bluuebunny/crossref_metadata_2025 huggingface dataset:
Role of trap clustering in the trapping kinetics
Authors: Makhnovskii Yurii A. , Berezhkovskii Alexander M. , Sheu Sheh-Yi , Yang Dah-Yen , Lin Sheng Hsien
Published: July 2002

The Smoluchowski theory describes the kinetics of trapping of Brownian particles by absorbers randomly placed without correlations between their positions. We generalize this theory to take trap correlations into account when traps occur in spherical clusters distributed in space in a noncorrelated manner. A cluster contains n traps uniformly distributed within the cluster. An effective medium treatment is used to handle trap-correlation effects. Explicit expressions are obtained for the time-dependent rate coefficient and the particle survival probability valid for the entire range of n and cluster radius R. We analyze how the trap clustering manifests itself in the kinetics. In particular, we show that there exists a domain of the parameters n and R, where the kinetics is well fitted by a stretched exponential function for more than 99% of the decay. Such behavior should be contrasted to the essentially exponential kinetics predicted by Smoluchowski theory for noncorrelated traps.
Evidence of a perpendicular component in the photodissociation of BrCN at 213 nm
Authors: Franks Karen J. , Li Hongzhi , Kong Wei
Published: July 2002

A strong, uniform electric field was used to achieve brute force orientation of supersonically cooled cyanogen bromide. Laser-induced fluorescence of the CN fragment from photolysis of the sample at 213 nm was detected. A 20% enhancement in signal was observed when the dissociation laser was polarized parallel, rather than perpendicular, to the orientation field. This indicates the dominance of a transition dipole moment parallel to the permanent dipole of the molecule. However, the degree of enhancement falls well below that predicted for a pure parallel transition at a rotational temperature of 1.5 K. Thus, evidence of contribution from a perpendicular transition is seen. Based on quantitative analysis, the direction of the effective transition dipole, and thereby the amount of contribution from a transition dipole perpendicular to the permanent dipole of the molecule, can be determined. Using this method, approximately 33% perpendicular character was found in the dissociation process of BrCN at 213 nm.
Dynamics of ions in liquid water: An interaction-site-model description
Authors: Chong Song-Ho , Hirata Fumio
Published: July 2002

We present a molecular theory for investigating the dynamics of ions in polar liquids. The theory is based on the interaction-site model for molecular liquids and on the generalized Langevin equation combined with the mode-coupling theory. The velocity autocorrelation function, diffusion and friction coefficients of ions in water at 25 °C and at infinite dilution are studied. The theoretical results for the velocity autocorrelation functions exhibit a gradual change from oscillatory to monotonic decay as the ion size increases. The diffusion (friction) coefficients of ions in aqueous solutions pass through a maximum (minimum) as a function of the ion size, with distinct curves and maxima (minima) for positive and negative ions. These trends are in complete accord with those of the molecular dynamics simulation results performed on the same system by Rasaiah and co-workers [J. Phys. Chem. B 102, 4193 (1998)]. It is worthwhile to mention that this is the first molecular theory that is capable of describing the difference in the dynamics of positive and negative ions in aqueous solutions. A further analysis of the friction coefficients of ions in water is presented in which the friction is decomposed into the “Stokes,” dielectric and their cross terms. The Stokes and dielectric terms arise from the coupling of the ion dynamics to essentially the acoustic dynamics of the solvent via the short-range interaction, and from the coupling to the optical mode of the solvent via the long-range interaction. The most striking feature of our results is that the Stokes friction so defined does not increase monotonically with increasing ion size, but decreases when ions are very small, implying a formation of a molecular “complex” comprising the ion and its nearest neighbor solvent molecules. Interesting observations concerning the cross term are: (1) its magnitude is rather large for small ions and cannot be neglected at all, and (2) the cross term for small ions seems to cancel out the Stokes part, and consequently the total friction for small ions seems to be to a large extent determined by its dielectric component.
Photoinitiated unimolecular decomposition of NO2: Rotational dependence of the dissociation rate
Authors: Bezel I. , Ionov P. , Wittig C.
Published: July 2002

Photoinitiated unimolecular decomposition rate constants of rotationally excited NO2 molecules have been measured near dissociation threshold (D0) by employing a double resonance technique. Rotational selectivity has been achieved by using narrow-linewidth (0.015 cm−1) infrared excitation to prepare specific rotational levels (N′=1,3,…,15, Ka′=0) of the (1,0,1) vibrational level. The picosecond-resolution pump–probe technique has then been used to photodissociate the molecules thus tagged and to monitor the appearance of the NO product. Data have been obtained for two progressions of average excess energies, 〈E〉−D0: (i) 10 cm−1+E101rot and (ii) 75 cm−1+E101rot, where 〈E〉 denotes an average over the pump laser linewidth and E101rot is the rotational energy of the (1,0,1) X̃ 2A1 intermediate vibrational level. The measured rate constants do not display any noticeable dependence on N′, which is a reflection of significant rovibronic interaction. Spin–rotation interaction, which has been implicated as the main source of rovibronic coupling for small values of N′, is not likely to yield such a result. A model is proposed to describe the influence of rotation on the dissociation rate. The experimental data are consistent with a Coriolis coupling mechanism causing transitions to occur between Ka levels.
The tetrahydrofuran⋯hydrogen chloride complex: Rotational spectrum and theoretical analysis
Authors: López J. C. , Alonso J. L. , Lorenzo F. J. , Rayón V. M. , Sordo J. A.
Published: July 2002

