bluuebunny/crossref_metadata_2025 huggingface dataset:
Matrix isolation study of the interaction of excited neon atoms with CCl4: Infrared spectra of the ion products and of Cl2CCl⋅⋅Cl
Authors: Lugez Catherine L. , Jacox Marilyn E. , Johnson Russell D.
Published: July 2002

When a Ne:CCl4 sample is codeposited at approximately 5 K with a beam of neon atoms that have been excited in a microwave discharge, the infrared spectrum of the resulting deposit includes prominent absorptions not only of CCl3 but also of several other neutral and ionic species. The absorptions assigned to CCl3+, CCl2+, and CCl+ are consistent with previous spectroscopic identifications of these species. As at lower energies, the Cl2CCl⋅⋅Cl isomer of CCl4 contributes to the product spectrum. Higher level ab initio calculations of the fundamental vibrations of this isomer and of its carbon-13 substituted counterpart give improved agreement with the experimentally observed infrared spectrum and, together with the results of a supplementary experiment, provide further support for the assignment of a structured absorption near 500 cm−1 to this species, rather than to Cl3+. Uncharged Cl2CCl⋅⋅Cl is readily destroyed by visible radiation, with concomitant growth in the absorptions of CCl4. Photodestruction of ionic species occurs in the ultraviolet spectral region. Evidence is presented for the stabilization of Cl2CCl⋅⋅Cl− and of Cl3CCl− in these experiments. The latter species undergoes photodestruction in the near infrared spectral region. Two absorptions are tentatively assigned to the ClCCl⋅⋅Cl− fragment ion.
Solvation state selective excitation in resonance Raman spectroscopy. I. Experimental study on the C=N and the C=O stretching modes of phenol blue
Authors: Yamaguchi T. , Kimura Y. , Hirota N.
Published: July 2002

We have measured the excitation photon energy dependence of the resonance Raman spectra of a solvatochromic dye, phenol blue (PB) in various solvents from nonpolar to dipolar ones, including supercritical fluids of trifluoromethane and carbon dioxide. We have found that the band peak positions of the C=N stretching mode of PB appear to change with the excitation laser frequency in polar solvents. On the other hand, no dependence of the band peak positions on the excitation frequency is found in nonpolar solvents. The peak positions of the C=O stretching mode also show small excitation energy dependence in chloroform and in methanol. The extent of the excitation energy dependence is correlated with the amount of the fluctuation of the local field on PB molecule exerted by the solvent, which had been estimated from the bandwidths of the absorption and the Raman spectra of PB in solution. The IR absorption spectra of PB are measured in chloroform and carbon tetrachloride, and compared with the Raman spectra in the same solvents. The excitation energy dependence is explained by the solvation state selective excitation in the resonance Raman spectroscopy, that is, the inhomogeneity that appears in the absorption spectra of PB also appears in the resonance Raman process.
The first electronic states of Ar2+ studied by high resolution photoelectron spectroscopy
Authors: Signorell R. , Merkt F.
Published: July 2002

The pulsed-field-ionization (PFI) zero-kinetic-energy (ZEKE) photoelectron spectrum of Ar2 has been recorded between 116 500 cm−1 and 128 200 cm−1. The spectrum consists of a long progression of transitions to the vibrational levels of the ground A 2Σ1/2u+ state of Ar2+ with v+ up to 52, a shorter progression of four bands attributed to transitions to the first four vibrational levels of the C 2Π1/2u state and of a single sharp line assigned to the C 2Π3/2u(v+=0)←X 10g+(v=0) transition. Rotational constants of several vibrational levels of the A 2Σ1/2u+ state have been determined from high resolution PFI-ZEKE photoelectron spectra. From these measurements new information on the first electronic states of Ar2+ has been extracted. An analytical potential energy function has been derived for the A 2Σ1/2u+ state which extends to large internuclear distances (beyond 5 Å) and reproduces all measured vibrational energy levels up to v+=52. The adiabatic ionization potential for the photoelectronic transitions to the A 2Σ1/2u+, C 2Π3/2u and C 2Π1/2u states are determined to be 116 591.1±6 cm−1, 126 883.9±3 cm−1 and 128 004.1±5 cm−1, respectively, from which dissociation energies (D0+) of 10 603.7±6 cm−1, 310.8±3 cm−1, and 622.5±5 cm−1 are obtained. The vibrational levels of the C 2Π1/2u state can be described by a Morse potential with ωe=58.9±0.8 cm−1 and ωexe=1.40±0.27 cm−1, respectively. Associative ionization and collisional ionization processes involving argon atom Rydberg states induce spurious peaks in the PFI-ZEKE photoelectron spectrum. Ways to unambiguously identify these spurious peaks are discussed.
Photodissociation dynamics of trifluoroethylene at 157 nm excitation
Authors: Lin J. J. , Hsu T. C. , Hwang D. W. , Lee Y. T. , Yang X.
Published: July 2002

Photodissociation of trifluoroethylene (F2CCFH) at 157 nm has been investigated using photofragment translational spectroscopy. Four dissociation channels have been experimentally observed: molecular HF elimination, H atom elimination, F atom elimination, and double bond breaking. Double bond breaking is found to be the most important channel, while molecular HF elimination and H atom elimination are found to be significant. Contribution from F atom elimination is minor. Product translational energy distributions for all dissociation channels have been determined. The translational energy distributions for all four dissociation channels are peaked away from zero energy. This is quite similar to that of 1,1-difluoroethylene. Branching ratios and averaged energy partitions for all dissociation channels have also been estimated.
Periodic surfaces and cubic phases in mixtures of oil, water, and surfactant
Authors: Ciach Alina , Hołyst Robert
Published: July 2002

