bluuebunny/crossref_metadata_2025 huggingface dataset:
On the theory of rate processes: The role of coherent mechanical vibrations
Authors: Fain V. M.
Published: February 2003

The influence of the nonequilibrium degrees of freedom of the electron–nuclear system on the rate of relaxation processes is investigated. The electron–nuclear system is modeled by two potential energy hypersurfaces—electronic terms, plus the perturbation causing transitions between these terms. A harmonic approximation with displaced equilibrium positions and with identical frequencies is assumed for a description of the terms. The equations describing rate processes in such a system are derived. These equations determine the change in time not only of mean occupation numbers of electronic levels but of the amplitudes of nuclear vibrations as well. The dependence of the rate constants on the amplitudes of nonequilibrium excited vibrations is investigated under various assumptions about nonequilibrium of the system. The possibility of excitation of coherent nuclear vibrations in the course of nonradiative transition and in the presence of a constant input on the upper electronic level is investigated as well. The conditions under which such coherent vibrations may arise are obtained, and the stationary amplitude of these vibrations is derived. Attempts are made to apply the theory for a description of enzyme catalysis.
Self-avoiding walks interacting with an interface
Authors: Torrie Glenn M. , Middlemiss Keith M. , Bly Stephen H. P. , Whittington Stuart G.
Published: February 2003

This paper is concerned with self-avoiding walks with at least one vertex in an interfacial plane and interacting with the interface via a short range potential. We present some rigorous results on the form of the partition function and examine exact enumeration data for several lattices in an attempt to locate the point at which adsorption sets in.
Diffusion in protein solutions at high concentrations: A study by quasielastic light scattering spectroscopy
Authors: Phillies George D. J. , Benedek George B. , Mazer Norman A.
Published: February 2003

Quasielastic light scattering spectroscopy was used to study the mutual diffusion coefficient Dm of bovine serum albumin over the pH range 4.3–7.6 for concentrations ranging as high as 334 g/l. Using literature measurements of the tracer diffusion coefficient DT and the osmotic compressibility (∂π/∂c)P,T, we find that Dm depends on concentration and pH as predicted by the generalized Stokes–Einstein relation Dm=DT(∂π/∂c)P,T (1−φ)/KBT, φ being the volume fraction of solute and KB being Boltzmann’s constant. Our data show that Dm and DT depend quite differently on solute concentration; DT can be a factor of 3 smaller than Dm at protein concentrations exceeding 200 g/l.
Wideline NMR studies of ammonia intercalation complexes. II. TaS2(ND3) and TiS2 (ND3)
Authors: Silbernagel B. G. , Gamble F. R.
Published: February 2003

The 2D NMR of ND3 molecules in the intercalation complexes TaS2(ND3) and TiS2(ND3) has been studied at temperatures from 5–300 K by wideline techniques. The magnitude and temperature dependence of the observed electric quadrupolar coupling constant (e2qQ/h)eff provides information about the configuration of the intercalated ND3 molecules and their dynamics. The similarity of (e2qQ/h)eff to previously reported values for solid ND3 suggests that intercalation entails only small changes in molecular geometry or electronic configuration. The molecules exhibit several types of motion, including spinning about their threefold (C3) symmetry axis and a complex reorientation of that axis. The rates of these motions are reduced as the temperature is lowered, with reorientation falling to ∼106 s−1 near 140 K. C3 spinning persists at rates greater than 106 s−1 even at temperatures of 5 K. The quadrupole interaction does not vanish at high temperatures, ∼300 K, showing that reorientation of the C3 axis is not isotropic and the form of motion is deduced from the high temperature value of the coupling constant. A central component of the spectrum is observed at low temperatures and attributed to ND3 molecules residing near molecular vacancies in the layers. The present results are compared in detail with 1H observations in the ND3 analogues.
The affinity in local chemical equilibrium
Authors: Berrondo M. , García-Colín L. S. , Robles-Domínguez J.
Published: February 2003

We prove a generalized version of the H theorem for a Boltzmann-like equation which includes a reactive term for a bimolecular reaction. We find the hydrodynamic equations for the reactive system as well as the distribution function assuming local chemical equilibrium. The resulting chemical affinity vanishes identically as a consequence of the constraints imposed by the reaction. Finally we derive Gibbs relation for the local entropy production in terms of the chemical components, with the result that there appears no term in the affinity.
Molecular dynamics studies of Lennard-Jones liquid mixtures. I. Role of attractive term in the behavior of one different particle
Authors: Toukubo Kazuo , Nakanishi Koichiro
Published: February 2003

