bluuebunny/crossref_metadata_2025 huggingface dataset:
Time-dependent chemical reactions: A revision of monomer–excimer kinetics?
Authors: Sienicki K. , Durocher G.
Published: July 2002

From the kinetic point of view, the fundamental quantity of a chemical reaction is the rate constant. The abundance of chemical reactions is governed by first-order kinetics. However, in many cases the diffusion of molecules in the medium leads to a diffusion-controlled chemical reaction. Among many, the monomer–excimer kinetics frequently encountered in photophysical studies is a classical example. An analysis of the kinetics of the monomer–excimer diffusionally-controlled reaction has been presented. It was shown that the theoretical results previously obtained by us using a specific technique of convolutions are in direct relation to the differential kinetic equations, provided that their solution is obtained by means of ensemble averaging. It was shown that the convolution method does not represent by itself any new method and, that it is in strict relation to more fundamental differential kinetic equations. In order to establish this relation, one cannot rely on a simple formal mathematical analysis of the differential equations, and the importance of the averaging procedure while solving the differential kinetic equations is shown. A discussion of the several aspects of differential kinetic equations is also presented.
Geometries and potential energy curves of InSb2, SbIn2, GaAs2, AsGa2, and their ions
Authors: Das Kalyan K. , Balasubramanian K.
Published: July 2002

Complete active space self-consistent field calculations (CASSCF) are carried out on the potential energy curves of three electronic states (2B2, 2A1, and 2B1) of InSb2 and GaAs2, the 1A1 state of InSb+2, seven electronic states of SbIn2, and six electronic states of SbIn+2. In addition, permutational isomers of AsGa2, GaAs2, and InSb2 are investigated. We use multireference singles+doubles CI calculations to optimize the equilibrium geometries and to compute the dissociation energies of all these species. The ground states of InSb2 and SbIn2 were found to be of 2B2 and 2B1 symmetries, respectively, with isosceles triangular geometries. The linear permutational isomer, In–Sb–Sb was found to be 16 kcal/mol above the bent isosceles triangular structure. We find that AsGa2 and GaAs2 also to have permutational isomers ∼16 kcal/mol above the bent (C2v) structures. However, the linear permutational isomers convert to the more stable bent isosceles triangular forms without barriers. The ground states of InSb+2 and SbIn+2 were found to be 1A1 with isosceles triangular geometry and 3Σ−g with linear geometry, respectively. The atomization energies and ionization potential of InSb2 are computed as 4.42 and 5.83 eV, respectively, while the corresponding values for SbIn2 are 2.73 and 5.65 eV, respectively. The atomization energy of AsGa2 is 3.7 eV. The ground state InSb2 (GaAs2) exhibits enhanced Sb–Sb (As–As) bonding, while the ground state of SbIn2 (AsGa2) exhibits enhanced In–Sb (Ga–As) bonding and nonbonding interactions between In (Ga). Our theoretical calculations of mixed III–V trimers explain the experimental relative abundance of trimers and odd–even alternation in the IPs observed by O’Brien et al. [J. Chem. Phys. 84, 4074 (1986)] for GaxAsy (x+y=3).
Electronic states of Ga4 and In4
Authors: Balasubramanian K. , Feng P. Y.
Published: July 2002

Complete active space–multiconfiguration self-consistent-field (CAS–MCSCF) followed by configuration interaction calculations, which included up to 250 000 configurations, are carried out on 16 electronic states of Ga4 and In4. Three nearly-degenerate electronic states of 3B3u, 3Au, and 1Ag symmetries with rhombus, square, and square geometries, respectively, are found as candidates for the ground states of Ga4 and In4. The tetramers of Ga and In are bound by 1.2 and 1 eV, respectively, compared to the trimers.
Directed energy transfer in Langmuir–Blodgett multilayers with asymmetrical forward and reverse transfer rates and energy migration
Authors: Sienicki K.
Published: July 2002

The transport of electronic excitation energy between chromophores randomly distributed in Langmuir–Blodgett multilayers is described. In theoretical considerations it has been assumed that excitation energy can migrate within each layer and can be transferred forward and reverse between layers with asymmetrical rate constants. The decay profiles of donor fluorescence have been calculated for an arbitrary number of layers. The analysis of donor fluorescence profiles resulting from forward and reverse energy transfer has been presented. It was shown that for a large separation of layers and short time the reverse energy transfer has a negligible influence on donor fluorescence profiles. In the case of layers being infinitely close the reverse energy transfer has a very substantial influence on donor fluorescence profiles.
Surface diffusion of molecules physisorbed on dielectric substrates
Authors: Lakhlifi A. , Girardet C.
Published: July 2002

Semiempirical potential calculations are performed to determine the potential surfaces experienced by a X molecule (X=Xe,CH4,N2,CO,NH3,CH3F) adsorbed on the (100) face of dielectric substrates (MgO, NaCl) and on the (111) dense face of xenon. The diffusion mechanism of X on these surfaces is then described, within the transit time concept, as a successive jump process along the diffusion coordinate, including the statical and dynamical influence of the other molecular motions. The calculated diffusion constants can serve as a test of the surface corrugation and of the admolecule dynamics.
Theoretical study of GaAs surface passivation with Se
Authors: Gayen Sanjukta , Ermler Walter C. , Sandroff Claude J.
Published: July 2002

