bluuebunny/crossref_metadata_2025 huggingface dataset:
Core excitation, decay, and fragmentation in solid benzene as studied by x-ray absorption, resonant Auger, and photon stimulated desorption
Authors: Menzel D. , Rocker G. , Steinrück H.-P. , Coulman D. , Heimann P. A. , Huber W. , Zebisch P. , Lloyd D. R.
Published: July 2002

For condensed benzene ice layers, core photoabsorption near edge structure (x-ray absorption; recorded by Auger electron yield measurements), decay electron spectra for resonant and nonresonant excitation, and fragmentation as evident in yields of hydrogen and other ions, have been measured in the C1s region. The absorption spectrum is better resolved than most previously published spectra, exhibits some new features, and shows a high degree of parallelity to the spectrum of isolated molecules. Interestingly, the hydrogen ion yield indicates a particular dissociativeness of a certain core excitation resonance, X, which in the molecule has previously been assigned to a Rydberg state. This selective dissociation suggests that the responsible excitation is strongly antibonding for the carbon–hydrogen bond, while the degenerate Rydberg states broaden into a conduction band in the solid; and that the bond breaking probably occurs or at least starts in the core-excited state, thus proceeding on an extremely short time scale, similarly to observations for other hydrogen-containing molecules. The decay spectra are analyzed in terms of autoionization vs normal Auger decay and indicate that, apart from the first strong π resonance (which leads to pure autoionization) and the X resonance, the core resonances partly or fully ionize before core decay takes place. For the X resonance, the decay spectrum contains a contribution which cannot be assigned to intact benzene; this is taken as additional evidence for ultrafast dissociation, i.e., competitive with core decay. We use these results for a discussion of the influence of condensation on excitation, decay, and fragmentation.
Kinetics of nonstationary, diffusion-influenced reversible reactions in solution
Authors: Molski Andrzej , Keizer Joel
Published: July 2002

The statistical nonequilibrium thermodynamic theory of diffusion-influenced reactions is extended to nonstationary situations. Coupled dynamic equations for the average concentrations and the radial distribution function are derived, and, in the low density limit, applied to study the approach of the reversible reaction A+B⇄C to equilibrium. Two types of rate coefficients for the bimolecular reaction are discussed: (i) molecular rate coefficient describing the rates of elementary reactive events, and (ii) phenomenological rate constants defined via the macroscopic rate equations. In contrast to the phenomenological rate constant, the molecular forward rate coefficient ceases to depend on diffusion when the reaction reaches equilibrium. If the relaxation time for the reaction is much greater than that for diffusion, the classical expressions of Eigen for the linearized relaxation rate near equilibrium are recovered. A close relationship between the classical approach, the pseudo-steady-state approximation, and Onsager’s regression hypothesis is demonstrated. The relation between the present results and those recently put forward in the literature is discussed.
A method of molecular simulation of free energy
Authors: Watson Bradley S. , Chao Kwang-Chu
Published: July 2002

A method of molecular simulation of free energy is proposed to sample the Boltzmann exponential functional of molecular interaction energy exp[−u/kT] by the Metropolis Monte Carlo algorithm. To obtain the Helmholtz energy at (N,V,T), the average of the samples in the canonical ensemble is calculated at 2T, 4T, 8T,... . The residual Helmholtz energy of a molecule (ar/kT) is given by the sum of the logarithms of the reciprocals of the averages extended to a high temperature, plus (ar/kT)T→∞. The latter quantity is equal to (ar/kT) of the hard core of the molecule, or zero if the ideal gas state is obtained at high temperature. The method avoids the insertion of a molecule in a fluid, which is difficult at high density, especially for polyatomic molecules.
A far wing line shape theory and its application to the water vibrational bands (II)
Authors: Ma Q. , Tipping R. H.
Published: July 2002

A far wing line shape theory based on the binary collision and quasistatic approximations that is applicable for both the low and high frequency wings of vibration–rotational bands is presented. This theory is applied in order to calculate the frequency and temperature dependence of the continuous absorption coefficient for frequencies up to 10 000 cm−1 for pure water vapor. The calculations are made assuming an interaction potential consisting of an isotropic Lennard-Jones part with two parameters that are consistent with values obtained from virial data and the anisotropic dipole–dipole part, together with measured line strengths and positions of allowed transitions. The results are compared with existing laboratory data in the 2400–2700 cm−1 window and in the 3000–4300 cm−1 band center region, with field measurements in the 2000–2225 cm−1 region and with a recent experimental measurement near 9466 cm−1. The overall good agreement, together with even better agreement with the more accurate laboratory data in the 300–1100 cm−1 window presented previously, allows us to conclude that both the magnitude and temperature dependence of the water vapor continuum can be accounted for by our comprehensive far wing line shape theory without the introduction of any adjustable parameters. Refinements of the present theory and extension to foreign-broadened absorption are also discussed briefly.
Molecular dynamics investigation of dynamic scaling for dilute polymer solutions in good solvent conditions
Authors: Pierleoni Carlo , Ryckaert Jean-Paul
Published: July 2002

