bluuebunny/crossref_metadata_2025 huggingface dataset:
Simulations of sputtering induced roughening and formation of surface topography in deposition of amorphous diamond films with mass separated kiloelectronvolt ion beams
Authors: Koponen I. , Sievänen O.-P. , Hautala M. , Hakovirta M.
Published: July 2002

Atomic scale simulations are performed for ion bombardment induced roughening of amorphous diamond films in a process where deposition is done by using kiloelectronvolt mass separated ion beams operated in the upper limit of practical deposition energies up to 20 keV. Resulting surfaces are shown to be self-affine and they have low surface roughness. The experimentally observed extremely low roughness is obtained when moderate surface relaxation within the distance of next nearest neighbors is taken into account in simulations. It is found that there is also another mechanism, originating from the ballistic movement of redeposited atoms, which can maintain the eroding surfaces smooth. High-energy, off-normal deposition is shown to lead in pattern formation on a mesoscopic scale.
Response to “Comment on ‘A new approach to optimum design in thermoelectric cooling systems’ ” [J. Appl. Phys. 82, 6368 (1997)]
Authors: Yamanashi M.
Published: July 2002

The relevant results [M. Yamanashi (J. Appl. Phys. 80, 5494 (1996)] are correct because the numerical simulations were performed by using dimensionless thermoelectronic cooling (TEC) system equations. These equations can be used to optimize a TEC in a TEC system when p- and n-type thermoelements have different geometrical factors (lp/Sp≠ln/Sn). Usefulness of exergetic efficiency and the definition of the maximum dimensionless entropy flow are also explained.
Investigation of L-related indirect transitions in GaAs/GaAlAs multiquantum wells under hydrostatic pressure
Authors: Dai N. , Huang D. , Liu X. Q. , Mu Y. M. , Lu W. , Shen S. C.
Published: July 2002

Intersubband transitions in GaAs/GaAlAs multiquantum wells (QW) under hydrostatic pressure have been investigated with photomodulated transmission spectroscopy and free-standing QW samples. In addition to the direct transitions associated with the Γ critical points, several L-related transitions between the L conduction band and the Γ valence band of the GaAlAs barriers and between the L conduction subbands and the Γ valence subbands inside the GaAs quantum wells are observed. The distinct L-related spectral features are a clear indication that photomodulated transmission is particularly useful for enhancement of the weak phonon-assisted indirect transitions.
Two-dimensional percolation phase transition in ultrathin Co/Cu(110)
Authors: Hope S. , Tselepi M. , Gu E. , Parker T. M. , Bland J. A. C.
Published: July 2002

Near the critical region for the onset of ferromagnetic order, the magnetic susceptibility χ follows a power law with a critical exponent γ=2.39±0.08, in excellent agreement with the theoretical value for a perfect two-dimensional geometric percolation phase transition. Scanning tunneling microscopy studies reveal that the Co grows via the formation of three-dimensional elongated island structures, forming an array of Co “nanomagnets.” The ordering transition is a consequence of direct exchange developing between Co islands. The in-plane uniaxial anisotropy in the ferromagnetic phase is predominantly attributed to the internal dipolar field of the Co islands.
Sorption in pulsed laser deposition of multicomponent materials: Experiment versus modeling
Authors: Tyunina M. , Levoska J. , Leppävuori S.
Published: July 2002

Deposition rate, elemental composition, and spatial distribution of both deposition rate and of composition in the deposit were studied experimentally during the room temperature pulsed laser ablation of multicomponent targets of PbZr0.65Ti0.35O3, Bi2Sr2CuO6, and CuInSe2. Different material dependent modes of behavior of the deposition rate, composition, and their spatial distribution were observed with increasing pressure of ambient gas (oxygen, argon). Comparison of the obtained experimental data with the results of the previous modeling of pulsed laser deposition in a gas revealed a number of discrepancies. The possible reason for the discrepancies between the experimental observations and the model predictions was suggested to arise due to an assumption of the unit sticking probability of the species in the modeling. Qualitative phenomenological analysis of the deposition process as a sorption of ablated species on the substrate surface was performed for the studied multicomponent materials with respect to the nature and pressure of ambient gas. Good agreement between the experimental data and the conclusions of such an analysis indicated considerable influence of sorption on the deposition process.
Evaluation of residual stresses in chemical-vapor-deposited diamond films
Authors: Fan Qi Hua , Grácio J. , Pereira E.
Published: July 2002

In this article we report the determination of residual stresses in diamond films grown on Si (100) by using a plate bending theory and a bimetal theory combined with micro-Raman spectroscopy. Raman spectra show that with increase in the film thickness the characteristic diamond line shifts from higher wave numbers (>1332 cm−1) to lower (<1332 cm−1), indicating a change in the nature of residual stress with the film growth. A plate bending theory and a bimetal theory are used to determine the distribution of residual stress induced by thermal mismatch. The modeled results show that the thermal stress decreases linearly along the film growth direction and the stress at the film/substrate interface decreases once the film becomes thicker. The difference from the Raman results is attributed to intrinsic stress, originating from lattice mismatch between the diamond and Si at the growth temperature and from a variation in microstructure of the film with its growth evolution.
Room temperature magnetic properties of quenched-disordered FeAl alloys: A simulational study
Authors: Restrepo J. , Pérez Alcázar G. A. , Landau D. P.
Published: July 2002