The hydrogen bonded heterodimer tetrahydrofuran⋯HCl has been investigated using molecular beam Fourier transform microwave spectroscopy in combination with ab initio calculations. The rotational spectrum, observed in the range 6–18 GHz, shows a complex pattern originated by the existence of small tunneling splittings in addition to the Cl-nuclear quadrupole coupling hyperfine structure. The vibrational energy difference between the members of the doublet, ΔE=3.550(25) MHz, has been determined from the analysis of the a-type Coriolis coupling interaction between them. Doublets of the same magnitude are also present in the spectra of the different HCl isotopomers analyzed. These tunneling splittings were not observed for the species C4D8O⋯H35Cl. The analysis of all the available data has allowed us to conclude that these splittings are due to pseudorotation within the tetrahydrofuran subunit. The spectroscopic constants have been interpreted in terms of a geometry in which tetrahydrofuran has a conformation close to the twisted ring-form with HCl lying on the plane bisector to the COC ring angle. The potential energy surface for the interaction between tetrahydrofuran and hydrogen chloride has been explored by using ab initio methodologies at the correlated level [MP2, MP4(SDTQ)] with Pople’s 6-31G** and Dunning’s aug-cc-pVDZ basis sets. One minimum and three transition structures were located and characterized at the MP2/6-31G** level. The geometry parameters and rotational constants of the minimum agree quite well with those determined from the spectroscopic data. The transition structures correspond to interconversions between equivalent conformations, the first one via an inversion motion and the remaining two via pseudorotation movements. One of these latter two is responsible for the splittings detected in the microwave spectroscopy study. The tetrahydrofuran⋯hydrogen chloride interaction can be seen as a combination of electrostatic and charge transfer contributions both consistent with the angular geometry exhibited by the complex.
Emission spectroscopy of photodissociating N2O4 excited near 200 nm to the πnb,OπNO2*/nσN–N* avoided crossing
Authors: Parsons B. F. , Curry S. L. , Mueller J. A. , Ray P. C. , Butler L. J.
Published: July 2002

These experiments and complementary electronic structure calculations seek to probe the early dissociation dynamics of N2O4 excited in the strong 186 nm ultraviolet absorption band. Laser photons of 199.7, 203, and 205 nm are used to dissociate N2O4 molecules expanded in a free jet. The emission from the dissociating molecules is dispersed in a spectrometer and collected with an optical multichannel analyzer (OMA). We observe a strong progression in ν3, the N–N stretching mode, consistent with electronic structure calculations and a previous observation of N–N bond fission leading to NO2 photoproducts in this band. We also observe emission to combination bands in ν4, the torsion, and ν5, the out-of-phase antisymmetric NO2 stretch, which we attribute to vibronic coupling to a σσ* excited state configuration. Analysis of our data allows for an assignment of the torsional frequency of N2O4. Around 200 nm nitric acid, nitromethane, and N2O4 all exhibit a similar absorption to an adiabatic surface that changes electronic character from πnb,OπNO2* at short O2N–R (R=CH3, OH, NO2) internuclear distances to nσ* at extended internuclear distances. We compare our present emission spectra of N2O4 with previous emission experiments on nitromethane to understand how the character of the Franck–Condon region of the excited states in these two molecules differ.
Iterative and direct methods employing distributed approximating functionals for the reconstruction of a potential energy surface from its sampled values
Authors: Szalay Viktor
Published: July 2002

The reconstruction of a function from knowing only its values on a finite set of grid points, that is the construction of an analytical approximation reproducing the function with good accuracy everywhere within the sampled volume, is an important problem in all branches of sciences. One such problem in chemical physics is the determination of an analytical representation of Born–Oppenheimer potential energy surfaces by ab initio calculations which give the value of the potential at a finite set of grid points in configuration space. This article describes the rudiments of iterative and direct methods of potential surface reconstruction. The major new results are the derivation, numerical demonstration, and interpretation of a reconstruction formula. The reconstruction formula derived approximates the unknown function, say V, by linear combination of functions obtained by discretizing the continuous distributed approximating functional (DAF) approximation of V over the grid of sampling. The simplest of contracted and ordinary Hermite-DAFs are shown to be sufficient for reconstruction. The linear combination coefficients can be obtained either iteratively or directly by finding the minimal norm least-squares solution of a linear system of equations. Several numerical examples of reconstructing functions of one and two variables, and very different shape are given. The examples demonstrate the robustness, high accuracy, as well as the caveats of the proposed method. As to the mathematical foundation of the method, it is shown that the reconstruction formula can be interpreted as, and in fact is, frame expansion. By recognizing the relevance of frames in determining analytical approximation to potential energy surfaces, an extremely rich and beautiful toolbox of mathematics has come to our disposal. Thus, the simple reconstruction method derived in this paper can be refined, extended, and improved in numerous ways.
Molecular dynamics of iso-amyl bromide by dielectric spectroscopy, and the effects of a nonpolar solvent, 2-methylpentane, on the spectral features
Authors: Kalinovskaya O. E. , Vij J. K.
Published: July 2002

