bluuebunny/crossref_metadata_2025 huggingface dataset:
Theoretical analysis of the vibrational structure of the T←N transition in ethylene
Authors: Siebrand Willem , Zerbetto Francesco , Zgierski Marek Z.
Published: July 2002

A theoretical analysis is reported of the ground state-to-triplet absorption spectrum of ethylene, observed as an electron-loss spectrum. The analysis is based on ab initio MO calculations of the vibrational force fields in the ground state and in the two stationary configurations (planar and staggered) of the triplet state. The calculations use the gamess program with the 6-31++G** basis set. The transition turns the double bond into a single bond and thus increases the CC distance. This affects modes ν2 and ν3 , corresponding to mixed CC stretching and CH scissoring, and leads to a long ν3 progression together with some ν2 structure. The transition also excites torsional levels (mode ν4 ), preferentially those just above and below the torsional barrier. This relatively narrow torsional spectrum is superimposed on each of the CC-stretch peaks. The systematic change in this structure observed along the ν3 progression is due to anharmonic ν3 ν4 coupling. Because of this coupling, a meaningful assignment of ν4 quantum numbers to the spectrum is not possible. The significance of this interpretation for the enigmatic V←N absorption spectrum is briefly indicated.
A general model of electron spin polarization arising from the interactions within radical pairs
Authors: Norris J. R. , Morris A. L. , Thurnauer M. C. , Tang J.
Published: July 2002

We present a general description of electron spin polarization observed for interacting radical pairs. Unlike previous treatments, both chemically induced dynamic electron polarization and correlated radical pair polarization are included for ST0 mixing. A key feature of this model is that the members of the pair can remain interacting throughout the time sequence of electron spin polarization. Explanations of the time-dependent evolution of the electron spin polarization are presented using the density matrix and its associated vector diagram. A new vector method is formulated for calculating and visualizing chemically induced electron spin polarization. The approach is based on the vector, Δρ(t) which represents the displacement of the density vector ρ(t) from its direction at the time of its ‘‘birth.’’ In addition, these electron spin polarization phenomena are described in spectroscopic terms as well as in the usual mathematical manner.
Structures, stabilities, and intermolecular vibrational frequencies of small ammonia complexes by molecular mechanics for clusters analysis
Authors: Dykstra Clifford E. , Andrews Lester
Published: July 2002

The electrically based molecular mechanics for clusters (MMC) approach has been applied to complexes of ammonia, including the ammonia dimer, trimer, and tetramer as well as mixed, binary complexes with hydrogen, water, hydrogen fluoride, and hydrogen cyanide. The MMC approach is designed for the generation of potential energy surfaces for weakly interacting assemblies, and equilibrium structures, stabilities, dipole moments, and harmonic frequencies for the intermolecular vibrational modes are obtained directly. This information is reported for the ammonia clusters that were studied and serves to characterize the nature of ammonia’s weak interactions. Generally, the good comparison with available spectroscopic data indicates that the weak bonding of ammonia is principally a consequence of its electrical properties and its shape. The trimer and tetramer are found to be symmetric, cyclic structures and the relative orientations of a pair of ammonias progress smoothly from the dimer to the trimer to the tetramer.
X-ray diffraction study of some liquid alkanes
Authors: Habenschuss A. , Narten A. H.
Published: July 2002

The structure of the liquids n-heptane, n-decane, n-pentadecane, and n-eicosane was investigated using x-ray diffraction. The data are analyzed using group scattering factors for -CH2 and -CH3 sites which permits extraction of the carbon–carbon structure and distribution functions. Analysis has yielded the short-range intramolecular distances and bond angles, the average torsional angles for the gauche and trans rotational isomer segments, and the gauche/trans populations. Intramolecular carbon–carbon distances at 3.11±0.03 Å and 3.94 ±0.01 Å are assigned to gauche and trans molecular segments, respectively. The average torsion angle of the gauche segments is near 64±2°, and the trans segments appear to be twisted from a planar conformation by 10±2°. The average fraction of gauche segments in the liquid phase of these alkanes is approximately 45±8%, slightly higher than in the gas phase. The conformational equilibrium of the n-alkanes is only minimally perturbed by the intermolecular interactions in the liquid phase, and the results are inconsistent with the possibility of a large fraction of extended chains (or chain segments) in the liquid.
The structures of C2H+ and C2H+2 as measured by Coulomb explosion imaging
Authors: Zajfman D. , Vager Z. , Naaman R. , Mitchell R. E. , Kanter E. P. , Graber T. , Belkacem A.
Published: July 2002

The structures of ethynyl and acetylene cations were investigated utilizing the Coulomb explosion imaging method. The geometrical probability density functions for the bending modes were obtained for the protonated and deuterated forms of each species. Detailed analyses of the data reveal that both the measured linear equilibrium geometries and the observed extent of the correlated bending vibrations agree with ab initio calculations for C2H+2 and C2D+2. C2H+ and C2D+ are found to be linear, but the extent of the bending amplitudes are by far larger than the corresponding theoretical predictions.
Comment on: Rheology of n-alkanes by nonequilibrium molecular dynamics
Authors: Chynoweth Simon , Klomp Ulfert C. , Michopoulos Yanos
Published: July 2002

