bluuebunny/crossref_metadata_2025 huggingface dataset:
Classical dynamics of t
r
a
n
s-diimide: Intramolecular vibrational relaxation involving an active torsion
Authors: Spears L. Gene , Hutchinson John S.
Published: July 2002

The classical dynamics of vibrationally excited trans-diimide (N2H2) were studied, focusing on the interaction of the torsion with the local NNH bends and NH stretches. Energy flow to and from the torsion is facilitated by nonlinear resonances between the NH stretch and a bend-torsion combination mode, as has been found for other types of vibrational motions. The significant frequency ratios for such combination resonances have been predicted by a simple, analytic model and verified using a local harmonic oscillator potential surface. Several of these resonances were observed for trans-diimide. Since the dominant couplings are primarily kinetic, these resonances should also be important in the torsional vibrational dynamics of other molecules.
The isomerization of 1-methylcyclopropene: A multichannel unimolecular reaction induced by overtone excitation
Authors: Baggott J. E. , Law D. W.
Published: July 2002

The distribution of products resulting from the isomerization of gas phase 1-methylcyclopropene (1) induced by C–H stretch overtone excitation has been studied in a search for mode specific effects. Quantum yields for the disappearance of 1 have been determined as a function of the total pressure of a mixture of 1 and methylenecyclopropane and of the added bath gases SF6 and Ar using a combination of intracavity dye laser photolysis and end product analysis by gas chromatography. These data, together with the ratios of the yields of the major isomerization products, are adequately reproduced by RRKM theory and a weak collisional energy transfer model: no evidence for mode specificity is obtained. The experimental data demonstrate the importance of inefficient collisional energy transfer but allow some conclusions to be drawn concerning the applicability of RRKM theory which are not strongly dependent upon the details of the energy transfer model.
Test of closure approximations in equilibrium classical many-body theory
Authors: Root Leslie J. , Stillinger Frank H. , Washington Gary E.
Published: July 2002

Many integral equations for numerically predicting pair correlation functions g(2) in classical fluids have been proposed, each based on its own ‘‘closure approximation.’’ We have investigated a procedure for evaluating such closures, particularly their capacity to describe clustering under the influence of attractive interparticle forces. Our approach utilizes the Gaussian core model in a large closed system which, under sign change of the coupling constant λ, undergoes a collapse to form a single compact aggregate with known properties. In the infinite system limit this phenomenon causes inverse temperature (β) expansions, such as that for g(2), to diverge. By contrast available closures lead to convergent expansions for the Gaussian core model, but with a ‘‘critical point’’ on the negative real βλ axis. Specific calculations have been performed illustrating this behavior for the BGYK and the PY integral equations. We suggest that the proximity of the artifactual negative-axis critical point to the origin (where the collapse singularity should actually appear) provides a measure of accuracy of closure approximations.
Brillouin light scattering in low temperature water–ethanol solutions
Authors: Benassi P. , D’Arrigo G. , Nardone M.
Published: July 2002

Brillouin light scattering spectra of water–ethanol mixtures in a wide temperature and concentration range are reported and analyzed on the basis of a generalized hydrodynamic theory. The obtained sound velocities and longitudinal kinematic viscosity coefficients are compared with the analogous results obtained from ultrasonic measurements. In particular, light scattering spectra of 23% molar water–ethanol solution show direct evidence of a relaxation process in the GHz region which appears to be well separated from the lower frequency relaxation process determining the anomalous ultrasonic properties of these solutions. This high frequency relaxation involves both shear and volume viscosities and closely resembles the viscoelastic relaxation found in many associated liquids. The role played by the microscopic variables driving the relaxation is briefly discussed.
A computer and analytic study of the metallic liquid–glass transition. II. Structure and mean square displacements
Authors: Li D. H. , Moore R. A. , Wang S.
Published: July 2002

A study, using both computational and analytic techniques, of the extent to which the cage concept of liquids applies to the metallic liquid–glass transition is presented. Monte Carlo calculations on a Ca0.7Mg0.3 system yield the angular distribution functions required to determine the cage structure and the atomic motion. The analysis parallels that for normal liquids. The results indicate that the cages are icosahedral, that they exist from the melting temperature down into the glass state, and that the cage concept applies quantitatively to the static properties of metallic binary alloys. Finally, the cage concept itself does not appear to describe fully atomic diffusion in metallic systems.
Thermodynamic characterization of the low-temperature phase transformations in KOH and KOD
Authors: White Mary Anne , Perrott Allyson , Britten Debra , Van Oort Michiel J. M.
Published: July 2002

The heat capacities of KOH and its deuterated analog KOD have been measured over the temperature range 17<T<342 K and 32<T<295 K, respectively. The thermodynamics of the phase transformations to the low-temperature antiferroelectric phases have been characterized: Ttr (KOH)=226.7±0.2 K; Ttr (KOD)=253.1±0.1 K; ΔStr (KOH)=0.121±0.002 R; ΔStr (KOD)=0.1258±0.0007 R. The thermodynamic changes are consistent with tunneling of the hydroxide (deuteroxide) ions between two minima in a double-well potential.
Termolecular recombination: Coupled nearest-neighbor limit and uncoupled intermediate levels limit
Authors: Flannery M. R. , Mansky E. J.
Published: July 2002

