bluuebunny/crossref_metadata_2025 huggingface dataset:
Investigations of pyroelectric generation of x rays
Authors: Brownridge James D. , Raboy Sol
Published: July 2002

Experiments to study the influences of the environment, external to pyroelectric crystals, on certain measurements to discern the properties of crystals, such as LiTaO3, LiNbO3, and CsNO3, are discussed. During increasing temperature and at appropriate pressures electrons in the vacuum system are accelerated to the −z base of the pyroelectric crystal and are repelled from the +z base of the crystal. The electrons striking the crystal may have sufficient energy to excite x-ray absorption edges of the elements in the crystal and the electrons repelled to a target may have sufficient energy to excite x-ray absorption edges in the elements of the target. During decreasing temperature electrons in the vacuum system are repelled from the −z base whereas the electrons are accelerated to the +z base. The light produced, associated with the pyroelectric effect, appears to come from recombination processes in the gases in the vacuum chamber. The polarization charge produced during the change of temperature of the crystals appears to follow the changing temperature in a reproducible manner.
Exciton saturation in GaAs multiple quantum wells at room temperature
Authors: Miller A. , Riblet P. , Mazilu M. , White S. , Holden T. M. , Cameron A. R. , Perozzo P.
Published: July 2002

Pump-probe experiments in GaAs/AlGaAs multiple quantum well samples are described using both picosecond and femtosecond laser pulses in different polarization configurations. The excitonic and free carrier components of exciton absorption saturation were analyzed as a function of well width. The relative importance of phase space filling, Coulomb screening and broadening contributions was determined from polarization and laser bandwidth dependencies via spin-dependent and spin-independent components. A five-level model was used to fit the data and nonlinear coefficients for the individual contributions were determined for each well width. The Coulomb contributions arising from screening and broadening of the excitons was found to dominate the absorption bleaching using picosecond pulses, whereas phase space filling was largest in the femtosecond regime. Exciton phase space filling was found to be four times larger than free carrier phase space filling at narrower well widths. A sublinear dependence of exciton broadening with carrier density was observed.
Dense fully 111-textured TiN diffusion barriers: Enhanced lifetime through microstructure control during layer growth
Authors: Chun J.-S. , Petrov I. , Greene J. E.
Published: July 2002

Low-temperature deposition of TiN by reactive evaporation or sputter deposition onto amorphous substrates leads to highly underdense layers which develop mixed 111/002 orientations through competitive growth. In contrast, we demonstrate here the growth of low-temperature (450 °C) fully dense polycrystalline TiN layers with complete 111 texture. This was achieved by reactive magnetron sputter deposition using a combination of: (1) highly oriented 25-nm-thick 0002 Ti underlayers to provide 111 TiN orientation through texture inheritance (local epitaxy) and (2) high flux (JN2+/JTi=14), low-energy (EN2+≃20 eV), N2+ ion irradiation in a magnetically unbalanced mode to provide enhanced adatom diffusion leading to densification during TiN deposition. The Ti underlayers were also grown in a magnetically unbalanced mode, in this case with an incident Ar+/Ti flux ratio of 2 and EAr+≃11 eV. All TiN films were slightly overstoichiometric with a N/Ti ratio of 1.02±0.03. In order to assess the diffusion-barrier properties of dense 111-textured TiN, Al overlayers were deposited without breaking vacuum at 100 °C. Al/TiN bilayers were then annealed at a constant ramp rate of 3 °C s−1 to 650 °C s−1 and the interfacial reaction between Al and TiN was monitored by in situ synchrotron x-ray diffraction measurements. As a reference point, we find that interfacial Al3Ti formation is observed at 450 °C in Al/TiN bilayers in which the TiN layer is deposited directly on SiO2 in a conventional magnetically balanced mode and, hence, is underdense with a mixed 111/002 orientation. However, the onset temperature for interfacial reaction was increased to 610 °C in bilayers with fully dense TiN exhibiting complete 111 preferred orientation.
Magnetoelastic coupling in Ni and Fe monolayers on Cu(001)
Authors: Gutjahr-Löser Th. , Sander D. , Kirschner J.
Published: July 2002

The correlation between mechanical stress and magnetic anisotropy of Ni and Fe films on Cu(001) is investigated. The magnetoelastic coupling and the film stress during the growth are measured in situ with a highly sensitive optical bending beam technique. For Ni a dramatically reduced magnetoelastic coupling of B1=3.5 MJ/m3 is found for films thinner than 10 ML, roughly one third of the bulk value of 9.4 MJ/m3. This change is explained by a strain correction to the magnetoelastic coupling. The influence of the interfaces does not significantly contribute to the magnetoelastic coupling. A very small magnetoelastic coupling of 0.4 MJ/m3 for Fe films in the range from 12 ML to 25 nm is attributed mainly to the crystallographic orientation of the bcc–Fe.
Interface dilution and morphology of CdTe/MnTe superlattices studied by small- and large-angle x-ray scattering
Authors: Eymery J. , Hartmann J. M. , Baumbach G. T.
Published: July 2002

