bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental evidence for metastable states of D3O and its monohydrate by neutralized ion beam spectroscopy
Authors: Gellene Gregory I. , Porter Richard F.
Published: February 2003

The oxonium radical (H3O) has been generated in its ground state by neutralizing a fast beam of ions in the near resonant electron transfer reaction H3O++K(g)→H3O*+K+. Analysis of neutral beam scattering profiles and collisionally reionized mass spectra indicate that the fully deuterated species (D3O) can be formed in a distribution of dissociative and metastable states (τ>0.6 μs). Thermalization of the precursor D3O+, prior to electron transfer, is required for production of metastable D3O. Neither H3O nor D2HO is observed in metastable states. These isotope effects support earlier theoretical predictions of a shallow local minimum on the oxonium potential surface. The ionization potential of D3O is calculated to be 4.3±0.1 eV. Some spectroscopic implications for this radical are discussed. The oxonium monohydrates (H3O⋅H2O) are also observed to exist in metastable states for several H/D isotopic variants. The ionization potential of D3O⋅D2O is estimated to be ≥3.4 eV.
Quantum mechanical differential and integral cross sections for state-to-state vibrational excitation of I2 by He
Authors: Schwenke David W. , Truhlar Donald G.
Published: February 2003

We have performed scattering calculations for vibrationally inelastic collisions of He with I2. We employed close coupling to treat vibration, the infinite-order-sudden approximation for rotation, and a recently reported potential energy surface based on ab initio calculations. We have calculated state-to-state and rotationally summed differential and integral cross sections for three impact energies. The energy dependence of the integral cross sections agrees reasonably well with the recent experiments of Hall et al.
Bimodal vibrational distribution of BaBr in the reaction Ba+CF3Br
Authors: Munakata Toshiaki , Kasuya Takahiro
Published: February 2003

Nascent vibrational distributions of BaBr were studied for the reaction Ba+CF3Br →BaBr+CF3. In a crossed beam configuration of supersonic CF3Br beam and effusive Ba beam, the product state distribution was measured by the laser induced fluorescence as a function of collision energy. The vibrational distribution of BaBr clearly showed a bimodal feature. At low collision energy, BaBr was produced preferentially around ν″=32, while at some higher collision energy, another vibrational component gained intensity at around ν″=15. The average vibrational energy of the higher vibrational component was about 80% of the total available energy, and that of the lower, about 35%. The bimodality of vibrational distribution indicates that the reaction Ba+CF3Br proceeds along two reaction pathways. In one pathway, Ba approaches in the direction of Br–CF3, and forms BaBr in low vibrational states. Whereas in the other, the atom approaches in the CF3–Br direction, and leads to a high vibrational excitation of BaBr. The branching ratio to the former pathway increases with collision energy.
A model for orientation effects in electron-transfer reactions
Authors: Siders Paul , Cave Robert J. , Marcus R. A.
Published: February 2003

A method for solving the single-particle Schrödinger equation with an oblate spheroidal potential of finite depth is presented. The wave functions are then used to calculate the matrix element TBA which appears in theories of nonadiabatic electron transfer. The results illustrate the effects of mutual orientation and separation of the two centers on TBA. Trends in these results are discussed in terms of geometrical and nodal structure effects. Analytical expressions related to TBA for states of spherical wells are presented and used to analyze the nodal structure effects for TBA for the spheroidal wells.
Proton magnetic resonance study of the H2–He potential energy surface
Authors: Lemaire Claude , Armstrong Robin L. , McCourt Frederick R.
Published: February 2003

Measurements of the proton spin-lattice relaxation time (T1) carried out at 18.775 MHz in two H2–He mixtures are presented. Values of T1/ρ)∞lin, accurate to within 2%, are deduced for ten temperatures between 85 and 300 K. These data are compared with theoretical values deduced from various H2–He potential surfaces. It is shown that the data are quite capable of distinguishing between the various surfaces and that the most recent H2–He surface of Kohler and Schaefer gives an excellent representation of the data.
Model based calculations of the lattice mode spectra of ice Ih and amorphous solid water
Authors: Nielson Glen , Townsend R. Michael , Rice Stuart A.
Published: February 2003

We report calculations of the translational and librational mode regions of the Raman and infrared spectra of ice Ih and amorphous solid water. The models used for these materials are those proposed by Bergren and Rice [J. Chem. Phys. 77, 583 (1982)] and Madden et al. [J. Chem. Phys. 69, 3497 (1978)]. There is good agreement between the calculated and observed librational mode spectra. The disagreement between the calculated and observed translational mode Raman and infrared spectra, together with agreement between calculated and observed translational mode densities of states, supports the suggestion by Klug and Whalley [J. Glaciology 21, 551 (1978)] that electric field induced mixing of translational states is important in ice Ih.
Optical dephasing and photon echoes from energetically and substitutionally disordered crystals
Authors: Root L. , Skinner J. L.
Published: February 2003

