bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic properties and microstructure of mechanically milled Sm2(Co,M)17-based powders with M=Zr, Hf, Nb, V, Ti, Cr, Cu and Fe
Authors: Chen Zhongmin , Meng-Burany X. , Okumura Hideyuki , Hadjipanayis G. C.
Published: July 2002

The structure, microstructure, and magnetic properties of nanostructured Sm2Co17-based powders synthesized by mechanically milling and subsequent annealing have been systematically studied. It has been found that a nanoscale 2:17 phase with an average grain size of about 30 nm is developed within the powders, which have an average particle size of about 5 μm. Optimum magnetic properties of Ms=110.5 emu/g, Mr=66.2 emu/g, Mr/Ms=0.60, Hc=9.6 kOe, and (BH)m=10.8 MGOe have been obtained in stoichiometric Sm2Co17 powders milled for 6 h and annealed at 800 °C for 30 min. The observed magnetic hardening is believed to arise from the high anisotropy of the Sm2Co17 phase and its nanoscale grain size. A small amount of Zr substitution for Co significantly increases the coercivity by increasing the anisotropy field of the Sm2Co17 phase. Cu substitution in Zr-contained samples further increases the coercivity by introducing a nanoscale 1:5 phase which forms a uniform mixture with the 2:17 nanograins. The highest coercivity of 20.6 kOe has been obtained in the Sm12(Co0.92Cu0.06Zr0.02)88 powders. Fe substitution enters the Co lattice sites of the 2:17 structure, leading to an increase of the magnetization but a decrease of the coercivity. An optimum maximum energy product of 14.0 MGOe is obtained in the Sm12(Co0.7Fe0.3)88 powders.
Magnetic and crystallographic properties of La–Zn substituted Sr-ferrite thin films
Authors: Liu Xiaoxi , Bai Jianmin , Wei Fulin , Yang Zheng , Takei Shigeto , Morisako Akimitsu , Matsumoto Mitsunori
Published: July 2002

La–Zn substituted Sr-ferrite thin films were prepared by rf diode sputtering. The magnetic and crystallographic properties of the films were investigated by vibrating sample magnetometry and x-ray diffractometry. It is found that the texture of the films sensitively depends on the amount of substitution. Both the saturation magnetization Ms and coercivity Hc increase with the increase of La–Zn substitution. The increase of Hc is caused, in part, by the increase of K1 due to the substitution.
Infrared photoluminescence from Er doped porous Si
Authors: Henley W. , Koshka Y. , Lagowski J. , Siejka J.
Published: July 2002

Intense infrared photoluminescence with the characteristic maximum at about 1.55 μm was observed at room temperature in Er-doped porous silicon [Er related infrared photoluminescence (ErIR PL)]. Porous Si layers of different controlled porosity were fabricated by electrochemical anodization of n and p Cz–Si wafers as well as at p+/n silicon junction. Er was introduced into the porous Si using a spin-on doping technique. Rutherford backscattering spectroscopy measurements show that annealing up to 1000 °C does not influence the Er depth distribution in the porous silicon, although it strongly influences the oxygen content of the Si skeleton and the ErIR PL intensity. For the spin-on-doped samples annealed at 1000 °C, the ErIR PL intensity is increased by two orders of magnitude compared to Er implanted and annealed porous Si layers. It was found that a strong ErIR PL intensity was only observed in the spin-on-doped porous Si layers formed on p and p+/n substrates, which exhibit, simultaneously, an intense PL intensity in the visible range. The mechanism of Er related IR luminescence in porous silicon is discussed.
Ion-beam modification of Co/Ag multilayers I: Structural evolution and magnetic response
Authors: Veres T. , Cai M. , Cochrane R. W. , Roorda S.
Published: July 2002

We describe the effects of 1 MeV Si+ ion-beam irradiation on a sputtered Co/Ag multilayer with layer thicknesses of 5 Å for Co and 25 Å for Ag, thicknesses for which the magnetoresistance is maximum in the as-deposited sample. X-ray diffraction, magnetization, and magnetoresistance measurements all point to the conclusion that the Co is initially dispersed through the Ag and segregates completely upon ion-beam bombardment. Throughout the process both Ag and Co grains maintain a high degree of texture, essentially face centered cubic (111). The magnetization behavior evolves from superparamagnetic to mixed superparamagnetic–ferromagnetic with ion dose whereas the room-temperature magnetoresistance decreases from 12% to 1.5% upon irradiation up to 5×1016 Si+/cm2. Simple models taking into account the size distribution of the Co particles have been used to analyze these phenomena in order to quantify the particle size distribution.
A method for the calibration of magnetic force microscopy tips
Authors: van Schendel P. J. A. , Hug H. J. , Stiefel B. , Martin S. , Güntherodt H.-J.
Published: July 2002

A method is presented for the analysis of magnetic force microscopy (MFM) measurements that allows the quantitative determination of the sample stray field. It is shown how measurements on an easily obtainable calibration sample can be used to calibrate the MFM measurement as a function of the wavelength of the sample stray field at the tip apex. It is demonstrated that the thus obtained MFM calibration can be used to determine the stray field distribution of the tip. Furthermore, the effect of the tip on the MFM imaging mechanism is analyzed by comparing it to some simple tip models. From the analysis, it is shown that the point–monopole and dipole models do not accurately describe the MFM imaging mechanism.
Collision complex formation in the reaction of C+ with H2O
Authors: Sonnenfroh D. M. , Curtis R. A. , Farrar J. M.
Published: July 2002

