bluuebunny/crossref_metadata_2025 huggingface dataset:
Collisional orientation of the benzene molecular plane in supersonic seeded expansions, probed by infrared polarized laser absorption spectroscopy and by molecular beam scattering
Authors: Pirani F. , Bartolomei M. , Aquilanti V. , Scotoni M. , Vescovi M. , Ascenzi D. , Bassi D. , Cappelletti D.
Published: June 2003

The large number of elastic and inelastic collisions which take place during supersonic gaseous expansions produce not only acceleration and internal cooling of molecules, but also their alignment or orientation. The collisional alignment of the rotational angular momentum, corresponding to the orientation of the benzene molecular plane, in supersonic seeded expansions with lighter carrier gases is demonstrated via two complementary experiments: one interrogating benzene via polarized laser light IR absorption the other one probing its orientation via molecular beam scattering on rare gas targets. Typical seeding gases are helium, neon and their mixtures, and molecular hydrogen. Total stagnation pressures are of the order of ∼1 bar and ∼0.1 mm nozzle. A propensity is demonstrated for benzene molecules in seeded molecular beams to fly with the molecular plane preferentially oriented parallel to the molecular beam propagation direction. The analysis of the experimental results has been carried out using a phenomenological model which provides the fraction of molecules traveling in such a “frisbee” mode. A frisbee propensity function is defined and found to range between 0.71 and 0.85, corresponding to IR and to scattering experiments, respectively. These values are significantly higher than the value 2/3, expected for a random distribution of molecular plane orientations. The trend in the measured values is discussed in terms of different angular cones of the supersonic expansion sampled in the two experiments and evidence is provided that the orientation is higher the narrower is the sampled cone aperture.
The diradical (CH3)2CHN and its isomeric molecule (CH3)2C=HN: Generation and characterization
Authors: Zheng Sun , Dong Wang , Rui Ding , Maofa Ge , Dianxun Wang , Foo-Tim Chau , Mok D. K. W.
Published: June 2003

A continuously flowing (CH3)2CHN beam is generated by pyrolysis of (CH3)2CHN3 at 113(±0.5)°C using normal inlet system with an 8 mm bore of the exit of the quartz tube under the presence of molecular sieve (30 Å) and stabilizing NO gas, and its HeI photoelectron (PE) spectrum is also recorded in situ. A spectrum recorded further away from the pyrolysis catalyst or by using a 0.4 mm bore of the quartz tube is the PE spectrum of (CH3)2C=NH, which comes from the isomerization of (CH3)2CHN. The ionization energies of (CH3)2CHN and (CH3)2C=NH are determined for the first time by the photoelectron spectroscopy experiment, and Gaussian 2 and improved density functional theory calculations. Experimental and theoretical results agree reasonably well, and show that (CH3)2CHN is a diradical with Cs symmetry and has a 3A″ ground state, and (CH3)2C=NH is a closed shell molecule with Cs symmetry.
Vacuum ultraviolet mass-analyzed threshold ionization spectroscopy of benzene: Vibrational analysis of C6H6+ and C6D6+ in the X̃ 2E1g state
Authors: Kwon Chan Ho , Kim Hong Lae , Kim Myung Soo
Published: June 2003

Vibrational spectra of C6H6+ and C6D6+ in the ground electronic state have been measured by one-photon mass-analyzed threshold ionization (MATI) spectroscopy using coherent vacuum ultraviolet radiation generated by four wave mixing in Kr gas. The ionization energies of C6H6 and C6D6 determined by one-photon MATI, 74551±5 and 74579±5 cm−1, respectively, are similar to those reported previously. Vibrational spectra are much simpler than the previous zero kinetic energy photoelectron and MATI spectra obtained by two-photon excitation. Almost complete vibrational assignments for the cations have been possible, which will be useful for future theoretical studies of the Jahn-Teller effect in these cations. Implication from the present one-photon spectra agrees with the previous suggestion that the geometry of benzene cation in the ground electronic state belongs to the D6h symmetry.
Photodissociation dynamics of ethyl ethynyl ether: A new ketenyl radical precursor
Authors: Krisch M. J. , Miller J. L. , Butler L. J. , Su H. , Bersohn R. , Shu J.
Published: June 2003

The work presented here investigates the dynamics of the photodissociation of ethyl ethynyl ether at 193.3 nm with photofragment translational spectroscopy and laser-induced fluorescence. The data from two crossed laser-molecular beam apparatuses, one with vacuum ultraviolet photoionization detection and one with electron bombardment detection, showed that only cleavage of the C–O bond to form a C2HO radical and a C2H5 (ethyl) radical occurs. We observed neither cleavage of the other C–O bond nor molecular elimination to form C2H4+CH2CO (ketene). The C2HO radical is formed in two distinct product channels, with 37% of the radicals formed from a channel with recoil kinetic energies extending from about 10 to 70 kcal/mole and the other 63% formed from a channel with lower average recoil energies ranging from 0 to 40 kcal/mole. The measurements using photoionization detection reveal that the C2HO radical formed in the higher recoil kinetic-energy channel has a larger ionization cross section for photon energies between 10.3 and 11.3 eV than the radical formed in the lower recoil kinetic-energy channel, and that the transition to the ion is more vertical. The radicals formed in the higher recoil kinetic-energy channel could be either X̃(2A″) or Ã(2A′) state ketenyl (HCCO) product and the shape of the recoil kinetic-energy distribution fitting this data does not vary with ionization energy between 10.3 and 11.3 eV. The C2HO formed in the channel with the lower kinetic-energy release is likely the spin forbidden ã(4A″) state of the ketenyl radical, reached through intersystem crossing. The B̃ state of ketenyl is energetically inaccessible. We also consider the possibility that the lower kinetic-energy channel forms two other C2HO isomers, the CCOH (hydroxyethynyl) radical or the cyclic oxiryl radical. Signal from laser-induced fluorescence of the HCCO photofragment was detected at the electronic origin and the 510 band. The fluorescence signal peaks after a 20 μs delay, indicating that HCCO is formed with a significant amount of internal energy and then subsequently relaxes to the lowest vibrational level of the ground electronic state. The data show that the photodissociation of ethyl ethynyl ether produces C2HO with unit quantum yield, establishing it as the first clean photolytic precursor of the ketenyl radical, a key species in combustion reactions.
Infrared action spectroscopy and time-resolved dynamics of the OD–CO reactant complex
Authors: Pollack Ilana B. , Tsiouris Maria , Leung Helen O. , Lester Marsha I.
Published: June 2003

