bluuebunny/crossref_metadata_2025 huggingface dataset:
Monte Carlo simulations of phase equilibria for a lattice homopolymer model
Authors: Mackie Allan D. , Panagiotopoulos Athanassios Z. , Kumar Sanat K.
Published: July 2002

Vapor–liquid phase equilibria for lattice homopolymer systems are simulated in the Gibbs ensemble for chains of length n=1, 8, 16, 32, 64, and 128 using a newly proposed methodology for volume change moves [Mackie et al., Europhys. Lett. 27, 549 (1994)]. This is the first time that extensions of the Gibbs ensemble methodology for direct calculation of phase coexistence are presented for lattice models. The simulation results show, in agreement with experiment, that the chain length dependence of the critical temperature of polymer-hole systems follows the Schultz–Flory form. The critical densities obey an n−0.32 scaling relationship over this limited range in chain lengths, an exponent somewhat less than is found from experimental data. We show that both the Flory model and the Guggenheim theory do not agree with the simulation results, although the Guggenheim model permits better agreement in all cases.
Vibrational dynamics of the Si–H stretching modes of the Si(100)/H:2×1 surface
Authors: Guyot-Sionnest P. , Lin P. H. , Hiller E. M.
Published: July 2002

The lifetime T1 of the symmetric Si–H stretching mode for the Si(100)/H:2×1 surface is significantly longer (T1≳6 ns at 100 K) than for Si(111)/H:1×1. T1 is strongly sample dependent and temperature dependent. Samples with the longest lifetimes also show the smallest inhomogeneous width. The difficulty in reproducing the longer lifetimes at low temperature indicates that the measured T1 may not be intrinsic and may by due to defects. The deuterated surface has a lifetime of 250±30 ps with little variation from sample to sample and a weak temperature dependence. It is expected to be the intrinsic lifetime. On the hydrogenated surface, the energy transfer time between the symmetric and asymmetric mode is measured to be 90±15 ps at 100 K. From photon-echo measurement, the measured dephasing time at 100 K is 75±5 ps. We propose that the dephasing on that surface is induced by a low-energy silicon phonon (200–300 cm−1) as for the Si(111)/H:1×1 surface. We derive an expression relating the dephasing time and the energy transfer time for the general case of two coupled oscillators and discuss its application to this system.
V, Nb, and Ta hydride molecules in deuterium and rare-gas matrices: Infrared and electron spin resonance spectra
Authors: Van Zee R. J. , Li S. , Weltner W.
Published: July 2002

The VH2 molecule was found to have S=3/2 and a 4Σ ground state with a zero-field splitting parameter ‖D‖=0.315 cm−1. 51V (I=7/2) hyperfine splitting was observed in the ESR spectra, but the hydrogen hyperfine was unresolved. The narrowest lines were observed for VD2 in solid deuterium at ∼2 K. Efforts to observe VH4 via ESR were not successful presumably because it could not be prepared in a high enough concentration. The infrared spectra of NbH2 molecules and their deuterated isotopomers were observed in solid deuterium and rare-gas matrices. NbH2 was found to be bent at 130°, in accord with theoretical calculations. NbH2 was not observed in ESR spectra although it presumably has a 4B1 ground state; this is attributed to a large (≳4 cm−1) negative D value. NbH4 was observed via ESR as a tetragonally distorted (D2d) tetrahedral molecule in its 2B1 lowest state resulting from a static Jahn–Teller effect. 93Nb (I=9/2) and also H hyperfine splittings were observed. A corresponding TaH4 spectrum was attributed to a similarly distorted tetrahedral S=1/2 molecule.
NO v–j vector correlations in the photodissociation of 2-chloro-2-nitrosopropane
Authors: Uberna Radoslaw , Cline Joseph I.
Published: July 2002

The correlation between the velocity and angular momentum vectors of the NO X 2ΠΩ=1/2,3/2(v=0) fragment from the photodissociation of 2-chloro-2-nitrosopropane, CH3CCl(NO)CH3, following excitation to the S1Ã 1A″ electronic surface is measured. The experiments are performed in a molecular beam and a scheme for detecting NO by polarized 1+1′ resonance-enhanced multiphoton ionization and pulsed-extraction time-of-flight mass spectrometry is described. The vector correlation is a function of the NO rotational state j, and the bipolar moment describing the correlation has a maximum value of β00(22)=−0.13(±0.01) at j≊18.5–22.5, indicating a preferred perpendicular orientation of the NO angular momentum and velocity vectors. The correlation diminishes as j decreases. This result is consistent with impulsive dissociation from a barrier along the reaction coordinate on the T1ã 3A″ surface, previously postulated to produce the highest angular momentum states of NO.
van der Waals binding energies and intermolecular vibrations of carbazole⋅R (R=Ne, Ar, Kr, Xe)
Authors: Droz Thierry , Bürgi Thomas , Leutwyler Samuel
Published: July 2002

