bluuebunny/crossref_metadata_2025 huggingface dataset:
On the statistical thermodynamics of interacting charged particles of arbitrary shape and concentration
Authors: Rosenfeld Yaakov , Gelbart William M.
Published: February 2003

A great deal of effort has been devoted for many years to the statistical thermodynamics of interacting charged particles having spherical symmetry, e.g., simple electrolyte solutions, fluid plasmas, and micellized suspensions of ionic surfactants. In the present work we provide a straightforward and physically suggestive scheme for treating the bulk properties of interacting particles characterized by arbitrary charge, axial ratio and concentration. The basic idea involves a marriage of Onsager’s charge ‘‘smearing’’ optimization with a variational statement of the mean spherical approximation. In particular the direct correlation function is shown to be described accurately by the pair interaction energy between smeared charge distributions. After illustrating this approach for point particles (one and multicomponent plasmas), we generalize it to line charges of arbitrary length l. A simple analytical theory of the isotropic→nematic transition in this system is presented for the large l, high density regime. Results are also discussed for the weak-coupling (Debye–Hückel) and ‘‘high salt’’ limits. Finally, we stress the special features of our theory which enable ‘‘crossover’’ between dimensionalities and inclusion of polydispersity in both charges and lengths.
A continuous thermodynamics approach to chemical equilibrium within an isomer group
Authors: Alberty Robert A. , Oppenheim Irwin
Published: February 2003

As the number of carbon atoms increases in an homologous series the number of isomers becomes so large that it becomes advantageous to use standard thermodynamic properties for the isomer group and to use probability density functions to represent the distribution of properties within the isomer group. The Pearson type III probability density function is used to represent the distribution of the standard Gibbs energy of formation within an isomer group. The standard Gibbs energy of formation of the isomer group is expressed in terms of the minimum, mean, and standard deviation of this probability density function. The probability density function for the equilibrium mole fraction within the isomer group is expressed in terms of these same properties for the isomer group and the temperature. Two types of equations are derived for the calculation of the other standard thermodynamic properties of an isomer group from the probability density function for the standard Gibbs energy of formation. The use of these equations is illustrated by the calculation of isomer group properties for the decanes at 1000 K in the ideal gas state.
The effects of rotational diffusion on the fractal dimensionality of structures formed by cluster–cluster aggregation
Authors: Meakin Paul
Published: February 2003

The effects of rotational diffusion on cluster–cluster aggregation have been investigated using two-dimensional off lattice simulations. Earlier work has shown that, in the absence of rotational motion, diffusion limited cluster–cluster aggregation in two dimensions leads to structures with a fractal dimensionality (D) of 1.4–1.45. In the limit of very fast rotational diffusion (as compared to translational diffusion), the fractal dimensionality is 1.0 (for all Euclidean dimensionalities). For the model used in this paper to investigate the effects of rotational diffusion D varies continuously from a value of 1.40–1.45 to a limiting value of 1.0 as the rate of rotational diffusion is increased. Similar results are expected in higher dimensional simulations.
Rate coefficient temperature dependence and branching ratio for the OH+ClO reaction
Authors: Hills Alan J. , Howard Carleton J.
Published: February 2003

The rate constant for the reaction OH+ClO →HO2+Cl (1a) GFIX→HCl+O2 (1b) has been measured in a discharge-flow system with far infrared laser magnetic resonance detection of OH, ClO, and HO2. Measurements were made at low pressure (1.0–1.1 Torr) and at temperatures between 219 and 373 K under pseudo-first-order conditions. The measured rate constant is k1=(8.0±1.4)×10−12 exp[(235±46)/T] cm3 molecule−1 s−1. The branching ratio at 293 K is, k1a/k1=0.86±0.14. The rate constant to channel (1a) was used with that for the reverse reaction to calculate the heat of formation of HO2. ΔH°f,298 (HO2)=3.0±0.4 kcal mol−1.
Studies of negative ion formation in fluoroethers and fluorosulphides using low-energy (≤10 eV) electron beam and electron swarm techniques
Authors: Spyrou S. M. , Hunter S. R. , Christophorou L. G.
Published: February 2003

The attachment of low-energy (≤10 eV) electrons to four fluoroethers (CF3OCF3, CF3OCF2H, CF2HOCF2H and CF3OCH3) and two fluorosulphides (CF3SCF3 and CF3SCH3) has been studied using a time-of-flight mass spectrometer (TOFMS) and a high pressure electron swarm technique. The relative cross sections as a function of incident electron energy for all observed anions were measured by employing the former, and the total absolute electron attachment rate constants were measured by employing the latter technique. All six molecules were found to attach electrons dissociatively. The types and relative intensities of the fragment anions depend strongly on the number and relative positions of the F atoms in the molecule and on the presence of O or S atoms in the molecule. The fluorosulphides attach lower energy electrons than do the fluoroethers. The magnitude of the total electron attachment rate constants increases with increasing number of F atoms in the molecule. The observed negative ions (in decreasing order of intensity) and the positions of the peak intensities (given in parentheses in eV) are: F−(5.3) and CF3O−(4.8) from CF3OCF3; CF3O−(3.7), F−(5.7), HF−2(6.1), CFO−(5.9), and CF−3(6.7) from CF3OCF2H; CF3O−(3.0), HF−2(4.7), F−(5.0), and CFO−(4.9) from CF2HOCF2H; F−(6.7) from CF3OCH3; CF3S−(0.6) and F−(3.8) from CF3SCF3; CF3S−(∼0.0) from CF3SCH3. Energetic considerations were employed to identify possible fragmentation mechanisms of the negative ion states (NISs) leading to the production of the observed fragment negative ions and to deduce values for the electron affinities of the radicals CF3O and CF3S. CNDO/2 and MNDO molecular orbital calculations were performed on all six molecules investigated in an effort to rationalize the types, relative intensities, and positions of the maxima in the cross sections of the observed anions.
Photodissociation dynamics of 1,3,5-triazine
Authors: Ondrey G. S. , Bersohn R.
Published: February 2003

