bluuebunny/crossref_metadata_2025 huggingface dataset:
Low-energy collision-induced energy transfer in the HeI*2 system
Authors: Kim Young Sik , Hutchinson Michael , George Thomas F.
Published: July 2002

An investigation is made of vibrational energy transfer in low-energy collisions between He and vibrationally excited I2. Taking into account the open-channel as well as closed-channel resonances and the coupling between the rotational and the vibrational motions, it is shown that the dynamics is not sensitive to the resonance position, and that the enhancement of the vibrational relaxation cross section at low collision energies is due to resonances (both closed-channel and open-channel), the anharmonicity of the I2 molecule, and the long-range attractive force of the intermolecular potential.
Solvation forces and phase separation of water in a thin film of nonpolar liquid between mica surfaces
Authors: Christenson Hugo K. , Blom Christina E.
Published: July 2002

We present direct measurements of the force between two mica surfaces across a thin film of nonpolar liquid (octamethylcyclotetrasiloxane) containing water at activities in the range 0–0.75. The oscillatory solvation force at zero water content has been measured down to molecular contact for the first time. As the water activity is increased there is a reduction in the amplitude of the solvation force at larger separations, but progressively more of the inner oscillations become experimentally inaccessible as the interaction in this regime is replaced by a strong attraction. The adhesion at contact increases dramatically with water activity. We suggest that the results may be explained in terms of two related but separable effects. With increasing activity the increase in surface excess of water disrupts the layering of the molecules of the liquid between the surfaces which smears out the solvation force at long range. Phase separation of water occurs at progressively larger separations, at least for activities of 0.5 and above, and this gives rise to the sudden attraction that replaces the solvation force. The very large adhesion reflects the presence of condensed water around the surfaces at contact.
Alkali promoted CO bond weakening on aluminum: A comparison with transition metal surfaces
Authors: Paul J. , Hoffmann F. M.
Published: July 2002

Data on the adsorption and decomposition of carbon monoxide on alkali promoted Al(100) are presented. CO dissociates on the potassium or sodium promoted surface and aluminum oxide and aluminum carbide form after annealing to 700 K. At intermediate temperatures EELS show alkali–CO complexes with vibrational frequencies ranging from 1060 to 2060 cm−1. A band at 1750 cm−1 was assigned to CO molecules coordinated to bulk potassium. CO vibrational spectra as well as work function measurements reveal an altered alkali dispersion as a function of preannealing temperature. Comparisons are made between the surfaces of aluminum and transition metals with respect to (i) alkali adsorption, (ii) hybridization between metal d states and CO π orbitals, (iii) the magnitude of unscreened (long-range) perturbations, and finally (iv) the energetics of carbide and oxide formation. Potassium but not sodium atoms bind strongly to aluminum carbide (Td>700 K). We suggest that potassium is rare among alkali metals not in its ability to promote CO dissociation but in preventing a downshift of the C 2pz orbital and thus carbide to graphite transformation.
Distribution functions of a one-dimensional phase equilibrium with an interface
Authors: Robert M. , Viswanathan R.
Published: July 2002

We calculate the exact expression of the pair distribution function in the interface of the one-dimensional lattice gas, in which the separation of phases is achieved by imposing an external field which changes sign at the middle of the lattice. The exact distribution functions of arbitrary order can then be obtained from an earlier theorem of Percus. The pair distribution function is shown to decay exponentially as a function of interparticle distance, with a decay length equal to the correlation length of the spontaneous density fluctuations in either bulk liquid or gas phases. This exact result enables us to test approximate theories of the pair distribution function of nonuniform fluids. The exact expression of the direct correlation function is obtained from recent studies of Percus, and of Robert and Widom, and it also is used to test approximate theories of the direct correlation function of nonuniform fluids. Extremely severe disagreement is found between exact results and all commonly used approximations based on local thermodynamics à la van der Waals, while excellent, nearly perfect agreement is found for the dynamic capillary wave-like approximation of Kalos, Percus, and Rao.
Time-domain formulation of photofragmentation involving nonradiatively coupled excited states, and its implementation via wave packet perturbation theory
Authors: Coalson Rob D.
Published: July 2002

A time-domain formulation of photodissociation theory is given for the case that two or more excited states are involved due to nonradiative coupling. We show how to compute the fragment distributions onto the various potential surfaces involved. In addition, we give an alternative prescription for calculating the total absorption cross section onto each surface (‘‘branching ratio’’) as a function of irradiation frequency. The alternative technique avoids the necessity of explicitly summing over all energetically allowed fragment state cross sections; it is therefore expected to be simpler to implement in many-dimensional applications then the fragment summation procedure. Our formalism is successfully implemented via localized (Gaussian) wave packet perturbation theory for a model problem involving two coupled one-dimensional excited state potentials. In particular, rather strong nonradiative coupling effects are accommodated.
Effect of an external electric field on the rate of diffusion-controlled reactions
Authors: Tachiya M.
Published: July 2002

The relation between the fractal geometry of reactant trajectories and the rate of diffusion-controlled reactions is considered. A new possible mechanism for the effect of an external electric field on the rate of reactions is proposed on the basis of this consideration. The proposed mechanism predicts an increase in the rate constant with increasing electric field strength. Experiments in the literature which may be related with the proposed mechanism are surveyed. Finally directions in future extensions of the present study are suggested.
The shear melting of colloidal crystals: A long wavelength driven transition
Authors: Harrowell Peter , Fixman Marshall
Published: July 2002

