bluuebunny/crossref_metadata_2025 huggingface dataset:
Rotational tunneling of partially deuterated methyl groups: Optical studies of dimethyl-s-tetrazine in different crystalline matrices
Authors: Johnson M. , Orth K. , Friedrich J. , Trommsdorff H. P.
Published: July 2002

Rotational tunneling of monodeuterated methyl groups of dimethyl-s-tetrazine guest molecules in n-octane and tetramethylbenzene crystalline matrices is characterized by optical spectroscopy including hole burning methods. An analysis of the level structure shows that the asymmetric substitution leads to a partial localization of one of the methyl groups in n-octane, while in the tetramethylbenzene host both groups are equivalent and are strongly localized. The relaxation, by incoherent tunneling, between different orientations proceeds on a time scale of >107 s−1 in n-octane at temperatures below 2 K. The coupling of the orientation of the methyl group with the environment is estimated from information on inhomogeneous broadening attributed to random strain fields in the crystal. The strength of this coupling is related to the observed relaxation behavior. The origin of the asymmetry of the rotational potential is discussed in terms of static and dynamic contributions which are due to changes of bond length upon isotopic substitution and the dependence of vibrational frequencies on orientation.
Thermally activated traversal of an energy barrier of arbitrary shape
Authors: Berezhkovskii Alexander M. , Talkner Peter , Emmerich Jens , Zitserman Vladimir Yu.
Published: July 2002

The thermally activated escape of a Brownian particle over an arbitrarily shaped potential barrier is considered. Based on an approximate solution of the corresponding Fokker–Planck equation a rate expression is given. It agrees in the limiting case of high friction with the rate following from the corresponding Smoluchowski equation and, in the limit of weak friction with the rate obtained from transition state theory. For a parabolic barrier the approximate rate expression deviates less than 16% from the known result. The results for cusp shaped and quartic barriers agree with known expressions which have been obtained by other means. Estimates of the rates from numerical simulations are compared with the approximate rate expressions for the cusp and quartic barrier.
Geometry optimization of atomic microclusters using inverse-power distance coordinates
Authors: Baker Jon , Pulay Peter
Published: July 2002

Using redundant inverse-power distance coordinates we present a series of optimizations on randomly generated argon clusters containing 12, 24, and 48 argon atoms. Our results clearly show that inverse distance coordinates perform significantly better for optimizing the geometries of atomic clusters than either Cartesian or natural/delocalized internal coordinates. The potential utility of these coordinates for global optimization is discussed.
Second-order Epstein–Nesbet correction to ‘‘dynamic’’ configuration interaction energies
Authors: Mitrushenkov A. O.
Published: July 2002

The zero threshold estimate of our dynamic configuration interaction procedure, is used to obtain the correlation energy correction for nonactive electron pairs in multireference configuration interaction treatments. For exact eigenvectors, the expression is shown to be equivalent to second order perturbation theory with the Epstein–Nesbet partitioning of the reference Hamiltonian. We describe our code which has been efficiently implemented on parallel computers. Few tests have been performed, to show the accuracy of our approach by comparison with the related multiconfigurational, Mo/ller–Plesset-type treatment.
Dielectric constant of polarizable water at elevated temperatures
Authors: Mountain Raymond D.
Published: July 2002

The dielectric constant of a polarizable water model is estimated for several thermodynamic states at elevated temperatures and reduced densities relative to ambient conditions using molecular dynamics simulations. An extended Lagrangian formulation of induced polarization is used in these simulations in conjunction with thermostat variables. This approach to the simulation of induced polarization is found to require about one-half the computational effort needed to determine directly the adiabatic values for the induced dipole moments. As the temperature increases and the density decreases, the difference between the model and experimental values of the dielectric constant is positive and significantly larger than the uncertainty in the model values.
Comparison of classical and quantal calculations for the reaction O+H2(υ=0, J)→OH(υ′, J′)+H near threshold
Authors: Song Ju-Beom , Gislason Eric A.
Published: July 2002

State-to-state reaction probabilities have been calculated for the reaction O+H2(υ=0,J) →OH(υ′,J′)+H near threshold using the quasiclassical trajectory technique. In most cases the total classical angular momentum 𝒥 of the system was held equal to zero. The procedure for ‘‘quantizing’’ the product vibrational energy is somewhat ambiguous, and two procedures were used. The results were compared with the quantal calculations of Chatfield et al. on the same system for 𝒥=0. Both the classical and quantal calculations give very state-specific product distributions. The agreement between the classical and quantal calculations is reasonably good, particularly for the classical procedure which conserves the total product internal energy during the quantization procedure. The agreement is worst for J=0. For the case J=14 we have also carried out a calculation for all 𝒥 values (i.e., all impact parameters and initial orientations) of the product density distribution P(Evib′,Erot′) and compared it with the comparable distribution for 𝒥=0. Here, Evib′ and Erot′ are the product vibrational and rotational energies. The results for all 𝒥 show a peaked product distribution, but it is broader than that for 𝒥=0 and it has a somewhat different correlation between Evib′ and Erot′.
Interaction of Cs and CO on Ru(0001) for Cs submonolayer coverages
Authors: He P. , Jacobi K.
Published: July 2002

