bluuebunny/crossref_metadata_2025 huggingface dataset:
Shocked molecular solids: Vibrational up pumping, defect hot spot formation, and the onset of chemistry
Authors: Dlott Dana D. , Fayer Michael D.
Published: July 2002

A model and detailed calculations are presented to describe the flow of energy in a shocked solid consisting of large organic molecules. The shock excites the bulk phonons, which rapidly achieve a state of phonon equilibrium characterized by a phonon quasitemperature. The excess energy subsequently flows into the molecular vibrations, which are characterized by a vibrational quasitemperature. The multiphonon up pumping process occurs because of anharmonic coupling terms in the solid state potential surface. Of central importance are the lowest energy molecular vibrations, or ‘‘doorway’’ modes, through which mechanical energy enters and leaves the molecules. Using realistic experimental parameters, it is found that the quasitemperature increase of the internal molecular vibrations and equilibration between the phonons and vibrations is achieved on the time scale of a few tens of picoseconds. A new mechanism is presented for the generation of ‘‘hot spots’’ at defects. Defects are postulated to have somewhat greater anharmonic coupling, causing the vibrational temperature in defects to briefly overshoot the bulk. The influence of the higher defect vibrational temperature on chemical reactivity is calculated. It is shown that even small increases in defect anharmonic coupling have profound effects on the probability of shock induced chemistry. The anharmonic defect model predicts a size effect. The defect enhanced chemical reaction probability is reduced as the particle size is reduced.
Freezing of binary mixtures of colloidal hard spheres
Authors: Bartlett P. , Ottewill R. H. , Pusey P. N.
Published: July 2002

The freezing phase transition in a binary suspension of colloidal hard spheres of diameter ratio α=0.61 was studied by light scattering and scanning electron microscopy. The suspensions consisted of sterically stabilized poly(methyl methacrylate) spheres of diameters about 670 and 407 nm suspended in a near refractive indexed matched suspension medium composed of carbon disulphide and cis-decalin. With increasing volume fraction, binary suspensions of number fraction of larger component A xA>0.43 crystallized to give irregularly stacked close packed crystals containing almost entirely component A. As the number fraction xA decreased, the rate of crystallization decreased. Suspensions of xA≊0.28 remained amorphous and showed glassy behavior. Suspensions of xA≊0.057 showed a complex sequence of phase behavior with coexistence of crystals of component B, the ordered binary alloy phase AB13, and a binary fluid. In suspensions with xA<0.057, the only solid phase observed was irregularly stacked close packed crystals of component B. The observed phase behavior is compared with the predictions of a model for freezing of a mixture of hard spheres which are assumed to be immiscible in the solid phase.
CO diffusion on Pt(111) with time-resolved infrared-pulsed molecular beam methods: Critical tests and analysis
Authors: Reutt-Robey J. E. , Doren D. J. , Chabal Y. J. , Christman S. B.
Published: July 2002

The microscopic diffusion of CO on stepped Pt(111) crystal surfaces has been investigated with pulsed molecular beam–time-resolved surface infrared methods. Following a rapid exposure to CO, we record the time evolution of the CO surface vibrational spectra as the CO diffuse from the initial random distribution to the thermodynamically favored step sites. The data are simulated with a model that describes the kinetics of diffusion across the terraces, as well as the filling of step sites. We critically evaluate this model and the general experimental approach by extending our previous measurements of CO diffusion on Pt(28(111)–(11̄0)) to a surface with higher step density, Pt(l2(111)–(11̄0)), with varying coverages. The model describes both sets of data with the same parameters, confirming the original determination of the diffusion barrier (ΔET) and prefactor (AT) for microscopic surface hopping of CO/Pt(111). This further provides a quantitative means to estimate systematic errors. We report ΔET=4.0±0.7 kcal/mol and AT=109.2 s−1, with minimum and maximum possible values of 107.0 and 1010.4 s−1 for AT.
Optical dephasing in solution: A line shape and resonance light scattering study of azulene in isopentane and cyclohexane
Authors: Nibbering Erik T. J. , Duppen Koos , Wiersma Douwe A.
Published: July 2002

Results of a line shape and resonance light scattering study of the S1←S0 and S2←S0 electronic transitions of azulene in isopentane and cyclohexane are reported. The results are analyzed using two different non-Markovian master equations that make different assumptions about the statistical properties of the bath. For both these origin transitions we find that the solution dynamics fall in the so-called intermediate modulation regime. If exponential decay is assumed for the bath correlation function we obtain a correlation time of the bath of 25 fs for the S1←S0 transition and of 13 fs for the S2←S0 transition at room temperature. From the frequency dependence of the ratio of fluorescence to Raman yields of the S1←S0 transition we calculate an excited state lifetime of 1.4±0.2 ps using the parameters of the bath derived from the line shape analysis, and irrespective of which master equation is used.
An electronic Hamiltonian for origin independent calculations of magnetic properties
Authors: Helgaker Trygve , Jo/rgensen Poul
Published: July 2002

