bluuebunny/crossref_metadata_2025 huggingface dataset:
Statistical effects in the skeletal inversion of bicyclo(2.1.0) pentane
Authors: Sorescu Dan C. , Thompson Donald L. , Raff Lionel M.
Published: July 2002

A semiempirical potential-energy surface for bicyclo(2.1.0) pentane which includes bond stretching, bending, and torsional terms is reported. The bond dissociation energies have been estimated using the available thermochemical data and results of ab initio molecular orbital calculations performed at the fourth order Mo/ller–Plesset (MP4) perturbation theory level using a 6-31G** basis set. The predicted equilibrium geometry of bicyclo(2.1.0) pentane and of the 1,3-cyclopentanediyl radical, the barrier for the ring inversion, and the fundamental frequencies of bicyclo(2.1.0) pentane are in fair-to-good agreement with the measured and ab initio calculated values. Using a projection method of the instantaneous Cartesian velocities onto the normal mode vectors and classical trajectory calculations, the skeletal inversion and the intramolecular energy flow in bicyclo(2.1.0) pentane are studied for different types of excitation. For random energization of the vibrational modes, the results of trajectory calculations agree with the predictions of statistical unimolecular theory. The same statistical behavior is supported by the results of power spectra calculated at different energization levels. The significant broadening and overlapping of the spectral bands, together with the disappearance of characteristic spectral features in the power spectra of the flap angle, indicate high intramolecular vibrational redistribution rates and global statistical behavior. The total intramolecular vibrational relaxation rates for the energy flow from the flap mode have been extracted from the time dependence of the average total normal-mode energy in this mode. For initial excitation of the flap mode in the range 30–60 kcal/mol, the calculated total intramolecular vibrational relaxation rates are found to be significantly larger than the microcanonical ring inversion rates. This result further supports the statistical character of the ring inversion in bicyclo(2.1.0) pentane.
Photoinduced intermolecular electron transfer reaction between N,N-dimethylaniline and anthracene in acetonitrile solution: A theoretical study
Authors: Ando Koji
Published: July 2002

The photoinduced intermolecular electron transfer (ET) reaction between N,N-dimethylaniline and excited state anthracene in acetonitrile solution is studied theoretically. A solvation coordinate s which represents stochastic one-dimensional dynamics of the solution phase reaction is defined and a Hamiltonian in terms of s and perpendicular bath modes is derived from the spin-boson Hamiltonian. This has an advantage that the dynamics of the transferring electron is influenced by the bath only through coupling with the coordinate s. Intra- and intermolecular potentials are constructed by using ab initio molecular orbital methods, and a series of molecular dynamics simulation analysis is performed. Mean force potentials as a function of the donor–acceptor distance R are computed and the bimolecular encounter dynamics is investigated. Diabatic free energy curves for the coordinate s are computed and shown to be well approximated by parabolas, indicating that the dielectric saturation effect is negligible. The dependence of the free energy relationships on R is examined. It is shown that the present system corresponds to the increasing region of the rate constant, in contrast with the conventional picture. The electronic coupling of the ET is evaluated by the method of corresponding orbitals. The R dependence of the ET rate is evaluated and the reaction adiabaticity and mechanism are discussed. Dynamical solvent effects are taken into consideration in terms of the generalized Langevin equation formalism.
Zero-kinetic-energy photoelectron spectroscopy of the hydrogen-bonded phenol-water complex
Authors: Dopfer Otto , Reiser Georg , Müller-Dethlefs Klaus , Schlag Edward W. , Colson Steven D.
Published: July 2002

Two-photon, two-color (1+1′) zero-kinetic-energy (ZEKE) photoelectron spectra are presented for the 1:1 phenol-water complex, a prototype system for hydrogen bonding between an aromatic molecule and a simple solvent. ZEKE spectra via different (intermolecular) vibrational intermediate S1 levels of the fully protonated complex (C6H5OH–H2O, h3) as well as the ZEKE spectrum via the vibrationless S1 state of the threefold deuterated complex (C6H5OD–D2O, d3) have been recorded. The spectra are rich in structure, which is mainly attributable to intermolecular vibrations of the ionic complex. Progressions of the intermolecular stretch vibration (240 cm−1) in combination with different intermolecular and intramolecular vibrational levels are the dominant feature of all ZEKE spectra obtained and indicate a large change in the complex geometry along the hydrogen-bond coordinate on ionization. Comparison between the spectrum of the d3 complex and the spectra via different intermediate intermolecular levels of the h3 complex has allowed a more detailed analysis of the intermolecular features compared to previously reported results. Finally, the vibrational assignments obtained are compared with ab initio results for the phenol-water cation reported in the following paper in this issue.
Diffusion of hydrogen atoms on a Si(111)-(7×7) reconstructed surface: Monte Carlo variational phase-space theory
Authors: Sorescu Dan C. , Thompson Donald L. , Raff Lionel M.
Published: July 2002