The infrared action spectrum of the linear OD–CO reactant complex has been recorded in the OD overtone region near 1.9 μm using an infrared pump-ultraviolet probe technique. The pure overtone band of OD–CO (2νOD) is observed at 5148.6 cm−1 and combination bands involving the simultaneous excitation of OD stretch and D-atom bend are identified 160.0 and 191.2 cm−1 to higher energy. Band assignments and spectroscopic constants are derived from the rotationally resolved structure of the spectra. The change in the ground state rotational constant upon deuteration demonstrates that the H/D-atom of the hydroxyl radical points toward CO in the OH/D-CO complex. Direct time-domain measurements yield a lifetime of 37(4) ns for OD–CO (2νOD) prior to decay via inelastic scattering or chemical reaction. This is significantly longer than the laser-limited lifetime of ⩽5 ns observed for OH–CO (2νOH), and is attributed in part to the closing of a near-resonant vibration to vibration energy transfer channel upon deuteration. Vibrational predissociation of OD–CO (2νOD) proceeds by a vibration to rotation and/or translation mechanism that yields highly rotationally excited OD (v=1) fragments. Intermolecular D-atom bend excitation, which drives the structural transformation from the reactant complex to the transition state for reaction, results in a dramatic shortening of the lifetime to ⩽6 ns (laser-limited). Excitation of the D-atom bend also supplies sufficient energy to reopen the near-resonant vibrational energy transfer channel, resulting in minimal rotational excitation of the OD (v=1) fragments. Finally, a ground state binding energy for OD–CO of D0⩽456 cm−1 is established from the OD (v=1) product state distribution.
Relaxation strength of localized motions in D-sorbitol and mimicry of glass-softening thermodynamics
Authors: Power G. , Johari G. P. , Vij J. K.
Published: June 2003

The dielectric relaxation strength, ΔεJG, the relaxation rate, fm,JG and the distribution parameter, αJG, of the faster relaxation process in D-sorbitol have been studied as a function of temperature and the cooling rate. Amongst these, fm,JG and αJG of the glass and the supercooled liquid change smoothly with the temperature, T, but ΔεJG of the glassy state increases slowly on heating until the glass-softening range is reached and thereafter it increases rapidly at T above the glass-softening temperature, Tg. Thus its plot against T has an elbow-shape, remarkably similar to that observed for the volume, enthalpy and entropy. The derivative (dΔεJG/dT) increases relatively abruptly at Tg like the thermal expansion coefficient and the heat capacity of a glass. Thus ΔεJG is a function of the state’s entropy and volume. The distribution of relaxation times became narrower as T was increased, and fm,JG increased according to the Arrhenius equation, fm,JG=2.992×1014 exp[−5.312×104/RT], where R=8.314 J (K mol)−1. It is deduced that fm,JG increases on structural relaxation of D-sorbitol. The results indicate that the relaxation mechanism involves motions of segments of the D-sorbitol molecules or of the whole molecule in local regions.
Depletion force and torque on an ellipsoid
Authors: Li Weihua , Ma H. R.
Published: June 2003

The depletion force and torque acting on a hard rotational ellipsoid near a hard wall or two hard walls, induced by a small hard-sphere fluid, are calculated by Monte Carlo simulations. The depletion potential is determined by the acceptance ratio method, while the force and the torque on the ellipsoid are obtained by numerical differentiation of the differentiation of the depletion potential.
Boundary condition effects in the simulation study of equilibrium properties of magnetic dipolar fluids
Authors: Wang Zuowei , Holm Christian , Müller Hanns Walter
Published: June 2003

In this paper we investigate the equilibrium properties of magnetic dipolar (ferro-) fluids and discuss finite-size effects originating from the use of different boundary conditions in computer simulations. Both periodic boundary conditions and a finite spherical box are studied. We demonstrate that periodic boundary conditions and subsequent use of Ewald sum to account for the long-range dipolar interactions lead to a much faster convergence (in terms of the number of investigated dipolar particles) of the magnetization curve and the initial susceptibility to their thermodynamic limits. Another unwanted effect of the simulations in a finite spherical box geometry is a considerable sensitivity to the container size. We further investigate the influence of the surface term in the Ewald sum—that is, due to the surrounding continuum with magnetic permeability μBC—on the convergence properties of our observables and on the final results. The two different ways of evaluating the initial susceptibility, i.e., (1) by the magnetization response of the system to an applied field and (2) by the zero-field fluctuation of the mean-square dipole moment of the system, are compared in terms of speed and accuracy.
Application of density functional theory to tethered polymer chains: Effect of intermolecular attractions
Authors: Ye Yuan , McCoy John D. , Curro John G.
Published: June 2003

