bluuebunny/crossref_metadata_2025 huggingface dataset:
Cluster expansion for concentration dependence of self-friction coefficients for suspensions of interacting spheres
Authors: Muthukumar M. , Freed Karl F.
Published: February 2003

We present a general cluster theory for the concentration dependence of the translational and rotational self- (tracer) friction coefficients for interacting colloidal spheres in a suspension at vanishing shear rate in the steady state limit. The sphere–sphere interactions enter through the Mayer cluster expansion of the various sphere distribution functions, and the calculation at any order of concentration is reduced to a series of quadratures. The contributions at each order of concentration involve integrals of these equilibrium distribution functions multplied by hydrodynamic functions. The latter are represented in an infinite multiple scattering series which is shown to involve an expansion in higher inverse powers of the interparticle separations and, therefore, to be convergent at low concentrations. This represents the first systematic derivation of the cluster expansion beyond the leading concentration dependence. The general theory is tested by the evaluation of the leading concentration dependent terms which are found to be in good agreement with earlier calculations for translational friction. The concentration dependent rotational friction has not been given previously. Explicit formulas are also presented for the calculation of the leading three-body hydrodynamic contributions.
Cratering due to surface defects in the Gaussian model
Authors: Nowak Ernst , Deutch J. M. , Berker A. Nihat
Published: February 2003

Surface defects on freshly applied liquid films like the phenomenon of cratering are characterized by almost discontinuous interfacial profiles. Our purpose is to model the effect of surface inhomogeneities and impurities that influence the equilibrium configuration of the surface film. We therefore employ the Gaussian model which is known to provide a simple equilibrium statistical thermodynamics description of the liquid–vapor interface. By modifying previous work by Weeks, we allow for additional external forces acting on the interface and discuss the resulting shape of the surface profile. Even local application of external forces leads to deviation from the plane surface decaying on a macroscopic length scale. This does not accurately model the cratering phenomenon. In systems, which can be described by our Gaussian model, surface tension forces will always lead to a smooth interfacial profile.
Dilute solutions of macromolecules in a rectilinear Poiseuille flow
Authors: Stasiak Wojciech , Cohen Claude
Published: February 2003

Rheological results for a dilute solution of elastic and rigid dumbbells in a rectilinear Poiseuille flow are obtained within the framework of the kinetic theory of polymeric liquids. The values of the stresses and viscometric functions resulting from the two models of a macromolecule are calculated and compared. In both cases some of the stress tensor components show contributions due to the coupling of Brownian motion to convection in the nonhomogeneous velocity field of the solvent. The coupling parameter ε∼λγ̄L/B is assumed small and used to develop a perturbation scheme. Here, γ̄ is the shear rate at the wall, B is the half-width of the channel, and L and λ are the characteristic length and relaxation time of the macromolecule. Unlike the elastic dumbbell model, the rigid dumbbell model predicts a viscosity increase with increasing channel width and a negative value for the second normal stress difference.
Dynamics of helical wormlike chains. III. Eigenvalue spectra and time-correlation functions
Authors: Yamakawa Hiromi , Yoshizaki Takenao
Published: February 2003

Some preliminary numerical results for the eigenvalue spectra and the time-correlation functions computed from the formulas derived in the preceding paper (paper II) are presented. First, it is claimed that certain conditions must be imposed on the translational and rotatory friction coefficients ζt and ζr of each of the sub-bodies comprising the model. The parameter ζt must lie in the range over which the diffusion matrix is positive definite. For the present model, the eigenvalues λ01,k in the lowest branch of the spectrum for the diffusion operator may possibly become negative at small wave numbers k because of the preaveraging approximation used. However, it is possible to make λ01,0 very small in magnitude, and thus to neglect it, by choosing ζr properly. For flexible chains, this enables one to remove the negative eigenvalues completely and recover the Rouse–Zimm values for λ01,k at small k in the coil limit in a very good approximation. Then, after the examination of the accuracy of the block-diagonal approximation, numerical results for the eigenvalues λjL,k (L=1,2;‖j‖≤L) in this approximation are presented, taking, as examples of flexible chains, isotactic and syndiotactic polystyrenes and isotactic and syndiotactic poly(methyl methacrylate)s, and as an example of stiff chains DNA all with proper assigned values of the model parameters. An empirical noncrossing rule is found. With these eigenvalues, the time-correlation functions relevant to the dielectric susceptibility are computed for the flexible chains, and the ones relevant to the fluorescence polarization anisotropy for DNA. The associated correlation times are also estimated, and a good agreement with experiment is found. Finally, a detailed discussion of some general aspects of the present theory is given. In the Appendix, an interpolation formula for the mean reciprocal distance between two subbodies is given.
Highly excited states of HCN: The probable applicability of classical dynamics
Authors: Farrelly David , Reinhardt William P.
Published: February 2003

The conclusion of Lehmann, Scherer, Klemperer1 regarding the applicability of classical mechanics to the HCN problem is discussed further and several suggestions are made. (AIP)
The dielectric properties of alcohols–water solutions. I. The alcohol rich region
Authors: Bertolini D. , Cassettari M. , Salvetti G.
Published: February 2003