A gauge origin independent formalism for the calculation of molecular magnetic properties is presented. Origin independence is obtained by using London’s gauge invariant atomic orbitals, expanding the second quantization Hamiltonian in the external magnetic field and nuclear magnetic moments, and using the resulting expansion terms as perturbation operators in response function calculations. To ensure orthonormality of the molecular orbitals, a field-dependent symmetrical orthonormalization is employed. In this way the gauge dependence of the London orbitals is transferred to the Hamiltonian. The resulting perturbation operators may be used to calculate magnetic properties from any approximate ab initio wave function.
Electric properties of diatomic interhalogens. A study of the electron correlation and relativistic contributions
Authors: Sadlej Andrzej J.
Published: July 2002

The recently developed polarized basis sets for halogen atoms are used to calculate several basic electric properties of diatomic interhalogen molecules. In the valence approximation the electron correlation contribution is treated at the level of the fourth-order many-body perturbation theory approximation. The corresponding results are corrected for the core-polarization and relativistic effects. It is shown that the dipole moments of heavy interhalogen diatomics are considerably affected by the relativistic contribution, while the core-polarization effects are of lesser importance. The calculated, relativistically corrected, dipole moments are in agreement with experimental data and are used to discuss the reliability of the effective core potential approximation. The calculated quadrupole moments and parallel components of the dipole polarizability tensor are only marginally affected by contributions due to core-polarization and relativistic terms. The geometry derivatives of dipole moments and dipole polarizabilities computed in this study can be used for the evaluation of intensities in infrared and Raman spectra of interhalogen diatomics.
Vibrational dependence of the anisotropic intermolecular potential of Ar–HF
Authors: Hutson Jeremy M.
Published: July 2002

A new intermolecular potential for Ar–HF is obtained by fitting to results from high-resolution microwave, far-infrared, and infrared spectroscopy. The new potential, designated H6(4,3,2), is a function of the diatom mass-reduced vibrational quantum number η=(v+ (1)/(2) )/(μHX)1/2 as well as the intermolecular distance R and angle θ, and has 22 adjustable parameters. It reproduces all the available spectroscopic data for levels of Ar–HF correlating with HF, v=0, 1, and 2, and DF, v=0 and 1. The H6(4,3,2) potential is qualitatively similar to previous potentials, with a linear Ar–H–F equilibrium geometry and a secondary minimum at the linear Ar–F–H geometry. Compared to the potential of Nesbitt et al. [J. Chem. Phys. 90, 4855 (1989)], obtained from spectra of Ar–HF (v=1), the H6(4,3,2) potential is rather deeper near the equilibrium geometry (Ar–H–F), but shallower around the secondary minimum (Ar–F–H). The absolute well depth increases by 19 cm−1 between HF v=0 and v=1. The vibrationally averaged induction energy is calculated to be substantially (8.1 cm−1 ) greater for v=1 than for v=0, and is responsible for most of the observed red shift in the complex. Predictions of additional spectroscopic properties that would test the new potential are given, including far-infrared and overtone spectra of Ar–DF and dipole moments of excited states of Ar–HF and Ar–DF.
Toward a molecular theory of vapor-phase nucleation. II. Fundamental treatment of the cluster distribution
Authors: Ellerby H. Michael , Reiss Howard
Published: July 2002

A statistical mechanical theory of the cluster distribution involved in homogeneous vapor-phase nucleation is presented based on the stochastic nature of physical clusters. This new approach supports the work of Ellerby, Weakliem, and Reiss (Paper I of this series) by deriving their results by a different, though equally rigorous method. Furthermore, the present treatment is more direct and transparent, avoiding completely the need to construct the partition function for the entire vapor, requiring instead the consideration only of the probability of an individual cluster. This circumvents the problem of complicated configuration counting involved in the partition function approach, and therefore eliminates the potentially tedious explanations corresponding to it. The increased simplicity and clarity of the method presented here result in new insights and discoveries concerning the cluster distribution. For example, the distribution is now understood in relation to fluctuation theory, and is cast in terms of an equilibrium constant for cluster formation. Beyond this, the meaning of the free energy of cluster formation in the theory is now thoroughly understood. This approach complements that of Ellerby, Weakliem, and Reiss, and advances the understanding and exposition of a viable theory of vapor-phase nucleation.
Viscosity and solvent dependence of low-barrier processes: Photoisomerization of cis-stilbene in compressed liquid solvents
Authors: Nikowa L. , Schwarzer D. , Troe J. , Schroeder J.
Published: July 2002