Mass-selective ground-state vibrational spectra of jet-cooled carbazole⋅R (R=Ne, Ar, Kr, and Xe) van der Waals complexes were obtained by populating ground-state intra- and intermolecular levels via stimulated emission pumping, followed by time delayed resonant two-photon ionization of the vibrationally hot complex. By tuning the dump laser frequency, S0 state vibrational modes were accessed from ≊200 cm−1 up to the dissociation energy D0. Upon dumping to ground-state levels above D0, efficient vibrational predissociation of the complexes occurred, allowing us to determine the S0 state van der Waals binding energies very accurately. The D0(S0) values are <214.5±0.5 cm−1 (R=Ne), 530.4±1.5 cm−1 (R=Ar), 687.9±4.0 cm−1 (R=Kr), and 890.8±1.6 cm−1 (R=Xe). In the S1 state, the corresponding binding energies are larger by 9% to 12%, being <222.9±1.0 cm−1, 576.3±1.6 cm−1, 756.4±4.5 cm−1, and 995.8±2.5 cm−1, respectively.
Quantum Monte Carlo studies of anisotropy and rotational states in HeNCl2
Authors: McMahon Michele A. , Whaley K. Birgitta
Published: July 2002

We investigate ground and rotationally excited states of HeNCl2 (N=1,6,20) using variational (VMC) and diffusion Monte Carlo (DMC). The structure of these clusters is dominated by the He–Cl2 interaction, which has a well depth of about 30 K and a minimum in the perpendicular orientation. Results using the full, anisotropic potential are compared with those using an isotropic approximation. The effect of anisotropy is notable for the N=6 cluster in which the perpendicular well enhances the formation of a ring of six He atoms around the Cl–Cl bond axis. Because of the stability of this ring, the He6Cl2 structure is not significantly affected by rotational excitation to the L=2 state. However, such an excitation does cause both the He and Cl2 densities to delocalize slightly in the He20Cl2 cluster. For all of these cluster sizes, the Cl2 density is distributed on and about the cluster center. This behavior is similar to SF6 in HeN, N<112, although the Cl2 is not as localized as the more strongly bound SF6.
Crossover to entangled dynamics in polymer solutions and melts
Authors: Schweizer Kenneth S. , Szamel Grzegorz
Published: July 2002

A statistical dynamical theory of the crossover from unentangled Rouse dynamics to entangled behavior is constructed for chain polymer solutions and melts. Both time and spatial crossovers in long chain fluids, and the degree of polymerization crossover for short polymers, are treated. The analysis is based on a microscopic theory of the perturbative dynamical corrections to Rouse theory arising from chain connectivity and intermolecular excluded volume forces. The dependence of crossover properties such as the plateau shear modulus and entanglement time and length scale on solution density, solvent quality, and chain statistical segment length are derived by combining the dynamical theory with equilibrium liquid state integral equation methods. Scaling relations are obtained which appear to be in general accord with most experiments on both solutions and melts. The physical origin of the predicted scaling behaviors is the fractional power law temporal decay of the entanglement friction memory function on intermediate time scales, and power law reduced density dependence of the equilibrium force correlations. The theory is also applied to compute the dependence of the chain normal mode relaxation times on polymer density and chain length. Favorable qualitative comparisons with recent neutron spin echo experiments are made.
A modified self-consistent-field theory: Application to a homopolymer melt near a hard wall
Authors: Nath Shyamal K. , McCoy John D. , Donley James P. , Curro John G.
Published: July 2002

A modified self-consistent-field (SCF) theory is introduced and applied to hard-site Gaussian chains at meltlike density in contact with a hard wall. The traditional Edwards, quantum theoretical approach to inhomogeneous polymeric systems is used, but the solvent induced potential is calculated by density functional (DF) methods where the polymer reference interaction site model (PRISM) theory is used to calculate the ‘‘input’’ for the potential calculation. Specific calculations were performed with DF theory, Helfand–Tagami SCF theory, and modified-SCF theory. It is seen that the modified-SCF theory is capable of capturing the major structural features predicted by DF theory, and it promises to be a useful bridge between the SCF and DF theories.
On the role of bond functions in interaction energy calculations: Ar⋅⋅⋅HCl, Ar⋅⋅⋅H2O, (HF)2
Authors: Burcl Rudolf , Chal/asiński Grzegorz , Bukowski Robert , Szczȩśniak Mal/gorzata M.
Published: July 2002

We analyze the effect of an extended set of bond functions on the SCF and MP2 interaction energies, and their SAPT perturbation components; electrostatic, induction, dispersion, and exchange. The electrostatic, induction, and exchange terms at the SCF level prove to be largely independent. The dispersion energy is substantially improved and the improvement did not depend much on the bond-function location. In contrast, the electrostatic-correlation term is usually seriously distorted and the distortion strongly dependent on the bond-function location. It was also shown that the distortion may be significantly reduced by appropriate shifting of the location. Only then the interaction energies obtained with bond functions may be considered reliable. It is strongly recommended to control the electrostatic-correlation term. We also present samples of accurate results (within 5% error bar) for the Ar–HCl, Ar–H2O, and (HF)2 complexes.
A hybrid density functional study of the first-row transition-metal monocarbonyls
Authors: Adamo Carlo , Lelj Francesco
Published: July 2002

The results of a systematic density functional study, carried out using a self-consistent hybrid approach including exact exchange contribution, on the all monocarbonyls of first-row transition metals, are reported. Geometries, harmonic wave numbers, and binding energies, obtained using both standard generalized gradient corrected and hybrid functionals, are compared with previous published theoretical data and the available experimental findings. It is shown that hybrid functionals give results close to highly correlated post Hartree–Fock approaches and which are sensibly different from those obtained by standard local, even gradient corrected, methods. A nice agreement has been also found between theoretical and experimental binding energies. A natural bond orbital analysis confirms the role of the π interaction in the metal-carbonyl bond and gives an explanation to the preference for bent structures found in chromium and copper monocarbonyls.
Void structure and vapor–liquid condensation in dilute deionized colloidal dispersions
Authors: Yoshida Hiroshi , Ise Norio , Hashimoto Takeji
Published: July 2002

