bluuebunny/crossref_metadata_2025 huggingface dataset:
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o theoretical studies of the cation radicals 12C2 16O+2, 12,13C2 16O+2, 13C2 16O+2, 12C2 16,17O+2, 12C2 17O+2, and 12,13C2 16,17O+2 isolated in neon matrices at 4 K. The use of matrix isolation for trapping ion–neutral reaction products
Authors: Knight Lon B. , Steadman J. , Miller P. K. , Bowman D. E. , Davidson Ernest R. , Feller David
Published: February 2003

An experimental procedure for generating and trapping the products of ion–neutral reactions has been developed. The method has been applied to a neon matrix ESR study of the C2O+2 radical (X 2Bu) formed during deposition at 4 K by the reaction CO++CO. Six different isotopic combinations of C2O+2 were studied which allowed a complete characterization of the 13C and 17O nuclear hyperfine structure. The experimental A tensors were compared with the results of an extensive SCF and CI theoretical calculation. A full discussion of the theoretical procedure utilized is presented. The electronic ground state was determined by a CI calculation to be the planar trans configuration with a CCO bond angle of 141°. A description of the MO containing the unpaired electron is presented and compared with CO+ and the isoelectronic anion radical C2N−2. The effects of noncoincidence between the g and A tensors are considered in detail for this powder sample of C2O+2 which exhibited relatively narrow ESR lines in a neon matrix at 4 K. The observed g values were gx=2.0034(2), gy=2.0019(2), and gz=1.9912(2). The effective 13C A parameters observed in the principal g tensor axis system were Ax=577(1), Ay=606(1), and Az=583(1) MHz. Experimental estimates of Ayz varied from about 7 to 32 MHz. The 17O A tensor had components of Ax≊0, ‖Ay‖=74(1) MHz and Az≊0. Analysis of the g tensor for C2O+2 indicates the presence of a low lying excited electronic state (2Au) which is predicted at ≊18 000 cm−1 by an SCF theoretical calculation.
Lifetimes of N2O+(A), CS+2(A), and CS+2(B) states produced by pulsed EUV photons
Authors: Ibuki Toshio , Sugita Nobuyuki
Published: February 2003

Pulsed Ne i resonance lines (73.6 and 74.4 nm) were generated by a dc discharge lamp consisting of a charging resistor, a coaxial capacitor, and a spark gap. By a single-photon counting technique, the radiative lifetimes of N2O+(A 2Σ+, v′1=0 and 1), CS+2(A 2Πu), and CS+2(B 2Σ+u) states were measured. The Stern–Volmer treatments gave the quenching rate constants for the excited ions in the collision with the neutral parent triatomic molecules.
Femtosecond study of the recovery dynamics of malachite green in solution
Authors: Erskine D. J. , Taylor A. J. , Tang C. L.
Published: February 2003

The ground state recovery process of malachite green in water and ethylene glycol has been studied in the time domain with ∼50 femtosecond resolution using the equal-pulse correlation technique. The observed recovery law is consistent with a single exponental decay with time constant t21=0.93+/−0.05 ps for malachite green in water and t21=4.46+/−0.05 ps for malachite green in ethylene glycol.
Application of cluster expansion techniques to open shells: Calculation of difference energies
Authors: Haque Md. Azizul , Mukherjee Debashis
Published: February 2003

In this paper, we have tested the efficacy of a recently developed nonperturbative open-shell formalism in generating such difference energies as ionization potential (I. P.), electron affinity (E. A.) and excitation energy (E. E.). In the formalism, the difference energies come out directly as eigenvalues of a non-Hermitian eigenproblem. Two different kinds of cluster expansion about multideterminant ‘‘model’’ wave functions have been considered: (a) an ordinary Ursell–Mayer type exponential form of cluster expansion; and (b) a normally ordered exponetial cluster ansatz. The key theoretical concept underlying our development is a generalization of the ‘‘core-valence separability’’ concept of the open-shell manybody perturbation theory which we have termed the ‘‘subsystem embedding condition’’ (SEC). SEC allows us to start with the zero valence problem and offers an unambiguous way of building up the successive one, two, ..., n-valence problems hierarchically furnishing the difference energies. I. P., E. A., and E. E. of a selection of nitrogen heterocycles under the π-electron approximation have been calculated and the performance of the methods has been assessed against the ‘‘model exact’’ full CI results. The trend of the results has been found to be encouraging.
Energy formulas for atoms and molecules
Authors: Palke William E.
Published: February 2003

Exact expressions are derived for the nonrelativistic energy of an atom in terms of its chemical potential and the electrostatic potential at the nucleus. The formulas do not require the consideration of fractional numbers of electrons. Generalizations to multinuclear systems are included.
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o quantum chemical study of the dimerization of silicon monoxide
Authors: Snyder Lawrence C. , Raghavachari Krishnan
Published: February 2003

