bluuebunny/crossref_metadata_2025 huggingface dataset:
A localization threshold for macromolecular quasispecies from continuously distributed replication rates
Authors: McCaskill J. S.
Published: February 2003

Localization in the space of mutant sequences of an equilibrium distribution of independently self-replicating macromolecules or quasispecies, as described by the deterministic evolution equations of Eigen (1971) is necessary for a dynamically stable, finite population and simple Darwinian selection of a particular genotype. In this work, the localization properties are investigated as a functional of the probability distribution (possibly continuous) of replication rates for the different mutants, including the neutral mutation extreme. The central result is the existence and evaluation of a threshold sequence length (for fixed monomer copying fidelity) below which the quasispecies is localized with unit probability around the particular mutant with maximum replication rate. This localization threshold differs in two respects from the error threshold of Eigen, which it confirms in the limit of identical superiority of the wild type over all other mutant replication rates: It is independent of mutant population variables and it predicts a localization threshold even in the presence of mutants arbitrarily close in exact replication rate to the maximum. General conclusions about the threshold are made on the basis of extreme value theory. Similarities of the problem with that of Anderson localization in disordered metals and quantum spin systems are exploited in the probability analysis and renormalization of the perturbation theory.
A simple pseudomolecular fluid model. Exact and approximate structural properties
Authors: Smith William R. , Nezbeda Ivo , Labik Stanislav
Published: February 2003

A simple model mimicking a molecular fluid is introduced, whose properties can be calculated easily and essentially exactly. A range of properties, including the spherical harmonic expansion coefficients, both of the pair correlation function and of the background correlation function, are calculated. The model is a useful tool to assess the accuracy of molecular fluid theories, and we demonstrate this use by considering two approaches currently available. It is also demonstrated that the model can be used to estimate the properties of realistic molecular fluid models.
Ellipsoidal model of polyelectrolyte solutions
Authors: Bratko D. , Dolar D.
Published: February 2003

The cell model of polyelectrolyte solutions is described, whereby the linear polyions are represented as prolonged ellipsoids of revolution, placed in the cells of ellipsoidal symmetry. The simple ions are distributed throughout the cell according to the nonlinear, two dimensional Poisson–Boltzmann equation. Applying usual relations, several thermodynamic properties are calculated from the numerical solution of the Poisson–Boltzmann equation, obtained by the finite difference method. The influence of the degree of polymerization and of the concentration on thermodynamic behavior of polyelectrolyte solutions is studied. A brief comparison with a few disposable experimental results is given.
Perturbation theory for a polymer chain with excluded volume interaction
Authors: Muthukumar M. , Nickel Bernie G.
Published: February 2003

We present a simple derivation of the mean square end-to-end distance 〈R2〉 of a linear flexible chain as a perturbation series in the dimensionless excluded volume parameter zd. Our results, to orders six and four in space dimension d=3 and 2, respectively, are 〈R2〉= Ll[1+ (4)/(3)  z3−2.075 385 396 z23+6.296 879 676 z33 GFIX−25.057 250 72 z43+116.134 785 z53 GFIX−594.716 63 z63+⋅⋅⋅],  d=3, 〈R2〉= Ll[1+ 1/2 z2−0.121 545 25 z22+0.026 631 36 z32 GFIX−0.132 236 03 z42+⋅⋅⋅],  d=2, where z3=(3/2πl)3/2 wL1/2 and z2=wL/πl with L the contour length of the chain, l the effective or Kuhn segment length, and wl2 the effective binary cluster integral for a pair of segments. Our method uses in an essential way Laplace transforms with respect to the contour length L; the resulting graphical expansion, when combined with the field theoretical methods, is far simpler than that in the conventional cluster expansion approach. Furthermore, we prove that 〈R2〉 is free of ln L terms to all orders in the perturbation theory in both d=2 and 3.
Total and orbital SCF energies in molecules
Authors: Schaad Lawrence J. , Hess B. Andes , Schwartz Maurice E.
Published: February 2003

Results of extensive calculations are given for the SCF electronic energy in molecules. The total SCF energies were calculated with the 3−21G basis set. (AIP)
Determining the anisotropic interaction potential of D2Ar from rotationally inelastic cross sections
Authors: Buck U. , Meyer H. , LeRoy R. J.
Published: February 2003

In a crossed molecular beam experiment, time-of-flight distributions of oD2 molecules scattered from Ar have been measured in a center-of-mass angular range from 50° to 100° at a collision energy of E=85 meV. The data show clearly resolved 0→2 rotational transitions of D2 in the backward direction. From a combined analysis of these inelastic cross sections and the measured total differential cross sections for D2+Ar at E=83 meV, the complete repulsive potential surface is derived. Comparison with the potential which LeRoy and Carley determined from spectroscopy shows that the repulsive part of its anisotropic V2 term is steeper, whereas the isotropic parts V0 agree within the experimental error. A combined analysis of the present inelastic scattering data and the original spectroscopic data yields an improved version of the three-dimensional stretching-dependent potential surface of LeRoy and Carley. Cross sections calculated from the semiempirical ‘‘HFD’’ potentials reported by Tang and Toennies and by Rodwell and Scoles are also fairly close to experiment, with the latter potential performing somewhat better than the former. The comparison with other hydrogen molecule–rare gas interactions reveals a maximum in the effective strength of the repulsive anisotropy for Ne–H2 and a nearly complete conformality of the reduced V0 and V2 terms to each other for He, Ne, and Ar–H2.
Shell binding energies of neutral atoms: Comparison between theory and experiment
Authors: Dutt Ranabir , Varshni Y. P.
Published: February 2003

