bluuebunny/crossref_metadata_2025 huggingface dataset:
Semiempirical theory of chemisorption on narrow d-band metals
Authors: Doyen G. , Ertl G.
Published: February 2003

A previous developed model is applied to chemisorption of various adsorbates on Ni, Cu, Pd, and Ag surfaces. The aim of the calculations was mainly to predict the trends of various physical properties over a series of different adsorption systems. Adsorption energies, ionization energies, and energy profiles are evaluated and compared with experiment. The difference photoelectron spectra are derived to a first approximation by including the optical matrix elements for excitation into plane wave final states. Chemisorption of atomic hydrogen and oxygen is treated in detail. In the case of hydrogen chemisorption the results agree qualitatively well with conclusions drawn from more elaborate numerical calculations published recently. For oxygen chemisorption the importance of the multiplet structure of the O atom is stressed. In this model the large exchange splitting of the O 2p level is preserved in the adsorbed state; the main effect caused by coupling to the metal is a screening shift by about 8 eV towards lower ionization energies. The earlier treated adsorbates CO, NO, and N2 are included in the concluding discussion which attempts to extract the dominating physical aspects. Comparison with experimental data is generally encouraging.
Wien effect in mixed strong electrolytes
Authors: Chen Mou-shan
Published: February 2003

A method of Fourier transformations and matrix representations is formulated for the computation of Wien effect in diluted solutions of mixed strong electrolytes. The method permits the extension of the computation to systems containing any number of species of ions of any valence type. Thus, for the first time Wien effect of mixed electrolytes is computed from the fundamental equations of ion–ion interactions. At very low field strengths, where Ohm’s law is obeyed, the integral representation of Onsager and Kim is reproduced. At extremely high field strengths, the relaxation effect and the electrophoretic effect are computed to the second order term. At intermediate field strengths, the equations for the computation of the relaxation effect and the electrophoresis by triple integrations are derived.
G-intensity sum rule applications: XY3 molecules
Authors: Bruns Roy E. , Bassi A. B. M. S.
Published: February 2003

Using the pyramidal XY3 (X=N, P; Y=H, F) molecules as examples, the G-intensity sum rule of Crawford, expressed in terms of effective charges, is shown to be an effective test of the numerical results of vibrational analyses. Errors in previously reported effective charge values for NF3 and PF3 are corrected. Values for the as yet unreported polar tensors of these molecules are presented. Comparisons of these results with those previously reported for other molecules are made.
Studies in fluctuation analysis. II. Injected white noise as a solution to the phase problem of relaxation spectrometry
Authors: Romine William O.
Published: February 2003

As a continuation of the first paper in this series, we extend the analysis of the theory for obtaining kinetic rate constants, relaxation times, and Onsager flow coefficients to a macroscopic ’’fluctuation’’ level. The concept is based on the injection of a stochastic Markoffian (Gaussian) white noise as a thermodynamic perturbation in lieu of the classical relaxation approaches involving deterministic signals. As found with the case of microscopic statistics studied in the previous paper, we find that certain relations hold between the rate coefficients and the power spectral matrix, these being as follows: (1) the matrix of rate coefficients is four times the inverse of the ’’normalized’’ power spectrum of the measured reactant fluctuations in reduced (exactly determined) form at the zero frequency extrapolation; (2) the eigenvalues of the normalized spectral matrix at the zero frequency point are the relaxation times; (3) the coefficients of the Onsager ’’flow’’ or ’’conductance’’ matrix L are readily obtained from the spectral matrix inverse, the externally induced second moments, and the calculated intrinsic statistical mechanical second moments. The implications at the experimental level, notably with regard to this approach being a suitable solution to the ’’phase problem’’ of relaxation spectrometry, are discussed.
Studies of the chemiluminescent reaction Ho+N2O → HoO*
Authors: Tang S. P. , Wicke B. G. , Friichtenicht J. F.
Published: February 2003

The chemiluminescent spectrum of the reaction Ho+N2O →HoO*+N2 has been obtained, identifying three excited electronic states populated by the reaction. Each excited state is shown to result from a bimolecular reaction, and the relative chemiluminescent cross section vs kinetic energy for producing each excited state is obtained. The total scattering cross section for Ho+N2O is obtained vs kinetic energy. The total reactive cross section is estimated to be 16±4 Å2.
The OH and OD laser: Collision-induced energy transfer pumping
Authors: Smith John H. , Robinson Dean W.
Published: February 2003

Detailed studies of the relative intensities of pure rotational lasing in the v=0, 1, 2, 3 states of OH(2Π1/2) and the v=0, 1, 2, states of OD(2Π1/2) are presented. Differences in the laser patterns of these two molecules, such as variation in intensity maxima for specific J levels and double pulsed lasing phenomena, are explained on the basis of a computer simulation model incorporating R→T and V→R energy transfer mechanisms. The probability of collisionally induced R→T transitions is assumed to be given by exponential gap model in which the rotational parameter is evaluated by information theory (sum rule) approximations. The V →R relaxation is described in a two step process: first, the change of oscillator vibrational energy directly into oscillator rotational energy, and second, the absorption or removal of the energy defect (mismatch) by collisional R→T processes. The appropriate parameters for this transfer mechanism are also calculated with sum rule assumptions for the two molecules. The temporal evolution of nascent populations relaxing by the mechanisms lead directly to population inversions which corroborate in specific detail the lasing patterns observed experimentally.
Pumping iron: A KrF laser pumped atomic iron laser
Authors: Trainor Daniel W. , Mani Siva A.
Published: February 2003

