bluuebunny/crossref_metadata_2025 huggingface dataset:
Exchange coupling through spin density wave Cr(001) using Fe whisker substrates
Authors: Heinrich B. , Cochran J. F. , Monchesky T. , Urban R.
Published: July 2002

Exchange coupling through spin density wave in Fe whisker/Cr/Fe(001) structures was studied by Brillouin light scattering (BLS) and longitudinal magneto-optical Kerr effect (MOKE) techniques. It will be shown that interface alloying at the Fe whisker/Cr interface profoundly affects the behavior of short wavelength oscillations. The first crossover to antiferromagnetic coupling occurs at 5 monolayers (ML), the phase of short-wavelength oscillations is reversed compared to that expected for the spin density wave in Cr(001), and the strength of coupling is significantly decreased from that obtained from first principle calculations. Using Cu and Ag atomic layers between the Cr(001) and Fe(001) films, heterogeneous interfaces showed that the exchange coupling in Cr(001) is strongly affected by electron multiple scattering. It appears that electron quantum well states in the Fe film play no important role in the strength of the exchange coupling when the Fe film is bounded on one side by Au, but they become important when the Fe film is bounded by Cr on both sides.
Magnetic properties of Fe-based alloys
Authors: Schumann F. O. , Tobin J. G.
Published: July 2002

The magnetic properties of fcc FexNi1−x and FexCo1−x alloys grown on Cu(100) were investigated in an element-specific fashion. The technique employed was linear dichroism in photoemission, which by varying the chirality can also determine the magnetization axis. We observed a different behavior for the two alloys at Fe concentrations above 60%. At this concentration the FexNi1−x alloy shows a strong reduction of the Fe dichroism associated with the invar instability. This is in contrast to the FexCo1−x alloy, where the Fe dichroism stays essentially constant across the concentration. Despite these differences both systems show a change of the easy axis at roughly the same electron count. For small Fe concentrations the easy axis is in-plane along the [011] direction. This changes into the [001] direction at Fe63Ni37, which is at 0.7 excess electrons per atom when compared with Fe. This is different to the bulk, where a change occurs at Fe25Ni75. We find the easy axis change for FexCo1−x to occur at 49% Fe. This would be equivalent to 0.5 excess electrons when compared with Fe.
Magneto-optic Kerr effect study of a two-step reorientation transition of an ultrathin magnetic film
Authors: Arnold C. S. , Pappas D. P. , Popov A. P.
Published: July 2002

The magnetic reorientation transition for ultrathin Fe films on Gd is studied. At low temperatures, the films are magnetized in plane and at intermediate temperatures they undergo a reorientation to out of plane. The reorientation is attributed to the reducing magnetization of Gd as it approaches its Curie temperature and the perpendicular magnetic anisotropy at the Fe surface. Detailed measurements of the reorientation were made in situ using the magneto-optic Kerr effect (MOKE). The first step, at low temperature, is a continuous, reversible reorientation of the surface moment from in plane to canted out of plane; a corresponding peak in the susceptibility identifies it as a second-order phase transition. The second step is a discontinuous, irreversible rotation from this canted direction to perpendicular to the film plane; thermal hysteresis of the magnetization identifies it as a first-order phase transition. The MOKE thermal hysteresis loop shows that the Gd surface region participates in the first-order phase transition.
Point-contact studies of current-controlled domain switching in magnetic multilayers
Authors: Myers E. B. , Ralph D. C. , Katine J. A. , Albert F. J. , Buhrman R. A.
Published: July 2002

We present measurements demonstrating current-induced magnetic domain switching, and also other magnetic excitations, in point-contact devices containing alternating ferromagnetic (F) and noble metal (N) layers, for perpendicular currents ∼109 A/cm2. For F/N/F trilayers in which one F layer is much thinner than the other, we can controllably switch the magnetic moments in the two F layers parallel with a current bias of one sign, and switch them antiparallel with a reversed current. For thicker magnetic films, and for thin films in the presence of a saturating magnetic field, we observe nonhysteretic current-induced changes in resistance, which can be understood as current-induced spin-wave excitations. These observations are in agreement with a model of current-induced magnetic reorientations caused by local exchange forces between conduction electrons and the magnetic moments.
Effect of temperature on the dissociative and nondissociative electron attachment to C3F8
Authors: Spyrou S. M. , Christophorou L. G.
Published: July 2002

The rate constant for total electron attachment to C3F8 has been measured as a function of the mean electron energy in the range 0.76–4.81 eV and the temperature T in the range 300–750 K using a swarm apparatus; it was found to first decrease and then increase with increasing T. The former behavior is due to nondissociative and the latter due to dissociative electron attachment processes whose rate constants, respectively, decrease and increase with increasing T. The measured dependence of the total attachment rate constant on the gas pressure and temperature was used to quantify the contribution of dissociative and nondissociative electron attachment in C3F8 as a function of electron energy.
The electronic absorption spectra of ClCN+, BrCN+, and ICN+ in neon matrices
Authors: Leutwyler Samuel , Maier John P. , Spittel Uta
Published: July 2002

The optical absorption spectra of the B̃ 2Π3/2 ← X̃ 2Π3/2 transitions of chloro-, bromo-, and iodocyanide cations in neon matrices at 4.5 K have been obtained and vibrationally analyzed. The origin band of the Ã 2Σ+ ← X̃ 2Π3/2 transition of ICN+ is also observed as well as the absorption bands of the CN radical in the spectra of ClCN+ and BrCN+. These features and the vibrational frequency data are discussed.
Clifford algebra unitary group approach to many-electron correlation problem
Authors: Paldus J. , Sarma C. R.
Published: July 2002

