bluuebunny/crossref_metadata_2025 huggingface dataset:
Renormalization group study of Rouse–Zimm model of polymer dynamics through second order in ε
Authors: Wang Shi-Qing , Freed Karl F.
Published: July 2002

Gell–Mann–Low style renormalization group methods are employed in conjunction with dimensional regularizations and ε expansions to study the bead-spring model of polymers with excluded volume in dilute solutions. Perturbation calculations are carried out through order ε2 for the translational diffusion coefficient D (ε=4−d, d being the spatial dimension) both with and without the use of the preaveraging approximation. Polymer excluded volume interactions are systematically incorporated into the Rouse–Zimm model by using the well-known two parameter model along with the ε expansion. The nondraining limit hydrodynamic interaction fixed point is found to be unaffected by preaveraging through order ε2 and is identical to that obtained previously from the order ε2 preaveraged Kirkwood–Riseman formalism. Our work indicates that the nonpreaveraged renormalization group perturbation expansions appear more well behaved (controllable) in the Rouse–Zimm model than in the Kirkwood–Riseman rigid body model.
Color center laser spectroscopy of vibrationally excited C2H
Authors: Yan Wen-Bin , Hall Jeffrey L. , Stephens J. W. , Richnow Marilyn L. , Curl R. F.
Published: July 2002

Infrared transitions involving vibrationally excited C2H radicals produced by excimer laser (193 nm) flash photolysis of acetylene were observed in transient absorption by color center laser probing. Three bands with origins at 3702, 3562, and 3320 cm−1 were found, assigned, and analyzed. All three bands are of 2∑–2∑ symmetry with the lower state of each transition the vibrationally excited (0,0,1) level of the ground electronic state. The frequencies and fitted spectroscopic constants for the three bands are reported. Possible vibronic assignments of the upper states are discussed.
Measurements of the Rayleigh ratio of some pure liquids at several laser light wavelengths
Authors: Moreels E. , De Ceuninck W. , Finsy R.
Published: July 2002

The Rayleigh ratios Rv,v for vertically polarized incident and scattered radiation of benzene, toluene, carbon tetrachloride, and cyclohexane have been measured at a scattering angle of 90° and at 25 °C using argon–ion and krypton–ion lasers at wavelengths 488, 514.5, and 647.1 nm. The problem of the change of the scattering geometry with the refractive index of the investigated liquid was solved by a suitable experimental procedure. Errors are estimated to be in the range of 1%–2%.
The dynamics of H2 dissociation on Ni(100): A quantum mechanical study of a restricted two-dimensional model
Authors: Jackson Bret , Metiu Horia
Published: July 2002

A quantum mechanical study of the dynamics of H2 dissociation on Ni is presented. The H2 molecule approaches the surface and is held parallel to the surface. The center of mass is atop a Ni atom and the dissociated atoms have minimum energy at bridge binding sites. This restricted molecular configuration allows us to propagate the molecular wave function in time numerically, using fast Fourier transform techniques. The probability for dissociative adsorption is computed as a function of initial molecular kinetic energy, for a variety of model gas–surface potentials. The way in which the height of the barrier to dissociation affects this energy dependence, as well as the nature of the transfer of energy from the center of mass into the relative motion of the H atoms is examined. By including effects due to H atom mobility it is demonstrated how barriers to surface diffusion can dominate dissociation rates by controlling the extent of recombination. Activation barriers to adsorption in the entrance channel are shown to attenuate the incoming molecular beam, and temporarily trap H2 near the surface. The dissociation of H2 is fairly nonclassical, particularly at thermal energies where much reflection occurs at barrier crossing even when the incident energy is above the barrier.
Comparison of a
b i
n
i
t
i
o and semiempirical pseudopotentials for Ca in calculations for CaO
Authors: Igel-Mann G. , Dolg M. , Wedig U. , Preuss H. , Stoll H.
Published: July 2002

Two recently reported pseudopotentials, an ab initio and a semiempirical one, both simulating the Ar-like core of the Ca atom, are compared in calculations for CaO. While the former pseudopotential leads to a collapse of the molecule in the X 1Σ+ ground state, such a collapse does not occur with the latter one. The reasons for this discrepancy are discussed and are attributed mainly to differences in the treatment of the higher l components of the pseudopotentials.
Fine-structure electronic predissociation in van der Waals molecules. I. Theory
Authors: Jouvet Christophe , Beswick J. Alberto
Published: July 2002

A quantum-mechanical model for fine-structure predissociation in van der Waals molecules is presented. The process corresponds to: A(2S+1LJA )⋅⋅⋅M→A(2S+1LJ′A) +M where A is an atom and M an atom or a diatomic molecule A(2S+1LJA) ⋅⋅⋅M represents the van der Waals complex in a state which correlates to the 2S+1LJA electronic manifold of A. The general formalism based on close-coupling equations is presented first. The limiting case of slow predissociation is then considered and treated by perturbation theory. Finally, the application to the atom–atom and the atom–diatom van der Waals complexes for L=1 and S=1/2 or S=1 is discussed. This simple analysis allows predictions to be made for predissociation rates and lifetimes in cases where the spin–orbit interaction is larger than the van der Waals binding energy.
Phase diagrams of charged colloidal particles
Authors: Shih Wan Y. , Aksay Ilhan A. , Kikuchi Ryoichi
Published: July 2002

