bluuebunny/crossref_metadata_2025 huggingface dataset:
Direct Monte Carlo simulation of chemical reaction systems: Internal energy transfer and an energy-dependent unimolecular reaction
Authors: Dunn Shannon M. , Anderson James B.
Published: July 2002

We report a direct Monte Carlo simulation of an energy-dependent unimolecular reaction system of the type A→B in which the rates of the reactions of specific states Ai of A increase with their internal energies. Intermolecular exchange of translational and internal energies among colliding species Ai+Aj→Ak+Al is incorporated with use of a simple flexible statistical model satisfying all requirements of momentum and energy conservation, microscopic reversibility, and equilibrium. The required calculations are straight forward and rapid. The observed variation of overall reaction rate with pressure deviates from that of a simple Lindemann–Christiansen mechanism. For unrestricted energy exchange allowing transitions between all levels of A the deviations are small. For restricted (ladder) energy exchange limited to transitions up or down one level per collision the deviations are larger and the behavior is similar to that observed in a number of experimental studies.
Electron paramagnetic resonance spectroscopic study of the gallium trimer, Ga3
Authors: Howard J. A. , Joly H. A. , Mile B.
Published: July 2002

The 69Ga3 cluster has been prepared and trapped in an adamantane matrix at 77 K using a rotating cryostat. It has been examined by EPR spectroscopy and has three magnetically equivalent 69Ga nuclei. Attempts to simulate the powder spectrum recorded at 4 K gave ‖a⊥(69)‖=240±3 MHz, g⊥=1.9651±0.002, and g∥=1.9568±0.0025. Spectra were less sensitive to the magnitude of ‖a∥(69)‖ but a value of 120±10 MHz appeared to give the best fit. There were no other fine structure lines in the spectrum suggesting that the electronic ground state for this group 13 trimer is probably  2A1′ although a  4A2 ground state cannot be completely excluded.
Composition fluctuation induced depolarized Rayleigh scattering from diblock copolymer melts
Authors: Jian T. , Semenov A. N. , Anastasiadis S. H. , Fytas G. , Yeh F.-J. , Chu B. , Vogt S. , Wang F. , Roovers J. E. L.
Published: July 2002

Dynamic light scattering in the depolarized geometry has been used to investigate the pretransitional behavior of diblock copolymer melts near the order–disorder transition (ODT). Besides the segmental orientational dynamics, an additional slow relaxation process contributes to the dynamic depolarized intensity. This new relaxation mode exhibits an almost exponential shape, a wave vector (q) independent relaxation rate, and a strongly temperature dependent amplitude that compares with the temperature dependence of the maximum of the static structure factor at q*, measured with small-angle x-ray scattering. An account for the characteristics of this new relaxation process is based on the coupling between orientation and order parameter fluctuations. The depolarized intensity is due to orientation and/or extension of the copolymer chains in the pretransitional state above the ODT, with the relevant length scale of the slow orientation fluctuations being close to 2π/q*.
The influence of guest–host interactions on the guest molecule in intercalated alumino-silicate compounds
Authors: Holtz M. , Park Ta-Reong , Amarasekera J. , Solin S. A. , Pinnavaia T. J.
Published: July 2002

We have intercalated Cr(en)3+3 and Co(en)3+3 molecules (en=ethylenediamine), in known solid-solution fractional quantities, into the highly purified layered alumino-silicate fluorohectorite. Both molecules have like chemistry, but different physical sizes as they function to prop open the interlamellar gallery. As the fractional component x in [Co(en)3+3]x[Cr(en)3+3] 1−x–fluorohectorite is systematically varied, we observe the effect of intercalation on the host crystal basal spacing and the effects of intercalation on the guest molecules. Typical of the intercalation of rigid-layer compounds with agents differing in size, the basal spacing shows a sigmoidal dependence on fractional concentration. This is due to the large, but not infinite rigidity of the host and guest constituents. The lifetime of the fluorescence emission from the Cr(en)3+3 molecule exhibits the same sigmoidal dependence, indicating that the uniaxial stress exerted on the molecule, as it props open the gallery, is the primary factor dictating the guest fluorescence properties.
Polaron theory of an excess electron in xenon
Authors: Chen Jiqiang , Miller Bruce N.
Published: July 2002

This paper concerns the behavior of an electron thermalized in supercritical xenon. It is known that the electron can become localized (or self-trapped) in certain ranges of density and temperature. In the case of xenon, localization is enhanced by the strong polarizability. Previous investigations have employed path integral Monte Carlo methods to study the quantum states of the electron and the local properties of the fluid where the electron is treated quantum mechanically and the xenon atoms are treated classically. In this paper we present a reference-interaction-site model (RISM)–polaron calculation for the system of an excess electron in xenon at T=309 K and 248 K. The system is characterized by a Lennard-Jones fluid in which the electron interacts with xenon atoms through a soft potential: a repulsive core with an attractive long range tail. The HNC (hypernetted chain) closure is imposed on the RISM–polaron equation. Significant improvements over the primitive hard sphere model are achieved. The calculation shows that, above the critical density, the electron–solvent radial distribution function g(r) predicted by the RISM–polaron theory is in good agreement with its path integral Monte Carlo counterpart.
Cooperative ultrafast nonlinear optical response of molecular nanostructures
Authors: Wang Ningjun , Chernyak Vladimir , Mukamel Shaul
Published: July 2002

