bluuebunny/crossref_metadata_2025 huggingface dataset:
Rotational conformational energetics of stiff aromatic polyimides: Effects of exchange repulsions, dipole-moiety interactions, and π-conjugations
Authors: Cui Chunzhi , Cho Seung Joo , Kim Kwang S. , Baehr Christoph , Jung Jin Chul
Published: July 2002

To study the conformational stability of aromatic polyimides which are highly thermally stable and retain their useful physical and electrical properties over 200 °C, we investigated four pristine compounds (N-phenyl-1,8-naphthalenedicarboximide, N-phenylmaleimide, N-2-pyrrolylmaleimide, and N-3-pyrrolylmaleimide) using ab initio calculations. Among these, the polyimides having the last two pristine compounds have not been synthesized yet. The four compounds representing the aromatic polyimides were comprised of two adjoining rings of, respectively, 6-6, 5-6, 5-5, and 5-5′, where each number denotes the number of atoms in each ring-skeleton, and 5′ represents a ring having an N–H group (instead of a C–H group in 5) to interact with the C=O group of the imide ring. The (internal rotation) angles (φ) between two rings of the global minimum energy conformers for the systems of 6-6, 5-6, 5-5, and 5-5′ are predicted to be 90° (perpendicular), 46°, 0° (coplanar), and 0°, respectively. The conformational energetics are governed mainly by the competition between the exchange repulsions, dipole–moiety interactions, and π conjugations. For rings/6-6 and rings/5-6, the dipole interactions and π conjugation effects in favor of coplanar structures are overwhelmed by the exchange repulsions which make the coplanar structures unstable. On the other hand, for rings/5-5 and rings/5-5′, the dipole–moiety interactions just overcome the exchange repulsions, hence the π conjugation effects would become relevant in accounting for the statibility of the coplanar conformation. It is worthwhile investigating these new types of coplanar polyimides for possible developments of new types of polyimides with useful electrical, optical, and physical properties.
Direct determination of localized Hartree–Fock orbitals as a step toward N scaling procedures
Authors: Rubio Jaime , Povill Angels , Malrieu Jean Paul , Reinhardt Peter
Published: July 2002

A method is proposed for the solution of the self-consistent field equations that can lead to localized occupied and virtual molecular orbitals, avoiding the need for solving for the canonical molecular orbitals. The method starts with strongly localized “guess molecular orbitals”, it is nonperturbative and proceeds through the diagonalization of single configuration interaction matrices which may be rendered size-consistent through appropriate coupled electron pair approximation or coupled-cluster-type dressings. We see a potential utility for the method in applications to large systems where localized orbitals will improve the scaling of the computational resources required with increasing system size.
Fundamental measure theory for mixtures of parallel hard cubes. I. General formalism
Authors: Cuesta José A. , Martı́nez-Ratón Yuri
Published: July 2002

This article, the first of a series of two, describes the formulation of Rosenfeld’s fundamental measure theory for a mixture of parallel hard cubes, a model recently introduced to study the demixing transition for additive hard core potentials. Special emphasis is put on the good performance of the functional when reducing the dimensionality of the system, a necessary feature to give reasonable results in highly inhomogeneous situations. This property allows for an extremely simple formulation of the theory in arbitrary dimensions. In a subsequent article we will describe the properties of the mixture as they are predicted by the theory, in particular the demixing in presence of the freezing transition.
The C̃ 2A2 excited state of NO2: Evidence for a Cs equilibrium structure and a failure of some spin-restricted reference wavefunctions
Authors: Crawford T. Daniel , Stanton John F. , Szalay Péter G. , Schaefer Henry F.
Published: July 2002

The geometry and harmonic vibrational frequencies of the the C̃ 2A2 electronic state of nitrogen dioxide have been determined using coupled cluster (CC) theory in conjunction with large basis sets. The minimum energy C2v-constrained N–O bond distance predicted by the highest level of theory is significantly shorter than the experimentally inferred value, and it is unlikely that further refinement in the theoretical treatment would reduce the discrepancy. However, this work suggests that a prevailing assumption of C2v symmetry may be incorrect; the equilibrium structure of the C̃ state may instead have Cs symmetry. In addition, and contrary to expectation, CC calculations based on variationally optimal spin-unrestricted or spin-restricted reference determinants give qualitatively different results for this system, unless very high levels of electron correlation are included in the calculation.
Reactions of size-selected positively charged nickel clusters with carbon monoxide in molecular beams
Authors: Vajda Štefan , Wolf Sebastian , Leisner Thomas , Busolt Ulrike , Wöste Ludger H. , Wales David J.
Published: July 2002

Reactions of small thermalized positively charged nickel clusters with carbon monoxide were studied in a molecular beam experiment. The nickel clusters were produced in a high intensity cluster ion source and thermalized in a large helium-filled quadrupole ion guide. The clusters were size selected by a quadrupole mass spectrometer. The mass- and charge-selected nickel clusters then passed through a linear quadrupole drift tube filled with a mixture of helium buffer gas and carbon monoxide. The reaction products were then analyzed by a quadrupole mass-spectrometer. Using this technique, saturation limits for Nin+ clusters with n=4–31 were measured and the competitive reaction channels were identified. Under certain experimental conditions carbide formation was observed in the case of the nickel tetramer, pentamer, and hexamer. The structure of the nickel carbonyl clusters is discussed within the framework of the polyhedral skeletal electron pair theory. The cluster growth may be explained by a pentagonal sequence of structures for n=4–7, capping of the pentagonal bipyramid to buildup an icosahedron at Ni13+, and further capping of this icosahedron to form a double icosahedron at Ni19+.
Aligning symmetric and asymmetric top molecules via single photon excitation
Authors: Weida Miles J. , Parmenter Charles S.
Published: July 2002

