bluuebunny/crossref_metadata_2025 huggingface dataset:
Analytic solution of relaxation in a system with exponential transition probabilities. IV. Decay at early times
Authors: Forst Wendell , Xu Guo-Ying
Published: July 2002

Analytic solution of the master equation using the exponential transition probability has been obtained previously in part III [J. Chem. Phys. 80, 2504 (1984)] in the form of an infinite series eigenfunction expansion for c(x,t), the population distribution. While the number of terms that effectively contribute to the sum is only one at equilibrium, it increases to infinity at time zero. Thus such eigenfunction expansion is not useful for describing the bulk properties [i.e., averages over c(x,t)] of the relaxing system at early times. It is nevertheless possible to solve the relaxation problem at early times by noting that the final (postcollision) energy distribution resulting from the nth collision is in fact the initial energy distribution for the next [(n+1)th] collision. It is shown that in this way simple analytical expressions can be obtained for various bulk properties of the relaxing system from the first collision onward—but not all the way to equilibrium—if the initial (at time zero) energy distribution is a delta function. It turns out that for the first several hundred collisions or so the (bulk-) average energy 〈〈y〉〉 decays linearly with time, and as a result the average energy transferred per collision is an energy-independent constant. The relaxation time decreases linearly with time and after only a few collisions c(x,t) becomes a Gaussian. The limitations of this approach are noted and discussed.
Ionization and electron thermalization distances in isomeric pentanes: Effects of molecular shape and density
Authors: György István , Freeman Gordon R.
Published: July 2002

Free ion yields were measured in n-, iso-, and neo-pentane at densities d=1–600 kg/m3 along the vapor–liquid coexistence curve. The total yield of ionization Gtot is 4.3 ± 0.1 in the gases at d≤100 kg/m3. The free ion yield extrapolated to zero electric field strength G0fi decreases continuously with increasing density in n-pentane, whereas in iso- and neo-pentane it goes through a maximum at d≈230 and 350 kg/m3, respectively. The density-normalized thermalization distances bGPd, go through a maximum in neo-pentane and iso-pentane in the low-density liquids, while in n-pentane they remain relatively constant (∼4×10−6 kg/m2). The values of bGPd correlate with the density normalized mobilities nμ of hot electrons (at high field strengths) in both the gas and liquid phases of the isomeric pentanes. The effect of molecular shape (sphericity) on electron scattering is much greater in the liquid than in the gas phase, and much greater at thermal than at epithermal electron energies. The direction of the shape (sphericity) effect reverses on going from the gas to the liquid phase.
The calculation of single inclusion probability functions
Authors: Gillette Gregory J. , McCoy John J.
Published: July 2002

The single inclusion probabilities for circular and infinite slit inclusion shapes are calculated explicitly, using geometrical methods which stem from a new procedure for obtaining multipoint correlation functions for two-phase suspension materials. A numerical method is then developed for treating more general inclusion shapes; as an example the numerical results for elliptical inclusions are presented. The numerical approach renders computational the determination of multipoint correlation functions for general two-phase suspensions.
Proton and carbon-13 relaxation and molecular motion in glassy bisphenol-A polycarbonate
Authors: Connolly John J. , Inglefield Paul T. , Jones Alan Anthony
Published: July 2002

An interpretation of proton and carbon-13 spin–lattice relaxation in glassy polycarbonate is developed which is consistent with the geometry, time scale, and amplitude determined from chemical shift anisotropy line shape collapse. The line shape data indicate π flips and libration about the same axis as the predominant motions. A correlation function incorporating these motions is developed to quantitatively interpret the proton spin–lattice relaxation data and the line shape collapse. The π flip process is described as an inhomogeneous distribution of correlation times using the Williams–Watts fractional exponential. An apparent activation energy of 46 kJ/mol is determined with the fractional exponent remaining constant at 0.15. The librational motion is described by the Gronski formalism where the amplitude increases with the square root of temperature; and the rotational diffusion constant, linearly with temperature. Rotational diffusion constants fall in the range of 108 to 109 s−1 which is comparable to those observed in solution in sterically hindered polycarbonates. The librational motion only contributes to spin–lattice relaxation at the higher temperatures so that only an order of magnitude estimate of the restricted rotational diffusion constant results. This correlation function is then applied to carbon-13 T1 data taken at various positions across the chemical shift anisotropy line shape on an isotopically enriched system. Little change in spin–lattice relaxation with position is observed which is consistent with the broad distribution of π flip correlation times. The rate of carbon-13 spin–lattice relaxation is also fairly well predicted. Comparisons are made with magic angle sample spinning spin–lattice relaxation both in the laboratory and rotating frame. The former is fairly well approximated by the correlation function while the latter requires a significant spin–spin contribution to be reconciled with the rest of the interpretation.
Cooperative lengths and elastic constants in lipid bilayers: The chlorophyll a/dimyristoyllecithin system
Authors: Watnick Paula I. , Dea Phoebe , Nayeem Akbar , Chan Sunney I.
Published: July 2002

