bluuebunny/crossref_metadata_2025 huggingface dataset:
Cluster catalyzed chemisorption of H2 on Si(111)(1×1): Effects of collision speed and cluster geometry
Authors: Beauregard John N. , Mayne Howard R.
Published: July 2002

We have carried out classical dynamical simulations of collisions of Ar12H2 clusters with a rigid Si(111)(1×1) solid surface for a variety of collision speeds and two different cluster geometries. At low cluster temperatures, the most stable cluster geometry is that with the H2 on the outside of the cluster. It is found that dissociative chemisorption of the hydrogen can occur with this cluster geometry at H2 collision energies as low as 0.05 eV; this is markedly lower than the 0.4 eV threshold for ‘‘bare’’ hydrogen on this surface. The reactivity increases with collision energy until a steric limit of roughly 33% reaction is attained. This corresponds to that fraction of collisions in which the H2 can be caged by the Ar cluster on the surface. The other isomer, with the H2 inside the Ar cluster, is less reactive at low energies, since the H2 does not have free access to the surface. However, the reactivity begins to rise steeply away from threshold as the H2 becomes sufficiently energetic to rearrange the Ar ‘‘solvent’’ shell. Studies of the details of the dynamics reveal that the kinetic energy of the Ar cluster can be ‘‘pooled’’ to allow access to relatively high energy channels, such as chemisorption, dissociative trapping, and even collision-induced dissociation.
Adiabatic ionization energy and electron affinity of CH2Br
Authors: Ma Z.-X. , Liao C.-L. , Ng C. Y. , Ma Ngai Ling , Li Wai-Kee
Published: July 2002

The photoionization efficiency spectrum of supersonically cooled CH2Br has been measured near its ionization threshold. The adiabatic ionization energy (IE) of CH2Br is determined to be 8.61±0.01 eV, in excellent agreement with the value obtained previously using the He i photoelectron spectroscopic method. We have also performed Gaussian-2 (G2) calculations on CH2Br+, CH2Br, and CH2Br− which yield values of 8.47 and 0.97 eV for the IE and electron affinity of CH2Br, respectively. The G2 electron affinity is in accord with the literature value of 1.0±0.3 eV calculated from the acidity of CH3Br.
Vibration-internal rotation coupling in trifluoronitromethane
Authors: Tolles W. M.
Published: July 2002

The barrier to internal rotation for CF3NO2 has previously been determined in the early 1960s by two independent methods (1) electron diffraction, and (2) rotational microwave spectroscopy. These two methods have indicated the barrier to rotation to be 3 kcal/mol and 74 cal/mol, respectively. This discrepancy has not been resolved in 25 years. In an attempt to explain this discrepancy, a model has been developed to incorporate the effects of coupling between the internal rotation motion and the vibration of the NO2 group. This coupling appears to be present based on symmetry considerations and based upon geometries obtained from electronic calculations of the molecule. Both electron diffraction radial distribution curves and rotational microwave spectra are simulated and compared with experimental data. A surprising result indicates that the coupling between internal rotation and the vibrational motions may be strong, but minimally visible in the rotational spectrum. Such coupling can introduce interatomic correlation which affects the appearance of diffraction experiments, yet is not apparent in microwave spectra. This model is consistent with the behavior observed by the two methods, and is offered as the explanation for the otherwise contradictory interpretations.
Determination of consecutive bond energies by photoionization of SbHn (n=1–3)
Authors: Ruscic B. , Berkowitz J.
Published: July 2002

A photoionization mass spectrometric study of SbH3 is presented. The adiabatic ionization potential (IP) of SbH3 is ≤9.40±0.02 eV. The lowest energy fragment ion, SbH+ (+H2), has an appearance potential (0 K) of 9.730±0.008 eV, while SbH2+ has an AP of 11.66±0.02 eV. The transient species SbH2 and SbH are generated in situ by reacting F atoms with SbH3. The IP of SbH2, forming SbH2+ (X 1A1), is 8.731±0.012 eV. The IP of SbH (X 3Σ−,0+) to form SbH+ (X 2Π1/2) is probably 8.753±0.009 eV, but certainly <8.79 eV. Autoionizing structure in the photoion yield curve of SbH+ (SbH) is interpreted as Rydberg series converging to SbH+ (a 4Σ−), which appears to be split into 1/2 and 3/2 components, with IP’s of 10.843±0.011 eV and 10.866±0.011 eV. The difference in IP’s (Sb–SbH, SbH–SbH2) appears to conform to the extended Goddard–Harding model, when adjusted for spin–orbit splittings. The derived heats of formation are ΔHf00(SbH)=59.1±0.3 kcal/mol and ΔHf00(SbH2)=52.5±0.6 kcal/mol. These values lead to D0(SbH)=56.4±1.0, D0(HSb–H)=58.3±0.6, D0(H2Sb–H)=67.5±0.5 (in kcal/mol). The differences in successive bond energies, 1.9±1.2 and 9.2±0.8 kcal/mol, depart significantly from the constant value (4.44 kcal/mol) predicted by the Goddard–Harding model. A rationalization is presented, that incorporates relativistic effects. This relativistic picture implies that for BiHn, D0(BiH)≳D0(HBi–H), a conclusion for which some experimental evidence exists. However, relativistic ab initio calculations, which agree rather well in their calculated differences in successive bond energies for SbHn, do not predict this reversal in BiHn.
Photodissociation dynamics of Ar+3
Authors: Ikegami T. , Kondow T. , Iwata S.
Published: July 2002

