bluuebunny/crossref_metadata_2025 huggingface dataset:
Theoretical and experimental total state-selected and state-to-state cross sections. III. The (Ar+H2)+ system
Authors: Baer M. , Liao C. -L. , Xu R. , Flesch G. D. , Nourbakhsh S. , Ng C. Y. , Neuhauser D.
Published: July 2002

A detailed three-dimensional quantum mechanical study of the (Ar+H2)+ system along the energy range 0.4 eV≤Etot≤1.65 eV is presented. The main difference between this new treatment and the previously published one [J. Chem. Phys. 87, 465 (1987)] is the employment of a new version of the reactive infinite-order sudden approximation (IOSA), which is based on the ordinary inelastic IOSA carried out for an optical potential. In the numerical treatment we include three surfaces (only two were included in the previous treatment), one which correlates with the Ar+H+2 system and two which correlate with the two spin states of Ar+(2Pj); j=3/2,1/2. The results are compared with both trajectory-surface-hopping calculations and with experiments. In most cases, very good agreement is obtained.
Decoupling in the line mixing of acetylene infrared Q branches
Authors: Pine A. S. , Looney J. P.
Published: July 2002

The Q-branch profiles of the ν1 +ν5 , ν3 +ν4 and ν2 +2ν4 +ν5 Πu –Σg combination bands in the 2.5 μm C–H stretch-bend region of acetylene have been recorded with a difference-frequency laser spectrometer at pressures from 1 to 500 Torr (0.13 to 66.7 kPa). The broadening coefficients, obtained from the ν1 +ν5 band at pressures low enough to avoid significant spectral overlap, can be well fit with empirical rotationally inelastic energy-gap scaling laws or satisfactorily modeled with semiclassical line broadening theory using known intermolecular potential parameters. At pressures when lines are overlapped, collisional interference or line mixing is manifest as a deviation of the Q-branch profiles from an additive superposition of individual transition components. However the line coupling given by the state-to-state collisional scaling laws used to fit the broadening coefficients predicts far more collisional narrowing or Q-branch collapse than is observed. We find that only about one-third of the collisions that broaden the individual lines effectively couple the lines within the f sublevel of the l-doubled excited Π vibrational state observed in the Q branch. This decoupling indicates that there is little or no propensity for preserving the vibrational angular momentum sublevel upon collision, and that elastic reorientational and dephasing collisions may also be-significant. Additionally, we find that the collisional parameters and decoupling are independent of the vibrational state despite dramatically different spectral overlaps exhibited by the three bands studied and a close Fermi resonance between the lower two vibrations. This implies that vibrational relaxation and dephasing collision rates are negligible compared with rotationally inelastic and reorientational rates and usually can be ignored for infrared spectral broadening.
Line mixing and finite duration of collision effects in pure CO2 infrared spectra: Fitting and scaling analysis
Authors: Hartmann Jean-Michel , Boulet Christian
Published: July 2002

We present experimental and calculated pure CO2 infrared spectra in the 2.0, 4.0, and 5.0 μm regions for pressures and temperatures in the 10–60 bar and 300–950 K ranges, respectively. Our measurements are in good agreement with previous ones. The inaccuracy of the Lorentzian model is demonstrated in both the wings and central regions of absorption bands. Models which account for line mixing within the impact approximation are also tested; they are based on nondiagonal relaxation operators built by using fitting laws and the energy corrected sudden scaling analysis. These approaches fail in modeling absorption in far wings, due to the breakdown of the impact approximation. Correction functions, which roughly account for the finite collision duration through a wave-number-dependent relaxation operator, are deduced from measurements beyond the ν3 bandhead. They enable accurate modeling of the P-branch side of this band and have wave-number and temperature dependences consistent with available data. Spectra at high densities in the 2.0 and 5.0 μm regions show that calculated absorptions are very sensitive to the amount of interbranch mixing. Good agreement with experimental results is obtained when intra- and interbranch line mixings are of the same order.
Electronic transition moment for the B 2Σ+–X 2Σ+ emission of CN. Analysis of dependence on the internuclear distance
Authors: Ito Haruhiko , Suzuki Kaoru , Kondow Tamotsu , Kuchitsu Kozo
Published: July 2002

The dependence of the electronic transition moment Re(R) for the B 2Σ+–X 2Σ+ transition of CN on the internuclear distance R is determined by an analysis of the relative intensity of its emission spectrum. In the present analysis of the spectrum, the relative values of Re(R) are directly evaluated by use of the vibrational integrals, 〈v′‖Rn‖v″〉 (n=1,2,...), instead of the r-centroid approximation, which is found to be inadequate particularly in the tail bands (v′≥10). The validity of the r-centroid approximation is discussed on the basis of the Rydberg–Klein–Rees (RKR) potentials of the B 2Σ+ and X 2Σ+ states. The R dependence of the Re(R) value is interpreted in terms of the electronic wave function of B 2Σ+; the configuration-interaction (CI) coefficients for the electronic configurations of the B 2Σ+ state are estimated. The CI coefficients obtained in the present study, as well as those for the A 2Πi state derived previously from a similar analysis of the electronic part of the spin–orbit constant A(R) for A 2Πi, are used to evaluate the electronic part of the spin–orbit matrix element between the B 2Σ+ and A 2Πi states HSOel. The HSOel values are predicted to be −25 and −21 cm−1 at R=0.97 and 1.3 Å, respectively, being consistent with those determined experimentally in our previous study, −31 and −21 cm−1, respectively.
Gaussian fluctuation formula for electrostatic free-energy changes in solution
Authors: Levy Ronald M. , Belhadj Mahfoud , Kitchen Douglas B.
Published: July 2002