Density functional theory (DFT) was used to study polymer chains, tethered to a surface and in the presence of a solvent. For reasons of computational practicality, it is common practice to remove the explicit solvent molecules from the problem. Contact was made with two such models, which we call the “implicit-solvent” and “continuum-solvent” approximations. First, DFT was applied to tethered chains in an implicit solvent. Using the equation of state of bead–spring chains as input, we found excellent agreement of the theory with density profiles obtained in molecular dynamics simulations on the same model as a function of temperature. Next, DFT was applied to tethered chains in an incompressible, continuum solvent. Using the Flory–Huggins theory as input, our DFT equations reduced to conventional self-consistent field theory. From our DFT formalism, we demonstrated that the implicit-solvent problem, at a given temperature, is equivalent to the continuum-solvent problem, provided the χ parameter and total density are interpreted appropriately.
On the entropy equations for a liquid’s relaxation time at high pressures
Authors: Johari G. P.
Published: June 2003

The use of the configurational entropy equation for the dielectric relaxation time has been extended to include the change in the free energy of activation with pressure and the relevant thermodynamic functions. Its consequences have been examined in relation to the current manner of fitting the equation to the high-pressure relaxation time data, and compared against another fitting in which the total entropy is used.
A potential energy surface construction scheme for accurate reaction rate calculations: General approach and a test for the H+CH4→H2+CH3 reaction
Authors: Wu Tao , Manthe Uwe
Published: June 2003

An approach for the construction of accurate potential energy surfaces for reaction rate calculations is presented. It employs Shepard interpolation with reference points randomly selected from range of geometries relevant for the reaction rate. Quantum dynamics calculations, which use the multiconfigurational time-dependent Hartree approach and flux correlation functions to obtain thermal rate constants, monitor the convergence of the potential energy surface with increasing number of reference points. As a test of the approach, the H+CH4→H2+CH3 reaction is studied and the analytic Jordan–Gilbert potential energy surface is reproduced by the interpolation scheme. About 40 reference points are required in the interpolation to obtain a converged interpolated surface which reproduces the thermal rate constants with errors smaller than 20%.
Restricted density functional theory of linear time-dependent properties in open-shell molecules
Authors: Rinkevicius Zilvinas , Tunell Ingvar , Sałek Paweł , Vahtras Olav , Ågren Hans
Published: June 2003

In this paper we report the derivation and the performance of a spin-restricted density functional formalism for linear time-dependent properties in open-shell molecules. The formalism is based on an exponential parameterization of the density operator with the response functions defined through Ehrenfest’s principle. In addition to the derivation of formulas, details of implementation are given as well as a discussion of numerical results for excitation energies and dynamic polarizabilities for a selected set of radicals.
Decomposition pathways of dinitramic acid and the dinitramide ion
Authors: Alavi Saman , Thompson Donald L.
Published: June 2003

Gas-phase dinitramic acid HN(NO2)2 [HDN] decomposes along two pathways, one involving a molecular rearrangement, HDN→HNO3+N2O, and a second initiated by N–N bond fission, HDN→HṄNO2+ṄO2. A molecular rearrangement pathway for the gas phase dinitramide ion N(NO2)2−[DN−], DN−→NO3−+N2O, can also occur. The rates and pathways for the decomposition of HDN and the corresponding dinitramide ion are subjects of the present work. Density functional theory calculations at the B3LYP/6-311G(d,p) level are carried out to determine the geometries, vibrational frequencies, and zero-point energies of the reactants, products, and transition states involved in the gas phase decomposition of HDN. These geometries are then used in the modified Gaussian-2 method (G2M) to calculate energies to sufficient accuracy to predict the rates of the decomposition reactions. The lowest energy pathway for N2O formation initially involves an internal proton transfer in the HDN molecule. The system then passes through a four-center transition state that has a protonated bridge oxygen atom. The energy of this geometry is 35.2 kcal/mol higher than the reactant from which it is formed. This path has not been previously identified. The rates of the N2O elimination pathways are calculated using the RRKM theory. The rates of HDN and DN− decomposition are compared to each other and to the rate of N–N bond fission in dinitramic acid.
Statistical evaporation of rotating clusters. I. Kinetic energy released
Authors: Calvo F. , Parneix P.
Published: June 2003

Unimolecular evaporation in rotating atomic clusters is investigated using phase space theory (PST) and molecular dynamics simulations. The rotational densities of states are calculated in the sphere+atom approximation, and analytical expressions are given for a radial interaction potential with the form −C/rp. The vibrational densities of states are calculated using Monte Carlo simulations, and the average radial potential at finite temperature is obtained using a recent extension of the multiple range random-walk algorithm. These ideas are tested on simple argon clusters modeled with the Lennard-Jones interaction potential, at several excitation energies and angular momenta of the parent cluster. Our results show that PST successfully reproduces the simulation data, not only the average KER but its probability distribution, for dissociations from LJ14, for which the product cluster can effectively be considered as spherical. Even for dissociations from the nonspherical LJ8, simulation results remain very close to the predictions of the statistical theory.
Singlet–triplet excitation spectrum of the CO–He complex. I. Potential surfaces and bound–bound CO(a 3Π←X 1Σ+) transitions
Authors: Zeimen W. B. , Groenenboom G. C. , van der Avoird A.
Published: June 2003