We have performed small- and large-angle x-ray scattering experiments on CdTe/MnTe superlattices. The Fresnel optical method and the distorted wave Born approximation were used to extract from small-angle measurements out-of-plane and in-plane information about the interfaces. Specular reflectivity shows that the interface roughness is quite high (about 7 Å) for all superlattices. The effective MnTe concentration, directly determined from the refractive index profile, is successfully used to simulate the structured nonspecular scattering, and to determine the lateral correlation length of the interface roughness (about 1500±750 Å). Moreover, it is shown that the layers are almost completely correlated over the sample thickness. The thickness fluctuations along the growth direction are estimated from the analysis of the large-angle (004) reflection, and the effective MnTe profile is also checked by dynamical simulation. It is shown that the small- and large-angle results are in good agreement. The MnTe profile width deduced from x-ray reflectivity is slightly overestimated due to the large integration area of this technique. An estimation of the local MnTe profile is given.
Chemical trends of the rattling phonon modes in alloyed germanium clathrates
Authors: Dong Jianjun , Sankey Otto F. , Ramachandran Ganesh K. , McMillan Paul F.
Published: July 2002

Alloys based on Ge clathrates are promising thermoelectric materials because of their expected “rattling” properties. We have incorporated the elements of columns I and II into the cages of (Ge, Ga)46 type-I clathrates as cation guests and have theoretically examined their “rattling” behavior using density functional theory. The potential energy curves of guest atoms in the cages are evaluated to understand the nature of the weak guest–framework interaction. Some atoms are unstable at the center of the cages, while others appear to be bonded by weak restoring forces. We calculate the phonon modes and the Raman spectra and find that heavy alkali-earth elements, such as Sr and Ba, induce low-frequency “rattling” phonon modes as predicted by Slack’s model, while heavy alkali metal atoms (K, Rb, and Cs) are less “rattler-like” since they interact less with the acoustic modes of the Ge-based clathrate framework.
Phonon dispersion effects and the thermal conductivity reduction in GaAs/AlAs superlattices
Authors: Bies W. E. , Radtke R. J. , Ehrenreich H.
Published: July 2002

The experimentally observed order-of-magnitude reduction in the thermal conductivity along the growth axis of (GaAs)n/(AlAs)n (or n×n) superlattices is investigated theoretically for (2×2), (3×3) and (6×6) structures using an accurate model of the lattice dynamics. The modification of the phonon dispersion relation due to the superlattice geometry leads to flattening of the phonon branches and hence to lower phonon velocities. This effect is shown to account for a factor-of-three reduction in the thermal conductivity with respect to bulk GaAs along the growth direction; the remainder is attributable to a reduction in the phonon lifetime. The dispersion-related reduction is relatively insensitive to temperature (100<T<300 K) and n. The phonon lifetime reduction is largest for the 2×2 structures and consistent with greater interface scattering. The thermal conductivity reduction is shown to be appreciably more sensitive to GaAs/AlAs force constant differences than to those associated with molecular masses.
Semiclassical calculation of cross sections for vibration–rotation energy transfer in HF–HF collisions
Authors: Billing G. D.
Published: July 2002

Cross sections for HF(10)+HF(00) → HF(v1 j1)+HF(v2 j2) are calculated using a semiclassical method in which the relative translational motion is treated classically, whereas the vibrational and rotational motion of both molecules are quantized. In order to decouple the rotation projection states we introduce the large j approximation in the coupling elements. The results of 98-quantum state calculations at three energies are reported and qualitatively compared with recent laser experiments.
Raman spectrum and structure of thermally treated silica aerogel
Authors: Walrafen G. E. , Hokmabadi M. S. , Holmes N. C.
Published: July 2002

Raman spectra have been obtained from untreated SiO2 aerogel at 25 °C, and after vacuum heating between 350 and 620 °C. The untreated aerogel displays a strong Raman peak near 478 cm−1 due to eight-membered (4-SiO) surface rings, but no measurable intensity at 600 cm−1 from six-membered (3-SiO) rings. However, an intense 600 cm−1 peak develops upon heating between 350 and 620 °C as the 478 cm−1 peak is replaced by a 490 cm−1 peak characteristic, in fused silica, of eight-membered internal rings. Vibrations involving surface CH3O groups disappear along with the fluorescence at 350 °C, well before the 478 cm−1 Raman intensity is gone. The untreated aerogel is protected from water absorption by the hydrophobic surface methoxy groups, but the thermally treated aerogel reacts readily to form Si–OH groups as shown by the 970–980 cm−1 Si vs OH and 3750 cm−1 surface OH stretches. Raman contour shapes near 800 cm−1 and between 900–1300 cm−1 are sensitive to the conditions of thermal treatment. The Hokmabadi–Walrafen vibrational correlation relating decrease of the mean Si–O–Si bridging angle to increase of the mean Si–O bond length is obeyed for both treated and untreated aerogels.
The structure of several electronic states of the Hg–Ar complex as determined by laser double resonance in a supersonic jet
Authors: Duval Marie-Christine , D’Azy Odile Benoist , Breckenridge W. H. , Jouvet Christophe , Soep Benoit
Published: July 2002