To gain insight into the effects of the weakening of the electrostatic interactions on molecular dynamics when polar molecules are dissolved in a nonpolar solvent, the dielectric polarization and relaxation behaviors of iso-amylbromide and its 50 mol % solution in 2-methylpentane have been studied in detail over the frequency range, 1 mHz–1 MHz, and a temperature range approaching their liquid to glass transition. Features of the (i) α-relaxation spectrum, (ii) the Johari–Goldstein relaxation process in the liquid state at low temperatures, with an asymmetric spectral shape, and (iii) the temperature dependence of the relaxation dynamics have been determined and the effects of weakening of the electrostatic interaction on these features examined. The high-frequency wing of the loss spectrum of the α-relaxation is proportional to ω−β. The dynamics of its α-relaxation follows the Arrhenius equation initially at high temperatures and thereafter the Vogel–Fulcher–Tamman equation. Alternative equations for the change in the relaxation rate have been discussed. A decrease in the dipole–dipole interaction and reduction in the internal field in a solution with a nonpolar solvent leads to a remarkable change in the shape of the relaxation spectra at high frequencies such that the dielectric loss for the α-relaxation becomes proportional to ω−αβ, with α, β<1. The relaxation spectra of iso-amyl bromide dissolved in 2-methylpentane follows the H–N function and therefore behaves similar to a polymer, whereas for pure iso-amyl bromide follows the Davidson–Cole behavior.
S
1
–S
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vibronic coupling in trans-1,3,5-hexatriene. I. Electronic structure calculations
Authors: Woywod Clemens , Livingood William C. , Frederick John H.
Published: July 2002

The potential-energy surfaces of the 1 1Ag, 2 1Ag, and 1 1Bu states of trans-1,3,5-hexatriene (THT) are explored in the vicinity of the ground state equilibrium structure. The S0 geometry optimization and force field calculation have been carried out with the restricted Hartree–Fock plus Mo/ller–Plesset second-order perturbation theory method. Vibronic coupling constants for the normal coordinates of ag and bu symmetry were computed with the complete-active-space self-consistent-field (CASSCF) and single state multiconfigurational second-order perturbation theory (CASPT2) electronic structure models. The CASSCF/CASPT2 method unequivocally places the vertical excitation energy of the dark 2 1Ag “phantom state” below the 1 1Bu level and predicts an energy difference of ca. 0.5 eV. The results are consistent with time-resolved photoionization yield and photoelectron spectroscopy experiments that indicate the existence of a low lying S1–S2 conical intersection which induces rapid 1 1Bu→2 1Ag internal conversion on a time scale of 40 fs to 50 fs [Cyr and Hayden, J. Chem. Phys. 104, 771 (1996)]. Based on the vibronic coupling constants five totally symmetric vibrations with high Franck–Condon and/or tuning activity have been identified. The S1 and S2 states interact primarily via the two bu normal modes ν24 and ν26. Other ag and bu normal vibrations do not appear to couple significantly to the lowest lying π→π* transition. The modeling of the ultrafast relaxation processes following optical excitation of the 1 1Bu state of THT and the calculation of absorption and resonance Raman spectra are discussed in the following paper.
Ab initio configuration interaction study of the predissociation of the (4s), (4pσ) 1,3Π, and (4pπ) 3Σ+ Rydberg states of HCl and DCl
Authors: Li Yan , Bludsky Ota , Hirsch Gerhard , Buenker Robert J.
Published: July 2002

Ab initio configuration interaction (Cl) calculations are carried out for the lowest-lying six 1,3Π and three 3Σ+ states. In addition, the first and second radial couplings 〈φi(R,r)|∂/∂R|φj(R,r)〉 and 〈φi(R,r)|∂2/∂R2|φj(R,r)〉, spin-orbit, and rotational couplings are also evaluated based on the resulting highly correlated wave functions. Energy positions and predissociation linewidths of rovibrational levels associated with the above electronic states are determined in the adiabatic representation by means of the complex scaling method employing a basis of complex scaled Hermite polynomials and Gauss–Hermite quadrature. The present treatment correctly reproduces a number of observed trends in energy and line broadening for individual rovibrational levels of the (4s)b 3Π and C 1Π states of HCl and DCl. The calculated linewidths for the v=0, J=2, and J=8 rovibrational levels of the (4s)C 1Π state of HCl are 1.2 and 2.1 cm−1, respectively, which is in good agreement with the recently observed rotational dependence of line broadening for the v=0 level of this state [M. H. Alexander et al., Chem. Phys. 231, 331 (1998)]. The present calculations find that the linewidths of the (4pσ)D 1Π state are greater than for the (4pσ)d 3Π state. The distinctive predissociation mechanisms of the D 1Π and the d 3Π states are discussed on the basis of the calculated radial couplings. The calculations predict very broad lines for the v=0 and 1 vibrational levels of the lowest adiabatic bound 3Σ+ state, consistent with the fact that the corresponding state has not yet been identified by spectroscopic means.
A small-system ensemble Monte Carlo simulation of supersaturated vapor: Evaluation of barrier to nucleation
Authors: Oh K. J. , Zeng X. C.
Published: July 2002

A small-system grand canonical ensemble Monte Carlo method is developed to evaluate cluster size distribution and barrier to the nucleation in a supersaturated Lennard-Jones vapor. The theoretical foundation is a physical cluster theory in which the Stillinger cluster is used as a prototypical physical cluster. Using method of Mayer’s cluster expansion, the cluster–vapor interaction is effectively taken into account. From a separate canonical ensemble Monte Carlo simulation using a test particle method, the averaged volume of the cluster is obtained and is also incorporated in the small-system ensemble simulation. By this implementation our simulation is computationally more efficient compared to that based on the n/v Stillinger cluster theory in that instead of searching the saddle point on a two-dimensional free energy surface (a function of cluster size n and volume v) one needs only to find the peak on a free energy curve (a function of n only). A comparison with the height of barrier obtained from a large-system ensemble Monte Carlo simulation [K. Oh and X. C. Zeng, J. Chem. Phys. 110, 4471 (1999)] shows that omission of the vapor–cluster attraction can cause an overestimation of the height by several kBT.
Time-dependent quantum mechanical study of photodissociation of molecular oxygen in the Schumann–Runge continuum
Authors: Balakrishnan N. , Jamieson M. J. , Dalgarno A. , Li Y. , Buenker R. J.
Published: July 2002

