bluuebunny/crossref_metadata_2025 huggingface dataset:
Large scale Monte Carlo simulations of center-adsorbed star polymers
Authors: Shida Kazuhito , Ohno Kaoru , Kimura Masayuki , Kawazoe Yoshiyuki
Published: July 2002

A star-shaped polymer whose center unit is adsorbed on a surface offers a peculiar example of surface-grafted polymers. When it is isolated in a good solvent, it has been conjectured that several distinct scaling relations hold for the monomer and end-point density profiles. Especially, the density decay in a direction parallel to the surface is described by a new critical exponent λ(f) as ρ(r,z=0)∼r−d+λ(f). However, the precise values of the exponent as a function of the number of arms were still unclear. Another interesting quantity is the total number of configurations behaving as N∼lγs(f)−1μfl. Here, l is the length of the arm, μ the effective coordination number for a single chain, and λs(f) a new surface critical exponent yet to be known. We perform large scale Monte Carlo simulations of such an adsorbed star with the number of arms, f, ranging from 2 to 15, to verify the predicted scaling theory and to calculate various static properties and exponents. Estimates of γs(f ) are presented. The validity of the scaling relations is clearly shown, and the first estimation of the value of λ(f ) is given also. Furthermore, an empirical form of the exponent λ(f ) as a function of f is proposed.
Low energy cluster ion–atom collision: Quantum mechanical molecular dynamics simulation of Ar+n+Ar
Authors: Ichihashi Masahiko , Ikegami Tsutomu , Kondow Tamotsu
Published: July 2002

The molecular dynamics method combined with a quantum mechanical calculation was used to simulate the collision of an argon atom with an argon cluster ion, Ar+n (n=3–23), containing a given amount of internal energy. Two processes were observed; (i) evaporation caused by collisional energy transfer to the internal degrees of freedom vs (ii) fusion of the target atom with the cluster ion via complex formation. The total reaction cross sections were compared with those experimentally obtained. It is found that the branching fractions of the evaporation and the fusion depend critically on the impact parameter.
Accurate vertical ionization potentials of CH2, NH2, and H2O using the multiconfigurational spin tensor electron propagator method (MCSTEP)
Authors: Yeager Danny L.
Published: July 2002

The multiconfigurational spin tensor electron propagator method (MCSTEP) is used to determine accurate low-lying vertical principal ionization potentials (IPs) for the free radicals CH2 and NH2 and the closed shell molecule H2O. In previous MCSTEP calculations we had compared MCSTEP IPs with full configuration interaction (FCI) values for CH2 using a double zeta with polarization (DZP) basis set. Those MCSTEP results were in very good agreement with ΔFCI. An alternative complete active space (CAS) choice used in this paper gives even better agreement—differing from ΔFCI values by 0.03 eV and 0.01 eV for the lowest two vertical principal IPs from the 3B1 neutral ground state. These MCSTEP results are extended by also using pVDZ, pVTZ, and pVQZ basis sets at both the previous (FCI) geometry and the experimental geometry. Choosing the CAS in the same manner for NH2 yields very good principal vertical IPs compared with experiment. Comparisons are again made using pVDZ, pVTZ, and pVQZ basis sets. Low-lying principal vertical MCSTEP IPs are obtained for H2O and compared with recent ΔFCI values using the same basis set and geometry as the FCI calculations. Low-lying principal vertical MCSTEP IPs are also obtained using pVDZ, pVTZ, and pVQZ basis sets at the experimental geometry.
Avoiding long propagation times in wave packet calculations on scattering with resonances: A hybrid approach involving the Lanczos method
Authors: Kroes Geert-Jan , Neuhauser Daniel
Published: July 2002

We investigate the usefulness of a hybrid method for scattering with resonances. Wave packet propagation is used to obtain the time-dependent wave function Ψ(t) up to some time T at which direct scattering is over. Next, Ψ(t) is extrapolated beyond T employing resonance eigenvalues and eigenfunctions obtained in a Lanczos procedure, using Ψ(T) as starting vector to achieve faster convergence. The method is tested on one two-dimensional (2D) and one four-dimensional (4D) reactive scattering problem, affected by resonances of widths 0.1–5 meV. Compared to long time wave packet propagation, the hybrid method allows large reductions in the number of Hamiltonian operations NH required for obtaining converged reaction probabilities: A reduction factor of 24 was achieved for the 2D problem, and a factor of 6 for the 4D problem.
Quantum analysis of absolute collision-induced scattering spectra from bound, metastable and free Ar diatoms
Authors: Chrysos Michael , Gaye Omar , Le Duff Yves
Published: July 2002

Motivated by recent experimental measurements on the low-frequency binary (Ar)2 depolarized spectrum, a fully quantum frequency-resolved analysis of spectral intensities from bound, metastable and free argon diatoms is performed in collision-induced light scattering. The methodology is based on an analytical discrete variable representation (DVR) allowing the reliable and systematic computation of all bound and resonance state energies and wave functions as well as of the discretized low-energy continuum associated to any rotational level.
The photodissociation of ClO2: Potential energy surfaces of OClO→Cl+O2
Authors: Peterson Kirk A. , Werner Hans-Joachim
Published: July 2002