For a crystal with both diagonal disorder (inhomogeneous broadening) and substitutional disorder we have calculated the contribution to the photon echo decay due to purely electronic intersite interactions. We have considered the two important limiting cases of macroscopic and microscopic inhomogeneous broadening. In the case of macroscopic broadening we find that the optical dephasing rate constant 1/T2∼f J, where  f is the fractional concentration of impurities and J is the nearest-neighbor intersite interaction in frequency units. This result is valid for  f≪1. In the opposite limit of microscopic inhomogeneous broadening, we find that 1/T2∼f J2/δ̄, where δ̄ is the width in frequency units of the inhomogeneous distribution. This result is valid for arbitrary  f, including  f=1, and for J≪δ̄. In the case that  f≪1 and J≪δ̄, 1/T2 is much smaller for microscopic broadening than for macroscopic broadening. We discuss our results in relation to several experiments on mixed molecular crystals and to an experiment on the stoichiometric material EuP5O14.
Microwave spectrum, structure, and π conjugation of isocyanocyclopropane
Authors: Taylor William H. , Harmony Marlin D. , Cassada David A. , Staley Stuart W.
Published: February 2003

The microwave spectrum of the normal isotopic species of isocyanocyclopropane and its three unique mono-substituted 13C isotopic varieties have been investigated in their natural isotopic abundances. For the normal isotopic species, Stark modulation and RFMDR techniques lead to the following rotational constants: A=15 827.303±0.009 MHz, B=3734.004±0.002 MHz, and C=3515.892±0.002 MHz. These data, along with those for the three isotopic species, lead to the following heavy atom structural parameters: C1C2=1.521±0.007 Å, C2C3=1.513±0.005 Å, C1N=1.377±0.008 Å, and C≡N=1.176±0.005 Å. To within a precision of ±1°, the C–N≡C group is linear and is inclined at an angle of 123.4±0.6° to the plane of the ring. The structural parameters are compared to those of related molecules and interpreted in terms of a frontier molecular orbital model for the π conjugation of the cyclopropyl ring.
Theory of photon echoes from interacting impurities in crystals with inhomogeneously broadened absorption spectra
Authors: Loring Roger F. , Andersen Hans C. , Fayer M. D.
Published: February 2003

We present a theory of photon echoes from impurity molecules in mixed crystals that takes into account electronic interactions among impurity molecules and inhomogeneous broadening of the electronic absorption spectrum. The system is modeled as a collection of randomly placed interacting two level systems with a distribution of transition energies. The observable of the echo experiment is expanded in powers of the density of two level systems, and the first two terms of this expansion are calculated. A scaling argument suggests that the density expansion is rapidly convergent for concentrations of experimental interest, and the density expansion is used to construct a Padé approximant. At times comparable to or less than the inverse of the width of the inhomogeneous distribution of resonance frequencies, the time dependence of the signal is determined primarily by the interactions between the impurities and is independent of the inhomogeneous width. At longer times the inhomogeneity becomes significant and the echo signal displays a much weaker time dependence. We examine the effect of different microscopic models of inhomogeneous broadening on the echo observable. We demonstrate that the photon echo experiment can provide a sensitive probe of interactions between impurities in mixed crystals.
Diode laser detection of the bending mode of HCO+
Authors: Davies P. B. , Rothwell W. J.
Published: February 2003

The infrared laser absorption spectrum of the bending fundamental of HCO+ has been detected using the velocity modulation technique. Assignment and measurement of 23 transitions yields a band center of 829.721(2) cm−1.
Theoretical study of the correlation and relativistic effects in the shakeup in the photoelectron spectrum of cesium
Authors: De Alti G. , Decleva P. , Lisini A.
Published: February 2003

Quasidegeneracy effects in the transitions in cesium are considered by relativistic and nonrelativistic CI calculations, and the interplay between correlation and relativistic effects is discussed. Dynamical correlation effects in the core shakeup are treated by nonrelativistic MCSCF calculations. Good agreement with experimental data is obtained and a revision of some features is suggested.
Theory of relaxation in viscous liquids and glasses
Authors: Brawer S. A.
Published: February 2003

A theory of relaxation (the time-dependent change of macroscopic properties) of nonpolymeric viscous liquids and glasses is presented. The fluid is described by a set of quasiequilibrium structures, and a master equation gives the transitions among these structures. Any structural change is presumed to require a cooperative rearrangement involving many atoms, and this rearrangement entails a fluctuation to a high-energy transition state. The structure of the fluid varies from point to point, and the rate of this transformation depends crucially on the local structure. The resulting kinetic equation describes very well the main features of observed relaxation—namely, the broad distribution of relaxation times and the nonlinearity (in ΔT) of relaxation following a temperature jump ΔT, where the apparent activation energy for relaxation depends on time. The kinetic equation is solved exactly, and the resulting solution is exhibited for a particular set of the parameters. The resulting relaxation function for energy relaxation goes as e−t1/4, except at the very shortest times. (The precise exponent— 1/4 in this case—will depend on the parameters used in the particular calculation.) A detailed comparison with other theories is made, and suggestions of how the theory can be used to develop a phenomenological model of relaxation of glass are given.
Theoretical approach to the reaction C(3P)+NO(X 2Π)
Authors: Halvick P. , Rayez J. C. , Evleth E. M.
Published: February 2003

