ALBERT

Notizen: The far infrared spectrum of gaseous 2-fluoropropenoyl fluoride CH2CFCFO, has been recorded at a resolution of 0.10 cm−1 in the 350–35 cm−1 region. The asymmetric torsional fundamental of the more stable s-trans (two double bonds oriented trans to one another) and the high energy s-cis conformations have been observed at 84.1 and 68.5 cm−1, respectively, each with excited states falling to lower frequencies. From these data, the asymmetric torsional potential function governing internal rotation about the C–C bond has been determined. The potential coefficients are V1=−201±4, V2=2106±16, V3=382±6, V4=−41±5, and V5=−74±3 cm−1. The s-trans to s-cis and s-cis to s-trans barriers have been determined to be 2232 and 2124 cm−1, respectively, with an enthalpy difference between the conformations of 108±26 cm−1 (309±74 cal/mol). From variable temperature studies of the Raman spectrum, the conformational enthalpy difference has been determined to be 187±39 cm−1 (535±112 cal/mol) and 370±82 cm−1 (1058±234 cal/mol) for the gas and liquid, respectively. A complete assignment of the vibrational fundamentals observed from the infrared (3500–50 cm−1) spectra of the gas and solid and the Raman (3200–10 cm−1) spectra of all three physical states is proposed. All of these data are compared to the corresponding quantities obtained from ab initio Hartree–Fock gradient calculations employing both the 3-21G and 6-31G* basis sets. Additionally, complete equilibrium geometries have been determined for both rotamers. The results are discussed and compared with the corresponding quantities obtained for some similar molecules.

Notizen: The photodissociation dynamics of some alkali halides are explored via the method of femtosecond transition-state spectroscopy (FTS). The alkali halide dissociation reaction is influenced by the interaction between the covalent and the ground state ionic potential energy surfaces (PES), which cross at a certain internuclear separation. Depending upon the adiabaticity of the PES, the dissociating fragments may be trapped in a well formed by the avoided crossing of these surfaces. Here, we detail the FTS results of this class of reactions, with particular focus on the reaction of sodium iodide: NaI*→[Na---I]°* →Na+I. As in our first report [T. S. Rose, M. J. Rosker, and A. H. Zewail, J. Chem. Phys. 88, 6672 (1988)], we observe the dynamical motion of the wave packet along the reaction coordinate and the crossing between the covalent and ionic surfaces. The studies presented here characterize the effects of various experimental parameters, including pump and probe wavelengths, on the dynamics of the dissociation and its detection. Comparisons of the results with classical and quantum mechanical calculations are also presented.

Notizen: A previous paper gave the results of a full calculation involving free ion and crystal–field interaction within the 4 f2 and 4 f15d1 configurations. Presently, the calculation is extended to 4 f16s1 and 4 f16p1 as well. As a result, the usual experimental/calculated discrepancy of the 1D2 multiplet is completely eliminated. A complete Hamiltonian, including the magnetic contribution, is utilized to reproduce the Zeeman effect in PrCl3.

Notizen: The photofragment internal and translational energy distributions resulting from the 193 nm photolysis of acetone have been measured. Vacuum-ultraviolet laser-induced fluorescence was used to probe the CO fragment, and multiphoton ionization time-of-flight mass spectrometry was used to probe the CH3. A Boltzmann distribution was observed to fit each degree of freedom with the following characteristic temperatures: CO@B: Tvib =2700 K, Trot =3000 K, Ttrans =3000 K; CH3@B: Tvib =800 K, Trot =500 K, Ttrans =3500 K. No evidence was found for two distinct CH3 populations, as might be characteristic of a stepwise reaction. Energy partitioning between the fragments was fit well by a simple impulsive model in which the available energy is divided equally between the two dissociating C–C bonds, the two bonds cleaving in rapid succession on a time scale short enough to allow little redistribution of energy into the methyl degrees of freedom.

Notizen: Photolysis of NCO(X 2Π) at 193 nm leads to the production of CN(X 2Σ+ )+O(3 P). The CN(X) was detected by monitoring the CN X 2Σ+ →B 2Σ+ laser-induced fluorescence excitation spectrum. A spectral simulation calculation was used to obtain vibrational and rotational population distributions in the CN fragments. The fractional vibrational populations obtained in this manner are 0.43, 0.32, 0.21, and 0.04 for v=0, v=1, v=2, and v=3, respectively. The near-nascent rotational distributions in the different CN(X) vibrational levels are not characterized by a Boltzmann rotational temperature, but rather are bi-modal with maxima at both high and low N. The high N rotational excitation of the CN fragment suggests the existence of an excited dissociative state of NCO which is bent. A lower limit for the heat of formation of NCO, ΔHf 〉37 kcal/mol, is derived from the upper limit on the internal excitation of the CN(X) fragments. The spectrum of prompt emission produced by the 193 nm photolysis indicates the existence of a bound excited state of NCO which radiatively relaxes to the A 2Σ+ state. From the spectrum, this state is thought to be linear.

Notizen: We have employed direct two-photon laser excitation of specific vibrational levels of N2(a 1 Πg) to measure the lifetime of this state. Direct observation of emission from the a 1Πg –X 1∑+g transition in a large cell was employed to follow fluorescence decays. Experiments were conducted to verify that the effects of collisional transfer and diffusion were not contributing to the observed lifetime. Our experiments showed that the radiative lifetime of vibrational levels 0–2 is 56±4 μs and is independent of vibrational level, within experimental error. The observed lifetimes are in good agreement with recently reported theoretical calculations.

Notizen: We show how the Kohn variational principle for the K matrix is free from anomalies when complex short-range basis functions are used. Using only the real part of the Kv matrix increases the accuracy of the calculation and guarantees the unitarity of the resulting S matrix.

Notizen: From the far infrared spectrum of 2-methylpropanal (isobutyraldehyde), (CH3)2CHCHO, in the gaseous state, the asymmetric torsion for the gauche conformer was observed as a series of Q branches at 75.0, 80.9, 85.8, and 90.3 cm−1 with similar transitions observed for (CD3)2CDCHO. Also from an investigation of the Raman spectrum of the gas a transition at ∼110 cm−1 has been tentatively assigned as the "double jump'' of the asymmetric torsion for the trans conformer. The microwave spectrum of the excited vibrational states for the light molecule has been investigated in the region from 12.5 to 40 GHz and, from relative intensity measurements, the frequencies of the asymmetric torsions were determined to be 72±7 and 50±20 cm−1 for the gauche and trans rotamers, respectively. Additionally, "tunneling'' splitting of 2.0±0.4 and 60.0±0.3 MHz has been observed for the second and third excited states, respectively, of the asymmetric torsion of the gauche conformer. From these data the asymmetric potential function has been calculated and the following potential constants have been evaluated: V1=−131±35, V2=−225±21, V3=555±10, V4=−70±5, V5=95±5, and V6=−140±10 cm−1 where the 1←0 transition of the gauche has been assigned at 75.0 cm−1. This potential function leads to an enthalpy difference of 250±66 cm−1 (715±189 cal/mol) which is in good agreement with the value of 248±35 cm−1 determined from variable temperature studies of the Raman spectrum of the gas. Analysis of the transitions observed between 250 and 150 cm−1 in the far infrared spectrum of the gas indicated that the two methyl rotors are coupled for the gauche conformer and the barriers to internal rotation of the methyl tops have been calculated to be 987 cm−1 (2.82 kcal/mol) and 1132 cm−1 (3.24 kcal/mol).

Notizen: Cyanoacetylene underwent polymerization reaction in a solid phase at pressures above 1.5 GPa. The Raman study of the reaction product showed that the polymer had a conjugated linear backbone with CN pendant groups. The Raman spectra for this substituted polyacetylene demonstrated a resonance behavior similar to that reported for trans-polyacetylene. The optical gap associated with the π–π* transition in the conjugated system was smaller than that of trans-polyacetylene, probably due to the resonance interactions between the CN triple bonds and the conjugated double bonds.

Notizen: Resonant eigenvectors are calculated within the framework of the complex rotation theory using a continuous deformation and extension of the localized bound states through the potential barrier. The vector is obtained by gradually increasing the basis size and defining for each new basis a lesser perturbation path which best verifies the complex extension of the virial theorem. The modifications resulting from the increase of the basis set N and from the variations of the optimal rotation angle θ(N) are well handled using a Bloch wave operator formulation.

Notizen: We present a formalism for molecular liquids in which the interatomic force depends upon both the internuclear separation and on internal, quantum degrees of freedom. The internal degrees of freedom are spin and orbital angular momenta, and they are eventually approximated to yield a purely classical function for the interatomic interaction. We show that the interpolative character of the classical energy function is derived from a resolution of a model Hamiltonian. The formal derivation of the potential energy function is illustrated with an explicit calculation for some nonpolar and polar systems, and with discussion for ionic systems. Our derivation emphasizes an alternative to the ever more popular three-body potential energy functions for statistical mechanics studies of liquids. We outline the path to be followed in computer simulations and analytical studies of liquids with our pairwise-additive, spin-dependent potential energy functions.

Notizen: The structural characteristics of the β phase of solid nitrogen have been studied for large samples by the molecular-dynamics simulation technique. An effective interatomic potential appropriate to this phase was developed including nonbonded and quadrupole terms. Several properties of the crystal structure have been studied as functions of both temperature and pressure. In particular, a detailed investigation of the molecular orientational order has been made as a function of these parameters. At high temperatures the simulation sample is found to have a hexagonal structure with randomly oriented molecules. As the temperature decreases an orientational ordering transition is found to occur at ≈35 K. The effects of increasing pressure are found to be analogous to those of reducing the temperature. The nature of the local transition in the sample is investigated by calculating correlation parameters which specify twinning and tetrahedral ordering. The variation of the latter indicates that tetrahedral ordering increases strongly with decreasing temperature or increasing pressure, suggesting that the low-temperature phase is a condensation of this arrangement.

