bluuebunny/crossref_metadata_2025 huggingface dataset:
A test of partial third order electron propagator theory: Vertical ionization energies of azabenzenes
Authors: Ortiz J. V. , Zakrzewski V. G.
Published: July 2002

Vertical ionization energies of pyradine, pyridazine, pyrimidine, pyrazine, s-triazine, and s-tetrazine are calculated with partial third order electron propagator theory. Extensive reorderings of final states are produced by correlation corrections to Koopmans’s theorem results. The partial third order (P3) quasiparticle method succeeds in producing the correct order of final states and close agreement with photoelectron spectra. Because P3 is more efficient than the outer valence Green’s function and other methods based on the third order self-energy, it shows considerable promise for predicting photoelectron spectra of large molecules.
Polymer chain in a good solvent between attracting walls. A scaling approach
Authors: Allegra Giuseppe , Colombo Emanuele
Published: July 2002

We present a scaling approach to the problem of a single chain in a good solvent between attracting walls that yields a simple analytical expression for the chain free energy as a function of the wall-to-wall distance, together with the amount of chain expansion parallel to the walls and with the effective layer thickness. Using the Domb–Gillis–Wilmers interatomic distribution function whereby the probability of a small interatomic distance r is ∝rθ, the method yields the critical exponents ν and θ for the general dimensionality d through self-consistent optimization of the chain free energy. For d=2 we get ν=0.707 and θ=0.828. An infinitely long, ringlike chain comprised between attracting parallel walls is investigated. Wall attraction on the chain atoms is effectively reduced by the solvent-mediated excluded-volume effect. The partition function is factorized into a bidimensional component projected on the walls and a monodimensional orthogonal component, using the thermal-blob concept. The resulting self-consistent free energy per monomer is a universal function of the effective wall-interaction free energy and of the wall-to-wall distance L. Remarkably, the numerical value of the exponents only affects the front factor of the free energy. Below the critical temperature T* at which wall attraction exactly compensates for the chain entropy loss at the wall boundary, the chain free energy attains a minimum at a wall-to-wall distance L̄ below and above L̄ the walls are respectively repelled and attracted by the chain. The minimum originates from excluded-volume effects; if L̄ is smaller than L̄ the chain is merely squeezed by the walls, if L≳L̄ it collapses on them with an increased compression, if L=L̄ it is pulled apart by the attracting walls, thus relieving the excluded-volume repulsion. At T=T* the distance L̄ goes to infinity, and at T≳T* the repulsive force on the walls jumps to larger values, constant with temperature. Experimental measurements of the force exerted by polymer chains on confining surfaces, obtained by Klein and co-workers, appear to be consistent with these theoretical results.
Photodissociation spectroscopy of Ca+–CO2
Authors: Scurlock C. T. , Pullins S. H. , Duncan M. A.
Published: July 2002

Electronic spectra are observed for the metal cation complex Ca+–CO2, using resonance-enhanced photodissociation spectroscopy. The complexes are produced in a laser vaporization/supersonic expansion source, size selected and excited on resonance, and the mass-analyzed product is measured in a time-of-flight mass spectrometer. Both Ca+ and CaO+ dissociation channels are observed to have sharp resonances. Spectra from two isotopomers, the 40Ca+ and 44Ca+ species, are recorded and analyzed. Transitions from the X 2Σ+(v″=0) ground vibronic state to several vibrational levels in the D 2Πr excited electronic state are measured. The structure of the complex is confirmed to be linear by the presence of prominent spin–orbit multiplets. Spectroscopic constants for the 40Ca+–CO2 complex are determined: ν00=22 099.1 cm−1, Aso′=136.3 cm−1, ωe′=258.9 cm−1, and ωexe′=4.23 cm−1.
High-resolution infrared spectroscopy of isotopic impurity Q1(0) transitions in solid parahydrogen
Authors: Weliky David P. , Kerr Karen E. , Byers Teresa J. , Zhang Yu , Momose Takamasa , Oka Takeshi
Published: July 2002

We have made a high-resolution infrared spectroscopic study of the Q1(0) (v=1←0, J=0←0) vibrational transitions of the isotopic impurities D2 and HD in solid parahydrogen. Each impurity has a spectrum composed of ∼100 sharp lines spread over ∼0.4 cm−1. The linewidths vary, but are on the order of 10 MHz. These spectra make clear: (1) the infrared Q1(0) transitions of J=0 isotopic impurities are induced by the quadrupolar fields of nearby impurity J=1 molecules; and (2) the spectral pattern of strong Q1(0) lines is due to the splitting of the M-orientational levels of J=1/J=0 o-D2 or J=1/J=0 HD nearest-neighbor (nn) impurity pairs. With the aid of several theoretical works, the strong lines in the D2 and HD spectra can be individually and unambiguously assigned as specific quantum state Q1(0) transitions of nn impurity pairs containing p-D2/o-D2 or o-H2/o-D2, and o-H2/HD, respectively. The assigned transitions of nn impurity pairs containing o-H2 are confirmed by combination differences which agree to within 5×10−4 cm−1, the instrumental precision. These assignments yield complete Q1(0) energy level diagrams for the nn impurity pairs o-H2/o-D2 and o-H2/HD embedded in solid parahydrogen. The experimental energy level splittings are fit to a two parameter model which describes anisotropic interactions in the parahydrogen crystal. These experimental parameters appear to have significant contributions from the changes in renormalization and lattice constant around the heavier isotopic impurity. We have also assigned a few of the weaker spectral features as Q1(0) transitions of more distant impurity pairs, but the bulk of these transitions are yet to be assigned. They do form a distinctive pattern and are thought to be the Q1(0) transitions of impurity triples and larger clusters. This study is one of the few cases for which high-resolution laser spectroscopy has been successfully applied to the condensed phase and for which many of the transitions have rigorous quantum state assignments.
Photochemistry of adsorbed molecules. XV. Localized atomic scattering in the photolysis of HI/LiF(001) and HI/NaF(001)
Authors: Barclay V. J. , Hung W.-H. , Keogh W. J. , Kühnemuth R. , Polanyi J. C. , Zhang G. , Zeiri Y. , Jennison D. R. , Li Y. S.
Published: July 2002

