bluuebunny/crossref_metadata_2025 huggingface dataset:
High resolution absorption spectrum of jet-cooled CS2 between 65 000 and 71 000 cm−1: Assignment of bent …5σu3πu and linear …2πg3 3d and 5s gerade states
Authors: Cossart-Magos Claudina , Lefebvre-Brion Hélène , Jungen Martin , Launay Françoise
Published: July 2002

The absorption spectrum of jet-cooled CS2 was photographed between 65 000 and 71 000 cm−1 at a resolution limit of 0.0008 nm. In the first half of the energy interval considered, a bending vibrational progression is assigned corresponding to the transition between the linear ground state and a bent excited state …6b29a11B2 correlating with the …5σu3πu1Πg state of the linear molecule. The same progression is observed in the (3+1) resonance enhanced ionization (REMPI) spectrum of Baker and Couris [J. Chem. Phys. 103, 4847 (1995); 104, 6130 (1996); 105, 62 (1996)]. Another observed bending progression in the [(1+1)+1] REMPI spectrum for the same region is here assigned to the other, less bent state …6b23b11A2 issuing from the …5σu3πu1Πg linear state. In both progressions, Δv1=1 transitions are also observed. In the upper half of the energy range considered, the absorption spectrum consists essentially of 210, 201, and 203 bands associated with excitation of …2πg3 3d and 5s Πg1 states. The corresponding origin bands, as well as those of all the other two-photon allowed transitions related to the same configurations, are assigned to bands observed in the [(1+1)+1] REMPI spectra. The rotational band profile associated with two-photon one-color excitation of the 3d, 5s supercomplex of CS2, is calculated using a program based on Hund’s case (e) representation. The band positions and relative intensities in the simulated contour are in excellent agreement with those assigned to origin transitions in the two-color parallel polarized REMPI spectrum. All other bands of the experimental two-photon spectrum can be assigned as the 101 bands associated with the observed 3d electronic origins. The quantum defect values used in the final band contour calculation are consistent with those obtained in an ab initio calculation. A calculation of the same type is performed for the excitation energy from 2πu and 5σu orbitals to 7σg (4sσg) and from 6σg to the valence 3πu orbital. These transitions were suggested by several authors as possible assignments in this spectral region but are indeed at much higher energy. The 4p 3Σu− and 5p 1Σu+←X̃ 1Σg+ transition bands near, respectively, the lower and higher limits of the interval studied here, are also assigned.
Quantum dissociation dynamics of H2 and D2 on a Ni13 cluster
Authors: Álvarez Yazmı́n L. , López Gustavo E. , Cruz Astrid J.
Published: July 2002

The dissociation dynamics of H2 and D2 on a rigid Ni13 cluster has been investigated using a quantum mechanical model. The model is based on the spectral grid/fast Fourier transform technique and includes three variables which are treated quantum mechanically; the translational motion of the molecule normal to the cluster, the vibrational coordinate, and the polar orientation angle. The remaining three variables are fixed during the simulations. The dependence of the dissociation probability on the incident beam energy, initial molecular state and impact site has been examined. The probabilities for rovibrational excitation of the scattered flux have also been computed as function of incident beam energy and impact site. In addition, the dissociation probability has been averaged over the remaining three variables that define the impact site configuration.
Elongational viscosities from nonequilibrium molecular dynamics simulations of oscillatory elongational flow
Authors: Todd B. D. , Daivis Peter J.
Published: July 2002

We present a simple new technique to simulate the elongational flow of a simple atomic fluid by nonequilibrium molecular dynamics (NEMD). This technique involves simulating elongational flow by applying a frequency-dependent strain rate that ensures that the system attains a temporally periodic steady state. For a given magnitude of the strain rate, quantities of interest, such as the diagonal elements of the pressure tensor, and hence elongational viscosities, are then calculated by extrapolating their frequency-dependent values down to zero frequency. The zero frequency results are in excellent agreement with independent conventional NEMD calculations of these quantities. The main advantage of this technique is that it provides a convenient and consistent means of extrapolating to the zero-frequency (steady elongation) elongational viscosity, unlike the standard method, in which it may be difficult to distinguish between the transient response and the steady-state response.
Rare gas scattering from molten metals examined with classical scattering theory
Authors: Muis André , Manson J. R.
Published: July 2002

Classical limit expressions of the differential reflection coefficient for atoms scattering from a surface are compared with recent experiments for the scattering of monoenergetic beams of Ne, Ar, and Xe with incident energies in the eV range from the molten metal surfaces Ga, In, and Bi. We find that single collision events usually make the greatest contribution to the backscattered intensity, double collision events make a significant but smaller contribution, and scattering of atoms that are completely trapped and subsequently thermally desorbed has a small probability. In the case of large mass incident projectiles and small mass target atoms we find some evidence for collective mass effects of the target. An analysis of the temperature dependence of the intensities shows that these surfaces act neither as a smooth continuous repulsive barrier nor as a collection of discrete scattering centers; rather they exhibit a behavior in between these two extremes.
Gibbs ensemble Monte Carlo simulation of the properties of water with a fluctuating charges model
Authors: Medeiros Milton , Costas Marı́a Eugenia
Published: July 2002