We study a ternary mixture of oil, water, and surfactant in the case of equal volume fractions of oil and water using the Landau–Ginzburg model derived from a lattice model of this ternary mixture. We concentrate on a phase region close to a coexistence line between microemulsion and cubic phases. In our model the bicontinuous cubic phases exist in a narrow window of the volume fraction of surfactant ρs≈0.6. The sequence of phase transitions is L→G→D→P→C as we increase ρs along the cubic-microemulsion bifurcation line. Here L stands for the lamellar phase and C for the cubic micellar phase. The gyroid G, primitive P, and diamond D phases are all bicontinuous. The transitions weakly depend on the temperature. The increase of ρs is accompanied by the increase of the surface area per unit volume. In the case of fluctuating monolayers the interface is diffused and the average area of the monolayer per unit volume is larger than the “projected area,” i.e., the area of the surface describing the average position of the monolayer, per unit volume. The effect is the strongest in the L and the weakest in the C structure.
Computer simulation study of liquid CH2F2 with a new effective pair potential model
Authors: Jedlovszky Pál , Mezei Mihaly
Published: July 2002

A new effective pair potential model is proposed for computer simulations of liquid methylene fluoride and used in Monte Carlo simulations on the isothermal-isobaric ensemble at two different temperatures. The new model is able to reproduce the thermodynamic (internal energy, density, heat capacity, vapor-liquid equilibrium) and structural (neutron diffraction data) properties of liquid methylene fluoride with good accuracy. The structure of liquid methylene fluoride is analyzed in detail on the basis of the present simulation at 153 K. It is found that, unlike in liquid water, the preferential location of the nearest neighbors is in the direction of the face centers of the tetrahedron of the central molecule. However, the four nearest neighbors do not surround the central molecule in a highly tetrahedral arrangement: the obtained distribution of the tetrahedral angular order parameter is rather similar to that in liquid argon. Preferential head-to-tail type orientation is found for nearest neighbors, accompanied by a slight preference for antiparallel dipole–dipole arrangement. The orientational correlation of the molecules is found to be rather long ranged, extending over the first coordination shell. The observed preferential nearest neighbor arrangement is resulted from the competition of steric and electrostatic interactions. No evidence for C–H⋅⋅⋅⋅F type hydrogen bonding is found in liquid methylene fluoride.
A statistical rate constant calculation method based on Monte Carlo transition state theory and application to unimolecular dissociation of HFCO
Authors: Okuno Yoshishige
Published: July 2002

A statistical calculation method is proposed for a classical reaction rate constant expressed as the equilibrium flux of systems through a dividing surface that corresponds to the hypersurface at the transition state perpendicular to the reactive normal coordinate. In this method, an efficient microcanonical Monte Carlo sampling method is employed to approximately compute the flux integral for the classical rate constant expression. To determine whether or not each configuration generated by a Markov walk in the Monte Carlo sampling lies on the dividing surface, we present a method to evaluate the value of the reactive normal coordinate at the configuration by means of a transformation of internal coordinates into normal coordinates. We applied the present statistical calculation method to the unimolecular dissociation of HFCO. It was found that the calculated rate constant is much more correct than the rate constant evaluated as the equilibrium flux through an intuitive dividing surface specified by an HCF angle. In addition, a separable and harmonic approximation for vibrational–rotational modes on a rate constant was found to overestimate the rate constant for the dissociation.
State to state He–CO rotationally inelastic scattering
Authors: Antonova Stiliana , Lin Ao , Tsakotellis Antonis P. , McBane George C.
Published: July 2002

Relative integral cross sections for rotational excitation of CO in collisions with He were measured at energies of 72 and 89 meV. The cross sections are sensitive to anisotropy in the repulsive wall of the He–CO interaction. The experiments were done in crossed molecular beams with resonance enhanced multiphoton ionization detection. The observed cross sections display interference structure at low Δj, despite the average over the initial CO rotational distribution. At higher Δj, the cross sections decrease smoothly. The results are compared with cross sections calculated from two high quality potential energy surfaces for the He–CO interaction. The ab initio SAPT surface of Heijmen et al. [J. Chem. Phys. 107, 9921 (1997)] agrees with the data better than the XC(fit) surface of Le Roy et al. [Farad. Disc. 97, 81 (1994)].
Crystallization of power-law fluids: A modified weighted density approximation model with a solid reference state
Authors: Wang Dean C. , Gast Alice P.
Published: July 2002

We investigate the freezing behavior of particles interacting with an inverse nth power potential under the modified weighted density approximation (MWDA) formalism of Denton and Ashcroft [A. R. Denton and N. W. Ashcroft, Phys. Rev. A 39, 470 (1989)]. We model the liquid state with the perturbative hypernetted chain (PHNC) integral equation [H. S. Kang and F. H. Ree, J. Chem. Phys. 103, 3629 (1995)], chosen for its small computational time and high degree of accuracy. The deterioration of MWDA predictions of equilibrium properties with decreasing n is traced to its inability to accurately estimate the free energy in the static solid limit. An improvement in the MWDA theory is suggested by incorporating information on the static lattice into the model. This is done by moderating the direct correlation function via the Ornstein–Zernicke equation. It is found that this new model can drastically improve results for the coexisting densities for inverse nth power fluids. Other properties, such as the Lindemann parameter, are also improved in this new scheme.
Dissociative scattering of fluorocarbon ions from a liquid surface at hyperthermal incident energies
Authors: Koppers Wim R. , Gleeson Michael A. , Lourenço João , Weeding Tina L. , Los Joop , Kleyn Aart W.
Published: July 2002