The stationary nonlinear reflection and the time resolved four wave mixing signal from a molecular monolayer are calculated using Green function techniques. Cooperative resonant nonlinear response found in small aggregates suggests the existence of coherence size of order of optical wavelength. A new peak in the nonlinear reflection spectrum is predicted, which is missed by the local field approximation. For an infinite two dimensional molecular monolayer with transition dipole moments in the lattice plane, the momentum-dependent two exciton decay rate is found to be larger than the sum of the single exciton radiative decay rates, as predicted by the local field approximation.
Characteristics of electron movement in variational Monte Carlo simulations
Authors: Sun Zhiwei , Soto M. M. , Lester William A.
Published: July 2002

Improving the efficiency of quantum Monte Carlo (QMC) to make possible the study of large molecules poses a great challenge. Evaluating the efficiency of Monte Carlo sampling, however, is at a rudimentary level and in need of new algorithms. Instead of the autocorrelation time as an efficiency measure for Monte Carlo simulations, we propose a direct method to characterize the movement of electrons in atoms or molecules during variational Monte Carlo computations. Further, the approach makes possible an efficient diagnostic tool to understand objectively many interesting issues in QMC. The usefulness of the method is demonstrated by comparisons among improved Metropolis algorithms and the original Metropolis algorithm. We also present an optimization method for choosing step sizes for Monte Carlo walkers. These step sizes are governed by the acceptance ratio of the electrons closest to the heaviest nucleus. Step sizes obtained for Ne and Ar are consistent with those obtained by the autocorrelation approach. Our study shows no evidence to support distinctions of core and valence electrons during simulations, and confirms that, in most cases, moving electrons individually is more efficient than moving all the electrons at once. We find that ‘‘trapped’’ or ‘‘stale’’ configurations are due to a large quantum force, and a solution to this problem is suggested.
Potential energy curves of M(np 2P)⋅RG(2Π) excited states and M+⋅RG ground states (M=Li, Na; RG=He, Ne)
Authors: Bililign Solomon , Gutowski Maciej , Simons Jack , Breckenridge W. H.
Published: July 2002

It has been established for some time that the bond energies for any given valence pπ excited state of Group 1 (2P) and Group 2 (1,3P) metal-atom/rare-gas (M⋅RG) van der Waals complexes tend to increase with the polarizability of the RG atom. It is also known that the binding energies of the corresponding M+⋅RG ground state ions are generally greater than those of the neutral M(pπ)⋅RG excited states with the same RG atom. However, there are two stark exceptions to these trends, both involving Group 1 metal atoms and the rare gas He; Li(2p 2P)⋅He(2Π) and Na(3p 2P)⋅He(2Π), which are the focus of the present study. We have conducted ab initio calculations of the potential energy curves of M(np 2P)⋅RG(2Π) and M+⋅RG states, where M=Li, Na and RG=He, Ne. We find that the unusual behavior of the pπ Group 1 metal atom states is due to (i) the lack of M(pπ)–RG(pπ) repulsive orbital overlap in the He case, and (ii) substantial additional attraction due to correlated motion of the RG atom’s electrons and the diffuse M(npπ) electron which is absent in the M+RG cases.
Dynamics and spectroscopy of higher bending vibrational levels of PH2(Ã 2A1)
Authors: Nguyen Xuan Chieu , Margani Alessandro
Published: July 2002

The zero-pressure lifetimes of the bending vibrational levels from v2′ = 5,6, and 7 of PH2(Ã 2A1) have been measured and are found to be 720, 35 (PQ1), 26 (PP1), and 3 ns, respectively. The self-quenching constants are (11.0 ± 0.9)×10−10 cm3 molecule −1 s−1 for v2′ = 5 and (7.4 ± 1.4) [PQ1] and (7.7 ± 1.4) [PP1]×10−10 cm3 molecule−1 s−1 for v2′ = 6. Evidence of predissociation of PH2(Ã 2A1) is shown by the sudden drop of its lifetime at v2′ = 6, which however should start at v2′ = 4. A model of indirect predissociation mechanism has been proposed. Composite emissions due to simultaneous excitation of v2′ = 5 and v2′ = 6 and resulting in biexponential fluorescence decay curves have been identified in the laser-induced fluorescence (LIF) excitation spectrum of v2′ = 5 where the spectroscopic assignment concerning some rotational lines and reported in literature has also been found to likely be incorrect. Besides, the origin of some unassigned peaks recorded in the same spectrum has been found, through fluorescence decay studies, to itself be v2′ = 5.
Adsorption kinetics on chemically modified or bimetallic surfaces
Authors: Xu Chen , Koel Bruce E.
Published: July 2002

Surface adatom modifiers that act as promoters or poisons on transition metal catalysts can have a large influence on adsorption kinetics along with their influence on adsorption energetics. Accounting for this effect requires recognition of the role of a new kind of intrinsic precursor — the modifier precursor state. A mathematical description of the adsorption kinetics of molecules on chemically modified or bimetallic surfaces incorporating this state has been derived based on Kisliuk’s statistical model [P. Kisliuk, J. Phys. Chem. Solids 3, 95 (1957)]. The resulting equation can be used to explain satisfactorily data on CO adsorption kinetics on chemically modified (Cl-, P-, S-, H-, O-, and K-precovered) Pt(111) surfaces. This concept should have broad implications for many systems in heterogeneous catalysis, such as bimetallic and alloy surfaces and coadsorbed layers, and for the multicomponent surfaces that are involved in materials growth and processing.
Rotational diffusion of rhodamine 6G depends on structural features of water and polyvinyl pyrrolidone mixtures
Authors: Ko Dong-Seob
Published: July 2002