We propose an analytic approximation scheme to obtain a simple formula for the shell binding energies Ens of neutral atoms which gives accurate results over the entire range of the atomic number Z up to 84. Inclusion of Fermi–Amaldi correction leads to significant improvement of our numerical results for the lighter atoms as compared to other theoretical predictions.
Exact-exchange extension of the local-spin-density approximation in atoms. II. The iron series
Authors: Baroni Stefano
Published: February 2003

s-d interconfigurational energies, s-spin flip energies, and ionization potentials for atoms in the first transition series are calculated within a local-spin-density scheme, where the exchange is treated exactly. The results so obtained are in better agreement with experiments than those obtained by the Hartree–Fock (HF) or local-spin-density approximations (LSDA), while they are of the same quality as those obtained by the self-interaction-corrected (SIC) version of the LSDA. The merits of the proposed scheme with respect to the other mentioned approximations are discussed in detail.
Fluctuation growth during a slow transit across a chemical instability
Authors: Lin J. , Siri R. , Mizraji E. , Kahn P. B.
Published: February 2003

We study the behavior of homogeneous fluctuations near an instability point of the Brusselator as the concentration of substrates changes slowly in time. We compute the transient and asymptotic behavior of the covariance matrix in the extensive regime. If the system is quenched to a distance ∼(system size)−1/2 near the instability point an enhancement of the fluctuation to macroscopic size is observed. Using Suzuki’s approach, the onset time for macroscopic behavior and the probability density for the amplitude are estimated. Further extensions are discussed.
Potential energy barriers in the kinetic theory of nucleation
Authors: Ziabicki Andrzej , Jarecki Leszek
Published: February 2003

The problem of partition of the transition free energy δf among the potential barriers for cluster growth (U+i−1) and dissociation (U−i) is discussed. The proposed simple solutions U+i−1=ε+zδfi, U−i=ε−(1−z)δfi, and z= 1/2 [1+sign(δfi)] satisfy all requirements imposed on the problem. Consequently, in the nucleation theory two different critical cluster sizes appear: i*0, which corresponds to the change of sign of δfi and transition in the potentials U+i−1, U−i, and i* (generally different from i*0 ), which accounts for nucleation rate.
Spectroscopic studies of the jet-cooled nickel dimer
Authors: Morse M. D. , Hansen G. P. , Langridge-Smith P. R. R. , Zheng Lan-Sun , Geusic M. E. , Michalopoulos D. L. , Smalley R. E.
Published: February 2003

We report the first gas-phase electronic spectrum of nickel dimer. Ni2 is produced by laser vaporization of metallic nickel in the throat of a supersonic nozzle. Using resonant two photon ionization, bands previously observed in inert matrices and attributed to Ni2 are conspicuous in their absence. Further to the red, an abrupt onset of complicated spectral structure indicates rapid predissociation above 16 680 cm−1. We argue that this represents the true dissociation limit, and places D0=2.068±0.01 eV. A congested pattern of spectral features from 6000 to 9000 Å confirms theoretical predictions of a large number of low-lying electronic states in nickel dimer. Rotationally resolved bands near 8500 Å are indicative of a ΔΩ=+1 transition, with Ω″=4, Ω′=5. Rotational analysis of these bands yields a bond length of 2.200±0.007 Å for the ground state of Ni2, which must be of either 1Γg or 3Γu electronic symmetry species. Both the long bond length of 2.20 Å and the high value of Ω″ are in agreement with theoretical predictions, and confirm that no substantial 3d participation contributes to the chemical bonding of Ni2.
Electron-coupled nuclear spin–spin interactions in cubic PbF2 and the 19F chemical shift
Authors: Sears R. E. J. , Guo Q. Zh. , Mackey H. J.
Published: February 2003

The 19F NMR multipulse spectrum of a single crystal of cubic PbF2 was found to vary with the orientation of the crystal in the static magnetic field. This enabled the isotropic J and anisotropic (or pseudodipolar) J′ electron-coupled spin–spin interactions to be determined. J is ±2.15±0.05 kHz with the negative sign being most likely according to the theory of Jameson and Gutowsky. J′ is found to be negative with a magnitude 0.07±0.05 kHz or 2.71±0.05 kHz, the latter value being consistent with a negative sign for J and with the expectations of simple localized-bond models and the extent of the non-s bonding in cubic PbF2. For J=−2.15 and J′=−2.71 kHz, the spin–spin coupling anisotropy is +5.42±0.2 kHz. The 19F chemical shift of cubic PbF2 is −127.7±0.5 ppm at room temperature, with respect to C6F6. This shift fits the correlation of isotropic chemical shifts with cation radii previously established for the group IIa difluorides.
Trajectory study of the reaction O(1D2)+HCl→OH+Cl on a fitted ab initio surface
Authors: Schinke R.
Published: February 2003

We performed a classical trajector study of the reaction O(1D)+HCl→Cl+OH using an analytical potential energy surface fitted to extensive ab initio calculations. The reactions proceed via long-living HOCl complexes before breaking up into products. Because no barrier is present for insertion pathways the total cross section is a decreasing function of the energy. The vibrational OH distribution is inverted and peaks at ν′=2–3. The ratio P(ν′=1)/P (ν′=0) is 1.42 and agrees very well with the experimental value of 1.5. The rotational OH distribution within the vibrational ground state is highly inverted, peaks at about j′=28, and is in good agreement with recent experimental data. The angular OH distribution is slightly forward peaked. The results for O(1D)+HCl→Cl+OH are compared with those for the similar reaction O(1D)+H2→H+OH.
Intramolecular vibrational relaxation in 1,4 dioxane
Authors: Kulp Thomas , Ruoff Rodney , Stewart George , McDonald J. D.
Published: February 2003