The dissociative scattering of CF+, CF2+ and CF3+ ions from a perfluoropolyether (PFPE) surface is explored across the hyperthermal incident energy range 50 to 220 eV. A comparative study on internal energy uptake efficacy and on translational energy transfer to the surface in scattering from the PFPE surface is presented. In addition, measurements are performed on C+ scattering in order to examine the energy transfer to the PFPE liquid surface in more detail. The CF3+ ions take up more internal energy as a function of translational energy transfer in the collision than the CF2+ and the CF+ ions. Furthermore, a detailed analysis and discussion is presented on the surface effective mass as a function of incident (fluoro)carbon ion mass. The analysis is based on elastic scattering from a single group. The results are remarkably different from ion/metal surface scattering.
Inter- and intrachain attractions in solutions of flexible polyelectrolytes at nonzero concentration
Authors: Chu J. C. , Mak C. H.
Published: July 2002

Constant temperature molecular dynamics simulations were used to study solutions of flexible polyelectrolyte chains at nonzero concentrations with explicit counterions and unscreened Coulombic interactions. Counterion condensation, measured via the self-diffusion coefficient of the counterions, is found to increase with polymer concentration, but contrary to the prediction of the Manning theory, the renormalized charge fraction on the chains decreases with increasing Bjerrum length without showing any saturation. Scaling analysis of the radius of gyration shows that the chains are extended at low polymer concentrations and small Bjerrum lengths, while at sufficiently large Bjerrum lengths, the chains shrink to produce compact structures with exponents smaller than a Gaussian chain, suggesting the presence of attractive intrachain interactions. A careful study of the radial distribution function of the center-of-mass of the polyelectrolyte chains shows clear evidence that effective interchain attractive interactions also exist in solutions of flexible polyelectrolytes, similar to what has been found for rodlike polyelectrolytes. Our results suggest that the broad maximum observed in scattering experiments is due to clustering of chains.
Crystal-field and charge transfer transitions due to Cr3+ ions in fluorides
Authors: Aramburu J. A. , Moreno M. , Doclo K. , Daul C. , Barriuso M. T.
Published: July 2002

The metal-ligand equilibrium distance, Re, vibrational frequencies, crystal-field (CF) and charge transfer (CT) transition energies, Huang–Rhys factors, Stokes shift, as well as oscillator strengths of allowed CT transitions due to Cr3+ in fluorides, have been investigated using density functional theory. CrF63− and CrF6K8K611+ clusters simulating Cr3+ in the K3CrF6 lattice have been used. In the CrF6K8K611+ cluster the computed equilibrium distance using a generalized gradient approximation functional is Re=1.88 Å and an overall agreement with experimental spectroscopic parameters is achieved. As salient feature it is pointed out that 6% errors in Re can lead to errors of ∼40% and ∼25%, respectively, in the calculated vibrational frequencies and the 10 Dq parameter. Moreover the force constant, kT, involved in the A1g symmetrical mode at the equilibrium distance is shown to be mainly determined by the interaction between the central ion and the ligands while the interaction between the latter ones and K+ neighbors leads to a contribution which is about 25% of kT. The calculated oscillator strengths confirm that the transitions involving the ligand t1u orbital and the antibonding eg*↑ and eg*↓ ones should be the most intense among the nine allowed CT transitions. Moreover, CT transitions connecting two orbitals which do not have the same σ or π character are found to display an oscillator strength much smaller than for the rest of the cases. This idea is shown to be also valid for other transition-metal complexes. The computed Huang–Rhys factor associated with the A1g mode, SA, for the lowest CT transition is shown to be about ten times higher than that corresponding to the T2g4 CF state and explains the experimental bandwidth. The origin of this important difference is discussed. Finally, the electronic relaxation in the first T2u4 CT state is shown to induce an important decrement (∼40%) of the A2g4→A2u4 transition energy and an increase of the total charge on the central ion of only 0.2 e.
Preliminary nuclear magnetic resonance studies of the use of stray magnetic fields to increase the rate of polarization of dipolar nuclei
Authors: Homer John , Robson John H. , Müller Detlef R.
Published: July 2002

For nuclear magnetic resonance (NMR) studies of slowly relaxing dipolar nuclei, a method is proposed to achieve a reduction in the nuclear polarization times. The method relies on the magnetic field dependence of the spin-lattice relaxation times for dipolar nuclei. For different polarizing fields the magnetization relaxation characteristics differ. By using progressively increasing magnetic fields, but lower than that used for analytical purposes, it is possible to transfer from one relaxation pathway to another in order to achieve the optimum rate of polarization. The principles of the proposed method are shown to be borne out by C13 studies of diamonds.
Six-dimensional quantum calculation of the intermolecular bound states for water dimer
Authors: Chen Hua , Liu S. , Light J. C.
Published: July 2002