It is found that 1,3,5-triazine in a molecular beam can be photodissociated into three HCN molecules at 248 and 193 nm. Such a photodissociation must take place from the 1A1 ground state of the parent and is therefore likely to generate nearly equal speeds in all three fragments. Most of the available energy is released into vibration but fragments have an average translational energy/HCN of 10 kcal and 2 kcal/mol at the two photon energies. This anomaly can possibly be explained by assuming that dissociation is too fast to permit the attainment of vibrational relaxation in the ground state and thus the molecules retain a ‘‘memory’’ of the region of configuration space in which they crossed over to the ground state.
A field-theoretic model Hamiltonian for the proper dissociation of multiple bonds
Authors: England Walter B. , Silver David M. , Steinborn E. Otto
Published: February 2003

Lipkin’s modified eigenvalue problem is derived for a BCS function which violates general conservation laws. The results allow for the proper description of multiple bond formation and dissociation, and provide a zeroth-order problem for ordinary many-body perturbation theory. The method is illustrated for the proper dissociation of N2.
Relativistic scattered wave calculations of hexachloro- and hexabromoiridate (IV)
Authors: Lopez Jesus P. , Case David A.
Published: February 2003

Dirac scattered-wave (DSW) calculations are reported for the ground and several excited states [IrCl6]2− and [IrBr6]2−, and comparisons are made to electron spin resonance, optical absorption, and magnetic circular dichorism spectra. Metal and ligand-centered contributions to the Zeeman and hyperfine tensors differ from those estimated from ligand field theory, although both calculations are in qualitative agreement with experiment. The predicted symmetries and spin-orbit splittings of ligand-to-metal charge transfer transitions are approximately correct, but the estimated transitions energies are too low by 0.2–0.7 eV. The implications of this behavior for future relativistic Xα calculations are discussed, with an emphasis on various approximate methods of including spin-polarization effects in relativistic calculations.
The dependence of the 13C and the 1H nuclear magnetic shielding on bond extension in methane
Authors: Osten H.-Jörg , Jameson Cynthia J.
Published: February 2003

The mean bond displacements 〈Δr〉 in the methane isotopic homologous series 13/12CX4−nYn  (X, Y=H, D, T) at 300 K, and the temperature dependence of 〈ΔrCH〉 in 13CH4 from 250 to 350 K were calculated. With the assumption that the linear terms are sufficient to account for the isotope shifts, we determine from the 2/1H-induced 13C isotope shift an empirical value of(∂σC/∂ΔrCH)e=−35±3 ppm/Å. This predicts a temperature dependence in the 13C resonance in CH4 gas in the zero-pressure limit of 2.7×10−2 ppm over 100 °, which explains why it could not be observed. We observed the 13/12C-induced 1H isotope shift in CH4, −0.0024 ppm. With the same mean bond displacements, this isotope shift gives an estimate of (∂σHi /∂ΔrCHi )e=−38±3 ppm/Å. From the reported 2/1H-induced two-bond 1H isotope shift in CH4, −0.016 ppm, using the mean bond displacements and the derivative obtained from the 13/12C-induced one-bond isotope shift, we get an estimate of (∂σHi/ ∂ΔrCHj) e=−1.3±0.2 ppm/Å.
High pressure Raman study of intermolecular interactions and Fermi resonance in liquid ethylene carbonate
Authors: Schindler W. , Zerda T. W. , Jonas J.
Published: February 2003

The Raman band of the ring vibration (ν5) and the Fermi resonance between the carbonyl stretching (ν2) and the first overtone of the ring breathing vibration (2ν7) have been studied in liquid ethylene carbonate in the pressure range between 1 and 3000 bar and at temperatures from 40 to 160 °C. The relative changes of the transition dipole moments of both bands in resonance are estimated from the measurements of their intensity ratios. The knowledge of these parameters enables a comparison of the experimental frequency noncoincidence effect between isotropic and anisotropic components of the bands and the theory based on strong dipole moment coupling. The noncoincidence effect for the ν5 vibration are also explained in terms of this theory. The pressure induced frequency shifts of the bands are interpreted qualitatively by the permanent dipole–dipole coupling model.
A spectroscopic study of the structure and dynamics of the urea inclusion adduct of stearic acid
Authors: Casal Hector L. , Cameron David G. , Kelusky Eric C. , Tulloch A. P.
Published: February 2003

The urea inclusion adduct of stearic acid and derivatives deuterated at positions 2, 4, 12, 16, and 18 have been prepared. The temperature dependence of the Raman, infrared, and 2H NMR spectra reveals the existence of a phase transition centered at 234 K. The distortion of the urea host is completed within a 4 K interval while the order–disorder transition of the included stearic acid occurs over 30 K. The changes in motional freedom of the included chain are accompanied by a change in the geometry of the dimeric carboxyl moiety. Chain motion is not completely frozen in the low-temperature crystal form and evidence is presented for position-dependent off axis and torsional motions.
Infrared spectra of the PH3, AsH3, and SbH3--HX hydrogen bonded complexes in solid argon
Authors: Arlinghaus Robert T. , Andrews Lester
Published: February 2003