The diffusion of hydrogen atoms on a reconstructed Si(111)-(7×7) surface has been investigated using variational phase-space theory methods. The dimer–adatom-stacking (DAS) fault model of the reconstructed Si(111)-(7×7) surface proposed by Takayanagi et al. is employed to describe a four-layer lattice structure containing 292 atoms. The lattice potential is that developed by Bolding and Andersen; the gas–lattice interaction potential is described by a sum of Morse functions and bending terms between the hydrogen adatom and the Si atoms in the first and second layers. Canonical Markov walks with importance sampling are used to evaluate the flux across a set of dividing surfaces separating different adsorption sites. The minimum jump frequencies are then used as input to a set of coupled phenomenological kinetics equations that describe the diffusion rates of adatoms between adjacent adsorption sites. The diffusion coefficients D at different temperatures 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. The results at 300, 500, and 800 K yield D=0.023 exp(−1.54 eV/kT) cm2/s. The calculated activation energy of 1.54 eV is in excellent agreement with the experimental results obtained by Reider et al. using an optical second-harmonic diffraction technique. The coordinates corresponding to the minimum energy diffusion path suggest that hydrogen-atom diffusion between atop sites occurs along paths that involve lattice penetration. Calculated upper limits for the tunneling rates at 300, 500, and 800 K show that tunneling processes make only a small contribution to the total diffusion rate.
Characterization of a molecular beam containing metastable nitrogen and its use in scattering experiments with xenon
Authors: Cappelletti David , Liuti Giorgio , Luzzatti Emilio , Pirani Fernando
Published: July 2002

A thermal molecular beam containing metastable nitrogen, N2*, produced in a microwave discharge in He–N2 mixtures, is characterized by magnetic analysis and, after velocity selection, is used for scattering experiments with Xe. The magnetic analysis reveals that the excited N2 molecules are essentially in the A 3Σ+u metastable state but also evinces the presence of a small concentration of A′ 5Σ+g molecules, increasing with beam velocity. This allows us to estimate the mean collision free lifetime of the 5Σ+g metastable molecules by an experiment completely different from those usually performed to this purpose. Absolute total cross sections are measured both for the metastable and the ground state N2 by passing the molecular beam through a cooled scattering chamber containing Xe. From the analysis of the scattering data, the characterization of the intermolecular potential for both systems is obtained and from the potential parameters the average molecular polarizability of metastable N2 is obtained.
The reaction of NH2 (X 2B1) with O (X 3P): A theoretical study employing Gaussian 2 theory
Authors: Yang D. L. , Koszykowski M. L. , Durant J. L.
Published: July 2002

The ground 2A″ and excited 4A′ surfaces for the reaction of NH2 (X 2B1) with O (X 3P), have been characterized by calculating energies, geometries, and frequencies for all important stationary points connecting reactants and products. The Gaussian 2 methodology was used for all calculations with further refinement for transition state properties made by calculating energies using QCISD/6–311G(d,p) geometries and frequencies. The results predict that, on the 2A″ surface, an H2NO intermediate is formed which is 87.6 kcal/mol below the separated NH2+O reactants. This intermediate may either fragment to form H+HNO or H2+NO, or undergo a 1,2 hydrogen shift to form trans-HNOH. This second intermediate may dissociate to either NH+OH or H+HNO, or isomerize to cis-HNOH which, in turn, may dissociate into the same products. The abstraction reaction NH2+O→NH+OH was found to have a transition state 6.7 kcal/mol above the energy of the separated reactants.
Excess electron surface states on helium clusters
Authors: Rosenblit M. , Jortner Joshua
Published: July 2002

In this paper we report on quantum mechanical calculations for the ground and the excited electronic surface states of an excess electron on (He)N clusters (N=3.5×105–6×1023), exploring the cluster size dependence of the excess electron localization and the bridging between the properties of the electron on cluster microsurfaces and on flat macrosurfaces. Representing the e-(He)N potential by a short-range repulsive model potential or by a pseudopotential, together with a long-range attractive dielectric image potential, we have shown that the electronic energies are relatively insensitive (i.e., within 20% for N=106 and within 6% for N≥107) to the details of the short-range repulsive interactions. The model potential results in a ‘‘critical’’ radius R(1,0)c=148 Å with a number of constituents N(1,0)c=3.0×105 for electron localization in the ground n=1, l=0 electronic state, while with a further increase of the cluster radius R above R(1,0)c, higher n,l states become localized at cluster radii R(n,l)c, with Rc(n,l′) ≳ Rc(n,l) for l′≳l and Rc(n′,l′) ≳ Rc(n,l) for n′≳n and for all values of l and l′.

The energies En,l of the n,l electronic states above the localization threshold are characterized by the scaling relations En,l(R)∝(R−R(n,l)c)η(l) with η(l)=2 for l=0 and η(l)=1 for l≠0. The charge distribution in this size domain for l=0 is characterized by the moments 〈rJ〉∝(R−R(n,0)c)−J, while for l=1, 〈r〉∝(R−R(n,1)c)−1/2. The ‘‘critical’’ cluster radii for localization obey algebraic relations, which result in the cluster size dependence of the number of bound electronic states. Cluster surface size equations were obtained for R→∞ providing a quantitative description of the convergence of the electronic energies to those for a flat surface. Information on electronic spectroscopy was inferred from the cluster size dependence of the transition energies and oscillator strengths for the 1,0(1s)→n,1(np) electronic excitations. The 1s→1p electronic transition is characterized by a transition energy and an oscillator strength which both decrease as R−2, manifesting the onset of l degeneracy for macrosurfaces. Finally, electric field effects provide information on field-induced ionization and huge polarizabilities αc≂ (109–1011)αH (where αH is the polarizability of the hydrogen atom) of these giant excess electron states.
Observation of unprotonated ammonia cluster ions generated via multiphoton ionization mass spectrometry
Authors: Xia Ping , Lyktey Michael Y. M. , Garvey James F.
Published: July 2002

