bluuebunny/crossref_metadata_2025 huggingface dataset:
Field emission from phosphorus-doped polycrystalline diamond films
Authors: Sugino Takashi , Kuriyama Kenji , Kimura Chiharu , Yokota Yuuko , Kawasaki Seiji , Shirafuji Junji
Published: July 2002

Field emission characteristics have been investigated for P-doped polycrystalline diamond films. It is demonstrated that the turn-on voltage of the electron emission decreases with increasing temperature for the P-doped diamond film, while no variation in the turn-on voltage occurs for the undoped diamond film. The temperature-dependent field emission characteristics are found to be inherent to the P-doped diamond film. A behavior of the field emission characteristics can be well explained by means of the thermionic field emission model combined with the temperature dependence of the ionized donor concentration. This means that an increase of the ionized donor concentration with increasing temperature may lead to a reduction in the tunnel barrier width at the interface between the diamond and the cathode, resulting in an enhancement of the internal emission current. It is suggested that the internal electron emission is important to the field emission characteristics of the P-doped diamond films. A variation in the field emission characteristics of P-doped diamond films with various cathode metals and with various P-doping concentrations can be consistently understood on the basis of the internal electron emission model.
Transient electroluminescence of polymer light emitting diodes using electrical pulses
Authors: Pinner D. J. , Friend R. H. , Tessler N.
Published: July 2002

Detailed experimental and theoretical analysis of the pulsed excitation of polymer light emitting diodes is presented. We find a set of universal transient features for a variety of device configurations (different polymers/cathodes) which can be reproduced using our phenomenological numerical model. We find that the temporal evolution of the electroluminescence in response to a step voltage pulse is characterized by: (i) a delay followed by; (ii) a fast initial rise at turn-on followed by; (iii) a slow rise (slower by at least one order of magnitude). The large mobility mismatch between holes and electrons in conjugated polymers allows us to separately time resolve the motion of holes and electrons. We suggest a method for extracting mobility values that takes into account the possible field-induced broadening of carrier fronts, and which is found to be compatible with mobilities determined from constant wave measurements. By using appropriate device configurations it is possible to determine the mobilities of both holes and electrons from a single device. Mobilities for holes and electrons are extracted for a poly(p-phenylenevinylene) copolymer and poly(di-octyl fluorene).
Heavy doping effects in Mg-doped GaN
Authors: Kozodoy Peter , Xing Huili , DenBaars Steven P. , Mishra Umesh K. , Saxler A. , Perrin R. , Elhamri S. , Mitchel W. C.
Published: July 2002

The electrical properties of p-type Mg-doped GaN are investigated through variable-temperature Hall effect measurements. Samples with a range of Mg-doping concentrations were prepared by metalorganic chemical vapor phase deposition. A number of phenomena are observed as the dopant density is increased to the high values typically used in device applications: the effective acceptor energy depth decreases from 190 to 112 meV, impurity conduction at low temperature becomes more prominent, the compensation ratio increases, and the valence band mobility drops sharply. The measured doping efficiency drops in samples with Mg concentration above 2×1020 cm−3.
Effects of strained layer near SiO2–Si interface on electrical characteristics of ultrathin gate oxides
Authors: Eriguchi Koji , Harada Yoshinao , Niwa Masaaki
Published: July 2002

Ultrathin gate oxides formed by different process technologies are investigated in detail. The following important evidence is found: the discrepant result on the two time-dependent dielectric breakdown (TDDB) lifetime measurements, the constant-current stress, and the constant-voltage stress. The discrepancy is due mainly to the difference in the oxide leakage characteristics. Apparent changes in the activation energy and the defect generation rate during the TDDB testing are also experimentally observed for the two oxides formed by different process technologies. From the analysis based on the x-ray photoelectron spectroscopy by means of the oxide etch by dilute HF and the Fourier-transform infrared attenuated total reflection method, we consider that the above phenomena are induced by the difference in the built-in compressive strain of the Si–O network near the SiO2 and Si interface.
Cross phase modulation artifact in liquid phase transient absorption spectroscopy
Authors: Ekvall K. , van der Meulen P. , Dhollande C. , Berg L.-E. , Pommeret S. , Naskrecki R. , Mialocq J.-C.
Published: July 2002

We present experimental results for the cross phase modulation (xpm) induced transient absorption signal in a 1 mm thick fused silica plate using a white light continuum as a probe. The fused silica plate mimics the entrance window of a commercial flow cell commonly used in liquid-phase transient absorption measurements. The experimental results are compared with those obtained theoretically by numerically solving the set of nonlinear coupled wave equations describing the propagation of the pump and the probe. The simulations allow for the different group velocities of the pump and probe pulses, and include the influence of the first and second order dispersion on the continuum probe. From the calculations the physical origin of the complex oscillatory feature observed around the zero time delay of each wavelength of the (chirped) continuum has been accurately identified. The influence of propagation effects arising from the finite thickness of the sample is discussed in great detail, and the necessity to work with thin samples, preferably free-flowing jets, is emphasized. The good agreement between theory and experiment indicates that the xpm artifact may be useful for characterizing the continuum probe, in particular its chirp.
Large field induced strain in single crystalline Ni–Mn–Ga ferromagnetic shape memory alloy
Authors: Murray S. J. , Marioni M. A. , Kukla A. M. , Robinson J. , O’Handley R. C. , Allen S. M.
Published: July 2002