Void formation and vapor–liquid condensation in deionized dilute aqueous colloidal dispersions were studied for the same latex sample. The former was observed under density-matched conditions, while the latter was observed when the particles were denser than the dispersion medium. The critical concentration for void formation was found by direct observation using a confocal laser scanning microscope. Vapor–liquid condensation was observed for a dispersion of the same latex particles below the critical concentration. The critical concentrations for both phenomena were in agreement. The results clearly suggest an important role of a long-range electrostatic attractive interaction on the stability of the colloidal dispersion.
Reversible reactions of metastable reactants
Authors: Burshtein A. I. , Lukzen N. N.
Published: July 2002

The non-Markovian encounter theory is reformulated and used to find the quantum yields and kinetics of reversible energy transfer. Assuming a short range interaction between the particles, the kinematic approximation is employed to determine the integral kinetic equations and following from them differential equations of encounter theory. The differential variant of the theory is shown to be less appropriate since it fails to describe delayed luminescence during encounter: The reaction constant diverges with time when the acceptor of energy decays slower than the energy donor. On the contrary, integral encounter theory is very suitable for calculating the quantum yields and dissipation kinetics at any ratio between decay times.
Molecular dynamics simulations of the glass former ortho-terphenyl
Authors: Kudchadkar S. R. , Wiest J. M.
Published: July 2002

An 18-site, three-ring model has been developed for the van der Waals system ortho-terphenyl (OTP) which has been studied extensively experimentally because of its glass forming ability. The method of constraints has been used to freeze out the fast internal modes of the molecule, but the model retains some internal motion in the form of side-ring torsions. When used in molecular dynamics simulations, the model provides a reasonable representation of the properties of OTP in the liquid and supercooled liquid states, including the volume–temperature behavior and diffusion coefficients. The glass transition temperature has been obtained from the break in the slope of the volume–temperature curve and found to agree with experimental values, given the high cooling rates of the simulations. The short time dynamics of the system have been probed using velocity autocorrelation functions, mean-square displacements, van Hove correlation functions, and intermediate scattering functions. The dynamics of the model can be interpreted consistently within a molecular cage framework. It is found that the lifetimes of the transient cages increase significantly with decreasing temperature until, in the glass, cage breakup can no longer occur on the time scale of the simulations. Two distinct regimes are seen in the single particle motion in the liquid; these correspond to localized motion within the cage and the diffusive motion that sets in after cage breakup. Around the supercooled liquid region, a subdiffusive behavior occurs between the two regimes—indicative of the increasing difficulty encountered by the cage breakup mechanisms.
Electron correlation and relativistic effects in the coinage metal compounds. II. Heteronuclear dimers: CuAg, CuAu, and AgAu
Authors: Kellö Vladimir , Sadlej Andrzej J.
Published: July 2002

Electric properties of heteronuclear dimers of the coinage metals are calculated at the level of the CCSD(T) approximation applied to 38 electrons of the valence and next-to-valence atomic shells. The relativistic effects are accounted for by using the scalar approximation to the Pauli hamiltonian. Both the pure relativistic and mixed relativistic-correlation contributions to energies and electric properties are computed. All calculations have been carried out by using the recently developed first-order polarized basis sets of the coinage metal atoms. In the non-relativistic approximation all studied dimers show only a moderate degree of polarity; the non-relativistic CuAg turns out to be the most polar dimer with the Cu(−)Ag(+) polarity. The relativistic effects considerably reduce the negative value of the CuAg dipole moment, change the sign of the CuAu dipole moment, and make the AgAu molecule the most polar species in the series. Simultaneously, the parallel component of the dipole polarizability shows only a small relativistic contraction. The calculated quasirelativistic interaction potentials have a correct behavior in the vicinity of their minima and give the Re and ωe values in complete agreement with experiment. Much less satisfactory are the dissociation energy data which seem to suffer from the single reference configuration approximation.
Modeling solvation contributions to conformational free energy changes of biomolecules using a potential of mean force expansion
Authors: Pellegrini Matteo , Doniach Sebastian
Published: July 2002

The standard free energy perturbation (FEP) techniques for the calculation of conformational free energy changes of a solvated biomolecule involve long molecular dynamics (MD) simulations. We have developed a method for performing the same calculations many orders of magnitude faster. We model the average solvent density around a solute as the product of the relevant solute–solvent correlation functions (CF), following the work of García, Hummer, and Soumpasis. We calculate the CF’s by running Monte Carlo simulations of a single solute atom in a box of explicit water molecules and also angular dependent CF’s for selected pairs of solute atoms. We then build the water shell around a larger solute (e.g., alanine dipeptide) by taking the product of the appropriate CF’s. Using FEP techniques we are able to calculate free energy changes as we rotate the dihedral angles of the alanine dipeptide and we find they are in close agreement with the MD results. We also compute the potential of mean force as a function of distance between two solvated methanes and calculate the contribution of the solvent to the free energy change that results from rotating n-butane about its dihedral angle.
Interpretation of the principal diffraction peak of liquid and amorphous water
Authors: Elliott S. R.
Published: July 2002

