ALBERT

Notizen: Raman scattering measurements on K2ZnCl4 have been performed from 20 up to 250 K in the lattice mode frequency range and in the Zn–Cl stretching mode region. The splitting of certain modes in the low-frequency region and a change in the slope of the frequency vs temperature plot at 105 K show the existence of a phase transition at this temperature. Characteristic phenomena are also observed for the phase transition near 150 K.

Notizen: The rotational spectra of the four monosubstituted isotopomers of protonated cyanogen HN13CCN+, HNC13CN+, H15NCCN+, and HNCC15N+, in the ground vibrational state have been investigated in the spectral region from 130–270 GHz, using a negative glow discharge production method. From the moments of inertia of the five monosubstituted species, including the DNCCN+ isotopomer [G. Cazzoli et al., Chem. Phys. Lett. 194, 297 (1992)] and of the parent species HNCCN+ [T. Amano and F. Scappini, J. Chem. Phys. 95, 2280 (1991)], the complete rs structure has been obtained. The alchemy of the ion production has been further improved compared to the previous works.

Notizen: Time domain diode laser absorption spectroscopy has been used to measure vibrational, rotational, and translational excitation of CO2 and CO following excimer laser photolysis of iodine in a low pressure mixture of CO2 and I2 or CO and I2. Nascent rotational population distributions have been measured in a number of low-lying CO2 vibrational levels, including 0001, 1000, 0200, 0220, and 0002 as well as the v=1 level of CO. In addition, measurements of CO2 translational excitation have been obtained for the majority of the rovibrational states probed. Significant vibrational excitation of CO2 has been observed with almost no increase in rotational and translational energy of the molecule. These results are consistent with the production of vibrationally excited CO2 via collisions with hot electrons which arise from multiphoton ionization of I2. Direct detection of the electrons has been accomplished using a time-resolved magnetic induction technique.

Notizen: Time-dependent quantum-mechanical theories and simulations provide a clear and intuitive description of molecular processes. Due to ensuing simplification of the theory and the generally employed numerical algorithms, the vast majority of these treatments are based upon perturbation theory. Especially in light of the current level of experimental sophistication, with experiments being realized which are influenced by the spectral, temporal, and spatial shape of the laser pulse, it is important to move beyond treatments limited to weak fields or idealized δ-function wave forms. Various methods to examine the results of high-field simulations are presented. All of the techniques are shown to have the familiar linear response form in the weak-field limit. In a time-dependent framework the difference between the linear and nonlinear response expressions can be seen from expectation values over stationary versus nonstationary states. The high-field photodissociation of methyl iodide illustrates this approach. Methyl iodide represents a physical system well suited for examining the effects of such exciting laser-field characteristics as strength, linewidth, and frequency upon the photodissociation dynamics. Its dissociation occurs upon coupled repulsive excited electronic potential-energy surfaces which have recently been revised to fit the most current experimental data. The effect of the surface intersection has previously been typically studied by examining the branching and the internal state distributions of the products in the two channels as a function of excitation frequency only.The collinear photodissociation dynamics is examined using a numerically exact time-dependent quantum-mechanical method. The equations of motion for the amplitudes upon the ground and two coupled excited electronic surfaces, explicitly incorporating the laser field, are integrated by a scheme which employs a low-order polynomial approximation to the evolution operator. The effects of the three field characteristics upon the branching ratio and internal state distributions of the products and the spectroscopy of the process are delineated. The course of the photodissociation dynamics is shown to be affected by these characteristics. The results demonstrate the causal connections between the pulse shape and the resulting photoprocesses. Practical manifestations of strong fields (power broadening, sub-threshold absorption, higher harmonic generation, emission shaping of the ground state, temporal development) are stressed.

Notizen: The complex coordinate scattering theory for the calculation of T-matrix elements, as was introduced by Engdahl, Moiseyev, and Maniv [J. Chem. Phys. 94, 1636 (1991)] and by Peskin and Moiseyev [J. Chem. Phys. 96, 2347 (1992)], is shown to satisfy the complex version of the Kohn variational principle introduced by Nuttall and Cohen [Phys. Rev. 188, 1542 (1969)]. This theory and the related S-matrix version of the Kohn variational principle, developed by Zhang, Chu, and Miller [J. Chem. Phys. 88, 6233, (1988)] are combined to formulate a generalized variational basis set approach for quantum scattering calculations. In this approach the Kohn variational procedure to optimize the linear parameters in the T matrix is followed by an optimization of the complex nonlinear parameters. This enables the application of the complex coordinate analytical continuation of the T matrix to the calculation of scattering probability amplitudes for long range potentials. Illustrating numerical applications to short and long range potentials are given.

Notizen: State-to-state total cross sections for rotational excitation and inversion of NH3 by collisions with Ar have been calculated within the accurate close coupling framework. The inversion motion in NH3 was included both via a delta function model and by taking the inversion coordinate explicitly into account. We used an ab initio potential and a potential in which one term in the angular expansion of the ab initio potential is scaled in order to reproduce spectroscopic data. At the energies of these calculations the delta function model is found to be in nearly quantitative agreement with the "exact'' inversion results. Comparison with experiment shows the original ab initio potential to be better than the scaled one. The state-to-state cross sections for ortho-NH3 are in general accord with the measurements. For para-NH3 the agreement is good also, but the relative magnitudes of the cross sections for transitions to the ± inversion states of the same rotational level are not reproduced correctly for all levels.

Notizen: Ab initio energy calculations using the Møller–Plesset perturbation theory up to fourth order with the 6-31G(D,P) basis set have been performed for the H5O2+ ion cluster with and without a hydrogen molecule attached to it. In agreement with experimental results, a C2 structure which gives rise to only two high O–H stretching frequencies is predicted to be the only minimum of the isolated ion, whereas when H2 is present, again only one minimum is found which can be roughly assigned to be of the Cs type. This structure is predicted to have four active O–H stretching frequencies, again in agreement with the experimental spectrum.

Notizen: We present the results of quasiclassical trajectory (QCT) and quantum centrifugal sudden hyperspherical (CSH) scattering calculations for the Cl+HCl→ClH+Cl reaction using a semiempirical potential energy surface. In particular, we report state-to-state integral and differential cross sections in the vicinity of a transition state resonance that occurs at a total energy E of 0.642 eV. This resonance, which is labeled by the transition state quantum numbers (0,0,2), strongly perturbs the cross sections for the initial rovibrational state HCl(v=1, j=5), which was therefore considered in all our calculations. For E≥0.680 eV, which is well removed from the resonance energy, the QCT and CSH results are in good agreement, but for E near the resonance energy, important quantum effects are found in the integral cross sections, product state distributions, and differential cross sections. The CSH integral cross sections show smooth steplike increases for E≈0.642 eV, which are not seen in the QCT results. Associated with these steps are increased branching to the v'=0 product HCl vibrational state, and a strong propensity for the production of rotational states with j'=15 and 16 for v'=0. These features of the product energy partitioning are not present in the QCT results, although the correct rotational distributions are approximately recovered if the final vibrational action is constrained to match its quantum value. The CSH differential cross sections show a sudden shift from backward to sideward scattering between 0.642 and 0.660 eV, while the QCT cross sections remain backward peaked. An analysis of the "number of atom–diatom encounters,'' during the course of a reactive collision, shows that there are chattering trajectories. These are associated with sideward scattering, but their probability is low and as a result they do not produce distinct features in the angular distributions. However, if the classical deflection function is weighted by the quantum reaction probability, angular distributions are obtained that are in reasonable agreement with the CSH angular distributions (including resonance features).

Notizen: The results of the first variational and Green's function Monte Carlo calculations of the vibrational ground states of He2Cl2 and He3Cl2 van der Waals (vdW) clusters are presented in this paper. The quantum dynamics of all internal degrees of freedom are treated exactly. The ground state wave function of He2Cl2 is characterized by means of the probability distribution functions of the intermolecular degrees of freedom, which reveal an exceptionally fluxional vdW complex. A simple model for the ground state of HenCl2 vdW clusters was developed. The zero-point energies of He2Cl2 and He3Cl2 predicted by this model are in remarkable agreement (to within 0.6%) with the accurate results.

Notizen: A class of approximate projection operators is used to reduce the variance in path integral Monte Carlo calculations in a formally exact manner. Paths are classified according to the projection operators, allowing the identification of paths whose contribution to the variance is negligible. The approach is applied to two canonical systems: Two like-spin electrons in a three dimensional harmonic well and the two dimensional Hubbard model, representing localized and extended electronic states, respectively. Time savings of 15–900 over straightforward Monte Carlo calculations are observed.

Notizen: We report time-resolved optical double resonance spectroscopic experiments in which gas-phase acetylene molecules are selectively prepared and monitored in discrete rotational states of the v2=1 (C≡C stretch, 1974 cm−1) vibrational level. This is achieved by pulsed coherent Raman excitation and laser-induced fluorescence detection. State-selective spectra of single rovibrational states are presented under effectively collision-free conditions. Several new rovibronic bands in the A˜←X˜ absorption system of acetylene are identified in this way, owing to the enhanced sensitivity and spectral simplification of our Raman-optical double resonance technique. Investigations of C2H2(g) concentrate on rotationally resolved vibronic bands of the form 21030x (where x=1,2,3,...), exploring spectroscopic subtleties such as axis switching. The method has also been extended to the 21030x410 vibronic bands of C2H2(g), by Raman excitation in the (ν2+ν4−ν4) hot band, and to studies of the deuterated isotopomers, C2HD(g) and C2D2(g). Two distinct experimental strategies are demonstrated, in terms of their utility for spectroscopic assignment and energy transfer applications. One such approach comprises a rovibronic fluorescence excitation spectrum, recorded with fixed Raman excitation frequency. The alternative approach yields state-selected Raman spectra, with the Raman excitation frequency varied and the rovibronic excitation wavelength fixed.

Notizen: Molecular beam depletion spectroscopy has been employed to study the dissociation of small methyl fluoride clusters upon excitation of the ν3 C–F stretch vibration at 1048.6 cm−1. Size selection has been achieved by dispersing the (CH3F)n cluster beam by a secondary rare gas beam. For the methyl fluoride dimer only very weak dissociation signals could be observed. The corresponding spectrum features a single, 13.4 cm−1 broad absorption line. This observation is explained with a symmetric dimer structure, in which both monomer units reside at equivalent positions, and an inefficient coupling of the molecular vibration to the intermolecular bond. For the trimer and tetramer very strong dissociation yields are observed. Whereas the trimer shows a complicated spectrum which is attributed to its nonsymmetric structure, the tetramer spectrum is again characterized by a single peak. In order to obtain supplementary information, dissociation spectra have also been measured for small methyl fluoride clusters residing inside or on the surface of large Arx host clusters. These matrixlike spectra are consistent with the free gas-phase cluster data. Finally, in a computational approach, the structures of the methyl fluoride dimer, trimer, and tetramer have been determined by total energy minimization. The theoretical results are in perfect agreement with the experimental findings.

