bluuebunny/crossref_metadata_2025 huggingface dataset:
Nanometer-sized Ge particles in GeO2–SiO2 glasses produced by proton implantation: Combined effects of electronic excitation and chemical reaction
Authors: Hosono Hideo , Kawamura Ken-ichi , Kameshima Yoshikaza , Kawazoe Hiroshi , Matsunami Noriaki , Muta Ken-ichi
Published: July 2002

It was reported [H. Hosono et al., Appl. Phys. Lett. 65, 1632 (1994)] that nanometer-sized crystalline (nc) Ge colloid particles were formed by implantation of protons into 0.1 GeO2–0.9 SiO2 glasses at room temperature. The depth profiles of Ge colloids and the density of Si–OH or Ge–OH created by the implantation were measured and compared with those of energy deposition in order to examine the formation mechanism of Ge colloids by proton implantation. The depth region of nc-Ge particles was found to correspond to the overlapped region between the OH distribution and the peak of electronic energy deposition. Transmission electron microscopic observation revealed that the size of Ge colloid particles created by proton implantation was close to that of GeO2-rich particles occurring in the substrate glasses. These results indicate that GeO2-rich particles are converted into Ge particles by a combined effect of the electronic excitation and the chemical reaction of implanted protons. A mechanism was proposed consisting of displacement of bridging oxygen into interstitials by electronic excitation and subsequent trapping of the oxygen interstitials by a formation of OH groups.
Lattice dynamics of GaN/AlN superlattices
Authors: Wei Guanghong , Zi Jian , Zhang Kaiming , Xie Xide
Published: July 2002

Phonon dispersion curves of cubic (001)-(GaN)n/(AlN)n (n=1–4) superlattices for various propagating directions are calculated. Short-range interactions up to the second-nearest neighbors are described by using a two-parameter Keating model, and the long-range Coulomb interactions are included via an Ewald summation. Raman spectra are calculated by a bond-polarizability model. The confinement of optical modes is discussed. The anisotropies of the zone-center optical phonons are investigated.
Profile broadening of high dose germanium implants into (100) silicon at elevated temperatures due to channeling
Authors: Nejim A. , Knights A. P. , Jeynes C. , Coleman P. G. , Patel C. J.
Published: July 2002

(100) Silicon wafers were implanted with 120 keV germanium ions at substrate temperatures up to 600 °C. The germanium profile was monitored together with the crystalline fraction of the implanted silicon by Rutherford backscattering spectroscopy channeling in the 〈100〉 direction. Extensive profile broadening was seen with elevated temperature implants accompanied by a peak shift of 50 nm away from the surface for the highest temperature implant at 600 °C. A germanium tail was also seen extending deeper than 300 nm for this implant together with a reduction in the peak germanium concentration of more than 60% when compared with similar implants at room temperature. Radiation enhanced diffusion and enhanced sputtering are ruled out as causes for the profile broadening. Instead we demonstrate channeling along the 〈100〉 direction to be the cause of both the observed broadening and the increase in the peak depth. This was confirmed by eliminating such profile broadening in a sample implanted at 600 °C by tilting the implant away from major axial and planar channels. Positron annihilation spectroscopy was used to monitor the profile of open volume defects in the implanted material. Data from this analysis show that a defect concentration of greater than 1016/cm3 extends to a depth of ≈1.2 μm for 1×1014 Ge/cm2 at room temperature. A higher concentration of such defects appear deeper for similar implants at 450 °C. These deep open volume defects can be eliminated by using tilted implants through an amorphous 0.5 μm SiO2 mask layer. Their formation is consistent with germanium ion channeling during the initial stages of room temperature implantation and extensive channeling for elevated temperature implantation.
Buried, self-aligned barrier layer structures for perovskite-based memory devices comprising Pt or Ir bottom electrodes on silicon-contributing substrates
Authors: Saenger K. L. , Grill A. , Kotecki D. E.
Published: July 2002

The integration of noble metal electrodes into semiconductor memory devices incorporating ferroelectric or high dielectric constant ε materials is expected to require deposition of a conductive, oxidation-resistant barrier material between the noble metal and the silicon contact. Described is an alternative type of barrier layer structure which is formed as buried, self-aligned layer during oxygen-ambient annealing after noble metal deposition on silicon-contributing substrates. Reactions of Pt(20 nm) and Ir(20 nm) films with substrates of single crystalline silicon (c-Si), polycrystaline silicon (poly-Si), and tungsten silicide (WSix/Si with x=2.4–2.8) were examined after anneals in atmospheric pressure ambients of oxygen or nitrogen at temperatures of 640–700 °C, a temperature range of interest for high-epsilon materials deposition. While Pt(20 nm) films reacted with silicon and WSix/Si during oxygen annealing to form a mixture of Pt silicides and Pt, Ir(20 nm) films on the same substrates did not form any iridium silicides during oxygen annealing. In all cases, unreacted noble metal M was left due to the formation of an oxygen-containing M–O–Si barrier which interfered with the silicidation reaction. In contrast to these results for oxygen annealing, the Pt and Ir films were completely consumed by silicidation reactions during anneals in nitrogen. Qualitative through-film resistance measurements indicated that the M–O–Si barrier layers formed during oxygen annealing were at least moderately conductive for the cases of M=Ir on silicon and M=Ir or Pt on WSi2.8(300 nm)/Si, a prerequisite for the use of these electrode barrier structures in high-density dynamic random access memory.
Nonuniform vertical charge transport and relaxation in quantum well infrared detectors
Authors: Perera A. G. U. , Silvestrov V. G. , Matsik S. G. , Liu H. C. , Buchanan M. , Wasilewski Z. R. , Ershov M.
Published: July 2002