We present here the first results of a systematic study of the dielectric properties of alcohols–water solutions. The complex dielectric constant ε̃=ε′−iε′′ and the dielectric relaxation time τ of MeOH, EtOH, 2-PrOH alcohols, and their aqueous solutions are reported for some values of the water mole fraction xw in the range 0≤xw≤0.5, at various temperatures (from 30 °C down to −43 °C for MeOH and its solutions). τ has been measured by a two coaxial resonators spectrometer in a significant frequency range for the principal relaxation region of these liquids (0.47–4 GHz). Measurements of ε′ at 13 MHz with a different experimental apparatus have also been made. Our data do not confirm previous determinations of τ vs xw at 30 °C. In all the investigated temperature range, τ is a decreasing monotone function of xw for the aqueous solutions of EtOH and 2-PrOH. On the contrary, the dielectric relaxation time of MeOH aqueous solution is practically constant up to xw=0.1 at 30 °C; at 0° and −25 °C, τ increases at the beginning and a maximum is present; at lower temperature, τ increases with xw. The observed behavior of τ is discussed and a model for the dielectric relaxation in H-bond associated liquids is suggested.
Energy states of CH3D molecules in the phase II structure of solid methane
Authors: Lushington K. J. , Morrison J. A. , Maki Kazuo , Heidemann A. , Press W.
Published: February 2003

A high resolution neutron scattering study has been made of a solid solution of CH3D (15 mol %) in CH4 at temperatures down to T=1.5 K in order to obtain information about the energy states of CH3D molecules in the phase II structure, i.e., on sites of O and D2d symmetries. Measurements were made with spectrometers IN5 and IN10 at ILL, Grenoble, in the energy range 0<ε<1.3 meV. ‘‘Free’’ rotation and tunneling transitions were observed for both CH3D and CH4 molecules. The results obtained for the tunneling transitions are used to refine the parameters of a model used previously to correlate neutron scattering and thermodynamic data for partially deuterated methanes in the more complicated phase III structure.
Melting and surface tension in microclusters
Authors: Natanson Grigory , Amar Francois , Berry R. Stephen
Published: February 2003

Approximate partition functions are constructed from approximate quantum mechanical models of near-rigid, solidlike and nonrigid, liquidlike clusters. The Helmholtz free energies are evaluated for these clusters as functions of temperature T and cluster size N. The thermodynamic temperatures of melting, at which the free energies of the two forms are equal, match well with the melting temperatures determined from classical simulations of Ar clusters. Moreover, the effective coexistence ranges of temperature for solid- and liquidlike forms are approximately the same as the range of ‘‘supercooling’’ and ‘‘superheating’’ found in molecular dynamics computations of Ar. The surface tension is determined from an expansion of the free energy in powers of N−1/3. The concept of melting in small clusters, as a transition from a near-rigid, small-amplitude vibrator to a nonrigid molecule capable of frequent passage from one local equilibrium geometry to another, is related to the recent proposal by Stillinger and Weber of the nature of the melting transition.
A study of the critical cluster size for water monolayer clusters on a model AgI basal substratea)
Authors: Ward Richard C. , Hale Barbara N. , Terrazas Sergio
Published: February 2003

We present a formalism and estimate a critical cluster size for water monolayer formation on a (rigid) model AgI basal substrate. The formalism is modified from that developed for vapor clusters [B. N. Hale and R. C. Ward, J. Stat. Phys. 28, 487 (1982)] and uses a Metropolis Monte Carlo method developed by Squire and Hoover [J. Chem. Phys. 50, 701 (1969)] to determine (Helmholtz) free energy differences for clusters containing n and n−1 molecules. Calculations for clusters of n=1, 2, 3, 4, 6, and 24 water molecules on a model AgI basal face at 265 K are used in a statistical mechanical formalism which assumes that the adsorbed clusters form a mixture of noninteracting ideal gases; the adsorbed monomer concentration is related to the vapor concentration at the same temperature. At water saturation and 265 K a critical cluster size of n*=3 molecules and a steady state nucleation rate (for monolayer formation) of 1023 cm−2 s−1 is predicted. The implications of this for ice nucleation on the model AgI substrate under atmospheric conditions are discussed.
Constant photoelectron energy spectroscopy of acetylene
Authors: Holland D. M. P. , West J. B. , Parr A. C. , Ederer D. L. , Stockbauer R. , Buff R. D. , Dehmer J. L.
Published: February 2003

Constant photoelectron energy (CPE) spectra of acetylene are reported for electron kinetic energies of 0, 0.1, 0.5, and 1.0 eV in the spectral range 11≤hν≤22 eV. This form of photoelectron spectroscopy involves measuring the intensity of photoelectrons with a particular kinetic energy as a function of the wavelength of the incident radiation. Such measurements may be carried out using small wavelength increments and can distinguish between direct and indirect, e.g., autoionization and photoionization processes. Autoionization features in the range 12.5≤hν≤16 eV are observed, which populate high vibrational levels of the ground state far beyond the Franck–Condon region.
Photophysics of infrared multiphoton excitation in thiophosgene
Authors: Brenner D. M. , Spencer M. N. , Steinfeld J. I.
Published: February 2003

IR multiphoton absorption (IRMPA) in thiophosgene has been studied by IR-visible double resonance. In particular, the probability of absorption has been measured in both collision-free (molecular beam) and collision-perturbed environments by monitoring the depopulation of the ground state level (000000). Although no evidence for true multiphoton absorption is found, a number of important observations have been made. (1) No correlation exists between the IRMPA spectrum under collision-free conditions and the low energy IR absorption spectrum. (2) Homogeneous depletion of rotational populations occurs at all CO2-laser frequencies. (3) Bottlenecks to absorption do not occur in the pumped-mode ladder. (4) The probability of absorption depends inversely on pressure and is affected dramatically by long-range collisions. These results are interpreted in the framework of an optical Bloch equation model.
Breakdown of Kramers theory description of photochemical isomerization and the possible involvement of frequency dependent friction
Authors: Velsko Stephan P. , Waldeck David H. , Fleming Graham R.
Published: February 2003