As part of a theoretical program to understand the fundamental molecular constituents of silicate glasses, we have carried out accurate quantum chemical computations to describe silicon monoxide (SiO), its dimer (Si2O2) and the process of dimerization. The dimer Si2O2 is found to be cyclic with alternating silicon and oxygen atoms in the ring. Each silicon atom in the dimer is bonded to two oxygen atoms; It is in a divalent state, in contrast to the tetravalent state of silicon in silicate glasses. The equilibrium SiO bond length in the dimer is computed to be 1.683 Å and OSiO bond angle to be 85.7° as compared with 1.59 Å bond lengths and 109.5° bond angles in typical silicate glasses. The heat of formation of the dimer is predicted to be −43.0 kcal/mole, in excellent agreement with the gas phase expeimental value of −44.6±3.0 kcal/mole. The normal vibrational modes and frequencies of the cyclic dimer have been computed, and are in qualitative agreement with the frequencies of infrared absorptions attributed to the dimer in matrix isolation experiments. We have computed potential energy surfaces for the reaction of two SiO monomers to form the cyclic dimer, and find no barrier to dimerization. The SiO bond length and OSiO bond angle of the dimer are close to those observed for amorphous silicon monoxide by x-ray diffraction. This suggests that amorphous silicon monoxide also contains divalent silicon.
On the Mg(3P)–He(1S) interaction using SA-MCSCF/ICF-CI wave functions
Authors: Pouilly Brigitte , Lengsfield Byron H. , Yarkony David R.
Published: February 2003

State averaged-multiconfiguration self-consistent-field (SA-MCSCF)/interacting correlated fragment-configuration interaction (ICF-CI) wave functions developed from an extended basis of Slater type orbitals are used to determine the portion of the nonrelativistic Born–Oppenheimer 3Σ+ and 3Π potential energy curves (PEC’s), relevant to fine structure changing collisions in the system Mg(3P)-He(1S). Using the finite perturbation method the parallel and perpendicular polarizabilities of Mg(3P) at the SA-MCSCF level are found to be α∥ =120 a.u.3 and α⊥ =84.5 a.u.3 The PEC’s exhibit the requisite (weak) long-range attraction, with E(3Σ+)α⊥. The 3Σ+ and 3Π curves, which are rigorously degenerate at R(Mg–He)=∞, cross near R(Mg–He)=14 a.u. The 3Σ+ curve exhibits a very shallow minimum (∼1.5 cm−1) near R(Mg–He)=13.7 a.u. while a deeper minimum ∼16 cm−1 is found for the 3Π curve near R(Mg–He)=7.9 a.u. The effects of spin-orbit coupling are incorporated in a semiempirical manner. The SA-MCSCF/ICF-CI approach is compared with the results of a more standard MCSCF/ICF-CI treatment.
Monte Carlo simulation of electron thermalization in gases. II. Subexcitation electrons in molecular hydrogen
Authors: Koura Katsuhisa
Published: February 2003

The Monte Carlo simulation (MCS) of the thermalization of low-energy electrons (ε≲0.1 eV) due to the rotationally inelastic and elastic collisions in normal H2 [J. Chem. Phys. 79, 3367 (1983), referred to as I] is extended to high-energy subexcitation electrons (ε∼1 eV) by taking into account the vibrationally inelastic collisions and using available experimental cross section data. The MCS is performed for the thermalization of subexcitation electrons with the initial Maxwell, δ function, or Platzman velocity distribution at the initial effective electron temperature 103≤Te(0)≤3×104 K in normal H2 at the gas temperature 77≤T≤103 K. The electron velocity distribution deviates significantly from the local Maxwell distribution (MD) even for the initial Maxwell distribution owing to the vibrationally and rotationally inelastic collisions. Consequently, the degradation of the effective electron temperature Te (reduced mean electron energy) is slower than that obtained with the MD assumption. The thermalization time τth when Te/T=1.1 is insensitive to the initial electron velocity distribution and effective electron temperature. At T≳300 K, τth is about 70% larger than that for the MD, where τth is dominated by the rotationally inelastic collisions. At the low gas temperature T=77 K, τth is about 160% larger than that for the MD, where τth is dominated by the elastic collisions. The MCS value of τth p=3.4  μs Torr at T=300 K, where  p is the gas pressure, is about 130% larger than the experimental value of 1.5  μs Torr obtained by Warman and Sauer using the assumption of the constant energy exchange rate coefficient and about 40% larger than the theoretical value of 2.37  μs Torr obtained by Tembe and Mozumder using the approximate theory based on the MD assumption.
Studying the nonintegrability of Hamiltonian systems using the Kolmogorov entropy: Application to the Hénon–Heiles system
Authors: Termonia Yves
Published: February 2003

The present work proposes to use the Kolmogorov entropy K as a criterion for the determination of the critical energy above which an Hamiltonian system becomes nonintegrable. Application to the Hénon–Heiles system shows that this system is totally nonintegrable, i.e., K>0 at all energies, though none of the other criteria used so far were sensitive enough to detect this fact at energies below 1/12.
Application of the semiclassical perturbation approximation to scattering from surfaces. Generalization to include phonon inelasticity
Authors: Hubbard Lynn M. , Miller William H.
Published: February 2003

The semiclassical perturbation (SCP) approximation of Miller and Smith is applied to the inelastic scattering of atoms from solid surfaces. Earlier applications of this approach have been to diffraction (i.e., elastic scattering) of He from LiF and to diffraction and rotationally inelastic scattering of H2 from LiF, and the present work is an extension which allows for motion of the surfaces atoms (and thus phonon inelasticity). Therefore, phenomena that involve energy transfer between the surface and gas molecules can be described. In the present work sticking probabilities are calculated for He–W(110) and Ne–W(110), and the results show that a simple model such as this one can be quite useful in predicting energy transfer in gas–solid scattering systems.
Molecular dynamics study of the two-dimensional Lennard-Jones equation of state
Authors: Bruin C. , Bakker A. F. , Bishop Marvin
Published: February 2003