The nonadiabatic trajectory calculations were performed for the photodissociation process of Ar3+. Two methods—hemiquantal dynamics and Tully’s surface-hopping method—were applied and the results were compared. The calculated velocities of the photofragments had slow and fast bimodal distributions, as were experimentally observed. The ratio of the slow Ar+ fragment to the fast one decreased with the excitation wavelength, also in good agreement with the experimental results. It was shown that the slow component of Ar+ was produced only through the nonadiabatic transition during the photoissociation process, and that the nonadiabatic transition was dependent on the excitation energy. In addition, the vibrational motion, especially the bending motion, was shown to play an important role in the nonadiabatic process.
Viscosity renormalization in colloidal suspensions
Authors: Allain C. , Cloitre M. , Lacoste B. , Marsone I.
Published: July 2002

We report on systematic measurements of the viscosity of concentrated colloidal silica suspensions as a function of the volume fraction of particles and of the ionic strength. The experimental results are rescaled on a universal curve using a direct renormalization procedure which is described; the scaling ratios are interpreted in terms of an effective volume fraction which depends on the electrostatic repulsion between the particles. The master curve is compared with models existing for the viscosity of hard spheres suspensions.
Monte Carlo simulation of acetonitrile clusters [CH3CN]N, N=2–256: Melting transitions and even/odd character of small clusters (N=2–9), heat capacities, density profiles, fractal dimension, intracluster dimerization, and dipole orientation
Authors: Wright D. , El-Shall M. Samy
Published: July 2002

The thermodynamic and structural properties of acetonitrile clusters [CH3CN]N, N=2–15, 20, 30, 60, 128, and 256 have been investigated using Monte Carlo simulation. Interactions in the small clusters (N≤9) are dominated by antiparallel pairing of the molecular dipoles. The simulations reveal rigid ↔ fluid (melting) transitions with a remarkable even–odd alternation in the transition temperatures for the N=2–9 clusters. The higher melting temperatures of the even-N clusters arise as consequences of the antiparallel paired dipoles which provide favorable electrostatic interactions. Even–odd alternation has also been observed in the configurational energies and heat capacities and the percentage of molecules possessing an antiparallel nearest neighbor. These observations are consistent with the fact that Coulomb potential terms dominate the interaction energies in clusters with N<12. The average density in clusters with N≳60 is fairly well represented by the bulk liquid density. Order parameters characterizing dipole orientation indicate that the molecular dipoles tend to lie flat on the cluster surface for N≥30. Significant dimerization within the clusters suggests evaporation of molecules via dimers and an enhancement of evaporative loss over condensation and this may explain the slower nucleation rates observed for acetonitrile compared to the predictions of the classical nucleation theory.
Three-photon resonance-enhanced ionization spectroscopy of CO(A 1Π←X 1Σ+). II. Line shift/profile and the ac Stark effect
Authors: Xu Su , Sha Guohe , He Jinbao , Zhang Cunhao
Published: July 2002

The influence of the ac Stark effect on the line shift and line shape is studied in the three-photon resonance enhanced ionization spectroscopy of COA 1Π(v′)←X 1Σ(v″=0), where v′=1, 2, or 3. Experimental results show that the line shift varies from one vibrational band to another and differs for various lines in the same band, although it is to a lesser degree. This is given an adequate theoretical elucidation. A detailed calculation, based on the ac stark effect with evaluation of the dipole matrix element, gives −0.66, −0.31, and 0.87 cm−1 for P11 of the (1−0), (2−0), and (3−0) band, respectively, where the negative values stand for red shift while the positive one for blue shift. These results are very close to the experimental observations, viz., −0.48, −0.13, and 0.77 for P11 of the (1−0), (2−0), and (3−0) bands, respectively. Furthermore, the line shape is found to be asymmetrically broadened. For the blue-shift line, the red side is much steeper than the blue side, whereas for the red-shift line, the reverse situation appears. This is again explained as due to the ac Stark effect coupled with the spatial inhomogeneity of the laser field. According to this model, a numerical calculation for the line shape is provided which gives fairly good agreement with the experimental observation.
Properties of size and composition selected gas phase alkali fulleride clusters
Authors: Weis Patrick , Beck Rainer D. , Bräuchle Götz , Kappes Manfred M.
Published: July 2002

Neutral and positively charged clusters Mx(C60)z, where M=K/Rb and z=1–3 were prepared in pulsed molecular beams by laser desorption (266 nm) from preformed alkali metal fulleride thin films. Experimental results encompassed mass spectral characterization of beam abundance and in selected cases, determination of rough ionization potential brackets and elucidation of fragmentation pathways upon (i) unimolecular dissociation in gas phase and (ii) surface collision induced dissociation from Si(111). Results are contrasted with electrostatic model calculations as well as ab initio self-consistent field computations.
The thionitroxyl free radical (H2NS) and its ionic counterparts (H2NS+ and H2NS−): A theoretical and experimental study
Authors: Nguyen Minh Tho , Vanquickenborne Luc G. , Flammang Robert
Published: July 2002