We have studied ammonia cluster ions utilizing multiphoton ionization and a reflection time-of-flight mass spectrometer. The observed intensity distributions of {NH3}+n and {NH3}+n/{NH3}n−1H+ indicate that the formation of protonated ammonia cluster ions is due to a fast intracluster proton transfer reaction following ionization, similar to that which occurs in single photon ionization.
Evidence for predissociation of N2 a 1Πg(v≥7) by direct coupling to the A′ 5Σ+g state
Authors: van der Kamp A. Barbara , Siebbeles Laurens D. A. , van der Zande Wim J. , Cosby P. C.
Published: July 2002

Predissociation of the (v=7–10) levels of the a 1Πg state in N2 is studied with translational spectroscopy. These levels are produced by the charge-transfer neutralization of a 4 keV energy N+2 beam in sodium vapor and the dissociation fragments are monitored by a time- and position-sensitive detector. Lifetimes of 300, 300, 100, and 230 (±20) ns are measured for a 1Πg(v=7, 8, 9, and 10), respectively. Quantum mechanical calculations based on Fermi’s golden rule give evidence that the predissociation is caused by direct coupling to the continuum of the A′ 5Σ+g state rather than by an indirect mechanism. The a 1Πg–A′ 5Σ+g electronic coupling matrix element is estimated to be 0.44 (±0.03) cm−1. The required change in spin multiplicity suggests that this coupling arises from a spin–spin interaction.
Effect of chain stiffness on the conformational properties, pair correlations, and equation of state of polymer melts
Authors: Yethiraj Arun
Published: July 2002

The effect of molecular stiffness on the local structure and thermodynamic properties of semiflexible polymer melts is investigated via Monte Carlo simulation. Contrary to what might be expected from simple packing arguments, increasing the stiffness tends to decrease the tendency for segments on different chains to layer against each other, although the value of the pair correlation function at contact does increase. The pressure is relatively insensitive to the stiffness of the molecules because an increase in the two-body contribution to the pressure is compensated for by a decrease in the three-body contribution. Comparison of self-consistent polymer reference interaction site model predictions to the simulations shows that the accuracy of the theory for the pair correlation function diminishes somewhat as the chains become stiffer.
A comparative study of Ar/Ag(111) potentials
Authors: Kirchner E. J. J. , Kleyn A. W. , Baerends E. J.
Published: July 2002

The potential energy surface for the Ar/Ag(111) system is studied within the local density approximation. Results for N atom clusters, where N=4, 10, and 19, are compared with the potentials obtained with slabs with summation of pair potentials. This last method predicts the threefold hollow site as equilibrium site. However, the larger clusters find the top site as adsorption site. On the top site, the adsorption height and energy are calculated as 3.2 Å and 0.10 eV, respectively. The physisorption energy is in reasonable agreement with experimental data, in spite of the LDA level of theory used. Thus the Ag(111)–Ar system is a new example where LDA appears to describe a van der Waals system with reasonable accuracy. A correction to the sum of pair potentials is given that effectively reproduces the calculated potentials at threefold hollow site and top site.
The Raman and vibronic activity of intermolecular vibrations in aromatic-containing complexes and clusters
Authors: Maxton Patrick M. , Schaeffer Mark W. , Ohline Shane M. , Kim Wousik , Venturo Vincent A. , Felker Peter M.
Published: July 2002

Theoretical and experimental results pertaining to the excitation of intermolecular vibrations in the Raman and vibronic spectra of aromatic-containing, weakly bound complexes and clusters are reported. The theoretical analysis of intermolecular Raman activity is based on the assumption that the polarizability tensor of a weakly bound species is given by the sum of the polarizability tensors of its constituent monomers. The analysis shows that the van der Waals bending fundamentals in aromatic–rare gas complexes may be expected to be strongly Raman active. More generally, it predicts strong Raman activity for intermolecular vibrations that involve the libration or internal rotation of monomer moieties having appreciable permanent polarizability anisotropies. The vibronic activity of intermolecular vibrations in aromatic-rare gas complexes is analyzed under the assumption that every vibronic band gains its strength from an aromatic-localized transition. It is found that intermolecular vibrational excitations can accompany aromatic-localized vibronic excitations by the usual Franck–Condon mechanism or by a mechanism dependent on the librational amplitude of the aromatic moiety during the course of the pertinent intermolecular vibration. The latter mechanism can impart appreciable intensity to bands that are forbidden by rigid-molecule symmetry selection rules. The applicability of such rules is therefore called into question. Finally, experimental spectra of intermolecular transitions, obtained by mass-selective, ionization-detected stimulated Raman spectroscopies, are reported for benzene–X (X=Ar, –Ar2, N2, HCl, CO2, and –fluorene), fluorobenzene–Ar and –Kr, aniline–Ar, and fluorene–Ar and –Ar2. The results support the conclusions of the theoretical analyses and provide further evidence for the value of Raman methods in characterizing intermolecular vibrational level structures.
Effect of light intensity on the fluorescence quenching kinetics probed by frequency-domain fluorometry
Authors: Sung Jaeyoung , Shin Kook Joe , Lee Sangyoub
Published: July 2002