Intramolecular vibrational energy transfer in isolated gas phase molecules of dioxane has been studied by spectrally resolving the IR laser induced fluorescence from C–H stretch fundamentals, combination bands, and overtones in the region from 2800 to 3000 cm−1. By comparing the experimentally observed relative energy content of the IR active modes with that calculated from a statistical model it is concluded that substantial randomization of the energy initially deposited in the zeroth order optically excited state occurs. This statistical distribution differs from a thermal distribution due to fluctuations in the local state density. The energy content distribution is best described by the model if coupling over a range of 4 cm−1 is assumed. Dilution factors for the various C–H stretch modes were observed which vary from 0.018 to 0.13. These values are dependent on the rotational state excited. Collisional time dependent spectra of several relaxed modes are presented. The experiments are performed by exciting molecules cooled in a nozzle expansion with light from an infrared optical parametric oscillator. The fluorescence spectra are recorded with a cryogenic Michelson interferometer.
Dispersion spectroscopy of optical electron transfer in solution
Authors: Delahay P. , Dziedzic A.
Published: February 2003

The effect of dielectric dispersion of the solvent on optical electron transfer is interpreted as a shift ΔGd of the free energy level of the ground state in the photoionization process. The shift ΔGd is derived for transparent and absorbing solvents by application of the Marcus theory of nonequilibrium polarization of a continuous medium. Only the inner-sphere solvation shell contributes significantly to the dispersion correction ΔGd. Application is made to photoelectric emission by solutions. The effect of dispersion on the energetics of emission causes deviation from the expected emission law. The resulting dispersion spectra are obtained for photoelectron emission by 17 inorganic anions in aqueous solution in the 7 to 10 eV range of photon energies. The spectra primarily result from dispersion of the solvent and are not strongly affected by the nature of the photoionized anion. Experimental dispersion spectra agree well with the theoretical spectrum predicted from the real and imaginary dielectric constants obtained from reflectance spectroscopy data of liquid water. Dispersion spectroscopy of optical electron transfer developed in this work provides an experimental probe of the inner-sphere solvation shell.
Pressure-induced splitting and collapsing of the CN stretching vibration band in the Raman spectrum of crystalline Hg(CN)2
Authors: Wong P. T. T.
Published: February 2003

Raman spectra of crystalline Hg(CN)2 have been measured as a function of pressure up to 45.1 kbar. The CN stretching band appears as a singlet in the pressure range 0–16 kbar and its frequency exhibits a nonlinear pressure dependence. It becomes a doublet and shifts to lower frequency at 20 kbar where a structural phase transition takes place. The intensity of the high-frequency component of the doublet decreases with increasing pressure and totally disappears at 40 kbar where a second structural phase transition takes place. The phase transition at 20 kbar is driven by the intramolecular distortion and the intermolecular coordination processes and the transition at 40 kbar is triggered by an increase of symmetry of the molecular configuration.
Stimulated emission spectroscopy: A complete set of vibrational constants for X̃ 1A1 formaldehyde
Authors: Reisner David E. , Field Robert W. , Kinsey James L. , Dai Hai-Lung
Published: February 2003

A complete set of 27 normal mode vibrational constants ω0i and xi j as well as six harmonized vibrational frequencies ωi is obtained for the H2CO X̃1A1 state: ω01 =2811.42(15), ω02 =1755.858(40), ω03 =1500.32(49), ω04 =1170.224(30), ω05 =2861.30(14), ω06 =1250.565(63) x11=−28.95(14), x12=1.15(19), x13=−23.03(14), x14=−10.099(65), x15=−193.32(24), x16=−49.78(33) x22=−9.926(23), x23=−8.26(11), x24=−7.199(39), x25=−17.23(23), x26=6.581(49), x33=−0.164(97), x34=−1.769(52), x35=6.00(37), x36=−29.861(88), x44=−3.157(12), x45=−13.35(17), x46=−2.860(70) x55=−17.97(13), x56=−17.63(33), x66=−1.567(56), ω1=2977.91(31), ω2=1778.26(16), ω3=1528.95(54), ω4=1191.02(11), ω5=2997.04(36), ω6=1298.91(26) (1σ uncertainty in parentheses). These vibrational constants are derived primarily from stimulated emission pumping (SEP) spectra of more than 50 4500–9300 cm−1 vibrational levels of the X̃1A1 state, supplemented by partial rotational analyses of 12 4000–8100 cm−1 FTIR overtone and combination bands. This is the first time that the SEP technique has been systematically applied to a traditional but seldom achievable objective of high resolution vibrational spectroscopy, determination of a complete set of ω0i and xi j constants. Insofar as the rotationless vibrational levels of H2CO X̃1A1 can each be unambiguously assigned a set of normal mode quantum numbers and reproduced by a minimal set of vibrational constants, the X̃ state of formaldehyde remains vibrationally well organized up to at least 9300 cm−1.
Scaling of state multipoles in rotationally inelastic transfer
Authors: Proctor M. J. , McCaffery A. J.
Published: February 2003