We present new data on the ground state isomerization rate of DODCI and compare our results to previously obtained excited state data. We find that the one-dimensional Kramers expression does not fit the data over the whole viscosity range when the friction is measured by either the overall rotation time or the solvent viscosity. The deviation is qualitatively similar to that previously observed in diphenyl butadiene when the viscosity was used as the friction measure. Deviations from the simple hydrodynamic theory appear to depend on the reduced frequency for isomerization. We show how our results are consistent with the one-dimensional Kramers theory when a frequency dependent friction is used to describe the influence of the solvent.
Magnetic susceptibility and electron paramagnetic resonance of chromocene and 1,1′-dimethyl-chromocene: Evidence for the dynamic Jahn–Teller effect
Authors: Desai V. P. , König E. , Kanellakopulos B.
Published: February 2003

The magnetic susceptibility of 1,1′-dimethyl-chromocene, Cr(C5H4CH3)2, has been studied between 1.00 and 321.6 K. The data are well reproduced by a calculation of the dynamic Jahn–Teller effect for the 3E2g(a1ge32g) ground state of D5d symmetry. Two sets of ‘‘best fit’’ parameter values are obtained: ζ=160 cm−1, δ=100 cm−1, kJT=1.10, κ=0.70 (set I); ζ=200 cm−1, δ=200 cm−1, kJT=1.15, κ=0.70 (set II). The magnetism of chromocene Cr(C5H5)2 is well approximated by the same set of parameters above ∼10 K. The calculated values of the lowest energies are in good agreement with recent measurements of the electronic Raman effect in chromocene. The results have been utilized to interpret the EPR spectra of the compounds in methyltetrahydrofuran solution.
The mean spherical approximation for the surface density profile of the one-component plasma. II
Authors: Rosinberg M. L. , Badiali J. P.
Published: February 2003

The MSA/MSA solution of the wall-particle Ornstein–Zernike equation for the one-component plasma is formulated as a fastly convergent expansion, convenient for numerical calculations, which avoids the usual zone-by-zone representation of the profile ρ(z). ρ(z) is then a sum of exponential terms of complex arguments tn, where the tn are the zeros of the Baxter function Q̃(k). A typical relation between the contact value ρ(o), the total potential drop, and the isothermal compressibility is derived. This new treatment of the MSA/MSA equation is used to extend previous work in two directions: (i) the wall is placed at an arbitrary distance from the edge of the background profile; and (ii) the ions and the background interact via a pseudopotential. The influence of the pseudopotential radius is discussed numerically and the free-surface situation is also considered. The effect of the pseudopotential is formally equivalent to the introduction of an adsorption potential at the wall.
Hydrogen molecule ion in superintense laser fields
Authors: Lima C. A. S. , Miranda L. C. M.
Published: February 2003

The structure of H+2 under an intense laser field is discussed. It is shown that the dissociation energy decreases on increasing the laser field strength and a discussion on the validity criteria of the theory is also given.
The Fourier transforms of some exponential-type basis functions and their relevance to multicenter problems
Authors: Weniger E. Joachim , Steinborn E. Otto
Published: February 2003

We analyze the properties of Slater-type functions (STFs) and B functions with respect to the Fourier transformation which is one of the most important methods for the evaluation of multicenter integrals. Although B functions have a much more complicated structure than STFs, their Fourier transform is probably the simplest of all expontential-type basis functions. Accordingly, in multicenter integrals, B functions seem to have much more attractive properties than STFs. We demonstrate this by analyzing shifting operators which make it possible to increase quantum numbers of the orbitals. Again we find that the shifting operators of B functions can be applied much more easily and yield much more compact results than the corresponding operators for STFs. We also show that the extremely compact convolution and Coulomb integrals of B functions are direct consequences of the simple Fourier transform of B functions.
Dissociation energies of diatoms related to molecular electron density gradients
Authors: Mucci J. F. , March N. H.
Published: February 2003

In earlier work, we have demonstrated that the difference between the total energy E for molecules at equilibrium and (3/2)Es, where Es is the sum of orbital energies, involves (a) the chemical potential μ and (b) the lowest-order density gradient correction to the kinetic energy, proportional to ℱ(∇ρ)2/ρ dr, ρ(r) being the electron density in the molecule. It is stressed here that a consequence of Teller’s theorem is that good molecular binding energies can only be obtained by inclusion of density gradients. This has prompted a plot of the measured dissociation energy per electron D/N for diatoms against E−(3/2)Es+(3/2)Nμ, the latter quantity being estimated from self-consistent field calculations already available in the literature. A striking correlation between these two quantities is revealed. That this plot is indeed closely related to one of D/N vs ℱ(∇ρ)2/ρ dr is apparent not only from Teller’s theorem but also from: (i) the approximate equation E−(3/2)Es+(3/2)Nμ=− (ℏ2/72m)ℱ[(∇ρ)2/ρ]dr, which is the result of the lowest-order gradient expansion theory; (ii) a study of the electron density contours for several diatomic molecules.
Polarized emission from doublet state organic free radicals
Authors: DiMauro Louis F. , Wood David E. , Lloyd Roger V.
Published: February 2003

