bluuebunny/crossref_metadata_2025 huggingface dataset:
Measurement of the molecular content of binary nuclei. IV. Use of the nucleation rate surfaces for the n-nonane-n-alcohol series
Authors: Viisanen Y. , Wagner P. E. , Strey R.
Published: July 2002

In a previous paper the molecular content of binary water-n-alcohol nuclei has been determined from nucleation rate measurements. A strong mutual enhancement of water and alcohol in forming the nuclei was observed, although macroscopically the higher alcohols are only partially miscible with water. In this paper we replace water by n-nonane, that is, we examine n-nonane-CiH2i+1OH systems with i=2–6. Using the nucleation pulse technique nucleation rates in the range 105<J/cm−3 s−1<109 are measured. Ranging from pure n-nonane to pure n-alcohol the n-nonane and n-alcohol activities, a1 and a2, respectively, are varied for each system with about eight intermediate compositions at a constant temperature of T=230 K. A rather reluctant conucleation of the n-alcohols with n-nonane is found, the most stubborn being ethanol. However, one observes that with increasing alcohol chain length the nucleation process tends to become more ideal. We present the full experimental nucleation rate surface J(a1,a2) for n-nonane-n-propanol as an example. From the nucleation rate surface for each system the onset activities corresponding to a reference nucleation rate of J0=107 cm−3 s−1 are determined. From the slopes of the nucleation rate surfaces one obtains the individual numbers of molecules in the critical cluster ni*≈∂ ln J/∂ ln ai. As noted previously, determining the molecular content this way does not involve any particular theoretical model, nor does it depend on the structure of the critical cluster. Accordingly, the average composition of the critical clusters is obtained. An alcohol depletion of the nuclei at low alcohol activity fraction is found for all alcohols examined, the degree diminishing with increasing alcohol chain length. Macroscopically all alcohols are miscible with nonane. Similarly, a depletion of n-nonane in the nuclei is observed at low n-nonane activity fractions. The approach towards a macroscopic miscibility gap for the shorter alcohols is reflected in quantitative but no qualitative changes of the composition of the microscopic nuclei.
The largest angle generalization of the rotating bond order potential: Three different atom reactions
Authors: Laganà A. , Ochoa de Aspuru G. , Garcia E.
Published: July 2002

The LAGROBO functional representation of the atom-diatom interaction has been extended to the case of three different atom reactive systems having two open product channels and an atom in an electronically excited state. The analytic formulation of the LAGROBO model is given and its application to the construction of the potential energy surface of the Li+FH and O(1D)+HCl reactions are discussed. Reactive properties calculated on these surfaces using quasiclassical methods are compared with experimental findings.
Efficient recursive implementation of the modified Broyden method and the direct inversion in the iterative subspace method: Acceleration of self-consistent calculations
Authors: Kawata M. , Cortis C. M. , Friesner R. A.
Published: July 2002

An efficient recursive procedure to solve N-dimensional nonlinear equations using the modified Broyden method is described. This procedure is extended to include the direct inversion in the iterative subspace (DIIS) method to further improve the rate of convergence in iterative calculations. In the recursive procedures. the approximate solutions are constructed as linear combinations of n vectors of length N. The calculations are reduced to determine the appropriate coefficients of the linear combinations. The coefficients are evaluated through small matrix operations, the size of which are at most (n+1)×(n+1) except for the generation of a n×n matrix, where n is the iteration number. Storage is required only for the n vectors and the small matrices. The procedures described below can be applied to large systems. To examine the efficiency of the methods, some numerical results are presented in the context of self-consistent calculations of liquid structure using the reference interaction site model (RISM) integral equation and a molecule-site form of the Ornstein–Zernike integral equation. The results indicate that significant acceleration with respect to the Picard iteration method has been achieved by the recursive procedures: The converged solution is obtained in a very small number of iterations and in a fraction of the CPU time. Moreover, the extended method which includes the DIIS approach has further improved the rate of convergence. It also enables solutions to be obtained in an otherwise divergent case.
Rotational analysis and tunnel splittings of the intermolecular vibrations of the phenol–water complex by high resolution UV spectroscopy
Authors: Helm R. M. , Vogel H.-P. , Neusser H. J.
Published: July 2002

Highly resolved (Δν<100 MHz) UV-REMPI (ultraviolet-resonantly enhanced multiphoton ionization) spectra of different vibronic bands in the phenol–water complex are presented. The torsional splitting caused by the hindered rotation of the water moiety in the hydrogen-bonded system is investigated. An autocorrelation procedure reveals torsional subbands, a correlation automated rotational fitting (CARF) of the spectra yields the rotational constants: The analyzed vibrations are classified by the rotational constants of the corresponding vibronic band and the symmetry of the torsional state. The transition to the stretching vibration at 156 cm−1 excess energy is shown to consist of two different torsional transitions similar to the electronic origin. The torsional splitting in the origin band is 0.8491(2) cm−1 and that of the stretching vibration is 0.8915(3) cm−1, demonstrating a very small coupling between the stretching and the torsional motion. We assign the 121 cm−1 band as the negative parity component of the transition to the wagging vibration β2 while the band at 125 cm−1 is tentatively assigned as the positive parity component of the same band. The resulting large torsional splitting of −4.596(3) cm−1 points to a strong coupling to the torsional motion. The band at 95 cm−1 has only one torsional parity component within its rotational envelope. The observed large change of its rotational constants does not fit to the pattern of the other vibrations and the band is tentatively assigned as an overtone of the torsional vibration τ with positive parity.
A quantum mechanical description of vibrational motion in benzene in terms of a completely symmetrized set of complex vibrational coordinates and wave functions
Authors: Rashev S. , Stamova M. , Djambova S.
Published: July 2002

