bluuebunny/crossref_metadata_2025 huggingface dataset:
The initial product vibrational energy distribution in the reaction between O(1D2) and H2
Authors: Aker P. M. , Sloan J. J.
Published: July 2002

We report the first determination of the initial vibrational distribution in the OH product of the reaction between O(1D2) and H2. The measurement was made using a novel time-resolved Fourier transform spectroscopic technique which permits the observation of spectra on a microsecond time scale at a known time after the initiation of a reaction. The result is P(v′=1:2:3:4)=0.29:0.32:0.25:0.13 suggesting that the reaction dynamics involve very large attractive energy release during reagent approach followed by an extremely short-lived interaction leading to products within a few vibrational periods.
Measurement of the radii of low axial ratio ellipsoids using cross correlation spectroscopy
Authors: Rarity J. G.
Published: July 2002

The intensity cross correlation from a system of Brownian rotating ellipsoids is calculated using the Rayleigh–Gans–Debye (RGD) approximation. In principle, estimates of the rotational diffusion coefficient, radii, and axial ratio can be made from one measurement. Close to the Rayleigh–Gans–Debye minimum the technique appears sensitive to very low axial ratios (∼1.025). The theory is tested well below the RGD minimum using cross-linked and swollen polystyrene latex particle approximating an ellipsoid of revolution of low axial ratio (∼1.5). Measurements of the initial decay of the field autocorrelation function at various angles are used to confirm the estimated radii and axial ratio and illustrate some advantages of the cross-correlation technique.
Coverage dependent structural changes during chlorine adsorption on Ag{110}
Authors: Moon D. W. , Bleiler R. J. , Winograd Nicholas
Published: July 2002

The adsorption of Cl2 on Ag{110} at room temperature has been studied using angle-resolved static mode secondary ion mass spectrometry (SIMS), Auger spectroscopy, and LEED. The system has been examined over a coverage range from near zero to the p(2×1) overlayer structure observed by LEED. This structure could be produced by exposing the Ag{110} crystal to 2.5 L of Cl2. The Cl LMM Auger spectra exhibited a distinct change in shape at about 1.0 L exposure which is attributed to a change in the electronic structure of the Ag–Cl bond. In addition, the SIMS Cl− secondary ion yield deviated at this exposure from the expected exponential dependence on work function predicted by theory. An analysis of the secondary Cl− ion kinetic energy distributions, and the polar and azimuthal angle distributions also suggests that the Cl atom is highly charged in the limit of zero coverage with an extended Ag–Cl bond length over that of bulk AgCl. As the coverage approaches the 1.0 L Cl2 exposure point, however, there is significant weakening of surface dipoles due to adlayer interactions. This depolarization appears to be sufficient to allow significant contraction of the Ag–Cl bond length, such that the Cl atom may actually fall into the valley of the {110} surface. A submonolayer Cs overlayer appears to stabilize the negatively charged Cl adatoms, inhibiting the changes observed on the undoped Ag{110} surface. It is suggested that adlayer interactions which influence the formation of ordered overlayer structures mask important electronic and structural features of adsorbates which may only be observed in the single atom limit.
Ionization of liquid hydrocarbons and tetramethylsilane by 241Am alpha particles
Authors: Muñoz Raul C. , Cumming James B. , Holroyd Richard A.
Published: July 2002

The charge spectra resulting from alpha particle ionization of tetramethylsilane, neopentane, 2,2,4,4-tetramethylpentane, 2,2,4-trimethylpentane, and 2,2-dimethylbutane were measured as a function of field. The data are evaluated in terms of the track models proposed by Jaffé and by Kramers. These models explain the observed increase in asymmetry of the spectral shape with increasing field in terms of the orientation of the alpha tracks. However, they fail to account for all the results in detail. From a consideration of the deficiencies of these models we formulate several modifications which should be considered in developing a correct model. The most important among these is the internal field arising from the difference between positive and negative ion densities which should be explicitly included along with the contribution of delta rays.
Study of monolayer polymerization using nonlinear optics
Authors: Berkovic G. , Rasing Th. , Shen Y. R.
Published: July 2002

Optical second harmonic generation has been used to study the polymerization of a monolayer of vinyl stearate or octadecyl methacrylate spread at the air/water interface. Rapid polymerization occurred when monolayers were irradiated with ultraviolet light, while some slower polymerization occurred when certain redox initiators were added to the water subphase. Information regarding polymerization kinetics and the orientation of molecules at the interface were also obtained using this method.
Higher order adiabatic separation of strongly coupled systems. I. Two-body systems and dressed hydrogen atom
Authors: Nguyen-Dang T. Tung , Bandrauk Andre D.
Published: July 2002

The phase corrected adiabatic separation method proposed recently by Marechal [J. Chem. Phys. 83, 247 (1985)] is reexamined. We have been able to show that a state independent differential equation exists, which governs the phase coupling function f of this method, if f further satisfies a set of constraints, Eqs. (7) of the present paper. The method is illustrated by its application to three simple, exactly soluble two-body systems: the free hydrogen atom, the linearly forced hydrogen atom, and the system of two harmonically coupled oscillators. We show that for these systems, the present method, which employs an adiabatic separation procedure, after phase correcting the Hamiltonian, leads to exact results, suggesting its utility for more general problem, involving nonseparable potentials. A nonseparable system of particular interest is the dressed hydrogen atom, which is a representative of all dressed systems, molecular or atomic, with respect to radiative couplings. The method of phase corrected adiabatic separation is shown to give, for field frequencies lying below an IR threshold, a better representation for the exact eigenstates of this dressed systems, as ensuing residual couplings are estimated to be of second order in the original radiative coupling constant.
Collisional energy transfer in unimolecular reactions induced by vibrational overtone excitation
Authors: Baggott J. E. , Law D. W.
Published: July 2002

