bluuebunny/crossref_metadata_2025 huggingface dataset:
Vibrational energy transfer from the 61 level of S1 (1B2u) benzene in a supersonic expansion. I. Monatomic collision partners
Authors: Waclawik Eric R. , Lawrance Warren D.
Published: July 2002

Vibrational energy transfer has been monitored from the 61 level of 1B2u(S1) benzene seeded in rare gas supersonic free jet expansions at X/D=5, where the temperature is calculated to be ∼10 K. The monatomic collision partners helium, neon, argon, and krypton form the subject of this study. Consequently, transfer is limited to one mechanism, transfer of vibrational energy in benzene to translational energy of the collision pair. The vibrational energy transfer is followed using time resolved, dispersed fluorescence spectroscopy. While there are five possible destination levels, only three are found to be important. These are transfer to the 162 level and transfer to the spectrally unresolved 111 and 161 levels. Negligible transfer is observed to both of the remaining two accessible levels, 00 and 41. It is found that the branching ratio for the two destination channels is insensitive to the identity of the collision partner. The branching ratios are reproduced by calculations based on SSH-T theory. The calculations suggest that the insensitivity of the branching ratio to the collision partner is fortuitous: while the combined 111/161 channel retains approximately the same ratio to 162 for all collision partners, the relative importance of the 111 and 161 levels themselves is collision partner dependent. Evidence is presented suggesting that there is significant rotational excitation accompanying the vibrational energy transfer in the case of heavy collision partners. This study, by establishing the behavior of vibration to translation transfer, forms the basis for further studies of the role of vibration to rotation and, subsequently, vibration to vibration mechanisms in vibrational energy transfer in benzene at low temperatures.
Competition between intermolecular and intramolecular association in flexible hard chain molecules
Authors: Ghonasgi Dhananjay , Chapman Walter G.
Published: July 2002

A new theory to explain the competition between inter- and intramolecular association in flexible hard chain molecules is presented. The theory has been tested through comparisons with Metropolis Monte Carlo simulation results. For intermolecular association we use Wertheim’s theory which has been shown to be accurate for intermolecular association in flexible associating hard chain molecules. For intramolecular association we use a theory we developed for intramolecular association in the absence of intermolecular association. These two theories are combined to develop a theory for the competition between inter- and intramolecular association. The new theory is in good agreement with simulation results and is able to predict some salient features of associating chain molecules. The theory predicts that intermolecular association becomes more important at high densities and that intramolecular association dominates at low density and low temperatures. In addition, theory and simulation show a minimum in the compressibility factor when plotted against the association energy at low density. This minimum is due to the presence of intramolecular association and is not observed for intermolecularly associating fluids.
Structure, disorder, and phase transition in ternary crystals of anthracene, phenanthrene, and tetracyanobenzene, AxPh1−xTCNB
Authors: Lefebvre J. , Rohleder K. , Mierzejewski A. , Luty T.
Published: July 2002

The structure of ternary, charge-transfer crystals of anthracene, phenanthrene, and tetracyanobenzene, AxPh1−xTCNB, in a wide range of concentrations has been studied. We have focused on the problem of substitutional and orientational disorder of anthracene and phenanthrene molecules and its influence on the orientational instability. The crystal structure determination has been performed at room temperature and 120 K, and the refinement procedures allowed us to determine the orientational disorder for different concentrations. The phase diagram for the orientational phase transition with symmetry change, C2/m to P21/a, has been determined and a critical concentration (xc=0.74) was found. The disorder and the phase diagram has been successfully modeled by a minimization of the crystal energy using the ‘‘split molecule’’ concept and the virtual crystal approximation to mimic both orientational and substitutional disorders. The ternary crystals, their structure, disorder, and phase transitions are well understood in terms of a competition between orientational disorder of anthracene molecules and local stresses created by the substitutional disorder (with statistical orientations) of phenanthrene molecules. A possibility for an existence of molecular orientational glassy state is suggested for concentrations below the critical one.
Main factors influencing the recoil energy distribution in the products of three-atom reactions governed by long-range forces and proceeding through long-lived complexes
Authors: Bonnet L. , Rayez J. C.
Published: July 2002

We propose a simplified version of the classical statistical theory of three-atom reactions governed by long-range forces. This version is based on a partial treatment of total angular momentum conservation. We focus the developments on the determination of the recoil energy distribution of reactions performed in supersonic crossed-beam experiment. This distribution function is directly linked to the maximum values of the moduli of rotational and orbital angular momenta of the products consistent with their recoil energy and the total angular momentum disposal. Due to the simplicity of the present version, we can pinpoint the main factors which play a role in statistical reaction dynamics. It is shown that the shape of the translational energy distribution can be estimated essentially in terms of two mechanical parameters; the first one represents the available fraction of maximum angular momentum disposal in the products and the second gives a measure of the fraction of total angular momentum due to the rotation of the newly formed bond AB. Comparisons with exact calculations and experimental results are presented. They show that in addition to being able to determine propensity rules, our simple approach can even provide satisfying quantitative results.
Many-body correlations and the cancellation effect in the Green–Kubo time correlation functions for the shear viscosity
Authors: Stassen H. , Steele W. A.
Published: July 2002