Two extreme limits of collisional coupling in termolecular recombination are investigated. The coupled nearest neighbor (CNN) limit includes only couplings between neighboring excited energy levels of the associating species AB*, while the uncoupled intermediate levels (UIL) limit includes only couplings between the fully dissociated reactants A+ and B− and each of the (assumed uncoupled) excited levels of AB*, which are then coupled to the fully associated products AB. Comparison is made with results of previous exact and diffusion treatments.
Dynamic dipole polarizability of a dielectric sphere in an electrolyte
Authors: Vogel E. , Pauly H.
Published: July 2002

The response of a suspension of spherical particles to a periodic electric field is given by the real and imaginary part of the effective dielectric number εeff(ω). In order to describe its dispersion within an enhanced dipole residual interaction model, the polarizability α(ω) of a single sphere has to be found first. This calculation is facilitated by the smallness of the ratio of the Debye length δ to the radius a of the sphere. In a thin double layer approximation, an improved expansion in δ/a is found for α(ω) in the long wavelength limit k→0. In contrast to Schwarz (Ref. 1) who assumed tangential diffusion effects around the spheres, the volume diffusion is the dominant feature here. A comparison with a simple relaxation time ansatz is given in order to show the significance of diffusion.
Molecular Rydberg states: Classical chaos and its correspondence in quantum mechanics
Authors: Lombardi M. , Labastie P. , Bordas M. C. , Broyer M.
Published: July 2002

The Rydberg spectrum of Na2 has been shown previously to alternate when increasing energy between ‘‘stroboscopic fringes’’ which correspond to a well known separable Hund’s coupling case (a), and a complex, unidentifiable intermediate coupling. We use this system as a prototypic example to test some current ideas on the correspondence between classical chaos and properties of quantum spectra. We first determine the phase space structure and transition to chaos in classical mechanics. We then determine the change in line intensities and level spacing statistics in quantum mechanics. We show that this system has the expected behavior in the semiclassical limit in the presence of classical chaos, except for a peculiarity in level spacing statistics, but that this behavior is not a signature of chaos, since the same system shows similar behavior for some values of the parameters which correspond to a nonchaotic situation in classical mechanics. We discuss also some problems related to the nonvalidity of the semiclassical limit.
Unitary-group approach (UGA) to the configuration interaction (CI) problems of the many-electron systems. II. Weyl graphical method of two-body operator matrix elements
Authors: Lin Hai-Lun , Cao Yu-Fang
Published: July 2002

A recursive graphical approach for the evaluation of two-body operator matrix elements is presented. This new approach is simpler as well as clearer compared to other approaches.
Instabilities in closed chemical systems under nonideal conditions. II. Debye–Hückel interactions
Authors: Dutta Chitra , Das Jyotirmoy
Published: July 2002

The integrated form of second law has been used to examine the global stability properties of closed chemical systems in weak ionic environments where Debye–Hückel limiting law is valid. Beyond a threshold value of the ionic strength, the system exhibits multiple equilibria. Selection of any particular equilibrium state is governed by (i) initial conditions, (ii) the concentrations of reacting ionic species, and (iii) apparently nonreacting ionic species comprising the system. Under marginal stability conditions, where linear stability theory fails, the stability behavior of the system can be predicted from the present analysis. The regions in the phase space, where the system might exhibit complex dynamic phenomena like damped oscillation and chaos have been identified. The present analysis has demonstrated for the first time that excitation of stable equilibrium state is possible even in a mass-closed system.
Multitime correlation functions and susceptibilities for a Brownian two-dimensional rotor
Authors: Fulton Robert L. , Lim Raymond
Published: July 2002

General formulas are found for all the multitime correlation functions and susceptibilities of a simple two-dimensional rotor, the motion of which is governed by a Langevin equation containing Gaussian white noise as the torque. A sufficient condition that the equal time Poisson bracket of the angle with its conjugate momentum be satisfied at all times is inferred from the fluctuation–dissipation theorem applied to the fluctuating torque. The formulas include inertial effects, a necessity if high frequency behavior is to be correctly reproduced. The Fourier transforms of the two- and four-time dipole susceptibilities are extracted from the general results in the large friction constant limit. For the particular case of the rotor subjected to a strong constant background field and a weak time dependent probe field, we find that at low frequencies, the rotor exhibits saturation as measured by the real component of the four-time susceptibility. The imaginary part of the parallel component of the four-time susceptibility is found to be negative over the entire range of frequencies, this merely indicating that the overall dissipation in the system is decreased in the presence of a strong field. The Appendix determines the most general form of the correlation functions of the torque when it is specified that the two-time Poisson bracket of the torque is independent of the torque and of the system variables and that the fluctuation–dissipation theorem holds. They are a bit more general than Gaussian.
Smoluchowski fluctuation theory of ion transport in solutions
Authors: Kim Hyung J. , Friedman Harold L.
Published: July 2002

