bluuebunny/crossref_metadata_2025 huggingface dataset:
Excited state spectroscopy of 1,5-naphthyridine: Identification of the lowest energy excited singlet state as 1B
g(1nπ*)
Authors: Boldridge David W. , Scott Gary W.
Published: February 2003

The electronic absorption spectra of excited-state 1,5-naphthyridine from 400 to 650 nm are reported in several solvents. The time evolution of these spectra indicate that they arise from one or more Sn←S1 transitions. Comparison of these observed spectra with calculated excited state absorption spectra obtained using the INDO molecular orbital formalism demonstrates excellent agreement of the transition energies and oscillator strengths with Sn(1Au)←S1(1Bg,nπ*) transitions. These results conclusively identify the lowest excited singlet state of 1,5-naphthyridine as 1Bg(1nπ*).
System-bath decomposition of the reaction path Hamiltonian. II. Rotationally inelastic reactive scattering of H+H2 in three dimensions
Authors: Schwartz Steven D. , Miller William H.
Published: February 2003

Earlier work of the authors [J. Chem. Phys. 77, 2378 (1982)] has shown how the reaction path Hamiltonian of Miller, Handy, and Adams [J. Chem. Phys. 72, 99 (1980)] can be divided into a ‘‘system’’ of the reaction coordinate and modes strongly coupled to it, plus a ‘‘bath’’ of more weakly coupled modes. Quantum mechanical perturbation theory was used to show how one can combine an exact description of the system dynamics with an approximate (perturbative) treatment of the effect of the bath. The present paper applies this approach to the 3d H+H2 reaction, where the two collinear degrees of freedom constitute the system, and the two bending modes the bath. Comparison with the accurate scattering calculations of Schatz and Kupermann [J. Chem. Phys. 65, 4668 (1976)] shows it to provide a good description of the coupling between bending (i.e., rotational) and collinear modes.
Exact optical rotation for a nearly helicoidal model
Authors: Leuliette-Devin E. , Locqueneux R. , Tillieu J.
Published: February 2003

We determine the exact expression of optical rotation for free electrons constrained to move on two semicircular arcs. The obtained expression is compared with the result of other authors. Curves are plotted for helicoidal model and circle model.
Spin-coupled VB description of the potential energy surfaces for the reaction Be++H2 → BeH++H
Authors: Raimondi M. , Gerratt J.
Published: February 2003

The spin-coupled VB method is used to study potential energy surfaces of the reaction Be++H2 → BeH++H, BeH+H+. Four collinear paths on the potential were chosen corresponding to (i) Be+H2, R(Be–H2) varied with R(H–H) fixed, (ii) BeH+H, R(BeH–H) varied with R(Be–H) fixed, (iii) R(H–H) varied with Re(Be–H2) fixed, and (iv) R(Be–H) varied with R(BeH–H) fixed. The spin-coupled VB theory provides a compact description of the system: only 58 nonorthogonal configurations were used, giving rise to 83 spin-coupled structures. The various features of the potential energy surfaces are discussed, and it is hoped that these will stimulate ion beam reactive scattering experiments similar to those carried out on the B++H2 system.
An economical technique for forcing convergence in conventional SCF methods
Authors: Rico J. Fernández , García de la Vega J. M. , Paniagua M. , Fernández-Alonso J. I.
Published: February 2003

We present an economical technique for ensuring convergence of the open- and closed-shell SCF methods. In this technique, the number of operations required is proportional to the square of the number of basis functions and all the employed quantities are present in any conventional SCF procedure. We test their efficacy with several numerical calculations.
Substituent effects on intermolecular hydrogen bonding from a lattice gas theory for lower critical solution points: Comparison with experiments on aqueous solutions of alkylpyridines
Authors: Goldstein Raymond E.
Published: February 2003

The lattice-gas models of reentrant phase transitions in binary mixtures with hydrogen bonding, introduced and studied by Walker, Vause, and Goldstein, are shown to be sensitive to subtle changes in the molecular properties of the mixture components. Specifically, the phase equilibria in aqueous mixtures of alkylpyridines (methyl, dimethyl, and ethyl) are matched with the theoretical parameters describing the strength and orientational specificity of the intermolecular hydrogen bonding. The parametric trends are found to agree with those expected from consideration of the electronic and steric effects of ortho-, meta-, and para-alkyl substitution in aromatic compounds. These trends also correlate well with other thermodynamic and kinetic data on these alkylpyridines. Critical exponent renormalization in these systems is also discussed.
Perturbed angular correlation studies of indium metalloporphyrin complexesa)
Authors: Mullins Oliver C. , Kaplan Morton
Published: February 2003

Electric field gradients (EFG) at the metal center were studied in seven chloroindium porphyrin complexes and in chloroindium phthalocyanine by the technique of perturbed angular correlation of gamma rays. These complexes were synthesized with the probe nucleus 111In for use in polycrystalline and solution sources. The magnitudes of the EFGs of the metalloporphyrins span a wide range and are shown to correlate with the electron donating properties of the peripheral substituents. For the phthalocyanine complex the derived EFG is quite large in spite of the electron withdrawing properties of the phthalocyanine ring. The porphyrin complexes all exhibited appreciable distributions in the magnitudes of the EFGs, which may be correlated with the deformable nature of the porphyrin macrocycle in contrast to the relatively planar, inflexible structure of the phthalocyanine ring which showed no such spread. The PAC spectra of metalloporphyrins dissolved in organic solvents exhibited two attenuation components, whose parameters were consistent with porphyrin properties and provide evidence for metalloporphyrin aggregation.
Photoeffects on the fluorination of silicon. I. Influence of doping on steady-state phenomena
Authors: Houle F. A.
Published: February 2003