Correlation diagram and extended ab initio calculations are used to obtain the main features of the dynamical aspects of the reaction C(3P)+NO(X 2Π). The lowest surface of 2Π symmetry leading to the ground state via an expected intermediate CNO is found to be 4.2 eV below the reactant energies. There is no potential energy barrier for the collinear approach along the 2Π surface. The first excited state of CNO has 2Σ+ symmetry. The high energy barrier appearing for the collinear C+NO approach yielding this state disappears for a bent configuration. A spin orbit avoided crossing between the 2Σ+ and 4Π surfaces is computationally investigated for the collinear approach. It is concluded that any such surface crossing will not play a significant role in the generation of excited CN(A 2Π). Due to the deep wells for the ground and first excited state of CNO, the vibrational populations in the products CN(X 2Σ+) and CN(A 2Π) are expected to be thermalized. However, the rotational populations are expected to be somewhat nonstatistical due to a bent approach along the first excited 2 2A′ surface and a relatively flat entrance valley for the ground state process.
Probabilities of vibrational energy exchange between carbon dioxide and carbon tetrafluoride
Authors: Manzanares I Carlos , Colón Emilio
Published: February 2003

The deactivation rate constant of CO2(ν3) in mixtures with CF4 has been measured at 298 K. A pulsed CO2 laser was used to excite the CO2 molecules. To interpret the mechanism of energy exchange, the probabilities of V–V energy transfer for CO2–CF4 mixtures and the V-T self-relaxation of CF4(ν2) were calculated. The model used considers a Morse potential to describe the interaction between the molecules and the quantum mechanical first order distorted wave approximation to calculate the probabilities. Order of magnitude agreement between the experimental and calculated results for CO2–CF4 mixtures and for the CF4(ν2) self-relaxation is obtained.
An improved potential for Ne–Ar
Authors: Aziz Ronald A. , van Dalen A.
Published: February 2003

Literature interatomic potentials for the Ne–Ar system are analyzed and critically assessed as to their abilities to predict dilute gas bulk (second virial and transport coefficients) and microscopic properties (differential and total collision cross sections). One potential determined by Ng et al. gives a good description of the Ne–Ar interaction. But it does not accurately predict some of the newer data presented in the literature since its development. The other potentials give an inadequate description of the Ne–Ar interaction. An improved potential of the HFD form with individually damped dispersion terms is presented which accurately predicts a wide range of properties.
Deviations from the linear mixture rule in nonequilibrium chemical kinetics
Authors: Dove John E. , Halperin Stephen , Raynor Susanne
Published: February 2003

In experimental studies of vibrational relaxation, dissociation, or isomerization of molecular gases, it is common to use mixtures of the reacting gas with inert diluents. Rate constants for the reaction in pure reactant gas or pure diluent gas are then evaluated by extrapolation using the linear mixture rule (LMR): kLMR=∑ixiki, where xi is the mole fraction of gas i and ki is the rate constant for the reaction in pure component i. However, this rule is only obeyed rigorously for first order or pseudo-first-order processes having a single rate-determining step or which proceed at thermal equilibrium. We demonstrate theoretically that deviations from the linear mixture rule are always positive or zero (ktrue≥kLMR) and show with model calculations that the neglect of these deviations resulting from the use of the linear mixture rule may lead to large overestimates of the true pure component rate constants.
Optoacoustic detection of photodissociation and termolecular recombination in Cl2
Authors: O’Connor M. T. , Diebold G. J.
Published: February 2003

In a spectrophone cell at pressures on the order of 1 atm, photodissociation of a homonuclear diatomic molecule gives two radicals which ultimately recombine in a three-body process to release an amount of heat equal to the energy of the incident photon. If the radiation producing the photodissociation is amplitude modulated, a periodic heating of the gas takes place that gives rise to a sound wave. Since the recombination process that liberates heat is bimolecular in the radical concentration, the equations describing the optoacoustic effect are nonlinear. The nonlinear response causes the phase lag in the acoustic signal to be dependent on the intensity as well as modulation frequency of the incident radiation. Additional phase shifts are caused by heat release from thermalization of the recoil energy of the nascent photofragments and an increase in mole number immediately following photofragmentation. To test the calculated amplitude and phase dependences of the optoacoustic effect produced by photodissociation, experiments with Cl2 irradiated at a wavelength of 488 nm were done in a nonresonant spectrophone cell. Several effects depending on modulation frequency and radiation intensity were found that are characteristic of the mechanism of energy release in the system. At low frequencies the acoustic phase lag increases with modulation frequency but decreases with light intensity. At high modulation frequencies the component of the acoustic signal from the mole number increase and photofragment recoil dominates the response giving a phase lag that decreases with increasing modulation frequency.
Excited states deactivating exclusively through correlated electron tunneling
Authors: Berezin Alexander A.
Published: February 2003

The consideration of polycenter excited states in impurity complexes in which N electrons are distributed over 2N sites is given. We demonstrate that for specific spatial arrangements of these sites, the system as a whole can undergo into the ground state only through the simultaneous correlated tunnel transition of all N electrons. Cases for N=2, 3, and 4 are considered explicitly.
Defects in amorphous silica: Ab initio MO calculations
Authors: O’Keeffe M. , Gibbs G. V.
Published: February 2003