A theoretical treatment of a colloidal crystal undergoing shear flow is based on the assumption of small displacements of particles from the lattice sites of a deforming crystal. Long wavelength transverse modes are amplified by the shearing solvent to produce fluctuations in adjacent particle separations which diverge with the size of the crystal L as L1/2. These fluctuations are used with the Lindemann criterion to predict a shear melting transition in which the melting point depends on the size of the crystal. The consequences of this result for recent extensions of thermodynamics to systems under shear are discussed.
Vibrational frequencies and intensities of H-bonded systems. 1:1 and 1:2 complexes of NH3 and PH3 with HF
Authors: Kurnig Ingrid J. , Szczȩśniak M. M. , Scheiner Steve
Published: July 2002

Frequencies and intensities are calculated by ab initio methods for all vibrational modes of the 1:1 H3X–HF and 1:2 H3X–HF–HF complexes (X=N,P). The HF stretching frequencies are subject to red shifts, roughly proportional to the strength of the H bond, and to manyfold increases in intensity. Although the intramolecular frequency shifts within the proton acceptors are relatively modest, the intensities of the NH3 stretches are magnified by several orders of magnitude as a result of H bonding (in contrast to PH3 which exhibits little sensitivity in this regard). The frequencies and intensities corresponding to bending of the H3X–HF H-bond rise with increasing H-bond strength while the properties of the other intermolecular modes appear somewhat anomalous at first sight. The intensity patterns are analyzed by means of atomic polar tensors which reveal that intensification of the proton donor stretch is chiefly due to increasing charge flux associated with H-bond formation. The different behavior of the N–H and P–H stretching intensities is attributed to the opposite sign of the hydrogen atomic charges in the two molecules. As a general rule, low intensities can be expected for intermolecular modes with the exception of those which involve motions of hydrogens that appreciably alter the magnitude or direction of a subunit’s dipole moment.
Dynamics of dissociative attachment reactions in electron stimulated desorption: Cl− from condensed Cl2
Authors: Azria R. , Parenteau L. , Sanche L.
Published: July 2002

Energy analysis of Cl− ions produced by dissociative attachment in electron stimulated desorption from Cl2 condensed on a platinum substrate is reported. The electron energy dependence of the Cl− signal exhibits two peaks around 2 and 5 eV which arise, respectively, from the 2Πg and 2Πu core-excited Cl−*2 resonant states. At higher Cl2 coverages, a third peak is observed around 11.5 eV. From kinetic energy distributions, it is possible to ascribe this latter peak to Cl− ions formed via the 2Πu resonance by electrons which have suffered energy losses through the excitation of low-lying electronic states of molecular chlorine. In the energy range of the 2Πu Cl−*2 resonance, we observe that multiple scattering processes are also important and that the curve representing the kinetic energy of Cl− ions formed via a single scattering process as a function of incident electron energy is a straight line with a slope 1/2. This indicates that the chlorine lattice is not involved in the dissociation dynamics.
A variable energy photoelectron study of the valence and Hg 5d levels of Hg(CH3)2
Authors: Bice J. E. , Tan K. H. , Bancroft G. M. , Yates B. W. , Tse J. S.
Published: July 2002

Using monochromatized synchrotron radiation, the gas phase photoelectron spectra of the Hg 5d and valence levels of Hg(CH3)2 have been obtained between 21 and 100 eV photon energies. Experimental valence band branching ratios have been compared with theoretical branching ratios from MS-Xα calculations. Our results strongly suggest that the ordering of valence levels is 2a″2<3a1<2e′<2e″, rather than 2a″2<3a1<2e″<2e′ from previous theoretical calculations. For the 3a′1 orbital, two shape resonances are predicted at 30 and 50 eV kinetic energy. Only the one at 30 eV is observed. Interchannel coupling with Hg 5d photoemission is important for the outermost 2a″2 and 3a′1 orbitals. Shape resonances and interchannel coupling strongly suggest that He ii:He i cross section ratios cannot generally be used to infer orbital character. The Hg 5d intensity ratio in Hg(CH3)2 differs from that of atomic Hg in the low photon energy region. This effect is probably due to the differences in the outgoing f wave potential between Hg and Hg(CH3)2.
Monte Carlo simulation of electron thermalization in gases. VII. Imaginary component of microwave conductivity of Ar
Authors: Koura Katsuhisa
Published: July 2002

A direct comparison is made between the transient imaginary component of the microwave conductivity during the electron thermalization in Ar obtained by a Monte Carlo simulation (MCS) and a microwave frequency-shift measurement. Both the MCS and experimental conductivities indicate the peak pattern due to the existence of the Ramsauer minimum, and the overall agreement between theory and experiment shows considerable improvement as compared with the previous discrepancy found in the comparison of the electron temperature (energy) degradation in rare gases with Ramsauer minima. The agreement is better for the Margenau conductivity formula than for the alternative one, contrary to the case of He. There still exists, however, some disagreement; the MCS value of the thermalization time for the conductivity to reach within 1% of the thermal value is 24 μs, while the experimental value is 16 μs for the gas temperature T=291 K, pressure p=309 Torr, and the radian microwave frequency ω/2π=9.461 GHz. Some possible causes of the disagreement are briefly discussed.
Monte Carlo simulation of electron thermalization in gases. VIII. Thermalization distance and microwave conductivity in rare gases
Authors: Koura Katsuhisa
Published: July 2002