Using large multireference configuration interaction wave functions, potential energy surfaces involved in the photodissociation of symmetric ClO2 to Cl+O2 are investigated. The production of atomic chlorine from OClO, which may have important implications for stratospheric ozone chemistry, is predicted to occur via the excited 1 2B2 electronic state after initial excitation to the A 2A2 state. A calculated C2v transition state connecting 1 2B2 OClO to Cl+O2 is strongly bent and has a barrier height relative to the X 2B1 ground state of 2.86 eV (2.75 eV with zero-point vibrational corrections). However, this is only a 2nd-order transition state with imaginary vibrational frequencies along both the OClO→Cl+O2 and OClO→ClO+O reaction paths (symmetric bending and asymmetric stretching modes, respectively). Thus, the present theoretical work suggests that only a small amount of Cl+O2 will be formed in the photodissociation of ClO2 due to the dominance of the ClO+O channel. Much of the O2 that is produced is predicted to be in the a 1Δg state, since the 1 2B2 potential energy surface in C2v symmetry correlates with this state of O2. However, other nearby electronic states of OClO, namely the 1 2A1 and 2 2B2, interact in the exit channel and will facilitate the production of especially X 3Σ−g and perhaps b 1Σ+g O2, respectively. The present results are in very good accord with the recent photofragment translational energy spectroscopy experiments of Davis and Lee [J. Chem. Phys. 105, 8142 (1996)].
Gaussian basis density functional theory for systems periodic in two or three dimensions: Energy and forces
Authors: Jaffe John E. , Hess Anthony C.
Published: July 2002

We describe a formulation of electronic density functional theory using localized Gaussian basis functions for systems periodic in three dimensions (bulk crystals) or two dimensions (crystal slabs terminated by surfaces). Our approach generalizes many features of molecular density functional methods to periodic systems, including the use of an auxiliary Gaussian basis set to represent the charge density, and analytic gradients with respect to nuclear coordinates. Existing quantum chemistry routines for analytic and numerical integration over basis functions can be adapted to our scheme with only slight modifications, as can existing extended Gaussian basis sets. Such basis sets permit accurate calculations with far fewer basis functions (and hence much smaller matrices to diagonalize) than plane-wave based methods, especially in surface calculations, where in our approach the slab does not have to repeat periodically normal to the surface. Realistic treatment of molecule–surface interactions is facilitated since both molecule and surface can be treated at the same level of theory. Our real-space method also offers opportunities to exploit matrix sparsity, since in a large unit cell many pairs of basis functions will be essentially nonoverlapping and noninteracting. Longer-ranged Coulomb interactions are summed by a form of the Ewald technique that guarantees absolute convergence. We also give a straightforward extension to periodic systems (both two- and three-dimensional) of the usual molecular formalism for analytic nuclear first derivatives (forces).
Dynamics of coarse-grained helical wormlike chains. II. Eigenvalue problems
Authors: Yoshizaki Takenao , Osa Masashi , Yamakawa Hiromi
Published: July 2002

The eigenvalue problem for the matrix representation of the diffusion operator appearing in the diffusion equation previously derived for the coarse-grained helical wormlike (HW) chain model is considered. Following the procedure already established in the study of the original diffusion equation for the discrete HW chain, the problem is solved in the subspace and block-diagonal approximations in the subspace L(1), where L is the ‘‘total angular momentum quantum number’’ and (1) indicates the one-body excitation basis set. The 1(1) eigenvalues λ1,kj thus obtained form three branches of the eigenvalue spectrum specified by the index j as before, and it is shown that λ1,k0 in the j=0 (lowest) branch at small wave number k is approximately proportional to the corresponding Rouse–Zimm eigenvalue in the Hearst version. The behavior of the eigenvalue spectra is also numerically examined taking atactic polystyrene with the fraction of racemic diads fr=0.59 and atactic poly(methyl methacrylate) with fr=0.79 as typical examples of flexible polymers. It is found that the ratio λ1,k0/λ1,10 at small k depends somewhat (appreciably for large k) on chain stiffness and local chain conformation.
Nuclear quadrupole resonance line shape study of the orientationally disordered phase of p-chloro nitrobenzene
Authors: González Cecilia E. , Pusiol Daniel J.
Published: July 2002

An experimental study of the nuclear quadrupole resonance (NQR) line shape of p-chloro nitrobenzene in the temperature range 77–150 K is reported. The molecules of p-ClC6H4NO2 in the solid phase are arranged following an orientationally disordered pattern. The experimental NQR spectra at several temperatures are unusually broad for a molecular crystal and show a structure of peaks. Our measurements suggested that the line broadening is due to a static distribution of disordered molecules. In order to explain the nontypical resonance frequency dispersion, a simulation of the NQR line shapes is carried on. The model for the simulated crystal assumes that each molecule is an electric dipole and that the crystal is a disordered binary alloy of dipoles having two possible orientations. By calculating the intermolecular contribution to the electric field gradient (EFG) at the resonant nuclei sites, we get the expected EFG distribution in the sample. The approach we propose here explicitly involves the symmetry of the lattice.
An ab initio analytical potential energy surface for the O(3P)+CS(X 1Σ+)→CO(X 1Σ+)+S(3P) reaction useful for kinetic and dynamical studies
Authors: González Miguel , Hijazo J. , Novoa J. J. , Sayós R.
Published: July 2002