We have compared the ‘‘elastic’’ H atom angular-scattering, Pel(θ′), of HI(ad) photolyzed at 248 nm for LiF and NaF substrates, both experimentally and theoretically. The observed Pel(θ′) for elastic scattering were similar for LiF and NaF, with a peak in the range 43°–48°. The Pel(θ′) for H from HI/NaF was broader than that for H from HI/LiF. Energy distributions, P(ET′), for H recoiling from HI/LiF and HI/NaF are also reported. A semiempirical potential-energy surface (pes) is given for HI adsorbed on NaF, based on an earlier one for LiF [J. Chem. Phys. 94, 978 (1991)]. Ab initio ‘‘density function theory’’ (DFT) calculations were performed to verify the suitability of this pes; the DFT points were in satisfactory agreement with the semiempirical pes. For HI/LiF the DFT calculation gave a heat of adsorption of about 9 kcal/mol, the semiempirical model gave 5 kcal/mol, and experiment gave 7 kcal/mol. We have computed H atom scattering by the classical trajectory method for two different models one involving dense and the other perforated arrangements of the HI adsorbate molecules. Comparison with the experimental results showed that the second model gave a better description of both Pel(θ′) and P(ET′) for HI/LiF and HI/NaF. The calculations showed that the angular and energy distributions of the scattered H were sensitive to the size of the vacancies between adjacent adsorbate molecules. The study also indicates the importance of unit-cell size in relation to the size of the adsorbate molecule in determining adsorbate geometry and hence the energy and angular distributions of the scattered photofragments.
Saddle-node bifurcations in the LiNC/LiCN molecular system: Classical aspects and quantum manifestations
Authors: Borondo F. , Zembekov A. A. , Benito R. M.
Published: July 2002

A classical-quantum correspondence study of a saddle-node bifurcation in a realistic molecular system is presented. The relevant classical structures (periodic orbits and manifolds) and its origin are examined in detail. The most important conclusion of this study is that, below the bifurcation point, there exists an infinite sequence of precursor orbits, which mimic for a significant period of time the (future) saddle-node orbits. These structures have a profound influence in the quantum mechanics of the molecule and several vibrational wave functions of the system present a strong localization along the saddle-node periodic orbits. A striking result is that this scarring effect also takes place well below the bifurcation energy, which constitutes a manifestation of the so-called ‘‘ghost’’ orbits in configuration and phase space. This localization effect has been further investigated using wave packet dynamics.
Studies of spin relaxation and molecular dynamics in liquid crystals by two-dimensional Fourier transform electron spin resonance. II. Perdeuterated-tempone in butoxy benzylidene octylaniline and dynamic cage effects
Authors: Sastry V. S. S. , Polimeno Antonino , Crepeau Richard H. , Freed Jack H.
Published: July 2002

Two-dimensional Fourier transform (2D-FT)-electron spin resonance (ESR) studies on the small globular spin probe perdeuterated tempone (PDT) in the liquid crystal solvent 4O,8 (butoxy benzylidene octylaniline) are reported. These experiments, over the temperature range of 95 °C to 24 °C, cover the isotropic (I), nematic (N), smectic A (SA), smectic B (SB), and crystal (C) phases. The 2D-ELDOR (two-dimensional electron–electron double resonance) spectra confirm the anomalously rapid reorientation of PDT, especially in the lower temperature phases. The model of a slowly relaxing local structure (SRLS) leads to generally very good non-linear least squares (NLLS) global fits to the sets of 2D-ELDOR spectra obtained at each temperature. These fits are significantly better than those achieved by the standard model of Brownian reorientation in a macroscopic orienting potential. The SRLS model is able to account for anomalies first observed in an earlier 2D-ELDOR study on PDT in a different liquid crystal in its smectic phases. Although it is instructional to extract the various spectral densities from the COSY (correlation spectroscopy) and 2D-ELDOR spectra, the use of NLLS global fitting to a full set of 2D-ELDOR spectra is shown to be more reliable and convenient for obtaining optimum model parameters, especially in view of possible (incipient) slow motional effects from the SRLS or dynamic cage. The cage potential is found to remain fairly constant at about kBT over the various phases (with only a small drop in the SB phase), but its asymmetry increases with decreasing temperature T. This value is significantly larger than the weak macroscopic orienting potential which increases from 0.1 to 0.3kBT with decreasing T. The cage relaxation rate, given by Rc is about 3×107 s−1 in the I phase, but increases to about 108 s−1 in the SA, SB, and C phases. The rotational diffusion tensor for PDT shows only a small T-independent asymmetry, and its mean rotational diffusion coefficient is of order 1010 s−1, with however, a small increase in the SB phase. These results are consistent with a model previously proposed for PDT in benzylidene liquid crystal solvents, that as T is reduced the PDT molecules are partially expelled from the hard core (dipolar) region of the liquid crystalline molecules toward the more flexible aliphatic chain region as a result of increased core packing from smectic layer formation, and it thus experiences a more fluid (for a given temperature) local cage structure.
Dissociative chemisorption of H2 on Cu(100): A four-dimensional study of the effect of parallel translational motion on the reaction dynamics
Authors: Kroes G. J. , Wiesenekker G. , Baerends E. J. , Mowrey R. C. , Neuhauser Daniel
Published: July 2002