Laser excitation of molecules in thermal cells gives rise to an anisotropic distribution of m states and thus fluorescence from these states, and from levels populated by inelastic collisions, is polarized. Neglect of fluorescence polarization can give rise to misleading conclusions concerning population transfer in the excited state. Precise measurements of the transfer of population, orientation, and alignment, through polarization ratios, can provide an excellent means for testing the various fitting laws for rotationally inelastic transfer through modified ‘‘sudden’’ scaling laws. The justification for this is fourfold: rotational line strength factors and laser power fluctuations do not affect the degree of polarization; thirdly, the effect of multiple collisions is less significant on the inelastic polarization transfer than it is on the population transfer as measured in conventional total fluorescence intensity measurements; and fourthly, the leading parameters present in fitting laws cancel out, reducing the number of fitting parameters by one. Scaling laws appropriate to polarization measurements are derived and various fitting relations are tested against Li*2/He and Li*2/Ar rotationally inelastic polarization data.
New aspects of the ‘‘channel three’’ problem in benzene, as revealed by multiphoton ionization photoelectron spectroscopy
Authors: Achiba Yohji , Hiraya Atsunari , Kimura Katsumi
Published: February 2003

Combining a photoelectron spectroscopic technique with a (1+1) resonant ionization method, we have investigated intramolecular decay processes of benzene in its S1 state under collision-free conditions. Photoelectron spectra were obtained by selective excitation of benzene with a pulsed UV laser at several single vibronic levels of the S1 state up to an internal energy (ΔE) of 5000 cm−1. These spectra strongly suggest that the excitation of benzene at the vibronic bands above the onset of the ‘‘channel three’’ is followed by intramolecular vibrational redistribution within the S1 state. It is concluded that there are no decay channels faster than this redistribution process up to ΔE=5000 cm−1 at the first decay stage. The results of the integrated multiphoton ionization intensity distribution over the vibronic bands, as well as the internal-energy dependent spectral changes observed in the photoelectron spectra, also strongly suggest that the channel three is initiated by the redistributed vibrational modes, which lead to a fast internal conversion leads to the ground electronic state.
An alternative to the stoichiometric factor in the Oregonator model
Authors: Noyes Richard M.
Published: February 2003

The original Oregonator model to the Belousov–Zhabotinsky oscillating reaction used three composition variables, five rate constants, and one stoichiometric factor involving five irreversible reaction steps. As a result of objections to the stoichiometric factor by Noszticzius, Farkas, and Schelly, we have developed an alternative model of the same complexity based on six irreversible steps. The revised model eliminates some stoichiometric difficulties, and by minor modification of one of the six steps it handles situations in which the final oxidation state of the reduced bromine is +1, 0, or −1. An alternative skeleton model called the Explodator and proposed by Noszticzius, Farkas, and Schelly should not be considered a viable alternative to the Oregonator unless a computational prerequisite and an experimental kinetic prerequisite can both be satisfied.
CO2 and CO laser microwave double resonance spectroscopy of OCS: Precise measurement of dipole moment and polarizability anisotropy
Authors: Tanaka Keiichi , Ito Hajime , Harada Kensuke , Tanaka Takehiko
Published: February 2003

Laser–microwave double resonance (LMDR) with high electric field was applied to the OCS molecule. Stark Lamb-dip spectra due to the infrared transitions of the 2ν2(0200–0000, 9.6 μm), 2ν1 (2000–0000, 5.8 μm), and ν1+2ν2 (1200–0000, 5.3 μm) bands were observed with the CO2 and CO lasers. The spectra due to the corresponding hot bands; 0310–0110, 0400–0200, 1200–1000, 1310–1110; 2110–0110, 3000–1000; 1400–0200, 1420–0220, 1510–0310, 2200–1000; and a few bands of OC34S and O13CS were also identified. Associated with these infrared transitions, more than 90 LMDR signals were detected and assigned to rotational transitions in the 11 vibrational states 0000, 1000, 2000, 0110, 0200, 0220, 0310, 0400, 1200, 1420, and 2200 of the normal species, and in the two vibrational states 0000 and 0200 of both OC34S and O13CS. Dipole moments were determined with accuracies (2.5σ) better than 2×10−5 D for all these vibrational states. Polarizability anisotropies were also obtained for some states. The data for the ground ν1 and ν2 vibrational states are, with the 2.5σ uncertainties in parentheses.From the Stark Lamb-dip spectra the origins of various vibrational bands were determined, among which those for the 0200–0000 and 0310–0110 bands are 31 389 530.4(25) and 31 566 477.57(67) MHz, respectively. The dipole moments and band origins obtained in the present study agree well with the available accurate values from molecular beam electric resonance and heterodyne measurement, respectively. A procedure for the calibration of electric field in laser Stark and double resonance spectroscopy, in which the dipole moment of OCS is used as the standard, is described.
Visible and near IR Si–H vibrational overtones in SiH4
Authors: Bernheim R. A. , Lampe F. W. , O’Keefe J. F. , Qualey J. R.
Published: February 2003

Absorption spectra in the 12 000 to 18 000 cm−1 range have been recorded for gaseous SiH4 using intracavity photoacoustic detection with cw dye lasers. The observed transitions correspond to the Δv=6–9 overtones of the Si–H local mode stretch and show considerable rotational structure.
ENDOR of benzil in bibenzyl: The dominance of crystal forces
Authors: Chan I. Y. , Sandroff C. J.
Published: February 2003

A proton ENDOR investigation has been conducted on the lowest triplet state of benzil in bibenzyl. The triplet molecule was shown to possess a precise center of inversion. The dicarbonyl fragment therefore has to be precisely trans-planar. The orientation of the two phenyl rings with respect to the dicarbonyl plane was semiquantitatively determined. These results, in comparison with a previous study on the geometry of triplet benzil in its own lattice, demonstrate the dominant role of lattice packing on determining the conformation and spectroscopy of flexible chromophores.
The b 1Σ+ → X 3Σ− transition in PH: A measurement of the term energy, bond length, and vibrational frequency of a phosphinidene metastable
Authors: Droege A. T. , Engelking P. C.
Published: February 2003

