bluuebunny/crossref_metadata_2025 huggingface dataset:
Theory of the free surface of copolymer melts: Application to symmetric diblock copolymers
Authors: Hariharan Arvind , Harris Jonathan G.
Published: July 2002

We develop a mean-field lattice theory of a compressible liquid copolymer in the framework of the Scheutjens and Fleer theory of polymer solutions and apply it to study the structure and thermodynamics of the free surface of a symmetric diblock copolymer film. The theory predicts that the difference in the interaction energies between the two constituent components causes the segregation of the block component with the weaker interactions to the free surface. The resulting segment density depth profiles of the individual components are oscillatory owing to the connectivity between the blocks. A detailed study of chain shape and bond orientation characteristics provides a microscopic description of the interfacial structure in the disordered as well as the microphase separated states. The inclusion of compressibility causes the disordered to lamellar microphase transition to occur at higher temperatures and shorter chain lengths than predicted by incompressible theories using the same interaction parameters. A calculation of the surface tension when the copolymer is disordered in the bulk reveals that there exists an interesting and potentially useful situation where the surface tension of an A–B copolymer is significantly lower than that of each of the homopolymers A and B of the same chain length.
Analysis of two ion swarms in dynamic equilibrium: Simultaneous determination of two drift velocities and two reaction frequencies
Authors: Iinuma K. , Hamano T. , Takebe M.
Published: July 2002

In a drift-tube study, four simple formulas, based on a dynamic equilibrium analysis of two gaseous ion swarms, are proposed to determine simultaneously the forward and reverse reaction frequencies together with two drift velocities. The formulas are put to the test for (N+4,N+2)/N2 system with two arrival time spectra (ATS) generated from the experimental data previously reported at E/N=150 Td (the ratio of electric field strength to gas number density: 1 Td=10−17 V cm2). The percent differences between the data and the determined values are estimated to be 0.8%, −1.4%, 6.8%, and 4.9%, for the drift velocities of N+4 and N+2 ions and the forward and reverse reaction frequencies, respectively. For the same system, two experimental ATS obtained at 180 Td by McKnight et al., being supposedly in near equilibrium, are directly examined by this method.
The quantum theory of the effects of bound pairs on the transport properties of gases
Authors: Curtiss C. F.
Published: July 2002

A quantum form of the Boltzmann equation which applies to a gas of atoms which interact through a potential with an attractive portion is developed. It is shown that the existence of discrete negative eigenvalues of the Hamiltonian describing the relative motion leads to modifications of the usual equation. It is then shown that in the classical limit, the limit in which ℏ goes to zero, the present results lead to the results previously obtained through a discussion in which the modifications which arise from a potential involving an attraction portion were considered from a classical point of view.
Comment on ‘‘A quantum mechanical representation in phase space’’ [J. Chem. Phys. 98, 3103 (1993)]
Authors: Hu Xu-Guang , Li Qian-Shu
Published: July 2002

In this comment questions about the improper eigenfunctions in J. Chem. Phys. 98, 3103 (1993) are addressed and the other suitable eigenfunctions are suggested at the same time.
Rotation–vibration state resolved unimolecular dynamics of highly excited CH3O (X̃ 2E). II. Intramolecular vibrational dynamics of excited ‘‘C–O stretch’’ states
Authors: Geers A. , Kappert J. , Temps F. , Wiebrecht J. W.
Published: July 2002

The stimulated emission pumping (SEP) spectra of highly excited CH3O (X̃ 2E) reported in the preceding paper [A. Geers, J. Kappert, F. Temps, and J. Wiebrecht, J. Chem. Phys. 101, 3618 (1994); paper I] are analyzed to extract quantitative data on the collision-free intramolecular vibrational dynamics of the molecules. Attention is focused on the spectra and dynamics of the excited C–O stretch vibration (ν3) states at energies between 3 000 cm−1≤E≤10 000 cm−1. The spectra are found to exhibit a rather sudden transition at E≊5000 cm−1 from the regular, assignable properties at low energies to the ‘‘statistical’’ region at high energies. This IVR ‘‘threshold’’ is approximately 2000 cm−1 below the asymptotic H–H2CO dissociation limit of the molecule. The observed densities of states at these energies approach the full J- and symmetry-sorted rovibronic densities estimated from harmonic state counts. The nearest-neighbor level spacing distribution in the spectra and the magnitude of the inferred root mean square level coupling matrix elements support the assumption of nearly ‘‘ergodic’’ molecular properties in this region. From computed time autocorrelation functions, the IVR lifetimes of the highly excited C–O stretch states are found to be of the order of τ≊0.2–0.3 ps (v3=5 to 6). Evidence is obtained from some spectra for two-tier IVR processes with characteristic time scales of 0.2 and 2 ps, respectively. The highest excited C–O stretch states at energies above ≊8000 cm−1 (v3≥8) appear to be coupled only weakly to neighboring background states. This last observation hints at the existence of quasistable periodic orbits, which are embedded in the quasicontinuum of dissociative states at these energies.
The refractive index of colorless liquids in the visible and infrared: Contributions from the absorption of infrared and ultraviolet radiation and the electronic molar polarizability below 20 500 cm−1
Authors: Bertie John E. , Lan Zhida
Published: July 2002