The interaction of Cs and CO was studied for submonolayer coverages of Cs on Ru(0001) by means of high-resolution electron energy-loss spectroscopy (HREELS), thermal desorption spectroscopy (TDS), and low-energy electron diffraction (LEED). From the HREEL spectra and the 2×2 LEED pattern it is concluded that two-dimensional islands of Cs+2CO stoichiometry are formed first. For high CO doses CO adsorbs on the bare Ru surface between the islands as concluded from the ν(C–O) stretch-mode frequency and the (∛×∛)R30° LEED pattern. Besides some local interaction between CO and Cs, there is strong evidence found for a long range, through substrate interaction. Our data can be understood in terms of a charge redistribution model; Cs donates a given amount of charge to the surface which is then backdonated to and shared between the adsorbed CO molecules within the (Cs+2CO)-islands and outside of them on the bare surface. The desorption temperature of CO with θCO→0 increases nearly linearly with θCs indicating CO desorption out of a two dimensional Cs gas in which the lateral interaction increases with θCs.
Fluctuation, relaxation and rearrangement dynamics of a model (H2O)20 cluster: Non-statistical dynamical behavior
Authors: Baba Akinori , Hirata Yoshihiro , Saito Shinji , Ohmine Iwao , Wales David J.
Published: July 2002

Relaxation phenomena in a model (H2O)20 cluster are investigated by considering energy fluctuations and dielectric relaxation, making connections with the underlying hydrogen-bond rearrangement dynamics. In particular, we relate these effects to the potential energy surface, including rearrangement mechanisms, minimum energy pathways and normal mode excitations. The statistical behavior of structural transitions in this water cluster is also examined. The transition rates estimated from RRKM theory are compared with those obtained by molecular dynamics (MD) simulations. The RRKM rates are 1–3 orders of magnitude larger than the MD values and the difference is larger at low energy. To find the origin of this discrepancy, the mechanism of coupling among various modes is examined. We find that there is significant coupling only among a subset of the normal modes in the low energy dynamics. A modified statistical theory, assuming that only a subset of modes contributes to the reaction rate, reproduces the energy dependence of the MD transition rates quite well. It is also found that the energy fluctuations and dielectric relaxation are non-Debye in character, and instead exhibit so-called 1/f spectra.
Equilibrium properties of polyampholytes in electric fields
Authors: Winkler Roland G. , Reineker Peter
Published: July 2002

An analytically tractable model for the description of a charged polymer chain (polyampholyte) in an external electric field is presented. The chain is comprised of mass points and their connectivity is taken into account by harmonic constraints. In particular the finite extensibility of a chain is captured by our approach. Equilibrium properties for chains of various charge distributions are calculated, especially for intermediate and strong electric fields. Particularly, chains of alternating positive and negative charges are considered within a discrete chain model. For intermediate electric fields such chains exhibit stretched configurations, whereas for strong electric fields a collapse or stretching occurs depending on the number of mass points. In addition a continuous chain with random charge density is considered. Our investigations demonstrate that the equilibrium properties of polyampholytes in strong electric fields decisively depend on the charge distribution along the chain.
Impact induced vibrational excitation in surface scattering of hyperthermal neutral C60 molecule
Authors: Tsipinyuk B. , Budrevich A. , Grinberg M. , Kolodney E.
Published: July 2002

A mass-spectrometry based method for measuring the average vibrational energy (vibrational thermometry) of large and hot polyatomic molecules is presented. The method is applied to C60 inelastically scattered off nickel with impact energies of 10–50 eV. Both the vibrational cooling effect on the supersonically expanded C60 and the collisional excitation upon surface impact are measured within an experimental accuracy of ±0.25 eV. Under nearly normal beam incidence conditions and impact energy of 33.0 eV (out of which 30.6 eV are in the normal energy component), vibrational excitation was found to be below ∼2% of the impact energy, showing that the recoiled C60 is nearly nondeformed at these collision energies. This value is much lower than estimated before for higher energies C60+ ion surface scattering. The implication of this result in relation with the low energy scattering dynamics of C60 is discussed.
Molecular dynamics simulation studies of the density profiles of water between (9-3) Lennard-Jones walls
Authors: Spohr E.
Published: July 2002

The results of molecular dynamics computer simulations of water laminae confined between two (9-3) Lennard-Jones walls are reported. The interaction potential is identical to the one used in a recent singlet integral equation study [M. J. Booth, D.-M. Duh, and A. D. J. Haymet, J. Chem. Phys. 101, 7925 (1994)]. In the simulation studies the internal pressure, system size, and the treatment of long-range Coulomb interactions has been varied. Large density oscillations similar to the integral equation results have not been observed. Simulated pair correlation functions are found to exhibit only little anisotropy near the interface.
Cavitation model of electron solvation dynamics: Effect of energy dissipation
Authors: Rips Ilya
Published: July 2002

The effect of energy dissipation on the electron solvation dynamics is studied within the framework of the cavitation model. Two different energy dissipation mechanisms, associated with the sound emission and viscous friction, are analysed. The radiational damping of the cavity boundary dynamics is described using the Herring equation. The model is applied to analysis of the electron solvation dynamics in water. Sound emission constitutes the most efficient mechanism of energy dissipation. Energy dissipation does not change the theoretical prediction concerning the small isotope effect on the electron solvation dynamics in water.
Dynamics of coarse-grained helical wormlike chains. III. First cumulant of the dynamic structure factor
Authors: Yoshizaki Takenao , Osa Masashi , Yamakawa Hiromi
Published: July 2002