Molecular dynamics simulations of the time-correlation functions that appear in the Green–Kubo expression for the shear viscosity are reported for liquid argon over a wide range of densities. The potential part of the microscopic stress tensor is separated into contributions due to the repulsive and attractive branches of the (12/6) Lennard-Jones potential. The separate correlation functions for these parts of the interactions as well as their cross term are further broken down into two-, three- and four-body parts. Each of these shows the same kind of cancellation effect as is found in simulation studies of dipole-induced dipole light scattering. We compare our results to the light scattering findings and find a very consistent picture of the cancellation effect in these time-correlation functions.
Toward determining the true hard-sphere interfacial free energy
Authors: Marr D. W. M.
Published: July 2002

Due to the lack of exact simulation results, several approaches are used to estimate the interfacial free energy between solid and fluid phases in the hard-sphere system. All of the estimates presented agree well with one another and center around γ=0.55 kT/σ2.
State-selected ion-molecule reactions: Statistical calculations with constraints
Authors: Uiterwaal C. J. G. J. , van Eck J. , Niehaus A.
Published: July 2002

For the two reactive systems, NH3+(Eint)+N H3→NH4++NH2 and  H2+(Eint)+H2→H3++H, for which the relative cross sections were measured earlier in our group for Ec.m.≊40 meV we calculated the relative cross section as a function of internal energy using the statistical Rice–Ramsperger–Kassel–Marcus (RRKM) theory that implicitly conserves total energy and total angular momentum. We found satisfactory agreement between theory and experiment by imposing rather mild constraints upon the loose transition state configuration. These constraints involve inactive vibrations and steric hindrance. The steric hindrance imposed in case of the (NH3–NH3)+ system is interpreted as being due to the anisotropic interaction of the ionic charge with the permanent electric dipole of the respective neutral collision partner in the two dissociation channels. We cannot be absolutely sure that the specific combination of modifications we propose for each of the two systems is the only one that agrees well with experiment. However, we find it striking that an agreement can be obtained by such weak and physically meaningful modifications, and we take this as a strong indication that the two studied systems do behave statistical.
Determination of force field parameters for molecular simulation by molecular simulation: An application of the weak-coupling method
Authors: Njo Swie Lan , van Gunsteren Wilfred F. , Müller-Plathe Florian
Published: July 2002

A method for automatic adjustment of force field parameters to known liquid properties is described. The method works by coupling the time derivative of the force field parameter to the deviation of the actual value of the observable from prescribed reference value. The method is generally applicable, if (i) a strong relationship between a parameter and an observable exists, and (ii) if this relationship is monotonic within the convergence interval of the parameter. The method is applied to the Lennard-Jones potential energy function. It is shown computationally that it is feasible to couple the Lennard-Jones ε parameter to the heat of vaporization or to the chemical potential, and the Lennard-Jones σ parameter to the pressure or the diffusion coefficient of a liquid. It is also shown that it is possible to couple simultaneously two parameters to two observables. With the range of applicability established, the technique is then tested on liquid mercury.
An ab initio investigation of structure and energetics of clusters MgnCl2n
Authors: Eichkorn Karin , Schneider Uwe , Ahlrichs Reinhart
Published: July 2002

Results of a systematic theoretical study of stoichiometric clusters MgnCl2n up to n=24 are reported within the self-consistent-field (SCF) approximation. Calculations within second-order Mo/ller–Plesset perturbation theory and single- and double-excitation coupled cluster method with a perturbational estimate of triples excitations are performed for the smaller clusters to check accuracies. The clusters MgnCl2n do not show a consistent preference for geometries which are fragments of the bulk lattice as it is known for the alkali halide systems: Small clusters MgnCl2n prefer linear chain structures corresponding to the SiS2 crystal lattice, the most stable medium sized clusters (5≤n<20) are high-symmetry ‘‘cyclic’’ SiS2 structures, and only for large systems (n≥20) we realize a transition to the magnesium chloride lattice structure. The computed binding energies of the larger clusters allow for an extrapolation of the lattice energy which is in good agreement with experiment.
Nonadiabatic dynamical studies of the rotational Raman spectrum of H2 in water
Authors: Xiao L. , Coker D. F.
Published: July 2002

The rotational Raman spectrum of H2 in liquid H2O and D2O have been calculated using our nonadiabatic time correlation function results presented in earlier work [L. Xiao and D. F. Coker, J. Chem. Phys. 100, 8646 (1994)]. The rotational potential experienced by the H2 molecule in this solvent environment is so anisotropic that classical solvent fluctuations not only drive transitions between orientational basis states, but mixing of states from different total angular momentum levels is also considerable. Our nonadiabatic calculations are able to quantitatively reproduce the experimental rotational Raman line shapes and their trends with solvent isotopic substitution. We demonstrate that nonadiabatic transitions between adiabatic rotor states play a key role in smoothing out artificial structures predicted in the static and adiabatic limit spectra.
Asymmetric line shapes in the indirect predissociation of the f 1Σu+ Rydberg state of O2
Authors: Lewis B. R. , Banerjee S. S. , Gibson S. T.
Published: July 2002

