bluuebunny/crossref_metadata_2025 huggingface dataset:
An ESR, ENDOR, and ELDOR study of tunneling rotation of a hindered methyl group in x-irradiated 2,2,5-trimethyl-1,3-dioxane-4,6-dione crystals
Authors: Geoffroy M. , Kispert L. D. , Hwang J. S.
Published: February 2003

Electron spin resonance (ESR), electron nuclear double resonance (ENDOR), and electron-electron double resonance (ELDOR) spectra are reported for radical (I) in x-irradiated 2,2,5-trimethyl-1,3-dioxane-4,6-dione single crystals over the temperature range 10–64 K. The β-methyl group is observed to undergo tunneling rotation with a coalescence temperature of 30 K. The height of the hindering barrier has been deduced from the temperature dependence of the ESR spectrum and also from the solution to the Mathieu equation. All measurements are consistent with a tunneling splitting equal to 830±40 MHz.
Two-dimensional equations of state: A comparison with experiment
Authors: Glandt Eduardo D. , Myers Alan L. , Fitts Donald D.
Published: February 2003

The theoretical equations of state for a Lennard-Jones fluid as determined by a variety of theories (Percus–Yevick, hypernetted-chain, Barker–Henderson, Weeks–Chandler–Andersen, Andrews, and virial expansion) are compared with experimental data for both argon and krypton monolayers. The experimental two-dimensional equation of state for argon is obtained from adsorption isotherms for argon on graphitized carbon black and is reported here over a range of temperatures from 120 to 166 K. The Percus–Yevick-energy equation of state is found to give the best overall agreement with experimental results.
Rate of recombination of nitrogen atoms
Authors: Yamashita Takuro
Published: February 2003

Decays of the concentration of N(4S) atoms in the Lewis–Rayleigh nitrogen afterglow were measured in a static mode, with a mass spectrometer that was calibrated against NO titration. The rate constant for recombination of N(4S) atoms with N2 molecules as the third body [N(4S)+N(4S)+N2→N2+N2] was determined as 7.2×10−33 cm6 molecule−2 sec−1. This value is compared with the values obtained by previous investigators. The rate constant for the wall recombination process is also given.
Electronic excited state transport in solution
Authors: Gochanour C. R. , Andersen Hans C. , Fayer M. D.
Published: February 2003

The transport of electronic excitations between randomly distributed sites is examined. The Green function solution to the master equation is expanded as a diagrammatic series. Topological reduction of the series results in an expression for the Green function which is equivalent in form to the Green function solution of a generalized diffusion equation. The diagrammatic technique used suggests an interesting class of self-consistent approximations. This self-consistent method of approximation is applied to the specific case of the Förster transfer rate. The solutions obtained are well-behaved for all times and all site densities and indicate that transport is nondiffusive at short times and diffusive at long times. The mean squared displacement of the excitation and the time derivative of the mean squared displacement are calculated. These calculations illustrate that the time regime in which diffusive transport occurs is dependent on density. For low density systems transport becomes diffusive only at very long time, i.e., more than a few lifetimes. For high densities transport becomes diffusive within one lifetime. The application of the picosecond transient grating technique to the study of this problem is briefly discussed.
Theory of nonprimitive electrolyte solutions: generalized electrostatic model for the equilibrium properties of ionic–polar mixtures
Authors: Adelman S. A. , Chen Jing-Huei
Published: February 2003

A new electrostatic method for the equilibrium properties of nonprimitive electrolyte solutions is proposed. The theory is developed in detail for a 1-1 symmetric electrolyte composed of equal radius rigid spheres with embedded charges and dipoles. The theory is based on the following key ideas. The microscopic electrostatic equation for the ionic–polar mixture is reduced by averaging to a coupled hierarchy of effective electrostatic equations which depend, respectively, on the coordinates of 0, 1, 2, 3, etc., molecules. The first member of the hierarchy, which determines the macroscopic electrostatics in the solution, involves a salt concentration dependent dielectric constant ε and a generalized Debye inverse screening length κ. These constitutive parameters are determined self-consistently from the solutions of the second set of hierarchy equations. These are rendered tractable by a mean field closure of the hierarchy which is tantamount to assuming the constitutive parameters of the solution when one molecule is fixed are identical to the macroscopic constitutive parameters. The resulting first hierarchy equations are of generalized Poisson–Boltzmann form and involve dielectric and Debye screening functions which mirror the reference fluid hard sphere distribution function gHS(r). The theory is solved analytically by assuming gHS(r) has its low-density limit to yield a finite ionic strength generalization of the Onsager polar fluid theory. A more realistic approximation to gHS(r) yields a theory including local structure (i.e., Kirkwood) corrections to the Onsager model. The ion–ion potential of mean force predicted by this latter model is calculated at both zero and finite ionic strength. The significant differences between results of the nonprimitive electrolyte theory and an analogous primitive model theory also developed here are identified and discussed qualitatively for low, intermediate, and high dielectric constant solvents. Salt concentration and local structure effects on the dielectric behavior of the solution are also examined quantitatively.
Two-photon absorptivities of the all t
r
a
n
s α, ω-diphenylpolyenes from stilbene to diphenyloctatetraene via three wave mixing
Authors: Anderson Richard J. M. , Holtom Gary R. , McClain Wm. M.
Published: February 2003

