bluuebunny/crossref_metadata_2025 huggingface dataset:
Investigating bonding in small silicon–carbon clusters: Exploration of the potential energy surfaces of Si3C4, Si4C3, and Si4C4 using ab initio molecular dynamics
Authors: Bertolus Marjorie , Finocchi Fabio , Millié Philippe
Published: February 2004

A theoretical investigation of the properties of the Si3C4, Si4C3, and Si4C4 clusters is reported. Systematic explorations of the potential energy surfaces of the three clusters are performed using a combination of ab initio molecular dynamics and local energy minimizations using density functional theory. A large number of isomers with a large variety of geometries has been found. The geometries, energies, and vibrational frequencies yielded are discussed. Furthermore, a quantitative analysis of the interatomic distances, angles, and coordination numbers observed, as well as the conclusions on the bonding properties, are presented. The cluster properties are then compared to those of solid SiC and of the smaller Si–C clusters (with size up to 6) obtained in a previous study. Analysis of our results and comparison with bulk properties show that even clusters as small as Si3C4, Si4C3, and Si4C4 exhibit properties similar to those of the amorphous bulk, in particular as for the structures and bonds formed by C atoms.
First principles study of CO oxidation on TiO2(110): The role of surface oxygen vacancies
Authors: Wu Xueyuan , Selloni Annabella , Nayak Saroj K.
Published: February 2004

The reactivities of the stoichiometric and partially reduced rutile TiO2(110) surfaces towards oxygen adsorption and carbon monoxide oxidation have been studied by means of periodic density functional theory calculations within the Car–Parrinello approach. O2 adsorption as well as CO oxidation are found to take place only in the presence of surface oxygen vacancies (partially reduced surface). The oxidation of CO by molecularly adsorbed O2 at the O-vacancy site is found to have an activation energy of about 0.4 eV. When the adsorbed O2 is dissociated, the resulting adatoms can oxidize incoming gas-phase CO molecules with no barrier. In all studied cases, once CO is oxidized to form CO2, the resulting surface is defect-free and no catalytic cycle can be established.
Low-energy electron scattering by cubane: Resonant states and Ramsauer–Townsend features from quantum calculations in the gas phase
Authors: Gianturco F. A. , Lucchese R. R. , Grandi A. , Sanna N.
Published: February 2004

Calculations are carried out, using a nonempirical modeling of the interaction potential and solving the quantum scattering coupled channel equations, for low energy electron scattering from cubane (C8H8) molecules in the gas phase. Total integral cross sections are obtained and partial contributions are analyzed for the most important irreducible representations that describe the continuum electron in the Oh molecular symmetry. Several trapping resonances are found and analyzed in terms of the molecular-type features of the resonant electron states associated with them. A Ramsauer–Townsend minimum is also found and its possible behavior related to features of the scattering length as k→0.
Electric conductivities of 1:1 electrolytes in liquid methanol along the liquid–vapor coexistence curve up to the critical temperature. I. NaCl, KCl, and CsCl solutions
Authors: Hoshina Taka-aki , Tsuchihashi Noriaki , Ibuki Kazuyasu , Ueno Masakatsu
Published: February 2004

The molar conductivities Λ of NaCl, KCl, and CsCl in liquid methanol were measured in the concentration range of (0.3–2.0)×10−3 mol dm−3 and the temperature range of 60–240 °C along the liquid–vapor coexistence curve. The temperature range corresponds to the solvent density range of (2.78–1.55)ρc, where ρc=0.2756 g cm−3 is the critical density of methanol. The concentration dependence of Λ at each temperature and density (pressure) has been analyzed by the Fuoss–Chen–Justice equation to obtain the limiting molar conductivity Λ0 and the molar association constant KA. For all the electrolytes studied, Λ0 increased almost linearly with decreasing density at densities above 2.0ρc, while the opposite tendency was observed at lower densities. The relative contribution of the nonhydrodynamic effect on the translational friction coefficient ζ was estimated in terms of Δζ/ζ, where the residual friction coefficient Δζ is the difference between ζ and the Stokes friction coefficient ζS. At densities above 2.0ρc, Δζ/ζ increased with decreasing density though ζ and Δζ decrease, and the tendencies are common for all the ions studied. The density dependences of ζ and Δζ/ζ were explained well by the Hubbard–Onsager (HO) dielectric friction theory based on the sphere-in-continuum model. At densities below 2.0ρc, however, the experimental results cannot be explained by the HO theory.
W3 theory: Robust computational thermochemistry in the kJ/mol accuracy range
Authors: Boese A. Daniel , Oren Mikhal , Atasoylu Onur , Martin Jan M. L. , Kállay Mihály , Gauss Jürgen
Published: February 2004

