bluuebunny/crossref_metadata_2025 huggingface dataset:
Step structure and ordering in Zn-doped GaInP
Authors: Lee S. H. , Fetzer C. M. , Stringfellow G. B. , Choi C.-J. , Seong T. Y.
Published: July 2002

GaInP grown on (001) substrates by organometallic vapor phase epitaxy is typically highly ordered. The driving force is due to the [1̄10] oriented P dimers on the surface. There are apparently additional kinetic factors related to surface steps that also play a key role in the ordering mechanism. However, the mechanism remains undetermined. This work presents the effects of Zn on the step structure and ordering during epitaxial growth. The degree of order is estimated from the low temperature photoluminescence peak energy to be approximately 0.5 for undoped epitaxial layers and the layers are completely disordered at Zn doping concentrations [from dimethyzinc (DMZn) addition to the system] of >1.7×1018 cm−3. This is verified by transmission electron diffraction results. As a consequence, the band gap energy increases by 110 meV as the Zn doping level is increased from 3×1017 to 1.7×1018 cm−3. The [1̄10] and [110]-step spacing as well as the root-mean-square roughness are found to be unchanged over the range of doping that produces disordering for both singular (001) and vicinal substrates. This indicates the disordering mechanism induced by Zn does not involve the step edge adatom attachment kinetics as previously reported for Te. The disordering is believed to be caused by the intermixing of Ga and In due to the increase in diffusion coefficient caused by the introduction of Zn. Modulation of the DMZn flow rate during growth has been used to grow heterostructures and quantum wells. No well boundaries were observed by transmission electron microscopy for thin wells, although both ordered and disordered regions are observed in 50 nm “wells.” This is believed to result from Zn diffusion between the layers during growth.
Band-gap renormalization and band filling in Si-doped GaN films studied by photoluminescence spectroscopy
Authors: Yoshikawa M. , Kunzer M. , Wagner J. , Obloh H. , Schlotter P. , Schmidt R. , Herres N. , Kaufmann U.
Published: July 2002

We have studied band-gap renormalization and band filling in Si-doped GaN films with free-electron concentrations up to 1.7×1019 cm−3, using temperature-dependent photoluminescence (PL) spectroscopy. The low-temperature (2 K) PL spectra showed a line-shape characteristic for momentum nonconserving band-to-band recombination. The energy downshift of the low-energy edge of the PL line with increasing electron concentration n, which is attributed to band-gap renormalization (BGR) effects, could be fitted by a n1/3 power law with a BGR coefficient of −4.7×10−8 eV cm. The peak energy of the room-temperature band-to-band photoluminescence spectrum was found to decrease as the carrier concentration increases up to about 7×1018 cm−3, followed by a high-energy shift upon further increasing carrier concentration, due to the interplay between the BGR effects and band filling. The room-temperature PL linewidth showed a monotonic increase with carrier concentration, which could be described by a n2/3 power-law dependence.
Charge transfer and hybridization effects in Ni3Al and Ni3Ga studies by x-ray-absorption spectroscopy and theoretical calculations
Authors: Chang Y. K. , Lin K. P. , Pong W. F. , Tsai M.-H. , Hseih H. H. , Pieh J. Y. , Tseng P. K. , Lee J. F. , Hsu L. S.
Published: July 2002

This work investigates the charge transfer and Al(Ga) p-Ni d hybridization effects in the intermetallic Ni3Al (Ni3Ga) alloy using the Ni L3,2 and K edge and Al (Ga) K x-ray absorption near edge structure (XANES) measurements. We find that the intensity of near-edge features at the Ni L3 edge in the Ni3Al (Ni3Ga) alloy decreased with respect to that of pure Ni, which implies a reduction of the number of unoccupied Ni 3d states and an enhancement of the Ni 3d state filling in the Ni3Al (Ni3Ga) alloy. Two clear features are also observed in the Ni3Al (Ni3Ga) XANES spectrum at the Al (Ga) K edge, which can be assigned to unoccupied Al 3p-(Ga 4p-) derived states in Ni3Al (Ni3Ga). The threshold at the Al K-edge XANES for Ni3Al shifts towards the higher photon energy relative to that of pure Al, suggesting that Al loses some p-orbital charge upon forming Ni3Al. On the other hand, the Ni K edge shifts towards the lower photon energy in Ni3Al (Ni3Ga) relative to that of pure Ni, suggesting a gain of charge at the Ni site. Thus both Al and Ni K-edge XANES results imply a transfer of charge from Al 3p orbital to Ni sites. Our theoretical calculations using the spin-polarized first-principles pseudofunction method agree with these results.
Neutron diffraction study of mechanically alloyed and in situ annealed Al75Mo25 powders
Authors: Enzo S. , Frattini R. , Canton P. , Monagheddu M. , Delogu F.
Published: July 2002

