bluuebunny/crossref_metadata_2025 huggingface dataset:
Asymmetric giant magnetoimpedance in field-annealed Co-based amorphous ribbon
Authors: Kim Cheol Gi , Jang K. J. , Kim H. C. , Yoon S. S.
Published: July 2002

The giant magnetoimpedance (GMI) has been measured in weak-field-annealed Co-based amorphous ribbon as functions of both the annealing field, Ha, and the measuring frequency. The GMI profile measured at 0.1 MHz shows a typical two-peak behavior for the sample with Ha less than 50 mOe. The GMI peak in the region where the applied field is antiparallel to the annealing field decreases with Ha, showing an asymmetry in the GMI profile. Eventually, the GMI peak in the antiparallel-field region disappears, and a drastic step-like change in the GMI peak is revealed for an applied field which is parallel to the annealing field of 500 mOe. The asymmetric GMI phenomenon for the 0.1 MHz measuring frequency, the so-called “GMI valve,” is due to a peculiar domain structure, which occurs in the sample surface during the field annealing. The GMI peak in the parallel-field region appears again for measuring frequencies over 0.5 MHz due to the remarkable contribution of the magnetization rotation to the GMI.
Origin of exchange field reductions in diluted magnetic garnets
Authors: Dionne Gerald F.
Published: July 2002

New theory on the source of magnetic frustration that causes canting of spin alignments in diluted ferrimagnetic systems is presented, providing a fundamental basis for the widely used semiempirical relations between molecular field coefficients and sublattice dilution fractions in the ferrimagnetic garnets. The theory explains the remarkable linearity and symmetry found in the original relations between the coefficients and the sublattice dilution fractions. It also distinguishes between the dilution and canting contributions to the weakening of the exchange fields, which has remained an unresolved issue. As part of the analysis, a probability is defined for each sublattice to characterize the canting effect on a particular spin caused by the removal of a neighboring magnetic ion from the opposing sublattice. The canting probabilities are found to be almost identically 0.07 for both the octahedral and tetrahedral garnet Fe3+ sublattices, thereby confirming the overall reciprocity of the garnet magnetic system.
Improvement of the electron density in the channel of an AlGaAs/GaAs heterojunction by introducing Si δ doping in the quantum well
Authors: Bouzaïene L. , Sfaxi L. , Sghaeïr H. , Maaref H.
Published: July 2002

The electronic subband of δ-doped AlGaAs/GaAs heterostructure has been studied theoretically by the finite differential method. We use an efficient self-consistent analysis to solve simultaneously the Schrödinger and Poisson equations. The results show the possibility to increase the electron density in the channel by the introduction of the silicon δ doping in a quantum well, where the Al concentration is smaller than in the barrier. The effect of the quantum well width is studied on the electron density. To test the validity of our calculation, we have grown, by molecular beam epitaxy, a series of δ-doped AlGaAs/GaAs heterojunctions having various alloy compositions seen by the silicon. If we consider the spreading of the silicon in space during the δ-layer growth, we show that the theoretical model explains well the experimental Hall data for all aluminum concentrations.
The effect of the interface on the dc transport properties of nonlinear composite materials
Authors: Lipton Robert , Talbot D. R. S.
Published: July 2002

The effects of the interface separating two strongly nonlinear electric conductors is investigated. The interface may either be highly conducting or exhibit an electric contact resistance. Our analysis and results are based upon new variational principles for nonlinear composites with surface energies. For monodisperse suspensions of spheres separated from the matrix by a highly conducting interface, a critical direct-current (dc) applied field strength is found for which the electric potential inside the sample is the same as for a sample containing no spheres. For this field strength the overall electric current passing through the sample is the same as for a sample containing pure matrix conductor. When there is a contact resistance between the matrix and sphere phase, a critical dc applied current density is found for which the current density inside the sample is the same as for a sample containing no spheres. These results are shown to be independent of the location of the spheres within the sample. Moreover, this effect is independent of the concentration of spheres in the sample even beyond the onset of interface percolation.
Patterned growth on GaAs (311)A substrates: Engineering of growth selectivity for lateral semiconductor nanostructures
Authors: Fricke Jörg , Nötzel Richard , Jahn Uwe , Niu Zhichuan , Schönherr Hans-Peter , Ramsteiner Manfred , Ploog Klaus H.
Published: July 2002

The growth selectivity on patterned GaAs (311)A substrates differs qualitatively from that on low-index (100) and (111) substrates. During molecular beam epitaxy of (Al,Ga)As, [01-1] oriented mesa stripes develop a fast growing convex sidewall. A continuous transition occurs towards the slow growing concave sidewall upon turning the mesa along the perpendicular [-233] direction without breaking up the growth front into microfacets. This allows their systematic combination at the corner or edge of intersecting mesa stripes appropriately inclined from [01-1] among which we highlight those involving fast growing sidewalls. The scenario, which is unique for patterned GaAs (311)A substrates, offers a novel degree of flexibility for the design of lateral functional semiconductor nanostructures.
Dynamical studies of the ferroelectric domain structure in triglycine sulfate by voltage-modulated scanning force microscopy
Authors: Likodimos V. , Orlik X. K. , Pardi L. , Labardi M. , Allegrini M.
Published: July 2002