Rotational diffusion of rhodamine 6G has been measured in an aqueous solution of polyvinyl pyrrolidone by the time-correlated single photon counting method. The rotational relaxation times do not scale linearly with viscosity when the solvent composition is changed at a constant temperature. The change in relaxation time with composition is interpreted in terms of an associated complex formation in the polymer solution system.
Line mixing effects in the 00°3–00°0 band of CO2 in helium. I. Experiment
Authors: Thibault F. , Boissoles J. , Doucen R. Le , Menoux V. , Boulet C.
Published: July 2002

The shape of the 00°3–00°0 CO2 band in helium has been investigated at room temperature over an extended range of perturber pressures (0–140 atm). Various and strong deviations from an additive superposition of Lorentzian lines have been observed, due to important line mixing effects enhanced by the specific structure of the R branch in this band.
The effect of pressure on the fluorescence lifetime of pentacene in para-terphenyl at low temperature
Authors: Baer Bruce J. , Chronister Eric L.
Published: July 2002

Time resolved fluorescence measurements are presented as a function of pressure for pentacene in para-terphenyl at low temperature. A photosite specific pressure effect is observed at low temperature for the triclinic (P≤5.5 kbar) phase of para-terphenyl. In addition, the fluorescence lifetime of pentacene in the monoclinic (high pressure) crystal phase at low temperature is obtained. We also observe unique fluorescence decay dynamics in the pressure region (5.5–6.5 kbar) intermediate to the triclinic and the monoclinic crystal phases. The effect of pressure on the triclinic host crystal is discussed in terms of a lowering of the energy of the lowest singlet excited state (S1) relative to the nearby triplet level (T2) with a corresponding decrease in the intersystem crossing rate. The fluorescence dynamics also provide information on the nature of the pentacene environment in the higher pressure (i.e.≳5.5 kbar) structural phases of the p-terphenyl host crystal.
Correlations between angular momentum orientation and exit velocity in gas–surface scattering: A probe of the dependence of collision dynamics on the position of impact
Authors: Masson Denis P. , Hanisco Thomas F. , Nichols William L. , Yan Chun , Kummel Andrew C. , Tully John C.
Published: July 2002

The scattering of rotationally cold N2 from Ag(111) results in angular momentum alignment and orientation of the scattered molecules; measurement of the angular momentum polarization as a function of exit angle, final J state, and exit translation energy provides direct information on the dynamics of the collisions. In this paper, the orientation of the angular momentum vector of the scattered N2 molecules, A{1}1−(J) has been measured for slow, medium, and fast groups of molecules in single rotational states at fixed exit angles. With normal incidence scattering (θi=0°) and off-normal detection, for a given final J state, the ‘‘slow’’ molecules have a higher probability of tumbling backwards (‘‘back spin’’) than the ‘‘fast’’ molecules. Conversely, for glancing incidence scattering (θi=30°) with quasi-specular detection, the opposite trend is observed: the slow molecules have a higher probability of tumbling forwards (‘‘top spin’’) than the fast molecules. These experiments were simulated and analyzed using molecular dynamics trajectory calculations. The calculations show that the amount of gas kinetic energy transferred to the surface is sensitive to the narrow dispersion of impact sites and molecular orientations that lead to scattering into a given final rotational state at a given exit angle. The calculations demonstrate that for both incident angles, collisions near the top of a surface atom lead to slower final velocities than collisions with the hollow sites in analogy with the simple case of two colliding spheres. Therefore, the experimentally observed dependence of the angular momentum orientation on the exit velocity results from the correlation between the initial molecular bond angle and the impact site for scattering into a given J state and at a fixed exit angle.
X-ray diffraction study of poly(3-n.decylpyrrole), polypyrrole, and their monomers
Authors: Petrillo C. , Borra S. , Cagnolati R. , Ruggeri G.
Published: July 2002

The structure of the organic intrinsically conducting polymers polypyrrole and poly(3-n.decylpyrrole) was investigated by x-ray diffraction. The liquid monomers pyrrole and 3-n.decylpyrrole were also measured. From the analysis of all the data, information on the molecular and intermolecular correlations in the polymers was obtained. Both the polymeric systems are found to maintain a substantial planarity of the aromatic rings, i.e., the alkylic substituent in poly(3-n.decylpyrrole) does not affect the planarity of the compound. Possible effects related to the size of the counterion are also discussed.
Time-resolved two-photon induced anisotropy decay: The rotational diffusion regime
Authors: Wan Chaozhi , Johnson Carey K.
Published: July 2002

