bluuebunny/crossref_metadata_2025 huggingface dataset:
A normal-mode study of a polymer glass containing a chromophore impurity
Authors: Forrest Bruce M. , Leontidis Epameinondas , Suter Ulrich W.
Published: July 2002

We examine the vibrational density of states of atomistic models of polypropylene glasses containing a single impurity molecule of s-tetrazine. We discuss existing methods and develop new ones to achieve significant data reduction and navigate through the complex spectrum of the normal modes of the glass. By calculating the participation ratio, the distribution of the kinetic energy of each mode on the atomic coordinates, and a mode-proximity index to the solute it is possible to identify impurity-related, polymer-related, and mixed modes and assess their relative contributions to the vibrational density of states. Activation of specific modes using molecular dynamics allows the observation of anharmonicities in the doped glass, even at very low temperatures.
The scaling behavior of critical adsorption in critical polymer solutions
Authors: Caylor Craig L. , Law Bruce M.
Published: July 2002

The critical adsorption ellipsometric measurements of five solutions of polystyrene in cyclohexane for different polystyrene molecular weights collapse to a single universal curve when scaled as a function of nBξ/λ, where nB is the polymer solution refractive index, ξ=ξ0Nnt−ν is the correlation length, and λ is the wavelength of incident light in vacuum. From this universal feature we deduce the value of the polymerization critical exponent n=0.258±0.017. We consider both the volume fraction order parameter (φ) and a symmetrized order parameter (ψs) together with both the renormalization group (RG) and Monte Carlo (MC) simulation forms for the surface scaling function P+(x). The symmetrized order parameter gives significantly better agreement with experiment than the volume fraction order parameter. The combination of RG and ψs provides better agreement with experiment than does the combination of MC and ψs.
H/D fragment ratio in Lyman-α photolysis of CH2D2
Authors: Heck Albert J. R. , Zare Richard N. , Chandler David W.
Published: July 2002

The photofragment imaging technique is used to study the formation of H and D in the photolysis of CH2D2. From the images different reaction channels are distinguished that lead to hydrogen fragments. For the channel that leads to hydrogen atoms concomitant with methyl fragments a H/D ratio of 3.5±0.6 is found.
Incoherent four-wave-mixing on nile blue and cresyl violet in glass and polymer at 5 K: Single-site line shape analysis
Authors: Zhang Yiping , Hartmann S. R. , Moshary F.
Published: July 2002

Time-delayed-four-wave-mixing (TDFWM) experiments performed on nile blue and cresyl violet at 5 K using incoherent laser radiation yield a response which depends dramatically on excitation conditions. Fast (femtosecond) and slow (picosecond) processes, variable peak shifts, and type I and II quantum beats are all observed according to the laser frequency ωL and bandwidth ΔωL. We model the TDFWM response on the basis of a site-independent molecular response and an associated inhomogeneous distribution function. These functions are constructed by combining information from TDFWM and fluorescence-line-narrowing (FLN) experiments.
The structure and intermolecular dynamics of the nitrous oxide–ethylene complex: Experiment and ab initio theory
Authors: Bemish R. J. , Rhee W. M. , Pedersen L. G. , Miller R. E.
Published: July 2002

Optothermal infrared spectroscopy has been used to study the vibrational modes of N2O–C2H4 that correlate with ν9 of the ethylene monomer and ν1+ν3 in the N2O subunit. The results show that the N2O lies above the plane of the ethylene and in a rigid picture has its axis parallel to the C=C bond. However, both bands show tunneling splittings that indicate that the two molecules rotate with respect to one another about the axis through their centers-of-mass. Ab initio calculations are in agreement with this, indicating that this is the only feasible tunneling path. The tunneling splittings are used in conjunction with a single axis, two rotor tunneling model to estimate the height of the barrier to internal rotation.
Relativistic effects in the cationic platinum carbene PtCH+2
Authors: Heinemann Christoph , Schwarz Helmut , Koch Wolfram , Dyall Kenneth G.
Published: July 2002

Fully relativistic four-component Dirac–Fock Coulomb calculations in conjunction with a second-order perturbational estimate for the correlation energy have been performed in benchmark calculations on geometric and electronic structures as well as the binding energy of the cationic platinum carbene complex PtCH+2. The relativistic stabilization of this species amounts to as much as 50 kcal/mol and the combination of relativistic and correlation effects shorten the Pt–C bond length by nearly 1 bohr, changing the bond order from one to two. The relative importance of spin-free and spin-dependent relativistic effects on the geometry, the electronic structure, and the binding energy is evaluated by comparison to the Douglas–Kroll method. Relativistic effective core potentials are shown to describe the spin-free effects reliably. The best theoretical estimate for the bond dissociation energy underestimates the experimental value by 13% due to truncation errors in the one- and n-particle space treatments. The mixed Hartree–Fock/density-functional method Becke3-Lee-Yang-Parr performs surprisingly well with respect to the structure and binding energy of the target molecule.
A tight-binding model for calculations of structures and properties of graphitic nanotubes
Authors: Molina Jose Molina , Savinsky S. S. , Khokhriakov N. V.
Published: July 2002

A parameterized tight-binding model is applied to calculate the equilibrium structures, energies, band gaps and elastic properties of graphitic nanotubes with various helicities and radii. The structures, energies and band gaps for the tubes of large radii are in agreement with more accurate calculations. The elastic moduli depend only on the tube radius and follow the continuum model predictions.
Millimeter-wave spectra of CaSH and CaSD
Authors: Taleb-Bendiab Amine , Scappini Flavio , Amano Takayoshi , Watson James K. G.
Published: July 2002

