis set expansion for the rotational degrees of freedom and a 2D L-shaped grid for the vibrational and translational coordinates. The time propagation is carried out by expanding the time-evolution operator in a series of Chebyshev polynomials. The molecule–surface interaction is described using a modified London–Eyring–Polanyi–Sato (LEPS) potential with parameters chosen to represent the H2/Ni(100) system. The dissociation probability was calculated for different incident energies and initial rotational and vibrational states and compared to the results of other theoretical calculations. Higher incident energies are required for D2 dissociation than for H2. The barrier height and zero-point energies at the saddle point can be determined from the energy dependence of the dissociation probabilities for H2 and D2.
Dipole hyperpolarizability surfaces of ammonia
Authors: Spirko Vladimir , Luo Yi , Ågren Hans , Jo/rgensen Poul
Published: July 2002

Full-dimensional first dipole hyperpolarizability surfaces of NH3 are determined by means of multiconfigurational quadratic response theory. On the basis of the two-dimensional, symmetric stretching vs inversion, hyperpolarizability functions, and approximate vibrational wave functions, effective inversional hyperpolarizability moments are evaluated for the D3h ammonia isotopomers. The calculations have revealed their strong vibrational and frequency dependence. A fairly close reproduction of the first hyperpolarizability ESHG constant β (632.8 nm) by theory indicates that the major correction to the one-particle hyperpolarizability is obtained by variational wave functions including a comparatively small set of active orbitals. We find that a meaningful comparison with experimental results can only be obtained by an appropriate evaluation of both the vibrational and the correlated dispersion dependencies.
Competing mechanisms for intramolecular vibrational redistribution in the ν14 asymmetric methyl stretch band of trans-ethanol
Authors: Bethardy G. A. , Perry David S.
Published: July 2002

The extensively perturbed spectrum of the asymmetric methyl stretching vibration of trans-ethanol near 2990 cm−1 has been reinvestigated via direct absorption infrared spectroscopy at a resolution of 30 MHz. A ground state combination difference analysis of the vibrational state mixing is presented for the upper state levels Ka′ = 0–2 and J′=0–4. The analysis indicates that the rotationless 000 level is anharmonically coupled to the dark bath states. The effective number of perturbing states in each rovibrational transition increases with both J and Ka providing evidence for rotational involvement in intramolecular vibrational redistribution (IVR). The decrease of the average dilution factor from φd=0.41 at Ka′ = 0 to φd=0.09 at Ka′ = 2 and the increase of the average interaction width from Δε=0.04 cm−1 at Ka′ = 0 to Δε=0.19 cm−1 at Ka′ = 2 indicate an a-type Coriolis component to the bright-bath coupling. In the Ka′ = 0 series the dilution factor decreases rapidly from φd=0.92 at J′=0 to φd=0.14 at J′=3 indicating that b,c-type Coriolis coupling also plays a significant role in the IVR process. The effective level density ρeffc for all of the observed transitions lie above the total vibrational state density ρvib=9 levels per cm−1 and most are closer to the total rovibrational state density ρrovib=(2J+1)ρvib. This suggests that following a coherent preparation of the asymmetric methyl stretching vibration, the ensuing dynamics explores all of the energetically accessible vibrational phase space of both the gauche and trans forms and much of the accessible rovibrational phase space, i.e., that the Ka quantum number is at least partially destroyed. The C–H stretch is deduced to decay with a 59 ps IVR lifetime to the asymptotic probability of 0.24.
Monte Carlo simulation of the solid to super liquid phase transition of Langmuir monolayers using cross-section potentials
Authors: Swanson D. R. , Hardy R. J. , Eckhardt C. J.
Published: July 2002

A cross-section potential which models the interaction of two planar objects of prescribed shape is developed and used to model the interactions of amphiphiles in vertical phases of Langmuir monolayers. The results of Monte Carlo simulations of an isobaric–isothermal ensemble show qualitative agreement with experiment. The model system undergoes a continuous, bound-to-free rotator transition between phases of the same symmetries as the solid (S) and super liquid (LS) phases exhibited by fatty acid films.
Wavelength dependence of the photochemistry of O2 on Pd(111) and the role of hot electron cascades
Authors: Weik F. , de Meijere A. , Hasselbrink E.
Published: July 2002

