bluuebunny/crossref_metadata_2025 huggingface dataset:
Suppression of deposition rate variations and of structural changes in the characteristics of magnetic films deposited by magnetically controlled magnetron sputtering
Authors: Furuya Akinori , Hirono Shigeru
Published: July 2002

This article describes the effect of the target magnetic field on the plasma process and the physical properties of films deposited by magnetically controlled rf and dc planar magnetron sputtering techniques. The target magnetic field, which is parallel to the target surface, is controlled and its’ strength is a key parameter in controlling the deposition rate in both rf and dc magnetron sputtering. Based on this technique, we propose a way of suppressing variations in a magnetic film’s deposition rate and suppressing changes in its structural characteristics during rf magnetron sputtering. For dc magnetron sputtering, we clarified the mechanism that automatically stabilizes the deposition rate even if the target magnetic field is changed. We also clarified that controlling the target magnetic field (controlling the self-bias voltage) is very important for controlling the physical properties of sputter-deposited films in both rf and dc magnetron sputtering.
Resonance modes of cubic single crystal thin film with exchange anisotropy: A theoretical study
Authors: Layadi A.
Published: July 2002

Ferromagnetic resonance (FMR) modes of exchange coupled bilayer thin films have been studied. The interfacial exchange anisotropy is modeled as a magnetic field HE. The ferromagnetic film is taken to be a single crystal with cubic and uniaxial magnetocrystalline anisotropies. The effect of the applied magnetic field H, the exchange anisotropy field HE, and the cubic anisotropy on the mode behavior will be shown. It will be shown how the FMR linewidth relates to the exchange anisotropy. The angular dependence of the resonance frequency and frequency linewidth will be studied.
Shear force interaction in the viscous damping regime studied at 100 pN force resolution
Authors: Schmidt J. U. , Bergander H. , Eng L. M.
Published: July 2002

We report a very good force resolution for tuning fork based shear force microscopy as used for feedback regulation in scanning near-field optical microcopy (NSOM). The sensitivity and dynamics of fiber tips attached to 100 kHz tuning forks are investigated both experimentally and theoretically applying a finite element analysis. Operating the tuning fork at vibration amplitudes smaller than 10 nm allows to discriminate between viscous damping due to capillary wetting, and fiber bending upon tip-sample approach to hydrophilic sample surfaces indicating the direct transition from “noncontact operation” (pure viscous damping due to contamination layer) down to tip-sample contact. Viscous damping manifests in frequency shifts of less than 50 mHz, as deduced from resonance curves recorded under feedback control. For relative amplitude changes of less than 0.5% the viscous damping force acting lateral on the fiber tip is calculated to ∼100 pN using the finite element method. This detection limit proves that tuning fork based shear force control is superior to other feedback mechanisms employed in NSOM.
Raman gain and optical loss in GaP–AlGaP waveguides
Authors: Saito Takao , Suto Ken , Kimura Tomoyuki , Watanabe Akiyoshi , Nishizawa Jun-ichi
Published: July 2002

The stimulated Raman gain coefficient and the optical loss in GaP/AlGaP semiconductor Raman amplifiers are measured for both double path and single path structures. For the double path structure, the highest gain per waveguide length is 3.27 dB/cm for a waveguide with a cross-sectional area of 4 μm2, under continuous wave pumping. A long waveguide with a length of 10 mm is demonstrated to have the highest gain of 2.57 dB. For a single path structure, the gains of backward scattering and forward scattering are separately measured and it is found that the gain of backward scattering is dominant. From these results, the backward gain coefficient is estimated to be 12.3×10−8 W cm−1. This value is about five times that of CS2 and LiNbO3, which is known to have the highest Raman scattering gain. However, the optical losses of the waveguides are estimated from the finesse measurement. The lowest one way loss corresponds to an effective absorption coefficient of αeff=0.28 cm−1.
Spin-dependent scattering in semiconducting ferromagnetic (Ga,Mn)As trilayer structures
Authors: Akiba N. , Chiba D. , Nakata K. , Matsukura F. , Ohno Y. , Ohno H.
Published: July 2002

The spin-dependent scattering in ferromagnet/nonmagnet/ferromagnet (Ga,Mn)As/(Al,Ga)As/(Ga,Mn)As trilayer structures was studied. An increase of sheet resistance was observed when the magnetizations of the two ferromagnetic (Ga,Mn)As layers were aligned anti-parallel, which was realized by the different coercivity of the two (Ga,Mn)As layers with different compositions. This is the first demonstration of spin-dependent scattering in magnetic multilayer structures made of semiconductor-materials alone.
Tensile strained InGaAs/InP multiple-quantum-well structures studied by magneto-optical spectroscopy
Authors: Shao Jun , Haase Dieter , Dörnen Achim , Härle Volker , Scholz Ferdinand
Published: July 2002

Low-temperature (1.8 K) magneto-optical absorption experiments have been performed on two tensile strained In1−xGaxAs/InP multiple-quantum-well samples. By using derivative spectra and the effective-mass equation of a quasi-two-dimensional exciton, we are able to determine the exciton ground state energy more accurately and to identify higher excited exciton states up to the 5S level. Data of the effective masses for the light hole and the heavy hole are given. Furthermore, the effect of nonparabolicity is discussed.
Exchange biasing and thermal stability of CoFe/PtPdMn films
Authors: Hung C.-Y. , Mao M. , Funada S. , Schneider T. , Miloslavsky L. , Miller M. , Qian C. , Tong H.-C.
Published: July 2002