The temperature dependence of capacitance and ac conductance for GaAs/AlGaAs multiquantum well (MQW) structures with 4, 8, 16, and 32 wells has been studied at different bias and frequency ranges. The dominant contribution to ac conductance depends on temperature and frequency, and changes from thermally assisted tunneling through the first excited state to thermionic emission with activation energies of 100±15 meV and 170±10 meV, respectively. In the temperature interval 80–100 K, conductance and capacitance have two distinct regions of frequency dispersion due to tunneling and thermionic emission respectively, giving rise to two relaxation times. Above 100 K, increased conductance makes electrical response of the MQW structure so fast that at frequencies up to 1 MHz relaxational properties are masked. A model based on a series circuit comprised of barrier resistance and capacitance in parallel shows results in good agreement with experimental data, indicating strong nonuniform conductance of MQW structures.
Analysis of carrier concentration, lifetime, and electron mobility on p-type HgCdTe
Authors: Yoo Sang Dong , Kwack Kae Dal
Published: July 2002

Minority carrier transport characteristics of vacancy-doped p-type HgCdTe such as carrier concentration, lifetime, and mobility are investigated. In the calculation of the carrier concentration two acceptor levels—a donor level and a trap level—were taken into account. The acceptor levels have been described by two models—two independent singly ionized levels and a divalent level with two ionization energies. When each model was examined by calculating electron mobility as a function of temperature, the latter was found to be more accurate. Electron mobility as a function of majority carrier concentration was also presented for both n-type and p-type HgCdTe with 0.225 Cd mole fraction. Steady state electron lifetime was computed assuming the acceptor levels and the trap level would act as Schokley–Read–Hall type recombination centers. The calculated results using the divalent acceptor model were in good agreement with the experimental data.
One- and two-photon-excited time-resolved photoluminescence investigations of bulk and surface recombination dynamics in ZnSe
Authors: Wang H. , Wong K. S. , Foreman B. A. , Yang Z. Y. , Wong G. K. L.
Published: July 2002

Measurements of femtosecond time-resolved one- and two-photon-excited photoluminescence of bulk ZnSe enable us to distinguish the surface and bulk contributions to recombination dynamics. A photoluminescence lifetime of several nanoseconds or longer is measured for the bulk. A fast relaxation component with a decay time constant τT of a few tens of picoseconds observed in one-photon-pumped time-resolved spectra is identified as the result of diffusion and rapid surface recombination. A one-dimensional model taking into account surface nonradiative recombination and carrier diffusion is able to describe the observed behavior. The temperature dependence of τT shows good agreement with the theory. At room temperature, a surface recombination velocity S=5.8×105 cm s−1 and a diffusion constant D=10 cm2 s−1 are found using this model.
Nonlinear luminescence spectroscopy in a Pt:ethynyl compound
Authors: McKay T. J. , Staromlynska J. , Wilson P. , Davy J.
Published: July 2002

We report pulsed laser studies of nonlinear luminescence in a platinum ethynyl complex which, from 560 to 700 nm, shows strong two-photon absorption from the ground singlet state to an excited singlet state. The spectral dependence of the photoluminescence peaks indicates that in the region from 560 to 700 nm the two-photon absorption cross section is independent of excitation wavelength. Through the use of a simple calibration we have been able to estimate the number of states (NS1) formed from two-photon absorption, and have found good agreement with theory using the value of β=0.34 cm/GW measured previously.
Magnetization canting in epitaxial Cu/Ni/Cu/Si(001) films
Authors: Ha Kin , O’Handley Robert C.
Published: July 2002

The magnetic anisotropy of epitaxial Cu(50 Å)/Ni(tNi/Cu(2000 Å)/Si(001) films as a function of the nickel film thickness (30 Å⩽tNi⩽150 Å) has been studied using a torque magnetometer. It is found that the magnetization rotates continuously from out-of-plane to in-plane as the nickel film thickness increases. For the 150 Å film, the magnetization cants at 42° with respect to the film normal. The canting state is possible because K4eff is positive. The magnitude of K4eff (≈105 ergs/cm3) is an order of magnitude greater than the corresponding magnetocrystalline anisotropy constant of nickel. The origin of the fourth-order anisotropy in these films is believed to be magnetoelastic in nature.
Thermal effusivity profile characterization from pulse photothermal data
Authors: Krapez Jean-Claude
Published: July 2002

An inversion method is proposed for depth profiling of the thermal effusivity from the surface temperature evolution after an excitation. Focus is on pulse heating. The effusivity profile is obtained through a Laplace inversion. The Stehfest method is implemented and we propose to use the number of elements in the Stehfest series N as a regularizing parameter. The optimum N value is defined by plotting a characteristic C curve with the norm of the solution derivative and the norm of the residues of the so-called apparent effusivity function. Examples of inversion results are given for linear and Gaussian effusivity profiles. For this purpose, we extended the formalism of thermal quadrupoles to the case of linearly varying effusivity. A statistical analysis is performed to assess the influence of noise on the inversion results. Recommendations on the thermogram minimum duration are derived therefrom. Experimental results obtained with a case-hardened steel sample show the potential of the present nondestructive approach.
Molecular beam electric resonance study of the ground and excited states of cyanoacetylene
Authors: DeLeon Robert L. , Muenter J. S.
Published: July 2002