Using the interference between the Raman resonances of the solvent and the two-photon absorption resonances of the solute in a three wave mixing experiment (CARS), we have extracted the absolute two-photon absorptivities at single energies for solution samples of the all -trans isomers of α, ω-diphenyl-ethylene, -butadiene, -hexatriene, and -octatetraene. Their absorptivities, for two identical linearly polarized photons in multiples of 10−50 cm4 sec photon−1 molecule−1 at the appropriate total energies (2?, cm−1), are as follows: stilbene, 12.1(0.9) at 38 873 cm−1, diphenylbutadiene, 14.4 (2.0) at 32 900 cm−1, diphenylhexatriene, 43.4 (4.0) at 32 900 cm−1, and diphenyloctatetraene, 61.0 (23.0) at 32 900 cm−1. Numbers in parentheses are standard deviations.
Lifetimes of fine structure levels of metastable H2 in the c 3Πu state
Authors: Chiu Lue-Yung Chow , Bhattacharyya Dilip K.
Published: February 2003

Lifetimes of fine structure levels of the v=0 level of the metastable c 3Πu state of H2 are calculated by considering both the forbidden predissociation and the forbidden radiative transition to the same dissociative b 3Σu+ state as the competing decay processes. The decay rate Wp due to forbidden predissociation is calculated for the rotational level N=1 of ortho-H2 and N=2 of para-H2. The weak spin–orbit and spin–spin interactions are the perturbative interactions to mix the metastable c 3Πu state with the repulsive b 3Σu+ state. For the fine structure levels J=N, the forbidden predissociative decay rate Wp calculated is approximately 14 times larger than the previously calculated radiative decay rate Wr. The resulting lifetimes for these J=N levels (which are 0.12 msec for N=1 and 0.11 msec for N=2) are shorter than the detection limit of Johnson’s time of flight experiment. For the fine structure levels J=N±1, Wp is approximately 1/3 of Wr (except for case J=N−1=0, where Wp=0) and the resulting lifetimes vary from 1.3 to 1.4 msec. The lifetimes of non-metastable levels which predissociate via the strong Λ-doubling (or orbit–rotation) interaction are also calculated. They are 1.36×10−9 sec for N=1 of para-H2 and 4.52×10−10 sec for N=2 of ortho-H2.
Electron paramagnetic resonance of divalent manganese in ferrous fluosilicate at 4.2 K
Authors: Rubins R. S.
Published: February 2003

EPR spectra of Mn2+ in single crystals of FeSiF6⋅6H2O have been studied at 16.5 and 25.0 GHz in the parallel orientation at 4.2 K. The spectra were fitted to a standard spin Hamiltonian with gz=2.0005±0.0005, D=−(254±1) ×10−4 cm−1, E=−(40±10) ×10−4 cm−1, a−F=+(8±1) ×10−4 cm−1, A=−(91.1±0.5) ×10−4 cm−1. Independent calculations of the line positions and the relative intensities of the ’’forbidden’’ hyperfine transitions both gave the value ϑ= (8±1) ° for the angle between the crystallographic c axis and the z axis of a Mn2+ ion.
Infrared laser enhancement of chemical reactions via collision induced absorption
Authors: Orel A. E. , Miller W. H.
Published: February 2003

We have pointed out previously that an infrared laser will in general enhance the rate of a chemical reaction via a collision induced absorption, even if the reactants themselves are infrared inactive. This paper examines this phenomenon more fully by presenting a simple analytically solvable model which illustrates it and also by presenting the results of classical trajectory calculations we have carried out for the reactions X+H2?HX+H, for X=H, F, Cl. One new feature which is revealed by these calculations is that the polarization of the laser is an important parameter, i.e., certain polarizations are much more effective in enhancing the rate of the reaction than others.
Water: Anomalous compressibilities to 1.9 kbar and correlation with supercooling limits
Authors: Kanno H. , Angell C. A.
Published: February 2003

A new high pressure volumetric technique, which employs a fine glass capillary as joint sample chamber and pressure vessel is described. Because of the small samples used, it is suitable for supercooled liquid studies. The compressibilities κT of water and of D2O have been determined over the applied pressure range 0.1–190 MPa (1–1900 bar) at temperatures in the range of 25–30°C. The anomalous low temperature increases in κT reported earlier for H2O at 1 bar are found at higher temperatures in D2O, as expected. In both liquids, increases in pressure cause the anomalous regions to be displaced to lower temperatures. The displacement per unit pressure change increases with increasing pressure. As found previously, the low temperature data conform to an empirical equation κT=A (Ts/T−1)−γ where Ts is a characteristic temperature now found strongly dependent on pressure. High pressure data are inadequate to yield both Ts and γ parameters reliably but, when γ is assigned a pressure-independent value, the best fit Ts values vary in each case with pressure in a manner strikingly similar to that of the homogeneous nucleation temperature, confirming a previously suspected relation between the two quantities. Preliminary attempts to separate the total compressibility into ’’normal’’ and ’’anomalous’’ parts suggest that the exponent γ is close to unity, as found previously for the anomalous component of the expansivity.
Energy disposal in thermal-energy charge transfer reactions determined by kinetic energy measurements of product ions: Ne++O2→O++O+Ne and Ar++O2→O2++Ar
Authors: Mauclaire G. , Derai R. , Fenistein S. , Marx R.
Published: February 2003