Two sets of PtPdMn exchange biased films Ta 50 Å/CoFe 100 Å/PtPdMn (tAF) Å/Ta 50 Å, with PtPdMn thickness, tAF=350, 600 Å, were deposited on Si substrates by dc magnetron sputtering techniques. After magnetic annealing, these two sets of films exhibited values of exchange bias field, Hex=229 and 254 Oe, respectively. The PtPdMn layer was then thinned to various thicknesses from 600 down to 50 Å by ion beam etching. Hex does not retain its original value. It decreases with decreasing tAF and becomes zero at tAF∼75 Å. In addition, we have observed that the training effect or the anomalous hysteresis loss becomes more pronounced with decreasing tAF. This confirms that not only face-centered-tetragonal phase but, more critically, tAF plays role in determining exchange biasing and its thermal stability. The blocking temperature, TB, appears unaffected by the thinning of the PtPdMn layer, and no apparent change occurs in the local blocking temperature distribution, as suggested by the finite size effect.
Effect of oxygen stoichiometry on properties of La0.815Sr0.185MnO3+d
Authors: Bukowski Z. , Dabrowski B. , Mais J. , Klamut P. W. , Kolesnik S. , Chmaissem O.
Published: July 2002

The effect of oxygen stoichiometry on properties of La0.815Sr0.185MnO3+d was studied for samples with oxygen content controlled by quenching from high temperatures (700–1400 °C) in air, cooling under low oxygen partial pressure, and annealing at high oxygen pressure. The oxygen content determined from thermogravimetric analysis was found to vary within a range of 3.00–3.10. At room temperature the lattice parameters of the rhombohedral structure (R3̄c) decrease linearly with increasing oxygen content. The rhombohedral-orthorhombic structural transition temperature decreases abruptly with increasing oxygen content while the Curie temperature increases with increasing oxygen content, reaching a maximum at d∼0.04, and then decreases. The saturation magnetization decreases linearly with increasing oxygen content indicating increasing Mn4+ concentration, i.e., hole doping by cation vacancies. Finally the magnetoresistance effect decreases with increasing d.
Magnetic structure and giant magnetoresistance in granular metals
Authors: Kechrakos D. , Trohidou K. N.
Published: July 2002

The effect of dipolar interactions on the giant magnetoresistance (GMR) of a granular metal is studied numerically. The equilibrium magnetic configuration of the system is obtained by classical Monte Carlo simulation and the conductance is calculated using the real space Kubo–Greenwood formula and a single band tight-binding Hamiltonian. The numerical results are compared with experimental finding.
Gas phase reactions of iron clusters with hydrogen. I. Kinetics
Authors: Richtsmeier S. C. , Parks E. K. , Liu K. , Pobo L. G. , Riley S. J.
Published: July 2002

The kinetics of the gas phase reactions of hydrogen and deuterium with iron clusters in the range Fe6 to Fe68 have been investigated. It is found that reaction rate constants are a strong function of cluster size, varying by more than five orders of magnitude in this size range. The largest rate constants correspond to approximately 3% of a hard sphere cross section. Abrupt changes in the rate constant from one cluster to the next are seen. Qualitative temperature dependencies of cluster reactivity have been determined. The more reactive clusters show decreased reactivity with increased tempeature, while the least reactive clusters become more reactive. Strong isotope effects are seen only in the Fe10 to Fe14 size range. Mechanisms for the reactions of H2 and D2 with iron clusters are discussed in light of these observations.
Laser-excitation spectra of the chloroacetylene and deuterochloroacetylene cations, ClCCH+ and ClCCD+
Authors: King M. A. , Maier J. P. , Ochsner M.
Published: July 2002

The Ã 2Π←X̃ 2Π laser-excitation spectra of chloroacetylene and deuterochloroacetylene cations are reported. Analysis of the band systems yields values for all three stretching fundamentals (ν1–ν3) in the excited elecronic state. Rotationally resolved spectra of the 000Ω =3/2 subbands have been recorded and analyzed. The derived rotational constants allow estimates of the H–Cl distances in both electronic states to be made.
Raman spectroscopic investigations on molecular crystals: Pressure and temperature dependence of external phonons in naphthalene-d8 and anthracene-d1
Authors: Häfner W. , Kiefer W.
Published: July 2002

The molecular crystals naphthalene-d8 and anthracene-d10 have been investigated at pressures in the range of ambient to 0.8 GPa and simultaneously at temperatures from room temperature down to 15 K. The accurate frequency measurement of the lattice modes allowed the determination of a small temperature dependence in some of the mode-Grüneisen parameters. A qualitative comparison of the temperature dependence with theory is made.
The excitation of N(2P) by N2(A 3∑+u, v′=0,1)
Authors: Piper Lawrence G.
Published: July 2002

