bluuebunny/crossref_metadata_2025 huggingface dataset:
Adsorption dynamics of monoenergetic iodine monobromide (IBr) on the Si(111)-7×7 surface
Authors: Liu Yong , Komrowski Andrew J. , Taylor Peter R. , Kummel Andrew C.
Published: July 2002

The adsorption of monoenergetic IBr molecules on the Si(111)-7×7 surface has been studied using scanning tunneling microscopy, mass spectrometry, Auger electron spectroscopy, and supersonic molecular beam techniques. The adsorption proceeds predominantly via the direct abstractive adsorption mechanism and preferentially occurs at the center Si adatoms. The IBr abstraction probabilities at the incident energies of 0.15 and 0.82 eV have been determined to be 0.90±0.03 and 0.77±0.03, respectively. The minor dissociative adsorption channel of IBr can be enhanced at the expense of the abstractive adsorption channels by raising the incident energy. Most importantly, no atomic selectivity for iodine or bromine was observed. A reaction mechanism involving two types of transition states, Si⋯I⋯Br(s) and Si⋯Br⋯I(s), has been proposed to interpret the experimental observations. The attractive interaction between the nearly symmetric highest occupied molecular orbitals (HOMO, π* antibond) of IBr and the partially-filled Si adatom dangling bonds governs the surface site selectivity and the atomic selectivity of IBr adsorption on Si(111). Comparison with the adsorption of ICl on the surface has also been made to clarify the role of the asymmetric molecular bonding in adsorption dynamics.
Hyperquantization algorithm. II. Implementation for the F+H2 reaction dynamics including open-shell and spin-orbit interactions
Authors: Aquilanti Vincenzo , Cavalli Simonetta , De Fazio Dario , Volpi Alessandro , Aguilar Antonio , Giménez Xavier , Lucas José Mariá
Published: July 2002

This work focuses on numerical aspects and performances of the hyperquantization algorithm, presented in the preceding paper, for a prototypical atom–diatom reaction. Here we provide also the extensions which allow the treatment of excited electronic surfaces. Test calculations have been carried out on the reaction F+H2 at a total nuclear angular momentum equal to zero, the fine structure of the fluorine atom being also explicitly taken into account. The technique presented is shown to be simple and effective for applications to reactive scattering problems, and the results are competitive with those obtained applying other current methods, especially in the strong triatomic interaction region.
High-resolution absorption spectrum of jet-cooled CS2 between 70 500 and 81 550 cm−1: np and nf Rydberg series converging to the first ionization potential
Authors: Cossart-Magos Claudina , Jungen Martin , Launay Françoise
Published: July 2002

The absorption spectrum of jet-cooled CS2 was photographed between 70 500 and 81 550 cm−1 at a resolution limit of 0.0008 nm, i.e., 0.4 to 0.5 cm−1. Wave numbers of over 200 features are reported and assigned in terms of ⋯2πg3np (n=5–25) and nf (n=4–17) allowed transitions—electronic origin bands and vibronic 101 bands corresponding to excitation of the totally symmetric stretching vibrational mode in the excited electronic states—as well as forbidden ⋯2πg3nsσ (n=5–7), ndσ (n=4 and 5), and ndδ (n=3–5) g−g transitions—201 and 212 bands involving excitation of the bending vibrational mode in the corresponding excited 1,3Πg states—, and the (⋯2πu3Ã2Πu)4sσg 1Πu allowed transition, first term of a series converging to the second ionization limit. Ab initio calculations of the electronic energies and transition moments for effective principal quantum numbers about 4 and 5 are carried out for all the observed series. Rotational band contours are calculated using Hund’s case (e) representation for one- and three-photon excitation of the ⋯2πg3np and nf Rydberg complexes for every observed n value. An approximate Rydberg formula is obtained which allows the calculation of the principal peak wave numbers for n>12 to within 1 cm−1. This work completes the study of CS2 Rydberg series converging to the first ionization limit initiated previously with the ⋯2πg34s and 4p complexes [C. Cossart-Magos et al., J. Chem. Phys. 104, 7412 (1996)] and the ⋯2πg33d, 5s supercomplex [C. Cossart-Magos et al., J. Chem. Phys. 107, 1308 (1997)]. Systematic comparison with the (3+1) resonance enhanced multiphoton ionization (REMPI) spectra recorded by Morgan et al. [J. Chem. Phys. 104, 6117 (1996)] and by Berger et al. [J. Chem. Phys. 107, 8866 (1997)] reveals that only exceptionally the same transition is observed both in the absorption and the REMPI experiments: the two techniques give complementary information, with the absorption providing a much greater volume of data. Previous tentative assignments of the REMPI spectra by Morgan et al. and by Berger et al. are completed, with a few reassignments being necessary. Detailed comparison of the present absorption spectrum with the vacuum-ultraviolet laser and synchrotron radiation photoionization recorded by Huang et al. [J. Chem. Phys. 106, 864 (1996)], shows that, in the common part (n=14–25), the same transitions are observed, and that, for the electric field intensities used (up to 1070 V/cm), the Stark shifts are less than 1 cm−1.
Critical behavior and partial miscibility phenomena in binary mixtures of hydrocarbons by the statistical associating fluid theory
Authors: Blas Felipe J. , Vega Lourdes F.
Published: July 2002

