bluuebunny/crossref_metadata_2025 huggingface dataset:
Characteristics of transmission of electrons in a bent quantum waveguide with inhomogeneous magnetic fields
Authors: Gu Ben-Yuan , Lin Yuh-Kae , Chuu Der-San
Published: July 2002

We present numerical studies of the transmission properties in an L-shaped quantum waveguide (LQW) subject to an inhomogeneous magnetic field perpendicular to the LQW plane. The magnetic field remains zero at the corner region, thus, a magnetically defined cavity is formed in this LQW. We find that transmission characteristics of electrons in the LQW depend strongly on geometric parameters and magnetic configurations. Sharp peaks with unity amplitude and deep dips are observed in transmission as a function of the Fermi energy of the incident electron at some high fields. The mode–mode coupling between the wires and the cavity and multiple reflection of electrons in the cavity lead to the appearance of these structures in transmission spectrum. We also study the variation of transmission with magnetic field for different magnetic confinement configurations when fixing incident electron energy. In the magnetic depletion process of the propagating modes in wires, the transmission exhibits various patterns, such as stepped drop, wide valley, deep dips, large oscillations, or without any structure, sensitively dependent on incident electron energy and magnetic confinement configurations. It is expected that one can flexibly modify transmission spectrum of the LQW by applying an inhomogeneous magnetic field to match practical requirements.
Cu sputtering and deposition by off-normal, near-threshold Cu+ bombardment: Molecular dynamics simulations
Authors: Abrams Cameron F. , Graves David B.
Published: July 2002

Sputter yields Y and sticking coefficients S are essential inputs for feature profile evolution studies. Molecular dynamics simulations are used to compute sputter yields and sticking coefficients for Cu+ ions impinging on a Cu surface at various incident energies 15<Ei<175 eV, and incident angles 0<θi<85°. Threshold energies for sputtering Eth are also predicted and shown to vary with θi. We show that for energies below what is experimentally considered as threshold for physical sputtering (Eth(expt)∼60 eV) a yield between 0.01 and 0.1 Cu/ion is observed for some off-normal angles of incidence [C. Steinbrüchel, Appl. Phys. Lett. 55, 1960 (1989)]. We show that Y∝Ei−Eth below Eth(expt) when Y is a maximum with respect to θi (at θi=45°). We find that Y∝Ei1/2−Eth1/2 at other angles of incidence. We show that S is sensitive to Ei and θi in this regime. In particular, when θi=85°, we see that ln S∼1/Ei for Ei⩾20 eV. We discuss some assumptions commonly used in profile simulation studies which may now be relaxed, with an eye toward improving the predictive power of those simulations.
Contactless thermally stimulated lifetime measurements in detector-grade cadmium zinc telluride
Authors: Kessick Royal , Tepper Gary , Lee Ed , James Ralph
Published: July 2002

Contactless thermally stimulated lifetime measurements were performed on detector-grade Cd1−xZnxTe (x∼0.1) crystals using a pulsed laser microwave cavity perturbation method. The carrier lifetime decreased from approximately 30 μs at 110 K to 4 μs at 160 K, and then remained relatively constant from 160 to 300 K. The sudden drop in carrier lifetime within a particular temperature range is consistent with the thermal activation of a charge trap with a detrapping time longer than the carrier lifetime. The maximum trap activation temperature and the minimum detrapping time are estimated from the lifetime versus temperature curve to be approximately 160 K and 10−6 s, respectively.
X-ray magnetic circular dichroism at the rare earth L2,3 edges in R2Fe14B intermetallics
Authors: Miguel-Soriano J. , Chaboy J. , Garcı́a L. M. , Bartolomé F. , Maruyama H.
Published: July 2002

We present a systematic x-ray magnetic circular dichroism (XMCD) study performed at the rare-earth (R) L2,3 edges on the R2Fe14B series. The identification of multipolar contributions to the signal allows the application of sum rules to the obtained XMCD spectra. The results show that both orbital and spinorial components of the R(5d) magnetic moment are proportional to the 4f ones. Hence, the magnetic moment of 5d shell derived from a sum-rules analysis evidences an important nonquenched orbital contribution. This result is in contradiction with Campbell’s model for R–Fe intermetallic compounds. Consequently, both XMCD sum rules and Campbell’s model have to be revised in the atomic level.
Spin-dependent electron interferometry
Authors: Egger S. , Back C. H. , Pescia D.
Published: July 2002

We use interference of electron waves in a quantum well to switch the spin polarization of electrons. The spin quantum resonator consists of a Cu-film of variable thickness sandwiched between vacuum and a magnetic Co-film. Electrons are injected into the resonator from the vacuum side. The Co-film provides a spin dependent reflector. Varying the resonator thickness results in periodic modulation of the spin polarization of the incoming electrons. We use spin interferometry to measure the divergence of the period of interlayer exchange coupling upon approaching a zone boundary. We propose that interferometric spin selection should be observable in spin polarized scanning tunneling spectroscopy.
Current–voltage analysis of a tunneling emitter-undoped single quantum well infrared photodetector
Authors: Gurusinghe K. K. M. N. , Premaratne K. , Andersson T. G. , Bandara S. V.
Published: July 2002