Molecular dynamics calculations have been made for three kinds of fluid systems each containing 108 Lennard-Jones particles. In the first reference system, the values of the potential parameters for all the particles were assumed to be those of argon. In order to examine the effect of the attractive term, each particle (referred to as the solute) in the other two systems was assumed to have different values for the energy parameter of the potential, ε12. The ε12 value assigned was either twice or half that of the ε11 value for all the other particles (referred to as the solvent), while the σ value was common to all the particles. Both equilibrium and transport properties, as well as cooperative and relaxation phenomena, were calculated and discussed. Among others, the following important results were obtained. The position and height of the first peak of the solute–solvent radial distribution function change appreciably and systematically with ε12. The self-diffusion coefficient of the solute becomes larger as ε12 decreases, and in contrast to the somewhat gaslike motion of the 0.5ε solute, the oscillatory nature of the motion of the 2.0ε solute was clearly seen from the velocity autocorrelation function for each solute. The time dependence of the pair correlation functions and the correlation between solute and neighboring solvents were also investigated. The correlations of the motion of neighboring solvents with that of the central solute, especially of 2.0ε solute, were found to be unexpectedly large for the first neighbors as compared with those for the particles outside them.
Photoionization with molecular beams. I. Autoionization structure of nitric oxide near the threshold
Authors: Ng C. Y. , Mahan B. H. , Lee Y. T.
Published: February 2003

The photoionization efficiency curve of NO+  (X1Σ+) was obtained using a molecular beam technique in the wavelength range from 1350 to 1185 Å (9.18–10.46 eV). Due to the lowering of the rotational temperature by supersonic expansion, the autoionization structure superimposed on the vibrational steps is well resolved. This structure is attributed to vibrational autoionization. The splitting of the autoionization peaks due to the spin–orbit ground state (2Π1/2, 2Π3/2) of nitric oxide is observed. The strong intensity peaks of the vibrational autoionization are found to correspond to a vibrational quantum number change of Δv=−1. The autoionization structure observed on the first four vibrational steps obeys the propensity rule proposed by Berry.
Second order hyperpolarizability of a polypeptide α-helix: Poly-γ-benzyl-L glutamate
Authors: Levine B. F. , Bethea C. G.
Published: February 2003

We have measured the second order β and third order γ hyperpolarizabilities of the PBLG polypeptide α-helix and find it to have an extremely large nonlinearity. The decay time of the generated harmonic has been directly determined and found to be τ=5.0 msec which is consistent with Kerr and dielectric experiments. The nonlinear saturation at large static fields has been observed and is in quantitative agreement with theory.
Langevin forces in chemically reacting multicomponent fluids
Authors: Grossmann S.
Published: February 2003

The correlation properties of noise in reacting multicomponent fluids are given. Particular emphasis is put on the combined effects of reaction, heat flow, and diffusion in a two-component single-reaction system and in a multispecies fluid at rest. The applicability of the method to steady states far from equilibrium is discussed.
Structure and dynamics of methylene iodide in solution from nuclear spin–lattice relaxation studies
Authors: Mayne Charles L. , Grant David M. , Alderman Donald W.
Published: February 2003

The nuclear spin–lattice relaxation of carbon-13 enriched methylene iodide (diiodomethane) dissolved in benzene-d6 has been studied both with and without proton decoupling and using various pulse techniques to perturb the AX2(13CH2) spin system from thermal equilibrium. The return of the spin system to steady state was monitored using carbon-13 Fourier transform nuclear magnetic resonance techniques. It is shown that the equation of motion of the spin density matrix reduces in general to the master equation for populations in terms of interlevel transition rates, and that a linear transformation based on the complete set of irreducible spherical tensor operators which span the spin space further simplifies the equation of motion. The relaxation was modeled as intramolecular dipole–dipole interactions modulated by rotational reorientation of the molecule plus other mechanisms which can be treated collectively as external random magnetic fields interacting with the nuclear spins of interest. Extreme narrowing is assumed. The model was parameterized in terms of the spectral densities of fluctuations in the molecular rotational reorientation and in the external random field along with other necessary parameters characterizing the operation of the NMR spectrometer. A best least squares fit of this model to all of the relaxation data was calculated. From the values of the spectral densities thus obtained all of the interlevel transition rates were calculated. Assuming a rotational diffusion model for the molecular rotational reorientation, the following structural and dynamical parameters were calculated from the dipole–dipole spectral densities: &HCH=104°±2°, Dxx/Dzz =0.205±0.0033, and Dyy/Dzz=0.079±0.057, where Dαα, α=x,y,z, are the elements of the diagonalized rotational diffusion tensor.
Rapid intramolecular V–V energy transfer in SF6
Authors: Frankel Donald S.
Published: February 2003

The infrared double resonance technique has been used to observe V–V energy transfer from the ν3 mode of SF6 to other vibration–rotation levels lying 1000 cm−1 or more above the ground state. Results obtained with nanosecond time resolution indicate that the major portion of the transfer occurs in the absence of collisions. Relevance to laser induced, isotope selective, collisionless dissociation is discussed.
Measurement of the rate coefficients for the bimolecular and termolecular charge transfer reactions of He2+ with Ne, Ar, N2, CO, CO2, and CH4
Authors: Lee F. W. , Collins C. B. , Waller R. A.
Published: February 2003

