bluuebunny/crossref_metadata_2025 huggingface dataset:
The one-atom cage effect in I2(B)–Ar: Evidence that caging is inefficient for the T-shaped isomer
Authors: Burroughs Amy , Van Marter Todd , Heaven Michael C.
Published: July 2002

Caged recombination is observed when the binary I2–Ar complex is excited to energies above the B state dissociation limit. The recombination mechanism has been the subject of several previous investigations. Two alternative models have been proposed; Nonadiabatic recombination in a T-shaped complex, or adiabatic recombination in linear I2–Ar. In the present study, fluorescence depletion measurements were used to test these models. Fluorescence resulting from caged recombination could not be depleted using the well-known resonances of T-shaped I2–Ar. Instead, depletion via a diffuse band system previously assigned to the linear I2–Ar was observed. These results are consistent with the adiabatic recombination model. Geminate recombination of I2 in a He supersonic expansion is also reported.
Rotational state-to-state rate constants and pressure broadening coefficients for He–C2H2 collisions: Theory and experiment
Authors: Heijmen Tino G. A. , Moszynski Robert , Wormer Paul E. S. , van der Avoird Ad , Rudert Armin D. , Halpern Joshua B. , Martin José , Gao Wen Bin , Zacharias Helmut
Published: July 2002

Converged close-coupling and coupled-states calculations were used to obtain state-to-state rate constants and pressure broadening coefficients for the collisional rotational (de-)excitation of C2H2 by He. The ab initio potential used in these calculations was previously computed by symmetry-adapted perturbation theory. The computed pressure broadening coefficients and total rate constants agree well with the available experimental data. In the experimental part of the paper stimulated Raman-pumping has been used to prepare acetylene in selected rotational states (ji=2 to 18 and ji=1 to 19 of the C≡C stretching mode). The population decay in the prepared state and the transfer to other rotational states was monitored by laser induced fluorescence. The experimental data can be described by an infinite-order-sudden power law (IOS-P) or directly compared with the ab initio derived rate constants. The influence of multiple collisions possible at the relatively large pressure-delay-products employed has been taken into account by simulating the rotational energy transfer with a master equation. Experimentally we obtain a total rate constant for depopulation of ktot=10.89±0.07μs−1 Torr−1 and IOS-P fitting parameters of A=5.58 and γ=0.96 in very good agreement with the ab initio calculated values. There we obtain ktot=10.69±0.09μs−1 Torr−1 and IOS-P fitting parameters of A=6.18 and γ=0.96. Also the experimental state-to-state rotational energy transfer constants, which vary from 5.83 to 2.32 μs−1 Torr−1 for endothermic Δj=2 collisions, agree very well with the ab initio data.
Polymer collapse in supercritical solvents
Authors: Dua Arti , Cherayil Binny J.
Published: July 2002

We show analytically that in dilute solutions of high molecular weight polymers, a collapse transition of the chain can be induced by proximity to the critical point of the solvent. The transition is driven by the fluctuations in the medium, which lead to an effective attractive interaction of long range between different parts of the polymer. At the critical point itself, however, the chain adopts the same average conformations that characterize its size in the off-critical limit. In other words, on approach to the critical point, the polymer is found first to contract and collapse, and then subsequently to return to its original dimensions. This behavior has recently been observed in simulations of polymer-solvent mixtures near the lower critical solution temperature of the system, and it is also known to be characteristic of solutions of polymers in bicomponent solvent mixtures near the critical consolute point of the two solvents.
State selected unimolecular dissociation of HOCl near threshold: The 6νOH vibrational state
Authors: Dutton Gregory , Barnes Rhett James , Sinha Amitabha
Published: July 2002

The spectroscopy and unimolecular dissociation dynamics of HOCl are examined by accessing rotational resonances of the 6νOH vibrational level over the Ka=0–5 manifolds using overtone–overtone double resonance. The spectroscopic analysis indicates that state mixing between the zeroth-order “bright” O–H stretching overtone state, 6 0 0, and “dark” background vibrational levels is incomplete as the bright state couples to only a fraction of the available states. The coupling of 6 0 0 to a set of nearby dark states is mediated primarily by anharmonic coupling with the fourth-order vibrational resonance k1,223 playing a particularly important role through its ability to couple the 6 0 0 state directly to the 5 2 1 vibration and indirectly to the 4 4 2 vibration. The measured state-specific unimolecular dissociation rates for 6 0 0 show large fluctuations with J and Ka and are substantially slower than that expected on the basis of statistical theory. The rate fluctuations are interpreted on the basis of spectroscopic data which suggest that the fluctuations arise as a result of variation in state mixing as different dark vibrational states come in and out of resonance with the bright state for different values of J and Ka.
Real time quantum correlation functions. I. Centroid molecular dynamics of anharmonic systems
Authors: Krilov Goran , Berne B. J.
Published: July 2002