We have studied the electronic energy transfer reaction between N2(A 3∑+u, v′=0,1) and N(4S) in a discharge-flow reactor. Monitoring the decay of the forbidden Vegard–Kaplan emission, N2(A 3∑+u−X 1∑+g ), as a function of time and of atomic nitrogen number density allows determination of the total reaction rate coefficients for removal of vibrational levels 0 and 1 by N(4S). Simultaneously observing the temporal profiles of the Vegard–Kaplan bands and of the forbidden N(2P−4S) line at 346.6 nm allows the branching fraction into atomic nitrogen excitation to be determined. The total quenching rate coefficient for both v′=0 and 1 is (4.0±0.5)×10−11 cm3 molecule−1 s−1 at 300 K. The apparent rate coefficient for exciting N(2P) by N2(A, v′=0) is (19±3)×10−11 cm3 molecule−1 s−1, at 300 K, while that for excitation by N2(A,v=1) is (5±1)×10−11 cm3 molecule−1 s−1. We interpret the large discrepancy between the N(2P) excitation and N2(A) destruction rate coefficients as evidence that the currently accepted value for the N(2P−4S) transition probability is a factor of 4 to 5 too small, or else that our source of N2(A) metastables, energy transfer from metastable argon atoms to molecular nitrogen, is contaminated by the presence of a second nitrogen metastable species with an energy in excess of 3.6 eV.
A reevaluation of forces measured across thin polymer films: Nonequilibrium and pinning effects
Authors: Horn R. G. , Hirz S. J. , Hadziioannou G. , Frank C. W. , Catala J. M.
Published: July 2002

We have measured forces between molecularly smooth solid surfaces separated by thin films of molten polydimethylsiloxane. We show that a long-range repulsion reported in earlier work is not an equilibrium force, but can be attributed to viscous drag effects. Consistent with previous results, the viscosity of the film can be modeled by assuming that a layer of polymer molecules is immobilized or ‘‘pinned’’ at each surface for a time longer than the time scale of the measurements. We propose that this pinning is a result of entanglement-like effects in the vicinity of a wall.
Interference dips in molecular absorption spectra calculated for coupled electronic state potential surfaces
Authors: Reber Christian , Zink Jeffrey I.
Published: July 2002

Interference effects in electronic absorption spectra caused by coupling between excited states are calculated and interpreted by using numerical integration of the time-dependent Schrödinger equation and the time-dependent theory of electronic spectroscopy. The interference between nearby spin-forbidden doublet and spin-allowed quartet states of chromium (III) and vanadium (II) metal complexes causes sharp decreases of intensity or dips in the envelopes of absorption bands. The states are coupled by spin–orbit coupling. The origin of the dips is explained in terms of interference between wave packets moving on potential energy surfaces representing the states. The importance of curve crossing and amplitude transfer between the states is analyzed quantitatively. The theory is applied to the absorption spectrum of a chromium (III) complex.
Reflection and transmission of waves by a complex potential—a semiclassical Jeffreys–Wentzel–Kramers–Brillouin treatment
Authors: Vibók Á. , Balint-Kurti G. G.
Published: July 2002

In this paper, the reflection and transmission of plane waves are examined from a complex potential. Such potentials have the property of absorbing wave packets incident on them and are used widely in time-dependent quantum scattering theory. The purpose of the study is to determine the optimal form of potential to be used for absorbing wave packets near the edges of finite grids in coordinate space. The best potentials for such purposes lead to the minimum possible transmission and reflection of the incident wave packet. The Jeffreys–Wentzel–Kramers–Brillouin (JWKB) theory is used to address this problem and a new form for the optimal complex potential is proposed. A scaled dimensionless form of the Schrödinger equation is also derived, so that the parameters of any optimized potential obtained for a particular collision energy and mass combination may be readily converted to apply to a new set of masses and energies.
Lifetime of the hypothetical charge transfer state in the phenothiazine crystal
Authors: Petelenz Piotr , Danielak Roman , Gorczyca Bogusl/aw , Michalak Artur
Published: July 2002

Absorption and electroabsorption spectra suggest that a long-lived charge transfer state may be the lowest singlet state of the phenothiazine crystal. A simple theoretical model proposed previously for other crystals is applied to estimate the lifetime of that state. The result is discussed in the context of recent time-resolved two-photon photocurrent experiments.
Experimental study of the surface structure of diblock copolymer films using microscopy and x-ray scattering
Authors: Cai Zhong-hou , Huang Kegang , Montano P. A. , Russell T. P. , Bai J. M. , Zajac G. W.
Published: July 2002

The surface structure of a model system was measured in real space by atomic force and optical microscopies, and compared with that obtained from measurements in reciprocal space by x-ray reflectivity and off-specular scattering at grazing incidence. Experiments were performed on films of symmetric diblock copolymers of polystyrene and polymethylmethacrylate, whose surfaces were covered by micrometer-size islands or holes (domains) of uniform height. The correlation functions extracted from the images of the film surfaces show weak peaks in the real-space domain distribution. The corresponding structures were also found in reciprocal space. The height of the domains obtained from the scattering measurements was found to be in excellent agreement with that obtained by atomic force microscopy. We developed a formalism using the kinematical approximation for the analysis of the x-ray-scattering measurements. We used a multilayer film model with roughness at each interface and relief domains at the surface. We extracted the domain–domain correlation functions for the x-ray-scattering analysis from the atomic force and optical microscopy images.
Electronic relaxation induced by interaction between diatomics in low temperature matrices
Authors: Chergui M. , Schwentner N. , Tramer A.
Published: July 2002

Fluorescence (phosphorescence) excitation spectra as well as luminescence spectra and decay times were investigated for NO in N2 and mixed N2/Kr matrices. All Rydberg states rapidly relax to the ground state in mixed matrices with the N2 molar fraction exceeding xN2=0.35. This dependence on the N2 concentration is attributed to the complete fluorescence quenching of all NO molecules with at least one N2 nearest neighbor. The vibrational levels of the B 2Π valence states strongly coupled to the Rydberg state are also depopulated. The general picture of relaxation channels is discussed.
Experimental measurement of stochastic potentials and relative stabilities in a bistable chemical system
Authors: Hjelmfelt A. , Schneider F. W.
Published: July 2002

