bluuebunny/crossref_metadata_2025 huggingface dataset:
A large-scale and long-time molecular dynamics study of supercritical Lennard-Jones fluid. An analysis of high temperature clusters
Authors: Yoshii Noriyuki , Okazaki Susumu
Published: July 2002

A number of large-scale and long-time molecular dynamics calculations, N=10 976 and t=1 ns, have been performed for the supercritical Lennard-Jones fluid at various densities along an isotherm at T=1.03Tc in order to investigate the structure and dynamics of clusters formed in the fluid as well as the fluid itself. The calculations successfully demonstrated both Ornstein–Zernike behavior of structure factor at small k region and critical slowing down in dynamic structure factor near the critical density. A molecular based picture for these phenomena in real space has been presented by newly defining several statistical mechanical correlation functions and distribution functions. A few static and dynamic properties of clusters such as generation and annihilation dynamics and single particle and collective energy fluctuations are also given.
Turbidity of critical solutions of polymethylmethacrylate in 3-octanone
Authors: An X.-Q. , Shen W.-G. , Xia K.-Q.
Published: July 2002

We have measured the turbidity for a series of solutions of polymethylmethacrylate in 3-octanone (PMMA/3-OCT) with various polymer molecular weights. The obtained correlation length (ξ) and the osmotic compressibility (χ) show power law dependence on both the reduced temperature ε=(T−Tc)/Tc and the degree of polymerization N of the polymer chains, i.e., ξ∼N0.15ε−0.63 and χ∼N−0.06ε−1.23, with the associated scaling exponents in good agreement with theoretical predictions. When the results of the present experiment are combined with those from an earlier coexistence curve measurement [K.-Q. Xia, X.-Q. An, and W.-G. Shen, J. Chem. Phys. 105, 6018 (1996)], it is found that the concept of two-scale-factor universality applies to systems consist of the same polymer/solvent pair but with different molecular weights.
Biased reptation of polyampholytes: Trapping and enhancement effects
Authors: Loomans D. , Schiessel H. , Blumen A.
Published: July 2002

We consider the dynamics of polyampholytes (PAs, polymers containing positive and negative charges) in a fixed network (gel). Under the influence of an external electrical field a PA undergoes a biased reptation; the electrophoretic mobility of the chain depends crucially on the given charge distribution. This effect, which does not occur for equally charged polymers (polyelectrolytes, e.g., DNA) leads to an effective separation of PAs according to their charge distribution—even for PAs of the same length and the same overall charge.
Deexcitation of Ne(3P1) and Ne(3P2) in collisions with Ar, Kr, and Xe
Authors: Khadka Deba Bahadur , Fukuchi Yuji , Kitajima Masashi , Hidaka Koji , Kouchi Noriyuki , Hatano Yoshihiko , Ukai Masatoshi
Published: July 2002

The cross sections for the deexcitation of Ne(3P1) and Ne(3P2) by Ar, Kr, and Xe have been measured as a function of the mean collisional energy in the range of 17–38 meV or in the temperature range from 133 K to 295 K using a pulse radiolysis method combined with time-resolved optical absorption spectroscopy. The deexcitation cross sections are in the range of 18–26 Å2 and 11–20 Å2 for Ne(3P1) and Ne(3P2), respectively, and increase slightly with increasing the collisional energy. The absolute experimental cross sections for the Ne(3P1) deexcitation are in relatively good agreement with the total Penning ionization cross sections obtained by an optical model calculation, which shows the importance of an electron-exchange interaction in the deexcitation of Ne(3P1) in collisions with Ar, Kr, and Xe. The absolute cross section values for the deexcitation of resonant Ne(3P1), which are only slightly larger than those for the deexcitation of metastable Ne(3P2), have also supported the discussion on the deexcitation mechanism of Ne(3P1) by Ar, Kr, and Xe.
Yield determination of OH (v=0,1) radicals produced by the electron-ion recombination of protonated molecules
Authors: Gougousi Theodosia , Johnsen Rainer , Golde Michael F.
Published: July 2002

A flowing afterglow apparatus in conjunction with laser induced fluorescence (LIF) diagnostics has been used to determine the yields of OH(X2Π, v=0,1) produced by the dissociative recombination (DR) of HCO2+, HN2O+, HCO+ ions with electrons at 300 K. The yield for v=0 radicals was determined relative to the known OH yield from the recombination of H3O+ ions while the production of vibrationally excited OH in v=1 was obtained relative to that in v=0 by comparing LIF spectra.
Gas-phase thermochemical stabilities of cluster ions [(N2)m(Ar)n]+ with (m+n)=1–5
Authors: Hiraoka K. , Fujimaki S. , Nasu M. , Minamitsu A. , Yamabe S. , Kouno H.
Published: July 2002