The pure rotational spectra of CaSH and CaSD were observed using millimeter-wave absorption spectroscopy. The gas phase free radicals were produced in a high-temperature cell by the reaction of Ca vapor with H2S or D2S in the presence of an electrical discharge. The spectra were analyzed using an S-reduced Hamiltonian. The molecular constants including the rotational constants, centrifugal distortion constants, and spin-rotation constants were extracted from the spectra. The off-diagonal spin-rotation term, ‖εab+εba‖/2, is 3.38(12) MHz and 4.59(4) MHz for CaSH and CaSD, respectively. The moment of inertia data of CaSH and CaSD yielded the r0 structure. The three structural parameters are Ca–S=2.564(2) Å, S–H=1.357(17) Å, and ∠Ca–S–H=91.0(18)°.
Quantum calculations for line shapes in Raman spectra of molecular nitrogen
Authors: Green Sheldon , Huo Winifred M.
Published: July 2002

Using previously described close coupling (CC) and coupled states (CS) cross sections for N2–N2 collisions [J. Chem. Phys. 104, 7572 (1996)], we have calculated CARS linewidths at room temperature and below. Agreement with experimental values at room temperature is quite good but predictions become increasingly too large at lower temperatures, with errors reaching 10%–30% at 113 K. We believe these low temperature discrepancies reflect errors in the intermolecular potential used here. To obtain linewidths at higher temperatures we have used the energy corrected sudden (ECS) approximation, taking the fundamental cross sections, σ(0.0→J1,J2), from the CC–CS calculations extended to higher collision energies with additional CS and infinite order sudden (IOS) calculations: the ECS scaling distance, lc was chosen by fitting to the 300 K CC–CS results. In general, we find rather good agreement with experimental values to 1500 K, although it appears that smaller values of lc are more appropriate for higher temperatures and for higher rotational levels. This variability of lc is reasonable from physical arguments but somewhat diminishes the predictive utility of this approach. Agreement of these purely ab initio predictions with experimental data is nearly as good as that obtained from the best rate law model whose parameters were fitted to these data.
A numerical investigation of the liquid–vapor coexistence curve of silica
Authors: Guissani Yves , Guillot Bertrand
Published: July 2002

In using a modified version of the interionic potential of Tsuneyuki, Tsukada, Aoki, and Matsui [Phys. Rev. Lett. 61, 869 (1988)] for silica we have evaluated by molecular dynamics simulation the liquid–vapor coexistence curve of this model fluid. Although the critical point is located at a very high temperature (Tc=11 976 K, ρc=0.58 g/cm3, and Pc∼2 kbar) as expected for an ionic fluid, the overall shape of the coexistence curve is found to be very similar to that of a molecular fluid and especially close to that of water in the low temperature region (i.e., near and below the boiling point). A detailed comparison with available experimental data on silica melts and glasses is also presented. An analysis of the evolution of the silica structure with temperature shows that the tetrahedral SiO4 units subsist into the simulated melt up to approximately 8000 K and that the network formed by corner-sharing SiO4 tetrahedra is disrupted by a considerable extent only above this temperature. It is stressed that similar behavior has been recently reported in another network former liquid (i.e., water). Finally it is shown that the nature of the structural transformations exhibited by the simulated silica under very high pressure (up to the megabar range) may provide some clues to improve our knowledge about the constitution and the electrical conductivity of the earth’s mantle.
Numerical calculation of the rate of crystal nucleation in a Lennard-Jones system at moderate undercooling
Authors: Rein ten Wolde Pieter , Ruiz-Montero Maria J. , Frenkel Daan
Published: July 2002

We report a computer-simulation study of the rate of homogeneous crystal nucleation and the structure of crystal nuclei in a Lennard-Jones system at moderate undercooling. The height of the nucleation barrier has been determined using umbrella sampling, whereas the barrier crossing rate is calculated using molecular dynamics simulation. The simulations clearly show that the barrier crossing is a diffusive process. Nevertheless, the kinetic prefactor in the nucleation rate is found to be some two orders of magnitude larger than predicted by classical nucleation theory. The height of the barrier is in good agreement with the theoretical prediction. Although the Lennard-Jones system has a stable face-centered cubic (fcc) phase below the melting line, the precritical nuclei are found to be mainly body-centered cubic (bcc) ordered. As they grow to their critical size, they become more fcc ordered in the core. However, the critical and postcritical nuclei retain a high degree of bcc ordering in the interface. Furthermore it is found that in the interface the density falls off faster than the structural order parameter, which is in agreement with the predictions of density functional calculations. [P. Harrowell and D. W. Oxtoby, J. Chem. Phys. 80, 1639 (1984)].
Rotational spectrum, internal rotation barrier and ab initio calculations on 1-chloro-1-fluoroethane
Authors: Hinze R. , Lesarri A. , López J. C. , Alonso J. L. , Guarnieri A.
Published: July 2002

