bluuebunny/crossref_metadata_2025 huggingface dataset:
A cluster theory for electrolytes
Authors: Tani A. , Henderson D.
Published: February 2003

A simple extension of the Bjerrum theory is formulated and examined. Clusters of trimers as well as dimers are included. The intercluster partition function is evaluated in the mean spherical approximation. At low concentrations this cluster theory gives good results for the thermodynamic functions and for the radial distribution function for unlike pairs. The radial distribution function for like pairs is less satisfactory. The theory is less successful at higher concentrations but is always at least as good as the mean spherical approximation.
A Monte Carlo off-lattice method: The slithering snake in a continuum
Authors: Brender Chava , Lax Melvin
Published: February 2003

An off-lattice Monte Carlo simulation of a polymer chain using the slithering snake method is introduced. The mean square end-to-end distance is calculated and compared with previous studies. Results indicate that small angular deviations from the tetrahedral lattice models produce large conformational change in the polymer chain.
Diffusion-controlled flocculation: The effects of attractive and repulsive interactions
Authors: Meakin Paul
Published: February 2003

A Monte Carlo computer model has been used to investigate the effects of repulsive interactions (sticking probabilities less than 1.0) and attractive interactions (particle capture) on diffusion-controlled aggregation processes. We find that particle capture leads to clusters with lower densities, whereas small sticking probabilities lead to higher density clusters. The radii of gyration (Rg) of clusters grown under diffusion-controlled conditions can be related to the number of particles in the cluster by Rg=ANβ. Despite the considerable difference in densities, we find that clusters grown with small sticking probabilities or large capture radii give quite similar values for the radius of gyration exponent (β) in the limit of large cluster sizes. The model details are expressed in the radius of gyration amplitude factor A. The results reported in this paper were obtained using a two-dimensional nonlattice model. We expect that similar results will be found in three-dimensional simulations.
Temperature dependence of the Maier–Saupe order matrix from 2H–NMR of fluorene-d1 in the nematic, smectic A, B, and C phases
Authors: Vaz Nuno A. P. , Doane J. William
Published: February 2003

From 2H–NMR of perdeuterated fluorene (diphenylenemethane-d10) dissolved in 4-n-butyloxybenzylidene-4-n-octylaniline (4O.8) and in a mixture of 4O.8 with 4-4′-bis-n-heptyloxyazoxybenzene (HOAB, 15.1 wt. %) we have measured each of the orientational order parameters of the Maier–Saupe order matrix and their temperature dependence in the nematic, smectic A, smectic B, and smectic C phases, respectively. The fluorene-d10 molecule, being a rigid structure of known conformation and one which contains a sufficient number of differently oriented deuterated sites, allows for the complete determination of the order matrix in terms of the measured 2H–NMR spectral splittings. The results show: (1) The principal molecular z axis (most ordered axis) is parallel to a direction which joins the centers of the six membered rings at all temperatures and in each phase studied; (2) nonlinearities are present in the temperature dependence of the ratios of the splittings and are a result of the temperature dependence of the elements of the order matrix, Szz and Sxx–Syy, demonstrating that a change in molecular conformation is not necessarily the cause of such nonlinearities as suggested in the literature; (3) splitting ratio vs ratio plots are linear throughout all phases of 4O.8 and 4O.8+HOAB mixtures; (4) the degree of ordering of the solute (fluorene) is significantly reduced in the smectic phases, in confirmation of that suggested by other solute studies reported in the literature.
Renormalization group theory of transport properties of polymer solutions. II. Rotatory friction coefficient vs intrinsic viscosity
Authors: Oono Y. , Kohmoto M.
Published: February 2003

The rotatory friction coefficient fr is calculated with the aid of the renormalization group theory. The present calculation shows that the ratio of fr to the intrinsic viscosity [η] depends on the excluded volume and the draining effects. The calculated ratio fr/[η] is about ∼5%–7% less than the conventional value which is obtained by using the preaveraging approximation of the Oseen tensor. The conventional value is independent of the excluded volume and draining effects.
Reactions of energetic tritium atoms with ethyl fluoride over an extensive pressure range
Authors: Tang Y.-N. , Wu E.-C. , Anderton J. W. , Clark R. R.
Published: February 2003

Recoil tritium reactions with ethyl fluoride have been studied over a pressure range of six orders of magnitude. The results show that subsequent to T-for-H substitution, the fraction of collisionally stabilized C2H4TF ranges from 0.05 at 10−2 Torr to 0.91 in the liquid. The low pressure results indicate that 95% of the C2H4TF molecules being formed possess an excitation energy of 65 kcal mol−1 or higher. The high pressure and liquid phase studies suggest that about 15% of the excited molecules have energies distributed in a very long and narrow tail at the high energy end.
Application of scaling theory to vibrational relaxation in linear anharmonic triatomic molecules
Authors: Clary D. C. , DePristo Andrew E.
Published: February 2003

The energy-corrected-sudden (ECS) scaling theory is extended to vibrational relaxation in the collisions of anharmonic linear triatomic molecules with atoms. Application is made to the collisions of He atoms with 12C 16O2, 14C 16O2, and 12C 18O2. By combining the rate constants for the (0110 → 0000) transitions, calculated using the vibrational close-coupling rotational infinite-order-sudden (VCC-IOS) method, with the ECS scaling theory, we predict rate constants for the transitions (1000 → 0110), (0220 → 0110) and (0200 → 0110). These agree very well with the rate constants computed directly using the VCC-IOS technique. This comparison presents a particularly severe test of the accuracy of the ECS scaling theory for anharmonic polyatomics since Fermi resonance effects are large for the 1000 and 0200 levels in CO2.
Site dependence of the 1 1A
g to 2 1A
g transition of all-trans-1,3,5,7-octatetraene
Authors: Kohler Bryan E. , Snow Judith Bannon
Published: February 2003

