ALBERT

Notes: The dephasing and energy relaxation contributions to the line width in infrared (IR) and sum-frequency generation (SFG) spectra of adsorbates are derived from the generalized master equation approach. Expression for the line shift is also obtained. The anharmonic interaction between the adsorbate and the substrate is expanded in a polynomial in terms of the adsorbate and phonon coordinates, and the dephasing is shown to be mainly due to two-phonon processes, while two-phonon, three-phonon or four-phonon processes can contribute to energy relaxation, depending on the relative values of the adsorbate vibrational and the phonon frequencies. The temperature-dependence data of the IR absorption for C(111):H is found to be consistent with the theory, and the large line width for C(111):D can be accounted for by the efficient two-phonon energy relaxation process which is not available for C(111):H due to the higher adsorbate vibrational frequency for C(111):H. © 1997 American Institute of Physics.

Notes: The mobility of ions drifting in polar gases is explored both theoretically and experimentally. New experimental results are presented for (i) NO+ ions drifting in H2O (the reduced zero-field mobility K0(0) is 0.66±0.07 cm2 V−1 s−1), (ii) H3O+(H2O)3 ions drifting in H2O (K0(0)=0.43±0.06 cm2 V−1 s−1), and (iii) NO+(CH3COCH3)n ions (n=2,3) drifting in CH3COCH3 (K0(0)=0.041 ±0.010 cm2 V−1 s−1 for n=2 and K0(0)=0.050±0.015 cm2 V−1 s−1 for n=3). A number of theoretical models for ion mobilities in polar gases are described. The models are compared with the available experimental data and a reasonable agreement is obtained. For larger cluster ions the measured mobilities are considerably smaller than the calculated values. Some possible reasons for the discrepancies are discussed, including momentum transfer outside the capture cross section, dipole–dipole interactions, ligand exchange, inelastic collisions, and the validity of Blanc's law. © 1997 American Institute of Physics.

Notes: We have investigated the electronic-to-vibrational (E-V) energy transfer between electronically excited rubidium atoms (Rb 5 2P1/2,3/2) and hydrogen molecules (H2). We have used the CARS (Coherent Anti-Stokes Raman Scattering) spectral technique to probe the internal state distribution of collisionally-populated H2 molecules. Both scanned CARS spectra and activated CARS spectra reveal that during E-V energy transfer processes H2 molecules are produced only at the v=1 and v=2 vibrational levels. From scanned and activated CARS spectral peaks two possible population ratios (n1/n2) are obtained. Through shape simulations of the time-resolved CARS profiles under a simple kinetic model, the actual population ratio n1/n2 is unambiguously determined to be of 0.59 (σ=0.05). This n1/n2 ratio indicates that the H2 molecules produced by the E-V energy transfer process are 37% populated at the v=1 vibrational level and 63% at v=2, and that the efficiency of the E-V energy transfer is 0.489 for the Rb 5 2P1/2−H2 system and 0.481 for the 5 2P3/2−H2 system, coincident with the highest E-V transfer fraction 0.489 under the impulsive model and a collinear collision geometry. © 1997 American Institute of Physics.

Notes: An extensive quasiclassical trajectory study of the dynamics of the CN+H2→HCN+H reaction has been undertaken on two of the potential energy surfaces reported by ter Horst, Schatz, and Harding [J. Chem. Phys. 105, 558 (1996)] with the goal of converging product state distributions. The effect of zero-point energy violations on the behavior of the reactive cross section near threshold has been examined leading to an improved estimate of the thermal rate constant on ter Horst–Schatz–Harding potential energy surface 3 (3.01±0.24×10−14 cm3/s at 300 K). The calculated HCN vibrational product state distribution is not statistical and exhibits a systematic over population in the stretching vibrations of the ground state bend manifold indicating that the –C–N does not behave like a "spectator bond" in this reaction. There is also significant population in modes with bending excitation, but these vibrations are under populated relative to prior statistical expectations. The sensitivity of the distribution on the size of the barrier and its location in the entrance channel has been undertaken by comparing results on the ter Horst–Schatz–Harding potential energy surfaces 2 and 3. Similar to the case of exoergic atom-diatom reactions, it is found that the earlier barrier on ter Horst–Schatz–Harding potential energy surface 3 gives rise to more excitation in the –C–H stretching vibration. The rotational distributions of the HCN product appear similar to the thermal distribution of CN reagents from which they are born indicating that the abstraction of the light H atom perturbs the rotational motion of the cyano radical very little. The dependence of the average HCN rotational quantum number, 〈J〉, on the bending quantum number, v2, exhibits an interesting alternation such that the points for even values of v2 are larger than those for odd. There is a corresponding alternation in the dependence of the average scattering angle, 〈θ〉, on v2 in the opposite sense. These observations suggest that for the odd bending states (which are primarily l=1) the energy diverted into exciting motion perpendicular to the reaction path at the transition state is not available to excite product rotation or to produce reactive trajectories with large impact parameters which lead to small scattering angles. © 1997 American Institute of Physics.

Notes: It is proposed to analyze the magnetic shielding tensor as a continuous function of space point coordinates in atoms and molecules. Such a three-dimensional magnetic shielding surface should be very helpful for better understanding of nuclear magnetic shieldings, which is not a trivial problem. The magnetic shielding surface provides more information about molecular electronic structure than nuclear magnetic shieldings alone. A connection between magnetic shielding at an arbitrary point and nuclear magnetic shielding can be made by "performing" a hypothetical NMR experiment using a neutron as a virtual probe, instead of nuclei with nonzero magnetic moments. The theory is illustrated by the numerical results of the magnetic shielding surface calculations for atoms and small linear molecules. © 1997 American Institute of Physics.

Notes: This paper considers optically inactive molecules possessing a symmetry plane. Degenerated excited electronic states in such molecules may, in principle, differ in symmetry with respect to mapping onto the symmetry plane. Should this prove the case, the parity-nonconserving electron-nuclear interaction (PNI) causes the degenerated electronic level to suffer a splitting linear in the Weinberg constant. The paper analyzes from this standpoint the lowest-lying excited states in the ten-electron HF, NH3, B2, and H2O molecules. Two of them, namely HF and NH3, possess the necessary and sufficient symmetry properties for such a linear splitting to occur. Factors are discussed that augment the PNI-induced splitting of the excited states under consideration in comparison with the splitting of the ground state in left- and right-handed modifications of optically active molecules. Computations confirm the occurrence of a great ((approximate)10−13 eV) splitting of the levels being considered due to the PNI effect. A similar effect can also occur in the electronic ground state of paramagnetic molecules, such as NO. The computation uses the consistent multiple-electron perturbation theory with a model single-electron central field bare potential. The computer code used is a modification of the original code developed for precision atomic calculations. All the computations boil down to the solution of a single set of ordinary differential equations, i.e., a unidimensional procedure. © 1997 American Institute of Physics.

Notes: An ab initio molecular dynamics simulation technique is developed employing the Born–Oppenheimer (BO) approach in the framework of a Gaussian implementation of Kohn–Sham density functional theory (DFT). Simulation results for H5O2+ at 200 K are reported. The density profiles, autocorrelation functions and power spectra are presented. The anharmonic frequencies at 200 K are found to be close to the harmonic frequencies calculated directly from quantum methods at 0 K. Structures of large hydrated proton clusters are optimized. Simulated annealing techniques were employed to search for low energy structures and found to be very useful for clusters with 7–8 water molecules. A few very different structures with ground state energy 1–2 kcal/mol apart are shown. H3O+ is found to be the central unit of a few structures optimized. The ionic hydrogen bond was responsible for the stability of the H9O4+ unit in the large hydrated proton clusters. We also find structures with nascent H5O2+ units at the center whose energy is close to, sometimes even lower than that of the H3O+ centered structures. This can be used to explain the solvation facilitated proton transfer in clusters and in solution. The vibrational frequencies of the structures we optimized are tabulated and compared with the experimental results of Price et al. Questions are raised regarding their prediction of a new feature due to water molecules in the third solvation shell. Some new features have been observed for large clusters with heretofore unpredicted structures. © 1997 American Institute of Physics.

Notes: A recently suggested mixture theory for the prediction of mixture properties is formulated for nonspherical molecules. The theory is capable of handling equations of state that are not conformal (do not obey the corresponding states principle). The exact composition dependence of the density expansion of the mixture equations, plus other features of the original theory, are retained. Two methods for evaluating the cross molecular parameters for nonspherical molecules are suggested. They become identical for spherical molecules, and give similar predictions for mixture properties of nonspherical molecules at high densities. The two methods predict the properties of mixtures of convex bodies and fused hard spheres within simulation uncertainty. Properties of diatomic Lennard-Jones mixtures, and ethane–carbon dioxide are predicted fairly accurately. © 1997 American Institute of Physics.

Notes: DNA orientation measurements by linear dichroism (LD) spectroscopy and single molecule imaging by fluorescence microscopy are used to investigate the effect of DNA size (71–740 kilo base pairs) and field strength E (1–5.9 V/cm) on the conformation dynamics during the field-driven threading of DNA molecules through a set of parallel pores in agarose gels, with average pore radii between 380 Å and 1400 Å. Locally relaxed but globally oriented DNA molecules are subjected to a perpendicular field, and the observed LD time profile is compared with a recent theory for the threading [D. Long and J.-L. Viovy, Phys. Rev. E 53, 803 (1996)] which assumes the same initial state. As predicted the DNA is driven by the ends into a U-form, leading to an overshoot in the LD. The overshoot-time scales as E−(1.2−1.4) as predicted, but grows more slowly with DNA size than the predicted linear dependence. For long molecules loops form initially in the threading process but are finally consumed by the ends, and the process of transfer of DNA segments, from the loops to the arms of the U, leads to a shoulder in the LD as predicted. The critical size below which loops do not form (as indicated by the LD shoulder being absent) is between 71 and 105 kbp (0.5% agarose, 5.9 V/cm), and considerably larger than predicted because in the initial state the DNA molecules are housed in gel cavities with effective pore sizes about four times larger than the average pore size. From the data, the separation of DNA by exploiting the threading dynamics in pulsed fields [D. Long et al., CR Acad. Sci. Paris, Ser. IIb 321, 239 (1995)] is shown to be feasible in principle in an agarose-based system. © 1997 American Institute of Physics.

Notes: We report the first experimental investigation devoted to analyze the influence of a ferrofluid on the surface properties of lyotropic liquid crystals doped with magnetic particles. By means of optical methods, the bulk orientation of the director vs the temperature is determined. The theoretical analysis shows that near the nematic–isotropic transition, the thermodynamical model for the temperature surface transition works reasonably well. On the contrary, in the low temperature region near the lamellar–nematic transition, important deviations from the theoretical behavior are observed. In order to interpret our experimental data, we propose a simple phenomenological extension of the thermodynamical model that takes into account the different contributions to the surface energy and of the residual lamellar order in the nematic phase. We show, furthermore, that the effective splay-bend elastic constant depends on the ferrofluid doping concentration, in agreement with a recently proposed theory. © 1997 American Institute of Physics.

Notes: The complete vibrational spectroscopy of pyrrole is addressed within the one-dimensional algebraic framework. Particular attention is devoted to the study of the CH/NH stretching levels, including their possible anharmonic resonance mechanisms with other ring modes. © 1997 American Institute of Physics.

Notes: Results are presented for Doppler-resolved laser-induced fluorescence measurements of collision-induced rotational alignment of N2+(v″=0) drifted in He in a drift-tube apparatus. A single-frequency ring dye laser is used to probe the R1(N′′=15) rotational line of the B 2Σu+–Χ 2Σg+ system both parallel and perpendicular to the drift field at three different field strengths and at several different Doppler-selected velocities. A strong correlation is found between the degree of rotational alignment and the velocity subgroup probed along the field direction. For field strengths of 8 and 16 Td and laser probe parallel to the drift field, there is a monotonic increase in the quadrupole alignment parameter A0(2) with higher velocity subgroup, up to a maximum value of A0(2)=−0.150 (6) for 16 Td at the high-velocity tail. There is evidence that the correlation between alignment and velocity increases with increasing field strength. The mechanisms of the alignment are discussed and these results are attributed primarily to the change in anisotropy of the relative velocity vector distribution of the N2+–He pair with field strength. © 1997 American Institute of Physics.