Unitary group approach (UGA) to the many-electron correlation problem is generalized by embedding the unitary group U(n) in a much larger group U(2n) via the rotation groups SO(m) with m=2n or 2n+1 and their covering group Spin (m). Exploiting the spinorial Clifford algebra basis associated with Spin (m), it is shown that an arbitrary N-electron configuration state can be represented as a linear combination of two-box Weyl tableaux of U(2n), and the explicit representation for U(n) generators as simple linear combinations of U(2n) generators is given. The problem of U(n) generator matrix element evaluation for two-column irreducible representations then reduces to an elementary problem of evaluation of generator matrix elements for the totally symmetric two-box representation of U(2n). Thus a general N-electron problem is effectively reduced to a number of two-boson problems. The proposed formalism also enables us to exploit other than Gelfand–Tsetlin coupling schemes and particle nonconserving operators.
On the use of the Breit–Pauli approximation for evaluating line strengths for spin-forbidden transitions. II. The symbolic matrix element method
Authors: Yarkony David R.
Published: July 2002

A procedure for constructing HsoCI using the symbolic matrix element method has been developed. Here Hso is the spin-orbit portion of the Breit–Pauli interaction and CI satisfies (H0−E0I) CI=O, where H0 is the nonrelativistic electronic Born–Oppenheimer Hamiltonian. When this procedure is used in conjunction with a recently developed algorithm for the determination of the corresponding first order perturbed wave function WI =VI +CI, using (H0 −E0I) VI =HsoCI [J. Chem. Phys. 83, 1168 (1985)] line strengths for spin-forbidden radiative processes can be determined for the large configuration state function expansions tenable within the context of direct configuration interaction procedures. Thus for systems for which the Breit–Pauli approximation is valid this methodology will permit determination of lifetimes for spin-forbidden transitions with an accuracy previously available only for spin-allowed processes. The potential of this methodology is illustrated by considering the spin-forbidden radiative process b1Σ+ → X 3Σ− in NF.
Circular intensity differential scattering of light by hierarchical molecular structures
Authors: Patterson Chris W. , Singham Shermila B. , Salzman Gary C. , Bustamante Carlos
Published: July 2002

We show that circular intensity differential scattering (CIDS) of light by macromolecules with different levels of chiral structure is the superposition of the CIDS from each individual level. As an example, we treat a model superhelix with anisotropic polarizability.
Thermodynamics of the hydride chemical heat pump: Model (I)
Authors: Dantzer P. , Orgaz E.
Published: July 2002

The efficiencies of a solid absorption heat pump are calculated with the help of chemical models. The first one is based on a reversible analysis where only the energy contributions to regenerate and insure continuous operations of the engine are taken into account. In the second one, the complete thermochemical behavior is described following the formalism of classical thermodynamics, in which no constraints on time or rate are allowed. Chemical constraints only have been introduced; they are derived from equilibrium studies. This model has been developed for two operating modes: Isotherm or isobar, and has been tested with the LaNi4.77Al0.22/LaNi5 system. The calculated efficiencies amount to 80% to 90% of the Carnot efficiency.
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U(2) coordinate geometry for semiclassical theory of rotors and oscillators
Authors: Harter William G.
Published: July 2002

A spinor coordinate geometry is described which unifies a number of theoretical approaches to rotational and vibrational dynamics. Visualization aids previously developed for describing complex rotor spectra are extended and related to vibrational models. The relation is based upon analogies with optical polarization ellipsometry and spin rotation or precession. Rotational energy (RE) surfaces are used to describe a transition between normal and local modes in the presence of simple forms of Coriolis coupling and anharmonicity.
Ionic conduction in one dimension: A structural study of the hollandite K1.54Mg0.77Ti7.23O16 over the range 133≤T≤919 K
Authors: Weber Hans-Peter , Schulz Heinz
Published: July 2002

Joint probability density functions (pdf) of the mobile K atom (and their corresponding pseudopotentials) have been obtained from seven x-ray (MoKα) intensity data sets collected on a one-dimensional conductor with the hollandite structure (K1.54Mg0.77Ti7.23O16; space group I4/m; Z=1). The data were collected at high resolution [(sin θ/λ)max ≥0.9 Å−1] and at several temperatures (133, 298, 387, 459, 540, 707, and 919 K) in order to determine the conduction mechanism. Two models of the K distribution within the strand of cavities making up the conduction tunnel have been refined. In the most complex model B [Rw(F) =0.02], up to four atomic positions are needed to describe the observed electron density. The pdfs show that—already at room temperature and within undamaged segments of the conduction tunnel—potassium ions diffuse at a high rate across the bottleneck between two neighboring cavities. Below about 430 K the potassium ions preferentially occupy sites shifted off the cavity center in the direction of the bottleneck; above this temperature the cavity center becomes the preferred location. At 298 K the potential barrier at the bottleneck amounts to 0.032(4) eV. In a plot of the cell constants vs T an inflection shows up at 387<T<459 K, indicating a second order transition. Within the same temperature interval we also observe a sharp break in a plot of U33 (refined from the one-atom model A) vs T. A comparative study of two other hollandites K1.92V8O16 and (Ba0.98Ca0.03 Zr0.02) [Al1.1 Ni0.48 Ti6.4] with different tunnel stoichiometry underscores the importance of the values of ionic density and charge in the conduction column to the conductivity.
Dissipation, tunneling, and adiabaticity criteria for curve crossing problems in the condensed phase
Authors: Wolynes Peter G.
Published: July 2002