Linear-response theory is used to derive a microscopic formula for the free-energy change of a solute-solvent system in response to a change in the charge distribution of the solutes. The formula expresses the change in the solvent polarization energy as a quadratic function of the changes in the partial charges at the atomic centers of the solute atoms. The average electrostatic potential at the sites of the solute charges and the second moment of the fluctuations in the electrostatic potential at these sites enter as parameters in the formula. These parameters can be obtained from computer simulations of a reference system with fixed solute charges and the results then compared with explicit free-energy simulations of the corresponding processes or with experiment. The formula provides a microscopic definition of the dielectric response function for the combined solute plus solvent system which can be related to standard formulas for the dielectric response. A simple numerical example involving a simulation of the charging free energy of two ions in aqueous solution is discussed. The change in the solvent polarization with solute charge predicted using the molecular dielectric response function agrees remarkably well with the results of corresponding free-energy simulations for large changes in the solvent polarization energy. Some applications of the molecular dielectric response formula are discussed.
All-electron molecular Dirac–Hartree–Fock calculations: The group IV tetrahydrides CH4, SiH4, GeH4, SnH4, and PbH4
Authors: Dyall Kenneth G. , Taylor Peter R. , Faegri Knut , Partridge Harry
Published: July 2002

We describe a basis-set-expansion Dirac–Hartree–Fock program for molecules. Bond lengths and harmonic frequencies are presented for the ground states of the group IV tetrahydrides CH4, SiH4, GeH4, SnH4, and PbH4. The results are compared with relativistic effective core potential (RECP) calculations, first-order perturbation theory (PT) calculations and with experimental data. The bond lengths are well predicted by first-order perturbation theory for all molecules, but none of the sets of RECPs considered provides a consistent prediction. Perturbation theory overestimates the relativistic correction to the harmonic frequencies; the RECP calculations underestimate the correction.
The 31P shielding in phosphine
Authors: Jameson Cynthia J. , de Dios Angel C. , Jameson A. Keith
Published: July 2002

The temperature dependence of the phosphorus shielding in phosphine has been remeasured in the range 300–400 K in samples with densities in the range 7–30 amagat. The shielding surfaces are calculated using the localized orbital–local origin (LORG) method of Hansen and Bouman in terms of the symmetry coordinates for the molecule. These surfaces are used to calculate the rovibrationally averaged 31P shielding. The calculated temperature dependence and the deuterium-induced isotope shift for phosphine are in agreement with experiment. The shapes of the 31P in PH3 and the 15N in NH3 shielding surfaces are very similar. With the exception of the inversion coordinate, the remarkable similarity of the surfaces becomes obvious when the shielding functions are scaled by the values of 〈r−3〉np for the ground states of the neutral P and N atoms.
The enthalpy change and the detailed rate coefficients of the equilibrium reaction OH+C2H2=MHOC2H2 over the temperature range 627–713 K
Authors: Lai Lih-Huey , Hsu Yen-Chung , Lee Yuan-Pern
Published: July 2002

The reaction between OH and C2H2 in He has been studied over the pressure range 414–864 Torr and the temperature range 627–713 K by means of the laser-photolysis/laser-induced-fluorescence technique. Analysis of the temporal profile of [OH] yielded the forward and the reverse rate coefficients, hence the equilibrium constant for the reaction OH+C2H2+M=HOC2H2+M. The temperature dependence of the equilibrium constant yielded the enthalpy of reaction ΔH(670±43 K)=−(34.2±1.0) kcal mol−1 and the entropy of reaction ΔS(670±43 K)=−(30.5±1.2) cal K−1 mol−1 near 670 K; consequently, ΔH0 (298 K)=−(33.6±1.2) kcal mol−1 and ΔS0 (298 K)=−(28.9±1.5) cal K−1 mol−1 were derived for the reaction, in agreement with theoretical prediction. The temperature dependence of the rate coefficients for the forward, the reverse, and the adduct-loss (isomerization/decomposition other than the reverse) reactions near 1 atm have been determined to be kf = (1.1 ± 0.2) × 10−13 exp[(1460 ± 500)/T] cm3 molecule−1 s−1, kr = (2.2 ± 0.4) × 1012 exp[ − (15 100 ± 600)/T] s−1, and ka= (7.4+0.2−5.6) × 1010 exp[ − (13 200+700−1700)/T] s−1, respectively.
Evolution of the core and unoccupied orbitals of biphenyl and bithiophene on Cs doping
Authors: Ramsey M. G. , Netzer F. P. , Steinmüller D. , Steinmüller-Nethl D. , Lloyd D. R.
Published: July 2002