The electronic states of the Hg–Ar complex have been studied in a supersonic free jet expansion using a laser double resonance technique. Detailed observations were made of the states correlating with the mercury 6 3P levels 3P0, 3P1, and 3P2. The states correlating with the 3P0 and 3P2 metastable states had not been studied previously since they are optically inaccessible from the ground state. A model was developed which accounts for the structures of the various states. The binding energies can be related simply to the average orientation of the 6p mercury orbital with respect to the internuclear axis. In addition, the Hg(7 3S1)–Ar Rydberg state was reinvestigated and shown conclusively to exhibit a double minimum potential, with a deep well similar to the Hg–Ar+ ion and a shallow van der Waals minimum at larger internuclear distances.
The electronic structure of α-quartz: A periodic Hartree–Fock calculation
Authors: Dovesi R. , Pisani C. , Roetti C. , Silvi B.
Published: July 2002

The electronic structure of α-quartz has been calculated using the periodic Hartree–Fock method. Charge density maps, band structure, and density of states have been computed and are discussed. An overall agreement with available experimental results and with previous calculations performed within different theoretical approaches is found. In particular it is shown that ‘‘cluster type’’ calculations on molecules such as H6Si2O7 accurately reproduce the charge density in the neighborhood of the bridging oxygen. Geometry optimizations yield realistic values of the SiO bond length and of the SiOSi angle.
Convergence of the Magnus expansion for time dependent two level systems
Authors: Maricq M. Matti
Published: July 2002

This paper derives criteria for the convergence of the Magnus expansion, applied to an arbitrary time dependent two level system, in terms of the eigenvalues of the Hamiltonian for that system. When a weak to moderate intensity field perturbs a two level system the resonance frequency of the system must lie below the frequency of the applied field for the series to converge. An alternative solution for the exponential form of the evolution operator is developed that converges for all values of the expansion parameter and clearly shows the singularities that lead to the failure of the Magnus solution. Finally, a simple unitary transformation is presented that extends considerably the radius convergence of the Magnus series, as illustrated by a spin 1/2 in a linearly polarized field.
Molecular dynamics simulation of dielectric properties of water
Authors: Anderson Jon , Ullo John J. , Yip Sidney
Published: July 2002

The dielectric properties of liquid water are studied using an empirical potential model treating the monomers as flexible molecules with short-range interactions between oxygens and Coulomb interactions between charges situated on the individual atoms. Satisfactory results for the static dielectric constants at 259, 300, and 350 K are obtained from long simulation runs; these constitute a significant improvement over recent calculations using other potential models. Time correlation functions of dipole moment and single molecule orientation are evaluated. In the supercooled state an enhanced slowing down in the temporal decay of polarization fluctuations occurs, a behavior essentially absent in the single molecule reorientations. Good agreement is found in the self-diffusion coefficient over the entire temperature range.
Complex CO–potassium interactions on Cu(100): An electron energy loss, thermal desorption, and work function study
Authors: Dubois L. H. , Zegarski B. R. , Luftman H. S.
Published: July 2002

A detailed examination of the adsorption states of carbon monoxide on a potassium-dosed Cu(100) surface is reported. High resolution electron energy loss spectroscopy (EELS) studies on surfaces with a variety of potassium precoverages indicate the presence of at least seven different C–O vibrational modes ranging in frequency from 900 to 2090 cm−1 with as many as five modes observed simultaneously. Thermal desorption spectroscopy (TDS) demonstrates at least five states of CO coadsorbed with potassium with desorption maxima occuring over a range from 120 to 600 K. Low resolution EELS and work function changes are discussed as well. The complexity of the high resolution EELS and TDS measurements precludes models involving only one or two types of potassium–CO interactions. In this system we suggest that potassium influences CO adsorption by, (1) indirectly increasing 2π* backbonding, (2) indirectly changing bonding sites, and (3) effecting a direct interaction of adsorbed CO with one, two, or possibly three neighboring potassium adatoms. Although we find no conclusive evidence for the reductive coupling of carbon monoxide to form discrete (CO)n–K polyhedral complexes at high potassium surface coverages, we cannot rule out the formation of such species.
Localizability and dephasing of dipolar excitons in topologically disordered systems
Authors: Logan David E. , Wolynes Peter G.
Published: July 2002

We develop a self-consistent theory for the localizability of dipolar excitations, and the interplay between localization and dephasing mechanisms such as exciton–phonon coupling, within the framework of a tight-binding model of topologically disordered systems. When the dephasing rate vanishes the pure Anderson localization problem may be examined, and we find that dipolar excitations are always extended at any finite density. For sufficiently small number density, however, it is found that while no state in the band is ever strictly localized, the excitation transfer rate is exponentially small, and on the time scale of many experimental probes the excitation would effectively appear to be localized. In the limit of rapid dephasing the present theory correlates with the usual master equation treatments of incoherent (Förster) transport. For sufficiently large disorder, examination of the excitation transfer rate as a function of the dephasing rate predicts a nonmonotonic but continuous crossover from coherent to incoherent transport.
The structure of Jahn–Teller surfaces
Authors: Ceulemans Arnout
Published: July 2002