The electrical behavior of a tunneling emitter-undoped single quantum well infrared photodetector structure was studied theoretically and compared with experimental results. In order to understand the current generation and transport mechanisms, and to estimate the quantum well electron density variation and the applied bias voltage distribution, the current–voltage dependence was modeled and numerically analyzed using the known parameters of the structure. In this study, resonance and nonresonance currents from the emitter contact and the field emission and the thermionic emission currents from the quantum well, were taken into account. A simple analytical expression for resonance tunneling current was derived using the transfer Hamiltonian description of resonance tunneling. Effective mass variation of different materials in the heterostructure was considered. The model was developed by balancing the current across the quantum well layer, where the carrier density reached to a steady value. Resulting calculation accurately predicted the resonance behavior of the dark current through the device as bias voltage changed. The carrier density variation in the quantum well region and barrier band bending, as a function of bias voltage, were also calculated.
Space lateral transfer and negative differential conductance regimes in quantum waveguide junctions
Authors: Polizzi E. , Abdallah N. Ben , Vanbésien O. , Lippens D.
Published: July 2002

Nonlinear conduction properties of multiport quantum waveguide-based devices are theoretically investigated. A two dimensional finite element solver for the time independent Schrödinger equation combined with Poisson equation has been developed. It handles arbitrary geometrical designs and potential profiles in the device active zone. Starting from transmission spectra calculated out of equilibrium, current–voltage characteristics of a quantum branch line directional coupler are derived as a function of the chemical potential of the injection region and of the applied bias between the input and output terminals of the structure. Under the approximation of ballistic transport, a detailed analysis of mono- and multimode propagation regimes shows that pronounced negative differential conductance effects combined with real space lateral transfers can be obtained. The robustness of the current–voltage characteristics as a function of temperature is also addressed.
Photodissociation spectroscopy and dynamics of negative ion clusters. II. CO3−⋅(H2O)1,2,3
Authors: Hunton D. E. , Hofmann M. , Lindeman T. G. , Albertoni C. R. , Castleman A. W.
Published: July 2002

Energy resolved photodissociation studies of CO3−⋅(H2O)n, n=1,2,3 are reported for photon energies ranging from 1.95 to 2.2 eV. The only dissociation channel observed is the loss of all attached water molecules to give unclustered CO3− as the sole photofragment ion. The cross section for this mechanism is substantially higher than that for the bare ion, and the sharp structure observed in the spectrum of the bare ion is nearly lost in the clusters. Analysis of the kinetic energy distributions for the photofragment ions places an upper limit of 20 μs on the lifetime of the excited clusters, and demonstrates that approximately 95% of the excess energy in the cluster remains in the CO3− containing fragment rather than being partitioned into relative translation of the photofragments or into internal motion of the water fragments. The dissociation mechanism begins with a bound–bound 2A1←2B1 transition within the core CO3− ion. Internal conversion returns the core ion to the electronic ground state with substantial vibrational excitation; redistribution of this vibrational energy results in vibrational predissociation of the cluster. The relations of this mechanism to those that occur in the bare ion and to other vibrational predissociation experiments on clusters are discussed.
Energy distributions in the HF and CO products of the reaction of F atoms with HCO
Authors: Donaldson D. J. , Sloan J. J.
Published: July 2002

The initial energy distributions in both products of the title reaction have been measured using a variant of the low-pressure infrared chemiluminescence technique. The vibrational populations of both products decrease with increasing vibrational level, indicating of the formation of a long-lived HFCO intermediate. The HF vibrational population is well represented by a (restricted phase space) statistical calculation. The CO vibrational excitation is considerably colder than this calculation predicts. These results, combined with ab initio reaction coordinate calculations suggest that free exchange of energy among the modes of the HFCO intermediate ceases early in the exit channel. Thereafter, an excess of energy is partitioned into relative translation of the products.
The temperature dependence of ion–molecule association reactions
Authors: Patrick R. , Golden D. M.
Published: July 2002

A simplified version of unimolecular rate theory is shown to provide a good method for predicting and interpreting the temperature dependence of rate constants for ion–molecule association reactions. Specific calculations are presented for the reactions N2++N2→ N4+, O2++N2→O2N2+ and O2++O2→O4+ and comparisons are made with experimental data. It is shown that substantial deviations from third order behavior would be expected for the N2++N2 rate constant in the commonly used experimental pressure range, 1–3 Torr.
Conformation of polydiacetylene macromolecules in solution: Field induced birefringence and rotational diffusion constant
Authors: Lim K. C. , Kapitulnik A. , Zacher R. , Heeger A. J.
Published: July 2002

Application of an external electric field induces birefringence in a solution of polydiacetylene (poly-4BCMU) in toluene due to orientation of the macromolecules. The field induced orientation in the red phase indicates an anisotropic polarizibility tensor characteristic of a rod-like conformation. The birefringence due to the polymer in its coil conformation (yellow phase) is extremely weak. The transient response of the anisotropic light scattering was studied in the red phase after switching the electric field on or off. We find a free rotational relaxation time of ∼0.1 s, consistent with the rotational diffusion constant expected for rod-like polydiacetylene macromolecules. These results rule out the large aggregate interpretation proposed to explain the color change transition. The field induced birefringence data independently demonstrate the existence of the rod-coil conformational transition for polydiacetylene 4BCMU macromolecules in solution.
Spectroscopic and light scattering studies of the conformational (rod-to-coil) transition of poly(diacetylene) in solution
Authors: Lim K. C. , Heeger A. J.
Published: July 2002