A Gibbs ensemble Monte Carlo calculation of the properties of water using the four point transferable intermolecular potential with fluctuating charges model (TIP4Pfq) is presented. The model yields very good results for the thermodynamic properties, comparable with the ones obtained with fixed charges models and with the experimental data. Remarkable results for the liquid phase chemical potential at room temperature are obtained. The dielectric constant calculation is improved with respect to the model with fixed charges, and gives similar results when compared with a molecular dynamics simulation for the same water model. The intermolecular potential model used here seems to be a promising one for studying the properties of aqueous solutions, where the dielectric behavior of the solvent makes a direct and substantial contribution to the solution properties.
Density fluctuations of assemblies of irreversibly deposited particles on solid surfaces
Authors: Bafaluy J. , Schaaf P. , Senger B. , Voegel J.-C. , Pagonabarraga I.
Published: July 2002

For general irreversible deposition processes, a relation between the variance σ2 of the number of deposited particles on subsystems out of a large surface and the available surface function Φ is obtained. This relation is based on a mean field assumption and follows the resolution of a master equation system. It is valid at low to intermediate values of the surface coverage θ as shown by comparison with exact results and with numerical simulations for special deposition models. In the low coverage limit, if the available surface function is written as a series expansion of the coverage θ, whose first nontrivial term varies as θk, the reduced variance has a similar expansion. However, the prefactor of θk derived in this article is in general different in both series expansions. This result has also been obtained by a rigorous argument based on the evolution of the k-particle distribution function with the coverage.
Microwave spectroscopic and ab initio studies of the hydrogen-bonded trimethylamine–hydrogen sulfide complex
Authors: Tubergen Michael J. , Flad Jonathan E. , Del Bene Janet E.
Published: July 2002

Rotational spectra have been recorded for six isotopomers of the trimethylamine–hydrogen sulfide complex using a Fourier-transform microwave spectrometer. The spectra are characteristic of a symmetric top (B+C)/2=1395.463 (1) MHz, and are indicative of free internal rotation of H2S about the trimethylamine symmetry axis. A structure with a single, linear hydrogen bond, with an S–N distance of 3.36 (5) Å, best reproduces the moments of inertia of six isotopomers, including three distinct deuterated complexes. The experimental structure is compared to an ab initio structure optimized at the MP2/6-31+G(d,p) level which predicts an S–N distance of 3.328 Å. MP2/aug′-cc-pVTZ calculations were used to determine the binding energy of the complex (ΔEe=−5.8 kcal/mol) and the barrier to an internal tunneling motion which exchanges the two H2S hydrogens (3.0 kcal/mol).
Monte Carlo simulation studies of electrolyte in mixture with a neutral component
Authors: Reščič J. , Vlachy V. , Bhuiyan L. B. , Outhwaite C. W.
Published: July 2002

Monte Carlo simulations are obtained for a multicomponent primitive model mixture consisting of a symmetric 1:1 electrolyte and a neutral particle. The equilibrium structure and thermodynamics are studied as a function of the concentration of the electrolyte and concentration of the neutral species. For the unequal-sized mixture the simulations clearly indicate a transformation in the nature of interaction between two like ions from being repulsive at low concentrations of the neutral component to being attractive at higher concentrations of the neutral component. The computer results are used to assess the accuracy of the corresponding predictions from the symmetric Poisson–Boltzmann, the modified Poisson–Boltzmann, and the hypernetted chain theories. Comparison of the pair distribution functions and thermodynamics shows that the theoretical results are in semiquantitative or better agreement with the simulation results. While the good agreement of the modified Poisson–Boltzmann and the hypernetted chain results with the Monte Carlo data is expected, the results from the relatively simple symmetric Poisson–Boltzmann theory shows a surprisingly good overall consistency with the simulations.
Pair potential for water from symmetry-adapted perturbation theory
Authors: Mas Eric M. , Szalewicz Krzysztof , Bukowski Robert , Jeziorski Bogumil
Published: July 2002

The interaction energies of over a thousand water dimer configurations have been calculated using the symmetry-adapted perturbation theory. Effective, interaction optimized bases were used leading to 0.2 kcal/mol accuracy near the minimum of the dimer potential. The computed points were then fitted to two types of analytic potential energy surfaces, a site-site form and an expansion in functions dependent on the vector connecting the centers of mass and on the Euler angles defining the orientation of each monomer. The second virial coefficient was calculated from these surfaces including the quantum correction and isotopic dependence, as well as the molar heat capacity at constant pressure. Comparison of these data to experiment shows that both of our surfaces are superior to any previously available.
Electronic structure aspects of the spin-forbidden reaction CH3(X 2A2″)+N(4S)→HCN(X 1Σ+)+H2(X 1Σg+)
Authors: Sadygov Rovshan G. , Yarkony David R.
Published: July 2002