The possibility of deducing reaction mechanisms from kinetic energy released in the products is discussed for thermal energy ion-molecule reactions. An experimental method based on ICR spectrometry is described, allowing for the first time kinetic energy measurements of ions produced in thermal energy reactions. Application to charge transfer from Ne+ and Ar+ to O2 is presented. Ne+ produces O+ ions with 0.55 eV mean KE, corresponding to the O+(4S°)+O(1D) dissociation limit of O2+, while Ar+ produces O2+ molecular ions with 1 eV mean KE corresponding to ground state ions in rather high vibrational states (?∼8–10). With Ne+ the results can be accounted for in the framework of a (quasi) resonant charge transfer process while with Ar+ charge transfer involves electronic into kinetic energy transfer.
Multiple IR photon laser induced reactions of cyclopropane
Authors: Hall Richard B. , Kaldor Andrew
Published: February 2003

The multiple photon IR laser induced reactions of cyclopropane resulting from excitation of either of two fundamental vibrational modes has been demonstrated. In each case the products are typical of those formed with thermal excitation but the relative yields are not. The yield of the lowest energy barrier channel, isomerization, relative to higher channels, fragmentation, is dependent on the mode excited. Excitation of the C–H asymmetric stretch at 3.22 μm produces propylene with almost no fragmentation; excitation of the CH2 wag at 9.50 μm produces roughly equal yields of propylene and fragmentation products. Collisions play a critical role in this process. At 3.22 μm, the effect of collisions with argon is to increase yields of the fragmentation channel. As these collisions cannot add any significant amount of energy to the cyclopropane, it is concluded that the upper channel is opened due to intermode vibrational energy redistribution. At 9.50 μm, argon tends to favor the lower energy pathway. These results are consistent with an excitation that is initially nonstatistical or mode selective.
Triplet acetylene: Near infrared electronic absorption spectrum of the c
i
s isomer, and formation from methylene
Authors: Wendt H. R. , Hippler H. , Hunziker H. E.
Published: February 2003

The electronic absorption spectrum in the gas phase of the metastable 1 3B2 state of acetylene has been observed. Its’ features, including rotational and vibronic structure as well as deuterium isotope effects, agree remarkably well with ab initio theoretical predictions for the 1 3A2–1 3B2 electronic transition of the lowest triplet state in its cis geometry. The metastable species was generated in the Hg-photosensitized reactions of acetylene, ketene, and diazomethane. In the latter two cases it is probably formed by the reaction 3CH2+3CH2 →C2H2(1 3B2)+H2.
Molecular and electronic structures of transition metal trifluorides
Authors: Yates J. H. , Pitzer R. M.
Published: February 2003

Quantum mechanical calculations of the ab initio restricted-Hartree–Fock type have been carried out on the molecules MF3 where M varies from Sc to Ni. Several electronic states were investigated for each molecule and in all cases the ground states were found to have high spin. The molecular structures were optimized and found to be trigonal planar (D3h) except for CrF3, found to be pyramidal (C3v), and MnF3, not investigated. Ionization potentials, electronic properties, and population analyses of the ground states have been obtained, and vibrational analyses of the out-of-plane modes have been carried out. The Jahn–Teller distortion of a low-lying electronic state of CoF3 was worked out in detail.
The dissociation dynamics of internal energy selected C6H6+ a)
Authors: Baer Tomas , Willettb) Gary D. , Smithc) Deborah , Phillipsd) J. Sanford
Published: February 2003

Absolute fragmentation rates and average energy released in the gas phase dissociation of 2,4 and 1,5 hexadiyne have been measured by photoion–photoelectron coincidence. The decay rates over the metastable energy range, as measured by the asymmetric time-of-flight distributions of C4H4+ and C3H3+ fragments are identical, and equal to those measured by others for benzene. This indicates that over this energy range all three C6H6+ isomers rearrange to a common ion structure prior to dissociation. Calculated rates based on the statistical theory suggest that the C6H6+ ions fragment to all product channels by competitive pathways and that the existence of isolated states or noncommunicating isomeric structures is highly unlikely. Above this energy range, the branching ratios to the C3H3+ and C4H4+ fragments indicate that the 1,5 hexadiyne ion no longer isomerizes prior to dissociation.
Depolarized scattering of atomic fluids with variable polarizability
Authors: Alder B. J. , Beers J. C. , Strauss H. L. , Weis J. J.
Published: February 2003

The calculated band shapes and intensities of the depolarized light scattering from dense systems of atoms interacting with either hard-sphere or Lennard–Jones potentials are shown to agree qualitatively with experiment, if an empirical form for the pairwise polarizability distortion anisotropy is used.
Determination of the structure of ArCO2 by radio frequency and microwave spectroscopy
Authors: Steed J. M. , Dixon T. A. , Klemperer W.
Published: February 2003

The species ArCO2 produced by adiabatic expansion is structurally characterized by molecular beam electric resonance spectroscopy. ArCO2 is shown to have C2v symmetry. The spectroscopic constants obtained are: The derived structural constants for ArCO2 are: Comparison is made with a number of proposed Ar–CO2 intermolecular potentials. The determined force constants are shown to be poorly fitted by a spherical argon–oxygen interaction.
Determination of the electric field gradients for the two different nuclear sites of the cadmium atoms in b
i
s (thiosemicarbazide) cadmium(II) sulfate using gamma ray perturbed angular correlation spectroscopy on the 111Cd isotope
Authors: Bauer Rogert , Christensen Claus , Larsen Erik
Published: February 2003