Predictions of critical lines and partial miscibility of binary mixtures of hydrocarbons have been made by using a modified version of the statistical associating fluid theory (SAFT). The so-called soft-SAFT equation of state uses the Lennard-Jones potential for the reference fluid, instead of the hard-sphere potential of the original SAFT, accounting explicitly for the repulsive and dispersive forces in the reference term. The mixture behavior is predicted once an adequate set of molecular parameters (segment size, dispersive energy, and chain length) of the pure fluid is available. We use two sets of such parameters. The first set is obtained by fitting to the experimental saturated liquid density and by equating the chemical potential in the liquid and vapor phases for a range of temperatures and pressures. The second set is obtained from the previous one, by rescaling the segment size and dispersive energy to the experimental critical temperature and pressure. Results obtained from the theory with these parameters are compared to experimental results of hydrocarbon binary mixtures. The first set gives only qualitative agreement with experimental critical lines, although the general trend is correctly predicted. The agreement is excellent, however, when soft-SAFT is used with the rescaled molecular parameters, showing the ability of SAFT to quantitatively predict the behavior of mixtures. The equation is also able to predict transitions from complete to partial miscibility in binary mixtures containing methane.
A semiclassical study of tunneling effects in aziridine
Authors: Guo Yin , Wilson Angela K. , Chabalowski Cary F. , Thompson Donald L.
Published: July 2002

The tunneling effects in the molecular inversion of aziridine are investigated. A full-dimensional potential was constructed based on ab initio results and used to study the effect of vibrational excitations on tunneling. Using the semiclassical approach that incorporates tunneling into classical trajectory calculations, it is found that excitations of all the modes except the reaction coordinate have negligible effects on tunneling. This allows accurate thermal rate calculations carried out by using a one-dimensional model. (This is in accord with the conclusions of the work by Rom et al. [Chem. Phys. Lett. 204, 175 (1993)] and by Smedarchina et al. [J. Chem. Phys. 102, 7024 (1995)].) A one-dimensional Weutzel–Kramers–Brillouin (WKB) formula was employed to obtain the level splitting and was found to be very accurate based on comparisons with the quantum results. Furthermore, the calculated thermal rates are in good agreement with the experimentally measured values. The results provide insight into the reaction mechanism and explanations for the experimental findings of non-Rice–Ramsperger–Kassel–Marcus (RRKM) behavior and incoherent tunneling.
Solvent–solute interactions and the Raman CH stretching spectrum of cyclohexane-d11. II. Density dependence in supercritical carbon dioxide
Authors: Pan Xun , Cooper McDonald J. , MacPhail Richard A.
Published: July 2002

We have measured the isotropic Raman CH stretching spectrum of cyclohexane-d11 in supercritical CO2 at 49.7 °C and in liquid CO2 at room temperature over a range of densities from 0.2ρc to 2ρc, where the critical number density ρc for CO2 is 6.4 nm−3. The axial and equatorial CH stretching bands in the spectrum shift to lower frequencies and broaden with increasing density. As was the case in an earlier study of cyclohexane-d11 in liquid solvents [G. J. Remar and R. A. MacPhail, J. Chem. Phys. 103, 4381 (1995)], the “perturbed hard-fluid model” of Ben-Amotz and Herschbach provides a satisfyingly consistent description of the observed shifts in terms of competing contributions from repulsive and attractive solute–solvent forces along the CH bond. In particular, when the repulsive contribution to the shift is calculated according to the prescription developed in the liquid solution study, the attractive contribution is found to scale linearly with the density and with the polarizability derivative of the CH bond, as predicted by the model. The ratio of the equatorial to axial linewidths has a density-independent value of 1.2, nearly the same value found for the liquid solutions and numerically equivalent to the ratio of polarizability derivatives for the CH bonds. This equivalence is consistent with Schweizer and Chandler’s theoretical result for the width of a band that is inhomogeneously broadened by attractive force fluctuations, but the density dependence is not; their result would predict a nonlinear density dependence with a maximum near ρc, whereas the observed linewidths show a nearly linear dependence on density. Neither the frequency shifts nor the linewidths show any clear evidence for a “local solvent density enhancement” that would be predicted for this mixture near the critical point. In the accompanying paper, Frankland and Maroncelli describe molecular-dynamics simulations of cyclohexane in supercritical CO2 that reproduce the observed linewidths nearly quantitatively. They show convincing evidence that the linewidths are dominated by binary, collisional interactions between the hydrogen and the solvent, and they discuss the apparent absence of a density enhancement.
A simple algorithm for conservation of invariants of motion in reaction dynamics calculations: Application to the NH3++NH3 system
Authors: Tachibana Akitomo , Fuju Masahiro
Published: July 2002

This paper presents a new simple algorithm that guarantees simultaneous conservation of energy, linear momentum, and angular momentum of a whole system in reaction dynamics calculations, employing atomic Cartesian coordinates. We apply this algorithm to the reaction dynamics in the NH3++NH3 system. We show that along the intrinsic reaction coordinate (IRC) of the hydrogen abstraction (HA) channel of the reaction, the geometries of local minima and transition state (TS) change appreciably with the rotational energy due to the angular momentum. Reaction dynamics calculations in the vicinity of the IRC reveal the dynamical effects that the angular momentum promotes or suppresses the reaction.
Rotation–vibration interactions in (HF)2. I. Using parallel supercomputers to calculate rotation–vibration energy levels
Authors: Wu Xudong T. , McCoy Anne B. , Hayes Edward F.
Published: July 2002