In this work a fully symmetrized quantum mechanical description of vibrational motion in terms of complex vibrational coordinates and complex basis wavefunctions is outlined, designed for studying vibrational level mixing and intramolecular vibrational energy redistribution (IVR) around CH stretch overtone states in benzene. Symmetrized local mode (LM) formalism has been applied to the CH stretch system, while the remaining benzene vibrations (including out-of-plane modes) were treated as normal modes (NM). Using the outlined approach a model calculation of the absorption spectrum of the first overtone state CH (n=2) at ∼6000 cm−1 has been carried out.
Rotationally resolved photoelectron study of O2: Identification of the vibrational progressions for O2+(2 2Πu,2Σu−) at 19.6–21.0 eV
Authors: Hsu C.-W. , Evans M. , Stimson S. , Ng C. Y.
Published: July 2002

On the basis of rotationally resolved pulsed field ionization photoelectron measurements, we have shown that the weak vibrational progressions in the energy region of 19.6–21.0 eV observed in recent high resolution photoelectron spectroscopic studies of O2 are associated with transitions to the 2 2Πu 1/2 (2 2Πu 3/2) and 2Σu− states of O2+, with ionization energies of 19.6268±0.0005 eV (19.6329±0.0005 eV) and 20.3528±0.0005 eV, respectively.
Electron transfer driven by conformational variations
Authors: Pudlak Michal
Published: July 2002

In this paper is given a general formulation of electron transfer (ET) in the system where the conformational transitions are present. The conformation changes of the system were described as a classical telegraphic noise. In the work was assumed that electron transfer reaction can be completely interrupted by the fluctuation of the electronic coupling. A functional-integral approach to the dynamics of a two-state system was used. We have got exact analytical nonperturbative expression for the probability to find electron on donor at time t. We derived two limiting cases for the electron transfer—the nonadiabatic limit and the conformational-controlled adiabatic ET case.
Time correlation functions for mixed quantum-classical systems
Authors: McWhirter J. Liam
Published: July 2002

We consider the time correlation function of observables pertaining to a (quantum subsystem +bath), where the bath is coupled to a reservoir with many degrees of freedom. Integrating over the coordinates of this reservoir and assuming no initial correlations between the (quantum subsystem+bath) and the reservoir, we obtain an expression for the time correlation function that contains an influence functional. We then take the semiclassical and Fokker–Planck limits while modeling the reservoir with an Ohmic continuum of harmonic oscillators coupled bilinearily to the coordinates of the bath. The semiclassical limit is taken using a variant of Pechukas’ stationary phase analysis of the reduced propagator that yields a time correlation function written in terms of connected “classical” paths. These paths are got by solving the concatenation of several short-time interval Pechukas equations; as a result, the determination of these paths is more feasible than the determination of the “classical” path associated with a single long-time interval Pechukas equation. This concatenation includes the dissipative and stochastic forces associated with a classical Brownian particle. We then use decoherence arguments derived from an inspection of the influence functional to eliminate the phase interference structure of the bath. This elimination yields a mixed quantum-classical time correlation function that can be evaluated using nonadiabatic mixed quantum-classical dynamics schemes similar to those proposed recently by Webster and Tully.
Scattering state spectroscopy of the reaction Mg*(3s3p 1P1)+CH4→MgH(v=0,1;N)+CH3
Authors: Wong T. H. , Freel C. , Kleiber P. D. , Sando K. M.
Published: July 2002

We report scattering state spectroscopic studies of the chemical quenching dynamics of Mg*(3p(1P)) by CH4. We have measured the final-state resolved action spectra for the MgH(v=1,N) reactive product channels, following excitation of the Mg*(3p)–CH4 transient bimolecular collision complex. As in earlier work on the ground vibrational state of the product, we have found a strong electronic orbital alignment effect: Reaction to the vibrationally excited product follows only on the attractive excited potential-energy surfaces in “Π-like” symmetry. For both MgH(v=0 and 1) product channels we have found that the rotational quantum state distribution is independent of laser excitation wavelength, indicating that the rotational energy partitioning is determined by exit channel dynamics. However, our results show that the product vibrational energy disposal is a function of excitation laser wavelength, suggesting that the vibrational energy partitioning is correlated with the collisional impact parameter. We have also carried out a careful search for the MgCH3 reactive product in this system, finding no evidence for any observable branching to this product. We discuss the implications of these results for the chemical dynamics of this metal-alkane reaction system.
Photoionization spectroscopy of Ga-rare gas complexes
Authors: Stangassinger A. , Knight A. M. , Duncan M. A.
Published: July 2002

New electronic states, F 2Δ3/2, G 2Δ5/2, H 2Π1/2, and I 2Π3/2, are investigated for the complexes GaAr, GaKr, and GaXe with resonant two-photon photoionization spectroscopy. These excited states correlate to the 2D←2P (4d←4p) atomic transition of gallium. Vibronic structure in these spectra are used to obtain vibrational constants, and extrapolated progressions are used to determine dissociation energies. The upper 2Δ states are more than twice as strongly bound as the corresponding 2Π states. Excited state values of dissociation energies are used in energetic cycles to determine ground-state dissociation energies for GaAr, GaKr, and GaXe. In all three cases, the values obtained are significantly lower than previous estimates. The ground state of GaAr is extremely weakly bound, with D0″=20±20 cm−1, while the corresponding value for GaKr is only 35±20 cm−1. The B 2Σ+ excited states of the Ga–RG complexes are confirmed to have substantial barriers in their long-range potentials.
Study of the total and partial fragmentation dynamics of Ar–HCl after uv photodissociation
Authors: Garcı́a-Vela A.
Published: July 2002