In order to study the slow, cooperative motional modes induced by the addition of chlorophyll a (chl a) to dimyristoyllecithin (DML) bilayers, the T1, T1ρ, and T2 deuterium relaxation rates in 1,2-dimyristoyl(9,9′,10,10′-d4) phosphatidyl choline have been measured as a function of increasing chl a content. These data have been interpreted in terms of a model of fluctuating order directors. A fit of the T1ρ and T2 relaxation rates using this model reveal a trend of decreasing elastic constants and cooperative lengths as chl a is added.
Quasiclassical theory of two and three body collisions in rare gases, with application to krypton excimer formation
Authors: Janssens H. , Vanmarcke M. , Desoppere E. , Bouciqué R. , Wieme W.
Published: July 2002

A quasiclassical model for two and three body collisions in excited rare gases under atomic excitation is proposed. Process rates are calculated for collision induced radiation, excimer formation, and atomic excitation transfer processes in kyrpton. The analysis shows that three body collision processes are to be interpreted as two successive two body collisions in a reversible two step decay system. Conditions allowing the reduction of the two step decay system into an overall one step process are determined. Examples of such three body processes with quadratic pressure dependence are excimer formation and interatomic excitation transfer. Decay rates are derived from the calculated process rates and loss rates of the Kr*(3P2,1) excited states. These decay rates are compared with results from our own alpha excitation experiments and with literature values. Taking into account the diffusion and resonance radiation processes, but neglecting the quenching processes, the resulting krypton loss rates are: ν1(3P2)=35.3/Λ2p+15p+23.2p2 and ν2(3P1)=1.7×105+2050p+82.0p2 (p in mbar).
Ion-induced desorption of (H2O)nH+ ion clusters
Authors: Christiansen J. W. , Tsong I. S. T. , Lin S. H.
Published: July 2002

The mass distribution of (H2O)nH+ ion clusters desorbed from ice at 78 K was measured for 1≤n≤40. The desorption was induced by bombardment of the ice by three different primary ions, Ne+, Ar+, and Kr+ at 6 keV. The mass distribution can be described by RRK theory which fits the relative ion yields Yn/Y1, i.e., the ratio of the yield of the nth cluster to that of the first cluster, as a function of n. The energy deposited per normal mode required for desorption caused by each bombarding ion species is calculated from our theoretical fits to the experimental data. This is then compared with the energy deposited per mode estimated from the energy loss due to nuclear stopping of the bombarding ion in the ice. Reasonable agreement between the two is obtained. The existence of the ‘‘magic number’’ cluster ion (H2O)4H+ was observed in each of the three bombardment schemes.
A novel representation of the dense-fluid pressure equation
Authors: Phillies George D. J.
Published: July 2002

A new nonanalytic representation for the pressure of the nonideal gas is proposed, namely PV/NKBT=exp[a(N/V)v], where PV=NKBT is the ideal gas law and v and a are scaling coefficients. The form agrees well (often to within reported sampling errors) with Monte Carlo calculations on hard disks, hard spheres, and soft spheres. For hard-core bodies, v=3d/4 to good accuracy, d being the system’s dimensionality.
Exactly solvable long-range three-body interactions
Authors: Koga Toshikatsu , Uji-ie Mitsuru
Published: July 2002

Using the accurate first order perturbation wave functions for the two-body H(1s)–H(1s) and H(1s)–H+ van der Waals interactions, we derive exact leading terms of the nonadditive three-body long-range interactions in the H(1s)–H+–H+ and H(1s)–H(1s)–H+ systems. The nonadditive contribution is given by a product of geometrical factor and interaction constant as is the case of the H(1s)–H(1s)–H(1s) system examined previously, but the present interaction constants are found to be fractional numbers rigorously.
Crystal structure of polydiacethylene TCDU revisited
Authors: Kobayashi Akiko , Kobayashi Hayao , Tokura Yoshinori , Kanetake Tatsuo , Koda Takao
Published: July 2002

The crystal structure of the as-prepared form of poly(5,7-dodecadiyne-1,12-diyl-bis-phenylurethane), customarily called TCDU, has been reexamined by an x-ray analysis. Bond lengths of the conjugated polymer backbone chain presently determined indicate that the structure of the as-prepared form of TCDU crystal is more close to that predicted for the acetylenic structure than the hitherto presumed butatrienic one.
A rotationally resolved LIF study of the N+2 products of the thermal energy Penning ionization reaction: Ne*(3P2)+N2
Authors: Sonnenfroh David M. , Leone Stephen R.
Published: July 2002

Multiple temperature rotational distributions are observed in the N+2 products of the Penning ionization of N2 by Ne*(3P2). The rotational distributions for N=2–21 are well fit by a two-component Boltzmann distribution having a low temperature of 50 K±10 K and a high temperature ranging from 220 to 490 K±50 K. The relative contribution of each component varies smoothly with vibrational level from 62% Tlow:38% Thigh for v=0 to 43% Tlow:57% Thigh for v=4(±5%). We interpret the observed multiple rotational distributions as evidence for exit channel effects arising from details of the impulsive interaction in the electron transfer process. Since the overall degree of rotational excitation is small, observation of the multiple distributions requires suppression of the reactant initial rotation by using a supersonic molecular beam.
A dissociative electron attachment, electron transmission, and electron energy-loss study of the temporary negative ion of acetylene
Authors: Dressler R. , Allan M.
Published: July 2002