We present a Monte Carlo study of the room temperature magnetic properties of quenched-disordered Fe1−qAlq alloys with 0⩽q⩽0.6. Simulations were performed using a random site-diluted classical Heisenberg model with nearest neighbor interactions, and using Metropolis dynamics. Results for the Al concentration of the magnetization and susceptibility predict the occurrence of a critical concentration qc at around 40 at. % Al, at which a transition from a ferromagnetic state (q<qc) to a paramagnetic one (q⩾qc) takes place. The simulated dependence of the average magnetization on the Al content is compared with experimental results of the average hyperfine field obtained by Mössbauer spectroscopy. Finally, an analysis of the finite size scaling properties of this system is presented.
Compositional dependence of the soft magnetic properties of the nanocrystalline Fe–Zr–Nb–B alloys with high magnetic flux density
Authors: Makino A. , Bitoh T. , Kojima A. , Inoue A. , Masumoto T.
Published: July 2002

The compositional dependence of the soft magnetic properties of the nanocrystalline Fe–Zr–Nb–B alloys has been investigated. The magnetostriction (λs) and the grain size of the (Fe90Zr7B3)1−x(Fe84Nb7B9)x alloys, which are two typical ternary alloys mixed with the best soft magnetic properties, show intermediate values between those of the Fe90Zr7B3 with negative λs and the Fe84Nb7B9 with positive λs. However, the soft magnetic properties of the Fe–(Zr, Nb)7–B alloys are inferior to those of the Fe90Zr7B3 and the Fe84Nb7B9 alloys. The best soft magnetic properties have been obtained at Zr+Nb=6 at. %. The Fe85.5Zr2Nb4B8.5 alloy shows a high μe of 60 000 at 1 kHz, a high Bs of 1.64 T, and zero λs, simultaneously. The alloy also exhibits a very low core loss of 0.09 W/kg at 1.4 T and 50 Hz, which is extremely lower than that of Fe–Si–B amorphous alloys. The nanocrystalline Fe–Zr–Nb–B alloys with improved soft magnetic properties are therefore suitable for pole transformers.
Spin-coupled VB theory of molecular electronic structure. Ground and low-lying 1Σ+ states of CH+
Authors: Gerratt J. , Manley J. C. , Raimondi M.
Published: July 2002

The spin-coupled VB theory is applied to the ground and low-lying 1Σ+ states of CH+. Using 500 spin-coupled structures, the calculated values of the spectrosopic constants for the ground states De(eV)=4.14 (4.26), ωe(cm−1)=2845 (2858), Re(Å) =1.135 (1.131); experimental results are given in parentheses. The form of the ground state potential energy curve is very close to the MCSCF-CI curve of Green et al. [Phys. Rev. A 5, 1614 (1972)], and similarly the excited states are very similar to those obtaind by MCSCF-CI methods by Saxon et al. [J. Chem. Phys. 73, 1873 (1980)]. The only exception is in the 4 1Σ+ state below R=3.5a0 where there is an avoided intersection with a Rydberg state. At least for small systems of this kind, the spin-coupled VB approach is capable of the same level of accuracy as the best MCSCF-CI methods, but uses an order of magnitude fewer functions. Each state in the spin-coupled VB calculation is dominated by one or two structures. This provides the wave function with much visuality, but without the usual attendant loss in accuracy.
Multiple trapping sites and symmetry splitting in cryogenic matrices: Infrared spectroscopy of HCN in Ar, Kr, and Xe
Authors: Abbate Alison D. , Moore C. Bradley
Published: July 2002

Infrared spectra of HC14N and HC15N trapped in cryogenic matrices of Ar, Kr, and Xe and of DCN in Xe have been measured. In addition to the fundamental frequencies, overtone and combination bands along with their anharmonic constants are reported. In the Kr and Xe matrices several of the bands exhibit multiple peaks. Changes in the bands with temperature, concentration, annealing, and host indicate that there are two trapping sites. In Xe the degeneracy of the C–H bend ν2 is split in one site. The spectral data in Xe may be understood in terms of HCN in a substitutional site of the face-centered-cubic rare gas lattice with some molecules oriented along the 〈111〉 axis and others along the 〈110〉 axis. The low symmetry about the 〈110〉 axis removes the degeneracy between the two bending coordinates of an HCN oriented along it. Two sites occur in Kr. Although a splitting of the ν2 degeneracy is not resolved, the same sites are probably involved in both Kr and Xe. A single unsplit site is observed for Ar.
The rotational spectrum and properties of N2 ⋅⋅⋅ HCN
Authors: Goodwin Elizabeth J. , Legon A. C.
Published: July 2002

The ground state rotational spectra of eight isotopic species of a weakly bound dimer N(1)N(2) ⋅⋅⋅ HCN(3) formed between molecular nitrogen and hydrogen cyanide have been detected and measured by the technique of pulsed-nozzle, Fourier-transform microwave spectroscopy. Rotational constants B0, centrifugal distortion constants DJ, and, where appropriate, nuclear quadrupole coupling constants χn(14N) and χ(D) have been determined. For the five isotopic species containing 14N nuclei the results are:               An analysis of the B0 values shows that the equilibrium geometry is linear, or nearly so, with the nuclei in the order shown. The DJ values lead to kσ=2.39 N m−1 for the intermolecular stretching force constant while the difference χ1(14N)–χ2(14N) is interpreted in terms of a transfer of 0.016 e from N(1) to N(2) when the complex is formed in the zero-point state.
Superconducting and other phases in organic high polymers of polyacenic carbon skeletons. I. The method of sum of divergent perturbation series
Authors: Kimura M. , Kawabe H. , Nishikawa K. , Aono S.
Published: July 2002