This work reports the measurement of termolecular charge transfer reactions of He2+ into nonassociative product channels. In this study ion destruction frequencies have been experimentally determined from the selectively excited fluorescence of N2+ in high pressure afterglows of mixed gases excited by an intense electron beam discharge. Data have been obtained as functions of helium pressure over the range from 300 to 1500 torr and as functions of the partial pressure of reactant from 25 to 400 μ Hg. From these data, pressure-dependent rate coefficients have been extracted and subsequently resolved into contributions from bimolecular and termolecular components for reactions of He2+ with Ne, Ar, N2, CO, CO2, and CH4. The bimolecular components have been found to agree with the ESSA flowing afterglow results to within experimental error, and in some cases the values reported here represent an improvement in precision. The sensitivity of the method has been sufficient to detect termolecular components as small as 2×10−30 cm6/sec and values were found to range widely from 2×10−30 for Ne to 67×10−30 cm6/sec for CO2. The size of these termolecular rates suggests the general importance of three-body ion–molecule reactions in higher pressure plasmas such as those found in e-beam lasers.
Power broadening of vibrational–rotational transitions of CO2
Authors: Burak Itamar
Published: February 2003

Power broadening of the P (18) (100) – (001) vibrational–rotational transition was studied by irradiating low pressure CO2 gas by CO2 laser pulses. The number of molecules excited to the upper level was related to the intensity of the 4.3 μ laser-induced fluorescence. Under conditions where collision effects may be neglected, the number density of the excited molecules is determined by the intensity of the laser field. The CO2 sample was placed both inside and outside the laser cavity. The excitation to the (001) level was studied by measuring the fluorescence intensity as a function of the laser intensity. The interactions of a molecule with running and standing wave fields are considered theoretically. In the running wave case the Rabi strong-field solution is integrated over the molecular velocities and orientations with respect to the field polarization. An exact solution for the density matrix in the standing wave case is derived. The total number of molecules excited to the upper level is calculated for both running and standing wave fields and compared to the experimental data. Specific interesting features of the standing wave solution are discussed.
Threshold photoelectron spectra of HF, DF, and F2
Authors: Guyon Paul-Marie , Spohr Reimar , Chupka William A. , Berkowitz Joseph
Published: February 2003

A simple threshold photoelectron energy selector was installed in the source of a conventional photoionization mass spectrometer to measure photoionization thresholds. In this work, HF, DF, and F2 were investigated. A number of vibronic levels of the HF+ and DF+ ions, not observed by HeI photoelectron spectroscopy, have been detected, from which spectroscopic constants of the molecular ions could be inferred for comparison with recent emission spectroscopy data. The threshold photoelectron spectra show some complicated structure associated with resonant autoionization of molecular Rydberg states belonging to series converging to the various vibronic levels of the ions.
Rate coefficient for deactivation of O(21S) by H2O
Authors: Bingham Felton W. , Johnson A. Wayne , Rice James K.
Published: February 2003

We have measured the absolute rate coefficient for collisional deactivation of O(21S) by H2O. The experimental method involved electron-beam irradiation of mixtures of H2O and O2 in high-pressure Ar; the irradiation energy initially deposited in the argon transferred to O2 to produce O(21S) atoms. Observation of the O(21S) lifetime as a function of H2O pressure permitted deduction of a quenching coefficient of 1.27±0.15×10−10 cm3 s−1.
Moments and distribution function of polymer chains
Authors: Fixman Marshall , Skolnick Jeffrey
Published: February 2003

Methods are presented for facilitating the calculation of even moments 〈R2p〉 of the end to end vector R and for the inference of the probability distribution in the absence of excluded volume effects. The gain in efficiency of moment calculations is several hundredfold for the first 10 moments, 0⩽p⩽10. The proposed inference scheme is similar to the Hermite expansion from a least square standpoint but differs in choice of weight function. Tests on freely rotating chains exhibit quantitatively useful convergence for all R, including chains with too few bonds to permit ring closure.
Vib–rotational energy distributions and relaxation processes in pulsed HF chemical lasers
Authors: Ben-Shaul A. , Kompa K. L. , Schmailzl U.
Published: February 2003

The rate equations governing the temporal evolution of photon densities and level populations in pulsed F+H2→HF+H chemical lasers are solved for different initial conditions. The rate equations are solved simultaneously for all relevant vibrational–rotational levels and vibrational–rotational P-branch transitions. Rotational equilibrium is not assumed. Approximate expressions for the detailed state-to-state rate constants corresponding to the various energy transfer processes (V–V, V–R,T, R–R,T) coupling the vib–rotational levels are formulated on the basis of experimental data, approximate theories, and qualitative considerations. The main findings are as follows: At low pressures, R–T transfer cannot compete with the stimulated emission, and the laser output largely reflects the nonequilibrium energy distribution in the pumping reaction. The various transitions reach threshold and decay almost independently and simultaneous lasing on several lines takes place. When a buffer gas is added in excess to the reacting mixture, the enhanced rotational relaxation leads to nearly single-line operation and to the J shift in lasing. Laser efficiency is higher at high inert gas pressures owing to a better extraction of the internal energy from partially inverted populations. V–V exchange enhances lasing from upper vibrational levels but reduces the total pulse intensity. V–R,T processes reduce the efficiency but do not substantially modify the spectral output distribution. The photon yield ranges between 0.4 and 1.4 photons/HF molecule depending on the initial conditions. Comparison with experimental data, when available, is fair.
The spectrum and ground state potential curve of Ar2
Authors: Colbourn E. A. , Douglas A. E.
Published: February 2003