Rotationally-resolved Beutler–Fano line shapes observed in the photoabsorption spectrum of the (2,0) band of the 3pπuf 1Σu+←X 3Σg− Rydberg system of O2 are interpreted using a coupled-channel Schrödinger equations model. It is found that the f 1Σu+ state is indirectly predissociated by the B 3Σu− continuum, and that the f←X transition borrows oscillator strength primarily from dipole-allowed transitions into the mixed Rydberg-valence states of 3Σu− symmetry. Both the predissociation linewidth and oscillator strength of the (2,0) resonance are controlled by the spin-orbit interaction between the 1Σu+ and 3Σu− components of the 3p-complex. There is some evidence for a destructive quantum interference between the transition amplitude borrowed from the 3pπuE 3Σu−←X 3Σg− transition and that borrowed weakly from the f 1Σu+←b 1Σg+ transition through spin-orbit mixing between the b 1Σg+ and X 3Σg− states.
The reactive flux method in the energy diffusion regime. II. Importance of the solvent’s spectral profile
Authors: Reese Sabine K. , Tucker Susan C. , Schenter Gregory K.
Published: July 2002

The dependence of energy-diffusion-limited unimolecular-rate constants upon the form of the solvent’s spectral profile is studied using generalized Langevin equation (GLE) dynamics. We find that the initial energy relaxation of the solute as it leaves the barrier region and the subsequent vibrational relaxation into the solute reactant well are governed by different frequency regions of the solvent’s spectral profile. Additionally, we find that for the case of a slowly relaxing bath the rate can depend quite dramatically upon the form of the friction kernel used in the GLE. Specifically, while the initial solute energy relaxation is observed to be similar for the Gaussian and exponential friction cases studied, there is a bottleneck to solute vibrational energy relaxation in the Gaussian friction case that is not present in the exponential friction case. In the Gaussian friction case, we find that neither the reactive flux method nor the Pollak–Grabert–Hänggi turnover theory (PGH) correctly predict the overall rate. As predicted in paper I [S. C. Tucker, J. Chem. Phys. 101, 2006 (1994)], the reactive flux in this case has two plateaus corresponding to two phenomenological rate constants. Mean first passage time calculations confirm that only the first of these two plateaus—which corresponds to the PGH estimate of the rate constant—is observed in the reactive flux simulations.
Line and boundary tensions at the wetting transition: Two fluid phases on a substrate
Authors: Perković S. , Blokhuis E. M. , Han G.
Published: July 2002

We develop and analyze a mean-field model free energy that describes two fluid phases on a substrate in order to calculate the (numerically) exact line and boundary tensions, on approach to the first-order wetting transition. A theory based on the van der Waals theory of gas–liquid interfaces is used. We implement a multigrid algorithm to determine the two-dimensional spatial variation of the density across the three-phase and boundary regions, and hence, the line and boundary tensions. As the wetting transition is approached, the tensions approach the same, finite, positive limit with diverging slopes. We compare our results with those of recent related work.
Cavity ring-down overtone spectroscopy of HCN, H13CN and HC15N
Authors: Romanini Daniele , Lehmann Kevin K.
Published: July 2002

This paper reports the results of our use of Cavity Ring Down Spectroscopy to extend the study of highly excited vibrational states in HCN. We extend our previous study of the H12C14N isotopomer, reporting on some weaker bands between 17 500 and 19 500 cm−1. We also report spectra of overtone and combination bands with six, seven, and eight quanta of stretching vibration in the isotopomers H12C15N and H13C14N in the interval from 17 500 to 23 000 cm−1. The observed spectroscopic constants and band intensities are compared with calculated values. All but one of the observed bands can be fit to within experimental accuracy (∼0.02 cm−1) to the standard distortable–rotor Hamiltonian. The one perturbed band has been successfully analyzed in terms of three anharmonically coupled levels. An anomalous line intensity distribution has been observed in the 1115 and 0116 Π←Σ bands, which we believe is produced by Coriolis coupling.
Spectroscopy and quantum dynamics of the 1,2-dimethylnaphthalene⋅Ar van der Waals complex
Authors: Droz Thierry , Leutwyler Samuel , Mandziuk Margaret , Bačić Zlatko
Published: July 2002

We report a detailed experimental and theoretical study of the intermolecular vibrational levels of the 1,2-dimethylnaphthalene⋅Ar van der Waals complex in the S1 electronic state. Due to the low symmetry of the aromatic molecular substrate, excitations in all three van der Waals modes (two in-plane, one out-of-plane) are allowed in the electronic spectrum, leading to the most complete data set of intermolecular vibrational transitions measured so far for an atom-large molecule complex. Two-color resonant two-photon ionization spectra reveal twelve bands within ≊95 cm−1 of the electronic origin. Accurate quantum three-dimensional calculations of the van der Waals vibrational levels were performed, using a method based on the 3D discrete variable representation. Combination of theory and experiment allowed a complete and quantitative interpretation of the level structure, as well as an accurate parametrization of the intermolecular potential energy surface (PES), modeled as sum of atom–atom Lennard-Jones pair potentials. The parametrization was partly transferable from other aromatic molecules, but methyl group parameters had to be adapted. Quantum number assignments of the vibrational states were possible for the low-lying levels, but difficult or impossible for states at higher energies, due to strong mode coupling, mainly between the two in-plane modes.
Microwave detected, microwave-optical double resonance of NH3, NH2D, NHD2, and ND3. I. Structure and force field of the Ã state
Authors: Henck Steven A. , Mason Martin A. , Yan Wen-Bin , Lehmann Kevin K. , Coy Stephen L.
Published: July 2002