The role of dissipation in curve crossing phenomena in condensed phases is discussed. Adiabaticity criteria are found in two regimes via path integral arguments. In the high temperature regime a stochastic quantum Langevin approach is developed. In the low temperature region an instanton method is used. In both cases the curve crossing time scale competes with the time scale of molecular motion. Dissipation causes the process to be more likely to be adiabatic.
Dielectric properties of new stable ferroelectric liquid crystals with large spontaneous polarization
Authors: Ozaki Masanori , Yoshino Katsumi , Sakurai Takao , Mikami Naoko , Higuchi Ryo-ichi
Published: July 2002

New series of ferroelectric liquid crystals with two chiral carbons or bulky molecular components at the chiral carbon are synthesized and are found to possess extremely large spontaneous polarization exceeding 2×10−7 C/cm2 and a dielectric constant larger than 7000. These compounds are stable and exhibit a novel dielectric behavior due to the large spontaneous polarization and strong interaction with surfaces.
Phase transitions in resorcinal
Authors: Ebisuzaki Y. , Askari L. H. , Bryan A. M. , Nicol M. F.
Published: July 2002

Optically clear crystals of resorcinal [C6H4(OH)2] were grown and employed in the Raman and calorimetric studies of the phase transitions. The enthalpy and entropy change for the solid–solid (α–β) phase transition at 369±6 K are 1.370±0.007 kJ mol−1 and 3.71±0.05 J K−1 mol−1, respectively. The slope (dp/dT) of the α–β phase boundary at 1 atm is negative (−16.7 atm K−1). The characteristic Raman features of the α–β transition are the disappearance of the librational mode at 102 cm−1 and one of the O–H stretching modes at 3060 cm−1 (298 K and 1 atm), while other vibrations remain continuous in both phases. The pressure induced α–β phase transition at room temperature was observed in a diamond anvil cell at 5 kbar. The enthalpy of fusion of β-resorcinal at 382.8 K is 20.89±0.07 kJ mol−1 and the entropy change is 54.59±0.18 J K−1 mol−1.
Structure and vibrational dynamics of the CO2 dimer from the sub-Doppler infrared spectrum of the 2.7 μm Fermi diad
Authors: Jucks K. W. , Huang Z. S. , Miller R. E. , Fraser G. T. , Pine A. S. , Lafferty W. J.
Published: July 2002

Sub-Doppler infrared spectra of two Fermi resonance coupled bands of carbon dioxide dimer have been obtained at 3611.5 and 3713.9 cm−1 using an optothermal molecular beam color-center laser spectrometer. The band origins for the complexes are red shifted by approximately 1 cm−1 from the corresponding ν1+ν3/2ν02+ν3 CO2 bands. The higher frequency band is perturbed while the lower frequency band appears free of extraneous perturbations as determined from a precision fit to a Watson asymmetric rotor Hamiltonian. This fit and the observed nuclear spin statistical weights reveal that the complex is planar with C2h symmetry. The C--C separation and C--C–O angle are determined to be 3.599(7) Å and 58.2(8)°, respectively. The nearest neighbor O--C distance is 3.14 Å which is the same as that found in the crystal. From the centrifugal distortion analysis the weak bond stretching and symmetric bending frequencies are estimated to be 32(2) and 90(1) cm−1. No interconversion tunneling is observed.
Tunable far infrared laser spectroscopy of van der Waals bonds: Vibration–rotation–tunneling spectra of Ar–H2O
Authors: Cohen R. C. , Busarow Kerry L. , Laughlin K. B. , Blake Geoffrey A. , Havenith M. , Lee Y. T. , Saykally R. J.
Published: July 2002

The first high resolution spectra of a rare gas–H2O cluster have been observed using a tunable far infrared laser to probe the vibration–rotation–tunneling levels of Ar–H2O formed in a continuous planar supersonic jet. The high sensitivity of this spectrometer facilitated extensive measurements of two perpendicular subbands which are assigned to transitions from the ground state to the upper component of a hydrogen exchange tunneling doublet (c-type) at 21 cm−1, and to vb1 =1+ (b-type) at 25 cm−1, the lower tunneling component of a bending vibration which is perpendicular to the tunneling coordinate. The tunneling splitting is shown to be in the range 2.5–7 cm−1 and the lower tunneling component of the excited bending vibration lies between 39 and 43 cm−1 above the ground state of the complex. The experimentally determined center-of-mass separation (Rc.m. =3.75 Å) and harmonic stretching force constant (ks =0.0134 mdyn/Å) are compared to those of related first and second row hydrides. The large amplitude motions occurring within this complex make it difficult to establish its structure.
State-to-state rotational phase coherence effect on the vibration–rotation band shape: An accurate quantum calculation for CO–He
Authors: Boissoles J. , Boulet C. , Robert D. , Green S.
Published: July 2002

Accurate coupled state calculations of line coupling are performed for infrared lines of carbon monoxide perturbed by helium. Such calculations lead to both real and imaginary line couplings. For the first time, the effect of this imaginary line couplings, connected with state-to-state rotational phase coherences, on infrared band shape, is analyzed. An extension of detailed balance principle to the complex plane is suggested from the present computed off-diagonal cross sections. This allows us to understand the physical mechanism underlying the weak effect of phase coherences on CO–He infrared band shape.
The interacting correlated fragments model for weak interactions, basis set superposition error, and the helium dimer potential
Authors: Liu B. , McLean A. D.
Published: July 2002

