bluuebunny/crossref_metadata_2025 huggingface dataset:
Quantum-classical molecular dynamics as an approximation to full quantum dynamics
Authors: Bornemann Folkmar A. , Nettesheim Peter , Schütte Christof
Published: July 2002

This paper presents a mathematical derivation of a model for quantum-classical molecular dynamics (QCMD) as a partial classical limit of the full Schrödinger equation. This limit is achieved in two steps: separation of the full wave function and short wave asymptotics for its ‘‘classical’’ part. Both steps can be rigorously justified under the same smallness assumptions. This throws some light on the time-dependent self-consistent-field method and on mixed quantum-semiclassical models, which also depend on the separation step. On the other hand, the theory leads to a characterization of the critical situations in which the QCMD model is in danger of largely deviating from the solution of full Schrödinger equation. These critical situations are exemplified in an illustrative numerical simulation: the collinear collision of a classical particle with a harmonic quantum oscillator.
Ultrafast rise of translational temperature after photoexcitation to electronic excited state in solution: Transient lens study of Ni2+ aqueous solution
Authors: Terazima Masahide
Published: July 2002

An increase of the translational temperature after photoexcitation to the electronically excited state of Ni2+ in aqueous solution is detected using a time-resolved transient lens method from the picosecond to millisecond time scale. Besides a large lens signal due to the volume expansion of water in the submicrosecond time range, two other lens signals are observed at an initial part of the signal. During the pump laser pulse, a convex lens signal is observed, which is attributed to the optical Kerr lens signal. After the pulse, a concave lens is created, which remains a nearly constant amplitude until the density lens signal appears. The signal is attributed to the temperature lens signal, which originates from the heat energy by the nonradiative transition of the excited state. It is found that the signal rises within 3 ps after the photoexcitation, which indicates that the temperature rise is very fast. After the fast rise, another slower dynamics (500 ps) is observed and the deactivation processes of Ni2+ in aqueous solution are discussed based on these results.
The T1(nπ*)←S laser induced phosphorescence excitation spectrum of acetaldehyde in a supersonic free jet: Torsion and wagging potentials in the lowest triplet state
Authors: Liu H. , Lim E. C. , Muñoz-Caro C. , Niño A. , Judge R. H. , Moule D. C.
Published: July 2002

The laser induced T1(nπ*)←S0 phosphorescence excitation spectrum of jet-cooled acetaldehyde has been observed for the first time with a rotating slit nozzle excitation system. The vibronic origins were fitted to a set of levels that were obtained from a Hamiltonian that employed flexible torsion-wagging large amplitude coordinates. The potential surface extracted from the fitting procedure yielded barriers to torsion and inversion of 609.68 and 869.02 cm−1, respectively. Minima in the potential hypersurface at θ=61.7° and α=42.2° defined the equilibrium positions for the torsion and wagging coordinates. A comparison to the corresponding S1-state parameters showed that the torsion barrier (in cm−1) does not greatly change, S1/T1=710.8/609.7, whereas the barrier height for the wagging-inversion barrier increases dramatically, 574.4/869.0.
Accurate quantum Monte Carlo calculations for hydrogen fluoride and the fluorine atom
Authors: Lüchow Arne , Anderson James B.
Published: July 2002

Quantum Monte Carlo calculations of the ground state energies of the fluorine atom and the hydrogen fluoride molecule have been carried out using both fixed-node and released-node methods. The fixed-node total energies for both F and FH are among the most accurate available. The fixed-node dissociation energy De=141.3(4) kcal mol−1 is in excellent agreement with experimental data. The cancellation of the nodal error was achieved with trial wave functions having nodal surfaces determined by Slater determinants of the near Hartree–Fock limit quality. With the released-node Green’s function quantum Monte Carlo method the lowest upper bounds to the ground state energies to date for both F [E0=−99.731(4) Hartree] and FH [E0=−100.458(5) Hartree] have been obtained.
Reliable anisotropic dipole properties, and dispersion energy coefficients, for O2 evaluated using constrained dipole oscillator strength techniques
Authors: Kumar Ashok , Meath William J. , Bündgen Peter , Thakkar Ajit J.
Published: July 2002

Constrained anisotropic dipole oscillator strength techniques are used to obtain reliable values for a wide range of anisotropic and isotropic dipole properties of O2, including the dipole–dipole dispersion energy coefficients for the interaction of O2 with O2, H2, N2, CO, He, Ne, Ar, Kr, and Xe. Some of the anisotropic constraints required for our calculations are obtained via dipole sum rules from ab initio, multireference configuration interaction (CI) wave functions for the ground state of O2. The individual dipole properties of O2 considered include the dipole oscillator strength sums Sk, k=2,1,0(−1/2)−2,−3,−4,..., the logarithmic dipole sums Lk and mean excitation energies Ik, k=2(−1)−2, and, as a function of wavelength, the dynamic polarizability and its anisotropy, the total depolarization ratio, the Rayleigh scattering cross section, and the Verdet constant. Our constrained dipole oscillator strength results are often the only reliable, and sometimes the only available, ones for many of the properties and dispersion energies considered.
Transformation kinetics in one-dimensional processes with continuous nucleation: The effect of shielding
Authors: Weinberg Michael C. , Birnie Dunbar P.
Published: July 2002