The kinetics of the gas phase isomerisation of cyclobutene initiated by direct single photon excitation of C–H stretching overtones have been studied and the observed behavior has been interpreted using a treatment which combines RRKM theory and models accounting for weak intermolecular energy transfer. Results obtained from the excitation of five different overtone states spanning an energy range of 5070 cm−1 have allowed us to examine the form of the dependence on energy of the energy transfer step sizes 〈ΔE〉 deduced from numerical solutions of the collisional master equation using an exponential-down model, for the collider gases cyclobutene, SF6, CHF3, CH4, CO2, H2, CO, N2, Ar, and He. Our calculations indicate very small energy transfer step sizes, particularly for the most inefficient collider molecules. We discuss the factors which might contribute to the determination of low values for 〈ΔE〉 and the sensitivity of the master equation calculations to details of the RRKM model. Our results, when expressed in terms of absolute changes in the magnitudes of 〈ΔE〉 as a function of energy, appear to be in agreement with the recent direct studies of Hippler et al. [J. Chem. Phys. 83, 3906 (1985)] for a number of collider molecules over the energy range of the present work. Mean collision efficiency parameters calculated directly from the slopes of the Stern–Volmer plots are shown to be linearly correlated with mean values of 〈ΔE〉. In future studies of unimolecular reactions induced by overtone excitation, our results should enable full account of weak collisional energy transfer to be taken, allowing for a more rigorous comparison of experimental energy-specific rate constants with RRKM predictions.
On the universality of the bridge function in simple classical fluids
Authors: Robert M. , Viswanathan R.
Published: July 2002

The bridge function of the one-dimensional lattice gas with interactions between nearest-neighbor and next-nearest-neighbor particles is determined exactly. This result is compared with the recently proposed hypothesis of universality of the bridge function of simple classical fluids. It is found that in contrast to three-dimensional systems previously studied, the bridge function of the one-dimensional lattice gas is a strongly nonmonotonic function of the interparticle distance, being negative at very short distances and vanishing positively at infinity. Consequently, the one-parameter family of positive and monotonic bridge functions of the hard-sphere (rod) system, recently suggested to be universal, cannot be used for the one-dimensional lattice gas. Dedicated to Thomas Leland.
The excited state dynamics of triplet 1,8-dichloroanthraquinone in solutions at room temperature
Authors: Hamanoue Kumao , Nakayama Toshihiro , Shiozaki Masaya , Funasaki Yasutaka , Nakajima Kazuo , Teranishi Hiroshi
Published: July 2002

The transient absorption spectra of the title compound (1,8-DCAQ) in solutions at room temperature have been measured by the picosecond laser photolysis, and the buildup and decay of a new absorption were observed on a picosecond time scale. The decay of this absorption was accompanied by a rate matching increase in the triplet–triplet absorption of the lowest triplet state of a ππ* character. Since the addition of 2,5-dimethyl-2,4-hexadiene (1 M) or triethylamine (2 M), as a triplet energy acceptor, quenched the new absorption, it was assigned to the second nπ* triplet state of 1,8-DCAQ with an intramolecular charge–transfer character between the chlorine and oxygen atoms.
Classical dynamics of intramolecular energy flow and overtone-induced dissociation in HO2H and HO2D
Authors: Uzer T. , Hynes James T. , Reinhardt William P.
Published: July 2002

A detailed classical trajectory study of the overtone-induced dissociation of hydrogen peroxide HO2H and its isotopic variant HO2D is presented. The factors affecting intramolecular energy flow, such as nonlinear resonances and the various couplings, are examined in detail. The dissociation lifetimes are found to be on the order of picoseconds and comparable with statistical lifetimes, although the intramolecular energy redistribution is not complete within the lifetime of the molecules. Lifetime broadening contributes very little to the rather large width of the overtone excitation feature, which we conclude is in the main inhomogeneously broadened by rotational structure instead. The implications of our results are discussed.
Nuclear relaxation and molecular motions in liquid trifluoroacetonitrile
Authors: Huang Shaw-Guang , Rogers Max T.
Published: July 2002

1 9F and 14N nuclear magnetic spin-lattice relaxation rates in liquid CF3CN have been measured at 14.1 kG. From 14N relaxation rates, the correlation times of the tumbling motion were directly determined. By separating 19F relaxation rates into terms contributed from the dipolar and the spin-rotational interactions, the correlation times of the spinning motion were also obtained. Spin-rotational relaxation rates were used to estimate the angular momentum correlation times. Gordon’s J diffusion model was found to be appropriate to describe the molecular reorientations of this molecule. Some 13C spin-lattice relaxation rates have also been measured and used to estimate the magnitude of 13C spin-rotational constants and chemical shielding anisotropies.
The coadsorption of nitrogen with carbon monoxide and oxygen on the Ru(001) surface: Local chemical interactions in mixed overlayers
Authors: Anton A. B. , Avery N. R. , Madey T. E. , Weinberg W. H.
Published: July 2002

