bluuebunny/crossref_metadata_2025 huggingface dataset:
Kinetic theory of thermal transpiration and mechanocaloric effect. V. Flow of polyatomic gases in a cylindrical tube with arbitrary accommodation at the surface
Authors: Lo S. S. , Loyalka S. K. , Storvick T. S.
Published: February 2003

Thermal transpiration and mechanocaloric effect in a long cylindrical tube are studied by use of the Hansen and Morse polyatomic gas model of the linearized Wang–Chang and Uhlenbeck equation and Maxwellian diffuse–specular reflection model for the gas–surface interaction. For all rarefactions, the dimensionless phenomenological coefficients for mass and energy flows due to axial pressure and temperature gradients are calculated as a function of the fraction of incident molecules that are reflected diffusely at the surface. The calculated thermal transpiration effect ratios were compared with the experimental data to obtain α for Xe, Ar, Ne, He, and H2, and CO2 in the transition flow regime, and Kr, Ar, Ne, and He in the near-continuum regime. The values of α were found to lie in the range of 0.8–1.0 and are in general agreement with the reported experimental results.
Sensitivity of biphotonic systems to light intensity fluctuations: Experimental evidence in the thermoluminescence of fluoresceine in boric acid glass
Authors: Micheau J. C. , Horsthemke W. , Lefever R.
Published: February 2003

We study the response of biphotonic systems to fluctuations of the incident light intensity. A convenient experimental model for such a system is the thermoluminescence of fluoresceine in boric acid glass. The location of the steady states of this system as a function of the incident light intensity is determined for constant and fluctuating modes of illumination. A comparison of the results for these two modes reveals that the light intensity fluctuations can significantly modify the curve of the steady states. For low values of the average intensity the efficiency of the photoionization is lowered, whereas for high values of the average light intensity the efficiency is enhanced.
The influence of molecular motions on the stabilities and shapes of solid particles
Authors: Searcy Alan W.
Published: February 2003

A conjecture of Gibbs, that the relative weakness of bonds at crystal edges and corners would cause rounding which is not predicted by his thermodynamic expression for stable crystal shapes, is evaluated in two ways. First, the vacancy distribution necessary for dynamic equilibrium between adjacent crystal rows or layers of different bond energies is derived. Then it is shown that minimization of the total free energy of two parts of a crystal with respect to their total vacancy concentration leads to the same vacancy distribution. Generalization of these results shows that the stabilities of particles which have a fixed number of sites are greatest when vacancies are so distributed as to equalize molecular fluxes between every pair of regions of different bonding energies. Rounding will occur when the mole fractions of vacancies required at corners and edges to maintain the molecular flux balance approach unity.
Accurate a
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o calculations of radiative transition probabilities between the A 3Σ+u, B 3Πg, W 3Δu, B′ 3Σ−u, and C 3Πu states of N2
Authors: Werner Hans-Joachim , Kalcher Josef , Reinsch Ernst-Albrecht
Published: February 2003

Potential energy and electronic transition moment functions for the lowest five triplet states of N2 have been calculated from highly correlated multiconfiguration-reference CI wave functions. From the calculated transition moments and RKR potential energy functions, radiative transition probabilities and lifetimes have been evaluated which are believed to be accurate within 15%. The theoretical lifetime of 36.7 ns of the C 3Πu state is in close agreement with the most recent experimental values. The calculated lifetimes of the B 3Πg state decrease from 13.4 μs for v′=0 to 4.82 μs for v′=12. The values for v′=5 to v′=12 are in good agreement with recent LIF measurements. The lifetimes for v′=0 to v′=4, however, are considerably larger than most previously measured values. The fact that the calculated values are very stable with respect to improvements of the electronic wave functions leads us to conclude that the experimental values are too low. Empirical transition moment functions for the B–A system are apparently too steep, and the moments at short distances much too large. Previous estimates of the lifetimes of the W 3Δu vibrational levels are three times smaller than our calculated values, whereas estimates for the B 3Σ−u state agree reasonably well with the theoretical results.
Electron spin echo modulation studies of the structure of solvated electrons in ethylene glycol glass
Authors: Narayana M. , Kevan Larry , Samskog P. O. , Lund A. , Kispert L. D.
Published: February 2003

Electron spin echo deuterium modulation patterns have been analyzed for radiation-produced solvated electrons in the specifically deuterated ethylene glycol glasses: DOCH2CH2OD and HOCD2CD2OH. The analyses suggest that both the hydroxyl deuterons and the alkyl deuterons fall in two nonequivalent groups in the first solvation shell of the electron. A possible structural model is discussed.
Picosecond excitation and selective intramolecular rates in supersonic molecular beams. II. Intramolecular quantum beats and IVR
Authors: Lambert W. R. , Felker P. M. , Zewail A. H.
Published: February 2003