Cyanoacetylene was studied using molecular beam electric resonance spectroscopy. Ground state results are μ=3.731 72 D, eQq=−4319.24 kHz, CN=0.976 kHz. Zero field spectra were observed for the three singly excited bending modes and Stark data were taken for the lower energy of two of these three states. Excited state results include ν7; μ=3.7225 D, eQqaa=−4302.0 kHz, eQ(qbb−qcc)=−28.8 kHz, CN(J=2)=1.47 kHz; ν6; μ=3.7263 D, eQqaa=−4369.7 kHz, eQ(qbb−qcc)=177.1 kHz, CB(J=2)=1.30 kHz; ν5; eQqaa=−4325.7, eQ(qbb−qcc)=96.8 kHz, CN(J=2)=1.33 kHz. l-doubling constants were also obtained for the degenerate vibrations.
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o calculations in exchange coupled copper (II) dimers: Influence of structural and chemical parameters in modeled copper dimers
Authors: Astheimer Harald , Haase Wolfgang
Published: July 2002

The influence of structural and chemical parameters on the singlet–triplet splitting (2J=ΔEST ) in modeled exchange coupled oxygen-bridged dimeric copper (II) complexes is studied, using an ab initio method followed by a perturbation development of the configuration interaction. The effect of varying the Cu–O–Cu′ angle and the Cu–O distance in the Cu2O2 core of modeled dimers is calculated. The results are comparable to the experimental ones. Changes in the geometric coordination around the copper (II) atom and around the oxygen atom of the core are studied. The results show that the magnitude of the exchange coupling strongly depends on both of these factors. Electronegativity effects are studied by varying the counterion in the model and by changing from alkoxo-bridged to hydroxo-bridged copper (II) dimers.
Langevin modes of macromolecules
Authors: Lamm Gene , Szabo Attila
Published: July 2002

The standard gas phase normal mode analysis is generalized to condensed phases within the framework of the classical Langevin equation. The solution of this equation for a system of 3N atoms moving on a harmonic potential surface and subject to viscous damping described by a friction matrix (which is nondiagonal in the presence of hydrodynamic interactions) is reduced to the diagonalization of a 6N×6N real nonsymmetric matrix obtained from the mass-weighted force constant and friction matrices. Alternative formulations of the problem requiring either the diagonalization of a complex symmetric matrix or the solution to a generalized eigenvalue problem involving real symmetric matrices are also considered. The resulting eigenvalues, in general complex, are real when the motion is overdamped and occur in complex conjugate pairs in the underdamped case. It is shown that when the eigenvectors are normalized in a particular way, all quantities of interest (e.g., correlation functions) have simple spectral representations and can be expressed as summations over the eigenvalues and eigenvectors (‘‘Langevin modes’’) of the system. To treat situations in which the exact matrix solution is computationally prohibitive, a perturbation approach is developed which utilizes the gas phase normal mode results for the system. In zeroth-order each normal coordinate is a Langevin oscillator with a friction constant equal to a diagonal element of the normal mode-transformed friction matrix. This description is analytically tractable, can be improved perturbatively, and is exact in the limit that all atoms experience the same friction. The formalism developed in this paper provides a viable way of studying the influence of solvent on the dynamics of collective motions in macromolecules.
Flow-induced scission of isolated macromolecules
Authors: Odell J. A. , Keller A. , Rabin Y.
Published: July 2002

We review experimental results on the degradation of stretched polymer molecules in strong extensional flow fields. Atactic polystyrene and polyethylene oxide both show closely central scission along the backbone of the chain. We present a theoretical approach based upon a modified thermally activated barrier to scission model. This successfully describes the scission process as a function of molecular weight and stiffness and strain rate.
Geminate recombination in excited-state proton-transfer reactions: Numerical solution of the Debye–Smoluchowski equation with backreaction and comparison with experimental results
Authors: Pines Ehud , Huppert Dan , Agmon Noam
Published: July 2002

The well-known phenomenon of proton dissociation from excited-state hydroxy-arenes is analyzed by the Debye–Smoluchowski equation which is solved numerically with boundary conditions which account for the reversibility of the reaction. The numerical solution is then compared with the measured dissociation profiles which were obtained by picosecond time-resolved fluorescence spectroscopy. The intrinsic rate constants thus determined are used to predict steady-state rates, yields, and pK values, in agreement with experiment.
Electronic states of semiconductor clusters: Homogeneous and inhomogeneous broadening of the optical spectrum
Authors: Alivisatos A. P. , Harris A. L. , Levinos N. J. , Steigerwald M. L. , Brus L. E.
Published: July 2002