We present a time-dependent quantum mechanical investigation of predissociation of molecular oxygen following absorption in the Schumann–Runge continuum. We calculate the total and partial photoabsorption cross sections in a wave packet formalism in which the predissociation of the B 3Σu− molecular state is attributed to spin-orbit interactions with the Πu1, a 3Πu, Πu5, and 2 3Σu+ repulsive states. We calculate the branching ratio, describing the relative rates of production of excited (1D) and ground-state (3P) oxygen atoms, and find close agreement with a recent measurement.
Nonadditive three-body polarizabilities of molecules interacting at long range: Theory and numerical results for the inert gases, H2, N2, CO2, and CH4
Authors: Champagne M. H. , Li X. , Hunt K. L. C.
Published: July 2002

Collision-induced light scattering spectra of the inert gases and hydrogen at high densities provide evidence of nonadditive three-body interaction effects, for which a quantitative theory is needed. In this work, we derive and evaluate the three-body polarizability Δα(3) for interacting molecules with negligible electronic overlap. Our results, based on nonlocal response theory, account for dipole-induced-dipole (DID) interactions, quadrupolar induction, dispersion, and concerted induction-dispersion effects. The contribution of leading order comes from a DID term that scales as α3d−6 in the molecular polarizability α and a representative distance d between the molecules in a cluster. Quadrupolar induction effects are also large, however, ranging from ∼35% to 104% of the leading DID terms for equilateral triangular configurations of the species studied in this work, at separations approximately 1 a.u. beyond the van der Waals minima in the isotropic pair potentials. For the same configurations, the dispersion terms range from 2% to 7% of the total Δᾱ(3). The dispersion and induction-dispersion contributions are derived analytically in terms of integrals over imaginary frequency, with integrands containing the polarizability α(iω) and the γ hyperpolarizability. For H, He, and H2, the integrals have been evaluated accurately by 64-point Gauss–Legendre quadrature; for heavier species, we have developed approximations in terms of static polarizabilities, static hyperpolarizabilities, and van der Waals interaction energy coefficients (C6 and C9). In the isotropic interaction-induced polarizability Δᾱ, the three-body terms are comparable in magnitude to the two-body terms, due to a cancellation of the first-order, two-body DID contributions to Δᾱ. For the heavier species in this work (Ar, Kr, Xe, N2, CH4, and CO2) in the configurations studied, the three-body contributions to Δᾱ range from −7 to −9% of the two-body terms for equilateral triangular arrays and from 35% to 47% of the two-body terms for linear, centrosymmetric systems.
Vibronic structure of alkoxy radicals via photoelectron spectroscopy
Authors: Ramond Tanya M. , Davico Gustavo E. , Schwartz Rebecca L. , Lineberger W. Carl
Published: July 2002

Ultraviolet photoelectron spectra of CH3O−, CH3CH2O−, (CH3)2CHO−, (CH3)3CO−, as well as CD3O− and CD3CD2O− are presented, providing improved electron affinities and new information on vibronic coupling in the corresponding neutral radicals. Jahn-Teller vibronic transitions are assigned to e vibrational modes of X̃ 2E CH3O. The excitation energy of the Ã 2A′ state of CH3CH2O with respect to the X̃ 2A″ state is observed directly at 355±10 cm−1. Vibronic coupling between these low-lying electronic states perturbs the observed vibronic levels. Features of the (CH3)2CHO− photoelectron spectrum are assigned. The splitting between the X̃ 2A′ and Ã 2A″ states of (CH3)2CHO is determined to be 1225±65 cm−1. Significant vibronic coupling is not observed in (CH3)2CHO. Vibrational assignments are made for the spectral features of (CH3)3CO and no Jahn-Teller effects are observed directly. Electron affinities (EAs) for the neutrals (RO) are (in eV); EA(CH3O)=1.572±0.004; EA(CD3O)=1.559±0.004; EA(CH3CH2O)=1.712±0.004; EA(CD3CD2O)=1.699±0.004; EA((CH3)2CHO)=1.847±0.004; EA((CH3)3CO)=1.909±0.004. Bond dissociation energies of ROH, D300(RO–H), along with ΔfH300(RO), ΔfH300(RO−), and ΔfH300(RO+) are derived.
Computer simulation of the chemical potential of binary Lennard-Jones mixtures
Authors: Borówko M. , Zagórski R. , Malijevský A.
Published: July 2002

The chemical potentials of binary Lennard-Jones mixtures are calculated by means of the modified scaled particle Monte Carlo method (SP-MC) and using the conventional test particle procedure (TPI). Grand canonical Monte Carlo (GCMC) simulations are also performed as an independent verification of the SP-MC technique. The agreement of the SP-MC and GCMC results is very good.
Quantum mechanical canonical rate theory: A new approach based on the reactive flux and numerical analytic continuation methods
Authors: Rabani Eran , Krilov Goran , Berne B. J.
Published: July 2002