A molecular dynamics (MD) investigation of dynamic properties of a single freely jointed N mer (N=6,9,20,30) in explicit solvent at high dilution and in good solvent conditions is presented. Our aim is to establish the validity of scaling laws for short chains so that all experiments are performed at the same temperature and overall density. In order to unravel the periodic boundary conditions (PBC) artifacts of standard MD, we first consider a nine beads chain in solvent and compare data obtained for various cubic boxes of edge L in the range 0.1<Rg/L<0.3, where Rg is the chain radius of gyration. In this range, static properties of the chain do not present PBC effects, while global dynamical properties, such as the chain intermediate scattering function S(k,t) in the low k regime, are strongly box size dependent. This effect, ascribed to hydrodynamic interactions (HI) due to image chains, does not show up on the local chain fluctuations (high k regime), because the typical relaxation times are shorter than the time HI need to spread a distance d≊(L−2π/k). This explains our earlier observation of the high k universal behavior of S(k,t)/S(k) for different chain lengths and box sizes [C. Pierleoni and J.-P. Ryckaert, Phys. Rev. Lett. 61, 2992 (1991)], the limits of which are now discussed in detail. In the small k regime, the important finite size effect can be identified as an increase of the center of mass diffusion with the box size. As recently pointed out [B. Dunweg and K. Kremer, Phys. Rev. Lett. 61, 2996 (1991)], this dependence can be predicted with the aid of the Kirkwood formula for the chain diffusion coefficient after suitable modification to incorporate image chains influence. In the present range of chain lengths and box sizes, we observe a very good quantitative agreement between Kirkwood formula predictions and direct measurements of the diffusion. Finally, as the main outcome of this work, we find a consistent dynamic scaling picture emerging from our low and high k regimes analyses: our MD experiments yield ν=0.584±0.002 and ν′=0.52±0.01 for the static and the dynamic exponent, respectively.
Near-critical light scattering of an ionic fluid with liquid–liquid phase transition
Authors: Weingärtner Hermann , Wiegand Simone , Schröer Wolffram
Published: July 2002

We report on the first static and dynamic light scattering measurements of near-critical behavior of an ionic fluid. The model system tetra-n-butylammonium picrate-tridecanol exhibits a liquid–liquid phase transition with an upper critical point near 342 K. The correlation lengths and scattering intensities scale with temperature corresponding to mean-field behavior. The data are also concordant with nonclassical 3D-Ising exponents if suitable Wegner corrections are applied. The spherical model recently suggested to apply for ionic fluids can be ruled out, thus resolving a current controversy in the literature.
Measurement of (00v3) levels in X 2Π NCO by stimulated emission pumping spectroscopy
Authors: Northrup F. J. , Wu Ming , Sears Trevor J.
Published: July 2002

Laser induced fluorescence measurements of NCO formed in the reaction CN+O2→NCO+O in a free jet expansion are reported. The rotational temperature achieved in the experiments was typically 10–15 K and absorption from vibrationally excited levels was very much reduced in intensity. Two bands in the Ã–X̃ system, the v1, v2, v3=200–000 and 120–000, of the radical were rotationally resolved for the first time, and wavelength resolved laser excited fluorescence measurements have been used to characterize many ground state vibronic energy levels below 6000 cm−1 in energy. Stimulated emission pumping experiments were carried out on vibronic levels associated with the ν3 vibration with 1≤v3≤3 in the electronic ground state. Harmonic and anharmonic constants were determined, together with the spin–orbit coupling parameters and rotational constants for these levels.
Persistence length and related conformational properties of semiflexible polymers from Dirac propagator
Authors: Kholodenko A. L.
Published: July 2002

A recently discovered connection between Dirac’s fermions and semiflexible polymers [Ann. Phys. (NY) 202, 186 (1990)] is developed further. The scattering form factor, diffusion and viscosity coefficients, and relaxation times are obtained for the arbitrary stiffness of the chain in the limit of infinite dilution. It is demonstrated explicitly how the excluded volume effects can be incorporated in calculations of the above observables. New methods of experimental determination of the persistence length in the presence of excluded volume effects are proposed along with a new method of experimental evaluation of the excluded volume parameter. Obtained results can be used for both homo and random semiflexible copolymers.
Linear and nonlinear polarizabilities of t
r
a
n
s-polysilane from a
b i
n
i
t
i
o oligomer calculations
Authors: Kirtman Bernard , Hasan Muhammad
Published: July 2002

Linear and nonlinear static electronic polarizabilities of the σ-conjugated polymer trans-polysilane (t-PSi) are obtained at the Hartree–Fock level by extrapolation of ab initio calculations on the oligomers H3Si–(SiH2)N–SiH3 (N odd). For the longitudinal components, in particular, a 6–31 G basis is shown to yield accurate values if the chains are long enough. No computational advantage is found in using effective core potentials. Chain lengths through N=13 are sufficient to give limiting infinite chain polarizabilities with relatively small uncertainty. The calculated longitudinal linear polarizability per SiH unit is about the same as the corresponding property in the prototype π-conjugated polymers polyacetylene (PA) and polydiacetylene (PDA), but the longitudinal second hyperpolarizability is significantly less in the σ-conjugated polymer. In t-PSi the conjugation length is about half that in either PA or PDA. Frequency-dependent vibrational distortion contributions are estimated in the doubly harmonic approximation and turn out to be important in trisilane and pentasilane even at optical frequencies. An analysis is developed which sheds light on the expected behavior for increasing chain lengths. Reasonable agreement with experiment is obtained for both the electronic and vibrational longitudinal second hyperpolarizabilities.
Preferential in-plane rotational excitation of H2O (001) by translational-to-vibrational transfer from 2.2 eV H atoms
Authors: Lovejoy Christopher M. , Goldfarb Leah , Leone Stephen R.
Published: July 2002