A total of 816 ab initio points at the PUMP4/6-311G(2d) level were used to derive an analytical expression for the lowest 3A′ adiabatic potential energy surface (PES) of the reaction O(3P)+CS(X 1Σ+)→CO(X 1Σ+)+S(3P). Thermal rate constants calculated using the variational transition state theory and semiclassical tunneling correction were used as a tool to determine the optimum analytical surface. This was done by comparing the calculated rate constant at room and lower temperatures with the experimental values. The best analytical surface (PES 3) reproduces the rate constant at low temperatures well. However, it has not been possible to obtain an analytical PES capable of reproducing both the rate constant at 300 K and the activation energy (150–300 K range). At higher temperatures, the contribution of the lowest 3A″ adiabatic potential energy surface to the rate constant seems to be important to reproduce the experimental data. At present, the PES 3 is the most suitable analytical surface to be used for kinetic and dynamical single surface studies.
One- and two-color resonance-enhanced multiphoton ionization spectroscopy of the Kr⋅NO complex via the Ã 2Σ+ state
Authors: Bush Andrew M. , Dyke John M. , Mack Paul , Smith David M. , Wright Timothy G.
Published: July 2002

A structured spectrum of the Ã state of the Kr⋅NO complex has been obtained for the first time, employing the techniques of one-color (1+1) and two-color (1+1′) resonance-enhanced multiphoton ionization spectroscopy. The T0 value of the Ã←X̃ transition is 44 201±1 cm−1, and the spectrum allows estimates to be made of the dissociation energies (D0) of both the X̃ and Ã states of ∼110 cm−1. Of significance is that the spectrum appears in both the Kr⋅NO+ mass channel and (when high photon intensities are used) in the Kr+ mass channel. Non-Rydberg behavior of the Ã states of Ar⋅NO and Kr⋅NO is noted.
Characterization of CH stretches on diamond C(111) single- and nanocrystal surfaces by infrared absorption spectroscopy
Authors: Cheng C.-L. , Lin J.-C. , Chang H.-C. , Wang J.-K.
Published: July 2002

The stretching motions of CH on diamond C(111) single- and nanocrystal surfaces have been investigated using Fourier transform infrared spectroscopy. The observations at 800 K indicate a band center of 2835.7 cm−1 and a heterogeneous broadening of 3 cm−1 for the monohydride CH stretch on ideally hydrogen-terminated C(111)-1×1 surfaces at 0 K.
Geometric dephasing in zero-field magnetic resonance
Authors: Jones J. A. , Pines A.
Published: July 2002

Geometric phases acquired randomly can give rise to coherence dephasing in nuclear spin systems, equivalent to spin relaxation. We calculate the form and extent of this geometric dephasing in a number of model systems involving the motion of 131Xe nuclei in shaped containers. The dephasing is calculated in two ways: first, using an analytical treatment of the diffusive motion of individual nuclei, and second, using ensemble averaged propagators. The effects of applying additional magnetic fields to these systems are discussed briefly.
Structure, binding energy, and intermolecular modes for the aniline/ammonia van der Waals clusters
Authors: Fernandez J. A. , Bernstein E. R.
Published: July 2002

Aniline(ammonia)n, n=1,2,3, formed in a supersonic jet are investigated spectroscopically, using two-color mass resolved excitation spectroscopy (MRES), hole burning spectroscopy (HB), and by model potential energy calculations. The large shift in the cluster origin (ca. 875 cm−1 for n=1) with respect to the aniline bare molecule origin, and the extent (about 1500 cm−1 for the one-to-one cluster) of the aniline cluster spectra indicate strong interactions between the solute and solvent molecules in both S0 and S1 electronic states. A Lennard-Jones–Coulomb 6-12-1 potential is used to model the intermolecular interactions and predict minimum energy cluster geometries, binding energies, and van der Waals modes. Comparison between experimental results and calculations shows the importance of hydrogen bonding interactions for cluster properties. A minimum energy structure for the cluster is proposed based upon this comparison.
Model study of near-threshold photoionization of large molecules: The effect of vibrational relaxation
Authors: Thoss Michael , Domcke Wolfgang
Published: July 2002

The influence of intramolecular vibrational relaxation in large molecules on near-threshold photoabsorption and photoionization processes is investigated. The vibrational relaxation results from coupling of the initially excited modes to a large number of inactive bath modes. Starting from a model Hamiltonian including all vibrational modes, the bath degrees of freedom are eliminated within the Markov approximation using Hilbert-space projection-operator techniques. Additional Feshbach projection techniques and a threshold expansion of Coulomb Green’s function are used to cast the resulting expressions into a numerically tractable form. Predissociation channels are included in a phenomenological manner. The numerical results allow us to study the characteristic effects of vibrational relaxation on absorption and ionization spectra. In particular, the competition between autoionization, predissociation and vibrational relaxation of the ion core is investigated. The suppression of the ionization quantum yield above the lowest ionization threshold as a result of these two decay channels is demonstrated. It is shown that the quenching of autoionization by vibrational relaxation or predissociation can be experimentally distinguished on account of the different scaling behaviour of both processes.
Kinetics of superstructure formation in block copolymers
Authors: Vakulenko S. , Vilesov A. , Stühn B. , Frenkel S.
Published: July 2002