We report the LM-2 helium dimer interaction potential, from helium separations of 1.6 Å to dissociation, obtained by careful convergence studies with respect to configuration space, through a sequence of interacting correlated fragment (ICF) wave functions, and with respect to the primitive Slater-type basis used for orbital expansion. Parameters of the LM-2 potential are re=2.969 Å, rm=2.642 Å, and De=10.94 K, in near complete agreement with those of the best experimental potential of Aziz, McCourt, and Wong [Mol. Phys. 61, 1487 (1987)], which are re=2.963 Å, rm=2.637 Å, and De=10.95 K. The computationally estimated accuracy of each point on the potential is given; at re it is 0.03 K. Extrapolation procedures used to produce the LM-2 potential make use of the orbital basis inconsistency (OBI) and configuration base inconsistency (CBI) adjustments to separated fragment energies when computing the interaction energy. These components of basis set superposition error (BSSE) are given a full discussion.
The rotational spectrum and structure of chlorine peroxide
Authors: Birk Manfred , Friedl Randall R. , Cohen Edward A. , Pickett Herbert M. , Sander Stanley P.
Published: July 2002

The products of the ClO self-reaction have been studied in a flowing chemical reactor using submillimeter wave spectroscopy. The complete spectrum between 415 to 435 GHz has been measured as well as selected transitions in the range 285 to 415 GHz. The major products have been identified as the ClO dimer (Cl2O2) and chlorine dioxide (OClO). The observed rotational b-type spectra of the most abundant isotopic species35 ClOO35Cl and 37ClOO35Cl have been analyzed. The observed nuclear spin statistics for the main species, the relative abundance of the lesser species, and the structure determination demonstrate unambigiously that the ClO dimer must possess identical chlorine atoms in a peroxide structure. The rotational constants as well as a complete set of quartic centrifugal distortion constants have been determined. Structural parameters for the vibronic ground state have been calculated: rOO=142.59(21) pm, rClO=170.44(4) pm, ∠ClOO=110.07(1)° and dihedral angle=81.03(1)°. Rotational transitions in the first excited torsional state have been measured and analyzed for the isotopic species 35,35 and 35,37. An effective molecular structure for the first torsional excited state has been calculated. A torsional frequency has been derived from relative intensity measurements and the cis and the trans barrier heights estimated.
Stability and potential energy surface of the three low lying electronic states of Al3
Authors: Tse John S.
Published: July 2002

The potential energy surfaces for the low lying 2A1, 4A2, and 4B1 electronic states of Al3 have been calculated with correlated wave functions. Structural parameters, vibrational frequencies, and spectroscopic constants of the respective states were evaluated from the quartic force field determined by fitting the energy data points to a polynomial in internal coordinates. The 2A1 and 4A2 states are found to be minima whilst the 4B1 state is a transition state for pseudorotation. Single point calculations at the optimized geometries with a large basis set employing the coupled-cluster method that includes single and triple substitutions [CCD+ST(CCD)], predict the following order of stability: 2A1≊4A2<4B1. The theoretical results are consistent with the observations made from electron spin resonance (ESR) and magnetic deflection experiments.
Spin-dependent unitary group approach. I. General formalism
Authors: Gould M. D. , Paldus J.
Published: July 2002

A new spin-dependent unitary group approach to the many-electron correlation problem is investigated. It is demonstrated that the matrix elements of the U(2n) generators, in the U(n)×U(2) adapted electronic Gel’fand basis, are determined by the matrix elements of a single U(n) adjoint tensor operator, herein denoted by Δij(1≤i, j≤n), where Δ is a polynomial of degree two in the U(n) matrix E=[Eij]. The method is then applied, in the second paper of the series, to derive a simple segment level formula for the matrix elements of all U(2n) generators. The advantages of this new procedure for computer implementation are discussed and the central role played by the matrix Δ for the determination of molecular spin densities is highlighted.
Tunable far infrared laser spectroscopy of van der Waals bonds: The intermolecular stretching vibration and effective radial potentials for Ar–H2O
Authors: Cohen R. C. , Busarow Kerry L. , Lee Y. T. , Saykally R. J.
Published: July 2002

Measurements of the fundamental van der Waals stretching vibration Σ(000,vs=1) ←Σ(000,vs=0) of Ar–H2O [ν0=907 322.08(94) MHz] and a transition from the lowest excited internal rotor state Σ(101,vs=0) to the Σ(101,vs=1) level [ν0=1019 239.4(1.0) MHz] are presented. A simultaneous rotational analysis of the new stretching data with the internal rotor bands observed by us previously [J. Chem. Phys. 89, 4494 (1988)], including the effects of Coriolis interactions, provides experimental evidence for the new assignment of the internal rotor transitions suggested by Hutson in the accompanying paper. Fits to the rotational term values for the vs=0 states are used to derive effective radial potential energy surfaces for each of the Σ internal rotor states. The results show the well depth (153.4 cm−1) of the effective radial potential for the Σ(101,vs=0) level to be approximately 25 cm−1 deeper than that for the Σ(000,vs=0) ground state of the complex, indicating that the former is stabilized considerably more by the anisotropic intermolecular potential energy surface than is the ground state.
An intersection seam between the ground state of ozone and an excited state of like symmetrya)
Authors: Xantheas S. , Elbert S. T. , Ruedenberg K.
Published: July 2002