We report the experimental measurement of a stochastic potential in a bistable chemical system, the iodate–arsenous acid reaction. The reaction, run in a continuous flow stirred tank reactor, is driven by a fluctuating reactant flow which causes the system to move randomly between the two stable steady states. We calculate the probability of the system being in a particular state, and the probability distributions depend on the average flow rate. The experimental results are in agreement with predictions of the Fokker–Planck equation.
Molecular dynamics-modified neglect of diatomic overlap study of the thermal isomerization mechanism from cis to trans form in polyacetylene
Authors: Lombardo G. M. , Piccitto G. , Pucci R.
Published: July 2002

In the present work we have studied the thermal isomerization process of polyacetylene (PA) from the trans-cisoid (TC) and cis-transoid (CT) forms to the trans-transoid (TT) form. We have used the molecular dynamics-modified neglect of diatomic overlap (MD-MNDO) method, which combines a molecular dynamics algorithm with the MNDO approximation. The thermal isomerization mechanism from TC-to-TT form is fully described. In our simulation, starting from the CT form in its ground state and heating the system up to T=450 K, there is no evidence of an isomerization process. However, a model, which simulate the subtraction of some charge from the double bonds of the PA chains, is able to give important insights into the isomerization mechanism from the CT to the TT form. This charge subtraction is essential to induce a CT-to-TC local transformation followed by the TC-to-TT isomerization.
Diffusional desaturation of electron transfer
Authors: Burshtein A. I.
Published: July 2002

Noncontact electron transfer (in the Marcus inverted zone) during encounters of neutral reagents in solution is considered. It is shown by means of generalized encounter theory that the saturation of the transfer rate at short distances due to vibrational friction or the dynamical solvent effect is removed when diffusion is sped up and never appears in the kinetic limit.
Search for SmAd–SmAd′ transitions
Authors: Wu Lei , Garland C. W. , Shashidhar R.
Published: July 2002

High-resolution heat capacity measurements have been made on two mixtures of 4-nonyloxybenzoyloxy-4′-cyanoazobenzene (9OBCAB) and 4-nitrophenyl-4′-(4″-n-hexyloxybenzoyloxybenzoate) (6ONPBB). The goal of this work was to establish the thermal behavior near a proposed SmAd–SmAd′ transition between two different partial bilayer smectic-A phases. Although nematic-smectic-A transitions with small Cp peaks were observed, there was no calorimetric evidence of smectic–smectic transitions. Any SmAd–SmAd′ transition must be first order due to symmetry, and it seems doubtful that true thermodynamic transitions of this type exist.
Ab initio studies of open-shell complexes of CO+ with rare gases
Authors: Hamilton P. A. , Hughes A. N. , Sales K. D.
Published: July 2002

The open-shell complexes HeCO+, NeCO+, and ArCO+ have been studied using ab initio methods in both the ground and first excited states. Calculations were performed using the 6-311G** and 6-311++G** basis sets with unrestricted Hartree–Fock and Mo/ller–Plesset perturbation theory to fourth order (UMP4). The results predict that all the complexes are bound but that only the Ar species in the ground state is rigid. The most stable equilibrium conformation is linear X–CO+ in the ground state and T-shaped or linear X–OC+ in the first excited state. Observations of these complexes by electronic spectroscopy may be difficult because of the prediction of a geometry change on excitation. The results from several other methods, including restricted open-shell Hartree–Fock with Mo/ller–Plesset, restricted Mo/ller–Plesset for open shells, and quadratic configuration interaction with singles, doubles, and triples, were compared to check that the UMP calculations give a reasonable description of these open-shell molecules.
A classical trajectory study of CD2HNC isomerization
Authors: Marks Alison J.
Published: July 2002

Classical trajectory studies of CD2HNC isomerization have been carried out using a simple potential energy function. Mode-specific effects are found to be small, and the fastest rate of reaction is observed for a microcanonical ensemble. Comparison with the CH3NC isomerization reaction reveals an unexpected inverse secondary isotope effect. Rotational excitation about the molecular axis is found to enhance the rate of reaction more than rotation perpendicular to this axis.
C60–C+60 collisions: Semiempirical molecular dynamics simulations
Authors: Long Xiping , Graham Richard L. , Lee Chengteh , Smithline Shepard
Published: July 2002

Semiempirical molecular dynamics is used to study the collision of C60 and C+60. Particles are propagated classically using forces calculated from the modified neglect of differential overlap (MNDO) Hamiltonian. By assigning different collision energies (Ec) and impact parameters (b) to the Buckminster fullerenes (buckyballs), we simulated six collision events: four head on collisions with impact parameter b=0 and collisions energies of 100, 150, 200, and 400 eV, and two collisions with b=1.5 Rb (Rb=buckyball radius) and Ec=100 and 400 eV. The head on collisions show that at 100 eV the two buckyballs scatter off one another and at 200 and 400 eV they fuse, while at 150 eV they either scatter or form a metastable dimer depending on how the simulation is prepared. This barrier is consistent with recent experiments. In addition, we observe tetrahedral bonding in the C+120 structure formed in the 200 eV, b=0 collision, while at 400 eV, b=0 we see large rings and chains of carbon atoms. The off center collisions also display interesting structural features. In the 100 eV b=1.5 Rb collision, the buckyballs graze one another, distorting their cage structure and scattering at an angle relative to their incident velocities. The buckyballs in the 400 eV, b=1.5 Rb collision also scatter, but in contrast to the 100 eV, b=1.5 Rb collision, the individual buckyballs are severely distorted, forming what we refer to as an ‘‘open mouth’’ structure.
Response to ‘‘Comment on: ‘The ν1+ν3 combination mode of C3 in Ar and Kr matrices: Evidence for a bent structure’ ’’ [J. Chem. Phys. 101, 5413 (1994)]
Authors: Szczepanski J. , Vala M.
Published: July 2002