Voltage-modulated scanning force microscopy has been employed to investigate the dynamics of ferroelectric domains as a function of time and temperature in triglycine sulfate (TGS) single crystals. Branching of the domain structure and nucleation of fine domain patterns by internal fields of thermal origin have been directly observed on the submicron scale. Domain coarsening after quenching TGS samples from the paraelectric phase into the ferroelectric one has been studied, revealing a nonlinear time dependence of the characteristic correlation domain length and supporting the validity of dynamical scaling law and the conservation of the total surface charge. Substantial differences in the evolution of the domain structure have been detected during cooling or heating treatment in the ferroelectric phase. Domain contrast is shown to monitor the ferroelectric phase transition through its temperature dependence, reflecting the competition between electrostatic and piezoelectric effects.
Fe-based magnetic-semiconductor hybrid structures for photocarrier-induced magnetism
Authors: Haneda S. , Munekata H. , Takatani Y. , Koshihara S.
Published: July 2002

Preparation of GaAs–Fe composite structures has been studied by molecular-beam epitaxy. Enhanced magnetization has been observed under the red-light irradiation at 160 K. Through the study of both magnetization and magnetotransport characteristics, we believe that enhanced magnetization is strongly related to the interaction between photogenerated carriers and magnetic inclusions.
Variable color temperature fluorescent lamp
Authors: Ravi J. , Maya J.
Published: July 2002

Color temperature change in a mercury-rare gas low pressure discharge has been investigated. Different pulse waveforms have been employed to increase the ratio of mercury upper level transitions with respect to the resonant 254 nm radiation. Low pressure fluorescent light sources were made with coatings consisting of a blue phosphor, sensitive to 365 nm ultraviolet radiation, blended with the standard 254 nm excited red/green phosphors. With a fast rise excitation waveform, a color temperature rise of as much as 1700 K was realized although at a cost of 26% in relative luminous efficacy. An improved scheme for greater color temperature change is proposed based on a phosphor that is excitable by the mercury 185 nm ultraviolet radiation but which does not absorb 254 nm radiation.
X-ray diffraction and phenomenological studies of the engineered monoclinic crystal domains in single crystal relaxor ferroelectrics
Authors: Durbin M. K. , Hicks J. C. , Park S.-E. , Shrout T. R.
Published: July 2002

We present x-ray diffraction data, which demonstrate that after high field poling, solid solutions of lead zinc niobate with 8% lead titanate (PZN–PT) consist of a mixture of a dominant monoclinic phase and a remnant tetragonal phase. To understand the origin of this engineered monoclinic phase we studied a Devonshire phenomenological model that predicts the field induced phase transitions in PZN–PT. Although previous attempts to use this model for these materials have matched the phase diagram and dielectric behavior, we show that the coefficients used in those models cannot be reconciled with recent data on electric field behavior. By modifying the coefficients we are able to explain the phase diagram, dielectric behavior, and high field phase transitions in compositions near the morphotropic phase boundary. Using these coefficients, we find no stable monoclinic phase present in the model. This contradiction is explained by noting that within the engineered domain state the tetragonal regions strain the rhombohedral regions, resulting in a metastable monoclinic phase.
Separation of temperature and density effects on collision-induced Rayleigh and Raman line shapes of liquid carbon disulfide
Authors: Hegemann B. , Jonas J.
Published: July 2002

Quantitative results for the separate temperature and density effects on collision-induced scattering (CIS) line shapes for the dense molecular fluid CS2 are presented for the temperature range T=293–353 K and density range ρ=1.25–1.48 g cm−3. The depolarized Rayleigh scattering (DRS) and ν1, ν2, and ν3 mode Raman scattering line shapes are specifically investigated. The three distinctive CIS line shape regions observed, corresponding to low, intermediate, and high frequency line shape ranges, are interpreted qualitatively in terms of available theoretical models. The results generally support the multipole–induced-dipole interaction mechanism for all CIS line shapes at all frequencies, and the dominance of translational over orientational dependence of the induced dipole moment at intermediate and high frequencies of CIS line shape.
A simple kinetic theory model of reactive collisions of rigid nonspherical molecules
Authors: Evans Glenn T. , She Richard S. C. , Bernstein Richard B.
Published: July 2002

The classical kinetic theory for dilute gases of rigid convex molecules, as developed by Hoffman (1969), is now applied to the calculation on the bimolecular rate coefficient, the energy-dependent reaction cross section σR(E), and the orientation-dependent differential cross section, for general diatom–diatom reactions. Incorporated in the theory are the angular momentum and the convex shape of the colliding molecules, as well as the dependence of the barrier height upon mutual orientation. Several simple collision systems are considered, including that of two reactive ellipsoidal molecules. For atom–diatom scattering, it is found that, in the post-threshold region (E≳E0), σR(E) has quadratic and higher-order terms in E−E0 but no linear term. Like σR, the differential cross section depends sensitively upon the shape of the colliding molecules, as well as upon the angle-dependent threshold energy. For the near-spherical case, one obtains simple formulas that display explicitly the dependence of the cross sections and the rate coefficient upon the pair-anisotropy parameter λ. In the spherical limit (λ=0), the results reduce to those for the steric model of Levine and Bernstein (1984) and of Smith (1982).
Vibrational excitation in the CO2 product of the oxidation of CO on platinum: Coverage dependence and implications on reaction dynamics
Authors: Brown L. S. , Bernasek S. L.
Published: July 2002

