bluuebunny/crossref_metadata_2025 huggingface dataset:
Photolytic spectroscopy of simple molecules. V. Prompt and delayed photolysis of Cs2 excited at yellow wavelengths
Authors: Davanloo F. , Collins C. B. , Inamdar A. S. , Mehendale N. Y. , Naqvi A. S.
Published: July 2002

This report belongs to a series concerned with the correlation of photolysis bands observed in electronic transition spectra of simple molecules with their dissociation products. In this work a time-delayed, double resonance application of optical impedance spectroscopy was used for the study of the state selective photolysis of Cs2 excited in the yellow range of visible wavelengths. Two independently tunable dye lasers were synchronously pulsed, the first being used to dissociate the constituents of the sample along the various possible channels and the second, delayed pulse being used to identify the products by exciting them selectively into easily detected Rydberg states. The time between photolysis and detection could be varied, starting from a value too short to permit collisional mixing or radiative cascading of the products. Particular attention was paid to the photolytic production of the fine structure components of the first and second electronically excited state of atomic cesium and to the subsequent processes which tended to degrade the selectivity produced in the initial distributions of product populations among the available states. Data were fit to a quantitative model from which rate coefficients could be extracted for various mixing processes. Reported here is what appears to be a first value for the fine-structure mixing cross section (5 2D5/2→5 2D3/2) of 17 Å2±60%. In addition a delayed process for the selective production of Cs(5 2D) atoms was found to result from the excitation of a state having a lifetime less than 100 ns that is tentatively identified as a molecular state correlated with Cs(5 2D), possibly 5d 3Δu.
Spectroscopic study of jet-cooled fluoranthene
Authors: Chan I. Y. , Dantus Marcos
Published: July 2002

Fluorescence excitation and single vibronic level (SVL) fluorescence spectra of fluoranthene seeded in an Ar free-jet are presented. The S1 excitation spectrum builds on a strong 427 cm−1 vibronic false origin. Fluorescence spectra exciting SVL with vibrational energy 0, 427, and 751 cm−1 contain sharp lines, whereas with 1345 cm−1 the fluorescence spectrum is a featureless continuum. The sharp lines have been analyzed and assigned with the help of Raman and IR data. The broad spectrum when exciting 1345 cm−1 above the S1 origin is ascribed to energy redistribution (IVR). The behavior of fluoranthene in light of current understanding of the IVR process is discussed.
Rotational assignment using phase relationships in optical–optical double resonance: The BaI C 2Π–X 2Σ+ system
Authors: Johnson M. A. , Zare Richard N.
Published: July 2002

We describe an optical–optical double resonance scheme in which a lower vibration-rotation level is labeled. One laser is fixed in frequency and probes the population of the labeled level via the resulting laser-induced fluorescence; a second laser is scanned in frequency through the same vibronic band excited by the probe. A double resonance signal results when the population in the labeled level is either increased or decreased by the action of the second laser. The positions and phase pattern of the double resonance spectrum reveal the J numbering of the labeled level and permit a good approximation to be made for the upper and lower state rotational constants. This information allows the J value of the labeled level to be systematically changed, permitting the spectrum to be unraveled. This technique is proven by applying it to the highly congested C–X spectrum of the BaI molecule, for which no rotational information was previously available for any of its states.
Product vibrational and rotational state distributions from the surface reaction F(ad)+H2(ad) → HF(g) (v′, J′)+H(ad/g)
Authors: Barnes J. A. , Polanyi J. C. , Reiland W. , Thomas D. F.
Published: July 2002

Product vibrational and rotational state distributions, ks(v′, J′), have been measured for the reaction F(ad)+H2(ad) → HF(g)+H(ad/g) in which both reagents are adsorbed on a 20 K surface consisting of polycrystalline CF4. The relative rate constants into vibrational levels v′=1 and v′=2, summed over the rotational levels, are [ks (v′=2)/ks (v′=1)]=0.58. The surface reaction therefore exhibits less vibrational excitation in the products than does the same reaction in the gas. Rotational excitation within each vibrational level is, however, comparable to that for the gas phase reaction. The product vibrational and rotational state distributions for surface reaction are markedly non-Boltzmann. Both the vibrational and the rotational excitation in the product of the surface reaction greatly exceed the equilibrium value for the surface temperature; ks (v′=2)/ks (v′=1) corresponds to a vibrational population ratio for 10 000 K, though TS=20 K.
Correlation energy generating potentials for molecular hydrogen
Authors: Sharma B. S. , Thakkar Ajit J.
Published: July 2002

A variety of local correlation energy functionals are currently in use. All of them depend, to some extent, on modeling the correlation energy of a homogeneous electron fluid. Since atomic and molecular charge densities are neither uniform nor slowly varying, it is important to attempt to use known high accuracy wave functions to learn about correlation energy functionals appropriate to such systems. We have extended the definition of the correlation energy generating potentials Vc introduced by Ros. A charge density response to correlation has been allowed for by inclusion of an electron–nuclear component Venc in addition to the electron–electron component Veec. Two different definitions of Venc are given. We present the first calculations of Vc for a molecular system—H2. The results show that Venc, in either definition, is by no means negligible. Moreover, Veec and both forms of Venc show significant nonlocal dependence on the charge density. Calculations with ten different model correlation energy functionals show that none of them is particularly sensitive to the charge density. However, they are quite sensitive to the parametrization of the electron fluid correlation energy. The schemes which include self-interaction corrections (SIC) are found to be superior to those of Kohn–Sham type. The correlation energy generating potentials implied by the SIC type and empirical correlation energy functionals are found to correspond roughly to averages of one of the accurate potentials.
Structure and bonding of chemisorbed ethylene and ethylidyne on Pt(111) from near edge x-ray absorption fine structure spectroscopy and multiple scattering calculations
Authors: Horsley J. A. , Stöhr J. , Koestner R. J.
Published: July 2002