The effect of visible light on the reaction between silicon and XeF2 has been studied as a function of pressure, light intensity, and dopant. Steady-state etch rates are sensitive to all three variables. Two major light-induced processes have been identified. The reaction can be suppressed by diffusion of fluorine into unreactive sites and enhanced by an increased rate of addition of fluorine to silicon centers relative to the rate of desorption of the intermediate fluorosilyl species. Experiments indicate that these unreactive sites are located beneath the surface. The data have been interpreted in terms of a model in which adsorption of electronegative fluorine causes band bending in the semiconductor, the direction of which depends on whether the material is doped n or p type. Photogenerated charge carriers induce faster fluorine insertion rates into Si–Si bonds, and enhance diffusion below the surface. N- and p-type silicon differ mainly in their relative ability to stabilize subsurface fluorine. The data have important implications for the mechanism of the spontaneous reaction of fluorine with silicon. They may also be relevant to photo-oxidation of clean semiconductor surfaces and issues related to the stability of amorphous Si:H:F alloys.
The adiabatic and diabatic reactions of S(1D) atoms with OCS: Internal state distribution of the S2 products
Authors: van Veen N. , Brewer P. , Das P. , Bersohn R.
Published: February 2003

OCS at low pressure was photodissociated at 248 nm. It was shown by two photon laser induced fluorescence (LIF) that the concentration of the S(1D) atoms produced decays with time due to collisions with undissociated OCS molecules. In contrast, the S(3P) atom concentration rises from zero or nearly zero to a maximum and then subsequently decays. The S(1D) atoms react with OCS molecules to give S2 (X 3Σg), S2(a 1Δg), and S(3P). The rotational and vibrational distribution of the diatomic products was studied by LIF. The diabatic product S2(X 3Σg) was entirely in the v=0 state and was rotationally cold. The adiabatic product S2(a 1Δ) was probed in the v=1 state and had a fairly narrow non-Boltzmann type rotational distribution peaking at J=65. We speculate that the reaction takes place through a C2v COS2 intermediate. The adiabatic and diabatic reactions are characterized by sequential and concerted C–S bond breaking, respectively.
A variation of the alkoxy radical
Authors: Box Harold C. , Budzinski Edwin E.
Published: February 2003

A free radical species is described which has an ESR spectrum similar to that of alkoxy radicals, yet with distinct differences. It is produced by x rays in single crystals of sucrose irradiated at low temperature. The characterization of this species from ESR and ENDOR measurements is described. The radical is identified as an alkoxy radical whose variant properties are caused by partial delocalization of the unpaired electron.
Critical behavior of the diameters of liquid–liquid coexistence curves
Authors: Greer S. C. , Das B. K. , Kumar A. , Gopal E. S. R.
Published: February 2003

We have analyzed the diameters of the liquid–liquid coexistence curves of several binary liquid mixtures in search of the critical anomaly predicted by current theories. We find that while the data are consistent with the predicted functional form, the evidence for such an anomaly is not compelling.
The radial distribution function at low densities: Exact results for small and large separations for smooth potentials
Authors: Friend Daniel G.
Published: February 2003

Explicit expressions are obtained for the r→0 and r→∞ limits of the first density coefficient of the radial distribution function. This double integral has also been evaluated numerically by the Clenshaw–Curtis quadrature for Lennard-Jones and inverse power potentials. Our numerical results agree closely with previous work where overlap exists and confirm our analytic results in the r→0 and r→∞ regions.
The phase transition in sodium azide
Authors: Aghdaee S. R. , Rae A. I. M.
Published: February 2003

Sodium azide undergoes a transition from a high-temperature rhombohedral phase to a low-temperature monoclinic phase at 292.2 K. Results of x-ray diffraction studies of the low-temperature phase are reported which indicate that the transition is of the elastic type with the order parameter corresponding to a linear combination of the strains e13 and (e22−e11) along with a rotation of the azide ions in the xz plane. The temperature dependence of the order parameter in the vicinity of the transition is consistent with Landau theory for a weakly first-order change, and this model is also shown to explain a wide range of experimental data previously obtained on the transition. A microscopic model is developed on the basis of the quasi-harmonic approximation and this is shown to be in quantitative agreement with the experimental data on the transition.
Electron terminal times in irradiated helium and hydrogen
Authors: Douthat D. A.
Published: February 2003

The times required for moderation of keV electrons in gaseous helium and hydrogen to terminal energies in the range of 0.1 to 1 eV are calculated. Data used in the calculation include the subexcitation electron energy distributions and the stopping cross sections. The calculated terminal times are compared with the theoretical results of Mozumder and with results extracted from the experimental data of Warman and Suer.
Surface amplification of photoionization signal
Authors: Naaman R. , Petrank A. , Lubman D. M.
Published: February 2003