We developed a Smoluchowski-level theory of time-dependent ion concentration fluctuations at equilibrium, including higher-order correlations, but with hydrodynamic interactions suppressed. A partial sum over the higher-order correlations, a chain sum, eliminates divergences. From the resulting renormalized self-van Hove function we recover Onsager’s limiting law for the self-diffusion coefficients of the ions. The limiting law comes from the chain sums that are neglected in a ‘‘linearization’’ approximation used in some earlier work. The corresponding development for the distinct van Hove functions is merely indicated. We apply these results to a solution of a single spherical polyion with many small ions, to obtain the self-diffusion coefficient for the polyion in near-limiting law conditions, and to simple models for aqueous NaCl and CuSO4 solutions, to calculate the self-diffusion coefficients of the ions and the Maxwell (Debye–Falkenhagen) relaxation, all in the concentration range up to 1 M.
Determination of the intrinsic reaction coordinate: Comparison of gradient and local quadratic approximation methods
Authors: Ischtwan Josef , Collins Michael A.
Published: July 2002

The determination of the intrinsic reaction coordinate (IRC) by gradient methods often fails or is very inefficient when gradients vary markedly between coordinates as occurs in asymptotic regions of bimolecular surfaces. The alternative local quadratic approximation (LQA) approach, based on second derivatives of the surface, is shown to be generally accurate and efficient.
Vibration–rotation spectroscopy by stimulated emission pumping in a supersonic beam: a-Axis Coriolis coupled v4=1 and v8=1 levels of X̃ 1A
g glyoxal
Authors: Frye Donald , Lapierre Luc , Dai Hai-Lung
Published: July 2002

In combination with supersonic cooling, stimulated emission pumping (SEP) can be used to obtain vibrational spectra of medium-size molecules in their electronic ground state with single rotational level selectivity. The greatly reduced rotational congestion in the spectra of 0.04 cm−1 resolution makes the vibration–rotation assignment straightforward. Two vibrational levels, C–C stretching v4=1 with vibrational term values determined at 1065.80(1) cm−1 and CH wagging v8=1 at 1048.07(1) cm−1, of glyoxal were studied. These vibrational levels are symmetry forbidden in the IR spectroscopy, but can be detected by SEP. Their rotational constants are determined from primarily the low J, K levels, as A=1.9394(25), B=0.1594(25), and C=0.1426(25) cm−1 for 41 and A=1.7335(14), B=0.1594(10), and C=0.1464(8) cm−1 for 81. The A constants and the symmetries indicate that the two vibrational levels are strongly coupled to each other through the a-axis rotation. A deperturbation analysis yielded the A constants as 1.8414(126) for 41 and 1.8317(127) for 81, as well as the Coriolis coupling constant ζa48 =0.3642(226). These, and other previous observations, indicate that Coriolis interactions are important in promoting intramolecular vibrational relaxation in the glyoxal molecule.
Photothermal lensing spectroscopy of supersonic jet expansions of acetylene
Authors: Hineman Max F. , Rodriguez Rene G. , Nibler Joseph W.
Published: July 2002

Photothermal lensing effects are observed in the early expansion stages (X/D=1–7) of a supersonic free jet and narrow P and R branch rotational lines have been recorded at 100 MHz resolution for the ν2 +5ν3 band of acetylene. Rotational Raman loss spectra were also recorded and, from the collision-broadened linewidths of these, it is estimated that about 100 collisions occur in the beam transit from pump to probe positions. This is sufficient to produce rotational and some vibrational relaxation in forming the lens. The photothermal intensities are consistent with a Boltzmann rotational distribution in the ground state and a rotational temperature of 70 K is deduced for 100 PSI expansions of 20% C2 H2 in helium. Charac- teristics of the photothermal signal in the jet are described and the time and spatial dependence of the signal is used to measure a flow velocity 625 m/s for the jet. This photothermal method may prove useful as a diagnostic probe of beam properties and as a spectroscopic measure of weak one- and two-photon transitions of molecules cooled in jets.
Optically pumped atomic spin orientation and the law of mass action for 2Cs⇄Cs2
Authors: Ho J. J. , Wang Chongye , Bernheim R. A.
Published: July 2002

The effect of optically pumped atomic spin orientation on the atom–dimer equilibrium in cesium vapor has been observed. Such a shift in equilibrium was predicted over two decades ago and is due to the redistribution of spin-determined bonding and antibonding collision trajectories. Orientation of the total spin angular momentum of 6 2S1/2 ground state Cs atoms in the vapor phase was produced by optical pumping with a tunable cw dye laser. A decrease in the equilibrium molecular density resulted and was measured using laser induced fluorescence from the B 1Πu state. The shift in equilibrium is a function of the degree of atomic orientation and could be observed in the temperature range 383–421 K for cesium vapor in a glass cell which also contained a He–N2 buffer gas mixture with a composition ratio of 9:1.
Multiplet structure in the 35,37Cl ligand electron nuclear double resonance spectra of single crystals of Cu(II)-doped histidine hydrochloride monohydrate
Authors: McDowell C. A. , Sastry D. L.
Published: July 2002

Multiplet structure was observed at 4.2 K in the 35,37Cl ligand electron nuclear double resonance (ENDOR) spectra of single crystals of Cu(II)–doped histidine hydrochloride monohydrate. By using second-order perturbation theory we show clearly that the observed multiplet structure arises from the 63Cu and 35,37Cl indirect nuclear spin–spin interaction. Our theoretical analysis in general because it encompasses the anisotropies of both the g and hyperfine tensors, and also allows for the quantization of the nuclear spins along the effective field at each nucleus.
Mean field approach to molecule–surface scattering at finite temperature: One phonon theory
Authors: Jackson Bret
Published: July 2002