A derivation is presented for the fraction of material transformed as a function of time, X(t), for one-dimensional phase transformations which occur via continuous nucleation and growth and which produce anisotropic particles. This derivation, which accounts for shielding effects and does not assume site saturation, is the first such derivation given for this type of problem. The analytical formulas which are found are evaluated in an approximate fashion for two different values of growth rate anisotropy. The results are compared with numerical simulations, and it is shown that the expressions derived are highly accurate.
Shear-flow effects on self-assembly of semidilute solutions of off-critical polymer mixtures: Shear-hysteresis effects
Authors: Asakawa Koji , Hashimoto Takeji
Published: July 2002

We use flow light scattering to study the phase transitions in semidilute solutions of polystyrene and polybutadiene in dioctylphthalate under simple shear flow. The phase-separated solution was brought into a single-phase state by increasing the shear rate, γ̇, above a critical shear rate, γ̇c,i. The solution was then brought back into a two-phase state by lowering γ̇ below a critical shear rate, γ̇c,d. We previously reported a large hysteresis effect in solutions with off-critical compositions; γ̇c,i is always higher than γ̇c,d. Shear-drop experiments were conducted to illuminate the origin of this hysteresis effect. The experimental results showed that at temperatures close to the cloud point temperature the formation of phase separated structures did not occur until up to 22 h after lowering the shear rate below γ̇c,i. Thus the hysteresis effect was found to be due to a surprisingly slow ordering process at γ̇ close to γ̇c,i. The ordering induced by the shear drop needs a much longer time than the homogenization induced by increasing shear. If the time scale of observation is sufficiently long, the hysteresis effect disappears, yielding the drop of cloud point temperature with shear, ΔTc(γ̇), given by ΔTc(γ̇)∝γ̇1.0±0.1 for the off-critical mixtures, rather than ΔTc(γ̇)∝γ̇0.5±0.1 found previously for the near critical mixtures. Finally, the incubation time was found to initially increase with γ̇ and then decrease with a further increase of γ̇, suggesting that the ordering mechanisms are different in the low and high shear-rate regimes.
Theory of x-ray emission of conjugated molecules
Authors: Gel’mukhanov Faris , Yang Li , Ågren Hans
Published: July 2002

A theory of nonresonant x-ray emission spectra of conjugated molecules is presented. The role of relaxation effects on the spectral shape of x-ray emission is advanced and examined in detail for the transpolyenes. In some contrast to unconjugated systems, the relaxation of valence charge around the core hole is shown to influence the distribution of x-ray intensities significantly. Site and size dependences of the spectral shape of x-ray emission are predicted; the site dependence as being alternant among the π levels. Comparative calculations of x-ray emission spectra at the ab initio Hartree–Fock and the semiempirical levels show that the relaxation qualitatively changes the spectral distribution and shifts intensity from the σ to the π parts of the spectra, creating excitonlike features in the latter. The exciton character is site and size dependent; for longer polyenes it is stronger for mid-atoms than for end-atoms, and is found stronger at the top than at the bottom of the π band. Complication/simplification trends of the spectra along the oligomer sequence are predicted for both the relaxed and frozen orbital models.
A rigid core-flexible chain model for mesogenic molecules in molecular dynamics simulations of liquid crystals
Authors: La Penna Giovanni , Catalano Donata , Veracini Carlo Alberto
Published: July 2002

A model of a mesogenic molecule, built up as a rigid anisotropic Gay–Berne site mimicking the aromatic core, connected to an array of isotropic sites mimicking a flexible chain, is proposed and tested in molecular dynamics calculations. Simulations have been performed on a system composed of 256 molecules with three different numbers of methylenic units in the chain, in order to explore the effect of chain length on static and dynamic properties. The systems are all at the same mass density and temperature and result in nematic liquid crystalline phases. The order parameters for various molecular fragments and the T1z nuclear magnetic resonance (NMR) relaxation times of deuterons are in agreement with previous molecular dynamics simulations on atomistic systems and, at least qualitatively, with 2H-NMR experimental results. The intermolecular interactions are always dominated by the anisotropic site simulating the molecular core. The influence of the phase order on the chain static and dynamic properties is put in evidence. Extensions of the model are suggested in order to have a better reproduction of the dynamical features of such systems.
Theoretical study of properties of H− and NH2− complexes with neutral ammonia solvent molecules
Authors: Roszak S.
Published: July 2002

The experimental results from photoelectron spectroscopy yielding detailed information on interactions of H− and NH−2 with one and two molecules of NH3 are supplemented by theoretical studies. The structure of complex anions involved has been determined and the nature of their interactions has been discussed. Electron affinities, dissociation energies, and basicity of H−(NH3)n and NH−2(NH3)n (n=1,2) anions have been calculated for theoretically determined structures. The excellent agreement between experimental and theoretical results confirms conclusions derived from experimental studies. A recently proposed complete basis set method has been proven to be a useful tool to study weak molecular complexes.
Anomalous height increases upon bending for an Alexander–de Gennes polymer brush
Authors: Sevick E. M. , Williams D. R. M.
Published: July 2002

We examine the bending of an Alexander–de Gennes polymer brush over a wide range of curvature from weak to strong. Two different models are used, a blob model and a simpler Flory mean field model. In both cases the height change upon bending is anomalous. In the case of a blob model the height increases for weak bending. In the Flory model there is no height change to first order in the curvature. This is in sharp contrast to more sophisticated theories for brushes with free ends, and may have applications to real-life Alexander–de Gennes brushes which have been synthesized recently.
Optical pumping magnetic resonance in high magnetic fields: Characterization of nuclear relaxation during pumping
Authors: Augustine Matthew P. , Zilm Kurt W.
Published: July 2002