The m- or p-fluorobenzyl radicals were prepared by x-irradiation of the precursor fluorotoluenes in argon at 10 K. The samples in an external magnetic field were excited with circularly polarized light obtained by passing the output of a He–Cd laser through a λ/4 plate. The right and left circular polarization components of the fluorescence output were alternately sampled by means of a photoelastic modulator and a digital synchronous computer. The results showed that there is net circularly polarized emission from the excited electronic states of doublet organic free radicals, thus demonstrating that the populations of the excited state magnetic sublevels are related to the population distribution in the ground state. The intensity of the effect is linearly dependent upon external magnetic strength because of the increase in the Zeeman splitting.
ESR investigations of H2O+, HDO+, D2O+, and H2 17O+ isolated in neon matrices at 4 K
Authors: Knight Lon B. , Steadman J.
Published: February 2003

The H2 16O+, HD 16O+, D2 16O+, and H2 17O+ ion radicals have been generated by photoionization, trapped in neon matrices at 4 K, and investigated by ESR (electron spin resonance) spectroscopy. The magnetic parameters are giso=2.0093(3), Aiso(H)=73.7(6) MHz, and Aiso(17O)=83.5(6) MHz. A weak electronic absorption feature at 6036(3) Å was observed which is tentatively assigned to a known gas phase band. The H2O+ radical appears to be rotating in neon and exhibits an unusual intensity increase as the matrix temperature is lowered from 9 to 4 K. Comparisons between H2O+ and the isoelectronic radicals NH2, NH+3, and CH3 are made.
Conformational mapping of fatty acids in ordered and disordered phases: A spectroscopic study
Authors: Zerbi Giuseppe , Minoni Giuseppe , Tulloch A. P.
Published: February 2003

The infrared active CD2 rocking vibration of selectively deuterated palmitic and stearic acids is used as a probe for the mapping of the local conformation of these molecules in the solid and liquid phases. The infrared spectra at different temperatures are analyzed and interpreted by means of normal coordinate calculations and conformational energy calculations. Evidence is collected that in the C solid modification molecules are fully trans planar. The cooperatively distorted local conformation of the carboxyl group in the liquid phase is discussed.
Photochemical and photophysical dynamics of I2 isolated in a rare gas cage
Authors: Beeken Paul B. , Hanson Eric A. , Flynn George W.
Published: February 2003

The photochemical and photophysical properties of I2 in Ar, Kr, and Xe matrices have been investigated and are reported here. Following laser excitation high into the B 3π(0+u) and 1πlu dissociative states, prompt emission is observed from the B 3π(0+u), A 3π1u, and A′ 3π2u electronic states depending on the matrix host. In all matrices the emission attributed to the B electronic state lies 200 nm to the red of the excitation wavelength and is generally featureless. The time resolved emission for this state has a detector limited rise and fall. The emission attributed to the A 3πlu electronic state was resolved with a monochromator and the source determined to be the v′=0 level of the A state. The lifetime for this emission is 260 μs and was found to be roughly the same in all matrices. Emission from the A′ 3π2u state was also detected but only in Xe matrices. The measured lifetime for this state was 6.3 ms. Data from a number of previous experiments on matrix isolated halogens in Ar, Kr, and Xe are reviewed and a consistent model for the relaxation pathways for intramolecular electronic energy transfer is presented. The implications of this model for studies of the solvent cage effect on I2 photodissociation and the development of the I2 chemical laser are also discussed.
Test of variational transition state theory with a large-curvature tunneling approximation against accurate quantal reaction probabilities and rate coefficients for three collinear reactions with large reaction-path curvature: Cl+HCl, Cl+DCl, and Cl+MuCl
Authors: Bondi D. K. , Connor J. N. L. , Garrett Bruce C. , Truhlar Donald G.
Published: February 2003

The large-curvature ground-state model for the transmission coefficient of generalized transition state theory [presented in a previous paper by B. C. Garrett, D. G. Truhlar, A. F. Wagner, and T. H. Dunning, J. Chem. Phys. 78, 4400(1983)] is tested against accurate quantal calculations of the rate coefficients for collinear reactions with very large inertial effects, namely Cl+HCl→ClH+Cl, Cl+DCl→ClD+Cl, and Cl+MuCl→ClMu+Cl. The tests cover the temperature range 200–2400 K. The accurate rate calculations are based on reaction probabilities obtained by a new numerical method for solving Schrödinger’s equation in Delves’ coordinates. Improved canonical variational transition state theory predicts rate coefficients 5.0–18 times smaller than those predicted by conventional transition state theory for the H transfer; for the D transfer, the ratio is 2.0–3.4; and for Mu it is 22–2.8×107. The large-curvature model predicts transmission coefficients as large as 41, 8, and 206 for the H, D, and Mu-transfer cases at 200 K, decreasing to 1.2, 1.1, and 1.4 at 2400 K. Despite the large effect of variationally optimizing the transition state location and the large size of the tunneling effect and the wide variation of both effects with temperature, improved canonical variational transition state theory with large-curvature ground-state transmission coefficients (ICVT/LCG) is accurate within a factor of 1.7 over a temperature range of a factor of 8, 300–2400 K, for all three reactions. At 200 K, the ICVT/LCG model underestimates the rate coefficients, by factors of 2.3, 1.9, and 1.5 for H, D, and Mu, respectively.
Nonphotochemical hole burning in a hydrogen bonding glass: Dependence on deuteration
Authors: Fearey B. L. , Stout R. P. , Hayes J. M. , Small G. J.
Published: February 2003

