bluuebunny/crossref_metadata_2025 huggingface dataset:
Liquid structure under shear: Comparison between computer simulations and colloidal suspensions
Authors: Hanley H. J. M. , Rainwater James C. , Clark N. A. , Ackerson B. J.
Published: February 2003

Simulated scattered light intensity plots are calculated for a soft sphere inverse-12 system subjected to a shear and are compared to experimental plots for a colloidal suspension under approximately equivalent conditions. The simulated plots were obtained by a Fourier transform of the radial distribution function. The two sets show points of striking similarity: The Debye–Scherrer rings become elliptical when both systems are subjected to the shear, and the light intensity around the rings is a function of polar angle. An interesting feature is the degree to which the experimental plots display non-Newtonian characteristics of the suspension. Overall, the work is a direct comparison of the results of a computer simulation with real experimental data. Suggestions for future work are given.
The relation between the changes of thermodynamic quantities at the glass transition and the long-wavelength fluctuations of glass structure
Authors: Jäckle Josef
Published: February 2003

The thermodynamic relation between the discontinuity of the longitudinal elastic compliance at the glass transition and the long-wavelength density fluctuations in the glass is rederived using the formal framework of linear response theory. Analogous new relations are derived for the discontinuities of the specific heat and the thermal expansion coefficient. From these relations a new interpretation of the Prigogine–Defay ratio in terms of the correlation between enthalpy and volume fluctuations is obtained.
Vibrational studies and phase transition in mercury(II) perchlorate hexahydrate
Authors: Patel M. B. , Bist H. D.
Published: February 2003

The infrared spectra of polycrystalline mercury(II) perchlorate hexahydrate in the temperature range 100–300 K are reported for the region 4000–200 cm−1. A complete vibrational assignment is proposed using the infrared spectra of the deuterated analog and room temperature Raman spectra of the hydrate. The observed spectral changes in the thermosensitive IR bands are attributed to a transformation of the normal hydrate [Hg(OH2)6]2+ (ClO−4)2 at room temperature to a pseudohydrate [Hg(OH−)3 (H3O+)3]2+ (ClO−4)2 below 243 K.
Study of prenucleation ion clusters: Correlation between ion mobility spectra and size distributions
Authors: Suck (Salk) S. H. , Hagen D. E. , Kassner J. L. , Stoddard L. E.
Published: February 2003

Additional studies regarding our earlier electrothermodynamic theory are presented. Comparisons to recent expansion cloud chamber ion mobility measurements are made, indicating general agreement with observations. This theory predicts more stable and ordered structure for prenucleation ion–water cluster systems than accounted for by the classical Thomson’s theory. In the limiting case of the dielectric constant ε=1, our monopole electrostatic energy term contributed by the foreign ion center precisely converges to that of Thomson. Predicted ion cluster size distributions are found to correlate well with ion cluster size spectra obtained from the ion mobility measurements of hydrated ion clusters and Champman–Enskog theory. In view of good correlation between the theory and observation, we believe that ion mobility study at sufficiently low electric field is a powerful tool for studying prenucleation dynamics.
The microwave spectrum of the N 35Cl radical in the X 3Σ− state
Authors: Yamada Chikashi , Endo Yasuki , Hirota Eizi
Published: February 2003

The microwave spectrum of the N 35Cl radical in the ground vibronic state has been observed by using a source frequency modulation spectrometer with a 3.5 m long free space absorption cell. The NCl radical was generated directly in the cell by a dc discharge in a mixture of N2 and Cl2 flowing through the cell. The electric-dipole allowed rotational transitions of N up to 4 ← 3, each being resolved into a few spin components, were observed in the frequency region 41.5–155 GHz. Each transition was found to consist of many hyperfine components. To analyze the observed spectrum, the matrix elements of magnetic hyperfine and electric quadrupole hyperfine Hamiltonians were derived for a diatomic molecule in the triplet state of which both nuclei have nonzero spins. The rotational, spin–spin interaction, and spin-rotation interaction constants were determined to be B=19 383.4655(42) MHz, λ=56 390.850(16) MHz, and γ=−208.6306(96) MHz, with three standard errors in parentheses. The hyperfine parameters b, c, and eQq for both the nitrogen and chlorine atoms were precisely determined together with the nuclear-spin rotation interaction constant for the chlorine atom. The spin density of the unpaired electrons was estimated to be 76% and 22%, respectively, on the nitrogen and chlorine atoms from the observed hyperfine coupling constants.
Magnetic field and temperature effects on the luminescence quenching in quasi-one-dimensional mixed crystals: Ba[Pt1−xNix(CN)4] ⋅  4H2O
Authors: Schultheiß R. , Hidvegi I. , Gliemann G.
Published: February 2003

The optical emission of Ba[Pt(CN)4] ⋅ 4H2O single crystals (MCP) is quenched by doping with [Ni(CN)4]2− complexes. The quenching process drastically reduces the E⊥c-polarized emission (E=electric field vector, c=stack axis). We have studied the E⊥c-polarized emission of mixed crystals Ba[Pt1−xNix (CN)4] ⋅ 4H2O with 0.01≲x≲0.1 as a function of temperature (1.7 K≲T≲30 K) and of magnetic field strength (0≤H≤5 T). The experimental results are explained by assuming two types of [Pt(CN)4]2− species within a linear system of quasi-localized states.
Interference between vibration-to-translation and vibration-to-vibration energy transfer modes in diatomic molecules at high collision energies
Authors: Shin H. K.
Published: February 2003