An experimental and theoretical characterization has been carried out for the thionitroxyl radical (H2NS) and its ionic counterparts (H2NS)+ and (H2NS)−. Experimentally, (H2NS)+ and (H2NS) have been detected by a combination of collisional activation and neutralization-reionization mass spectrometry. Thiourea has been used as a precursor. Theoretically, the neutral and ionic potential energy surfaces have been explored using (PU)MP4/6-311++G(3df,2p) energies based on (U)MP2/6-311++(d,p) geometries. Predictions for some spectroscopic and thermochemical properties have been made on the basis of molecular orbital calculations and empirical corrections. The rearrangements leading to the (HNSH) and (NSH2) isomeric species and the loss of hydrogen or other fragmentations have also been examined. The (HNSH) and (NSH2) species are less stable than their respective thionitroxyl isomers in all states considered. For the (H2NS)+ cation, an H loss constitutes the lowest energy fragmentation followed by an H2 elimination giving NS+ and a cleavage of the N–S bond. The thermochemical data are predicted as follows: ΔH0f,0 (H2NS)=31.2±3 kcal/mol, IEa(H2NS)=88.±0.2 eV, EAa(H2NS)=1.4±0.2 eV, PA(H2NS−)=357.5±3 kcal/mol as compared with the experimental value of 356.8±3 kcal/mol, ΔH0f,0(HNS)=52.6±3 kcal/mol and ΔH0f,0(H2NSH)=13.4±3 kcal/mol.
The structure and dynamics of the CS2 molecular ion
Authors: Sohlberg Karl , Chen Yibai
Published: July 2002

Several common elementary methods of computing molecular properties, prerequisite to molecular dynamics studies, are tested for their validity for CS+2. Judged sufficiently accurate, these methods are then used to aid in investigating the collisionally activated dissociation of CS+2 upon impact with xenon. Rice–Ramsperger–Kassel–Marcus (RRKM) unimolecular decay rate calculations are presented and compared to experimental studies employing collisional activation. RRKM theory is shown to reproduce the experimental results for collision energies near threshold. When corrected for vibrational anharmonicity, the RRKM calculation shows agreement with the experimental results over a slightly wider range of energies. A discussion is given on the applicability of the ‘‘adiabatic criteria’’ to describing electronic excitations in the collisional activation process. This model is shown to provide a better description of the activation process than direct vertical excitation.
Chemical reactions of silicon clusters
Authors: Ramakrishna Mushti V. , Pan Jun
Published: July 2002

Smalley and co-workers discovered that chemisorption reactivities of silicon clusters vary over three orders of magnitude as a function of cluster size. In particular, they found that Si33, Si39, and Si45 clusters are least reactive towards various reagents compared to their immediate neighbors in size. We explain these observations based on our stuffed fullerene model. This structural model consists of bulk-like core of five atoms surrounded by fullerene-like surface. Reconstruction of the ideal fullerene geometry gives rise to fourfold coordinated crown atoms and π-bonded dimer pairs. This model yields unique structures for Si33, Si39, and Si45 clusters without any dangling bonds and thus explains their lowest reactivity towards chemisorption of closed shell reagents. This model is also consistent with the experimental finding of Jarrold and Constant that silicon clusters undergo a transition from prolate to spherical shapes at Si27. We justify our model based on an in depth analysis of the differences between carbon and silicon chemistry and bonding characteristics. Using our model, we further explain why dissociative chemisorption occurs on bulk surfaces while molecular chemisorption occurs on cluster surfaces. We also explain reagent specific chemisorption reactivities observed experimentally based on the electronic structures of the reagents. Finally, experiments on SixXy (X = B, Al, Ga, P, As, AlP, GaAs) are suggested as a means of verifying the proposed model. We predict that Six(AlP)y and Six(GaAs)y (x=25,31,37;y=4) clusters will be highly inert and it may be possible to prepare macroscopic samples of these alloy clusters through high temperature reactions.
Vibrational energy transfer between isotopes of CO and isotopes of CO2 in the gas phase and in liquid Kr solution
Authors: Wilson G. J. , Turnidge M. L. , Reid J. P. , Simpson C. J. S. M.
Published: July 2002

Rate constants are presented for (VV) energy transfer between CO(ν=1) and CO2(0001) in the gas phase down to 115 K and in liquid Kr solution at 118 and 130 K. Four isotopically substituted systems were investigated for which the energy mismatches varied between 104 and 306 cm−1. The gas and liquid phase data show several systematic effects with changing energy mismatch. In particular it was found that the ratio of the liquid and gas phase rate constants at the same temperature, kL/kG, increased with decreasing energy mismatch. This is not predicted by current theories of liquid phase energy transfer and is in contrast to previous work using liquid Kr as the solvent.
Kinetic study of the 308 and 345 nm emissions of the molecule XeCl
Authors: Rives P. , Teyssier J. L. , Galy J. , Birot A. , Brunet H. , Asselman H.
Published: July 2002

The light pulses of two emissions of the molecule XeCl, centered at 308 and 345 nm, both have decay time constants that behave in a very similar way with respect to variations in the partial pressures of xenon and HCl; this is explained by collisional coupling between the states B and C of XeCl which are at the origin of the two emissions. The identification of the different collision processes and the determination of the reaction rate constants particularly underline the occurrence of a three-body collision process between XeCl (B,C), Xe, and HCl; the rate constant is kM=(5.85±2.70)×10−5 Torr−2 ns−1. Measurements were also made of the coupling constants B→C and C→B which were found to be kBC=(2.56±1.05)×10−4 Torr−1 ns−1 and kCB=(1.45±1.05)×10−4 Torr−1 ns−1. The deduced energy gap between the two states (EB−EC=119 cm−1) is in very good agreement with the values in the literature.
Three-dimensional Cartesian finite element method for the time dependent Schrödinger equation of molecules in laser fields
Authors: Yu Hengtai , Bandrauk André D.
Published: July 2002

