bluuebunny/crossref_metadata_2025 huggingface dataset:
Molecular dynamics simulations of the effects of truncation of the Taylor expansion of the potential energy on the thermodynamic properties of a crystal
Authors: Lacks Daniel J. , Shukla Ramesh C.
Published: July 2002

Molecular dynamics simulations are carried out on a Lennard-Jones crystal, for the potential energy surfaces generated from the full Hamiltonian and the Taylor expansion of the potential energy truncated at the quartic term, to determine the accuracy of the quartic truncation with regard to the thermodynamic properties of a crystal. The results show that the errors arising from the quartic truncation become significant only for temperatures T≳0.2Tm, and are only on the order of 5% at T=0.8Tm, where Tm is the melting temperature. The quartic truncation represents a significant improvement over the quadratic (harmonic) truncation, and the errors associated with the quadratic truncation are decreased by 75%. The sources of error in the λ2 perturbation theory are investigated; the errors are found to arise from the truncation of the potential energy expansion at low temperatures, and primarily from the truncation of the perturbation expansion at high temperatures.
Rotational autoionization and interseries coupling of high Rydberg states by the anisotropy of the molecular core: The quantal long time evolution
Authors: Remacle F. , Levine R. D.
Published: July 2002

Using exact matrix elements for the coupling, the effect of the anisotropy of the core on high molecular Rydberg states is studied by quantum dynamics. It is found that on the time scale which can be probed by zero kinetic energy spectroscopy there is extensive interseries mixing. In particular, the long decay times are due to the sojourn in Rydberg series which are not directly effectively coupled to the continuum. These are series built on higher rotationally excited states of the core and a dynamical bottleneck controls the coupling to the bound series directly coupled to the ionization continuum. The computations are carried out for realistic molecular parameters and in the presence of a weak external dc field. The quadrupolar coupling is often more effective in interseries coupling than the dipolar anisotropy even though the latter has a far higher range. The external field exhibits the expected ‘‘dilution’’ or ‘‘time stretching’’ effect at short times (of the order of the Stark period) but enhances the interseries mixing at longer times. An incomplete l mixing is the origin of another dynamical bottleneck. The time evolution is described both by exact quantum propagation and by a reduced description where degenerate states (i.e., states which differ only in the magnetic quantum numbers) are taken to be equally populated, on the average. This grouping, valid at longer times, facilitates the quantal computations which include several series with the full complement of angular momentum states of the electron. Such computations are possible by taking advantage of the conservation of the (total projection) quantum number M. For higher values of M the coupling to the continuum is very much hindered and the bound Rydberg series exhibit extreme stability. The paper concludes by an analysis of the three bottlenecks which can give rise to longer decays.
Long-range potential of B1(3Σ+) state based on a near-dissociation expansion of the mercury–argon molecules
Authors: Koperski J.
Published: July 2002

A long-range potential of B1(3∑+) state of the mercury–argon molecules was derived using a LeRoy–Bernstein procedure. The procedure was applied to B1(3∑+)←X0(1∑+) excitation spectrum that was recorded previously. A dissociation energy, an effective vibrational index at the dissociation limit, and an asymptotic behavior of the B1(3∑+) state potential energy curve were determined. The result was compared with others available in the literature showing versatility of the method.
Rotating around the quartic angular momentum barrier in fast multipole method calculations
Authors: White Christopher A. , Head-Gordon Martin
Published: July 2002

The fast multipole method (FMM) evaluates the potential interactions of point charges (or masses) in time scaling linearly with their number. The FMM requires the ability to manipulate multipole and Taylor expansions. These manipulations are conventionally performed using a set of O(L4) translation operators, where L is the degree of the expansion. We introduce a method which achieves O(L3) scaling by rotating the expansions such that translation is always along the quantization or Z axis. It retains the FMM error bound. An overall speedup approaching a factor of 3 for relatively high orders of multipoles (L=21) is demonstrated. This procedure makes high accuracy potential evaluations substantially more efficient.
Fluctuation-dominated A+B→0 kinetics under short-ranged interparticle interactions
Authors: Oshanin G. , Sokolov I. M. , Argyrakis P. , Blumen A.
Published: July 2002

In the present paper we analyze the kinetics of irreversible A+B→0 reactions in situations appropriate to electrolyte solutions. We consider diffusing species which experience short-range repulsive (attractive) interactions between like (unlike) particles and we highlight the role of fluctuations in the particles’ spatial distributions. We focus mainly on one-dimensional systems and devise a many-particle description of the reaction kinetics. In terms of our analytical approach we show that at intermediate times the interplay between fluctuations and short-ranged interactions gives rise to unusual behaviors, characterized by novel dynamical exponents of the particles’ mean concentrations. For batch reactions at longer times, when the concentrations drop off significantly, the usual fluctuation-induced behavior is recovered. We also analyze systems with external, steady sources of particles and we show that in such situations the effects of the interactions become decisive at long times. Our analytical findings are in good agreement with the results of numerical simulations, which we also present.
First-row hydrides: Dissociation and ground state energies using quantum Monte Carlo
Authors: Lüchow Arne , Anderson James B.
Published: July 2002