The dynamic structure factor S(k,t) as a function of the magnitude k of the scattering vector and time t and its first cumulant Ω(k) are evaluated in the Gaussian approximation on the basis of the coarse-grained helical wormlike chain model. In this approximation, S(k,t) and therefore Ω(k) may be expressed in terms of the solutions of the 1(1) eigenvalue problem previously obtained. It is then found that the eigenvalues in the j=0 branch of the 1(1) eigenvalue spectrum at small wave numbers make main contribution to Ω(k). The numerical results for typical flexible polymers show that the so-called “universal” plot of η0Ω(k)/kBTk3 against 〈S2〉1/2k depends on the kind of polymer, where η0 is the solvent viscosity, kB the Boltzmann constant, T the absolute temperature, and 〈S2〉 the mean-square radius of gyration. It is also shown that for semiflexible chains, the plot depends appreciably on the reduced total contour length. A comparison of theory with experiment is made with respect to Ω(k) and also the coefficient C introduced by Stockmayer and co-workers. It is then found that the theory may give a rather satisfactory explanation of experimental results for both flexible and semiflexible polymers despite the fact that the preaveraged Oseen tensor is used along with the Gaussian approximation.
Transition state barrier height for the reaction H2CO→H2+CO studied by multireference Mo/ller–Plesset perturbation theory
Authors: Nakano Haruyuki , Nakayama Kenichi , Hirao Kimihiko , Dupuis Michel
Published: July 2002

The second-order multireference Mo/ller–Plesset perturbation method (MRMP) was applied to the accurate estimation of the transition state barrier height of H2CO→H2+CO reaction. The best estimate for the classical barrier height is 84.5 kcal/mol at the highest level of MRMP theory with the quadruple zeta plus triple polarization basis set and with the active space of 12 electrons in 11 active orbitals. The inclusion of zero-point vibrational energy correction reduces the activation energy to 79.1 kcal/mol, which is in excellent agreement with the experimental value of 79.2±0.8 kcal/mol [Polik, Guyer, and Moore, J. Chem. Phys. 92, 3453 (1990)]. Analysis of the second-order energies in terms of internal, semi-internal, and external contributions shows that the present MRMP provides a well balanced treatment for the estimation of the energy difference between the equilibrium and transition state structures.
Calculation of electronic tunneling matrix element in proteins: Comparison of exact and approximate one-electron methods for Ru-modified azurin
Authors: Daizadeh Iraj , Gehlen John N. , Stuchebrukhov Alexei A.
Published: July 2002

In recent years several theoretical methods have been developed for evaluation of the magnitude of electronic coupling between distant donor and acceptor complexes mediated by a protein molecule. Most detailed studies have been carried out within the one-electron tight-binding (extended Hückel) approximation for electronic structure of the protein medium. In this paper different approximate and exact one-electron methods such as perturbation theory, exact diagonalization, and method of tunneling currents are reviewed and results of calculations are compared for three HisX-Ru-modified azurin molecules, where X=122, 124, and 126. These systems have been recently synthesized and studied experimentally by Gray and co-workers. The calculations show that perturbation theory results are in excellent agreement with exact calculations if the symmetry of the zeroth-order wave functions of the donor and acceptor metal ions are chosen correctly. A simple computational procedure for construction of such correct zeroth-order functions is proposed.
Incorporation of angular momentum anisotropy into the analysis of resonant four-wave mixing spectroscopy
Authors: Wasserman Thierry A. W. , Vaccaro Patrick H. , Johnson Bruce R.
Published: July 2002

A state-multipole expansion of the zero-order density operator has been used to derive analytical signal expressions for near-resonant degenerate four-wave mixing spectroscopy performed on systems that possess angular momentum anisotropy. The predictions of this formalism are found to be in good agreement with experimental optical–optical double resonance measurements involving aligned intermediate states.
Rayleigh–Schrödinger perturbation theory for coupled oscillators. III. Curve-crossing problems
Authors: Špirko Vladimı́r , Čı́žek Jiřı́
Published: July 2002

The Rayleigh–Schrödinger perturbation theory approach developed previously for evaluating nonadiabatic corrections to the adiabatic energy levels of a system of two coupled oscillators is generalized to the case of the so-called “mixed” representations which arise from the diabatic representation of a given problem by performing a unitary transformation on the diabatic potential energy matrix (the adiabatic representation is obtained as a special case with a purely diagonal potential energy matrix). Different representations provide different coupling conditions and, consequently, different bases for evaluation of the perturbation corrections. This is reflected, quite generally, in the convergence and summability properties of the perturbation series and can thus be used to improve the accuracy and stability of the perturbation calculations. The latter possibility is especially important in the case of closely coinciding levels. Model calculations have revealed that changing representations may allow the determination of the energies of these levels to a high degree of accuracy even in the case of strong perturbation resonances.
Competition between singlet and triplet channels in the photoinitiated decomposition of HNCO
Authors: Zyrianov M. , Droz-Georget Th. , Reisler H.
Published: July 2002