A theory for describing the frequency dependence of the fluorescence response stimulated by a sinusoidally modulated light is developed starting from a hierarchical system of many-body Smoluchowski equations for the reactant molecule distribution functions. A new aspect of the present theory is that it predicts the dependence of phase angle and modulation on the light intensity. Under weak light intensity and at low quencher concentration, the present theory agrees with the theories proposed by Zhou and Szabo and by Molski and Keizer. At high quencher concentration, however, the discrepancy in the predicted values of phase angle and modulation is noticeable even under weak light intensity.
Structure and dissociation of the methyldiazenyl radical: A quadratic configuration interaction computational study
Authors: Andrews B. Kim , Weisman R. Bruce
Published: July 2002

The structure and properties of CH3NN, the methyldiazenyl radical, have been studied using quadratic configuration interaction (QCI) ab initio calculations. Fully optimized equilibrium and transition state geometries were located at the QCISD/6-311G** level and the energies at these points were evaluated using QCISD(T)/6-311G*. The intrinsic reaction coordinate (IRC) for dissociation of the methyldiazenyl radical to form N2 plus CH3 has been traced at the QCISD/6-31G* level. These IRC results show that extension of the C–N bond is accompanied by shortening of the N–N bond, contraction of the N–N–C angle, and planarization of the methyl group. Dissociation is found to be exoergic and activated, with a small activation barrier whose height is estimated to be 630 cm−1 in a vibrationally adiabatic process. The standard heat of formation of methyldiazenyl radical is calculated to be 262 kJ/mol. Tunneling calculations based on the computed IRC profile predict the methyldiazenyl radical to be bound, but short lived, with a maximum lifetime between 10−11 and 10−9 s.
Hyperfine structure in the electronic spectra of the CdH and CdCH3 radicals
Authors: Tan Xue Qing , Cerny Timothy M. , Williamson James M. , Miller Terry A.
Published: July 2002

Magnetic hyperfine interactions in the ground and first excited states of CdH and CdCH3 were studied using high resolution electronic spectroscopy. Hyperfine splittings associated with the H nucleus were observed in the X state of CdH; hyperfine splittings associated with 111Cd and 113Cd were observed in both the ground and first excited states of CdH and CdCH3. The hyperfine parameters of CdH were found to be similar to the corresponding parameters of CdCH3. Comparison of the ground state molecular hyperfine constants with values determined in an Ar matrix electron paramagnetic resonance (EPR) study [L. B. Knight, Jr. and W. Weltner, Jr., J. Chem. Phys. 55, 2061 (1971)] indicates clearly a shift of unpaired electron spin density towards Cd, in the matrix. The excited state molecular hyperfine constants indicate that the unpaired electron essentially resides in a Cd 5pπ orbital.
Two primary product channels in OClO photodissociation near 360 nm
Authors: Bishenden Elizabeth , Donaldson D. J.
Published: July 2002

In the near UV region, there are two product channels possible for the photodissociation of OClO—OCl+O and Cl+O2. We have performed a number of experiments which confirm that Cl atoms are a primary product of OClO photodissociation near 360 nm. We use (2+1) resonance-enhanced multiphoton ionization to detect the Cl(2P3/2 and 2P1/2) and OCl (X 2Π) products of the photolysis of jet-cooled OClO. The two product channels have action spectra which have the same peak positions as the OClO Ã←X̃ absorption spectrum, but which have different relative peak intensities, showing that the two product channels are dynamically different. Kinetic studies show that the Cl atom signal disappears at a rate consistent with the reaction Cl+OClO→2OCl, also indicating that Cl is a primary photodissociation product. A double-resonant excitation of OClO might take place, and could explain the disparate photodissociation results reported to date.
Conformation change-driven anchoring transition of liquid crystals on crown ether liquid crystal Langmuir–Blodgett films
Authors: Zhu Yang-Ming , Wei Yu
Published: July 2002

The monolayers of a double-armed crown ether liquid crystal at the air–water interface show a first-order phase transition, accompanied by a conformational change. The crown ether liquid crystal Langmuir–Blodgett (LB) films have been prepared in different phases, which can produce homogeneous and homeotropic alignments of liquid crystals, depending upon the preparation conditions. The anchoring transition of liquid crystals can be interpreted as the result of a conformational change of the aligning agents.
Theory of spectral line shape. II. Collision time theory and the line wing
Authors: Roney P. L.
Published: July 2002

In this second part of a theory on spectral line shapes, applicable to conditions of atmospheric transmission, a means of evaluation of the frequency dependence of the collision operator is developed. An extension of formal scattering theory relates the off-the-energy shell matrix elements of the transition operator to their on-shell values through an operator Φ which is a function of the energy increment, a collision time operator T(E) and a free propagation time operator 𝒯(E). In the spectral line near-wing application, a first-order perturbation solution is obtained in terms of Φ as a function of the displacement from the line center. The real and imaginary parts of the (reduced) Fano collision operator are expressed in terms of shift and width functions and the impact approximation shift Δ(0) and width Γ(0) parameters. In addition, other impact approximation quantities are required which do not play a role in line shape in the impact approximation theory. A preliminary comparison with experiment for the CO2 continuum near 4.3 μm shows excellent agreement. The derived values of the collision times are in the expected picosecond range. There also seems to be little temperature dependence for N2–CO2 broadening over a wide range of temperatures.
Formula for the calculation of integral excitation cross sections at small scattering angles
Authors: Chen Zhifan , Msezane A. Z.
Published: July 2002