Time evolution of the mean-square displacement and the thermalization distance Rth of subexcitation electrons in rare gases are obtained by the null-collision Monte Carlo simulation (MCS) with the mass scaling rule and compared with available experimental and theoretical results. The pressure normalized distance Rthp, especially for Ar, is one order of magnitude larger than the experimental one. Time evolution of the imaginary component of the microwave conductivities of Kr and Xe is also obtained by the MCS and compared with the experimental results of the microwave frequency-shift measurement. A serious discrepancy between theory and experiment again appears. Because Rth can be comparable with the radius of the microwave cavity, the electron cooling by the cavity wall is considered to be a possible cause of the discrepancy.
Distance dependence in photoinduced intramolecular electron transfer. Additional remarks and calculations
Authors: Larsson Sven , Volosov Andrey
Published: July 2002

Rate constants for photoinduced intramolecular electron transfer are calculated for four of the molecules studied by Hush et al. The electronic factor is obtained in quantum chemical calculations using the CNDO/S method. The results agree reasonably well with experiments for the forward reaction. Possible reasons for the disagreement for the charge recombination process are offered.
Generalized van der Waals analysis of critical drops and cavities in simple fluids
Authors: Hooper Malcolm A. , Nordholm Sture
Published: July 2002

The classical treatment of critical drops and cavities in metastable bulk fluids is reexamined and extended by the use of simple generalized van der Waals (GvdW) approximations. Step function particle density profiles are assumed but the density of the nucleating phase is allowed to vary. The effect of metastability and surface curvature on the surface tension contribution to the excess free energy of the critical drop or cavity is examined. Significant reductions are found which may play an important role in the determination of nucleation rate phenomena.
Spectroscopy and photophysics of 1Πg and 3Πg Rydberg, ion-pair states of Cl2, revealed by multiphoton ionization
Authors: Li Leping , Lipert Robert J. , Park Haiyoon , Chupka William A. , Colson Steven D.
Published: July 2002

Mass-resolved, two-photon resonant, three-photon ionization of jet-cooled Cl2 has been used to study the spectroscopy and photophysics of the lowest energy, double-well 1Πg and 3Πg states of mixed Rydberg (4sσ) and ion-pair character. Inner well levels from v′=0 (64 027 cm−1) to v′=15 (73 343 cm−1) have been observed for 1Πg of the mass 70 molecular chlorine isotope. Inner well levels for 3Πg have been observed from v′=0 at 63 472 cm−1 to the top of the barrier, and well beyond that to levels spanning both wells. Observation of these long progressions is attributed to excitation via a repulsive continuum at a well-phonon level. The photophysics of this latter process as well as the mechanism for copious atomic ion formation are discussed. Variation of linewidth with vibrational quantum number due to the predissociation of inner well levels is observed and attributed to repulsive valence states. The data are in good agreement with the theoretical calculations of Peyerimhoff and Buenker, and Tuckett and Peyerimhoff.
Energies and lifetimes of predissociative states of van der Waals molecules: Self-consistent field calculations for I2(v)He, I2(v)Ne
Authors: Schatz G. C. , Gerber R. B. , Ratner M. A.
Published: July 2002

The self-consistent field (SCF) approximation for coupled anharmonic vibrations is applied to the calculation of vibrational energy levels (predissociation resonances) of collinear models of I2 (v)He and I2 (v)Ne, with vibrational quantum number v between 5 and 30. The predissociation lifetimes of these same states are obtained from a distorted wave Born approximation calculation with self-consistent field states taken as the initial states of the complex, and with correlation between the modes taken as the interactions leading to decay. Although the binding energies of the van der Waals complex are very small (order of several cm−1 ), the SCF eigenvalues are in remarkable agreement with the exact numerical values. The lifetimes obtained from the SCF-distorted wave Born approximation (DWBA) are compared with calculations in which the initial state is treated more simply, assuming separability of the modes involved. Results then show that the DWBA with SCF initial states is considerably more accurate than with the more primitive initial state choice. We conclude from these results that the self-consistent field method offers a very accurate description of large-amplitude vibrational motions in van der Waals clusters, with good quantitative results for both the energy levels and predissociation dynamics of these species.
Coulomb shifts for intra- and intershell electron pairs in LiH
Authors: Banyard K. E. , Mobbs R. J.
Published: July 2002

The influence of electron correlation on the interparticle momentum distribution f ( p12), where p12=‖p1−p2‖, is examined for the individual intra- and intershell electron pairs in LiH. These changes Δ f ( p12) vs p12, known as Coulomb shifts, are compared with a similar and earlier analysis of correlation effects in momentum space for the united-atom Be. Selected expectation values are also reported at both the correlated and Hartree–Fock levels of approximation. The use of momentum space for such a study is particularly advantageous for a molecule since, unlike an analysis in position space, the calculation is reduced in complexity by being effectively single-centered. Consequently, atomic-based concepts such as ‘‘radial’’ and ‘‘angular’’ correlation remain applicable. For this initial example of a many-electron molecule, the Coulomb shifts for LiH emphasized the ionic components of the two intrashells, as well as their reduced interdependence by comparison with Be. Correlation produced an increase in momentum, especially for the more loosely bound electrons. This corresponds to a contraction of the charge density in the outer regions of the molecule when viewed in position space.
Numerical solution of two new integral equations for the electrical double layer
Authors: Colmenares Pedro J. , Olivares Wilmer
Published: July 2002