The theory developed by Zare [Ber. Bunsenges. Phys. Chem. 86, 422 (1982)] for using electric-dipole-allowed photoexcitation with linearly polarized light to align linear and symmetric top molecules via parallel transitions is extended to include perpendicular transitions, as well as the alignment of asymmetric tops via a-, b-, or c-type transitions. Analytical expressions for the spatial distribution of a symmetric top figure axis following a parallel or perpendicular transition are presented. A prescription is developed for determining the spatial distribution of each principal axis of an asymmetric or symmetric top following parallel or perpendicular type transitions. The degree of alignment obtainable via photoexcitation for symmetric and asymmetric tops is discussed, with the somewhat surprising result that all three principal axes of an asymmetric top can be highly aligned via photoexcitation. A simple computer program for calculating the degree of alignment of each principal axis of a symmetric or asymmetric top following an a-, b-, or c-type transition is described and made available.
Quantum-chemical study of the vibronic structure of the phosphorescence and T1→Tn absorption spectra of naphthalene
Authors: Zgierski Marek Z.
Published: July 2002

The vibronic structure of the phosphorescence spectrum of naphthalene is calculated from ab initio and semiempirical geometries and force-fields of the S0 and T1 states. An excellent agreement with the experiment is obtained. The final triplet state in the triplet→triplet absorption spectrum is identified as 3 3B3g state and the vibronic structure of the 1 3B1u→3 3B3g transition is found in a good agreement with the experimental spectrum. The wavefunction of the 3 3B3g state is dominated by the same electronic configurations as that of the lowest triplet of the same symmetry.
Semiclassical quantization of fragmented tori: Application to saddle-node states of LiNC/LiCN
Authors: Zembekov A. A. , Borondo F. , Benito R. M.
Published: July 2002

A new implementation of the EBK method for the semiclassical quantization of partially destroyed tori is presented. The application to the calculation of some quantum states of the LiNC/LiCN molecule which are influenced by a saddle-node bifurcation is discussed. In this quantization surrogates of invariant tori, computed using gap filling saddle-node orbits, are used. These orbits are obtained from a very detailed study of the islands around islands structure existing in the relevant classical phase space region.
On the validity of a variational principle for multicomponent systems
Authors: Marı́n C. , Garzó V.
Published: July 2002

The validity of a variational principle for nonequilibrium steady states proposed by Evans and Baranyai [Phys. Rev. Lett. 67, 2597 (1991)] is investigated in the case of a dilute binary mixture described by the well-known Groos–Krook kinetic model. We construct a perturbation solution around the unconstrained shear flow state and evaluate the phase-space compression factor, the temperature ratios, and the nonlinear shear viscosity up to the first-order approximation. All these quantities are nonlinear functions of the shear rate and the parameters of the mixture (particle masses, concentrations, and force constants). It is shown that this principle does not hold exactly, although deviations from it are small in some situations for not very large shear rates. The calculations presented here extend previous results derived for a single dilute gas.
Spectra of rare gas hydrides. VI. The 4f complex and the electronic structure of KrD
Authors: Dabrowski I. , Sadovskiı́ D. A.
Published: July 2002

The complete rotational analysis of the 4f(2Φ,2Δ,2Π,2Σ+)→4d 2Δ (6600 cm−1), 4f(2Φ,2Δ,2Π,2Σ+)→4d 2Π (8500 cm−1), and 4d 2Δ→5p 2Π (7100 cm−1) Rydberg electronic transitions of KrD is reported. The two newly observed states, the doublet 4f complex and 4d 2Δ together with the np(2Π,2Σ+), nd 2Σ+, ns 2Σ+, and nd 2Π studied earlier by Dabrowski et al., [Mol. Phys. 63, 269 (1988)], and by Dabrowski and Sadovskiı́ [Mol. Phys. 81, 291 (1994)], form a set of states which permits the whole Rydberg electronic structure of the molecule to be determined from the experimental data.
Molecular structure, vibrational frequencies, and energetics of the HOCO+ ion
Authors: Francisco Joseph S.
Published: July 2002

Equilibrium geometries have been optimized and harmonic vibrational frequencies obtained for each of the neutral and protonated structures of HOCO employing augmented coupled cluster methods. Calculations on CO and CO2 have been performed to assist in calibrating the computed results. The proton affinity is estimated to be 130.7 kcal mol−1 and is in good agreement with experiments. Adiabatic ionization potential for HOCO is estimated to be 184.4 kcal mol−1 (8.01 eV) and is found to be discrepant with the measurements of Ruscic, Schwarz, and Berkowitz [J. Chem. Phys. 91, 6780 (1989)].
Hyperpolarizability depolarization ratios of nitroanilines
Authors: Norman Patrick , Luo Yi , Jonsson Dan , Ågren Hans , Sylvester-Hvid Kristian O. , Mikkelsen Kurt V.
Published: July 2002