The polarization of 129Xe gas by spin exchange with optically pumped Rb vapor is investigated in high magnetic field. Operation in a high field provides added spectral dispersion via Zeeman shifts of the electronic transitions. This simplifies the physics of the pumping process in such a way as to make it more amenable to treatment with simple rate theory. A relationship between the steady state 129Xe polarization and the applied laser power is derived which agrees with experimental results. This theory serves as a guide to the manufacture of very high polarizations of 129Xe gas.
Design of optimized photorefractive polymers: A novel class of chromophores
Authors: Wortmann Rüdiger , Poga Constantina , Twieg Robert J. , Geletneky Christian , Moylan Christopher R. , Lundquist Paul M. , DeVoe Ralph G. , Cotts Patricia M. , Horn Hans , Rice Julia E. , Burland Donald M.
Published: July 2002

It is demonstrated that the microscopic mechanism of the photorefractive (PR) effect in organic composites with low glass transition temperatures involves the formation of refractive index gratings through a space-charge field-modulated Kerr effect. A tensorial formulation of the macroscopic aspects of the PR Kerr effect and its microscopic interpretation is presented. The second-order dipole orientation term containing the anisotropy of the first-order optical polarizability α(−ω;ω) is shown to yield the dominant contribution to the Kerr susceptibility χ(3)(−ω;ω,0,0). A class of special chromophores having negligible second-order polarizabilities β(−ω;ω,0) and large dipole moments μ has been identified in order to optimize this term. These chromophores are not subject to the efficiency-transparency tradeoff typically encountered with second-order nonlinear optical (NLO) chromophores, providing highly transparent materials with large PR Kerr response. Contrary to previous approaches in this field, the best-performing PR polymers are then expected to employ chromophores that would be useless for second- order applications (negligible β). We report PR of the material 30% 2,6-di-n-propyl-4H-pyran-4-ylidenemalononitrile (DPDCP): 15% N,N′-bis(3-methylphenyl)- N,N′-bis(phenyl)benzidine (TPD):55% poly(methyl methacrylate) (PMMA):0.3% C60 as an illustration of this principle. A 100 μm thick film of this material exhibits a steady-state diffraction efficiency of η=25% and net two-beam coupling of Γ=50 cm−1 at a bias field of 100 V/μm and a wavelength of 676 nm. The macroscopic Kerr susceptibility of the material is related to molecular electronic properties of the chromophore DPDCP which were independently determined by experiments in solution and by quantum chemical calculations.
Rotational spectra, structure, and internal dynamics of Ar–H2S isotopomers
Authors: Gutowsky H. S. , Emilsson T. , Arunan E.
Published: July 2002

Several groups have studied the rotational spectra of various isotopomers of the pseudolinear Ar–H2S dimer. We have found two additional K=0 progressions, for Ar–H2 32S and Ar–H2 34S, which show that the symmetrical isotopomers (with H2S or D2S) exist in lower and upper states giving progressions with B̄ differing by about 50 MHz. When assigned and analyzed on this basis, the overall spectra are quite similar to those of the Ar–H2O analogue where the doubling is caused by 000 and 101 internal rotor states of the H2O or D2O. Such states of the H2S most likely cause the doubling found in the Ar–H2S isotopomers. However, some of the details differ in interesting ways, indicating substantial differences in their potential energy surfaces. The lower states of Ar–D2 32S and D2 34S have B̄’s about 28 MHz larger than those for H2S, while the isotopomers of the upper states exhibit a more normal decrease of about 12 MHz. Somewhat related to this is the effect of deuteration on the difference in B̄ between lower and upper states. Perdeuteration of Ar–H2O decreases ΔB̄ from 76 to 68 MHz. The corresponding change for Ar–H2S is much larger, from 44 to 5 MHz. For the internal rotor model, analysis of the hfs for Ar–H2O/D2O assigns its upper and lower states to 000 and 101 rotor states of the water, respectively. But this assignment is reversed in Ar–H2S/D2S. Also, we have observed and fitted the S33 quadrupolar hfs for the lower and upper states of Ar–H2 33S, finding values for χaa(33S) of −7.89 and −17.36 MHz.
Operator-splitting integrators for constant-temperature molecular dynamics
Authors: Zhang Fei
Published: July 2002

The Gaussian thermostatting technique has been widely used in constant-temperature molecular dynamics. In this paper we develop operator-splitting integrators for the Gaussian thermostated equations of motion. The new integrators are explicit, very simple to program, and require minimum computer memory. In particular, they can preserve the constancy of the system’s kinetic energy. Numerical experiments show that the present integrators are much more efficient than conventional integrators such as the Runge-Kutta methods. Extension of the integrators to multiple timescale MD simulations is also discussed.
Multireference configuration interaction study of the mixed Valence–Rydberg character of the C2H4 (π,π*)1V state
Authors: Krebs Stefan , Buenker Robert J.
Published: July 2002

The spatial extension of the C2H4 (π,π*)1V state is investigated by means of low selection threshold multireference configuration interaction (CI) calculations employing two atomic orbital (AO) basis sets with different numbers of polarization and Rydberg functions. The results are shown to be nearly independent of the choice of one-electron basis (ground N, triplet T, and singlet V self-consistent field molecular orbitals (SCF MOs)) in forming the many-electron basis for the configuration interaction indicating that the AO basis limit has been closely approached in each case. The calculations indicate that the value for the 〈ΨV|Σxi2|ΨV〉≡〈x2〉V matrix element falls in the 18±1 a02 range, 50% larger than the corresponding values computed for 〈x2〉N and 〈x2〉T, respectively, for the corresponding N and T states. This result is interpreted to be a consequence of the mixing of diabatic (π,π*)1 valence and (πd,π)1 Rydberg states in the Franck–Condon region of the V–N transition. The corresponding excitation energy is computed to lie in the 7.90–7.95 eV range, indicating that there is a distinct nonverticality in the measured absorption spectrum which is caused in part by nonadiabatic interactions between the V and (π,3py)1 Rydberg states as a result of torsional motion of the C2H4 molecule.
Integral equation approaches to mixtures of atomic and molecular fluids
Authors: Anta J. A. , Lomba E. , Alvarez M. , Martı́n C. , Lombardero M.
Published: July 2002