Gamma ray perturbed angular correlation spectra of 111Cd2+ in bis (thiosemicarbazide)cadmium(II) sulfate and in bis (thiosemicarbazide)zinc(II) sulfate have been recorded. From the measurements of 111Cd in the cadmium complex of known structure two different sets of electric field gradient parameters have been evaluated. These observed quantities have been interpreted in a general ligand field model suitable to translate information about electric field gradients of metal ions of nd10 electronic configruation into chemical bonding properties.
Species-specific Raman spectroscopic measurements of concentration fluctuations in unsteady flow
Authors: Chabay Ilan , Rosasco G. J. , Kashiwagi T.
Published: February 2003

Concentration fluctuations of specific chemical species in unsteady mixed gas flow were studied by fast Fourier transform (FFT) analysis of Raman intensity. Average concentration and the amplitude and frequency distribution of fluctuations about that average (at a specific Raman shift) can be determined by the FFT/Raman technique. The fluctuation spectra can be modeled to yield information on the lifetimes, periodicity, amplitudes, and the statistical distribution of these in unsteady or turbulent motion. A simple burner with a concentric outer air sheath and inner CH4 jet was used in the preliminary experiments for both cold gas and flame studies. Interference from flame emission (uncorrelated to Raman scattering) was minimized by chopping the Raman exciting beam and observing the FFT signal near the chopper frequency. Peak height at the chopper frequency indicates average concentration, while the sidebands around this frequency contain information on the fluctuations in concentration of the species under observation. With the present system, fluctuations in the frequency range of 0–1 kHz having an amplitude of 5% of the average concentration of certain species whose average partial pressure is as little as 1% of atmospheric pressure could be measured. The fluctuations are observed from sampling volumes as small as ∼10−4 mm3 at any position in the flow. Improvements in the technique and extensions to other systems are discussed.
Rotational energy transfer in Na*2–Xe collisions: Level to level dynamics
Authors: Brunner Timothy A. , Driver Richard D. , Smith Neil , Pritchard David E.
Published: February 2003

Using the method of laser fluorescence we have measured thermally averaged rate constants for rotation changing collisions of Na*2 (A 1Σ) with Xe at six values of initial rotational quantum numbers ranging from 16 to 74. We propose a new scaling law based on the T matrix formalism which fits all these rates to within 20%. This scaling law shows that the square of the T matrix has a power law dependence on the amount of energy transferred, and that mj is conserved in the collision.
Theory of high-spin?low-spin transitions in transition metal compounds induced by the Jahn–Teller effect
Authors: Kambara Takeshi
Published: February 2003

A model for high-spin?low-spin transitions in transition metal compounds is presented. The Jahn–Teller coupling between the d electrons and local distortion is considered as the driving force for the transition. The system built up of octahedrally coordinated Fe(II) complexes is studied microscopically on the basis of the ligand field theory. The first-order phase transition, in which the order parameter is the local displacement Q, occurs more than once over a certain range of the coupling strength, a. When the second order coupling is introduced, in some cases all of the three possible patterns of the transition (Q≳0?Q=0, Q≳0?Q<0, and Q<0?Q=0) appear in the system. The phase transitions occur not only between high-spin and low-spin states but also between the two high-spin states split by the distortion Q. The calculated temperature dependences of effective magnetic moment μeff reproduce the essential feature of observed results for the case of discontinous high-spin?low-spin transition with reasonable values of E0 and a, where E0 is the energy separation between the high-spin and low-spin states in the undistorted phase. These values also give consistently reasonable values of axial ligand field splitting of the high-spin state and the abrupt change of Fe–ligand bond length. It is suggested that the observed peculiar behavior of μeff at lower temperatures in the high-spin compounds may be due to the first-order phase transitions between the high-spin states.
Bounds on dynamic polarizabilities and dispersion force coefficients
Authors: Luyckx R. , Coulon Ph. , Lekkerkerker H. N. W.
Published: February 2003

A simple derivation and extension of the bounds on dynamic polarizabilities obtained by Langhoff, Karplus, and Gordon are given. A new matrix method to solve the relevant moment equations is presented.
Mechanisms of energy transfer in the deuterium fluoride systema)
Authors: Wilkins Roger L.
Published: February 2003

A three-dimensional trajectory study has been employed to determine rate coefficients as a function of temperature for the important energy-transfer processes that occur in DF(v1)+DF(v2) collisions. From this study, it was predicted that the v→v energy-transfer processes occur by means of Δv=±1 transitions and that the rate coefficients for the v→v processes DF(v1=1)+DF(v2) →DF(v′1=0)+DF(v′2=v2+1) with v2±1 through 5, respectively, decrease with increasing vibrational quantum number v. The computed rate coefficients for the v→v processes are k (v1=1, v2=1; v′1=0, v′2=2) =1.3×1013 cm3/mole sec and k (v1=1, v2; v′1=0, v′2=v2+1) =1.611−Hv2k (1,1;0,2) at 300° K. These v→v processes correspond to near-resonant vibration-to-vibration (v→v) intermolecular energy transfer. The v→R energy-transfer processes occur by converting multiple quanta of vibrational energy of a vibrationally excited DF molecule into rotational energy of the same molecule. This process is nonresonant v→R intramolecular energy transfer. These multiquantum v→R transitions provide more ways to distribute the vibrational energy of the vibrationally excited DF molecule into rotational energy and thereby populate its high rotational states. The high rotational quantum states are relaxed slowly by R→ (R′,T) processes. A rotational nonequilibrium model is used to calculate quenching rate coefficients for vibrational relaxation of DF(v1=1) by DF(v2=0). The results are in good agreement with available experimental data.
Laser magnetic resonance spectroscopy of the ν2 fundamental band of DO2 at 9.8 μm
Authors: McKellar A. R. W.
Published: February 2003