The Delft MD processor has been used to simulate a two-dimensional Lennard-Jones fluid for a variety of densities at the reduced temperature of 1.00. The equation of state obtained is described excellently by the Padé approximant of Barker–Henderson–Abraham.
Diffusion of aromatic compounds: An investigation on the effects of molecular shape, mass, and dipole moment
Authors: Chan T. C.
Published: February 2003

Diffusion coefficients of pseudoplanar molecules p-chlorotoluene, (o-, m-, and p-) dichlorobenzene, and (o-, m-, and p-)xylene at infinite dilution in acetone, ethanol, and n-tetradecane have been precisely measured at 298.2 K by using the Taylor dispersion (chromatographic broadening) technique. It is found that the limiting mutual diffusion coefficients are generally insensitive to the mass and dipole moment of the solutes. The data are, however, slightly dependent on the shape of the solute molecules. The effects of the solvent are reported.
Rational fraction analytic continuation method for complex resonance energies in multidimensional systems
Authors: Thirumalai Devarajan , Thompson Todd C. , Truhlar Donald G.
Published: February 2003

The method of analytic continuation of stabilization eigenvalues to calculate resonance energies and widths is improved by incorporating rational fraction fitting techniques. The refined method is applied successfully to seven collisional resonances in collinear chemical reactions.
A comparison of the predictions of various model N2–He potential energy surfaces with experiment
Authors: McCourt Frederick R. , Fuchs R. R. , Thakkar A. J.
Published: February 2003

Predictions of beam scattering and bulk gas phenomena based upon five different model potential energy surfaces for the N2–He system are compared with experiment. The surfaces considered are our recent HFD1 and HFD2 surfaces based on the Hartree–Fock plus damped dispersion (HFD) model, the surface of Habitz, Tang, and Toennies (HTT) based upon the Tang–Toennies model, the surface of Keil, Slankas, and Kuppermann (KSK) and a modification (KKM3) of the KSK surface. The physical observables against which these surfaces are tested include total differential scattering cross sections, state-to-state inelastic differential scattering cross sections, interaction second virial coefficients, shear viscosity and binary diffusion coefficients, and the relaxation cross section for the Senftleben–Beenakker effect on the shear viscosity. None of the surfaces is in complete agreement with all of these observables. For the interaction second virial coefficients, the shear viscosity and binary diffusion coefficients, the HFD1 surface is the only one to predict values within most of the experimental error bars. The relaxation cross section is correctly predicted only by the KKM3 surface which was essentially fitted to it. The HFD1, HFD2, and HTT surfaces are all in good agreement with the state-to-state inelastic cross sections. The KSK surface gives the best agreement with the total differential cross section. It appears that an accurate N2–He surface cannot be obtained from simple models, and its determination will require multiproperty fits.
Electron impact spectroscopy of methane and methane–d4
Authors: Dillon M. A. , Wang R.-G. , Spence David
Published: February 2003

Electron impact spectra of methane and methane-d4 have been obtained employing electrons of 200 eV incident energy at scattering angles of 2° to 15° over an energy-loss range that includes the second ionization potential. In the bound state spectrum evidence is presented for the existence of at least one additional state in the region of the 3s transition. A new vibrational progression has been found on the side of a conjectured 4s band lending support to that assignment. One diffuse structure and three new optically forbidden vibrational progressions have been found in a 4 eV range below the 2A1 ion threshold. Evidence indicates that the first three transitions belong to the symmetry forbidden Rydberg series (2a1, ns)←(2a1)2 where n=3, 4, and 5. It is suggested that the fourth transition is the first member of a series converging to an excited ion state.
Dynamics of liquid state chemical reactions: Photolysis of molecular iodine in liquid xenon
Authors: Brooks C. L. , Adelman S. A.
Published: February 2003

The photodissociation and geminate recombination dynamics of molecular iodine present at infinite dilution in liquid xenon are studied via MTGLE classical stochastic trajectory simulations. The experimentally measured transient picosecond electronic absorption spectrum and geminate quantum yields are computed from the trajectories. The excitation pulse wavelength dependence of the photodynamics is studied. From the wavelength dependence of the dissociation pathway, it is inferred that there are observable consequences of curve-crossing phenomena which manifest themselves in the short-time behavior of the I2 transient electronic absorption spectrum. Also because of the presence of slow vibrational relaxation of the recombining iodine atoms, the apparent asymptote of the transient spectra may not accurately reflect the geminate quantum yield. The probe pulse wavelength dependence of the transient absorption spectrum is also studied. Here it is observed that: (i) Longer wavelength interrogation light yields shorter time scale dynamical information concerning the energy relaxation of nascent I2 molecules. (ii) Interrogation at several probe wavelengths can yield fairly detailed information about the X state time-dependent vibrational level distribution of nascent I2 molecules.
Excitation spectrum of the chemical reaction F2+O2 in the red spectral range
Authors: Frei Heinz
Published: February 2003