A technique for producing stimulated emission in an optically pumped atomic iron system at room temperature is described. The required iron density (∼1014 atoms/cm3) for single pass amplified spontaneous emission was produced at room temperature by two techniques: a low pressure (50 torr) discharge of iron pentacarbonyl and neon and by the flash photodecomposition of Fe(CO)5 in an argon buffer. A commercial KrF laser producing output powers near 51 Mw was modified to improve the beam quality, and the beam was focused into the reaction cell. Resonant processes involving the 3d6 4p 4p (5F°) intermediate state of iron and the KrF laser field (λ?248 nm) produced stimulated emission near 300 and 304 nm in agreement with theoretical predictions.
Piecewise polynomial basis functions for configuration interaction and many-body perturbation theory calculations. The radial limit of helium
Authors: Silverstone Harris J. , Carroll Dennis P. , Silver David M.
Published: February 2003

Highly accurate configuration interaction and many-body perturbation theory calculations are shown to be possible with piecewise polynomial basis functions. In a preliminary calculation on the helium atom ground state, a new upper bound of −2.8790286 a.u. is found for the radial limit.
Dynamical effects in unimolecular decomposition: A classical trajectory study of the dissociation of C2H6.
Authors: Grant Edward R. , Bunker Don L.
Published: February 2003

The unimolecular dissociation of ethane via C–C and C–H scission has been simulated by trajectory calculation on a realistic and well coupled potential energy surface. Detailed lifetime distributions were obtained from this calculation for both C–C and C–H fragmentation channels at several energies (180, 210, and 240 kcal/mole) and for several energization patterns. The potential surface was also used to determine vibrational frequencies for the activated molecular model and various critical configurations. These frequencies were applied in the accurate census of phase space volumes for precisely tailored RRKM calculations. The trajectory results for the different energization patterns do not agree with corresponding statistical predictions nor with each other. The details of these deviations are discussed with regard to the importance of dynamical effects in intramolecular relaxation and unimolecular dissociation.
Hole photogeneration in binary solid solutions of triphenylamine and bisphenol-A-polycarbonate
Authors: Borsenberger P. M. , Contois L. E. , Hoesterey D. C.
Published: February 2003

By means of potential discharge techniques, the photogeneration of holes has been investigated in solid solutions of triphenylamine and bisphenol-A-polycarbonate. The electric field, temperature, and concentration dependences of the quantum efficiency were measured with 300-nm radiation. The experimental results have been analyzed within the framework of the Onsager theory of geminate recombination. According to this theory, the photogeneration efficiency is determined by the separation distance of the photoexcited electron–hole pair and the quantum yield of pair formation. Depending on the concentration, these values were determined as between 22 and 27 Å and approximately 10−2, respectively. Both are independent of temperature and increase with increasing triphenylamine concentration. From these results, it is concluded that the low intrinsic photogeneration efficiency is due to both the low primary quantum yield and the high recombination probability of the bound electron–hole pair.
Rapid and accurate evaluation of inelastic molecular cross sections
Authors: Tarr Susan M. , Rabitz Herschel
Published: February 2003

An inexpensive and accurate technique for computing inelastic cross sections has been developed by combining an exponential distorted wave (EDW) approximation and an effective Hamiltonian. This procedure is implemented through the use of uniformized WKB wavefunctions. It is shown to yield cross sections in semiquantitative agreement with those from exact calculations representing a wide range of physical parameters. The EDW cross sections can be used to compute rate constants, since good results are obtained even near threshold. The EDW procedure therefore provides a practical means for studying physical trends in cross sections, rate constants, and relaxation.
Electron spin resonance studies on Cu2+ doped at tetrahedrally coordinated Zn2+ sites in the single crystals of guanadinium zinc sulfate
Authors: Krishnan V. G.
Published: February 2003

Electron spin resonance studies have been made on Cu2+ doped single crystals of guanadinium zinc sulfate and incorporated at tetrahedrally coordinated Zn2+ sites at 295 °K. The spectra could be fitted to an axial spin Hamiltonian and spectral parameters characterize large g∥ value and unusually small hyperfine splitting for the paramagnetic ion. The metal–ligand bonding is predominantly ionic and the 3d–4p mixing is an important feature in explaining the small hyperfine coupling constant.
Kinetics of aerosol formation in irradiated gaseous NO2–SO2 mixtures
Authors: Iwamoto Kazutoshi , Presser Nathan , Ross John
Published: February 2003

We observe in gaseous mixtures of NO2 and SO2 the formation of white particles upon irradiation with 488 nm laser light and have measured the time of irradiation necessary for first detecting visually these particles at various total pressures, mole ratio of SO2 to NO2, and light intensities. A proposed simple reaction mechanism, up to the first reaction step of photoexcited NO2 and SO2, is consistent with the observations. The quenching coefficient of NO2* by SO2 is 5.7×1013 cm3/mole sec and the coefficient of the assumed rate-determining initial step, NO2*+SO2, is 8.2×1010 cm3/mole sec.
Revised central force potentials for water
Authors: Stillinger F. H. , Rahman A.
Published: February 2003