We present results of six-dimensional bound-state calculations of the vibrations of rigid water dimer based on two older and two very recent potential energy surfaces. The calculations are done by a new sequential diagonalization–truncation approach using the symmetrized angular basis presented most recently by Althorpe and Clary [J. Chem. Phys. 101, 3603 (1994)] and a potential optimized discrete variable representation (DVR) in the monomer–monomer distance coordinate. The lowest ten or so states of each symmetry are apparently converged to 0.5 cm−1 using a coupled angular basis of Wigner rotation functions with Jmax=11 and mmax=5 on each monomer. The results differ significantly from the results presented by Leforestier et al. [J. Chem. Phys. 106, 8527 (1997)] and demonstrate that the ASP-S potential yields more accurate tunneling splittings than the more recent ASP-Wx potentials [C. Millot et al., J. Phys. Chem. A 102, 754 (1998)].
Photodetachment photoelectron spectroscopy of doubly charged anions: S2O82−
Authors: Ding Chuan-Fan , Wang Xue-Bin , Wang Lai-Sheng
Published: July 2002

A photodetachment photoelectron spectroscopy study of a doubly charged anion (S2O82−) in the gas phase is reported at three photon energies: 193, 266, 355 nm. Adiabatic and vertical electron binding energies of S2O82− were measured to be 1.7±0.2 and 2.2±0.1 eV, respectively. Several excited states were observed for the S2O8− singly charged anion at 193 nm. The repulsive Coulomb barrier that binds multiply charged anions was clearly shown in the detachment spectra of S2O82−. The effects of the Coulomb barrier to the detachment spectra and electron tunneling through the barrier are presented. The barrier height, which is equivalent to the magnitude of the Coulomb repulsion between the two excess charges in S2O82− was estimated to be ∼3 eV.
Femtosecond photoelectron spectroscopy of the I2− anion: Characterization of the Ã′ 2Πg,1/2 excited state
Authors: Zanni Martin T. , Batista Victor S. , Greenblatt B. Jefferys , Miller William H. , Neumark Daniel M.
Published: July 2002

A potential energy curve for the Ã′ 2Πg,1/2 state of I2− is constructed based on femtosecond photoelectron spectroscopy of the I2− Ã′ 2Πg,1/2←X̃ 2Σu+ transition at 780 nm. The experiment is sensitive to the slope of the repulsive potential wall, the well depth, equilibrium bond length, and the long-range attractive portion of the upper state potential. The Ã′ 2Πg,1/2 potential is fit to a piecewise potential which is flexible in each of these regions. Simulations of the spectrum using a previously determined Morse potential for the X̃ 2Σu+ state of I2− [J. Chem. Phys. 107, 7613 (1997)] yields a well depth (De) of 0.017±0.010 eV for the Ã′ 2Πg,1/2 state with an equilibrium bond length (Re) of 6.2±0.6 Å. These values differ significantly from previous semiempirical results.
All the nonadiabatic (J=0) bound states of NO2
Authors: Salzgeber R. F. , Mandelshtam V. A. , Schlier Ch. , Taylor H. S.
Published: July 2002

We calculated all 3170 A1 and B2 (J=0) vibronic bound states of the coupled electronic ground (X̃ 2A1) and the first excited (Ã 2B2) surfaces of NO2, using a modification of the ab initio potentials of Leonardi et al. [J. Chem. Phys. 105, 9051 (1996)]. The calculation was performed by harmonic inversion of the Chebyshev correlation function generated from a DVR Hamiltonian in Radau coordinates. The rms error of the eigenenergies is about 2.5 cm−1, corresponding to a relative error of 10−4 near the dissociation energy. The results are compared with the adiabatic and diabatic levels calculated from the same surfaces, with experimental data, and with some approximations for the number of states function N(E). The experimental levels are reproduced fairly well up to an energy of 12 000 cm−1 above the potential minimum while the total number of bound levels agrees to within 2% with that calculated from the phase space volume.
A natural orbital diagnostic for multiconfigurational character in correlated wave functions
Authors: Gordon Mark S. , Schmidt Michael W. , Chaban Galina M. , Glaesemann Kurt R. , Stevens Walter J. , Gonzalez Carlos
Published: July 2002

The natural orbitals and their corresponding occupation numbers are constructed for several interesting problems to demonstrate that the existence of negative natural orbital occupation numbers for single reference correlation methods provides a simple diagnostic for the need for a multiconfigurational description of the wave function.
The photodissociation of the vinyl radical (C2H3) at 243 nm studied by velocity map imaging
Authors: Ahmed Musahid , Peterka Darcy S. , Suits Arthur G.
Published: July 2002

The technique of velocity map imaging (VELMI) has been applied to study the photodissociation of the vinyl radical (C2H3) at 243.2 nm in a molecular beam. Using momentum conservation, we show that the primary product is singlet vinylidene [H2CC(X̃ 2A′)], or singlet acetylene at energies where interconversion between the H2CC and HCCH geometries is facile. In addition, a minor contribution is seen which is assigned to triplet acetylene [C2H2(ã 3B2)]. We argue that out-of-plane motion of the third H atom is necessary to bring the excited states, of A″ symmetry, to an A′ symmetry leading to products, and the observed tranlsational energy distribution may show evidence of the barrier to this process. The heat of formation of vinylidene is derived to be 100.3±4.0 kcal/mol, in agreement with literature values. From the translational energy release, we derive the T0 for triplet acetylene C2H2(ã 3B2) to be 28 900 cm−1, which does not agree well with recent ab initio calculations. Possible reasons for the disagreement are discussed.
Quantitative characterization of the (D2O)3 torsional manifold by terahertz laser spectroscopy and theoretical analysis
Authors: Viant Mark R. , Brown Mac G. , Cruzan Jeff D. , Saykally Richard J. , Geleijns Michel , van der Avoird Ad
Published: July 2002