A room temperature free shear strain of 5.7% is reported in a single crystal of Ni–Mn–Ga having a composition close to the Heusler alloy Ni2MnGa. A twin boundary was created in a 2 mm×2 mm×25 mm single crystal using a permanent magnet with surface field strength of about 320 000 A/m. A sharp 6.5° bend occurs in the sample at the twin boundary. The surface magnetization changes abruptly across this boundary. By moving the sample relative to the edge of the magnet, we were able to sweep the boundary back and forth along the crystal length. Surface magnetization was measured using a Hall probe and the results confirm that the easy axis is the tetragonal c axis. Powder x-ray diffraction shows that the fcc to body-centered-tetragonal bct martensitic transition of this material involved a 6% reduction of the bct cell c/a ratio, from √ to about 1.33. The maximum achievable strain is thus estimated to be 6.2%. The twin planes in the system are the {112}bct and were observed to lie almost normal to the long axis of the sample tested.
Magnetic ordering in M(ox)(bpy) system (M=Fe, Co, Ni; ox=C2O42−; bpy=4,4′-bipyridine)
Authors: Yuen Tan , Lin C. L. , Mihalisin T. W. , Lawandy Michael A. , Li Jing
Published: July 2002

The temperature and field dependence of the magnetization have been measured to investigate the magnetic properties of the first oxalate–bpy mixed-ligand framework with a general formula M(ox)(bpy) (M = Fe(II), Co(II), Ni(II); ox = C2O42−; bpy = 4,4′-bipyridine). All compounds in the M(ox)(bpy) system crystallize in an orthorhombic structure with space group Immm (No. 71), in which the magnetic M ions form one-dimensional chains along the a axis. Spontaneous magnetic orderings with transition temperatures of 12, 13, and 26 K have been found for Fe(ox)(bpy), Co(ox)(bpy), and Ni(ox)(bpy), respectively. The nature of the ordering in Ni(ox)(bpy) is antiferromagnetic, however it may exhibit a metamagnetic transition at high magnetic fields. The type of ordering in Fe(ox)(bpy) and Co(ox)(bpy) is also antiferromagnetic but with canting, or so-called weak ferromagnetism. The residual moment resulting from the canting is larger in Fe(ox)(bpy) than in Co(ox)(bpy). The values of μeff obtained by fitting χ(T) indicate a high spin state for M ions in all three compounds. The origin of the magnetic ordering in these compounds is attributed to antiferromagnetic exchange interactions between the M ions through the bridging oxalate molecules along the magnetic chain and interchain M–M couplings along the [111] direction.
Effect of Ni addition into Co ferrite thin films for perpendicular recording media
Authors: Zhang F. , Kitamoto Y. , Abe M. , Naoe M.
Published: July 2002

The present article describes effects of doping Ni2+ ions into Co ferrite plated films on film structure, magnetic properties, and read/write characteristics. While thinner films are required for low noise level even in perpendicular recording media, average surface roughness Ra of the Co ferrite films increased from 4 to 6 nm with decreasing the film thickness from 100 to 50 nm. Adding Ni2+ ions had an effect on controlling the grain growth, resulting in a reduction in Ra to 3–4 nm for the 50-nm-thick films by adding a small amount of Ni2+ ions {rm[=CNi/(CCo+CNi)]=0.1}. Perpendicular coercivity Hc⊥ of the Co–Ni ferrite film at rm=0.1 was 2500 Oe, which is almost the same as that of the Co ferrite films, and Hc⊥ decreased with increasing rm. Atomic force microscope images showed that the Co–Ni ferrite films were composed of uniform and isolated grains without coagulation. As a result, a Co–Ni ferrite medium fabricated at rm=0.1 exhibited lower noise level than a Co ferrite medium without Ni addition.
Effective sulfur passivation of an n-type GaN surface by an alcohol-based sulfide solution
Authors: Huh Chul , Kim Sang-Woo , Kim Hyun-Soo , Lee In-Hwan , Park Seong-Ju
Published: July 2002

The sulfur passivation of an n-GaN surface was investigated by employing an aqueous (NH4)2S solution and (NH4)2S+t-C4H9OH solution. Photoluminescence and Auger electron spectroscopy revealed that treatment with (NH4)2S+t-C4H9OH results in a more effective passivation of the n-GaN surface than that with (NH4)2S due to a higher chemical reactivity of sulfur species in the former solution. The (NH4)2S+t-C4H9OH-treated sample shows a stronger photoluminescence intensity by a factor of 2.5 with respect to an untreated sample. In addition, improved Ohmic characteristics of the sample are evident from current–voltage measurements. This result can be attributed to the effective removal of an insulating layer on the n-GaN surface.
Tunneling magnetoresistance of a GaMnAs-based double barrier ferromagnetic tunnel junction
Authors: Hayashi T. , Tanaka M. , Asamitsu A.
Published: July 2002

Introduction of a ferromagnetic quantum well in a ferromagnetic tunnel junction is shown to greatly enhance the tunneling magnetoresistance (TMR) effect, due to spin filtering as well as energy filtering. We have theoretically analyzed resonant transmission probability of a magnetic double barrier heterostructure consisting of III–V based ferromagnetic semiconductor GaMnAs, and nonmagnetic semiconductor AlAs. Experimentally, we have observed very large TMR effect of such a system grown by low-temperature molecular-beam epitaxy, and have shown that some of the measured tunneling features are attributed to spin-dependent resonant tunneling.
Structure and magnetic properties of RCo7−xZrx (R=Y, Gd, Nd, or Ho, x=0–0.8)
Authors: Huang M. Q. , Sankar S. G. , Wallace W. E. , McHenry M. E. , Chen Q. , Ma B. M.
Published: July 2002