The kinetics of order formation in symmetric diblock copolymers is investigated using a nonlinear dynamic equation. The equilibrium solution is the periodic pattern of the ordered state. The theory describes both the regime of weak and strong segregation. In the latter case we find structure formation to proceed on two time scales: a rapid formation of kinks in the concentration profile is followed by a slow ordering of the kinks on to a periodic lattice. Explicit formulae for the temperature dependence of the relaxation times are derived. The results are found to agree qualitatively with experimental observations.
The effects of triplet perturbers on photophysical processes in the Ã1Au state of acetylene
Authors: Drucker Stephen , O’Brien Jonathan P. , Patel Paresh , Field Robert W.
Published: July 2002

Rotationally resolved excitation spectra of the acetylene Ã 1Au←X̃ 1∑g+ transition were recorded by measuring separately the total emission in two spectral regions: UV (200–400 nm) and NIR (0.9–1.5 μm). UV emission is due to Ã→X̃ transitions. The source of the NIR signal is suggested by the observation that the NIR:UV intensity ratio is approximately constant throughout the V02K01 subband. (V denotes the trans-bending mode.) This is consistent with C2H Ã 2Π→X̃ 2∑+ NIR transitions occurring as a result of resonance-enhanced two-photon excitation of predissociated Rydberg states. The V03K01 subband shows significant fluctuations in the NIR:UV intensity ratio. The increase in trans-bending excitation from 2ν3′ to 3ν3′ enables accidental resonances with background states (both triplet and S1) to alter the NIR emission intensity. Triplet perturbers can facilitate Tn (n>1)→T1 NIR transitions or modify the resonance enhancement of C2H Ã̃-state production. Extra lines in the V03K01 subband, due to an S1 anharmonic perturber, show dramatically diminished NIR but enhanced UV emission intensity. This indicates that the perturbing state is anomalously resistant to photodissociation.
Exciton effects in the nonlinear optical susceptibilities of molecular crystals
Authors: Munn R. W.
Published: July 2002

The frequency dependence of quadratic and cubic susceptibilities given by the conventional expressions for molecular crystals is analyzed for resonances (poles) in the light of the dielectric theory of excitons. Exciton frequencies correspond to poles in the local-field tensors; in the nonlinear susceptibilities the poles in these tensors cancel out the poles that occur in the hyperpolarizability where the output and input frequencies coincide with molecular excitation frequencies, and replace them by poles at the exciton frequencies, including wavevector dependence and any Davydov splitting. In the cubic susceptibility, there are additional poles, through cascading, where the sum of any two input frequencies coincides with an exciton frequency.
Quantum dynamics of a model system with a conical intersection
Authors: Ferretti Alessandro , Lami Alessandro , Villani Giovanni
Published: July 2002

We study the quantum dynamics of a two-electronic state model system with a conical intersection involving two vibrational degrees of freedom, x and y. The two diabatic surfaces are displaced only along x, while the interstate coupling is linear in y and contains Gaussian damping factors along x and y. The role of the initial momentum and of the difference of the slopes at the conical intersection is discussed. In particular, the analysis of the numerical results shows that the diabatic transition at the conical intersection is accompanied mainly by a massive flow of vibrational quanta from the x to the y oscillator of the same electronic state. Furthermore, we propose a simplified time-dependent monodimensional model in which the x oscillator is assumed to move as an independent classical oscillator, while the y oscillator is described quantum-mechanically. Despite the crude approximations involved, this simplified model reproduces quite well the exact results for the transition probabilities at the first passage through the conical intersection and candidates itself as a useful tool for treating multidimensional systems with low computational effort.
A Gibbs ensemble Monte Carlo study of phase coexistence in model C60
Authors: Caccamo C. , Costa D. , Fucile A.
Published: July 2002

Extensive Gibbs ensemble Monte Carlo (GEMC) simulations of a rigid molecule model of C60, characterized by a deeply attractive short-ranged interaction potential, are performed with the aim to establish the effect of the system size on the existence and location of the liquid–vapor binodal line and of its critical point. The results obtained with N=300, 600, and 1500 particles indicate that the position and the overall shape of the binodal is only minorly influenced by finite size effects. The estimated critical temperature and density at the different N fall in the ranges 1920–1940 K, and 0.4–0.45 nm−3, respectively. The results are discussed by making reference to previous studies of finite size effects in GEMC simulations. The GEMC predictions are also compared with previous computer simulation and theoretical calculations for the same model fluid. The agreement is on the whole satisfactory for both the liquid–vapor coexistence line and the critical point parameters. On the basis of previously determined freezing lines of C60, and of the actual binodal line, different estimates of the location of the triple point are also made. Triple point temperatures are found, in any case, definitely lower (by at least 150 K) than the critical temperatures, thus confirming the existence of a relatively narrow liquid phase region in the phase diagram, as predicted in previous molecular dynamics and theoretical works. The existence of such a liquid phase for the adopted model potential is discussed and assessed in the more general framework of liquid–vapor coexistence conditions in fluids interacting through short-ranged forces. The possibility to get the liquid phase of “real life” C60, hitherto not observed experimentally, is also discussed in connection with recent high temperature experimental results on fullerite samples.
Rotational spectrum of the Ar–HCO+ ionic complex
Authors: Ohshima Yasuhiro , Sumiyoshi Yoshihiro , Endo Yasuki
Published: July 2002

Rotational spectra of four isotopomers of Ar–HCO+ have been studied in a jet-cooled condition by using a Fabry–Perot-type Fourier-transform microwave spectrometer. The ionic complex has a linear proton-bound form, with approximate intermolecular stretching and bending frequencies of 130 cm−1 and 110 cm−1, respectively.
A molecular theory for surface forces adhesion measurements
Authors: Frink Laura J. Douglas , van Swol Frank
Published: July 2002

