bluuebunny/crossref_metadata_2025 huggingface dataset:
Mössbauer spectroscopic studies of the moment distribution in Fe73.5Cu1Nb3Si13.5B9
Authors: Aghamohammadzadeh H. , Williams J. M. , Gibbs M. R. J. , Parmar B. S. , Davies H. A.
Published: July 2002

Fe 57 Mössbauer spectroscopy can be used to determine the distribution of moment directions in magnetic materials. Here, we have applied this technique to a study of Fe73.5Cu1Nb3Si13.5B9 (isocompositional with FINEMET). We are particularly interested in establishing the distribution of moment directions in the nanophase material, obtained by devitrification from an amorphous precursor. The nanophase material consists of Fe–Si grains embedded in an amorphous matrix. Current understanding of the resulting magnetic properties, which involves application of the random anisotropy model to these materials, assumes that the magnetization in the grains is exchange coupled to neighbors, giving a long exchange length compared with the nanometer grain size. We present data for the amorphous precursor, the standard nanophase material, and nanophase material devitrified in an external magnetic field. If σu and σv represent the Gaussian widths of the out-of-plane and in-plane moment direction distributions respectively, we have found that σu=14° and σv=68° for the amorphous precursor, σu=14° and σv=92° for the standard nanophase material, and σu=22° and σv=79° for a magnetic field aligned specimen. We interpret these data as demonstrating that standard devitrification does result in grains with a wide spread of moment direct ions over the volume sampled by this technique, even allowing for exchange coupling. These results and the implications for macroscopic magnetic properties such as anisotropy will be discussed.
Magnetic spectroscopy and characterization of La0.65Pb0.35MnO3
Authors: Chipara M. I. , Adenwalla S. , Dowben P. A. , Xu Qin Lan , Liou Sy-Hwang , Shoemaker R.
Published: July 2002

Crystalline films of La0.65Pb0.35MnO3 grown on Si (100) substrates by rf sputtering have been investigated by ferromagnetic resonance and MOKE. The dependence of the ferromagnetic resonance line parameters on the sample orientation and temperature is investigated. An expression for the angular dependence of the resonance field on the orientation of the sample is proposed, based on the assumption that close to the Curie temperature the Zeeman and demagnetizing terms are dominant. MOKE data support this hypothesis. The temperature dependence of the magnetization at saturation, in relative units, estimated from ferromagnetic resonance agrees with SQUID data.
Magnetoresistance in magnetite films prepared from aqueous solution at room temperature
Authors: Nishimura K. , Kohara Y. , Kitamoto Y. , Abe M.
Published: July 2002

Polycrystalline iron oxide films, 0.1–1.1 μm in thickness, are synthesized on glass substrates at room temperature directly from an aqueous solution of FeCl2 by ferrite plating. The films have a single phase spinel structure of mixed solution (Fe3O4)1−x–(γ-Fe2O3)x, as revealed by x-ray diffractometer and conversion electron Fe57 Mössbauer spectroscopy. At room temperature the films exhibit negative magnetoresistance (MR) of ratio ΔR/R=−3.3% to −7.4%, which is ascribed to intergranular tunneling of spin polarized electrons of Fe3O4. The resistivity R of the films is 2.2×100–1.3×103 Ω cm, much higher than that (∼10−2 Ω cm) reported for bulk samples of Fe3O4; R and ΔR/R depend on the plating conditions and, therefore, the composition x in (Fe3O4)1−x–(γ-Fe2O3)x of the samples.
Annealing behavior of hydrogen-defect complexes in carbon-doped Si quenched in hydrogen atmosphere
Authors: Fukata Naoki , Suezawa Masashi
Published: July 2002

Optical absorption spectra and the annealing behavior of hydrogen (H)-point defect complexes in carbon (C)-doped Si after hydrogenation were investigated. Specimens of C-doped Si (C concentration: 1.7×1017 cm−3) were sealed in quartz capsules together with H2 gas and were annealed at a high temperature for 1 h followed by quenching in water. We measured the optical absorption spectra at about 7 K with a Fourier-transform infrared spectrometer. The VH4 (V: monovacancy) defect was almost annealed out at 600 °C. The formation energy of the VH4 defect in C-doped Si was estimated to be about 3.2 eV from the quenching temperature dependence of the 2223 cm−1 peak. The observed 2192 and 2203 cm−1 peaks are probably due to the VH3 defect, which captures one H atom during annealing and become the VH4 defect. After annealing at 700 °C, we observed two absorption peaks at 2093 and 2086 cm−1, which are probably due to Si–H stretching vibration of H on internal surfaces of voids. From these assignments, it was found that V is introduced into C-doped Si at high temperatures, although it is known that C introduces I into Si at high temperatures.
Formation of voids in Ti-salicided BF2+-doped submicron polysilicon lines
Authors: Chua H. N. , Pey K. L. , Lai W. H. , Siah S. Y.
Published: July 2002