The uv photolysis of the Ar–HCl cluster is studied applying an exact time-dependent wave packet method in three dimensions, assuming zero-total angular momentum. The photodissociation process is found to occur via two different fragmentation mechanisms, depending on the initial excitation energy of the cluster. One mechanism leads to total dissociation of the complex, producing three fragments, Ar–HCl+hν→H+Ar+Cl. The fragmentation dynamics in this case is governed by resonance states at relatively low energies of the cluster, in which the H atom collides a number of times with Ar and Cl before dissociating. Manifestations of these collisions are found in the final kinetic energy distribution of the photofragments, which is redshifted in the case of the H fragment, and blueshifted in the Ar and Cl cases. The second type of mechanism consists of a fast and direct photodissociation of the hydrogen, leading to a partial fragmentation of Ar–HCl into hot H fragments and bound Ar–Cl radical molecules. This mechanism dominates at higher energies, which are those mostly populated by the wave packet initially prepared in the present calculations. The experimental implications of the results are discussed.
Quadratic electro-optic effects in bacteriorhodopsin: Measurement of γ(−ω;0,0,ω) in dried gelatin thin films
Authors: Yamazaki Mikio , Goodisman Jerry , Birge Robert R.
Published: July 2002

Quadratic electro-optic effects (dc or low frequency Kerr effect) of bacteriorhodopsin dispersed in dried gelatin thin films are examined in the near resonance region at three wavelengths: 633, 647, and 676 nm. The films show relatively large quadratic electro-optic effects compared to other molecular dispersed systems. The purple membrane is fixed within the polymerized gelatin matrix, and we show that the electronic contribution to γ dominates over possible orientational contributions. At 676 nm, the quadratic electro-optic coefficient s1133(−ω;0,0,ω) is 6.7×10−20 m2/V2 and the third order nonlinear susceptibility χ1133(3)(−ω;0,0,ω) is 7.0×10−13 cm4 statCoulomb−2, with both values obtained for a protein concentration of 6.9×1018 cm−3. The orientationally averaged second molecular hyperpolarizability 〈γ(−ω;0,0,ω)〉 determined from the quadratic electro-optic coefficients at 676 nm assuming an Onsager ellipsoidal local field factor is (10.8±5.1)×10−32 cm7 statCoulomb−2 [(1.34±0.63)×10−56 F3 m4 C−2]. The 〈γ(−ω;0,0,ω)〉 value increases roughly tenfold when the probe wavelength is decreased to 633 nm. The behavior of γ(−ω;0,0,ω), when fit to a two-state model, predicts that γ(−ω;0,0,ω) is strongly enhanced via type III processes. Thus, the magnitude of γ(−ω;0,0,ω) is dominated by a term (Δμ102×μ102)/(ω10−ω)3, where Δμ10 is the change in dipole moment, μ10 is the transition moment, and ω10 is the transition energy of the lowest-lying allowed 1Bu*+-like π,π* state. We calculate that Δμ10 is 12.8±1.2 D, in good agreement with previous Stark and two-photon experimental values. Time-dependent Hartree–Fock methods based on the MNDO Hamiltonian yield reasonable agreement with experiment, underestimating γ(−ω;0,0,ω) by factors of only 2–4, with the error increasing as the frequency approaches resonance.
Critical adsorption in the undersaturated regime: Scaling and exact results in Ising strips
Authors: Ciach A. , Maciolek A. , Stecki J.
Published: July 2002

Critical adsorption for weak surface field h1 is reconsidered. On the basis of physical heuristic arguments, approximate behavior of the scaling function is derived. New form of a scaling for weak h1 in finite systems is proposed and verified by testing against exact results obtained for this purpose in the 2D Ising strips. For weak h1 we find the approximate behavior of adsorption Γ∼τβ−Δ1 for the reduced temperatures h11/Δ1∼τ≪1. This behavior is consistent with experimental data [N. S. Desai, S. Peach, and C. Franck, Phys. Rev. E 52, 4129 (1995)] obtained for τ=10−5, and is in a very good agreement with exact results in the 2D Ising strip.
Monte Carlo simulations of a surface reaction model showing spatio-temporal pattern formations and oscillations
Authors: Gelten R. J. , Jansen A. P. J. , van Santen R. A. , Lukkien J. J. , Segers J. P. L. , Hilbers P. A. J.
Published: July 2002

Results of dynamic Monte Carlo simulations of a model for CO oxidation on a reconstructing Pt(100) surface are presented. A comparison is made between simulations that explicitly include surface diffusion of adsorbed CO and simulations without diffusion. Oscillatory behavior as well as spatio-temporal pattern formation are studied as a function of system size. In the absence of diffusion the amplitude of kinetic oscillations decreases with grid size and oscillations are not stable. Spatio-temporal patterns appear, as expected for an excitable medium. Such patterns become stabilized by structural substrate defects. The length scale of the patterns is in the order of 10–100 nm, the temporal period of the oscillations is around 200 seconds. Inclusion of diffusion stabilizes and synchronizes oscillations. Spatio-temporal features now appear with larger spatial dimensions.
Relaxation process of CO stretching mode of metal carbonyls adsorbed on Y zeolites studied by pump–probe experiment
Authors: Onda Ken , Yaginuma Michio , Yokota Toshiya , Wada Akihide , Domen Kazunari , Hirose Chiaki , Kano Satoru S.
Published: July 2002

