bluuebunny/crossref_metadata_2025 huggingface dataset:
Recombination of geminate species in the prescence of a scavenger
Authors: Tachiya M.
Published: February 2003

We present a general method for calculating the recombination probability of geminate radicals or ions in the presence of a scavenger. By use of this method the recombination probabilities of geminate radicals in one-, two-, and three-dimensional spaces are analytically calculated as a function of scavenger concentration. The so-called square-root law holds in one- and three- dimensional spaces, but not in a two-dimensional space. The present method is also applied to the recombination probability of geminate ions. The recombination probability of geminate ions in the presence of both a scavenger and an external electric field has been rigorously derived for the first time.
Raman spectrum of β-CH4 in the intramolecular and lattice regions
Authors: Medina F. D. , Daniels W. B.
Published: February 2003

The Raman spectrum of β-CH4, or phase II of solid methane, has been observed in the intramolecular and lattice regions. The ν4 mode and the combination bands 2ν4 and ν2+ν4 have been observed for the first time. The spectrum of the 2ν4 band provides further evidence of free rotation in β-CH4. However, the rotation–vibration spectrum of Oh molecules was not observed in the ν4 band due to its low intensity. In the lattice region, three first-order Raman lines were observed, as predicted by theory. A broad, asymmetric line in the lattice region, identified as a librational mode, gives direct evidence for the existence of a group of excited librational states about 50 cm−1 from the librational ground state of D2d molecules. The measured temperature dependence of the frequency and width of this line is compared to previous theoretical calculations.
A theoretical study of resonance Raman scattering from molecules
Authors: Fujimura Y. , Lin S. H.
Published: February 2003

An expression for the resonance Raman scattering (RRS) cross section from large molecules is derived by means of a time independent Green function method. This expression is obtained in the adiabatic basis set, and both the radiative and nonradiative damping effects are explicitly considered, assuming the statistical limit in the excited vibronic states. Expanding the nuclear coordinate dependence of the transition moment, an analytical expression for the RRS cross section is derived for a displaced potential energy surface model. The expression obtained is applicable to the resonance Raman scattering not only in the weak coupling but also in the strong coupling case, and consists of the sum of the nth order vibrational transitions. It is shown that in the weak coupling case, the fundamental cross section may be explained as a succession of the absorption to the lowest level in the excited electronic state followed by the emission from it. Multimode effect on the RRS cross section which has not been reported before is studied and the analytical expression for the RRS cross section including this effect is obtained. The dimensionless displacement Δ dependence on the excitation energy profile is calculated. The effect of the nonadiabatic coupling (or the Born–Oppenheimer coupling) on the RRS cross section is also investigated. It is shown that using the adiabatic basis set, the BO coupling appears only in the high order approximations in most cases.
Molecular dynamics study of the hydration of Lennard-Jones solutes
Authors: Geiger A. , Rahman A. , Stillinger F. H.
Published: February 2003

In order to clarify the nature of hydrophobic interactions in water, we have used the molecular dynamics simulation method to study a system comprising two Lennard-Jones solute particles and 214 water molecules. Although the solutes were placed initially in contact, forces in the system drive them slightly apart to permit formation of vertex-sharing solvent ’’cages.’’ Definite orientational preferences have been observed for water molecules in the first solvation layer around the Lennard-Jones solutes; these preferences are loosely reminiscent of structure in clathrates. Nevertheless, substantial local disorder is obviously present. The dynamical data show that translational and rotational motions of solvation–sheath water molecules are perceptibly slower (by at least 20%) than those in pure bulk water.
An O−2 molecule ion in CaF2: Structure and dynamical features
Authors: Bill H.
Published: February 2003

This experimental study reports EPR and ENDOR results of an O−2 molecule ion in natural yellow fluorites. The O−2 is associated with a sodium neighbor and a calcium neighbor. This cluster performs a rapid switching movement between energetically equivalent positions. The resulting dynamical features in the EPR spectrum are investigated with the aid of the stochastic Liouville equation.
An improved model for a three dimensional pure substance with two critical points
Authors: Hecht Charles E. , Sapse Anne-Marie , Shenkin Peter
Published: February 2003

Equations of state are derived for the hard-sphere model using first order perturbation theory. (AIP)
Comment on explicit integration method for the time-dependent Schrödinger equation
Authors: Rubin Robert J.
Published: February 2003

It is pointed out that the numerical solution proposed by Askar and Cakmak is neither original nor unconditionally stable, as the authors claim. (AIP)
Depolarized light scattering from dense noble gases
Authors: An S. C. , Fishman L. , Litovitz T. A. , Montrose C. J. , Posch H. A.
Published: February 2003

Depolarized Rayleigh spectra for four different thermodynamic states of krypton are reported. At large liquidlike densities and for low frequency shifts (<20 cm−1) the existence of a Lorentzian component previously discovered in liquid argon is confirmed. For large ν the line shapes are mainly exponential with the exception of a weak shoulder located at around 50 cm−1. These spectral features are interpreted in the light of a recent theory by Madden. In comparing the results for krypton and argon the law of corresponding states is found to hold. The dependence of the Lorentzian half-width on the diffusion coefficient is not the same as inferred from the Madden theory.
Momentum dependent dielectric response of polystyrene
Authors: Ritsko John J.
Published: February 2003