We present the reactive flux analytic continuation (RFAC) method, based on the quantum reactive flux formalism combined with a numerical analytic continuation approach to calculate quantum canonical rates in condensed phase systems. We express the imaginary time reactive-flux correlation function in terms of a frequency dependent rate constant, and use path integral formalism to derive a working expression suitable for Monte Carlo simulation techniques. The imaginary time data obtained by simulation is analytically continued to the real time using the maximum entropy method to obtain the reaction rate. Motivated by the success of the method to predict the rates for a simple one dimensional parabolic barrier model, we assess its accuracy for a condensed phase reaction modeled by a double-well coupled to a harmonic bath. We note that the method is applicable to a more general Hamiltonian as long as the reaction coordinate can be identified. The reaction rates computed in this fashion are in very good agreement with analytic and numerically exact results. We demonstrate the applicability of the method for a wide range of model parameters and temperatures.
A heat capacity estimator for Fourier path integral simulations
Authors: Neirotti J. P. , Freeman David L. , Doll J. D.
Published: July 2002

Previous heat capacity estimators useful in path integral simulations have variances that grow with the number of path variables included. In the present work a new specific heat estimator for Fourier path integral Monte Carlo simulations is derived using methods similar to those used in developing virial energy estimators. The resulting heat capacity estimator has a variance that is roughly independent of the number of Fourier coefficients (kmax) included, and the asymptotic convergence rate is shown to be proportional to 1/kmax2 when partial averaging is included. Quantum Monte Carlo simulations are presented to test the estimator using two one-dimensional models and for Lennard-Jones representations of Ne13. For finite kmax, using numerical methods, the calculated heat capacity is found to diverge at low temperatures for the potential functions studied in this work. Extrapolation methods enable useful results to be determined over a wide temperature range.
Stereo-selective binding of chlorobenzene on Si(111)-7×7
Authors: Cao Y. , Deng J. F. , Xu G. Q.
Published: July 2002

The adsorption and binding of chlorobenzene (C6H5Cl) on clean and D-modified Si(111)-7×7 surfaces have been investigated using high resolution electron energy loss spectroscopy (HREELS) and thermal desorption spectroscopy (TDS). On a clean surface, both chemisorbed and physisorbed C6H5Cl are observed at an adsorption temperature of 110 K. The HREEL spectra show direct evidence for the presence of both sp2 and sp3 carbon atoms in chemisorbed C6H5Cl molecules on Si(111)-7×7. Upon D-modification, the chemisorption of C6H5Cl decreases rapidly with increasing D-coverage (θD). At θD=1/3 ML, only physisorbed chlorobenzene is detected, which strongly suggests the involvement of rest-atoms in the chemisorption of C6H5Cl. Combined with the scanning tunneling microscopy (STM) results by Chen et al. [Surf. Sci. 340, 224 (1995)] showing the participation of adatoms in the binding, we propose that the 2,5-carbon atoms in C6H5Cl are stereo-selectively di-σ bonded to a pair of adjacent adatom and rest-atom on the Si(111)-7×7 surface, yielding a 2,5-chlorocyclohexadienelike surface adduct.
Helium scattering cross section of adsorbates on highly corrugated surfaces
Authors: Farı́as D. , Patting M. , Rieder K. H.
Published: July 2002

The properties of different definitions of the cross section for helium scattering of perfectly diffuse adsorbates on highly corrugated surfaces have been investigated experimentally. For the adsorption of CO on Rh(311) we found that the normalized cross section associated with each diffraction beam scatters strongly with the angle of incidence, the incident energy, and the intensity of the diffraction beam. In particular, our results demonstrate that, contrary to the suggestion of a recent work, no simple relation exist between the intensity of a given diffraction beam and the associated cross section. On the other hand, a total cross section defined by including all diffraction beams was found to vary smoothly as a function of incident parameters, in a way similar to that observed on low corrugated surfaces and in gas phase scattering.
Influence of the treatment of electrostatic interactions on the results of free energy calculations of dipolar systems
Authors: Brunsteiner Michael , Boresch Stefan
Published: July 2002

Use of atom based truncation schemes in thermodynamic integration leads to hydration free energies of water that depend strongly on the cut-off radius. By contrast, Ewald summation, a reaction field technique, and group based truncation of the potential give the same result (6.5±0.1 kcal/mole), independently of the cut-off radius and system size.
Classical survival probability for ozone photodissociation in the Hartley band
Authors: Parlant Gérard
Published: July 2002

The survival probability for photodissociation of ozone in the Hartley band is computed using classical trajectory techniques with Wigner sampling of the initial conditions. In the general Cs symmetry, the survival probability exhibits notable recurrences, in qualitative agreement with quantum dynamics results. The present work confirms the analysis by Johnson and Kinsey [J. Chem. Phys. 91, 7638 (1989)], who postulated the role of unstable quasiperiodic orbits in the experimental autocorrelation function.
Vibrational excitation of carbon oxysulfide molecules by positron and electron impacts
Authors: Kawada M. K. , Sueoka O. , Kimura M.
Published: July 2002