In chemical applications of the Jahn–Teller effect, the shape of the adiabatic potential near the instability point is of primary importance. The present article examines this shape, both in the usual representation in the space of the nuclear configurations and in the less current projective representation in the space of the electronic functions. The general structural aspects of these representations are viewed as the results of a symmetry breaking process, starting at the continuous invariance group of the degenerate coupling limit, and ending with the finite point group of the representation space. The various vibronic coupling mechanisms involved in this process are characterized by irreducible tensor functions of the full orthogonal groups. The leading tensorial rank of the unequal linear coupling mechanism is shown to be four, while the bilinear coupling mechanism also involves an irreducible component of the sixth rank. This tensorial analysis provides a grouptheoretical foundation for the recently formulated epikernel principle, which predicts the location of extremal points on a Jahn–Teller surface.
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o force field for the planar vibrations of benzene
Authors: Guo Hong , Karplus Martin
Published: July 2002

A theoretical force field for in-plane vibrations of benzene has been determined from ab initio calculations at both the Hartree–Fock level with 4-21G, 6-31G, and 6-31G* basis sets and the MP2 level with 4-21G and 6-31G basis sets. The average error of the calculated frequencies at the MP2 level is between 2% and 3%. The reliability of the force field and vibrational frequency predictions of the calculations are analyzed. All diagonal stretching force constants obtained at the MP2 level are in quantitative agreement with Ozkabak–Goodman experimental force field, while the diagonal force constants involving ring deformation and CH rock are somewhat overestimated by the theory. Most of the off-diagonal force constants agree with the Ozkabak–Goodman results in sign but there are some significant quantitative differences in magnitude. Comparisons are made with other force fields, including results obtained by scaling ab initio calculations or introducing modified Hamiltonians. A simple extrapolation method for introducing correlation corrections into Hartree–Fock force constants gives excellent results for benzene.
The theory of semipermeable vesicles and membranes: An integral-equation approach. II. Donnan equilibrium
Authors: Zhou Yaoqi , Stell George
Published: July 2002

Integral equations that yield the charge and density profiles are derived for a Donnan system, in which an ionic solution is separated into two regions by a semipermeable membrane (SPM) or a spherical semipermeable vesicle (SPV). These equations are obtained from the Ornstein–Zernike (OZ) equation. We show how quantitative results can be obtained from either the mean spherical approximation (MSA) closure or the hypernetted-chain (HNC) closure for profiles. Use is made of bulk-correlation input obtained by means of the Debye–Hückel approximation, the MSA approximation, or the HNC approximation. The resulting approximations will be referred as MSA/DH, HNC/DH, MSA/MSA, etc. The system on which we focus contains three charged hard-sphere species: cation, anion, and a large ion (a protein or polymer ion) separated by a plane SPM, through which the large ion cannot pass, and to one side of which all large ions are confined, or a spherical SPV, outside of which the large ions are confined. Analytical expressions for the bulk density ratio between the two sides of a plane membrane as well as the membrane potential in various approximations are obtained. Results obtained from these expresssions are compared with the results obtained by equating electrochemical potentials. A new contact-value theorem is provided for the plane SPM system. Analytical solutions for the charge profile and the potential profile in the MSA/DH approximation are obtained. It turns out that results obtained in the HNC/DH approximation are exactly the same as those obtained by using 1D nonlinear Poisson–Boltzmann equations if the repulsive cores of the macroions are neglected.
Molecular dynamics simulation of a weakly interacting system, quinuclidine–benzene. II
Authors: Maliniak A. , Laaksonen A. , Kowalewski J. , Stilbs P.
Published: July 2002

A system corresponding to a 6 mol % quinuclidine solution in benzene is simulated using the molecular dynamics technique. Results are reported for the structure of the liquid and the dynamics of the two components in the solution. The translational diffusion coefficients are reported and compared with the experiments. Both the tumbling and the spinning motion of the molecules are investigated and compared with previously reported NMR data. The reorientations are analyzed using several models for dynamics in liquids.
The computed spectrum of AlC
Authors: Bauschlicher Charles W. , Langhoff Stephen R. , Pettersson Lars G. M.
Published: July 2002

The low-lying (up to about 40 000 cm−1) doublet and quartet states of AlC have been studied at the CASSCF and MRCI levels of theory. The bonding in the X 4Σ− state is shown to involve three one-electron bonds. The first excited state, a 2Π, lies about 8700 cm−1 higher and like the X 4Σ− state is derived from ground state Al and C, but has a two-electron π bond leading to a much shorter bond length. Those states derived from Al 3s13p2 are much higher in energy, since the promotion energy is not recovered by the formation of two-electron bonds. Transition moments are presented for all the dipole-allowed transitions. The very strong B 4Σ−–X 4Σ− transition near 22 600 cm−1 is probably the best way to observe AlC in either emission or absorption.
Deexcitation electron spectroscopy of core-excited O2
Authors: Carroll T. X. , Thomas T. D.
Published: July 2002

The electron–electron coincidence technique has been used to measure the spectrum of autoionizing (Auger) electrons that are emitted following excitation of a core electron in O2 to the half-filled 1πg orbital. The spectrum is dominated by transitions to doublet states, with very little intensity in quartet peaks. The kinetic energy spectrum calculated with inclusion of the effects of vibrational overlap in excitation and deexcitation as well as the effects of lifetime-vibrational interference agrees well with experiment. Comparison of the spectrum with the normal Auger spectrum shows significant similarities between the major features of each.
Rotational structure near the local mode limit in the (3000) band of germane
Authors: Zhu Qingshi , Thrush Brian A.
Published: July 2002