Notizen: The discrete variable representation (DVR) is used to calculate vibrational energy levels of H2O and SO2. The Hamiltonian is written in terms of bond length–bond angle coordinates and their conjugate momenta. It is shown that although these coordinates are not orthogonal and the appropriate kinetic energy operator is complicated, the discrete variable representation is quite simple and facilitates the calculation of vibrational energy levels. The DVR enables one to use an internal coordinate Hamiltonian without expanding the coordinate dependence of the kinetic energy or evaluating matrix elements numerically. The accuracy of previous internal coordinate calculations is assessed.

Notizen: The (X 2Σ+g)np Rydberg states converging to the X 2Σ+g, v+≥9 ionization thresholds of molecular hydrogen were excited by double-resonance excitation via the E,F 1Σ+g, v = 6 level. The energy region of interest (139 500–140 000 cm−1) included the X 2Σ+g, v+=9, N+=0–3 ionization thresholds as well as the energetic threshold for ion pair formation. The decay of the autoionizing Rydberg states was studied by both conventional and constant-ionic-state photoelectron spectrometry. The results support the Δv=minimum propensity rule for vibrational autoionization of Rydberg states with a high level of vibrational excitation. However, the relative intensities of the (X 2Σ+g)np, v'≥ 9 Rydberg series converging to different rotational levels of the ion are dramatically different from those for the analogous Rydberg series with v'≤2. In addition, the two-color photoelectron spectrum via the E,F 1Σg+, v=6 level shows considerable intensity in the v+=7 and 8 photoelectron bands both on and off resonance, in marked contrast with the single-color photoelectron spectrum obtained for the same intermediate levels. These observations may be due to the dependence of the transition dipole matrix elements on the internuclear distance and to the presence of doubly excited electronic states that cross the H2+ X 2Σ+g potential curve at a total energy close to that accessed by the two-color excitation scheme.

Notizen: Infrared spectra (3100–2600 cm−1) of HCl dissolved in liquid argon (94–124 K), liquid krypton (117–167 K), and liquid xenon (161–221 K) at concentrations varying from 0.8×10−3 to 2.8×10−2 M are reported. At low concentrations in all three solvents, only the spectrum due to monomeric species is observed. For solutions in liquid argon, the observed rotational fine structure was assigned. For all solvents, the monomeric stretching frequency shows a linear relation to the relative density of the solvent, extrapolating to the dilute vapor phase frequency. At higher concentrations in liquid argon and liquid krypton, bands due to oligomeric species are found. Factor analysis shows at least three oligomeric species are present. The band profile analysis of the oligomer absorptions allows the assignment of observed bands to dimer, trimer, and tetramer. From the temperature dependence of the oligomer band intensities, the enthalpy difference for dimerization is found to be 3.78±0.33 kJ mol−1 in liquid argon and 5.09±0.55 kJ mol−1 in liquid krypton. The trimerization and tetramerization enthalpy differences in liquid argon were measured to be 12.6±1.9 and 18.7±3.2 kJ mol−1, respectively.

Notizen: Ground state rotational spectra of the three isotopomers (CH2)2O...H35Cl, (CH2)2O...H37Cl, and (CH2)2O...D35Cl of a short-lived hydrogen-bonded dimer have been detected in the reactive mixture of oxirane and hydrogen chloride by using a fast-mixing nozzle in conjunction with a Balle–Flygare Fourier-transform microwave spectrometer. Rotational constants, centrifugal distortion constants and Cl-nuclear quadrupole coupling constants were determined for each isotopomer. In particular, all four components χaa, χbb, χcc, and χac of the coupling tensor were obtained. A detailed analysis of the rotational constants allows the conclusion that the dimer has Cs symmetry, with a steeply pyramidal arrangement completed at oxygen by the hydrogen bond with HCl. Diagonalization of the complete Cl-nuclear quadrupole coupling tensor leads to the principal axis components χxx, χyy, and χzz (where z is the HCl direction in the dimer). The angle of rotation α is the angle between the HCl (z) direction and the a-axis direction in the equilibrium conformation of the dimer. It is larger by ∼10° than the angle γ between the O...Cl internuclear line and the principal inertial axis a in each case and implies that the hydrogen bond is bent by 180-θ=∼16.5° from the collinear arrangement O...H–Cl (θ=0). The angle 180-θ and the angle φ=76.2° made by the O...Cl internuclear line with the extension of the oxirane local C2 axis are interpreted in terms of a simple model of the hydrogen bond.

Notizen: Spontaneous radiative lifetimes of the observed electronic states of CuCl have been calculated using a model including spin–orbit interaction mixings within the Cu+(3d94s)Cl−(3s23p6) structure. The required wave functions have been determined semiempirically and locations of the as-yet unobserved components have been estimated. For the A 3Σ1+, C 3Π0e, D 1Π1, E 1Σ0+, and F 3Δ1 components, the lifetimes calculated in the simple single-configuration pure-precession approximation with a slight modification in the Ω=0e block are in good agreement with earlier experimental measurements. On the contrary, for the B 3Π1 component, there is a marked discrepancy that cannot be reduced except by assigning very unlikely values to the off-diagonal spin–orbit parameters.

Notizen: A new approach for many-body perturbation theory (MBPT) built upon a restricted open-shell Hartree–Fock (ROHF) reference function is presented. ROHF-MBPT is shown to give much improved results compared to unrestricted Hartree–Fock (UHF) MBPT in cases where there is large spin contamination of the UHF reference function, and to converge much more rapidly to the infinite-order coupled-cluster result. Equations for analytical gradients at the MBPT(2) level are described and implemented. ROHF-MBPT and restricted open-shell Hartree–Fock single- and double-excitation coupled cluster (ROHF-CCSD) applications are presented for several difficult cases. These include the structure and electron affinity of the CN radical; structure, binding energy, and vibrational frequencies of Li3; the structure and vibrational frequencies for the unobserved FCS molecule; and the multiplet structure of the Ni atom.

Notizen: The successive ionization potentials (IP's) of atoms He–Zn are calculated using the relativistic and correlated local-density RCXi method. The contribution of correlation energy to IP's of these atoms are reported. It is found that these correlation contribution to IP's are different for different IP's of the same atom. It is also different for a given IP for different atoms. This behavior is qualitatively explained on the basis of the results of pair-correlation energy. A simple approximate expression to calculate the pair-correlation energy proposed earlier is discussed.

Notizen: The energy surface for the interaction between water and the close-pack Hg surface was computed at the relativistic core potential Hartree–Fock+second-order many-body perturbation theory level. The binding energies were found to be 13.1, 12.2, and 11.6 kcal/mol for the binding of a water molecule to the ontop, bridging, and hollow sites, respectively. The equilibrium surface-to-oxygen distances were found to be 5.33, 4.89, and 4.86 bohrs for the ontop, bridging, and hollow sites, respectively. The water molecule physisorbs with the hydrogens pointing away from the surface. The mechanism of the physisorption bonding and physical explanation of the binding-site preference is also presented.

Notizen: The model system of the rotor chain is considered in order to analyze the coupling between conformational transitions and the librational motions, i.e., the small amplitude displacements within the energy wells of the stable states. The master equation for site (conformer) distributions (MESD) on the torsional angle displacements is the suitable formalism which allows one to describe at the same time both types of motions. While in the previous work the MESD was proposed on a phenomenological ground, the formal derivation of the MESD is now provided by means of a projection procedure. The ingredients of the MESD, in particular the coordinate-dependent rate kernel, are determined unambiguously in this way. A reduced equation without site indeces is recovered by applying the MESD to the rotor chain with equivalent potential minima. A new procedure based on the Gaussian approximation for the site distribution is introduced to solve the reduced equation. This allows one to calculate rather easily correlation functions for different observables. A good agreement is found in the comparison with the available Brownian molecular dynamics simulations on the same system. The behavior of the orientational correlation functions is examined in detail in order to disentangle in the overall relaxation the contributions of the conformational dynamics from the effects of the librational motions.

Notizen: In this paper we present calculations of the yield and polarization of the fluorescence from the Hg λ253.7 nm transition following photodissociation of the Hg–Kr van der Waals molecule. We have used a coupled channel wave packet method to predict the alignment produced in photodissociation [J. Chem. Phys. 95, 8124 (1991)]. We show that the alignment effects depend sensitively on the repulsive part of potential near photodissociation threshold. We have made predictions for three different sets of interatomic potentials for Hg–Kr proposed by different authors [M. Findeisen and T. Grycuk, J. Phys. B 22, 1583 (1988); M. Findeisen, T. Grycuk, and J. Staby, The B and X State Potentials for Hg–Ar, –Kr, –Xe (to be published); M. Okunishi, H. Nakazawa, K. Yamanouchi, and S. Tsuchiya, J. Chem. Phys. 93, 7526 (1990); D. M. Segal and K. Burnett, J. Chem. Soc. Faraday Trans. 2 85, 925 (1989)].

Notizen: Deexcitation cross sections of He(2 1P) by N2 and O2 have been measured using a pulse radiolysis method in a region of the mean collisional energy between 18 and 38 meV. Fairly large deexcitation cross sections ((approximately-equal-to)100 A(ring)2) by N2 and the negative slope of the curve of cross section vs energy are interpreted in terms of the energy transfer cross section based on a dipole–dipole interaction. Validity of theoretical formula based on a semiclassical approach with rectilinear trajectories, i.e., the Watanabe–Katsuura formula, is discussed. The deexcitation cross section of He(2 1P) by O2 and its collisional energy dependence are not very different from that by N2. Optical model calculation of the deexcitation cross section has been also presented for collisions between He(2 1P) and M (M=H2 or N2) with an isotropically averaged complex potential optimized through a fitting procedure to the experimental cross sections. It has been found by the model calculation that an electron exchange interaction is not negligible in the deexcitation process.

Notizen: We report the first observation of infrared multiphoton dissociation of a strongly bound diatomic molecule, HCl+. The dissociation is explained by a charge–resonance coupling of electronic states of the molecular ion HCl+. This coupling results in Stark shifts which depend on the internuclear separation thereby changing the molecular bonding. Using a barrier suppression model, we obtain good agreement with the observed dissociation threshold. We show the close relationship between barrier suppression and chaotic dissociation. We also report the first quantitative theoretical and experimental study of infrared multiphoton ionization of a small molecule, HCl. Based on tunnel ionization, we develop a molecular ionization model that incorporates both the large Stark shifts of the molecular ion and the associated large induced dipole moments. The model agrees with the experiment for the multiphoton ionization of HCl. It allows us to derive a general expression for the maximum intensity which can be applied to a neutral molecule without ionizing it.

