bluuebunny/crossref_metadata_2025 huggingface dataset:
Green function theory of charge transfer processes in solution
Authors: Newton Marshall D. , Friedman Harold L.
Published: July 2002

By using a Green function Q to characterize the linear response of a dielectric body to electric charges, we obtain a theory for the solvent dielectric contribution to relaxation along the reaction coordinate RC(t) in an electron transfer process. For an electron transfer reaction model, in which the ions are embedded in a dielectric continuum, the theory gives, at t=0, the reorganization free energy derived by Marcus in 1956. For the same model the characteristic time τQ associated with RC(t) is evaluated in terms of the dielectric function εω of the medium. How the rate constant ket for an electron transfer process depends on τQ is illustrated for both high-barrier and low-barrier cases by approximating RC(t) as a Smoluchowski process on a potential surface. Applying the theory to a molecular model (charged hard sphere ions in a dipolar hard sphere solvent), treated in the mean spherical approximation for the response at any frequency (Wolynes, 1987), indicates that the effects of the molecular structure of the solvent on τQ are large even if the ion is several times larger than a solvent molecule.
Structure of hard-particle fluids near a hard wall. III. y
w(z) for hard-sphere mixtures
Authors: Labik S. , Smith William R.
Published: July 2002

Two methods of predicting the wall–cavity concentration profiles nwi(z) for hard-sphere mixtures at a hard wall are investigated. The first, which is an extension of the procedure for the pure fluid derived in part II of this series, calculates nwi(z) by means of thermodynamic properties of the bulk fluid (for which precise expressions are available), and contact values of the wall–sphere distribution functions (for which we propose simple analytical expressions). The second method, based on scaled-particle theory, is new. In this approach, nwi(z) may be predicted using only thermodynamic properties of the bulk fluid. All predictions are tested against a new set of computer simulation data for binary hard-sphere mixtures at a hard wall. Both theories yield good accuracy at distances up to contact. We also use these theories to examine the concentration enhancement effect of each component at the wall relative to the bulk values, due to the presence of the wall in the system.
Density functional theory of freezing for hexagonal symmetry: Comparison with Landau theory
Authors: Laird Brian B. , McCoy John D. , Haymet A. D. J.
Published: July 2002

Density functional theory, studied recently by us [J. Chem. Phys. 87, 5449 (1987)] is used to study the freezing of hard disks and hard spheres into crystals with hexagonal symmetry. Two different numerical techniques are used, namely a Gaussian approximation to the crystal density and a more general Fourier expansion of the crystal density. The results from these methods are compared with each other, more approximate versions of density functional theory, and computer simulations. In addition, we compare density functional theory with Landau theories of first order transitions, in which the free energy is expanded as a power series, usually in just one order parameter. We find that traditional Landau theory has little validity when applied to the freezing transition.
Nuclear spin statistical weights for the trimers and tetramers of C2H2, C2HD, and CO2
Authors: Weber Alfons
Published: July 2002

Nuclear spin statistical weights have been calculated for the rotational levels of C2H2 trimers and tetramers for various geometries having D3h,C3h,D4h,C4h, and D2d symmetries as well as for the several possible planar trimer and tetramer forms of C2HD to assist in the interpretation of high resolution rotation–vibration spectra of these complexes. Results are also given for the trimers and tetramers of CO2. The weights obtained for the 12C2H2 trimer with C3h symmetry as well as the weights for the trimers of 12C2HD are directly applicable to the trimers of HF and H12CN, respectively. Reference is made to earlier calculations of the nuclear spin statistical weights for monomer molecules, the results of which may sometimes be transferred to van der Waals and hydrogen-bonded complexes. A discussion is presented on the role of the separable degeneracies occuring in the classification of energy levels of molecules belonging to one of the Cnh or other point groups with complex characters.
Intermultiplet and angular momentum transfers of excited sodium atoms in collisions with molecules. II. Models
Authors: Desfrancois C. , Astruc J. P. , Barbe R. , Schermann J. P.
Published: July 2002

The use of ionic intermediate models for the description of collisions involving excited alkali atoms and molecules is discussed. It is shown that the multiple curve crossing and the quasifree electron models can be expressed with the same formalism. The results of these models are compared to experimental electronic to electronic transfer cross sections in the case of oxygen and nitrogen molecules for which detailed informations upon the resonant free electron–molecule scattering data are available.
Relative efficiency of long range nonradiative energy transfer among tryptophan residues in bacteriophage T4 lysozyme
Authors: Ghosh Sanjib , Zang Li-Hsin , Maki August H.
Published: July 2002

The three tryptophan (trp) residues located at positions 126, 138, and 158 in wild-type bacteriophage T4 lysozyme have been investigated by low temperature phosphorescence and ODMR spectroscopy. Assignment of spectral properties to individual trp residues was facilitated by measurements on mutant proteins containing one or two trp→tyr (tyrosine) substitutions. It has been observed that (i) the triplet state energies (ET) of the trp residues lie in the order: ET(126)>ET(158)>ET(138) and (ii) S→S nonradiative energy transfer takes place predominantly from trp 126 to trp 158. Calculated orientation factors in Förster’s equation using crystal structure data support this result, suggesting a specific direction for the S1←S0 transition moment of trp.
Liquid crystalline order in self-assembling systems: Orientation dependence of the particle size distribution
Authors: Herzfeld Judith
Published: July 2002