The near edge x-ray absorption fine structure (NEXAFS) spectra of ethylene on Pt(111) at 90 and 300 K have been measured and compared to the spectra calculated using the multiple scattering Xα method for cluster models of adsorbed ethylene and ethylidyne. The results indicate that ethylene is adsorbed in the di-σ configuration (sp3 hybridization) at 90 K, and that the π* orbital retains its identity in this configuration, with no component along the C–C axis. The spectrum obtained at 300 K is attributed to ethylidyne (CCH3) and the calculated spectrum for a cluster consisting of this species bonded symmetrically to three Pt atoms is in good agreement with experiment. The calculations show that the multiple scattering molecular cluster approach is a useful technique for interpreting the NEXAFS spectra of adsorbed molecules.
A low energy crossed beam study of the reactions of C+ with CH4
Authors: Curtis R. A. , Farrar J. M.
Published: July 2002

We present a crossed molecular beam study of the reactions of C+ with CH4 over the relative energy range from 0.66 to 2.38 eV. CH+3 production involves a direct rebound collision mechanism. Production of C2H+3 and C2H+2 proceeds through unimolecular decay of long-lived collision complexes. Hydrogen atom emission proceeds through a rotating ethylene cation and yields a polarized angular distribution of reaction products. The kinetic energy distributions for vinyl cation production place nearly three times as much of the available energy in translation as a statistical model predicts. At low collision energy, C2H+2 production proceeds through molecular hydrogen elimination and the angular distribution for reaction products is polarized perpendicularly to the initial relative velocity vector. At a higher collision energy of 2.38 eV, the complex decays in a fraction of a rotational period. The lifetime of 0.03 ps inferred from the asymmetry in the angular distribution is in good agreement with a statistical calculation incorporating nonadiabatic couplings among the excited and ground states of C2H+4. The recoil distribution at this energy is dramatically narrower than at the lower collision energy and is consistent with the sequential ejection of two hydrogen atoms through a metastable C2H+3 intermediate.
Alternancy symmetry: A unified viewpoint
Authors: Koutecký Jaroslav , Paldus Josef , Čížek Jiří
Published: July 2002

A general formulation of the alternancy symmetry adaptation for the semiempirical Pariser–Parr–Pople (PPP) type Hamiltonians is presented at both the spin-orbital and spin-adapted many-electron levels. The derivation of the general form of the alternancy symmetry conjugation operators is based solely on the tight-binding approximation for the short range one-particle part of the Hamiltonian considered. It starts by a simple formulation of the desired invariance properties of the PPP type Hamiltonian. Using algebraic properties of the unitary group generators and of their particle number nonconserving extensions, it leads to a completely explicit and general form for the alternancy symmetry conjugation operators. In this way the prior descriptions, which become special cases of this general formulation, are interrelated and unified. The spin and quasispin character of certain components of these operators are also pointed out and explicitly derived. The spin-adapted version is based on the unitary group formulation of the valence bond-type approach. A completely general formulation is given which applies to many-electron states of an arbitrary multiplicity of neutral systems, either of the normal kind with an identical number of starred and nonstarred sites, or of the radicaloid character with different numbers of starred and nonstarred sites. An explicit form of the dependence of the relative phase factor of the alternancy symmetry conjugation operator on the total spin, total electron number and on the number of starred and nonstarred sites is also given. General rules for the construction of spin and alternancy symmetry-adapted states are illustrated on a few simple examples. Finally, a brief discussion of the implications of the alternancy symmetry is presented.
CO chemisorption on Cr(110): Evidence for a precursor to dissociation
Authors: Shinn Neal D. , Madey Theodore E.
Published: July 2002

High resolution electron energy loss spectroscopy (EELS), electron stimulated desorption ion angular distributions (ESDIAD), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES) have been combined to study CO chemisorption on the Cr(110) surface. At 120 K, two molecular binding modes are sequentially populated. The first, α1CO, is a novel binding mode for a clean metal surface in which the molecules are bound with the C–O axis nearly parallel to the Cr(110) surface. These α1CO molecules exhibit a νCO of 1150–1330 cm−1, no detectable ion emission in ESDIAD, and form a c(4×2) α1CO overlayer at saturation (θCO≈0.25). A model is proposed in which the α1CO are π bonded to the Cr(110) surface at twofold symmetric surface hollow sites. Annealing experiments demonstrate that the α1 binding mode is the precursor to dissociation on Cr(110). At higher CO coverages, 0.25<θCO<0.35, the α1CO overlayer is disordered by the addition of the second binding mode, α2CO. Three new vibrations, at 495, 1865, and 1975 cm−1, are observed by EELS while O+ ion emission along the [110] direction is seen in ESDIAD. These α2CO results are indicative of terminally bound molecules adsorbed at atop and bridge sites and aligned approximately perpendicular to the surface. Annealing studies of CO and O/CO adlayers provide evidence for a surface reaction in addition to direct dissociation above 170 K. In contrast, adsorption at 300 K leads to direct dissociative chemisorption with no detectable intermediate molecular adsorbate. The implications of these results to catalytic reactions of CO are considered and comparisons to CO adsorption on clean and ‘‘promoted’’ transition metals are made.
A generalization of the Müntz–Szász theorem to floating exponents with applications to Gauss- and Slater-type functions
Authors: Klahn Bruno
Published: July 2002