A crossed beam study of the title reaction at collision energies of 0.62 and 2.14 eV demonstrates that the production of mass 29 ions, either HCO+ or COH+, occurs through the formation of a transient intermediate HCOH+ complex which lives approximately one rotational period at the lower collision energy. The asymmetry of the angular distribution at a relative energy of 0.62 eV indicates that the lifetime of the transient intermediate is ∼10−13 s. The widths of the kinetic energy distributions at both collision energies are consistent with production of a significant fraction (≳20%–30%) of the more stable formyl HCO+ isomer. These observations suggest the formation of a hydroxycarbene cation insertion complex at the lower energy which may decay by C–H cleavage to form isoformyl cation COH+ or by O–H bond cleavage to form the more stable formyl cation HCO+. At the higher collision energy, a smaller fraction of the mass 29 products are constrained to be HCO+. This observation and the form of the product flux distribution suggest the increasing dominance of a direct, ‘‘knockout’’ mechanism at higher energy. The internal energy of the mass 29 product as inferred from the kinetic energy distributions is above the HCO+/COH+ isomerization barrier for approximately one-third of the products at the lower collision energy, and for two-thirds of the products at the higher energy.
Accurate quantum yields by laser gain vs absorption spectroscopy: Investigation of Br/Br* channels in photofragmentation of Br2 and IBr
Authors: Haugen Harold K. , Weitz Eric , Leone Stephen R.
Published: July 2002

A two-laser pulse-and-probe technique is used to study photofragmentation of Br2 and IBr over the wavelength range 450–530 nm. The metastable Br(2P1/2–2P3/2) transition is probed by time–resolved laser gain vs absorption spectroscopy using a tunable color center laser. This new approach to the measurement of quantum yields provides highly accurate absolute values for Br*(2P1/2) production. The peak quantum yield for Br2 photodissociation is φ=87% at λ=500 nm. The difference between the spectral variation of φ and the total absorption spectrum characterizes the A state of bromine, which contributes ≲14% to the absorption spectrum at λ=510 nm. The peak in the Br* yield from photofragmentation of IBr is φ∼73% at λ=500 nm. The present absolute IBr data together with the previous molecular beam studies suggest a reassessment of the contributions of the continuum states in IBr. The laser gain vs absorption method for obtaining quantum yields is readily generalized to other atoms and molecules.
Energy transfer and fine structure axes determination in a hybrid porphyrin dimer oriented in a liquid crystal. Time resolved triplet EPR spectroscopy
Authors: Gonen Oded , Levanon Haim
Published: July 2002

A study of laser photoexcitation-EPR diode detection of the transient triplet state of the hybrid covalenty linked porphyrin dimer [ZnP–H2P] in a uniaxial liquid crystal and a glass matrix at 100 and 10 K, is reported. This time resolved technique enables monitoring the transient behavior of the magnetization until thermal relaxation sets in. Selective singlet excitation of the dimer components reveals a singlet energy transfer process [ZnP*−H2P]–––ket→ZnP–H2P*, whose rate at 10 K is calculated to be [1.0±0.2]×109 s−1. Triplet line shape analysis of the subunit monomer triplet and the dimer yields: (a) the location of the fine structure tensor axes in the molecular frame (X, Y along the pyrrole nitrogens and Z perpendicular to the porphyrin plane); consequently, the signs of the parameters D and E are found to be positive for the monomeric free base and the dimer; (b) the ratio of the sublevel population rates; and (c) the spin-lattice relaxation rate.
Charge–transfer organic crystals: Molecular vibrations and spectroscopic effects of electron–molecular vibration coupling of the strong electron acceptor TCNQF4
Authors: Meneghetti Moreno , Pecile Cesare
Published: July 2002

A fundamental aspect in the study of the charge–transfer (CT) organic crystals with ionic or partially ionic ground states is the investigation of the spectroscopic effect of the electron–molecular vibration coupling. 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (TCNQF4), an electron acceptor much stronger than TCNQ, is an outstanding component of many interesting CT systems. A thorough vibrational analysis of the title compound and of its monomeric radical anion is reported. The analysis is based on the Raman depolarization ratio measurements and on infrared data of solutions of both neutral and ionic species as well as on polarized infrared spectra of oriented crystals of the neutral molecule. The vibrational assignment, completed by a normal coordinate analysis (NCA), brings to the identification of the ionization frequency shifts and to the choice of the fundamentals (b1uν19 and b2uν33) whose frequencies are diagnostic of the degree of charge transfer for a TCNQF4 moiety partner of a CT system. Through a combined use of a RHF-CNDO/S(CT) electronic calculation and of the eigenvectors of the radical anion given by the NCA, the numerical evaluation of the linear e–mv coupling constants is carried out. The relative values of these constants are extracted, by applying the dimer model, from the vibronic features of the infrared spectrum of the low temperature phase of Rb–TCNQF4. This compound is recognized, by comparison with the alkali salts of TCNQ, to display a phase transition at T≂130 K from a regular to a dimerized stack structure. From the analysis of all these data it is possible to characterize the spectroscopic vibronic effects related to e–mv coupling in CT systems of TCNQF4. This is successfully verified by the segregated dimerized stack system of DBTTF–TCNQF4 and applied to the not yet characterized system TSF–TCNQF4 which is recognized here as a fully ionic complex with segregated dimerized stack motif. As a consequence, through the vibrational spectra, TCNQF4 can be fruitfully used as a sensitive probe for structural and electronic characterization of its CT complexes.
A numerical investigation of nematic ordering based on a simple hard-rod model
Authors: Lee Sin-Doo
Published: July 2002