Thermochemical stabilities of nitrogen-argon cluster ions [(N2)m(Ar)n]+ were measured using a pulsed-electron beam mass spectrometer. The thermochemical data obtained for the exchange reactions N4++Ar=N2Ar++N2 (ΔH0=−1.0 kcal/mol) and Ar2++N2=N2Ar++Ar (ΔH0=−2.1 kcal/mol) lead to the determination of the bond dissociation energies (D), D(N2Ar+→N2++Ar)=26.8, D(N2Ar+→Ar++N2)=30.5, and D(Ar2+→Ar++Ar)=28.4 kcal/mol. For the mixed cluster ions [(N2)m(Ar)n]+, the irregular decreases in bond dissociation energies are found with (m+n)=3→4. This fall-off indicates that the core ions in the mixed cluster ions are trimer cations, [(N2)m(Ar)n]+ with (m+n)=3 in agreement with the experimental results by Magnera and co-workers [Chem. Phys. Lett. 192, 99 (1992); J. Chem. Soc. Faraday Trans. 86, 2427 (1990)]. The most stable cluster ions of [(N2)m(Ar)n]+ are found to be those composed of the core ion N2Ar+N2 solvated by further N2 ligands. The rate of exchange reaction (N2)m++Ar=N2Ar+N2(N2)m−3+N2 was found to become slower with decrease of temperature due to the existence of energy barrier. Ab initio [G2(MP2)] calculations on some cluster ions [(N2)m(Ar)n]+ were performed. Trimer cations have linear geometries, and the fourth neutral molecule is coordinated in the bridged (bidentate) form to the trimer cation.
Circular dichroism at microwave frequencies: Calculated rotational strengths of selected transitions for some oxirane derivatives
Authors: Salzman W. R.
Published: July 2002

A summary and selected results are presented for calculations of the rotational strengths of circular dichroism transitions in the microwave for the three chiral deuterated oxiranes, methyl- and trans-dimethyloxirane, and methylthiirane. The calculations, up to J=10, were carried out using a modified version of the spectral cataloging programs of Pickett. A selection of transitions, with rotational strengths between 10−44 and 100×10−44 esu2 cm2, which are considered to be the most likely candidates for observation of rotational circular dichroism, is given.
The OBrO C(2A2)←X(2B1) absorption spectrum
Authors: Miller Charles E. , Nickolaisen Scott L. , Francisco Joseph S. , Sander Stanley P.
Published: July 2002

The highly structured visible absorption spectrum of the bromine dioxide radical, OBrO, has been observed in the 15 500–26 000 cm−1 region. The spectrum is dominated by a long progression in the Br–O symmetric stretching motion (ν1′) and a series of short progressions built on the bending mode (ν2′); there are no features associated with the excitation of the antisymmetric stretching mode (ν3′). The spectrum also contains numerous transitions originating from the (0,1,0) and (1,0,0) vibrational levels of the electronic ground state, X(2B1). A simultaneous fit to all of the observed vibronic features yielded the frequencies ν1″=799.4 cm−1, ν2″=317.5 cm−1, ω1′=641.5 cm−1, ω2′=210.7 cm−1, and a band origin T0=15 863 cm−1. Franck–Condon simulations combined with ab initio calculations of the four lowest OBrO doublet electronic states identify the spectrum as arising from the C(2A2)←X(2B1) electronic transition.
Bound states and time-dependent dynamics of the N2H+ molecular ion in its ground electronic state. I. 2D treatment
Authors: Mahapatra S. , Vetter R. , Zuhrt Ch. , Nguyen H. T. , Ritschel T. , Zülicke L.
Published: July 2002

The ground-state potential energy surface (PES) for linear arrangements of the N2H+ molecular ion is numerically computed by the multireference single- and double-excitation configuration interaction (MRD-CI) technique. An analytical representation of the potential energy function is obtained by fitting a power series in the Simons–Parr–Finlan coordinates to the numerical data. For investigating the intramolecular dynamics we describe the nuclear motion by a Gaussian wave packet located initially in the strong interaction region of the PES. The vibrational eigenvalue spectrum is calculated by Fourier transforming the time autocorrelation function. The spectrum is then analyzed statistically in the light of random matrix theory (RMT) to understand the nature of the intramolecular dynamics. We examine the short-range correlation in the spectrum through the nearest neighbor level spacing distribution P(s) and the long-range correlation through Δ3 and Σ2 statistics. The spectrum in the time domain is analyzed by computing the ensemble averaged survival probability 〈〈P(t)〉〉. The above four quantities obtained from the spectrum are compared with the distribution predicted for regular, irregular, and mixed (intermediate) spectra by the RMT. We find the system is of mixed type and the fractional irregularity is 0.7±0.05. In order to reveal a possible correspondence to the classical dynamics, we have carried out the spectral analysis of the dynamical variables for classical trajectories over a wide range of internal energies. In addition the classical dynamics of proton collisions with N2 molecules has also been preliminarily studied on the same PES, in particular the dependence of the final vibrational action nf on the initial vibrational phase φi of N2 and, furthermore, the Poincaré surface-of-section superimposed with the zero-order separatrix; we find a large number of trapped trajectories.
A theory for nonisothermal unimolecular reaction rates
Authors: Pollak Eli , Talkner Peter , Berezhkovskii A. M.
Published: July 2002

A simple expression is derived for the survival probability of a reactive chemical species which is initially prepared at a temperature which differs from its surrounding. The competition between relaxation of reactants back to the external equilibrium and the possibility of reaction may prevent the usual single exponential kinetics for the survival probability. The theory is accurate for activated reactions with moderate (V‡/kBT⩾3) to high reduced barrier heights. It is especially relevant for multidimensional systems where the characteristic energy at which a molecule dissociates is greater than the barrier height.
Use of Gauss–Hermite quadrature in the treatment of predissociation resonances with the complex-scaling method
Authors: Li Yan , Bludsky Ota , Hirsch Gerhard , Buenker Robert J.
Published: July 2002