Alloys with the composition RCo7−xZrx (R=Y, Gd, Nd, or Ho, x=0–0.8) were synthesized and characterized in the temperature range from 10 to 1273 K and in fields up to 5 T. As with the R=Sm, Pr, or Er systems studied earlier in our laboratory, the effects of Zr doping on the stability of TbCu7 structure phase and changes in the magnetocrystalline anisotropy HA are observed in the systems in which the rare earth is Y, Gd, Nd, or Ho. Nearly single phase TbCu7 structure materials were formed in the as-cast alloys when x=0.1 and 0.2. In the case of R=Y or Gd, a large increase in HA (which is mainly contributed by Co sublattice) by Zr doping was observed. For R=Y, HA increases from 18 kOe for x=0 to 74 kOe for x=0.2 at 300 K and from 20 kOe for x=0 to 82 kOe for x=0.2 at 10 K. For R=Gd, HA increases from 35 kOe for x=0 to 140 kOe for x=0.2 at 300 K and to 182 kOe for x=0.2 at 10 K. In the case of R=Ho and Nd, the R-sublattice favors a planar anisotropy at room temperature, presumably due to the negative second order Stevens’ coefficient. For R=Ho, the anisotropy changes from planar for x=0 to axial with HA∼60–70 kOe at 300 K when x=0.1 and 0.2. For R=Nd, HA is dominated by R sublattice which results in a planar anisotropy. Other magnetic properties such as magnetization, Curie temperature, and spin reorientation temperature for these alloys are presented.
Generalized discrete variable approximation in quantum mechanics
Authors: Light J. C. , Hamilton I. P. , Lill J. V.
Published: July 2002

The formal definition of the generalized discrete variable representation (DVR) for quantum mechanics and its connection to the usual variational basis representation (VBR) is given. Using the one dimensional Morse oscillator example, we compare the ‘‘Gaussian quadrature’’ DVR, more general DVR’s, and other ‘‘pointwise’’ representations such as the finite difference method and a Simpson’s rule quadrature. The DVR is shown to be accurate in itself, and an efficient representation for optimizing basis set parameters. Extensions to multidimensional problems are discussed.
Free-ion, crystal-field, and spin-correlated crystal-field parameters for lanthanide ions in Cs2NaLnCl6 and Cs2NaYCl6:Ln3+ systems
Authors: Reid Michael F. , Richardson F. S.
Published: July 2002

Free-ion and crystal-field energy parameters for trivalent lanthanide ions (Ln3+) in Cs2NaLnCl6 and Cs2NaYCl6 are presented and discussed. Spin-correlated crystal-field parameters are determined for those systems for which sufficient data are available. The superposition model is used to compare the crystal-field interactions in the Cs2NaYCl6:Ln3+ and LaCl3:Ln3+ systems.
A molecular dynamics study of barium meta-fluorozirconate glass
Authors: Kawamoto Y. , Horisaka T. , Hirao K. , Soga N.
Published: July 2002

Molecular dynamics simulation was performed on barium meta-fluorozirconate, BaZrF6, glass by using the Born–Mayer–Huggins pair potential. The cumulative F coordination numbers and the F correlation functions of Zr and Ba ions revealed that, in the simulated BaZrF6 glass, the number and the mean distance of the nearest neighboring F were about eight at 2.12 Å for Zr ions and about ten at 2.75 Å for Ba ions. This is consistent with the previous x-ray diffraction result of the glass. A little uncertainty of the cumulative coordination number, however, may admit the presence of small amounts of seven F-coordinated Zr, as deduced in the previous studies. On the other hand, the Zr–Zr pair correlation function disclosed that the F polyhedra of Zr are linked by sharing the edges and/or the corners, the edge-sharing/corner-sharing ratio in number being approximately 3:2. The glass transition temperature and the Zr–F symmetric stretching vibration spectrum generated for the simulated glass were favorably compared with the observed ones. The mean square displacements and the trajectories of F, Zr, and Ba ions clearly showed that fluorozirconate glasses might exhibit fluoride-ion conduction. The present successful simulation indicates the validity of the calculation procedure, especially of the evaluation of the repulsive parameters of Zr and Ba ions. Based on the previous Raman spectroscopy, differential thermal analysis, and x-ray diffraction studies and the present study, the overall structure of BaZrF6 may be described as follows: In BaZrF6 glass, most of Zr ions are dodecahedrally coordinated by eight F and a small amount of them, monocapped-trigonal prismatically by seven F. These polyhedra connect with each other by sharing the edges and/or corners to build up three-dimensional fluorozirconate network. Ba ions are present as the modifier ions in the interstices of the fluorozirconate network, being surrounded on the average by ten F.
Photoionization of water clusters at 11.83 eV: Observation of unprotonated cluster ions (H2O)+n (2≤n≤10)
Authors: Shinohara Hisanori , Nishi Nobuyuki , Washida Nobuaki
Published: July 2002