The homogeneous (single-cluster) and inhomogeneous contributions to the low temperature electronic absorption spectrum of 35–50 Å diameter CdSe clusters are separated using transient photophysical hole burning. The clusters have the cubic bulk crystal structure, but their electronic states are strongly quantum confined. The inhomogeneous broadening of these features arises because the spectrum depends upon cluster size and shape, and the samples contain similar, but not identical, clusters. The homogeneous spectrum, which consists of a peak 140 cm−1 (17 meV) wide, with a phonon sideband and continuum absorption to higher energy, is compared to a simple molecular orbital model. Electron–vibration coupling, which is enhanced in small clusters, contributes to the substantial broadening of the homogeneous spectrum. The inhomogeneous width of the lowest allowed optical transition was found to be 940 cm−1, or seven times the homogeneous width, in the most monodisperse sample.
Layering in colloidal fluids near a smooth repulsive wall
Authors: Van Winkle David H. , Murray C. A.
Published: July 2002

We have directly observed layering in three-dimensional colloidal fluids caused by the presence of a smooth repulsive glass wall. The system we study is a colloidal suspension of highly charged monodisperse latex spheres in water, with diameter 0.3 μm, charge ∼104 electronic charges, and mean separation ∼0.8–2 μm. This allows direct atomic resolution of the colloid using ordinary optical microscopy and digital imaging techniques. The layering of the colloid is manifested as a density modulation perpendicular to the wall. For fluids with low bulk densities, we find the density profile perpendicular to the wall to be essentially identical to the pair distribution function in a thin slab of spheres parallel to the wall, with the exception of the spacing of the first peak. For a fluid with bulk density approaching that of the crystal, we find a smaller peak spacing than that of the in-plane pair distribution function, indicating incipient crystallization.
Rotational spectrum and structure of the linear CO2–HCN dimer: Dependence of isomer formation on carrier gas
Authors: Klots T. D. , Ruoff R. S. , Gutowsky H. S.
Published: July 2002

A linear hydrogen-bonded dimer, OCO–HCN, has been identified and characterized via its microwave rotational spectrum. The study was made using the pulsed nozzle Fourier transform method with the Flygare/Balle Mark II spectrometer. A T-shaped HCN–CO2 dimer was reported earlier by the Klemperer group. Rotational constants have been determined for all seven monoisotopically substituted species of the linear form. B0 , DJ , and χaa (14 N) for the normal isotopic dimer are 1057.9397(2) MHz, 1.372(8) kHz, and −4.2466(5) MHz, respectively. The average torsional displacements of the OCO and HCN monomers about their center of mass (c.m.) are found to be 7.66° and 12.40°, based on the substitution O–C and C–N bond distances for the dimer. With these values for α and γ, the B0 for the normal isotopic dimer corresponds to a c.m. to c.m. distance R=5.035 Å. Bending and stretching force constants and the well depth (ε∼590 cm−1 ) are estimated from the centrifugal distortion. The relative concentrations of the linear and T-shaped isomers are unusually sensitive to the carrier gas used in the supersonic jet expansion. The linear form could not be detected at all with argon as the carrier gas but gave a strong signal in neon first run (70% Ne, 30% He). In contrast, the T form gave strong signals in both carrier gases. However, a carrier-gas effect was not found for the N2 O/HF dimer pair, which has a high barrier between the bent NNO–HF and linear FH–NNO isomers. Similar results were obtained for chlorocyclohexane (CCH) and ethyl formate (EF), which have two conformational isomers. In CCH which has a high barrier to a↔e interconversion, the two conformers gave strong signals in both Ar and He. In EF, with a low barrier, the gauche conformer could not be detected in Ar but gave a strong signal in He, while the trans form gave strong signals in both carrier gases.
Lattice theory of polymer blends and liquid mixtures: Beyond the Flory–Huggins approximation
Authors: Pesci Adriana I. , Freed Karl F.
Published: July 2002

Lattice model calculations of corrections to the Flory–Huggins mean field approximation from the preceding paper are applied to the thermodynamic properties of polymer blends. We describe the variation of the noncombinatorial entropy of mixing with composition and monomer structure by considering an extended lattice model in which monomers extend over several lattice sites and therefore have differing sizes and shapes. Composition and temperature dependences of heats of mixing or the equivalent Flory χ parameters are in accord with the magnitude observed experimentally. It is only because we treat the idealized limit of an incompressible blend, for simplicity, that the heat of mixing and Flory χ parameter depend on one effective interaction parameter that is represented in terms of differences in van der Waals energies. The corrections to the Flory–Huggins approximation produce a much lower critical temperature in general agreement with recent Monte Carlo simulations by Sariban and Binder. Concentrated polymer solutions and mixtures of small flexible molecules follow as simple limits of the blend theory and are used to compute the surface volume fractions that appear as empirical parameters into previous theories of liquid mixtures. Comments are made concerning recent observations of a cross link dependence of the Flory χ parameters for slightly swollen polymer networks.
Vibrational relaxation of OH (X 2Πi, v=2)
Authors: Rensberger Karen J. , Jeffries Jay B. , Crosley David R.
Published: July 2002

Vibrational relaxation rates for the v=2 level of the X 2Πi state of the OH radical have been measured in a low pressure flow system, using a novel two-laser pump-and-probe technique. The OH is prepared in the v=2 level by overtone pumping (2←0) and monitored by ultraviolet laser-induced fluorescence in the (1,2) band of the A–X system. Scanning the time delay between the lasers at a given collider pressure produces exponential decay whose rate as a function of collider pressure yields the rate constant. We determine values (all cm3 s−1 units) for NH3: (1.20±0.15)×10−10; CH4: (2.3±0.2)×10−12; CO2: (6.7±1.1)×10−13; N2O: (4.6±0.6)×10−13; O2: (2.6±0.54)×10−13; N2 and H2: ≤10−14. Except for ammonia, these are two to three orders of magnitude smaller than those measured for relaxation of v=1 in the A 2Σ+ excited state of OH, where attractive forces appear to play a role.
Higher-order polarizabilities for the helium isoelectronic series
Authors: Bishop David M. , Rérat Michel
Published: July 2002

