ALBERT

Notes: CO adsorption on small cationic, neutral, and anionic Aun (n=1–6) clusters has been investigated using density functional theory in the generalized gradient approximation. Among various possible CO adsorption sites, the on-top (one-fold coordinated) is found to be the most favorable one, irrespective of the charge state of the cluster. In addition, planar structures are preferred by both the bare and the CO-adsorbed clusters. The adsorption energies of CO on the cationic clusters are generally greater than those on the neutral and anionic complexes, and decrease with size. The adsorption energies on the anions, instead, increase with cluster size and reach a local maximum at Au5CO−, in agreement with recent experiment. The differences in adsorption energies for the different charge states decrease with increasing cluster size. © 2002 American Institute of Physics.

Notes: We study the process of charged polymer translocation, driven by an external electric potential, through a narrow pore in a membrane. We assume that the number of polymer segments, m, having passed the entrance pore mouth, is a slow variable governing the translocation process. Outside the pore the probability that there is an end segment at the entrance pore mouth, is taken as the relevant parameter. In particular we derive an expression for the free energy as a function of m, F(m). F(m) is used in the Smoluchowski equation in order to obtain the flux of polymers through the pore. In the low voltage regime we find a thresholdlike behavior and exponential dependence on voltage. Above this regime the flux depends linearly on the applied voltage. At very high voltages the process is diffusion limited and the flux saturates to a constant value. The model accounts for all features of the recent experiments by Henrickson et al. [Phys. Rev. Lett. 85, 3057 (2000)] for the flux of DNA molecules through an α-hemolysin pore as a function of applied voltage. © 2002 American Institute of Physics.

Notes: The 364 nm negative ion photoelectron spectrum of AgO− is reported. Transitions to the A 2Σ+ state and the X 2Π1/2 and X 2Π3/2 spin–orbit states of AgO are observed. The electronic ground state of AgO− is found to have a vibrational frequency of 497 (20) cm−1 and an equilibrium bond length of 1.935 (15) Å. The electron affinity of AgO is found to be 1.654 (2) eV. © 2002 American Institute of Physics.

Notes: It is shown that the work by Farago and Gradzielski [J. Chem. Phys. 114, 10105 (2001)] is based on incorrect expressions for the scattering functions, contains a number of other serious defects, and should be revised. © 2002 American Institute of Physics.

Notes: We present a method for computing intermolecular energies of large molecules based on a suitable fragmentation scheme, which allows one to express the complete interaction energy as a sum of interaction energies between pairs of fragments. The main advantage consists in the possibility of using standard ab initio quantum methods to evaluate the fragment energies. For the 4-n-pentyl-4′-cyanobiphenyl (5CB) dimer, the present results indicate that the most favorite arrangement corresponds to an antiparallel side-by-side geometry with a stabilization energy of about 16 kcal/mol. It is shown that, by the present method, the interaction energy of the 5CB dimer can be evaluated for all geometrical conformations and, in principle, it can be used for bulk simulations. © 2002 American Institute of Physics.

Notes: Second-order quantized Hamiltonian dynamics (QHD-2) is mapped onto classical mechanics by doubling the dimensionality. The mapping establishes the classical canonical structure for QHD-2 and permits its application to problems showing zero-point energy and tunneling via a standard molecular dynamics simulation, without modifying the simulation algorithms, by introducing new potentials for the extra variables. The mapping is applied to the family of Gaussian approximations, including frozen and thawed Gaussians, which are special cases of QHD-2. The mapping simplifies numerous applications of Gaussians to simulations of spectral intensities and line shapes, nonadiabatic and other quantum phenomena. The analysis shows that frozen Gaussians conserve the total energy, while thawed Gaussians do not, unless an additional term is introduced to the equation of motion for the thawed Gaussian momentum. The classical mapping of QHD-2 is illustrated by tunneling and zero-point energy effects in the harmonic oscillator, cubic and double-well potential, and the Morse oscillator representing the O–H stretch of the SPC-F water model. © 2002 American Institute of Physics.

Notes: In this paper we present a new approach to simulation methods for classical statistical mechanics relying on a field-theoretical formalism. It is based on applying the complex Hubbard–Stratonovich transformation to the canonical and grand-canonical partition function, which allows one to reexpress their particle representation in terms of a functional integral over a fluctuating auxiliary field. The thermodynamic averages from the resulting field representations can then be calculated with a conventional Monte Carlo algorithm. We explored the applicability of the auxiliary field methodology for both the canonical and grand-canonical ensemble using a system of particles interacting through a purely repulsive Gaussian pair potential in a broad range of external parameters. In the grand-canonical case this technique represents an alternative to standard grand-canonical Monte Carlo methods. Generally providing a framework for simulating classical particle systems within a continuum formalism can be useful for multiscale modeling where the field or continuum description naturally appears within quantum mechanics on smaller length scales and within classical mechanics on larger ones.© 2002 American Institute of Physics.

Notes: We construct exchange energy functionals that depend on the non-interacting square kinetic energy density τ2(r)=〈fraction SHAPE="CASE"〉14∑k|Δφk(r)|2 rather than the electron density. Since τ2 is a non-local functional of the electron density, it may describe non-local effects beyond standard generalized gradient approximation models. These effects may be essential in dealing with such a non-local quantity as the exchange energy. The problem of v-representability of τ2 for a slightly perturbed electron gas and Coulomb systems is discussed. A gradient expansion technique that permits construction of gradient-corrected functionals of τ2 is developed, and the performance of the new exchange functionals is analyzed. © 2002 American Institute of Physics.

Notes: The near-dissociation microwave rovibronic spectra of HeN+ [Carrington et al., Chem. Phys. Lett. 262, 598 (1996)] are used to obtain coupled potential energy curves for the six electronic states correlating with He+N+ 3P0, 3P1, and 3P2. High-quality ab initio calculations are carried out, using a spin-restricted open-shell coupled-cluster method with an augmented correlation-consistent quintuple-zeta basis set (aug-cc-pV5Z). Fully coupled calculations of bound and quasibound states are performed, including all six electronic states, and suggest two possible assignments of the observed transitions. The potentials are then morphed (scaled) to reproduce the experimental frequencies. One of the two assignments, designated SH1, is preferred because it gives a more satisfactory explanation of the observed hyperfine splittings. The corresponding morphed potential has well depths of 1954 cm−1 and 192 cm−1 for the spin-free 3Σ− and 3Π curves, respectively. © 2002 American Institute of Physics.

Notes: Dispersed fluorescence (DF) and stimulated emission pumping (SEP) experiments have been carried out on the jet-cooled 7Li16O7Li molecule. Rotationally resolved SEP bands to υ2l(l=0,2) vibrational angular momentum levels show that LiOLi is a linear molecule. The DF experiments have resulted in the measurement of 54 (υ1,υ2l,0) levels of the X˜ 1Σg+ ground state, up to 6000 cm−1. The energy levels determined experimentally are amazingly close to those calculated very recently for a high-level ab initio LiOLi(X˜ 1Σg+) potential surface. The DF vibrational level energies are analyzed and discussed in terms of a global fit to an 11-term power series. From the SEP experiments, effective rotational constants Bυ1,υ2l,0 for several of these vibrational levels [including σg+(l=0) and δg(l=2) vibrational angular momentum states for (υ1,υ2l≥2(even),υ3=0) levels] have also been determined. The bond distance R0 is estimated to be 1.611±0.003 Å from an extrapolation of Bυ1,0,0 values. Finally, the unusual "Li+O−2Li+" ionic bonding and the low ω2=112 cm−1 bending frequency for LiOLi(X˜ 1Σg+) are briefly discussed. © 2002 American Institute of Physics.

Notes: The ionic dissociation of H–X acids (X=F, Cl, Br, I) in water was examined by conducting a theoretical study on the properties of the clusters formed by the acids with up to five water molecules: X–H(H2O)n (n=1–5). Calculations were done using the DFT/B3LYP and MP2 methods in conjunction with the TZVP basis set and allowed the identification of several minima on the potential surfaces for the clusters. Based on the results, the MP2 method predicts a lower tendency to ionization than does the DFT/B3LYP method; however, both methods provide similar results. The dissociation characteristics of the acids were examined in terms of various parameters including the lengths of the bonds involved in the proton transfer and the frequencies associated with the X–H and O–H stretching modes in the bonds taking part in the proton transfer. The successive incorporation of water molecules to the cluster was found to lengthen X–H distances and simultaneously decrease O(centered ellipsis)H distances. In addition, the X–H stretching frequency underwent a marked redshift; the signal disappeared in the ionized structures, at the expense of a new series of bands around 2800 cm−1 due to stretching vibrations of the O–H bond in the H3O+ ion. Hydrogen fluoride failed to dissociate in the clusters considered; in fact, while some structures were ionized, they were not the most stable configurations for the corresponding clusters. In HCl and HBr, the dissociated structure was the most stable in the clusters of four or more water molecules (n=4–5); however, HBr exhibited a stronger tendency to dissociating above n=3. Finally, HI exhibited dissociation at n〉2.© 2002 American Institute of Physics.

Notes: Midinfrared spectra of the 81Br−-H2 and I−-H2 anion complexes are measured in the H-H stretch region by monitoring the production of halide anion photofragments. The spectra, which are assigned to complexes containing ortho H2, exhibit rotationally resolved ∑-∑ bands whose origins are redshifted from the molecular hydrogen Q1(1) transition by 110.8 cm−1 (Br−-H2) and 74.1 cm−1 (I−-H2). The complexes are deduced to possess linear equilibrium structures, with vibrationally averaged intermolecular separations between the halide anion and H2 center of mass of 3.461 Å (Br−-H2) and 3.851 Å (I−-H2). Vibrational excitation of the H2 subunit causes the intermolecular bond to stiffen and contract by 0.115 Å (Br−-H2) and 0.112 Å (I−-H2). Rydberg–Klein–Rees inversion of the spectroscopic data is used to generate effective radial potential energy curves near the potential minimum that are joined to long-range potential energy curves describing the interaction between an H2 molecule and a point negative charge. From these curves the dissociation energies of Br−-H2 and I−-H2 with respect to isolated H2 (j=1) and halide fragments are estimated as 365 and 253 cm−1, respectively. © 2002 American Institute of Physics.

Notes: The depolarized light scattering spectra of the glassforming liquid ortho-terphenyl have been calculated in the low frequency region using molecular dynamics simulation. Realistic system configurations are produced by using a recent flexible molecular model and combined with two limiting polarizability schemes, both of them using the dipole-induced-dipole contributions at first and second order. The calculated Rayleigh spectral shape are in good agreement with the experimental results in a large temperature range. The analysis of the different contributions to the Rayleigh spectra emphasizes that the orientational and the collision-induced (translational) terms lie on the same time scale and are of comparable intensity. Moreover, the cross correlation terms are always found to be an important contribution to the scattering intensity. © 2002 American Institute of Physics.

