bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental determination of dipole moments for molecular ions: Improved measurements for ArH+
Authors: Laughlin K. B. , Blake Goeffrey A. , Cohen R. C. , Saykally R. J.
Published: July 2002

An improved value for the dipole moment of ArH+ has been obtained from new measurements of the rotational g factors of ArH+ and ArD+ made with tunable far-IR laser spectroscopy. Systematic errors present in earlier measurements have been eliminated. The new result (μ=3.0±0.6 D) is slightly higher than the ab initio value of Rosmus (2.2 D) at the 2σ limits of precision.
Multiphoton ionization studies of NO: Spontaneous decay channels in the (4pπ)K 2Π(v=2) Rydberg state
Authors: Miller R. J. , Li Leping , Wang Yumin , Chupka William A. , Colson Steven D.
Published: July 2002

Time-delayed optical double resonance multiphoton ionization (MPI) studies involving (4pπ)K 2Π(v=2)←(3sσ)A 2Σ+(v=1) ←←X 2Π(v=0) NO show dramatic loss of the ionization signal amplitudes on a time scale that is very short relative to the lifetime of the A state. From direct comparisons of MPI and laser induced fluorescence (LIF) dip temporal profiles, it is concluded that loss of the signal amplitude results from inefficient ionization of the K state. MPI temporal profiles measured in a supersonic jet are identical with those measured in a cell indicating that collisional effects are not important. For delay times τ≥20 ns, ionization signals can be observed only at high probe laser intensities. The resultant spectra exhibit marked power broadening and a pronounced dip appears in the center of each of the power broadened resonances. The observed line shapes are rationalized in terms of the spatial and temporal distributions of the probe laser field in conjunction with a dynamical competition between photoionization and spontaneous decay channels in the K state. Such arguments lead to an upper bound on the K 2Π(v=2) lifetime of ≊2 ns. The short lifetime of K 2Π(v=2) is attributed to indirect heterogeneous predissociation through the (4pσ)M 2Σ+ state and/or to homogeneous predissociation via the a 4Π continuum.
Rovibrational intensities for the Δv=1 bands of the X 3Σ− NH radical: Experiment and theory
Authors: Chackerian C. , Guelachvili G. , López-Piñeiro A. , Tipping R. H.
Published: July 2002

The vibrational transition dipole moment for the highly reactive radical species, NH, in its ground electronic state is obtained via the Herman–Wallis effect manifest in emission spectra produced in a plasma reactor. The results of these experiments on the five lowest Δv=1 bands, are in good agreement with high quality ab initio calculations of the electric dipole moment function.
A quantum theoretic model of vibrational relaxation of a diatomic molecule adsorbed on a surface
Authors: Benjamin I. , Reinhardt William P.
Published: July 2002

The vibrational relaxation of a diatomic molecule adsorbed on a surface is studied using a quantum mechanical model which explicitly takes into account the coupling to the anharmonic adbond vibration. The intramolecular vibrational frequency is assumed to be much higher than the phonon frequency, and it is described as a two-level system. The molecule–surface adbond is taken to be a Morse oscillator which is anharmonically coupled to the fluctuating bath. The bath is a Debye distribution of harmonic oscillators. A closed expression is derived for the relaxation rate which is shown to include contributions from different channels. The most important channel for the energy flow consists of near (nonlinear) resonance energy transfer from the molecular vibrational mode to the adbond, accompanied by low-order (one or two) emission/absorption of phonons. The theory is applied to the relaxation of OH on silica and can reproduce quantitatively the measured relaxation rate and its temperature dependency.
Difference frequency laser spectroscopy of the ν3 fundamental band of NH+2
Authors: Okumura M. , Rehfuss B. D. , Dinelli B. M. , Bawendi M. G. , Oka T.
Published: July 2002

The ν3 band of NH+2 in the X̃ 3B1 ground electronic state was observed in direct absorption with a tunable difference frequency laser spectrometer in the 3 μ region, using velocity modulation detection. NH+2 and NH+3 ions were generated in an ac discharge of He and NH3, or of He, N2, and H2. Fifty-three rovibrational transitions were measured and fit to a triplet A-reduced Hamiltonian to determine rotational, centrifugal distortion, and spin–rotation constants. The band origin was found to be ν0=3359.932 cm−1, in excellent agreement with a recent calculation of Jensen, Bunker, and McLean. Indirect evidence from the spectrum suggested that NH+2 is quasilinear, but selection rules prevented a determination of the A rotational constant.
Vibrational predissociation in OH–Ar
Authors: Berry Mary T. , Brustein Mitchell R. , Lester Marsha I.
Published: July 2002

Results are presented showing that the vibrational predissociation dynamics of OH−Ar differs greatly in its ground and excited electronic states. Measurements of the vibrational predissociation lifetime and the nascent internal state distribution of the OH product.(AIP)
Time dependent three-dimensional body frame quantal wave packet treatment of the H+H2 exchange reaction on the Liu–Siegbahn–Truhlar–Horowitz (LSTH) surface
Authors: Neuhauser Daniel , Baer Michael , Judson Richard S. , Kouri Donald J.
Published: July 2002

Converged vibrational state-resolved reactive scattering probabilities for the three-dimensional H+H2 exchange reaction, with total angular momentum J=0 have been obtained using a rotating (body) frame, time dependent quantal wave packet approach. The results are the first obtained by a numerically exact, fully quantal 3D wave packet approach, and yield close agreement with those obtained earlier by Zhang, Kouri, Haug, Schwenke, Shima, and Truhlar.
Analysis of the bond energy of ScCO
Authors: Frey Regina F. , Davidson Ernest R.
Published: July 2002