SCF-MO calculations have been carried out on a number of silicon oxy-hydroxides chosen to simulate possible defects in amorphous silica. It is concluded that Si=O double bonds and four-membered SiOSiO rings are not plausible high-concentration defects. Support is provided for Galeener’s assignment of the D1 and D2 ‘‘defect’’ bands in the Raman spectrum of amorphous silica to eight- and six-membered rings, respectively.
Computer experiments of aqueous solutions. V. Monte Carlo calculation on the hydrophobic interaction in 5 mol % methanol solution
Authors: Okazaki Susumu , Touhara Hidekazu , Nakanishi Koichiro
Published: February 2003

Monte Carlo calculation has been carried out for 5 mol % aqueous solution of methanol at 298.15 K and experimental density with the Metropolis scheme in NTV ensemble. The total number of molecules is 216, of which 11 are methanol. The three kinds of pair potential function used are all based on SCF MO calculations, namely, water–water interactions with MCY (Matsuoka–Clementi–Yoshimine) potential, water–methanol, and methanol–methanol interactions with those proposed by Okazaki et al. and Jorgensen. Totally 5 800 000 configurations have been generated and final 4 200 000 configurations have been used to the calculation of thermodynamic quantities and various distribution functions. It is found that the mixing is slightly exothermic which can be ascribed to the promotion of water structure around methanol rather than the formation of hydrogen bonding between water and methanol. Evidence is given for the existence of hydrophobic interaction effect and the self-association of methanol with or without one water layer in between.
Cadmium-113 NMR investigations of cadmium oxide
Authors: Shatlock Michael P. , Maciel Gary E.
Published: February 2003

1 1 3Cd NMR has been used to examine the Cd–CdO solid solution. A series of samples prepared under different conditions was analyzed for percent Cd(0) using a direct chemical analysis. Conduction electrons arising from the ionization of Cd(0) appear to be affected by the chemisorption of water on the CdO surface. The conduction electrons influence the resonance frequencies and spin-lattice relaxation times of the Cd(II) moieties detected in Bloch decay experiments. The low-shielding signals observed for CdO samples have a variable linewidth, which correlates directly to the temperature of sample preparation. Selective saturation, Hahn spin echo and two-dimensional NMR experiments showed that this line width is produced by an inhomogeneous broadening mechanism, probably resulting from a distribution of CdO crystallites having different Cd(0) concentrations. Korringa-relation predictions of spin-lattice relaxation as a function of resonance frequency confirm the importance of conduction electron concentrations in determining the 113Cd NMR parameters.
Repulsive interactions of closed-shell ions with Ar, Kr, and Xe atoms: Comparison of beam and transport measurements
Authors: Viehland Larry A. , Mason E. A.
Published: February 2003

Measurements of repulsive interactions of closed-shell alkali and halogen ions with Ar, Kr, and Xe atoms by ion-beam scattering are compared with measurements of ion mobility and diffusion in drift tubes at high electric field strengths. Both measurements probe the repulsive interaction and have a region of overlap which can be tested for consistency. The transport data are generally consistent with the ion-beam measurements, but the latter should probably not be analyzed as a single exponential repulsion, which ignores the effects of attraction at larger separations. As a by-product, generalized Einstein relations for calculating ion diffusion coefficients from measured mobilities are found to give good agreement with direct measurements.
The vibrational modes of a nonrigid molecule: The C̃ 1B2 electronic state of sulfur dioxide
Authors: Jaffé Charles
Published: February 2003

Classical mechanics is used to study the dynamical nature of the vibrational modes of a nonrigid molecule. This study reveals the existence of two qualitatively different sets of vibrational modes. One set is identified as local modes, while the other is identified as normal modes. These results are used to assign the observed vibrational structure of the C̃ 1B2←X̃ 1A1, electronic transition of SO2. The implications for NO2, ClO2, and SeO2 are discussed.
Spectroscopic evidence of vibrational dipole–dipole coupling between nitrosyl groups in some nitroprussides
Authors: González S. R. , Aymonino P. J. , Piro O. E.
Published: February 2003

The nature and magnitude of the intermolecular vibrational coupling between nitrosyl groups in Sr[Fe(CN)5NO]⋅4H2O and Ba[Fe(CN)5NO]⋅2H2O have been studied quantitatively by IR spectroscopy. The isotopic dilution technique has been used by incorporating in the first compound up to 40% of 18O as N18O ligand in the crystal. The results can be interpreted by postulating a dynamic electric dipole–dipole interaction which operates between the polar NO stretching vibrations. The main features of the fine structure displayed by the NO stretching band in the infrared spectra of polycrystalline Sr[Fe(CN)5N(16O/18O)]⋅4H2O, and in single crystals of Ba[Fe(CN)5NO]⋅2H2O can be explained in terms of a linear-chain model of nitrosyls vibrationally coupled through transition dipole–dipole forces, with interactions between nearest neighbors. The value ‖(∂m/∂S)0‖=13 D/Å calculated on the basis of this model for the NO dipole-moment derivative agrees satisfactorily with the value reported from integrated IR absorption intensity measurements. The coupling force constants between neighboring nitrosyls f1=+0.172 mdyn/Å obtained for barium nitroprusside, and the average f̄1=+0.189 mdyn/Å value obtained for the strontium salt, show that these interactions are the strongest thus far reported for ionic solids.
Fluorescence polarization as a probe of the rotational dynamics of isolated highly excited molecules
Authors: Nathanson Gilbert M. , McClelland Gary M.
Published: February 2003