Unprotonated cluster ions (H2O)+n (2≤n≤10) and (Ar)m(H2O)+n (1≤m≤3; 2≤n≤7) are detected in the mass spectra, for the first time, in addition to normally observed protonated ions (H2O)n−1H+ when supersonic cluster beams of water–argon mixtures (P≳2 atm) are photoionized with the vacuum-UV resonance lines at 11.83 and 11.62 eV. The protonated water cluster ions (H2O)n−1H+ are produced by ionization of neutral water clusters (H2O)n followed by intracluster proton-transfer reactions, whereas the unprotonated cluster ions (H2O)+n are generated via the photoionization of water–argon binary clusters: (Ar)m(H2O)n+hν →(H2O)+n+mAr+e−. The excess energies on ionization can be randomized within the [(Ar)m(H2O)+n]*vip clusters (‘‘intracluster excess energy dissipation’’) and finally converted to the decompositions of argon atoms, giving rise to the stable (H2O)+n and various (Ar)k(H2O)+n ions. The (H2O)+n ions have the distinct potential energy minima along the proton transfer reaction coordinates.
Velocity modulation laser spectroscopy of negative ions: The infrared spectrum of hydroxide (OH−)
Authors: Rosenbaum Neil H. , Owrutsky Jeffrey C. , Tack Leslie M. , Saykally Richard J.
Published: July 2002

We report the measurement and analysis of the fundamental vibration–rotation bands of 16OH− and 18OH− near 3550 cm−1 by velocity modulation spectroscopy with a color center laser. These bands were analyzed with a least squares fit yielding vibration–rotation constants through sextic distortion terms. From the band origins of the two isotopes, harmonic frequencies and anharmonicities were calculated and a dissociation energy was estimated assuming a Morse potential. Comparisons with high level ab initio calculations are presented. The OH− concentration is found to be dramatically dependent on the presence of metal sputtered on the discharge cell wall.
An equation of state for dense rigid sphere gases
Authors: Ma Danning , Ahmadi Goodarz
Published: July 2002

Based on the existing expressions for virial series, molecular-dynamics data, and the expected asymptotic behavior, a new improved equation of state for dense rigid-sphere gases is developed. Comparisons with the existing equations and computer simulation data are made. It is shown that the developed equation of state is in excellent agreement with the simulation results. In particular, the equation is well suited for describing the state of dense-sphere gases at extreme high concentration limits as long as the transition to an order state does not occur.
Einstein coefficients and transition moment variation for the NO(A 2Σ+–X 2Π) transition
Authors: Piper Lawrence G. , Cowles Lauren M.
Published: July 2002

Branching ratio measurements for the NO γ bands excited by energy transfer from metastable nitrogen molecules show that the electronic transition moment for the NO(A 2Σ+–X 2Π) transition varies by about 40% over the r-centroid range of 1.13–0.97 Å. Combining this transition-moment variation with radiative lifetime measurements provides a complete set of Einstein coefficients for NO(A–X) transitions from v′=0–2.
The discrete variable–finite basis approach to quantum scattering
Authors: Lill J. V. , Parker Gregory A. , Light John C.
Published: July 2002

A discrete variable representation for scattering problems is developed. In this representation the potential matrix is diagonal, with elements being the potential evaluated at the proper quadrature points. The angular momentum operators may be treated exactly up to truncation of the basis set and provide the coupling in the coordinate-labeled discrete variable representation. The definition of the inner product over the internal coordinates as quadratures rather than integrations allows a discrete matrix transformation to be used to diagonalize any potential matrix. This framework allows one to obtain approximate solutions in a particularly simple and efficient manner and is presented in detail for atom–diatom collisions. At large values of the scattering distance the coupled equations may be solved to a high degree of accuracy using the distorted wave approximation in the finite basis representation. At small values of the scattering distance an exactly analogous technique may be used to obtain an approximate solution in the discrete variable representation. The solutions may be matched exactly and the exact scattering boundary conditions applied. Numerical results from atom–rigid rotor collisions are presented.
Polymer shapes in three dimensions
Authors: Bishop Marvin , Michels J. P. J.
Published: July 2002

The asymmetry and degree of prolate or oblateness of three-dimensional linear and ring polymers, with and without excluded volume, are investigated by Brownian dynamics. It is found that the simulation results are consistent with theoretical predictions and that the asymmetry is a better measurement parameter of polymer shapes than the degree of prolate or oblateness because it is easier to obtain accurate data about it.
Theory of oriented symmetric-top molecule beams: Precession, degree of orientation, and photofragmentation of rotationally state-selected molecules
Authors: Choi Seung E. , Bernstein Richard B.
Published: July 2002

A theoretical study of the properties of rotationally state-selected symmetric top molecules has been carried out. Quantum mechanical and classical calculations are presented for the orientational probability density function PJKM( ρ), characterizing the molecular precessional motion for various rotational states ‖JKM〉 in terms of the degree of orientation ρ (≡μ̂⋅ℰ̂) of the dipole moment μ in a (weak) electric field ℰ. For all J>0, the breadths of the quantum mechanical distributions PJKM( ρ) are significantly greater than those of the classically calculated ones Pcl( ρ). Their first moments, i.e., 〈 ρ〉≡KM/(J2+J), are, of course, identical. P( ρ) is, in principle, directly observable through measurements of polarized laser-induced photofragmentation of oriented molecule beams. Expressions are derived to analyze experimentally observable angular distributions of photofragments. It is shown that IJKM/IJK=2PJKM( ρ), where IJKM and IJK are the photofragment intensities from oriented (JKM) and unoriented (JK) state-selected beams, respectively. From this result, one can determine the quantum number J purely from experimental photofragmentation data, i.e., I( f)JKM/IJK =(2J+1)δK,M. The f denotes that the molecules are oriented with their dipole moment parallel to the orienting field in the ‘‘favorable’’ direction. Expressions and calculations are presented appropriate to the description of the angular distribution of the photofragments of oriented symmetric-top molecules.
Infrared spectra of carboions. II. ν3 band of acetylene ion C2H+2(2Πu)
Authors: Crofton Mark W. , Jagod Mary-Frances , Rehfuss Brent D. , Oka Takeshi
Published: July 2002