Surface forces have been measured by others in undersaturated vapors to determine the adhesive energy of the solid (mica) as well as to probe the limits of the Laplace-Kelvin equation in micropores. The measured pull-off forces are complex requiring an intimate understanding of the underlying oscillatory solvation forces, adsorption, and surface deformation. While the elastic energy of the solid has been taken into account in previous theoretical studies of adhesion, the Laplace-Kelvin assumption of a uniform bulk-like fluid has always been applied. In this paper we present the first application of a modern molecular theory—a nonlocal density functional theory—to the prediction of pull-off forces with the surface forces apparatus. In this theory, the confined fluid is allowed to be nonuniform, and oscillatory solvation forces may be predicted. For rigid surfaces, it is demonstrated that the separation of forces most often used to analyze the surface forces apparatus measurements is highly accurate only when adsorption is properly treated and when the relative pressure is p/po>0.2−0.4. The limiting value of the relative pressure decreases as the strength of the fluid-surface interaction increases. In addition, the range over which the vacuum limit of the solid surface free energy, γs may be measured is strongly dependent on the strength of various molecular interactions. We predict, as observed in experiments, that the saturation limit of the pull-off force is given by the Laplace pressure alone if there are at least two fluid layers between the surfaces. Finally, we show that using pull-off forces to test the limits of the Laplace-Kelvin theory is misleading because the measurements by design minimize solid-liquid contributions to the total force.
First observation and electronic spectroscopy of chromium mononitride: The A 4Πr←X 4Σ− transition near 745 nm
Authors: Balfour Walter J. , Qian Charles X. W. , Zhou Chi
Published: July 2002

We report the first gas phase spectroscopic study of chromium mononitride. CrN molecules were generated in our laser vaporization molecular beam source and studied using laser induced fluorescence techniques. Twelve vibronic subbands of Cr14N have been rotationally analyzed in the 745–647 nm region. These bands are assigned to the A 4Πr←X 4Σ− transition. The assigned ground state electron configuration 1δ29σ1 and symmetry are identical to those of the isovalent VO, MoN, and WN molecules. The energy observed for the excited 4Πr state matches closely the first 4Π state energy derived from a recent ab initio calculation [J. F. Harrison, J. Phys. Chem. 100, 3513 (1996)]. In addition to the A-X transition, 11 other bands of Cr14N have been identified and vibrationally assigned to transitions involving two other excited states, presumably the B4Σ−, and a 2Π state. Comparisons with the isovalent VO, MoN, and WN systems have provided more insights into the bonding and electronic structure of the CrN molecule.
Photoinduced Rydberg ionization spectroscopy of phenol: The structure and assignment of the B̃-state of the cation
Authors: LeClaire J. E. , Anand R. , Johnson P. M.
Published: July 2002

The newly developed technique of photoinduced Rydberg ionization (PIRI) spectroscopy has been successfully applied to study the B̃←X̃ transition in the phenol and phenol-d6 cations. Vibrationally resolved spectra have been obtained for the B̃ state in phenol ion via the origin and the ν6, and ν12 vibrations of the ground ionic state. Similarly, vibrationally resolved spectra for the B̃ state in phenol-d6 ion have been obtained via the origin and ν6 vibration. Calculations to date have suggested the character of the half-filled orbital is π type, and experimental evidence for the B̃-state assignment so far has been inconclusive. In contrast to previous featureless photoelectron spectra, the main feature in all of the spectra presented here is the presence of several long, low frequency Frank–Condon progressions, suggestive of a large geometry change in the transition. Configuration interaction singles 6-31G* calculations, allowing full geometry optimization, show that the first excited σ state has the OH group rotated 90° from the planar ground state. Therefore, the symmetry for the B̃ state in phenol cation is assigned to be 2pσ instead of π, corresponding to that of benzene and several other monosubstituted benzenes. Further support for this assignment is found in a calculation of the normal mode vibrations, based on the geometry optimized for the excited σ state. These show three low frequency normal modes having a large amount of OH torsion, one of which has a ring motion identical to one of the two normal modes that induces the B̃←X̃ transition in the benzene cation. This calculated normal mode is, therefore, assigned to the most intense and most extensive progression observed in the photoinduced Rydberg ionization spectra.
A Chebyshev method for calculating state-to-state reaction probabilities from the time-independent wavepacket reactant-product decoupling equations
Authors: Althorpe Stuart C. , Kouri Donald J. , Hoffman David K.
Published: July 2002

Recently, Peng and Zhang have introduced the reactant-product decoupling (RPD) equations. These are an exact formulation of quantum mechanical reactive-scattering, whereby the Schrödinger equation is partitioned into a set of uncoupled equations, each of which describes the dynamics in one arrangement of the reaction. In this paper we derive an efficient method for solving the RPD equations which is based on the Chebyshev propagator. The derivation makes use of the recently derived time-independent wavepacket version of the RPD equations. We test the method by applying it to the collinear H+H2 reaction.
Theoretical study of vibrational overtone spectroscopy and dynamics of methanol
Authors: Halonen Lauri
Published: July 2002