High resolution electron energy loss spectroscopy and thermal desorption mass spectrometry have been employed to investigate the molecular chemisorption of N2 on both disordered and ordered overlayers of atomic oxygen on the Ru(001) surface, as well as the chemisorption of CO on overlayers of N2 on Ru(001). Pertinent results obtained for the adsorption of N2 on the clean Ru(001) surface are also presented for comparison. Disordered oxygen poisons a fraction of the surface to the subsequent adsorption of N2 whereas the N2 that does adsorb is indistinguishable from N2 on clean Ru(001). The fraction of the surface that is poisoned to the adsorption of N2 is approximately twice the fractional surface coverage of disordered oxygen. The p(2×2) overlayer of ordered oxygen adatoms, which is formed at a fractional surface coverage of 0.25, stabilizes the chemisorption of N2 into a new binding state with a heat of adsorption that is approximately 1.5 kcal/mol greater than any one observed for the adsorption of N2 on the clean surface. Coverage measurements indicate that this state results from the stoichiometric addition of one N2 molecule to each unit cell of the p(2×2)–O overlayer. Electron energy loss spectroscopic results suggest that this N2 binding state results from stabilization of the dominant σ donor contribution to the Ru–N2 bond, due to the presence of the electronegative oxygen adatoms of the p(2×2) overlayer. Measurements of the adsorption of CO on saturated overlayers of N2 show that N2 is displaced from the surface by increasing coverages of subsequently adsorbed CO. For low coverages of CO in the presence of N2, the observed value of ν(CO) is lower than observed under any conditions for the adsorption of CO alone on the Ru(001) surface. The N2 admolecules enhance the ability of the surface ruthenium atoms to backdonate electron density into the 2π orbital of coadsorbed CO under these conditions. At coverages of CO in excess of 0.10 monolayer, the results are consistent with CO island formation and segregation of N2 and CO admolecules into different local regions on the surface.
Nonequilibrium structure of the homogeneous phase of block copolymers under steady flow
Authors: Fredrickson Glenn H.
Published: July 2002

A theory is developed to describe the scattering function of a diblock copolymer in the presence of a steady homogeneous flow field. The nonequilibrium steady state under flow conditions is analyzed within a mean field approximation that is expected to accurately describe the disordered phase of block copolymer melts. In the absence of flow the theory reduces to the equilibrium theory of Leibler. Simple shear and extensional flows are seen to render the scattering function highly anisotropic and to attenuate the peak intensity. The instability at the spinodal is predicted to disappear for very strong flows, such as a uniaxial extensional flow. The theory provides an expression for the structure factor that will allow for quantitative interpretation of neutron and x-ray scattering experiments on diblock melts under steady macroscopic flow.
A new EPR study of the OH2− molecular ion in CaO
Authors: Sanjuán M. L. , Orera V. M.
Published: July 2002

The OH2− defect in CaO has been studied by EPR. The 77 K static spectrum has been reinterpreted and new values are given for the spin-Hamiltonian parameters and principal axes orientation. The averaging features due to the motion of the defect at higher temperatures are studied by means of the stochastic Liouville formalism. Inclusion of pseudosecular Hamiltonian terms within the theory explains most of the experimental details. The hopping process follows the classical exponential temperature dependence, with an activation energy of 0.23±0.01 eV. A superhyperfine (shf) structure resolved at room temperature permits identification of a nearest-neighbor substitutional Mg2+ ion as the perturbation that stabilizes the defect. The 25Mg shf and quadrupolar parameters, including their absolute signs, are also given.
Polymers with excluded volume in various geometries: Renormalization group methods
Authors: Wang Zhen-Gang , Nemirovsky A. M. , Freed Karl F.
Published: July 2002

Renormalization group (RG) methods are generalized to study a single polymer chain with excluded volume in various geometries with different boundary conditions (or polymer–surface interactions) on the limiting surfaces. Methods for the renormalization of these theories are presented and are used to derive the RG equations which dictate the generalized scaling behavior as a function of the several interaction and geometrical parameters. We illustrate the general theory by studying a polymer chain confined between two parallel plates with three different (Neumann, Dirichlet, and periodic) boundary conditions to one-loop order. We show that ε expansions are well behaved as long as the radius of gyration of the chain is smaller than the interplate separation L. The finite size corrections to the full space (bulk) limit are found to be proportional to L−1 for free boundaries, while they are exponentially small for periodic boundary conditions. The presence of several lengths and/or interactions produces interesting crossovers, which we illustrate for a Gaussian polymer chain attached to the exterior surface of a repulsive sphere where full crossover scaling functions are obtained for the partition function and moments of the end-vector distribution function. A new exponent associated with the radius of the sphere is predicted on the basis of scaling arguments which are supported by the RG equations. This work provides the necessary input ingredients for extension of the theory to treat semidilute concentrations.
Determination of nuclear shielding tensor orientations in polycrystalline materials by dipolar modulation. The dilute I2S system
Authors: Munowitz M. , Huang T.-H. , Griffin R. G.
Published: July 2002