In this paper, the self-consistent-field (SCF) and correlated bond energies in ScCO were examined. At the SCF level, the bond energy was determined to consist primarily of the s→d promotion energy of the metal, the CO distortion energy, and the SCF interaction energy which is the combined effect of the σ and π interactions. Corrections to the SCF bond energy to obtain the correlated bond energy of the complex were determined to be mainly the dispersion energy of the complex, the correlation corrections to the SCF CO distortion energy, and the correlation and relativistic corrections to the SCF scandium promotion energy.
Laser ionization spectroscopy of CD3 via the 3p
z 2A″2 Rydberg state
Authors: Parker David H. , Wang Z. W. , Janssen Maurice H. M. , Chandler David W.
Published: July 2002

Nascent CD3 radicals produced by photodissociation of CD3I after absorption of 266 nm radiation by the Ã state are studied using the photofragment imaging technique. Two-photon, resonance-enhanced, multiphoton ionization [(2+1) REMPI] probes CD3 via the 3pz 2A′2 Rydberg excited electronic state. CD3I photodissociation at 266 nm is found to produce ground-electronic-state CD3 radicals with substantial vibrational excitation of the ν2 umbrella mode. Rotational constants are determined for v=0, 1, and 2 of ν2 in the 3pz excited state by analysis of the spectra. A first-order perturbation and diagonalization procedure is used to generate potential energy curves for the umbrella mode in both the ground and excited electronic states and Franck–Condon factors for various transitions between the states. These results should prove useful when employing REMPI to characterize methyl radicals in many environments.
A new numerical procedure for the construction of orthonormal hyperspherical functions
Authors: Wolniewicz L.
Published: July 2002

A method for the numerical solution of the grand angular momentum eigenvalue problem arising in quantum mechanics of three particles is discussed. Symmetric hyperspherical coordinates are used and the orthonormal eigenfunctions are labeled in an unambiguous way. Emphasis is put on applicability of the method in automatic computations and an algorithm is derived from elementary considerations. It can be used for any angular momentum and is easily vectorizable.
Photolysis of a simple molecule on a metal surface—adsorbed O2 dissociation and desorption on Pd(111)
Authors: Guo X. , Hanley L. , Yates J. T.
Published: July 2002

The observation of photon-induced dissociation and desorption of submonolayer molecular O2 chemisorbed on Pd(111) is reported. The cross sections estimated for photodissociation and photodesorption are each ∼3×10−21 cm2.
Theoretical study of low-lying 1Σ+ and 1Π states of CO. II. Transition dipole moments, oscillator strengths, and radiative lifetimes
Authors: Kirby Kate , Cooper David L.
Published: July 2002

Transition dipole moments between five low-lying adiabatic states of CO, X 1Σ+, A 1Π, 2 1Σ+ (B 1Σ+ and D′ 1Σ+), 3 1Σ+ (C 1Σ+ and C′ 1Σ+) and 2 1Π (E 1Π), have been obtained. Ab initio calculations of the electronic energies and wave functions for these states have been reported by us previously. The calculated oscillator strengths and radiative lifetimes are compared with various measurements of these quantities. These comparisons are complicated by the large discrepancies among the experimental measurements, apparent even in the most recent studies.
Assignment of the Raman active lattice vibrations in various phases of 1,2,4,5-tetrabromobenzene and 1,2,4,5-tetrachlorobenzene crystals
Authors: White K. M. , Eckhardt C. J.
Published: July 2002

Results of polarized Raman spectroscopy are reported and used to obtain complete assignment of the optical lattice modes of 1,2,4,5-tetrabromobenzene crystals for both the beta and gamma phases. Similar information is provided for the beta phase of 1,2,4,5-tetrachlorobenzene crystal. The results are compared with lattice-dynamics calculations. Discrepancies with previously reported measurements and calculations are discussed and rationalized.
The intermolecular potential of Ne–HCl: Determination from high-resolution spectroscopy
Authors: Hutson Jeremy M.
Published: July 2002

A new intermolecular potential for Ne–HCl with HCl in its v=1 state is obtained by least-squares fitting to molecular-beam near-infrared laser spectra of the Ne–HCl and Ne–DCl van der Waals complexes. In agreement with previous potentials, the equilibrium geometry is found to be linear Ne–H–Cl, with a well depth around 64 cm−1. There is also a secondary minimum at the linear Ne–Cl–H geometry, with a depth around 57 cm−1, which arises because the Ne atom can approach closer to the Cl atom at the linear geometry than at a T-shaped geometry. The new potential is slightly less anisotropic than earlier potentials for HCl in its v=0 state. The fitted potential is used to calculate additional bound states of the Ne–HCl van der Waals complex, which have not yet been observed spectroscopically.
Bond-orientational order in molten salt: Molecular dynamic simulation
Authors: Cheng Zhaonian , Lou Xuecai , Ma Jianpeng , Shao Jun , Chen Nianyi
Published: July 2002