Infrared spectra have been observed for several group V base–HX (X=F, Cl) hydrogen bonded complexes formed by codeposition of Ar/base and Ar/HX samples. The decrease in displacement of the H–X stretching fundamentals for the heavier bases parallels a similar decrease in base proton affinities, indicating decreasing hydrogen bond strengths. A single, sharp degenerate librational mode was observed, which indicates C3v structures for the complexes. The decrease in librational fundamentals observed in the order PH3>AsH3>SbH3 and decreasing 2νl/νl ratios indicate increasing anharmonicity and less rigid structures for the heavier complexes. Polarization of the phosphine lone pair and concentration of the P-H bonding electrons is suggested by blue-shifted νc3 and νc1 complex modes. OFF
Charge transfer half-collisions: Photodissociation of the Kr⋅O+2 cluster ion with resolution of the O2 product vibrational states
Authors: Jarrold Martin F. , Misev Liubomir , Bowers Michael T.
Published: February 2003

The photodissociation of the Kr⋅O+2 cluster ion has been studied in the visible and UV regions of the spectrum (350–580 nm) using a crossed high energy ion beam/laser beam experiment. Photodissociation of Kr⋅O+2 occurs by a charge transfer transition to Kr+⋅O2 state(s). The only ionic product observed was Kr+. A value for the dissociation energy of the Kr⋅O+2 cluster of D00(Kr–O+2) ∼0.33 eV was estimated from the results. A compilation of ion–molecule cluster dissociation energies is given. The product angular distributions indicate that the lifetime of the excited state(s) is less than a rotational period. In the visible region of the spectrum the products are Kr+(2P3/2)+O2(2Σ−). For the UV it is argued that one of the products is probably electronically excited [i.e., either Kr+(2P1/2) or O2(1Δg)]. The product relative kinetic energy distributions show resolved features that can be assigned to production of the product O2 in specific vibrational states. Information on the product rotational excitation was also derived from these results. The potential surfaces of the [Kr⋅O2]+ system are discussed along with literature data on the charge transfer reaction between Kr+ and O2.
Direct absorption spectroscopy of jet-cooled polyenes. II. The 1 1B+u←1 1A−g transitions of butadienes and hexatrienes
Authors: Leopold D. G. , Pendley R. D. , Roebber J. L. , Hemley R. J. , Vaida V.
Published: February 2003

In the present paper, we report the direct absorption spectra of the 1 1B+u←1 1A−g transitions of gas phase butadiene, deuterated and methylated butadienes, and the cis and trans isomers of hexatriene cooled to low rotational and vibrational temperatures in supersonic molecular jets. These jet absorption spectra allow the more accurate determinations of Franck–Condon factors, upper state vibrational intervals and vibronic band homogeneous widths. We discuss the experimental constraints that the measurements reported here and in the previous paper of this series impose on theoretical models of the equilibrium structures and relaxation dynamics of the 1 1B+u excited states of the small linear polyenes.
A Raman study of pressure-densified vitreous silica
Authors: McMillan Paul , Piriou Bernard , Couty René
Published: February 2003

We have obtained polarized Raman spectra for vitreous silica densified at 3.95 GPa and 530 °C. The glass spectra show considerable changes on densification, which are discussed in terms of the observed reduction in average intertetrahedral bond angle and the central force model for connected networks. The results of this densification experiment are compared with previous work at high pressure but room temperature. This comparison suggests a model for the densification mechanism, and indirectly supports recent structural interpretations of the Raman spectrum of fused silica.
Rotational collisional narrowing in the NO fundamental Q branch, studied with cw stimulated Raman spectroscopy
Authors: Lempert W. , Rosasco G. J. , Hurst W. S.
Published: February 2003

Self-broadened NO Q-branch spectra were obtained in the pressure region ∼20–100 kPa. We determined J- and Ω-dependent pressure broadening coefficients. The observed collisional narrowing was fitted by means of a relaxation matrix theory, incorporating recent experimental and theoretical values of NO state-to-state rates. A ‘‘fitting law’’ representation of the state-to-state rates yielded good agreement with both the measured broadening coefficients and the observed spectrum.
Kinetic studies on the CO oxidation on a Rh(111) surface by means of angle-resolved thermal desorption
Authors: Matsushima Tatsuo , Matsui Toshiji , Hashimoto Masanori
Published: February 2003

The kinetics of the reaction of adsorbed CO with oxygen adatoms was studied in the temperature range of 100–600 K with LEED and angle-resolved thermal desorption. At small oxygen coverages the CO2 formation peaked from 500 to 400 K with increasing CO exposure. The activation energy decreased from 45 to 35 kcal/mol. When the oxygen coverage was large, a new CO2 formation peak appeared around 400 K. The activation energy was 30 kcal/mol. LEED observations revealed that the surface was covered by separate domains of CO and oxygen. The former CO2 is produced outside the domains or on the perimeters, whereas the latter is formed in the oxygen domains. The angular distribution of the desorption of CO2 in the former varied as cos4 θ, where θ is the desorption angle. The desorption of CO2 in the latter showed a sharper angular distribution of cos15 θ.
Collisions of polyatomic molecules with solid surfaces: A semiclassical stochastic trajectory approach
Authors: Clary D. C. , DePristo Andrew E.
Published: February 2003