Highly resolved emission and one-photon fluorescence excitation spectra and fluorescence lifetimes for all-trans-1,3,5,7-octatetraene substituted in a series of n-alkane crystals maintained at liquid helium temperatures have been measured. In the n-octane host, the 1 1Ag to 2 1Ag transition is strictly symmetry forbidden: In other hosts, the spectra exhibit both allowed and symmetry forbidden components. While the relative intensity of the allowed (built on the 0–0) and the symmetry forbidden (built on odd symmetry vibrational modes) components of the spectra are sensitive to local perturbations, the energies of the excited 1 1Ag and 1 1Bu states and the vibrational development of the 1 1Ag to 2 1Ag transition are nearly the same in all of the hosts studied. The increase in the allowed component of the spectrum with increasing site perturbation correlates with a decrease in the measured fluorescence lifetime.
On the vibrational frequency correlation in liquids: Raman study of aqueous thiocyanate solutions
Authors: Katō Toshiko
Published: February 2003

The vibrational frequency correlation functions 〈ω(0) ω(t)〉 are calculated from isotropic Raman spectra for the CN stretching mode of the SCN− anion in aqueous LiSCN, NaSCN, and KSCN solutions, at concentrations 1–10 M, and at temperatures of 30, 55, and 80 °C. The experimental 〈ω(0) ω(t)〉 shows rapid relaxation within a particular environment on a subpicosecond time scale, accompanied by constant positive correlation. It is concluded that the homogeneous dephasing is due to the fast relaxation of 〈ω(0) ω(t)〉, and that the inhomogeneous dephasing comes from the static positive correlation, which should be attributed to a distribution of environments persisting over the time scale of Raman probe. The analysis of 〈ω(0) ω(t)〉 confirms that the inhomogeneous broadening increases with decreasing temperature, with increasing salt concentration, and in the order KSCN<NaSCN<LiSCN.
Theoretical interpretation of the resonance Raman spectrum of gaseous chlorine
Authors: Ghandour F. , Jacon M. , No/rby Svendsen E. , Oddershede Jens
Published: February 2003

A theoretical calculation of the Raman band profiles of gaseous chlorine excited by a 3638 Å line is performed. The resonance part of the Raman amplitude is calculated within a semiclassical approach and the nonresonant part by an ab initio calculation. The profiles of the overtones are well described by considering only the resonant scattering via the excited electronic state C 1Π(1u) and the anomalous value of the fundamental band intensity compared to the first overtone is shown to be due to virtual transitions to higher electronic states (normal Raman effect).
Comment on ‘‘Force in SCF theories’’
Authors: Pulay Péter
Published: February 2003

It is shown that the method suggested by Nakatsuji et al. [J. Chem. Phys. 77, 3109 (1982)], i.e., the calculation of Hellmann–feynman forces in conjunction with an extended ‘‘family’’ basis set, is not competitive with the regular quantum chemical gradient method.
Quantitative interpretation of the absorption and emission spectra of 1,8-diphenyl-1,3,5,7-octatetraene
Authors: Henneker W. H. , Siebrand W. , Zgierski M. Z.
Published: February 2003

Diagonalization of two-state, two-mode Hamiltonian matrix accounts quantitatively for the absorption and emission spectra of 1,8-diphenyl-1,3,5,7-octatetraene observed by Ikeyama and Azumi [J. Chem. Phys. 76, 5762 (1982)].
Energy and phase relaxation of phosphorescent F centers in CaO
Authors: Glasbeek M. , Smith D. D. , Perry J. W. , Lambert Wm. R. , Zewail A. H.
Published: February 2003

In this paper we study the temperature-induced homogeneous broadening of the no-phonon line in the emission spectrum of the F center in CaO. The linewidth can be fitted to n̄(n̄+1), where n̄ is the thermally averaged occupation number of phonons with a frequency of 90 cm−1. The results are characteristic of elastic scattering of pseudolocalized phonons at the defect site. These phonons also appear to dynamically couple the Jahn–Teller components of the F center in the photoexcited 3T1u state and thus give rise to a temperature dependence of the lifetime of this phosphorescent state. Finally, from experiments using laser-selective excitation it is concluded that the zero-phonon emission peaking at 571.1 nm does not originate in the F center.
Reaction velocities and the affinity decay rate
Authors: Garfinkle M.
Published: February 2003

Reaction velocities are calculated for several chemical reactions with diverse mechanisms from thermodynamic considerations alone. These are compared to reaction velocities computed in the conventional manner from mechanistic considerations. The comparisons indicate that a linear phenomenological relationship between chemical affinity and reaction velocity cannot be justified and that there exists a basic compatibility between the thermodynamic and the mechanistic approach to reaction kinetics.
State-to-state dissociation dynamics in CF3NO
Authors: Bower R. D. , Jones R. W. , Houston P. L.
Published: February 2003