The mechanism for the quenching of surface states of GaAs by adsorbed elemental Se is studied using atomic clusters in ab initio Hartree–Fock calculations. Both the Ga- and the As-terminated surfaces are tied off with Se atoms in a 2×1 reconstruction. The most stable structure is established as two Se atoms bridging two As atoms. It is found that after termination of Ga and As dangling orbitals with H atoms and passivating the surface with Se, the density of surface states in the bandgap region is appreciably reduced.
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o calculations of dissociative electronic states of ClCN: Implications to the photodissociation dynamics of the cyanogen halides
Authors: Bai Y. Y. , Segal G. A. , Reisler H.
Published: July 2002

A b initio configuration interaction calculations with full geometric optimization along the Cl–CN dissociation coordinate have been carried out for several electronic states of ClCN. The calculations treat all low-lying singlet and triplet states and yield the vertical transition probabilities and oscillator strengths, as well as angular dependences near the Franck–Condon (FC) region and least energy paths for several electronic states as a function of rCCl. We find that the low-lying excited states derive from three electronic configurations: π3σ*, π3π*, and σπ4σ*. The lowest excited triplet and singlet states derive from the π3σ* configuration and give rise to bent 1,3A′ and 1,3A″ states. States arising from the π3π* configuration are linear (Σ and Δ states). There is evidence of surface crossings along the reaction coordinate between triplet states arising from the π3π* configuration and those arising from the π3σ* and σπ4σ* configurations. These crossings can be induced by slight bending of ClCN causing lowering of the symmetry. The calculated vertical excitation energies are in good agreement with features of the absorption spectrum, and suggest that the A continuum of ClCN involves transitions to the 2 1A′ and 1 1A″ states. The implications to the photodissociation dynamics of the cyanogen halides are discussed.
Production of fluorine 2,4P→2P resonance radiation by electron impact on SF6, CF4, NF3, and CCl2F2
Authors: Roque M. B. , Siegel R. B. , Martus K. E. , Tarnovsky V. , Becker K.
Published: July 2002

We studied the emission of the fluorine (2p43s) 2,4P→(2p5) 2P resonance lines in the vacuum ultraviolet (VUV) at 95.5 and 97.5 nm, respectively, following dissociative excitation of SF6, CF4, NF3, and CCl2F2 by controlled electron impact under single collision conditions. Absolute photoemission cross sections and appearance potentials have been determined for the 95.5 nm 2P→2P multiplets for all four target gases. The apparent cross sections, which include cascade contribution from higher states, were measured to range from 2 to 5×10−18 cm2 at 200 eV impact energy. The cross sections were found to be heavily influenced by 3p→3s cascading with contributions ranging from 30% (SF6) to 75% (CF4). Subtraction of the cascade contributions yielded direct cross sections of 0.5× 10−18 cm2 (CF4), 1.4×10−18 cm2 (CCl2F2), 1.5×10−18 cm2 (SF6), and 2.9×10−18 cm2 (NF3) at 200 eV. The fluorine emissions are the result of the total fragmentation of the parent molecule for the targets SF6, CF4, and NF3. A much lower appearance potential and a different energy dependence of the cross section in the case of CCl2F2 indicates that partial fragmentation of this molecule plays a very important role. The findings for the 97.5 nm 4P→2P multiplet were essentially similar to those for the 2P→2P multiplet with the exception that accurate apparent emission cross sections could not be determined from a measurement of the 97.5 nm photon emission intensity. Due to their longer lifetime an appreciable fraction of the excited fluorine (2p43s) 4P atoms was found to drift out of the viewing area of our detection system before decaying radiatively. An attempt was made to extrapolate values for the 97.5 nm 4P→2P cross sections at 200 eV from a detailed analysis of the near-threshold region of the cross section for SF6, CF4, and NF3 This approach yielded FI 4P →2P cross sections comparable in magnitude to the FI 2P→2P cross sections.
Effects of preadsorbed oxygen on the bonding and desorption kinetics of CO on Ni(110)
Authors: Feigerle C. S. , Overbury S. H. , Huntley D. R.
Published: July 2002

The effects of oxygen preadsorption on CO adsorption and CO desorption kinetics have been studied by temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS). It is found that preadsorbed oxygen decreased total CO uptake at 120 K by no more than 20% even though the adsorption and desorption energies are strongly affected. The presence of oxygen causes several new desorption states to appear in the TPD whose populations depend sensitively upon oxygen predose. Using published results for the structure of oxygen covered Ni(110) and the present HREELS measurements, the desorption states have been assigned to structurally distinct bonding sites. Using the variation in heating rates method, the first order desorption energies and preexponential factors are obtained as a function of CO coverage for various oxygen predoses. The relationship between the desorption energies and the bonding sites, especially with regard to the distance of CO from neighboring oxygen atoms, is discussed.
Semiclassical quantization of vibrational systems using fast-Fourier transform methods: Application to HDO stretches
Authors: Pickett Thomas J. , Shirts Randall B.
Published: July 2002

Based on work by Martens and Ezra and partially developed independently by Eaker, we apply an improved method of approximating the quantum energy levels of a system of coupled oscillators using the fast-Fourier transform of classical coordinates and momenta to find quantizing trajectories. Application is made to a two-dimensional system modeling the stretching motions of the HDO molecule. The results are in excellent agreement with quantum calculations. This method is useful because: (1) it gives results which are independent of any separability of the Hamiltonian, (2) it is not limited in the number of degrees of freedom that can be handled, and (3) no zero-order approximation to the system is necessary. Results are equally valid inside and outside of resonance zones.
The influence of self-consistency on nonlocal density functional calculations
Authors: Fan Liangyou , Ziegler Tom
Published: July 2002