We are proposing a new computational thermochemistry protocol denoted W3 theory, as a successor to W1 and W2 theory proposed earlier [Martin and De Oliveira, J. Chem. Phys. 111, 1843 (1999)]. The new method is both more accurate overall (error statistics for total atomization energies approximately cut in half) and more robust (particularly towards systems exhibiting significant nondynamical correlation) than W2 theory. The cardinal improvement rests in an approximate account for post-CCSD(T) correlation effects. Iterative T3 (connected triple excitations) effects exhibit a basis set convergence behavior similar to the T3 contribution overall. They almost universally decrease molecular binding energies. Their inclusion in isolation yields less accurate results than CCSD(T) nearly across the board: It is only when T4 (connected quadruple excitations) effects are included that superior performance is achieved. T4 effects systematically increase molecular binding energies. Their basis set convergence is quite rapid, and even CCSDTQ/cc-pVDZ scaled by an empirical factor of 1.2532 will yield a quite passable quadruples contribution. The effect of still higher-order excitations was gauged for a subset of molecules (notably the eight-valence electron systems): T5 (connected quintuple excitations) contributions reach 0.3 kcal/mol for the pathologically multireference X 1Σg+ state of C2 but are quite small for other systems. A variety of avenues for achieving accuracy beyond that of W3 theory were explored, to no significant avail. W3 thus appears to represent a good compromise between accuracy and computational cost for those seeking a robust method for computational thermochemistry in the kJ/mol accuracy range on small systems.
Quantum and classical studies of the O(3P)+H2(v=0–3,j=0)→OH+H reaction using benchmark potential surfaces
Authors: Braunstein M. , Adler-Golden S. , Maiti B. , Schatz G. C.
Published: February 2004

We present results of time-dependent quantum mechanics (TDQM) and quasiclassical trajectory (QCT) studies of the excitation function for O(3P)+H2(v=0–3,j=0)→OH+H from threshold to 30 kcal/mol collision energy using benchmark potential energy surfaces [Rogers et al., J. Phys. Chem. A 104, 2308 (2000)]. For H2(v=0) there is excellent agreement between quantum and classical results. The TDQM results show that the reactive threshold drops from 10 kcal/mol for v=0 to 6 for v=1, 5 for v=2 and 4 for v=3, suggesting a much slower increase in rate constant with vibrational excitation above v=1 than below. For H2(v>0), the classical results are larger than the quantum results by a factor ∼2 near threshold, but the agreement monotonically improves until they are within ∼10% near 30 kcal/mol collision energy. We believe these differences arise from stronger vibrational adiabaticity in the quantum dynamics, an effect examined before for this system at lower energies. We have also computed QCT OH(v′,j′) state-resolved cross sections and angular distributions. The QCT state-resolved OH(v′) cross sections peak at the same vibrational quantum number as the H2 reagent. The OH rotational distributions are also quite hot and tend to cluster around high rotational quantum numbers. However, the dynamics seem to dictate a cutoff in the energy going into OH rotation indicating an angular momentum constraint. The state-resolved OH distributions were fit to probability functions based on conventional information theory extended to include an energy gap law for product vibrations.
A density-functional study of Al-doped Ti clusters: TinAl (n=1–13)
Authors: Xiang J. , Wei S. H. , Yan X. H. , You J. Q. , Mao Y. L.
Published: February 2004

Equilibrium geometries, stabilities, and electronic properties of TinAl (n=1–13) clusters have been studied by using density-functional theory with local spin density approximation and generalized gradient approximation. The ground-state structures of TinAl clusters have been obtained. The resulting geometries show that the aluminum atom remains on the surface of clusters for n<9, but is slowly getting trapped beyond n=9, meanwhile, the Al atom exhibits a valent transition from monovalent to trivalent. The geometric effects and electronic effects clearly demonstrate the Ti4Al cluster to be endowed with special stability. The studies on the bonds indicate the change from ionic to metalliclike.
An ab initio potential energy surface and predissociative resonances of HArF
Authors: Li Hui , Xie Daiqian , Guo Hua
Published: February 2004

A three-dimensional potential energy surface of the ground electronic state HArF is constructed from more than 2000 ab initio points at the multireference averaged quadratic coupled-cluster level employing an augmented large basis set. The calculations indicate that the linear HArF molecule is metastable with a barrier of 0.643 eV in the atomization (HArF→H+Ar+F) channel and a barrier of 1.017 eV in the dissociation (HArF→Ar+HF) channel. Variational calculations of low-lying predissociative resonances of both HArF and DArF are performed on the three-dimensional potential energy surface using a complex-symmetric Lanczos propagation method, which yields both positions and widths of the resonance states. The resonance lifetime generally decreases with energy, but strong mode selectivity exists. Reasonably good agreement with experiment confirms the accuracy of our potential. These calculations provide valuable information on the stability and dynamics of HArF/DArF in its ground electronic state.
Ab initio configuration interaction study of the low-lying Σ+1 electronic states of LiCl
Authors: Weck P. F. , Kirby K. , Stancil P. C.
Published: February 2004

Ab initio configuration interaction calculations have been performed for the X 1Σ+ and B 1Σ+ electronic states of LiCl. Potential energy curves, dipole moment functions, and dipole transition moments have been computed for internuclear distances between R=2.5a0 and 50a0. Single- and double-excitation configuration interaction wave functions were constructed using molecular orbitals obtained from a two-state averaged multiconfiguration self-consistent-field calculation. This procedure yielded an accurate energy splitting between the covalent and ionic separated-atom limits. The calculated avoided crossing of the X and B state curves occurs at R=16.2a0, in close agreement with previous calculations using a semiempirical covalent–ionic resonance model. X 1Σ+ state spectroscopic constants are in excellent agreement with experimental values.
The microwave and infrared spectroscopy of benzaldehyde: Conflict between theory and experimental deductions
Authors: Speakman Lucas D. , Papas Brian N. , Woodcock H. Lee , Schaefer Henry F.
Published: February 2004