A search has been made for new, low-energy pathways of the reaction F2+O2→O2F+F or O2F2 through excitation of F2 or O2 in an oxygen matrix with red and near infrared photons. By monitoring the product yield as a function of laser irradiation frequency, a reaction excitation spectrum of O2+F2 was recorded which begins at 14 500 cm−1 and extends through the entire red spectral range. It revealed two discrete absorptions O2 1Σ+g, v′=1←3Σ−g, v″=0 and (O2)2(1Δg,1Δg)0,0 ←(3Σ−g, 3Σ−g)0,0, and a continuous, oxygen-enhanced absorption of F2 which is assigned to excitation to the repulsive part of the weakly bound ΠOu+3 state. The F2 absorption detected in this experiment extends more than 200 nm to longer wavelengths than any electronic absorption of molecular fluorine reported so far.
Localization and delocalization. II. Role of overlap in interbond interactions
Authors: Mayer István , Surján Péter R.
Published: February 2003

A special perturbational theory for intramolecular electron delocalization is presented taking into account the nonorthogonality of basis orbitals. First, a biorthogonal formulation is proposed in which no series expansion is made for the overlaps. Then a partition of the overlap matrix S is also introduced which permits to incorporate the overlap effects into the perturbational expansion. The latter approach allows to discuss the role of overlap in ‘‘through space’’ and ‘‘through bond’’ interactions causing electron delocalization. Both formalisms are illustrated by some numerical examples.
Molecular motion in thermotropic liquid crystals: Deuterium relaxation and spectral densities of motion for a highly ordered spin probe in 4-octyl-4′-cyanobiphenyl
Authors: Selwyn Lyndsie S. , Vold Robert L. , Vold Regitze R.
Published: February 2003

Deuterium relaxation data obtained at 9.2 and 38.4 MHz are reported for a solution of p-diethynyl-d,d′-benzene (DEB) in 4-n-octyl-4′-cyanobiphenyl (8CB). The temperature and frequency dependences of the spectral densities of motion for this highly ordered spin probe in 8CB are markedly different from those observed for DEB in nematics without underlying smectic phases. J1(ω0) and J2(2ω0) are both frequency dependent and because of the high degree of ordering observed for the deuterons (Szz =0.28–0.67) we have interpreted the relaxation data in terms of ‘‘rotational diffusion in a cone.’’ Given the simplistic nature of this model, the semiquantitative agreement observed between theory and experiment is satisfactory.
Site geometry of trivalent iridium in AgBr
Authors: Deri R. J. , Spoonhower J. P.
Published: February 2003

The geometry of Ir3+ ions in AgBr has been investigated by vibrational spectroscopy. The results provide definitive evidence of a distorted IrBr3−6 complex. The nature of the distortion is tetragonal or lower, and probably not trigonal. Details of the distortion are discussed in terms of geometries involving charge-compensating silver ion vacancies.
Photon correlation spectroscopy of particle polydispersity: A cubic B-spline analysis
Authors: Lavery A. N. , Earnshaw J. C.
Published: February 2003

A cubic B-spline model has been applied to the analysis of photon correlation data from heterogeneous suspensions of particles. The analysis procedure has been tested with simulated data corresponding to unimodal and bimodal size distributions and used in experimental studies of aqueous suspensions of polystyrene latex microspheres.
Structure and dynamics of excited singlet states of isolated diphenylhexatriene
Authors: Kohler Bryan E. , Spiglanin Thomas A.
Published: February 2003

The fluorescence spectrum, one photon fluorescence excitation spectrum, and single vibrational level decay kinetics have been measured for 1,6-diphenyl-1,3,5-hexatriene seeded in helium in a supersonic free jet. The 2 1Ag doubly excited state (S1) is located more than 3400 cm−1 below the strongly absorbing 1 1Bu state (S2) which is 29 157 cm−1 above the ground state. The lowest energy observed vibronic band of S1 is at 25 741.8 cm−1; emission produced by excitation into this band has a lifetime of 90.7 ns. The vibronic development of the excitation spectrum shows that this band is a vibronically promoted false origin. Fluorescence excitation intensity for this band is smaller than that of the S0 to S2 fluorescence excitation origin by at least a factor of 104. Direct excitation of S2 in the isolated molcule is followed by relaxation, fast on the nanosecond time scale, to isoenergetic vibrationally excited levels of S1. This is apparent both from the observed fluorescence spectrum and the fluorescence decay time (48.1 ns at the S2 origin) which is more than an order of magnitude longer than the intrinsic fluorescence lifetime of S2.
The two-dimensional potential energy surface for the ring puckering and ring twisting of 1-silacyclopent-3-ene-d, 1-d1, and 1, 1-d2
Authors: Killough P. M. , Laane J.
Published: February 2003

The far-infrared spectra of 1-silacyclopent-3-ene and an isotopic mixture containing both 1-d1 and 1, 1-d2 species have been recorded and analyzed. In addition to the main ring-puckering series, a weak side band series, resulting from molecules in the ring-twisting excited state, was observed for the undeuterated molecule. In the 330–520 cm−1 region of the infrared spectrum of the isotopic mixture, combination bands of the puckering with the SiHD or SiD2 rocking were observed. The ring-twisting fundamental was also assigned to the 380.6 cm−1 band in this region. Analysis of the data for all three isotopic species results in the following two-dimensional potential energy surface: V(cm−1)=2.08×105x41−0.005×105x21 +0.547×105x22+1.70×105x21x22 where x1 and x2 are the ring-puckering and ring-twisting coordinates (in Å), respectively. The potential minimum corresponds to a planar conformation and the positive cross term shows the vibrational interaction to be anticooperative.
Collisional energy exchange in highly vibrationally excited molecules: The biased random walk model
Authors: Gilbert Robert G.
Published: February 2003