The Müntz–Szász condition for completeness of the monomials xλn in L2 is extended to floating functions, where the exponents λn may depend on the number of basis functions used in an approximation. The generalized condition is rewritten for Gauss- and Slater-type functions and then discussed, especially with respect to even-tempered functions.
Calculation of dynamic polarizabilities of He, H2, Ne, HF, H2O, NH3, and CH4 with MC-SCF wave functions
Authors: Reinsch Ernst-Albrecht
Published: July 2002

In order to calculate frequency dependent polarizabilities, time dependent perturbation theory for MC-SCF wave functions has been used and a significant part of the correlation contribution is accounted for. Explicit formulas for the coefficients of the system of linear equations to be solved have been given. The results reported for α(ω) of He, H2, Ne, HF, H2O, NH3, and CH4 show that in the case of the two electron systems excellent results can be obtained and that in the case of the ten electron systems with 45-configuration wave functions about 80% of the correlation contribution for ω=0 have been included. The absolute accuracy for the latter case is in the range of 4% to 9.5%. For large ω values the experimental polarizability increases slightly more than the calculated values.
Collisional dynamics of the IF B 3Π(0+) state. II. Electronic quenching at low pressures
Authors: Wolf Paul J. , Davis Steven J.
Published: July 2002

Electronic quenching of IF B 3Π(0+) was studied in a time resolved laser fluorescence experiment. IF (B) deactivation rate constants were determined as a function of vibrational quantum number for the following collision partners: He, Ne, Ar, Kr, Xe, N2, F2, I2, O2, and H2O. Quenching by the noble gases and N2 was extremely slow with all rate constants less than 1×10−14 cm3 molecule−1 s−1 for 3≤v′≤8. The quenching rate constants for F2 showed a weak dependence on vibrational quantum number, ranging from (3.4±0.5)×10−12 to (5.2±0.4)×10−12 cm3 molecule−1 s−1 for v′=3 and v′=7, respectively. Double exponential IF (B) fluorescence decays were observed with both O2 and H2O indicating two depletion processes occurring over the lifetime of IF (B). The quenching rate constants, extracted from the decays at long times, were (1.4±0.2)×10−12 cm3 molecule−1 s−1 for O2 and (7.6±1.6)×10−12 cm3 molecule−1 s−1 for H2O. At early times, the respective deactivation rate constants for O2 and H2O were (8.3±1.4)×10−12 and (2.4±0.7)×10−10 cm3 molecule−1 s−1. A possible quenching mechanism by O2 is discussed. The most efficient quencher was I2 with an estimated rate constant of 3.9×10−10 cm3 molecule−1 s−1.
Photoisomerization of stilbene in low viscosity solvents: Comparison of isolated and solvated molecules
Authors: Courtney S. H. , Fleming G. R.
Published: July 2002

The photoisomerization of stilbene has been studied in low viscosity liquid alkanes and in the gas phase. The barrier crossing process shows no evidence of a low-friction or energy controlled region even at the lowest liquid viscosities studied. We present evidence that the barrier crossing may be in the energy controlled region in the thermal vapor. We discuss the influence of intramolecular vibrational energy transfer on the observed dynamics and note that entropy effects should be considered in comparing experimental data with theoretical models.
Determination of orientational correlation functions in ordered fluids: Raman scattering
Authors: Kirov N. , Dozov I. , Fontana M. P.
Published: July 2002

Fourier band shape analysis is applied to obtain the reorientation time correlation functions in the short time limit from the Raman spectra of some ordered nematic liquid crystals. New equations for the simultaneous determination of the static order parameters 〈P2〉 and 〈P4〉, and the tumbling diffusion coefficient Dr⊥ are proposed. Their applicability and accuracy are demonstrated by Raman scattering measurements on two prototype aligned mesogens, namely 4, n-pentyl 4′-methoxytolane (PMT) and 4,n-octylethoxytolane (OET). The molecular dynamics in the investigated aligned nematics is also discussed.
The effect of temperature and pressure on excess electron mobility in n-hexane, 2,2,4-trimethylpentane, and tetramethylsilane
Authors: Muñoz Raul C. , Holroyd Richard A.
Published: July 2002

Measurements of excess electron mobility are reported for liquid n-hexane, 2,2,4-trimethylpentane, and tetramethylsilane for pressures from 1 to 2500 bar and for temperatures from 18 to 120 °C. For tetramethylsilane, a liquid that exhibits a high electron mobility, the mobility at constant density is proportional to T−0.9 between 25 and 100 °C. The results are compared with the Basak–Cohen deformation potential theory. For n-hexane, where the mobility is low, Arrhenius behavior is observed. The isochoric activation energy increases with density. The results in this case are consistent with both the two-state and hopping models. In 2,2,4-trimethylpentane the mobility increases with increasing pressure at room temperature and decreases at high temperature. At 2500 bar and at intermediate temperatures (70–80 °C) the mobility is approximately constant.
Deuteron Zeeman relaxation of CD4 in the isotropic liquid, the liquid crystalline, and the solid state of several substances
Authors: Beckmann Peter A. , Bloom Myer , Burnell E. Elliott
Published: July 2002