Results are reported of a four-dimensional dynamics study on the dissociation of H2 on Cu(100). The potential-energy surface was taken from density functional calculations, which employed the generalized gradient approximation and a slab representation for the surface. Reaction occurs preferentially in impacts near the bridge and hollow sites. Collisions near top sites promote vibrational excitation. The conclusion that vibrationally inelastic scattering and reaction occur preferentially on different sites can be generalized to other low index Cu surfaces. Resonances affect the reaction in the 4D model through a mechanism in which the molecule, trapped by excitation of the molecular bond which is weakened at the surface near top sites, is allowed more time to tunnel through the barrier to reaction. The calculated dependence of the diffraction probabilities on incidence energy suggests that a measurement of low-order diffraction would be able to determine whether the minimum barrier to reaction occurs for impacts on the bridge sites (as theory predicts) or on the hollow or top sites. The calculated reaction probabilities are in good agreement with experiment, further improvement being expected from including the rotational degrees of freedom in the model.
Examining the influence of the [Zn(H2O)6]2+ geometry change on the Monte Carlo simulations of Zn2+ in water
Authors: Sánchez Marcos Enrique , Martínez José M. , Pappalardo Rafael R.
Published: July 2002

Quantum chemical calculations have been carried out to estimate the variation in the interaction energy between one or two water molecules and the Zn2+ hydrated ion ([Zn(H2O)6]2+) when the Zn–O intramolecular distance is changed. Likewise, changes in the interaction energy between a second hydration shell formed by twelve H2O and the hydrated ion when shrinking the hexahydrate have been also examined, using a previously reported hydrated ion–water potential [Pappalardo et al., J. Phys. Chem. 97, 4500 (1993)] and the Matsuoka–Clementi–Yoshimine (MCY) potential for the water–water interactions. Using these potentials and that of Clementi for the Zn2+–H2O interaction, Monte Carlo simulations have been performed using two different Zn–O distances for the first hydration shell. Results show that influence of the geometrical relaxation on the solvation energy is not large, about 2.5%. Consequences on the use of a flexible or rigid hydrated ion are discussed.
Laser excited fluorescence study of reactions of excited Ca and Sr with water and alcohols: Product selectivity and energy disposal
Authors: Oberlander M. D. , Parson J. M.
Published: July 2002

Reactions of the metastable 3P0J states of Ca and Sr in atomic beams with H2O, D2O, and CH3OH yielding ground electronic state products have been observed by laser excited fluorescence of MOH, MOD, and MOCH3. The water reactions favor metal hydroxide products while methanol reactions favor methoxides. For SrOH product, spectral simulation of the B̃ 2Σ+–X̃ 2Σ+ transition based on coupled harmonic-oscillator Franck–Condon factors was used to determine crude vibrational energy distributions in the bending and metal-stretching modes, and simulation of a higher resolution scan of excitation of the ground vibrational level gave some information about the rotational energy distribution in that level. While excitation of metal stretching and rotation were considerable and not too far from the predictions of a prior model, bending was significantly colder. Limited spectroscopic constants and severe spectral congestion have precluded other successful simulations.
Diffusion in configuration space according to the choice of the solution of the Hamilton–Jacobi–Yasue equation and the role of boundary conditions
Authors: Battezzati M.
Published: July 2002

It is shown that by splitting the velocity of a diffusing harmonic oscillator into a deterministic spatially dependent part, plus a fluctuating component with zero mean, two basic diffusion operators result in correspondence to the two singular solutions of the related Hamilton–Jacobi–Yasue equation. The diffusion equations with time dependent coefficients which had been proposed before by different authors, are shown to result as linear combinations of these. It is proven the connection of the asymptotic propagator with the two-time transition probability density, in relation to the boundary conditions which are imposed to the system.
Theoretical studies of the electronic and optical properties of permethyl-oligosilanes
Authors: Liu Zhihua , Terakura Kiyoyuki , Abe Shuji , Harris John F.
Published: July 2002

A systematic study was made on the optical absorption spectra of permethyl oligosilanes, CH3[CH3–Si–CH3]NCH3, as a function of their backbone chain length from N=2 to 16 by using first principles [density functional with local density approximation (LDA)] electronic structure calculations. The general features of the calculated spectra agree with those of experimental measurements. Analysis of the nature of optical transition revealed the existence of two qualitative crossovers at chain lengths of N=2 and around N=10 and 12. The first one is associated with the molecular orbitals giving rise to the first ultraviolet absorption peak, while the second one is associated with the crossover from ‘‘molecular’’ to ‘‘polymeric’’ as the oligomers become larger. The broadening of the first absorption peaks is estimated by the vibration modes and electronic relaxation energies in the oligomers. Comparison with calculated results by a model Hamiltonian based on the Sandorfy C model is made and the relative merit of semiempirical and LDA calculations is discussed.
One-atom cage effect in collinear I2(B)–Ar complexes: A time-dependent wave packet study
Authors: Fang Jian-Yun , Martens Craig C.
Published: July 2002