A theory is developed for collisional energy exchange between a large highly vibrationally excited molecule and a rare gas, based on a diffusive motion for the internal energy of the molecule during the collision. The model is developed from a number of general trends deduced from the corpus of reliable experimental data and from trajectory studies. Since ‘‘energy diffusion’’ must be restricted to satisfy microscopic reversibility, one obtains a Smoluchowski equation for the distribution function for energy transfer in terms of one quantity s=(collision duration)1/2 × (‘‘energy diffusion coefficient’’)1/2. An expression for s is obtained (using linear response theory) in terms of properties such as the vibrational frequencies of the molecule. The simple model, with no adjustable parameters, is able to fit to ∼40% a very wide range of data: Some 50 experiments, with temperatures ranging from 300–1200 K and molecules ranging from haloethanes to C10–hydrocarbons. The model is improved by an empirical statistical treatment, and then offers a reliable means of estimating collisional energy transfer properties for rare gas/highly vibrationally excited molecule systems.
Electron–gas intermolecular potential of N2O–Ar. Comparison with mean square torques measurements
Authors: Dreyfus C. , Balou D. , Brigot-Dutartre N.
Published: February 2003

The intermolecular potential of the N2O–Ar system is computed using the Gordon–Kim electron–gas model for 12 distances and 17 angles. The electron–gas Hartree–Fock potential is compared to SCF pseudopotential calculations for three orientations: 0, π/2, and π. Taking into account the long range dispersive part of the potential, the mean square torque applied by a gaseous argon medium to the N2O molecule is computed at different temperatures and a comparison with experimental data is carried out.
Analysis of the optical spectrum of Np(BD4)4 diluted in Zr(BD4)4 and the magnetic properties of Np(BH4)4 and Np(BH3CH3)4
Authors: Rajnak K. , Banks R. H. , Gamp E. , Edelstein N.
Published: February 2003

The optical spectrum of Np(BD4)4/Zr(BD4)4 is reported and analyzed. The parameter values obtained are consistent with those for U(BD4)4/Hf(BD4)4. A total of 46 levels were fit with σ=84 cm−1. EPR data on Np(BD4)4/Zr(BD4)4 and Np(BH3CH3)4/Zr(BH3CH3)4 and the magnetic susceptibility of Np(BH3CH3)4 are reported. They could be fit with the eigenvectors from the optical analysis only by the inclusion of orbital reduction factors k=0.885 for Np(BD4)4 and 0.862 for Np(BH3CH3)4. These values indicate greater covalency for the methylborohydride.
Strong vibronic coupling in molecular Rydberg states
Authors: Whetten Robert L. , Grant Edward R.
Published: February 2003

We consider the vibronic properties of molecular Rydberg states in the strong coupling (degenerate) limit of the ion core, and show how the observed optical Rydberg spectrum for such a molecule is closely related to the Franck–Condon spectrum of its Jahn–Teller ion core, regardless of the symmetry labels attached to the Rydberg electronic species. These relationships permit the resolution of a quarter-century old paradox in the Rydberg spectrum of benzene and give rise to easily verifiable predictions concerning benzene’s vibronic spectrum, which are applied to a recently discovered new Rydberg series. We further suggest that observed deviations from the idealized Rydberg case discussed here will reveal additional details about the coupling of Rydberg electronic and core-vibronic motion.
On the mechanism of the CO self-exchange reaction
Authors: Rockwood A. , Bahler D. L. , McCullough E. A.
Published: February 2003

The gas phase exchange reaction with stoichiometry 13C 16O+12C 18O→13C 18O+12C 16O has been studied using a mercury photosensitization technique. The mechanism of the photosensitized exchange reaction is complex and almost certainly involves electronically excited CO(a 3Π). No evidence for a simple bimolecular exchange mechanism involving ground electronic state, vibrationally excited CO was found, although such a mechanism is not conclusively ruled out by the photosensitization experiments for vibrational levels higher than 9. Since such a mechanism has previously been proposed based on shock tube studies, a computer simulation of the shock tube exchange reaction was undertaken. One set of shock tube experiments can be reinterpreted quite well in terms of an atomic chain mechanism initiated by traces of O2 impurity. The mechanism of the second set remains a mystery.
Photoionization mass spectroscopy of Ne dimers
Authors: Trevor D. J. , Pollard J. E. , Brewer W. D. , Southworth S. H. , Truesdale C. M. , Shirley D. A. , Lee Y. T.
Published: February 2003

The photoionization efficiency curve of Ne2 as a function of wavelength has been obtained between 560 and 620 Å with a synchrotron radiation photon source. Detailed considerations of the autoionization mechanisms and symmetry of the excited states of Ne2 have led to the partial assignment of the excited electronic states of Ne2 which are responsible for the structure observed in its photoionization efficiency curve, and demonstrated the importance of Rydberg states with a B 2Πg core. The lower bound to the dissociation energy of Ne+2 was determined to be 1.24±0.08 eV (28.6±1.8 kcal/mol).
Complete determination of the state multipoles of rotationally resolved polarized fluorescence using a single experimental geometry
Authors: Bain A. J. , McCaffery A. J.
Published: February 2003