Nonequilibrium molecular dynamics simulations of n-butane for an increased system size and longer computational runs have been extended to both lower and higher shear rates. Results from these simulations are considerably more accurate than those previously reported in the literature and an alternative rheological interpretation of them is proposed.
Bond-specific chemistry: OD:OH product ratios for the reactions H+HOD(100) and H+HOD(001)
Authors: Bronikowski Michael J. , Simpson William R. , Girard Bertrand , Zare Richard N.
Published: July 2002

From laser-induced fluorescence measurements, the OD:OH product branching ratio is determined to be greater than 25:1 for the reaction H+HOD(001) and less than 1:8 for the reaction H+HOD(100). Fast H atoms are generated by the photolysis of HI at 266 nm and the HOD is excited to a specific vibronic state using an optical parametric oscillator. Here HOD(001) and HOD(100) correspond to O–H and O–D local-mode stretch fundamentals, respectively. Within our signal-to-noise level, reaction occurs exclusively with the extended hydrogen atom of HOD in each case.
The hard ellipse liquid: An integral equation study
Authors: Ferreira P. G. , Perera A. , Moreau M. , Telo da Gama M. M.
Published: July 2002

We investigate the structural and thermodynamic properties of the isotropic phase of the hard ellipse liquid for several length-to-breadth ratios (2,4,6), using the hypernetted chain (HNC) and Percus–Yevick (PY) integral equation theories. A comparison with the recent simulations of Cuesta and Frenkel [Phys. Rev. A 42, 2126 (1990)] is made. Satisfactory agreement is found with PY theory for small aspect ratios, while for larger ones, HNC results become superior. In addition, HNC (but not PY) predicts the orientational instability of the isotropic phase at a finite density for systems of aspect ratios 4 and 6, in good agreement with computer simulation results. In order to investigate the order of the orientational phase transition, density functional theories (DFT) calculations are carried out using as input the direct correlation functions of the isotropic liquid obtained in this work. No first-order transition is found within the version of DFT used herein.
Resonance Raman scattering of butadiene: Vibronic activity of a b
u mode demonstrates the presence of a 1A
g symmetry excited electronic state at low energy
Authors: Chadwick Richard R. , Zgierski Marek Z. , Hudson Bruce S.
Published: July 2002

Resonance Raman spectra of buta-1,3-diene-d0 and buta-1,3-diene-1,1,4,4-d4 have been obtained with ultraviolet excitation from 239.5 to 199.9 nm. Activity of the first overtone of mode 24, the bu symmetry CCC chain deformation mode, is observed with excitation energy below the origin of the 1 1Bu state. This vibronic activity of a nontotally symmetric mode is shown to be evidence of resonance with the 2 1Ag state of butadiene. A quantitative analysis of the ratio of intensities of 2ν24 to ν9, the ag symmetry CCC chain deformation mode, demonstrates that enhancement of 2ν24 cannot be due to resonance with the 1 1Bu state. The resonance enhancement behavior of this overtone band also shows that it is of vibronic origin rather than Franck–Condon allowed. The intensity pattern seen for the modes of bu symmetry is fully consistent with the results of a quantitative calculation of vibronic activity for the eight bu symmetry modes. The 2 1Ag electronic state is estimated to be ca. 0.25 eV below the 1 1Bu electronic state. Overtones of out-of-plane C–H bending and CH2 twisting modes are seen with excitation radiation near the peak of the transition to the 2 1Ag state, indicating that the 2 1Ag state of butadiene has appreciably lower resistance to deformation along out-of-plane coordinates than does the ground electronic state. This is consistent with the expectations of semiempirical calculations.
Thermodynamics and structure of the primitive model near its gas–liquid transition
Authors: González-Tovar E. , Lozada-Cassou M. , Mier-y-Terán L. , Medina-Noyola M.
Published: July 2002

The hypernetted-chain/mean spherical approximation (HNC/MSA) is employed to calculate the thermodynamic properties of the restricted primitive model (symmetric binary mixture of charged hard spheres) in the low density–high coupling region, where the gas–liquid phase transition is located. The spinodal curve is calculated from the virial and the compressibility equations. The HNC/MSA total correlation function does not show long-range behavior as the spinodal curve is approached. This is due to the hybrid nature of this approximation. However, a consistent HNC/MSA total correlation function is defined which does show the expected long-range behavior. This long-range behavior is found to be related to the phenomenon of ionic clustering, in accordance with previous studies.
A
b i
n
i
t
i
o study of the nonadditive effects in the trimer of ammonia
Authors: Szczȩśniak Mal/gorzata M. , Kendall Rick A. , Chal/asiński Grzegorz
Published: July 2002

The three-body potential in the trimer of ammonia is analyzed in terms of Heitler–London (HL)-exchange, self-consistent field (SCF)-deformation, induction, and dispersion nonadditivities. The nonadditive term is due largely to the SCF-deformation effect. However, its anisotropy resembles more that of the HL-exchange nonadditivity. Correlation effects do not contribute significantly to the nonadditivity. The trimer is of C3h symmetry. The geometry is determined at the level of pairwise interactions. Apart from a slight shortening of the N––N distance, the three-body effect has virtually no influence on the mutual orientations of subsystems in the trimer. Nonadditive properties of ammonia are compared to those of other trimers: (HF)3; (HCl)3; (H2O)3; and (CH4)3. An examination of the basis set dependence of the components of three-body effect leads us to believe that the present treatment yields three-body potential with accuracy better than 0.1 kcal/mol for the trimer of ammonia, as well as for other polar trimers.
Application of Floquet theory to the nuclear magnetic resonance spectra of homonuclear two-spin systems in rotating solids
Authors: Nakai Toshihito , McDowell Charles A.
Published: July 2002