The semiclassical stochastic trajectory method is extended to the study of vibrational excitation and relaxation of polyatomic molecules in collisions with nonrigid solid surfaces. The technique involves a quantum-mechanical treatment of the molecular vibrational modes and a classical treatment of the translational and surface motion. Surface temperature effects are incorporated in the method through use of the generalized Langevin equation. The sudden approximation is used to treat the molecular rotational motion. Calculations of vibrational transition probabilities are reported for the collisions of CO2 with a Pt(111) surface, and these probabilities, when relatively small, are found to be quite sensitive to surface temperature. The results are relevant to recent experiments on the excitation and relaxation of the vibrational modes of CO2 in collisions with surfaces.
Microwave and radiofrequency Stark spectrum of ArHCN: A highly nonrigid molecule
Authors: Leopold K. R. , Fraser G. T. , Lin F. J. , Nelson D. D. , Klemperer W.
Published: February 2003

The rotational spectrum of the weakly bound complex ArHCN has been observed using molecular beam electric resonance spectroscopy. The spectrum is superficially characteristic of that of a linear molecule with both unusually large centrifugal distortion (requiring a J6 dependent distortion term to fit the data) and an unexpectedly large bending amplitude. The spectroscopic constants are       The centrifugal distortion constant DJ is remarkably large and abnormally sensitive to isotopic substitution. Using the usual model, the stretching and bending force constants obtained from these data are an order of magnitude smaller than those similarly computed for the hydrogen halide complexes of argon. The calculated stretching and bending frequencies are 10 cm−1, predicting that excited vibrational levels should be populated in the beam. Three transitions have been observed which appear to correspond to an excited vibrational level of ArDCN, but poor signal-to-noise has prohibited their unambiguous assignment. While seemingly, the data support a hydrogen bonded structure for this complex, they require an anomalously short bond length of 2.7 A, despite the apparent weakness of the bond. The vibrationally averaged molecular geometry and force constants are seen to be unexpectedly sensitive to isotopic substitution. Our conclusion is that the observed behavior of this system is best interpreted in terms of strong coupling between the radial and angular degrees of freedom associated with the weak bond. The amplitude of oscillation in the two coordinates is large, making the ‘‘structure’’ of this complex nebulous.
Wavelength-dependent fragmentation in the resonance-enhanced ionization of bromobenzene
Authors: Koplitz Brent D. , McVey Jeffrey K.
Published: February 2003

The ionic fragmentation of bromobenzene is studied as a function of excitation energy into the 1B2 state, with two UV photons being necessary to ionize the neutral molecule. A mass-resolved analysis reveals an increase in the C6H+5 /C6H5Br+ ratio at lower excitation energies, which can be traced to a larger photodissociation cross section for the parent ion. Selected resonant and off-resonant features of the 1B2 state also have been excited. Surprisingly, a slightly smaller phenyl/parent ion ratio is observed for a resonant excitation as compared with an off-resonant excitation. An additional ionization pathway through a triplet state is discussed as a possible explanation of the observed reduction in the phenyl/parent ratio.
Fermi resonance in solid CO2
Authors: Bogani Franco , Salvi Pier Remigio
Published: February 2003

A theory of Fermi resonance in molecular crystals is described and applied to the ω1; 2ω2 Fermi resonance region of the CO2 crystal. It is shown by means of the Green functions method that bound states can originate out of the two-phonon manifold of ω2+ω2 vibrational states for sufficiently high values of the anharmonicity constant. Starting from the crystal harmonic Hamiltonian, Green functions for the excitations of interest are built. Introducing as anharmonic contribution to the Hamiltonian the intramolecular coupling between ω1 and 2ω2, renormalization of the ω2 density of states results. The profiles of the renormalized density of states are seen to change both with mechanical anharmonicity and the unperturbed ω1 frequency. Two cases of practical interest are discussed: in the first the ω1 state is immersed into the ω2+ω2 manifold while in the second ω1 is well separated from that. The intensity of the Raman spectrum is discussed in terms of a direct mechanism, i.e. scattering of light by ω2(k) and ω2(−k) phonons through second order terms of the polarizability tensor and an indirect mechanism, through the ω1 Raman active mode, and of interference between them. Using known gas phase values of anharmonicity constant and ω1, good agreement with experiment is found. The intensity and frequency of the bound states as well as the intensity, position and profile of the two-phonon continuum are well reproduced by the model.
Long and short range correlations in an ionic fluid near its critical point
Authors: Medina-Noyola Magdaleno
Published: February 2003

Extended versions of the mean spherical approximation are applied to the study of the structural properties of the restricted primitive model in the regime of large coupling and small density, where the gas–liquid phase transition of the model has been located. The numerical results indicate that the long-range behavior, expected in the neighborhood of the critical point, induces strong short-range correlations which point at an enhanced process of ionic association as the critical point is approached.
Improved electron-gas model calculations of solid N2 to 10 GPa
Authors: LeSar R.
Published: February 2003

Recently, LeSar and Gordon [J. Chem. Phys. 78, 4991 (1983)] reported calculations of the crystal structures of solid N2 and CO2 to 10 GPa using electron-gas short-range energies and pairwise dispersion energies that included only the anisotropic C6/r6 term. While the calculated zero temperature pressure-volume curve was in excellent agreement with the experimental results, the structure that was calculated to be most stable at high pressure (the β-O2 structure with space group R3̄m) is not consistent with experimental spectroscopic results. Here we report calculations on solid N2 using a pairwise damped-dispersion energy that includes terms up to C10/r10. While we fail to predict the low pressure, low temperature Pa3(α)–P42/mnm(γ) transition, we find a transition from the P42/mnm structure to one with a space group R3̄c at 1.9 GPa, in agreement with the experimental transition pressure. The calculated structure is consistent with the high pressure experimental results. Possible distortions of the R3̄c to a R3c structure were examined, but no distortions were found to 75 GPa.
Multiple pulse NMR in rotating solids
Authors: Olejniczak E. T. , Vega S. , Griffin R. G.
Published: February 2003