The mechanical treatment of a Al75Mo25 mixture of pure elemental powders in a high-energy mixer mill induces partial solid state reactivity and solid solution formation. This is suggested by a quantitative phase evaluation and by the changes of Al lattice parameter carried out with Rietveld analysis of diffraction patterns collected after extended time of processing (4, 8, 16, 32, 57, and 78 h). The small angle neutron scattering experiments showed that diffusion processes are the rate-controlling step for these solid state reactions. The formation of metastable phases is confirmed by in situ neutron diffraction annealing experiments on selected as-milled powders. In the specimen mechanically alloyed for 4 h, Al12Mo phase is formed at 590 °C (below the aluminum melting point). Soon after this temperature, the remaining Al is consumed to form the Al8Mo3 phase. In the alloy mechanically treated for 32 h, the Al12Mo phase appears after annealing at 430 °C, while the Al8Mo3 phase is found at 493 °C. After the disappearance of Al12Mo phase (500 °C), the solid state reaction proceeds to form a new tetragonal Al3Mo phase, not reported in the equilibrium phase diagram. In the specimen mechanically treated for 57 h, the total formation of Al8Mo3 is recorded at 430 °C. Conversely, in the powder ball milled for 78 h, the Al8Mo3 phase appears at 390 °C and coexists with tetragonal Al3Mo.
Disordered structure of D2O ice VII from i
n s
i
t
u neutron powder diffraction
Authors: Jorgensen James D. , Worlton Thomas G.
Published: July 2002

Neutron powder diffraction data have been collected for the ice VII phase of D2O at 26 kbar and 22 °C by the time-of-flight technique. The data confirm the cubic Pn3m space group with completely disordered hydrogen bonding. Rietveld analysis of the data to obtain a bond length and bond angle for the D2O molecule is hampered by combined static and thermal disorder of both the deuterium and oxygen atom positions. However, simple models give results which are consistent with the D2O molecule parameters for other ice structures.
Dirac scattered-wave calculations for Ag2+3, Auq+3, and Auq+4 (q=1, 2) clusters
Authors: Arratia-Perez Ramiro , Malli Gulzari L.
Published: July 2002

Dirac scattered-wave (DSW) calculations are presented for the Ag2+3, Au+3, Au2+3, Au+4, and Au2+4 clusters. The results show that relativistic effects in bonding are not negligible for the silver cluster; whereas for the gold clusters these are very significant and lead to appreciable s–d hybridization in the bonding molecular orbitals. Zeeman and hyperfine tensors have been calculated for the Ag2+3, which are in very good agreement with the experimental results. These tensors are also predicted for the Au2+3 cluster. First ionization potentials and excitation energies are predicted for all these cationic clusters using the spin-restricted transition state method. Contour diagrams which clearly reveal the significant relativistic effects in bonding are also presented.
Theoretical study of the diatomic alkali and alkaline-earth oxides
Authors: Langhoff Stephen R. , Bauschlicher Charles W. , Partridge Harry
Published: July 2002

A b initio calculations at the self-consistent field and singles plus doubles configuration-interaction level are used to determine accurate spectroscopic parameters (De,re,ωe) for the 2Π and 2∑+ states of the alkali oxides and the a 3Π (or A 1Π) states of the alkaline-earth oxides. Numerical Hartree–Fock (NHF) calculations performed on KO demonstrate that the extended Slater basis sets employed are near the Hartree–Fock limit. When the dissociation is referenced to the ionic limits, the differential correlation contribution is found to increase the D0 by a relatively small amount (0.0–0.2 eV). An accurate description of the alkali and alkaline-earth oxides requires correlating both the oxygen and metal electrons. The theoretical dissociation energies (D0) permit a critical assessment of the experimental literature and allow us to recommend D0 values that are accurate to 0.1 eV for all systems considered. There is a strong correlation between the dissociation energy (to ions) and re, because the bonding is predominantly electrostatic in origin. Theoretical 2Π–2∑+ energy separations are presented for the alkali oxides. An extensive study of the 2Π–2∑+ energy separation in KO reveals a 2∑+ ground state at all levels of theory. Basis set studies in combination with NHF calculations indicate different basis set requirements for the 2Π and 2∑+ states. In the NHF limit the 2∑+ state of KO is lower by about 250 cm−1. The separation is almost unaffected when the 15 valence electrons are correlated at the singles plus doubles level using an extended Slater basis.
X-ray diffraction study of liquid propane at 92 and 228 K
Authors: Habenschuss A. , Narten A. H.
Published: July 2002

X-ray diffraction data for liquid propane near the triple and boiling points are presented. The data are analyzed using scattering factors for –CH3 and –CH2 groups which permit extraction of intermolecular structure and distribution functions for these carbon-centered sites. The radial distribution functions indicate a qualitative structural change in the liquid from the low to the high temperature states.
Quadrupole echo distortion as a tool for dynamic NMR: Application to molecular reorientation in solid trimethylamine
Authors: Vega A. J. , Luz Z.
Published: July 2002

Deuterium NMR quadrupole echo spectra of trimethylamine-d9 (TMA-d9) were recorded in the solid phase from about −150 °C to the melting point (−117 °C). The spectra exhibit pronounced line shape variation with temperature which is interpreted in terms of two independent dynamic processes, (i) threefold methyl group reorientation about the respective C3 axes, and (ii) threefold whole molecule reorientation about the molecular C′3 axis. To derive quantitative kinetic parameters for these processes the experimental spectra are compared with simulated traces, placing particular emphasis on the effect of line shape distortion due to the delay time τ between the π/2 pulses in the quadrupole echo experiment. This approach improves the sensitivity of the fitting method and allows accurate determination of the kinetic parameters for both the C3 and C3 reorientation processes. The results are k(−150 °C)=3.0×104 s−1, k′(−150 °C)=0.1×104 s−1, ΔE=3.0±0.6 kcal/mol, and ΔE′=8.0±1.6 kcal/mol, respectively. In Appendix A a convenient procedure is described for calculating spin dynamics in I=1 systems and applied to a number of cases. In Appendix B the approach of Appendix A is applied to compute dynamic line shapes in the quadrupole echo experiment.
A study of nonrigid aromatic molecules by supersonic molecular jet spectroscopy. II. Propyltoluenes
Authors: Breen P. J. , Warren J. A. , Bernstein E. R. , Seeman Jeffrey I.
Published: July 2002

