bluuebunny/crossref_metadata_2025 huggingface dataset:
Kinetic study of the reaction of atomic chlorine with hydrogen bromide
Authors: Rubin Rachamim , Persky Avigdor
Published: February 2003

The reaction Cl+HBr→HCl+Br was studied by a competitive method using the reaction Cl+C2H6→HCl+C2H5 as a reference reaction. Experiments to determine ratios of rate constants kCl+HBr/kCl+C2H6 in the temperature range 222–504 K were carried out in a fast flow system with mass spectrometric analysis of the reaction mixture. The results fit the Arrhenius expression: kCl+HBr/kCl+C2H6=(0.53±0.03) exp [−(321±16)/T]. A comparison is made with values calculated from reported results for the individual rate constants kCl+HBr and kCl+C2H6.
Deuterium site occupation in the oxygen-stabilized η-carbides Zr3V3ODx. II. Application of a geometric model
Authors: Westlake D. G.
Published: February 2003

A geometric model has been applied to the η-carbides Zr3V3ODx, which belong to space group Fd3m. With criteria of minimum hole size and minimum distance between deuterium atoms and with the assumption that the larger interstices are preferred, it was possible to rationalize the experimentally observed preference, in decreasing order, for 32e sites, 192i sites, 96g1, and 96g2 sites. The respective numbers of coordinating metal atoms for these interstices are Zr3V, Zr2V2, Zr3V, and ZrV3. The largest interstices, which are the octahedral 8a sites with Zr6 coordination, remain empty for x=1.86, 2.85, and 4.93 because of their proximity to the more numerous 32e sites. The failure of deuterium atoms to occupy any 96g1 (Zr3V) sites at x=1.86 may indicate the existence of a short-range repulsive interaction between oxygen and deuterium. The possibilities for achieving higher values of x are also explored.
Ab initio calculation of the vibrational spectra of BeF2 glass simulated by molecular dynamics
Authors: Brawer Steven
Published: February 2003

An ab initio calculation of the density of states, infrared absorption, Raman scattering, and inelastic neutron scattering of BeF2 glass is reported and the calculated spectra compared with experiment. The model of the glass structure was obtained by molecular dynamics simulations. The spectra are obtained with no free parameters apart from an overall multiplicative constant. The force constants are obtained directly from the interaction potential used in the simulations. The shape of the spectra agree well with experiment but the frequencies of the band centers are 100–150 cm−1 too small. The spectra differ appreciably from the density of states, particularly for the infrared absorption. The calculated results are the same for glasses of 120 and 390 ions. None of the modes are localized.
Acetylene chemisorption on Pd(100): Evidence for a strongly distorted speciesa)
Authors: Kesmodel L. L.
Published: February 2003

High-resolution electron energy loss spectra for C2H2 and C2D2 chemisorption on a Pd(100) surface are reported. The vibrational frequencies provide the first evidence for a strongly distorted acetylene species on this surface and indicate a hybridization state near sp3.
Collision induced dissociation of molecular iodine on sapphire
Authors: Kolodney E. , Amirav A.
Published: February 2003

The energy dependence of the dissociation probability for iodine collisions with a sapphire surface is determined in the range 1−10 eV.(AIP)
A spectroscopic study of the ground states of CHCl+2 and CDCl+2 using a new multichannel detector photoelectron spectrometer
Authors: Andrews Lester , Dyke John M. , Jonathan Neville , Keddar Noureddine , Morris Alan
Published: February 2003

The first photoelectron bands of CHCl2 and CDCl2 produced by F atom reactions with CH2Cl2 have been observed with a new multichannel detector photoelectron spectrometer. Vibrational structure was resolved on each band; the vertical ionization energy and v′=0−1 ionic vibrational spacing were measured as 8.54±0.01 eV and 860±30 cm−1 for CHCl2 and 8.52±0.01 eV and 790±30 cm−1 for CDCl+2 vibrational interval is in agreement with a weak chlorine isotopic triplet absorption observed at 845 cm−1 in solid argon and assigned to the symmetric C–Cl2 stretching mode of CHCl+2.
Deuterium isotope effects on the nitrogen chemical shift and 1J(N,H) in the ammonium ion
Authors: Wasylishen Roderick E. , Friedrich Jan O.
Published: February 2003

The effect of successive deuterium substitution on the nitrogen-14 nuclear shielding constant and 1J(14N,H) in the ammonium ion has been measured. Each additional replacement of hydrogen by deuterium resulted in an approximately 0.30 ppm increase in the nitrogen shielding. This is less than half that previously reported for liquid ammonia and approximately twice that reported for methane. These observations can be qualitatively accounted for by the theoretical model of Jameson. A measurable secondary deuterium isotope effect of 1J(14N,1H) was observed: on going from NH+4 to NHD+3 , 1J(14N,1H) decreased by 0.15±0.03 Hz. The primary isotope effect on 1J(14N,H) is less than 0.2 Hz.
On the effective interaction of test multipoles in polar fluids
Authors: Marchildon Louis
Published: February 2003

An exact formula for the effective interaction energy of two test multipoles of arbitrary order immersed in a fluid of polar nonpolarizable molecules is derived. Coordinate space as well as Fourier space techniques are used. This allows for a careful specification of the range of validity of the results, which turn out to be rather general. The explicit dependence of the results on key parameters involved in microscopic dipole–dipole and dipole–multipole interactions is obtained, and a physical interpretation of these parameters is proposed.
Capillary rise, wetting layers, and critical phenomena in confined geometry
Authors: Moldover M. R. , Gammon R. W.
Published: February 2003