Notes: We analyze the correlated motions of hydrogen bonded clusters in liquid hydrogen fluoride, methanol, and water using the Instantaneous Normal Mode approach. In the case of hydrogen fluoride and methanol, which form a topologically linear hydrogen bond network, the relevant cluster is a triplet formed by a molecule and its two neighbors. In the case of water, whose hydrogen bond structure has a local tetrahedral symmetry, the basic unit considered is the pentamer formed by a molecule and its four neighbors. For each of these clusters we identify, using symmetry arguments, the a priori modes describing the relative motions of the cluster molecules and introduce suitable projections in order to evaluate how much these modes contribute to the actual motions at different frequencies. In the case of hydrogen fluoride we confirm the assignment of a 50 rad/ps peak observed in the single and collective correlation function spectra to the stretching of the hydrogen bonded network. In the case of methanol the analysis of the correlated motions of the triplets shows that in the intermediate frequency range (around 25 rad/ps) a picture similar to what is observed in hydrogen fluoride applies, whereas the high frequency properties of the liquid (beyond 50 rad/ps) are mostly due to the asymmetric stretching motion. In the case of water we demonstrate that the a priori modes, based on the full tetrahedral symmetry of the water pentamer, do indeed mix strongly under the effect of the interaction with the neighbors. The results are related to the spectroscopic measurement. © 2002 American Institute of Physics.

Notes: Compared with classical search algorithms, Grover quantum algorithm [L. K. Grover, Phys. Rev. Lett. 79, 325 (1997)] achieves quadratic speedup and Brüschweiler hybrid quantum algorithm [R. Brüschweiler, Phys. Rev. Lett. 85, 4815 (2000)] achieves an exponential speedup. In this paper, we report the experimental realization of the Brüschweiler algorithm in a three-qubit nuclear magnetic resonance ensemble system. The pulse sequences are used for the algorithms and the measurement method used here is improved on that used by Brüschweiler, namely, instead of quantitatively measuring the spin projection of the ancilla bit, we utilize the shape of the ancilla bit spectrum. By simply judging the downwardness or upwardness of the corresponding peaks in an ancilla bit spectrum, the bit value of the marked state can be read out, especially the geometric nature of this read-out can make the results more robust against errors. © 2002 American Institute of Physics.

Notes: First-principles density-functional approach was used to obtain the crystal parameters and phonon spectra of γ-Mg2SiO4 (spinel and ringwoodite) as functions of pressure. The Grüneisen parameters at the Γ point were obtained and compared with the experimental data. Using quasiharmonic approximation, the total free energy of the crystal was calculated as a function of volume for several temperatures, hence, the bulk modulus and thermal expansion were found as a function of temperature. A good agreement between calculated and experimental values was obtained in a wide temperature range. © 2002 American Institute of Physics.

Notes: The magnetic clusters are usually described by an effective Hamiltonian acting in a fixed S space. This single-spin description neglects the mixing between states with different total spin S produced by anisotropic interactions, such as the local crystal field and the magnetic dipole-dipole coupling. We have developed a general and simple procedure, based on a perturbational approach, where the S-mixing effects are included in some new terms to be added to the original effective Hamiltonian. These terms contain some new operators, which depend on the anisotropic interactions and on the difference ΔS between the spin of the states involved in the mixing. Interestingly, these operators are very similar (in some specific cases equal) to the well known Stevens operator equivalents. A list of them for ΔS=1 and ΔS=2 and for second-order anisotropic interactions is presented. The method has been applied to study the S-mixing in two iron nanomagnets, Fe4 and Fe8. © 2002 American Institute of Physics.

Notes: A model characterizing water-like fluids confined in cylindrical micropores is presented. The equation of state was derived based on perturbation theory assuming that the reference system of hard spheres confined in the cylindrical pores is homogenous. The perturbed state accounts for fluid–fluid, fluid–wall, and hydrogen bonding interactions. Fluid–fluid and fluid–wall interactions are modeled as the pairwise sum of Lennard-Jones potentials. The hydrogen bonding model accounts for the open structure of liquid water, as well as for the fact that the hydrogen bonding capabilities of confined molecules are distorted. This model was used to analyze the dependence of the density of water inside the micropores on the density outside the pores, the pore radius, and the affinity of the pore walls for water molecules. For gas-phase adsorption, the model predicts that the density of water inside the pores depends on the fluid–wall interactions. The state of the adsorbed phase varies from the density of vapor outside the pores (for the hard sphere wall) to a bulk liquid-like density (for a hydrophilic sample). The predicted behavior of confined water in the presence of bulk liquid outside the pores was more interesting. The model predicts that for small pores of hydrophobic materials, the density of fluid inside the pores is much smaller than the bulk liquid density, i.e., vapor-like. However, as the radius and/or hydrophility are increased, the fluid density inside the pores approaches the bulk liquid density very rapidly. © 2002 American Institute of Physics.

Notes: This article constructs trajectories associated with various boundary conditions for the Smoluchowski equation on an interval. Single-particle diffusion processes are first constructed by taking the diffusion limits of random walks. The diffusion limit gives both boundary conditions which enforce the single-particle constraint and properties of underlying trajectories at those boundaries. Mean-field diffusions are then obtained as limits of sums of single-particle processes. The results help to interpret the application of diffusion models to both ion channels and wider pores that facilitate molecular transport across membranes. Potential applications to Brownian dynamics simulations are discussed. © 2002 American Institute of Physics.

Notes: An improved density matrix functional (DMF) combining the properties of the "corrected Hartree" (CH) and "corrected Hartree–Fock" (CHF) approximations is proposed. Functionals of the CH/CHF type and the closely related natural orbital functional of Goedecker and Umrigar (GU) are tested in fully variational finite basis set calculations of light atoms, the lowest energy singlet methylene, and, for the first time, potential energy curves of diatomic molecules. Although CH/CHF-style DMFs may give reasonable energies for atoms and molecules near equilibrium geometries, they predict unrealistically shallow minima in the potential energy curves for diatomic molecules with more than two electrons. The calculated CH and CHF molecular dissociation curves exhibit the same patterns of over- and under-correlations as the corresponding correlation energy plots for the homogeneous electron gas undergoing a transition from high to low densities. In contrast, the GU functional yields not only accurate atomic and molecular energies but also plausible dissociation curves. The reasons behind the observed performance are analyzed. © 2002 American Institute of Physics.

Notes: The sequential importance sampling method and its various modifications have been developed intensively and used effectively in diverse research areas ranging from polymer simulation to signal processing and statistical inference. We propose a new variant of the method, sequential importance sampling with pilot-exploration resampling (SISPER), and demonstrate its successful application in folding polypeptide chains described by a two-dimensional hydrophobic-hydrophilic (HP) lattice model. We show by numerical results that SISPER outperformed several existing approaches, e.g., a genetic algorithm, the pruned-enriched Rosenbluth method, and the evolutionary Monte Carlo, in finding the ground folding states of 2D square-lattice HP sequences. In a few difficult cases, the new method can find the ground states without using any prior structural information on the chain. We also discuss the potential applications of SISPER in more general problems. © 2002 American Institute of Physics.

Notes: The Ewald-like method for quasi-two dimensional systems proposed by M. Kawata and co-workers [J. Chem. Phys. 115, 4457 (2001)] is examined and compared to the method proposed by Sphor, Yeh, Berkowitz and others. Both methods are found numerically equivalent. © 2002 American Institute of Physics.

Notes: We investigate on small world networks (SWN's) the survival probability of immobile targets, which get annihilated by random walkers at first encounter. On SWN's we find (distinct from regular lattices, Cayley trees, and regular ultrametric spaces) that in general the survival probability cannot be directly related to the average number of distinct sites visited. We underline this finding with arguments related to the structural disorder of SWN's and through the derivation of a lower bound for the targets' decay. © 2002 American Institute of Physics.

Notes: Previously developed technique of charge exchange ionization in a collision cell outside the ion source, which detects ions originating from the collision gas in the cell, was used to find long-lived excited electronic states of monosubstituted benzene cations. The technique is based on the criterion that charge exchange between polyatomic species is efficient only when the energy of reaction is close to zero or negative (ΔE≤0), or the exoergicity rule. The B˜ 2B2 states of chlorobenzene, bromobenzene, benzonitrile, and phenyl acetylene cations were found to have long lifetimes (10 microseconds or longer) while excited electronic states with long lifetime were not detected for fluorobenzene, iodobenzene, toluene, nitrobenzene, and styrene cations. The long-lived states found were those displaying well-resolved vibrational structures in the photoelectron spectra. In particular, these were the states generated by removal of an electron from the in-plane nonbonding p orbitals of halogens or in-plane π orbitals of the triple bonds. © 2002 American Institute of Physics.

Notes: Infrared spectra of the weakly bound complexes N2O–4He, N2O–3He, and OCS–3He have been observed using a tunable diode laser to probe a pulsed supersonic jet expansion. The rotational structure of the bands was analyzed using a conventional asymmetric rotor Hamiltonian. The N2O–3He and OCS–3He spectra are mostly a type (ΔKa=0) in structure, with very weak b-type (ΔKa=±1) transitions, but for N2O–4He the a- and b-type components are both prominent. The fitted rotational parameters are consistent with roughly T-shaped structures with intermolecular separations around 3.4–3.5 Å for N2O–He and 3.8–3.9 Å for OCS–He. The angle between the N2O or OCS axis and the He position is about 80° for N2O–He and 65° for OCS–He. The vibrational band origins are slightly blueshifted from those of the free molecule, with the N2O–He shifts (+0.2 cm−1) being about twice the magnitude of the OCS–He shifts (+0.1 cm−1). The results are of particular interest since N2O and (especially) OCS have both been used as probes in experiments on ultracold helium nanodroplets. © 2002 American Institute of Physics.

Notes: Results of the QCICD/6-311++G(3df,3pd) ab initio calculations on the ground state of Ar2H+ are presented. With accurate method and basis sets, the potential energy surface for the ground state was scanned with more than 7000 points, and an analytic global potential energy surface was constructed based on these points. The properties such as the potential minima, the transition state, and the dissociating paths of [Ar–H–Ar]+ were discussed. The influence of the three-body interaction in this system was also investigated, and it is found that a potential based on the two-body additive interaction is not good to represent the Ar2H+ system. © 2002 American Institute of Physics.

Notes: The photoelectron spectra of the weakly bound KrO− anion are simulated using a theory which combines the atoms-in-molecule model for molecular electronic wave functions and the Rau–Fano model for photodetachment intensities [J. Chem. Phys. 112, 5852 (2000)]. The nonrelativistic potential energy curves of the anion are obtained from ab initio calculations. The calculated spectra and their temperature variation agree with the experimental data presented in the preceding article. The strong selectivity of the photodetachment process with respect to the symmetry of fine-structure components is rationalized and quantified. © 2002 American Institute of Physics.

Notes: Angular distributions of selected rotational states of NO(A 2Σ+,ν=0) products obtained in the 213 nm photodissociation of (NO)2 have been determined in a molecular beam by using the photofragment ion imaging technique. Specifically, images of NO(A,ν=0) products in N=0, 11, and 19 have been recorded, for which the maximum energies available to the NO(X 2Π) products are 2038, 1774, and 1278 cm−1, respectively. The recoil anisotropy parameter of the photofragments, βeff, decreases significantly at low center-of-mass translational energies from its maximum value of 1.36±0.05, and depends strongly on the rotational angular momentum of the photoproducts. This behavior is described well by a classical model that takes into account the transverse recoil component mandated by angular momentum conservation. For each of the observed NO(A) N states, highly rotating NO(X) levels are produced via planar dissociation, and the angular momenta are established at an interfragment separation of about 2.6 Å. For most of the center-of-mass translational energy range, both corotating and counterrotating fragments are produced, but at the lowest energies, only the latter are allowed. The correlated rotational energy distributions exhibit deviations from the behavior predicted by phase space theory, suggesting that exit-channel dynamics beyond the transition state influences the product state distributions. In this study, a new method for image reconstruction is employed, which gives accurate angular distributions throughout the image plane. © 2002 American Institute of Physics.