The bond-orientational order in molten salt LiF, KF, and KCl systems has been studied by molecular dynamics simulation. The bond spherical harmonics have been calculated for the instantaneous local structures in computer-generated configurations of the molten salts. A new method of bond spherical harmonics calculation with equal neighbor number N has been proposed. For the standard structures with different coordination number, their quadratic invariant Ql histograms with same N have been calculated and a model with parameter of square deviation have been suggested. The Ql histograms with N=4 and N=12 for some standard structures have been given and the qualitative or semiquantitative estimations of the bond-orientational orders in those three molten salt systems have been obtained. The bond angle probability distributions have also been calculated and taken as the basis for evaluating the usefulness of the new method. The results show that the local structures in molten salt systems are somewhat similar to tetrahedron while the icosahedron structure is hardly found.
On the apparent spectroscopic rigidity of floppy molecular systems
Authors: Nesbitt David J. , Naaman Ron
Published: July 2002

There has been a wealth of recent infrared experimental data on van der Waals and hydrogen bonded complexes obtained under cooled, supersonic jet conditions where only a small fraction of the total bound quantum states can be elucidated. This partial set of data can often be well fit to a traditional Watson Hamiltonian derived from a rigid rotor perspective with low order centrifugal distortion effects included. In this paper we show that even in extremely floppy molecular systems with wide amplitude vibrational motion, the quantum term values are very well fit by a rigid or semirigid rotor Hamiltonian over the limited range of energy states accessible in a cooled beam. We provide explicit examples of this behavior by full quantum solutions in two extremes of floppy motion: (1) a symmetric triatomic with a square well bending potential (‘‘hinge’’) and (2) a nearly free internal rotor (‘‘pinwheel’’). These results show that potentials with fundamentally different topologies can be consistent with same data, and indicate that even the limits of nearly rigid and floppy internal motion may be difficult to distinguish from a limited set of rovibrational eigenvalues.
The ClHCl− anion: Its chemical bond, vibrations, and free energy
Authors: Ikuta Shigeru , Saitoh Toshikazu , Nomura Okio
Published: July 2002

The geometry of ClHCl− has D∞h symmetry, [Cl–H–Cl]−, as opposed to C∞v symmetry [Cl–H...Cl−], which is predicted to be favored at the Hartree–Fock (HF) level of calculation with a basis set of [11s9p2d1f(Cl)/5s2p1d(H)]. The chemical bond of the D∞h structure is a nonclassical four-electron three-center bond, while the C∞v geometry corresponds to an ion–dipole complex. The energy difference between the two is small. Calculations with inclusion of configuration revealed that both have predominantly a HF configuration. D∞h is realized by excitations from σ molecular orbitals (MO) to their antibonding σ* MO, whereas excitations to π MO are ineffective. Normal vibrational frequencies were calculated as follows (units are cm−1): G82(MP2): (ν1,σg): 346; (ν3,σu ): 547; (ν2,πu): 854; 1D Analysis(MP4): (ν1,σg ): 338; (ν3,σu): 830; (ν2, πu ):...; 2D Analysis(MP4): (ν1,σg): 310; (ν3,σu) : 709; (ν2,πu) : 877. The potential for the asymmetric vibration is shown to be quartic. The standard free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) changes (i.e., at 1 atm and 298.16 K) were evaluated from the reaction, HCl+Cl−→ClHCl−, to be as follows: ΔG°=−16.5 kcal/mol, ΔH°=−23.5 kcal/mol, and ΔS°=−23.5 cal/K/mol.
Two-photon spectroscopy at ultrahigh resolution: Fine structure and hyperfine structure of the (3sσ)A 2Σ+(v=1,N=3) Rydberg state of NO
Authors: Miller R. J. , Glab Wallace L. , Bushaw B. A.
Published: July 2002

The utility of ultrahigh resolution, continuous-wave laser sources in multiphoton spectroscopy is demonstrated by measurements of fluorescence excitation spectra resulting from two-photon pumping of individual rotational branches of (3sσ)A 2Σ+(v′=1,N′=3)←X 2Π(v′=0) NO. The general physical significance of fine structure and hyperfine structure interactions is briefly discussed emphasizing their relation to dynamical phenomena. Current observations yield the following fine structure and hyperfine structure parameters characterizing A(v=1,N=3) NO: γ=−80.10±0.04 MHz, b=40.9±0.2 MHz, c=1.5±4.5 MHz, and eQq=−7.2±0.5 MHz. The effective cross section for two-photon excitation of R11(J″=2.5) is estimated from experimental observables to be 1×10−44 cm4 s.
Modeling of the kinetic oscillations in the CO oxidation on Pt(100)
Authors: Andrade R. F. S. , Dewel G. , Borckmans P.
Published: July 2002

We analyze a model recently introduced by Imbihl et al. to describe the kinetic oscillations in the catalytic oxidation of CO on Pt(100). However we describe the surface reconstruction by a time dependent Ginzburg–Landau equation. With realistic values of the rate constants the region of oscillations in the PCO×PO2 diagram extends to very low values of the partial pressures (PCO=1.5×10−6 Torr, PO2=2.7×10−5 Torr) in agreement with the experimental results. In the investigation of the critical points and of the dynamics it has been possible to identify both saddle-node infinite period (SNIPER) and Hopf bifurcations.
On the relaxation between Debye and nonexponential relaxation in supercooled monohydric alcohols and water: A solution study
Authors: Floriano M. A. , Angell C. A.
Published: July 2002