The physical characteristics and distribution of voids in TiSi2/p+-Si system as a function of polycrystalline silicon (polySi) linewidths were investigated in detail using scanning electron microscopy, cross-sectional transmission electron microscopy, and the focused ion beam technique. Surface and subsurface voids were found in Ti-salicided BF2+-implanted polycrystalline silicon (polySi) lines that were annealed at temperatures ranging from 650 to 850 °C. Most of the subsurface voids were found in the TiSi2 film as well as at the TiSi2/polySi interface. The void density, shape irregularity, and void size increase with decreasing polySi linewidth, especially when it is smaller than 0.24 μm, indicating that stress associated with narrow polySi lines may play an important role in the voiding phenomenon. It was also shown that the TiN film overlying the TiSi2 film and the etch-back process could not be responsible for the void formation process. Instead the absence of voids in B+-implanted polySi and the segregation of fluorine into the TiSi2/polySi and TiN/TiSi2 interfaces from BF2+-implanted polySi after the first salicidation rapid thermal annealing suggest a fluorine-assisted voiding mechanism.
Comparison on the effect of (La0.5Sr0.5)MnO3 and (La0.5Sr0.5)CoO3 buffer layers on fatigue properties of (Pb0.6Sr0.4)TiO3 thin films prepared by pulsed laser deposition
Authors: Cheng Hsiu-Fung , Chen Yi-Chun , Chou Chen-Chia , Chang Kuang-Chung , Hou Chun-Shu , Lin I-Nan
Published: July 2002

The fatigue properties of (Pb0.6Sr0.4)TiO3 (PSrTO) thin films deposited using (La0.5Sr0.5)MnO3 (LSMO) materials as buffer layer were compared with those of the films grown using (La0.5Sr0.5)CoO3 (LSCO) materials as buffer layer. The extent of degradation induced by polarization switching for 1×1010 cycles with 210 kV/cm maximum field, which is four times of coercive field (Ec), is less pronounced for PSrTO/LSMO/Pt(Si) thin films than that for PSrTO/LSCO/Pt(Si) films. This phenomenon is ascribed to the smaller strain induced in PSrTO/LSMO/Pt(Si) materials. Moreover, the pulse response testing indicates that the degradation of the films mainly occurs at PSrTO-to-LSMO (or PSrTO-to-LSCO) interface.
The effect of excitons on CdTe solar cells
Authors: Karazhanov S. Zh. , Zhang Y. , Mascarenhas A. , Deb S.
Published: July 2002

Temperature and doping-level dependence of CdTe solar cells is investigated, taking into account the involvement of excitons on photocurrent transport. We show that the density of excitons in CdTe is comparable with that of minority carriers at doping levels ⩾1015 cm−3. From the investigation of the dark-saturation current, we show that the product of electron and hole concentrations at equilibrium is several orders of magnitude more than the square of the intrinsic carrier concentration. With this assumption, we have studied the effect of excitons on CdTe solar cells, and the effect is negative. CdTe solar cell performance with excitons included agrees well with existing experimental results.
Magnetic phase diagrams of the Ln(Mn1−xCox)O3 (Ln=Eu, Nd, Y) systems
Authors: Troyanchuk I. O. , Khalyavin D. D. , Lynn J. W. , Erwin R. W. , Huang Q. , Szymczak H. , Szymczak R. , Baran M.
Published: July 2002

We report magnetization measurements in the Ln(Mn1−xCox)O3+λ (Ln=Eu, Nd; 0⩽x<1) and Y(Mn1−xCox)O3 (0.35⩽x⩽0.5) perovskites as a function of temperature and magnetic field. Compositions in the range of 0.35⩽x⩽0.8 (Ln=Eu), 0.4⩽x⩽0.6 (Ln=Nd), and 0.35⩽x⩽0.45 (Ln=Y) exhibit metamagnetic behavior associated with a transformation of the magnetic structure from ferrimagnetic, where the magnetic moments of Co2+ and Mn4+ ions are antiparallel, to ferromagnetic where they are parallel. A ferromagnetic state is observed for the 0.1⩽x⩽0.15 (Ln=Nd) and 0.45<x⩽0.5 (Ln=Y) compositions. Doping and H-T phase diagrams are presented, and these diagrams resemble those of Ln1−xCaxMnO3 charge-ordered manganites. Neutron diffraction studies indicate site ordering of the Co and Mn ions in the Y(Co0.5Mn0.5)O3 system, which is presumably responsible for its ferromagnetic behavior.
Chemi-ionization reactions of state-selected electronically excited Ar(3P0) and Ar(3P2) atoms
Authors: Golde Michael F. , Ho Yueh-Se
Published: July 2002

Using a chemical method to separate the spin-orbit Ar(3P0) and Ar(3P2) metastable states in a discharge-flow system, branching fractions, f0 and f2, for ionization of several molecular reagents by these species have been measured. Experiments have been performed at room temperature and at 220 K. The large differences between f0 and f2 observed for CCl4, CF3Br, HBr, and DBr are ascribed to energy effects and it is concluded that the yield of chemi-ionization increases sharply as the electronic energy of the excited atom rises above the ionization potential of the molecule. This contrasts with the much smaller effect of reagent kinetic energy on the chemi-ionization yield. Branching fractions for some reaction channels leading to emission of light are also reported.
Differential nonreactive scattering of He*(2 1S, 2 3S) by D2 and H2: Anisotropic optical potentials and comparison with ab initio theory
Authors: Martin D. W. , Siska P. E.
Published: July 2002