We have calculated the phase diagrams of charged colloidal particles in the parameter space. The free energies of different phases, which were used to determine the phase boundaries, were calculated variationally. The Einstein oscillators and the hard sphere fluid were used as the reference systems for the solid phases and the liquid, respectively. The interparticle interactions were approximated to be the Debye–Hückel screened Coulomb potential with size correction. We show that the solid phases are stable at low salt concentrations: bcc is preferred only at high charges and at low densities while fcc is dominant at higher densities. The solid phases melt upon the addition of salt: bcc may or may not transform to fcc before melting, depending on the particle number densities. No reentrant transition is found upon the addition of salt. When the particles are extremely highly charged, the system may undergo fcc–bcc–fcc transitions at nonzero salt concentration when the particle number density is increased.
Solvent induced vibrational population relaxation in diatomics. II. Simulation for Br2 in Ar
Authors: Herman Michael F.
Published: July 2002

Results are presented from a computer simulation of the population relaxation of the lowest (1→0) vibrational transition for a system of Br2 in a dense Ar fluid at 300 K. The calculate relaxation time is 253 ps. The method of calculation is a mixed quantum–semiclassical–classical simulation procedure. The vibrational state energies and wave functions are obtained from perturbation theory for fixed values of the rotational and translational variables. The relaxation rate for the vibrational transition is evaluated using a semiclassical surface hopping theory of nonadiabatic processes. The configurations of rotational and vibrational variables are sampled from a classical canonical ensemble density using standard Monte Carlo sampling. The relative efficiency of the rotations and translations in promoting the vibrational population relaxation is examined, and the use of perturbation theory and some assumptions of the model are tested numerically.
Multilevel adaptive technique for quantum reactive scattering
Authors: Duneczky Csilla , Wyatt Robert E.
Published: July 2002

Discretization (with localized basis functions or grid points) of the coupled integral equations for molecular collisions leads to a very large system of linear algebraic equations. New methods, which are well adapted to vector supercomputers and parallel architectures, are developed for solving this large system. The multilevel adaptive technique (MLAT) is combined with recursive and iterative techniques. First, a multichannel solution is obtained on a low level grid. The basis is then adapted to this solution and the coarse solution is projected or interpolated onto the adapted basis. The scattering amplitudes (K-matrix elements) on the high level are then developed through use of either the recursion method (for single amplitudes, or a small batch of them) or the iterative technique (for all transitions from a specified initial state). In both of these methods, the original large system of algebraic equations is projected into a much smaller subspace (an orthonormalized Krylov space) spanned by a few basis vectors. Applications to very large systems are possible because it is not necessary to store or invert a large matrix. Computational results on a model chemical reaction are presented.
Global potential energy hypersurface for dynamical studies of energy transfer in HF–HF collisions
Authors: Redmon Michael J. , Binkley J. Stephen
Published: July 2002

The interaction energy of two HF molecules at 1332 individual points has been calculated with Moeller–Plesset (many–body) perturbation theory at the MP4-SDTQ level using a 6-311G** basis set. 293 of the points correspond to stretching of one HF molecule from its equilibrium geometry. No attempt was made to use a sufficiently fine grid to accurately describe the well region corresponding to hydrogen bonding. However, the location and minimum energy are consistent with experiment and other accurate theoretical results. An extensive global fit (rms error of 1 kcal/mol) is reported of 1319 points (below 10 eV of potential energy) using a modified London potential with corrections obtained using polynomials through four-body interactions. A model electrostatic potential represents the long-range interaction. In addition, the use of an expansion in products of three Legendre functions is discussed. It is shown that the latter approach, although accurately fitting the ab initio data, has difficulties interpolating in regions of the surface exhibiting diverse magnitudes of potential energy, and therefore must be used with caution. This surface should be useful for studies of T–V–R processes in this system.
Autodetachment spectroscopy and dynamics of CH2CN− and CD2CN−
Authors: Lykke K. R. , Neumark D. M. , Andersen T. , Trapa V. J. , Lineberger W. C.
Published: July 2002

The technique of autodetachment spectroscopy is utilized in a study of two electronic states of CH2CN− and CD2CN−. The ground electronic state is a normal valence state while the outermost electron in the excited state is bound by the dipole moment of the neutral radical. Autodetachment occurs from excited rotational levels of this dipole bound state, giving some 5000 sharp features near the photodetachment threshold. All of these features were assigned and spectroscopic constants for both states are reported. The binding energy of the electron in this latter state is determined to be ≲66 cm−1 and analysis of the autodetachment spectrum shows the electron affinities of CH2CN and CD2CN to be ∼12 500 and ∼12 430 cm−1, respectively. The dynamics of the autodetachment process is studied and various mechanisms for detachment are described.
Interionic potentials in multihalides
Authors: Patil S. H.
Published: July 2002

We develop perturbative expressions for the long-range and short-range interaction in a collection of ions. These are used to obtain the interionic potential in multihalides and deduce bond lengths, binding energies, bond angles, and force constants.
Positive ions of the first- and second-row transition metal hydrides
Authors: Pettersson Lars G. M. , Bauschlicher Charles W. , Langhoff Stephen R. , Partridge Harry
Published: July 2002