An analysis of magic angle sample spinning (MASS) and multiple pulse/MASS NMR experiments is presented. The approach relies on an examination of magnetization vectors of individual crystallites during the experiments, and is similar in spirit to the vector model employed in liquid state spectroscopy. Many features of MASS and multiple pulse/MASS experiments can be understood with this approach. For instance, it is easy to illustrate that vectors from the spin packets refocus once every rotor cycle leading to the formation of rotational echo trains. Furthermore, an examination of the magnetization’s path during a rotor cycle suggests a method to generate spin echoes in a rotating sample which involves two π pulses separated by a rotor period TR. A similar analysis leads to an explanation of the four pulse cycle used to suppress sidebands in MASS spectra (the so-called TOSS experiment) and to multiple π-pulse trains which generate echoes every second through fifth rotor period. In addition, it is shown that the rotor frequency lines which appear in multiple pulse/MASS arise because of errors in the multiple pulse train. Finally, when the cycle time in chemical shift scaling experiments becomes comparable to the rotor period, then distorted powder patterns and sideband patterns with a spacing ‖2νR−νSC‖ are observed, where νR and νSC are the rotation and scaling frequencies, respectively. The origin of these powder and sideband patterns is explained.
Temporal coherence effects in multiphoton excitation of collisionless molecules
Authors: Schek Israel , Jortner Joshua
Published: February 2003

In this paper we examine the conditions for the erosion of coherence effects in the multiphoton excitation of a collision-free sparse molecular level structure driven by a laser field, which is characterized by a stabilized amplitude and a fluctuating phase. The stochastic equations of motion for the molecule excited by a phase-diffusing laser field were solved within the framework of second-order cumulant expansion. The kinetic master equation was found to be valid for the description of the dynamics of the multiphoton excitation process provided that the correlation time of the laser phase is considerably shorter than the relevant molecular time scales and that the rate of the slow diffusion of the laser phase considerably exceeds the Rabi frequencies. We conclude that the experimental observation of the celebrated fluence dependence of the efficiency of high-order multiphoton excitation in isolated molecules may sometimes originate from the fluctuations in the laser field rather than from intramolecular vibrational energy redistribution.
Picosecond dynamics and photoisomerization of stilbene in supersonic beams. I. Spectra and mode assignments
Authors: Syage J. A. , Felker P. M. , Zewail A. H.
Published: February 2003

In this and the following paper, we present a full account of our earlier report [Syage et al., Chem. Phys. Lett. 88, 268 (1982)] on the spectra and picosecond dynamics of stilbene isomerization in supersonic jets. The jet-cooled excitation and dispersed fluorescence spectra of t-stilbene-h12 and -d12 are reported and assigned for the Bu ← Ag electronic excitation. The 000 wavelengths for h12 and d12 (in excitation) are 3101.4 and 3092.5 Å, respectively. Previously unidentified low frequency modes (as low as 20 cm−1 in S0 for -h12) have been observed and tentatively assigned as out-of-plane modes of au symmetry in C2h. This indicates that t-stilbene has a propeller-like geometry involving phenyl rotation (i.e., C2 symmetry). A Franck–Condon analysis of the low frequency modes and particularly the ag, ν25 in-plane symmetric bend mode indicates that a large geometry change takes place upon electronic excitation possibly due to a delocalization of double bond character from the Ce–Ce bond to Ce–φ bond. The geometry change of the in-plane Ce–Ce–φ between S1 and S0 was determined from the Franck–Condon and a normal mode analysis to be 1.3°±0.3°. The rms amplitude of this bend motion for the symmetric ν25 bend mode (for one quanta in S0) is ‖〈Δφ〉2‖1/2=1.0±0.2°. Most ag modes involving benzene-type vibrations (other than C–H stretch modes) have been assigned. Dispersed fluorescence spectra exhibited a broad background indicative of IVR which increased rapidly with S1 vibrational energy. The spectra were completely diffuse above 1200 cm−1 which is consistent with the barrier for isomerization being at about 1100–1200 cm−1. The excitation spectra show a rapid decline in intensity at higher energies due to the process of isomerization which competes with radiative decay. However, sharp (albeit weak) structure could still be discerned at energies well in excess of 2000 cm−1. In the accompanying paper, we present results on the dynamics of isomerization and its dependence on mode mixing and the nature of the reactive surface (adiabatic vs diabatic).
Vibrational intensities of lattice modes and the distributed dipole model: Crystalline acetylene
Authors: Gamba Z. , Bonadeo H.
Published: February 2003

The vibrational intensities of crystalline acetylene are calculated using models of increasing complexity for the molecular charge distributions and polarizabilities. It is found that it is necessary to include high order molecular multipole moments, represented by a distributed dipole model, to account for the absolute infrared absorption intensities of the two crystal phases. High order molecular polarizabilities, represented by distributed dipolar polarizabilities, have an important effect on the calculated absolute intensities and susceptibilities, although the relative intensity patterns are not too much model sensitive.
Temperature dependent Raman spectra of monoclinic and cubic Li2SO4
Authors: Cazzanelli Enzo , Frech Roger
Published: February 2003