An algorithm for calculating rotation–vibrational energy levels and wave functions for AB–CD tetra-atomic systems is presented. By transforming the wave equation into a large sparse eigenvalue problem, we can take advantage of the implicitly restarted Lanczos method developed by Sorensen and co-workers. The algorithm has been applied to calculations of the lowest 40 bound states of (HF)2, (DF)2 and HF⋅DF with even and odd parities. The lowest 40 energies and corresponding wave functions for (HF)2 with J=0 and even parity can be calculated in 10.5 minutes on 126 processors of a CRAY T3E. The resulting energy levels are found to be in excellent agreement with the previously reported values of Zhang, et al. [J. Chem. Phys. 102, 2315 (1995)].
Characterization of the lowest triplet states of linear form C2n+1 by anion photoelectron spectroscopy
Authors: Kohno Masamichi , Suzuki Shinzo , Shiromaru Haruo , Achiba Yohji
Published: July 2002

Photoelectron spectroscopy was applied to characterize electronically excited states of carbon clusters consisting of odd number atoms, C2n+1− (n=2–7). By optimizing the cluster generation condition so that the fraction of Cn− anions with a linear form increases, the energy difference between the ground and the first excited states of the carbon linear chains up to C15− was systematically examined. As a result, it was found that the photoelectron second bands are attributed to the first excited triplet states of neutral carbon linear chains. The monotonous decrease of the excitation energy found for C7–15 clearly indicates that these linear chains possess a common electronic property for the lowest triplet states. Furthermore, by analyzing the decrease of the excitation energy in the triplet manifold, it was found that the gap tends to converge to almost zero by increasing a length of carbon chain.
Dynamic heterogeneities of translational and rotational motion of a molecular glass former from computer simulations
Authors: Qian J. , Hentschke R. , Heuer A.
Published: July 2002

We employ classical molecular dynamics simulations to study the rotational and translational dynamics of propylene carbonate. The homogeneous and heterogeneous contributions to the nonexponential relaxation are studied. The non-Gaussian parameter is found to be strongly correlated with the degree of heterogeneity. Furthermore the coupling between translational and rotational motion is investigated. A dimensionless coupling constant is introduced which is found to increase with increasing time scale and decreasing temperature.
Atomically resolved adsorption and scanning tunneling microscope induced desorption on a semiconductor: NO on Si(111)-(7×7)
Authors: Rezaei M. A. , Stipe B. C. , Ho W.
Published: July 2002

Using a variable-temperature, ultrahigh vacuum scanning tunneling microscope (STM), we have studied the adsorption and STM induced desorption of NO from Si(111)-(7×7). NO adsorbs preferentially on faulted corner sites, followed by faulted center sites, unfaulted corner sites and unfaulted center sites. The preference for the different adsorption sites is independent of temperature and correlates well with the local density of states at these sites. NO can be desorbed from Si(111) with the STM. We present data that suggest the desorption is induced by the electric field under the STM tip. The threshold positive electric field for desorption of NO is 0.114 ± 0.009 V/Å. For sufficiently small tip–surface distances, NO can be desorbed locally without affecting the neighboring adsorbates.
Equations of state of freely jointed hard-sphere chain fluids: Theory
Authors: Stell G. , Lin C.-T. , Kalyuzhnyi Yu. V.
Published: July 2002

Using the analytical solution of a multidensity integral equation solved in our previous papers [J. Chem. Phys. 108, 6513, 6525 (1998)], we derive two compressibility and two virial equations of state (EOS) for freely jointed hard-sphere chain fluids on the basis of the approximations defined by the polymer Percus–Yevick (PPY) closure and of the PPY ideal-chain closure for the integral equations. We also extend a version of first-order thermodynamic perturbation theory to polymers, using a dimer fluid as the reference system, to treat mixtures of heteronuclear chain fluids and polymer solutions; the structural information of the dimer fluid is obtained from the PPY ideal-chain approximation in the complete-association limit. The attractive forces between monomers of chain molecules are treated using simple perturbation theory. We find that the compressibility EOS derived on the basis of the PPY approximation subject to the chain-connectivity condition reduces to the compressibility EOS based upon the PPY ideal-chain approximation in the complete-association limit, which is also equivalent to the EOS derived by Chiew [Mol. Phys. 70, 129 (1990)] and to the EOS derived by Kalyuzhnyi and Cummings [J. Chem. Phys. 105, 2011 (1996)]. On the other hand, the virial EOS derived on the basis of the PPY ideal-chain approximation coincides with Attard’s virial EOS [J. Chem. Phys. 102, 5411 (1995)] only in the zero-density limit. The advantages in numerical implementation of the EOS presented in this work are also discussed, but a full quantitative assessment of our results and a detailed numerical comparison among them are made in a companion paper, as is comparison with available simulation results.
Equations of state of freely jointed hard-sphere chain fluids: Numerical results
Authors: Stell G. , Lin C.-T. , Kalyuzhnyi Yu. V.
Published: July 2002