The state-to-state dissociation dynamics of CF3NO have been investigated by exciting the parent compound in a supersonic jet expansion with one tunable laser and monitoring the state distribution of the NO fragment by one- or two-photon laser-induced fluorescence using a second tunable laser. At the lowest levels of excitation, CF3NO dissociates to give an NO distribution which is consistent with a statistical distribution of energy in the products. As more highly excited vibronic states of S1 are selected, the product distribution begins to show signs of nonstatistical behavior, although the deviations are small. The appearance time of the NO decreases nearly monotonically with increasing energy above the S1 origin and is equal to the fluorescence lifetime of the CF3NO* to within experimental error. It is likely that both the appearance time of the NO and the fluorescence lifetime of the CF3NO* measure the rate of internal conversion from S1 to highly excited vibrational levels of S0, which then dissociate in a nearly statistical fashion on the S0 surface. The barrier to dissociation of the CF3–NO bond is 39.61±0.23 kcal/mol.
Calculation of product distributions by perturbation of the transition state
Authors: Pollak Eli , Pechukas Philip
Published: February 2003

Two algorithms are presented for a direct determination of the boundary between reactive and unreactive portions of phase space in collinear collisions. These algorithms provide a fast and highly accurate determination of classical reactant and product distributions. Since boundary trajectories originate at variational transition states, this method provides new insight to an old problem: the relationship between product and reactant state distributions and the transition state of a chemical reaction.
Translational temperature dependence of mode-to-mode vibrational energy flow in 1B3u naphthalene induced by low energy collisions with Ar
Authors: Moss David B. , Kable Scott H. , Knight Alan E. W.
Published: February 2003

Mode-to-mode vibrational energy flow from the 8̄1 (b1g) level in the 1B3u state of naphthalene (C10H8) has been mapped in the energy regime εvib≤800 cm−1. Vibrational state changes are induced by low energy collisions with the carrier gas Ar in the warm to cool regions of a supersonic expansion. The pattern of energy transfer is estimated from time-resolved dispersed fluorescence spectra obtained following laser excitation of the absorption transition 8̄10. Propensities for particular transfer channels are found to be in qualitative accord with expectations based on studies of single-ring aromatics such as benzene and aniline. One-quantum changes are preferred over two-quantum changes and an energy gap law is evident. The competition between certain vibrational energy transfer channels is examined as a function of the translational temperature Ttrans at specific distances X/D from the nozzle aperture. At X/D≤1.5, evidence is found for both endoergic and exoergic transfer channels. Ttrans at X/D=1.5 is determined from our data to be 80±10 K. It is established that the collision energy here is sufficient to support vibrational relaxation out of the 8̄1 level via some endoergic transfers. The most efficient endoergic channel involves the addition of a single quantum of a low frequency out-of-plane mode with εvib=127 cm−1. At X/D≳2, endoergic channels appear to close and only exoergic transfer is observed. The dominant exoergic channel from 8̄1 is found to be transfer to the 1B3u zero point level, reached through the loss of the ν8̄ quantum (ΔE=435 cm−1). The collision energies at which endoergic transfer ceases to occur are found to agree with expectations based on an assumption of Maxwell–Boltzmann statistics and the estimation of Ttrans for particular values of X/D using our spectroscopic data.
Theoretical Compton profile anisotropies in molecules and solids. IX. Chemical bonding and 0–90 anisotropies in the first-row diatomic hydrides AH
Authors: Pettitt B. M. , Matcha Robert L. , Ramirez B. I.
Published: February 2003

An analysis of the parallel–perpendicular Compton profile anisotropies in the first-row diatomic hydrides is presented. Both the total and molecular orbital anisotropies present trends that may be related to the nature of the chemical binding in the molecules.
Improved calculations on the outer-valence Auger spectrum of F2
Authors: Liegener Christoph-Maria
Published: February 2003

The outer-valence Auger spectrum of F2 has been calculated by methods of many-body perturbation theory. The results have been compared with the recently recorded experimental spectrum and some partially new assignments have been proposed.
A theoretical study of the properties of BH3NH3
Authors: Binkley J. S. , Thorne L. R.
Published: February 2003

Borane monoammoniate (BH3NH3) has been studied using several ab initio electronic structure methods and Gaussian basis sets. Equilibrium geometries have been computed at the Hartree–Fock level and, using the electron-correlated Mo/ller–Plesset perturbation method, carried out to third order (MP3) with double-zeta polarized quality basis sets. The computed MP3 geometry is in close agreement with recent microwave data; electron correlation is found to be necessary for a proper description of the B–N distance. Hartree–Fock dipole moments and harmonic vibrational frequencies are presented and discussed. Mo/ller–Plesset perturbation theory carried out to fourth order with triple-zeta plus polarization basis sets is used to compute a B–N dissociation energy of 34.7 kcal mol−1 and a (Hartree–Fock zero-point corrected) rotational barrier of 2.065 kcal mol−1, which is in excellent agreement with the experimental value. Analysis of the dissociation energy as a function of perturbation order indicates that terms involving triple and quadruple substitutions are required in the dissociation energy.
The properties of LiH in its ground and first excited electronic state
Authors: Bishop David M. , Cheung Lap M.
Published: February 2003

With the aid of recently produced potential curves for the X 1Σ+ and A 1Σ+ states of lithium hydride, the rovibronic energies of 7LiH have been calculated. From these energies the standard spectroscopic constants have been evaluated. The effect on these constants of inclusion of adiabatic corrections (for diagonal nuclear motion) in the potential curves is discussed. The expectation values of certain powers of the electronic coordinates are also tabulated.
Hartree–Fock and Hartree–Fock–Slater electric field gradients (H2, CH4) and their symmetry mode derivatives (CH4)
Authors: Snijders J. G. , van der Meer W. , Baerends E. J. , de Lange C. A.
Published: February 2003