Measurement of deuteron Zeeman relaxation rates of CD4 dissolved in benzene, hexane, and the liquid crystals MBBA, EBBA, and Merck ZLI-1132 and in pure CD4 gas as a function of temperature at 30.7 and 61.4 MHz shows that the CD4 is uniformly dispersed in the liquid solvents but resides in gas pockets when the solvents are in the solid state. Effects of centrifugal distortion were observed in the gas phase. The relaxation rate was found to be nearly independent of solvent, temperature, and pressure for the methane–liquid mixtures. This result is explained in terms of the extended diffusion model for the combined effects of free molecular rotation and collisions on the spectral density of quadrupolar interactions when the collisional and mean free rotational periods are of the same order of magnitude. It can also be interpreted in terms of the Fokker–Planck–Langevin model for rotational Brownian motion.
Vibrationally state-selected reactions of ammonia ions. I. NH+3(v)+D2
Authors: Morrison Richard J. S. , Conaway William E. , Ebata Takayuki , Zare Richard N.
Published: July 2002

Resonance enhanced multiphoton ionization has been applied to the production of vibrationally state-selected ion beams. Ammonia ions are selectively formed with a specific number of vibrational quanta in the ν2 umbrella bending mode. The effect of vibrational excitation of this mode on the reaction of NH+3(X̃, v=0 to 9) with D2 is examined over the 0.5 to 10 eV center-of-mass kinetic energy range in a tandem quadrupole mass spectrometer. Under these conditions, (1) abstraction of a D atom to form NH3D+ is the dominant reaction channel, (2) NH3D+ having sufficient internal energy may decompose to yield NH2D+ and this decomposition process is enhanced by vibrational excitation of the NH+3 reagent, and (3) NH2D+ is also formed by direct hydrogen–deuterium exchange of NH+3 with D2, but this channel appears as a minor contribution which is insensitive to the vibrational excitation of the NH+3. A spectator stripping model is able to account for the ratio of NH2D+ to NH3D+ as a function of the NH+3 translational and vibrational energy.
One-electron diatomics in momentum space. II. Second and third iterated LCAO solutions
Authors: Koga Toshikatsu , Kawa-ai Ryousei
Published: July 2002

Recurrence formulas are derived for the iterative LCAO solution of the one-electron two-center Schrödinger equation in the Fock representation. The results are applied to the second and third iterated LCAO solutions of the H+2 system at various internuclear distances R. For 0≤R≤20 (a.u.), the maximum errors in the electronic energy are reduced to 2.7% (second iterated) and 1.6% (third iterated), which should be compared with the previous errors of 28.2% (zeroth iterated) and 4.7% (first iterated).
Collision energy dependence of the cross sections for the dissociative excitation reactions: Rg(3P0,2) +NH3→Rg+NH(A 3Π, c 1Π)+H2 (Rg=Ar, Kr)
Authors: Tabayashi Kiyohiko , Shobatake Kosuke
Published: July 2002

The integral cross sections for the title reactions were determined for the relative collision energies between 0.3 and 1.4 eV by applying arc-heating metastable rare gas beams to a crossed molecular beam–time-of-flight (TOF) energy selection technique, in which product fluorescence intensities from both NH(A 3Π–X 3Σ−) and NH(c 1Π–a 1Δ) bands were monitored. With Ar(3P0,2 ) excitation, the relative branching fractions of nascent NH(A 3Π) and NH(c 1Π) formation were found to be invariant with the collision energy within the experimental error. With Kr(3P0,2) excitation, however, no apparent NH(c 1Π–a 1Δ) emission was observed, only emission from NH(A 3Π) was observed. The absolute cross section for formation of these nascent NH radicals was also determined. Collision energy dependence of the cross section for the reaction Ar (3P0,2) +NH3→Ar(1S0)+NH(A 3Π,c 1Π)+H2 was found to be negative, whereas that for the reaction Kr(3P0,2 )+NH3→Kr(1S0)+NH(A 3Π)+H2 showed positive dependence in the collision energy range studied. Possible mechanisms for the formation of NH(A 3Π,c 1Π) from Rg(3P0,2)+NH3 are discussed.
Dynamics of OH Λ-doublet production through photodissociation of water in its first absorption band. I. Formal theory
Authors: Balint-Kurti G. G.
Published: July 2002

The formal theory is presented for the photofragmentation of water into a hydrogen atom and an OH radical in its ground 2Π state. Full account is taken of the electron spin angular momentum, the electronic orbital angular momentum and the angular momentum generated by motion of the nuclei. A formula is presented which permits the calculation of the differential cross section for the production of individual OH(X 2Π)Λ-doublet states. From this formula others for less detailed cross sections are derived. In particular, an expression is presented for the integral photodissociation cross section for the production of OH Λ-doublet fragments. In all cases the formulas are specialized to apply to the Ã 1B1 ← X 1A1 photodissociative transition of water. The general formalism that is developed however, transcends this particular application. It is clearly apparent how the formalism may be extended to apply to many other cases of current experimental interest. Simplified formulas are derived to permit approximate Franck–Condon estimates of the photodissociation cross sections and product state distribution to be made.
Product energy disposal in the reaction of N+(3P) with O2(X 3Σ)
Authors: O’Keefe A. , Mauclaire G. , Parent D. , Bowers M. T.
Published: July 2002