Dispersed emission (DE) and time of flight mass spectra (TOFMS) are presented for supersonic molecular jet-cooled o-, m-, and p-n-propyltoluene. The spectra exhibit multiple origins which are assigned to anti and gauche conformations of the propyl group relative to the aromatic ring. The TOFMS of m-n-propyltoluene rules out an eclipsed propyl conformation as a populated conformation. Empirical force field (EFF) calculations are presented which support these assignments. The spectra also exhibit features, typically within 100 cm−1 of the origin, which are assigned to transitions associated with the internal rotation of the ring methyl group. A model which treats this methyl motion as a one-dimensional rigid rotor is used to solve for the rotational constant B of the methyl rotor, and for the size and shape of the barrier to rotation. The barrier for p-n-propyltoluene is found to be small in both the ground (S0−V6=5 cm−1) and excited (S1−V6=20 cm−1) electronic states. For m-n-propyltoluene, the ground state is again found to have a low barrier (V6=23 cm−1), but the excited state has a potential barrier of V3=75 cm−1. The barrier for o-n-propyltoluene in the ground

state is observed to be higher (V3=64 cm−1) than for the other two isomers. The first excited electronic state of this latter compound displays a multitude of levels in the TOFMS which are explained by a double-rotor model involving the ring methyl group (V3=72 cm−1, V6=−14 cm−1) and terminal methyl group of the propyl chain (V3=106 cm−1, V6=−13 cm−1). The results are indicative of an interaction between the ring methyl group and the propyl chain. In all three n-propyltoluene isomers, more than one origin peak is present in the TOFMS, indicating the presence of multiple stable propyl conformations for each isomer. The propyltoluenes thus demonstrate two types of nonrigid molecular behavior: internal rotational motion associated with the ring methyl group, and potentially three stable conformations of the propyl chain with respect to the aromatic ring.
Anomalous resonance enhanced quasielastic light scattering of cytochrome-c
Authors: Reinisch L. , Schomacker K. T. , Champion P. M.
Published: July 2002

We have measured anomalously large resonance enhanced quasi-elastic light scattering from reduced cytochrome-c using excitation frequencies throughout the ‘‘Soret’’ absorption band. The resonance enhancement is clearly observed in the perpendicularly polarized scattering. The parallel polarized scattering is consistent with a depolarization ratio near 1/8 for the resonant scattering from the heme group. In addition, the parallel data contain the expected large nonresonant scattering due to the globular protein material and solvent. Both of these contributions are extracted directly from the data and agree with other, independent, experimental observations. Absolute cross section measurements reveal that the resonance Rayleigh scattering is three times larger than predictions based on Kramers–Kronig dispersion theory. Interference with nonresonant background states can lead to increased scattering intensity, but is incompatible with the experimental excitation profile. Previous studies of the absorption line shape at low temperature indicate that very low frequency modes are not significantly coupled to the Soret excitation. Thus, very low frequency, nonresolved resonance Raman processes are not a likely explanation for the observed excess Rayleigh scattering. A distribution in the electronic relaxation of the resonant state can possibly account for some of the discrepancy.
Photochemical hole burning of phthalocyanine in polymer glasses: Thermal cycling and spectral diffusion
Authors: Schulte G. , Grond W. , Haarer D. , Silbey R.
Published: July 2002

In this paper we report an analysis of hole burning line shapes yielding both reversible and irreversible contributions to the linewidth which are due to spectral diffusion. These terms show a linear T dependence and can, through cycling experiments, be investigated in the temperature range of 0.5<T<25 K. The range 1<T<25 K is particularly interesting because, under normal conditions, Debye contributions and local mode contributions overwhelm the small spectral diffusion terms at these temperatures. Comparison between optical data and specific heat data supports our model of spectral diffusion.
Monte Carlo simulation of hard spheres near random closest packing using spherical boundary conditions
Authors: Tobochnik Jan , Chapin Phillip M.
Published: July 2002

Monte Carlo simulations were performed for hard disks on the surface of an ordinary sphere and hard spheres on the surface of a four-dimensional hypersphere. Starting from the low density fluid the density was increased to obtain metastable amorphous states at densities higher than previously achieved. Above the freezing density the inverse pressure decreases linearly with density, reaching zero at packing fractions equal to 68% for hard spheres and 84% for hard disks. Using these new estimates for random closest packing and coefficients from the virial series we obtain an equation of state which fits all the data up to random closest packing. Usually, the radial distribution function showed the typical split second peak characteristic of amorphous solids and glasses. High density systems which lacked this split second peak and showed other sharp peaks were interpreted as signaling the onset of crystal nucleation.
Distinguishing liquids from amorphous solids: Percolation analysis on the Voronoi network
Authors: Medvedev Nikolai N. , Geiger Alfons , Brostow Witold
Published: July 2002