Vibrational excitation to the symmetric stretching, bending and asymmetric stretching modes of OCS molecules by electron and positron impacts is investigated for impact energy from 2 to 6 eV theoretically and experimentally. Because the OCS molecule is a polar molecule, interactions that induce the vibrational excitation for the three modes are primarily long-range permanent dipole interactions. And hence, all excitation cross sections for three modes thus obtained are found to be similar in size both for electron and positron impacts, although for positron, the magnitude is generally slightly smaller by 25% than that of electron. This situation is markedly different from that of the CO2, where for the symmetric stretching mode, the difference between positron and electron impacts is a few orders of magnitude at 5 eV. Therefore, a comparative discussion of CO2 is useful for further understanding.
Observation of collisional ionization electron spectra of van der Waals clusters with metastable He*(2 3S) atoms: An evidence for autoionization from superexcited Ar clusters
Authors: Tanaka Hideyasu , Maruyama Ryo , Yamakita Yoshihiro , Yamakado Hideo , Misaizu Fuminori , Ohno Koichi
Published: July 2002

Penning ionization electron spectra (PIES) of van der Waals clusters have been observed for the first time by means of a newly developed magnetic bottle electron spectrometer, which is 1000 times more efficient than a conventional PIES apparatus. In the PIES of Ar clusters with He*(2 3S), two bands of Ar 3p hole characters with different ionization energies (IE), band A (IE=ca.15 eV) and band B (IE=ca.13.5 eV), were observed. Band A was assigned to vertical transitions in Franck–Condon regions corresponding to the band in the ultraviolet photoelectron spectra (UPS) of clusters reported by Carnovale et al. [J. Chem. Phys. 90, 1452 (1989)]. Band B, which is an extra band missing in the UPS of clusters, can be ascribed to the autoionization into a stable structure of Arn+ via superexcited states of Arn. Band B can be considered as an adiabatic ionization which is forbidden as a direct ionization. The energy difference of ca. 1.5 eV between the vertical IE for band A and the adiabatic IE for band B was found to agree well with the reported binding energy of the trimer ion core.
Guided ion beam studies of electron and isotope transfer in N+14+15N2 collisions
Authors: Glosik J. , Luca A. , Mark S. , Gerlich D.
Published: July 2002

Collisions of ground state N+ ions with N2 were investigated in a Guided Ion Beam (GIB) apparatus at center-of-mass (CM) collision energies up to 10 eV. Isotopic labeling of the target gas, N215, was used as one criterion to distinguish electron transfer, atom transfer, and isotope exchange channels. The absolute cross sections are in good agreement with previous measurements. By combining the GIB technique with time-of-flight analysis (acronym GIB-TOF) more detailed information on the reaction dynamics has been obtained. The product velocity distributions measured for N2+15 and N1514N+ indicate that both, electron and atom transfer, have to be divided into at least two parts, a direct one and one which seems to be characteristic for a long lived complex. The -- branching ratios derived for these four channels show a strong energy dependence. The mean product kinetic energy indicates that the remaining nitrogen atom is most probably formed in the metastable N(2D) state. For the nearly thermoneutral N+14–15N+ isotope exchange it was observed that a significant fraction of the translational energy is transferred into internal excitation of the neutral molecule.
Hot atom reaction yields in Mu*+H2 and T*+H2 from quasiclassical trajectory cross sections on the Liu–Siegbahn–Truhlar–Horowitz surface
Authors: Senba Masayoshi , Fleming Donald G. , Arseneau Donald J. , Mayne Howard R.
Published: July 2002

In order to provide an assessment of the “global” accuracy of the Liu–Siegbahn–Truhlar–Horowitz (LSTH) potential surface for H3, hot atom reaction yields, which are determined from collision processes over an energy range much wider than that of single-collision experiments, have been calculated for the Mu*+H2 and T*+H2 systems. The isotopic comparison of muonium (Mu=μ+e−), an ultralight isotope of hydrogen (mMu/mH≈1/9), with the heaviest H-atom isotope, tritium, is a novel approach in testing the global accuracy of the H3 surface. These reaction yields have been calculated using a formalism developed for (μ+) charge exchange, with input cross sections for elastic, inelastic (rovibrational excitation) and reactive collisions determined from quasi classical trajectories on the LSTH surface, in the center-of-mass energy range 0.5–11 eV. The rate of energy loss of the hot atom (Mu* or T*) due to elastic and inelastic collisions with the moderator (H2) drastically affects the hot atom reaction yield. In particular, the forwardness of the angular differential cross section for the elastic process plays a crucial role in determining the stopping power for hot atoms. Good agreement is obtained in the absolute yields for both Mu*+H2 and T*+H2, for the first time from microscopic cross sections, demonstrating that the LSTH surface remains surprisingly accurate over a wide range of energy and isotopic mass.
Pressure and temperature effects on the phase transition from a dense droplet to a lamellar structure in a ternary microemulsion
Authors: Seto Hideki , Okuhara Daisuke , Kawabata Youhei , Takeda Takayoshi , Nagao Michihiro , Suzuki Jiro , Kamikubo Hironari , Amemiya Yoshiyuki
Published: July 2002

A small-angle x-ray scattering (SAXS) study of a ternary microemulsion composed of AOT [sodium bis(2-ethylhexyl) sulfosuccinate], water and n-decane was undertaken in order to clarify the phase behavior and the feature of the corresponding structural transition from a dense droplet to a lamellar structure with increasing pressure and temperature. The volume fractions of water and decane were fixed to be equal and the volume fraction of AOT against the whole volume (φs) was selected to be 0.209 and 0.230 in order to compare results with those obtained by small-angle neutron scattering (SANS). The pressure was varied between 1 and 800 bar under controlled temperature at 20, 25, 29, or 33 °C. Under all conditions applied, the phase transition from the droplet structure to the lamellar structure was observed. The results of analysis of the SAXS profiles indicated that the short-range adhesive potential between droplets becomes more intense with increasing pressure.
A computer simulation study of the static structure and dynamic properties of liquid C60 using Girifalco's potential
Authors: Alemany M. M. G. , Rey C. , Diéguez O. , Gallego L. J.
Published: July 2002