Second order configuration interaction wave functions based on molecular orbitals determined from a state-averaged multiconfigurational self-consistent field procedure are used to investigate the intermediate complex driven model for the spin-forbidden reaction CH3(X 2A2″)+N(4S)→HCN(X 1Σ+)+H2(X 1Σ+). The minimum energy crossing point (MECP), the minimum energy point on the surface of intersection connecting the reactant channel, A′′3 potential energy surface, and product channel A′1 potential energy surface, is determined directly, i.e., without a priori characterization of the individual potential energy surfaces. The MECP is found to be 8.2 kcal/mol below of the reactants. The structure at the MECP clearly evinces the incipient formation of a H2 bond. Barrierless paths from the reactants to the intermediate complex-methylnitrene, from the intermediate complex to the MECP, and from the MECP to the products are established. The absence of a barrier on these paths supports the intermediate complex mechanism. Spin–orbit interactions are determined to be ∼30 cm−1 for points on the surface of intersection in the vicinity of the MECP. Spin–orbit interactions and the local potential surface topology at the MECP are used to probe the efficiency of the intersystem crossing using the Landau–Zener model. A reduced dimensionality model is proposed.
Intramolecular vibrational relaxation seen as expansion in phase space. III. The long-time limit
Authors: Pavlov-Verevkin V. B. , Lorquet J. C.
Published: July 2002

Asymptotic formulas that describe the behavior of the function N(T) measuring the phase space volume sampled by a nonstationary wave packet during its time evolution are derived. It is shown that, in the long-time limit, N(T)∼T−1 when the dynamics is regular, whereas N(T)∼T−2 ln T for the chaotic case.
Test of equations of state for the Weeks–Chandler–Andersen reference system of two-dimensional Lennard-Jones fluids
Authors: Mulero A. , Cuadros F. , Galán C.
Published: July 2002

Molecular dynamics results of pressure and potential energy for the reference system of the Weeks–Chandler–Andersen theory applied to two-dimensional fluids have been obtained in a wide range of temperatures and densities. These results are used to test the validity of six different analytical equations of state for hard-disk fluids proposed in the literature (some of which have been published recently), together with two different analytical expressions for the molecular diameter, to describe the properties of the above-mentioned fluids. The best choices for the combination of equations of state and molecular diameters in the calculation of the pressure and potential energy are given. It is shown how that choice may differ for different ranges of temperatures and densities, and how there is not a direct relation between the simplicity or complexity of the analytical expressions and their accuracy.
Quantum effects and the excess proton in water
Authors: Pavese Marc , Chawla Sanjay , Lu Dongsheng , Lobaugh John , Voth Gregory A.
Published: July 2002

The influence of nuclear quantum effects on the excess proton in water is examined through quantum dynamical computer simulations. Results are presented for the isotope effects on both the proton exchange process and the infrared absorption spectrum. A critical assessment of the accuracy of density-functional-theory-based molecular dynamics simulation methods for addressing this problem is also given.
The role of local density in the collisional deactivation of vibrationally highly excited azulene in supercritical fluids
Authors: Schwarzer D. , Troe J. , Zerezke M.
Published: July 2002

The collisional deactivation of vibrationally highly excited azulene was studied from gas into compressed liquid phase by pump-and-probe picosecond laser spectroscopy. Collisional deactivation rates were compared with solvatochromic shifts Δν of the azulene S3←S0 absorption band under identical conditions. Employing supercritical fluids at pressures between 0.03 and 4000 bars and temperatures between 298 and 640 K, measurements covering the complete gas–liquid transition were performed. For the energy transfer experiments, azulene with an energy of ∼20000 cm−1 was generated by laser excitation into the S1- and internal conversion to the S0*-ground state. The subsequent loss of vibrational energy was monitored by following the transient absorption at the red wing of the S3←S0 absorption band near 290 nm. Transient signals were converted into energy-time profiles using hot band absorption coefficients from shock wave experiments for calibration and accounting for solvent shifts of the spectra. Under all conditions, the energy decays were found to be exponential with phenomenological deactivation rate constants kc. kc and spectral shifts Δν showed quite similar density dependences: the low pressure linear increase of both quantities with density ρ at higher densities starts to level off, before it finally becomes stronger again. The parallel behavior of energy transfer rate constants and solvent shifts becomes particularly apparent near to the critical point: measurements in propane at 3 K above the critical temperature showed that kc and Δν are essentially constant over a broad density interval near to the critical density. These observations suggest that both quantities are determined by the same local bath gas density around the azulene molecule. By Monte Carlo simulations it is shown that kc(ρ) follows an isolated binary collision (IBC) model, if the collision frequency Z is related to the radial distribution function g(r) of an attractive hard-sphere particle in a Lennard-Jones fluid. Within this model, average energies 〈ΔE〉 transferred per ethane–azulene collision are temperature independent between 298 and 640 K and pressure independent between 0.03 and 4000 bars. By means of radial distribution functions the density dependence of Δν can be represented as well.
Two-photon spectroscopy of the low lying Rydberg states of NO. I. The 3p and 3d complexes
Authors: Meyer Henning
Published: July 2002