Experimentally observed coupling between OH and CH stretching modes in the high overtone region has been modeled successfully in terms of a curvilinear internal coordinate Hamiltonian including harmonic coupling between anharmonic OH and CH stretching oscillators and cubic Fermi resonance kinetic and potential energy couplings between CH stretches and HCH bends. The Hamiltonian matrices have been set up in block diagonal forms including only resonant states. The potential-energy parameters have been optimized by the least-squares method using experimental vibrational term values as data. The OH/CH stretch interaction parameter obtained agrees well with the one calculated by perturbation theory from a published ab initio harmonic force field. The model has reproduced well experimental band origins in the OH stretching overtone region, and it has provided assignments for the bending overtones in the CH stretching fundamental region. Finally, a unitary transformation is found from the internal coordinate representation to the corresponding normal coordinate representation providing a set of normal coordinate parameters like diagonal anharmonicity parameters, Darling–Dennison resonance constants and cubic Fermi resonance force constants. The results confirm the experimental finding of energy redistribution between the OH and CH stretching modes on subnanosecond time scale at 5νOH.
Compositional control of rovibrational wave packets in the E(1Σg+) “shelf” state of Li2 via quantum-state-resolved intermediate state selection
Authors: Williams Richard M. , Papanikolas John M. , Rathje Jacob , Leone Stephen R.
Published: July 2002

Compositional control in the preparation of rovibrational wave packets is demonstrated in the E(1Σg+) state of gas-phase Li2 molecules using ultrafast pump–probe laser spectroscopy combined with quantum-state-resolved intermediate state selection. The intermediate state, from which subsequent ultrafast excitation occurs, is a stationary rovibrational level in the A(1Σu+) state of Li2, produced by cw laser excitation from the ground X(1Σg+) state. The effect that the intermediate state has on the final composition of the wave packet is investigated by comparing the transients resulting from ultrafast pump–probe excitation of two different intermediate states (vA=14, JA=18 versus vA=13, JA=18). In these experiments the pump wavelength is compensated so that in each case the same E-state eigenstates (vE=13–18, JE=JA±1) make up the wave packet, but with different amplitudes. Theory predicts, and experiments confirm, that the relative amplitudes of the rovibrational eigenstates are strongly dependent upon the intermediate state and determine the spatial and temporal evolution of the wave packet. Evidence for this includes differences in the observed pump–probe transients and dramatically different amplitudes of the beat frequencies in the Fourier analysis of the time-domain transients. Theoretical three-dimensional wave packet simulations highlight how the composition of the wave packet is used to vary its spatial and temporal evolution.
Electric field-gradient contributions to the chemical shifts of liquid water
Authors: Nymand Thomas M. , Åstrand Per-Olof
Published: July 2002

Contributions to the gas-to-liquid chemical shifts of water arising from the electric field gradient of the surrounding molecules have been calculated as a function of the temperature. Since the theoretical model is based on perturbation theory, this part of the chemical shift may be calculated from quadrupole shielding polarizabilities and statistical mechanical ensemble averages of external electric field-gradients. The contributions from the electric field gradients are found to be substantial and are calculated to −9.7 ppm for the oxygen shift and 0.7 ppm for the proton shift at room temperature.
Surface reactions driven by cluster impact: Oxidation of Si(111) by (O2)n+ (n∼1600)
Authors: Klopcic Stephan A. , Jarrold Martin F.
Published: July 2002

The oxidation of room temperature Si(111) by oxygen clusters (O2)n+ (n∼1600) at impact energies of 1.5, 3.0, and 4.5 keV has been examined by x-ray photoelectron spectroscopy. The impact of an oxygen cluster on clean Si(111) at these energies results in the oxidation of an area approximately equal to the cross sectional area of the cluster. Both suboxide and SiO2 are formed. The amount of oxide produced increases with the impact energy. Further exposure of the oxidized surface to the impact of oxygen clusters results primarily in an increase in the amount of SiO2. The number of silicon atoms oxidized per cluster impact on an oxidized surface is substantially less than for clean Si(111) and shows a strong dependence on the impact energy.
p–T
diagrams of the system of CH3(CH2)n−1 self-assembled on the Au(111) crystal surface
Authors: Sadreev Almas F. , Sukhinin Yurii V.
Published: July 2002

Equilibrium states of the system of self-assembled monolayers (SAM’s) of n-alkanethiol molecules CH3(CH2)n−1 chemiabsorbed on the Au(111) crystal surface are considered in relation to temperature and external pressure applied normally to the surface. Couplings between the atoms (C, H) of the n-alkanethiols are approximated both by the Morse potential and by the Lennard-Jones one. Couplings between the n-alkanethiols and the crystal surface are approximated by the 12-3 potential. Because of the symmetry Z2 of the n-alkanethiols in the tilted state the system is reduced to the Ising model on the triangular lattice with two competing exchange parameters. Calculated p–T diagrams of the tilted system include the following phases: para, ferro, incommensurate, and structure 2×1. The incommensurate phase results from competition between the exchange parameters. It is shown that for some specific choices of the coupling constants the p–T diagram has tricritical points. The temperatures of phase transitions and equilibrium tilt, twist, and azimuthal angles depending on the coupling constants are found. All phase transitions are of the first order. Temperature behavior of heat capacity is calculated.
Angle-resolved photoelectron and photoion study on the shape resonance in the B 1s ionization region of BF3
Authors: Shimizu Y. , Ueda K. , Chiba H. , Ohmori K. , Okunishi M. , Sato Y. , Hayaishi T.
Published: July 2002