Electric field gradients in the molecules H2 and CH4 are calculated by the Hartree–Fock and Hartree–Fock–Slater methods. In CH4, the derivatives of the electric field gradient with respect to the vibrational symmetry modes are determined. It is shown that one C–H stretch symmetry mode derivative is completely dominant. This derivative has recently been used to explain the quadrupole splittings found in the NMR spectra of the deuterated methanes dissolved in liquid crystals.
Roles of the Rydberg transitions in fast excitation transfer studied in cyclohexane and n-heptane using synchrotron radiation
Authors: Kimura Kazuie , Hormes Josef
Published: February 2003

Excitation spectra of PPO (2,5-diphenyl oxazole) in cyclohexane solution show peaks at 63 000 and 78 000 cm−1, while those in n-heptane solution show a single peak near 66 000 cm−1. The 63 000 cm−1 peak for cyclohexane and 66 000 cm−1 peak for n heptane are ascribed to excitation transfer at respective valence-band absorption peaks. An extra peak at 78 000 cm−1 for cyclohexane solution is tentatively ascribable to the Rydberg transitions and it is situated near the eighth peak of the Rydberg series. The expanded reaction radius is, therefore, estimated to be about 30 Å. Taking account of the Rydberg series can explain the known fact that excitation transfer in cyclohexane is extraordinarily fast compared with that in n heptane. Kinetic analysis shows that these upper excited states at 63 000 and 78 000 cm−1 of cyclohexane make direct excitation transfer, competing with relaxation to the lowest excited state at 49 750 cm−1. It is briefly discussed that the Rydberg series, which can be observed only in cyclic (or round) alkanes, may also be one of the key processes that explain molecular structural dependence observed in the excitation transfer and electron mobilities in alkanes.
The P
V
T properties of nitrogen from −20 to +35 °C and 200 to 570 MPa, and some comparisons with argon and the Lennard-Jones (6-12) fluid
Authors: Morris Edwin C. , Wylie Russell G.
Published: February 2003

The density ρ(T, p) of nitrogen has been measured for T=−20 to +35 °C and p in the range 200 to 570 MPa, using a method already described in a paper which included results for argon. An improvement in the accuracy with which the quantities of gas are measured has reduced the overall uncertainty in ρ(T, p) to 0.03% at the 95% confidence level. The results are presented graphically and in the form of a fitted equation, and are compared with those of other workers. A comparison is made with the results of the argon study and a hypothetical Lennard-Jones fluid, in terms of the density, the internal energy and the entropy, and the internal kinetic energy of the molecules as calculated by Schottky’s method.
Toward a comprehensive semiclassical ergodic theory
Authors: Kay Kenneth G.
Published: February 2003

We identify quantum dynamical conditions that tend, as ℏ→0; to the classical conditions of ergodicity, weak mixing, mixing, and absolutely continuous spectrum. We use these conditions to derive asymptotic properties of wave functions, matrix elements, and energy levels of systems with various ergodic properties in the classical limit. Some of these properties are new while others modify older predictions of ‘‘quantum ergodic properties.’’ Thus, the present work serves to extend, unify, and refine a number of previous theories. One new prediction is that weak mixing leads to the Van Hove diagonal singularity condition for a certain class of operators. This result may have significant implications for the applicability of nonequilibrium statistical mechanics to finite systems.
Monte Carlo calculation of the radial distribution function of quantum hard spheres at finite temperatures using path integrals with boundary conditions
Authors: Jacucci G. , Omerti E.
Published: February 2003

The two body contribution to the radial distribution function of quantum hard spheres is calculated using the Monte Carlo method based on path integrals. Hard core boundary conditions are taken into account by means of an ‘‘image’’ approximation obtaining accurate numerical results.
Excluded volume effects in polymers attached to surfaces: Chain conformational renormalization group
Authors: Freed Karl F.
Published: February 2003

The chain conformational space renormalization group method is extended to treat polymers with one or both ends attached to a wall. The theory is illustrated for the case of reflecting boundary conditions at the wall, but the methods for treating general boundary conditions are outlined. The fixed end-vector partition functions are evaluated both in the asymptotic scaling limit and in the cross-over regime where there is an explicit dependence on the strength of the excluded volume interaction. The mean square chain sizes transverse 〈z2〉 and parallel 〈‖ρ‖2〉 to the surface are evaluated separately to illustrate the use of the theory for the direct computation of these moments (including the prefactors) and the exponents. The calculations in these cases are far simpler than those required to determine the distribution functions. The prefactors display 〈‖ρ‖2〉 to be more spread out than 〈z2〉 which, in turn, as expected, is more spread out than 〈R2〉 for a chain in free space.
Raman and infrared excitation of local mode states in neopentanea)
Authors: Henry Bryan R. , Tarr Allan W. , Sonnich Mortensen O. , Murphy W. F. , Compton D. A. C.
Published: February 2003

The Raman spectrum of neopentane is measured in the regions of ΔvCH=1 and 2. The results are compared to the absorption spectrum of neopentane in the region from ΔvCH=2 to 5. The peak positions are calculated within the local mode model. The intensity patterns observed in both types of spectra are explained. In particular, electrical anharmonicity is found to make a significant contribution to the observed peak intensities.
A reexamination of the magnetic susceptibility of the coordination complex μ4-oxohexa-μ-chlorotetrakis-(triphenylphosphine oxide) copper(II), Cu4OCl6(TPPO)4
Authors: Dickinson Richard C. , Baker W. A. , Black T. D. , Rubins R. S.
Published: February 2003