An explicit time dependent approach for simultaneous VT and VV energy transfer in diatom–diatom collisions is explored using the exponential form of ladder operators in the solution of the Schrödinger equation of motion. The collision of two hydrogen molecules is chosen to illustrate the extent of interference between VT and VV modes among various vibrational states. While vibrational energy transfer processes of nominally VT type can be treated with pure VT mode at low collision energies, the intermode coupling is found to be very important at collision energies of several ℏω. The occurrence of the coupling appears to be nearly universal in vibrational transitions at such energies. Exceptions to the coupling have been discussed.
Analysis of NMR line shapes of rigid-lattice multispin systems. V. 19F shielding tensors of CF3 groups bonded to carbon
Authors: Garg S. K. , Ripmeester J. A. , Davidson D. W.
Published: February 2003

The fluorine chemical shielding tensors, CF3 group geometries, and fluorine spin-rotation tensors in CF3CN, CF3CCl3, CF3COOD, and CF3CD2OD are determined from rigid-lattice 19F NMR line shapes for these molecules incorporated in glass matrices. In contrast to the variability shown by trifluoromethyl halides, the shielding components hardly vary with the substituents on the α carbon atom. Shielding is largest parallel to the CF bond and least perpendicular to C–F in the CCF plane. The full tensors agree fairly well with the partially defined tensors of similar molecules given by liquid crystal studies but they are far from being axially symmetric about the CF bonds.
X-ray photoelectron spectroscopy of the 3p level in Ni(II) compounds
Authors: Stickle W. F. , Hayes R. G.
Published: February 2003

We have measured the x-ray photoelectron spectra arising from 3p ionization of several Ni(II) compounds with different geometries and ground states. We have compared them with calculations in which the coupling among the 3p and 3d electrons by interelectronic repulsion is included in a crystal-field model. The calculations are able to account for the spectra satisfactorily.
Experimental and theoretical investigations of the EFG parameters at the Br site in Cd(BrO3)2 ⋅ 2H2O
Authors: Valli R. , Rama Rao K. V. S.
Published: February 2003

In Cd(BrO3)2 ⋅ 2H2O there are two chemically inequivalent sites of bromine in the unit cell which give rise to two nuclear quadrupole resonances for each isotope of bromine both at RT as well as at LNT. Single crystal Zeeman experiments carried out on the 79Br resonances at RT have revealed the presence of four physically inequivalent sites belonging to each crystallographic site of bromine and the EFG parameters at these sites have been obtained. The conventional point-charge model, when applied to this system for the evaluation of the field gradient at the Br sites, has yielded unsatisfactory results. In an attempt to include the effect of the covalent bonding in the bromate group, CNDO/2 MO calculations have been carried out on the two inequivalent bromate groups of the system and from the resulting population densities in the orbitals, intraionic EFG or qcov has been estimated. Later, interionic contribution qion, obtained from a point-charge approximation to the rest of the lattice has been added to qcov to obtain the total EFG. The results obtained from this model are in good agreement with the experimental values and it is found that intraionic EFG forms more than 95% of the total EFG at the Br site.
State selected ion–molecule reactions by a TESICO technique. VII. Isotope effect in the reactions O2+(X 2Πg, a 4Πu)+HD→O2 H+(O2D+)+D(H)
Authors: Tanaka Kenichiro , Kato Tatsuhisa , Guyon P. M. , Koyano Inosuke
Published: February 2003

The kinetic isotope effect in the reactions O2++HD→O2H+(O2D+) +D(H) (ΔE=1.78 eV) has been studied by selecting the vibronic states of O2+ using the threshold electron-secondary ion coincidence (TESICO) technique. Relative cross sections for the individual vibronic states, v=19 and 20 of the X 2Πg state and v=0–4 of the a 4Πu state of O2+, have been determined at a collision energy of 0.4 eV. The results for the a 4Πu state show that, regardless of the vibrational state, the isotope effect is consistent with that expected from a direct reaction mechanism. A similar isotope effect has also been observed for v=19 and 20 of the X 2Πg state, suggesting that the above reaction with O2+(X 2Πg) proceeds via a direct mechanism even at collision energies below the translational threshold when a sufficient vibrational energy is supplied to the reactant ion.
The positive muon as a tracer for the study of dynamic correlation effects in metal hydrogen systems
Authors: Richter D. , Hempelmann R. , Hartmann O. , Karlsson E. , Norlin L. O. , Cox S. F. J. , Kutner R.
Published: February 2003

In order to investigate whether the positive muon (μ+) can be used as a tracer in hydrogen diffusion studies, we have made what we believe to be the first systematic study of muon–hydrogen correlations in a metal hydride (NbHx) over a large range of hydrogen concentrations. The following observations were made from transverse field μSR experiments: (i) Loading with hydrogen has a strong impact on muon mobility in Nb, and shifts the motional narrowing region from 60 to above 170 K; (ii) in β-NbHx, the inverse muon correlation times τ−1μ are of the same order as the hydrogen jump rates τ−1H. However, the activation energies are distinctly different (EHa≂230 meV, Eμa≂140–150 meV). Moreover, the concentration dependence of τ−1μ exhibits strong negative deviations from a (1−c) behavior; (iii) the H/D isotope effects as measured by μ+ diffusion in β-NbH(D)x is significantly nonclassical. While the activation energy for τ−1μ is slightly larger in the deuteride, the ratio of jump rates is significantly larger than √2; (iv) at low temperature the μ+ mobility exhibits an irregular concentration dependence which can be associated with phase transitions to subhydride phases. They are characterized by an ordering of the vacancies on the H sublattice which apparently suppresses long-range muon motion; (v) single crystal experiments support tetrahedral site occupancy as expected for a muon on a substitutional site of the H sublattice. The data are interpreted with the aid of Monte Carlo calculations, where the diffusion of tracer particles in a concentrated lattice gas was simulated. There is clear evidence that repulsive muon–hydrogen interactions are an important feature of the dynamic muon–hydrogen correlations.
Unimolecular decomposition and vibrationally induced electron autodetachment of acetone enolate ion
Authors: Foster Robert F. , Tumas William , Brauman John I.
Published: February 2003