The translational-to-vibrational and rotational (T-V,R) excitation of H2O with 2.2 eV hydrogen atoms is studied by time-resolved Fourier-transform infrared spectroscopy. Up to 2900 cm−1 of rotational excitation is observed in (001), with a strong preference for in-plane rather than out-of-plane rotation for J≥7, which implies a unique collision process leading to rotational alignment.
Diffusion-limited polymerization of rigid rodlike molecules: Dilute solutions
Authors: Agarwal U. S. , Khakhar D. V.
Published: July 2002

A theoretical investigation of rates of diffusion-limited polymerization between rigid rodlike molecules in dilute solutions is presented, incorporating both translational and rotational diffusion of the molecules. Simplified criteria for reaction are proposed according to which molecules must approach within a specified reaction radius, and have a relative orientation angle in a specified range to react. The translational diffusion of the molecules is assumed to be isotropic. The results of the exact analytical solution, finite element computations, as well as approximate solutions for low rotational diffusivity, are presented. The match between the solutions is satisfactory in the appropriate limits, although the exact solution is found to be computationally intensive for low rotational diffusivities and rapid intrinsic kinetics. The reaction rate is found to decrease to a limiting value with decreasing rotational diffusivity, and the magnitude of the decrease in rate increases with intrinsic rate of reaction. The limiting value of rate is entirely due to the translational diffusive flux of the correctly oriented molecules from the far field, and an asymptotic expression for the rate in this limit is given. Comparison with reported experimental data is qualitative, since the data is only available for the semidilute regime for such systems, where the anisotropic translational diffusion is expected to play a prominent role.
Simple theory of diffuse structure in continuous ultraviolet spectra of polyatomic molecules. III. Application to the Wulf–Chappuis band system of ozone
Authors: Braunstein M. , T Pack R.
Published: July 2002

We apply a simple model for the photodissociation absorption spectra of bent symmetric triatomic molecules to the Wulf–Chappuis band system of ozone (10 000–22 000 cm−1 ) to assign the electronic states and the diffuse vibrational bands involved. The conical intersection between the two lowest 1A″ states is treated in an approximate way, and the role of the lowest excited triplet states is explored. The results indicate that the Wulf band is probably due to the 3A2 state of ozone which gains intensity through spin–orbit coupling. The 1 1A″ (1A2) state gives rise to the featureless red wing of the Chappuis band. Most of the structure in the Chappuis band is reproduced in the model and is due to the 2 1A″ (1B1) state as was previously supposed. A more complete treatment of the conical intersection and nonadiabatic effects will be necessary to quantitatively reproduce all features of the experimental spectra.
The C1s core shake-up spectra of alkene molecules: An experimental and theoretical study
Authors: Sjögren B. , Svensson S. , de Brito A. Naves , Correia N. , Keane M. P. , Enkvist C. , Lunell S.
Published: July 2002

The C1s core photoelectron spectra of a series of alkene molecules, ethene, propene, 1-butene, cis and trans 2-butene, 2-methyl-propene, and 1-pentene are discussed. The experimental spectra are assigned using intermediate neglect of differential overlap-configuration interaction (INDO-CI) calculations and comparative discussions. It is shown that hyperconjugation is a useful concept in the assignment of the transitions. INDO-CI is shown to give a reasonable description of the low energy part of the spectra. The results are used in the discussion of molecular models for the interpretation of the electronic structure of polyacetylene.
Enhanced double-quantum nuclear magnetic resonance in spinning solids at rotational resonance
Authors: Nielsen N. C. , Creuzet F. , Griffin R. G. , Levitt M. H.
Published: July 2002

The excitation efficiency of double-quantum (2Q) coherences between pairs of spin-1/2 nuclei in rotating powdered solids is greatly enhanced at rotational resonance, where an integer multiple of the spinning speed matches the difference in isotropic chemical shifts. In powdered samples it is possible to transfer up to 50% of the single-quantum magnetization through the 2Q transitions, an order of magnitude more than is achievable in magic-angle-spinning experiments performed off rotational resonance. The effect is simulated numerically by integration of the homogeneous evolution propagator and described analytically using first-order average Hamiltonian theory. Rotational resonance enhanced 2Q filtering is demonstrated experimentally for doubly 13C-labeled samples of zinc acetate and diammonium oxalate monohydrate.
Correlated chemisorption: The effect of coalescence and diffusion on chemisorption islands
Authors: Becker Oren M.
Published: July 2002

The theory of island formation in chemisorption [Becker and Ben-Shaul, Phys. Rev. Lett. 61, 2859 (1988)], which treats chemisorption as a combination of direct adsorption and cluster formation, is generalized to account for the effects of cluster–cluster coalescence and of surface diffusion. The effect of cluster–cluster coalescence is approximated by series expansion in increasing ‘‘overlaps.’’ Exact calculation of the second order term, which describes the two-cluster overlaps, yields a good agreement with Monte Carlo simulation results. By incorporating the surface diffusion process it is shown that upon increasing the diffusion rate (i.e., raising the surface temperature) the system changes its behavior from correlated chemisorption (cluster formation) to random chemisorption. The extended formalism includes, as specific realizations, both the case of immobile particles and the cases of rapid diffusing particles (Langmuir’s and Kisliuk’s models).
Vibrational relaxation of a dipolar molecule in water
Authors: Whitnell Robert M. , Wilson Kent R. , Hynes James T.
Published: July 2002