A finite element (FE) method in three-dimensional Cartesian coordinates is described to solve the time dependent Schrödinger equation for H+2, H2, and H+3 in the presence of time dependent electromagnetic fields. The ionization rates and harmonic generation spectra have been calculated for these molecules for field directions parallel or perpendicular to the molecular axis. Nonlinear optical susceptibilities of H+2 have been also obtained for different laser field directions. The time dependent Hartree–Fock results are compared to frozen core calculations for H2. Comparisons of present FE numerical results with previously published calculations show the FE method reproduces perturbative results and can also treat nonperturbatively the effect of intense short laser pulses as the method includes both bound and continuum electronic states.
Electric field effect observed on the infrared spectra of the N2O molecule adsorbed in NaA zeolite
Authors: Bras N. , Cohen de Lara E.
Published: July 2002

The spectrum of the N2O molecule adsorbed in the cavity of NaA zeolite presents two main components for each of the stretching vibrational modes. It is assumed that they correspond to molecules parallel and antiparallel to the electric field of the inner surface of the zeolite cavity. In order to verify this assumption, the frequency shifts and the intensities of these components have been calculated for the two orientations of the molecule with respect to the field, by applying the model of Bishop for the vibrational Stark effect. These calculations require the knowledge of molecular quantities such as derivatives of permanent electric moments and polarizability.
On the use of liquid helium cluster beams for the preparation and spectroscopy of the triplet states of alkali dimers and other weakly bound complexes
Authors: Stienkemeier F. , Ernst W. E. , Higgins J. , Scoles G.
Published: July 2002

Sodium dimers in their lowest triplet state are prepared by pick-up of sodium atoms by a beam of large (n≂104) liquid helium clusters when these cross a scattering box containing sodium vapor. Using laser induced fluorescence detection the vibrationally resolved laser excitation spectrum of the 1 3Σ+g←1 3Σ+u transitions of Na2 is obtained. The spectral shifts with respect to the corresponding gas phase values are surprisingly small. These data show the feasibility of using helium clusters as an almost ideal matrix for the synthesis and spectroscopic analysis of weakly bound complexes.
Photoneutral–photoneutral coincidence study of Ar+3
Authors: Jones A. B. , Buxey A. L. M. , Jukes P. R. , Smith J. A. , Stace A. J.
Published: July 2002

A coincidence technique has been used to correlate the motion of the two neutral fragments (argon atoms) that result from the photodissociation of Ar+3. An analysis of the results reveals the presence of two separate dissociation channels.
Quantum Monte Carlo for floppy molecules: Vibrational states of C3
Authors: Brown Willard R. , Glauser William A. , Lester William A.
Published: July 2002

Correlation function quantum Monte Carlo, a stochastic method for simultaneously computing ground and excited states, has been used to calculate vibrational states of C3 to ascertain the usefulness of the approach for weakly bound systems. Up to 14 low lying states of C3 were determined for two potential energy surfaces. Furthermore, the vibrational wave functions have been examined to ascertain the linearity of C3.
On the molecular recognition and associations between electrically conducting polyaniline and solvents
Authors: Ikkala Olli T. , Pietilä Lars-Olof , Ahjopalo Lisbeth , Österholm Heidi , Passiniemi Pentti J.
Published: July 2002

The specific interactions between sulphonic acid protonated polyaniline (PANI) and solvents are here studied both by the semiempirical AM1 method and experimentally. Phenolic solvents are shown to have a relatively large interaction with the sulphonate anions of the counterions and with the amines in PANI. In addition, a properly functionalized counterion may form cyclic associations provided that there is a steric match between the molecules concerned. This concept is called molecular recognition and it is a novel concept in the context of PANI. For example, the carbonyl group in (±)-10-camphor sulphonic acid (CSA) can form a hydrogen bond to the hydroxyl group of m-cresol, whereby the phenyl ring becomes coplanar with one of the PANI rings thus enabling enhanced van der Waals interaction. This additional specific interaction agrees with our observed increased solubility with CSA doped PANI in m-cresol, compared to its solubility in dimethyl sulphoxide or chloroform, or to tosylene sulphonic acid doped PANI in m-cresol. The above cyclic associations are suggested for dilute solutions and for the amorphous domains of solid films. In the latter case, they provide a potential mechanism to yield planar conformation in the crystalline domains: during the evaporation of m-cresol, stacking to crystal structure may twist the rings due to the planar m-cresol molecules on top of PANI rings. This is in agreement with the observed high conductivity. The present results indicate that computational methods combined with the concept of molecular recognition may open new possibilities to tailor the rigidities and solubilities of macromolecules.
Freezing transition temperature of random copolymers
Authors: Moskalenko A. , Dawson K. A.
Published: July 2002

We use the mean field replica theory to analyze the dependence of the freezing transition temperature on the composition and coupling constants for two letter random copolymers. Gaussian disorder is considered.
Boron nuclear quadrupole couplings and localized electronic wave functions in the boron clusters o-carborane and o-silaborane
Authors: Olliges Josef , Lötz Albert , Kilian Dieter , Voitländer Jürgen , Wesemann Lars
Published: July 2002