High resolution electron energy loss spectra as a function of momentum transfer have been measured in polystyrene. The strong E1u, π→π* excitation of the benzene rings at 7.0 eV shows an unexpected negative dispersion of 0.45 eV which is not observed in gas phase electron scattering measurements on benzene. It is suggested that the negative dispersion of the π→π* excitation is due to a solid state local field effect. The dielectric response function at a momentum transfer of 1.0Å−1 is calculated from the measured energy loss spectrum and a comparison is made with a simple theory of local field effects.
Master equation derivation of Keizer’s theory of nonequilibrium thermodynamics with critical fluctuations
Authors: Fox Ronald Forrest
Published: February 2003

A master equation derivation of Keizer’s theory of nonequilibrium thermodynamics is presented. It reduces the number of independent postulates in Keizer’s theory from three to one, clarifies delicate questions regarding ’’scaling’’, and extends Keizer’s theory to include fluctuations at a critical point. The critical fluctuations are not Gaussian, like the noncritical fluctuations, but still possess a universal character exhibited in a distribution function which is the exponential of a quartic form. Both Fokker–Planck and nonlinear Langevin formulations are utilized.
Trimer binding in two dimensions
Authors: Cabral F. , Bruch L. W.
Published: February 2003

The ground state energies of loosely bound boson and fermion trimers interacting via Lennard-Jones (12,6) pair potentials in two dimensions are evaluated with variational calculations. Threshold coupling constants for self-binding are determined by extrapolation to zero binding energy. The results are used to give estimates of the binding energy of the 4He trimer and in a discussion of the ordering of dimer and trimer thresholds in two dimensions.
Laser time-resolved studies of exciton migration in doped lithium niobate
Authors: Powell Richard C. , Freed Edwin E.
Published: February 2003

Time-resolved spectroscopy measurements were made on LiNbO3 and LiNbO3:Eu3+, Cr3+ crystals after pulsed nitrogen laser excitation. The results are found to be consistent with a model of host exciton migration and trapping at activator ion sites with efficient Eu3+–Cr3+ transfer also occurring. The properties of this host-sensitized energy transfer are characterized in a diffusion model and the results are discussed in terms of their possible implications on the use of this material in laser host and holographic storage applications.
Vibrational dependence of pressure induced spectral linewidths and line shifts: Application of the infinite order sudden scattering approximation
Authors: Green Sheldon
Published: February 2003

The infinite order sudden (IOS) approximation to molecular rotation is applied to simplify the theory of linewidths and shifts in vibration–rotation spectra. This approximation is expected to be most accurate for hard, short-range collisions and is therefore complementary to Anderson theory which is best for weak, glancing collisions. The IOS approximation predicts identical linewidths and shifts for P- and R-branch transitions with the same line number. It also predicts zero line shifts for pure rotational spectra. The dependence of linewidths and shifts on vibrational band is seen to be due mainly to variation in diagonal vibrational matrix elements of the intermolecular potential. Calculations are performed for the 0–0, 0–1, and 0–2 bands of CO perturbed by He, using a theoretical interaction potential with no semiempirical or adjustable parameters; results are in satisfactory accord with experimental data.
Exchange dephasing and motional narrowing of vibrational modes
Authors: Abbott Richard J. , Oxtoby David W.
Published: February 2003

A general formalism using cumulant expansion techniques is applied to the study of the band shapes of interacting or overlapping vibrational bands in dense media. The formalism is applied to the interaction between a fundamental and a hot band and that between two fundamentals. Both models appear to be capable of describing the recent experimental results of Harris and co-workers on the band shapes of modes in the C–H stretching region of solid durene.
Infrared spectra of the CH2F2+, CHF2+, CHF+, and FH– (CHF)− molecular ions in solid argon
Authors: Andrews Lester , Prochaska Frank T.
Published: February 2003

Argon resonance photoionization of argon–methylene fluoride samples during condensation at 15 K produced three groups of new infrared absorptions, which are separated by their mercury arc photolysis behavior and isotopic data. Four infrared absorptions assigned to CH2F2+ correspond closely with CH2F2 neutral absorptions both in position and carbon-13 and deuterium shifts. The C–H stretching modes of CH2F2+ are lowered by the removal of a C–H bonding electron. The high antisymmetric C–F2 stretching fundamental for CHF2+ at 1608 cm−1 is consistent with extensive pi bonding in this planar cation. A sharp 1368 cm−1 absorption is attributed to the C–F stretching mode of CHF+. Strong, broad absorptions showing small carbon-13 shifts are assigned to the H–F stretching and hydrogen deformation modes of the intramolecular hydrogen bonded electron-capture product (F–H) –– (CHF)−. A vibronic absorption from 413 to 544 nm in these samples is attributed to the CHF intermediate.
Droplet model for the analysis of the dielectric properties of critical binary mixtures
Authors: Goulon José , Greffe Jean-Louis , Oxtoby David W.
Published: February 2003