Angular distributions of He*(2 1S, 2 3S) scattered by D2 and H2 measured in crossed supersonic molecular beams at collision energies in the range 1.0–2.4 kcal/mol are analyzed to yield anisotropic optical potentials that simultaneously reproduce these data along with quenching rates and ionization cross sections. Comparison with ab initio calculations of the potentials by Cohen and Lane and Hickman, Isaacson, and Miller shows very good agreement. The results are combined with one-electron model potential calculations to probe the nuclear and electronic dynamics involved in these collisions. Quenching or Penning ionization is found to occur mainly through broadside attack of He* on H2, despite the fact that the occupied σg orbital on H2 has greater spatial extent along the bond axis than perpendicular to it. An implication is that Penning ionization electron spectroscopy (PIES) experiments on larger molecules cannot be interpreted simply on the basis of van der Waals radii and spatial extent of various molecular orbitals; the nature of the excited-state potential surface may play a dominant role in determining these spectra.
On the form of the adiabatic and diabatic representation and the validity of the adiabatic approximation for X3 Jahn–Teller systems
Authors: Thompson Todd C. , Truhlar Donald G. , Mead C. Alden
Published: July 2002

Starting with an expansion previously obtained for the adiabatic potential energy surfaces and derivative couplings for an X3 system in the vicinity of a D3h conical intersection, with the potential energy truncated at the second order in the distance from the intersection, and the derivative coupling carried to the highest order consistent with the existence of the ‘‘strictly diabatic’’ representation, we obtain the rigorous functional form to the same order of approximation for the diabatic representation of the same system. This diabatic representation is then applied to the reinterpretation of the recently observed laser-induced fluorescence spectrum of Cu3. In a more general context, the same functional form is used to estimate quantitatively the validity of the electronic adiabatic approximation, as a function of a unitless scale parameter, over a range of this parameter corresponding to several metal trimer systems. Some of the methods and results of this study are more general than the X3 systems to which most of the paper is addressed; we briefly indicate the possible extensions and implications for other systems.
Hard sphere treatment of binary diffusion in liquid at high dilution up to the critical temperature
Authors: Chen Bobby H. C. , Sun C. K. Jacob , Chen S. H.
Published: July 2002

Binary diffusion coefficients at high dilution are measured for ethane and ethylene dissolved in cyclohexane at temperatures from 280.5 to 363.2 K and for benzene in cyclohexane and n-hexane up to their critical temperatures. With the effective hard-sphere diameters backed out from self-diffusion data from both solute and solvent molecules and fitted to Protopapas, Anderson, and Parlee’s formula, the prediction with the Sung–Stell theory is found to agree to within ±7% of experimental data for wide ranges of solute-to-solvent mass and size ratios at temperatures up to the critical point.
The laser photoelectron spectrum of gas phase aniline
Authors: Meek Jon T. , Sekreta Ellen , Wilson Warren , Viswanathan K. S. , Reilly James P.
Published: July 2002

An effusive molecular beam of aniline vapor has been two-step ionized by irradiating it with the output from a frequency doubled dye laser. Photoelectrons ejected while exciting several different vibronic levels of its 1B2 electronic state have been kinetic energy analyzed. In all cases ions are found to be generated in their ground electronic state. Their vibrational state distribution is strongly wavelength dependent and differs remarkably from that obtained in conventional single photon photoelectron spectroscopy. The energies of several in-plane vibrational modes of aniline cations have been measured. In contrast with the ground electronic state of the neutral molecule, the ground state of the ion and 1B2 excited electronic state of the neutral molecule both appear to be nearly planar.
Calculation of NMR chemical shifts. V. The gauge invariant coupled Hartree–Fock calculation for H2O, H3O+, and OH−
Authors: Fukui H. , Miura K. , Yamazaki H. , Nosaka T.
Published: July 2002

Magnetic shielding constants are calculated for the hydrogens and oxygens in the isolated species of H2O, H3O+, and OH− using the coupled Hartree–Fock (CHF) method with gauge invariant atomic orbitals (GIAO). The difference from the H2O shielding is evaluated and compared with the shift relative to water in the gas phase measured experimentally. The calculated relative shift of the H3O+ protons has the same sign as the experimental result, but the shift to low field found experimentally for the OH− proton is not reproduced.
Fluorescence of NO2 in the 2 2B2 state and its application to the optical–optical double resonance (OODR) study of the complex visible spectrum
Authors: Tsukiyama Koichi , Shibuya Kazuhiko , Obi Kinichi , Tanaka Ikuzo
Published: July 2002

The fluorescence of NO2 in the 2 2B2 state, which is located at 1.86 eV above the dissociation limit (D0=3.12 eV), has been observed for the first time. The fluorescence spectrum consists of two band systems, one in the ultraviolet and the other in the visible. The ultraviolet bands correspond to the 2 2B2→X̃ 2A1 transition. By employing the 2 2B2 state as a final state, optical–optical double resonance spectroscopy through the complex visible absorption system of NO2 has been applied to assign the transition responsible for the visible fluorescence of NO2 2 2B2 and also to study the complex visible absorption spectrum of NO2. As a result, the visible emission bands have been assigned to the 2 2B2→1 2B2 transition. The vibrational analysis is partly reported in the range of 479–616 nm for the 1 2B2←X̃ 2A1 electronic transition.
Consistent analytic representation of the two lowest potential energy surfaces for Li3, Na3, and K3
Authors: Thompson Todd C. , Izmirlian Grant , Lemon Stephen J. , Truhlar Donald G. , Mead C. Alden
Published: July 2002