Two-dimensional time-dependent wave packet calculations are carried out on a collinear model of the I2(B)–Ar complex to investigate the possible kinematic origin of the one-atom cage effect in small van der Waals molecules. Three different excitation wavelengths are considered (496.5, 488, and 476.5 nm), and the dynamics are assumed to be restricted to the I2 B state electronic surface, with no nonadiabatic transitions following the pump excitation. Good agreement with experiment is obtained. To investigate the sensitivity of observable final state distributions on the weak intermolecular potential between I2 and Ar, three slightly different B state I–Ar interactions are employed for the case of 488 nm excitation. It is found that relatively small changes in the form and magnitude of the weak van der Waals interactions can have a large effect on the final state distributions. These results suggest that the experimental data on I2–Ar photodissociation–recombination can be explained by a purely kinematic one-atom cage effect on the B state electronic surface for a collinear population of I2–Ar clusters, without the need to introduce nonadiabatic electronic effects.
A single-theory approach to the prediction of solid–liquid and liquid–vapor phase transitions
Authors: Pourgheysar Payman , Mansoori G. Ali , Modarress Hamid
Published: July 2002

For simultaneous prediction of solid–liquid and liquid–vapor phase transitions it has been customary to apply two different theories for solid and fluid phases. A single-theory approach will be desirable to answer many of the fundamental problems of molecular theory and their relationship with macroscopic behavior of the matter. Based on a modified version of the cell model of statistical mechanics, a single-theory approach for simultaneous prediction of solid–liquid and liquid–vapor phase transitions is presented here. In developing this theory the order–disorder transition is considered as the essential feature of the fusion and a new function for the potential energy field inside a single-occupancy cell is derived. By reporting the variations of total pressure of the macroscopic system with respect to temperature and volume the nature of the various phase transitions in the system are evaluated and discussed. Variations of the radial distribution function of the molecules in the system with intermolecular distance, temperature, and volume are reported for various phases of matter.
Computer simulations of the Raman Q-branch of nitrogen in helium
Authors: Michels J. P. J. , Scheerboom M. I. M. , Schouten J. A.
Published: July 2002

Dynamical computer simulations have been performed on a single nitrogen particle in helium at room temperature and pressures ranging from 0.6 up to 8.7 GPa. Both the frequency and the linewidth have been determined. The ‘‘repulsive’’ contribution to the frequency is comparable to that in pure nitrogen. In contrast with the pure component case, the experimental values for the shift in the mixture are higher than those obtained from the simulations. From the difference between the experimentally determined shift and the simulation results, an estimate has been made of the change in interaction energy with the surrounding particles on excitation of a nitrogen molecule. A good fit is obtained by the addition of a weak Lennard-Jones potential. The results for the linewidth reveal a marked difference to those for a system of pure N2, which has been confirmed by experimental evidence. This behavior, is for the greater part, due to a significant difference in the time correlation of the vibration frequency and the resulting correlation time.
Extraction of the transition dipole matrix from the photodissociation of oriented/aligned parent molecules
Authors: Fuglesang Christopher D. , Baugh Delroy A. , Pipes Leonard C.
Published: July 2002

A formalism for the analysis of detailed photofragmentation experiments involving oriented/aligned parent molecules is presented. It is demonstrated for the photofragmentation of an oriented/aligned parent molecule that both the magnitudes and the relative-phases of the transition dipole matrix elements can be determined from angle integrated cross sections or the product orientation/alignment moments derived from them. Formulas are presented for extracting transition dipole matrix elements from the photodissociation of an aligned diatomic molecule to form aligned and/or oriented atomic products using linearly or circularly polarized photolysis lasers. While these detailed measurements are ideally performed using inhomogeneous field state-selected parent molecules, they can be also readily carried out by doing double-resonance photodissociation experiments with polarized lasers. The analysis presented here can, therefore, be used to extract the transition dipole matrix elements from these double-resonance photofragmentation experiments as well.
A simple model of the HNCO (1A′) excited state potential energy surface and a classical trajectory analysis of the vibrationally directed bond-selected photodissociation
Authors: Brown Steven S. , Cheatum Christopher M. , Fitzwater David A. , Crim F. Fleming
Published: July 2002

Recent state-selected photodissociation experiments on isocyanic acid, HNCO, have provided a wealth of data on its photochemistry and dissociation dynamics. The excited state potential energy surface on which the dissociation occurs is central to these observations but is relatively uncharacterized. We construct a two-dimensional analytical model for the excited state potential that is consistent with experimental observations, including the ultraviolet absorption spectrum and the dynamics of the C–N and N–H bond dissociations. We then test this surface by running classical trajectories on it, using Morse oscillator vibrational wave functions from the ground electronic state to determine the probability distributions of initial conditions. The trajectory calculation reproduces the experimentally observed variation in the photochemical branching with photolysis wavelength. It also reproduces the bond selectivity in the photodissociation of HNCO molecules containing three quanta of N–H stretching excitation (3ν1) that we observed experimentally. Although the model for the surface is very simple and includes only two degrees of freedom, it captures the essential features that determine the photochemical branching in a direct dissociation.
Long-range, collision-induced hyperpolarizabilities of atoms or centrosymmetric linear molecules: Theory and numerical results for pairs containing H or He
Authors: Li Xiaoping , Hunt Katharine L. C. , Pipin Janusz , Bishop David M.
Published: July 2002