We report the numerical solution of two new integral equations for the diffused electrical double layer referred to as WLMB/MSA and WLMB/HNC. The WLMB/MSA uses the homogeneous bulk MSA approximation for the direct correlation function and predicts surface potentials that fare within 5% of the reported Monte Carlo data. The effect of the ionic diameter on the surface potential is studied and a behavior similar to that reported for the HNC/MSA is found. The use of the HNC approximation for the bulk electrolyte direct correlation function improves the description of the density profile for a high concentraiton but gives poor results for low concentrations.
The influence of the density of the solvent on the static and dynamic properties of star polymers. A molecular dynamics study
Authors: Smit B. , van der Put A. , Peters C. J. , de Swaan Arons J. , Michels J. P. J.
Published: July 2002

The static and dynamic properties of a star polymer in a solvent at several densities have been studied by computer simulations using molecular dynamics. In addition to the repulsive Lennard-Jones potential which interacts between all particles we have used a harmonic potential to link 19 particles into a star polymer. It was found that both the static properties (e.g., the end-to-end distance and the radius of gyration) as well as the dynamic properties (e.g., the autocorrelation functions and the center-of-mass diffusion coefficient) are significantly influenced by the density of the solvent.
Observation and characterization of a new c(2)3Σ+ electronic state using Stark effect and perturbation analysis in NaK(B 1Π)
Authors: Derouard J. , Sadeghi N.
Published: July 2002

A new c(2)3Σ+ state of NaK connected to Na(3S)+K(4P) has been studied through its numerous, weak interactions with the nearby B(1)1Π state. Very small (>10 MHz) energy shifts in perturbed B 1Π, V, J levels were detected by measuring the Λ doubling using e–f Stark mixing spectroscopy. Fifty centers of perturbation were located and attributed to 21 different vibrational levels of the perturbing c(2)3Σ+ state. Comparison of preliminary molecular constants with existing electronic structure calculations shows good consistency with Stevens et al. (1984), but some inconsistency with Jeung et al. (1983), and indicates the index of the lowest observed vibrational levels V0≊20. The identification of this c(2)3Σ+ state has been deduced from (i) the observation of the infrared fluorescence to the lower triplet a(1)3Σ+ state, and (ii) the determination of spin–orbit interaction matrix elements and selection rules as deduced from the measurement of frequency shifts in B 1Π–X 1Σ+ lines connected to perturbed levels of the B 1Π state.
A 193 nm laser photofragmentation time-of-flight mass spectrometric study of CS2 and CS2 clusters
Authors: Tzeng W.-B. , Yin H.-M. , Leung W.-Y. , Luo J.-Y. , Nourbakhsh S. , Flesch G. D. , Ng C. Y.
Published: July 2002

A crossed laser and molecular beam photofragmentation apparatus is described. The apparatus is equipped with a rotatable molecular beam source and a translationally movable ultrahigh vacuum mass spectrometer for time-of-flight (TOF) measurements. Using this apparatus we have measured the TOF spectra of S and CS resulting from the photofragmentation processes, CS2+hν(193 nm)→CS(X,v)+S(1D or 3P). The translational energy distributions of photofragments derived from the S and CS TOF spectra are in good agreement. This observation, together with the finding that the TOF spectra of S and CS are independent of laser power in the 25–150 mJ range, shows that the further absorption of a laser photon by CS to form C(3P)+S(3P) within the laser pulse is insignificant. The TOF spectra of S obtained at electron ionization energies of 20 and 50 eV are indiscernible, indicating that the contribution to the TOF spectrum of S from dissociative ionization of CS is negligible at electron impact energies ≤50 eV. The thermochemical thresholds for the S(1D) and S(3P) channels are determined to be 18.7 and 45.0±0.4 kcal/mol, respectively, consistent with literature values. Structures found in the translational energy distribution can be correlated with vibrational structures of CS(X,v=0–5) associated with the S(1D) channel. The translational energy distribution supports the previous observation that the vibrational state distribution of CS(X,v) is peaked at v=3. The TOF experiment is also consistent with the S(3P)/S(1D) ratio of 2.8±0.3 determined in a recent vacuum ultraviolet laser induced fluorescence measurement on the S photofragment. Photofragments from CS2 clusters are observed at small laboratory angles with respect to the CS2 beam direction and are found to have velocity distributions peaked at the CS2 cluster beam velocity.
Angular dependence of 2H NMR relaxation rates in lipid bilayers
Authors: Jarrell Harold C. , Smith Ian C. P. , Jovall P. A. , Mantsch Henry H. , Siminovitch David J.
Published: July 2002