We report the measurement of two new perpendicular (D2O)3 torsional bands by terahertz laser vibration–rotation–tunneling (VRT) spectroscopy of a planar pulsed supersonic expansion. The first (28.0 cm−1) band corresponds to the k=±2l←0 transition, and is the lowest frequency vibrational spectrum observed for a water cluster. The second (81.8 cm−1) band originates in the first excited torsional state, and has been assigned as k=3u←±1l. An effective three-dimensional Hamiltonian is derived to describe the rotational structure of each torsional state. Degenerate torsional levels with k=±1 and k=±2 exhibit a Coriolis splitting linear in K implying the presence of vibrational angular momentum, and a second-order splitting from off-diagonal coupling between degenerate sublevels with +|k| and −|k|. With this effective Hamiltonian we fit a total of 554 rovibrational transitions in five different bands connecting the lowest nine torsional states, with a rms residual of 1.36 MHz. The data set comprises the two new VRT bands together with the 41.1 cm−1 parallel band, the 89.6 cm−1 parallel band, and the 98.1 cm−1 perpendicular band. This analysis provides a comprehensive characterization of the torsional energy levels in (D2O)3 up to 100 cm−1 above the zero-point energy, and confirms the torsional assignments for all five (D2O)3 VRT bands observed to date. Moreover, it vindicates the adiabatic separation of the trimer torsional and hydrogen bond stretch/bend vibrational modes which underlies the torsional model.
Formation free energy of clusters in vapor-liquid nucleation: A Monte Carlo simulation study
Authors: Oh K. J. , Zeng X. C.
Published: July 2002

The formation free energy of clusters in a supersaturated vapor is obtained by a constrained Monte Carlo technique. A key feature of this approach is to set an upper limit to the size of cluster. This maximum cluster size serves essentially as an extra thermodynamic variable that constrains the system. As a result, clusters larger than the critical cluster of nucleation in the supersaturated vapor can no longer grow beyond the limiting size. Like changing the overall density of the system, changing the maximum cluster size also results in a different supersaturation and thereby a different formation free energy. However, at the same supersaturation and temperature it is found that the formation free energy has a unique value, independent of the upper limit of cluster size. The predicted size of critical cluster of nucleation is found to be consistent with the nucleation theorem as well as previous results using different simulation approaches.
The photodissociation of tert-butyl nitrite at 193 nm
Authors: Finke H. , Spiecker H. , Andresen P.
Published: July 2002

The photodissociation of tert-butyl nitrite, (CH3)3CONO, at 193 nm, is examined by a new experimental setup for photofragment imaging. Probing nascent NO fragments time delayed (several μs) and spatially displaced (relative to the dissociation event) by planar laser-induced fluorescence (PLIF) yields quantum state specific velocity distributions. In this way the internal and the translational energy distributions are determined simultaneously, permitting a very detailed understanding of the energy partitioning of the available energy among the product degrees of freedom and allowing one to analyze correlated product formation. The results differ very strongly from previous photodissociation studies of aliphatic nitrites performed at smaller photon energies, since a different dissociation channel is taken.
Identifying physical clusters in vapor phase nucleation
Authors: Kusaka Isamu , Oxtoby David W.
Published: July 2002

We present a new approach to cluster simulation as a step toward a molecular theory of vapor phase nucleation. This approach does not involve a cluster criterion which is commonly introduced to determine whether a given molecule in the system belongs to the vapor or to a cluster for any instantaneous configuration of molecules. Instead, the stochastic evolution of the metastable vapor phase was explored in a grand canonical Monte Carlo (MC) simulation aided by the umbrella sampling technique. The physical clusters, i.e., density fluctuations, that lead to nucleation emerge naturally as we attain a coarse-grained description of this stochastic process by introducing proper order parameters, in particular the molecular content and the potential energy of the system suitably chosen in the vapor. Our method also allows an efficient evaluation of the free energy of cluster formation as a function of these order parameters. The method was applied to a Lennard-Jones fluid at temperatures above the bulk melting point. When the physical clusters thus identified were further subjected to a molecular dynamics (MD) simulation, the system was found to evolve along a quasi-continuous trajectory while closely following the valley passing through the saddle point of the free energy surface, thereby demonstrating the relevance of these clusters in the dynamics of nucleation.
On the question of empirical corrections in ab initio model chemistries
Authors: Merrill G. N. , Gordon M. S.
Published: July 2002

Model chemistries that employ additivity schemes have become increasingly popular within the chemical community. These approaches are predicated upon the assumption that less rigorous (and, therefore, less expensive) calculations may be combined to approximate a more accurate (and otherwise intractable) level of theory. Most of these models make some use of an empirical correction, the desirability of which is at best questionable. The present paper critically examines the importance of one such correction (the “higher-level correction”) to the most widely used additivity method (the Gaussian model). An alternative approach, which does not depend upon any sort of ad hoc empiricism, is also outlined. The current model is found to offer accuracy comparable to that of the Gaussian model.
Car–Parrinello molecular dynamics on excited state surfaces
Authors: Bittner Eric R. , Kosov D. S.
Published: July 2002