The (0,0), (1,1), and (2,2) vibrational bands of the weak, spin forbidden b 1Σ+ → X 3Σ− transition of the PH radical have been observed in a flowing afterglow of PH3 in He. The spectra yield the following constants for the upper b state: Te =(14 325.5±0.1) cm−1, Be =(8.587±0.003) cm−1, we =(2403.0±0.1) cm−1, Dv =(4.0±0.05×10−4) cm−1, αe =(0.0253±0.003) cm−1, and re =(1.4178±0.0004) Å. The intensity distribution is consistent with the mixing of the b 1Σ+ state almost exclusively with the A 3Π state.
A Raman study of matrix isolated methane
Authors: Regitz E. , Loewenschuss A. , Bier K. D. , Jodl H. J.
Published: February 2003

The Raman spectrum of methane in solid matrices is reported. The ν1 and ν3 modes were observed in argon, krypton, xenon, and nitrogen matrices, and the effect of temperature and concentration changes investigated. The ν2 transition was recorded in Ar, Kr, and Xe solids. For CD4 the ν1, ν2, 2ν2, and ν3 bands were measured in xenon matrices. The rotational band structure observed in the ν3 and ν2 modes is related to a theoretical model for matrix isolated methane, existing for ν3 and applied by us to ν2 also, and to the free molecular transitions. Our results are compared to previously published Raman spectra and differences and discrepancies are pointed out.
Failure of universality for translational diffusion of chain polymers
Authors: Fixman Marshall
Published: February 2003

Simulations of translational diffusion have been performed on models of chain polymers at the θ point. The combination of fluctuating hydrodynamic interaction and internal friction within small groups of  beads produces a decrease in the diffusion constant. The magnitude of the decrease approaches that obtained for completely rigid chains at moderate values of the internal friction constant, i.e., values comparable in magnitude to the bead friction constant. The dependence of the diffusion constant on local dynamical stiffness shows no sign of disappearing with increasing chain length at the largest chain length studied, 896 beads.
Rate coefficients at 300 K for the vibrational energy transfer reactions from N2(v=1) to O+2(v=0) and NO+(v=0)
Authors: Ferguson E. E. , Adams N. G. , Smith D. , Alge E.
Published: February 2003

Rate coefficients for the reactions N2(v=1)+O+2 (v=0)→N2(v=0)+O+2 (v=1) and N2(v=1)+NO+(v=0)→N2(v=0)+NO+(v=1) have been measured in a SIFT/flowing afterglow system at 300 K, the first measurements of thermal energy vibrational transfer from neutrals to ions. The values are k=1×10−12 and <10−13 cm3 s−1, respectively. These values are consistent with an intermediate complex vibrational predissociation model which has been successful in rationalizing earlier ion vibrational relaxations by neutral collisions. The NO+ result establishes that the efficient quenching of NO+(v=1) by N2 is not a resonant V→V process as was previously assumed.
Ba+(6p 2P) gas phase collisional quenching and intramultiplet mixing
Authors: Hovis Floyd E. , Gelbwachs Jerry A.
Published: February 2003

Collisional quenching and intramultiplet mixing of Ba+ (6p 2P1/2,3/2) has been studied by laser-pumping 6p 2P3/2 and measurement of the 6p 2P3/2 to 6p 2P1/2 population ratio under steady-state conditions. Gas phase Ba+ was generated by the methods of two photon ionization of Ba and by multiphoton dissociation and ionization of BaI2. The cross sections for quenching of 6p 2P1/2 by N2, CH4, C2H6, and C2H4 are 17 Å2, ≤3, ≤6, and 120 Å2, respectively. The cross sections for 6p 2P3/2→6p 6p 2P1/2 mixing by N2, CH4, C2H6, and C2H4 are 24, 70, 100, and 80 Å2, respectively. These cross sections are comparable to those of the alkali neutrals.
Unimolecular dissociation of methane: A trajectory study using Metropolis sampling
Authors: Raff L. M. , Viswanathan R. , Thompson Donald L.
Published: February 2003

Classical trajectories have been used to compute the total and individual microcanonical rate coefficients as a function of energy for CH4→CH2+H2 and CH4→CH3+H. Reaction mechanisms and product translational energy distributions for both reaction channels have also been obtained as a function of the initial CH4 internal energy. A semiempirical valence-force potential-energy surface was used throughout. Averaging over the phase space available to CH4 was accomplished by a Metropolis-type Monte Carlo procedure. Excellent first-order decay plots are obtained by the integration of 103 or less trajectories. The branching ratio for the two open dissociation channels obtained from the decay plots is near unity over the energy range studied (6.0–7.25 eV). The total microcanonical rate coefficient for CH4 reaction exhibits an RRK-type behavior as a function of energy. The computed distributions of CH2+H2 relative translational energy peak at energies well above zero as expected for a decomposition channel which has a back-reaction barrier of about 0.345 eV. However, the corresponding distributions for the CH3+H product are also found to peak above zero even though there is no back-reaction barrier. The results suggest that this effect is due to the fact that the total CH4 energy is well above the threshold for reaction. The mechanism for the three-center elimination is found to be a ‘‘half-reaction’’ of CH3+H.
A theoretical study of the isotope separation of Li2 by sequential two-photon ionization
Authors: Cremaschi Pietro
Published: February 2003