We have used an interferometric technique to measure the capillary rise of sulfur hexafluoride (SF6) between closely spaced, nearly parallel plates. The data span the range of plate separations 0.8–3.5 μm and span temperatures from 2 mK below the critical temperature (Tc=319 K) to 540 mK below Tc. The data demonstrate that a thin wetting layer of liquid SF6 intrudes between each interferometer plate and the SF6 vapor. Close to Tc, where the capillary rise is quite small, the wetting layer becomes as thick as 0.7 μm. These layers are a factor of 3 thicker than our theoretical estimates which are based on very simple models which assume that the thickness is governed by a competition between the gravitational force which tends to thin the layers and long-ranged van der Waals forces which tend to thicken the layers. Although our data are close to Tc, the correlation length in the SF6 is always much less than the separation between the interferometer plates; thus, we did not expect to observe strong departure from three-dimensional critical behavior towards two-dimensional critical behavior. In agreement, we find that the sample of SF6 between the interferometer plates has the same critical temperature as that of bulk SF6 (±1 mK). Furthermore the capillary rise data are consistent with the hypotheses that the surface tension and the difference between the liquid density and the vapor density of this confined sample of SF6 are the same as those of bulk SF6 at the same temperature. These results concerning critical phenomena contrast with results obtained in certain earlier studies of binary liquid mixtures near their consolute temperatures. The earlier experiments were interpreted without consideration of wetting layers to indicate that large critical temperature shifts and a crossover to two-dimensional behavior did occur in a confined geometry and temperature range similar to the one we use. We briefly discuss the effects that wetting layers have on other experiments near the critical point of SF6.
EPR power pattern analysis for cubic sites of Fe3+ in MgO
Authors: Boldú J. L. , Muñoz P. E. , Chen Y. , Abraham M. M.
Published: February 2003

A complete electron paramagnetic resonance power pattern characterization of Fe3+ in cubic sites in presented. A one-to-one correspondence among the peaks appearing in the powder pattern and the outer fine-structure transitions (M≠ 1/2 ) observed in the single crystal along the 〈100〉, 〈111〉, and 〈110〉 directions is shown. It is shown that the process of mechanically grinding the single crystal to a powder (particle size ∼1–10 μ) does not remove the cubic symmetry sites. No axial or lower symmetry sites which may be induced by lattice distortion of the crystallites due to strain have been observed.
A further test on an effective nuclear charge model for the prediction of valence force constants
Authors: Barbiric D. A. , Castro E. A. , Fernández F. M.
Published: February 2003

An effective nuclear charge model, capable of predicting valence force constants, has been recently presented. The method includes in its development some ad hoc expressions. The expressions are examined here and alternative formulas are derived. Numerical results are compared with previous theoretical ones and with available experimental data.
An electron gas treatment of the potential curve and polarizability tensor of the lowest 3Σ+u state of H2
Authors: Cina Jeffrey A. , Harris Robert A.
Published: February 2003

A modification of the electron gas model of Gordon and Kim which is applicable to an electronic system with all of its spins parallel is used to determine the internuclear potential and the polarizability tensor of the 3Σ+u state of H2. The electron gas model together with the usual assumption of additive electron density gives results which compare favorably with the configuration interaction results in the region of intermediate internuclear separation.
Electronic symmetry breaking in polyatomic molecules. Multiconfiguration self-consistent field study of the cyclopropenyl radical C3H3
Authors: Hoffmann Mark R. , Laidig William D. , Kim Kwang S. , Fox Douglas J. , Schaefer Henry F.
Published: February 2003

For equilateral triangle geometries (point group D3h), the C3H3 radical has a degenerate 2E″ electronic ground state. Although the 2A2 and 2B1 components separate in energy for C2v geometries, these two components should have identical energies for equilateral triangle structures. In fact, when approximate wave functions are used and the orbitals not required to transform according to the D3h irreducible representations, an energy separation between the 2A2 and 2B1 components is observed. At the single configuration self-consistent field (SCF) level of theory this separation is 2.8 kcal with a double-zeta basis set and 2.4 kcal with double-zeta plus polarization. It has been demonstrated that this spurious separation may be greatly reduced using multiconfiguration self-consistent field (up to 7474 variationally optimum configurations) and configuration interaction (up to 60 685 space and spin adapted configurations) techniques. Configurations differing by three and four electrons from the Hartree–Fock reference function are found necessary to reduce the 2A2-2B1 separation to below 0.5 kcal.
Dipole moment and potential energy functions of the X 1Σ+ and A 1Σ+ states of NaH
Authors: Zemke W. T. , Olson R. E. , Verma K. K. , Stwalley W. C. , Liu B.
Published: February 2003

Calculated radiative transition probabilities between all vibrational levels in the X 1Σ+ state (v=0–21) and in the A 1Σ+ state (v=0–32) of NaH are given. In addition, the calculated spontaneous emission lifetimes for vibrational–rotational levels (J=0,1) and the calculated dipole moments for these same levels are given. The calculations use new hybrid potential energy curves based on Rydberg–Klein–Rees (RKR) and ab initio potential curves. The RKR curve for the X state is new. The calculations use new ab initio dipole moment functions of internuclear distance.
The Enskog theory for multicomponent mixtures. III. Transport properties of dense binary mixtures with one tracer component
Authors: López de Haro M. , Cohen E. G. D.
Published: February 2003