Two-photon excitation (TPE) of randomly oriented chromophores in solution generates an anisotropic distribution. In a previous paper [Chem. Phys. 179, 513 (1994)], the polarization dependence of the TPE signal probed by a secondary spectroscopic transition (fluorescence or transient absorption) was determined. In this paper, the time dependence of anisotropic two-photon induced fluorescence or transient absorption signals due to rotational diffusion is treated in spherical tensor formalism. The two-photon signal in general contains isotropic (orientation independent) and anisotropic (orientation dependent) contributions. The latter decay with up to five exponential components. Four time-dependent anisotropy parameters can be defined and measured, allowing additional information, not available in conventional one-photon fluorescence depolarization measurements, to be determined. The special case of one-color TPE is discussed in particular. It is shown that by measurement of the linear and circular anisotropies r1(t) and r2(t), more than one rotational correlation time can be determined in many cases, providing information on rotational diffusion parameters not readily determined by analogous one-photon methods and leading in some cases to resolution of rotational motion about the principal diffusion axes of the molecule.
Experimental and theoretical studies of the (C 1s−1,π*)3Π state of CO: Momentum transfer dependence and vibrational structure
Authors: Francis J. T. , Kosugi N. , Hitchcock A. P.
Published: July 2002

The intensity of the X 1Σ+→(C 1s−1,π*)3Π transition of CO has been measured by electron energy loss spectroscopy using a range of scattering angles (0°–45°) and impact energies (376 to 1806 eV) in order to investigate the momentum transfer dependence of a spin forbidden inner-shell excitation. A Franck–Condon factor analysis of the vibrational structure of the singlet and triplet (C 1s−1,π*) states was used to quantify differences in the potential energy curves of these states. Ab initio self-consistent field configuration interaction (SCF-CI) calculations were carried out to generate the potential curves of the 1Π and 3Π(C 1s−1,π*) states. The electronic and vibrational energies and Franck–Condon factors are in good agreement with the experimental results. The calculations indicate that the difference in the 1Π and 3Π potential curves are related to differences in relaxation of both the (active) π* and other (passive) valence electrons.
A perturbation theory calculation of the exchange energy of the HeH++ molecular ion
Authors: Patil S. H. , Tang K. T. , Toennies J. P. , Yiu C. L.
Published: July 2002

The problem of a heteronuclear one electron molecule with different nuclear charges z1 and z2 has been analyzed by a perturbation treatment. Starting from a homonuclear system in which each nucleus has the average charge (z1+z2)/2, the ±(z1−z2)/2 difference charges with opposing signs at the two nuclei are used as a perturbation. For HeH++ second-order perturbation calculations of the ground and first excited states are in very good agreement with exact calculations. The model predicts the correct behavior for R→0 and for R→∞ when there is no crossing of the energy levels. The electronic energies of ten low-lying excited states are also in good agreement with the exact numerical results. This analysis clarifies the role of exchange in a simple heteronuclear system.
Spectroscopic determination of the intermolecular potential energy surface for Ar–NH3
Authors: Schmuttenmaer C. A. , Cohen R. C. , Saykally R. J.
Published: July 2002

The three-dimensional intermolecular potential energy surface (IPS) for Ar–NH3 has been determined from a least-squares fit to 61 far infrared and microwave vibration–rotation–tunneling (VRT) measurements and to temperature-dependent second virial coefficients. The three intermolecular coordinates (R,θ,φ) are treated without invoking any approximations regarding their separability, and the NH3 inversion–tunneling motion is included adiabatically. A surface with 13 variable parameters has been optimized to accurately reproduce the spectroscopic observables, using the collocation method to treat the coupled multidimensional dynamics within a scattering formalism. Anisotropy in the IPS is found to significantly mix the free rotor basis functions. The 149.6 cm−1 global minimum on this surface occurs with the NH3 symmetry axis nearly perpendicular to the van der Waals bond axis (θ=96.6°), at a center-of-mass separation of 3.57 Å, and with the Ar atom midway between two of the NH3 hydrogen atoms (φ=60°). The position of the global minimum is very different from the center-of-mass distance extracted from microwave spectroscopic studies. Long-range (R≳3.8 Å) attractive interactions are greatest when either a N–H bond or the NH3 lone pair is directed toward the argon. Comparisons with ab initio surfaces for this molecule as well as the experimentally determined IPS for Ar–H2O are presented.
Line mixing effects in the 003–000 band of CO2 in helium. III. Energy corrected sudden simultaneous fit of linewidths and near wing profile
Authors: Boissoles J. , Thibault F. , Le Doucen R. , Menoux V. , Boulet C.
Published: July 2002

Line coupling induced by collisions leads to drastic modifications of the spectral profile of the 0003–0000 of CO2 pressurized by helium. Calculation of these modifications have been performed by using two recent energy corrected sudden (ECS) formalisms. The two formulations lead to theoretical predictions rather similar and in good agreement with the available data over extended ranges of frequency and perturber pressure. It has been shown that a simultaneous fit of the pressure broadened linewidths and the near wing profile allows a more accurate determination of the basic ECS parameters. For that purpose, it has been necessary to extend the measurement of the broadened widths to high J values (up to J≊90).
Photochemistry and reaction dynamics of HBr in xenon matrices: Photodissociation of HBr and production of Br atoms
Authors: LaBrake Dwayne , Ryan E. Todd , Weitz Eric
Published: July 2002