The vibrational energy relaxation of a model methyl chloride molecule in water is studied through equilibrium and nonequilibrium molecular dynamics simulations. Previous work [Whitnell, Wilson, and Hynes, J. Phys. Chem. 94, 8625 (1990)] has demonstrated the validity of a Landau–Teller formula for this system in which the relaxation rate is equal to the frequency-dependent friction that the solvent exerts on the bond. In the present work, an analysis of this friction is used to test the isolated binary interaction (IBI) approximation for vibrational energy relaxation. In this system, where long-range electrostatic Coulomb forces dominate the interaction between the water solvent and the CH3Cl molecule, we show that the binary approximation to the friction only partially accounts for the rapid relaxation of the vibrational energy. We attribute the importance of cross correlations between different solvent molecules to the overlap of the CH3Cl vibrational frequency with the librational band of the water solvent. The dominance of the long-range Coulomb forces is further explored in nonequilibrium simulations. The vibrational energy relaxation is effected by a hysteresis in the Coulomb forces that the solvent exerts on the solute such that the force as the CH3Cl bond compresses is different from that as it expands. The non-Coulomb forces do not show this hysteresis to any significant extent. This hysteresis is reflected in the spatial distributions for the average dipole moment of the water solvent molecules. These spatial distributions also show that a large number of solvent molecules participate in the energy flow out of the CH3Cl molecule and that most of these important molecules are at positions perpendicular to the CH3Cl bond. The overall picture we develop here is of a process that is more complex than a simple binary interaction description can accurately portray.
Stimulated soft electron spin echo envelope modulation spectroscopy
Authors: Hustedt E. J. , Schweiger A. , Ernst R. R.
Published: July 2002

An improved technique of electron spin echo envelope modulation (ESEEM) spectroscopy using soft pulses is proposed. It is functionally related to the standard three-pulse stimulated echo experiment, but has an improved bandwidth. The general class of soft-ESEEM experiments is analyzed theoretically by using a product operator formalism. The predicted features are verified experimentally, in particular, the lack of blind spots and the observation of high modulation frequencies.
Quantum-mechanical calculations of the dissociation of H3 Rydberg states
Authors: Krause Jeffrey L. , Kulander Kenneth C. , Light John C. , Orel Ann E.
Published: July 2002

We present three-dimensional, time-dependent quantum-mechanical calculations of the dynamics of the dissociation of H3 Rydberg states at total energies up to 6 eV. The method used in this work employs a Chebychev propagator in time, and computes the kinetic-energy operators in the discrete variable representation. We calculate the total dissociation cross section, as well as partial vibrational and rotational cross sections, and compare our results to previous two-dimensional calculations and to experiment. The results display clear three-dimensional effects, and indicate the importance of including both sheets of the H3 ground potential-energy surface in the dynamics.
Dynamic light scattering study of concentrated microgel solutions as mesoscopic model of the glass transition in quasiatomic fluids
Authors: Bartsch E. , Antonietti M. , Schupp W. , Sillescu H.
Published: July 2002

This paper presents a light scattering study of the dynamics of concentrated solutions of nearly monodisperse (σ≊0.16) spherical micronetwork particles consisting of highly cross-linked polystyrene dissolved in carbon disulfide, i.e., a ‘‘good’’ solvent. Above volume fractions of φ=0.50 the intermediate scattering function, measured over a time window of 10−7 to 103 s using the ALV5000 correlator, decays in two steps and shows indications of nonergodic behavior for φ≥0.64. Such behavior is typical for glass forming systems and has recently been found close to the glass transition of a hard sphere colloidal system [W. van Megen and P. N. Pusey, Phys. Rev. A 43, 5429 (1991)]. Thus the introduced system can be used for modeling the glass transition of atoms on a mesoscopic scale. The traditional analysis of structural relaxation in terms of a Kohlrausch–Williams–Watts distribution yields a mean relaxation time which follows the empirical Mooney equation as a function of concentration and thus corresponds to Vogel–Fulcher–Tammann behavior. However, the necessity to add an unspecified ‘‘intermediate’’ process between the short and long time KWW decays demonstrates the limitations of this ‘‘pragmatic’’ approach. The mode coupling theory of the glass transition interprets the intermediate scattering function consistently over nearly seven decades in time, the intermediate region corresponding to the crossover from β to α relaxation (von Schweidler law). The critical volume fraction of 0.636 derived by this analysis corresponds to a value of 0.59 for an ideal monodisperse system which is well in accord with other experimental and computer simulation studies of the glass transition of atomic systems.
Dielectric and orientational relaxation in a Brownian dipolar lattice
Authors: Zhou Huan-Xiang , Bagchi Biman
Published: July 2002