Solid-state NMR methods of dipolar modulation and separated local field spectroscopy are extended to magnetically dilute, heteronuclear I2S moieties in polycrystalline systems, and then used to measure the principal directions of the 15N chemical shielding interactions in an NH2 group. Characteristic features of the dipolar-modulated chemical shift anisotropy powder spectrum are analyzed, and the dependence of the modulation pattern on the relative orientation of the two tensors is discussed. The effect of homonuclear interactions is considered briefly.
Phase space dynamics of bimolecular reactions and the breakdown of transition state theory
Authors: Davis Michael J.
Published: July 2002

We investigate the phase structure of the reaction H+H2 restricted to collinear geometry. When variational transition state theory is exact, the phase space structure is relatively simple and a phase space approach is in agreement with this theory which is centered in configuration space. At energies where variational transition state theory breaks down, a phase space approach leads to the resolution of an interaction region. All trajectories which violate transition state theory enter this region during their propagation. In addition, all trajectories which are not violations miss the region. The resolution of the interaction region allows us to model the reaction even at energies where variational transition state theory fails by up to a factor of 2. The phase space approach also defines conditions under which variational transition state theory is exact which are stronger than those previously put forth. Other aspects of the reaction dynamics which can be understood from the phase space dynamics are also discussed in this paper.
Thermodynamics of electron transport in amorphous insulators
Authors: Baird James K. , Rehfeld Ross H.
Published: July 2002

Excess electrons can be introduced into insulators by ionizing radiation or electrical discharge. We present a theory of the electron drift velocity in insulating liquids and glasses which is based upon the assumption of a dynamic equilibrium existing between electrons in traps and electrons in the conduction band. Using statistical thermodynamics, we relate the electron drift mobility to parameters which can be operationally defined in terms of experiments. Among these parameters are the electron effective mass in the conduction band, the Hall mobility, the photocurrent threshold energy, the volume fraction of the traps, and the coordination number and vibrational frequencies of the atoms binding the electron to the trap.
Bistable limit cycle oscillations in chemical systems. I. Basins of attraction
Authors: Celarier Edward A. , Kapral Raymond
Published: July 2002

When bistable oscillations occur in nonlinear chemical systems, the concentration space can be separated into regions that asymptotically tend to each of the oscillatory states. The structure of such basins of attraction is studied for Rössler’s model of a far-from-equilibrium chemical system. Starting with a set of local pictures of the basins, their three-dimensional structure is deduced. The idea of a phase basin is introduced. For a multilooped limit cycle, the phase basin is the set of those initial phase points that tend to the same discrete phase of the cycle, defined in a plane that cuts the cycle transversely. The nature of such basins of attraction is important for studies of noise-induced transitions between coexisting limit cycles or loss of phase coherence in a single limit cycle.
Electron impact excitation cross section studies of methane and acetylene
Authors: Pang K. D. , Ajello J. M. , Franklin B. , Shemansky D. E.
Published: July 2002

We have measured the electron impact emission cross sections for CH4 and C2H2 at 200 eV in a crossed beam laboratory system. Included in the study are all vacuum ultraviolet (VUV) emission features from 40 to 200 nm. The features are entirely from the atomic dissociation fragments (C i, C ii, and H). The Lyman series of H is observed to truncate near principal quantum number n=10 due to the long lifetime and to the high kinetic energy of the excited H fragments. The threshold region of the excitation functions has been measured at an energy resolution of 0.2–1.0 eV for the Lyman-α and Lyman-β transitions of H and C i (165.7, 193.1 nm) multiplets; and several distinct appearance potentials (AP) have been detected. For example, appearance potentials of Lyman-α from dissociation of CH4 and C2H2 are noted at several energies, including the first observations of a Lyman-α AP from C2H2 at 16.3 eV.
Dimensional expansions for two-electron atoms
Authors: Loeser J. G. , Herschbach D. R.
Published: July 2002

Approximate expansions in inverse powers of the dimensionality of space D are obtained for the ground-state energies of two-electron atoms. The method involves fitting polynomials in δ=1/D to accurate eigenvalues of the generalized D-dimensional Schrödinger equation. To the maximum order obtainable from the data, about δ7, the power series for nuclear charges Z=2, 3, and 6 all diverge at D=3. Asymptotic summation yields an energy for the Z=2 atom 1% in excess of the true value at D=3. However, expansions with a shifted origin, i.e., expansions in (δ−δ0), show improved convergence. Of particular interest is the case δ0=1, because the expansion coefficients can in principle be calculated by perturbation theory applied to the one-dimensional atom. Series in powers of (δ−1) appear to converge rapidly. Also the series in (δ−1) can be evaluated even for the hydride ion, with Z=1. For helium, this series is quite comparable to the more familiar expansion in powers of λ=1/Z, with errors in the partial sums decreasing by roughly an order of magnitude per term. Thus, for Z=2 the first four terms of the expansion in (δ−1) yield an energy within 0.02% of the true value at D=3. Similar results are found in an analogous treatment of accurate eigenvalues for the Hartree–Fock approximation. This provides a rapidly convergent dimensional expansion for the correlation energy.
Solvent induced attractions in micelles and microemulsions
Authors: Pincus P. A. , Safran S. A.
Published: July 2002