Notes: The Br2 fragment rotational distributions that result from the vibrational predissociation of NeBr2 in the B electronic state have been measured for several initial vibrational levels. In each case, the rotational distributions extend to the effective energetic limit determined by the amount of energy available (Eavl) for disposal into the fragment rotational and translational degrees of freedom. Analysis of the data allows refinement of the NeBr2 dissociation energy; we find that D0=70.0±1.1 cm−1 for the X electronic state, v=0. Both Δv=−1 and −2 dissociation events have been examined. For dissociation pathways with approximately the same value of Eavl the Δv=−2 pathways are observed to have a higher fraction of the fragment energy in rotational excitation. The overall shape of the Δv=−1 distributions are insensitive to the value of Eavl, suggesting that a Franck–Condon model for the dissociation may have some validity, though quantitative quantum mechanical calculations demonstrate that this model does not reproduce the large degree of fragment rotational excitation. Two classical models for the dissociation also fail to reproduce the extent of fragment rotational distribution. This result is discussed in light of previous experimental and theoretical investigations, focusing on the apparent agreement of classical models with the IBr fragment rotational distributions that result from the dissociation of NeIBr. © 1997 American Institute of Physics.

Notes: Absolute photoabsorption and fluorescence cross sections of gaseous SiCl4 have been measured in the energy region 6.2–31 eV using synchrotron radiation as the light source. Higher order light from the 1-m Seya monochromator was suppressed by use of an Ar gas filter in the energy range 11.3–15.5 eV (110–80 nm) and a LiF window at hν〈11.8 eV (105 nm). Emissions have been observed for the C˜ 2T2→A˜ 2T2 and C˜ 2T2→X˜ 2T1 processes of the SiCl4+ molecular ion and the A˜ 1B1→X˜ 1A1 and 3B1→X˜ 1A1 of the SiCl2 radical. The total fluorescence cross section has been determined to be 22.4±1.0 Mb at 21.22 eV. By comparing with the partial cross section for formation of the C˜ 2T2 ionic state [Carlson et al., J. Chem. Phys. 84, 641 (1986)], it has been concluded that the SiCl4+(C˜ 2T2) formed decays via radiative processes with quantum yield φ(approximate)1. The experimental results provide information on the breakdown pathways of the SiCl4+(C˜ 2T2) ion state. The bands observed in photoabsorption and fluorescence excitation spectra have been assigned as the (4s, np, and np)←3t2, (4s and np)←1e, (4s and np)←1e, (4s and 4p)←2t2, and (np and nd)←2a1 Rydberg series. The ionization energies for the (2t2)−1 and (2a1)−1 processes have been found to be 15.04±0.03 and 18.17±0.03 eV, respectively. © 1997 American Institute of Physics.

Notes: Two-dimensional photoelectron spectroscopy is performed for studying autoionization of acetylene in the Franck–Condon gap between the X 2Πu and A 2Ag states of C2H2+. The photoelectron spectrum in the photon energy range from 12.8 to 13.6 eV shows exclusive vibrational excitation of the symmetric C–H stretching mode ν1 of C2H2+(X 2Πu), which results from autoionization of the valence state (3σg)−1(3σu)1. Vibrational frequencies with anharmonicities of the ν1 and ν2 (the symmetric C–C stretch) modes are determined by a least-squares fit of the ionization energies of the observed peaks to a second order expansion. At the photon energy of 14.120 eV, autoionization of the Rydberg state (3σg)−1(3pπu)1 leads to a complicated photoelectron spectrum where probably the trans-bending mode ν4 of C2H2+(X 2Πu) as well as ν1 is excited, reflecting a substantial geometrical change during autoionization. Furthermore, a similar excitation of the ν4 mode is observed at ∼13.8 eV. An excellent agreement in positions of the vibrational levels between the spectra at 13.821 and 14.120 eV suggests the presence of the Rydberg state (3σg)−1(3pσu)1 at ∼13.8 eV which has not been identified previously in the photoabsorption or photoionization cross section curves. The constant-ionic-state spectra for the ν1=0–4 levels of C2H2+(X 2Πu) show two spectral features: (a) a weak shoulder (v1=0) or a small maximum (ν1=1–4) at 13.8 eV and (b) two groups of peaks in the range of 14.0–14.4 eV. The ratio of the integrated intensity of the 13.8 eV maximum to that of the two groups differs from level to level. This observation is interpreted in terms of a strong interaction between the Rydberg (3σg)−1(3pσu)1 and valence (3σg)−1(3σu)1 states. © 1997 American Institute of Physics.

Notes: The reaction between ground state carbon atoms and propylene, C3H6, was studied at average collision energies of 23.3 and 45.0 kJ mol−1 using the crossed molecular beam technique. Product angular distributions and time-of-flight spectra of C4H5 at m/e=53 were recorded. Forward-convolution fitting of the data yields a maximum energy release as well as angular distributions consistent with the formation of methylpropargyl radicals. Reaction dynamics inferred from the experimental results suggest that the reaction proceeds on the lowest 3A surface via an initial addition of the carbon atom to the π-orbital to form a triplet methylcyclopropylidene collision complex followed by ring opening to triplet 1,2-butadiene. Within 0.3–0.6 ps, 1,2-butadiene decomposes through carbon–hydrogen bond rupture to atomic hydrogen and methylpropargyl radicals. The explicit identification of C4H5 under single collision conditions represents a further example of a carbon–hydrogen exchange in reactions of ground state carbon with unsaturated hydrocarbons. This versatile machine represents an alternative pathway to build up unsaturated hydrocarbon chains in combustion processes, chemical vapor deposition, and in the interstellar medium. © 1997 American Institute of Physics.

Notes: Transient diffusion-controlled reactions in a binary suspension of equal-sized spheres are studied in a region of the equilibrium phase diagram, where a separation of time scales occurs. In Ising model language, the suspension consists of hard spheres with an internal classical spin taking two values, with a square well Ising interaction and with a magnetic field present. Transitions can occur whenever a pair of spheres approaches within the range of the Ising interaction. A separation of time scales is observed near the ferromagnetic phase transition. This allows a simplified phenomenological description of the relaxation of magnetization. © 1997 American Institute of Physics.

Notes: The infrared chemiluminescence of vibrationally excited H2O and HDO from the highly exothermic reactions of OH and OD radicals with HI and GeH4 was observed in the 2200–5500 cm−1 range. The experiments utilized a fast-flow reactor with 0.3–1 Torr of Ar carrier gas at 300 K; the OH(OD) radicals were produced via the H(D)+NO2 reaction and the H or D atoms were generated by a discharge in a H2(D2)/Ar mixture. The H2O and HOD vibrational distributions were determined by computer simulation of the emission spectra in the 2200–3900 cm−1 range. The total vibrational energy released to H2O and HOD molecules is, respectively, 〈fv〉=0.36 and 0.41 from HI and 〈fv〉=0.46 and 0.51 from GeH4. These values are significantly smaller than for the reactions of OH and OD with HBr, 〈fv〉=0.61 and 0.65. The populations of the O–H stretching vibration of HOD and the collisionally coupled ν1 and ν3 stretching modes of H2O decrease with increasing vibrational energy. In contrast, the vibrational distribution from the HBr reaction is inverted. The bending mode distributions in all stretching states of H2O and HOD extend to the thermodynamic limit of each reaction. A surprisal analysis was made for H2O(HOD) distributions from the title reactions and compared with that for OH(OD)+HBr. The surprisal analysis tends to confirm that the dynamics for the HI and GeH4 reactions differ from the HBr reaction. The HI reaction may proceed mainly via addition-migration, while the GeH4 reaction may involve both direct abstraction and addition-migration. A rate constant for the OH+GeH4→H2O+GeH3 reaction was evaluated by comparing the H2O emission intensities with that of the OH+HBr→H2O+Br reaction, kGeH4/kHBr=6.5±0.9. Secondary kinetic-isotope effects, kOH/kOD=1.4±0.1, 1.0±0.2, and 1.3±0.2, were determined for reactions of OH and OD with GeH4, HI, and HBr, respectively, by comparing the relative H2O and HOD emission intensities. © 1997 American Institute of Physics.

Notes: In order to determine the lowest energy isomer of the hydroperoxyl radical dimer, H2O4, ab initio quantum mechanical methods were employed to predict the geometrical structures, relative energies, harmonic vibrational frequencies, and associated IR intensities of both open chain and cyclic isomers. Two minima were located on the open chain potential energy surface, one of C2 symmetry and one of C1 symmetry. The relative energies of the different H2O4 structures vary strongly with level of theory. The most reliable treatment used in the present study predicted that the global minimum is the closed-shell C1 chain isomer which is lower in energy than the planar C2h triplet cyclic isomer by 1.6 kcal mol−1 including zero point vibrational energy corrections. It is argued that both structures should be observable, depending on the method of preparation. © 1997 American Institute of Physics.

Notes: Ab initio averaged relativistic effective core potentials (AREP) and spin-orbit (SO) operators are reported for the elements Am through element 118. Two sets have been calculated for certain elements to provide AREPs with varying core/valence space definition, thereby permitting the treatment of core/valence correlation interactions. The AREPs and SO operators are tabulated as expansions in Gaussian-type functions (GTF). GTF valence basis sets are derived for the lowest energy state of each atom. The reliability of the AREPs and SO operators is gauged by comparing calculated atomic orbital eigenvalues and SO splitting energies with all-electron relativistic values. © 1997 American Institute of Physics.

Notes: A theory, which has been advanced by the author and his co-workers, for frequency shifts of electronic absorption spectra of a nonpolar rodlike molecule in dilute nonpolar and polar solutions is extended so that it is applicable to a polar rodlike solute molecule and a nonpolar or polar disklike solute molecule. The theory takes into account higher-order multipole moments (in addition to a dipole) of the solute molecule, and its solution-phase polarizabilities are properly taken into account. Furthermore, it explains how the dispersive interaction is altered by the presence of permanent dipoles of the solvent. © 1997 American Institute of Physics.

Notes: We have developed robust and very efficient algorithms for solving the reference interaction site model (RISM) equations for salt solutions in the bulk and near a solute atom of noble gases. The theory of dielectric consistency recently developed for solutions at finite salt concentrations is employed in the formalism. The change in water structure in the bulk caused by addition of salts have been examined for model 1–1 salt solutions (LiCl, NaCl, KCl, KF, KBr, KI, and CsI). The density and orientational structures of each salt solution near a solute atom have been analyzed. The water model employed is the extended simple point charge (SPC/E) model. Ions characterized by positive hydration (F−, Li+, and Na+) are strongly hydrated in the bulk and stay significantly far from the atom. Those of negative hydration (Cl− and Br−) or hydrophobic hydration (Cs+ and I−) are excluded from the bulk to the atom. Due to a specific orientational order of water molecules adjacent to the solute atom, there is a trend that cations stay less closer to the atom than anions. Overall, cations indirectly affect the solubility of noble gases via the change in water structure induced by addition of those ions. On the other hand, anions affect the solubility not only indirectly but also directly by interacting with solute atoms. The agreement between the calculated and experimental values for the salting coefficient is excellent for He. However, the discrepancy becomes larger as the number of electrons of the solute atom increases (the calculated value is larger), which implies that the ion-induced dipole interaction neglected has significantly large effects. © 1997 American Institute of Physics.

Notes: The interfacial curvature free energy is shown to cause a significant barrier height correction to the classical nucleation rate. This correction is found to be temperature dependent, but independent of nucleus size. Density functional (DF) calculations are presented for a nonuniform spherical droplet model of the nucleus. Calculations for the surface tension, as a function of nucleus size, and for the interfacial curvature free energy support theoretical predictions and provide an explanation for systematic discrepancies between classical and DF nucleation theories and between the classical theory and experiment. © 1997 American Institute of Physics.