A recent extension to mixtures of Verlet’s closure is applied in conjunction with the Ornstein–Zernike relation to solve the structure and thermodynamics of mixtures of hard-spheres and homonuclear hard-dumbbells. This integral equation (IE), which is proven to be very accurate when compared with simulation data, is used to explore the possibility of phase separation in an asymmetric mixture. While our results do not show evidence of such phase separation in the asymmetric binary hard-sphere mixture studied by Biben and Hansen [T. Biben and J. P. Hansen, Phys. Rev. Lett. 66, 2215 (1991)], an equivalent mixture of hard-dumbbells and large hard-spheres seems to exhibit a certain tendency to phase separate as far as the integral equation results are concerned. Finally, given the ability of this integral equation to reproduce the hard-core system, we have incorporated these results into a previous Reference Hypernetted Chain scheme to treat a mixture of N2 and Ar modeled by means of site–site Lennard-Jones potentials. In consonance with the results for pure fluids, the parameterization of a hard-core reference system with the same molecular shape leads to excellent results both for the structure and thermodynamics of real systems.
Photochemistry of adsorbed molecules. XVII. Photodissociation at 193 nm of CH3Br adsorbed on LiF(001) and NaCl(001)
Authors: Garrett S. J. , Heyd D. V. , Polanyi J. C.
Published: July 2002

The adsorption and photodissociation of CH3Br(ad) on LiF(001) and NaCl(001) have been studied by temperature programed desorption (TPD) and angle-resolved time-of-flight mass spectrometry. The results of TPD on LiF(001) indicated that CH3Br(ad) formed a first adsorbed layer in which repulsive adsorbate–adsorbate interactions dominated. At high coverages, the TPD profiles on LiF(001) were characteristic of multilayer (“zero-order”) desorption but at no time were separate monolayer and multilayer TPD peaks observed. In contrast, on NaCl(001) CH3Br(ad) formed a distinct monolayer phase before growth of the second and subsequent layers. Methyl radicals were produced by photodissociation of CH3Br(ad) at 193 nm on both surfaces at coverages from submonolayer to >10 monolayers. The CH3(g) translational energy distribution, P(ET′), exhibited two coverage-dependent photodissociation channels; the first, termed the “direct” (DIR) channel, observed at multilayer coverages, had a P(ET′) centered at 2.4 eV, and the second, termed the “indirect” (IND) channel, had a P(ET′)⩽1.8 eV. The mean energy and width of the DIR methyl, P(ET′), was similar to that from CH3Br(g) photodissociation. This DIR channel peaked at approximately 25° off-normal for LiF(001) and 38° off-normal for NaCl(001), giving the most probable C–Br bond directions. The IND channel was comprised of CH3(g) which escaped from the adlayer after suffering an inelastic collision (including sometimes reaction) with neighboring CH3Br(ad) molecules. The DIR channel dominated at all CH3Br(ad) coverages on LiF(001) but the IND channel dominated at all coverages on NaCl(001), consistent with a model in which CH3Br(ad) on LiF(001) predominantly “stands up” and CH3Br(ad) on NaCl(001) predominantly “lies down.” Polarized IR spectroscopy performed in this laboratory supports these geometries. There was a peak in the LiF(001) IND energy distribution centered at 0.9 eV and there were two peaks in the NaCl(001) IND energy distribution, IND(1) and IND(2), centered at 1.1 and 0.5 eV, respectively. These substantial but specific energy losses (compared with the energy of the directly photorecoiling CH3) are indicative of a strongly inelastic encounter with constrained dynamics, believed to be the exchange reaction CH3+BrCH3′(ad)→CH3Br+CH3′ with retention of direction in the CH3′. The LiF(001) and NaCl(001) IND angular distributions, P(Θ), were generally broad and of the form cosn Θ centered on the surface normal but there was evidence for retention of direction of methyls in the IND channel at multilayer coverages on LiF(001), characteristic of exchange reaction.
Off-resonant coherent hyper-Raman scattering spectroscopy
Authors: Cho Minhaeng
Published: July 2002

A theoretical description of the off-resonant coherent hyper-Raman scattering spectroscopy is presented. By considering two pump fields whose frequencies and corresponding wave vectors are ω1, ω2(=2ω1), k1, and k2, respectively, the effective Hamiltonian is derived to show that the spatial transient grating can be created by the third-order interaction with the pump fields. A delayed probe field with frequency of ω1 and wave vector of k1 is scattered by the spatial transient grating to produce the macroscopic polarization with wave vector 4k1−k2. The expressions for the measured signals by homodyne and heterodyne method are derived and compared with that of the spontaneous hyper-Raman scattering signal.
Simultaneous measurement of dielectric constant and refractive index of SF6 and CO2 as a function of pressure
Authors: Adjouri C. , Balu N. , Obriot J. , Bose T. K.
Published: July 2002