When laser radiation is used to prepare single rovibronic levels in molecules, the excited state Mj distribution is invariably polarized. In many such experiments the polarization of the excited state is ignored, which is an inadequate basis for accurate work as much valuable detail is lost. A better approach is a completely polarization resolved experiment in which the preparation, dynamics, and detection of the excited state polarization components (the state multipoles JJρKQ) are fully described. A treatment of polarized excitation in terms of the state multipoles JJρKQ is presented and consideration of excited state symmetry indicates that a common experimental geometry for linearly and circularly polarized excitation is feasible. A complete determination of the state multipoles (K=0,1,2) is shown to be possible within a single experimental geometry. It is shown that neglect of polarization phenomena can lead to ambiguities in the interpretation of some experiments.
A spectroscopic study of the F(0+u) ion-pair state of Br2 by the double resonance method
Authors: Shinzawa Tsutomu , Tokunaga Atsuto , Ishiwata Takashi , Tanaka Ikuzo , Kasatani Kazuo , Kawasaki Masahiro , Sato Hiroyasu
Published: February 2003

A new ion-pair state of Br2 has been observed by an optical–optical double resonance technique utilizing ultraviolet fluorescence detection. The OODR excitations proceed in an one-photon resonant three-photon absorption through the B3Π (0+u) state. The effects of polarization on their transition strengths are used to determine the symmetry of excited molecular state formed by two-photon absorption from the B state. The results indicate that the new state has the 0+u symmetry with Te=53 899.6(7) cm−1, ωe=155.8(2) cm−1, and re=3.276(8) Å for 79Br2. This state is expected to correlate with Br+(3P0)+Br−(1S) and named F (0+u) after the well-defined I2 molecule. The single rovibronic fluorescence spectra of the F(0+u) state show several transitions terminating on low-lying valence states.
Vibrational predissociation in the hydrogen fluoride dimer
Authors: DeLeon Robert L. , Muenter J. S.
Published: February 2003

A color center IR laser was used to excite the proton-accepting monomer of a HF dimer in a molecular beam. The maximum excess linewidth arising from vibrational predissociation was 10 MHz, indicating a vibrational predissociation lifetime of ≥30 ns. The maximum lifetime was estimated to be 300 ns from intensity considerations.
State selected removal of vibrationally excited NH2[X 2B1(0,ν2,0)] radicals
Authors: Gericke Karl-Heinz , Torres Lourdes M. , Guillory William A.
Published: February 2003

The influence of vibrational excitation on the decay rate of NH2 radicals in the presence of selected substrates has been studied using the laser-induced fluorescence (LIF) technique. The NH2 radicals were generated by infrared multiple photon dissociation (IRMPD) of selected precursors (N2H4, CH3NH2), and the state selected NH2(v″2 =0,1) decay was observed by means of the sensitive LIF measurement of [NH2]. The reactions studied were of the type: NH2(v]2 =0,1)+R→products, with R=NO, CH3NH2, N2H4. The reaction rates were determined under pseudo-first-order conditions, and were found to be strongly dependent on the vibrational state of the NH2 radical, e.g., kNO(v″2 =0)=1.4×10−11 cm3 molecule−1 s−1 and kNO(v]2 =1)=3.2×10−11 cm3 molecule−1 s−1. This influence of vibrational excitation on the total decay rates is discussed and compared with previously reported data on thermal excitation. In thermal studies of these same chemical reactions, a negative temperature coefficient was found for the reaction rate constants. The contribution of diffusion and of vibrational relaxation processes is also considered, as well as the relative importance of the two-body vs the three-body collision reaction channels: NH2+NH2→NH3+NH and NH2+NH2+M→N2H4+M for M=O2, N2, and Ar.
Semiclassical calculations of (Z+e−+e+) energy spectra
Authors: Grujić Petar V.
Published: February 2003

A modified Einstein–Keller–Maslov quantization, based on a quasi-one-particle model has been applied in calculating resonance energy levels for a heavy charge Z plus electron and positron systems. The method starts with the strict correlation plane configuration and then allows for the vibrational modes excitation. Calculations have been carried out for [H;e+] and [He+;e+] transient systems, and series of autoionizing levels are obtained. Possible improvements of the method as well as further extensions to other few-particle systems are considered, too.
Salt effects on the surface tensions of dilute electrolyte solutions: The influence of nonzero relative solubility of the salt between the coexisting phases
Authors: Nichols Albert L. , Pratt Lawrence R.
Published: February 2003

Extant theories of salt effects on the surface tensions of dilute fluid electrolyte solutions explicitly assume that the solubility of the salt in one of the coexisting phases is negligible. It is shown here that this assumption can influence both the sign and the analytical form of the dependence of the surface tension on the salt concentration in the limit of low concentrations. The variability of the low concentration behavior of the surface tension is traced to short ranged ion–solvent interactions involved in determining the relative solubility of the salt in the two coexisting phases. In particular, if there is appreciable solubility of the salt in both phases, and if the short ranged ion–solvent forces are dissimilar, then we predict that the surface tension should decrease proportional to the square root of the salt concentration ρ1/2 in the limiting region of low concentrations. This is contrasted to the ρ ln ρ limiting law which results when the solubility in one of the phases is vanishingly small.
The relative motion of ions in solution. I. Microdynamical models and intermolecular dipolar spin relaxation
Authors: Fries P. H. , Patey G. N.
Published: February 2003