The transport properties of dense binary fluid mixtures with one tracer component are discussed in the context of the revised Enskog theory (RET). Except for the mutual and thermal diffusion coefficients, the transport coefficients are those of the excess component alone. The kinetic diffusion coefficient is given exactly by its low density (Boltzmann) value divided by the magnitude of the radial distribution function at contact. The values of the mutual and thermal diffusion coefficients can be estimated to within 1% for all mass ratios μ. A comparison with earlier work on the limiting cases of the Rayleigh ( μ=∞) and Lorentz ( μ=0) gases is made.
Absolute rate constants for the reaction of O(3P) atoms with ethylene, propylene, and propylene-d6 over the temperature range 258–861 K
Authors: Perry Robert A.
Published: February 2003

Absolute rate constants for the reaction of O(3P) with ethylene, propylene, and propylene-d6 were determined over the temperature range 258–861 K using a laser photolysis-chemiluminescence technique. The following empirical expressions are the best fits to the data: kethylene=2.12×10−13T−63e−1370/RT, kpropylene=3.40×10−19T2.56e1130/RT, and kpropylene−d6=3.40×10−19 T2.53 e1210/RT cm3 molecule−1 s−1. A simple transition state theory model is shown to provide a reasonable explanation for non-Arrhenius temperature behavior.
Orange chemiluminescence from NO2
Authors: Kenner R. D. , Ogryzlo E. A.
Published: February 2003

A spectroscopic and kinetic investigation of the visible and near infrared luminescnece produced in a system containing nitric oxide, oxygen atoms, and nickel is presented. The emission spectrum is shown to be a continuum extending from 500 nm to beyond 900 nm with a maximum near 800 nm. The rate law governing this emission is shown to be I=k[O2(A, A′)][NO][O2]/[M], where k=7×10−14 cm3 molecule s−1, and [O2(A, A′] represents the combined concentrations of the 3Σu+ and 3Δu states of oxygen.
Vibrational analysis of the lowest 1B+u state of trans,trans 1,3,5,7-octatetraenea)
Authors: Kohler Bryan E. , Spiglanin Thomas A. , Hemley Russell J. , Karplus Martin
Published: February 2003

The vibronic structure of the 1 1A−g to 1 1B+u transition of trans,trans-1,3,5,7-octatetraene is measured in alkane matrices at 4.2 K and the results compared with Franck–Condon factors obtained from quantum mechanical calculations of the ground and excited state potential energy surfaces. Excellent agreement between theory and experiment is found for the vibrational intensity distribution in the spectrum, which is characterized by the activity of two carbon–carbon stretching modes with weaker involvement of two low frequency bends. The agreement between theory and experiment suggests that the calculated 1 1B+u excited state equilibrium geometry is quantitatively correct. The excited state normal modes are examined in detail, with particular attention to the degree of Duschinsky mixing upon electronic excitation.
Observation of 5S nitrogen metastable state in charge exchange collisions
Authors: McAfee K. B. , Szmanda C. R. , Hozack R. S.
Published: February 2003

The charge-transfer reactions of excited metastable nitrogen atomic ions with argon are reported. The principal contributor to the reaction cross section is N+(2s,2p3; 5S) while small contributions are made by the 1S and 3P states of N+. The experiment quite uniquely allows a clear identification of the specific electronic state which undergoes a charge-transfer reaction. Excited atomic nitrogen ions are important in interstellar and synthetic plasmas. The N+(5S) state is thought to be responsible for auroral emissions in the ultraviolet.
Time resolved studies of pentacene triplets by electron spin echo spectroscopy
Authors: Yu Hsiang-Lin , Lin Tien-Sung , Weissman S. I. , Sloop David J.
Published: February 2003

The triplet kinetics of pentacene imbedded in p-terphenyl and benzoic acid crystals have been studied by electron spin echo spectroscopy in the temperature range of 77–300 K. Population inversions have been observed for both H0‖‖x and H0‖‖z in the benzoic acid crystal but only for H0‖‖z in the p-terphenyl crystal at room temperature. Detailed analyses showed that the triplet population and decay of the pentacene are very much host and temperature dependent. Dimer formation and tautomerization (proton transfer of the benzoic acid dimer) may be responsible for the peculiar triplet kinetics of the pentacene in the benzoic acid crystal.
ESR examination of XeO4
Authors: Huston J. L. , Brodbeck C. M.
Published: February 2003

An ESR examination of XeO4 in the vapor and solid states suggests that the ground state of this molecule is not a spin triplet as has been proposed for this molecule based on a semiempirical molecular orbital scheme. Solid XeO4 does exhibit paramagnetic states when subjected to ultraviolet radiation of wavelength less than 420±5 nm. The g values of these photoinduced states are consistent with the production of O2− ions and trapped electrons in the solid.
Local fields and infrared absorption intensities of H2O in hydrated β-aluminaa)
Authors: Bates J. B. , Wang J. C. , Dudney N. J.
Published: February 2003

Local electric fields were calculated in the region of the conduction layer occupied by water molecules in hydrated b-alumina, and the dipole derivatives for the H2O normal vibrations in Li and Na0.3Li0.7 β-alumina were determined from the local-field-corrected absorption intensities. The local field has a significant effect on the relative magnitudes of ν1 and ν3. However, a calculation of the polar tensors of H2O and Li β-alumina shows that the large intensity of ν1 compared to the gas phase value is due to hydrogen bonding which results in a large increase in the component of the dipole moment change along the O–H bond with displacement of the proton.
The kinematic apse and j
z-preserving propensities for nonreactive, dissociative, and reactive polyatomic collisions
Authors: Hoffman D. K. , Evans J. W. , Kouri D. J.
Published: February 2003