Accurate ground state energies comparable to or better than the best previous ab initio results can be obtained using the fixed-node quantum Monte Carlo (FN-DQMC) method. The residual energy, the nodal error due to the error in the nodal structure of a trial wave function, is examined in this study using nodal surfaces given by near HF-limit wave functions. The study is aimed at better understanding of the nodal error and the cancellation of nodal errors in calculating energy differences. Calculations have been carried out for the first-row hydrides LiH to FH and the corresponding atoms. The FN-DQMC ground state energies are among the lowest to date. The dissociation energies De have been calculated with accuracies of 0.5 kcal mol−1 or better. For all hydrides, the dissociation energies are consistent with experimental values. The fixed-node quantum Monte Carlo method can therefore offer a very straight-forward way to calculate highly accurate dissociation energies.
The rotational spectrum of the CrF radical in the X 6Σ+ state
Authors: Okabayashi Toshiaki , Tanimoto Mitsutoshi
Published: July 2002

The rotational spectrum of the CrF radical in the X 6Σ+ state was observed by employing a source modulation microwave spectrometer. The CrF radical was generated in a free space cell by a dc glow discharge in CF4 and He. The chromium atom was supplied by the sputtering reaction from chromium powder placed over a lower surface of the cylindrical electrodes. The transitions with N=12−11 to 20−19 were measured in the region between 270 and 460 GHz. The rotational, centrifugal distortion and several fine-structure constants were obtained by a least squares analysis.
Site-specific fragmentation in condensed (CH3S)2 by sulfur K-edge photoexcitation
Authors: Baba Y. , Yoshii K. , Sasaki T. A.
Published: July 2002

The site-specific fragmentation by core-to-valence resonant photoexcitation has been investigated for condensed (CH3S)2 (dimethyl disulfide). The data presented are the desorption yields of the fragment ions from multilayered (CH3S)2 around the sulfur K-edge photoexcitation as well as the x-ray absorption and Auger decay spectra. The x-ray absorption spectrum exhibits the double structure of the S 1s→σ* resonance peak corresponding to the S 1s→σ*(S–S) and S 1s→σ*(S–C) photoexcitations. It was found that the S 1s→σ*(S–C) excitation is predominantly followed by the CH+3 desorption, but the CH+3 and S+ ions are desorbed in comparable intensity at the S 1s→σ*(S–S) excitation. The Auger decay spectra around the S 1s→σ* excitation revealed that there exist two kinds of sulfur KL2,3L2,3 spectator Auger decay originating from the S 1s→σ*(S–S) and S 1s→σ*(S–C) resonant excitations. The observed site-specific fragmentation is interpreted in terms of the localization of the spectator electron in each antibonding orbital, i.e., σ*(S–S) and σ*(S–C).
Multiconfiguration time-dependent Hartree (MCTDH) study on rotational and diffractive inelastic molecule-surface scattering
Authors: Ehara Masahiro , Meyer H.-D. , Cederbaum L. S.
Published: July 2002

The multiconfiguration time-dependent Hartree (MCTDH) method is applied to rotational and diffractive inelastic molecule-corrugated surface scattering. The molecule is treated as a rigid rotor, hence there are five degrees of freedom included in the calculation. The model systems H2/rectangular lattice and N2/LiF (001) are investigated for scattering with normal incidence. The performance and reliability of the MCTDH method is critically examined with respect to the structure of the MCTDH wave function and the choice of the basis set representation. The MCTDH reproduces the fine details of the state-to-state transition probabilities calculated by the numerically exact close-coupled wave packet (CCWP) method. We show that it is useful to represent two of the internal degrees of freedom by one set of single-particle functions when these degrees are strongly coupled, or when their MCTDH-contraction efficiency is low.
Coverage, lateral order, and vibrations of atomic nitrogen on Ru(0001)
Authors: Dietrich H. , Jacobi K. , Ertl G.
Published: July 2002

The N/Ru(0001) system was studied by thermal desorption spectroscopy (TDS), low-energy electron diffraction (LEED), and high-resolution electron energy-loss spectroscopy (HREELS). Atomic nitrogen was prepared by NH3 decomposition at sample temperatures decreasing from 500 to 350 K during NH3 exposure. A maximum N coverage of θN=0.38 could thus be achieved. ∛, split 2×2 and 2×2 LEED patterns were observed for decreasing θN. After NH3 decomposition and before annealing the sample to a temperature above 400 K, the surface is composed of adsorbed N, H, and NH species. This composite layer exhibits a split ∛ LEED pattern due to domains of size 4 with heavy walls. This phase decays through dissociation of NH leading to sharp first-order type desorption peaks of H2 and N2. From the weak intensity of the ν(Ru–NH) stretch mode it is concluded that NH is adsorbed at threefold-hollow sites. The energy of the ν(Ru–N) mode shifts from 70.5 to 75.5 meV when θN is increased from 0.25 to 0.38.
Shell structure and shapes of fermion microsystems: A comparative study of 3He and Na clusters
Authors: Yannouleas Constantine , Landman Uzi
Published: July 2002

A semiempirical shell-correction method including ellipsoidal deformations is used to determine binding energies of open-shell, unpolarized 3HeN clusters. Shell effects, shapes, and other ground-state properties (like the chemical potential) are determined. 3HeN clusters are found to be substantially less deformed due to their relatively high surface energy as compared to that of alkali-metal clusters (e.g., sodium clusters). As a result, the size-evolutionary patterns associated with 3HeN clusters are significantly different than the corresponding ones for NaN clusters. In particular, odd–even oscillations and signatures of subshell closures are absent in the case of 3HeN clusters, while they are prominent in the case of alkali-metal clusters.
Model for slow relaxation in supercooled liquids
Authors: Das Shankar P.
Published: July 2002