There is now much evidence that the vibrational modes of isolated highly excited molecules are strongly mixed, but little is known about the rotational motion of such molecules. The basic question is whether rotation is approximately separable from vibration or is strongly coupled with vibration by Coriolis and centrifugal forces. In an effort to develop experimental methods to answer this question, we present computations of the polarization of fluorescence from electronically excited symmetric top molecules undergoing two extreme types of rotational motion: Regular rotation, for which the rotational motion is that of a rigid body, and statistical motion, in which the nuclear energy is partitioned statistically between rotational and vibrational motion within the constraints of total angular momentum and energy conservation. This statistical assumption yields a particular distribution for the angle between the angular momentum and molecular symmetry axis, which in turn determines the fluorescence polarization. Both quantum and classical theories for the fluorescence polarization are given, and we present extensive numerical calculations of the polarization in the classical limit. In these calculations, the fluorescence wavelength is not resolved but we allow for several degrees of resolution of the excitation spectrum. We find that the fluorescence polarization is in general much different in the regular and statistical limits, and that the statistical polarization is generally much less than the regular polarization. For statistical rotors, fluorescence polarization decreases as the rovibrational energy of the excited molecule is increased, while the polarization from regular rotors is unchanged. We find that polarization experiments can be interpreted even if no rotational structure is resolved in absorption. This technique thus does not require nozzle cooling to simplify spectra, and it can be applied to rotationally hot molecules. It is shown that electronic radiationless transitions can act either to enhance or reduce fluorescence polarization. We conclude that polarization experiments are excellently suited to probing the rotational motion of highly excited molecules.
Vibronic structure of nonadiabatic and fluxional states: Two-photon absorption spectroscopy of jet isolated 3s 1E′ s
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m-triazine
Authors: Whetten Robert L. , Grant Edward R.
Published: February 2003

The nature of nonadiabatic effects in the Rydberg and ground ionic states of jet-cooled sym-triazine is investigated by means of ultraviolet two-photon absorption spectroscopy. A highly resolved band system is observed in the region from 55 000 to 60 000 cm−1, which can be associated with excitation from an e′ lone pair to the 3s Rydberg orbital. A complete assignment of the low-energy bands of this system, as determined by isotope effects and comparison to model calculations, reveals a clear example of the dynamical Jahn–Teller effect for the case where only one mode, the ν6 ring distortion, is significantly active. On the basis of simple limiting models we derive approximate vibronic coupling parameters and present a quantitative description of vibronic motion in terms of adiabatic molecular coordinates. We conclude that sym-triazine, in its ground state one of the most rigid of medium-sized polyatomics, becomes strikingly fluxional in its Rydberg and ground ionic states, as the dynamic Jahn–Teller effect introduces a ring-distortion vibronic pseudorotation which can be characterized adiabatically in terms of a rotor frequency no greater than 80 cm−1. This is a limit, however, which the data shows is valid only well below a vibronic energy of 1100 cm−1, at which point the conical intersection is reached and all levels become intrinsically nonadiabatic.
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o assignment of the UV spectra of the ethyl, isopropyl, and t-butyl radicals
Authors: Lengsfield Byron H. , Siegbahn P. E. M. , Liu B.
Published: February 2003

A b initio SCF and CI calculations are used to assign the UV spectra of a series of alkyl radicals. Contrary to earlier speculation, the low-lying states are shown to be predominantly Rydberg in character. The spectra of the methyl, ethyl, isopropyl, and t-butyl radicals are found to be successively red shifted in accordance with the trend observed in their ionization potentials. The nature of the first valence transition in this series of radicals is also investigated and its transition energy placed relative to the Rydberg transitions.
Stabilization of unstable states and oscillatory phenomena in an illuminated thermochemical system: Theory and experiment
Authors: Zimmermann E. C. , Schell Mark , Ross John
Published: February 2003

A simple external feedback mechanism is employed to investigate theoretically and experimentally the unstable states, and transitions between stable states, of a bistable illuminated thermochemical system. Bifurcations leading to oscillatory phenomena are predicted and experimentally verified Predictions of chaos are also made. Experimental evidence is reported for the occurrence of tristability and metastability, in the absence of external feedback. OFF
Quenching of I*(5 2P1/2) by DF
Authors: Pritt A. T. , Patel D.
Published: February 2003

The method of time-resolved, infrared fluorescence was used to determine the total rate constant for deactivating I* by DF, HF, and HCl. The rate constants are <1.05×10−4, 8.9×10−13, and 3.2×10−15 in units of cm3 molecule−1 s−1, respectively. Moreover, no evidence was found for collisionally enhancing the radiative rate of I* with partial pressures of up to 50 Torr of DF or 100 Torr of HCl.
Angular correlation of the N+ ions produced in the dissociative double ionization of nitrogen
Authors: Ezell R. L. , Edwards A. K. , Wood R. M.
Published: February 2003