Static dielectric constant ε and refractive index n have been measured simultaneously over a large pressure range (≈23 MPa). The measurements are carried out at temperatures in the supercritical region both for SF6 and CO2. From these static measurements, we determined the coefficients γ1, γ2,…, related to the linear, quadratic,…, density dependence. It is shown that the meaningful physical coefficients γ1 are related to the integrated absorption by means of the Kramers–Kronig relation. For SF6, the results are compared with the earlier dielectric and optical results. For CO2, the binary coefficients γ2 are in good agreement with the static and the absorption results. Furthermore, our results together with the accurate existing integrated absorption data demonstrate the temperature dependence of the γ2 component.
Dissociation dynamics of HNCO and DNCO after laser photoexcitation in the vacuum ultraviolet
Authors: Brownsword R. A. , Laurent T. , Hillenkamp M. , Vatsa R. K. , Volpp H.-R.
Published: July 2002

Using the laser photolysis–laser-induced fluorescence “pump–probe” technique, the gas-phase dissociation dynamics of HNCO(X̃ 1A)+hν→H+NCO and DNCO(X̃ 1A)+hν→D+NCO after photoexcitation at the Lyman-α wavelength were studied under collision-free conditions at room temperature. In the vacuum ultraviolet photodissociation experiments narrow band tunable Lyman-α laser radiation (λ≈121.4–121.6 nm) was used both to photodissociate the parent molecules and to detect the produced nascent H and D atom products via (2p 2P←1s 2S) laser induced fluorescence. The following quantum yields ΦH–D for H–D atom formation were determined by a photolytic calibration method: ΦH=(0.62±0.15) and ΦD=(0.51±0.17). For HNCO and DNCO the measured H–D atom Doppler line shapes can be well described by a single Gaussian function, which corresponds to a statistical Maxwell–Boltzmann-like distribution of the translational energy. From the measured H and D atom Doppler profiles the average H and D atom kinetic energy was determined to be ET(H)=(137±10) kJ/mol and ET(D)=(115±4) kJ/mol, respectively. The average kinetic energies were found to be in reasonable agreement with results from simple statistical calculations in which it is assumed that H–D atoms are produced in combination with NCO in the ground electronic state (X̃ 2Π). A dissociation mechanism is suggested in which H–D atom formation proceeds via a statistical unimolecular decay of a hot H–DNCO intermediate formed by a radiationless transition of the optically excited bound H–DNCO state to a lower-lying dissociative state.
Density functional study of mononitrosyls of first-row transition-metal atoms
Authors: Blanchet Catherine , Duarte Hélio A. , Salahub Dennis R.
Published: July 2002

The mononitrosyls of the transition metals of the third row of the periodic table were calculated by means of the density functional theory. The geometries and frequencies of the MNO complexes in different states are reported. A bent geometry is found for the systems with higher multiplicities from vanadium through copper and linear geometries for lower multiplicities. The binding mechanism is described through an analysis of the molecular orbitals (MO). The bent geometry is obtained when the σ antibonding MO resulting from the interaction of the 5σ MO of NO and the dσ orbital of the metal center is occupied. The electronic configuration of all complexes studied is presented in a diagram that allows one to predict qualitatively whether the complex is bent according to its multiplicity. The interaction of the transition metals with other diatomic molecules is briefly discussed.
Mobility and formation kinetics of NH4+(NH3)n cluster ions (n=0–3) in helium and helium/ammonia mixtures
Authors: Krishnamurthy M. , de Gouw Joost A. , Ding Li Ning , Bierbaum Veronica M. , Leone Stephen R.
Published: July 2002

NH4+(NH3)n (n=0–3) cluster ions are produced in a field-free flow tube section of a selected ion flow–drift tube (SIFDT) apparatus. Cluster ion mobilities are measured in mixtures of He and NH3 and used to obtain the individual mobilities in helium and in ammonia by applying Blanc’s law to the mixtures. Mobilities of the cluster ions are also measured in pure helium by producing the ions in the ion source of a flowing afterglow, selected ion flow–drift tube apparatus (FA-SIFDT). The measurements in pure helium compare well with the mobilities in helium obtained by applying Blanc’s law to the mixtures. The zero field mobilities of the cluster ions in helium are 22.1±0.4 cm2 V−1 s−1 for NH4+, 16.6±0.4 cm2 V−1 s−1 for NH4+(NH3), 12.2±0.4 cm2 V−1 s−1 for NH4+(NH3)2, and 12.1±0.4 cm2 V−1 s−1 for NH4+(NH3)3. The decrease with increasing size of the cluster can be explained in terms of the sizes of the geometric cross sections. The zero-field mobilities in NH3 are 0.94±0.35 cm2 V−1 s−1 for NH4+, 0.83±0.22 cm2 V−1 s−1 for NH4+(NH3), 0.50±0.27 cm2 V−1 s−1 for NH4+(NH3)2, and 0.25±0.20 cm2 V−1 s−1 for NH4+(NH3)3. The small values of the mobilities in these polar gas systems are understood in terms of the strong ion–dipole interactions. Calculated mobilities in NH3 are obtained by computing the collision cross section with the ion–dipole interactions taken into account; the results compare well with the measurements for NH4+ and NH4+(NH3). However, the measured mobilities of the larger cluster ions are smaller than the computed values. The discrepancies may be due to several factors including dipole–dipole interactions, ligand exchange reactions, formation of long-lived quasibound complexes, and efficient transfer of kinetic energy into internal energy of the cluster ion and the ammonia molecules.
The effect of correlation on molecular magnetizabilities and rotational g tensors
Authors: Ruud Kenneth , Helgaker Trygve , Jo/rgensen Poul
Published: July 2002