Notes: The kinetic energy dependence of the reactions of Fen+ (n=2–18) with O2 are studied in a guided ion beam mass spectrometer. A variety of FemO2+, FemO+, and Fem+, where m≤n, product ions are observed, with the dioxide cluster ions dominating the products for all larger reactant cluster ions. Reaction efficiencies are near unity for all but the smallest clusters. The energy dependence of the products is analyzed in several different ways to determine thermochemistry for both the first and second oxygen atom binding to iron cluster ions. The trends in this thermochemistry are discussed and compared to bulk phase values. © 1997 American Institute of Physics.

Notes: Analysis of the energy dependence of the cross sections for collision-induced dissociation reactions has permitted the determination of quantitative thermodynamic information for a variety of ionic clusters. As such clusters become larger, the rate at which the decomposition occurs becomes comparable to the instrumental time available for observing the reaction. A method for incorporating statistical theories for energy-dependent unimolecular decomposition in this threshold analysis is reviewed and updated. The revision relies on the fact that for most ionic clusters, the transition state is a loose association of the products that can be located at the centrifugal barrier. This permits a straightforward estimation of the molecular parameters needed in statistical theories for the transition state. Further, we also discuss several treatments of the adiabatic rotations of the dissociating cluster. The various models developed here and previously are compared and used to analyze a series of data for Li+(ROH) complexes, where ROH=methanol, ethanol, n-propanol, i-propanol, n-butanol, i-butanol, s-butanol, and t-butanol. The trends in the bond energies derived by these various models are compared and their accuracy evaluated by comparison with relative values determined by equilibrium methods. © 1997 American Institute of Physics.

Notes: The intermolecular potential energy surface of the Ar⋅NO+ cationic complex has been calculated using ab initio methods: RHF, MP2, MP3, MP4, CCSD, and CCSD(T), using the cc-pVDZ and cc-pVTZ basis sets. An additional surface was calculated at the MP2/cc-pVQZ level. All calculations were performed with a fixed NO+ bond length, but in one set of calculations the effect of variation of the NO+ bond length was studied. Finally, the MP2/cc-pVQZ intermolecular potential energy surface was recalculated by performing a point-by-point correction for basis set superposition error. All of these surfaces were used to extract anharmonic intermolecular vibrational frequencies, in order to compare to previous calculations, and experimental results. Rotational energy levels were also calculated. © 1997 American Institute of Physics.

Notes: We report accurate ground state energies and structural properties for small clusters of 4He computed with the diffusion quantum Monte Carlo (DMC) method combined with high quality trial wave functions and using the recent analytical pair potential of Tang, Toennies, and Yiu [Phys. Rev. Lett. 74, 1546 (1995)]. Calculations based on the older HFD-B(He) potential are reported for comparison. The clusters are found to be extremely floppy and to be characterized by very diffuse wave functions. The DMC results for 4He2 and 4He3 are in excellent agreement with other calculations using conventional methods. 4He3 is found to have a noticeable contribution from nearly linear geometries. The internal structure of the clusters is described by a three particle correlation function which reveals a significantly non-spherical internal cluster structure. The energies for all cluster sizes are found to be slightly higher than those obtained with the HFD-B(He) pair potential. Exploratory calculations on the helium trimer indicate that the effects of three body interactions do not exceed the uncertainty margin provided by different state of the art pair potentials. © 1997 American Institute of Physics.

Notes: Simulations by isothermal and isoergic molecular dynamics show that vibrationally-cold clusters, initially at some high-energy point on their potential surfaces, relax monotonically down to low-lying minima where they are trapped. The most significant point regarding this relaxation is that species with sawtoothlike and staircaselike potentials show the same qualitative behavior, equilibrating their vibrations after each major saddle crossing. This result justifies the use of transition state kinetics for constructing coarse-grained master equations to describe the well-to-well flow of population distributions on complex potential energy surfaces, even in cases such as (KCl)32 and some protein models which have staircase topographies in which large drops in potential energy from one well to the next might suggest nonthermal kinetics could occur. © 1997 American Institute of Physics.

Notes: Within the context of a standardlike Enskog theory, we prove the existence of both a global and a local H-theorem for a system consisting of a binary mixture of dissimilar hard spheres. The semipositive character of the entropy production, in addition to previous results on the proof of Onsager's theorem, exhibits the complete compatibility of our theory with linear irreversible thermodynamics. © 1997 American Institute of Physics.

Notes: Raman spectra of oligo-p-phenylenes, oligorylenes, and oligoacenes of different chain lengths have been obtained in the solid state and in solution. Among the properties studied, particular attention is devoted to frequency and intensity dispersion of the Raman bands with increasing conjugation length and to the vibrational second order hyperpolarizability γ r. The results obtained are compared with those relative to polyenic systems. The behavior of the various classes of molecules studied is in some cases different both in absolute values and trends. This fact is discussed in order to clarify the influence of the topology of the π-electron system on the properties of conjugated materials and to determine whether the presence of aromatic rings in the main chain can confine π electrons and so reduce delocalization. Oligorylenes turn out to be the compounds with the largest vibrational γ r. The results also indicate that absolute Raman intensity shows strong intensity dispersion with conjugation length and can be used as a powerful tool in characterizing conjugated compounds. © 1997 American Institute of Physics.

Notes: Two methods for studying tunneling dynamics are compared, namely the instanton model and the approach of Truhlar and co-workers, which are based on the direct output of electronic structure calculations and thus are parameter free. They are employed to evaluate the zero-level tunneling splitting due to intramolecular hydrogen exchange in the glycolate anion. The first method was developed in a series of recent studies and presents a combination of the instanton theory with quantum-chemically computed potentials and force fields. For the compound at hand, which has 21 internal degrees of freedom, a complete potential-energy surface is generated in terms of the normal modes of the transition-state configuration. It is made up of the potential-energy curve along the tunneling coordinate and harmonic force fields at the stationary points. The level of theory used is HF/6–31++G**. All modes that are displaced between the equilibrium configuration and the transition state are linearly coupled to the tunneling mode, the couplings being proportional to the displacements in dimensionless units. These couplings affect the instanton trajectory profoundly and, depending on the symmetry of the skeletal modes, can enhance or suppress the tunneling. In the glycolate anion all modes have such displacements and thus are included in the calculation. Based on the similarity with malonaldehyde, it is argued that tunneling prevails in the studied process, and the zero-level tunneling splitting is predicted. The latter is found within the computational scheme developed earlier, which avoids explicit evaluation of the instanton path and thus greatly simpli-fies the tunneling dynamics. These results are tested by the method of large-curvature tunneling of Truhlar and co-workers implemented in a dual-level scheme. The potential energy surface needed for the dynamics calculations is generated at the semiempirical PM3 level of theory and then corrected by interpolation with high-level HF/6–31++G** results for the stationary points. The code corresponding to this approximation is in the package MORATE 6.5. The tunneling splittings found by the two approaches are in quantitative agreement. We have found that the computational scheme based on the instanton model is much less time consuming both in the static and dynamics part. This computational efficiency, also demonstrated in a number of earlier studies, merits future application of the method to fairly large systems of practical interest, such as clusters and organic compounds with excited-state proton transfer. © 1997 American Institute of Physics.

Notes: van der Waals complexes Na(centered ellipsis)XCH3 (X=F, Cl, and Br) have been generated by crossing a beam of sodium with the expansion region of a supersonic jet of the appropriate halide, seeded with a rare gas. The identity of these complexes was determined by photoionization time-of-flight mass spectrometry. The primary route for photodepletion of these complexes is thought to be the excitation of the Na chromophore followed by a charge-transfer dissociation: Na(centered ellipsis)XCH3+hν→[Na*(centered ellipsis)XCH3]

Notes: The potential energy surface (PES) for the gas-phase NH3+H↔NH2+H2 reaction is constructed with suitable functional forms to represent the stretching and bending modes, and using as calibration criterion the reactant and product experimental properties and the ab initio saddle point properties. This surface is then used to calculate rate constants with variational transition-state theory over the temperature range 300–2000 K. While the forward rate constants agree with experimental results, the reverse ones are lower by factors of between 4 and 6. Since the same PES is used and these rates are related by detailed balance, this disagreement could indicate an uncertainty in the few available experimental studies for the reverse reaction. We also provide a detailed analysis of the equilibrium constants and of the kinetic isotope effects and compare the results of this analytical PES with earlier ab initio reaction-path calculations. Finally, for the vibrational frequency calculations, we analyze the consequences of the choice of different coordinate systems (curvilinear or rectilinear) on various kinetic magnitudes. © 1997 American Institute of Physics.

Notes: We present ab initio calculations for the N and O 1s→2π photoexcitation spectra and the deexcitation electron spectra which result in the decay of these core excited states. The photoexcitation spectra are simulated with potential energy curves and transition dipole moments calculated with the multiconfiguration coupled electron pair approximation method. The energy lifetime width of the core excited states, which influences the form of these excitation spectra via the broadening of the vibrational progressions, was calculated by the one center approximation. The theoretical spectra are compared to recent experimental data. The deexcitation electron spectra, which monitor the autoionization of the core excited states, are reproduced by a combination of valence configuration interaction calculations to obtain the relative energies and the one center approximation which yields decay rates to the singly charged final states. Furthermore, we included lifetime vibrational interference effects using the recently proposed moment method of Cederbaum and Tarantelli [J. Chem. Phys. 98, 9691 (1993)]. The calculated deexcitation spectra compare extremely favorably with experimental data. © 1997 American Institute of Physics.

Notes: High order perturbation energies are computed for excited 1A1 states of BeH2 at geometries near the Be→H2 symmetric insertion transition state. The equations of multireference perturbation theory are solved through 30th order to study the difficulties in selecting the appropriate zeroth order Hamiltonian, orbitals, orbital energies, and reference functions for the computations of smooth molecular potential energy surfaces. The origin of the perturbative divergence produced by Möller–Plesset and Epstein–Nesbet partitionings is analyzed using a conceptually simple two-state model constructed using one state each from the reference and orthogonal spaces. The optimized zeroth order partitioning scheme (OPT) for double reference space computations with configurations 1a122a123a12 and 1a122a121b22 produces a truly convergent perturbation expansion through 30th order. The OPT energies are accurate in low orders as compared to the exact (197 dimensional) solution within the basis. The forced valence orbital degeneracy partitioning method (FD) also generates a truly convergent expansion for the same double reference space calculation, with slightly poorer low order energies than the OPT scheme. The BeH2 system facilitates the consideration of larger reference spaces (constructed using three through six orbitals) where the FD method produces highly accurate energies in low orders despite the asymptotic nature of the FD perturbation expansion. The "delayed'' perturbative divergence behavior with the FD partitioning scheme (for large reference spaces) is shown to occur due to the incorrect ordering between the zeroth order energies of some reference and complementary space levels. © 1997 American Institute of Physics.

Notes: Electronic structure methods and nonperturbative resonance theory are applied to study the radiative and radiationless decay mechanisms of the MgBr (A 2ΠΩ) vibrational levels. The X 2Σ+ and 1,2 2ΠΩ adiabatic electronic states are characterized using ab initio state-averaged multiconfigurational self-consistent field/second order configuration interaction wave functions. Interstate derivative couplings between the 2Π states have been calculated and used to construct a rigorous diabatic basis. The nonrelativistic potential energy curves are modified in the first order of degenerate perturbation theory to take account of the spin–orbit interactions treated within Breit–Pauli approximation. All vibrational levels in the A 2ΠΩ manifold are resonances predissociated by the repulsive 2 2Π state. A recently developed computational approach [S. Han and D. R. Yarkony, Mol. Phys. 88, 53 (1996)] based on a Feshbach formalism is employed to determine energies, linewidths, and radiative and radiationless decay rates in a coupled diabatic states basis within a Hund's case (a) approximation. Large nonadiabatic interactions cause significant energy shifts in the resonances levels. It is shown that a pronounced Ω-dependence in the radiationless decay rates results from the large fine structure splitting in the 2 2ΠΩ diabatic state which corresponds to Mg(1S)Br(2P). Comparisons with absorption and fluorescence spectra reveal important insights into A 2ΠΩ state decay. The spectroscopic constants of the A 2ΠΩ, Ω=3/2 and 1/2 states and the A 2Π3/2 state predissociation are well described in a Hund's case (a) approximation. However it is found that the A 2Π1/2 state predissociation is significantly underestimated in this limit. Rather the A 2Π1/2 state is indirectly predissociated by the 2 2Π3/2 state through rotational coupling to the A 2Π3/2 state. © 1997 American Institute of Physics.