High purity samples of Cu4OCl6 (triphenylphosphine oxide)4 have been prepared for a reexamination of the χ vs T behavior of this compound. EPR studies on single crystals of the compound have revealed the ground state to be S′=0 and an observed spectrum from an excited S′=2 manifold with a g factor of 2.10±0.01. The ordering of the energies of the spin manifolds is proposed to be S′=0, S′=2, S′=1. On this basis, experimentally determined χ vs T data were fitted to a modified Heisenberg exchange model for g=2.10, E(S′=1)−E(S′=0)=85±13 cm−1, E(S′=2)−E(S′=0)=10±1 cm−1, and z′J′=0±1 cm−1. Magnetic susceptibility measurements were also made on samples prepared by ‘‘doping’’ the tetramer into the pores of a crosslinked polymer.
Raman spectra of 7Li2SO4 and 6Li2SO4
Authors: Cazzanelli Enzo , Frech Roger
Published: February 2003

The polarized Raman spectra of single crystal 7Li2SO4 and 6Li2SO4 at room temperature are reported. Symmetry-based vibrational band assignments are given for the external and internal optic modes. Comparison of spectra from the two isotopically substituted crystals identifies higher frequency external modes originating in the lithium ion motion. The interaction of these modes with the intramolecular modes of the sulfate ion is discussed.
Ammonium ion rotation in ammonium perchlorate as studied by infrared spectroscopy
Authors: Corn Robert M. , Strauss Herbert L.
Published: February 2003

The infrared spectra of the ND-stretching bands of dilute NH3D+ in NH4ClO4 are presented. The spectra are studied over the temperature range from 3 K to room temperature. At low temperatures, the NH3D+ is preferentially oriented. The extent of the orientation at a given temperature depends on the deuterium concentration and thus on the concentration of NH3D+ ions. As the temperature is raised, the various ND-stretching bands collapse due to a variety of dephasing processes. At low temperature, the dephasing process is phonon-assisted tunneling. As the temperature is raised, dephasing is accomplished by interaction among librational states. At still higher temperatures, the bandwidth is due to stochastic motion of the ammonium ions.
Critical fluctuations at the phase transition in benzil
Authors: Yoshihara A. , Bernstein E. R. , Raich J. C.
Published: February 2003

New Brillouin scattering data are presented for benzil single crystals near the phase transition at 83.5 K. These data demonstrate that for the c11 governed longitudinal acoustic (LA) mode at ∼15 GHz, critical fluctuations are quite large near the phase transition and dominate the behavior of this mode within +40 K of the transition. These observations are analyzed in terms of four contributing soft modes; an optical soft mode and two transverse acoustic (TA) soft modes at the zone center and a zone boundary M-point soft mode. It is argued that the zone boundary mode is the major contributor to the width and elastic constant anomalies of the LA mode. Calculations of these properties support these conclusions. Critical exponents are evaluated for Δc11 and ΔΓa, the critical contributions to the elastic constant and width of the c11 governed LA mode, based on the experimental data.
Expectation values for Morse oscillators
Authors: Tipping R. H. , Ogilvie J. F.
Published: February 2003

Several exact recursion relations for expectation values of different operators are derived for the Morse potential using the hypervirial and Hellmann–Feynman theorems. These results enable one to express the expectation values of other useful operators as series expansions in terms of the dimensionless anharmonicity parameter ωeXe/ωe. The present results, obtained without explicit use of the eigenfunctions, are compared with similar results derived by other techniques, and it is illustrated how accurate values can be obtained with very little computational effort.
Time resolved photoacoustic spectroscopy applied to properties of picosecond transients
Authors: Rothberg Lewis J. , Bernstein Mark , Peters K. S.
Published: February 2003

Time resolved photoacoustic spectroscopy represents a promising new approach to the study of time resolved absorption with picosecond time resolution. The sensitivity of photoacoustic detection permits investigation of dilute and fluorescent samples which are difficult to study by conventional flash photolysis. Theory shows that quantitative measurements of important transient properties and new insights into photochemical and photophysical dynamics can be obtained by measuring heat deposition as a function of probe fluence. Experimental determinations of absorption and stimulated emission cross sections, and lower limits for rates of nonradiative relaxation of highly excited states, are presented.
Spectroscopic properties and relaxation processes of impurity molecules in solids. II. Vibrational relaxations
Authors: Kono H. , Lin S. H.
Published: February 2003

We have calculated the vibrational relaxation rates for HCl (DCl)/Ar matrix systems. The quantitative agreement between our calculations and experimental data is fairly good. Our approach based on the adiabatic approximation can consistently treat the eigenvalue problems and vibrational relaxation processes for diatomic molecules embedded in monatomic crystals. The adiabatic approximation is used to separate high (intramolecular vibration) and low frequency modes (molecular rotation and lattice vibration). The nonadiabatic couplings (the kinetic energy operators for the low frequency modes) induce the vibrational relaxation processes. Our numerical calculations support the mechanism proposed by Bondybey and Brus that for small hydrides molecular rotation is the dominant accepting mode. The A1g totally symmetric lattice modes in the classification by the irreducible representations of the substitutional site symmetry Oh are shown to be dominant to accept the energy mismatch between initial and final rotational levels. For the DCl/Ar system, we have found that the mixing of initial rotational levels due to the rotation-lattice vibration coupling is responsible for the vibrational relaxation. The calculated relaxation rate for DC1 is smaller than that of HCl by one order of magnitude.
Diffusion of pseudospherical molecules: An investigation on the effects of dipole moment
Authors: Chan T. C.
Published: February 2003