A theory is presented for the phonon inelastic scattering of light atoms and molecules from surfaces. Both the gas species and the thermal fluctuations of the solid are treated in a fully quantum fashion. A self-consistent field method is used to reduce the evolution of the reduced density matrix to the propagation of a single wave function and a set of coefficients describing phonon excitation and annihilation. The method allows one to extend recent time dependent molecule–surface scattering theories to finite temperature, with only a small increase in computer time. Agreement is found with experimental data for the thermal attenuation of diffraction peaks for He scattered from Cu. Energy transfer is found to be sensitive to the steepness of the repulsive potential, the molecular kinetic energy, and the angle of incidence, and only weakly dependent on the well depth. The ‘‘Beeby correction’’ is examined and shown to be invalid, except at very low beam energies where there is a small correlation between well depth and inelastic scattering. For this model, energy transfer does not scale with the normal component of the beam energy.
Model polarizabilities and multipoles for ionic compounds. Alkaline-earth monohalides
Authors: Davis Stephen L.
Published: July 2002

The Törring–Ernst–Kindt (TEK) model for the alkaline-earth monohalide molecules was extended to predict molecular polarizabilities and quadrupole moments in addition to dipoles. Calculations were carried out for the 19 molecules for which experimental bond lengths and vibrational frequencies are known. It was found that for many of these molecules the TEK model predicts negative polarizability anisotropies, α⊥>α∥. This is in disagreement with the dipole-induced dipole, or Silberstein, formula, but in agreement with an empirical correlation between the anisotropy and the molecular asymmetry noted by Winicur. The TEK polarizabilities give rise to a much smaller second-order Legendre anisotropy in the CaCl–Ar interaction potential than was found in earlier work using the Rittner model. The D-shell model applied earlier to the alkali halides was also applied to the 19 alkaline-earth monohalides. With a slight modification it was found to describe both classes of ionic compounds successfully, although less successfully for the alkaline-earth compounds. The D-shell model was shown to be a generalization of the TEK model which includes the effect of the shell overlap on the polarizing fields at the ions. Nevertheless, the TEK model predicts better dipole moments, unless the shell charge is treated as an additional, adjustable parameter in the D-shell model.
Photophysics of atomic magnesium isolated in solid methane and perdeuteromethane. III. Evidence for a kinetic isotope effect in the competitive process of atomic magnesium insertion into a C–H bond
Authors: McCaffrey John G. , Parnis J. Mark , Ozin Geoffrey A.
Published: July 2002

The formation of the C–H bond insertion product methylmagnesium hydride (CH3MgH) and the simultaneous emission of atomic triplet magnesium are observed following photoexcitation of the first allowed singlet resonance transition of atomic magnesium isolated in solid methane matrices at 12 K. Isotopic variation of the solid methane hosts produces observable differences in the relative branching ratios into the photophysical (atomic triplet emission) and photochemical (insertion product formation) channels. In solid perdeuteromethane (CD4), the intensity of the atomic emission is approximately five times that in solid methane (CH4) while the rate of formation of the insertion product shows the opposite behavior in the two solids. No singlet atomic magnesium emission is observed in the solid Mg/methane systems and the rise time of the atomic triplet emission is deduced to occur on a time scale of less than 10 ns. A simple model derived from spin and orbital correlations between reactants and products is presented which considers the effect of the low symmetry of an insertive reaction coordinate in the approach geometry of atomic magnesium to methane yielding the linear product CH3MgH. Using this model, the absence of the singlet atomic emission is explained in terms of the attractive nature of the singlet surface with respect to the formation of a bent, inserted intermediate. The observation of an enhanced rate of atomic magnesium intersystem crossing is thought to occur as a result of the symmetry-induced participation of the repulsive triplet surface in the process leading to the linear insertion product. The model also suggests an origin for the observed kinetic isotope effects. Differences in the observed behavior of the 1P state of atomic magnesium in gas-phase and solid-phase quenching experiments (explicitly the formation of fragmented products only with no unreacted atomic triplet in the former case and the formation of the insertion product with intense atomic triplet emission in the latter) are discussed in relation to the presence of efficient relaxation pathways in the solid phase and the absence of such pathways in the single-collision conditions of the gas-phase experiments.
Laser induced vibration–rotation excitation of hydroxyl ions: OH− and OH+
Authors: Sharma Bhupat , Prasad Vinod , Mohan Man
Published: July 2002

The quantum dynamics of vibration–rotation excitation of hydroxyl ions (e.g., OH− and OH+ in the gaseous phase) in the presence of a laser beam is investigated. The nonperturbative Floquet method is used to solve the equation of motion with an explicitly time-dependent Hamiltonian. Convergent results are obtained for the long-time averaged transition probabilities for various transitions by including a sufficiently large number of rovibrational and photon states. In order to understand the roles played by power broadening, dynamic stark shifts, and non-Lorentzian line shapes, higher laser intensities (in the GW/cm2 range) are considered in the molecular excitation process.
Vibration–rotation interactions and ring-puckering potential function in trimethylene sulphone
Authors: López Juan C. , Alonso José L.
Published: July 2002