The nonphotochemical hole burning of ρ electron molecules has been studied in a variety of polar hydrogen−bonding glasses, hydrocarbon glasses and polymers. The polymers studied include PMMA, polycarbonate and polyvinylcarbazole. (AIP)
Welding of polymers of nonuniform molecular weight
Authors: Prager S. , Adolf D. , Tirrell M.
Published: February 2003

The chain dynamics for the welding which occurs when two pieces of the same polymer are brought together is discussed. The molecular weight dependence of healing rates is discussed in particular. (AIP)
The theory of nonlinear dielectrics. Polar, polarizable molecules
Authors: Fulton Robert L.
Published: February 2003

Some aspects of the nonlinear dielectric behavior of a simple fluid composed of polar, polarizable particles having spherical hard cores are discussed. In particular, the connection between the lowest order nonvanishing nonlinear dielectric constant and fluctuations in the polarization and dipole densities is found. By ignoring certain high order fluctuations, the nonlinear dielectric constant is related to two, three, and four body correlation functions. A further approximation involving the neglect of short range angular correlations produces the analog of the Onsager connection for the linear dielectric constant. The connections given here differ from all previous proposals. Some examples are given to illustrate the relative importance of electrostriction, anisotropic polarizabilities, and permanent dipole moments and the difference with previous proposals.
Cluster partition functions for a two-dimensional hard core square well gas
Authors: de la Selva S. M. T. , Dickman R. , Schieve W. C. , Canestaro C.
Published: February 2003

We study physical (or Hill’s) clusters in a two-dimensional hard core square well (HCSW) system. Partition functions for clusters of five or fewer particles have been determined, using both ‘‘proximity’’ and ‘‘negative energy’’ bond definitions. The cluster size distributions calculated on the basis of the partition functions are in good agreement with molecular dynamics simulations of the HCSW system.
Gelation in finite polycondensation systems
Authors: Yan Johnson F.
Published: February 2003

Flory’s theory of gelation is modified with the consideration of the conservation of both polymers and monomers. This results in an implicitly renormalized truncated weight distribution applicable to the entire system. Weight-average degree of polymerization is obtained by using the cascade theory with a finite number of generations. The dispersity of the weight distribution reaches a maximum at the gel point. In the post-gel region, a new relation for calculating the extinction probability is obtained. Results on weight-average DP and the gel fraction are in excellent agreement with those obtained by Donoghue and Gibbs, and Ziff and Stell.
CO/N2 solid solutions: Head–tail reorientations
Authors: Liu Shang-Bin , Conradi Mark S.
Published: February 2003

The rates of CO reorientation in α-phase solid solutions of CO/N2 were determined with dielectric spectroscopy. The activation energy is found to be an increasing and approximately linear function of the CO concentration x, similar to the transition temperature Tαβ. The extrapolated activation energy for CO reorientation in α-N2 is compared to that in pure α-CO; the transition temperatures and libration frequencies of α-N2 and α-CO are also compared. Evidently, the same interactions control Tαβ, the librations, and the energy barriers to reorientation. The CO dipole moment magnitudes determined from the dielectric data are all about 30% below the free molecule value. Not all of this discrepancy is ascribable to incomplete filling of the cell by the solid sample and librational reduction of the effective moment.
Attenuation of fluctuations by an external field in a correlated walk
Authors: Godoy S. V.
Published: February 2003

Correlated walk in the presence of an external field is applied to the study of diffusion coefficients in simple cubic lattices. A continuous time distribution is introduced with the aid of the pausing time distribution developed by Montroll. An exact expression for the diffusion coefficient is derived for perfect cubic lattices. It is found that the external field attenuates the effects of the rms deviations. These results are used to discuss the atomic diffusion in cubic lattices with impurities, which act as traps. Comparison with previous experimental and theoretical results is made and discussed.
Energy redistribution among internal states of nitric oxide molecules upon scattering from Pt(111) crystal surface
Authors: Asscher M. , Guthrie W. L. , Lin T.-H. , Somorjai G. A.
Published: February 2003

The internal states distributions rotation and vibration of nitric oxide molecules scattered from the Pt(111) crystal surface are reported. These distributions were measured as a function of crystal temperature (Ts) and scattering angles. The extent of equilibration of the three degrees of freedom, e.g., translation, rotation, and vibration with the metal surface is discussed in terms of the energy accommodation coefficients γ. It was found that γtrans≳γvib≳γrot. A model is suggested to account for the cold rotational distribution of molecules scattered via a trapping desorption mechanism. The vibrational distributions are determined by kinetic effects and by the competition between vibrational excitation of the adsorbed molecules and their desorption.
Abstraction reactions of Ca+ and Sr+ ions
Authors: Murad Edmond
Published: February 2003