Proteins and surfactants can associate reversibly into large asymmetric aggregates that can form liquid crystalline phases. A general theoretical treatment is given of such systems, allowing for spherical aggregates of arbitrary diameter and composition that can grow indefinitely in one direction to form rods, or in two directions to form disks. Assuming that interparticle interactions may be approximated by hard-core repulsions, a fine-mesh lattice model is used to derive the configurational free energy. Assuming further a simple phenomenological form for the intraparticle interactions, the equilibrium size and orientation distribution may be expressed in terms of a small number of parameters. The distribution function is a generalization of the well-known most probable distribution for linear condensation polymers in ideal solutions. The generalization allows for the unequal growth of particles in different orientations under the influence of interparticle repulsions.
ESR investigation of the cation radicals 14N2 13CO+, 15N2 12CO+, and 15N2 13CO+: The trapping of ion–neutral reaction products in neon matrices at 4 K
Authors: Knight Lon B. , Steadman J. , Miller P. K. , Cleveland J. A.
Published: July 2002

ESR results are reported for the cation radicals 15 N212 CO+ , 15 N213 CO+ and 14 N213 CO+ trapped as isolated ions in neon matrices at 4 K. The N2CO+ radical was generated by codepositing N2 and CO into a neon matrix under ionizing conditions (both photoionization at 16.8 eV, and 50 eV electron bombardment). A complete resolution of the 14 N, 15 N, and 13 C A tensors reveal that the radical is planar and nonlinear (NNC O). Electronic structure changes that occur as N+2 and CO (or CO+ with N2) combine to form N2CO+ are analyzed by comparing the nuclear hfs of the diatomic reactants with that of the product radical. The 13C hfs is extremely large with Ax =1376(1); Ay =1407(1), and Az =1403(1) MHz. The A tensor for the inner 14 N atom is: Ax,y =200.2(6) and Az =226.6(3) MHz. The outer 14 N has ‖Ax,y‖ =4(1) and Az =9.4(2) MHz. The nuclear g tensor appears axially symmetric with gx,y=2.0007(3) and gz =2.0002(3). SCF calculations also show N2 CO+ to be nonlinear and yield A values in reasonably good agreement with experiment. These ESR results for N2CO+ are compared with similar measurements for the isoelectronic ions C2O+2, N+4, and C2N−2.
Extraction and utilization of dynamically embedded information and dual Lanczos transformations in the investigation of spectral and temporal properties of globally linear dynamical systems
Authors: Wassam W. A. , Nieto-Frausto Juan , Torres-Vega Go.
Published: July 2002

The concept of extraction and utilization of dynamically embedded information in dual Lanczos transformation theory is put on a sound theoretical foundation. Also, our earlier work on dual Lanczos transformations is cast in a different form that may be readily applied to problems with an initial representation characterized by either discrete or continuous indices. This provides a more lucid and flexible interpretation of their significance, giving the theory a simpler and more coherent formal structure. We show that dual Lanczos transformations represent a particular mechanism for carrying out the process of extraction and utilization of dynamically embedded information and correspond to the transformation of the global dynamics of a system into the closed subdynamics of a dynamically invariant subspace. Although this subdynamics is not necessarily equivalent to the global dynamics, it is shown that knowledge of the subdynamics is sufficient for determining certain spectral and temporal properties of a system depending on the bias built into the dynamically invariant subspace. We report a closed set of equations for carrying out dual Lanczos transformations by means of symbolic manipulation on computers in the treatment of local and nonlocal classical models in the classical phase space representation.
Ionic rotational branching ratios in resonant enhanced multiphoton ionization of NO via the A 2Σ+(3sσ) and D 2Σ+(3pσ) states
Authors: Rudolph H. , Dixit S. N. , McKoy V. , Huo W. M.
Published: July 2002

We present the results of ab initio calculations of the ionic rotational branching ratios in NO for a (1+1) REMPI (resonant enhanced multiphoton ionization) via the A 2Σ+(3sσ) state and a (2+1) REMPI via the D 2Σ+(3pσ) state. Despite the atomic-like character of the bound 3sσ and 3pσ orbitals in these resonant states, the photoelectron continuum exhibits strong l mixing. The selection rule ΔN+l=odd (ΔN≡N+−Ni) implies that the peaks in the photoelectron spectrum corresponding to ΔN=odd (±1,±3) are sensitive to even partial waves while those corresponding to even ΔN probe the odd partial waves in the photoelectron continuum. Recent experimental high resolution photoelectron studies have shown a strong ΔN=0 peak for ionization via the A 2Σ+ and the D 2Σ+ states, indicating a dominance of odd-l partial waves. While this seems natural for ionization out of the 3sσ orbital, it is quite anomalous for 3pσ ionization. Based on extensive bound calculations, Viswanathan et al. [J. Phys. Chem. 90, 5078 (1986)] attribute this anomaly to a strong l mixing in the electronic continuum caused by the nonspherical molecular potential. We have performed ab initio calculations of the rotational branching ratios and compared them with the experimental results. The electronic continuum shows a significant p-wave component which leads to the large ΔN=0 peak in both cases. Calculations are performed for both rotationally ‘‘clean’’ and ‘‘mixed’’ branches. The relative heights of the peaks are very sensitive to the photoelectron kinetic energy for the D 2Σ+ state and less so for the A 2Σ+ state. This is a direct consequence of the l mixing in the continuum.
What is the solvent longitudinal time scale controlling electron transfer rates?
Authors: Sparpaglione Massimo , Mukamel Shaul
Published: July 2002