The instabilities of a normal molecular orbital state of polyacenic materials are studied within RPA with a g model for an electronic interaction. The condensed states predicted are singlet superconducting (SSC), charge density wave (CDW), and spin density wave (SDW) ones, and their phase diagram is shown. In contrast to usual one-dimensional (1D) conductors, there reveals a wide range of superconducting state, which is not overcome by CDW transition. Weakness of Peierls distortion of the present model is also contrasted with the case of polyacetylene.
The nature of the bonding in XCO for X=Fe, Ni, and Cu
Authors: Bauschlicher Charles W. , Bagus Paul S. , Nelin Constance J. , Roos Björn O.
Published: July 2002

The bonding in the 5,3Σ− and 3Δ states of FeCO, the 3,1Σ+, 3Δ, and 3 Π states of NiCO, and the 2Σ+ state of CuCO are analyzed using the constrained space orbital variation (CSOV) technique for both (CASSCF) and SCF wave functions. The bonding is discussed in terms of σ repulsion between the metal 4s and the CO 5σ, CO to metal σ donation when there is an empty or partly occupied dσ orbital and metal to CO 2π * backdonation. The bonding is compared for the different metals and between the different states.
Adiabatic separations of stretching and bending vibrations: Application to H2O
Authors: Johnson Bruce R. , Reinhardt William P.
Published: July 2002

A detailed investigation is made into the use of adiabatic approximations for describing excited stretching and bending vibrations of the water molecule. The goal is to determine precisely how effective this approach can be in a fully quantum mechanical triatomic calculation which incorporates anharmonicities to all orders in each of the modes. Great care is taken to avoid introducing unnecessary limitations or approximations: (i) Curvilinear coordinates are used rather than the Cartesian coordinates which form the starting point for normal mode calculations; (ii) the exact quantum kinetic energy operator in these coordinates is used as the basis for both the adiabatic and full three-dimensional calculations; (iii) a Sorbie–Murrell-type potential energy surface is used, giving a reasonable representation of the ground electronic surface for large excursions from the equilibrium configuration. In addition to the bond and bond-angle variables of earlier local mode investigations, a slightly different set of fully curvilinear coordinates is also investigated. These coordinates are shown to provide a more nearly separable description in both the exact and adiabatic treatments of this specific problem. The conventional adiabatic approach, in which the slower bending mode experiences an effective force due to averaging over the faster stretching modes, is reaffirmed to be accurate for excited stretching states. For states with any appreciable bending excitation, however, it turns out that the adiabatic calculations quickly erode in reliability. In answer to this problem, the reverse adiabatic procedure (with the bend treated first) is also implemented here. While counterintuitive, this latter method is found to yield a significant improvement for the calculated bending overtones, as well as many of the combination bands. Thus, by thorough consideration of both the coordinates and order of averaging employed, the adiabatic method is shown to be very effective for either bending or stretching overtones in a realistic, fully anharmonic, triatomic vibrational problem. In addition, introduction of a new orthonormal set of basis functions for the bending angle overcomes some of the problems associated with use of the less flexible Legendre basis.
Analytic energy gradients for general coupled-cluster methods and fourth-order many-body perturbation theory
Authors: Fitzgerald George , Harrison Robert J. , Bartlett Rodney J.
Published: July 2002

Energy gradient equations are presented for the coupled-cluster model with all possible excitations. By taking advantage of the equations for the coupled-cluster amplitudes, the gradient formulas may be expressed without explicit reference to the first-order changes in the amplitudes, in contrast to all earlier work. The coupled-cluster doubles (CCD) and coupled-cluster singles, doubles, and triples (CCSDT) models are treated as special cases of the general theory. Finally, by limiting the model to finite orders in perturbation theory, the gradient equations for the full fourth-order many-body perturbation energy are derived. Like the fourth-order energy itself, the gradient procedure is shown to be an n7 process in the number of basis functions. The computational implementation of this fourth-order energy gradient is discussed in detail.
Study of the plastic phases of norbornene and bicyclooctene by means of muonic radicals
Authors: Riccò M. , Bucci C. , De Renzi R. , Guidi G. , Podini P. , Tedeschi R. , Scott C. A.
Published: July 2002

We have observed the formation of muonium adduct radicals in norbornene and bicyclooctene. The behavior of the hyperfine constants and of the μSR linewidths have been investigated as a function of temperature; they have been correlated with the reorientational dynamics of the molecules, which have already been partially determined by means of Raman, NMR, and neutron scattering data. Our measurements have also been interpreted in terms of the electronic configuration of the radicals. In the case of norbornene at the plastic–brittle transition, an abrupt change of τc of 13 orders of magnitude was determined.
Relativistic effective potentials in quantum Monte Carlo calculations
Authors: Hurley M. M. , Christiansen P. A.
Published: July 2002

The frozen-core approximation was introduced into quantum Monte Carlo calculations by means of relativistic effective potentials. The conventional semilocal effective potentials were converted to local form using the same trial wave function used for importance sampling. Test calculations on Li, K, and their negative ions were readily carried out on a MicroVAX computer. The errors in the computed electron affinities (due to the effective potential and local potential approximations) were well under 0.1 eV.
A theoretical determination of the rovibrational energy levels of the water molecule
Authors: Carter S. , Handy N. C.
Published: July 2002