We present here some further developments of the droplet model recently proposed by Oxtoby, which appears quite suitable for an extensive phenomenological analysis of the dielectric behavior of binary mixtures in the vicinity of a consolute critical point. Along the critical isochore, one may expect the static permittivity εE(T) to remain finite but to exhibit a cusp, whereas the nonlinear dielectric effect, the Kerr effect, and the Cotton–Mouton effect all are predicted to diverge strongly with the universal exponent Ψ=2γ−dν. These predictions are supported by recent experimental evidence.
Laser induced photodissociation of O2+
Authors: McGilvery D. C. , Morrison J. D. , Smith D. L.
Published: February 2003

The photodissociation spectrum of O2+, over the energy range 1.7–2.6 eV, has been recorded using tandem quadrupole mass filters and a tunable dye laser. Many of the peaks in the spectrum have been examined at higher resolution and fine structure is observed. A discussion is given of the states involved in the photodissociation. It is believed that processes of both direct dissociation and at least one, but possibly two, types of predissociation are occurring.
Effects of density and temperature on mobilities of electrons in vapors and liquids of pentane isomers: Molecular structure effects
Authors: György István , Freeman Gordon R.
Published: February 2003

The mobility of electrons in low density gaseous n-pentane is independent of applied electric field strength up to E/n=2 Td. The mobility in isopentane increases at E/n≳0.4 Td, and in neopentane at ≳0.2 Td. The ratio of the threshold drift velocity for electron heating to the velocity of sound, vd(threshold)/co, increases as the molecules become less spherelike. The value of the ratio in low density xenon is ∼0.7, in neopentane ∼5, isopentane ∼15, and n-pentane ≳100. The high field mobilities in neopentane and isopentane vapors seem to approach the field independent value in n-pentane. Representing the velocity dependence of the scattering cross section as σv=e/mbjv1−j, the value of 1−j increases with increasing sphericity of the molecules. Values of 1−j and σav=〈v〉/〈v/σv〉 for thermal electrons in the dilute gases at 300 K are respectively: n-pentane, 1.1, 1.3×10−15 cm2; isopentane, 1.5, 1.8×10−15 cm2; neopentane, 3.2, 4.0×10−15 cm2. The variation of (1−j) and σav with sphericity for the isomeric pentanes can be interpreted in terms of either: a) a Ramsauer–Townsend effect, the σv minimum occurring at lower v for less spherelike molecules; or b) a transient negative ion state lying near zero energy, which makes a stronger contribution to scattering from more spherelike molecules. The apparent values of (1−j) and σv increase in the high density gases, due to multibody interactions. Mobilities in the liquids n-pentane and isopentane are interpreted in terms of thermally activated electron transitions between localized and extended states. The extent of localization is greater for less spherelike molecules. Behavior in neopentane is more complex. The structure factor S (0) does not have the influence suggested earlier on electron scattering in the fluid far removed from the triple point.
Mixed and fermionic helium trimers
Authors: Duffy K. , Lim T. K.
Published: February 2003

The existence of mixed and fermionic helium trimers is investigated via the Faddeev–UPE method. Each three-atom system is assumed to interact through phenomenological pairwise local potentials. The ground-state binding energy of (4He)2–3He is evaluated to be about 0.000 002–0.0008 °K while (3He)2–4He is found to be unbound as is (3He)3.
Heavy rare-gas trimers and tetramers: Testing ground for few-body techniques in chemical physics
Authors: Lim T. K. , Duffy K. , Nakaichi Shuji , Akaishi Yoshinori , Tanaka Hajime
Published: February 2003

The ground-state energies of systems of three and four identical heavy rare-gas atoms, treated as spinless bosons and assumed to interact via pairwise Morse potentials, are calculated using five different methods. Self-bound trimers and tetramers are found in every case with upper and lower bound formulas yielding energies which are extremely close. For these near-classical and massive systems, the Faddeev–UPE method is decidedly poor. On the other hand, the simple generalization of Bruch–Sawada’s upper bound is particularly effective. The plight of Faddeev–UPE here implies that great caution must be exercised in using the unitary pole expansion in chemical physics. ATMS is excellent and appears the method of choice for bound-state problems.
Concerning the anisotropy of the helium diatom polarizability
Authors: Frommhold Lothar , Proffitt Michael H.
Published: February 2003

The spectral distribution and absolute intensity of the collision-induced Raman spectrum of He is measured and compared to wave mechanical calculations. (AIP)
A spectrum of triatomic hydrogen
Authors: Herzberg G.
Published: February 2003

Rydberg states were studied in H3 and D3 using a hollow cathode discharge tube. (AIP)
Muonium atoms and intraspur processes in water
Authors: Walker D. C. , Jean Y. C. , Fleming D. G.
Published: February 2003

Since muonium atoms come into existence as a result of stopping energetic positive muons, it has been suggested that intraspur reactions (primarily with thermalized electrons or OH radicals, in water) are responsible for: (i) muonium formation, (ii) direct generation of a fraction of the muons in diamagnetic molecules, and (iii) the ’’missing’’ muon polarization. These postulates are examined in detail by adding scavengers which can preclude intraspur reactions without reacting with muonium. The results show convincingly that intraspur processes are insignificant in (i) and (ii). Furthermore, they could only be important in (iii) if a spin-exchange process occurred with a frequency which was much greater than the ordinary diffusion-limited encounter frequency. Rather than this ’’spur model’’, the data obtained are consistent with the ’’hot model’’, whereby hot muonium atoms form beyond the spurs of the muon track.
Nuclear magnetic resonance studies of 1H in zirconium halide hydrides: Shielding tensors
Authors: Dubois Murphy P. , Gerstein B. C.
Published: February 2003