The depolarization ratios for the first hyperpolarizabilities of para-, meta-, and ortho-nitroanilines have been calculated in gas and solution phases. The results resolve ambiguities in the interpretation of experimental results for these quantities, and pinpoint the actual problems in previous simulations. It is argued that extraction of individual components of β of low symmetry molecules from experimental depolarization measurements alone is difficult, and that the support from simulations to extract the proper information is indispensable in most cases.
Laser light scattering studies on thermodynamics of C8-lecithin and monovalent salt solutions
Authors: Huang Yao-Xiong
Published: July 2002

The purpose of this paper is to investigate the effect of monovalent salts on the thermodynamics of micelle formation in aqueous C8-lecithin solution. The critical micelle concentration (CMC) and mean hydrodynamic radius (R̄h) of the micelles as functions of amphiphile concentration, temperature, ionic identity, and strength were determined using laser light scattering technique. Then the standard Gibb’s free energy (ΔG0), enthalpy (ΔH0), and chemical potential (Δμ) of the micelle formation and growth were deduced from the experimental data. It is found that the micellization is a spontaneous process. Compared with the situation in a salt free solution, the reaction of micelle formation in KI added solutions is less exothermic, and the micelles in the solutions are less likely to form, and are smaller and less stable. The reaction of micelle formation in KCl added solutions, however, is more exothermic, and the micelles in the solutions are facile to form, and are larger and more stable. This indicates that monovalent salts affect the micelle formation and growth in the way predicted by the tendency of Δμ to vary with ionic identity and strength of the salts.
A further theoretical exploration of the surface-aligned photo-initiated H+CO2 reaction: Surface motion and temperature dependence
Authors: Setzler Josie V. , Bechtel Jason , Guo Hua , Schatz George C.
Published: July 2002

We report a further theoretical investigation of a model surface-aligned photoreaction with a moving surface approximation. The reaction is initiated by the photodissociation of a well-aligned HBr adsorbed on the LiF(001) surface. The collision of the dissociating H fragment with a coadsorbed CO2 leads to the OH and CO products. In an earlier theoretical study with a static surface model, it has been shown that the reactivity can be significantly enhanced relative to the corresponding gas phase reaction for some adsorption alignments. In this work, we address the roles played by surface motion and temperature. Our results indicate that some (∼0.3 eV) energy can be lost to the surface either from the adsorbed HOCO complex or from one of the products when it collides with the surface during the final disintegration of the HOCO complex. However, the energy transfer has a minor effect on the reactivity. The final state distributions of the products are found to be similar to those produced with the static surface model. On the other hand, a significant temperature effect is predicted for one adsorbate configuration. Apparently, the lowering of temperature for a well-aligned system results in a more narrowly focused alignment and higher reactivity.
Hydration pressure induced by solid surface dissolution in ion adsorption from aqueous media: Role of structural force in proton/oxydryle isotherms of α-tricalcium phosphate/water systems
Authors: Mezzasalma Stefano A.
Published: July 2002

By experimental titration measurements—which were conducted on α-Ca3(PO4)2/H2O (l) dispersions to follow H+/OH−adsorption from aqueous solution onto the solid surface—an unexpected shape transition (S→L, in the Giles classification) was observed in the adsorption isotherm when the solid mass concentration reached the (critical) value cNc=0.54±0.01. This phenomenon did not occur when acid/basic aqueous solutions containing Ca2+ and PO43−ions were employed as titrants. Formulation and application of an Ising-based model suggested that solid solubility phenomena can influence the adsorption mechanism by inducing an additional (inhomogeneous) hydration pressure, which seems to be related to interfacial order perturbations produced by the surface dissolution. In the end, the predicted critical solid mass concentration was cNc&dbigwig;=0.53±0.01and, correspondingly, extrapolated values from the model of water correlation length were in agreement with the current literature concerning TCP (tricalcium phosphate) wettability and inhomogeneous structural forces.
A subpicosecond pump-probe laser study of ionization and geminate charge recombination kinetics in alkane liquids
Authors: Siebbeles Laurens D. A. , Emmerichs Uli , Hummel Andries , Bakker Huib J.
Published: July 2002

We report on time resolved optical pump-probe investigations of the electron-cation recombination in alkane liquids. The alkanes were excited via two-photon excitation, using intense sub-picosecond pulses at 266 nm. Adding an electron scavenger to the liquid and probing the transient absorption at three different wavelengths (800 nm, 1500 nm, and 2250 nm) made it possible to distinguish the transient absorption due to excess electrons from that due to other absorbing species. The experimentally observed charge-recombination kinetics in n-hexane and n-octane could be fairly well reproduced by computer simulations in which the initial electron thermalization distance distribution was taken to be f(r)r2dr=(1/b)exp(−r/b)dr. Other distributions, such as a Gaussian, gave unsatisfactory results. The average electron thermalization distance in n-hexane was found to be 35±5 Å and in n-octane it was found to be 70±10 Å. The results for isooctane could be described either by the distribution (1/b)exp(−r/b)dr with an average thermalization distance of 25±5 Å or by a Gaussian distribution with an average thermalization distance 50±10 Å. The transient absorption in n-hexane and isooctane was observed to exhibit a red shift during the first 2 ps after the onset of the absorption. This spectral relaxation could be due to the slowing down of the ejected electron to thermal energies.
Dynamics and instabilities near the glass transition: From clusters to crystals
Authors: Nayak Saroj K. , Jena Puru , Ball Keith D. , Berry R. Stephen
Published: July 2002