The mutual arrangement of the Delaunay simplices (configurations of four nearest atoms) has been studied for molecular dynamic (MD) models of liquid and quenched rubidium obtained by M. Tanaka [J. Phys. Soc. Jpn. 55, 3108 (1986)]. The Delaunay simplices with large circumradii and low local density of atoms, the simplices with small circumradii representing dense atomic configurations, and the simplices close in shape to perfect tetrahedron representing ‘‘rigid’’ arrangements of atomic quadruplets were delineated. The Delaunay simplices form clusters; consideration of the latter constitutes a site percolation problem on the Voronoi network [N. N. Medvedev, V. P. Voloshin, and Yu. I. Naberukhin, J. Phys. A: Math. Gen. 21, L247 (1986)]. Analysis of the MD results in these terms shows that low density atomic configurations in the liquid phase form a percolative cluster. Such a cluster does not occur in a solid phase. On the contrary, there is a percolative cluster in the solid sample, but formed by high density configurations which are nearly tetrahedral.
Pressure tensor of a spherical interface
Authors: Blokhuis Edgar M. , Bedeaux Dick
Published: July 2002

In this paper we show how the use of the Irving–Kirkwood expression for the pressure tensor leads to expressions for the pressure difference, the surface tension of the flat interface, and the Tolman length which agree with the expressions found using microscopic sum rules. The use of the Schofield–Henderson expression for the pressure tensor for a particular contour different from the contour that leads to the Irving–Kirkwood expression is found to give incorrect results for the pressure difference and, in particular, also for the Tolman length. The distance between the so-called mechanical surface of tension and the Gibbs dividing surface is found not to be given by Tolman’s length. Using an approximate expression for the pair density it is possible to find values for the location of the mechanical surface of tension and for Tolman’s length which are in reasonably good agreement with values found by Nijmeijer et al. in molecular dynamics simulations.
Rotational spectra, structure, Kr-83 nuclear quadrupole coupling constant, and the dipole moment of the Kr-benzene dimer
Authors: Klots T. D. , Emilsson T. , Gutowsky H. S.
Published: July 2002

Rotational spectra were observed for the Kr-82, Kr-83, Kr-84, and Kr-86 isotopic species of the Kr-benzene dimer with the Mark II Balle-Flygare Fourier transform microwave spectrometer. The spectra are those of a symmetric top, with the Kr on the symmetry axis of the benzene. In the most abundant Kr-84 benzene dimer the rotational constants B0, DJ, and DJK were found to be 795.6821(1) MHz, 1.315(1) kHz, and 7.895(4) kHz, respectively. R, the Kr to benzene center-of-mass distance, is 3.663 Å and the zero-point vibrational bending of the benzene with respect to R gives an average angular displacement of 16.6°. The Kr-83 quadrupole coupling constant was determined experimentally to be −5.201(5) MHz and the dipole moment, 0.136(2) D. A theoretical prediction of these electrically induced properties gives results for a distributed multipole model which are superior to those using central multipoles.
The photoionization and dissociation dynamics of energy-selected acetylene dimers, trimers, and tetramers
Authors: Booze Jon A. , Baer Tomas
Published: July 2002

The photoionization and dissociation characteristics of (C2H2)n (n=2, 3, and 4) have been studied by the threshold photoelectron–photoion coincidence technique applied to a molecular beam. For photon energies between 10.20 and 11.27 eV, only dissociative ionization pathways were observed for the clusters; no direct ionization of (C2H2)2, (C2H2)3, and (C2H2)4 to the stable ions C4H4+, C6H6+, and C8H8+ was observed. The shape of the time of flight peaks is consistent with statistical dissociation dynamics, and statistical analysis of the fragment peak shapes suggests that the dimer, trimer, and tetramer produce the fragments C4H2+/C4H3+, C4H4+, and C6H6+, respectively. Consistent with these observations, ab initio calculations are presented which indicate that the neutral T-shaped dimer and triangular trimer geometries are unstable on the ionic potential energy surface, and indeed have vertical ionization energies well above the dissociation limits of many of their possible product channels. The statistical modeling of the C4H4+ peak width is consistent with a process in which the trimer neutral produces a C4H4+–C2H2 complex, which then dissociates via monomer evaporation without exploring any of the covalent C6H6+ potential energy surface, i.e., benzene ion is not formed. The heat of formation of the observed C4H4+ daughter is estimated to be 1240±15 kJ/mol, indicating that it is cyclobutadiene, butatriene, or vinylacetylene ion. The peak width of C6H6+ is also analyzed, and suggests a process whereby the tetramer produces a C6H6+–C2H2 complex, which dissociates via monomer evaporation without exploring any of the covalent C8H8+ potential energy surface. The heat of formation of the C6H6+ daughter fragment is estimated as 1160±15 kJ/mol, which is consistent with the formation of benzvalene ion.
Quasiclassical trajectory study of collisional energy transfer in toluene systems. II. Helium bath gas: Energy and temperature dependences, and angular momentum transfer
Authors: Lim Kieran F.
Published: July 2002