A new formula to calculate integral excitation cross sections at small scattering angles has been derived using the recently developed universal function. The Xe 5p6(1S)→5p5(2P3/2)6s transition is used to demonstrate the importance of small angle contribution to the integral cross sections. Additionally, measurements of differential cross sections for the resonance transition in Na are contrasted at 54.4 eV. The results show that a correct asymptotic form and an accurate total excitation cross section can be obtained by normalizing the measurement to the universal function.
Many-electron explicitly correlated Gaussian functions. II. Ground state of the helium molecular ion He+2
Authors: Cencek Wojciech , Rychlewski Jacek
Published: July 2002

Explicitly correlated Gaussian functions have been used to compute variationally the Born–Oppenheimer (B–O) potential energy curve for the ground 2Σ+u state of He+2. The energy values are much lower than all previously reported over the whole range of the internuclear distance R. The strict lower bound for the binding energy De amounts to 2.4730 eV and the true value (within the B–O approximation) is estimated to be 2.4742±0.0006 eV.
Photoinduced electron transfer processes in oligothiophene/C60 composite films
Authors: Janssen R. A. J. , Christiaans M. P. T. , Pakbaz K. , Moses D. , Hummelen Jan C. , Sariciftci N. S.
Published: July 2002

We present near steady-state photoinduced absorption (PIA) spectroscopy and steady-state light-induced electron spin resonance (LESR) studies on photoinduced electron transfer reactions in composite films of well defined α-oligothiophenes (Tn, n=6, 7, 9, and 11) as electron donor with buckminsterfullerene (C60) and a methanofullerene derivative (1-(3-cholestanoxycarbonyl)- propyl-1-phenyl-[6.6]C61) as electron acceptors. Dispersion on a molecular level of these fullerenes in oligothiophene films causes quenching of both the photoluminescence and intersystem crossing after photoexcitation across the π–π* energy gap as a result of a fast electron transfer reaction. The PIA spectra exhibit bands due to T+•n radical cations, fullerene radical anions and electroabsorption oscillations of the π–π* band edge resulting from strong local electric fields set up by the photoinduced charges. LESR spectra give additional unambiguous evidence of the photoinduced electron transfer reaction. Using PIA and LESR, we do not observe an effect of the conjugation length of the photoinduced electron transfer reaction or on the metastability of the charge-separated state.
Ab initio potential energy surface and near-infrared spectrum of the He–C2H2 complex
Authors: Moszynski Robert , Wormer Paul E. S. , van der Avoird Ad
Published: July 2002

Symmetry-adapted perturbation theory has been applied to compute the intermolecular potential energy surface of the He–C2H2 complex. The interaction energy is found to be dominated by the first-order exchange contribution and the dispersion energy. In both contributions it was necessary to include high-level intramolecular correlation effects. Our potential has a global minimum of εm=−22.292 cm−1 near the linear He–HCCH geometry at Rm=8.20 bohr and ϑm=14.16°, and a local minimum at a skew geometry (Rm=7.39 bohr, ϑm=48.82°, and εm=−21.983 cm−1). The computed potential energy surface has been analytically fitted and used in converged variational calculations to generate bound rovibrational states of the He–C2H2 molecule and the near-infrared spectrum, which corresponds to the simultaneous excitation of the vibration and hindered rotation of the C2H2 monomer within the complex. The nature of the bound states and of the spectrum predicted from the ab initio potential are discussed.
Theory and calculation of singlet excitation energy transfer in mixed molecular crystals
Authors: Gherban Doru , Kryschi Carola , Kupka Hans
Published: July 2002

A theoretical study of excitation energy transport among dipole–dipole interacting guests in mixed molecular crystals is carried out. To describe the temporal evolution of the excited state population, we derive a microscopic theory which treats an ensemble of dipole–dipole interacting guest molecules homogeneously distributed among two inequivalent sites of a host lattice. The theory is based on a generalized effective Hamiltonian accounting for intramolecular excited state depopulation and excitation energy transfer. The results are applied to the analysis of experimental data obtained from transient grating experiments in p-terphenyl:pentacene mixed molecular crystals.
Orbital-based direct inversion in the iterative subspace for the generalized valence bond method
Authors: Ionova Irina V. , Carter Emily A.
Published: July 2002