Microwave detected, microwave-optical double resonance was used to record the Ã state electronic spectrum of NH3, NH2D, and NHD2 with both vibrational and rotational resolution. To investigate ND3 with the same resolution as we had with our hydrogen containing isotopomers, a strip-line cell was constructed allowing the simultaneous passage of radio-frequency and ultraviolet radiation. Rotational constants were obtained as a function of ν2 excitation and an Ã state equilibrium bond length was estimated at 1.055(8) Å. In addition, the harmonic force field for the Ã state has been experimentally determined. fhh, fαα−fαα′, and frr were found to be 1.06(4) aJ/Å2, 0.25(2) aJ, and 4.9 aJ/Å2, respectively. This calculated harmonic force field predicts that the asymmetry observed in the NH3 24 band is due to a strong anharmonic interaction with the 43 level and the broad feature observed in the dispersed fluorescence spectrum previously assigned to the 11 band is more likely attributable to the 42 level.
Two hard sphere models for the reaction A+BC
Authors: Song Ju-Beom , Gislason Eric A. , Sizun Muriel
Published: July 2002

In an attempt to understand the mechanism of chemical reactions such as A+BC→AB+C or AC+B, we have constructed two models based upon the assumption that A, B, and C are hard spheres with B and C initially touching. Both models include an accurate estimate of the total cross section for A+BC collisions, the proper angular dependence of the activation energy, and a set of reasonable procedures for selecting the product state (AB+C, AC+B, A+BC, or A+B+C) once the final velocities of the three atoms have been computed. The two models differ in that one uses the sequential impulse model to calculate the final velocities, whereas the other uses the direct interaction with product repulsion (DIPR) model to obtain the velocities. The two models are used to study the O(3P)+H2 system, and the results are compared with quasiclassical trajectory (QCT) calculations on this system. At high energies the DIPR model appears to give better overall agreement with the QCT results. In particular, the QCT calculations show that at high energy the major product channel is the knockout reaction, where the O atom first hits one H atom but then goes on to react with the other atom. This effect is predicted by the DIPR model, but not by the sequential impulse model.
Computation of interior eigenstates of large matrices using the quasiadiabatic evolution of instantaneous eigenvectors
Authors: Jolicard Georges , Killingbeck John P.
Published: July 2002

A two-stage iterative scheme is proposed to handle a central problem of molecular dynamics, the computation of interior eigenvalues of large Hamiltonian matrices. The proposed method involves an initial propagation process for a time-dependent wave operator which is then inserted in an iterative process (recursive distorted wave approximation or single cycle method) to yield the exact stationary wave operator. The merits of the wave operator formalism for quasiadiabatic propagation are analyzed, and possible improvements such as the use of partial adiabatic representations and spectral filters, are outlined. The proposed algorithm is applied to the test case of two coupled oscillators with variable coupling strength, and yields accurate results even with small switching times.
Quantum time evolution in time-dependent fields and time-independent reactive-scattering calculations via an efficient Fourier grid preconditioner
Authors: Peskin Uri , Miller William H. , Edlund Åke
Published: July 2002

A numerical scheme is suggested for accurate large-scale quantum dynamics simulations. The time-dependent Schrödinger equation with finite time-dependent interaction terms is replaced by an inhomogeneous equation with imaginary boundary operators applied along the time axis. This equation is solved globally for a finite time interval using recent Krylov subspace-based iterative methods that are accelerated by a Fourier grid preconditioner. The same scheme is applied also to time-independent reactive-scattering calculations with absorbing boundary operators where the operation of the Green’s function is carried out by solving an inhomogeneous time-independent equation. The scheme is economic in terms of both memory requirement and computation time. It is especially favorable when high grid densities are required, e.g., for representation of highly oscillatory fields or high-energy wave functions. Illustrative applications are given for representative models of bound and dissociative systems driven by time-dependent pulsed fields, and for time-independent calculations of the cumulative reaction probability for the generic reaction H+H2 at high collision energies.
Vibrational analyses of trans,trans-1,3,5,7-octatetraene and all-trans-1,3,5,7,9-decapentaene based on ab initio molecular orbital calculations and observed infrared and Raman spectra
Authors: Hirata So , Yoshida Hiroshi , Torii Hajime , Tasumi Mitsuo
Published: July 2002

Normal coordinate analyses are performed for trans,trans-1,3,5,7-octatetraene and all-trans-1,3,5,7,9-decapentaene on the basis of ab initio molecular orbital (MO) calculations and observed infrared and Raman spectra. The infrared and Raman spectra of all-trans-1,3,5,7,9-decapentaene are measured in this study. Ab initio MO calculations are carried out at the second-order Mo/ller–Plesset perturbation (MP2) level with the 6-31G* basis set. The calculated force fields are scaled by using the scale factors determined to obtain good fits between the calculated and observed frequencies of trans-1,3,5-hexatriene. The calculated frequencies are in satisfactory agreement with the observed for all the modes of both octatetraene and decapentaene including the in-phase C=C stretches, for which the calculated frequencies at the Hartree–Fock (HF) level deviate considerably from the observed. It is concluded that good force fields of all-trans oligoenes can be obtained from ab initio MP2 calculations by use of the scale factors derived in this study. From the comparison of the force fields at the MP2/6-31G* and HF/6-31G* levels, it is clear that electron correlation has large effects not only on the absolute values of diagonal and off-diagonal force constants of the C=C and C–C stretches but also on the chain-length dependence of these force constants. Important features of the vibrational modes are also discussed.
Lattice theory of solvation in macromolecular fluids. III. Monte Carlo simulations
Authors: Olender Roberto , Nitzan Abraham , Knödler D. , Dieterich W.
Published: July 2002