A self-consistent nonlocal density functional method (NL-SCF) based on the gradient corrected exchange term by Becke [Phys. Rev. A 33, 2786(1988)] and a gradient corrected correlation term by Perdew [Phys. Rev. B 33, 8822(1986)] has been implemented into the hfs-lcao-program by Baerends et al. [Chem. Phys. 2, 41(1973)]. An expression for the variational potential corresponding to Becke’s gradient correction has been derived. A detailed analysis is given of the change in density δρ induced by the gradient corrections. It is shown that δρ is 2 orders of magnitude smaller than the deformation density Δρ=ρmolecule−ρatom. The dipole moments calculated by NL-SCF differ as a consequence only modestly from dipole moments obtained by the local density approximation (LDA), in which the gradient corrections are absent. The importance of self-consistency for calculations on bond distances, bond energies, and vibrational frequencies are assessed by comparing results from NL-SCF calculations with data obtained by a perturbative approach (NL-P), in which densities obtained from LDA calculation were used in the evaluation of the gradient corrections. It is found that self-consistency effects are negligible for bond energies and vibrational frequencies. SCF effects were not found to be important for bond distances neither, with the possible exception of metal–ligand bonds.
Semiflexible planar polymeric loops
Authors: Camacho Carlos J. , Fisher Michael E. , Singh Rajiv R. P.
Published: July 2002

Two-dimensional polymeric loops of N hard disk monomers are subject to a bending rigidity κ. The mean square radius of gyration 〈R2G〉, mean area 〈A〉, shape parameter Σ=〈R2Gmin〉/〈R2Gmax〉, and specific heat are studied as a function of N and κ. Scaling behavior is governed by the combination y∼N/κ when N, κ→∞. In the rigid limit, y→0, 〈R2G〉 scales as N2 and 1–Σ varies as yψ with ψ=1/2. Conversely, in the soft limit, y→∞, 〈R2G〉 scales as y−ω with ω=2(1−ν)=1/2 and Σ→0.39. The scaling domain is extended beyond the soft limit by using a nonlinear scaling field incorporating an effective rigidity κ̃(κ).
Quantum mechanical effects in the three dimensional low energy D+H2→HD+H reaction
Authors: Kornweitz H. , Persky A. , Baer M.
Published: July 2002

In this work possible quantum mechanical effects for the three dimensional reactive D+H2→HD+H reaction are discussed. The study is carried out by comparing quantum mechanical and quasiclassical backscattered differential cross sections as a function of collision energy. A strong quantum effect is detected for the v=0–>v′=0 transition in the energy range around Etr=13 kcal/mol.
Hylleraas and Kinoshita wave functions: Revision and correction
Authors: Koga Toshikatsu
Published: July 2002

As is the case of the Hylleraas six-term wave function, it is shown that several Hylleraas and Kinoshita wave functions for two-electron atoms given in the literature are not sufficiently accurate particularly in their exponents ζ. For example, Kinoshita reported that his 80-term helium wave function has the minimum energy of −2.903 723 7 a.u. for ζ=1.855 199, while we obtain −2.903 724 347 a.u. for ζ=2.245 896.
Jahn–Teller coupling in Rydberg series of benzene
Authors: Staib A. , Domcke W.
Published: July 2002

Jahn–Teller coupling effects in doubly degenerate Rydberg series (npx,y and ndxz,yz, respectively) converging towards the doubly degenerate X̃2E1g ion core of benzene are analyzed in a multichannel quantum-defect description. The model includes Jahn–Teller coupling (via the lowest-frequency E2g mode ν6 ) in the ion core and in the Rydberg orbitals, as well as the interaction of degenerate electronic channels. Fast internal-conversion processes in the Rydberg manifold of benzene are included in a phenomenological manner via an absorptive imaginary part of the reactance matrix. The model parameters are determined from the analysis of existing multiphoton absorption spectra of low Rydberg members. Absorption spectra of the complete npx,y and ndxz,yz series and near-threshold photoionization cross sections are calculated, including hot-band spectra (one quantum of ν6 excited in the initial electronic state). The results are of relevance for the assignment of the enigmatic vibronic structure of the 3p1A2u and 3p1E2u Rydberg states and for the understanding of autoionization dynamics in benzene.
Diode laser spectroscopy of the four radicals, CdD, ZnD, BaD, and SrD, in their 2∑ ground state
Authors: Birk Helmut , Urban Rolf-Dieter , Polomsky Petra , Jones Harold
Published: July 2002

The infrared spectra of several isotopic forms of the monodeuterides of the metals cadmium, zinc, barium, and strontium have been measured in their ground electronic state (2∑) in the gas phase using a diode laser spectrometer. The analysis of the experimental data was carried out in two ways: (a) a complete set of Dunham parameters was determined for each isotopic species independently and (b) all the available data over both deuterated and protonated species of a particular compound were simultaneously fitted to obtain a set of mass-independent parameters. Mass scaling coefficients, which take into account effects such as a breakdown of the Born–Oppenheimer approximation, were determined for the hydrogen atom.
Numerical study of irreversible aggregation process in three dimensional polymer and surfactant systems
Authors: Sintes Tomàs , Toral Raúl , Chakrabarti Amitahba
Published: July 2002