The intensities of the sequential two-photon ionization of Li2 are evaluated starting from ab initio CI electronic wave functions and using orthogonalized Coulomb functions for the continuum. The rotovibrational energies and the Franck–Condon factors are computed using a spline procedure both for the fitting of the electronic potentials and for the definition of the vibrational basis set. In agreement with experimental results, a substantial isotope separation is found for the three molecular isotopes of Li2 and a wide spectrum of wavelengths useful for effective isotope enrichment is suggested. The modulation of the line intensities plays an important role in the fractionation process, while the energy matching criterion alone is not enough to establish which resonances are responsible for isotope enrichment. The importance of the Franck–Condon factor contributions to the spectrum modulation makes it possible to extend these results to other molecules.
Quasiperiodic quantum states
Authors: Ramaswamy Ramakrishna
Published: February 2003

We show that the Hose–Taylor criterion of quantum quasiperiodicity can be recovered from low-order nondegenerate perturbation theory. It is seen that this mnemonic, which purports to identify energy levels which can be obtained by quantizing classical quasiperiodic motions, can lead to contradictions when applied to systems which are more semiclassical than that treated previously. These discrepancies arise since the criterion is both perturbation scheme and basis set dependent: the correlation between the semiclassical quantization and such a definition of quantum quasiperiodic behavior is not straightforward. As the underlying search is for a quantum KAM-like theory (in particular for near-separable systems which are typical of molecular vibrational Hamiltonians) some possibilities are discussed.
A self-consistent microscopic theory of hydrogen bond melting with application to poly(d
G)⋅poly(d
C)
Authors: Gao Y. , Devi-Prasad K. V. , Prohofsky E. W.
Published: February 2003

In this paper we develop a modified self-consistent phonon approximation model which displays effects which indicate melting. The model is applied to hydrogen bond melting of poly (dG)⋅poly(dC) and predicts melting at 340 K. The calculation has no parameters adjusted to fit the melting, the hydrogen bond potentials are adjusted to fit data at room temperature.
Resonance fluorescence of a two-level atom near a metal surface
Authors: Huang Xi-Yi , Lin Jui-teng , George Thomas F.
Published: February 2003

A derivation of surface-dressed optical Bloch equations, involving a treatment of surface-reflected photons and a surface plasmon resonance, is presented for a collision-damped two-level atom near or adsorbed on a metal surface. Effects of the laser bandwidth are included by means of a phase-diffusion model for the driving field. In the weak-field or large detuning limit, the population inversion and resonance fluorescence spectrum are analytically obtained. These quantities along with the surface-induced phase-decay constant of the adatom show strong oscillatory behavior as a function of the adatom–surface distance.
Experimental study of the density profile in the liquid–vapor interface of mercury
Authors: Bosio Louis , Oumezine Mohamed
Published: February 2003

The x-ray reflectivity (for Mokα radiation) of liquid mercury as a function of angle of incidence has been measured up to 10 θc (the critical angle θc being equal to 3.76 mrad). Since the x-ray reflectance is a probe of the properties of the surface, the data can be used to discriminate among models of density profiles in the liquid–vapor interface.
ND4 Schüler band absorption observed by laser FM spectroscopy in a photochemical reaction
Authors: Whittaker E. A. , Sullivan B. J. , Bjorklund G. C. , Wendt H. R. , Hunziker H. E.
Published: February 2003

Twenty-one lines of the Schüler band of ND4, previously observed in emission from high pressure ND3 discharges and assigned to a transition between excited states, have been observed in absorption in the Hg-photosensitized reaction of ND3. In this system ND4 is probably formed by a reaction of the HgND*3 exciplex. The observed transition originates from either the 3s, 2A1 ground state or the 3p, 2F2 excited state, the former being more likely.
Decomposition rate of benzyl chloride excited at 266 nm in the vapor phase
Authors: Ichimura T. , Mori Y. , Sumitani M. , Yoshihara K.
Published: February 2003

Picosecond laser photolysis of benzyl chloride vapor has been examined by monitoring the absorption time profile of benzyl radicals formed in the 266 nm photodecomposition. The rise time of benzyl radical absorption 6 ns was attributed to the decomposition rate of benzyl chloride molecules under collision free conditions.
ESR study of Sr(BrO3)2⋅H2O single crystals γ irradiated at 300 and 77 K
Authors: Sastry D. L. , Rama Rao K. V. S.
Published: February 2003

The paramagnetic defect centers observed in Sr(BrO3)2⋅H2O single crystals γ irradiated at 300 K are identified as pseudo Vk centers of the type (A) (bBr–aBrO2)− and (B) (bBrO3–aBrO2)−. Both these centers occur at one site each. A comparison of the experimentally observed direction cosines with those calculated from the crystal structure shows that the separation between the secondary bromine (bBr) and primary bromine (aBr) nuclei is 0.366 nm in the case of radical (A) and 0.369 nm in the case of radical B. In addition, O3− and O− paramagnetic centers and a BrO− diamagnetic center have also been observed in the crystals γ irradiated at 300 K. After γ irradiation at 77 K, BrO32− centers occuring at two magnetically inequivalent sites were observed. Thermal and UV light bleaching effects on these centers are discussed.
Optical spectrum of matrix-isolated Cu2
Authors: Kolb D. M. , Rotermund H. H. , Schrittenlacher W. , Schroeder W.
Published: February 2003