We continue our series of studies in which the equations of state (EOS) are derived based on the product-reactant Ornstein–Zernike approach (PROZA) and first-order thermodynamic perturbation theory (TPT1). These include two compressibility EOS, two virial EOS, and one TPT1 EOS (TPT1-D) that uses the structural information of the dimer fluid as input. In this study, we carry out the numerical implementation for these five EOS and compare their numerical results as well as those obtained from Attard’s EOS and GF-D (generalized Flory-dimer) EOS with computer simulation results for the corresponding chain models over a wide range of densities and chain length. The comparison shows that our compressibility EOS, GF-D, and TPT1-D are in quantitative agreement with simulation results, and TPT1-D is the best among various EOS according to its average absolute deviation (AAD). On the basis of a comparison of limited data, our virial EOS appears to be superior to the predictions of Attard’s approximate virial EOS and the approximate virial EOS derived by Schweizer and Curro in the context of the PRISM approach; all of them are only qualitatively accurate. The degree of accuracy predicted by our compressibility EOS is comparable to that of GF-D EOS, and both of them overestimate the compressibility factor at low densities and underestimate it at high densities. The compressibility factor of a polydisperse homonuclear chain system is also investigated in this work via our compressibility EOS; the numerical results are identical to those of a monodisperse system with the same chain length.
Photolysis of hydrogen chloride embedded in the first argon solvation shell: Rotational control and quantum dynamics of photofragments
Authors: Žďánská Petra , Schmidt Burkhard , Jungwirth Pavel
Published: July 2002

Under standard conditions reaction yields are connected with terms like free energy differences and thermal distributions. However, many modern experimental techniques, such as supersonic beam expansion or matrix isolation, deal with cryogenic temperatures and isolated reactants in inert clusters or solid matrices. Under these conditions the photochemical reaction mechanism is in many cases strongly dependent on the shape of delocalized initial vibrational or rotational wave functions of the reactants which can be employed for an efficient reaction yield control. Here, we apply, using quantum molecular dynamics simulations, such a scheme to the rotational control of photolysis of the HCl molecule embedded in an icosahedral Ar12 cluster. First, the HCl molecule is preexcited into a specific low lying rotational level. Depending on the rotational state, the hydrogen probability is enhanced in different directions within the cluster. In a second step, the HCl molecule is photolyzed by an UV pulse. The rapidly dissociating hydrogen atom then reaches primarily either the holes in the solvent shell or the argon atoms, depending on the rotational preexcitation. Starting either from the ground or from the first totally symmetric excited rotational states, the direct dissociation and the delayed process accompanied by a temporary trapping of the hydrogen atom have very different relative yields. As a consequence, differences up to a factor of 5 in the temporary population of the hydrogen atom inside the cluster after the first hydrogen-cage collision are observed. In the energy domain a significant difference in the structure of the kinetic energy distribution spectra, connected with the existence of short-lived vibrational resonances of the hydrogen atom, is predicted.
Lifetimes of the CO metastable a 3Π(v⩽3,Ω,J) levels
Authors: Sykora Thomas , Vidal C. R.
Published: July 2002

We reanalyze the theoretical lifetimes of the metastable a 3Π(v=0,Ω,J) levels as well as the spontaneous a 3Π(v=0) decay rates into the X 1Σ+(v) ground state levels, clearly favoring the measurements of Jongma et al. [J. Chem. Phys. 107, 7034 (1997)] as compared to the theoretical results of James [J. Chem. Phys. 55, 4118 (1971)]. Theoretical lifetimes of the vibrationally higher lying metastable levels a 3Π(v=1–3,Ω,J) are listed for the first time. The lifetimes of the two Λ components are shown to be different. A new technique for measuring the lifetime of individual rovibrational metastable molecular levels is applied to the CO rovibrational level a 3Π(v=3,Ω=1,J=2). Our result τ=3.04±0.38 ms is in good agreement with the theoretical lifetime of 2.97 ms. The metastable population is deduced to be Nmeta=(2.3±1.2)⋅106 molecules/shot. Finally, our ms lifetime measurement technique is compared with that developed by Jongma et al. [J. Chem. Phys. 107, 7034 (1997)].
A surprising asymmetric structure for the short-lived excited S1 state of 4,4′-bipyridine
Authors: De Waele V. , Buntinx G. , Poizat O. , Flament J-P. , Kassab E.
Published: July 2002

The combination of ab initio calculation using the CIS method and experimental vibrational data previously obtained from picosecond time-resolved Raman measurements [C. Didierjean, V. De Waele, G. Buntinx, and O. Poizat, Chem. Phys. 237, 169 (1998)] leads to a confident picture of the structure of the short-lived excited singlet state S1 of 4,4′-bipyridine. The nπ* nature of S1 is confirmed. The excitation is confined in one of the pyridyl rings, breaking the initial ground state symmetry and leading to an asymmetric structure equivalent to an excited pyridine substituted by an unperturbed pyridyl ring. This structure is in complete agreement with the experimental data.
The inner valence photoionization of acetylene
Authors: Wells M. , Lucchese Robert R.
Published: July 2002