Notes: The theoretical treatment in Paper I [D. W. Miller and S. A. Adelman, J. Chem. Phys. 117, 2672, (2002), preceding paper] of the vibrational energy relaxation (VER) of low-frequency, large mass dihalogen solutes is extended to the VER of the high-frequency, small mass molecular hydrogen solutes H2 and D2 in a Lennard-Jones argon-like solvent. As in Paper I, values of the relaxation times T1 predicted by the theory are tested against molecular dynamics (MD) results and are found to be of semiquantitative accuracy. To start, it is noted that standard Lennard-Jones site–site potentials derived from macroscopic data can be very inaccurate in the steep repulsive slope region crucial for T1. Thus, the H–Ar Lennard-Jones diameter σUV is not taken from literature values but rather is chosen as σUV=1.39 Å, the value needed to make the theory reproduce the experimental H2/Ar gas phase VER rate constant. Next, by MD simulation it is shown that the vibrational coordinate fluctuating force autocorrelation function 〈F˜(t)F˜〉0 of Paper I decays roughly an order of magnitude more rapidly for the molecular hydrogen solutions than for the dihalogen solutions. This result implies a relatively slow decay for the molecular hydrogen friction kernels β(ω)=(kBT)−1∫0∞〈F˜(t)F˜〉0 cos ω tdt, yielding for the H2/Ar and D2/Ar systems at T=150 K physical millisecond values for T1=β−1(ωl) despite the high liquid phase vibrational frequencies ωl of H2 and D2. The rapid decay of 〈F˜(t)F˜〉0 is due to both the steepness of the repulsive slope of the H–Ar potential and the small masses of H and D. Thus, the small value chosen for σUV is needed to avoid unphysically long T1's. Next, an analytical treatment of the H2/D2 isotope effect on T1, based on the theory, is found to predict that the H2/Ar and D2/Ar T1's are close in value due to the compensating effects of lower ωl but slower decay of 〈F˜(t)F˜〉0 for D2/Ar, a result in qualitative agreement with experiments. Applying the theory to numerically study the isothermal ρ dependencies of the VER rate constant k(T,ρ)=T1−1 at 150 K reveals that for both H2/Ar and D2/Ar, as for the solutions of Paper I, k(T,ρ) can be factorized as in the isolated binary collision (IBC) model. Moreover, the molecular theory and IBC rate isotherms differ only slightly for both solutions, a result interpreted in terms of the form of the H–Ar pair correlation function. The theoretical and experimental rate isotherms at 150 K are then compared. Agreement is very good for the H2/Ar solution, but for the D2/Ar solution the theoretical rates are about four times too large. Finally, the isochoric T dependencies of k(T,ρ) in the range 200–1000 K are found for both solutions to conform to an Arrhenius rate law. © 2002 American Institute of Physics.

Notes: We report a molecular dynamics investigation of the glass transition temperature in selenium at pressures ranging from 0 to 6 GPa as a function of the quench rate, Qr. For moderate pressures the specific volume of the glass depends strongly on the quench rate, whereas the specific enthalpy varies only little. We find for both volume and energy a linear dependence on the quench-rate-dependent glass transition temperature. The slopes of these curves reflect the different energy scales of void formation, inter- and intrachain interactions. The extrapolated glass transition temperatures for quench rates of order K/s agree with the experimental ones within 20%. Applying a pressure of 1 GPa the glass transition temperature is raised by 37 K. For the same Qr, the transition temperature Tg is much higher for simulations using fixed volume conditions (NVT ensemble) than for the ones using fixed pressure (NPT ensemble) when one compares results for equal pressure at T=0. © 2002 American Institute of Physics.

Notes: We present an efficient transfer matrix formalism for obtaining the quantum conductivity of single-wall carbon nanotubes (SWCN's) based on a nonorthogonal tight-binding scheme. The formalism is used to calculate conductivity in the presence of topological defects and H adsorbates. I-V characteristics show large oscillatory behavior as a function of the number of H adatoms for both (10,0) and (5,5) SWCN's. Furthermore, the conductivity is found to depend sensitively on structural relaxation. © 2002 American Institute of Physics.

Notes: We report a new method to generate focused phonon polaritons inside ferroelectric crystals. Using an excitation pulse in the form of a ring we generate a phonon-polariton wave packet that propagates inward toward a focal point at the center. This results in a sharp increase in the polariton intensity. It also permits direct visualization and quantitative measurements of the Gouy phase shift and of anisotropy in polariton propagation. Since vibrational and electromagnetic modes are coupled, coherent vibrational amplitudes can be increased substantially through exploitation of electromagnetic wave propagation. © 2002 American Institute of Physics.

Notes: Catalytic formation of N2O and NO2 were studied employing density functional theory with generalized gradient approximations, in order to investigate the microscopic reaction pathways of these catalytic processes on a Pt(111) surface. Transition states and reaction barriers for the addition of chemisorbed N or chemisorbed O to NO(ads) producing N2O and NO2, respectively, were calculated. The N2O transition state involves bond formation across the hcp hollow site with an associated reaction barrier of 1.78 eV. NO2 formation favors a fcc hollow site transition state with a barrier of 1.52 eV. The mechanisms for both reactions are compared to CO oxidation on the same surface. The activation of the chemisorbed NO and the chemisorbed N or O from the energetically stable initial state to the transition state are both significant contributors to the overall reaction barrier Ea, in contrast to CO oxidation in which the activation of the O(ads) is much greater than CO(ads) activation. © 2002 American Institute of Physics.

Notes: The adiabatic ionization potential of methylene has been determined to be 83772±3 cm−1 from a rotationally resolved photoelectron spectroscopic study of the CH2+ X˜ 2A1 (0,0,0)←CH2 X˜ 3B1(0,0,0) transition. This value was used to determine thermochemical quantities such as the 0 K dissociation energy of the ketene cation in CO and CH2+ D0(CH2(Double Bond)CO+)=33202±7 cm−1, the 0 K dissociation energy of the methyl radical D0(CH2–H)=38179±49 cm−1, the 0 K dissociation threshold of methane in CH2 and H2 D0(CH2–H2)=38232±50 cm−1 and the 0 K enthalpy of formation of CH2 ΔfH(minus sign in circle)(CH2,T=0 K)=390.73±0.66 kJ mol−1. © 2002 American Institute of Physics.

Notes: A new method for calculating the ground-state tunneling splitting is presented. It is based on the semiclassical theory including recently derived corrections and it is the first method, which explicitly takes into account the whole conformational space between the minima and the transition state. The density-functional theory is used to determine the qualitative shape of the potential energy surface (PES) and high level ab initio calculations provide information about the stationary points. With a dual level scheme, the low-level energy surface is mapped onto the high-level points to get a good quantitative description of the high-level PES. Therefore, the new method requires no adjustment of additional parameters like scaling of the energy barrier as is necessary in other methods. Once the high-level PES is calculated, the most probable tunneling paths are determined with a global optimization procedure. Along this representative tunneling path, the tunneling splitting is calculated with additional consideration of zero-point vibrational effects. The method is applied to three molecular systems, namely hydrofluoric acid dimer, malonaldehyde, and tropolone. These systems were chosen because their energy barriers differ strongly (1 kcal/mol–7 kcal/mol). The predicted tunneling splittings agree very well with the experimental ones, therefore, we expect our method to be generally applicable, independent of the magnitude of the energy barrier. © 2002 American Institute of Physics.

Notes: We present an ab initio theoretical study of five low-energy isomers of the water hexamer {Chair, Cage(du)[1], Book, Prism, and Boat}, their intramolecular vibrations, binding energies De and dissociation energies D0. Møller–Plesset second order perturbation calculations using the aug-cc-pVTZ basis set at aug-cc-pVDZ optimized geometries including vibrational zero point energy corrections predict Chair to be the most stable isomer, followed closely by Cage(du)[1] (+0.02 kcal/mol) and Book (+0.05 kcal/mol), while Prism is 0.15 kcal/mol higher. The Boat conformer is least stable at both the De and D0 levels. The main focus is on the intramolecular normal modes of the five isomers. The calculated O–H stretching frequencies and intensities are compared to recent infrared spectra of water hexamer in supersonic jets, liquid-helium droplets and solid para-hydrogen matrices. The IR spectra indicate that Book and Chair are major species in the latter two environments and may also exist in supersonic jets. The (H2O)6 gas phase interconversion equilibria are calculated and predict that the most abundant isomer is Chair below 8 K, Cage between 8–26 K, and Book above 26 K. Several of the low-frequency vibrational modes are identified as low-amplitude precursors of the Chair↔Book↔Cage isomerization pathways. © 2002 American Institute of Physics.

Notes: Singlet excited states and ionized states of aniline are studied by the symmetry adapted cluster/configuration interaction method. Absorption bands of states that have mainly π–π* nature are assigned as 1A″ (∼1B2), 1A′ (∼1A1), 1A″ (∼1B2), 1A′ (∼1A1), 1A″ (∼1B2) in increasing-energy order. An s-Rydberg state is predicted to lie between the first and second valence states, in agreement with recent experimental results. The lowest band has a charge-resonance character with a slight charge-transfer (CT) character (CT is defined as NH2→C6H5); third and fifth valence bands have back-CT (BCT) nature, and second and fourth are local excitations within the benzene ring. The extent of CT of excited states depends on amino group conformation. In the planar form, CT characters of several states were altered; however, spectral shapes are very similar to that of the equilibrium form. On the other hand, amino group twisting altered both the spectrum and nature of excited states. Third and fourth lowest valence states exhibited strong CT character, while fifth to eighth states are of the strong BCT type, implying that the CT nature of excited states of aniline can be changed by amino group twisting. For ionized states, the lowest three states are assigned to 2A′ (∼2B1), 2A″ (∼2A2), 2A′ (∼2B1) in increasing-energy order, all being π-ionizations. The sixth one is also due to π-ionization (∼2B1) and the others are σ-ionizations. Ordering was the same as Koopmans' case. © 2002 American Institute of Physics.

Notes: A sensitive experimental method for ion spectroscopy and state specific reaction dynamics is described, briefly called laser induced reactions (LIR). The technique is based on (i) trapping ions over a long time in a cold 22-pole rf ion trap followed by mass spectrometric detection, (ii) providing a suitable low density gas environment for collisions, (iii) modifying the low temperature chemical kinetics using selective excitation via a tunable radiation source. In this paper, the H-atom transfer reaction C2H2+ (v3=1,J)+H2→C2H3++H, is used to monitor the infrared excitation of acetylene ions. Rotationally resolved spectra are presented for the antisymmetric C–H stretching vibration. For recording a spectrum, it is sufficient to fill the trap with a few thousand parent ions. Differences with respect to conventional IR spectroscopy are discussed, especially the processes which influence the LIR signal. From the measured intensities and their dependence on parameters such as storage time, laser fluence and target gas density, information on state specific rate coefficients has been obtained at an ambient temperature of 90 K. Based on a model simulating the kinetics, rate coefficients for various inelastic and reactive collisions are derived. Vibrational excitation of C2H2+ (v3=1,J) increases the rate of the title reaction by more than three orders of magnitude, while rotation hinders the reaction. The fine-structure state of the parent ion does not affect its reactivity. Ways are pointed out to apply the method to various classes of molecular ions. © 2002 American Institute of Physics.