The microwave spectrum of trimethylene sulphone has been studied in the ground and five vibrational excited states of the ring-puckering vibration. Very large deviations from the rigid rotor behavior were observed for the lowest pair of states. These perturbations resulted from a strong Coriolis coupling between them and have been analyzed using a reduced axis system Hamiltonian. The energy separation between the v=0 and v=1 levels has been determined from this analysis to be of 0.9106±0.0004 cm−1. Small deviations from rigid rotor spectrum were observed for the v=2 to v=5 ring-puckering states that were found to fit semirigid rotor theory where the quartic centrifugal distortion constants account for the vibration–rotation interaction effects. From the variation of the rotational constants and the vibrational separation between v=0 and v=1 states, a reduced ring-puckering potential function has been determined to be V(X)=6.62(X4−9.22X2), which gives a barrier to the planar configuration of 140±35 cm−1. With this potential function the vibration–rotation interaction effects observed for states v=2 to v=5 have been interpreted. Considerations about the ring conformation and the dynamics of the ring-puckering motion have also been made. The μa component of the electric dipole moment has been determined to be 4.8 D.
Complex structures in the calculated photoionization spectrum of CO in the 17–17.5 eV energy region
Authors: Leyh Bernard , Raşeev Gheorghe
Published: July 2002

Complicated spectral features appear in the CO photoionization spectra between 17 and 17.5 eV photon energy. In order to interpret these processes, we refine in the present paper our previous model [B. Leyh and G. Raşeev, Phys. Rev. A 34, 2920 (1986)] by introducing all the vibrational levels of the first three electronic states of CO+. This more accurate treatment leads to two major results. The resonances at 17.1 and 17.3 eV are complex resonances resulting from the interaction between the Rydberg series converging to the A 2Π and B 2Σ+ ionic states: they can no more be assigned simply to Rydberg states converging to CO+B 2Σ+. The intermediate experimental feature at 17.2 eV, missing in our previous calculation, is now present in the theoretical spectrum. Mainly, this feature, which we have called composite resonance, is a broadened (by the experimental resolution) superposition of Rydberg states converging to the A 2Π ν+=4 state. In this paper, we present and analyze the vibrationally resolved cross sections and asymmetry parameter for the X 2Σ+ν+=0–3 and A 2Π ν+=0–4 states. These theoretical results are compared to recent experimental data by Leyh et al. [Chem. Phys. 115, 243 (1987)] and by Hardis and co-workers (the preceeding paper).
The vibrational circular dichroism of dimethylcyclopropane in the C–H stretching region
Authors: Amos Roger D. , Handy Nicholas C. , Drake Alex F. , Palmieri Paolo
Published: July 2002

The vibrational circular dichroism (VCD) of dimethylcyclopropane is studied by ab initio techniques. The equilibrium geometry, the molecular force field, and the matrix elements for the infrared (IR) and VCD intensities are evaluated using the derivative method for molecular wave functions of SCF type. An alternative derivation of Stephens’ expression for the rotational strength is presented. The results are compared with the available experimental information for the C–H stretching modes. This is an excellent demonstration of the validity of this theory for the VCD intensity of the C–H vibrations, which are easily rationalized using group theoretical arguments.
Calculation of the cooperative friction coefficient in a dilute suspension of spheres. I. Two-body effects
Authors: Jones Dumont M. , Muthukumar M.
Published: July 2002

We apply diagrammatic techniques of quantum-mechanical angular momentum algebra to the efficient calculation of transport properties in a solution of spheres, based on the hydrodynamic scattering formalism of Muthukumar and Freed. The method is illustrated by computing the two-body correction to the cooperative friction coefficient in a dilute suspension.
Methylnitrene free radical. Ground state vibrational fundamentals and harmonic force field from jet-cooled emission spectra of the Ã 3E–X̃ 3A2 band system
Authors: Chappell E. L. , Engelking P. C.
Published: July 2002

The Ã 3E–X̃ 3A2 UV emission spectra of the jet-cooled methylnitrene radical CH3N and the isotopomers 13CH3N and CD3N have been examined with moderate resolution. Analysis of the vibrational band progressions gives a complete set of all six ground state vibrational fundamentals for CH3N: The symmetric hydrogen stretch, ν″1=2943(04) cm−1; the symmetric hydrogen bend, ν″2=1349(04) cm−1; the C–N stretch, ν″3=1040(04) cm−1; the asymmetric hydrogen stretch, ν″4=2989(04) cm−1; the scissors, ν″5=1490(04) cm−1; and the rock, ν″6=903(08) cm−1. A Jahn–Teller distortion in the 3E excited state activates ground state progressions of the e-type modes. These vibrational frequencies, along with those from 13CH3N and CD3N, determine the 12 coefficients of a ground state harmonic force field. The resulting force constants indicate a strong C–N bond and weak C–H bonds, accompanying considerable departure from a usual methyl structure. The structure and force field are discussed in the context of a model involving sigma bond optimization of the C–N bond along with hyperconjugation.
A theoretical study of the energies of BHn compounds
Authors: Curtiss Larry A. , Pople John A.
Published: July 2002