The double ionization of N2 by He+ projectiles was studied by measuring the angular correlation between the two N+ ions emitted in the dissociation of N2+2 molecular ions. If there were no recoil velocity imposed on the N2+2 ion in the initial ionizing collision, and thermal effects were neglected, the pair of N+ ions would have equal and opposite velocity vectors in the laboratory frame of reference. Measuring the coincidence yield of pairs of N+ ions as a function of the angle between their velocity vectors permits an estimate to be made of the component of momentum transferred to the N2+2 parent ions in the beam direction. The results presented in this report show the recoil velocity to be considerably less than the mean thermal velocity of N2 molecules at room temperature. We also report mesurements of the relative cross section for N+ production from N2+2 as a function of the orientation of the N2 target molecules relative to the projectile beam axis.
HF multiquantum V→R relaxation rates with N2 and CO
Authors: Yang Xue-Feng , Pimentel George C.
Published: February 2003

Through photolysis of NF3/HI/M=1.5/0.5/38 Torr mixtures (M=N2,CO,Ar) in a grating cavity, HF rotational laser emission is produced for (v=0, J=21→20), (1,23→22), (2,24→23), and (3,18→17). Known attenuators are placed in the laser cavity and used to estimate the absolute population inversions responsible for the laser gains. The results confirm the role of selective V→R resonant pumping of these high J rotational lasers. Further, they provide rough estimates for the Δv=2, Δ J=10, and Δv=3, Δ J=20 near-resonant V→R rate constants for HF relaxation by N2 from v=4, J=14, and v=5, J=3, 4, and 5 to v=2, J=24. These estimates fall in the range 10−13 to 10−14 cm3/molecule s, indicating that for N2 collision partners, HF multiquantum processes with Δv=3 may contribute a few percent of the total v=5 vibrational relaxation rate. They also suggest that, for both N2 and CO, V→R near-resonant channels (including Δv=1) provide the dominant relaxation process.
Resonances by the complex coordinate method with Hermitean Hamiltonian. III. Autoionization
Authors: Froelich P. , Moiseyev N.
Published: February 2003

By using the Hermitean representation of the complex coordinate method that recently has been developed, the 1S autoionization resonance of helium and its corresponding error estimates are calculated. The results obtained by the Hermitian representation of the complex coordinate method are in good agreement with previous results which were obtained solving the complex eigenvalue problem obtained within the conventional framework of the complex coordinate method.
Diatomics in molecules: A simplified approach
Authors: Olson J. A. , Garrison B. J.
Published: February 2003

A simplified procedure for obtaining matrix elements in the diatomics in molecules scheme is presented. An essential advantage in this approach is that it avoids partitioning the antisymmetrizer. This allows the components of the valence bond wave functions, i.e., determinants, to be used as fundamental quantities and leads to a transparent method of obtaining matrix elements of the atomic and diatomic Hamiltonians. Two examples, one with five electrons and five nuclei and one with six electrons and five nuclei, are formally treated.
Rotational potential surface for alkanes: Basis set and electron correlation effects on the conformations of n-butane
Authors: Raghavachari Krishnan
Published: February 2003

The rotational potential surface of n-butane is explored by means of accurate ab initio molecular orbital methods. Complete geometry optimizations are performed to determine the structures of the two minima (trans and gauche) and the two transition states (C2 and C2v) at several theoretical levels. Basis set effects are investigated in a systematic manner by performing single point computations with several progressively larger basis sets (up to triple zeta plus polarization functions on all atoms). Electron correlation effects are included by means of Mo/ller–Plesset perturbation calculations to third order. Vibrational frequencies and zero point energies are evaluated for all the stationary points. The best computed 0 K relative energies for the two minima and the two transition states are 0.0, 0.7, 3.3, and 6.0 kcal/mol, respectively. The results suggest that some of the previously accepted values need some revision.
Cubic ice, snowflakes, and rare-gas solids: Surface energy, entropy, and the stability of small crystals
Authors: Kiefte H. , Clouter M. J. , Whalley E.
Published: February 2003

Recent studies of the relation between the cubic and hexagonal forms of ice and of crystals of the rare gases show a common feature, in that, surface energy and entropy are important in determining the relative stability of crystal nuclei and of macroscopic crystals.
May low-density expansions for imperfect gases contain information about condensed phases?
Authors: Aguilera-Navarro V. C. , Fortes M. , de Llano M. , Rojo O.
Published: February 2003

The question posed is examined within the context of two simple models, the Ford problem and the van der Waals equation, both of which possess two phases: (i) a low-density phase for which all the virial coefficients in the density power-series expansion are known and (ii) a high-density ‘‘condensed’’ phase. Padé extrapolation techniques are applied to the virial series in an attempt to predict the condensed phase branch.
Revisiting the GR method for scattering from a corrugated hard wall: Improved stability, convergence, and new results
Authors: Stoll E. , Baumberger M. , García N.
Published: February 2003