A new effect involving the interaction of electromagnetic radiation with adsorbed molecules is presented. A long pulsed, low peak power laser has been used in order to induce ionization of several organic compounds adsorbed on thin metal films. An enhancement in the electron signal of several orders of magnitude is observed, as compared to conventional gas phase multiphoton ionization. Two experimental configurations have been used. The surfaces were illuminated either through external or internal reflection. In this study the ionization spectra of molecules on surfaces is presented and the mechanism is discussed. It is believed that this effect relies upon the production of positive ions on the metal surface which lowers the work function sufficiently to allow efficient generation of photoelectrons by the photon flux.
Dielectric spectra of supercooled halogenobenzene-decalin solutions: A single particle site model for relaxation and resonant behaviors
Authors: Reid C. J. , Vij J. K.
Published: February 2003

Dielectric loss spectra for 10% V/V decalin solutions of PhBr, PhCl, and PhF have been measured using a precision, coaxial three-terminal cell at temperatures from 77 to 150 K and for a frequency range 200 Hz–5 MHz. These results which are in agreement and which extend previous measurements, are interpreted within the context of cosinal itinerant oscillation. Data for PhBr decalin are simulated using a model of a single Brownian particle rotating in the presence of a cosinal potential, which is a special case of the cosine itinerant oscillator model.
Reply to ‘‘A Comment on the order of the wetting transition at a solid–fluid interface’’
Authors: Benner R. E. , Teletzke G. F. , Scriven L. E. , Davis H. T.
Published: February 2003

Evans and Tarazona have raised a disagreement with the results of Teletzke et al. concerning wetting transitions predicted by van der Waals’ integral theory with exponential fluid potential and two different fluid-solid potentials. We reexamined these results and in agreement with Evans and Tarazona we conclude that the wetting transition is first order when a Lennard-Jones 9-3 fluid–solid potential is chosen. The results are supported by the predictions of a simple model, a discontinuous thick film.
Mass spectrometric determination of partial and total electron impact ionization cross sections of SO2 from threshold up to 200 eV
Authors: Orient O. J. , Srivastava S. K.
Published: February 2003

A new approach to quadrupole mass spectrometric measurement and analysis has been used to study the electron impact ionization of SO2 to determine the cross sections for the production of O+, S+, SO+, and SO+2 from SO2. The relative flow technique is utilized to determine the cross section values. The threshold potentials are found to be 12.5 eV (SO+2), 16.5 eV (SO+), 16.5 eV (S+), and 23.5 eV (O+). The partial cross section values at 100 eV electron impact energy are 4.20×10−16 cm2 for SO+2, 1.65×10−16 cm2 for SO+, 0.90×10−16 cm2 for S+, and 0.25×10−16 cm2 for O+.
Electronic and vibrational spectra of MgO–CoO solid solutions
Authors: Zecchina A. , Spoto G. , Coluccia S. , Garrone E.
Published: February 2003

The UV-VIS diffuse reflectance and IR transmission spectra of MgO–CoO solid solutions show that the frequency of both electronic and vibrational transitions changes with x molar fraction of CoO. In particular: (i) the energy of the 4T1g(P)←4T1g(F), 4A2g(F)←4T1g(F), and 4T2g(F)←4T1g(F) transitions varies linearly with x (compression effect); (ii) the energy of the transition 4T1g→(conduction band) shows a parabolic dependence upon x; (iii) the transverse optical mode (TO) shows a one-mode vibrational behavior, whose dependence upon x is well described by the modified random isodisplacement model (MREI).
Effect of variable excluded volume on dynamics of lattice chains
Authors: Romiszowski Piotr , Stockmayer Walter H.
Published: February 2003

The Kranbuehl–Verdier computer model for single-chain dynamics on a simple cubic lattice is extended to short chains with partial (weighted) overlap to simulate the effects of varying excluded volume on free-draining chains in dilute solution. As the probability of self-intersection decreases, the relaxation time derived from the autocorrelation function of the end-to-end vector increases strongly. For a given chain length, the increase is more rapid than that of the mean square end-to-end length. As in previous studies of lattice chains, the results depend on the types of allowed motion. For completely nonintersecting chains the observed relaxation time is roughly proportional to the cube of the chain contour length, in agreement with Kranbuehl and Verdier but in disagreement with scaling predictions. It thus appears that either the effects of long-lived traps have not been completely eliminated or topological restrictions play a role.
Vibrationally excited nitrogen molecules formed in the catalytic decomposition of ammonia on platinum
Authors: Foner S. N. , Hudson R. L.
Published: February 2003

Catalytic decomposition of ammonia on polycrystalline platinum was studied with a modulated molecular beam mass spectrometer. Threshold ionization measurements on N2 have established the production of significant quantities of vibrationally excited nitrogen molecules (N*2 ). Although the overwhelming majority of molecules have excitation energies less than 1.3 eV, a few of the molecules have energies as high as 2.4 eV, requiring excitation to at least the v=9 vibrational level of N2. It is suggested that the observed vibrational energy is provided by two reactions involving recombination of chemisorbed NH radicals: NHad+NHad→N*2+H2 and NHad+NHad→N*2+2Had. The ionization data on N*2 indicate that during ionization vibrational energy is being efficiently converted into electronic energy via autoionization processes. Measurements on the appearance potential of NH+2 show no evidence for the evolution of NH2 radicals from NH3 decomposition, confirming recently published results. Pulsed heating experiments show that N*2 molecules have, as expected, shorter lifetimes than N2 molecules. Reaction mechanisms for N*2 production are discussed.
Reflectivity of a shock front in water and nitromethane
Authors: Harris Paul , Presles Henri-Noel
Published: February 2003