Acetone enolate ion trapped in a pulsed ion cyclotron resonance spectrometer cell was irradiated with a pulsed CO2 laser. Both unimolecular decomposition to give HC2O− and CH4, and vibrationally induced electron autodetachment were observed. The data are consistent with a scheme in which a vibrationally excited enolate ion decomposes through an adiabatic (decomposition) pathway and a nonadiabatic (autodetachment) pathway. From an analysis of the branching using statistical reaction rate theory the autodetachment rate constant ka is estimated to be ∼107 s−1 at ∼1 eV above the electron detachment threshold.
Comment on: ‘‘An improved analytical approximation to n!’’
Authors: Mohanty S. , Rummens F. H. A.
Published: February 2003

It is shown here that n!≂(2π)1/2(n+1)n+1/2exp{[1/12(n+1)]−(n+1)} gives very accurate values even for n<10 and is far superior to the classical approximation suggested by Weissman.
Electron spectroscopy of hydrogen chloride from 5 to 19 eV
Authors: Wang R.-G. , Dillon M. A. , Spence David
Published: February 2003

Using an electron energy loss spectrometer, we have measured the spectra of electrons inelastically scattered from HCl for an incident electron energy of 200 eV, energy loss between 5 and 19 eV, and scattering angles of 0° to 19°. The low-angle scattering spectra, corresponding to photoabsorption, confirm many optically allowed energy levels in HCl, and locate many others, including six levels of the σ2 π3 nsσ 1Π Rydberg series. Series analysis yields an HCl ionization potential of 12.790 eV, midway between the known HCl+ 2Π3/2, 1/2 energies 12.750 and 12.830 eV. In the ionization continuum, we locate a new vibrational progression associated with previously observed (negative ion) Feshbach resonances, which has configuration 3pσ3pπ4 4sσ, and is the lowest Rydberg state associated with the first excited ionic state A 2Σ+.
Laser induced fluorescence of NH2(Ã 2A1) in the supersonic free jet
Authors: Mayama Shinya , Hiraoka Susumu , Obi Kinichi
Published: February 2003

The fluorescence excitation spectra of the NH2 Ã 2A1←X̃ 2B1 transition have been measured in the supersonic free jet. The rotational cooling of the ground state NH2 radical results in the observation of only the Π vibronic subbands in the excitation spectra. The fluorescence lifetimes are obtained for single rovibronic levels of the Ã 2A1 state in the jet. The radiative lifetime is shortened from 10.3 μs for Π(0,8,0) 11,0 to 4.4 μs for Π(0, 14, 0) 11,0 with decreasing the excitation wavelength, which is explained by the cubic transition energy dependence of the Einstein A coefficient. Little difference is observed between the lifetimes of the spin components belonging to the same rotational level and between those of the Fermi resonance pairs. The transition moment of the Ã 2A1–X̃ 2B1 transition is experimentally determined to be 0.037±0.004e2α20.
Picosecond studies of the temperature dependence of homogeneous and inhomogeneous vibrational linewidth broadening in liquid acetonitrile
Authors: George S. M. , Harris A. L. , Berg M. , Harris C. B.
Published: February 2003

The temperature dependence of homogeneous and inhomogeneous vibrational linewidth broadening is reported for the symmetric CH3-stretching vibration in acetonitrile over its entire liquid range at P=1 atm. A selective excite-and-probe vibrational dephasing experiment based on transient stimulated Raman scattering in high laser depletion is used to measure the homogeneous dephasing times T2. The separation of homogeneous and inhomogeneous broadening processes is accomplished using the combined results of isotropic spontaneous Raman studies and selective picosecond vibrational dephasing experiments. As a function of temperature, the relative contributions of homogeneous and inhomogeneous broadening are shown to change significantly in opposing directions. Agreement between experiment and theory supports previous suggestions that homogeneous broadening is caused by rapidly varying processes which affect the vibration via short range repulsive forces. The results also suggest that inhomogeneous broadening is caused by slowly varying local density sites which interact with the vibration through long range attractive forces.
Thermodynamics of the lanthanide trifluorides. V. The heat capacities of dysprosium trifluoride, DyF3, and erbium trifluoride, ErF3, from 5 to 350 K
Authors: Flotow Howard E. , O’Hare P. A. G.
Published: February 2003