This paper addresses the separation of the contributions to the visible refractive index of colorless liquids from electronic (ultraviolet) and vibrational (infrared) absorption. The goal is to find the most accurate infrared values of nel(ν̃), the refractive index that results solely from electronic absorption, by fitting and extrapolating currently available visible refractive index data. These values are needed, interalia, to improve the accuracy of infrared real refractive index spectra calculated by the Kramers–Kronig transform of infrared imaginary refractive-index spectra. The electronic molar polarizability αel(ν̃) is calculated from the values of nel(ν̃) at wave numbers between 20 500 and 0 cm−1. The methods are applied to ten liquids: H2O, D2O, CH3OH, CH3COOH, CH3CN (CH3)2CO, CH2Cl2, C6H6, C6H5Cl, and C6H5CH3. The visible refractive indices are expressed as power series in wave number, by expansion of the Kramers–Kronig integral. Terms in ν̃+2m, m=1,2, are due to the electronic contribution and terms in ν̃−2m are due to the vibrational contribution.

The vibrational contribution to the visible refractive index is also calculated from experiment by Kramers–Kronig transformation of the known infrared imaginary refractive index spectrum of the liquid. It is shown that the vibrational absorption contributes ≥0.001 to the visible refractive index only for the four hydrogen-bonded liquids, and that, for all ten liquids, at least 25% of the vibrational contribution arises from absorption below 2000 cm−1. If the vibrational intensities are not known, the available visible refractive indices yield the most accurate infrared values of nel for all liquids except H2O if they are fitted to the equation n=a0+a2ν̃2+a4ν̃4. A similar equation, with the additional term a2ν̃−2, is theoretically superior because the latter term adequately describes the vibrational contribution to the visible refractive indices, but only for H2O are the currently available visible refractive indices sufficiently accurate and sufficiently extensive to allow the four coefficients in the equation to be determined with useful accuracy. For H2O, D2O, CH3OH, CH2Cl2, C6H6, C6H5Cl, and C6H5CH3, corrections are given to slightly improve the accuracy of the previously published infrared real refractive-index spectra.
Pump–dump coherent control with partially coherent laser pulses
Authors: Jiang Xue-Pei , Shapiro Moshe , Brumer Paul
Published: July 2002

The theory of coherent control of photodissociation with partially coherent laser pulses is developed and applied to the pump–dump control scenario of a collinear model of DH2. The coherence characteristics of the pump pulse are shown to be crucial for maintaining control over the product yield, whereas the coherence properties of the dump pulse are only of secondary importance. Control is shown to survive for partially coherent laser pulses, but only for a range of incoherence which precludes control with typical nanosecond pulsed dye lasers.
Pressure-induced amorphization of covellite, CuS
Authors: Peiris Suhithi M. , Sweeney Jeffrey S. , Campbell Andrew J. , Heinz Dion L.
Published: July 2002

CuS, or covellite (hexagonal symmetry), was compressed in a diamond anvil cell at room temperature up to a pressure of 45 GPa, and studied using x rays from both a Mo Kα source and a synchrotron. The x-ray diffraction spectrum of CuS disappears by about 18 GPa. The presence of Cu fluorescence lines in all spectra and the reappearance of diffraction lines upon decompression confirm that CuS undergoes reversible pressure-induced amorphization at this pressure. A third-order Birch–Murnaghan equation of state fit to the diffraction data below 11 GPa yields a bulk modulus of 89±10 GPa with a pressure derivative of −2±2 for covellite. Further compression up to 45 GPa shows three to four diffraction lines of very low intensity, implying some high pressure ‘‘ordering’’ of the amorphous phase. The Raman spectra obtained indicate that the changes in structure are probably due to the twisting or the distortion of covalently bonded CuS4–CuS4 units in different directions.
Electron correlation effects on the theoretical calculation of nuclear magnetic resonance spin–spin coupling constants
Authors: Perera S. Ajith , Nooijen Marcel , Bartlett Rodney J.
Published: July 2002

The equation-of-motion coupled cluster singles and doubles (EOM-CCSD) method for general second-order properties is derived providing a quadratic, CI-like approximation and its linked form from coupled cluster (CC) energy derivative theory. The effects of the quadratic contribution, of the atomic basis set employed, and of electron correlation on NMR spin–spin coupling constant calculations using EOM-CCSD methods are investigated for a selected set of difficult molecules, notably CH3F, B2H6, CH3CN, C2H4, and CH3NH2. We find that the quadratic contribution is insignificant for the couplings in the molecules considered in this study and in addition the quadratic contribution only slightly depends on the basis set used. Therefore it seems well justified to use the less expensive CI-like approximation or its linked-diagram form to evaluate spin–spin coupling constants. The Fermi-contact contribution shows the largest variation with the change of basis sets. The diamagnetic spin–orbit (DSO) and the spin–dipole (SD) contribution vary little, seemingly being converged at the DZP level while the paramagnetic spin–orbit (PSO) term shows moderate variations. Except for very few cases, the FC contribution is dominant in all the couplings in the selected set of molecules and it is also most sensitive to the inclusion of electron correlation. The other contributions are less affected by electron correlation. Although of lesser importance, the significance of the noncontact contributions and electron correlation effects on accurate calculation of coupling constants such as 1J(13C19F) in CH3F and 1J(13C15N) in CH3CN is clearly demonstrated.
Theoretical study of the vibrational spectra of the transition-metal carbonyl hydrides HM(CO)5 (M=Mn, Re), H2M(CO)4 (M=Fe, Ru, Os), and HM(CO)4 (M=Co, Rh, Ir)
Authors: Jonas Volker , Thiel Walter
Published: July 2002