In recent years there have been extensive experimental searches for bond selectivity. However, actual energy localization has not been achieved due to very fast intramolecular vibrational relaxation (IVR). We have recently reported [Zhu, Thrush, and Robiette, Chem. Phys. Lett. 150, 181 (1988)] the spectrum of (3000) stretching overtone band of germane near 6130 cm−1 in which the transition exhibits striking pseudosymmetric-top structure. A detailed analysis is presented here. The P, Q, and R branches with J up to 10 were assigned, and molecular constants of (3000) state were derived for 74GeH4, 72GeH4, and 70GeH4. The results indicate that the (3000) state of germane is very near the local mode limit and that physically the molecule can be thought of as vibrating with the three GeH stretching quanta localized in one bond. Since there are no strong couplings between this state and any other state, the energy will only be slowly delocalized by the IVR process.
Rotational cooling in associative desorption
Authors: Yang Kai , Rahman Talat S.
Published: July 2002

The energy distributions of H2 molecules desorbing from Cu surface are explored in the context of a model in which molecular rotations with the angular momentum vector along the surface normal (the helicopter mode) are assumed to be free, while rotations for which the angular momentum vector lies in the plane parallel to the surface (the cartwheel mode) are taken to be hindered. In addition, a coupling is introduced between the cartwheel types of rotations and translations away from the surface. Using the classical trajectory method, it is found that the cartwheel type of rotations are ‘‘cooled’’ below their thermal value implying that the desorbing molecules should be predominantly of the helicopter type. These results are more generally applicable and are in qualitative agreement with available data.
Interfacial studies of incompressible binary blends
Authors: Tang Hai , Freed Karl F.
Published: July 2002

Density functional theory is used to investigate two of the possible reasons for the discrepancy between theory and experiment for the interfacial width and tension of phase separated binary blends. The two reasons are the inadequacy of the density functionals used previously and the neglect of the composition dependence of the Flory interaction parameter χ. For the sake of algebraic simplicity, our computations are restricted to symmetric, incompressible binary blends with correlation functions taken in the random phase approximation. We extend the analysis in Paper I [Tang and Freed, J. Chem. Phys. 94, 1572 (1991)] on the validy of density functionals for binary blends to the strong segregation limit with narrow interfaces by studying the conditions under which the Roe free energy functional is appropriate. Our derivation in this limit resums a rigorous Landau expansion to show that the Roe functional is appropriate only for infinite molecular weights. Nevertheless, even in the strong segregation limit this functional becomes incorrect in the wings of the concentration profile where a Landau expansion is valid despite the current unavailability of a local density functional.

Interfacial properties in the theoretically simplest weak segregation limit are in accord with those of previous workers, but we are able to suggest an interpolation form for the interfacial width and tension of symmetric blends that crosses over between the weak and strong segregation limits. The composition dependence of the Flory interaction parameter χ poses several problems when experimental χeff are substituted into theories that ignore this composition dependence. We therefore investigate the errors incurred by the use of different experimental measures of χ and by the use of χ measured at different compositions for two simple models in which χ varies linearly or quadratically with blend composition. Situations arise where there exists an experimental χeff that yields the correct interfacial width in the composition independent theory, but other examples produce substantial errors. Dimensionless combinations of the interfacial width and tension are constructed that should be weakly dependent on molecular weight and temperature unless ignored factors, such as blend compressibility, have a significant influence on interfacial properties. The relevance of the composition dependece of χ to interfacial profiles suggests a similar relevance of blend compressibility because compressibility influences χ and its composition dependence.
Direct inelastic scattering of oriented NO from Ag(111) and Pt(111)
Authors: Tenner Manfred G. , Kuipers Edgar W. , Kleyn Aart W. , Stolte Steven
Published: July 2002

A pulsed supersonic and cold oriented beam of NO molecules is incident upon the (111) face of clean Ag and Pt single crystal surfaces. The steric effect in the scattered density distributions is determined by a quadrupole mass spectrometer. It is found that the steric effect in the peak in the distribution of direct inelastically scattered molecules depends linearly on the reflection angle. In all circumstances O-end collisions lead to scattering angles more inclined towards the surface than N-end collisions. For the Pt(111) surface a much stronger steric effect is measured than for the Ag(111) surface. The steric effect seems to scale with the incident normal velocity. These strong steric effects can be explained by the larger trapping probability for the N-end orientation and a leverage effect due to the high trapping probability.
Physisorption in confined geometry
Authors: Derycke I. , Vigneron J. P. , Lambin Ph. , Lucas A. A. , Derouane E. G.
Published: July 2002

The physisorption of molecules in confined geometry, i.e., in pores of atomic size such as found in zeolites, has been investigated using a simple pairwise-additive Lennard-Jones potential and an effective-medium model. In a spherical geometry, it is found that the equilibrium distance D corresponding to the lowest equilibrium energy is reduced to about 90% of the pair equilibrium distance σe. This originates from the increased dominance of long-range forces in the condensed state. The enhancement of the physisorption energy due to surface curvature and confinement effects reaches its maximum value of 5.05, relative to the flat surface, when D=0.899σe. This value must be compared to the factor of 8 which was derived previously [D. H. Everett and P. C. Powl, J. Chem. Soc. Faraday Trans. 1 72, 619 (1976); E. G. Derouane, J.-M. André, and A. A. Lucas, Chem. Phys. Lett. 137, 336 (1987)] using a simple van der Waals model neglecting repulsion forces. It is also concluded that molecules can be strongly trapped in pores which are substantially narrower than their free (gaseous phase) sizes, the situation of lowest energy corresponding to R=D=0.899σe and the sorption energy remaining negative down to R=D=0.749σe (R denotes the pore radius).
A new strong principle of corresponding states for nonpolar fluids
Authors: Ihm G. , Song Yuhua , Mason E. A.
Published: July 2002