An efficient numerical integration procedure based on the Gauss–Hermite quadrature is developed for evaluation of rovibrational Hamiltonian matrix elements in a basis of complex-scaled harmonic oscillator functions. By scaling the basis functions rather than the Hamiltonian itself, it is possible to employ molecular potential energy and coupling data directly in numerical form without first having to fit them to analytical functions. The new method is employed for the treatment of the CO B-D 1Σ+ system by employing model diabatic potentials and coupling elements from the literature. Calculations are carried out in both the original diabatic and the corresponding adiabatic representation of the electronic states. Because of the sharp oscillations in the nonadiabatic coupling functions it is found that the convergence properties in the diabatic basis are somewhat better than in the corresponding adiabatic treatment, but very good agreement is obtained between the two sets of energy and linewidth results for the lowest 11 vibrational states. Comparison is also made with earlier results for the same system obtained by employing the optical potential and close coupling methods, respectively. The second-derivative G12 coupling matrix elements are found to have an important effect on the computations in the adiabatic representation and are essential for obtaining a high level of agreement with the corresponding diabatic results. The present method is well-suited for applications based on ab initio potential energy surfaces and couplings since it requires neither that the pointwise computed data be fitted to polynomials nor that they be subjected to a diabatic transformation.
An ab initio study of OCCN and OCCN+
Authors: Francisco Joseph S. , Liu Ruifeng
Published: July 2002

The structure and vibrational frequencies are reported for the OCCN radical and its cation using quadratic configuration interaction and complete active space self-consistent field methods. The heat of formation for the OCCN radical is estimated as 50.2 kcal mol−1. The adiabatic ionization potential is estimated as 8.5 eV. Vertical excitation energies for low-lying excited states of OCCN are also reported.
Diffusion quantum Monte Carlo calculation of positronium affinity of lithium
Authors: Yoshida Takashi , Miyako Gensho
Published: July 2002

The diffusion quantum Monte Carlo (DQMC) method simulating the Green’s function to quadratic accuracy is applied to the computation of the positronium affinity of the lithium atom. The result suggests that the positronium lithium system, [e+﹍Li−] is a stable entity with respect to positronium detachment as well as positron dissociation.
Asymptotic method for polarizabilities and dispersion coefficients: With applications to hydrogen and helium systems
Authors: Patil S. H. , Tang K. T.
Published: July 2002

The asymptotic behavior of the atomic wave functions is used to show that high-order multipole moment matrix elements have a sharp peak when the excitation energy is equal to twice the ionization energy. This makes it possible to deduce the correct forms of the weighted sums involving these matrix elements. In this paper we demonstrate in the case of hydrogen and helium how these correct forms can be used to obtain accurate polarizabilities αL and dispersion coefficients C2n of arbitrary high L and n.
Electron–phonon interactions in CsCdBr3:Yb3+
Authors: Hehlen Markus P. , Kuditcher Amos , Rand Stephen C. , Tischler Michael A.
Published: July 2002

Pronounced electron–phonon coupling is observed for the F7/22↔F5/22 4f transitions of Yb3+ doped into CsCdBr3. A comparison of the Raman spectrum and the luminescence excitation sideband accompanying the F7/22(0)→2F5/2(2′) crystal-field transition reveals vibrational properties of the [YbBr6] coordination unit that differ markedly from those of the CsCdBr3 host. In particular, the vibronic transition associated with the totally symmetric [YbBr6] stretching mode appears as a very weak feature at 191 cm−1 in the Raman spectrum, whereas the totally symmetric [CdBr6] stretching mode of the CsCdBr3 bulk, which appears as a strong feature at 162.5 cm−1 in the Raman spectrum, is only weakly discernible in the sideband. This is direct evidence for a large contribution from [YbBr6] local modes and a small contribution from bulk modes to the vibronic intensity. The intensity of the local mode is enhanced by approximately a factor of 2 in the Raman spectrum when the laser is tuned into resonance with the F7/22(0)→2F5/2(2′) absorption of Yb3+, providing direct confirmation of its assignment. The observation of the first and second members of a Franck–Condon progression for both the local and the bulk totally symmetric modes indicates that a Δ process, rather than an M process, induces the vibronic intensity. Huang–Rhys factors of Slocal=0.010±0.002 and Sbulk=0.15±0.03 were determined from the data, and reflect quite different electron–phonon coupling strengths. These results suggest that multiphonon relaxation of excited electronic states proceeds by the excitation of local modes of [YbBr6] followed by energy transfer to bulk modes of the lattice, possibly through a nonlinear coupling mechanism which is discussed briefly.
Infrared laser induced isomerization of methanol polymers trapped in nitrogen matrix. I. Trimers
Authors: Coussan S. , Loutellier A. , Perchard J. P. , Racine S. , Peremans A. , Tadjeddine A. , Zheng W. Q.
Published: July 2002