We present the results of numerical simulations of the irreversible aggregation process for self-avoiding flexible polymers in three dimensions. We have paid special attention to two experimentally interesting systems: telechelic ionomers and reverse micelles. The dynamics used in this paper to simulate the clustering process is based on Eden’s method. This dynamics was previously introduced by us for two-dimensional systems. We find a scaling relation between the radius of gyration of the cluster Rg, the chain length n and the number of chains N in the cluster in the form Rg∼nαNβ. The values of the exponents α and β calculated from the numerical simulations are somewhat lower than those predicted by Flory’s classical theory. We argue that the lower values of the exponents are possibly due to the finite size of the clusters and the presence of higher order interaction terms not considered in Flory’s theory.
A semiclassical study of He, Ne, and Ar sticking on metal surfaces
Authors: Jackson Bret
Published: July 2002

A semiclassical study is made of He, Ne, and Ar sticking on a model Cu surface. A fully quantum formulation for gas-phonon scattering is modified to describe the motion of the gas particle semiclassically, via the use of a Gaussian wave packet. The phonons are treated quantum mechanically. Probabilities for energy transfer and trapping are derived. Trapping is studied as a function of particle mass, energy, surface temperature, the depth of the attractive well, and the shape of the repulsive part of the potential energy surface. The limit of low surface temperature and beam energy is discussed. A comparison is made with the forced oscillator model, another semiclassical formulation.
Polarization dependent translational energy release observed in the photodissociation of C2F5I at 304.7 nm
Authors: Hwang Hyun Jin , El-Sayed Mostafa A.
Published: July 2002

Using a state-selective photofragment translational spectroscopy, we determined the angular distribution and the polarization dependence of the velocity distribution for the iodine atom in the ground 2P3/2 and spin–orbit excited 2P1/2 states produced in the photodissociation of C2F5I at ∼304 nm. Consistent with theoretical and experimental results on other alkyl iodides, the excited state iodine is found to be produced predominantly from the parallel 3Q0←N absorption (βlab =1.63±0.06) with a high fraction of available energy released in translation (∼67%). The substantially lower anisotropy parameter (βlab =1.08±0.03) and the polarization dependent velocity distribution observed for the ground state iodine atoms, however, suggest that they are formed from two different excited states, by direct dissociation from the 3Q1 state (∼22%) and indirect dissociation via curve crossing from the 3Q0 to 1Q state (∼78%). The dissociation along the 3Q1 state is found to release about 3.1 kcal/mol more energy in translation than the dissociation via curve crossing from the 3Q0 state. This was blamed on the difference in the vibrational energy redistribution along the potential energy surfaces involved.
Polyelectrolyte properties of proteoglycan monomers
Authors: Li Xiao , Reed Wayne F.
Published: July 2002

Light scattering measurements were made on proteoglycan monomers (PGM) over a wide range of ionic strengths Cs, and proteoglycan concentrations [PG]. At low Cs there were clear peaks in the angular scattering intensity curve I(q), which moved towards higher scattering wave numbers q, as [PG]1/3. This differs from the square root dependence of scattering peaks found by neutron scattering from more concentrated polyelectrolyte solutions. The peaks remained roughly fixed as Cs increased, but diminished in height, and superposed I(q) curves yielded a sort of isosbestic point. Under certain assumptions the static structure factor S(q) could be extracted from the measured I(q), and was found to retain a peak. A simple hypothesis concerning coexisting disordered and liquidlike correlated states is presented, which qualitatively accounts for the most salient features of the peaks. There was evidence of a double component scattering autocorrelation decay at low Cs, which, when resolved into two apparent diffusion coefficients, gave the appearance of simultaneous ‘‘ordinary’’ and ‘‘extraordinary’’ phases. The extraordinary phase was ‘‘removable,’’ however, by filtering. At higher Cs the proteoglycans appear to behave as random nonfree draining polyelectrolyte coils, with a near constant ratio of 0.67 between hydrodynamic radius and radius of gyration. The apparent persistence length varied as roughly the −0.50 power of ionic strength, similar to various linear synthetic and biological polyelectrolytes. Electrostatic excluded volume theory accounted well for the dependence of A2 on Cs.
Delayed ionization following photoexcitation of small clusters of refractory elements: Nanofilaments
Authors: Amrein Andreas , Simpson Richard , Hackett Peter
Published: July 2002

Photoionization of small clusters of the refractory elements niobium, tantalum, and tungsten at 308 nm is interpreted in terms of a sequential two-photon excitation mechanism in which a rapid coupling between electronic and vibrational degrees of freedom is proposed. Ionization is slow and may compete with dissociation. The analogy with thermionic emission is made.
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o study of the collinear reaction of Fe+ (4F) and Fe+ (6D) with H2
Authors: Sodupe M. , Lluch J. M. , Oliva A. , Illas F. , Rubio J.
Published: July 2002

The collinear reaction of Fe+ (4F) and Fe+ (6D) with H2 leading to FeH+ and H has been studied at the configuration interaction (CI) level of theory, using a relativistic pseudopotential approach. The 6D electronic state of Fe+ reacts with H2 through a very large energy barrier that slightly exceeds the endothermicity of the reaction. The 4F electronic state appears to be much more reactive than the 6D state, in good agreement with the experimental results.
The anisotropy of the proton momentum distribution in KHCO3: A deep inelastic neutron scattering study
Authors: Postorino P. , Fillaux F. , Mayers J. , Tomkinson J. , Holt R. S.
Published: July 2002