Molecular dynamics simulation has been used to explore the evolution, kinetics, and dynamics of a liquid–glass transition in clusters and bulk matter. We demonstrate a dynamical indicator that characterizes the onset of the glass transition in clusters and is consistent with other indicators of glass transitions in bulk systems. This criterion, based on changes in chaotic behavior as measured by the largest Liapunov exponent, reveals aspects of the microscopic processes associated with the phase change from liquid to glass, and provides a connection between the thermodynamic and dynamical behavior of systems and their multidimensional potential surfaces.
Solvation dynamics: The role of hydrogen bonding
Authors: Sesé G. , Padró J. A.
Published: July 2002

Molecular dynamics computer simulations have been performed to analyze the dynamics of ionic solvation. Dilute solutions of Na+ and Cl− in methanol and in a nonassociated model polar solvent whose molecules have the same polarity as the methanol molecules, but that cannot establish hydrogen bonds between themselves, have been studied. An analysis of the solvent response to step changes of the electrical charge of the solute in the linear response approximation has been carried out by calculating the autocorrelation functions of the fluctuations of the electric potential at the position of the ion. It is proven that most of the relaxation occurs in a short time period, especially in the nonhydrogen bonded system. The long-time regime of the solvent response is considerably diminished in the nonassociated liquid. In order to investigate the role of the rotational motions of solvent molecules on the relaxation processes, the angular velocity correlation functions have also been evaluated.
Laser spectroscopy and crystal field analysis of Cm3+ diluted in Cs2NaYCl6
Authors: Murdoch K. M. , Cavellec R. , Simoni E. , Karbowiak M. , Hubert S. , Illemassene M. , Edelstein N. M.
Published: July 2002

Laser selective excitation, excited state absorption, two-photon absorption, and fluorescence line narrowing techniques have been used to investigate the electronic energy level structure of the Cm3+ ion diluted in the host crystal Cs2NaYCl6. In this crystal the Cm3+ ion replaces a Y3+ ion at an octahedral site (Oh symmetry). Analysis of these spectra resulted in the assignment of 52 energy levels, which were fitted to the parameters of an empirical Hamiltonian with a rms deviation of 57 cm−1. Many of the Γ8 symmetry levels exhibited small splittings, which was indicative of a site symmetry lower than Oh. The energies of the most prominent vibronic features have been measured and assigned to specific vibrational modes of the CmCl63− moiety or to lattice modes. The lifetimes of four fluorescing levels have been obtained at room and liquid helium temperatures. These were analyzed to determine the processes which contribute to the relaxation of an excited Cm3+ ion in this host. Blue 6P5/2→8S7/2 upconversion fluorescence was observed when pumping the orange 8S7/2→6D7/2 absorption transitions and explained by a phonon-assisted energy transfer mechanism.
High resolution Raman spectroscopy from vibrationally excited states populated by a stimulated Raman process: 2ν2–ν2 of C212H2 and C213H2
Authors: Bermejo D. , Cancio P. , Di Lonardo G. , Fusina L.
Published: July 2002

A generalization of a previously reported technique for investigating vibrationally excited states via high resolution Raman spectroscopy is proposed. In a first step, a vibrational state, typically not accessible by a dipole moment transition from the ground state, is populated in a pulsed stimulated Raman process. After a delay of several nanoseconds, a high resolution spectrum corresponding to transitions from this long lived state is recorded following a quasi-cw stimulated Raman spectroscopy scheme. The use of a pulsed dye laser for providing one of the pumping radiations overcomes some of the limitations of the previously proposed scheme, where this radiation was generated by a Raman shifter filled at relatively high pressure with the gas under study. Experimental aspects are discussed and the spectra of the Q branches of 2ν2(Σg+)–ν2(Σg+) of C212H2 and C213H2 are presented. The analysis of the measured transitions yielded accurate values of the spectroscopic parameters for the v2=2 vibrationally excited state for both isotopomers and improved values of the corresponding anharmonicity constants x220 are derived.
Double-resonance spectroscopy of the high Rydberg states of HCO. III. Multiple pathways in the vibrational autoionization of the bending overtone
Authors: Mayer Eric E. , Hedderich Hartmut G. , Grant Edward R.
Published: July 2002

Ultraviolet first-photon absorption selects individual rotational levels in the Renner–Teller split (020) vibronic band system in the 3pπ 2Π Rydberg state of HCO. These gateway states serve as originating levels for vertical second-photon transitions to vibrationally autoionizing Rydberg series converging to individual rotational levels associated with the (0200) and (0220) states of HCO+. Linewidths of assigned series convey information on autoionization dynamics. Resonances throughout the (020) autoionization spectrum match sharp profiles seen earlier for series converging to HCO+(010). Linewidths for autoionization via relaxation both of the bending fundamental and its overtone are measurably narrower than resonances built on CO stretch, (001), which indicates that mode-selectivity inhibiting bending autoionization relative to stretch extends to the bending overtone. Features in the (020) spectrum that fall below the threshold energy for decay by autoionization to the (010) continuum appear with diminished intensity and yet narrower linewidth. This provides evidence that vibrational autoionization in HCO conforms with a Δv=1 propensity rule that favors relaxation of (020) states to the (010) continuum over the two-quantum decay to produce HCO+(000). Analysis of detailed line shapes and intensities associated with individual vibrational angular momentum components of the relaxing-core overtone point to the way in which vibrational and orbital-electronic symmetries factor in the bound molecule and cation plus free electron, and fix the order in vibrational coordinates of necessary coupling terms.
Measurement of mass diffusion coefficients using nonexponential forced Rayleigh scattering signals
Authors: Spiegel D. R. , Marshall A. H. , Jukam N. T. , Park H. S. , Chang T.
Published: July 2002