Recently, it has been proposed that ab initio calculations cannot accurately treat molecules comprised of a benzene ring with a π-conjugated substituent, for example, benzaldehyde. Theoretical predictions of the benzaldehyde barrier to internal rotation are typically a factor of 2 too high in comparison to the experimental values of 4.67 (infared) and 4.90 (microwave) kcal mol−1. However, both experiments use Pitzer’s 1946 model to compute the reduced moment of inertia and employ the experimentally observed torsional frequency to deduce benzaldehyde’s rotational barrier. When Pitzer’s model is applied to a system with a nonconjugated functional group, such as phenol, the model and theoretical values are in close agreement. Therefore, we conclude the model may not account for conjugation between the substituent and the π-system of benzene. The experimental values of the benzaldehyde rotational barrier are therefore misleading. The true rotational barrier lies closer to the theoretically extrapolated limit of 7.7 kcal mol−1, based on coupled cluster theory.
An ab initio study of the ground and valence excited states of GaF
Authors: Yang Xinzheng , Lin Meirong , Zhang Baozheng
Published: February 2004

Ab initio calculations on the ground and valence excited states of the GaF molecule have been performed by using the internally contracted multireference electronic correlation methods (MR-CISD, MR-CISD+Q, and MR-AQCC) with entirely uncontracted all-electronic basis sets and Douglas–Kroll scalar relativistic correction. The potential energy curves of all valence states and the spectroscopic constants of bound states are fitted. It is the first time that the 12 valence Λ–S states of GaF molecule and all of the 23 Ω states generated from the former are studied in a theoretical way. Calculation results well reproduce most of the experimental data. The effects of the size-extensivity correction and the avoided crossing rule between Ω states of the same symmetry are analyzed. The transition properties of the A 3Π0+, B 3Π1, C 1Π1, and Σ1+3 states are predicted, including the transition dipole moments, the Franck–Condon factors and the radiative lifetimes. The radiative lifetime of the C 1Π1 state of GaF molecule is of the order of nanosecond, implying that it is a rather short-live state. The lifetimes of the B 3Π1 and Σ1+3 states are of the order of microsecond, while the lifetime of the A 3Π0+ state are the order of millisecond.
The one-particle Green’s function method in the Dirac–Hartree–Fock framework. I. Second-order valence ionization energies of Ne through Xe
Authors: Pernpointner M. , Trofimov A. B.
Published: February 2004

The one-particle Green’s function theory in its various implementations is a well-established many-body approach for the calculation of electron ionization and attachment energies in atoms and molecules. In order to describe not only scalar-relativistic effects but also spin–orbit splitting on an equal footing an embedding of this theory in the four-component framework was carried out and fully relativistic ionization energies of the noble gas atoms Ne through Xe were calculated using the second-order algebraic diagrammatic construction [ADC(2)] approximation scheme. Comparison with nonrelativistic ADC(2) results and experimental data was made.
On the zirconium oxide neutral cluster distribution in the gas phase: Detection through 118 nm single photon, and 193 and 355 nm multiphoton, ionization
Authors: Matsuda Y. , Shin D. N. , Bernstein E. R.
Published: February 2004

Zirconium oxide clusters are generated in the gas phase by laser ablation of the metal into a flow of ca. 5% O2/95% He at 100 psig and supersonic expansion into a vacuum chamber. Mass spectra of neutral gas phase zirconium oxide clusters are obtained through photoionization at three different laser wavelengths: 118, 193, and 355 nm. Ionization of the clusters with 118 nm laser radiation is through a single photon ionization mechanism, while ionization by 193 and 355 nm laser radiation is through a multiphoton (three or more photon) mechanism. Fragment ion features are observed in the mass spectra of ZrmOn+ for only the 193 nm and 355 nm ionization schemes. The true neutral ZrmOn cluster distribution is obtained only through 118 nm single photon ionization, as verified by mass spectral peak linewidths and calculations of the cluster binding energies, ionization energies, and fragmentation rates. The neutral cluster distribution consists mainly of the series ZrmO2m and ZrmO2m+1 for m=1,…,∼30.
On the iron oxide neutral cluster distribution in the gas phase. I. Detection through 193 nm multiphoton ionization
Authors: Shin D. N. , Matsuda Y. , Bernstein E. R.
Published: February 2004

Iron oxide (FemOn) neutral clusters are generated in the gas phase through laser ablation of the metal and reaction with various concentrations of O2 in He. The mixture of expansion gas and neutral FemOn cluster species is expanded through a supersonic nozzle into a vacuum system, in which the clusters are ionized by an ArF excimer laser at 193 nm, and the ions are detected and identified in a time-of-flight mass spectrometer. In this report, the experimental parameters that influence the observed cluster distributions, such as ablation laser power, expansion pressure, vacuum system pressure, and 193 nm ArF ionization laser power, are explored. In the second paper in this series, the effect of the ionization laser wavelength (355 nm, 193 nm, 118 nm) on the observed cluster ion distribution is explored. The cluster ion distribution observed employing 193 nm laser ionization, is sensitive to the neutral cluster distribution as evidenced by the change in the observed time-of-flight mass spectra with changes in laser power, growth conditions, and expansion conditions. The thermodynamically stable neutral clusters for saturated O2 growth conditions are suggested to be of the forms FemOm, FemOm+1, and FemOm+2; which one of these series of neutral clusters is most stable depends on the size of the cluster. For m<10, FemOm is the most stable neutral cluster series, for 10⩽m⩽20, FemOm+1 is the most stable neutral cluster series, and for 21⩽m⩽30, FemOm+2 is the most stable neutral cluster series. Some neutral cluster fragmentation is clearly present for 193 nm ionization due to multiphoton absorption in both the neutral and ionic cluster species.
On the iron oxide neutral cluster distribution in the gas phase. II. Detection through 118 nm single photon ionization
Authors: Shin D. N. , Matsuda Y. , Bernstein E. R.
Published: February 2004