An account of observations pertinent to quantum beat-modulated fluorescence decays of jet-cooled anthracene excited to S1+1380 cm−1 is given. Based on both spectral and temporal characteristics of the beats, combined with theoretical expectations of such characteristics, it is shown that the interference phenomenon arises from the coupling of zero-order vibrational levels within the S1 manifold. (Consistent with this interpretation is the observed absence of a magnetic field effect on the beats.) As such, it is shown to be a manifestation of restricted IVR. The results of measurements of rotational band contours and the effect of carrier gas on beat parameters are used to assess the role of rotations in the coupling between vibrational levels. Specific vibration–vibration coupling schemes are briefly discussed within the context of the observations.
Picosecond excitation and selective intramolecular rates in supersonic molecular beams. III. Photochemistry and rates of a charge transfer reaction
Authors: Syage J. A. , Felker P. M. , Zewail A. H.
Published: February 2003

The picosecond state-selective dynamics and photochemistry of the molecule A–(CH2)3–φ, where A and φ are aromatic chromophores, was studied under collision-free conditions in a supersonic beam. Time-resolved fluorescence measurements of the reactant and the charge transfer (exciplex) product were undertaken as a function of specific vibrational energy above the zero point level of S1. From these studies along with an analysis of the excitation spectra, dispersed flourescence, and quantum yields, the following results and conclusions were reached: (i) IVR is much faster than reaction at all excess energies studied. (ii) The energy threshold for product formation is E0≂900 cm−1 (2.6 kcal/mol). The analysis of the rates using an effective temperature model gives a frequency factor of A0≊1.2×1010 s−1. Four torsions were identified as critical to the reaction dynamics which were modeled according to a multidimensional reaction coordinate using an RRKM scheme. (iii) The thermodynamics of the isolated charge transfer product indicates strong stabilization ΔH=−9.2 kcal/mol and extensive charge transfer, the static dipole moment is 13 D, and the charge transfer contribution to the total electronic wave function ‖c2‖2 is 0.86. (iv) A comparison of the present work to solution phase studies of A–(CH2)3–φ indicates similar static properties but different dynamics. The calculated thermal (room temperature) reaction time for exciplex formation in the vapor (540 ps) was compared to the shortest observed value in solution (1.4 ns) to assess the role of the solvent on the chain motions which lead to product formation.
Comment on vibrational energy redistribution in the isolated dimethyltetrazine dimer
Authors: Smith Duane D. , Lorincz Andras , Siemion Jeanne , Rice Stuart A.
Published: February 2003

Vibrational energy redistribution from the 6a1 intramolecular ring mode to the intermolecular van der Waals modes in the dimethyltetrazine dimer was measured directly using time resolved spectroscopy. The relaxation time is 35±10 ps, in fairly good agreement with the 66±22 ps obtained from line broadening data.
Dicke narrowing and collisional broadening of spectral lines in dilute molecular gases
Authors: Corey Gregory C. , McCourt Frederick R.
Published: February 2003

A unified description of pressure broadening and Dicke or diffusional narrowing of spectral lines is presented from the point of view of a quantum mechanical kinetic theory. The present description of Dicke narrowing includes the correlation between collision-induced changes in the internal energy levels and the molecular velocity. The spectral line shape function is expressed in terms of relaxation coefficients which are directly proportional to generalized collision cross sections obtained from a linearized kinetic collision (super)operator. Liouville space techniques are utilized to obtain exact expressions for the collision cross sections in terms of S-matrix elements in the total-J representation. These generalized cross sections are then simplified through the introduction of the coupled states and the infinite-order sudden approximations. The description of the generalized cross sections is valid for any linear molecule in a multiplet-Σ electronic state, infinitely dilute in a bath of structureless perturbers. Using the Hund’s case (b) coupling scheme for the molecular wave function, the equivalence of the scattering dynamics for paramagnetic and diamagnetic molecules is emphasized. All dependence of the generalized collision cross sections on the nonzero electronic spin of open shell molecules is contained inWigner 6-j symbols, which multiply spin-independent S-matrix elements. Explicit expressions for a diamagnetic molecule in a 1Σ electronic state are also presented. Connections between the present theory and earlier theoretical treatments of pressure broadening are made.
Semiclassical vibrational eigenvalues of triatomic molecules: Application of the FFT method to SO2, H2O, H+3, and CO2
Authors: Eaker Charles W. , Schatz George C.
Published: February 2003

We have applied a recently developed fast Fourier transform method for determining vibrational action variables to the calculation of semiclassical eigenvalues for SO2, H2O, H+3, and CO2 (all for zero rotational angular momenta). The results for SO2 and H2O are essentially equivalent in accuracy to results from previous semiclassical applications. H+3 and CO2, on the other hand, have not been studied previously by semiclassical methods, in part because of the 1:1 and 2:1 resonances respectively present in these molecules. The results of the present calculations are in reasonable agreement with quantum eigenvalues which have been calculated for H+3 and CO2, with errors which are consistent with those obtained for H2O and SO2.
Calculated properties of matrix isolated muonium in solid xenon
Authors: Etters R. D. , Derbyshire M.
Published: February 2003