The 11B nuclear quadrupole couplings in ‘‘o-silaborane’’ [1,2-dimethyl-1,2- disila-closo- dodeca- borane(12), B10Si2(CH3)2H10] were measured at 77 K with a nuclear quadrupole double resonance spectrometer with rapidly switched magnet. Three 11B transitions were found which were assigned by comparison with ab initio calculations of the electric field gradients. The surprising difference in the order of the quadrupole couplings of equivalent borons in o-silaborane and the corresponding carbon compound o-carborane (B10C2H12) is explained by the different valence bond structures of the two boron clusters which were obtained by localization of the electronic wave functions.
Improved Roothaan–Hartree–Fock wave functions for atoms and ions with N≤54
Authors: Koga Toshikatsu , Watanabe Shinya , Kanayama Katsutoshi , Yasuda Ryuji , Thakkar Ajit J.
Published: July 2002

Improved Roothaan–Hartree–Fock wave functions are reported for the ground states of all the neutral atoms from He to Xe, singly charged cations from Li+ to Cs+, and stable singly charged anions from H− to I−. Our neutral atom wave functions are an improvement over those of Clementi and Roetti [At. Data Nucl. Data Tables 14, 177 (1974)], Bunge et al. [Phys. Rev. A 46, 3691 (1992)] and Koga et al. [Phys. Rev. A 47, 4510 (1993)]. The ion wave functions are an improvement over those of Clementi and Roetti, and Koga et al. [J. Phys. B 26, 2529 (1993)]. In all cases, the current wave functions predict energies within 1.3×10−5 hartrees of the numerical Hartree–Fock limit.
Nuclear magnetic resonance studies of domain growth in the late stage of phase separation of a binary liquid mixture
Authors: Cau Franco , Lacelle Serge
Published: July 2002

The domain growth processes occurring during the gravity-dominated regime of the phase separation of a cyclohexane/aniline critical mixture, following a temperature quench in the unstable region below the temperature-composition coexistence curve, were investigated with high-resolution 1H NMR. Diffusive exchange of cyclohexane molecules between a small volume fraction of immiscible droplets and the surrounding fluid with a different cyclohexane concentration leads to a domain-size dependent 1H NMR cyclohexane chemical shift in the continuous phase. In this case, the equations of motion for the transverse magnetization density reduce to a first-order exchange between two immiscible phases with rate coefficients depending on the droplet size and the transport process across the liquid–liquid interface. Application of this model to the data yields a power law for the growing droplets, R(t)=Atφ, where R is a typical domain size, with the growth exponent φ=0.30±0.01 and the amplitude A≂0.9×10−6 ms−φ. These values are in close agreement with previous visualization studies on the same binary mixture [F. Cau and S. Lacelle, Phys. Rev. E 47, 1429 (1993)]. The results also confirm the presence of a new domain growth regime involving Brownian coagulation and sedimentation in the very late stage of the phase separation processes of binary liquid mixtures. A strong linear composition dependence of the 1H NMR cyclohexane chemical shift in one-phase cyclohexane/aniline mixtures was interpreted on the basis of solvent shift effects induced by intermolecular interactions and susceptibility effects. In each phase of the two-phase equilibrium mixture, inhomogeneous line broadening, arising from the magnetic susceptibility discontinuity at the interface between phases, is also investigated.
The influence of shear on the ordering temperature of a triblock copolymer melt
Authors: Nakatani Alan I. , Morrison Faith A. , Douglas Jack F. , Mays Jimmy W. , Jackson Catheryn L. , Muthukumar M. , Han Charles C.
Published: July 2002

The effect of shear on the ordering temperature of a triblock copolymer melt of polystyrene-polybutadiene-polystyrene (SBS) is examined by in situ small angle neutron scattering (SANS). Results obtained by SANS are compared to the rheologically determined order–disorder transition temperature, TRODT=115±5 °C. The SANS measurements from a Couette geometry shear cell are then used to construct a ‘‘dynamical phase diagram’’ based on characteristic changes in the scattering with temperature and shear rate, γ̇. A shear rate dependent ordering temperature, Tord(γ̇), is identified as the system is sheared isothermally from the disordered state. The scattering behavior is shown to be highly strain dependent. We compare our findings on the shear rate dependence of the ordering transition in triblock materials with previous observations on diblock copolymer materials and theoretical expectations for the shear rate dependence of the order–disorder transition temperature. A simple scaling argument leads to a good description of the shear rate dependence of Tord(γ̇) in both diblock and triblock copolymer measurements over the range of shear rates examined.
Nanosecond time-resolved degenerate four-wave mixing measurements of small Pd particles: Thermal phase grating analysis
Authors: Tatsu Yoshiro , Yamamura Soichiro , Sakaguchi Toru , Miya Masaru , Fukumi Toshio , Kadono Kohei , Ohta Koji
Published: July 2002

Reflection by the thermal phase grating formed in methanol containing small Pd metal particles has been measured by using a degenerate four-wave mixing optics equipped with a nanosecond Nd:YAG laser and a streak camera. Although the integrated intensity of the reflection depended on the cubic power of the laser intensity, the temporal profile of the reflection in a pulse duration showed the delayed response, the theoretical profile of which has been formulated by considering the formation of thermal phase grating. The theoretical calculation based on heat conduction equation was in good agreement to the experimental results with respect to the shape of the profile, the intensity, and the polarization dependence.
Nonequilibrium interfacial tension measurements on polymer solutions
Authors: Peach Sarah , Franck Carl
Published: July 2002