The absorption spectrum of Ar2 has been photographed at a dispersion sufficiently high to allow a rotational analysis of one band system. From the analysis, the vibrational and rotational constants of the five lowest vibrational levels of the ground state have been determined. These spectroscopic data, together with information from long-range forces and from the second virial coefficient, have been used to calculate the potential curve of the ground state. This calculated curve is compared with other curves which have been proposed.
Transient emitting species in phosphorus chemiluminescence
Authors: VanZee Richard J. , Khan Ahsan U.
Published: February 2003

The phosphorescence of phosphorus oxidation is the oldest and the best known chemiluminescing reaction, but a definitive spectroscopic study of this classic system has been lacking. In this paper we report the results of an investigation of the oxidation of P4 vapor under atmospheric conditions, with added H2O or D2O vapor. In spectral studies of small horizontal rectangular cross sections of the phosphorus flame, the relative ratios of emission from the transient species remained the same regardless of the cross section chosen, which permits accurate spectral analysis of the chemiluminescence. We have analyzed the visible and ultraviolet spectrum of the reaction, consisting of discrete band structure in the 228.8–272.1 nm region and a broad continuum onsetting at 335 nm and extending to 800 nm and longer, upon which are superimposed a number of weak bands from 450–650 nm. Discrete band emissions enabled the identification of the bands at 228.8–272.1 nm with the PO γ-system transitions, PO(? 2Σ+) →PO(X 2Π). Spectral changes arising from the substitution of D2O vapor for H2O vapor in the reaction led to the assignment of the weak bands at 450–650 nm to HPO (or DPO), ? (1A″) →? (1A′). The main band emission in the visible region, the broad continuum which cannot be identified with any simple electronic transition, exhibits the kinetic and spectral characteristics of an excimer, an excited state dimer with only one member of the molecular pair being electronically excited. In the case of phosphorus chemiluminescence the dimer is (PO*⋅⋅⋅PO) and the equilibrium reaction for its formation is (PO*⋅⋅⋅PO) ?PO(4Π)+PO(X 2Π) ?PO(B 2Σ+)+PO(X 2Π) The existence of the (PO) *2 excimer has been confirmed by affecting the dynamics of this equilibrium through dilution with buffer gas and through thermal dissociation of the excimer, resulting in the appearance of the formerly quenched (0,0) transition of the PO β emission in the spectrum. Our investigation has eliminated the possibility that the visible continuum arises from either PO2 or HOPO. Enough information has been extracted from temperature dependent studies to construct the approximate shapes of the potential energy surfaces of the ground (Δ?g=35±200 cm−1) and the first excited state (Δ?ex=846±200 cm−1) of the (PO)2* excimer.
Self-diffusion in compressed liquid heavy water
Authors: Wilbur D. J. , DeFries T. , Jonas J.
Published: February 2003

The self-diffusion coefficients D in liquid heavy water have been measured by the NMR spin-echo method over the temperature range 10–200°C and the pressure range 1 bar to 9 kbar. A good agreement with tracer diffusion studies by Woolf performed over a more limited range of temperatures and pressures has been obtained. As in other studies, we find an anomalous initial increase in the diffusion coefficients with pressure at temperatures below approximately 30°C. At high temperatures compression leads to a monotonic decrease of the self-diffusion. The fact that the activation energies at constant density decrease both with increasing temperature and increasing density indicates that both high temperature and high compression significantly affect the hydrogen bond network and produce a more ’’normal’’ liquid. We find that the Stokes–Einstein equation is accurate to within 10% over the range of experimental conditions studied. Phenomenological interpretation of plots of D/T vs molar volume supports the view that at high compression and high temperature the dynamic behavior of heavy water resembles that of a hard sphere liquid. All experimental results are in agreement with our conclusions obtained in our earlier study of NMR relaxation in compressed liquid water and heavy water.
Proton spin relaxation of the alkoxybenzoic acid series liquid crystals in the solid phase
Authors: Thompson R. T. , Pintar M. M.
Published: February 2003

The molecular dynamics in the solid phases of some members of a liquid crystalline homologous series were analyzed using nuclear spin thermometry. At low temperatures (77–200 K the relaxation is dominated by the C3 reorientation of the CH3 groups. At high temperatures (≳300 K), self-diffusion is evident. Between 200 and 300 K various possibilities for the molecular motions are discussed. Some systematic trends in the homologous series are evident.
Approximations to the rate constant for dissociation of diatomic molecules
Authors: Snider Neil S.
Published: February 2003

A number of results which pertain to the rate of collisionally induced gas phase dissociation of diatomic molecules were derived. In the standard model for this process collisions bring about random transitions between internal states of the diatom and ultimately lead to dissociation. The rate constant kd for such a model is simply related to λ1, the smallest eigenvalue of a matrix κ which is expressed in terms of rate constants for detailed transitions between states of the diatom. A standard approximation for λ1 is the solution of the linearized characteristic equation for κ. It was shown that the kd so obtained is very nearly the same as the kd obtained from the steady state approximation. If the states of the diatom are assumed to form a continuum, then κ becomes the kernel of an integral operator. Relations between discrete and continuum models were established. It was shown that equivalent diffusion equations which have been derived for the continuum case have analogs in the discrete case. These discrete analogs pertain to stepladder models, models for which transitions are allowed only between adjacent states. It was further shown how discrete (quantum) models and continuum (classical) models are linked by quasicontinuum models which, while still discrete, are formulated in terms of elements of the continuum rather than in terms of individual quantum states. Some of the foregoing results were illustrated by a treatment of the separable exponential model. Parameters α and β for this model have a straightforward physical interpretation. The rate constant and the state distribution for the model are obtainable in closed form via the steady state approximation. Their dependence on α and β is readily interpretable physically. The state distribution for the model was shown to be potentially useful in variational calculations of kd for other models. In such calculations, α and β would be chosen so as to maximize the variational lower bound to λ1 for the model in question.
On the electron detachment of Cl− by argon
Authors: Champion R. L. , Doverspike L. D.
Published: February 2003