Notes: Within the scope of joint experiments by the international Nucleation Workshop Group, nucleation experiments on n-pentanol were carried out using a pulse-expansion wave tube. Data were obtained for nucleation at temperatures between 240 K and 260 K. Total pressures of the carrier gas (helium) during nucleation varied from 89 to 109 kPa. The results are presented in tabular form, to facilitate future comparison. Our results are consistent with existing data by Hrubý et al. Comparisons are made to the Kinetic Classical Theory (KCT) as well as to the semiphenomenological theory by Kalikmanov and Van Dongen (KvD–SPT). Although both theories predict nucleation rates that are apparently too low in the temperature range of interest, the KvD–SPT is approximately two orders of magnitude closer to the experimental results. © 1997 American Institute of Physics.

Notes: A version of the statistical associating fluid theory (SAFT) is developed for chain molecules of hard-core segments with attractive potentials of variable range (SAFT-VR). The different contributions to the Helmholtz free energy are evaluated according to the Wertheim perturbation theory. The monomer properties are obtained from a high-temperature expansion up to second order, using a compact expression for the first-order perturbation term (mean-attractive energy) a1. Making use of the mean-value theorem, a1 is given as the van der Waals attractive constant and the Carnahan and Starling contact value for the hard-sphere radial distribution function in terms of an effective packing fraction. The second-order perturbation term a2 is evaluated with the local compressibility approximation. The monomer cavity function, required for the calculation of the free energy due to the formation of the chains and the contribution due to association, is given as a function of a1. We analyse the equation of state for chain molecules with three different types of monomer hard-core potentials with variable attractive range: square-well (SW), Yukawa (Y), and Sutherland (S). The theory for the hard-core potentials can easily be generalised to soft-core systems: we develop a simple equation of state for Mie m−n potentials, of which the Lennard-Jones (LJ) 6-12 potential is a particular case. The equations of state, expressed in terms of reduced variables, are explicit functions of the reduced temperature, the packing fraction, the number of monomers segments forming the chain, and the parameter λ which characterises the range of the the attractive potential. The relevance of the last parameter in the application of the theory to n-alkanes and n-perfluoroalkanes is explicitly shown with the SW expressions. An accurate description of the vapour pressure and the saturated liquid densities is obtained, with a simple dependence of the parameters of the monomer potential on the number of carbons. The extension of our SAFT-VR expressions to mixtures is also presented in terms of a simple expression for the mean-attractive energy for mixtures, based on a straightforward generalisation of the theory for pure components. © 1997 American Institute of Physics.

Notes: A theoretical study is made on He scattering from three basic classes of disordered adlayers: (a) translationally random adsorbates, (b) disordered compact islands, and (c) fractal submonolayers. The implications of the results to experimental studies of He scattering from disordered surfaces are discussed, and a combined experimental-theoretical study is made for Ag submonolayers on Pt(111). Some of the main theoretical findings are: (1) The scattering intensities from the three disorder classes differ significantly, and can be used to distinguish between them. (2) Structural aspects of the calculated intensities from translationally random clusters were found to be strongly correlated with those of individual clusters. (3) For fractal islands, just as for all surfaces considered here, the off-specular intensity depends on the parameters of the He/Ag interaction, and does not follow a universal power law as previously proposed in the literature. In the experimental-theoretical study of Ag on Pt(111), we use experimental He scattering data from low-coverage (single adsorbate) systems to determine an empirical He/Ag–Pt potential of good quality. Then, we carry out He scattering calculations for high coverage and compare with experiments for these systems. The conclusion is that the actual experimental phase corresponds to small compact Ag clusters of narrow size distribution, with partial translational disorder. © 1997 American Institute of Physics.

Notes: The reaction of H2 on Cu(100) is investigated using a four-dimensional (4D) quantum dynamical fixed-site model to assess the influence of molecular rotation on dissociation over the most reactive (the bridge) site. The potential energy surface (PES) is a fit to the results of density functional calculations performed using a generalized gradient approximation treating a Cu slab with a periodic overlayer of H2. Dissociation probabilities for molecules with "helicoptering'' (mj=j) and "cartwheeling'' (mj=0) rotational motions are here found to be comparable because of the strong corrugation in the azimuthal coordinate. The calculations indicate that reaction is accompanied by significant rotationally inelastic scattering. Surprisingly, vibrational excitation is also found to be an efficient process in collisions with the reactive bridge site. In these collisions, the molecular axis is tilted away from the orientation parallel from the surface. Considering the approximate nature of the 4D model used, the calculated reaction probabilities are in good agreement with experiment, indicating that the PES that was used is accurate. © 1997 American Institute of Physics.

Notes: Previous investigations have demonstrated that the formation of chemical waves in the NO+H2 reaction on Rh(110) involves a cyclic transformation of the surface structure via various N,O-induced reconstructions, i.e., starting form the c(2×6)-O a cycle is initiated comprising the formation of a (2×3)/(3×1)-N and a mixed c(2×4)-2O,N structure. The stability and reactivity of these structures has been investigated in titration experiments as well as under stationary reaction conditions employing LEED, work function, rate measurements, and thermal desorption spectroscopy. It was shown that the c(2×6)-O and c(2×4)-2O,N structures exhibit a low reactivity whereas the (2×1)/(2×1)-N displays only a small to moderate decrease in catalytic activity ((approximate)20%–30%) compared to the clean surface. On the basis of these results, an excitation mechanism for pulses in the NO+H2 reaction on Rh(110) was constructed consisting of the sequence c(2×6)-O, (2×1)/(3×1)-N c(2×4)-2O,N, c(2×6)-O. © 1997 American Institute of Physics.

Notes: Absolute scattering probabilities of nearly monoenergetic D2 and He beams are compared for the highly reactive clean Ni(110) surface at a surface temperature Ts=700 K along the more corrugated [001] direction. At incident energies between 20 and 110 meV the total reflectivity of D2 is about a factor 200 smaller than for He, whereas the first order diffraction intensities relative to the specular peak are a factor 7 larger. The D2 angular distributions also show clear evidence of rotationally inelastic diffraction peaks. The diffraction intensities of both He and D2 can be accounted for by a conventional hard wall model with reasonable values of the corrugation amplitudes of 0.060 Å for He and 0.091 Å for D2 without including a lateral variation in the probability for chemisorption. The reflectivity results when extrapolated to Ts=0 indicate that for He only 33% of the incident atoms are coherently reflected. For D2 only 9% are coherently scattered and approximately 24% are chemisorbed. The coherently scattered fraction is attributed to D2 molecules with orientations not sufficiently parallel to the surface plane to permit chemisorption to occur. © 1997 American Institute of Physics.

Notes: Laser-induced vibrational predesorption of molecules physisorbed on insulating substrates is theoretically investigated based on the Markoff master equation. The system vibrations, which consist of intramolecular vibrations of an admolecule and admolecule-surface vibrations, are divided by the adiabatic approximation, whereby the predesorption is represented by the nonadiabatic transitions from the bound states to the desorption continuum. By using the projection operator in the double(Liouville)-space representation, the bound-continuum couplings due to the nonadiabatic and the optical interactions are explicitly included in the master equation. The adiabatic theory is applied to CO physisorbed on a NaCl(100) surface, in which CO stretching and CO-surface vibration are chosen as the system vibrations. This two-dimensional model with a shallow Morse potential for the CO-surface potential gives a desorption rate of ∼10−4 s−1, which agrees with the experimentally measured rate by Chang and Ewing [Chem. Phys. 139, 55 (1989) and Phys. Rev. Lett. 65, 2125 (1990)]. The rate equations explicitly derived from the master equation are used to analyze the desorption dynamics. It is shown that predesorption is considerably enhanced by the incoherent phonon-assisted predesorption mechanism, i.e., by thermal excitation of the CO-surface stretching in the manifold of the excited CO stretching. Excitation by a single laser is extended to the two-laser excitation scheme to accelerate the predesorption. © 1997 American Institute of Physics.

Notes: The quantum energy gap law for electron transfer (ET) reactions in water is examined. Molecular dynamics (MD) simulation analysis is carried out to obtain the solvent reorganization energies, time correlation functions (TCF), spectral density functions, and quantum rate constants. Their dependence on the reaction free energy and on the donor–acceptor distance is explored along with the solvent isotope effects. Properties of the imaginary-time saddle-point for the TCF expression of the ET rate formula are also examined. The high-frequency intramolecular vibrational modes of the solvent water are found to present marked quantum effects on the ET rate, while their contribution to the static reorganization energy is small (less than 6%). The energy gap dependence of the quantum activation free energy is shown to become nearly independent of the donor–acceptor distance when renormalized by the reorganization energy. Approximations to compute quantum rate constants from MD simulation data are briefly discussed in light of of the present results. © 1997 American Institute of Physics.

Notes: The molecule SiNN was investigated with the highly correlated coupled cluster method with single and double excitations and corrections for the connected triples [CCSD(T)], and with the multireference single and double excitations configuration interaction approach (MRCI). A series of basis sets ranging from the simple 6-31G*, 6-311G*, and Dunning's double-zeta (DZ) plus polarization basis sets to the more extended correlated consistent cc-pVTZ and cc-pVQZ ones were employed to check the dependence of the geometry optimization and of the frequency evaluation on the basis sets. Our highest level result of 1859 cm−1 for the harmonic stretching frequency of the NN bond differs considerably from a previous CCSD(T)/DZ prediction of 1726 cm−1. In fact, most of the results analyzed in this study point to a frequency value greater than 1810 cm−1. Reexamining the scant experimental evidence, we estimate the harmonic frequency to be very close to 1830 cm−1. Surprisingly, an extended internally contracted MRCI calculation with the cc-pVTZ basis predicts a frequency 144 cm−1 higher than the corresponding CCSD(T)/cc-pVTZ result. Comparisons with existing density functional studies and with a previous MRCI calculation are also carried out. At the CCSD(T)/cc-pVTZ level, other isomers of SiN2 are further investigated for the first time. One symmetric linear (3∑g−), and one symmetric bent (1A1) structure are found to lie relatively high in energy: 84.60 and 102.23 kcal/mol, respectively, relative to SiNN (3Σ−). However, one cyclic 1A1 and the singlet asymmetric isomer SiNN (1Σ+) are only 6.09 and 15.81 kcal/mol above the global minimum. Although the higher frequencies of the former isomers do not fall in the region of relevance to the experimental assignment discussed in this work, that of the lowest lying 1A1 state (1850 cm−1) is practically identical to the CCSD(T)/cc-pVTZ frequency computed for the 3Σ− state. © 1997 American Institute of Physics.

Notes: We present the main characteristics of the energy and coupling surfaces for the BeH24+ quasimolecule, that are relevant to the dynamics of electron capture in Be4++H2 collisions in the 50 eV amu−1〈E〈1 keV amu−1 energy range. To construct the wave functions, we implemented a block-diagonalization method using the many electron description standard (MELD) program, which was recently modified to calculate nonadiabatic couplings. © 1997 American Institute of Physics.