We report the anisotropy of the proton momentum distribution in potassium hydrogen carbonate, KHCO3. Using the deep inelastic neutron scattering technique, the mean proton momenta along, and across, the O–H–O bond have been determined. While the momentum across the bond corresponds to a proton in a harmonic potential, that along the bond shows this potential is consistent with a previously proposed double well shape.
Lifetime measurement of a collision complex using ion cyclotron double resonance. H2C6N+2
Authors: Anicich Vincent G. , Sen Atish D. , Huntress Wesley T. , McEwan Murray J.
Published: July 2002

In the ion–molecule reaction between HC3N+ and HC3N, the lifetime of the collision complex (H2C6N+2)* was long enough that ion cyclotron double resonance techniques could be used to probe the distribution of the lifetimes of the collision complex. The mean lifetime of the collision complex at room temperature was measured as 180 μs with a distribution ranging from 60 to 260 μs as measured at the half-heights in the distribution. Lifetimes of this magnitude with respect to unimolecular dissociation allow for some stabilization of the collision complex by the slower process of infrared photon emission.
Hot H photochemistry of H2S and CO coadsorbed on Cu(111) at 68 K
Authors: Chakarov D. V. , Ho W.
Published: July 2002

Photochemistry involving two types of molecules coadsorbed at monolayer coverage on a catalytically active single crystal metal surface at low temperatures has been observed with mass spectrometry and high resolution electron energy loss spectroscopy. Irradiation in the wavelength range 240–365 nm of H2S and CO coadsorbed on Cu(111) at 68 K leads to the desorption of H2, CO, H2S, HCO, H2CO, and the formation of HCO, H2CO, and OCS on the surface. The primary step of the photoreaction involves the selective photodissociation of H2S, generating a hot H atom (significantly more energetic than in thermal equilibrium) and HS fragment in ground and excited vibrational states. Subsequent collisions with coadsorbed species give rise to the observed photoproducts. The wavelength dependence for CO and HCO formation generally follows that of the H2 signal. The cross sections at 240 nm for photoinduced desorption of the two most abundant products, H2 and CO, are 2.4 ± 0.7 × 10−20 and 1.2 ± 0.6 × 10−20 cm2, respectively.
Molecular dynamics study of rotational reorientation of tryptophan and several indoles in water
Authors: Hu Yi , Fleming Graham R.
Published: July 2002

Molecular dynamics simulations of the rotational reorientation of tryptophan and a series of indole derivatives in water were carried out. The majority of the simulations used CHARMm and produced results that are quite similar to the experimental data on these systems. Decomposition of the calculated correlation functions shows that the indole reorientation is dominated by in-plane spinning. For systems with side chains the motion is quite different and is dominated by motion around the side chain axis. The issue of solvent attachment is addressed and the origin of the stickier reorientation of tryptophan as compared to indole discussed. Finally memory functions are calculated and briefly discussed.
A new model for polymer melts and concentrated solutions
Authors: Schieber Jay D. , Biller Peter , Petruccione Francesco
Published: July 2002

A new mesoscopic model is presented for polymer melts and concentrated solutions. It is a single Kramers chain model in which elementary motions of the Orwoll–Stockmayer type are allowed. However, for this model, the bead jumps are no longer given by a Markovian probability, but rather are described by ‘‘a waiting time distribution function.’’ Such a distribution is supposed to occur when the chain is ‘‘frozen’’ in space until a ‘‘gap’’ in the solution or melt meets with the bead or chain segment. The time a bead must wait to jump is given by a distribution function with a single adjustable parameter β, which describes the long-time behavior of the distribution: ∼1/t1+β . We find that the model predicts non-Fickian diffusion in agreement with experimental data and Fickian diffusion for longer times which scales with chain length as 1/N2/α−1, where α is a function of β. For β=1.3, D∼1/N2.28. The autocorrelation of the end-to-end vector of the chain is a stretched-exponential form with a time constant which scales as the length of the chain to the 3.3 power for β=1.3.
Convergence of multiple scattering series for two-body hydrodynamic effects on shear viscosity of suspensions of spheres
Authors: Thomas Cristina Urdaneta , Muthukumar M.
Published: July 2002

An efficient computational scheme has been developed that includes many-body hydrodynamics for viscosity in a suspension of spheres following the approach of Jones and Muthukumar [J. Chem. Phys. 89, 6406 (1988)] for friction coefficients. The method combines the hydrodynamic scattering formalism of Freed and Muthukumar [J. Chem. Phys. 76, 6186 (1982)] with angular momentum diagrammatic techniques from quantum mechanics. The concentration dependence of the viscosity is expressed as a series of finite integrals labeled by the equivalent of angular momentum quantum numbers. This series is smoothly convergent in contrast to the case of the alternate formalism of a series of terms of inverse powers of the sphere separation. A comparison between different methods and the slowness of convergence of the series are addressed in detail. At the two-body level, the virial expansion for the shear viscosity η of a suspension of hard spheres of volume fraction Φ is determined to be η/η0=1+2.5Φ+4.8292Φ2+O(Φ3), where η0 is the viscosity of the fluid and the correction for the Brownian motion [G. K. Batchelor, J. Fluid Mech. 83, 97 (1977)] has been ignored. Our value for the two-body coefficient differs in 3.4% from the extrapolated value 5.0019 obtained by Cichocki and Felderhof [J. Chem. Phys. 89, 1049 (1988)].
Electronic spectra of naphthalene⋅Arn solvent clusters (n=1–30)
Authors: Troxler Thomas , Leutwyler Samuel
Published: July 2002