Computer simulation by the molecular dynamics technique has been used to investigate a modification of the previously introduced ’’central force model’’ for liquid water. The simulation involved 216 molecules, with periodic boundary conditions and Ewald summation, at 29.5°C and 1 g/cm3. In all respects considered (pressure, energy, pair correlation functions, self-diffusion rate) the revised set of interactions represents water more accurately than the earlier set.
The Raman spectrum of NF3: Coriolis coupling and transition dipole interaction in the liquid
Authors: Gilbert M. , Nectoux P. , Drifford M.
Published: February 2003

First order Coriolis coupling in an e-symmetry mode of a symmetric top and its consequences on the associated Raman line in the liquid are investigated for classical rotations and a classical harmonic vibration. Except for quantum asymmetries, the overall features of the Raman line are reproduced. The second and fourth classical moments of the e-symmetry Raman line are given together with an expression of the linewidth for Debye diffusion. The relevant autocorrelation function <Trβ (0) β (t) ≳ is obtained for free rotation and J rotational diffusion. The Raman spectrum of gaseous NF3 was recorded and the band contours of ν3(e) and ν4(e) were analyzed to determine the intensity ratio between the contributions of γ±1 and γ±2. The Raman profiles were studied in the plastic crystal and in the liquid between 60 and 185 K. The intense infrared lines ν1(a1) and ν3(e) show complex structures made up of two peaks at low temperature, similar to the LO-TO structures of the intense infrared lines of crystals and which are due to a particularly strong transition dipole coupling. The separation between peaks, which is proportional to the integrated infrared intensity of the lines, can be reasonably well predicted by the theory of cubic crystals. The isotropic part of ν1 looses its pseudo-LO-TO structure above 100 K and narrows as the temperature increases, due to rotational averaging of transition dipole coupling. The weak infrared lines ν2(a1) and ν4(e) have temperature-sensitive, symmetrical Raman spectra which were analyzed in terms of molecular rotations. The NF3 molecules rotate more easily about the main axis with a ratio between the mean angles Φ∥ and Φ⊥ of rotation about the main axis and the other axis, respectively, equal to ∼3 at 78 K. The Coriolis perturbation is found to reduce the contribution of the spinning motion to the linewidth of ν4, by a factor of 3 owing to the large value of the Coriolis constant ζ4=−0.895.
Selected-state photodissociation of glyoxal: Vibronic effects in the quantum yields of CO
Authors: Atkinson G. H. , McIlwain M. E. , Venkatesh C. G.
Published: February 2003

The absolute quantum yield for the dissociation of glyoxal into carbon monoxide following excitation to seven (00, 71, 51, 81, 81 72, 21, 81 41) single vibronic levels (SVL) in the 1Au state are presented. Samples in the 1–10 torr pressure regime were studied. Significant vibronic and pressure dependencies were observed for all seven SVL examined. Dependencies arising solely from the characteristics of specific vibronic modes were found following excitation to two levels (81 and 81 41). The results are interpreted in terms of an excited-state mechanism which includes dissociation from vibrationally excited 3Au levels.
Heat capacity of the binary liquid system triethylamine–water near the critical solution point
Authors: Thoen J. , Bloemen E. , Dael W. Van
Published: February 2003

The specific heat Cpx has been measured for the binary system triethylamine–water as a function of temperature near the critical solution temperature at the critical concentration. Simple power law fits of the data resulted in rather large values (around 0.25) for the critical exponent α. For a fixed value of α equal to the Ising value αI=0.125 good fits could only be obtained if more complex expressions were used in fitting the data. For this case the best result was obtained by including a correction-to-scaling term. Combining our result for the amplitude of the specific heat singularity with the correlation length amplitude allowed us to test two-scale-factor universality. Our result strongly indicates that, in contrast to earlier findings, two-scale-factor universality is also valid for binary systems.
A semiclassical determination of the energy levels of a rigid asymmetric rotor
Authors: Colwell S. M. , Handy N. C. , Miller W. H.
Published: February 2003

Recent applications of semiclassical methods to vibrational energy levels have renewed interest in finding a semiclassical method for rotational levels. Here a simple semiclassical method is used to determine the 2J+1 energy levels of an asymmetric rigid rotor.
Computation of transport coefficients by cell hydrodynamics
Authors: Visscher P. B.
Published: February 2003

A new technique for numerically calculating transport coefficients in classical fluids is described, based on a formulation of hydrodynamics in terms of discrete cells. It appears to have significant conceptual and practical advantages over present techniques, such as the Green–Kubo (GK) formulas. Like the GK formulas, it gives transport coefficients in terms of time correlations computed from molecular dynamics simulations. Unlike GK, however, it requires from molecular dynamics only short-time (a few collision times) correlations, among small cells. The large-scale motions causing the ’’long-time tail’’ and convergence difficulties in the GK approach are taken in account by a scaling procedure. This implies a substantial efficiency advantage; numerical calculations are underway to verify this directly.
Theoretical studies in photoelectron spectroscopy: Extraction of dynamical and structural information from the angular distributions for oriented or rotationally resolved unoriented molecular samples
Authors: Ritchie Burke , Tambe B. R.
Published: February 2003