Notes: The hyperfine coupling constants (hfcc) and electron-spin g-factors (magnetic moments) calculated for B2+, Al2+, Ga2+, BAl+, BGa+, and AlGa+ are reported. The hfcc's are obtained with single-reference configuration interaction, second-order Møller–Plesset, density functional (B3LYP, PW91PW91) methods, and 6-311+G(2df ) basis sets. The 2σg/3σ SOMOs of X 2Σg+(1σg21σu22σg)/X 2Σ+(1σ22σ23σ) mainly have a pσ–pσ composition, leading in most cases to similar values of Adip and Aiso. As a result, |A(parallel)| is up two orders of magnitude larger than |A⊥|. The A⊥'s are slightly negative (ca. −10 MHz) for Al2+, Ga2+, and AlGa+. The g-shifts (Δg=g−ge) are evaluated with multireference CI wave functions, perturbation expansions up to second-order, and 6-311+G(2d) basis sets. Both Δg(parallel) and Δg⊥ are negative, but Δg(parallel) lies close to zero. The Δg⊥'s of B2+, Al2+, Ga2+ are about −1 300, −12 800, −97 300 ppm, respectively, while for BGa+, BAl+, AlGa+, they are much smaller (−800, −2 800, −47 400 ppm). The reduced Δg⊥'s for XY+ result from the mutual cancellation between a positive contribution from the 1 2Π(3σ→1π) state but a negative one from 2 2Π(3σ→2π). The positive contribution is at variance with the rule-of-thumb stating that SOMO→virtual MO excitations should contribute negatively. The variation of the hfcc's with bond distance is analyzed for all systems, and that of the Δg⊥ component for B2+ and BAl+. Experimental or previous theoretical electron-spin resonance data are not available for comparison. © 2002 American Institute of Physics.

Notes: We propose two realistic schemes for producing electron spin-polarized alkaline-earth ions. One of the schemes is based on one-photon resonant two-photon ionization via fine structure manifolds of triplet states, while the other is based on one-photon resonant one-photon ionization. For both schemes we find that the ionization must take place from a triplet state to produce spin-polarized photoions. Photoions produced from a singlet state are always unpolarized regardless of the laser polarization and a magnetic sublevel from which ionization takes place. We carry out specific analysis for both schemes with Sr atoms, and find 85%–90% and 66% polarizations for the former and the latter schemes, respectively, if the wavelengths and polarizations of lasers are judiciously chosen. In particular, if all the produced photoions are in the ground s state, we find an exact one-to-one correspondence between the spin-polarization of photoions and ejected photoelectrons. © 2002 American Institute of Physics.

Notes: We propose a detailed investigation of the schematic mode-coupling approach to experimental data, a method based on the use of simple mode-coupling equations to analyze the dynamics of supercooled liquids. Our aim here is to clarify different aspects of this approach that appeared so far uncontrolled or arbitrary, and to validate the results obtained from previous works. Analyzing the theoretical foundations of the approach, we first identify the parameters of the theory playing a key role and obtain simple requirements to be met by a schematic model for its use in this context. Then we compare the results obtained from the schematic analysis of a given set of experimental data with a variety of models and show that they are all perfectly consistent. A number of potential biases in the method are identified and ruled out by the choice of appropriate models. Finally, reference spectra computed from the mode-coupling theory for a model simple liquid are analyzed along the same lines as experimental data, allowing us to show that, despite the strong simplification in the description of the dynamics it involves, the method is free from spurious artifacts and provides accurate estimates of important parameters of the theory. The only exception is the exponent parameter, the evaluation of which is hindered, as for other methods, by corrections to the asymptotic laws of the theory present when the dynamics is known only in a limited time or frequency range. © 2002 American Institute of Physics.

Notes: The reorientational relaxation of nonlinear molecules in liquids is treated using the site–site generalized Langevin/mode-coupling theory. We found an inconsistency between the rank-1 reorientational correlation functions of different vectors on a molecule when the molecule is nearly planar. We show that it is because the coupling between the torque and the acceleration of different rotational modes is missing in the theory. A modification of the theory is proposed to incorporate this coupling, and the inconsistency between the reorientational correlation functions is remedied by the modification. We also apply the modified theory to the reorientational motion of water. The rotational part of the memory function becomes greater compared with the conventional theory, and it approaches to that from the molecular dynamics simulation. The charge-current spectrum of water is also shown to be improved by the modification. © 2002 American Institute of Physics.

Notes: We report on a Monte Carlo study of the liquid crystal phases of two model fluids of linear elongated molecules: (a) hard spherocylinders with an attractive square-well (SWSC) and (b) purely repulsive soft spherocylinders (SRS), in both cases for a length-to-breadth ratio L*=5. Monte Carlo simulations in the isothermal–isobaric ensemble have been performed at a reduced temperature T*=5 probing thermodynamic states within the isotropic (I), nematic (N), and smectic A (Sm A) regions exhibited by each of the models. In addition, the performance of an entropy criterion to allocate liquid crystalline phase boundaries, recently proposed for the isotropic–nematic transition of the hard spherocylinder (HSC) fluid, is successfully tested for the SWSC and the SRS fluids and furthermore extended to the study of the nematic–smectic transition. With respect to the more extensively studied HSC fluid, the introduction of the attractive square well in the SWSC model brings the I–N and N–Sm A transitions to higher pressures and densities. Moreover, the soft repulsive core of the SRS fluid induces a similar but quite more significant shift of both of these phase boundaries toward higher densities. This latter effect is apparently in contrast with very recent studies of the SRS fluid at lower temperatures, but this discrepancy can be traced back to the different effective size of the molecular repulsive core at different temperatures. © 2002 American Institute of Physics.

Notes: Athermal, tethered chains are modeled with density functional (DFT) theory for both the explicit solvent and continuum solvent cases. The structure of DFT is shown to reduce to self-consistent-field theory in the incompressible limit where there is symmetry between solvent and monomer, and to single-chain-mean-field (SCMF) theory in the continuum solvent limit. We show that by careful selection of the reference and ideal systems in DFT theory, self-consistent numerical solutions can be obtained, thereby avoiding the single chain Monte Carlo simulation in SCMF theory. On long length scales, excellent agreement is seen between the simplified DFT theory and molecular dynamics simulations of both continuum solvents and explicit-molecule solvents. In order to describe the structure of the polymer and solvent near the surface it is necessary to include compressibility effects and the nonlocality of the field. © 2002 American Institute of Physics.

Notes: Atomic layer deposition (ALD) of hafnium oxide (HfO2) using HfCl4 and H2O as precursors is studied using density functional theory. The mechanism consists of two deposition half-reactions: (1) HfCl4 with Hf-OH sites, and (2) H2O with Hf-Cl sites. Both half-reactions exhibit stable intermediates with energies lower than those of the final products. We show that increasing the temperature reduces the stability of the complex. However, increasing temperature also increases the dissociation free-energy barrier, which in turn results in increased desorption of adsorbed precursors. Both half-reactions are qualitatively similar to the corresponding reactions of ZrO2 ALD using ZrCl4 and H2O. © 2002 American Institute of Physics.

Notes: Electron pair correlation contribution from individual electron pairs of the molecular orbitals is defined and used in analyzing the correlation patterns of F2, O22−, and CH3CH3 isoelectronic covalent systems. Based on our simple strategy so-called "separating large system into smaller ones" for estimating the correlation energies by investigating both the ionic and the diradical partitioning schemes for covalent systems, a simple scaling scheme is presented for estimating the total correlation energy. It is achieved by summing the scaled correlation energies of its ionic fragments derived from the ionic partitioning scheme rather than from the diradical partitioning scheme. Of the three estimated results, the absolute deviations are less than ±0.29 kcal/mol, however, using this simple scaling approach, at least 90% of computational work can be saved. At the present condition with the computational demand for calculating the electron correlation energies of large covalent molecules, it is hopeful that this simple scaling approach could be useful to estimate the correlation energies of large CH-containing alkaline compounds. © 2002 American Institute of Physics.

Notes: A simple relation between magnetic shielding and magnetizability has been found. Its validity has been first shown by numerical calculations and than proven analytically. This fundamental relation shows that the magnetic shielding tensor integrated over a whole space is simply proportional to the magnetizability tensor with a constant factor equal to two-thirds of the vacuum permeability with a negative sign. © 2002 American Institute of Physics.

Notes: The reaction dynamics of ground-state atomic oxygen [O(3P)] with allyl radicals (C3H5) has been investigated by applying a combination of crossed beams and laser induced fluorescence techniques. The reactants O(3P) and C3H5 were produced by the photodissociation of NO2 and the supersonic flash pyrolysis of precursor allyl iodide, respectively. A new exothermic channel of O(3P)+C3H5→C3H4+OH was observed and the nascent internal state distributions of the product OH (X 2Π:υ″=0,1) showed substantial bimodal internal excitations of the low- and high-N″ components without Λ-doublet and spin–orbit propensities in the ground and first excited vibrational states. With the aid of the CBS-QB3 level of ab initio theory and Rice–Ramsperger–Kassel–Marcus calculations, it is predicted that on the lowest doublet potential energy surface the major reaction channel of O(3P) with C3H5 is the formation of acrolein (CH2CHCHO)+H, which is consistent with the previous bulk kinetic experiments performed by Gutman et al. [J. Phys. Chem. 94, 3652 (1990)]. The counterpart C3H4 of the probed OH product in the title reaction is calculated to be allene after taking into account the factors of reaction enthalpy, barrier height and the number of intermediates involved along the reaction pathway. On the basis of population analyses and comparison with prior calculations, the statistical picture is not suitable to describe the reactive atom-radical scattering processes, and the dynamics of the title reaction is believed to proceed through two competing dynamical pathways. The major low N″-components with significant vibrational excitation may be described by the direct abstraction process, while the minor but extraordinarily hot rotational distribution of high N″-components implies that some fraction of reactants is sampled to proceed through the indirect short-lived addition-complex forming process. © 2002 American Institute of Physics.

Notes: Prospects for the existence and detection of yet unknown dicyanoacetylene (NCCCCN) isomers are discussed, based on quantum-chemical calculations for linear, hexagonal and branched C4N2 structural variants. It is concluded that apart from dicyanoacetylene itself and its two already discovered isomers, NCCCNC and CNCCNC, at least two other species are of importance: linear CCCNCN and Y-shaped CC(CN)CN (dicyanovinylidene). Combined CCSD(T) and MP4 calculations predict CC(CN)CN and CCCNCN to be 57 kcal/mol and 66 kcal/mol less stable than dicyanoacetylene, respectively. The height of the energy barrier for dicyanoacetylene←dicyanovinylidene isomerization is about 5 kcal/mol. Density functional theory calculations indicate that CCCNCN should give rise to prominent IR absorption bands, two orders of magnitude stronger than those of dicyanoacetylene. © 2002 American Institute of Physics.