The collisional deactivation of highly vibrationally excited toluene-d0 and toluene-d8 by helium bath gas has been investigated using quasiclassical trajectory simulations. Collisional energy transfer was found to increase with initial toluene internal energy, in agreement with the experiments of Toselli and Barker [J. Chem. Phys. 97, 1809 (1992), and references therein]. The temperature dependence of 〈ΔE2〉1/2 is predicted to be T(0.44±0.10), in agreement with the experiments of Heymann, Hippler, and Troe [J. Chem. Phys. 80, 1853 (1984)]. Toluene is found to have no net angular-momentum (rotational-energy) transfer to helium bath gas, although 〈ΔJ2〉1/2 has a temperature dependence of T(0.31±0.07). Re-evaluation of earlier calculations [‘‘Paper I:’’ Lim, J. Chem. Phys. 100, 7385 (1994)] found that rotational energy transfer could be induced by increasing the mass of the collider, or by increasing the strength of the intermolecular interaction: in these cases, angular-momentum transfer depended on the initial excitation energy. In all cases, the final rotational distributions remained Boltzmann.
Electron conduction in molecular wires. I. A scattering formalism
Authors: Mujica V. , Kemp M. , Ratner M. A.
Published: July 2002

We extend a model originally intended for the description of the scanning tunneling microscope (STM) current in molecular imaging of one-dimensional systems, to encompass the more general process of electron transfer between two reservoirs of states. In the STM problem, the reservoirs are naturally associated with the metal density of states of the electrodes. In the molecular electron transfer problem, the identification of the reservoirs with the Franck–Condon weighted density of vibrational states allows a number of fruitful connections with the theory of nonadiabatic electron transfer (ET) in molecules to be established. In this article, we present an exact procedure, based on Löwdin’s partitioning technique, to determine the Green’s function and the T matrix, relevant to the transport process. We obtain compact expressions for the conductance and the density of states in the limit of small applied voltage and low temperature, and discuss the important case where the molecular wire is described by a tight-binding Hamiltonian. Finally, we discuss some physical implications of the model.
Fluctuations, temperature, and detailed balance in classical nucleation theory
Authors: McGraw Robert , LaViolette Randall A.
Published: July 2002

The role of temperature in classical nucleation theory is examined. It is shown that while even small clusters are assigned a temperature in the classical theory, this must be a fluctuating quantity. Stochastic simulations of cluster evaporation and growth are presented to track the temperature fluctuations in time. The relation 〈‖δT‖2〉=kT20/Cν for the mean square temperature fluctuation is confirmed, where k is the Boltzmann constant, Cν is the cluster heat capacity, and T0 is the bath temperature. For small capillary drops (50–100 molecules), the resulting rms temperature fluctuations of 10°–20° might be expected to have a significant effect on the nucleation rate. However, the simulations reveal a cluster temperature distribution that is centered several degrees below T0. A theory is presented to explain this effect. To first order, which includes Gaussian fluctuations of the cluster temperature T, we find that the effective temperature for cluster evaporation is T−h/2Cν, where h is the latent heat. This temperature correction is precisely that required by detailed balance and results both in a centering of the cluster temperature distribution on T0 and a cancellation of any significant effect of temperature fluctuations on the nucleation rate.
Adsorption and reaction of CH3COOH and CD3COOD on the MgO(100) surface: A Fourier transform infrared and temperature programmed desorption study
Authors: Xu Chen , Koel Bruce E.
Published: July 2002

The adsorption of acetic acid (CH3COOH and CD3COOD) on the (100) surface of a MgO single crystal has been studied using primarily Fourier transform infrared transmission absorption spectroscopy and temperature programmed desorption (TPD). Acetic acid dissociates upon adsorption on this surface, even at 120 K, forming an adsorbed acetate (CH3COO) species. Upon heating in TPD, some of the acetate recombines with surface hydrogen and desorbs as acetic acid but most undergoes a disproportionation reaction to form acetic acid and ketene (CH2CO) products which desorb. The IR intensities of the vibrational modes of chemisorbed acetate over the temperature range of 425–720 K show a strong polarization dependence on the incident IR light indicating an oriented, monodentate adsorbed species. Consideration of the vibrational frequencies of this species reveals new insight concerning the structure and bonding of the acetate radical on the MgO(100) surface and supports our proposal of a novel bimolecular surface reaction between two adsorbed acetate radicals to form ketene and water.
Direct observation of polymer gel surfaces by atomic force microscopy
Authors: Suzuki A. , Yamazaki M. , Kobiki Y.
Published: July 2002

We report here, for the first time, the direct observation of the submicron structure of gel surfaces in water by using an atomic force microscope (AFM). We present also its change in response to external stimuli; we investigated, among the variables that affect the topography of the gel surface, the effect of the network density of poly(acrylamide) gels and the effect of the temperature change of poly N-isopropylacrylamide gels. Gels were prepared with disklike shape of thickness ranging from 10 to 50 μm, and one of the gel surfaces was chemically adhered onto a glass plate. Spongelike domains of submicrometer scale were found here on the gel surfaces, which was strongly affected by the cross-linking density (nature of the gel network) as well as the osmotic pressure (environmental condition), and also thickness (condition of constraint). The qualitative properties of the surface microscopic structure of gels are discussed in relation to a hypothetical model of two-dimensional gels based on the Flory–Huggins theory. These results disclose that the surface microstructures of polymer gels in solvent as well as the nanometer scale structural changes are associated with the gel phase transition. Moreover, they indicate that the potential for a new technology to control the domain size of the gel surface as well as its function by external stimuli could emerge, which would find a variety of applications in many fields, such as engineering, medicine, and biology.
Density functional calculation for doubly excited autoionizing states of helium atom
Authors: Singh Ranbir , Deb B. M.
Published: July 2002