The heat capacities of pure, well-characterized samples of DyF3 and ErF3 were measured by adiabatic calorimetry from 5 to 350 K. The heat capacities C○p, the entropies S°, the enthalpy increments H°(T)−H°(0), and the Planck functions −[G°(T)−H°(0)]/T are presented in tabular form at selected temperatures to 350 K. The values of the last three functions were adjusted for the removal of degeneracies associated with the magnetic ordering of DyF3 and ErF3 below 5 K. Also presented are recommended thermochemical functions at 298.15 K and tables of recommended high-temperature thermodynamic functions from 400 K to the melting temperatures. The excess heat capacities due to the internal electronic energy levels of Dy+3 in DyF3 and of Er+3 in ErF3 were calculated from calorimetric results. In the case of DyF3, the eight Kramer’s doublets derived from spectroscopic measurements were used to calculate an excess heat capacity which is in good agreement with the calorimetric data.
The orientation of CO at the step sites of the Pt(321) surface and its implications for catalytic oxidation
Authors: Trenary M. , Tang S. L. , Simonson R. J. , McFeely F. R.
Published: February 2003

Angle-resolved photoemission spectroscopy employing polarized He ii (40.8 eV) radiation has been used to characterize two distinct types of CO bonding, associated with terrace and step sites, that were shown to exist on the Pt(321) surface in an earlier TDS study. CO was found to be oriented normal to the (111) terraces of Pt(321) as it is to Pt(111). The step site CO was found to be oriented asymmetrically with respect to the Pt(321) normal and to be 40°±20° from [321] in the direction of [21̄4̄]. This existence of such a tilted CO orientation has been postulated to explain the dynamics of the CO oxidation reaction on Pt surfaces in several recent studies.
Electronically excited states of chemisorbed molecules
Authors: Avouris Ph. , DiNardo N. J. , Demuth J. E.
Published: February 2003

The electronic excitations of CO, NO, and pyridine chemisorbed on a variety of surfaces have been studied with low energy (10–20 eV) electron energy loss spectroscopy. By comparing the observed electronic transitions of CO on Ni(100), Cu, and Si(111) surfaces, the ∼6 and ∼8.5 eV transitions of chemisorbed CO are reassigned as singlet and triplet coupled 5σ̃→2π̃* excitations, respectively. By simple arguments we show that screening (both image and charge transfer) and spin quenching by a metallic substrate should not significantly affect the transition energies of neutral adsorbate excitations. This behavior is discussed in relation to photoemission measurements and, in particular, the limitations of using photoemission data to estimate electronic excitation energies of adsorbed molecules. The dependence of electronic spectra on adsorbate bonding configurations are considered for NO and pyridine. Vibrational spectra of NO on Ni(100) reveal changes in bonding configuration as a function of NO surface coverage, but only small changes are observed in the electronic spectra. Coverage or temperature dependent bonding phase transitions are found for pyridine on Ni(100). In this case, however, intensities of the π→π* excitations of the aromatic ring are found to be very sensitive to the nature (n vs π bonding) of the chemisorption bond.
Chemical formation and spectroscopy of S2 in a free jet expansion
Authors: Heaven M. , Miller Terry A. , Bondybey V. E.
Published: February 2003

H2S seeded in a free jet expansion of Ar is photolyzed by an ArF laser. The liberated free radical fragments react to form S2 molecules, cooled rotationally by the jet to ≲20 K. A detailed rotational analysis of the laser induced fluorescence spectrum of the 2–3, B–X band of S2 reveals localized perturbations due to the B″ 3Πu state of S2. A deperturbation analysis for both 32S2 and 32S34S spectra yields, in conjunction with recent work by Patino and Barrow, values for the rotational constant, spin-orbit coupling constant, and Te for B″ 3ΠuS2.
Chemical, electronic, and structural properties of conducting heterocyclic polymers: A view by XPS
Authors: Pfluger P. , Street G. B.
Published: February 2003

XPS techniques have been used to characterize electrically conducting films of polypyrrole and polybithiophenes as grown electrochemically. These techniques have yielded information not only on the nature of the polymeric cation and its counter anion but also on the structural disorder in these polymers. In particular the N1s spectra have revealed a nonuniformity in the distribution of the anions relative to the nitrogen atoms and the C1s spectra clearly show that as many as 1/3 of the pyrrole rings can be different from the rest. Significant improvement in the structural order is achieved by blocking the β sites on the pyrrole ring with methyl groups to ensure a completely α,α′ bonded polymer. Similar experiments on polybithiophene polymers show that this polymer is considerably more ordered than polypyrrole. The XPS results are best interpreted by assigning a nonmetallic character to the pyrrole polymers, in accordance with other experiments.
Penning ionization of a van der Waals molecule: He*(2 1S, 2 3S)+Ar2→He+Ar+2 +e−
Authors: Siddiqui H. R. , Bernfeld D. , Siska P. E.
Published: February 2003

The collision energy dependence of the cross sections for Penning ionization in the title systems has been measured in a crossed beam experiment in the range 1.5–6.0 kcal/mol. Comparison with the corresponding monomer ionization and with theory based on a sudden approximation indicates unusual behavior in the 3S system.
Multiconfigurational electron propagator (MCEP) ionization potentials for general open shell systems
Authors: Nichols Jeffrey A. , Yeager Danny L. , Jo/rgensen Poul
Published: February 2003