X-ray photoelectron spectroscopy (XPS) and near-edge x-ray absorption fine structure (NEXAFS) have been used to study the core and unoccupied orbitals of condensed multilayers of biphenyl and bithiophene on Cs dosing. The CK XPS shifts, observed on dosing are understandable in terms of repositioning of the Fermi level due to creation of states in the band gap, and at higher Cs concentrations to the high polarizability of the charge transfer complexes formed. The behavior of the CK XPS satellites on increasing Cs exposure are related to changes observed in the electron energy loss spectra (EELS). The CK NEXAFS of biphenyl and the SL23 NEXAFS of bithiophene indicate that an unoccupied molecular orbital is filled by charge transfer from Cs. In the case of bithiophene, both the SL23 XPS and NEXAFS features shift by 2 eV to lower binding and adsorption energy, respectively. This strong chemical shift suggests significant localization of the transferred charge on the S sites. Comparison of the SL23 NEXAFS of doped and undoped bithiophene allows the identification of the unoccupied orbital with S character and leads to a reassignment for the NEXAFS features of thiophene and its oligomers.
Solvation structure and dynamics of the Fe2+–Fe3+ ion pair in water
Authors: Kumar P. Vijaya , Tembe B. L.
Published: July 2002

Computer simulations are performed to study the structure of the coordination shells of Fe+2 and Fe+3 ions fixed at a very close reactive separation. The simulations show that it is possible for the two octahedral aquo complexes, i.e., Fe(H2O)62+ and Fe(H2O)63+, to come as close as 5 Å without disrupting their coordination shells. The reorientational dynamics within the hydration shells of these ions at this separation is examined by studying the time correlation functions (TCFs) of the unit vectors on the water molecules and along the iron–oxygen vector. The quantities related to the solvent polarization relaxation during a change in the charge, the dipole moment, and the quadrupole moment located at the solute ions are examined by studying the corresponding TCFs in the system. The TCFs exhibited a bimodal response, with a very fast initial relaxation due to inertial motions of the solvent, followed by a long tail corresponding to a diffusive component. The polarization fluctuations are also estimated via the cavity field time correlation function (CFTCF), which is useful in the theory of electron transfer processes. The memory kernel or the time dependent friction (TDF) of the solvent is also estimated from the force–force time correlation function. The cross correlations between the Coulombic and the non-Coulombic components of the forces at the ions contribute significantly to the TDF.
A detailed state-to-state low-energy dynamics study of the reaction O(3P)+OH(2Π)→O2(X̃ 3Σg−)+H(2S) using a quasiclassical trajectory–internal-energy quantum-mechanical-threshold method
Authors: Varandas A. J. C. , Marques J. M. C.
Published: July 2002

The quasiclassical trajectory (QCT) method has been used for a detailed study of the state-to-state dynamics of the reaction O(3P) + OH(2Π)→O2(X̂33Σ−g) + H(2S) over the range of translational energies 0.125 ≤ Etr/kcal mol−1≤2.0, corresponding to the temperature range 40≤T/K≤680. A novel variant of this method insuring that trajectory calculations properly account for the zero-point energy of the diatomic molecules, the so-called quasiclassical trajectory–internal-energy quantum-mechanical-threshold method, is also suggested and applied to the title reaction. The most recent and accurate double many-body expansion potential-energy surface for the ground doublet state of the hydroperoxyl radical has been employed in all calculations. The computed reactive cross sections for initial quantum rotational states of OH varying from J=0 to J=10 (the vibrational quantum number is kept fixed at v=0) are shown to have a marked decreasing dependence on translational energy, thus suggesting that long-range forces play a major role on the dynamics of the O(3P) + OH(2Π) reaction. A comparison of the thermalized rate coefficients with the results of direct experimental measurements is shown to agree best with the data of Howard and Smith over the whole range of temperatures covered by experiment. It is also shown that nonstatistical recrossing effects are important for all translational energies and rotational states, while a nearly linear dependence is shown over the range of translational energies that have been studied. Finally, appropriate averaging over the initial conditions shows that the dependence on temperature of the calculated recrossing factor is in good agreement with the corresponding estimate from recent direct thermalized QCT calculations using the same potential-energy surface while fitting well by a recently proposed model for this recrossing factor.
Continuous wave multiquantum electron paramagnetic resonance spectroscopy. III. Theory of intermodulation sidebands
Authors: Mchaourab Hassane S. , Hyde James S.
Published: July 2002

The interaction of a spin 1/2 system with two continuous wave transverse electromagnetic fields is studied using the dressed-atom formalism and the Floquet theory. The equation of motion of the density matrix in the presence of two fields is solved and used to analyze the response of the spin system to double irradiation under steady state. In the average frequency rotating frame, the diagonal elements of the density matrix oscillate at even harmonics of the frequency difference, while the off-diagonal elements oscillate at odd harmonics. The spectral response of the spin system can be predicted by applying the conservation rules: The frequency spectrum is a consequence of the conservation of total angular momentum, while the resonance condition is the result of the conservation of energy. The interpretive and predictive nature of the theoretical framework presented is illustrated by the treatment of the classical Anderson experiment and the simulation of the splitting of the multiquantum signal at high frequency difference. Approximate expressions for the population differences and coherences are derived and graphic representation is used to study the general nonlinear dependence on spectral parameters. At low values of the saturation parameter S, the n-quantum absorption is proportional to dT2S(n−1)/2, where d=1/2γH. Therefore, the signal amplitude is proportional not only to T2, but also to powers of T1T2, which makes the multiquantum signals more sensitive to relaxation rates than conventional one-photon displays. The frequency difference swept line shape of the multiquantum signal depends on both T1 and T2. However, when T2≪T1, the new spectroscopic dimension, namely, the frequency difference, can be used to determine the spin–lattice relaxation time. Several spectroscopic features of multiquantum signals are discussed in the context of the general mathematical equivalence of double irradiation and amplitude modulation spectroscopy.
Electronic structure of Li(9-crown-3)2: A molecule with a Rydberg-type ground state
Authors: Rencsok R. , Kaplan T. A. , Harrison J. F.
Published: July 2002