We investigate the accuracy of the recently proposed centroid molecular dynamics (CMD) method [J. Cao and G. A. Voth, J. Chem. Phys. 100, 5106 (1994)] in the presence of highly anharmonic steep short range repulsive potentials. Such potentials are often present in condensed phases and govern collisions between solvent particles. We compare the results of CMD simulations with exact quantum results for several model one- and two-dimensional nondissipative systems and a one-dimensional system under isobaric conditions. We show that, for nondissipative systems, CMD is accurate only for very short times, and is unable to reproduce the effects of quantum coherences, which play an important role in these few-dimensional systems. CMD gives much better results under isobaric conditions. The correlation functions and the general lineshape of the absorption cross-section in the dipole limit are well reproduced. This is primarily due to dephasing of quantum coherences through inhomogeneous broadening.
Reaction dynamics of O(1D)+HD. I. The insertion pathway
Authors: Hsu Yen-Tsung , Liu Kopin , Pederson Lisa A. , Schatz George C.
Published: July 2002

The doubly differential cross sections (angle and speed) for the reactions O(1D)+HD→OH(OD)+D(H) at Ec=2.05 kcal/mol were directly mapped out using the Doppler-selected time-of-flight technique in a crossed-beam experiment. Except for the previously noted discrepancy in the H/D isotope branching ratio, good agreement with QCT (quasiclassical trajectory) results on a high quality ab initio surface are found for all dynamical features, including the product angular and translational energy distributions and the very detailed angle-specific translational energy release.
Extraction of potentials and dynamics from condensed phase pump–probe spectra: Application to I2 in Kr matrices
Authors: Bargheer M. , Dietrich P. , Donovang K. , Schwentner N.
Published: July 2002

The many-body dynamics in the condensed phase often leads to structureless absorption spectra while vibrational recurrences in localized coordinates are preserved. A scheme is presented to directly extract anharmonicities and energy relaxation from the varying vibrational periods in pump–probe spectra. For the B state of I2 a constant anharmonicity and a vibrational energy relaxation rate decreasing by one order of magnitude in the wavelength range from 540 to 583 nm are obtained. An analytical expression for compensating the wave packet dispersion due to the anharmonicity is derived and it is applied to the control of wave packets by chirped pulses. Focusing of the wave packet is experimentally observed in accordance with the anharmonicity and even after significant energy relaxation. Predissociation rates are determined from the envelopes of the pump–probe spectra.
Assignment of the infrared spectra of the methanol trimer
Authors: Tschumper Gregory S. , Gonzales Jason M. , Schaefer Henry F.
Published: July 2002

Second-order Möller-Plesset perturbation theory (MP2) and density functional theory (DFT) were employed to optimize two cyclic conformations of the methanol trimer, denoted “bowl” and “chair,” as well as a third noncyclic, “open-chain” conformation. This is the first time all three methanol trimer conformations have been studied at the same level of theory. Harmonic vibrational frequencies confirm that all stationary points are minima on the intermolecular potential energy hypersurface. The chair is consistently predicted to be the most stable conformer. The bowl and open-chain conformers are only slightly less stable and lie within 3 kcal/mol. Results indicate that the bowl structure contains a threefold rotational axis of symmetry which resolves a discrepancy over its symmetry in previous theoretical investigations. Further, this finding precludes the possibility that only the cyclic conformers of (CH3OH)3 are the source of the five bands observed in the OH stretching spectrum of the methanol trimer recently obtained by infrared cavity ringdown laser absorption spectroscopy. From the ab initio frequencies, it is concluded that the bowl, chair, and open-chain conformations of (CH3OH)3 must be present in the supersonic expansion in order to account for all five observed bands.
Structure of poly(γ-benzyl-L-glutamate) monolayers at the gas–water interface: A Brewster angle microscopy and x-ray scattering study
Authors: Fukuto Masafumi , Heilmann Ralf K. , Pershan Peter S. , Yu Seungju M. , Griffiths Jennifer A. , Tirrell David A.
Published: July 2002

This paper reports Brewster angle microscopy (BAM), x-ray specular reflectivity (XR), grazing incidence diffraction (GID) and off-specular diffuse scattering (XOSDS) measurements of Langmuir monolayers formed on water by both mono- and polydisperse samples of α-helical poly(γ-benzyl L-glutamate) (PBLG) as a function of area/monomer A. The microscopic behavior does not exhibit any discernible effects due to differing dispersity. At low surface densities (A>∼21 Å2/monomer, surface pressure Π=0), BAM images reveal partial surface coverage by solidlike monolayer islands. GID measurements show an interhelix peak corresponding to a local parallel alignment of rodlike PBLG molecules, indicating their tendency to aggregate laterally without external pressure. Compression to A<21 Å2/monomer first leads to full and uniform surface coverage by the monolayer, followed by a steep rise in Π that is accompanied by a decrease in the interhelix distance. Further compression results in a plateau of constant Π in the Π-A isotherm (∼11.5<A<∼18.5 Å2/monomer, Π∼9 dyn/cm), which has previously been attributed to a first-order monolayer–bilayer transition. The interfacial electron density profiles determined by the XR measurements on both sides of the coexistence plateau provide direct evidence for this transition. On the basis of x-ray scattering results, the film on the high-density side of the plateau is shown to consist of a newly formed incomplete and incommensurate second layer that sits on top of and has lower average density than a homogeneous first layer. GID measurements indicate that the second layer can be characterized by larger interhelix d-spacing than the first layer, while XOSDS measurements on the bilayer suggest that the second layer is microscopically inhomogeneous. For both mono- and bilayers, the analysis of observed GID peak widths indicates that the extent of lateral positional correlations between parallel PBLG rods ranges from a few to no more than ∼15 interhelix distances, implying short-range order.
Kinetics of the condensation of Cs from its supersaturated vapor: Comments on the entropy of a small Cs cluster
Authors: Bauer S. H. , Zhang Yi-Xue , Wilcox C. F.
Published: July 2002