The photoisomerization of cis-stilbene in liquid solution was studied by time-resolved excited-state absorption spectroscopy using 306 nm pump and 612 nm probe pulses of 100 fs width. Transient absorption signals were found to decrease exponentially with time. Decay rate constants were determined over the pressure range 1–4000 bars at temperatures of 295 and 390 K in a series of alkane solvents as well as in methanol, acetonitrile, and in polymethylmethacrylate (PMMA). The viscosity dependence confirms the existence of two pathways of the reaction, one leading to ground-state cis- and trans-stilbene (C/TS), the other to dihydrophenanthrene (DHP). Whereas the DHP component shows only little viscosity dependence up to 4 kbar, the C/TS component is characterized by rate constants which are inversely proportional to the solvent viscosity. This is in contrast to earlier conclusions from studies with solvents at 1 bar. The C/TS process shows practically no temperature dependence apart from that of the viscosity, whereas the DHP process has an apparent activation energy of 8.5 kJ mol−1. The results are interpreted in terms of a Kramers–Smoluchowski equation for low-barrier processes as well as by solvent modifications of the low-barrier potential. Alternative interpretations by rate theory for barrierless processes are considered as well.
Simulation of the adsorption of symmetric diblock copolymers at the interface of the two monomeric homopolymers
Authors: Wang Yongmei , Mattice Wayne L.
Published: July 2002

A lattice model is presented to study a block copolymer compatibilizing two immiscible monomeric homopolymer blends. Each monomeric homopolymer occupies a single site on the lattice. The symmetric diblock copolymer AB is modeled by a self-avoiding walk on a cubic lattice. The two monomeric homopolymer fields interacting with the diblock copolymer are modeled by introducing two different kinds of vacancies in the lattice. These vacancies have the reduced pairwise interaction with A or B segments representing the homopolymer fields, namely εAS1 = 0, εBS2= 0, εAS2 = εBS1= εAB = ε ≥ 0. The two kinds of vacancies each occupy half of the lattice sites divided along the z direction, and forming a sharp interface at z=Lz/2. The equilibrium of copolymers dispersed in either phases vs localization at the interface is studied for a range of copolymer concentrations and different interaction energies. The number of chains per surface area, σ, was found to depend on the product of φb exp(εNc), where φb is the equilibrium concentration of the symmetric diblock copolymers in two phases, and Nc is the degree of polymerization of the copolymers. The dependence of σ on φb exp(εNc) is close to the Langmuir type adsorption. These results hold both at the cases that the copolymer chains in the bulk do not form micelles, and do form micelles. Micellization processes do not modify the overall function very much. That is partly due to the fact there is a large entropy loss when forming the micelles compared to the localization of the AB copolymer chains at the flat interface between the two monomeric homopolymer fields.
Calorimetric heats of adsorption for CO on nickel single crystal surfaces
Authors: Stuckless J. T. , Al-Sarraf N. , Wartnaby C. , King D. A.
Published: July 2002

An adsorption calorimeter for studies on well-defined single crystal surfaces under ultrahigh vacuum conditions is now available, based on supersonic molecular beam dosing onto ultrathin metal single crystals. Here we discuss the relationship between the calorimetric heat of adsorption as measured in this system and the related parameters: the differential heat of adsorption, the isosteric heat, and the Arrhenius desorption energy. Coverage-dependent calorimetric heats of adsorption and sticking probabilities for CO on Ni{111}, {110}, and {100} are presented, and comparisons made with literature values for isosteric heats and Arrhenius desorption energies. At intermediate coverages some significant discrepancies occur which are attributed to a temperature-dependent adlayer structure. By combining sticking probability with heat measurements at high coverage, at 300 K, where significant desorption occurs, the desorption preexponential has been accurately determined; differential entropies of adsorption are also obtained. Differences in initial heats of adsorption and in the coverage dependencies for the three crystal planes are discussed, particularly in relation to surface stoichiometry, and to CO–CO interactions.
Adiabatic pseudospectral methods for multidimensional vibrational potentials
Authors: Friesner Richard A. , Bentley Joseph A. , Menou Michel , Leforestier Claude
Published: July 2002

We describe a new algorithm for computing eigenvalues, spectral intensities, and selected eigenvectors of multidimensional vibrational potential surfaces. The method involves a synthesis of pseudospectral and sequential adiabatic reduction methods and merges the storage and computational advantages of the former with the improved basis set generated by the latter. The recursive residue generation method, which utilizes a Lanczos-based diagonalization procedure, is employed to calculate the observables. As a test case, we apply the method to computation of the infrared and stimulated emission pumping spectra for the HCN molecule and demonstrate a very large (one to three orders of magnitude) reduction in computational effort (for comparable accuracy) as compared to discrete variable representation (DVR)/adiabatic reduction or standard collocation approaches. We expect that this advantage will be increased considerably for larger (e.g., tetra-atomic) systems and will permit accurate basis set calculations on such systems to be carried out in a straightforward fashion.
Adiabatic and approximate diabatic potential energy surfaces for the B...H2 van der Waals molecule
Authors: Alexander Millard H.
Published: July 2002