The ν2 fundamental band (bending vibration, ν0=1020.162 cm−1) of the deuteroperoxyl radical DO2 has been studied using the technique of laser magnetic resonance, in which molecular transitions are tuned into resonance with fixed laser lines by means of the Zeeman effect. The DO2 was produced in a flow system by reacting the products of a discharge in O2 with methanol (CH3OD or CD3OD) or ethanol (C2H5OD). About 150 resonances, observed using 19 different CO2 laser lines in the 1009–1038 cm−1 region, were assigned to DO2 transitions with 1⩽N⩽9 and 1⩽Ka⩽6. Their analysis yielded the ν2 band origin as well as rotation, spin–rotation, and centrifugal distortion parameters for the ground and ν2 excited vibrational states. It was necessary to include in the analysis the effects of Coriolis interaction between ν2 and ν3, which lies about 100 cm−1 higher in energy. Thus the Coriolis coupling constant and some effective ν3 band parameters were also determined, though ν3 was not observed directly.
Calculations of surface structures for naphthalene crystals
Authors: Filippini Giuseppe , Gramaccioli Carlo M. , Simonetta Massimo
Published: February 2003

Models for surface structures on (001) and (100) planes have been calculated for naphthalene crystals, assuming semiempirical atom–atom potentials. Relaxation of the bulk structure in proximity of surfaces leads to small shifts, with molecular rotations of the order of 0.5–2° and translations of the molecular centers of 0.007–0.1 Å, without appreciable energy variations; on going inside the crystals these effects fall off rapidly. The perfect cleavage of the crystal on (001) is easily explained on grounds of packing energy and of the most likely surface model.
A study of laser-assisted surface ionization of lithium
Authors: Mariella R. P.
Published: February 2003

An experimental study on the influence of electronic energy in surface ionization has been performed using hot tantalum and rhenium ribbons with electronically excited lithium. Upper limits of quantum yield for this process are 10−4 for the tantalum and 4×10−2 or less for the rhenium.
A Monte Carlo study of the dense polarizable dipolar hard sphere fluid
Authors: Patey G. N. , Torrie G. M. , Valleau J. P.
Published: February 2003

In this paper we report Monte Carlo results for a fluid of polarizable dipolar hard spheres. Both isotropic and anisotropic particles are considered. It is shown that the induced forces make a large contribution to the thermodynamic properties and significantly influence the fluid structure. The various approximate treatments of polarizable systems are evaluated and a recent theory proposed by Wertheim is found to be very successful.
Heat capacity and entropy of CuF2 and CrF2 from 10 to 300 °K. Anomalies associated with magnetic ordering and evaluation of magnetic contributions to the heat capacity
Authors: Boo W. O. J. , Stout J. W.
Published: February 2003

The low temperature heat capacity of CuF2 and of CrF2 is reported from 10 to 300 °K. Peaks in heat capacity associated with the onset of long-range magnetic order are observed at 70.90±0.05 °K for CuF2 and 49.26±0.01 °K for CrF2. Magnetic contributions to the heat capacity and entropy are evaluated and tabulated. Smoothed values of the total heat capacity, entropy, enthalpy, and Gibbs energy are tabulated between 10 and 300 °K. Values at 298.15 °K are: CuF2, C °P=15.65 cal °K−1 mole−1, S °=18.506 cal °K−1 mole−1, H°T−H°0=2891.8 cal mole−1; CrF2, C °P=15.48 cal °K−1 mole−1, S°=20.762 cal °K−1 mole−1, H°T−H°0−2911.5 cal mole−1.
Chemisorption on perfect surfaces: Hydrogen and nitrogen on tungsten and rhodium
Authors: Polizzotti R. S. , Ehrlich Gert
Published: February 2003

Chemisorption on highly perfect low index surfaces, prepared by low temperature field evaporation, is compared in simultaneous field emission measurements with chemisorption on the atomically rough regions which dominate the total electron current from the emitter. For the (110) plane of tungsten, the most densely packed of this lattice, it is found that contrary to some earlier reports, nitrogen does chemisorb after high exposures at room temperature. The nitrogen has no significant effect upon the work function for electrons; instead, chemisorption manifests itself by a change in the absolute emission intensity. Hydrogen also chemisorbs on W(110). However, at T?144 °K, this plane is populated only after the rougher, stepped regions have been substantially covered by a chemisorbed layer. At still lower temperatures, at T?38 °K, a layer of molecularly bound gas forms on exposure of W(110) to hydrogen. The layer is removed by warming to T<77 °K, without conversion into the chemisorbed state. This suggests that on the (110) plane, dissociative chemisorption of hydrogen is activated, and must occur over a barrier ? 3.5 kcal/mole. Chemisorption occurs preferentially on the stepped, atomically rough regions. It is by transport from these regions, and not by direct dissociation on W(110), that this surface is filled up. Measurements have also been made on the (111), (100), and (110) planes of rhodium at various temperatures. On all of these, hydrogen chemisorbs at a rate comparable to that on rough planes, and there is no indication that steps play an important role in the adsorption process.
H2O quenching of the transitions H(n=3→n=2) and O(3p 3P→3s 3S) in pulse radiolytically excited water vapor
Authors: Quickenden T. I. , Trotman S. M. , Irvin J. A. , Sangster D. F.
Published: February 2003