The rotational structure and polarization dependence of two-photon spectra of aligned ensembles of open shell diatomics is investigated in terms of the spherical tensor components of the two-photon absorption operator. The formalism allows the straightforward incorporation of state interactions and perturbations. It is applied to the two-photon spectroscopy of NO, in particular to the excitation of the Rydberg states derived from the 3p and 3d complexes. All states investigated show a nearly quadratic power dependence indicating the saturation of the ionization step. Transitions dominated by a zeroth rank tensor component (e.g., C 2Π–X 2Π or H 2Σ, H′ 2Π–X 2Π) are insensitive to a possible angular momentum alignment in the ensemble. These transitions are ideally suited to determine degeneracy averaged observables, e.g., collision cross sections in a molecular beam scattering experiment or product velocity anisotropies in a single color photodissociation experiment. Rotational alignment data must be determined using two-photon transitions which are carried by a second rank tensor component (e.g., D 2Σ–X 2Π or F 2Δ–X 2Π).
Semiclassical tunneling in the initial value representation
Authors: Kay Kenneth G.
Published: July 2002

Tunneling in the one-dimensional Eckart system is treated by a semiclassical method that describes the S-matrix in terms of an integral over the initial momenta of real-valued classical trajectories. The results are found to be sensitive to a certain parameter γ which is expected to be essentially arbitrary for classically allowed processes. Analysis of the semiclassical error allows formulation of conditions for the validity of the tunneling treatment. This, in turn, leads to an explanation for the sensitivity of the results to γ and an understanding of how this parameter should be chosen. With an optimized choice, the semiclassical method is found to yield very accurate tunneling results even for probabilities as small as 10−10. The relationship between the present method and the conventional uniform semiclassical treatment of barrier tunneling is discussed.
Effects of dynamic spatial disorder on ionic transport properties in polymer electrolytes based on poly(propylene glycol)(4000)
Authors: Ferry Anders
Published: July 2002

The equivalent ionic dc conductivity (Λ) generally exhibits a dramatic concentration dependence in electrolytic systems based on the host polymer poly(propylene glycol) of molecular weight 4000 (PPG4000). In particular, Λ typically increases rapidly with increasing salt concentration passing through a temperature dependent maximum at high concentration. Prompted by recent reports on a microscopic phase separation occurring in these electrolytes, we here report vibrational spectroscopic, ionic conductivity, and restricted diffusion data for ion-conductors based on PPG4000 complexed with the lithium salts LiCF3SO3 and LiN(CF3SO2)2, in an attempt to resolve seemingly contradictory results concerning ionic transport phenomena in these complexes. We find that the differential salt diffusion coefficient Ds, describing bulk salt motion over long time scales, exhibits a qualitatively similar concentration dependence as Λ. This is contrary to recent F19 pfg-NMR diffusion results for the PPG4000-LiCF3SO3 system which show that the anionic diffusion coefficient decreases monotonically with increasing salt concentration and is inversely proportional to solution shear viscosity. As determined from analyses of characteristic vibrational modes of the [CF3SO3]− and [(CF3SO2)2N]− anions, respectively, the spectroscopic data show very small changes in the distribution of anionic species over the range of electrolyte compositions corresponding to a sharp enhancement of Λ. The results are interpreted in terms of slowly fluctuating salt-rich electrolyte microdomains in equilibrium with salt-depleted polymer regions.
Theoretical study of the atomic-force-microscopy imaging process on the NaCl(001) surface
Authors: Tang Hao , Bouju Xavier , Joachim Christian , Girard Christian , Devillers Jean
Published: July 2002

Based on a fully relaxed molecular-mechanics approach, we present a complete series of atomic force microscopy images of the NaCl(001) surface. For a given atomic structure of a diamond probe-tip [111] oriented, we analyze the results obtained from the two usual imaging modes either based on the monitoring of lateral or vertical forces. By accounting for the atomic relaxation of the whole system for any tip-sample configuration, we were able to get new insights about the achievement of atomic resolution on such substrates. Depending on the tip-sample distance range, the scanning of the NaCl(001) surface results in completely different behaviors associated with these two imaging modes. At very short distance we observed an increase of the apparent corrugation when working with the lateral force mode. The perturbations generated in the images by the presence of localized defects (vacancy and monoatomic step) are also analyzed within this model. These last calculations clearly indicate the fundamental role played by the relaxation phenomena on the observed topography when the tip scans the surface defects.
Structural and transport properties of (Bi,Pb)4Sr3Ca3Cu4−mFemOx (m=0–0.06) glasses: Precursors for high Tc superconductors
Authors: Chatterjee S. , Bhattacharya S. , Chaudhuri B. K.
Published: July 2002