Using Girifalco's potential to describe the interaction between two C60 molecules, we performed computer simulations to analyze the static structure and dynamic properties of liquid C60, and calculated its diffusion constant and shear viscosity. Our dynamical calculations predict that this yet-unobserved liquid does not support collective phenomena.
Thermodynamic consistency of reaction mechanisms and null cycles
Authors: Schmitz Guy
Published: July 2002

At equilibrium, the relationship K=kf/kr between the equilibrium constant and the rate constants of an elementary reaction is a consequence of the principle of detailed balancing. Out of equilibrium, it remains valid for most elementary reactions. In general, it is not valid for complex reactions out of equilibrium but its validity for the elementary steps of the mechanism may have important consequences for the interpretation of the experimental results. A related criterion of thermodynamic consistency of reaction mechanisms is the absence of null cycles defined as sets of different reactions taking place under given conditions and leading to no net stoichiometric change.
Resonance enhanced multiphoton ionization of the hydrogen halides: Rotational structure and anomalies in Rydberg and ion-pair states of HCl and HBr
Authors: Kvaran Ágúst , Wang Huasheng , Logadóttir Áshildur
Published: July 2002

(2+1) resonance enhanced multiphoton ionization spectra have been recorded, simulated, and used to derive energies of rovibrational levels in the F(1Δ), E(1Σ+), and V(1Σ+) states for HCl (H35Cl and H37Cl) and HBr (H79Br and H81Br). Spectroscopic parameters derived for the F states compare nicely with those derived by others using conventional analysis methods. Clear evidence for near resonance interactions between the F and the V states is seen for the first time, both in HCl and HBr. Shape of curves for rotational level energy spacings versus rotational quantum numbers are found to depend characteristically on the nature of off-resonance interactions observed between the E and the V states. Model calculations for state interactions, based on perturbation theory, are performed for HCl. These prove to be useful to interpret observed perturbations, both qualitatively and quantitatively. Interaction strengths are evaluated for F to V and E to V state interactions. Variations observed in the intensity ratios of O and S line series to Q line series in vibrational bands of the V state for HCl are discussed and mechanisms of two-photon excitation processes are proposed.
Photoinduced localized atomic reaction (LAR) of 1,2- and 1,4-dichlorobenzene with Si(111) 7×7
Authors: Lu P. H. , Polanyi J. C. , Rogers D.
Published: July 2002

We report a comparative study of the photoinduced localized atomic reaction, LAR, of 1,2- and 1,4-dichlorobenzene (1,2- and 1,4-diClPh) on Si(111) 7×7, studied by scanning tunneling microscopy (STM). We observe different chlorine nearest-neighbor separations for the Cl–Si photoformed chemical “imprints” for the two adsorbates; the most probable separation of neighboring Cl’s in the case of 1,2-dichlorobenzene being 8±3 Å, whereas that for 1,4-dichlorobenzene is 14±3 Å. These differing separations can be understood if the two C–Cl bonds in the respective adsorbates extend roughly linearly to chlorinate a nearby Si dangling-bond; for 1,2 diClPh the two C–Cl bonds are at 60° to one another leading to the chlorination of closely neighboring Si, whereas for 1,4 diClPh the two C–Cl bonds are at ∼180° leading to chlorination of sites twice as far apart, located to either side of the 1,4 diClPh adsorbate.
Spectroscopy and relaxation kinetics of the perturbed CO(b 3Σ+,v′=0,1,2) and CO(a′ 3Σ+, v′=31–36, 40, and 41) levels and reinterpretation of CO(a′ 3Σ+, v′=34 and 35) formation in the Kr(5s′[1/2])+CO reaction
Authors: Zikratov G. , Setser D. W. , Sadeghi N.
Published: July 2002

The spectroscopic and kinetic consequences associated with the strong homogeneous interactions between (i) CO(b 3Σ+,v′=0) and CO(a′ 3Σ+, v′=30, 31, and 32), (ii) CO(b 3Σ+,v′=1) and CO(a′ 3Σ+, v′=34, 35, and 36), (iii) CO(b 3Σ+,v′=2) and CO(a′ 3Σ+, v′=40 and 41) levels are evaluated. Mixing of b 3Σ+ character into the a′ 3Σ+ levels reduces the radiative lifetimes of the latter, because the lifetimes of pure b 3Σ+ and a′ 3Σ+ states are approximately 50 ns and 3 μs, respectively. The strength of the interaction changes with rotational level, and the rotational line intensities and the radiative branching to different v″ levels of the CO(a′ 3Σ+–a 3Π) transition are strongly affected. Comparison of high resolution CO(a′, v′=31, 34, and 35) experimental spectra with calculated spectra shows a marked underestimation of the rotational temperature of these CO(a′,v′) levels unless the mixing is explicitly recognized. With benefit of this knowledge, some results from the Kr(5s′[1/2]0)+CO excitation-transfer reaction need to be reinterpreted. Emission spectra for 300 K rotational distributions from CO(b,v′=0, 1, and 2) and CO(a′,v′=30, 31, 34, and 35) were used to obtain vibrational band intensities for comparison with model calculations. Analysis of the pressure and time dependence of the laser-induced fluorescence data permit the electronic relaxation mechanism of the CO(b,v′=0, 1, 2) and CO(a′,v′=31 and 35) levels in He buffer gas to be discussed. The experimental radiative lifetimes of CO(b,v′=0,1,2) were measured as 60±6, 63±4, and 58±4 ns, respectively. The role of the homogeneously perturbed levels in the collisional relaxation mechanism is discussed.
Spontaneous motion of droplets during the demixing transition in binary fluids
Authors: Kumaran V.
Published: July 2002