The reflectivity of a 5.8 kbar shock front in water is compared to the reflectivity of a 6.0 kbar shock front in liquid nitromethane and to theoretical predictions containing secondary reflection. While agreement between theory and experiment is good for 70° optical angle of incidence, it is not good for 82°. There is also an unexplained systematic difference between the water and nitromethane data which increases as the optical angle of incidence grows to 82°.
Electron affinities from electron transfer equilibria in the gas phase and the electron affinity of SO2
Authors: Caldwell Gary , Kebarle P.
Published: February 2003

Measurements of gas phase electron transfer equilibria [Eq. (1)] A−+B=A+B− were performed with a pulsed electron beam high ion source pressure mass spectrometer. The equilibrium constants K1 were used to evaluate ΔG01=−RT ln K1. The ΔG01 were found to change little with temperature such that ΔG01≊ΔH01. Good agreement was observed between the present results and earlier measurements by Fukuda and McIver, which used ion cyclotron resonance and worked at much lower pressures. Absolute electron affinities were obtained by anchoring the relative electron affinities (ΔH1 values) to known absolute E.A.’s from the literature. The relative results agree well with differences predicted by absolute measurements. The resulting E.A. are in eV: CH3NO2, 0.49; CS2, 0.54; (C6H5)CO, 0.64; 2, 4, 6-trimethyl nitrobenzene 0.68; 2, 3-butanedione 0.71; 2, 3-dimethyl nitrobenzene, 0.83; 2-methyl nitrobenzene, 0.88; 3-methyl nitrobenzene, 0.94; nitrobenzene, 0.97; 1-fluoro-4-nitrobenzene, 1.06; SO2 1.11; 1-chloro-3-nitrobenzene, 1.23; maleic anhydride, 1.42; 1,3-dinitrobenzene, 1.6; 2, 5-dimethyl-p-benzoquinone, 1.72; methyl-p-benzoquinone, 1.78; p-benzoquinone, 1.86. The assignment of the present absolute values disagrees with that by Fukuda and McIver based on ICR bracketing experiments. It is suggested that the bracketing experiments were misleading due to presence of excitation in the ionic reactants used.
Classical barrier height for H+H2→H2+H
Authors: Liu B.
Published: February 2003

A b initio configuration interaction calculations on H3 and H2 show that the classical barrier for H+H2→H2+H must be less than 9.86 kcal/mol and most likely lies between 9.53 and 9.65 kcal/mol.
Fluorescence of excited ethynyl radicals produced by pulsed vacuum ultraviolet photolyses of C2H2, C2D2, and C2HBr
Authors: Saito Y. , Hikida T. , Ichimura T. , Mori Y.
Published: February 2003

Electronically excited ethynyl radicals were produced by pulsed vacuum ultraviolet photolyses of C2H2, C2D2, and C2HBr. The broad fluorescence spectra of the ethynyl radical extending from 410 to wavelengths longer than 750 nm were found to be insensitive to parent molecules photolyzed, their pressures, and also pressures of an added gas, He. Time profiles of the fluorescence intensity were found to be a superposition of two components. The dominant component is due to excited ethynyl radicals formed within the excitation pulse duration of ∼20 ns. At higher pressures, however, some minor contribution from a long-lived precursor to the fluorescence was detected. Zero-pressure decay rates and quenching rate constants were obtained. Formation mechanism of fluorescent ethynyl radicals is discussed briefly.
Trigonometric polynomial interpolation of quantum mechanical properties by mutual perturbation approach
Authors: Laforgue Alexandre , Čížek Jiří , Paldus Josef , Pellégatti Alain
Published: February 2003

A general procedure is formulated which enables an interpolation between two exactly solvable limiting cases of quantum mechanical Hamiltonians. This procedure employs trigonometric polynomials and its main advantages stem from its finite character, which avoids the singular behavior often found when interpolating using Padé approximants. The basic equations determining the coefficients of interpolating trigonometric polynomials from one- or two-point perturbation theory expansions are derived for the general case as well as for certain special cases in which the interpolated property possesses certain symmetry properties. The method is illustrated on simple examples from the Hückel theory of heteroanalogs of planar conjugated hydrocarbons.
s-cis octatetraene: Photoproduction and spectroscopic properties
Authors: Ackerman John R. , Forman Stuart A. , Hossain Musharraf , Kohler Bryan E.
Published: February 2003

Irradiation of solutions of all-trans octatetraene in n-octane and cyclohexane at temperatures below 10 K efficiently produces a photoproduct whose fluorescence and absorption spectra are shifted ∼1200 cm−1 to lower energy than the corresponding features in the all-trans precursor. High resolution excitation and fluorescence spectra and fluorescence lifetimes for this product have been determined. These data establish that the photoproduct is a noncentrosymmetric octatetraene isomer. The spectroscopic properties and thermal lability of this isomer are consistently interpreted if it is assumed that the photoproduct is generated from all-trans octatetraene by isomerization about one of the single bonds.
Electronic structure determination of iron(II) phthalocyanine via magnetic susceptibility and Mössbauer measurements
Authors: Labarta A. , Molins E. , Viñas X. , Tejada J. , Caubet A. , Alvarez S.
Published: February 2003