We have developed a multiconfigurational electron propagator (MCEP) technique for the theoretical determination of ionization potentials for general open shell and highly correlated atomic and molecular systems. In order to do this, we have used and extended the generalized spin-symmetry adapted operators of Pickup and Mukhopadhyay. To properly account for correlation effects we have additionally included ionization and electron affinity operators analogous to the ‖Γ〉〈0‖ state transfer operators necessary in multiconfigurational linear response. MCEP ionization potentials and ionization process probabilities have been evaluated for both O2 and N2 and used to carry out detailed examination and interpretation of the respective PES and ESCA spectra. The MCEP results are extremely encouraging for both principal and shake-up I.P.′s. For example, using 〈5s4p1d〉 contracted Gaussian basis sets the principal valence ionization potentials to bound ionic states are calculated within ±0.3 eV of experiment for both N2 and O2. For several I.P.′s the breakdown of the single particle picture is calculationally demonstrated, e.g., the intensity of the 2σu electron removal process to give 2Σ−u states in O2 is almost equally divided between two ESCA peaks. One of these peaks is the previously unassigned A peak in the ESCA spectrum.
The HO+2 molecular ion. Geometrical structure and vibrational frequencies
Authors: Raine Gwendolyn P. , Schaefer Henry F. , Handy Nicholas C.
Published: February 2003

The equilibrium geometry and harmonic vibrational frequencies of HO+2 have been predicted at seven distinct levels of ab initio theory. In this regard, self-consistent-field (SCF) and configuration interaction (CI) methods were used in conjunction with double zeta (DZ), double zeta plus polarization (DZ+P), and extended basis sets of contracted Gaussian functions. The molecular structure of HO+2 predicted at the DZ+P CI level of theory is re(H–O) =1.005 Å, re(O–O) =1.239 Å, θe(HOO) =111.5°, compared to the experimental structure of neutral HO2, r0(H–O)=0.977 Å, r0(O–O)=1.334 Å, θ0(HOO)=104.2°. Anharmonic vibrational frequencies were also predicted at four different levels of theory, with the DZ+P CI results being ν1=3204 cm−1, ν2=1572 cm−1, ν3=1144 cm−1. For comparison, the observed fundamentals for the neutral HO2 molecule are ν1=3436 cm−1, ν2=1392 cm−1, and ν3=1098 cm−1.
Calculated properties of alkali metal clusters with fivefold symmetry
Authors: Fantucci Piercarlo , Koutecký Jaroslav , Pacchioni Gianfranco
Published: February 2003

A b initio calculations indicate a high stability for the pentagonal pyramid Li6 and the pentagonal bipyramid Li7 clusters. The pivotal role of the Jahn–Teller or pseudo-Jahn–Teller effect in determining the optimal geometrical shape of small neutral and anionic Li clusters is demonstrated. The calculated electronic properties of the pentagonal bipyramids Li7, Na7, and K7 are in satisfactory agreement with recent ESR measurements.
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o calculations on the structure and nature of the hydrogen bonded complex H2S ⋅⋅⋅ HF
Authors: Singh U. Chandra , Kollman Peter A.
Published: February 2003

A b initio calculations employing an extended basis set and correlation energy estimates at the MP2 (second order Möller–Plesset) level have been used to estimate the binding energy, minimum energy S ...F distance, and minimum energy bisector angle θ (angle between the twofold axis of H2S and the S ...F line) for the hydrogen bonded complex H2S ... HF. The calculated distance and bisector angle θ are in reasonable agreement with experiment; the calculated binding energy can be used to provide a good estimate of the (as yet unknown) experimental value. Morokuma component analyses and simple electrostatic (molecular mechanics) calculations have been carried out on the complex as a function of bisector angle and they demonstrated that, despite suggestions to the contrary, the H2S ... HF bisector angle is predominantly determined by the electrostatic energy.
A high efficient single line absorber for iodine Mössbauer spectroscopy
Authors: Pasternak M. , Van der Heyden M. , Langouche G.
Published: February 2003

A new single-line absorber in the form of Na5IO6 has been employed for 129I Mössbauer spectroscopy. It is characterized by a relatively large significantly larger than that of CuI[f=0.027(20)], the universally adopted single line 129I absorber. The linewidth Γ measured for zero effective thickness is 0.98 mm s−1, somewhat larger than that of CuI (Γ=0.78 mm s−1). The spectral features are compared with those of Na3H2IO6, previously considered as a single-line absorber. The synthesis, chemical properties, and the potential applications of this absorber are described.
Some energy formulas within the approximation of zero chemical potential
Authors: Politzer Peter
Published: February 2003

Two exact formulas for the nonrelativistic total energies of atoms and molecules, derived earlier, are now modified by introducing the assumption of zero chemical potential. As a result, the integration required by these formulas need no longer be carried out over isoelectronic series. The new approximate molecular formula is tested for a group of ten neutral diatomic molecules and yields energies that are, with two exceptions,within an average 0.4% of the experimental values. The new formulas are rigorously valid for any model in which the chemical potential does equal zero, such as the Thomas-Fermi theory for neutral systems.
Ordered structures in hydrogen–niobium systems: Ground states analysis
Authors: Futran Mauricio , Hall Carol K.
Published: February 2003

A ground states analysis appropriate to the ordered structures found in hydrogen–niobium systems is presented. The purpose of this study is to elucidate the nature of the hydrogen–hydrogen interactions in these systems using knowledge of their phase-change behavior. Hydrogen–hydrogen interactions out to sixth neighbors on a tetragonal sublattice of the lattice of tetrahedral interstitial sites of the bcc lattice are considered. It is shown that the interaction parameters must satisfy certain constraints in order for the ordered phases β and ε to be stable at low temperatures. The hydrogen–tantalum system is also briefly discussed.
On the relation between entropy and energy versions of thermodynamic length
Authors: Salamon P. , Nulton J. , Ihrig E.
Published: February 2003