Several doubly excited autoionizing states of He have been calculated within the density functional framework by employing the Harbola–Sahni exchange potential. Correlation effects have been incorporated in the total effective potential through a Wigner-type correlation potential. Although continuum functions are not explicitly incorporated into these calculations, resonance energies of these states are in satisfactory agreement with other theoretical results.
Rotationally resolved spectrum of the C 2Πu←X 2Πg electronic transition of C−4 via resonant two-photon detachment spectroscopy
Authors: Zhao Yuexing , de Beer Esther , Neumark Daniel M.
Published: July 2002

The C 2Πu←X 2Πg electronic transition of C−4 has been studied by both one-color and two-color resonant two-photon detachment (R2PD) spectroscopy. The one-color spectrum reveals vibrational structure in the excited anion state. Transitions due to excitations in one of the symmetric stretching modes as well as the bending modes are observed. Spectral resolution in the one-color experiment is limited by power broadening; using two-color R2PD, rotationally resolved spectra of the origin and 210 bands of the C 2Πu←X 2Πg transition are obtained. Molecular constants determined by fitting the rotationally resolved spectra are generally in good agreement with a recent ab initio calculation by Schmatz and Botschwina [Int. J. Mass. Spec. Ion. Proc. 149, 621 (1995)]. Perturbations in the 210 band are attributed to Fermi resonance interactions in the C 2Πu state.
Spectroscopy and electron detachment dynamics of C−4, C−6, and C−8
Authors: Zhao Yuexing , de Beer Esther , Xu Cangshan , Taylor Travis , Neumark Daniel M.
Published: July 2002

Resonant multiphoton detachment spectroscopy has been used to obtain vibrationally resolved spectra of the C 2Π←X 2Π electronic transitions in C−4, C−6, and C−8. Transitions due to vibrational excitations in the totally symmetric stretching modes as well as the bending modes are observed. The electron detachment dynamics subsequent to multiphoton absorption are studied by measuring the electron emission time profiles and electron kinetic energy distributions. The observation of delayed electron emission combined with the form of the electron kinetic energy distributions indicates that these species undergo the cluster equivalent of thermionic emission. This interpretation is supported by comparing the experimental results to a microcanonical model for cluster thermionic emission.
Two-dimensional Raman spectroscopy of the intermolecular modes of liquid CS2
Authors: Tokmakoff A. , Fleming G. R.
Published: July 2002

The two-dimensional response of the intermolecular modes of CS2 has been measured using nonresonant fifth order Raman spectroscopy. Whereas third order Raman spectroscopies only observe the spatially averaged spectrum of solvent motions, the fifth order experiment allows the microscopic composition of this spectral density to be probed. Within the limitations of the theory, the results confirm a picture of the liquid CS2 in which dynamics are dictated by a distribution of fixed structures at short times (<300 fs), and by spatially uniform structural relaxation at long times (>700 fs). The data suggest that the microscopic dynamics show highly damped oscillatory behavior at intermediate times. The full sensitivity to the details of the molecular dynamics is obtained by virtue of the full two-dimensional response. The ability to make such observations has important general implications for understanding the broad range of time scales in the study of dynamics in complex condensed phases.
Mixing of Rydberg states induced by interaction with moving ion
Authors: Dorofeev D. L. , Zon B. A.
Published: July 2002

Interaction between the Rydberg electron and a moving ion in the presence of a stray dc electric field is considered. Due to m-mixing and changes in time dynamics of l-mixing, slow thermal motion of the ion has a dramatic effect on the time evolution of the Rydberg electron wave function. This effect can significantly increase the lifetime of the Rydberg level.
Very large amplitude intermolecular vibrations and wave function delocalization in 2,3-dimethylnaphthalene⋅He van der Waals complex
Authors: Bach Andreas , Leutwyler Samuel , Sabo Dubravko , Bačić Zlatko
Published: July 2002

We report a combined experimental and theoretical study of the intermolecular vibrations and van der Waals isomerism of the 2,3-dimethylnaphthalene⋅He van der Waals complex. Two-color resonant two-photon ionization spectra of the S0→S1 electronic transition of 2,3-dimethylnaphthalene⋅He exhibit five bands within 30 cm−1 of the electronic origin. The intermolecular potential energy surface was modeled as a sum of atom–atom Lennard-Jones pair potentials; it exhibits two equivalent global minima on each side of the naphthalene moiety, and a single shallower local minimum adjacent to the two methyl groups. Based on this surface, accurate three-dimensional quantum calculations of the van der Waals vibrational levels using the discrete variable representation method were performed. Careful optimization of the potential parameters lead to a quantitative reproduction of four observed bands as intermolecular vibrational excitations, a vibrationally averaged He atom distance from the aromatic plane 〈z0〉=3.22 Å, and a dissociation energy D0(S1)=−60.3 cm−1, compatible with experiments. The fifth band is assigned as a van der Waals isomer, corresponding to the local minimum. The quantum calculations were extended up to the dissociation limit, yielding ≈173 van der Waals vibrational states. Above 70% of D0, many vibrational states are completely delocalized over the potential surface, with root-mean-square vibrational amplitudes up to 6 Å parallel to and up to 1.5 Å perpendicular to the molecular surface. Calculated tunnelling splittings range from <10−4 cm−1 for localized states, to >3 cm−1 for highly delocalized ones.
The solvation of ions in acetonitrile and acetone: A molecular Ornstein–Zernike study
Authors: Richardi J. , Fries P. H. , Krienke H.
Published: July 2002