A theory for electron transfer rates in polar solvents is developed using an expansion of the density matrix in Liouville space and utilizing the analogy with the problem of nonlinear optical line shapes. The relevant solvent longitudinal time scale which controls the rate and the adiabaticity of electron transfer processes is precisely defined in terms of the solvent dielectric function ε(ω) for an arbitrary non-Debye solvent.
The transport properties of rod-like particles. II. Narrow slit pore
Authors: Magda Jules J. , Tirrell Matthew , Davis H. Ted
Published: July 2002

We investigate the behavior of rigid macromolecules in narrow slit pores. Molecular dynamics studies are reported for hard, infinitely thin rods between semi-infinite pore walls. The rod rotational motion exhibits an ‘‘entangled’’ regime at high concentrations similar to that observed in the bulk fluid. However the transition to the entangled regime takes place at a higher concentration in the pore. At concentrations well below the transition concentration, the pore translational diffusion coefficient exceeds the corresponding bulk diffusivity. At concentrations near or above the transition concentration, the pore diffusivity drops below the bulk diffusivity, and is only weakly dependent upon concentration. The translational motion is ‘‘Knudsen-like’’ in this regime, with the diffusion coefficient primarily determined by rod–wall collisions.
Methane in vapor deposited argon: An inelastic neutron scattering study
Authors: Prager M. , Langel W.
Published: July 2002

The inelastic neutron scattering spectrum of a sample of 1% CH4 in Ar was recorded in the energy range below 3 meV. Two strong quantum transitions at 0.90 and 0.59 meV and a broad distribution in the energy range up to 1.2 meV are found in the unannealed sample. Weaker transitions appear at 1.3, 1.7, and 2.2 meV, the latter with a shoulder at 2.3 meV. In the annealed sample (annealing temperature Ta=80 K, annealing time 1 h) the broad distribution and the line at 0.59 meV have disappeared. A new line at 0.45 meV and a shoulder at 0.77 meV merge from the broad distribution. The sharp transitions originate from CH4 molecules in well defined neighborhoods. They are attributed to molecules in an undisturbed fcc site (0.90, 1.3, and 2.2 meV), a fcc site neighboring a stacking fault (0.77 meV), an hcp site in hcp crystallites (0.59 meV), and hcp site within a single stacking fault (0.45 meV). Sites at grain boundaries, lattice distortions, etc., cause the broad background. The annealed sample still contains one stacking fault every 20 lattice planes.
Determination of the ionization threshold of azulene in hydrocarbon liquids by multiphoton ionization
Authors: Faidas H. , Christophorou L. G.
Published: July 2002

The ionization threshold of azulene is determined in a number of nonpo liquids, using a new multiphoton ionization technique. The nature of the various multiphoton ionization mechanisms are discussed.(AIP)
Reply to the ‘‘Comment on: ‘Laser and phosphor host La1−xMgAl11+xO19 (x=0.050): Crystal structure at 295 K’ ’’
Authors: Abrahams S. C. , Brandle C. D.
Published: July 2002

The comments by A. Kahn et alia (ref. 1) on the authors’ paper  Laser phosphor host.....  (ref. 2), are answered. The authors agree that the citations were incomplete.They also agree that the precise congruent melting composition is crucial to crystal growth. The analysis of the x−ray diffraction patterns by aluminium and magnesium atoms in the title material, and the determination of Mg:Al ratio is also discussed.(AIP)
Theoretical study of structures and energies of acetylene, ethylene, and vinyl radical and cation
Authors: Curtiss Larry A. , Pople John A.
Published: July 2002

A systematic study of the structures and energies of acetylene, ethylene, and vinyl radical and cation using high levels of ab initio molecular orbital theory, including correlation effects beyond fourth order perturbation theory, is presented. In agreement with previous theoretical studies, the present theory predicts that the bridged form of C2H+3 is more stable than the classical form with an energy difference of 3.1 kcal/mol (including zero point energies). The bond dissociation energy of C2H4,D0(C–H), is calculated to be 110.2 kcal/mol in agreement with the recent photoionization study of Berkowitz et al. and in significant disagreement with several previous experimental studies. Finally, triplet vinyl cation is studied for the first time and is calculated to be 54.5 kcal/mol higher in energy than the singlet ground state.
Nitric oxide photofragment rotational distribution from UV dissociation of cyclopentadienylnickel nitrosyl
Authors: Georgiou Savas , Wight Charles A.
Published: July 2002

The rotational state distribution for NO produced by dissociation of (η5–C5H5)NiNO near 225 nm has been obtained by [1+1] resonance-enhanced multiphoton ionization. The distribution is approximately characterized by a Boltzmann distribution at T=900±200 K. The relative populations of the two spin–orbit components of the ground 2Π manifold are also consistent with this temperature. No preference is observed for populating either of the lambda–doublet components, even at high rotational quantum numbers. Comparison of the results with statistical models shows that the state distribution is significantly colder than expected. One plausible explanation for this is that the parent molecule dissociates to produce cyclopentadienylnickel fragments in an excited electronic state.
Transition rates in a bistable system driven by external dichotomous noise
Authors: L’Heureux Ivan , Kapral Raymond
Published: July 2002