Mössbauer and magnetic susceptibility measurements of iron(II) phthalocyanine in the temperature range between 77 and 300 K have been carried out. The crystal field parameters and the electronic structure of the Fe(II) ion have been obtained using a least-squares fitting procedure of the dependence on the temperature of both magnetic susceptibility and quadrupole splitting. It is concluded that the ground state has major 3A character.
s-cis octatetraene: Ground state barrier for s-cis to s-trans isomerization
Authors: Ackerman John R. , Kohler Bryan E.
Published: February 2003

At temperatures below 10 K photolysis of solutions of all-trans octatetraene in n-octane efficiently produce a photoproduct which has been identified as an s-cis isomer. This paper reports quantitative measurements of the relative concentrations of this photoproduct and the all-trans precursor as a function of irradiation and temperature. These data establish that the primary photochemical channel open to all-trans octetraene under these conditions leads to the production of the s-cis isomer which, at temperatures above 50 K, quantitatively reverts to the all-trans form. This thermal reversion is well described as an activated first order process proceeding over a barrier of 4.0±0.2 kcal/mol. A model for computing isomerization rate constants from a simple one-dimensional potential also quantitatively fits the data. In this model the barrier height for s-cis to s-trans conversion is 3.16 kcal/mol, the enthalpy of the s-cis species is 2.88 kcal/mol above that of the all-trans and the internal rotor is a terminal ethylene group.
Dissociation of vibronic states of C–1A′ DCN: Quantum treatment
Authors: Chuljian David T. , Ozment Judy , Simons Jack
Published: February 2003

The rates of dissociation of several vibrational states of nπ* excited (C–1A′) DCN have been determined via quantum dynamical means in which only the CD stretching and DCN bending motions are treated. The ab initio configuration interaction potential energy surface used in our earlier classical trajectory study of these same dissociation rates was employed in the present study. The results of this quantal study tend to support our earlier prediction that v2→v1 (bending-to-stretching) energy transfer plays an important role in determining the dissociation rates of these vibronic states. Surprisingly, the absolute rates obtained via the quantum method are in quite close agreement with a certain component of the classically determined rates.
OH(A 2Σ+ → X 2Π) emission from dissociative excitation of HO2 at 147 nm
Authors: Suto Masako , Lee L. C.
Published: February 2003

Photodissociation processes of the HO2 radical were studied using the Xe resonance line at 147 nm as an excitation source. HO2 was produced by a three-body reaction process H+O2+He → HO2+He in a flow tube. The photoexcitation processes for various chemical species in the flow tube that could produce optical emission were investigated. An emission band in the 310±10 nm region has been observed, which is attributed to the OH(A 2Σ+ → X 2Π) system produced from photodissociation of HO2.
Picosecond study of an intermediate in the trans to cis isomerization pathway of stiff stilbene
Authors: Doany F. E. , Heilweil E. J. , Moore R. , Hochstrasser R. M.
Published: February 2003

A picosecond spectroscopic study of trans 1,1′-biindanylidene (‘‘stiff’’ stilbene) has exposed a transient absorption at 351 nm which is not an electronically excited state of the trans or cis forms. The signal strength increases with decreasing viscosity in a manner that is consistent with the decay of the excited trans molecules being the precursor of the new transient. The data suggest that after the viscosity dependent formation of the intermediate, it decays with a time constant of 10±3 ps into cis stiff stilbene. It is suggested that the intermediate is the ‘‘phantom’’ state of stilbene photochemistry.
Two new potential energy surfaces for the F+H2 reaction
Authors: Truhlar Donald G. , Garrett Bruce C. , Blais Normand C.
Published: February 2003

We have adjusted a new potential energy surface for the reaction F+H2→FH+H and isotopic analogs such that, as compared to the widely used Muckerman no. 5 potential energy surface, rate constants and activation energies are in better agreement with experiment, and the bending potential is in better argreement with ab initio calculations, while the product vibrational energy distribution for thermal collisions is not changed greatly. The adjustments are carried out using a combination of variational transition state theory, the correlation of product vibrational energy distributions with mixed energy release as defined by Polanyi and co-workers, and quasiclassical trajectory calculations. The new surface has a classical barrier height of 0.74 kcal/mol.
Time-resolved laser induced fluorescence measurement of excited states relaxation constants: Application to the quenching of mercury atoms by NO molecules
Authors: Bras N. , Brunet J. P. , Butaux J. , Jeannet J. C. , Perrin D.
Published: February 2003

A time-resolved laser induced fluorescence arrangement is described, which allows direct study of energy transfers and of relaxation processes. A first application to the direct measurement of quenching cross sections of mercury excited (6 3P1) and metastable (6 3P0) atoms by NO molecules is reported. For Hg(6 3P1) our result: σ=80±5 Å2 is in good agreement with previous works. On the contrary, our result for Hg(6 3P0): σ=85±5 Å2 strongly disagrees with previous ones. The reasons for this discrepancy are reviewed. We emphasize that our measurement is direct and that it has to do with a system in which only Hg and NO are present. On the contrary, in the previous works nitrogen was present, generally at pressures several orders of magnitude greater than NO pressures.
Electron–exciton complexes and the maxima in low-energy electron transmission spectroscopy
Authors: Caron L. G. , Bader G.
Published: February 2003