We consider the generalization of the kinematic apse to nonreactive polyatom–polyatom impulsive collisions, dissociative atom–molecule impulsive collisions, and (partially) impulsive reactive atom–diatom collisions. Appropriate generalizations of the kinematic apse are obtained along which there is a classical propensity for preserving the projection of the total intrinsic spin. In the case of reactive scattering, we discuss several different situations for which such a propensity occurs. For reactive systems in which no such propensity exists, the analysis may still provide a basis for classifying reactions.
Photoacoustic detection of stimulated emission pumping in p-difluorobenzene
Authors: Moll D. J. , Parker G. R. , Kuppermann Aron
Published: February 2003

Photoacoustic detection has been used to monitor a stimulated emission pumping process in p-difluorobenzene. Using the Ã 1B2u51 state as an intermediate, several vibrational levels of the ground electronic state were populated. The photoacoustic method is an attractive alternative to other detection techniques because of its sensitivity, simplicity, and its ability to differentiate between stimulated emission pumping and excited state absorption. An example of excited state absorption in aniline is given.
Application of fitting laws to rotationally inelastic rate constants: Li*2(A1Σ)+Ne, Ar, Xe.
Authors: Scott Thomas P. , Smith Neil , Pritchard David E.
Published: February 2003

Using the method of LIF we have measured with an average absolute error of 8% rate constants for rotationally inelastic collisions from level ji=8, 22, and 42 to level jf=ji+Δj for Li*2 (AΣ) colliding with Xe, Ar, and Ne. Values of ‖Δj‖ up to 32 were studied, involving rotational energy charges as large as 1.5 kT and collisions during which the Li2 molecule rotated as much as 3 rad. These extremes are several times larger than the corresponding values in any previous experimental studies of rotationally inelastic collisions, and present a formidable challenge both to the experimental technique and to any general relationships used to fit the data. Four parameter fits with average deviation from the data of 11% have been found; these fits combine the energy corrected sudden approximation of DePristo et al. with our recently proposed power law-exponential expression for the basis rates kj→0 .
Study of the reactions N+2+2N2→N+4+N2 and O+2+2O2→O+4+O2 from 20 to 160 K by the CRESU technique
Authors: Rowe B. R. , Dupeyrat G. , Marquette J. B. , Gaucherel P.
Published: February 2003

Rate coefficients k1 and k2 have been determined for the association reaction N+2+2 N2→k1N+4+N2 and O+2+2 O2→k2O+4+O2 in the temperature range 20–160 K, using a supersonic jet apparatus (CRESU), which is described in detail. For the highest temperatures where other measurements exist, the measured values of the rate coefficients are in excellent agreement with previous measurements. The temperature dependence of k1 can be represented by a power law of the form k1=6.0×10−29 (300/T)1.85 from 20 K to higher temperatures. For k2, only lower limits for 20 and 45 K are reported but the present data do not confirm the existence of a maximum in k2 as suggested by other authors. These results are examined in the light of recent theoretical arguments.
Temperature dependence of the ion–molecule reactions N++CO, C++NO, and C+, CO+, CO+2+ O2 from 90–450 K
Authors: Miller Thomas M. , Wetterskog Rodney E. , Paulson John F.
Published: February 2003

A variable-temperature, selected-ion flow tube was used to study the ion–molecule reactions N++CO, C++NO, C++O2, CO++O2, and CO+2+O2 over the temperature range of 90–450 K. We have measured reaction rate coefficients and have determined branching ratios for different reaction channels. The two C+ reactions are found to be essentially gas kinetic in rate for 90–450 K. The other three reaction rate coefficients are found to decrease with temperature.
Molecular dynamics in intense fields. II. Adiabatic approximations and coupled equations
Authors: Nguyen-Dang T. T. , Bandrauk André D.
Published: February 2003

Electrons, nuclei, and photons are treated simultaneous quantum mechanically for molecular multiphoton processes. Coupled equations are derived for these processes in weak and intense electromagnetic fields. Different gauges are shown to give rise to different adiabatic separations for all three constituents. The Bloch Nordsieck transformation of QED which was applied in a previous paper is extended further and is discussed in relation to the other gauge representations.
Variational calculation of lower vibrational energy levels of formaldehyde X̃ 1A1
Authors: Maessen Bärbel , Wolfsberg Max
Published: February 2003

The energies of lower lying vibrational states (J=0) of formaldehyde in its ground electronic state are calculated variationally for H2CO and D2CO. The functions of the basis set correspond to products of harmonic oscillator functions. The full Watson Hamiltonian is used and integrals are evaluated by Gauss–Hermite numerical quadrature. Two different potential functions in internal displacement coordinates, which have been given in the literature, are employed in the calculations. Calculations are carried out for A1, B2, and B1 symmetries with 196, 165, and 108 basis functions, respectively. The forms of some of the eigenvectors are reported.
High resolution, cw laser induced fluorescence study of the A 2Πu–X 2Σ+g system of N+2
Authors: Miller Terry A. , Suzuki Tetsuo , Hirota Eizi
Published: February 2003