We analyze the angular distributions of molecular photoelectrons for three different experimental situations: (i) photoionization in unoriented samples for rotationally unresolved transitions, (ii) photoionization in unoriented samples for rotationally resolved transitions, and (iii) photoionization in oriented samples. It is known that (iii) provides a means of determining the molecular orientation relative to the direction of polarization of the incident light and a probe of the anisotropy of the initial molecular orbital greater than that of (i). We show that (ii) also provides a probe of molecular orbital anisotropy greater than that of (i) and a probe of dynamical effects such as ’’l spoiling’’ and the penetration of waves of higher l into the inner regions of the molecular field greater than that of (i) or (iii). Calculations are carried out for the photoionization of H2, for which experimental results exist for processes (i) and (ii). Since molecular orbital anisotropy and the above mentioned dynamical effects are small in H2, it is a good example for the study of enhanced molecular effects available in the angular distribution for process (ii). However, the anisotropy of H2 (and in the field of H+2) is sufficiently small that the information in the angular distribution for process (iii) is overwhelmingly geometrical rather than dynamical or structural.
ESR of sulfur-substituted purines and nucleosides; sulfur- and oxygen-centered radicals in mercaptopurine crystals at 77 K
Authors: Sagstuen Einar , Alexander Chester
Published: February 2003

Single crystals of the sulfur-substituted adenosine analog, 6-methylmercaptopurine riboside, have been irradiated by 4.0 MeV electrons and studied by electron spin resonance techniques at 77 K. Three resonances with large g-value variations were observed. Two of these, R3 and R5, showed no hyperfine structure, but exhibited g-tensor principal values of: R3 as 2.0608, 2.0211, and 2.004; and R5 as 2.154, 2.003, and 1.989. These are assigned to a π-electron disulfide radical (R3) and a monosulfide RS, (R5). The third resonance (R4) exhibited a 41.5 G isotropic doublet splitting and the g-tensor principal values were 2.086, 2.001, and 1.989. This resonance is ascribed to a radical formed by hydrogen abstraction from the O3′ atom in the ribose sugar. The electronic and molecular structures of the radicals are discussed in some detail.
Formulation of the direct configuration interaction method for triplet spin states. Applications to glyoxal
Authors: Lucchese Robert R. , Schaefer Henry F.
Published: February 2003

The direct CI method developed by Roos and Siegbahn has been extended to triplet spin states. Included in the CI are all single excitations plus all doubly excited configurations having nonzero Hamiltonian matrix elements with the Hartree–Fock reference configuration. The new method has been implemented as a part of the berkeley system of minicomputer-based programs. Applications to the 3Au(n→π*), 3Bg(n→π*), and 3Bu(π→π*) states of glyoxal (HCO)2 are presented. Some modifications, following Bender’s vector method, of the original Roos method for closed-shell singlets are also discussed. These have allowed us to determine fully variational CI wavefunctions including as many as 15 006 configurations (42 456 Slater determinants).
Depolarized light scattered near the gas–liquid critical point of Xe, SF6, CO2, C2H4, and C2H6
Authors: Garrabos Yves , Tufeu Roland , Le Neindre Bernard
Published: February 2003

We report an experimental study of depolarized Rayleigh light scattered at 90° by two isotropic fluids (xenon and sulfur hexafluoride) and three anistropic fluids (carbon dioxide, ethane, and ethylene) near their respective gas–liquid critical points. All measurements were performed along the critical isochore, in the single phase region, that is to say for T≳Tc. The depolarization ratios, as shown by theory, are separable into two contributions. Far from the critical temperature single scattering of light is predominant but near the critical point double and multiple scatterings become the main phenomena. From measurements of polarized intensities in the temperature range corresponding to double scattering, the cross sections or the compressibility factors were calculated when a choice of a theoretical expression for the Rayleigh ratio was made.
High-resolution infrared spectrum of the ν4 band of POF3 by laser Stark spectroscopy
Authors: Amano T. , Schwendeman R. H.
Published: February 2003

Transitions in the ν4 band of POF3 have been observed by means of an infrared laser Stark spectrometer resonating on the CO2 R (38) to R (54) laser lines near 10.1 μm. Many POF3 transitions were observed for this perpendicular band with both ΔM=0 and ΔM=±1 selection rules. Many of the lines were observed as Lamb dips. The spectral assignment was confirmed by the observation of a number of infrared-microwave double resonances. Molecular constants obtained from analysis of the laser Stark spectra are ν4=29768578.6±1.0 MHz, A4=4803.06±0.16 MHz, B4 =4584.67±0.03 MHz, (Aζ)4=2429.0±0.2 MHz, μ4 =1.8394±0.0004 D, and μ0=1.8696±0.0004 D.
Quantum corrections to the equation of state of a fluid using hard-sphere basis functions
Authors: Singh B. P. , Sinha S. K.
Published: February 2003

We develop the expansion for the Slater sum, using a hard-sphere potential as a reference potential and the hard-sphere wavefunction as a basis set, which replaces the usual Wigner–Kirkwood series. Expressions are given for the first and second quantum corrections to the free energy arising due to perturbation potential. Results are given for the first and second quantum corrections to the second and third virial coefficients. We find that the quantum corrections to the higher virial coefficients depend on the potential at the core as well as the shape of the well. The general expression for the third virial coefficient of a gas at high temperature is given by Bq3 =Bc3+(5π2d6/18)[(3/√2) BI3(λ/d) +1.708 BII3(λ/d)2+⋅⋅⋅], where BI3 and BII3 are constants depending upon the shape and nature of the perturbation potential and equal to unity in the limit of the hard sphere [i.e., up(r) →0].
The electronic structure of trigonal Se and Te for pressure near the phase transition points
Authors: Starkloff Th. , Joannopoulos J. D.
Published: February 2003