The angular dependences of 2H Zeeman (T1Z) and quadrupolar (T1Q) relaxation times have been used to assess the relative contribution of order director fluctuations (ODF) to the spin-lattice relaxation rate of saturated phospholipid acyl chains. In order to circumvent orientational averaging due to the effects of rapid lateral diffusion [Brown and Davis, Chem. Phys. Lett. 79, 431 (1981)], relaxation measurements were performed on macroscopically oriented multibilayers of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC). 2H T−11Z values of both DMPC abd DPPC in the liquid crystalline (Lα) phase are relatively insensitive to the orientation of the bilayer normals with respect to the magnetic field. From a comparison of the observed angular dependences of both spin-lattice relaxation rates with those predicted if ODF were the only relaxation mechanism, we conclude that although ODF cannot be ruled out as a relaxation mechanism, they do not provide the dominant relaxation pathway in the MHz regime, as previously suggested [Brown, J. Chem. Phys. 77, 1576 (1982)]. While the frequency dispersion of relaxation rates may in principle provide a critical test for models of molecular dynamics in these systems, the angular dependence of 2H spin-lattice relaxation rates can provide a useful alternative test, particularly when the 2H relaxation time data cover less than a decade of Larmor frequencies.
Solution reaction path Hamiltonian for reactions in polar solvents. I. Formulation
Authors: Lee Sangyoub , Hynes James T.
Published: July 2002

A solution reaction path Hamiltonian (SRPH) is constructed for heavy particle charge transfer reactions in a nondissipative polar solvent. This formulation includes anharmonicities arising from, e.g., potential anharmonicity and reaction path curvature. The SRPH is used in conjunction with variational transition state theory (VTST) to obtain an expression for the reaction rate constant. This expression generalizes the van der Zwan–Hynes rate theory to include the influence of anharmonic effects as well as nonequilibrium solvation. A discussion of the reaction path in solution and its departure from an equilibrium solvation perspective is given.
Adsorption and diffusion at rough surfaces: A comparison of statistical mechanics, molecular dynamics, and kinetic theory
Authors: Thurtell John H. , Thurtell George W.
Published: July 2002

The effect of surface structure on adsorption, diffusion, and the local velocity distributions has been studied using computer simulations and Brownian diffusion theory. The results of these two studies are in nearly perfect agreement both qualitatively and quantitatively. In contrast they are in sharp disagreement with statistical mechanic theories due to a wall structure induced local deviation of the equilibrium velocity distributions from the usual Maxwell–Boltzmann distribution.
Dipole moments of highly excited vibrational states of HCN
Authors: Dodd James A. , Smith Alice M. , Klemperer William
Published: July 2002

Measurement of the Stark effect by intracavity photoacoustic spectroscopy of weak visible vibrational overtone transitions of HCN has produced dipole moments for several excited vibrational levels: μ(005) (15 552 cm−1)=3.142(3) D; μ(105) (17 550 cm−1)=3.157(7) D; μ(402) (14 654 cm−1)=3.075(1) D; μ(104) (14 670 cm−1)=3.095(7) D. The resultant dipole moments are compared with both ab initio theory and linear extrapolation from lower states. The agreement between observation and these predictions is fair. No evidence is seen for asymptotic behavior in the region of CH stretching investigated. The use of additive bond dipole moments to describe molecular dipole moments in HCN is problematic at best.
The sticking of O2 on a Pt(111) surface
Authors: Luntz A. C. , Williams M. D. , Bethune D. S.
Published: July 2002

This paper reports detailed molecular beam measurements of the sticking coefficient at zero coverage for O2 on a Pt(111) surface as a function of initial energy (Ei), angle of incidence (θi), and surface temperature (Ts). Under most conditions the sticking coefficient measures the probability for dissociative chemisorption. These results demonstrate that both precursor mediated and quasi-direct dissociation can be observed, depending upon the initial conditions. The quasi-direct process is revealed by a step increase in the sticking with Ei. This feature scales intermediately between Ei and the normal component En, and is weakly dependent on Ts. The precursor mediated sticking is well described by standard precursor kinetic models. At low Ei and Ts, sticking measures trapping into a molecularly adsorbed state. This trapping decreases more rapidly with Ei than anticipated from simple models and scales intermediately between Ei and En. The sticking results are discussed in terms of likely dynamic processes occurring on a potential energy surface which contains an intermediate molecularly adsorbed species formed by charge transfer from the metal to the O2.
Chain entanglements. II. Numerical results
Authors: Fixman Marshall
Published: July 2002

Numerical methods are described for the calculation of time correlation functions and the stress relaxation moduli for the model developed in the preceding paper [J. Chem. Phys. 89, 3892 (1988)]. Illustrative results are presented for the storage and loss modulii, the shear compliance, and the translational diffusion constant, with parameters appropriate to polybutadiene.
CH2I2 photodissociation: Dynamical modeling
Authors: Zhang Jinzhong , Heller Eric J. , Huber Daniel , Imre Dan G. , Tannor David
Published: July 2002

A full quantum mechanical calculation is carried out on the first excited state of CH2I2 to model the absorption and emission spectra and examine the photodissociation dynamics from a time dependent point of view. The dissociation at 355 nm is direct in the sense that the wave packet does not revisit the Franck–Condon region. The initial motion of the excited molecule is mainly along the CI2 symmetric stretch coordinate while simultaneously spreading in the antisymmetric stretch coordinate. The molecule then dissociates along a C–I ‘‘local’’ mode; no I2 can be formed in this energy region. Vibrationally hot CH2I radical in the C–I mode is predicted. The model is in good agreement with available experimental results. A simple and intuitive method is presented to construct model potential energy surfaces for two chromophore systems from the potential energy surface and information known for the corresponding single chromophore. CH3I and CH2I2 are used as numerical illustrations.
Short-time behavior of electron thermalization in gases
Authors: Nishigori Takeo , Shizgal Bernie
Published: July 2002