The microwave spectrum of 1-chloro-1-fluoroethane (CFC 151-a) has been studied in the frequency region 8–250 GHz using waveguide Fourier transform, Stark, and source modulation spectrometers. Accurate rotational, quartic, and sextic centrifugal distortion and quadrupole coupling constants have been obtained from a global fit for the ground, v17=1 (Cl–F skeletal bending mode), and v18=1 (CH3 torsional) vibrational states of the 35Cl isotopomer and for the ground state of the 37Cl isotopomer. The larger off-diagonal element of the χ tensor was also determined for the 35Cl isotopomer. Assignment of the v17=1 and v18=1 states was confirmed by the presence of small A–E internal rotation splittings in the v18=1 state, in agreement with ab initio calculations, but in contradiction with previous assignment of the microwave spectrum by Thomas et al. [J. Chem. Phys. 61, 5072 (1974)]. The barrier height for the internal rotation of the methyl group was determined to be 3814(11) cal/mol, and compared with the result of ab initio calculations made for 1-chloro-1-fluoroethane and other related chlorine or fluorine substituted ethanes.
Scanning tunneling microscopy of the effect of incident energy upon chemisorption sites for O2/Si(111)-7×7
Authors: Yan Chun , Jensen John A. , Kummel Andrew C.
Published: July 2002

Scanning tunneling microscopy and molecular beam techniques were used to investigate the chemisorption sites of O2 onto Si(111)-7×7 surfaces as a function of O2 incident energy (Ei). It is shown that there are two different O-containing sites being formed, i.e., bright and dark sites. The distribution of dark sites is not sensitive to Ei. However, the distribution of bright sites is a strong function of O2 incident energy. At Ei=0.02 eV, there is no preference between bright corner-adatom and center-adatom sites (about 1.1:1) while at Ei≳0.06 eV, the bright sites are preferentially located at corner-adatom sites over center-adatom sites by about 2:1. It is proposed that difference in the spatial distribution of adsorbates at different incident energies results from different chemisorption mechanisms: precursor-mediated (physisorption) chemisorption at low incident energy and direct activated chemisorption at high incident energies.
A study on ion–molecule reactions in the H+3 system with the trajectory-surface-hopping model
Authors: Ichihara Akira , Shirai Toshizo , Yokoyama Keiichi
Published: July 2002

Cross sections for ion production in the D++H2, D++D2, and H++D2 collisions have been calculated in the center-of-mass collision energy range of 2.5 to 8.0 eV by using trajectory-surface-hopping method on ab initio potential energy surfaces. For the production of H+ and HD+ ions in the D++H2 collisions the present results agree with experiments. For the production of D+2 ions in the D++D2 collisions the calculations show agreement with the experimental results at energies below 4.5 eV, but lead to an overestimate above 5.0 eV. For the production of D+ and HD+ ions in the H++D2 collisions, agreement between the calculations and experiments is good in the whole energy range, while a deviation comes out between both the results for the charge transfer H++D2→D+2+H above 5.0 eV. As far as the energy dependence of cross sections is concerned, the present calculations well reproduce all the experiments, owing to the use of the ab initio surfaces, instead of the diatomics-in-molecules surfaces.
Infrared spectra of the reaction products of laser ablated lead atoms and oxygen molecules in condensing argon and nitrogen
Authors: Chertihin George V. , Andrews Lester
Published: July 2002

Reactions of laser ablated lead atoms with oxygen molecules in condensing argon and nitrogen streams gave the PbO, OPbO, Pb(O2), OPb(O2), PbOPb, Pb2O2, PbPb(O2), and Pb4O4 molecules. The formation of lead monoxide and linear dioxide molecules is the main difference with thermal evaporation experiments. The linear OPbO isomer was identified from oxygen-18 and lead (natural abundance) isotopic substitution in the strong antisymmetric stretching absorption at 764.8 cm−1; a weak ν1+ν3 combination band was observed at 1423.4 cm−1. Two bands [ν1=728.7 cm−1 (Ar) and 730.6 cm−1 (N2) and ν2=437.3 cm−1 (Ar) and 449.6 cm−1 (N2)] are assigned to the cyclic isomer Pb(O2) based on oxygen isotopic substitution. The position of the O–O stretching fundamental shows that Pb(O2) is a peroxidelike molecule. The secondary reaction Pb(O2)+Pb gives a large yield of cyclic Pb2O2 and allows observation of very strong antisymmetric Pb–O stretching fundamentals and combination bands with the two symmetric stretching fundamentals. Density functional frequency calculations with effective core potentials support assignment of the observed infrared absorptions.
Collisional deactivation of K in the high-lying 2S and 2D states by He, Ne, and Ar
Authors: Chou Ching-Su , Lin King-Chuen
Published: July 2002

By measuring time-resolved fluorescence signals, we have obtained radiative lifetimes and total cross sections for collisional deactivation by He, Ne, and Ar of K n 2S and (n−2)2D states (n=7–11). The radiative lifetimes obtained agree with those previously reported. The cross sections, in the range of 15–180 Å2, generally increase with increasing the principal quantum number. Their magnitudes for the n 2S and (n−2)2D states are similar. The results are closely related to the geometric size of the excited K states, and weakly dependent on their orbital angular momentum. Our case differs from the K*–H2 collision, in which the electron harpoon mechanism should be involved. Our case also differs from the collision of Na* with rare gas; the latter easily induces l-mixing (l≥2) of the same n. We have also found that the relative efficiency for He and Ar is different in quenching of the low-lying and the high-lying K states. When n is small, the total cross section of deactivation by He collision is larger than that by Ar collision, but is reversed for the high-lying states. Detailed discussion on these phenomena is presented.
Wetting on an attractive compact spherical substrate
Authors: Hadjiagapiou Ioannis
Published: July 2002