Using a self-consistent pseudopotential approach, band structures and charge densities have been calculated for trigonal Se and Te at hydrostatic pressures just below the transition to the metallic phase. The results are compared to zero pressure calculations and experimental data. Very good agreement between experimental and theoretical results for the pressure dependence of the fundamental gap is found in both substances. Remarkable similarity is obtained between the Se and Te band structures at their respective phase transition points. This suggests that the transition leads to very closely related crystal structures.
Mass and momentum transport in dilute reacting gases
Authors: Xystris Nicolas , Dahler John S.
Published: February 2003

A formal kinetic theory and illustrative model calculations are presented for a mixture of four species coupled together by the two chemical reactions A+BC?AB+C. The cross sections for reaction are not restricted to be small and so the reactive collisions can not be treated as if they were small perturbations, but must be dealt with on an equal par with nonreactive events. To describe the dynamics of the reactive collisions we use an extension of the DIPR (direct interaction, product repulsion) model which takes into account rotational–translational exchanges of energy and, in a cruder way, vibrational excitation as well. Because the dynamics of these reactive events are not derived from a Hamiltonian, care must be exercised to insure that the condition of microreversibility is satisfied. A moment method is used to extract from the kinetic equation estimates for the nonequilibrium corrections to the reaction rates and for the changes of viscosity and diffusion coefficients due to the chemical reaction. Calculations reveal that the reaction rates can differ by as much as 80% from estimates based upon the assumption of an equilibrium distribution over reactant states. Furthermore, reaction can alter the coefficients of viscosity and diffusion by 10% or more. It is shown that the Onsager relations for the diffusion coefficients are valid only when chemical equilibrium prevails.
Enskog theory for chemically reacting fluids
Authors: Xystris Nicolas , Dahler John S.
Published: February 2003

Presented here is a theory of nonequlibrium processes for a dense fluid composed of chemically reactive, rigid-sphere molecules. Explicit consideration is given to the relocations of the molecular centers of mass that result from the occurrence of reaction and the accompanying rearrangements of chemical bonds. The collision operators are separated into source and flux parts in order to produce equations of motion which faithfully mimic those of continuum mechanics. As a consequence of this we are led naturally to the discovery of ’’collisional transfer’’ contributions to the mass fluxes of the reactive species. To transform these concepts into a computationally tractable theory we invoke the approximate Enskog functional relationship between the singlet and pair-space distribution functions. This is followed by the development of a procedure for estimating the nonequilibrium corrections to the reaction rates as well as the coefficients of viscosity and diffusion, with special attention being devoted to the collisional transfer contributions to the species’ mass fluxes.
The electronic states of Ar+2, Kr+2, Xe+2. I. Potential curves with and without spin–orbit coupling
Authors: Wadt Willard R.
Published: February 2003

The low-lying states of Ar+2, Kr+2, and Xe+2 have been investigated using the POL CI method. Spin–orbit coupling has been included with a simple atoms-in-molecule approach. The calculated dissociation energies for the ground I (1/2)u states of Ar+2 and Kr+2 are in good agreement (10% error) with experiment, while the agreement is slightly worse (20% error) for Xe+2. The well depth decreases from 1.19 eV in Ar+2 to 0.79 eV in Xe+2 mainly because of spin–orbit effects. As expected, the calculated bond distances increase from Ar+2 to Xe+2 as the atoms increase in size. The first excited state, I (3/2)g, possesses a small well (0.12 eV) at larger distances (3–4 Å) in all the rare gas dimer ions. The higher excited states arising from the lowest asymptote are more repulsive. There are three dipole-allowed transitions from the ground state. The I (1/2)u→I (3/2)g transition, which occurs in the near infrared, is very weak in accordance with propensity rules based on changes in Ω. The I (1/2)u→I (1/2)g transition occurs between 700–800 nm and grows dramatically in intensity from Ar+2 to Xe+2 because of spin–orbit effects. The predicted increase in intensity is in excellent agreement with experiment. Finally, the I (1/2)u→II (1/2)g transition is very strong with the intensity decreasing slightly for the heavier rare gas dimer ions. The absorption maxima are predicted at 319, 339, and 375 nm for Ar+2, Kr+2, and Xe+2, respectively, which are to be compared with the experimental values of 292 and 325 nm for Ar+2 and Kr+2, respectively. The theoretical values for the peak absorption cross sections are in good agreement with experiment for Ar+2 and Kr+2. Thus, although the calculated wavelengths for the peak absorption are too long by 20–30 nm, the size and shape of the calculated absorption bands should be in good agreement with experiment. Finally, a comparison is made between the ab initio SCF potentials for the 1Σ+g state of Ar2, Kr2, and Xe2 with the potentials predicted by the electron gas model.
A quasiclassical trajectory study of the energy transfer in CO2–rare gas systems
Authors: Suzukawa Henry H. , Wolfsberg Max , Thompson Donald L.
Published: February 2003