This paper describes a method to do ab initio molecular dynamics in electronically excited systems within the random phase approximation (RPA). Using a dynamical variational treatment of the RPA frequency, which corresponds to the electronic excitation energy of the system, we derive coupled equations of motion for the RPA amplitudes, the single particle orbitals, and the nuclear coordinates. These equations scale linearly with basis size and can be implemented with only a single holonomic constraint. Test calculations on a model two level system give exact agreement with analytical results. Furthermore, we examined the computational efficiency of the method by modeling the excited state dynamics of a one-dimensional polyene lattice. Our results indicate that the present method offers a considerable decrease in computational effort over a straight-forward configuration interaction (singles) plus gradient calculation performed at each nuclear configuration.
Characterization of the ground X̃  2Π state of the complexes R⋅SH (R=Ne,Ar,Kr)
Authors: Yang Min-Chieh , Carter Christopher C. , Miller Terry A.
Published: July 2002

Information characterizing the X̃  2Π state of the R⋅SH (R=Ne,Ar,Kr) complexes has been obtained from two complementary experimental techniques. The spin-vibronic energy levels have been determined by wavelength resolved fluorescence spectroscopy subsequent to laser excitation of specific vibrational levels of the Ã 2Σ+ state. In addition, several “hot” bands from excited spin-vibronic levels of X̃ 2Π Ne⋅SH have been observed and assigned. The experimental data have been used to construct a simple model for the ground state potential energy surface for each complex. These models show that the most stable conformation for each complex is linear H-bonded, but the barrier to isomerization to the S-bonded complex is quite low. The overall bonding is somewhat weaker and more isotropic than the corresponding hydroxyl complexes.
Electronic isomers in [(CO2)nROH]− cluster anions. I. Photoelectron spectroscopy
Authors: Tsukuda Tatsuya , Saeki Morihisa , Kimura Ryoichi , Nagata Takashi
Published: July 2002

Photoelectron spectra of [(CO2)n−1ROH]− (R=H and CH3) with 2⩽n⩽7 have been measured at a photon energy of 4.66 eV. Analysis of the photoelectron band envelopes has revealed that the spectra of [(CO2)n−1H2O]− with 3⩽n⩽5 consist of two band components. The maximum of each component corresponds to the vertical detachment energy (VDE) of the relevant anionic species. In each spectrum the VDE values for the two components differ by ≈1 eV. For example, the [(CO2)4H2O]− spectrum is characterized by two VDE values of 2.63±0.04 and 3.71±0.06 eV. From the VDE difference, we conclude that the observed two components arise from isomers having different electronic structures, and that these “electronic isomers” can be designated as C2O4−⋅H2O(CO2)n−3 and CO2−⋅H2O(CO2)n−2. Coexistence of electronic isomers occurs also in [(CO2)n−1CH3OH]−, but only at n=3. The [(CO2)n−1CH3OH]− anions with n≠3 display photoelectron spectra composed of a single broad band, which corresponds to photodetachment from CO2−⋅CH3OH(CO2)n−2 structure. The CO2−⋅ROH(CO2) isomers (n=3) are found to be distinguishable from C2O4−⋅ROH by selective photodepletion at 532 nm during the time of flight in the mass spectrometer, implying that the two electronic isomers of [(CO2)2ROH]− coexist without interconversion. In contrast, it is revealed that [(CO2)4H2O]− (n=5) are fluctuating between C2O4−⋅H2O(CO2)2 and CO2−⋅H2O(CO2)3 structures. Based on these experimental findings, combined with results obtained by recent ab initio calculations, a possible isomerization mechanism operative in [(CO2)n−1ROH]− is proposed.
A viscoelastic continuum model of nonpolar solvation. III. Electron solvation and nonlinear coupling effects
Authors: Berg Mark A.
Published: July 2002

The viscoelastic (VE) continuum model of solvation developed in the first paper of this series [J. Phys. Chem. A 102, 17 (1998)] is applied to solvation of the electron in water and is compared to the computer simulations of Rossky and co-workers. The theory correctly predicts both the inertial and diffusive solvation times for both injected electrons and electrons excited to the p state. These times are associated with the speed of phonon propagation and the rate of shear relaxation respectively. The ability of the VE model to predict the inertial solvation time shows that continuum models are a valuable first approximation, even at very short times. The full solvation response function, the time-dependent cavity shape and the effect of deuteration are also all reproduced accurately for solvation of the p state. The effect of a shape change in the excited state of the electron is compared to the effect of a size change. A shape change produces a low amplitude, picosecond tail in the solvation response function, which is not present with a purely spherical size change. The theory is extended to include quadratic terms in the solvation difference potential. This nonlinearity accounts for the largest differences between the solvation response function in the ground and excited states of the electron. All the major features seen in the simulations can be accounted for by mechanical relaxation of the solvent. At present, there is no compelling indication of a significant role for dielectric relaxation, although the issue merits further investigation.
Improved treatment of the lateral-reptative mixing and approximate analytical expressions for the lateral motion model of polymer melts
Authors: Herman Michael F. , Panajotova Biliana V.
Published: July 2002