Limiting mutual diffusion coefficients of carbon tetrachloride, 1,1,1-trichloroethane, 2,2,-dichloropropane, and 2-chloro-2-methylpropane in acetone, methanol, ethanol, and n-decane at 298.2 K have been determined by the chromatographic broadening method. The mutual diffusion data for each solvent are approximately equal and independent of the mass and dipole moment of the pseudospherical solute molecules.
Critical demixing in fluid bilayers of phospholipid mixtures. A neutron diffraction study
Authors: Knoll Wolfgang , Schmidt Günther , Sackmann Erich , Ibel Konrad
Published: February 2003

Critical lateral demixing in fluid bilayers of lecithins differing in chain length by four CH2 groups is established for the first time by small angle neutron scattering, exploiting the high scattering contrast between deuterated and protonated lipids. Good agreement with the Ornstein–Zernicke theory allows one the estimation of the correlation length of concentration fluctuations. The interesting aspect of critical demixing phenomena is their possible role in membrane processes such as facilitated trans-membrane transport, long range protein–protein interaction, and the control of enzyme activity.
Critical behavior of the density, heat capacity, and refractive index for triethylamine + water
Authors: Furrow G. P. , Greer S. C.
Published: February 2003

We have measured the mass density as a function of temperature for a mixture of triethylamine + water of critical composition in the one-phase region below the lower critical solution point. We find an anomalous decrease in density near the critical point of about 10−3 of the density. The functional form predicted by renormalization group theory represents the data within error. We have reanalyzed the heat capacity measurements of Thoen, Bloemen, and Van Dael [J. Chem. Phys. 68, 735 (1978)] on this system and have shown thermodynamic consistency with the density results. We have reanalyzed the refractive index measurements of Beysens and Bourgou [Phys. Rev. A 19, 2407 (1979)] on this system and have shown that these data do not have the same critical behavior as the density. We have calculated the amplitude of the correlation length divergence from the density results and found reasonable agreement with light scattering results.
The influence of attractions on the static and dynamic properties of simulated single and multichain systems
Authors: Bishop Marvin , Kalos M. H. , Frisch H. L.
Published: February 2003

The influence of the attractive portion of the Lennard-Jones potential on the statics and dynamics of both single chain and multichain systems is investigated by reptation and molecular dynamics simulations. There is no significant effect at a reduced temperature of 10.0. At a temperature of 1.7 the single chain and multichain system at low densities (0.1) indicate that the attractions cause both the chains to be significantly more compact and a slowing of the time autocorrelation functions of the square of the end-to-end distance and radius of gyration. At a moderate density of 0.5, the attractions have almost no effect on the static structure, but they still influence the dynamic properties.
Absolute coverages and hysteresis phenomena associated with the CO-induced Pt(100) hex⇄(1×1) phase transition
Authors: Jackman T. E. , Griffiths K. , Davies J. A. , Norton P. R.
Published: February 2003

Temperature hysteresis in the hex⇄(1×1) phase transition in the Pt(100)–CO system has been investigated using Rutherford backscattering spectroscopy, nuclear microanalysis, work function measurements, LEED, and thermal desorption spectroscopy. The onset of the hex→(1×1) transition occurs at an average surface coverage of 0.08±0.05 monolayers while the onset of the (1×1)→hex transition occurs at 0.25±0.05 monolayers. The results are consistent with a recently proposed model for the phase transition.
Multiphoton ionization spectroscopy of Al2Me6: Detection of the np 2P
J and nf 2F
J Rydberg states of aluminium
Authors: Hackett P. A. , John Phillip
Published: February 2003

The multiphoton ionization spectrum of Al (I) generated from the from the multiphoton dissociation of trimethyl aluminium, has been investigated in the range 410–650 nm. Rydberg states of Al (I) in the np 2PJ and nf 2FJ series have been identified for n≤18 and 17, respectively. (AIP)
Intense hot Raman transitions within Renner–Teller states
Authors: Band Yehuda B. , Scharf B.
Published: February 2003

We analyze the hot Raman transitions within the split components of the first excited vibrational level of a Renner–Teller active mode in linear molecules. These transitions are predicted to be almost as intense as Rayleigh transitions and we expect them to be among the most prominent features of the Raman spectra. We describe the spectroscopic pattern, excitation profile, and depolarization ratio of Raman transitions within 2Π Renner–Teller active states via 2Σ, 2Π, and 2Δ intermediate electronic states. We discuss these Raman transitions for the Renner–Teller case in the absence and presence of spin-orbit coupling.
The laser photoelectron spectrum of gas phase benzene
Authors: Long S. Randolph , Meek Jon T. , Reilly James P.
Published: February 2003