Notes: A two-dimensional model for hydrogen pair exchange in transition metal trihydrides is used to interpret NMR data observed for [cp(PPh3)IrH3]+. Inspired by quantum chemical results for [cp(PH3)IrH3]+, the model describes a combined process of rotational tunneling and IrH2 bending that merges into an H2 "lift-off'' motion at a small proton–proton distance. The condensed environment with which the tunneling system interacts is represented by a heat bath. A second-order perturbation treatment yields a master equation for the populations of the vibrational states within each of the rotational symmetry species A and B and for the respective AB coherences. A theoretical basis is provided for the evolution of the tunneling (AB) coherence as a damped oscillation in agreement with an independent treatment very recently published by Szymanski [J. Chem. Phys. 104, 8216 (1996)]. A simplified model assumption, containing one adjustable parameter, is made for the system–bath interaction. The temperature-dependent frequency of the tunneling process is found to be close to the Boltzmann average of the tunnel frequencies in the individual vibrational states. Both the calculated temperature-dependent coherence damping-rate constant and the tunnel frequency fit the experimental data after adjustment of three parameters describing the potential energy surface and of the parameter representing the system–bath interaction strength. © 1997 American Institute of Physics.

Notes: Using synchrotron radiation, very high resolution electron spectra at the 2p→6a1 and 2p→3b2 resonances of H2S have been recorded. Auger transitions from molecular field split S 2p hole states of HS* fragment, created by fast dissociation of the core excited H2S, are found to reveal different partial rates depending on the symmetry of the final state. The experiments have made it possible to determine, for the first time, the molecular field splitting of the core levels in a free radical. The molecular field splitting between the 3σ1/2 and 1π3/2 ("2p3/2'') levels in the HS* fragment is found to be 90(5) meV, which significantly differs from the molecular field splitting of the "S 2p3/2'' levels in the H2S molecule. Resonant Auger decay in the molecule before dissociation is also observed. © 1997 American Institute of Physics.

Notes: We present the calculation of phase diagrams for fluids in disordered porous materials using theories based on the replica symmetric Ornstein–Zernike equations. We consider molecular models in which the porous medium is described by quenched disordered configurations of spheres and the fluid-fluid and matrix intermolecular potentials are the sum of a hard-sphere core and an attractive tail. Such models account for the combined effect of confinement, wetting, and disorder that are expected to be important to describe recent experimental observations. We use the replica method to derive the expressions relating the thermodynamic properties of the fluid inside the porous material to the pair distribution functions within the mean-spherical approximation and the optimized random-phase approximation (ORPA). We also consider higher-order corrections within the optimized cluster theory developed by Andersen and Chandler for bulk fluids. In most cases a vapor–liquid coexistence curve, similar to that observed for the bulk fluid, although displaced and somewhat narrowed, is obtained. The improved ORPA+B2/EXP approximation also predicts the appearance of a second fluid–fluid phase transition at a lower temperature. © 1997 American Institute of Physics.

Notes: In this paper we demonstrate that a small transverse temperature gradient of a few mK/cm induces the nucleation of a wetting layer at the liquid–vapor surface of the critical binary liquid mixture hexadecane+acetone. The nucleation phenomenon is well described by a classical surface nucleation theory [B. M. Law, Phys. Rev. Lett. 72, 1698 (1994)] where far from the critical temperature the height of the energy barrier is independent of the magnitude of the temperature gradient while the characteristic nucleation time A=1/(attempt frequency) is approximately inversely proportional to the transverse temperature gradient which induces laminar fluid flow. © 1997 American Institute of Physics.

Notes: The adsorption of amphiphilic oligomers from solution to different interfaces has been studied using a lattice model. The oligomers consist of one hydrophobic and one hydrophilic block and can form micellar aggregates in solution at sufficiently high concentrations. We have investigated the relationships among the amphiphile bulk concentration, its chemical potential, and adsorbed amount. Below the critical micellization concentration (cmc) the amphiphile chemical potential is a logarithmic function of the concentration, above the cmc it becomes constant. When the amphiphile solution is in contact with a hydrophobic surface, the hydrophobic blocks adsorb to the surface and the hydrophilic blocks protrude into the solution. As the adsorbed amount increases, the adsorbed molecules become laterally more confined and stretch away from the interface. The adsorption isotherm shows a plateau at high bulk concentration, which is due to the constant chemical potential above the cmc. The influence on the adsorption of both the interaction between the amphiphile and the solvent, and between the amphiphile and the interface are investigated. Finally, the adsorption at a hard hydrophobic surface is compared with the adsorption at a fluid interface. © 1997 American Institute of Physics.

Notes: Here we present a theory for predicting the effect of interparticle interactions on the nonequilibrium dynamics of concentrated colloidal dispersions. A configuration-space conservation equation for the pair density P2 provides a fundamental basis for calculating the nonequilibrium microstructure; however, it includes pairwise additive three-body couplings. The resulting forces depend on the three-particle distribution function, necessitating an additional equation to completely specify P2. In this paper nonequilibrium Percus–Yevick and hypernetted chain closures complete the formulation by relating these forces to the interparticle force and pair distribution function. A computational algorithm exploiting Fast Fourier Transforms solves the resulting integro-differential equations for weak perturbations from equilibrium, yielding the perturbed pair density as a function of the volume fraction φ and the interparticle potential. The advantage of a fundamental approach is that clearly defined approximations lead from the characteristics of the individual colloidal particles to the nonequilibrium structure and macroscopic properties. The calculation of all dynamic properties, both rheological stresses and diffusion coefficients, is accomplished with the same approximations. Detailed predictions of the structure provide an additional comparison with simulation and experiment lacking in theories that calculate only bulk properties. The numerical methods demonstrated here allow efficient solution of a class of models more sophisticated than previously attempted. To test the merits of nonequilibrium closures we present predictions of the low-shear viscosity and long-time self-diffusion coefficient as a function of volume fraction for various repulsive potentials without hydrodynamic interactions. Comparison with results available from computer simulations demonstrates that the closures capture the trends in the transport properties with volume fraction and interparticle potential and yield realistic predictions for the nonequilibrium structure. The hypernetted chain closure yields the best agreement with the available data for bulk properties at moderate volume fractions (φ〈0.4), but significant quantitative deviations appear at φ≥0.45. © 1997 American Institute of Physics.

Notes: An instantaneous normal mode (INM) theory is presented for quantum time correlation functions. It is argued that the INM formalism for classical correlation functions is particularly amenable to quantum correction. The intermolecular (Rayleigh) and allowed vibrational Raman spectra of liquid CS2 are calculated as an illustration. The Applequist–Quicksall polarizability model is employed, yielding the correct values for both the molecular polarizability and its derivatives with respect to the normal coordinates. Agreement with experiment is reasonable for the intermolecular Raman and for some aspects of the allowed Raman, but not for the linewidth. A brief discussion is given regarding the future developments which will be needed for an accurate INM theory of vibrational line shapes. © 1997 American Institute of Physics.

Notes: 2ω–2ω bleaching measurements of the ground state absorption and 2ω–1ω fluorescence dip measurements were carried out on the s–p transitions of Ag atoms in Xe matrices by ps laser pulses with 2ω corresponding to about 320 nm and 1ω to 640 nm. The absorption spectrum is analyzed in terms of a dynamic Jahn–Teller (JT) effect with a depth of the bound −JT state of about 30 meV and a pseudorotation frequency of about 20 ps. The −JT state seems to be rather long lived with a depopulation time of 3(±1) ps and a small energy dissipation rate of about 1 phonon per ps. Fluorescence occurs after a significant static lattice deformation accompanied by an energy relaxation of 0.4 eV. This static deformation and the energy relaxation to the emitting states proceed on a similar time scale of 3 ps which is very fast if the involved energies and the dissipation rate of about 30 phonons per ps are considered. The different rates are related to the different coupling to the lattice in the pseudorotating and the statically deformed geometry. Transient absorption is only observed in the relaxed state with σ=1.6(±0.5)⋅10−16 cm−2. © 1997 American Institute of Physics.

Notes: We report studies of the structure and dissociation dynamics of the Mg2H2O+ ion–molecule complex. The weakly bound clusters are formed in a supersonic molecular beam equipped with a laser vaporization source, mass-selected, and studied by laser photofragmentation spectroscopy in a tandem time-of-flight mass spectrometer. Broad structureless molecular absorption bands are observed in the red (610–745 nm), the green (515–595 nm), and the near UV (335–390 nm) spectral regions, and are assigned to transitions localized on the Mg2+ chromophore. Three daughter ions (Mg+, Mg2+, and MgH2+) have been observed. We have studied the competitive branching into accessible product channels as a function of photolysis wavelength in these bands. In order to understand the dissociation mechanisms we have carried out ab initio calculations of the ground and low lying excited states of the Mg2H2O+ complex. These studies give insight into the dissociation dynamics. Results from these experiments are compared and contrasted with previous work on the photodissociation of Mg2CO2+. © 1997 American Institute of Physics.

Notes: Nascent Doppler profiles of CN (X 2Σ+) fragments from the 193 nm photodissociation of NCCN have been measured using high-resolution transient frequency modulated (FM) absorption spectroscopy. This new method is highly suited for Doppler spectroscopy of nascent photoproducts. The experimental line shapes suggest an asymptotic available energy of 5300±100 cm−1 and are well described by a model in which the available energy is partitioned between a statistical reservoir (4700 cm−1) and a modest exit barrier (600 cm−1). We have determined state dependent v⋅j correlations. A trend of j becoming increasingly perpendicular to v for the higher rotational states is in accord with phase space theory, although the observed correlations are more than twice as strong. The v⋅j correlations can be quantitatively modeled by further restricting the phase space model with an approximate conservation of the K-quantum number, the projection of total angular momentum about the linear axis of NCCN. Global rotational and vibrational product distributions have also been measured. The highest accessible rotational states are underpopulated, compared to a phase space calculation. The global vibrational distribution is substantially colder than the phase space theory predictions. Vibrational branching ratios for coincident fragments have been measured as a function of the detected CN state from a close analysis of high signal-to-noise Doppler profiles. The correlated vibrational distribution, P(v1,v2), shows an excess of vibrationless coincident fragments, at the expense of dissociation to give one ground state and one vibrationally excited CN fragment. The correlated formation of two vibrationally excited CN fragments is as likely as the phase space prediction, yet the formation of v=2 is strongly suppressed. The fragment vector and scalar correlations provide a highly detailed view of the loose transition state typical for reactions well described by statistical reaction theories. © 1997 American Institute of Physics.

Notes: The photodissociation of dimethylsulfoxide [(CH3)2SO] at 193.3 nm has been investigated using the molecular beam time-of-flight (TOF) mass spectrometric technique. In addition to CH3 and SO, CH3SO is also observed as a stable primary product, indicating that CH3SO+CH3 is an important product channel for the 193.3 nm photodissociation of (CH3)2SO. The analysis of the TOF data provides evidence that SO is formed via a stepwise mechanism: (CH3)2SO+hν (193.3 nm)→CH3SO+CH3→2CH3+SO. The analysis also indicates that (approximate)53% of the primary CH3SO radicals undergo further dissociation to produce CH3+SO, yielding a quantum yield of (approximate)1.53 for CH3. Within the sensitivity of our experiment, the product channel of CH3SCH3+O is not found. The angular distribution for the formation of CH3SO+CH3 is found to be isotropic, an observation consistent with a predissociation mechanism, in which the dissociation of photoexcited (CH3)2SO is slow compared to its rotational period. The energetics for selected dissociation reactions of (CH3)2SO have also been investigated by ab initio calculations at the G2(MP2) level of theory. The experimental dissociation energy at 0 K (53±2 kcal/mol) for the CH3–SOCH3 bond obtained here is in excellent agreement with the theoretical prediction of 52.6 kcal/mol. © 1997 American Institute of Physics.