Neutral clusters of iron oxide are created by laser ablation of iron metal and subsequent reaction of the gas phase metal atoms, ions, clusters, etc., with an O2/He mixture. The FemOn clusters are cooled in a supersonic expansion and detected and identified in a time-of-flight mass spectrometer following laser ionization at 118 nm (10.5 eV), 193 nm (6.4 eV), or 355 nm (3.53 eV) photons. With 118 nm radiation, the neutral clusters do not fragment because single photon absorption is sufficient to ionize all the clusters and the energy/pulse is ≈1μJ. Comparison of the mass spectra obtained at 118 nm ionization (single photon) with those obtained at 193 nm and 355 nm ionization (through multiphoton processes), with regard to intensities and linewidths, leads to an understanding of the multiphoton neutral cluster fragmentation pathways. The multiphoton fragmentation mechanism for neutral iron oxide clusters during the ionization process that seems most consistent with all the data is the loss of one or two oxygen atoms. In all instances of ionization by laser photons, the most intense features are of the forms FemOm+, FemOm+1+, and FemOm+2+, and this strongly suggests that, for a given m, the most prevalent neutral clusters are of the forms FemOm, FemOm+1, and FemOm+2. As the value of m increases, the more oxygen rich neutral clusters appear to increase in stability.
Semiclassical tunneling splittings from short time dynamics: Herman–Kluk-propagation and harmonic inversion
Authors: Giese Kai , Kühn Oliver
Published: February 2004

We investigate a recently proposed method [J. Chem. Phys. 108, 9206 (1998)] to obtain tunneling splittings from short time cross-correlation matrices that were propagated according to the semiclassical propagator of Herman and Kluk. The energy levels were extracted by harmonic inversion of the cross-correlation matrix using the filter diagonalization technique. The aim of this study is twofold: First, the short time behavior of the Herman–Kluk-propagator and the meaning of using cross-correlation matrices rather than autocorrelation functions is addressed. Numerical examples are given for one- and two-dimensional model potentials. Second, the performance of the method is investigated for a system with considerable anharmonicity and coupling. Here the proton transfer in 3,7-dichlorotropolone is considered using an ab initio reaction surface Hamiltonian approach. For this example also the extension to more dimensions is critically discussed.
Neutral and zwitterionic glycine.H2O complexes: A theoretical and matrix-isolation Fourier transform infrared study
Authors: Ramaekers Riet , Pajak Joanna , Lambie Bert , Maes Guido
Published: February 2004

The H-bond interaction between glycine and H2O has been studied by a combined theoretical (DFT(B3LYP)/6-31++G**) and experimental (matrix-isolation FT-IR) methodology. The 1:1 and 1:2 complexes of the most stable conformation (I) of glycine appear to be neutral complexes which have been vibrationally characterized in detail. The higher stoichiometry complexes (glycine).(H2O)n with n larger than 3 are demonstrated to be zwitterionic H-bonded complexes. A set of characteristic IR absorption bands for this zwitterionic structure has been observed in low-temperature Ar matrices. This would be the first experimental IR evidence for proton transfer occurring between the NH2 and COOH groups of amino acids by a H-bonded water molecular channel in isolated conditions.
Electronic ground states of the V2O4+/0/− species from multireference correlation and density functional studies
Authors: Pykavy Mikhail , van Wüllen Christoph , Sauer Joachim
Published: February 2004

The molecular and electronic structures of the V2O4+/0/− species are examined by multireference averaged coupled-pair functional (MR-ACPF) and density functional B3LYP calculations. For all three species, new conformers have been found. Shallow potential energy curves imply high mobility of the oxygen atoms in the neutral and anionic species for which antiferromagnetic coupling of the weakly interacting 3dV electrons is found. Good agreement between the MR-ACPF and B3LYP results for the molecular structures and the relative energies of states with different spin multiplicity, as well as for the ionization energy and electron affinity, is observed. For the computation of the height of the transition barriers between different conformers elaborated MR-ACPF calculations are required.
Ab initio study of the reactions of Ga(2P, 2S, and 2P) with methane
Authors: Pacheco-Sánchez J. H. , Luna-Garcı́a H. , Castillo S.
Published: February 2004