A discrepancy in the experimental results of Mandl [J. Chem. Phys. 64, 903 (1976)] concerning the collisional detachment of Cl− ions by argon is pointed out. AIP
Response: On Cl− detachment by Ar
Authors: Mandl A.
Published: February 2003

The experimental results of a previous measurement of collisional detachment of Cl− ions with argon are experimental. AIP
I* (5 2P1/2) quenching by hydrogen halides
Authors: Pritt A. T. , Coombe Robert D.
Published: February 2003

The total quenching rates of electronically excited iodine (I*,5 2P1/2) by HF, HCl, HBr, and HI have been measured. Two techniques were used to observe the temporal behavior of the I*: (1) absorption of the 206.2 nm resonance line and (2) I* emission at 1.315 μm. Close agreement between the two methods supports the modified form of the Beer–Lambert relation for data analysis in the absorption technique. Rate constants for quenching by HF, HCl, HBr, and HI are 3±1×10−12, 6.5±0.9×10−15, 1.58±0.07×10−13, and 5±1×10−14 cm3 molecule−1 sec−1, respectively. I* quenching by HF and HCl is shown to be consistent with the efficient electronic to vibrational (E–V) energy transfer mechanism known to exist for Br* (4 2P1/2) quenching by hydrogen halides. Quenching by HBr and HI is too rapid to be consistent with the E–V argument and is probably dominated by reactive channels.
Towards a molecular theory of freezing. II. Study of bifurcation as a function of density
Authors: Raveché Harold J. , Stuart Charles A.
Published: February 2003

We study, as a function of density, crystalline solutions for the single particle probability density which bifurcate from the fluid solution of a hard sphere system. The BBGKY equation is used to describe the fluid phase, with its closure taken from computer simulations. As the density increase from zero, crystalline solutions bifurcate from the fluid with a periodicity determined by the density. Bifurcation is found to be characteristic of metastable states, and in general it does not occur at the equilibrium coexistence of two phases. When compared to the known hard disk and hard sphere isotherms, the bifurcation points are seen to be remarkably consistent with the density at which the metastable extension of the crystalline branch meets the equilibrium fluid branch. Metastable crystalline states are also predicted in one dimension. The controversial Kirkwood instability criterion is interpreted in terms of our theory. We show that our analysis applies to a large class of potential energy functions. The possibility that the bifurcation near the jamming densities observed in the amorphous packing of incompressible spheres and disks is also discussed.
Atomic and molecular calculations with the model potential method. V
Authors: Bonifacic V. , Huzinaga S.
Published: February 2003

Model potentials to be used with double-zeta Slater-type basis sets are reported for Li through Ne, and also for Vanadium.
Solutions of the Yvon–Born–Green equation for the square-well fluid at very high densities
Authors: Co King U. , Kozak John J. , Luks Kraemer D.
Published: February 2003

The Yvon–Born–Green equation with superposition approximation is solved for g(2)(x) for a system of square-well molecules at very high densities for the three isotherms ϑ=0, 0.2, 0.45. The solutions were obtained for a range of 30 diameters with the customary boundary condition of g(2)(x) ≡1 for x≳30 and, in one instance, for a devised periodic boundary condition. At all isotherms studied, beyond a density denoting a lower bound on the limit of stability of the pure liquid or dense fluid regime, the g(2)(x) functions demonstrated infinite periodicity. This ’’periodic’’ region extended beyond the density of close packing without any indication of termination. Throughout this region, the isothermal compressibility function is singular. Properties of these g(2)(x) functions are also discussed.
Vibrational energy transfer in methane and methane–rare-gas mixtures
Authors: Hess Peter , Moore C. Bradley
Published: February 2003

The vibrational relaxation time for the asymmetric stretch ν3 of CH4 is 3.9±0.6 μsec torr in pure CH4 and 12, 13, 10, 5.9, 5.5, 5.0, 11, and 9.5 μsec torr (±20%) for CH4 dilute in 3He, 4He, Ne, Ar, Kr, Xe, H2, and HD, respectively, at 295°K. The ν3 fundamental was pumped by a tunable infrared laser. The rise time for bending fluorescence following ν3 excitation is 30%–50% shorter than the ν3 decay. In pure CH4, measurements of emission from 2ν4 and ν2+ν4 indicate that a large fraction of the vibrational energy is channeled through these two levels. The main relaxation path appears to be transfer from ν3 through 2ν4 to ν4, with the ν2 bending vibration being excited mostly in the later stages of V→V transfer.
Reaction of C+(4P) with molecular hydrogen
Authors: Jones C. A. , Wendell K. L. , Kaufman Joyce J. , Koski W. S.
Published: February 2003