A redetermination of the quartic force field of the water molecule surface has been recently calculated by the method of Hoy, Mills, and Strey. A novel form of the bending coordinate has been introduced into this force field to ensure that the pure bending potential has the correct form at θ=π, and a few select force constant have been rerefined to the J=0 vibrational spectrum of H2O. The vibrational–rotational energy levels of the resulting surface have been determined by the variational method. Two particular sets have been examined: (i) low vibrational quanta, J≤9 and (ii) all levels below 12 000 cm−1 (relative to zero point), J≤2. The results are compared with the experimental levels in the literature.
Very short-range interatomic potentials
Authors: Pathak Rajeev K. , Thakkar Ajit J.
Published: July 2002

First-order united atom perturbation theory calculations of very short-range interactions between rare gas atoms are reported. These should prove useful because many of the state-of-the-art potentials have a spurious maximum at very small distances. Fits to these calculations are provided along with interpolation functions to smoothly connect them to the best available potentials for larger distances. Thirring bounds are computed for the first time. They readily reveal gross inadequacies in some potentials at very short range, but they are too weak to distinguish among potentials with the correct limiting behavior at R=0.
Connected-diagram expansions of effective Hamiltonians in incomplete model spaces. II. The general incomplete model space
Authors: Mukherjee Debashis , Kutzelnigg Werner , Koch Sigurd
Published: July 2002

We generalize here the formalism of the preceeding paper to encompass the case of the general incomplete model space. The classification of operators as diagonal or nondiagonal depends in this case upon the specific m-valence model space. It is stressed that even then one has to work in Fock space in order to get connected-diagram expansions, since connectedness is a Fock space property. Two choices of separable normalization of the wave operator W leading to a connected Heff are discussed. It is shown that the intermediate normalization is not separable in general and hence not compatible with a connected-diagram expansion. We also discuss how to generate ‘‘subduced’’ incomplete model spaces of lower particle rank such that Heff remains a valid effective Hamiltonian for these subduced model spaces as well. We discuss the nature of the various disconnected diagrams encountered in many-body formalisms and point out which of these are really worth worrying about. We finally comment on the question of the separability of the wave function into proper fragments.
On the 1A1–3B1 separation in CH2 and SiH2
Authors: Bauschlicher Charles W. , Langhoff Stephen R. , Taylor Peter R.
Published: July 2002

We have determined the 1A1–3B1 separation (Te) in both CH2 and SiH2 using very large Gaussian basis sets (including g functions) and second-order CI wave functions. Complete geometry optimizations have been performed, and relativistic effects have been included using first-order perturbation theory. This treatment yields Te values for the 1A1–3B1 separation of 9.07 kcal/mol in CH2 and −20.58 kcal/mol in SiH2. Using a combination of theoretical and experimental values to estimate the contribution of zero-point vibration to the separation yields T0 values of 8.9 kcal/mol for CH2 and −20.9 kcal/mol for SiH2, in excellent agreement with the experimental values of 9.02 and −21.0 kcal/mol. A corollary to the small zero-point vibrational contribution to the separation is that the symmetric stretching fundamental in CH2(3B1) must be near 3100 cm−1, much less than a recently suggested value of around 3400 cm−1. Our accurate Te value for SiH2 establishes the ionization potential of the 1A1 state as 9.15 eV, the higher of two recent experimental values.
An exact trajectory surface hopping procedure: Comparison with exact quantal calculations
Authors: Parlant Gérard , Gislason Eric A.
Published: July 2002

A new exact trajectory surface-hopping procedure is presented. The method is used to run test calculations on two classic (Landau–Zener – and Demkov-type) atom–atom systems. Transition probabilities as a function of impact parameter show an excellent agreement with quantal results.
Rectilinear diameters and extended corresponding states theory
Authors: Singh Rajiv R. , Pitzer Kenneth S.
Published: July 2002

The amplitude of the rectilinear diameter is examined for a large number of normal fluids and a few other fluids. For the normal fluids, the slope of the diameter shows a linear dependence on the acentric factor, as do other fluid properties on a reduced basis. Thus, we conclude that the shape of the pair potential is the primary factor in determining the slope of the diameter rather than the relative strength of three-body forces, as has been suggested by analogy to recent results for rubidium and cesium. The situation for the near-critical singularity in the diameter remains ambiguous for normal fluids and a suggestion is made for its resolution.
Searching for the N2 A′ 5Σ state
Authors: Young R. A. , Bower R. D.
Published: July 2002

Using a calibrated laser induced fluorescence method employing the Herman infrared band system in the nitrogen atom afterglow and in a new short-lived nitrogen afterglow it is demonstrated that less than 4×107 cm−3 N2(A′ 5Σ) are present. It is concluded that if the nitrogen atoms statistically expected to recombine into the 5Σ state do so then the nitrogen quenching rate constant of the 5Σ state must be greater than 1×10−11 cm3 s−1. Also, the 5Σ state is not the precursor to the short lived N2 afterglow recently described by Piper [J. Chem. Phys. 88, 6911, 232 (1988)].
Application of the thermodynamic cycle perturbation method to systems with nonadditive potentials
Authors: Wang Jian , Jordan Peter C.
Published: July 2002