On the basis of a simple functional scaling, we have constructed the direct generalization of the Carnahan–Starling equation for the free energy of a hard sphere fluid to a nematic liquid crystal. The orientational degrees of freedom are completely decoupled from translational ones under that scaling, which is equivalent to the well-known decoupling approximation for the pair correlation function of hard-core fluids. For long rods the generalized free energy reduces exactly to Onsager’s in the low density limit. Numerical calculations have been performed for a variety of the length-to-diameter ratios of hard spherocylinders, with the aid of an accurate iterative solution to the nonlinear integral equation for the orientational distribution function. The predictions made by our calculations are in fairly good agreement with the results of Monte Carlo simulations on a system of rather short rods. We present the calculations of the jump in the order parameter, critical packing fraciton, and the other thermodynamic properties at the isotropic–nematic transition.
CO chemisorption on free gas phase metal clusters
Authors: Cox D. M. , Reichmann K. C. , Trevor D. J. , Kaldor A.
Published: July 2002

Pulsed fast flow reactor techniques have been used to study the reactivity of CO toward clusters of many different transition metals; V, Fe, Co, Ni, Cu, Nb, Mo, Ru, Pd, W, Ir, and Pt as well as Al, for clusters containing up to 14 atoms. Clusters are produced by pulsed laser vaporization of metal substrates, injected into the reactor, formed into a molecular beam, and detected by photoionization time of flight mass spectrometry. Our results show that CO is readily chemisorbed on most transition metal clusters containing five or more metal atoms, and that the reactivity for larger clusters varies by a factor of 2 or 3, depending on both cluster size and metal type. Depending on the metal, certain atoms, dimers, trimers, and tetramers exhibit little evidence of reactivity toward CO. This observation is explained in terms of a competition between unimolecular decomposition and collisional stabilization, and leads to a prediction of the ordering of the metal–CO bond strengths.
Influence of H2O2 internal motion on scalar and vector properties of OH photofragments
Authors: Grunewald A. U. , Gericke K.-H. , Comes F. J.
Published: July 2002

The formation of ground state OH(X) radicals from the photolysis of jet cooled H2O2 at 193 nm is studied by Doppler and polarization spectroscopy. The features of the process are characterized by a complete analysis of the scalar and vector properties of the fragments. In the dissociation process 85% of the available energy is released into fragment translation. The remaining part emerges as rotational excitation that performs a narrow Gaussian-like distribution peaking at N=12 with a FWHM of ΔN≂5. The vector correlations between the transition dipole moment μ of the H2O2 and recoil velocity v as well as angular momentum J of the products were evaluated in terms of four bipolar moments. The observed 〈μ ⋅ v〉 correlation was used to determine the state specific contribution of both the Ã 1A and B̃ 1B dissociative states to the overall product rotational distribution. On the average, 65% of the OH fragments are formed via the 1A state. A comparison of data obtained from the photolysis of room temperature and jet cooled H2O2 molecules indicates that transfer of parent rotation causes a symmetric broadening of the product distribution and a small increase in the 〈v ⋅ J〉 correlation [βvJ(T≂20 K)=0.5, βvJ(T=300 K)=0.7] of the fragments. In order to describe the influence of initial parent motion on the product state distribution and on vector correlations a model is used where the formation of two OH radicals in the same microscopic event is considered.
Electronic dephasing, vibrational relaxation, and solvent friction in molecular nonlinear optical line shapes
Authors: Yan Yi Jing , Mukamel Shaul
Published: July 2002

The role of solvation dynamics in molecular nonlinear optical line shapes is analyzed using a reduced description based on the time evolution of the density matrix in Liouville space. Langevin equations are used to treat the coupling of the solvent to the molecular electronic and nuclear degrees of freedom. Electronic dephasing is calculated using a solvation coordinate which satisfies a generalized Fokker–Planck equation, and vibrational relaxation is related to the solvent viscosity and friction. The combined effect of both processes is incorporated into appropriate multitime correlation functions which are evaluated using a Liouville-space generating function. The present eigenstate-free approach is particularly suitable for calculating spectral line shapes as well as rate processes (isomerization, electron transfer) of large polyatomic molecules in condensed phases.
The vibrational spectra of molecular ions isolated in solid neon. II. O+4 and O−4
Authors: Thompson Warren E. , Jacox Marilyn E.
Published: July 2002

When a relatively concentrated Ne:O2 sample is codeposited at ∼5 K with a beam of excited neon atoms, prominent infrared absorptions appear which are assigned to O+4 and O−4. Absorptions of O3 and O−3 are also present, and their product distributions in isotopic substitution experiments indicate that O-atom production and reaction is a minor channel in this experimental system. Detailed isotopic substitution experiments require that both O+4 and O−4 possess two equivalent O2 units. Analysis of the isotopic shifts strongly favors a planar trans configuration (C2h) for both molecules. Several combination bands of O+4 are observed, and give evidence regarding the position of ν1(ag), which is infrared inactive, and regarding perturbations by combinations of low-frequency fundamentals. The mechanism of photodestruction of the ions in this system is also considered.
Mass spectrometric evidence for icosahedral structure in large rare gas clusters: Ar, Kr, Xe
Authors: Miehle W. , Kandler O. , Leisner T. , Echt O.
Published: July 2002