The reaction of molecular hydrogen with C+ in its first excited state was studied over the energy range 0.5–9.0 eV in the laboratory system. The angular and energy distribution and the energy spectra indicated a direct mechanism for the reaction. The experimental measurements coupled with spin and symmetry considerations indicated that the CH+ product was in the a 3Π electronic state.
Two methods for calculating the effect of vacancy defects on the free energy of harmonic solids
Authors: Kincaid Robert L. , Huckaby Dale A.
Published: February 2003

Two methods are developed for calculating the change in vibrational free energy ΔF of a solid with first-neighbor interactions which results from the introduction of a vacancy defect. One method employs a summation of central moments calculated from closed walks. From this well-defined method a more intuitive cell cluster technique is derived. As an example, both methods are used to calculate ΔF for the triangular lattice. Each method yields the result ΔF/kT=−1.9.
High resolution two-photon spectroscopy in the 1B2u state of benzene
Authors: Lombardi J. R. , Wallenstein R. , Hänsch T. W. , Friedrich D. M.
Published: February 2003

Highly resolved rotational spectra of two-photon excited vibrational levels in the 1B2u state of C6H6 and C6D6 are recorded for linear and circular polarization of the exciting laser radiation, generated with a powerful narrow-band dye laser system. The fitted theoretical spectra are in excellent agreement with the experimental results and provide molecular parameters (rotational constants, Coriolis constants, and structural parameters) of considerably improved accuracy.
The high temperature vaporization of defect scandium monosulfide
Authors: Tuenge R. T. , Laabs F. , Franzen H. F.
Published: February 2003

A high temperature vaporization study of the scandium–sulfur system has shown that a defect scandium monosulfide phase with composition Sc/S=0.8065 vaporizes congruently over the temperature range 1875–2220 K. High temperature mass spectrometric results show that the congruently subliming phase has two principle vaporization reactions producing ScS(g)+S(g) and Sc(g)+S(g). Thermodynamic results for Sc0.8065S(s) were obtained from both mass spectrometric and target collection investigations. Based on Mann’s cross sections for atoms, additivity for molecules, and the assumption of no fragmentation, the third-law mass-spectrometric enthalpy of vaporization at 298 K of Sc0.8065S to form Sc(g)+S(g) is 223.4±4.5 kcal and to form ScS(g)+S(g) is 133.8±4.5 kcal; the dissociation energy D°298 obtained for ScS(g) is 111.1±4.5 kcal. The third-law mass-spectrometric results are in good agreement with the target collection results. Ion intensity data were collected for ScS+ as a function of time upon vaporization of stoichiometric ScS(s) at 2035 K. The third-law value obtained for the enthalpy of atomization at 298 K of Sc1.0S(s) is 240.2±5 kcal.
Kinetic energy release distributions for the dissociation of internal energy selected CH3I+ and CD3I+ ions
Authors: Mintz Donald M. , Baer Tomas
Published: February 2003

Product translational energy distributions have been measured from an analysis of symmetrically broadened time-of-flight peaks of CH3+ and CD3+ ions in photoion–photoelectron coincidence spectra of CH3I and CD3I, respectively. The reactions were investigated at various precursor ion internal energies up to 1.7 eV above the dissociation threshold. We found that the experimentally obtained kinetic energy release distributions can be fitted accurately by simple exponential functions over the energy range considered. The experimental results are compared to predictions of the statistical theory of unimolecular reactions. An angular momentum conserving version of the quasiequilibrium theory, formulated in terms of the Langevin collision cross section model for the reverse association reaction, predicts kinetic energy release distributions which agree very well with the experimental results at precursor ion energies up to 0.65 eV above the dissociation onset. At approximately this energy, the average kinetic energy released begins to deviate sharply from the theoretically predicted average, possibly indicating the accessibility of a new reaction channel.
Probabilities for classically forbidden transitions using classical and classical path methods
Authors: Muckerman J. T. , Rusinek Isak , Roberts R. E. , Alexander Millard
Published: February 2003

Limits are established for the applicability of purely classical methods for calculating nonreactive, inelastic transition probabilities in collinear collisions of a structureless atom and a harmonic oscillator. These limits, obtained by comparison with previous exact quantum mechanical results, indicate that such methods are inappropriate not only for ’’classically forbidden’’ but for many ’’classically allowed’’ transitions (in spite of the fact that they are widely used to calculate probabilities for such processes). A classical path method in the context of infinite-order time-dependent perturbation theory is described which yields extremely accurate transition probabilities even for the most classically forbidden transitions in the collinear atom–harmonic oscillator system. The essential features of this method are: (1) the use of the expectation value of the total interaction potential in determining the atom–oscillator (central force) trajectory, and (2) the use of the arithmetic mean of the initial and final velocities of relative motion in the (elastic) central force trajectory. This choice of interaction potential allows the relative motion to be coupled to changes in the internal state of the oscillator. The present classical method is further applied to three-dimensional atom-breathing sphere collisions, and exact quantum mechanical calculations are also carried out. Comparison of the classical path and exact quantum results shows excellent agreement both in the specific inelastic cross section and in the individual partial-wave contributions.
Effects on hyperpolarizabilities of molecular interactions in associating liquid mixtures
Authors: Levine B. F. , Bethea C. G.
Published: February 2003