This paper is the first in a series involving the theoretical and experimental study of the relative motion of ions in solution. In this paper we describe the general theoretical approach and make explicit calculations for model aqueous electrolytes. In all models the ions are considered to be charged hard spheres and in order to compare with real solutions effective hard sphere diameters must be estimated from crystal radii or space-filling molecular models. Our object is to provide theoretical results for these model solutions which can be tested with nuclear magnetic resonance (NMR) experiments. Therefore, we have calculated the dimensionless spectral density j̄2(ωτ) by solving the Smoluchowski equation including a force term dependent upon the ion–ion potential of mean force. Experimentally, j̄2(ωτ) can be determined from NMR measurements of the interparticle dipole–dipole relaxation rate of the nuclear spins I located on a diamagnetic ion and interacting with the electronic spins S of a paramagnetic species. Theoretically, we examined j̄2(ωτ) as a function of frequency and concentration for pairs of physically realistic ions. The influence of varying the ion size and charge as well as spin eccentricity effects are discussed. It is shown that the ionic charge strongly influences the relative motion giving values of j̄2(ωτ) which are much larger (i.e., by one or two orders of magnitude) for pairs of attractive ions and smaller for the repulsive case than those obtained for neutral particles. Furthermore, for attractive ion pairs the molecular and continuum solvent models yield very different results. The molecular solvent values for j̄2(ωτ) are frequently much larger than those given by the corresponding primitive model and for many physically realistic ion pairs the discrepancy between the models lies well outside the range of uncertainty introduced by the choice of hard sphere diameter. Hence for such systems NMR experiments should easily and clearly distinguish between these different theoretical pictures. For pairs of repulsive ions both models give similar results for dilute solutions, but the discrepancies become larger with increasing ionic strength as Debye screening of the Coulombic interaction increases the importance of the short-range potential.
The relative motion of ions in solution. II. An NMR relaxation study of attractive ions in water at low ionic strength
Authors: Fries P. H. , Jagannathan N. R. , Herring F. G. , Patey G. N.
Published: February 2003

In this paper we describe a nuclear magnetic resonance study of the relative motion of the attractive ion pairs (CH3)4N+/⋅ON(SO3)−22 and (CH3)4N+/ in dilute D2O solutions of varying ionic strength. Our major purpose is to test previous theoretical calculations for different model electrolytes. We have measured the interparticle dipole–dipole spin-lattice relaxation rate at 400 MHz of the (CH3)4N+ protons due to their interaction with the electronic spins of the paramagnetic nitroxide ions. At 400 MHz this relaxation rate is proportional to the reduced spectral density j̄2(ωpτ) which depends upon the relative ion distribution and motion, and can be directly compared with theoretical results. The present experiments verify the large charge and ionic strength effects predicted by the theory and allow us to distinguish between molecular and continuum solvent models. For the molecular solvent picture good agreement is obtained between the experimental and theoretical results. This is true for both ion pairs and over a range of ionic strengths. The primitive or continuum solvent model is found to be very poor. This is particularly evident for the (CH3)4N+/⋅ON(SO3)−22 pair where the continuum theory underestimates j̄2(ωpτ) by roughly a factor of 4 at low ionic strength.
The role of electron–hole pair excitations in desorption dynamics
Authors: Korzeniewski Gregory E. , Hood Eric , Metiu Horia
Published: February 2003

We carry out model calculations to compare the role of phonon excitations and electron-hole pair excitations in thermal desorption from metals. Phonon effects are computed by using a model developed previously [J. Chem. Phys. 79, 2436 (1983)]. The rate of energy transfer from the adsorbate to electrons (caused by Coulomb coupling to the effective charge of the adsorbate) is computed within the Random Phase Approximation. Our main result is that the effect of electron-hole pair excitations on the rate of desorption is negligible, except for the few cases in which the vibrational frequency of the chemisorption bond is many times (i.e., five to ten times) larger than the surface Debye frequency.
MS-Xα calculation of the elastic electron scattering cross sections and x-ray absorption spectra of CX4 and SiX4 (X=H,F,Cl)
Authors: Tossell J. A. , Davenport J. W.
Published: February 2003

Multiple scattering Xα bound state and continuum calculations are used to study low energy elastic electron scattering cross sections and pre-edge features in the x-ray absorption spectra (XAS) of (C,Si)X4, X=H,F,Cl. Maxima in the electron scattering cross section are predicted to occur at energies below 4 eV in the t2 channel for CF4, CCl4, SiH4, and SiCl4. These maxima can be assigned to final state orbitals which are bound in XAS and well localized in space and a quantitative correspondence of such scattering ‘‘resonance’’ energies and XAS energies may be obtained using the transition state approach. For CH4 and SiF4 even those bound state orbitals giving the greatest XAS intensity are very diffuse, being essentially of Rydberg character. Broad electron scattering maxima are found at energies above 4 eV in these molecules which cannot be directly associated with the bound state orbitals dominating the XAS. The results thus establish that low energy electron scattering resonances and pre-edge XAS are closely related only for orbitals bound and reasonably well localized in XAS. The MS-Xα results almost always reproduce experimental trends along the molecular series but quantitative discrepancies from experiment are sometimes substantial, particularly for the broad high energy scattering maxima in CH4. The narrow t2 resonance calculated for CF4 is found to vary greatly in position and intensity as the C–F distance is varied by small amounts but the stronger, broader t2 resonance in SiH4 is little affected by bond distance variation.
Solubility of solids in supercritical gases. II. Extension to molecules of differing sizes
Authors: Vezzetti David J.
Published: February 2003