Microstructure and transport properties (between 80–450 K) of the Fe doped as-quenched Bi3Pb1Sr3Ca3Cu4−mFemOx (m=0.0, 0.02, 0.04, 0.05, and 0.06) type multicomponent glassy precursors for high-temperature superconductors were first reported in this paper. It has been shown from x-ray diffraction, scanning electron microscopy, thermal analysis, density, and oxygen molar volume measurements that single phase homogeneous glasses are formed. Even with small addition of Fe, a large change of glass transition temperature (Tg) indicates appreciable change of glass network structure of the undoped (Bi,Pb)4Sr3Ca3Cu4Ox glass which is reflected in the properties of the corresponding high-temperature annealed glass termed as glass-ceramic (GC) samples. These GC samples are found to be superconductors. Addition of Fe is considered to break up some of the original Cu–O–Cu bonds forming some new Cu–O–Fe and Fe–O–Fe type bonds. A nonlinearity (increase) in the dc conductivity (σdc) of the as-quenched glass sample is observed around m=0.02. This increase of dc conductivity is ascribed to be due to the additional hopping between Fe2+ and Fe3+ which finally destroys superconductivity for m⩾0.06. This is also responsible for the nonlinear variation of activation energy of the Fe doped glass and hence change in the superconducting properties of the glass ceramics. The high temperature (above θD/2, θD being the Debye temperature) σdc data of the glasses were explained by considering small polaron hopping conduction mechanism. The dc conductivity of these glasses are found to follow Greaves’ variable range hopping model in the intermediate range of temperature. The polaron hopping model of Schnakenberg and Emin can predict the conductivity data in the high-temperature regime. All the Fe-doped glass ceramics showed superconducting transitions with Tc between 100 and 110 K. The corresponding zero resistance temperature, Tco which is maximum (79 K) for m=0, decreases consistently with increase of Fe-concentration and finally superconducting behavior is destroyed for m⩾0.06. This behavior is consistent with-pair breaking mechanism.
Ultrafast reaction dynamics in cluster ions: Simulation of the transient photoelectron spectrum of I2−Arn photodissociation
Authors: Faeder J. , Parson R.
Published: July 2002

Combining an effective Hamiltonian model of electronic structure with nonadiabatic molecular dynamics simulations, we calculate the recently measured transient photoelectron spectrum of I2− dissociated inside a cluster of argon atoms. We find good agreement between calculated and experimental spectra. The transient spectral shifts reflect the dynamics of both the I2− and argon degrees of freedom, revealing pathways and time scales for dissociation, recombination, and vibrational relaxation.
Free-volume distribution and positronium formation in amorphous polymers: Temperature and positron-irradiation-time dependence
Authors: Wang C. L. , Hirade T. , Maurer F. H. J. , Eldrup M. , Pedersen N. J.
Published: July 2002

Positron annihilation lifetime spectroscopy was used to study the free-volume size distribution and the o-Ps (ortho-positronium) formation in two amorphous polymers. We performed positron lifetime measurements on poly(vinyl acetate) (PVAc) in the temperature range of 84–414 K and on poly(methyl methacrylate) (PMMA) in the temperature range of 84–454 K and as a function of time (<200 h) at four temperatures (T=84, 149, 224, and 249 K). The glass transition temperature Tg and secondary transition temperature were determined from the average o-Ps lifetime τ3 versus temperature. The width of the o-Ps lifetime distribution was evaluated using the maximum entropy method (MELT). Analysis by the MELT program on the spectra shows that (i) the standard deviation σ(τ3) of the o-Ps lifetime distribution in PVAc and PMMA increases slightly with temperature in the glassy state, and then increases significantly with temperature above the glass transition temperature Tg; (ii) the σ(τ3) in PMMA has no observable variation as a function of positron irradiation time, which suggests that the measured free-volume distribution is not influenced by the positron irradiation. The o-Ps formation probability shows large variations with temperature, thermal history, and measuring time. These effects are discussed in the framework of the spur model.
Effect of charge upon metal cluster chemistry: Reactions of Nbn and Rhn anions and cations with benzene
Authors: Berg Christian , Beyer Martin , Achatz Uwe , Joos Stefan , Niedner-Schatteburg Gereon , Bondybey Vladimir E.
Published: July 2002

The reactions of anionic niobium and rhodium clusters Mn−, M=Nb, Rh, n=3–28, with C6H6 are investigated under single collision conditions in a Fourier-transform ion-cyclotron-resonance mass spectrometer and compared with the results of previous studies on corresponding cationic species. This reveals strong effects of the cluster charge state on hydrocarbon activation as a function of cluster size. Both differences and parallels are observed for reactions of anions and cations. Niobium clusters with a given number of atoms react quite differently than those with a single atom more or less. The fact that almost identical such effects are in the present work found for anion clusters, as for cations with the same number of atoms but two less electrons, suggests that the observed reactivity patterns are more a function of the cluster shape and geometry, than of the details of their electronic structure. The variety of interesting trends and effects observed is interpreted in terms of simple physical models.
Quantum mechanical calculation of inelastic scattering of an atom by a large anharmonic cluster: Application to He+Ar13
Authors: Rom A. Y. , Neuhauser D. , Gerber R. B.
Published: July 2002

An approximate quantum mechanical method is proposed for the calculation of inelastic scattering of an atom from a large anharmonic cluster or molecule. The method is based on: (a) computing the vibrational states of the cluster (or molecule) in the vibrational self-consistent field approximation; (b) treating the scattering of the atom to a first approximation as taking place from a vibrationally frozen cluster; (c) obtaining inelastic transitions by a distorted wave approximation, where the coupling is the vibrationally dependent part of the atom/cluster potential. Computationally convenient expressions are worked out. The method is applied to He scattering from Ar13 and the results are compared to experimental data for size-dispersed clusters. Good qualitative agreement is found. The merits of the proposed method compared with alternative approaches are discussed.
Variance and width of absorption lines of single molecules in low temperature glasses
Authors: Pfluegl Wolfgang , Brown Frank L. H. , Silbey Robert J.
Published: July 2002