The prominent first peak in the x-ray or neutron-derived structure factor of the low-density amorphous (LDA) form of water, prepared by vapor deposition or hyperquenching of the liquid, is identified as a ‘‘first sharp diffraction peak’’ (FSDP), as found in other covalent network glasses. The origin of this peak can be ascribed to the ordering of interstitial voids around the oxygen atoms comprising the tetrahedrally coordinated continuous-random network structure that is believed to describe LDA water.
Ab initio calculations of van der Waals interactions in one- and two-dimensional infinite periodic systems
Authors: Fink Karin , Staemmler Volker
Published: July 2002

A new CEPA-PNO (coupled electron pair approach with pair natural orbitals) method for the calculation of correlation energies in infinite periodic systems is proposed and applied to one- and two-dimensional He. The method starts from a crystal orbital Hartree-Fock (COHF) wavefunction with the occupied Bloch orbitals transformed into Wannier orbitals. The coupled-cluster equations for the infinite system are simplified by CEPA-type approximations: A CEPA-0 (or linear coupled-cluster) formula is applied for the small intercell contributions to the total correlation energy while CI-SD, ACPF or other CEPA variants are used for the large intracell contributions. The enormous number of single and double excitations into the virtual space is greatly reduced by the use of pair natural orbitals (PNOs), which leads to large savings in the necessary computer time and disk storage. First applications to the van der Waals interaction in the linear chain and the hexagonal plane of He atoms, performed with medium size and large atomic basis sets, show that an accuracy can be reached for the infinite systems which is comparable to the accuracy of the corresponding calculations for small He clusters. Because of the extended use of the translational symmetry of the Wannier orbitals, the calculations for the linear infinite systems are even considerably faster than those for the oligomers He5 and He7.
Simulations of quantum crystals by classical dynamics
Authors: Sterling M. , Li Z. , Apkarian V. A.
Published: July 2002

Classical molecular dynamics simulations of quantum crystals, using a simple pseudopotential, are reported. The method is implemented for calculating both equilibrium and dynamical properties. As a test, the radial distribution functions for pure solid H2 and Li doped solid H2 are computed, and found to be in excellent agreement with prior results obtained by the variational Einstein model [D. Li and G. A. Voth, J. Chem. Phys. 96, 5340 (1992)]. The method also yields a realistic phonon density of states, which is obtained by normal mode analysis. As an implementation in dynamics, the rotational motions of O2 isolated in solid D2 are investigated. In agreement with recent experiments, it is found that O2 does not rotate in solid D2.
Alignment of gas phase molecules by dynamic Stark effect with coherent narrow-band ultraviolet laser pulses
Authors: Neuhauser R. , Neusser H. J.
Published: July 2002

Alignment of vibronically excited benzene (C6 D6) along a lab-fixed axis in a selected rotational J, K, and m state is obtained by a UV–UV optical double-resonance experiment without additional, static electric or magnetic fields. The dynamic Stark effect caused by a narrow band, coherent laser pulse leads to an energetic separation of the individual m levels and renders the selective excitation of ‖m′‖=J′ levels by a weaker probing laser pulse.
A two state lattice model of membrane proteins: Configuration as a function of sequence
Authors: Seagraves Caren , Reinhardt William P.
Published: July 2002

A two state lattice model of soluble proteins is extended to model membrane proteins. The relationship between the structure of model proteins and their sequences is investigated as a function of the relative energy of hydrophobic type interactions. Relative energies of the interactions of hydrophobic and hydrophilic subunits with the solvent, the membrane, and with one another were chosen to mimic, within the simple model, their experimental counterparts. It is found that this reasonable energy parameterization produces model membrane proteins which share many characteristics with real membrane proteins, while other parameter sets fail in this regard. Consideration of the results obtained with the reasonable parameter sets leads to predictions about membrane proteins. Among these are that a single sequence may give a proteinlike native state in both aqueous and membrane environments.
On the low-lying Rydberg states of azabenzenes
Authors: Dion C. F. , Bernstein E. R.
Published: July 2002

Mass resolved excitation spectra of supersonic expansion cooled mono- and diazabenzenes are reported for the low lying Rydberg states. Transitions are located for pyridine, pyrazine, and pyridazine, but not pyrimidine. The Rydberg state lifetimes of these molecules are estimated, based on a Lorentzian line shape analysis, to be ca. 500 fs. Ab initio calculations for pyrazine at the complete active space self-consistent-field (CASSCF) and CASSCF many-body second-order perturbation theory (CASSCF/MBPT2) levels show that extensive configuration interaction and dynamic electron correlation are necessary to account for the excited states of these systems.
Evidence of the existence of dissociated water molecules in water clusters
Authors: Lee Chengteh , Sosa Carlos , Novoa Juan J.
Published: July 2002

The dissociation of water molecules in the water cluster was investigated using ab initio methods and density functional theory. A stable minimum energy configuration of a cluster containing H3O+ and OH− ions was located for a water cluster with five water molecules, (H2O)5. There are six hydrogen bonds in the dissociated water cluster to form the minimum energy structure. A similar structure with H3O+ and OH− ions was also found for a (H2O)8 cluster.
Zero kinetic energy proton and deuteron production from photoionization of H2 and D2
Authors: He Z. X. , Cutler J. N. , Southworth S. H. , Hughey L. R. , Samson J. A. R.
Published: July 2002