The magnetic shielding tensors and T*2’s of 1H have been determined at room temperature in ZrClH0.5, ZrBrH1.0, and ZrBrH0.5 utilizing multiple pulse NMR. In addition, stoichiometries have been determined by spin counting. Each proton shielding tensor exhibits a large anisotropy compared to those found in hydrocarbons and salts, and is similar to that found in ZrClH1.0. Site occupations of the protons in these ’’sandwichlike’’ compounds has been inferred from isotropic shifts and from anisotropies.
Photoelectron spectroscopy using a supersonic molecular beam source. The 2B3u+e−←1A
g transition in ethylene
Authors: Dehmer P. M. , Dehmer J. L.
Published: February 2003

We show that a supersonic molecular beam source will significantly improve the quality of molecular photoelectron spectra by reducing both Doppler and rotational broadening of the target gas, and that the benefits of such a source are realized for spectrometers operating at resolutions of ?20 meV. The photoelectron spectra of the O2+ 2Πg,v′=0+e−←O2 3Σg− transition in molecular oxygen and the C2H4+ 2B3u +e−←C2H4 1Ag transition in ethylene are presented as examples of the technique. Newly resolved vibrational structure in the ethylene spectrum is compared to recent theoretical calculations and to the vibrational structure of the 3s Rydberg transition at 1744 Å.
Laser photoelectron spectrometry of Co− and Ni−
Authors: Corderman Reed R. , Engelking P. C. , Lineberger W. C.
Published: February 2003

The photoelectron spectra of the negative ions Co− and Ni− have been obtained with a fixed frequency cw ArII laser (488 nm). The electron affinity of cobalt is found to be 0.662±0.010 eV, while that of nickel is 1.157±0.010 eV, in excellent agreement with a previously reported value E.A.(Ni) =1.15±0.15 eV determined by threshold photodetachment. Resolution of both negative ion and neutral fine structure intervals is obtained, and results in the determination of Co−: 3F4−3F3=910±50 cm−1, 3F3−3F2=650±50 cm−1 and Ni−: 2D5/2−2D3/2=1470±100 cm−1. The intensities of individual fine structure components are calculated from an angular momentum analysis of the photodetachment process and are compared with the experimental results. The fine structure intensities do not follow the (2J″+1) (2J′+1) statistical weighting of the negative ion (J″) and neutral (J′) spin–orbit states, but are much better represented by geometrical factors. The detachment of s electrons is preferred to the detachment of d electrons at 488 nm.
ESR and ENDOR at 9 and 35 GHz on a powder of the enzyme methanol dehydrogenase from H
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Authors: de Beer R. , van Ormondt D. , van Ast M. A. , Banen R. , Duine J. A. , Frank J.
Published: February 2003

ESR and ENDOR at 9 and 35 GHz have been applied to a powder of the enzyme methanol dehydrogenase from Hyphomicrobium X. The observed g-tensor and ESR linewidth suggest that the protein-bonded free radical originates from a quinone. ENDOR measurements at 35 GHz show that there are two strongly coupled protons in the free radical. The ENDOR response of the proton with the largest hyperfine coupling has been simulated by means of a line shape model. From this simulation the principal values of the hyperfine tensor could be extracted. The derived values indicate that the proton concerned is attached directly to a carbon of the quinoid ring.
Absolute rate coefficients for F + H2 and F + D2 at T =295 Ka)
Authors: Heidner R. F. , Bott J. F. , Gardner C. E. , Melzer J. E.
Published: February 2003

The rate coefficients for the F+H2 and F+D2 reactions were determined to be (1.8±0.3) ×1013 and (9.5±1.7) ×1012 cm−3/mol-s, respectively. The absolute values and the resultant ratio of H-atom to D-atom abstraction of 1.92±0.19 are in agreement with earlier determinations. Fluorine atoms were generated under isothermal conditions in excess argon by means of the infrared multiphoton dissociation of SF6. The reaction between F and H2(D2) was monitored by means of the time-resolved infrared emission from HF+(DF+). The validity of the experimental method is discussed.
The shear viscosity of a hard-sphere fluid via nonequilibrium molecular dynamics
Authors: Naitoh Toyoaki , Ono Syu
Published: February 2003

The calculations of shear viscosity were performed for the four cases of the particle number N=108, 256, 500, and 1372. The periodic boundary condition has been generalized to the system in which the stationary Couette flow is present. The viscosity was calculated from the temporal increase in the temperature due to dissipation as well as from the pressure tensor. The values of the viscosity were obtained at the different shear rates over the entire fluid density range. The calculated values are rather in good agreement with the Alder–Gass–Wainwright results. A general tendency of the shear viscosity to decrease with shear rate is observed. The decrease in the viscosity with N due to the size effect was observed at freezing density. The Stokes relation was dealt with, and the approximate nonspherical pair distribution functions in the presence of Couette flow were calculated.
Bond lengths of vibrationally hot AXn molecules. A simple treatment
Authors: Bartell L. S.
Published: February 2003