The interaction of He with metastable CO(a 3Π) gives rise to two adiabatic potential surfaces of reflection symmetry A′ and A″ which were calculated with the partially spin-restricted open-shell single and double excitation coupled cluster method with perturbative triples, RCCSD(T). Two diabatic potentials were constructed and fitted analytically; the appropriate form of the angular expansion functions was derived from general invariance properties. From variational calculations on these diabatic potential surfaces we obtained the quasibound vibration-rotation-spin levels of the CO–He complex in its lowest triplet state. Only the lower spin–orbit levels of this complex with approximate quantum number Ω=0 of the CO(a 3Π) monomer were found to be stable with respect to dissociation into He and triplet CO. The potential and the bound van der Waals levels of the ground state CO(X 1Σ+)–He complex were recalculated and used in combination with the triplet excited state wave functions to compute the line strengths and the bound–bound part of the singlet–triplet excitation spectrum of the CO–He complex. The spin-forbidden singlet–triplet transitions access mainly the higher spin–orbit levels with |Ω|=1, but these were found to undergo rapid predissociation. The companion Paper II explicitly studies this process, predicts the excited state lifetimes, and generates the bound-continuum part of the CO–He singlet–triplet spectrum.
Singlet–triplet excitation spectrum of the CO–He complex. II. Photodissociation and bound-free CO(a 3Π←X 1Σ+) transitions
Authors: Zeimen W. B. , Groenenboom G. C. , van der Avoird A.
Published: June 2003

The dissociating states of the triplet–excited CO–He complex are studied by means of scattering calculations on ab initio diabatic potential energy surfaces produced in the preceding paper (Paper I). With the aid of an effective transition dipole function and the bound states of the CO–He complex in the ground singlet state we obtain the photoabsorption cross section as a function of the excitation energy and generate the full UV spectrum of the singlet–triplet transition. It was found that the dominant contributions to the spectrum, in the energy range from −5 to +10 cm−1 relative to the band origin at 48 473.201 cm−1, originate from resonances that correspond to higher spin–orbit levels of the excited CO(a 3Π)–He complex with approximate quantum number |Ω|=1. Rapid predissociation, with the triplet CO fragment decaying into its lower spin–orbit levels with Ω=0, limits the lifetime of these excited levels to, typically, 10–700 ps. We also predict the rotational and spin–orbit state distribution of the triplet CO fragment and the maximum deflection angle of the photodissociation products in a molecular beam experiment.
Hidden integrability in ideal magnetohydrodynamics: The Pohlmeyer–Lund–Regge model
Authors: Schief W. K.
Published: June 2003

It is shown that the equilibrium equations of ideal magnetohydrodynamics reduce to the integrable Pohlmeyer–Lund–Regge model subject to a volume-preserving constraint if the Maxwellian surfaces are assumed to coincide with the constant total pressure surfaces. It is demonstrated that any solution of the constrained Pohlmeyer–Lund–Regge model gives rise to a multiplicity of solutions of the magnetohydrodynamic system which share the streamline and magnetic field line geometry. Explicit solutions given in terms of elliptic functions and integrals are constructed.
Fabrication and characterization of C60 thin-film transistors with high field-effect mobility
Authors: Kobayashi S. , Takenobu T. , Mori S. , Fujiwara A. , Iwasa Y.
Published: June 2003

We report an improvement in performance of C60 thin-film field-effect transistors (TFTs) fabricated by molecular-beam deposition. Devices, fabricated and characterized under a high vacuum without exposure to air, routinely showed current on/off ratios >108 and field-effect mobilities in the range of 0.5–0.3 cm2/V s. The mobility obtained is close to that derived from the photocurrent measurements on C60 thin films and comparable to a very high value among n-type organic TFTs.
Charge-injection-controlled organic transistor
Authors: Nakayama Ken-ichi , Fujimoto Shin-ya , Yokoyama Masaaki
Published: June 2003

A promising type of organic transistor based on the charge injection process at an organic/metal interface was proposed and demonstrated. The device was composed of an organic deposited film sandwiched between two metal electrodes, with a third stripe-shaped electrode embedded in the organic film. The output current, corresponding to the collector current, was modulated by an applied voltage on the third electrode, corresponding to the base electrode, and the current amplification factor, hFE, exceeded 70. The operation of the device as a transistor was attributed to electron injection from the accumulation of holes supplied by the third electrode.
Density of vibration states and ferroelectric properties of complex perovskites
Authors: Gvasaliya S. N. , Lushnikov S. G. , Sashin I. L. , Shaplygina T. A.
Published: June 2003

We studied the effect of temperature on density of vibrational states of model complex perovskites PbMg1/3Nb2/3O3 (PMN), PbMg1/3Ta2/3O3 (PMT), and BaMg1/3Ta2/3O3 (BMT) by inelastic neutron scattering. It has been found that PMN and PMT relaxor ferroelectrics exhibit an anomalous temperature evolution of the density of vibrational states G(E), while G(E) in the BMT compound does not demonstrate any temperature evolution. It is shown that there is a correlation between the temperature behaviors of the dielectric response and vibrational spectra of all the complex perovskites studied. Possible reasons for variation in G(E) in these materials are discussed.
Photochemistry of caged molecules: CD3Cl@Ice
Authors: Lilach Y. , Asscher M.
Published: June 2003