The second derivative matrices of internal energy or of entropy may be used to define a metric structure on the set of equilibrium states of a thermodynamic system. When expressed relative to the same coordinates, the metric matrices are proportional to each other with constant of proportionality given by the negative of the absolute temperature. This establishes the conformal equivalence of the two metric structures.
Pressure melting of ice
Authors: Weber Thomas A. , Stillinger Frank H.
Published: February 2003

A 250-molecule ice Ih crystallite has been melted at a pressure of about 2 kbars using molecular dynamics computer simulation. The ST2 potential has been used to represent molecular interactions. Melting was observed to begin at the crystallite surface and to proceed inward until the entire crystal was converted to an amorphous droplet. The melting point was found to be depressed by about 23 °C at 2 kbars in comparison with a corresponding calculation at ambient pressure. Quenches from various thermodynamic states were created to study the inherent structures in the fluid. Approximately 80%–90% of the latent heat of melting is due to the difference in the packing structures of the fluid compared to the crystal; the remainder may be ascribed to the greater anharmonicity of the fluid over the crystal.
Lattice theory of reaction efficiency in compartmentalized systems
Authors: Musho Matthew K. , Kozak John J.
Published: February 2003

We consider the problem of reaction efficiency in compartmentalized systems. In our model, we position a target molecule (active site) at a centrosymmetric location in a reaction space defined by N-1 locations; the coordinates of these sites are specified by their positions on a d-dimensional lattice of valency ν. We then consider the factors that may influence the efficiency of an (eventual) irreversible reaction at the reaction center between a diffusing coreactant A and the target molecule B, i.e., we consider the bimolecular reaction A+B → C. We suppose there exists a biasing potential of the form v(r)=r−s centered at the active site and then explore the interplay between this longer-range biasing potential and other short-range (nonspecific) chemical or cage effects in influencing the efficiency of the underlying reaction–diffusion process. These effects are studied as a function of the geometry of the compartmentalized system (i.e., its spatial extent, dimensionality d, and valency ν), the strength of the biasing potential (values of the potential exponent s ranging from s=1 to s=6 are studied), the temperature and finally the role of the initial conditions. Our calculations are carried out using an approach developed recently wherein group theoretic arguments are coupled with theorems from the theory of finite Markov processes to produce exact resuls on the efficiency of reaction–diffusion processes taking place on finite, d-dimensional networks of valency ν. That is, all results presented in this paper represent the exact solution of the underlying lattice statistical problem, and may be taken as an accurate documentation of the effects studied. Our calcuations also have relevance to the related problem of reaction–diffusion processes in systems where a set of reaction centers is distributed uniformly throughout an extended system, and this relationship and its implications will be brought out in the text.
The reaction O+2+CH4 → CH3O+2+H studied from 20 to 560 K in a supersonic jet and in a SIFT
Authors: Rowe B. R. , Dupeyrat G. , Marquette J. B. , Smith D. , Adams N. G. , Ferguson E. E.
Published: February 2003

Rate coefficients k have been determined for the reaction O+2+CH4 → CH3O+2+H at several temperatures in the uniquely wide temperature range 20–560 K using a new expanding jet (CRESU) apparatus and a selected ion flow tube (SIFT) apparatus. In the overlapping temperature range of the experiments the rate coefficients are in excellent agreement. A minimum is observed in the rate constant near 300 K but the outstanding feature is the rapid increase at low temperatures from the minimum value k (290 K)=5.4×10−12 cm3 s−1 to k (20 K)=4.7×10−10 cm3 s−1, the latter being about half of the collisional limiting value kc for the reaction (kc=1.16×10−9 cm3 s−1). Indeed the data show that k → kc as T → 0 K and the low temperature values can be fitted to a power law of the form k=1.1×10−7 T−1.8. The results strongly indicate that the CH3O+2 product ion is formed via rearrangement in a long-lived intermediate (CH4O2)+ complex, the lifetime against unimolecular decomposition of which largely controls the rate of the reaction. It is suggested that this rearrangement proceeds via H− abstraction within the complex forming an electrostatically bound (CH+3 ⋅ HO2) complex which can then undergo concerted molecular rearrangement leading to CH3O2++H products. These data and other available data on exothermic ion–molecule reactions at low temperatures suggest that complex lifetimes may be the controlling parameter in many reactions at low temperatures.
Dependence of reaction attributes, including differential cross sections and resonance features, on changes in the potential energy surface for the F+D2 reaction
Authors: Walker Robert B. , Blais Normand C. , Truhlar Donald G.
Published: February 2003

We present a series of comparisons of dynamical reaction attributes for the reaction F+D2→FD+D. We employ three potential energy surfaces, Muckerman surface no. 5 and two surfaces suggested more recently. These surfaces differ in barrier height and bending potential. The dynamical calculations are based on the quasiclassical trajectory method and on the bend-corrected rotating linear model. We find that new surface no. 2 does allow for significant reaction probability at 0.8 kcal/mol, as required to agree with molecular beam experiments. The wider bending potential of new surface no. 2 does not, however, have a large effect on the angular distribution. Although it is known from previous work that the barrier in the entrance channel is important for the compound-state resonance in this reaction, lowering the barrier from 1.06 kcal/mol in Muckerman surface no. 5 to 0.74 kcal/mol in the new surfaces does not make a qualitative change in the resonance features of the results. We conclude, on the basis of this paper and previous studies of the thermal rate constants, that new surface no. 2 is more accurate than the widely studied Muckerman surface no. 5 and provides a better starting point for future work.
The pressure dependence of the rate constant of the reaction of OH radicals with CO
Authors: Paraskevopoulos George , Irwin Robert S.
Published: February 2003