The three title electron-impact techniques are used to study the ground and excited states of acetylene negative ion and their decay processes. The π* resonance at 2.6 eV predissociates into C2H− and H. Four narrow resonances are observed in the transmission spectrum in the 7.5–9.5 eV region and assigned to Feshbach and core-excited shape resonances with double occupation of Rydberg orbitals and ground state positive ion core. These four resonances decay into low-lying Rydberg states of neutral acetylene, the first two undergo quasiresonant autodetachment ejecting low energy (<0.7 eV) electrons, and the lowest also dissociates to yield C−2. One additional resonance is observed in the C−2 yield, which is not observable in other decay channels. The shapes of the dissociative attachment bands differ qualitatively from the band shapes of the parent and grandparent states, indicating either a strong v dependence of the dissociation rate or the admixture of σ* orbitals in some of the dissociating anion states.
Picosecond Raman studies of dynamic charge localization following metal to ligand charge transfer excitation in tris (2,2′-bipyridyl) ruthenium (II)
Authors: Chang Y. J. , Orman L. K. , Anderson D. R. , Yabe T. , Hopkins J. B.
Published: July 2002

Direct measurements of dynamic charge localization in tris (2,2′-bipyridyl) ruthenium (II) have been performed in fluid solution for the first time. Using picosecond Raman spectroscopy it has been determined that charge localization occurs on a time scale of ≪30 ps for low-viscosity solvents. In high-viscosity solvents such as glycerol at room temperature, the charge localized species Ru(bpy)2(bpy)− appears on a time scale of 100 ps. It is clear from these data that solvent reorganization is one of the dominant mechanisms in electron trapping.
On self-diffusion in a water-in-oil microemulsion
Authors: Genz U. , Dhont J. K. G. , Klein R.
Published: July 2002

The self-diffusion coefficient Ds is calculated to first order in volume fraction for a water-in-oil microemulsion model. Assuming spherical droplets, the interaction consists of hard-core repulsions, an attractive square-well potential due to overlapping surfactant tails, and hydrodynamic interactions. The latter are described by high-order series expansions outside the range of the attractive potential, whereas simplifying assumptions are made inside the range of overlapping. Within this model, expressions for Ds are derived in terms of the depth V0 of the attractive potential, the ratio of the range of overlapping to the radius of the hard cores, and a parameter describing the hydrodynamic interaction. Assuming a linear relation between V0 and the droplet radius the initial slope of Ds vs the volume fraction becomes more negative when V0 increases. A similar behavior is found if V0 increases at constant droplet radius. These results are compared to those obtained without hydrodynamic interactions.
Transient behavior of liquid crystalline polymers
Authors: Honerkamp J. , Seitz R.
Published: July 2002

We study the stochastic differential equation for the motion of rod-like polymers in the isotropic and liquid crystalline phase for simple shear flow and elongational flow. We find a simple interpretation of the approximation given by Doi and we are able to treat the exact equations numerically. Comparing the results with the Doi approximation we find qualitative agreement.
Collisional energy flow in weakly bound complexes
Authors: Shin H. K.
Published: July 2002

Collisional energy transfer dynamics in weakly bound complexes is studied at collision energies above and below the fragmentation threshold in classical mechanics. The collinear collision of a linear complex C⋅⋅⋅B–A, where C is an atom or a diatomic molecule, with an incident atom is considered. For the collision energy range 0.01–10 eV, energy transfer to the complex is efficient, and essentially all of the energy transferred to the complex is localized in the weak bond. The energy initially stored in the molecular bond remains in it when the collision is over, and the bond gains essentially no energy from translation. The collision model is formulated such that the incident atom hits the outer atom A of the molecular unit B–A, so the efficient energy buildup in the weak bond indicates an easy flow of energy across the stiff molecular bond from the initial impact region. Energy flows from the molecular bond to the weak bond through a sequence of energy gain–loss steps on a subpicosecond time scale. A brief discussion of the collision of a diatom–diatom complex with an atom is presented to show the effects of vibration of the second molecular unit on energy flow.
Relaxation rates in chaotic and quasiperiodic systems
Authors: Dumont Randall S. , Brumer Paul
Published: July 2002

Relaxation times of arbitrary initial distributions in conservative systems are considered in both quasiperiodic and chaotic systems as modeled by simple two-dimensional maps. Two meaningful characteristic times are introduced: τF, a Fourier space estimate of the longest relaxation time, and τP, a phase space estimate of the time of certain passage. These times, analytically given in terms of the properties of the initial distribution and the system Lyapunov exponents, are shown to be in good agreement with computational results. Extensions to multidimensional systems, cross correlation functions, and continuous time systems are described.
New ionic contributions to the vapor pressure of urania
Authors: Magill J. , Hyland G. J. , Rand M. H.
Published: July 2002

The gas phase equilibrium reaction between uranium dioxide and uranium trioxide is studied.The implications of the high electron affinity of the uranium trioxide molecule on the total equilibrium vapor pressure and on the congruently vaporizing composition of urania are discussed.(AIP)
Traversal time for tunneling: Local aspects
Authors: Kotler Zvi , Nitzan Abraham
Published: July 2002

The relationship between inelastic tunneling processes and the traversal time for tunneling is studied with emphasis on the local aspects of the tunneling time. Viewed in this framework, the local tunneling time is shown to be a dominant factor in determining the inelastic tunneling probability. It is shown that the Buttiker–Landauer semiclassical formalism, when generalized to the case of local interactions and applied to the calculation of inelastic tunneling probabilities, gives results identical to other perturbation theory calculations such as the Bardeen formula. Analytical results derived for square potential barrier are shown to hold also for strongly biased barriers. Application to inelastic tunneling in typical scanning tunneling microscope configuration are discussed.
Bonding and reactivity of acetonitrile on W(100)-(5×1)-C
Authors: Friend C. M. , Serafin J. G.
Published: July 2002