For atoms or molecules of D∞h or higher symmetry, this work gives equations for the long-range, collision-induced changes in the first (Δβ) and second (Δγ) hyperpolarizabilities, complete to order R−7 in the intermolecular separation R for Δβ, and order R−6 for Δγ. The results include nonlinear dipole-induced-dipole (DID) interactions, higher multipole induction, induction due to the nonuniformity of the local fields, back induction, and dispersion. For pairs containing H or He, we have used ab initio values of the static (hyper)polarizabilities to obtain numerical results for the induction terms in Δβ and Δγ. For dispersion effects, we have derived analytic results in the form of integrals of the dynamic (hyper)polarizabilities over imaginary frequencies, and we have evaluated these numerically for the pairs H...H, H...He, and He...He using the values of the fourth dipole hyperpolarizability ε(−iω; iω, 0, 0, 0, 0) obtained in this work, along with other hyperpolarizabilities calculated previously by Bishop and Pipin. For later numerical applications to molecular pairs, we have developed constant ratio approximations (CRA1 and CRA2) to estimate the dispersion effects in terms of static (hyper)polarizabilities and van der Waals energy or polarizability coefficients. Tests of the approximations against accurate results for the pairs H...H, H...He, and He...He show that the root mean square (rms) error in CRA1 is ∼20%–25% for Δβ and Δγ; for CRA2 the error in Δβ is similar, but the rms error in Δγ is less than 4%. At separations ∼1.0 a.u. outside the van der Waals minima of the pair potentials for H...H, H...He, and He...He, the nonlinear DID interactions make the dominant contributions to Δγzzzz (where z is the interatomic axis) and to Δγxxxx, accounting for ∼80%–123% of the total value. Contributions due to higher-multipole induction and the nonuniformity of the local field (Qα terms) may exceed 15%, while dispersion effects contribute ∼4%–9% of the total Δγzzzz and Δγxxxx. For Δγxxzz, the α term is roughly equal to the nonlinear DID term in absolute value, but opposite in sign. Other terms in Δγxxzz are smaller, but they are important in determining its net value because of the near cancellation of the two dominant terms. When Δγ is averaged isotropically over the orientations of the interatomic vector to give Δγ̄, dispersion effects dominate, contributing 76% of the total Δγ̄ (through order R−6) for H...H, 81% for H...He, and 73% for He...He.
Molecular beam infrared spectroscopy of the HCCCN–HCCH and HCN–HCCCCH van der Waals complexes
Authors: Yang X. , Pearson R. Z. , Lehmann K. K. , Scoles G.
Published: July 2002

High resolution IR spectra of the linear HCCCN–HCCH and HCN–HCCCCH hydrogen bonded complexes have been obtained using optothermal detection molecular beam techniques. Two vibrational bands have been observed for each complex, which correspond to the terminal ‘‘free’’ C–H stretch vibrations (ν2) of the cyano units and the hydrogen bonded vibrations (ν3) of the acetylenic CH stretches. For both complexes, accurate molecular constants have been obtained. Furthermore, predissociation lifetimes for the ν3=1 states of the both complexes have been determined. The results are compared with those of the linear HCN–HCCH complex obtained by Block et al. [Chem. Phys. 139, 15 (1989)].
Quantum mechanical analysis of photofragment alignment near asymmetric resonances
Authors: Lee Sungyul
Published: July 2002

Quantum mechanical analysis is presented for the alignment of the oxygen atoms produced from the photodissociation of OH. The alignment parameters are predicted to be independent of energy across the isolated Lorentzian resonances, when only one channel contributes to indirect dissociation. When more than one channel interferes with one another, they may change very slowly. Across the asymmetric resonances, the alignments exhibit rapid variations due to the quantum interference between the indirect and the direct dissociation pathways. The alignments of O(3P2) and O(3P1) exhibit different variations, both of which are asymmetric across the asymmetric resonances. It is also shown that photoexcitation to repulsive states, coupled with bound electronic states, can give asymmetric resonances and sharp variations of the alignment, suggesting that the analysis of the dynamics of direct photodissociation by measurements of vector properties could be complicated by the effects of quantum interference.
Phase separation in mixtures of a rodlike colloid and two or more rodlike polymers
Authors: Sear Richard P. , Frenkel Daan
Published: July 2002

A suspension of rodlike colloidal particles and rodlike liquid crystalline polymers is modelled as a mixture of thick (colloidal) and thin (polymeric) hard rods. Extensive immiscibility in the fluid phase is observed in the mixtures. For two species of polymers, one species much longer than the other, we observe two demixing critical points. For three polymer species we find three critical points. Polymer molecules of length l induce an attractive interaction of range l between colloidal rods. Two different polymers induce effective attractions of two different ranges. The range of the attraction determines the density at which the demixing occurs. The attractions of different ranges create demixing at different densities.
The mobilities of NO+(CH3CN)n cluster ions (n=0–3) drifting in helium and in helium–acetonitrile mixtures
Authors: de Gouw Joost A. , Ding Li Ning , Krishnamurthy M. , Lee Hack Sung , Anthony Eric B. , Bierbaum Veronica M. , Leone Stephen R.
Published: July 2002