An improved treatment of the coupling between the lateral and reptative motions in the lateral motion model of polymer melt dynamics is presented. Approximate analytical formulas are also provided for the monomer and center of mass mean squared displacements and for the diffusion constant. Scalings are given for all predicted regions, and expressions are presented for the corresponding crossover times and chain lengths. Results from the full calculations and the analytical expressions are compared with published data from simulations and experimental systems. The theoretical predictions are in excellent agreement with the simulation and experimental data. The results also demonstrate that the approximation analytical formulas agree extremely well with the full calculations. Given the simplicity and accuracy of these analytical expressions, they should facilitate the implementation of the theory and its comparison to experimental data.
Exploring the reaction dynamics of nitrogen atoms: A combined crossed beam and theoretical study of N(2D)+D2→ND+D
Authors: Alagia M. , Balucani N. , Cartechini L. , Casavecchia P. , Volpi G. G. , Pederson L. A. , Schatz G. C. , Lendvay G. , Harding L. B. , Hollebeek T. , Ho T.-S. , Rabitz H.
Published: July 2002

In the first successful reactive scattering study of nitrogen atoms, the angular and velocity distribution of the ND product from the reaction N(2D)+D2 at 5.1 and 3.8 kcal/mol collision energies has been obtained in a crossed molecular beam study with mass spectrometric detection. The center-of-mass product angular distribution is found to be nearly backward–forward symmetric, reflecting an insertion dynamics. About 30% of the total available energy goes into product translation. The experimental results were compared with those of quasiclassical trajectory calculations on an accurate potential energy surface obtained from large scale ab initio electronic structure computations. Good agreement was found between the experimental results and the theoretical predictions.
Quantum effects in the dissociative adsorption of hydrogen
Authors: Groß Axel
Published: July 2002

Three-dimensional quantum and classical dynamical calculations of the dissociative adsorption of hydrogen have been performed, in which, besides one reaction path coordinate, the lateral degrees of freedom of the hydrogen center of mass were taken into account. These calculations were compared to results obtained by classical and quantum sudden approximations, which assessed the importance of tunneling, zero-point effects, and also steering in the dissociative adsorption of hydrogen. For energies below the minimum barrier height, tunneling is of course the relevant mechanism for dissociation, but above the minimum barrier height quantization and zero-point effects become more prominent. Zero-point effects suppress the dissociation probability; however, for energies slightly above the minimum barrier height, steering of the particles is only operative in the quantum dynamics and can thereby almost compensate the suppression of the quantum sticking probabilities due to zero-point effects, compared to the classical calculations. The consequences of these findings with respect to the concept of zero-point corrections in order to obtain effective quantum barrier heights are discussed. The results presented in this study should be relevant for the reaction and propagation dynamics in all systems containing hydrogen.
Hydrogen atom release from methyl groups of energized molecules
Authors: Min Zhiyuan , Wong Teh-Hwa , Bersohn Richard
Published: July 2002

Three diverse molecules, 2,3,4,5,6-pentafluorotoluene (PFT), trimethylamine (TMA), and methyl bromide were irradiated by weak 193.3 nm light. In each case hydrogen atoms were released, as the major channel for PFT and as a minor channel for the two others. The low average translational energy release, the isotropic angular distribution, and the linear excimer power dependence all lead to the conclusion that the dissociation is from a very hot ground state. The unimolecular dissociation is less selective than in the more usual unimolecular processes at much lower energies. During these experiments an unusual multiphoton dissociation was found. Focusing 205.1 nm light on TMA produces H atoms with an average kinetic energy of 4.2±0.5 eV.
On the consistent definition of spin–orbit effects calculated by relativistic effective core potentials with one-electron spin–orbit operators: Comparison of spin–orbit effects for Tl, TlH, TlH3, PbH2, and PbH4
Authors: Han Young-Kyu , Bae Cheolbeom , Lee Yoon Sup
Published: July 2002

The spin–orbit effects for Tl, TlH, TlH3, PbH2, and PbH4 are evaluated by two-component calculations using several relativistic effective core potentials (RECP) with one-electron spin–orbit operators. The used RECPs are shape-consistent RECPs derived by Wildman et al. [J. Chem. Phys. 107, 9975 (1997)] and three sets of energy-consistent (or adjusted) RECPs published by Schwerdtfeger et al. [Phys. Scr. 36, 453 (1987); J. Chem. Phys. 90, 762 (1989)], Küchle et al. [Mol. Phys. 74, 1245 (1991)], and Leininger et al. [Chem. Phys. 217, 19 (1997)]. The shape-consistent RECP results are in very good agreement with the Küchle et al. energy-consistent RECP results for all the molecules studied here and all-electron results for TlH. The RECPs of Schwerdtfeger et al. and Leininger et al. seem to provide qualitatively different spin–orbit effects. If one defines spin-free RECP as the potential average of the corresponding two-component RECP, all RECPs give very similar spin–orbit effects for all the cases. Most of the discrepancies of molecular spin–orbit effects among various RECPs reported in the literature may originate from different definitions of RECPs with or without a spin–orbit term and not from the inherent difference in spin–orbit operators.
Accurate structures and binding energies for small water clusters: The water trimer
Authors: Nielsen Ida M. B. , Seidl Edward T. , Janssen Curtis L.
Published: July 2002