The recently published measurements of temperature-dependent critical supersaturation levels for avalanche condensations of Cs vapor are interpreted via the recently revised kinetic molecular model that incorporates accretions with dimers as well as monomers. The kinetic parameters required to reproduce the reported condensation rate of, approximately, unit cluster per cm3 s permits estimations of the entropies of clusters. Therefrom the mean intermolecular (bulk) frequency derived for Cs13 (466 K) fits well with a model proposed by Hoare for Xe13.
Transient instability upon temperature quench in weakly ordered block copolymers
Authors: Qi Shuyan , Wang Zhen-Gang
Published: July 2002

We report a novel transient instability upon temperature quench in weakly ordered block copolymer microphases possessing a soft direction or directions, such as the lamellar and hexagonal cylinder (HEX) phases. We show that reequilibration of the order parameter is accompanied by transient long wavelength undulation of the layers or cylinders—with an initial wavelength that depends on the depth of the temperature quench—that eventually disappears as the structure reaches its equilibrium at the new temperature. Such undulation leads to a transient transverse broadening of the scattering peaks near the Bragg positions. We argue that this instability might be responsible for the experimentally observed unusual ordering dynamics of the HEX phase of a diblock copolymer after quenching from the disordered state.
A simple semiclassical approach to the Kramers’ problem
Authors: Chaudhuri Jyotipratim Ray , Bag Bidhan Chandra , Ray Deb Shankar
Published: July 2002

We show that the Wigner–Leggett–Caldeira equation for Wigner phase space distribution function which describes the quantum Brownian motion of a particle in a force field in a high temperature, ohmic environment can be identified as a semiclassical version of Kramers’ equation. Based on an expansion in powers of ℏ, we solve this equation to calculate the semiclassical correction to Kramers’ rate.
One- and two-body densities for the beryllium isoelectronic series
Authors: Gálvez F. J. , Buendı́a E. , Sarsa A.
Published: July 2002

One- and two-body densities in position space have been calculated for the atomic beryllium isoelectronic series starting from explicitly correlated multideterminant wave functions. The effects of electronic correlations have been systematically studied by comparing the correlated results with the corresponding Hartree–Fock ones. Some expectation values such as 〈δ(r)〉, 〈rn〉, 〈δ(r12)〉, 〈r12n〉, 〈δ(R)〉, and 〈Rn〉, where r, r12, and R stand for the electron–nucleus, interelectronic, and two electron center of mass coordinates, respectively, have been obtained. All the calculations have been carried out by using the Monte Carlo algorithm.
Optimal orbitals from energy fluctuations in correlated wave functions
Authors: Filippi Claudia , Fahy Stephen
Published: July 2002

A quantum Monte Carlo method of determining Jastrow–Slater and correlated multideterminant wave functions for which the energy is stationary with respect to variations in the single-particle orbitals is presented. A potential is determined by a least-squares fitting of fluctuations in the energy with a linear combination of one-body operators. This potential is used in a self-consistent scheme for the orbitals whose solution ensures that the energy of the correlated wave function is stationary with respect to variations in the orbitals. The method is feasible for atoms, molecules, and solids and is demonstrated for the beryllium, carbon, and neon atoms and for the solid diamond.
Phase transformations and electronic properties in mixed-metal oxides: Experimental and theoretical studies on the behavior of NiMoO4 and MgMoO4
Authors: Rodriguez José A. , Hanson Jonathan C. , Chaturvedi Sanjay , Maiti Amitesh , Brito Joaquin L.
Published: July 2002

Mixed-metal oxides play a relevant role in many areas of chemistry, physics, and materials science. We have examined the structural and electronic properties of NiMoO4 and MgMoO4 by means of synchrotron-based time-resolved x-ray diffraction (XRD), x-ray absorption near-edge spectroscopy (XANES), and first-principles density functional theory (DFT) calculations. Nickel molybdate can exist in two phases (α and β). Mo is in a near tetrahedral environment in the β-phase, whereas in the α-phase the metal exhibits a pseudo-octahedral coordination with two very long Mo–O distances (2.3–2.4 Å). The results of DFT calculations indicate that the α-phase of NiMoO4 is ∼9 kcal/mol more stable than the β-phase. On the other hand, in the case of magnesium molybdate, an α-NiMoO4-type phase is ∼13 kcal/mol less stable than β-MgMoO4. These trends in stability probably result from variations in the metal–metal repulsion within the α-phases of the compounds. For the α→β transition in NiMoO4, the DFT calculations predict an energy barrier of ∼50 kcal/mol. An apparent activation energy of ∼80 kcal/mol can be derived from the time-resolved XRD experiments. The degree of ionicity in MgMoO4 is larger than that in NiMoO4. The nickel molybdate displays a large density of states near the top of the valence band that is not observed in the magnesium molybdate. This makes NiMoO4 more chemically active than MgMoO4. A similar type of correlation is found between the electronic and chemical properties of NiMoO4, CoMoO4, and FeMoO4. The DFT results and Mo LII-edge XANES spectra show big differences in the splitting of the Mo 4d orbitals in the α- and β-phases of the molybdates. The line shape in the O K-edge essentially reflects the behavior seen for the 4d orbitals in the Mo LII-edge (i.e., mainly O 1s→Mo 4d electronic transitions). The Mo LII- and O K-edges in XANES can be very useful for probing the local symmetry of Mo atoms in mixed-metal oxides.
Laser spectroscopy of jet-cooled ethyl radical: Infrared studies in the CH2 stretch manifold
Authors: Davis Scott , Uy Dairene , Nesbitt David J.
Published: July 2002