The mobilities of NO+(CH3CN)n cluster ions (n=0–3) drifting in helium and in mixtures of helium and acetonitrile (CH3CN) are measured in a flow-drift tube. The mobilities in helium decrease with cluster size [the mobility at zero field, K(0)0, is 22.4±0.5 cm2 V−1 s−1 for NO+, 12.3±0.3 cm2 V−1 s−1 for NO+(CH3CN), 8.2±0.2 cm2 V−1 s−1 for NO+(CH3CN)2 and 7.5±0.5 cm2 V−1 s−1 for NO+(CH3CN)3] and depend only weakly on the characteristic parameter E/N (electric field strength divided by the number density of the buffer gas). The size dependence is explained in terms of the geometric cross sections of the different cluster ions. The rate constants for the various cluster formation and dissociation reactions have also been determined in order to rule out the possibility that reactions occurring in the drift region influence the measurements in the mixtures. Since high pressures of acetonitrile are required to form NO+(CH3CN)2 and NO+(CH3CN)3, the mobilities of these ions are found to be dependent on the acetonitrile concentration, as a result of anomalously small mobilities of these ions in acetonitrile [K(0)0=0.041±0.004 cm2 V−1 s−1 for NO+(CH3CN)2 and 0.044±0.004 cm2 V−1 s−1 for NO+(CH3CN)3]. These values are at least an order of magnitude smaller than any previously reported ion mobility, which can be partly explained by the large ion-permanent dipole interaction between the cluster ions and acetonitrile. The remaining discrepancies may be the result of momentum transfer outside the capture cross section, dipole–dipole interactions, ligand exchange, the formation of long-lived collision complexes or the transfer of kinetic energy into internal energy of the cluster ion and the acetonitrile molecule.
The vibrational energy pattern in 12C2H2(II): Vibrational clustering and rotational structure
Authors: Temsamani Mohammed Abbouti , Herman Michel
Published: July 2002

We achieve a systematic modeling of all rovibrational levels in the 12C2H2 (X̃ 1Σ+g) molecule, which is tested up to the near infrared range. It is based on the cluster picture, which was demonstrated to block diagonalize the full vibrational energy matrix, and to allow unraveling the vibrational energy pattern in 12C2H2, up to 12 000 cm−1 [see M. Abbouti Temsamani and M. Herman, J. Chem. Phys. 102, 6371 (1995)]. Each of those clusters, which are called here V-clusters, is made of pure vibrational type diagonal and off-diagonal matrix elements. That model is extended to take care of the rotational structure, defining the V/l/C-cluster model. In a first step J-dependent terms are included in the diagonal elements of the V-clusters, and rotational l resonance off-diagonal matrix elements are included, leading to couple specific V-cluster matrices, resulting into so-called V/l-clusters. This extension is quantitatively demonstrated to reproduce the reported effective principal rotational constant and effective higher order distortion constants, for four selected clusters of levels: those containing V1+V3, V1+V2+V3, 3V3 and V2+3V3. In the case of the 3ν3 range, new FTIR spectra recorded around 9700 cm−1 are used. The related experimental conditions and new observed spectral features are briefly presented. A further extension of the model is then accomplished to include Coriolis-type interaction, by coupling V/l-clusters using a systematic mechanism. That step, defining the model of V/l/C-clusters, allows to suggest assignment for extra rovibrational lines observed around 3ν3. Those various steps are supported by a consistent picture involving constants of the motion, starting with three pseudoquantum numbers in the case of V- cluster, {ns,nr,k}, from which two, {ns,nr} and then one {nr} remain when defining respectively the V/l-cluster and V/l/C-cluster matrices.
Auger decay of the molecular field split S 2p core excited states in HS radical
Authors: Naves de Brito A. , Svensson S. , Osborne S. J. , Ausmees A. , Kivimäki A. , Sairanen O.-P. , Nõmmiste E. , Aksela H. , Aksela S. , Saethre L. J.
Published: July 2002

Using synchrotron radiation, very high resolution electron spectra at the 2p→6a1 and 2p→3b2 resonances of H2S have been recorded. Auger transitions from molecular field split S 2p hole states of HS* fragment, created by fast dissociation of the core excited H2S, are found to reveal different partial rates depending on the symmetry of the final state. The experiments have made it possible to determine, for the first time, the molecular field splitting of the core levels in a free radical. The molecular field splitting between the 3σ1/2 and 1π3/2 (“2p3/2’’) levels in the HS* fragment is found to be 90(5) meV, which significantly differs from the molecular field splitting of the “S 2p3/2’’ levels in the H2S molecule. Resonant Auger decay in the molecule before dissociation is also observed.
Reactions of translationally excited and thermal fluorine atoms with CH4 and CD4 molecules in solid argon
Authors: Misochko E. Ya. , Benderskii V. A. , Goldschleger A. U. , Akimov A. V. , Benderskii A. V. , Wight C. A.
Published: July 2002