The observation of critical phenomena in polymer solutions is hindered by the slow approach of such systems to equilibrium. We have addressed this problem with a variation of the sessile drop technique that allows equilibrium interfacial properties to be obtained from systems far from equilibrium. Following a temperature jump, the interfacial region of a drop responds rapidly compared with the bulk phases, and, despite significant flux of polymer and solvent across it, the interface can reveal information about the equilibrium state well before bulk equilibration is complete. Interfacial data from these systems show good agreement with data from equilibrated samples. The kinetics of solvent and polymer transport across the interface following a temperature perturbation are also examined.
Fragmentation process of size-selected aluminum cluster anions in collision with a silicon surface
Authors: Terasaki Akira , Tsukuda Tatsuya , Yasumatsu Hisato , Sugai Toshiki , Kondow Tamotsu
Published: July 2002

Dynamical processes involved in the collision of aluminum cluster anions, Al−N (4≤N≤25), with a silicon surface were investigated. Intact and fragment cluster anions, Al−n (n≤N), were produced upon the collision. The surf02ace-tangent and surface-normal recoil velocity components of these product a0n0ions were determined. The tangential recoil velocities of the fragment cluster anions were considerably slow, ranging from 5% to 30% of the velocity of the incident parent cluster anion, while the normal velocities were conserved relatively well. These results are explained in such a manner that the fragment cluster anion is evaporated from the parent cluster anion while it interacts with the surface and loses its tangential momentum. The dynamics and the energetics derived from these results show further that the fragmentation process involves not only sequential evaporation of aluminum atoms but also simultaneous production of several small fragment clusters. Comparison of the present result with that of the collision-induced dissociation by a rare-gas atom 88lends a further support on this nonsequential fragmentation 1/1mechanism.
Simple one-particle diffusional model to mimic some properties of the glass transition
Authors: Ivanov V. A. , Jung B. , Semenov A. N. , Nyrkova I. A. , Khokhlov A. R.
Published: July 2002

A new simplified model is proposed to mimic some properties of the glass transition. The physical system undergoing glass transition is modeled as Brownian particle diffusing in one- or two-dimensional space with obstacles. In one dimension obstacles are points which cannot be crossed by Brownian particles, in two dimensions obstacles are randomly distributed sections of straight lines which are impenetrable for the diffusing particle. The obstacles have a finite lifetime τ. After time τ the obstacle disappears and reappears in some new random position. In another modification of the model the obstacle barrier can be opened for short time and then closed again. Both cases are studied for one-dimensional diffusion, while in two dimensions only the first modification of the model is considered. The main feature of the model is that the mean lifetime of obstacles τ is connected with the diffusion coefficient of the Brownian particle through the coupling equation Dτ=K, with K being the coupling constant. This idea is borrowed from the theory of reptations in polymer liquids. Both analytical calculations and results of computer simulations are presented. The model is shown to reflect some of the features of glass transition. It was found that a slight change in the model, i.e., opening and closing of a barrier in the same position versus transfer of a barrier to a new position, leads to a drastic change in the diffusion kinetics of the system.
Electronic transition moment and rotational transition probabilities in CH. II. B 2Σ−–X 2Π system
Authors: Luque Jorge , Crosley David R.
Published: July 2002

Vibrational transition probabilities of the B 2Σ−–X 2Π band system of the CH radical, for the bound levels v′=0 and 1, have been measured in an 8 Torr flame using laser-induced fluorescence. These new branching ratio measurements together with Rydberg–Klein–Rees wave functions furnish an empirical electronic transition moment for this system. A quadratic form compares very well with previous ab initio calculations. The model predicts significant variations in lifetime with rotational quantum number. As a consequence, the Einstein emission and absorption coefficients are strongly dependent on rotational quantum number, and are needed for quantitative temperature and nonthermal population distribution determinations.
Coherent population transfer and dark resonances in SO2
Authors: Halfmann T. , Bergmann K.
Published: July 2002

Highly efficient population transfer between the (0,0,0) and the (9,1,0) vibrational levels of the electronic ground state X̃ 1A1 of SO2 is demonstrated. The process relies on stimulated Raman scattering with adiabatic passage induced by two suitably delayed ultraviolet laser pulses with nearly transform-limited bandwidth. A transfer efficiency of 100% is achieved. The associated dark resonance is observed. Properties of the latter are compared for delayed and fully overlapping pulses.
Molecular exchange-correlation Kohn–Sham potential and energy density from ab initio first- and second-order density matrices: Examples for XH (X=Li, B, F)
Authors: Gritsenko Oleg V. , Leeuwen Robert van , Baerends Evert Jan
Published: July 2002

The molecular Kohn–Sham exchange-correlation potential vxc and the energy density εxc have been constructed from ab initio first- and second-order density matrices for the series XH (X=Li, B, F). The way various effects of electronic structure and electron correlation manifest themselves in the shape of vxc and εxc has been analyzed by their decomposition into various components; the potential of the exchange-correlation hole, the kinetic component and (in the case of vxc) the ‘‘response’’ component. The kinetic energy of noninteracting particles Ts, the kinetic part of the exchange-correlation energy Tc, and the energy of the highest occupied molecular orbital εN have been obtained with reasonable accuracy and the effect of bond formation on these functionals has been studied.
Picosecond dynamics of cresyl violet H-aggregates adsorbed on SiO2 and SnO2 nanocrystallites
Authors: Liu Di , Kamat Prashant V.
Published: July 2002