We report unrestricted Hartree–Fock (UHF) results for the molecule Li(9C3)2, obtained with several basis sets, including diffuse functions, up to 6-31G++**. These results show that (1) the spatial distribution of the highest occupied molecular orbital (HOMO), or valence, electron is predominantly ‘‘outside’’ the Li(9C3)2 complex. That is, the complex is rather accurately described by the formula Li+(9C3)2e−; (2) the spin density at the Li nucleus drops by about three-orders of magnitude, as compared to the free lithium atom; (3) the character of the ground state, and the first few excited states, is predominantly Rydberg-type, centered on the lithium, with the valence electron ‘‘loosely’’ bound to the complex. We argue that this complex is thermodynamically stable, and show graphically what happens to the spin density as the 9-crown-3 (9C3) molecules complex the neutral lithium atom. As far as we are aware, this is the first example of a neutral molecule with a Rydberg-type ground-state. We also discuss the relation of our results to the properties of the crystalline electrides.
On the internal field correction in far-infrared absorption of highly polar molecules in neat liquids and dilute solutions
Authors: Vij J. K. , Kalmykov Yu P.
Published: July 2002

Far-infrared absorption spectra for liquid acetone, methylene chloride, acetonitrile, methyl iodide, and their dilute solutions in cyclohexane at 20 °C are measured by molecular laser spectrometer. Measurements of dielectric loss of polar liquids and solutions in the frequency range 2–300 GHz are made using a number of different techniques. These two sets of measurements are combined with those made using a Fourier transform spectrometer in order to cover the frequency range up to 250 cm−1 and total integrated absorption intensities are calculated. It is shown that the discrepancy between experimental integrated absorption and the theoretical results given by Gordon’s sum rule with the Polo–Wilson internal field factor can be explained in the context of Bossis’ theory. This theory gives a better agreement with the experimental integrated absorption intensity for these liquids.
On the use of divergent series in vibrational spectroscopy. Two- and three-dimensional oscillators
Authors: Čížek Jiří , Špirko Vladimír , Bludský Ota
Published: July 2002

Prospects for higher order perturbation theory in evaluating accurate eigenvalues and eigenvectors of the vibrational Hamiltonian are investigated by performing calculations on a series of two- and three-dimensional quartic anharmonic oscillators imitating vibrational motions in the H2O, H2S, NO2, SO2, and HOF triatomic molecules. Despite the fact that most of the corresponding perturbation expansion series diverge strongly they can be summed efficiently using the same methods as in previous studies on one-dimensional oscillators. Hence, higher order perturbation theory appears as a reasonable alternative to accurate variational methods in the case of small molecular systems and as a very promising tool for a proper treatment of larger systems.
Femtosecond dynamics of hydrogen-bonding solvents. Formamide and N-methylformamide in acetonitrile, DMF, and water
Authors: Chang Yong Joon , Castner Edward W.
Published: July 2002

We have used optical-heterodyne-detected optical Kerr effect (OHD-OKE) spectroscopy to measure the ultrafast dynamical profile of the amide series formamide (FA), N-methylformamide (NMF), and N,N-dimethylformamide (DMF). We have studied the concentration dependence of binary solutions of FA and NMF using DMF, water, and acetonitrile as cosolvents. Our results show an underdamped librational motion with a period of about 100 cm−1 in both FA and NMF, which also occurs in the binary solutions. The significance of these results for ultrafast chemical reactions in solution is discussed.
A new model for atom–atom potentials
Authors: Cvetko D. , Lausi A. , Morgante A. , Tommasini F. , Cortona P. , Dondi M. G.
Published: July 2002

A new model potential for van der Waals atomic pairs is proposed and shown to be in excellent agreement with ab initio calculations and experimental potentials. The model allows the potential energy of a given pair to be estimated with good accuracy from ab initio density functional calculations of the free-atom electron densities.
Line mixing effects in the 00°3–00°0 band of CO2 in helium. II. Theoretical analysis
Authors: Boissoles J. , Thibault F. , Doucen R. Le , Menoux V. , Boulet C.
Published: July 2002

In paper I of this series, important deviations from an additive superposition of Lorentzian profiles were experimentally evidenced in the 00°3–00°0 band of CO2 in He. All the observed deviations are explained by the collision-induced line mixing effects which schematically transfer intensity from the wing of the band to its central part. The IOS approximation has been found to be insufficient while, the ECS approximation leads to theoretical predictions in good agreement with the experimental data over extended ranges of frequency and perturber pressure. However it must be emphasized that it has been necessary to resort to the method in current use for the determination of the fundamental rates, an ad hoc adjustement starting from the observed linewidths.
Carbon monoxide adlayer structures on platinum (111) electrodes: A synergy between in-situ scanning tunneling microscopy and infrared spectroscopy
Authors: Villegas Ignacio , Weaver Michael J.
Published: July 2002

The spatial structure of compressed carbon monoxide adlayers on Pt(111) in aqueous acidic solution has been explored by means of in-situ scanning tunneling microscopy (STM) along with infrared reflection–absorption spectroscopy (IRAS). Besides offering a detailed structural picture of this electrochemical interface in comparison with the well-studied Pt(111)/CO system in ultrahigh vacuum (uhv) environments, the real-space structural information provided by STM allows an assessment of the obfuscating influence of dynamic dipole coupling upon IRAS binding-site assignments. In turn, the latter data provide an important crosscheck on the validity of binding-site assignments deduced from the STM images. Emphasis is placed on the structures formed from near-saturated CO solutions, encouraged by the electrode potential-induced adlayer phase transition at ca. 0 V vs SCE observed previously under these conditions by IRAS. At potentials below 0 V, a hexagonal close-packed (2×2)–3CO adlayer is observed, with a CO coverage, θCO, of 0.75.