The endothermic reactions Ca+ (O2, O) CaO+, Ca+ (CO2, CO) CaO+, Ca+(H2O, H) CaOH+, Sr+ (O2, O)SrO+, and Sr+ (H2O, H) SrOH+ have been studied in a tandem mass spectrometer. The measured thresholds yield D00(Ca+–O)=3.30±0.1 eV, D00 (Ca+–OH)=4.51±0.15 eV, D00(Sr+–O)=3.07±0.15 eV, and D00(Sr+–OH)=4.57±0.1 eV. These dissociation energies lead to I.P.(CaO)=6.9±0.15 eV, I.P. (SrO)=7.0±15 eV, I.P.(CaOH)=5.6±0.2 eV, and I.P.(SrOH)=5.1±0.2 eV, when combined with auxiliary thermodynamic data. These ionization potentials agree with previous measurements, except for I.P. (CaO) which is considerably lower. This discrepancy is briefly discussed.
Collisional removal of CH2(1A1): Absolute rate constants for atomic and molecular collisional partners at 295 K
Authors: Langford Andrew O. , Petek Hrvoje , Moore C. Bradley
Published: February 2003

The technique of cw laser resonance absorption has been used to monitor the time evolution of individual CH2(1A1) rotational levels following the excimer laser photolysis of CH2CO. Absolute rate constants for the removal of CH2(1A1) by He, Ar, Kr, N2, CO, O2, CH4, C2H6, C3H8, C2H4, i-C4H8, and CH2CO have been determined at 295 K. Removal efficiencies range from nearly gas-kinetic for the higher hydrocarbons and CH2CO to 10−2 for He. For He, Ar, and CH2CO removal rates were measured for the v2=1 excited bending vibrational level and found to be identical to the ground state rates. Pseudo-first-order rate constants for equilibration of the nascent rotational distribution in collisions with He and CH2CO were found to be factors of 17 and 2.7 faster than the respective removal rates.
Photophysics of laser-excited thioformaldehyde Ã(1A2)
Authors: Bruno A. E. , Steer R. P.
Published: February 2003

Fluorescence has been excited in the rotational subband heads of low lying vibrational levels in the Ã(1A2) state of thioformaldehyde. Single exponential fluorescence decays measured as a function of pressure yield linear Stern–Volmer plots from which gas kinetic quenching rates and collision-free lifetimes of between 14 (000 band) and 4 μs (420 band) have been obtained. The results are interpreted in terms of solely radiative decay of the vibronic states investigated under isolated molecule conditions.
BKLT equations for reactive scattering. I. Theory and application to three finite mass atom systems
Authors: Shima Y. , Kouri D. J. , Baer Michael
Published: February 2003

The BKLT equations for reactive scattering are considered in detail, both from a formal and computational point of view. The equations are very attractive because they do not require any matching of wave functions. It is shown how these equations may be solved for a general collinear three-finite mass atom system. Special care is taken to treat subleties in the theory arising from restrictions on the ranges of the vibrational coordinate of the various diatoms due to the skewing angle being less than 90°. In addition, the structure of the equations is explored in detail since this has significance for their optimum solution. It is found that the structure of the equations for asymmetric systems leads to important redutions in the size of the matrix which must be inverted within the present, nonpropagative method. Other solution methods are also discussed to some extent. Finally, the method is illustrated by an application to the H+H2 exchange reaction with the Porter–Karplus potential surface. The results obtained agree well with those obtained earlier by Diestler using a close coupling, propagation procedure.
Energy-dependent energy transfer: Deactivation of azulene (S, Evib) by 17 collider gases
Authors: Rossi Michel J. , Pladziewicz Jack R. , Barker John R.
Published: February 2003

Collisional deactivation of highly vibrationally excited azulene in the electronic ground state was investigated using infrared fluorescence detection. Azulene (S0, E) was prepared with E≂17 500 cm−1 and E≂30 600 cm−1 by laser excitation at 600 and 337 nm, respectively. Advantage was taken of the fast internal conversion rate to S0 azulene from S1(600 nm) and S2(337 nm) electronic states. The collider gases investigated are He, Ne, Ar, Kr, Xe, H2, D2, N2, CO, O2, CO2, H2O, NH3, CH4, SF6, n-C4H10, and unexcited azulene. The results are expressed in terms of 〈ΔE(E)〉, the average energy transferred per collision, which can depend on the vibrational excitation energy E of the azulene. Using previously obtained knowledge of the dependence of infrared fluorescence intensity on E [M. J. Rossi and J. R. Barker, Chem. Phys. Lett. 85, 21 (1982)], two methods were used to obtain 〈ΔE(E)〉 values from the fluorescence decay curves: (1) an approximate method that considered only the average energy, and (2) solution of the full collisional master equation. Both methods gave 〈ΔE(E)〉 values that depend strongly on E. The limited experimental information on the identity of the energy-transfer processes operative in the deactivation of azulene is discussed. Additional experimental results on vibration-to-vibration energy transfer from azulene to CO2 are presented, which indicate that the emission at 4.3 μm observed previously [J. R. Barker, M. J. Rossi, and J. R. Pladziewicz, Chem. Phys. Lett. 90, 99 (1982)] originates not only from CO2(001), but from other states with one quantum of excitation in ν3. The experimental results are discussed in terms of models for energy transfer, which have appeared in the literature. It is concluded that only a superficial understanding exists and theory has lagged far behind experiments on energy transfer.
On the reaction Mg+N2O→MgO+N2
Authors: Yarkony David R.
Published: February 2003