The spin-forbidden channel, NH(X 3∑−)+CO, has been observed directly in the photodissociation of jet-cooled HNCO following S1←S0 excitation. The NH(X 3∑−) yield spectrum is obtained in the energy regimes near the thresholds to H+NCO and NH(a 1Δ)+CO channels. The spectrum is similar to the NCO yield spectrum when direct dissociation on S1 is not significant. At photolysis energies >43 400 cm−1, state specific differences between the NH(X 3∑−), NH(a 1Δ) and NCO yield spectra are observed, and at energies >44 000 cm−1 all structure in the NH(X 3∑−) yield spectrum is lost, while the NH(a 1Δ) yield spectrum remains structured. The results are interpreted in terms of the different time scales for intersystem crossing and dissociation.
The Toeplitz approach to treating three-dimensional reactive exchange processes: Quantum mechanical cross sections and rate constants for the D+H2 and H+D2 reactions
Authors: Charutz David M. , Last Isidore , Baer Michael
Published: July 2002

Recently a new approach to treating reactive (exchange) processes was introduced. It is based on the ability of negative imaginary potentials to decouple products arrangement channels without affecting the final results and on the behavior of the Hamiltonian which in the asymptotic region becomes a Toeplitz operator. This approach, developed within the discrete variable representation, is extended here to three dimensions and applied to calculate state-selected cross sections and rate constants for the two title reactions.
Improved estimates of the total correlation energy in the ground state of the water molecule
Authors: Lüchow Arne , Anderson James B. , Feller David
Published: July 2002

Two new calculations of the electronic energy of the ground state of the water molecule yield energies lower than those of any previously reported variational calculations. A fixed-node quantum Monte Carlo calculation gives −76.420(1) hartrees and an analytic variational calculation gives −76.4274 hartrees. These values lie only 17 and 11 mhartrees, respectively, above the “experimental” value.
Revision of the experimental electron affinity of BO
Authors: Rienstra Jonathan C. , Schaefer III Henry F.
Published: July 2002

The experimental electron affinity of BO has proven questionable. We obtained the electron affinity of BO using the large aug-cc-pVQZ basis with SCF, CISD, CISD+Q, CCSD, and CCSD(T) methods and predict a value of 2.57 eV, or 0.55 eV smaller than the latest experimental value. The Σ2+ to Π2 excitation energy of BO has also been obtained with the CCSD(T) method and found to be 2.82 eV.
The free energy difference between simple models of B- and Z-DNA: Computer simulation and theoretical predictions
Authors: Gil Montoro J. C. , Abascal J. L. F.
Published: July 2002

A method recently proposed to calculate by computer simulation the relative free energy between two conformational states of a polyelectrolyte is used for the case of the salt induced B- to Z-DNA transition. In this method, the calculation of the free energy may be split in two steps, one corresponding to the setup of the uncharged conformer in solution while the other one is the charging process of such a structure. Following the description of the method, simulations are reported to compute the free energy difference between the above mentioned DNA conformers in presence of monovalent added salt. We use a simple DNA solution model—the DNA is represented by charged spheres at the canonical positions of the phosphate groups, water by a dielectric continuum of appropriate permittivity and counterions and coions are modeled as soft spheres of equal ionic radius—for which theoretical approximations have been proposed. It is seen that the charging term is much more important than the setup contribution at any of the investigated salt concentrations. The variation of the free energy of each conformer as a function of the added NaCl concentration has been calculated. Both the B and Z conformers increase noticeably their stabilities with higher salt concentrations but the effect is more pronounced for the latter. As a consequence, the relative population of B-DNA, which is clearly prevalent at moderate ionic strengths, decreases with the addition of salt. However, up to 4.3 M NaCl a B→Z transition is not predicted for this DNA solution model. Additionally, the theoretical calculations are checked for the first time against computer simulation results. In particular, we have tried to assess the foundations and predictive ability of (especially) the Soumpasis potential of mean force theory and, in a lesser extent, the counterion condensation theory of Manning and the polymer reference interaction site model theory of Hirata and Levy.
Ultraviolet photoelectron spectroscopy of the chromium dioxide negative ion
Authors: Wenthold Paul G. , Jonas Karl-Ludwig , Lineberger W. C.
Published: July 2002

The 351 nm photoelectron spectrum of chromium dioxide negative ion has been measured. Two vibrational frequencies of neutral chromium dioxide are observed in the spectrum, at 220±20 and 895±20 cm−1, and are assigned to the bending mode and symmetric stretch, respectively. From hot bands in the spectrum, a frequency of 205±40 cm−1 is determined for the bending mode in the negative ion. The electron affinity of CrO2 is found to be 2.413±0.008 eV. The measured anisotropy parameter for photodetachment indicates that the electron is detached from an orbital with significant 4s character.
Cavity-enhanced fluorescence decay rates from microdroplets
Authors: Arnold Stephen
Published: July 2002

We show that enhanced fluorescence decay rates in microdroplets due to weak coupling to Mie resonances should not follow an inverse radius dependence, 1/a, as has been assumed in several publications, but should contain both 1/a and (1/a)2 components. The correspondence with experiment is very good.
Photon correlation spectroscopy of interacting and dissociating hemoglobin
Authors: Beretta Sabrina , Chirico Giuseppe , Arosio Daniele , Baldini Giancarlo
Published: July 2002