Theoretical dissociation energies for the first- and second-row transition metal hydride positive ions are critically compared against recent experimental values obtained from ion beam reactive scattering methods. Theoretical spectroscopic parameters (re,ωe) and dipole moments are presented for the ground and several low-lying excited states. The calculations employ large Gaussian basis sets and account for electron correlation using the single-reference single- and double-excitation configuration interaction and coupled-pair-functional methods. The Darwin and mass-velocity contributions to the relativistic energy are included in the all-electron calculations on the first-row systems using first-order perturbation theory, and in the second-row systems using the Hay and Wadt relativistic effective core potentials. Owing to the smaller correlation effects in the second-row systems, the theoretical D0 values are probably at least as accurate as for the first row. Therefore, the theorectical D0 values for the second-row transition metal hydride positive ions should provide a critical measure of the experimental values, which are not as refined as many of those in the first transition row.
Ground state properties of alkali and alkaline–earth hydrides
Authors: Fuentealba P. , Reyes O. , Stoll H. , Preuss H.
Published: July 2002

The ground state potential energy curves of alkali (LiH to CsH) and alkaline–earth monohydrides (BeH to BaH) have been calculated. A pseudopotential formalism including a core-polarization potential has been used. For the valence correlation energy, two different methods, the local spin-density functional and the configuration interaction with single and double excitations, have been employed. Dissociation energies, bond lengths, vibrational frequencies, anharmonicity constants, and dipole moments are reported. The agreement with experimental values, where available, is very good. A discussion and a comparison with other theoretical values, at different levels of approximation, are also included.
IR diode laser study of vibrational energy distribution in CO2 produced by UV excimer laser photofragmentation of pyruvic acid
Authors: O’Neill James A. , Kreutz Thomas G. , Flynn George W.
Published: July 2002

Time domain absorption spectroscopy using a tunable, infrared diode laser has been used to monitor the vibrational excitation of CO2 produced in the 193 nm excimer laser photolysis of gas phase pyruvic acid (CH3COCOOH). Nascent vibrational populations were measured in the following ten vibrational states of CO2: 0000, 0110, 0220, 0330, 0440, 0001, 0002, 0003, 0111, and 0221. Approximately 97% of the CO2 photoproduct is observed to be directly formed in the vibrational ground state. The remaining molecules are formed with a significant degree of vibrational excitation, having mode temperatures T(ν2)=1800±150 K, T(ν3)=3700 ±1000 K, T(ν2+ν3)=2000±400 K. The present experimental data suggest that the 193 nm photolysis may proceed through more than a single dissociation channel and involve a number of different photofragments.
Formation energies and concentrations of microclusters for homogeneous nucleation
Authors: Kobraei H. R. , Anderson B. R.
Published: July 2002

The Gibbs free formation energies and the energies of formation for argon clusters having 3–19 atoms have been computed. These calculations have been carried out for the free cluster (atomistic) model and the model of interacting monomers and clusters (IMC). In the IMC model the interactions among background monomers and monomer–cluster interactions are considered. However, for the results presented in this work, only the effect of the interactions among background monomers has been computed. Comparison of the free cluster model with the IMC model indicates that even for argon the interactions among background monomers should not be ignored. The results of both models become almost identical at high temperatures. The depletion of monomers has also been considered. A simple relation is given which describes how background monomers are depleted in a supercooled system. The depletion of background monomers can easily be incorporated into the rate of nucleation and, when performed, this introduces a temperature-dependent correction to the nucleation rate.
Two-photon absorption spectroscopy of mass-selected ions: N2O+ and CS+2
Authors: Danis P. O. , Wyttenbach T. , Maier J. P.
Published: July 2002

A technique for the spectroscopic characterization of mass-selected ions in ground and bound excited electronic states involving two-photon absorption and fragment ion detection is demonstrated. A triple quadrupole mass analyzer system is used. Collisionally relaxed ions (M+) from a high pressure source are mass selected, then excited sequentially by two laser colors (λ1,λ2) according to M+→λ1M+*→λ2M+**→F+i and fragment ions (F+i) are detected. Stable ionic states are characterized by scanning λ1 with constant λ2 and this is illustrated on the known transitions of N2O+ (Ã 2Σ+←X̃ 2Π) and of CS+2 (B̃ 2Σ+u←X̃ 2Πg). New spectroscopic information—vibrational frequencies and rotational constants—and dynamic parameters—lifetimes and fragmentation branching ratios—on N2O+(B̃ 2Σ+) and CS+2 (C̃ 2Σ+g) are obtained by scanning λ2 when λ1 is chosen to populate several or individual rotational levels of the intermediate state.
Chemical reactions in isolated clusters: Excited state electron transfer in 3- and 4-dimethylaminobenzonitrile
Authors: Warren J. A. , Bernstein E. R. , Seeman Jeffrey I.
Published: July 2002