The photochemistry of O2 adsorbed on Pd(111) has been studied as a function of the energy of the exciting photons in the range of hν=3.9–6.4 eV. The obtained data are reproduced by a proposed kinetic model considering photostimulated desorption and dissociation, whereby the latter gives rise to additional displacement processes. This modeling yields rate constants and, hence, cross sections for the respective processes. It is found that the cross sections for all processes rise exponentially with photon energy. This result motivates a study of the distribution of hot electrons generated by laser irradiation, and its decay via electron-hole pair scattering events resulting in a broad secondary electron distribution. The energetic and spatial distribution of these electrons is calculated. The probability for these electrons to tunnel into an adsorbate affinity level is modeled and compared to the observed photo cross sections.
Nuclear magnetic resonance relaxation enhancements produced by paramagnetic solutes: Effects of rhombicity in the zero field splitting tensor with the S=2 spin system as an example
Authors: Bovet Jean-Marc , Sharp Robert R.
Published: July 2002

Effects due to the nonuniaxial part of the zero field splitting (ZFS) tensor on NMR relaxation enhancements produced by paramagnetic species in solution (the NMR PRE) has been studied theoretically and experimentally in the ZFS limit, i.e., in the limit where the ZFS energy is large compared to the Zeeman energy. In the ZFS limit, the precessional motion of the electron spin is quantized with respect to molecule-fixed coordinate axes. The uniaxial part of the ZFS tensor induces precessional motion in the transverse (x,y) components of the electron spin vector S, and x,y anisotropy in the ZFS tensor (i.e., a nonzero ZFS parameter E) induces precessional motion in the z component of S. The NMR-PRE phenomenon is particularly sensitive to the motion of Sz and hence also to ZFS anisotropy in the xy plane. Mathematical expressions have been derived which describe the motion of the spin vector evolving under the influence of a general rhombic ZFS Hamiltonian and the influence of this motion on the NMR PRE in the ZFS limit. It is shown that oscillations in Sz occur at the transition frequencies of the S spin system; the frequencies and amplitudes of the precessional components of Sz can be calculated by diagonalizing the general ZFS Hamiltonian. These motions and their consequences with respect to the behavior of the NMR PRE are described in detail for the S=2 spin system. A parametrization of NMR-PRE data is proposed which gives a clear criterion for the conditions under which rhombic parts of the ZFS tensor significantly affect the relaxation enhancements produced by an S=2 spin system. This criterion is of considerable practical importance for the analysis of NMR-PRE data, since it defines conditions under which data may be analyzed without the need for independent experimental information concerning the magnitude of the ZFS tensor.
Optical control of molecular dynamics: Molecular cannons, reflectrons, and wave-packet focusers
Authors: Krause Jeffrey L. , Whitnell Robert M. , Wilson Kent R. , Yan YiJing , Mukamel Shaul
Published: July 2002