The structures of absorbed acetonitrile and the surface intermediate resulting from the formation of an N–H bond and the cleavage of one C–H bond on W(100)-(5×1)-C under ultrahigh vacuum conditions have been investigated using high resolution electron energy loss and x-ray photoelectron spectroscopies. At temperatures below 200 K, a weakly bound molecular state and a species with a strongly perturbed C–N bond are present on the surface. The molecular state coordinates to a single tungsten atom via the nitrile nitrogen and undergoes competing desorption and reaction below 400 K. Acetonitrile is hydrogenated at the nitrile nitrogen and dehydrogenated at the methyl carbon to form a CH2CNH surface intermediate in the temperature range of 200–400 K in the competing reaction. The C–N stretching frequency for the RCNH intermediate is approximately 1400 cm−1 corresponding to a weakening of the C–N bond proposed to result from bonding of both the nitrile carbon and nitrogen to the surface. The RCNH intermediate reacts further above 400 K. Gaseous acetonitrile is reformed in the range of 400–600 K with competing irreversible decomposition yielding gaseous H2, HNC( HCN) and surface carbon and nitrogen. The acetonitrile formation is proposed to result from an intramolecular isomerization of the CH2CNH on the surface. Only atomic carbon and nitrogen are present on the surface after heating to 700 K.
Theory of the phonon perturbed photon echo experiment: Direct determination of electronic excitation–phonon coupling
Authors: Wilson William L. , Wäckerle G. , Fayer M. D.
Published: July 2002

A theory is presented describing the proposed phonon perturbed photon echo experiment. This experiment is a combination of picosecond time scale stimulated Raman pumping and photon echo experiments. The theory demonstrates that the phonon perturbed photon echo can directly measure electronic excitation–phonon coupling matrix elements by observing the influence of a well defined coherent phonon wave on the photon echo signal. The theory predicts that the echo pulse area (integrated intensity) is reduced. The size of this change is related to the strength of the excitation phonon coupling. In addition, a realistic estimate is made of the size of the effect, and it is shown that the experiment is feasible with available laser equipment.
Unexpected critical points in the nematic behavior of a reversibly polymerizing system
Authors: Herzfeld Judith , Taylor Mark P.
Published: July 2002

A variety of amphiphilic molecules reversibly aggregate to form rod-like particles that spontaneously align at sufficiently high concentrations. A model combining a lattice description of excluded volume effects and a phenomenological description of aggregate assembly is used to calculate the phase behavior of such a system. An unexpectedly rich phase diagram is predicted with critical points dependent on end effects in the aggregates. When the aggregation is very weak, there is an apparent multicritical point in the isotropic–nematic transition. This situation applies to sickle cell hemoglobin and provides the first theoretical basis for the apparent critical point observed in that system. When aggregation is not quite so weak, a first-order nematic–nematic transition is predicted, either in addition to or merged with, the isotropic–nematic transition. The denser nematic phase contains extremely long, closely packed rods, suggestive of the hexagonal phase formed by cylindrical surfactant micelles.
Rotationally resolved double resonance spectra of NO Rydberg states near the first ionization limit
Authors: Biernacki D. Therese , Colson Steven D. , Eyler E. E.
Published: July 2002

Optical–optical double resonance multiphoton ionization spectroscopy is used to study several Rydberg states of nitric oxide with the A(3sσ) 2Σ+, v=1 state as an intermediate level. These v=1 Rydberg states include the 8s, 9s, 7f, 8f, and 8p, but no d states are observed. The states are rotationally resolved due to the capability of optical–optical double resonance experiments to select particular rotational levels of the intermediate state. A rotationally analysis of the data for the 8s and 9s states yields values for the rotational constant, the centrifugal distortion constant, and the term energy of the lowest rotational level for each state. Analysis of the data for the 7f and 8f states yields values for the ion core quadrupole moment, polarizability and rotational constant, and a correction for core penetration. These constants are obtained by a generalized least-squares fit to a long-range interaction model for electrostatic forces between the ion core and the Rydberg electron. The 8p, v=1 state is perturbed, probably by an interaction with the 5f, v=2 state. One example of quantum interference between the 5f level with v=2, N=4, and ℒ=−3, and the 8p 2Σ+, v=1, N=4 level has been analyzed.
A novel wave function factorization simplifying the matrix representation of the Schrödinger equation
Authors: Stechel E. B. , Webster Frank , Light J. C.
Published: July 2002

In quantum scattering theory, coordinate systems with nontrivial Jacobians may arise and cause difficulty in the reduction of close coupled equations to the desired, simple, obviously Hermitian form {−I(d2/dx2)+W(x)}f=0 with W=W°. We consider x to be the translational coordinate, orthogonal to the surface coordinates, and the Schrödinger equation is represented in a basis of surface functions. We introduce a novel wave function factorization which permits reduction to the above form if the basis is (locally) independent of x for arbitrary Jacobian and general weight function for the surface functions. This factorization is compared with the more common factorization where all coordinates are treated equally. Applications to wave function matching and the calculation of surface integrals are mentioned. Several three-dimensional, orthogonal coordinate systems provide examples simplified by the novel factorization.
Photochemical reactions in doped fluorene single crystals involving hydrogen abstraction in different reaction geometries
Authors: Steidl P. , von Borczyskowski C. , Fujara F. , Prass B. , Stehlik D.
Published: July 2002