Notes: The photodissociation dynamics of jet-cooled BrCl molecules have been investigated at four different wavelengths in the range 425–485 nm by high-resolution velocity map ion imaging. Four images of the Cl(2P3/2) atomic fragments are recorded at each photolysis wavelength with the probe laser polarization, respectively, linearly aligned and vertical (i.e., perpendicular to the detection axis), right circularly polarized, horizontally linearly polarized (i.e., parallel to the detection axis) and left circularly polarized on successive laser shots, thereby ensuring automatic mutual self-normalization. Appropriate linear combinations of these images allow quantification of the angular momentum alignment of the Cl(2P3/2o) fragments [i.e., the correlation between their recoil velocity (v) and their electronic angular momentum (J)] in terms of the alignment anisotropy parameters s2, α2, η2, and γ2, and determination of the "alignment-free" recoil anisotropy parameter, β0, as a function of parent excitation wavelength. Both incoherent and coherent contributions to the alignment are identified, with both simultaneous parallel and perpendicular excitations to the B 3Π(0+) and C 1Π(1) states and excitations to the Ω=±1 components of the C state contributing to the latter. The deduced values of the alignment-free β parameters indicate (wavelength dependent) contributions from both parallel and perpendicular parent absorptions in this wavelength range. Such a conclusion accords with approximate deconvolutions of the parent absorption spectrum that are currently available, and with determinations of the orientation parameter γ1′ obtained by fitting the difference image obtained when using left and right circularly polarized radiation to probe the ground state Cl atoms arising in the 480.63 nm photodissociation of BrCl when the photolysis laser radiation is polarized linearly at 45° to the detection axis. © 2002 American Institute of Physics.

Notes: Resonantly-enhanced two photon ionization and mass-analyzed threshold ionization (MATI) spectra of 2-aminopyridine (2AP–NH2) and its deuterated analogs have been obtained using two-photon (1+1′) excitation process via S1 intermediate states for ionization. Ionization energies of 2AP–NH2 and 2AP–ND2 are both precisely and accurately determined to be 8.1086±0.0005 and 8.1027±0.0005 eV, respectively. Two geometrical isomers, 2AP–NHD or 2AP–NDH, of which H or D is hydrogen-bonded to the nitrogen atom on the pyridine ring, respectively, are spectroscopically well isolated using the hole-burning spectroscopy in the S1 states. Corresponding ionization energies are thus separately determined to be 8.1067±0.0005 or 8.1048±0.0005 eV for 2AP–NHD or 2AP–NDH, respectively. Vibrational bands of 2-aminopyridine ions associated with various aromatic ring-skeletal modes are identified in the MATI spectra and appropriately assigned with the aid of ab initio calculation. All of the ring-skeletal vibrational frequencies observed in this work become slightly higher than those in the S1 states when the molecules are ionized, consistent with the fact that the S1–S0 excitation is due to π*–π transition. According to ab initio calculation, the amino group is in the molecular plane both in the S1 and D0 states, while it is slightly distorted in the ground state of 2-aminopyridine. Inversion modes in 2AP–ND2 and 2AP–NDH in S1 states are split into two bands due to their strong coupling with the other mode, which is most probably due to torsional motion of the amino group. Strong mode couplings are clearly manifested in interferencelike patterns observed in vibrational band structures of MATI spectra taken via those two bands in S1 states as intermediate states. A new spectroscopic scheme, in which MATI signals are used for obtaining mode-resolved spectra for the intermediate state is introduced. The vibrational band at 911 cm−1 from the S1–S0 origin that has been previously assigned as the inversion mode of 2AP–NH2 is found to actually consist of two closely-spaced different modes giving two clearly-resolved different Franck–Condon active modes in corresponding MATI spectra. © 2002 American Institute of Physics.

Notes: Transport properties of pure carbon dioxide have been calculated from the intermolecular potential using the classical trajectory approach. Results are reported for shear viscosity, viscomagnetic coefficients, and self-diffusion in the dilute-gas limit and in the temperature range of 200–1500 K for the three recently proposed carbon dioxide potential energy hypersurfaces. Agreement with the measurements is, in general, within the experimental error. The calculations indicate that the corrections in the second-order approximation and those due to the angular-momentum polarization for the viscosity are small, 〈1% in the temperature range considered. The very good agreement of the calculated values for the Bukowski et al. potential energy hypersurface (1999) with the experimental viscosity data is consistent with the rigid-rotor assumption made in the calculations being reasonable for the three properties considered. © 2002 American Institute of Physics.

Notes: The effect of hydrogen–deuterium substitution on the rate of rotational diffusion of two species of amino-substituted anthraquinone dyes dissolved in the isotropic liquid phase of 4′-n-pentyl-4-cyanobiphenyl (5CB) and in a liquid mixture of alkanes (paraffin) has been studied as a function of temperature. The rotational dynamics was probed by time-resolved detection of fluorescence depolarization. In 5CB we observe a reduced rotational mobility of the deuterated species with respect to the protonated ones by up to 43% (at 311 K) and a corresponding increase of 4–5 kJ/mol in the activation energy, as deduced from the temperature dependence. To our knowledge, this is the largest isotopic effect ever reported for the molecular rotational diffusion in liquids. In liquid paraffin the effect vanishes. We attribute our findings to an isotopic effect in the breaking kinetics of the hydrogen-bond between the amino groups of dye molecules and the cyano group of 5CB that cannot be explained with the known isotopic effect of hydrogen-bond stability. © 2002 American Institute of Physics.

Notes: The effects caused by the chemical reactions on the transport coefficients of a mixture of ideal gases which undergo a simple reversible symmetric reaction of the type A+A(r harp over l)B+B are analyzed within the framework of a kinetic theory based on the Boltzmann equation. The analyzed system is closed to the final stage of a chemical reaction where the affinity is considered to be a small quantity and the system tends to the chemical equilibrium. This kind of reaction is known as "fast" reactions, because the reactive processes are of the same order as the elastic ones. The internal degrees of freedom of the molecules of the gas are not taken into account. This enables to represent the gas molecules as rigid spheres and therefore to determine the coefficients of thermal conductivity, diffusion, thermal-diffusion ratio, shear viscosity, and the forward reaction rate analytically. It was verified that some factors—among others, the concentration of the reagents (or products of the reaction), the activation energy, the heat of reaction, and the steric factor—can affect in a significant way the transport coefficients in reactive gas mixtures. © 2002 American Institute of Physics.

Notes: Density functional theory has been used to study the interaction of molecular and atomic hydrogen with (5,5) and (6,6) single-wall carbon nanotubes. Static calculations allowing for different degrees of structural relaxation are performed, in addition to dynamical simulations. Molecular physisorption inside and outside the nanotube walls is predicted to be the most stable state of those systems. The binding energies for physisorption of the H2 molecule outside the nanotube are in the range 0.04–0.07 eV. This means that uptake and release of molecular hydrogen from nanotubes is a relatively easy process, as many experiments have proved. A chemisorption state, with the molecule dissociated and the two hydrogen atoms bonded to neighbor carbon atoms, has also been found. However, reaching this dissociative chemisorption state for an incoming molecule, or starting from the physisorbed molecule, is difficult because of the existence of a substantial activation barrier. The dissociative chemisorption deforms the tube and weakens the C(Single Bond)C bond. This effect can catalyze the shattering and scission of the tube by incoming hydrogen molecules with sufficient kinetic energy. © 2002 American Institute of Physics.

Notes: The NO/NiO(100) system represents an excellent test case for the theory of surface chemical bond since accurate information about geometry, adsorption strength, and spin properties is available from experiments performed on NiO and Ni-doped MgO powders, single crystals, and thin films. We used cluster models to describe the NO/NiO interaction in combination with density functional theory (DFT) and wave function-based methods. We have identified four major aspects of the interaction: (1) the bonding cannot be described by a single determinant; (2) a spin-polarized DF-B3LYP approach gives reasonable adsorption properties at the price of a physically incorrect spin distribution; (3) a key ingredient of the interaction is the Coulomb repulsion within the Ni 3d shell; since this term is described very differently depending on the exchange-correlation functional it can result in overbound generalized gradient approach or Becke, Lee, Yang, and Parr or in strongly unbound (HFLYP) systems depending on the DFT approach; (4) the proper inclusion of the dynamical correlation is essential to treat the on-site Coulomb repulsion within the Ni 3d shell and to provide an accurate bond strength. In fact, the explicitly correlated complete-active-space second-order perturbation theory method gives results in overall agreement with the experiment. This shows the importance of treating on the same footing spin and electron correlation as well as the multiconfiguration character of the wave function. © 2002 American Institute of Physics.

Notes: A new density functional theory is developed for inhomogeneous mixtures of polymeric fluids by combining Rosenfeld's fundamental-measure theory for excluded volume effects with Wertheim's first-order thermodynamic perturbation theory for chain connectivity. With no adjustable parameters, theoretical predictions are in excellent agreement with Monte Carlo simulation data for the density distributions and for the adsorption isotherms of hard-sphere chains near hard walls or in slit-like pores. This theory is applied to calculate the force between two parallel hard walls separated by hard-sphere chains at different densities. Calculated results indicate that the chain-mediated force is attractive and decays monotonically with separation at low chain densities, it oscillates at high chain densities and in between, it is attractive at small separation and repulsive at large separation. This new density functional theory is simpler than similar theories in the literature and is directly applicable to mixtures. © 2002 American Institute of Physics.

Notes: A femtosecond pump-probe study of the peculiar amide I band of acetanilide, a molecular crystal with hydrogen bonded chains of peptide units, is presented. The almost perfect harmonicity of the 1666 cm−1 subpeak is related to significant delocalization of this state at low enough temperatures (93 K). The "anomalous" peak (1650 cm−1), on the other hand, is strongly anharmonic, and hence assigned to a self-trapped state. This assignment is in agreement with a more indirect previous work. With increasing temperature, thermal disorder localizes the 1666 cm−1 band (Anderson localization) and at the same time destroys the self-trapping mechanism. Both the self-trapped state and the delocalized state decay on a 2 ps time scale into states outside the spectral window of this study. The excitation energy reappears on a much slower 35 ps time scale in the form of an increased lattice temperature. © 2002 American Institute of Physics.

Notes: This work addresses the construction of slow manifolds for chemically reactive flows. This construction relies on the same decomposition of a local eigensystem that is used in formation of what are known as Intrinsic Low Dimensional Manifolds (ILDMs). We first clarify the accuracy of the standard ILDM approximation to the set of ordinary differential equations which model spatially homogeneous reactive systems. It is shown that the ILDM is actually only an approximation of the more fundamental Slow Invariant Manifold (SIM) for the same system. Subsequently, we give an improved extension of the standard ILDM method to systems where reaction couples with convection and diffusion. Reduced model equations are obtained by equilibrating the fast dynamics of a closely coupled reaction/convection/diffusion system and resolving only the slow dynamics of the same system in order to reduce computational costs, while maintaining a desired level of accuracy. The improvement is realized through formulation of an elliptic system of partial differential equations which describe the infinite-dimensional Approximate Slow Invariant Manifold (ASIM) for the reactive flow system. This is demonstrated on a simple reaction-diffusion system, where we show that the error incurred when using the ASIM is less than that incurred by use of the Maas-Pope Projection (MPP) of the diffusion effects onto the ILDM. This comparison is further done for ozone decomposition in a premixed laminar flame where an error analysis shows a similar trend. © 2002 American Institute of Physics.