A study of the energies of BHn compounds (n=1–3), cations and neutrals, using high levels of ab initio molecular orbital theory, including correlation effects beyond fourth-order perturbation theory, is presented. The appearance potentials of BH+, BH+2, and BH+3 are in good agreement with experiment. The results suggest that the experimental dissociation energy and ionization potential of BH may be in error.
Variational principle for termolecular recombination in a gas
Authors: Flannery M. R.
Published: July 2002

A variational principle for the rates of termolecular processes is proposed and then applied to recombination between atomic ions with excellent results. The variational expression when minimized with respect to stabilization probabilities is capable of providing rates identical to those determined from the quasi-steady-state solution of the full Master equation. Connection is made with electrical networks and with the principle of least dissipation.
Semiclassical localization in a one-dimensional random analytic potential
Authors: Dumont Randall S. , Pechukas Philip
Published: July 2002

A careful semiclassical analysis of ‘‘above-barrier’’ localization is provided for the case of one-dimensional random analytic potentials. The result is a practical semiclassical formula for the localization length, which shows exponential unboundedness in the classical limit. The algebraic, ℏ→0, asymptotic series for the density of states is also determined. In the process of deriving the latter result an interesting link between the density of states and the localization length (via the Lyapunov exponent) is revealed. In addition, the results of this paper provide some of the groundwork for the semiclassical analysis of ‘‘nonadiabatic transport’’ associated with time-varying potentials.
Rotationless molecular vibrations
Authors: Shipsey E. J.
Published: July 2002

Relative coordinates convenient for variational calculations are discussed. The coordinates, for suitable representations of the potential, allow matrix elements to be expressed as sums of products of one-dimensional integrals for product basis functions. A small vibration limit is reviewed in these coordinates. Examples are given for application to the linear triatomic, water, ammonia, acetylene, and finally methane.
Understanding dephasing in mixed molecular crystals. I. Photon echoes from dimers revisited
Authors: Spano Frank C. , Warren W. S.
Published: July 2002

We show that a fundamental assumption used in analyzing photon echo decays from interacting molecules—the assumption that the ‘‘pure’’ dephasing rate can be obtained by subtracting one half of the fluorescence decay rate (2T1)−1 from the photon echo decay rate—is generally inconsistent with a fully quantum mechanical treatment. The density matrix evolution for a system of noninteracting dimers is calculated using the retarded electromagnetic coupling between two oscillating dipoles. We are able to properly include the effects of spontaneous emission and Dicke superradiance on the echo decay.
Dielectric properties of condensed fluoromethanes and fluoromethane mixtures
Authors: Böhmer R. , Loidl A.
Published: July 2002

Dielectric measurements were performed on liquid and solid (CF4)1−x (CMF3)x with M=H, Cl, and Br. The dielectric behavior of the polar molecules in the liquid state is well described by an Onsager equation. At the melting point, pure hydrogenated and brominated fluoromethanes condense into dipolar rigid phases, while pure tetrafluoromethane forms a plastic crystal. CClF3 exhibits dipolar relaxational phenomena in the solid state. The data indicate that near the melting point this compound is close to a transition into a plastic phase. Mixtures of CF4 with CHF3 and CBrF3 exhibit monotectic phase diagrams with a limited solubility in the liquid state and complete immiscibility in the solid state. However, formation of mixed crystals is found in (CF4)1−x (CClF3)x which exhibits a eutectic phase diagram.
Symptoms of chaos in observed oscillations near a bifurcation with noise
Authors: Harding Robert H. , Ross John
Published: July 2002

We examine an experimental transition from periodic to aperiodic and back to periodic dynamics in the combustion of acetaldehyde(ACH) in a continuous stirred tank reactor (CSTR) with power spectra, autocorrelation functions, phase portraits, Poincaŕe sections, the Wolf–Swift–Swinney–Vastano (WSSV) method for determining the largest Lyapounov exponent, and the Grassberger–Procaccia (GP) method for determining correlation dimension. Each technique gives some indications of a transition to chaos, but there are discrepancies in that the largest Lyapounov exponent is positive but does not converge and the GP method results in a correlation dimension between one and two for two aperiodic data sets. We explore in instructive detail possible explanations for false indications of chaos by comparing our results with calculations on the Rössler chaotic attractor and the van der Pol periodic attractor modified to examine the effects of uneven point distribution and three types of experimental noise. An uneven distribution of points results in a decreased range of length scales for convergence and a larger required embedding dimension for the GP method, but does not explain our experimental results. Observation noise (a Gaussian noise added to each term in the time series but not entering in the equations of motion) and constraint shift (the motion relaxes to an attractor but a constraint changes monotonically during the course of measurement) added to a periodic attractor both result in a low length scale cutoff below which the attractor dimension does not converge with embedding dimension, and above which it converges to 1. Constraint variation noise (a Gaussian noise is added to each term in the time series and enters into the equations of motion as a stochastic perturbation) does yield correlation dimensions between 1 and 2. The experimental transition shows many similarities to a Hopf bifurcation found in another experiment on the same system and to a theoretical Hopf bifurcation with constraint variation noise. A modification of the WSSV Lyapounov exponent analysis for this experimental transition shows the random walk separation of trajectories expected for constraint variation noise

added to the dynamics of a periodic attractor with a Hopf bifurcation. We therefore identify the experimental transition as an arc in constraint space which does not cross but is nearly tangent to a Hopf bifurcation set.
Observation of the formation of C+, O+, and ArC+ in the collisions of Ar+(2P3/2,1/2) with CO
Authors: Flesch G. D. , Ng C. Y.
Published: July 2002