The reaction of N+(3P) with O2(X 3∑) has been studied using the technique of kinetic energy ion cyclotron resonance spectroscopy. The portion of reaction exoergicity channeled into product kinetic energy is measured and used to infer the relative distribution of product internal energy. Both the atom transfer reaction, producing NO+, and the charge transfer reaction, producing O+2, have been studied. The combination of the present kinetic energy measurements with recent low resolution spectroscopic studies of the NO+ atom transfer product permit a detailed analysis of the reaction pathways. Through such an analysis both the ionic product state distribution and the neutral atomic product distribution are determined, demonstrating substantial vibrational and electronic excitation in the ionic and neutral products, respectively. The charge transfer reaction channel, however, produces predominantly ground state product N atom and mild vibrational excitation in the O+2 product.
Analysis of saturation transfer electron paramagnetic resonance spectra in terms of amplitude and phase
Authors: Shimoyama Y. , Watari H.
Published: July 2002

A systematic analysis of saturation transfer (ST) electron paramagnetic resonance spectroscopy was developed to separate an observed absorption signal either into an amplitude and a phase spectrum or into an in-phase and an ST spectrum. We established a phase reference procedure, replacing the low-power phase null method by a high-power phase maximum method. This is an extension of the finding that a peak-to-peak height and a single integration of the second harmonic absorption display vary sinusoidally as functions of the modulation phase [Y. Shimoyama and H. Watari, Appl. Spectrosc. 39, 170 (1985)]. Based on the sinusoidal variation in the peak-to-peak height, we found a phase shift at each magnetic field and could evaluate the amplitude and phase of a signal vector. The sinusoidal variation of the single integrated value allowed the absorption signal to be separated into an in-phase and an ST spectrum. The high-power phase maximum method provides a remedy of the phase setting problem in ST spectroscopy by which the in-phase signal can be detected at any microwave field.
Measurement of chemiluminescence polarization as a function of collision velocity by time-of-flight spectroscopy: Reactions of Xe* with HCl, HBr, Cl2, Br2, and I2
Authors: de Vries Mattanjah S. , Tyndall George W. , Cobb Cathy L. , Martin Richard M.
Published: July 2002

A new method is described for obtaining chemiluminescence polarization as a function of reactant relative velocity, employing a beam-gas time-of-flight technique. These experiments provide information about product alignment as a function of velocity. Results are reported for the formation of xenon halide excimers from the reactions of Xe(3P2,0) with HCl, HBr, Cl2, Br2, and I2. The HX reactions give strong excimer polarization, primarily due to kinematic constraints, whereas with the X2 reactions the excimers are polarized due to the collision dynamics. In this work we have made the first observation of cases in which the products rotate preferentially in planes normal to the initial relative velocity vector. This is observed for the Xe*+Cl2 and Br2 reactions at low velocities, and is attributed to a mechanism which gives preferential repulsive energy release perpendicular to the reactant relative velocity vector. The direct interaction product repulsion (DIPR) model does not give agreement with the data, showing that a more realistic potential surface model will be necessary to explain the present results.
Raman spectral studies of the dynamics of ions in molten LiNO3–RbNO3 mixtures. I. Rotational relaxation
Authors: Katō Toshiko , Takenaka Tohru
Published: July 2002

The Raman spectra of the ν1(A′1) mode of NO−3 ions in molten LiNO3–RbNO3 mixtures are measured at various concentrations and at temperatures from 478 to 750 K. Rotational correlation functions are calculated from the Raman spectra. Reorientation of the trigonal axis of an NO−3 ion is found to be represented by the inertial rotation of an associated ion cluster for short times and by the rotational diffusion at longer times. The speeds of inertial rotation and rotational diffusion are found to increase as the temperature rises and as the mole fraction c(Li+) of LiNO3 decreases.
Spectral measurements along the path of N ions impinged into dense He gas (400 Torr and 5.2 K). I. Emission spectra and decay measurements
Authors: Kimura Kazuie
Published: July 2002

Emission spectra and decay from 4 MeV/amu N-ion impinged dense helium gas (400 Torr, 5.2 K) were measured in a wavelength range from 3500 to 8000 Å. Most emission peaks can be assigned to He excimers except for new triplet bands at 4599, 4587, and 4575 Å. An excimer–dimer is proposed to interpret the triplet. All the emission bands have a fast initial-rise component which is completed within a N-ion pulse duration (2 ns). Emission peaks can be classified into two groups regarding their decays. Group I has a fast decay component with a lifetime of a few tens of nanoseconds and a very slow component corresponding to the afterglow. The fast decay component was explained as being caused by reproducing of the excimer by excimer–excimer reactions, analogous to the known reactions between the metastable a3(2sσ 3Σ+u). On the other hand, group II has a much faster decay component of lifetime of a few nanoseconds and gives no afterglow. In this case, the regenerating reactions were negligible.
Analytic solution of the molecular Ornstein–Zernike equation for nonspherical molecules. Spheres with anisotropic surface adhesion
Authors: Cummings Peter T. , Blum Lesser
Published: July 2002

The molecular Ornstein–Zernike equation is solved for a fluid of hard spheres with anisotropic surface adhesion yielding a small set of nonlinear algebraic equations which must be solved numerically in order to obtain structural and thermodynamic properties. It is argued that such a model may mimic the short-ranged, anistropic interactions encountered in hydrogen-bonded fluids such as water.
The dynamic Jahn–Teller effect in s
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m-triazine: Nonadiabatic wave functions and hindered fluxionality
Authors: Whetten Robert L. , Haber Kenneth S. , Grant Edward R.
Published: July 2002