Monte Carlo simulations are used to calculate the energy, free energy, and entropy of solvation in a lattice model of polymer host. The solute particle interacts with specific beads in the host chain at nearest neighbor sites. The results are used to check the accuracy of the quasichemical approximation (QCA) recently used [Olender and Nitzan, J. Chem. Phys. 101, 2338 (1994)] to study ion solvation and ion pair dissociation in polymer hosts. For noninteracting chains we find that the QCA is very accurate when the solvent consists of homogeneous chains (all beads interact equally with the impurity), and give errors of up to 20% when nonhomogeneous chains (with some of the beads interacting with the impurity) are used. For interacting chains this trend is reversed and the QCA works better for nonhomogeneous chains. Deviations of the QCA prediction from the ‘‘exact’’ numerical results are traced to three-body and higher order correlations. The success of the QCA for interacting solvents of nonhomogeneous chains is associated with cancellation of opposing effects of such correlations.
Pyrazine: Supercollisions or simple reactions?
Authors: Chesko James D. , Stranges Domenico , Suits Arthur G. , Lee Yuan T.
Published: July 2002

Photodissociation of 1,4 diazine (pyrazine) following 248 nm (S2←S0) and 308 nm (S1←S0) excitation has been studied by photofragment translational spectroscopy. While the latter wavelength was inefficient at generating reactive products, the former wavelength afforded a rich variety of fragments, the most translationally energetic being HCN carrying up to 36 kcal/mol. The implications of this reactive mechanism toward interpretation of vibrational energy transfer experiments from hot ground state pyrazine are considered. The absolute quantum yields φ(248 nm, 27-A)=0.40±0.10, φ(248 nm, 27-B)=0.08±.02, φ(308 nm, 27-B′)=0.003±.001 and the two corresponding translational energy distributions are presented.
Monte Carlo simulation of sequential decay processes: Application to argon cluster evaporation at zero pressure
Authors: Dumont Randall S. , Jain Shashi
Published: July 2002

Monte Carlo simulation of argon cluster evaporation is in reasonable accord with molecular dynamics computations. The new Monte Carlo algorithm is based on assumptions of ergodicity and mixing. Our results affirm these statistical assumptions for argon cluster dynamics. Observations of algebraic long time decay in evaporation lifetime distributions are also in support of the assumptions. The Monte Carlo algorithm provides an easily implemented statistical approximation to decay dynamics in many degree of freedom systems. It is formulated for application to any sequential unimolecular decay process.
The resonance Raman spectrum of cyclobutene
Authors: Negri Fabrizia , Orlandi Giorgio , Zerbetto Francesco , Zgierski Marek Z.
Published: July 2002

The vibronic pattern of the resonance Raman spectrum of cyclobutene is simulated by ab initio molecular orbital and by density functional theory calculations. Both Franck–Condon and Herzberg–Teller contributions are included in the analysis of the spectrum. The results suggest an initial dynamics of cyclobutene in the S1 excited state in which the molecule attempts a cis–trans isomerization of the ethylene moiety.
Performance of close-coupled wave packet methods for molecule-corrugated surface scattering
Authors: Kroes G. J. , Snijders J. G. , Mowrey R. C.
Published: July 2002

The H2+LiF(001) system was used to investigate the performance of the hybrid close-coupling wave packet (CCWP) method and of a symmetry adapted, fully close-coupled wave packet (SAWP) method for a molecule–surface problem characterized by fairly high corrugation. In the calculations, a realistic, φ-dependent model potential was used. The calculations were performed for a collision energy of 0.2 eV, with H2 initially in its j=0 rotational state at normal incidence to the surface. Large increases in the computational efficiencies of both wave packet methods were achieved by taking advantage of the potential coupling matrices associated with both methods becoming sparser with increasing molecule–surface distance. For the present model problem and employing this increased sparseness at longer range, the SAWP method is faster than the CCWP method by a factor of 2. The potential usefulness of the SAWP method for dissociative chemisorption problems is discussed.
Measurement of the state-specific differential cross section for the H+D2→HD(v′=4, J′=3)+D reaction at a collision energy of 2.2 eV
Authors: Xu Hao , Shafer-Ray Neil E. , Merkt Frédéric , Hughes Daniel J. , Springer Michael , Tuckett Richard P. , Zare Richard N.
Published: July 2002

HI and D2 are coexpanded into a vacuum chamber. The photolysis of HI at 212.8 nm initiates the H+D2 reaction. The HD(v=4,J′=3) velocity distribution is determined by analyzing the time-of-flight profile of HD+ ions produced by delayed pulsed field ionization of long-lived Rydberg states. The angular distribution is deduced using the law of cosines (photoloc technique).
Measurement of the molecular content of binary nuclei. III. Use of the nucleation rate surfaces for the water-n-alcohol series
Authors: Strey R. , Viisanen Y. , Wagner P. E.
Published: July 2002