Recent reports on mass-diffusion forced Rayleigh scattering (FRS) experiments have emphasized that the detected signal arises as a difference between two exponentially decaying fields diffracted from “complementary” ground-state and photoproduct population gratings. A mass-diffusion coefficient has nevertheless often been extracted by forcing a single-exponential fit to the data, especially in cases where the decay appears to be monotonic. In this paper, we use simulations and experiments to evaluate the accuracy of single-exponential fits for FRS profiles, and we propose a useful alternative method for obtaining a meaningful rate constant in cases in which the error in the single-exponential analysis is large. We begin by noting from the complementary grating model that (1) severe deviations from single-exponential decay can occur for an arbitrarily small (but nonzero) difference in the ground-state/photoproduct rate constants, and (2) the first cumulant of a FRS decay—in contrast to that of a dynamic light scattering profile—does not (in general) represent a physically useful decay rate. These statements apply to both monotonic and nonmonotonic decays. We then show that a combination of the first two FRS cumulants provides a physically useful mean rate constant. Finally, to address these issues experimentally, we have reexamined the diffusion of methyl red (MR) through 2-propanol at room temperature, a system previously analyzed using single-exponential fits. The new experiments, carried out at higher sensitivity than the previous studies, show that the MR/2-propanol signal is nonmonotonic. The geometric-mean diffusion coefficient obtained from the curvature of the local maximum is compared to the diffusion coefficient inferred from single-exponential fits, and it is found that the latter is larger by nearly a factor of 2. The results reported here should prove important in improving the accuracy of the FRS technique.
Perturbation theory for the breakdown of mean-field kinetics in oscillatory reaction-diffusion systems
Authors: Velikanov Mikhail V. , Kapral Raymond
Published: July 2002

Spatially distributed, nonequilibrium chemical systems described by a Markov chain model are considered. The evolution of such systems arises from a combination of local birth-death reactive events and random walks executed by the particles on a lattice. The parameter γ, the ratio of characteristic time scales of reaction and diffusion, is used to gauge the relative contributions of these two processes to the overall dynamics. For the case of relatively fast diffusion, i.e., γ≪1, an approximate solution to the Markov chain in the form of a perturbation expansion in powers of γ is derived. Kinetic equations for the average concentrations that follow from the solution differ from the mass-action law and contain memory terms. For a reaction-diffusion system with Willamowski-Rössler reaction mechanism, we further derive the following two results: (a) in the limit of γ→0, these memory terms vanish and the mass-action law is recovered; (b) the memory kernel is found to assume a simple exponential form. A comparison with numerical results from lattice gas automaton simulations is also carried out.
Long-range electron transfer driven by two lasers: Induced irradiance
Authors: Dakhnovskii Yuri , Lubchenko Vassiliy , Coalson Rob D.
Published: July 2002

Kinetics and nonlinear optical properties of a long range electron transfer complex in a polar solvent are studied in the presence of two perpendicularly directed, independent (incoherent) lasers with electric fields aligned along the electron path. The rate constant and absorption/emission cross sections in both direction are computed on the basis of the channel approach, whose validity is justified by a rigorous derivation. It is found that despite strong dissipation due to interaction with the polar environment, emission can be stimulated in one (“primary”) direction. The emission spectra are found to depend on the frequency of the auxiliary laser propagating perpendicular to the direction of the primary laser, the intensities of both lasers, the frequency and reorganization energy of quantum modes in the solvent, and the reaction heat. The emission spectrum is only slightly sensitive to the laser intensities in certain high frequency regimes. The latter property is important for potential applications in which the reaction system would be employed as the working system for a laser with tunable frequency. Calculations suggest that a long range electron transfer system can be also used as a nonlinear optical device which transfers energy from one laser to another.
The internal energy of CO2 produced from catalytic oxidation of CO by NO
Authors: Bald Daniel J. , Bernasek Steven L.
Published: July 2002

Internal energy distributions for CO2 produced in the catalytic oxidation of CO by NO on polycrystalline platinum have been directly measured using a high-resolution tunable diode laser spectrometer. Absorption spectra have been collected for CO2 produced in a flow cell reactor as a function of surface temperature and reactant ratio. Vibrational excitation of the product CO2 is observed for all reaction conditions. The vibrational energy of the asymmetric stretching mode is higher than the bending and symmetric stretching modes. The level of vibrational excitation for all normal modes is sensitive to the NO:CO ratio. The observed internal energy distribution of the product CO2 is compared to that of CO2 produced from CO oxidation by O2, using the same method. The similarities in the vibrational excitation and its dependence upon oxygen coverage suggest that the transition state for the CO2 formation step is the same for both the CO+NO and CO+O2 catalytic oxidation reactions. This similarity of likely transition states suggests that the CO+NO reaction proceeds by dissociative adsorption of NO, producing adsorbed oxygen atoms which oxidize CO to form CO2. This is the same route to the formation of CO2 which is observed for the CO+O2 reaction on Pt.
Investigation of photoexcitations of conjugated polymer/fullerene composites embedded in conventional polymers
Authors: Brabec C. J. , Dyakonov V. , Sariciftci N. S. , Graupner W. , Leising G. , Hummelen J. C.
Published: July 2002