Notizen: We have studied the wetting and drying transitions in a solid–fluid interace with truncated Lennard-Jones interactions, for which there have been reported molecular dynamics results by Sikkenk et al. [Phys. Rev. Lett. 59, 98 (1987)]. We consider in detail the differences resulting from the use of a "real'' solid substrate instead of the "inert wall'' model which has been used in all previous calculations.

Notizen: The relaxation behavior of the bistable iodate–arsenous acid reaction near a hysteresis limit has been investigated. Outside the bistability region, relaxations to the unique stable steady state may exhibit a plateau in concentration as a function of time, corresponding to a local minimum in rate of reaction. Very close to the saddle-node bifurcation defining the bistability limit, the relaxing system remains in the plateau for long periods of time. Experimental measurements of plateau behavior are compared to behavior predicted by a one-variable model of the iodate–arsenous acid system.

Notizen: The gas phase infrared spectra of the hydrated hydronium cluster ions H3O+⋅(H2O)n(n=1, 2, 3) have been observed from 3550 to 3800 cm−1. The new spectroscopic method developed for this study is a two color laser scheme consisting of a tunable cw infrared laser with 0.5 cm−1 resolution used to excite the O–H stretching vibrations and a cw CO2 laser that dissociates the vibrationally excited cluster ion through a multiphoton process. The apparatus is a tandem mass spectrometer with a radio frequency ion trap that utilizes the following scheme: the cluster ion to be studied is first mass selected; spectroscopic interrogation then occurs in the radio frequency ion trap; finally, a fragment ion is selected and detected using ion counting techniques. The vibrational spectra obtained in this manner are compared with that taken previously using a weakly bound H2 "messenger.'' A spectrum of H7 O+3 taken using a neon messenger is also presented. Ab initio structure and frequency predictions by Remington and Schaefer are compared with the experimental results.

Notizen: The decay rates of optically excited triplet states of pyrazine in supersonic expansion were measured by using three different methods. The excess energy dependence of the radiationless rate constants in the energy range between the T1 and the S1 electronic origins of the isolated molecule was explored. Decay rates between 7×102 –2.5×104 s−1 were found in the 1500 cm−1 range of excess vibrational energy from the origin of the T1 state. The decay rates are free of mode specificity and rotational effects. The pure radiative lifetime in the measured range is rovibronic independent. The results support a model which suggests that certain vibrational modes, those which undergo large frequency changes in the excited state, control the strong vibrational energy dependence of the T1 →S0 intersystem crossing of pyrazine.

Notizen: Using multipulse techniques, principal values of the proton nuclear magnetic resonance (NMR) chemical shift tensors were measured at room temperature for the monoclinic form of oxalic acid, potassium hydrogen oxalate, and potassium hydroxide. The isotropic and perpendicular shifts of the first two compounds, which are of medium hydrogen bond strength, were found to fit the linear correlation with O⋅⋅⋅O distance, RO⋅O, established by Rohlfing, Allen, and Ditchfield for medium and strongly O–H⋅⋅⋅O hydrogen bonded solids. Although KOH is very weakly hydrogen bonded, the shifts were also found to conform to the correlation, at least as well as does the other data, thus extending this to weakly hydrogen bonded solids. An empirical correlation of the isotropic and perpendicular shifts with exp(−RO⋅O/ρ), where RO⋅O is in A(ring) and ρ is 0.94 A(ring), is given here which has better agreement with the data and has an interpretation in terms of a simple ionic model of an O–H⋅⋅⋅O bond.

Notizen: The application of the algorithm of Zeiger and McEwen to the analysis of noisy photon correlation data is investigated. For the particular case where the data are sampled at equidistant time intervals a complete solution is given allowing reliable reconstruction of the spectrum of exponential decay rates without any a priori knowledge. A particular attractive feature of the method is that the singular value analysis of the Hankel matrix of autocorrelation functions offers a practical criterion for the decomposition of the data into a signal and a noise part. Some tests of the method are illustrated with experiments on monodisperse latices, gold sols, and binary mixtures of monodisperse latices. In the latter case comparable and even better results are obtained in significantly shorter computing time when compared to an analysis with Contin and the maximum entropy method. Since the present method does not require any a priori parameter setting, it is also complementary to these methods.

Notizen: The dissociation reaction of HgI2 is examined experimentally using femtosecond transition-state spectroscopy (FTS). The reaction involves symmetric and antisymmetric coordinates and the transition-state is well-defined: IHgI*→[IHgI]

Notizen: Zero-point energy excitation has a profound effect on the relaxation of benzene CH and CD overtone states. Only adding a fraction of the zero-point energy for each normal mode in the initial conditions results in smaller overtone relaxation rates. If no zero-point energy is added to C6H6, the n=3 and 5 CH overtones do not relax within 1 ps. Adding zero-point energy to different types of normal modes has nonequivalent effects on overtone relaxation. Zero-point excitation of modes with HCC bend character is particularly effective in enhancing relaxation of the overtones.

Notizen: The gas-phase reactions of Nb2+ with small alkanes at thermal energies are reported. For methane and ethane, dehydrogenation is a prominent reaction pathway. For propane and butane, charge transfer is virtually the only reaction pathway observed (〉99%). NbCH2+2 and NbC2H2+2 formed in the reactions of Nb2+ with methane and ethane are thermodynamically stable with D(Nb2+–CH2)=197±10 kcal/mol, D(Nb+–CH+2)=107±10 kcal/mol, D(Nb2+–C2H2)≥74 kcal/mol, and D(Nb+–C2H+2)≥7 kcal/mol. The stability of these ions is most likely due to the charge-stabilizing effect of the metal center. Collision-induced dissociation of these ions results in charge-splitting reactions as well as reactions in which both charges remain on the metal center. Hydride transfer is observed to be competitive in the primary reactions of Nb2+ with alkanes. The hydride- and charge-transfer results are in qualitative agreement with a simple curve-crossing model.

Notizen: High-resolution near-infrared spectra are reported for all of the O–H stretch vibrational bands of the water dimer. The four O–H vibrations are characterized as essentially independent proton-donor or proton-acceptor motions. In addition to the rotational and vibrational information contained in these spectra, details are obtained concerning the internal tunneling dynamics in both the ground and excited vibrational states. These results show that for tunneling motions which involve the interchange of the proton donor and acceptor molecules, the associated frequencies decrease substantially due to vibrational excitation. The predissociation lifetimes for the various states of the dimer are determined from linewidth measurements. These results clearly show that the predissociation dynamics is strongly dependent on the tunneling states, as well as the Ka quantum number, indicating that the internal tunneling dynamics plays an important role in determining the dissociation rate in this complex.

Notizen: The photofragment kinetic energy spectrum of Ar+3 has been recorded in a crossed-beam apparatus at 532 nm; the only photofragment product observed is Ar+. For the purposes of analysis, a spectrum for the photofragmentation of Ar+2 was recorded under similar experimental conditions. In each case, the ions were prepared by the electron impact ionization of a neutral argon cluster beam. The Ar+3 spectrum consists of two quite distinct features, a high-energy component which closely resembles the result observed for Ar+2, and a second, low-energy feature, which is peculiar to Ar+3 alone. The two high-energy wings appear to arise from a very rapid dissociation process where approximately 70% of the excess energy appears as Ar+ kinetic energy. A computer simulation of this region of the spectrum gives an anisotropy parameter, β, of 1.1±0.2. The low-energy, component to the spectrum arises from a two-step dissociation process, in which a weakly bound atom carries away a relatively large fraction of the available excess energy to leave a quasibound dimer ion. The energetics associated with this latter process can be accounted for using a partitioning scheme proposed by Baer et al. [J. Chem. Phys. 76, 5917 (1982)]. A computer simulation of the low-energy component gives β(approximately-equal-to)0.0. A detailed discussion of the results in terms of electronic structure, photofragmentation dynamics, and allowed electronic transitions, concludes that Ar+3 takes the form of a stable dimer ion and a weakly bound atom, and that it is most probably nonlinear.

Notizen: We demonstrate that the surface structure of organic monolayers can be determined by low energy helium diffraction at low surface temperatures. This uniquely surface-sensitive and nondestructive technique shows that the CH3-terminated surface of a monolayer of docosane thiol (CH3(CH2)21SH) on Au(111) is composed of small, ordered domains (lattice constant 5.01±0.02 A(ring)), a large fraction of which share a common orientation. The helium diffraction intensities decrease monotonically with increasing temperature and vanish around 100 K, due to thermal motion of the CH3 groups. Surface order is observed for chains as short as ten carbons (CH3(CH2)9SH) but a shorter chain, (CH3(CH2)5SH), gave no diffraction.

Notizen: Details are given of a new type of crossed beam experiment that is designed to study state-resolved vibrationally inelastic scattering of neutral atoms from neutral diatomics or polyatomics. The molecule is initially excited to a selected vibrational level in an excited electronic state by laser pumping. Dispersed fluorescence from molecules that have undergone inelastic scattering is used to observe the final vibrational state distributions. The technique is used here to characterize the vibrationally inelastic scattering of He from each of three initial levels υ'=15, 25 and 35 within the B0+u state of I2 . The center-of-mass collision energy is 89 meV (720 cm−1). Thirty cross sections (relative, not absolute) for vibrationally inelastic scattering are obtained. The cross sections characterize processes with vibrational quantum changes extending to Δυ=±3 for the initial level υ'=15, to Δυ=±5 for υ'=25, and to Δυ=±7 for υ'=35. Relative cross sections for pure rotationally inelastic scattering within the initially pumped υ' levels are also reported. The data analysis incorporates much new information about I2 predissociation, spectroscopy, and Franck–Condon factors that has become available within the last ten years. An error analysis of the cross sections has been performed so that they are suitable benchmarks for theoretical calculation. Procedures are given for estimating fluorescence signal intensities in experiments of this type. The signal expectations are consistent with the I2 observations and suggest that extension to certain polyatomics is quite feasible.