We present new analytic representations of ab initio interaction potentials for Li3, Na3, and K3. The analytic representations are based on a functional form that has the correct analytic behavior in its dependence on the nuclear coordinates, even in the vicinity of D3h conical intersections and for collinear geometries, and that reduces, when one atom is removed to infinity, to an accurate diatomic potential energy curve. We show that the new representation can be used to predict excited-state energies by analytic continuation of ground-state energies to a second Riemann sheet. We also report pseudorotation barriers, Jahn–Teller stabilization energies, and harmonic vibration frequencies.
The electronic structure of the calcium monohalides. A ligand field approach
Authors: Rice Steven F. , Martin Hans , Field Robert W.
Published: July 2002

The electronic structure of the calcium monohalides is addressed using a ligand field model which approximates the halide as a polarizable negative charge perturbing the one electron valence structure of the Ca+ ion. A simple, zero-free-parameter model is shown to predict accurately electronic energies, transition moments, permanent dipole moments, and several other molecular constants that have been experimentally determined. The molecular properties and electronic wave functions are interpreted in terms of the polarization (s/p/d/f mixing) and radial expansion (nl/n+1l mixing) of the low lying, free ion, basis functions caused by the electric field of the ligand.
Adsorption states of light atoms (H, D, He) on quantum crystals (H2, D2, He, Ne)
Authors: Pierre L. , Guignes H. , Lhuillier C.
Published: July 2002

A study is made of the adsorption states of light atoms (H,D,He) on perfect quantum crystals (H2,D2,He,Ne). The quantum nature of the crystal is taken into account at the outset of the theory and the adsorption states are calculated as the bound states of a single atom in the potential created by the semi-infinite solid (rigid lattice approximation). The three-dimensional Schrödinger equation is solved to obtain the binding energies of the various hydrogen and helium isotopes on the (111) and (100) faces of the following quantum crystals: H2, D2, He, Ne. The major sources of uncertainty in the results are discussed in the paper: by increasing importance they are the neglect of many body interactions, the neglect of the coupling of the adatom with the surface, and finally the uncertainties in most of the pair potentials. A good agreement is obtained with available experimental results.
Exciton dynamics and transient grating experiments
Authors: Garrity D. K. , Skinner J. L.
Published: July 2002

The theory of the dynamics of Frenkel excitons is discussed within the context of transient grating experiments. Our results are based on the stochastic model of Haken and Strobl. We show that the transient grating signal is very sensitive to the nature of the initial state prepared by the crossed excitation beams, especially when the exciton motion is ‘‘coherent’’ (when there is no exciton–phonon scattering). If the initial density matrix is assumed to be site diagonal, we find agreement with previous theoretical work of Wong and Kenkre. Our results are consistent with experiments performed by the Fayer group on anthracene, where it appears that the exciton motion is incoherent. However, based on their results and our theory, we suggest that it may be possible to observe partially coherent transport at T=2 K with a grating spacing of 2000 Å.
Model potential calculations for second-row transition metal molecules within the local-spin-density method
Authors: Andzelm Jan , Radzio Elżbieta , Salahub Dennis R.
Published: July 2002

The model potential (MP) method originally proposed by Huzinaga and Bonifacic is extended to spin-polarized local-spin-density calculations, including scalar relativistic effects. The theoretical justification of the MP method in this case is studied and the method of optimization of the basis functions and MP parameters is given. The validity of the frozen core approximation is studied for Mo2, Ru2, and Ag2. It is found that the MP can very accurately reproduce all-electron (AE) results if the 4p electrons of Ag and the 3d electrons of Mo are also considered as valence electrons, although inclusion of these electrons in the core still yields a useful level of accuracy. It is shown that the present MP results are not sensitive to basis set superposition errors (BSSE). Upon inclusion of the scalar relativistic effects the calculated bond length and vibrational frequency of Ag2 are in near perfect agreement with experiment, while the dissociation energy is overestimated by 23% with the ‘‘best’’ local potential (VWN). MP calculations have also been performed for AgH, AgO, and AgF. The same level of agreement with experiment as for Ag2 was found, with the exception of the bond length for AgO. Our calculated bond length is 0.05 Å shorter than the presently accepted experimental value. Since some uncertainty is associated with the spectroscopic assignments for AgO we believe an experimental reexamination would be in order.
Theoretical studies of fast H atom collisions with NO
Authors: Colton Mitchell C. , Schatz George C.
Published: July 2002

This paper presents a detailed theoretical study of the NO vibration/rotation distributions produced in nonreactive H+NO collisions in the 1–3 eV range of relative translational energies. The collision dynamics is studied by applying the quasiclassical trajectory method to each of the four potential surfaces (1A′, 1A″, 3A′, 3A″) which correlate to H(2S)+NO(2Π), followed by a statistical average of the cross sections over the four surfaces. Each surface is generated by fitting a flexible empirical function (a sum of Morse functions with coordinate dependent parameters) to available ab initio and experimental data on the surfaces. The resulting vibrational distributions are in excellent agreement with measured laser induced fluorescence results at 0.95 and 2.2 eV. In addition, the average fraction of energy transferred into vibration shows the same flat dependence on translational energy in the 1–3 eV range that is seen experimentally, and which contrasts with the linear proportionality seen for the corresponding H+CO system. Details of the collisions are analyzed, and it is found that complex formation plays a much more important role in H+NO than in H+CO, with roughly 2/3 of the cross section at 0.95 eV and 1/5 that at 2.2 eV due to complex formation. In fact, at both energies, the trajectory vibrational distributions are quite close to statistical. At 2.2 eV, however, direct collisions make a substantial contribution to the vibrational excitation process, so the agreement with statistical theory is partly accidental. The rotational distributions are found to be substantially colder than statistical, particularly at higher energy, with an average rotational quantum number which is independent of vibrational state except for the highest three states allowed by energy conservation. Agreement between experimental and theoretical rotational distributions is poorer than for the vibrational distributions, with the theoretical rotational distributions being hotter. This presumably reflects errors in the anisotropy of the potential energy surfaces.
Analysis of the reaction He++He+He→He+2+He
Authors: Russell J. E.
Published: July 2002