Benzene vapor in both effusive and supersonic molecular beams has been irradiated with the frequency doubled output from a tunable dye laser. Photoelectrons ejected as a result of two-step ionization have been kinetic energy analyzed by a time-of-flight method. Electron energy resolution is between 3 and 10 meV depending on beam conditions. Spectra resulting from the excitation of ten different vibronic bands of C6H6 and three of C6D6 are presented. Ions are found to be generated in their 2E1g ground electronic state in vibrational state distributions that are strongly wavelength dependent. Analysis of these distributions leads to an improved understanding of Jahn–Teller effects in benzene cation and observation for the first time of Jahn–Teller splitting in C6H+6 and C6D+6. Normal mode ν16 is conclusively assigned as 39.5 meV (319 cm−1) and 34.5 meV (278 cm−1) in C6H+6 and C6D+6. ν4 is tentatively assigned as 51.5 meV (415 cm−1) and 43.5 meV (351 cm−1) in these species, respectively. An explanation for the anomalously large frequency of ν6 previously measured with conventional photoelectron spectroscopy is provided. It is also demonstrated that information contained in laser photoelectron spectra can be of assistance in assigning vibronic features in near ultraviolet molecular absorption spectra.
Two photon resonant, four photon ionization of CO via the A 1Π state with photoelectron energy analysis
Authors: Pratt S. T. , Dehmer P. M. , Dehmer J. L.
Published: February 2003

We report measurements of photoelectron spectra from resonantly enhanced four photon ionization of CO via a two photon transition to the R-branch heads of the A 1Π, v=1–3 ← X 1Σ+, v=0 bands. In addition, we present preliminary data on the rotational state dependence of the photoelectron spectra. The results are compared with our earlier measurements of the photoelectron spectra obtained in the six photon ionization of CO via a three photon resonant transition to the same intermediate levels. The observed vibrational branching ratios in the photoelectron spectra differ both from those observed in the earlier study and from the pattern predicted by the Franck–Condon overlap between the intermediate A 1Π state and the ionization continua. It is shown that accidental resonances with the J 1Σ+ state at the three photon energy account for the observed photoelectron spectra obtained at the wavelengths of the A 1Π, v=2,3 R-branch bandheads. Finally, we present conclusive evidence for the production of electronically excited CO+ A 2Π and discuss possible mechanisms for its formation.
Radiationless transitions in a new light
Authors: Heller Eric J. , Brown Robert C.
Published: February 2003

Radiationless transitions in polyatomic molecules prove to be quite amendable to a semiclassical treatment both below and above crossings between the potential surfaces involved in the transition. Below such crossings, tunneling integrals are easily performed which give good estimates of the dependence of the nonradiative rate on the energy gap and excess energy in the electronic state. Above the surface crossing, the transitions become classically allowed and a Tully–Preston surface hopping model suffices. We find that a nonlinear dependence of ln(knr) vs E plots is the rule rather than the exception. The ln(knr) vs E plots tend to flatten out with increasing energy. This effect can occur below surface crossings, but is most dramatic when a surface crossing is reached. The recent beam results of Smalley and co-workers on pyrazine and pyrimidine are seen to be a possible case of this simple behavior.
Nonequilibrium electron velocity distribution and temperature in thermalization of low-energy electrons in molecular hydrogen
Authors: Koura Katsuhisa
Published: February 2003

The thermalization of low-energy electrons (ε≲0.1 eV) due to the rotational and elastic collisions in normal H2 at the gas temperature T=300 or 77 K is studied by the Monte Carlo simulation, where electrons are so diluted in molecules that the electron–electron collision is neglected as compared with the electron–molecule collision. The accuracy of the approximate theory based on the assumption of the local Maxwell velocity distribution (MD) is examined using, for simplicity, the rotational cross section of the Gerjuoy–Stein formula and the elastic cross section of the hard-sphere model, which are a little larger than the experimental cross sections at low electron energy (ε≲0.1 eV); the initial electron velocity distribution is taken to be the MD. The electron velocity distribution significantly deviates from the MD especially at low gas temperature (T=77 K); consequently the degradation of the electron temperature Te is slower than that for the MD and the thermalization time τth when Te/T=1.1 is larger than that for the MD to the extent of 20% at T=300 K, where τth is dominated by the rotational collision, and 140% at T=77 K, where τth is dominated by the elastic collision. τthp≂1.9 μs Torr at T=300 K, where p is the gas pressure, is about 27% larger than the experimental value of 1.5 μs Torr at T=296 K for the higher initial electron energy (ε∼1 eV).
Nonrelativistic all electron SCF, MCSCF, and CI calculations on the AgH, AuH, and Ag2 molecules
Authors: McLean A. D.
Published: February 2003

Near basis set limit SCF and proper dissociating MCSCF nonrelativistic potential curves are given for AgH, AuH, and Ag2. The importance of f functions in the Slater Ag basis, and low exponent f functions with the same spatial extent as the outer lobe of the Au 5d shell in the Slater Au basis, is demonstrated. Their inclusion adds ∼0.1 eV/heavy atom to dissociation energies and is absolutely necessary for determining basis set limit dipole moments. In CI calculations which correlate the bonding electrons and the outer d shells of the heavy atoms, the inclusion of f functions is shown to be even more important. For AgH, correlation effects are shown to decrease Re from a Hartree–Fock value of 1.774 to 1.702 Å (expt 1.618 Å), and to increase De from a Hartree–Fock value of 1.224 to 2.16 eV (expt 2.39 eV). Relativistic effects are responsible for the bond shortening not accounted for in these calculations. In AuH, remarkably parallel results are obtained. Correlation decreases the bond length from a Hartree–Fock value of 1.824 to 1.753 Å (expt 1.528 Å) and increases De from a Hartree–Fock value of 1.109 to 2.02 eV (expt 3.36 eV). The much larger remaining differences from experimental values reflect the much larger relativistic effects in gold containing molecules. Less complete calculations on Ag2 give a Hartree–Fock bond length of 2.791 Å and De=0.38 eV. Partial inclusion of bond and d-shell correlation effects shortens the bond length to 2.724 Å which is probably ∼0.08 Å longer than the true nonrelativistic value. A correlated De=1.13 eV (expt 1.67 eV) is computed, but this value is probably ∼0.4 eV too low because of limitations in basis set and correlation effects. Molecular quadrupole moments of all three molecules are discussed.
Analytical solution of coupled nonlinear rate equations. II. Kinetics of positive catalytic feedback loops
Authors: Phillipson Paul E. , Schuster Peter
Published: February 2003