We have measured the second and third order hyperpolarizabilities β and γ using the technique of electric field induced second harmonic generation in a variety of weakly associating (e.g., nitrobenzene–benzene) and strongly associating (e.g., water–methanol) liquid mixtures. These experiments are shown to yield interesting information on dipolar forces, hydrogen bonding, charge transfer interactions, and other solute–solute, solute–solvent effects.
Hyperpolarizability of the pyridine–iodine charge transfer complex
Authors: Levine B. F. , Bethea C. G.
Published: February 2003

The second order hyperpolarizability β for the classic pyridine–I2 charge transfer complex dissolved in benzene has been measured using the technique of electric field induced second harmonic generation in solution. The measured β was found to be very large, being in fact ∼30 times larger than β for pyridine. By using Mulliken’s charge transfer theory for the ground and excited state wavefunctions and substituting them into the quantum mechanical expression for β we were able to accurately account for the experimental results.
13C–{1H} nuclear Overhauser enhancement and 13C spin lattice relaxation in molecules undergoing multiple internal rotations
Authors: London Robert E. , Avitabile John
Published: February 2003

The effects of multiple internal rotations on the 13C–{1H} nuclear Overhauser enhancement (NOE) and 13C spin lattice relaxation has been considered. It is shown that the 5×5 matrices which arise from a treatment using the Wigner rotation matrices of order 2 can be replaced by 3×3 matrices by making use of the inherent symmetry of the problem without any loss of generality. The cross correlations which arise in chains of methylene carbons are also taken into account. Calculations are given for the NOE values for carbons in a methylene chain for several values of the internal diffusion coefficients Di, assumed constant along the chain, as a function of D0. It is found that the effect of cross correlations on the NOE values is relatively small except in the region where the isotropic diffusion coefficient of the end of the chain satisfies the condition 6D0∼ωc. In this region the effect of cross correlations depends on the internal diffusion coefficient Di, being insignificant for Di=109 sec−1 and somewhat more important for Di=1010 and 1011 sec−1. The spin lattice relaxation is also most markedly nonexponential in the region where 6D0∼ωc. As D0 increases or decreases beyond this value the relaxation becomes more exponential. The effect of the isotropic D0 motion on both the NOE and the spin lattice relaxation decreases rapidly for carbons further from the isotropically tumbling end of the molecule. An example involving a galactolipid phospholipid suspension is briefly discussed.
Electron nuclear double resonance of triplet diphenylmethylene in P21 benzophenone: Conformation and spin distribution
Authors: Anderson Richard J. M. , Kohler Bryan E.
Published: February 2003

The angular variation of the ENDOR transition frequencies has been studied for all the protons of the ground state triplet molecule molecule diphenylmethylene in a single crystal of benzophenone in its P21 phase at T?2 °K. The hyperfine coupling tensor for each proton has been determined from the experimental data. The molecular confirmation and spin densities are given which best reproduce the anisotropic components of these tensors under the constraint of a distributed point dipole model. There is a slight but readily discerned imbalance in the spin distribution between the two rings, and one phenyl ring is shown to be rotated out of the plane of the divalent carbon hybridization more than the other phenyl ring.
Classical trajectory study of internal energy distributions in unimolecular processes
Authors: McDonald J. D. , Marcus R. A.
Published: February 2003

The method of classical trajectories has been used to study the flow of energy in a molecular system (similar to the molecules CD3Cl and CD3H) representing a chemical activation experiment. Energy distributions are obtained both before and after the breakup of the activated molecule by means of a correlation function technique. Four different potential energy surfaces are employed. It is found that the initial distribution of energy in the activated molecule may or may not be random, depending on the details of the particular surface. This distribution becomes random in less than 5×10−12 sec. The distribution of energy in the final product (CD3) is found to be randomly distributed (as predicted by RRKM theory including angular momentum considerations) for a surface with no exit channel barrier or strong intermode couplings. When these special forces are present nonrandom energy distributions result. Product channel barriers result in an excess of translational energy and exit channel intermode couplings result in nonrandom vibrational distributions. Angular momentum considerations are found to be important in matching the predictions of RRKM theory with the calculations.
Coupled-channel study of rotational excitation of a rigid asymmetric top by atom impact: (H2CO,He) at interstellar temperatures
Authors: Garrison Barbara J. , Lester William A. , Miller William H.
Published: February 2003

A quantum mechanical scattering study is carried out to test a collisional pumping model for cooling the 6 and 2 cm doublets of interstellar formaldehyde. The Arthurs and Dalgarno formalism is extended to the collision of an s-state atom with a rigid asymmetric top molecule and applied to rotational excitation of ortho formaldehyde by helium impact. Using a previously determined configuration interaction potential energy surface, the coupled-channel (CC) equations are integrated at 12 scattering energies between 20 and 95°K. Up to 16 ortho formaldehyde states, yielding a maximum of 62 CC equations, are retained to test convergence of computed cross sections. Resonance structure is obtained at ∼20.2, 32.7, and 47.7°K. The computed inelastic cross sections are averaged over a Maxwell–Boltzmann distribution and the resultant rates used to solve the equations of statistical equilibrium for the relative populations. The 6 and 2 cm doublets are found to be cooled only upon inclusion of the j=3 doublet.
Theoretical studies of atmospheric molecules: SCF and correlated potential surface results for the X 2B1 and ? 2B2 states of H2O+
Authors: Fortune P. J. , Rosenberg Bruce J. , Wahl Arnold C.
Published: February 2003