The recent finding that vapor phase C3 is linear, and not quasilinear, is shown to be immaterial to the interpretation of the C3 infrared combination band in matrices, where the molecule is found to be bent.
A model for electron nuclear dynamics of a monatomic chain
Authors: Calais Jean-Louis , Deumens Erik , Öhrn Yngve
Published: July 2002

The electron nuclear dynamics (END) approach is developed for a linear chain in a parametrized model inspired by the PPP (Pariser–Parr–Pople) model. Particular attention is given to the model parameters, and the choice of basis functions in this time-dependent theory. The resulting equations of motion include electronic–vibrational couplings. Explicit analysis of the simplest model leads to coupling between the highest frequency longitudinal vibrational mode and the electrons.
Study of collisional effects on band shapes of the ν1/2ν2 Fermi dyad in CO2 gas with stimulated Raman spectroscopy. III. Modeling of collisional narrowing and study of vibrational shifting and broadening at high temperature
Authors: Lavorel B. , Millot G. , Fanjoux G. , Saint-Loup R.
Published: July 2002

High resolution stimulated Raman spectra (SRS) of the ν1/2ν2 Fermi dyad of CO2 have been recorded in the 0.4–22. amagat density range at 700 K and in the 0.6–16 amagat density range at 900 K. The data have been successfully analyzed with a theoretical model taking into account both rotational line mixing through the energy corrected sudden-polynomial (ECS-P) law for the relaxation matrix and vibrational line broadening γv and line shifting δv. The two vibrational parameters have been accurately determined from fit to the experimental spectra. These new high temperature data together with previous data obtained at 295 and 500 K [B. Lavorel et al., J. Chem. Phys. 93, 2176 (1990); B. Lavorel et al., J. Chem. Phys. 93, 2185 (1990)] entirely determine the temperature dependence in the 295–900 K range of Raman band shapes of ν1 and 2ν2 in a large pressure range. These results will be very useful in CARS thermometry. A few coherent anti-Stokes Raman spectra (CARS) recorded in the 1100–1300 K domain are consistent with extrapolations of the 295–900 K data.
Phase behavior of tethered polymers with lateral mobility in poor solvents
Authors: Tang Hai , Carignano Marcelo A. , Szleifer Igal
Published: July 2002

We present a scaling theory (ST) for the phase behavior of tethered polymers with lateral mobility in poor solvents. The coupling between inter- and intrachain interactions is included to describe the crossover between the ‘‘mushroom’’ and the layer regimes. The macroscopic phase separation and the associated changes in the chain configurations along the coexistence curve are obtained. The coupling between the thermodynamic state and the configurational behavior is described in detail. Good agreement between the ST and a molecular approach (single-chain mean-field theory) is obtained for the thermodynamic behavior and most of the conformational properties of the chains. Based on the ST, the renormalization group (RG) analysis is employed to derive the degree of polymerization, N, exponents in the critical amplitudes of the phase separation. It is found that some of the critical amplitudes have no N dependence, as expected in the semidilute regime, while others do show a dependence. These findings are in line with the fact that the critical region is in the crossover between the dilute and the semidilute regimes. The N exponents also differ from those for 2D polymer solutions, due to the fact that the thickness of the tethered chain layer has a power law dependence on N.
Polymer chains with correlations between adjacent segments: Qualitative differences to the freely jointed chain
Authors: Glatting G.
Published: July 2002

We generalize the freely jointed chain used for the description of polymer chains by inclusion of arbitrary correlations between adjacent segments. The partition function of a single chain is analytically calculated after projecting it on a one-dimensional lattice and the force extension relation and the mean square end-to-end distance are investigated. New quantitative and qualitative features are obtained. Therefore not all macromolecular chains are correctly modelled by the widely used freely jointed chain.
Radiative dissociation of C2H2, C2HD, and C2D2 superexcited at 50–90 nm region
Authors: Ibuki Toshio , Horie Yasuhiko , Kamiuchi Akira , Morimoto Yoshikazu , Tinone Marcia C. K. , Tanaka Kenichiro , Honma Kenji
Published: July 2002

Radiative dissociation of superexcited acetylene was studied at hν=13.8–24.8 eV (90–50 nm) by using C2H2, C2HD, and C2D2. The observed emission bands were d 3Πg→a 3Πu, e 3Πg→a 3Πu, C 1Πg→A 1Πu, and D 1∑+u→X 1∑−g of C2 radical, and A 2Δ→X 2Πr, B 2Σ−→X 2Πr, and C 2Σ+→X 2Πr of CH and CD radicals. The fluorescence cross sections of the electronically excited C*2 radicals showed a hydrogen isotope effect, i.e., the cross sections were in order of σf[C2(C2H2)]≳σf[C2(C2HD)]≳σf[C2(C2D2)]. Reverse is true for the fluorescence cross sections of CH* and CD*, i.e., σf(C2H2)<σf(C2HD)<σf(C2D2). These isotope effects were interpreted by the competition of some decay processes from the superexcited states. Hydrogen isotope effect in simple C–H and C–D bond dissociation is important for the C*2 formation. As a result of the competition with this C*2 formation, the ‘‘reverse’’ isotope effect emerges in the CH* and CD* formations. Another important competing process is the isomerization followed by formation of nonradiative fragments. Since H atom migrates more easily than D atom through a cyclic cavitated complex and the nonradiative fragmentation competes with the CH* and CD* formation, the radiative intensities of the CH* and CD* radicals inevitably show the apparent inverse hydrogen isotope effect. The isomerization seems to be specially important in the wavelength region, λ≳80 nm, where a trans-bent superexcited state is formed.
An electron spin echo envelope modulation study of the primary acceptor quinone in Zn-substituted plant photosystem II
Authors: Astashkin Andrei V. , Kawamori Asako , Kodera Yoshio , Kuroiwa Shigeki , Akabori Kozo
Published: July 2002