Closed positive feedback loops of catalytic reactions between macromolecules provide a kinetic mechanism whereby each species serves to catalyze self-reproduction of its successor in the loop. The dynamics of these catalytic networks, hypercycles as they are known, is described by coupled nonlinear differential equations for which the present study develops approximate analytic solutions. The method involves transformation of the rate equations to suitable generalized coordinates and subsequent iterative solution of derived integral equations expressed in these coordinates. The integral equations and their solutions reveal solution of hypercycles in time as the unfolding of memory functions which reflect at any instant accumulated past dynamical history. Quantitatively, the analytical solutions are sufficiently close to computer solutions of the differential equations to justify their providing a reliable picture of the dynamics. The three species system is shown to be stable and to exhibit oscillatory exponential decay towards its fixed point. The four species hypercyle is quasistable in that the fixed point is approached likewise in an oscillatory manner, but asymptotically at the slow rate of inverse square root of time. The five species hypercycle is unstable and so evolves into a limit cycle characteristic of a biochemical clock whose period and structure is highly nonlinear, yet traceable analytically.
Relative stability among metastable steady state structures in chemical reaction systems
Authors: Shimizu Hiroshi , Sawada Yasuji
Published: February 2003

In a nonlinear chemical reaction system, which consists of coupled reaction cells in one dimensional space, various spatial structures characterized by the peak number Np, and the spacing between the peaks λp, were shown to exist stably for a proper region of parameters. The relative stability of the steady states was studied by fluctuating the value of a parameter in computer simulation. The steady state entropy production for each metastable state was calculated. These results have shown that the entropy production is maximized for the most stable steady state. The present result that the relative stability of steady states can be described by means of entropy production at steady states is consistent with the variational principle previously proposed for the relative stability among the metastable states of nonlinear nonequilibrium systems.
Viscoelastic twist properties of some nematic liquid crystalline azoxybenzenes
Authors: van Dijk J. W. , Beens W. W. , de Jeu W. H.
Published: February 2003

The static and dynamic behavior of the Frederiks transition for twist of a uniform planar nematic sample in an external magnetic field has been studied by monitoring the associated conoscopic figure. In particular, the decay of this figure after the field has been switched off is considered in detail in order to obtain results for the twist viscosity γ1. Results for the twist elastic constant K2 and for the viscosity γ1 are reported for the homologous series of p, p′-dialkylazoxybenzenes. K2 is found to be smaller than both K1 and K3, while K2 varies little along the series. The temperature dependence of γ1 can be described by a combination of an activated process and a term depending on the orientational order parameter.
Measurements of mobilities and longitudinal diffusion coefficients of Na+ ions in Ne, Ar, and CH4 at room temperature by a continuous guard-ring system
Authors: Iinuma K. , Takebe M. , Satoh Y. , Seto K.
Published: February 2003

The mobility of Na+ ions has been measured at 313 K in Ne, at 314.5 K in Ar, and at 311 K in CH4 in the pressure range 0.05–1.2 Torr, over the E/N range 3–123 Td in Ne, 6–409 Td in Ar, and 13–520 Td in CH4 (E is the electric field strength, N the neutral gas number density, and 1 Td=10−17 V cm2). The measurements were made with a continuous guard-ring system which is different from conventional thin or thick type. The zero-field reduced mobilities are found to be 7.90±0.19 in Ne, 3.01±0.07 in Ar, and 2.58±0.06 cm2/V s in CH4, which are about +17.3%, +6.24%, and −8.53% higher than the Langevin polarization-limit mobilities, respectively. The mobility data in Ne and Ar have been compared with those of others over the whole E/N range. The longitudinal diffusion coefficients were also measured and compared with the data compiled by Ellis et al. and the theoretical values calculated by the generalized Einstein relations.
13C spin diffusion of adamantane
Authors: Bronniman Charles E. , Szeverenyi Nikolaus M. , Maciel Gary E.
Published: February 2003

Two-dimensional exchange spectroscopy of natural abundance 13C–13C spin diffusion in solid adamantane illustrates the influence that 13C–1H dipole–dipole coupling exerts on 13C spin diffusion by determining spectral overlap in the 13C system. 2D 13C spectra were obtained for several values of mixing time τm and compared with spectra calculated in the limit of nearest-neighbor coupling. Good agreement is obtained for short τm, during which the equilibration of neighboring spins dominates. For longer τm, slower spin diffusion that is not acounted for by the simple model is seen; after nearest-neighbor spins equilibrate, communication over larger distances produces further mixing. It is possible to modify spin diffusion rates by altering experimental conditions, e.g., magic-angle spinning, low-power 1H decoupling, or spin locking 13C in the rotating frame during τm.