Experimental and theoretical analysis of the effect of both intermolecular interactions and dissociation on the diffusional properties of carbon–monoxide bovine hemoglobin in solution are reported here. When performing accurate photon correlation spectroscopy measurements versus protein concentration, even on dilute solutions, one finds that the first cumulant diffusion coefficient of the macromolecule has a relevant dependence upon pH (5⩽pH⩽9.5), ionic strength (10–100 mM), and, particularly, on hemoglobin concentration (1⩽c⩽20 mg/ml). The results cannot be interpreted by considering the occurrence of either protein dissociation or intermolecular interactions only. As a consequence a simple theoretical expansion of the first cumulant diffusion coefficient, to first order in concentration, is derived here with the inclusion of protein interactions and dissociation. A fit procedure based on this expression leads to a good description of the dissociation and an accurate estimate of the native protein charge and of its single particle diffusion coefficient in the full range of solution parameters.
Self-diffusion in single-file pores of finite length
Authors: MacElroy J. M. D. , Suh S.-H.
Published: July 2002

Molecular dynamics simulation results are reported for single-file self-diffusion in pores of infinite and finite length. The model system involves moderately dense, nonovertaking, hard-sphere fluids in cylindrical pores, and the particle mobility is investigated as a function of time and pore length. It is shown that, while stationary Fickian diffusion in infinitely long pores with diffusely reflecting pore walls is nonexistent, a diffusivity may be defined for single-file pores of finite length. For pores of moderate length, it is also demonstrated that the self-diffusion coefficient scales as 1/√L where L is the pore length.
Validation of trajectory surface hopping methods against accurate quantum mechanical dynamics and semiclassical analysis of electronic-to-vibrational energy transfer
Authors: Topaler Maria S. , Hack Michael D. , Allison Thomas C. , Liu Yi-Ping , Mielke Steven L. , Schwenke David W. , Truhlar Donald G.
Published: July 2002

The validity of the quasiclassical trajectory surface hopping method is tested by comparison against accurate quantum dynamics calculations. Two versions of the method, one including electronic coherence between hops and one neglecting this effect, are applied to the electronically nonadiabatic quenching processes Na(3p)+H2(v=0,j=0 or 2)→Na(3s)+H2(v′,j′). They are found to agree well, not only for quenching probabilities and final-state distributions, but also for collision lifetimes and hopping statistics, demonstrating that electronic coherence is not important for this system. In general the accurate quantum dynamical calculations and both semiclassical surface hopping models agree well on the average, which lends credence to applications of semiclassical methods to provide insight into the mechanistic details of photochemical processes proceeding on coupled potential surfaces. In the second part of the paper the intimate details of the trajectories are analyzed to provide such insight for the present electronic-to-vibrational energy transfer process.
Wave packet investigation of the Renner–Teller-induced predissociation of DCO(Ã2A″)
Authors: Loettgers Andrea , Schinke Reinhard
Published: July 2002

We present the results of 3D time-dependent wave packet calculations for the Renner–Teller induced dissociation of DCO following excitation in the first absorption band (Ã2A″←X̃2A′). They complement our investigation of the photodissociation of HCO [J. Chem. Phys. (in press)]. The excitation energies and the decay widths agree well with available experimental data.
Vibrational dynamics of large clusters from helium atom scattering: Calculations for Ar55
Authors: Schröder T. , Schinke R. , Krohne R. , Buck U.
Published: July 2002

Vibrational excitation of an Ar55 cluster in collisions with He (Ekin=25 meV) is investigated by using classical trajectories and a highly approximate quantum mechanical method (vibrationally sudden approximation). The energy transfer from the helium atom to the cluster (ΔE) is calculated as a function of the scattering angle θ. It is found (i) that predominantly the modes corresponding to the cluster atoms in the outer shell are excited and (ii) that the probability for multi-phonon (Δn⩾2) excitations steadily increases with the scattering angle. The results of both sets of calculations are generally in good agreement with measured energy loss spectra over the entire range of scattering angles. In the region θ≈15°–30°, which—because single-phonon transitions dominate—is most important for determining the frequency distribution of the cluster, the quantum mechanical calculations are superior to classical mechanics; the latter fails to conserve zero-point energy and therefore leads to unrealistic energy transfer below ΔE≲2 meV.
Transient hole-burning spectroscopy of associated ethanol molecules in the infrared: Structural dynamics and evidence for energy migration
Authors: Laenen R. , Rauscher C.
Published: July 2002

Hydrogen-bonded ethanol molecules in internal positions of oligomers are investigated in solutions of carbon tetrachloride (concentration 0.17 M). After resonant excitation of the OH-stretching vibration by a 2-ps infrared pump pulse at 3340 cm−1, transient spectral hole burning within the inhomogeneously broadened band is observed with a delayed, independently tunable probing pulse of 1 ps duration. Evidence is presented that the vibrational excitation migrates along the oligomer chain with subsequent breaking of H bonds so that additional ethanol dimers and trimers are formed. Correspondingly, the number of ethanol monomers or hydroxilic groups with proton acceptor function is found to increase. Several time constants describing the various processes, e.g., the proposed migration of vibrational quanta along the H bonded chain, breaking of hydrogen bonds, and the reassociation of the generated shorter species, are determined by a comparison of the experimental data with model computations.
Classical dynamics simulations of SiMe3+ ion–surface scattering
Authors: Schultz David G. , Wainhaus Samuel B. , Hanley Luke , de Sainte Claire Pascal , Hase William L.
Published: July 2002