The vibrational relaxation lifetimes of Mo(CO)6 and Cr(CO)6, adsorbed in a cage of HY, DY, and NaY zeolites, were measured at various temperatures by the pump–probe method using picosecond infrared laser pulses. The lifetimes at 110 K of the v=1 level of the CO stretching mode were 70±4, 53±5, and 150±9 ps for Mo(CO)6 on HY, DY, and NaY zeolite, respectively, and 87±6, 74±7, and 140±12 ps for Cr(CO)6, respectively. It was shown by comparison of the lifetimes at 110 K, as well as the temperature dependence, that the rate of vibrational relaxation is determined by the coupling of the CO stretching mode with four quanta of the accepting modes comprised of: (i) modes with displacement of central metals Cr or Mo in all of the three zeolite systems, (ii) modes associated with the OH and OD groups on zeolite surface in the HY and DY zeolite systems, and probably (iii) the frame vibrational modes of zeolite with energies at 300–800 cm−1.
In situ SFG spectroscopy of film growth. II. Deposition of formic acid on Ni(110) surface
Authors: Ishida Hirokazu , Iwatsu Katsuhiko , Kubota Jun , Wada Akihide , Domen Kazunari , Hirose Chiaki
Published: July 2002

Infrared-visible sum-frequency generation (SFG) spectroscopy was used to monitor in situ the growth of multiple layers of formic acid on a Ni(110) surface. The signal by the CH stretching band displayed a characteristic interference pattern as the deposition proceeded and the feature was analyzed by the formula presented in the preceding article. The effect of substrate structure was examined to reveal that the deposition of a SFG active layer on alien surfaces requires the substrate temperature higher than 138 K with the optimum at 158 K, but the same growth as under the optimum condition was sustained at 143 K once the active layer had been formed beforehand. No SFG-wise change was observed when the sample temperature was changed in vacuum either from the SFG undetectable 143–153 K or to the opposite direction to indicate that the layers formed at the two temperatures are not related with phase transition. However, the deposition of the SFG-active layer took place at 143 K, when the sample was precovered by the active layer or the SFG-inactive layer was heated to 153 K. Covering by a DCOOD layer, which has no SFG signal at the examined frequency, gave a significant suppression of the interference feature. An interpretation is given on the crystal structure and molecular orientation of the SFG-active and SFG-inactive layers.
First-principles calculation of bulk susceptibility for second-harmonic generation in crystalline C60
Authors: Munn R. W. , Shuai Z. , Brédas J.-L.
Published: July 2002

The hyperpolarizabilities for electric dipole second-harmonic generation (SHG) due to the bilinear combination of an electric field and either the electric-field gradient (μeeq) or the associated magnetic field (τeem), and for electric quadrupole SHG quadratic in the electric field (μqee) have been calculated for the C60 molecule at pump photon energies from 0.5 to 2.2 eV using the valence effective Hamiltonian method with a minimal STO-3G basis; magnetic dipole SHG quadratic in the electric field (τmee) is zero by symmetry in C60. The coefficients exhibit resonances at photon energies near 1.9 for μeeq, 1.4 for μqee, and 2.0 eV for τeem. They are combined with empirical local-field factors and a theoretical local field-gradient tensor to calculate the composite susceptibility χ(2) for SHG from crystalline C60 in the same energy range. The susceptibility shows several resonances that arise from those in μeeq, μqee, and τeem. The results are able to reproduce the magnitudes and signs reported experimentally, subject to a sensitive dependence on photon energy near resonances.
Segmental dynamics and incompatibility in hard/soft polymer blends
Authors: Karatasos K. , Vlachos G. , Vlassopoulos D. , Fytas G. , Meier G. , Du Chesne A.
Published: July 2002

We report on the segmental dynamics of the binary polymer blend polystyrene (PS)/poly (methylphenylsiloxane) (PMPS) in the two-phase region using dielectric spectroscopy that essentially probes the PMPS component. Based on the experimental orientation relaxation functions, the average glass transition temperature Tg controls phase separation. When the spinodal temperature Ts exceeds Tg, the PMPS segmental relaxation displays two distinct decays characteristic of a merely pure and a mixed, roughly at the initial composition, PMPS regions. On the contrary, when Ts falls in the proximity of Tg, the PMPS relaxation is strongly nonexponential and its average time reflects mixed regions rich in PMPS due to incomplete phase separation, which drives only the glassy phase out of local thermodynamic equilibrium. Distinct morphological differences in the two-phase state of these blends, inferred from their segmental dynamics, are revealed by transmission electron microscopy.
Photoionization spectra and ionization energies of HSCl, HSSSH, SSCl, and HSSCl formed in the reaction system Cl/Cl2/H2S
Authors: Eberhard Jürg , Chen Wei-Chen , Yu Chin-hui , Lee Yuan-Pern , Cheng Bing-Ming
Published: July 2002