Clusters of argon, krypton, and xenon are grown in a free jet and ionized by electron impact. The size of these clusters, (Rg)+n, extends up to n≂1000. Individual cluster sizes are mass resolved up to n≂570 in the case of Ar+n. The well known, but puzzling differences in the size distributions of Kr and Xe clusters disappear beyond n≂130, while those between Ar and Xe disappear beyond n≂220. The most pronounced ‘‘magic numbers’’ in the distributions of large cluster ions occur at n=147 (148 for Ar), 309, and 561, in striking agreement with the number of atoms required to build icosahedral clusters with 3, 4, and 5 complete coordination shells, respectively. Closure of the 6th icosahedral coordination shell is indicated by another strong intensity drop at n≂923 in the unresolved part of the spectra. Several additional intensity extrema are observed between major shell closures. A simple structural model, assuming an icosahedral core decorated by the additional atoms, accounts for these anomalies reasonably well up to n=561.
Quantum scattering studies of electronically inelastic collisions of CN (X 2Σ+, A 2Π) with He
Authors: Werner Hans-Joachim , Follmeg Bernd , Alexander Millard H. , Lemoine Didier
Published: July 2002

Using recent ab initio interaction potential energy surfaces for the CN (X 2Σ+, A 2Π)+He system [H.-J. Werner, B. Follmeg, and M. H. Alexander, J. Chem. Phys. 89, 3139 (1988)], we have calculated fully quantum cross sections for inelastic transitions between individual rovibrational levels of the A 2Π and the X 2Σ+ states of CN. We have concentrated on the transitions studied experimentally by Dagdigian and co-workers for CN+Ar, namely transitions between the rotational levels of the A, v=8 and X, v′=12, the A, v=7 and X, v′=11, and the A, v=3 and X, v′=7 vibrational manifolds. In the case of the 8→12 and 7→11 transitions the cross sections are large (0.1–1 Å2), and the dependence on initial Λ doublet level and on final rotational quantum number displays the same subtle alternations as seen experimentally. In the case of the 3→7 transitions, for which the vibrational levels are energetically much more separated, the calculated cross sections for CN+He are extremely small (10−5 Å2), far smaller than observed experimentally for CN+Ar. In order to resolve this discrepancy, we have carried out some additional ab initio calculations for the CN+Ar system, but the change in the interelectronic coupling potential appears not to be large enough to explain the magnitude of the experimental cross sections.
Quenching of N(2D) by O(3P)
Authors: Fell C. , Steinfeld J. I. , Miller S.
Published: July 2002

A definitive measurement of the rate coefficient for the quenching of N(2D) by O(3P) is reported. The O(3P) atoms were generated by titrating NO directly into the active nitrogen flow. Analysis of the results required that the rate coefficient for the reaction N(2D)+NO→N2+O be known accurately, and this was also determined. A finite mixing time correction is also necessary. The best estimate of the rate coefficient from this work is (6.9+0.7, −1.1)×10−13 cm3 s−1 at T=298 K, considerably smaller than a previous measurement [J. Phys. Chem. 92, 5977 (1988)] and in much better accord with values required by atmospheric models.
Unitary group approach to reduced density matrices
Authors: Gould M. D. , Paldus J. , Chandler G. S.
Published: July 2002

A fully spin-adapted approach to many-electron density matrices is developed in the context of the unitary group approach to many-electron systems. An explicit expression for the single-electron spin-density operator, as a polynomial of degree two in the orbital U(n) generators, is derived for the case of spin-independent systems. Extensions to spin-dependent systems are also considered, leading to the appearance of total-spin transition densities, whose general properties are investigated. A corresponding formalism for the two-electron density matrix, which is capable of further generalization, is also developed. The results of this paper, together with recent developments on the matrix elements of the U(2n) generators in the electronic Gel’fand basis, afford a versatile method for the direct calculation of one- and two-body density matrices in the unitary group approach framework.
Correlated Monte Carlo wave functions for the atoms He through Ne
Authors: Schmidt K. E. , Moskowitz J. W.
Published: July 2002

We apply the variational Monte Carlo method to the atoms He through Ne. Our trial wave function is of the form introduced by Boys and Handy. We use the Monte Carlo method to calculate the first and second derivatives of an unreweighted variance and apply Newton’s method to minimize this variance. We motivate the form of the correlation function using the local current conservation arguments of Feynman and Cohen. Using a self-consistent field wave function multiplied by a Boys and Handy correlation function, we recover a large fraction of the correlation energy of these atoms. We give the value of all variational parameters necessary to reproduce our wave functions. The method can be extended easily to other atoms and to molecules.
A classical theory of pump–probe photodissociation for arbitrary pulse durations
Authors: Fried Laurence E. , Mukamel Shaul
Published: July 2002