We present a new strong principle of corresponding states that reduces the entire pressure–volume–temperature (p–v–T) surface of a nonpolar fluid to a single curve; this curve corresponds to an effective pair distribution function at contact as a function of reduced density. This reduction of a surface to a curve is based on statistical-mechanical theory, which also furnishes the algorithms for calculating, from the intermolecular pair potential, the three temperature-dependent parameters needed for the reduction. If the pair potential is not known, data on the second virial coefficient as a function of temperature can be used instead. The principle is tested on a computer-simulated Lennard-Jones (12,6) fluid, on the noble-gas fluids (except He), on N2, CO2, CF4, SF6, and on the first four alkanes. A suitable reciprocal plot yields virtual straight lines for all the real fluids, which differ in shape from the expected Carnahan–Starling curve that describes the (12,6) fluid; we suggest that these shape differences are caused by many-body forces in the real fluids. By curve fitting straight lines to the empirical data for the real fluids, we obtain a simple analytic equation of state, cubic in the density, that can be characterized by three constants: the Boyle temperature, the Boyle volume, and a slope constant. This equation is not accurate in the nonanalytic critical and two-phase regions, but otherwise describes the volumetric behavior of nonpolar fluids very accurately over the entire range from the dilute gas to the dense liquid. It has considerable predictive power, since it permits the construction of the entire p–v–T surface from just the second virial coefficient plus a few liquid densities.
Vibrational relaxation of H2O by H2, HCl, and H2O at 295 K
Authors: Zittel P. F. , Masturzo D. E.
Published: July 2002

A laser induced fluorescence method has been used to measure rate constants for vibrational relaxation of the equilibrated ν1 and ν3 stretching level reservoir, the 2ν2 bending overtone level, and the ν2 bending level of H2O by H2 and HCl at 295 K. The rate constants for relaxation by H2 were found to be (8.0±1.1), (29±4), and (13±2)×10−13 cm3 molecule−1 s−1, respectively. For relaxation by HCl the rate constants were (8.4±1.2), (47±7), and (24±4)×10−12 cm3 molecule−1 s−1, respectively. Relaxation by intermolecular V→V transfer was experimentally determined to account for <8% of the rate constant for relaxation of the H2O stretching levels by HCl and was estimated to contribute <16% of the rate constant for relaxation of the bending overtone level. For both collision partners, the H2O stretching levels were relaxed predominantly to the 2ν2 level and the 2ν2 level was relaxed predominantly to ν2. Following direct laser excitation, the rate constant for relaxation of the equilibrated ν1+ν2 and ν3+ν2 combination vibrational levels of H2O by H2O was measured to be (1.6±0.2)×10−10 cm3 molecule−1 s−1. Direct relaxation to the ν1 and ν3 fundamental levels was experimentally determined to account for <40% of the relaxation rate constant.
A Born–Green–Yvon integral equation treatment of a compressible fluid
Authors: Lipson J. E. G. , Andrews S. S.
Published: July 2002

Previous work using the Born–Green–Yvon (BGY) integral equation approach has been extended to investigate the case of a pure compressible lattice fluid, yielding the equation of state as our main result. We show that the BGY equation does a very good to excellent job at fitting the experimental pressure–volume–temperature surface for both small molecules and polymers; the fit parameters are then used to make predictions about thermodynamic properties for the system of interest. We note that two other equations of state can easily be obtained from the BGY equation, and compare results using BGY and other equations of state. We also comment on the agreement between the BGY description in the athermal limit, which is equivalent to Guggenheim’s treatment of random mixing, and some lattice Monte Carlo results.
Electronic structure of the III–V tetramer clusters and their positive ions
Authors: Liao D. W. , Balasubramanian K.
Published: July 2002

We study mixed group III–V tetramers and their positive ions, in particular, Al2P2, Ga2As2, In2Sb2, Tl2Bi2, and their positive ions. We employ ab initio complete active space multiconfiguration self-consistent field followed by multireference singles + doubles configuration interaction (MRSDCI) calculations. We also include the effects of spin–orbit coupling for Tl2Bi2 and In2Sb2 tetramers. The mixed III–V diatomics (AlP–TlBi) are also studied for comparison. Among the states and geometries considered here, we find that the lowest states of all four tetramers are 1Ag with rhombus equilibrium geometries. We considered several excited states of the positive ions and found that the 2B1u state with a rhombus geometry is the lowest for the positive ions. The binding energies and adiabatic ionization energies of all four tetramers are computed at the MRSDCI level and periodic trends are obtained. The Tl2Bi2 tetramer and TlBi dimer differ from their lighter analogs due to relativistic effects. We find that the binding energies per atom for the tetramers are larger than the trimers but comparable to the dimers.
Resonance Raman studies of the ground and lowest electronic excited state in CdS nanocrystals
Authors: Shiang J. J. , Risbud S. H. , Alivisatos A. P.
Published: July 2002