Photoionization-efficiency (PIE) spectra in the wavelength range 110–140 nm were measured for products of the reaction system Cl/Cl2/H2S in a discharge-flow reactor coupled to a photoionization mass spectrometer employing a synchrotron as source of radiation. According to PIE spectra of HSCl, HSSSH, SSCl, and HSSCl, obtained for the first time, the ionization energies (IE) derived are (9.887±0.016), ⩽9.09, (9.04±0.03), and (9.266±0.014) eV, respectively. Ab initio calculations of these IE with the GAUSSIAN-2 method agree well with experimental results. Other products observed in the system include S2, HSSH, S3, and SCl2. Their PIE spectra and IE were also measured; in some cases discrepancies with previous reports are found. The formation mechanism of the observed products is discussed.
Chaos and dynamical coexistence in Lennard-Jones clusters
Authors: Calvo F.
Published: July 2002

The chaotic dynamics in Lennard-Jones microclusters is revisited for sizes ranging from 3 to 147 monomers. We calculate the largest Liapunov exponent λ in classical molecular dynamics simulations at several total energies, carefully looking at the convergence of λ. Contrary to previous results, no evidence for any distinct signature of the solid-liquid phase transition is found in the variations of λ versus E for clusters smaller than 38 atoms (except LJ3). Larger clusters with 38, 55 and 147 atoms undergo a clear but continuous increase of λ above the freezing energy. This increase occurs simultaneously with a drop in the kinetic temperature between the freezing and melting energies. Hence dynamical coexistence appears to smooth the strong variations of λ near melting.
Ionic interactions and transport in a low-molecular-weight model polymer electrolyte
Authors: Ferry Anders , Orädd Greger , Jacobsson Per
Published: July 2002

AC impedance, FT-Raman and pulsed field gradient (pfg) NMR measurements have been conducted on solutions of poly(ethylene oxide) dimethyl ether (MW 400) complexed with LiCF3SO3 as a function of temperature and salt concentration. From an analysis of the νS(SO3) and δS(CF3) vibrational band envelopes of the CF3SO3 anion, respectively, the relative concentrations of anions in various chemical environments have been calculated. We find spectroscopic evidence for a redissociation of associated ionic species into spectroscopically “free” anions with increasing salt concentration in dilute solutions. The relative abundance of associated ionic species increases with increasing temperature. Pfg-NMR measurements show that D−(19F) and D+(7Li) are very similar for all concentrations (i.e., O:Li⩾53:1) and temperatures (25–80 °C) investigated. Most notably, the diffusivity of the oligomer solvent, D(1H), is significantly faster than the self-diffusion coefficients of the dissolved ions in all cases. Predicted values for the ionic conductivity were obtained from the NMR diffusivities, using the Nernst–Einstein relation, and compared with those from direct measurement. We find that the calculated values are higher for all concentrations; the discrepancy increases with decreasing salt concentration and increasing temperature. A good correlation is found between the concentration dependence of the ionic redissociation pattern, as determined from the νS(SO3) Raman band envelope, and an increase in equivalent ionic conductivity with increasing salt concentration in dilute solutions (i.e., O:Li⩾110:1). We suggest that fluctuating, salt-rich heterogeneities of dissolved ions and polymer segments form at low salt concentrations, and that this may be a general behavior of dilute polymer–salt complexes.
Electrostatic calculations and multiple time scales in molecular dynamics simulation of flexible molecular systems
Authors: Procacci Piero , Marchi Massimo , Martyna Glenn J.
Published: July 2002

In this paper, we deal with the handling of the electrostatic forces in complex molecular systems. In particular, we focus on instabilities experienced by reversible multiple time step algorithms when used in conjunction with Ewald summation techniques for periodic systems. We show that energy conservation is negatively affected by the intra-molecular energy term due to electrostatic excluded contacts required by the most frequently used of the modern force fields for biomolecular systems. These effects are due to a non-complete cancellation of the intra-molecular electrostatic energy and forces at intermediate or long time steps.
Self consistent Ornstein–Zernike approximation compared with exact results for lattice gases in one and two dimensions
Authors: Ho/ye J. S. , Borge A.
Published: July 2002

We evaluate numerically results for the self consistent Ornstein–Zernike approximation (SCOZA) for the Ising model or the lattice gas in one and two dimensions where exact results are known. The cases we consider thus include the Ising model with nearest-neighbor interaction in two dimensions, and in one dimension the cases with a Kac interaction or exponential potential in the infinite range limit and the one with nearest- and next-nearest neighbor interactions. As earlier found for the three-dimensional Ising model, results with high general accuracy are found, although the phase transition of the two-dimensional Ising model is smeared out a bit, as SCOZA at least in its present form, does not yield a phase transition in two dimensions. In the two-dimensional case more long- range interactions are also considered to see to what extent SCOZA approximates the expected universal critical behavior. By extrapolation we find our numerical results quite consistent with a value near the exact one γ=1.75 for the supercritical exponent of isothermal susceptibility. In the case with the nearest- and next-nearest neighbor interactions a situation that clearly favors ferromagnetic configurations is needed. Otherwise the present version of SCOZA will fail, i.e., the solution becomes less accurate and finally ceases to exist.
Accurate density functional calculation of core-electron binding energies with a scaled polarized triple-zeta basis set. IV. Application to isomers of C3H6O, C3H3NO, and C6H6
Authors: Chong Delano P. , Hu Ching-Han
Published: July 2002