Vibrational energy distributions in CO2 molecules formed in the catalytic oxidation of CO on platinum have been measured by using a variety of filtering techniques to analyze infrared chemiluminescent emission. For surface temperatures in the range of 650–1100 K the product molecules were vibrationally excited substantially beyond thermal equilibrium with the surface. Emission spectra observed in the 4.3 μm region were significantly red shifted from the fundamental of the asymmetric stretch at 2349 cm−1, indicating that much of the emission originated from higher lying bend–stretch combination states. The vibrational energy of the product, particularly in the asymmetric stretching mode, was sensitive to the coverage of oxygen present on the catalyst surface. These results are consistent with a model in which bending and asymmetric stretching motions contribute strongly to the reaction coordinate for CO oxidation.
Intramolecular vibrational dynamics including rotational degrees of freedom: Chaos and quantum spectra
Authors: Dai Hai-Lung , Field Robert W. , Kinsey James L.
Published: July 2002

A criterion for chaos in quantum spectra developed by Heller et al. is applied to the vibration-rotation spectrum at about 8000 cm−1 of the X̃ 1A1 state of formaldehyde. It is found that the observed increase in complexity in the spectrum as the angular momentum quantum numbers are increased corresponds nonetheless with decreasingly stochastic behavior. This can be understood in terms of an available phase space volume that expands more rapidly than the occupied phase space volume in J increases.
Theoretical investigation of the low-lying electronic states of Cu, Zn, and their ions
Authors: Sunil K. K. , Jordan K. D.
Published: July 2002

The ab initio SCF-CI procedure is utilized to calculate the energies of the low-energy excitation and ionization processes in Cu and Zn, as well as the electron affinity of Cu. Although tight f functions are required for describing d2 → f2 contributions to the correlation energy of individual states, diffuse f functions are found to be generally more important for describing the differential correlation in the electronic excitation and ionization processes.
Zeeman quantum beat spectroscopy applied to the Ã 1A2–X̃ 1A1 transition of SO2
Authors: Watanabe Hajime , Tsuchiya Soji , Koda Seiichiro
Published: July 2002

Zeeman quantum beats were observed in the fluorescence from a number of rovibronic levels of SO2 in the Ã 1A2 state. Rotationally cooled SO2 was prepared by supersonic expansion of its seeded mixture in Ar and was excited to the first electronically allowed 1A2 state by a frequency-doubled dye laser in the wavelength range 260–320 nm. In the time-resolved fluorescence measurements, a number of rovibronic lines were found to exhibit beating fluorescence decays in the presence of a weak magnetic field. The beating phenomena were analyzed on the basis of Zeeman quantum beat theory; of particular concern are the oscillation amplitude of the beating and the g value. It was found that almost all rotational levels of upper vibronic states of various bands (D, F, L, and N in Clements’ notation) possess sizable magnetic moments. On the other hand, only a limited number of rovibronic lines of several less perturbed bands (E and G) showed beating fluorescence decays. The derived g values and fluorescence lifetimes distribute rather irregularly over a wide range, but an almost linear relationship between the g value and the lifetime was found for the rR0(0) lines of various vibronic transitions. The intramolecular coupling mechanism needed to explain the anomalous behaviors of these excited rovibronic states are discussed. It is suggested that the dominant mechanism is spin-orbit coupling.
The 266 nm photolysis of ICN: Recoil velocity anisotropies and nascent E,V,R,T excitations for the CN+I(2P3/2) and CN+I(2P1/2) channels
Authors: Nadler I. , Mahgerefteh D. , Reisler H. , Wittig C.
Published: July 2002

We report the detection of nascent CN(X 2Σ+, v″=0) following the 266 nm photodissociation of 300 K ICN, using sub-Doppler resolution laser-induced fluorescence, and polarized photolysis and probe lasers. When monitoring a particular CN internal state, the translational energies of the I+CN and I*+CN channels differ by the iodine spin-orbit splitting 7603 cm−1. This is used to determine the separate contributions from each channel. For I+CN, high N″ are selectively produced, with little population below N″=20 (〈Erot〉 =3300±300 cm−1), whereas the I*+CN channel is associated with a distribution peaked sharply at low N″(〈Erot〉 =355±35 cm−1). It is clear that the low and high N″ derive from linear and bent exit channel geometries, respectively. The spatial anisotropy is high (βI =1.3±0.2; βI* =1.6±0.2) and initial excitation is via a parallel transition(s), probably to a state which begins correlating with I*+CN in the linear configuration. Nascent spin-rotation states (F1 and F2) are also resolved for each channel, and for the case of I+CN, and F1 and F2 populations are quite different. There is very little vibrational excitation (<2%), and the rotational distributions and translational energies of v″=1 and 2 correspond to those of the I+CN channel. Subsequent to initial excitation, both adiabatic and/or nonadiabatic processes can ensure access to potential surfaces not excited directly, and a model is discussed which rationalizes the present experimental results, as well as the known variation of nascent E, V, R, T excitations with the photolysis wavelength.
Double well isomerization in solution: A new comparison of theory and experiment
Authors: Flom S. R. , Brearley A. M. , Kahlow M. A. , Nagarajan V. , Barbara P. F.
Published: July 2002