Selective vibrational excitations of OH stretching modes of methanol aggregates trapped in solid nitrogen were carried out in the range 3500–3370 cm−1. This proved an efficient means of interconverting trimers and possibly branched polymers absorbing in this domain. Irradiation in the absorption band of the cyclic trimer (CT) around 3450 cm−1 leads to a cycle opening and to the appearance of several groups of bands assigned to different forms of open chain trimer (OTi, i=1–3). Irradiations at the absorption frequencies of OT1 and OT2 interconverted these open species and partially regenerate CT. Isotopic dilution experiments enable to improve vibrational analysis of the open trimers, previously advanced from data obtained with irradiations performed with polychromatic light [Chem. Phys. Lett. 217, 123 (1994)]. Two other bands at 3424 and 3370 cm−1, growing upon annealing as the high polymer bands around 3250 cm−1, also turn out to be sensitive to selective irradiation. OH/OD and 16O/18O isotopic substitutions lead to the conclusion that the species (X) responsible for these absorptions is a branched trimer generated by fixation of one extra molecule on a cyclic trimer. Among the various products of photolysis one unstable species, with a lifetime of 8 min at 7 K, has been identified as a branched open chain trimer with one methanol molecule acting as proton biacceptor. Isomerization kinetic studies were followed in the ν CO region. According to the nature of the precursor, more or less complicated conversion schemes were identified. A quantitative analysis was carried out in only two cases, namely the photolysis of CT and OT1 upon irradiation at 3450 and 3389 cm−1, respectively. In both cases the kinetics were interpreted in term of first order processes, leading to quantum yield values between 10−2 and 10−3.
Ab initio study of the reactions of Zn(1S, 3P, and 1P) with SiH4
Authors: Luna-Garcı́a H. , Castillo S. , Ramı́rez-Solı́s A.
Published: July 2002

The reactions of Zn(1S, 3P, and 1P) with SiH4 have been studied through multiconfigurational self-consistent field (with relativistic effective core potentials) followed by extensive variational and perturbational second-order multireference Möller–Plesset configuration interaction by perturbation selected by iterative process calculations using extended Gaussian basis sets. The Zn atom in the 3P(4s14p1) state breaks the Si–H bond of silane spontaneously, leading directly to the ZnH+SiH3 final products, in agreement with experimental results. The 1P(4s14p1) Zn atom is also inserted in the Si–H bond and the corresponding interaction surface shows an avoided crossing with the lowest-lying X 1A′ potential surface, adiabatically correlated with the Zn(1S:4s2)+SiH4 reactants. This interaction leads also to the ZnH+SiH3 products. The structure of the HZnSiH3 intermediate product was carefully studied as well as the dissociation channels leading to the ZnH+SiH3 and H+ZnSiH3 products. Accurate energy differences between these species are also reported. The qualitative difference in the behavior of the 3P(4s14p1) Zn reaction with methane and silane has been explained by analyzing the corresponding potential energy surfaces; the present results confirm the C–H bond steric hindrance hypothesis advanced by Wang et al. [J. Chem. Phys. 104, 9401 (1996)].
Hydrogen-bond structure in an aqueous solution of sodium chloride at sub- and supercritical conditions
Authors: de Jong P. H. K. , Neilson G. W.
Published: July 2002

The method of neutron diffraction and isotopic substitution was used to determine the hydrogen-bond structure as defined by gOH(r) in a 1 m aqueous electrolyte solution of sodium chloride at two thermodynamic states, one subcritical with T=300 °C and P=120 bar, and the other supercritical T=460 °C and P=800 bar (the critical temperature of 1 m sodium chloride is 425 °C). The results are similar to those found in pure water substance, and confirm the progressive disruption of the hydrogen bond network brought about by an increase in temperature.
Collisions of rare gas ions with C60: Endohedral formation, energy transfer, and scattering dynamics
Authors: Basir Yousef , Anderson Scott L.
Published: July 2002

Scattering of rare gas cations from C60 has been studied in a guided-beam tandem mass spectrometer. Charge transfer (CT) is observed to be the dominant channel over the collision energy range from 0 to 100 eV, but dissociative CT and endohedral complex formation are significant at high collision energies. The threshold energies for endohedral penetration are found to be proportional to rare gas atom size. Our CT and dissociative CT data allow us to make several conclusions about the nature of energy transfer in rare gas-fullerene collisions. Surprisingly, the conclusion is that the energy transfer distribution must be sharply bimodal, with ∼85% of collisions resulting in little collision-to-internal energy transfer, and ∼15% of collisions being essentially 100% inelastic. The results indicate that the dissociative CT and endocomplex formation channels are closely related.
Kink grain boundaries in a block copolymer lamellar phase
Authors: Matsen M. W.
Published: July 2002

We introduce a method for examining the boundaries between periodic block copolymer morphologies using self-consistent field theory (SCFT). The technique is illustrated on kink grain boundaries in the AB diblock copolymer lamellar phase. In agreement with experiment, the A and B domains evolve from a simple “chevron” shape to a complex “omega” shape as the kink angle θ increases beyond about 90°. The transformation begins with the formation of an “intermediate” shape where protrusions develop symmetrically from the A and B domains at the center of the grain boundary. Following that, a continuous symmetry-breaking transition occurs forming the omega boundary where protrusions extend only from either the A or B domains. At low angles, in the chevron region, the grain boundary tension ΓK obeys the scaling relation ΓK∼θ3, and at high angles, in the omega region, the tension peaks and then decreases with increasing angle. Raising the segregation in the melt causes a monotonic increase in the grain boundary tension; at strong segregations, the tension should scale as ΓK∼χ1/2, where χ is the Flory-Huggins interaction parameter. Both compositional and conformational asymmetries in the diblock molecule produce a decrease in ΓK.
Physical aspects and quantitative theory of time resolved spectroscopy of high molecular Rydberg states
Authors: Remacle F. , Levine R. D.
Published: July 2002