A Fe2+ ion on the acceptor side of plant photosystem II has been substituted by Zn2+ and an anion radical of the primary acceptor quinone, Q−A, has been studied by electron spin echo method. The electron spin echo modulation shows the interaction of the unpaired electron of Q−A with nitrogen nuclei of the histidine and, probably, alanine residues situated nearby. The comparison of the modulation spectra of Q−A with those of the anion radical of plastoquinone-9 stabilized in protonated and deuterated isopropanol matrices allows one to distinguish between the spectrum lines due to the quinone protons and due to the protons of other molecules that form hydrogen bonds with the oxygen atoms of the quinones.
Inference of crystal properties from cluster magnitudes
Authors: Francisco E. , Recio J. M. , Martín Pendás A.
Published: July 2002

Using ab initio pairwise potentials, we have minimized the total energy of cubelike neutral clusters of NaCl containing from eight up to eight thousand ions. Different size coordinates have been used to describe the progressive emergence of the crystalline behavior of the lattice parameter, cohesive energy, bulk modulus, surface energy, and surface tension. Our analysis has revealed that the crystal limit may also be succesfully obtained from relatively small clusters, provided that only the atoms displaying the coordination index found in the crystal are used to compute the magnitude examined. The validity of both approaches has been demonstrated by atomistic simulations in the NaCl crystal using the same interatomic potentials.
Preferential molecular ordering at the surface of a liquid perfluoropolyether revealed by x-ray photoelectron spectroscopy
Authors: Ramasamy S. , Pradeep T.
Published: July 2002

Surface chemical constitution of a liquid perfluorinated polyether, F[CF(CF3)CF2O]27(ave)CF2CF3 has been investigated by x-ray photoelectron spectroscopy. It is shown that the surface is primarily composed of the terminal –CF3 groups implying preferential alignment of molecules at the liquid surface. In the bulk of the liquid, the helical structure exposes more –CF than –CF3 groups. The surfaces do not adsorb any species implying that the only adsorption site, namely oxygen, is beneath the surface.
The influence of surface heterogeneity on thermodynamics of adsorption. Hard rods on a line
Authors: Bakaev V. A. , Steele W. A.
Published: July 2002

A system of hard rods on a line in an inhomogeneous external field is considered. This is a model of adsorption on a heterogeneous surface or, more closely, adsorption at a heterogeneous step on a surface. The external field (adsorption potential) is a Gaussian distribution of energy corresponding to a single adsorption site. A numerical method of calculation of the microdensity and its derivatives with respect to chemical potential and temperature from the Percus integral equation is described. These derivatives make it possible to calculate the differential entropy and energy (heat) of adsorption. The phenomenon of redistribution of adsorbate with increase of chemical potential is discussed. It is shown that adsorbed phase local density decreases on some parts of the interval and increases on other parts as the total adsorption increases. The differential heat of adsorption is close to the inverse cumulative energy distribution of adsorbate. It is also shown that the variation of the differential entropy of adsorption is suppressed in comparison to adsorption in an uniform external field.
Molecular dynamics simulations of ion clustering and conductivity in NaI/ether solutions. I. Effect of ion charge
Authors: Payne Vilia Ann , Xu Jian-hua , Forsyth Maria , Ratner Mark A. , Shriver Duward F. , de Leeuw Simon W.
Published: July 2002

Model systems of sodium iodide dissolved in dimethyl ether were studied in order to investigate the structural and dynamic properties of ionic solutions in small and polymeric ethers having low dielectric constants. Full molecular dynamics simulations were performed at ion charges ranging from 0.5 to 0.1 e, and an algorithm designed to assign ions to clusters and calculate all the terms contributing to ionic conductivity was implemented. Quantitative results were obtained for the contributions of various ionic species to the conductivity. These model systems are stable for ion charges at or below 0.3 e, and a maximum in conductivity is observed at 0.3 e. A range of ion cluster sizes is observed in each system, but the current giving rise to ionic conductivity is due primarily to the movement of free ions and the relative movement of ions within loosely bound pairs.
Electron transfer in high n Rydberg states
Authors: Smith Jonathan M. , Chupka William A.
Published: July 2002

A model is developed to determine the rate of electron transfer between high n molecular Rydberg states and ions of the same species under zero kinetic energy pulsed field ionization (ZEKE-PFI) experimental conditions. A simple hydrogenic model is used considering the two particles to be at rest with respect to each other. The results of accurate calculations for the u–g splitting in H+2 at low principal quantum numbers (separated atom) as a function of radius are extrapolated to large values of n relevant to typical ZEKE-PFI experimental conditions and used to determine the rate of electron transfer. Radius ≊5n2 is arrived at as a simple expression to estimate the radius for a charge transfer rate of ≊108 s−1 for lowest energy members of the Stark manifold (the fastest). Expressions are derived for other members of the manifold. Results are compared with the recent observation of 3% charge transfer in a ZEKE-PFI experiment by Alt et al.
Critical behavior of the non-Newtonian shear viscosity of colloidal systems
Authors: Dhont Jan K. G.
Published: July 2002