In two preceding papers the molecular content of binary ethanol-hexanol and water-ethanol nuclei, respectively, was determined from nucleation rate measurements. While nucleation of ethanol-hexanol mixtures behaved nearly ideal, a strong mutual nucleation enhancement for water-ethanol was observed. Here we extend the investigations to include the longer chain alcohols, that is water −CiH2i+1OH systems with i=2 to 6. Using the nucleation pulse technique developed in the past few years nucleation rates in the range 105<J/cm−3 s−1<109 are measured. Ranging from pure water to pure alcohol the water and alcohol activities, a1 and a2, respectively, are varied for each system with about ten intermediate compositions at constant temperature T=260 K. Aside from a remarkably similar behavior of the various alcohols, one observes that with increasing alcohol chain length the mutual nucleation enhancement decreases. Since all water-alcohol systems behave qualitatively similar, we confine ourselves to present the full experimental nucleation rate surface J(a1,a2) for one system, water-n-pentanol, as an example. From the nucleation rate surface for each system the onset activities corresponding to a reference nucleation rate of J0=107 cm−3 s−1 are determined. From the slopes of the nucleation rate surfaces one obtains the individual numbers of molecules in the critical cluster ni* because ∂ ln J/∂ ln ai≊ni*. As noted previously, determining the molecular content this way does not involve any particular theoretical model, nor does it depend on the structure of the critical cluster. Accordingly, the average composition of the critical clusters can be obtained. An alcohol enrichment of the nuclei at low alcohol activity fraction is found for all alcohols examined, the degree diminishing with increasing alcohol chain length. The appearance of a macroscopic miscibility gap for the higher alcohols is not reflected in any qualitative change of the composition of the microscopic nuclei.
Ab initio molecular orbital study of potential energy surface for the reaction of C2H3 with H2 and related reactions
Authors: Mebel Alexander M. , Morokuma Keiji , Lin M. C.
Published: July 2002

The potential energy surface of the reaction C2H3+H2→C2H4+H→C2H5 has been investigated using various theoretical methods including QCISD(T), CCSD(T), RCCSD(T), Gaussian-2 (G2), and the density-functional B3LYP approach. The reaction of the vinyl radical with molecular hydrogen is shown to take place through the hydrogen atom abstraction channel leading to the formation of C2H4+H with the activation energy of 10.4 kcal/mol at all the G2, QCISD(T)/6-311+G(3df,2p), and CCSD(T)/6-311+G(3df,2p) levels. The rate constant, calculated using the variational transition state theory with tunneling correction, k=3.68⋅10−20⋅T2.48⋅exp(−3587/T) cm3 molecule−1 s−1, is in good agreement with the experimental estimates. C2H5 cannot be formed directly by inserting C2H3 to H2, but can only be produced by addition of H to C2H4, with a barrier of 4.5–4.7 kcal/mol calculated at high levels of theory. In order to match the experimental rate constant, the activation energy needs to be adjusted to 2.8 kcal/mol. Generally, the B3LYP method is found to predict well the geometries and vibrational frequencies of various species. However, it is less reliable for energy calculations than the QCISD(T) and CCSD(T) methods.
Anisotropic dynamical effects on two-dimensional potential energy surface reactions: Bond breaking electron transfer reactions
Authors: Spirina O. B. , Cukier R. I.
Published: July 2002

The effect of solvent and intramolecular dynamics on the rate of a bond breaking electron transfer reaction is investigated. The reaction takes place on a two-dimensional potential energy surface with one coordinate the solvent’s polarization and the other the breaking bond’s displacement. The dynamics are governed by overdamped spatial diffusion along the polarization coordinate and by energy diffusion along the bond coordinate. A scheme is presented that treats the transition from rate control by the equilibrium rate constant kr (as evaluated by, e.g., a Golden Rule calculation) to dynamical control, where the rate is controlled by diffusion on the surface, with rate constant kd, that accounts for the different character of the dynamics in the two directions. The overall rate constant has the form appropriate to a consecutive reaction mechanism: k−1=k−1d+k−1r. The kd rate constant is analyzed numerically and the results compared with a number of approximation schemes. A method of analysis is developed for situations where one dynamics is fast/slow compared with the other. Which time controls kd depends strongly on the relation between the fast diffusion rate and kr.
Infrared spectroscopy of Ar2CO2 trimer: Vibrationally averaged structures, solvent shifts, and three-body effects
Authors: Sperhac Jeanette M. , Weida Miles J. , Nesbitt David J.
Published: July 2002