The infrared spectrum of the ν3 band of C2H+2(2Πu) has been observed and analyzed. This is the first observation of this fundamental molecular ion in any spectral range.
Intrinsic viscosity from the Green–Kubo formula
Authors: Jagannathan A. , Oono Y. , Schaub B.
Published: July 2002

The Green–Kubo formalism and the conventional Kirkwood–Riseman formalism (with and without hydrodynamic preaveraging approximation) are compared in the calculation of the intrinsic viscosity of a Gaussian chain. It is discussed that the Green–Kubo-type formula for the intrinsic viscosity is correct to order ε(=4−d, d being the spatial dimension). Then it is shown that without self-avoiding interaction the two formalisms give identical results to order ε; i.e., the Kirkwood–Riseman formalism is as reliable as the Green–Kubo formalism without self-avoiding interactions. With self-avoiding interaction it is argued that the validity of the Kirkwood–Riseman formalism cannot be expected.
Photoionization of the valence orbitals of OH
Authors: Stephens J. A. , McKoy V.
Published: July 2002

We report the results of studies of the photoionization cross sections and asymmetry parameters for the 3σ and 1π levels of OH, corresponding to the production of the A 3Π, c 1Π, a 1Δ, b 1Σ+, and X 3Σ− molecular ions. The calculations employed multiplet-specific Hartree–Fock potentials and numerical photoelectron continuum orbitals, obtained using the iterative Schwinger variational method. Noticeable nonstatistical behavior of the cross sections is seen, mainly for the 1π level, although deviations are not as pronounced as in other open-shell systems. Comparison with fragmentary experimental data is encouraging, although synchrotron radiation studies are needed to fully assess the accuracy of the calculated cross sections.
Theoretical study of the alkali and alkaline–earth monosulfides
Authors: Partridge Harry , Langhoff Stephen R. , Bauschlicher Charles W.
Published: July 2002

A b initio calculations are used to determine accurate spectroscopic constants (re, ωe, De ) for the X 2Π and A 2Σ+ states of the alkali sulfides (LiS–RbS) and the X 1Σ+, a 3Π, and A 1Π states of the alkaline–earth sulfides (BeS–SrS). In contrast to the alkali oxides, all of the alkali sulfides have X 2Π ground states owing to the larger electrostatic interaction. For the X 1Σ+ states of the alkaline–earth sulfides, our recommended dissociation energies (D0 in eV) are BeS(3.27), MgS(2.32), CaS(3.29), and SrS(3.41), in overall good agreement with the (uncertain) experimental values. Our computed Te(a 3Π) and Te(A 1Π) values for BeS of 7269 and 8244 cm−1 are in excellent agreement with experiment. For the heavier alkaline–earth sulfides, our calculated Te(a 3Π) values (cm−1) are MgS(4218), CaS(6528), and SrS(6870), and our calculated Te(A 1Π) values are MgS(5068), CaS(6901), and SrS(7179). Core correlation increases the Te values for the a 3Π and A 1Π states of MgS, CaS, and SrS.
The determination of molecular structures by density functional theory. The evaluation of analytical energy gradients by numerical integration
Authors: Versluis Louis , Ziegler Tom
Published: July 2002

An algorithm, based on numerical integration, has been proposed for the evaluation of analytical energy gradients within the Hartree–Fock–Slater (HFS) method. The utility of this algorithm in connection with molecular structure optimization is demonstrated by calculations on organics, main group molecules, and transition metal complexes. The structural parameters obtained from HFS calculations are in at least as good agreement with experiment as structures obtained from ab initio HF calculations. The time required to evaluate the energy gradient by numerical integration constitutes only a fraction (40%–25%) of the elapsed time in a full HFS-SCF calculation. The algorithm is also suitable for density functional methods with exchange-correlation potential different from that employed in the HFS method.
Empirical three-body potential for vitreous silica
Authors: Feuston B. P. , Garofalini S. H.
Published: July 2002

A three-body potential suitable for molecular dynamics (MD) simulations has been developed for vitreous silica by adding three-body interactions to the Born–Mayer–Huggins (BMH) pair potential. Previous MD simulations with the BMH potential have formed glassy SiO2 through the melt-quench method with some success. Though bond lengths were found to be in fair agreement with experiment, the distribution of tetrahedral angles was too broad and the model glass contained 6%–8% bond defects. This is indicative of a lack of the local order that is present in the laboratory glass. The nature of the short range order is expected to play an important role when investigating defect formation, surface reconstruction, or surface reactivities. An attempt has been made to increase the local order in the simulated glass by including a directional dependent term in the effective potential to model the partial covalency of the Si–O bond. The vitreous state obtained through MD simulation with this modified BMH potential shows an increase in the short range order with a narrow O–Si–O angle distribution peaked about the tetrahedral angle and a low concentration of bond defects, typically ∼1%–2%. The static structure factor S(q) is calculated and found to be in good agreement with neutron scattering results. Intermediate range order is also discussed in reference to the distribution of ring sizes.
Quantum Monte Carlo dynamics: The stationary phase Monte Carlo path integral calculation of finite temperature time correlation functions
Authors: Doll J. D. , Beck Thomas L. , Freeman David L.
Published: July 2002