The theory for the nuclear magnetic resonance (NMR) spectra of homonuclear two-spin systems under sample spinning has been developed. The propagator describing the time-evolution of the systems, which is driven by the homogeneous Hamiltonian composed of the chemical-shift difference and the flip–flop parts of the dipolar and J couplings, is treated using Floquet theory; the Floquet eigenvalues and eigenvector components determine the resonance frequencies and intensities, respectively. Nondegenerate Rayleigh–Schrödinger perturbation theory is used to solve the Floquet secular equation. Recurrence equations for the perturbation corrections have been derived, allowing us to evaluate efficiently very high-order terms, such as 20th-order terms. Analytical expressions for the resonance frequency obtained in the low-order approximations provide an intuitive understanding of the main spectral features; the second-order equations can describe the conspicuous solid-state effects on the magic-angle spinning (MAS) spectra, such as the unique line shapes and the additional shifts, while the zeroth and first-order approximations yield spectra similar to those in solution-state NMR. The spectra should, however, be calculated up to the perturbation order where line shape function converges, to reproduce precisely the experimental results, especially the change of the additional shifts with the spinning frequency which were obtained in the MAS spectra for 13C doubly-labeled sodium acetate. The theoretical treatment shows that off-magic angle spinning (OMAS) spectra of two-spin systems should exhibit line shapes dependent on the mutual orientations of the dipolar and the chemical-shift tensors of the two nuclei. Also, the nondegenerate perturbation treatment yields criteria for the conditions under which the normal and n=0 rotational resonance occur.
The demixing and gas–liquid instability of a binary Yukawa fluid
Authors: Chen X. S. , Forstmann F.
Published: July 2002

For a binary mixture we relate the second order expansion of the grand canonical potential Ω to direct correlation functions and to thermodynamic expressions. The matrix of second functional derivatives of Ω with respect to the particle density fluctuations is diagonalized. The zero of the smallest eigenvalue indicates a phase instability vs particle density fluctuations. The related eigenvector characterizes the instability uniquely. Calculations of correlation functions via the reference hypernetted chain integral equations are performed for a binary mixture of hard core particles with attractive Yukawa interactions; the spinodals are established for different concentrations and different interaction potentials using the calculated correlation functions. Along the spinodal we find a continuous change from predominantly demixing to mainly condensation instabilities.
Quantum mechanical reaction probabilities via a discrete variable representation-absorbing boundary condition Green’s function
Authors: Seideman Tamar , Miller William H.
Published: July 2002

The use of a discrete variable representation (DVR) and absorbing boundary conditions (ABC) to construct the outgoing Green’s function G(E+)≡limε→0(E+iε−H)−1, and its subsequent use to determine the cumulative reaction probability for a chemical reaction, has been extended beyond our previous work [J. Chem. Phys. 96, 4412 (1992)] in several significant ways. In particular, the present paper gives a more thorough derivation and analysis of the DVR-ABC approach, shows how the same DVR-ABC Green’s function can be used to obtain state-to-state (as well as cumulative) reaction probabilities, derives a DVR for the exact, multidimensional Watson Hamiltonian (referenced to a transition state), and presents illustrative calculations for the three-dimensional H+H2 reaction with zero total angular momentum.
High resolution 1.3 μm overtone spectroscopy of HF dimer in a slit jet: K
a=0←0 and K
a=1←0 subbands of vacc=2←0
Authors: Suhm Martin A. , Farrell John T. , McIlroy Andrew , Nesbitt David J.
Published: July 2002

Continuous wave difference frequency mixing of a single mode Nd:YAG laser at 1.06 μm and a scanning, single mode ring dye laser (R6G) in a LiNbO3 crystal generates a novel source of widely tunable near infrared radiation in the 1.2–2.2 μm region. In conjunction with the high sensitivity of a pulsed slit nozzle expansion with multipass optics (0.48 m path length), this narrow band source of tunable ir light allows the high resolution study of overtone (v=2←0) spectra for a wide variety of molecular complexes with H stretching vibrations. In this paper, we report the first rotationally resolved spectra of (HF)2 in the first HF stretching overtone region. In particular, we observe Ka=1←0 and 0←0 subbands for a vibrational state from one member of the v=2 overtone triad in (HF)2 with a band center of 7682.8228(5) cm−1. We tentatively assign this state as the hydrogen bond acceptor (i.e., free) HF stretching overtone 2νacc based on predissociation line widths and excellent agreement with predictions based on an anharmonic local mode description of (HF)2. Splittings of 0.2119(5) cm−1 (K′a = 0) and 0.0942(3) cm−1 (K′a = 1) due to interconversion tunneling are found.