A simple power series expansion in time is shown to complement nicely the eigenfunction expansion in electron thermalization problems. With the hot-electron zero-field mobility as an example, a recursion relation is derived to facilitate the calculation of higher-order short-time expansion coefficients. The electron–atom momentum transfer cross section is modeled by a velocity polynomial. Numerical results for the mobility in helium are discussed to see the usefulness of the present expansion.
Disordered phases in vapor deposited rare gases
Authors: Langel W. , Schuller W. , Knözinger E. , Fleger H.-W. , Lauter H. J.
Published: July 2002

Neutron and x-ray diffraction from vapor deposited krypton is reported. Strong diffuse intensity between the positions of the (111) and (200) reflections and between the (311) and (222) reflections of the fcc lattice is observed and ascribed to long range perturbations of the lattice.
Theoretical spectrum of AlN
Authors: Langhoff Stephen R. , Bauschlicher Charles W. , Pettersson Lars G. M.
Published: July 2002

The electronic states of AlN with excitation energies below about 35 000 cm−1 have been studied at the CASSCF and MRCI levels of theory. Less elaborate calculations have been performed for the low-lying states of AlN+. Although the results are not definitive, they strongly suggest a 3Π ground state for AlN with a very low-lying 3Σ− state, in analogy with the Al2 molecule. All of the strong transitions however involve the lowest 3Π state. The 3Π–3Π AlN band system observed in emission is assigned as the C3Π–X 3Π transition. The C 3Π radiative lifetime is estimated as 151, 163, and 175 ns for v′=0,1,2, respectively. However, these lifetimes may be significantly shortened due to predissociation by the (1) 5Π state. The calculated De of 2.35 eV for the ground state is about half that of the isovalent BN molecule. Thus the bonding in AlN is significantly weaker than for BN, owing to the smaller overlaps of the valence orbitals. Finally, transition moment functions are presented for many of the dipole-allowed transitions to aid further spectroscopic studies of AlN.
Electron degradation and yields on initial products. II. Subexcitation electrons in molecular nitrogen
Authors: Kowari Ken-ichi , Kimura Mineo , Inokuti Mitio
Published: July 2002

Subexcitation electrons lose their kinetic energy through vibrational excitation, rotational excitation, and elastic collisions in molecular gases. Initial yields of vibrationally and rotationally excited states of nitrogen molecules are calculated by using the Spencer–Fano equation (SFE) and its simplification, the continuous-slowing-down approximation (CSDA), both in time-independent and time-dependent representations. One focus of the present study is a close comparison of the CSDA with the rigorous treatment of the SFE in the subexcitation domain. The present result reveals for the first time distinct energy regions in which either vibrational excitation or rotational excitation dominates. This recognition explains the different time dependence of the yields of vibrational and rotational excitation.
Quantitative absorption and fluorescence spectroscopy of BCl3 in vacuum ultraviolet
Authors: Suto Masako , Ye Chao , Han J. C. , Lee L. C.
Published: July 2002

The absorption and fluorescence cross sections and the fluorescence quantum yields of BCl3 were measured in the 106–190 nm region. A visible emission band appears at excitation wavelengths shorter than 173 nm and two ultraviolet emission bands appear at wavelengths shorter than 126.5 nm. Fluorescence spectra were dispersed to identify emitting species that are attributed to BCl*2. The upper limits of the potential energies for the observed emitting states are 2.56, 5.19, and 5.70 eV, as determined from the fluorescence thresholds. The photodissociation process of BCl3 is discussed in accord with the fluorescence observed. The radiative lifetimes of the visible band vary over the 5–24 μs range, and the quenching rate constants of the excited state by BCl3 vary in (1.2–2.7)×10−10 cm3/s, both depending on fluorescence wavelength.
Thermodynamics of isotopic polymer mixtures: Significance of local structural symmetry
Authors: Bates F. S. , Muthukumar M. , Wignall G. D. , Fetters L. J.
Published: July 2002

The dependence on composition (Φ) and degree of polymerization (N) of the effective segment–segment interaction parameter, χ̃=− 1/2 ∂2Γ/∂Φ2, has been determined for mixtures of perdeuterated and normal (protonated) poly(vinylethylene) and poly(ethylethylene) by small angle neutron scattering (SANS), where Γ represents the excess free energy of mixing. Owing to the small reduction in length and polarizability of carbon–hydrogen bonds with deuterium substitution, isotopic polymer mixtures are characterized by small positive χ̃ parameters. However, deuterium substitution does not significantly influence the local structural symmetry (i.e., the liquid structure or reduced thermodynamic properties) of polymer isotopes, consistent with the assumptions of Flory–Huggins (FH) lattice theory. Nevertheless, we find that χ̃ depends significantly on both Φ and N, contrary to the predictions of FH theory. A general theory, which corrects FH theory for monomer concentration fluctuations, is shown to closely account for these experimental results. In understanding the previous experimental investigations of χ̃ for mixing chemically different species, the monomer concentration fluctuations discussed here must be addressed in addition to the effects due to local structural asymmetry.
Effects of coadsorbed carbon monoxide on the surface diffusion of hydrogen on Ru(001)
Authors: Mak C. H. , Deckert A. A. , George S. M.
Published: July 2002