Laser induced fluorescence detection of the N+2 ion has been accomplished using a single mode, cw dye laser, thereby demonstrating the feasibility of continuous ion monitoring and measurement of ion Doppler profiles. A detailed spectral analysis has been performed for 173 transitions of the 4–0 2Πu↔2Σ+g Meinel band system of N+2 . Molecular constants for both the X and A states are obtained. By combining the present results with earlier optical emission work on other vibrational transitions of the Meinel system, equilibrium molecular constants are obtained.
Sodium nuclear quadrupole interactions in sodium hydroxide. I. The orthorhombic phase
Authors: Amm D. T. , Segel S. L.
Published: February 2003

Using nuclear magnetic resonance techniques the sodium nuclear quadrupole couplings have been observed in anhydrous NaOH and in the monohydrate. They have the values, respectively, of e2qzzQ/h=3.54±0.08 MHz, η=0 and e2qzzQ/h=2.11±0.04 MHz, η=0.62±0.03 at 293 K. The fact that η=0 for the anhydrous salt agrees with a lattice gradient calculation although it differs from previous Zeeman quadrupole resonance experiments.
Fluorescence from photofragments as an aid in identifying new molecular states: The N2 case
Authors: Wu C. Y. R. , Lee L. C. , Judge D. L.
Published: February 2003

Synchrotron radiation has been used to measure the excitation function for producing fluorescence in the 1050–1800 Å region through photodissociative excitation of N2 in the 400–620 Å region and the photoabsorption cross section of N2 in the 600–670 Å region. From the fluorescence excitation function we have found: (i) a molecular state(s) in the 21–22.5 eV region which has not been observed in absorption and photoionization spectra, (ii) structures which correlate with known two-electron excited Rydberg states, and (iii) a decrease in fluorescence cross section at photon energies >23.8 eV, which indicates the existence of new competing processes. The observed molecular state(s) as well as the weak absorption structure in the 19–21 eV region are tentatively assigned as a Rydberg series leading to the D 2Πg of N+2. The newly observed Rydberg states in the 21–22.5 eV region may account for the unusual behavior recently reported in photoelectron spectroscopy results.
The infrared diode laser spectrum of the ν2 band of the FO2 radical
Authors: Yamada Chikashi , Hirota Eizi
Published: February 2003

The ν2 (F–O stretching) band of the FO2 radical was observed in the gas phase at Doppler-limited resolution by an infrared diode laser spectrometer. The FO2 radical was generated in a flow cell by the reaction of O2 with microwave discharge products of a F2 and He mixture. The observed spectrum was analyzed to obtain the rotational, centrifugal distortion, and spin-rotation interaction constants for the first time. The band origin was determined to be 579.318 39(35) cm−1, which was found to be lower than the values in matrices previously reported by 5 to 7 cm−1. Intensity alternation was observed in neither Ka nor N, and the possibility that the molecule takes a symmetrical OFO structure was eliminated. A preliminary analysis of the observed data indicates that the F–O and O–O bonds are close to 1.65 and 1.20 Å, respectively.
Classical and quantum mechanical studies of overtone and multiphoton absorption of HF in an intense laser field
Authors: Dardi Peter S. , Gray Stephen K.
Published: February 2003

Classical trajectories and exact quantum dynamics are used to study overtone and multiphoton absorption of HF in an intense laser field. Transition probabilities and energy absorbed are calculated for nonrotating HF at a fixed frequency (ν̄=3922 cm−1), and the initial vibrational state is varied. Quite good agreement between the classical and quantum results is found in this case. Calculations are also performed about the first overtone frequency for both rotating and nonrotating HF. In this case the quantum and classical comparison is much worse with the absorption maxima of the two results shifted by 200 cm−1 from each other. Poincaré surfaces of section are constructed to help interpret and characterize the classical results for nonrotating HF.
An ESR–ENDOR study of l-asparagine⋅H2O single crystals x-irradiated at 77 K
Authors: Moulton Grace C. , Coleman John M.
Published: February 2003

ESR and ENDOR spectroscopy were used to determine the effects of irradiation on single crystals of l-asparagine and partially deuterated l-asparagine x-irradiated at 77 K. Hyperfine tensors were determined from the ENDOR data for two anion radicals, one at the carboxyl end of the molecule and the other at the amide end, and one cation radical, the decarboxylation radical. The ESR spectra of the carboxyl anion radical are the most intense. In addition to several hydrogen ENDOR lines, ENDOR lines for one nitrogen and two deuterium lines were observed, and tensors were determined for the nitrogen and one of the deuterium nuclei in addition to the hydrogen hyperfine tensors. These nitrogen and deuterium couplings were necessary in order to explain the ESR spectra of the decarboxylation radical. None of these radicals were stable when the crystal was warmed to room temperature.
Observation of the cubic-field splitting of an excited S=2 manifold in a cubic copper tetramer
Authors: Black T. D. , Rubins R. S. , De D. K. , Dickinson Richard C. , Baker W. A.
Published: February 2003

EPR measurements on single crystals of Cu4OCl6(triphenylphosphine oxide)4 at liquid helium temperatures in the frequency ranges 14–17 and 34–35 GHz were fitted to a simple cubic S=2 spin Hamiltonian with g=2.10±0.01 and a zero-field splitting of (0.53±0.01) cm−1. From the decrease in intensity of the S=2 spectrum on cooling below 4.2 K and the absence of an S=1 spectrum, the S=2 manifold was deduced to lie (14±1) cm−1 above a nonmagnetic ground state. The EPR results are used as a test of the various theories developed to explain the magnetic susceptibility of copper tetramers.
Dynamic light scattering studies of concentration fluctuations in aqueous t-butyl alcohol solutions
Authors: Euliss G. W. , Sorensen C. M.
Published: February 2003