Notizen: The free energy of polymer chains connected by slip links has been investigated in this work. For the problem of two network chains connected by a single slip link a degeneracy of the free energy surface above the slip-link configuration space was obtained. The degeneracy depends on the lateral separation d of the chains. In the case of d≥dc, where dc is some critical distance, the degeneracy vanishes. The reason for this nontrivial free energy behavior is the concurrence of two entropic forces. One is the usual force against stretching of the chains parts and the other is the attempt by system to reduce the influence of the constraint caused by the slip link. For chain configurations with more than one slip link, a tendency for the slip links to form clusters to get a higher amount of entropy can be shown.

Notizen: Dissolution (mixing or melting) of inhomogeneities formed during spinodal decomposition in binary polymer mixtures is studied both experimentally and theoretically. The details of the dissolution experiment with time-resolved light scattering on polystyrene/poly(vinylmethylether) are presented. The theoretical approach differs from that of Langer, Bar-on, and Miller in the way the fluctuations are treated in the nonlinear theory, and in the details of the calculations arising from the chain connectivity (polymer effect). The effect of mode coupling arising from nonlinearity on the relaxation rate is discussed. It is found both experimentally and theoretically that the wave number corresponding to peak intensity decreases in time asymptotically following a t−0.5 power law.

Notizen: A dynamic Monte Carlo technique was applied to gas-surface reactions simulating diamond growth under chemical vapor deposition. A combined methyl-and-acetylene reaction mechanism was assumed, where the additions of methyl radicals and acetylene molecules are allowed to occur only when no steric interferences arise. The sterically resolved computations demonstrate nonlinear kinetic coupling: methyl and acetylene additions occur simultaneously and interdependently on each other−adsorption of CH3 creates sites for C2H2 addition, and addition of C2H2 creates sites for CH3 adsorption. It is also shown that the incorporation of acetylene by three-center additions only, irreversible on physical grounds, is capable of explaining the rate of diamond growth, thus dismissing the argument of reaction reversibility advanced against our proposed mechanism of acetylene addition.

Notizen: As in the preceding paper (Paper I) we study here a model of chains with excluded volume enclosed in a "box'' on a square lattice. The system is simulated by the Metropolis Monte Carlo method and the entropy is extracted from the samples by using the "hypothetical scanning method.'' With this method each system configuration is treated as if it has been generated step by step with the scanning method (studied in Paper I). The transition probabilities are reconstructed and three approximations of the entropy are obtained. Thus the pressure and the chemical potential are calculated directly from the results of the entropy as in Paper I using standard thermodynamic relations. These results are found to be in a very good agreement with those obtained in Paper I, which are considered to be exact within the statistical error.

Notizen: The diffuse scattering of x rays by thin films limited by rough interfaces is presented theoretically in terms of height–height correlation functions. The method employed, analogous to the use of Green functions, allows a rigourous treatment down to grazing incidences. An expression for the scattered intensity is obtained from the scattering cross section. The results are then discussed using a model correlation function, and the interpretation of x-ray reflectivity experiments is reexamined. We show that off-specular scans yield direct information about the correlations between interfaces which, in case of liquid thin films, can be used to determine elastic parameters. The generalized case of stratified media is examined in an appendix. This theory is compared to experimental results in the case of soap films in a companion article.

Notizen: Using the scanning simulation method we study a system of many chains with excluded volume contained in a "box'' on a square lattice. With this method an initially empty box is filled with the chains monomers step by step with the help of transition probabilities. The probability of construction, P of the whole system is the product of the transition probabilities used and hence the entropy S is known, (S∼ln P). Thus the pressure and the chemical potential can be calculated with high accuracy directly from the entropy using standard thermodynamic relations. In principle, all these quantities can be obtained from a single sample without the need to carry out any thermodynamic integration. Various alternatives for performing the scanning construction are discussed and their efficiency is examined. This is important due to the fact that for lattice polymer models the scanning method is ergodic (unlike some dynamical Monte Carlo techniques). The computer simulation results are compared to the approximate theories of Flory, Huggins, Miller, and Guggenheim and to the recent improved theories of Freed and co-workers.

Notizen: The adsorption of a fluid of hard spheres of diameter σ onto a planar surface containing a triangular lattice of adsorption sites, spacing σ, was studied using a model which is equivalent to a lattice gas with n-body interactions that are related to the n-body correlation functions of the fluid. The present paper extends our previous work, which included only pairwise interactions, to also include triplet interactions. We discovered a simple but accurate analytical approximation to the triplet correlation function for three spheres in mutual contact, which, when combined with a Müller–Hartmann and Zittarz approximation of the critical point in the equivalent lattice gas, yields the estimate ρcσ 3=0.8409 of the fluid density at the critical point of the condensation transition which occurs at the fluid–crystal interface. This estimate, which includes the effects of both pair and triplet correlations in the fluid, is significantly higher than the value ρcσ 3=0.6678 obtained if only the effects of pair correlations are included in the calculation.

Notizen: The diffuse scattering of x rays by black-soap films has been investigated. The results were analyzed using a treatment of surface scattering developed in a companion paper. We show that the fluctuations of the surfaces limiting the film are correlated at wavelengths on the order of 100 nm. The results are consistent with the Derjagin–Landau–Verwey–Overbeek theory when applicable. Reflectivity results have been reinterpreted within this frame, showing that the Newton black film can be seen as a single fluctuating membrane.

Notizen: We have determined an empirical potential energy function for the interaction of xenon with the Pt(111) surface which is consistent with a wide range of dynamical and equilibrium experimental data. These include scattering measurements, with detailed angular distributions and energy transfer data, at incidence energies from 0.5 to 14.3 eV. Also used are thermal desorption rates and trapping probabilities, as well as thermodynamic properties of monolayer phases including the "energy jump'' at the transition from the commensurate to the uniaxially compressed incommensurate phase. The potential also agrees with an experimental value for the frequency of vibration normal to the surface, and has the correct asymptotic behavior at large distances from the surface (V=−c3/z3, with an experimental estimate of c3). The equilibrium position for a single Xe atom lies directly above a surface platinum atom, and the calculated height above this atom is 3.35 A(ring).

Notizen: The dielectric permittivity and loss tangent of hyperquenched glassy water (HGW) have been measured for fixed frequencies of 1 and 10 kHz from 80 K to its crystallization temperature and corresponding measurements have been made of the crystallized forms. The effect of thermal cycling has been investigated. Except for a shoulder at T〈Tg for 10 kHz measurement, there are no features in the loss tangent until the beginning of crystallization when the increase in tan δ of the water above Tg is dominated by the decrease on the formation of cubic ice and a peak appears. The dielectric loss of HGW at 80 K is ≈5 times more than that of cubic ice formed on crystallization by heating to 195 K and 2.8 times higher at Tg. The dielectric loss of the cubic ice formed on crystallization tends towards a plateau value prior to rapidly increasing with increasing temperature, which is evidence for a low temperature relaxation which vanishes on conversion to hexagonal ice. The plateaulike region indicates the presence of topologically disordered structure of intergranular water. The loss tangent lacks any feature that can be attributed to the presence of "dangling'' OH groups. A comparison of these results with those of in vacuo sintered vapor-deposited amorphous solid water (ASW) [Johari et al., J. Chem. Phys. 95, 2955 (1991)] shows that the presence of ≈5% cubic ice in HGW causes its crystallization to occur more rapidly, leading to larger grains or less intergranular space and a lower value of dielectric loss of the cubic ice formed by heating it, than that formed by heating ASW. Implications of these results for the heat evolved in the phase transformation process have been discussed.

Notizen: UV laser irradiation of ammonia adsorbed on GaAs(100) leads to molecular desorption, with a mean translational temperature of 〈Etrans/2k(approximately-greater-than)=300 K, independent of photon energy and isotope substitution. However, the photodesorption cross section depends strongly on isotope substitution: σNH3/σND3=4.1 at hν=6.4 eV. This isotope effect is too large to be accounted for by the mass difference in the leaving particles (NH3 vs ND3), but can be successfully explained in terms of an isotope effect in the internal N–H(D) coordinates. We take this as evidence for uv-driven photodesorption from electronically quenched, but vibrationally hot ground state ammonia.

Notizen: Recent research has demonstrated that optimal electromagnetic fields capable of producing selective vibrational excitation in molecules can be designed employing linear quadratic control methods using a cost functional that balances the energy distribution in the molecule, the fluence of the optical field, and a final cost to insure the desired excitation. Practical computations of molecular control theory for large molecules especially with anharmonic potentials become difficult to obtain due to the increased dimensionality and the accompanying uncertainty in the Hamiltonian. In this paper we reduce the complexity of the problem by treating a portion of the molecule including the target and optical dipoles in full detail, while the remainder of the molecule is modeled as an external disturbance of bounded energy. The optimal control field now minimizes the cost functional which is simultaneously maximized with respect to the energy constrained external disturbance to assure robustness. This optimal design process is commensurate with taking the most pessimistic view of the disturbance. This conservative view was born out in the numerical calculations such that practical laboratory studies should reach results much improved over the worst case design. As an illustration we investigate disturbances of varying energy content for a truncated 20 atom molecular chain where the uncontrolled remainder of the chain is the source of the system disturbance. The sensitivity of the system with respect to the disturbance was found to be strongly dependent on the distance of the disturbance to the target bond and the dipole arrangement. In addition, in the range of physically reasonable disturbance energy the optimal field could be accurately predicted from an asymptotic expansion involving only the reference undisturbed case. Although the present paper takes advantage of linear system techniques, the same robust optimal control procedure can be generalized to nonlinear systems by a variety of means.

Notizen: 2H-nuclear magnetic resonance(NMR)-spin–lattice relaxation experiments have been performed for studying the crossover from viscous (α process) to secondary (β processes) dynamics in the van der Waals liquid orthoterphenyl and the H-bridged network glycerol. The essential and general features, observed in both systems, are the following: (a) a dominating α process in the liquid and viscous regime; (b) a change from exponential to nonexponential spin–lattice relaxation as the temperature is lowered below a characteristic temperature above Tg; (c) the existence of a slow (〉10−9 s) secondary reorientational process in the highly viscous regime; and (d) the existence of a fast (∼10−12 s) local process in the glassy state. Whereas the slower process is shown to be the one known from dielectric studies, we attribute the fast mode to a β process found in quasielastic neutron scattering.

Notizen: We study, by an adiabatic dynamical simulation technique, a mixed classical-quantum model for strongly H-bonded complexes in polar solvents. The solvent influence on the adiabatic proton dynamics is interpreted in terms of a protonic polarization effect, usually referred to as the Zundel polarization. The relation to the solvent-induced proton transfer and the consequences on the broadening of the infrared absorption spectrum are discussed. We show that for increasing solvent-complex coupling, the system passes from an "oscillatory'' to a "reactive'' behavior, whereas the Zundel polarization passes from a familiar electronic-like regime to a saturated regime. In the latter case, a large band broadening, comparable to experimental observations, is obtained in the calculated infrared spectrum.