The interactions of Ga(2P:4s24p1, S2:4s25s1, and P2:4s25p1) with CH4 is studied by means of Hartree–Fock self-consistent-field (SCF) calculations using relativistic effective core potentials and multiconfigurational-SCF plus multireference variational and perturbational on second-order Möller-Plesset configuration interaction calculations. The Ga atom P2(4s25p1) state can spontaneously insert into the CH4. In this interaction the 4 2A′ potential energy surface is initially attractive and becomes repulsive only after meeting with the 3 2A′ surface, adiabatically linked with the Ga(2S:4s25s1)+CH4 fragments. The Ga atom S2(4s25s1) excited state inserts in the C–H bond. In this interaction the 3 2A′ potential energy surface initially attractive, becomes repulsive after meet the 2 2A′ surface linked with the Ga(2P:4s24p1)+CH4 fragments. The two A′2 curves (2 2A′ and X 2A′) derived from the interaction of Ga(2P:4s24p1) atoms with methane molecules are initially repulsive. The 2 2A′ curve after an avoided crossing with the 3 2A′ curve goes smoothly down and reaches a minimum: after this point, it shows an energy barrier. The top of this barrier is located below the energy value of the Ga(2S:4s25s1)+CH4 fragments. After this energy top the 2 2A′ curve goes down to meet the X 2A′ curve. The 2 2A′ curve becomes repulsive after the avoided crossing with the X 2A′ curve. The X 2A′ curve becomes attractive only after its avoided crossing with the 2 2A′ curve. The lowest-lying X 2A′ potential leads to the HGaCH3 X 2A′ intermediate molecule. This intermediate molecule, diabatically correlated with the Ga(2S:4s25s1)+CH4 fragments, which lie 6 kcal/mol, above the ground-state reactants, the dissociation channels of this intermediate molecule leading to the GaH+CH3 and H+GaCH3 products. These products are reached from the HGaCH3 intermediate without activation barriers. The work results suggest that Ga atom in the first excited state in gas-phase methane molecules could produce better quality a-C:H thin films through CH3 radicals, as well as gallium carbide materials.
On the copper oxide neutral cluster distribution in the gas phase: Detection through 355 nm and 193 nm multiphoton and 118 nm single photon ionization
Authors: Matsuda Y. , Shin D. N. , Bernstein E. R.
Published: February 2004

The distribution of neutral copper oxide clusters in the gas phase created by laser ablation is detected and characterized through time-of-flight mass spectroscopy (TOFMS). The neutral copper oxide clusters are ionized by two different approaches: Multiphoton absorption of 355 and 193 nm radiation; and single photon absorption of 118 nm radiation. Based on the observed cluster patterns as a function of experimental conditions (e.g., copper oxide or metal sample, ablation laser power, expansion gas, etc.) and on the width of the TOFMS features, one can uncover the true neutral cluster distribution of CumOn species following laser ablation of the sample. Ablation of a metal sample generates only small neutral CumOn clusters for m≲4 and n∼1, 2. Ablation of copper oxide samples generates neutral clusters of the form CumOm (m⩽4) and CumOm−1 (m>4). These clusters are directly detected without fragmentation using single photon, photoionization with 118 nm laser radiation. Using 355 and 193 nm multiphoton ionization, the observed cluster ions are mostly of the form Cu2mOm+ for 4⩽m⩽10 (193 nm ionization) and CumO1,2 (355 nm ionization) for copper oxide samples. Neutral cluster fragmentation due to multiphoton processes seems mainly to be of the form CumOm,m−1→CumOm/2,m/2+1. Neutral cluster growth mechanisms are discussed based on the cluster yield from different samples (e.g., Cu metal, CuO powder, and Cu2O powder).
Photodissociation of hydrogen iodide on the surface of large argon clusters: The orientation of the librational wave function and the scattering from the cluster cage
Authors: Slavı́ček Petr , Jungwirth Pavel , Lewerenz Marius , Nahler N. Hendrik , Fárnı́k Michal , Buck Udo
Published: February 2004

A set of photodissociation experiments and simulations of hydrogen iodide (HI) on Arn clusters, with an average size 〈n〉=139, has been carried out for different laser polarizations. The doped clusters are prepared by a pick-up process. The HI molecule is then photodissociated by a UV laser pulse and the outgoing H fragment is ionized by resonance enhanced multiphoton ionization in a (2+1) excitation scheme within the same laser pulse at the wavelength of 243 nm. The measured time-of-flight spectra are transformed into hydrogen kinetic energy distributions. They exhibit a strong fraction of caged H atoms at zero-kinetic energy and peaks at the unperturbed cage exit for both spin–orbit channels nearly independent of the polarization. At this dissociation wavelength, the bare HI molecule exhibits a strict state separation, with a parallel transition to the spin–orbit excited state and perpendicular transitions to the ground state. The experimental results have been reproduced using molecular simulation techniques. Classical molecular dynamics was used to estimate the HI dopant distribution after the pick-up procedure. Subsequently, quasi-classical molecular dynamics (Wigner trajectories approach) has been applied for the photodissociation dynamics. The following main results have been obtained: (i) The HI dopant lands on the surface of the argon cluster during the pick-up process, (ii) zero-point energy plays a dominant role for the hydrogen orientation in the ground state of HI–Arn surface clusters, qualitatively changing the result of the photodissociation experiment upon increasing the number of argon atoms, and, finally, (iii) the scattering of hydrogen atoms from the cage which originate from different dissociation states seriously affects the experimentally measured kinetic energy distributions.
Energy distributions in multiple photon absorption experiments
Authors: Mehlig K. , Hansen K. , Hedén M. , Lassesson A. , Bulgakov A. V. , Campbell E. E. B.
Published: February 2004

Photofragmentation experiments on molecules and clusters often involve multiple photon absorption. The distributions of the absorbed number of photons are frequently approximated by Poisson distributions. For realistic laser beam profiles, this approximation fails seriously due to the spatial variation of the mean number of absorbed photons across the laser beam. We calculate the distribution of absorbed energy for various laser and molecular-beam parameters. For a Gaussian laser beam, the spatially averaged distributions have a power-law behavior for low energy with a cutoff at an energy which is proportional to fluence. The power varies between −1 for an almost parallel laser beam and −5/2 for a divergent beam (on the scale of the molecular beam). We show that the experimental abundance spectra of fullerenes and small carbon clusters can be used to reconstruct the distribution of internal energy in the excited C60 molecule prior to fragmentation and find good agreement with the calculated curves.
Trajectory approach to dissipative quantum phase space dynamics: Application to barrier scattering
Authors: Hughes Keith H. , Wyatt Robert E.
Published: February 2004