Notes: We consider the problem of the calculation of the intramolecular electron transfer (ET) rate for molecules in solution and focus on the case of rapid, almost activationless processes. We assume that the weak-coupling, nonadiabatic limit holds and utilize the Fermi golden rule expression for the ET rate, avoiding the introduction of phenomenological data. The Fermi Golden Rule is elaborated in the Liouville space formalism taking into account at second order the coupling to the bath of instantaneous normal modes (INM) of the solvent as well as to the intramolecular bath responsible for relaxation in the isolated molecule. The couplings among the principal modes (the ones more directly involved in the ET process), mainly intramolecular, are taken into account exactly. The main inputs are weighted densities of states which can be, at least in principle, calculated. For those concerning the solvent we take advantage from the recent progresses in the INM approach to the description of the short time dynamics. We compute the ET line shape (i.e., the ET rate as a function of the electronic energy gap E) for some model cases, with one, two or more principal modes, investigating the influence of the solvent and of the temperature. The ET rates show a complex, but not dramatic, dependence on the solvent and are quite sensitive to the energy gap E. The temperature dependence is generally weak. The results seem to be in general agreement with recent experimental data on molecular systems exhibiting rapid ET. © 1997 American Institute of Physics.

Notes: Recently the possible divergence of many-body perturbation theory or many-body green function (propagator) methods with lattice summations in extended systems has been raised. The convergence of these methods with lattice summations is not only the key to establishing their correct scaling properties in inhomogeneous systems, but is also a necessity if numerical calculations are to be meaningful. In this report, it is rigorously shown that many-body perturbation theory (MBPT), coupled cluster theory (CC), and many-body green function (MBGF) methods all converge uniformly with lattice summations, although the integrand for the integration over the reciprocal lattice vector, k, could become infinite at special k values. Our proof is given not only for infinite polymers but also for crystals. In our proof, only the continuity of the zeroth-order band structure and Bloch orbitals with k is used. We show that MBPT, CC, and MBGF methods converge with the radius of the lattice summation range R at least as fast as 1/R1/n where n is the dimensionality of the system. It could be much faster if the zeroth-order band structure and Bloch orbitals have continuous first- or even higher-order derivatives with k. In practical numerical calculations, one should not pursue the convergence of the integrand, but the convergence of the correction considered with lattice summations. © 1997 American Institute of Physics.

Notes: We present a new theoretical framework for a statistical mechanical and thermodynamic description of any general inhomogeneous system (not necessarily polymeric) in the presence of surfaces. The framework is an extension of a lattice theory recently developed for a homogeneous system and requires approximating the original lattice by a recursive lattice which, for simplicity, we take to be a modified tree structure (see Fig. 4), TM as described in the text. The tree is formed recursively by two basic elements, the main tree T and the surface tree T¯. The model is solved exactly using a recursion technique. The technique allows us to account for connectivity, architecture, excluded-volume effects, interactions, etc. exactly. The resulting description goes beyond the random-mixing approximation used in most mean-field theories. We consider a general model of a multicomponent system and its exact solution on the modified tree TM provides us with an approximate theory of the inhomogeneous system on the original lattice. We provide a general discussion of the theory and principles involved. Our method produces results similar to those of Monte Carlo simulations but can even be applied to cases where Monte Carlo simulations are not possible. We also obtain surface free energy and the surface entropy that is not easily obtained in a Monte Carlo simulation. Our method is more reliable than the mean-field method of Scheutjens and Fleer, whose predictions are, in many cases, in direct contradiction with the Monte Carlo simulations. Our method is fast by at least three orders of magnitude compared to rival methods. © 1997 American Institute of Physics.

Notes: The nonequilibrium molecular dynamics generated by the SLLOD algorithm [so called due to its association with the DOLLS tensor algorithm (D. J. Evans and G. P. Morriss, Statistical Mechanics of Nonequilibrium Liquids (Academic, New York, 1990)] for fluid flow is considered. It is shown that, in the absence of time-dependent boundary conditions (e.g., shearing boundary conditions via explicit cell dynamics or Lees–Edwards boundary conditions), a conserved energy, H exists for the equations of motion. The phase space distribution generated by SLLOD dynamics can be explicitly derived from H. In the case of a fluid confined between two immobile boundaries undergoing planar Couette flow, the phase space distribution predicts a linear velocity profile, a fact which suggests the flow is field driven rather than boundary driven. For a general flow in the absence of time-dependent boundaries, it is shown that the SLLOD equations are no longer canonical in the laboratory momenta, and a modified form of the SLLOD dynamics is presented which is valid arbitrarily far from equilibrium for boundary conditions appropriate to the flow. From an analysis of the conserved energy for the new SLLOD equations in the absence of time-dependent boundary conditions, it is shown that the correct local thermodynamics is obtained. In addition, the idea of coupling each degree of freedom in the system to a Nosé–Hoover chain thermostat is presented as a means of efficiently generating the phase space distribution. © 1997 American Institute of Physics.

Notes: We report nucleation-rate measurements in metastable liquid argon–krypton solutions at pressures of 1.0 and 1.6 MPa over a wide temperature and concentration range. These measurements were performed with the use of a superheated liquid lifetime measurement method. The experimental results are compared with the homogeneous nucleation theory data both using a macroscopic (capillary) approach and taking into account the dependence of critical bubble surface tension on interface curvature. The size effect in nucleation is considered in the framework of the Van-der-Waals, Cahn–Hilliard method. The experimental data indicate that the homogeneous nucleation theory quantitatively describes the kinetics of a first order phase transition in binary solutions of simple liquids if the size effect is taken into account and nucleation rates are J(approximately-greater-than)106 m−3 sec−1. At J(approximately-less-than)106 m−3 sec−1 there is initiated nucleation. A diffusion spinodal of a solution is approximated. The attainable superheating temperature data are presented. © 1997 American Institute of Physics.

Notes: A molecular dynamics study of the adsorption of CO2 on NaCl (100) is presented. The model potential includes short-range Buckingham atom–atom and charge–charge terms for the molecule–molecule interaction and Lennard-Jones atom–atom and charge–charge terms for the molecule substrate interactions. Calculations have been performed for finite patches and for samples with periodic boundary conditions. The adlayer can form domains with the 2×1 herringbone arrangement. The structure of the adlayer as a function of temperature is discussed in terms of various distribution functions. The vibrational properties of the adlayer (internal and external vibrations) are calculated and compared with available experimental data. © 1997 American Institute of Physics.

Notes: Two mobile cylinders embedded in a lipid monolayer exhibit two types of short-range attractions, a depletion-induced attraction in the range r〈σL, where σL is the diameter of a lipid and r is the distance between the surfaces of the two cylinders, and a fluctuation-induced attraction in the range 1〈r/σL〈4. Using Monte Carlo methods we have calculated the force between the cylinders for various diameters σP. Evidence is given that the strength of attraction increases with decreasing curvature of the cylinders. © 1997 American Institute of Physics.

Notes: Both B(3Πu0) and B′′(1Π1u) surfaces contribute to resonance Raman scattering of iodine, and the interference between these two channels leads to modulation of the intensity profile of overtone progressions, as observed experimentally. The effect is simulated through mixed order semiclassical molecular dynamics.© 1997 American Institute of Physics.

Notes: We derive a capture cross section which takes into accounts the effect of evaporative losses. We suggest that the coagulation cross sections measured for van der Waals impurities reflect the mass and energetic stability of the guest cluster, rather than incomplete coagulation of the impurity molecules. © 1997 American Institute of Physics.

Notes: Third order nonlinear response of liquid water is investigated by using a classical molecular dynamics simulation. The calculated response function yields a marked agreement with the recent experimental results. The physical origins of the third order spectrum is analyzed in terms of the translational and the librational motions. Its temperature dependence and isotope effect are also analyzed. © 1997 American Institute of Physics.

Notes: We comment on the explanation offered by Salian et al. for the abrupt onset of damping in collective excitations of an atomic cluster. We note that this effect cannot be due to the cluster escaping from the global minimum potential well because the critical energy lies below that of the lowest known transition state.© 1997 American Institute of Physics.

Notes: Thermalization of high-energy electrons in gaseous Ar at room temperature has been investigated by analyzing the imaginary component of the transient microwave conductivity produced by pulse radiolysis. The conductivity signal amplitude showing a peak due to the Ramsauer minimum has been correlated with the amplitude derived from calculations of the effective collision frequency using Margenau's formula assuming Maxwellian velocity distribution of electrons. Two approaches, using the peak and the plateau of the signal, for absolute normalization of the conductivity amplitude give results consistent with each other. It has been found that the excess mean electron energy drops very rapidly to about 0.2 eV and then decreases exponentially with a constant relaxation time. The thermalization time for 1 eV electrons to relax to 10% thermal energy has been determined to be 5.8 ms at 1 Torr Ar. Electron energy loss rate coefficients have been derived as a function of the mean electron energy. © 1997 American Institute of Physics.

Notes: Certain copolymeric hydrogels such as poly(N-ispropylacrylamide)/poly(acrylic acid) show first-order volume phase transition on heating. By using the lattice-fluid-hydrogen-bond theory we have shown earlier that both hydrogen bonding interactions and hydrophobic interactions are responsible for this behavior in homopolymer gels. In this paper we demonstrate that the transition temperature of copolymer gels can be varied by changing the ratio of the two monomer concentrations in the copolymerization and also by slightly varying the hydrophobic nature of the gels. Thus it might be possible to "tailor-make" gels with desired transition temperatures. Such gels can find novel applications in biotechnology, soft robotics, separations, and sensors. © 1997 American Institute of Physics.

Notes: The effects of chain volume and connectivity upon the motions of flexible polymers in dilute solution have been studied by computer simulation of simple off-lattice bead-flip models of from 9 to 99 beads. Long internal relaxation times are given for free-draining chains with bead diameters from zero to 0.93 times the stick lengths. Moves are forbidden which would result either in bead overlap (excluded volume) or in one stick passing through another (chain connectivity). In the extreme case of zero bead diameter, where there is no expansion of the chains by excluded volume, the long relaxation time varies as about the 2.1 power of chain length, as opposed to the 2.0 power for similar chains without connectivity constraints. As bead diameter is increased until it equals stick length, the exponent increases to the value of 2.48 established by previous work. Over the range of bead diameters employed, the chain-length dependence of long relaxation times and translational diffusion constants can be described by the sum of two terms, the first due to chain swelling by excluded volume and consistent with the predictions of scaling theory, the second due only to chain connectivity. © 1997 American Institute of Physics.

Notes: Rapid adsorbate diffusion into the solid is known to suppress the desorption yield measured in a thermal desorption experiment. We show that this suppression can be controlled (at least partly) by pulsed-laser heating at rates in excess of 1010 K/s. As an example, we analyze the D/Zr system. In this case, deuterium adsorbed on a surface rapidly diffuses into the bulk of Zr with increasing temperature, and the deuterium desorption probability measured with conventional heating rates (β≤100 K/s) is as low as (approximate)10−4 for polycrystalline Zr foils (deuterium desorption is not observed at all from single-crystal Zr from which dissolved H/D has been removed). Heating the Zr(0001) surface by pulsed-laser thermal excitation with β(similar, equals)1011 K/s is demonstrated to result in the increase of the deuterium desorption probability up to approximately 0.01. To interpret this observation, general equations for describing associative desorption accompanied by adsorbate diffusion into the solid are simplified by employing the specifics of the temperature-programmed kinetic regimes with a linear increase of temperature. The desorption yield calculated without any adjustable parameters is in good agreement with the experimental results. © 1997 American Institute of Physics.

Notes: The optical response of CdSe semiconductor nanocrystals is investigated using the reduced single-electron density matrix in real space, calculated by means of the time-dependent Hartree–Fock technique. The spectroscopic signatures of exciton confinement are analyzed using the frequency-dependent electronic coherence matrix (off-diagonal density-matrix elements). The effects of Hartree and the Fock (exchange) type Coulomb interactions on the exciton binding energy are discussed. The latter result in almost dark excitons situated energetically below the main transition. Off-diagonal Coulomb matrix elements lead to larger exciton binding energies compared with previous calculations, and result in a better agreement of the size dependence of the lowest optical transition with experiment. © 1997 American Institute of Physics.