This paper presents classical dynamics simulations of Si(CD3)3+scattering from a hexanethiolate self-assembled monolayer on Au(111) and from a clean Au(111) surface. Simulations are performed with a united atom model using purely repulsive scattering potentials. These simulations predict the partitioning of the incident ion kinetic energy into the scattered ion kinetic energy and the internal modes of both the surface and the ion. For the organic surface, the simulations predict energy transfer to surface, ion internal, and ion kinetic energies of 0.78, 0.11, and 0.12 of the collision energy. The corresponding transfer efficiencies of 0.12, 0.21, and 0.65 were calculated for the Au(111) surface. These computational results compare well with the experimental results on the same systems which are reported in the preceding paper. The simulations predict near specular scattering for both surfaces. They also demonstrate that the ion penetrates only the topmost two to three layers of Me atoms of the organic surface and that it spends up to 250 fs in contact with the surface. Finally, these calculations determine the dependence of energy transfer on the incident ion angle.
Branching ratio for the production of OD(A) and OH(A) by controlled electron impact on HOD
Authors: Furuya Kenji , Koba Fumihiro , Ogawa Teiichiro
Published: July 2002

The OH(A–X) and OD(A–X) emission spectra have been measured by 17 eV electron impact on the mixtures of H2O and D2O with several mixing ratios. The intensity ratio of the OH(A–X) emission to that of OD(A–X), q, in the spectra of the mixture has been estimated using the OH(A–X) and OD(A–X) spectra obtained from pure H2O and pure D2O. As a result, q is proportional to the ratio of the H-atom quantity to the D atom in the mixture, indicating that q is independent of the fraction of HOD in the mixture. The following emission intensity ratios are obtained. σHOD→OH/σHOD→OD=σH2O→OH/σD2O→OD=0.53±0.02,σD2O→OD/σHOD→OD=1.88±0.24, where σHOD→OH means the total emission cross section of OH(A–X) by electron impact on pure HOD, and the others are similar.
Energy transfer and surface-induced dissociation for SiMe3+ scattering off clean and adsorbate covered metals
Authors: Wainhaus Samuel B. , Lim Hanjo , Schultz David G. , Hanley Luke
Published: July 2002

We scatter 10–70 eV SiMe3+ from clean Au(111), a hexanethiolate self-assembled monolayer on Au(111) (C6), and a NiO(111) layer grown on top of Ni(111). We examine both the scattered ion fragmentation patterns and the kinetic energy distribution spectra (KEDS) as a function of the incident ion energy E. Surface infrared and KEDS data indicate that we have prepared a saturated monolayer of hexanethiolate (C6) on Au(111) where the C6 carbon backbone is predominantly upright on the surface. C6 monolayers with a mixture of prone and upright C6 can also be prepared, but only the upright C6 monolayers are used for ion scattering experiments. The fragment ion distributions and the KEDS are then used to determine the channeling of the incident SiMe3+ion energy into the scattered ion internal energy Eint, and the scattered ion kinetic energy Escat. Overall, we find the order of Eint/E for SiMe3+ to be Au(111)≫NiO(111)>C6. From the Escat values, we find that MiSe3+ scattering off C6 is highly inelastic while scattering off Au is much more elastic. We use this information to calculate the energy (Esurf) which is channeled into the surface as a result of the collision for C6 and Au(111). By comparing the KEDS for the parent and fragment ions, we determine the importance of unimolecular dissociation off the various surfaces. In the accompanying paper, we present a series of classical dynamics simulations to explain and supplement these experimental results.
Vibrational energy relaxation in the condensed phases: Quantum vs classical bath for multiphonon processes
Authors: Egorov S. A. , Berne B. J.
Published: July 2002

We develop a theory for relating quantum and classical time correlation functions in the context of vibrational energy relaxation. The treatment is based on the assumption that both the quantum and the classical systems are characterized by effective harmonic Hamiltonians with identical normal modes; and the solute-solvent interaction is taken to be linear in the solute vibrational coordinate, but nonlinear in the bath coordinates. We propose an approximate “quantum correction” which allows the determination of the quantum energy relaxation rates from the classical force-force time correlation functions in the limit of large solute’s vibrational frequency. We test the accuracy of this approximate correction against exact numerical results for two forms of the solute-solvent interaction (exponential and power law), and find it to be accurate for a wide range of solute vibrational frequencies and for different solvent thermodynamic states. A simple form of the “quantum correction” is proposed for the models based on Lennard-Jones interactions. In all cases it is found that the vibrational relaxation time in a fully quantum system is better approximated by a fully classical theory (classical oscillator in classical bath) than by a mixed quantum-classical theory (quantum oscillator in classical bath).
Slow relaxation in interpenetrated networks
Authors: Chikina J. , Daoud M.
Published: July 2002