The global minimum on the water trimer potential energy surface has been investigated by means of second-order Mo/ller-Plesset (MP2) perturbation theory employing the series of correlation-consistent basis sets aug-cc-pVXZ (X = D, T, Q, 5, 6), the largest of which contains 1329 basis functions. Definitive predictions are made for the binding energy and equilibrium structure, and improved values are presented for the harmonic vibrational frequencies. A value of 15.82±0.05 kcal mol−1 is advanced for the infinite basis set frozen core MP2 binding energy, obtained by extrapolation of MP2 correlation energies computed at the aug-cc-pVQZ MP2 geometry. Inclusion of core correlation, using the aug-cc-pCV5Z basis set, has been found to increase the binding energy by 0.08 kcal mol−1, and after consideration of core correlation and higher-order correlation effects, the classical binding energy for the water trimer is estimated to be 15.9±0.2 kcal mol−1. A zero-point vibrational correction of −5.43 kcal mol−1 has been computed from aug-cc-pVTZ MP2 harmonic vibrational frequencies. The accuracy of different computational schemes for obtaining the binding energies of the water dimer and trimer has been investigated, and computationally feasible methods are suggested for obtaining accurate structures and binding energies for larger water clusters.
Photoinitiated electron transfer to selected physisorbed alkyl bromides: The effects of alkyl chain length on dissociation cross sections
Authors: Khan K. A. , Camillone N. , Osgood R. M.
Published: July 2002

We report the results of measurements of the cross section as a function of wavelength (351, 248, and 193 nm) for photoinitiated dissociative electron attachment to three normal alkyl bromides [CH3(CH2)n−1Br, n=1, 2, and 3] physisorbed on GaAs(110). Upon UV exposure, the molecules undergo C–Br bond cleavage due to a substrate-mediated electron-transfer process. The cross sections for all three molecules increase monotonically with decreasing wavelength. Our results suggest a ∼1 eV higher threshold for dissociation of ethyl and propyl bromide than for methyl bromide. A simple model of the electron-transfer process is employed to estimate the peak per-electron cross section for dissociative attachment in the monolayer. We find that the cross sections for the physisorbed molecules are approximately five times smaller than those for gas-phase molecules, due to a reduction in the lifetime of the molecular anion in the vicinity of the surface. In addition, we also find an increase in cross section with chain length very similar to that observed in the gas phase; the gas-phase behavior has been explained by an increase in the anion lifetime with chain length. Our results suggest that while quenching of the molecular anion at the surface is important, it does not eliminate the progression of anion lifetime with chain length.
Adsorption of ethylene on the Ge(100)-2×1 surface: Coverage and time-dependent behavior
Authors: Lal Pratik , Teplyakov Andrew V. , Noah Yétundé , Kong Maynard J. , Wang George T. , Bent Stacey F.
Published: July 2002

Studies of the adsorption and thermal chemistry of ethylene on the Ge(100)-2×1 surface have been performed. The results of multiple internal reflection Fourier transform infrared (MIR-FTIR) spectroscopy and temperature programmed desorption (TPD) show that ethylene chemisorbs molecularly on the Ge(100)-(2×1) surface at room temperature. Infrared spectroscopy gives evidence for only one adsorbate configuration on this surface at room temperature, consistent with a structure in which ethylene bridges across a germanium dimer. However, TPD measurements show two molecular desorption features at almost all coverages, indicating that at least two adsorption states can be formed. Further shifts in one of the peaks with both coverage and time, paralleled by changes in the vibrational spectrum, suggest the presence of attractive intermolecular interactions or cooperative effects. The complex time- and coverage dependence of ethylene adsorption on Ge(100)-2×1 is analyzed using a two-state kinetic model.
Direct molecular simulation of gradient-driven diffusion of large molecules using constant pressure
Authors: Thompson Aidan P. , Heffelfinger Grant S.
Published: July 2002

Dual control volume grand canonical molecular dynamics (DCV-GCMD) is a boundary-driven nonequilibrium molecular-dynamics technique for simulating gradient-driven diffusion in multicomponent systems. Two control volumes are established at opposite ends of the simulation box. Constant temperature and chemical potential of diffusing species are imposed in the control volumes (i.e., constant-μ1⋯μn−1μnVT). This results in stable chemical potential gradients and steady-state diffusion fluxes in the region between the control volumes. We present results and detailed analysis for a new constant-pressure variant of the DCV-GCMD method in which one of the diffusing species for which a steady-state diffusion flux exists does not have to be inserted or deleted. Constant temperature, pressure, and chemical potential of all diffusing species except one are imposed in the control volumes (i.e., constant-μ1⋯μn−1NnPT). The constant-pressure method can be applied to situations in which insertion and deletion of large molecules would be prohibitively difficult. As an example, we used the method to simulate diffusion in a binary mixture of spherical particles with a 2:1 size ratio. Steady-state diffusion fluxes of both diffusing species were established. The constant-pressure diffusion coefficients agreed closely with the results of the standard constant-volume calculations. In addition, we show how the concentration, chemical potential, and flux profiles can be used to calculate local binary and Maxwell–Stefan diffusion coefficients. In the case of the 2:1 size ratio mixture, we found that the binary diffusion coefficients were asymmetric and composition dependent, whereas the Maxwell–Stefan diffusion coefficients changed very little with composition and were symmetric. This last result verified that the Gibbs–Duhem relation was satisfied locally, thus validating the assumption of local equilibrium.