We consider the line shapes of single molecules in low temperature glasses due to dipole–dipole interaction between the molecules and other particles of the system. Motivated by computer simulations, we employ a simplified formula for the absorption lines and derive an analytic expression for the distribution of the variance of such absorption lines. The simplest version of this distribution—derived for pointlike particles without cutoff of the interaction at small distances—already accounts for the qualitative features of width histograms measured experimentally or gained by computer simulation. We further analyze the effect of the minimal approach distance between the chromophore and the perturbing particles and employ an approximate relation between the variance and the full width at half maximum of the absorption lines. We find that the main characteristics of the histogram of widths stem from the (homogeneous) distribution of interactions in space and do not reflect the distribution of the internal parameters of the underlying model.
Tilted n-fold symmetric radio frequency pulse sequences: Applications to CSA and heteronuclear dipolar recoupling in homonuclear dipolar coupled spin networks
Authors: Gross J. D. , Costa P. R. , Griffin R. G.
Published: July 2002

We describe new solid-state NMR methods for measuring the magnitude of the chemical shift anisotropy (CSA) and/or the heteronuclear dipolar coupling in the presence of homonuclear dipolar coupled spin networks under magic-angle spinning (MAS). The techniques employ 2π/5 phase shifts of a frequency switched Lee–Goldburg irradiation scheme which recouples the CSA and/or the heteronuclear dipolar coupling while simultaneously decoupling the homonuclear dipolar interactions. The induced spin dynamics are sensitive to the magnitude of the CSA and its asymmetry and may be implemented under conditions where the MAS rate exceeds the size of the anisotropic interactions. These approaches could find use in measurements of the magnitudes and relative orientations of CSA and/or heteronuclear dipolar couplings to extract torsion angles in uniformly labeled systems.
Degenerate four-wave mixing spectroscopy as a probe of orientation and alignment in molecular systems
Authors: Wasserman Thierry A. W. , Vaccaro Patrick H. , Johnson Bruce R.
Published: July 2002

Degenerate four-wave mixing (DFWM) spectroscopy is shown to provide a facile means for probing angular momentum (or rotational) anisotropy in nonequilibrated ensembles of gaseous molecules, with judicious selection of experimental conditions permitting quantitative determination of population distributions and Zeeman coherences for magnetic sublevels of the target species. A theoretical description of the nonlinear response induced under such circumstances is obtained by incorporating a state multipole expansion of the zero-order density operator into a perturbative (weak-field) treatment for the DFWM interaction. Aside from allowing the effects of incident field polarizations and phase-matching geometries to be considered in detail, this compact spherical tensor formalism provides guidelines for the extraction of spatial information from rovibronically resolved spectral data. Furthermore, these analyses have identified unusual polarization schemes that lead to signal generation only in the presence of rotational anisotropy, thereby suggesting a new class of four-wave mixing measurements that permit the selective detection of molecular orientation and alignment.
Photolysis of ICl causes mass-dependent interference in the Cl(2P3/2) photofragment angular momentum distributions
Authors: Rakitzis T. Peter , Kandel S. Alex , Zare Richard N.
Published: July 2002

We have studied the photodissociation of ICl using linearly polarized light at 532 nm. This wavelength excites several electronic states. Using linearly polarized probe light, we have measured the angular momentum distributions of the Cl35(2P3/2) and Cl37(2P3/2) photofragments, which are found to differ significantly. These distributions are expressed in a multipole moment expansion having four terms. These terms can be classified as incoherent contributions from parallel and perpendicular transitions, and coherent contributions from interference that arises from these interacting states. Analysis of these angular momentum distributions using this formalism shows that the incoherent contributions from the dissociating surfaces are mass independent, whereas the coherent contributions are mass dependent.
Hyperspherical elliptic coordinates for the theory of light atom transfer reactions in atom-diatom collisions
Authors: Tolstikhin Oleg I. , Nakamura Hiroki
Published: July 2002

We formulate and demonstrate a new method for quantum 3D calculations of light atom transfer reactions in atom-diatom collisions. The method follows a general scheme of the hyperspherical method, in common with other hyperspherical formulations in the field. The main novelty consists in the hyperspherical elliptic coordinates (ξ,η) used to parametrize the hypersphere. These coordinates have been introduced recently for studying three-body Coulomb systems, and here we apply them to study a system of three atoms. The coordinates are defined and their relation with the Smith-Whitten and Delves coordinates is explored. On account of a big difference between vibrational and rotational excitation energies in molecules, the hyperspherical adiabatic Hamiltonian allows adiabatic separation between ξ and η. This not only greatly facilitates solution of the hyperspherical adiabatic eigenvalue problem, but also provides an approximate classification of the states by a pair of indices (nξ,nη) representing vibrational and rotational quantum numbers simultaneously for a reagent and a product. Another novel technology exploited here is the Slow/Smooth Variable Discretization (SVD) method. The SVD is used for treating nonadiabatic couplings between the ξ and η motions, as well as between the motions with respect to the hyperradius and the hyperangular variables. The whole scheme is illustrated by calculations for the reaction O(3P)+HCl→OH+Cl for zero total angular momentum. It is shown to be very efficient, accurate, and providing a framework of choice for elucidating light atom transfer reaction mechanisms.
Experimental evidence for K-conservation in the dissociation of singlet ketene
Authors: Mellinger Axel , Ashikhmin Michael V. , Moore C. Bradley
Published: July 2002