Irreversible hydrogen abstraction reactions have been observed in dihydrophenazine doped single crystals of fluorene after photoexcitation of the guest molecules. Additional hydrogen abstraction from one fluorene host molecule in any of several inequivalent nearest-neighbor sites leads to the production of stable phenazinyl–fluorenyl radical pairs in the triplet state. Their electron paramagnetic resonance spectra have been analyzed for the different reaction geometries. There are indications that the abstracted hydrogen atoms remain in the crystal as an integral part of the radical pair geometry.
Equation of state of 1-butene shocked to 54 GPa (540 kbar)
Authors: Hamilton D. C. , Mitchell A. C. , Ree F. H. , Nellis W. J.
Published: July 2002

Hugoniot equation-of-state data for liquid 1-butene were measured in the shock pressure range 12–54 GPa (120–540 kbar) using a two-stage light-gas gun. The data are compared with previous data for polybutene, a stoichiometrically equivalent liquid with a smaller initial specific volume. The data for both butenes are in agreement with chemical equilibrium calculations which assume that shock-compressed hydrocarbons dissociate and form a two-phase mixture consisting of molecular hydrogen and carbon in a stiff, diamond-like phase.
Extraction of differential cross sections by the method of moments. I. Photodissociation
Authors: Bandukwalla G. , Vernon M. F.
Published: July 2002

We describe a general way of extracting the center-of-mass frame differential photodissociation cross section from the space-fixed fragment time-of-flight distributions obtained in laboratory experiments. Our approach is a variant of the method of moments and involves only elementary numerical computations. Experimental resolution and error analysis are naturally incorporated in the methodology. Several numerical examples are provided for the purpose of illustration.
Diffusion controlled reactions at an interface
Authors: Durning Chris , O’Shaughnessy Ben
Published: July 2002

This paper analyzes diffusion controlled reactions between Brownian particles in d dimensions separated by a d−1-dimensional hyperplane. We calculate the rate coefficient k(t) defined by Ṅ=k(t)ρAρB, where Ṅ is the reaction rate per unit interface area, ρi is the number density of i far from the interface on the side containing i, and t is the reaction time. k(t) is expressed in terms of the particle diffusion coefficients Di and the distance of closest approach between the components before a reaction. The critical dimension dc at which reaction kinetics cross over from compact (ddc) behavior is reduced by the presence of an interface to dc =1 from dc =2 for reactions without an interface. Thus, for d>1, k(t) exhibits all the characteristics of noncompact reactions. For example, k(t) has a constant long time limit k. For noncompact cases with d>2 we find that when DA/DB≫1 the more mobile species dominates (k∝DA) while for d=2 the slow species enters logarithmically and k vanishes in the limit DB →0 [k∝DA/ln(DA/DB)]. The crossover case d=dc =1 is very different and exhibits some features of compact behavior; e.g., k(t) is quenched logarithmically at long times. In this case we find, as in the d=2 noncompact case, that k(t) vanishes in the limit DB/DA→0. This effect is entirely absent for reactions without an interface and results from the rate being limited by diffusion of the slow species to the interface.
The open-shell coupled-cluster method: Effect of single excitations on electronic transition energies
Authors: Kaldor Uzi
Published: July 2002

Ionization potentials (IP) of H2O and alkali atoms, excitation energies (EE) of H2O, and electron affinities (EA) of alkalies are calculated by the open-shell coupled-cluster method with double excitations only (CCD). Comparison with previously reported computations including single and double excitations (CCSD) gives the contributions of the singles. These are very small (0.02 eV) for alkali atom IPs, larger (up to 0.09 eV, or 20%) for alkali EAs, and about 0.2 eV for the water molecule IPs and EEs. One-half of the effect in water is assigned to single excitations from the closed shells.
Structure and dynamics of 3s
E′ cyclopropane: A very fluxional multimode Jahn–Teller system
Authors: Zwanziger Josef W. , Ghelichkhani Abrahim , Grant Edward R.
Published: July 2002

The two-photon absorption spectra of the 3sE′ states of cyclopropane-h6 and cyclopropane-d6 are obtained by two-photon resonant three-photon ionization spectroscopy. Bands are vibronically resolved by virtue of a free-jet expansion. The hot bands of these spectra suggest that the excited state is distorted in both the carbon ring deformation coordinate and the CH2 wag coordinate. These are the lowest e′ modes, and thus their activity can be interpreted as arising from Jahn–Teller interactions. A satisfactory fit of both line positions and intensities is achieved within this model. The dynamics and structure predicted by the Hamiltonian used are described. It is concluded that, in the ground vibronic state, nonadiabatic interactions are small, but the electronically excited molecule is nevertheless very fluxional, sampling a wide range of geometries. Morever, while only minimal barriers to pseudorotation exist in each coordinate individually, there is some locking of the phases of the two modes, so the pseudorotations are not completely independent.
Infrared spectra of ClF, Cl2, and Cl complexes with HCl in solid argon
Authors: Andrews Lester , Hunt Rodney D.
Published: July 2002