A glow discharge, slit supersonic expansion in conjunction with direct infrared laser absorption methods has been utilized to record high resolution vibration–rotation spectra of the CH3–CH2 ethyl radical. The slit supersonic expansion results in efficient rotational cooling from discharge temperatures down to Trot≈14 K, permitting unambiguous rotational assignment and spectral analysis for the first time. Furthermore, a discharge on/discharge off data collection scheme permits clean discrimination between spectral contributions from radical vs precursor absorption. Spectra for both symmetric and asymmetric CH2 stretch manifolds are observed. Least-squares fits of transition frequencies out of the K=0 ground state manifold to a near prolate top model Hamiltonian reproduce the data to within the 7 MHz experimental uncertainty and provides rotational constants for both ground and vibrationally excited symmetric/asymmetric CH2 stretch states. The band origins for the CH2 stretch vibrations [3037.018 96(12) cm−1 and 3128.693 69(13) cm−1] are in reasonable agreement with ab initio theory; though predictions for relative intensities of the two bands are off by nearly an order of magnitude and indicate that the transition moment vector is tilted 33° away from each C–H bond toward the C–C bond axis. Structural analysis based on the measured B and C rotational constants imply a C–C bond distance of 1.49 Å. This is consistent with partial (≈15%) double bond character for the ethyl radical carbon frame and in excellent agreement with theoretical predictions.
Coupling a polarizable water model to the hydrated ion–water interaction potential: A test on the Cr3+ hydration
Authors: Martı́nez José M. , Hernández-Cobos Jorge , Saint-Martin Humberto , Pappalardo Rafael R. , Ortega-Blake Ivan , Marcos Enrique Sánchez
Published: July 2002

A strategy to build interaction potentials for describing ionic hydration of highly charged monoatomic cations by computer simulations, including the polarizable character of the solvent, is proposed. The method is based on the hydrated ion concept that has been previously tested for the case of Cr3+ aqueous solutions [J. Phys. Chem. 100, 11748 (1996)]. In the present work, the interaction potential of [Cr(H2O6)]3+ with water has been adapted to a water model that accounts for the polarizable character of the solvent by means of a mobile charge harmonic oscillator representation (MCHO model) [J. Chem. Phys. 93, 6448 (1990)]. Monte Carlo simulations of the Cr3+ hexahydrate plus 512 water molecules have been performed to study the energetics and structure of the ionic solution. The results show a significant improvement in the estimate of the hydration enthalpy [ΔHhydr(Cr3+)=−1109.6±70 kcal/mol] that now matches the experimental value within the uncertainty of this magnitude. The use of the polarizable water model lowers by ∼140 kcal/mol the statistical estimation of the [Cr(H2O6)]3+ hydration enthalpy compared to the nonpolarizable model. (−573 kcal/mol for the polarizable model vs −714 kcal/mol for the nonpolarizable one.) This improvement reflects a more accurate treatment of the many-body nonadditive effects.
Theoretical electronic absorption and natural circular dichroism spectra of (−)-trans-cyclooctene
Authors: Pedersen Thomas Bondo , Koch Henrik
Published: July 2002

Using the random phase approximation and coupled cluster singles and doubles linear response theory in conjunction with two basis sets of augmented double-zeta quality and two nuclear geometries, we have calculated electronic absorption and natural circular dichroism spectra of (−)-trans-cyclooctene. We present a density functional theory optimized nuclear geometry whose ground state electric dipole moment and harmonic vibrational spectrum compare well with experimental data. The coupled cluster results obtained with this nuclear geometry are in good agreement with experimental electronic spectra, although the original interpretation of the most intense low-lying band as a π→π* transition is contradicted.
A transition state real wave packet approach for obtaining the cumulative reaction probability
Authors: Forsythe Kelsey M. , Gray Stephen K.
Published: July 2002

We show how the transition state wave packet method of Zhang and Light can be applied within a real wave packet formalism. We also implement random superpositions into the approach, as in the recent work of Matzkies and Manthe, which can significantly reduce the number of propagations at higher temperatures. The net result is a very efficient approach for calculating the cumulative reaction probability, and hence the thermal rate constant, for bimolecular chemical reactions. Full dimensional quantum calculations, including all relevant total angular momenta, of the cumulative reaction probability and thermal rate constant for the D+H2→HD+H are used as illustration.
Photoinitiated H2CO unimolecular decomposition: Accessing H+HCO products via S and T1 pathways
Authors: Valachovic L. R. , Tuchler M. F. , Dulligan M. , Droz-Georget Th. , Zyrianov M. , Kolessov A. , Reisler H. , Wittig C.
Published: July 2002