We consider the mechanism for very slow relaxation in a supercooled liquid in the framework of the self-consistent mode coupling model. The short time dynamics in the model is taken into account through the inclusion of the bare transport coefficient while the cooperative dynamics over different time and length scales is approximated through the mode coupling terms. A lower cutoff time t0 for the memory functions representing the long time dynamics is considered in order to account for the contribution coming from the bare transport coefficients. In the present work, we have investigated the implications of this cutoff time (t0) for the asymptotic dynamics in the supercooled regime. Our calculation shows that the self-consistent approach results in slow relaxation times typical of what is seen in the fragile liquids at temperature high compared to the glass transition temperature with a power law increase around a critical temperature. It however does not give rise to the very sharp increase in relaxation time scales for any reasonable approximation for t0 in a fully self-consistent calculation.
Field and temperature dependencies of free carrier photogeneration efficiencies of molecular glasses
Authors: Lin L.-B. , Jenekhe S. A. , Borsenberger P. M.
Published: July 2002

The field and temperature dependencies of free carrier photogeneration efficiencies of vapor deposited molecular glasses have been studied by time-of-flight photocurrent techniques. The measured photogeneration efficiencies were analyzed by a theory of geminate recombination due to Onsager. In spite of the wide distribution of the charge mobilities and molecular dipole moments of the materials studied, thermalization distances and primary quantum yields were 27–36 Å and 10−3 to 10−2, respectively. The results suggest that the thermalization process in molecular glasses is not driven by processes that determine bulk transport properties.
Investigation of molecular structure in solids by two-dimensional NMR exchange spectroscopy with magic angle spinning
Authors: Tycko Robert , Weliky David P. , Berger Alan E.
Published: July 2002

An approach to the investigation of molecular structures in disordered solids, using two-dimensional (2D) nuclear magnetic resonance (NMR) exchange spectroscopy with magic angle spinning (MAS), is described. This approach permits the determination of the relative orientation of two isotopically labeled chemical groups within a molecule in an unoriented sample, thus placing strong constraints on the molecular conformation. Structural information is contained in the amplitudes of crosspeaks in rotor-synchronized 2D MAS exchange spectra that connect spinning sideband lines of the two labeled sites. The theory for calculating the amplitudes of spinning sideband crosspeaks in 2D MAS exchange spectra, in the limit of complete magnetization exchange between the labeled sites, is presented in detail. A new technique that enhances the sensitivity of 2D MAS exchange spectra to molecular structure, called orientationally weighted 2D MAS exchange spectroscopy, is introduced. Symmetry principles that underlie the construction of pulse sequences for orientationally weighted 2D MAS exchange spectroscopy are explained. Experimental demonstrations of the utility of 2D MAS exchange spectroscopy in structural investigations of peptide and protein backbone conformations are carried out on a model 13C-labeled tripeptide, L-alanylglycylglycine. The dihedral angles φ and ψ that characterize the peptide backbone conformation at Gly-2 are obtained accurately from the orientationally weighted and unweighted 2D 13C NMR exchange spectra.
Feasibility of using photophoresis to create a concentration gradient of solvated molecules
Authors: Space Brian , Rabitz Herschel , Lörincz András , Moore Preston
Published: July 2002

The objective of this work is to estimate the feasibility of creating a measurable concentration gradient of molecules in a solvent by a laser driven photophoresis process. The molecules are dissolved in a suitable solvent that is not significantly absorbing at the applied radiation frequency. The molecule is anisotropic, or ideally propeller shaped, and has an appropriate transition dipole capable of driving rovibrational motion. The polarization of the laser can be taken as rotating slowly with the molecules. The resulting torque driven hindered rotation imparts a forward thrust, and thus creates a net flow of the molecules which can set up a concentration gradient in a finite cell. The relevant physical parameters are estimated with the aid of instantaneous normal mode and molecular dynamics simulation methods on a prototype system, and the results indicate that a detectable concentration gradient may be established. A practical issue is to treat heating and resultant mixing or turbulence in the medium. Laboratory experiments are needed to further explore the photophoresis process.
Acrylonitrile on Cu(100): A density functional theoretical study of adsorption and electrochemical grafting
Authors: Geskin V. M. , Lazzaroni R. , Mertens M. , Jérôme R. , Brédas J. L.
Published: July 2002

We present a density functional theoretical study of neutral and negatively charged complexes of acrylonitrile (AN) with several clusters representing the Cu(100) surface. An external electric field is used to induce electron transfer onto the adsorbed molecule, in order to model the initial stage of electrochemical polymerization of AN on a copper electrode. We compare different possibilities of AN interaction with the metal and determine the most favorable configurations of the adsorbates in the absence and in the presence of the electric field. We discuss the influence of the electrochemical polarization on the type of AN adsorption, in relation with the experimental data on the adsorption of AN and electrochemical grafting of polyacrylonitrile.
TDMP2 calculation of dynamic multipole polarizabilities and dispersion coefficients of the triplebonded molecules CO, N2, CN−, and NO+
Authors: Hättig Christof , Hess Bernd Artur
Published: July 2002