We examine one of a number of possible classes of exceptions to the usual rule that non-Arrhenius behavior in supercooled liquids is accompanied by a departure from exponential relaxation kinetics. The exceptions we study are the dielectric relaxations of monohydric alcohols and supercooled water, in which also the dielectric relaxation times may greatly exceed their mechanical relaxation counterparts. This paper gives evidence that the exceptional behavior is due to a clustering or self-micellization phenomenon by showing how both the relaxation time ratio and Debye relaxation anomalies can be removed by small additions of ionic solutes. These compete with the hydrogen bonding interactions responsible for the clustering. The study, which uses the electrical modulus formalism for data analysis, is restricted by the rapid merging of conductivity and dielectric loss peaks at salt content increases. The relaxation times extracted from a given data set depend on the formalism employed in the data analysis, and the importance of consistency in this respect when comparing mechanical and electrical pheonomena is emphasized. We conclude that comparisons between different responses are most appropriately made in the susceptibility formalism, that the dielectric response in n-propanol is ∼160 times slower than the mechanical response at ∼130 °C, and that the difference is due to the fact that slowly relaxing hydrogen bonded molecular clusters dominate the dielectric susceptibility, hence also the dielectric relaxation. Using the susceptibility formalism for comparsions, we then infer that in water the dielectric relaxation process is considerably slower than the mechanical relaxation process, and that this fact, as well as the fact that the dielectric relaxation in supercooled water remains exponential while uniquely non-Arrhenius in temperature dependence, is to be explained by the dominance of the dielectric relaxation process by ‘‘network clusters.’’
Theory for first super-strong liquid-crystalline polymers
Authors: Dowell F.
Published: July 2002

This paper presents a unique microscopic molecular theory for liquid-crystalline polymers (LCPs). In the following paper, this theory is used to predict and design (atom by atom, bond by bond) the first super-strong (SS) LCPs. This paper here compares the results calculated from this theory with available experimental data for different kinds of existing LCPs. The theory presented in this paper treats backbone LCPs, side-chain LCPs, combined LCPs, and low-molecular-weight LC materials in the isotropic liquid phase, in the nematic LC phase, and in multiple smectic-A LC phases. In this paper here, we see that results calculated using this theory are in very good agreement with available experimental data for various existing LCPs. Specifically, the relative deviations between the theoretical and experimental results presented here vary from 0% to less than 6.2%. This agreement gives us considerable confidence in the use of this theory in the following paper to predict and design the first SS LCPs.
Experimental measures of the electron distribution and bonding in bis(η5-cyclopentadienyl) osmium from He(I) and He(II) valence photoelectron spectroscopy
Authors: Lichtenberger Dennis L. , Copenhaver Ann S.
Published: July 2002

The He(I) and He(II) high resolution photoelectron spectra of osmocene are reported. Vibrational fine structure is observed in all the valence metal-based ionizations and in the cyclopentadienyl π ionizations which derive from the e1g and e1u symmetry combinations. Analysis of the vibrational progressions found in the metal-based ionizations provides a measure of the force constants and vibrational frequencies for the metal–ring stretch in the positive ions. The vibrational analysis for the 2E2(5/2) and 2E2(3/2) states of the osmocene cation [derived from the spin–orbit split ionization of the metal e2g(dx2−y2, dxy) set] indicates an 0.12 Å greater metal–ring bond length in the cation compared to the neutral molecule. The sharp 2A1(1/2) ionization [correlating with removal of an electron from the metal a1g(dz 2) orbital] is observed to have a much shorter vibrational progression. The adiabatic ionization is the most intense (vertical) band of the series, indicating that there is no appreciable change in metal–ring bond distance upon ionization from the nonbonding a1g(dz 2) orbital to produce the 2A1(1/2) cationic state. A large spin–orbit coupling is observed in the metal ionization region and a moderate amount in the cyclopentadienyl ring π ionization region. From evaluation of the spin–orbit coupling, the ionization band which correlates primarily with the e1g combination of the cyclopentadienyl π orbitals is shown to contain about 27% metal character. The spin–orbit coupling effects are negligible in the ionization which corresponds to the e1u combination of the ring orbitals, as expected from the lack of symmetry interaction with the metal d orbitals. The ionizations of this complex also provide experimental measures of relative metal-based and carbon-based ionization cross sections with He(I) and He(II) energy sources which are compared with theoretical calculations. All of the observations indicate that covalent bonding is more prevalent in osmocene

than in ferrocene.
Calculation of resonant states using an optimal path of continuous extension of the bound states: The cubic and quartic energy distortion of the harmonic oscillator
Authors: Jolicard Georges , Perrin Marie-Yvonne
Published: July 2002

Resonant eigenvectors are calculated within the framework of the complex rotation theory using a continuous deformation and extension of the localized bound states through the potential barrier. The vector is obtained by gradually increasing the basis size and defining for each new basis a lesser perturbation path which best verifies the complex extension of the virial theorem. The modifications resulting from the increase of the basis set N and from the variations of the optimal rotation angle θ(N) are well handled using a Bloch wave operator formulation.
Molecular weight dependence of polymer interfacial tension and concentration profile
Authors: Helfand Eugene , Bhattacharjee Somendra M. , Fredrickson Glenn H.
Published: July 2002

Measurements of the interfacial tension γ between immiscible polymers of low molecular weight have not resolved the question of how γ depends on the degree of polymerization N. In this paper the issue is addressed theoretically by solving equations derived earlier, but previously solved only in the infinite molecular weight limit. The basic origin of molecular weight dependence is entropic adsorption of chain ends to the interfacial region. It is found that the interfacial tension and concentration profile across the interface approach the infinite degree of polymerization limit like N−1. No sign is found of the N−2/3 approach reported to fit the data, although not unambiguously. If correct, N−2/3 may be a law applicable over a limited range, and not asymptotically.
Photolysis of chemisorbed dioxygen on Pd(111): Dependence on photon energy
Authors: Hanley Luke , Guo Xingcai , Yates John T.
Published: July 2002