We have extended the thermodynamic perturbation cycle approach to compute thermodynamic properties of systems with nonadditive potentials. A rapid iterative method was developed and tested by application to systems of water microclusters of alkali metal cations and halogen anions interacting by means of a model potential incorporating polarization. The perturbation cycle calculations are in agreement with the results of direct calculation as long as both initial and final microclusters are unambiguously stable. For marginally stable cationic microclusters, e.g. Rb(H2O)+6 or Cs(H2O)+5, the perturbation cycle approach appears to provide more reliable estimates of thermodynamic properties. Transmutation entropy estimates entail substantial uncertainty, even for transitions between stable microclusters. They are in fair agreement with those of other theoretical studies but differ noticeably from the experimental values.
Vibrational spectra of oligothiophenes as model of polythiophenes
Authors: Zerbi G. , Chierichetti B. , Ingänas O.
Published: July 2002

The vibrational spectra of a few oligomers of thiophene in the solid and solution are studied with the help of quantum mechanical calculations and of the theory of the effective conjugation coordinate recently developed. The aim of the work is to extract from the spectra structural probes to be used in the understanding of the spectra, structure, and thermochromic effects in polyalkylthiophene treated in a paper which follows (paper II).
Peptides in ionic solutions: A comparison of the Ewald and switching function techniques
Authors: Smith Paul E. , Pettitt B. Montgomery
Published: July 2002

The methodological dependence of observed ion–peptide associations in molecular dynamics simulations is investigated. We compare the results from several simulations of a pentapeptide in explicit solvent and salt ions which differ in the their treatment of the long ranged electrostatic interactions. Results for both the Ewald and switching function techniques are presented. It was found that there were important differences between the two methods for the water dipole–dipole temporal and spatial correlations, total dipole moment fluctuations, and self-diffusion constants. Electrostatic potentials calculated in the region of the peptide are also used to illustrate the large differences that can arise from different treatments of the electrostatic interactions. It appears that the switching function distorts the molecular electrostatic potential experienced by the salt ions to such a degree that their behaviour becomes highly dependent on the initial conditions. In summary, the use of a switching function is not recommended for the simulation of ions and their interactions with biomolecules.
A remedy for zero-point energy problems in classical trajectories: A combined semiclassical/classical molecular dynamics algorithm
Authors: Alimi R. , García-Vela A. , Gerber R. B.
Published: July 2002

A new method is proposed for dealing with difficulties in molecular dynamics (MD) simulations caused by nonpreservation of zero-point energies (ZPE) in classical dynamics. Specifically addressed is a difficulty, for molecules held in weakly bound clusters, of energy flow from the initial ZPE of stiff molecular vibrations into soft cluster modes, causing unphysical dissociation or melting of the cluster. The remedy proposed is a classicallike MD algorithm, which treats the stiff modes by semiclassical Gaussian wave packets and the soft modes by classical dynamics, using the time-dependent self-consistent field (TDSCF) approach to couple the classical and the semiclassical modes. The resulting algorithm is very similar in form to classical MD, is computationally simple, stable, and appears free of unphysical effects. The method is illustrated by test applications to models of the clusters I2He and (HBr)2 in the ground states, which dissociate at the expense of their ZPE classically, but remain stable in the new method.
Ultrafast solvation dynamics in a Stockmayer fluid
Authors: Perera Lalith , Berkowitz Max L.
Published: July 2002

Computer simulations of charge solvation dynamics in Stockmayer solvent display the existence of two time regimes. In this paper we show that the short time regime is due to inertial independent motion of solvent particles.
The nature and role of quantized transition states in the accurate quantum dynamics of the reaction O+H2→OH+H
Authors: Chatfield David C. , Friedman Ronald S. , Lynch Gillian C. , Truhlar Donald G. , Schwenke David W.
Published: July 2002

Accurate quantum mechanical dynamics calculations are reported for the reaction probabilities of O(3P)+H2→OH+H with zero total angular momentum on a single potential energy surface. The results show that the reactive flux is gated by quantized transition states up to the highest energy studied, which corresponds to a total energy of 1.90 eV. The quantized transition states are assigned and compared to vibrationally adiabatic barrier maxima; their widths and transmission coefficients are determined; and they are classified as variational, supernumerary of the first kind, and supernumerary of the second kind. Their effects on state-selected and state-to-state reactivity are discussed in detail.
Theoretical investigation of differential cross sections for vibrational excitation and vibronic charge transfer in H++H2 collisions
Authors: Florescu A. , Sizun M. , Sidis V.
Published: July 2002

The multitrajectory semiclassical method combined with the fixed rotor approximation is used to investigate the differential state-to-state scattering in H++H2 collisions at Ec.m.=20 eV. Little differences are found with respect to previous quantal infinite order calculations of Baer et al. [J. Chem. Phys. 91, 4169 (1989)] when similar DIM (diatomics in molecules) potential energy surfaces are used. Adjustment of the ground state DIM potential energy surface to ab initio data considerably improves the comparison with the experiment of Niedner et al. [J. Chem. Phys. 87, 2685 (1987)].
Magnetic and conductivity studies of poly(vinylene disulfide)
Authors: Nguyen Thien-Phap , Giffard Michel , Molinié Philippe
Published: July 2002