Notes: The frequency dependence of quadratic and cubic susceptibilities given by the conventional expressions for molecular crystals is analyzed for resonances (poles) in the light of the dielectric theory of excitons. Exciton frequencies correspond to poles in the local-field tensors; in the nonlinear susceptibilities the poles in these tensors cancel out the poles that occur in the hyperpolarizability where the output and input frequencies coincide with molecular excitation frequencies, and replace them by poles at the exciton frequencies, including wavevector dependence and any Davydov splitting. In the cubic susceptibility, there are additional poles, through cascading, where the sum of any two input frequencies coincides with an exciton frequency. © 1997 American Institute of Physics.

Notes: Recently, two works were reported that the permanent electric dipole moment of the ICl A state has the same direction as that of the X state, through the analysis of electric and magnetic pendular spectra and through the direct analysis of the hyperfine structures under the Stark effect, respectively. We determined clearly and sensitively the direction of the dipole moment in the A (v=27) state of I35Cl, through the line shape analysis of the Stark modulation spectra (SMS). While the result was the same as those of previous two works, the analysis of the line shape of the SMS was so useful to determine the relative direction of the dipole moment of the upper electronic state to that in the lower state, whichever of the first-order or the second-order Stark effect the state may show. © 1997 American Institute of Physics.

Notes: The Cartesian coordinates for the structure H clathrate unit cell have been determined to within 5% error. The unit cell, consisting of 34 water molecules and 6 hydrocarbon molecules, was replicated to create a system amenable to molecular dynamics simulation. Radial distribution functions were determined for this crystal. The method of molecular dynamics was employed to model the interface between the crystal and liquid water. It was found that the interfacial behavior spanned a region of 15 Å on both sides of the interface. Both orientational and translational motion in the liquid phase became more vibrational as the interface was approached, while the temperature profile remained constant within about 15° across the interface. In addition, there was a small tendency for the molecules in the liquid phase of the interfacial region to align themselves in an ordered, solidlike configuration. The solid region within about 10 Å of the interface demonstrated persistent steady-state disorder compared with the solid bulk phase. © 1997 American Institute of Physics.

Notes: This work is focused on explicit finite size corrections in the calculation of the fluctuation in the number of hard disks and parallel (aligned) hard squares deposited on a finite flat surface through a random sequential adsorption process. Explicit size effects are made evident by using a finite-system pair correlation function for calculating the fluctuation. The method is based on the relation between this pair correlation function and its infinite-system counterpart. A diagrammatic density (coverage) expansion of the corresponding infinite-system pair correlation function is used to calculate the low-coverage behavior of the fluctuation. Results also include border effects due to consider a finite size region for evaluating the fluctuation. A comparison with Monte Carlo computer simulations shows an excellent agreement between theoretical and simulation results. © 1997 American Institute of Physics.

Notes: Hartree–Fock solutions of the Pariser–Parr–Pople and MNDO Hamiltonians are shown to give reasonable predictions for the ionization potentials and electron affinities of gas-phase polyenes. However, the energy predicted for formation of a free electron-hole pair on an isolated chain of polyacetylene is much larger than that seen in the solid state. The prediction is 6.2 eV if soliton formation is ignored and about 4.7 eV if soliton formation is included. The effects of interchain interactions on the exciton binding energy are then explored using a model system consisting of one solute and one solvent polyene, that are coplanar and separated by 4 Å. The lowering of the exciton binding energy is calculated by comparing the solvation energy of the exciton state to that of a single hole (a cationic solute polyene) and a single electron (an anionic solute polyene). It is argued that when the relative timescales of charge fluctuations on the solute and solvent chains are taken into account, it is difficult to rationalize the electron–electron screening implicit in the parametrization of a single-chain Hamiltonian to solid-state data. Instead, an electron–hole screening model is developed that includes the time scales of both the electron–hole motion and the solvent polarization. The predicted solvation energies, which are saturated with respect to solute and solvent chain length, are 0.07 eV for the exciton and 0.50 eV for a well separated electron–hole pair. Given this large, 0.43 eV reduction in the exciton binding energy due to interaction with a single chain, it seems likely that interchain interactions play a central role in establishing the solid-state exciton binding energy. © 1997 American Institute of Physics.

Notes: In this paper we discuss a new generalized time-dependent Ginzburg-Landau theory for the numerical calculation of polymer phase separation kinetics in 3D. The thermodynamic forces are obtained by a mean-field density functional method, using a Gaussian chain as a molecular model. The method is especially aimed at describing the formation kinetics of the irregular morphologies which are typical for many industrial systems. As proof of concept we present the formation of irregular morphologies in quenched symmetric and asymmetric block copolymer melts. © 1997 American Institute of Physics.

Notes: We present a new simulation algorithm for minimizing empirical contact potentials for a simplified model of protein structure. The model consists of backbone atoms only (including Cβ) with the φ and ψ dihedral angles as the only degrees of freedom. In addition, φ and ψ are restricted to a finite set of 532 discrete pairs of values, and the secondary structural elements are held fixed in ideal geometries. The potential function consists of a look-up table based on discretized inter-residue atomic distances. The minimization consists of two principal elements: the use of preselected lists of trial moves and the use of a genetic algorithm. The trial moves consist of substitutions of one or two complete loop regions, and the lists are in turn built up using preselected lists of randomly-generated three-residue segments. The genetic algorithm consists of mutation steps (namely, the loop replacements), as well as a hybridization step in which new structures are created by combining parts of two "parents'' and a selection step in which hybrid structures are introduced into the population. These methods are combined into a Monte Carlo simulated annealing algorithm which has the overall structure of a random walk on a restricted set of preselected conformations. The algorithm is tested using two types of simple model potential. The first uses global information derived from the radius of gyration and the rms deviation to drive the folding, whereas the second is based exclusively on distance-geometry constraints. The hierarchical algorithm significantly outperforms conventional Monte Carlo simulation for a set of test proteins in both cases, with the greatest advantage being for the largest molecule having 193 residues. When tested on a realistic potential function, the method consistently generates structures ranked lower than the crystal structure. The results also show that the improved efficiency of the hierarchical algorithm exceeds that which would be anticipated from tests on either of the two main elements used independently. © 1997 American Institute of Physics.

Notes: Equilibrium structures, solvation forces, and conformational dynamics of thin confined films of n-hexadecane and squalane are investigated using a new grand canonical ensemble molecular dynamics method for simulations of confined liquids. The method combines constant pressure simulations with a computational cell containing solid surfaces and both bulk and confined liquid regions in equilibrium with each other. For both molecular liquids layered density oscillations in the confined films are found for various widths of the confining gap. The solvation force oscillations as a function of the gap width for the straight chain n-hexadecane liquid are more pronounced exhibiting attractive and repulsive regions, while for the branched alkane the solvation forces are mostly repulsive, with the development of shallow local attractive regions for small values of the gap width. Furthermore, the nature of the transitions between well-formed layered configurations is different in the two systems, with the n-hexadecane film exhibiting solid-like characteristics portrayed by step-like variations in the number of confined segments occurring in response to a small decrease in the gap width, starting from well-layered states of the film. On the other hand the behavior of the squalane film is liquid-like, exhibiting a monotonic continuous decrease in the number of confined segments as the gap width is decreased. These characteristics are correlated with structural properties of the confined films which, for n-hexadecane, exhibit enhanced layered ordering and in-plane ordered molecular arrangements, as well as with the relatively high tendency for interlayer molecular interdigitation in the squalane films. Reduced conformational (trans-guache) transition rates in the confined films, compared to their bulk values, are found, and their oscillatory dependence on the degree of confinement is analyzed, showing smaller transition rates for the well-formed layered states of the films. © 1997 American Institute of Physics.

Notes: In a crossed molecular beam arrangement helium atoms are scattered from ammonia clusters of the averaged sizes n¯=18, 745, and 1040 which are generated by isentropic expansions with conical nozzles. The inelastic energy transfer is detected by time-of-flight analysis of the scattered helium atoms with a resolution of less than 5 meV at a collision energy of 95 meV. The energy transfer increases with increasing deflection angle and extends to 65 meV. Intensity maxima are observed between 11 and 16 meV, around 27 meV, and at 33 meV. The one in the middle is attributed to the vibration of a specific cluster network, while the other two occur in the energy regime of the translational or librational modes of the solid with a preference for small or large clusters, respectively. © 1997 American Institute of Physics.

Notes: Theoretical studies of monolanthanum carbides, LaCn for n=2–6, are presented. The fan structures were found as ground states in most cases studied. The computed enthalpies of formation of LaCn and atomization energies of these species are close to the corresponding experimental data. The agreement is even closer when experimental Gibbs energy functions are corrected using theoretical ground state structures and partition functions. The La–C bond is strongly ionic due to electronic charge transfer from lanthanum to carbon atoms. © 1997 American Institute of Physics.

Notes: Rotational and vibrational energy transfer rate constants have been measured for excited rovibrational levels of I2(X). Stimulated emission pumping was used to excite the levels v=23, J=57, and v=38, J=49 via the B–X transition. Laser induced fluorescence from the D–X system was used to follow the collision dynamics. Energy transfer processes induced by collisions with He,Ar,N2,O2,Cl2,I2, and H2O were investigated. Rotational energy transfer was found to be efficient for all collision partners. In accordance with classical models, the total rotational transfer rate constants were proportional to the collision momentum (except for H2O). The total transfer rate constants and the distributions of rotational levels populated by collisions were not dependent on the initial vibrational state. For colliders that are not good quenchers of I2(B), the rotational energy transfer dynamics of the X and B states were found to be very similar. For colliders that are good quenchers, comparisons of the X and B state dynamics show that quenching competes with rotational energy transfer in the B state. Vibrational energy transfer was characterized for all collision partners with the exception of I2, which appears to have a low vibrational transfer efficiency. Vibrational transfer was dominated by Δv=−1 steps. Multiquantum vibrational transfer was not observed. The dependence of the vibrational transfer rate constants on the initial vibrational state appeared to be weaker than the linear scaling predicted by the Landau–Teller model. Vibrational deactivation of I2(X) plays an important role in chemically driven oxygen–iodine lasers. Effective deactivation rate constants have been derived from the vibrational transfer rate constants. Estimates for the deactivation rate constants for O2 and H2O differ from those currently in use by almost an order of magnitude. © 1997 American Institute of Physics.

Notes: Optical potentials are used in a quantum mechanical treatment of loss processes, e.g., ionization, where the loss of flux is described by the imaginary part. We present a numerical method for calculating two-center two-electron integrals necessary to construct the imaginary part of the optical potential. By introducing Slater-type orbitals with complex-valued exponents (CSTOs), we are able to represent the free electron wave with a limited number of CSTOs. For the representation of free electron wave functions with many oscillations, i.e., in a large r range or for a high kinetic energy, these new CSTOs form a more natural set of basis functions. The introduction of CSTOs is inevitable for the calculation of integrals concerning collisions in the mK energy range, where the interaction acts over large internuclear distances. Extensive numerical checks show that the final imaginary part of the optical potentials can be calculated with an accuracy better than 2%. © 1997 American Institute of Physics.

Notes: A model for a chemically associating fluid, adsorbed in a disordered porous media, is proposed. The formation of the associates occurs through the directional bonding between the fluid particles. For simplicity, we restrict our attention to the dimerization of particles. In the absence of association, this model reduces to that of Kaminsky and Monson (KM) for the adsorption of methane in a xerosilica gel. This model is studied by means of the replica Ornstein–Zernike ROZ equations, with the hypernetted chain approximation, extended for associating fluids. It follows from a comparison with the computer simulation data that this theory yields a very good description of the structural properties of the KM model. The influence of the fluid density, the matrix packing fraction, and the association energy on the dimerization in the disordered matrix is studied. The fluid compressibility for the KM model and for the dimerizing fluid in a disordered matrix is obtained via the compressibility equation. © 1997 American Institute of Physics.