In order to test the performance of the recently developed time-dependent second-order Mo/ller–Plesset perturbation theory (TDMP2) for anisotropic frequency-dependent multipole polarizabilities, we have studied the isoelectronic series CO, N2, CN−, and NO+. The polarizabilities of these triple-bonded diatomics are an interesting test for the TDMP2 method, because it is known that Mo/ller–Plesset perturbation theory has its difficulties describing multiple bonds. We selected these molecules as test systems because, especially for N2 and CO, accurate experimental and other ab initio data are available to compare with and their dynamical polarizabilities are needed for dispersion coefficients of van der Waals complexes, which are presently under intensive investigation. To get reliable results near the TDMP2 basis set limit we used large one-particle basis sets, optimized for polarizability calculations at the coupled Hartree–Fock level. The results show that the TDMP2 method is capable to improve for the isotropic as well as for the anisotropic polarizabilities considerably on the TDHF approximation, with the exception of the dipole polarizabilities of N2 and NO+, for which the static correlation effects are too strong to be treated by second-order Mo/ller–Plesset perturbation theory. However, we find, that the TDMP2 method, due to the use of coupled (TDHF) first-order orbital rotation parameters, is somewhat more stable with respect to static correlation effects than the singles-doubles-triples many-body perturbation theory to second order [SDT-MBPT(2)] double perturbation theory. Where reference data are available, the TDMP2 results for static polarizabilities and for the first Cauchy moments are in good agreement with the best theoretical and experimental data. We also calculated dispersion coefficients for the (N2)2 and the (CO)2 dimer and, utilizing the results of previous TDMP2 studies for the atoms He through Xe, also for the respective rare gas complexes of N2, CO, CN−, and NO+. We estimate the results to be the most accurate ab initio data available for these van der Waals coefficients.
On the heat of formation of carbonyl fluoride, CF2O
Authors: Asher Robert L. , Appelman Evan H. , Ruscic Branko
Published: July 2002

In light of the recent controversy surrounding its heat of formation, CF2O was reexamined by photoionization mass spectrometry. In particular, the CO+ fragment ion yield curve from CF2O was interpreted in terms of a retarded CO++F2 process, and a more facile two-step fragmentation to CO++2F. The former process produces a weak, slowly growing tail region without a clear onset, while the latter occurs at higher energy and causes a pronounced growth with a conspicuous onset, which was found to occur at ≤20.87+0.03/−0.07 eV at 0 K by fitting with a model curve that incorporates ‘‘fluctuations’’ associated with second-generation fragments. This onset leads to ΔHf° 298(CF2O) ≥−149.1+1.4/−0.7 kcal/mol, and indicates that the older experimental values for this quantity are too low by at least 3–4 kcal/mol. While the F2 elimination is retarded by competition with lower energy processes, the two-step process derives its strength from the FCO+ fragment, which assumes the role of a pseudoparent. Thus, the onset of CO++2F is expected to appear reasonably close to the thermochemical threshold. Such an interpretation indicates that recently calculated ab initio values of −145.3±1.7 kcal/mol and −145.6±1.0 kcal/mol are very likely too high by 3–4 kcal/mol. In addition, the adiabatic ionization potential of CF2O was refined to 13.024±0.004 eV, while the 0 K appearance potential of the FCO+ fragment was found by fitting to be ≤14.752±0.005 eV. Together with the suggested value for ΔHf°(CF2O), these two onsets lead to ΔHf° 298(CF2O+)=151.2+1.4/−0.7 kcal/mol and ΔHf° 298(FCO+)=173.5+1.4/−0.7 kcal/mol.
State-to-state collision energy transfer of 7Li2 within high-lying triplet states: Gateway effect of mixed levels in energy transfer between singlet and triplet states
Authors: Li Li , Antonova S. , Yiannopoulou A. , Urbanski K. , Lyyra A. M.
Published: July 2002

State-to-state collision energy transfer in the 2 3Πg, 3 3Πg, 3 3Σ+g, 1 3Σ−g, and 1 3Δg states and from singlet to triplet states of 7Li2 has been studied experimentally by continuous wave optical–optical double resonance (OODR) resolved fluorescence spectroscopy. Propensity rules of collision-induced transitions within a triplet state have been observed. When a rovibrational (v,J) level of the F 1Σ+g or G 1Πg state was excited by OODR transition, fluorescence from high-lying triplet gerade states to the a 3Σ+u and/or b 3Πu states was always observed. This indicates that population can transfer from F 1Σ+g and G 1Πg states to triplet states by collision. Irrespective of the J of the initially excited singlet level, the most intense fluorescence from the triplet state will occur at the N or J corresponding to that of the F 1Σ+g (and/or G 1Πg)∼3Λg mixed levels. Molecular population moves to the mixed levels and then transfers to the triplet state. This clearly indicates that singlet–triplet mixed levels played the role of a gateway, through which the population moves from singlet to triplet states. The gateway levels can be perturbed levels in the same vibrational level of the F 1Σ+g or G 1Πg state, or the perturbed levels in different vibrational levels.
The effect of rotation on resonances: Application to HCO
Authors: Qi Jianxin , Bowman Joel M.
Published: July 2002