We present an algorithm that is a new combination of the direct inversion in the iterative subspace (DIIS) and the generalized valence bond (GVB) methods. The proposed algorithm is based on applying the DIIS directly to the orbitals updated via the GVB scheme as opposed to the conventional approach of applying DIIS to a series of composite Fock matrices (CFMs). The new method results in GVB convergence in situations where the CFM-based GVB-DIIS cannot be applied at all, e.g., when the original GVB method diverges. When both the new and the conventional methods converge, the former achieves the same reduction in the number of self-consistent field (SCF) iterations as the latter, but using considerably less storage and DIIS-related CPU time. Also, the orbital-based GVB-DIIS is less sensitive to the proximity of an initial guess to the exact wave function, and it does not depend on empirical criteria used in the CFM-based GVB-DIIS. Finally, the orbital-based DIIS formulation is not limited to GVB; it can be easily incorporated into any SCF approach that involves an iterative updating of the orbitals, such as, e.g., multiconfiguration SCF or Kohn–Sham density-functional theory.
Infrared spectroscopy, infrared photoconversion, and ultraviolet photodissociation of NO dimers in neon and nitrogen matrices
Authors: Legay F. , Legay-Sommaire N.
Published: July 2002

The nitric oxide dimers isolated in neon or nitrogen matrices have been irradiated in the infrared by selected lines of a cw CO laser and in the UV at 193 nm by an ArF excimer laser. IR excitation of the antisymmetric mode of the cis dimer in neon induces a conversion to a special van der Waals dimer. At T<6 K UV irradiation dissociates the dimers mainly to NO monomers and, in smaller proportion, to NO2 in the neon matrix or to N2O in the nitrogen matrix. NO2 is formed exclusively from the cis dimer and N2O from the trans dimer. At higher temperature in nitrogen, the oxygen atoms produced in the N2O formation migrate and react with NO to form NO2. The vibrational spectroscopy in nitrogen reveals several new features. The symmetrical vibrational mode of planar trans-(NO)2, normally IR inactive, becomes observable at 5.8 K owing to the matrix strain. cis-(NO)2 is trapped in two sites and the energy difference between them changes abruptly around 14 K, suggesting a structural transformation in the neighborhood of the sites. Cooling at 5.8 K induces a reversible decrease of the cis dimer bands, correlated with the appearance of vdW dimer bands. In both matrices, the antisymmetric band of the cis dimer is broadened by a predissociation effect as in the gas phase.
Simulation of forces on entangled polymers due to slippage
Authors: Deutsch J. M. , Yoon Hyoungsoo
Published: July 2002

We consider the force on the end of a polymer chain being pulled through a network at velocity v, using computer simulations. We develop algorithms for measuring the force on the end of the chain using lattice models of polymers. Our algorithm attaches a spring to the end being pulled and uses its average extension to calculate the force. General problems associated with the use of lattice models in obtaining forces are discussed. Variants of this method are used to obtain upper and lower bounds to the force. The results obtained are in agreement with recent analytical predictions and experiments.
Ionic and covalent electronic states for K adsorbed on Cu5 and Cu25 cluster models of the Cu(100) surface
Authors: Bagus Paul S. , Pacchioni Gianfranco
Published: July 2002

The chemisorption of K on the fourfold hollow site of the Cu(100) surface has been theoretically investigated by means of Cu5–K and Cu25–K cluster models. We have analyzed Hartree–Fock self-consistent field (SCF) wave functions for various electronic states of the two clusters. Four different measures have been used to establish the degree of ionicity of each state: (1) the analysis of the dipole moment curve for the variation of the Cu–K distance; (2) a constrained variation of the SCF orbitals to separate electrostatic, polarization, and charge transfer contributions; (3) the projection of the K valence orbitals onto the cluster wave function to measure the orbital occupancies, and (4) an energetic analysis of the cost and benefit of forming an ionic bond. We found different properties for the two clusters. All the considered electronic states of Cu25–K show large ionic character, suggesting that the bonding of K to a Cu(100) surface is indeed ionic at low coverage. The bonding character of the lowest states of Cu5–K is different, ranging from dominantly ionic to dominantly covalent. This behavior for Cu5–K is related to the small size of the cluster but it can be useful for modeling the transition from ionic to metallic bonding as the coverage of the alkali metal increases.
Initial probability of dissociative chemisorption of oxygen on iridium(110)
Authors: Kelly D. , Verhoef R. W. , Weinberg W. H.
Published: July 2002

The dissociative chemisorption of oxygen on Ir(110) has been investigated using supersonic molecular beam techniques. The initial probability of dissociative chemisorption (in the limit of zero surface coverage) as a function of incident kinetic energy between 1 and 28 kcal/mol and surface temperature from 85 to 1000 K is reported. For beam kinetic energies less than approximately 4 kcal/mol, the measured values of the initial probability of dissociative chemisorption are explained by a trapping-mediated adsorption mechanism. In this adsorption regime initial probabilities of dissociative chemisorption decrease with both increasing beam energy and surface temperature. The trapping probability of oxygen into the physically adsorbed state on Ir(110) as a function of incident beam energy is presented. For beam kinetic energies greater than ∼4 kcal/mol, a direct chemisorption mechanism dominates. In the direct adsorption regime, initial probabilities of dissociative chemisorption increase with increasing beam energy, and they are dependent on surface temperature, with the dependence decreasing with increasing surface temperature. This behavior is attributed to direct chemisorption into a molecularly chemisorbed state, from which there is a thermally activated kinetic competition between desorption and dissociation. A pseudo-steady-state kinetic model including physically adsorbed oxygen, molecularly chemisorbed oxygen, and atomically chemisorbed oxygen is applied to find that the activation barrier to desorption from the physically adsorbed molecular state is 1.6±0.1 kcal/mol higher than the barrier to conversion to the molecularly chemisorbed state. The activation barrier for desorption from the molecularly chemisorbed state is 1.5±0.15 kcal/mol greater than the barrier to dissociation from this state.
A laser-induced fluorescence study of OH desorption from Pt(111) during oxidation of hydrogen in O2 and decomposition of water
Authors: Fridell Erik , Elg Alf-Peter , Rosén Arne , Kasemo Bengt
Published: July 2002