The first-excited-singlet state of each of the compounds 2-vinylanthracene (2VA) and 2-(2′-propenyl) anthracene (22PA) undergoes an electronically adiabatic torsional double-well isomerization which interconverts a s-cis conformation and a s-trans conformation. The forward rate constant kf for the isomerization was measured by picosecond fluorescence spectroscopy as a function of temperature and solvent viscosity η in the range 0.23–3.25 cP, i.e., in the C5–C16 normal alkanes. For 2VA, kf is independent of η within experimental uncertainty (∼5%), while for 22PA kf≂cη−0.23. An effective torsional isomerization potential of the form V(φ)=(V1/2)(1−cos φ)+(V2/2)(1−cos 2φ) was determined from thermochemical data on the isomerization. Torsional well (ω0) and barrier (ωb) frequencies were calculated from the effective potential and the appropriate moments of inertia. With an estimate in hand for ωb, it was possible to directly evaluate theoretical predications from the (i) theory of Kramers and alternatively from the (ii) theory of Grote and Hynes [employing the Zwanzig–Bixon–Metiu–Oxtoby–Freed (ZBMOF) expression for the frequency dependent friction ζ̂(λ)]. Kramers’ theory incorrectly predicts a large η dependence of kf for both 2VA and 22PA. In contrast, Grote–Hynes, with the ZBMOF ζ̂(λ), correctly predicts the absence of a significant η dependence of kf for 2VA, but underestimates the η dependence of kf for 22PA.
A two-laser pulse-and-probe study of T-R, V energy transfer collisions of H+NO at 0.95 and 2.2 eV
Authors: Wight Charles A. , Donaldson D. J. , Leone Stephen R.
Published: July 2002

Vibrational, rotational, and spin-orbit state distributions are obtained for inelastic collisions of H+NO at 2.2 and 0.95 eV. The H atoms are generated by excimer laser photolysis of H2S at 193 and 248 nm, respectively, and the excited states of the NO molecules are probed by laser-induced fluorescence using a tunable dye laser. The rotational state distribution accompanying the T-V excitation of v=1–3 at 2.2 eV is approximately characterized by a Boltzmann distribution at 1275 K, and is essentially independent of the vibrational level excited. At 0.95 eV, the rotational populations are approximately characterized by a 1050 K distribution. In each case, the temperatures of the spin-orbit state populations and the rotational states are the same. No selective population of Λ-doublet states is observed. The results are discussed in terms of chemical interactions between these two open-shell species on the HNO potential energy surfaces.
The harmonic and anharmonic force fields and vibration–rotation constants of ClNO, FNO, FSN, and ClSN from SCF and CI a
b i
n
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t
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o computations
Authors: Bendazzoli G. L. , Cazzoli G. , Esposti C. Degli , Fano G. , Ortolani F. , Palmieri P.
Published: July 2002

The harmonic and cubic force field of the triatomic molecules ClNO, FNO, ClSN, and FSN are evaluated by using SCF and various CI methods. The vibration–rotation constants derived from the force field are compared to the experimental values. The computed cubic force field is found to be accurate at the SCF level for ClNO and FNO, despite severe limitations of the HF description for the latter molecule. CI effects are important for some of the harmonic components of the force field of FNO and for its α vibration–rotation constants. The values of the vibration–rotation interaction constants have been predicted for ClSN and FSN.
Crossover behavior for Brownian motion
Authors: Matsuura Shu , Tsurumi Satoshi , Imai Nobuhisa
Published: July 2002

The trajectories of molecular motions of polystyrene latex and bacteriophage T4, the latter of which was visualized by dye DAPI, were observed in solutions under light microscopes. The experimental plots of log L(r) vs log r, where L(r) is the trajectory length and r is the observation scale, showed a broad transition-like behavior between the fractal states of the fractal dimension D=1 and D=2, when r is increased. We presented here an experimental formula of D(r) or L(r) which connected the two fractal states, and discussed this transition character on the basis of this formula.
Intramolecular vibrational energy redistribution and the gas phase overtone spectra of selectively deuterated neopentanes
Authors: Tarr Allan W. , Henry Bryan R.
Published: July 2002

The gas phase overtone spectra of neopentane-d6 [(CH3)2C(CD3)2] and neopentane-d9 [(CH3)C(CD3)3] are measured in the regions of ΔvCH =2–5. The peak positions are analyzed in terms of the local mode model with the assumption of methyl group independence. The overtone linewidths are shown to be consistent with intramolecular vibrational decay of the overtone states. The coupling scheme for the vibrational decay is similar to that applied to the linewidth data of benzene by Sibert, Reinhardt, and Hynes.
Photoionization studies of HgKr and HgXe
Authors: Liao C. L. , Ng C. Y.
Published: July 2002