Angle resolved photoelectron and photoion spectroscopy has been used to investigate the e′ shape resonance just above the B 1s ionization threshold of BF3. The B 1s photoelectron asymmetry parameter β is ∼0 in the vicinity of the peak of the shape resonance and increases with increase in photon energy, illustrating good agreement with a theoretical prediction. From the angular distribution measurement of energetic F+ fragments following B 1s photoionization, we have decomposed the photoionization cross section into two components σe′ and σa2″ for the B 1s→e′ parallel and B 1s→a2″ perpendicular transitions and confirmed that the shape resonance is in the e′ symmetry component.
Symmetries and dissociation dynamics of F 1s core excited states of BF3
Authors: Shimizu Y. , Ueda K. , Chiba H. , Okunishi M. , Ohmori K. , West J. B. , Sato Y. , Hayaishi T.
Published: July 2002

Using the photoelectron–energy-selected photoion coincidence technique, we find that photoions of BF3 with kinetic energy larger than 4 eV are predominantly F+ in the F 1s ionization region whereas B+ is more intense than F+ at the F 1s→2a2″ excitation. Two partial cross sections σ‖ and σ⊥ for A1′→E′ parallel and A1′→A2″ perpendicular transitions are obtained from the angular distribution measurement of energetic photoions and the symmetries of some F 1s core excited states are confirmed. The dissociation dynamics of the F 1s−12a2″ core excited state are discussed in detail.
Ab initio calculations on the electronic states of GaAr and GaAr+
Authors: Park Su Jin , Kim Myeong Cheol , Lee Yoon Sup , Jeung Gwang-Hi
Published: July 2002

The 1-2 2Π1/2, 1-2 2Π3/2, and 1-3 2Σ1/2+ states of GaAr and the 1 1Σ+ and 1 3Σ− states of GaAr+ are calculated by ab initio methods. Spectroscopic properties for these states are compared with experimental data, and the existence of potential barrier for the Rydberg Σ1/22 states is explained. We have also proved the nonbonding character of the 1 2Π1/2(X) state of GaAr. The bond energy of the doubly excited 1 3Σ− state, 12 557 cm−1, is much larger than that of the ground state (X 1Σ+) of GaAr+, 584 cm−1, which in turn is more strongly bound than the neutral species (70 cm−1 for the ground state, 340 cm−1 for the 2 2Π3/2 state). The nonbonding character of the X 2Π1/2 state of GaAr can be attributed to the spin–orbit interaction. Transition properties between these states are also calculated.
Near IR photoluminescence in mixed films of conjugated polymers and fullerenes
Authors: Hasharoni K. , Keshavarz-K. M. , Sastre A. , González R. , Bellavia-Lund C. , Greenwald Y. , Swager T. , Wudl F. , Heeger A. J.
Published: July 2002

The photoinduced electron transfer between conjugated polymers and a series of functionalized fullerenes was studied. A new photoluminescence signal was observed in the near IR (∼1.4 eV). This weak IR photoluminescence does not result from direct excitation of the fullerene, but from radiative electron-hole recombination between the fullerene excited state and the polymer ground state. The intensity of this recombination luminescence depends on the electrochemical nature of the functional group; it is observed only for fullerenes with first reduction potential higher than that of C60.
Is laser heating advantageous for thermal desorption of large polar molecules?
Authors: Handschuh Martin , Nettesheim Stefan , Zenobi Renato
Published: July 2002

A systematic study on the desorption and dissociation kinetics of ethylene glycol and poly-(ethylene glycol) oligomers (PEG) with masses ranging from 62 to 35 000 Da from a silica surface was undertaken. The measurements include characterization of the PEG-silica adsorption system using temperature programmed desorption, investigation of the surface coverage, and chemical composition during slow surface heating by real time x-ray photoelectron spectroscopy, and determination of translational energy distributions following nanosecond CO2 laser desorption. We found that even for heating rates exceeding 109 K/s desorption can be described by a thermal model. Intact desorption was limited to PEGs with masses ⩽1000 Da for classical as well as for laser heating.
State selected unimolecular dissociation of HOCl
Authors: Barnes Rhett James , Sinha Amitabha
Published: July 2002

The unimolecular decomposition of HOCl is investigated by exciting the molecule to the region of the sixth overtone of its OH stretching vibration (7νOH) using overtone–overtone double resonance. The excitation scheme is sufficiently selective to allow preparation of a single angular momentum quantum state within the 7νOH vibrational manifold lying ∼2500 cm−1 above the dissociation limit. From the measured linewidths associated with the rotational features appearing in the action spectrum, we obtain an upper limit estimate for the dissociation rate of kuni<9.4×109 s−1 which is substantially slower than that expected on the basis of RRKM theory. The nascent OH fragment product state distribution exhibit strong oscillations which depend on the rotational quantum numbers of the parent HOCl molecule. From the measured energy release associated with the OH fragment, the heat of formation of HOCl is estimated to be ΔH00(0 K)=−16.7±0.6 kcal/mol.
Photodissociation spectroscopy of Mg2CH4+
Authors: Cheng Y. C. , Chen J. , Kleiber P. D. , Young M. A.
Published: July 2002