We performed both single and coupled-channel scattering calculations of the photoionization of acetylene using an iterative Schwinger variational method. A comparison of the resulting single-channel and multichannel cross sections shows that the two results differ at photon energies of up to several hundred eV, which indicates that interchannel coupling and/or nonorthogonality terms may continue to be important even at these high photon energies. We compared the energy dependent satellite branching ratios with the corresponding ratios of spectroscopic intensity factors (SIFs) and found that the theoretically predicted high energy convergence of the former to the latter occurs by 1000 eV in only half of the cases studied. When the satellite provenance results from more than one main line, we do not observe this convergence behavior. We compare our theoretical results to experimental synchrotron radiation photoelectron spectroscopy (SRPES) results and illustrate how the superposition of one or more low intensity satellite lines may lead to the erroneous identification of convergence behavior. Based on our theoretical results for the photon energy dependence of the branching ratios of the four principle satellites of acetylene, results which are in excellent agreement with the experimental results, we conclude that all four of the satellites are dynamically correlated according to a phenomenological classification system. This conclusion opposes conclusions of a recent study that three of the satellites are intrinsically correlated, conclusions based upon the same experimental data that we use here, the same classification system, but differing in that the conclusion was formed in part on the basis of SIFs, i.e., static theoretical results. The results presented here underscore the difficulties inherent in analyzing the dynamics of photoionization using static theoretical results such as SIFs, and illustrate how a dynamic theory of photoionization can be used to interpret experimental data.
Evaluation of gradient corrections in grid-free density functional theory
Authors: Glaesemann Kurt R. , Gordon Mark S.
Published: July 2002

The Almlöf–Zheng approach to grid-free density functional theory (DFT) uses the resolution of the identity (RI) instead of a finite grid to evaluate the integrals. Application of the RI can lead to stability problems, particularly when gradients are involved. The focus of the current work is on choosing a stable method of evaluating the gradient correction using the RI. A stable method is compared to several unstable methods.
Exponentially correlated Gaussian functions in variational calculations: Energy expectation values in the ground state helium dimer
Authors: Komasa Jacek
Published: July 2002

Exponentially correlated Gaussian wave functions have been employed to compute expectation values of energy operators in the electronic ground state of the helium dimer. The expectation values are calculated for a wide range of internuclear distances, 0.0⩽R/a0⩽15.0, with particular regard to small R. The results include the total and the interaction energy, the energy derivative with respect to R, and components of the kinetic and the Coulomb energy. The variation of the expectation values of the kinetic and Coulomb energy yields information on the electron cloud dynamics upon the geometry change. The electronic energy and its derivative are analyzed with respect to rigorous theoretical constrains which they should fulfill. The Thirring upper bound is evaluated from an accurate electrostatic potential computed for the beryllium atom. This potential is also used to check the accuracy of the united atom perturbation theory. Smooth transition of all the expectation values to the limit of united atom verifies the validity of the Born–Oppenheimer approximation in large energies. As the wave function used is presently the most accurate variational wave function obtained for the He2, the results reported may serve as benchmarks.
N-chlorodifluoromethanimine, CF2=NCl: Quadrupole coupling and r structure
Authors: Groner P. , Durig J. R. , DesMarteau D. D. , Hwang S.-H.
Published: July 2002

The microwave spectra of CF212=15N35Cl, CF212=15N37Cl, and CF213=14N35Cl have been recorded and analyzed. Rotational, quartic centrifugal distortion and quadrupole coupling constants were determined. The off-diagonal element of the chlorine quadrupole coupling tensor, χab, and the principal element of that tensor, χz, were obtained as 63.7 and −103.5 MHz, respectively, for Cl35 (average), and 51 and −82 MHz, respectively, for Cl37 in CF212=15N37Cl. During the analysis of the spectrum of the C13 species, it was necessary to diagonalize the complete exact Hamiltonian of the asymmetric rotor with two quadrupole nuclei because of irregular hyperfine patterns of some transitions caused by previously neglected matrix elements from the N14 quadrupole. Earlier data for the normal isotopic species were re-evaluated because neglected off-diagonal matrix elements of the Hamiltonian led to data for χab and χz inconsistent with the results obtained during this investigation. The rotational constants of all five isotopic species were used to determine the r0 structure. All structural parameters for the planar molecule except for the difference between the CF distances could be determined. The results for the distances r(C=N) and r(NCl) and the angle α(CNCl) were 1.254(27) Å, 1.713(17) Å, and 113.62(26)°, respectively. The experimental data for the structure, the inertial defect, and the centrifugal distortion constants are compared to results from ab initio calculations.
Broadening of CH3F in presence of Stark fields. II. Collisional coupling between the Stark components
Authors: Lemaire V. , Dore L. , Cazzoli G. , Buffa G. , Tarrini O. , Baldanzi E. , Belli S.
Published: July 2002

The collisional line shape of methyl fluoride rotational transitions J,K=2,2→3,2 and 5,5→6,5 is observed under the effect of a static electric field. The collisional coupling between partially overlapping Stark components is studied under two particularly favorable conditions: first, the degree of overlapping can be set by both the electric field and the pressure, second, we have a calculated collisional relaxation matrix checked on both well isolated and completely unresolved Stark components. This allows us to obtain better insight into the collisional coupling effect and to show definitely that this effect explains the difference between collisional linewidths of the CH3F rotational transitions and of their Stark components.
State-selected ion-molecule reactions: The charge-transfer N2+(X,v+)+O2→O2++N2
Authors: Knott W. J. , Proch D. , Kompa K. L.
Published: July 2002