Given the form of an anharmonic, modified Urey–Bradley molecular force field for AXn molecules, it is shown how to express the mean vibrational displacements of bond lengths from equilibrium lengths re simply in terms of elementary quantities. The dominant terms isolated are due to Morse stretching anharmonicity, nonbonded interactions, and the centrifugal stretching of bonds encountered as the atoms swing over arced trajectories in bending vibrations. Illustrative calculations are presented for CH4 and CD4.
Inference of vibrational anharmonicity in hot SF6: An electron diffraction study
Authors: Bartell L. S. , Doun S. K. , Goates S. R.
Published: February 2003

In an investigation of gaseous SF6 at temperatures ranging from 298 to almost 1000 K, the observed thermal expansion of the mean bond length as a function of the mean-square amplitude of vibration was found to be much too large to be accounted for by the commonly invoked model of Morse anharmonicity. Experimental results can be reproduced quantitatively, however, with the aid of existing potential constants fed into an anharmonic, modified Urey–Bradley force field proposed previously.
A perturbation method for classical atomic fluids which have a repulsive pair potential
Authors: Johnson Elijah
Published: February 2003

A new method for choosing a reference pair potential for atomic fluids is presented. This method and the optimized cluster theory are used to calculate pair distribution functions for a number of classical fluids. The results indicate that pair distribution functions obtained by this method are very accurate for fluids which have a repulsive pair potential.
Kinetics of Fe2+ oxidation and Fe3+ reduction in MgO single crystals
Authors: Sonder E. , Stratton T. G. , Weeks R. A.
Published: February 2003

Electron paramagnetic resonance measurements have been used to determine the rate of change of the Fe3+ concentration in MgO:Fe crystals due to reduction and oxidation with ambient gases at temperatures between 1000 and 1500 K. The results indicate that diffusion of vacancies with an activation energy of 3.23 eV is the rate-limiting step. There is some evidence that when a diffusion coefficient is obtained from a concentration profile, a contribution due to anomalously high mobility (presumably along dislocations) can be detected.
The magnetic field dependence of the delayed fluorescence in p-terphenyl between 130 and 300 K
Authors: Fünfschilling J. , Altwegg L. , Zschokke–Gränacher I. , Chabr M. , Williams D. F.
Published: February 2003

The magnetic field dependence of delayed fluorescence in single crystal p-terphenyl has been measured from 300–130 K; that is, through its phase transition region. Current theories predict satisfactorily the observed resonance positions with respect to field orientation. The field dependence of the fluorescence intensity however increases with increasing field. Such behavior is described in terms of a magnetic field dependent trap induced spin relaxation.
Resonant dissociative photoionization of H2
Authors: Kirby K. , Guberman S. , Dalgarno A.
Published: February 2003

The absorption of photons with an energy of 26.9 eV by H2 produces H+ ions with kinetic energies ranging up to 4.4 eV. We attribute the energetic H+ ions to absorption into a 1Σu+ resonance state of H2 which autoionizes into a continuum corresponding to a free electron moving in the field of the 1sσg core of H2+. We present calculations of the real and imaginary parts of the complex potential energy surface of the resonance state and of the transition dipole moment between the ground state of H2 and the resonance state. The resulting cross section for the production of energetic H+ ions and the calculated ratio of H+ to H2+ ions produced are in agreement with measurements. The energy distribution of the H+ ions has a maximum at about 3.2 eV. The maximum cannot be reproduced by a classical description of the nuclear motion in the autoionizing resonance state, but it does arise when a quantal treatment is used, in which the nuclear motion is governed by the real part of the complex potential.
Kinetic theory of reacting fluid mixtures. I. The BBGKY hierarchy in arrangement channel quantum mechanics
Authors: Hoffman D. K. , Kouri D. J. , Top Z. H.
Published: February 2003

The statistical mechanics of reacting fluid mixtures is considered using arrangement channel quantum mechanics. In this theory, the many body wave function, ψ, is expressed in terms of arrangement channel components ψα which satisfy coupled time dependent equations. Using the properties of the ψα, we obtain the corresponding generalization of the density matrix ρ denoted ραβ and the quantum ’’Liouville equation’’ which it satisfies. Reduced arrangement channel density matrices are introduced and are found to satisfy a generalization of the BBGKY hierarchy for reactive mixtures. The properties of the hierarchy are explored, and it is shown that the streaming terms in succeeding members of the hierarchy have increasing connectivity appropriate to the number of atoms in the contributing arrangement channels. The resulting hierarchy can serve as the rigorous starting point for developing statistical theories of reactive fluids. For example, it can be used to derive a Boltzmann equation appropriate to reacting gaseous mixtures.
Emission spectra of the radical cations of difluorodiacetylene, perfluoropentadiyne-1,3, and perfluorohexadiyne-2,4 in the gas phase: ?→? band systems and the lifetimes of the ? states
Authors: Allan Michael , Maier John P. , Marthaler Oskar , Stadelmann Jean-Pierre
Published: February 2003