The effects of coadsorbed carbon monoxide on the surface diffusion of hydrogen on Ru(001) were studied using laser-induced thermal desorption techniques. The surface mobility of hydrogen was measured as a function of CO surface coverage at 260 and 280 K. At both temperatures, the surface diffusion of hydrogen displayed an abrupt reduction at a coadsorbed CO coverage of ΘCO =0.12 ML. LEED studies revealed that a CO coverage of ΘCO =0.12 ML corresponded to the onset of the formation of ordered √3×√3 CO islands at 260 and 280 K. Temperature programmed desorption results demonstrated that a lateral repulsive interaction exists between hydrogen and carbon monoxide on Ru(001). Assuming that the lateral repulsive interaction leads to a hydrogen exclusion area around each CO admolecule, a hydrogen exclusion radius of rCO =2.2–2.7 Å was determined. The lateral repulsive interaction would also result in hydrogen exclusion from the interior of ordered √3×√3 CO islands and provide an explanation for the hydrogen surface diffusion results. According to this interpretation, the rigid √3×√3 CO islands obstruct the hydrogen surface diffusion pathways and thereby reduce the hydrogen surface mobility on Ru(001).
Chemisorption of hydrogen on the nickel (111) surface
Authors: Yang Hong , Whitten Jerry L.
Published: July 2002

The chemisorption of atomic H on the [111] surface of Ni is treated by use of a many-electron embedding theory to describe the electronic bonding. The lattice is modeled as a 26-atom, three layer cluster which is extracted from a larger cluster by an orbital localization transformation. Ab initio valence orbital CI calculations carried out on a local surface region permit an accurate description of bonding at the surface. H atoms are found to bind strongly to the Ni(111) surface at threefold and bridge sites. Calculated H binding energies are in the range 2.50–2.66 eV, for optimized Ni–H bond lengths ranging from 1.81 to 1.87 Å. Vibrational frequencies are in the range 1043–1183 cm−1. The binding energy of H at an atop atom site is 1.94 eV. The work function increases by ∼0.2 eV on H adsorption.
Interatomic potentials of the C 11 state of HgNe, HgAr, HgKr, and HgXe as studied by tunable vacuum ultraviolet laser spectroscopy
Authors: Tsuchizawa Tai , Yamanouchi Kaoru , Tsuchiya Soji
Published: July 2002

The laser induced fluorescence spectra of the C–X transitions of HgRg (Rg=Ne, Ar, Kr, and Xe) van der Waals complexes formed in a supersonic jet have been observed in the longer wavelength side of the Hg(1P1−1S0) resonance transition by use of a tunable vacuum ultraviolet laser, which is generated by the two-photon resonant four-wave sum mixing in Sr vapor. On the basis of vibrational quantum numbers assigned to the upper levels of the observed C–X transitions,potential parameters of the C state are determined. The interatomic potentials for the C state are well approximated by a Morse function and the dissociation energies for HgNe, HgAr, HgKr, and HgXe are 89, 486, 1410, and 3463 cm−1 , respectively.
On the dynamics of the associative desorption of H2
Authors: Harris John , Holloway Stephen , Rahman Talat S. , Yang Kai
Published: July 2002

The dynamics of activated associative desorption is discussed with particular reference to the system H2–Cu and to the partitioning of the energy released among the various product degrees of freedom. It is argued that a simple theory based on transition-state concepts should hold for this system because the potential energy surface (PES) divides naturally into reactant and product regions, separated by a ‘‘seam’’ or ‘‘ridge’’ at which it is reasonable to assume a thermal distribution of desorbing trajectories. Using a PES constructed in accordance with available electronic structure calculations we consider the angular distributions and translational, vibrational, and rotational energy distributions of the desorbing molecules. It is shown that, whereas the rotational energy reflects the surface temperature, the vibrational energy is markedly enhanced because the energetically low-lying regions of the ridge in the PES correspond to an H–H bond distance that is distended as compared with the gas-phase equilibrium separation. The enhancement is found to be a strong function of the surface temperature. The translational energy, however, is found to be only very weakly dependent on the temperature. These results are discussed in connection with available data.
Solid–solid phase transitions and molecular motions in long-chain paraffins studied by positron annihilation techniques
Authors: Lévay B. , Lalović M. , Ache H. J.
Published: July 2002

Positron annihilation lifetime and Doppler-broadening measurements were performed on n-dotriacontane (n-C32H66) and n-tritriacontane (n-C33H68) between 80 K and their melting points. The annihilation parameters plotted as a function of temperature show irregularities above the room temperature, which reflect solid–solid phase transitions in these materials. The transition temperatures derived from the positron annihilation measurements are in excellent agreement with literature values determined by other techniques. In the case of n-C32H66 additional phase transitions not reported previously in the literature were observed. The lifetime vs temperature curves could be reasonably well interpreted within the framework of the free volume model taking into account the effect of special types of molecular motions and structural defects. On the basis of our measurements, linear dependence of ortho-positronium (o-Ps) lifetime on the size of the cavity around o-Ps could be expected. The proposed empirical relationship proved to fit well to lifetime data for molecular crystals as a function of their molecular volume. The o-Ps yield showed an abrupt decrease with increasing temperature at 190 and 165 K in the case of n-C32H66 and n-C33H68, respectively. This effect was attributed to the onset of rotation of the methyl end groups of the paraffin chains.
The effects of bending and stretching vibration on the reaction of acetylene cations with methane
Authors: Orlando Thomas M. , Yang Baorui , Anderson Scott L.
Published: July 2002