The extent of K-mixing in the dissociation of ketene on its S0 potential energy surface has been investigated. Using a two-step photodissociation scheme for ketene and laser-induced fluorescence (LIF) to detect the CH2 radical, photofragment excitation (PHOFEX) spectra of high energy selectivity were recorded. The ratio of the step heights shows that K is strongly mixed for K>0 and that for K=0 the extent of mixing increases with J.
Ab initio and density functional theory calculations on the protonated species of As4 clusters
Authors: Alcamı́ M. , Mó O. , Yáñez M.
Published: July 2002

High level ab initio and density functional theory approaches have been used to study the structure and bonding of the stationary points of the As4H+ potential energy surface. The geometries and the harmonic vibrational frequencies of the different species investigated were obtained through the use of all-electron MP2(full) and B3LYP methods using 6-311G(d) basis expansions. Final energies were obtained in G2(MP2) and B3LYP/6-311+G(3df,2p) calculations. The global minimum is the result of the side protonation of As4 tetrahedral molecule. The corner-protonated species is significantly less (∼23 kcal/mol) stable, while the face-protonated form is a second-order saddle point. The open structure 5 resulting from the fission of two As–As bonds lies 18 kcal/mol above the global minimum. In the global minimum the proton is covalently attached to two As atoms through the formation of a three-center delocalized bonding orbital. The relative stability of this “nonclassical” structure is enhanced with respect to that of the homologous P4H+ system, indicating that the heavier elements prefer divalent arrangements. As a consequence, the As4 cluster is predicted to be about 8 kcal/mol more basic than P4 tetramer in the gas phase. The harmonic vibrational frequencies of the As4H+ side-protonated species is consistent with the existence of the three-center As–H–As covalent bond.
Infrared predissociation spectra of Nen–HN2+ clusters (n=1–5)
Authors: Nizkorodov Sergey A. , Meuwly Markus , Maier John P. , Dopfer Otto , Bieske Evan J.
Published: July 2002

Infrared predissociation spectra of Nen–HN2+ (n=1–5) cluster ions have been recorded in the vicinity of the N–H stretching vibration (ν1) of HN2+. Several bands of the Ne–HN2+ dimer are rotationally resolved and provide direct information on the geometry, intermolecular stretching and bending frequencies, and complexation induced frequency shift of the ν1 vibration. The Ne–HN2+ complex has a linear, proton bound structure with an average separation between the Ne atom and the HN2+ center-of-mass of 〈1/R2〉−1/2=3.28 Å. The observed characteristics of the dimer are reproduced by a two-dimensional intermolecular potential energy surface calculated at the MP2 level, that is adiabatically corrected to account for the coupling of the intramolecular N–H stretching and intermolecular motions. The binding energy in the vibrational ground state of the complex is found to be 795 cm−1 and increases to 1005 cm−1 upon excitation of the N–H stretching vibration. Although the spectra of the larger complexes are not rotationally resolved, shifts in the ν1 transition frequency suggest that the larger clusters possess structures where the Ne atoms are weakly attached to the side of a linear Ne–HN2+ core.
Comment on “Frequency-dependent equation-of-motion coupled cluster hyperpolarizabilities: Resolution of the discrepancy between theory and experiment for HF?” [J. Chem. Phys. 107, 10823 (1997)]
Authors: Hättig Christof , Koch Henrik , Jo/rgensen Poul
Published: July 2002

In J. Chem. Phys. [107, 10823 (1997)], Rozyczko and Bartlett report an open-ended formula for equation-of-motion coupled cluster (EOM-CC) hyperpolarizabilities. We demonstrate that this formula is incompatible with the generic definition of EOM-CC properties and converges to a wrong full configuration limit. We derive the correct expressions for the EOM-CC quadratic and cubic response functions.
Generalized thermodynamic perturbation theory for polyatomic fluid mixtures. I. Formulation and results for chemical potentials
Authors: Smith William R. , Nezbeda Ivo , Strnad Martin , Třı́ska Bohumil , Labı́k Stanislav , Malijevský Anatol
Published: July 2002