Time resolved spectral and kinetic studies at the nanosecond level were carried out on water vapor excited by a 0.53 MeV pulsed electron beam. Three electronically excited species: H(n=3), O(3p 3P), and O(3p 5P), were identified in the 600–900 nm emission region investigated. The rate constants for the very fast quenching by H2O at 294 K of H(n=3) and O(3p 3P) were determined to be (9.1±1.6) ×10−9 cm3 molecule−1 s−1 and (9.4±1.5) ×10−10 cm3 molecule−1 s−1, respectively. The rate constant for O(3p 3P) has not been determined previously.
Pure rotational lasing in four electronic states of NH: Impulsive to adiabatic collisional pumping
Authors: Smith John H. , Robinson Dean W.
Published: February 2003

Pure rotational chemically pumped lasing transitions are reported for the free radical NH, as formed by flash photolysis of HN3 in a large excess of rare gas. The bulk of excited nascent population occurs in the a 1Δ (v=0) state but smaller concentrations reaching the c 1Π state result in the eventual population of higher vibrational states of X 3Σ−, a 1Δ, b 1Σ+, and A 3Π. These latter states all exhibit pure rotational lasing. A computer simulation model is constructed that details the full relaxation history of vibrationally and rotationally excited NH particles formed from c 1Π transitions to the a 1Δ state. This model incorporates V→R and R→R,T transfer mechanisms and can accommodate collisions from purely impulsive (with the possibility of V→T energy loss) to purely adiabatic (as affected by vibration, rotation, and mass of the collision partner). The model successfully explains the relative intensities of lasing transitions in the a 1Δ state, the relative intensities observed using different rare gases as buffers and the phenomenon of double pulsed lasing in the same transition, again using various rare gases as collision partners. The lasing transitions observed in A 3Π were found to be enhanced by the presence of the heavier rare gases, and are discussed in terms of collision induced intersystem crossing from the c 1Π state.
The quartic force field of H2O determined by many-body methods that include quadruple excitation effects
Authors: Bartlett Rodney J. , Shavitt Isaiah , Purvis George D.
Published: February 2003

Many-body perturbation theory (MBPT) and coupled cluster methods are employed in an investigation of the potential energy surface of H2O in the vicinity of its equilibrium geometry. The basis set of 39 Slater-type orbitals is the same as that previously used in a configuration interaction study (limited to all single and double excitations, SD-CI) of this surface, and is capable of accounting for 80% of the total correlation energy of the molecule. Detailed comparisons among the results of the various methods are presented, with particular reference to the role of size extensivity in providing a reliable model for the prediction of the shape of the surface. The predicted quartic force field obtained by the coupled cluster doubles (CCD) and by several MBPT models is in very good agreement with experiment. The inclusion of quadruple excitations, which account for about 5% of the correlation energy, is found to have a significant effect on the shape of the surface, bringing the predicted force field into substantially better agreement with experiment than that obtained with SD-CI.
Pressure induced changes in liquid alkane chain conformation
Authors: Schoen P. E. , Priest R. G. , Sheridan J. P. , Schnur J. M.
Published: February 2003

Conformations of n-alkane molecules in the liquid state have been studied as a function of temperature and pressure using Raman scattering. Observations of conformationally sensitive Raman bands in hexane, heptane, octane, and hexadecane reveal no sign of the pressure-induced effects in polyethylene. Instead the Raman intensities indicate chain kinking as a function of pressure—a dramatic effect in the shorter chains, a slight effect for hexadecane. A theory based on considerations of conformational energy and translational entropy is presented which explains these effects qualitatively.
Multiphonon and energy transfer relaxation in charge compensated crystals
Authors: Miller Marvin P. , Wright John C.
Published: February 2003

Alkaline earth fluoride crystals doped with trivalent rare earth contain both isolated and clustered rare earths. Electronic nonradiative relaxation for isolated rare earths proceeds solely by multiphonon relaxation while for clustered rare earths, it proceeds by both efficient nonresonant energy transfer and multiphonon relaxation. We have measured the rates of all the relaxation processes for representative sites in CaF2, SrF2, and BaF2 in order to obtain a comprehensive understanding of relaxation in the materials. Exponential dependences of the multiphonon and energy transfer rates on energy gap have been found. The rates do not appear to scale simply with the lattice phonon energies but depend upon the details of the phonon distribution and coupling. Temperature dependence of the rates indicate a complex mixture of phonons may contribute to the observed relaxation.
Kinetic theory of thermal transpiration and mechanocaloric effect. III. Flow of a polyatomic gas between parallel plates
Authors: Loyalka S. K. , Storvick T. S.
Published: February 2003

The phenomenological coefficients for one-dimensional mass and energy flows between infinite parallel plates containing polyatomic gases at all degrees of rarefaction are reported. The polyatomic gas model equations developed by Hansen and Morse were used in the linearized Wang Chang and Uhlenbeck equation with Maxwell’s diffuse scattering boundary conditions. The solutions reproduce the molecular flow results at large Knudsen numbers and the continuum fluid properties for small Knudsen numbers. The phenomenological coefficients are functions of the Eucken factor, the internal constant volume heat capacity, and the translational Eucken factor. Calculations for the Gross–Jackson and the BGK model are reported and compared to the polyatomic gas results. We find the thermal transpiration effect depends on the translational Eucken factor and is independent of the inelastic collisions. The monatomic gas results can be used to estimate the thermal transpiration effect by ’’scaling’’ the Knudsen numbers used to compute the 𝒥MQ coefficient. This scaling is not uniform and errors become large for large Knudsen numbers. The cross coefficients 𝒥MQ=𝒥QM for all calculations but the energy flux near the wall was found to be negative for inverse Knudsen numbers greater than 3.
The hydrogen atom within spherical boxes with penetrable walls
Authors: Ley-Koo E. , Rubinstein Sergio
Published: February 2003