Hydrocarbons formation following UV photo-induced dissociation of CD3Cl trapped and caged inside thin amorphous solid water (ASW) layers on Ru(001) has been measured for the first time under well-defined UHV conditions. Stable products such as C2D6, CHD3, CD3CD2Cl, CD3OH were detected via post-irradiation temperature programmed desorption. Specific reactivity pathways for the various photo-products were identified based on excitation wavelengths, ASW layer thickness, and parent molecules initial coverage dependence. Cross sections of (1–6)×10−19 cm2 and (1–3)×10−20 cm2 at 193 nm and 248, respectively, were measured. These photo-induced phenomena of caged molecules are discussed as a possible mechanism for the formation of hydrocarbons in interstellar space.
Relationship between the frequency of the main LO mode of silica glass and angle of incidence
Authors: Wang T. B. , Liu Z. G. , Tan C. Z.
Published: June 2003

The infrared reflection spectra of silica glass were monitored at different angles of incidence and different polarization directions of the incident light. The frequency of the transverse optic mode in the vicinity of 1129 cm−1 remains a constant under different conditions, whereas the frequency of the longitudinal optic mode near 1260 cm−1 and the ratio RLO/RTO are functions of the angle of incidence and polarization direction of the plane polarized incident light, where R denotes the reflectance. These observations are due to the gradient in the vibration amplitude in the direction perpendicular to the glass surface.
Quantification of lateral repulsion between coadsorbed CO and N on Rh(100) using temperature-programmed desorption, low-energy electron diffraction, and Monte Carlo simulations
Authors: van Bavel A. P. , Hopstaken M. J. P. , Curulla D. , Niemantsverdriet J. W. , Lukkien J. J. , Hilbers P. A. J.
Published: June 2003

Temperature programmed desorption of CO coadsorbed with atomic N on Rh(100), reveals both long- and short-range interactions between adsorbed CO and N. For CO desorption from Rh(100) at low coverage we find an activation energy Ea of 137±2 kJ/mol and a preexponential factor of 1013.8±0.2 s−1. Coadsorption with N partially blocks CO adsorption and destabilizes CO by lowering Ea for CO desorption. Destabilization at low N coverage is explained by long-range electronic modification of the Rh(100) surface. At high N and CO coverage, we find evidence for a short-range repulsive lateral interaction between COads and Nads in neighboring positions. We derive a pairwise repulsive interaction ωCO–NNN=19 kJ/mol for CO coadsorbed to a c(2×2) arrangement of N atoms. This has important implications for the lateral distribution of coadsorbed CO and N at different adsorbate coverages. Regarding the different lateral interactions and mobility of adsorbates, we propose a structural model which satisfactorily explains the observed effects of atomic N on the desorption of CO. Dynamic Monte Carlo simulations were used to verify the experimentally obtained value for the CO–N interaction, by using the kinetic parameters and interaction energy derived from the temperature-programmed desorption experiments.
Statistics of low-frequency plasma fluctuations in a simple magnetized torus
Authors: Rypdal K. , Ratynskaia S.
Published: June 2003

Statistical analysis is performed on time series of plasma potential and electron pressure fluctuations for two particular plasma states in a simple magnetized torus. In spite of the occurrence of cyclic trends in the signals, the analysis demonstrates the existence of long-range dependence on much longer time-scales than the plasma confinement time, and both Gaussian and non-Gaussian self-affinity is demonstrated in the probability distribution functions (PDFs) of increasingly coarse grained time series. Non-Gaussian PDFs for electron pressure fluctuations p are believed to arise from a nonlinear relationship between p and the plasma potential V. Non-Gaussian PDFs for V are well approximated by distributions derived from extreme value statistics.
Variable coupling in n-type silicon–germanium double quantum dots
Authors: Ferguson Andrew J. , Hasko David G. , Ahmed H. , Williams David A.
Published: June 2003

We report low-temperature electron transport behavior of trench-isolated silicon–germanium (SiGe) double quantum dots. By sweeping the potentials on side gates, we find a hexagonal charging diagram in the conductance measurements. It is also found that the gates allow the parameters of individual tunnel barriers to be changed over a wide range. Charging effects may therefore be observed in different coupling regimes, for example, the two dots that comprise the double dot can be induced to merge together yielding a large single dot.
Diffusion and vibration of CO molecules adsorbed on a Cu(100) surface: A periodic density functional theory study
Authors: Fouquet Peter , Olsen Roar A. , Baerends Evert Jan
Published: June 2003

The potential energy surface (PES) describing the diffusion and vibration of CO molecules adsorbed on a Cu(100) surface has been calculated using density functional theory within two different generalized gradient approximations (GGAs), employing a slab representation of the surface. One goal of the study was to investigate the origin of the discrepancy between a recently published PES [J. Chem. Phys. 114, 1053 (2001)] and inferences that had been made from various experiments. A further aim was to create a PES that could provide a better basis for modeling and understanding of the diffusive and vibrational motion in the CO/Cu(100) system. We found that the calculated PES has a global minimum on-top of the substrate atoms in line with the experimentally determined adsorption site. Diffusion is preferred across the bridge site, a saddle point, with a diffusion barrier of 95±30 meV and 125±30 meV for the two GGAs. Vibrational frequencies deduced from the PES agree with experimental results to within 10 meV.
The effects of anion and cation substitution on the ultrafast solvent dynamics of ionic liquids: A time-resolved optical Kerr-effect spectroscopic study
Authors: Giraud Gerard , Gordon Charles M. , Dunkin Ian R. , Wynne Klaas
Published: June 2003