The geometries of the title compounds have been optimized at the levels of restricted Hartree–Fock (RHF) theory, second-order Mo/ller–Plesset perturbation theory, and gradient-corrected density functional theory (DFT) using all-electron and effective core potential wave functions in conjunction with polarized double- and triple-zeta basis sets. The harmonic force fields have been determined from RHF and DFT calculations. The DFT results are generally in very good agreement with the available experimental data. The computed frequencies are accurate enough to be of predictive value, especially when correcting for small systematic errors. Several new assignments of bands in the observed vibrational spectra are proposed. Infrared intensities, dipole moments, and charge distributions are also discussed.
Dimensional perturbation theory for vibration–rotation spectra of linear triatomic molecules
Authors: Suvernev Andrei A. , Goodson David Z.
Published: July 2002

A very efficient large-order perturbation theory is formulated for the nuclear motion of a linear triatomic molecule. All coupling between vibration and rotation is included. To demonstrate the method, all of the experimentally observed rotational energies, with values of J almost up to 100, for the ground and first excited vibrational states of CO2 and for the ground vibrational states of N2O and of OCS are calculated. The perturbation expansions reported here are rapidly convergent. The perturbation parameter is D−1/2, where D is the dimensionality of space. Increasing D is qualitatively similar to increasing the angular momentum quantum number J. Therefore, this approach is especially suited for states with high rotational excitation. The computational cost of the method scales only in proportion to JNv5/3, where Nv is the size of the vibrational basis set.
The phosphorescence excitation spectrum of biacetyl: An analysis of the acetyl and methyl torsional mode structure
Authors: Moule D. C. , Sharp A. C. , Judge R. H. , Liu H. , Lim E. C.
Published: July 2002

The T1(n,π*)←S0 laser-induced phosphorescence excitation spectrum of biacetyl has been recorded with a rotating slit nozzle excitation apparatus. The 000 system origin was observed for the first time as a weak band at 19 529 cm−1. Built on this band were the activities of the three large amplitude modes: υ16(au) acetyl torsion, υ15(au) gearing methyl torsion, and υ21(bg), the antigearing methyl torsion. The potential surface for methyl internal rotation extracted from fitting the observed levels yielded a barrier to methyl torsion of 359.6 cm−1.
Two-dimensional vibrational potential energy surface for phthalan: The effect of large coupling on vibrational quantum states
Authors: Sakurai Sachie , Meinander Niklas , Laane Jaan
Published: July 2002

The low-frequency analysis of the phthalan ring-puckering vibration shows a highly irregular pattern of energy levels which cannot be adequately described by a one-dimensional potential energy function. Because the ring-puckering mode interacts strongly with the ring-flapping vibration of the same symmetry, a two-dimensional analysis involving these two motions is required. Two-dimensional kinetic energy (reciprocal reduced mass) expansions were calculated for the puckering, flapping, and interaction terms and these were utilized in the calculation of the two-dimensional potential energy surface. This surface does an excellent job of reproducing the irregular pattern of observed energy spacings for the ring-puckering vibration in both the ground and excited flapping states. The potential-energy surface has a barrier to planarity of 35 cm−1 and energy minima at x1=0.09 Å, x2=−0.03 Å, and x1=−0.09 Å, x2=0.03 Å where x1=puckering and x2=flapping. Although the minima correspond to puckered conformations with a slight amount of flapping, the molecule for practical purposes is planar since the energy barrier is so small. The calculations demonstrate that the unusual irregular pattern of the puckering levels arises primarily from the kinetic-energy interactions of the two vibrations.
Diffusion of liquid n-alkanes: Free-volume and density effects
Authors: von Meerwall E. , Beckman S. , Jang J. , Mattice W. L.
Published: July 2002

In support of our numerical simulations of self-diffusion D of chain molecules in the melt, we have used the pulsed-gradient spin-echo NMR method to make detailed D measurements in 15 liquid n-paraffins (8–60 carbons; molecular weight M=114–844) at eight temperatures T from 30 to 170 °C. We find that D∼Mβ, with β changing approximately linearly from −2.72 to −1.85 as T increases. Thus the apparent activation energies also rises linearly with log M. In the absence of molecular entanglements, Rouse kinetics predicts β=−1, but Cohen–Turnbull–Bueche free-volume effects due to molecular chain ends add a further nonpower-law term, enhancing D increasingly at low M. The combined D vs M theory does, however, closely mimic a power law with the exponents observed at all our measurement temperatures. To obtain this result it is necessary to include in the free-volume term the dependence of the liquid’s density on M and T, analytically modeled based on the incomplete literature data. The fitted density model itself is useful as a guide for the molecular modeling effort. The success of this approach makes it possible to deduce a small intrinsic thermal activation energy contained within the larger, free-volume dominated, apparent values.
Theoretical study of hydrogen bond dynamics of methanol in solution
Authors: Staib A.
Published: July 2002