An intersection seam between the ground state of O3 and its first excited state of the same symmetry 1A′ in Cs, 1A1 in C2v) has been found by ab initio calculations in the neighborhood of the transition state between the ring and open equilibrium structures of the ground state.
Microscopic simulation of chemical oscillations in homogeneous systems
Authors: Baras F. , Pearson J. E. , Mansour M. Malek
Published: July 2002

A microscopic computer experiment is set up to investigate the statistical properties of far from equilibrium homogeneous chemical systems undergoing instabilities. Sustained periodic behavior of limit cycle type is observed. Both the frequency and the amplitude of the oscillations are found to be in good agreement with the macroscopic description. A comparison with the stochastic theory of chemical systems based on master equation formalism is also carried out. The dynamical and static correlation functions obtained by these two procedures are in very good agreement.
Cellular dynamics: A new semiclassical approach to time-dependent quantum mechanics
Authors: Heller Eric J.
Published: July 2002

A new semiclassical approach that constructs the full semiclassical Green’s function propagation of any initial wave function directly from an ensemble of real trajectories, without root searching, is presented. Each trajectory controls a cell of initial conditions in phase space, but the cell area is not constrained by Planck’s constant. The method is shown to be accurate for rather long times in anharmonic oscillators, indicating the semiclassical time-dependent Green’s function is clearly worthy of more study. The evolution of wave functions in anharmonic potentials is examined and a spectrum from the semiclassical correlation function is calculated, comparing with exact fast Fourier transform results.
Electron stimulated desorption via dissociative attachment in amorphous H2O
Authors: Rowntree P. , Parenteau L. , Sanche L.
Published: July 2002

Low energy (0–12 eV) electron impact on condensed amorphous H2O and D2O films is shown to induce electron stimulated desorption of H− and D−, respectively, via dissociative electron attachment. The onsets for H− and D− detection are at 5.5 eV, with a maximum yield for anion desorption at ∼7.4 eV. The kinetic energy distributions of the desorbing anions are peaked near 0 eV, indicating that the anions suffer post-dissociation collisions at or near the surface, with a large probability of anion trapping on the surface. The present results provide direct information on the dissociation products, prior to the interferences of subsequent reaction processes in the condensed film.
Molecular dynamics in perfluoro-n-eicosane. II. Components of disorder
Authors: Albrecht Thomas , Elben Helmut , Jaeger Raimund , Kimmig Martin , Steiner Ralf , Strobl Gert , Stühn Bernd , Schwickert Heinz , Ritter Clemens
Published: July 2002

The defect structure in the rhombohedral modification R of perfluoro-n-eicosane has been analyzed in a temperature-dependent x-ray and neutron scattering study. The analysis was based on an evaluation of Bragg-reflection intensities and of the diffuse scattering. Different components of disorder were detected and characterized. At the transition from the low temperature phase to modification R, rotation of the molecules about their long axes sets in. As shown by the azimuthal distribution of the diffuse scattering, rotations occur cooperatively with correlations between neighbors. The Debye–Waller factors measured for the layer reflections are indicative for a moderate roughening of the interfaces due to a restricted longitudinal diffusive motion. Molecules are perturbed by the formation of helix reversal defects. Evaluation of the diffuse scattering yields the associated finite azimuthal correlation length. There is also evidence for a continuous slow untwisting of the helix. With increasing temperature, it approaches the all-trans form.
On the mechanism of the reaction CH(X 2Π)+N2(X 1Σ+g)→HCN(X 1Σ+) +N(4S ). I. A theoretical treatment of the electronic structure aspects of the intersystem crossing
Authors: Riad Manaa M. , Yarkony David R.
Published: July 2002

The reaction CH(X 2Π)+N2(X 1Σ+g)→HCN(X 1Σ+) +N(4S) has been suggested as the initial step in the formation of ‘‘prompt’’ NO in flame fronts. Since the reaction is spin-forbidden an intersystem crossing must occur in the vicinity of the allowed crossing hypersurface of the lowest doublet and quartet potential energy surfaces. In this work the electronic structure aspects to this intersystem crossing are considered using multireference configuration interaction wave functions. The key to this treatment is a new implementation of a constrained analytic gradient search algorithm which is used to locate regions of the doublet–quartet crossing hypersurface. In those regions of nuclear coordinate space the spin–orbit coupling (matrix elements of HSO) is determined using the full microscopic Breit–Pauli spin–orbit interaction (that is both the spin–orbit and spin–other–orbit contributions are included). Included in this treatment are the largest configuration state function expansions, 700 000–900 000 terms, used to date to evaluate matrix elements HSO within the full Breit–Pauli approximation. Our conclusions are as follows. The lowest energy point on the doublet–quartet crossing hypersurface corresponds, approximately, to a C2v nuclear configuration in which the HC moiety has been inserted into a highly stretched N2 bond. The spin–orbit coupling in this region is approximately 12 cm−1. This region is estimated to be endoergic with respect to the reactant channel asymptote by approximately 7.5 kcal/mol. A second region of the crossing hypersurface corresponds to a perturbed nitrogen atom collinearly adjacent to the nitrogen side of the HCN moiety which is in its ground electronic state. The spin–orbit coupling in this region is considerably larger, approximately 44 cm−1. However, this region is estimated to be endoergic with respect to the reactant channel asymptote by over 50 kcal/mol. A qualitative Landau–Zener analysis of the electronic structure data provides the first computational evidence supporting the role of this reaction in the production of prompt NO in flame fronts.
Reaction geometry from orbital alignment dependence of ion pair production in crossed-beam Ba(1P1)–Br2 reactions
Authors: Suits Arthur G. , Hou Hongtao , Davis H. Floyd , Lee Yuan T. , Mestdagh Jean-Michel
Published: July 2002