Argon mixtures of ClF, Cl2, and Cl atoms have been codeposited with Ar/HCl at 5 and 12 K. Infrared spectra in the HCl stretching region give evidence for the FCl--HCl, Cl2--HCl, and Cl--HCl hydrogen-bonded complexes. The molecular complexes are more stable in the hydrogen-bonded than antihydrogren-bonded forms as no evidence for the latter were observed. The Cl--HCl radical hydrogen-bonded complex is considerably different from the centrosymmetric (Cl–H–Cl)− anion.
Dielectric relaxation spectra for the bulk and concentrated solutions of c
i
s-polyisoprene
Authors: Imanishi Yasuo , Adachi Keiichiro , Kotaka Tadao
Published: July 2002

The dielectric relaxation spectra for the bulk and concentrated solutions of cis-polyisoprene with narrow distribution of molecular weight were calculated based on a novel method. It was found that the relaxation spectra for the normal mode process exhibit a sharp peak in the range of the longest relaxation time. A weak secondary peak was observed in the time domain about one decade shorter than the main peak. The intensity of the secondary peak increased with increasing molecular weight and concentration. The molecular mechanisms for the secondary process are discussed. The reliability of this new method for calculating the relaxation spectrum was tested by using the loss curves calculated from model spectra. It was found that the reproducibility is better than Tschoegl’s second approximation method.
Statistical mechanics of a quenched bonded fluid
Authors: Singh Rajiv R. , Kovac Jeffrey
Published: July 2002

A new approach to the statistical mechanics of glasses is presented. The method is based on a physical picture of the glass as a quenched state in which certain aspects of the liquid structure are frozen into the system. The free energy is calculated as a function of the constraints imposed by that structure and the free energy is then averaged over the probability that a particular configuration of constraints occurs. The general method is applied to a one-dimensional bonded fluid model and the thermodynamic properties of the equilibrium and quenched fluid are calculated. The behavior of the quenched fluid is like that expected for a glass with a single ordering parameter.
Refractivity virial coefficients of C2H4 and C2H4–Ar mixtures
Authors: Bose T. K. , Boudjarane K. , Huot J. , St-Arnaud J. M.
Published: July 2002

Refractivity virial coefficients of pure ethylene and five mixtures of ethylene–argon have been determined at 303 K. Combination of the dielectric second virial coefficient (Bε) and the refractivity second virial coefficient (BR) gives the contribution of the quadrupole induced dipole effect (BOR =Bε −BR) which leads to the determination of the average quadrupole moment of (3.33±0.16)×10−26 esu cm2 for pure C2H4 and (3.23±0.38)×10−26 esu cm2 for C2H4–Ar. These values agree well with the values of the quadrupole moment determined by other methods.
Molecular flexibility of polymethylene molecules: A Raman spectroscopic study
Authors: Zerbi Giuseppe , Roncone Paolo , Longhi Giovanna , Wunder Stephanie L.
Published: July 2002

The Raman spectrum in the CH stretching region of molecules containing long alkyl residues shows characteristic features which provide direct information on their molecular flexibility, as probed by the C–H stretching oscillator which vibrates on the picosecond time scale. From their Raman spectra, librational motions of the (CH2)n units can be studied for molecules in the solid and liquid phases. In the latter case, segmental motions must be considered. In this paper, the spectra of hydrocarbons in the solid, liquid and as clathrates in urea and perhydrotriphenylene are studied in terms of their overall mobility. Using molecular dynamical calculations we show the existence of a selective coupling between CH stretchings and skeletal torsions, which is modulated by the collective mobility of the carbon skeleton. In particular, we account for the frequency dependence of the antisymmetic CH stretching mode using a model which allows for selective coupling between this high frequency mode and the low frequency torsional oscillations about the C–C bonds in the chain.
Vibrational energy relaxation of highly compressed N2–D2 mixtures
Authors: Khalil-Yahyavi B. , Chatelet M. , Oksengorn B.
Published: July 2002

Vibrational energy relaxation time T1 of nitrogen in highly compressed mixtures of D2–N2 has been measured at room temperature as a function of gas density and mixture ratio. The first vibrational state of N2 has been populated by a rapid V–V transfer from D2 which has been vibrationally excited by collision-induced absorption. The decay of the N2 vibrational excited population has been time resolved by monitoring the infrared collision-induced fluorescence. The data are compared with attractive hard-sphere models developed for energy relaxation in dense fluids.
Hydrodynamic radii of diffusion-limited aggregates and bond-percolation clusters
Authors: Chen Zhong-Ying , Weakliem Paul C. , Meakin Paul
Published: July 2002

The hydrodynamic radii of two types of aggregates, diffusion-limited aggregation clusters (DLA) and bond-percolation clusters (BPC), are calculated by numerically solving the hydrodynamic interaction between different particles in the cluster. Though they have almost the same fractal dimensionality, DLA and BPC clusters exhibit different effective hydrodynamic scaling behaviors. For BPC, the ratio between the hydrodynamic radius and the radius of gyration, Rh/Rg, remains almost constant (1.14) for clusters of up to 900 particles; while for DLA the hydrodynamic radius Rh increases faster than the radius of gyration Rg, with Rh∼N0.45 for the same range of N.
Vibrational cooling in large molecular systems: Pentacene in naphthalene
Authors: Chang Ta-Chau , Dlott Dana D.
Published: July 2002