Van der Waals (vdW) clusters of naphthalene with argon were synthesized in supersonic beam expansions. Mass-selective absorption spectroscopy was carried out by using fragmentation-free two-color resonant two-photon ionization with mass-spectrometric detection. Electronic spectra of naph⋅Arn (n≤6) were recorded at the vicinity of the naphthalene S1←S0 electronic origin (32 020 cm−1 ), and corresponding spectra for the 8̄10 transition up to n=30. For n=3, the spectra due to two different isomers could be separated. Clear evidence for the existence of different cluster isomers was also found for n=6, 8, 9, 12–14, and 25–28. In comparison with most other solvent clusters M⋅Arn (M is the aromatic molecule), the spectral red shifts of 000 and vibronic bands of the naph⋅Arn clusters are very small. Analysis of the size dependence of the electronic spectral shifts indicates that the stepwise solvation of the naphthalene molecule proceeds predominantly on one side of the molecule. Two semicyclic trends in the spectral shifts are interpreted as successive wetting-nonwetting transitions which occur with increasing solvent cluster size. In the proposed mechanism for these transitions, part of the innermost solvent layer ‘‘unwets’’ the substrate, and moves to the second or third layer, thereby forming a nonwetting dropletlike cluster on one side of the naphthalene substrate.
Electron mobilities in low density hydrogen and carbon monoxide gases: Momentum transfer cross sections at very low energies
Authors: Ramanan G. , Freeman G. R.
Published: July 2002

Values of the momentum transfer cross sections σm of hydrogen and carbon monoxide molecules for very low energy electrons (1–300 meV, 0.2–50 zJ; z=zepto=10−21 ) have been refined by electron mobility measurements in the gases over wide ranges of temperature. This is part of an ongoing study of effects of molecular shape on electron transport in fluids. Electron mobilities in hydrogen gas at temperatures 17–450 K are roughly consistent with previously reported values of σm, but do not exclude the possibility of a σm minimum of 5.2×10−20 m2 at an electron energy ξ≤0.7 zJ. Carbon monoxide molecules have a minimum σm of 5.8×10−20 m2 at ξ=5 zJ. A new treatment of low energy electron scattering by CO is needed.
Thermodynamic properties of solid hydrogen at 150 GPa
Authors: Lorenzana Hector E. , Jeanloz Raymond
Published: July 2002

A simple thermodynamic model accounts for the strong curvature experimentally observed in the ∼150 GPa phase transition boundary of solid hydrogen, yielding estimates for the average phonon vibrational frequency at the transition and the zero temperature transition pressure of 19.7 (±2.5) THz and 152 (±3) GPa, respectively. The volume decrease at the transformation is also found to be small, less than ∼2 parts in 104.
Comment on: Structures, stabilities, and intermolecular vibrational frequencies of small ammonia complexes by molecular mechanics for clusters analysis
Authors: Greer J. C. , Ahlrichs R. , Hertel I. V.
Published: July 2002

We calculate binding energies for the molecular mechanics for clusters (MMC) geometries obtained by Dykstra and Andrews [J. Chem. Phys. 92, 6043 (1990)] for neutral ammonia clusters and compare these energies with those obtained from previous calculations by Sagarik, Ahlrichs, and Brode [Mol. Phys. 57, 1247 (1986)] and Greer, Ahlrichs, and Hertel [Chem. Phys. 133, 191 (1989)]. All geometries are compared energetically near the Hartree–Fock limit; for the dimer, correlation effects are included by means of the coupled pair functional (CPF) method.
Mobility variance determined from ac electrophoretic light scattering
Authors: Robertson Baldwin
Published: July 2002

The homodyne correlation function for light scattered from an aqueous suspension of particles in a uniform oscillating electric field is calculated assuming the mobility varies from particle to particle. The correlation function has an exponential decay that is modulated at frequency ω/2π with amplitude (q⋅E/ω)2 〈Δμ2〉, where q is the scattering vector, E and ω/2π are the amplitude and frequency of the oscillating field, and 〈Δμ2〉 is the variance of the mobility of the particles. The variance can be determined by fitting the theoretical correlation function to homodyne photon autocorrelation measurements. This technique has the advantage of insensitivity to convection of the particles from heating by the applied electric field.
Rotational-level-dependent radiative lifetimes and branching ratios in NO(B 2Π) (v=7, Ω=1/2,3/2)
Authors: Gadd G. E. , Huestis D. L. , Slanger T. G.
Published: July 2002

The previous study [G. E. Gadd and T. G. Slanger, J. Chem. Phys. 92, 2194 (1990)] of the v=0–6 levels of the NO(B 2Π) valence state has been extended to the v=7 level, which is of particular interest because it lies at the first dissociation limit of the molecule, and also in a region where strong homogeneous mixing with the C 2Π Rydberg state manifests itself. Thus, the v=7 radiative lifetime can be affected by both these interactions, which are dependent on rotational level. It has often been argued that the B 2Π3/2 spin–orbit component interacts strongly with C 2Π1/2,3/2, whereas the B 2Π1/2 component is unperturbed. We show here that ‘‘unperturbed’’ is a relative term and that a sensitive way to demonstrate that even the B 2Π1/2 levels are mixed with the C 2Π state is to measure the intensity branching in the B–X 7–v″ vibrational progression from fluorescent spectra, for different rotational levels. In the present study, excitation of these levels is carried out on the B–X 7–0 band at 191–192 nm. We have investigated the lifetimes of both spin–orbit components of B 2Π(v=7) and find that the decreasing interaction with the C 2Π state with increasing rotational level, in spite of the increasing energy above the dissociation limit, leads to a steady increase in radiative lifetime. The value reached at high J is 350–400 ns, whereas the lifetimes of the Ω=1/2 levels that lie below the dissociation limit approach a value of 600–700 ns, consistent with the extrapolation of lifetime data for v=0–6.
Electron spin resonance study on the structure and dynamics of propane cation radical: The intrinsic form with a C
S symmetry
Authors: Toriyama Kazumi , Okazaki Masaharu , Nunome Keichi
Published: July 2002