Interpenetrated networks are made by cross linking linear polymer chains of two different chemical nature. We consider the kinetics of the microphase separation that occurs when such cross linking is performed on a blend where composition fluctuations are present. We assume a simple exponential relaxation between the known initial and final states. The relaxation time however is assumed to be dependent on the spatial scale, and to diverge for large enough distances. We find that the scattered intensity in a radiation scattering experiment exhibits a minimum that varies slowly with time. This is interpreted as the very slow relaxation of the initial fluctuations that are present in the initial blend and are trapped by the cross links.
Trapping a precursor to chemisorption at 32 K—NH3 adsorption on Cu(110)
Authors: Mocuta D. , Ahner J. , Yates J. T.
Published: July 2002

The adsorption of NH3 onto Cu(110) at 32 K results in the trapping of a metastable state with a distinctive bonding structure compared to chemisorbed NH3. Heating to 55 K causes the metastable NH3 species to convert to the more stable chemisorbed species. The binding energy for the metastable state is estimated to be 0.2 eV, compared to 0.77 eV for the chemisorbed state.
Sub-Doppler study of the v3=2 state of SF6 by infrared–infrared double resonance with a sideband spectrometer
Authors: Khelkhal M. , Rusinek E. , Legrand J. , Herlemont F. , Pierre G.
Published: July 2002

The v3=2 state of SF6 has been investigated using two independently tunable sidebands of a CO2 laser. An electro-optic modulator, based on microstrip concept, is excited by a wave in the range of 0–4 GHz to create a sideband resonant with the cold band transition used as pumping radiation. A second electro-optic modulator working in the 8–18 GHz range is used to monitor the disturbance in the 2ν3−ν3 band. About 250 lines of both the cold band ν3 and the hot band 2ν3–ν3 have been assigned on the basis of a calculated spectrum. Their frequencies have been determined with an accuracy better than 50 kHz. The spectrum is analyzed and the lines are fitted using an effective Hamiltonian developed through the sixth order for the ground state, the seventh order for the v3=1 state and the sixth order for the v3=2 state. The standard deviation is a few tens of kHz for ν3 lines and a few hundreds of kHz for 2ν3–ν3 lines.
Laser excited metastable states of acetylene in the 5.5–5.7 eV region
Authors: Humphrey Susan J. , Morgan Christopher G. , Wodtke Alec M. , Cunningham Kevin L. , Drucker Stephen , Field Robert W.
Published: July 2002

Experimental results are presented which show UV excitation to metastable states of C2H2 above and below the dissociation limit. A strong S1–T3 interaction, located close in energy to the S13ν3 (trans-bend) vibrational level is detected. A “gateway mediated intersystem crossing” singlet–triplet coupling mechanism explains the results.
Photofragmentation of chlorotoluenes and dichlorobenzenes: Substituent effects on the dissociation mechanism, and angular distribution of the Cl fragment
Authors: Ichimura Teijiro , Mori Yuji , Shinohara Hisanori , Nishi Nobuyuki
Published: July 2002

Time-of-flight spectra of the Cl photofragments were measured for molecular beams of o-, m-, and p-chlorotoluene (ClC6H4CH3) and o-, m-, and p-dichlorobenzene (ClC6H4Cl) irradiated by a 193 nm excimer laser pulse. The observed translational energy distributions of photofragments revealed that these chlorinated benzene derivatives dissociate via three different channels: (1) very fast predissociation and/or a direct dissociation, (2) predissociation through the triplet state, and (3) predissociation via highly excited vibrational levels of the ground electronic state (hot molecules). The three dissociation channels for dichlorobenzene have similar probabilities (∼0.3) in accord with those for chlorobenzene, indicating no significant change caused by the additional chlorine atom. The methyl substituent on chlorobenzene (chlorotoluene), however, remarkably induces dissociation through triplet states, probably due to the enhanced intersystem crossing by the methyl group. The angular distribution of the photofragment was also measured for p-chlorotoluene and p-dichlorobenzene excited by linearly polarized laser light. Angular distributions of the Cl fragments via the 2nd and 3rd channels were isotropic, while the fastest fragment via the 1st channel has an anisotropic distribution, confirming that the dissociation rate of the 1st channel is shorter than a molecular rotation.
Ab initio potential-energy surface and rotationally inelastic integral cross sections of the Ar–CH4 complex
Authors: Heijmen Tino G. A. , Korona Tatiana , 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 Ar–CH4 complex. The interaction energy, including high-level intramonomer correlation effects, is found to be dominated by the first-order exchange contribution and the dispersion energy. The ab initio potential has four equivalent minima of εm=−144.30 cm−1 at Rm=7.00 bohr, for structures in which the argon atom approaches the face of the CH4 tetrahedron. The computed potential-energy surface has been analytically fitted and used in converged close-coupling calculations to generate state-to-state integral cross sections for rotational excitation of CH4 in collisions with argon. The computed cross sections are generally in good agreement with the experimental data [W. B. Chapman et al., J. Chem. Phys. 105, 3497 (1996)]. Some discrepancies for the smallest cross sections can be explained by the influence of sequential collision channels, with the use of a master equation approach.
Transient infrared spectrum of vibrationally excited Rh6(CO)16
Authors: Childs Jeffrey , Beckerle John D.
Published: July 2002