In this article, both numerical and analytical studies of dielectric and orientational relaxation in a simple model system are presented. The system consists of point dipoles fixed at the sites of a simple cubic lattice. The dipoles are free to undergo rotational Brownian motion in the force field of all the other dipoles of the system. The dynamics in this simple model is described essentially by only one parameter η=1/3ρμ2/kBT, where ρ is the number density, μ is the dipole moment, and kBT is the Boltzmann constant times the temperature. Extensive Brownian dynamics simulations are carried out to obtain the frequency-dependent dielectric function of this system. The dielectric function becomes progressively non-Debye as the polarity of the system is increased. A comparison between single particle and collective moment–moment time correlation functions is made. It is found that at high polarities, the collective correlation function decays much faster than the single particle function, although a slowly decaying component develops for the collective function. The projection operator technique is used to derive a perturbative equation of motion for the dipole moment at any lattice site−the equation is exact to the second order in η. This approach is quite successful in explaining the single particle dynamics. The perturbative equation also accounts for the progressively faster initial decay of the collective correlation function as η increases. However, it fails to describe the slowly decaying component and consequently the observed non-Debye behavior at large η. We also make a detailed comparison of the simulation results with the self-consistent continuum model of Nee and Zwanzig. It is found that the generalized diffusion equation is reliable if an accurate dielectric friction is used (from simulation, for example). However, the other ingredients of the continuum model are inadequate to describe the dielectric relaxation in this system. In particular, the predicted torque–torque correlation function decays too rapidly compared to the simulation results.
Vector correlations in the vibrational predissociation of hydrogen fluoride dimer
Authors: Marshall Mark D. , Bohac E. J. , Miller R. E.
Published: July 2002

The influence of laser polarization direction on the photofragment angular distributions resulting from vibrational predissociation of HF dimer has been studied in an effort to establish the correlation between the direction of the transition moment in the parent molecule (μ) and the recoil velocity vector of the fragments (ν). This is possible, even for the long-lived predissociative states considered here, since the dimer is initially excited to a single rotational level, with a well-defined time-independent, anisotropic rotational wave function. The mathematical framework appropriate for treating the photodissociation of this asymmetric rotor is presented and used to extract anisotropy parameters for the various initial states. The results clearly show that the recoil velocity vector of the fragments is preferentially aligned along the a axis of the dimer. The implications of this for the associated dynamics are discussed. Data of this type is directly related to the center-of-mass differential photodissociation cross section and thus can be used to establish further points of comparison with the theoretical calculations that are becoming possible for systems of this type.
The nonresonant-two-photon zero kinetic energy photoelectron spectrum out of the 2Π1/2 electronic ground state of nitric oxide
Authors: Strobel Andreas , Fischer Ingo , Staecker Jan , Niedner-Schatteburg Gereon , Müller-Dethlefs Klaus , Bondybey Vladimir E.
Published: July 2002

The nonresonant-two-photon (1+1) zero kinetic energy (ZEKE) photoelectron spectrum out of the 2Π1/2 electronic ground state of nitric oxide is reported. The ZEKE spectra, obtained for different initial temperatures in the jet, show complete rotational resolution of the X 2Σ+ electronic ground state (v+=0) of the ion. Angular momentum transfers up to ΔJ=±7/2 are observed. The rotational J+←J″ branching ratios determined for this ionization process from a valence state differ remarkably from those observed in two-color resonant ZEKE spectra of electronically excited Rydberg states.
Isotopic substitution as a probe of cluster structure, aufbau and isomerism in (benzene)N, N=11–20
Authors: Easter David C. , Khoury Joseph T. , Whetten Robert L.
Published: July 2002

Isotopically substituted benzene clusters (dilute C6H6 in C6D6) in the N=11–20 range have been formed in a helium jet and interrogated through the C6H6 molecular B2u←A1g 000 and 601 transitions. The isotopic shift for perdeuterated vs hydrogenated benzene is sufficiently large that the former is spectroscopically transparent in the region of investigation. Line shapes for the singly substituted clusters are remarkably simple in appearance, being dominated by a few sharp lines at large spectral shift. This indicates that the predominant observed absorption for each size derives from the cluster’s interior site. Analysis of the data strongly corroborates the icosahedral aufbau hypothesis and, in particular, a shell closing at N=13. Some aspects of the spectral simplification offer compelling new evidence for distinct stable isomers for several cluster sizes.
Predissociation of oxygen in the B 3Σ−u state
Authors: Chiu S. S-L. , Cheung A. S-C. , Finch M. , Jamieson M. J. , Yoshino K. , Dalgarno A. , Parkinson W. H.
Published: July 2002

The predissociation linewidths and level shifts of vibrational levels of three oxygen isotopic molecules 16O2, 16O18O, and 18O2 arising from the interactions of the B 3Σ−u state with the four repulsive states 5Πu, 3Σ+u, 3Πu, and 1Πu, have been calculated. A set of parameters characterizing these interactions has been determined. Good agreement between calculated and experimental predissociation widths and shifts has been obtained for all the three isotopic molecules.
Photodissociation of ethylene at 193 nm
Authors: Balko B. A. , Zhang J. , Lee Y. T.
Published: July 2002