We report the results of magnetic [electron-spin resonance (ESR) and susceptibility] and conductivity measurements on a new type of semiconducting polymer: poly(vinylene disulfide)–(S—CH=CH—S)n synthesized from (E)-1,2 dithiocyanatoethene. ESR spectra of pristine and protonic acid doped samples have four g factor values which can be explained by the nonplanar structure of the polymer. Decrease in paramagnetism and enhancement of the spin density by doping are observed to be in close relation with the increase of the conductivity. A conduction mechanism assuming direct protonation of the polymer backbone is proposed to explain the behavior of these polymers upon chemical doping by protonic acid.
Structure and thermodynamics of flexible polymer gels
Authors: Geissler Erik , Horkay Ferenc , Hecht Anne-Marie
Published: July 2002

Unlike the situation for polymer solutions, for swollen polymer networks, there is no generally accepted relation between the macroscopic osmotic properties and the scattering behavior. Complementary measurements involving osmotic and elastic observations, small angle neutron and x-ray scattering, as well as quasielastic light scattering, can, however, yield detailed information on the relationship between the structure and the thermodynamics in polymer gels. Here, we apply such a complementary investigation to two types of networks, differing in the mechanism of cross linking. The neutron scattering spectra of the gels contain two components, dynamic and static. The former is governed by the macroscopic osmotic and elastic moduli of the swollen network, and is associated with the thermodynamic concentration fluctuations. The static component of the scattered intensity is generated by local variations of the cross-link density. The amplitude of the static concentration fluctuations 〈δφ2〉/φ2 is determined by the detailed mechanism of the cross-linking procedure. Its concentration dependence can be expressed in terms of the same macroscopic osmotic and elastic variables as govern the thermodynamic properties of the gel.
Dielectric constant effects on the energetics of oppositely charged colloids: A lattice field theory study
Authors: Ben-Tal Nir , Coalson Rob D.
Published: July 2002

A recently developed lattice field theory formulation of the statistical mechanics of a classical Coulomb gas interacting with a fixed charge distribution [R. D. Coalson and A. Duncan, J. Chem. Phys. 97, 5653 (1992)] is extended to deal with systems having a spatially varying dielectric constant. To illustrate the utility of the formalism, electrostatic free energies of two oppositely charged macroions suspended in ionic solution are calculated for several geometric configurations. In many cases, it is found that the free energy is minimized at a separation between macroions corresponding to a ‘‘nontouching’’ configuration. The value of the dielectric inside the macroions has an important effect on the free energy of near-touching configurations. In particular, lowering the dielectric constant inside the macroion relative to the solvent dielectric constant leads to higher free energies of near-touching configurations, and hence promotes a stable state where the macroions are farther apart. Our findings also indicate that a large difference between the magnitude of the charges on the positive and negative macroions enhances the stability of a ‘‘nontouching’’ configuration.
Coadsorption of hydrogen and CO on Pt(335): Structure and vibrational Stark effect
Authors: Wang Hong , Tobin R. G. , Lambert David K.
Published: July 2002

We have studied CO and H coadsorbed on the stepped Pt(335) surface in vacuum to learn about their interaction at steps and to compare with previous electrochemical studies. Both electroreflectance and conventional reflectance absorption vibrational spectra were obtained of atop bonded CO. Its stretch vibrational frequency ν, Stark tuning rate (dν/dE), and integrated absorbance S were all studied as functions of CO and H coverage. With CO only on step edges, S for atop CO decreases to zero with increasing H coverage. The CO affected by H is in a mixed phase and is bridge bonded. Atop CO’s (dν/dE) decreases with increasing CO coverage but is unaffected by coadsorbed H. In the low CO coverage limit (dν/dE)=88±9 cm−1/(V/Å), in agreement with theory and with previous measurement. The standard model of dipole–dipole screening is unable to explain the CO coverage dependence of both (dν/dE) and S. The vibrational polarizability of CO increases with coverage. We also compare our results with spectroelectrochemical data from CO on Pt(335) in aqueous electrolyte. Our measured (dν/dE) is too small to explain the variation of CO’s ν with electrode potential φ in the electrochemical experiments if (dE/dφ) in the double-layer is taken from conventional models. The effect of coadsorbed H is also different; in vacuum H has no effect on CO’s (dν/dE) but in electrolyte, at low CO coverage, (dν/dφ) drops to zero at the potential where H adsorption begins.
Some predictions relevant to future spectroscopic observation of S1 vinylidene
Authors: Stanton John F. , Gauss Jürgen
Published: July 2002

The equation-of-motion coupled cluster method is used to obtain a precise estimate of the lowest adiabatic singlet–singlet excitation energy (S1←S0) of vinylidene. As photodetachment spectroscopy offers a promising means for producing the first excited singlet state, harmonic vibrational frequencies of S1 and the extent of mixing between the corresponding normal modes and those of the anion are also calculated. To calibrate the calculations and provide a basis for empirical correction, parallel studies of the previously observed T1 and T2 triplet states are reported. The adiabatic excitation energy [S1←S0] is estimated to be 3.12±0.05 eV, suggesting a photodetachment threshold energy of 3.61±0.05 eV. Progressions in the totally symmetric C–C stretch and H–C–H bending modes should be pronounced in the spectrum, with the extent of mode mixing similar to that found for the S0←anion and T1←anion processes, but differing in phase.
Applicability of valence-universal multireference coupled-cluster theories to quasidegenerate electronic states. II. Models involving three-body amplitudes
Authors: Jankowski K. , Paldus J. , Grabowski I. , Kowalski K.
Published: July 2002