The Wigner–Keck Monte Carlo trajectory method is employed to calculate the rate coefficient k for He+ ions recombining in gaseous helium at temperatures in the range 80<T<320 K. The attractive (repulsive) He+2 interaction is approximated by the sum of a Morse (anti-Morse) potential and a term taking into account the long range ion–induced dipole interaction. The relevant three body interaction is represented by an expression that is, in effect, an approximation to a simple variational calculation. This expression depends on the same parameters that specify the He+2 interactions. A method is devised for sampling points on the Wigner–Keck surface that permits the determination of a value of k to within a statistical error of about 10% by computing less than 1600 trajectories, of which less than 500 represent actual formation of a He+2 molecule. The computed values of k are found to be in fair agreement with experimental results obtained by Biondi and his collaborators. The temperature dependence is reproduced rather satisfactorily, but the magnitude is 10% to 20% too low. The computed values of k are found to be fairly insensitive to reasonable variations in the behavior of the two and

three body potentials. In particular, the results are found to be totally insensitive to the behavior of the three body potential if more than one of the internuclear separations are relatively small. It is also found that long range ion–induced dipole forces, while playing a very important role, do not appear to dominate the recombination process to quite the extent that had apparently been assumed in some previous investigations. The results generally support a model proposed by Mahan, who had suggested that charge transfer is the mechanism primarily responsible for the reaction. It is suggested that the remaining discrepancy with experiment might be largely due to the formation of highly excited quasibound states by inward tunneling through the rotational barrier, which is a mechanism that had originally been proposed by Dickinson et al. as the dominant one in He+ recombination. Some qualitative arguments are presented that indicate that this mechanism, which is not taken into account by classical trajectory calculations, might be less important than had originally been thought.
The microwave spectrum of the vinoxy radical
Authors: Endo Yasuki , Saito Shuji , Hirota Eizi
Published: July 2002

The pure rotational spectrum of the vinoxy radical, CH2CHO, was observed for the first time in the millimeter-wave region. Both the C2H4+O and CH3CHO+F reactions yielded the radical, the latter being four to five times more efficient. Many a-type R-branch transitions were identified in the frequency region of 80 to 176 GHz with resolved fine structure and partially resolved hyperfine structure. An accidental degeneracy between the 624 J=6.5 and 716 J=6.5 levels enabled us to accurately determine the off-diagonal components of the fine and hyperfine interaction constants. A small positive inertial defect Δ0=0.057 58 (16) amu Å2 calculated from the observed rotational constants indicates the radical to be planar in the ground electronic state. The observed hyperfine coupling constants of the protons clearly demonstrate that the formyl methyl type resonance form ⋅CH2CH=O makes dominant contribution to the ground electronic state, while the ethenyloxy form CH2=CHO⋅ is relatively unimportant.
The infrared spectra of NH3⋅H2O and ND3⋅D2O at 100 K
Authors: Bertie John E. , Shehata Mohamed R.
Published: July 2002

The infrared spectra of ND3⋅D2O and hydrogen or deuterium impurity in NH3⋅H2O or ND3⋅D2O at 100 K are reported for the first time. Their interpretation is aided by a D2h pseudosymmetry caused by the heavy atoms being nearly coplanar in the P212121, D42, crystal. No evidence of the possible disorder of the hydrogen atoms is observed. Oriented gas model calculations gave the approximate relative intensities of the unit cell components of the molecular vibrations. The site splitting and correlation splitting are comparable for ν3 and, probably, ν4 of ammonia; three unit cell components are observed instead of the six predicted under D2, three components being calculated to have near-zero intensity. The symmetric deformation of ammonia, ν2, yields two unit cell modes with significant intensity, separated by 37 cm−1 for NH3 and 23 cm−1 for ND3. The isotope frequency ratio for ν2 is lower than

for any other mode, so this large splitting must be due to intermolecular coupling, probably transition dipolar in origin. The two strong νOH (HDO) absorptions are 140 cm−1 further apart than the two strong νOH (H2O) absorptions, a surprising result because intramolecular coupling is negligible because νO–H⋅⋅⋅N is ∼400 cm−1 below νO–H⋅⋅⋅O. In contrast, νOD (D2O) yields four strong absorptions approximately centered with respect to the two strong νOD (HDO) absorptions. The O–D⋅⋅⋅N doublet is due to the B1 and B2 unit cell group components, the B3 component being too weak to see, as is the case for ν2 of ammonia. The corresponding O–H⋅⋅⋅N doublet is unresolved. Use of the O–D⋅⋅⋅O–D′interaction force constant of the ice phases, −0.10 mdyn Å−1, and oriented gas model calculations of the relative intensities shows that the O–D⋅⋅⋅O bands at 2390 and 2459 cm−1 are due to the in-phase (B3) and out-of-phase (B2) motions of the two O–D bonds in each chain of water molecules in each unit cell. In NH3⋅H2O the out-of-phase O–H⋅⋅⋅O vibration interacts, probably with 2ν4 of ammonia, and its intensity is dissipated among several weak features. We are unable to explain the difference between the coupled and uncoupled O–H⋅⋅⋅N frequencies, 2887 and 2825 cm−1, in NH3⋅H2O. The bending mode of water is tentatively assigned at 1696, 1467, and 1243 cm−1 for H2O, HOD, and D2O, respectively. The lattice absorptions are assigned to rotational or translational motion. The rotational modes of ammonia are assigned to the B1, B2, and B3 unit cell modes that have significant intensity.
Organic polymers based on aromatic rings (polyparaphenylene, polypyrrole, polythiophene): Evolution of the electronic properties as a function of the torsion angle between adjacent rings
Authors: Brédas J. L. , Street G. B. , Thémans B. , André J. M.
Published: July 2002