The H-aggregates of cresyl violet dye which are formed on the negatively charged SiO2 and SnO2 nanocrystallites, exhibit relatively short-lived excitonic singlet state with a lifetime of about 35 ps. The difference absorption maximum (λmax=470 nm) is blue shifted compared to the corresponding singlet excited state of monomer (λmax=515 nm). Time-resolved transient absorption measurements show that these dye aggregates are capable of injecting electrons from the triplet excited state into SnO2 nanocrystallites. The rate constant for heterogeneous electron transfer as measured from the formation of cation radical and electron trapping in SnO2 nanocrystallites was 2.0×108 s−1.
Absolute partial and total cross-section functions for the electron impact ionization of C60 and C70
Authors: Matt S. , Dünser B. , Lezius M. , Deutsch H. , Becker K. , Stamatovic A. , Scheier P. , Märk T. D.
Published: July 2002

Electron impact ionization of C60 and C70 was studied using a molecular/electron beam ion source in combination with a two sector field mass spectrometer operated in the ion beam deflection mode. Relative partial ionization cross sections for the production of singly and multiply charged parent ions (up to charge state z=4) and fragment ions (down to C+44 in the case of C60 and down to C2+50 in the case of C70) were determined from threshold up to 1000 eV electron energy. Absolute partial and total ionization cross sections are obtained using a novel approach for the absolute calibration involving an intercomparison of the cation with the anion yield. The results obtained reveal not only an anomalous large parent ion cross section as compared to other ionization channels [e.g., σ(C+60/C60) is more than a factor of 30 larger than σ(C+58/C60)] but also anomalies for the production of multiply charged parent and fragment ions. For instance, the maximum cross section for the formation of C2+60 amounts to 30% of the maximum C+60 cross section and that of C2+70 to about 50% of C+70. Moreover, for all fragment ions, the formation of the doubly charged fragment ions has a larger cross section than that of the respective singly charged fragment ion. These peculiar features of the kinetics of electron impact ionization of C60 and C70 are related to the specific electronic and geometric structures of these fullerenes. The present absolute cross-section data for the summed up partial cross sections are in good agreement with a recent semiclassical calculation from our laboratory.
On the inclusion of the Breit interaction term in the closed-shell generator coordinate Dirac–Fock formalism
Authors: Jorge F. E. , da Silva A. B. F.
Published: July 2002

The generator coordinate Dirac–Fock formalism with the inclusion of the frequency-independent Breit interaction term is presented. The integration of the Dirac–Fock equations is performed through an integral discretization technique in order to preserve the continuous character of the original generator coordinate formalism. The inclusion of the Breit interaction term in the generator coordinate Dirac–Fock formalism is assessed through Dirac–Fock–Breit calculations for the relativistic closed-shell atoms with d and f orbitals, i.e., from zinc (Z=30) up to nobelium (Z=102), by using a universal Gaussian basis set generated previously by employing the generator coordinate Dirac–Fock method. The Dirac–Fock–Breit energy results presented in this work are the best, so far, obtained for the relativistic closed-shell atoms from Zn (Z=30) through No (Z=102) by using Gaussian-type functions, and show the efficacy of the generator coordinate Dirac–Fock method in performing accurate Dirac–Fock–Breit calculations.
Extensions of the quasi-Gaussian entropy theory
Authors: Amadei A. , Apol M. E. F. , Berendsen H. J. C.
Published: July 2002

In this paper we present the quasi-Gaussian entropy theory in a comprehensive and consistent way, introducing a new derivation of the theory very suited for applications to molecular systems, and addressing its use in the case of multi-phase systems. A general derivation of the possible confinement of the system within a part of phase space is given, and for water it is shown that for this a hard sphere excluded volume model can be used. To obtain the temperature dependence of the pressure, a new differential equation is derived, and besides the previously introduced Gaussian and Gamma states, in this paper we also describe a new statistical state, the Inverse Gaussian state. We discuss the properties of these different statistical states and for water compare their thermodynamics with experimental data, finding that both the Gamma and Inverse Gaussian states are excellent descriptions.
Wave selection and lateral instabilities in mixed autocatalytic systems
Authors: Milton R. A. , Scott S. K.
Published: July 2002

Chemical systems with mixed quadratic and cubic autocatalytic feedback support simple reaction-diffusion fronts under suitable circumstances. If reactant and autocatalyst species have equal diffusivities, then from typical initial seedings, the system selects between two different types of fronts depending on the relative strengths of the two channels. In terms of the parameter μ=kq/(kq+kc[A]0) where kq,c are the rate coefficient for the quadratic and cubic channels and [A]0 is the initial reactant concentration, then with μ>1/3, the system selects a “quadratic type” front determined only by the quadratic channel, but for μ<1/3, the system adopts a “cubic type” front in which both channel contribute. For pure cubic autocatalysis (μ=0) these fronts become geometrically unstable if the diffusivity of the reactant exceeds that of the autocatalyst. The present paper examines the selection of wave type and wave velocity and the onset of lateral instabilities of smooth waves for mixed autocatalytic systems for arbitrary reactant and autocatalyst diffusivities.
On the formation and vibronic spectroscopy of α-halobenzyl radicals in a supersonic expansion
Authors: Yao Jianlong , Bernstein Elliot R.
Published: July 2002