The complex vibronic structure resolved in the two-photon absorption spectrum of the 3s 1E′ Rydberg state of sym-triazine is quantitatively assigned in terms of a simple Jahn–Teller two-state electronic Hamiltonian coupled to second order by a single active mode, ν6. For Jahn–Teller linear and quadratic terms, k=2.14, and g=0.046 (in units of the zeroth-order frequency), eigenvalues of this Hamiltonian fit positions and splittings of more than 25 measured bands to within an average deviation of 0.5%. Eigenvectors show evidence of strong mixings of adiabatic states and of linear Jahn–Teller wave functions by quadratic (localization) effects, both of which are confirmed by quantitative agreement between measured and calculated band intensities. Adiabatic potential energy surfaces are calculated, and exact nonadiabatic quantum mechanical results are compared with various levels of approximation. This comparison shows that the simple model of an adiabatic free rotor/radial oscillator serves well to qualitatively describe the structure and dynamics of the lowest few states. Lower surface adiabatic Born–Oppenheimer (or Born–Huang) calculations also give a good approximate account of energy level structure for these deep states. Interestingly, compared with exact results, wave functions of the adiabatic approximations appear to underestimate potential-energy localization of nuclear density over surface depressions, and overestimate above barrier reflection. Cone resonances are identified for high energy states of triazine’s linear coupling parameters but it is shown that higher order coupling tends to disrupt such localizations.
The particle scattering function and the distribution functions in the interior of fully swollen polymer molecules
Authors: Tsunashima Yoshisuke , Kurata Michio
Published: July 2002

The particle scattering function and the distribution function for the spatial distance between two interior elements were determined by light scattering measurements on fully swollen polymers, polystyrenes in toluene, and poly (α-methylstyrenes) in benzene. Measurements were made in the large q region such that x=q2〈s2N〉 was 20 to 100. Here 〈s2N〉 is the mean-square radius of gyration and q the magnitude of the scattering vector. The distribution function is well represented by the generalized Domb–Gillis–Wilmers form W(r,n)=Cnrl exp[−(r/σn)t] with t=2.40 and l=2.80. Here n is the length of a polymer segment, r is the end-to-end distance of the segment, Cn is the normalization constant, and σn is the scaling factor, respectively. The value of l is considerably high as compared with 2.0 for a Gaussian chain and 2.27 for the end-to-end distance of the self-avoiding chain, but is favorably compared with des Cloizeaux’s estimate, 2.71, for the distance between interior elements. Thus the effect of dangling ends is detectable by the study of the particle scattering function.
Systematic determination of MCSCF equilibrium and transition structures and reaction paths
Authors: Jensen Hans Jo/rgen Aa. , Jo/rgensen Poul , Helgaker Trygve
Published: July 2002

The restricted step optimization algorithm is applied to potential energy surfaces calculated from multiconfiguration self-consistent-field wave functions. Equilibrium and transition-state geometries are determined by iteratively solving a set of level-shifted Newton–Raphson equations. At each geometry the molecular gradient and Hessian are calculated analytically, and a first-order prediction of the wave function at the next geometry is obtained by combining the geometrical derivatives of the wave function with the geometrical step vector. The usefulness of this prediction is discussed and illustrated by test calculations. The numerical accuracy which is required in the wave function and its geometrical derivatives in order to maintain quadratic convergence in the optimization of the molecular geometry is analyzed. It is demonstrated that the Newton–Raphson step vector and the wave function prediction may be determined without calculating the molecular Hessian explicitly. Sample calculations are carried out for the potential energy surfaces of diazene (N2H2) and the diazenyl radical (N2H). Equilibrium geometries are determined in less than five iterations and the optimization of transition states requires typically ten iterations.
Photoionization dynamics of excited molecular states. Photoelectron angular distributions and rotational and vibrational branching ratios for H2 C 1Πu, v=0–4
Authors: Pratt S. T. , Dehmer P. M. , Dehmer J. L.
Published: July 2002

Photoelectron angular distributions following three photon resonant, four photon (3+1) ionization of H2 via the C 1Πu, v′=0–4 ← X 1Σ+g, v″=0 Q(1) transitions are reported. The observed angular distributions are generally more isotropic for v+≠v′ than for v+=v′. Photoelectron spectra obtained along the polarization axis of the laser following (3+1) ionization via the C 1Πu, v′=4 ← X 1Σ+g, v″=0 R(0) and R(1) transitions are also reported. These spectra are rotationally resolved and exhibit strongly v+-dependent rotational branching ratios. The comparison of the angular distribution data with available theoretical calculations indicates good agreement for some transitions and poor agreement for others, suggesting the need for substantial progress in understanding the photoionization dynamics of even the simplest excited molecular states.
Production of OH from the collision-free photodissociation of nitromethane at 266 nm
Authors: Zabarnick S. , Fleming J. W. , Baronavski A. P.
Published: July 2002