A rod-to-coil conformational transition has been demonstrated for polydiacetylene, 4-butoxy-carbonyl-methylurethane (4BCMU) in solution. The transition can be induced by changing either the temperature or the quality of the solvent. The light scattering and spectroscopic data as a function of polymer concentration have shown that the transition is a single chain (intramolecular) phenomenon. However, because of the large end-to-end length (L≂1.2 μm) of the fully extended polymer, the dilute limit is not reached until concentrations below 10−5 g/cm3. At higher concentrations evidence of cluster growth and aggregation are observed prior to gelation which occurs above a critical concentration c0≂5×10−4 g/cm3. This cluster growth occurs as a result of the rod-like conformation of the individual molecules, but it is not the cause of the transition. The large increase in scattering intensity (at fixed polymer concentration) on going from coil to rod follows directly from the change in dielectric constant due to the spectral shift of the π–π* absorption; no significant increase in molecular weight is implied by the data. A theoretical model of the transition has been developed in which the ordered rod-like conformation is the low temperature phase. Conformational kinks (to a coil phase) cost energy through interruption of the π-electron delocalization and through the breaking of H bonds between R groups. Nevertheless, the increase in entropy associated with the many degrees of freedom of the coil-like conformation is sufficient to lead to the observed transition.
Propagating and stationary structures in chemical reaction–diffusion systems
Authors: Shyldkrot Haim , Ross John
Published: July 2002

We consider propagating fronts and stationary patterns in chemical reaction–diffusion systems with nonlinear rate mechanisms maintained far from equilibrium. We study analytically and numerically the dependence on diffusion coefficients of the direction of propagation of the concentration profile which is obtained when two homogeneous steady states are placed in contact under identical constraints. We analyze the possible concentration profiles in a two-variable system with two stable stationary states for various values of diffusion coefficients and reaction time scales, and show that the direction of propagation depends on the diffusion coefficients. Finally, we show that a stationary pattern can develop behind the propagating concentration profile, a process for which there is some experimental evidence.
Lattice dynamics and vibrational spectra of undoped and doped polyparaphenylene
Authors: Zannoni Giuseppe , Zerbi Giuseppe
Published: July 2002

Lattice dynamical calculations of undoped and doped polyparaphenylene are carried out and the results compared with the vibrational spectra presently available in the literature. The purpose of the work is to determine from vibrational spectra the extent of delocalization of π electrons along the chain. Moreover, it is suggested that the doping induced spectra are consistent with the formation of bipolarionic type defects.
Quantum and semiclassical description of a triply degenerate anharmonic oscillator
Authors: Patterson Chris W.
Published: July 2002

Quantum and semiclassical energies are compared as a function of anharmonicity using the Hecht Hamiltonian for the triply degenerate anharmonic oscillator for octahedral and tetrahedral molecules. Semiclassical energies are found by turning on the potential adiabatically and the corresponding classical trajectories are described in terms of an adiabatic vibrational energy (VE) surface. Accurate semiclassical energies are obtained for chaotic trajectories near the separatrix of the VE surface. The quantum wave functions corresponding to the semiclassical trajectories after the onset of chaos are used to disprove a recently proposed quantum analog to classical quasiperiodic motion.
Rotational state selective photoionization of the H2 molecule from B 1Σ+u(v ′=0,3) states
Authors: Meier W. , Rottke H. , Zacharias H. , Welge K. H.
Published: July 2002

Selective photoionization from single rotational states of the B 1Σ+u(v′=0 and 3) excited state of the H2 molecule has been investigated with fixed frequency and tunable laser radiation around λ=266 nm. State selection of the B-state levels was achieved by tunable VUV laser excitation around λ=110 and 106 nm, respectively. The ionization cross section has been determined for different states, and found to be critically dependent upon the initial rotational quantum state and to show pronounced spectral structure with the ionization wavelength. At λI=266.05 nm the cross section varies between 0.23×10−18 and 1.2×10−18 cm2, depending on the initial state.
Quasielastic light scattering and viscometric measurements on dilute ionomer solutions
Authors: Kim Mahn Won , Peiffer D. G.
Published: July 2002

The hydrodynamic radius and intrinsic viscosity of a number of dilute solutions containing lightly sulfonated polystyrene ionomers and its unmodified analog have been measured in both polar and nonpolar solvents. Since these ionic interactions are known to significantly modify the physical properties of a polymer through relatively strong intra- and intermolecular interactions, it is important to examine how the individual chain characteristics of a charged polymer are influenced with changes in solvent quality and sulfonation level. In a polar solvent, such as dimethylsulfoxide, a short and a long correlation time is observed indicative of at least two relaxation processes. The long correlation time is attributed to the translational motion of the entire polymer chain, while the short relaxation process is correlated with the motion of the polystyrene segments situated between ionic groups. This result may be due to the ability of dimethylsulfoxide to dissolve lightly sulfonated polystyrene, but not its nonionic analog. On the contrary, in a relatively nonpolar solvent, such as cyclohexanone, only a single correlation function is noted. Again correlation with viscometric measurements shows that this is entirely due to translational motion of the individual coil. In this particular solvent, both polystyrene and sulfonated polystyrene are soluble. These results are useful as a basis for interpreting similar measurements in the semidilute region where the ‘‘universal’’ solution characteristics of associating polymers more specifically ionomers, may be observed.
Shortcomings of current theories of non-Markovian activated rate processes
Authors: Straub John E. , Borkovec Michal , Berne Bruce J.
Published: July 2002