We present ab initio Hartree–Fock and valence effective Hamiltonian (VEH) calculations on polyparaphenylene, polypyrrole, and polythiophene dimers and polymer chains. These polymeric materials are among the most studied compounds in the field of conducting polymers. We examine, as a function of the torsion angle between consecutive rings, the evolution of electronic properties such as ionization potential, bandgap and width of the highest occupied bands and of the carbon–carbon bond length between rings. This investigation is motivated by the fact that many derivatives of these compounds have substituents that lead to an increase of the torsion angle between adjacent rings, as a result of steric interactions. As expected, on going from a coplanar to a perpendicular conformation, the ionization potential and bandgap values increase and the width of the highest occupied bands decreases. This makes it more difficult to ionize or reduce the polymer chains and can result in achieving lower maximum conductivities on doping. However, since the evolution of the electronic properties is found to follow a cosine law (related to the decrease of the overlap between the π orbitals on adjacent rings), the modifications up to a ∼40° torsion angle are not very large. For instance, in all three polymers, the ionization potential value for a 40° torsion angle is about 0.4 eV larger than the coplanar conformation value. Therefore, substituents that lead to torsion angles between consecutive rings smaller than 40° are quite acceptable. Finally we discuss the importance, for the substituted compounds, of the possibility of achieving a coplanar conformation upon doping, in order to permit high intrachain mobilities of charge carriers such as bipolarons.
Variational calculation of the tunneling system interacting with a heat bath. II. Dynamics of an asymmetric tunneling system
Authors: Harris R. A. , Silbey R.
Published: July 2002

The dynamics of tunneling of an asymmetric double well interacting with a heat bath is reexamined in the binary collision dilute gas phase limit and investigated in the harmonic bath case. (a) In both cases dynamic and static asymmetries are additive. (b) In both cases, when the tunneling amplitude is not renormalized to a value of zero, the effect of asymmetry on the approach to equilibrium due to incoherent tunneling only quantitatively differs from the symmetric double well. (c) When the renormalized tunneling amplitude is zero, the asymmetric tunneling turns dephasing into population relaxation. (d) Variational methods again give results in agreement with renormalization group and instanton calculations.
A study of the isomeric dependence of low-energy (<10 eV) electron attachment: Perfluoroalkanes
Authors: Spyrou S. M. , Hunter S. R. , Christophorou L. G.
Published: July 2002

The attachment of low-energy (<10 eV) electrons to the open-chain i-C4F10, i-C5F12, neo-C5F12, and to the cyclic c-C4F8 and c-C5F10 perfluoroalkanes has been studied using a time-of-flight mass spectrometer (TOFMS) and a high pressure electron swarm technique as well as the TOFMS results reported earlier on c-C4F8 and i-C4F10. Both long-lived parent anions (autodetachment lifetimes τa >10−6 s) and dissociative attachment fragment anions were observed for these molecules in the mass spectrometric study. The relative cross sections for negative ion formation and the τa of the long-lived parent anions were measured as a function of electron energy in the mass spectrometric study. The absolute total electron attachment rate constants as a function of the mean electron energy have been measured in the swarm study and used to calculate the total electron attachment cross sections as a function of electron energy. Both studies have shown a strong dependence of dissociative and nondissociative electron attachment processes on the structure of the molecules studied. For neo-C5F12, the predominant anion is the fragment C4F−9. In contrast, for i-C4F10, i-C5F12, c-C4F8, and c-C5F10, the parent anions are stronger than any of the fragment anions. Energetic considerations have been employed to identify fragmentation mechanisms of the negative ion states (NISs) leading to the formation of the observed fragment anions and to deduce some thermochemical data. The positions of the NISs, the observed differences in the cross sections for parent and fragment anion formation, and the types of the fragment anions formed for the various molecules are discussed.
Direct-inelastic and trapping-desorption scattering of NO(v=0, J) from Ir(111): Angular, velocity, and rotational energy distributions
Authors: Hamers R. J. , Houston P. L. , Merrill R. P.
Published: July 2002

Angular, velocity and rotational energy distributions are reported for the scattering of NO from IR(111) at surface temperatures from 100–700 °K. (AIP)
The convergence of CI calculations for atomic and molecular electronic bound states in a basis of floating Gauss orbitals
Authors: Klahn Bruno
Published: July 2002