Raman spectra of an oriented crystal of anhydrous Li2SO4 are reported in the temperature range 25–775 °C which includes the monoclinic-fcc phase transition at 575 °C. Qualitative differences in the general spectra in the two solid phases are discussed. A quantitative analysis has been carried out for the internal ν1 band of the SO2−4 ions, the high frequency component of the ν2 band, and the lithium lattice modes, both in Bg and in Ag symmetry. The broadening and frequency shift of the ν1 band are discussed in the light of current theories. In the ‘‘pretransitional’’ range below Tc the frequency of ν1 seems to exhibit critical behavior with the exponent equal to 1/6. A temperature-dependent coupling between lithium modes and the high frequency component of the ν2 band is revealed by analysis of the temperature dependence of the frequency and bandwidth of both modes in Bg symmetry. Comparison is made with similar phenomena in other lithium-containing sulfate crystals.
The carbon dioxide–hydrogen chloride complexes. A matrix isolation study and an a
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o calculation on the 1–1 species
Authors: Fourati N. , Silvi B. , Perchard J. P.
Published: February 2003

Vibrational (IR and Raman) spectra are reported for binary mixtures of H(D)Cl with CO2 in argon matrices at 10 K. At high dilution in both dopants the 1:1 complex is identified by three of its modes. In the presence of an excess of HCl with respect to CO2, a 2:1 complex (HCl)2CO2 with two non equivalent HCl molecules is evidenced. For CO2/HCl molar ratios larger than one a series of lines in the HCl region area characteristic of 1:n aggregates (n=2–4). Finally, for HCl/CO2 molar ratio of the order of one and Ar/HCl=500, two bands in the HCl region are believed to characterize the 2:2 species. For all of these complexes HCl vibrational perturbations are weak, all the bands being in the frequency range 2854–2790 cm−1. Base submolecule modes show small displacements towards high as well as low frequency, depending upon the structure of the complexes. Ab initio calculations on the 1:1 complex have been performed to obtain the equilibrium geometry, the hydrogen-bond dissociation energy and the two first multipole moments of CO2⋅⋅⋅HCl. Moreover, vibrational properties of the HCl submolecule (frequency shift, dipole moment derivative) are predicted in relatively good agreement with experimental data.
Laser induced fluorescence of the HgH and HgD A1 2Π1/2 state in the supersonic free jet
Authors: Mayama Shinya , Hiraoka Susumu , Obi Kinichi
Published: February 2003

The fluorescence excitation spectra and lifetimes of the HgH and HgD A1 2Π1/2 are obtained in the supersonic free jet. The rotational temperature of the ground state HgH and HgD radicals is cooled down to about 10 K in the free jet. The Morse potential parameters of the HgH and HgD A1 2Π1/2 states are determined. The fluorescence lifetimes of the A1 state are obtained to be 101 (v′=0) and 41 ns (v′=1) for HgH, and 131 (v′=0), 127 (v′=1), and 126 ns (v′=2) for HgD. The isotope effect of the fluorescence lifetimes is explained by the weak predissociation to the ground state atoms caused by the tunneling effect through the left-hand limb of the potential curves.
Classical trajectory-quantum forced oscillator study of gas–surface phonon scattering: He/Si(100)–(2×1)
Authors: Park Seung C. , Bowman Joel M.
Published: February 2003

Inelastic atom–surface phonon scattering for a model He/Si(100)–(2×1) system is investigated by the classical trajectory-quantum forced oscillator (DECENT) method. Energy transfer between the atom and the phonon modes is examined. Single and multiphonon transition probabilities of the normal modes are calculated for several initial beam orientations and one initial kinetic energy. A high degree of group-mode specificity for energy transfer is seen.
Thermal conductivity of polyacetylene
Authors: Schweizer R. J. , Menke K. , Roth S.
Published: February 2003

The thermal conductivity of polyacetylene has been measured in the temperature range of 0.8 to 300 K. For T<70 K we present also data of AsF5-doped samples and of doped samples which have been compensated with (CH3)2NH into the insulating state. The undoped sample shows a T2-temperature dependence up to 12 K followed by a decreasing power law at higher temperatures and a thermal conductivity value of 4 mW/K cm at room temperature. Doping and even compensating increases the thermal conductivity below 12 K. This additional conductivity is therefore explained by a change in the phonon boundary scattering at the interfaces between crystalline and amorphous regions of polyacetylene and not by a contribution of charge carriers to the thermal conductivity.
Electron energy loss spectroscopy of H2S adsorbed on Ni(100)
Authors: Baca A. G. , Schulz M. A. , Shirley D. A.
Published: February 2003

H2S adsorption has been studied on Ni(100) at temperatures between 110–295 K by means of electron energy loss spectroscopy, low-energy electron diffraction and Auger spectroscopy. Coverage dependence of the H2S adsorption has been studied at 110 K. At low coverages H2S dissociates to Sads+Hads. At higher coverages two distinct molecular species are observed; H2S with C2v symmetry (highest coverages) and either HS or H2S with Cs symmetry (intermediate coverages). Hads coexists with the two molecular species and Sads, and a H–Ni stretching frequency of 650 cm−1 suggests a four-fold hollow site for Hads. A saturation coverage is reached which produces a faint c(2×2) LEED pattern, and a comparison with D2S is made at this coverage. The temperature-dependent spectra shown dissociation of the molecular species at ∼170 K and Hads recombining and leaving the surface at 220 K.
An elastic hinge model for dynamics of stiff chains. III. Viscoelastic and Kerr-effect behavior of bent molecules
Authors: Roitman Daniel B.
Published: February 2003