We calculate the barrier crossing rate constants for a Brownian particle in a double well potential experiencing a non-Markovian friction kernel using full stochastic simulation. We compare the simulation results with recently proposed interpolation formulas which are based on the Grote–Hynes relation and energy diffusion mechanism. We find that such formulas fail in the intermediate regime by orders of magnitude. Therefore we question interpretations of dependence of isomerization rate data on the viscosity based on such ideas.
A scattering resonance description of very low energy collision induced vibrational relaxation
Authors: Gray Stephen K. , Rice Stuart A.
Published: July 2002

We report a study of very low energy collision induced vibrational relaxation using an approximate resonant state formalism which relates the inelastic cross section to the properties of metastable states. A study of models loosely based on the He+I2 (B 3Π0+u) system reveals that the combined effect of a low collision energy resonance and high initial diatomic vibrational excitation can lead to a large enhancement of the vibrational relaxation cross section. However, even in the absence of resonances the cross section, for very low energies, increases with decreasing energy. It is found that this effect increasingly enhances the contribution of a resonance to the cross section the closer the resonance is to zero collision energy. Because the density of resonances near zero collision energy is small the collision dynamics near zero collision energy is very sensitive to the nature of the potential energy surface.
Light scattering and viscosity studies of a ternary mixture with a double critical point
Authors: Sorensen C. M. , Larsen G. A.
Published: July 2002

We have performed viscosity and dynamic light scattering measurements on the system 3-methylpyridine, water, and heavy water. This system displays a closed-loop coexistence curve which shrinks with increasing H2O/D2O ratio until the upper and lower consolute points merge at a double critical point. Our measurements were performed to study critical phenomena in this mixture in both the phase-separating and non-phase-separating regimes. We have found that the correlation length derived from our measurements diverges in both regimes with a power-law dependence on temperature relative to the double critical point temperature. The power-law exponent was double that usually found in ising-like liquid systems when the system was asymptotically far from the double critical point. Implications of this work for other non-phase-separating yet nonideal systems is discussed.
Binary nucleation at low temperatures
Authors: Zahoransky R. A. , Peters F.
Published: July 2002

The onset of homogeneous condensation of binary vapors in the supersaturated state is studied by means of the unsteady expansion in a shock tube. The experiments are concerned with low temperatures down to 220 K which are of particular interest for atmospheric conditions. Two representative systems, ethanol/n-propanol and water/ethanol, are investigated since they allow a sufficient theoretical description. The first system forms a nearly ideal solution, whereas the latter one is an example of a strongly nonideal mixture. For either system vapor mixtures of various compositions are diluted in dry air at small mole fractions and expanded in the driver section from room temperature. The onset of homogeneous condensation is detected optically and the corresponding thermodynamic state is evaluated. The experimental results are compared with the binary nucleation theory and the particular problems of the theoretical evaluation at the low temperatures are discussed. The use of surface tension values derived for small binary droplets1 in the nucleation theory is mandatory to restore agreement between theory and experiment for the surface active system water/ethanol.
The stability of the bifluoride ion (HF−2) in the gas phase
Authors: Heni Martin , Illenberger Eugen
Published: July 2002

The bifluoride ion (HF−2) has been generated by dissociative electron attachment to trans-1,2-difluorethylene and cis-1,2-difluoroethylene. Evaluation of the energetics for these reactions leads to ΔH0f (HF−2) ≤−6.9 eV and E. A. (HF2)≥4.8 eV.
Limiting forms of the tunneling matrix element in the long distance bridge mediated electron transfer problem
Authors: Beratan David N. , Onuchic José Nelson , Hopfield J. J.
Published: July 2002

A simple model is presented for long distance electron transfer through a bridging medium. Assumptions about the bridge mediated interaction, inherent in many other models, are shown to be limits of the more general problem. The relative importance of through bond and through space coupling is discussed.
A three-dimensional semiclassical quantization of H2O
Authors: Sibert Edwin L.
Published: July 2002

The semiclassical mechanics of the vibrations of H2O is examined. The analysis makes use of the work of Sibert, Reinhardt, and Hynes [J. Chem. Phys. 77, 3595 (1982)] in which the two-dimensional stretching dynamics of ‘‘local mode’’ molecules was examined. There an approximate but accurate Hamiltonian was derived via the techniques of nonlinear mechanics. Quantization of this Hamiltonian, which has the form of a hindered rotor, gives vibrational energies in excellent agreement with direct quantum calculations of the H2O model Hamiltonian. In the present treatment we significantly advance the techniques developed in the earlier work to incorporate the bending degree of freedom and to generalize the treatment to be able to examine more realistic potential energy surfaces. Our results, which extend to energies as high as 22 000 cm−1, are in good agreement with the corresponding quantum mechanical calculation of Child and Lawton [Chem. Phys. Lett. 87, 217 (1982)] and demonstrate that a twofold hindered rotor Hamiltonian provides a good description of the three-dimensional vibrational eigenvalues.
Spectroscopic constants of alkali halides and hydrides using modified T-Rittner model
Authors: Kumar M. , Kaur A. J. , Shanker J.
Published: July 2002