Computational methods are presented for the study of collisions between a linear, symmetric triatomic molecule and an atom by three-dimensional quasiclassical trajectory calculations. Application is made to the investigation of translational to rotational and translational to vibrational energy transfer in the systems CO2–Kr, CO2–Ar, and CO2–Ne. Potential-energy surfaces based on spectroscopic and molecular beam scattering data are used. In most of the calculations, the CO2 molecule is initially in the quantum mechanical zero-point vibrational state and in a rotational state picked from a Boltzmann distribution at 300°K. The energy transfer processes are investigated for translational energies ranging from 0.1 to 10 eV. Translational to rotational energy transfer is found to be the major process for CO2–rare gas collisions at these energies. Below 1 eV there is very little translational to vibrational energy transfer. The effects of changes in the internal energy of the molecule, in the masses of the collidants, and in the potential-energy parameters are studied in an attempt to gain understanding of the energy transfer processes.
Molecular theory of solvated ion dynamics
Authors: Wolynes Peter G.
Published: February 2003

We present a simple molecular approach for understanding the dynamics of a single ion in a polar solvent. The approach is based on a heuristic development starting from a resolution of the force–force correlation function of a Brownian ion, into parts depending on the attractive ion–solvent interaction and a repulsive, hard core ion–solvent interaction. We show that in the limit of infinitely weak, long range attractive ion–solvent interactions the molecular theory reduces to the phenomenological dielectric friction picture for the drag on an ion. The molecular theory also gives sensible results in the case of strong, short range attractive interactions. In this case the theory essentially indicates that the drag on the ion reduces to the drag on a composite body, i.e., a solventberg consisting of the ion and the molecules in the first solvation shell.
Spinodal decomposition in a binary liquid mixture
Authors: Goldburg Walter I. , Shaw Ching-Hao , Huang John S. , Pilant Michael S.
Published: February 2003

Light scattering was used to study phase separation near the critical temperature Tc in a critical mixture of 2,6-lutidine and water. This system has an inverted coexistence curve, so that a quench into the two-phase region is produced by an upward jump in temperature. The scattered intensity ℐ (q,t) was recorded at various angles and at a number of quench depths ΔTf =Tf−Tc in the range 0.5≲ΔTf≲2.5 mK. Here Tf denotes the final temperature. The initial temperature was also varied, and no initial-state effects were observed. One set of experiments employed a cell of very short optical path (0.1 mm) to minimize multiple scattering at the sacrifice of quenching speed. In another set, emphasis was placed on achieving a temperature jump in roughly 0.1 sec so that phase separation could be followed in its early stages. For ΔTf≲1 mK, the change in the measured ring diameter (qm−1) with time, is in fair agreement with the nonlinear theory of Langer, Bar-on, and Miller. However, the intensity of the ring, ℐ (qm,t), increases faster than this theory predicts. The measurements also reveal that the results depends significantly on the quench rate as well as quench depth.
Optical and elastic properties of linear metal–chain compounds: The 1,2-benzoquinonedioximates (bqd) of nickel, palladium, and platinum, M(bqd)2
Authors: Brill J. W. , Mégnamisi-Bélombé M. , Novotny M.
Published: February 2003

Polarized single crystal reflectance spectra of the linear metal–chain complexes Pt(bqd)2, α-Pd(bqd)2, β-Pd(bqd)2, Pd(bqd)2I0.55, Ni(bqd)2, and Ni(bqd)2I0.5 (bqd−=ion of 1,2-benzoquinonedioxime, bqdH) have been recorded at room temperature. Well-defined sharp absorption bands polarized parallel to the metal atom chains are observed in the frequency region from 0.5 to 2.5 eV, and are most reasonably identified as the interband electronic transitions between the ndz2 valence and the (n+1) pz conduction bands in a direct optical excitation process (n=3, 4, 5). The elastic behavior (Young’s modulus and internal friction) examined using a vibrating reed technique in the temperature range 17–300 K reveals, in the case of α-Pd(bqd)2, a peculiar phase transformation occurring along the metal atom chains below 100 K. From powder x-ray diffraction patterns recorded at ambient and at liquid nitrogen temperatures, this phase transition is interpreted as a lattice distortion resulting in the formation of stable chains of dimerized neutral metal complexes.
The charge transfer in some alkaline salts of tetracyanoquinodimethane (TCNQ): A Zeeman perturbation study of the 14N nuclear quadrupole resonance spectrum of polycrystalline Rb–TCNQ
Authors: Murgich Juan
Published: February 2003

Two 14N nuclear quadrupole resonance absorption lines (2904.4 and 2967.7 kHz) detected in Rb–tetracyanoquinodimethane (TCNQ) at room temperature were studied by the Zeeman perturbation effect and the results interpreted in terms of the theory of Pissanetzky [J. Chem. Phys. 59, 4197 (1973)]. The two lines were identified as ν+ with η of 0.40±0.04 and 0.57±0.05 and coupling constants of 3490 and 3254 kHz, respectively. The observed spectrum is compatible with a crystal structure with two inequivalent nitrogen sites such as the monoclinic phase of Rb–TCNQ II. Using the Townes and Dailey theory and an estimation of the effects of the crystal field, and by comparison of the NQR data of K, Rb, and neutral TCNQ, we found an increase of about 10% to 14% in the πx population and a decrease of about 5% in the σ population at the nitrogen. A two-way charge transfer within the CN group seems to take place upon the molecular charge transfer as predicted by Clementi and Klint [J. Chem. Phys. 50, 4899 (1969)].
Lattice model studies of the effect of chain flexibility on the nematic–isotropic transition. II. Rigid cores having semiflexible tails
Authors: Dowell F. , Martire D. E.
Published: February 2003