The conditions for choosing the R and G vectors to obtain regular stable matrices for the GR method are reformulated. It is shown that the practical range of validity of the method can be extended to corrugation amplitudes beyond the Rayleigh limit and that it converges faster than was discussed in earlier papers. As an illustration of the capability of the method, we analyzed He diffraction from the large Ni(110)+H(2×6) unit cell.
Vibrational excitation and deexcitation rates of molecules adsorbed on metal surfaces
Authors: Asscher Micha , Somorjai G. A. , Zeiri Yehuda
Published: February 2003

The rates for vibrational excitation and deexcitation of adsorbed molecules on metal surfaces are reported. The desorption of the adsorbed molecules is shown to compete with the excitation process, resulting in ‘‘vibrationally cold’’ desorbing molecules. Simple rate equations, together with the available experimental data, are used to estimate the rates for vibrational excitation and deexcitation of NO molecules on Pt(111) surfaces as a function of the crystal temperature. The application of this approach to other systems is discussed.
Spectroscopy of 4d and 5d transition metal ions in alkali halide crystals
Authors: Powell Richard C. , Schweitzer Robert H. , Martin Joel J. , Venikouas George E. , Hunt Charles A.
Published: February 2003

The fluorescence spectra and lifetimes of divalent Rh, Ru, Pt, and Ir ions in alkali halide crystals are measured using pulsed nitrogen laser excitation. Emission from both the charge transfer states and the excited d levels is observed. Changes in the relative intensities and lifetimes are monitored as a function of temperature, annealing, and radiation treatment. The results are interpreted in terms of the possible application of these materials for tunable solid state lasers.
Indole: A model system for one-photon threshold photoionization in polar media
Authors: Lee Jamine , Robinson G. Wilse
Published: February 2003

Indole in a sufficiently polar medium, such as water, presents a relatively simple example of the one-photon near-threshold photoionization process in the condensed phase. This photoejection mechanism is the main object of study in this paper. To carry out this study, dynamic processes of indole in its first excited singlet state S1 are explored as a function of temperature in mixed water/methanol solvents. In mixed solvents having high water content, the dominant radiationless transition in the higher temperature regimes is a localized photoejection process. In pure methanol, electron photoejection is unimportant, indicating a distinction between aqueous solvents and other hydrophilic solvents that goes far beyond dielectric constant considerations. A water ‘‘cage,’’ composed of 4±1 water molecules, and probably quite close to the parent cation, is the electron acceptor. The apparently unique structure of this water cluster, different from that in normal water, is thought to be responsible for the large activation energy (43.4 kJ/mol−1) for this photoejection mechanism. A Markov random walk matrix method is adopted in order to analyze the photoelectron process in different mixture configurations. Good agreement between observed and theoretical results over the entire solvent concentration range for both the fluorescence decay rates and the fluorescence quantum yields at various temperatures between −15 and +75 °C supports the assumed model. It is interesting to speculate about possible roles of threshold electrons in chemical and biological long range electron transfer reactions.
Intramolecular vibrational relaxation and spectra of CH and CD overtones in benzene and perdeuterobenzene
Authors: Sibert Edwin L. , Reinhardt William P. , Hynes James T.
Published: February 2003

A theoretical model is presented for the vibrational dynamics of highly excited CH and CD overtones in benzene and perdeuterobenzene. The origin, path, and time scale for the overtone relaxation are described. The critical near resonant interaction responsible for the energy flow from an excited CH(D) oscillator to the ring is a Fermi resonance coupling, identified by Sibert, Reinhardt, and Hynes [Chem. Phys. Lett. 92, 455 (1982)]. Quantum overtone spectra are calculated both from time independent and time dependent perspectives and good qualitative agreement is found with the experimental overtone spectra of Reddy, Heller, and Berry [J. Chem. Phys. 76, 2814 (1982)]. Some expected consequences for future experiments on benzene and related systems are indicated.
Classical dynamics of highly excited CH and CD overtones in benzene and perdeuterobenzene
Authors: Sibert Edwin L. , Hynes James T. , Reinhardt William P.
Published: February 2003

The classical decay of highly excited CH and CD overtones of benzene and perdeuterobenzene is examined by trajectory techniques. The character, mechanism, and time scale of these decays are in striking qualitative agreement with those found in the preceding quantum study by Sibert, Reinhardt, and Hynes. The classical coupling mechanism for the decay is shown to be nonlinear Fermi resonance involving the CH(D) stretch and the in-plane CCH(D) wag. The decay path is described in terms of overlapping nonlinear resonance interactions.
Temperature dependence of thermal positive ion production from sodium bromide molecules incident upon a glowing rhenium surface
Authors: Kawano Hiroyuki , Kenpō Tsutomu
Published: February 2003