A zero ion kinetic energy spectrometer has been developed to study the production of near zero energy protons and deuterons from dissociative photoionization of H2 and D2. Both H+ and D+ spectra show four peaks on top of a continuum. The continuum was found to be in excellent agreement with the single center Coulomb calculation for the direct dissociation through the X 2Σ+g state of H+2. The observed structures were shown to originate from autoionization of the doubly excited Q1 1Σ+g(1), Q1 1Σ+u(1), Q1 1Σ+u(2), and Q2 1Σ+u(1) states, of which the Q1 1Σ+g(1) state is dipole forbidden.
Homopolymers and block copolymers at the interface of two different phases
Authors: Kosmas M.
Published: July 2002

We study the macroscopic behavior of homopolymers and block copolymers at the interface of two different phases focusing on the random walk nature of the polymer chains. The macromolecules or parts of them can belong to either one of the phases and are described by a simple model of random walks according to which the chain parts can have different average sizes depending on the phase they belong. Though the exact description of polymer chains under additional potentials need non-ideal walks, the present effort can be considered as an initial study seeking the effects of the differentiation of the size of the chains in the two phases because of the different interactions. The way of settling of chains in the two phase system is determined by means of the connection of the weights of the macrostates of localized chains with the probability distributions of the positions of the chain units. Density profiles of both homopolymers and diblock copolymers along the direction perpendicular to the interface are well described and the study is extended to miktoarm star macromolecules made from different homopolymeric arms.
Photoelectron spectroscopy of manganese–sulfur cluster anions
Authors: Zhang Nan , Kawamata Hiroshi , Nakajima Atsushi , Kaya Koji
Published: July 2002

Manganese–sulfur cluster anions (MnnS−m, n=1–10, m=1–10) have been studied using a magnetic-bottle type photoelectron spectroscopy (PES). The MnnS−m cluster anions were formed in a laser vaporization cluster source. The most stable cluster anions were found to have the compositions of n=m and n=m±1. The electron affinities of neutral manganese sulfide clusters were measured from the onsets of the PES spectra. A new electronically excited state at 0.75 eV above the ground state was found for MnS. From the size dependence of the PES spectra, it was found that Mn2S−2, Mn3S−3, and Mn4S−4 are structural frameworks in MnnS−2, MnnS−3, and MnnS−4 series, respectively. The electronic properties and geometrical structures of the clusters are discussed.
Direct vibrational energy transfer in zeolites
Authors: Brugmans Marco J. P. , Bakker Huib J. , Lagendijk Ad
Published: July 2002

With two-color picosecond infrared laser spectroscopy the dynamics of O–H and O–D stretch vibrations in zeolites are investigated. Zeolites appear to be good model systems to study transfer of vibrational energy in a solid. For the O–D vibrations, transient spectral holes are burnt in the inhomogeneously broadened absorption bands by saturating the absorption with a strong pump pulse. From the spectral hole widths the homogeneous absorption linewidths are obtained. The excited population lifetimes are determined using a time-resolved pump–probe technique, and in combination with the homogeneous linewidth the pure dephasing time is revealed as well. For high concentrations of O–H oscillators the vibrational stretch excitations are found to diffuse spectrally through the inhomogeneous absorption band. This spectral diffusion process is explained by direct site-to-site transfer of the excitations due to dipole–dipole coupling (Förster transfer). The dependences of the transient spectral signals on oscillator concentration and the results of one-color polarization resolved experiments confirm this explanation. The spectral transients are satisfactorily described by simulations in which the site-to-site transfer by dipole–dipole coupling is taken into account.
Theoretical study of potential energy surfaces for interactions of Pd2 with CO
Authors: Dai Dingguo , Roszak S. , Balasubramanian K.
Published: July 2002

Potential energy surfaces for the low-lying electronic states of the Pd2CO complex are studied using the second order perturbation theory of Mo/ller and Plesset (MP2) and the complete active space multiconfiguration self-consistent-field method (CASSCF) followed by large scale multireference singles+doubles configuration interaction (MRSDCI) computations that included up to 3.95 million configuration spin functions (CSFs). The bridged structure was found to be more stable by 1.3–1.5 eV compared with the linear structure depending on the level of theory. The calculated MP2 vibrational frequencies for Pd2CO are compared with the experimentally determined values for different chemisorptive sites. The properties of Pd2CO are compared with the corresponding properties of Pt2CO.
Line shape analysis of Doppler broadened frequency-modulated line spectra
Authors: North Simon W. , Zheng Xiaonan S. , Fei Ruian , Hall Gregory E.
Published: July 2002

We present a method for recovering Doppler broadened absorption line shapes from frequency modulated (FM) line spectra. The method of analysis is calibrated and demonstrated with thermalized CN radicals produced by photodissociation of cyanogen (NCCN), probed on the A–X system near 800 nm with a frequency modulated Ti: sapphire ring laser. Nonthermal, Doppler broadened lines from translationally nascent photofragments can also be recovered by direct transformations of experimental FM line profiles acquired with a time resolution exceeding 100 ns. The superior signal-to-noise afforded by FM spectroscopy, relative to other direct absorption methods, should encourage the application of transient FM spectroscopy to problems in photoinitiated reaction dynamics.
Low energy electron scattering from CH3Cl
Authors: Shi X. , Chan V. K. , Gallup G. A. , Burrow P. D.
Published: July 2002