We present a numerically exact procedure for the calculation of an important class of finite temperature quantum mechanical time correlation functions. The present approach is based around the stationary phase Monte Carlo (SPMC) method, a general mathematical tool for the calculation of high dimensional averages of oscillatory integrands. In the present context the method makes possible the direct numerical path integral calculation of real-time quantum dynamical quantities for times appreciably greater than the thermal time (βℏ). Illustrative applications involving finite temperature anharmonic motion are presented. Issues of importance with respect to future applications are identified and discussed.
Computer simulation of the dynamics of aqueous solvation
Authors: Maroncelli Mark , Fleming Graham R.
Published: July 2002

Equilibrium and nonequilibrium molecular dynamics computer simulations have been used to study the time dependence of solvation in water. The systems investigated consisted of monatomic ions immersed in large spherical clusters of ST2 water. Relaxation of the solvation energy following step junction jumps in the solute’s charge, dipole moment, and quadrupole moment have been determined from equilibrium molecular dynamics (MD) simulations under the assumption of a linear solvation response. The relaxation times observed differ substantially depending on the type of multipole jump and the charge/size ratio of the solute. These results could not be quantitatively understood on the basis either of continuum or molecular theories of solvation dynamics currently available. Even the qualitative picture of a distribution of relaxation times which monatonically increases with distance away from the solute is not correct for the systems studied. This lack of agreement is partially explained in terms of the structured environment of the first solvation shell of aqueous solutes. However, translational mechanisms of polarization decay and effects due to the finite distribution of charge within solvent molecules, which should be operative in less structured solvents as well, also contribute to deviations from theoretical predictions. The validity of a linear response approach has been examined for the case of charge jumps using nonequilibrium simulations. The observed dynamics are not generally independent of the size of the charge jump and thus linear response theories are not strictly applicable. In most cases, however, predictions based on a linear response calculation using the equilibrium dynamics of the appropriate reference system still provide a reasonable description of the actual nonequilibrium dynamics.
6-311G is not of valence triple-zeta quality
Authors: Grev Roger S. , Schaefer Henry F.
Published: July 2002

We have discovered a weakness in the construction of the 6-311G basis set. The second set of three contracted s-functions is, in fact, not a valence orbital but is instead a 1s function. Thus in the s space this basis set is better represented as 63-11. Coupled with an earlier observation that additional diffuse p functions are required to yield results comparable to those of the Huzinaga–Dunning (10s6p/5s3p) triple-zeta basis set in certain cases, this suggests that greater caution should be exercised in the use of the 6-311G basis set.
A study of the singlet and triplet states of vinylidene by photoelectron spectroscopy of H2C=C−, D2C=C−, and HDC=C−. Vinylidene–acetylene isomerization
Authors: Ervin Kent M. , Ho Joe , Lineberger W. C.
Published: July 2002

The X̃ 1A1, ã 3B2, and b̃ 3A2 states of vinylidene are observed in the ultraviolet (351.1–364.0 nm) photoelectron spectra of X̃ 2B2 H2CC−, X̃ 2B2 D2CC−, and X̃ 2A′ HDCC−. The X̃ 1A1 state exhibits vibrational structure well above the barrier for isomerization to acetylene. A strict lower bound to the lifetime of the singlet state against rearrangement is τ>0.027 ps, with an estimate of τ≊0.04–0.2 ps based on a simulation of the line shapes including rotational broadening. A vibrational analysis of the singlet and lower triplet state bands provides vibrational frequencies and estimates of the changes of molecular geometries between the anion and the neutral species. A qualitative potential energy surface for the CH2 rock mode, which closely corresponds to the reaction coordinate for isomerization, is extracted from the experimental data. The adiabatic electron affinity is EA(X̃ 1A1 H2CC)=0.490±0.006 eV and the triplet term energies are T0(ã 3B2 H2CC)=2.065±0.006 eV and T0(b̃ 3A2 H2CC)=2.754±0.020 eV. Experimental values for the bond dissociation energy of vinyl radical, D0(H2CC–H)=80.0±5.0 kcal/mol, and the acetylene–vinylidene isomerization energy, ΔHI=46.4±5.5 kcal/mol, are derived. Combining the latter value with the upper limit of Field and co-workers, ΔHI≤44.1–44.7 kcal/mol, yields ΔHI≊41–45 kcal/mol.
The coupled-cluster single, double, and triple excitation model for open-shell single reference functions
Authors: Watts John D. , Bartlett Rodney J.
Published: July 2002

The CCSDT model for general single determinant reference functions for open and closed-shell electronic states has been implemented for the first time and has been used to compute the electron affinity of the F atom, the CH2, 3B1–1A1 energy difference, and the ionization potentials of 1A1 CH2. The results compare very well with FCI and are markedly superior to those of simpler coupled-cluster methods.
Properties of atoms in molecules: Atomic polarizabilities
Authors: Laidig Keith E. , Bader Richard F. W.
Published: July 2002