A study of dichotomous noise-induced transitions is presented for a simple one-dimensional system exhibiting deterministic bistability between two steady states. A phenomenological rate law for the kinetics of such transitions is derived and the corresponding rate coefficient is evaluated. Critical slowing down for such transitions is shown to be possible and an asymptotic scaling form for the rate coefficient is derived. Finally, memory effects and the breakdown of the phenomenological rate law due to the magnitude of the noise correlation time are discussed.
Anomalies in the heat-capacity signatures of submonolayer adsorbates with attractive lateral interactions
Authors: Kim Young Sik , Battaglia Franco , George Thomas F.
Published: July 2002

The analytic closed form of the heat-capacity signatures previously derived for the McQuistan–Hock (MQH) model of a lattice gas is applied to various adsorbed systems for which the lateral interaction varies from a few meV to about 300 meV. It is shown that whenever the adsorption system can be described by a two-dimensional gas on which the substrate effects are less important than the adatom–adatom interactions, the computed temperatures at which the heat-capacity signatures display their maximum are in excellent agreement with the experimental measurements.
Electric dipole moment functions from vibrational absorption intensities of fundamental and overtone transitions
Authors: Longhi G. , Zerbi G. , Ricard L. , Abbate S.
Published: July 2002

We have measured the intensities of fundamental and overtone transitions of CH stretching vibrations for a few selected paraffins, ketones, ethers, aromatic compounds, and haloforms. We have noticed some trends in the data, which we have correlated to the nature of the chemical bonds in the molecules. In order to account for both the fundamental and overtone intensity data, three different models for the CH bond electric dipole moments have been employed.
Optical–optical double resonance (stimulated emission pumping) spectroscopy of HCF
Authors: Suzuki Tetsuo , Hirota Eizi
Published: July 2002

Optical–optical double resonance or stimulated emission pumping spectroscopy has been applied to the HCF molecule to obtain precise molecular constants in excited vibrational states associated with the ground electronic state. A cw dye laser excites HCF molecules to a single rotational level of the Ã1 A″(000) state, and transitions induced by a second cw dye laser downward to rotational levels in excited vibrational states of the ground electronic state are observed as dips of the fluorescence from the Ã1 A″(000) state. The signals are observed at sub-Doppler resolution (about 0.002 cm−1 FWHM), but their wave numbers are determined only to the precision 0.003 cm−1 of the dye laser wave number measurement. Molecular constants in the (010) and (020) states are determined precisely by a conventional least-squares analysis of the observed signals. The (100) state is found to interact with the (011) state through Coriolis coupling, and the spectral data obtained for the two states are analyzed simultaneously. The ν1 band origin thus determined for the first time is unusually low, i.e., 2643.0393 (26) cm−1 with one standard deviation in parentheses.
Role of free volume on the interpretation of effective blend interaction parameters from neutron scattering data
Authors: Freed Karl F.
Published: July 2002

The liquid nature of a blend implies the existence of free volume that facilitates liquid-type freedom of molecular motion and that, as noted by Sariban and Binder in a specialized context, can lead to significant alteration in the perceived composition and molecular weight dependence of the effective Flory–Huggins interaction parameter χ for the blend. We analyze the contributions of free volume to extrapolated zero angle neutron scattering data and apply the theory to data from the isotopic blends of Bates et al. The resultant χ parameters are found to have a substantially different molecular weight and composition dependence than that obtained using the customary model of an incompressible blend.
Stark and Zeeman effects in ethylene observed by sub-Doppler infrared spectroscopy
Authors: Chen Yit-Tsong , Oka Takeshi
Published: July 2002

The recent development of sub-Doppler spectroscopy using microwave modulation sidebands on CO2 laser radiation has enabled us to perform ultrahigh resolution tunable infrared spectroscopy and study the Stark and Zeeman effects in ethylene. Clear Stark splittings were observed in low J vibration–rotation transitions of the ν7 band by applying an electric field of up to 50 kV/cm. The Stark shift is caused by the polarizability of ethylene as well as by the second order Stark effect between the accidentally degenerate ν7 and ν8 vibrational states (ν07−ν08 =8.91 cm−1). Analysis of the observed Stark patterns gives the polarizability anisotropies for the ground state to be αaa− 1/2 (αbb+αcc) =1.91(13) Å3 and αbb−αcc =0.07(19) Å3 and for the ν7 state to be αaa− 1/2 (αbb+αcc) =1.78(11) Å3 and αbb−αcc =0.15(14) Å3. The dipole moment along the a axis of ethylene induced by the ν7 and ν8 vibrations was determined to be ∂2μa/∂q7 ∂q8 =0.0791(4) D. Zeeman splittings of ethylene in a magnetic field of approximately 2.5 kG were also observed. Since the infrared radiation was right-handed circularly polarized and the axial magnetic field was along the direction of the beam, only the Δm=+1 transitions were observed. The tensor components of the rotational g factors were determined to be gaa =−0.424(8), gbb =−0.123(8), and gcc =0.037(6). A qualitative discussion on the relationship of the electronic structure of ethylene and the rotational g factors is given.
Some issues of thermodynamic consistency in binary nucleation theory
Authors: Wilemski Gerald
Published: July 2002

The ‘‘generalized’’ Kelvin equations used to determine the critical nucleus composition in some binary nucleation theories are shown to be thermodynamically inconsistent. Also, the surface tension calculated by the method of Flageollet-Daniel, Garnier, and Mirabel [J. Chem. Phys. 78, 2600 (1983)] is shown to be thermodynamically consistent with respect to the dynamic surface tension, contrary to the recent assertion of Spiegel, Zahoransky, and Wittig.
Excitation and decay of a C–H overtone attached to a linear hydrocarbon chain: A simple quantum-mechanical model
Authors: Lami Alessandro , Villani Giovanni
Published: July 2002