The charge-transfer process, which is the subject of this paper, is an important link in the chain of reactive events which occur in the E- and F-regions of the earth’s atmosphere. The reaction is suspected to proceed via the formation of a short-lived intermediate complex (N2⋅O2)+. The transient existence of this van der Waals molecule has been inferred from molecular orbital and ab initio arguments. To the present day, however, none of the numerous studies focusing on this system has provided proof of its existence. The positive evidence obtained from this experiment is of an indirect nature, based on measurements of the kinetic energy of the released O2+ as a function of the expendable center-of-mass collision energy. The results point to an indirect reaction (i.e., via an intermediate) in the low energy regime, and a direct process once the center-of-mass energy exceeds the bond energy of the complex. A model description which contracts both pictures matches the complete set of energy-dependent cross section data in a very persuasive manner. A competing reactive channel which has also been given attention results in the formation of NO+. The initiation of this reaction requires a cleavage of the N–N bond, which should be eased by vibrational excitation of the reagent ion. The cross section data, however, turn out to be unaffected by such details of reagent state preparation.
Molar excess volumes of liquid hydrogen and neon mixtures from path integral simulation
Authors: Challa Sivakumar R. , Johnson J. Karl
Published: July 2002

Volumetric properties of liquid mixtures of neon and hydrogen have been calculated using path integral hybrid Monte Carlo simulations. Realistic potentials have been used for the three interactions involved. Molar volumes and excess volumes of these mixtures have been evaluated for various compositions at 29 and 31.14 K, and 30 atm. Significant quantum effects are observed in molar volumes. Quantum simulations agree well with experimental molar volumes. Calculated excess volumes agree qualitatively with experimental values. However, contrary to the existing understanding that large positive deviations from ideal mixtures are caused due to quantum effects in Ne–H2 mixtures, both classical as well as quantum simulations predict the large positive deviations from ideal mixtures. Further investigations using two other Ne–H2 potentials of Lennard–Jones (LJ) type show that excess volumes are very sensitive to the cross-interaction potential. We conclude that the cross-interaction potential employed in our simulations is accurate for volumetric properties. This potential is more repulsive compared to the two LJ potentials tested, which have been obtained by two different combining rules. This repulsion and a comparatively lower potential well depth can explain the positive deviations from ideal mixing.
Photoelectron angular distributions as a probe of alignment evolution in a polyatomic molecule: Picosecond time- and angle-resolved photoelectron spectroscopy of S1 para-difluorobenzene
Authors: Reid Katharine L. , Field Thomas A. , Towrie Michael , Matousek Pavel
Published: July 2002

We demonstrate that picosecond time-resolved photoelectron angular distributions (PADs) provide a sensitive probe of an evolving alignment in an excited polyatomic molecule. Such an evolving alignment can be caused by pure rotational recurrences or by rotation–vibration coupling. If a molecule is chosen for which the rotational recurrence times are well-known the method provides a means of establishing the mechanism of intramolecular vibrational energy redistribution (IVR). In the case of S1 para-difluorobenzene we observe striking alignment changes as a function of pump–probe time delay which we attribute to rotationally mediated IVR.
Dependence of the self-diffusion coefficient on the sorbate concentration: A two-dimensional lattice gas model with and without confinement
Authors: Bhide Shreyas Y. , Yashonath S.
Published: July 2002

Previous pulsed-field gradient (PFG) nuclear magnetic resonance (NMR) measurements on different sorbate–zeolite systems suggest that there exist at least five different types of dependence of self-diffusivity of the sorbate, D, on the concentration, c, of the sorbate. Sorbate–zeolite systems have been modeled as a two-dimensional lattice gas and studied by carrying out Monte Carlo simulations under different conditions. Among the different factors that have been varied are the arrangement of adsorption sites, hop length, nature and strength of the sorbate-sorbate interaction, and the degree of confinement. Surprisingly, even the simple 2-D lattice gas model could yield more than one type of D vs c dependence. The present study provides insight into the possible reasons for a given type of D vs c dependence.
A comparison of methods for the calculation of NMR chemical shifts
Authors: Gregor Thomas , Mauri Francesco , Car Roberto
Published: July 2002

A theory (MPL) to compute the NMR chemical shifts in condensed matter systems using periodic boundary conditions was presented by F. Mauri, B. Pfrommer, and S. G. Louie [Phys. Rev. Lett. 77, 5300 (1996)]. The MPL method has been implemented so far within a pseudopotential formulation in which the wave functions are expanded in plane waves. In this paper, we compare analytically the MPL approach within the density functional theory to existing methods for the calculation of the chemical shifts such as GIAO (gauge-including atomic orbitals), CSGT (continuous set of gauge transformations), and IGAIM (individual gauges for atoms in molecules). To this end we apply the MPL approach to molecules since the latter methods are conceived only for finite systems. We show theoretically the equivalence between a variant of the CSGT and the MPL method applied to finite systems. Moreover, we analyze numerically the efficiency of the different methods when atomic orbital basis sets are employed, by comparing the basis-set convergence properties. We find that the CSGT and IGAIM approaches have the same convergence properties as GIAO, whereas their computational time is significantly smaller. In the MPL method, the contribution of the valence electrons to the chemical shift converges rapidly with respect to the size of the basis set, whereas the convergence properties of the core contribution are poor. We improve the convergence by separating the core and the valence contributions in a gauge-invariant manner, by applying the MPL method only to the valence contribution, and by treating the core contribution with IGAIM. The performances of the resulting approach compare favorably with those of the other methods. Finally we find that the core contribution to the chemical shift is independent of the chemical environment, in contrast to what is sometimes found in the literature. In conclusion, our results indicate that, to compute the chemical shifts in both molecules and solids, using atomic orbital basis sets, one could use the MPL method to evaluate the valence contribution and add to it a rigid core contribution as obtained, for instance, from an atomic calculation.
Photoconductivity and singlet oxygen generation in illuminated polymer in the air atmosphere
Authors: Burshtein A. I. , Igoshin O. A.
Published: July 2002