The classical Condon picture for linear absorption spectroscopy is a time-honored approximation known to give accurate results for broad spectral lines. The generalization of the classical Condon picture to nonlinear spectroscopies, however, is a problem under current investigation. Bersohn and Zewail have proposed a phenomenological formula applicable to the pump–probe photodissociation of iodine cyanide (ICN). We undertake an investigation of classical approximations in pump–probe spectroscopy from first principles. We present a new classical formula for the probe absorption that is derived using the density matrix in phase space and is valid for arbitrary pulse durations. Conditions whereby the signal depends only on field spectral or temporal intensities are specified. The classical formula presented here is compared with exact quantum mechanical calculations of the probe absorption for a simple model of ICN. Our formula shows very good agreement with the quantum calculations. We recover the formula of Bersohn and Zewail in the case of pulses that are very short compared to the time scale of nuclear dynamics.
Laser-induced fluorescence in the B̃–X̃ system of the HCO radical
Authors: Sappey Andrew D. , Crosley David R.
Published: July 2002

The spectroscopy of the B̃ 2A′–X̃ 2A′ system of the formyl radical has been studied by laser-induced fluorescence. HCO was generated by photolysis of acetaldehyde, and a tunable laser operated near 245 nm excited eight bands of B̃–X̃. The (0,0,2)–(0,0,0) band has been rotationally analyzed, yielding A′=14.46 cm−1 and (B′+C′)/2=1.13 cm−1 for this slightly asymmetric top; asymmetry splitting and spin doubling are observed. The intense branches have ΔK=0 but there also are weaker perpendicular components with the transition moment near the b axis. Vibronic transition energies agree with those from matrix absorption but with a 130 cm−1 blueshift. Resolved fluorescence spectra to X̃ levels as high as 15 000 cm−1 furnish vibrational constants for the ground state.
Chemical reactivity of iron atoms near room temperature
Authors: Mitchell S. A. , Hackett P. A.
Published: July 2002

The reactivity of ground state iron atoms with respect to atom transfer and adduct formation reactions with a variety of simple molecules in Ar buffer gas near room temperature has been investigated. Iron atoms are produced by visible multiphoton dissociation of iron pentacarbonyl or ferrocene, and their removal by added gases under pseudo-first-order conditions is monitored by resonance fluorescence excitation at variable time delay following the photolysis pulse. Upper limits for second-order rate constants for reaction of ground state iron atoms with O2, CO, H2O, (CH3)2O, C2H2, C2H4, N2O, C2H4O, and CF3Cl at room temperature and 100 Torr total pressure are estimated to be in the range (2–10)× 10−15 cm3 molecule−1 s−1, which corresponds to reaction probabilities of less than ≂10−5 per hard sphere collision. Pseudo-second-order rate constants in the range (2–60)× 10−14 cm3 molecule−1 s−1 are found for 1:1 adduct formation reactions of ground state iron atoms with C6H6, 1,3-butadiene, NH3, and NO. The formation of such complexes at room temperature implies that the binding energies are greater than ≂7 kcal mol−1.

For the complex FeNH3, the kinetic data allow measurements of dissociation equilibrium constants at 278.5 and 295 K, from which a binding energy ΔH°0 = 7.5 ± 1 kcal mol−1 is derived. Measurements of the dependence of the pseudo-second-order rate constant for formation of FeNO on the pressure of Ar buffer gas at 296 K allow estimates to be made of the limiting low- and high-pressure recombination rate constants: k0 =2.3×10−32 cm6 molecule−2  s−1 and k∞=1×10−12 cm3 molecule−1 s−1. Comparisons are made between the present results which indicate a remarkably low reactivity for iron atoms, and previously reported results for reactions of iron atoms in low temperature matrices and gas phase reactions of other transition metal atoms. An interpretation of our results is given in terms of schematic potential energy curves which are based on the results of previous ab initio calculations for the complex FeCO. The qualitative features of these curves are consistent with low reactivity of ground state iron atoms with respect to adduct formation due to repulsive ground state interactions and low probabilities for transitions from repulsive to attractive potential surfaces.
Angle-resolved photoelectron spectroscopy of the core levels of N2O
Authors: Schmidbauer M. , Kilcoyne A. L. D. , Randall K. J. , Feldhaus J. , Bradshaw A. M. , Braunstein M. , McKoy V.
Published: July 2002

We have measured photoionization cross sections and photoelectron asymmetry parameters for each of the core levels of N2O. We have also carried out frozen- and relaxed-core Hartree–Fock studies of these cross sections so as to better understand the underlying shape resonant structure and the role of electronic relaxation in these processes. A broad shape resonance is observed in each of the core-hole cross sections at 10-20 eV kinetic energy and there is some evidence of a second shape resonance near the thresholds, an energy region which is not accessible experimentally. The cross sections also exhibit site-specific behavior with maxima at widely separated photoelectron kinetic energies. These differences probably arise from the fact that photoelectron matrix elements for different core orbitals probe different regions of the shape resonant orbital which extends over the entire molecule. Although the higher energy shape resonances appear quite similar, Hartree–Fock studies show that the central nitrogen resonance is more sensitive to effects of electronic relaxation than the terminal nitrogen or oxygen resonances. Large differences are also seen between the photoelectron asymmetry parameters for the central and terminal atoms.
A molecular dynamics study of the far infrared spectrum of liquid water
Authors: Guillot Bertrand
Published: July 2002