Ultrafast solvent dynamics of room-temperature ionic liquids have been investigated by optical heterodyne-detected Raman-induced Kerr-effect spectroscopy (OHD-RIKES) by studying the effects of cation and anion substitution on the low frequency librational modes. The spectra of two series of imidazolium salts are presented. The first series is based on the 1-butyl-3-methylimidazolium salts [bmim]+ containing the anions trifluoromethanesulfate [TfO]−, bis(trifluoromethanesulfonyl)imide [Tf2N]−, and hexafluorophosphate [PF6]−. The second series is based on [Tf2N]− salts containing the three cations 1-butyl-2,3-dimethylimidazolium [bmmim]+, 1-methyl-3-octylimidazolium [omim]+, and [bmim]+. It is found in all five samples that the signal is due to libration of the imidazolium ring at three frequencies around 30, 65, and 100 cm−1 corresponding to three local configurations of the anion with respect to the cation.
Photophysical properties of coumarin-120: Unusual behavior in nonpolar solvents
Authors: Pal Haridas , Nad Sanjukta , Kumbhakar Manoj
Published: June 2003

Photophysical properties of coumarin-120 (C120; 7-amino-4-methyl-1,2-benzopyrone) dye have been investigated in different solvents using steady-state and time-resolved fluorescence and picosecond laser flash photolysis (LFP) and nanosecond pulse radiolysis (PR) techniques. C120 shows unusual photophysical properties in nonpolar solvents compared to those in other solvents of moderate to higher polarities. Where the Stokes shifts (Δν̄=ν̄abs−ν̄fl), fluorescence quantum yields (Φf), and fluorescence lifetimes (τf) show more or less linear correlation with the solvent polarity function Δf={(ε−1)/(2ε+1)−(n2−1)/(2n2+1)}, all these parameters are unusually lower in nonpolar solvents. Unlike in other solvents, both Φf and τf in nonpolar solvents are also strongly temperature dependent. It is indicated that the excited singlet (S1) state of C120 undergoes a fast activation-controlled nonradiative deexcitation in nonpolar solvents, which is absent in all other solvents. LFP and PR studies indicate that the intersystem crossing process is negligible for the present dye in all the solvents studied. Photophysical behavior of C120 in nonpolar solvent has been rationalized assuming that in these solvents the dye exists in a nonpolar structure, with its 7-NH2 group in a pyramidal configuration. In this structure, since the 7-NH2 group is bonded to the 1,2-benzopyrone moiety by a single bond, the former group can undergo a fast flip-flop motion, which in effect causes the fast nonradiative deexcitation of the dye excited state. In moderate to higher polarity solvents, it is indicated that the dye exists in an intramolecular charge-transfer structure, where the bond between 7-NH2 group and the 1,2-benzopyrone moiety attains substantial double bond character. In this structure, the flip-flop motion of the 7-NH2 group is highly restricted and thus there is no fast nonradiative deexcitation process for the excited dye.
Structural dynamics of hydrogen bonded methanol oligomers: Vibrational transient hole burning studies of spectral diffusion
Authors: Piletic I. R. , Gaffney K. J. , Fayer M. D.
Published: June 2003

Frequency resolved pump-probe experiments have been conducted on the deuterated hydroxyl stretch of methanol-d in a solution containing 0.8% methanol-d/23% methanol-h in carbon tetrachloride. Methanol-d molecules that both donate and receive hydrogen bonds have an inhomogeneously broadened hydroxyl stretch absorption line centered at 2487 cm−1. With a laser tuned to 2513 cm−1, the high-frequency side of the absorption spectrum is excited. The equilibration of the excited state peak and the ground-state hole results in the time-dependent shift in the frequency of the signal, which is used to monitor the dynamics of spectral diffusion. Model calculations were conducted to address the influence of spectral diffusion in the ground and excited states on the experimental observables when the vibrational lifetime is comparable to the spectral diffusion time. The model calculations illustrate the influence on the signal of absorbers in the ground state that have relaxed from the excited state. This aspect of the problem has not been addressed in previous descriptions of frequency resolved pump-probe spectroscopy. The calculations were used to fit the time-dependent peak maximum, resulting in a bi-exponential frequency–frequency correlation function, with a fast time constant of roughly 0.1 ps and a slower time constant of 1.6±0.3 ps. The observed dynamics have been compared with the predictions of dielectric continuum theory. The inability of a simple dielectric continuum theory to predict the observed spectral diffusion dynamics suggests that these dynamics do not result from the long-wavelength, collective orientational relaxation of the solvent. Instead the dynamics are attributed to fluctuations in the local hydrogen bond network, which is consistent with recent molecular-dynamics simulations of vibrational transient hole burning in water.
Photoinduced intermolecular electron transfer from aromatic amines to coumarin dyes in sodium dodecyl sulphate micellar solutions
Authors: Kumbhakar Manoj , Nath Sukhendu , Pal Haridas , Sapre Avinash V. , Mukherjee Tulsi
Published: June 2003