Photoionization efficiency data for HgKr+ and HgXe+ have been obtained in the region of 730–1290 Å. The ionization energies (IE) of HgKr and HgXe are determined to be 10.056±0.012 (1233±1.5 Å) and 9.709±0.011 eV (1277±1.5 Å), respectively. Using these values, the known dissociation energies of HgKr and HgXe, and the IE of Hg, the binding energies for the ground-state mercury-rare gas molecular ions are calculated to be 0.393±0.013 eV for HgKr+ and 0.748±0.013 eV for HgXe+. By analyzing the shifts in energy between corresponding autoionization peaks observed in the Kr+, HgKr+, Xe+, and HgXe+ spectra and by assuming the charge induced-dipole interaction to be the dominant interaction at the equilibrium bond distances for HgKr and HgXe, the equilibrium bond distances for HgKr and HgXe are deduced to be 3.98 and 4.23 Å, respectively. The latter values are in excellent agreement with values determined by previous spectroscopic studies.
Methyl fluoride–13C in nematic liquid crystals: Anisotropy of the indirect 13C–19F spin–spin coupling and of the 1H, 13C, and 19F chemical shieldings
Authors: Jokisaari J. , Hiltunen Y. , Lounila J.
Published: July 2002

The anisotropy of the indirect 13C–19F spin–spin coupling tensor of methyl fluoride–13C in the liquid crystals ZLI 1167, EBBA, their mixtures, phase IV, and phase 1221 was studied by applying 1H and 19F NMR spectroscopy. The relative anisotropy ΔJCF/JCF gets values between −4.3 (in ZLI 1167) and +30.7 (in EBBA) when determined in the conventional way from the experimental dipolar coupling constants taking into account only harmonic vibrational corrections. The inclusion of the deformational corrections in both the direct and indirect C–F coupling tensors leads to a constant, solvent independent relative anisotropy of −2.5±0.2. This result is also obtained when a mixture of the liquid crystals ZLI 1167 and EBBA is used which mixture gives an undistorted geometry for methyl fluoride. The chemical shielding anisotropies ΔσH, ΔσC, and ΔσF for methyl fluoride were determined by applying the method of mixing two thermotropic nematogens (ZLI 1167 and EBBA) with opposite anisotropies of diamagnetic susceptibility. The results ΔσH =+5.2±0.2 ppm, ΔσC =+87±4 ppm, and ΔσF =−90±4 ppm are in fair agreement with theoretical calculations.
A model for imperfect mixing in a CSTR
Authors: Bar-Eli Kedma , Noyes Richard M.
Published: July 2002

When a chemical reaction is carried out in a continuously stirred tank reactor, the behavior may be significantly affected by the efficiency with which the entering chemicals are mixed with the main contents of the reactor. We have developed a model for this effect which assumes that a feed of premixed chemicals remains for a while in totally segregated packets before they are rapidly and perfectly mixed with the rest of the system. The time of this initial segregation is affected by the efficiency of stirring in the reactor. The model has been tested by computations on a mechanism developed by Roelofs et al. for a reaction which would oscillate even in a closed system. It has also been tested by computations on the rapid autocatalytic oxidation of cerous ion by bromate in the presence of a small amount of bromide. The results are qualitatively consistent with effects observed experimentally and in computations with other models including a somewhat similar one by Kumpinsky and Epstein. More quantitative tests should recognize the difference whether two streams of chemicals enter the reactor independently or are premixed before they do so.
Theory of the transient electric birefringence of rod-like polyions: Coupling of rotational and counterion dynamics
Authors: Szabo Attila , Haleem Matheen , Eden Don
Published: July 2002

The transient electric birefringence (TEB) of polyelectrolytes reflects not only the rotational motion of the macroions but also the dynamics of the surrounding ion atmosphere. A rigorous formulation of the coupled rotational and ion atmosphere dynamics of a rod-like macroion with no permanent dipole moment is presented under the assumptions that (1) distortions of the ion atmosphere along the long axis of the molecule are the only source of induced dipole moments and (2) the dynamics of the ion atmosphere can be described by a single relaxation time. A coupled diffusion equation for the joint probability that the macroion has a certain orientation and a certain instantaneous dipole moment is derived and solved in the low field limit. In this way, simple analytic expressions for the TEB are obtained when the electric field is (1) turned on, (2) reversed, and (3) oscillates sinusoidally. These expressions reduce to the classical permanent dipole results when the ion atmosphere relaxation is slow and to the instantaneously induced dipole (polarizable) results when the relaxation is rapid compared to rotation. An illustrative application to the analysis of experimental TEB data on a short DNA restriction fragment is presented and various extensions of the theory are discussed.
Algebraic resonance quantization of coupled anharmonic oscillators
Authors: Kellman Michael E. , Lynch Eric D.
Published: July 2002

A method called algebraic resonance quantization (ARQ) is presented for highly excited multidimensional systems. This approach, based on the Heisenberg form of the correspondence principle, is a fully quantum mechanical matrix method. At the same time, it uses modern nonlinear classical mechanics to greatly simplify the Hamiltonian matrix. For a model system of coupled Morse oscillators, a nonlinear resonance analysis shows that the Hamiltonian matrix is dominated by a few leading terms. This leads to an effective truncated sparse matrix whose diagonalization yields eigenvalues in excellent agreement with the exact values, even high in the chaotic regime. A new finding is that quantum couplings corresponding to rapidly oscillating, nonresonant terms can be important, and not just the higher-order resonant terms. The generalization of ARQ via a numerical semiclassical technique to many-dimensional systems with arbitrary couplings is outlined. The applicability of contemporary vector methods from quantum chemistry to ARQ of high vibrational levels is considered. The feasibility of sparse matrix ARQ methods for fitting spectra in the highly chaotic, ‘‘unassignable’’ regime is discussed.
Diode laser probing of vibrational product state distributions in metal–molecule collisions: Hg(6 3P1)–CO2(m
n
l p)
Authors: Brady Brian B. , Spector Garry B. , Chia Lucille , Flynn George W.
Published: July 2002