SCF and MCSCF/CI calculations were carried out on the X 2B1 and ? 2B2 states of H2O+. Vibrational analyses based on second order perturbation theory were also performed. Computed equilibrium geometries, force constants, and low-lying vibrational frequencies for both the SCF and MCSCF/CI surfaces are reported.
Measurements of dielectric properties by time domain spectroscopy
Authors: Chahine R. , Bose T. K.
Published: February 2003

The dielectric response function of severals alcohols is calculated from TDR measurements in the temperature range 0 to 50 °C. Using simple formulas derived by real time analysis, this function was computed from the time integral and self-convolutions up to the second order of the reflections produced by an incident step voltage pulse on a finite sample inserted in a low-loss coaxial line. The calculations take account of finite amplitude of the reflection and the response function in third order approximation and of finite rise time. Precision of the analysis and the experiments are discussed.
Unified statistical model for ’’complex’’ and ’’direct’’ reaction mechanisms
Authors: Miller William H.
Published: February 2003

A unified statistical theory for bimolecular chemical reactions is developed. In the limit of a ’’direct’’ mechanism it becomes the usual transition state theory, which is correct for this situation, and if the reaction proceeds via a long-lived collision complex it reduces to the statistical model of Light and Nikitin. A general criterion for locating the ’’dividing surfaces’’ that are central to statistical theory is also discussed. This prescription (Keck’s variational principle) is shown not only to locate the usual dividing surfaces that pass through saddle points and minima of the potential energy surface, but it also selects the critical surfaces relevant to the ’’orbiting’’ and ’’nonadiabatic trapping’’ models of complex formation.
Self-avoiding random walks subject to external spatial constraints
Authors: Wall Frederick T. , Mandel Frederic , Chin John C.
Published: February 2003

Self-avoiding random walks, used to simulate macromolecules, have been investigated under circumstances involving external spatial constraints. Calculations have been made for two and three dimensional systems in which the available room for a chain is finite in one direction but infinite in other orthogonal directions. A representative statistical sample is obtained using the ’’slithering snake’’ Monte Carlo technique on chains so confined. Theoretical equations have been developed for dimensions of chains that are limited, in one direction, to a magnitude at least as great as that of the stretched chain. Agreement between observation and theory is satisfactory, and the overall results disclose the effect of such confinement.
Computer molecular dynamics studies of a chemical instability
Authors: Ortoleva Peter , Yip Sidney
Published: February 2003

Computer molecular dynamics studies are carried out on a binary mixture of hard disks in which one species can convert to the other by photoinduced transition or by collision-induced reactions. Phenomenological rate laws are postulated for a simple model system which is then shown to possess multiple steady-state concentrations under suitable conditions. The computer results for a 450-disk system show that such solutions do exist, and that in general the postulated rate equations adequately describe the system behavior. Concentration fluctuations are examined and found to agree with the predictions of a master equation. Fluctuations are found to be enhanced near a point of marginal stability.
Variational time-dependent Hartree–Fock calculations. III. Potential curves for two-electron molecular systems
Authors: Stewart R. F. , Watson D. K. , Dalgarno A.
Published: February 2003

Time-dependent Hartree–Fock calculations of excitation energies, oscillator strengths, and related properties are reported for the molecules H2 and HeH+. For a wide range of internuclear separations, the results are in accord with more refined comparison data. The comparison demonstrates that the time-dependent Hartree–Fock method yields accurate molecular properties over a range of internuclear distances.
14N quadrupole resonance of some liquid crystalline compounds in the solid
Authors: Seliger J. , Osredkar R. , Mali M. , Blinc R.
Published: February 2003

Using proton–nitrogen double resonance and cross relaxation in the laboratory frame, the 14N NQR spectra of solid p-azoxyanisole (PAA), diheptyloxyazoxybenzene (HOAB), ethoxybenzilidene butylaniline (EBBA), and p-anisalazine have been measured. The room temperature 14N quadrupole coupling constant of p-anisalazine is e2qQ/h=4410 kHz whereas the asymmetry parameter is η=0.213. In EBBA, e2qQ/h=3990 kHz and η=0.186. In PAA and HOAB there are more than two chemically nonequivalent nitrogens in the unit cell. The 14N quadrupole coupling constants lie in the range between 4320–4370 kHz (PAA) and 4160–4400 kHz (HOAB) whereas the asymmetry parameters lie between 0.37 and 0.5. A comparison of the results for solid and nematic PAA shows that the largest principal axis of the 14N electric field gradient tensor is very nearly parallel to the N=N bond and that the central part of the PAA molecules containing the N=N bonds is rigid in the solid whereas it rotates around the long molecular axis in the nematic phase.