Absolute rate constants for the reaction OH+CO+M have been determined in the gas phase, at 298 K, in the pressure range 20–700 Torr of He, N2, CF4, and SF6 and 0–20 Torr of H2O. Hydroxyl radicals were generated by flash photolysis of H2O vapor in the vacuum UV, and monitored by time-resolved resonance absorption at 308.2 nm [OH (A 2Σ+→X 2Π)]. The second-order rate constant was found to depend on the pressure and identity of the third body M. Approximate relative efficiencies of the gases as third bodies were estimated to be: H2O: SF6: CF4: N2: He≊ 1.0: 0.5: 0.3: 0.1: 0.02. In the presence of small amounts of O2 the OH decay became nonexponential and slower. Computer simulation of our system indicated that the effects of O2 are adequately explained by reactions of HO2 radicals formed in the presence of O2. A mechanism of the reaction which is consistent with the pressure effect is discussed. The values of the rate constant obtained with M=N2 are recommended for use in modeling atmospheric processes.
Inhomogeneity corrected Thomas–Fermi theory for atoms
Authors: Cummins Peter L. , Nordholm Sture
Published: February 2003

The utility of the Thomas–Fermi theory is restricted by the large errors that arise in the treatment of atoms and molecules due to the very rapid variation in the electron densities particularly in the neighborhood of a nucleus. We consider here a number of simple inhomogeneity corrections which can reduce the error. Most of these are modifications of the gradient correction of the kinetic energy as proposed by von Weizsäcker. The most successful is based on the separation of the kinetic energy into ground and excited state contributions. Simple model calculations using exponential or hydrogenic functional forms for the electron densities have been carried out for light atoms. The results show that binding energies and electron densities can be substantially improved by use of simple inhomogeneity corrected kinetic energy functionals.
Triplet state nonradiative lifetimes of collision-free aniline and aniline–argon complexes above the S1 origin
Authors: Knee J. L. , Johnson P. M.
Published: February 2003

Triplet state lifetimes have been measured for collision-free aniline and aniline–argon complexes in a pump-probe photoionization experiment. The triplet state populations are due to intersystem crossing from optically pumped S1 vibronic states. The measured triplet decay rates as a function of excess energy above the T1 origin show behavior which is not exponential with respect to excess energy, increasing much more rapidly than similar molecules studied by this technique. It is postulated that this could be due to increased spin-orbit coupling caused by the T2 charge transfer state. The results for the aniline–argon complex are used to estimate a ∼275 cm−1 van der Waals bond energy in the highly excited triplet as well as to aid the understanding of both the S1–T and the T-S0 nonradiative processes.
Observation of the lowest triplet transitions 3Σ+g–3Σ+u in Na2 and K2
Authors: Huennekens J. , Schaefer S. , Ligare M. , Happer W.
Published: February 2003

We report here the observation and definite identification of the lowest triplet transition 3Σ+g–3Σ+u in Na2 and K2. The identification of these bands is based upon direct comparison of K2 absorption, thermal emission, and laser-induced fluorescence, and Na2 laser-induced fluorescence, with spectra calculated from recent, accurate, ab initio potential curves. These data also serve as a sensitive check on the accuracy of these potentials.
Photoeffects on the fluorination of silicon. II. Kinetics of the initial response to light
Authors: Houle F. A.
Published: February 2003

When a silicon surface etched by XeF2 is illuminated with visible laser radiation, strong changes are observed in product formation rates. Initial rates of change of the products SiF, SiF2, SiF3, and SiF4 have been determined as a function of doping, pressure, and light intensity. Analysis of the kinetics has provided new insight into the mechanism by which radiation influences the etch rate. In particular, it has been found that initial rates of the species SiF4, SiF2, and SiF exhibit an approximately quadratic dependence on the laser intensity, while SiF3 appears to depend on its fourth power. It is proposed that trapping of electrons and holes by fluorine and silicon fluoride centers at or near the surface greatly increases their reaction rate. Modeling confirms this picture and provides some estimates for the relative magnitudes of rate constants for essential processes.
Translational energy dependence of cross sections for reactions of OH− (H2O)n with CO2 and SO2
Authors: Hierl Peter M. , Paulson John F.
Published: February 2003

A tandem mass spectrometer has been used to measure cross sections for reactions of the solvated negative ions OH−(H2O)n, where 0≤n≤3, with the neutral molecules CO2 and SO2 over the range of reactant translational energy 0.15–25.0 eV (LAB). The reactions observed include solvent switching, collisional dissociation, and charge transfer. The exoergic solvent switching reactions are very rapid, having cross sections which exceed a hundred square Angstroms at low energies. These cross sections decrease approximately as (energy)−0.5 up to 1 eV and then decrease much more rapidly at higher collision energies. Estimates of bond dissociation energies for the cluster ions are derived from the measured translational energy thresholds for the endothermic collisional dissociation reactions.
Charge transfer of O++ in helium at thermal energies
Authors: Butler S. E. , Heil T. G. , Dalgarno A.
Published: February 2003