The desorption of OH radicals from Pt(111) at high temperature, ≳1000 K, during the water formation (H2+1/2 O2→H2O) and water decomposition reactions, respectively, was investigated using the laser-induced fluorescence technique. The results are compared with corresponding data from our laboratory for polycrystalline Pt. The OH desorption rate in H2+O2 at 1–100 mTorr total pressure has its maximum at 8%–9% relative H2 concentration for surface temperatures between 1100 and 1400 K. With H2 replaced by D2, the OD desorption rate maximizes at somewhat higher relative hydrogen content. The apparent activation energy for OH desorption increases from about 1.4 eV at low relative hydrogen concentration to about 2.0 eV at hydrogen contents of 25% or more. For the water decomposition reaction, the apparent activation energy for OH desorption was found to be 1.7±0.2 eV at 0.5 Torr and 1.9±0.2 eV at 1 Torr. These differences in apparent activation energies are primarily due to kinetic effects. The results are analyzed within a kinetic model previously developed by Hellsing et al. [J. Catal. 132, 210 (1991)], and are also compared with previous data for polycrystalline Pt. The kinetic model calculations give good overall agreement with the measured OH desorption rates as functions of temperature, H2/O2 mixture and H2O pressure, respectively. A nonuniqueness problem, with respect to the choice of kinetic parameters, is encountered in the simulation of the measured data; quite different sets of two of the kinetic constants, namely the activation energy for water formation (via OH+H→H2O) and the activation energy for OH desorption can reproduce the data as long as their difference is constant.

This nonuniqueness problem, which is a consequence of the steady-state nature of the measurements, is analyzed and discussed in some detail, as are some apparent contradictions in the absolute values of reported kinetic constants in the literature. From this analysis two important conclusions are drawn. (i) The apparent contradictions in the literature about absolute values of activation energies for the water formation reaction and for OH-desorption may be less severe than believed or nonexistent. (ii) Coverage dependent activation energies must be considered and experimental exploration of such coverage dependencies are needed to create a firmer base for the kinetic modeling of the H2/O2 reaction on Pt.
Coherence effects in the polarization of Lyman-α fluorescence following photodissociation of H2 and D2
Authors: Flemming E. , Wilhelmi O. , Schmoranzer H. , Glass-Maujean M.
Published: July 2002

The polarization of the Lyman-α fluorescence following photodissociation of H2 and D2 into fragments in the 1s and 2l states has been determined as a function of the excess energy of the fragments. The predicted oscillations of the polarization as a function of excess energy were clearly observed. In addition, the theoretical polarization was recalculated including the stronger B–B′ coupling scheme as well as collisional effects on the polarization. The collisional effects include the quenching processes for H(2s) and H(2p) and disaligning collisions. The calculations reproduced the experimental data quite well giving a cross section for the disaligning collisions of (1.0±0.2)⋅10−14 cm2 for H2 and D2 at a gas temperature of 137 K in the relative velocity range of 2 to 7 km s−1. This can be considered as a proof of the theoretical predictions on the interference effects between the continua excited coherently.
Study of the overtone C–O stretching band of methanol by multiple resonance spectroscopy
Authors: Xu Li-Hong , Andrews A. M. , Fraser G. T.
Published: July 2002

Two microwave-sideband CO2 lasers have been used with a molecular-beam electric-resonance spectrometer to study the overtone C–O stretching vibration of methanol. Infrared-infrared double-resonance results have been obtained for levels involving the K=1 and 2, A symmetry, and the K=2, E2 symmetry species. In the A torsional symmetry case, radio frequency-infrared multiple resonance was used to obtain accurate asymmetry splittings for the vco=1 and 2, C–O stretching states. The asymmetry splitting constants determined for these states are in good agreement with the literature values for the first excited C–O stretching states. However, the nearly factor-of-2 change in the K=2 asymmetry splitting constant for the vco=2 level compared to the vco=0 and 1 level results suggests that this state is weakly perturbed. The overtone transition frequencies obtained in this work were combined with previous overtone Fourier-transform results in a global fit to a torsion–rotation Hamiltonian to refine the fundamental molecular constants for the second-excited C–O stretching state. The vco=2 torsional barrier height is found to be 372.227(3) or 374.984(7) cm−1 depending on data set used. In the analysis the overtone vibrational energy origin is constrained to 2054.831 13 cm−1. This barrier can be compared to the v0=0 and 1 values of 373.5421 and 392.35 cm−1, respectively.
Band oscillator strengths of the Herzberg I bands of O2
Authors: Yoshino K. , Esmond J. R. , Murray J. E. , Parkinson W. H. , Thorne A. P. , Learner R. C. M. , Cox G.
Published: July 2002