We study a model for the hydrogen atom confined within spherical boxes with penetrable walls. The potential consists of the Coulomb potential inside the box and a constant potential outside the box; the Schrödinger equation admits analytical solutions in both regions. The energy eigenvalues and eigenfunctions for the lowest states of the system are determined numerically for boxes of different sizes and penetrabilities. In addition, we also evaluate the hyperfine splitting, nuclear magnetic shielding, polarizability and pressure of the system and investigate the effect of the confinement on these atomic properties.
A quasiclassical trajectory study of vibrational predissociation of van der Waals molecules: Collinear He⋅⋅⋅I2(B 3Π)
Authors: Woodruff Susan Beatty , Thompson Donald L.
Published: February 2003

Vibrational predissociation of the van der Waals molecule He⋅⋅⋅I2(B 3Π) has been investigated using quasiclassical trajectories. The study was restricted to collinear motion to allow comparisons with the quantum mechanical calculations of Beswick and Jortner [J. Chem. Phys. 68, 2277 (1978); 69, 512 (1978)]. The unimolecular dissociation of He⋅⋅⋅I2(B 3Π) confined to a single adiabatic electronic potential-energy surface was studied as a function of the initial I2 vibrational quantum state, where a zeroth-order approximation of the separation of the I–I and He–I2 oscillators was made for the purpose of assigning initial states. The computed trajectory results show that the unimolecular decay as a function of time obeys the exponential decay law quite well. The computed decay rates are in accord with the quantum mechanical values and with experimental measurements.
The halogen hexafluoride radicals: A variable temperature study
Authors: Boate A. R. , Morton J. R. , Preston K. F. , Strach S. J.
Published: February 2003

The temperature variation of the central-atom hyperfine interaction in the halogen hexafluorides has been studied in various matrices. By comparing the parameters of hypercotangent and Debye fits to the data, it is concluded that an intramolecular tlu vibration offers the best explanation of the phenomena.
A re-examination of the use of rate equations to account for fluence dependence, intramolecular relaxation, and unimolecular decay in laser driven polyatomic molecules.
Authors: Stone James , Goodman Myron F.
Published: February 2003

A general set of rate equations including intramolecular relaxation and laser-induced excitation in a polyatomic molecule is proposed. These equations differ from relaxation equations transcribed from nmr theory and applied recently to polyatomic molecules in that they explicitly account for the dynamics of heat bath feedback on the pump mode excitation. Feedback of heat bath excitation to the pump mode is shown to result in decomposition which depends exclusively on laser fluence only when T1 relaxation is rapid, but which depends more generally on peak laser intensity. Analytic expressions for stimulated absorption and emission coefficients, consistent with heat bath feedback when T1 relaxation is rapid, are derived and incorporated in a degenerate rate equation. Previous arguments advanced to justify the calculation of decomposition rates from RRKM theory with a thermal distribution of absorbed energy are shown to fail precisely because they require absorption and emission coefficients which are not consistent with the dynamics of heat bath feedback.
Production and relaxation cross sections for the shear viscosity Senftleben–Beenakker effect. I. Formal expressions and their coupled-states and infinite-order–sudden approximations for atom–diatom systems
Authors: Liu W.-K. , McCourt F. R. , Fitz D. E. , Kouri Donald J.
Published: February 2003

Starting from kinetic theory collision integrals obtained from a generalized Boltzmann equation for a linear molecule in a bath of atomic perturbers and using Liouville (vector) space algebra, general expressions are derived for the three cross sections determining the shear viscosity Senftleben–Beenakker effects. These expressions are presented in terms of S-matrix elements in the total-J representation since this representation is especially useful for dynamical calculation and approximation procedures. Coupled-states and infinite-order–sudden dynamical approximations are then introduced and expressions obtained for the three cross sections in initial-l, final-l, and average-l labeling schemes. All cross sections simplify greatly when initial or final-l labeling is employed but little or not at all when average-l labeling is used. Nonetheless, even when the latter choice is made, less work will be involved than would be required for the corresponding full close-coupled or coupled-states calculation.
Small angle neutron scattering from polymer networks
Authors: Ullman Robert
Published: February 2003

The small angle neutron scattering of labeled chains in a polymer network has been calculated for several possible models of the network. Cases of isotropic and anisotropic deformation are considered. The influence of entanglements is examined. Results are presented in graphical form, and low angle and high angle limiting behavior is treated.
The electronic structure and spectra of the X 1Σ+g and A 1Σ+u states of Li2
Authors: Konowalow Daniel D. , Olson Mark L.
Published: February 2003

The potential energy curves for the X 1Σ+g and the A 1Σ+u states of Li2 have been calculated on the single configuration Hartree–Fock–Roothaan (HF) level, and on the multiconfiguration self-consistent-field (MCSCF) level. The MCSCF results give binding energies De (X 1Σ+g) =8297 cm−1 and De (A 1Σ+u) =9299 cm−1; a semiempirical scaling which reproduces the experimental vibrational energy level spacings suggests ’’most-likely’’ dissociation energies De′ (X 1Σ+g=8450±100 cm−1 and De′ (A  (1Σ+u) =9400±100 cm−1.
Multiscale propagation phenomena in reaction–diffusion systems
Authors: Pismen L. M.
Published: February 2003