The wetting phenomena on a compact spherical substrate of radius R, immersed in an one-component bulk vapor, are studied as functions of R and εW (a parameter characterizing the adsorbent) within the context of Sullivan’s model. The substrate acts as an external potential Vext(r) on fluid molecules; Vext(r) is found by integrating over the adsorbent’s volume and its strength depends strongly on R. The fluid molecules are of constant diameter d. The density functional theory is used to analyze the structure of the wall–vapor interface by properly generalizing Sullivan’s model. For any value of R, the substrate is either incompletely wet or nonwet; the density profiles, labeled by εW, display only one growth mode, that from thin to thick film as εW increases (in this case adsorption is always positive); for larger values of R, but small εW’s, there is, in addition, another transition from nonwetting to wetting, adsorption now changes from negative to positive as εW increases. In all cases, the thickness l of the wetting layer grown on the substrate is of the order of ln R. In conclusion, the effect of a spherical substrate on an one-component bulk vapor in comparison with that of a planar substrate is to reduce the three wetting classes of the planar adsorbent to two which are identified with class III, nonwetting, and class II, partial wetting with thin and thick films; class I, complete wetting, is absent.
On the vacuum ultraviolet radical photolysis CH2(1 3B1)+hν→CH(A 2Δ)+H(1 2S): A combined experimental and theoretical investigation
Authors: Kassner Ch. , Stuhl F. , Luo M. , Lehner M. , Fink R. , Jungen M.
Published: July 2002

We present photoexcitation experiments indicating that efficient CH(A) emission from irradiated ground state methylene can be observed only if energies of more than 9.5 eV have been deposited in the CH2(1 3B1) radical although according to energy considerations and adiabatic potentials 7.3 eV should be sufficient to generate CH(A). Theoretical studies on the photodissociation process using potential energy surfaces obtained with highly correlated quantum chemical methods and surface hopping techniques have been applied to understand the experimental observations.
Experimental investigation of binary nucleation rates of water–n-propanol and water–n-butanol vapors by means of a pex-tube
Authors: Rodemann T. , Peters F.
Published: July 2002

Measurements of homogeneous nucleation rates of two binary vapor mixtures of water and alcohol carried in nitrogen have been performed with a new instrument for nucleation and droplet growth studies, the piston expansion tube (pex-tube). The pex-tube subjects the vapors to short supersaturation pulses generating a limited number of nuclei which grow into monodisperse droplets. The droplets are detected in number and size by Mie light scattering. The results provide nucleation rates between 105 and 109 cm−3 s−1 at constant temperature as a function of the vapor phase activities a1 and a2 of water and alcohol, respectively. Comparison with published experimental data exhibits good agreement.
A determination of the phase diagram of relaxed Langmuir monolayers of Behenic acid
Authors: Bommarito G. M. , Foster W. J. , Pershan P. S. , Schlossman M. L.
Published: July 2002

Grazing incidence x-ray scattering (GIXS) and Brewster angle microscopy (BAM) are used to determine the π–T phase diagram of Behenic acid monolayers supported on the surface of water (pH=2.0) over the temperature range of 3 °C to 20.6 °C. The phase diagram is constructed from measurements taken during isothermal compressions in which the surface pressure relaxed to a stable value at each surface density, and during temperature scans at fixed average surface density. The phase diagram is different than those previously reported for Behenic acid primarily because of the surface pressure relaxation. For temperatures less than 12 °C the phase diagram exhibits similar phases and topology as the published diagrams, although the location of the phases in the π–T plane is different. Temperature scans combined with the isotherms, and the Clausius–Clapeyron relation are used to determine three coexistence lines that meet in a triple point. Changes in entropy across the phase boundaries are determined. Near room temperature (20.6 °C) only one phase is measured over the range of surface pressure from 0 dynes/cm to the collapse pressure in contrast to reported measurements on monolayers out of equilibrium (i.e., when the surface pressure is not allowed to relax) that exhibit several phases near room temperature at pressures higher than the collapse pressure. Discrepancies are observed between the average area per molecule (AT) and the area per molecule determined from measurements of the unit cell (AX) in the close packed regions of the phase diagram. It is conjectured that the π–AX plane is a better representation of the ordered equilibrium monolayer phases than the π–AT plane. Isotherms plotted in the π–AX plane are used to determine the compressibility of the ordered phases and the nature of the phase transitions.
State-to-state differential cross sections for rotationally inelastic collisions of NO(2Π1/2,j=0.5) with Ar at kinetic energies between 117 cm−1 and 1694 cm−1
Authors: Jons Steven D. , Shirley Jeffrey E. , Vonk Matthew T. , Giese Clayton F. , Gentry W. Ronald
Published: July 2002

State-to-state differential cross sections have been measured for scattering of NO(2Π1/2,j=0.5) by Ar at kinetic energies of 117, 149, 442, and 1694 cm−1. The differential cross sections at each collision energy are presented as a function of final state (Ω′,j′) at constant center-of-mass scattering angle. Center-of-mass angular distributions are also given for final rotational states (2Π1/2, j′=1.5, 2.5, 8.5, 12.5, and 14.5) at a kinetic energy of 442 cm−1, and for j′=18.5 at a kinetic energy of 1694 cm−1. Rotational rainbow structure is seen in both types of data. The results are generally in good agreement with quantum scattering calculations carried out by Alexander [J. Chem. Phys. 99, 7725 (1993)] using newly calculated ab initio potential energy surfaces, and thus may serve as a new benchmark for the microscopic dynamics of molecular energy transfer in open-shell molecules.
Independent center, independent electron approximation for dynamics of molecules and clusters
Authors: McGuire J. H. , Straton Jack C. , Wang J. , Wang Y. D. , Weaver O. L. , Corchs S. E. , Rivarola R. D.
Published: July 2002