A universal mechanism which results in short-range attractive interactions between globules in micellar, vessicular, or microemulsion systems is described. The perturbation of the surfactant concentration profile by the globules is the physical origin of this attraction.
The nitrogen–hydrogen fluoride dimer: Infrared spectroscopy and vibrational predissociation
Authors: Jucks K. W. , Huang Z. S. , Miller R. E.
Published: July 2002

The ν1 infrared spectrum of N2–HF has been observed at sub-Doppler resolution using an infrared laser–molecular beam apparatus which is based on the optothermal detection method. This spectrum has been used to obtain the following molecular constants: B1=0.107 19±0.0001 cm−1 and ν0=3918.2397±0.005 cm−1. In addition, the Stark spectrum of the R1 transition was recorded in order to determine the ground (μ0=1.991±0.008 D) and excited vibrational (μ1=2.106±0.008 D) state dipole moments of the complex. Spectra recorded at the highest resolution clearly show homogeneous broadening of the individual rotational transitions giving a predissociation lifetime of 44±10 ns.
Vibrational energy transfer in collisions of He atoms with para-difluorobenzene
Authors: Clary D. C.
Published: July 2002

Vibrational energy transfer in the three-dimensional collisions of He atoms with para-difluorobenzene (pDFB) has been studied theoretically. A quantum-mechanical scattering method has been employed which uses the close-coupling method for the molecular vibrations and the infinite-order-sudden approximation for rotational motion. Both V–T and V–V processes are studied. The normal mode coefficients for pDFB were obtained from an SCF gradient program. The computations showed a strong propensity for excitation and relaxation of the v30 vibrational mode, which is the mode of lowest frequency. This finding is in agreement with molecular beam experiments of Gentry and co-workers, and laser-fluorescence measurements of Parmenter and co-workers. When the different vibrational modes were given the same frequency in the calculations, modes involving atomic displacements out of the molecular plane had distinctly larger vibrational excitation cross sections than those for in-plane modes.
Excited state dynamics of guaiazulene using laser-saturated fluorescence
Authors: Carrabba Michael M. , Woudenberg Timothy M. , Kenny Jonathan E.
Published: July 2002

We report the results of a laser-saturated fluorescence study of the S2←S0 000 vibronic bands of two conformers of guaiazulene in a supersonic jet. Photophysical parameters were derived from the saturation study using a three-state kinetic model which explicitly accounted for heterogeneous broadening. Since heterogeneous broadening (rotational and Doppler) is significant in heavy polyatomics, this method could become important in extending the utility of saturation studies beyond the atomic and diatomic systems to which the technique has usually been applied. To assess the utility of our kinetic model, we compared the results to those obtained with another commonly used kinetic model which ignores all frequency dependence in a spectroscopic resonance. We also collected direct absorption and unsaturated fluorescence excitation spectra in the jet, and extensive laser beam diagnostics, to parametrize the kinetic model and to provide an independent determination of the absorption cross section. The kinetic fit yielded cross sections around 3 Å2 for the transitions, in good agreement with the direct absorption measurement, and excited-state lifetimes of 760±230 ps, in reasonable agreement with literature values from condensed-phase experiments.
A multiconfiguration self-consistent-field (MCSCF) study of the bent and linear conformations of HCCN
Authors: Rice Julia E. , Schaefer Henry F.
Published: July 2002

An MCSCF procedure of the complete active space (CAS) genre has been used to predict the structures and energetics of the HCCN molecule. Full configuration interaction (CI) within the π-electron space (1πx, 1πy, 2πx, 2πy, 3πx, 3πy orbitals, six electrons) was used in these MCSCF studies. With double zeta plus polarization (DZP) and triple zeta plus double polarization (TZ2P) basis sets, the bent structure lies 6.2 and 5.7 kcal, respectively, below the linear conformation. Multireference CI wave functions reduce this energy difference significantly.
Orbitally correlated crystal field parametrization for lanthanide ions
Authors: Yeung Y. Y. , Newman D. J.
Published: July 2002

Analogous to the treatment of the spin-correlated crystal field, a new type of lanthanide correlation crystal field, which depends on the relative orientations of the individual 4f electron orbital angular momenta and the total orbital angular momentum, is postulated. Associated sets of parameters have been obtained for TmCl3⋅6H2O, Er3+:LaCl3, and Pr3+:LaCl3 from the quadratic moments of the J-multiplet energy levels. The results show that this field is noticeable in the Pr3+:LaCl3 spectrum in which the particularly well known deviance of the 1D2 multiplet found in conventional crystal field fitting is largely eliminated.
Calculation of photoelectron angular distributions from resonant multiphoton ionization of atomic carbon
Authors: Tang X.
Published: July 2002

I have calculated the photoelectron angular distributions following two-photon resonant, three-photon ionization of atomic carbon. The results provide a unified interpretation of existing experimental data.
ESR spectrum of IO2−4
Authors: Byberg J. R.
Published: July 2002