The photoinitiated unimolecular decomposition of formaldehyde via the H+HCO radical channel has been examined at energies where the S0 and T1 pathways both participate. The barrierless S0 pathway has a loose transition state (which tightens somewhat with increasing energy), while the T1 pathway involves a barrier and therefore a tight transition state. The product state distributions which derive from the S0 and T1 pathways differ qualitatively, thereby providing a means of discerning the respective S0 and T1 contributions. Energies in excess of the H+HCO threshold have been examined throughout the range 1103⩽E†⩽2654 cm−1 by using two complementary experimental techniques; ion imaging and high-n Rydberg time-of-flight spectroscopy. It was found that S0 dominates at the low end of the energy range. Here, T1 participation is sporadic, presumably due to poor coupling between zeroth-order S1 levels and T1 reactive resonances. These T1 resonances have small decay widths because they lie below the T1 barrier. Alternatively, at the high end of the energy range, the T1 pathway dominates, though a modest S0 contribution is always present. The transition from S0 dominance to T1 dominance occurs over a broad energy range. The most reliable value for the T1 barrier (1920±210 cm−1) is given by the recent ab initio calculations of Yamaguchi et al. It lies near the center of the region where the transition from S0 dominance to T1 dominance takes place. Thus, the present results are consistent with the best theoretical calculations as well as the earlier study of Chuang et al., which bracketed the T1 barrier energy between 1020 and 2100 cm−1 above the H+HCO threshold. The main contribution of the present work is an experimental demonstration of the transition from S0 to T1 dominance, highlighting the sporadic nature of this competition.
Steady state response of the nonlinear dielectric relaxation and birefringence in strong superimposed ac and dc bias electric fields: Polar and polarizable molecules
Authors: Déjardin J. L. , Kalmykov Yu. P.
Published: July 2002

Both nonlinear dielectric relaxation and dynamic Kerr effect responses of an assembly of polar and anisotropically polarizable molecules acted on by strong superimposed external dc E0 and ac E1(t)=E1 cos ωt electric fields are evaluated in the context of the rotational diffusion model in the noninertial limit. The relaxation functions fn(t) (the expectation value of the Legendre polynomials Pn), which are appropriate to describe these nonlinear relaxation phenomena, are calculated by expanding them as a Fourier series in the time domain. An infinite hierarchy of recurrence relations for these Fourier amplitudes of fn(t) is obtained, the solution of which is expressed in terms of an infinite matrix continued fraction, so allowing us to evaluate the dynamic characteristics of the electric polarization and birefringence. For a weak ac field, the results predicted by the theory are in complete agreement with previous solutions obtained by perturbation methods. The solutions for the particular cases, where only either permanent or induced dipole moments are taken into account, can easily be extracted from the general solution. Diagrams of the frequency behavior of the in-phase and out-of-phase components of the electric birefringence and polarization are presented showing pronounced nonlinear effects due to the high ac field.
Rigidity constants from mean-field models
Authors: Oversteegen S. M. , Blokhuis E. M.
Published: July 2002

The interfacial tension of the planar interface and rigidity constants are determined for a simple liquid–vapor interface by means of a lattice-gas model. They are compared with results from the van der Waals model and from an analytical expansion around the critical point. The three approaches are in agreement in the regions where these theories apply.
Absorption spectra of dipolar Frenkel excitons in two-dimensional lattices with configurational disorder: Long-range interaction and motional narrowing effects
Authors: Domı́nguez-Adame F. , Malyshev V. A. , Rodrı́guez A.
Published: July 2002

We present results of numerical simulations of optical absorption line shape of Frenkel excitons in two-dimensional disordered lattices. Disorder is generated by Gaussian randomness in the molecular positions. The intersite interaction is considered to be of dipole origin, including coupling to far neighbors. Results of simulations are compared with those obtained in the frame of the nearest-neighbor approximation, showing remarkable differences in the absorption line shape. The motional narrowing effect is found to be essentially different from that previously reported for the case of diagonal disorder as well as for that produced by randomness in nearest-neighbor hopping integrals.
Protein dynamics at low temperatures
Authors: Schlichter J. , Friedrich J. , Herenyi L. , Fidy J.
Published: July 2002

The low temperature conformational dynamics of the heme type protein mesoporphyrin-IX-substituted horseradish peroxidase is investigated by spectral diffusion waiting time/aging experiments. Spectral diffusion broadening is governed by a power law in time. There is a small but significant aging effect. It is assumed that the conformational dynamics of the protein which leads to the spectral broadening of the burnt-in holes is governed by a diffusion type equation. In this case the shape of the spectral diffusion kernel is Gaussian. This model is contrasted with spectral diffusion phenomena as described by the TLS-model (TLS, two level system).
Hard sphere perturbation theory of dense fluids with singular perturbation
Authors: Mon K. K.
Published: July 2002

Hard sphere perturbation theories (HSPT) played a significant role in the fundamental understanding of fluids and continues to be a popular method in a wide range of applications. The possibility of difficulty with singular perturbation for some classical soft core model fluids appears to have been overlooked or ignored in the literature. We address this issue in this short note and show by analysis that a region of phase space has been neglected in the standard application of HSPT involving singular perturbation.
On the long-range and short-range behavior of potentials from reproducing kernel Hilbert space interpolation
Authors: Soldán Pavel , Hutson Jeremy M.
Published: July 2002