The unrestricted generalized transition-state model using a gradient-corrected density functional was shown in previous papers to be a reliable procedure for calculating core-electron binding energies. Relativistic corrections were estimated. Recently, a more efficient basis was proposed and tested. The results indicated that the new scaled polarized valence triple-zeta basis performs as well as the much larger cc-pV5Z basis set. This procedure is followed in the present study of the known isomers of C3H6O, C3H3NO, and C6H6. The results demonstrate that x-ray photoelectron spectroscopy, complemented by accurate theoretical calculations, can indeed be used to help chemical analysis.
On the properties of polymer globules in the high density limit
Authors: Witelski Thomas P. , Grosberg Alexander Yu. , Tanaka Toyoichi
Published: July 2002

We re-examine quantitative mean-field theory for the collapsed globule phase of a polymer chain taking full account of its finite compressibility. The mathematical properties of the nonlinear mean-field equations describing the structure of the globule are formulated. Our results explain findings and observations of the recent computer simulation and experimental studies. While the effects are due to the restricted compressibility, they are seen well before the globule reaches its dry maximal density.
Phase and amplitude control in the formation and detection of rotational wave packets in the E 1Σg+ state of Li2
Authors: Uberna Radoslaw , Khalil Munira , Williams Richard M. , Papanikolas John M. , Leone Stephen R.
Published: July 2002

Femtosecond laser pulse amplitude/phase masking techniques are employed to control the formation and detection of rotational wave packets in the electronic E 1Σg+ state of lithium dimer. The wave packets are prepared by coherent excitation of rovibronic E 1Σg+(νE,JE) states of Li2 from a single intermediate state, A 1Σu+(νA=11, JA=28), and probed by time-resolved photoionization. In the detection step, the wave packet is projected onto the X 2Σg+ state of Li2+. New resonance structure in the X 2Σu+ ionic state continuum is obtained by measuring the wave packet signal modulation amplitude as a function of the frequencies removed from the spectrally dispersed probe pulse by insertion of a wire mask in a single-grating pulse shaper. A split glass phase mask inserted into the pulse shaper is used to produce step function changes in the spectral phase of the pulse. The phase relation among the wave packet states is varied by changing the relative phases of spectral components in the pump pulse and is monitored by measuring the changes in the phase of the rotational wave packet recurrences using an unmodified probe pulse. By altering the relative phases among the wave packet components, the spatial distribution of the initial wave packet probability density is varied, resulting in phase-dependent “alignment” of the probability density in angular space. Phase changes in the signal recurrences are also observed when a phase modified pulse is used in the wave packet detection step after wave packet preparation with an unmodified pulse. The formation and detection of the wave packets is discussed in terms of quantum interference between different excitation routes. The relative phase factors encoded in a single optical pulse (pump or probe) are transferred into the interference term of the measured signal through the molecule–photon interaction.
Vibrational overtone spectroscopy and internal dynamics in gaseous nitromethane NO2CH2D
Authors: Cavagnat D. , Lespade L.
Published: July 2002

The CH-stretching overtone spectra of the methyl group in gaseous nitromethane NO2CH2D have been recorded with conventional Fourier transform near-infrared spectroscopy in the ΔvCH=1–4 regions and by intracavity laser photoacoustic spectroscopy in the ΔvCH=5 and 6 regions. All spectra exhibit a complex structure; they have been analyzed with a theoretical model which takes into account, within the adiabatic approximation, the coupling of the anharmonic CH stretch vibrations, described by Morse potentials, with the quasifree internal rotation of the methyl group and with isoenergetic combination states involving methyl bending modes. Most of the parameters of this model, and their variation with the internal rotation coordinate (θ), are identical to ones used to account for the overtone spectra of the monohydrogenated and perhydrogenated species. Fermi resonance phenomena, also modeled with θ dependent parameters, lead to only weak intramolecular vibrational energy redistribution. This simple calculation successfully describes the relative intensity and frequency of each peak within a given overtone. The vibrational energy is seen to be localized at lower energy in the dihydrogenated compound [at the second overtone (v=3)] than in the perhydrogenated one [at the third overtone (v=4)]. The overtone vibrations obtained from the calculations can be considered as normal modes up to Δv=2 and as local modes from Δv=3 to 6. However, at Δv=3, the existence of a transitional regime, where normal and local modes coexist, can be demonstrated. The CH/CD interbond coupling shifts the overtone spectra toward high for some additional weaker features in the high overtone spectra (Δv=5 and 6).
Electronically nonresonant coherent Raman scattering using incoherent light: Two Brownian oscillator approaches
Authors: Kirkwood Jason C. , Ulness Darin J. , Albrecht A. C.
Published: July 2002

The theory for electronically nonresonant coherent Raman scattering using incoherent light (I(2)CRS) is presented for a Brownian oscillator material response. The exploration of Raman line shapes is based on two different approaches, both of which incorporate the Brownian oscillator model. The first takes the Raman transition as a two-level system that is indirectly coupled to the bath through “primary” Brownian oscillators. The I(2)CRS signal in the overdamped, high temperature regime is calculated analytically using this approach. The second approach is the standard one in which the Raman transition itself is represented by primary Brownian oscillators that are directly coupled to the bath. This method permits analytic calculation of the I(2)CRS signal for the entire parameter space and also for a distribution of oscillators to account for inhomogeneous linewidth broadening. These two methods allow for different interpretations of Raman line shapes using noisy light spectroscopy.
Description of noble gas diffusion in a polymer matrix by a hopping model
Authors: Hartmann Alexander K. , Heermann Dieter W.
Published: July 2002