The hydrogen-bonded methanol dimer dissolved in carbon tetrachloride is investigated with molecular dynamics simulations which provide a quantum-mechanical description of the hydroxyl stretch vibration. Effects of hydrogen bonding on the infrared absorption spectrum and on the population relaxation of the OH stretch mode are analyzed by contrasting theoretical results for monomer and dimer. The exchange dynamics of hydrogen donor and acceptor in the dimer is studied and its role for near-resonant transfer of vibrational excitation among methanol molecules is discussed.
The description of the multiquantum effects in electron paramagnetic resonance spectroscopy using the Bloch equation
Authors: Jeleń Małgorzata , Froncisz Wojciech
Published: July 2002

The Bloch equations are applied to describe the response of the two-level system, irradiated with bichromatic microwave field under condition of electron paramagnetic resonance in the wide range of saturation condition and relaxation times. The generation of the intermodulation effects in such systems has been demonstrated. Numerical procedures of solving a set of stiff, nonlinear differential equations are applied. The line shapes of the generated signals have been simulated for different microwave amplitudes and relaxation rates. The dependence of the intermodulation signals on ω2−ω1, the frequency difference between two incident microwave fields, is studied. The influence of a significant saturation factor on this dependence is demonstrated and explained. Then an estimation of the relaxation rates from the intermodulation sidebands is examined on the basis of the classical model. In the case when the frequency separation between two alternating fields is less than relaxation rates, analytical formulas for the intermodulation signals have been found.
Diels–Alder reactions of butadienes with the Si(100)-2×1 surface as a dienophile: Vibrational spectroscopy, thermal desorption and near edge x-ray absorption fine structure studies
Authors: Teplyakov Andrew V. , Kong Maynard J. , Bent Stacey F.
Published: July 2002

The mechanism and intermediates of a Diels–Alder-type cycloaddition reaction between dienes and the silicon dimers of a Si(100)-2×1 surface, which was theoretically predicted by Doren and Konecny, have been investigated. The reactions of 1,3-butadiene and 2,3-dimethyl-1,3-butadiene were studied using multiple internal reflection infrared spectroscopy, thermal desorption spectrometry, and near edge x-ray absorption fine structure (NEXAFS) measurements. The results show that the compounds physisorb on Si(100)-2×1 at cryogenic temperature. Infrared studies of the room temperature adsorption of both dienes indicate that reaction leads to the formation of stable, chemisorbed Diels–Alder adducts. By NEXAFS measurements on 2,3-dimethyl-1,3-butadiene, we determine that the angle between the π orbitals of the reaction product and the Si(100)-2×1 surface is near 40°. Upon heating, the chemisorbed butadienes primarily decompose to form adsorbed carbon and hydrogen at the surface. Hydrogenation of chemisorbed butadienes by atomic hydrogen was also investigated, and conversion from the Diels–Alder geometry to [2+2] bonding is observed. This effect is attributed to cleavage of the Si–Si dimer bond upon atomic hydrogen exposure. Temperature dependent studies suggest the presence of an activation barrier for chemisorption at low temperature. The parallels between our experimental results and the theoretical studies of these reactions are discussed.
Effects of diffusion on magnetic resonance imaging of laser-polarized xenon gas
Authors: Song Yi-Qiao , Goodson Boyd M. , Sheridan Bhima , de Swiet Thomas M. , Pines Alexander
Published: July 2002

Molecular diffusion during the application of magnetic field gradients can distort magnetic resonance images. A systematic characterization of these distortions in one dimension was performed using highly spin-polarized xenon gas. By varying the strength of the applied gradient and the geometric dimension of the sample, the evolution of these image distortions between the regimes of strong and weak diffusion was observed. These results are compared with numerical simulations. By directly measuring the displacement distribution of the polarized xenon atoms, it is shown that in the weak-diffusion regime the image distortions originate from the restricted diffusive motion near the sample boundaries, in agreement with previous theoretical work. Additionally, it is shown that the effects of diffusion can be utilized to enhance the contrast between the boundaries and bulk in the images of polarized gas samples, and thus may be exploited as a means of boundary detection in such systems.
Coordinates scaling and multiple time step algorithms for simulation of solvated proteins in the NPT ensemble
Authors: Marchi Massimo , Procacci Piero
Published: July 2002

Constant pressure and temperature algorithms have been derived based on a reversible multiple time step (r-RESPA) approach and a new modification of the particle mesh Ewald method. As such they provide very fast and accurate tools for simulation of complex molecular systems in ensembles other than the microcanonical. We have also developed a novel scaling scheme named atomic group scaling which is similar to atomic scaling, but has important computational advantages when used in conjunction with bond constraints. The investigation of the molecular and atomic group scaling schemes against static and dynamic properties of a test system (a solvated bovine pancreatic trypsin inhibitor) has confirmed their equivalence also for large molecules such as proteins. For the specific test system the simulation box cell volume upon changes of pressure decays to its equilibrium value within 5 ps of simulation for both approaches. This goes against theoretical arguments that molecular scaling would not be suitable for simulation of biomolecular systems.
Quantum mechanical study of the CH(v=2) overtone in 30-mode benzene
Authors: Wyatt Robert E.
Published: July 2002