We consider the control of molecular dynamics using tailored light fields, based on a phase space theory of control [Y. J. Yan et al., J. Phys. Chem. 97, 2320 (1993)]. This theory enables us to calculate, in the weak field (one-photon) limit, the globally optimal light field that produces the best overlap for a given phase space target. We present as an illustrative example the use of quantum control to overcome the natural tendency of quantum wave packets to delocalize on excited state potential energy curves. Three cases are studied: (i) a ‘‘molecular cannon’’ in which we focus an outgoing continuum wave packet of I2 in both position and momentum, (ii) a ‘‘reflectron’’ in which we focus an incoming bound wave packet of I2, and (iii) the focusing of a bound wave packet of Na2 at a turning point on the excited state potential using multiple light pulses to create a localized wave packet with zero momentum. For each case, we compute the globally optimal light field and also how well the wave packet produced by this light field achieves the desired target. These globally optimal fields are quite simple and robust. While our theory provides the globally optimal light field in the linear, weak field regime, experiment can in reality only provide a restricted universe of possible light fields. We therefore also consider the control of molecular quantum dynamics using light fields restricted to a parametrized functional form which spans a set of fields that can be experimentally realized. We fit the globally optimal electric field with a functional form consisting of a superposition of subpulses with variable parameters of amplitude, center time, center frequency, temporal width, relative phase, and linear and quadratic chirp. The best fit light fields produce excellent quantum control and are within the range of experimental possibility. We discuss relevant experiments such as ultrafast spectroscopy and ultrafast electron and x-ray diffraction which can in principle detect these focused wave packets.
Comment on ‘‘A modified superposition approximation to the three-body distribution function’’ [J. Chem. Phys. 97, 5142 (1992)]
Authors: Taylor Mark P. , Lipson J. E. G.
Published: July 2002

Hernando and Gamba have recently proposed a simple modification to the Kirkwood superposition approximation which they suggest may be universally applicable to all simple fluids [J. Chem. Phys. 97, 5142 (1992)]. While this ansatz was shown to work well for a Lennard-Jones fluid we find it to be less than satisfactory for the hard sphere fluid, thus weakening the proposal of universality.
Effect of dielectric friction on the perpendicular reorientation of the nitrate ion in water and organic solvents
Authors: Nakahara Masaru , Emi Keiji
Published: July 2002

The perpendicular orientational correlation time τ2R has been determined for the nitrate ion in tetrabutylammonium nitrate (Bu4NNO3) solutions in such an aprotic solvent as acetonitrile (MeCN) and such protic solvents as water (H2O), methanol (MeOH), ethanol (EtOH), and 1-propanol (PrOH) as a function of concentration (c) between 0.01 and 1 M at 30 °C by measuring the 14N nuclear magnetic resonance spin-lattice relaxation times. To ensure the presence of the common limit of τ2R at infinite dilution, the countercation effect on τ2R has been investigated for the potassium ion in water and for the lithium ion in ethanol. In all cases, the rotational correlation time depends linearly on concentration (c); τ2R=τ2R0(1+αc). The reduced slope α depends on the solvent and the cation size; α increases with a decrease in the solvent polarity and the countercation size. The effect on α of the solvent polarity and the surface charge density of the countercation can be explained qualitatively well by our hydrodynamic interaction model previously presented; the anomalously large slope for the Bu4N+ ion in water indicates its promoting effect on the water structure. The limiting correlation times (τ2R0) are 1.08±0.01, 1.14±0.01, 3.38±0.03, 6.12±0.06, and 11.6±0.1 ps, respectively, in MeCN, H2O, MeOH, EtOH, and PrOH. The solvent effect on τ2R0 is much stronger than that expected from viscosity, except for water. The friction coefficient which exceeds the viscosity contribution indicates a key role of the dielectric friction (DF), as predicted by the Hubbard–Onsager DF theory for ionic rotation.
Rotational alignment by stimulated Raman pumping: C2H2(ν″2=1, J″)
Authors: Dopheide R. , Zacharias H.
Published: July 2002

Rotational alignment in a polyatomic molecule is produced by stimulated S-branch Raman pumping. This technique, applied to pump the (ν2″=1) vibration of acetylene, generates a nearly perfect quadrupole aligned MJ distribution (A0(2)=−1) for Jf=2. First results concerning the collisional alignment decay rate constant are reported.
Approximate solution of the classical Liouville equation using Gaussian phase packet dynamics: Application to enhanced equilibrium averaging and global optimization
Authors: Ma Jianpeng , Hsu D. , Straub John E.
Published: July 2002