The behavior of linear bent macromolecules with some flexibility is considered. The ‘‘trumbell’’ model with a restoring force at the hinge is extended by shifting the minimum energy configuration of the spring to an arbitrary bend angle Δ. The results show three main contributions: (1) the slowest relaxation λ1 corresponds to nearly pure end-over-end rotation. (2) the next relaxation λ2 is associated with the fastest rotational mode of the model, (3) the fastest relaxation λ3 is a mixed bend-rotation proportional to the relative force constant Z of the spring. The two rotational modes dominate the Kerr-effect behavior of the model while the fast bending-related mode becomes dominant at high frequencies in the viscoelastic behavior of the model.
Adsorption of hydrogen and deuterium on Ru(001)
Authors: Conrad H. , Scala R. , Stenzel W. , Unwin R.
Published: February 2003

The adsorption of hydrogen and deuterium has been studied with high resolution electron energy loss spectroscopy (HREELS). At maximum coverage at 105 K the H adlayer exhibits two dominant loss features at 102 (ν1) and 141 (ν2) meV which show pronounced intensity variations as a function of the primary electron energy (maximum between 5 and 6 eV). The hydrogen is assigned to the threefold coordinated site of the Ru (001) surface leading to a C3v symmetry of the adsorption complex. The angular dependence of both loss peaks shows only a smooth intensity decrease in off-specular directions thus preventing the separation of dipole and impact scattering contributions. Three additional loss peaks with considerably lower intensities are identified with overtone and combination excitations. The anharmonicity coefficients derived from the respective energies are used to fit a two-dimensional potential function for the hydrogen atom located in a plane perpendicular to the surface with first order perturbation theory. The resulting differences in the potential gradient allows the assignment of the ν1 and ν2 fundamentals to the vibrational modes parallel and perpendicular to the surface, respectively. As a function of increasing coverage both ν1 and ν2 shift to higher frequencies with a concomitant decrease in the peak width, the shift of the parallel mode being distinctively larger. At low coverages a third loss peak is observed which is ascribed to a different adsorption state of hydrogen. For deuterium the two fundamentals and the lowest overtone are observed and exhibit reasonably accurately the expected isotopic shift. The adsorption properties are found to be similar.
Recombinative desorption of H2 and D2 from Cu(110) and Cu(111): Determination of nonequilibrium rovibrational distributions
Authors: Kubiak Glenn D. , Sitz Greg O. , Zare R. N.
Published: February 2003

Rotational and vibrational state distributions have been determined for H2 and D2 recombinatively desorbing from clean Cu(110) and Cu(111) surfaces in ultra-high vacuum. A 2+1 resonance-enhanced multiphoton ionization technique is employed. Rotational distributions for both isotopes are found to be mildly non-Boltzmann and having mean rotational energies less than the surface temperature Ts and equal for both surfaces. The (v″=1)/(v″=0) vibrational population ratio for both H2 and D2, however, is ∼50 times greater than the value expected for an equilibrium ensemble at Ts for desorption from Cu(110) and ∼100 times greater from Cu(111).
The effect of pressure on the optical absorption edge of sulfur to 300 kbar
Authors: Peanasky M. J. , Jurgensen C. W. , Drickamer H. G.
Published: February 2003

The optical absorption edge of sulfur has been measured to 300 kilobar pressure. A diamond anvil cell with ruby fluorescence for pressure calibration was used.(AIP)
Magnetotropic cholesteric-to-nematic transition in lyotropic poly(γ-benzyl L-glutamate) liquid crystals in m-cresol
Authors: Toriumi Hirokazu , Matsuzawa Kazufumi , Uematsu Ichitaro
Published: February 2003

Magnetically induced cholesteric-to-nematic phase transition is examined for a lyotropic poly (γ-benzyl l-glutamate) liquid crystal in m-cresol, which udergoes a thermally induced cholesteric-sense inversion. The twist elastic constnat K22 in this system remains essentially constant in the polymer molecular-weight range of 9.6–24.6×104, and exhibits a slight decrease as the solution concentration is increased. The ratio of K22 to the diamagnetic anisotropy Δχ is obtained as K22/Δχ=13 emu cgs for a right-handed cholesteric phase at 30 °C, and 73 emu cgs for a left-handed one at 83 °C, both having similar cholesteric pitches. This result, indicating an exceeding reduction of Δχ at an elevated temperature than that of K22, is discussed in terms of the change in the sidechain secondary structure. The observed value of sidechain diamagnetic anisotropy and its temperature dependence are reproduced by using a phenyl-ring orientational order parameter determined from an independent deuterium NMR measurement.
A one molecular fluid approximation for diatomic fluid mixtures
Authors: Waisman Eduardo M. , Lebowitz J. L. , MacGowan David
Published: February 2003

We investigate a one component molecular fluid approximation for conformally similar molecules. We test this scheme on (two) mixtures of rigid homonuclear diatomic Lennard-Jones (LJ) fluids for which a limited amount of information from molecular dynamics simulations is available. For two components of approximately equal bond length but different LJ parameters our results compare favorably with the machine computations. From the very few simulation data available for equimolar mixtures of molecules differing only in their bond lengths we cannot reach any firm conclusion. Alternative procedures for treating general molecular fluid mixtures are discussed.
Energetics of long alkyl chains embedded in a crystalline matrix: (n-C18H37NH3)2CdCl4
Authors: White Mary Anne
Published: February 2003