Diode laser probing has been used to follow the time dependent changes in the populations of low lying vibrational states of CO2 produced via quenching of Hg(6 3P1) initially excited by a pulse from a doubled dye laser. The conversion of mercury electronic energy into CO2 vibrational energy is quite efficient and mode specific. The ratio of the number of bending mode quanta to the number of asymmetric stretch quanta produced by the quenching process is 40±12. The rate of filling of both the bending and asymmetric stretch levels is identical and corresponds to 80 CO2/CO2 gas kinetic collisions. This data, taken in conjunction with the known quenching rate of Hg(6 3P1) by CO2 (four gas kinetic collisions) suggests that the quenching process produces a metastable state of CO2 which is either a bent electronic triplet or a highly excited vibrational level of the ground electronic state.
Infrared fluorescence relaxation of photoexcited gas-phase ions by chopped-laser two-photon dissociation
Authors: Dunbar Robert C. , Chen Jyh Horung , So Hun Young , Asamoto Bruce
Published: July 2002

Two-photon photodissociation in the ion cyclotron resonance ion trap, using repetitively pulsed laser excitation at low pressure, was exploited to measure the noncollisional energy dissipation rate for a number of gas-phase ions. Since the photoexcited ions initially contain 2–5 eV of excess vibrational energy, the observations give cooling rates for the ions at very high vibrational temperatures. Calculated as first-order rate constants for internal energy dissipation, the values lie in the range of 3–16 s−1, with the more polar molecules generally showing faster relaxation. Under these collisionless conditions, it is presumed that the cooling mechanism is infrared fluorescence. The observed rates of cooling are compared with the rates that would be observed if the infrared radiative characteristics of the ions were the same as those of the corresponding neutrals. The ions all show faster radiative relaxation than the neutrals, often being faster by about a factor of 2. This is consistent with a general increase in vibrational transition dipole moment due to charge movement involving the free positive charge on the ion.
Integral equation and Monte Carlo study on hydrophobic effects: Size dependence of apolar solutes on solute–solute interactions and structures of water
Authors: Tanaka Hideki
Published: July 2002

Hydrophobic effects for various solute sizes have been investigated by the RISM equation and Monte Carlo simulation. It is shown that the association of solute molecules is enhanced by the hydrogen bonds among water molecules. The degree of association depends on the solute size. For a large apolar solute, the structure of water is enhanced, resulting in the exothermic hydration and the negative entropy change in hydration. For a solute comparable with the water size, the hydrogen bonds among water molecules are strengthened but the net hydrogen bonds number does not increase. These results are also confirmed by the analysis of the geometric patterns formed by the hydrogen bond network. It is shown that the ordered structure of water in the solution of the large solutes gives rise to the larger enthalpy of the hydration, and thus they are more soluble in water than the smaller solutes.
Vibrationally state-selected reactions of ammonia ions. III. NH+3(v)+ND3 and ND+3(v)+NH3
Authors: Conaway William E. , Ebata Takayuki , Zare Richard N.
Published: July 2002

The role of vibrational excitation of the ν2 umbrella-bending mode of ND+3 (X̃,v=0 to 10) and NH+3 (X̃,v=0 to 9) on the reaction with NH3 and ND3 is examined over the 2 to 12 eV center-of-mass kinetic energy range. Isotopic substitution permits the investigation of charge transfer, proton/deuteron transfer, and neutral atom abstraction. The charge transfer channel is moderately enhanced by excitation of the ammonia ion ν2 vibrational mode. The proton/deuteron channel is suppressed by vibrational excitation of the ion at low kinetic energies. The hydrogen/deuterium atom abstraction channels show nearly a factor of 6 vibrational enhancement relative to NH+3 (v=0). The ion vibrational and translational energy play inequivalent roles in the reactions, indicating that nonstatistical factors are important in the transition from reactant to product channels. A simple dynamical model is offered to explain both the proton/deuteron transfer and the neutral atom abstraction reactions; it is based on the argument that vibrational motion along the particle transfer coordinate promotes reaction while vibrational motion perpendicular to the transfer coordinate hinders reaction.
ESR and a
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t
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o theoretical studies of the cation radicals 14N+4 and 15N+4: The trapping of ion–neutral reaction products in neon matrices at 4 K
Authors: Knight Lon B. , Johannessen Karl D. , Cobranchi Daryl C. , Earl Edward A. , Feller David , Davidson E. R.
Published: July 2002