Notes: A systematic ab initio investigation has been carried out to examine the effects of a basis set and correlation method on barrier height to linearity of bent triatomic CH2 and H2O in their ground electronic states. The theoretical models employed varied from Hartree–Fock with minimal STO-3G to highly correlated quadratic configuration interaction method QCISD(T) with Dunning's correlation-consistent aug-cc-pVQZ basis set. It is shown that the barrier height to linearity is very sensitive to the choice of the model and it is necessary to employ a proper theoretical model with large enough basis set to be assured of the convergence of the calculated barrier height. We found that calculated barrier height to linearity tends to be higher than experimentally determined barrier height in most cases, which is caused by the slower convergence of energy at linear saddle point geometry than at bent equilibrium geometry. The effect of frozen core approximation in correlated calculations is shown to always increase the barrier height to linearity slightly (less than 3% of total amount) from the value with the full activation of orbitals, regardless of the basis set or correlation method used in the study. © 1997 American Institute of Physics.

Notes: The isotropic hyperfine coupling constants of several organic radicals including CH3, CH2, CH2−, C2H5, C2H3, H2CN, C6H7, and C3H5 are calculated analytically using the coupled cluster (CC) "relaxed density'' matrix approach. We employ three different commonly used basis sets with CCSD and CCSD(T) in order to calibrate expected accuracy. The Chipman basis set combined with the CCSD(T) method performs best for carbon isotropic hyperfine coupling constants with a mean absolute deviation within 8% compared to experiment. The corresponding mean absolute deviation for hydrogen isotropic hyperfine coupling constants from experiment is 12%. We show that the UHF, ROHF, and quasi (QRHF) reference function CCSD spin densities are effectively numerically equivalent in the notorious case of the allyl radical. © 1997 American Institute of Physics.

Notes: Geometries and energy separations of 32 low-lying electronic states of Rh5 with different structures have been investigated. Complete active space multi-configuration self-consistent field method followed by large scale multireference singles + doubles configuration interaction computations that included up to 2.09 million configurations were used. Three nearly degenerate electronic states, namely, 2B2(C2v) with a distorted trigonal bipyramid, 4A2(C2v) with a distorted trigonal bipyramid, and 4A2(C2v) with a distorted tetragonal geometry were found as candidates for the ground state of Rh5. The calculated ionization potential from the ground state structure was found to be within the observed experimental range. The atomization and dissociation energies have also been computed and the results are compared with the smaller clusters. © 1997 American Institute of Physics.

Notes: R-matrix theory combined with generalized quantum defect theory is used to calculate the electronic spectrum of the CaF and BaF molecules from the ground state up near the ionization limit. The approach, an effective one-electron method similar in spirit to the ligand-field model of Rice, Martin, and Field [S. F. Rice, H. Martin, and R. W. Field, J. Chem. Phys. 82, 5023 (1985)] and to the electrostatic polarization model of Törring, Ernst, and Kändler [T. Törring, W. E. Ernst, J. Kändler, J. Chem. Phys. 90, 4927 (1989)] removes many of the limitations inherent in the previous work. The resulting level energies (effective principal quantum numbers) are in good agreement with the available experimental data and constitute the first quantitative theoretical calculation of the full electronic spectrum of CaF and BaF. Limitations and possible extensions of the theory are discussed, and quantum defects of high orbital angular momentum states are predicted. © 1997 American Institute of Physics.

Notes: Using systematic sequences of correlation consistent basis sets, the accuracy of calculated bond energies De(CH) and equilibrium geometries (re, θe) has been investigated for the CHn and C2Hn series (n=1–4). Perturbation theory (MP2, MP3, MP4), coupled cluster [CCSD, CCSD(T)], and single and multireference configuration interaction (HF+1+2, CAS+1+2) methods have been investigated. Except for the vinyl radical, all of the calculated bond energies showed significant basis set dependence with average errors (standard deviations) of 5.6 (±3.0) kcal/mol for the cc-pVDZ set, 1.4 (±0.8) kcal/mol for the cc-pVTZ set, and 0.5 (±0.4) kcal/mol for the cc-pVQZ set with CCSD(T) wave functions. For the vinyl radical the total variation with basis set was just 0.6 kcal/mol. Strong basis set dependence was also observed for the equilibrium geometries, e.g., for re(CH) the average error decreased from 0.020 Å (cc-pVDZ) to 0.003 Å (cc-pVTZ) to 0.002 Å (cc-pVQZ). The effect of including the core electrons in the correlated calculations was also investigated for the two series. Inclusion of core correlation in the CHn series increased De(CH) by 0.13 (CH) to 0.61 kcal/mol (CH2) and decreased the equilibrium CH bond lengths by approximately 0.0015 Å. For the C2Hn series, correlation of the core electrons increased De(CH) by 0.18 (C2H4) to 1.01 (C2H) kcal/mol, but decreased De(CH) in C2H2 by 0.25 kcal/mol. Predictions are also made for the equilibrium geometries of C2H, H2CC, and C2H3, as well as the CH bond strength of vinylidene and the acetylene–vinylidene isomerization energy. © 1997 American Institute of Physics.

Notes: The paper deals with the relationships between the total rate of a relaxation process occurring in a system with static disorder and the decay rates attached to the different individual reaction channels. It is proven that the models of relaxation constructed on the basis of these two types of rates are equivalent to each other. From an experimentally observed relaxation curve it is possible to evaluate only the density of channels characterized by different relaxation rates and the overall probability distribution of the total relaxation rate. For evaluating the probability density of the individual relaxation rates attached to different channels an approach based on the maximum information entropy principle is suggested. A statistical thermodynamic formalism is developed for the relaxation time of a given channel, i.e., for the reciprocal value of the individual relaxation rate. The probability density of the relaxation time is proportional to the product of the density of channels to an exponentially decreasing function similar to the Boltzmann's factor in equilibrium statistical mechanics. The theory is applied to the particular case of stretched exponential relaxation for which the density of channels diverges to infinity in the limit of large relaxation times according to a power law. The extremal entropy of the system as well as the moments and the cumulants of the relaxation times and of the relaxation rates are evaluated analytically. The probability of fluctuations can be expressed by a relationship similar to the Greene–Callen generalization of Einstein's fluctuation formula. In the limit of large rates the density of channels and the probability density of individual rates have the same behavior; both functions have long tails of the negative power law type characterized by the same fractal exponent. For small rates, however, their behavior is different; the probability density tends to zero in the limit of very small rates whereas the density of channels displays an infrared divergence in the same region and tends to infinity. Although in the limit of small rates the density of channels is very large the probability of occurrence of these channels is very small; the compensation between these two opposite factors leads to the self-similar features displayed by the stretched exponential relaxation. The thermodynamic approach is compared with a model calculation for the problem of direct energy transfer in finite systems. The connections between stretched exponential relaxation and the thermal activation of the channels are also investigated. It is shown that stretched exponential relaxation corresponds to a distribution of negative and positive activation energies of the Gompertz-type. © 1997 American Institute of Physics.

Notes: A functional integral formalism is used to derive the extension of a stiff chain subject to an external force. The force versus extension curves are calculated using a mean-field approach in which the hard constraint u2(s)=1 is replaced by a global constraint 〈u2(s)〉=1 where u(s) is the tangent vector describing the chain and s is the arclength. The theory quantitatively reproduces the experimental results for DNA that is subject to a constant force. We also treat the problems of semiflexible chain in a nematic field. In the limit of weak nematic field strength our treatment reproduces the exact results for chain expansion parallel to the director. When the strength of the nematic field is large, a situation in which there are two equivalent minima in the free energy, the intrinsically mean-field approach yields incorrect results for the dependence of the persistence length on the nematic field. © 1997 American Institute of Physics.

Notes: Using integral formalism we developed the encounter theory of reversible photoionization followed by charge recombination. This is a problem that can not be approached with conventional (differential) formalism, unless ionization is highly exothermic and thus irreversible. In this limit, the integral theory supplemented by the recipe for calculating the ion distribution may be successfully reduced to the differential theory used in our previous work. However, there is no alternative to integral theory when ionization is quasiresonant and the back electron transfer to the excited state should be accounted for. Using the contact approximation we calculated the free-energy dependence of the Stern-Volmer constant of reversible photoionization accompanied by charge recombination. © 1997 American Institute of Physics.

Notes: The kinetics of the oxidation of carbon monoxide on Pt(111) surfaces was studied isothermally by using an effusive directional molecular beam in an arrangement based on a variation of the dynamic method originally devised by King and Wells. Three temperature regimes were identified for this reaction on surfaces precovered with atomic oxygen. Below 300 K no reaction is observed, and the presence of preadsorbed atomic oxygen on the surface does not significantly affect the initial sticking coefficient of CO but only reduces its saturation coverage by less than half, which it does by preferentially blocking the bridge sites. Above 400 K, on the other hand, the desorption of CO2 from oxygen-covered surfaces is controlled by the impinging frequency of the incoming CO. The most interesting temperature range is that between 300 and 400 K, where the rate of surface recombination of CO with oxygen competes with that of CO adsorption; under those conditions the overall dynamic behavior is fairly complex, and not all the surface oxygen is reactive. Furthermore, the reaction rates in this regime not only depend on the coverages of the reactants, but also on how the surface is prepared. Two kinetically distinct types of oxygen atoms develop during the course of reaction in spite of the fact that they all sit on identical sites at the start of the kinetic runs, suggesting that the reactivity of chemisorbed CO depends on the local oxygen coverage of neighboring sites. We propose that such local arrangements modify the adsorption energy for atomic oxygen, and that this in turn changes the activation energy for the oxidation reaction. Previous reported molecular beam experiments were also extended to cover a wider range of surface coverages in order to better determine the dependence of the rate constant for the surface oxidation step on the coverages of CO and oxygen. It was found that while the presence of oxygen on the surface helps the production of CO2, increasing CO coverages augment the activation barrier for this reaction, an observation that is in direct contrast with previous reports. Finally, the adsorption sites for CO during the surface CO+O recombinatory reaction were characterized by reflection–absorption infrared spectroscopy. The data reported here is analyzed and discussed in terms of possible kinetic models. © 1997 American Institute of Physics.

Notes: Prewetting transitions in polymer blends near a hard surface which favors one of the phases in the blend are studied by both mean-field and Monte Carlo methods. The mean-field results predict for systems that have a first-order wetting transition at a bulk density of (ρ∞)W, there exists first-order prewetting transitions for (ρ∞)W≤ρ∞〈(ρ∞)PW. For ρ∞〉(ρ∞)PW there exist second-order transitions so that (ρ∞)PW may be identified as the prewetting critical point. Monte Carlo simulations of the bond fluctuation model on a simple cubic lattice between two hard walls H lattice spacings apart are performed and qualitative agreement is found with the mean-field predictions. © 1997 American Institute of Physics.

Notes: The reactive behavior of catalytic CO oxidation on Pt(210) is studied by means of combined reaction rate measurements and photoelectron emission microscopy (PEEM). These methods allow an investigation of the phenomena at macroscopic and mesoscopic level, respectively. The external control parameters (flow rate, CO and oxygen partial pressures, surface temperature and scanning rates of pressure and temperature) are systematically varied to reveal various reactive regions in parameter space. The macroscopic measurements for a given temperature and flow rate (under isothermal conditions) show that lower pressures lead to a pronounced clockwise hysteresis in the production rate of CO2, while increasing pressures cause a systematic narrowing leading to a crossing of the two hysteresis branches into a region of counterclockwise hysteresis. A further pressure increase leads to macroscopic temporal oscillations. Mesoscopic spatiotemporal oscillations appear at the same conditions. The resulting macroscopic isothermal kinetic phase diagram exhibits a cross-shaped characteristic similar to that previously obtained for the Pd(110) surface. The mesoscopic lateral distribution of CO and oxygen adsorbed on the surface is monitored with the photoelectron emission microscope during the reaction at isothermal conditions and different constant oxygen pressures. The observed mesoscopic spatiotemporal patterns, such as islands, waves, target patterns and spirals, are correlated via the external control parameters with different regions in the macroscopic isothermal phase diagram. The results are compared with previous data of CO oxidation on other surfaces, like Pd(110) and Pt(110). © 1997 American Institute of Physics.