Our original integral encounter theory is used to describe the photoconductivity of p-phenilene vinelene (PPV) accompanied by singlet oxygen generation in the polymer volume. This theory allows calculating (a) the free carrier generation due to the forward electron transfer to oxygen from the excited singlet of PPV and (b) production of singlet oxygen by energy transfer from triplet PPV generated by intersystem crossing. We obtained the stationary concentration of the free carriers, with account of their geminate recombination before separation, as well as the stationary rate of singlet oxygen generation, affected by preliminary quenching of nearest excitations in the course of ionization. Both effects are related to off-diagonal elements of the matrix integral equations for the particle concentrations that can be represented in terms of the pair distributions of charged products and phantoms of triplet states. The stationary rate constants of electron and energy transfer can be easily obtained by the contact approximation, while the charge separation quantum yield and recombination rate constant are accessible for analytic calculations within the rectangular model of the remote reaction layer for the backward electron transfer. The suppression factor for singlet oxygen generation was obtained in the contact approximation as well as numerically for the exponential energy transfer rate. The role of the finite lifetimes of singlet and triplet excitations was especially emphasized.
Topology of electron–electron interactions in atoms and molecules. III. Morphology of electron intracule density in two 1Σg+ states of the hydrogen molecule
Authors: Cioslowski Jerzy , Liu Guanghua , Rychlewski Jacek , Cencek Wojciech , Komasa Jacek
Published: July 2002

The differences in electronic structures of two 1Σg+ states of the hydrogen molecule are vividly reflected in their intracule densities I(r). The ground-state wave function of H2 is associated with two distinct topologies of I(r) (one of which pertains to the united atom limit), whereas no fewer than 11 unequivalent sets of critical entities are found for I(r) of the EF state that involves multiple electronic configurations. These sets and the catastrophes that interrelate them, which arise from conflicts between topological features of I(r) pertinent to different configurations, are characterized in detail. The usefulness of topological analysis of I(r) in the detection and characterization of various types of electron correlation is demonstrated.
A new variational coupled-electron pair approach to the intermolecular interaction calculation in the framework of the valence bond theory: The case of the water dimer system
Authors: Specchio R. , Famulari A. , Sironi M. , Raimondi M.
Published: July 2002

A general nonorthogonal coupled-electron pair approach based on the intermediate optimization of virtual orbitals is presented. The resulting procedure, similar to the independent electron pair approximation scheme, is developed in the framework of the valence bond (VB) theory, where the effect of the overlap is directly taken into account. Nonorthogonal virtual orbitals optimal for intermolecular correlation effects were determined starting from the self-consistent field for molecular interaction wave function. These were used in the context of a general ab initio variational multistructure VB wave function consisting of double excitations arising from simultaneous single excitations localized on each monomer. The basis set superposition error is excluded in an a priori fashion and geometry relaxation effects are naturally taken into account. As an application example, the equilibrium structure and binding energy of the water dimer system were determined. The equilibrium oxygen–oxygen distance results to be 2.954 Å, in good agreement with the experimental values (2.946 or 2.952 Å) corrected for anharmonicity of the dimer vibrations. The estimated equilibrium interaction energy is −5.02 kcal/mol, thus comparing favorably with the experimental value of −5.44±0.7 kcal/mol. Taking zero-point vibrational effects into account, the calculated binding enthalpy is −3.22 kcal/mol, in accordance with the experimental estimate of −3.59±0.5 kcal/mol, determined from measures of thermal conductivity of the vapor. The importance of employing basis sets that include diffuse polarization functions in correlated calculations on hydrogen-bonded systems is confirmed.
Exploring the spin–orbit reactivity in the simplest chlorine atom reaction
Authors: Lee Shih-Huang , Liu Kopin
Published: July 2002

The reaction of Cl(2P)+H2→HCl+H serves as a benchmark for understanding the abstraction mechanism and has been the subject of numerous experimental and theoretical studies. Despite such intensive investigations, the reactivity of the low-lying spin–orbit excited Cl*(2P1/2) is unknown and has always been assumed to be negligibly small. By exploiting two different sources for generating the Cl(2P) beam, it is now found that the excited Cl*(2P1/2) atom is surprisingly more reactive to H2 than the ground state Cl(2P3/2) reagent, overturning conventional wisdom. By using a technique called Doppler-selected time of flight to directly map out the doubly differential cross sections (angle and speed), the detailed dynamical attributes for both spin–orbit states are also elucidated and contrasted for the first time for any bimolecular reaction.
Ab initio ground potential energy surface and quasiclassical trajectory study of the O(1D)+CH4(X 1A1)→OH(X 2Π)+CH3(X 2A2″) reaction dynamics
Authors: González Miguel , Hernando Jordi , Baños Irene , Sayós R.
Published: July 2002