The T-Rittner model derived from quantum mechanical exchange perturbation theory is modified by incorporating (i) new values of molecular state polarizabilities, (ii) the recent values of van der Waals potentials estimated from the Kirkwood–Muller (KM) formula, and (iii) the repulsive hardness parameters evaluated from the overlap integrals using the exchange charge model (ECM). With the help of this modified potential model we calculate values of binding energy W, dipole moment μ, vibrational frequency ωe, vibrational anharmonicity constant ωeXe, and rotational constant αe for diatomic molecules of alkali halides and hydrides. The results obtained in the present study are compared with the experimental data. A good agreement between calculated and experimental values supports the validity of the modified T-Rittner model used in the present study.
Infrared photodissociation of benzene dimers in the 1000 cm−1 frequency region
Authors: Johnson Russell D. , Burdenski Siegfried , Hoffbauer Mark A. , Giese Clayton F. , Gentry W. Ronald
Published: July 2002

The infrared photodissociation of benzene dimers has been investigated in the frequency region of the CO2 laser. A single absorption peak is observed with a maximum at 1038 cm−1, and a full width at half-maximum of about 2 cm−1. The laser fluence dependence of the dissociation yield and two-laser hole burning experiments both indicate that the linewidth is mostly homogeneous. The benzene monomer fragments from the photodissociation emerge with an isotropic angle distribution. A relatively large fraction of the available energy goes into product translation, and the remainder into rotation.
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o studies of the low-lying electronic states of ketene
Authors: Allen Wesley D. , Schaefer Henry F.
Published: July 2002

Features of the potential energy surfaces of the X̃ 1A1, 3A2(3A″), 1A2(1A″), 3A1(3A′), 1B1, and 2 1As1 low-lying electronic states of ketene have been investigated using self-consistent-field (SCF) and configuration interaction singles and doubles (CISD) methods with double zeta plus polarization (DZP) and DZP+Rydberg (DZP+R) basis sets. The DZP+R CISD vertical excitation energies are in excellent agreement with observed transition energies and suggest assignments for the X̃ 1A1→1B1 and X̃ 1A1→2 1A1 transitions in the electronic spectrum of ketene. Stationary points have been located at the DZP SCF level of theory for the first four states listed above, and SCF quadratic force constants and harmonic vibrational frequencies have been computed analytically at these stationary points. The X̃ 1A1 geometry and vibrational frequencies compare favorably with experimental values, the agreement being typical of DZP SCF results. Due to curve crossings and conical intersections of potential surfaces, the four lowest theoretical excited state surfaces have only two valid (double) minima, corresponding to 3A″ and 1A″ electronic states. At the DZP SCF geometries, Davidson-corrected CISD adiabatic excitation energies of 16 700 and 19 000 cm−1 have been obtained for the 3A″ and 1A″ states, supporting the previous experimental T0 upper bounds of Laufer and Keller. Finally, the X̃  1A1 state is predicted to lie only 5500–7000 cm−1 below the 1A″ state at the 1A″ optimum geometry and appears to have a significant effect on the 1A″ out-of-plane frequencies.
Adiabatically reduced coupled equations for intramolecular dynamics calculations
Authors: Voth Gregory A. , Marcus R. A.
Published: July 2002

‘‘Adiabatically reduced’’ coupled equations are derived to obtain an approximate quantum mechanical solution for the dynamics of nonstationary states in isolated polyatomic molecules. Under suitable conditions, the number of such equations is considerably less than the number of coupled equations needed in practice for the exact calculation. The relationship of the present technique to several other methods, including the partitioning method, is discussed, and specific applications of the present treatment are given.
Rotational alignment of the CN fragment of ICN photodissociation
Authors: Hall G. E. , Sivakumar N. , Houston P. L.
Published: July 2002

The alignment of the CN fragment following dissociation of ICN has been measured as a function of wavelength by varying the polarization of the tunable laser used for dissociation relative to that of the tunable laser used for laser-induced fluorescence detection of the CN product. The results show that the angular momentum vector of the CN rotor is aligned predominantly perpendicular to the polarization axis of the dissociation laser, in agreement with the expectation for a predominantly parallel absorption in the ICN. However, the degree of alignment is not perfect. The most plausible explanation appears to be that the absorption occurs via a transition that is a mixture of parallel and perpendicular components. Alternate explanations for the imperfect alignment involving either rotation of the ICN parent molecule prior to dissociation or orbital angular momentum during the dissociation are examined and rejected. Near 263–266 nm the transition moment appears to be a mixture of 85% parallel and 15% perpendicular; it becomes more perpendicular to shorter and longer wavelengths.
Absolute resonance Raman intensities demonstrate that the spectral broadening induced by the β-ionone ring in retinal is homogeneous
Authors: Myers Anne B. , Trulson Mark O. , Pardoen Johannes A. , Heeremans Carola , Lugtenburg Johan , Mathies Richard A.
Published: July 2002