Ultrafast laser experiments are conducted on low temperature crystals of pentacene in naphthalene (PTC/N) to study the process of vibrational cooling. A vibration of the excited singlet state, denoted Sν1, is excited, and the decay out of this state, as well as the subsequent arrival at the vibrationless ground state S01, are monitored by photon echoes, absorption recovery, and a new technique, pump-induced coherent Stokes Raman scattering [T.-C. Chang and D. D. Dlott, Chem. Phys. Lett. 147, 18 (1988)]. Eight vibrational modes of PTC, ranging from 260 to 1350 cm−1 are studied. The experimental results are interpreted using a previously developed model of vibrational cooling [J. R. Hill and D. D. Dlott, J. Chem. Phys. 89, 830 (1988)]. This model predicts the dependence of the vibrational cooling rate on the amount of excess vibrational energy and the temperature. The motion of the vibrational probability distribution toward the ground state is predicted to occur with a temperature independent ‘‘vibrational velocity’’ which describes the rate of vibrational energy dissipation. Using the model, we fit all eight data sets with a single adjustable parameter, the vibrational velocity, and we obtain the value V0=10±2 cm−1/ps. The prediction of a nearly temperature independent V0 is confirmed over the temperature range 1.5 to 35 K. Finally, we discuss the application of these measurements to the problem of heme cooling in optically excited heme proteins.
Electron transfer quenching of rhodamine-6G in polymer films
Authors: Palazzotto M. C. , Sahyun M. R. V. , Serpone N. , Sharma D. K.
Published: July 2002

Electron transfer quenching of the fluorescence of rhodamine-6G in polyvinyl acetate and poly-α-methylstyrene films by aromatic amine quenchers has been examined, using both steady-state and time-resolved methods. Observed quencher concentration dependences do not conform to expectations from either the simple Perrin or Stern–Volmer models. The steady-state quenching results are consistent with long-range electron (tunnel) transfer, with collective quenching at higher quencher concentration, but existing models of this process do not quantitatively rationalize our results. Some potential difficulties with these models are accordingly discussed. Time-resolved (streak camera) experiments confirm that all the quenching events are of dynamic origin, to the exclusion of the involvement of a fractional population of fluorescence-inactive absorbers owing to ground-state complex formation.
Unique optical changes in cholesteric liquid crystals using guest mediated single laser beam excitation
Authors: Becker Ralph S. , Chakravorti Sankar , Das Suresh
Published: July 2002

The dynamics of three distinctively different optical changes of three different cholesteric liquid crystal mixtures of different pitch doped with a guest molecule are seen for the first time utilizing single laser beam excitation. The guest molecule acted as a light absorber and subsequent fast heat radiator. Two of the three cholesteric mixtures had opposite directions of pitch change from the third with increasing temperature. In the first optical process (also the fastest), just after laser excitation, ≤200 ns, the reflectance band always moved to longer wavelength irrespective of the magnitude of the original pitch or the directional sensitivity of the pitch change to increasing temperature. This nanosecond time domain phenomenon is assigned as originating from formation of a transient grating caused by modulation of the index of refraction associated with heat instantaneously (≤10 ps) released by the guest molecules to their immediate neighborhood. The second process, in the time domain of 30–600 μs, is mainly the result of the characteristic pitch change of the helix of the cholesteric mixtures with corresponding reflection band shift to longer or shorter wavelengths according to the type of mixture. The final process (>1000 μs) is the consequence of the pitch of the cholesteric returning to the original value after the heat is dissipated.
Photodissociation of gas-phase halogenated benzene radical cations
Authors: So Hun Young , Dunbar Robert C.
Published: July 2002

Photodissociation was observed and studied for a series of gas-phase cations of fluoro- and chlorobenzenes, using trapped-ion cyclotron resonance techniques. All of these dissociations were interpreted as involving absorption of two or more photons. Photodissociation (PDS) spectra of trichlorobenzene and trichloro-trifluorobenzene ions were compared with literature fluorescence emission spectra. In both cases, the sharp fluorescence peak at the 0-0 transition energy was not reflected in the PDS spectra; this was interpreted as showing high fluorescence quantum yield, and correspondingly low internal conversion quantum yield, for ions in the B̃ state vibrational ground state. Collisionless (infrared radiative) relaxation rates for several of the ions were determined by chopped-laser two-photon dissociation, showing that the presence of several chlorines, and particularly several fluorines, enhances the rate of infrared emission. The highest radiative cooling rate constant observed was 31 s−1 for trifluorobenzene ion. For those ions whose visible–UV fluorescence quantum yields have been reported as near unity, the observation of photodissociation was accounted for by a combined mechanism involving fluorescence-pumping heating of the ions, followed by two-photon photodissociation.
Comprehensive examination of radiationless energy transfer models in dyes: Comparisons of theory and experiment
Authors: Ali M. A. , Ahmed S. A.
Published: July 2002

Experimental data on radiationless energy transfer, covering a wide range of physical conditions, was compared with predictions developed from three different theoretical models, to define and determine their respective validities and range of applicability.
Hydrodynamic interactions and the viscosity of suspensions of freely moving spheres
Authors: Ladd Anthony J. C.
Published: July 2002