Using multiconfigurational self-consistent field (MCSCF) wave functions and perturbation-dependent basis sets, the effect of electron correlation on molecular magnetizabilities and rotational g tensors is investigated. The eight molecular systems considered (H2O, NH3, HF, C2H2, CO, H2CO, O3, and LiH) vary in the importance and relative magnitudes of the static and dynamic correlation contributions. The results for O3 are the first correlated calculations of the rotational g tensor of this system. We confirm previous findings that, except for systems with large static correlation effects, the effect of correlation on molecular magnetizabilities is small. A somewhat larger correlation contribution is usually observed for the rotational g tensor, although this property is also rather insensitive to the correlation treatment. Agreement with experimental rotational g tensors is only fair and estimates of rovibrational corrections are needed to assess properly the accuracy of theoretically calculated rotational g tensors.
Isomerization of stilbene in the gas phase: Theoretical study of isotopic and clustering effects
Authors: Gershinsky Gidon , Pollak Eli
Published: July 2002

This paper presents a continuation of our previous theoretical studies on the rate of isomerization of trans-stilbene from the first excited electronic state based on the potential energy surface of Vachev et al. [J. Phys. Chem. 99, 5247 (1995)]. Harmonic RRKM computations and molecular dynamics and Monte Carlo based classical rates are presented for deuterated isotopes of stilbene as well as hexane clusters of stilbene of varying size. Good agreement with experiment is found for energy dependent rates of d12 vs h12 stilbene. However, we find that the rate for d2 stilbene is greater than for d10 stilbene in contradiction to the experimental observations. For the hexane clusters we find that addition of hexane molecules causes a systematic decrease in the rate, in agreement with experiment.
Theoretical study of CH4 photodissociation on the Pt(111) surface
Authors: Akinaga Yoshinobu , Taketsugu Tetsuya , Hirao Kimihiko
Published: July 2002

The photodissociation of CH4/Pt(111) is studied by density functional theory and the state-averaged complete active space self-consistent field (SA-CASSCF) method using a cluster model Ptn (n=1,4,6,7,10). With the small clusters (n⩽4), the equilibrium molecule–surface distances (H3CH–Pt) are less than 2.3 Å and the binding energies are 4–14 kcal/mol, the order of the chemisorption. With larger clusters, the molecule–surface distance and the binding energy are calculated to be 3.00 Å and 0.67 kcal/mol, respectively, of the order of the physisorption, which coincides with the experiments. The SA-CASSCF calculations verify that, in spite of the weak interaction between CH4 and Ptn in the ground state, the first excited state of CH4 (Rydberg type) interacts with Ptn unoccupied states strongly, resulting in the charge-transfer state and finally leading to the dissociation to CH3+H(−Pt); on the Pt(111) surface, the excitation energy to the Rydberg state of CH4 decreases by ∼3 eV compared to that in an isolated CH4 molecule. These results support the experimental results that the direct excitation of CH4 is invoked on the Pt(111) surface by irradiation of the 193 nm photon, leading to the dissociation to CH3 and H.
Phase separation dynamics in mixtures containing surfactants
Authors: Melenkevitz J. , Javadpour S. H.
Published: July 2002

The phase separation process in mixtures containing two immiscible liquids and a surfactant was investigated in two dimensions by numerically integrating a time dependent Ginzburg–Landau model. The model free energy was constructed from two scalar and one vector order parameter fields. The scalar fields describe, respectively, the local density difference of the immiscible liquids and the local surfactant density, while the vector field provides the local average orientation of the surfactant molecules. The time evolution of the characteristic domain size was studied as a function of the mean surfactant density, no, in systems having a 1:1 ratio of the two immiscible liquids. At low no, the growth law for the domain size follows nearly power law behavior, with the growth law exponent decreasing with increasing surfactant concentration. As no was further increased, the growth rate for the characteristic domain size at intermediate to late times was found to be significantly slowed, in agreement with previous theoretical investigations. The slow growth is attributed to the accumulation of surfactant at the interface between the immiscible liquids, which leads to a reduction in the surface tension between the immiscible liquids. We found that the surfactant moves to the interfacial region very early in the phase separation process; however, the interfaces are not uniformly coated with surfactant. Dynamic scaling was observed at late times for the range of mean surfactant densities considered.
Theoretical study of bifurcating reaction paths
Authors: Yanai Takeshi , Taketsugu Tetsuya , Hirao Kimihiko
Published: July 2002

Theoretical study has been given on bifurcating reaction paths where the intrinsic reaction path (IRP) has a valley-ridge inflection (VRI) point on the way from transition state to product, and leads to another first-order saddle point which connects two symmetrically equivalent products. To extract dynamically important regions in the configuration space between the VRI point and products, a group of steepest descent paths are calculated starting from zero-point energy regions at the VRI point, and the obtained configuration space is analyzed in terms of the reaction coordinate and the normal coordinate. The method is applied to Berry pseudorotations in SiH4F− and PH4F, H3CO→H2COH, and cyclopropylidene→allene, by employing the second-order Mo/ller–Plesset and complete active space self-consistent field ab initio molecular orbital calculations. It is shown that the extension of bifurcating reaction paths largely depends on the position of the VRI point on the IRP. For the respective reactions, non-totally symmetric excited states are calculated along the IRP to understand the valley-ridge inflection mechanism in terms of the second-order Jahn–Teller effect. The isotope effect on bifurcating reaction paths is also investigated.
Bounds to information entropies for atomic systems
Authors: Tao Jianmin , Li Guobao , Li Jianmin
Published: July 2002