The results of large-scale quantum mechanical calculations of the CH(v=2) 1st overtone spectrum for 30-mode benzene are reported. This overtone was chosen for investigation because of its high degree of fragmentation and resulting complexity compared to spectra for the fundamental and higher overtones. These calculations use the best available ab initio force field supplemented by higher-order terms for the CH stretch–wag interaction. The dynamical calculations were conducted in large active spaces with 12 000 or 16 000 vibrational basis functions. The recursive residue generation method was used to compute residues (intensities) and eigenvalues. From these quantities, the lineshape function, survival probabilities, and vibrograms were computed. Wherever possible, these results were compared to experimental overtone spectra and to other computational results.
Infrared emission spectroscopy of NH: Comparison of a cryogenic echelle spectrograph with a Fourier transform spectrometer
Authors: Ram R. S. , Bernath P. F. , Hinkle K. H.
Published: July 2002

The high-resolution emission spectrum of NH has been observed in the near infrared using a Fourier transform spectrometer (FTS) and a cryogenic echelle spectrograph (called Phoenix) at the National Solar Observatory at Kitt Peak. By using a large format InSb array detector, the newly constructed Phoenix is calculated to offer a large increase in sensitivity over a Fourier transform spectrometer for measurements near 5 μm (2000 cm−1). In order to test the performance of Phoenix, we recorded vibration–rotation emission spectra of the free-radical NH. The infrared bands of NH were produced in a microwave discharge of a mixture of NH3 and He. The rotational structure of five bands, 1–0, 2–1, 3–2, 4–3, and 5–4 in the 2200–3500 cm−1 region has also been measured using two FTS spectra. An analysis of these bands combined with the previous electronic, vibration–rotation, and pure rotation measurements provides improved molecular constants for the ground electronic state. In particular, we have extended the range of measured J values so that the new constants are suitable for predicting line positions in high-temperature sources such as stellar atmospheres and flames. A comparison of the Phoenix spectra with the FTS spectra confirms the higher sensitivity of the Phoenix spectrometer. The relative advantages and disadvantages of instruments like Phoenix are discussed. Although designed for astronomical work, cryogenic echelle spectrographs have applications in the ultrasensitive detection of molecules in chemical physics.
Static structure factor of a suspension of charge-stabilized colloids: Application to liquid-glass transition phase diagram and to micellar solution
Authors: Lai S. K. , Wang J. L. , Wang G. F.
Published: July 2002

The charge-stabilized colloidal dispersion is modeled by a mixture of spherical charged hard spheres whose static partial structure factors were analytically solved by the mean spherical approximation (MSA). For point-like small ions (counterions and electrolyte), this so-called primitive model (PM) can be shown to yield exactly the same macroion–macroion structure factor S(q) as that of the effective one-component model (OCM). Such structural equivalence permits the use of the PM S(q) as input data to the idealized version of mode-coupling theory and hence the determination of the liquid-glass transition loci for a charge-stabilized colloidal dispersion. Numerically it is found that, for the whole boundary of the predicted liquid-glass transition loci, the portion of the line along 0<η≲0.43 reveals an inadequacy in the S(q) since its corresponding pair correlation function near the distance of contact approaches a negative value. This inherent shortcoming of the MSA has previously been noted mostly for the low-density (η≲0.1) and highly charged colloids, but now it is manifested in highly charged colloidal dispersions having a large η. This MSA problem, in principle, can be remedied by the technique of rescaling the macroion size, provided in the course of rescaling one can deal concurrently the nonadditive contact radii relation between the macroions and small ions. Unfortunately, there are still technical difficulties and ambiguities in the handling of this latter kind of problem within the PM. This prompts us to suggest using the S(q) of the effective OCM of Belloni where such problem of nonadditive contact radii can be taken into account approximately. We contrast the liquid-glass transition phase boundary determined from the latter model with that of the PM, where the additive contact radii property is preserved throughout, to reveal qualitatively the uncertainties in the liquid-glass transition loci within the PM. Further evidences in support of this OCM can be seen from its successful interpretation for the S(q) of a micellar solution and for the charges of polystyrene spheres which are predicted in this work to agree reasonably with the theoretical values deduced from the density functional theory.
Laser-induced fluorescence studies of excited Sr reactions: II. Sr(3P1)+CH3F, C2H5F, C2H4F2
Authors: Teule J. M. , Janssen M. H. M. , Bulthuis J. , Stolte S.
Published: July 2002

The vibrational and rotational energy distributions of ground state SrF(X 2Σ) formed in the reactions of electronically excited Sr(3P1) with methylfluoride, ethylfluoride, and 1,1-difluoroethane have been studied by laser-induced fluorescence. Although the reactions of ground state Sr with these reactants are exothermic, no SrF products are observed for those reactions in this study. The fraction of available energy disposed into the sum of rotational and vibrational energy of the SrF(X 2Σ) product is approximately the same for all three reactions, i.e., 40%. The reaction of Sr(3P1) with CH3F results in very low vibrational excitation in the SrF reaction product. The product vibration increases in going to C2H5F and C2H4F2. It is concluded that the alkyl group influences the energy disposal mechanism in these reactions, and some suggestions are given for a partial explanation of the observations.
Calculated paramagnetic properties of the acute GaAs2 and obtuse Ga2As clusters
Authors: Arratia-Pérez Ramiro , Hernández-Acevedo Lucı́a , Weiss-López Boris
Published: July 2002