The z-corrugation pattern evident in the STM images indicates the presence of two threefold hollow and one atop CO per unit cell. This binding-site assignment is supported by the corresponding IRAS data which yield C–O vibrational bands at ca. 2065 and 1775 cm−1. The relative intensities of these two νCO bands, ca. 2:1, differs markedly from the 1:2 binding site occupancy deduced from STM. This apparent disparity, however, can be accounted for by dynamic dipole coupling effects between the atop and multifold CO oscillators. At potentials above 0 V (up to the onset of CO electrooxidation at ca. 0.25 V), a markedly different adlayer arrangement is formed, having a (√19×√19)R23.4°–13CO unit cell, with θCO=13/19. This hexagonal structure features CO binding in predominantly asymmetric sites inbetween atop and bridging geometries. A distinction between several alternate adlayer arrangements sharing (√19×√19) symmetry was achieved on the basis of the z-corrugation pattern along with the corresponding IRAS data upon consideration of dipole-coupling effects. Another CO adlayer structure, having a (√7×√7)R19.1°-4CO unit cell (θCO=4/7), was commonly observed at potentials below 0.2 V after the removal of solution-phase CO. These adlayer arrangements are distinctly different to the compressed Pt(111)/CO structures found in uhv. The increased accommodation of CO in multifold sites observed for the former can be understood chiefly from the markedly (ca. 1 V) lower surface potentials (and excess electronic surface charges) characterizing the electrochemical interface.
Structure and properties of the metal–liquid interface
Authors: Bérard D. R. , Kinoshita M. , Ye X. , Patey G. N.
Published: July 2002

Theoretical results are given for simple dipolar liquids in contact with a metallic slab. The metal is treated by employing a jellium model together with density functional (DF) theory. The liquid structure at the interface is given by the reference hypernetted-chain (RHNC) approximation. The liquid and metal interact electrostatically and the coupled DF/RHNC equations are solved iteratively to obtain electron density distributions and metal–liquid correlation functions which are completely self-consistent. The electron density, liquid structure, and potential drop across the interface are discussed in detail. It is found that dipoles in contact with the metal prefer to orient perpendicular to the surface with their positive ends out. This is in accord with earlier calculations for dipolar monolayers on metal surfaces. Further from the surface, the dipolar orientations oscillate and the liquid structure rapidly decays to the bulk fluid limit.
The vibrationally resolved participator Auger spectra of selectively excited C 1s(2σ)−12π1 vibrational states in carbon monoxide
Authors: Osborne S. J. , Ausmees A. , Svensson S. , Kivimäki A. , Sairanen O.-P. , de Brito A. Naves , Aksela H. , Aksela S.
Published: July 2002

The fully vibrationally resolved participator Auger spectra originating from the decay of the C 1s(2σ)−12π1 resonance in CO are presented. The C 1s(2σ)−12π1 v′=0 resonance has been excited with a 75 meV monochromator bandpass, i.e., in Auger resonant Raman conditions, and the participator Auger spectrum observed. The C 1s(2σ)−12π1 v′=1 resonance is also excited and the corresponding participator Auger spectrum observed with a monochromator bandpass slightly larger than the inherent width. The results are compared to theoretical simulations using coherent lifetime-vibrational interference theory which accounts for the details of the spectrum. We have observed an interference shift on the transitions to different vibrational sublevels in the final state. A high resolution C 1s photoelectron spectrum of CO is also presented. The lifetime width of the C 1s core–hole state is determined to be 97(10) meV, whereas the C 1s(2σ)−12π1 resonance is measured to have a width of 86(10) meV.
A new coarsening mechanism of droplet spinodal decomposition
Authors: Tanaka Hajime
Published: July 2002

We propose here a new coarsening mechanism (‘‘diffusion-coupling mechanism’’) of droplet phase separation. For a system of high droplet density, the inhomogeneous distribution of droplets and the resulting anisotropic, selective coupling between the diffusion fields around neighboring droplets lead to the formation of the closed iso-concentration lines including more than two droplets. The excess energy associated with the concentration gradient in the matrix phase around neighboring droplets tries to reduce the length of the iso-concentration lines and causes the attractive interaction between the droplets. This mechanism could accelerate the droplet coarsening in spinodal decomposition.
On the propensity rules for inelastic NH3–rare gas collisions
Authors: van der Sanden G. C. M. , Wormer P. E. S. , van der Avoird A. , Schleipen J. , ter Meulen J. J.
Published: July 2002