Differential cross section measurements for the elastic scattering of electrons from CH3Cl at energies from 0.5 to 9.5 eV are reported for scattering angles of 30° and 100°. The angular scattering dependence is determined at selected energies over this range. At energies below 1.0 eV, the cross sections are in excellent agreement with calculations using the Born dipole approximation. At large angles and higher energies, the scattering is dominated by a 2A1 temporary negative ion state near 3.5 eV. Energy loss data at 3.5, 5.0, and 8.5 eV are reported and the relative contributions of various vibrational modes determined. Differential cross sections for vibrational excitation of the υ3(a1)C–Cl and υ4(e)CH stretching modes have been measured. The latter reveals a broad shape resonance of 2E symmetry peaking near 5.5 eV. Angular distributions for excitation of these same modes are also reported. Using fittings to the vibrational excitation functions, the resonance parameters have been extracted and used in a mixed semiempirical ab initio calculation to compute the relative strengths of the vibrational energy loss peaks. These compare favorably with the results of the experiment. Elastic cross sections integrated over angle are reported for low energies. They are substantially larger than the results from recent measurements of the total cross section.
The viscosity anomaly near the critical consolute point of the ionic ethylammonium nitrate-n-octanol mixture
Authors: Oleinikova A. , Bonetti M.
Published: July 2002

Measurements of the temperature dependence of the shear viscosity of the ionic binary mixture ethylammonium nitrate in n-octanol near the critical consolute point are reported. The kinematic viscosity is measured with a capillary viscometer in the range of reduced temperature 3×10−5≤t=(T−Tc)/Tc≤6.2×10−2. The temperature dependence of the density is measured by a volume dilatometer. No critical density anomaly is observed within the temperature range 5×10−4≤t≤4×10−2. Shear viscosity data are consistent with a power law divergence η=η0 (Qξ0)−y/νt−yF predicted by the mode-coupling and the dynamic renormalization group theories. The value of the critical exponent y agrees with the theoretical value y=0.032 when the critical temperature Tc is fixed at the experimental value. When Tc is set as a free parameter the fits give 0.0385≤y≤0.0438, depending on the forms chosen for the background viscosity η0 and the crossover function F. Estimation of the wave number Q is found in agreement with data obtained for neutral fluids.
Assignment of the 3Πu←X 3Σ−g bands of O2 observed in the region 1040–1200 Å
Authors: England J. P. , Lewis B. R. , Gibson S. T. , Ginter M. L.
Published: July 2002

A comprehensive vibronic assignment of the 3Πu states in the energy region approaching the first ionization threshold of O2 is presented for the first time. Measurements of seventeen 3Πu←X 3Σ−g bands in the 85 800–93 000 cm−1 region of the ground-state photoabsorption spectra of 16O2 and 18O2 at 79 K are reported, including six bands in the spectrum of 18O2 for the first time. Irregularities in vibrational spacings and in the extent of broadening by predissociation are shown to result from Rydberg-valence interactions. The observed transitions have been assigned, using semiempirical modeling of the 3Πu Rydberg-valence interactions based on the coupled-channel Schrödinger equations technique, to two vibrational progressions with upper states which we call F 3Πu and F′ 3Πu. The F and F′ states are formed from the avoided crossings of the npσu (n=3–5) Rydberg potential-energy curves by the lowest repulsive 3Πu valence potential-energy curve.
Liquid theory for the instantaneous normal modes of a liquid. II. Solutions
Authors: Larsen Ross E. , Goodyear Grant , Stratt Richard M.
Published: July 2002

There are a number of different ways of thinking about the intermolecular vibrations present in liquids. The approach suggested by instantaneous normal modes is a particularly interesting one, not just because of its connections with short-time dynamics, but because these modes can be analyzed and computed using the statistical mechanical ideas of standard liquid theory—or at least they can for neat, atomic liquids. We show in this paper that the instantaneous normal modes of atomic mixtures can be handled in virtually an identical fashion. We construct a renormalized mean-field theory that allows us to predict not only the total density of states of the mixture’s instantaneous normal modes, but also its projections into species-specific parts. This projection then allows us to predict the separate dynamics of all the species present in the mixture. We illustrate these results by applying them first to mixtures of Ar and Kr and then to binary isotopic mixtures with far more extreme mass differences, comparing in both cases with simulation. For mixtures of atoms not much more disparate than Ar and Kr, we find that the solution densities of states can be described quantitatively, over the entire range of compositions, merely by regarding the system as an effective neat liquid in appropriately scaled units. When the masses of the components differ by an order of magnitude or more, this simple scaling no longer holds, but what is interesting is that the liquid’s behavior is also quite different from what one would have seen in substitutionally disordered crystals with this same mass ratio. The dynamics of a light solute in a liquid makes an especially sharp contrast with that of an analogous light impurity in a crystal lattice.
Semiempirical methodology for simulating covalently bonded materials: Application to silicon
Authors: Chen Ai , Corrales L. René
Published: July 2002

A recently introduced semiempirical methodology is used to model and simulate silicon via molecular dynamics. This approach is capable of grasping essential qualitative and quantitative features of the coupling between the electronic coordinates and the geometric structure. Properties of the bulk diamond crystal, the melt and amorphous solid states are obtained using optimization techniques and molecular dynamics simulations. The pair distribution function of the amorphous state is in excellent agreement with experimental and other molecular dynamics simulation results.
A dynamic rotational isomeric state approach for extension of the time scale of the local dynamics observed in fully atomistic molecular dynamics simulations: Application to polybutadiene
Authors: Haliloǧlu Türkan , Bahar Ivet , Erman Burak , Kim Eung-Gun , Mattice Wayne L.
Published: July 2002