Photoabsorption cross section measurements of the Herzberg I bands (A3Σu+−X3Σg−) of O2 have been made by Fourier transform spectrometry with a resolution of 0.06 cm−1 in the wavelength region 240-270 nm. Precise band oscillator strengths of the (4,0)–(11,0) bands are obtained by direct measurement, and for some of the strong bands, they are significantly higher than the previous experimental values. The rotational line strengths and the branching ratios are also presented for the same bands. The dissociation energy of O2 is discussed.
Carbon-13 chemical shift tensor correlation via spin diffusion in solid tropolone using switched-angle spinning spectroscopy
Authors: Larsen R. G. , Lee Y. K. , He B. , Yang J. O. , Luz Z. , Zimmermann H. , Pines A.
Published: July 2002

In switched-angle spinning spectroscopy (SAS) a sample is spun about different angles, β, relative to the magnetic field, during various periods of the experiment. In the present work, SAS is combined with two-dimensional exchange spectroscopy in order to correlate carbon-13 chemical shift tensors of the carbonyl (1) and hydroxyl (2) carbons of tropolone. Experiments were performed on a sample enriched to 25 at. % in each of these sites (at different molecules). At this level of enrichment the dominant exchange mechanism between the two sites involves spin diffusion. The experiment consists of a preparation period during which the sample spins at the magic angle and the magnetization of one of the sites is quenched by means of a selective pulse sequence. During the rest of the experiment the sample spins with its axis away from the magic angle except for a short period just before the detection where the axis is switched to the magic angle in order to select the magnetization to be detected. Experiments were performed for all four possible combinations of the initial and final magnetizations, thus providing chemical shift correlations between carbons 1,1′, 2, and 2′ in the two magnetically inequivalent (but symmetry related) molecules in the unit cell. Combining these results with the known crystal structure of tropolone (neglecting a small tilt between the perpendicular to the molecular plane and the crystallographic c-axis) provides information on the orientation and magnitude of the chemical shift tensors of the two types of carbons. The principal values (in ppm) are σ1xx=65, σ1yy=33, σ1zz=−98, σ2xx=77, σ2yy=17, and σ2zz=−94. Assuming σzz to be perpendicular to the molecular plane, the orientations of the σyy’s are 12° off the C1=0 bond (toward the hydroxyl carbon) for carbon 1 and 10° off the C3=C2 bond (away from the carbonyl carbon) for carbon 2.
Molecular dynamics study of liquid carbon disulfide and benzene: Effect of pressure on the far-infrared collision-induced absorption
Authors: Fujita Yoshitaka , Ikawa Shun-ichi
Published: July 2002

We have performed molecular dynamics calculations of liquid carbon disulfide and liquid benzene, intending to examine the effect of pressure on the far-infrared collision-induced absorption as well as on the local structure and on the molecular reorientational dynamics. The calculations reproduced well the experimental results of the reorientational correlation times of liquid carbon disulfide at pressures, of the reorientational anisotropy of liquid benzene, and of the density dependence of the collision-induced dipole moments and their time derivatives of both liquids. Contributions of various interaction terms to the collision-induced dipole moments were examined as a function of density. The experimental results that the collision-induced dipole moments of liquid benzene decreases and those of liquid carbon disulfide changes little with increase in density are explained well by pressure-dependent change in degree of the cancellation between the positive two-molecule interaction term and the negative three-molecule interaction terms.
Synchronization in the discrete chemical oscillation system
Authors: Miyakawa Kenji , Okabe Tadao , Mizoguchi Michiko , Sakamoto Fumitaka
Published: July 2002

The properties of the coupling between chemical oscillators were studied in the discrete chemical oscillation system which was realized by immersing cation exchange beads loaded with ferroin in the Belousov–Zabotinskii reaction solution. A phase diagram of coupling states was obtained as a function of natural frequencies of oscillators and the distance d between oscillators. The synchronization was found not to be attributed to a simple entrainment of the slower oscillator by the faster one. Various entrainments between oscillators occurred depending on ratios of natural frequencies in the uncoupled state. A chaotic behavior was found at the boundary between stably coupled regions with frequency ratios of n/1 where n is an integer. Furthermore, effects of external perturbation on the coupling were investigated. Irregular oscillations were induced by illumination with a He–Ne laser light, which strongly depended on the phase of oscillator at the beginning of illumination. Such irregular behaviors were localized within the illuminated bead. This indicates that two oscillators are decoupled by illumination.
Product fine structure state populations and nonadiabatic dissociation dynamics: Cl*(2P1/2)/Cl(2P3/2) branching ratio in the visible and near ultraviolet photodissociation of nitrosyl chloride
Authors: Cao Jianying , Wang Yifei , Qian Charles X. W.
Published: July 2002

Chlorine atoms in their 2Pj states were observed using resonance-enhanced multiphoton ionization and time-of-flight techniques, following the photodissociation of nitrosyl chloride at preselected wavelengths (i.e., 600, 562, 478, and 355 nm). The fine structure branching ratio and the spatial anisotropy parameter were determined. Combining these observations with results from previous studies, a complete picture about product fine structure state populations is obtained. The observed fine structure populations are explained with the aid of a new correlation diagram in the Cs point group. The adiabatic, nonadiabatic, and diabatic dissociation dynamics involving several avoided crossings at large interfragment separations are discussed.