A formalism is developed for evaluating probabilities and cross sections for multiple-electron transitions in scattering of molecules and clusters by charged collision partners. First, the molecule is divided into subclusters each made up of identical centers (atoms). Within each subcluster coherent scattering from identical centers may lead to observable phase terms and a geometrical structure factor. Then, using a mean field approximation to describe the interactions between centers we obtain AI∼∑k∏keiδkIAIk. Second, the independent electron approximation for each center may be obtained by neglecting the correlation between electrons in each center. The probability amplitude for each center is then a product of single electron transition probability amplitudes, aIki, i.e. AIk≊∏iaiki. Finally, the independent subcluster approximation is introduced by neglecting the interactions between different subclusters in the molecule or cluster. The total probability amplitude then reduces to a simple product of amplitudes for each subcluster, A≊∏IAI. Limitations of this simple approximation are discussed.
Phase behaviour of a symmetric binary mixture of hard rods
Authors: Sear Richard P. , Mulder Bela M.
Published: July 2002

The phase behaviour of long hard rods is independent of their length to breadth ratio in the limit that this ratio is very large. We form a binary mixture of rods with different length to breadth ratios but the same second virial coefficient. As the second virial coefficient is the same for both components, their phase behaviour in the pure state is identical. However, the difference in their shapes—one is longer and thinner than the other—results in an increased interaction between a pair of rods of different components. As the difference in shape of the two components is increased, first isotropic–isotropic coexistence is observed (with a critical point), then in addition nematic–nematic coexistence. At first there is a nematic–nematic critical point but this point reaches the isotropic–nematic transition, creating a four phase region. Gibbs’ phase rule, as usually stated, permits a maximum of three phases to coexist simultaneously in a binary athermal mixture. Here, the symmetry between the two components allows four to coexist.
Absence of high-density consolute point in nematic hard rod mixtures
Authors: van Roij René , Mulder Bela
Published: July 2002

In this paper we resolve a controversy related to the demixing transition in the nematic phase of binary hard rod mixtures. Previous analyses do not agree on the existence of a consolute point closing the nematic-nematic coexistence region. We definitely rule out the existence of this critical point, at least in the Onsager theory. Our analysis requires the determination of self-consistent orientation distribution functions, for which we develop an efficient numerical scheme. This scheme is based on the scaling properties of the high density limit of the stationarity condition on the free energy functional. We illustrate this method in some detail for the monodisperse thin hard rod system.
Long-time behavior of the diffusion-controlled A+B→0 reaction with hopping energy relaxation
Authors: Baranovskii S. D. , Hensel F. , Golub J. E. , Thomas P.
Published: July 2002

The effect of hopping energy relaxation on the diffusion-limited reaction A+B→0 is considered. The main effect of the relaxation is to make the diffusion constant a decreasing function of time. We find that fluctuation effects radically alter the decay law predicted from the rate-equation approach, as is already known to be the case for diffusion in thermal equilibrium. New long-time behavior is predicted for the concentration of reacting particles as compared to the thermal equilibrium case.
Single particle and collective orientational relaxation in an anisotropic liquid near the isotropic–nematic transition
Authors: Perera A. , Ravichandran S. , Moreau M. , Bagchi B.
Published: July 2002

We report molecular dynamics simulation results for the single particle and collective orientational correlations for Gay–Berne fluid of anisotropy parameters κ=3 and κ=5. The following significant results are reported. (i) The decay of collective orientational relaxation of rank two is found to slow down appreciably while approaching the isotropic–nematic (I–N) transition. It eventually becomes even slower than the single particle orientational relaxation of rank one. Surprisingly, even the latter shows a marked slowing down near the I–N transition. (ii) Analysis of the long-time behavior shows a breakdown of the rank dependence predicted by the Debye law in the same region. (iii) An analysis of the memory functions of the orientational time-correlation functions shows that for the collective relaxation surprisingly approaches a Markovian behavior near the I–N transition.
Molecular beam study of the collisions of state-monitored, metastable noble gas atoms with O2(X 3Σg−)
Authors: Rickey Dawn , Krenos John
Published: July 2002