Absolute total cross sections for the formation of C+ , O+ , and ArC+ in the collisions of Ar+ (2 P3/2,1/2 )+CO have been measured over the center-of-mass collision energy (Ec.m. ) range of ∼4–123 eV. The observed appearance energies for C+ and O+ are in agreement with the thermochemical thresholds for the C+ (2 P)+O(3 P) and O+ (4 S0 )+C(3 P) product channels, respectively. The cross sections for C+ are significant compared to those for CO+ . At Ec.m. =12.9 eV, the analysis of the kinetic energy distributions of C+ and CO+ supports the conclusions that many excited vibronic states of CO+ are populated and that the C+ ions are formed by predissociation of electronic excited CO+ .
Nuclear magnetic resonance line shapes of exchanging spin 3/2 nuclei
Authors: Eliav Uzi , Baram Asher , Navon Gil
Published: July 2002

We present here a calculation of nuclear magnetic resonance (NMR) spectral line shapes of spin 3/2 nuclei exchanging between sites with different quadrupole interactions and different reorientational correlation times. The calculation, which is based on the solution of the semiclassical Liouville equation, is performed for the whole range of exchange rates and reorientational correlation times including very slow motions. It is shown that the linewidth at half-height does not vary in a simple way as a function of the exchange rate, but rather passes through two maxima.
Crossed beam study of the I2+F2→IF(B)+IF(X) reaction: Spectrum of the chemiluminescence and rovibrational distribution of IF(B)
Authors: Billy N. , Girard B. , Gouédard G. , Vigué J. , Di Caprio D. , Caillaud B. , Le Gall J. L. , Venturi M.
Published: July 2002

The chemiluminescence of IF produced by the I2+F2 reaction has been the subject of several works. In this paper, we present a study of this reaction under crossed beam conditions. The chemiluminescence spectrum has been recorded at several collision energies and the rovibrational distribution of the IF molecules has been deduced from the spectra by a linear least square fit. This distribution presents a strong vibrational inversion and extends to very high J values. It is very different from all the previously reported rovibrational distributions observed in the IF chemiluminescence. This result confirms that at least two different mechanisms contribute to populate the IF B state in the I2+F2 reaction, and our experiment is the first one to give the spectrum of the I2+F2 chemiluminescent reaction occurring in a single collision regime.
Orientational order of Langmuir–Blodgett films as determined by fluorescence anisotropy
Authors: LeGrange Jane D. , Riegler Hans E. , Zurawsky Walter P. , Scarlata Suzanne F.
Published: July 2002

The orientational order parameters 〈P2〉 and 〈P4〉, of cadmium stearate Langmuir–Blodgett multilayers have been calculated from steady state fluorescence anisotropy experiments. It has been shown that it is valid to model the polarization components using the assumptions that the fluorescent probes are axially symmetric, the film is azimuthally symmetric within the plane, and that the rotational motion is slow enough to be neglected. Although the data do not preclude a dependence of anisotropy on thickness, within the sample-to-sample variations, there is no significant effect of thickness on orientational order. The order parameters for newly deposited films are 〈P2〉=0.33 and 〈P4〉=0.02. The orientational order decreases in the first 600 h after deposition, and then remains stable for at least several months.
Surface phonon induced Raman scattering: A new mechanism for intense inelastic scattering
Authors: Mamiche-Afara Suzanne , Dignam Michael J.
Published: July 2002

This paper forms the third in a series on the optical properties of a system consisting of a uniform, radially uniaxial coating of molecules on a small isotropic sphere. In this paper, the induction of Raman activity in a coated metal sphere by the fields generated by the coupled surface phonon modes of the coating is considered. Thus, the surface phonon modes induce surface plasmons in the metal that result in its polarizability being modulated. The phenomenon, which requires dipole active rather than Raman active molecular modes to be operative, we have termed surface phonon induced Raman scattering (SPIRS). Scattering intensities predicted via SPIRS and surface enhanced Raman scattering (SERS) have been compared for CO on a spherical silver particle. That for SPIRS is found to be of the order of or greater than that for SERS, depending on exactly how the surface region of the metal sphere is modeled. Some predictions based on SPIRS are reminiscent of data previously assigned to SERS but not explained by current models.
From isolated HCN molecules to the crystal: Inelastic neutron scattering, far infrared spectroscopy, and normal coordinate analysis
Authors: Langel W. , Kollhoff H. , Knözinger E.
Published: July 2002