Electron–exciton complexes have already been proposed to explain small bumps in the low-energy electron transmission spectra of some hydrocarbon films. We discuss two mechanisms by which these should be maxima. The first is a scattering model which incorporates the electron ↔ electron–exciton complex transformation. The analysis allows us to set bounds to the various mean-free paths involved for a maximum in transmission. The second, which assumes radiative decay of the complex, seems somewhat inefficient.
State-selected photodissociation dynamics: Complete characterization of the OH fragment ejected by the HONO Ã state
Authors: Vasudev R. , Zare R. N. , Dixon R. N.
Published: February 2003

T r a n s-HONO is optically prepared in specific −N=O stretching vibrational levels (2n, n=1,2,3) of the Ã state at 369, 355, and 342 nm. The ejected OH fragment is completely characterized by Doppler and polarization laser excitation spectroscopy. In this manner the OH translational energy, angular distribution, rotational alignment, and internal state distribution (vibration, rotation, spin-orbit and Λ-doubling components) are probed through the OH A–X system. The OH fragment is found to be translationally hot (∼0.5 eV) with a nearly sin2 θ angular distribution about the electric vector of the photolysis laser. The corresponding line shapes are Doppler split. However, the line shapes and widths do not noticeably depend on either fragment rotation or parent vibration. The internal motion of the OH fragment is vibrationally and rotationally cold; the spin-orbit components and the Λ doublets are not in equilibrium. The OH fragment is aligned and its π lobe lies preferentially in the plane of rotation. With increasing rotational excitation, these effects become more pronounced. This information allows us to construct a detailed photodissociation mechanism. The fragmentation is prompt and the trajectories of the recoiling fragments lie close to the initial HONO plane. The impulse associated with the central O–N bond fission contributes predominantly to OH translation while the rotation appears to arise from the zero-point motion of the parent in-plane bending and torsional vibrations. The OH energy content is found to be quite insensitive to the parent ν2 vibration, suggesting that the Ã state surface is rather ‘‘flat’’ along the −N=O stretch compared to the steep fragmentation coordinate.
Fluorescence of the Na*–N2 collision complex
Authors: Kamke W. , Kamke B. , Hertel I. , Gallagher A.
Published: February 2003

The fluorescence spectrum of Na(3p) atoms in N2 gas has been measured in the far wings of the Na resonance lines, from 550–790 nm. We observe a broad continuum from 630–790 nm that is independent of N2 pressure and gas temperature, and which agrees with that calculated from the theoretical NaN2 potentials of Habitz if one includes only free collision states of the Na*N2 molecule. We conclude that this continuum is due to an attractive Na(3p)–N2 interaction, but that bound states in this potential well predissociate to Na(3s) and vibrationally excited N2 in much less than the Na* radiative lifetime of ∼16 ns. The intensity on the blue wing of the Na resonance lines decreases rapidly with decreasing wavelength, and shows temperature dependence corresponding to a repulsive Na(3p)–N2 interaction. Good agreement is obtained with a blue-wing spectrum calculated from the X̃ and B̃ states of Habitz.
Energy disposal in the thermal and near-thermal energy charge exchange reactions: N+(3P)+CO(X1Σ+) → N(4S)+CO+(X 2Σ) and N+(3P)+CO(X 1Σ+) → C(3P)+NO+(X 1Σ)
Authors: O’Keefe Anthony , Parent Denise , Mauclaire Gérard , Bowers Michael T.
Published: February 2003

The reaction of N+(3P) with CO (X 1Σ) has been studied using the technique of kinetic energy ion cyclotron resonance spectroscopy. The amount of reaction exothermicity which is channeled into product kinetic energy is measured and used to infer the distribution of product internal energy. The two lowest energy reaction pathways, producing CO+ or NO+, have been examined. The final vibrational state distribution of product CO+(X 2Σ) has been studied as a function of reactant center of mass collision energy and a strong dependence has been found. This strong dependence may arise from the energy resonant production of excited vibrational levels of product CO+.
Hard sphere sequential impulse model: Angular and velocity distributions of knock-out reactions
Authors: Safron S. A. , Coppenger G. W.
Published: February 2003

Angular and velocity distributions for the reaction A+BC→AB+C, derived from the hard sphere sequential impulse model (SIM), are given for the general case where energy can either be given up or taken up in each collision. This model is then used to obtain the angular and velocity distributions for the complementary ‘‘knockout’’ reaction A+BC→AC+B which takes place by the same sequence of collisions. A steric factor is introduced to make the model more realistic for knockout reactions. Example contour diagrams are shown for several endoergic reaction cases.
Comparison of quantum, classical, and statistical behavior in dissociating triatomics
Authors: Rai Sachchida N. , Kay Kenneth G.
Published: February 2003

Time-dependent calculations are presented which compare the dynamics of quantum and classical one-dimensional triatomic systems undergoing intramolecular vibrational energy transfer and dissociation. The purpose of these calculations is to determine whether statistical dissociative behavior in classical systems implies similar behavior in the analogous quantum systems and to test for the presence of quantum mechanical effects that reduce the tendency of the systems to decompose statistically. The intramolecular vibrational energy transfer is monitored by computing the probability for the systems to remain in their initial, coarsely grained states, and the dissociation is followed by calculating the time-dependent decomposition probability and the product distribution. The classical calculations are performed by a version of the quasiclassical technique while the quantum calculations are carried out by an R-matrix method. The results show that the two forms of dynamics usually result in similar intramolecular evolution and unimolecular decay. Since the behavior of the classical systems is statistical in a well-defined sense, it is argued that the behavior of the quantum mechanical systems can likewise be labeled as statistical in these typical cases. Important exceptions to the generally good quantum-classical agreement occur, however, when the systems are prepared with high energy in a dissociable bond and low energy in the other bond. In such cases, the quantum behavior differs significantly from the classical behavior; the quantum dynamics of decomposition is nonstatistical even though the classical dynamics is statistical. It is found that the ‘‘quantum trapping’’ states which lead to the nonstatistical quantum mechanical behavior are associated with narrow Feshbach resonances which accumulate in certain specific energy regions. It is further concluded that these states occupy a significant proportion of the classical phase space available to molecular complexes with energy below the first vibrational threshold.
Shock tube study of the thermal decomposition of cyanogen
Authors: Szekely Attila , Hanson Ronald K. , Bowman Craig T.
Published: February 2003