An approximate method for integrating the Liouville equation to obtain the time-dependent classical phase space density distribution at constant energy or temperature is presented. The density distribution of each degree of freedom is represented by a single Gaussian phase packet (GPP) whose center and width obey variationally optimized equations of motion. The constant energy dynamics is applied to the calculation of equilibrium thermodynamic averages for a Lennard-Jones cluster and fluid to demonstrate the feasibility and utility of this approximate method for the simulation of many-body condensed phase systems. The rate of kinetic energy equipartitioning is examined for GPP dynamics using a generalization of the ergodic measure and found to be significantly faster than for standard molecular dynamics simulation. A global optimization algorithm is developed based on simulated annealing of the phase space density distribution. This method is applied to the global energy minimization of Lennard-Jones clusters and found to be superior to simulated annealing methods employing classical point particles.
A theoretical study of the dissociation of NO2
Authors: Klippenstein Stephen J. , Radivoyevitch Tomas
Published: July 2002

A combination of ab initio quantum chemical and variational Rice–Ramsperger–Kassel–Marcus (RRKM) theory calculations are employed in a detailed theoretical modeling of the NO2 dissociation process. Estimates of the interaction energies between NO and O in the transition state region are obtained at the multireference singles and doubles configuration interaction level employing a 6-31G* basis set. A two-dimensional variational optimization of the transition state number of states is performed employing an analytic potential energy function obtained from a fit to the present quantum chemical data. The resulting theoretical estimates of the energy resolved rate constants and product vibrational distributions are compared with the corresponding experimentally determined values. The effect of quantum mechanics on the number of states is considered via a comparison of quantum and semiclassical evaluations for an assumed center-of-mass separation distance reaction coordinate.
The long-time behavior of reversible binary reactions: Theory, Brownian simulations and experiment
Authors: Agmon Noam , Edelstein Arieh L.
Published: July 2002

Many-body effects on reversible pseudo-unimolecular reactions are investigated using a combination of theory, simulation, and experiment. Theoretically, we rederive the superposition approximation starting from the fundamental N-particle equations. All the relations obtained are actually rigorous, except for a requirement that the concentration profile outside a vacant trap obeys a diffusion equation. Our derivation also yields a new numerical procedure for evaluating the superposition solution. Brownian dynamics simulations of one-dimensional competitive binding are presented over an unprecedented time regime. Comparison with the superposition approximation shows that this mean-field theory is exact at infinite dilution, but breaks down at high particle concentration. The main discrepancy is not at asymptotically long times as previously suspected, but rather at intermediate times, where a new power law-phase emerges. This is reflected in a maximum in the logarithmic derivative of the survival probability, which is more pronounced in our simulation as compared with the approximate theory. Finally, we show that the transient fluorescence data from an excited dye molecule which transfers a proton reversibly to water, develops a similar maximum in its logarithmic derivative at low pH values.
Steric effects in total integral reaction cross sections for Sr+HF(v=1,j=1,m=0)→SrF+H
Authors: Loesch H. J. , Stienkemeier F.
Published: July 2002

In a crossed beam study we have investigated the reactive scattering of Sr atoms from state prepared HF molecules. All measurements have been performed for two different directional distributions (alignments) of the molecular axis of HF favoring either end-on or side-on attacks. Using a surface ionization detector we have measured angular distributions of the total SrF flux at various collision energies ranging from Etr=230 to 626 meV. We found that the total product flux as well as the relative total integral reaction cross sections deduced from these data are markedly influenced by the prepared alignments (steric effect). The energy dependence of the steric effect has been quantitatively rationalized using the angle-dependent line-of-centers model. The resulting anisotropic height of the potential energy barrier features a minimum at a bent transition state. Ab initio potential energy surfaces calculated for the homologous systems Mg+HF and Ca+HF substantiate this result.
On the interpretation and rotational assignment of degenerate four-wave mixing spectra: Four-photon line strengths for crossover resonances in NO A 2Σ+–X 2Π
Authors: Friedman-Hill Ernest J. , Rahn Larry A. , Farrow Roger L.
Published: July 2002