An ab initio study of the ground potential energy surface (PES) of the O(1D)+CH4→OH+CH3 reaction has been performed using the second and fourth order Mo/ller–Plesset methods with a large basis set. From the ab initio data a triatomic analytical ground PES with the methyl group treated as an atom of 15.0 amu has been derived. This PES has been employed to study the dynamics of the reaction by means of the quasiclassical trajectory (QCT) method. A good agreement between the experimental and QCT OH rovibrational distributions at a collision energy of 0.212 eV with the methane molecule at 298 K has been obtained. The analysis of the microscopic reaction mechanism shows that the reaction takes place almost exclusively through the insertion of the O(1D) atom into a C–H bond, due to the presence of the deep (CH3)OH minimum, and the resulting trajectories may be direct or nondirect (short-lived collision complexes mainly) with about the same probability. The OH vibrational distribution arising from the direct mechanism is inverted, while the nondirect mechanism leads to a noninverted one. There is some tendency to give broader OH rotational distributions peaking at higher N′ values, particularly for the vibrational levels v′=0–1, in the case of the nondirect trajectories. The PES derived here may be used in dynamics studies under conditions where the methyl group motions are not strongly coupled to the motions leading to reaction.
Many-body potential and structure for rhodium clusters
Authors: Chien Chang-Hong , Blaisten-Barojas Estela , Pederson Mark R.
Published: July 2002

The many-body potential for ferromagnetic and paramagnetic rhodium clusters proposed in this work has 11 parameters (14 for the paramagnetic case) that are fitted on the energy surface of Rh2 through Rh6 clusters calculated from first principles within the generalized gradient approximation (GGA) of density functional theory. Under this potential the most stable ferromagnetic and paramagnetic cluster structures are generated up to Rh58. Additionally, the growth under several symmetries is pursued up to N=400. The face-centered-cubic (fcc) growth path is the most stable at that cluster size regime. An effective measure of the cluster stiffness is calculated as a function of cluster size displaying a monotone increase towards the bulk value. The melting temperature is about constant up to clusters with 45 atoms, presenting a sharp increase towards the bulk value at larger sizes.
Quantum-phase and information-entropy dynamics of a two-state molecular system interacting with strongly amplitude- and phase-squeezed fields
Authors: Nakano Masayoshi , Yamaguchi Kizashi
Published: July 2002

We perform numerically exact calculations of quantum dynamics for a two-state molecular model system interacting with initially single-mode strongly amplitude- and strongly phase-squeezed fields (average photon number 〈n̂〉=8). The second-order correlation function for these fields are taken to be 2, which is equivalent to that for the chaotic field with the identical average photon number. Although the responses of the molecule for these two squeezed fields are relatively similar to those for the chaotic field as shown in a previous paper, there are found to be some significant differences among these fields in the dynamical behavior of quantum-phase distributions of photons, i.e., the Pegg–Barnett phase and the Q function distributions. These differences are also elucidated by the dynamics of the information entropy for the molecule, i.e., molecular entropy, which represents the degree of entanglement between the molecule and the photon field.
Double-resonance spectroscopy of autoionizing states of ammonia
Authors: Raptis C. A. , Bacon J. A. , Pratt S. T.
Published: July 2002

New double-resonance ionization data are presented on the autoionizing Rydberg states of NH3 excited via selected rotational levels of the C̃′ 1A1(0100) and (0200) states. Extrapolation of the Rydberg series provides a determination of the adiabatic ionization energy of NH3 that is in excellent agreement with that obtained by using zero kinetic energy photoelectron spectroscopy (ZEKE-PES). A comparison is presented between the field-free spectrum obtained by using a time-of-flight mass spectrometer and the 1-Tesla-spectrum obtained by using a magnetic-bottle electron spectrometer. Finally, the rotational and vibrational autoionization mechanisms are discussed.
Tracer diffusion of polyorganosiloxane nanoparticles in solution: Effects of tracer topology and particle concentration
Authors: Graf Christina , Schärtl Wolfgang , Maskos Michael , Schmidt Manfred
Published: July 2002

Spherical polyorganosiloxane nanoparticles containing photoreactive dye labels have been used to study self-diffusion in concentrated colloidal dispersions by forced Rayleigh scattering (FRS). Samples studied contained about 0.5 wt. % of these tracer particles and the order of 50 wt. % nonfunctional host particles dissolved in cyclohexane. To avoid any influence of the label molecules on the particle mobility, the tracers have been equipped with a nonfunctional protective shell. The effect of the thickness of this shell as well as of the label content on particle diffusion has been examined carefully. A much lower tracer mobility has been found for particles with protective shell of thickness less than 1 nm. In some cases, even complementary grating effects have been found in the FRS signals, indicating a different tracer mobility of bleached and nonbleached particles. By diffusion studies using tracers with protective shell sufficiently thick to avoid such unwanted effects, the single particle mobility for polyorganosiloxane microgel dispersions in the highly concentrated regime has been determined. Experimental results show that polyorganosiloxane nanoparticles in organic solvent do not exhibit strong volume swelling as most colloidal model systems but are porous rigid spheres filled with only about 25 wt. % solvent. Therefore, they behave like a very realistic hard sphere system.