Notizen: A projection operator formalism is used to quantify the normal/resonant character of eigenstates of a vibrational Hamiltonian with two degrees of freedom. We show how the application of the formalism quantifies the resonant character of these eigenstates, in the form of a probability, and how this probability is related to the width of the resonance in classical phase space. Normal and resonant quantum numbers shared by a given eigenstate are found to be canonically related.

Notizen: The scaling of the Schrödinger equation with spatial dimension D is studied by an algebraic approach. For any spherically symmetric potential, the Hamiltonian is invariant under such scaling to order 1/D2. For the special family of potentials that are homogeneous functions of the radial coordinate, the scaling invariance is exact to all orders in 1/D. Explicit algebraic expressions are derived for the operators which shift D up or down. These ladder operators form an SU(1,1) algebra. The spectrum generating algebra to order 1/D2 corresponds to harmonic motion. In the D→∞ limit the ladder operators commute and yield a classical-like continuous energy spectrum. The relation of supersymmetry and D scaling is also illustrated by deriving an analytic solution for the Hooke's law model of a two-electron atom, subject to a constraint linking the harmonic frequency to the nuclear charge and the dimension.

Notizen: The mechanism of the radiative species Si*(1P0) formation from SiH4 was investigated with ab initio molecular orbital–configuration interaction calculations on two reaction routes: (a) the simplest one-step reaction mechanism SiH4→Si+2H2 and (b) the second simplest two-step reaction mechanism SiH4→SiH2+H2→Si+2H2. The conclusions obtained are as follows: (i) the one-step mechanism (a) operates effectively in the generation of the radiative species Si*(1P0), (ii) no effective path was found for the formation of Si*(1P0) in the two-step mechanism, (iii) the radiative species Si*(1P0) originates from the fifth lowest excited singlet state 11T1 of SiH4 at the Td structure in the one-step mechanism (a), whereas the nonradiative species Si(1D) is produced spontaneously from the fourth lowest excited singlet state 11E in both reaction mechanisms of (a) and (b), (iv) the nonradiative species SiH2(11B1) is spontaneously generated from the lowest excited singlet state 11T2 of SiH4, (v) the electronic state of the radiative Si*(1P0) has Rydberg character and the emission of Si*(1P0) at 288.2 nm is approximately expressed by 3p4s(1P0)→3p2(1D) where 4s is a Rydberg atomic orbital (AO). It was also concluded that the contributions of 3d AOs are important for making a description of the quantitative energy separations among the lower lying singlet states, 1D, 1S, and 1P0 of Si atoms.

Notizen: The shapes of the real-phonon and pseudophonon sideband holes (PSBH) which occur in hole-burned spectra in amorphous solids are both wavelength and burn time dependent. The theory previously proposed to simulate hole shapes in the short burn time limit is extended to examine the PSBH shapes at arbitrary times and at different burn wavelengths. The simulated spectra are compared with experimental data for tetraphenyl porphin in polystyrene. It is also shown how the simulated spectra may be used to deconvolve the antihole spectrum from the hole spectrum.

Notizen: Vibrational excitation and relaxation cross sections of He colliding with SO2 molecule are evaluated. The results obtained rely on several approximate methods. The infinite-order-sudden approximation provides the framework for treating the rotational motion, while the vibrational processes are described using both exact close-coupling and approximate distorted wave techniques. The latter scheme permits the computations to be extended to much higher collisional energies than so far reported in the literature. Vibrational excitation and relaxation cross sections are presented over a range of collision energies.

Notizen: We have measured the absolute rates of internal conversion and of intersystem crossing in isoquinoline excited to its S2 vibronic states (3131–3062 A(ring)) in supersonic beams. The combination of direct detection of the triplets, absorption and excitation spectra allowed us to extract these rates. The intersystem crossing rate barely depends on the excess vibrational energy in the S2 manifold, being about 2×109 s−1. The internal conversion rates grow by more than two orders of magnitude over the range of 0–800 cm−1 of excess vibrational energy in the S2 state, up to 5×1010 s−1. At the S2 electronic origin intersystem crossing is the dominant nonradiative channel, while at vibrational energy exceeding 400 cm−1 internal conversion becomes dominant.

Notizen: Se3 and Te3 exist as the S3 molecule in an open C2v and a closed D3h form with different electronic configurations but nearly the same total energy. The ionization spectra of Se3 and Te3 are calculated in both the C2v and D3h forms by many-body Green's function methods. The D3h forms behave quite regularly, but very strong correlation effects are found in the C2v forms. If a theory accurate to third order is used, satellite lines appear at the ionization threshold. If higher order terms are included the satellite lines at the ionization threshold experience a slight change in energy, but a considerable reduction of intensity. Se3 has a positive electron affinity in both the C2v and D3h forms (2B1, 2A'2, states), and Te3 appears to have two positive electron affinities (2B1, 2A1 for the C2v form and 2A2 and 2E' for the D3h form).

Notizen: A simple approximation scheme is given for the association constant of a family of models of associating particles introduced and considered by Cummings and Stell, Rasaiah et al., and Stell. The result correctly reduces to the law of mass action at low concentrations. For the ionic-species version, the theory requires accurate assessment of the cavity function of charged hard spheres, for which we develop a recipe. We show the close similarity (and some differences) between the above models and those considered by Wertheim, through a simple extensions of his models. We also note the equivalence of our approximation and Wertheim's first-order thermodynamic perturbation theory.

Notizen: Recently we have shown that the lowest triplet state (T0) of pyridine, incorporated in a single crystal of benzene, may be studied by electron spin-echo (ESE) spectroscopy. From the nitrogen hyperfine structure in the ESE detected electron paramagnetic resonance (EPR) spectra, we were able to conclude that pyridine, a planar molecule in the ground state, becomes nonplanar upon excitation into T0. Here we report the results of a detailed investigation of this distortion and of the electronic nature of the lowest triplet state of pyridine-d5. We have performed electron spin–echo envelope modulation (ESEEM) spectroscopy. From the modulation spectra, the electron-nuclear double resonance (ENDOR) frequencies corresponding to the various deuterium nuclei are obtained. Analysis of the dependence of these frequencies on the orientation of the magnetic field with respect to the triplet fine-structure axes system allows for a determination of the deuterium hyperfine and quadrupole tensors. From these tensors and the known nitrogen hyperfine tensor, the structure and spin-density distribution of pyridine in its lowest triplet state are deduced. Pyridine adopts upon excitation into T0 a boatlike structure, in which the nitrogen atom is tilted by about 40° with respect to the plane through the ortho- and meta-carbon atoms and the para-carbon/para-deuterium fragment by about 10° with respect to this plane. Thereby the hybridization of the ortho-carbon atomic orbitals strongly deviates from that for aromatic hydrocarbons and becomes almost sp3; the hybridization of the atomic orbitals on the other carbon atoms changes much less, while the nitrogen atomic orbitals remain sp2 hybridized. Approximately half of the spin density is found to be localized on the nitrogen atom with a remarkable distribution over the atomic orbitals: the π orbital carries 40%, the n orbital 10%. The rest of the spin density is distributed over the para- (30%) and ortho-carbon atoms (10% each). The lowest triplet state of pyridine is neither an nπ* state nor a ππ* state, but a state of mixed character as the result of a strong vibronic coupling between the 3B1 (nπ*) and 3A1 (ππ*) states.

Notizen: A method is presented for calculating energy levels of atom–rigid-diatom systems for various values of the total angular momentum (J) of the complex. The technique is based upon the collocation method for the vibrational motions of the system and the Galerkin approach for the total rotation. Unlike the Rayleigh–Ritz variational principle, the method does not require the evaluation of integrals over the Hamiltonian and so is very simple to implement. An important feature of the method is that the wave function is obtained in an analytic form and so it is a simple matter to calculate many quantities of spectroscopic interest such as rotational constants and spectral intensities. It is also shown that contracted basis sets can be used in conjunction with the collocation method to enhance the efficiency of the calculation. The method is demonstrated by calculating rovibrational levels of the van der Waals complex Ar–HCl for J up to 10.

Notizen: This study focuses on van Hove-type spikes in the sticking probability of light particles on crystalline surfaces. The spikes result from singularities in the density of surface phonons subject to the constraints of the energy and the momentum conservation. The sharp features in sticking probability as a function of incoming angle and energy are derived and demonstrated for a model H2/Cu(100) system.

Notizen: The near-infrared spectrum of ArHF prepared in a slit supersonic expansion is recorded with a difference frequency infrared laser spectrometer. By virtue of the high sensitivity of the technique, and the lack of appreciable spectral congestion at the 10 K jet temperature, we observe 9 of the 11 vibrational states with energies below the Ar+HF(v=1, j=0) dissociation limit. These include (1000), the lowest bound HF (v=1) state, the singly, doubly, and quadruply van der Waals stretch excited states (1001) (1002), and (1004), both the Σ bend (1200) and Π bend (111e,f 0), and the multiply excited, Π bend plus van der Waals stretch (111e,f 1). Two Ar+HF(v=0) states, (0000) and (0001), are also characterized. This spectroscopic information is quite sensitive to the Ar+HF potential energy surface away from the equilibrium configuration, and thus provides a rigorous test of trial potential energy surfaces. Excellent agreement is obtained between experiment and the predictions of a recently reported Ar+HF(v=1) potential.