Translationally excited and mobile H atoms are produced by the 193 and 248 nm photodissociation of HBr in xenon matrices. In concentrated matrices the loss of HBr and production of Br atoms follows second order kinetics due to reactive depletion of the remaining HBr by photolytically produced H atoms. The second order rate constant for this process can be directly related to the laser fluence, photodissociation cross-section for HBr, and initial HBr concentration. At higher dilutions the average distance between HBr molecules becomes large compared to the mean displacement of an H atom and reactions between H atoms and HBr molecules are unlikely; the loss of HBr follows first order kinetics as does the production of Br atoms. The mean displacement of H atoms, as a result of 193 nm photolysis, is on the order of 100 Å. The Br atom co-fragment is thermally stable and does not exhibit significant photoinduced mobility at temperatures below 40 K. Formation of Xe2H+ ions is observed but does not measurably effect the HBr loss or Br formation kinetics.
Influence of transport limitations on the kinetics of homopolymerization reactions
Authors: Oshanin G. , Moreau M.
Published: July 2002

We discuss the influence of transport limitations on the kinetics of homopolymerization reactions. We show that in the systems in which the polymer’s diffusivity decreases faster than the first inverse power of its radius of gyration, the transport limitations represent the rate-controlling step and induce anomalous behavior of the characteristics of the process. For such systems we compute the evolution of the molecular weight distribution and its zeroth moment explicitly.
Perturbation facilitated optical–optical double resonance spectroscopy of the 2 3Σ+g, 3 3Σ+g, and 4 3Σ+g Rydberg states of 7Li2
Authors: Yiannopoulou A. , Urbanski K. , Lyyra A. M. , Li Li , Ji B. , Bahns J. T. , Stwalley W. C.
Published: July 2002

This paper reports the experimental observation of the 2 3Σ+g, 3 3Σ+g, and 4 3Σ+g states of 7Li2 by cw perturbation facilitated optical–optical double resonance spectroscopy. Molecular constants and RKR potential curves have been obtained. Our experimental Te and Re for the 2 3Σ+g state are 27 297.45(16) cm−1 and 3.0797(18) Å, respectively, and for the 3 3Σ+g state are 31 043.93(53) cm−1 and 3.0378(19) Å, respectively. The above values are in very good agreement with theoretical calculations. Hyperfine splitting for both states has been resolved. Both states follow Hund’s case (bβS) hyperfine coupling scheme. The experimental Fermi contact parameter, bF, is approximately 96±2 MHz for the 2 3Σ+g state and 95.6±3 MHz for the 3 3Σ+g state. These values are in good agreement with the previously obtained value 98.6±4 MHz [Li et al., J. Chem. Phys. 96, 3342 (1992)]. One level of the 4 3Σ+g state has been observed and its hyperfine structure has been resolved and characterized with Hund’s coupling case (bβS).
Improved quantum Monte Carlo calculation of the ground-state energy of the hydrogen molecule
Authors: Chen Bin , Anderson James B.
Published: July 2002

We report an improved Green’s function quantum Monte Carlo calculation of the nonrelativistic ground-state energy of the hydrogen molecule, without the use of the Born–Oppenheimer or any other adiabatic approximations. A more accurate trial function for importance sampling and the use of the exact cancellation method combine to yield an energy which is a factor of 10 more accurate than that of previous quantum Monte Carlo calculations. The energy is less accurate than that of recently improved analytic variational calculations. The calculated energy is −1.164 0239 ±0.000 0009 hartree. Expressed as the dissociation energy and corrected for relativistic and radiative effects, the result is 36 117.84±0.20 cm−1, a value in agreement with the most recent experimental value 36 118.11±0.08 cm−1 obtained by Balakrishnan et al.
High-level ab initio prediction of the structure and infrared spectra of formaldehyde–water radical-cation complexes
Authors: Coitiño Elena L. , Pereira Alberto , Ventura Oscar N.
Published: July 2002

In a previous work we have identified two possible structures for the radical cation obtained by ionization of hydrogen-bonded formaldehyde–water complexes [Coitiño et al., J. Am. Chem. Soc. 115, 9121 (1993)], a hydrogen-bonded and an addition-like complexes. We observed that the results were highly dependent on the method of calculation employed. Inclusion of correlation was crucial for obtaining the correct structures of some of the complexes. In this work we used high-level ab initio calculations in order to predict the equilibrium structure of these two complexes, the possibility of its existence in gas phase, and the infrared spectrum to be expected in each case.

A series of progressively more sophisticated basis sets was used to assess the effect of the quality of the calculations on the expected results. Also, full geometry optimization with each basis set was performed at the second-order Mo/ller–Plesset level, and correlation energy was calculated at the fourth-order Mo/ller–Plesset level to assess the contribution of this factor to the global result. Confirming our previous results, we found that correlation affects the hydrogen-bonded radical-cation complex more than the addition one, due to the different bonding patterns in each of them. Both complexes are stable—toward decomposition to the fragments or to CO+H+H3O+—by several kcal/mol at all levels of theory. The hydrogen-bonded complex is more stable than the additional one by a respectable amount (13 kcal/mol at the highest level used here), lending support to our previous analysis of the reactions of the former as the main channels for evolution of the formaldehyde–water radical cation. The H-bonded complex [H3O+...HCO⋅] presents two characteristics, very intense absorptions which should allow identification of this radical cation if present in the experimental setup. These transitions are also present in the HCO⋅ radical but their intensity is enhanced by an order of magnitude due to the coupling with the proton in H3O+. We conclude that the combination of stability and characteristic infrared transitions should make this radical-cation complex a relatively easy target for experimental determination.
Exact solution to linearized Poisson–Boltzmann equation: Ion-penetrable membranes bearing nonuniformly distributed fixed charges
Authors: Kuo Yung-Chih , Hsu Jyh-Ping
Published: July 2002