A paramagnetic species prepared from IO−4 embedded in solid KBrO4 by x irradiation at 26 K is identified as the radical-ion IO2−4 because its spin Hamiltonian is closely analogous that of BrO2−4. Labeling with 17O shows that IO2−4 is a σ radical with the electron spin located primarily on two oxygens atoms and on the iodine atom, whereas the nuclear quadrupole interaction indicates that the nuclear geometry of IO2−4 resembles that of the parent IO−4. Due to the large hyperfine splitting (∼4 GHz) only one allowed ESR transition and an NMR transition appear at 9.3 GHz, whereas the six allowed ESR transitions are all observed at 35 GHz. A similar signal observed in x irradiated KIO4 is also assigned to IO2−4.
A time resolved EPR study of the lowest excited triplet states of α-chloroanthraquinones
Authors: Yamauchi Seigo , Hirota Noboru
Published: July 2002

Time resolved EPR and optically detected magnetic resonance (ODMR) experiments were performed on the lowest excited triplet (T1) states of α-chloroanthraquinones at low temperatures (77–1.4 K). In contrast to the case of anthraquinone smaller zero field splittings (zfs; 4–6 GHz) were obtained for 1-chloroanthraquinone (1ClAQ) and 1,5-dichloroanthraquinone (1,5DClAQ). And unusually small zfs (≲1.2 GHz) were obtained for 1,8-dichloroanthraquinone (1,8DClAQ). The vibrational analyses of the phosphorescence spectra of 1 ClAQ and 1,5DClAQ were made. Long progressions of n×184(201) cm−1 bands were ascribed to the C–Cl bending mode, which indicates distorted structures of the T1 states. It is concluded that the T1 states of 1ClAQ and 1,5DClAQ show intermediate characters between those of 3nπ* AQ and T1 of 1,8DClAQ. The possibility of T1 1,8DClAQ being a 3ππ* or a mixed 3ππ*–3nπ* state is denied on the basis of the obtained results, particularly very small zfs. A nπ* (n=an0−bnCl; a>b>0) state with a delocalized n electron is suggested as a possible candidate for T1 of 1,8DClAQ.
Diffusion Monte Carlo simulation of condensed systems
Authors: Coker D. F. , Watts R. O.
Published: July 2002

Diffusion Monte Carlo calculations are reported for liquid 4He using a Lennard-Jones potential and for solid molecular hydrogen using a reliable spherical pair potential and Axilrod–Teller three body interactions. Calculated thermodynamic and structural properties are in good agreement with experiment over a range of densities. Discrepancies between calculated and experimental values for hydrogen at the highest densities are attributed to the neglect of anisotropy in the calculations.
Crossed beam study of the reaction of van der Waals molecule O+(NO)2→NO*2+NO
Authors: Honma Kenji , Kajimoto Okitsugu
Published: July 2002

The reaction O+(NO)2 was studied by use of a crossed-beam apparatus. An (NO)2 beam generated by the supersonic expansion crossed at right angles with a collimated effusive beam containing oxygen atoms which were formed by microwave discharge. The product NO*2 was detected by the chemiluminescence. The angular distribution of the product was measured by an angle-resolved emission detection technique. The distribution has forward and backward peaks with respect to the oxygen atom incidence and indicates the occurrence of an intermediate complex in the course of the reaction. The angular velocity distribution, as well as the onset of the chemiluminescence, indicates that the reaction exothermicity appears mostly as internal energy of NO*2 . The remaining energy flows into the product translation and rotation; the vibrational freedom of the third-body molecule is not effective as an energy absorber. The emission intensity was found to decrease with increasing relative collision energy. A sharp drop of the intensity was observed at 2 kcal/mol of collision energy, which is in good agreement with the bond energy of the NO dimer.
Nonadiabatic effects in multiphoton transitions: A coupled equations study
Authors: Bandrauk André D. , Gélinas Nadia
Published: July 2002

A coupled equations method is used to examine nonadiabatic effects on two- and three-photon transitions in diatomics. Using C12 as a test case, it is shown that nonadiabatic corrections are less important for virtual (nonresonant) transitions than in the case of resonant transitions. The contributions from virtual transitions diminish with the order of the multiphoton transition. Finally, the present method enables one to calculate transition amplitudes. The phase information in such amplitudes are shown to be useful to identify photon pathways in some multiphoton transitions especially at high intensities where the coupled equations approach enable us to go beyond perturbative methods.
Fluid dynamical effects of polymers adsorbed to spherical particles
Authors: Anderson John L. , Kim Jeen-Ok
Published: July 2002

The effects of adsorbed macromolecules (polymers) on certain transport coefficients of spherical particles are determined using methods of matched asymptotic expansions to solve the Stokes flow equations within and outside the polymer layer. The main advantage of this analysis is that it allows direct substitution of a linear, viscous model for flow through the polymer segments to obtain the desired transport coefficient without solving the entire set of fluid dynamics equations. The results, expressed as an effective ‘‘hydrodynamic thickness’’ (Lh) of the adsorbed polymer, are accurate to O(λ) where λ is the ratio of the polymer-layer length scale to the particle radius. Three flow configurations are considered: steady translation and steady rotation of a single particle, and pure straining flow around a single particle that produces stresses leading to enhanced viscosity of a suspension (intrinsic viscosity). The Reynolds number is assumed to be small in all cases. The O(λ0) term for Lh is the same for all three flows, but the O(λ) corrections differ. Sample calculations show that (1) the O(λ) correction is negative, meaning Lh increases as particle radius increases assuming all other conditions are constant; (2) a free-draining model for flow through the polymer chains is a very good approximation for calculating Lh if the Stokes radius of the polymer segments is much smaller than the length scale of the polymer scale; and (3) the presence of only a small amount of adsorbed polymer ‘‘tails’’ can make a significant contribution to Lh if the length scale of the extended tails exceeds the length scale of the ‘‘loops’’ by a factor of 2 or more. Finally, we derive the hydrodynamic response of a particle having polymer nonuniformly adsorbed to its surface and demonstrate through a sample calculation how an ‘‘adsorption dipole’’ can lead to strong orientational bias of the particle as it moves in response to an applied force.
A universal Gaussian basis set for atoms Li through Ne based on a generator coordinate version of the Hartree–Fock equations
Authors: Mohallem J. R. , Trsic M.
Published: July 2002