Strong orbital alignment dependence was observed for Ba+ produced in crossed-beam reaction of Ba(1P1) with Br2. The peak of this dependence varied strongly with scattering angle for alignment of the p orbital in the scattering plane, with the maximum flux seen for perpendicular alignment with respect to the relative velocity vector. The measured Ba+ was always favored by alignment of the orbital in the scattering plane, regardless of laboratory scattering angle. The experimental results suggest that this charge-transfer process is dominated by large impact-parameter collisions which achieve collinear nuclear geometry and Σ orbital alignment at the crossing point. Orbital locking is probably not important owing to the large internuclear distance of the crossing region.
Adiabatic separatrix crossing theory for heavy–light–heavy chemical reactions in three dimensions
Authors: Skodje Rex T.
Published: July 2002

The beautifully regular dynamics observed for the collinear I+HI reaction appears to be largely irrelevant for the three-dimensional reaction. The pronounced oscillations in the collinear reaction probability and other variables are suppressed in three dimensions due to the extreme instability of the collinear dynamics in directions orthogonal to the collinear subspace. A new theory is formulated for the three-dimensional classical dynamics of heavy–light–heavy (HLH) reactions. This theory is based on three ideas. First, the fastest time scale motion can be adiabatically eliminated with high accuracy. The fast motion corresponds to diatomic vibration in the asymptotic channels and to asymmetric stretch motion in the strong collision region. A composite set of ‘‘α’’ and ‘‘β’’ channel Jacobi coordinates properly captures the correct separation of time scales. Second, the reactive separatrix can be easily defined within the adiabatic approximation and is crucial in modeling the reactive dynamics. The separatrix is the boundary in phase space between the trajectories where the light atom is dynamically bound to one of the heavy atoms and those trajectories where the light atom is exchanging back and forth between the two heavy atoms. Third, trajectories which cross the separatrix behave statistically in three dimensions. For the I+HI reaction with J=0, it is found that the reaction probability is very accurately modeled by PR= (1)/(2) Px, where Px is the probability for trajectories to cross the separatrix in the adiabatic approximation. Numerical simulations on the I+HI reaction strongly support a statistical adiabatic separatrix crossing theory and suggest widespread chaotic scattering for reactive orbits.
Luminescence measurements of Ar++H2O and N+2+H2O suprathermal charge transfer collisions: Product state distributions from H2O+Ã 2A1–X̃ 2B1 analysis
Authors: Dressler Rainer A. , Gardner James A. , Salter Richard H. , Murad Edmond
Published: July 2002

Luminescence measurements of the Ar++H2O and N+2+H2O charge transfer systems are reported at collision energies ranging from 0.6 to 783 eV at a maximum resolution of 0.5 nm (FWHM). Both systems produce H2O+Ã 2A1–X̃ 2B1 emissions throughout the measured energy range. Approximate Ã state vibrational populations are determined using known spectroscopic constants. At the highest energies investigated, the Ã state population resembles a Franck–Condon distribution. At low collision energies, near-resonant vibrational levels of the Ã state are preferentially populated indicating dominance of large impact parameter charge transfer collisions. Population of high K vibronic sublevels, corresponding to high rotational excitation about the A rotational axis of H2O+, is observed in the nearest-resonant vibrational levels. A drop to near-zero population is observed for off-resonant levels at low collision energies that is associated with the dominance of the atom pickup channel at small impact parameters.
Laser spectroscopy in a pulsed jet of AlH: Ionization-detected ultraviolet absorption spectra of the transitions C 1Σ+–X 1Σ+ and b 3Σ−–X 1Σ+
Authors: Zhu Yi Fei , Shehadeh Rana , Grant Edward R.
Published: July 2002

We report the production of radical transients AlH and AlD in a filament-source pulsed free-jet expansion. In this source, hydrogen gas from a pulsed valve passes over hot tungsten filament wrapped by a layer of aluminum wire. Liquid aluminum wetting the filament reacts rapidly to form (AlH3)x oligomer. Depending on filament temperature, either AlH or AlH2 sublimes from this oligomeric coating to be entrained in subsequent pulses of H2. The source has been characterized by laser induced fluorescence and ionization-detected one-photon absorption with time-of-flight mass resolution. Laser-induced fluorescence measurements focus on the well-known A–X transition of AlH. Higher energy ionization-detected absorption spectra resolve the C 1Σ+–X 1Σ+ transition in AlH and AlD, and for the first time the forbidden transition, b 3Σ−–X 1Σ+, in AlH. Spectroscopic constants determined from an analysis of bands assigned to v=0 and v=1 of the b 3Σ− state are Te = 40 524 cm−1, Be = 7.0675 cm−1, De = 6.1 × 10−5 cm−1, αe = 0.637 cm−1, βe = 6.38× 10−4, and γe = 1.5725, with v=0 spin–orbit and spin–rotation coupling parameters λ0=0.29 cm−1, and γ0=0.03 cm−1, inducing structure in a transition that appears to be predominantly parallel. For v=1, spin–orbit coupling is much stronger, and rotational branch intensities suggest interference, indicating that both parallel and perpendicular components of the transition moment contribute to the total oscillator strength. Results for the C–X (0–0) transitions give B0 = 6.3869 cm−1 and D0 = 5.329 × 10−4 with T0 = 44 594.05 cm−1 for the AlH C state, and B0 = 3.3457 cm−1 and D0 = 1.380× 10−4 cm−1 with T0 = 44 631.09 cm−1 for the AlD C state.
Contrast variation in surface plasmon field-enhanced Raman spectroscopy
Authors: Nemetz Axel , Knoll Wolfgang
Published: July 2002