The 14N+4 and 15N+4 molecular cation radicals have been generated by the ion–neutral reaction N+2 +N2 and isolated in solid neon matrices at 4 K for detailed ESR (electron spin resonance) investigation. Both photonionization at 16.8 eV and electron bombardment (50 eV) were used in conjunction with the neon matrix trapping technique to produce the N+4 dimer cation. The ESR results clearly show that N+4 is linear and has a 2Σμ ground electronic state. The magnetic parameters in neon are: g∥=2.0016(4) and g⊥=1.9998(2); A∥(14N)=311(1) MHz and A⊥(14N)=264(1) MHz for the central atoms and ‖A∥‖=10.4(5) MHz and ‖A⊥‖=20.4(1) MHz for the outer or terminal 14N atoms. Electronic structure information for N+4 was obtained from the ESR results and compared with ab initio CI calculations. The unpaired electron resides primarily on the inner nitrogen atoms with significant 2pσ and 2s character. Orbital characters obtained from the commonly applied free atom comparison method (FACM) were compared with the results of a Mulliken type spin population analysis conducted on the calculated wave function. The calculated nuclear hyperfine parameters (A tensors) showed reasonable agreement with experiment except for the very small Aiso parameter for the outer nitrogen atoms. Benchmark calculations employing large basis sets were conducted for the free nitrogen atom; these efforts demonstrate the difficulty in computing the Aiso parameter when inner shell effects are important.
The intensities of HCN overtone transitions from 12 600–18 400 cm−1
Authors: Smith Alice M. , Jo/rgensen Uffe G. , Lehmann Kevin K.
Published: July 2002

The absolute intensities of 12 visible overtone bands of HCN have been determined using intracavity photoacoustic detection and a long pathlength absorption cell. The relative intensities of nine of these weak overtone bands (from 14 600–18 400 cm−1) were obtained by using intracavity photoacoustic detection. The absolute intensity of the (0,0,5) band, obtained previously using a multipass cell with an optical pathlength×pressure of about 34 km Torr, was used to calibrate the photoacoustic response, and thus to obtain the absolute intensities for eight more HCN overtone bands. The absolute intensities of three HCN overtone bands between 12 600–13 800 cm−1 were measured using a multipass cell. Optical pathlengths×pressure from 10 to 15 km Torr were used for these three bands. The band origins in cm−1, the band labels (CN, bend, CH), and the intensities in cm/mol are:12 636, (0,0,4), 154(3); 12 658, (3,0,2), 9.7(3); 13 702, (2,0,3), 3.1(2); 14 654, (4,0,2), 8.0(4); 14 670, (1,0,4), 7.2(4); 15 552, (0,0,5), 17.5(4); 15 711, (3,0,3), 0.05(1); 16 164, (0,11,5), 0.047(8); 16 640, (5,0,2), 0.40(4); 16 674, (2,0,4), 1.7(1); 17 550, (1,0,5), 13.9(4); 18 377, (0,0,6), 2.4(2). This work represents the most complete study to date of the overtone intensities of a polyatomic molecule. We have shown that the typical method of intracavity laser power correction is likely to introduce serious errors in relative intensity measurement. We also have found evidence, namely the anomalously large intensity of the (1,0,5)–(0,0,0) transition, for the breakdown of the typically invoked bond dipole model for overtone and combination band intensities. As well as discussing the astromonical interest in predicting overtone intensities of HCN, we present a thorough comparison of the measured absolute overtone intensities with ab initio results.
Electromagnetic nuclear shielding tensors and their relation to other second-order properties. A study of the methane molecule
Authors: Lazzeretti P. , Zanasi R.
Published: July 2002

The results of an accurate calculation of second-order properties of the methane molecule are reported, with particular emphasis on dynamic shielding tensors, which enable one to evaluate the effective fields acting on the nuclei of a molecule in the presence of an electromagnetic field. The present theoretical study provides an extended numerical test, showing the reciprocal relations among different properties, i.e., sum rules and formulas explicitly linking molecular response tensors. A scheme for partitioning molecular properties into atomic contributions is discussed, which seems to imply transferability of average electric dipole polarizabilities.
On the theory of barrierless electronic relaxation in solution
Authors: Bagchi Biman
Published: July 2002

We report a theoretical study of the barrierless electronic relaxation in solution. The existing theoretical models are divided into two classes: (A) Instantaneous death models where the excited state decays with unit probability as soon as it attains certain critical conformations, and (B) finite decay models, where the decay from the critical geometries occur at a finite rate (=k0). All the models belonging to class (A) predict inverse friction (ζ) dependence of the nonradiative decay rate if the radiative relaxation is neglected. These models predict that the long time decay rate is independent of the excitation wavelength although the average rate (so the quantum yield) show a strong dependence. The relaxation behavior of models (B) is controlled by the dimensionless parameter k̃0(=k0ζ/ω2μ, where ω is the frequency of the excited state surface and μ is the effective mass of the reactive motion). A fractional friction dependence of the long time rate is obtained at low to intermediate values of k̃0, but an inverse friction dependence is predicted for large values of this parameter. The fractional dependence of the average rate on viscosity is found to depend critically on the wavelength of the exciting light. A small negative activation energy is found at small values of k̃0. A delta-function sink (DFS) is introduced to simplify numerical calculations. This DFS model retains the essential features of the more realistic Gaussian sink model. The DFS model is studied both analytically and numerically. We find that the asymmetry in the position of the sink function, with respect to the minimum of the excited state potential surface, can play an important (hidden) role in the relaxation process. Especially, it can give rise to a significant temperature coefficient of the rate and can lead to an erroneous conclusion on the ‘‘molecular’’ activation energy. Finally, systematics of fitting the experimental data to the present theories are discussed.
Quasidissipative intramolecular dynamics: An adiabatically reduced coupled equations approach
Authors: Voth Gregory A.
Published: July 2002