The Caldeira–Leggett master equation, expressed in Lindblad form, has been used in the numerical study of the effect of a thermal environment on the dynamics of the scattering of a wave packet from a repulsive Eckart barrier. The dynamics are studied in terms of phase space trajectories associated with the distribution function, W(q,p,t). The equations of motion for the trajectories include quantum terms that introduce nonlocality into the motion, which imply that an ensemble of correlated trajectories needs to be propagated. However, use of the derivative propagation method (DPM) allows each trajectory to be propagated individually. This is achieved by deriving equations of motion for the partial derivatives of W(q,p,t) that appear in the master equation. The effects of dissipation on the trajectories are studied and results are shown for the transmission probability. On short time scales, decoherence is demonstrated by a swelling of trajectories into momentum space. For a nondissipative system, a comparison is made of the DPM with the “exact” transmission probability calculated from a fixed grid calculation.
Infrared laser spectroscopy of jet cooled cobalt tricarbonyl nitrosyl
Authors: Trauth Kyle S. , Burns William A. , Berry Ginger , Reeve Scott W.
Published: February 2004

Rotationally resolved infrared absorption spectra for the 101 band of jet cooled cobalt tricarbonyl nitrosyl have been observed and analyzed. Several longitudinal modes of a Pb-salt diode laser were utilized to measure 105 rovibrational transitions for this particular vibrational band centered near 2112 cm−1. Spectra were optimized using both argon and helium carrier gases and these experiments eventually led to rovibrational transitions being assigned to four different K subbands, specifically the K=0, 3, 6, and 9 subbands. An iterative least-squares analysis of the spectroscopic data yielded the following molecular parameters ν̃0=2111.7457(9) cm−1, B0=0.034747(12) cm−1, B1=0.034695(15) cm−1, C1=0.03380(9) cm−1, and D1K=6.3(9)×10−6 cm−1 (where 3σ uncertainties are listed in parenthesis).
A new perspective on the coarse-grained dynamics of fluids
Authors: Ayton Gary S. , Tepper Harald L. , Mirijanian Dina T. , Voth Gregory A.
Published: February 2004

A computational methodology is presented that is designed to model, at a coarse-grained level, the mesoscale dynamics of fluids and potentially other forms of soft matter. Within a molecular dynamics simulation, “ghost” particles of a specific size, corresponding to the fundamental length-scale of coarse-graining, are used as micro-probes designed to respond to local mesoscale fluid flows and stress gradients. A subsequent coarse-grained model is then developed that incorporates both the coarse-grained mesoscale dynamics and isothermal compressibility of the original microscopic system. The method is applied to water and methanol. A contrast with dissipative particle dynamics (DPD) is also presented.
The incomplete beta function law for parallel tempering sampling of classical canonical systems
Authors: Predescu Cristian , Predescu Mihaela , Ciobanu Cristian V.
Published: February 2004

We show that the acceptance probability for swaps in the parallel tempering Monte Carlo method for classical canonical systems is given by a universal function that depends on the average statistical fluctuations of the potential and on the ratio of the temperatures. The law, called the incomplete beta function law, is valid in the limit that the two temperatures involved in swaps are close to one another. An empirical version of the law, which involves the heat capacity of the system, is developed and tested on a Lennard-Jones cluster. We argue that the best initial guess for the distribution of intermediate temperatures for parallel tempering is a geometric progression and we also propose a technique for the computation of optimal temperature schedules. Finally, we demonstrate that the swap efficiency of the parallel tempering method for condensed-phase systems decreases naturally to zero at least as fast as the inverse square root of the dimensionality of the physical system.
Case study of enthalpy–entropy noncompensation
Authors: Graziano Giuseppe
Published: February 2004

Enthalpy–entropy noncompensation characterizes the relative changes in the hydration thermodynamic functions upon “transforming” ethane into fluoromethane, chloromethane, bromomethane, and iodomethane. An analysis grounded on a simple statistical mechanical theory of hydration allows a plausible rationalization of such enthalpy–entropy noncompensation. It is shown that increasing the strength of solute–water attractive interactions modifying the chemical nature of a part of the solute molecule, but not its size, is a largely noncompensating process for the hydration of noncharged and nonhydrogen bonding species, and dominates the compensating contribution coming from the reorganization of water H bonds.
Investigation of vapor-deposited amorphous ice and irradiated ice by molecular dynamics simulation
Authors: Guillot Bertrand , Guissani Yves
Published: February 2004

With the purpose of clarifying a number of points raised in the experimental literature, we investigate by molecular dynamics simulation the thermodynamics, the structure and the vibrational properties of vapor-deposited amorphous ice (ASW) as well as the phase transformations experienced by crystalline and vitreous ice under ion bombardment. Concerning ASW, we have shown that by changing the conditions of the deposition process, it is possible to form either a nonmicroporous amorphous deposit whose density (∼1.0 g/cm3) is essentially invariant with the temperature of deposition, or a microporous sample whose density varies drastically upon temperature annealing. We find that ASW is energetically different from glassy water except at the glass transition temperature and above. Moreover, the molecular dynamics simulation shows no evidence for the formation of a high-density phase when depositing water molecules at very low temperature. In order to model the processing of interstellar ices by cosmic ray protons and heavy ions coming from the magnetospheric radiation environment around the giant planets, we bombarded samples of vitreous ice and cubic ice with 35 eV water molecules. After irradiation the recovered samples were found to be densified, the lower the temperature, the higher the density of the recovered sample. The analysis of the structure and vibrational properties of this new high-density phase of amorphous ice shows a close relationship with those of high-density amorphous ice obtained by pressure-induced amorphization.
193-nm photodissociation of acryloyl chloride to probe the unimolecular dissociation of CH2CHCO radicals and CH2CCO
Authors: Szpunar D. E. , Miller J. L. , Butler L. J. , Qi F.
Published: February 2004