Notizen: Three-dimensional potential energy and electric dipole moment functions for the electronic ground state of H2O+ have been calculated from highly correlated multiconfiguration reference configuration interaction (MRCI) electronic wave functions. The analytic representations of these functions have been used in vibrational and perturbational calculations of the rovibrational absorption spectrum of H2O+. The quartic force fields in normal coordinates have been employed in the evaluation of the equilibrium spectroscopic constants in H2O+, D2O+, and HDO+ by perturbation theory. The equilibrium structure, vibrational band origins, centrifugal distortion constants and rotational energy levels agree very well with the available experimental data. Absolute vibrational band intensities have been calculated from the dipole moment functions and are compared with theoretical integrated band intensities. The radiative lifetimes of excited vibrational states exhibit mode specific variations. The rotationally resolved room temperature absorption spectra have been evaluated ab initio for the pure rotational and the ν2, 2ν2, ν1, ν3, and 3ν2 transitions. The rovibrational electric dipole transition matrix elements and absolute line intensities are given for the most intense transitions. These data take full account of anharmonicity effects and vibration–rotation coupling.

Notizen: A simple model is proposed for correcting problems with zero point energy in classical trajectory simulations of dynamical processes in polyatomic molecules. The "problems'' referred to are that classical mechanics allows the vibrational energy in a mode to decrease below its quantum zero point value, and since the total energy is conserved classically this can allow too much energy to pool in other modes. The proposed model introduces hard sphere-like terms in action–angle variables that prevent the vibrational energy in any mode from falling below its zero point value. The algorithm which results is quite simple in terms of the cartesian normal modes of the system: if the energy in a mode k, say, decreases below its zero point value at time t, then at this time the momentum Pk for that mode has its sign changed, and the trajectory continues. This is essentially a time reversal for mode k (only!), and it conserves the total energy of the system. One can think of the model as supplying impulsive "quantum kicks'' to a mode whose energy attempts to fall below its zero point value, a kind of "Planck demon'' analogous to a Brownian-like random force. The model is illustrated by application to a model of CH overtone relaxation.

Notizen: The photoion–photoelectron coincidence spectra for C2H+ and C2H+2 have been measured in the wavelength range of 645–765 A(ring). The C2H+2(A˜ 2Ag,B˜ 2∑+u) ions prepared with internal energies above 17.39 eV are found to dissociate completely into C2H++H in the temporal range 〈12 μs. An upper bound of 17.33±0.05 eV is determined for the appearance energy of the process C2H2+hν→C2H++H+e− at 0 K.

Notizen: Vibrationally mediated photodissociation, in which one photon prepares a highly vibrationally excited molecule by vibrational overtone excitation and a second photon dissociates the vibrationally excited molecule, is a means of studying the spectroscopy and photodissociation dynamics of highly vibrationally excited states. Applying this dissociation scheme to nitric acid (HONO2) excited in the region of the third overtone of the O–H stretching vibration (4νOH) and detecting the OH fragment by laser induced fluorescence determines the energy partitioning and identifies the influence of vibrational excitation prior to dissociation. Vibrationally mediated photodissociation using 755 and 355 nm photons deposits more energy in relative translation than the isoenergetic single photon dissociation with 241 nm light. The former process also produces three times more vibrationally excited OH fragments, and both processes form electronically excited NO2, which receives over three-quarters of the available energy. In these experiments, vibrational overtone excitation enhances the cross section for the electronic transition by about three orders of magnitude. The observed differences are consistent with the motion of the vibrationally excited molecule on the ground electronic state surface strongly influencing the dissociation dynamics by allowing access to different electronic states in the photolysis step.

Notizen: A procedure which offers computational advantages over the customary formalism is presented for calculating the shifts of the energy levels of discrete states embedded in a continuum. Numerical procedures are described and applied to the vibrational level shifts of the B 3Σ−u state of 16O2 interacting with the 5Πu continuum.

Notizen: A mechanism for the enhanced splitting detected in the millimeter-wave rotational spectra of the first excited S–S stretching state of HSSH (disulfane) has been studied. The mechanism, which involves a potential coupling between the first excited S–S stretching state and excited torsional states, has been investigated in part by the use of ab initio theory. Based on an ab initio potential surface, coupling matrix elements have been calculated, and the amount of splitting has then been estimated by second-order perturbation theory. The result, while not in quantitative agreement with the measured splitting, lends plausibility to the assumed mechanism.

Notizen: Structures for hydride–water, hydroxide–H2 and double-Rydberg isomers of OH−3 are optimized at the MBPT(2)/6-311++G(d,p) level. While the first two isomers have nearly equal total energies, the double-Rydberg isomer is 1.68 eV less stable. Vertical ionization energies of the isomers are calculated with electron propagator theory and a 6-311++G(2d,2p) basis augmented with extra diffuse functions. The result for the hydride–water complex, 1.50 eV, is in excellent agreement with a recent photoelectron experiment, while the value for the hydroxide–H2 structure, 2.27 eV, is substantially different. Calculations are performed on a double-Rydberg anion with a pyramidal, C3v structure, yielding a vertical ionization energy of 0.43 eV. Corresponding neutral structures are optimized and provide adiabatic ionization energies. Harmonic vibrational frequencies are calculated for the anionic structures.

Notizen: We present a theoretical analysis of the effects that lead to distortion or symmetry restoration of the nuclear configuration in ions of weakly bound molecular systems and clusters. The vibronic coupling mechanisms involved are reviewed and discussed in terms of simple and general models. Guided by a qualitative characterization of the wave function of a loosely bound system, we study the symmetry determining effects for such ions in terms of first order (Koopmans'), relaxation and correlation distortion energies associated with electron removal from the parent system. Our discussion suggests that the Koopmans' approximation can reasonably be expected to provide a qualitatively correct dependence of the ion energy on nuclear motion. Exceeding these expectations, it is found in numerical applications to the He+3 and He+4 ions that the relaxation and correlation energy changes accompanying symmetry breaking tend to cancel exactly. This implies, and we show in practice, that while insufficiently accurate correlation energy estimates may easily lead to incorrect geometries, the inexpensive Koopmans' approximation can often suffice to obtain the correct energy curvature along symmetry breaking coordinates. Further preliminary results for N3 and CO+2 suggest that these findings have a much wider applicability than anticipated.

Notizen: The enthalpy of sublimation of fermium (Fm), element 100, has now been determined directly by measuring the partial pressure of Fm over alloys, for the temperature range of 642 to 905 K. The partial pressures were determined using Knudsen effusion and target collection techniques. Dilute (10−5–10−7 atom %) solid alloys of Fm and mixtures of Fm and Es in both Sm and Yb solvents were studied. The presence of Es in two of the alloys allowed a direct comparison of the behavior of Fm and Es, where the latter could be used as a reference. It was possible to calculate enthalpies of sublimation and a hypothetical vapor pressure/temperature relationship for pure Fm metal by selecting Yb as the solvent most likely to form a nearly ideal alloy with Fm. From the experimental vapor pressure data, we derived average Second Law values of 33.8±3 kcal/mol and 23.5±3 cal/mol deg for the enthalpy and entropy of sublimation of Fm at 298 K. Third Law enthalpy values were also calculated using the experimental partial pressure data and entropies estimated from derived free energy functions and heat capacities for the solid and gaseous forms of Fm. The average Third Law values (34.8 kcal/mol and 25.1 cal/mol deg, respectively, at 298 K) are in agreement with those obtained via the Second Law. These results establish that Fm, like Es (element 99), is a divalent metal. The finding that Fm metal is the second divalent actinide element experimentally establishes the trend towards metallic divalency expected in the second half of the actinide series.

Notizen: The scattering of light by a dilute monodisperse population of random-coil molecules is approximately described by the well-known function P(θ)=(2/u2)(e−u+u−1), where u=R2gq2, Rg is the radius of gyration of the random coils, and q=(4πn/λ)sin(θ/2) is the magnitude of the scattering vector. We show that if the molecules in this population undergo random scission, then P(θ) is still given by the above formula where, however, u is now equal to R2goq2+r, where r is the average number of scissions per molecule in the originally monodisperse population and Rgo is the original radius of gyration. It is suggested that this could be useful for determining depolymerization rate constants, and for investigating whether various forms of scission are actually random. Results from initial experiments which apply the theory to the acid hydrolysis of hyaluronic acid are presented. The rate constants found are in reasonable agreement with values in the literature.

Notizen: The changes of enthalpy at the irreversible transformation of high-density amorphous ice to low-density amorphous ice and of low-density amorphous ice to ice Ih have been measured in a Tian–Calvet heat-flow calorimeter. The equilibrium pressures of the two transformations have been estimated from the measured enthalpy and volume changes, neglecting the unknown, but small, entropy changes, as about 5 and 2 kbar, respectively. The course of the transformations has been followed in a piston-cylinder apparatus having an internal diameter of either 50 or 12 mm. The transformations are extraordinarily sharp when they are done slowly, the transformation pressure being constant to ±50 bar in 5 or 10 kbar in several transformations, although sometimes a sample transformed in two steps that were a few hundred bars apart. It seems that the activation volumes of the transformations are large and negative, and so the mechanism of the transformations is highly cooperative.

Notizen: A theory is presented to describe the nucleation and subsequent growth of weakly inhomogeneous lamellar phases from a supercooled disordered phase. Detailed calculations are performed for the case of near-symmetric diblock copolymer melts, although the results have relevance for other physical systems in the Brazovskii "universality class.'' Nucleation phenomena in such systems are particularly interesting because the bulk phase transitions have a very weak, fluctuation-induced first order character. We find unusually small nucleation barriers and critically slowed growth kinetics. For a diblock copolymer melt with degree of polymerization N and undercooling δ∝(Tt−T)/Tt(very-much-less-than)1, the barrier is of order ΔF*/kBT∼N−1/3δ−2. Our estimate for the completion time of the nucleation and growth processes is θc ∼N1/12 δ−3/4 τd exp(ΔF*/4kBT), where τd is the copolymer terminal relaxation time.