Ar2CO2 is studied using direct absorption infrared spectroscopy. The van der Waals molecules are formed when a mixture of CO2 and Ar gases is expanded in a supersonic slit jet. To probe the clusters, the ν3 asymmetric stretch of the CO2 monomer is then monitored in absorption. Sixty-one trimer transitions are assigned and fit to a Watson asymmetric top Hamiltonian. Rotational constants for the upper and lower vibrational states permit determination of vibrationally averaged molecular structures, which indicate that the Ar atoms lie in the plane that bisects CO2 and is perpendicular to the CO2 intramolecular axis. These geometries are consistent with an equivalent ‘‘T-shaped’’ ArCO2 geometry for each Ar atom. Vibrational origins for the ν3 CO2 asymmetric stretch frequency in ArnCO2 are found to shift approximately linearly for zero, one, and two Ar atoms. Calculations using pair potentials are used to extrapolate these red shifts out to the bulk phase and to compare the results to experimental matrix data. Finally, the slight nonlinearity in the red shift between ArCO2 dimer and Ar2CO2 trimers is interpreted in the context of three-body forces.
A detailed study of the predissociation dynamics of the 1B2(1Σ+u) state of CS2
Authors: Mank A. , Starrs C. , Jego M. N. , Hepburn J. W.
Published: July 2002

The photodissociation dynamics of the 1B2 state of CS2 have been studied for excitation energies between 46 600 and 50 500 cm−1 (214–198 nm). These experiments used a tunable pulsed uv laser to photodissociate jet-cooled CS2 and the resulting atomic S products were detected by single photon VUV laser-induced fluorescence. These data allow for the measurement of the S(1D2):S(3PJ) product branching ratios as a function of vibrational level in the 1B2 state. Because of the low rotational temperature of the jet cooled CS2, we were able to resolve vibrational bands, and simulate the rotational contours for K=0 and K=1 vibrational bands up to 50 100 cm−1 excitation energy, obtaining homogeneous linewidths for these levels. For vibrational levels close to the zero point, the dissociation dynamics, both lifetimes and product branching ratios, were very different for K=0 and K=1 levels at the same energy. In particular, K=1 levels have shorter lifetimes, and a substantially higher yield of S(1D2) product. We propose a model for the dissociation dynamics of the 1B2 state of CS2, based on our results and preliminary ab initio calculations of the potential energy curves for excited states of CS2.
Basis set superposition problem in interaction energy calculations with explicitly correlated bases: Saturated second- and third-order energies for He2
Authors: Bukowski Robert , Jeziorski Bogumil/ , Szalewicz Krzysztof
Published: July 2002

Explicitly correlated basis set of Gaussian-type geminals has been employed in supermolecular calculations of the interaction energy of two helium atoms using the second- and third-order of the many-body perturbation theory and the Mo/ller–Plesset partitioning of the Hamiltonian. A geminal extension of the counterpoise procedure of Boys and Bernardi has been proposed to correct for the basis set superposition error. Performance of the proposed correction scheme has been analyzed at the second-order level using a sequence of geminal bases varying in the degree of completeness in representing the intra- and intermonomer correlation effects. The nonlinear parameters of these bases were optimized by minimizing the second-order energy of the helium atom and the second-order dispersion energy of the He dimer. The best upper bounds to date have been obtained for both quantities. The numerical results show that the counterpoise procedure should be used at all levels of basis set completeness. By employing the union of the largest of the obtained bases and reoptimizing some of the nonlinear parameters using the complete second-order energy functional for the dimer, the best estimates to date of the second- and third-order supermolecular interaction energies for He2 have been computed. At the minimum interatomic separation these energies are estimated to be accurate to 0.01 K or better. Adding higher-order terms computed using orbital bases, leads to a helium dimer interaction potential with the depth of 11.00 K, somewhat larger than current experimental results.
Photodissociation spectroscopy of Ca+–H2O and Ca+–D2O
Authors: Scurlock C. T. , Pullins S. H. , Reddic J. E. , Duncan M. A.
Published: July 2002

Electronic spectra are observed for the monosolvated metal cation complexes Ca+–H2O and Ca+–D2O using resonance enhanced photodissociation spectroscopy. The clusters are produced in a laser vaporization/supersonic expansion source and the mass-analyzed product is observed using a time-of-flight mass spectrometer. Both Ca+ and CaOH+ (or CaOD+) dissociation channels are observed on sharp resonances. Transitions from the ground electronic state to two excited electronic states are assigned, with vibrational progressions in the Ca–OH2 stretching mode. Spectroscopic constants are Ca+–H2O: (2) 2B2←X 2A1 (T0=21 464 cm−1, ΔG1/2=357.9 cm−1) and (2) 2B1←X 2A1 (T0=23 273 cm−1, ΔG1/2=335.9 cm−1); and Ca+–D2O: (2) 2B2←X 2A1 (T0=21 447 cm−1, ΔG1/2=350.9 cm−1) and (2) 2B1←X 2A1 (T0=23 261 cm−1, ΔG1/2=324.1 cm−1). These transitions are rotationally resolved, confirming the structure of the complex to be C2v. The Ca+–H2O bond distance is 2.22 Å and the H–O–H bond angle is 106.8° in the ground state. Comparisons with theoretical calculations are also made.
Perturbation theory and dynamic reaction path analysis of intramolecular vibration mixing: An application to the case of H2O
Authors: Taketsugu Tetsuya , Hirano Tsuneo
Published: July 2002

A perturbational method in terms of cubic force constants was proposed to predict how intramolecular vibration mixing among normal modes occurs through the anharmonicity of the potential energy. The method was applied to H2O and it was predicted that symmetric and antisymmetric stretching normal modes having similar frequencies should couple strongly with each other, even at the equilibrium point moiety, as the antisymmetric normal vibration develops. Dynamic reaction path calculations were also performed for H2O, and the strong vibration coupling between those predicted normal vibration pair was confirmed.
Diode laser absorption study of internal energies of CO2 produced from catalytic CO oxidation
Authors: Bald Daniel J. , Kunkel Ralf , Bernasek Steven L.
Published: July 2002