A steric, mean field, three-dimensional, simple cubic lattice model (using DiMarzio lattice statistics), with and without attractive forces, for molecules composed of rigid cores and semiflexible pendant tails is presented. The calculated trends and absolute values of the following properties at the nematic–isotropic transition for cores of length-to-breadth ratios of 4 and 5 with tails of varying length are compared with relevant experimental data: temperature, core order parameter, intramolecular and intermolecular tail order parameters, densities, relative density change, and entropy change. With results intermediate between those for the completely flexible tail model and those for the rigid rod models, this semiflexible tail model reasonably describes the general features and trends observed experimentally in homologous series of liquid crystals at the transition. Tail chain flexilibity (important in the transition) at a given temperature is rather adequately treated as an entirely internal property in this model. Steric repulsions are sufficient to generate the transition; attractive interactions, while not essential, do allow some adjustments in trends and some improvements in energetics. Preliminary calculations indicate that a softened repulsive potential leads to somewhat improved values for the core order parameter and the relative density change.
Stochastic model for induced consecutive transitions with application to Rydberg angular momentum transfer
Authors: Herrick David R.
Published: February 2003

Theory is offered for a stochastic model describing induced excitation and transfer, decay, and subsequent detection of particles occupying a finite manifold of states. In intense excitation-transfer fields k the observed particle density for excited states becomes ρ″∝kt−1/σΔl, where t is the excitation time and σΔl is a phenomenological cross section characteristic of transfer between states in the manifold. Formulas are derived which relate σΔl simply to parameters governing the experiment, and thus provide insight to values of σΔl measured for electron impact excitation and angular momentum transfer of high-Rydberg atoms. A problem is posed wherein the model is generalized to include a variable dimensionality as one way of introducing nonlinearity for transition cross sections connecting states in the manifold.
Overtone fluorescence as a probe of intermode energy flow in laser pumped molecules
Authors: Sheorey R. S. , Slater R. C. , Flynn G. W.
Published: February 2003

Laser induced fluorescence from the overtone 2ν3 of the pumped ν3 C–F stretch fundamental in CH3F has been observed. The risetime of this fluorescence, which corresponds to equilibration of ν3 and 2ν3 by a resonant collision process, is very fast. The fluorescence decay, on the other hand, reflects loss of population from ν3 into the other CH3F modes since on the time scale of the decay, ν3 and 2ν3 are tighty coupled. Specifically, three decay rates have been observed, the slowest of which is simple vibration–translation/rotation relaxation. The two fast decay rates are controlled largely by direct vibration to vibration energy transfer processes coupling the ν3, ν6 and ν2, ν5 levels. The measurement of these two V–V rates provides two internal rate constants which describe the collisional coupling between ν3 and ν6 and between ν6 and ν2, ν5. The breathing sphere model predicts too small a value for the probability of ν3→ν6 transfer and too large a value for the ν6→ν2, ν5 transfer.
Comparison of the cross sections for reaction of methyl iodide with potassium and rubidium
Authors: Wu K. T. , Pang H. F. , Bernstein R. B.
Published: February 2003

The ratio of the cross sections σR for the reactions of CH3I with K and Rb has been determined over the range of translational energies Etr up to 1.7 eV. The existence of a threshold energy, an Arrhenius-like positive energy dependence of σR just above threshold, and a postmaximum decline of σR to a minimum with increasing Etr are characteristic features of these reaction systems. Combining the present experimental results with those previously published, it has been possible to construct best overall smooth curves of σR(Etr) for each of the two reactions, suitable for comparison with theoretical calculations.
Effect of pressure on molecular conformations. I. Internal rotation angle in trithiane
Authors: Lewis G. J. , Whalley E.
Published: February 2003

The effect of hydrostatic pressure on the Raman spectrum of polycrystalline trithiane up to 17.8 kbar has been measured to test a suggestion based on the infrared spectrum that the molecule goes from the puckered to the flat conformation at pressures of a few tens of kilobars. The disappearance of four infrared bands is consistent with flattening, but only after three of them have been reassigned, and the lack of disappearance in the C–H stretching region is also consistent after some reassignment. At 17.8 kbar the A1, ν2 internal rotation, or ring flattening, vibration decreased in intensity relative to its neighbors to about one quarter of its value at zero pressure, which suggests that the molecule is about halfway towards being flat. A first approximation to a theory suggests that the flattening is plausible.
Semiclassical calculation for collision induced dissociation. II. Morse oscillator model
Authors: Rusinek Isak , Roberts Robert E.
Published: February 2003