Photolysis of chemisorbed dioxygen bound in the peroxo and superoxo configurations to Pd(111) has been studied and cross section vs photon energy measurements have been made. Three photoprocesses occur: (1) photodesorption of O2(g); (2) photoconversion between different O2(a) species; and (3) photodissociation to produce O(a). The threshold photon energy for O2(a) photodesorption or photoconversion is 3.4±0.3 eV or lower; for photodissociation of O2(a) to produce O(a), the threshold photon energy is 3.7±0.3 eV. The cross sections for photodissociation and photodesorption of chemisorbed dioxygen on Pd(111) at a photon energy of 5.2 eV are (3.5±1.2)×10−20 and (1.3±0.1)×10−19 cm2 , respectively.
Primitive subunits: Models for cooperative motion in glass-forming liquids. Application to an idealized polymer melt
Authors: Mazur Stephen
Published: July 2002

A family of mechanistic models are proposed to describe cooperative molecular displacements in glass-forming liquids. These models assume that within each cooperative domain motion occurs by sequences of discrete, localized events, but that each of these events involves synchronous displacement of a smaller cluster of neighboring components. The size and properties of this cluster, described as a ‘‘primitive subunit,’’ are assumed to reflect intrinsic details of local structure, not sensitive to external degrees of freedom. However, the length of the sequence of events by which each subunit moves is assumed to be a statistical function of some communal degree of freedom such as the free volume. Two examples are explored for the case of a polymer melt in which idealized conformational rearrangements are constrained by steric interferences. The distribution of lengths for cooperative sequences of events required to remove these interferences is derived as a function of a parameter β related to free volume. It is shown that the mean length of such sequences diverge to infinity for some nonzero (critical) value of β, and that the divergence obeys a scaling law. The divergence is a time-invariant feature of the model, similar in physical significance to the equilibrium phase transition which has been proposed as the underlying basis for the glass transition. However, in the present models it follows from mechanistic constraints, independent of any thermodynamic considerations.
On the gas–liquid critical curves of different size hard-spheres binary mixtures interacting via the Lennard-Jones potential
Authors: Zarragoicoechea G. J. , Scalise O. H. , Rodríguez A. E. , Gianotti R. D.
Published: July 2002

The critical behavior of binary mixtures represented by a potential model of different size hard spheres plus a Lennard-Jones interaction is studied by the application of thermodynamic perturbation theory. The Lennard-Jones potential function is the perturbing potential to the hard-sphere reference system. It is found that convexity of the critical curves of type I is sensitive to small changes of the unlike interaction parameter values. Results for Ar–Kr and CH4–Kr mixtures are in good agreement with experimental data.
Nonlocal electrostatic effects on polar solvation dynamics
Authors: Kornyshev A. A. , Kuznetsov A. M. , Phelps D. K. , Weaver M. J.
Published: July 2002

A phenomenological theory of polar solvation dynamics in electron transfer that accounts for the spatial- and frequency-dependent dielectric function of the solvent is developed and described in a format appropriate to time-dependent fluorescence Stokes shifts. The basic features of the relaxation dynamics are explored by using various analytical expressions for the dielectric function. The presence of spatial correlations persisting to frequencies higher than those corresponding to longitudinal solvent relaxation, τ−1L, yields significant or even substantial decay components with relaxation times shorter than τL. These are associated with motions of individual molecules within the solvent structural network. The implications of these predictions for solvation dynamics in activated charge-transfer processes are noted.
Fluorescence lifetimes of SD(A 2Σ+,v′=0,N′ ) radicals and rotational distribution of SD(X 2Π3/2,v″=0,J″) photofragments generated in photodissociation of D2S and C2H5SD at 193 nm
Authors: Kawasaki Masahiro , Sato Hiroyasu , Inoue Gen , Suzuki Makato
Published: July 2002

Fluorescence lifetimes of SD(A 2Σ+,v′=0,N′=0–13) have been measured as a function of rotational energies. They decrease monotonously from 198 ±7 ns to 44 ±1 ns with increasing rotational energies by predissociation through tunneling mechanism. Nascent rotational distributions were measured for SD(X 2Π3/2,v″=0,N″) generated in photodissociation of D2S, and C2H5SD at 193 nm. For D2S and C2H5SD, the rotational temperatures of SD are 250 and 650 K, respectively. Combined with time-of-flight data, dynamics of photodissociation is discussed.
A photoionization study of the COOH species
Authors: Ruscic B. , Schwarz M. , Berkowitz J.
Published: July 2002

A photoionization mass spectrometric study has been performed on the products of the F+HCOOH, F+DCOOH, and F+HCOOD reactions. Hydrogen/deuterium abstraction on the carbon site yields the COOH/COOD radical which can be photoionized and detected. Abstraction on the oxygen site also occurs, but does not yield a detectable isomeric HCOO/DCOO product. Instead, CO2 is readily observed, implying that HCOO/DCOO is unstable to decomposition under the conditions of this experiment. The detection of COOH is believed to be the first experimental evidence for the stability of this species in the gas phase. An adiabatic ionization potential of 8.486±0.012 eV is obtained. Combined with the recently obtained ΔHf00 (COOH+)=143.2±0.5 kcal/mol, this ionization potential leads to ΔHf00 (COOH)=−52.5±0.6 kcal/mol. Hence, COOH is stable with respect to the lowest energy dissociation products (H+CO2) by 10.2±0.6 kcal/mol.
Time dependent nucleation
Authors: Shizgal B. , Barrett J. C.
Published: July 2002