From the observed intensity alternation due to nuclear spin statistical weights, the overall vibrational symmetry for K′a = 0 and 1 is unambiguously determined to be Γvib=A+ and B+ for the lower and upper tunneling levels, respectively. These A+ and B+ symmetry designations correspond to irreducible representations of the MS4 molecular symmetry group, which allows for large amplitude motion and exchange of the identical HF subunits. Predissociation line broadening is observed in each of the four upper vibrational levels which varies between 56(20) and 175(25) MHz and depends sensitively on both K′a and the tunneling symmetry. This tunneling symmetry dependence, together with the unusual K′a dependence of the tunneling splitting and the anomalously large intensity ratio between the parallel and perpendicular transitions, indicates the presence of vibrational resonances in the overtone region not clearly evidenced in the analysis of the corresponding fundamental HF stretch region. Our results are discussed in the context of earlier static cell FTIR spectra and recent ab initio predictions for this overtone state. The data suggest that the overtone dynamics in (HF)2 can not be satisfactorily described as an oscillator pair connected by a 1D interconversion pathway, and may instead involve substantial coupling to other intermolecular vibrational degrees of freedom.
Kinetics of oxygen adsorption, absorption, and desorption on the Cu(111) surface
Authors: Bloch J. , Bottomley D. J. , Janz S. , van Driel H. M. , Timsit R. S.
Published: July 2002

The interaction of oxygen with the Cu(111) surface in ultrahigh vacuum (UHV) has been studied in the temperature range 400<T<800 K using second-harmonic generation (SHG) and x-ray photoelectron spectroscopy (XPS). When the clean surface is exposed to oxygen at pressures between 5×10−8 and 10−5 Torr and for T<500 K, the SHG intensity decreases monotonically with exposure by more than one order of magnitude to a value which has no measurable temperature dependence. For T≳500 K, the SHG intensity passes through a minimum before achieving this constant value. The observation of this minimum is interpreted in terms of an outward relaxation of the Cu(111) surface as oxygen penetrates the subsurface region. When UHV conditions are restored for T≳600 K, the SHG intensity reverses its temporal dependence. These observations are consistent with initial incorporation of atomic oxygen into the subsurface region at a rate which is dependent on surface temperature and oxygen pressure and subsequent backdiffusion in UHV, driven by the oxygen concentration gradient near the surface. From experiments performed at different oxygen pressures and sample temperatures we establish rate constants for oxygen incorporation and surface outward relaxation as a function of temperature. The kinetics of oxygen incorporation determined from changes in the SHG intensity are compared with those derived elsewhere from ellipsometry studies. Differences yielded by the two experimental techniques are related to differences in monitored depths. Complementary XPS experiments suggest that sites occupied by the subsurface oxygen are characterized by tetrahedral symmetry.
Polymer chain between attractive walls: A second-order transition
Authors: Allegra Giuseppe , Colombo Emanuele
Published: July 2002

A matrix approach is adopted to evaluate the partition function of random-walk chains between parallel walls. Assuming an attractive potential ε at the sites contiguous to the walls, a second-order transition is deduced at the temperature T* that makes ε/T equal to the entropy loss of the chain bonds reaching those sites. At T=T*, the walls act as reflecting barriers to the chain, whereas at T≥T* and at T≤T*, they change to repelling and attracting barriers, respectively. In the limit of an infinitely long chain comprised between infinitely distant walls, T* takes the role of a tricritical temperature, as we have three distinct states at T≥T* (the chain is repelled from the walls), at T=T* (the chain density is uniformly distributed between the walls), and at T≤T* (the chain collapses on either wall). Denoting as r the ratio between the probability of an end atom and that of a middle-chain atom touching the walls, we obtain r≫1, r=6/5, and r≤1 at T≥T*, T=T*, and T≤T*, respectively.
The low-lying bending vibrational levels of the CCH (X̃ 2Σ+) radical studied by laser-induced fluorescence
Authors: Hsu Yen-Chu , Lin Jim Jr-Min , Papoušek D. , Tsai Jaw-Jer
Published: July 2002

The uv spectrum of the CCH radical was recorded using the laser-induced fluorescence technique on the 193 nm photolysis product of acetylene. Four 2Π–2Π bands at 38 805, 37 946, 37 010, and 36 075 cm−1 of CCH were rotationally analyzed and assigned as transitions from the (0,v21,0) (v2=1, 3, 5, 7) vibrational levels of the X̃ 2Σ+ state to a common upper vibronic state (denoted as U), which possibly belongs to the 2 2Π state. A simultaneous nonlinear least squares fit of the uv bands, in combination with the infrared transitions previously observed in the X̃ 2Σ+ state, provided improved spectroscopic parameters for the U state and the (0,31,0), (0,51,0), and (0,71,0) levels of the X̃ state.
Monte Carlo study of associative polymer networks. I. Equation of state
Authors: Groot Robert D. , Agterof Wim G. M.
Published: July 2002

The equation of state of associative polymer networks has been studied by Monte Carlo simulation. To describe the associations, an algorithm is introduced which for dilute monatomic systems reduces to the well-known mass action law. For the polymers, the simple bead spring model was employed. The incorporation of a finite volume of the beads is essential to prevent phase separation once the associative interaction is turned on, and the system is quenched into the gel state. The excess pressure of this quenched state is well described by two exponents of the density ρ. The repulsive part of the excess pressure scales proportional to ρα, the attractive part is proportional to ρβ. For the nonassociating polymers we measured the first exponent as α=2.339±0.006, and for associating polymers far away from the critical point we found β=2.06±0.05. For certain values of the density and the association constant the measured pressure was negative, in these cases we find microphase separated configurations. The simulations also enabled the establishment of the percolation transition. The association constant at the gel point has been obtained as a function of density and polymer length. Together with the equation of state this resulted in a phase diagram for polymer gels.
On the possible role of nonelectrostatic interactions in the mechanism of the slow polyelectrolyte mode observed by dynamic light scattering
Authors: Sedlák Marián
Published: July 2002