Heat capacity measurements of (n-C18H37NH3)2CdCl4 from 10 to 370 K are reported. By analogy with homologs of shorter chain lengths, the heat capacity of a CH2 unit as a function of temperature has been derived. In addition, four phase transitions in the title compound are reported: at 349.6±0.1, 356.0±01, 359.5±0.3, and 365.6±0.3 K, with transition entropies of 132±1, 6.5±0.3, 9.6±0.3, and 95±1 J K−1 mol−1, respectively.
The dielectric properties at 9.1 GHz and nature of small polarons in nonstoichiometric uranium oxides
Authors: Tateno Jun
Published: February 2003

The dielectric constants of UO2, UO2+x, and U4O9 have been measured using the standing wave method at 9.1 GHz in the temperature range 233–300 K. The conductivity is calculated from the imaginary part of the dielectric constant. The conduction mechanism is attributed to small polaron hopping, since the mobility is very small (1.2×10−5 cm2/V s for x=0.15 at 300 K). From the analysis of the dependence of the conductivity on x and the activation process, it is concluded that there are two types of charge carriers; free carriers at low concentrations (x=0.07, 0.10) and carriers bound to interstitial oxygen ions at high concentrations (x=0.15, 0.20). Ea =0.15±0.01 eV and J=0.01±0.001 eV are obtained for free carriers and Ea =0.25±0.01 eV and J=0.08±0.005 eV for bound carriers. The dielectric relaxation time is estimated to be 2.7×10−11 s.
Excluded volume effects on stress transmission in rubber elasticity
Authors: Gao J. , Weiner J. H.
Published: February 2003

The generally accepted view of the nature of excluded volume effects in rubber elasticity is that they serve only to provide an isotropic pressure which maintains the system at constant volume. This question is reexamined here by the computer simulation of an idealized model representing one chain of a network enclosed in a tube which provides the confining effect of the remaining chains. This model demonstrates that intramolecular excluded volume effects make a substantial contribution to the stress transmitted across an arbitrary interior plane of the system. Furthermore, the magnitude of this contribution depends upon the chain extension. It is therefore conjectured that intramolecular excluded volume effects may make an anisotropic contribution to the stress tensor in rubber elasticity. The model results also cast light on the possibility of a statistical mechanical formulation for rubber elasticity in which only the atoms of an interior portion of the network are treated explicitly, while the surrounding network serves as heat bath and loading device.
The statistical mechanics of a liquid of two-state molecules. II. The competition between quantum mechanics and a condensed phase
Authors: Desjardins Steven G. , Stratt Richard M.
Published: February 2003

In this paper we study the behavior of liquids whose molecules have explicitly quantum mechanical internal degrees of freedom, with the aim of finding out how such internal degrees of freedom affect, and are affected by, their environment. After a review of the way in which general problems of this kind can be treated with discretized path integrals, we specialize to a model system in which the intramolecular dynamics is that of a tunneling system. The model problem is then solved in the mean field and mean spherical approximations. The internal degrees of freedom are always found to become more classical because of the bath, and the intermolecular interactions are always found to be weakened because of the quantum mechanics—a situation we summarize as a competition between quantum mechanics and the condensed phase. The nature of the phase transition for our model system, and the implications of our findings for other types of quantal degrees of freedom, are discussed. In particular, it is pointed out how the observed disappearance of tunnel splitting in ammonia vapor provides an experimental confirmation of our ideas.
Electron bombardment enhancement of the reactivity of graphite with atomic hydrogen: Threshold-like effects
Authors: Brice D. K. , Ashby C. I. H.
Published: February 2003

The chemical reactivity of graphite surfaces exposed to atomic hydrogen is enhanced under electron bombardment by an enhancement factor ε′ which depends on the incident electron energy. A sharp threshold-like increase in ε′ is observed for crystalline graphite at an energy Eth which depends on the angle of incidence θ of the electron beam with respect to the basal-plane surface normal. Measurements of the dependence of Eth on θ are reported here. Model calculations indicate that the effect is due either to a direct excitation of the electronic transition responsible for the enhancement or to plasmon production which subsequently drives the transition. Further experiments are suggested to distinguish between these mechanisms.
On migration effects in circular Couette flow
Authors: Goh C. J. , Phan-Thien N. , Atkinson J. D.
Published: February 2003

We report some Monte Carlo simulation data illustrating the migration effects produced by a circular Couette flow of a dilute suspension of dumbbell molecules. Our data support an approximate theory due to Aubert and Tirrell [J. Chem. Phys. 72, 2694 (1980)] in the central region of the flow field. By appending to Aubert and Tirrell’s solution, a boundary-layer correction, we obtain approximate expressions for the concentration profile of the dumbbell and the conditional shear stress which are well supported by our simulation data.
Collapse transition of a stiff chain
Authors: Muthukumar M.
Published: February 2003

We have derived the mean square end-to-end distance of a semiflexible chain with the two-body interactions using perturbation theory. Following the usual arguments, we have generalized the fifth-power type relation for the chain extension factor to arbitrary chain stiffness and the three-body interactions. Similar to the flexible chain, a stiff chain of finite length undergoes the coil–globule transition if the two (three)-body interaction is above (below) a certain critical value. The stiffness dependence of the critical values of the two-body and three-body interaction parameters is calculated.