The Poisson–Boltzmann equation governing the electrostatic potential distribution of an ion-penetrable particle immersed in an arbitrary a:b electrolyte solution is discussed. The particle, which mimics biological cells and some artificial membranes, carries nonuniformly distributed fixed charges. Three types of particle, including planar, cylindrical, and spherical, and two classes of nonuniform fixed charge distributions are investigated. These distributions all yield the same amount of fixed charges in the membrane phase, which is independent of the geometry of a particle. Under the condition that the potential is low, the exact distribution of the electrostatic potential and the free energy of the system under consideration are derived.
Accidental predissociation phenomena in theE 1Π,v=0 andv=1 states of 12C16O and 13C16O
Authors: Cacciani Patrice , Hogervorst Wim , Ubachs Wim
Published: July 2002

We have studied v=0 and v=1 levels of the E 1Π state of CO in excitation from the ground state by one- and two-photon transitions, thus probing e and f Λ-doublet components. New accidental predissociations were found in the E 1Π, v=0 state for high J values (Je=41, 44 for 12C16O and Je=41, 50 for 13C16O). The predissociation phenomenon in the E 1Π, v=1 state at J=7 was reinvestigated and for both e and f components also J=9, 10, and 12 were found to be perturbed. Perturbations by all three spin components of a k 3Π, v=5 state were deduced. Furthermore the accidental predissociation in E 1Π, v=0 Je=31 was reinvestigated. Measurements of spectral line shifts were modeled assuming a spin-orbit coupling between E 1Π and the 3Π1 component of the k 3Π state. Relative predissociation lifetimes of k 3Π, v=3 and 5 with respect to E 1Π, v=0 and v=1 are deduced from an analysis of observed intensity effects. For the E 1Π, v=1 state rotational state dependent lifetimes were determined at low-J values. Line positions of CO lines were calibrated on an absolute frequency scale within 0.05 cm−1 against the tellurium and iodine standard in the visible. Accurate molecular constants for the E 1Π, v=0 and v=1 states are determined for both 12C16O and 13C16O. The E 1Π, v=1 state of 12C17O is observed for the first time.
Auger electron spectroscopy of molecules: Angular and spin correlation with photoelectrons from rotating linear molecules
Authors: Chandra N. , Sen S.
Published: July 2002

A detailed theoretical framework is developed for studying sequential emission of a photoelectron and an Auger electron from a rotating linear molecule with information on momenta and spin-polarization of the departing particles. Identical expressions, except, of course, for different dynamical amplitudes, in both the Hund’s coupling schemes (a) and (b) are obtained for the three different correlation functions considered in this paper. The use of the parity adapted wave functions for the molecular states involved results in, among other things, the presence of only a finite number of harmonics for each of the directions included in the correlation function. Several specific photon-propagation, electron-detection configurations are suggested for which the general correlation functions derived herein become particularly simple. The correlation between the Auger and the photoelectrons is shown to become, under specific conditions, completely isotropic for all bound molecular orbitals, whatever may be their symmetries, from which pair of electrons comes out. This analysis is independent of any dynamical calculations which can be performed in a hierarchy of approximations beginning from semiempirical phenomenological models to sophisticated ab initio methods.
Concentration dependence of the vibrational band shape and frequency in binary mixtures
Authors: Moser G. , Asenbaum A. , Barton J. , Döge G.
Published: July 2002

We propose a stochastic model which describes vibrational broadening of Raman lines in binary mixtures. The line broadening by a dynamic environment is included by considering two-particle exchange processes in the nearest neighbor shell of the reference molecule. Different environment states influence the vibration of the reference molecule. A finite interaction time between the molecules leads to finite correlation times in the vibrational relaxation. As a result the symmetric band shapes in the neat liquids deviate from Lorentzian profiles. Experimentally this is observed for the symmetric C–H stretching vibration in neat CH2I2. The model represents an extension of the model proposed by Knapp and Fischer, which reproduces Lorentzian shapes in the neat liquids. A quantitative comparison of both models with experimental data for the C–H stretching vibration in CH2I2/CCl4 mixtures is performed at several mole fractions.
Multistate interactions in nitric oxide probed by laser-induced grating spectroscopy
Authors: McCormack E. F. , Dehmer P. M. , Dehmer J. L. , Pratt S. T.
Published: July 2002

Two-color laser-induced grating spectroscopy (TC-LIGS) via the A 2Σ+, v′=0 state has been used to study the B 2Π, v=28 and L 2Π, v=8 valence states and the Q 2Π, v=0 5pπ Rydberg state of NO. Energies for previously unobserved or unresolved transitions to the perturbed L 2Π1/2,3/2, v=8 and Q 2Π1/2,3/2, v=0 states are reported for the first time. The interaction between these states is reconsidered in light of the new assignments. A comparison of different detection methods made for the B 2Π1/2,3/2, v=28←A 2Σ+, v′=0 transitions demonstrates the ability of TC-LIGS to detect states that are difficult to detect by ionization or fluorescence-dip detection.
Interchain interactions in one-dimensional periodic systems: An analysis of second-order effects causing deformation
Authors: Yoshizawa Kazunari , Hoffmann Roald
Published: July 2002