Notes: A modified ab initio potential energy surface (PES) is used for calculations of ozone recombination and isotopic exchange rate constants. The calculated low-pressure isotopic effects on the ozone formation reaction are consistent with the experimental results and with the theoretical results obtained earlier [J. Chem. Phys. 116, 137 (2002)]. They are thereby relatively insensitive to the properties of these PES. The topics discussed include the dependence of the calculated low-pressure recombination rate constant on the hindered-rotor PES, the role of the asymmetry of the potential for a general X+YZ reaction (Y≠Z), and the partitioning to form each of the two recombination products: XYZ and XZY. © 2002 American Institute of Physics.

Notes: We report the first rotationally resolved observation of the infrared spectrum of the molecular complex C2H4–HCl. The complex was produced by a supersonic expansion through a pulsed slit jet. By means of a high-resolution tunable diode laser spectrometer, we have recorded the spectrum of the HCl stretching vibration for the isotopes C2H4–H35Cl and C2H4–H37Cl. From the analysis of the spectra, we determined the rotational constants and vibrational frequencies of both isotopes. These experimental results have been interpreted in view of obtaining information on the intermolecular interaction. The experimental data have been complemented by standard coupled cluster singles and doubles model including connected triple excitations with a correlation consistent polarized valence triple zeta basis set ab initio followed by grid calculations, in order to study the anharmonicity, the coupling between intramolecular and intermolecular motions, and the basis set superposition error effects. The results obtained in this study are compared to those of our previous work on the C2H2–HCl [J. Chem. Phys. 113, 4876 (2000)]. © 2002 American Institute of Physics.

Notes: Vibrational dynamics of highly excited SO2(X˜ 1A1) is studied based on an algebraically expanded effective Hamiltonian constructed from the generators of U(2) Lie algebras. A total of 512 experimental vibrational-level energies of SO2 are fitted to the full three-dimensional model Hamiltonian with a root-mean-square deviation of 2.34 cm−1. The vibrational wave functions are constructed from the eigenvectors of the best-fit Hamiltonian. An examination of the probability distribution of the wave functions reveals that the normal-to-local transition is promoted first, and then suppressed as the bend quantum number increases. The origin of these two competing effects of the bend excitation is discussed on the basis of the structure of the algebraic Hamiltonian. © 2002 American Institute of Physics.

Notes: We suggested a simple method for obtaining the quantitative transition rate constants of trans–cis isomerization in the pseudo-stilbene-type of azobenzene molecule. The absorbances of the solution and film of a poly(malonic ester) containing disperse red 1(PDR1) were measured with the pumping beam intensity, and analyzed by the rate equation for the fractions of trans and cis conformers. The rate constants of PDR1 solution and film were successfully determined. © 2002 American Institute of Physics.

Notes: In order to analyze the properties of the ideal lines of thermodynamic parameters use is made of some analytical equations of state, approximate integral equations of the theory of liquids, and computer simulations data. The findings of this study reveal that the ideal lines are not linear functions in the density–temperature phase diagram as was previously believed. It is also shown that the nonlinear behavior of the ideal lines is associated with the structure of the fluid, that is with the inherent features of correlation functions in different domains of the phase diagram. Changes from one linear branch of the ideal line to another are found to occur in the vicinity of the Fisher–Widom line, around which the asymptotically monotonic damping of the total correlation function alters to damping oscillations. © 2002 American Institute of Physics.

Notes: In a recent paper, standard hard-sphere variational theory has been applied to pressure dissociation in dense fluid hydrogen (Juranek and Redmer, J. Chem. Phys. 112, 3780 (2000)). The correlation contributions to the dissociation equilibrium were determined from the free energy functional using effective pair potentials and minimization with respect to the hard-sphere reference system. For simplicity, the Berthelot mixing rule was used to determine the H–H2 pair potential (which entails additive effective hard spheres), and single-component reference pair correlation functions were employed for evaluating the correlation integrals. In this paper, we employ multicomponent reference pair correlations, and we study the sensitivity of the results with respect to nonadditivity of the effective hard spheres. We compare our results with available ab initio simulation data. © 2002 American Institute of Physics.

Notes: Molecular dynamics (MD) simulations were performed to study nanoscale friction for alkyl monolayers terminated with -CH3 (hydrophobic) and -OH (hydrophilic) on Si(111) in the presence of water molecules. Variation of the -OH/-CH3) ratio results in different surface hydrophobicities. To calculate friction in MD simulations, two Si(111) surfaces coated with the alkyl monolayers were slid against each other. It was shown that the friction coefficient decreased quickly for hydrophilic monolayers, whereas it remained almost the same for hydrophobic monolayers, as the number of water molecules (relative humidity) increased. Simulation results are in good agreement with those from scanning force microscopy measurements for both hydrophilic and hydrophobic monolayers. The behavior of water molecules confined between hydrophobic or hydrophilic monolayers was also discussed. A fundamental understanding of nanoscale friction is critical to the design of coatings for microelectromechanical systems. © 2002 American Institute of Physics.

Notes: In this paper, using the density functional theory, we present a consistent study of the thermodynamics of the Si–SiO2 system. This includes the formation of oxygen interstitials in silicon and of oxygen vacancies in silicon dioxide (cristobalite). Relations between calculated and measured quantities are analyzed. The composition dependence of the Si–Si and Si–O bond energies are discussed, which has implications on the modeling of the Si–SiO2 interface. © 2002 American Institute of Physics.

Notes: We propose a theory for the effective interaction between soft dendritic molecules that is based on the shape of the monomer density profile of the macromolecules at infinite dilutions. By applying Flory-type arguments and making use of the experimentally measured density profiles, we derive a Gaussian effective interaction whose parameters are determined by the size and monomer number of the dendrimers that are derived from small-angle neutron scattering (SANS) measurements. By applying this theory to concentrated dendrimer solutions we calculate theoretical structure factors and compare them with experimental ones, derived from a detailed analysis of SANS-data. We find very good agreement between theory and experiment below the overlap concentration, where drastic shape deformations of the dendrimers are absent. © 2002 American Institute of Physics.

Notes: A density functional theory for polymer solutions is generalized to cases where the monomers have a different diameter to the solvent. An appropriate free energy functional is obtained by integration of the generalized Flory equation of state for such systems. This functional predicts that entropic demixing may occur in polymer solutions in which the solvent particles are smaller than the monomers. Demixing is promoted not only by a large size disparity, but also by a high pressure as well as by polymer length. The existence of two separate phases in the bulk solution suggests the possibility of capillary-induced phase transitions, even when the confining surfaces are hard, but otherwise inert. We examine such phase transitions and their relation to surface forces and colloidal stability. The density functional theory also predicts that under certain conditions, layering transitions will occur at hard and flat surfaces. A transition from a thin to a thick polymer-rich surface layer may take place as the separation between two surfaces is decreased, and we study the concomitant change on the surface force. Stable thick phases are predicted even at very large undersaturations, and they give rise to a profound increase of the range and strength of the surface force. We furthermore include comparisons with predictions from a model in which the solvent only enters the description implicitly. Responses of the surface forces to changes in monomer diameter, solvent diameter, polymer density, and chain length are investigated. © 2002 American Institute of Physics.

Notes: This paper investigates the ability of closed loop quantum learning control techniques to meet a posed physical objective while simultaneously steering the dynamics to lie in a specified subspace. Achievement of successful control with reduced space dynamics can have a number of benefits including a more easily understood control mechanism. Judicious choices for the cost functional may be introduced such that the closed loop optimal control experiments can steer the dynamics to lie within a subspace of the system eigenstates without requiring any prior detailed knowledge about the system Hamiltonian. Learning control with reduced space dynamics takes advantage of the expected existence of a multiplicity of fields that can all give acceptable quality control outcomes. The procedure eliminates the hard demands of following a specific dynamical path by only asking that the dynamics reside in a subspace. Additional measurements characterizing the subspace are necessary to monitor the system evolution during the control field learning process. This procedure is simulated for optimally controlled population transfer experiments in systems of one and two degrees of freedom. The results demonstrate that optimal control fields can be found that successfully derive the system to the target state while staying within the desired subspace. © 2002 American Institute of Physics.

Notes: Two-photon excitation was used to excite a number of vibronic bands within the B 1E″ state of ammonia, including a number of combination bands involving the umbrella mode, ν2, and the asymmetric bend, ν4. Photoelectron spectra following single-photon ionization of these levels by the same laser provide insight into the character of the intermediate levels, and resolve questions raised by previous photoelectron studies via the B 1E″ state. Two-color double-resonance spectra were also recorded via selected rotational levels of the B 1E″ 2341 state. These spectra show complex series of resonances that appear to converge to the X 2A2″ 2341 state of the ion. Photoelectron spectra for a large number of these resonances show that Δv2=−1 and Δv4=−1 processes dominate, but that the branching between the two processes depends strongly on the specific resonance. © 2002 American Institute of Physics.

Notes: The double photoionization of HBr molecules, by synchrotron radiation in the energy range between 25 and 55 eV, has been studied in a mass spectrometric experiment. The HBr2+ and Br2+ product ions have been detected by a photoelectron-photoion-coincidence technique, while the H++Br+ formation, which follows the double ionization of HBr, has been studied by photoelectron-photoion-photoion-coincidence technique. HBr2+ ions are produced with a threshold of 32.4±0.4 eV, while the dissociative channel leading to H++Br+, shows a threshold around 33 eV. The production of H+Br2+ occurs with a threshold of 40.2±0.4 eV. These results appear to be in a fairly good agreement with earlier nonrelativistic calculations of potential energy curves and also with values indirectly obtained from experimental Auger spectra. © 2002 American Institute of Physics.

Notes: The speed and angular distributions of I+ ions, produced when ICl molecules were exposed to both ultraviolet and visible radiation at 304+608 nm, 355+608 nm, and 304+532 nm, were measured by velocity map imaging. An intense central feature in the I+ images was observed to be very sensitive to the polarization of the ultraviolet light and is attributed to a dissociative ionization mechanism involving three-body fragmentation: ICl+hv (visible)+3hv (ultraviolet)→I++Cl+e−. The effect of varying the delay between the visible and ultraviolet radiation on the I+ images suggests that an intermediate gateway state of ICl reached by absorption of one photon of visible light mediates the transition to the superexcited dissociative ionization state. © 2002 American Institute of Physics.

Notes: Relativistic core-potential calculations have been carried out on Ω states resulting from the interaction of Xe* (5p56s, 3P, 1P) with ground-state Kr atoms as well as for the system Ar* (3p54s, 3P, 1P) with ground-state Ne, using different basis sets and configuration interaction procedures. The present calculations on ArNe, employing larger sets of Rydberg functions than those of the previous calculations, yield totally repulsive potentials for the excited states of ArNe. Similar calculations on XeKr obtain shallow minima (600–860 cm−1) in the potential energy curves of the excited states at large internuclear distances (6.9–7.8 bohr). Dipole transition moments have been calculated and strong radiative transitions are predicted from excited states to the ground state. The 1(I) state, correlating with the metastable 3P2 state of Xe is found to have a small but nonzero dipole transition moment at short and intermediate nuclear distances leading to a radiative lifetime for the v=0 level of this state of 21.0 μs. © 2002 American Institute of Physics.