The short-range and long-range asymptotic behavior of potential functions obtained from the reciprocal power reproducing kernel Hilbert space interpolation procedure [Ho and Rabitz, J. Chem. Phys. 104, 2584 (1996)] is analyzed. In the short-range region, the potential functions are polynomial in form: the method should not be used for extrapolation in this region. General formulae for the short-range and long-range expansion coefficients are presented. Potentials for He-Ar+ are discussed as examples.
The slowly-varying noninteracting electron gas in terms of its kinetic energy density
Authors: Ernzerhof Matthias , Scuseria Gustavo E.
Published: July 2002

The noninteracting kinetic energy density τ(r)=12∑iocc|∇φi(r)|2, calculated from the Kohn–Sham orbitals φi, has recently been used to model the exchange energy (EX). The question arises whether it is in principle possible to obtain EX or, more general, all the properties of a noninteracting ground-state system exclusively in terms of τ(r). We produce evidence to counter this possibility by showing that a one-to-one correspondence between local potentials v(r), which differ by more than a constant, and τ(r) does not exist. Furthermore, we show that EX cannot be obtained exactly in terms of τ. However, in the slowly varying limit a one-to-one mapping between τ and v can be established. Thus we are able to derive the gradient expansion of the electron density and of EX in terms of τ.
Effect of small-scale architecture on polymer mobility
Authors: Luettmer-Strathmann Jutta
Published: July 2002

Processes on different length scales affect the dynamics of chain molecules. In this work, we focus on structures on the scale of a monomer and investigate polyolefins, i.e., hydrocarbon chains with different small-scale architectures. We present an exact enumeration scheme for the simulation of interactions and relative motion of two short chain sections on a lattice and employ it to deduce the probability for segmental motion for polymers of four different architectures in the melt. The probability for segmental motion is inversely proportional to the monomeric friction coefficient and hence the viscosity of a polymer. Combining our simulation results with an equation of state for the thermodynamic properties of the polymers, we are able to make predictions about the variation of the friction coefficient with temperature, pressure, and small-scale architecture. To compare our results with experimental data, we have determined monomeric friction coefficients from experimental viscosity data for the four polyolefins considered in this work. For temperatures well above the glass transition temperature, we find that our simple approach gives a good qualitative representation of the variation of the friction coefficient with chain architecture, temperature, and pressure.
Photodissociation spectroscopy of Ca+(C2H4)
Authors: Holmes J. H. , Kleiber P. D. , Olsgaard D. A. , Yang K.-H.
Published: July 2002

We have studied Ca+(C2H4) by photodissociation spectroscopy in a reflectron time-of-flight mass spectrometer over the spectral range 440–790 nm. Ca+ is the only photofragment observed. We find four absorption bands of the complex and assign them to metal-centered transitions correlating with excitation of Ca+(3d and 4p). Spectral assignment is supported by ab initio electronic structure calculations of the complex and isotope substitution experiments. Calculations find a weakly bound ground state equilibrium structure with C2V π-bonding geometry and a dissociation energy of De″=0.506 eV. Theoretical and experimental results show the 4pπ(2 2B2 & 2 2B1) excited states to be relatively weakly bound at long range. Spectral analysis gives vibrational constants for the Ca+--C2H4 intermolecular a1-stretch in the 1 2A1, 2 2B1, and 2 2B2 states, and for the CH2–CH2 a1-wag and the HCH a1-bend in 2 2B2. The results offer an interesting comparison with previous studies of similar weakly bound bimolecular complexes of light metal ions with alkene or alkane hydrocarbons.
Adsorption of water on the NaCl(001) surface. III. Monte Carlo simulations at ambient temperatures
Authors: Engkvist Ola , Stone Anthony J.
Published: July 2002

Adsorption of water on NaCl(001) is studied at room temperature using recently constructed intermolecular potentials. Coverages of 0.5 and 3.0 water molecules per NaCl were studied in molecular simulations. At low coverage water molecules cluster on the surface to form islands, while at higher coverage a layered structure appears. These results are in agreement with recent Fourier transform infrared spectroscopy measurements.
Time-resolved dissociative intense-laser field ionization for probing dynamics: Femtosecond photochemical ring opening of 1,3-cyclohexadiene
Authors: Fuß W. , Schmid W. E. , Trushin S. A.
Published: July 2002

The concerted photochemical ring opening of 1,3-cyclohexadiene was investigated in the gas phase by low-intensity pumping at 267 nm and subsequent probing by high-intensity photoionization at 800 nm and mass-selective detection of the ion yields. We found five different time constants which can be assigned to traveling times along consecutive parts of the potential energy surfaces. The molecule is first accelerated in the spectroscopic state 1B along Franck–Condon active coordinates, then alters direction before changing over to the dark state 2A. All constants including that for leaving the 2A surface are below 100 fs. These times are shorter than appropriate vibrational periods. Such a maximum speed is evidence that the pathway is continuous leading from surface to surface via real crossings (conical intersections) and that the molecule is accelerated right into the outlet of the 2A/1A funnel. On the ground state it arrives as a compact wave packet, indicating a certain degree of coherence. The experimental method promises a high potential for investigating dynamics, since many consecutive phases of the process can be detected. This is because the fragmentation pattern depends on the location on the potential energy surface, so that monitoring several different ions permits to conclude on the population flow through these locations. Ionization at the intensities used is normally considered to be an effect of the electric field of the radiation. But in our case it is enhanced by resonances in the neutral molecule and in particular in the singly positive ion, and it is not sensitive for the length of the molecule (different conformers of the product hexatriene). The ionic resonances explain why hexatriene has a much richer fragmentation pattern than cyclohexadiene. Coulomb explosion is observed from an excited state of a doubly positive ion. Its mechanism is discussed.
Two-dimensional vibrational spectroscopy. V. Novel 2-dimensional surface vibrational spectroscopies of adsorbed molecules on surfaces or at interfaces
Authors: Cho Minhaeng
Published: July 2002