Notizen: X-ray powder patterns obtained from three different types of 35–40 A(ring) CdSe nanoclusters are analyzed. We simulate the effects of thermal fluctuations, stacking faults, surface reconstructions, and bond compressions on powder patterns of CdSe clusters using the Debye formula. We find that the spectra of capped and annealed CdSe particles grown in inverse micelles are best fit by a mixture of crystalline structures intermediate between zinc-blende and wurtzite. We describe a new preparation for CdSe clusters, the structure of which appears to be well-defined wurtzite with, on average, less than one stacking fault per cluster. Thermal effects are found to be important and to mask the subtle effects of likely surface and core reconstructions.

Notizen: An experimental study is presented of the reactions Na+HCl/DCl from 590 to 820 K. Na atoms were produced in an excess of HCl/DCl and He bath gas, by the pulsed photolysis of NaCl vapor. The metal atom concentration was then monitored by time-resolved laser induced fluorescence of Na atoms at λ=589 nm. A fit of the data to the Arrhenius form yields (2σ uncertainty): k(Na+HCl)=(2.1±0.5)×10−9 exp[(−41.8±1.5 kJ mol−1)/RT] cm3 molecule−1 s−1; k(Na+DCl)=(2.2±1.0)×10−9 exp[(−45.5±3.8 kJ mol−1)/RT] cm3 molecule−1 s−1. The large activation energies are shown to be consistent with vibrational excitation of the hydrogen halides greatly enhancing the reaction cross-sections. A pair of ab initio potential surfaces for these reactions are then calculated at constant angle cuts through the lowest 2 A' hypersurface, including a collinear surface and the surface containing the lowest saddle point which is found to occur at a bent configuration with θNaClH=54.7°. Both surfaces exhibit a late reaction barrier. The effect of reactant vibrational excitation is then demonstrated by using quasiclassical trajectories on the collinear surface.

Notizen: The band-Lanczos algorithm is used to compute a few matrix elements of a resolvent, i.e., to compute 〈ψk||(ω−H)−1||ψl〉. Here H denotes a Hermitian matrix of large dimension. It is shown that the proposed method which is based on the band-Lanczos algorithm is more efficient than a method based on simple-Lanczos algorithm, e.g., than the residue algebra method. The performance of the Lanczos algorithm (both simple and band) for evaluating resolvents or other functions of H is discussed in some detail.

Notizen: Isotropic chemical shifts and shift anisotropies for carbon, nitrogen, oxygen, and fluorine in first-row-atom molecules have been calculated in the perturbed Hartree–Fock gauge including atomic orbital scheme for both experimental and optimized molecular structures using a 6-311G* basis for heavy atoms and a scaled 4-31G basis for hydrogen. Structure optimization leads to the expected shortening of bond lengths, which is accompanied by an increase in the isotropic chemical shifts. The increased shifts show much improved agreement with gas phase experimental values for nitrogen and oxygen, while the results for carbon are only mildly affected and remain good; shift anisotropies for all species tend to decrease in magnitude and also generally improve. Fluorine is anomalous, its increasing shifts upon structure optimization moving further away from experiment at this level of basis set. The trend in the optimized isotropic shifts is explained in terms of the general tendency for atoms in the right-hand portion of the first row of the periodic table to have negative shift derivatives with bond extension.

Notizen: We present a theoretical description of the aggregation equilibrium of nonswollen and water-swollen micelles in oil (or vice versa). While we specialize on spherical, noninteracting aggregates, we treat the competition between phase separation, dissolution in monomers and formation of micelles with variable extent of swelling. The present model is based on an interfacial free energy of the surfactant monolayer which includes stretching and bending contributions. This free energy allows one to desribe both, the macroscopic interface in a two-phase system and the internal interface surrounding the aggregates on equal footing. The extent of swelling of the micelles at the CMC is determined by the bending energy of the saturated surfactant monolayer. Increasing the splay modulus or decreasing the spontaneous curvature favors swollen micelles (microemulsion droplets) and ultralow interfacial tensions. We present explicit results for the size distributions of the aggregates and interfacial tensions. Considering shape fluctuations of the aggregates we conclude that spherical micelles may be stabilized by small interfacial tension alone, while stability of strongly swollen microemulsion droplets requires a finite splay modulus of the monolayer.

Notizen: A few years ago, P. J. Flory [J. Am. Chem. Soc. 61, 1518 (1939)] solved a statistical problem analogous to determining the random-irreversible saturation formation of a 1×2 overlayer on a linear n lattice, by deriving an expression for the fraction of empty lattice points. Methods are presented here for both the enumeration and construction of saturation configurations. This alternate approach is derived from the theory of partitions and incorporates shift and sum-rule operators for the calculation of configurational probabilities. The fraction of filled lattice points is evaluated, and the configurational probabilities provide distribution functions for saturation configurations having different saturation levels (for fixed n). These distribution functions can be used to calculate sequence selective properties in addition to average saturation levels. Applications are illustrated in the analysis of the randomness of dehydrochlorination of polyvinylchloride and the distribution of UV-induced cyclobutyl-thymine dimers in Tetrahymena pyriformis DNA.

Notizen: A quasi-isochoric scanning method has been used to study the phase diagram of nitrogen from 150 to 550 K and up to 85 kbar in a diamond anvil cell in order to make a comparison with previous measurements of the binary phase diagram He–N2. It has been confirmed that there is only one solid-solid-fluid triple point in N2 up to 85 kbar. However, both the δ–β transition line and the melting line have shifted appreciably towards lower pressures. The present experiment shows that, as a result of this, the triple point is located at 555±5 K and 80±2 kbar, which is 20% lower in pressure than previous data.

Notizen: Recently, a simple mean-field theory was developed to describe structural and thermodynamic properties of fluid phases in monolayer films of flexible-chain surfactants [J. Chem. Phys. 91, 4423, 4431 (1989)]. This approach is extended here to examine films of mixed chain length. The high-density (LC-LE) phase transition predicted for the single component monolayers is found to exhibit nonideal mixing behavior, ranging from positive azeotropy to phase separation (in the LC phase) as the difference in chain lengths is increased. In contrast, the phase diagram in the LE-G transition region is predicted to exhibit almost ideal behavior, with a nearly linear bubble-point curve.

Notizen: Brownian dynamics computer simulations are used to investigate the shape of the mean-squared end–to–end distance distribution function for two-dimensional linear chains in three regimes: (1) excluded volume, (2) θ, (3) collapsed. It is found that Mazur's function fits regime (1), that the Gaussian function fits regime (3), and that the changeover in behavior between these two functional forms appears to occur at temperatures well above the expected θ point.

Notizen: A neutron scattering study was performed on K1−x(NH4)xI mixed crystals (x≤0.44) at temperatures T≤20 K. Our results show in a direct fashion that the NH+4–NH+4 interaction contributes significantly to the crystal-field potential. A quantitative discussion in terms of a locally perturbed field is applied to the x=0.02 results. The barrier increase due to the presence of a single NH+4 ion cannot be ascribed to octopole–octopole interactions alone. A qualitative model, which includes dipole–dipole interactions, is presented. The temperature study on a sample with x=0.28 shows a continuous transition from a quantum regime to a classical regime of reorientational motions.

Notizen: Recent application of the reference hypernetted-chain theory to the calculation of the structure of electrical double layers in wholly molecular models of aqueous electrolytes [J. Chem. Phys. 90, 4513 (1989)] is extended to NaCl solutions. The strong interaction of the small Na+ ion with the solvent molecules leads to a number of novel features, notably a very high adsorption of Na+ into a region about 4 A(ring) from the surface next to the first solvent layer. This is sufficient to produce a positive potential of zero charge of about 29 mV and to cause the effective surface charge experienced by the diffuse layer to be positive even when the surface carries a substantial negative charge. In most cases, however, the surface potential is found to be remarkably insensitive to the details of the structure of the double layer for a given surface charge.

Notizen: A microscopic model of lyotropic systems is presented. Amphiphile and water molecules are described by simple intermolecular potentials which correctly include important excluded volume effects and the relative energy scales in the system. The present model represents an attempt to bridge the gap between more primitive lattice models and highly complex all-atom descriptions of amphiphilic systems. A constant-temperature molecular dynamics study in which the divergence of the pressure tensor is constrained to zero is discussed. Preliminary calculations on the phase diagram, order parameters, and dynamical observables of the model are reported. Some comments on improvements to the model and the theoretical techniques used to study it are made in the final section of the paper.

Notizen: The nondissociative sticking of CO on Pt(111) has been measured as a function of incident energy and angle. Both these sticking measurements and prior molecular beam scattering experiments are incompatible with simple theoretical models that describe successfully weak physisorption interactions. For strong chemical interactions, such as for CO–Pt(111), lateral and rotational corrugations in the interaction potential are large and play an important role in the scattering and sticking. Both translational to rotational and normal to parallel energy conversions cause substantial energy scrambling in the interaction. Classical trajectory calculations that incorporate these effects realistically predict a rather gentle fall off in sticking with incident energy, a scaling of sticking with total rather than normal energy and directly scattered distributions which consist of broad lobular components about the specular superimposed on an even broader diffuse component. All of these findings are in accord with experiment.