The sensitivity of absolute resonance Raman cross sections to the excited state homogeneous width is used to examine whether the electronic spectral breadth induced by the β-ionone ring of retinal is primarily a homogeneous or an inhomogeneous effect. Resonance Raman intensities have been measured for both all-trans retinal and its 5,6-dihydro analog. The dihydro analog has a structured absorption spectrum because the broadening effect of the C6–C7 bond is lost when the π system does not extend into the ionone ring. While the two molecules have similar relative Raman intensities, the absolute cross sections are about a factor of two lower in all-trans retinal, indicating that the ‘‘effective homogeneous linewidth’’ is significantly larger in all-trans retinal than in the dihydro analog. Calculations carried out to model both the absorption spectra and the resonance Raman intensities require an effective homogeneous linewidth that is more than twice as large for all-trans retinal (∼1200 cm−1) as for the 5,6-dihydro analog (∼500 cm−1). The additional width is most simply attributed to Franck–Condon activity in the C6–C7 torsional degree of freedom; however, no intense resolved lines are observed in the Raman spectrum between 20 and 800 cm−1. The torsional modes involved must have very low ground-state frequencies and may be observable only through their contribution to the apparent Rayleigh and Raman linewidths.
Reaction kinetics on ultrametric spaces
Authors: Zumofen G. , Blumen A. , Klafter J.
Published: July 2002

We study the kinetics of the A+A→0 and the A+B→0 diffusion-limited reactions by modeling the dynamics through random walks on ultrametric spaces, which allow to account for energetic randomness and to incorporate effects due to changes in temperature. We treat both pseudounimolecular (target and trapping) problems, as well as bimolecular reactions, and we compare the results to those which obtain for reactions on regular lattices and on fractals. Furthermore we analyze the possibility of describing reactions on ultrametric spaces through extensions of Smoluchowski-type approaches, and we show the limitations of such schemes when, as a function of temperature and of the reaction progress, the fluctuations in the particle densities get large.
Effect of elongational flow on the isotropic–nematic phase transition in rod-like systems
Authors: Thirumalai D.
Published: July 2002

The effect of the elongational flow on the isotropic–nematic phase transition in monodisperse rigid rod-like molecules is examined. The critical density at which a strongly ordered state starts to form decreases in the presence of the external field. The width of the biphasic region decreases with increasing strain rate. It is shown that there is a critical strain rate above which only the ordered state is stable even in the infinite dilution limit. The critical strain rate is found to be inversely proportional to the cube of the number of monomer units that constitute the rod molecule.
Hindered rotation and site structure of methane trapped in rare gas solids
Authors: Jones Llewellyn H. , Ekberg Scott A. , Swanson Basil I.
Published: July 2002

High resolution infrared absorption spectra have been recorded for the ν4 vibrational mode of CH4 trapped in rare gas solids. In argon, krypton, and xenon hosts two dominant sites are observed which we call site R and site A,B. For both sites we believe that the CH4 molecule replaces one host atom. The fine structure of site R at a temperature of 5 K is that predicted for a tetrahedral molecule rotating in an octahedral field; singlet peaks for P(1), Q(1), R(0), and a doublet for the R(1) transitions. The barrier for this rotation is not clearly defined. Site A,B shows site symmetry splitting and orientational ordering suggestive of C3v symmetry. Rotational structure is also associated with site A,B; it is not clear whether the hindered rotation for this site takes place about only the C3 axis or about axes perpendicular to C3 as well. From annealing studies and the effects of deposition temperature we conclude that site R is for CH4 molecules trapped in the normal cubic-close-packed environment while site A,B is for CH4 trapped in hexagonal-close-packed pockets induced by stacking faults formed during deposition. Temporal changes of peak intensities while the matrices stand at 5 K are indicative of a slow relaxation of nuclear spin from a triplet state (J=1) to a quintet state (J=0). For site A,B the triplet to quintet spin relaxation is considerably faster than for site R; for example, for a matrix of CH4 in krypton (1/10 000) at 5 K the spin relaxation half-lives are 87 minutes and 26 minutes for sites R and A,B, respectively.
On the primary process in the plasma-chemical and photochemical vapor deposition from silane. Mechanism of the radiative species SiH*(A 2Δ) formation
Authors: Oikawa Setsuko , Tsuda Minoru , Yoshida Junko , Jisai Yasuhiro
Published: July 2002

A b initio molecular orbital–configuration-interaction (CI) calculations at full valence CI level as well as at the SDTQ-CI level on valence and Rydberg MOs were carried out on the potential energy hypersurfaces in the lower singlet states along one probable reaction path of the decomposition from SiH4 to 2SiH+2H+1H2. The result shows that there exists a one-step decomposition route which forms the radiative species SiH*(A 2Δ) from SiH4 spontaneously. The adiabatic potential energy hypersurfaces following the reaction path explain straightforwardly various observations in the plasma- and photo-CVD experiments of SiH4. The generation routes of SiH in other lower excited states were also clarified.
Use of an ensemble intermediate between the generalized ensemble and the isothermal–isobaric ensemble to calculate the equilibrium distribution of hydrocarbons in homologous series
Authors: Alberty Robert A.
Published: July 2002