Detailed calculations are described of the cross sections and rate coefficients at thermal energies for the charge transfer of O2+ (3P) ions in helium and upper limits are obtained to the rate coefficients of the metastable O2+ (1D) and O2+(1S) ions. At 1000 K, the rate coefficient for the ground state ion is 3.1×10−11 cm3 s−1 and for the metastable ions less than 10−15cm3 s−1. Charge transfer of O2+(3P) in helium occurs preferentially into the metastable O+1(2P) ion.
Differential cross sections for (R,T) energy transfer in He–HF collisions: Comparing theory with experiments
Authors: Battaglia F. , Gianturco F. A. , Palma A.
Published: February 2003

Starting from a recently proposed potential energy surface, the relative occurrence of rotational inelasticity during He–HF collisions at thermal energies is examined as a function of scattering angle by employing both rigorous ab initio calculations (CC) and approximate solutions of the scattering equations (IOSA). The chosen range of energy is the same as one recently studied in highly accurate molecular beam experiments, so that rather close scrutiny of the computational outcome is made possible by comparison with measured differential cross sections (DCS) over a wide angular region well beyond the rainbow angle. The possible effect of isotopic substitution and the reliability of energy sudden schemes are investigated by computing both total and individual DCS involving rotational transitions. A very satisfactory fit to the measured total DCS is found within the IOSA approximation, thus providing further checks on the quality level of the employed potential surface. The general inefficiency in this system of energy deposition into rotations at thermal collision energies is here indicated by both theory and experiments.
Collision-induced dissociation of the cesium halides
Authors: Parks E. K. , Pobo L. G. , Wexler S.
Published: February 2003

Total cross sections vs the relative collision energy are reported for collision-induced dissociation (CID) of CsF and CsBr to ion pairs on collision with beams of accelerated projectiles M, where M includes Ar, Kr, and Xe atoms as well as SF6 molecules. Measurements of the CID of CsCl with Ar and SF6 are also presented. Partial cross sections for both three body dissociation and for the formation of the weakly bound molecular ions CsM+ are determined. The cross sections for CID of CsF are found to be considerably smaller than those for the other halides for all projectiles but particularly for SF6. For Xe colliding with CsF the CID cross sections are determined to be only weakly dependent on the CsF internal energy, in contrast to earlier studies on CsI where a strong dependence was obtained. Two-dimensional in-plane trajectory calculations for Xe colliding with the four cesium halides are performed to identify the important dynamical effects governing the efficiency of CID. For CsCl, CsBr, and CsI the dissociation efficiency is found to depend mainly on the effects of relative mass, whereas for CsF, the relative size of the projectile vs the F− ion strongly affects the dissociation efficiency. A surprisingly strong influence of the weak attractive forces between the projectile and the cesium or halogen product ion is found which affects an increase in the energy transfer, and is related to the formation of either bound or transient molecular ions. A general discussion of the experimental results for the four cesium halides, as well as the dynamical effects involved in both three body dissociation and molecular ion formation, is presented.
Investigations of the Stokes and anti-Stokes resonance Raman scattering of cytochrome c
Authors: Schomacker K. T. , Bangcharoenpaurpong O. , Champion P. M.
Published: February 2003

We present the first Stokes and anti-Stokes Raman intensity measurements of a molecular system using laser excitation throughout a resonant electronic transition. Experiments on oxidized and reduced cytochrome c at both room and cryogenic temperatures are reported. We employ a Kramers–Kronig transform method in our analysis and use the absorption and Raman cross sections on an absolute scale in order to find the linear Franck–Condon coupling strengths of the observed low frequency modes. A general expression for the Boltzmann factor is also derived in terms of the resonant Stokes and anti-Stokes scattering intensities. This expression is appropriate for the determination of temperatures in absorbing media. The traditional Stokes/anti-Stokes ratio used for this purpose is shown to fail both experimentally and theoretically when the laser excitation is in resonance with an electronic transition of the material under study. Finally, we discuss the frequency domain approximations necessary for application of the transform method at finite temperature and quantitatively treat the dispersion of the coupling strength across the Raman spectrum of cytochrome c. This latter phenomenon leads naturally to the concept of a coupling strength ‘‘spectrum.’’ This spectrum is calculated and subsequently used to estimate a parameter of interest (the Stokes shift, Δ) in recent theories of vibrationaly assisted electron transport.
Production and quenching of excited photofragments of N2O
Authors: Lee L. C. , Suto Masako
Published: February 2003

The photoabsorption and fluorescence cross sections of N2O are measured in the 105–160 nm region using synchrotron radiation as a light source. The quantum yields for producing the N2(B 3Πg →A 3Σ+u) and NO(B 2Πr →X 2Πr) emissions are determined. The quenching of emission intensity by various quenchers is investigated. The quenching rate constants of N2(B 3Πg) by H2 and N2 are measured to be 2.4×10−11 and 2.1×10−11 cm3/s, respectively. For the 120–140 nm excitation band the precursor of the NO β emission is attributed to ‘‘hot’’ O(1S), and for the 140–155 nm band it is N(2D). The ratios for the rate constant of hot O(1S) quenched by N2O, N2, Ar, H2, and He are measured to be 1:0.77:0.61:0.38:0.36, respectively. The lower limit for the rate constant of hot O(1S) quenched by N2O is estimated to be (2±0.8)×10−10 cm3/s. The quantum yield for producing the NO β emission from the reaction of hot O(1S) (produced at 125 nm) with N2O is about 0.4%. The O(1S) requires about 1.9 eV of kinetic energy to overcome the potential barrier between the reactants [O(1S)+N2O(X 1Σ+)] and the products [NO(B 2Πr) +NO(X 2Πr) ]. Based on the hot O(1S) kinetics, all the previous measurements as well as the current observations can be explained consistently.