The thermal decomposition of cyanogen behind incident shock waves has been studied in the temperature range 2500–3450 K, at pressures between 0.23 and 0.58 bar. The course of the reaction was followed by monitoring the CN (B 2Σ+, v=0←X 2Σ+, v=0) absorption at 388 nm. The low-pressure rate coefficient for the reaction C2N2+Ar→2CN+Ar was fitted by the Arrhenius expression k1=1016.8±0.17 exp[−(50 040±1070)/T]cm3/mol s. Comparisons of the present work with previously published results are discussed.
Thermal unimolecular decomposition of formic acid
Authors: Saito Ko , Kakumoto Terumitsu , Kuroda Hiroyuki , Torii Shigeaki , Imamura Akira
Published: February 2003

The initial steps involved in the thermal decomposition of formic acid in the temperature range of 1370–2000 K have been investigated by monitoring the IR emission intensities at 3.4 and 4.6 μm, corresponding to the reactant and carbon monoxide, respectively, behind reflected shock waves in mixtures diluted in Ar (0.1–1.5 mol%, total densities 5.3×10−6–3.2×10−5 mol cm−3). It was found that the decomposition proceeded via channel (1) HCOOH+Ar→CO+H2O+Ar dominantly and the contribution of channel (2) HCOOH+Ar→CO2+H2+Ar was very small. An Arrhenius expression of the second order rate constant was obtained as k1,0 =1014.32  exp(−40.4 kcal mol−1/RT) cm3 mol−1 s−1. Ab initio calculations were performed for probable transition states (TS) of channels (1) and (2). The results showed that the potential energy for a TS of channel (1) was lower than that of channel (2) by 20–30 kcal mol−1. On the basis of a RRKM weak collision, k2,0 values were estimated which was smaller than k1,0 by about two orders, being consistent with the experimental results.
A vibronic analysis of the lower Ã 1A″←X̃ 1A′ singlet–singlet and ã 3A″←X̃ 1A′ triplet–singlet band systems of thioacrolein (2-propenethial)
Authors: Judge R. H. , Moule D. C.
Published: February 2003

The visible spectrum of thioacrolein has been photographed under conditions of long path lengths 98 m and modest dispersion (15 nm cm−1). Two electronic systems have been identified in the spectrum, namely, the ã 3A″←X̃ 1A′ spin forbidden but orbitally allowed n→π* transition and its fully allowed companion transition Ã 1A″←X̃ 1A′. The vibrational fine structure in both transitions is assigned in terms of a single quantum of ν10(CS), ν11(CC), ν12(CCS), and sequence transitions in ν18 (torsion). The structural changes which accompany the electronic excitation are interpreted in terms of a shortening of the CC bond, a lengthening of the CS bond and a stiffening of the skeleton to torsional distortion. No evidence for cis thioacrolein has been found.
Transient infrared spectroscopy on the picosecond time scale by coherent pulse propagation
Authors: Hartmann H.-J. , Laubereau A.
Published: February 2003

A novel time-resolved IR spectroscopy is accomplished studying coherent propagation of ultrashort pulses at low-intensity level. A detailed theoretical discussion is presented for electric dipole interaction between a resonant coherent pulse and a molecular transition. Homogeneous broadening (single line) and also more realistic situations with several adjacent transition frequencies are considered. Drastic changes of the pulse wings are predicted depending on several important parameters; e.g., absorption length αl, pulse duration tp, and a molecular dephasing time T2. It is shown that coherent superposition of neighboring transitions leads to a beating phenomenon, which is readily observable for αl≲1 and T2/tp>1. The theoretical results are substantiated by experimental data on HCl gas in the pressure range 0.5 to 3 bar. Tunable pulses of 4 ps produced by a parametric generator system are used and two ultrafast IR light gates. Quantitative agreement between theory and experiment is obtained. The dephasing time T2 of the R(3) vibration-rotation transition is measured to be T2×p=21.5 ps bar in good agreement with spectroscopic data. Collective beating of the R(3) transition of the two isotopic species HCl35 and HCl37 is observed for the first time in coherent pulse propagation. The linear intensity dependence of the propagation effect favors a wide range of applications in spectroscopy.
Homogeneous linewidths of single rotational lines in the ‘‘channel three’’ region of C6H6
Authors: Riedle E. , Neusser H. J.
Published: February 2003