Continuum approximations to the discrete birth and death equations for classical nucleation are investigated. The discrete equations are parametrized by rate coefficients αi and βi for a cluster of size i to lose or gain a monomer, respectively. The continuum equations considered for the distribution function f(x,t) of clusters containing x monomers at time t are all of the form of a Fokker–Planck equation: ∂f/∂t=∂/∂x[Bf eq∂(f/f eq)/∂x], where f eq(x) is the equilibrium distribution and B(x) is a diffusion coefficient. The dependence of B(x) on various continuum approximations to the rate coefficients is discussed at length. Three different forms of B(x) are considered; that used by Frenkel [Kinetic Theory of Liquids (Oxford, Oxford, 1946)], that suggested by Goodrich [Proc. R. Soc. London Ser. A 371, 167 (1964)], and a third form proposed here. Steady state distributions and time lags obtained from the continuous and discrete equations are compared. The time-dependent Fokker–Planck equation is solved by an eigenfunction expansion and the eigenfunctions and eigenvalues of the Fokker–Planck operator are compared with those of the birth and death equations. A change of variables transforms the Fokker–Planck equation into a Schrödinger equation and permits the interpretation of the eigenvalues as energy levels in a potential function. Since the potential is approximately quadratic near its minimum, the lower eigenvalues are close to the harmonic oscillator results. The results show that the choice for B(x) suggested by Goodrich and the one proposed in this paper generally give better agreement with values from the discrete equations than the Frenkel form, used previously by most workers.
Potential energy surfaces for NbH+2 and MoH+2
Authors: Das Kalyan K. , Balasubramanian K.
Published: July 2002

Electronic structures, potential energy surfaces (PES), and some one-electron properties of 12 electronic states of the NbH+2 and MoH+2 ions are studied using the complete active space MCSCF (CASSCF) followed by multireference singles plus doubles configuration interaction (MRSDCI) calculations. The 3B2 ground state of NbH+2 (re =1.687 Å, θe =105.2°) is formed by the spontaneous insertion of Nb+(a3F, 4d35s1) into H2 while the lowest a 5F(4d35s1) state of the Nb+ ion has to surmount a barrier to 56 kcal/mol to insert into H2. The ground state (4B2) potential energy curve of MoH+2 contains two minima with geometries: re =1.637 Å, θe =37° and re =1.626 Å, θe =115.7°. The a4G state of Mo+ inserts spontaneously into H2 to form the 4B2 state of MoH+2, while the ground state (a 6S, 4d5) of the Mo+ ion has to overcome a barrier of 74 kcal/mol to form the linear 6Πu state of the MoH+2 ion. In the collinear mode of interaction, the ground state of Mo+ forms a van der Waals complex which is only 1.2 kcal/mol more stable than Mo++H2. In general, all the low-lying states of NbH+2 and MoH+2 are formed from the excited low-spin states of the metal ions. The PES of NbH+2 were found to be similar to the neutral surfaces confirming Smalley and coworkers experimental findings. The addition of f-type diffuse functions does not alter the geometries much. The vertical ionization potentials of NbH2 and MoH2 are calculated as 7.57 and 8.04 eV, respectively.
Quantum reactive scattering via the log derivative version of the Kohn variational principle: General theory for bimolecular chemical reactions
Authors: Manolopoulos David E. , D’Mello Michael , Wyatt Robert E.
Published: July 2002

The log derivative version of the Kohn variational principle is reviewed in the context of a general bimolecular chemical reaction. The basis of this review, namely, the Wigner and Eisenbud general formulation of rearrangement scattering, has been well known for many years. Therefore, so as to avoid any unnecessary confusion, the relationship between their equally famous ℛ matrix theory and Kohn’s variational derivation is carefully described. The log derivative matrix is then eliminated from a basis set representation of Kohn’s principle to leave a unitary and symmetric variational expression for the scattering matrix S. This new expression is expected to find its most fruitful application in the iterative solution of very large quantum scattering problems for which transitions from only a few initial states are required.
Langevin dynamics simulation of ions in solution: Influence of the solvent structure
Authors: Trullàs J. , Giró A. , Padró J. A.
Published: July 2002

The Langevin dynamics simulation method has been used to study the structural and dynamical properties of ions in electrolyte solutions. Time evolutions of Na+ and Cl− ions in aqueous solutions have been simulated using different solvent averaged ion–ion interaction models. The changes observed in the properties of ions when nonoscillatory solvent averaged potentials are exchanged for more realistic ones including the solvent structure effects are discussed. We have also investigated the formation of ionic associations, paying special attention to the lifetime of the detected dimers.
Projected dynamics: Analysis of a chemical reaction model
Authors: Wu Xiao-Guang , Kapral Raymond
Published: July 2002

The rate processes corresponding to barrier crossing dynamics in a symmetric double-well potential driven by a white-noise source are investigated. The rate kernels in the generalized phenomenological rate law describing the reaction involve time evolution through a projected Fokker–Planck operator. The spectral properties of the projected Fokker–Planck operator are studied, and the rate kernels are expressed in terms of the eigenvalues and eigenfunctions of this operator. The formal results are illustrated with numerical calculations. The results provide information on the nature of the generalized chemical rate law for fast processes.
Photolysis and nonequilibrium reaction kinetics
Authors: Kelly K. , Kostin M. D.
Published: July 2002