The theory of atoms in molecules is applied to the determination of the atomic and group contributions to the molecular polarizability in diatomic and polyatomic systems, covering a wide range of atomic interactions. The calculations are performed through the use of coupled-perturbed Hartree–Fock theory. An applied field polarizes the atomic charge distributions and induces a transfer of charge between them. The resulting changes in the dipole moment per unit applied field determine the corresponding contributions to the polarizability. In general, the component of the polarizability along the internuclear axis in a linear molecule or in the direction of a chain of bonds in a nonlinear polyatomic molecule, exceeds the perpendicular component because of the transfer of electronic charge between the two ends of the molecule, between the methyl groups in ethane, and between the methylene groups in ethene, for example. Atoms shielded from an applied field, carbon in methane for example, polarize in a direction counter to that of the applied field, responding instead to the opposing field created by the transfer of charge between the exterior atoms. The same behavior is found in vibrationally induced polarizations of a molecular charge distribution. The extent of atomic polarization and of interatomic charge transfer caused by an external field are readily related to known properties of the atomic charge distributions and the corresponding contributions to the polarizability may be understood and predicted. The mean molecular polarizability and its group contributions are found to be linearly proportional to the corresponding sum of atomic volumes, making possible the prediction of molecular polarizabilities from tabulated atomic volumes as determined by the theory of atoms in molecules.
Gaussian-2 theory for molecular energies of first- and second-row compounds
Authors: Curtiss Larry A. , Raghavachari Krishnan , Trucks Gary W. , Pople John A.
Published: July 2002

The Gaussian-2 theoretical procedure (G2 theory), based on ab initio molecular orbital theory, for calculation of molecular energies (atomization energies, ionization potentials, electron affinities, and proton affinities) of compounds containing first- (Li–F) and second-row atoms (Na–Cl) is presented. This new theoretical procedure adds three features to G1 theory [J. Chem. Phys. 90, 5622 (1989)] including a correction for nonadditivity of diffuse-sp and 2df basis set extensions, a basis set extension containing a third d function on nonhydrogen and a second p function on hydrogen atoms, and a modification of the higher level correction. G2 theory is a significant improvement over G1 theory because it eliminates a number of deficiencies present in G1 theory. Of particular importance is the improvement in atomization energies of ionic molecules such as LiF and hydrides such as C2H6, NH3, N2H4, H2O2, and CH3SH. The average absolute deviation from experiment of atomization energies of 39 first-row compounds is reduced from 1.42 to 0.92 kcal/mol. In addition, G2 theory gives improved performance for hypervalent species and electron affinities of second-row species (the average deviation from experiment of electron affinities of second-row species is reduced from 1.94 to 1.08 kcal/mol). Finally, G2 atomization energies for another 43 molecules, not previously studied with G1 theory, many of which have uncertain experimental data, are presented and differences with experiment are assessed.
The melting curve of neon at high pressure
Authors: Vos W. L. , Schouten J. A. , Young D. A. , Ross M.
Published: July 2002

We have measured the melting curve of neon to 54.5 kbar and 328 K using a diamond-anvil cell. The measured points together with earlier low-pressure data are fitted accurately with a Simon-Glatzel function. Theoretical calculations of the melting curve using lattice dynamics and variational fluid theory with exponential-six potentials fitted to solid isotherm data are in good agreement with the experimental data. The law of corresponding states is tested for the melting curves of He, Ne, and Ar, and is found to be obeyed.
Vibrationally resolved spectra of C2–C11 by anion photoelectron spectroscopy
Authors: Arnold D. W. , Bradforth S. E. , Kitsopoulos T. N. , Neumark D. M.
Published: July 2002

Anion photoelectron spectroscopy has been employed to obtain vibrationally resolved spectra of the carbon molecules C2–C11. The spectra of C−2–C−9 are dominated by linear anion to linear neutral photodetachment transitions. Linear to linear transitions contribute to the C−11 spectrum, as well. From these spectra, vibrational frequencies and electron affinities are determined for the linear isomers of C2–C9 and C11. The term value is also obtained for the first excited electronic state of linear C4. The spectra of C−10 and C−11 show evidence for transitions involving cyclic anions and/or neutrals. Similar types of transitions are identified in the spectra of other smaller molecules, specifically C−6, C−8, and to a lesser extent C−5.
Laser-induced fluorescence spectroscopy of jet-cooled SiC2
Authors: Butenhoff Thomas J. , Rohlfing Eric A.
Published: July 2002

We present a spectroscopic study of the Ã 1B2←X̃ 1A1 system of jet-cooled SiC2 that includes both laser-induced fluorescence (LIF) excitation and dispersed fluorescence (DF) spectra. Rotational analysis of several previously unobserved bands in the LIF spectrum enables us to reassign the vibronic structure in the Ã 1B2←X̃ 1A1 system and gives an accurate term energy for the low-frequency, antisymmetric mode in the ground state, G(0,0,1)=196.37(4) cm−1. Analysis of DF spectra from ten different Ã-state levels yields vibrational term energies for 43 levels in the ground state, including (0,0,v″3) levels as high as v″3=16. The rapidly decreasing level spacings in the (0,0,v″3) progression, coupled with a dramatic decrease in the small inertial moment, give insight into the extreme anharmonic potential surface of the ground state. These data also provide indirect evidence for the onset of large-amplitude pseudorotation that samples both cyclic and linear minima. In contrast, the Ã state is quite harmonic at the cyclic minimum, indicative of a more substantial barrier between the two isomers.
Multiphoton ionization studies of clusters of immiscible liquids. II. C6H6–(H2O)n, n=3–8 and (C6H6)2–(H2O)1,2
Authors: Garrett Aaron W. , Zwier Timothy S.
Published: July 2002

Resonant two-photon ionization (R2PI) time-of-flight mass spectroscopy is used to record S0–S1 spectra of the neutral complexes C6H6–(H2O)n with n=3–8 and (C6H6)2–(H2O)1,2. Due to limitations imposed by the size of these clusters, a number of vibronic level arguments are used to constrain the gross features of the geometries of these clusters. Among the spectral clues provided by the data are the frequency shifts of the transitions, their van der Waals structure, the fragmentation of the photoionized clusters, and the complexation-induced origin intensity and 610 splitting. In the 1:3 cluster, simple arguments are made based on the known structures of the 1:1 and 1:2 clusters which lead to the conclusion that all three water molecules reside on the same side of the benzene ring.