Dirac molecular orbital calculations are reported for the bent (C2v) gallium arsenide GaAs2 and Ga2As clusters. The calculated clusters electronegativity (χ) and the calculated highest-occupied molecular orbital–lowest unoccupied molecular orbital (HOMO-LUMO) gap suggest that GaAs2 should be softer and more reactive than Ga2As. In GaAs2, the unpaired electron spin spend 33.8% of its time on the apical Ga atom and 33.1% of its time on each basal As atom. In Ga2As, the unpaired electron spin spend 69.4% of its time on the apical As atom and 15.3% of its time on each basal Ga atom. The calculations of the Zeeman (Δgi) interaction for both clusters suggest that Δg⊥>Δg∥ for GaAs2, while Δg⊥<Δg∥ for Ga2As. We also calculated the hyperfine interactions (Ahfi) of the Ga69 and As75 nuclei for both clusters, and we have made an approximate decomposition of the total relativistic hyperfine tensors into Fermi, spin-dipolar, and orbital contributions, allowing us to identify some interesting features of relativistic effects on hyperfine interactions. The orbital contributions of each nuclei are substantial, thus providing an important contribution to the overall hyperfine coupling constants.
Do cylinders exhibit a cubatic phase?
Authors: Blaak Ronald , Frenkel Daan , Mulder Bela M.
Published: July 2002

We investigate the possibility that freely rotating cylinders with an aspect ratio L/D=0.9 exhibit a cubatic phase similar to the one found for a system of cut spheres. We present theoretical arguments why a cubatic phase might occur in this particular system. Monte Carlo simulations do not confirm the existence of a cubatic phase for cylinders. However, they do reveal an unexpected phase behavior between the isotropic and crystalline phase.
Vibrational and rotational collisional relaxation in CO2–Ar and CO2–He mixtures studied by stimulated Raman-infrared double resonance
Authors: Deroussiaux A. , Lavorel B.
Published: July 2002

The collisional relaxation among vibrational levels of the Fermi dyad of CO2 mixed with Ar and He (10% CO2, 90% rare gas) has been studied at room temperature with a double resonance experiment. Stimulated Raman effect from the ground state achieved the pumping process with a Nd:YAG laser and a pulse amplified dye laser. After pumping the v1 or 2v2(Σ+g) level, a cw CO2 laser was used to probe either the depopulation rates of the pumped levels (vibrationally or rotationally resolved) or the energy transfer rates to neighboring states. The vibrational energy relaxation has been studied from experimental depopulation of v1 and population of 2v2 levels through a five-level model. A full set of vibrational rates has been determined and compared with other experimental and theoretical works when available. Our measurements could stimulate new accurate calculations. The rotational energy transfer among the J-resolved rotational levels of 2v2 has been observed for J=14 to 34 with a maximum difference between pumped and probed levels of ΔJ=20. The numerous experimental data have been modeled through a relaxation model based on the energy corrected sudden (ECS) approximation. The adjustable parameters of the ECS matrix have been directly determined by fitting the experimental time-dependent population curves. The diagonal elements of this matrix agree well with experimental linewidths. The resulting ECS relaxation matrix can be used to calculate line shapes in high-pressure coherent Raman spectra of v1 and 2v2 bands or infrared absorption spectra. In contrast, the inelastic linewidths of 2v2 deduced from depopulation data are in disagreement with available experimental linewidths, as already shown in numerous previous studies of gases and mixtures.
Microscopic solvent structure of subcritical and supercritical methanol from ultraviolet/visible absorption and fluorescence spectroscopies
Authors: Bulgarevich Dmitry S. , Sako Takeshi , Sugeta Tsutomu , Otake Katsuto , Takebayashi Yoshihiro , Kamizawa Chiyoshi , Uesugi Masayuki , Kato Masahiro
Published: July 2002

Ultraviolet/visible absorption and fluorescence spectroscopies at different temperatures and pressures were applied to investigate the microscopic solvent structures of subcritical and supercritical methanol using 4-nitroanisole, ethyl-(4-dimethylamino)benzoate, Reichardt’s dye, and anthracene as the probe molecules. It was found that at temperatures higher than 150 °C the long winding chains of sequentially hydrogen-bonded methanol molecules were probably broken, but the small hydrogen-bonded aggregates possibly existed in methanol even at higher temperature. It was also found that the solvation process of the anthracene molecule in the S0-ground state obeyed the Langmuir adsorption model. However, in the case of fluorescence measurements in supercritical methanol, we detected deviations from the simple Langmuir adsorption model. These deviations were explained in terms of preferential solvation of the solvent molecules around photoexcited anthracene. Judging from the experimental results, it was concluded that the local density augmentation of the supercritical methanol around the nonpolar solute was a short-ranged effect, which did not correspond directly to the large isothermal compressibility of fluid near the critical point.
Electron transfer in a slow relaxation bath: Fast and slow degrees of freedom: Application to primary electron transfer in photosynthetic bacteria
Authors: Dakhnovskii Yuri
Published: July 2002