Notizen: Light and small angle neutron scattering results are presented for a series of poly(dimethyl siloxane) (PDMS) gels, cured in the dry state by endlinking with a trifunctional crosslinking agent, ethyl triacetoxy silane (ETAS). For gels swollen in octane, the scattered light and the smallest angle neutron response are dominated by scattering from aggregates of polymerized ETAS, the mean radius of which was found in this system to be ∼800 A(ring), and which have a smooth surface in the length scale between 500 and 100 A(ring). The internal surface area of this precipitate was roughly 200 cm2 per cm3 of the swollen gel. Only about one third of the free ETAS participates in this condensed phase. The rest is disseminated as oligomers or monomers throughout the gel. The gel itself appears as a solution-like structure with in addition permanent waves in the polymer concentration distribution. In this system, the permanent waves could be modeled by a Gaussian function of characteristic width ΔR=80 A(ring) and relative amplitude 〈Δc2〉1/2/〈c〉≈0.1. The polymer–polymer correlation length ξ describing the shorter range fluctuations is substantially modified in comparison with an unperturbed polymer solution at the same overall concentration. For the uncrosslinked solution at volume fraction φ=0.16 it is found that ξ=10.3 A(ring), while for the crosslinked gel swollen to the same degree, ξ=18 A(ring).

Notizen: We compare the predictions of the capillary wave model for fluid interfaces with known exact expressions, due to Triezenberg, Zwanzig, and Wertheim, that relate the surface tension to the interfacial structure, and find agreement in all dimensions. It is shown that contrary to the case d〈3, the interfacial pair correlation function does not obey scaling in d=3 dimensions on length scales less than the capillary length Lc. Nevertheless, once the sensitive dependence on the gravitational field and the short distance cutoff is properly taken into account, we find no evidence for recent allegations that the capillary wave model leads to inherently inconsistent results for the direct correlation function in d=3. Several issues regarding the connection with density functional theory and the interpretation of the Triezenberg–Zwanzig formula, on which these results touch, are discussed.

Notizen: Two energy estimators, the Barker estimator and the Berne virial estimator, commonly used in path integral simulations of quantum systems are compared with respect to statistical accuracy. It is found that the accuracy of these estimators is strongly affected by the algorithm used. Four common algorithms are considered here: (1) the pure primitive algorithm, (2) the primitive algorithm augmented by whole chain moves, (3) the normal-mode algorithm, and (4) the staging algorithm. The error of the mean of the Barker energy estimator is found to grow as (P)1/2, where P is the number of discretization points of the quantum paths (or the number of chain particles in the isomorphic classical chain), for all of the algorithms above. The error of the mean of the Berne virial energy estimator is independent of P for algorithms 2, 3, and 4, and increases as (P)1/2 for algorithm 1. It is concluded that the virial estimator is far more accurate than the Barker estimator for algorithms 2, 3, and 4, and is at least as accurate for algorithm 1. Because the error analysis depends strongly on the temporal correlations in the sequence of values of the energy estimator generated during Monte Carlo or molecular-dynamics simulations, we review the general question of error analysis in simulations.

Notizen: A combined neutron diffraction and molecular dynamics study is reported of the Dy3+ ion environment in vitreous NaF–DyF3–BeF2, using a special version of the isotopic substitution procedure known as the null technique. To investigate the effects of both NaF and DyF3 on the basic beryllium fluoride glass network, complementary measurements and simulations have been performed for vitreous NaF–BeF2, with the same NaF:BeF2 ratio as the DyF3 containing glass, and also for pure vitreous BeF2. The neutron diffraction data indicate that for glasses simulated using pair potentials the BeF4 tetrahedra are much more distorted than those in the real materials and the mean Be–F–Be angle is too high. Both of these deficiencies in the simulations are attributed to the use of simple ionic potentials with no bond angle restoring forces. Experimentally, the distribution of Dy–F first neighbor distances is found to be narrow, having a root mean square deviation of 0.110±0.003 A(ring) about the mean value of 2.290±0.003 A(ring). The Dy(F) coordination number is 7.3±0.2 which is slightly less than that in crystalline DyF3.

Notizen: Classical molecular dynamics simulations of sputtering using large samples of up to 15 500 particles are demonstrated for Lennard-Jones solids. Two features are introduced to represent the effect of the surrounding solid substrate. First, the outermost particles on each side of the sample are subjected to an additional harmonic potential restraining them close to the equilibrium position. Second, these outermost particles are maintained at a preset temperature by assigning velocities at each time step randomly from the corresponding Gaussian distribution. This boundary thermostat produces a "sandbag'' effect and provides a means for excess kinetic energy to dissipate out of the sample. Preliminary results were obtained for single trajectories at 45° incidence and these illustrate that the sputtering yield from these model rare gas solids is strongly dependent on both the length of the simulation and the size of the sample. Simulations were continued to times in excess of 25 ps after which time for the largest sample there was some recoalescence of particles remaining in the surface region of the substrate. The long time sputtering yield was about 500 atoms which is comparable with experimental data on rare gas solids. It is demonstrated that particles within a cluster are just as likely to arise from nonadjacent sites as from near neighbors in the original lattice.

Notizen: An algorithm for simulating the Doi–Edwards and Curtiss–Bird models for concentrated polymer solutions and melts in homogeneous flow situations is developed. For steady shear flow, it is demonstrated that this simulation algorithm is capable of supplying high-precision results for the viscometric functions. The proposed simulation algorithm is not only useful for studying arbitrary time-dependent flows and various generalizations of the original Doi–Edwards and Curtiss–Bird models, but it also further illuminates the mathematical and physical content of the basic equation describing the polymer dynamics in these reptation theories.

Notizen: The dynamics of silicon-atom diffusion on a reconstructed Si(111)-(7×7) surface have been investigated using variational phase-space theory methods with a previously described [J. Chem. Phys. (to be published)] potential-energy surface. A four-layer lattice model of the Binnig et al. (7×7) reconstruction containing 291 atoms is employed for the surface. Canonical Markov walks with importance sampling incorporated are used to evaluate the flux across both right-circular and right-elliptical cylindrical dividing surfaces separating adsorption sites. This flux is minimized with respect to the parameters of the dividing surface to obtain the best estimate of the classical jump frequencies. The minimum jump frequencies so obtained are corrected for recrossings of the dividing surface by the calculation of trajectories that start from phase-space points obtained in the random walk that lie within a specified distance w of the dividing surface. The corrected jump frequencies are then used as input to a set of 225 diffential equations that describe the diffusion rates across the (7×7) surface. Diffusion coefficients D are computed from the slope of plots of the time variation of the root-mean-square displacements obtained from the solution of the rate equations. Arrhenius plots of results obtained at 300, 600, and 1000 K yield D=2.15×10−3 exp (−1.51 eV/kBT) cm2/s. The calculated activation energy of 1.51 eV is in excellent accord with the result obtained by Farrow from molecular-beam pyrolysis data on SiH4 deposition. An examination of the details of the diffusion shows that it is not isotropic on the (7×7) surface. We find that preferential directions of flow exist. These directions correspond to "gateways'' at three of the four corners of the (7×7) unit cell. The results suggest that diffusion rates are a sensitive function of the geometry of the (7×7) reconstruction so that careful measurements of diffusion rates and associated activation energies may be able to serve as a means of differentiating different proposed models of the Si(111)-(7×7) reconstruction.

Notizen: Continuum approximations to the discrete birth and death equations for classical nucleation are investigated. The discrete equations are parametrized by rate coefficients αi and βi for a cluster of size i to lose or gain a monomer, respectively. The continuum equations considered for the distribution function f(x,t) of clusters containing x monomers at time t are all of the form of a Fokker–Planck equation: ∂f/∂t=∂/∂x[Bf eq∂(f/f eq)/∂x], where f eq(x) is the equilibrium distribution and B(x) is a diffusion coefficient. The dependence of B(x) on various continuum approximations to the rate coefficients is discussed at length. Three different forms of B(x) are considered; that used by Frenkel [Kinetic Theory of Liquids (Oxford, Oxford, 1946)], that suggested by Goodrich [Proc. R. Soc. London Ser. A 371, 167 (1964)], and a third form proposed here. Steady state distributions and time lags obtained from the continuous and discrete equations are compared. The time-dependent Fokker–Planck equation is solved by an eigenfunction expansion and the eigenfunctions and eigenvalues of the Fokker–Planck operator are compared with those of the birth and death equations. A change of variables transforms the Fokker–Planck equation into a Schrödinger equation and permits the interpretation of the eigenvalues as energy levels in a potential function. Since the potential is approximately quadratic near its minimum, the lower eigenvalues are close to the harmonic oscillator results. The results show that the choice for B(x) suggested by Goodrich and the one proposed in this paper generally give better agreement with values from the discrete equations than the Frenkel form, used previously by most workers.

Notizen: A transition that breaks the m-fold rotational symmetry of a chemical pattern of traveling waves is reported. The chemical waves travel around a thin annular gel, the inner and outer edges of which are in contact with continuous flow chemical reservoirs. Observations of the dependence of the rotating wave pattern as a function of temperature reveal several distinct states, including a wavelength-doubled state in which successive waves alternate in shape.

Notizen: The ethyl radical has been produced by the F+C2H6 reaction, and studied by photoionization mass spectrometry. The adiabatic ionization potential (IP) is found to be 8.117±0.008 eV. The radical and cation have been studied by the G1 level of molecular orbital theory. The calculated adiabatic ionization potential is 8.06 eV. On the basis of both experiment and theory, it is concluded that the cation in its ground state has a bridged structure. The adiabatic value resolves a previous inconsistency involving ΔH(open circle)f(C2H5), ΔH(open circle)f(C2H+5) and IP (C2H5). Broad autoionizing structures are observed at higher energy. These features can be rationalized as Rydberg states converging to the first excited state 3A‘ calculated to be at 12.05 eV.