The ensemble first introduced by Stockmayer to study the relation between light scattering and composition fluctuations of solutions provides the basis for calculating equilibrium mole fractions of isomer groups in homologous series. The number of molecules of one component in a system at a particular temperature and pressure is held constant, and another component is provided through a semipermeable membrane at a specified chemical potential. The equilibrium alkylation of benzene may be calculated by fixing the number of molecules of benzene in the system and providing ethylene through a semipermeable membrane. A catalyst for the formation of all possible isomers of all possible alkyl benzenes, or some subset of these species, is assumed to be present. The advantage of using this approach is that exponential distributions are obtained that are a function of the ethylene partial pressure for ideal mixtures of perfect gases. The Gibbs energy of formation obtained from the Stockmayer ensemble can be used to calculate the equilibrium constant of a reaction written in terms of isomer groups.
The lifetimes for spontaneous emission from the X 3Σ−(v=1) and a 1Δ states of CH−
Authors: Okumura M. , Yeh L. I. , Normand D. , Lee Y. T.
Published: July 2002

The radiative decay of excited CH− trapped in a radio frequency ion trap was measured, and the total excited state population was probed by observing the depletion of trapped CH− caused by photodetachment at 1.16 eV, below the expected electron affinity for the ground 3Σ− state. The signal decayed biexponentially with time. We assigned the long lived state (lifetime 5.9+0.8, −0.6 s) as the metastable a 1Δ state previously identified in the photoelectron spectrum. The fast decay, with a lifetime of 1.75±0.15 ms, was attributed to the first excited vibrational level of the ground 3Σ− state, in good agreement with a theoretical result by Manz, Zilch, Rosmus, and Werner. These results support the electron affinity of 1.238 eV for CH− obtained by Kasdan, Herbst, and Lineberger from photoelectron spectroscopy, and contradict the value of 0.74 eV determined by Feldmann from photodetachment spectroscopy.
Electronic states of PbS
Authors: Balasubramanian K.
Published: July 2002

Relativistic configuration interaction calculations of six ω–ω states [0+, 1, 0−, 0−(II), 0+(II), 0+(III)] and six λ–s states (1∑+, 3∑+, 3Π, 1Π, 3∑−, and 1∑−) are carried out. These calculations enable assignment of the observed X, a, A, B, and C states. The nature of these states as well as other states which are yet to be observed is discussed. The properties of low-lying electronic states of PbS are compared with those of PbO. The dissociation energy of the ground state of PbS is calculated to be 3.83 eV.
The rotational spectrum and geometry of the heterodimer oxirane ⋅ ⋅ ⋅ hydrogen cyanide in the vibrational ground state and the vβ(0)=1 state
Authors: Goodwin Elizabeth J. , Legon A. C. , Millen D. J.
Published: July 2002

The rotational spectrum of the heterodimer formed by oxirane and hydrogen cyanide has been observed by pulsed-nozzle, Fourier-transform microwave spectroscopy. Two sets of rotational transitions of similar intensity have been identified for each of the isotopic species (CH2)2O ⋅⋅⋅ HC14N, (CH2)2O ⋅⋅⋅ HC15N, and (CH2)2O ⋅⋅⋅ DC14N. Each set of rotational transitions was fitted to give rotational constants B and C, centrifugal distortion constants ΔJ, ΔJK, δJ, and HJK, and, where appropriate, 14N–nuclear quadrupole coupling constants χaa and χbb. The rotational constants for each of the two states (labeled A and B) were interpreted in terms of a molecule of Cs symmetry, with a hydrogen bond O ⋅⋅⋅ HCN and a pyramidal geometry at the oxygen atom. For the species (CH2)2O ⋅⋅⋅ HC14N, the angle φ between the bisector of the COC angle and the HCN axis increases from 52.2(3)° in state A to 61.3(3)° in state B, while the distance r(O ⋅⋅⋅C) correspondingly increases from 3.035(4) to 3.130(4)Å. Similar changes are observed in (CH2)2O ⋅⋅⋅ HC15N and (CH2)2O ⋅⋅⋅ DC14N between states A and B. It is concluded that the potential energy barrier to inversion of the configuration at O by means of the low frequency hydrogen bond bending mode νβ(0) is relatively low and that states A and B correspond to the ground state and the state having vβ(0)=1, respectively.
Theory of laser catalyzed reactions. I. Potential surfaces and transition dipoles in the Li+HCl reaction
Authors: Shapiro Moshe , Zeiri Yehuda
Published: July 2002

Potential surfaces and electronic transition dipoles for the Li+HCl→LiCl+H reaction are calculated using a semiempirical valence bond (SEVB) approach. Enhancement of the transition dipole when the colliders are close together is predicted and explained. We argue that this fact and our finding that the second excited electronic state is weakly binding, would make laser catalysis of this and similar alkali+hydrogen–halide reactions, possible. In particular, irradiation in the 32 000–36 000 cm−1 range is predicted to preferentially enhance the Li+HCl reaction.
Theory of charge–transfer excitations at the neutral–ionic interface
Authors: Soos Z. G. , Kuwajima S. , Harding R. H.
Published: July 2002

Diagrammatic valence bond (DVB) theory is applied to charge–transfer (CT) excitations in ion–radical organic solids that, like TTF–chloranil, are close to the neutral–ionic (n–i) interface. Small but finite Mulliken integrals ‖t‖ in mixed regular donor–acceptor stacks lead to charge resonance among segments containing 2r adjacent ions and to a linear lowering of the CT absorption. Finite t and long range Coulomb interactions also lead, up to a critical value, to an ionicity jump Δq=qi−qn <1 between the neutral and ionic phases. The TTF–chloranil ionicities and CT absorption are fit with ‖t‖=0.23 eV. Neither attractive nor repulsive interstrand Coulomb interactions affect the CT spectra at fixed ionicity in the mean field approximation. Direct DVB solutions to finite (2N≤18) donor–acceptor strands yield the n–i interface while 2N≤16 strands suffice for the CT absorption and intensity.
Transient effects in nucleation for two-dimensional systems
Authors: Edrei I. , Gitterman M.
Published: July 2002