The role of second-order perturbations in interchain interactions of one-dimensional electronic systems is examined. The general features of such interchain interactions are deduced from a simple two-chain model. From a perturbation-theoretic analysis, the second-order term originating from two different bands near the Fermi level leads to an important out-of-phase coupling of charge-density-waves on neighboring chains. The preferred distortion is predicted for various electron counts, using a transition-density analysis. Specific geometrical conclusions are derived for polyacene systems.
Molecular dynamics simulation of infrared spectra for potassium palmitate B-form crystal
Authors: Ishioka Tsutomu , Murotani Satoru , Kanesaka Isao , Hayashi Soichi
Published: July 2002

The intra- and intermolecular potentials of potassium palmitate B-form crystal were estimated by a normal mode analysis and a molecular dynamics simulation. Based on these potentials, we calculated the time history of the dipolemoment in the nine unit cells (3a×3b) containing 18 molecules and obtained the polarized infrared spectra by a Fourier transformation. The frequencies and the intensities of the intense bands agreed well with the observed ones, and were consistent with the crystal structure.
Simulated polymer melt stress relaxation. II. Search for entanglements
Authors: Gao J. , Weiner J. H.
Published: July 2002

Simulations, described in the previous paper, by nonequilibrium molecular dynamics of stress relaxation in a model polymer melt of freely jointed chains with N=200 bonds exhibited an incipient plateau, followed by a period in which the relaxing stress corresponded to the entropic stress with an entanglement length of Ne=40 bonds. This paper describes two types of diagnostic subroutines used to search for the entanglements or obstacles responsible for these phenomena. The first examined the rate of separation of initially neighboring interchain atom pairs but did not serve to uncover many topological entanglements. The second introduced a measure of time-averaged atomic mobility and found that intrachain atoms of relatively low mobility tended to cluster in groups along the chain. The average spacing between clusters appears to introduce a new length scale in polymer melts that is independent of the conventional picture of interchain entanglements.
An exact lattice model of complex solutions: Chemical potentials depend on solute and solvent shape
Authors: Krukowski Anton E. , Chan Hue Sun , Dill Ken A.
Published: July 2002

For the theoretical modeling of physical transformations such as boiling, freezing, glassification, or mixing, it is necessary to know how the partition function of a system depends on its density. Many current treatments rely either on low density expansions or they apply to very simple and symmetric molecular shapes, like spheres or rods. Here we develop an exact analytical lattice theory of materials and mixtures that applies to arbitrarily complex molecular shapes over the full range of densities from gas to crystal. The approach is to compute partition functions for small numbers of shapes and to explore the dependence on density by varying the volume of the system. Recently a question has been raised about whether entropies of dissolution are better treated using classical solvation theories or Flory–Huggins theory. We explore this for a range of molecular sizes and shapes, from lattice squares and cubes, to rods, polymers, crosses, and other shapes. Beyond low densities, the entropic component of the chemical potential depends on shape due to the different degrees to which molecules ‘‘interfere’’ with each other. We find that neither Flory–Huggins nor classical solvation theories is correct for all shapes. Molecules that are ‘‘articulated’’ are remarkably well treated by Flory–Huggins theory, over all densities, but globular molecules are qualitatively and quantitatively different, and are better treated by the classical chemical potential, consistent with experiments of Shinoda and Hildebrand. These results confirm that the Flory–Huggins theory differs from classical theory not because of molecular size differences per se; it accounts for the coupling between translations and conformational steric interference.
Magnetic-field induced N2(B→A) emission from a beam of metastable nitrogen molecules and precise term energy of the N2(A′ 5Σ+g) state
Authors: Ottinger Ch. , Vilesov A. F.
Published: July 2002

Magnetic field anticrossing measurements have been made in order to identify unambiguously the N2(A′ 5Σ+g) level responsible for the recently observed perturbation of the N2(B 3Πg, v=10, 3Πe2(12)) level [Ch. Ottinger and A. F. Vilesov, J. Chem. Phys. 100, 4862 (1994)]. When a weak magnetic field (H=0–3000 G) was applied across the N2 metastable beam, new field-induced perturbations were observed in B,v=11, which were ascribed to interactions with A′, v=2. This multiple perturbation pattern permits a determination of the relative energy of the A′ and B states with a precision of 0.02 cm−1.
Uniqueness of the microscopic stress tensor
Authors: Wajnryb Eligiusz , Altenberger Andrzej R. , Dahler John S.
Published: July 2002

Schofield and Henderson have shown that the balance equation for the density of linear momentum does not provide a unique definition for the microscopic stress tensor. It is argued here that additional conditions of symmetry and physical interpretability, which reasonably can be imposed upon this tensor, are sufficient to establish the uniqueness of the familiar Irving–Kirkwood formula.
Electron stimulated desorption of O− and metastable CO* from physisorbed CO2
Authors: Huels M. A. , Parenteau L. , Cloutier P. , Sanche L.
Published: July 2002