Notes: We report the reinvestigation of the S1–S0 electronic transitions of (benzene)n clusters by two-color mass-selective resonantly enhanced two-photon ionization (R2PI) and UV–UV (ultraviolet) hole burning spectroscopies. The present paper describes the band system that has been assigned to the trimer for two decades. Hole burning measurements by monitoring the trimer ion isotopomer channels in the expansion of a mixture of C6H6 and C6D6 have shown the contribution of six spectral carriers in the R2PI spectra, two of which are isotopically pure clusters. The other hetero isotopic species containing at least one C6H6 moiety appear in two adjacent isotopomer channels. It is argued that the band system should be reassigned to the neutral tetramer having four equivalent sites, which are detected in daughter ion mass channels due to efficient fragmentation after ionization even with two-color excitation. The experimental results are consistent with a distorted tetrahedral structure with S4 symmetry predicted as the global minimum by several model calculations. This conclusion is further supported by an analysis of exciton splittings in the C6H6-localized origin band, and this analysis provides coupling constants for the excitation exchange in the S1 state. Other experimental results reported so far pertaining to this species are reconsidered on the basis of the new assignment. © 2002 American Institute of Physics.

Notes: We present a reexamination of the S1–S0 transition of the (benzene)n cluster that appears only in the dimer ion channel and thus has been assigned to an isomer of the neutral dimer other than the extensively studied T-shaped form. Mass-selective resonantly enhanced two-photon ionization (R2PI) excitation and UV–UV (ultraviolet) hole burning spectra are measured in the 000 and 601 vibronic regions. It is established from the observed spectra monitoring three isotopomer channels, (C6H6)2+, [(C6H6)(C6D6)]+, and (C6D6)2+, that efficient fragmentation following ionization prohibits one to observe the two-color R2PI spectra in the parent ion channels, similar to the case for the benzene tetramer as presented in Paper I [J. Chem. Phys. 117, 3656 (2002)]. Three neutral isotopomers containing at least one C6H6 moiety are identified, and this result argues the reassignment of the band system to the neutral benzene trimer with a cyclic form, where the three benzene sites are equivalent. The spectra of the homo isotopomers [(C6H6)3 and (C6D6)3] in the two vibronic regions exhibit small splittings due to the excitation exchange interactions, and this observation is discussed on the symmetry of the cluster geometry. Energetics pertaining to neutral and ionic benzene clusters are also discussed with previous experimental studies to assess a reason of the efficient fragmentation in the ionic states after R2PI for the trimer and larger clusters but not for the dimer. © 2002 American Institute of Physics.

Notes: A general method is presented for the reconstruction of interatomic vector orientations from nuclear magnetic resonance (NMR) spectroscopic data of tensor interactions of rank 2, such as dipolar coupling and chemical shielding anisotropy interactions, in solids and partially aligned liquid-state systems. The method, called PRIMA, is based on a principal component analysis of the covariance matrix of the NMR parameters collected for multiple alignments. The five nonzero eigenvalues and their eigenvectors efficiently allow the approximate reconstruction of the vector orientations of the underlying interactions. The method is demonstrated for an isotropic distribution of sample orientations as well as for finite sets of orientations and internuclear vectors encountered in protein systems. © 2002 American Institute of Physics.

Notes: A new method for calculation of the radial distribution functions (RDFs) at contact from a specified hard-sphere mixture equation of state (EOS) is proposed. The method is applied to two available accurate EOS expressions and new analytical formulas for the RDFs are obtained for binary additive hard-sphere mixtures. The results of the new formulas are compared with available computer simulation data and with those of other RDF expressions from the literature. The evidence to date suggests that the new formula is more accurate than alternative formulas currently available. © 2002 American Institute of Physics.

Notes: The virial expansion of the collective mobility (sedimentation) coefficient is considered for hard sphere suspensions at equilibrium. The term of the second order in volume fraction, which involves three-particle hydrodynamic interactions, is calculated with high accuracy. To achieve that we represent the collective mobility coefficient as the sum of convergent integrals over particle configurations. In this way the short-wave-number limit k→0 is avoided. Moreover, an efficient numerical procedure is applied to evaluate the hydrodynamic interactions. The algorithm is based on the multipole expansion, corrected for lubrication. The method allows us to analyze contributions to the collective mobility coefficient from different configurations of three particles and to select the dominant part. This suggests a general approximation scheme. © 2002 American Institute of Physics.

Notes: The electron transfer (ET) process has been studied in a three sites system: the donor and the acceptor of electrons separated by a bridge. We have considered an isolated system in order to understand the characteristics of the process itself without introducing a bath. The ET process has been studied considering both the electronic and the vibrational part. This complete vibronic analysis has been done in a three electronic-n vibrational model. Two questions are put into evidence. First of all we have analyzed the relevance of the vibrational part in modifying a pure electronic description both in the electron transfer time and in the site populations. A second aspect underlined is the difference between a tight-binding system (donor and acceptor without a direct coupling) and a full coupling one. The drastic difference between these two cases has been rationalized. © 2002 American Institute of Physics.

Notes: On the basis of the recently determined low temperature crystal structure the lattice dynamics of 2-butyne and a single particle methyl rotational potential are calculated using pair potential parameters given by Williams in 1974 within the model of semirigid molecules. In the regime of lattice modes the existence of four methyl librational bands with significant dispersion can explain the measured density of states. The single particle librational energy obtained for the rotational potential from the Schrödinger equation of the methyl rotor coincides well with the mean librational band energy. The calculated single particle tunneling frequency is only 17% lower than observed in experiment. Similarly well the activation energy is reproduced. Although the pair potential parameters developed for aromatic compounds need some scaling they are useful for materials with triple bond carbons. © 2002 American Institute of Physics.

Notes: We have investigated the thermal evolution of the structure and the dynamics of poly(vinyl chloride) (PVC) in a wide temperature range. Corroborating earlier findings, small angle neutron scattering revealed the presence of structural heterogeneities. On the other hand, the single chain form factor corresponds to that of Gaussian chains. Gradually with increasing temperature the system becomes homogeneous. A simple description of the heterogeneities in terms of microcrystallites is forwarded. The dynamical behavior of PVC has been investigated combining broadband dielectric spectroscopy (DS) with coherent and incoherent neutron scattering. In a wide temperature range broadband DS facilitated a precise determination of the dynamic response related to the segmental relaxation. Close to the glass transition temperature the line shape strongly deviates from the usual Kohlrausch–Williams–Watts functional form of common glassforming systems. Moreover, the characteristic relaxation time observed by incoherent scattering displays an anomalous dependence on momentum transfer indicating the possible existence of heterogeneities in the sample. Based on the structural and dynamical results, a model is proposed, that considers the coexistence of regions with different dynamical properties leading to a distribution of characteristic relaxation times. The model accounts for the experimental observations, assuming for all regions the same functional form for the α-relaxation. It may be univocally determined from the coherent scattering data at the first static structure peak. The distribution of relaxation times found is compatible with the distribution of only one variable, the glass transition temperature. © 2002 American Institute of Physics.

Notes: A state-selective multireference coupled-cluster algorithm is presented which is capable of describing single, double (or higher) excitations from an arbitrary complete model space. One of the active space determinants is chosen as a formal Fermi-vacuum and single, double (or higher) excitations from the other reference functions are considered as higher excitations from this determinant as it has been previously proposed by Oliphant and Adamowicz [J. Chem. Phys. 94, 1229 (1991)]. Coupled-cluster equations are generated in terms of antisymmetrized diagrams and restrictions are imposed on these diagrams to eliminate those cluster amplitudes which carry undesirable number of inactive indices. The corresponding algebraic expressions are factorized and contractions between cluster amplitudes and intermediates are evaluated by our recent string-based algorithm [J. Chem. Phys. 115, 2945 (2001)]. The method can be easily modified to solve multireference configuration interaction problems. Performance of the method is demonstrated by several test calculations on systems which require a multireference description. The problem related to the choice of the Fermi-vacuum has also been investigated. © 2002 American Institute of Physics.

Notes: In the present paper we describe a stochastic quadrature method that is designed for the evaluation of generalized, complex averages. Motivated by recent advances in sparse sampling techniques, this method is based on a combination of parallel tempering and stationary phase filtering methods. Numerical applications of the resulting "stationary tempering" approach are presented. We also examine inherent structure decomposition from a probabilistic clustering perspective. © 2002 American Institute of Physics.

Notes: In this work, a pulsed laser photolysis/chemiluminescence (PLP/CL) technique was used to measure absolute rate coefficients for the reaction of C2H+H2→products over the temperature range 295–666 K. Ethynyl radicals were produced pulsewise by excimer laser photolysis of acetylene at 193 nm and real-time pseudo-first-order decays of C2H were monitored by the CH(A 2Δ→X 2Π) chemiluminescence resulting from their reaction with O2. Over the experimental temperature range, the results indicate that the rate coefficient exhibits a non-Arrhenius behavior in line with theoretical predictions, khydrogen(T)=3.92×10−19 T2.57±0.30 exp[−(130±140) K/T] cm3 molecule−1 s−1. Experiments were supplemented by ab initio molecular orbital calculations up to the coupled-cluster theory including all single and double excitations plus perturbative corrections for the triples, UCCSD(T), with the 6-311++G(d,p) basis set for geometry optimizations and the aug-cc-pVTZ for electronic energy single points, revealing that the direct hydrogen abstraction yielding HC(Triple Bond)CH+H is the only product channel of any importance. There is also no important crossing between the doublet and quartet energy surfaces. Finally, geometry optimizations at the UCCSD(T)/6-311++G(2df,2p) level have shown that the transition structure for H-abstraction is linear; harmonic vibration frequencies at this level, and single-point UCCSD(T)/aug-cc-pVTZ energies for these geometries result in an adiabatic barrier height for H-abstraction, including harmonic vibration zero point energies, of 12.8 kJ/mol, while the classical potential energy barrier is 9.2 kJ/mol. © 2002 American Institute of Physics.

Notes: The polarization dependence and temporal profile of the fifth-order Raman response function and corresponding correlation function in liquid Xe are studied both analytically and numerically. Based on the symmetry of an isotropic sample, the fifth-order Raman response function has twelve distinct tensor elements, ten of which are independent, and the corresponding correlation function has twelve distinct tensor elements, seven of which are independent. The coefficients for decomposition into independent components are calculated explicitly based on the tensor property of an isotropic sample and are used to identify different coupling mechanisms in liquid Xe. The two-dimensional profile of the fifth-order Raman response function is evaluated by a simple hydrodynamic expression derived using the Gaussian factorization scheme. An alternative approach reduces the fifth-order Raman response function to time correlation functions that are easy to compute. © 2002 American Institute of Physics.

Notes: The intermolecular nuclear magnetic resonance (NMR) chemical shifts of Xe in nanochannels of various dimensions and shapes are considered. Predictions of the line shapes that result from anisotropic averaging of the shielding tensor for Xe atoms in various limiting cases of nanochannels are made, based on ab initio calculations of 129Xe shielding surfaces. Variation of the line shapes with channel cross sections, Xe loading, and with temperature are predicted for channels having cross-sectional areas that do not permit two or more Xe centers to be located on the same cross-sectional plane. It is shown that Xe in effectively one-dimensional channels should be expected to exhibit signature line shapes in 129Xe NMR spectra, provided that the cross-sectional dimensions of the channels are sufficiently small that Xe atoms do not pass each other during diffusion. These predictions are tested against experimental examples of anisotropic Xe line shapes in various cavities and nanotubes. © 2002 American Institute of Physics.