A promising approach to improve the processability of semiconducting polymers is their incorporation into host matrices formed by conventional polymers such as polyethylene (PE) or polystyrene (PS). We have characterized the linear optical properties of several guest–host systems by absorption and luminescence measurements and probed the excited states by photoinduced absorption measurements (PIA) and light induced electron spin resonance (LESR). The interesting photophysics of conjugated polymer/fullerene donor–acceptor system, showing an ultrafast photoinduced electron transfer and a metastable charge separation in the pristine state, has been investigated in such composites where the photoactive components are further embedded into a conventional polymer matrix. The composition of the blend and the relative concentration of the components are found to strongly influence the photoinduced interaction between the conjugated polymer and the fullerenes (C60 and functionalized fullerenes). Photoinduced electron transfer between a soluble polyphenylenevinylene (PPV) derivative and C60 is observed in a system with PS as host when the concentration of the both electroactive components is 33%. In diluted composites with PS as matrix we find strong luminescence quenching of the PPV derivative but no electron transfer upon adding C60. Dominant photoexcitations in these PS systems are triplet states as follows from intensity dependencies and lifetime measurements. In diluted systems with PE as matrix neither charge transfer nor luminescence quenching has been observed.
Density dependence of self-diffusion in liquid hydrogen fluoride
Authors: Karger N. , Lüdemann H.-D.
Published: July 2002

Previously published self-diffusion data of HF are analyzed with newly measured densities of the liquid, from 258 K to 373 K, and at pressures up to 200 MPa. The results confirm the importance of hydrogen bonding upon the translational mobility in liquid hydrogen fluoride.
Near-infrared spectroscopy of CH2 by frequency modulated diode laser absorption
Authors: Marr Andrew J. , Sears Trevor J. , Chang Bor-Chen
Published: July 2002

A diode laser spectrometer incorporating a multi-pass Herriott type cell and frequency modulation detection was used to record a previously unaccessed region of the near-infrared singlet←singlet absorption spectrum of methylene between 10 000 cm−1 and 10 600 cm−1. With this spectrometer, signal-to-noise ratios close to the quantum noise limit have been attained. Identification of rovibronic transitions to five previously unobserved levels, K=1 ã(0,9,0), K=2 b̃(0,1,0), K=2 ã(1,6,0), K=3 b̃(0,1,0) and K=3 ã(0,10,0), was made. Despite the fact that the present spectra access levels within approximately 1300 cm−1 of the barrier to linearity, the spectrum is dense and perturbed, characteristics in common with spectra recorded in many previous studies at shorter wavelengths. Recent spectroscopic observations of halomethylenes [J. Mol. Spectrosc. 188, 68 (1998)] had suggested that the CH2 spectrum might become simpler at longer wavelengths, but this was not evident in the observed spectra. The mixed nature of the singlet states is evidenced by the assignment of rovibronic transitions to levels containing primarily ã 1A1 state character. The new measurements provide a stringent test for modern theoretical models for CH2 and will enable refinement of the electronic potential surfaces.
Photodissociation dynamics of (C6H6)3+: Role of the extra benzene molecule weakly bound to the dimer core
Authors: Ohashi Kazuhiko , Nishi Nobuyuki
Published: July 2002

The benzene trimer ion has a charge-localized structure, (C6H6)2+⋯C6H6, where (C6H6)2+ is the dimer core. The trimer ion is photodissociated by excitation of the charge resonance transition of the dimer core. Branching ratios and translational energies of the product ions, (C6H6)2+ and C6H6+, are measured as functions of photon energies (hν=0.99–1.80 eV). At the lowest photon energy studied, the dominant dissociation channel is the formation of (C6H6)2+ and C6H6. In this process, only a small fraction (7%–8%) of the available energy is partitioned into the translational energy of the products. The branching ratio of the (C6H6)2+ product decreases smoothly with increasing photon energy from 0.90 at hν=0.99 eV to 0.04 at 1.80 eV. The behavior is consistent with the following two-step model for the formation of C6H6+. The photoexcited (C6H6)3+ ion first ejects one C6H6 molecule, yielding the transient dimer ion. If the dimer ion has sufficient internal energy, it further dissociates into C6H6+ and C6H6. Statistical theories for unimolecular reactions are applied to predict the translational energies and the branching ratios. A comparison of the theoretical branching ratios with the experimental data suggests that a part (30%–35%) of the product internal energy is distributed to the intramolecular vibrations of the extra C6H6 molecule. As far as the energy partitioning is concerned, the extra C6H6 molecule is no longer a spectator.
Nucleation in n-alkanes: A density-functional approach
Authors: Seok Chaok , Oxtoby David W.
Published: July 2002

A density-functional theory for a polyatomic system is applied to gas–liquid nucleation in n-butane and n-heptane, employing an interaction site model and the rotational isomeric state approximation. Effects of chain length and flexibility on equilibrium properties and nucleation are discussed. It is shown that the n-alkane systems cannot be well approximated by a system with the spherically symmetric Lennard-Jones potential, giving a nucleation rate scaled by the classical rate smaller by six orders of magnitude.
Evidence for a strong intermolecular bond in the phenol⋅N2 cation
Authors: Haines S. R. , Geppert W. D. , Chapman D. M. , Watkins M. J. , Dessent C. E. H. , Cockett M. C. R. , Müller-Dethlefs K.
Published: July 2002