Solid state reactions of fluorine with methane have been studied by ultraviolet laser photolysis of dilute mixtures of the two reagents in solid argon at 13–30 K. Using a combination of EPR and FTIR spectroscopies, three distinct mechanisms of product formation have been identified. At temperatures below 18 K, product formation is dominated by direct photolysis of F2–CH4 heterodimers, resulting in formation of closed-shell complexes HF–CH3F. A small fraction of reaction intermediate, a nonplanar methyl radical trapped in the reaction cage with HF and F, is also formed. This intermediate decays to product on a time scale of 103 s at 13 K after the photolysis period. Above 20 K, photogenerated F atoms undergo thermally activated diffusion in argon, and they react with isolated CH4 molecules to form CH3–HF complexes. The rate constant for this process is ∼10−25 cm3 s−1 at 20 K, and exhibits an activation energy of 1.7 kcal/mol. A third reaction channel involves reaction of F with CH4 to generate isolated planar methyl radicals by escape of the HF product from the reactive site. This channel has been observed for reactions of translationally excited F atoms (below 20 K) and thermally diffusing F atoms (during post-photolysis reactions above 20 K).
Computation of the static structure factor of the path-integral quantum hard-sphere fluid
Authors: Sesé Luis M. , Ledesma Ricardo
Published: July 2002

A calculation of the quantum static structure factor of the path-integral hard-sphere fluid is presented. The starting point is an approximate path-integral partition function factorized into two parts: one depending upon the necklace center-of-mass coordinates and the other consisting of independent bead packets associated with each particle in the sample. This produces two Ornstein–Zernike equations, namely linear response (true particle) and center of mass, formally identical to the well-known classical version and that can be solved with the aid of Baxter’s equations and Dixon–Hutchinson’s variational procedure. Linear response and center-of-mass direct correlation functions and their corresponding structure factors are computed from r-space radial distribution functions obtained with several propagators: crude, Barker’s, Jacucci–Omerti’s, and Cao–Berne’s. The results show: features of Baxter’s equations; the proximity between the quantities arising from the efficient propagators; the convergence of the crude propagator quantities to the latter; and the usefulness of the Feynman–Hibbs Gaussian picture (even in this context) to get one-particle quantum functions. The validity of the approximate partition function under the present working conditions is also stated, and an assessment of the quantum effects on the structure factor is made by comparing the classical Percus–Yevick and the path-integral results. Isothermal compressibilities are compared with Yoon–Scheraga’s data and the agreement is excellent.
Structure of uniform hard-sphere fluid: A density functional approach
Authors: Kim Soon-Chul
Published: July 2002

A simple weighted-density approximation (SWDA) based on both local average and bulk densities is used to investigate the equilibrium structure of a uniform hard-sphere fluid. The main advantage of SWDA is computationally much simpler than the WDA of Tarazona based on the local density. The scheme which was pointed out by Percus has been used to interconnect between the structure of a uniform fluid and that of its nonuniform counterpart. The weighting function of the uniform hard-sphere fluid which is the requirement input is taken from the free energy functional approximation based on the truncated density expansion. The calculated results for structure-related functions such as the radial distribution function, cavity function, and cavity function are found to be in good agreement with the computer simulations. Comparisons with other approximations show that the SWDA results are a significant improvement upon those of the Percus–Yevick approximation and of the WDA of Denton–Ashcroft [A. R. Denton and N. W. Ashcroft, Phys. Rev. A 44, 1219 (1991)] based on the higher-order weighted-density approximation, and comparable to those of the WDA of Tarazona [G. P. Brenan and R. Evans, Mol. Phys. 73, 789 (1991)]. These results also provide that the SWDA provides an accurate description of inhomogeneous hard-sphere fluids.
Recombination dynamics of photodissociated I2− in size selected Ar and CO2 clusters
Authors: Vorsa Vasil , Nandi Sreela , Campagnola Paul J. , Larsson Mats , Lineberger W. C.
Published: July 2002

We report time-resolved photodissociation and geminate recombination dynamics of I2− in size-selected I2− Arn and I2−(CO2)n cluster ions by using ultrafast pump–probe techniques at 790 nm in conjunction with a tandem time-of-flight mass spectrometer. The absorption recovery, which reflects the time scale for photodissociation followed by recombination and vibrational relaxation of I2− inside the cluster, shows a strong dependence on the composition of the surrounding cluster solvent. The absorption recovery time for I2−(CO2)16 is ∼1 ps, whereas for I2−Ar20 it is ∼130 ps. This difference is discussed in terms of electrostatic and hard sphere interactions. We also observe the time dependence of the destruction of the Ar solvent cage for I2−Ar16. Finally, absorption recovery data for I2−(CO2)n cluster ions taken with 790 nm pump–probe wavelengths are compared with the greater energy release 720 nm data.
Equal reaction rates for all recombination pathways
Authors: Wei G. W. , Alavi Saman , Snider R. F.
Published: July 2002

Formal collision theory indicates that the absolute termolecular recombination reaction rate can be expressed in a form associated either with direct or indirect mechanisms for the recombination. Moreover, the same exact rate constant can be calculated using either of the four mechanisms. These results are to be contrasted to the rate constants standardly calculated by approximate methods.
Paramagnetism of caesium titanium alum
Authors: Dubicki Lucjan , Riley Mark J.
Published: July 2002