We calculate the effect of rotation on resonance energies and widths of HCO using the adiabatic rotation method [J. M. Bowman, Chem. Phys. Lett. 217, 36 (1994)]. We test this approximate method against previous exact calculations for the even-parity resonances for total angular momentum J=1, and then apply the method for a range of J. We focus on the shifts in position and width of numerous resonances relative to nonrotating HCO for J=1, 6, and 20 and the body-fixed projection quantum number K=0 and 1. The variation of both resonance energies and widths is much greater for K=1 than K=0, and the implications for simple energy-shifting models are assessed. Some results are also presented for J=6, K=4 and 6 which illustrate the transformation of high-lying bound states for J=0 to resonances for J greater than zero. The implication of this transformation to theories of recombination is discussed.
Computer simulation study of the Rayleigh light scattering in the isotropic phase of PCH5
Authors: Yakovenko Sergei Ye. , Muravski Anatoli A. , Eikelschulte Frank , Geiger Alfons
Published: July 2002

Configurations of 200 p-n-pentyl-(p-cyanophenyl)-cyclohexane (PCH5) molecules from molecular dynamics simulations were used to calculate the Rayleigh light scattering spectra of the isotropic phase. The interaction induced contribution is calculated to the first order dipole–induced dipole terms in the point molecular polarizability approximation. The effect of the size of the simulated system is checked from different points of view. Molecular flexibility is explicitly taken into account and comparisons in terms of time correlation functions are performed. In contrast to systems of small-sized molecules no cancellation effect is observed for the interaction induced contribution to the scattered light intensities of this mesogen. Studies of the different contributions induced by the isotropic and the anisotropic part of the molecular polarizability, evidenced the negligibility of the second one and showed the importance of orientational-translational intermolecular correlations for spectra formation in mesogen systems. The problem of comparing anisotropic and isotropic components of Rayleigh spectra is outlined.
Luminescent charge transfer in a beam of CO++ colliding with Ar, N2, H2, D2 and CO
Authors: Ehbrecht A. , Mustafa N. , Ottinger Ch. , Herman Z.
Published: July 2002

Luminescent charge transfer of CO++(3Π,1Σ+) ions in a beam with several atomic and molecular target species in a scattering cell was observed at 60–2000 eVlab projectile energies. Emission from CO+(B 2Σ+→X 2Σ+) was spectrally analyzed with a resolution of up to 1 Å full width at half maximum. In the case of CO+++CO collisions, CO+(B) emission due to electron capture by the projectile and to target ionization were distinguished using isotopic substitution. CO+(A 2Π→X 2Σ+) emission from the ionized target was also observed in this reaction. Similarly, with N2 as a target, strong N+2(B 2Σ+u→X 2Σ+g) emission was observed. The absolute cross sections for CO+(B) formation are 1–5 Å2. The CO+(B) vibrational excitation at 2000 eV can be explained by Franck–Condon transitions from the CO++(3Π)/(1Σ+) beam components in the ratio 1:2. At low energy the product CO+ vibrational excitation is higher, especially from collisions with Ar. The vibrational population distribution differs from that derived from earlier translational energy spectra, possibly due to a different CO++(3Π/1Σ+) beam composition. The rotational excitation is in all cases strikingly low, even at low impact energy. This is ascribed to the large electron capture radius (∼2.8 Å).
Extreme metastability in Ar–HCl (v=1) and Ar–DF (v=1)
Authors: Oudejans L. , Nauta K. , Miller R. E.
Published: July 2002

High resolution optothermal spectra have been obtained for the Ar–HCl and Ar–DF complexes which conclusively show that they do not dissociate on H/D–X vibrational excitation. The observed linewidths place a lower limit on the lifetimes of 500 ns, while time-of-flight considerations indicate that the lifetimes are in excess of 0.4 ms. Stark spectra are also reported, which result in the determination of the (v=1) excited state dipole moments of these complexes.
Scattering matrix elements by a time independent wave packet complex scaling formalism
Authors: Rom Naomi , Pang John W. , Neuhauser Daniel
Published: July 2002

A time-independent approach to calculate scattering matrix elements using the complex coordinate method is examined. This approach is based on a combination of the expressions derived by Tannor and Weeks [J. Chem. Phys. 98, 3884 (1993)] and by Kouri, Huang, Zhu, and Hoffman [J. Chem. Phys. 100, 3662 (1994)], with an analytic continuation of the Hamiltonian, while keeping the initial and final wave packets unscaled. The procedure is examined using a one dimensional Eckart barrier representing the H+H2 reaction, and a comparison between two complex scaling schemes and an optical potential one shows good convergence of the method. In addition, a one-dimensional electron scattering from a barrier is calculated, showing an advantage here of the complex-scaling approach over the optical potentials method when very light particles are involved in the dynamics. The complex-scaling version enables the use of iterative techniques, hence is a promising tool for calculating dynamics in large systems of light particles.
‘‘Gentle recoil’’ synthesis of free-radical clusters via unimolecular photolysis of closed shell complexes
Authors: Mackenzie S. R. , Votava O. , Fair J. R. , Nesbitt D. J.
Published: July 2002