For general mixtures of polyatomic molecules and their constituent atoms, we first rigorously derive an exact statistical mechanical result relating the background pair correlation function y(1,2,…,m) to a certain excess chemical potential difference involving its components, βΔμe, extending and generalizing our previous results. Second, using only thermodynamic methods, we develop a perturbation theory for the equation of state (EOS) which involves βΔμe; we then express this EOS in an alternative form involving y(1,2,…,m). The latter form coincides with results recently obtained by Zhou and Stell using a different approach and with the EOS of the Wertheim first-order perturbation theory (TPT1); our approach explicitly exposes the underlying thermodynamic approximations involved. Third, we show for the case of tangent fused-hard sphere (FHS) systems, under the approximation that βΔμe is independent of composition, that implementation of the former form of the theory yields results analytically equivalent to those obtained from the Boublik–Nezbeda (BN) EOS; and that the alternative implementation is only slightly less accurate, due to a (numerically small) internal inconsistency in this EOS. This sheds light on the remarkable accuracy obtained for several previous implementations of TPT1 for such systems. We present new computer simulation results for a particular ternary tangent FHS heteronuclear diatomic mixture, which support the approximation that βΔμe for mixtures of such molecules is nearly composition independent. Finally, for several FHS mixture model systems, we test the Lewis–Randall rule and several other approximations for calculation of the mixture chemical potentials. The Lewis–Randall rule is generally superior for the individual chemical potentials, and is competitive for βΔμe.
The ultraviolet photodissociation of Cl2O at 235 nm and of HOCl at 235 and 266 nm
Authors: Tanaka Yoshiki , Kawasaki Masahiro , Matsumi Yutaka , Fujiwara Hisashi , Ishiwata Takashi , Rogers Leon J. , Dixon Richard N. , Ashfold Michael N. R.
Published: July 2002

The primary photochemistry of gas phase dichlorine monoxide (Cl2O) and of hypochlorous acid (HOCl) following excitation at 235 nm has been investigated using photofragment ion imaging to obtain the recoil velocity and angular distributions of the ground (2P3/2) and spin-orbit excited (2P1/2) atomic chlorine products. In the case of Cl2O, both Cl spin-orbit products exhibit angular distributions characterized by an anisotropy parameter, β=1.2±0.2, consistent with previous interpretations of the ultraviolet (UV) absorption spectrum of Cl2O which associate the broad intense absorption feature peaking at λ∼255 nm with excitation to a (bent) dissociative state of B21(C2v) symmetry. The recoil velocity distributions of the two Cl spin-orbit products are markedly different. The ground state atoms (which constitute >90% of the total Cl atom yield) are partnered by ClO fragments carrying significantly higher average levels of internal excitation. The slowest Cl atoms are most readily understood in terms of three body fragmentation of Cl2O to its constituent atoms. These findings are rationalized in terms of a model potential energy surface for the 1 1B2 state, which correlates diabatically with ClO(X) radicals together with a spin-orbit excited Cl atom, with efficient radiationless transfer to one (or more) lower energy surfaces at extended Cl-O bond lengths accounting for the dominance of ground state Cl atom fragments. The image of the ground state Cl atoms resulting from photolysis of HOCl at 235 nm is consistent with parent excitation via a transition for which the dipole moment is closely aligned with the Cl-O bond, followed by prompt dissociation (β=1.7±0.2) with the bulk of the excess energy partitioned into product recoil. Such conclusions are consistent with the results of laser induced fluorescence measurements of the OH(X) products resulting from 266 nm photodissociation of HOCl which reveal OH(X) products in both spin-orbit states, exclusively in their zero-point vibrational level, and carrying only modest levels of rotational excitation (well described by a Boltzmann distribution with Trot∼750±50 K).
Resonance-enhanced multiphoton electron detachment (REMPED) spectra of C10− and C11−
Authors: Ohara M. , Suwa M. , Ishigaki T. , Shiromaru H. , Achiba Y. , Krätschmer W.
Published: July 2002

We report on resonance-enhanced multiphoton electron detachment (REMPED) spectra of C10− and C11−. Both species were obtained by laser evaporation of graphite in a flow of helium. The spectra obtained under several different evaporation conditions suggest that both anions occur as well in a ring as in a chain form. The ring form dominates at low laser fluences. For the species C10−, our data indicate a third isomeric form which may consist of a somehow distorted ring structure.
All electron density functional study of neutral and ionic polybromine clusters
Authors: Schuster Peter , Mikosch Hans , Bauer Günther
Published: July 2002

The structural and electronic properties of homoatomic chains consisting of up to five Br-atoms are studied by means of the linear combination of Gaussian-type orbitals–local spin density method including nonlocal corrections to the exchange and correlation energy. A highly flexible basis set is used and the effects of introducing additional diffuse basis functions are examined. By comparison of the results for atomic Br and Br2 with those from very accurate correlated ab initio calculations the quality of the present method is established. Based on these results neutral and singly charged Br3, Br4 and Br5 are investigated, for which very few accurate data exist in literature. Geometries, harmonic vibrational frequencies, ionization potentials, electron affinities and charge distributions are reported and found in satisfactory agreement with available experimental data.
The 16 valence electronic states of nitric oxide dimer (NO)2
Authors: East Allan L. L.
Published: July 2002

Sixteen electronic states of nitric oxide dimer are investigated using various ab initio levels of theory and various orientations of the dimer. These are the states which arise from the mixing of the singly occupied πNO* orbitals of the monomers, and include all eight states which directly correlate to the Π2 ground states of the monomers. Twelve of the sixteen states are significantly multiconfigurational in character, which cause incorrect state orderings at low levels of theory. At several plausible geometries, eight low-lying states are predicted (four singlets and four triplets) within a 1 eV span, hence corresponding to excitations in the infrared, while the other eight states (six singlets and two triplets) lie much higher in the far ultraviolet, and in the realm of numerous other electronic states. The results imply, but do not confirm, that the only potential minimum lying below the lowest dissociation asymptote is the cis-ONNO geometrical conformation of the X̃ A11 ground state.