Two-color time-of-flight mass spectra of 3- and 4-dimethylaminobenzonitrile (3- and 4-DMABN) bare molecules and clusters with methane, water, acetone, dichloromethane, and acetonitrile are reported and discussed. The clusters and molecules are isolated and cooled in a supersonic expansion. Both 3- and 4-DMABN bare molecules display significant changes in geometry, associated with rotation–inversion coordinates of the dimethylamino group, upon excitation from the ground electronic state S0 to the first excited singlet state S1. Cluster spectra for the monosolvates [CH4, H2O, (CH3)2CO, CH2Cl2, CH3CN] of 3- and 4-DMABN evidence two general types of behavior. (1) Cluster spectra have both red and blue shifts from their respective bare molecule origins which are relatively small (in general less than 200 cm−1). These cluster spectra are nearly identical with the bare molecule spectra; the solvation process seems to have little effect on the DMABN molecule, especially the –N(CH3)2 moiety, for these clusters or cluster states. (2) Cluster spectra have shifts that are large (∼500–1000 cm−1) and to low energy of the bare molecule spectra. These cluster spectra are composed of both sharp and broad features which bear little resemblance to those observed in the respective bare DMABN molecule spectra. Results of calculations of cluster geometry suggest that thedifferent cluster spectra may be associated with clusters of different geometry. These data are interpreted in terms of solution behavior of 3- and 4-DMABN. The relationship between the twisted intramolecular charge transfer (TICT) state model proposed for 4-DMABN in dipolar aprotic solvents and these cluster results is explored.
Cavity distribution functions of pure and mixed hard-sphere systems
Authors: Labík S. , Smith William R.
Published: July 2002

A new set of computer simulation results for the cavity concentration profile n(r), which is related to the cavity distribution function y(r), are presented for both pure and mixed binary hard-sphere systems. Several theoretical methods for predicting n(r) are compared with the simulation data. The most successful theory for pure fluids, due to Boublík, is tested for hard-sphere mixtures. It is found that this theory, which requires only thermodynamic data for its implementation, accurately describes the cavity concentration profiles in the overlap region for a wide range of densities, sphere volume ratios, and compositions. The theory also successfully predicts the radial distribution functions up to the first minimum.
Scalar and vectorial properties in the photodissociation of tert-butyl nitrite from the S1 and S2 states
Authors: Schwartz-Lavi D. , Rosenwaks S.
Published: July 2002

A detailed comparison between the dynamics of photodissociation of (CH3)3CONO from its first two excited singlet states is presented. The fragmentation processes are studied by exciting the molecule at 365.8 and 351.8 nm [S1(nπ*)←S0 transition] and at 250 nm [S2(ππ*)←S0 transition] and probing the NO fragment by single photon laser induced fluorescence combined with polarization and sub-Doppler spectroscopy. The μ, v, and J vector correlations, Λ-doublet and spin–orbit populations, and the vibrational, rotational, and translational energy content of the NO fragment are determined. The scalar and vectorial properties point on different mechanisms of fragmentation from the S1 and S2 states, but both are highly selective. The findings of this study, especially those concerning the less studied S2 state, can be utilized to predict the behavior of other alkyl nitrites and demonstrate the power of the techniques mentioned above in characterizing the dynamics of photodissociation, even for large molecules.
Correlations between angular momenta of coincident product pairs
Authors: Gericke K.-H. , Grunewald A. U. , Klee S. , Comes F. J.
Published: July 2002

The correlation between angular momenta of the coincident product pairs formed in an elementary dissociation process has been investigated. Doppler shift measurements are used in connection with state-specific detection of scattered photofragments by laser-induced fluorescence. The method is demonstrated in the photodissociation of H2O2 and D2O2 at 193 and 266 nm where Doppler profiles of OH (OD) absorption lines are used to determine the (average) rotational energy of the OH (OD) partner molecule coincidently formed in the same elementary fragmentation process. At high product rotations the partner molecules are generated with comparable angular momenta, while at low rotational excitation a more complex correlation is observed. In addition to the partner fragment distribution, dissociation energies were determined to be 197 kJ/mol for HO–OH and 204 kJ/mol for DO–OD, which are significantly lower than the previously recommended values.
Nascent rotational distribution and the relaxation of the N+2 ion produced by double resonant multiphoton ionization
Authors: Fujii Asuka , Ebata Takayuki , Ito Mitsuo
Published: July 2002

Laser induced fluorescence (LIF) detection has been applied to measure the rotational distribution of the N+2 ion produced by double resonant multiphoton ionization of the N2 molecule. By analysis of the LIF spectra of the generated N+2 ion, the rotational propensity rules of the photoionization of N2 have been determined, which agree with theoretical prediction. The observed rotational intensity distribution shows relatively good agreement with the calculated result. Rotational relaxation of the N+2 ions by N2 collision has also been measured. The rotational relaxation rate constant is almost equal to that of the vibrational relaxation and the selection rule ‘‘symmetric’’ (+)↔‘‘symmetric’’(+) has been found to be obeyed.
Phase diagram of a lattice microemulsion model
Authors: Dawson K. A. , Lipkin M. D. , Widom B.
Published: July 2002

We derive the phase diagram of a recently proposed model of microemulsions. The model is equivalent to an Ising model with nearest-neighbor interaction parameter J, diagonal-neighbor interaction parameter 2M, and further-neighbor interaction parameter M. We find the regions of stability of the various phases in the j(=J/kT), m(=M/kT) plane, both by solution of the mean-field equations at finite j and m and by an exact analysis at T=0(m→−∞, j→±∞). The disordered (paramagnetic) region is bounded by an ellipse and two of its tangents. We concentrate our attention on the phases of periodic structure near the paramagnetic boundary, on the low-temperature phases, and on the connections between them.
High pressure solid phases of benzene. I. Raman and x-ray studies of C6H6 at 294 K up to 25 GPa
Authors: Thiéry M. M. , Léger J. M.
Published: July 2002