The phenol⋅N2 complex cation has been studied with a combination of two-color resonant zero kinetic energy (ZEKE) and mass analyzed threshold ionization (MATI) spectroscopies to probe the interaction of a polar cation with a quadrupolar solvent molecule. Extended vibrational progressions are observed in three modes which are assigned as the in-plane bend (35 cm−1), the stretch (117 cm−1), and in-plane wag (130 cm−1) intermolecular vibrations, and are consistent with a structure where the N2 forms a directional bond to the phenol OH group in the plane of the aromatic ring. Ab initio calculations at the UMP2/6-31G*, UHF/cc-pVDZ, and UMP2/cc-pVDZ levels of theory support this assignment. The spectra also provide a value for the adiabatic ionization energy (67 423 cm−1±4.5 cm−1) and an estimate of the dissociation energy of the cluster (1650±20 cm−1) which illustrate that the quadrupolar nitrogen molecule binds considerably more strongly to the phenol cation than a rare gas atom. These results constitute the first report of an aromatic⋅N2 complex where the interaction can be described in terms of weak hydrogen bonding, rather than in terms of a van der Waals bond to the π-system of the benzene ring.
Charge transfer and fragmentation of liquid helium clusters that contain one or more neon atoms
Authors: Ruchti Thomas , Förde Kirk , Callicoatt Berton E. , Ludwigs Henrik , Janda Kenneth C.
Published: July 2002

An investigation of the electron impact ionization and fragmentation of helium clusters that contain Ne atoms and Nek subclusters has been performed. The charge transfer probability from He+ to Ne and the branching ratios for fragmentation of the Nek subclusters were found by analyzing the dependence of the ion signal intensities on the Ne pressure in the “pickup” region. The measured charge transfer probability from He+ to Ne ranges from 0.06±0.01 for clusters of mean original size 〈N〉=3300 to 0.43±0.02 for 〈N〉=1100. Charge transfer to a single Ne atom within the helium clusters never yields bare Ne+ ions. Instead, fragments of the type NeHen+ are produced. The charge transfer from He+ to Ne2 subclusters yields mainly Ne2+ for smaller initial cluster sizes, but NeHen+ or Ne2Hen+ fragments are more probable for larger clusters. This shows that He droplets of a few thousand atoms are able to cage Ne2 subclusters by dissipating the entire energy released by charge transfer and formation and vibrational relaxation of the Ne2+ ion. Interestingly, it was found that in these relatively small helium clusters the Ne3 and Ne4 subclusters never survive the charge transfer from He+. Fragments such as Ne2+ and Ne2Hen+ are more likely to survive than are Ne3+ and Ne4+. In general, the results presented here are qualitatively similar to those for a recent study of the ionization of Ar in helium droplets. In both cases fragmentation to the bare ion is rare, while fragmentation to the dimer ion dominates. However, the helium cluster caging effect is more efficient for Ne subclusters than for Ar subclusters. Also, there is no evidence for shell structures in the NeHen+ ion fragment distributions.
Investigation of the short-time photodissociation dynamics of trans-1-bromo-2-iodoethane in the
A
-band absorption
Authors: Zheng Xuming , Phillips David Lee
Published: July 2002

We have obtained resonance Raman spectra and absolute Raman cross section measurements at five excitation wavelengths within the A-band absorption for 1-bromo-2-iodoethane in cyclohexane solution. The resonance Raman spectra have most of their intensity in the fundamentals, overtones, and combination bands of six Franck–Condon active vibrational modes; the nominal CCI bend, C–I stretch, C–Br stretch, C–C stretch, CH2 wag with the Br atom attached to the CH2 group, and CH2 wag with the I atom attached to the CH2 group. The resonance Raman intensities and A-band absorption spectrum were simulated using a simple model and time-dependent wave packet calculations. The simulation results and normal mode descriptions were used to find the short-time photodissociation dynamics in terms of internal coordinate displacements. The A-band short-time photodissociation dynamics for trans-1-bromo-2-iodoethane show that the C–I, C–Br, and C–C bonds as well as the CCI, CCBr, HCC, ICH, and BrCH angles have significant changes during the initial stages of the photodissociation reaction. This indicates the photodissociation reaction has a large degree of multidimensional character and suggests that the bromoethyl photofragment receives substantial internal excitation in so far as the short-time photodissociation dynamics determines the energy partitioning. Comparison of our results for 1-bromo-2-iodoethane with the A-band short-time dynamics of iodoethane, 1-chloro-2-iodoethane, and 1,2-diiodoethane and the trends observed for their A-band absorption spectra suggest that both the C–I and C–Br bonds experience a noticeable amount of photoexcitation.
Shape and size of simple cations in aqueous solutions: A theoretical reexamination of the hydrated ion via computer simulations
Authors: Martı́nez José M. , Pappalardo Rafael R. , Marcos Enrique Sánchez
Published: July 2002