α-halobenzyl radicals, such as C6H5CHCl, C6H5CHBr, and C6H5CCl2, along with C6H5CH2, are generated by pyrolysis and photolysis methods. The mechanism of the formation of the α-halobenzyl radicals in a pyrolysis nozzle is analyzed. Secondary reactions play an important role in the pyrolysis process. Spectra of these supersonic jet cooled radicals are detected by laser induced fluorescence and multiphoton ionization techniques in the 21 200–23 500 cm−1 range. Assignment of important vibronic bands of the α-halobenzyl radicals is made by comparing their spectra to that of the benzyl radical and by employing rotational contour and vibrational mode analyses and ab initio calculations. The similarity between the vibronic structure of the benzyl radical and α-halobenzyl radicals suggests a similar vibronic coupling scheme for them. Substitution of halogen atoms on the methylene group of the benzyl radical has only a modest affect on the vibronic transitions studied.
One- and two-pulsed field ionization spectra of NO: High-lying Rydberg states near ionization threshold
Authors: Sato Shin-ichiro , Kimura Katsumi
Published: July 2002

High-lying rotational Rydberg states (p series) of NO near the ionization threshold were prepared by two-color (1+1′) resonant excitation, and then detected by a ZEKE (zero kinetic energy) electron technique as well as by a MATI (mass analyzed threshold ion) technique using one- and two-pulsed field ionization (abbreviated by 1PFI and 2PFI, respectively). The MATI-2PFI spectrum has been interpreted in terms of a single progression due to p-series Rydberg states (n=37–48) converging into the ion core with N+=11, perturbed by the d-series Rydberg states converging into N+=12. From a ZEKE-1PFI spectrum obtained at a high field strength of F=−33 V/cm, it has been found that (1) the band width (ΔE) is narrower than expected from the relation ΔE=C(F)1/2 (C=4∼6) [W. A. Chupka, J. Chem. Phys. 98, 4520 (1993)], and (2) the field ionization threshold was observed at n=82 (principal quantum number) for the p-series Rydberg states at a static dc field of 0.25 V/cm. From a MATI-2PFI spectrum obtained at F1=20 V/cm and F2=−33 V/cm, it has been found that the lifetimes of the p-series Rydberg states (n⩾37) below the field ionization threshold are lengthened at the same static electric field. These experimental results may be interpreted in terms of two factors; one is the lifetime lengthening due to the Stark mixing between the lower and the higher ℓ states, and the other is the rotational coupling of between a low-n high-N+ state and a high-n low-N+ state, lengthening the apparent lifetime of the low-n high-N+ state.
Normal mode analysis of a double-stranded DNA dodecamer d(CGCGAATTCGCG)
Authors: Lin Donghai , Matsumoto Atsushi , Go Nobuhiro
Published: July 2002

The normal mode analysis in dihedral angle space is carried out on a famous double-stranded DNA dodecamer d(CGCGAATTCGCG) to reveal the dynamical characteristics and internal motion of this molecule. We have observed the coupled motion of sugar and base. We have also noted the dependence of internal motion in the dodecamer on location of the nucleotide relative to the helical ends, and the dependence of atomic fluctuations on distance from the helix axis. The similarity of atomic fluctuations between two strands suggests the presence of a twofold pseudosymmetry axis with its origin located at the center of the molecule. Our results show that the atomic fluctuations of duplex DNA are mainly determined by a small number of low-frequency normal modes, the normal modes with frequencies below 30 cm−1 make major contributions to the site-dependence of atomic fluctuations.
Nonadiabatic dynamics for processes involving multiple avoided curve crossings: Double proton transfer and proton-coupled electron transfer reactions
Authors: Fang Jian-Yun , Hammes-Schiffer Sharon
Published: July 2002

The extension of the surface hopping method “molecular dynamics with quantum transitions” (MDQT) to double proton transfer and proton-coupled electron transfer reactions is tested by comparison to fully quantum dynamical calculations for simple model systems. These model systems each include four potential energy surfaces and three or four avoided curve crossings. The agreement between the MDQT and fully quantum dynamical calculations provides validation for the application of MDQT to these biologically important processes.
Accurate relativistic effective potentials for the sixth-row main group elements
Authors: Wildman S. A. , DiLabio G. A. , Christiansen P. A.
Published: July 2002

Errors in predicted bond lengths in effective potential calculations involving heavy main-group elements are shown to be the result of the poor representation of the f-shell space. Test calculations for PbO and the hydrides of thallium, lead, and bismuth demonstrate that with the inclusion of the 5d, 6s, and 6p subshells in the valence space and the proper partitioning of the f-shell valence spinors to form pseudospinors, accurate bond lengths are attainable. The previous reasonable bond lengths from 6s6p potentials appear to be the result of fortuitous error cancellations. New relativistic effective potentials in standard form are provided for Tl, Pb, Bi, At, and Rn in electronic format at www.clarkson.edu/∼pac/reps.html.
The hindering of the inversion motion in the van der Waals aniline-Arn clusters: An adiabatic molecular dynamics simulation for n=1–3
Authors: Parneix Pascal , Bréchignac Philippe
Published: July 2002

An adiabatic quantum molecular dynamics simulation has been performed for the van der Waals clusters aniline-Arn (with n=1–3) to understand the influence of the intramolecular inversion mode on the static properties (rotational constants). The effect of deuteration of the amino group in the chromophore (C6H5NH2 or C6H5ND2) on the rotational constants of the complexes has been evaluated. The results are in good agreement with recent experimental values [W. E. Sinclair and D. W. Pratt, J. Chem. Phys. 105, 7942 (1996)]. In the ground electronic state S0, calculations show that the −NH2 (or −ND2) plane of the aniline molecule tends to be blocked on the same side as the argon atom in the complex. On the other hand, for larger clusters (aniline-Ar2 and aniline-Ar3), it appears that the −NH2 (or −ND2) group is more localized on the side opposite to the argon atoms for the isomers containing at least one argon atom bound in a site near the nitrogen atom.