Crystalline benzene has been investigated at room temperature as a function of pressure up to 25 GPa in diamond anvil cells by Raman scattering and powder x-ray diffraction techniques. The concomitant spectroscopic and crystallographic results show the existence of numerous pressure-induced phases. Changes in the profiles of the Raman spectra and in the x-ray diffraction patterns, as well as changes in the variations of the Raman frequencies and the cell parameters with pressure indicate two first-order phase transitions at 1.4±0.1 and 4±1 GPa and a second-order one at 11±1 GPa. At 24 GPa the x-ray diffraction pattern seems to indicate the existence of a new phase. Two monoclinic structures are proposed for the phases above 1.5 GPa, in addition to the already known one. From these data, molar volume has been determined as a function of pressure and the Grüneisen parameters have been inferred in the different phases. Their pressure dependences are analyzed in the light of theoretical predictions. Arguments are given for a phase transformation at normal pressure and below 140 K or at room temperature below 1 GPa. A schematic P–T phase diagram is suggested and a controversy on the nature of the triple points located on the melting curve is clarified.
Equation of state of polymer melts: General formulation of a microscopic integral equation theory
Authors: Schweizer Kenneth S. , Curro John G.
Published: July 2002

A microscopic statistical mechanical theory for the virial equation of state of polymer liquids is developed by combining reference interaction site model (RISM) integral equation methods for flexible chain molecules with a superposition approximation for three-body orientational correlation functions. A compact expression for the pressure is obtained for athermal (hard core) fluids by neglecting explicit chain end effects. An analytical analysis of three-body contributions to the equation of state is carried out for flexible polymers and the scaling dependence on chain length and monomer density is derived. The merits and disadvantages of the compressibility route to the equation of state are briefly discussed, along with the inclusion of attractive intermolecular forces via thermodynamic perturbation theory.
A resonance Raman intensity study of electronic spectral broadening mechanisms in CS2/cyclohexane
Authors: Myers Anne B. , Li Bulang , Ci Xiaopei
Published: July 2002

Complete resonance Raman spectra, including absolute cross sections, have been measured for CS2 in cyclohexane using four excitation wavelengths from 223 to 204 nm, on resonance with the strongly allowed S3←S0 electronic transition. Absolute intensities have also been measured in CS2 vapor using 200 nm excitation. These total Raman cross sections, together with the Rayleigh cross sections calculated from the Kramers–Kronig transform of the absorption spectrum, are used to determine the homogeneous linewidth Γ for the electronic transition under the assumption that the solvent induced broadening may be partitioned into a static, inhomogeneous part and a very rapidly modulated, homogeneous component. Γ is found to be ∼13 cm−1 in the vapor and ∼200 cm−1 in cyclohexane solution, indicating that homogeneous broadening is responsible for much of the increase in spectral breadth between vapor and solution phases. Direct modeling of the solution phase absorption spectrum and the intensities of the lower lying Raman transitions gives a best fit to the absolute cross sections with a somewhat smaller Γ of ∼115 cm−1, probably due to the approximate nature of the model employed for the highly anharmonic excited state potential surface. The possible importance of solvent ‘‘memory’’ effects (finite solvent correlation time) on the electronic spectral broadening is discussed.
Vibration–rotation spectrum of the carbon dioxide–acetylene van der Waals complex in the 3 μ region
Authors: Prichard Diana G. , Nandi R. N. , Muenter J. S. , Howard B. J.
Published: July 2002

The infrared absorption spectrum of the carbon dioxide–acetylene van der Waals complex has been observed in a pulsed molecular beam. A color center laser was used to excite the vibration associated with the antisymmetric hydrogen stretching mode of the acetylene monomer. The vibrational origin is 3281.740 cm−1 and the rotational constants are A″=8876, B″=2859, C″=2155, A′=8864, B′=2855, and C′=2154 MHz. The axes of the two monomer are parallel to one another and the complex has C2v symmetry. The separation of the monomer units is 3.285 Å. The angular part of the intermolecular potential function is discussed in terms of electrostatic interactions between distributed multipole moments.
The ice/water interface: A molecular dynamics simulation study
Authors: Karim Omar A. , Haymet A. D. J.
Published: July 2002

The structure and dynamics of the ice/water interface have been investigated using molecular dynamics and the TIP4P model of water. In the bulk liquid phase, the pair correlation functions for this model compare well with recent neutron scattering results, and unlike most water models, the density at room temperature and pressure is close to the experimental value. For the basal plane of ice 1h exposed to liquid water, we have simulated an interface which is stable on the time scale of at least 100 ps. The density profile, molecular orientation, and diffusion constants have been calculated as a function of the distance normal to the interface. The interface is found to be broad, approximately 10 Å in extent, with the precise width determined by the particular property under investigation. Finite size effects have been studied. Dynamical molecular trajectories have been used to explore the loss of translational order from the solid through the interface to the liquid.
(e, 2e) momentum spectroscopic study of the interaction of –CH3 and –CF3 groups with the carbon–carbon triple bond
Authors: Goruganthu R. R. , Coplan M. A. , Moore J. H. , Tossell J. A.
Published: July 2002