The quasidissipative intramolecular dynamics of experimentally prepared nonstationary states in large, isolated polyatomic molecules are examined using an adaptation of the adiabatically reduced coupled equations approach [J. Chem. Phys. 84, 2254 (1986)]. Under appropriate conditions, the present method predicts an exponential decay of the initial state probability which has a ‘‘generalized’’ Golden Rule-like decay rate constant. The present dynamical approach is complementary to approximate resolvent operator techniques commonly employed in radiationless transitions theory.
Time resolved luminescence spectroscopy of alkaline earth oxides after pulsed electron beam irradiation. I. Spectra and thresholds of CaO and CaO:Mg
Authors: Grant Janice L. , Cooper Ronald , Boas John F.
Published: July 2002

Time resolved luminescence spectroscopy has been used to study the emissions occurring in CaO and CaO:Mg in the range 20 ns to 100 ms after irradiation with nanosecond duration pulses of electrons of energy between 0.2 and 3.0 MeV. An emission at 375 nm in pure CaO with a threshold energy of 0.29±0.2 MeV is attributed to processes resulting from the displacement of oxygen anions and the formation of F+ centers. A related emission in CaO:Mg is attributed to displacements involving oxygen anions and the formation of F+Mg centers. An emission at 560 nm observed in both CaO and CaO:Mg with a threshold energy ∼0.7 MeV is attributed to processes resulting from cation displacement. A surprising consequence of these observations is that the displacement energies of oxygen ions in the CaO lattice appear to be lower (∼50 eV for CaO and ∼35 eV in CaO:Mg) than that of the calcium cations (∼65 eV).
Langevin dynamics of Rouse chains under flow
Authors: Wang Shi-Qing , Freed Karl F.
Published: July 2002

A systematic approach is developed for describing the hydrodynamics of flowing polymer solutions by using a microscopic Langevin model for which the inertial nonlinearities and solvent advection are ignored. The influence of polymer motion on the solution velocity field is evaluated by averaging over the polymer degrees of freedom at a time in the distant past in order to derive an effective hydrodynamic equation of motion for the averaged polymer solution. The polymer’s contribution to the fluid stress tensor is computed from the total solution stress tensor whose divergence appears in the averaged effective hydrodynamic equation. Introduction of the Rouse chain and long wavelength limits enables the analytical evaluation of this stress tensor for any time-dependent linear flows. All the material functions for the polymer solution are evaluated in a simplified fashion for these flows, recovering some known results and deriving several new ones.
Monte Carlo studies of diffusion on inhomogeneous surfaces
Authors: Mak C. H. , Andersen Hans C. , George S. M.
Published: July 2002

A kinetic lattice gas model is applied to study collective surface diffusion of adsorbates on inhomogeneous surfaces. The diffusion coefficient is extracted from Monte Carlo (MC) simulations by observing the decay of the autocorrelation functions for density fluctuations. Calculations are presented for diffusion on a surface with various different coverages of randomly placed blocks and to diffusion on a surface with various distributions of traps of different binding energies. In the cases where analytical expressions for the collective diffusion coefficient can be derived, the MC results show excellent agreement with analytical predictions.
Theoretical study of electric dipole and transition moments of GeH, SnH, and PbH
Authors: Chapman Douglas A. , Li Junqing , Balasubramanian K. , Lin S. H.
Published: July 2002

Electric dipole and transition moments are calculated in the relativistic CI formalism for five low-lying states of GeH, SnH, and PbH. In addition, results of complete active space MCSCF/CI calculations of the ground state dipole moments of these molecules are described. The ground state dipole moments (μe) of GeH, SnH, and PbH are calculated to be 0.09, 0.39, and 0.94 D, respectively, with polarity (M+H−) in all cases. Transition moments for the ω-ω components of the 2Π→4Σ− and 2Π→2Δ transitions in these molecules are calculated and compared with available experimental findings. The spin-orbit interaction is found to make a significant contribution to the dipole and transition moments of PbH.
On the density expansion of the pair distribution function of nonuniform fluids
Authors: Robert M. , Jeng J. F. , Viswanathan R.
Published: July 2002

The density expansion method is used to determine the pair distribution function of a nonuniform fluid in one dimension in which a two-phase interface is induced by an external field. When the fluid is modeled by the lattice gas with nearest-neighbor forces and the external field by the step function that changes sign at the middle of the chain, terms of arbitrary order in the density expansion can be determined exactly. The kernels of the first- and second-order terms are evaluated in closed form and the pair distribution function is evaluated at each order. These results are tested against the exact expression of the pair distribution function of the same model interface, which was recently obtained by the transfer matrix method. It is found, especially for strong intermolecular forces, that the first- and second-order approximations are not only unable to improve the severe disagreement between the zeroth-order approximation and the exact result, but further aggravate this disagreement, suggesting that the density expansion of the pair distribution function is at best asymptotically convergent.
The local correlation treatment. II. Implementation and tests
Authors: Saebo Svein , Pulay Peter
Published: July 2002

Computational results are reported which demonstrate the performance of the local correlation method, including convergence behavior and supercomputer timing. Calculations at the local MP4(SDQ) level with the 6-311G** basis have been carried out for molecules as big as octatetraene, demonstrating that, using the local correlation approach, highly accurate calculations are feasible for quite large molecules.