Photoinduced intermolecular electron transfer interaction between coumarin dyes and aromatic amines has been investigated in sodium dodecyl sulphate micellar solutions using steady-state and time-resolved fluorescence quenching measurements. Steady-state fluorescence quenching of the coumarin dyes by the amine quenchers always shows a positive deviation from linear Stern–Volmer relationship, which arises due to the localized high quencher concentrations at the micellar Stern layer. In time-resolved fluorescence measurements, the analysis of the fluorescence decays following a micellar quenching kinetics model assuming a unified quenching constant (kq′) per quencher occupancy does not give satisfactory results, especially for the higher quencher concentrations used. The observed fluorescence decays are, however, seen to fit reasonably well following a bi-exponential analysis for all the quencher concentrations used. The average fluorescence lifetimes of the coumarin dyes in the micellar solution as estimated from the bi-exponential decay analysis are seen to undergo a systematic reduction with the effective mean quencher concentrations. The bimolecular quenching constants (kq) thus estimated are seen to be much smaller than those reported in the homogeneous solutions (e.g., in acetonitrile), indicating that the electron transfer in the micellar media is inherently inefficient. Correlation of the observed kq values in the micellar solutions with the free-energy changes (ΔG0) for electron transfer reactions show an inversion in the observed rates as predicted by Marcus’ outer sphere electron transfer theory at exergonicities more that ∼0.65 eV. To the best of our knowledge this is the first report on the Marcus inverted region observed for the electron transfer reactions in micellar solution.
Diffusion assisted end–to–end relaxation of a flexible Rouse polymer chain: Fluorescence quenching through a model energy transfer
Authors: Bandyopadhyay Tusar , Ghosh Swapan K.
Published: June 2003

The diffusion-influenced end–to–end conformational relaxation of a flexible polymer chain molecule (within the Rouse model) is investigated theoretically in the Markovian limit utilizing a generalized diffusion equation for the probability distribution of the end–to–end distance of a chain molecule, which has its origin in the Zwanzig’s treatment of Onsager’s theory of irreversible processes. The end–to–end diffusion dynamics of the chain molecule is considered to be probed by fluorescence resonance energy transfer between two chromophores, attached to the chain ends. The resulting diffusion equation with a sink term representing this energy transfer through a suitably modified Förster rate expression is solved numerically for equilibrium close/open conformations, and end–to–end irreversible ring opening or closure of the chain. Illustrative calculations and comparisons between the kinetics arising out of these conformations are presented to demonstrate the competitive interplay between the transfer sink strength, diffusion, and chain relaxation. From the numerical results for a variety of transfer parameters and end–to–end diffusion coefficients, it is found that a nonmonotonic distribution of reaction times is a confirmative signature for a cyclization transition, whereas a monotonic decay of distribution of the reaction times, as is always the case with the chain opening transition or with the equilibrium configuration (either closed or opened) of the chain, cannot also be ruled out for the cyclization transition. The knowledge of the distribution of energy-transfer distances is utilized to help delineate the features associated with the reaction time distribution during the end–to–end relaxation.
Femtosecond photoelectron imaging of pyridazine: S1 lifetime and (3s(n−1),3p(n−1)) Rydberg state energetics
Authors: Matsumoto Yoshiteru , Kim Sang Kyu , Suzuki Toshinori
Published: June 2003

The first real-time study on pyridazine in the S1(n,π*) state is presented. The S1 state is found to dephase with a time constant of 323±17 ps at its origin, and the electronic dephasing mechanism is attributed to the S1–S0 internal conversion. The S1 lifetime is found to decrease rather sharply as the internal energy increases. The 3s (n−1) and 3p (n−1) Rydberg states of pyridazine are clearly identified in angle- and energy-resolved photoelectron images obtained in the (1+2′) photoionization scheme, providing their respective term values of 5.68±0.03 and 6.28±0.04 eV.
Electron inertia effects on m=1 instability in a cylindrical tokamak with a negative central current
Authors: Ishizawa A. , Sato M. , Wakatani M.
Published: June 2003

The formation of a current hole by a series of nonlinear m=1 magnetic reconnections due to electron inertia in a tokamak is demonstrated numerically. If a negative current exists around the magnetic axis of a tokamak plasma, an unstable m=1 magnetic reconnection shifts the negative current outward but a nonlinear secondary magnetic reconnection induced by electron inertia attempts to recover the initial current profile. A partially recovered current profile is again unstable to the m=1 mode and therefore subject to a subsequent nonlinear magnetic reconnection. Through such repeated magnetic reconnections, it is shown that the negative current density is gradually diminished, thus forming a current hole.
Turbulent flux and the diffusion of passive tracers in electrostatic turbulence
Authors: Basu Ronni , Jessen Thomas , Naulin Volker , Rasmussen Jens Juul
Published: June 2003

The connection between the diffusion of passive tracer particles and the anomalous turbulent flux in electrostatic drift-wave turbulence is investigated by direct numerical solutions of the 2D Hasegawa–Wakatani equations. The probability density functions for the point-wise and flux surface averaged turbulent particle flux are measured and compare well to a folded Gaussian, respectively a log-normal distribution. By following a large number of passive tracer particles we evaluate the diffusion coefficient based on the particle dispersion. It is found that the particle diffusion coefficient is in good agreement with the one derived from the turbulent E×B-flux by using Fick’s law. Employing the Lagrangian conservation of the “Potential Vorticity” in the Hasegawa–Wakatani equations, the analytical support for this result is obtained. The transport estimated by passive tracer dispersion and turbulent plasma flux are found to coincide.
Quaternionic roots of SO(8), SO(9), F4 and the related Weyl groups
Authors: Koca Mehmet , Koç Ramazan , Al-Barwani Muataz
Published: June 2003