Notizen: A classical Boltzmann equation which describes the time evolution of the distribution function in the phase space of a single molecule is developed for a low-density gas made up of nonreacting molecules of arbitrary structure. This development is based on the Liouville equation, the equation describing the time evolution of the distribution function in the phase space of the entire system through the Bogoliubov–Born–Green–Kirkwood–Yvon set of equations, the equations which describe the time evolution of the contracted distribution functions. The set of equations is truncated by use of the molecular chaos assumption, the assumption that the dynamical states of a pair of molecules are uncorrelated before a collision. From the general result the form of the collision integral arising from collisions between two diatomic molecules is considered as a special case. This result is then further reduced to the idealized rigid-rotor, harmonic-oscillator model. This result is then shown to be a generalization of an earlier development.

Notizen: This series is concerned with the quantum dynamics of overtone relaxation in planar benzene and in reduced mode planar benzene fragments. In these studies, ultralarge direct product primitive vibrational spaces (of dimension up to 1010) are contracted to active spaces of dimension 5000–10 000. The contractions are carried out via artificial intelligence tree pruning algorithms, or a new iterative wave operator pruning algorithm. The exact dynamics within the active space is then developed via the recursive residue generation method. In part I of this series, emphasis is placed upon v=3 CH overtone dynamics in the 5 and 9 mode benzene fragments C3H and C3H3. Neither system undergoes complete relaxation, but the survival probability in C3H undergoes large amplitude oscillations with a period characteristic of stretch–wag interaction in the CH chromophore. For C3H3, the two initially nonexcited CH stretch modes do not play a significant role in the dynamics for t〈1 ps. However, modes in both systems that have a high degree of wag motion for the initially excited chromophore play a significant role at short times. Comparisons with earlier classical trajectory studies show good correspondence between the classical and quantum results only at short times, t〈0.1 ps.

Notizen: A large number of carbon cluster monoanions, C−n, have now been detected by negative ion photoelectron spectroscopy. In addition, evidence for carbon cluster dianions, C2−n, as small as C2−7 has been obtained mass spectrometrically. In this research we report results of theoretical calculations of structures and energetics of formation of linear carbon cluster monoanions and dianions containing up to ten carbon atoms. A number of different electronic states have been investigated. Self-consistent field (SCF) theory, many-body perturbation theory, and coupled-cluster theory including triple excitations have been used with basis sets containing polarization and diffuse functions. Considerably larger basis sets have also been used in calculations on some of the smaller species. For the monoanions, the observed electron detachment energies and the even–odd alternation thereof are well reproduced by the calculations. For the dianions, the even numbered species are found to be more easily formed than the odd numbered species, in accord with the intensity pattern observed in the mass spectrometric experiments, and with the availability of partially occupied π orbitals. C2−10 is established to be vertically and adiabatically stable to electron loss, while C2−8 is found to be vertically stable but adiabatically unstable to electron loss. Improved calculations may be sufficient to make C2−8 also stable to adiabatic electron loss. C2−7 and C2−9 are both found to be unstable to vertical electron loss, although both have negative highest occupied molecular orbital (HOMO) eigenvalues and C2−9 is stable to vertical electron loss at the SCF level. The geometry changes resulting from the addition of two electrons are significant, especially for the even numbered clusters. Addition of two electrons to the partially occupied π orbitals of the latter leads to strong single–triple bond alternation, which may be rationalized by noting that the dianions are products of double deprotonation of HC2nH. Such an "accordion'' mechanism may have a role in the ability of carbon clusters to conduct electricity.

Notizen: Surface diffusion of Sb on Ge(111) has been measured with the newly developed technique of optical second harmonic microscopy. In this method, concentration profiles at submonolayer coverage are imaged directly by surface second harmonic generation with 5 μ spatial resolution. A Boltzmann–Matano analysis yields the coverage dependence of the diffusivity D without parametrization. Experiments were performed at roughly 70% of the bulk melting temperature Tm. In the coverage range 0≤θ≤0.6, the activation energy Ediff remains constant at 47.5±1.5 kcal/mol, but the pre-exponential factor D0 decreases from 8.7×103±0.4 to 1.6×102±0.4 cm2/s. Both Ediff and D0 are quite large, which is consistent with high-temperature measurements in other systems. The inadequacies of current theories for high-temperature surface diffusion are outlined, and a new vacancy model is proposed for low-coverage diffusion. The model accounts semiquantitatively for the large values of Ediff and D0, and suggests that these quantities may be manipulated using doping levels and photon illumination. An islanding mechanism is proposed to explain the decrease in D0 with θ.

Notizen: Following the recent resolution of the longstanding problem of reconciling constant frequency nuclear-spin lattice relaxation (SLR) activation energies and d.c. conductivity activity energies in ion conducting glasses, we point out a new problem which seems not to have been discussed previously. We report conductivity data measured at a series of fixed frequencies and variable temperatures on a lithium chloroborate glass and compare them with SLR data on identically prepared samples, also using different fixed frequencies. While phenomenological similarities due to comparable departures from exponential relaxation are found in each case, pronounced differences in the most probable relaxation times themselves are observed. The conductivity relaxation at 500 K occurs on a time scale shorter by some 2 orders of magnitude than the 7Li SLR correlation, and has a significantly lower activation energy. We show from a literature review that this distinction is a common but unreported finding for highly decoupled (fast-ion conducting) systems, and that an inverse relationship is found in supercoupled salt/polymer "solid'' electrolytes. In fast-ion conducting glasses, the slower SLR process would imply special features in the fast-ion motion which permit spin correlations to survive many more successive ion displacements than previously expected. It is conjectured that the SLR in superionic glasses depends on the existence of a class of low-lying traps infrequently visited by migrating ions.

Notizen: We have performed ab initio algebraic diagrammatic construction [ADC(3)] Green's function calculations of the valence photoemission spectra of PF3 and NiPF3. We obtained overall good agreement with experiment for both the free PF3 molecule and the PF3 molecule chemisorbed on a Ni(111) metal surface. A comparison to NiCO shows that there are certain similarities between NiPF3 and NiCO, not only in the σ donor–π acceptor bonding mechanism in the ground state, but also in the metal–ligand CT excitations associated with the creation of a valence hole in the ligand. However, it appears that the many-body effect, such as the configuration interactions in the final ionized state, seems to be weaker for NiPF3 than for NiCO, judging from the main line spectral intensity. The quasiparticle picture of the 4e level breaks down completely as in the case of the 1π level of NiCO.

Notizen: The dynamics of an adsorbate described by a two-band generalization of the small-polaron model [J. Chem. Phys. 95, 8599 (1991)] are investigated. Lattice induced mixing of the bands can result in the adsorbates becoming self-trapped at one end of an adsorption site or the other. Tunneling between adjacent sites can either preserve or change the end of the site in which the adsorbate resides. The associated rates have very different temperature and mass dependences, and contribute to the overall diffusion in very different ways. The observed equilibrium and transport properties of the H/W(110) system can be consistently explained within our model.

Notizen: We have investigated the interface formation of Ca with poly(p-phenylene vinylene) (PPV) using x-ray photoemission spectroscopy (XPS). The most astonishing result of the investigations is that the Schottky barrier formation in Ca/PPV is a slow process possibly caused by the oxygen and sulfur impurities segregated on the PPV surface.

Notizen: An infrared double resonance technique for the study of jet-cooled polyatomics is reported which offers state selection, access to one-photon-forbidden vibrations, sub-Doppler resolution, and high sensitivity. Molecular eigenstate spectra of the propyne 2ν1 band reveal a predicted doorway state which mediates the two-stage IVR coupling mechanism.

Notizen: Occurrence of chaos and fractals in the solution to the problem of turning points for a rotating vibrator is pointed out.

Notizen: We have employed picosecond pump–probe techniques in conjunction with a tandem time-of-flight mass spectrometer to investigate the caging dynamics of photodissociated I2− solvated with a specific number of CO2 molecules. In this paper, we report the observation of a recurrence at ≈2 ps in the I2− absorption recovery, a feature which is attributed to coherent I...I− nuclear motion following I2− photoexcitation.

Notizen: We determine Ec(J), the angular momentum dependent energy of onset of classical chaos in HD2+, from the divergence of pairs of trajectories started from microcanonical initial conditions. Ec(J) drops from 0.66 eV at J=0 to 0.03 eV at about J=30, and has a secondary maximum at J=50.

Notizen: We present rotationally resolved data for the v'=0 and v'=1 levels of N2+(B 2Σu+) produced via 2σu−1 photoionization of N2. The data are obtained over a broad photon energy range (19≤hνexc≤35 eV). This is made possible by using synchrotron radiation excitation in conjunction with dispersed fluorescence detection. The results exhibit both resonant and nonresonant effects.

Notizen: For the set of finite-difference equations of Becker–Döring an exact formula for the induction time, which is expressed in terms of rapidly convergent sums, is presented. The form of the result is particularly amenable for analytical study, and the latter is carried out to obtain approximations of the exact expression in a rigorous manner and to assess its sensitivity to the choice of the nucleation model. The induction time, tind, is found to be governed by two main nucleation parameters, Φ*/kT, the normalized barrier height, and g*, the number of molecules in the critical cluster. The ratio of these two parameters provides an assessment of the importance of discreteness effects. We study the exact expression in both the continuous (g*→∞) and the asymptotic (Φ*/kT→∞) limits. Asymptotic results for tind are compared with those previously reported from simulation studies as well as with tind obtained numerically from the exact expression in the present study. Also, the accuracy of the Zeldovich equation, which is produced in the continuous limit, is discussed for several nucleation models.