Accurate and explicitly electron-correlated wave functions have been used to calculate the dipole polarizability (α), second hyperpolarizability (γ), dipole–dipole–quadrupole (B), and quadrupole–quadrupole (C) polarizabilities of the helium isoelectronic series He,...,Ne8+. It is believed that our results, which in nearly all cases are new, are very accurate and should serve as a benchmark for future experiments and calculations.
C
6 dispersion coefficients in multiconfiguration self-consistent field linear response theory
Authors: Fowler P. W. , Jo/rgensen P. , Olsen J.
Published: July 2002

An algorithm for the evaluation of correlated dipole–dipole dispersion coefficients by direct MCSCF linear response theory is presented. Stepwise construction of a pseudo-state basis using eigenvectors of successive (Cauchy) moments of the linear response function gives an efficient scheme for obtaining polarizabilities at imaginary frequency. The scheme is also useful for polarizabilities on the real axis below the frequency of the first dipole-allowed transition. Sample calculations of C6 coefficients are described for a range of two-electron (H−, He, Li+, Be2+, H2, and H+3 ) and many-electron (Be, N, N2 ) closed- and open-shell systems.
Theory of polyampholyte solutions
Authors: Higgs Paul G. , Joanny Jean-François
Published: July 2002

We consider polyampholyte polymers containing both positive and negative monomers randomly dispersed along the chain. Neutral chains collapse into a globule due to attractive electrostatic interactions. The behavior of the charges inside the globule is similar to that of charges in a small volume of simple electrolyte. A screening length κ−1p coming from the polymeric charge may be defined as in Debye–Hückel theory. The internal structure of the globule is that of close packed blobs of radius equal to the screening length. When salt is added this further screens the interactions and reduces the attractions. The globule begins to increase in size when the concentration of salt becomes larger than the concentration of charge on the polymer itself. Screened Coulomb interactions in a neutral chain behave like a negative contribution to excluded volume. For a chain in a good solvent there is a θ salt concentration at which the net excluded volume becomes zero. Chains are swollen above this concentration of salt, and collapsed below this concentration. For small sections of chain the Coulomb interactions are unscreened and cannot be treated as a modification to excluded volume. Chains with a strong net charge of one sign tend to behave as conventional polyelectrolyte with charges of only one sign. We determine the criterion for the value of the net charge at which the repulsions (polyelectrolyte effect) begin to dominate the attractions (polyampholyte effect). The predictions are found to be in good qualitative agreement with experiments.
Picosecond–microsecond structural relaxation dynamics in polypropylene glycol: Impulsive stimulated light-scattering experiments
Authors: Duggal Anil R. , Nelson Keith A.
Published: July 2002

Picosecond time-resolved impulsive stimulated light-scattering experiments are conducted on polypropylene glycol at temperatures above the glass transition temperature Tg. Through the use of a wide range of scattering angles, longitudinal acoustic waves are characterized in the 20 MHz–5 GHz frequency range. In addition, time-dependent thermal expansion is observed on nanosecond–microsecond time scales. The results are consistent with an empirical description of structural relaxation dynamics in terms of a stretched exponential relaxation function with exponent β=0.4 and with the average relaxation time given by the Vogel–Tamman–Fulcher form. Comparisons to Brillouin scattering and photon correlation spectroscopy results indicate that this description holds for a dynamic range of over 10 orders of magnitude. Comparison to dielectric relaxation measurements which probe mainly orientational motions of polymer segments indicates that density fluctuations involve different segmental motions with significantly faster dynamics for T>Tg.
Luminescence properties of CdSe quantum crystallites: Resonance between interior and surface localized states
Authors: Bawendi M. G. , Carroll P. J. , Wilson William L. , Brus L. E.
Published: July 2002

We use time-, wavelength-, temperature-, polarization-resolved luminescence to elucidate the nature of the absorbing and ‘‘band edge’’ luminescing states in 32 Å diameter wurtzite CdSe quantum crystallites. Time-resolved emission following picosecond size-selective resonant excitation of the lowest excited state shows two components—a temperature insensitive 100 ps component and a microsecond, temperature sensitive component. The emission spectrum, showing optic phonon vibrational structure, develops a ∼70 wave number red shift as the fast component decays. Photoselection shows the slow component to be reverse polarized at 10 K, indicating this component to be the result of a hole radiationless transition. The 100 ps emitting state is repopulated thermally as temperature increases from 10 to 50 K. All available data are interpreted by postulating strong resonant mixing between a standing wave molecular orbital delocalized inside the crystallite and intrinsic surface Se lone pair states. The apparent exciton transition is assigned to a ∼130 wave number wide band of eigenstates with the hole localized principally on the surface. The band contains strongly emitting ‘‘doorway’’ states and weakly emitting ‘‘background’’ states. The hole becomes mobile among these states as T increases to 50 K. It is suggested that such resonant mixing may be general in II–VI and III–V crystallites.
Spectroscopy of metastable species in a free-jet expansion: The D′←A′ transition of I2
Authors: Zheng Xiaonan , Fei Suli , Heaven Michael C. , Tellinghuisen Joel
Published: July 2002