We have studied the dissociation dynamics of Mg2CH4+ ion–molecule clusters through mass-resolved photodissociation spectroscopy, coupled with translational energy spectroscopy. We have observed distinct molecular absorption bands in the red (690–615 nm) and green (580–545 nm) spectral regions. Mg+ is the dominant fragmentation product in each band. We observe a significant energy release into relative translation and a pronounced photofragment anisotropy (β>0), consistent with a rapid dissociation. Based on the observed anisotropy and the result of an ab initio structure calculation, we assign the red band to the parallel transition 2 2A′←1 2A′ and the green band to a combination of 1 2A′′←1 2A′ and 3 2A′←1 2A′ transitions, all in Cs symmetry. These results are compared with earlier results from the photodissociation spectroscopy of the more strongly bound Mg2CO2+ and Mg2H2O+ bimolecular complexes.
Wigner spectrogram representations of heterodyne-detected four-wave-mixing and fluorescence up-conversion
Authors: Mukamel Shaul
Published: July 2002

Gated spontaneous emission and four-wave-mixing signals are expressed using a mixed time-frequency representation of the fields (Wigner spectrograms) and of the material response functions. Well-separated and overlapping pulses are described using two-sided (noncausal) and one-sided (causal) spectrograms, respectively. Pump–probe and fluorescence spectra are recast in an anologous form which facilitates the direct comparison of the underlying microscopic dynamics.
Nonlinear dynamics of torsion–rotation interactions: A model study of toluene
Authors: Vivian James T. , Lehn Scott A. , Frederick John H.
Published: July 2002

The internal rotation, or torsion, of a methyl group has been implicated in the acceleration of intramolecular vibrational redistribution (IVR) in numerous experimental studies. In the present work, we investigate its interaction with overall molecular rotation. To isolate the effects of torsion–rotation coupling, a simple two-degree-of-freedom model, including only torsion and three-dimensional rotation, is constructed and its dynamics at j=45 for several energies are studied. Investigation of other values of angular momentum indicate that the results reported are largely independent of j. Two primary effects are observed: (i) a shifting of the stable and unstable axes of rotation due to free methyl torsion, and (ii) a limited degree of weakly chaotic dynamics for trajectories whose torsional energy is near the top of its barrier. Chaos is first observed at the lowest energy at which torsion can surmount its barrier, but then disappears from the system at higher energies. Model toluene exhibits only narrow, high-order nonlinear resonances due to the frequency disparity between torsion and rotation, and these resonances are reduced in number by “selection rules” that arise in part from the symmetry of the Hamiltonian. These observations are analyzed in depth and simple, but quantitative, models are proposed to explain the magnitude of the rotational axis shift, the limited extent of chaos, and the resonance selection rules.
Ab initio diffusional potential energy surface for CO chemisorption on Pd{110} at high coverage: Coupled translation and rotation
Authors: Hu P. , King D. A. , Crampin S. , Lee M.-H. , Payne M. C.
Published: July 2002

The ground state potential energy surface for CO chemisorption across Pd{110} has been calculated using density functional theory with gradient corrections at monolayer coverage. The most stable site corresponds well with the experimental adsorption heat, and it is found that the strength of binding to sites is in the following order: pseudo-short-bridge>atop>long-bridge>hollow. Pathways and transition states for CO surface diffusion, involving a correlation between translation and orientation, are proposed and discussed.
A stochastic cage model for linear solutes
Authors: Moro Giorgio J. , Polimeno Antonino
Published: July 2002

A stochastic cage model describing a linear solute reorienting under the action of a librational potential due to neighboring solvent molecules is presented. The fluctuations of the cage structure are taken into account by means of a suitable distribution of the librational frequency. Moreover, a detailed description of the cage dynamics is introduced by considering both the cage rotation and its restructuring through randomizations of the librational frequency and of the equilibrium orientation of the solute. With a suitable choice of the basis functions for the representation of the time evolution operator, the cage model can be solved numerically in order to compute different types of dynamical observables: angular momentum and orientational correlation functions, frequency dependent dielectric permittivity, and far-infrared spectra. Typical behavior of such observables in normal liquids is recovered from the cage model, thus demonstrating its capability of describing experimental observations at quite different time scales.
Methanol–sulfur dioxide van der Waals complexes: A theoretical study
Authors: Rayón V. M. , Sordo J. A.
Published: July 2002

The potential energy surface for the interaction between methanol and sulfur dioxide has been extensively explored by using ab initio methodology at the correlated level [MP2, MP4, QCISD(T)] with Pople’s 6-31 G** and Dunning’s aug-cc-pVDZ basis sets. Three van der Waals complexes (minima) were located and characterized on the potential energy surface. The structural and energetic parameters of one of them correspond to the ones reported by Kuczkowski and co-workers from microwave spectroscopic studies. Ten transition structures connecting equivalent and inequivalent equilibrium configurations of the van der Waals minima were located and characterized. Two of them are associated to the large amplitude motions responsible for the splittings observed in the rotational spectra: (a) an inversion motion involving both the methanol and sulfur dioxide subunits (mostly the former) and (b) the methyl group internal rotation (torsion). In the first case, theoretical calculations give an estimate of the inversion barrier and provide structural information that, together with the experimental data, may well help to make an experimental prediction of such a barrier. Regarding the methyl group torsional barrier, the theoretical results give support to the idea that the drastic reduction experimentally observed in this barrier height when passing from bare methanol to the methanol⋯sulfur dioxide complex should be ascribed in part to dynamical effects.