The qualitative physical aspects and the quantitative description of time and frequency resolved absorption spectroscopy of high molecular Rydberg states are discussed. The frequency is that of the excitation laser and the time is the independently variable delay before detection. The discussion allows for the presence of a weak external electrical field. The essential new ingredient is the finite slice of Rydberg states that are detected (=are in the detection window) and the variation of this population with time due to the coupling of the Rydberg electron with the molecular core. Line shapes are provided showing the effect of the depth of the detection window and the advantages and limitations imposed by the finite width of the excitation laser. The sharpening of the spectrum as the delay time to detection is increased is also illustrated. The quantitative theory is expressed in terms of the expectation value of a detection operator, describing the range of states that can be ionized by the delayed field, taken over a wave function. This wave function is the state of the system at the time of detection. However, even just at the end of the excitation stage, due to the interseries coupling, this wave function is not identical to the state that is directly optically accessed. The time correlation function of this wave function, obtained as a Fourier transform of the frequency resolved spectrum, is shown to provide further insight into the dynamics, the more so when the excitation laser has a narrow width in frequency.
Many-particle effects on the relaxation kinetics of fast reversible reactions of the type A+B⇄C
Authors: Sung Jaeyoung , Shin Kook Joe , Lee Sangyoub
Published: July 2002

Effects of pair correlations between bound and unbound molecules on the relaxation kinetics of fast reversible reactions of the type A+B⇄C are investigated for one-, two-, and three-dimensional reaction systems in a unified manner. Starting from the many-body Smoluchowski equations for reactant molecule distribution functions, which are coupled in a hierarchical manner, we derive a set of reaction kinetic equations by using the dynamic superposition approximation. While most of previous theories are applicable only to the pseudo-first order case, the present theory is applicable to the second-order case as well. In the pseudo-first order case with B molecules present in excess of A molecules, we can consider two limiting situations where either A or B molecules are static. The present theory can deal with both limiting cases within a single theoretical framework. Previously, the two cases have been approached by using quite different theoretical formalisms. Simple analytic solutions are obtained that are applicable irrespective of the dimensionality of reaction system, and are shown to agree well with the full numerical solutions. The present theory shows that in the time dependence of the reactant concentration a transient rapid relaxation period precedes the well known long-time t−d/2 power-law decay phase, in agreement with the computer simulation results obtained by Edelstein and Agmon for the one-dimensional case.
Local frequency analysis and the structure of classical phase space of the LiNC/LiCN molecular system
Authors: Losada J. C. , Estebaranz J. M. , Benito R. M. , Borondo F.
Published: July 2002

The phase space structure of a generic Hamiltonian model, describing the vibrational dynamics of the LiNC/LiCN molecular system, is studied using a frequency analysis method. The results obtained for the regular region constitute a true parametrization of the corresponding invariant tori on which the trajectories are located. By performing the frequency analysis locally, much richer information about chaotic trajectories is obtained, since it clearly reveals the dynamical characteristics of trajectory fragments hidden behind the t→∞ ergodic property.
Anomalous autoionization lifetimes of Rydberg states in a circularly polarized microwave field
Authors: Bellomo Paolo , Farrelly David , Uzer T.
Published: July 2002

Recent experiments by Jones, Fu, and Gallagher [J. Chem. Phys. 106, 3578 (1997)] designed to mimic collisional effects on the lifetimes of ZEKE Rydberg states find that, in a circularly polarized microwave field, autoionization lifetimes of atomic Rydberg states exceed those in a linearly polarized field. Surprisingly, however, the lifetimes not only do not display the expected n5 scaling but are essentially independent of n for most of the states prepared in the experiment. Similar observations were made by Vrakking and Lee [J. Chem. Phys. 102, 8818 (1995)] in studies of NO Rydberg states. We present a classical model that predicts n-independent behavior of autoionization lifetimes over essentially the same range of n-values as in the experiments of Jones, Fu, and Gallagher. The origin of this behavior is traced to the particular combinations of field strengths and frequency used in the experiment.
Continuum elastic theory of adsorbate vibrational relaxation
Authors: Lewis Steven P. , Pykhtin M. V. , Mele E. J. , Rappe Andrew M.
Published: July 2002

An analytical theory is presented for the damping of low-frequency adsorbate vibrations via resonant coupling to the substrate phonons. The system is treated classically, with the substrate modeled as a semi-infinite elastic continuum and the adsorbate overlayer modeled as an array of point masses connected to the surface by harmonic springs. The theory provides a simple expression for the relaxation rate in terms of fundamental parameters of the system: γ=mω̄02/AcρcT, where m is the adsorbate mass, ω̄0 is the measured frequency, Ac is the overlayer unit-cell area, and ρ and cT are the substrate mass density and transverse speed of sound, respectively. This expression is strongly coverage dependent, and predicts relaxation rates in excellent quantitative agreement with available experiments. For a half-monolayer of carbon monoxide on the copper (100) surface, the predicted damping rate of in-plane frustrated translations is 0.50×1012 s−1, as compared to the experimental value of (0.43±0.07)×1012 s−1. Furthermore it is shown that, for all coverages presently accessible to experiment, adsorbate motions exhibit collective effects which cannot be treated as stemming from isolated oscillators.
Comment on “A study of HeI photoelectron spectroscopy on the electronic structure of the nitrate free radical NO3” [J. Chem. Phys. 106, 3003 (1997)]
Authors: Monks P. S. , Stief L. J. , Krauss M. , Kuo S. D. , Zhang Z. , Klemm R. B.
Published: July 2002