Photodissociation of nitromethane, CH3NO2, at 266 nm under collision-free conditions results in the single photon production of electronic ground state OH(X 2Πi). A rotational ‘‘temperature’’ of 1750±50 K describes the rotational distribution of the nascent OH radical. A quantum yield of 0.004±0.001 is determined for the photolytic pathway producing the OH radical. Within experimental uncertainty, no Λ-doublet population preference was observed for OH production from H2O2, HNO3, or CH3NO2 photolysis. If a saturation correction is not utilized apparent Λ-doublet population differences are found. This incorrect saturation compensation may account for previous literature Λ-doublet population ratios, fi/ f′i, that are greater than one.
The first ionization potential of zirconium atoms determined by two laser, field-ionization spectroscopy of high lying Rydberg series
Authors: Hackett P. A. , Humphries M. R. , Mitchell S. A. , Rayner D. M.
Published: July 2002

An accurate value for the first ionization potential of zirconium of 53 506.0(3) cm−1 has been obtained from convergence limits of high lying Rydberg series observed using two laser, field- ionization spectroscopy of an atomic beam of zirconium. The new value is consistent with analysis of s-electron binding energies of other second transition series elements.
Rotational analysis of some vibronic bands in the 3A
u←1A
g transition of glyoxal. Spin splittings in the lowest triplet state of the isolated molecule
Authors: Spangler Lee H. , Pratt David W. , Birss Fraser W.
Published: July 2002

A rotational analysis of three vibronic bands (000, 720, and 510) in the phosphorescence excitation spectrum of glyoxal (CHO–CHO) has been performed. From this analysis we deduce the rotational, spin-rotation, and spin–spin (plus spin-orbit) constants of the lowest triplet state of the isolated molecule. 3Au glyoxal has been shown to exist in nearly the same trans-planar (C2h) configuration as the lowest excited singlet state. Both geometries are very similar to that of the ground state. The dynamic implications of these structural findings are discussed. We also compare the spin splittings determined in this work with those previously measured in the condensed phase, using optically detected magnetic resonance methods.
Grand canonical Monte Carlo simulation of liquid argon
Authors: Ruff Imre , Baranyai András , Pálinkás Gábor , Heinzinger Karl
Published: July 2002

A grand canonical Monte Carlo procedure with fixed values of the chemical potential μ, volume V, and temperature T, is described which is suitable to simulate simple fluids with only a minor increase in computer time in comparison with canonical (N,V,T) simulations and considerably faster than (N,p,T) ones. The method is rapidly convergent for rather dense systems with a reduced density of about ρσ3=0.88. The rapid convergence is attained by decreasing the vain attempts in the regime when new particles are added. The chance to find a place for an additional particle is increased by locating the cavities suitable to house a particle with the aid of the Dirichlet–Voronoi polyhedra. As an example, liquid argon is simulated with Lennard-Jones potentials at T=86.3 K and μ=−73.4 J/mol. The simulated density has been found to be 1.468 g/cm3 which is to be compared with the experimental value of 1.425 g/cm3. The same density was obtained by starting the procedure with both 216 and 250 particles in the simulation box of length 2.1895 nm. The pair correlation function is also in very good agreement with both earlier (N,V,T) simulations and diffraction experiments. The configurations obtained are analyzed by the second- and third-order invariants of the even-l spherical harmonics as order parameters characterizing the nearest neighbors of argon atoms. These results as well as some other statistics on the geometry of the coordination sphere indicate that the prevailing cluster geometry in liquid argon is a distorted hexagonal close packed arrangement which is nevertheless distinguishable from face centered cubic or icosahedral clusters distorted to the same degree or more. The surroundings of vacancies, however, are completely random with no characteristic symmetry properties.
The lambda-doubling spectrum of 13CH, studied by microwave optical double resonance
Authors: Steimle Timothy C. , Woodward David R. , Brown John M.
Published: July 2002

Lambda-doubling transitions in 13CH in its X 2Π state have been observed using the technique of microwave optical double resonance (MODR). The frequencies of hyperfine components of five separate lambda-doubling transitions have been measured and used to determine the lambda-doubling and hyperfine parameters in the molecular Hamiltonian. The 13C hyperfine parameters are discussed in terms of the electronic wave function for CH in the X 2Π state.
Color center laser spectroscopy of ν2 HCN---HF
Authors: Kyrö E. K. , Eliades M. , Gallegos A. M. , Shoja-Chagervand P. , Bevan J. W.
Published: July 2002

A continuously tunable single frequency color center laser spectrometer has been applied to analyze the rovibrational gas phase spectra of the fundamental ν2 (C–H stretching vibration) and its hot bands ν2+ν4−ν4 and ν2+ν17−ν17 in the linear dimer HCN---HF. The l-type doubling constants q″v and qv of the Π←Π band are both determined to be 12.4(8) MHz and anharmonic cross terms X024 and X027 are evaluated as −0.1656(17) and +0.1310(35), respectively from the available data. Excited state amplitude lifetimes in the ν2 band are demonstrated to be 1.3(4)×10−8 s.
Scaling in two-dimensional linear and ring polymers
Authors: Bishop Marvin , Michels J. P. J.
Published: July 2002

Bead size effects on the excluded volume of two-dimensional linear and ring polymers are investigated with Brownian dynamics. It is found that the mean dimensions of the chains follow a scaling relation with scaling variable X=N(σ/l)d/φ, where N is the number of units on the chain, σ is the size of the unit, l is the link length, d is the dimension, and φ is the crossover exponent. The scaling law is 〈R2〉/〈R2〉0 or 〈S2〉/〈S2〉0∼X2ν−1 for X→∞. Here ν is the critical exponent for the mean dimensions of an isolated polymer chain and the subscript 0 denotes the nonexcluded volume case.
An irregular dependence of the total electronic energy of clusters on their size
Authors: Cioslowski J. , Kertesz M.
Published: July 2002