The far infrared spectrum of liquid water at room temperature is calculated by molecular dynamics simulation over the spectral range 0.5–1000 cm−1. It is shown that the experimental absorption intensity can be reproduced satisfactorily provided that the dipole induced dipole mechanism is conveniently implemented in the calculation and the classical profile corrected for quantum effects. The contribution due to exchange overlap dipoles between O and H atoms is also investigated but its role in the genesis of the far infrared (FIR) spectrum is negligible. Although the dipole induced dipole (DID) mechanism is found to be responsible for the peculiar band shape near 200 cm−1 by revealing the intermolecular oscillations of the hydrogen bond network, no other translational band is detected in the region 10–60 cm−1, a result in contradistinction with data put forward recently. Moreover, it is shown that the absorption spectrum is the seat of various cancellation effects between permanent and induced dipoles, effects which are described in detail.
Transport-collisional master equations for termolecular recombination as a function of gas density
Authors: Flannery M. R.
Published: July 2002

Sets of transport-collisional master equations are developed for the microscopic distribution n(R, E, L) of pairs over internal separation R, energy E, and orbital angular momentum L of (A–B) pairs in a background gas M of variable density. Expressions are also provided for the rate of recombination of A and B as a function of gas density in transport and collisional forms which, respectively, involve microscopic probabilities for association of dissociated (A–B) pairs and probabilities for collisional stabilization of bound pairs. Analytical solutions for the pair distributions n and microscopic probabilities for recombination are obtained in the classical absorption limit. They pertain to exact (A–B) trajectories under general symmetric interaction V(R) between A and B and are applied to ion–ion and electron–ion collisional recombination in a gas. A classical variational method is also presented. Useful expressions for the segments of hyperbolic and general trajectories enclosed by a sphere are derived in an Appendix.
Slit-jet near-infrared spectroscopy and internal rotor dynamics of the ArH2O van der Waals complex: An angular potential-energy surface for internal H2O rotation
Authors: Lascola Robert , Nesbitt David J.
Published: July 2002

Near-infrared vibration-rotation spectra of jet-cooled Ar-H2O complexes are detected for the first time via direct absorption of tunable difference frequency infrared radiation in a slit supersonic expansion source. Transitions from both the lowest para and ortho complexes are observed which correlate to 000 and 101 rotational levels of free H2O, respectively, and permit spectroscopic characterization of the complex in both the ground (ν3=0) and asymmetric stretch excited (ν3=1) levels. From hot bands involving excited internal rotor states correlating with the 101 H2O rotational level, we determine the Σe(101)-Πf(101) energy splitting to be 11.3333(3) cm−1 (J=1). In conjunction with far-infrared measurements of the Σ(110)-Π(101) and Π(110)-Σ(101) energy splittings, this information permits determination of a two-dimensional (2D) angular potential-energy surface of the complex as a function of the H2O orientation. The barriers to internal rotation of an ArH2O differ for in-plane (19 cm−1) and out-of-plane (33 cm−1 ) rotation of the H2O subunit. However, both barriers are only slightly higher than the ground-state energies and, hence, the behavior of H2O in the complex is that of a nearly free rotor. Agreement with recent ab initio calculations by Chalasinski et al. [J. Chem. Phys. 94, 2807 (1991)], as well as with 3D fits solely to far-infrared data by Cohen and Saykally [J. Chem. Phys. 94, 7991 (1991)], is remarkably good. Predictions based on this angular potential-energy surface are made for the internal rotor states of ArHDO and ArD2O and compared with recent far-infrared measurements by Suzuki et al. [J. Phys. Chem. 94, 824 (1991)].
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o theoretical study of small GaAs clusters
Authors: Graves Richard M. , Scuseria Gustavo E.
Published: July 2002

Theoretical calculations for the closed-shell ground state of small GaxAsy clusters (x=y; x=2–4) are carried out at the self-consistent field (SCF) Hartree–Fock level of theory, using analytic energy gradients for rapid geometry optimization. In addition, for Ga2As2, the SCF results are compared with theoretical predictions obtained at the coupled cluster level of theory including all single, double and perturbative triple excitations, CCSD(T). The equilibrium structures for Ga2As2, Ga3As3, and Ga4As4 are found to be of D2h, C1, and Ci symmetry, respectively. The cluster binding energies with respect to GaAs dimers (3Σ−) and Ga(2P)+As(4S) atoms are also obtained.
On the rotational diffusion of asymmetric molecules in liquid crystals
Authors: Tarroni R. , Zannoni C.
Published: July 2002

We present a nonperturbative solution of the rotational diffusion equation for an asymmetric rotor reorienting in a uniaxial liquid crystal, such as a nematic, smectic A or membrane bilayer covering the full range of order parameters from isotropic to complete order for rod-like and plate-like molecules. We obtain explicit expressions for the rotational diffusion matrix elements for a potential containing arbitrary rank interactions. For the actual numerical computations we consider the rotor with biaxial diffusion tensor to be subject to a second rank biaxial orienting potential. We study in detail the influence of changes in diffusion tensor and ordering matrix biaxiality on correlation functions and correlation times of first and second rank and we comment on experiments that can be sensitive to biaxiality effects in the diffusion tensor. We give a comparison with approximate results that have appeared in the literature.
Oscillatory CO oxidation on Pt(110): Modeling of temporal self-organization
Authors: Krischer K. , Eiswirth M. , Ertl G.
Published: July 2002