The anomalous behavior of the (zero frequency) shear viscosity of colloidal systems on approach of the gas–liquid critical point is analyzed. As a result of hydrodynamic interactions, the anomalous behavior is found to be qualitatively different from that of molecular systems. On the basis of the asymptotic solution of the Smoluchowski equation for the shear rate dependent pair-correlation function, an expression for the non-Newtonian zero frequency viscosity of a near-critical suspension is derived. The viscosity depends on two dimensionless groups: on ξ−1d via a cutoff function and on λ∝γ̇ξ4 via the structurefactor (ξ is the correlation length in the equilibrium system, d is the core diameter, and γ̇ is the shear rate). The transition from weak to strong shear occurs at λ≊1. The anomalous behavior of both the zero shear viscosity and the non-Newtonian characteristics is formally due to the fact that close to the critical point, where ξ is large, a very small shear rate γ̇ is sufficient to make λ a large number. The critical exponent for the zero shear viscosity is found to be equal to that of the correlation length. This exponent is much larger than for molecular systems, which is known to be very small (≊0.03). The exponential behavior sets in at ξ/d≊3.
Probability distribution of wormlike polymer loops
Authors: Liverpool T. B. , Edwards S. F.
Published: July 2002

We present a functional integral formulation of the probability distribution of the worm-like polymer chain which not only takes into account the skeletal stiffness but also explicitly includes the differentiability condition of Kratky and Porod while remaining mathematically tractable. We get closed form expressions for the probability distribution of loops of size L unlike the previous work of Yamakawa et al. [J. Chem. Phys. 57, 2843 (1972)] which gave the results as an infinite series.
Solids modeled by ab initio crystal field methods. X. Structure of α-glycine, β-glycine, and γ-glycine using a 15-molecule cluster
Authors: Peeters Anik , Van Alsenoy C. , Lenstra A. T. H. , Geise H. J.
Published: July 2002

The structure of three polymorphic forms of glycine in the crystal phase, α-glycine (P21/n), β-glycine (P21), and γ-glycine (P32), was completely optimized with standard gradient procedures using a point charge model and using a model constituted of a 15-molecule cluster surrounded by point charges. The calculations were performed with a 6-31G basis set and in the SCF step of the calculations the MIA approach was used. The results for the 15-molecule cluster are in better agreement with the experimental results than the results obtained with the point charge model.
Homogeneous vibrational dynamics and inhomogeneous broadening in glass-forming liquids: Infrared photon echo experiments from room temperature to 10 K
Authors: Tokmakoff A. , Fayer M. D.
Published: July 2002

A study of the temperature dependence of the homogeneous linewidth and inhomogeneous broadening of a high-frequency vibrational transition of a polyatomic molecule in three molecular glass-forming liquids is presented. Picosecond infrared photon echo and pump–probe experiments were used to examine the dynamics that give rise to the vibrational line shape. The homogeneous vibrational linewidth of the asymmetric CO stretch of tungsten hexacarbonyl (∼1980 cm−1) was measured in 2-methylpentane, 2-methyltetrahydrofuran, and dibutylphthalate from 300 K, through the supercooled liquids and glass transitions, to 10 K. The temperature dependences of the homogeneous linewidths in the three glasses are all well described by a T2 power law. The absorption linewidths for all glasses are seen to be massively inhomogeneously broadened at low temperature. In the room temperature liquids, while the vibrational line in 2-methylpentane is homogeneously broadened, the line in dibutylphthalate is still extensively inhomogeneously broadened. The contributions of vibrational pure dephasing, orientational diffusion, and population lifetime to the homogeneous line shape are examined in detail in the 2-methylpentane solvent. The complete temperature dependence of each of the contributions is determined. For this system, the vibrational line varies from inhomogeneously broadened in the glass and low temperature liquid to homogeneously broadened in the room temperature liquid. The homogeneous linewidth is dominated by the vibrational lifetime at low temperatures and by pure dephasing in the liquid. The orientational relaxation contribution to the line is significant at some temperatures but never dominant. Restricted orientational relaxation at temperatures below ∼120 K causes the homogeneous line shape to deviate from Lorentzian, while at higher temperatures the line shape is Lorentzian.
The infrared photodissociation spectra of Fen(CH3OH)m complexes and their deuterated analogs near 10μ
Authors: Knickelbein Mark B.
Published: July 2002

The infrared photodissociation spectra of Fen(CH3OH)m, Fen(CH3OD)m, Fen(CD3OH)m, and Fen(CD3OD)m (n=5–15; m=1,2) have been recorded from 920–1090 cm−1. Analysis of the spectral features indicates that in all cases, molecularly adsorbed methanol is the absorbing chromophore, with no evidence of methoxy formation. The observed frequencies for the C–O stretching vibration and other vibrational modes are shifted ∼40 cm−1 to the red of the corresponding fundamentals of gas-phase methanol. The spectral shifts and thus the magnitude of the cluster–methanol interaction are observed to be independent of iron cluster size over the size range studied, implying an insensitivity of the interaction to the underlying cluster structure.
Comment on ‘‘Ab initio calculation of [OH−; e+] system with consideration of electron correlation effect’’ [J. Chem. Phys. 101, 5925 (1994)]
Authors: Schrader D. M.
Published: July 2002