The dynamic rotational isomeric state (DRIS) formalism has been utilized to predict the local dynamics of amorphous cis- and trans-polybutadiene at bulk density from short-time molecular dynamics (MD) simulations at 425 K. The rates for transitions between rotational isomeric states have been calculated from the initial slopes of time-delayed transition (conditional) probability curves extracted from the MD simulation. First- (independent), second- (pairwise dependent), and third- (triplewise dependent) order conformational transitions have been incorporated into the DRIS formalism. Conformational and orientational correlation functions have been evaluated. The comparison of DRIS results with MD simulations indicates that this approach may be advantageously used to predict the time evolution of bond isomeric states and the contribution of transitions between these states to conformational correlation functions. The first-order conformational kinetics is the major factor controlling the relaxation in the latter. Cross-correlation functions are not reproduced as well, because they are dominated by torsional librations within rotational isomeric states, and such librations are not incorporated in the DRIS analysis. The prediction of anisotropic character of the segmental motions is also satisfactory to an important extent, but it still awaits some more consideration in the choice of the size of the kinetic segment, along with the accurate input of cooperative motions arising from both intra- and intermolecular interactions in a MD simulation. Information which would otherwise be extracted from the statistical analysis of very long trajectories of MD simulations may become readily obtainable from DRIS.
Fluorescence-line-narrowing spectroscopy of nile blue in glass and polymer at 5 K: Determination of a single-site line shape function
Authors: Zhang Yiping , Hartmann S. R. , Moshary F.
Published: July 2002

Fluorescence-line-narrowing (FLN) spectra of the oxazine dye nile blue are obtained at 5 K in both ethanol glass and PVOH polymer. Multiple spectra, each associated with a different excitation wavelength setting, are presented. We analyze these spectra assuming a site-independent molecular response. Excellent agreement obtains in the ethanol glass host.
Structural relaxations in H2-substituted myoglobin observed by temperature-cycling hole burning
Authors: Shibata Yutaka , Kurita Atusi , Kushida Takashi
Published: July 2002

A spectral hole-burning experiment has been carried out on H2-protoporphyrin-substituted myoglobin by cycling temperature from 4 K up to 70 K. When the excursion temperature Tc is below 30 K, the spectral diffusion kernel (SDK) has been found to show a Lorentzian shape. A steep increase in the hole width has been observed around 20 K, which is attributed to a narrow distribution of the barrier height of two-level systems (TLS’s) in myoglobin. When Tc is raised to ∼50 K, on the other hand, the major broadening has occurred in the tail of the hole, and the SDK has deviated significantly from a Lorentzian line shape. The hole profiles after the temperature cycling have been analyzed by the stochastic model which assumes that the spectral diffusion is induced by random flips of TLS’s. Both the non-Lorentzian SDK for Tc around 50 K and the Lorentzian SDK around 20 K have been found to be reproduced well by this model, if the finite size of the protein is taken into account. The details of the fitting procedure and the determined values of the number and the coupling constant of TLS’s in myoglobin are presented.
Anomalous impurity spectra in low-temperature glasses
Authors: Kikas J. , Suisalu A. , Zazubovich V. , Vois P.
Published: July 2002

Spectral holes have been measured for chlorin dopant educt and photoproduct states in five different glasses, o-terphenyl, benzophenone, polystyrene, ethanol, and triethylamine at 5 K. In contrast to the conventional picture of zero-phonon holes and phonon sideholes observable in the educt state, spectral holes in the photoproduct state reveal rather unconventional features. No zero-phonon hole is observable yet the spectra exhibit a peculiarity resonant with the burning frequency (‘‘Λ-spectra’’). The low-temperature vibronic spectra of an impurity in a 1D elastic lattice are calculated and demonstrated to match qualitatively well the Λ-spectra in the case of weak electron-phonon coupling. The applicability of a 1D model is argued to be due to the formation of a local structural defect (planar crack) caused by dopant phototransformation.
A molecular beam investigation into the dynamics and kinetics of dissociative O2 adsorption on Pt{100}-(1×1)
Authors: Bradley J. M. , Guo X.-C. , Hopkinson A. , King D. A.
Published: July 2002

The dissociative chemisorption of oxygen on Pt{100}-(1×1) has been investigated using supersonic molecular beams with incident translational energies between 0.04 and 0.90 eV over the surface temperature range 300 to 380 K. The adsorption process on the Pt{100}-(1×1) surface is found to differ from that on the reconstructed Pt{100}-hex-R surface in both magnitude and mechanism. The initial dissociative sticking probability is ≳0.2 on the (1×1) surface for all beam energies and surface temperatures investigated, whereas on the reconstructed Pt{100}-hex-R surface, under corresponding conditions, the initial sticking probability never exceeds 0.003. The initial sticking probability on the (1×1) surface at first decreases with increasing incident energy and then increases as the incident energy exceeds 0.1 eV, in a manner typical of an adsorption process that is precursor mediated at low incident energies and direct at higher incident energies. Precursor-mediated adsorption at low energies is also indicated by scattering measurements and by the angular variation of the initial sticking probability. At high incident energies the angular variation of the initial sticking probability indicates that adsorption is an activated process. Sticking probability measurements as a function of coverage indicate no extrinsic precursor involvement over the energy range studied; repulsive lateral interactions exist between adsorbed oxygen atoms. Thermal energy atom scattering measurements show an absence of island formation upon adsorption, in contrast to adsorption on the hex-R face.
Zero kinetic energy-pulsed field ionization and resonance enhanced multiphoton ionization photoelectron spectroscopy: Ionization dynamics of Rydberg states in HBr
Authors: Wales N. P. L. , Buma W. J. , de Lange C. A. , Lefebvre-Brion H. , Wang Kwanghsi , McKoy V.
Published: July 2002