The rate constant of a bimolecular gas-phase chemical reaction is determined by the energy distribution of the reactants and the cross section for the reaction. Systems in which photolysis and other nonequilibrium processes occur may have energy distributions which differ significantly from the equilibrium distribution. Analytical solutions are derived for the rate constant as a function of temperature for certain systems in which nonequilibium processes occur. The analytical solutions are compared with the numerical solutions and the agreement is found to be excellent, confirming the validity of the approximations that were made. Chemical reactions are simulated with the Monte Carlo method and the results again confirm the validity of the analytical solutions. Under certain nonequilibrium conditions, it is shown that the complex expression for the rate constant as a function of temperature reduces to a simple form, which resembles the Arrhenius equation. In this special case, the dependence of the results on the shape of the reaction cross section is investigated.
CO desorption kinetics from clean and sulfur-covered Ru(001) surfaces
Authors: Brand J. L. , Arena M. V. , Deckert A. A. , George S. M.
Published: July 2002

The desorption of CO from clean and sulfur-covered Ru(001) surfaces was studied using laser-induced thermal desorption (LITD) and temperature programmed desorption (TPD) techniques. CO was observed to desorb from clean Ru(001) with coverage-dependent kinetics. The isothermal desorption of CO was monitored with LITD measurements. The rates for CO desorption were determined using a simple Padé approximant method to evaluate coverage-dependent kinetic parameters. On the clean Ru(001) surface, the desorption activation energy and preexponential dropped sharply from Ed=34 kcal/mol and νd=5×1015s−1 for ΘCO<0.33 ML to Ed=23 kcal/mol and νd=2×1012s−1, for ΘCO>0.33 ML. The clean Ru(001) surface results agreed very well with earlier studies of CO desorption. The presence of surface sulfur shifted the TPD peaks for CO on Ru(001) to lower temperatures. Likewise, isothermal LITD measurements revealed that the CO desorption parameters at ΘCO=0.06 ML decreased from Ed=36 kcal/mol and νd=1×1016s−1 to Ed=22 kcal/mol and νd=5×1010s−1 as the sulfur coverage increased from ΘS=0.00 to 0.24 ML. The kinetic parameters derived from the isothermal LITD experiments were able to predict accurately the CO TPD spectra as a function of CO coverage, sulfur coverage and heating rate. The dramatic effect of sulfur on the CO desorption kinetics at low sulfur coverages argued for a long-ranged electronic effect for sulfur interacting with CO on Ru(001).
Structure of the polymer–solvent interface
Authors: Miles Danny G. , Schmidt James W.
Published: July 2002

The first measurements of ellipticities ρ̄ from the liquid–liquid interface between polymer and solvent mixtures near their consolute points are reported. When scaled according to theory the ellipticities of high molecular weight polystyrene + solvent mixtures are consistent with those from simple, low molecular weight mixtures. The universal value of the scaled ellipticity is approximately 20% lower than that predicted by present theories.
Kinetic model for heterogeneous catalysis: Cluster and percolation properties
Authors: Kolb M. , Boudeville Y.
Published: July 2002

A lattice model for heterogeneous catalysis on a flat metal surface (i.e., oxidation of carbon monoxide on platinum) has been investigated in mean field approximation and by Monte Carlo simulations. The cluster structure has been determined as a function of a single parameter YCO, directly related to the partial pressure of the carbon monoxide. At low YCO the surface is covered by a percolating oxygen cluster with small dispersed CO islands. As YCO is increased, the oxygen ceases to percolate. In parallel the CO clusters resp. the clusters of unoccupied sites increase in size and percolate just about when the system gets poisoned by the CO. The structure of the large clusters of all three types can be modeled by correlated percolation. The slowing down of the kinetics near the poisoning transition has been investigated and is shown to be related to the diffusion of clusters of unoccupied sites.
Experimental one and two photon absorption properties of acetone
Authors: Gedanken Aharon , McDiarmid Ruth
Published: July 2002

The 2 photon resonant multiphoton ionization (2RMPI) spectrum of the 3s Rydberg←X transition of acetone has been measured. The intensities of this, the 2RMPI spectra of the 3p Rydberg ←X transitions and the optical spectra of all four transitions have been compared with recently calculated cross sections for these transitions.
Cluster fragmentation in electrolyte induced aggregation of latex
Authors: Pefferkorn E. , Stoll S.
Published: July 2002

The scaling description of the aggregate size distribution cg (t), the number average N(t), and weight average S(t) cluster sizes are given for typical time-dependent coagulation–fragmentation processes. In initially concentrated suspensions, two modes of cluster growth are displayed. In a first mode, the process develops with a reduced size distribution that can be described in terms of irreversible diffusion-limited aggregation. In a second mode, the cluster growth develops in accordance with a reduced size distribution of a typical reaction-limited aggregation process (RLA). In diluted suspensions, the clusters only grow with the reduced size distribution of the RLA type. In all situations, two regimes are determined. In the first regime, N(t) and S(t) scale with tz , where the exponent z is dependent on the colloid concentration. The second regime is characterized by the presence of a constant number of clusters with a monotonically increasing mean cluster mass. These observations result from the fact that the breakup of aggregates mainly generates fragments of very low size.
Cross sections and NO product state distributions resulting from substrate mediated photodissociation of NO2 adsorbed on Pd(111)
Authors: Hasselbrink E. , Jakubith S. , Nettesheim S. , Wolf M. , Cassuto A. , Ertl G.
Published: July 2002