A recently formulated generator coordinate version of the Hartree–Fock equations is applied to atoms Li through Ne. The Griffin–Hill–Wheeler–Hartree–Fock equations are integrated numerically generating a universal Gaussian basis set for these atoms. The results for the total energy are compared with Hartree–Fock–Roothaan (both with Slater-type and Gaussian orbitals) calculations from the literature.
Instability of the trimeric antiferromagnetic chain
Authors: Zaspel C. E.
Published: July 2002

It is shown that the trimerized xy chain with only nearest-neighbor exchange interactions is unstable. The stable structure is a chain of dimers interacting through an intermediate spin.
Evidence for resonant intermolecular coupling in liquid benzene and pyridine from Raman difference spectroscopy of isotopic mixtures
Authors: Meinander N. , Strube M. M. , Johnson A. N. , Laane J.
Published: July 2002

Raman difference spectroscopy (RDS) affords a powerful tool for measuring very small frequency differences between similar samples. This technique has been used to measure the changes upon isotopic dilution in band frequencies and bandwidths for a number of vibrations of liquid benzene and liquid pyridine. Both blue and red shifts of the vibrational frequencies were observed, and these were interpreted in terms of additive resonant intermolecular coupling and volume effects. Both effects may provide either positive or negative contributions to the frequency shifts. The frequency shifts observed in mixtures containing both benzene and pyridine have also been interpreted in terms of resonance and volume effects.
The microwave spectrum of the HBF+ ion
Authors: Saito Shuji , Yamamoto Satoshi , Kawaguchi Kentarou
Published: July 2002

Rotational transitions of the HBF+ ion were observed in the region up to 380 GHz by using a source-modulated microwave spectrometer combined with a hollow cathode free space cell. The HBF+ (or DBF+) ion was generated by a dc discharge in a mixture of BF3 and H2 (or D2). The ion was identified on the basis of the linear-molecule spectral pattern with the nuclear hyperfine structure of the boron nucleus, supported by the spectral-intensity lowering by an external magnetic field. The rotational constant, the centrifugal distortion constant, and the nuclear quadrupole coupling constant were precisely determined for three isotopic species: H11BF+, H10BF+, and D11BF+. From the observed ground-state rotational constants the combined rs parameters were obtained; rs(BH) =1.173 47(2) Å and rs(BF) =1.210 28(2) Å.
Primary ion dependence of LiF direct recoil intensities and ion fractions
Authors: Chen J. N. , Shi M. , Rabalais J. W.
Published: July 2002

Time-of-flight (TOF) spectra of the scattered and recoiled particles resulting from 1–10 keV He+, Ne+, Ar+, Kr+, and Xe+ ions impingent on surfaces of LiF thin films have been obtained. Measurements of directly recoiled (DR) neutrals plus ions and neutrals alone are used to calculate positive and negative ion fractions Y+,− from DR events. The oppositely charged ion fractions have a distinctly different behavior as a function of kinetic energy. The Y+ values exhibit a threshold at low energy followed by a plateau region at higher energy while the Y− values are maximum in the low energy region followed by a decreasing yield as energy increases. The energy dependence of Y+,− is interpreted in terms of the recently developed model [J. Chem. Phys. 85, 3615 (1986)] for electronic charge exchange in keV ion/surface collisions which considers electron promotions in the close atomic encounter and resonant and Auger transitions along the outgoing trajectory. The ionization potential of the primary ion relative to the energy levels of the target atom is shown to have a large influence on charge exchange in the close encounter. The ratio of direct recoil to scattering particle flux increases by a factor of >102 from He to Xe; scattering and recoil cross sections are used to model this process.
Alkane activation: Adsorption and reaction of n-butane on clean and carbided W(100)
Authors: Liu A. C. , Friend C. M.
Published: July 2002