The energy convergence of CI calculations for atomic and molecular electronic bound states in a basis set of floating orbitals, which may depend on the number of basis functions used in a calculation, is investigated. Applying a generalization of the Müntz–Szász theorem, a condition for the choice of floating orbital exponents with respect to Gaussians results, which guarantees convergence of energies and wave functions to the right limits in any state, as the number of basis orbitals increases to infinity. This condition is discussed for the special choice of even-tempered Gaussians.
Calculated electric dipole moment of NiH X 2Δ
Authors: Walch Stephen P. , Bauschlicher Charles W. , Langhoff Stephen R.
Published: July 2002

A calculated dipole moment of 2.39 D is reported for the X 2Δ state of NiH (based on extensive CASSCF/CI calculations). The computed dipole moment is found to be a sensitive function of the 3d population of the wave function; thus, the excellent agreement with the experimental dipole moment, 2.4±0.1 D, provides strong confirmation of the mixed state character of NiH (computed 3d population of 8.66 electrons).
Local group modes and the dynamics of intramolecular energy transfer across a heavy atom
Authors: Lopez Vicente , Fairen Victor , Lederman Steven M. , Marcus R. A.
Published: July 2002

The dynamics of energy transfer is discussed for a model system in which two ligands are separated by a heavy atom. Numerical and analytical results are given for the case that each ligand is a CC. In the quasiperiodic regime, the dynamics are interpreted using perturbation theory. Local group modes involved in an intramolecular energy localization which can occur in this regime are identified. An approximate separation of the primarily ligand–ligand motions from the primarily ligand–metal–ligand ones occurs in the clearly quasiperiodic regime and also at an energy where the power spectra of the bond coordinates are ‘‘grassy.’’ The overall analysis is used to make predictions for systems with larger ligands, when the primarily metal atom–ligand modes are, as above, approximately separable from the primarily intraligand ones.
Use of nonlinear convergence accelerators for the efficient evaluation of GTO molecular integrals
Authors: Grotendorst Johannes , Steinborn E. Otto
Published: July 2002

Different methods for accelerating convergence of known series expansions for the auxiliary functions Fm(t) encountered in molecular integrals over Gaussian-type orbitals have been tested and compared. We have investigated the numerical properties of the Shanks transformation and the Levin transformations. It turns out that Levin’s t transformation is the best accelerator, whereas the Shanks transformation, which nearly yields the same results as the continued-fraction method, is less effective.
Laser induced excitation spectroscopy of copper trimer in various stages of supersonic expansion: Observation of fluorescence from dissociative levels
Authors: Crumley W. H. , Hayden J. S. , Gole J. L.
Published: July 2002

Gas phase spectroscopic studies of copper trimer are reported. Using laser induced excitation spectroscopy and a trimer source producing concentrations some three to six orders of magnitude in excess of those available from alternative devices, we obtain the visible spectrum of the trimer. This spectrum has been associated with a 2E″–2E′ transition of a D3h molecule in which both electronic states undergo Jahn–Teller distortion. Spectra obtained under a wide variety of experimental conditions encompass the wavelength range 6050–5180 Å corresponding to a range of effective temperatures. Data obtained in the present study are correlated with the recent studies of Morse et al. (laser vaporization, R2PI, and depletion spectroscopies) and Moskovits and co-workers (resonance Raman-matrix isolation). The observed spectroscopic features are in one-to-one correspondence with the detailed excited state vibronic calculations of Thompson et al. and in some respects may agree with the ‘‘first order’’ ground state vibronic calculations of Moskovits. The generated concentration of trimer molecules is such as to allow the characterization of strongly predissociative levels lost to the R2PI technique. The extent of the observed spectra allow the separation of level structure associated on the one hand with the excited state symmetric stretch mode and on the other with the vibronic levels resulting from the coupling of the asymmetric stretch and bending modes. Significant changes in those features which dominate observed excitation spectra as a function of effective temperature are interpreted in terms of a change in the ground state population distribution and hence an alteration in the configuration space connecting populated ground state and accessed excited state levels. These temperature dependent effects and observed hot band structure imply that the pseudorotation barrier in the ground electronic state of the trimer is considerably smaller than previously suggested.
Renormalization group treatment of excluded volume effects in a polyelectrolyte chain in the weak electrostatic coupling limit. II. Decomposition of interactions and calculation of properties
Authors: Bawendi M. G. , Freed Karl F.
Published: July 2002

A theoretical description of excluded volume in polyelectrolytes is considered in the flexible chain limit. The approach generalizes the work of Kholodenko and Freed and removes some deficiencies, present in their work, which become apparent in further extensions of the theory. Renormalization group calculations are pursued to order ε=4−d for the mean square end-to-end distance 〈R2〉, the radius of gyration 〈S2〉, and the second virial coefficient A2. The importance of including both a short range excluded volume interaction as well as a long range electrostatic one is stressed. The electrostatic interaction is effectively treated as a perturbation, so that the results are valid only in the weak electrostatic coupling region. The theory shows that polyelectrolyte excluded volume effects cannot generally be described in a scaling limit, and generalized scaling laws are derived which contain several parameters including the two interaction parameters. A qualitative comparison of the theory with available experimental data is included, and a program for evaluating effective charge distributions on polyelectrolytes is proposed based upon comparison of experiments with extensions of the theory to higher concentrations.
Methods for analyzing spectroscopic line shapes. NMR solid powder patterns
Authors: Alderman D. W. , Solum Mark S. , Grant David M.
Published: July 2002