The decay of a C–H overtone at one end of a linear chain of identical C–C bonds is investigated. It is assumed that only the C–H and the adjacent C–C bond may be sufficiently excited to exhibit anharmonicity, while the remaining part of the chain behaves harmonically. The limit of an infinite number of C–C bonds is considered and the time decay of the C–H overtone is calculated. The results (quite unexpected on the basis of classical calculations on finite chains) show that two time scales appear in the decay, one much slower than the other. This manifests also in the coexistence of sharp and broad peaks in the absorption spectra. The possible consequences on the reactivity of the terminal part of long-chain molecules are discussed. The mechanism of optical preparation of overtones is also analyzed in some detail.
The steady state and equilibrium approximations: A geometrical picture
Authors: Fraser Simon J.
Published: July 2002

We present a geometrical picture of the steady state (SSA) and equilibrium (EA) approximations for simple chemical reactions. Geometrically, SSA and EA correspond to surfaces in the phase space of species concentrations. In this space, chemical reaction is represented by trajectory motion governed by first order differential equations. Initially, during transient decay, trajectories move quickly into the (narrow) region ℛ between the equilibrium (ℰ) and steady state (𝒮) surfaces, puncturing ℰ or 𝒮 orthogonal to a variable axis. This is rigorously what the differential equations say. Once in ℛ, trajectories approach another surface, the slow manifold ℳ, sandwiched between ℰ and 𝒮. SSA or EA is the restriction of the system motion to 𝒮 or ℰ as a ‘‘shadow’’ of its true, nearby motion along ℳ. We illustrate these ideas in relation to the Lindemann mechanism, showing how ℳ is related to 𝒮 and ℰ, and how it may be obtained by iteration, yielding higher order SSA and EA formulas. This procedure is asymptotic with an interesting error analysis. We briefly discuss the generalization of these ideas.
A theoretical investigation of 2Σ+u resonance states of H−2
Authors: DeRose Eugene F. , Gislason Eric A. , Sabelli Nora H. , Sluis Kathleen M.
Published: July 2002

We have applied a new method for computing properties of molecular negative ion resonances to calculate the potential curves of the first three 2Σ+u states of H−2. The energy widths and lifetimes of the resonances are also calculated. The first and third resonances correlate to H+H− asymptotes at R=∞, but the second resonance disappears near R=6.0 a.u. The resonance properties are shown to be insensitive to variations in the basis set. The results are compared to recent calculations on this system.
The holographic investigation of diffusion of methyl red in poly(caprolactone) diol
Authors: Xia J. L. , Wang C. H.
Published: July 2002

The translational diffusion process of methyl red dissolved in poly(caprolactone) diol has been investigated as a function of temperature, using the laser induced holographic grating relaxation technique. The tracer diffusion coefficients of methyl red in trans and cis forms are found to follow the WLF type of temperature dependence with the WLF coefficients identical to that associated with the shear modulus of the polymer host. The cis isomer of methyl red although about 8% smaller in size diffuses about three times slower than the trans isomer. This is interpreted as due to a stronger interaction between the cis isomer and the polymer host.
A distributive mechanism for two-photon mean-frequency absorption
Authors: Andrews David L. , Hopkins Kevin P.
Published: July 2002

This paper describes a new mechanism for the concerted mean-frequency absorption process A+A+ℏω1+ℏω2→A*+A*. In contrast to a mechanism described previously, this effect can be subject to the normal selection rules for single-photon absorption, and is mediated by virtual photon coupling. Rate equations are given for molecules in a fluid, van der Waals molecules, and chromophore pairs in polyatomic molecules. The dependence of the absorption rate on the separation of the interacting pair is then examined in detail. An analogy is drawn between the long-range limit of the absorption rate and a process of hyper-Raman scattering followed by single-photon absorption. It is demonstrated that the two processes are equivalent where the molecular separation is large enough that the virtual photon can be considered as real and physically identifiable.
Absolute intensities of H2CO in the 9–11 μm region
Authors: Dang-Nhu M. , Deroche J.-C. , Allegrini M.
Published: July 2002

Line intensities of H2CO are analyzed between 1148.4 and 1192.8 cm−1 by taking into account the Coriolis interaction involving the ν4 and ν6 fundamentals. Finally two intensity parameters can be significantly determined: ∂μy/∂q4=0.0592±0.0018 D and ∂μx/∂q6=0.0772±0.0019 D. From these results, the ‘‘deperturbed’’ band strenghts can be calculated: S04=20.9±1.3 cm‘u−2 atm−1 and S06=38.0±1.9 cm−2 atm−1 at T=297 K.
Application of adiabatic switching to vibrational energies of three-dimensional HCO, H2O, and H2CO
Authors: Sun Qiyan , Bowman Joel M. , Gazdy Bela
Published: July 2002