The root systems of SO(8), SO(9) and F4 are constructed by quaternions. Triality manifests itself as permutations of pure quaternion units e1, e2 and e3. It is shown that the automorphism groups of the associated root systems are the finite subgroups of O(4) generated by left-right actions of unit quaternions on the root systems. The relevant finite groups of quaternions, the binary tetrahedral and binary octahedral groups, play essential roles in the construction of the Weyl groups and their conjugacy classes. The relations between the Dynkin indices, standard orthogonal vector and the quaternionic weights are obtained.
Prolongation methods and Cartan characters for the three-dimensional Riemann–Lanczos problem
Authors: Gerber A.
Published: June 2003

The Riemann–Lanczos problem in two dimensions is in involution and its general solution is known. The three-dimensional Riemann–Lanczos problem is not in involution and needs to be prolongated. We write the three-dimensional Riemann–Lanczos problem as an exterior differential system and prolong it to second order. Using algebraic computing we find that we have to add an integrability condition to make it a system in involution. We also suggest a prolongation of the three-dimensional Riemann–Lanczos problem in the same way as Bampi and Caviglia did for four dimensions. After supplementing this three-dimensional system with an integrability condition, it becomes involutive with Cartan characters (17,8,2) or (20,10,3) if no cyclic conditions are imposed. We present the relevant sections of REDUCE computer codes by means of which these results were obtained.
O–H flipping vibrations of the Cage water hexamer: An ab initio study
Authors: Losada Martin , Leutwyler Samuel
Published: June 2003

In the Cage[1] isomer of the water hexamer, the free O–H bonds of the two end-cap water molecules can flip between “up” (u) and “down” (d) orientations, giving four conformers denoted uu, ud, du, and dd. Using the Møller–Plesset second order perturbation method and large basis sets, we calculate fully relaxed potential energy curves as a function of both u↔d torsional angles, denoted φ1, φ2. These predict du as the lowest conformer, with uu nearly degenerate and ud and dd at 30–40 and 50–70 cm−1 higher energy, respectively. Along φ1 the torsional barriers are about 200 cm−1, along φ2 between 80 and 110 cm−1. The torsional zero-point energies are high, the vibrational ground states are strongly delocalized and averaging of the cluster properties is important along both φ1 and φ2. The dipole moment components vary strongly along both φ1 and φ2: μa changes from +0.8 to +2.2 D, μb from +0.5 to +1.2 D, and μc from +1.4 to −0.9 D. The φ2 torsional fundamental of (H2O)6 is predicted in the range 65–72 cm−1 with an intensity of ≈0.5 D2, the φ1 fundamental is in the 22–32 cm−1 range, with an intensity of ≈0.3 D2. Both excitations are b/c hybrid perpendicular bands with a dominant b component. The torsional overtones should be very weak.
A “perfect” hydrodynamic similarity and effect of the Reynolds number on the global scale motion
Authors: Ryutov D. D. , Remington B. A.
Published: June 2003

In a number of astrophysical systems and high-energy-density laboratory experiments designed to imitate them, the motions that are essentially dissipationless on the global scale may develop small-scale turbulence which is affected by dissipation. This may bring up a dependence of the global scale motion on the Reynolds number, even if it is very large. The difficulty in studying this issue in high energy density laboratory experiments lies in that equations of state and transport coefficients are usually poorly known, so that the subtle effect of the Reynolds number may be easily obscured by experimental uncertainties. An approach is suggested that allows one to circumvent this difficulty and isolate the effect of the Reynolds number.
Magnetic islands and perturbed plasma equilibria
Authors: Nührenberg Carolin , Boozer Allen H.
Published: June 2003

The confinement of plasmas in toroidal magnetic configurations is sensitively dependent on the quality of the magnetic surfaces. Existing numerical techniques for studying islands in plasma equilibria are computationally demanding and require far more computer power than equilibrium codes that presuppose the existence of perfect surfaces. Here new techniques are developed so equilibrium codes that assume perfect surfaces can be used with an ideal magnetohydrodynamics (MHD) stability code to find the modifications to the plasma shape that are required to make the assumption of perfect surfaces an accurate approximation. The method also allows one to determine what magnetic field distributions that are produced by the coils must be controlled to have good surfaces. Methods for calculating the island widths produced by external currents are also given. For an error-field-induced 1/1 island and the natural 5/5 islands in a finite-plasma-β Wendelstein 7–X variant [C. Nührenberg, Phys. Plasmas 3, 2401 (1996)], the island widths resulting from such an MHD calculation have been found to be approximately the same as the corresponding ones from a vacuum field analysis.
Amorphous carbon films prepared by the “dip” technique: Deposition and film characterization
Authors: Ray S. C. , Fanchini G. , Tagliaferro A. , Bose B. , Dasgupta D.
Published: June 2003

Several nondestructive characterization techniques (optical and photothermal deflection spectroscopy in the visible photon energy range, Raman spectroscopy, profilometry, photoluminescence, electron-spin-resonance characterization of the paramagnetic centers, etc.) are used to study the properties of the a-C:H:OH films deposited by the “dip” technique. With such a preparation method, the substrates are dipped in viscous mixtures of liquid carbon compounds. The subsequent baking in an oven at different temperatures (300–500 °C) allows the formation on a substrate of submicrometric thin solid films. The as-prepared material exhibits several characteristics that are comparable to that of floppy, polymer-like amorphous carbons. Transitions on many film properties (thicknesses, optical gaps, photoluminescence intensities, spin densities, g factors, relaxation times, etc.) were found between 300 and 350 and between 450 and 500 °C.