Ultraviolet irradiation of NO2 adsorbed on top of a NO saturated Pd(111) surface causes the photodissociation of NO2/N2O4 and results in the desorption of NO molecules. This process has been studied using excitation energies between 3.5 and 6.4 eV. At a photon energy of 6.4 eV, a cross section of 3×10−18 cm2 is found. Using laser-induced fluorescence to detect the desorbed NO molecules, fully state-resolved data detailing the energy channeling into different degrees of freedom has been obtained. Two desorption channels are found, one characterized by nonthermal state populations, and one showing accommodation to the surface. The yield of the fast channel shows a marked increase above 4 eV photon energy. The slow channel is interpreted as being due to NO molecules which, after formation, undergo a trapping–desorption process. A polarization experiment indicates that the photodissociation is initiated by excitation of metal electrons rather than direct absorption by the adsorbate.
A
b i
n
i
t
i
o potential energy surfaces of charge-transfer reactions: F++CO→F+CO+
Authors: Yamashita Koichi , Morokuma Keiji , Shiraishi Yasushi , Kusunoki Isao
Published: July 2002

A b initio potential energy surfaces (PESs) of the charge-transfer reaction F++CO→F+CO+, studied experimentally by Kusunoki and Ishikawa [J. Chem. Phys. 82, 4991 (1985)], are calculated by the MRSD-CI method using the DZP basis set. Six low-lying triplet A′ and six A″ states, correlated to the initial state F+(3P)+CO(X 1Σ+), the final state F(2P) + CO+(A 2Π) and the ground state F(2P)+CO+ (X 2Σ+ ), are investigated for full three-dimensional interactions between the F atom and the CO molecule. The mechanism of charge transfer is discussed based on the PES characteristics and the nonadiabatic coupling elements between the initial and relevant final states. A simple theoretical model which emphasizes the importance of the nonadiabatic transition along the CO vibrational coordinate is presented to interpret the vibrational excitation of the product CO+ (A).
Surface diffusion of CO on silica-supported Ru particles: 13C nuclear magnetic resonance studies
Authors: Duncan T. M. , Thayer A. M. , Root T. W.
Published: July 2002

Portions of CO adsorbed on Ru particles, selected by the orientation of the C–O bond relative to an external magnetic field, are labeled by inversion of the 13C nuclear magnetic dipole. Changes in the orientation of the CO bond of these labeled molecules are then observed with 13C NMR spectroscopy. The temperature dependence and rate of reorientation are consistent with surface diffusion on Ru particles with small numbers of flat faces. The insensitivity to CO pressure in the range 0.5–100 Torr discounts stimulated desorption by gas-phase CO.
Spatially resolved nuclear magnetic resonance study of polymer chains mobility during the swelling process
Authors: Tabak F. , Corti M.
Published: July 2002

A recent method to perform spatially resolved nuclear magnetic resonance (NMR) and relaxation measurements in narrow slices of a solid is applied to the study of the mobility changes of polymer chains during the swelling process. Spin density and T1 profiles are obtained during the swelling of polymethylmethacrylate and polystyrene bars exposed to vapors of deuterated chloroform and carbontetracloride. The diffusion coefficients and their concentration dependences have been determined and the nondestructive character of NMR for liquid–solid interface studies is established.
Sound absorption mechanism of an aqueous solution in nonelectrolyte
Authors: Endo Harumi
Published: July 2002

The expression on sound absorption and sound dispersion in aqueous solutions of nonelectrolyte has been obtained based on the idea due to hydrophobic interactions. The calculations are shown to be in agreement with the observation on butyl cellosolve (ethylene glycol monobutyl ether) aqueous solutions. It is shown that the relaxation curve is expressible in terms of two relaxation times—the upper relaxation time, corresponding to the usual intermolecular relaxation time hitherto employed, and the lower relaxation time, shortened under the influence of the diffusion of the molecules.
Negative ion production in collisions between K(n
d) Rydberg atoms and CF3Br and CF2Br2
Authors: Kalamarides A. , Marawar R. W. , Ling X. , Walter C. W. , Lindsay B. G. , Smith K. A. , Dunning F. B.
Published: July 2002

Negative ion production via electron transfer in collisions between K(nd) Rydberg atoms and CF3Br and CF2Br2 has been investigated over a wide range of of n (10≤n≤70). For both species, the major negative ion observed is Br−, although with CF2Br2, a small Br−2 signal is also detected. Kinematic studies show that Br− production via dissociative attachment is accompanied by sizable translational energy release (∼0.35 eV for CF3Br; ∼0.2 eV for CF2Br2). These studies further show that, despite this large energy release, post-attachment interactions between the product ions are important at intermediate values of n(n≲20) and suggest that the transient CF3Br−* and CF2Br2−* ions initially formed by electron capture have quite different lifetimes against dissociation. Cross sections for free-electron attachment to both species are derived from measurements of rate constants for electron transfer at high n. These cross sections are in reasonable agreement with results obtained in free-electron studies using the threshold photoelectron spectroscopy technique.
Method of measuring the Ham reduction factor q in C3v molecules
Authors: Bent Gary D.
Published: July 2002

This paper applies to molecules which have (i) a doubly degenerate electronic state in C3v symmetry, (ii) a single unpaired electron, (iii) a dynamic Jahn–Teller effect, (iv) spin–orbit coupling less than the Jahn–Teller active vibrational frequency, and (v) nonzero magnetic moments for the equivalent nuclei. Two Fermi contact terms can be measured in the hyperfine interaction of such molecules. A proof is given that the ratio of the Fermi contact terms is the Ham reduction factor q. Recent measurements for CH3O and CH3S are compared to the predictions of this paper.