We describe a new molecular beam-luminescence method for measuring state-resolved cross sections for the quenching of metastable noble gas atoms, and report values for “dark” collisions of Ng*(3P2,3P0) with O2(X 3Σg−), where Ng=Ar, Kr, and Xe. Cross sections for quenching σQ and, in some cases, cross sections for excited products σ* are also given for a number of state-specific, luminescent monitor reactions. The elastic reaction of Ng* with He or Ne is employed to correct the total disappearance cross section QT for viewing losses caused by nonquenching processes. The velocity-averaged, quenching cross section QQ is obtained by subtracting the nonquenching cross section QN from QT. Values of QQ measured at average relative velocity ḡ (average relative kinetic energy Ē) are deconvoluted to yield σQ(ḡ). For Ar* collisions with O2, we find σQ[3P2] values of 35.6±1.8 Å2 for ḡ (Ē) between 690 and 2000 m/s (50 and 350 meV) that gradually decrease above 2000 m/s, and σQ[3P0] values of 46±4 Å2 between 690 and 830 m/s (50 and 70 meV). For Kr* collisions, we report σQ[3P2] values of 38.1±2.5 Å2 between 575 and 810 m/s (46 and 87 meV) with no apparent velocity dependence, and a σQ[3P0] value of 56±7 Å2 at 576 m/s (46 meV). For Xe* collisions, we find σQ[3P2] values of 48±3 Å2 at 535 m/s (44 meV) and 38±2 Å2 at 697 m/s (73 meV), and a σQ[3P0] value of ∼125 Å2 at 535 m/s (44 meV). Comparisons with σQ values obtained with other techniques that do not require a viewing loss correction are excellent. We also use the Ionic-Intermediate-Curve-Crossing Model (IICCM) to calculate cross sections for the Ar*(3P2)+O2→Ar+O*(1D)+O(3P) reaction. In our application of the model, the product state dissociative continuum is coupled to Ar+O2− through the predissociating O2*(E 3Σu−) state that is valence Rydberg in character. Values of σQ derived from the model are in good agreement with our experiment.
Resonance Raman study of solvent dynamics in electron transfer. I. Betaine-30 in CH3CN and CD3CN
Authors: Zong Yaping , McHale Jeanne L.
Published: July 2002

Raman excitation profiles for nine vibrational modes of the solvatochromic dye betaine-30 have been measured in CH3CN and CD3CN solution at wavelengths that span the S0→S1 charge transfer transition. Though the absorption spectra of the dye are the same in the protonated and deuterated forms of the solvent, Raman cross sections for all modes were found to be lower in CD3CN solution than in CH3CN. Wave packet theory has been applied to model the absorption and Raman profiles, using a solvent dephasing model which accounts for bimodal solvent dynamics. The solvent isotope effect on the Raman intensities is interpreted in terms of different amplitudes of the fast solvent response in the protonated and deuterated solvents, and possible coupling of low-frequency solute and solvent modes. The solvent reorganization energy greatly exceeds that due to internal vibrational modes of betaine-30, and most of the internal reorganization is due to the torsional modes of the solute.
Optical properties of one-dimensional exciton systems: Beyond the Heitler-London approximation
Authors: Bakalis Lisette D. , Knoester Jasper
Published: July 2002

We study the properties of one-dimensional exciton systems in which the commonly made Heitler-London approximation (HLA) is relaxed. The nonresonant interaction terms which then exist, mix the multi-exciton bands of the HLA. Our approach is based on the exact diagonalization of the Hamiltonian, which is possible using the Jordan-Wigner and Bogoliubov transformations. Exact expressions for transition dipoles between multi-particle states are given. Results of our exact theory for the ground state and one-particle energies, the superradiant enhancement, the pump-probe spectrum, and the linear absorption to multi-particle states are compared quantitatively to the HLA, to the Bose approximation (where the excitons are treated as bosons), and to perturbation theory. In this comparative study, we use parameter values that are relevant to much studied quasi-one-dimensional J aggregates, such as PIC and TDBC. We find that for these systems the strongest effects of the HLA occur in the oscillator strengths of the various optical transitions. In particular, the exciton delocalization length derived from the experimentally observed superradiant enhancement is overestimated by roughly 10% due to the HLA. Also, the transition between the ground state and three-particle states, which is strictly forbidden in the HLA, does obtain a finite oscillator strength due to the non-resonant interactions.
Determination of the structure of HBr DBr
Authors: Chen Wei , Walker A. R. Hight , Novick Stewart E. , Tao Fu-Ming
Published: July 2002

We have investigated the structure and dynamics of the weakly bound complex of the four bromine isotopomers of HBr DBr, using pulsed-jet Fourier transform microwave spectroscopy. In addition, the equilibrium geometry, intermolecular energy, and vibrational force constants for the dimer have been calculated at the MP2 level using a large basis set containing bond functions. In the hydrogen halide dimers, allowed transitions are across the inversion doublet caused by the geared interchange of the donor and acceptor of the hydrogen bond. While in the classic study of (HF)21 it was observed that this splitting results in an inversion frequency in the microwave region of the spectrum, in (HCl)2 and (HBr)2 that the lower inversion barrier, results in an inversion frequency in the infrared. In order to investigate the hydrogen bromide dimer with the high precision allowed by an FT microwave experiment, we have “quenched” the inversion motion by substituting deuterium for one of the hydrogens. The Br–Br distance in the complex is 4.136 Å. Using measurements of the nuclear quadrupole coupling constants of the bromine nuclei, the wide amplitude bending motions of the hydrogen-bonded deuterium have been determined as have the wide amplitude bending angle of the nonbonding hydrogen atom. The Br–Br–D angle undergoes excursions of 28° about zero, and the H–Br–Br angle oscillates 25° about its 90° equilibrium angle.
First passage times, correlation functions, and reaction rates
Authors: Bicout D. J. , Szabo Attila
Published: July 2002