Multiphoton ionization (MPI) has been used to create C2H+2 in selected vibrational states, with excitation in either ν2 (C–C stretch) or a bending vibration (ν5-cis bend). Reactions of the state selected ions with CH4 and CD4 have been studied in a guided beam mass spectrometer. Absolute cross sections for formation of C3H+n (n=3,4,5) and C2H+3 products are reported for collision energies ranging from 60 meV to 5 eV. Evidence for both complex formation and direct reaction mechanisms is found. Vibrational excitation has different effects on the various product cross sections in different collision energy regimes. Bending excitation has also been found to cause somewhat different effects than C–C stretching vibration.
Hindered internal rotation and o
r
t
h
o-H2 enrichment in t
r
a
n
s-stilbene–H2/D2 complexes
Authors: DeHaan David O. , Zwier Timothy S.
Published: July 2002

A supersonic free jet expansion has been used to prepare trans-stilbene–H2 and D2 complexes. The cooling in the jet collapses most of the ortho and para H2 and D2 rotational population to the lowest rotational levels of a given nuclear spin symmetry: j=0 and j=1. The laser-induced fluorescence excitation spectrum of stilbene–D2 shows a well-resolved doublet at the origin due to stilbene–D2( j=0) and stilbene–D2( j=1) complexes. The 4.9 cm−1 splitting of these transitions indicates that the D2 molecule is undergoing hindered internal rotation in the complex and that the barrier to internal rotation changes upon electronic excitation. The relative intensities of the stilbene–D2( j=0) and stilbene–D2( j=1) origins depend on the D2 concentration in the jet. At low D2 flows the transitions arising from stilbene–D2( j=1) are favored while at high D2 flows the ( j=0)/(j=1) transition intensities approach the 2:1 intensity ratio given by their nuclear spin statistical weights. By contrast, in stilbene–H2 we observe only a single transition at the origin which we assign to stilbene–H2( j=1). We are able to place an upper bound on the stilbene–H2( j=0) transition intensity of 5% of the stilbene–H2( j=1) intensity. Dispersed fluorescence spectra are used to bracket the binding energies of the stilbene–H2/D2 complexes in both ground and excited states. In the ground state of stilbene–H2( j=1) and stilbene–D2( j=0,1) complexes, 169≤D′0 ≤249 cm−1, so that the stilbene–H2( j=0) transitions cannot be missing by virture of the complex being unbound. We propose a simple kinetic scheme which supports the possibility that selective exchange reactions, in which more strongly bound H2( j=1) preferentially displaces H2( j=0) from stilbene, contribute to the enrichment of stilbene–H2( j=1) complexes in the jet. We also propose that H2( j=2) molecules may contribute to the suppression of stilbene–H2( j=0) complexes by virture of the fact that

the H2( j=2) level is above the dissociation threshold for the stilbene–H2 complex while the D2( j=2) level is likely not.
High-order multiphoton ionization photoelectron spectroscopy of nitric oxide
Authors: Carman H. S. , Compton R. N.
Published: July 2002

Resonance-enhanced and nonresonant five- and six-photon ionization of NO was studied using angle resolved photoelectron spectroscopy. The (3+3) resonance-enhanced multiphoton ionization photoelectron spectrum (REMPI-PES) of NO via the A 2 Σ+ (v=0) level yielded a distribution of electron energies corresponding to all accessible vibrational levels (v+ =0–6) of the nascent ion. The observed energy distributions suggest near resonant enhancement due to vibrational levels of the D 2 Σ+ and C 2 Π states at the fourth photon level. Angular distributions of photoelectrons corresponding to v+ =0 and v+ =3 (rotationally unresolved) were significantly different, perhaps reflecting these different pathways. The (3+2) REMPI via the A 2 Σ+ (v=1) level produced only one low-energy electron peak corresponding to v+ =1. Nonresonant MPI at 532 nm yielded a distribution of photoelectron energies corresponding to both four- and five-photon ionization. Prominent peaks in the five-photon ionization photoelectron spectrum show that near resonant enhancement occurs via the A 2 Σ+ (v=5), the C 2 Π(v=2) and the D 2 Σ+ (v=1) states at the three-photon level. This study amply demonstrates the utility of angle resolved MPI-PES in understanding high-order multiphoton ionization processes. Finally, measurements of the ratio of NO+ ion signal for circular and linear polarized light when tuning near the A 2 Σ+ (v=0) and A 2 Σ+ (v=1) levels are reported and compared with theory.
Cross sections and coherence terms for associative ionization of two differently excited Na(3p) atoms
Authors: Meijer H. A. J. , Pelgrim T. J. C. , Heideman H. G. M. , Morgenstern R. , Andersen N.
Published: July 2002

The associative ionization process in thermal Na(3p)–Na(3p) encounters has been studied in a series of crossed-beam experiments where the light polarization of the two laser beams preparing the excited atoms before collision was varied independently. It is shown how in this way maximum possible information for our geometry is extracted about the dependence of the ion formation process on the shape and spatial orientation of the electron clouds of the two approaching atoms, including all coherence terms. The experimental findings are discussed in the light of recent theoretical results for the states of the Na2 molecule. It is concluded that just a few of the possible geometrical approaches are favorable for molecular–ion formation.