Semiclassical vibrational energies are calculated for nonrotating, three-dimensional HCO, H2O, and H2CO by the adiabatic switching method. For HCO and H2O the Hamiltonian is given in a body-fixed frame in terms of Jacobi coordinates. Several zero-order Hamiltonians for adiabatic switching to the full Hamiltonian are considered. The simple, ‘‘obvious’’ choices do not yield stable results; however, a nonseparable zero-order Hamiltonian, which can be transformed to a simple normal form, does give stable results. Semiclassical quantization is done with the transformed zero-order Hamiltonian, initial conditions are obtained in terms of the Jacobi coordinates, and adiabatic switching is done in these coordinates. The Watson Hamiltonian is used for H2CO in a straightforward fashion. In all cases, the semiclassical energies are in good agreement with quantum mechanical results, and for H2O in excellent agreement with other semiclassical results.
Influence of the autoionization in the calculation of the fluorescence polarization for the A 2Πu –X 2Πg transition in O+2
Authors: Lefebvre-Brion H.
Published: July 2002

An ab initio calculation of the fluorescence polarization including autoionization is made in the case of O+2, using the multichannel quantum defect approach. If only the states belonging to Rydberg series converging to the b 4∑−g state of O+2 are included, due to their narrow ab initio calculated autoionization widths, the values of the polarization of the A 2Πu −X 2Πg fluorescence is found to be negative in the 73–69 nm wavelength region, in contradiction with recent experiments [Keller et al., J. Chem. Phys. 87, 3299 (1987)]. Good agreement with experimental results is obtained only in taking into account the influence of the 3∑−u Rydberg state (B 2∑−g) 4pσu, which enlarges the autoionized peaks either by perturbation or by predissociation. There results are compared to the qualitative theoretical interpretation of Keller et al. in terms of two 3∑−u valence states.
Preexisting traps for electrons in polar liquids. Statistical distributions of trap energy and structure
Authors: Hilczer Maria , Bartczak Witold M. , Kroh Jerzy
Published: July 2002

The statistical distributions of the electron energy level and the structural parameters of the preexisting traps in polar liquids have been investigated. The Monte Carlo calculations were performed for the TIPS intermolecular potentials with the parameters corresponding to the simple alcohols, methanol and ethanol, in liquid phase. The distributions of the trapping level obtained for the TIPS potential seem to be better correlated with the data provided by experiments than the distributions obtained for the point–dipole model of alcohol molecules.
Prediction of fluid–fluid and fluid–solid equilibria in the molecular system helium–hydrogen up to 1 Mbar
Authors: van den Bergh Luc C. , Schouten Jan A.
Published: July 2002

By using variational theory together with an extended van der Waals one-fluid model, as has been developed by Ree, fluid–fluid coexistence surfaces of He–H2 mixtures are calculated up to 1 Mbar. The exponential-six potential parameters of the system are chosen such that good agreement is obtained with experimental data up to 75 kbar and 300 K. Moreover, using these parameters, fluid–solid equilibria and the three-phase line are calculated assuming that the hard-sphere packing fraction of the fluid along the freezing surface is a linear function of composition. The calculated triple point composition along the three-phase line agrees with experiment up to the highest pressure.
Dynamics of photofragmentation of dimethylnitrosamine from its first two excited singlet states
Authors: Lavi R. , Rosenwaks S.
Published: July 2002

The photofragmentation of dimethylintrosamine (DMN) from its first two excited singlet states was studied by monitoring the scalar and vector properties of the nascent NO via one-photon laser induced fluorescence, combined with polarization and sub-Doppler spectroscopy. The DMN was fragmented following irradiation at 363.5 nm [S1←S0(π*←n)] and 250 nm [S2←S0(π*←π)]. The photofragmentation is characteristic of a direct dissociation mechanism on a repulsive potential surface for both dissociation wavelengths. The NO fragment ejects with its velocity along the bond that breaks, and its angular momentum vector tends to be perpendicular to the plane of the C2NNO frame of the parent molecule. The experiments corroborate that the transition dipole moment is perpendicular to the plane of the parent molecule for the S1←S0 transition and lies parallel to this plane, along the bond which breaks, for the S2←S0 transition. The Λ-doublet population ratio obtained for the two dissociation wavelengths is consistent with an A″ symmetry for the S1 and an A′ symmetry for the S2 state. Finally, a comparison between the photodissociation of tert-butyl nitrite (TBN) and DMN is presented. In particular, it is shown that in both molecules, for both states, the fragmentation is largely planar with the main forces acting approximately along the bond which breaks. On the other hand, retainment of vibrational energy in the NO fragment is observed only for TBN S1. Also, a preference of the antisymmetric Λ component in NO from TBN S2 and of the symmetric component in DMN S2 is found.
The fluorescence lifetime of excited cyclohexane in an argon matrix and in solid state cyclohexane at low temperature
Authors: Preses J. M.
Published: July 2002

Electronic fluorescence has been observed from cyclohexane isolated in an Ar matrix in the temperature range 16–30 K. Excitation spectra have been obtained, and the behavior of the fluorescence lifetime has been determined as a function of excitation wavelength. The variation of the fluorescence lifetime as a function of excitation wavelength in the matrix resembles that seen in the gas phase. The fluorescence of neat, solid cyclohexane has also been observed at low temperature, and its decay rate can be used to refine the parameters obtained near room temperature which describe the decay rate. The fluorescence lifetime (τ) of excited cyclohexane can be described by an expression of the form 1/τ=[4.1×108+8.8×1011 exp(−4100/1.987 T)] s−1.
A kinetic theory of collision-induced depolarized light scattering
Authors: Madden Paul , Evans Glenn T.
Published: July 2002