The properties of dilute muonium impurities substitutionally trapped in solid Xe at low temperatures are calculated. It is shown that a variational Hartree approximation in which the first 16 shells around the impurity are relaxed to equilibrium provides a good description of the system. The results indicate that muonium forms a localized defect in the limit of zero temperature and, by comparison with experimental and theoretical studies of H impurities in solid H2, muonium should remain trapped for a period approaching its lifetime at temperatures below approximately 3 K. The distortion of the lattice parameters, ground state wave function, rms fluctuations of the muonium and Xe atoms about their equilibrium lattice positions, the energy required to substitutionally trap a muonium atom, and the local mode frequency of the impurity, are also calculated.
Cu and Ag as one-valence-electron atoms: Pseudopotential CI results for CuO and AgO
Authors: Igel G. , Wedig U. , Dolg M. , Fuentealba P. , Preuss H. , Stoll H. , Frey R.
Published: February 2003

Semiempirical pseudopotentials, representing singly ionized X+ cores (X=Cu, Ag), are used to determine bond lengths, dissociation energies, and dipole moments of the 2Π ground states of CuO and AgO. In addition, X 2Π→A 2Σ+ excitation energies are given for both molecules. The influence of core polarization and valence correlation is discussed.
Stationary transport with partially reflecting boundary conditions
Authors: Cole R. G. , Keyes T. , Protopopescu V.
Published: February 2003

The stationary monoenergetic transport equation (the Lorentz model) is considered in a semiinfinite geometry with partially reflecting boundary conditions at x=0. The extrapolation length, the density, and the space-dependent diffusion coefficient are studied as functions of the accommodation coefficient α, both for constant and renormalized current. The hydrodynamical description at the level of the evolution equation is practically unaffected by α and is still valid at distances of several mean free paths (mfp) from the boundary. Yet, when consistently applying the boundary condition, the boundary layer has to be considered in terms of the Milne extrapolation length, which varies like (1−α)−1. The case α=1 is a singular limit in the sense that the boundary layer solution disappears completely.
Inclusion of frequency shifts in the treatment of N-photon vibronic processes involving uncoupled harmonic surfaces: A simple and exact prescription
Authors: Coalson Rob D.
Published: February 2003

An exact prescription is given for averaging an n propagator string involving single mode harmonic oscillator Hamiltonians Ĥ1,Ĥ2,...,Ĥn over a Boltzmann distribution of the eigenstates of Ĥ1. Such averages govern finite temperature multiphoton vibronic spectra, and we calculate the averages relevant to (one photon) electronic absorption and (two photon) Raman scattering in the case where only two electronic surfaces are involved. Our solution is compact and computationally mechanizable. It is valid for arbitrary upper and lower surface frequencies ωu,ω1, and in the ωu=ω1 (‘‘displaced oscillator’’) limit agrees with previous solutions of this special case. An extension to systems moving according to arbitrary quadratic potentials is sketched. A sum rule connecting electronic absorption and Raman spectra of finite temperature samples is given.
Diffusion with back reaction
Authors: Agmon Noam
Published: February 2003

The diffusion equation for a constant and a linear potential is solved with boundary conditions which account for back-reaction (desorption). The solution is given in terms of Green’s function, from which expressions for the survival probability are derived. Inclusion of back reaction generally results in an ultimate survival probability of unity.
A study of the temperature dependence of a surface catalyzed and field enhanced formation of H3 and NH3 on metal surfaces
Authors: Ai C. F. , Tsong T. T.
Published: February 2003

In pulsed-laser field desorption of hydrogen, H+3 can be detected. In an H2–N2 mixed gas, the pulsed-laser field desorption mass spectrum contains NH+3 . The formation of H+3 and NH+3 on about 20 metal surfaces has been studied in a pulsed-laser imaging atom probe. The signal intensity and the relative abundance of these species depend on the surface temperature, the emitter material, the laser intensity, and the applied field strength. From these dependencies we conclude that the H+3 and NH+3 observed are produced by field ionization above the critical distance of field ionization of the pulsed-laser, thermally desorbed H3 and NH3 molecules. These molecules are produced by surface catalyzed chemical reactions, and exist in field adsorbed states.
Time evolution of fluorescence and strong nonadiabatic effects
Authors: Meyer H.-D. , Köppel H.
Published: February 2003

In this paper we study the spontaneous emission of systems with intersecting potential energy curves and strong nonadiabatic effects from first principles. Within the dipole approximation for the spontaneous emission process and a two-state description of the vibronic coupling system we derive expressions for the transition operator and the intensity of fluorescence which are virtually exact in the small molecule limit. The concept of a radiative damping matrix is introduced and formulated as an operator in vibrational space. Apart from energy prefactors, the probability of photon emission is found to be governed by the probability of occupying the upper of the vibronically interacting electronic states. It follows that the usual autocorrelation function is no longer an appropriate measure of the fluorescence in systems with strong nonadiabatic effects. As an application we study the Ã→X̃ internal conversion in C2H+4 for broadband excitation and find an ultrafast nonradiative decay (decay time ≊3 fs).
Three-body ion–molecule association rate coefficients as a function of temperature and cluster size: NO−3(HNO3)n+HCl→MNO−3 (HNO3)n(HCl)
Authors: Viggiano A. A.
Published: February 2003