We report multireference configuration-interaction calculations for the lowest potential energy surfaces of the B(2s22p 2P)...H2 van der Waals molecule. The degeneracy of the 2p orbital implies that there exist three adiabatic potential energy surfaces (two of A′ symmetry and one of A″ symmetry in Cs geometry) which become degenerate at large B–H2 separation. By assuming that the two adiabatic states of A′ symmetry correspond primarily to an orthogonal transformation of the in-plane B 2p orbitals, one can use calculated matrix elements of the electronic orbital angular momentum to transform to an approximate diabatic representation, which involves four potential energy functions. The proper angular expansion of these functions in terms of reduced rotation matrix elements is discussed and an analytic representation of the calculated points is obtained. The minimum energy of the B(2s22p 2P)...H2 van der Waals molecule is predicted to occur in C2v geometry with an electronic symmetry of 2B2, at a B–H2 distance of 3.11 Å, and a dissociation energy De of 121 cm−1. For the interaction of B(2P) with p-H2, assumed spherical in j=0, the zero-point corrected dissociation energy is D0=25 cm−1.
Application of the integral equation theory of polymers: Distribution function, chemical potential, and mean expansion coefficient
Authors: Gan Hin Hark , Eu Byung Chan
Published: July 2002

A recursive integral equation for the intramolecular correlation function of an isolated linear polymer of N bonds is derived from the integral equations presented in the preceding paper. The derivation basically involves limiting the density of the polymer to zero so that polymers do not interact with each other, and thus taking into account the intramolecular part only. The integral equation still has the form of a generalized Percus–Yevick integral equation. The intramolecular correlation function of a polymer of N bonds is recursively generated by means of it from those of polymers of 2, 3,..., (N−1) bonds. The end-to-end distance distribution functions are computed by using the integral equation for various chain lengths, temperatures, and bond lengths in the case of a repulsive soft-sphere potential. Numerical solutions of the recursive integral equation yield universal exponents for the mean square end-to-end distance in two and three dimensions with values which are close to the Flory results: 0.77 and 0.64 vs Flory’s values 0.75 and 0.6 for two and three dimensions, respectively. The intramolecular correlation functions computed can be fitted with displaced Gaussian forms. The N dependence of the internal chemical potential is found to saturate after some value of N depending on the ratio of the bond length to the bead radius.
Vibrational studies on electronic structures in metallic and insulating phases of the Cu complexes of substituted dicyanoquinonediimines (DCNQI). A comparison with the cases of the Li and Ba complexes
Authors: Yamakita Yoshihiro , Furukawa Yukio , Kobayashi Akiko , Tasumi Mitsuo , Kato Reizo , Kobayashi Hayao
Published: July 2002

Electronic structures in metallic and insulating phases of the Li, Cu, and Ba complexes of 2,5-R1,R2-DCNQI [R1=R2=Br (abbreviated as DBr) or R1=R2=CH3 (abbreviated as DMe); DCNQI=N,N′-dicyanoquinonediimine; 2,5- is usually omitted] have been studied by observing temperature dependencies of their infrared absorption bands between 295 and 23 K. At room temperature, the wave numbers (ν̃i) of infrared absorption bands of R1,R2-DCNQI and its Li and Ba complexes are linearly correlated with the degrees of charge transfer (ρ) (ρ=−0.5 and −1.0e for the Li and Ba complexes, respectively). The ν̃i–ρ relationships indicate that the ρ value for the Cu complexes is −0.67e. This result is consistent with the previously established view that the Cu cations in the Cu complexes at room temperature are in a mixed-valence state of Cu1.33+. In the infrared spectrum of Cu(DBr–DCNQI)2 at room temperature, no electron–molecular vibration (EMV) coupling bands are observed.

Below the metal–insulator (M–I) transition temperature (TMI), EMV bands grow continuously and the ordinary infrared bands observed at room temperature gradually split into three bands with decreasing temperature. Similarly, the infrared bands of Li(DBr–DCNQI)2 split into two bands. These splittings are due to an inhomogeneous charge distribution in the DCNQI columns produced by the freezing of charge-density wave (CDW). The peak-to-peak amplitudes of CDWs in the DCNQI columns estimated by use of the ν̃i–ρ relationships are 0.08±0.04 and 0.40±0.04e, respectively, for the Li and Cu complexes of DBr–DCNQI. The state of the frozen CDW is inferred from the number of split bands. Based on the observed continuous change of the infrared spectra of Cu(DBr–DCNQI)2 and the discontinuous changes of other quantities such as x-ray satellite reflections, lattice parameters, and magnetic susceptibilities, the M–I transition in Cu(DBr–DCNQI)2 may be described as follows: (1) above TMI the charges on Cu cations (two Cu1+’s: one Cu2+) are dynamically averaged to +1.33e through the Cu...N≡C bridge. (2) At TMI the charges abruptly localize in the order of (Cu1+...Cu2+...Cu1+...)n. At the same time, the CDWs begin to be frozen in the DCNQI columns. (3) As temperature decreases below TMI, the order of the frozen CDW develops gradually. In contrast to these changes in Cu(DBr–DCNQI)2, neither EMV bands nor band splittings are observed in the infrared spectra of Cu(DMe–DCNQI)2 at low temperatures. Instead, almost all bands show negative absorption lobes on their low-wave number sides and become asymmetric. This asymmetrization is due to interactions between the vibrational levels and low-lying continuous electronic levels responsible for a broad band observed in the 1600–800 cm−1 region.
Response and instabilities of the lamellar phase of diblock copolymers under uniaxial stress
Authors: Wang Zhen-Gang
Published: July 2002