The mean lifetime of a particle in the presence of an absorbing boundary is the mean first passage time to the boundary. For diffusive dynamics on a one dimensional potential, we establish an exact relation between the mean first passage time to x=a, averaged over a (pseudo) equilibrium distribution in the region a⩽x<∞, and the time integral of certain equilibrium correlation functions determined by dynamics on the entire (−∞<x<∞) surface in the absence of an absorbing boundary at x=a. As an application, we show that the sum of the forward and reverse rate constants for diffusive barrier crossing obtained using mean first passage times and obtained from the time integral of the normalized number correlation function, are identical for an arbitrary choice of the dividing surface.
Electron tunneling through water layers: Effect of layer structure and thickness
Authors: Benjamin Ilan , Evans Deborah , Nitzan Abraham
Published: July 2002

The effect of thickness and molecular structure on the probability of electron tunneling through water layers is investigated using a recently developed method. Water configurations of 1–4 layers are prepared between two parallel slabs of the Pt(100) surface, using equilibrium molecular dynamics and the polarizable simple point charge water model. Electron tunneling probabilities through the different water layers are computed as functions of energy using the absorbing boundary conditions Green function method and employing either an effective two-body water–electron interaction or a many-body polarizable water–electron potential. As long as the electron incident energy is below the barrier and far from a resonance state, the tunneling probabilities can be reasonably fitted to a one-dimensional rectangular-barrier model. However, near and over-barrier transmission probabilities cannot be reasonably described using a one-dimensional model, and the three-dimensional discrete structure of the water plays an important role. In all systems, the many-body electronic polarizability of the water significantly affects the transmission probability. The role played by the first adsorbed water layer is also discussed.
A study of the structure and bonding of small aluminum oxide clusters by photoelectron spectroscopy: AlxOy− (x=1–2, y=1–5)
Authors: Desai Sunil R. , Wu Hongbin , Rohlfing Celeste M. , Wang Lai-Sheng
Published: July 2002

The structure and bonding of aluminum oxide clusters, AlxOy (x=1–2, y=1–5), are studied with anion photoelectron spectroscopy (PES) and are compared with preliminary ab initio calculations. The spectra were obtained at four detachment photon energies: 2.33, 3.49, 4.66, and 6.42 eV. The 6.42 eV spectrum for AlO− reveals the X 2Σ+ ground state and two excited states of AlO. The 6.42 eV spectrum for AlO2− also shows three states for AlO2: X 2Πg ground state and the A 2Πu and BΣg+2 excited states. The spectra for Al2Oy− clusters show vibrationally resolved ground states which come from Al sp-type orbitals and also high binding energy excited states, which are mainly of oxygen 2p character. Al2O2, which has a D2h rhombus structure, has an electron affinity (EA) of 1.88 eV and its singlet–triplet excitation energy is measured to be 0.49 eV. Much higher EAs are measured for the larger Al2Oy clusters. The PES spectra of Al2O3−, Al2O4−, and Al2O5− show very similar electronic and vibrational structure. Furthermore, the ground state vibrational frequencies of these three molecules are also similar. These observations lead us to suggest that these molecules all have a rhombuslike structure, similar to Al2O2, with the oxygen atoms sequentially attaching to the terminal aluminum atoms. The spectra are consistent with an ionic bonding view of these clusters and the vibrational frequencies are in good agreement with the theoretical results. Significant information about the structure and bonding of these small aluminum oxide clusters is obtained and discussed.
Spectroscopic studies of the B̃ 2A′′-X̃ 2A′′ system of the jet-cooled vinoxy radical
Authors: Brock L. R. , Rohlfing Eric A.
Published: July 2002

We apply several techniques to the study of the B̃ 2A′′-X̃ 2A′′ band system of the jet-cooled vinoxy radical, CH2CHO. Vibronically resolved excitation spectra are obtained using both laser-induced fluorescence (LIF) and a two-color resonant four-wave mixing (TC-RFWM) scheme that provides the nonlinear equivalent of hole-burning spectra. Rotationally resolved LIF spectra recorded at low temperatures (⩽3 K) provide rotational constants for 9 B̃-state levels. We also measure the fluorescence lifetimes of 19 B̃-state levels and obtain high-quality dispersed fluorescence (DF) spectra from seven of the most strongly fluorescing levels in the B̃ state. The excitation and DF spectra reveal far more vibrational levels in the two electronic states than have been previously observed. In total, we provide assignments for 54 levels observed in the first 3650 cm−1 of the B̃ state and for 57 levels in the first 3100 cm−1 of the X̃ state. These assignments include the identification of the a′ fundamentals for ν4 through ν9 and all three a′′ overtones, 2ν10 through 2ν12, in both states. The differences between the TC-RFWM and LIF spectra and the measured lifetimes indicate a dramatic increase in the predissociation rate of the B̃ state beginning at 1190 cm−1 above the origin. The predissociation rate is markedly mode-specific and is enhanced by out-of-plane excitation, possibly due to vibronic coupling with either the Ã 2A′ or C̃ 2A′ electronic states. The congestion and complexity of the DF spectra at high energies provides direct evidence of extensive intramolecular vibrational redistribution on the ground-state potential surface.
A 250 GHz ESR study of o-terphenyl: Dynamic cage effects above Tc
Authors: Earle Keith A. , Moscicki Jozef K. , Polimeno Antonino , Freed Jack H.
Published: July 2002