Relative momentum densities have been measured for the outermost occupied π orbitals of acetylene, propyne, 2-butyne, and perfluoro-2-butyne using the (e, 2e) technique with an improved multiple detector spectrometer. The results, when compared with suitably averaged quantum mechanical calculations, show systematic differences between the molecules which can be interpreted in terms of wave function averages and differences using the ΔB(r) functions. A striking feature of the data is the significant momentum density at zero momentum for all of the molecules, in contrast to the calculations that all show zero momentum density. Ground state molecular distortions are discussed as a possible cause of the effect.
Molecular wave functions and properties calculated using floating Gaussian orbitals
Authors: Helgaker Trygve , Almlöf Jan
Published: July 2002

The calculation of molecular electronic wave functions and properties using floating Gaussian orbitals (i.e., orbitals whose positions are optimized in space) is described. The wave function is optimized using a second-order convergent scheme (the trust-region method), and molecular properties up to second order are calculated analytically. The method is applied to a series of small molecules (HF, H2O, NH3, CH4, CO, H2CO, and C2H4) at the Hartree–Fock level using four different floating basis sets (double zeta, double zeta plus polarization, double zeta plus diffuse, and double zeta plus polarization and diffuse). Geometries are fully optimized, and dipole moments, static polarizabilities, harmonic frequencies, and double-harmonic infrared intensities are calculated at the optimized geometries. The results are compared with those obtained using the corresponding fixed basis sets, and also with the results from a large basis of near-Hartree–Fock quality [6–311++G(3df,3pd)]. Floating produces only minor changes in the electronic energy, but other properties are often significantly improved. In particular, properties involving external field variations (dipole moments, polarizabilities, and intensities) converge considerably faster to the Hartree–Fock limit when floating is allowed. Properties calculated using the floating double-zeta basis set augmented with one set of polarization functions and one set of diffuse orbitals are close to the Hartree–Fock limit.
State mixing and vibrational predissociation in large molecule van der Waals complexes: t
r
a
n
s-stilbene–X complexes where X=He, H2, Ne, and Ar
Authors: DeHaan David O. , Holton Alicia L. , Zwier Timothy S.
Published: July 2002

We report a detailed study of vibrational predissociation and intramolecular–intermolecular state mixing in the first excited singlet state of trans-stilbene van der Waals complexes with helium, hydrogen, neon, and argon. We present evidence that the helium atom in stilbene–He and the H2 molecule in stilbene–H2 possess very low frequency van der Waals bending levels involving delocalization of the complexed species over both phenyl rings. In stilbene–He, the mode-selective, strong coupling of the out-of-plane phenyl ring modes with the pseudotranslation van der Waals modes leads to a dramatic, inhomogeneous broadening of the transitions to several times their breadth in in-plane vibrations. The observed dispersed fluorescence spectra give product state distributions and internal clock lifetime estimates which can only be made consistent with direct lifetime measurements by assuming extensive state mixing of the intramolecular levels with the van der Waals levels in which the states accessed by the laser are actually only about 30% intramolecular in character. We conclude that in these complexes the processes of intramolecular–intermolecular state mixing (static IVR) and vibrational predissociation are not independent processes but are

closely tied to one another. In fact, the vibrational product state distributions observed for the out-of-plane phenyl ring levels can best be interpreted as reflecting the percentage van der Waals character in the initially prepared state. In stilbene–H2 the mode selective coupling exhibits itself as a splitting of the out-of-plane transitions into a set of 5–6 closely spaced transitions separated by only about 1 cm−1. The sequence of transitions is suggestive of an in-plane potential for H2 motion which is nearly flat across the entire length of the stilbene molecule with a small barrier presented by the ethylenic carbons through which the H2 molecule can tunnel. Dispersed fluorescence spectra from these levels point to a two-tiered coupling scheme with the bound van der Waals levels. In contrast, the out-of-plane phenyl transitions in stilbene–Ne and stilbene–Ar possess unusual shifts, but the transitions are narrow once again. In these cases the complexed atom appears to be largely localized over a single phenyl ring.
State mixing and internal rotation predissociation in large-molecule van der Waals complexes: p-methyl-t
r
a
n
s-stilbene–X where X=He, H2
Authors: Zwier Timothy S.
Published: July 2002

In this paper we extend our study of state mixing and predissociation in large-molecule van der Waals complexes by considering the p-methyl-trans-stilbene–X complexes where X He, H2. The addition of the methyl group in the para position makes the two phenyl rings inequivalent but does not significantly change the ν37 vibrational frequency. It also introduces internal rotation levels as an added source of low frequency transitions which can be coupled to the van der Waals modes. The p-methyl-trans-stilbene molecule possesses two clear progressions on which we focus much of our attention: an internal rotation progression 2e→3a1 →4e, and a progression in ν37 activity: 3a1,37103a2, 3720. The p-methyl–He transitions built on the internal rotor levels exhibit narrow rotational band contours. Internal rotation predissociation from these levels appears to be efficient. The transitions in the ν37 progression in p-methyl–He show both increasing breadth and decreasing shift as one increases the out-of-plane vibrational excitation. The band contour of the 3720 0a1 transition is very