Bounds to atomic information entropies and their sum in position and momentum spaces have been derived in terms of radial and momentum expectation values 〈rn〉 and 〈pn〉. Numerical studies on atomic systems show that the bounds presented in terms of 〈r〉 and 〈p〉, the first moments of the position and momentum space densities are sharper than those given by Gadre and Bendale in terms of 〈r2〉 and 〈p2〉, the second moments of the position and momentum space densities. Within the BKCM procedure due to Burkhardt, Kónya, Coulson, and March, several relationships between the information entropies and the average electron densities 〈ρ〉 and 〈γ〉 in position and momentum spaces have been established.
Electric quadrupole second-harmonic generation spectra in epitaxial vanadyl and titanyl phthalocyanine films grown by molecular-beam epitaxy
Authors: Hoshi Hajime , Yamada Toshiki , Ishikawa Ken , Takezoe Hideo , Fukuda Atsuo
Published: July 2002

For epitaxial vanadyl and titanyl phthalocyanine films with Laue symmetry 4/mmm, second-harmonic generation (SHG) spectra due to only electric quadrupole and electric quadrupole coupling mechanisms were obtained under a suitable geometry, while the contributions of electric dipole, magnetic dipole, and magnetic dipole coupling mechanisms were eliminated. In addition to a resonant peak previously reported, two peaks were observed at higher energies. An oriented gas model analysis showed that in-plane components of molecular hyperquadrupole polarizabilities cause the resonances. On the basis of the molecular electronic structure calculated by the density functional theory, the resonant processes are attributed to electric quadrupole transitions at SH frequencies.
An ab initio molecular orbital study of the potential energy surface of the HO2+NO reaction
Authors: Sumathi R. , Peyerimhoff S. D.
Published: July 2002

The potential energy surface of the HO2+NO reaction has been investigated at second order Moller–Plesset perturbation (MP2) and density functional (DFT) methods with the 6-311++G** basis set and at complete active space [CAS(8,8)] self-consistent field level using the 6-31G** basis set. The reaction is shown to give three different groups of products, viz., HO–NO2, NO2+OH, and HNO+O2. The thermodynamically stable HO–NO2 can be formed from the energized ONOOH adduct by the 1,2 migration of the OH group via a loose transition state (referred to as TS2) with a relatively higher barrier height compared to O–O bond fission. The other exothermic product, NO2+OH, arises from a direct O–O dissociation of ONOOH and is expected to be the most favorable process on account of its low barrier height. HNO+O2 can be formed by two different channels: (i) the direct hydrogen abstraction and/or (ii) the barrierless association of the reactants to form the peroxynitrous acid, ONOOH, which then undergoes 1,3 hydrogen migration, giving rise to the HN(O)OO biradical followed by N–O dissociation. Of the two channels, channel (i) has been found to be dominant. Owing to their higher barrier heights, HNO formation is expected only at high temperatures. NOH+O2 and HONO+O are not expected to compete in the kinetics of the HO2+NO system. The energetic of the key reactions, namely HO2+NO→HO–NO2 and HO2+NO→NO2+OH, has also been obtained at the QCISD/6-311++G(2df,2pd)//MP2/6-311++G** level.
Photodissociation of ozone in the Chappuis band. II. Time-dependent wave-packet calculations and interpretation of diffuse vibrational structures
Authors: Flöthmann Heiner , Beck Christian , Schinke Reinhard , Woywod Clemens , Domcke Wolfgang
Published: July 2002

We present time-dependent wave-packet calculations describing the photodissociation of ozone in the Chappuis band, which evolves in the two lowest states of A″1 symmetry. The calculations are performed in the diabatic representation and include the coupling between the two relevant states. All three nuclear degrees of freedom are taken into account. The two potential-energy surfaces, the coupling potential, and the two transition-dipole-moment functions with the electronic ground state have been calculated previously by ab initio methods [Woywod et al., J. Chem. Phys. 107, 7282 (1997)]. The coupling between the two diabatic states is exceedingly strong, resulting in very fast dissociation into O+O2 on the time scale of only one symmetric stretch period. A small portion of the initially created wave packet is temporarily trapped leading to three tiny recurrences, which reflect basically symmetric stretch motion plus some amount of bending motion. The experimentally observed diffuse vibrational structures superimposed to the broad absorption spectrum are satisfactorily reproduced and discussed both in the time-independent and the time-dependent picture of spectroscopy. In view of the very short lifetime in the excited states, the correspondingly large widths of the diffuse structures, and the participation of all three vibrational modes we conclude that an unique assignment in terms of three quantum numbers is not possible. The main structures are due to symmetric stretch excitation, but bending and even asymmetric stretch motion are also involved. A one-state model, in which only the upper (bound) adiabatic potential is employed, provides a qualitatively correct explanation of the absorption spectrum and the diffuse structures.
Critical unmixing of polymer solutions
Authors: Frauenkron Helge , Grassberger Peter
Published: July 2002