Notizen: We have measured and assigned more than 800 new far-infrared absorption lines and 12 new microwave absorption lines of the ammonia dimer. Our data are analyzed in combination with all previously measured far-infrared and microwave spectra for this cluster. The vibration–rotation–tunneling (VRT) states of the ammonia dimer connected by electric-dipole-allowed transitions are separated into three groups that correspond to different combinations of monomer rotational states: A+A states (states formed from the combination of two ammonia monomers in A states), A+E states, and E+E states. We present complete experimentally determined energy-level diagrams for the Ka=0 and Ka=1 levels of each group in the ground vibrational state of this complex. From these, we deduce that the appropriate molecular symmetry group for the ammonia dimer is G144. This, in turn, implies that three kinds of tunneling motions are feasible for the ammonia dimer: interchange of the "donor'' and "acceptor'' roles of the monomers, internal rotation of the monomers about their C3 symmetry axes, and quite unexpectedly, "umbrella'' inversion tunneling.In the Ka=0 A+E and E+E states, the measured umbrella inversion tunneling splittings range from 1.1 to 3.3 GHz. In Ka=1, these inversion splittings between two sets of E+E states are 48 and 9 MHz, while all others are completely quenched. Another surprise, in light of previous analyses of tunneling in the ammonia dimer, is our discovery that the interchange tunneling splittings are large. In the A+A and E+E states, they are 16.1 and 19.3 cm−1, respectively. In the A+E states, the measured 20.5 cm−1 splitting can result from a difference in "donor'' and "acceptor'' internal rotation frequencies that is increased by interchange tunneling. We rule out the possibility that the upper state of the observed far-infrared subbands is the very-low-frequency out-of-plane intermolecular vibration predicted in several theoretical studies [C. E. Dykstra and L. Andrews, J. Chem. Phys. 92, 6043 (1990); M. J. Frisch, J. E. Del Bene, J. S. Binkley, and H. F. Schaefer III, ibid. 84, 2279 (1986)]. In their structure determination, Nelson et al. assumed that monomer umbrella inversion tunneling was completely quenched and that "donor–acceptor'' interchange tunneling was nearly quenched in the ammonia dimer [D. D. Nelson, G. T. Fraser, and W. Klemperer, J. Chem. Phys. 83, 6201 (1985); D. D. Nelson, W. Klemperer, G. T. Fraser, F. J. Lovas, and R. D. Suenram, ibid. 87, 6364 (1987)]. Our experimental results, considered together with the results of six-dimensional calculations of the VRT dynamics presented by van Bladel et al. in the accompanying paper [J. Chem. Phys. 97, 4750 (1992)], make it unlikely that the structure proposed by Nelson et al. for the ammonia dimer is the equilibrium structure.

Notizen: Salt effects on the photophysics of 3-aminofluoren-9-one (3-AF) are examined in acetonitrile solution. Infrared spectroscopy reveals no specific interactions between 3-AF and the dissolved ions. Combined steady state and time-resolved studies are used to determine the S0←S1 nonradiative and radiative rate constants. Added salt has a small effect on the radiative rate, which can be quantitatively accounted for by the Strickler–Berg equation. In contrast, electrolytes strongly influence the nonradiative decay rate which are quantitatively accounted for in terms of the energy gap law. No correlation is observed between the energy gap and the ionic strength of the solution. The results are compared to theoretical models for molecular solvation in ionic solutions.

Notizen: A theoretical expression giving the far-infrared spectrum of a dilute solution of diatomic polar molecules in a nonpolar liquid solvent is derived in order to analyze the influence on this spectrum from the J=1 and J=2 components of the Legendre polynomial expansion of the anisotropic solute–solvent potential. A non-Markovian spectral theory incorporating finite correlation of the interaction, mixing effects between rotational lines and quantum intermolecular potential correlation functions is used. This theory allows one to analyze the influence of each anisotropic part of the interaction in terms of a very reduced set of parameters: the strength and the width of its time correlation function. The zero and finite frequency components of the J=2 contribution are also studied. In particular, we show that the use of quantum potential correlation functions gives a zero frequency component not only in the width, but also in the shift of the different rotational lines. Numerical results for DCl, HCl, and HF in SF6 liquid at 273 K are obtained.

Notizen: In this paper we report on the electronic spectroscopy of mass-resolved tetracene⋅Arn (n=1–26) and tetracene⋅Krn (n=1–14) heteroclusters, utilizing two-photon, two-color near-threshold ionization in conjunction with mass-spectrometric detection. The spectra of the S0 → S1 transition and the ionization threshold of these heteroclusters were monitored. The structured spectral features of the S0 → S1 transition of small- and medium-sized (n=1–8) heteroclusters were attributed to the electronic origins of structural isomers and to their intermolecular vibrations. The S0 → S1 spectra of large (n≥9) heteroclusters are broad and were assigned to inhomogeneous broadening due to the coexistence of isomers, with the spectral feature(s) of each distinct isomer being homogeneously broadened. Isomer-specific inhomogeneous line broadening was interrogated by the observation of isomer-specific ionization potentials for medium-sized (n=6–7) heteroclusters and of the dependence of the relative intensities of the spectral features on the conditions of the supersonic expansion.The ionization thresholds of the tetracene⋅An (A=Ar,Kr) reveal a linear (or superlinear) n dependence, being qualitatively different from the sublinear n dependence of the spectral shifts. These different patterns of the size dependence can be traced to the different intermolecular interactions which govern excitation and ionization and to the difference in the charge distribution in S0 and in the positive ion. The experimental spectroscopic data for the spectral shifts and the spectral linewidths were simulated in terms of the first and second moments of the classical line shape, which were obtained from Monte Carlo (MC) constant temperature simulations, in conjunction with a two-parameter fit of the excited-state tetracene–rare-gas potential. The Monte Carlo simulations of the structural fluctuation parameters identified several isomerization phenomena, i.e., correlated restricted and unrestricted surface motion, adcluster isomerization, surface melting and side crossing, and characterized the size dependence of the temperature onsets of these processes for small and medium sized n=2–20 clusters. These isomerization processes could not be interrogated by the investigation of the size dependence of the spectral shifts and linewidths. The size dependence and the isomer specificity of the spectral shifts are well accounted for by the MC simulations. The homogeneous spectral linewidths of small (n〈8) clusters pertain to the spectroscopy of "static'' isomers, while the line broadening of large (n≥20) clusters manifests inhomogeneous line broadening due to the coexistence of wetting and nonwetting isomers. The temperature dependence of the spectral shifts and inhomogeneous linewidths of large (n≥20) clusters provides means for internal cluster thermometry.

Notizen: The adsorption of water on Ni(110) has been studied by thermal desorption spectroscopy (TDS), work function (ΔΦ), Fourier transform infrared reflection–absorption spectroscopy (FTIR-RAS), low energy electron diffraction (LEED), and electron-stimulated desorption ion angular distribution (ESDIAD). The major findings of this study are the following: (1) Water molecules in the chemisorbed c(2×2) half-monolayer do not cluster and the plane of the molecules is highly inclined to the surface normal; (2) no ESDIAD evidence of oriented water dimers is observed at 130 K and no FTIR activity is observed following adsorption at 80 K until multilayers are populated; (3) water has been measured to partially dissociate at a minimum temperature of 205±2 K; (4)the binding energy of water to the Ni(110) surface is increased by H-bonding to adsorbed hydroxyls (produced by the partial dissociation) in a linear OH(ad)/H2O complex with planar symmetry in the [001] direction. This bonding results in a higher temperature desorption state (A1).

Notizen: Cobalt–sodium bimetallic clusters (ConNam, n=3–48) were produced by a two independent laser-vaporization method. Ionization potentials of the ConNam clusters were measured up to m=3 using a tunable ultraviolet laser combined with a time-of-flight (TOF) mass spectrometer. In general, the ionization potentials decrease monotonically with the number of sodium atoms, and the ionization potentials of ConNam+1 decrease by 0.2–0.8 eV compared to those of the corresponding ConNam cluster. However, the amount of IP decrement by the Na doping is relatively large at n≤17 whereas it is small and constant at n≥18. This feature can be explained by a geometric rearrangement; at small n, the Na doping induces a large geometric rearrangement of the cluster, but at large n, the geometric change is small. Reactivity of ConNam cluster toward H2 was also measured and the effect of the Na doping was studied. The reactivity also suggests the geometric change induced by the Na doping. Moreover, we examined the anticorrelation between IP and reactivity of the ConNam clusters, and no anticorrelation between them could be revealed.

Notizen: The inhomogeneities in gels, in particular the mechanism of the formation of the inhomogeneities, were studied. The gels used in this report are water based poly(acrylamide) gels (AA gels) and poly(N-isopropylacrylamide) gels (N-IPA gels). It is known that the cross linkers in AA gels are water insoluble at low temperature and that the linear components in N-IPA gels are water insoluble at high temperature. By changing the gelation temperature at which gels are made, we can control the formation of the inhomogeneous structure in the samples. In order to investigate the inhomogeneities in the gels, two experiments were performed by using the standard laser light scattering method. For the characterization of the degree of inhomogeneities in gels, the position dependence of scattered light intensity from homogeneous and inhomogeneous gels were measured. It was found that the position dependence was a very good indicator of gel inhomogeneities. We also observed the change of the autocorrelation function of scattered light intensity from a sample undergoing the gelation process. It was found that the profile of the correlation functions measured from pregel solution, which is going to be inhomogeneous gel, were different from the correlation functions measured from pregel solution which is going to be homogeneous gel. From the correlation function measured from the sample which will be inhomogeneous gels, two relaxation modes were observed−one is the fast decaying mode which is expected to be observed from ideal gels or ideal semidilute solutions (the cooperative diffusion) and another mode is the slow decaying mode which is not expected to be observed from ideal gels or ideal semidilute solutions. From the experimental results, we discussed the mechanism of the formation of inhomogeneities in gels.

Notizen: The influence of relativity on the binding in the Pt–O molecule is investigated using density functional calculations and proves to be quite important, not only for the potential well but also for the repulsive wall. Using a Born–Mayer fit to this interatomic gas phase potential, we perform a classical trajectory study on high energy O2/Pt(111) scattering (Ei=80 eV). The Born–Mayer form of the interatomic potential leads to a higher degree of dissociation for O2/Pt(111) than for O2/Ag(111) which is also experimentally found. The role of relativity turns out to be significant. The dissociation mechanism, however, does not change when going from O2/Ag(111) to O2/Pt(111). The molecules were found to first gain primarily rotational energy, which is largely transfered to vibration at the turning point of the second atom, in the case of finally dissociating molecules. Since the calculated dissociation in the case of platinum is still less than found experimentally, we investigate the influence of better fits to the interatomic potential, as well as inclusion of the potential well. It is possible to improve agreement with the experimental results by directly reducing the long range of the Born–Mayer potential.

Notizen: Self-, N2-, O2-, H2-, Ar-, and He-broadening coefficients, pressure shifts, and integrated intensities have been measured for most transitions in the Q branch of the ν3 fundamental band of methane using a difference-frequency laser spectrometer. A systematic dependence of the broadening coefficients on the tetrahedral symmetry species and order index is observed with striking similarities for N2, O2, and Ar and for H2 and He buffer gases. Comparison with earlier measurements on other bands and branches of methane indicates very little vibrational, branch, or carbon isotope dependence. Dicke narrowing is evident at intermediate pressures, yielding an average narrowing coefficient and an optical diffusion constant for each gas mixture. A small amount of line mixing is evident for strongly overlapped lines near atmospheric pressure from nonlinear deviations of the observed spectra from the contours extrapolated from lower pressure measurements.