The lowest singlet (1A′) and triplet (3A′) potential energy surfaces of the reaction Mg+N2O(1Σ+)=MgO+N2(1Σ+g) are investigated using ab initio SCF, two configuration MCSCF and CI wave functions. The reactivity of Mg(1S) and Mg(3P) is contrasted. These surfaces can be qualitatively partitioned into three contiguous, nondisjoint regions characterized by distinct values of an approximate reaction coordinate (ζ); a reactant region in which ζ≡ζ1≂R(Mg–O), a bending region in which ζ≡ζ2≂∢NNO, and a product region in which ζ≡ζ3≂(N–O). Evolution into region 3 requires Mg to N2O charge transfer which facilitates, and is facilitated by, bending of N2O. On the 3A′ surface which correlates with MgO(a 3Π), this process is largely downhill and involves an orbital reorganization in the N2O moiety for ζ=ζ3. A small barrier to vertical charge transfer is predicted at the CI level. On the 1A′ surface, which correlates with MgO (X 1Σ+) approach is uphill for ζ=ζ1. Possible explanations for the profound differences in the reactivity of calcium and magnesium with N2O are suggested.
Sub-Doppler infrared–infrared double resonance spectroscopy of NH3 using Stark tuning and a diode laser
Authors: Weber W. H. , Terhune R. W.
Published: February 2003

Double resonance experiments on NH3 are described in which a CO laser pumps a 6 μm ν4 transition while a tunable diode laser probes a 10 μm ν2 transition having a common lower level. Four different combinations of pump–probe transitions are studied. The CO laser is Lamb-dip stabilized on the pump transitions, which are tuned into coincidence with it using a precision intracavity Stark cell. The pump and probe beams overlap colinearly in the Stark cell. The double resonance signals appear as narrow transmission peaks on the diode laser scans. The narrowest observed widths are ≂3 MHz (FWHM), a large portion of which is due to unresolved hyperfine structure. An analysis of the various broadening mechanisms indicates that the diode laser contributes less than 1 MHz to the widths. Resonances due to velocity-preserving but state-changing collisions are seen. Asymmetries between co- and counterpropagating linewidths are shown to arise from a combination of field inhomogeneity and coherent narrowing effects. The data are recorded using a computer-aided, rapid-scan, digital signal averaging technique.
Rotational Zeeman effect in ArHCl and ArDF
Authors: Campbell E. J. , Read W. G.
Published: February 2003

The molecular g values and magnetic susceptibility anisotropies of ArH35Cl, ArH37Cl, and ArDF have been measured using pulsed Fourier transform microwave spectroscopy carried out in a Fabry–Perot cavity located in the bore of a superconducting solenoid magnet. The measured magnetic parameters for these three molecules, obtained from rotational Zeeman splittings of K=0 R-branch transitions are           The relationship between these measurements and the signs of the electric dipole moments, the molecular quadrupole moments, and the anisotropy in the second moments of the electronic charge distribution is discussed in detail, with particular emphasis on the effects of the large zero point bending motions present in these molecules. The relationships between the measured Zeeman parameters and the properties of the individual rare gas and hydrogen halide subunits is treated. We conclude that the contribution of the rare gas atom to the measured g⊥ values is negligibly small, but that the rare gas atom apparently does make a measurable contribution to the magnetic susceptibility anisotropies of at least the HF containing systems.
Infrared spectra of OC–HX hydrogen-bonded complexes in solid argon
Authors: Andrews Lester , Arlinghaus Robert T. , Johnson Gary L.
Published: February 2003

The hydrogen-bonded complexes OC–HX (X=F, Cl, Br) has been prepared by condensing Ar/HX and Ar/CO reagent mixtures at 12 K. FTIR spectra of the complexes are characterized by strong H–X absorptions (νs) displaced below the isolated HX fundamental, strong C–O absorptions displaced above the isolated CO fundamental, and sharp degenerate librational modes (νl) in the far infrared. Observation of a single degenerate νl mode indicates linear structures for the complexes. The displacements Δνs and ΔνCO for the complexes decrease in magnitude in the series HF≳HCl≳HBr in direct relationship with the trend in hydrogen bonding strength for the hydrohalic acids.
Semiclassical calculation of electronic spectra of supercooled anharmonic molecules
Authors: Warshel Arieh , Stern Peter S. , Mukamel Shaul
Published: February 2003

The electronic absorption spectrum of supercooled methyl benzene is calculated using a semiclassical procedure which avoids the calculation of the vibronic eigenstates. Classical trajectories are used to generate the time dependent potential energy gap [U(τ)] for the electronic transition. This energy gap is utilized to calculate the spectrum. This procedure may be easily applied to large anharmonic polyatomic molecules. (AIP)
Low temperature specific heat of Tb6Mn23 and Ho6Mn23
Authors: Graves Tompson R. J. , White H. W. , Kebe B. , James W. J.
Published: February 2003

Specific heat measurements below 20 °K are reported for the rare earth intermetallic compounds Tb6Mr23 and Ho6Mn23. The main purpose of this investigation is to ascentain if the specific heat data of the ‘‘heavy’’ rare earth intermetallics shows any evidence for changes in the magnetic spin structures at low temperature. (AIP)
Comment on the classical models of electrons and nuclei and the generalizations of classical Poisson brackets to include spin
Authors: Lakshmanan M. , Daniel M.
Published: February 2003

We point out the close analogy between the canonical equations of motion for molecular systems consisting of electrons and nuclei interacting with an electromagnetic field discussed recently by Yang and Hirschfelder and the equations of motion for spin systems in condensed matter physics. Specifically, we treat the Heisenberg ferrmagnetic spin chain and discuss the close connection with the equations derived for a Breit–Pauli Hamiltonian. (AIP)
Theoretical studies on low-lying electronic states of the CCl+, SiCl+, and GeCl+ ions
Authors: Nishimura Y. , Mizuguchi T. , Tsuji M. , Obara S. , Morokuma Keiji
Published: February 2003