The effect of a poor solvation of a polyelectrolyte chain backbone in a polar solvent was investigated as a possible origin of large multichain domains in polyelectrolyte solutions giving rise to the slow polyelectrolyte mode observed by dynamic light scattering. It was investigated if these domains could be interpreted as polymeric aggregates formed due to the poor backbone solvation. Static and dynamic light scattering experiments on polyions in solvents of different quality with respect to the polyelectrolyte chain backbone, and the temperature dependence of the slow mode, showed that this hypothesis is not realistic. Poor solvation of the polyelectrolyte chain backbone in polar solvents is therefore not the origin of domain formation in solutions of strongly charged polyelectrolytes. This conclusion was confirmed also by analysis of literature data in view of the investigated issue.
High-pressure neutron diffraction on liquid sulphur hexafluoride and interpretation by statistical-mechanical theories and computer simulations
Authors: Strauss G. , Zweier H. , Bertagnolli H. , Bausenwein T. , Tödheide K. , Chieux P.
Published: July 2002

High pressure neutron diffraction measurements of the static structure factor of fluid SF6 are presented. The density of the liquid was varied in a range of ρ=0.85 g cm−3 up to ρ=1.85g cm−3 at the supercritical temperature 398 K. Aspects of the measurements and data evaluation, as far as they are specific for high pressure studies, are discussed. The intramolecular structure parameters are determined. The measurements were interpreted with statistical mechanical calculations (XRISM) as well as reverse Monte Carlo (RMC) calculations. The individual atom pair correlation functions are given and their dependence on density is discussed. An intermolecular potential of (12–6) LJ-type with added point charges is deduced. The density dependent measurements proved to be useful for appraising the intermolecular potential.
Evaporation and condensation at a liquid surface. II. Methanol
Authors: Matsumoto Mitsuhiro , Yasuoka Kenji , Kataoka Yosuke
Published: July 2002

The rates of evaporation and condensation of methanol under the vapor–liquid equilibrium condition at the temperature of 300 and 350 K are investigated with a molecular dynamics computer simulation. Compared with the argon system (reported in part I), the ratio of self-reflection is similar (∼10%), but the ratio of molecule exchange is several times larger than the argon, which suggests that the conventional assumption of condensation as a unimolecular process completely fails for associating fluids. The resulting total condensation coefficient is 20%–25%, and has a quantitative agreement with a recent experiment. The temperature dependence of the evaporation–condensation behavior is not significant.
Theoretical study of the mechanism of recombinative hydrogen desorption from the monohydride phase of Si(100): The role of defect migration
Authors: Nachtigall P. , Jordan K. D. , Sosa Carlos
Published: July 2002

Density functional theory with nonlocal corrections is used together with cluster models to examine various pathways for H2 desorption from the Si(100)2×1 surface. The barrier calculated for direct desorption of H2 from the doubly-occupied dimer is appreciably larger than the experimentally observed activation energy at submonolayer coverages. We propose a mechanism in which surface defects are converted into dihydride (SiH2) species from which H2 desorption occurs. The barrier calculated for this process (57 kcal/mol) is in excellent agreement with the measured activation energy. The barrier for defect migration is predicted to be only 14 kcal/mol, so that a single defect can account for the desorption of H2 from a large number of monohydride sites. Single-point calculations for several of the optimized structures are carried out using the quadratic configuration interaction (QCI) method. The reaction energies and barrier heights calculated with the QCI and density functional theory (DFT) methods are in excellent agreement.
Neutron diffraction studies of H2O/D2O at supercritical temperatures. A direct determination of gHH(r), gOH(r), and gOO(r)
Authors: Tromp R. H. , Postorino P. , Neilson G. W. , Ricci M. A. , Soper A. K.
Published: July 2002

Neutron diffraction studies are reported on H2O at temperatures of 300 and 400 °C. The method of isotopic substitution is applied to three mixtures of H2O and D2O, and the diffraction data are used to determine the three radial distribution functions gHH(r), gOH(r), and gOO(r). These results can be used to discuss changes in nearest neighbor structure between water molecules, and to assess the degree of usefulness of representative (usually pairwise) model potentials.
Pulsed field ionization zero kinetic energy photoelectron spectroscopy of the vanadium dimer molecule
Authors: Yang Dong Sheng , James Andrew M. , Rayner David M. , Hackett Peter A.
Published: July 2002

The technique of pulsed-field-ionization zero-kinetic-energy (PFI-ZEKE), photoelectron spectroscopy was employed to probe the electronic structure of the V+2 cation. Rotationally resolved PFI-ZEKE spectra of the V+2 ground state were obtained by two color excitation via the 700 nm A 3Πu←X 3Σ−g system. The observation of transitions from the A 3Π2u state to two spin–orbit components with Ω=1/2 and Ω=3/2, confirms that the cation ground state has 4Σg− symmetry, in accordance with previous experimental and theoretical work. Striking differences were observed in the rotational selection rules for the 4Σg−←A 3Π1u and the 4Σg−←A 3Π2u transitions. The adiabatic ionization potential of V2 was determined to be 51 271.14(50) cm−1. From an analysis of the rotational structure of the PFI-ZEKE spectra, the following molecular constants were determined for the 4Σg− state: r0=1.7347(24) Å, second order spin–orbit splitting, λ=5.248(17) cm−1, spin–rotation constant, γ=0.0097(87) cm−1, T0=51 282.20(50) cm−1 (1σ error bounds).
Thermodynamic properties of mixtures of dipolar and quadrupolar hard spheres: Theory and simulation
Authors: Jiang Shaoyi , Pitzer Kenneth S.
Published: July 2002