The work presented here uses photofragment translational spectroscopy to investigate the primary and secondary dissociation channels of acryloyl chloride (CH2=CHCOCl) excited at 193 nm. Three primary channels were observed. Two C–Cl fission channels occur, one producing fragments with high kinetic recoil energies and the other producing fragments with low translational energies. These channels produced nascent CH2CHCO radicals with internal energies ranging from 23 to 66 kcal/mol for the high-translational-energy channel and from 50 to 68 kcal/mol for the low-translational-energy channel. We found that all nascent CH2CHCO radicals were unstable to CH2CH+CO formation, in agreement with the G3//B3LYP barrier height of 22.4 kcal/mol to within experimental and computational uncertainties. The third primary channel is HCl elimination. All of the nascent CH2CCO coproducts were found to have enough internal energy to dissociate, producing CH2C:+CO, in qualitative agreement with the G3//B3LYP barrier of 39.5 kcal/mol. We derive from the experimental results an upper limit of 23±3 kcal/mol for the zero-point-corrected barrier to the unimolecular dissociation of the CH2CHCO radical to form CH2CH+CO.
Persistence rewarded: Successful observation of the B̃ 2Σu+–X̃ 2Πg electronic transition of jet-cooled BS2
Authors: He Sheng-Gui , Clouthier Dennis J.
Published: February 2004

The B̃–X̃ electronic transition of jet-cooled BS2 has been observed using laser-induced fluorescence techniques. The boron disulfide radical was produced in a pulsed electric discharge jet using a mixture of BCl3 and CS2 in high-pressure argon as the precursor. The spectrum consists of a strong 000 band with the Σ2–2Π3/2 component at 24 393.2 cm−1 and short progressions in the symmetric stretching (ν1′=506.7 cm−1) and bending (ν2′=303.2 cm−1) modes. A rotational analysis of both spin–orbit components of the 000 band gave an upper state B value of 0.093 277 9(19) cm−1 and a ground-state spin–orbit coupling constant of A=−405.163(4) cm−1. The ground-state bond length of 1.6649(2) Å increases to 1.6812(1) Å on σu→πg electronic excitation. The B̃–X̃ data have been used to further refine the Renner–Teller analysis, which is in good agreement with our previous work [J. Chem. Phys. 119, 2047 (2003)].
Amide I vibrational circular dichroism of dipeptide: Conformation dependence and fragment analysis
Authors: Choi Jun-Ho , Cho Minhaeng
Published: February 2004

The amide I vibrational circular dichroic response of alanine dipeptide analog (ADA) was theoretically investigated and the density functional theory calculation and fragment analysis results are presented. A variety of vibrational spectroscopic properties, local and normal mode frequencies, coupling constant, dipole, and rotational strengths, are calculated by varying two dihedral angles determining the three-dimensional ADA conformation. Considering two monopeptide fragments separately, we show that the amide I vibrational circular dichroism of the ADA can be quantitatively predicted. For several representative conformations of the model ADA, vibrational circular dichroism spectra are calculated by using both the density functional theory calculation and fragment analysis methods.
New results for the OH (ν=0,j=0)+CO (ν=0,j=0)→H+CO2 reaction: Five- and full-dimensional quantum dynamical study on several potential energy surfaces
Authors: Valero Rosendo , McCormack Drew A. , Kroes Geert-Jan
Published: February 2004

Full- [six-dimensional (6-D)] and reduced-dimensional [five-dimensional (5-D)] quantum wave packet calculations have been performed for the title reaction to obtain reaction probabilities deriving from the ground rovibrational states of OH and CO with total angular momentum J=0. Three potential energy surfaces (PES) are studied, namely, those of Bradley and Schatz (BS), Yu, Muckerman, and Sears (YMS), and Lakin, Troya, Schatz, and Harding (LTSH). 6-D calculations are performed only for the BS PES, while 5-D results are reported for all three PES’. The 6-D results obtained in the present work improve on those previously reported, since a larger vibrational basis and a better representation of the OH and CO bonds has been introduced. In particular, we now employ a generalized Lanczos–Morse discrete variable representation for both the OH and CO vibrations. In a further improvement, the generalized discrete variable representation of the CO vibration is based on different CO intramolecular potentials for the asymptotic and product grids employed in our projection formalism. This new treatment of the vibrational bases allows for a large reduction in computation time with respect to our previous implementation of the wave packet method, for a given level of accuracy. As a result, we have been able to extend the range of collision energies for which we can obtain converged 6-D results to a higher energy (0.8 eV) than was possible before (0.5 eV). The comparison of the new 6-D and previous 5-D results for the BS PES shows good agreement of the general trend in the reaction probabilities over all collision energies considered (0.1–0.8 eV), while our previous 6-D calculation showed reaction probabilities that differed from the 5-D results by up to 10% between 0.5 and 0.8 eV. The 5-D reaction probabilities reveal interesting trends for the different PES’. In particular, at low energies (<0.2 eV) the LTSH PES gives rise to much larger reactivity than the other PES’, while at high energies (>0.3 eV) its reaction probability decreases with respect to the BS and YMS PES’, being more than a factor of 2 smaller at 0.8 eV. A 5-D calculation on a modified version of the LTSH surface shows that the van der Waals interaction in the entrance channel, which is not correctly described in the other PES’ is largely responsible for its larger reactivity at low energies. The large difference between the 5-D reaction probabilities for the YMS and LTSH PES’ serves to emphasize the importance of the van der Waals interaction for the reactivity at low energies, because most of the stationary point energies on the YMS and LTSH PES are rather similar, being in line with high-level ab initio information.
Dissociation mechanism of 2-propanol on a Si(111)-(7×7) surface studied by scanning tunneling microscopy
Authors: Tanaka Ken-ichi , Nomoto Yuka , Xie Zhao-Xiong
Published: February 2004