Application of critical dynamics allows one to avoid model assumptions in the phenomenological theory of nucleation. The Fokker–Planck equation for the distribution function of nuclei follows from the Langevin equation for the radius of nuclei which, in turn, is an immediate consequence of the equations of critical dynamics. The transient behavior essentially depends on the initial condition (fast vs slow quench). The time lag determining the duration of transient processes following the fast quench is exponentially larger than that for a slow quench. The time dependence of the distribution function and flux of nuclei are significantly different for slow and fast quenches. Numerical estimates and graphic representation of the results are performed.
Rotational effects on intramolecular vibrational relaxation in dimethyl ether and 1,4 dioxane
Authors: Kulp T. J. , Kim Hong Lae , McDonald J. D.
Published: July 2002

The effect of rotational excitation on intramolecular vibrational relaxation in isolated gas phase molecules of dimethyl ether and 1,4 dioxane has been studied. Frequency resolved infrared fluorescence spectra were obtained from molecules isolated in a supersonic expansion and excited in the CH stretching region with an infrared optical parametric oscillator. By altering the expansion conditions the average J of the molecules was controlled. Measurements performed on both molecules at two different rotational temperatures indicate that the dilution factors of the CH stretch modes have a significant J dependence. It is demonstrated that the measured energy contents of the relaxed modes can be satisfactorily modeled by a statistical calculation which assumes equal coupling of all modes with 1 cm−1 of the optical state which conserve J and symmetry. It is shown that the ultimate distribution of the energy within the molecule is J dependent.
Semiclassical quantization of slightly nonresonant systems: Avoided crossings, dynamical tunneling, and molecular spectra
Authors: Farrelly David , Uzer T.
Published: July 2002

An important problem in the spectroscopy of isolated molecules is the passage of a system through an isolated internal nonlinear resonance as the total energy is changed. The spectral signature of such a passage may involve an avoided crossing together with pre-resonant attraction and/or post-resonant repulsion of levels. While most previous semiclassical studies have made a clear distinction between resonant and nonresonant problems, we develop an approach which allows for a quantitative description of the passage through an avoided crossing which levels may experience as the total energy of the molecule (as opposed to an external perturbation parameter) is varied. Stimulated by the quantum mechanical and time dependent classical studies of Heller and co-workers we have developed a time independent semiclassical formalism which is found to provide excellent quantitative agreement with quantum energy levels and splittings as the total energy is changed. The time independent semiclassical approach provides large savings in computational effort as compared to quantum calculations while providing the quantitative agreement with quantum results which is difficult to obtain from time dependent semiclassical studies. Application is made to nearly resonant 1:1 and 2:1 (Fermi resonant) systems and implications for dynamical tunneling in large molecules are discussed.
Fluorescence polarization theory for asymmetric rotors: Rotational dynamics in the regular and statistical limits
Authors: Nathanson Gilbert M. , McClelland Gary M.
Published: July 2002

Classical and quantum theories of ensemble averaged fluorescence polarization are presented for the asymmetric rotor in the regular, rigid rotor limit and in the limit of statistical vibration–rotation energy transfer in the isolated molecule. Planar asymmetric rotors are explored in detail, with special emphasis on p-difluorobenzene and pyrimidine. The classical polarizations are nearly identical to the quantum results, but are 1000 times faster to calculate. For nearly all molecular geometries, our results predict the transition from regular to statistical rotational motion will sharply reduce the fluorescence polarization, signaling the onset of intramolecular vibration–rotation energy transfer.
Quantum chemistry by random walk: Methane
Authors: Garmer D. R. , Anderson J. B.
Published: July 2002

The total energy of tetrahedral CH4 at the equilibrium bond length of 2.05 bohr is calculated by the fixed-node random walk method with importance sampling. The best result is obtained with a single-determinant trial function of double-zeta STO-basis-set quality. This yields an upper bound to the total energy of −40.506 hartree which is 0.048 hartree below the expectation value of the energy from the best variational calculations and only 0.008 hartree above the experimental nonrelativistic energy. Extended calculations with array processors and extrapolation to zero time step were used to obtain a statistical precision of 0.002 hartree (1.4 kcal/mol) for the random walk result.
Towards a density functional theory of the pair correlation function
Authors: Moore Steven M. , Raveché Harold J.
Published: July 2002

A new differential equation for the pair correlation function of a uniform liquid is derived from density functional theory. Its utility is probed by studying various regimes analytically, since the numerical solution is complicated by the fact that the coefficients depend on the unkown. It is shown that the behavior of the solution at large interparticle separation agrees with the Ornstein–Zernike differential equation and generalizes the equation obtained by Fisher et al. for the asymptotic behavior of the Yvon–Born–Green equation. Correct behavior is also obtained near the limit of zero separation, at least at low density. At higher densities, this limit of the equation is probed through computer simulation data. Extension beyond the square-gradient approximation leads to a result in agreement with that originally predicted by Hart.