The parameters entering the kinetics for the mechanism of catalytic CO oxidation have been adapted for a Pt(110) surface, giving rise to a two-variable model correctly predicting bistability. Oscillations are obtained when, in addition, the adsorbate-driven 1×2–1×1 structural phase transition of Pt(110) is taken into account. Mixed-mode oscillations can be qualitatively explained by including the faceting of the surface as a fourth variable. The limitations of the model essentially stem from the fact that only ordinary differential equations have been analyzed so far neglecting spatial pattern formation. It is discussed which dynamic phenomena observed experimentally in the CO oxidation on Pt(110) will probably not be adequately describable without taking spatial effects into account.
Gas phase solvation of Na+ with methanol
Authors: Selegue Thomas J. , Moe Neil , Draves Jeffrey A. , Lisy James M.
Published: July 2002

Cluster ions of the form Na+(CH3OH)N, N=3–25 have been studied using experimental and theoretical methods. The cluster ions were prepared in a molecular beam by combining a thermionic alkali ion emitter with a continuous expansion of methanol in argon, and were found to contain a substantial amount of internal energy. A cw CO2 laser was used to record the vibrational spectrum of mass-selected cluster ions in the 1020–1060 cm−1 region. Information on the stepwise solvation of the sodium ion by methanol is gained by comparing changes in the spectra as a function of cluster size. The first solvation sphere of the sodium ion is occupied by six methanol molecules. Further solvent shell structure is seen for N>6, including evidence of methanols resembling ‘‘bulk’’ solvent. The microscopic structure was investigated by Monte Carlo simulations of Na+(CH3OH)N, N=6–24. Radial distribution functions display clear minima that indicate the spatial extent and occupation numbers of solvation shells. No hydrogen bonding takes place between methanol molecules in the first solvent shell but is instrumental in determining the framework of the rest of the cluster ion. The internal energies of the cluster ions were estimated using time-of-flight measurements and calculations within the evaporative ensemble formalism.
State-to-state differential cross sections for rotationally inelastic multiplet-conserving and multiplet-changing collisions of NO(2Π1/2, j=0.5) with Ar
Authors: Jons Steven D. , Shirley Jeffrey E. , Vonk Matthew T. , Giese Clayton F. , Gentry W. Ronald
Published: July 2002

Crossed, pulsed molecular beam measurements of state-to-state differential cross sections have been carried out for scattering of NO(2Π1/2, j=<cf10;1;2≳) by Ar, at a kinetic energy of 442 cm−1. Differential cross sections are presented (1) as functions of center-of-mass (c.m.) scattering angle for multiplet-conserving collisions 2Π1/2→2Π1/2 and final states j′=1.5, 12.5, and 14.5, and (2) as functions of final state j′ for both multiplet-conserving and multiplet-changing collisions 2Π1/2→2Π3/2, at a constant c.m. scattering angle of 50°. Rotational rainbow structure is seen in both types of data. Some of the results are compared with the coupled-states calculations of Joswig, Andresen, and Schinke, J. Chem. Phys. 85, 1904 (1986).
Comparison of quasiclassical and quantum dynamics for resonance scattering in the Cl+HCl→ClH+Cl reaction
Authors: Jakubetz W. , Sokolovski D. , Connor J. N. L. , Schatz G. C.
Published: July 2002

We present the results of quasiclassical trajectory (QCT) and quantum centrifugal sudden hyperspherical (CSH) scattering calculations for the Cl+HCl→ClH+Cl reaction using a semiempirical potential energy surface. In particular, we report state-to-state integral and differential cross sections in the vicinity of a transition state resonance that occurs at a total energy E of 0.642 eV. This resonance, which is labeled by the transition state quantum numbers (0,0,2), strongly perturbs the cross sections for the initial rovibrational state HCl(v=1, j=5), which was therefore considered in all our calculations. For E≥0.680 eV, which is well removed from the resonance energy, the QCT and CSH results are in good agreement, but for E near the resonance energy, important quantum effects are found in the integral cross sections, product state distributions, and differential cross sections. The CSH integral cross sections show smooth steplike increases for E≊0.642 eV, which are not seen in the QCT results. Associated with these steps are increased branching to the v′=0 product HCl vibrational state, and a strong propensity for the production of rotational states with j′=15 and 16 for v′=0. These features of the product energy partitioning are not present in the QCT results, although the correct rotational distributions are approximately recovered if the final vibrational action is constrained to match its quantum value. The CSH differential cross sections show a sudden shift from backward to sideward scattering between 0.642 and 0.660 eV, while the QCT cross sections remain backward peaked. An analysis of the ‘‘number of atom–diatom encounters,’’ during the course of a reactive collision, shows that there are chattering trajectories. These are associated with sideward scattering, but their probability is low and as a result they do not produce distinct features in the angular distributions. However, if the classical deflection function is weighted by the quantum reaction probability, angular distributions are obtained that are in reasonable agreement with the CSH angular distributions (including resonance features).
Square-well and square-shoulder fluids: Simulation and equations of state
Authors: Heyes D. M. , Aston P. J.
Published: July 2002

We explore the properties of a simple analytic equation of state, proposed by Heyes and Woodcock [D. M. Heyes and L. V. Woodcock, Mol. Phys. 59, 1369 (1986)] which is a development of the van der Waals equation of state, improved with an analytically simple yet accurate hard-core component for the compressibility factor. We call this the hard-sphere van der Waals (HSvdW) equation of state. We show, for the first time, that the HSvdW equation of state gives analytic values for the critical parameters and that it gives significantly more realistic values for the critical density than the van der Waals and other cubic equations of state. For single-component square wells of arbitrary well width, the HSvdW equation of state also produces a critical temperature and pressure in agreement with those from a recent Gibbs Monte Carlo simulation study. The chief advantage over a similar equation of state with an alternative form for the hard-core compressibility factor, such as Carnahan–Starling, is that it leads to more tractable analytic expressions for thermodynamic properties without loss of accuracy.