Electron transfer in a bath with slow and fast degrees of freedom is described by a theory based on a microscopic spin-boson Hamiltonian, where the spectral function has two different relaxation times. Evolution dynamics is rigorously found, under the assumption that ωcs/ΓErs/kT≪1. Here ωcs is the cutoff frequencies in the spectral function for the slow modes, Ers is the reorganization energy of slow degrees of freedom, and Γ−1 is the reaction time. This is a short-time approximation for slow modes and a long-time approximation for the reaction. It is found that the time-dependent probability is presented as the averaged probability with a random Gaussian reaction heat with the mean value ε−Ers, where ε is the original reaction heat. The partial dynamics (when the random heat is fixed) is determined by the parameters of the fast degrees of freedom. The time-dependent probability is a nonexponential function. At later times, P(t)∼t−Erf/Ers for activationless reactions (here Erf is the reorganization energy of the fast degrees of freedom). Experimental dependence of P(t) in a log–log scale reveals the ratio Erf/Ers. The reaction is shown to be almost insensitive to temperature. It is pointed out that the nonexponential dependence of the time-dependent probability can be fitted by two exponential functions, and, consequently, be incorrectly interpreted as a two step process. The theory can explain the main features (nonexponentiality and temperature insensitivity) of primary electron transfer in photosynthetic bacteria.
Magnetic and microwave field effects for single rotational levels of the -band of oxalylfluoride in cooled jet conditions
Authors: Makarov Vladimir I. , Khmelinskii Igor V. , Kochubei Sergei A , Ishchenko Valerii N.
Published: July 2002

Fluorescence intensity and decay in oxalylfluoride vapors ((COF)2), excited to single rotational levels (SRLs) of the 1 Au(00) state of the Ã 1Au←Ã 1Ag transition, were measured as a function of an external magnetic field. On excitation to these levels, dynamics in zero field may be described in the small-molecule limit, with fluorescence exhibiting an almost exponential decay. However, at increased field strength B the initial fluorescence decay becomes faster, the decay profile becoming biexponential at higher fields. Thus, a magnetic field-induced change of dynamics occurs in the Ã 1Au state, from that of a small molecule, to the intermediate case. The decay rate constant of the fast component was measured for different SRLs, being independent on the magnetic field strength, while the slow component lifetime is field dependent, increasing at higher fields. Both the fast and slow decay lifetimes depend on the studied SRL. At higher fields, the slow component amplitude decreases, while that of the fast component increases with subsequent saturation at high fields. Halfwidth value (B1/2) of the field dependence of the slow component amplitude increases linearly with (2J′+1)−1. Structure of the OD EPR spectrum of (COF)2 excited to the J′=4 level was resolved. Experimental data are interpreted using the indirect mechanism theory in the low level density limit.
Six-dimensional quantum treatment of the vibrations of diatomic adsorbates on solid surfaces: CO on Cu(100)
Authors: Bahel Atul , Bačić Zlatko
Published: July 2002

Computational methodology for exact quantum 6D calculations of the vibrational eigenstates, energy levels, and wave functions of a diatomic molecule adsorbed on a rigid corrugated surface is presented. It is intended for adsorbates executing coupled, strongly anharmonic large-amplitude vibrations. Surface nonrigidity is introduced in an approximate way, by means of a simplified surface-mass model. Using this methodology, we calculate the vibrational levels of CO/Cu(100) for all four isotopomers of CO, C12O16, C13O16, C12O18, and C13O18. The empirical potential by Tully and co-workers [J. C. Tully, M. Gomez, and M. Head-Gordon, J. Vac. Sci. Technol. A 11, 1914 (1993)] is employed. Our calculated fundamental frequencies of CO/Cu(100) vibrations are compared to those from earlier theoretical treatments on the same potential, as well as with the experimental frequencies and isotope frequency shifts. In addition to 6D calculations, we perform 5D (rigid CO) and 4D (fixed-site) quantum calculations, which provide information about the couplings among the vibrational modes of CO on Cu(100). Excited levels of the lowest-frequency in-plane (doubly degenerate) frustrated translation mode are analyzed and assigned.
Anharmonicity contributions to the vibrational second hyperpolarizability of conjugated oligomers
Authors: Champagne Benoı̂t , Luis Josep M. , Duran Miquel , Andrés Jose Luis , Kirtman Bernard
Published: July 2002

Restricted Hartree–Fock 6-31G calculations of electrical and mechanical anharmonicity contributions to the longitudinal vibrational second hyperpolarizability have been carried out for eight homologous series of conjugated oligomers—polyacetylene, polyyne, polydiacetylene, polybutatriene, polycumulene, polysilane, polymethineimine, and polypyrrole. To draw conclusions about the limiting infinite polymer behavior, chains containing up to 12 heavy atoms along the conjugated backbone were considered. In general, the vibrational hyperpolarizabilities are substantial in comparison with their static electronic counterparts for the dc-Kerr and degenerate four-wave mixing processes (as well as for static fields) but not for electric field-induced second harmonic generation or third harmonic generation. Anharmonicity terms due to nuclear relaxation are important for the dc-Kerr effect (and for the static hyperpolarizability) in the σ-conjugated polymer, polysilane, as well as the nonplanar π systems polymethineimine and polypyrrole. Restricting polypyrrole to be planar, as it is in the crystal phase, causes these anharmonic terms to become negligible. When the same restriction is applied to polymethineimine the effect is reduced but remains quantitatively significant due to the first-order contribution. We conclude that anharmonicity associated with nuclear relaxation can be ignored, for semiquantitative purposes, in planar π-conjugated polymers. The role of zero-point vibrational averaging remains to be evaluated.
Collisional energy transfer probabilities of highly excited molecules from kinetically controlled selective ionization (KCSI). I. The KCSI technique: Experimental approach for the determination of P(E′,E) in the quasicontinuous energy range
Authors: Hold Uwe , Lenzer Thomas , Luther Klaus , Reihs Karsten , Symonds Andrew C.
Published: July 2002