The rate coefficients for the three-body association of NO−3(HNO3)n to HCl have been measured for n=0, 1, and 2. The rates were measured from room temperature to 153 K. The data were taken in three buffers: helium, nitrogen, and hydrogen. The magnitude of the temperature coefficients in the standard power law fits was found to increase with increasing cluster size. Significant variations were found in the different buffers. The association rate coefficient of NO−3(HNO3)(HCl) to HC1 was also measured at 153 K and found to have the same value as that for NO−3(HNO3)2 clustering to HCl. Implications for stratospheric negative ion chemistry are discussed.
Water–water interaction potential: An approximation of the electron correlation contribution by a functional of the SCF density matrix
Authors: Carravetta V. , Clementi E.
Published: February 2003

The approximate expression for the correlation energy suggested by Colle and Salvetti (1975), but modified with a different set of parameters obtained by a more accurate description of the Coulomb hole, has been applied to approximate the electron correlation in the water two-body interaction potential. The results obtained are in good agreement with previous configuration interaction and perturbative computations, indicating that the approach can be applied successfully to calculate the correlation contribution in interaction potentials between large molecules with a reasonable computational effort. With the new two-body potential, a Monte Carlo simulation of liquid water has been carried out and a comparison of computed and experimental results is presented for the internal energy, the oxygen–oxygen radial distribution function, and the neutron scattering structure function.
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o study of FH–PH3 and ClH–PH3 including the effects of electron correlation
Authors: Latajka Z. , Scheiner Steve
Published: February 2003

A b initio calculations are carried out for FH–PH3 and ClH–PH3 using a basis set including two sets of polarization functions. Electron correlation is incorporated via Mo/ller–Plesset perturbation theory to second and (in part) to third orders. The basis set is tested and found to produce satisfactory treatments of subsystem properties including geometries and dipole moments as well as the proton affinity and inversion barrier of PH3. Electron correlation is observed to markedly enhance the interaction between PH3 and the hydrogen halides. Its contribution to the complexation energy is 30% for FH–PH3 and 50% for ClH–PH3. Moreover, the equilibrium geometries of the complexes at correlated levels are quite different than SCF structures.
Time dependence of the fourth-order correlation function in colloidal and polymer solutions
Authors: Tough R. J. A. , Pusey P. N. , Ackerson B. J.
Published: February 2003

We discuss the time dependence of the fourth-order correlation function F(2)(K,Q,τ) which can be measured by ‘‘new’’ dynamic light scattering methods involving the cross correlation of light intensity signals received at two spatially separated detectors which both observe the same small number of correlated regions in a colloidal or polymer solution. F(2)(K,Q,τ) can be regarded as a four-particle generalization of the more familiar two-particle intermediate scattering function F (1)(K,τ).
Angular parametrizations in the hyperspherical description of elementary chemical reactions
Authors: Grossi Gaia
Published: February 2003

The two basic angular parametrizations in the hyperspherical coordinate approach to elementary chemical reactions as three-body problems are useful to establish relationships between the descriptions in terms of the configuration of the reactants and that of the products. These connections follow from our previous SO(4) analysis; their usefulness for approximations of the infinite order sudden type is described.
A temperature independent spherical potential which reproduces exactly the second virial coefficient of nonpolar molecules
Authors: Shaw M. S. , Johnson J. D.
Published: February 2003

An expression is derived for a temperature independent spherical potential which reproduces exactly the second virial coefficient for nonpolar molecules given the full anisotropic potential. Comparison is made with the median average over angles and the arithmetic average.
‘‘Ordered’’ structure in dilute solutions of sodium polystyrenesulfonates as studied by small-angle x-ray scattering
Authors: Ise Norio , Okubo Tsuneo , Kunugi Shigeru , Matsuoka Hideki , Yamamoto K. , Ishii Yasuo
Published: February 2003

The small angle x-ray scattering measurements were performed for aqueous solutions of sodium polystyrenesulfonates having relatively narrow molecular weight distributions. As was observed for other synthetic macroions, polynucleotide and proteins, a single, broad peak was observed. The scattering vector at the peak position (Sm) was shifted toward larger values with increasing polymer concentration and toward lower values with increasing salt concentration, which confirmed earlier observations with polyacrylate and poly-L-lysine. The molecular weight dependence of the scattering behavior, which was earlier observed, was confirmed to be true for samples with Mw of 74 000, 18 000, and 4600. The mixture of two fractions with different Mw’s gave a scattering curve which was again different from the composite curve obtained with the parent curves before mixing. A similar situation was observed for the mixture of polystyrenesulfonate and polyacrylate. Thus, it was concluded that the observed single peak indicates the presence of an intermolecular ordering, not an intramolecular ordering. The intermacroion distance (2Dexp) was thus calculated by using the Bragg equation. 2Dexp decreased with increasing polymer concentration and increased (not decreased) with increasing concentration of added salt and Mw. The 2Dexp values thus obtained were smaller beyond the experimental error than 2D0, a theoretical distance calculated from the concentration by assuming the uniform distribution of the macroions throughout the solution and the 2D0/2Dexp value amounted to 3.5 for high molecular weight samples. This fact indicates the presence of an intermacroion attractive interaction. When two fractions with different Mw’s were compared at a given number concentration of macroions, the 2Dexp value for the sample of a larger Mw was smaller than that for the fraction of a smaller Mw. This implies that the attraction must be intensified, though unexpectedly, with increasing valency of the macroions, confirming our earlier interpretation that the attraction could be brought into existence through the intermediary of counterions present between macroions. The larger the number of the counterions, the stronger the intermacroion attraction. With increasing temperature, the peak was slightly lowered and the 2Dexp became smaller, though very slightly. Rough agreements between x-ray and neutron scattering data were noted. On the basis of the direct microscopic observation of a similar ordering of polymer latex particles, it was suggested that the macroions in the ordered regions were subject to violent thermal motion, and that paracrystalline distortion of the ordered structures allowed us to observe only a single broad peak.
CH2OH+O2 reaction rate constant measured by detecting HO2 from photofragment emission
Authors: Wang W. C. , Suto Masako , Lee L. C.
Published: February 2003