We report electron stimulated desorption (ESD) measurements of O− yields produced by dissociative electron attachment (DEA) to physisorbed CO2. The molecules are condensed at about 17–20 K on polycrystalline Pt, either as pure multilayer films, or in submonolayer (ML) quantities onto thick rare gas substrates. For the pure disordered multilayer solids, we observe four peaks in the O− yield function at incident electron energies, E(e), of about 4.1, 8.5, 11.2, and 15 eV. The lowest two are assigned, respectively, to the 2Πu and 2Πg resonance states of CO−2, which dissociate into O−(2P)+CO(X 1Σ+), and are known to dominate the gas phase DEA O− production cross section for E(e)≤20 eV. Measurements of ESD CO* metastable yields from similar CO2 multilayer solids on Pt(111), also presented here, suggest that the 11.2 and 15 eV O− peaks are associated with the manifold of close-lying CO2*− states which dissociate into O−(2P)+CO* (a 3Π, a′ 3Σ+, or d 3Δ). For 0.15 ML of CO2 physisorbed on 20 ML thick rare gas substrate films significant sharp enhancements (fwhm ≤0.5 eV) are observed in the ESD O− yields at about 0.3–0.4 eV below the lowest substrate exciton energy. These enhancements are attributed to a coupling of the (electron plus exciton) core-excited anion resonances of the rare gas atoms to the dissociative Rydberg anion states of the coadsorbed CO2 at the solid’s surface. This is followed by a transfer of the charge and excitation energy to the coadsorbate.
Charge renormalization at the large-D limit for diatomic molecules
Authors: Bleil R. , Faliks A. , Miletic M. , Kais S.
Published: July 2002

The charge renormalization procedure for the calculation of the correlation energy of atoms utilizing the analytically known large-D limit solutions for the exact and Hartree–Fock equations is extended to diatomic molecules. This procedure is based on the variation of the nuclear charge, Z, and internuclear distance, R, of the Hartree–Fock Hamiltonian such that the Hartree–Fock energy will be significantly closer to the exact energy. We calculate to first order in δZ the leading contribution to the correlation energy by changing the nuclear charge to some renormalized nuclear charge, ZRi→Zi+δZi. To first order in δZ, this leads to an approximate expression, Ecorr(Za,Zb,R)=(∂EHF/∂Za)δZa+ (∂EHF/∂Zb)δZb, for the correlation energy based on the charge renormalization parameter δZ, which is fixed systematically from the large-D limit. The theory is applied to diatomic molecules. Near the equilibrium, we are predicting the correlation energy typically with 80% or greater accuracy in a completely self-consistent and systematic way with no additional cost to the Hartree–Fock calculation. An improved approach to estimating the correlation energy for all R is outlined.
Simple modeling of line-mixing effects in IR bands. I. Linear molecules: Application to CO2
Authors: Hartmann J. M. , L’Haridon F.
Published: July 2002

A simple approach is developed in order to model the influence of collisions on the shape of infrared absorption by linear molecules. It accounts for line-mixing effects within, as well as between, the different branches (P,Q,R) of the band. It is based on use of the strong collision model, of a classical representation of rotational levels, and of the rigid rotor approximation. The absorption coefficient then has a very simple analytical expression; its wave number and pressure dependencies are computed by using eight parameters which depend on the considered vibrational transition, the temperature, and the nature of the perturber only. These quantities are band-averaged values of the detailed spectroscopic and collisional parameters of the molecular system. Tests of the model are presented in the ν3 and 3ν3 bands of CO2 perturbed by He and Ar at elevated pressures. They demonstrate the accuracy of our approach in accounting for the effects of collisions on the spectral shape in a wide density range; indeed, the superposition of Lorentzian individual lines at low pressure, as well as the collapse (narrowing) of the band at very high pressure are satisfactory predicted.
Application of the Kubo–Anderson band shape equation to vibrational relaxation studies in the frequency domain and to an improved determination of spectral second moments from experimental data
Authors: Mariani L. , Morresi A. , Cataliotti R. S. , Giorgini M. G.
Published: July 2002

The well-known stochastic equation of Kubo–Anderson, widely adopted in the studies of time vibrational correlation functions, is put in a single-parametric form (defining a parameter α that describes the modulation regime of the dephasing) and Fourier transformed. It is shown how the corresponding analytical expression in the frequency domain can be compared with any experimental band shape, allowing the computation of the dynamical parameters. In particular, the more extensively addressed problems, in literature, in the field of vibrational relaxation studies, have been afforded and a contribution to their solution has been given; these are: the baseline positioning, the uncertainties of the dynamical variables involved in the stochastic model, and the frequency second moment computation. The algebraic development has been implemented in the KUBOFREQ© computer program; it has been applied in the fitting of two sets of experimental data: ν4 and ν5 mode of liquid CH3NO2 at various temperatures. The comparison of the dynamical variables computed with KUBOFREQ© and those previously obtained following the conventional time domain approach, shows a substantial agreement between the two methods: the former, however, gives more accurate values, because the baseline positioning and the second moment computation are based upon stringent criteria, allowing to correctly express the physical uncertainties of the variables. In the case of the ν5 mode, the uncertainty on α is about 5%, denoting that the Kubo–Anderson model is appropriate for the description of the vibrational relaxation of this mode; the uncertainties connected to the second moment are of the order of 10%–12%. The ν4 mode gives a band shape that still ‘‘wears’’ the theoretical profile, but in a worse way with respect to the ν5 profile: the α uncertainty is, in fact, of the order of 10%, while the second moment uncertainties are around 30%–40%. Finally, the analytical equation in the frequency domain may be regarded as a new representation of any band profile, directly dependent only on one parameter.