The collision-induced depolarized light scattering spectrum arising from the dipole–induced dipole (DID) interactions in atomic fluids has been calculated using kinetic theory methods. The theory contains no adjustable parameters and requires only the temperature, density, and collision diameter as input. Above 25 cm−1, the calculated line shape was found to be in excellent agreement with the spectrum obtained from a hard-sphere molecular dynamics simulation, with the same model for the interaction-induced polarizability. The domain of applicability of the approximations used in reducing the theory to a tractable form was carefully evaluated. The methods developed should be applicable to other intermolecular correlation functions, such as those observed in vibrational relaxation and interaction-induced infrared spectroscopy.
Relaxation cross sections for the rotational angular momentum vector in CF4
Authors: Jameson Cynthia J. , Jameson A. Keith
Published: July 2002

Nuclear spin–lattice relaxation times (T1) have been measured for 19F in the CF4 molecule in pure CF4 gas and in Ar, Kr, Xe, N2, CO, HCl, CO2, CH4, SF6, and C2H6, from which the following relaxation cross sections for the rotational angular momentum vector in CF4 have been obtained: σJ/Å2=19.2±0.7 (CF4–Ar), 29.2±0.8 (CF4–Kr), 34.3±0.8 (CF4–Xe), 12.7±0.2 (CF4–N2), 12.8±0.6 (CF4–CO), 22.0±0.1 (CF4–HCl), 29.7±0.7 (CF4–CO2), 12.2±0.2 (CF4–CH4), 39.4±0.8 (CF4–CF4), 58.0±1.2 (CF4–SF6), and 20.8±0.7 (CF4–C2H6). The temperature dependence of these cross sections in the range 210–400 K is essentially T−1 except for (CF4–Ar) and (CF4–CO, N2) for which it is T−0.5 and T−0.7, respectively.
The electronic structure of a liquid of interacting hydrogenic atoms: A prototype for expanded liquid metals
Authors: Xu Bing-Chang , Stratt Richard M.
Published: July 2002

The electronic structure of a metal heated above its critical temperature (an ‘‘expanded liquid metal’’) shows dramatic changes as the density is increased. There is some experimental evidence that as one proceeds from insulating to metallic behavior the substance can even go through two separate phase transitions: from an ordinary insulator to a so-called excitonic insulator, and then from an excitonic insulator to a metal. In an effort to study how the metallic phase is approached, we have used discretized-path-integral methods to look at the statistical mechanics and the electronic structure of a model liquid. In the gas phase, the atoms in the model have a single valence electron constrained to occupy one of the s or p orbitals of the valence shell, but at higher densities, the orbitals hybridize, leading to instantaneous dipole–dipole interaction. We show that formulating this electronic structure in terms of occupation numbers allows us to monitor the hybridization evolution via an imaginary-time correlation function—which we calculate from an analytical solution to the mean-spherical approximation for the model. The numerical results strongly suggest that the model has a sudden hybridization transition, suggesting, in turn, that it might be profitable to think of the excitonic insulator transition in this language.
Stochastic simulation of the Boltzmann equation for the Lorentz gas
Authors: Palmer Bruce J. , Keyes Thomas
Published: July 2002

We propose a stochastic simulation for solving the Boltzmann equation for the Lorentz gas and develop a formalism to rigorously show the equivalence between the simulation and the Boltzmann equation. We also propose several simple simulations which offer a substantial improvement over the Boltzmann equation and discuss the simulations in the context of repeated ring equations.
Two-photon ionization cross section calculation for H2O
Authors: Cacelli I. , Carravetta V. , Moccia R. , Rizzo A.
Published: July 2002

The two-photon first ionization cross section was computed for H2O in the vertical transition approximation. A Stieltjes imaging technique was used to obtain from the results of a L2 basis set calculation, the two-photon oscillator strength density in the continuum. The calculations were performed in the independent particle static-exchange approximation for the final states. The ground-intermediate state transition matrix elements and excitation energies were computed by adopting the single channel static-exchange approximation or the multichannel random phase approximation; in the last case the intermediate-final state transition matrix elements were obtained by the equation of motion method. The calculated cross sections refer to the case of single beam of linearly or circularly polarized photons.
The effect of random fiber orientation on Knudsen permeabilities
Authors: Faley Timothy L. , Strieder William
Published: July 2002

Variational upper bound calculations of the permeability to Knudsen flow are performed for several beds of overlapping, long fibers. When the fiber axes are oriented at random, perpendicular to the direction of the net flow, the permeability factor 0.7489 is slightly smaller than the corresponding factor 0.7630 for fiber axes mutually parallel. For fibers with axes oriented at random in three dimensions, the well-known result 12/13 of Derjaguin is obtained for fiber beds.
Dynamics of the condensation reactions of C+ with C2H4 and C2H2
Authors: Sonnenfroh D. M. , Farrar J. M.
Published: July 2002

The condensation reactions of C+ with C2H4 and C2H2 yielding new carbon–carbon bonds have been studied with crossed beam methods in the collision energy range from 0.5 to 1.5 eV. The data show that the reactions take place through short-lived collision complexes living a fraction of a rotational period. These results are shown to be consistent with schematic potential energy surfaces constructed from heats of formation and molecular structure calculations indicating that the large exothermicities of the reactions in comparison with the stabilities of intermediate C3H+4 and C3H+2 complexes should yield lifetimes in the 10−13 s regime. The data for C3H+2 formation from the reaction of C+ with C2H4 suggest two distinct production channels. These results are consistent with photoion-photoelectron coincidence breakdown curves for C3H+2 formation in the dissociative ionization of C3H4 isomers. The least exothermic reaction, C3H+ from reaction with ethylene, shows behavior in closest agreement with statistical predictions, specifically product kinetic energy distributions that scale with the total energy accessible to the products. This result is also in qualitative agreement with the nature of the potential energy surface mediating the reaction and lifetimes expected from the reaction energetics.
Solid-state ‘‘magic-angle’’ sample-spinning nuclear magnetic resonance spectroscopic study of group III–V (13–15) semiconductors
Authors: Han Oc Hee , Timken Hye Kyung C. , Oldfield Eric
Published: July 2002