The HO2 radical was produced in a flow tube by the Cl+CH3OH→HCl+CH2OH and CH2OH+O2→HO2+CH2O reactions. HO2 was detected by the OH(A 2∑+→X 2Π) photofragment emission produced by photodissociative excitation of HO2 in the vacuum ultraviolet region. Using this detection method, the rate constant for the CH2OH and O2 reaction was measured to be (1.4±0.4)×10−12 cm3/s.
A lattice model for the electrical double layer using finite-length dipoles
Authors: Kenkel Stephen W. , Macdonald J. Ross
Published: February 2003

A lattice model of the metal electrode–electrolyte interface is presented which partitions the region into a number of planar layers. In each layer statistical competition occurs between ions and solvent molecules, represented as point charges and finite length dipoles, respectively. Differential capacitance and other results are compared with those of earlier models and with experiments. Agreement with experiments requires explicit inclusion of an inner layer with solvent properties which differ from the remainder of the system. The inner layer may consist of finite-length dipoles alone, or of both dipoles and charges. The use of finite length dipoles drastically changes the high field saturation properties of the layers, as compared with models involving infinitesimal (point) dipoles. Very little dielectric saturation occurs at physically realizable fields.
Temperature dependent averages of statistical mechanics
Authors: Kostin M. D.
Published: February 2003

An equation is derived which relates the change in statistical mechanical averages with temperature to other averages. It is shown that in a special case, the equation reduces to the virial theorem of statistical mechanics. The general equation is used to derive equations relating heat capacities to averages involving the potential energy. As a further application, equations are derived giving the way in which the moments 〈xn〉 change with temperature.
Solubility of xenon in liquid n-alkanols: Thermodynamic functions in simple polar liquids
Authors: Pollack Gerald L. , Himm Jeffrey F. , Enyeart John J.
Published: February 2003

Measurements of the Ostwald solubility L(T) as a function of temperature in the range 10.0–50.0 °C are reported for 133Xe gas in 13 liquid normal alcohols: methanol through n-dodecanol, and n-tetradecanol. From the data for each solute–solvent system we find the mole fraction solubility x2(T), and calculate the following thermodynamic functions of solution: chemical potential Δμ○2(T) =−RT ln x2, enthalpy ΔH̄○2, and entropy ΔS̄○2, all based on the mole fraction scale. We also calculate directly from the Ostwald solubility the corresponding quantities based on the number density scale. These quantities are the chemical potential Δμ○ρ2(T) =−RT ln L, enthalpy ΔH○ρ2, and entropy ΔS○ρ2. The solute was chosen as an inert monatomic gas and the solvents were chosen in a polar homologous series in order to construct prototype solute–solvent systems. Solubility results and the associated thermodynamic functions are analyzed and compared with simple models.
FT-IR investigation of proton transfer in irradiated ice at 90 K in the absence of mobile bjerrum defects
Authors: Devlin J. Paul , Richardson Hugh H.
Published: February 2003

Samples of cubic and amorphous ice containing isolated D2O molecules have been prepared using established methods. Radiolysis with 1.7 MeV electrons at 90 K has been observed to convert the D2O in H2O cubic ice to primarily coupled HOD pairs (HOD)2 rather than isolated HOD. Based on the assumption that radiolysis produces an abundance of mobile protons but relatively few mobile orientational defects this result was predictable, i.e., the motion of protons through D2O sites converts the D2O to coupled (HOD)2, while extensive conversion of (HOD)2 units to isolated HOD requires the passage of orientational defects. Thus, the vanishingly small amount of isolated HOD formed during irradiation has been interpreted as evidence that no significant number of mobile orientational defects are produced by radiolysis. The ability to observe (HOD)2 as a dominant spectroscopic species has added to the credibility of an earlier thermal study of the kinetics of proton motion in ice that was dependent on the quantitative determination of the (HOD)2 concentration as a function of reaction time. Furthermore, the observation that protons produced radiolytically fall rapidly into shallow traps, from which they are released by modest warming (to ∼130 K), supports a suggestion by Warman that, for a brief period following a radiolysis pulse, mobile protons are in pseudoequilibrium with protons immobilized by association with L defects. The D2O isolated in amorphous ice was similarly converted to HOD by mobile protons produced during radiolysis. However, the broader infrared bands of the amorphous samples have prevented assigning the HOD to a particular form [(HOD)2 or isolated HOD]. The interesting result was the indication that the proton mobility in amorphous ice is comparable to that for cubic ice. By contrast, radiolysis of ND3 isolated in crystalline cubic NH3 caused only very limited formation of mixed isotopically substituted ammonia molecules, a clear indication that either few protons (or NH+4) are formed or the protons produced lack mobility. The latter situation appears to be most likely.
Fluid adsorption up to the critical point. Experimental study of a wetting fluid/solid interface
Authors: Findenegg G. H. , Löring R.
Published: February 2003