Emission spectra of the radical cations of difluorodiacetylene, perfluoropentadiyne-1,3, and perfluorohexadiyne-2,4 have been observed in the gas phase following excitation with low energy electrons. The band systems, which are detected in the 450–630 nm wavelength region, are assigned to the ?2Πu→? 2Πg, ? 2E→? 2E, and ? 2Eu→?2Eg electronic transitions of these three cations, respectively, by reference to their photoelectron spectroscopic studies. The vibrational fine structure of the emission spectra is interpreted, and the frequencies of most of the totally symmetric modes in the ground state of these cations are inferred. The lifetimes of the lowest vibrational level of the ? excited electronic states have been measured to be 28±3 ns and 46±2 ns for the cations of difluorodiacetylene, perfluoropentadiyne-1,3, and perfluorohexadiyne-2,4 respectively.
Electron paramagnetic resonance as a probe of (SN)x polymerization
Authors: Love P. , Labes M. M.
Published: February 2003

Electron paramagnetic resonance (EPR) of partially polymerized (SN)x obtained via four different routes which involve different reactive precursors has been studied. Distinctive intermediate polymerization states are observed via EPR, although the final diamagnetic polymers from all states show only subtle differences in their structural parameters and electronic properties.
The electronic structure of UCl6: Photoelectron spectra and scattered wave Xα calculations
Authors: Thornton Geoffrey , Edelstein Norman , Rösch Notker , Egdell Russell G. , Woodwark David R.
Published: February 2003

Helium i and ii excited photoelectron (p.e.) spectra of UC16 are reported. Experimental ionization energies agree well with those predicted by a quasirelativistic scattered wave Xα calculation; further support for the proposed assignments is provided by a consideration of intensity patterns in the spectra. Attention is ultimately focused on the relationship between ionization energies and charge transfer electronic excitation energies in hexahalo complexes. Further experimental work is required to establish an assignment for the electronic absorption spectrum of UC16.
Surface tension of a simple fluid
Authors: Jhon Myung S. , Desai Rashmi C. , Dahler John S.
Published: February 2003

A formally exact relationship is derived between the Kirkwood–Buff formula for surface tension and the small-wave number limit of the difference between surface excesses of the autocorrelation integrals of force components directed normal and tangential to the planar interface. By expressing these correlation integrals in terms of fluctuating many-body coordinates, relationships are obtained between the Kirkwood–Buff formula and the density fluctuation formula due to Yvon, Triezenberg and Zwanzig, and Lovett, de Haven, Vieceli, and Buff.
Crystal nucleation and growth in liquid rubidium
Authors: Hsu C. S. , Rahman Aneesur
Published: February 2003

Molecular dynamics technique is used to study the homogeneous nucleation of an ordered stable phase from a liquidlike state of aggregation. The rubidium potential is used for a 500 particles system with reduced density 0.95. At the start of the calculation the system is quenched rapidly from a reduced temperature of 0.8 to 0.1. The moment of nucleation and the explosive growth of the nucleus to engulf the whole system have been monitored by studying the characteristics of Voronoi polyhedra in addition to quantities such as temperature, pressure, pair correlation functions, etc. After nucleation, the surface of the nucleus is found to advance at a rate of about 130 m/sec; the size of the critical nucleus is estimated to contain 40 to 70 particles. The ordered stable phase is found to have a bcc structure. In spite of the very rapid rate of cooling which was used to start the calculation, a metastable disordered structure was not obtained.
The reaction N(4S)+O3: An upper limit for the rate constant at 298 K
Authors: Stief L. J. , Payne W. A. , Lee J. H. , Michael J. V.
Published: February 2003

The reaction of N(4S) atoms with ozone was examined at 298 K using discharge flow and flash photolysis techniques, both coupled with resonance fluorescence detection of N(4S). No reaction was detected in either system, and an upper limit of 5×10−16 cm3 molecule−1 s−1 was estimated for the rate constant. This is 103 times lower than the presently accepted value of 5.7×10−13 cm3 molecule−1 s−1 which is based on a discharge flow-mass spectrometric study of the reaction. Potential complications in the latter experiment are discussed.
Vibrational excitation and surface reactivity: An examination of the ν3 and 2ν3 modes of CH4
Authors: Brass Stephen G. , Reed David A. , Ehrlich Gert
Published: February 2003

The effect of vibrational excitation upon the chemisorption of methane on thin films of rhodium has been investigated. Vibrational modes of the gas are selectively excited by infrared radiation. The rate of reaction is measured by following the pressure decrease in a closed system containing CH4 in contact with the metal surface. Neither irradiation with a He–Ne laser at 3.39 μ to excite the ν3 mode, nor using an arc source with wavelengths in the range overlapping 2ν3, produces any detectable changes in the rate. From these experiments we estimate that the reaction probability of CH4 is <1×10−4 in the first excited state of ν3, and <7×10−2 in 2ν3. Other excitations must be involved in overcoming the barrier of ?7 kcal/mole which has previously been found for the chemisorption of CH4 on rhodium.
Crossed molecular beams kinetics: BaO recoil velocity spectra from spin-forbidden Ba+SO2
Authors: Freedman Andrew , Parr Timothy P. , Behrens Richard , Herm Ronald R.
Published: February 2003