The absorption spectrum of Cu2 isolated in Ne has been derived from a combined absorption and emission spectroscopic study over a photon energy range from 2 to 8 eV. It is demonstrated how emission yield spectroscopy can be used to discriminate the dimer absorption bands against those of the monomer. The Cu2 spectrum is discussed in the light of recent theoretical cluster calculations, and an energy level scheme for Cu2 is derived.
On the photofragmentation of Fe(CO)5. I. Infrared and Mössbauer evidence for the formation of Fe(CO)4 in polymer matrices
Authors: De Paoli Marco A. , de Oliveira Sonia M. , Baggio-Saitovitch Elisa , Guenzburger Diana
Published: February 2003

Pentacarbonyliron sorbed in low density polyethylene (LDPE) or polytetrafluorethylene (PTFE) and irradiated with UV light has been studied by infrared and Mössbauer spectroscopies. The main photofragment reacts with the residual pentacarbonyliron leading to the formation of Fe2(CO)9. This reaction, plus the information obtained from infrared and Mössbauer spectra, suggest that the new species could be assigned as tetracarbonyliron Fe(CO)4 and tricarbonyliron Fe(CO)3. We have proved that molecules sorbed in a polymer matrix can be used as Mössbauer absorbers at low temperature. The polymer matrix allowed the isolation of otherwise unstable species: The high permeability of LDPE to gaseous carbon monoxide precludes the reformation of pentacarbonyliron molecules, thus excluding the ‘‘cage effect’’ observed in frozen gas matrices.
Ab initio determination of bond length dependence of the correlated valence shell Hamiltonian of CH: Comparison with semiempirical theories
Authors: Sun Hosung , Freed Karl F.
Published: February 2003

The exact ab initio effective valence shell Hamiltonian , which is mimicked by semiempirical theories of valence, is calculated for CH at 11 bond lengths using quasidegenerate many-body perturbation theory to incorporate extensive correlation contributions. Least squares fits of the bond length dependence of the calculated CH matrix elements provide simple formulas which are compared with the intuitive forms introduced into semiempirical theories. Some of the semiempirical formulas, e.g., one-center, one-electron integrals and two-center, two-electron integrals, are in good agreement with our correlated ab initio calculations, while others display substantial departures. For example, the bond length dependence of one-center, two-electron integrals, which are assumed to be independent of bond length in semiempirical theories, is substantial but physically understandable. Corrections are found to the assumed proportionality of resonance and overlap integrals. The bond length dependence of nonclassical three-electron integrals is presented along with the hybrid and exchange integrals that are ignored in zero differential overlap methods.
Infrared intensities in chloroforma)
Authors: Kim Kwan , King W. T.
Published: February 2003

The integrated intensities of the ν2(A1), ν4(E), and ν5(E) modes in HCC13 and DCC13 have been measured by the Wilson–Wells–Penner–Weber method and the atomic polar tensors derived. The effective atomic charge for hydrogen was found to be χH/e=0.048±0.004, a value that is close to that in HCF3, χH/e=0.049±0.004, although there is a sign discrepancy in the H atom polar tensor between chloroform and fluoroform.
A direct approach to second-order MCSCF calculations using a norm extended optimization scheme
Authors: Jensen Hans- Jo/rgen Aa. , Jo/rgensen Poul
Published: February 2003

Using configuration amplitudes and the unitary generators of orbital rotation the NEO algorithm has been derived. NEO (acronym for norm extended optimization) can be implemented as a direct second-order restricted step MCSCF optimization procedure where the quadratic convergence is obtained through solving a Hessian-type eigenvalue problem instead of a set of linear equations. Because configuration amplitudes are used as variables, the computations in each iteration can be made comparable to those of a direct CI calculation. The NEO is especially promising because convergence is assured to a state with the desired number of negative eigenvalues of the Hessian. With the NEO procedure one achieves: (1) Any set of configurations used in a direct CI can also be used for MCSCF; (2) excellent convergence characteristics including guaranteed convergence in ground state calculations; and (3) the converged state has the desired number of negative Hessian eigenvalues.
The compression of solid N2 at 296 K from 5 to 10 GPa
Authors: Olinger Bart
Published: February 2003

The volume of solid N2 having Pm3n space group was measured at 296 K from 5 to 10 GPa using a high pressure, x-ray diffraction technique. The compression of NaF was used as the pressure gauge. From a fit of the compression of the Pm3n structure data converted to the velocity plane, the volumes and bulk moduli are calculated as a function of pressure. From this fit and simultaneous volume measurements of the β-N2 phase and the Pm3n structure, the pressure of transformation from the β-N2 phase to the Pm3n structure is found to be 4.8 GPa. Recently published a priori calculations are shown to agree well with the present data.
Thermochemical studies of the gaseous uranium chlorides
Authors: Lau K. H. , Hildenbrand D. L.
Published: February 2003

The gaseous uranium halides UCl, UCl2, UCl3, UCl4, and UCl5 were generated under equilibrium conditions using several types of effusive beam sources, and were identified and characterized thermochemically by mass spectrometry. Five independent gaseous equilibria involving these species were studied in sufficient depth so that both reaction enthalpy and entropy changes could be evaluated accurately from second law analysis. The enthalpy data yield the sequence of bond dissociation energies at 298 K in kcal/mol: D° (U–Cl)=108; D° (ClU–Cl)=117; D° (Cl2U–Cl)=117; D° (Cl3U–Cl)=101; and D° (Cl4U–Cl)=50, all ±2 kcal/mol. As expected, the absolute entropies of the gaseous chlorides show that the electronic partition functions of the lighter species are substantial at the experimental temperatures, leading to electronic entropies of 5–7 cal/deg mol at about 2100 K.
Critical exponents for surface interacting self-avoiding lattice walks. III. Infinite systems
Authors: Ishinabe Takao
Published: February 2003