The size dependence of the resonance Raman spectrum of CdS nanocrystals ranging in size from 10 to 70 Å radius has been studied. We find that while the lowest electronic excited state is coupled strongly to the lattice, this coupling decreases as the nanocrystal size is decreased. We demonstrate that the lifetime of the initially prepared excited state can influence the apparent strength of electron-vibration coupling. Absolute resonance Raman cross section measurements can be used to determine the value of the excited state lifetime, thus removing this parameter. The coupling to the lattice, while less in nanocrystals than in the bulk, is still greater than what is predicted assuming an infinite confining potential. The width of the observed LO mode broadens with decreasing size, indicating that the resonance Raman process is intrinsically multimode in its nature. The frequency of the observed longitudinal optic (LO) mode has a very weak dependence on size, in contrast to results obtained from multiple quantum well systems. The temperature dependence of the frequency and linewidth of the observed LO mode is similar to the bulk and indicates that the LO mode decays into acoustic vibrations in 2.5 ps.
High resolution rotational and ν3 coherent Raman spectra of C2H6
Authors: Al-Kahtani Abdullah , Montero Salvador , Nibler Joseph W.
Published: July 2002

Coherent anti-Stokes Raman spectroscopy has been used to study C2H6 in the rotational and ν3 CC stretching regions. Pure rotational transitions for torsionally excited molecules are seen for the first time and direct analysis of the peak maxima gives B values of 0.663 06 and 0.660 37 cm−1 for the υ4=0 and 1 torsional levels. The change in B is combined with ab initio calculations to show that the primary response of the molecule to the increased torsional amplitude is a CC extension of 0.0031 Å, plus a slight increase of 0.10 degrees in the CCH angle. For the ν3 vibrational Q branch, a complex mixture of four band systems is seen, a consequence of Fermi resonance interactions causing the predicted four torsional sublevels of this state to separate. Spectra of jet-cooled samples aid in the assignment of the spectrum and the four torsional components are determined as 994.973 (E3s), 994.878 (E3d), 994.864 (A1s), and 993.791 cm−1 (A3d). The A and B rotational constants decrease by 0.003 27 and 0.006 21 cm−1, respectively, when ethane is excited in the ν3 mode. These differences are used with the ab initio results to deduce that this transition results in a net increase of 0.010 Å in the average CC distance, accompanied by a decrease of 0.12 degrees in the CCH angle and a very small CH decrease of 0.0002 Å. The calculations also suggest that the V3 torsional barrier for the υ3=1 state is about 3% smaller than for the ground state.
Fluorescence properties of submonolayers of rhodamine 6G in front of a mirror
Authors: Cnossen Gerard , Drabe Karel E. , Wiersma Douwe A.
Published: July 2002

Fluorescence properties of submonolayers of rhodamine 6G have been measured as a function of distance to an aluminum mirror. For distances less than 5 nm (spacer layers of 1 nm thickness were used) a broadening of the emission spectrum is observed. The fluorescence lifetime has been measured for distances up to 6 nm. The broadening of the fluorescence spectra and the shortening of the lifetime observed when the sample is close to the mirror are attributed to efficient energy transfer from the monolayer to the mirror. The experiments clearly demonstrate that both volume and surface contributions must be considered in this energy-transfer process.
High resolution spectra of GeH4 v=6 and 7 stretch overtones. The perturbed local mode vibrational states
Authors: Zhu Qing-Shi , Campargue A. , Vetterhöffer J. , Permogorov D. , Stoeckel F.
Published: July 2002

Here we report the preliminary analysis of the high resolution spectra of 70GeH4 (6000) and (7000) stretch overtone bands obtained by intracavity laser absorption spectroscopy (ICLAS). The observed P(J+1), Q(J), and R(J−1) transitions agree well with the combination difference relations, indicating that the Coriolis coupling in the degenerate states is negligible. This is an important feature for a spherical top molecule near the local mode limit. The vibration–rotational energy levels of upper states as well as those of some perturber states are obtained. The improved local mode parameters of GeH4 are also derived.
Correlation effects on Raman scattering from low-energy vibrational modes in fractal and disordered systems. I. Theory
Authors: Duval E. , Garcia N. , Boukenter A. , Serughetti J.
Published: July 2002

In disordered fractal or nonfractal systems, the disorder induces rapid fluctuations of the acoustical vibrational wave functions, so that statistics can be used to resolve the problem of Raman scattering from localized low-energy vibrations. We derive the expressions of light-vibration coupling coefficient, C(ω), as a function of the frequency ω, in the case of the dipole-induced dipole mechanism of susceptibility fluctuation and for different types of correlation, from full correlation to no-correlation. The results are compared to the existing numerical calculations. The expressions of C(ω) are determined for other mechanisms of susceptibility fluctuations, depending on an interaction propagating in the fractal.
Interpolated variational transition state theory and tunneling calculations of the rate constant of the reaction OH+CH4 at 223–2400 K
Authors: Melissas Vasilios S. , Truhlar Donald G.
Published: July 2002