The adsorption and reaction of n-butane have been investigated on clean and carbided W(100) surfaces using temperature programmed reaction spectroscopy, Auger electron spectroscopy, low energy electron diffraction, and isotopic exchange methods. For the clean surface, the initial adsorption probability is 1.0 at a crystal temperature of 130 K. At low coverages, butane adsorbed at 130 K converts to a dissociated state with unit probability below 200 K during temperature programmed reaction. Higher exposures result in a decrease in the adsorption probability and the onset of molecular butane desorption in the range of 150–200 K. A weakly adsorbed molecularly bound butane state is observed for all coverages on the [13  −10] and (5×1) carbide surfaces. Studies using n-butane-d10 show a dramatic isotope effect. The low coverage dissociation probability of n-C4D10 is significantly less than unity and the saturation amount of decomposition in a temperature programmed reaction experiment is less than for n-C4H10. This is evidence for a kinetic isotope effect in the decomposition process, with the activation barrier for C–D bond scission greater than the barrier for C–H bonds. For butane exposures made above 200 K, the initial sticking probability is lowered, and the saturation value for decomposition is also decreased. Preadsorption of deuterium suppresses greater than 90% of the butane decomposition, while postadsorption of D2 under the same conditions blocks less than 5%. These observations suggest that for low butane exposures, there is a substantial amount of dehydrogenation upon adsorption which eventually leads to complete decomposition. As the crystal temperature is increased, buildup of surface hydrogen and possibly carbon from further decomposition rapidly inhibits dissociation of additional butane and results in molecular desorption. A limited amount of butane decomposition occurs on the intermediate coverage [13  −10] carbide surface formed following temperature programmed reaction of a saturation exposure of butane. The amount of butane decomposition on the [13  −10] carbide is an order of magnitude less than on clean W(100). Likewise, no decomposition of butane is observed on the W(100)–(5×1)-C surface.
A unified treatment of 2Π–2Π Rydberg–valence state interactions in NO
Authors: Raoult M.
Published: July 2002

Generalized quantum defect theory is used to study the 2Π–2Π Rydberg–valence state perturbations in the NO molecule. The work is a generalization of the two-step quantum defect treatment used previously by Giusti-Suzor and Jungen for the calculation of competing preionization and predissociation processes in NO. The existing high-resolution spectroscopic data, mainly from Miescher and co-workers, are used to extract a set of quantum defect parameters relating to the pπ Rydberg channel associated with the NO+ X 1Σ+ state, and to determine potential energy curves for the B 2Π and L 2Π states as well as Rydberg–valence interaction parameters. Based on these parameters, the observed term values including rotational and spin–orbit effects are reproduced with a mean deviation of 4 cm−1 for all the 2Π levels from 45 500 to 71 600 cm−1 (i.e., to within 200 cm−1 of the dissociation limit). The fit is better than that previously obtained by Gallusser and Dressler in a vibronic perturbation approach, particularly for the levels near the dissociation limit. The set of parameters is used for the calculation of competing electronic predissociation and vibrational preionization of Rydberg levels lying above the dissociation limit and the v+=0 ionization limit. The resonance widths and energies measured recently by Anezaki et al. by a MPI double resonance technique are better reproduced by the present calculation than in the previous work of Giusti-Suzor and Jungen.
Solvent induced vibrational relaxation in diatomics. I. Derivation of a local relaxation rate
Authors: Herman Michael F.
Published: July 2002

A local rate is derived for the vibrational population relaxation of a diatomic in a simple liquid or dense gas. The total relaxation rate of the system is obtained as the canonical ensemble average of the local rate. The rate expression is amenable to computer simulations in which the canonical average is performed by a Monte Carlo procedure. The vibrational motion is separated from the other degrees of freedom by an adiabatic approximation which treats the vibration as fast compared with the other motions. The adiabatic vibrational energies and the nonadiabatic couplings between vibrational states depend on the solvent configuration. These vibrational energies and couplings are obtained from quantum perturbation theory. The transitions between vibrational states are described semiclassically and the canonical averaging uses the classical canonical density for solvent configurations. The resulting procedure is a mixed quantum–semiclassical–classical simulation technique.
Absorption and circular dichroism due to bending vibrations of A2B2 molecules with C2 symmetry
Authors: Polavarapu P. L.
Published: July 2002

A formalism is developed to predict the relative absorption and circular dichroism intensities for the symmetric and antisymmetric bending vibrations of A2B2 molecules belonging to C2 symmetry. Predictions for three different situations are as follows: (a) when partial charges are assumed to follow the nuclei and contributions from the charge reorganization during the vibrations are negligible, the dissymmetry factor for symmetric bending vibration is about two orders of magnitude larger than that for antisymmetric bending vibration with opposite sign; (b) when the fixed partial charge contributions are negligible and charge flow along the B–B bond is of the same order as that along A–B bond, for molecules with ∼60° dihedral angle the dissymmetry factors for symmetric and antisymmetric bending vibrations are of nearly equal magnitudes with opposite signs; (c) when the fixed partial charge contributions are negligible and the charge flow along A–B bond is much greater than that along B–B bond the dissymmetry factors follow a simple relation. The latter two points are also applicable to the symmetric and antisymmetric A–B stretching vibrations.
A theory of electrolyte solutions near a polarizable surface
Authors: Vertenstein Mariana , Ronis David
Published: July 2002

The electrical double layer is studied using a cluster perturbation theory which incorporates the effect of images and fluctuations due to ion correlations. These equations are solved numerically and ion density profiles are determined for a wide range of surface charge and dielectric discontinuity. Two choices of the perturbation potential, corresponding to either the total electrostatic interaction or the polarization energy, are examined for a 1–1 electrolyte. Both theories exhibit excellent agreement with Monte Carlo results at low surface charge density and electrolyte concentration, but as the surface charge is increased for any dielectric discontinuity, the former theory overestimates the role of fluctuations, while the latter theory does better for intermediate values of the wall charge.