We report a comprehensive study of the thermodynamic properties (the Helmholtz energy, configurational energy, compressibility factor, and chemical potentials) for mixtures of dipolar hard spheres and quadrupolar hard spheres by computer simulation and from perturbation theory based on the Padé approximation. The dipole moment of dipolar component is fixed at μA*2=3.0 and the quadrupole moment of quadrupolar component is varied up to QB*2=3.0. The results show that the Padé approximation gives a good description of the Helmholtz energy and configuration energy of the mixtures, but some discrepancies are found for their derivatives, especially for compressibility factor. In view of very large deviations for the system from simple mixtures, the Padé approximation is rather successful for the system. These results are being used as a reference system in our effort to develop an equation of state for the CaCl2–H2O system.
Investigation of the ArN+2 ion by dissociative ionization of argon/nitrogen clusters
Authors: Mähnert Joachim , Baumgärtel Helmut , Weitzel Karl-Michael
Published: July 2002

The ArN+2 ion has been investigated by means of photoionization of an argon/nitrogen cluster beam in a threshold photoelectron photoion coincidence experiment. Two pathways for the formation of ArN+2 ions have been observed: (i) the nondissociative ionization of ArN2 neutrals and (ii) the dissociative ionization of Ar2N2. The two pathways are distinguished by the kinetic energy released (KER) in the dissociative ionization. The KER for the reaction Ar2N+2→ArN+2+Ar has been measured as a function of the excitation energy. The comparison of the measured KER with the statistically expected KER allows us to extrapolate to the thermochemical threshold of the reaction under investigation. A consistent picture is obtained under two assumptions: (i) the ArN+2 ion is linear and (ii) the ionization potential of ArN2 is 14.486±0.05 eV. The former assumption is confirmed by high level ab initio calculations (QCISD/6-311G*).
Evidence for stepwise dissociation dynamics in acetone at 248 and 193 nm
Authors: North Simon W. , Blank David A. , Gezelter J. Daniel , Longfellow Cheryl A. , Lee Yuan T.
Published: July 2002

The technique of molecular beam photofragment translational spectroscopy has been used to study the dissociation of acetone following S1←S0 (248 nm) and S2←S0 (193 nm) excitation. Excitation at 248 nm resulted in the production of CH3 and CH3CO with 14.2±1.0 kcal/mole on average of the available energy appearing as translation of the photofragments. Comparison of the measured 〈ET〉 with values reported at 266 nm suggest that the energy partitioning is dominated by the exit barrier caused by an avoided crossing on the potential energy surface. A substantial fraction (30±4%) of the nascent acetyl radicals from the primary dissociation contain sufficient energy to undergo spontaneous secondary decomposition. From the onset of the truncation of the CH3CO P(ET) a threshold of 17.8±3.0 kcal/mole for the dissociation of the acetyl radical has been determined in agreement with recent results on the photodissociation of acetyl chloride. The translational energy release in the dissociation of CH3CO closely matches the experimentally determined exit barrier. At 193 nm the only observed dissociation pathway was the formation of two methyl radicals and carbon monoxide. On average ∼38% of the available energy is found in product translation suggesting that significant internal energy resides in the nascent CH3 fragments consistent with the results of Hall et al. [J. Chem. Phys. 94, 4182 (1991)]. We conclude that the dynamics and energy partitioning for dissociation at 193 nm is similar to that at 248 nm.
Relativistic configuration interaction study of the low-lying electronic states of Bi2
Authors: Das Kalyan K. , Liebermann Heinz-Peter , Buenker Robert J. , Hirsch Gerhard
Published: July 2002

Relativistic effective core potentials have been employed in the framework of a spin–orbit configuration interaction treatment to compute potential curves, spectroscopic constants, and transition probabilities between pairs of vibrational states of the Bi2 molecule. The calculations find a steady increase in bond length for the lowest four states as a result of successive π→π* excitations en route from the X0+g ground state to the doubly excited 5Σ+g0+g, in good agreement with measured data. The corresponding 1g state with a Te value near 12 000 cm−1 has not yet been located experimentally. The next most stable λ–s state is found to be 3Δu, with Ω components increasing in energy in the order, 2u<3u<1u, of which only the latter has electric dipole-allowed transitions to X0+g. It is argued that the 1u species should be identified with the observed b state instead of the 3u component, especially since its calculated energy splitting relative to a2u is in much better agreement with the observed b−a separation than is the 3u−2u value. The radiative lifetime of the A0+u state is calculated to be 72 μs, whose result indicates that a previous experimental determination of this quantity in the presence of argon vapor needs to be reevaluated. In general it appears that the present computed Te values are accurate to within 500–1500 cm−1 of corresponding observed results, and that bond lengths are overestimated by 0.1 Å because of both deficiencies in the RECP employed as well as the failure to include the bismuth 5d electrons in the CI active space.
Computer simulation algorithms for molecules undergoing planar Couette flow: A nonequilibrium molecular dynamics study
Authors: Travis Karl P. , Daivis Peter J. , Evans Denis J.
Published: July 2002