Laboratory (lab) angular and time-of-flight (TOF) recoil velocity spectra of BaO product produced by crossing a thermal Ba beam with an SO2 nozzle beam have been measured in a molecular beam apparatus equipped with an electron bombardment ionizer–quadrupole mass filter detector. Product center-of-mass (c.m.) recoil angle and energy distributions have been fit to the data by convoluting the c.m.lab transformation over measured speed distributions of both beams and the bandpass of the TOF analyzer for four experiments which employed different sharp distributions in incident collision energy. By energy conservation, the BaO which is observed must be produced by the spin-forbidden Ba (1S)+SO2(1A1) →BaO(X 1Σ+)+SO(X 3Σ−) reaction, at least in the three lower collision energy experiments. Here, reaction proceeds via formation of a long-lived BaSO2 complex to give a symmetric product angular distribution which is very sharply peaked in the forward and backward directions. Results of this and of previous studies of this reaction would probably be consistent with a phase space theory analysis of the reaction energy partitioning because of the large uncertainty in the reaction exoergicity. In the highest collision energy experiment, BaO is scattered preferentially backward via a direct reaction mechanism.
Rotational diffusion in solvents with large characteristic lengths
Authors: Keyes T. , Ladanyi Branka M.
Published: February 2003

A theory, based upon mode coupling, is given for the rotational diffusion constant of a tagged particle in a solvent characterized by a large macroscopic length. The theory is used to explain rotational diffusion near a critical point or in the presence of polymer molecules.
Semiempirical potential surfaces for the alkali hydrogen-halide reactions
Authors: Shapiro M. , Zeiri Y.
Published: February 2003

Potential energy surfaces for 14 of the M+HX→MX+H reactions, (M=Li,Na,K,Rb; X=F,Cl,Br,I), were generated by a new VB semiempirical method. The dependence of barrier location and height on the bond angle were examined. The minimal barriers (=transition states) were found to occur in the bent configurations for the X=F reactions and in the collinear, or near-collinear, geometries for the X=Cl, Br, I reactions. This effect was accompanied by a shift of the barrier from the exit to the entrance channel and was shown to arise from a change in the effective HX distance at which an electron jump may be said to occur. A general tendency of increase in barrier height with increase in alkali atomic number was found. Relevant experimental observations including the energy disposal in the Ba+HX reactions, the role of vibrational excitations in the K+HCl, K+HBr reactions, the shape of the differential cross sections in the H+MX reactions and possible implications to the K+HBr, K+DBr, and K+TBr reactions are discussed.
Approximation method for molecular collisions in the turning point region
Authors: Shipsey Edward J.
Published: February 2003

The second order system of coupled differential equations in the close coupling approximation is approximately reduced to a first order unitary system. The procedure is illustrated numerically for a simple system in the vicinity of its turning points. The present work supplements some previous work. Taken together these works provide a fairly simple approximate means of reducing the second order close coupling equation to a unitary first order system over almost all of the range of the collision. The remaining portion of the problem, deep in the forbidden region, can be handled perturbatively.
Kinetic theory of a photolytically driven hot-atom reaction
Authors: Jhon Myung S. , Dahler John S.
Published: February 2003

Kinetic theory is applied to reactions of atoms excited translationally by photolysis. Chosen as examples are the hot deuterium atoms produced by photolysis of DI. A kinetic equation is derived for the momentum distribution of these hot atoms. This equation incorporates the moderating effects of nonreactive collisions as well as the perturbations caused by the two extraction reactions D+H2→HD+H and D+DI→D2+I. Relationships are established between the solution of this kinetic equation and the rates of formation of the two product species HD and D2. Finally, several properties of this equation and of its solutions are examined.
Polarized, rotationally state-selected cross sections and anisotropic intermolecular potentials for LiF on Ar
Authors: Tsou Len-Yuan , Auerbach D. J. , Wharton L.
Published: February 2003

High resolution differential cross sections of LiF in the J=1 rotational state in both a polarized condition and an unpolarized condition were experimentally studied with an Ar target for a range of LiF energies from 0.3 to 1.9 eV. The technique for performing such experiments is described in detail, including a description of the high resolution state selector and the seeded supersonic source. High frequency and rainbow features are observed in the differential cross sections. Manifestations of these oscillations are also observed in the significant difference in cross section due to polarization. The infinite order sudden approximation was used in a preliminary way to interpret the data. A model potential was used that had an inverse radial power series and appropriate angular dependent Legendere polynomials. The differential cross section is sensitive to the anisotropy parameters chosen. The cross section is generally larger for the (J, M) = (1, 1) polarization along the relative velocity for the energies and the small angles observed here.
Quasiclassical cross sections for the H+H2 (0, 0) →H+H2 reaction: Comparison of the Siegbahn–Liu–Truhlar–Horowitz and the Porter–Karplus potential surfaces
Authors: Mayne Howard R. , Toennies J. Peter
Published: February 2003

Integral and differential cross sections are calculated using the Porter–Karplus semiempirical potential and the Siegbahn–Liu–Truhlar–Horowitz analytic potential. (AIP)
Apparent faster electron solvation by laser flash photoionization than by pulse radiolysis in methanol–methyltetrahydrofuran glasses
Authors: Dolivo G. , Kevan Larry
Published: February 2003