Adsorption of 2-propanol, (CH3)2CHOH, on a Si(111)-7×7 surface was studied by scanning tunneling microscopy. (CH3)2CHOH adsorbs equally on the faulted and unfaulted half unit cells by forming Si–OCH(CH3)2 and Si–H on an adatom and rest atom pair. Si–OCH(CH3)2 is consecutively increased in each half unit cell, and the adsorption is saturated when every half unit cell has three Si–OCH(CH3)2, which corresponds to 0.5 of the adatom coverage. The sticking probability for the dissociation of (CH3)2CHOH is independent of the adatom coverage from 0 to 0.4, but it depends on coverage at higher than 0.4. By counting the darkened adatoms, Si–OCH(CH3)2 on the center adatom (m) and that on the corner adatom (n), it was found the m/n ratio is ca. 4 for the first dissociation of (CH3)2CHOH in virgin half unit cell, but it becomes ca. 1.9 and 1.8 when two and three Si–OCH(CH3)2 are contained in a half unit cell. This result reveals that the dissociation probability of (CH3)2CHOH at the adatom-rest atom pair site is influenced by the nearest Si–OCH(CH3)2 in the half unit cell.
Tractable molecular theory of transport of Lennard-Jones fluids in nanopores
Authors: Bhatia Suresh K. , Jepps Owen , Nicholson David
Published: February 2004

We present here a tractable theory of transport of simple fluids in cylindrical nanopores, which is applicable over a wide range of densities and pore sizes. In the Henry law low-density region the theory considers the trajectories of molecules oscillating between diffuse wall collisions, while at higher densities beyond this region the contribution from viscous flow becomes significant and is included through our recent approach utilizing a local average density model. The model is validated by means of equilibrium as well nonequilibrium molecular dynamics simulations of supercritical methane transport in cylindrical silica pores over a wide range of temperature, density, and pore size. The model for the Henry law region is exact and found to yield an excellent match with simulations at all conditions, including the single-file region of very small pore size where it is shown to provide the density-independent collective transport coefficient. It is also shown that in the absence of dispersive interactions the model reduces to the classical Knudsen result, but in the presence of such interactions the latter model drastically overpredicts the transport coefficient. For larger micropores beyond the single-file region the transport coefficient is reduced at high density because of intermolecular interactions and hindrance to particle crossings leading to a large decrease in surface slip that is not well represented by the model. However, for mesopores the transport coefficient increases monotonically with density, over the range studied, and is very well predicted by the theory, though at very high density the contribution from surface slip is slightly overpredicted. It is also seen that the concept of activated diffusion, commonly associated with diffusion in small pores, is fundamentally invalid for smooth pores, and the apparent activation energy is not simply related to the minimum pore potential or the adsorption energy as generally assumed.
Writing and reading bits on pre-patterned media
Authors: Moritz J. , Buda L. , Dieny B. , Nozières J. P. , van de Veerdonk R. J. M. , Crawford T. M. , Weller D.
Published: February 2004

We have written and read back bits on perpendicular pre-patterned media prepared by electron-beam lithography. Using a contact tester equipped with a standard longitudinal head, we are able to write different bit lengths on 100×200 nm2 dots spaced 100 nm apart. These tests demonstrate the role played by the distribution of the coercive fields of the dots during the write process. Signal-to-noise ratios (SNRs) are obtained by numerical analysis and are found to be on the same order of magnitude as in continuous perpendicular media, but remain constant at decreasing bit lengths. This property of the SNR is due to a partial reduction of transition noise. The potential to pattern these media with nanoimprint lithography and their good recording performance make them candidates for ultrahigh density recording.
Molecular cluster decay viewed as escape from a potential of mean force
Authors: Ford Ian J. , Harris Sarah A.
Published: February 2004

We show that evaporation from a quasistable molecular cluster may be treated as a kinetic problem involving the stochastically driven escape of a molecule from a potential of mean force. We derive expressions for the decay rate, and a relationship between the depth of the potential and the change in system free energy upon loss of a molecule from the cluster. This establishes a connection between kinetic and thermodynamic treatments of evaporation, but also reveals differences in the prefactor in the rate expression. We perform constant energy molecular dynamics simulations of cluster dynamics to calculate potentials of mean force, friction coefficients and effective temperatures for use in the kinetic analysis, and to compare the results with the directly observed escape rates. We also use the simulations to estimate the escape rates by a probabilistic analysis. It is much more efficient to calculate the decay rate by the methods we have developed than it is to monitor escape directly, making these approaches potentially useful for the assessment of molecular cluster stability.