A recently developed semiclassical procedure for calculating collision induced dissociation probabilities Pdiss is applied to the collinear collision between a particle and a Morse oscillator diatomic. The particle–diatom interaction is described with a repulsive exponential potential function. Pdiss is reported for a system of three identical particles, as a function of collision energy Et and initial vibrational state of the diatomic n1. The results are compared with the previously reported values for the collision between a particle and a truncated harmonic oscillator. The two studies show similar features, namely: (a) there is an oscillatory structure in the Pdiss energy profiles, which is directly related to n1; (b) Pdiss becomes noticeable (≳10−3) for Et values appreciably higher than the energetic threshold; (c) vibrational enhancement (inhibition) of collision induced dissociation persists at low (high) energies; and (d) good agreement between the classical and semiclassical results is found above the classical dynamic threshold. Finally, the convergence of Pdiss for increasing box length is shown to be rapid and satisfactory.
Theoretical study of wings of resonance lines of lithium and sodium perturbed by rare gases
Authors: Herman P. S. , Sando K. M.
Published: February 2003

A new quantum mechanical method for the evaluation of intensities in the wings of allowed atomic spectral lines is reported. Mathematical and computational comparisons between classical and quantum mechanical methods are made. Wing intensities predicted from three sets of interaction potentials are compared with observed intensities for the red wing of the Li resonance line perturbed by He. Discussions of qualitative features of alkali metal–rare gas potentials and of satellite bands in the wings of the spectral lines of Li and Na perturbed by rare gases are given.
Resonant Raman studies of pyrazine with tunable uv lasers: An analysis based on the generalized vibronic theory of Raman intensity
Authors: Hong Hwei-Kwan , Jacobsen Clayton W.
Published: February 2003

The resonant Raman effect of pyrazine is studied with tunable uv lasers. Polycrystalline samples of pure pyrazine and pyrazine in benzene were studied at low temperature to eliminate fluorescence. Genuine Raman spectra were obtained with this cryogenic technique as close as 40 cm−1 below the 0–0 of the 1B3u transition. Using the benzene 992 cm−1 band as the reference, absolute Raman cross sections were obtained from the known absolute Raman cross section of the benzene line. It was found that ν6a and ν10a have an enhancement factor of 106, corresponding to the strong vibronic coupling case. The data were analyzed by using the strong vibronic coupling theory of Raman intensity recently developed by Hong [J. Chem. Phys. 67, 801 (1977)]. In this generalized vibronic theory both the Franck–Condon effect (for ν6a) and the nonadiabatic Herzberg–Teller coupling (for ν10a) are treated in similar fashion through the use of the Green’s function. Vibronic coupling strengths were derived from analyses of the absorption spectra and used to calculate the absolute Raman cross sections. Agreement between theory and experiments is generally good. Comparison between the Green’s function approach and the traditional weak coupling theory is also made. It is emphasized that Raman spectroscopy and electronic spectroscopy are intimately related as is evident from the success of the theory to predict consistently not only the spectral features of absorption but also the absolute Raman cross sections.
The spin-system response to a dipolar spin coupling modulation. Theoretical description of NMR spectrum narrowing effects observed on polymer-solvent systems. Experimental illustration from polydimethylsiloxane chains
Authors: Cohen-Addad J. P. , Domard M. , Herz J.
Published: February 2003

The response of a spin system to a dipolar spin coupling modulation is quantitatively described to the second order in the spin interaction. The dependence of side band amplitudes upon the spinning frequency is given. Sample spinning effects are considered in different general cases where internal motion of nuclei, or exchange narrowing, are involved in the mechanism governing the loss of spin quantum coherence. Illustrations of some properties characterizing sample spinning effects are presented from entangled polydimethylsiloxane chains in solution and from a partly swollen polydimethylsiloxane network structure.
Absorption studies of Cs2 and Rb2 molecular bands in the visible and near visible
Authors: Gupta R. , Happer W. , Wagner J. , Wennmyr E.
Published: February 2003

By analyzing the temperature dependence of the absorption spectrum of Cs2 molecules in the visible region of the spectrum we have demonstrated that a group of unusually narrow bands near 7100 Å originate from a very shallow and possibly slightly repulsive part of a ground state potential curve. This may be part of the 3Σ ground-state potential curve. An analogous set of narrow bands occurs for Rb2 near 6000 Å. The remaining prominent visible absorption bands for Cs2 and Rb2 all originate from molecules near the bottom of the 1Σ ground state.
Ion pair and partially hydrated Li+NO3− ion pair structures: Correlation of molecular orbital results with matrix isolation data
Authors: Moore J. C. , Devlin J. Paul
Published: February 2003

The results of a molecular orbital study for the ion pair Li+NO3−, alone and hydrated, have been related to the vibrational data for the same species matrix isolated in argon. Initially it was established that a plot of the N–O bond overlap population vs empirical FNO force constants, for fourteen molecules, is impressively linear. Subsequently this plot has been used to convert the Gaussian 70-derived N–O overlap populations for the N–O bonds of Li+NO3−, alone or hydrated, to FNO values for the particular species. In the simple Li+NO−3 pair, for example, the minimum energy bidentate structure has a significantly smaller overlap population in the two metal-coordinated N–O bonds. These overlap populations indicate an N–O stretching force constant difference of 3.94×102 N m−1 which can be converted to a 245 cm−1 splitting of the otherwise degenerate ν3 antisymmetric stretching mode. The latter value compares well with the experimental value of 265 cm−1 and, together with equally encouraging comparisons for the hydrated ion pair, suggests that the Gaussian 70 results for these systems, even using the STO-3G minimum basis set, realistically depict the bonding for Li+NO3− as well as the influence of hydration on that bonding.