The A′2u 3Π state of I2 is observed in a free-jet expansion of I2 in Ar, where it is prepared by ArF laser irradiation. Laser excitation spectra are recorded for 37 bands in the υ′←0″ progression of the D′2g(3P2)←A′ transition. The spectra display rotational temperatures of ∼5 K. At the 0.08 cm−1 resolution of the probe laser, rotational congestion near the band origins necessitates analysis by a nonlinear least-squares contour simulation method. Subsequent correlated fits of the band-by-band results are combined with other results to produce global constants valid for υ′=0–86, υ″=0–32.
Theoretical study of the water/1,2-dichloroethane interface: Structure, dynamics, and conformational equilibria at the liquid–liquid interface
Authors: Benjamin Ilan
Published: July 2002

Very little is known about the structure and dynamics of the interface between liquid water and liquid 1,2-dichloroethane (DCE), despite the fact that a molecular level understanding of this and similar interfaces is of fundamental importance for the proper interpretation of many studies of charge transfer at interfaces. Molecular dynamics calculations are used to show that the interface is molecularly sharp with capillary wavelike distortions whose structure and dynamics closely resemble those expected from the capillary wave model. Molecular level structural information such as pair correlation functions and hydrogen bonding statistics is also consistent with this picture. The orientation of water at the interface is similar to what is known at other water interfaces. The dynamics of water and DCE translational and rotational motion are only slightly modified at the interface. The DCE gauche–trans isomerization reaction is investigated at the interface and in the bulk. A continuum electrostatic model for the torsional potential of mean force is developed. Although this model can qualitatively account for the different conformational population in the bulk and at the interface, the difference is significantly overestimated.
The use of systematic sequences of wave functions for estimating the complete basis set, full configuration interaction limit in water
Authors: Feller David
Published: July 2002

An assortment of 1- and 2-electron water properties were extracted from a systematic sequence of wave functions. The regularity inherent in this sequence permitted simple exponential fits of the resulting energies and, in many cases, the properties. To the extent the exponential fit accurately reflects the asymptotic convergence of a specific property, it provides an estimate of the complete basis set, full configuration interaction (CI) limiting value at a reduced computational expense. As a consequence of the vast reduction in the number of configurations that must be treated variationally, the proposed scheme may make possible improved estimates of the complete basis set, full CI limit beyond what could be obtained from explicit computations. In order to judge the accuracy of the procedure, we have carried out the highest level ab initio calculations to date on water, recovering in excess of 96% of the estimated valence correlation energy.
The one-atom cage effect: Continuum processes in I2–Ar below the B-state dissociation limit
Authors: Burke M. L. , Klemperer W.
Published: July 2002

Optical spectra recorded with Ar and I2 in a He expansion exhibit fluorescence from an excitation continuum through a broad region of the discrete B←X transitions of I2 and I2–Ar. This fluorescence emanates from B-state I2 and arises from excitations of a bimolecular I2–Ar van der Waals complex. These results were obtained in order to test a proposed mechanism for the one-atom cage effect in I2–Ar, whereby continuum excitation to the repulsive Πu state precedes coupling onto the B state, dissociation of the complex, and fluorescence from B-state I2. The variation of the relative intensity of the observed fluorescence with excitation wavelength can be adequately reproduced with this model, but the Πu←X transition is much too weak to explain the observed absolute intensities. We consider the possible existence of a linear I2–Ar isomer in the expansion along with the well-documented T-shaped isomer. Large geometry changes for the linear isomer upon B←X excitation would result in highly dispersed Franck–Condon factors and thus split this stronger transition over a continuum. Both absolute intensities and wavelength dependences observed for fluorescence from continuum excitation fit well to the linear isomer model. Linear isomers could also be responsible for the one-atom cage effect observed at higher excitation energies.
Calculation of size-intensive transition moments from the coupled cluster singles and doubles linear response function
Authors: Koch Henrik , Kobayashi Rika , Sanchez de Merás Alfredo , Jo/rgensen Poul
Published: July 2002

Coupled cluster singles and doubles linear response (CCLR) calculations have been carried out for excitation energies and dipole transition strengths for the lowest excitations in LiH, CH+, and C4 and the results compared with the results from a CI-like approach to equation of motion coupled cluster (EOMCC). The transition strengths are similar in the two approaches for single molecule calculations on small systems. However, the CCLR approach gives size-intensive dipole transition strengths, while the EOMCC formalism does not. Thus, EOMCC calculations can give unphysically dipole transition strengths, e.g., in EOMCC calculations on a sequence of noninteracting LiH systems we obtained a negative dipole strength for the lowest totally symmetric dipole allowed transition for 19 or more noninteracting LiH systems. The CCLR approach is shown to be a very attractive ‘‘black box’’ approach for the calculation of transition moments.
The choice of a zeroth-order Hamiltonian for second-order perturbation theory with a complete active space self-consistent-field reference function
Authors: Dyall Kenneth G.
Published: July 2002