We have obtained 27 Al, 69 Ga, 71 Ga, 113 In, and 115 In static and ‘‘magic-angle’’ sample-spinning (MASS) solid-state nuclear magnetic resonance (NMR) spectra of a series of polycrystalline III–V semiconductors (AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, and InSb) at magnetic field strengths of 8.45 and 11.7 T. Line-broadening mechanisms have been identified by comparing static and MASS NMR results. By applying the MASS technique, dipolar, first-order quadrupolar, and pseudodipolar interactions are removed. Thus, the MASS spectral linewidth (of the central, 1/2↔−1/2, transition) is dominated by exchange and/or second-order quadrupolar interactions. For compounds having the cubic zinc blende structure, the exchange interaction dominates, and exchange interaction constants can be determined. For AlP, AlAs, AlSb, GaP, GaAs, InP, and InAs, first-order quadrupole effects are evident as spinning sidebands (SSBs), due to the satellite transitions. These effects are due to a small distribution of electric field gradients caused by lattice defects, and result in an overall Lorentzian profile for the SSB envelope. For compounds having the hexagonal wurtzite structure (GaN and AlN), the second-order quadrupolar interaction is the main line-broadening mechanism, and we find isotropic chemical shifts δi of 333 ppm [from 1 M Ga(NO3 )3 ] for GaN, and of 115 ppm [from 1M Al(H2 O)6 Cl3 ] for AlN, and nuclear quadrupole coupling constant e2 qQ/h of 2.8 MHz for 69 GaN, 1.7 MHz for 71 GaN, and of 2.2 MHz for AlN.
Structure factor distortion for hard-sphere dispersions subjected to weak shear flow: Small-angle neutron scattering in the flow–vorticity plane
Authors: Johnson Shirley J. , de Kruif C. G. , May Roland P.
Published: July 2002

Small angle neutron scattering (SANS) was used as a probe of the structure factor distortion for concentrated hard-sphere dispersions subjected to steady shear flow. The dispersions consisted of stearically stabilized silica spheres of 40 nm radius suspended in cyclohexane, with volume fractions 0.36<φ<0.52. We subjected the dispersions to weak shear flow (Peclet number Pe<1) in a Couette flow cell, and collected intensity measurements in the flow–vorticity plane. We conducted the experiments using the D11 spectrometer at the Institut Laue-Langevin in Grenoble, France. The data permit a thorough evaluation of the existing theories for the shear-induced distortion of the structure factor. We find that the theories as they stand severely underpredict the distortion at low wave vectors (‖k‖<0.1 nm−1), but the theories can be made to qualitatively agree with the data by adjusting Pe in the equations to a higher value. The theories are based on a perturbation expansion that is linear in small Pe, but such an expansion is inadequate for describing our data. We show that the nonequilibrium contribution to the structure factor is proportional to fractional powers of Pe, with the exponent depending on volume fraction φ. The failure of the theories is probably due to their neglect of hydrodynamic interactions, which are important in concentrated dispersions. The distortion of the structure factor is greatest around its first maximum, with significant distortion being exhibited in both the flow and vorticity directions. The peak in the structure factor broadens, indicating that weak shear flow produces a more disordered structure relative to the equilibrium structure. The data reveal that the shear flow results in a decreased concentration of particles in the flow direction. The concentration of particles in the vorticity direction increases when shear flow is applied, and the formation of temporal clusters is probable.
Reaction dynamics on a fluctuating potential
Authors: Rodger P. Mark , Sceats Mark G.
Published: July 2002

The steady state rate constant for passage over a fluctuating potential barrier is derived from the Fokker–Planck equation using the stable states picture. The time scale of fluctuations in barrier height and position do not influence the reactive frequency; however, the reactive frequency does depend upon the time scale of fluctuations in the barrier curvature. If these are fast relative to [ω2m+(ξ/2)2]1/2, where ωm is the mean barrier frequency and ξ is the friction for motion along the reaction coordinate, then the crossing takes place on the potential of mean force. If the barrier fluctuations are slow on the same time scale, then the rate is an average over that for the distribution of barriers. In both of these limits the form of the Kramers solutions is recovered, whereas in the intermediate case, the rate also depends explicitly on the relaxation time scale of the fluctuations. A physically reasonable model for the decay of fluctuations in the potential is proposed and used to obtain an analytic expression for the rate that covers the range between these two limits.
Study on YAG:Nd3+ spectra. II. Fluorescence spectra
Authors: Song Zeng-fu , Hua Dao-hong , Wang Shu-kun , Gui You-xi
Published: July 2002

Several fluorescence line groups in the infrared, visible, and ultraviolet regions are observed by exciting YAG:Nd3+ crystal with Ar+ laser (5145 Å) and x-ray at liquid nitrogen temperature (LNT) and room temperature (RT), respectively. All of these line groups are assigned properly. Some new Stark levels are determined from the fluorescence spectra. The excited state absorption process is observed.