The total electronic energy of a special class of finite clusters is calculated using the backfolding theorem and the properties of the trapezoidal quadrature. The resulting formula consists of a linear term, proportional to the size of the molecule and the remainder. It is found that the remainder is an irregular (or periodic) function of the cluster’s size. As a consequence the validity of the extrapolation of properties from small-size clusters to infinite solid state is questioned.
The shapes of two-, four-, and five-dimensional linear and ring polymers
Authors: Bishop Marvin , Saltiel Craig J.
Published: July 2002

The shapes of two- , four- , and five-dimensional linear and ring polymers, with and without excluded volume, are investigated numerically via Brownian dynamics. It is found that the asphericity of nonexcluded volume linear polymers is in agreement with theoretical predictions in all dimensions. Agreement with available theoretical predictions is also obtained for two-dimensional nonexcluded volume ring polymers. Moreover, it is shown that excluded volume polymers become more ideal (i.e., closer to nonexcluded volume polymers) as the dimension is increased.
Stirring and premixing effects in the oscillatory chlorite–iodide reaction
Authors: Luo Yin , Epstein Irving R.
Published: July 2002

The reaction between chlorite and iodide has been studied in a CSTR at different stirring rates with and without premixing of the input reagents. Most of the experiments have been conducted under oscillatory conditions. Three steady states are found in the premixing configuration. With the state of mixing as the bifurcation parameter, phase portraits are obtained showing the disappearance of oscillations through various types of bifurcation. The mixing dependence of the system’s dynamical behavior is discussed in terms of the elementary steps in a recently proposed mechanism. The significant enrichment in the dynamical behavior observed on varying the mixing requires revision of the mechanism.
The effects of basis set selection on the vibrational transition frequencies obtained from SCF and correlated wave functions for an uncomplicated molecule, LiH
Authors: Liu Shi-yi , Daskalakis Michael F. , Dykstra Clifford E.
Published: July 2002

The pattern of sensitivity to basis set selection in computed vibrational frequencies of LiH is examined through potential curve calculations at the SCF and at the well-correlated ACCD level. LiH is electronically uncomplicated in the sense that it has only four electrons, and so it affords the best opportunity to isolate basis set effects from lingering correlation features that might not be so completely accounted for in a more complicated molecule. With a very large basis of 64 functions, the transition frequencies are found to be within 2.7 cm−1 for ω0, within 3.3 cm−1 for ω12, and within 3.8 cm−1 for ω23 of measured values. Convergence with respect to basis set size is demonstrated by a decreasing sensitivity to basis enhancement as the largest set is approached. However, small sets can give excellent potential curves if the basis sets maintain a balance in the flexibility on the two atomic centers.
Analysis of electron spin echo modulation in randomly oriented solids: 15N modulation of radical cations of bacteriochlorophyll a and the primary donor of photosynthetic bacterium R
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r
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l
l
u
m r
u
b
r
u
m
Authors: Lin C. P. , Bowman M. K. , Norris J. R.
Published: July 2002

The electron spin echo modulation spectrum of organic radicals in randomly oriented solids is analyzed in detail for axially symmetric hyperfine tensors. Numerical calculations of the modulation spectra are compared with experimental results from the radical cations of 15N labeled bacteriochlorophyll a and the primary donor of photosynthetic bacterium Rhodospirillum rubrum. Determination of hyperfine coupling constants from experimental spectra in the frequency domain is complicated by the anisotropy of modulation intensity, the effects of second and higher order hyperfine interactions, overlapping spectra from several nuclei, multiple quantum coherences among hyperfine levels, and the spectrometer deadtime. Nonlinear least-squares analysis of the time domain data [Tang et al., J. Chem. Phys. 83, 4197 (1985)] is the only reliable method for extracting coupling constants from randomly oriented organic radicals.
The rotational distribution in the mercury hydride molecules (X 2Σ+1/2;v″=0) produced in the reactions Hg(6 3P1)+H2, HD, or D2
Authors: Bras N. , Butaux J. , Jeannet J. C. , Perrin D.
Published: July 2002

Initial rotational distributions of the vibrationless HgH or HgD molecules produced in the exoergic reactions of Hg (3P1) with H2, D2, and HD have been determined. No isotope effects have been observed. In the case of the reaction with HD the ratio [HgH(v″=0)]/[HgD (v″=0)] as well as the summed over all v″ ratio [HgH]/[HgD] have been measured.
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o calculation of potential energy surfaces and spectroscopic properties of H2S and H3S+
Authors: Botschwina P. , Zilch A. , Werner H.-J. , Rosmus P. , Reinsch E.-A.
Published: July 2002

Potential energy surfaces and spectroscopic properties were calculated for H2S and H3S+ from highly correlated SCEP-CEPA wave functions. The equilibrium geometry of H3S+ is predicted to be re =1.350 Å and θe =32.2°. The vibrational frequencies of H323S+ (in cm−1) were calculated to be 2529 (ν1), 1050 (ν2), 2527 (ν3), and 1208 (ν4) which are all in close agreement with experimental values obtained for solid H3S+SbF−6. The computed proton affinity for H2S of PA298=716.7 kJ mol−1 is in very good agreement with experiment.