The convective interaction between a pair of droplets coarsening during the demixing transition of a binary fluid is examined. The starting point is the model H equation for binary fluids, and the droplet sizes are considered to be large enough that thermal fluctuations are neglected. Droplet motion is induced by the convective coupling in the concentration equation, where there is a flux of concentration due to the fluid velocity, and a reciprocal effect in the momentum equation. The effect of the convective force density is separated into two parts—one due to the sharp concentration gradients at the droplet interface, and the other due to the variation in the matrix. It is shown that the dominant contribution to the fluid velocity field is due to the sharp concentration variation at the interface, and this is proportional to the square of the droplet flux at the surface. The surface flux is determined by solving the diffusion equation in the matrix between the droplets, and matching the solution to that in the interfacial region. The analysis indicates that there is an attractive interaction if the two droplets have radii larger or smaller than the critical radius, while the interaction is repulsive if the radius of one droplet is larger and the other smaller than the critical radius. The magnitude of the induced droplet velocity is estimated.
On the covariance of Dirac’s modified canonical commutation relations for fermions interacting with gravity and bosons
Authors: Belinfante Frederik J.
Published: July 2002

For fermions in curved space-time, where iℏψ° is no longer the canonical conjugate of ψ, canonical quantization according to Dirac requires use of ‘‘modified’’ graded commutation relations, in which the canonical conjugates to the tetrad field components are no longer commutative with each other or with the fermion field. As shown earlier by the DeWitts, these modified commutation relations may be understood as a canonical quantization of new field variables that can conventionally be interpreted in terms of creation and annihilation operators. Because of the horrible transformation properties of these new variables, covariance of this quantization of fermion fields and of the gravitational field with which the fermions interact is best proved in terms of the old variables. Like canonical quantization, also this modified quantization requires alteration of the theory destroying general invariance of the Lagrangian. As discussed in a preceding paper, covariance of this modified quantization under a group of finite transformations will follow, under the conditions that (1) it can be proved for a generator T1, linear in infinitesimal descriptors of the transformations, that by these modified commutation relations it will generate the substantial variations of the canonical field variables, while (2) the second-order generator T12, constructed from T1 as previously discussed, will be conserved and invariant. It is here proved that these two conditions are satisfied, provided that the transformations leave the Lagrangian function L invariant. This restriction limits the proof of covariance of quantization to affine transformations only. It is discussed why this yet leaves the possibility of general covariance of the ‘‘physical part’’ of this quantization procedure.
Jet extension of a classical particle field: Principles of covariance and minimal coupling
Authors: Grassini Renato
Published: July 2002

Jet theory of a classical particle field is developed through a systematic transition from a local to a global formulation. Emphasis is laid on the role of gauge covariance and minimal coupling principles in producing a global geometrical framework which faithfully generalizes the one of gravitation theories.
A gravitational lens produces an odd number of images
Authors: McKenzie Ross H.
Published: July 2002

Rigorous results are given to the effect that a transparent gravitational lens produces an odd number of images. Suppose that p is an event and T the history of a light source in a globally hyperbolic space-time (M,g). Uhlenbeck’s Morse theory of null geodesics is used to show under quite general conditions that if there are at most a finite number n of future-directed null geodesics from T to p, then M is contractible to a point. Moreover, n is odd and 1/2 (n−1) of the images of the source seen by an observer at p have the opposite orientation to the source. An analogous result is noted for Riemannian manifolds with positive definite metric.
Collision-free gases in static space-times
Authors: Maharaj S. D. , Maartens R.
Published: July 2002

Collision-free gases in static space-times are analyzed by developing previous work in static spherically symmetric space-times and extending the analysis to include the cases of planar and hyperbolic symmetry. By assuming that the distribution function of the gas inherits the space-time symmetries, distribution solutions to the Einstein–Liouville equations, which are without expansion, rotation, shear, and heat flow, but which have an anisotropic stress are found. The conditions for the gas to behave like a perfect fluid are considered and the relation between equations of state and the distribution function are investigated. In particular, distribution functions that generate the γ-law equation of state are found. The solutions are extended to find invariant Einstein–Maxwell–Liouville solutions for a charged gas, subject to a consistency condition on the invariant electromagnetic potential. Finally, the general solution of Liouville’s equation in the static space-times is obtained and a particular nonstatic solution is considered, which can be shown to lead to a self-gravitating gas with expansion, shear, and heat flow.
The geometrical interpretation of the Faddeev–Popov determinant in Polyakov’s theory of random surfaces
Authors: Jaskolski Z.
Published: July 2002

In the context of Polyakov’s theory the geometrical approach, in which the functional measure is defined as a formal volume ∞-form, is developed. The special version of the Fubini theorem on manifolds is derived, which provides the geometrical interpretation of the Faddeev–Popov method. The conformal gauge is studied within this framework. As a result, the explicit form of functional measure in effective Liouville quantum field theory is obtained.