A new method is described for numerically computing theoretical NMR powder patterns which achieves a many-fold increase in speed and accuracy over previous techniques. The method incorporates a simple and efficient technique for selecting the set of crystal orientations over which the spectral frequencies are calculated. The orientation selection technique is then integrated with an interpolation scheme which transfers the intensities at these frequencies to the computed spectrum. The method will be useful whenever an average over a sphere is computed numerically. The new efficiency of the method makes practical least squares fitting of theoretical spectra to experimental NMR data. The fits provide unbiased estimates of the NMR parameters and their errors. The technique is illustrated by extracting chemical shift tensors from a proton decoupled carbon-13 NMR spectrum.
Dissociative single and double photoionization of N2: Analytical photoion spectroscopy
Authors: Masuoka Toshio , Fujikawa Hiroshi
Published: July 2002

Observed branching ratios for the fragment ions N+ produced in dissociative photoionization of N2 have been differentiated with respect to photon energy. The results indicate dissociation channels of the parent ions and the molecular Rydberg states into the fragment ions. The mechanisms of dissociative photoionization have been discussed by comparing the observed differential spectra with the known ionic states including those such as the multielectron bands observed in photoelectron spectra and the double ionization states observed in Auger spectra. It becomes evident that these ionic states play an important role in producing the fragment ions.
On the reaction Na(2P)+H2→Na(2S)+H2 nonadiabatic effects
Authors: Yarkony David R.
Published: July 2002

This work reports the first ab initio treatment of nonadiabatic coupling strengths for the quenching reaction, Na(2P)+H2→Na(2S)+H2. In particular, guided by the ab initio SCF-CEPA calculation of Botschwina et al. [J. Chem. Phys. 75, 5438 (1981)], the C2v allowed crossing seam of conical intersections of the 1 2A′ and 2 2A′ potential energy surfaces is located at the state averaged MCSCF/CI level. These crossings provide the path or funnel for channeling the Na(2P) nonradiatively to Na(2S). In order to determine the efficacy of this feature in inducing this transition, nonadiabatic coupling matrix elements arising from three internal coordinates are determined using a recently developed algorithm based on analytic gradient techniques.
On the manifestation of electronic structure effects in metal clusters
Authors: Kappes Manfred M. , Schär Martin , Radi Peter , Schumacher Ernst
Published: July 2002

Potasium (Kx x<40) and sodium (Nax, x<66) clusters were generated in molecular beams and probed by photoionization mass spectroscopy. Results obtained include measurements of neutral cluster abundances and determinations of ionization potentials. The I. P. values can be rationalized in terms of a global electrostatic model which extrapolates to the bulk work function. This model also applies to transition metals as can be shown by comparng it to the limited experimental data sets available. The I. P.’s of small clusters exhibit ‘‘quantum size’’ effects which can be understood by individual quantum chemical calculations. As previously found for sodium clusters, photoionization mass spectra obtained for potassium, mixed potassium/sodium and potassium/lithium exhibit abundance maxima at M+8 and M+20, where M is an alkali metal. This has been interpreted in terms of increased thermodynamic stability of the corresponding neutrals relative to neighboring clusters. We present data which show that a spherical jellium model, while providing a set of numbers correlating well with those of preferred stability in alkali clusters, is less successful in explaining other properties.
Intramolecular hydrogen transfer in the o-tolyl radical studied by time resolved absorption measurements
Authors: Ichimura Teijiro , Mori Yuji , Sumitani Minoru , Yoshihara Keitaro
Published: July 2002

The intramolecular hydrogen transfer rate of o-tolyl radicals has been examined by monitoring the absorption time profile of benzyl radicals formed in the picosecond 266 nm laser photolysis of o-chlorotoluene. The rise time of the benzyl radical absorption, 6.5 ns, was attributed mostly to the rearrangement rate of o-tolyl radicals to benzyl radicals.
Photoelectron spectroscopy of the NO dimer
Authors: Carnovale Frank , Peel J. Barrie , Rothwell Richard G.
Published: July 2002

He i photoelectron spectra are presented for the NO dimer, (NO)2, and for a mixture of higher clusters, (NO)nn≥3. The NO dimer shows a weak second band associated with a shake-up transition at a vertical I.E. of 10.8 eV, 1.9 eV above that of the first band. This disagrees with a theoretical estimate of 2.7 eV. The ratio of intensities of these two bands is measured as 1:0.18 in comparison with the theoretical estimate of 1:0.28. The spectrum of the mixture of higher clusters shows the analogous two bands at vertical I.E.s of 8.4 and 10.8 eV, respectively. The band separation of 2.4 eV is close to the value of 2.6 eV observed in the photoemission spectrum of condensed NO.
Semiclassical calculation of eigenvalues for higher order resonances in nonseparable oscillator systems
Authors: Noid D. W. , Marcus R. A.
Published: July 2002

It is shown how the ‘‘trajectory-close method’’ introduced in earlier papers of this series can be used to treat other resonant systems semiclassically. The method, which does not involve the use of any curvilinear coordinate system, is illustrated for two coupled oscillators which have 3:1, 4:1, 5:1, 3:2, and 5:2 internal resonances. It is readily executed and it is shown how it can be extended to the three-oscillator case. This work supplements our earlier studies of 1:1, 2:1, and 3:1 resonant systems using this technique. Shapes of eigentrajectories and of corresponding quantum mechanical wave functions are compared for each of these systems. The paper also contains a survey of and comparison with other semiclassical methods which have been applied to systems with internal resonances.