The simplest representation of monoatomic cations in aqueous solutions by means of a sphere with a radius chosen on the basis of a well-defined property (that of the bare ion or its hydrate) is reexamined considering classical molecular dynamics simulations. Two charged sphere–water interaction potentials were employed to mimic the bare and hydrated cation in a sample of 512 water molecules. Short-range interactions of trivalent cations were described by Lennard-Jones potentials which were fitted from ab initio calculations. Five statistically independent runs of 150 ps for each of the trivalent spheres in water were carried out in the microcanonical ensemble. A comparison of structural and dynamical properties of these simple ion models in solution with those of a system containing the Cr3+ hydrate ([Cr(H2O)6]3+) is made to get insight into the size and shape definition of simple ions in water, especially those that are highly charged. Advantages and shortcomings of using simple spherical approaches are discussed on the basis of reference calculations performed with a more rigorous hydrated ion model [J. Phys. Chem. B 102, 3272 (1998)]. The importance of nonspherical shape for the hydrate of highly charged ions is stressed and it is paradoxically shown that when spherical shape is retained, the big sphere representing the hydrate leads to results of ionic solution worse than those obtained with the small sphere. A low-cost method to generate hydrated ion–water interaction potentials taking into account the shape of the ionic aggregate is proposed.
The vinyl radical (Ã2A″←X̃2A′) spectrum between 530 and 415 nm measured by cavity ring-down spectroscopy
Authors: Pibel Charles D. , McIlroy Andrew , Taatjes Craig A. , Alfred Sterling , Patrick Katina , Halpern Joshua B.
Published: July 2002

High resolution (0.008 nm) transient absorption spectra have been measured for the vinyl radical (C2H3) Ã 2A″←X̃ 2A′ transition between 530 and 415 nm using cavity ring-down laser absorption spectroscopy. This is over 200 times better resolution than the only previous recorded spectrum of vinyl in this wavelength region by Hunziker, Kneppe, McLean, Siegbahn, and Wendt [Can J. Chem. 61, 993 (1983)]. With the improved resolution in the present study, new vibrational bands are detected, and some rotational structure of the bands is resolved. A calculation of the envelope of the origin band was carried out, using a model for a c-type transition of an asymmetric top. For a best fit to the data, the model requires a 1 cm−1 Lorentzian linewidth, which suggests a short-lived excited state.
An estimation of the isomerization energy of acetylene
Authors: Stanton John F. , Gauss Jürgen
Published: July 2002

An accurate prediction of the adiabatic energy difference separating acetylene and vinylidene is presented. The electronic energy contribution is estimated on the basis of CCSD(T) calculations using up to 400 contracted Gaussian basis functions together with extrapolation to the basis set limit; an additional correction is applied to account for residual electron correlation effects. Zero-point vibrational corrections are based on quartic force fields of both acetylene and vinylidene calculated at the CCSD(T) level. Also included are contributions for relativistic effects and the Born–Oppenheimer diagonal correction. The isomerization energy is estimated to be 15200±205 cm−1, which is somewhat lower than the best previous theoretical prediction.
Comment on “Dynamics of solvated ion in polar liquids: An interaction-site-model description” [J. Chem. Phys. 108, 7339 (1998)]
Authors: Biswas Ranjit , Bagchi Biman
Published: July 2002

Recently, Chong and Hirata have presented a molecular approach for friction [J. Chem. Phys. 108, 7339 (1998)] on a solvated ion in a dipolar liquid. While their work is interesting, we have found several unjustified statements about our work in the above article. In this Comment we address these statements to put our work in proper perspective.
Negative vibrational shift of nitrogen diluted in xenon at the fluid–solid transition
Authors: Kooi M. E. , Michels J. P. J. , Schouten J. A.
Published: July 2002

It is known from experimental evidence that the Raman shift of nitrogen and nitrogen in argon, measured as a function of the pressure at ambient temperature, reveals a positive jump at the transition from the liquid to the solid phase. Intuitively, this increase is sometimes attributed to the increase of density, but recently it has been shown that the change in order at the transition also plays a role. The present study deals with the behavior of nitrogen diluted in xenon. In this system, even a negative jump is found experimentally. Computer simulations on a model system for the N2–Xe mixture make clear that in this case the effect of the change in order is opposite.
Third-order transport coefficients of ions in electrostatic fields
Authors: Koutselos Andreas D.
Published: July 2002

The form of third-order transport coefficients of ions in electrostatic fields determined through molecular-dynamics simulation of the ion motion is examined using stochastic theory arguments. It is found that the simulation method determines components of the symmetric form of the transport coefficients defined through macroscopic considerations.
Cumulative reaction probability in terms of reactant-product wave packet correlation functions
Authors: Garashchuk Sophya , Tannor David J.
Published: July 2002

We present new expressions for the cumulative reaction probability (N(E)), cast in terms of time-correlation functions of reactant and product wave packets. The derivation begins with a standard trace expression for the cumulative reaction probability, expressed in terms of the reactive scattering matrix elements in an asymptotic internal basis. By combining the property of invariance of the trace with a wave packet correlation function formulation of reactive scattering, we obtain an expression for N(E) in terms of the correlation matrices of incoming and outgoing wave packets which are arbitrary in the internal coordinates. This formulation, like other recent formulations of N(E), allows calculation of the quantum dynamics just in the interaction region of the potential, and removes the need for knowledge of the asymptotic eigenstates. However, unlike earlier formulations, the present formulation is fully compatible with both exact and approximate methods of wave packet propagation. We illustrate this by calculating N(E) for the collinear hydrogen exchange reaction, both quantally and semiclassically. These results indicate that the use of wave packet cross-correlation functions, as opposed to a coordinate basis and flux operators, regularizes the semiclassical calculation, suggesting that the semiclassical implementation described here may be applied fruitfully to systems with more degrees of freedom.