We present a new heuristic model which describes the diffusion of noble gas atoms inside a polymer matrix. The motion of the gas atoms consists of movements inside small areas called traps and of hops between different traps. Whether an atom enters another trap during a molecular dynamics (MD) simulation is determined by observing a new quantity called cage overlap. This criterion is independent of size, time, and temperature scales of the system. The resulting motions are quantitatively described by probability distributions and correlation functions. We measure these distributions for three example systems of helium, argon, and krypton atoms inside a polyethylene matrix using MD simulations. Then we verify the model by comparing results from direct simulations of the hopping model with initial results from the MD.
Femtosecond β-cleavage dynamics: Observation of the diradical intermediate in the nonconcerted reactions of cyclic ethers
Authors: Scala A. A. , Diau E. W.-G. , Kim Z. H. , Zewail A. H.
Published: July 2002

Femtosecond (fs) dynamics of reactions of cyclic ethers, symmetric and asymmetric structures, are reported. The diradical intermediates and their β-cleavages, which involve simultaneous C-C, C-H σ-bond breakage and C-O, C-C π-bond formation, are observed and studied by fs-resolved mass spectrometry. To compare with experiments, we present density functional theory calculations of the potential energy surface and microcanonical rates and product distributions.
Extraction of tunneling splittings from a real time semiclassical propagation
Authors: Mandelshtam V. A. , Ovchinnikov M.
Published: July 2002

The long-standing question of whether the tunneling effects can be recovered from a real time semiclassical propagation is addressed. Even at very low energies below the barrier for symmetric one- and two-dimensional double wells the tunneling splittings are accurately computed using harmonic inversion of the time cross-correlation matrix by the filter-diagonalization method. The time propagation is carried out using the Herman–Kluk propagator.
A molecular dynamics simulation of water confined in a cylindrical SiO2 pore
Authors: Rovere M. , Ricci M. A. , Vellati D. , Bruni F.
Published: July 2002

A molecular dynamics simulation of water confined in a silica pore is performed in order to compare it with recent experimental results on water confined in porous Vycor glass at room temperature. A cylindrical pore of 40 Å is created inside a vitreous SiO2 cell, obtained by computer simulation. The resulting cavity offers water a rough hydrophilic surface and its geometry and size are similar to those of a typical pore in porous Vycor glass. The site-site distribution functions of water inside the pore are evaluated and compared with bulk water results. We find that the modifications of the site-site distribution functions, induced by confinement, are in qualitative agreement with the recent neutron diffraction experiment, confirming that the disturbance to the microscopic structure of water mainly concerns orientational arrangement of neighboring molecules. A layer analysis of MD results indicates that, while the geometrical constraint gives an almost constant density profile up to the layers closest to the interface, with a uniform average number of hydrogen bonds (HB), the hydrophilic interaction produces the wetting of the pore surface at the expenses of the adjacent water layers. Moreover, the orientational disorder together with a reduction of the average number of HB persists in the layers close to the interface, while water molecules cluster in the middle of the pore at a density and with a coordination similar to bulk water.
Quenching of interconversion tunneling: The free HCl stretch first overtone of (HCl)2
Authors: Liu K. , Dulligan M. , Bezel I. , Kolessov A. , Wittig C.
Published: July 2002

Cavity ringdown laser absorption spectroscopy has been used to record spectra of (H35Cl)2 and its Cl-substituted mixed dimers at the first overtone of the free hydrogen stretch (2ν1). The dimers were produced in pulsed planar supersonic expansions. Significant quenching of interconversion tunneling (i.e., which exchanges the roles of H-bond donor and acceptor) has been observed. Thus, the H35Cl–H37Cl and H37Cl–H35Cl heterodimers are distinguished in the 2ν1 eigenstates, which is not the case for the ground and HCl-stretch fundamental eigenstates because of facile tunneling mixing.
Dynamics of vibronically excited fluorene–Arn (n=4, 5) clusters
Authors: Pitts Jonathan D. , Knee J. L.
Published: July 2002

The fluorene–Ar4 cluster has been shown to exhibit two distinct isomers when formed in a molecular beam. Resonance enhanced multiphoton ionization and mass analyzed threshold ionization experiments have been performed to investigate the structural properties, energetics and dynamics of these clusters when excited to vibronic bands in the S1 electronic state, with a specific interest in measuring isomer interconversion. At 208 cm−1 excess energy in the S1 isomer interconversion is not observed in the Ar4 cluster. Dissociation of the Ar5 cluster from the 722 cm−1 band is shown to produce both Ar4 isomers.
Removing resonance effects from quantum mechanical vibrational partition functions obtained from perturbation theory
Authors: Isaacson Alan D.
Published: July 2002

This paper compares different methods for removing resonance effects from second-order perturbation theory calculations of vibrational energies in a variety of systems containing from two to six modes. Both the recently proposed method of Kuhler et al. and the standard approach of Nielsen yield stable energy levels even very close to resonance, with the latter giving smaller average errors in such cases. In addition, the method of Kuhler et al. is observed to affect the ground-state energy, unlike the standard approach. This generally worsens the accuracy of the vibrational partition function at room temperature, especially for systems close to resonance.
Electron and anion mobility in low density hydrogen cyanide gas. I. Dipole-bound electron ground states
Authors: Klahn Th. , Krebs P.
Published: July 2002