Three structures for the 1:3 cluster are proposed which are consistent with the available data. Of these, only one is also consistent with the remarkable similarity of the 1:4 and 1:5 spectra to those of the 1:3 cluster. This structure involves a cyclic water trimer in which one of the water molecules is near the sixfold axis in a π hydrogen-bonded configuration. This structure is then expanded in the 1:4 and 1:5 clusters to incorporate the fourth and fifth water molecules in cyclic structures which place the additional water molecules far from the benzene ring without disturbing the interaction of the other water molecules with the benzene ring. For 1:n clusters with n≥6, subtle and then significant changes are observed in the spectra which indicate changes in the way the water cluster interacts with the benzene ring. This development occurs at precisely the water cluster size which calculations predict that cagelike water cluster structures will begin to compete and eventually be favored over large cyclic structures. Finally, cursory scans of the 2:1 cluster show that this cluster also fragments efficiently upon photoionization by loss of a single water molecule and that it possesses a distinctly asymmetric structure with a sizable 610 splitting.

Two potential structures are proposed for the 2:1 cluster which can both reasonably explain the observations—a BBW structure in which the water molecule (W) is π hydrogen bonded to a single benzene molecule (B) and a BWB structure in which the water molecule bridges the two benzenes via π hydrogen bonds to both.
Nascent rotational quantum state distribution of NaH (NaD) from the reaction of Na*(4 2P) with H2, D2, and HD
Authors: Bililign Solomon , Kleiber P. D.
Published: July 2002

The nascent rotational quantum state distributions of NaH and NaD products resulting from the reactions of Na*(4 2Pj) with H2, D2, and HD have been determined using the laser pump–probe technique. We have observed a bimodal rotational distribution with a minor component peaking at low J and a major component peaking at high J. We have observed no evidence for a kinematic isotope effect on the product distribution. Our results are consistent with a model wherein the reaction occurs predominantly on the attractive 2B2 potential energy surface in near C2v geometry with the rotational distribution being determined late in the exit channel.
Takens–Bogdanov bifurcation in two-variable chemical reaction systems
Authors: Li Ru-Sheng , Liu Qing-Hua
Published: July 2002

The Extended Brusselator is shown to contain Takens–Bogdanov bifurcation point, the location of which in parameter space is determined analytically.
Monte Carlo simulation of irreversible polymer adsorption: Single chains
Authors: Konstadinidis Kariofilis , Prager Stephen , Tirrell Matthew
Published: July 2002

We are reporting results of dynamic Monte Carlo simulations on the irreversible adsorption of single polymer chains. We find that the irreversibility of the surface-segment interaction results in trapping of segments exclusively in trains and loops but no tails. Such nonequilibrium effects are also observed in reversible adsorption simulations for large adsorption energies per segment (≥2kT).
Protein core assembly processes
Authors: Fiebig Klaus M. , Dill Ken A.
Published: July 2002

How does a protein or HP (hydrophobic/polar) copolymer find its globally optimal (native) state without a globally exhaustive search? This is the Levinthal paradox. We consider three routes by which a copolymer might assemble a compact conformation with a maximum number of hydrophobic (HH) contacts: (i) the exhaustive search (ES) process, which assures the global optimum; (ii) a ‘‘maximum entropy string’’ (MES), a series of stepwise decisions each of which explores conformational space exhaustively for given prior contacts; and (iii) a ‘‘T-local string,’’ or ‘‘hydrophobic zippers’’ (HZ) process, which makes HH contacts opportunistically based on prior contacts. Using a two-dimensional HP short-chain lattice model, for which the partition function is exactly enumerable, we find that for many HP sequences, T-local strings lead to the globally optimal conformation, offering a resolution to the Levinthal paradox.
A quantum mechanical representation in phase space
Authors: Torres-Vega Go. , Frederick John H.
Published: July 2002

A quantum mechanical representation suitable for studying the time evolution of quantum densities in phase space is proposed and examined in detail. This representation on ℒ2 (2) phase space is based on definitions of the operators P̂ and Q̂ in phase space that satisfy various correspondences for the Liouville equation in classical and quantum phase space, as well as quantum position and momentum ℒ2 (1) spaces. The definitions presented here, P̂=p/2−iℏ∂/∂q and Q̂=q/2+iℏ∂/∂p, are related to definitions that have been recently proposed [J. Chem. Phys. 93, 8862 (1990)]. The resulting quantum phase space representation shares many of the mathematical properties of usual representations in coordinate and momentum spaces. Within this representation, time evolution equations for complex-valued functions (wave functions) and their square magnitudes (distribution functions) are derived, and it is shown that the coordinate and momentum space time evolution equations can be recovered by a simple Fourier projection. The phase space quantum probability conservation equation obtained is a good illustration of the quantization rule that requires one to replace the classical Poisson bracket between the Hamiltonian and the probability density with the quantum commutator between the corresponding operators. The possible classical analogs to quantum probabilities densities are also considered and some of the present results are illustrated for the dynamics of the coherent state.