Internal energy distributions for CO2 molecules produced in the catalytic oxidation of CO by O2 on polycrystalline platinum have been directly measured using a high resolution tunable diode laser (TDL) spectrometer. Absorption spectra have been collected for CO2 produced in a flow cell reactor for a wide range of temperatures and reactant ratios. Vibrational energies for CO2 produced under most reaction conditions are greater than would be expected from complete equilibration with the surface. The asymmetric stretching mode consistently exhibited a higher vibrational Boltzmann ‘‘temperature’’ than the bending or symmetric stretching modes. The level of vibrational excitation for all of the vibrational modes was sensitive to oxygen coverage, i.e, as either the O2:CO reactant ratio or the surface temperature is increased, the vibrational temperature of the product CO2 increases significantly.
Deuterium scattering from Rh(110) surface
Authors: Cvetko D. , Morgante A. , Santaniello A. , Tommasini F.
Published: July 2002

Thermal energy deuterium beam scattering (TEDS) from Rh(110) surface is reported and compared to helium atom scattering (HAS). Unlike for He, we find quite strong D2 off-specular diffraction. The D2 diffraction intensities are fairly isotropic along the [001] and [11̄0], reflecting the presence of a finite dissociation barrier in the potential energy surface (PES) rather than the anisotropic corrugation of the relevant Rh(110) surface electron density. Inelastic D2 scattering with strong J=0→2 kinetic-to-rotational energy conversion is also presented and discussed.
Theoretical study of gas-phase NanPb clusters and implications for liquid Na–Pb alloys
Authors: Chang Jianlin , Stott M. J. , Alonso J. A.
Published: July 2002

The production of an exceptionally abundant and stable intermetallic cluster Na6Pb has recently been reported. In an attempt to understand why this particular cluster appears so prominently in the experiments, we have performed ab initio total energy calculations on the clusters NanPb with n=3–8. The lowest energy structures and the evaporation energies for these clusters were determined. The energy to evaporate a Na atom from the octahedral cluster Na6Pb was found to be 1.58 eV, larger than that for bigger clusters. We propose that the high abundance of the Na6Pb cluster results from a cascade of evaporations from larger clusters which becomes blocked at Na6Pb because of the large evaporation energy. We also found that the cluster Na4Pb was particularly stable, and suggest how the relative stability of this cluster and Na6Pb might be related to the possible formation of complexes in Na–Pb liquid alloys.
Theory of lipid monolayers comprised of mixtures of flexible and stiff amphiphiles in athermal solvents: Fluid phase coexistence
Authors: Cantor Robert S.
Published: July 2002

Statistical thermodynamic theory developed recently to study self-assembled fluid monolayer films of short diblock copolymeric surfactants at the oil/water interface is applied to spread monolayers comprised of mixtures of amphiphiles that differ only in the flexibility of their chains. Although the oil and water solvents are assumed to be athermal for the two blocks of the amphiphiles, we predict large positive deviations from the ideal free energy of mixing that result in phase separation for sufficiently long chains. Pressure–composition phase diagrams and representative pressure–area isotherms are predicted for monolayer films over a range of chain lengths. By comparing predicted structural properties to the contributions to the mixing free energy, we explore the physical origins of the demixing, and consider its implications for experimental systems such as cholesterol/lipid mixtures in monolayers and bilayers.
Theoretical study of carbon doped small silicon clusters: Electron affinities of SinC (n=2–5)
Authors: Kishi Reiko , Gomei Motoki , Nakajima Atsushi , Iwata Suehiro , Kaya Koji
Published: July 2002

The geometries and energies of SinC and SinC− (n=2–5) were investigated with ab initio MO calculations including electron correlation. The Mo/ller–Plesset perturbation theory was used for n=3–5. The configuration interaction (CI) calculations were carried out for n=2 and 3. The most stable isomers of the neutral SinC clusters have the structure in which an Si atom having the largest sp hybridization in the corresponding Sin+1 cluster is replaced by a C atom. For their anions, the structures in which an excess electron is distributed avoiding a C atom become stable. Moreover, the adiabatic electron affinities of SinC and vertical detachment energies of anionic SinC− were computed. They reproduced the trends of photoelectron spectra of SinC− reported previously.
Radiative and nonradiative decay of electronically excited NCO
Authors: Wright Scott A. , Dagdigian Paul J.
Published: July 2002

A study to observe higher vibrational levels in NCO(Ã 2Σ+) and the onset of predissociation in this molecule has been carried out. Laser fluorescence spectra have been recorded over the wave number range 27 300–32 900 cm−1, from the Ã(0,0,2)–X̃(0,0,0) band up through the B̃(1,0,0)–X̃(0,0,0) band. Vibrational assignments have been made for a number of newly observed Ã–X̃ bands, and band origin wave numbers and upper level rotational constants have been derived from comparison of experimental spectra with simulations. Decay lifetimes for excitation of a large number of both assigned and unassigned excited vibronic levels have been determined. The onset of predissociation appears to occur at energies slightly below that of the B̃(0,0,0) level.