The photodissociation of ethylene at 193 nm was studied by measuring the product translational energy distributions for the H+C2H3 and H2+C2H2 channels. In agreement with previous workers, it was determined that atomic and molecular elimination occur in relatively equal amounts. Using 1,1 D2CCH2 and 1,2 cis HDCCDH, it was shown that both acetylene and vinylidene are formed and that the acetylene/vinylidene ratio is approximately 2/3 in the molecular elimination. This H2 elimination channel has a translational energy distribution peaked at around 20 kcal/mol, indicating that it is a concerted process with a substantial exit barrier. It was found that the H atom elimination channel is best described as a simple bond rupture occurring after internal conversion of the electronically excited molecule to the vibrationally excited ground state ethylene. Some of the primary C2H3 product has sufficient internal energy to spontaneously decompose to H+HC≡CH. At higher laser intensity a large fraction of the C2H3, however, absorbs another photon and fragments to H+H2C=C: (1A1 and 3B2).
Adiabatic three-dimensional simulations of the IHI−, BrHI−, and BrHBr− photoelectron spectra
Authors: Metz R. B. , Neumark D. M.
Published: July 2002

In order to better characterize the transition-state region for the I + HI, Br + HI, and Br + HBr reactions, the photoelectron spectra of IHI−, IDI−, BrHI−, BrHBr−, and BrDBr− have been simulated using a three-dimensional adiabatic approach. This method of simulation uses a Born–Oppenheimer separation in time scales between the fast hydrogen-atom motion and the slow halogen-atom motion to greatly simplify the computation of the photoelectron spectrum. The resulting simulations are compared to the experimental photoelectron and threshold photodetachment spectra of these anions, and to ‘‘exact’’ simulations of the IHI− and IDI− spectra. The comparison with the exact simulations shows that the adiabatic method is reasonably accurate, and is a considerable improvement over previous approximate simulation schemes. Potential-energy surfaces for the I + HI and Br + HI reactions are evaluated based on a comparison between the simulated and experimental spectra. A three-dimensional surface for the Br + HBr reaction that reproduces the experimental photoelectron spectrum is constructed by extending a fitted collinear surface to three dimensions.
A theoretical study of the parameters affecting the kinetics of gas adsorption on solid surfaces
Authors: Nordmeyer T. , Zaera F.
Published: July 2002

The nondissociative adsorption of gas phase molecules onto a spatially homogeneous square lattice was studied using a Monte Carlo computer simulation method which accounts for the existence of an extrinsic precursor state. The parameters affecting the adsorption kinetics were varied systematically in order to compare our simulations with a model for adsorption originally proposed by Kisliuk [J. Phys. Chem. Solids 3, 95 (1957)]. Our studies indicate that in the presence of precursor mobility, the sticking coefficients obtained from our simulations are consistently lower than those obtained using Kisliuk’s equation. Upon further investigation, we found two reasons for this discrepancy, namely, islanding of the adsorbates on the surface, and site revisiting in the precursor state. We found that some degree of islanding within the chemisorption layer occurs as a direct consequence of the nature of the precursor mediated adsorption process and that the extent of this clustering is controlled by the competition between mobility within the physisorbed layer and desorption; when mobility dominates, islanding formation is enhanced. Site revisiting within the precursor state was also found to affect the adsorption kinetics; we were able to derive here an analytical expression for the sticking coefficient that accounts for this factor and that correlates well with results from the corresponding simulation. Finally, we show that sticking coefficients are affected by surface geometry.
High-resolution infrared overtone spectroscopy of ArHF via Nd:YAG/dye laser difference frequency generation
Authors: Farrell John T. , Sneh Ofer , McIlroy Andrew , Knight Alan E. W. , Nesbitt David J.
Published: July 2002

The first high-resolution spectra of ArHF excited to the vHF=2←0 manifold near 7800 cm−1 are recorded via direct infrared absorption in a slit supersonic expansion. The tunable difference frequency light is generated via nonlinear subtraction of a cw Nd:YAG laser from a tunable cw ring dye laser in temperature phase matched LiNbO3, and permits continuous single-mode access to the 1–2 μm near-IR region. Rotationally resolved spectra are presented for the pure HF stretching overtone (2000)←(0000), as well as for combination band excitation into the Σ bend (2100)←(0000) and Π bend (2110)←(0000) internal rotor levels built on the vHF=2 overtone stretch. Local perturbations in the Π bend spectrum are observed which arise from a resonant crossing of rotational levels with the (2002) van der Waals stretch and allow spectroscopic analysis of this state. Nonresonant coupling between the Σ and Π bend vibrational levels is evidenced by anomalous P branch/R branch transition intensities and is analyzed as Coriolis interactions in a tumbling, hindered rotor. The spectra reveal Doppler limited line shapes [Δν=79(11) MHz] characteristic of the temperature and geometry of the slit expansion. An upper limit of Δνprediss≤2 MHz Lorentzian broadening is established, indicating an 80 ns lower limit to the predissociation lifetime. Comparison of intermolecular vibrational levels in ArHF vHF=0, 1, and 2 indicates a systematic increase in both angular anisotropy and radial well depth upon excitation of the high-frequency HF stretch. In conjunction with previous results from the vHF=1 and vHF=0 vibrational levels, these studies provide the necessary data for fitting an atom+diatom potential energy surface as a function of all intermolecular and intramolecular internal degrees of freedom.
Molecular-dynamics study of adiabatic proton-transfer reactions in solution
Authors: Laria Daniel , Ciccotti Giovanni , Ferrario Mauro , Kapral Raymond
Published: July 2002