Emission current of Na+ ions produced from NaBr molecules impinging with a flux of 6.55×1013 molecules cm−2 s−1 upon a heated polycrystalline rhenium surface in the residual gas pressure of about 2×10−7 Torr was measured as a function of surface temperature (T) in the range of about 900–2100 K, thereby indicating that the sticking probability of NaBr to the surface is virtually unity and that the ionization efficiency ( β+) is ∼1 and 0.23 for T≂1100–1500 and 1800–2100 K, respectively. These results are explained reasonably by our previous theory that the degree (γ) of dissociation of incident molecules generally depends upon the ionization coefficient (α+) and hence upon the effective work function (φ+) for the positive ionization unless α+ is much smaller than unity. The value of φ+ was 5.45–5.51 eV for T≳1800 K, below which φ+ gradually increased up to ∼6.8 eV according to a decrease in T. By this increase both β+ and γ were kept at virtually unity even at a low temperature down to about 1100 K, at which they began to decrease sharply with a reduction in T because φ+ had already turned to a decrease at about 1200 K.
Solution of Kramers–Moyal equations for problems in chemical physics
Authors: Knessl C. , Mangel M. , Matkowsky B. J. , Schuss Z. , Tier C.
Published: February 2003

We derive asymptotic solutions of Kramers–Moyal equations (KMEs) that arise from master equations (MEs) for stochastic processes. We consider both one step processes, in which the system jumps from x to x+ε or x−ε with given probabilities, and general transitions, in which the system moves from x to x+εξ, where ξ is a random variable with a given probability distribution. Our method exploits the smallness of a parameter ε, typically the ratio of the jump size to the system size. We employ the full KME to derive asymptotic expansions for the stationary density of fluctuations, as well as for the mean lifetime of stable equilibria. Thus we treat fluctuations of arbitrary size, including large fluctuations. In addition we present a criterion for the validity of diffusion approximations to master equations. We show that diffusion theory can not always be used to study large deviations. When diffusion theory is valid our results reduce to those of diffusion theory. Examples from macroscopic chemical kinetics and the calculation of chemical reaction rates (‘‘Kramers’’ models) are discussed.
Photoabsorption and photodissociation of HONO2 in the 105–220 nm region
Authors: Suto Masako , Lee L. C.
Published: February 2003

Photoabsorption and fluorescence cross sections were measured in the 105–220 nm region. The OH (A→X) emission was observed at wavelengths shorter than 153 nm with a maximum quantum yield of 1.6% at 122 nm. Emissions from excited NO2 were observed in the 105–220 nm region with an apparent quantum yield of 0.3% at 200 nm. The fluorescence excitation spectra reveal that the absorption continua consist of several repulsive states.
Reaction dynamics for O(3P)+H2 and D2. IV. Reduced dimensionality quantum and quasiclassical rate constants with an adiabatic incorporation of the bending motion
Authors: Bowman Joel M. , Wagner Albert F. , Walch Stephen P. , Dunning Thom. H.
Published: February 2003

Reduced dimensionality exact quantum and quasiclassical reaction probabilities, transmission coefficients, and rate constants are presented for the O(3P)+H2(ν=0,1) and O(3P)+D2(ν=0,1) reactions on an effective potential surface given by the ab initio MOD POLCI potential energy surface reported previously plus the adiabatic ground state bending energy eigenvalue obtained from the two (nondegenerate) nonlinear potential energy surfaces reported here. The new rate constants are compared to experiment. Good agreement is found for thermal rate constants and isotope effects and for vibrationally excited rate constants.
Electron density studies of porphyrins and phthalocyanines. III. The electronic ground state of iron(II) phthalocyanine
Authors: Coppens Philip , Li Liang
Published: February 2003

The electron density distribution in Fe–phthalocyanine is mapped using a set of accurate diffraction data collected at 110 K. Structural results are in agreement with an earlier room-temperature determination. Deformation electron density maps show a deficiency of the Fe electron density along the axis perpendicular to the molecular plane and excess along the diagonals of the Fe–N(pyrrole) directions. Other deformation density features are observed in the bonds of the macrocyclic ligand and in the nitrogen lone-pair regions. The distribution around the Fe atom indicates the 3EgA state to be the leading contributor to the ground state of FePc. This conclusion is further supported by the results of a multipole refinement in which spherical-harmonic electron density deformation functions are fitted to the distribution around each of the atoms. The calculated Mössbauer shift is in agreement with the spectroscopically observed value.
Internal density functional theory of molecular systems
Authors: Nalewajski Roman F.
Published: February 2003

A thermodynamiclike theory of internal equilibrium and constrained equilibrium states of individual molecular systems is formulated, based on the Legendre transformed density functional theory (LT DFT). The molecular system (nonrelativistic, field free, Born–Oppenheimer or non-Born–Oppenheimer) is treated as the closed composite thermodynamic system, consisting of very small, rigid (open) subsystems (simple systems) containing a multi-(m)-component charged fluid in the presence of an external field. The generalized Levy constrained search construction of various ‘‘thermodynamic’’ potentials of LT DFT is given and the local Maxwell relations are derived. The reduction of various second-order partial functional derivatives (system sensitivities) in terms of few independent, basic kernels is described, using the Jacobian determinants technique. The qualitative implications for the basic kernels of the theory, from the Maxwell relations and stability criteria (generalized Le Châtelier and Le Châtelier–Braun principles) are systematically examined. Finally, possible applications of the general formalism in the thermodynamic analysis of the chemical bond, molecular stability, and chemical reactivity are identified.