Dynamic light-scattering measurements using photon correlation spectroscopy have been performed on four different concentrations of t-butyl alcohol in water; 7.25, 13.2, 20.1, and 26.0 mol %. Temperatures ranged from as low as −16 °C in the supercooled regime to as high as 72 °C. Mutual diffusion constants of the concentration fluctuations were extracted from the light-scattering data. Viscosity measurements were also performed on these solutions over these temperature ranges. The correlation length of the concentration fluctuations determined from these measurements increased with increasing temperature, leveled off near room temperature and then showed another increase at lower, especially supercooled temperatures. These behaviors suggested critical demixing or consolute points should exist at both temperatures above the equilibrium boiling and below the freezing points of the mixture. The high temperature critical point is probably due to t-butyl alcohol and water association, whereas the low temperature critical point is most likely due to water–water self-associating. The results are discussed in terms of the unusual critical-phenomena-like properties of supercooled pure water and the possibility that these unusual properties are due to water–water association in the form of clathrate-type structure.
Measurements of homogeneous nucleation rates for n-nonane vapor using a two-piston expansion chamber
Authors: Wagner P. E. , Strey R.
Published: February 2003

The homogeneous nucleation rate J in supersaturated n-nonane vapor was measured as a function of supersaturation S and temperature T using a newly developed expansion chamber. Homogeneous nucleation is observed only during a short supercritical nucleation pulse so that nucleation and droplet growth are decoupled under the considered experimental conditions. Reproducible nucleation pulses with a duration of about 1 ms are achieved by means of two spring-operated pistons moving in a well-defined time sequence. The number concentration of the droplets growing in the expansion chamber is determined immediately after the end of the nucleation pulse by means of constant angle Mie scattering. Taking into account the duration of the nucleation pulse we measured homogeneous nucleation rates ranging from about 5×105 to 5×109 cm−3 s−1. Homogeneous nucleation in n-nonane vapor was observed in the temperature range from 199 to 241 K. Argon was used as the carrier gas. The measured nucleation rates were compared with the classical theory of nucleation. The slopes of the J–S curves were found to be in good agreement and thus for n-nonane the dependence of the nucleation rate on the supersaturation appears to be correctly described by the classical nucleation theory. However, the actual values of the experimental nucleation rates were found to be several orders of magnitude higher than the theoretical prediction and show a significantly different temperature dependence. A qualitative explanation of the obtained experimental results can probably be given by means of Burton’s recent microscopic calculations of the cluster free energy.
Dilute solutions of macromolecules in a time-dependent Poiseuille flow
Authors: Stasiak Wojciech , Cohen Claude
Published: February 2003

We have extended our previous results on the rheological properties of dilute solutions of elastic and rigid dumbbells to the general time-dependent Poiseuille flow. We present here the results for a few specific time-dependent flows that are typical of rheological experiments: Startup, cessation, and oscillatory flows. In all the cases considered for the rigid dumbbells, the inhomogeneity of the Poiseuille flow decreases the value of the viscosity at all times at the center of the channel.
Adsorption of ions on semiconductor surfaces. I. Silver and halide ions on silver halides
Authors: Zeiri Y. , Tenne R. , Shapiro Moshe
Published: February 2003

Interaction potentials of silver and halide ions adsorbed on the surfaces of silver halides crystals are calculated. The adsorption of such ions leads to a redistribution of charges at the interface between the semiconductor and an electrolyte (emulsion) containing such ions. This has important implications for such processes as photography and liquid-junction solar cells. We find that the adsorption of silver and halide ions occurs all over the crystal surface [(100) plane], with a pronounced tendency of Ag+ to be adsorbed onto X− sites, the X− onto Ag+ sites, and both to be adsorbed onto the midpoint site. The adsorption of Ag+ ions is found to be weaker than that of the X− ions, in accordance with experiments. The adsorption of ions when interstitial Ag+ ion and Ag+ vacancy (Frenkel defect) are present close to the surface is also considered. We find that adsorbed X− ions enrich the space-charge layer of the AgX crystal with extra positive charge, shifting the band edges towards a more cathodic potential. The reverse trend is found when Ag+ ions are adsorbed. In a forthcoming publication, we extend the present model to include model of charge transfer processes at these interfaces. From these calculations we obtain some guidelines for the construction of systems with high charge transfer probability. Surprisingly, these guidelines are satisfied by most photoelectrochemical cells as well as by other systems.
A theoretical study on the structure and stability of the nucleohistone complex. I. The interaction between the polypeptide backbone and single- and double-stranded B-DNA helices
Authors: Otto P. , Clementi E. , Ladik J. , Martino F.
Published: February 2003

The interaction energies between periodic single- and double-stranded DNA models, using the structural parameters of B-DNA, and polyglycine in various conformations are calculated with the help of the mutually consistent field method together with perturbation theoretical expressions. For the repeating units of DNA the four nucleotide bases adenine, guanine, thymine, and cytosine, the nucleotides adenylic acid and thymidine are chosen. The polypeptide component is represented by polyglycine assuming the fully extended β-pleated sheet and the α-helical conformations. In addition, several polyglycine helices are investigated, whose helix axes coincide with the one of DNA and whose turn lengths are a fraction, equal or a multiple of the length of a complete winding of B-DNA. For all DNA–polyglycine complexes, the structure is optimized with respect to the total energy of the combined system. It turns out that the complex, in which the agreement between the helical symmetry of B-DNA and polyglycine is realized, represents the most stable configuration. The results are discussed in the light of further improvements.
The vibrational spectra of chemisorbed molecular clusters: H2O on Ru(001)
Authors: Thiel P. A. , DePaola R. A. , Hoffmann F. M.
Published: February 2003