The solvation of alkali and halide ions in acetonitrile and acetone has been investigated via the molecular Ornstein–Zernike theory using the hypernetted chain approximation. Theoretical Gibbs solvation energies and solvation numbers are compared with experiments and numerical simulations. The calculated single-ion solvation energies are used to check the hypotheses serving to split-up the measured solvation energies of salts into their single-ion components. The solvation structure around the ions is discussed in detail and shown to be strongly influenced by the solvent–solvent spatial correlations. The calculated interionic potentials of mean force are presented and used to compute ion-ion association constants which are compared with experiment. The influence of the Lennard-Jones parameters of the ions upon the calculated properties is emphasized.
Primitive models of chemical association. III. Totally flexible sticky two-point model for multicomponent heteronuclear fixed-chain-length polymerization
Authors: Lin C.-T. , Kalyuzhnyi Yu. V. , Stell G.
Published: July 2002

A multidensity integral-equation theory for polymerization into freely jointed hard-sphere homonuclear chain fluids proposed earlier [J. Chem. Phys. 106, 1940 (1997)] is extended to the case of multicomponent heteronuclear chain polymerization. The theory is based on the analytical solution of the polymer Percus–Yevick (PPY) approximation for the totally flexible sticky two-point (S2P) model of associating fluids. The model consists of an n-component mixture of hard spheres of different sizes with species 2,…,n−1 bearing two sticky sites A and B, randomly distributed on its surface, and species 1 and n with only one B and A site per particle, respectively. Due to some specific restrictions imposed on the possibility of forming bonds between particles of various species, the present version of the S2P model represents an associating fluid that is able to polymerize into a mixture of heteronuclear chain macromolecules. The structural properties of such a model are studied in the complete-association limit and compared with computer-simulation results for homonuclear hard-sphere chain mixtures, symmetrical diblock copolymers, alternating copolymers, and homonuclear hard-sphere chains in a hard-sphere solvent. Some results for the case of partial association are also presented. The PPY theory represents a quantitatively successful theory for the mixtures of short homonuclear chains and the short copolymer systems studied here. We also expect that the theory will prove to be of the same order of accuracy in investigating the case of partial association.
Low-energy electron capture in group 14 methyl chlorides and tetrachlorides: Electron transmission and dissociative electron attachment spectra and MS-Xα calculations
Authors: Modelli Alberto , Guerra Maurizio , Jones Derek , Distefano Giuseppe , Tronc Michel
Published: July 2002

The electron transmission and dissociative electron attachment spectra in the series of compounds (CH3)4−nMCln, with M=C, Si, Ge, and Sn, are presented (except for the di- and trichloro germanium derivatives). The experimental resonance energies for temporary electron attachment to these compounds are compared with the total electron scattering cross sections obtained by means of continuum multiple-scattering (MS) Xα calculations and with the attachment energies calculated with the bound-state MS-Xα method, which also evaluates positive electron affinity values and provides the localization properties of the various empty orbitals. For all the compounds considered the LUMO is predicted to possess a1 symmetry. The silicon and tin derivatives display the smallest and largest electron–acceptor properties, respectively. The measurements of the fragment anion current, as a function of the incident electron energy, reveal quite different behaviors in the various compounds with respect to dissociative electron attachment from both the quantitative and qualitative points of view.
Ultrafast photodissociation dynamics of the S1 and S2 states of acetone
Authors: Owrutsky J. C. , Baronavski A. P.
Published: July 2002

The photodissociation dynamics for the two lowest excited electronic states (S1 valence state and the S2, 3s Rydberg state) of acetone (h6 and d6) have been studied using femtosecond mass-resolved photoionization spectroscopy. The S1 state dynamics was investigated by near ultraviolet (UV) pump (∼265 nm) and deep UV (205 nm) or visible (410 nm) probe. The primary dissociation time is instrument-limited, providing a 200 fs upper limit to the lifetime. The acetyl ion signal exhibits a subpicosecond decay and a persistent signal. The fast decay is consistent with results from Kim et al. [J. Chem. Phys. 103, 477 (1995)] for two-photon excitation to near the 4s state. The persistent signal is due to probe-induced ionization of acetyl radicals that are stable with respect to secondary dissociation. The S2 excited state lifetime measured for acetone-d6 using 194 pump and 259 nm probe is 13.5±1.0 ps. This is almost three times longer than we previously determined for this state in acetone-h6, 4.7±0.2 ps. The secondary dissociation time for acetyl-d3 measured with two-photon ionization probe at 388 nm is 3.0±1.0 ps. This is the same (within the experimental uncertainty) as our result for acetyl-h3 (3.1±0.5 ps), so that there is no apparent isotope effect. The calculated RRKM (Rice–Ramsperger–Kassel–Marcus) rate, however, is significantly faster for acetyl-h3 at the same internal energy, so that the isotopic dependence of the rate deviates from the RRKM predictions. Consequently, there is either an isotope dependence in the energy partitioning for primary dissociation or a reverse isotope effect in the secondary dissociation, or both. In the latter, more likely case, this indicates that the secondary dissociation does not conform to a statistical, RRKM-type unimolecular dissociation.
Comparison of different water models from ambient to supercritical conditions: A Monte Carlo simulation and molecular Ornstein–Zernike study
Authors: Jedlovszky Pál , Richardi Johannes
Published: July 2002