Results from extensive nonequilibrium molecular dynamics (NEMD) simulations are presented for liquid chlorine subject to planar Couette flow. Comparisons are made between the so-called atomic and molecular SLLOD algorithms [R. Edberg, G. P. Morriss, and D. J. Evans, J. Chem. Phys. 86, 4555 (1987)] with atomic and molecular thermostats, respectively. These two thermostats differ in the assumptions that are made regarding the streaming velocity. Both thermostats are responsible for the production of string phases characterized by a translational ordering at very high strain rates. In addition, the atomic thermostat is responsible for the existence of a nonvanishing antisymmetric stress and enhanced orientational ordering.
Rotational diffusion of rose bengal
Authors: Srivastava Arvind , Doraiswamy S.
Published: July 2002

Picosecond time resolved fluorescence depolarization spectroscopy has been used to measure the rotational reorientation times of rose bengal as a function of viscosity. The variation of viscosity has been effected in two different ways—using different solvents and different compositions of aqueous binary mixtures. While the Stokes–Einstein–Debye (SED) hydrodynamics theory is found to be reasonably satisfactory to explain the rotational diffusion of rose bengal in amides and aprotic solvents, the dielectric friction model provides a better appreciation of its motion in normal alcohols. It is possible to mimic the nonhydrodynamic behaviour of rose bengal in tertiary butanol–water (t-BuOH+H2O) mixture by incorporating the contribution due to dielectric friction, although the solvation effects appear to be important in the t-BuOH-rich zone of the binary mixture. In hexamethylphosphoramide–water (HMPA+H2O) binary mixture the looplike profile of the rotational reorientation time (τr) vs viscosity (η) can be understood only on the basis of significant solvation effects. The variation in the boundary condition as an alternate explanation for understanding the rotational dynamics in binary mixtures was not found to be satisfactory.
Collision-induced desorption of ammonia chemisorbed on Pt{111}: From direct measurement of the threshold energy to determination of the surface–adsorbate bond strength
Authors: Szulczewski Gregory , Levis Robert J.
Published: July 2002

We report the desorption of a chemisorbed polyatomic adsorbate from a transition metal surface by a beam of neutral, energetic Ar atoms. From direct measurement of the threshold energy for collision-induced desorption we calculate the surface–adsorbate bond energy. The absolute cross sections for NH3 desorption at one-quarter of a monolayer coverage were measured for Ar beam energies up to ∼3 eV at incident angles of 0°, 30°, 45°, and 60°. For the NH3/Pt{111} adsorbate–surface system, the threshold desorption energy is found to be 1.95±0.17 eV. Using a classical energy transfer mechanism this threshold energy corresponds to a bond energy of 1.1±0.1 eV using an effective mass of 1 Pt atom. The threshold desorption energy scales with the total energy of the noble gas atoms for each angle of incidence. This result is consistent with a strong lateral corrugation in the NH3/Pt{111} potential energy surface and a similar ejection mechanism at each angle.
Anomalous effects of weak chemisorption on desorption kinetics of alkenes: The desorption of propylene and propane from Ag(110)
Authors: Pawela-Crew Jacquelyn , Madix Robert J.
Published: July 2002

Temperature programmed desorption (TPD), using x-ray photoelectron spectroscopy for absolute coverage determinations, was used to compare the desorption kinetics of weakly bound propane and propylene from Ag(110). The dependence of the activation energy on coverage was quantified by a linear relationship obtained from a leading edge analysis of the TPD curves. Whereas propane shows very weak attractive lateral interactions, propylene shows clear evidence for repulsive interactions in desorption. Weak attractive interactions are expected for both the propane and propylene based on theories of physical adsorption, since the second virial coefficients for both in the gas phase are negative below 550 K. Adsorption of propylene introduces repulsive intermolecular forces that are not present in the gas phase. We suggest that these repulsive forces originate in local interactions resulting from weak chemical bonding interactions between the surface and adsorbed propylene, which give rise to a preferred orientation of the double bond perpendicular to the surface. Fourier transform infrared spectroscopy results indicate that two adsorption states of propylene exist on the surface simultaneously. In the high coverage region the increase in repulsive interactions gives rise to a change in the dominant binding configuration as crowding increases.
Nucleation on a solid substrate: A density functional approach
Authors: Talanquer V. , Oxtoby David W.
Published: July 2002

We extend the density functional approach to the statistical mechanics of inhomogeneous fluids to calculate the rate of heterogeneous nucleation of the gas-to-liquid transition by a planar solid substrate. Comparison with classical nucleation theory (extended to incorporate the line tension that results from three-phase contact) reveals the inadequacy of the latter approaches as the spinodal is approached. Wetting and drying transitions have a large effect on the usefulness of classical theory. Free energies of formation for critical heterogeneous nuclei and their shapes and density profiles are calculated from density functional theory.
The concentrations of electrolytes in charged cylindrical pores: The hydrostatic hypernetted chain/mean spherical approximation
Authors: Zaini Parisa , Modarress Hamid , Mansoori G. Ali
Published: July 2002