A molecular-dynamics study of adiabatic proton transfer between two ions in a polar solvent is presented. The proton is treated as a quantum particle in three dimensions and the polar solvent is composed of classical rigid, dipolar molecules. The coupled Schrödinger and Newton’s equations are solved to determine the proton charge density and solvent configuration. The rate coefficient for the proton transfer is computed from correlation function expressions and corrections to transition-state theory due to recrossing of a free-energy barrier are determined. The simulation results are compared with a simple two-state model.
Direct spectroscopic observation of the reactive catalytic site for CO oxidation on Pt(335)
Authors: Xu Jiazhan , Henriksen Peter , Yates John T.
Published: July 2002

We report the first direct spectroscopic identification of the reactive catalytic site for CO oxidation on a stepped Pt single crystal using infrared reflection–absorption spectroscopy. It is found that CO adsorbed on the (111) terrace sites is more reactive than CO on the (100) step sites.
Spin adapted restricted open shell coupled cluster theory. Linear version
Authors: Neogrády Pavel , Urban Miroslav , Hubač Ivan
Published: July 2002

We describe the linear version of the spin adapted coupled cluster theory with all single and double excitations for open shell systems in restricted Hartree–Fock formalism. The method is based on the partitioning of the Hamiltonian developed by Hubač and Čársky [I. Hubač and P. Čársky, Phys. Rev. A 22, 2392 (1980)] in restricted Roothaan open shell formalism. Correlation energy of some simple molecular systems is calculated.
The vibrational spectra of molecular ions isolated in solid neon. IX. HCN+, HNC+, and CN−
Authors: Forney Daniel , Thompson Warren E. , Jacox Marilyn E.
Published: July 2002

When a Ne:HCN sample is codeposited at ∼5 K with a beam of neon atoms that have been excited in a microwave discharge, infrared absorptions of HNC, HCN+, and CN− appear. The absorptions of HNC isolated in solid neon are considerably closer to the gas-phase band centers than are the previously reported argon-matrix absorptions. The CH-stretching fundamental of HCN+ is identified, and the two stretching force constants of HCN+ are determined, using the isotopic data obtained in this study and the CN-stretching frequencies of HCN+ and DCN+ obtained in earlier photoelectron spectroscopic studies. In the inert, nonpolar environment provided by the neon matrix, the infrared absorption of CN− appears at 2053.1 cm−1, very close to the position obtained in two recent ab initio calculations. HCN+ photoisomerizes to HNC+ with a threshold in the near-infrared spectral region, consistent with an earlier ab initio calculation of the position of the transition state for this process. The reverse photoisomerization has also been observed at somewhat higher energies. All three vibrational fundamentals of HNC+ have been identified, and the stretching force constants have been determined. In all but the most dilute samples, HCNH+ is also stabilized in detectable amounts. Electron photodetachment from CN− results in charge neutralization of HCN+ and HCNH+ and in growth of the infrared absorption of CN.
Semiclassical wave packet approach to reactive scattering in hyperspherical coordinates
Authors: Marković Nikola , Billing Gert D.
Published: July 2002

We present the theory for a semiclassical approach to reactive scattering of three particle systems in hyperspherical coordinates. The time dependent Schrödinger equation is solved using wave packet propagation methods in three quantum dimensions. Comparison with results obtained by other methods are carried out for J=0.
Freezing, melting, nonwetting, and coexistence in (KCl)32
Authors: Rose John P. , Berry R. Stephen
Published: July 2002

Binary clusters, notably salt clusters with their combination of attractive and repulsive long-range forces, exhibit structural and dynamical behavior different from that of homogeneous clusters. The melting and freezing, nonwetting, and the complexity of the potential surface of (KCl)32 are used to make the comparison. A new method to estimate the density of configurational states is described and applied to the evaluation of thermodynamic properties of (KCl)32. In particular, with this new method we compute for several temperatures the fraction or probability, P(φ,β), of clusters vibrating around a configuration with minimum energy φ. The behavior of P(φ,β) with temperature T is indicative of a coexistence of solidlike and liquidlike forms of (KCl)32 for a range of temperatures. The input data required by this new method can be obtained from constant temperature molecular dynamics simulations.
Surface tension of dilute polymer solutions. II. The second virial coefficient
Authors: Myers Kurt R. , Freed Karl F.
Published: July 2002

The second virial coefficient for the surface tension increment of polymer solutions is evaluated for repulsive (Dirichlet) and reflecting (neutral) impenetrable surfaces. Renormalization group theory is used to describe the influence of excluded volume. An interpolation function extends the calculations to intermediate values of the polymer–surface interaction strength and provides the surface pressure as a function of both polymer–polymer and polymer–surface interactions. Analysis is provided of some available experimental data, and suggestions are made concerning experiments necessary to provide a thorough test of the theory.
Characteristic properties of the nuclear magnetic resonance–paramagnetic relaxation enhancement arising from integer and half-integer electron spins
Authors: Sharp Robert R.
Published: July 2002

The influence of zero field splitting (zfs) interactions on the magnetic field dispersion profile of the nuclear magnetic resonance–paramagnetic relaxation (NMR–PRE) (i.e., the enhancement of nuclear magnetic relaxation rates that is produced by paramagnetic solute species in solution) has been explored systematically for S=1, 3/2, 2, and 5/2 spin systems using recently developed theory. To facilitate comparison of results for different spin values, the theory was expressed in a reduced form with Larmor frequencies in units of ωD (the uniaxial zfs parameter D in rad s−1), and correlation times and spin relaxation times in units of ωD−1. For S=1, the functional form of the profile can be described in terms of five types of qualitative features. Two of these are characteristic of Zeeman-limit [Solomon, Bloembergen, and Morgan (SBM)] theory and result from the magnetic field dependence of the spin energy level splittings. The remaining three have no analog in Zeeman-limit theory and arise from a change in the quantization axis of the electron spin precessional motion which, in the zfs limit, lies along molecule-fixed coordinate axes, and, in the Zeeman limit, lies along the external field direction.