Potential energy curves for the ground state X 1Σ+ and low-lying A 1Π and a 3Π states of the CCl+, SiCl+, and GeCl+ ions have been calculated by ab initio LCAO–SCF–MO configuration interaction method. Calculated molecular constants Te, ωe, and ωexe agree well with available experimental data. The assignments of A 1Π–X 1Σ+ and a 3Π–X 1Σ+ bands for CCl+ and a 3Π–X 1Σ+ band for SiCl+ have been confirmed.
The temperature dependence of the structural transformation rate in NiTiF6⋅6H2O: Evidence for a second phase transition
Authors: Lichti R. L.
Published: February 2003

The progression rate for the structural transformation in NiTiF6⋅6H2O has been measured under supercooled conditions by monitoring an EPR line of Ni2+ from the trigonal phase. The temperature dependence of the rate appropriate to the first thermal cycle for any sample indicates the presence of a second transition. These data yield an energy barrier of 4998±34 cal/mol and a heat of transformation of 666±84 cal/mol for the main trigonal→monoclinic transformation with a critical temperature of 136.3±0.5 K when fit to a nucleation and growth model. A plausibility argument suggests that the second transition which has nearly identical energy parameters and a critical temperature of 126.0±0.5 K, may be associated with an additional distortion of the [TiF6]2− ion known to occur in this temperature region.
On the stabilization of optical isomers through tunneling friction
Authors: Harris Robert A. , Silbey Robert
Published: February 2003

A handed molecule may be represented as a particle in a symmetric double well. When placed in contact with a low temperature medium, the mean position of the particle obeys an equation of motion identical to a damped harmonic oscillator. The standard relaxation theory is used to relate the damping to classical friction, in agreement with recent path integral methods; however, no renormalization of the tunneling matrix element occurs. The stability of handedness is explained in terms of the damped oscillator.
Pressure effects on the Raman spectra of micelles: Pressure induced phase transition and structural changes in aqueous sodium oleatea)
Authors: Wong P. T. T. , Mantsch H. H.
Published: February 2003

The Raman spectrum of an aqueous micellar solution of sodium oleate has been studied at pressures up to 5.05 kbar. It is shown that the acyl chain interactions are enhanced by pressure and that the number of gauche bonds in the micellar phase increases with increasing pressure. A first-order phase transition, from the micellar isotropic phase to a solidlike coagel phase was found to occur at 1.7 kbar. This phase transition has a very high pressure hysteresis. Significant differences were observed between the pressure and temperature effects on many structural properties of this aqueous sytem.
Visible spectroscopy of matrix isolated HCO: The 2A″(Π)←X  2A′ transition
Authors: van Ijzendoorn L. J. , Allamandola L. J. , Baas F. , Greenberg J. M.
Published: February 2003

The absorption spectrum of the 2A″(Π)←X 2A′ transition of HCO has been measured in Ar, Kr, Xe, CH4, CO, and N2 matrices and in the mixtures CO/H2O/NH3/CH4 (10:4:3:2) and (3:5:2:1). In the noble gas matrices, the spectrum appears with an alternating intensity pattern opposite that found in the gas phase. In a 10 K matrix, the transition originates from the K″=0 level instead of the K″=1 level and the ΔK=±1 selection rule is still important. Transitions into states normally strongly predissociated in the gas phase are measured indicating the importance of the cage effect. In the molecular matrices, the intensity alternation disappears and a strong underlying broad absorption is observed. This behavior is explained in terms of the complete breakdown of the ΔK=±1 selection rule. This interpretation is supported by calculations, using the Pople–Longuet–Higgins formulation, which show that the Renner effect is significantly greater in the molecular matrices then in the noble gas matrices. Upon warmup from 10 to 15 K the absorbance of HCO increases in Ar, Kr, and CH4 matrices indicating the release of H atoms which react with a low activation energy with CO. Simultaneously, the controversial infrared absorptions assigned either to matrix isolated hydrogen atoms or ArnH+ (KrnH+) disappear.
A self-consistent eikonal treatment of electronic transitions in molecular collisions
Authors: Micha D. A.
Published: February 2003

We develop an eikonal treatment of electronic transitions in many-atom collisions, in which classical nuclear trajectories are self-consistently coupled to quantal electronic transitions. The treatment starts with a discussion of the electronic representations required to assure that Hamiltonian matrices are Hermitian. The amplitudes of wave functions are found to satisfy coupled equations which are expanded in powers of a local de Broglie wavelength. Time-dependent equations are transformed to derive a Hamiltonian formalism that couples nuclear positions and momenta with electronic amplitudes. Cross sections are obtained from flux conservation and also from T-matrix elements.
Intermolecular multiple scattering of electrons. I. Theory
Authors: Bartell Lawrence S. , Jin Anding
Published: February 2003

Coherent intramolecular multiple scattering by free molecules has been investigated extensively in the literature. Scant attention has been paid to incoherent intermolecular multiple scattering, however, a potentially serious problem under some circumstances. Therefore, a treatment of this problem has been carried out, taking advantage of simplifications afforded by the predominance of forward scattering. Explicit expressions of elementary form are derived for the differential cross sections corresponding to double, triple, and higher scatterings, and for the fractional contribution of each to the total intensity. Illustrative calculations are presented for electrons diffracted at various sample pressures encountered in a recent diffraction study of collisionally assisted laser pumping of SF6.