The reliability study of the valence-universal coupled-cluster (VU-CC) methods [Jankowski et al., J. Chem. Phys. 97, 7600 (1992)] is continued by considering approximations involving one-, two-, and three-electron components of the cluster operator S, S≂S1+S2+S3. The explicit equations determining the cluster amplitudes for the quadratic VU-CCSDT approximation were derived and applied to the low lying quasidegenerate states of the model system consisting of two slightly stretched, interacting hydrogen molecules. The degree of quasidegeneracy of the two lowest, totally symmetric, singlet levels of this system may be varied over a wide range by changing its geometry. Along with the quadratic VU-CCSDT theory its linear version (VU-LCCSDT) is also examined. It is found that the effect of the S3 clusters is significant and is mainly due to the spectator part of this operator. The inclusion of this operator increases the number of physically meaningful multiple solutions of the equations for the cluster amplitudes. It also considerably improves the accuracy of the resulting energies, both in the strongly and weakly quasidegenerate regions, but not in the intermediately quasidegenerate region. It is shown that this behavior is a manifestation of an avoided crossing of the lowest excited states of the parent system in the valence-universal genealogy.
The equilibrium geometry and some spectroscopic constants of C5 from large-scale ab initio calculations
Authors: Botschwina Peter
Published: July 2002

An accurate equilibrium geometry for linear C5 (X̃ 1Σ+g) has been obtained from coupled-cluster single, double, and triple [CCSD (T)] calculations (all electrons correlated) with a large basis set of 275 contracted Gaussian-type orbitals: R1e (outer CC)=1.289 59 Å and R2e (inner CC)=1.281 90 Å. Analogous calculations for C3 and C2 in their electronic ground states yield 1.294 31 and 1.242 09 Å, respectively. The equilibrium dissociation energy C5→C3+C2 is predicted to be 597 kJ mol−1.
Nonoptical excited state spectroscopy of CF3Cl, CF2Cl2, and CFCl3: Bethe surfaces, and absolute transition probability measurement of preionization-edge valence and Rydberg transitions by angle-resolved electron energy loss spectroscopy
Authors: Ying J. F. , Leung K. T.
Published: July 2002

Absolute transition probabilities or generalized oscillator strengths (GOSs) of valence-shell electronic transitions of CF3Cl, CF2Cl2, and CFCl3 as functions of energy loss and momentum transfer (or Bethe surfaces) have been determined using angle-resolved electron energy loss spectroscopy (EELS) at an impact energy of 2.5 keV. Low-lying electronic excitation features in the energy loss region of 6.8–8.1 eV are observed. Using the results of single-excitation configuration interaction excited-state calculations, we show that these excitation features can be attributed predominantly as electronic transitions from the Cl 3p nonbonding (n) orbitals to a C–Cl σ* antibonding orbital (i.e., n→σ* transitions), some of which may lead to dissociation of the C–Cl bond. Moreover, the absolute GOS profiles of these low-lying excitation features have been determined for the first time. In particular, the GOS profile of the n→σ* feature of CF3Cl at 7.7 eV has been found to have a shape characteristic of a quadrupole transition. On the other hand, the GOS profiles of analogous excitation features for CF2Cl2 and CFCl3 are found to have more complicated structures. The combined profiles of the GOSs of these n→σ* excitation features in the CF4−mClm (m=1–4) series indicate increased dipole component with the number of chlorine atoms. The possible mechanisms and significance of this trend in the GOSs of these n→σ* transitions have been discussed. Furthermore, the GOS profiles of low-lying preionization-edge Rydberg transitions (that originate from the Cl 3p nonbonding orbitals) are also determined, and found to contain not only strong maxima at zero momentum transfer, which are characteristic of predominant dipole-allowed interactions, but also weak secondary maxima (and minima). The nature of these secondary extrema in the GOS profiles is discussed by considering the spatial overlaps of the initial-state and final-state orbital wave functions. Finally, we provide new tentative assignments for other valence-shell energy loss features of CF3Cl, CF2Cl2, and CFCl3 using the ionization potentials and term values reported in the literature.
Ab initio study of the optical spectra of C3, C5, and C7 chains
Authors: Kolbuszewski M.
Published: July 2002

Multireference configuration interaction calculations on the low lying singly excited states of linear C3, C5, and C7 clusters are reported. The results indicate that the 1Σ+u state of C3 has a term energy above 8.0 eV. The calculated vertical term energies of the low lying electronic states of these carbon chains are consistent with the photoelectron spectra of their parent negative ions. The predicted term energies are also compatible with the matrix spectra of carbon vapor. An assignment of strong spectral features in those spectra at ≊1950, 2227, and 2471 Å to the 1Σ+u←X̃ 1Σ+g transition in C5, C7, and C9, respectively, is proposed. A possible relation of carbon chains to diffuse interstellar bands is discussed.
The accuracy of the pseudopotential approximation. I. An analysis of the spectroscopic constants for the electronic ground states of InCl and InCl3 using various three valence electron pseudopotentials for indium
Authors: Schwerdtfeger Peter , Fischer Thomas , Dolg Michael , Igel-Mann Gudrun , Nicklass Andreas , Stoll Hermann , Haaland Arne
Published: July 2002