The choice of a zeroth-order Hamiltonian, Ĥ0, for second-order perturbation theory with a complete active space self-consistent-field (CASSCF) reference function is discussed in detail, in the context of the inclusion of the denominator shifts found to be important in recent single-reference high-spin open-shell theories and the formulation of a computationally efficient method. Using projection operators and second quantization algebra, an operator is constructed which consists of the complete active space Hamiltonian in the active space and the Mo/ller–Plesset zeroth-order Hamiltonian in the inactive and secondary spaces. This operator, designated CAS/A, has the reference as an eigenfunction without the necessity of projection, it naturally incorporates denominator shifts which appear in terms of active space Fock operators, it does not give rise to intruder states, and it costs little more than other CASSCF perturbation theories. The incorporation of the complete active space Hamiltonian introduces additional active space two-particle terms into the zeroth-order energies over the Fock operators, which may be regarded as an inconsistency. To achieve an approximate consistency, they may be removed or supplemented with other particle–particle and hole–hole terms. The results of test calculations indicate that supplementation is not advisable and that removal has only a modest effect. The test calculations are compared with other results and experiment, and support the effectiveness of the proposed CAS/A Ĥ0.
A molecular beam optical/Stark study of calcium monomethyl
Authors: Marr A. J. , Grieman F. , Steimle T. C.
Published: July 2002

Optical and optical Stark spectra of the 000 Ã 2E-X̃ 2A1 band system of a supersonic molecular beam sample of calcium monomethyl, CaCH3, have been recorded. Field free spectroscopic parameters were obtained on fitting the (|KR′|=1, |K′|=0 and 2)–(|KR″|=1) and the (|KR′|=0, |K′|=1)–(|KR″|=0) subbands. The branch features were reassigned and a resulting new set of spectroscopic parameters determined. The value of the a-principal axis component of the spin–rotation parameter, εaa(Ã 2E), is now consistent with the assumed nature of the low-lying excited electronic states. Dipole moments of 2.62±0.03 D and 1.69±0.02 D were determined for the X̃ 2A1 and Ã 2E states, respectively. A simple electrostatic model was adapted to predict dipole moments for CaCH3 and MgCH3.
Inert gas accumulation in sonoluminescing bubbles
Authors: Lohse Detlef , Hilgenfeldt Sascha
Published: July 2002

In this paper we elaborate on the idea [Lohse et al., Phys. Rev. Lett. 78, 1359–1362 (1997)] that (single) sonoluminescing air bubbles rectify argon. The reason for the rectification is that nitrogen and oxygen dissociate and their reaction products dissolve in water. We give further experimental and theoretical evidence and extend the theory to other gas mixtures. We show that in the absence of chemical reactions (e.g., for inert gas mixtures) gas accumulation in strongly acoustically driven bubbles can also occur.
A comparison of molecular hyperpolarizabilities from gas and liquid phase measurements
Authors: Kaatz Philip , Donley Elizabeth A. , Shelton David P.
Published: July 2002

The first and second hyperpolarizabilities (β and γ) of eight molecules at a fundamental wavelength of 1064 were measured by gas-phase electric-field-induced second-harmonic generation (EFISH), gas-phase hyper-Rayleigh scattering (HRS), and liquid-phase HRS experiments. The EFISH measurements give accurate values of β and γ for these molecules in the gas phase, and the HRS measurements show that the effective β of these molecules in the liquid is enhanced over the gas-phase value by a factor which varies from ≈0.4 to 2.0, over and above the Lorentz local field factors. Combining all of the measurements provides an accurate, absolute determination of the effective β for HRS in the liquid phase. The results for CCl4, suitable as reference standards, are 〈βVV2〉1/2=18.6±0.7 au in the liquid phase and βxyz=20.7±1.6 au in the gas phase. Comparison of measurements between hydrogenated and deuterated molecules indicates that vibrational contributions to β are small.
Phase equilibria of a nematic and smectic-A mixture
Authors: Chiu Hao-Wen , Kyu Thein
Published: July 2002

A phase diagram of a mixture consisting of nematic and smectic liquid crystals has been calculated self-consistently by combining Flory–Huggins (FH) theory for isotropic mixing and Maier–Saupe–McMillan (MSM) theory for smectic-A ordering. However, the MSM theory can be deduced to the original Maier–Saupe (MS) theory for nematic ordering. To describe the phase transitions involving induced smectic phase and nematic + smectic equilibrium, two nematic and two smectic order parameters for the nematic/smectic mixtures have been coupled through the normalized partition function and the orientation distribution function. Self-consistent numerical solution has been sought in establishing nematic/smectic phase diagrams involving (i) phase separation between nematic and smectic liquid crystals and (ii) occurrence of induced smectic in a nematic/smectic mixture. The predictive capability of this combined FH/MSM theory has been tested critically with a reported phase diagram of a nematic/smectic liquid-crystal mixture and also with our experimental phase diagram of a mixture consisting of a nematic side-on side-chain liquid-crystalline polymer and a smectic low molar mass liquid crystal.
Isomerization activation barrier of a three atom dipolar cluster
Authors: Cheng Vincent K. W.
Published: July 2002