The recent photoelectron spectroscopy (PES) paper by Wang, et al. confirms our earlier photoionization mass spectrometric (PIMS) determination of the adiabatic ionization energy of NO3. The PES results are also consistent with our conclusion that the ground-state neutral geometry for NO3 is D3h. However, the PE spectrum shows a second band (and higher bands) with a vertical I.E. (ionization energy) at 13.18 eV that was absent in our photoionization efficiency spectrum. A possible reason for the apparent discrepancy is suggested and further differences between the two studies are discussed.
An empirical valence bond model for proton transfer in water
Authors: Sagnella Diane E. , Tuckerman Mark E.
Published: July 2002

A new empirical valence bond model for proton transfer in bulk water that includes electron correlation effects is presented. The parameters of the model are based on ab initio calculations, in which electron correlation is treated at the MP2 level. Within this model, the properties of the gas-phase H5O2+ complex are in good agreement with recent ab initio path integral studies [M. E. Tuckerman, D. Marx, M. L. Klein, and M. Parrinello, Science. 101, 4878 (1994)] and ab initio molecular dynamics studies [D. Wei and D. R. Salahub, J. Chem. Phys. 106, 6086 (1997)]. Simulations of the solvated H5O2+ complex suggest that at room temperature, the quantum nature of the transferring proton does not affect the essential mechanism of proton transfer and only slightly affects the free energy profile of the asymmetric stretch within the strong hydrogen bond. The predictions of the model are consistent with ab initio molecular dynamics simulations of solvated hydronium using gradient-corrected density functional theory [M. E. Tuckerman, D. Laasonen, M. Sprik, and M. Parrinello, J. Chem. Phys. 103, 150 (1995)].
Surface and bulk diffusion of HDO on ultrathin single-crystal ice multilayers on Ru(001)
Authors: Livingston Frank E. , Whipple Galen C. , George Steven M.
Published: July 2002

The kinetics of HDO surface and bulk diffusion on ultrathin (25–192 BL; 90–700 Å) single-crystal H216O ice multilayers were studied using a combination of laser-induced thermal desorption (LITD) probing and isothermal desorption depth-profiling. The single-crystal hexagonal ice multilayers were grown epitaxially on a single-crystal Ru(001) metal substrate with the basal (001) facet of ice parallel to the Ru(001) surface. HDO surface diffusion on the single-crystal ice multilayer was not observed within the resolution of the LITD experiment at T=140 K. These LITD surface diffusion experiments yielded an upper limit to the HDO surface diffusion coefficient of Ds⩽1×10−9 cm2/s at T=140 K. The bulk diffusion coefficients were measured along the c axis of the hexagonal ice crystal which is perpendicular to the (001) plane. HDO was observed to diffuse readily into the underlying H216O ice multilayer. The measured HDO bulk diffusion coefficients ranged from D=2.2(±0.3)×10−16 cm2/s to D=3.9(±0.4)×10−14 cm2/s over the temperature range from 153 to 170 K. The HDO bulk diffusion coefficients were measured for H216O thicknesses of 25–192 BL (1 BL=1.06×1015 molecules/cm2) and initial HDO adlayer thicknesses of 2–9 BL. The HDO bulk diffusion was independent of H216O film thickness and initial HDO coverage. Arrhenius analysis of the temperature-dependent bulk diffusion coefficients yielded a diffusion activation energy of EA=17.0±1.0 kcal/mol and a diffusion preexponential of Do=4.2(±0.8)×108 cm2/s. Compared with extrapolations from macroscopic diffusion kinetics obtained earlier at temperatures close to the melting point, these bulk diffusion coefficients are larger and may reflect the perturbation of the ultrathin ice films induced by the nearby interfaces. The differences between these HDO diffusion kinetics and recently measured kinetics for H218O indicate that H/D exchange and molecular transport make comparable contributions to the HDO diffusion coefficient.
A model of a local dielectric constant in proteins
Authors: Voges Dieter , Karshikoff Andrej
Published: July 2002

A model for the calculation of the scalar, position-dependent static dielectric function of an ensemble consisting of polarizable sites and of free relaxing point dipoles at fixed positions is proposed. It is based on classical electrostatics and leads to an iteration equation or nonlinear partial differential equation for the local dielectric constant. The expressions contain the equations of Debye, Onsager, and Neumann as special cases and thus might be considered as an extension to inhomogeneous matter. The model may have applications in the case of biological macromolecules in particular proteins where the polar side chains can be identified with the model’s dipoles. The algorithm leads to a position dependent dielectric constant in the protein interior in contrast to the assumption of a homogeneous permittivity throughout the protein. By taking into account the dielectric fine structure inside the macromolecules we hope that our approach may help to improve continuum electrostatic models of these molecules. The relevant polarizabilities and dipole moments of the amino acids are given and their corresponding local dielectric constants are estimated as a first approximation based on the Onsager and Kirkwood equations.
Deuteron spin-lattice relaxation times in D2–Ar mixtures: A combined experimental and theoretical study
Authors: Sabzyan Hassan , Power William P. , McCourt Frederick R. W.
Published: July 2002