Notizen: A direct absorption, difference frequency, infrared laser spectrometer with 10−4 cm−1 resolution combined with slit supersonic jet optical pathlengths is presented as a tool for the study of mode–mode vibrational coupling in laser-excited hydrocarbons. These weak mode–mode couplings are evidenced in our frequency domain studies by virtue of transitions to isolated upper J states that are split into multiplets under sub-Doppler resolution. Instrument performance is demonstrated by investigating vibrational coupling in the 3000–3300 cm−1 C–H stretch fundamental region of 12C3 propyne, as well as the 12C213C propynes observed in natural isotopic abundance. No appreciable state mixing is observed in ν1=1←0 and ν6=1←0 spectra at T=4 K. However, near-resonant two-state mixing of ν2 and ν5+ν8+3ν10 in the ν2=1←0 transition of 12C3 propyne is detected and deperturbed to provide an anharmonic coupling matrix element of 0.096 41(38) cm−1. This matrix element is independent of J' and thus arises from purely anharmonic, non-Coriolis-mediated couplings. The implications of anharmonic coupling matrix elements of this magnitude in overtone vibrational dynamics are discussed.

Notizen: New results from pressure broadening measurements between 4.3 and 1.7 K for the J=2←1 and 3←2 transitions of CO broadened by helium are reported. Unlike the results of helium pressure broadening experiments at higher temperature, significant variations in cross section occur with changes in temperature. These measurements are compared with the results of close coupling calculations and show both the effects of resonances in the collision channels and energy defects at the very low collision energies. Room-temperature measurements of helium pressure broadening have also been made on CO, NO, CH3F, and H2S. These provide a normalization for our previous studies of these species at very low temperatures and a basis for the qualitative comparison of the diverse results we have obtained.

Notizen: The formation process of water clusters is investigated experimentally with a supersonic free molecular jet of water vapor mixtures of Ar, Xe, N2, CO2 and CO, especially noticing the effect of the solute molecules on the cluster formation. The terminal concentrations of clusters formed through a supersonic molecular expansion are measured with a mass spectrometer by changing the mole fraction of water vapor at the source. It is observed that, in the range of relatively small H2 O mole fraction of binary mixtures at the source, the formation process of water clusters is controlled mainly by the thermodynamical effects of the expansion. On the other hand, at larger H2 O mole fractions, the molecular dynamical behavior of mixture molecules must be taken into account for understanding the formation process of water clusters. In both cases, the binary species play an important role in the formation process of water clusters.

Notizen: Photoionization of singlet excitons in p-terphenyl crystal is studied with nanosecond laser excitation. The photoionization spectrum and the photoabsorption spectrum of the lowest excited state are measured. The ratio of the two spectra gives the efficiency with which higher excited states, generated by the two-step excitation, ionize. The observed efficiency is constant below 7.3 eV and is remarkably high (∼10−2 ). The results do not support the view that the excess energy is converted directly to the kinetic energy of the generated electrons in the crystal and indicate that the initially prepared higher excited states relax rapidly and the succeeding autoionization yields electrons with a fixed kinetic energy.

Notizen: The closed form of the solution to the master equation for unimolecular decomposition is investigated. Exact numerical values of the Rice–Ramsberger–Kassel–Marcus unimolecular rate coefficient are calculated for the decomposition of cyclobutene and the isomerization of quadricyclane. A comparison between the approximate strong collision low temperature expression now in general use and the high temperature strong collision exact expression shows that in some cases there are large deviations (∼40%) between the values calculated from the two expressions. The usage of the exact expression causes a shift of the falloff curves toward the high pressure limit. The larger the molecule, the larger the deviation; the higher the value of E0 the smaller the deviation. For E0/RT〉20 the approximate expression is sufficient. These deviations may be quite important in practical systems such as combustion and atmospheric chemistry and shock tubes.

Notizen: The range of validity of diffusive algorithms is studied by comparing the isomerization rates and rotational reorientation times from a series of Langevin dynamics trajectories of butane with the results of a Brownian dynamics (diffusive) trajectory, as well as with analytic approximations. It is found that inertial effects may be ignored for rotation at most liquid densities and contribute approximately 10% to the isomerization rate at water viscosities. For neat butane, or other short-chain alkane solvents at room temperature, the effect of inertial terms on isomerization rates is significant.

Notizen: A detailed analysis of the rotational and hyperfine structure of the (0,0) band of the B 3Φ–X 3Δ electronic transition of NbN has been performed from sub-Doppler spectra taken with linewidths of about 50 MHz. The Nb hyperfine structure is impressively wide in both states, but particularly so in X 3Δ where one of the unpaired electrons occupies a σ orbital derived from the metal 5s orbital. The electron spin and hyperfine structures do not follow the expected case (aβ ) coupling because of extensive second order spin-orbit effects. It is shown that the asymmetry in the spin–orbit structure of X 3Δ is explained almost quantitatively by interaction with a 1 Δ state from the same electron configuration (which lies at 5197 cm−1); also cross terms between the spin–orbit and Fermi contact interactions in the matrix element 〈3Δ2||H||1Δ〉 produce a large correction to the apparent coefficient of the I⋅L magnetic hyperfine interaction in X 3Δ2. The hyperfine structure in a triplet state turns out to be extremely sensitive to the details of the electron spin coupling, and reversals in the sense of the hyperfine structure in the 3Φ4–3Δ3 and 3Φ2–3Δ1 subbands are shown to be consistent with the3Δ state being a regular spin–orbit multiplet (A〉0). Particular care has been taken with the calibration, which has meant that extra terms have needed to be added to the magnetic hyperfine Hamiltonian to account for the spin–orbit distortions: instead of the usual three parameters needed in case (aβ ) coupling, the B 3Φ state has required four parameters and the X 3Δ state has required five. The model explains the data very well, and the standard deviation in the least-squares fit to more than 1000 hyperfine line frequencies was 0.000 58 cm−1 (17 MHz).

Notizen: The Chirikov pendulum is the standard Hamiltonian of nonlinear dynamics. While many resonant problems of this rich field can be mapped onto this paradigm, the accuracy of its predictions depends on the coupling strengths in the problem. In this article, we generalize the Chirikov pendulum by introducing an alternative reduction, namely to an asymmetric rotor. We also present a geometrical interpretation of both mappings, which is found to be helpful in displaying the dynamics, in particular local and normal mode formation in nonlinear oscillator systems. Our mapping proceeds by finding the Lie algebra for the particular resonance involved, and interpreting its generators as angular momentum components that generate a "constant energy surface'' in three dimensions.

Notizen: Using synchrotron radiation as a continuum light source, the absolute photoabsorption and direct dissociation cross sections of C2 H2 in the 1530–1930 A(ring) region have been measured at 295 and 155 K. In comparison with cross sections obtained at room temperature, the cross sections at absorption peaks typically increase by 10%–40% while those at absorption valleys decrease by as much as 30% at 155 K. Further, the absorption band profiles become narrower as the gas temperature decreases. In the presently studied wavelength region, there are at least three electronic states, namely, the A˜ 1 Au , the B˜ 1 Bu , and a continuum. Using the present low temperature data we have constructed the repulsive potential energy curve of the direct dissociation state according to the Franck–Condon principle. The potential energy curve thus constructed can be more accurate than that constructed by using room temperature cross section data because only the absorption cross section from the ν'=0 level contributes. The present cross section data have also been used in the analysis of oscillator strength distribution.

Notizen: A direct relationship between the N valence shell charge topology reflected in its Laplacian and the components of the electric field gradient (EFG) at the N nucleus in nitriles was found. The largest diagonal component of the N EFG tensor was showed to be determined by the combined effect of the bonded and nonbonded maxima of the Laplacian of the valence shell of the N atom along the C–N direction. In nonlinear nitriles, the asymmetry of the N field gradient around the C–N direction was showed to be determined by the difference in the values of the maxima of the Laplacian of the valence shell obtained in two mutually perpendicular planes that are also normal to the C–N direction and contained the N nucleus.

Notizen: The classical variational theory of chemical reaction rates gives the rate as the equilibrium flux of systems through a trial surface in the phase space of the reaction sysem. The surface divides the phase space into reactant and product regions and is varied to obtain a least upper bound for the rate of product formation. For atom–diatom reactions of the type A+BC→AB+C, we derived expressions which give the canonical rate coefficient and the microcanonical mean reaction cross section for the most general dividing surface defined by internal-configuration-space coordinates [J. Chem. Phys. 87, 5746 (1987)]. The dividing surface can be expressed as a power series in two of the internal coordinates and its flexibility can be systematically improved by introducing additional terms. We apply this variational formulation to the H+H2 and H+I2 reactions. Canonical rate coefficients are calculated using the downhill simplex algorithm to find the best values of three, six, and ten variational parameters in the first-, second-, and third-order expansions of the dividing surface. For the H+H2 reaction, canonical variational rate coefficients at 300 and 900 K show the expected improving trend for the first through third-order expansions of the dividing surface. The variational rate coefficient for the H+H2 reaction converges to the classical trajectory value at 300 K and exceeds the trajectory value at 900 K by a factor of 1.18±0.10. A reactivity map is devised to show the statistical importance of configurations on the dividing surface. For the quadratic dividing surface at 300 K, the most statistically important configuration on the dividing surface is nearly symmetric in terms of internuclear distances measured from the central H atom and has a "bond angle'' for the arrangement H–H–H of 166 deg. The power series dividing surface for both the canonical and microcanonical formulations converges to a position which is close to the symmetric dividing surface of conventional transition state theory. Canonical variational rate coefficientsfor the H+I2 reaction also show the expected improving trend with the expansion order of the dividing surface. However, the best variational rate coefficient for the H+I2 reaction exceeds the trajectory value by a factor of 1.767. The effective convergence of variational values of this ratio for the third-order expansion of the dividing surface shows that at this order, the dividing surface is nearly as good as it can be when its formulation is limited to configuration-space variables. For the quadratic dividing surface, the most statistically important configuration at 600 K has I–I and I–H internuclear separations of 5.10 and 4.65 a.u., respectively, and a bond angle for the arrangement I–I–H of 109 deg. The microcanonical formalism is applied to the H+I2 reaction and quadratic variational dividing surfaces are determined for seven values of the internal energy. The dividing surfaces show a weak dependence on the energy. The improvement obtained when the microcanonical results are used to evaluate the canonical rate coefficient at 600 K amounts to only 0.265%.