We present Monte Carlo simulations of semidilute solutions of long self-attracting chain polymers near their Ising type critical point. The polymers are modeled as monodisperse self-avoiding walks on the simple cubic lattice with attraction between nonbonded nearest neighbors. Chain lengths are up to N=2048, system sizes are up to 221 lattice sites and 2.8×105 monomers. These simulations used the recently introduced pruned-enriched Rosenbluth method, which proved extremely efficient, together with a histogram method for estimating finite size corrections. Our clearest result is that chains at the critical point are Gaussian for large N, having end-to-end distances R∼N. Also, the distance TΘ−Tc(N) [where TΘ=limN→∞Tc(N)] scales with the mean field exponent, TΘ−Tc(N)∼1/N. The critical density seems to scale with a nontrivial exponent similar to that observed in experiments. But we argue that this is due to large logarithmic corrections. These corrections are similar to the very large corrections to scaling seen in recent analyses of Θ polymers, and qualitatively predicted by the field theoretic renormalization group. The only serious deviation from this simple global picture concerns the N-dependence of the order parameter amplitudes, which disagrees with a minimalistic ansatz of de Gennes. But this might be due to problems with finite size scaling. We find that the finite size dependence of the density of states P(E,n) (where E is the total energy and n is the number of chains) is slightly but significantly different from that proposed recently by several authors.
Geometric and electronic structures of fluorine bound silicon clusters
Authors: Kishi Reiko , Negishi Yuichi , Kawamata Hiroshi , Iwata Suehiro , Nakajima Atsushi , Kaya Koji
Published: July 2002

Geometries and energies of SinF− and SinF (n=1–7) were investigated with ab initio MO calculations; the Mo/ller–Plesset perturbation and coupled cluster methods were used to take into account the electron correlation. The F atom is bound to the apex atom in the Sin frame, and the electronic structures of the Sin part in SinF− and SinF are similar to those of the corresponding Sin and Sin+, respectively. The calculated electron affinities (EAs) of SinF are in agreement with the experimental values. The size dependence of EAs of SinF is similar to that of the ionization energies of Sin at n⩾5, whereas it is different at n⩽4. In the HOMO of SinF (n⩽4), the weak antibonding nature between F and Si greatly makes the SinF− anions unstable, resulting in small EA. The vibrationally resolved photoelectron spectra (PES) of SinF− (n=1–3) were measured, and were completely reproduced theoretically.
Study of the low-lying states of NiO− and NIO using anion photoelectron spectroscopy
Authors: Moravec Vicki D. , Jarrold Caroline Chick
Published: July 2002

The 2.33 and 3.49 eV photoelectron spectra of NiO− obtained with a new apparatus using field-free electron energy analysis are reported. The electron affinity of NiO is determined to be 1.470(3) eV. A spin–orbit splitting of 260(40) cm−1 for the ground Π2 anion state is observed. A bond length of 1.668(4) Å and vibrational frequency of 660(40) cm−1 are determined from spectral simulations of the neutral Σ-3 ground state←anion Π3/22 state transition. A transition from an excited Σ-4 anion state [Te=900(40) cm−1, ω=760(40) cm−1] to the ground neutral state is assigned. New spectroscopic constants are obtained for the Φ3 state [Te=0.65(1) eV], the Δ1 state [Te=0.94(1) eV, ω=615(15) cm−1, and re=1.600(6) Å] and the second Π3 state [Te=1.194(10) eV, 2A=411(30) cm−1]. These findings are in general agreement with those recently reported by Wu and Wang [J. Chem. Phys. 107, 16 (1997)], although several reassignments of the photoelectron spectra (PES) are made based on comparison with calculations and newly resolved fine structure.
Exterior complex dilation for grid methods: Application to the cumulative reaction probability
Authors: Karlsson Hans O.
Published: July 2002

Exterior complex dilation (ECD) is introduced for the discrete variable representation (DVR) via a general coordinate mapping. The procedure leads to a computationally efficient and easily implemented approach for imposing outgoing boundary conditions, comparable with absorbing potentials in terms of minimizing the grid and parameters used. ECD relies on a rigorous mathematical framework in contrast to absorbing potentials. The DVR-ECD approach is illustrated by computation of the cumulative reaction probability for the H+H2 reaction in one and two dimensions.
Deuteration-induced phase transition in ammonium hexachloroplumbate
Authors: Kume Y. , Muraoka H. , Yamamuro O. , Matsuo T.
Published: July 2002

The heat capacities of (NH4)2PbCl6 and (ND4)2PbCl6 were measured at temperatures between 13 K and 300 K. A gradual phase transition was found at 77.8±0.3 K for (NH4)2PbCl6 with an associated transition entropy of 3.0 J K−1 mol−1. The transition temperature shifted to 81.0±0.3 K in (ND4)2PbCl6. A sharp first order transition occurred at 38.40±0.03 K for (ND4)2PbCl6 accompanied by a transition entropy of 7.5 J K−1 mol−1. There was no corresponding phase change in (NH4)2PbCl6. The mechanism of the deuteration-induced transitions in the present compound and similar transitions in (ND4)2TeCl6, (ND4)2PtCl6, (ND4)2PdCl6, and (ND4)2SeCl6 is discussed. Tunneling energies involved in the rotational motion of the ND4+ ion in the three-minimum potential were derived from the experimental entropy data. They correlate smoothly with the metal-chlorine bond distance for these five isomorphous compounds. Standard thermodynamic functions calculated from the heat capacity data are tabulated in the Appendix.
Density effect on molecular bond length in fluid I2 by x-ray absorption spectroscopy
Authors: Buontempo U. , Filipponi A. , Postorino P. , Zaccari R.
Published: July 2002

Precise measurements of x-ray absorption spectra above the I K-edge both in the low-density gas in a wide temperature range and in the liquid along the coexistence curve have been performed. By using an advanced data analysis procedure it was possible, for the first time, to reveal changes in the structure of the I2 molecule in the liquid as a function of density. The relevance of these changes and the possible correlations with other peculiar properties of liquid I2, and in particular with the metallization transition observed in the liquid under pressure, are discussed.