We have measured multilayer adsorption isotherms of propane on graphitized carbon black over a wide temperature range, corresponding to reduced temperatures T/Tc of the fluid from 0.7 to 1.004 and reduced densities ρ/ρc up to 1.4. Experimental isotherms of the surface excess concentration Γgs are analyzed in terms of the Frenkel–Halsey–Hill (FHH) model. The exponent n is somewhat less than 3 (2.55±0.30) and the amplitude parameter Δε/kT becomes nearly independent of temperature, up to T/Tc=0.98, when the simple one-step density profile of the original FHH model is replaced by a two-step profile, to account for the compression of the layer next to the solid substrate. Evidence for a compression of the liquid boundary layer comes from measurements of the surface excess concentration Γls at the liquid/substrate interface. Along the liquid–vapor coexistence curve, Γls0 diverges as T approaches Tc, in qualitative agreement with scaling law theory. The analogy of the present one-component fluid/solid–substrate interface near the critical point of the fluid, with a two-component liquid/vapor interface near the critical solution point of the liquid mixture is discussed.
A one-dimensional model for phonon-induced desorption. II. Numerical analysis of the desorption of noble gas atoms (argon, krypton, and xenon) from tungsten and carbon monoxide from copper
Authors: Hood Eric , Jedrzejek Czeslaw , Freed Karl F. , Metiu Horia
Published: February 2003

We use a quantum one-dimensional model to analyze numerically the kinetics of thermal desorption. The model consists of a master equation in which the adsorbed atom exchanges energy with the lattice and undergoes transitions between the states binding it to the surface. Desorption occurs when a transition promotes the atom into a continuum state. We analyze in detail both the properties of the transition rates from one state to another and the rate of desorption. The calculations use parameters which simulate noble gas desorption from tungsten and CO desorption from copper.
Vibrational predissociation, tunneling, and rotational saturation in the HF and DF dimers
Authors: Pine A. S. , Lafferty W. J. , Howard B. J.
Published: February 2003

The high-resolution spectra of the intramolecular stretching bands of the HF and DF dimers have been recorded with a tunable difference-frequency laser. These measurements yield considerable information about the dynamics of hydrogen bonding in these complexes. Vibrational predissociation is observed as a non-pressure-dependent excess linewidth for the ‘‘bound-H’’stretching band of the HF dimer, but no excess linewidth is observed for the ‘‘free-H’’ stretching band of the HF dimer or for either band of the DF dimer. An unusually large vibrational dependence to the interconversion tunneling frequency is observed for both species, with about a factor of three reduction from the ground state splitting upon excitation of any of the intramolecular stretches. The K subband origins obtained from the A/B hybrid free-H stretching band of the HF dimer exhibit an irregular pattern indicating anomalous centrifugal distortion effects suggestive of rotational saturation of the angular orientation of the hydrogen bond.
Partial wave analysis of the momentum density
Authors: Thakkar Ajit J. , Simas Alfredo M. , Smith Vedene H.
Published: February 2003

It is shown that a partial wave analysis provides a useful partitioning of the momentum density—especially for linear molecules. Convergence studies on 21 linear molecules indicate that such an expansion converges particularly rapidly for small momentum p and less rapidly for large momentum. If attention is restricted to the valence region (small p) alone, then all qualitative features of the anisotropic momentum density can be described by merely four partial waves. Empirical rules are given which enable one to predict the qualitative small p behavior of the contributions of various types of canonical molecular orbitals to these chemically dominant partial waves.
Triplet exciton transport in isotopic mixed naphthalene crystals. I. Kinetic analysis of trapping and fusion
Authors: Gentry Stuart T. , Kopelman R.
Published: February 2003

Time-resolved triplet exciton transport in binary mixed crystals has been studied for C10H8 in C10D8 (20%–100%) at liquid helium temperatures. The delayed fluorescence decays are exponential and range from milliseconds to nanoseconds as Cg goes from 0.2 to 1.0. The analysis is based on the presence of a BMN supertrap in large amounts (10−3) and on the fact that supertrapping of the guest excitons dominates the kinetics while heterofusion (triplet guest-triplet supertrap annihilation) dominates over the homofusion (guest–guest annihilation) and was used to monitor the kinetics. We also investigated in detail photodetrapping and the relative efficiencies of the various fusion channels. An analysis of the transport data is given in paper II.
Dissociation of a diatomic molecule to atomic fine structure states: Electronically nonadiabatic effects upon resonant two-photon dissociation
Authors: Singer Sherwin J. , Freed Karl F. , Band Yehuda B.
Published: February 2003