Electron energy loss spectroscopy and thermal desorption mass spectroscopy have been used to correlate vibrational spectra of H2O on Ru(001) with ESDIAD patterns reported recently. We find that the ‘‘extended bilayer’’ is characterized by a sharp fundamental O–H vibration at 3500–3565 cm−1 which is assigned to nonhydrogen-bonded OH bonds of molecularly adsorbed water. Hydrogen bonds within the well-ordered bilayer give rise to features at ∼3290–3450 cm−1 which are of relatively weak intensity in electron scattering due to the orientations of these bonds. In addition, very small clusters exhibit a weak vibrational mode at ∼2935 cm−1 which possibly represents H2O molecules with one O–H bond pointing into the surface. Isolated H2O molecules at low temperature and low coverage exhibit a fundamental O–H vibration at 3600 cm−1. A reinterpretation of the thermal desorption spectra is suggested.
Revised results for the pressure-induced rotation-libration transition in molecular solid hydrogen
Authors: Aviram I. , Goshen S. , Thieberger R.
Published: February 2003

The pressure at the transition point from free rotation to libration in solid molecular hydrogen isotopes (para-H2,and ortho-D2) at zero temperature is revised in accordance with a new equation of state proposed by Ross, Ree, and Young. The agreement with the experimental values is better than for previous calculations.
Comment on ‘‘IR-spectroscopic study of isotope effects on the NH/ND stretching bands of meso-tetraphenylporphine and vibrational hydrogen tunneling’’
Authors: Sarai Akinori
Published: February 2003

Recently, Limbach, Hennig, and Stulz reported the IR measurement of NH/ND stretching vibration in free-base tetraphenylporphine. The observation of narrow bandwidth in IR spectra lead them to conclude that the inner protons migrate without substantial coupling to the motion of skeleton. In this Comment, it is pointed out that the bandwidth in IR spectra is not relevant to the proton-skeleton coupling problem in the proton migration process, and some controversies on the mechanism of proton migration are discussed.
Reply to a ‘‘Comment on ‘IR-spectroscopic study of isotope effects on the NH/ND stretching bands of meso-tetraphenylporphine and vibrational hydrogen tunneling’’’
Authors: Limbach Hans Heinrich
Published: February 2003

In a comment, Sarai has suggested that the observation of a sharp NH stretching band in meso-tetraphenylporphine (TPP) may not be relevant to the proton-skeleton coupling problem of the hydrogen tunneling process in TPP, as was proposed by us previously. In this reply we point out that Sarais comment refers to the case where the top of the barrier is far above the first excited NH stretching level, as predicted by CNDO calculations, whereas our still valid statement refers to the low barrier case which is observed experimentally. The result of recent NMR experiments on the hydrogen migration in solid state porphines are discussed. It is shown that the asymmetry of the proton potential in triclinic TPP is caused by crystal packing forces and not by the pseudo-Jahn–Teller effect. The consequence is that there is, for the moment, no experimental evidence for a coupling of a heavy atom motion to proton tunneling in porphines.
Unimolecular dissociation as diffusion with a radiation boundary condition
Authors: Agmon Noam
Published: February 2003

Unimolecular decomposition of an isolated polyatomic molecule is discussed in terms of a diffusion equation with a radiation boundary condition, instead of a master equation with an absorbing boundary. This gives a short-cut to evaluating the mean first passage time. More important, it results in simple models, such as the model of diffusion in a linear potential solved here. This model is analytically solvable for the probability distribution and the survival probability, and may be helpful in analyzing future experiments.
The electronic structure of alkali bihalide molecules MX2. II. Excited states of LiF2, NaF2, LiCl2, and NaCl2 systems
Authors: Maessen Bärbel , Cade Paul E.
Published: February 2003

The low-lying valence excited states of the alkali bihalide molecules LiF2, NaF2, LiCl2, and NaCl2 are investigated in this research. This theoretical study is based on restricted-Hartree–Fock theory and employs large Gaussian basis sets in the calculations. This work complements previous work on the ground states of the same systems and uses identical methods and basis sets. A major focus is to examine the effect of the M+ counter ion on the vertical electronic transition energies between the ground state and the excited states of MX2 in comparison to parallel transitions in free X−2. We find the electronic transition energies in MX2 are very similar to parallel transitions in X−2 and that the M+ counterion has a small, but detectable, influence on the transition energy. A detailed comparative analysis is presented, i.e., the influence of M+, and pertinent geometric parameters, on the vertical electronic transitions from the isosceles triangle ground state (C2v) of MX2 systems. Only limited consideration is given to the detailed geometry of the excited state species, but it is apparent that a linear X––M––X structure is likely for certain excited states of MX2.
A molecular dynamics study of the overpopulation phenomena in a two-region system
Authors: Brey J. J. , Ordóñez J. Gómez , Santos A.
Published: February 2003