HCN was isolated in solid argon matrices in molar ratios of 1:200 to 1:15. Inelastic neutron scattering was recorded with an incident energy of 38 meV. The data sets were converted to densities of states. A normal coordinate analysis based on infrared and far infrared data of dimers and higher aggregates allowed to calculate the density of states of the transverse phonons of linear HCN-chains and to assign the observed neutron spectra to these modes. Inelastic neutron scattering (INS)and far infrared (FIR) spectra were also recorded from samples of pure HCN, which were either vapor deposited or frozen from the liquid. In the INS spectra a characteristic feature appeared, which shifted from 140 cm−1 at 6 K to 120 cm−1 at 200 K. In the vapor deposited samples it is essentially broadened as compared to HCN frozen from the liquid. Evidently the vapor deposition is related to an increased degree of disorder in the solid. Apart from this observation, the INS spectra do not reflect any major influence of the preparation conditions. In contrast, the FIR spectra were drastically depending on the history of the sample. This phenomenon is discussed as a superposition of Longitudinal optical/transverse optical (LO/TO) splitting and sidebands of optically inactive phonons to the optically active transitions.
Angular and temperature dependence of depolarized and polarized Rayleigh–Brillouin spectra of poly(methylphenylsiloxane)
Authors: Li B. Y. , Wang C. H.
Published: July 2002

The angular and temperature dependence of polarized and depolarized Rayleigh–Brillouin spectra of poly(methylphenylsiloxane) is studied with a Fabry–Perot interferometer. Considerable dispersion is observed in the polarized spectrum. The dispersion is interpreted as arising from structural relaxation. In the depolarized spectrum, evolution from an overdamped spectrum to the one showing shifted side peaks due to transverse acoustic waves are observed. The frequency of the transverse acoustic wave is found to decrease with increasing temperature and becomes vanishingly small at high temperature.
Methyl torsional barriers in different electronic states. Simultaneous determination from the rotationally resolved fluorescence excitation spectrum of a large molecule
Authors: Tan X.-Q. , Majewski W. A. , Plusquellic D. F. , Pratt D. W. , Meerts W. L.
Published: July 2002

Two types of splittings have been observed in the rotationally resolved fluorescence excitation spectrum of the 000 band in the S1 ←S0 electronic transition of 1-methylnaphthalene at ∼315 nm. One type is independent of J and K−1 and is produced by small differences in the A/E methyl torsional level spacings in the two electronic states. The second type is independent of J but dependent on K−1. It has its origin in a coupling of the methyl torsion with overall molecular rotation, which produces even smaller differences in the spacings of the S0 and S1 E levels. A simultaneous fit of both types of splittings yields the threefold barrier heights in the zero-point vibrational levels of both states, V3 (S0 )=295.2 and V3 (S1 )=292.6 cm−1. The ground state value of V3 is very different from that which has been reported earlier.
Brownian motion model of activated transitions in a periodic potential
Authors: Cartling Bo
Published: July 2002

The effects of the boundary conditions on the kinetics of activated processes are investigated by means of the nonstationary solutions of the Fokker–Planck equation for a particle moving in a bistable potential and coupled to a heat bath. The transition rate constant in a first-order rate equation is determined as a function of the damping constant, which describes the strength of coupling to the heat bath, for a periodic bistable potential and compared to the corresponding results for a bounded bistable potential previously obtained in Ref. 1. The mechanisms underlying the differences observed are discussed. The dependence of the rate constant on the potential barrier height is studied for both periodic and bounded potentials. Transient processes occurring after a nonequilibrium initial state are also investigated and proposed to be useful to extract information on the potential from time-resolved experiments. The transition rates determined are compared to those observed in trajectories simulated from the Langevin equation.
Vibrational quenching of NO+(v) ions in collision with H2, D2, and O2
Authors: Viggiano A. A. , Morris R. A. , Dale F. , Paulson J. F. , Ferguson E. E.
Published: July 2002

The vibrational quenching rate constants for NO+(v), predominantly in the v=1 state, have been measured at 200 and 293 K in collisions with H2 and D2 and at 200, 293, and 458 K with O2. The rate constants are all very low, corresponding to quenching probabilities ∼10−4. The low rate constants reflect very shallow attractive potential wells. In the case of H2 and D2 this is a consequence of their low polarizabilities. In the case of O2, repulsive chemical interactions offset the electrostatic attraction to yield a shallow attractive well. This is a consequence of the singlet NO+ and triplet ground state O2 not approaching on the attractive NO+3 ground state potential surface, which is a singlet. The temperature dependences of the quenching rate constants are generally slight, indicating that the collision energies are in a range comparable to the attractive well depth and that the quenching is not strongly dominated by either the attractive forces, which would give a negative energy dependence, or by the repulsive forces which would give a positive energy dependence.
Picosecond Raman investigations of interligand electron transfer in transition metal complexes
Authors: Orman L. K. , Chang Y. J. , Anderson D. R. , Yabe T. , Xu Xiaobing , Yu Soo-Chang , Hopkins J. B.
Published: July 2002

Two color picosecond Raman spectroscopy has been utilized to measure interligand electron transfer in the excited MLCT states of mixed ligand Ru(II) complexes. No contribution to electron transfer was found from hot vibrational levels produced by photoexcitation. For thermally relaxed vibrational levels the first direct measurement of the upper limit to the rate of interligand electron hopping is reported. It was found that the rate from the thermally equilibrated vibrational states was ≤2×106 s−1. Low temperature Raman spectra indicate that the slow rate of electron transfer creates a nonstatistical energy distribution between dissimilar ligands. The implications of these results are discussed within the framework of electron transfer theory.