We present the theory for two photon photodissociation of diatomic molecules into atomic fragments with nonvanishing angular momentum. Hund′s cases (a) and b) and any coupling scheme between them are explicitly treated. Various axial recoil limit approximations are derived in the limit for large fragment translational energy and are shown to simplify the description of the photodissociation. Expressions are derived for integral and differential photodissociation cross sections, for angular distributions of the fluorescence from excited atomic fragments, and for combined differential cross section and fluorescence photon angular distributions following the two photon dissociation. Anisotropy parameters and branching ratios in the J-independent axial recoil limit depend only on the symmetry of the molecular electronic states involved in the dissociation and on the atomic term limit. We tabulate these quantities for some common molecular electronic state symmetries and atomic term limits.
The structure of electronic excited states in t
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s-stilbene: Picosecond transient Stokes and anti-Stokes Raman spectra
Authors: Gustafson Terry L. , Roberts Dale M. , Chernoff Donald A.
Published: February 2003

We measured picosecond transient Stokes and anti-Stokes Raman spectra of S1 trans-stilbene in hexane. We observed 18 vibrational modes in the Stokes spectra and two modes in the low wave number anti-Stokes spectra. The band positions and band shapes of the transient Raman spectra suggest a distribution of molecular conformers in S1 having different single and double bond strengths. The band intensities suggest a redistribution of electron density in Sn that is concentrated in the olefinic region and in the ortho and meta region of the phenyl rings.
Kinetic isotope effects in hydrogen absorption by α-zirconium
Authors: Naito Shizuo , Hashino Tomoyasu , Kawai Takeshi
Published: February 2003

The absorption rates for gaseous hydrogen and deuterium and their solubility in polycrystalline α-zirconium metals have been investigated over the temperature range of 773–1073 K. A model is considered in which the absorption process is composed of three successive steps: the dissociative adsorption, the jump of the adsorbed atom into the interstitial site, and its diffusion into the metal bulk. The expressions for the isotope effects corresponding to these steps and to the solubility are formulated in terms of the absolute rate theory. The expressions show a dependence of the isotope effects on each other. The experimental results are adequately described by the expressions if the quantum corrections for the vibrations of hydrogen and deuterium in the gas phase, in the metal bulk, and on the metal surface are taken into account. The adsorption, as well as the diffusion, is slower and the solubility is smaller for deuterium than for hydrogen under the imposed experimental conditions in this investigation.
Laser-induced fluorescence measurement of nascent vibrational and rotational product state distributions in the charge transfer of Ar++N2→Ar+N+2 (v=0,1) at 0.2 eV
Authors: Hüwel Lutz , Guyer Dean R. , Lin Guang-Hai , Leone Stephen R.
Published: February 2003

A novel experimental technique couples a flowing afterglow ion source with a supersonic nozzle expansion in order to deliver high densities of relatively low kinetic energy ions into a low pressure chamber. The technique is used to study the charge transfer reaction Ar+(2P3/2)+N2(v=0) →Ar(1S0)+N+2 (v+0,1;N)+ΔE=0.18 eV under single collision conditions at 0.24 eV c.m. Nascent rotational and vibrational state distributions are obtained by the method of saturated laser-induced fluorescence probing. It is found that a substantial fraction of the available energy is partitioned into internal excitation of the N+2 product molecule. The higher of the two energetically accessible vibrational levels N+2 (v=1) is populated in 89±9% of the charge transfer collisions. The nascent rotational distribution in N+2 (v=1) is characterized by a Boltzmann distribution with Trot=700±50 K. The results are compared with a number of recent state-selected experiments on charge exchange in ArN+2 , ArH+2 , and NCO+ systems. It is suggested the experimental findings are best explained in terms of the detailed locations of potential surface crossing seams, rather than by the widely used energy resonance or diatomic molecule, Franck–Condon ionization models.
Molecular x-ray spectra: S-Kβ emission and K absorption spectra of SCO and CS2
Authors: Perera Rupert C. C. , LaVilla Robert E.
Published: February 2003

The sulfur Kβ emission in fluorescence and K absorption of SCO and CS2 in gas/vapor phase were measured with a double crystal spectrometer. The sulfur Kβ emission spectra were compared with the complimentary x-ray spectral data and with the MNDO and ab initio (STO-3G) MO calculations and with previous larger basis set ab initio calculations. A comparison with the x-ray spectra from CO2 was included for completeness. In addition the S-1s binding energy was estimated for SCO and CS2 as 2478.7 and 2478.1 eV, respectively. Using the MO calculations as a guide, a tentative asssignment of the prominent features in the absorption spectra was made and compared with the S-L2,3 absorption and energy loss spectra.
Dispersion of the Verdet constant of water: A band model interpretation
Authors: Grevendonk W. , Dauwen J. , Van den Keybus P. , Vanhuyse B.
Published: February 2003

Faraday rotation in water was measured at 30 wavelengths between 340 and 740 nm. The results are analyzed with dispersion equations, based on band models in solids. An energy gap of 8.4 eV is determined; this value is compared with data of refractive index, optical absorption, and dielectric function.
Constrained energy functionals obtained as solutions to a partial differential equation
Authors: Westhaus Paul
Published: February 2003