We have carried out ab initio calculations for the reaction OH+CH4→H2O+CH3 using second-order Mo/ller–Plesset perturbation theory, employing a very large basis set and scaling all correlation energy for the final energy calculation, but optimizing the equilibrium and transition state structures without scaling (MP-SAC2//MP2). We found that inclusion of correlation energy has an important effect on the geometry, barrier height, and vibrational frequencies of the transition state. The final calculated values for the forward and reverse classical barrier heights are 7.4 and 20.6 kcal/mol, respectively. We have used these with interpolated canonical variational transition state theory and the centrifugal-dominant small-curvature tunneling approximation, including information at the reactants, products, transition state, and two other points along the minimum energy path, to predict the rate constants for the above reaction in the temperature range from 223 to 2400 K. The calculated rate constants agree well with experiment over a wide temperature range.
The solution of the time-dependent Schrödinger equation by the (t,t′) method: Theory, computational algorithm and applications
Authors: Peskin Uri , Moiseyev Nimrod
Published: July 2002

A new powerful computational method is introduced for the solution of the time dependent Schrödinger equation with time-dependent Hamiltonians (not necessarily time-periodic). The method is based on the use of the Floquet-type operator in an extended Hilbert space, which was introduced by H. Sambe [Phys. Rev. A 7, 2203 (1973)] for time periodic Hamiltonians, and was extended by J. Howland [Math Ann. 207, 315 (1974)] for general time dependent Hamiltonians. The new proposed computational algorithm avoids the need to introduce the time ordering operator when the time-dependent Schrödinger equation is integrated. Therefore it enables one to obtain the solution of the time-dependent Schrödinger equation by using computational techniques that were originally developed for cases where the Hamiltonian is time independent. A time-independent expression for state-to-state transition probabilities is derived by using the analytical time dependence of the time evolution operator in the generalized Hilbert space. Illustrative numerical examples for complex scaled time periodic model Hamiltonians are given.
Classical and quantum proton vibration in a nonharmonic strongly coupled system
Authors: McDonald K. M. , Thorson W. R. , Choi J. H.
Published: July 2002

Classical and quantum descriptions of proton vibration are compared for a coupled nonharmonic model based on an ab initio potential for the bifluoride ion, [FHF]−. Accurate quantum calculations and exact classical dynamics are compared with quantum and classical versions of the self-consistent-field (SCF) approximation. Semiclassical and quantum SCF eigenvalues agree within JWKB-type errors. The SCF scheme closely approximates exact quantum states for the lowest 4–5 vibrational levels of each symmetry, except at avoided crossings where strong CI mixing of SCF levels occurs. True classical motion, however, is mainly irregular except at very low energies, and even where it remains regular it may be strongly reorganized by a 1:1 periodic resonance associated with major potential surface features. Strongly mixed CI states at systematic avoided crossings of SCF levels at higher energies do have classical analogs in the reorganized classical motions seen at low energies; stabilized CI components correspond to a stable periodic 1:1 orbit, destabilized components to an unstable periodic 1:1 elliptical orbit. Canonical perturbation theory is used to study further the sense in which the exactly separable classical SCF Hamiltonian is ‘‘close’’ to the true Hamiltonian. Where true motion is modal or SCF-like, first-order perturbed trajectories and second-order perturbed energies describe it very accurately. However since the dynamics can be strongly disturbed even at very low energies, correlation effects are obviously not ‘‘small’’ in the sense usually meant in classical dynamics, i.e., that regular trajectories mostly remain regular in the nonseparable perturbed system.
Statistical-mechanical equation of state for nonpolar fluids: Prediction of phase boundaries
Authors: Tao Fu-Ming , Mason E. A.
Published: July 2002

A perturbation correction term for the effect of attraction forces on the equation of state is calculated and combined with previous statistical-mechanical analytical equations of state proposed by Song and Mason and by Ihm, Song, and Mason. The major effect of the correction on the p–v isotherms occurs in the metastable and unstable regions (the ‘‘van der Waals loops’’), with the result that the vapor pressures and orthobaric densities predicted from the Maxwell equal-area construction are greatly improved in accuracy. Comparison is made with experimental data for 13 selected nonpolar fluids (Ar, Kr, Xe, N2, O2, CO2, CH4, C2H6, C3H8, n-C4H10, i-C4H10, C2H4, and benzene) and one slightly polar fluid (toluene). Densities in the stable region of the p–v–T surface are accurate to about 1%–2% in the dense fluid region, and to better than 1% in the low-density gas region; the accuracy is slightly better than that achieved without the perturbation correction. Vapor pressures are predicted with an accuracy of about 2%, with orthobaric densities that are accurate to about 2% for the saturated vapor and to better than 1% for the saturated liquid. As usual for analytical equations of state, the critical region is described less accurately. In principle, the entire fluid equation of state and its vapor–liquid phase boundaries can be calculated from the intermolecular potential plus a few liquid densities. If the potential is not known, measurements of the second virial coefficient as a function of temperature can be used instead; in the absence of any such measurements, the calculation can use as input only the critical temperature, the critical pressure, and the Pitzer acentric factor, with only slight loss of accuracy. Comparison is also made with several widely used empirical equations of state. The present equation of state can be extended to include mixtures, but numerical computations on mixtures are postponed for future work.