Notizen: The toluene/ammonia cluster system is studied by mass resolved excitation spectroscopy: like the toluene/water system, an extensive ion chemistry is found to exist generating ammonia solvated protons and (solvated) benzyl radicals. Extensive cluster fragmentation (dissociation) is observed even at "threshold ionization energies.'' Nozzle delay and deuteration studies are applied to elucidate the ion chemistry and dissociation of this cluster system. Proton transfer/cluster fragmentation reactions in this system are driven by the stability of the benzyl radical and (NH3)mH+ and solvation stabilization of the toluene ion. Cluster fragmentation by loss of a single ammonia molecule is not observed. Fragmentation is so extensive for this system that spectral features could not be identified with parent clusters.

Notizen: A number of the rovibrational levels of the B 1Π state are found to be strongly perturbed. The energy shifts of the e and f levels of the B 1Π state are studied, and the perturbations between the B 1Π and (2) 3Σ+ states and between the B 1Π and (1) 3Π1 states are identified. Forty-seven centers of perturbation are observed, and the molecular constants of the (2) 3Σ+ and (1) 3Π1 states are estimated. The hyperfine splitting of the (2) 3Σ+ state is observed. Each rovibrational level splits primarily into four lines, and each of the four lines splits into six lines. The splitting is identified as originating from the magnetic dipole interaction primarily with the nuclear spin of 23Na (I=3/2) and secondarily with the nuclear spin of 85Rb (I=5/2).

Notizen: We have measured the extended x-ray absorption fine structure spectra for dense selenium vapor at high temperatures and pressures up to 1600 °C and 150 bar for the first time. For the experiment, we have developed a high-pressure vessel and a polycrystalline sapphire cell of our own design. We have found that dense selenium vapor under the present experimental condition consists mainly of Se2 and the bond length is much shorter than that of liquid Se. With increasing pressure the bond length slightly increases. When the saturated vapor-pressure curve is approached, the bond is largely elongated.

Notizen: The b-type vibration–rotation band of N2⋅SO2 near the SO2 ν3 band origin was observed in a molecular-beam, diode laser direct absorption experiment. Rotational transitions and Stark effect data for this complex were additionally measured using molecular-beam electric resonance methods. The vibrational band origin was 1361.1440(2) cm−1, shifted by 0.9167(2) cm−1 from that of the SO2 monomer. Rotational constants were measured for the upper and lower vibrational states with A‘=8875.3(22) MHz, B‘=1620.3(22) MHz, C‘=1426.1(24) MHz, A'=8832.4(26) MHz, B'=1617.3(28) MHz, and C'=1431.6(15) MHz. The electric dipole moment components were determined, with μa = 0.0441(16) D and μc = 1.5884(29) D. The c component of the nitrogen quadrupole coupling component was found to be eqccQ = 1.30(21) MHz. A structure analysis gave the separation between the centers of mass of the monomers as 3.8925(28) A(ring). The angles between the symmetry axes of the SO2 and N2 units and the line connecting these monomers were calculated as 61.35° and 24.54°, respectively. Additionally, the SO2 monomer a axis was found to lie along the b axis of the complex. The electric dipole moment data indicate that the equilibrium angle for the SO2 is much closer to 90° than the rms result. These structural results were compared to model calculations of the binding energy of the complex.

Notizen: Resonance Raman spectra of polymer chains in the single crystals of the diacetylenes FBS (2,4-hexadiynylene-di-p-fluorobenzene sulfonate), TS/FBS [6-(p-toluenesulfonyloxy)-2,4-p-fluorobenzene sulfonate] and TS6 (2,4-hexadiynylene-di-p-toluene sulfonate) are described theoretically by means of a Franck–Condon model, which considers a chain length dependence. The electronic transition energies and matrix elements are calculated by means of a linear-combination-of-atomic-orbitals–molecular-orbital method calculation in the Hückel approach. We are able to describe the Raman excitation profiles as well as the Raman band profiles for two Raman active modes, i.e., the C+C-, and the C 3/4 C-stretching vibrations of the polymer chains. The model also contains a description of a side group vibrational mode which is enhanced by Fermi resonance with the C=C-stretching vibration. Observed Raman excitation profiles can be well simulated by these calculations. An evaluation of the parameters shows a strong influence of defects, which is described by including inhomogeneous broadening in the theoretical model. An interaction between ensemble and intramolecular properties due to defects can be shown. Simulation of the Raman band profiles yields information on the influence of chain length distribution. Changes of position and Raman band profile as well as of the strength of the Fermi resonance can be explained by the enhancement of modes belonging to polymer chains having different chain lengths. A considerable influence of the substitutents results in remarkable changes of several parameters on which the model calculation is based.

Notizen: The electron–electron coincidence method has been used to measure the deexcitation electron spectra of core-excited nitric oxide. Comparison of the deexcitation spectra with Auger spectra reveals the importance of participator and spectator decay in each case. Term splittings in the core-excited molecule and selection rules for Auger decay play important roles in simplifying the deexcitation spectra. Analysis of the spectra reveals that the core hole valence interaction is larger for nitrogen than for oxygen. As a result, the level orderings in the two core-excited molecules, N*O and NO*, are different.

Notizen: We present a stable and accurate inversion method for extracting potentials from spectroscopic data of diatomic molecules. The method, which was developed previously for inverting scattering data, is based on first-order functional sensitivity analysis in conjunction with the Tikhonov regularization, singular system analysis, and an exact transformation technique. Besides being numerically stable, it requires neither explicit functional forms nor special basis function expansions for the potential corrections when solving the corresponding linearized integral equation. Instead, we solve the linear equation directly in terms of the probability densities of the unperturbed vibrotation eigenstates. For illustration, we consider the ground electronic state of the H2 molecule. Inversions have been carried out for simulated data free of noise and for those with noise of magnitude comparable to realistic experimental errors. It is found that in both cases, a relatively large deviation of the starting reference potential from the truth may be tolerated to still accurately recover the intended one. The propagation of the spectral errors is analyzed in detail based on the linearization assumption. The variance of the recovered potential reveals the reliability of various regions of the recovered potential.

Notizen: Quasiclassical trajectories were used to study the energy transfer rates and mechanisms in collisions of HF(v,J) with CO. A potential-energy surface was formulated by using spectroscopic and ab initio information. We have computed state-to-state rates for HF (vi=3,5,7,9; Ji=2)+CO(vi=0)→HF(vf,Jf)+CO for a thermal distribution of translational energies and CO rotational states at 300 K. The relaxation is due predominantly to vibration-to-rotation energy transfer with Δv=−1. As the initial vibrational state is increased, multiquantum transitions (Δv≤−2) become increasingly important. The computed results are in good agreement with experimental results.

Notizen: We consider the problem of electron transfer between two symmetric redox states for cases in which the interstate coupling can be strong and the coupled harmonic bath can be nonadiabatic. We utilize an adiabatic bath coupled to the charge transfer species as a reference system and treat the solvation effects of nonzero frequency Fourier modes approximately, yielding an analytical theory for the activation free energy in terms of the spectral density of the bath. The theory is exact for both slow and fast bath modes. For small interstate coupling, the theory agrees with the golden rule result. We test the theory's accuracy at large couplings in the intermediate frequency regime by comparison with Monte Carlo simulation.

Notizen: The spectroscopy and dissociation dynamics of the NCO radical have been investigated by applying fast radical beam photodissociation spectroscopy to the B˜ 2Π←X˜ 2 Π electronic transition. Measurements of the photodissociation cross section as a function of dissociation wavelength show that even the lowest vibrational levels of the B˜ 2Π state predissociate. Analysis of fragment kinetic energy release reveals that the spin-forbidden N(4S)+CO(1Σ+) products are produced exclusively until 20.3 kcal/mol above the origin, at which point, the spin-allowed N(2D)+CO product channel becomes energetically accessible. The spin-allowed channel dominates above this threshold. By determining the location of this threshold, we obtain a new ΔHf0 for NCO of 30.5±1 kcal/mol, several kcal/mol lower than the previously accepted value.

Notizen: The ground-state potential for singly-charged barium ions interacting with argon atoms is inferred from laser-induced fluorescence measurements of the gaseous ion transport coefficients. The potential is used to compute velocity component distribution functions and fluorescence spectra. Comparison with the measured spectra provides insight into the accuracy with which the potential can be inferred and with which the distribution function and spectra can be calculated. Failure of a similar study of barium ions in helium gas is possibly due to inelastic collisions that invalidate a single-potential description of this system.

Notizen: Rate constants and product branching fractions have been measured for the gas-phase reactions of oxide (O−) and superoxide (O2−) anions with the halocarbons CF4, CF3Cl, CF3Br, CF3I, and C2F4 using a variable temperature–selected ion flow tube (VT–SIFT) instrument operated at 298 and 500 K. The reactions of O− with CF3X (X=Cl, Br, I) are fast and produce F−, XF−, and XO− for all X. For CF3Cl and CF3Br, X− is also formed. For CF3I, CF3− and IOF− are minor products. O− reacts rapidly with C2F4 producing F− as the major ionic product, along with contributions from reactive detachment and minor formation of FCO−, CF3−, and C2F3O−. The reaction of O2− with CF3Cl is slow, and both clustering and X− formation were observed. For CF3Br and CF3I, the reactions with O2− are fast, and nondissociative charge transfer was observed in addition to X− formation. O2− reacts rapidly with C2F4 by reactive detachment, in addition to producing F− as the major ionic product with smaller amounts of F2−, FCO−, FCO2−, CF3O−, and C2F4O−. O− and O2− were both found to be unreactive with CF4 at 298 and 500 K. The efficiencies of the reactions of both O− and O2− with CF3X are greater for the heavier halides at both 298 and 500 K. The rate constants for the reactions of O2− with CF3X appear to correlate both with the rates of thermal electron attachment to CF3X and with the electron affinities of CF3X, indicating that the O2−+CF3X reaction mechanism may involve initial electron transfer followed by dissociation. Thus the negative electron affinity of CF3Cl may explain the very slow rate for reaction with O2− despite the available exothermic pathways.

Notizen: Collisional removal of the v'=0 level of the A 3Πi state of the NH molecule has been studied as a function of rotational level with particular attention to high N'. Multiphoton dissociation of NH3 at 193 nm produces highly rotationally excited ground state NH (to N‘=30), which is subsequently electronically excited with a pulsed dye laser, and quenching rate coefficients determined by the pressure dependence of the time-resolved fluorescence. Colliders investigated are NH3, CO, CH4, H2, and D2. The quenching rates at first decrease with increasing N', but become nearly constant at highest N'. This is consistent with dynamic effects on an attractive, anisotropic surface. Radiative rates for A 3Πi were also determined and found to decrease with increasing N' at a rate in excellent agreement with recent ab initio calculations.