Previous results showed that pulse radiolysis of methanol–methyltetrahydrofuran (MTHF) glasses exhibits a spectral blue shift from 0.1–14 μs indicating predominant initial solvation by more abundant MTHF molecules converting to final solvation by the more polar methanol molecules. In contrast, laser flash photoionization of β-naphtholate in similar methanol–MTHF glasses at 110 K suggests that the electron is initially solvated in a methanol environment. It is suggested that the β-naphtholate is preferentially solvated by methanol clusters in the methanol–MTHF mixtures and that the electrons are directly generated and solvated in a methanol cluster.
Transition moment for ν3 of 74GeH4
Authors: Fox K. , Halsey G. W. , Daunt S. J. , Kennedy R. C.
Published: February 2003

A vacuum grating spectrometer was used to measure the absolute line strength of the vibration-rotation fundamental of GeHy. (AIP)
Laser induced fluorescence spectrum of gaseous C6H+6
Authors: Miller Terry A. , Bondybey V. E. , Zegarski B. R.
Published: February 2003

A laser induced fluorescence spectrum of the radical cation, C6H+6, of dimethyl-diacetylene has been observed in the gas phase. Vibrational structure is clearly resolved, analyzed, and compared with earlier data on C6H+6 from matrix isolation studies. Observations are reported that are relevant to the photodissociation of C6H+6 as well as possible Jahn–Teller distortion of the ? 2E1g and ? 2E1u states.
The uniform WKB theory of multichannel predissociation and inelastic scattering
Authors: Eu Byung Chan , Zaritsky Nava
Published: February 2003

In this paper the uniform WKB theory is applied to predissociations of N curve crossing systems as an extension of the study we made in a previous paper. We have obtained transition amplitudes (S-matrix elements) explicitly for three-state and N-state problems in terms of the transition factors and phase integrals used previously in our study of two-state problems. The expressions obtained in the local pairwise coupling approximation are extremely simple, physically transparent, and easy to compute. Within the framework of this model we have obtained the resonance characteristics for predissociative N-curve crossing problems. The level width formula is given in terms of the transition factors χi and is new. The transition amplitudes for the corresponding inelastic scattering problems were also derived.
The Poisson–Boltzmann equation and its application to polyelectrolytes
Authors: Fixman Marshall
Published: February 2003

The validity of the Poisson–Boltzmann (PB) equation is reconsidered on the basis of functional expansion techniques supplemented by the mean spherical approximation. In the application of greatest interest a strong Coulomb potential originating in an external source, such as a polyelectrolyte molecule, acts on a salt solution of small mobile ions. Where the local charge density of mobile ions is high, substantial errors may occur in the PB approximation that relates charge density to mean potential. However, the solution to the PB equation is nevertheless a good approximation in the indicated application because a quite small percentage change in the electrostatic potential can compensate large errors in the Boltzmann distribution. An application to DNA illustrates this compensation, and also its impending failure at bulk salt concentrations in excess of 0.1M. A two phase (or condensation) model is derived as an approximation to the PB equation and retains fair accuracy even at substantial salt concentrations, where the limiting laws lose theoretical validity.
General matrix ENDOR line shape model applied to the methyl radical in lithium acetate dihydrate using single crystal data
Authors: Kevan Larry , Narayana P. A. , Toriyama K. , Iwasaki M.
Published: February 2003

A new general matrix ENDOR line shape model is tested against ENDOR data on the methyl radical in γ-irradiated lithium acetate dihydrate. All parameters in the line shape model are determined or narrowly limited by experiment. These include the dipolar tensors of all protons within 5 Å of the methyl radical, the microwave and radiofrequency magnetic fields, and the electronic and nuclear spin–lattice and spin–spin relaxation times. The theoretical simulations agree satisfactorily with the experimental lineshape, microwave magnetic field dependence, radiofrequency magnetic field dependence and angular variation of single crystal ENDOR line intensities. The effective nuclear spin–lattice relaxation time is ∼10 ms at 77 K and seems to be dominated by an angularly independent nuclear relaxation mechanism. The angularly dependent electron–nuclear dipolar interaction is found to be of much less importance for the ENDOR response. It is noted that the angular variation of ENDOR intensities is an important parameter to assess the nuclear relaxation mechanisms that control the ENDOR response. Data on cw and pulsed matrix ENDOR are compared and found to be similar. It is also shown that a model based on idealized line shape theory for disordered systems can satisfactorily reproduce matrix ENDOR line shapes. Both models allow calculation of the relative contributions from angularly independent and dependent ENDOR responses.
IR laser sideband observations in GeH4 and CD4
Authors: Kreiner W. A. , Magerl Gottfried , Furch Bernhard , Bonek Ernst
Published: February 2003

We have used the technique of IR laser sideband spectroscopy to observe single ro–vibrational transitions in the ν2 fundamental of GeH4. The isotope structure has been resolved within the Doppler width of the lines. The isotopic frequency shift for the J″=7 transition in the R branch of ν2 was found to be 120 MHz/amu. This value is much smaller than the isotope shift in the ν4 fundamental (0.22 cm−1 =6.5 GHz/amu) which has been observed with a conventional high resolution grating spectrometer. For GeH4 a line width (FWHM) of 44.4 MHz was found which compares well with the Doppler width (41 MHz) at 300 K. The application of this spectroscopic method was extended to some transitions in the ν4 fundamental of CD4. In total 32 transitions have been observed in GeH4 and 17 in CD4. The absolute accuracy in frequency measurement is ? ±60 MHz.