We study the quasistatic behavior of the lamellar phase of diblock copolymers under uniaxial compression and tension along the normal direction of the layers, in both the weak segregation limit (WSL) and the strong segregation limit (SSL). In the SSL, we derive a (nonlinear) continuum free energy description of the system in terms of local displacement of the lamellar layers, and use this free energy to study the mechanical behaviors. While compression induces the usual Hookian elastic response (for strains or stresses that are not too large), tension leads to square-lattice wave undulations in the transverse directions when the strain exceeds a critical value. In the WSL close to the order–disorder transition temperature, compression can ‘‘melt’’ the lamellar phase to the isotropic phase; such a melting can take the form of three types of instabilities, a quasithermodynamic instability, a spinodal at controlled strain, and a mechanical instability at controlled stress. It is shown that the third instability always precedes the second one under controlled-stress conditions. For a weakly first-order transition, the quasithermodynamic instability precedes the mechanical instability; but for a (hypothetical) second-order transition, the mechanical instability appears first as the stress is increased. In the case of tension, a transverse square-lattice wave deformation again develops at a critical strain. This deformation can be followed by a subsequent melting of types similar to the compressional case, upon further increase of the stress or strain. In both the SSL and WSL, the modulus undergoes an abrupt decrease when layer undulation develops, to a value 7/15 of that before the onset of undulation. Because the critical strain for the onset of undulation is usually very small, the modulus for tension will appear different from the modulus for compression, the former being 7/15 of the latter. As a result of this decrease in the modulus, melting of the lamellar phase in the WSL will occur at larger strains under tension than under compression.
Atom scattering studies of liquid structure and dynamics: Collisions of Xe with a model of squalane
Authors: Lipkin N. , Gerber R. B. , Moiseyev N. , Nathanson G. M.
Published: July 2002

Molecular dynamics (MD) computer simulations are carried out for scattering of high-energy Xe atoms off liquid squalane, and the results are compared with those of molecular-beam scattering experiments. A crude model for squalane is adopted, describing the hydrocarbon chain molecule as a sphere, and ignoring the role of internal modes. Good overall agreement is found between the results of the simulations and experiment, both for angular distributions and for trends in energy transfer properties. In particular, excellent agreement is obtained for the dependence of the energy transfer on the deflection angle for in-plane scattering. Theory predicts less trapping events than found experimentally, probably due to the crude model adopted for the squalane molecules. The partial success of the model in predicting some properties and not others is discussed. The other main conclusions of the study are (1) The instantaneous local structure of the liquid surface is highly corrugated, giving rise to a broad angular distribution and to extensive out-of-plane scattering. (2) High-energy atoms undergo both a trapping desorption and also direct inelastic scattering, the latter yielding information on liquid structure. (3) The angular distribution of atoms at a selected final velocity is sensitive to the local structure and dynamics of the surface. (4) The direct scattering can be conveniently interpreted in terms of contributions from single, double, and multiple collision events, these being roughly equal in relative weight. Forward scattering at grazing angle is dominated by single collisions, while double and multiple collisions have higher contribution at other directions. The double collision contribution in particular contains structural information. (5) There is a substantial yield per collision for sputtering of the squalane-like soft spheres. These results provide insight into the dynamics of gas–liquid collisions, and indicate the usefulness of beam scattering as a tool for studying liquid structure and dynamics.
Reference interaction site model self-consistent field study for solvation effect on carbonyl compounds in aqueous solution
Authors: Ten-no Seiichiro , Hirata Fumio , Kato Shigeki
Published: July 2002

In the previous study, Chem. Phys. Lett. 214, 391 (1993), we developed a new computational procedure for the solvation effect on the electronic structure of solute based upon the reference interaction site model (RISM) integral equation and the Hartree–Fock equation. The method enables us to calculate the solvent distribution and solute electronic wave functions simultaneously, which is free from such empirical parametrizations as appeared in the usual models based on the dielectric continuum picture. In the present article, we have applied the method to several carbonyl compounds in aqueous solution. The SPC model was used to describe the liquid water. The vertical n→π*, π→π*, and σ→π* transitions of formaldehyde are examined by the RISM-self-consistent field formalism coupled with the restricted Hartree–Fock approximation, and then the free energy calculation was performed for the excited state in order to estimate the contributions for the optical fluorescence spectra. The intramolecular energy turned out to give significant contribution to the bathochromic shift of fluorescence relative to the absorption in the liquid phase. Furthermore the importance of the structural effect of the functional group was discussed by the calculations of three more carbonyl compounds, acetaldehyde, acetone, and acrolein.
Photodissociation pathways and conical intersections in the low-lying electronic states of SiH+2
Authors: Mort Steve P. , Jennings Neville A. , Balint-Kurti Gabriel G. , Hirst David M.
Published: July 2002