Extensive Monte Carlo computer simulations of square-well (SW) and square-shoulder (SS) fluids with variable well width have been performed. We have calculated the compressibility factor, internal energy, pair distribution functions, and coordination number (average number of particles within the soft-shell interaction range). We have also determined the gas–liquid coexistence curve using the particle insertion chemical potential method for the square-well case, λ=1.5σ, where λ is the diameter of the soft interaction shell. This agrees well with the Gibbs ensemble simulation coexistence curve. For the purpose of improving the attractive component of the HSvdW equation of state, we have made use of the SW and SS simulation data and that of other workers to develop another new simple equation of state based on the quasichemical local coordination number approximation (QCA) which is a lattice model of a fluid. The QCA is improved to include the local coordination of the hard-sphere fluid. Present adaptations extend the range of earlier QCA models to arbitrary well width and sign of the well (either attractive well or repulsive shoulder). The simulation coordination numbers are reproduced well by the present model, although there is little improvement in the overall equation of state over the simpler HSvdW.
Angular-momentum resolution in transitional-mode state counting for loose transition states
Authors: Smith Sean C.
Published: July 2002

The classical evaluation of the angular-momentum-resolved sum of states for the loosely hindered rotational degrees of freedom, i.e., the transitional modes, in loose transition states occurring in unimolecular dissociation, radical–radical recombination, ion–molecule, and other collision-complex-forming bimolecular reactions is considered. Exact analytic expressions are derived for the momentum-space volume available to the transitional modes at a given configuration q with energy E and total angular momentum vector J. The results are completely general with respect to the type of fragment rotors involved and their relative orientation within the loose transition state, and constitute a dramatically simplified technique for J-resolved classical state counting. The utility of the expressions lies in the fact that they obviate the necessity of numerical integration over the system’s momentum space, thus reducing substantially the computational effort involved in the exact evaluation of the transitional-mode sum of states. The present results verify expressions which were postulated to apply to arbitrary configurations in our earlier work.
Induced dipole moment and atomic charges based on average electrostatic potentials in aqueous solution
Authors: Gao Jiali , Luque F. J. , Orozco Modesto
Published: July 2002

A Monte Carlo quantum mechanical and molecular mechanical (QM/MM) simulation method using the combined Austin model 1 (AM1) and TIP3P potential is proposed to evaluate the average molecular electrostatic potentials (MEP) in aqueous solution. It was demonstrated that the average MEP in water can provide a quantitative account of the solvent polarization effect on molecular dipole moments and atomic charges. The computed induced dipole moments from the Monte Carlo AM1/TIP3P simulation method were found to be in good agreement with those predicted by continuum self-consistent reaction field theory using the AM1 and 6-31G(d) wave functions. The findings also indicate that the minimal basis set adopted by the AM1 method can capture the solvent effect effectively. Finally, good correlations in the MEP-fitted charges were obtained between different methods. These results should be useful for developing polarizable intermolecular potential functions (PIPF).
Spin-dependent unitary group approach to the Pauli–Breit Hamiltonian. I. First order energy level shifts due to spin–orbit interactions
Authors: Gould M. D. , Battle J. S.
Published: July 2002

In this paper, we investigate the application of the spin-dependent unitary group approach to many-electron systems in the Born–Oppenheimer and Pauli–Breit approximation. The unitary group approach form of the Pauli–Breit Hamiltonian is determined explicitly, which should enhance the usefulness of the spin-dependent formalism in applications. It is applied to determine the first-order energy level splitting due to spin–orbit and spin–other orbit interactions entirely in the unitary group framework. In particular, this work indicates the feasibility of a spin-dependent unitary group approach perturbation theory for the efficient calculation of spin-dependent operators. Our results are also discussed briefly in terms of the unitary group density matrix formalism. The energy level shifts due to spin–spin interaction will be investigated within the unitary group approach in the second paper of the series.
Optical properties of a chromophore embedded in a rare-gas cluster: Cluster size dependence and the approach to bulk properties
Authors: Adams John E. , Stratt Richard M.
Published: July 2002

One of the most intriguing aspects of the behavior of small clusters is the extent to which their physical and chemical properties depend sensitively on the size of the clusters. But for clusters that are ‘‘large enough,’’ surely their properties must approach those of the corresponding bulk systems. The property of special interest in the present work is the shift in the electronic absorption of a chromophore (benzene) deriving from interaction with nonpolar solvent atoms (Ar), a shift that can be calculated using a microscopic formalism described in this and in the preceding paper which is equally well suited to the study of cluster and condensed phase environments. We are able to identify the evolution of the collective character of the dielectric response of the solvent atoms as being the key feature of the optical properties of these clusters that determines the degree to which their behavior is bulklike. Furthermore, we can associate specific spectral features with particular classes of cluster geometries, and in doing so we derive support for our previous speculations concerning the evidence for the contribution of metastable, nonwetting cluster structures to the experimental spectra.