Caesium titanium alum, CsTi(SO4)2⋅12H2O, is a β alum and exhibits a large trigonal field and a dynamic Jahn–Teller effect. Exact calculations of the linear 2T2⊗e Jahn–Teller coupling show that in the strict S6 site symmetry the ground multiplet consists of a Kramers doublet 2Γ6 with magnetic splitting factors g∥=1.1 and g⊥=0, a Γ4Γ5 doublet at ∼60 cm−1 with g∥=2.51 and g⊥=0.06 and another Γ4Γ5 doublet at ∼270 cm−1 with g∥=1.67 and g⊥=1.83. The controversial g values observed below 4.2 K, g∥=1.25 and g⊥=1.14, are shown to arise from low symmetry distortions. These distortions couple the vibronic levels and induce into the ground state the off-diagonal axial Zeeman interaction that exists between the first excited and the ground vibronic levels.
The ordering of symmetric diblock copolymers: A comparison of self-consistent-field and density functional approaches
Authors: Nath Shyamal K. , McCoy John D. , Curro John G. , Saunders Randall S.
Published: July 2002

Polymer reference interaction site model (PRISM) based density functional (DF) theory is used to evaluate the structure and thermodynamics of structurally symmetric, freely jointed, diblock chains with 0.50 volume fraction. These results are compared to the results of self-consistent-field (SCF) theory. Agreement between the predictions of the SCF and DF theories is found for the lamella spacing well above the order–disorder transition (ODT) and for the qualitative behavior of the interfacial thickness as a function of both chain length and Flory–Huggins χ parameter. Disagreement is found for the magnitude of the interfacial thickness where DF theory indicates that the thickness is 1.7±0.2 times larger than that predicted by SCF theory. It appears that behavior on the monomer length scale is sensitive to system specific details which are neglected by SCF theory.
The hyperpolarizability of trans-butadiene: A critical test case for quantum chemical models
Authors: Norman Patrick , Luo Yi , Jonsson Dan , Ågren Hans
Published: July 2002

The conspicuous fact that the low-order method—the random phase approximation—has served as the best approach to obtain accurate one-photon spectra of the trans-polyenes is scrutinized by performing polarizability and hyperpolarizability calculations of the trans-butadiene molecule using a variety of state-of-the-art correlating methods. The observation persists also for these ground state properties, i.e., the random phase approximation provides results in excellent agreement with experimental data that cannot be matched by higher-order methods. The importance of this observation for simulations of conjugated molecules as non-linear materials is briefly discussed.
A simulation of the phase transition in monolayer CO/NaCl(001)
Authors: Vu N.-T. , Jakalian A. , Jack D. B.
Published: July 2002

We report a Monte Carlo simulation of the p(2×1)→p(1×1) phase transition in a monolayer of CO molecules adsorbed on a NaCl(001) surface. In the p(2×1) phase the CO molecules are tilted by 24° from the surface normal and have preferred azimuthal orientations whereas in the p(1×1) phase the molecules are oriented perpendicular to the surface with no preferred azimuthal orientation. The transition temperature is estimated to lie in the range 30–35 K. Multiple domains and phase coexistence are also observed.
Manipulating external degrees of freedom with intense light: Laser focusing and trapping of molecules
Authors: Seideman Tamar
Published: July 2002

The possibility of focusing and trapping molecules in the intensity gradient of a moderately intense laser field is shown. A fully quantum-mechanical and a hybrid quantum-classical model are developed and both the case of a near-resonance and that of a far-off-resonance field are studied numerically and analytically. Molecular focusing is obtained when the ratio between the kinetic energy and the field-matter interaction exceeds unity (but remains small). In this energy regime the laser beam serves as a lens for a beam of molecules. The properties of the “molecular lens” are analyzed and the experimental conditions required to focus molecules are examined. We propose the application of laser focusing to nanoscale deposition onto, and etching of a substrate and describe a possible setup to that end. Molecular trapping is obtained as the strong interaction limit of a general “molecular optics” scheme.
Real time study of bimolecular interactions: Direct detection of internal conversion involving Br(2P1/2)+I2 initiated from a van der Waals dimer
Authors: Tuchler Matthew F. , Wright Scott , McDonald J. Douglas
Published: July 2002

A reaction complex is formed from a van der Waals dimer precursor, HBr⋅I2, and is monitored with picosecond time resolution using standard pump–probe spectroscopy. The reaction is initiated in a slightly attractive region of an excited electronic state with insufficient energy to fragment and will eventually undergo an internal conversion to a lower electronic state via electronic to vibration energy transfer. A resulting product, highly vibrationally excited molecular I2, is monitored by resonance enhanced multiphoton ionization (REMPI) combined with time of flight mass spectroscopy. The HBr constituent of the precursor HBr⋅I2 is photodissociated at 220 nm. The H-atom departs instantaneously, allowing the remaining electronically excited Br(2P1/2) to form a collision complex, (BrI2)*, in a restricted region along the Br+I2 reaction coordinate determined by the precursor geometry. The evolution of this complex is probed in real time by tuning the probe to the REMPI line of the I atom: 298 nm. The resulting transients include I2+ and I+, with lifetimes of 55(±5) and 40(±5) ps, respectively. Similar results are obtained for initiation from DBr⋅I2, with risetimes of 43(±5) and 29(±5) ps measured for the I2+ and I+ transients, respectively. The originally formed (BrI2)* does not have enough internal energy to dissociate directly, but must undergo an internal conversion to a lower electronic state in order to continue to reactants or products. An isotope effect is also detected and explained with a simple kinetics model that is consistent with mechanism described above. Temporal discrepancies in the risetimes of I2+ and I+ imply that either the ground state process is also being observed or that differing vibrational states of the I2 product are formed at differing rates and detected with differing efficiencies.