Homogeneous linewidths of single rotational lines have been measured in the ‘‘channel three’’ region of C6H6. The necessary spectral resolution of about 5 MHz and the high sensitivity were achieved by Doppler-free two-photon spectroscopy within an external cavity setup. Linewidths produced by intramolecular relaxation were found to increase strongly with increasing J for K=0 and constant vibrational energy Eexcess =3412 cm−1. This result together with the investigated structure in the highly resolved spectrum of the 1410120 band is a strong indication that Coriolis coupling plays an important role in the intramolecular vibrational redistribution process.
Fast atom bombardment (FAB) generation of neutral and charged reactive intermediates for rare gas matrix isolation spectroscopic studies. An ESR investigation of 111,113CdOH and 111,113Cd+ in neon and argon matrices at 4 K
Authors: Knight Lon B. , Miller P. K. , Steadman J.
Published: February 2003

An experimental arrangement is presented which allows the combination of neutral atom bombardment in the 3–10 keV range with the rare gas matrix isolation technique for generating and trapping reactive intermediates and cation radicals for electron spin resonance (ESR) and other spectroscopic investigations. The potential of the experimental procedure for studying metal surface reactions is discussed. Neon and argon matrix results are reported for the bombardment of several gaseous and metal samples. The 111,113CdOH radical has been characterized by ESR and compared with previous findings for CdCN, CdF, and CdH. Neon matrix measurements for CdH and Cd+ have been obtained with the FAB method which allow a comparison to be made with previous argon matrix findings. This represents the first magnetic data comparison of neon and argon matrix results for a metal hydride type radical (MH) and a cation radical. These comparisons are especially important for gauging the ‘‘gas-like’’ nature of matrix results for such highly reactive radicals.
Spectroscopy and thermodynamics of ArHCl
Authors: Bernstein Lawrence S. , Wormhoudt Joda
Published: February 2003

A general model for analysis of the infrared spectrum of atom–diatom van der Waals molecules is developed. Using energy levels and spectroscopic parameters based on the Hutson and Howard M5 potential, this model quantitatively reproduces the previously observed ArHCl spectrum in the null gap of the monomer HCl spectrum. The thermodynamics of atom–atom and atom–diatomic systems is discussed. An approximate quantum mechanical method of evaluating bound and metastable partition functions is derived. Simple expressions for the partition functions are given which are shown to accurately reproduce the well established results for atom–atom complexes. The generalization of the approach to atom–diatomics is discussed. These partition functions are used to calculate thermodynamic properties such as heat capacities and equilibrium constants.
A non-Gaussian theory of rubberlike elasticity based on rotational isomeric state simulations of network chain configurations. III. Networks prepared from the extraordinarily flexible chains of polymeric sulfur and polymeric selenium
Authors: Mark James E. , Curro John G.
Published: February 2003

Distribution functions for the end-to-end separation of polymeric sulfur [-S-] and selenium [-Se-] are obtained from Monte Carlo simulations which take into account the chains’ geometric characteristics and conformational preferences. Comparisons with the corresponding information on polyethylene [-CH2-] demonstrate the remarkable equilibrium flexibility or compactness of these two molecules. Use of the S and Se distribution functions in the three-chain model for rubberlike elasticity in the affine limit gives elastomeric properties very close to those of simple Gaussian networks, even though their distribution functions appear to be significantly non-Gaussian. Specifically, unlike the case of polyethylene, even very short S and Se chains show only slight upturns in modulus at high elongations; this is consistent with their unusually high flexibility.
Laser-induced electron–phonon processes on metal surfaces
Authors: Murphy William C. , George Thomas F.
Published: February 2003

Electronic charge transfer from bulk to surface states in metals induced by laser radiation is examined. Due to the position of the Fermi energy, it is necessary to also include phonon excitations. Cross sections for this second-order process are presented. Comparison with the surface excitation in semiconductors is made. It is seen that for metals the required laser frequencies are larger and the cross sections substantially less than for semiconductors.
Constraints on electronic energy hypersurfaces of higher multiplicities
Authors: Mezey Paul G.
Published: February 2003

Upper and lower bounds are derived for electronic energy hypersurfaces of various multiplicities over a general nuclear configuration space. Whereas, in relations derived earlier, only energy hypersurfaces of the same multiplicity could be used as upper or lower bounds for each other, this restriction is now removed in the more general relations proposed in the present study.
Asymptotic region of a Thomas–Fermi atom
Authors: Patil S. H.
Published: February 2003

Thomas–Fermi model for an atom, is modified to include the correct asymptotic behavior of the charge density and electrostatic potential, in terms of its ionization energy. The predictions of the model, for the diamagnetic susceptibility, are in good agreement with the experimental values.
Second row homopolar diatomic molecules. Potential curves, spectroscopic constants, and dissociation energies at the basis set limit for SCF and limited CI wave functions
Authors: McLean A. D. , Liu B. , Chandler G. S.
Published: February 2003

Single configuration SCF calculations near the HF limit and CI calculations have been performed for the ground and selected excited states of the molecules Al+2 , Al2, Si+2 , Si2, P+2 , P2, S+2 , S2, Cl+2 , and Cl2. The CI calculation includes single and interacting double excitations from the valence orbitals of the single HF configuration. Potential energy curves from HF and CI wave functions, and with corrections for quadruple excitations, have been obtained for regions near the equilibrium bond lengths. Equilibrium bond lengths and vibrational separations for v=0–3 are calculated from these curves. Calculated ionization potentials are reported. Dissociation energies are calculated by determining the energies of the separated fragments in separate atomic calculations at the same level as the molecular calculations. The calculations are critically evaluated with emphasis on obtaining guide lines for more accurate work.