Notizen: An algebraic approach to molecular electronic spectra is presented. This approach is used to provide a simultaneous description of electronic, vibrational and rotational energy levels in the series of molecules LiH, BeH, BH, CH, NH, and OH.

Notizen: In this paper we provide an extension of the electrostatic model for the structure and energetics of large van der Waals ions consisting of an aromatic cation bound to rare-gas atoms. We consider the effects of intermolecular electronic overlap on the charge-induced dipole interactions. Model calculations for tetracene⋅R+n (n=1–3 and R=Ar, Kr) and for benzene⋅R+ provide a reasonable account for the ionization potentials of these vdW ions.

Notizen: A method for computation of dipole moments from the polarization propagator is presented and applied to the H2O molecule. Using extended basis sets it is shown that the coupled cluster singles doubles polarization propagator approximation (CCSDPPA) reproduces the experimental dipole moment to a high degree of accuracy

Notizen: A novel modification to the "slithering snake'' algorithm is used to enable Monte Carlo simulations of partially ionized polyelectrolyte chains where the charged beads interact via screened Coulomb interactions. The use of "stroboscopic'' averaging enables detailed shape characteristics of the chains to be calculated. The moment of inertia tensor, the distribution function of the end-to-end distance and the segment density distribution functions from the center of mass and both ends of the polyion are combined to give an overall characterization of the shape changes of a polyion as added salt concentration, chain length (up to 240 beads) and degree of ionization are varied. Some abrupt behavior seen for high degree of ionization may be smoothed once ionic degrees of freedom are included. The present work—by characterizing the shape of short chains—may also help to formulate models for very long polyions consisting of many segments, each of a few hundred beads.

Notizen: It is shown that the potential distribution of a strong test particle leads to the zero separation values of the cavity distribution functions yab(0) in a mixture. This relation furnishes a direct means of computing by Monte Carlo simulations the coincidence values of the cavity function ln yab(0) and the potential distribution 〈exp[−βΨa−βΨb]〉. Test particle simulations have been carried out for mixtures of Lennard-Jones molecules differing considerably in size [(σab/σbb)3 =0.25, 0.5, 0.75, 1.00, 1.25, 1.50, 1.75, and 2.00] and in strength of interaction (εab/εbb =0.5, 1.0, 1.5, and 2.0). Alternative Monte Carlo methods are employed to check the statistics. In order to predict the behavior of the potential distribution, a distribution function theory, the reference hypernetted chain (RHNC) equation, is solved based on the universality of the bridge functions. Hard sphere mixtures are taken as reference fluids. The criteria recently proposed by Rosenfeld and Blum are used to select the equivalent hard sphere diameters. Close agreement with MC results is attained for most of the states considered. This provides further evidence that the RHNC equation is a reliable theory for mixtures of simple fluids.

Notizen: We present a density functional approximation for the free energy of a system of hard bodies with arbitrary shape and orientational distribution. For systems with homogeneous density it reduces to existing treatments, which describe the isotropic liquid and the nematic liquid crystal. The treatment of the inhomogeneous density allows the study of smectic and crystal phases. We applied the approximation to a system of parallel hard spherocylinders, for which we compare the nematic to smectic-A phase transition with recent computer simulation data. We also study a system of oblique cylinders and show that smectic-C phase may appear as the result of packing effects in systems of hard bodies.

Notizen: Collision induced widths and shifts for Raman Q-branch transitions of D2 in He were calculated from S matrices obtained with converged close coupling scattering calculations on an accurate theoretical interaction potential. Results agree well with experimental values. Discrepancies between experimental line shifts and theoretical values from an earlier study are traced to a computational error in that work. The effects of vibrational inelasticity and of centrifugal distortion on energy levels and on potential matrix elements, all of which were ignored in the earlier study, are explicitly considered here and found to be small.

Notizen: The Langevin dynamics simulation method has been used to study the structural and dynamical properties of ions in electrolyte solutions. Time evolutions of Na+ and Cl− ions in aqueous solutions have been simulated using different solvent averaged ion–ion interaction models. The changes observed in the properties of ions when nonoscillatory solvent averaged potentials are exchanged for more realistic ones including the solvent structure effects are discussed. We have also investigated the formation of ionic associations, paying special attention to the lifetime of the detected dimers.

Notizen: A model structure for an annealed silica surface was obtained through the molecular dynamics simulation technique employing three-body interaction potentials. Nonbridging oxygen and edge-sharing tetrahedra were found to form on the oxygen-terminated surface with three-coordinated silicon, three-coordinated oxygen, and three-membered rings just below the outermost atoms. Four-membered rings were also created in relatively large concentrations during the surface relaxation. When considering the effect of removing periodic boundary conditions along the z direction, the concentration of larger rings, relative to the bulk, remained unchanged throughout the ∼11 A(ring) simulated surface region. A strong physical association between three-membered rings and three-coordinated oxygen was found which may account for the D2 defect peak observed in Raman scattering.

Notizen: Molecular dynamics simulations have been performed for systems of chains composed of Nc =8–20 units immersed in 343-Nc solvent molecules. Two different types of forces (rigid and elastic) between neighboring chain units are considered. Also, different values of the interaction between chain units and solvent molecules are introduced. Distribution functions of distances, dimensions, time-correlation functions and translational diffusion coefficients are calculated from the simulation results. The values of these properties show that the elastic model corresponds to a more expanded chain. More expanded chains are also simulated with more favorable polymer–solvent interactions. Slower relaxation times are obtained for the expanded chains. Scaling laws for dimensions and diffusion coefficients in terms of the chain lengths are investigated.

Notizen: The sticky electrolyte model in a dipolar solvent is studied in this paper. A detailed separation of the Ornstein–Zernike (OZ) equations and their solution in the mean spherical (MS) approximation for binding (or stickiness) at L=σ are given. The results derived earlier by Adelman and Deutch, Blum et al. and by Høye et al. in this approximation are reproduced when the stickiness is switched off. Also when the density of the solvent goes to zero, the results reduce to those of the sticky electrolyte model (SEM) in a continuum solvent. It is found that the PY/MS approximation gives negative solutions for the association parameter λ, while the HNC/MS approximation works in a narrow interval of the sticky potential well depth ε2 between the positive and negative ions. As expected, the ion association increases when sticky potential well becomes deeper, but the dipole moment of the solvent is found to have a strong screening effect on this association. The study of the radial distribution functions of this system shows that the probability of a free ion appearing near a counter ion is greatly decreased when binding occurs between the oppositely charged ions at the contact; the opposite happens for ions of the same sign. The absolute value of the ion solvation energy becomes smaller as the electrolyte concentration increases and when stickiness between oppositely charged ions is introduced.

Notizen: The existence of the stable lithium cluster Li5 has been proved experimentally by mass spectrometry. From the measurement of the gaseous equilibria Li5(g)+Li2(g)=Li4(g)+Li3(g) and Li5(g)+Li(g)=Li4(g)+Li2(g), an atomization energy D00(Li5) =110.0±2.5 kcal/mol was obtained. The ionization potential (IP) of Li5(g) was determined as IP(Li5)=4.56±0.3 eV, from which the atomization energy of the molecule ion Li+5, D00(Li+5) =130.0±5.0 kcal/mol is obtained for the reaction Li+5→Li++4 Li. The heat of formation was determined as ΔH00(Li5) =82.5±2.0 kcal/mol. By extrapolation from experimental results, the properties of neutral and ion clusters of hexamer lithium Li6 were predicted.

Notizen: The incoherent dynamic structure factor Fs has been measured on concentrated dispersions of particles with interactions approximating hard spheres. The particle mean squared displacements and self-diffusion coefficients are obtained from Fs at times and scattering vectors where the Gaussian approximation (for Fs ) is expected to be reasonable.

Notizen: Analytical expressions are derived for the NMR signal from isolated dipolar coupled 2H–14N spin pairs following a quadrupole echo pulse sequence. The expressions are used to calculate the pulse spacing dependence of the deuterium quadrupole echo intensity and the powder line shape. The theoretical predictions are in good agreement with experimental results obtained for urea-d4 which show that the rigid lattice powder spectra depend significantly on the pulse spacing.

Notizen: The interaction of CO, CO2, and CH3OH with stepped Rh field emitter surfaces has been studied by means of pulsed field desorption mass spectrometry (PFDMS) at 300 K and pressures at or below 1.3×10−4 Pa. A common feature in the PFD mass spectra of the different adsorbed layers is the occurrence of CO+ and Rh(CO)n+x ions (x=1–3, n=1,2). These species indicate that a surface reaction takes place between chemisorbed CO and Rh step sites to form adsorbed Rh-subcarbonyls. COad is formed by CO adsorption from the gas phase as well as CO2 and CH3OH decomposition on the surface. The relative and absolute abundances of the various species depend on the desorption field strength FD of the pulses. High index Rh(CO)x are observed in large quantities at low FD. Under these conditions only small amounts of chemisorbed CO are removed and the surface is kept close to the saturation coverage. At high FD, the surface layer is completely desorbed, so that the COad concentration which builds up during the field free reaction time, tR=1 ms, is insufficient for Rh-subcarbonyl formation. Coadsorbed oxygen and hydrogen formed during CO2 and CH3 OH decomposition, respectively, do not prevent Rh(CO)x production on the stepped Rh surface.