Notizen: We consider a model system of three weakly coupled, weakly anharmonic oscillators, containing two near 2:1 Fermi resonances. This system is initially reduced via a resonance approximation to one of two coupled pendula, each pendulum representing one of the interoscillator resonances. This leads to a natural description of the system in terms of analytically predictable resonance zones and their intersection in action space. A full numerical analysis of the classical dynamics of the two-pendulum system is given, with respect to both the appearance of stochasticity and the development of periodic orbits in the resulting two-dimensional area-preserving map. The success of the resonance approximation in describing adequately the behavior of the three-oscillator model is shown by comparison with a representative study of the classical dynamics of the full three-oscillator model.

Notizen: This paper illustrates an approach that can refine mechanisms and obtain information about rate constants from dynamical phase diagrams which show the regions of oscillation of a mechanism as a function of the experimental parameters. Possible mechanisms for the experimentally oscillating oxalate–persulfate–silver system are examined. Starting with a proposed mechanism by Ouyang and de Kepper, which they could not make oscillate, we show that some variations of the mechanism are stable for all nonnegative values of the rate constants. Other variations are unstable. For these variations, feedback cycles that lead to instability are compared with a conceptual picture of feedback in the experimental system. One unstable mechanism fits the picture well. Its unimportant reactions are omitted and an analytical solution for the unstable region using 13 adjustable parameters is obtained. The rate constants are adjusted to match this solution to the experimentally measured phase diagram. A good fit can only be obtained if [O2] is too low and k1 is much smaller than the known value. Both discrepancies are resolved if Ag2+ oxidizes water. The analysis predicts the width of the unstable region can increase when more O2 enters the reactor.

Notizen: We present a study of the intramolecular vibrational energy redistribution (IVR) from a highly excited C–H overtone of the CD3H molecule. The whole vibrational manifold has been explicitly considered in the calculations. Two different approaches have been used and compared. In a direct approach, we have kept all the states located below a given threshold energy, resulting in a basis set of 92 000 states. The second approach consisted in selecting the important states in order to define a dynamically relevant active space (AS) of much lower size (≈2000). The two approaches were first applied to the calculation of the n=6 C–H overtone spectrum, showing that the AS method is quite able to reproduce the exact results. More stringent test concerned the actual time evolution of the C–H stretch local mode ||6ν1(approximately-greater-than)0. Explicit time propagation has been carried out in the larger basis set, over a 15 ps interval. Results show that the dynamics is mainly governed by a few resonant states involving the C–H bending and C–D stretching motions. Very slow relaxation out of the C–H chromophore is observed over this 15 ps period. The AS method, using a much smaller basis set, was shown to reproduce the correct behavior of the C–H chromophore dynamics during the first 2 ps.

Notizen: The involvement of anharmonic interactions in the vibrational level mixing and IVR in S1 benzene at excess vibrational energy up to ∼5000 cm−1 was theoretically studied. The employed procedure is based on certain generally valid regularities governing the magnitude of anharmonic coupling matrix elements combined with detailed analysis of vibrational density computations. Definite conclusions concerning the rule of purely anharmonic interaction mechanisms in the vibrational mixing and redistribution behavior of S1 benzene have been arrived at.

Notizen: The excitation and ionization spectra of TiCl4 have been studied theoretically by the symmetry adapted cluster (SAC)/SAC-CI and multireference CI (MRD-CI) methods. The calculated spectra show good agreement with the observed spectra. The present results indicate several new assignments for the excitation spectrum. The peaks below 8.0 eV are assigned to valence excitations and those at 9.35 and 10.04 eV are assigned to Rydberg-type excitations within chlorine ligands. The ordering of the ionized states in the outer valence region is (1t1)−1 〈 (3t2)−1 〈 (1e)−1 〈 (2t2)−1 〈 (2a1)−1, which supports the result of earlier Green's function calculations.

Notizen: The theory of molecular structure, developed by Bader and co-workers, has met with considerable success and is now being applied to many aspects of chemistry. One finding, however, that has complicated the theory is that of non-nuclear attractors, that is, pseudoatoms. In this study we examine basis set and to some extent electron correlation effects on the appearance and disappearance of non-nuclear attractors in Li2, Na2, Na4, and Na5. The appearance of a pseudoatom in a lithium molecule correlates remarkably well with the size of the region, in an atomic calculation, of −∇2ρ(r) for the valence shell of the atom. This and the fact that the pseudoatom is also present in the promolecule indicates that the pseudoatoms are remnants of or, in fact, are portions of atoms that are not perturbed enough in the molecule to remove an essentially atomic characteristic.

Notizen: Quantum chemical ab initio calculations for the adsorption of CO on the NiO(100) surface have been performed at different levels of accuracy: self-consistent field (SCF), complete active space self-consistent field, and coupled electron pair approximation. Basis sets of double zeta and triple zeta + polarization (TZP) quality have been used. The NiO(100) surface is represented by a cluster containing one Ni2+ ion and the five adjacent O2− ions. The charge neutrality of the cluster and the saturation of the dangling bonds is achieved by adding eight protons, which gives the total composition Ni(H2O)3(OH)2. Alternatively, the Ni2+(O2−)5 unit is embedded in a lattice of point charges which correctly represent the half-infinite ionic crystal. In the most favorable configuration, CO is adsorbed linearly in the on-top position on the Ni2+ ion, with the C atom pointing toward the surface. The binding energies at the SCF level are rather small, only 0.08 eV and 0.03 eV for CO and OC bound to the cluster (TZP basis set, counterpoise correction for the basis set superposition error included). For the two configurations the equilibrium Ni–C and Ni–O distances are 5.40 and 5.50 a0, respectively. Electron correlation does not change these values markedly. Estimating the errors in our calculation we arrive at binding energies of 0.10±0.05 and 0.05±0.05 eV, respectively, for CO and OC on NiO(100). This is in agreement with the experimental estimates. The bonding is attributed predominantly to electrostatic and inductive forces. No genuine "chemical'' bond (overlap, charge transfer) exists between CO and the NiO(100) surface, i.e., CO is only physisorbed to this ionic surface. The harmonic vibration frequencies for the Ni–C stretching and the Ni–C–O bending vibrations are estimated to 52 and 139 cm−1, respectively.

Notizen: This paper demonstrates the close correspondence in phase-separation behavior of a variety of systems which contain charged particles. These include electron-hole fluids in semiconductors, oxide melts and glasses, and molten salts. Reduced critical parameters for the fifteen systems considered are calculated using the theory of corresponding states for ionic fluids. The reduced critical parameters are found to be in close agreement, indicating that the different systems exist in corresponding states at their critical points to a good approximation. A model of phase separation based on Debye–Hückel theory is presented. The usual limitations of the theory due to the linearization of the Poisson–Boltzmann equation as well as neglect of excluded volume effects due to hard core interactions are avoided by employing the dissociation/association equilibrium of Fuoss. The theoretical critical parameters which are derived are in excellent agreement with experimental data. The degree of dissociation is calculated to be about 3% at the critical point, which is in the expected range of a few percent.

Notizen: The dynamics of the methyl groups in the crystalline alanine dipeptide is examined over a wide range of temperatures using elastic and inelastic neutron scattering experiments and molecular dynamics (MD) simulations of the full crystal. Neutron scattering spectra are calculated from the simulations and directly compared to the experimental profiles. The N-ter and C-ter methyl groups in the molecule have low rotational barriers, i.e., (approximately-less-than)1 kcal/mol. They undergo rotations that are activated on time scales faster than ≈1 ns at temperatures as low as 50–100 K and are on the picosecond time scale at 150 and 300 K. At 300 K the rotational motion becomes strongly diffusive in the simulation. In contrast, the side-chain methyls possess a significant intramolecular intrinsic torsional barrier, ≈3 kcal/mol. As a result, their dynamics consists of librations and rare jumps between wells. The simulations are further analyzed to characterize in detail the motions giving rise to the calculated scattering. Using a quaternion-based method the simulated methyl dynamics is decomposed into rigid-body rotational and translational components. The decomposed motions and their contributions to the calculated neutron profiles are examined.

Notizen: We report resonance enhanced multiphoton ionization (REMPI) spectra of gas-phase gallium monochloride (GaCl) produced with laser light tuned between 330 and 430 nm. The spectra originate from one, two, and three photon resonances with electronic states that reside between 29 500 and 80 000 cm−1. We have assigned five Rydberg series comprised of 20 new electronic states. Each state exhibits a short vibrational progression. A least-squares fit of the unperturbed Rydberg state origins yields the adiabatic ionization potential, IPa(GaCl)=80 540±10 cm−1. We have also identified bands that originate from one-photon resonances with the previously known a 3Π state. We conservatively estimate the REMPI detection sensitivity for GaCl to be 109 molecules cm−3 (laser pulse)−1.

Notizen: Absorption and emission spectra are calculated by modeling cis-stilbene as a system of 12 displaced harmonic oscillators. We are able to obtain good agreement with the peaks of both the room temperature absorption and low temperature emission spectra using parameters from resonance Raman experiments by adjusting the position of the zero–zero transition energy (E0,0) and slightly adjusting the displacements along the normal modes. The best fit value for E0,0 is 29 000 cm−1. Using the displacements along the 12 degrees of freedom, and a normal mode description based on semiempirical quantum force field calculations (QCFF/PI), we determine a "relaxed'' excited state geometry which is twisted a maximum of approximately 37 deg along the ethylenic torsional coordinate relative to the relaxed ground state geometry. An extension of the spectral calculations is described which allows for one or more of the modes to be anharmonic and vibrationally unrelaxed. We apply this extension to show that cis-stilbene emission can not be originating from a geometry with a 90 deg twist along the ethylenic coordinate. Comparison of our results with a recently obtained room temperature solution phase emission spectrum suggest that this emission originates from vibrationally unrelaxed molecules.

Notizen: The velocity and recoil of bands of DNA containing 48.5 to 4800 kilobasepairs (kb) were measured during pulsed-field gel electrophoresis by a video imaging and analysis system. When a 10 V/cm electric field was first applied, the velocity showed an initial sharp peak after approximately 1 s whose amplitude depended on the molecular weight of the DNA and the rest time and polarity of the previous pulse. For example, G DNA (670 kb) exhibited an initial velocity peak of 13 μm/s. The velocity then oscillated through a shallow minimum and small maximum before reaching a 5.0 μm/s plateau. After the field was turned off, the bands moved backward (recoiled). The band position obeyed a stretched-exponential relation, x = x0 exp[ − (t/τ)β] with amplitude x0 equal to approximately 1/10th of the DNA contour length and β≈0.6; for S. pombe DNA, x0 was a remarkable 165 μm. Both the initial velocity spike and the recoil arise from the presence of a significant fraction of U-shaped molecules with low configurational entropy. The initial velocity spike is exploited in field-inversion gel electrophoresis to generate the "antiresonance,'' which is the basis of size-dependent mobility. Recent computer simulations which include tube-length fluctuations and tube leakage are in excellent accord with the measured velocities.