The observed and ab initio calculated propensity rules for collisions of NH3 with rare gas atoms are found to be in reasonable agreement for NH3–Ar, whereas for NH3–He they show large discrepancies. In order to examine these discrepancies we have calculated state-to-state integral cross sections for collisions of NH3 with He using the close coupling method. The NH3–He interaction potential has been obtained from SCF calculations, augmented by a multipole-expanded damped dispersion energy. Our calculations show that the discrepancies can be accounted for if the cross sections are corrected for the imperfect initial state preparation in the experiment. They also clarify why the discrepancies do not occur to the same extent for NH3–Ar. After comparing our new theoretical results with the experimental data we found that for one experimental cross section for NH3–He the earlier assignment must be corrected.
Formation of argon clusters by homogeneous nucleation in supersonic shock tube flow
Authors: Zahoransky R. A. , Höschele J. , Steinwandel J.
Published: July 2002

The formation of argon clusters by homogeneous nucleation of pure argon diluted in helium was investigated in the supersonic outflow of a shock tube, where the clusters were detected by laser light scattering from the particles in supersaturated states. The thermodynamic states for the onset of homogeneous nucleation varied between 48 and 85 K with corresponding argon partial pressures in the range 2–850 Torr. Values of adiabatic supercooling with respect to the argon phase equilibrium ranged between 3 and 10 K. The experiments are in agreement with previous investigations in subsonic shock tube and supersonic Ludwieg tube expansions but disagree with previous investigations in steady supersonic nozzle expansions. It is possible to explain qualitatively the observed differences in argon nucleation behavior on the basis of a thermodynamic and kinetic concept independent of the basic assumptions of existing nucleation theories.
Integral and differential state-to-state cross-sections for the reactions F+D2(v
i=0,j
i)→DF(v
f,j
f)+D: A comparison between three-dimensional quantum mechanical and experimental results
Authors: Baer M. , Faubel M. , Martinez-Haya B. , Rusin L. Y. , Tappe U. , Toennies J. P. , Stark K. , Werner H.-J.
Published: July 2002

In this letter we report quantum mechanical integral and differential cross sections for the title reactions as calculated on a new ab initio potential energy surface. The calculations, all carried out in the reagents arrangement channel employing negative imaginary potentials, were done within the coupled-states approximation. The final vibrational state-to-state differential and integral cross sections were compared with experiment. Altogether, a very encouraging agreement was obtained.
Translationally activated dissociation of CO2 on Si(100)2×1
Authors: Brown Kyle A. , Hu D. Q. , Ho W.
Published: July 2002

The reaction of hyperthermal (0.1 eV→2.6 eV) CO2 with Si(100)2×1 at ≤300 K has been studied with high-resolution electron energy loss spectroscopy (HREELS), low energy electron reflectivity (LEER), temperature programmed desorption (TPD), and Auger electron spectroscopy (AES). The initial sticking coefficient depends on the total kinetic energy of the incident molecule, but is insensitive to the vibrational state. Adsorption results in a dramatic reduction in specular electron reflectivity which is reversed upon desorption of one of the dissociatively adsorbed products, CO, at 400 K. The adsorbed oxygen remains on the surface after annealing to as high as 900 K. Continued exposure results in the population of a CO state which desorbs near 330 K as well as a molecular CO2 state desorbing at 110 K.
Multiconfiguration wave functions for quantum Monte Carlo calculations of first-row diatomic molecules
Authors: Filippi Claudia , Umrigar C. J.
Published: July 2002

We use the variance minimization method to determine accurate wave functions for first-row homonuclear diatomic molecules. The form of the wave function is a product of a sum of determinants and a generalized Jastrow factor. One of the important features of the calculation is that we are including low-lying determinants corresponding to single and double excitations from the Hartree–Fock configuration within the space of orbitals whose atomic principal quantum numbers do not exceed those occurring in the Hartree–Fock configuration. The idea is that near-degeneracy correlation is most effectively described by a linear combination of low-lying determinants whereas dynamic correlation is well described by the generalized Jastrow factor. All the parameters occurring in both the determinantal and the Jastrow parts of the wave function are optimized. The optimized wave functions recover 79%–94% of the correlation energy in variational Monte Carlo and 93%–99% of the correlation energy in diffusion Monte Carlo.
Line-mixing in the 106←000 overtone transition of HCN
Authors: Romanini Daniele , Lehmann Kevin K.
Published: July 2002

By using cavity ring-down spectroscopy (CRDS), we have obtained visible overtone absorption spectra of HCN which display a large collisional line-mixing effect in the proximity of the R branch band heads, for J∼18. We consider in detail the 106←000 (1=CN, 0=bend, 6=CH) parallel transition. The R branch profile was modeled using the modified-exponential-gap (MEG) and energy-corrected-sudden approximation (ECS) population transfer rate laws. We used the rates previously determined by Pine and Looney (PL) by fitting the self broadening coefficients measured for the Q branches of Π–Σ infrared perpendicular stretch–bend combination bands of HCN [J. Chem. Phys. 96, 1704 (1992)]. Contrary to what is found by these authors, in the present case the MEG law reproduces the R branch line-mixing satisfactorily, while the ECS model fails. This reflects an increasing propensity at higher J for collisional transitions with smaller ΔJ. Using the MEG law, we found we need to include, as had PL in their fits to the infrared Q branches, an empirical dephasing scale factor F∼0.6 for the coherence transfer rates to obtain a satisfactory simulation of the R band head. PL suggested that dephasing in the Q branch spectra are due to cross relaxation across l-type doublet levels of the Π state, but no such mechanism would be available in the present case. However, we have found that by using a 50/50 linear combination of the ECS and MEG rate laws, it is possible to fit our data even with F=1, which would imply no dephasing of coherence. We take this as a demonstration that the dephasing factor F cannot be reliably extracted from line-mixing studies alone but instead requires some independent source of information on the relative value for state to state inelastic collision rates.
Approaches to bifurcating reaction path
Authors: Taketsugu Tetsuya , Tajima Nobuo , Hirao Kimihiko
Published: July 2002