Spectroscopic constants for InCl and InCl3 are determined by a coupled cluster procedure using relatively large basis sets and an energy-consistent semilocal three valence electron pseudopotential for indium. Possible errors within the pseudopotential approximation are discussed in detail by comparison of available pseudopotentials adjusted through different techniques. Core-polarization corrections and the deviation from a point core approximation are discussed. These corrections, however, do not lead to more accurate bond distances as compared to the experimental results. Differently adjusted three valence electron pseudopotentials yield quite different results for the bond distances of InCl and InCl3. The single-electron adjusted energy-consistent pseudopotential of Igel-Mann et al. [Mol. Phys. 65, 1321 (1988)] yields the best results and therefore, this pseudopotential has been chosen for all further investigations on molecular properties. The Dunham parameters for InCl are calculated by solving the vibrational-rotational Schrödinger equation numerically. A finite field technique is used to determine the dipole moment and dipole-polarizability of diatomic InCl. The dependence of several molecular properties on the vibrational quantum state is determined by calculating the expectation value Pn=〈n‖P(R)‖n〉, where P(R) is the distance dependent molecular property. The P(R) curves show strong linear behavior and therefore, the shape of the Pn curve is mostly determined by anharmonicity effects in the InCl potential curve. For the vibrational ground state, ‖0〉, the calculated property P0 deviates only slightly from the property determined directly at the equilibrium distance, Pe. There is in general satisfying agreement of our calculated values with available experimental results. However, it is concluded that in order to obtain very accurate spectroscopic constants a small core definition for indium has to be preferred.
A density functional/molecular dynamics study of the structure of liquid nitromethane
Authors: Seminario Jorge M. , Concha Monica C. , Politzer Peter
Published: July 2002

A molecular dynamics simulation of liquid nitromethane has been carried out, in which a density functional (DF) procedure was used to compute the properties needed to establish the intermolecular force field. The latter was subsequently updated by further DF calculations. The first and second shells of neighbors are found to be at radii of about 6 Å and 11 Å from a given nitromethane molecule. OH partial pair correlation functions are determined, and indicate the presence of C–H...O hydrogen bonding.
Matrix isolation study of the interaction of excited neon atoms with BF3: Infrared spectra of BF2, BF+2, BF+3, and BF−3
Authors: Jacox Marilyn E. , Thompson Warren E.
Published: July 2002

When a Ne:BF3 sample is codeposited at approximately 5 K with a beam of neon atoms that have been excited in a microwave discharge, the infrared spectrum of the resulting solid deposit shows prominent absorptions of BF2. All three vibrational fundamentals and the ν1+ν3 combination band of BF2 isolated in solid neon are identified, and a normal coordinate analysis is conducted. In addition, infrared absorptions of the BF+3, BF+2, and BF−3 molecular ions are assigned, and the processes which occur when the solid deposit is exposed to visible and ultraviolet radiation are considered.
Reaction of molybdenum clusters with molecular nitrogen
Authors: Mitchell S. A. , Lian L. , Rayner D. M. , Hackett P. A.
Published: July 2002

Reactions of molybdenum clusters with molecular nitrogen have been investigated by using a fast-flow reactor equipped with a laser vaporization cluster source and laser ionization time-of-flight mass spectrometer for detection of clusters. Absolute rate coefficients are reported for reaction of clusters in the range 3–35 metal atoms, at temperatures 279, 300, and 372 K in He buffer gas at 1 and 2 Torr pressure. Rate coefficients generally decrease with increasing temperature, as expected for a mechanism involving formation of a weakly bound precursor complex. By comparison with surface chemistry studies of nitrogen adsorption on molybdenum and tungsten, the reactivity trend with cluster size is interpreted as reflecting the geometric configurations of metal atoms on the surfaces of the clusters. It is suggested that structural transitions occur in molybdenum clusters in the size range 13–20 metal atoms. Evidence is described that suggests that cluster reaction products have nitrogen bound in both atomic and molecular forms. Bulk surfacelike reactivity of the clusters appears to be established in the range 25–30 metal atoms.
Electronic absorption spectra of linear carbon chains in neon matrices. IV. C2n+1 n=2–7
Authors: Forney Daniel , Freivogel Patrick , Grutter Michel , Maier John P.
Published: July 2002

The 1Σ+u←X 1Σ+g electronic absorption system of the carbon chains C2n+1 (n=3–7) trapped in 5 K neon matrices has been identified. The 1Πu←X 1Σ+g transition has also been observed for C5 and C7, as well as a dipole forbidden one for all the species. The spectroscopic information was derived from absorption spectra recorded after neutralization of mass-selected anions and cations that were codeposited with an excess of neon to form a matrix. The assignment is based on the mass-selection, theoretical predictions, and trends observed for this homologous series. These results are related to published absorption spectra of graphite vapor trapped in rare gas matrices. The suggestion that such species may be potential carriers of diffuse interstellar bands is considered.
Vibrational coherence effects on electronic curve crossing
Authors: Jean John M.
Published: July 2002

Quantum dynamical simulations of vibrational phase coherence effects on ultrafast electronic curve crossing in condensed phases are presented for both symmetric and barrierless double well potentials. Reaction coordinate probability distributions are constructed from the reduced density matrix of the system, which provide considerable insight into the dynamics of curve crossing. The application of these ideas to retinal isomerization in rhodopsin is presented. Results suggest that coherence transfer processes are important for interpreting recent femtosecond transient absorption results on this system.