The method of kinetically controlled selective ionization (KCSI) for investigating collisional energy transfer in highly vibrationally excited molecules is presented in detail. In this first paper of a series the focus is on the key concepts and the technical realization of KCSI experiments to provide a common basis for following reports on our available results of KCSI studies on the vibrational relaxation of a variety of larger molecules. The KCSI technique directly monitors the energetic position and shape of the population distributions g(E,t) during the relaxation process by means of an energy selective two photon ionization process via an electronic intermediate state. Such measurements allow—for the first time—to extract complete and accurate experimental sets of transition probability distributions P(E′,E) even at quasicontinuous densities of states. Basic energy transfer quantities are already obtained from a straightforward analysis of the arrival time and width of the KCSI curves. A master equation formalism is outlined which is the basis of a data inversion providing a complete evaluation of the experimental information content. Various examples of characteristic KCSI data on collisional deactivation of highly vibrationally excited molecular populations are used to discuss important aspects of the quality and the general character of P(E′,E) parameters deduced from such measurements. The conditions for a successful modeling of these data are very tightly bound, and the resulting energy transfer parameters 〈ΔE(E)n〉 are therefore of high precision. In Paper II [J. Chem. Phys. 112, 4090 (2000), following article] we give a full account of the toluene KCSI experiments. We will deal with our completed studies on azulene, azulene-d8, pyrazine and pyridine in follow-up publications of this series.
The effect of strain on the adsorption of CO on Pd(100)
Authors: Wu M. W. , Metiu H.
Published: July 2002

We study how the binding energy, the vibrational frequencies and the adsorption isotherm of CO on Pd(100) are modified when the solid is subject to uniform strain. The parameters controlling the thermodynamics of adsorption (the adsorption energy, the vibrational frequencies of the adsorbed molecules and the interaction energy between the molecules) have been calculated by using density functional theory. These parameters are then used in a grand canonical Monte Carlo simulation that determines the CO coverage when the surface is in thermodynamic equilibrium with a CO gas, at a specified pressure and temperature. We find that this is substantially affected by the strain. Our results, along with those obtained by others, suggest that the development of “elastochemistry,” a study of the change in the chemical properties of a surface when subjected to strain will lead to interesting and measurable results. It also suggests that differences in chemical activity between clusters on a support and clusters in gas phase may be partly due to the strain induced when a cluster is placed on the support.
Witten index, axial anomaly, and Krein’s spectral shift function in supersymmetric quantum mechanics
Authors: Bollé D. , Gesztesy F. , Grosse H. , Schweiger W. , Simon B.
Published: July 2002

A new method is presented to study supersymmetric quantum mechanics. Using relative scattering techniques, basic relations are derived between Krein’s spectral shift function, the Witten index, and the anomaly. The topological invariance of the spectral shift function is discussed. The power of this method is illustrated by treating various models and calculating explicitly the spectral shift function, the Witten index, and the anomaly. In particular, a complete treatment of the two-dimensional magnetic field problem is given, without assuming that the magnetic flux is quantized.
On the structure of spatial infinity. I. The Geroch structure
Authors: Chruściel Piotr T.
Published: July 2002

Theorems on uniqueness and ‘‘quasi-uniqueness’’ of the differentiable structure of pointwise singular conformal structures are derived. This allows the classification of all ambiguities in the differentiable structure of conformally completed asymptotic three-dimensional ends.
Solving differential equations by a maximum entropy–minimum norm method with applications to Fokker–Planck equations
Authors: Baker-Jarvis James , Racine Michael , Alameddine Jihad
Published: July 2002

The method of maximum entropy–minimum norm is utilized to produce a general method of solving differential equations. The technique is a generalization and extension of previous work performed by Baker-Jarvis [J. Math. Phys. 30, 302 (1989)]. It is found that introducing an additional constraint on the norm of the solution vector produces a probability distribution that is integrable over the entire real axis. A number of simplifications occur. In this extended method the Lagrange multipliers and solution vector can be solved for explicitly, thus eliminating the necessity of solving systems of nonlinear equations for the Lagrange multipliers, as was required in the previous approach. It is shown that the solution obtained is equivalent to a minimum norm approximation. The maximum entropy solution of differential equations with Fourier moments is shown to be identical to a Fourier series solution. Additionally, the new method is applied to solving the random walk and Fokker–Planck equations.
Classical Yang–Baxter equations and quantum integrable systems
Authors: Jurčo Branislav
Published: July 2002

Quantum integrable models associated with nondegenerate solutions of classical Yang–Baxter equations related to the simple Lie algebras are investigated. These models are diagonalized for rational and trigonometric solutions in the cases of sl(N)/gl(N)/, o(N) and sp(N) algebras. The analogy with the quantum inverse scattering method is demonstrated.
The role of discontinuities in conservation laws
Authors: Cooperstock F. I. , Lim P. H. , Peters P. C.
Published: July 2002

Two types of discontinuities are considered in conjunction with the conservation laws: those that arise from a limit of finite and differentiable functions in a boundary layer and those that do not. It is shown that the former, which prevail in electromagnetism, do not play a role in the conservation laws, but the situation in other areas of physics, such as general relativity, may be more complex. A necessary and sufficient condition for the existence of the first type of discontinuity is derived. For the second type, the conservation laws are no longer of the conventional form.