By means of molecular dynamics, we have studied the time evolution of an isolated system composed of Lennard-Jones particles. The system was initially split into two regions, all the particles in the same region having the same speed. The dominant collisions leading to the overpopulation effects have been identified. It has also been shown that the more relevant features of the evolution of the system can be accurately described by means of a local equilibrium distribution function.
Effect of temperature and pressure on translational lattice vibrations and permittivity of ice
Authors: Johari G. P. , Chew H. A. M. , Sivakumar T. C.
Published: February 2003

Laser-Raman spectra of hexagonal H2O and D2O ice have been measured in the frequency range 100–350 cm−1, temperature range 25–272 K, and pressure range 1–1600 bar. The frequency of the main peak due to translational lattice vibrations νT, which appears near 220 cm−1, decreases with increasing temperature at an increasing rate. νT increases linearly with applied pressure. An analysis of the variation of νT with temperature and pressure shows that ∼55% increase in νT on cooling is due to the intrinsic dependence of translational vibrations on temperature and ∼45% due to the accompanied decrease in the distance between the nearest oxygen atoms in ice. For the same amount of decrease in the distance, isobaric cooling increases νT twice as much as isothermal compression near 255 K. The Grüneisen constant −(∂ ln νT/∂ ln V) is 1.84 at 255 K. The contribution to the refractive index and permittivity from translational vibrations has been calculated over a wide range of temperature. The decrease in the limiting high frequency dielectric permittivity of ice with decreasing temperature or increasing pressure is qualitatively consistent with the observed change in the main frequency of lattice vibrations.
Computer experiment on aqueous solution. IV. Molecular dynamics calculation on the hydration of urea in an infinitely dilute aqueous solution with a new urea–water pair potential
Authors: Tanaka H. , Touhara Hidekazu , Nakanishi Koichiro , Watanabe Nobuatsu
Published: February 2003

A molecular dynamics calculation on aqueous solution of urea has been carried out using constant temperature technique. The total number of molecules was 216, one of which was urea and the temperature was set to 298.15 K and an experimental value was used for the density. For water–water interaction, the MCY (Matsuoka–Clementi–Yoshimine) potential was used, whereas a new potential function was determined for urea–water interaction from SCF LCAO calculations for more than 800 different dimeric configurations with an STO-3G basis set and subsequent multiparameter fitting of the MO results thus obtained to an appropriate functional form by a nonlinear optimization method. The molecular dynamics calculation has been carried out up to 64 000 time steps and from the final 40 000 time steps, thermodynamic quantities, structural and energetic distribution functions, and time-dependent properties were obtained. The original water structure in the vicinity of urea molecule is slightly changed energetically by incorporation of the urea molecule. However, this energy difference is insignificant for the whole system. Instead of the possibility to form strong hydrogen bonding as estimated from the potential function, it is found that urea molecule could enter into the water structure without any appreciable distortion. This fact was confirmed by the angular dependence of any distribution function around the urea molecule. The hydrophilic region does not show a large energetic stabilization between water molecules and the system is stabilized slightly by including urea–water interaction. In contrast to this, the energy for water molecules in the hydrophobic region (above and below the plane containing urea molecule) becomes lower than that of pure water, although this region is small and water molecules cannot form a strong hydrogen bond with urea. This fact reveals that the role of each functional region, which may be either hydrophobic or hydrophilic, is similar to that of alcohol in aqueous solution, although the whole hydration structure of urea molecule is somewhat different from that of alcohol. Reflecting strong interaction of urea–water, the diffusion coefficient for shell water molecules in the vicinity of urea (within 5 Å from urea molecule) becomes smaller by 10%. Moreover, the hydration structure around urea continues for a long time (16 ps), though the energetic relaxation time is very short.
A measure of quantum chaos
Authors: Thiele Everett , Stone James
Published: February 2003

From any density matrix one can calculate an entropy. If by some process a given density matrix is transformed into another density matrix with higher entropy then that process causes a loss of information. We propose that the loss of information that occurs upon time averaging the density matrix of a given initial state be taken as a relative measure of quantum chaos. We also define a weighted time average of the density matrix, based on the assumption that the time interval between preparation and observation of the state of the system is distributed as for a randomly occurring event, with frequency λ. The loss of information that occurs with this weighted time average is a monotonic function of λ. This fact suggests a definition for an intramolecular relaxation time that is basis set independent.
Behavior of triplet correlation function at critical point of simple classical fluids
Authors: Senatore G. , March N. H.
Published: February 2003

Using an exact relation between the density dependence of the pair correlation function g and the triplet function g(3), it is demonstrated that ∫∫[g(3)(r,R)−g(r)]r(dφ/dr)dr dR, with φ the pair potential, diverges like S(0), the long wavelength limit of the structure factor, at the critical point. One current decoupling approximation, while yielding this property, is shown to be oversimplified in the way it relates S(0) to the correlation length.
Electron nuclear double resonance studies on polyacetylene
Authors: Baker Gareth J. , Raynor J. Barrie , Pron Adam
Published: February 2003

ENDOR measurements have been carried out on trans-polyacetylene and on 0.5 mol % ReF6 doped 70% cis-polyacetylene at 4 K. The pure trans-polyacetylene yields only a single line, but the ReF6 doped material yields two hyperfine couplings to hydrogens of 0.2 and 1.7 MHz. Calculations suggest the average chain length of the soliton is 93 carbon atoms. The absence of fluorine hyperfine coupling suggests that ReFn molecular units are in the C, H molecular plane of the trans-polyacetylene.