We present here a set of equations specifically adapted to simulation of fully resonant, high-resolution, phase-conjugate degenerate four-wave mixing (DFWM) in molecular gases. Signal-intensity dependence on molecular wave functions, lifetimes, and laser beam polarizations is explicitly included in these equations. The emphasis of the presentation is on both physically intuitive interpretation and a practical, ‘‘cookbook’’ approach to spectral simulation. We present experimental verification of our calculations drawn from the spectrum of dilute NO in N2 at low pressures. Both degenerate two-level and three-level (crossover) resonances were observed. The experimental spectral intensities are accurately reproduced by the expressions presented here. We point out some of the subtleties of DFWM spectra that could be used as aids to interpretation, especially the use of laser polarization as a probe for spectral line assignments.
A novel molecular orbital method for the calculations of polymer systems with local aperiodic part: The combination of the elongation method with the supercell method
Authors: Mitani Masaki , Aoki Yuriko , Imamura Akira
Published: July 2002

In this paper, we propose a new theoretical approach to treat polymer systems with local aperiodic part efficiently. This approach is the combination of the elongation method with the supercell method which has been developed in our group, and we developed this approach to obtain the electronic structure of a polymer having local aperiodicity by considering the locality of the interaction between periodic and local aperiodic parts. In order to confirm the validity of this method, several model calculations were performed at the complete neglect of differential overlap (CNDO/2) level. That is, we applied this method to all-trans polyacetylene interacting with a small molecule, and to all-trans polyacetylene with partial substitution of hydrogen atoms by fluorine atoms. We compared the results obtained by this calculation with those obtained by usual crystal orbital calculation concerning total energy, computational time, and electron density distribution. The charge extension on the polymer chain under the influence of aperiodicity was also discussed.
A Monte Carlo study of the liquid–vapor coexistence of charged hard spheres
Authors: Caillol J. M.
Published: July 2002

We determine the liquid–vapor coexistence curve of the restricted primitive model of electrolytes by means of Monte Carlo simulations in the Gibbs ensemble. Our results confirm earlier findings of Panagiotopoulos [Fluid Phase Equilibria 76, 97 (1992)]. A study of the electrical properties of the two coexisting phases reveals that the second Stillinger–Lovett condition is verified in the liquid but not in the vapor. This means that the latter could be dielectric.
The influence of nonadiabatic rotational transitions on the line shapes of the rotational Raman spectrum of H2 in liquid argon
Authors: Xiao L. , Coker D. F.
Published: July 2002

General expressions for time correlation functions of operators of a quantum subsystem being driven by a classical solvent are derived in the limit that the forces on the classical solvent from the quantal solute are independent of the state of the quantum subsystem [the zero back reaction (ZBR) limit]. These expressions are used to compute the rotational Raman spectrum of a quantal H2 rotor in liquid argon whose motion is described by nonadiabatic transitions between orientational sublevels resulting from solvent collisions. Line shapes computed in both the static and adiabatic limits are compared with those computed allowing for nonadiabatic transitions in an effort to understand the influence of these transitions on the spectrum.
The forbidden predissociation of metastable H2(c 3Πu−) molecules studied by state specific lifetime measurements
Authors: Berg Ch. , Ottinger Ch.
Published: July 2002

Using laser induced fluorescence detection of metastable H2(c 3Π−u, v=0–3, N=1–4) molecules in a beam at different distances from the source, the lifetime of resolved fine structure levels was determined. It was found to be much shorter for the F2 levels (∼150–10 μs for v=0–3, respectively) than for the F1 plus F3 components (several hundred to 70 μs). This is due to a forbidden (ΔN≠0) predissociation by the b 3Σ+u state, which operates preferentially on the F2 levels. Accurate bound–free Franck–Condon factor calculations reproduce the increasing trend of the predissociation rate in going from v=0–3. The absolute predissociation rates from previous theoretical work, however, can only be brought into agreement with the experimental results if they are multiplied with a common scale factor of 38.
Femtosecond electronic dynamics by time-delayed four-wave mixing with incoherent light: Iodine and azulene
Authors: Yang Tzyy-Schiuan , Zhang Ruihua , Myers Anne B.
Published: July 2002