Three nitroxide spin probes of different sizes and geometrical shape were used in a 250 GHz ESR study of the probe rotational dynamics in the fragile glass former ortho-terphenyl (OTP) over a wide temperature range from 380 to 180 K. Comparative studies at 9.5 GHz have also been performed. Perdeuterated 2,2′,6,6′-tetramethyl-4-methyl aminopiperidinyl-N-oxide (MOTA), and 3,3-dimethyloxazolidinyl-N-oxy-2′,3-5α-cholestane (CSL) are, respectively, comparable in size to and larger than the OTP host molecule, whereas Perdeuterated 2,2′,6,6′-tetramethyl-4-piperidine-N-oxide (PDT) is substantially smaller. The sensitivity of 250 GHz ESR to the details of the rotational tumbling for T≳Tc (where Tc is the crossover temperature) was exploited to show that the relaxation is fit by a model that is characteristic of a homogeneous liquid. A nonlinear least-squares analysis shows that below the melting point, Tm, CSL, and MOTA dynamics are well-described by a model of dynamic cage relaxation proposed by Polimeno and Freed wherein the probe relaxation is significantly influenced by a fluctuating potential well created by the neighboring OTP molecules. A model of simple Brownian reorientation does not fit the experimental spectra of CSL or MOTA as well as the dynamic cage model below Tm. Spectra of PDT do not show any significant non-Brownian dynamics for this probe. It was found that the characteristic rates of the cage model, viz., the reorientation of the probe and the cage relaxation, were describable by activated processes; however, the “average” rotational diffusion rates (defined in the usual manner as the time integral of the correlation function) derived from the dynamic cage parameters follow the Stokes–Einstein–Debye (SED) relation rather well, in agreement with previous studies by other physical techniques. It is then shown that the usual stretched exponential fit to the motional correlation function, interpreted in terms of an inhomogeneous distribution of simple reorientational rates, is clearly inconsistent with the observed ESR spectrum. The absence of a significant cage potential above Tm is discussed in terms of a model of frustration limited domain sizes proposed by Kivelson and co-workers. Evidence for the existence of substantial voids in OTP below Tm, especially from the spectra of the small PDT probe, is discussed in terms of the structure and packing of the OTP solvent.
Ligand and metal binding energies in platinum carbonyl cluster anions: Collision-induced dissociation of Ptm− and Ptm(CO)n−
Authors: Grushow Alexander , Ervin Kent M.
Published: July 2002

The collision induced dissociation (CID) of small gas phase platinum and platinum carbonyl cluster anions, Ptm− and Ptm(CO)n− (m=3−6), has been studied using a guided ion beam tandem mass spectrometer. The threshold energies obtained from analysis of the CID cross sections for carbonyl and metal atom loss are equated with binding energies. The binding energies of CO to the platinum cluster anions are indicative of two different binding sites, terminal (about 80–110 kJ/mol) and bridged (220–250 kJ/mol). Energies required to removed platinum atoms from the bare cluster anions are 410–470 kJ/mol, a substantial fraction of the bulk enthalpy of vaporization, 570 kJ/mol. Using threshold energies for both primary and secondary CID products in thermodynamic cycles, we have determined thermochemical stabilities of additional neutral and anionic platinum carbonyl species.
First principles simulations of fluid water: The radial distribution functions
Authors: Ortega José , Lewis James P. , Sankey Otto F.
Published: July 2002

We apply a recently developed first principles but simplified molecular dynamics method to the simulation of water at different conditions. The computational simplicity of this method allows its application to systems containing a significant number of molecules, yet still taking explicitly into account the quantum electronic structure of the system. In the present work we simulate a system of 216 H2O molecules with periodic boundary conditions at two different densities (ρ=1.0 g/cm3 and ρ=0.72 g/cm3 and temperatures ranging from ∼300 K to ∼580 K. The effect of density and temperature on the structure of water is analyzed by means of the partial radial distribution functions gOO, gOH and gHH. We find an important reduction of the hydrogen-bond peak for water at the supercritical conditions ρ= 0.72 g/cm3, T=580 K, in good agreement with recent experimental results.
Fluctuation theory of diblock copolymer/homopolymer blends and its effects on the Lifshitz point
Authors: Kielhorn L. , Muthukumar M.
Published: July 2002

We have considered the stability of lamellar, cylindrical, and spherical morphologies of a ternary melt system of A homopolymers, B homopolymers, and AB diblock copolymers in the weak segregation limit. We have extended the mean field analysis by including composition fluctuations in a one-loop self-consistent approximation. We find rich binary and ternary phase diagrams with multiphase equilibria which are highly sensitive to parameter variations. The order–disorder transition points (χN)ODT are shifted upward relative to the mean field theory (MFT) transition points and all second order MFT microphase separation transitions are changed to first order. Composition fluctuations modify the MFT microphase separation and macrophase separations differently. As a consequence, the MFT Lifshitz critical point is destroyed. This implies that observation of the scattering function by itelf does not suffice to infer the existence of Lifshitz critical points. We demonstrate this in a quantitative comparison between our results and experiments.
Fragmentation energetics and dynamics of fluorobenzene⋅Arn (n=1–3) clusters studied by mass analyzed threshold ionization spectroscopy
Authors: Lembach Gerhard , Brutschy Bernhard
Published: July 2002

The dissociation of fluorobenzene⋅Arn (n=1–3) complexes was investigated using mass analyzed threshold ionization spectroscopy. All ion state spectra of the cationic complexes exhibit low-frequency vibrational progressions of van der Waals bending modes, which indicate a significant structural change of the complexes upon ionization. Upper limits for the dissociation thresholds in the cationic state could be determined for all complexes. In the case of fluorobenzene⋅Ar2 and fluorobenzene⋅Ar3, two fragmentation thresholds could be observed.