Novel two-dimensional (2D) vibrational spectroscopies that are suitable for the investigation of the vibrational couplings of adsorbed molecules on surfaces or at interfaces are theoretically proposed. Depending on the sequence of the IR, Raman, and hyper-Raman transitions involved, there are seven distinctive nonlinear response functions associated with various 2D surface vibrational spectroscopies, which are either three- or five-wave-mixing processes. It is suggested that these novel techniques can be used to obtain information on the nonlinear coordinate dependencies of the dipole moment, polarizability, and hyperpolarizability as well as on the anharmonic couplings between any two vibrational degrees of freedom of adsorbed molecules.
On the hyperfine structure of NO2 levels near dissociation threshold
Authors: Xin Ju , Reid Scott A.
Published: July 2002

We report the first direct measurement of the hyperfine structure of single NO2 molecular eigenstates in the 2.53 cm−1 region below dissociation threshold (D0), using the method of polarization quantum beat spectroscopy (QBS). The hyperfine interaction of these loosely bound states is substantially weaker (on the average by a factor of ∼5) than found at energies below 22 000 cm−1, but is similar on the average to that observed 50–150 cm−1 below threshold. The implications of our results regarding the participation of additional electronic states near D0 are discussed.
Monte Carlo analysis of T1 pyrazine collisional vibrational relaxation: Evidence for supercollisions
Authors: Wu Fei , Weisman R. Bruce
Published: July 2002

The collisional loss of vibrational energy from polyatomic molecules in triplet electronic states has been studied in new detail through a variant of the competitive radiationless decay (CRD) method. Experimental transient absorption kinetics for T1 pyrazine vapor in the presence of helium relaxer reveals the competition between unimolecular radiationless decay and collisional vibrational relaxation. These data have been simulated with Monte Carlo stochastic calculations equivalent to full master equation solutions that model the distribution of donor vibrational energies during relaxation. The simulations included energy-dependent processes of T1→S0 radiationless decay, Tn←T1 optical absorption, and collisional energy loss. The simulation results confirm earlier findings of energy loss tendencies that increase strongly for pyrazine vibrational energies above ∼2000 cm−1. It is also found that the experimental data are not accurately simulated over a range of relaxer pressures if a simple exponential step-size distribution function is used to model collisional energy changes. Improved simulations are obtained by including an additional, low-probability channel representing large energy changes. This second channel would represent “supercollisions,” which have not previously been recognized in the vibrational relaxation of triplet state polyatomics.
Photodissociation spectroscopy of Mg+–C6H5X (X=H, F, Cl, Br)
Authors: Yang Xin , Gao Kelin , Liu Haichuan , Yang Shihe
Published: July 2002

Photodissociation of a series of complexes Mg+–C6H5X (X=H, F, Cl, Br) has been studied. The formation of Mg+ was found to be the predominant dissociation pathway. We monitored the photodissociation product as a function of the excitation wavelength in a broad spectral region. Experiments on the complexes with different halide substitutions showed similar action spectra. This is explained by a similar structure shared by these complexes with Mg+ being above the benzene ring. In the photodissociation of Mg+–(C6H5F), MgF+ is also formed besides Mg+. The action spectra corresponding to these two channels were found to be different, and the MgF+ channel is believed to be from a different isomer. The experimental results are consistent with our quantum ab initio calculations.
Probing the nature of surface intersection by ab initio calculations of the nonadiabatic coupling matrix elements: A conical intersection due to bending motion in C2H
Authors: Mebel Alexander M. , Baer Michael , Lin Sheng H.
Published: July 2002

Nonadiabatic coupling matrix elements between the 1 2A′, 2 2A′, and 1 2A″ electronic states of the C2H radical are computed using ab initio full valence active space CASSCF method. The line-integral technique is then applied to study possible geometric phase effects. The results indicate the existence of a unique conical intersection due to CCH bending between the 1 2A′ and 2 2A′ states at the linear configuration in the vicinity of rCC=1.35 Å and rCH=1.60 Å. The line-integral calculations with ab initio nonadiabatic coupling terms confirm that when a path encircles the conical intersection, the line integral always produces the value π for the topological (Berry) phase and when a path encircles the two (symmetrical) conical interactions or none of them, the line integral produces the value of zero for the topological phase.
Structural signature of the configurational entropy change during the glass formation of plastic crystal cyanoadamantane
Authors: Willart J. F. , Descamps M. , van Miltenburg J. C.
Published: July 2002

Calorimetric and x-ray diffraction experiments are combined to characterize the entrance of a cyanoadamantane plastic crystal into a glassy crystal state upon cooling. As usually observed for glass-forming liquids, the configurational entropy of this glassy crystal is shown to strongly decrease on approaching the glass transition. Taking advantage of the underlying lattice of this peculiar glass, we could identify the structural origin of this entropy change. It corresponds clearly to the development of an antiferroelectric short-range order which is the precursor of a transient metastable state recently described. The analysis of the results, in the framework of the Adam and Gibbs theory, provides a new picture of the cooperative processes expected in glasses.
Pendant group orientation of poly(2-vinylnaphthalene) thin film surface studied by near-edge x-ray absorption fine structure spectroscopy (NEXAFS) and angle-resolved ultraviolet photoelectron spectroscopy (ARUPS)
Authors: Morikawa E. , Saile V. , Okudaira K. K. , Azuma Y. , Meguro K. , Harada Y. , Seki K. , Hasegawa S. , Ueno N.
Published: July 2002

Angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) and near-edge x-ray absorption fine structure (NEXAFS) spectroscopy were applied to the investigation of the tilt angles of the naphthalene pendant groups at the surface of a poly(2-vinylnaphthalene) thin film. In contrast to NEXAFS, which provides only an average determination of the tilt angle, ARUPS combined with a sophisticated analysis of photoelectron angular dependence offers more detailed information. It was concluded that the naphthalene pendant groups are tilted randomly at the polymer surface, and that the tilt angle distribution is well described as a three-dimensional isotropic random orientation.
Purely theoretical electron-impact ionization cross-sections of silicon hydrides and silicon fluorides obtained from explicitly correlated methods
Authors: Malcolm N. O. J. , Yeager Danny L.
Published: July 2002

Electron impact total ionization cross-sections of small silicon hydrides, SiHn(n=1–4), and fluorides, SiFn(n=1–3), have been calculated by the application of a recently developed theoretical model. The binary-encounter-Bethe (BEB) model has a simple structure and requires information from calculations on the parent ground-state molecule only (binding energies, orbital kinetic energies, and occupation numbers). Previous applications of the BEB theory to the silicon hydrides and fluorides have employed a combination of experimental and Koopman’s theorem binding energies. In the current work binding energies have been calculated using the explicitly correlated multiconfigurational spin tensor electron propagator (MCSTEP) method which gives highly accurate ionization potentials for closed- and open-shell systems. Calculations have been performed using cc-pVDZ and cc-pVTZ basis sets with multiconfigurational self-consistent field (MCSCF) reference wave functions. Comparisons are made between our MCSCF/MCSTEP and previous Hartree–Fock (HF)/Koopman’s theorem results and available experimental data. The use of improved theoretical data does not have a significant effect on the resultant cross-sections; however, our new technique is a viable method for calculating electron impact ionization cross-sections for systems where Koopman’s theorem is known to be unreliable or no experimental data is available.
Krylov-space algorithms for time-dependent Hartree–Fock and density functional computations
Authors: Chernyak Vladimir , Schulz Michael F. , Mukamel Shaul , Tretiak Sergei , Tsiper Eugene V.
Published: July 2002

A fast, low memory cost, Krylov-space-based algorithm is proposed for the diagonalization of large Hamiltonian matrices required in time-dependent Hartree–Fock (TDHF) and adiabatic time-dependent density-functional theory (TDDFT) computations of electronic excitations. A deflection procedure based on the symplectic structure of the TDHF equations is introduced and its capability to find higher eigenmodes of the linearized TDHF operator for a given numerical accuracy is demonstrated. The algorithm may be immediately applied to the formally-identical adiabatic TDDFT equations.
Dipole moment surfaces and the mid- and far-IR spectra of N2-Ar
Authors: Wang Feng , McCourt Frederick R. W. , Le Roy Robert J.
Published: July 2002

An improved theoretical long-range dispersion plus induction dipole moment surface for N2-Ar is presented, and detailed numerical simulations are used to test its predictions against experimental far-IR and mid-IR spectra using two previously-reported potential energy surfaces. As was found in earlier work on the mid-IR spectrum using a pure induction dipole surface, the MMSVmod potential of Jäger et al. [J. Chem. Soc. Faraday Discuss. 97, 105 (1994)] yields distinctly better agreement with both experiments than does the XC-3 potential of Dham et al. [J. Chem. Phys. 103, 8477 (1995)]. However, the new dipole surface yields slightly poorer agreement with certain features of the experimental mid-IR and far-IR spectra, which suggests that the existing theoretical values and derivatives with respect to the bond length of some of the permanent moment and (hyper)polarizability properties of N2 need improvement.
Characteristics of instantaneous resonant modes in simple dense fluids with short-ranged repulsive interactions
Authors: Wu Ten-Ming , Ma Wen-Jong , Chang S. L.
Published: July 2002

We manifest the characteristics of the low-frequency, quasilocalized instantaneous normal modes, named as the instantaneous resonant modes (IRMs), in simple dense fluids with short-ranged repulsive interactions. The analyses include the potential energy profiles of the IRMs, and the local geometric structures and the number of the interacting neighbors of the particles at which the centers of the quasilocalization are located. We conclude that an IRM is created due to fluctuations in the local density, and has a barely-isolated center, which slightly interacts with one or two nearest neighbors, and the potential energy profile of an IRM is basically single-well with strong anharmonicity. The differences in character between the IRMs and the high-frequency localized instantaneous normal modes are also examined. Based on the barely isolated center picture, a necessary criterion for the occurrence of the IRMs is proposed. While only the imaginary-frequency IRMs are found in dense fluids with purely repulsive interactions satisfying the criterion, a tiny attractive well in the pair potential allows the occurrence of the real-frequency IRMs. The physical systems to detect the presence of the IRMs are discussed.