The transient IR spectrum of Rh6(CO)16 in CCl4 with one quanta of excitation in the CO-stretch modes has been measured by picosecond time-resolved IR saturation spectroscopy as a function of pump–probe time delay and polarization. At short time delays (<10 ps), the spectra can be consistently deconvoluted into a bleach of the ν6F2 fundamental at 2076.0 cm−1, and two hot-band transitions at 2068.6±0.4 and 2073.8±0.3 cm−1, which we tentatively assign to the A1 and E symmetry overtones of the ν6F2 fundamental. The broader hot-band feature observed at long time delay (tD=600 ps) is assigned to an inhomogeneous superposition of transitions to overtone and combination modes from an equilibrated population of v=1 CO stretch modes. A mean field model of coupled anharmonic oscillators substantially overestimates the magnitude of average frequency shift of the hot band in this intermediate size molecular system. The rapid (<10 ps) decay of the pump-induced dichroism in the sample suggests that vibrational coupling of the degenerate ν6F2 modes proceeds on a time scale similar to or faster than physical rotation of the molecule.
Measurement and analysis of the scattered light in the critical ionic solution of ethylammonium nitrate in n-octanol
Authors: Bonetti M. , Bagnuls C. , Bervillier C.
Published: July 2002

New measurements of the static light scattering as a function of temperature in the critical ionic mixture ethylammonium nitrate+n-octanol are reported. A basic analysis indicates that the system would belong to the Ising universality class. Attempts at characterizing the corrections to the Ising scaling are made. The conclusion is that, within the experimental uncertainty, the system shows a practically pure Ising behavior in the range 10−4⩽(T−Tc)/Tc⩽2×10−2 with, perhaps, a slight trend to display negative corrections (i.e., opposite to a partial crossover toward the classical behavior).
Structural study of liquid methylene chloride with reverse Monte Carlo simulation
Authors: Jedlovszky Pál
Published: July 2002

Reverse Monte Carlo simulation of liquid methylene chloride has been performed using five independent experimental datasets. The structure of the liquid has been analyzed in detail. Predictions for various total and partial structure factors of liquid methylene chloride has been made on the basis of the simulated configurations. Partial pair correlation functions, orientational correlations, and spatial distributions of the nearest neighbors have been analyzed. It is found that the nearest neighbors can be localized in three different positions around the central molecule. Preferences for both parallel and antiparallel orientations were found among these neighbors. The range of the orientational correlations between the molecules was found to be rather large, extending even to the second coordination shell of the molecules.
Eigensolution of a singular operator δ(x)d/dx+d/dxδ(x)
Authors: Park Tae Jun
Published: July 2002

The title operator can often be found in the flux operator which has been first introduced by Miller. From the numerical implementations, the flux operator is known to have such peculiar properties as follows: 1) it has two nonzero eigenvalues of same magnitude with opposite signs and all other eigenvalues being zero; 2) eigenvectors with nonzero eigenvalues are localized. In this work we have analytically solved the eigenvalue equation for the operator in the title whose eigensolutions reveal the properties mentioned above.
Structure and dynamics of the silacyclobutane radical cation, studied by ab initio and density functional theory and electron spin resonance spectroscopy
Authors: Fängström Torbjörn , Lunell Sten , Engels Bernd , Eriksson Leif A. , Shiotani Masaru , Komaguchi Kenji
Published: July 2002

The geometric structure, the proton isotropic hyperfine coupling constants and temperature dependence of the isotropic hyperfine coupling constants have been investigated for the silacyclobutane radical cation using Mo/ller-Plesset perturbation theory to second order, the coupled cluster approximation and density functional theory. The temperature dependence of the hyperfine coupling constants is explained employing a one-dimensional model for the description of the ring puckering motion. For the geometrical structure the study supports conclusions made in earlier work based on the measurements of the temperature dependence of the electron spin resonance (ESR) spectra, e.g. the silacyclobutane radical cation possesses an asymmetrically distorted C1 structure in which one of the Si–C bonds is considerably elongated, and the ring is puckered. A reassignment of part of the ESR spectrum is obtained on the basis of the theoretical results in combination with experimental measurements of the low temperature dynamical behaviour.
A computational study of 13-atom Ne-Ar cluster heat capacities
Authors: Frantz D. D.
Published: July 2002

Heat capacity curves as functions of temperature were calculated using Monte Carlo methods for the series of Ne13−nArn clusters (0⩽n⩽13). The clusters were modeled classically using pairwise additive Lennard-Jones potentials. The J-walking (or jump-walking) method was used to overcome systematic errors due to quasiergodicity. Substantial discrepancies between the J-walking results and those obtained using standard Metropolis methods were found. Results obtained using the atom-exchange method, another Monte Carlo variant for multi-component systems, also did not compare well with the J-walker results. Quench studies were done to investigate the clusters’ potential energy surfaces. Only those Ne-Ar clusters consisting predominately of either one or the other component had lowest energy isomers having the icosahedral-like symmetry typical of homogeneous 13-atom rare gas clusters; non-icosahedral structures dominated the lowest-energy isomers for the other clusters. This resulted in heat capacity curves that were very much different than that of their homogeneous counterpart. Evidence for coexistence behavior different than that seen in homogenous clusters is also presented.