A calculation of the positronium affinity of the hydroxyl radical (equivalently, the binding energy of positronium hydroxide, PsOH), recently reported by Tachikawa et al., is in error because an incorrect value of the electron affinity of OH was combined with their calculated positron affinity of the hydroxide anion. When the correct value of the electron affinity of OH is used, the calculated binding energy of PsOH changes sign, and PsOH is predicted to be unstable to dissociation. Thus, the conclusion reported by Tachikawa et al., namely, that PsOH is stable, is not supported by their calculations.
Matrix elements of U(2n) generators in a multishell spin–orbit basis. I. General formalism
Authors: Burton P. J. , Gould M. D.
Published: July 2002

This is the first in a series of papers which derives the matrix elements of the spin-dependent U(2n) generators in a multishell spin–orbit basis, i.e., a spin adapted composite Gelfand–Paldus basis. The advantages of such a multishell formalism are well known and well documented. The approach taken exploits the properties of the U(n) adjoint tensor operator denoted by Δij(1≤i,j≤n) as defined by Gould and Paldus [J. Chem. Phys. 92, 7394 (1990)]. Δ is a polynomial of degree two in the U(n) matrix E=[Eij]. The unique properties of this operator allow the construction of adjoint coupling coefficients for the zero-shift components of the U(2n) generators. The Racah factorization lemma may then be applied to obtain the matrix elements of all the U(2n) generators.

In this paper we investigate the underlying formalism of the approach and discuss its advantages and its relationship to the shift operator method of Gould and Battle [J. Chem. Phys. 99, 5961 (1993)]. The formalism is then applied, in the second paper of the series, to calculate the matrix elements of the del operator in a two-shell spin–orbit basis. This immediately yields the zero-shift adjoint coupling coefficients in such a basis. The del-operator matrix elements are required for the calculation of spin densities in a two-shell basis. In the third paper of the series we derive the remaining nonzero shift adjoint coupling coefficients all of which are required for the multishell case. We then use these coupling coefficients to obtain formulas for the matrix elements of the U(2n) generators in a two-shell spin–orbit basis. This result is then generalized, in the fourth paper, to the case of the multishell spin–orbit basis. Finally, we demonstrate that in the Gefand–Tsetlin limit the formula obtained is equivalent to that of Gould and Battle for a single-shell system.
Spectroscopy of CrF: Rotational analysis of the B 6Π–X 6Σ+ band system in the 1.2 μm region
Authors: Wallin S. , Koivisto R. , Launila O.
Published: July 2002

The B 6Π–X 6Σ+ band system of the CrF radical has been recorded in emission with FTS techniques in the region between 6965 and 9240 cm−1, using a resolution of 0.025 cm−1. A rotational analysis including the (0,0), (1,1), (1,0), (0,1) and (1,2) bands of this system has been carried out, and a set of molecular parameters has been derived for the B 6Π state. The present analysis has also resulted in substantially better determined values for the λ and γ parameters of the X 6Σ+ ground state. An improved set of molecular parameters has been derived for the previously analyzed, heavily perturbed A 6Σ+ state. The present analysis gives strong support for the interpretation that the local perturbations in the v=0–3 levels of A 6Σ+ are due to interactions with the vibrational levels v=3–6 of the B 6Π state. Earlier suggestions involving a low-lying 4Π state as the dominating perturber of the A 6Σ+ state must now be considered as erroneous. A previously suggested interpretation of the 7400 cm−1 band as being possibly due to a quartet transition has also been shown to be in error. The derived first order spin-orbit parameter value of B 6Π [A=47.0382(18) cm−1] and the equilibrium bond distances of X 6Σ+ [1.7839 Å], A 6Σ+ [1.8919 Å], and B 6Π [1.8277 Å] have been discussed, and it has been proposed that the 9σ (Cr4s–4p) molecular orbital of CrF is slightly bonding, while 4π (Cr3dπ) is nonbonding and 10σ (Cr3dσ) antibonding.
State-selective control for vibrational excitation and dissociation of diatomic molecules with shaped ultrashort infrared laser pulses
Authors: Korolkov M. V. , Paramonov G. K. , Schmidt B.
Published: July 2002

Ultrafast state-selective dynamics of diatomic molecules in the electronic ground state under the control of infrared picosecond and femtosecond shaped laser pulses is investigated for the discrete vibrational bound states and for the dissociative continuum states. Quantum dynamics in a classical laser field is simulated for a one-dimensional nonrotating dissociative Morse oscillator, representing the local OH bond in the H2O and HOD molecules. Computer simulations are based on two approaches — exact treatment by the time-dependent Schrödinger equation and approximate treatment by integro-differential equations for the probability amplitudes of the bound states only. Combination of these two approaches is useful to reveal mechanisms underlying selective excitation of the continuum states and above-threshold dissociation in a single electronic state and for designing optimal laser fields to control selective preparation of the high-lying bound states and the continuum states. Optimal laser fields can be designed to yield almost 100% selective preparation of any prescribed bound state, including those close to the dissociation threshold. State-selective preparation of the highest bound state may be accompanied by the appearance of a quasi-bound molecular state in the continuum with the kinetic energy of the fragments being close to zero. The respective above-threshold dissociation spectrum contains an additional, zero-order peak. The laser-induced dissociation from selectively prepared high-lying bound states is shown to be very efficient, with the dissociation probability approaching the maximal value. Flexible tools of state-selective laser control are developed which enable one to achieve selective control of the dissociation spectra resulting in time-selective and space-selective control of the dissociation fragments.