Potential energy surfaces are computed for the four lowest electronic states of SiH+2. The surfaces provide the data necessary for the discussion of the competitive photodissociation of SiH+2 to yield either SiH+ or Si+. The study has resulted in the ‘‘discovery’’ of a new conical intersection involving the ground state potential energy surface and a surface which had not previously been thought to lie at sufficiently low energies to be of interest in the photodissociation dynamics of the system. The three important conical intersections and their role in the photodissociation dynamics are discussed. Likely pathways leading from the photoexcited state created in the spectroscopic experiments to the observed SiH+ and Si+ ionic fragments are described. The ion, after initial excitation, has Ã 2A″ or odd symmetry and is thought to convert to a lower-lying even symmetry state (2A′) by making a transition from the upper- to the lower-half of a 2Π state through the mediation of a Renner–Teller coupling mechanism. The effect of the conical intersection on the strength and qualitative nature of the coupling between these two surfaces is examined.
Aggregation kinetics of paramagnetic colloidal particles
Authors: Promislow Joanne H. E. , Gast Alice P. , Fermigier Marc
Published: July 2002

Magnetic-field-induced chain formation of superparamagnetic latex particles is investigated via optical microscopy and digital image analysis. Our experiment is unique in that the aggregation process takes place in the bulk, where particles are free to diffuse in three dimensions, rather than on a surface. We find that the mean chain size 〈s(t)〉 has a power-law dependence on time, 〈s(t)〉∼tz, as predicted by the Smoluchowski equation and three-dimensional simulations of diffusion-limited aggregation of oriented particles. The value of the exponent z is found to have a weak inverse dependence on particle volume fraction φ and dimensionless dipole strength λ. We propose a new model for a characteristic time scale proportional to 1/λ and 1/φ. Comparison with experimental observations shows this scaling to be very effective but to break down suddenly above a critical volume fraction.
Theoretical study of the Cu(H2O) and Cu(NH3) complexes and their photolysis products
Authors: Pápai Imre
Published: July 2002

Equilibrium geometries, binding energies, harmonic vibrational frequencies, infrared intensities, and isotopic shifts have been calculated for the Cu(H2O) and Cu(NH3) complexes and their photolysis products [HCuOH, CuOH, HCu(NH2), and Cu(NH2)] using Kohn–Sham theory with a gradient-corrected nonlocal potential. Cu(H2O) and Cu(NH3) are weakly bound systems, their binding energies are estimated to be 3.7 and 12.0 kcal/mol, respectively. The HCuOH and HCu(NH2) insertion products are 2.4 and 6.3 kcal/mol less stable than Cu(H2O) and Cu(NH3), whereas H+CuOH and H+Cu(NH2) lie 49.7 and 58.0 kcal/mol above Cu(H2O) and Cu(NH3), respectively. The calculated harmonic frequencies agree remarkably well with matrix-isolation infrared data; the agreement is always within 50 cm−1 (30 cm−1 on average) and the mean relative deviation from the experimental frequencies is 2.8%. The calculated isotopic frequency shifts are in close agreement with experiment, except for normal modes, where two or more types of vibrations are coupled. For these modes, the sum of the isotopic shifts is accurately reproduced. The sensitivity of the calculated properties to the numerical integration grid has been investigated and it is found that the grid usually used for main-group molecules has to be extended to obtain numerically stable vibrational properties for transition metal-ligand systems.
Pattern formation arising from wave instability in a simple reaction-diffusion system
Authors: Zhabotinsky Anatol M. , Dolnik Milos , Epstein Irving R.
Published: July 2002

Pattern formation is studied numerically in a three-variable reaction-diffusion model with onset of the oscillatory instability at a finite wavelength. Traveling and standing waves, asymmetric standing-traveling wave patterns, and target patterns are found. With increasing overcriticality or system length, basins of attraction of more symmetric patterns shrink, while less symmetric patterns become stable. Interaction of a defect with an impermeable boundary results in displacement of the defect. Fusion and splitting of defects are observed.
Kinetic energy dependence of the reactions of Ru+, Rh+, Pd+, and Ag+ with O2
Authors: Chen Yu-Min , Armentrout P. B.
Published: July 2002

Reactions of Ru+, Rh+, Pd+, and Ag+ with molecular oxygen are studied as a function of kinetic energy by using guided ion beam mass spectrometry. By using a flow tube ion source, it has been possible to create Ru+, Rh+, Pd+, and Ag+ ions in their electronic ground state terms and primarily in the lowest spin–orbit levels. All reactions are observed to be endothermic. The reactivity of ground state Ag+ is found to be particularly inefficient and is believed to occur through an impulsive pairwise mechanism. Excited states of Ag+ are observed to react efficiently at thermal energies. Analyses of the endothermic reaction cross sections yield 0 K bond dissociation energies of D0(Ru+–O)=3.81±0.05 eV, D0(Rh+–O)=3.02±0.06 eV, D0(Pd+–O)=1.46±0.11 eV, and a speculative value of D0(Ag+–O)=1.23±0.05 eV. The reactivity differences among all four metal systems and the electronic states of Ag+ are explained by using simple molecular orbital concepts.
Vibrational force field of solid imidazole from inelastic neutron scattering
Authors: Loeffen P. W. , Pettifer R. F. , Fillaux F. , Kearley G. J.
Published: July 2002