A combined experimental and theoretical study of the NMR spin-lattice relaxation times for the deuteron of D2 in D2–Ar mixtures is presented. A gas-handling system and a sample cell have been designed and successfully employed in the experimental part of this study. Spin-lattice relaxation times for the deuteron of D2 in D2–Ar mixtures have been measured over the temperature range 180–420 K at several densities and mole fractions, and extrapolation to infinite dilution has been carried out. The quality of the results has been tested by analysis of the one-dimensional spectra. Theoretical values of the spin-lattice relaxation times associated with the D2–Ar interaction have been calculated using the XC(fit) potential energy surface obtained by Bissonnette et al. [J. Chem. Phys. 105, 2639 (1996)]. Two reliable methods have been proposed to compare the theoretical and experimental NMR spin-lattice relaxation times obtained for the equilibrium mixture of the two parity isomers of the D2–Ar system under conditions in which separate measurement of their deuteron relaxation times is not possible. The agreement between experimental and theoretical results is found to be relatively good only for higher temperatures. These results indicate that the anisotropies of the XC(fit) potential energy surface need refinement.
Correlated π-electronic states: Pyrene, 16-site polyene, and D2h symmetry adaptation
Authors: Wen G. , Soos Z. G.
Published: July 2002

Diagrammatic valence bond (DVB) theory is a general approach to electron correlations in quantum cell models that conserve total spin. VB diagrams are a convenient many-electron basis for combining spin, point-group, and other symmetries in oligomers with a large but finite basis. Half-filled Hubbard or Pariser–Parr–Pople (PPP) models with 16 sites have ∼34.7×106 singlet diagrams. Improved DVB methods yield exact low-lying states of the 16-site polyene in C2h symmetry and of pyrene in D2h symmetry. Several generalizations of symmetry adaptation are necessary for large bases, including new rules for linearly independent basis vectors and an iterative method for Hamiltonian matrix elements that avoids overlap and inversion. The number and dimensions of the disjoint invariant subspaces Sm encountered in symmetry adaptation depend on the connectivity. D2h symmetry adaptation is much simpler for acenes than for pyrene, linear stilbene, or polyphenyls. Standard PPP parameters account well for the 11 pyrene states identified in linear and two-photon spectra. Polyenes to N=16 show systematic deviations, with the 2 1Ag threshold decreasing less rapidly with N than experiment. We discuss perturbations that break electron-hole, parity, or point-group symmetry on the low-energy states, test the second π-electron approximation for pyrene, and comment on the feasibility of DVB for N=18 or 20 sites.
Monte Carlo studies of the thermodynamics and kinetics of reduced protein models: Application to small helical, β, and α/β proteins
Authors: Kolinski Andrzej , Galazka Wojciech , Skolnick Jeffrey
Published: July 2002

Employing a high coordination lattice model and conformational sampling based on dynamic and entropy sampling Monte Carlo protocols, computer experiments were performed on three small globular proteins, each representing one of the three secondary structure classes. The goal was to explore the thermodynamic character of the conformational transition and possible mechanisms of topology assembly. Depending on the stability of isolated elements of secondary structure, topology assembly can proceed by various mechanisms. For the three-helix bundle, protein A, which exhibits substantial helix content in the denatured state, a diffusion–collision mechanism of topology assembly dominates, and here, the conformational transition is predicted to be continuous. In contrast, a model β protein, which possesses little intrinsic denatured state secondary structure, exhibits a sequential “on-site” assembly mechanism and a conformational transition that is well described by a two-state model. Augmenting the cooperativity of tertiary interactions led to a slight shift toward the diffusion–collision model of assembly. Finally, simulations of the folding of the α/β protein G, while only partially successful, suggest that the C-terminal β hairpin should be an early folding conformation and that the N-terminal β hairpin is considerably less stable in isolation. Implications of these results for our general understanding of the process of protein folding and their utility for de novo structure prediction are briefly discussed.
Theoretical calculation of hydrogen molecule in silicon
Authors: Nakamura K. G. , Ishioka K. , Kitajima M. , Endou A. , Kubo M. , Miyamoto A.
Published: July 2002

First principle quantum chemical calculations have been performed on a hydrogen molecule in the silicon crystal using the cluster model of Si10H16. The ab initio molecular orbital theory and the density functional theory (DFT) calculations have been examined. In all calculations, the tetrahedral site is the most stable trapping site for the hydrogen molecule. The DFT calculations with generalized gradient approximation show that the bond length of H2 in the silicon crystal is comparable to that of gaseous H2. The calculated vibrational frequency of H2 in the silicon crystal agrees well with the experimental value obtained by Murakami et al. [Phys. Rev. Lett. 77, 3161 (1996)].
Microscopic solvent structure of supercritical carbon dioxide and its mixtures with methanol in the cybotactic region of the solute molecule
Authors: Bulgarevich Dmitry S. , Sako Takeshi , Sugeta Tsutomu , Otake Katsuto , Sato Masahito , Uesugi Masayuki , Kato Masahiro
Published: July 2002

UV/VIS absorption spectroscopy was used to estimate the microscopic solvent polarities of supercritical carbon dioxide alone and its mixtures with methanol. It was found that the local composition of methanol in the cybotactic region of 4-nitroanisole was larger than the corresponding bulk concentration. With increasing pressure the methanol molecules were pressed out of the solvation shell of 4-nitroanisole by the carbon dioxide molecules. From the solubility measurements the solvation numbers of the carbon dioxide molecules around 4-nitroanisole were calculated as a function of fluid density.
Electronic to vibrational and rotational energy transfer in S(1D)+CO quenching reaction: Ab initio MO and surface hopping trajectory studies
Authors: Tachikawa Hiroto
Published: July 2002