The intrinsic reaction path (IRP) often becomes unstable relative to some nontotally symmetric direction orthogonal to the path through a valley–ridge inflection point. We investigate geometric characters of the potential energy surface around the valley–ridge inflection boundary, and propose some ideas to determine a bifurcating reaction path, or to give a two-dimensional potential energy surface which connects bifurcating point and product regions. As a demonstration, bifurcating reaction paths are calculated for the isomerization reaction of methoxy radical (H3CO→H2COH) by the unrestricted Hartree–Fock (UHF) method.
Capillary condensation in structured pores
Authors: Röcken Petra , Tarazona Pedro
Published: July 2002

We investigate capillary condensation in slit-like pores with structured walls, described by a simple lattice gas model based on the mean-field approximation. Most theoretical workers did not go beyond very simple capillary models with perfect symmetry, thus being unable to map experiments in real pores with rather complex structure. In this paper we deal with chemical inhomogeneity, produced by a periodic wall potential, that causes fluid attraction and repulsion altering along the walls. We observe a new mechanism of capillary condensation, characterized by a splitting of the equilibrium ‘‘gas–liquid’’ transition, and we concentrate on the crucial role the typical length for the wall inhomogeneity, λ, plays compared with the pore width, H. Our microscopic results are confirmed by a macroscopic analysis that leads to a modified Kelvin equation, giving a quantitative prediction of capillary condensation in structured pores. Furthermore our model explains the nature of the critical isochore as measured in mesopore condensation experiments by Thommes and Findenegg.
Extended rotational isomeric model for describing the long time dynamics of polymers
Authors: Guenza Marina , Freed Karl F.
Published: July 2002

An extended rotational isomeric states (RIS) model is used in conjunction with the matrix expansion method for describing the long time dynamics of flexible polymers in solution. The extended RIS model is derived directly from the potential functions that contain hindered torsional potentials, nonbonded interactions, etc. The matrix expansion method for describing the long time dynamics contains equilibrium conformational averages which are evaluated here from the extended RIS model. The theory effectively assumes that the torsional barriers provide the dominant mechanism for the decay of orientational correlations in the polymer chains. The theory is applied to united atom alkane chain dynamics where previous Brownian dynamics simulations with the same potentials are available for an unambiguous, no-parameter test of the theory. The present computation of equilibrium averages with the extended RIS model represents a significant advancement over the prior treatments that evaluate the equilibrium averages using Brownian dynamics simulations. The comparison with the previous approach indicates the degree to which bond angle fluctuations affect the orientational time correlation functions.
A linear scaling method for Hartree–Fock exchange calculations of large molecules
Authors: Burant John C. , Scuseria Gustavo E. , Frisch Michael J.
Published: July 2002

We introduce the near-field exchange method for calculating Hartree–Fock exchange in time scaling near-linearly with system size. Benchmarks on polyglycine chains, water clusters, and diamond pieces show that microhartree accuracy and substantial speedups (up to 10×) over traditional calculations can be obtained for electrically insulating systems larger than 300 atoms.
Photodissociation studies on nitrosyl bromide: I. Photofragment spectroscopy and electronic structure
Authors: Loock Hans-Peter , Qian Charles X. W.
Published: July 2002

The UV-visible absorption spectrum of nitrosyl bromide (BrNO) was recorded. The spectrum exhibits several broad features in the 200–740 nm region. To aid the assignment of the spectrum, photodissociation experiments were carried out at several selected excitation wavelengths. Resonance-enhanced multiphoton-ionization spectra of both the NO and Br fragments as well as photofragment time-of-flight profiles were recorded. The electronic transitions were assigned based on the observed spatial anisotropy parameters (β), the Br spin orbit state population, and NO rotational, vibrational, spin-orbit, Λ-doublet state populations, as well as comparisons with isovalent ClNO system. The strongest UV band peaked around 210 nm was attributed to the parallel S5←S0 transition (σBr–N*←σBr–N) while all other features at longer wavelengths were assigned to transitions to excited triplet states. The intensity borrowing from the S5 state is responsible for the apparent parallel nature of these singlet–triplet transitions. Implications of this assignment on the electronic structure and the non-adiabatic dissociation dynamics are discussed.
The size of a polymer chain in an imperfect array of obstacles: Monte Carlo results
Authors: Slater Gary W. , Nixon Grant I.
Published: July 2002

Using Monte Carlo simulations, we test the prediction [Sommer and Blumen, Phys. Rev. Lett. 79, 439 (1997)] that the radius-of-gyration of a polymer chain moving in a periodic array of obstacles should be a minimum when a few defects are present. Our results agree qualitatively with this prediction, but the excluded volume interactions greatly reduce the effect.
Fourier transform infrared observation of the ν1(σ) mode of linear SiCH in Ar at 10 K
Authors: Han D. S. , Rittby C. M. L. , Graham W. R. M.
Published: July 2002