We describe a novel mechanism for efficient generation of open shell free-radical complexes via UV photolysis of the corresponding closed shell precursors. Ar–SH and Ar2–SH are detected following 248 nm excitation of Ar–H2S and Ar2–H2S. Of particular dynamical interest are the nonfragmentary channels in which no Ar atoms are ejected.
A high resolution spectroscopic study of the open-shell complex ArNO2
Authors: Low Russell J. , Brookes Matthew D. , Whitham Christopher J. , Howard Brian J.
Published: July 2002

Microwave and infrared spectra of the open-shell complex ArNO2 have been recorded. The microwave spectrum (6–18 GHz) consisted solely of the a-type transitions 505←404, 404←303, 303←202, and 202←101 involving the Ka=0 state and 523←422, 524←423, 422←321, 423←322, 321←220, 322←221 involving the Ka=2 state. These transitions showed structure due to fine, magnetic hyperfine and electric quadrupole interactions. The infrared spectrum, associated with the ν3 asymmetric vibrational mode of the NO2 monomer, consisted of three bands (RP0, RQ0, and RR0 and both K doublets of PP2, PQ2, and PR2) centered around 1615 cm−1. The data have been fitted to a semirigid Hamiltonian to determine the molecular parameters. The derived parameters are analyzed in terms of those of the free NO2 radical. Changes in these parameters upon complexation can be caused by a geometric effect due to the rotation of the inertial axes from the monomer to the complex, and an electronic effect caused by a distortion of the electronic wave functions on complex formation. The electronic changes (which may give an indication of incipient chemical bond formation) are shown to be very small. The absence of odd Ka″ states in both the infrared and microwave spectra was rationalized in terms of a high frequency tunneling motion of the NO2 within the complex. Both a dynamics calculation and a model potential based on atom–atom interactions provided additional support for a nonplanar equilibrium structure with a low barrier to planarity.
Quasiclassical trajectory studies of H(D)+HF(DF) collisions at 2 eV
Authors: Schatz George C.
Published: July 2002

This paper presents a quasiclassical trajectory study of nonreactive and reactive (F transfer) collisions involving H+HF, D+HF, H+DF, and D+DF, with emphasis on comparisons of product vibration/rotation distributions with those in an accompanying paper by Lindner, Lundberg, Lovejoy, and Leone. We find that the nonreactive rotational distributions for each isotope and each vibrational state are composed of a large peak at low J plus a long tail that extends to high J. These results are in general agreement with experiment. The low J peak arises from direct collisions of H with the F atom in HF, while the high J tail is due partly to failed reactive (F-exchange) collisions, and partly to collisions of H with the H atom in HF. The calculated reactive rotational distributions are controlled by the kinematic propensities expected for a heavy–light–heavy mass combination and a nearly isotropic potential. This result does not appear to match experiment, although limitations in the data preclude a detailed comparison.
Prediction of new sp2 and sp2/sp3 hollow carbon crystals
Authors: Benedek G. , Colombo L. , Gaito S. , Galvani E. , Serra S.
Published: July 2002

Among the hypothetical forms of fully covalent carbon lattices with either graphite-like sp2 or mixed sp2/sp3 bonding we investigated the possibility of monocrystals with a hollow structure. We have calculated the structural, elastic and electronic properties of two prototypical face–centered cubic lattices C20 and C22, respectively, making also a critical comparison to diamond and graphite. The first of these lattices belongs to a new class of sp2–bonded periodic solids which we call hollow graphites. We give a topological classification of such solids along with the algorithm to generate them. Both crystals, having a nanoporous lattice made of periodic sequence of adjacent cavities, rather than the tubular structure of schwarzites, are characterized by a large internal specific area and should be suitable to form reversible high capacity lithium insertion compounds.
Resonances in the predissociation of the A 2ΠΩ state of MgBr
Authors: Sadygov Rovshan G. , Rostas Joëlle , Taieb Guy , Yarkony David R.
Published: July 2002

Electronic structure methods and nonperturbative resonance theory are applied to study the radiative and radiationless decay mechanisms of the MgBr (A 2ΠΩ) vibrational levels. The X 2Σ+ and 1,2 2ΠΩ adiabatic electronic states are characterized using ab initio state-averaged multiconfigurational self-consistent field/second order configuration interaction wave functions. Interstate derivative couplings between the 2Π states have been calculated and used to construct a rigorous diabatic basis. The nonrelativistic potential energy curves are modified in the first order of degenerate perturbation theory to take account of the spin–orbit interactions treated within Breit–Pauli approximation. All vibrational levels in the A 2ΠΩ manifold are resonances predissociated by the repulsive 2 2Π state. A recently developed computational approach [S. Han and D. R. Yarkony, Mol. Phys. 88, 53 (1996)] based on a Feshbach formalism is employed to determine energies, linewidths, and radiative and radiationless decay rates in a coupled diabatic states basis within a Hund’s case (a) approximation. Large nonadiabatic interactions cause significant energy shifts in the resonances levels. It is shown that a pronounced Ω-dependence in the radiationless decay rates results from the large fine structure splitting in the 2 2ΠΩ diabatic state which corresponds to Mg(1S)Br(2P). Comparisons with absorption and fluorescence spectra reveal important insights into A 2ΠΩ state decay. The spectroscopic constants of the A 2ΠΩ, Ω=3/2 and 1/2 states and the A 2Π3/2 state predissociation are well described in a Hund’s case (a) approximation. However it is found that the A 2Π1/2 state predissociation is significantly underestimated in this limit. Rather the A 2Π1/2 state is indirectly predissociated by the 2 2Π3/2 state through rotational coupling to the A 2Π3/2 state.
Molecular theory of electronic spectroscopy in nonpolar fluids: Ultrafast solvation dynamics and absorption and emission line shapes
Authors: Stephens M. D. , Saven J. G. , Skinner J. L.
Published: July 2002