Structural, thermodynamic, and dielectric properties of three polarizable and two nonpolarizable water models are compared with experimental data at four different thermodynamic states from ambient to supercritical conditions. Pair-correlation functions and thermodynamic data are obtained from Monte Carlo simulations, performed both on the (N,V,T) and (N,p,T) ensembles. The dielectric constants are determined with the molecular Ornstein–Zernike theory. It is found that the polarizable models can reproduce the experimental structure considerably better than the nonpolarizable ones at the high-temperature states. In particular, the elongation of the hydrogen bonds with increasing temperature, which is observed by neutron diffraction measurements, in only seen in the simulations with the polarizable potential models. On the other hand, the polarizable models fail to describe the correct temperature dependence of the thermodynamic properties. Although at ambient conditions they overestimate both the density and the dielectric constant of the system, around the critical temperature they result in 10%–50% lower densities than the experimental values. The obtained magnitude of the internal energies as well as the dielectric constants are also considerably smaller than their experimental values at these thermodynamic state points. The results of this study point out the need of new polarizable water models which, besides the reasonable reproduction of the experimental pair-correlation functions, are also able to describe the dependence of the thermodynamic properties on the temperature and pressure.
A scaling approximation for structure factors in the integral equation theory of polydisperse nonionic colloidal fluids
Authors: Gazzillo Domenico , Giacometti Achille , Guido Della Valle Raffaele , Venuti Elisabetta , Carsughi Flavio
Published: July 2002

The integral equation theory of pure liquids, combined with a new “scaling approximation” based on a corresponding states treatment of pair correlation functions, is used to evaluate approximate structure factors for colloidal fluids constituted of uncharged particles with polydispersity in size and energy parameters. Both hard sphere and Lennard-Jones interactions are considered. For polydisperse hard spheres, the scaling approximation is compared to theories utilized by small angle scattering experimentalists (decoupling approximation and local monodisperse approximation) and to the van der Waals one-fluid theory. The results are tested against predictions from analytical expressions, exact within the Percus–Yevick approximation. For polydisperse Lennard-Jones particles, the scaling approximation, combined with a “modified hypernetted chain” integral equation, is tested against molecular dynamics data generated for the present work. Despite its simplicity, the scaling approximation exhibits a satisfactory performance for both potentials, and represents a considerable improvement over the above mentioned theories. Shortcomings of the proposed theory, its applicability to the analysis of experimental scattering data, and its possible extensions to different potentials are finally discussed.
Theoretical calculation of transition probability for N atom and ions
Authors: Zheng Nengwu , Wang Tao , Yang Ruyi , Wu Yonggang
Published: July 2002

The weakest bound electron potential model (WBEPM) theory is employed here to study the transition probability of N atom and ions. Some transition probabilities of allowed transitions are calculated. In the calculation, highly excited states are studied and spectral fine structure is also taken into consideration. Compared with the accepted values, results reported here are in excellent agreement with the accepted values, and the agreement is within experimental uncertainty, which is satisfying.
Freezing and melting of methanol in a single cylindrical pore: Dynamical supercooling and vitrification of methanol
Authors: Morishige Kunimitsu , Kawano Keizi
Published: July 2002

To study the freezing/melting behavior of a confined CH3OH, we performed x-ray diffraction measurements of CH3OH confined inside the cylindrical pores of seven kinds of regular mesoporous adsorbents (MCM-41 and SBA-15) with different pore radii (r=1.2, 2.1, 2.5, 3.9, 4.5, 5.3, and 7.0 nm) as a function of temperature. The freezing/melting behavior depends markedly upon the pore size. Within the pores of r⩽3.9 nm, the confined CH3OH vitrifies on freezing. On the other hand, cooling of the CH3OH confined to the pores of r⩾4.5 nm results in crystallization of the liquid. Within the pores of r=5.3 nm, the crystallization proceeds in two steps: prefreezing first occurs and then it transforms into a crystalline solid with the same structure as that of the bulk α phase. The prefreezing temperature seems to lower steeply with decreasing pore-size and to approach the freezing temperature for the pores of r=4.5 nm. Cooling of the CH3OH confined to the pores of r=7.0 nm results in formation of a crystalline solid with the same structure as that of the bulk β phase and it does not transform into the low temperature α phase on further cooling down to 30 K, leading to the appearance of a glassy crystal with the β phase structure. A large hysteresis effect between freezing and melting is observed. A mechanism of the vitrification is discussed.
An example of affine collineation in the Robertson–Walker metric
Authors: Bedran M. L. , Lesche B.
Published: July 2002

An affine collineation for the Robertson–Walker metric is found. It implies a condition on the metric that is compatible with Einstein’s equations for a perfect fluid satisfying the Hawking–Ellis energy conditions. It is shown how the geodesics of the metric are obtained from the constant of motion associated to the affine collineation.
Constraints, reduction, and quantization
Authors: Gotay Mark J.
Published: July 2002

Theorems are proved that establish the unitary equivalence of the extended and reduced phase space quantizations of a constrained classical system with symmetry. Several examples are presented.
Yang Baxterization
Authors: Li You-Quan
Published: July 2002

On the basis of the Yang–Baxter equation and the initial condition of Ř matrix, Yang Baxterization of representations of braid group and that of Temperley–Lieb algebra are discussed systematically. It is shown that there exist many Yang-Baxterization formulas beyond the knowns. New developments of Yang-Baxterization approach are presented. As examples some new solutions of the Yang–Baxter equation are given.