Self-avoiding walks interacting with a penetrable internal plane are investigated as a model of polymer adsorption at a liquid–liquid interface. The problem is closely related with the effect of a defect plane in a magnet. The critical exponent δa which is identical with the inverse of a surface crossover exponent φ, is estimated at 1.70±0.05 from the exact enumeration data for up to 20 steps on the tetrahedral lattice. The values of several other exponents are also estimated from the data; they are in agreement with the corresponding values for semi-infinite system except for γ. We correct the previous relation 1−ν=φ given by de Gennes and Bray and Moore.
Xenon adsorption on palladium. I. The homogeneous (110), (100), and (111) surfaces
Authors: Wandelt K. , Hulse J. E.
Published: February 2003

Systematic LEED, UPS, work function, and flash desorption investigations on the face specificity of xenon adsorption on macroscopic palladium single crystal surfaces are reported. The observed trends appear particularly suited to test current theoretical predictions about weak adsorption. With LEED, formation of hexagonally close-packed overlayers are observed on Pd(100) and Pd(110) and a 7/8 × 7/8 R 30° structure is detected on Pd(111), indicating complete registry on the (111) face, only one dimensional registry on the (110) face, and no registry on the (100) face. The latter two are in accord with earlier reports. UV-photoelectron spectroscopy reveals xenon induced face specific modifications of the Pd valence band emission which are regarded as fingerprints for electronic interaction between metal and absorbate. The electron binding energies of the Xe (5p3/2,1/2) photoemission peaks (in the limit of zero coverage) on the three Pd surfaces are found to obey the relationship EVB,0=EFB,0+φc, where EVB,0 and EFB,0 denote the binding energy with respect to the vacuum and Fermi level, respectively, and φc is the clean substrate work function. This substrate independence of EVB,0 , which is also observed on various surfaces of many other metals for both 5p-valence (UPS) and 3d-core levels (XPS) is explained as a pure surface electrostatic effect. The thermal desorption energies E0ad corresponding to the initial stage of adsorption decrease in the order Pd(110)>Pd(100)>Pd(111) and are estimated to be 10.2, 9.4, and 8.3 kcal/mol, respectively, using a preexponental of ν=1015 s−1. This ordering is in agreement with simple coordination arguments but not with an interpretation in terms of the charge-transfer-no-bond (CTNB) model for rare gas adsorption. Systematic results on the face specificity of the initial xenon dipole moment μ° are reported for the first time: μ°(111)=0.7D, μ°(100)=0.6D, and μ°(110)=0.44D. This ordering, which is reverse from that of E0ad, is incompatible with simple image potential arguments but, can instead be reconciled with a xenon–metal interaction model which assumes partial filling of an adsorption induces Xe (6s) resonance. The experimental data presented and their interpretation lay the basis for the understanding of the adsorption behavior of xenon on heterogeneous, i.e., polycrystalline, stepped, and sputter-roughened Pd surfaces which will be presented in part II of this work.
Photoconductivity of nonpolar liquids induced by vacuum-ultraviolet light
Authors: Böttcher Ernst-Holger , Schmidt Werner F.
Published: February 2003

The intrinsic photoconductivity (pc) of 11 liquid hydrocarbons and silanes induced by VUV light was measured. From the onset of the pc the following thresholds were estimated in eV at T=(294±2) K: isopentane 9.15; n-pentane 9.1; neohexane 8.75; 3-methyl pentane 8.85; cyclohexane 8.75; n-tridecane 9.25; n-pentene-1 8.33; cyclopentene 7.4; tetramethylethylene 6.8; hexamethyldisilane 6.75; triethylsilane 8.25. Absolute quantum yields were determines at 120 nm for a field strength of 2.5 kV cm−1.
Symmetry-dependent broadening parameters for methane
Authors: Fox Kenneth
Published: February 2003

Pressure-broadened widths of vibration-rotation lines in the infrared spectra of methane have been measured at very high resolution. Systematic variations of linewidths as a function of tetrahedral symmetry4 were found for a variety of foreign gases, and for both 12CH4 and 13CH4. These experimental results follow a pattern predicted in earlier calculations of scattering of argon from methane.
Photoconductivity measurements of x-ray absorption of liquids: Fe K-edge spectrum of ferrocene in 2, 2, 4, 2, 4-trimethylpentane
Authors: Sham T. K. , Holroyd R. A.
Published: February 2003

Using a conductivity technique, we have obtained EXAFS spectra of ferrocene Fe(C5H5)2 in 2,2,4-trimethylpentane under various conditions. The behavior of the collection efficiency of the conductivity cell affects the appearance of the photoconductivity spectrum. In thin cells, the photocurrent increases at the K edge and normal EXAFS structure is observed. In the black cell limit, i.e., for total absorption the photocurrent yield actually drops at the Fe K edge but still exhibits x-ray absorption fine structure (EXAFS) behavior. Both conductivity and transmission measurements give identical Fe(C5H5)2 bond lengths.
Iron K x-ray absorption-edge structures of FeS and FeS2
Authors: Sugiura Chikara
Published: February 2003

The Fe K absorption-edge structures of the NiAs-type FeS and the pyrite-type FeS2 have been measured with a high-resolution two-crystal spectrometer. The K absorption edges consist of a step-like structure and are interpreted in terms of the energy-band structures for these sulfides. The presence of the absorption structure is much clearer in Fes2 than in FeS. This difference is attributed to the difference of the iron 3d states in FeS and FeS2 and the distortion of the octahedron [FeS6]10− in these sulfides. The Fe K absorption spectrum of FeS2 is compared with the sulfur K absorption spectrum reported previously and a good agreement is obtained between them.