We present a theory of time- and frequency-domain spectroscopy of a dilute nonpolar solute in a nonpolar liquid or supercritical fluid solvent. The solute and solvent molecules are assumed to interact with isotropic pair potentials. These potentials, together with the solute and solvent masses, are the only input in the theory. We arrive at expressions for the absorption and emission line shapes, which include the possibility of motional narrowing, and for the time-resolved fluorescence and transient hole-burning observables, by assuming that the solute’s fluctuating transition frequency describes a Gaussian process. These expressions depend only on the average and variance of the transition frequency distributions in absorption and emission and on the normalized frequency fluctuation time-correlation functions. Within our formalism the former are obtained from the solute-solvent and solvent-solvent radial distribution functions, which are calculated using integral equations. The time-correlation functions involve the time-dependent solute-solvent Green’s function. Its solution depends upon the solute and solvent diffusion constants, which in turn are determined from the radial distribution functions. The theory compares favorably with computer simulation results of the same model. We then investigate the dependence of the various spectroscopic observables on the solvent density, the temperature, and the difference between the ground- and excited-state solute’s pair interaction with the solvent molecules. For example, since our theory for the time-correlation functions captures both their short- and long-time behavior, we can see how the crossover from inertial to diffusive dynamics depends on these variables. Our results are similar to a variety of experiments on solutes in both nonpolar and polar solvents.
Translational to vibrational and rotational (T→V,R) energy transfer and reactive exchange collisions of H(D)+HF(DF) in the energy range from 1 to 2 eV by time-resolved Fourier transform spectroscopy
Authors: Lindner Jörg , Lundberg James K. , Lovejoy Christopher M. , Leone Stephen R.
Published: July 2002

The product state distributions for hot atom collisions of H(D) with HF(DF) with a broad range of collision energies between 1.2  eV and 2.3 eV are measured with time-resolved Fourier transform spectroscopy and rotational resolution under multiple collision conditions. In most cases the vibrational distributions of reactive and nonreactive channels can be distinguished. All rotational distributions have a similar appearance with a maximum at J=5 and an additional pronounced higher component, which cannot be described by a single Boltzmann distribution. The results are compared with recent three dimensional quasiclassical trajectory calculations (accompanying paper by Schatz) by applying a multiple collision model for both the H(D) atom slowdown and the rotational and vibrational relaxation. The rotational distributions can also be interpreted by a three dimensional asymmetric breathing ellipsoid model.
Study of the origin of the phase lag between signals in coherent laser control
Authors: Lefebvre-Brion Hélène
Published: July 2002

It is shown that the origin of the phase lag between signals corresponding to photoionization and photodissociation of Rydberg levels of HI observed in a recent coherent control experiment [Zhu et al., Science 270, 77 (1995)] is not the difference in the molecular phase of the two processes.
The monolayer and multilayer structure of CO2/NaCl(001)
Authors: Hu W. , Saberi M.-A. , Jakalian A. , Jack D. B.
Published: July 2002

Results of a Monte Carlo simulation of monolayer and multilayers of CO2 molecules adsorbed on the (001) surface of NaCl at temperature T=55 K are presented. It is found that the monolayer adopts a (2×1) structure; bilayers adopt either a (2×1) or a c(2×2) structure with the latter configuration energetically favored; and trilayers adopt a c(2×2) structure owing to the instability of the (2×1) structure. These results are in accord with recent helium scattering work.
Instantaneous normal mode theory of quantum time correlation functions: Raman spectrum of liquid CS2
Authors: Keyes T.
Published: July 2002

An instantaneous normal mode (INM) theory is presented for quantum time correlation functions. It is argued that the INM formalism for classical correlation functions is particularly amenable to quantum correction. The intermolecular (Rayleigh) and allowed vibrational Raman spectra of liquid CS2 are calculated as an illustration. The Applequist–Quicksall polarizability model is employed, yielding the correct values for both the molecular polarizability and its derivatives with respect to the normal coordinates. Agreement with experiment is reasonable for the intermolecular Raman and for some aspects of the allowed Raman, but not for the linewidth. A brief discussion is given regarding the future developments which will be needed for an accurate INM theory of vibrational line shapes.
Density functional theory of adhesive hard sphere fluids
Authors: Choudhury Niharendu , Ghosh Swapan K.
Published: July 2002

We present a simple density functional approach to study the structure of homogeneous as well as inhomogeneous adhesive hard sphere fluid. Radial distribution function g(r) of the homogeneous adhesive hard sphere fluid is calculated by making use of the well known Percus identity which relates the density distribution of an inhomogeneous fluid to the g(r) of the corresponding homogeneous fluid when the external potential responsible for the inhomogeneity is the interparticle potential itself. We have also studied the local density distribution of the same fluid confined in a planar slit consisting of hard walls. The input required for the calculation is the two-particle direct correlation function of the bulk fluid, which is taken from the analytical results corresponding to the Percus Yevick approximation. Both perturbative and nonperturbative weighted density approaches are employed and the calculated radial distributions as well as the density profiles are shown on an average to compare quite well with results from computer simulation.
The ultraviolet photodissociation dynamics of 2-chloro-1,1-difluoroethylene
Authors: Huang Yibo , Gordon Robert J.
Published: July 2002