The collisional energy transfer reaction, S(1D)+CO→S(3P)+CO(v,J), has been studied by means of the ab initio MO (molecular orbital) and surface-hopping trajectory calculations. The potential-energy surfaces (PESs) calculated by the ab initio MO calculations showed that the singlet and triplet PESs [S(1D)+CO and S(3P)+CO] are composed of the attractive and repulsive potential curves, respectively. The strongly bond intermediate corresponding to SCO(1∑) is found on the collision region on the singlet PES. The singlet and triplet PESs are crossed each other at region of around r(S−C)=2.4 Å with the seam of the conical intersection. By using the fitted ab initio PESs, three-dimensional surface-hopping trajectory calculations are performed under the Landau–Zener approximation. The calculated rotational and vibrational state distributions of the product CO(v,J) are composed of two components due to the contributions from both direct and complex channels. The calculations show that the quenching probability decreases gradually with increasing collision energy. This decrease is due to the fact that the branching ratio of direct to complex channels is varied as a function of collision energy. The mechanism of the energy transfer was discussed on the basis of the theoretical results.
Femtosecond fifth-order nonlinear response of nuclear motion in liquids investigated by incoherent laser light. Part II. Experiment
Authors: Lau A. , Pfeiffer M. , Kozich V. , Tschirschwitz F.
Published: July 2002

A setup for detection of a fifth-order nonlinear Raman process based on incoherent laser light is developed, enabling the introduction of two independent delay times. Depending on phase matching conditions, interference of nonlinear processes of third- and fifth-order or a dominance of one of these processes can be obtained. Possibilities to distinguish between them are discussed and signals of fifth-order Raman-type processes are identified. Regarding the chosen time variable the delay curves are symmetric. Because the correlation time of the incoherent radiation is in the order of magnitude of the fast dephasing time in the liquids, a necessary deconvolution procedure is developed and applied to extract molecular data. A fast dephasing time obtained for nitrobenzene agrees with published data. In mixtures of nitrobenzene with trifluoroethanol the dephasing time decreases, additional excitation of an intramolecular vibration causes an increased effective dephasing times. Both effects could be simulated by independent molecular responses for both kinds of molecules or independent relaxation mechanisms, respectively. From the dependence of the measured FWHM times on the delay time between the two excitations, relations of two relaxation times can be deduced. This information is going beyond that obtainable from third-order coherent techniques.
Monte Carlo simulation of the phase separation dynamics of polymer blends in the presence of block copolymers. IV. Effects of chain length and composition of repulsive block copolymer
Authors: Kim Seung Hyun , Jo Won Ho
Published: July 2002

Monte Carlo simulation was used to study the effects of C-b-D block copolymers with repulsive interaction on the phase separation dynamics of A/B immiscible homopolymer blend. It is found that a repulsive block copolymer can significantly suppress the rate of phase separation of an immiscible binary blend when its interaction energies with the homopolymers satisfy a proper condition. It is also found that the longer the chain length of block copolymer, the better such a retardation effect. When the chain extension of block copolymer at the interface is examined, each block in the repulsive block copolymer is contracted in its corresponding homopolymer phase, whereas the attractive copolymer chain is extended at the interface. This may lead to less retardation effect of repulsive block copolymer as compared with attractive block copolymer. Unlike the attractive block copolymer, the repulsive block copolymer with an asymmetric composition (f=0.7) does not show better retardation than the symmetric one. However, in spite of some difference in retardation effect, both types of block copolymers (attractive and repulsive) obey the dynamic scaling law, and the scaled structure factors are universal with time, irrespective of copolymer structure.
Critical behavior of asymmetric polymer blend solutions: Poly(methyl methacrylate)/poly(dimethyl siloxane)/solvent
Authors: Miyashita Naoshi , Nose Takuhei
Published: July 2002

By means of light scattering, static and dynamic critical behavior was investigated for polymer blend solutions consisting of two polymers with highly asymmetric polymeric indices, that is, poly(methyl methacrylate)/poly(dimethyl siloxane)/benzene and poly(methyl methacrylate)/poly(dimethyl siloxane)/toluene solutions. Isothermal susceptibility ST and correlation length ξ in the benzene solution showed Fisher’s renormalized Ising behavior characteristic to ternary mixtures in the very limited region near the stability limit. On the contrary the toluene solution exhibited the renormalized behavior over the wide temperature range, reflecting asymmetric phase behavior arising from solvent selectivity of toluene. The temperature dependence of the critical-fluctuation decay rate Γ for both solutions exhibited “quasihydrodynamic” behavior far from the stability limit, which became the macroscopic hydrodynamic behavior with temperature approaching the critical point. We also measured the tracer-diffusion coefficient Dt of constituent polymers by pulsed-field-gradient nuclear magnetic resonance (NMR) to demonstrate that the quasihydrodynamic behavior may come from the dynamic asymmetry, i.e., difference in the translational mobility of Brownian motions between component molecules.
Generalized pathway model to compute and analyze tunneling matrix elements in proteins
Authors: de Andrade Paulo C. P. , Onuchic José N.
Published: July 2002

An iterative method is developed to generalize the original pathway model which identifies not simply the strongest pathway involved in the protein-mediation of electron transfer matrix elements but all the relevant ones. The generalized pathway model is a semianalytical, nonperturbative, and computationally tractable method that allows detail investigation of the electron tunneling propagation in the protein medium for any one-electron Hamiltonian, and to examine structure-function relationships in multiple pathway regimes that may be induced by hydrogen bond and through-space contacts in the folded protein structure. This method enables an understanding of how details of the 3-D protein structures control the tunneling process, and whether pathway interference in the protein structural motifs causes substantial deviation from standard pathway analysis.