Notizen: We have studied Langmuir monolayers of CH3(CH2)20OH (heneicosanol) using x-ray diffraction. Over the temperature range 7–55 °C, the monolayer isotherms show only one discontinuity, which may be either a "flat section'' or a "kink;'' we find that these are transitions between tilted and untilted phases. There are several untilted phases, with hexagonal and distorted-hexagonal structures, as in monolayers of the corresponding (heneicosanoic) acid. However, in contrast to the acid, when the pressure is lowered the molecules tilt only towards a next-nearest neighbor. The dependence of the lattice spacings on pressure and temperature indicate that the monolayer does not form a "tilted hexagonal'' phase, even at temperatures where the untilted phase is hexagonal (rotator-II).

Notizen: X-ray photoelectron spectroscopy (XPS) and near-edge x-ray absorption fine structure (NEXAFS) have been used to study the core and unoccupied orbitals of condensed multilayers of biphenyl and bithiophene on Cs dosing. The CK XPS shifts, observed on dosing are understandable in terms of repositioning of the Fermi level due to creation of states in the band gap, and at higher Cs concentrations to the high polarizability of the charge transfer complexes formed. The behavior of the CK XPS satellites on increasing Cs exposure are related to changes observed in the electron energy loss spectra (EELS). The CK NEXAFS of biphenyl and the SL23 NEXAFS of bithiophene indicate that an unoccupied molecular orbital is filled by charge transfer from Cs. In the case of bithiophene, both the SL23 XPS and NEXAFS features shift by 2 eV to lower binding and adsorption energy, respectively. This strong chemical shift suggests significant localization of the transferred charge on the S sites. Comparison of the SL23 NEXAFS of doped and undoped bithiophene allows the identification of the unoccupied orbital with S character and leads to a reassignment for the NEXAFS features of thiophene and its oligomers.

Notizen: The quartic free-energy contribution for a semi-infinite diblock copolymer system is demonstrated to be important near the bulk spinodal, even when the bulk is disordered. The retention of the quartic term leads to a finite-density profile decay length at the spinodal, contrary to the prediction of a divergent decay length at the spinodal that is derived with a quadratic theory. Variational methods are introduced to treat the quartic theory and are used to show that bulk ordering induces formation of surface structures, such as terraces, islands, and holes, even in the absence of surface interactions. We also present a necessary modification to the Ohta–Kawasaki Green's function for diblock copolymers.

Notizen: Detailed measurements of the macroscopic second-order optical nonlinearity χ(2)(2ω,ω,ω) of Langmuir–Blodgett dye-doped monolayers are reported. The observed deviations from a linear behavior of χ(2) with increasing surface density are shown to be due to local-field effects. In order to calculate these local-field factors for disordered systems, a novel Monte Carlo type calculation is introduced. This calculation not only accounts for density variations in the monolayers but also incorporates the effect of off-diagonal elements of the (microscopic) linear susceptibility tensor. Quantitative agreement is found between the calculations and the experimental results using only the molecular hyperpolarizability as a free parameter. A method is presented to determine the tilt angle of the chromophores in Langmuir–Blodgett monolayers from the anisotropy of the linear absorption. The tilt angle determined this way is in excellent agreement with a determination by second-harmonic generation.

Notizen: Electron drift mobilities μe and electron–ion recombination rate constants kr have been measured by the analysis of transient current induced with the irradiation of an x-ray pulse on gaseous or liquid krypton in a cell as a function of external electric field strength. The effects of an external dc electric field on kr up to 12 mTd (1 Td=10−17 V cm2/molecule) in liquid and 63 mTd in gas have been examined. The observed kr values in both liquid and gas phases have been found to be much smaller than those calculated by the reduced Debye equation. The deviation has been compared with recent theoretical studies, leading to the conclusion that recombination in liquid and gaseous krypton is not the usual diffusion-controlled reaction. The kr values in the gas phase increase with an external dc electric field up to a critical electric field strength below which μe values are nearly constant, and above which both kr and μe values descrease with further increase in the electric field for both gaseous and liquid phases.

Notizen: The morphological development in blends of bisphenol-A polycarbonate (PC) and poly(methylmethacrylate) (PMMA) blends during isothermal annealing above 200 °C has been investigated where competition between liquid–liquid phase separation by spinodal decomposition and interchange reactions take place. Interchange reactions between PC and PMMA occurs at temperatures above 200 °C and leads to the formation of in situ graft copolymers from an ester–ester interchange reaction. During spinodal decomposition, graft copolymers are produced mainly at the interface region between the interconnected microphase domains. Instead of the usual "coarsening'' process which is characteristic of the late-stage of spinodal decomposition, the mixture exhibits nearly monodisperse spherical domains as revealed by optical microscopy. This phenomenon is further studied through extensive small angle light scattering measurements. Resonance peaks up to fourth order are noted, a rare observation. The result clearly demonstrates that graft copolymers are formed in situ and can act as very effective "surfactants'' in polymer blends. Furthermore, an attempt is made to analyze the angular dependence of the scattering intensity from this morphology with the Percus–Yevick hard sphere liquid theory. These results are believed to be general and therefore applicable to a wide variety of blends containing one or more components capable of an interchange reaction.

Notizen: A microscopic molecular theory is developed for dielectric behavior at molecular crystal surfaces. Since dipolar interactions between layers of molecules decrease rapidly with separation, only layers within a short distance of the surface are affected by it. Algebraic results are obtained relating the local electric field and the macroscopic electric field in a surface layer to an applied uniform electric field, and hence to one another. These lead to expressions for the linear electric susceptibility of a layer and of an assembly of layers. The results provide a basis for calculations of various surface-induced and surface-modified properties.

Notizen: Planewise dipole sums are calculated for various lattice planes in anthracene, treated both as a single point and as three point submolecules. Conventional methods, generalized to treat submolecules, are used as well as a new method using partial reverse Fourier transformation of modulated bulk lattice dipole sums. The new method gives an independent check but is not otherwise preferable. The submolecule treatment makes the interaction between adjacent planes not much less than within a plane, essentially because of the molecular packing. Beyond adjacent planes, the sums decrease exponentially by typically a factor of 100 per 10 A(ring) separation. Calculations for arbitrary planes are conveniently performed by transforming the conventional crystal unit cell to contain the desired plane as a face. Wave vector-modulated planewise sums have also been calculated for point molecules and the (001) plane: They decrease less rapidly with separation than unmodulated sums.

Notizen: Pure metal and metal alloy clusters including Cun, Agn, CunAgm, CunAlm, CunInm, AgnAlm, AgnInm, and CunPbm are produced by a gas aggregation source and investigated by time-of-flight mass spectrometry following ionization with a KrF excimer laser. In the case of pure metal clusters (Cun,Agn,Inn), as well as alloy clusters composed of these metals, magic numbers are observed in their cluster ions which correspond to jellium shell closings (counting the total valence electrons from the component metals). These findings are in good agreement with their expected free-electron behavior. Interestingly, the abundance of pure Pbn+ corresponds to species which are expected to be especially stable due to their geometric structure. A similar situation also arises for the Pb-rich alloy clusters. By contrast, the metal alloy clusters CunPbm+ show magic numbers at jellium shell closing in the series of Cu-rich clusters.

Notizen: In this paper the Brownian dynamics technique for computer simulation of the fracture of polymer chains in a dilute solution subjected to elongational flow was used. Individual randomly coiled chains are placed at the stagnation point of a steady, uniaxial flow, and their evolution with time is monitored for chain fracture. The chains are modeled either as Rouse chains with a cutoff spring length, or as chains of springs obeying a Morse potential. Kinetic parameters are the first-order kinetic constant and the half-life time. The dependence of these parameters with elongational rate and chain length is expressed in the form of scaling laws, and the results are compared for the two models. Also studied was the size distribution of the resulting fracture fragments, finding that it may vary from sharply peaked to uniform, depending on the strength of the flow.

Notizen: The hopping integrals βam between the molecule and metal atomic orbitals are involved in both the description of the CO–metal chemisorption bond and the calculation of the infrared linewidth L of the C–O stretching vibration (related to an electron–hole pair process). Therefore, some information about the chemisorption bond may be extracted from the experimental values of L. The kinetic energy of the C and O nuclei may be transferred to the electrons using two channels: the 5σ orbital or the (1π, 2π) orbital pair. The βam integrals are assumed to be proportional to k0, the parameter "describing the chemisorption bond.'' When k0 is fitted to the experimental values of L, we find k0=0.19 for CO/Pt and k0=0.23 for CO/Ni. In both of these cases, the CO–metal bond and the electron–hole pair process involve only the 5σ orbital.

Notizen: The contribution of the π electrons to the polarizabilities of finite and infinite systems have been calculated for infinite one-dimensional chains modeling organic polymers, such as polyacetylene and various polyheterocycles. In the first part, results obtained previously using a finite-field technique within the Hückel and Pariser–Parr–Pople (PPP) models applied to polymers containing up to 400 atoms have been compared to results obtained from the sum-over-states (SOS) perturbation theory. The calculated polarizabilities depend on the type of monomer following polyacetylene ≥ polythiophene ≥ polypyrrole. This result is qualitatively independent on the type of model or formalism. In the second part, we present results of the application of the Genkin–Mednis formalism within the PPP model to the calculation of the π polarizability of infinite one-dimensional chains. Numerical difficulties arise from the phase of the linear-combination-of-atomic-orbital (LCAO) coefficients corresponding to different k points. It is necessary to monitor these phases in order to obtain reliable data. The Genkin–Mednis polarizabilities are in excellent agreement with the asymptotic values obtained using the SOS method on oligomers.

Notizen: A discretized model of globular proteins is employed in a Monte Carlo study of the helix–coil transition of polyalanine and the collapse transition of polyvaline. The present lattice realization permits real protein crystal structures to be represented at the level of 1 A(ring) resolution. Furthermore, the Monte Carlo dynamic scheme is capable of moving elements of assembled secondary and supersecondary structure. The potentials of mean force for the interactions are constructed from the statistics of a set of high resolution x-ray structures of nonhomologous proteins. The cooperativity of formation of ordered structures is found to be larger when the major contributions to the conformational energy of the low temperature states come from hydrogen bonds and short range conformational propensities. The secondary structure seen in the folded state is the result of an interplay between the short and long range interactions. Compactness itself, driven by long range, nonspecific interactions, seems to be insufficient to generate any appreciable secondary structure. A detailed examination of the dynamics of highly helical model proteins demonstrates that all elements of secondary structure are mobile in the present algorithm, and thus the folding pathways do not depend on the use of a lattice approximation. Possible applications of the present model to the prediction of protein 3D structures are briefly discussed.

Notizen: We report ionization threshold measurements for different isomers of Aniline–(argon)n clusters (n=1–3). The data are well represented by simple additivity rules of "site-specific'' solvent shifts, which can be generalized to other systems.