Propane cation radicals with CS symmetry were found by electron spin resonance in C3F8 and C2F6 at 4 K. They have one C–C bond elongated considerably, and the unpaired electron is in the 10a′ orbital of CS symmetry, which is derived from the 4b1 orbital of C2V symmetry by the deformation. The highest spin density appears in the elongated C–C bond, and the in-plane methyl proton at the other end has the largest hyperfine coupling (hfc) of about 9 mT. The degree of deformation from the C2V form is dependent on the matrices. From the general consideration on the structure of propane cation, it is deduced that the cation in SF6, which has been considered to have C2V symmetry, also has CS symmetry with a smaller distortion, and that distortion to CS symmetry is an intrinsic nature of the propane cation.
Systems of hard parallel rectangular particles. I. Exact cluster integrals for star graphs with 2,...,6 points
Authors: Foidl Ch. , Kasperkovitz P.
Published: July 2002

The cluster integrals of any D-dimensional system consisting of hard rectangular particles (squares, rectangles, cubes, etc.) can be obtained from the corresponding integrals for D=1. A new method is proposed how to calculate these elementary cluster integrals exactly. For periodic boundary conditions these functions are given for all star graphs with less than seven points, the range of density varying from dilute gas to close packing. For higher densities all integrals with more than two points are no more proportional to some inverse power of the periodicity length L. To study the cummulative effect of this anomalous volume dependence the equation of state was determined for a system consisting of six hard rectangles and the exact pressure was compared to the pressure obtained from various approximations (full and truncated virial series, anomalous volume dependence considered or neglected).
A three-dimensional lattice model of water molecules with and without solute molecules against a charged wall
Authors: Aloisi Giovanni , Guidelli Rolando
Published: July 2002

A three-dimensional lattice model of water molecules against a charged wall is presented, both in the absence and in the presence of nonpolar monomeric solute molecules. To account for water–water interactions, the water molecules are represented as spheres embodying three partial charges. The statistical-mechanical treatment of this model, which does not embody adjustable parameters, is carried out on the basis of Barker’s self-consistent field theory [J. A. Barker, J. Chem. Phys. 44, 4212, (1966)]. In the absence of solute molecules, the model provides the interfacial potential difference, the differential capacity, and the entropy surface excess as a function of the charge density σ on the wall; the qualitative behavior of these quantities compares favorably with that of the experimental, Gouy–Chapman corrected quantities at the interface between sp metals and aqueous solutions of nonspecifically adsorbed 1–1 valent electrolytes. Generalizing the model so as to include the presence of nonpolar monomeric solute molecules, yields calculated adsorption isotherms, both at constant charge and at constant potential, which compare favorably with the experimental adsorption behavior of simple, monofunctional aliphatic compounds on mercury from aqueous solutions.
Exoelectron emission during oxidation of Cs films
Authors: Böttcher A. , Grobecker R. , Imbeck R. , Morgante A. , Ertl G.
Published: July 2002

During oxidation of thin Cs films, a nonadiabatic surface reaction manifests itself in the emission of electrons. This effect was investigated in detail by combining measurements of the current and of energy distributions of these exoelectrons with studies on the electronic properties of the surface by means of ultraviolet photoelectron spectroscopy and metastable deexcitation spectroscopy. Exoelectron emission occurs via Auger deexcitation of the empty state derived from the O2 affinity level. This process is confined to the stage Cs2O2→CsO2 in which resonance ionization of the affinity level of the impinging O2 molecule upon crossing the Fermi level EF is efficiently suppressed due to the absence of metallic states near EF. A kinetic model based on the successive steps involved in the oxidation of Cs is developed which describes qualitatively well all the experimental findings.
Behavior of the rate constant for reactions in restricted spaces: Fluorescence probing of lipid vesicles
Authors: Argyrakis P. , Duportail G. , Lianos P.
Published: July 2002

The bimolecular reaction A+B→products, where [A]<[B], was studied by fluorescence probing of small unilamellar vesicles of dipalmitoylphosphatidylglycerol with pyrene, and by computer simulation on a square lattice containing nonpercolating clusters. The decay curves of the minority species were fitted with an equation obtained from the theory of random walks in fractal domains. The analysis of the data has allowed redefinition of the reaction rate in restricted geometries, which is now time dependent, and sets the basis for simple treatment of bimolecular reactions in organized assemblies. The values of the spectral dimension calculated from this work are in the range 0.35–0.66, where the upper limit reflects the Alexander–Orbach conjecture, and the lower values are used to monitor the solubilizate aggegation in vesicles.
The Cs(7P)+H2→CsH+H reaction. II. Rotationally resolved total cross sections
Authors: L’Hermite J.-M. , Rahmat G. , Vetter R.
Published: July 2002