Notes: By applying a corrugation-reducing procedure we have interpolated the six-dimensional (6D) potential energy surfaces for the H2/Pt(111) and H2/Cu(100) systems from data obtained by density functional theory (DFT) calculations. We have compared interpolated values with a large number of DFT results not used in the basis for the interpolation and we have obtained an average error below 20 meV and a maximum error of about 30 meV in the regions important for dissociative adsorption. Near the surface the corrugation-reducing procedure gives excellent results using only data from high-symmetry sites. However, we show that to reach the above mentioned accuracy level far from the surface, it is necessary to include information from at least one low-symmetry site. Care has been taken to demonstrate the quality of the interpolation along all degrees of freedom in different regions of the configuration space. The strengths of the method are shown together with the aspects requiring careful handling. A comparison with a direct interpolation method is also made. © 2002 American Institute of Physics.

Notes: The pump power dependence of the relaxation dynamics of CdSe nanoparticles (NPs) was studied with femtosecond pump probe spectroscopy at observation wavelengths of the first exciton transition at 560 nm, the near infrared (NIR) absorption at 2 μm, and the transient mid-infrared (IR) absorption at 4.5 μm. Excitation with less than one photon per particle leads to bleaching of the excitonic transitions, and the bleach intensity is initially linear to the pump power. At higher pump power the bleach intensity levels off, when complete saturation of the excitonic transition is reached. At the same time, increasing pump power causes an acceleration of the bleach decay, which is due to additional Auger processes when multiple excitons are formed in the NPs. In addition, the pump power effect was investigated for the NIR and IR regions, at 2 and 4.5 μm wavelength, respectively. Whereas the IR transients are very similar to the ones observed for the bleach, the NIR transients behave completely different. No pump power dependence was found for the transients at 2 μm when pumped in a power range from 0.5 to 5 μJ per pulse. The results show that the fs transients in the visible (bleach) and IR (absorption) regions are due to electron relaxation in the conduction band and the NIR transients are due to the relaxation of the hole. Furthermore, it suggests that in the investigated CdSe NPs, Auger processes act much more efficiently on the electrons than for the holes. © 2002 American Institute of Physics.

Notes: The orientational order parameters P¯2, P¯4, and P¯6 have been determined by neutron scattering from a mixture of normal hydrogenous and deuterium labeled molecules in the nematic, smectic A and smectic C phases of the mesogen 2′,3′-difluoro-4-heptyl-4″-nonyl-p-terphenyl. Since the method relies on a knowledge of the molecular structure, the influence of different molecular conformations resulting from the terminal chains on the values determined was examined in detail. It was shown that the choice of conformation could significantly effect the resulting values of the order parameters. The order parameters were, therefore, determined by assuming a distribution of conformations for the alkyl chains. The relative contributions of these conformations were governed by the Flory rotational isomeric state model. The values for P¯2 were consistent with those determined by nuclear magnetic resonance but all the order parameters were generally much greater than those predicted by the Maier–Saupe theory. © 2002 American Institute of Physics.

Notes: The polarizabilities of polyampholytes have been calculated in Monte Carlo simulations. For chains with random charge topologies, the polarizability increases with the chain length and the chain flexibility. For block copolymers, the polarizability initially increases with the block size to a maximum and then decreases. The polarizability is used to characterize the interaction between polyampholytes and external electric fields. As long as the field is not too strong, the polyampholyte response will be linear. This observation is used to describe the chain behavior near charged walls, and for low to moderately charged walls the results agree well with Monte Carlo simulations. At high field strengths the chain properties are dependent on the chain topology and several cases are discussed. © 2002 American Institute of Physics.

Notes: The effect of charging on the longitudinal second hyperpolarizability of polyacetylene (PA) chains containing up to nearly 70 carbon atoms has been investigated ab initio by characterizing chains with and without an explicit alkali atom (Li, Na, K) as dopant. Whereas charging dramatically enhances the static electronic and vibrational hyperpolarizabilities, γLe(0) and γLv, of an isolated chain at intermediate chain lengths, the presence of an alkali atom counterion substantially reduces this effect. As the size of the alkali atom increases, most properties, including the hyperpolarizabilities, approach those of the isolated chain. Detailed analysis shows that the behavior of γLe(0) is most simply explained in terms of a reduced electrostatic pinning potential due to increased distance between chain and counterion. At all chain lengths studied γLe(0) of PA is enhanced by alkali doping. For chains containing 50 carbon atoms (NC=50), the increase due to K doping is about 9×107 a.u., which more than doubles the value for an undoped chain of similar length. The normalized quantity γLe(0)/NC exhibits a maximum for the isolated soliton (at about NC=61) that is over four times that of the infinite undoped (and unbent) chain. When the alkali dopant is taken into account this maximum diminishes considerably and shifts to larger NC than we have considered. In comparison with the maximum for the undoped species (at NC=∞) there is a small enhancement of γLe(0)/NC for K doping, but none for either Li or Na doping at the coupled-perturbed Hartree–Fock (CPHF)/6-31G level of theory. Intermediate length isolated chains bearing a charged soliton show order of magnitude increases in γv for the degenerate four-wave mixing (DFWM) and, especially, electric field-induced second harmonic generation (dc-SHG) processes compared to undoped PA. As in the case of γLe(0) this enhancement persists, but is significantly reduced when the dopant atom is included. Vibrational anharmonicity, which contributes only to the dc Kerr effect at our level of treatment, is much more important in the doped than undoped species. © 2002 American Institute of Physics.

Notes: The convergence of the second-order Møller–Plesset perturbation theory (MP2) correlation energy with the cardinal number X is investigated for the correlation consistent basis-set series cc-pVXZ and cc-pV(X+d)Z. For the aug-cc-pVXZ and aug-cc-pV(X+d)Z series the convergence of the MP2 correlation contribution to the dipole moment is studied. It is found that, when d-shell electrons cannot be frozen, the cc-pVXZ and aug-cc-pVXZ basis sets converge much slower for third-row elements then they do for first- and second-row elements. Based on the results of these studies criteria are deduced for the accuracy of auxiliary basis sets used in the resolution of the identity (RI) approximation for electron repulsion integrals. Optimized auxiliary basis sets for RI-MP2 calculations fulfilling these criteria are reported for the sets cc-pVXZ, cc-pV(X+d)Z, aug-cc-pVXZ, and aug-cc-pV(X+d)Z with X=D, T, and Q. For all basis sets the RI error in the MP2 correlation energy is more than two orders of magnitude smaller than the usual basis-set error. For the auxiliary aug-cc-pVXZ and aug-cc-pV(X+d)Z sets the RI error in the MP2 correlation contribution to the dipole moment is one order of magnitude smaller than the usual basis set error. Therefore extrapolations towards the basis-set limit are possible within the RI approximation for both energies and properties. The reduction in CPU time obtained with the RI approximation increases rapidly with basis set size. For the cc-pVQZ basis an acceleration by a factor of up to 170 is observed. © 2002 American Institute of Physics.

Notes: In this paper a novel way to quantify "nonexponential" relaxations is described. So far, this has been done in two ways: (1) by fitting empirical functions with a small number of parameters, (2) by calculation of the underlying distribution function g(ln τ) of (exponential) relaxations using regularization methods. The method described here is intermediate, it does not assume a specific functional form but also does not aim at the complete distribution g(ln τ) but only its logarithmic moments 〈(ln τ)k〉. It is shown that these exist (in contrast to the linear moments) and can be calculated analytically for all currently used empirical descriptions of nonexponential relaxations. Therefore, the logarithmic moments represent a common basis for comparing literature data from authors which prefer different empirical formulas (e.g., those of Kohlrausch and Havriliak-Negami). The logarithmic moments are also shown to be related in a simple way to the (linear) moments of an underlying distribution of activation energies giving them a physical significance. © 2002 American Institute of Physics.

Notes: The first optical observation of a mixed germanium-silicon-carbon (GelSimCn) cluster has been made by trapping the products of the laser ablation of carbon/silicon/germanium rods in Ar at 10 K. The ν1(σ) C–C stretching mode of linear GeC3Si has been identified at 1939.0 cm−1. The assignment is confirmed by the very good agreement between Fourier transform infrared (FTIR) measurements of frequencies and isotopic shifts and predictions of our quantum theoretical calculations. © 2002 American Institute of Physics.

Notes: The results of an investigation into the liquid structure of fluorosulfuric acid measured using neutron diffraction with isotopic substitution are reported. The first-order neutron difference function is extracted from data recorded at 300 and 193 K from isotopically labeled FSO3H and FSO3D. Analysis of the derived radial distribution functions shows that unique, multisite hydrogen bonding is present in the liquid. These results suggest hydrogen bonding between the hydrogen and fluorine as well as oxygen occurs in the liquid. The fraction of molecules that is hydrogen bound in each way varies with temperature, with the 193 K data showing a more tightly bound system. © 2002 American Institute of Physics.

Notes: Electron transfer processes in Debye solvents are studied using a spectral analysis method recently proposed. Spectral structure of a nonadiabatic two-state diffusion equation is investigated to reveal various kinetic regimes characterized by a broad range of physical parameters; electronic coupling, energy bias, reorganization energy, and solvent relaxation rate. Within this unified framework, several kinetic behaviors of the electron transfer kinetics, including adiabatic Rabi oscillation, crossover from the nonadiabatic to adiabatic limits, transition from the incoherent to coherent kinetic limits, and dynamic bath effect, are demonstrated and compared with results from previous theoretical models. Dynamics of the electron transfer system is also calculated with the spectral analysis method. It is pointed out that in the large reorganization energy case the nonadiabatic diffusion equation exhibits a nonphysical behavior, yielding a negative eigenvalue. © 2002 American Institute of Physics.

Notes: We have critically examined the possibility of the existence of yet another process (often referred to as process II in the literature) of much smaller magnitude on the high frequency side in the case of 2-ethyl-1-hexanol and 4-methyl-3-heptanol using the dielectric relaxation technique. We have also studied the mixtures of 2-ethyl-1-hexanol with nonpolar methylcyclohexane and a mixture of 1-bromobutane in 4-methyl-3-heptanol for this purpose. In addition, the differential scanning calorimetry (DSC) has been used to study the structural relaxation in the vicinity of the glass transition temperature using the Tool–Narayanaswami–Moynihan procedure as discussed by Sartor et al. [J. Phys. Chem. 100, 6801 (1996)]. Our results show the presence of a clear process II on the higher frequency side whose freezing-out corresponds to the glass transition event at Tg in the DSC studies. It is suggested that process II is associated with free alcohol molecules and is connected to the structural relaxation in alcohols. The dielectric data have been analyzed to understand the thermodynamics of hydrogen bonding using the model of Dannhauser [J. Chem. Phys. 48, 1911 (1968)]. An attempt has been made to correlate the ratio of relaxation rates of processes I and II and the observed deviation from Debye behavior in the case of alcohols. © 2002 American Institute of Physics.