The reduced field dispersion profiles for the integer spin systems S=1 and S=2 were found to be very similar to each other, the principal difference being that the midfield positions of the requantization features (types 2, 3, and 4) are shifted for S=2 relative to S=1, the magnitude and sign of the shift depending on the position of the nuclear spin in the molecular coordinate frame. For half-integer spins, the dispersion profiles exhibit, in addition to the five features characteristic of integer spins, a sixth type of feature, which is centered somewhat to low field of ωSτc=1, where τc is the dipolar correlation time. The type-6 feature results from field-dependent level splitting of the mS=±1/2 Kramers doublet. It is present when ωDτc≥1. These theoretical predictions have been examined by means of reinterpretations of the NMR–PRE data for tris-(acetylacetonato)–metal complexes of V(III) (S=1), Cr(III) (S=3/2), Mo(III) (S=3/2), Mn(III) (S=2), and Fe(III) (S=5/2). As predicted, type-6 features are absent for the integer spin complexes, for which the T1 field dispersion profiles are nearly field independent.

The experimental profiles were successfully simulated quantitatively by the generalized theory, but not by Zeeman-limit theory. For the half-integer spin systems, the predicted zfs-related type-6 features appear to be present in the profiles, particularly for Mo(acac)3, for which the data deviate significantly from the Zeeman-limit profile in a manner that is explained by the generalized theory.
Effect of zero field splitting interactions on the paramagnetic relaxation enhancement of nuclear spin relaxation rates in solution
Authors: Sharp Robert R.
Published: July 2002

The enhancement of nuclear spin relaxation rate R1m that is produced by paramagnetic metal ions in solution (the NMR-PRE) has been investigated for electron spin systems with S=1 using recently developed relaxation theory that incorporates both Zeeman and zero field splitting (zfs) interactions of arbitrary magnitude in the electron spin Hamiltonian. The zfs interaction gives rise to important qualitative features which have no analog in the Zeeman-limit theory. The three principal physical phenomena responsible for these effects are (1) alterations in the geometry of the magnetic dipole–dipole coupling energy due to requantization of the electron spin from laboratory to molecular axes; (2) the crossing or ‘‘pinching’’ of spin energy levels that occurs in the regime of field strengths between the zfs and Zeeman limits; and (3) an effective magnetic field dependence in the reorientational correlation time that results from a change in the appropriate definition of this quantity in the intermediate regime. In the zfs limit and in the intermediate regime, the field dispersion profile depends strongly on the position of the nuclear spin with respect to the molecular coordinate axes. For equatorial positions of the nuclear spin, the principle qualitative feature of the dispersion profile is a strong increase in R1m with increasing field strength coupled, in most cases, with a shallow local R1m maximum; both features are centered near the cross-over field between the limits. For axial positions, the profile exhibits a feature that is superficially similar to those characteristic of Zeeman-limit theory, but which is fundamentally different in quantitative properties and in physical origin. As a test of theoretical predictions, the experimental magnetic field profile of the NMR-PRE of the hexaquo-Ni(II) cation, an S=1 model system that has previously been studied extensively, has been reinterpreted. It is shown that the major qualitative features of the experimental field profile result specifically from physical effects of the zfs interaction and are closely related to the phenomenon of requantization of the electron spin in the intermediate regime.
Angular-radial coupling in the tunneling motion of (HCCH)2
Authors: Suni Ian I. , Klemperer William
Published: July 2002

The torsion–rotation Hamiltonian is developed for the problem of two rods with a C2v equilibrium structure tunneling through a C2h saddle point, one rod rotating clockwise and the other counterclockwise with the two rods constrained to lie in a plane. The Hamiltonian is developed both for the case where the rod center-of-mass spacing is constant and the case where this spacing is an elliptical function of the torsional angle. The latter case is formally similar to Bunker’s semirigid bender. Both forms of the torsion–rotation Hamiltonian are then solved for (HCCH)2 and (DCCD)2 and spectral fits obtained. The results show that perturbation by an excited torsional state explains some spectral anomalies observed previously. The only spectral fit that includes all observed microwave and radiofrequency transitions and gives physically reasonable results is the model that allows for dependence of the intermolecular spacing on torsional angle. The best fit for (HCCH)2 yields an equilibrium C2v intermolecular distance of 4.465(2) Å and an intermolecular distance of 4.014(10) Å at the C2h saddle point. For (DCCD)2 intermolecular spacings of 4.450(2) and 3.991(15) Å are obtained. The radial shrinkage between the two configurations agree quite well for (HCCH)2 and (DCCD)2, 0.45(1) and 0.46(1) Å, respectively.