The results of rotationally resolved resonance enhanced multiphoton ionization photoelectron spectroscopy and zero kinetic energy-pulsed field ionization studies on HBr via various rotational levels of the F 1Δ2 and f 3Δ2 Rydberg states are reported. These studies lead to an accurate determination of the lowest ionization threshold as 94 098.9±1 cm−1. Observed rotational and spin–orbit branching ratios are compared to the results of ab initio calculations. The differences between theory and experiment highlight the dominant role of rotational and spin–orbit interactions for the dynamic properties of the high-n Rydberg states involved in the pulsed field ionization process.
Energy and state dependence in the quenching and predissociation of C2H2
Authors: Hashimoto Nobuhisa , Suzuki Toshinori
Published: July 2002

The energy and state dependence in quenching and predissociation of acetylene from Ã 1Au state has been investigated. Quenching is induced by efficient electronic relaxation prior to dissociation. The translational energy release in dissociation suggests an exit energy barrier of about 560 cm−1 and a tunneling effect.
Symmetry-breaking perturbations in the ν2+3ν3 rovibrational manifold of acetylene: Spectroscopic and energy-transfer effects
Authors: Milce Angela P. , Orr Brian J.
Published: July 2002

Time-resolved infrared–ultraviolet double resonance (IR-UV DR) spectroscopy is used to characterize complex rovibrational levels in the highly perturbed ν2+3ν3 region (∼11 600 cm−1) of gas-phase acetylene, C2H2. Here, very few of the known rovibrational levels have appreciable Franck–Condon factors linking them to accessible excited rovibronic levels, as is needed in the fluorescence-detected IR-UV DR excitation scheme; rovibrational levels that are ‘‘IR-bright’’ tend to be ‘‘UV-dark’’ and vice versa. The rovibrational states detectable by IR-UV DR in this region are strongly perturbed, such that IR-bright (but UV-dark) vibrational basis states are coupled to other states with more favorable Franck–Condon factors. The characterization of these perturbed rovibrational states (and their associated dynamical properties) is facilitated by a novel IR-UV DR technique in which the UV and IR laser frequencies are simultaneously scanned in opposite directions, with their sum held constant. From the observed IR-UV DR spectra, it is inferred that local perturbations tend to break symmetries and spoil quantum numbers (such as l, J, and possibly I) that are usually regarded as ‘‘good’’ in the C2H2 molecule. The most remarkable case entails an apparent collision-induced breaking of g/u symmetry that gives rise to rovibrational energy transfer with odd ΔJ (rather than the usual even-ΔJ situation). This observation is consistent with IR-UV DR kinetic measurements of collision-induced state-to-state energy transfer that are also briefly described. The supposed mechanism relies on Coriolis coupling to cause strong rovibrational perturbations by basis states with dominant bending character, such that the resulting perturbed state is then susceptible to dynamical breaking of g/u symmetry, with odd-ΔJ rovibrational transfer a direct consequence. Other possible mechanisms imply that excitation of C2H2 to a particular perturbed rovibrational level might cause facile interconversion of the ortho and para nuclear-spin modifications. One such interpretation of g/u symmetry-breaking in C2H2 invokes a combination of Coriolis coupling and nuclear hyperfine interaction, thereby mixing basis states that have a very close accidental coincidence in energy. Another (but energetically unlikely) possibility is that g/u symmetry is spoiled photochemically by intramolecular state-mixing involving the vinylidene isomer, thereby destroying the molecule’s center of symmetry.
Comparison of positive flux operators for transition state theory using a solvable model
Authors: Muga J. G. , Delgado V. , Sala R. , Snider R. F.
Published: July 2002

Several quantum operators representing ‘‘positive flux’’ are compared for the square barrier by examining their ability to reproduce the exact transmittance when traced with the exact microcanonical density operator. They are obtained by means of the ‘‘Weyl rule,’’ the ‘‘Rivier rule,’’ by symmetrizing the product of ‘‘flux’’ and ‘‘positive momentum projection’’ operators, and by a variational technique. Explicit expressions are given for all cases.
Vibration–torsion–rotation dipole moment operator for a molecule with an internal rotor
Authors: Duan Yun-Bo , Takagi Kojiro
Published: July 2002

The sequential contact transformation technique is applied to the vibrational–torsional–rotational dipole moment operator for a molecule containing a threefold symmetric internal rotor in order to be consistent with the transformed Hamiltonian to be used in the analysis of the spectra of the molecule. The detailed discussion is presented of various types of dipole moments and corresponding vibration–torsion–rotation transitions. The transition matrix elements of the transformed dipole moment operator are given for CH3OH-like molecule.