Plasmon surface polaritons (PSPs) are used for the enhancement of Raman scattered signals from an alkyl-thiol monolayer HS(CH2)21OH (with dielectric constant εT) at a metal/dielectric interface with dielectric constants εm and εamb, respectively. We investigate the intensity ratio of the symmetric and asymmetric CH2 Raman band which has been proven to be sensitive to the polarization of the exciting PSPs. Since the PSP-polarization also depends on the dielectric constant of the dielectric ambient εamb we show that a variation of εamb influences the CH2 Raman bands. In doing so we present experimental proof for a refined theoretical description of PSP-polarization in thin layers sandwiched between a metal and a dielectric in order to allow for contrast variation.
Density dependence of the effective mass of excess electrons in fluid methane
Authors: Plenkiewicz B. , Frongillo Y. , Jay-Gerin J.-P. , Jain Ashok
Published: July 2002

We report the calculation of the effective mass (m*) of excess electrons injected in fluid methane as a function of number density (n). The calculation is performed within the framework of the Wigner–Seitz model for nonpolar fluids, using an accurate molecular potential which satisfactorily describes the scattering of low-energy electrons in the gas phase. Our calculated m*(n) values are found to decrease monotonically from the free-electron mass (m0) in the dilute gas limit to 0.72 m0 in the high-density liquid near the triple point. Comparison is made with available data in the literature.
Mixed-mode oscillations in chemical systems
Authors: Petrov Valery , Scott Stephen K. , Showalter Kenneth
Published: July 2002

A prototype model is exploited to reveal the origin of mixed-mode oscillations. The initial oscillatory solution is born at a supercritical Hopf bifurcation and exhibits subsequent period doubling as some parameter is varied. This period-2 solution subsequently loses stability, but continues to exist−regaining stability to form the 11 mixed-mode state (one large plus one small excursion). Other mixed-mode states lie on isolated branches or ‘‘isolas’’ of limit cycles in the one-parameter bifurcation diagram and are separated by regions of chaos. As a second parameter is varied, the number of isola solutions increases and the ‘‘gaps’’ between them become narrower, leading to correspondingly more complete Devil’s staircases. An exactly comparable scenario is shown to arise in the three variable model of the Belousov–Zhabotinsky reaction proposed recently by Györgyi and Field [Nature 335, 808 (1992)].
Observation of charge-transfer excited states in the I−⋅CH3I, I−⋅CH3Br, and I−⋅CH2Br2SN2 reaction intermediates using photofragmentation and photoelectron spectroscopies
Authors: Cyr D. M. , Bishea G. A. , Scarton M. G. , Johnson M. A.
Published: July 2002

Photoelectron spectra of the I−⋅CH3I, I−⋅CH3Br, and I−⋅CH2Br2 complexes each display a relatively sharp main feature shifted from that of I− by the expected binding energies of the ion–dipole bound negative ions. We observe narrow photofragmentation bands in both I−⋅CH3I and I−⋅CH2Br2 upon excitation just above the photodetachment thresholds (≊3.5 eV), whose envelopes closely resemble their photoelectron spectra.
The dynamics of electronic to vibrational, rotational, and translational energy transfer in collision of Ba(1P1) with diatomic molecules
Authors: Suits A. G. , de Pujo P. , Sublemontier O. , Visticot J.-P. , Berlande J. , Cuvellier J. , Gustavsson T. , Mestdagh J.-M. , Meynadier P. , Lee Y. T.
Published: July 2002

Doppler measurements taken over a range of probe-laser angles in a crossed-beam experiment were used, in conjunction with forward convolution analysis, to obtain flux–velocity contour maps for Ba(3P2) produced in a collision of Ba(1P1) with H2, N2, O2, and NO. The contour maps suggest a general model for the dynamics of this process in which large impact parameter collisions result in a near-resonant transfer of initial electronic energy into final vibrational energy, while close collisions produce sideways scattering and effectively couple electronic energy to translation. The molecular collision partners fall into two categories: for one group, comprising O2 and NO, the existence of a well-defined molecular anion with favorable Franck–Condon factors linking excited vibrational levels to the ground vibrational state of the neutral results in greatly enhanced coupling for the near-resonant process. Molecules for which there exist no stable anions, such as N2 and H2, represent a second category. The electronically inelastic collision for this group is instead dominated by the nonresonant process yielding the ground vibrational state and large translational energy release.
The vibrational predissociation of Ar–CO2 at the state-to-state level. I. Vibrational propensity rules
Authors: Bohac E. J. , Marshall Mark D. , Miller R. E.
Published: July 2002

A modified version of the optothermal technique has been used to measure photofragment angular distributions resulting from the vibrational predissociation of Ar–CO2 from both members of the (1001)/(0201) Fermi diad. These angular distributions show resolvable structure which can be assigned to individual final states of the resulting fragments. The emphasis of the present paper is on the role of the vibrational degrees of freedom of the CO2 fragment as depositories for the excess energy. The dissociation energy (D0) of the complex has been determined to be 166±1 cm−1. This result is used in conjunction with the spectroscopic data already available in the literature to refine two previously reported two-dimensional Ar–CO2 potential energy surfaces.
Dynamics of analog-to-frequency transduction by excitable systems: Sensory receptors
Authors: Rovinsky Arkady , Menzinger Michael
Published: July 2002