similar in breadth and shape to the 3720 transition in stilbene–He. This broadening can only be accounted for by assuming, as we did in trans-stilbene–He, that the He atom can move almost freely between the two phenyl rings. The corresponding transitions in p-methyl–H2 are no longer split (as they were in stilbene–H2 ), but the dispersed fluorescence spectra from levels below the dissociation limit show less resonance fluorescence and more broadening. We argue that the states carrying the absorption strength from the ground state involve a H2 molecule localized on a single phenyl ring while background van der Waals levels built on lower lying vibrational levels are delocalized. The enhanced state mixing in p-methyl–X is a consequence of two factors. First, the reduced symmetry of the p-methyl–X complex makes coupling with all van der Waals states allowed. Second, the internal rotor levels act as added base levels on which stacks of van der Waals levels can be built, thus increasing the overall density of van der Waals states at a given energy.
Reaction-path analysis of the effect of monomer excitation on the tunneling splitting of the hydrogen fluoride dimer
Authors: Hancock Gene C. , Truhlar Donald G.
Published: July 2002

The tunneling splitting of (HF)2 has been calculated by semiclassical adiabatic reaction-path methods, employing a small-curvature approximation for the effective mass along the tunneling path. The calculations include all six vibrational degrees of freedom and are based on the partly ab initio, partly semiempirical potential energy surface proposed recently by the authors and Dykstra. The calculations are carried out for both unexcited and excited monomer vibrations, and they correctly predict that excitation of either monomer lowers the tunneling splitting, but they do not explain why the lower-energy monomer stretch is equally as effective as the higher-energy one.
A multiphoton ionization study of the photodissociation dynamics of the S2 state of CH3ONO
Authors: Winniczek J. W. , Dubs R. L. , Appling J. R. , McKoy V. , White M. G.
Published: July 2002

Two-color (1+1) REMPI (resonantly enhanced multiphoton ionization) photoelectron spectroscopy is used to probe the NO photofragments produced by the UV photodissociation of methyl nitrite, i.e., CH3ONO+hν→CH3ONO*(S2)→CH3O⋅(X)+NO(X, v, J). The photofragments are produced in their ground electronic states but with high rotational and translational energy. NO fragment angular distributions, rotational state distributions, and spatial alignment are determined by photoion and photoelectron detection. The initial state alignment is obtained by the CDAD (circularly dichroic angular distribution) technique for the first time. CDAD measurements for rotational levels with 35.5≤J≤46.5 result in alignment parameters at the classical high-J limit of 𝒜(2)0 =−0.4. This alignment is consistent with an ‘‘impulsive’’ dissociation mechanism in which photofragment recoil along the CH3O–NO bond imparts substantial rotational angular momentum to the NO molecule resulting in a high-J state distribution and preferential rotation in the plane of dissociation. These measurements clearly establish the utility of the CDAD method for probing chemical processes in which spatial alignment plays an important role. Photoion angular distributions are used to probe correlations between the CH3ONO transition dipole moment, NO fragment velocity, and angular momentum. These correlations reveal additional details of the photolysis mechanism.
Gaussian basis sets for use in correlated molecular calculations. I. The atoms boron through neon and hydrogen
Authors: Dunning Thom H.
Published: July 2002

In the past, basis sets for use in correlated molecular calculations have largely been taken from single configuration calculations. Recently, Almlöf, Taylor, and co-workers have found that basis sets of natural orbitals derived from correlated atomic calculations (ANOs) provide an excellent description of molecular correlation effects. We report here a careful study of correlation effects in the oxygen atom, establishing that compact sets of primitive Gaussian functions effectively and efficiently describe correlation effects if the exponents of the functions are optimized in atomic correlated calculations, although the primitive (sp) functions for describing correlation effects can be taken from atomic Hartree–Fock calculations if the appropriate primitive set is used. Test calculations on oxygen-containing molecules indicate that these primitive basis sets describe molecular correlation effects as well as the ANO sets of Almlöf and Taylor. Guided by the calculations on oxygen, basis sets for use in correlated atomic and molecular calculations were developed for all of the first row atoms from boron through neon and for hydrogen. As in the oxygen atom calculations, it was found that the incremental energy lowerings due to the addition of correlating functions fall into distinct groups. This leads to the concept of correlation consistent basis sets, i.e., sets which include all functions in a given group as well as all functions in any higher groups. Correlation consistent sets are given for all of the atoms considered. The most accurate sets determined in this way, [5s4p3d2f1g], consistently yield 99% of the correlation energy obtained with the corresponding ANO sets, even though the latter contains 50% more primitive functions and twice as many primitive polarization functions. It is estimated that this set yields 94%–97% of the total (HF+1+2) correlation energy for the atoms neon through boron.
Self-consistent reaction field calculations of photoelectron binding energies for solvated molecules
Authors: Medina-Llanos Carmen , Ågren Hans , Mikkelsen Kurt V. , Jensen Hans Jo/rgen Aa.
Published: July 2002