The zeroth order (hydrostatic) approximation for inhomogeneous system is applied in the hypernetted chain/mean spherical (HNC/MSA) equations for charged cylindrical pores. The derived equations are introduced as hydrostatic hypernetted chain/mean spherical approximation (HHNC/MSA). These equations are solved using the collocation version of the finite element method. Equilibrium density profiles and mean electrostatic potential profiles are presented and compared with the results of HNC/MSA equations. Density profiles and Exclusion coefficient profiles for 1:1 and 1:2 electrolytes are also compared with the grand canonical Monte Carlo (GCMC) data. Good agreement between the present calculations and GCMC data are observed. Quantitative differences between the present calculations and HNC/MSA are found which are especially significant for large pore diameters and high electrolyte concentrations.
Solvation dynamics study of 4-amino-N-methyl-phthalimide in n-alcohol solutions
Authors: Laitinen Eira , Salonen Keijo , Harju Timo
Published: July 2002

We have studied the solvation dynamics of 4-amino-N-methyl-phthalimide (4-ANMP) in n-butanol and n-decanol solutions. Solvation time correlation functions were evaluated from wavelength-dependent fluorescence lifetime distributions at two temperatures for the two solvents. Solvation dynamics of 4-ANMP was strongly temperature dependent in n-butanol, which is a more polar and less viscous solvent than n-decanol. A smaller effect was observed in n-decanol solution. We also measured the rotational correlation times of 4-ANMP in the same solutions and resolved two components indicative of a nonspherical structure of the molecule. The solvation times with relation to the dielectric relaxation times τL and τD for linear alcohols are discussed.
Vibronic excitations of large molecules in solution studied by two-color pump–probe experiments on the 20 fs time scale
Authors: Ashworth S. H. , Hasche T. , Woerner M. , Riedle E. , Elsaesser T.
Published: July 2002

The ultrafast vibronic response of organic dye molecules in solution is studied in pump–probe experiments with 30 fs excitation pulses resonant to S0–Sn transitions. The molecular dynamics is probed either by pulses at the same spectral position or by 20 fs pulses overlapping with both the S0–S1 absorption and emission bands. Three contributions on distinctly different time scales are observed in the temporally and spectrally resolved two-color measurements. In the regime below 50 fs, a strong coherent coupling of the S0–Sn and the S0–S1 transitions occurs that is due to coherent vibrational motions in the electronic ground state. This signal is superimposed on the fast bleaching of the electronic ground state, resulting in a steplike increase of transmission. In the range of the S0–S1 emission band, one finds a subsequent picosecond rise of transmission that is due to stimulated emission from vibronic S1 states. The data demonstrate that the relaxation of Sn states directly populated by the pump pulses is much faster than the buildup of stimulated emission. This gives insight into different steps of intramolecular vibronic redistribution and is compared to the Sn–S1 relaxation in other molecules.
Heat capacity anomaly near the lower critical consolute point of triethylamine–water
Authors: Flewelling Anne C. , DeFonseka Rohan J. , Khaleeli Nikfar , Partee J. , Jacobs D. T.
Published: July 2002

The heat capacity of the binary liquid mixture triethylamine–water has been measured near its lower critical consolute point using a scanning, adiabatic calorimeter. Two data runs are analyzed to provide heat capacity and enthalpy data that are fitted by equations with background terms and a critical term that includes correction to scaling. The critical exponent α was determined to be 0.107±0.006, consistent with theoretical predictions. When α was fixed at 0.11 to determine various amplitudes consistently, our values of A + and A− agreed with a previous heat capacity measurement, but the value of A+ was inconsistent with values determined by density or refractive index measurements. While our value for the amplitude ratio A+/A −=0.56±0.02 was consistent with other recent experimental determinations in binary liquid mixtures, it was slightly larger than either theoretical predictions or recent experimental values in liquid-vapor systems. The correction to scaling amplitude ratio D+/D −=0.5±0.1 was half of that predicted. As a result of several more precise theoretical calculations and experimental determinations, the two-scale-factor universality ratio X, which we found to be 0.019±0.003, now is consistent among experiments and theories. A new ‘‘universal’’ amplitude ratio RBcr± involving the amplitudes for the specific heat was tested. Our determination of RBcr+=−0.5±0.1 and R Bcr−=−1.1±0.1 is smaller in magnitude than predicted and is the first such determination in a binary fluid mixture.
Excited state properties of Cr3+ in Cs2NaYCl6 and Cs2NaYBr6: A density functional study
Authors: Gilardoni François , Weber Jacques , Bellafrouh Khalid , Daul Claude , Güdel Hans U.
Published: July 2002

The ground and excited state properties of the Cr3+ ion doped into the cubic host lattices Cs2NaYCl6 and Cs2NaYBr6 have been studied using density functional theory. A new symmetry based technique was employed to calculate the energies of the multiplets 4A2g, 4T2g, 2Eg, and 4T1g. The geometry of the CrX3−6 cluster was optimized in the ground and excited states. A Madelung correction was introduced to take account of the electrostatic effects of the lattice. The experimental Cr–X distance in the ground state can be reproduced to within 0.01 Å for both chloride and bromide systems. The calculated d–d excitation energies are typically 2000–3000 cm−1 too low. An energy lowering is obtained in the first 4T2g excited state when the octahedral symmetry of CrX3−6 is relaxed along the eg Jahn–Teller coordinate. The geometry corresponding to the energy minimum is in excellent agreement with the 4T2g geometry derived from high-resolution optical spectroscopy of Cs2NaYCl6:Cr3+. It corresponds to an axially compressed and equatorially elongated CrX3−6 octahedron.