This paper analyzes the dynamics of analog-to-frequency transduction occurring in spatially extended excitable media with particular attention to sensory receptors. It is shown that spatially extended excitable media generate periodic traveling waves when forcing a dynamical variable by a constant stimulus at a boundary (i.e., by a Dirichlet boundary condition). However, the dynamical range of frequency ω(S) as a function of stimulus S is usually too narrow for sensory transducers, unless special physical conditions are met. The crucial property of an excitable medium that endows it with a wide transduction range in frequency is shown to be the nonlinear slowing down of the recovery variable in the vicinity of the steady state. This property is illustrated by computation for a series of models of the encoder region of sensory neurons possessing increasing physiological relevance.
Ultraviolet absorption bands of ionic compound clusters: Onset of crystalline structures in [Csn+1In]+, n=1–13
Authors: Li Xiuling , Whetten Robert L.
Published: July 2002

Starting from the large photofragmentation cross sections recently reported for mass-selected alkali–halide cluster and nanocrystal beams [X. Li, R. D. Beck, and R. L. Whetten, Phys. Rev. Lett. 68, 3420 (1992)], we have obtained optical absorption spectra for ionic compound clusters in the ultraviolet charge-transfer bands. The smaller members of the CsI series, 3<n<14, show distinct absorption edges in the 4.6–5.8 eV region—just below the bulk exciton band near 5.9 eV—and the smallest cluster with an onset near the bulk is Cs14I13+, which is also a (3,3,3) nanocrystal. To help account for these features, we have computed minimum-energy structures for the entire series using a classical electrostatic potential, and have derived a simple model for the absorption energy, following deBoer. The locations of the spectral features are compared to the predicted energies, and good qualitative agreement is found.
Mechanism of quasi-one-dimensional electronic conductivity in discotic liquid crystals
Authors: Boden N. , Bushby R. J. , Clements J.
Published: July 2002

Recently, it has been shown that a new class of quasi-one-dimensional conductors can be created by doping discotic liquid crystals with appropriate oxidants. This paper reports the elucidation of the mechanism of conduction in these new materials. In particular, the ac conductivity of 2,3,6,7,10,11-hexahexyloxytriphenylene (HAT6) doped with the Lewis acid AlCl3, has been measured as a function of frequency (10−3–107 Hz), and temperature in its crystalline solid (K), hexagonal discotic liquid crystal (Dho), and isotropic liquid (I) phases. In all three phases the conductivity is independent of frequency at low frequencies, but shows a power law dependence on frequency [σ(ω)∼ωs, s∼0.7–0.8] at higher frequencies. This behavior is characteristic of charge carrier transport by a hopping mechanism. The conductivity data have been analyzed in terms of the Scher and Lax theory to obtain the parameters describing this process. In macroscopically aligned K and Dho phases, the conductivity measured along the column axes is approximately 103 greater than that in the perpendicular direction. The conduction along the columns is identified with a single charge transport process in which the carriers hop between localized states (radical cations) associated with AlCl4−-counterions linearly distributed off-axis along the columns. The charge carrier diffusion coefficient is independent of the concentration of dopant and has the value D∥∼3.4×10−10 m2 s−1 in the Dho phase at 343 K. In the isotropic melt phase, the conductivity measurements reveal the involvement of two distinct processes. One of these is identified with charge migration along supramolecular ‘‘stacks’’ some 200 nm in length, whilst the other appears to be associated with carriers hopping between these stacks. The conductivity behavior of the unaligned Dho and K phases is very similar to that of the isotropic liquid phase implying that the mesoscopic structure of the latter phase is ‘‘frozen-in’’ as a ‘‘defect structure’’ on cooling. This defect structure then dominates the conductivity behavior of the unaligned Dho and K phases.
Nuclear hyperfine interactions and dynamic state of H2O in Ar–H2O
Authors: Germann T. C. , Gutowsky H. S.
Published: July 2002

Rotational spectra for the Ar–H2O dimer have been measured with the Balle/Flygare Mark II pulsed nozzle, Fourier transform microwave spectrometer. The present study extends that of Fraser et al. [J. Mol. Spectrosc. 144, 97 (1990)] to higher resolution, enabling analysis of the hyperfine structure and spin statistics for the Σ(000) and Σ(101) internal rotor states of water in Ar–H2O and Ar–D2O and of the Σ(000) state in Ar–HDO. The results indicate the two internal rotor states to be quite similar, especially for Ar–D2O.
Grafted polymers under the influence of external fields
Authors: Marko J. F. , Johner A. , Marques C. M.
Published: July 2002

The structure of grafted polymer ‘‘brushes’’ may be profoundly modified by the action of ‘‘external fields’’—local shifts in chemical potential due to, e.g., interfacial effects at the grafting surface. We discuss the strong-stretching limit of the self-consistent mean-field theory for a brush exposed to an arbitrary external potential, a simple scaling law for the brush height arises in the case where the external field pushes monomers towards the surface. In contrast, repulsive interactions can lead to ‘‘exclusion zones’’ or regions from which polymer ends are repelled, leading to a breakdown of the simple scaling formula. We exactly solve the self-consistent theory of the brush in a repulsive square well where an exclusion zone always appears. Our results describe a brush in the case where a thin layer of one component of a binary solvent that is a worse solvent for the polymer than is the bulk mixture wets the grafting surface. We also discuss the effects of thermal fluctuations and chain polydispersity on our results, and estimate the effects of various interfacial phenomena on the brush structure.