The constrained energy functional, defined by E0(a) =minSa{a〈Ψ‖Ĥ0‖Ψ〉a}, ai=a〈Ψ‖Âi‖Ψ〉a, i=1, ..., M, ‖Ψ〉a∈Sa, with a={ai} a preassigned set of expectation values generated by any state vector ‖Ψ〉a in the class Sa, is shown to satisfy a first order, but nonlinear, partial differential equation. The equation results by replacing the Lagrange multipliers introduced in an earlier paper by the corresponding partial derivatives of the to-be-found energy functional with respect to the constraints. Two alternative expressions for E0(a), one focusing upon the ground state eigenvalue of a modified Hamiltonian operator and the other upon the corresponding eigenvector, lead via standard Rayleigh–Schrödinger perturbation theory to two approximate partial differential equations for the energy functional. Both equations together with the imposed subsidiary conditions lead in turn to the same approximate E0(a). An example based on the linear harmonic oscillator illustrates the concepts presented in the formalism.
Stability and other properties of Li13 clusters. An example of specific characteristics of clusters
Authors: Pacchioni Gianfranco , Koutecký Jaroslav
Published: February 2003

The icosahedron Li13 cluster has been found to be more stable than both the cubo-octahedron Li13 and a Li13 cluster section of the hcp packing employing the all electron MRD CI procedure. The ground states of these clusters are sextuplets with open shell electrons located entirely at the peripheral atoms. The contribution from the p orbitals to the one-electron density at the central atom is very large. The strength of the two-center bonds between central and peripheral atoms in icosahedron explains the larger stability of this cluster with respect to the fcc cubo-octahedron. The investigation of small Lin clusters shows that the most stable clusters for (n≤5) have planar geometries. It indicates the special stability of cluster geometries with fivefold rotational symmetry axis for Li6, Li7, and Li13 and gives further support to the conjecture that the clusters Li55 (Ih) are the seeds of the closed packed crystals of alkali metals.
Energetics of the protonation of CO: Implications for the observation of HOC+ in dense interstellar clouds
Authors: Dixon David A. , Komornicki Andrew , Kraemer W. P.
Published: February 2003

In this study, we report a series of large scale calculations for several studies on the H3CO+ potential energy hypersurface. This study was motivated by the recent observation of an abundance ratio of HCO+/HOC+ of at least 330 if the species HOC+ is indeed present in dense interstellar clouds. The calculations have focused on an accurate determination of the absolute proton affinities for the species H2, HCHO, and most importantly of CO to form both HCO+ and HOC+. These calcualtions have been performed using large polarized basis sets and include the effects of electron correlation at the CI(SDQ) level. Furthermore, the effects of zero point energy corrections have been incorporated by use of the calculated harmonic frequencies. This study has shown that the proton affinities of the species H2 and CO to form HOC+ are within 1 kcal of each other. The calculations demonstrate that there is no thermodynamic driving force to form HOC+ in collisions of H+3 with CO and that the formation of HCO+ in such collision is very exoergic. We have suggested a plausible mechanism to explain the differences observed between the laboratory and the interstellar medium. Finally, we have investigated the possibility of the formation of a molecular complex between H2 and HCO+/HOC+ as a sink of much of the HOC+ which may be formed.
Structure and hydrogen ordering in ices VI, VII, and VIII by neutron powder diffraction
Authors: Kuhs W. F. , Finney J. L. , Vettier C. , Bliss D. V.
Published: February 2003

The structures of deuterated ices VI, VII, and VIII have been studied under their conditions of stability by neutron powder diffraction. The mode of ordering of (tetragonal) ice VIII is clearly established, and no evidence is found of partial ordering as the temperature is raised. Ice VII is accurately cubic with D2O molecules disordered around their center of mass; there is no evidence of partial ordering at any temperature. The water molecule geometry is normal in both phases, and the hydrogen-bonded first neighbor in ice VIII is confirmed to be more distant than the first nonbonded neighbor. The transition temperature between the two phases occurs at 263±2 K, some 11 K lower than expected. Hydrogen bond lengths in both phases are equal at the transition. Although the ice VI data are not as good, we can see no evidence of the antiferroelectric ordering proposed by Kamb from work on recovered samples. Our results are consistent with thermodynamic measurements indicating disorder in ice VI at 193 K. We conclude that the orientational ordering behavior of these high pressure ices is simpler than thought previously. For our ice VI results to be consistent with those of Kamb, his antiferroelectric ordering must be an artifact of the recovery process, thus throwing doubt on the validity of structural ordering data obtained on samples recovered to ambient pressure.
Residence times in diffusion processes
Authors: Agmon Noam
Published: February 2003

We generalize the notion of ‘‘mean survival time’’ in diffusion processes, which characterizes the disappearance of probability density from the whole coordinate space into boundaries or sinks, by introducing ‘‘mean residence time,’’ characterizing the time spent in a portion of the coordinate space. In particular, the time integral of the transition probability (Green’s function) is the average residence time density at a point. It is a function of both initial and final variables and fulfills two differential equations, one for each variable. We demonstrate the solution of these equations and compare it to the time integral of the direct solution of the diffusion equation. We present the general solution for spherically symmetric diffusion, and compare it to results in the literature.