The rotational state distribution of HCl(v′=0 and 1) produced in the 193 nm photodissociation of the title molecule was measured in a pulsed pump-and-probe experiment. The product state population was found to have a Boltzmann-like distribution, in agreement with a simple phase-space model. The photodissociation mechanism is proposed to be a three-center elimination of HCl to produce the F2C=C: carbene. The barrier for rearrangement of this fragment is believed to be very large, so that any energy released in the eventual isomerization step does not affect the HCl state distribution. These results support our explanation of the nonstatistical state distribution of HCl produced in the photodissociation of vinyl chloride and dichloroethylene.
Optical measurements of the phase diagrams of Langmuir monolayers of fatty acid, ester, and alcohol mixtures by Brewster-angle microscopy
Authors: Teer Ellis , Knobler Charles M. , Lautz Carsten , Wurlitzer Stefan , Kildae John , Fischer Thomas M.
Published: July 2002

Surface pressure–temperature phase diagrams have been determined by Brewster-angle microscopy for Langmuir monolayers of heneicosanoic acid with the esters methyl and ethyl heneicosanoate and octadecanoic acid with methyl, ethyl, and propyl octadecanoate. The behavior is similar to that found previously in mixtures of an acid and an alcohol. In each case with increasing ester concentration the L2/L2′ phase boundary moves toward lower pressure and higher temperature while the L2/Ov boundary moves toward lower pressure and lower temperature. The L2′ and Ov phases eventually merge and the boundary with the L2 phase moves to zero pressure. The phase diagram of eicosyl acetate is similar to that of the fatty acids. We attribute the variations in the diagrams to the extent of hydrogen bonding between the head group and the subphase.
Pseudo-ensemble simulations and Gibbs–Duhem integrations for polymers
Authors: Escobedo Fernando A. , de Pablo Juan J.
Published: July 2002

Pseudo-ensemble simulations and Gibbs–Duhem integrations are formulated within the framework of the expanded grand canonical ensemble. Pseudo-isobaric–isothermal simulations are proposed in which volume moves are replaced by fluctuations in the number of molecular segments. For large systems of dense athermal polymers, this pseudo-isobaric–isothermal method is shown to achieve mechanical equilibration faster than both conventional volume moves and the recently proposed slab volume moves. Pseudo-ensembles are also discussed for Gibbs ensemble simulations and canonical simulation (of the chemical potential). It is shown that coexistence curves for pure homopolymers and polymer mixtures can be traced by performing a numerical integration of the Gibbs–Duhem equation based on (expanded) grand canonical simulations. The validity of the methods is demonstrated by tracing the vapor–liquid coexistence curve of pure square-well chains and the liquid–liquid binodal curve of a blend of square-well chains.
Ion solvation dynamics in binary mixtures
Authors: Day Tyler J. F. , Patey G. N.
Published: July 2002

The dynamics of selective ion solvation in binary Stockmayer solvents is investigated using molecular dynamics simulations. The dependence of the usual solvation response function, S(t), on solvent composition and on the relative polarity of the solvent species is examined and discussed. We also introduce particle solvation response functions which describe the compositional relaxation of the first solvation shell. It is shown that the selective solvation process can be well described by a simple phenomenological model based on the ideas of elementary chemical kinetics. This model is useful and helps in the identification of two distinct time scales present in the selective solvation process. These are associated with a rapid electrostriction step during which the total number of particles in the first shell increases to its equilibrium value, and a slower spatial redistribution process during which the composition of the first shell achieves equilibrium. The redistribution phase depends on the rate of mutual ion-solvent diffusion and also on the rate of particle exchange between the first and second shells. A detailed analysis of the exchange process indicates that exchanges occur on virtually a one-to-one basis with the insertion of a stronger dipole into the first shell being mirrored by an almost immediate ejection of a weaker one.
Two-dimensional transport and wall effects in the thermal diffusion cloud chamber. I. Analysis and operations criteria
Authors: Bertelsmann Anne , Heist Richard H.
Published: July 2002

In this paper we present results of a two-dimensional (z,r) treatment of the mass and energy transfer processes that occur during the operation of a thermal diffusion cloud chamber. The location of the wall is considered in solving the mass and energy transport equations, in addition to the vertical distance, z, between the upper and lower plate surfaces. We examine the effect of aspect (diameter to height) ratio on chamber operation; the effects of operation with either a dry or a wet interior chamber wall on temperature, supersaturation, nucleation rate, and total density profiles in the chamber; the effect of overheating the interior of the chamber wall on these conditions within the cloud chamber; and the effects associated with using different density background gases on the operation of the chamber. In a second paper, immediately following, we apply the formalism and the solutions developed in this paper to address the important problem of buoyancy-driven convection that can accompany (seemingly normal) operation of thermal diffusion cloud chambers in nearly all ranges of total pressure and temperature.