A lattice gas model, in which solvent and solute molecules may occupy different numbers of sites, is used to examine the solubility of pure solids in supercritical gases. Provided that the ratio of molecular sizes is ≲3, isotherms (solubility vs pressure) conform well to experimental data over the full range of gas densities. In most cases only one parameter is required aside from data on the pure components, and all parameters have direct, simple physical meanings.
Calculation of the shear viscosity of liquid rubidium by nonequilibrium molecular dynamics
Authors: Bansal R. , Bruns W.
Published: February 2003

We have calculated the shear viscosity coefficient of a liquid Rubidium-like system via nonequilibrium molecular dynamics method proposed by Ciccotti et al. with the Lees and Edwards boundary conditions. The simulations are performed for three different states of temperature and density and are based on the volume dependent effective pair potential developed by Price et al. Comparison of the calculated viscosity coefficient with the measured values is found to be satisfactory for all three states.
Fractal behavior in trapping and reaction
Authors: Klafter J. , Blumen A.
Published: February 2003

We study trapping and reaction processes on fractals and compare the direct reaction with acceptors vs the multistep migration of the excitation. For the direct mechanism both exact and approximate expressions for the survival follow. For migration the trapping probability is determined from the number of distinct sites visited. We conclude that for each mechanism a different dimension of the fractal is decisive.
Microstructure of two-phase random media. IV. Expected surface area of a dispersion of penetrable spheres and its characteristic function
Authors: Torquato S. , Stell G.
Published: February 2003

A new expression is derived for the expected surface area of a dispersion of spheres distributed with arbitrary degree of penetrability. A convenient representation of the characteristic function of the interfacial surface is also introduced.
Competing irreversible cooperative reactions on polymer chains
Authors: Evans J. W. , Hoffman D. K. , Burgess D. R.
Published: February 2003

We analyze model processes involving competition between several irreversible reactions at the sites of a 1D, infinite, uniform polymer chain. These reactions can be cooperative, i.e., the corresponding rates depend on the state of the surrounding sites. An infinite hierarchy of rate equations is readily derived for the probabilities of various subconfigurations. By exploiting a shielding property of suitable blocks of unreacted sites, we show how exact hierarchy truncation and solution is sometimes possible. The behavior of solutions is illustrated in several cases by plotting families of ‘‘reaction trajectories’’ for varying ratios of reactant concentrations. As a specific application, we consider competition between coordination of ZnCl2 to pairs of oxygen atoms and to single oxygen atoms in poly(propylene oxide). The observed glass transition temperature behavior is eludicated.
Temperature dependence of ultrasound and relaxation in liquid chloroform
Authors: Cardamone M. J. , Rhodes W. , Yeastedt M.
Published: February 2003

Spontaneous Brillouin scattering was used to determine the vibrational relaxation of liquid chloroform as a function of temperature. Comparison of experimental results with theoretical predictions leads to the conclusion that there is a simultaneous relaxation of all vibrational modes of the molecule.
Numerical simulations of accumulated stimulated photon echoes
Authors: de Vries Harmen , Wiersma Douwe A.
Published: February 2003

A theory has been developed describing probe pulse-detected accumulated stimulated photon echo experiments on inhomogeneously broadened electronic transitions. This theory gives a more quantitative description of these experiments than the theory previously developed by Hesselink and Wiersma [J. Chem. Phys. 75, 4192 (1981)]. At the same time, the present theory provides numerical instead of analytical results. A better founded insight is obtained in the possibilities and limitations of the accumulated photon echo method. The main conclusion is that the measured echo decay curves are still expected to give T2 directly, thus maintaining the value of the accumulated echo technique as a convenient method to determine picosecond dephasing times. It is concluded that the probe pulse detected accumulated echo method is not readily applied to accurate determinations of intersystem crossing yields.
Two dimensional Zeeman nuclear quadrupole resonance spectroscopy
Authors: Ramachandran R. , Oldfield Eric
Published: February 2003

We describe the first application of two-dimensional spectroscopy to the pure quadrupole regime. One- and two-pulse Zeeman perturbed experiments are outlined, and results of a one-pulse experiment on the two crystallographically nonequivalent 35Cl sites in 2,4,6-trichloro-1,3,5-triazine (‘‘cyanuric chloride’’) are presented. The results demonstrate that the overlapping Zeeman-perturbed resonances in a one-dimensional experiment are fully resolved in a two-dimensional experiment. The methods appear to have considerable utility for the determination of the quadrupole coupling constant e2qQ/h and asymmetry parameter η for spin I=3/2 nuclei in systems with several closely spaced pure nuclear quadrupole resonance lines.
Resonant multiphoton ionization of NO via the A 2Σ+ state: Photoelectron spectra and angular distributions
Authors: White M. G. , Chupka W. A. , Seaver M. , Woodward A. , Colson S. D.
Published: February 2003

Photoelectron spectra resulting from the two-photon resonant, four-photon ionization of NO via the A 2Σ+ state are reported for several rotational levels in the A, v=0 manifold. The observed final ionic state vibrational distributions are found to be consistent with and confirm spectroscopic studies which have assigned intense features in the four-photon A state MPI spectrum with accidental resonances in the energy range of the third photon. The upper states in the strong accidental double resonance transitions are assigned to Rydberg vibronic levels which are strongly perturbed by nearby valence states. Photoelectron angular distributions were also measured for two vibronic transitions and these are interpreted on the basis of a simple, one-electron description of the ionization step.