A computational method aimed at calculating the hydrodynamic interactions between spherical particles, within the framework of the ‘‘creeping-flow’’ approximation, is developed further. The basic fluid equations for a system with periodic boundary conditions have been rederived, paying particular attention to possible divergences in the fluid velocity field. Numerical results have been obtained for the viscosity of a suspension of freely moving hard spheres over a range of solid densities from the dilute limit to near the hard-sphere freezing transition.
Brownian dynamics study of the two-dimensional linear polymer collapse transition
Authors: Bishop Marvin , Croxton Clive A.
Published: July 2002

The collapse transition for continuum two-dimensional linear polymers is investigated by Brownian dynamics. Attractions are modeled by a Lennard-Jones force and the strength of the attraction is varied by changing the value of the reduced well depth, ε. The transition is followed by monitoring the change in the radius of gyration, 〈S2〉. It is found that the collapse transition occurs for ε≊1.375.
Molecular dynamics and Rayleigh–Benard convection
Authors: Given James A. , Clementi Enrico
Published: July 2002

A number of recent molecular dynamic studies have shown that transient convection cells can occur in hard disk systems containing as few as 5000 particles. We extend these studies, with special attention to the role of the various algorithms used to impose thermal boundary conditions. Finite-size effects generally destabilize vortex patterns obtained with a system of 5040 atoms, using either of two different thermal boundary algorithms. Long-lived, symmetric vortex patterns are acheived in a system of 10 200 atoms. We note, however, the fact that the class of stochastic boundary algorithms seem constrained to give vortex cells with unit aspect ratio. We also report simulations with a new cell boundary method that imposes thermal boundary conditions by making a local separation between translational and thermal energy and thermalizes groups of atoms at the same time. This algorithm represents a new hybrid between molecular dynamics and continuum methods of studying fluid behavior. Its efficiency for simulations of this kind is shown by the fact that it yields stable, long-lived convection cells whose horizontal wave number increases as the Rayleigh number is increased. A variant of this algorithm is used to simulate convection with no-slip boundary conditions. Implications for further research are discussed.
Glass formation in simple ionic systems via constant pressure molecular dynamics
Authors: Kumta Prashant N. , Deymier Pierre A. , Risbud Subhash H.
Published: July 2002

The utility of constant pressure molecular dynamics (MD) in studies of glass formation is demonstrated by choosing NaCl and ZnCl2 as examples of MX and MX2 salts. Rigid ion pair potentials have been used to model the two salts. Results of these simulations are in good agreement with general glass formation principles. The effect of the interatomic potential on the glass transition and the cooling rate on glass formation in both the salts is reported. The parameter characterizing the inertia of the borders of the MD simulation cell has a strong influence on the glass transition temperature.
Study of the ν6 band of CH3Br by infrared laser sideband and submillimeter-wave spectroscopy
Authors: Chevalier J. M. , Legrand J. , Glorieux P. , Wlodarczak G. , Demaison J.
Published: July 2002

About 136 transitions in the ν6 band of CH3Br have been measured using an infrared laser sideband spectrometer. The sidebands were generated by mixing the 10 μm radiation of a CO2 laser with the radiation of a tunable high power microwave source. Frequencies of transitions with J≤71 and K≤8 are reported. The accuracy of the measurements is estimated to be better than 10 MHz. Some high J rotational transitions in the v6=1 state have also been measured with a submillimeter-wave spectrometer. The new measurements have been combined with previous results to derive vibration–rotation parameters for the v6=1 state.
Conformational stability, barriers to internal rotation, vibrational assignment and a
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o calculations of fluoroacetyl chloride
Authors: Durig J. R. , Phan H. V. , Hardin J. A. , Little T. S.
Published: July 2002

The far infrared spectrum (375 to 35 cm−1) of gaseous fluoroacetyl chloride, CH2FC(O)C1, has been recorded at a resolution of 0.10 cm−1. The fundamental asymmetric torsions of the more stable trans (halogen atoms are trans) and the high energy cis conformations have been observed at 116.18 and 49.42 cm−1, respectively, each with several upper state transitions falling to lower frequency. From these spectral data, an asymmetric potential function has been calculated and the potential coefficients are: V1=43±6, V2=1039±36, V3=498±3, V4=149±21, and V6=−10±7 cm−1. The trans to cis and cis to trans barriers are 1455±25 cm−1 (4.16±0.07 kcal/mol) and 914±24 cm−1 (2.61±0.07 kcal/mol), respectively, with an enthalpy difference of 541±45 cm−1 (1.55±0.13 kcal/mol). From studies of the Raman spectra at variable temperatures, values of 509±37 cm−1 (1.46±0.10 kcal/mol) and 310±8 cm−1 (0.89±0.02 kcal/mol) have been determined for the enthalpy difference for the gas and liquid, respectively. The conformational stability, barriers to internal rotation, and fundamental vibrational frequencies which have been determined experimentally, are compared to those obtained from ab initio Hartree–Fock calculations employing both the 3-21G* and 6-31G* basis sets, and to the corresponding quantities obtained for some similar molecules.
Elastic constants of α-CO by Brillouin spectroscopy
Authors: Askarpour V. , Kiefte H. , Clouter M. J.
Published: July 2002

The technique of Brillouin spectroscopy has been used to determine the adiabatic elastic constants of oriented single crystals of α-CO at temperatures very close to the α–β phase transition. The values of the elastic constants at 60.9 K are c11=2.41, c12=1.44, and c44=1.06 (in units of 109 N m−2) and suggest that the k=0 acoustic phonon behavior is almost identical to that in a rare gas solid.