Two- and three-pulse time-delayed degenerate and nondegenerate four-wave mixing (photon echo) experiments have been performed on I2 in the vapor and in hexadecane solution and on azulene in cyclohexane by using ‘‘incoherent’’ light from a broadband dye laser to achieve femtosecond time resolution. Detailed theoretical analyses, including the cases of both parallel and perpendicularly polarized excitation pulses, are presented along with the experimental data. In I2 vapor, the signals as a function of delay time τ between the two excitation pulses exhibit beats at a frequency characteristic of the spacing between vibrational levels of the B state near the center laser frequency. The rate of decay of the beats and the symmetry of the signals with respect to τ are consistent with partial rotational energy randomization during the 10–20 ns waiting time between excitation and probe pulses. This can be considered as a form of spectral diffusion in the gas phase. The solution phase data on both I2 and azulene are consistent with no significant inhomogeneous component to the solvent-induced electronic spectral breadth on a time scale of several picoseconds or longer, but the theoretical signals from I2 are shown to be relatively insensitive to the partitioning of the breadth. The general utility of techniques based on incoherent light for examining femtosecond time scale dynamics in solution phase is evaluated.
The primitive model of ionic fluids near its critical point in the Poisson–Boltzmann and modified Poisson–Boltzmann theories
Authors: Bhuiyan L. B. , Outhwaite C. W. , Molero M. , González-Tovar E.
Published: July 2002

The Poisson–Boltzmann (PB) and modified Poisson–Boltzmann (MPB) theories are used to investigate the primitive model of ionic fluids in the low density–large coupling regime where the liquid–vapor transition is situated. The PB and MPB spinodal curves for the restricted primitive model are calculated from the virial route and compared with those from the mean spherical approximation (energy route) and the hybrid hypernetted-chain/mean spherical approximation (virial route). The effect of unequal ion sizes on the critical point and spinodal curves is also considered.
Transient changes in polarizability for centrosymmetric linear molecules interacting at long range: Theory and numerical results for H2...H2, H2...N2, and N2...N2
Authors: Li X. , Hunt K. L. C.
Published: July 2002

Transient, collision-induced changes in polarizability Δα on the subpicosecond time scale affect Rayleigh and rototranslational Raman scattering by diatomic molecules in dense gases and liquids, induced birefringence, impulsive stimulated scattering, and dielectric and refractivity virial coefficients. For pairs of D∞h molecules, this work gives the long-range contributions to Δα complete through order R−6 in the intermolecular separation R, including the first- and second-order dipole–induced-dipole (DID) interactions, higher-multipole induction, effects of the nonuniformity in the local field, hyperpolarization, and dispersion. We have used spherical tensor methods to cast Δα into the symmetry-adapted form needed for spectroscopic line shape analysis.

The polarization mechanisms included here give rise to isotropic rototranslational Raman scattering and to simultaneous rotational transitions on two interacting molecules; both are collision-induced phenomena. Transitions with ΔJ up to ±4 are produced by the R−5 and R−6 polarization mechanisms treated in this work (and the isotropic part of the pair potential). For the pairs H2...H2, H2...N2, and N2...N2, we have used ab initio results for permanent multipoles and susceptibilities to evaluate the classical induction contributions to Δα. For the dispersion contributions, we have derived exact results in the form of integrals of the polarizability and γ hyperpolarizability over imaginary frequencies, and we have approximated these numerically in terms of the static α and γ values, together with van der Waals energy coefficients. For the pairs and configurations studied, the first-order DID terms give the dominant contributions to Δα; the agreement between these terms and the full long-range results tends to be better for the anisotropic collision-induced polarizability ΔαM2 than for the isotropic part Δα00, particularly for T-shaped pairs.

The relative contributions to Δα from the other polarization mechanisms reach as high as 29% for second-order DID terms, 27% for E-tensor terms (higher-multipole induction and local field nonuniformity), 21% for dispersion, and 8% for hyperpolarization effects, for R values ∼0.5–1.0 a.u. outside the isotropic van der Waals minimum.