The Cs(7P)+H2→CsH(X 1Σ+)(v″=0,J″)+H harpooning reaction has been studied in a crossed-beam experiment, with electronic excitation of Cs atoms and laser-induced fluorescence detection of CsH products performed in a coherent saturation regime. Measurements of reactive cross sections have been achieved as a function of two parameters: The collision energy Ec (0.015≤Ec≤0.15 eV) and the rotational quantum number J″ of CsH products (0≤J″≤16, in v″=0). For Ec=0.09 eV, the total reactive cross sections corresponding to the two 7P fine structure levels of Cs are σ(Cs(7P1/2)+H2)=4.2×10−16 cm2 and σ(Cs(7P3/2)+H2)=10−16 cm2. These cross sections are much smaller than expected for a harpooning reaction. This is interpreted from ab initio potential energy and quantal dynamics calculations which show that the efficiency of the photochemical reaction (Cs atoms in the 7P state) is limited by the quenching processes associated with the underlying potential energy surfaces connected with Cs atoms in the 6S, 6P, 5D, and 7S states. From the present measurements, one deduces that the rate of these quenching processes is larger than 90%. The rotational distributions of products in v″=0 measured at two collision energies (Ec=0.045 and 0.09 eV) are close to ‘‘prior’’ statistical distributions, with the same ‘‘surprisal.’’ The energy dependence of rotationally resolved cross sections (J″ fixed, Ec variable) confirms the reaction thresholds and shows that there is no potential barrier higher than 0.015 eV on the entrance valley and the exit valley of the reactive system; it is in agreement with the dependence predicted by hemiquantal dynamical calculations performed in the frame of a harpooning model. The uncertainty on the measurement of absolute cross sections is larger than 70%, but of the order of 20% for relative cross sections.
Optical activity of oriented molecules
Authors: Cherepkov N. A. , Kuznetsov V. V.
Published: July 2002

Circular dichroism and optical rotation by oriented molecules in the electric dipole approximation are studied for molecules of different symmetry groups. It is shown that these effects are not connected with each other by the Kramers–Kronig relation and are irreversible, like the Faraday effect in a magnetic field. The optical rotation by the nematic liquid crystal of p-azoxyanisole, observed earlier by Williams [J. Chem. Phys. 50, 1342 (1969)], can be explained on the basis of this theory. Finally, the optical activity of an ensemble of molecules oriented in some direction and randomly oriented around this direction, is considered.
Investigation of internal conversion in chromyl chloride
Authors: Rashev S.
Published: July 2002

The nature of the experimentally observed biexponential fluorescence decay from single vibronic levels of the first excited electronic state S1 of chromyl chloride [D. S. Tinti, J. S. Baskin, and A. H. Zewail, Chem. Phys. Lett. 155, 243 (1989)] was investigated theoretically on the basis of a model proposed by these authors. Through explicit computations of nonadiabatic coupling matrix elements and effective vibronic level densities, an internal conversion process was shown to occur to a sparse set of highly vibrationally excited levels, belonging to S0. The rate constant of the process was determined. This sparse set served as an intermediate state in the overall redistribution process, decaying subsequently [through intramolecular vibrational energy redistribution (IVR)] into the dense set of vibrational levels from S0. Analysis showed that the sparse set was characterized by extremely high (strongly nonequilibrium) excitation of Cr–O stretch vibrational character. The computations are consistent with the model proposed by Tinti et al. for rationalizing the fluorescence characteristics of S1 chromyl chloride.
Colloid aggregation in the presence of polymers. Effects of mobility and reactivity of clusters on the flocculation kinetics
Authors: Elaissari A. , Pefferkorn E.
Published: July 2002

Small spherical lattices were used to model submicronic particles in a study of polyelectrolyte induced aggregation of colloids. The colloid concentration of the suspension was investigated with a particle counter, allowing determination of the aggregate size distribution and calculation of the average colloid characteristics. Dynamic scaling describes the aggregation kinetics: the number and the weight average sizes scale with flocculation time. The temporal variation of these characteristics is interpreted in the light of simulation results, for which the influence of the cluster mobility and cluster reactivity has been independently determined. Information relative to the variation of the aggregate compactness with the aggregate size is obtained from experimental and simulation studies. When colloid–polymer complexes aggregate by forming interparticulate polymer bridges, aggregation is generally found to develop with features of the reaction limited aggregation process. In the present case, this mode was disturbed by the unusual size dependent colloid mobility. Aggregates of small size were characterized by a relatively low mobility due to their loose structure. Aggregates of large size, which emerged during the terminal step of aggregation, displayed a relatively high mobility as a result of their increased compactness. We determined that, during flocculation, the process started with characteristics of diffusion limited aggregation and ended up with features of the reaction limited process. The transitory mode showed that aggregation developed in accordance with the assumption that aggregates of different sizes might have closely related mobilities.
Two-dimensional reactive systems: Rapid bimolecular processes on spherical surfaces
Authors: Swartz Michael W. , Peacock-López Enrique
Published: July 2002

Most of the approaches to surface kinetics assume an infinite two-dimensional plane. Although this condition can be met experimentally, in most of the practical cases the surface is usually in the form of small particles or spheres to maximize the total surface area. Here, we extend our previous two-dimensional results to dimerization where the reactants are constrained to move on a spherical particle. Using a statistical nonequilibrium thermodynamic approach, we find that under certain conditions the curvature of the sphere is an important parameter which significantly affects the rate coefficient.