Inelastic neutron scattering (INS) measurements are presented for solid imidazole at 20 K. Both powder and oriented platelets have been measured. Assignments of bands are consistent with earlier infrared and Raman studies except for a band at 961 cm−1 which is definitely assigned as A″. A fit with the proton-weighted vibrational density of states is presented and a full harmonic force field has been established. The mean deviation between measured eigenvalues and those predicted from the force field is ±3 cm−1. The potential energy distribution is compared to quantum chemistry calculations and is found to be in best agreement with an unscaled calculation [J. Sadlej, A. Jaworski, and K. Miaskiewicz, J. Mol. Struct. 274, 247 (1992)]. The INS spectra from a scaled ab initio force field [K. Fan, Y. Xie, and J. Boggs, Theochem, 29, 339 (1986)] are calculated both before and after refinement against only infrared frequencies and compared to experiment. It is demonstrated that comparison with the INS spectrum is a more stringent test of the force field than with just the infrared frequencies alone.
Ion condensation in salt-free dilute polyelectrolyte solutions
Authors: González-Mozuelos P. , de la Cruz M. Olvera
Published: July 2002

We study ion condensation in salt-free dilute solutions of polyelectrolyte chains of different fractal geometries, from random walks to rigid rods. In finite size chains with a discrete charge distribution, ion condensation is only observed in multiple chain systems. A thermodynamic analysis of ion condensation with linearized interactions among the condensed counterions and the monomers leads to rigid rod lowest free energy conformations, regardless of the values of the valency of the monomers (zm), the negative value of the ratio of the valency of the counterions to the valency of the monomers (Pz), and the ratio of the Bjerrum length (lB) to the distance between charges along the polyion (b). As z2mPzlB/b increases, however, the fraction of condensed counterions increases and the polyion concentration at which ion condensation appears decreases. Above this critical polyion concentration, ion condensation is observed even when lB/b<1/z2mPz. When correlations are included, representing the tendency of the condensed counterions to be surrounded by opposite sign charges, the chain conformation is strongly dependent on the valency of the counterions and lB/b. For large valency counterions and/or lB/b values, collapsed finite chains have lower free energies than rigid rods.
Excess chemical potential of small solutes across water–membrane and water–hexane interfaces
Authors: Pohorille Andrew , Wilson Michael A.
Published: July 2002

The excess chemical potentials of five small, structurally related solutes, CH4, CH3F, CH2F2, CHF3, and CF4, across the water–glycerol 1-monooleate bilayer and water–hexane interfaces were calculated at 300, 310, and 340 K using the particle insertion method. The excess chemical potentials of nonpolar molecules (CH4 and CF4) decrease monotonically or nearly monotonically from water to a nonpolar phase. In contrast, for molecules that possess permanent dipole moments (CH3F, CH2F, and CHF3), the excess chemical potentials exhibit an interfacial minimum that arises from superposition of two monotonically and oppositely changing contributions: electrostatic and nonelectrostatic. The nonelectrostatic term, dominated by the reversible work of creating a cavity that accommodates the solute, decreases, whereas the electrostatic term increases across the interface from water to the membrane interior. In water, the dependence of this term on the dipole moment is accurately described by second order perturbation theory. To achieve the same accuracy at the interface, third order terms must also be included. In the interfacial region, the molecular structure of the solvent influences both the excess chemical potential and solute orientations. The excess chemical potential across the interface increases with temperature, but this effect is rather small. Our analysis indicates that a broad range of small, moderately polar molecules should be surface active at the water–membrane and water–oil interfaces. The biological and medical significance of this result, especially in relation to the mechanism of anesthetic action, is discussed.
Conformations of freely jointed polyampholytes in external fields
Authors: Schiessel H. , Blumen A.
Published: July 2002

We extend our previous treatment [J. Chem. Phys. 103, 5070 (1995)] of polyampholytes (PAs, polymers with positively and negatively charged monomers) in an external field E, by modeling the PAs through freely jointed chains. Former works, based on Rouse-type models are linear, and hence are limited to sufficiently small values of E. The freely jointed chain model allows to consider the PAs conformations for E arbitrary. We calculate the PAs end-to-end distance for several charge distributions along the chain, which may be either ordered or random. For small E we recover previous results; large E-values lead to a crossover to stretched chain configurations, which depend crucially on the distribution of charges along the chain.
Gradient extremal bifurcation and turning points: An application to the H2CO potential energy surface
Authors: Bondensgård Kent , Jensen Frank
Published: July 2002

Gradient extremals are curves on potential energy surfaces connecting points where the derivative of the gradient norm, subject to the constraint that the energy is constant, is zero. The bifurcation of such curves is analyzed, and exemplified by tracing out some of the gradient extremals on the HF/STO-3G surface for H2CO. It is shown that gradient extremal following provides a semisystematic way of locating stationary points.
A thermodynamic distance criterion of optimality for the calculation of free energy changes from computer simulations
Authors: Schön J. C.
Published: July 2002