We measured the mobility of excess electrons in the polar hydrogen cyanide gas (D=2.985 D) at low densities as a function of density and temperature by the so-called pulsed Townsend method. Experiments were performed at 294 and 333 K in the gas number density range 1.23×1017⩽n⩽3.61×1018 cm−3. We found a strong density dependence of the “zero-field” density-normalized mobility (μn). Only about 10% of the observed density variation can be qualitatively explained by coherent and incoherent multiple scattering effects. With increasing gas density an increasing number of linear HCN dimers is formed which due to the high dipole moment (D=6.552 D) represent much stronger electron scatterers than the HCN monomers. It was found that the dimers may be only in part responsible for the observed density effect. Therefore, we consider a transport process where short-lived dipole-bound electron ground states (lifetime ⩾12 ps) as quasilocalized states are involved. For comparison the electron mobility in saturated 2-aminoethanol vapor with a dipole moment of similar size (D=3.05 D) does not show any anomalous density behavior in the temperature range 298⩽T⩽435 K. In contrast to this the electron mobility in saturated but also in nonsaturated CH3CN gas (D=3.925 D) shows a density behavior similar to that in HCN.
The dynamics of hydrogen abstraction reactions: Crossed-beam reaction Cl+n-C5H12→C5H11+HCl
Authors: Hemmi Naoki , Suits Arthur G.
Published: July 2002

We present a crossed molecular beam study of the dynamics of the metathesis reaction Cl+n-C5H12→HCl+C5H11 at a collision energy of 16.8 kcal/mol. The experiments were conducted on the Chemical Dynamics Beamline at the Advanced Light Source, using tunable undulator radiation to effect soft ionization of the pentyl radical product. Laboratory angular distributions and time-of-flight spectra at many laboratory angles were used to obtain the center-of-mass translational energy and angular distributions. These distributions were found to be strongly coupled, with the forward scattered pentyl radical formed extremely cold, while the backscattered radicals were formed leaving nearly 15 kcal/mol in internal energy in the products. These results are contrasted with recent studies on the analogous reaction with propane, suggesting in this case direct involvement of the carbon skeleton in the collision process.
The C–Cl bond fissions from the photolysis of CHCl=CCl2 at 193 nm
Authors: Lee Yu-Jinn , Lee Ya-Rong , Chou Chih-Chiang , Lin Shen-Maw
Published: July 2002

The photolysis of CHCl=CCl2 at 193 nm was investigated by translational spectroscopy. Two distinct product translational distributions were derived for the C–Cl bond fissions. The internally excited C2HCl2 fragment from the main dissociation channel is shown to decompose totally to produce the Cl+C2HCl products.
Double perturbation series in the differential equations of enzyme kinetics
Authors: Fraser Simon J.
Published: July 2002

The connection between combined singular and ordinary perturbation methods and slow-manifold theory is discussed using the Michaelis-Menten model of enzyme catalysis as an example. This two-step mechanism is described by a planar system of ordinary differential equations (ODEs) with a fast transient and a slow “steady-state” decay mode. The systems of scaled nonlinear ODEs for this mechanism contain a singular (η) and an ordinary (ε) perturbation parameter: η multiplies the velocity component of the fast variable and dominates the fast-mode perturbation series; ε controls the decay toward equilibrium and dominates the slow-mode perturbation series. However, higher order terms in both series contain η and ε. Finite series expansions partially decouple the system of ODEs into fast-mode and slow-mode ODEs; infinite series expansions completely decouple these ODEs. Correspondingly, any slow-mode ODE approximately describes motion on ℳ, the linelike slow manifold of the system, and in the infinite series limit this description is exact. Thus the perturbation treatment and the slow-manifold picture of the system are closely related. The functional equation for ℳ is solved automatically with the manipulative language MAPLE. The formal η and ε single perturbation expansions for the slow mode yield the same double (η,ε) perturbation series expressions to given order. Generalizations of this procedure are discussed.
Excitation spectrum of the A0+(5 3P1), B1(5 3P1)←X0+(5 1S0) transitions in the CdHe van der Waals molecule: Spectroscopic characterization of the X0+, A0+, and B1 electronic energy states
Authors: Koperski J. , Czajkowski M.
Published: July 2002

The first observation of an excitation spectrum of the A0+(5 3P1), B1(5 3P1)←X0+(5 1S0) transitions in the CdHe van der Waals molecule is reported. A continuous molecular-jet-expansion beam of CdHe seeded in helium was crossed with a pulsed-laser beam produced by a Nd:YAG laser-pumped-dye laser. The dye laser frequency was doubled using a KDP-C crystal and tuned to a frequency in the vicinity of the 5 3P1↔5 1S0 resonance transition in atomic cadmium (λres≈3262.0 Å). The assignment of vibrational bands was obtained with the aid of rigorous computer simulation of the A0+←X0+ and B1←X0+ transitions. The analysis of the excitation spectrum yielded potential parameters of the molecule: De″(X0+)=14.2 cm−1, ωe″(X0+)=9.6 cm−1, ωe″xe″(X0+)=1.63 cm−1 for the ground state, and De′(A0+)=41.2 cm−1, ωe′(A0+)=20.0 cm−1, ωe′xe′(A0+)=2.4 cm−1 as well as De′(B1)=6.1 cm−1, ωe′(B1)=3.6 cm−1, and ωe′xe′(B1)=0.53 cm−1 for the excited states. From a simulation procedure the differences between equilibrium internuclear separations ΔR=Re″(X0+)−Re′(A0+)=−1.50 Å and ΔR=Re″(X0+)−Re′(B1) =+0.12 Å were also obtained. The results were subjected to the LeRoy–Bernstein method to evaluate the C6/R6 long-range potential parameters for the X0+ and A0+ energy states.