A Fourier transform infrared (FTIR) study of the products of the vacuum ultraviolet (VUV) photolysis of a mixture of silane (SiH4) and methane (CH4) trapped in solid Ar at ∼10 K has resulted in the first detection of the linear SiCH radical by means of its silicon–carbon stretching mode ν1(σ), which has been observed at 1010.4 cm−1. The results of extensive measurements of D and 13C isotopic shifts are in good agreement with the predictions of new ab initio calculations performed at the CCSD(T)/6-31G** (coupled cluster with single, double and partial inclusion of triple excitations) level, and confirm the identification of the linear SiCH radical.
Electron spin resonance g tensors from general Hartree–Fock calculations
Authors: Jayatilaka Dylan
Published: July 2002

A method is described to calculate the electron spin resonance (ESR) g tensor as a derivative of the energy. The method is formally based on the second derivatives of the square of the Zeeman splitting energy. This can be reduced to a first derivative, and then a Hellmann–Feynman formula can be used. A general Hartree–Fock (GHF) method to calculate the g tensor is described and implemented. Formula for the one and two electron spin-orbit contributions are presented. The GHF method is used to calculate g tensor elements for CO+, CN, MgF, NO2, NF2, CO2−, O3−, OH2+, HCO, and H2CO+. In most cases, the agreement is within or close to the expected experimental accuracy of 500 ppm for neon matrix data, but there are still some significant discrepancies. The results are in better agreement with configuration interaction (CI) type calculations than density functional theory (DFT) calculations, although the differences are small. The results are significantly different from perturbation theory predictions. It is found that the diamagnetic contributions to the g tensor are not significant to within the experimental errors, if matrix isolation data is used. The g values for the CoCl42− anion are also studied to show the methodology can be extended to larger systems.
Optical properties of surface and bulk F centers in MgO from ab initio cluster model calculations
Authors: Illas Francesc , Pacchioni Gianfranco
Published: July 2002

The optical transitions of neutral and charged oxygen vacancies in the bulk and on the surface of MgO have been studied by means of first principles calculations. Using rather large cluster models representing the MgO F+ and F centers, oxygen vacancies with one and two electrons trapped in the cavity, respectively, we have performed extensive configuration interaction calculations on the ground and excited states. The computed optical transitions for bulk F and F+ centers occur at a similar energy, in agreement with the experimental measures. The absolute values of the transitions, ≈6 eV, are overestimated by about 1 eV compared to the experimental value of ≈5 eV, mainly because of basis set limitations. On the surface, the Fs and the Fs+ centers are found to absorb photons of lower energy than in the bulk. The expected transitions on the surface are smaller by about a factor of 2 than in the bulk. Thus, transition energies in the range 2–2.5 eV are predicted by the calculations. The results are compared with the few existing measurements of optical transitions at the surface of MgO.
Flow properties of liquid crystal phases of the Gay–Berne fluid
Authors: Sarman Sten
Published: July 2002

We have calculated the viscosities of a variant of the Gay–Berne fluid as a function of the temperature by performing molecular dynamics simulations. We have evaluated the Green–Kubo relations for the various viscosity coefficients. The results have been cross-checked by performing shear flow simulations. At high temperatures there is a nematic phase that is transformed to a smectic A phase as the temperature is decreased. The nematic phase is found to be flow stable. Close to the nematic–smectic transition point the liquid crystal model system becomes flow unstable. This is in agreement with the theoretical predictions by Jähnig and Brochard [F. Jähnig and F. Brochard, J. Phys. 35, 301 (1974)]. In a planar Couette flow one can define the three Miesowicz viscosities or effective viscosities η1, η2, and η3. The coefficient η1 is the viscosity when the director is parallel to the streamlines, η2 is the viscosity when the director is perpendicular to the shear plane, and η3 is the viscosity when the director is perpendicular to the vorticity plane. In the smectic phase η1 is undefined because the strain rate field is incommensurate with the smectic layer structure when the director is parallel to the streamlines. The viscosity η3 is found to be fairly independent of the temperature. The coefficient η2 increases with the temperature. This is unusual because the viscosity of most isotropic liquids decreases with the temperature. This anomaly is due to the smectic layer structure that is present at low temperatures. This lowers the friction because the layers can slide past each other fairly easily.
Effects of reagent rotation on the dynamics of the H2+OH reaction: A full dimension quantum study
Authors: Zhang Dong H. , Lee Soo-Y.
Published: July 2002

We have extended the time-dependent wave packet method to calculate cross sections and rate constants for rotationally excited initial states by using the centrifugal sudden (CS) approximation. A detailed study of the effects of rotational excitation of reagents on the title reaction on the WDSE PES has been carried out. It is found that (a) OH rotational excitation very mildly enhances the total cross section, (b) H2 rotational excitation quite substantially reduce the cross section, and (c) simultaneous OH and H2 rotational excitation has a largely uncorrelated effect. As a result, we found that the thermal rate constant can be obtained fairly accurately by only taking into account the effect of H2 rotation. A model calculation by changing the mass of an O atom reveals that the weak dependence of the cross section on OH rotation is not because the O atom is left relatively stationary by OH rotation. We speculate that it may be a general feature for the diatom-diatom reaction that the nonreactive diatom acts as a spectator not only vibrationally but also rotationally. It was also found that the “J-shifting” approximation works quite well for the reaction. On the other hand, the effect of K on the dynamics is found to be much stronger and more complicated than the J effect, making the “K-shifting” approximation not good for the reaction.