The calculation of the external work expended during the switching between two systems yields bounds on their free energy difference. The quality of these bounds depends on the choice of path along which the work is calculated. In this paper, it is shown that for sufficiently fast relaxations the optimal path will be characterized by a minimal thermodynamic length, and that it should be parametrized such that it is traversed with constant thermodynamic speed. As an example, the optimal switching of a magnetic field containing a paramagnet is considered. Finally, the effects of finite relaxation times are discussed.
The ionic strength dependence of the structure and dynamics of polyelectrolyte solutions as seen by light scattering: The slow mode dilemma
Authors: Sedlák Marián
Published: July 2002

The ionic strength dependence of the structure and dynamics of polyelectrolyte solutions was investigated by static and dynamic light scattering. Narrow molecular weight distribution sodium poly(styrenesulfonate) (NaPSS) standards in aqueous NaCl solutions were chosen as model systems. The study covers moderate polymer concentrations (∼0.5–50 g/L) where interparticle interactions rather than single-polyion properties are dominant. Two diffusive modes characterized as fast and slow were detected in most cases. Scattering amplitudes of these modes were evaluated from light scattering data, which was not done in previous investigations. Amplitudes were normalized by the scattering of a benzene standard. Attention was paid mainly to the occurrence of the slow diffusive mode, which is interpreted as the dynamics of large multichain domains. Experimental results show that the occurrence of the slow mode is not connected with any kind of a sharp transition at some critical conditions. The amplitude of the slow mode (scattering intensity associated with the slow mode) changes smoothly as a function of the ratio of the polymer concentration to the solution ionic strength. At sufficiently high polymer concentrations, the slow mode can even persist until theta conditions are reached (4.17 M NaCl at 25 °C). It was shown that the absolute scattering intensity associated with the slow mode increases upon decreasing ionic strength, which is a direct proof that the structures responsible for the slow mode (domains) really form in solution and cannot be associated with the presence of some impurities, stable aggregates due to the polymer incomplete solubility, etc. Multiangle dynamic light scattering data show that dimensions of the domains increase upon decreasing ionic strength, too. The fast and slow diffusive modes are always widely separated on the time scale and the ionic strength dependence of diffusion coefficient cannot be characterized as a ‘‘splitting of diffusion coefficient.’’
The superposition principle and cavity ring-down spectroscopy
Authors: Lehmann Kevin K. , Romanini Daniele
Published: July 2002

Cavity ring-down is becoming a widely used technique in gas phase spectroscopy. It holds promise for further important extensions, which will lead to even more frequent use. However, we have found widespread confusion in the literature about the nature of coherence effects, especially when the optical cavity constituting the ring-down cell is excited with a short coherence length laser source. In this paper we use the superposition principle of optics to present a general and natural framework for describing the excitation of a ring-down cavity regardless of the relative values of the cavity ring-down time, the input pulse coherence time, or the dephasing time of absorption species inside the cavity. This analysis demonstrates that even in the impulsive limit the radiation inside a high finesse cavity can have frequency components only at the natural resonance frequencies of the cavity modes. As an immediate consequence, a sample absorption line can be detected only if it overlaps at least one of the cavity resonances. Since this point is of particular importance for high resolution applications of the technique, we have derived the same conclusion also in the time domain representation. Finally, we have predicted that it is possible to use this effect to do spectroscopy with a resolution much higher than that of the bandwidth of the excitation laser. In order to aid in the design of such experiments, expressions are derived for the temporal and spatial overlap of a Fourier transform limited input Gaussian beam with the TEMmn modes of the cavity. The expressions we derive for the spatial mode overlap coefficients are of general interest in applications where accurate mode matching to an optical cavity is required.
On the vibrational fine structure in the near-threshold photofragmentation spectrum of the I−⋅CH3I complex: Spectroscopic observation of nonadiabatic effects in electron-molecule scattering
Authors: Dessent Caroline E. H. , Bailey Christopher G. , Johnson Mark A.
Published: July 2002

Photofragmentation of the I−⋅CH3I ion-molecule complex is observed to accompany photoexcitation in the vicinity of its electron detachment thresholds. The I− photofragment action spectrum displays a vibrational progression in the ν3 (largely C–I stretching) mode of neutral CH3I, the same mode which is excited upon photodetachment of the complex. The extent of this vibrational activity in the I−⋅CH3I photoelectron spectrum is found to strongly depend on the photodetachment energy, becoming very pronounced as the photon energy approaches the detachment threshold. This indicates that the vibrational features in the photoelectron spectrum arise from non-Franck–Condon effects. These observations of selective excitation of ν3 in both the photoelectron and photofragmentation spectra are correlated to nonadiabatic effects arising from the repulsive state of the CH3I− anion, which is thought to evolve into a resonance near the equilibrium separation of neutral CH3I. The I−⋅CH3I photochemistry is discussed in the context of the mechanisms postulated to govern electron-molecule scattering (i.e., vibrational inelastic and dissociative electron attachment) in bare CH3I. Finally, we cast the scattering mechanism in a spectroscopic picture and suggest that the threshold fragmentation in I−⋅CH3I can be viewed as the predissociation of a transient or virtual dipole-bound electronic state.