We searched for the minimum energy state of a three-atom dipolar cluster at a wide range of atomic dipole moment by simulated annealing and nongradient optimization. At the intermediate dipole moment (μ) range in which both the close pack and the linear states are of similar energy, the search for the lower energy close pack minimum requires a careful choice of initial guess. The isomerization pathway was followed along an assumed minimum energy path and the minimum energy cluster at a given isomerization coordinate was optimized with constraint imposed on the movement of atoms. The transition state was generally not located halfway along the assumed minimum energy path. The activation barrier was found to be lower than the inversion barrier of LJ3 (μ=0). The origin of the difficulty of searching for the close pack structure at intermediate μ appears to be its narrow range of phase space on the asymmetric potential surface. The contribution of the dipole energy to the total (%E) reveals the μ dependence for both isomers such that both isomers are dipole dominated structures at high μ.
Detection of the tunneling-rotation transitions of malonaldehyde in the submillimeter-wave region
Authors: Baba Takeshi , Tanaka Takehiko , Morino Isamu , Yamada Koichi M. T. , Tanaka Keiichi
Published: July 2002

The tunneling-rotation transitions of malonaldehyde, 43 a-type Q-branch lines, were observed in the submillimeter-wave region of 643–651 GHz. The proton tunneling splitting in the ground state, ΔE0=647 046.208±0.019 MHz, and the interaction constant, F=45.8965±0.0082 MHz, were determined, as well as the molecular constants for each tunneling sublevel.
Theoretical study of the electronic spectrum of
p
-benzoquinone
Authors: Pou-Amérigo Rosendo , Merchán Manuela , Ortı́ Enrique
Published: July 2002

The electronic excited states of p-benzoquinone have been studied using multiconfigurational second-order perturbation theory (CASPT2) and extended atomic natural orbital (ANO) basis sets. The calculation of the singlet–singlet and singlet–triplet transition energies comprises 19 valence singlet excited states, 4 valence triplet states, and the singlet 3s,3p, and 3d members of the Rydberg series converging to the first four ionization limits. The computed vertical excitation energies are found to be in agreement with the available experimental data. Conclusive assignments to both valence and Rydberg states have been performed. The main features of the electronic spectrum correspond to the ππ* 1 1Ag→1 1B1u and ππ* 1 1Ag→3 1B1u transitions, computed to be at 5.15 and 7.08 eV, respectively. Assignments of the observed low-energy Rydberg bands have been proposed: An n→3p transition for the sharp absorption located at ca. 7.4 eV and two n→3d and π→3s transitions for the broad band observed at ca. 7.8 eV. The lowest triplet state is computed to be an nπ* 3B1g state, in agreement with the experimental evidence.
Phase behavior of a simple model of globular proteins
Authors: Sear Richard P.
Published: July 2002

A simple model of globular proteins which incorporates anisotropic attractions is proposed. It is closely related to models used to model simple hydrogen-bonding molecules such as water. Theories for both the fluid and solid phases are presented, and phase diagrams calculated. The model protein exhibits a fluid–fluid transition which is metastable with respect to the fluid–solid transition for most values of the model parameters. This is behavior often observed for globular proteins. The model offers an explanation of the difficulty observed in crystallizing some globular proteins and suggests that some proteins may not have a solid phase at all under all but extreme conditions.
Temperature sensitive microgel suspensions: Colloidal phase behavior and rheology of soft spheres
Authors: Senff H. , Richtering W.
Published: July 2002

Rheological properties and the equilibrium colloidal phase behavior of concentrated dispersions of a temperature sensitive microgel were investigated. The temperature sensitive hydrogel particles consist of poly (N-isopropylacrylamid) (PNiPAM) chemically crosslinked with N,N′ methylenbisacrylamid (BIS). With increasing temperature the microgel particles decrease in size (hydrodynamic radius 142 nm at 10 °C and 58 nm at 35 °C) and with it the effective volume fraction, which leads to dramatic changes in rheology—vanishing yield stress and decreasing viscosity and elastic properties. The relative zero-shear viscosity and the plateau modulus at different temperatures superpose to mastercurves when plotted vs the effective volume fraction. The monodisperse hydrogel particles form colloidal crystals and glasses in concentrated solution but at higher effective volume fractions as compared to model hard sphere suspensions. Comparison of the experimental freezing point with soft sphere computer simulations indicate a repulsive interaction potential of the order 1/r12. The frequency independent shear modulus exhibited a power law concentration dependence which also agrees with the soft sphere behavior.
Reverse saturable absorption in C60-doped porous glasses studied by single- and double-pulse pump–probe experiments
Authors: Schell J. , Ohlmann D. , Brinkmann D. , Lévy R. , Joucla M. , Rehspringer J. L. , Hönerlage B.
Published: July 2002

We investigate the nonlinear absorption of C60-doped porous sol-gel glasses by single- and double-pulse pump–probe experiments. We find that the reverse saturable absorption (RSA) of these samples can be explained in the frame of a five level system as it is commonly used for C60 solutions. We observe a strong saturation of the RSA at high fluences, especially if the molecules are prepared in the triplet state. In a double-pulse pump–probe experiment we measure the triplet quantum yield for C60 solutions (0.8) and doped glasses (0.25) and the singlet and triplet absorption cross sections. In the first excited singlet state (S1) lifetimes of 1 ns and 65 ps are determined in solutions and glasses, respectively. We find that the dynamics of both depopulation processes, direct S1–S0 relaxation, and intersystem crossing is faster in glasses than in the case where C60 is in solution in a liquid aromatic solvent. We tentatively explain these findings by the absence of a stabilizing solvent and a perturbation of the molecular energy levels due to interaction with the solid glass matrix. We finally determine the lifetime of the triplet states in our glass samples to 2 μs.