Asymptotic methods and phase-plane analysis are combined to investigate existence and stability of various kinds of propagating fronts in nonlinear reaction–diffusion systems with widely separated characteristic time and length scales associated with different reactants. A basic difference is observed between two types of kinetic systems, called anticlinal and synclinal. This distinction is further traced down to processes of nucleation, excitation, and polarization of alternative steady states. The described phenomena include entrained, spearhead, shielding, and dragged fronts in infinite regions; travelling pulses and chaotic excitations in metastable systems; steady and undulating fronts in finite regions; mutual chase and split (synchronized) fronts in kinetic systems with several homogeneous steady states.
The vibrational infrared spectrum of CoO
Authors: Devore T. C. , Gallaher T. N.
Published: February 2003

The infrared spectrum of CoO has been observed in absorption by using a microwave discharge as the molecular source. The molecular constants are: υo(cm−1), 842.2(2); Be(cm−1), 0.522(5); αe×103(cm−1), 1.9. (2); re(nm), 0.160(1). The appearance of the spectrum indicates that this is a 4Σ− state.
Experimental and computational study of HF+Xe scattering
Authors: Becker C. H. , Tiedemann P. W. , Valentini J. J. , Lee Y. T. , Walker R. B.
Published: February 2003

Crossed molecular beam experiments have been performed measuring angular distributions I (ϑ) of HF(1Σ+) scattered off Xe(1S0) at collision energies of 0.044 and 0.153 eV. The I (ϑ) show rainbow and supernumerary rainbow structure, with diffractive oscillations just being resolved in the low energy data. A spherically symmetric potential V0(R) is fit to the data with well depth and minimum position 0.016 eV and 3.77 Å. Using this V0(R), four model potential surfaces V (R,γ) are constructed and scattering calculations are performed employing the centrifugal sudden approximation. The calculations treat HF as a rigid rotor and use a collision energy of 0.044 eV. The calculations emphasize the role of the initial rotational state as well as the effect of the type and degree of anisotropy of V (R,γ) upon total and state-to-state center-of-mass differential cross sections. In particular, it is found that when V (R,γ) is strongly attractive for one limited range of atom–diatom orientations, the elastically scattered rainbow is shifted to larger angles compared to the rainbow from V0(R) scattering—if the initial rotational state jin=0. For jin=1 and 2 the V (R,γ) scattering tends to resemble that for V0(R). Because the jin distribution in the HF beam is not well characterized, and because it is shown that very similar total differential cross sections at one energy can be generated from qualitatively different V (R,γ) with similar V0(R), even for the same jin, conclusions about the HF–Xe anisotropy of V (R,γ) from I (ϑ) are precluded. However, the extracted V0(R) is thought to be realistic based on the consistency of fits to the two I (ϑ) at significantly different collision energies and the likelihood of a relatively high rotational temperature of the HF beam due to heating of the nozzle source.
Solvent dependence of vibrational dephasing and reorientational relaxation
Authors: Moradi-Araghi A. , Schwartz M.
Published: February 2003

The polarized and depolarized Raman spectra of the ν1 band of chloroform and deuterochloroform were obtained as a function of concentration in the solvents carbon tetrachloride, acetonitrile, and acetone. The reorientational correlation functions were found to decay more rapidly in CCl4 but more slowly in the two polar solvents; these results contrast directly with the behavior predicted by models of rotational diffusion. The isotropic correlation functions were analyzed with the model of vibrational dephasing, as described recently by Rothschild. It was observed that the modulation (or collision) times increase in the nonpolar CCl4 and in acetone but remain constant in the second polar solvent, acetonitrile. An interpretation of the above results in terms of specific solute–solvent interactions is presented.
Deformation electron density of α-glycylglycine at 82 K. II. The x-ray diffraction study
Authors: Kvick Åke , Koetzle Thomas F. , Stevens Edwin D.
Published: February 2003

A study of the charge density in α-glycylglycine, C4H8N2O3, has been carried out, employing x-ray and neutron diffraction data measured at a temperature of 82 K. The crystal is monoclinic, space group P21/c, with a cell of the following dimensions a=8.025(2), b=9.543(4), c=7.788(2) Å, and β=106.50(2) ° at 82 K. X-ray data were collected in the sinϑ/λ range 0.00–1.07 Å−1, with Nb filtered MoKα radiation. The deformation densities were computed by an X–N method using positional parameters for all atoms from a neutron diffraction study and thermal parameters for nonhydrogen atoms from a high-order x-ray refinement. In the case of hydrogen atoms, the neutron thermal parameters were scaled up by 15% as suggested by a comparison of thermal parameters for nonhydrogen atoms from the neutron and the high-order x-ray refinements. Well-defined peaks of deformation charge density are found for all covalent bonds, and polarization of the density towards the more electronegative atom in the bonds can be noticed. The deformation densities in the four N–H⋅⋅⋅O hydrogen bonds are very similar, with excess charge in the N–H bonds and electron deficient regions on the sides of the hydrogen atoms pointing towards the acceptor oxygen atoms. There is also excess electron density close to the oxygen atoms in the direction of each hydrogen bond, but there does not appear to be any systematic correlation between the H...O distance in the hydrogen bonds and the peak heights in the ’’lone-pair’’ regions around the oxygen atoms.