The multiconfigurational self-consistent reaction field (MCSCRF) and the self-consistent reaction field (SCRF) methods are applied for solvation shifts of molecular photoelectron spectra. Calculations are performed for cavity wave functions of water, benzene, methanol, and formamide surrounded by dielectric continua corresponding to various solvents. The cavity wave functions for single- or multiconfigurational closed- and open-shell states are optimized self-consistently with their reaction fields, using either a continuum approach with one solute molecule embedded in the dielectric medium or a semicontinuum approach with one solute molecule and a solvation shell of molecules surrounded by the dielectric medium. The application of the MCSCRF/SCRF model gives new insight into the effects of a solvent on ionization spectra. The origin of both absolute and differential shifts upon solvation is investigated. This includes studies of local vs delocalized ionization, role of dielectric polarization vs reaction field contributions to the solvation shift, optical vs static dielectric response of the medium, and use of noncorrelated vs correlated solute wave functions. It is found that the use of the semicontinuum model with one solvation shell is often crucial to obtain experimental agreement. Comparatively large multipolar expansions of the solute charge distribution is often needed to adequately treat the solvent interaction. The results for differential core–valence and core–core shifts lead to some interesting observations that can be further explored using photoelectron and soft x-ray spectroscopic techniques.
Topographical features of the Laplacian of the momentum density of atoms and ions most indicative of shell structure
Authors: Sagar Robin P. , Ku Andrew C. T. , Smith Vedene H. , Simas Alfredo M.
Published: July 2002

The Laplacian of the spherically averaged momentum density, ∇2Π̄(p), is computed from nonrelativistic self-consistent-field wave functions for the neutral atoms from hydrogen to uranium, and the singly positive ions, from helium to barium and lutetium to radium. In particular, the topography of ∇2Π̄(p) reveals that there are seven distinct types of behavior for the studied atomic and ionic cases, based on the number of zeros and extrema present and the value of ∇2Π̄(p) at the origin. Shell structure within ∇2Π̄(p) is shown to be exhibited by the zeros that occur as ∇2Π̄(p) crosses from negative values to positive values. These zeros have been found to be the topographical feature most intimately related to the radii predicted by the Bohr theory of an atom. At most four shells per atom or ion are revealed based on this criterion.
Time resolved Stark spectroscopy in NaK(B 1Π; V=1–14). Measurement of permanent electric dipole, radiative lifetimes, and collisional rate constants
Authors: Derouard J. , Debontride H. , Nguyen T. D. , Sadeghi N.
Published: July 2002

The permanent dipole moment μ of NaK(B 1Π) has been determined for several vibrational levels by observing the electric field induced e–f mixing using either cw or pulsed laser excitation. In the latter case the spectrally resolved fluorescence decay is modulated by ‘‘Stark quantum beats.’’ We find μ(Re) =2.4 (2) Debye. The measurement of the fluorescence decay rates in zero electric field vs buffer gas pressure (rare gases, N2) yields radiative lifetimes and depopulation cross sections. The dependence of the lifetime τ with the vibrational index V can be represented by the expression: τ(V)=12.8+0.46 V ns. It is consistent with a transition dipole moment between the B 1Π and X 1Σ+ states almost independent of the internuclear distance, close to the value of the K(4P–4S) transition which corresponds to the dissociation limits of the molecular states.
Rotational analysis of the 7000 Å (A 3Φ→X 3Δ) electronic emission system of diatomic vanadium mononitride (VN)
Authors: Peter S. L. , Dunn T. M.
Published: July 2002

The (0,0) band of the electronic emission system of the diatomic molecule VN at ∼7000 Å has been generated and rotationally analyzed. The system is 3 Φr →3 Δr and is the vanadium analog of the niobium nitride system centered ∼6029 Å. The constants for the upper and lower (almost certainly the ground) states have been determined, including estimates of the spin–orbit coupling constants, despite the absence of satellite bands. The subband origins are somewhat asymmetrically located, probably due to the interaction of the 3 Δ2 and the higher lying (by ∼3000 cm−1 ) 1 Δ2 state. There is no evidence of localized perturbations in any of the subbands. VN has the shortest bond length (r0 =1.566 Å ) observed for any diatomic molecule containing a transition metal (apart from some hydrides). The (1,1) sequence bands have also been observed but have not been rotationally analyzed at this time.
Correlated product state distributions in the unimolecular reaction of NCNO
Authors: Qian C. X. W. , Ogai A. , Reisler H. , Wittig C.
Published: July 2002

Following excitation to S1, expansion-cooled NCNO undergoes nonradiative couplings to S0 and predissociates to CN and NO. Doppler profiles of selected CN B 2Σ+←X 2Σ+ rotational lines were recorded using LIF at several excess energies between 0 and 3000 cm−1. This yields NO V,R distributions associated with specific CN(X 2Σ+) rotational states. The profiles can be fit using the statistical PST/SSE model, and the correlated distributions show no evidence of dynamical bias or exit channel barriers. Doppler profiles generated with polarized lasers show little or no spatial anisotropy of recoil velocities, and are fit by anisotropy parameters β∼0, even at excess energies where predicted unimolecular lifetimes are ≤1 ps. Possible causes for the lack of spatial anisotropy are discussed. Analyses of NO fragment LIF spectra obtained at excess energies of 2348 and 2875 cm−1 show a slight preference for the Π(A′) Λ-doublet component for J″≥30.5, suggesting planar dissociation. An in-plane orientation of the singly occupied pπ lobe in NO is to be expected for dissociation on the ground (A′) electronic potential energy surface.