bluuebunny/crossref_metadata_2025 huggingface dataset:
Electronic energy transport in aromatic vinyl-polymers: Nonexponential picosecond trapping in poly-(N-vinylcarbazole)
Authors: Kauffmann Harald F. , Mollay Bernhard , Weixelbaumer Wolf-Dieter , Bürbaumer Josef , Riegler Martin , Meisterhofer Erich , Aussenegg Franz R.
Published: July 2002

The theoretical concept of nonexponential electronic S1 energy relaxation in nonrandom, polychromophoric polymers has been tested experimentally by means of picosecond time-resolved fluorescence spectroscopy. For the low-energy, sandwich-type excimer E2 of poly-(N-vinylcarbazole), p-N-VCz, in dilute liquid solution the fluorescence rise-profile FE2(t), collected at λem =460 nm, has been analyzed in terms of nonconventional relaxation kinetics. A time-dependent trapping function, k(t)=b+ct−1/2, which reflects both the ‘‘effective’’ diagonal disorder and the pronounced low dimensionality of carbazole hopping sites in the fluid regime has been used in a first attempt to model migrational sampling in a sequence of excited-state relaxation processes. The kinetic scheme consists of a distribution of transport states {X1}, a small ensemble of energy-relaxed monomeric chromophores X2, and a discrete state of the mobile excimer X3 (E2) coupled to X2. Exact solutions to the δ-pulse response behavior {X1}, X2, and X3, respectively, can be found which contain typically nonexponential terms of the form of time-dependent pre-exponentials  Aij(t). The functional forms of  Aij(t) as well as their relevancy to picosecond and nanosecond time scales have been demonstrated by synthetic data simulation. The excimer δ-pulse trial function based upon this scheme has been shown to recover satisfactorily the experimental data. The limitations of the model, the uncertainties of rise curve analysis, in general, and the main problems encountered in rationalizing excited state transport and trapping parameters in the presence of rotational sampling have been discussed.
The polarization-function counterpoise method. An application of the diagrammatic perturbation theory to the He–H2 molecule in the region of the van der Waals minimum
Authors: Olivares del Valle F. J. , Tolosa S. , Ojalvo E. A. , Esperilla J. J.
Published: July 2002

The results of the calculation on the C∞v He–H2 van der Waals molecule by employing the diagrammatic many-body theory to treat the electron correlation effects are reported in this paper. The use of the partial counterpoise method to compute the self-consistent field (SCF) and correlation energies, when a moderate basis set is employed, may lead to a reliable description of the potential curve for the van der Waals (VDW) system.
Excited state, charge transfer absorption spectrum of HgBr in the blue (λ∼447 nm)
Authors: Greene D. P. , Killeen K. P. , Eden J. G.
Published: July 2002

Absorption from the lowest-lying ion pair state of HgBr (i.e., Hg+–Br−: B 2∑+1/2 ) has been observed by transient absorption spectroscopy in a pulsed, transverse discharge containing a mixture of Ne, N2, and HgBr2 vapor. A highly structured band peaking at ∼447 nm is shown to be associated with charge–transfer transitions from the B 2∑+1/2 state to a higher-lying, covalently bonded (Hg*–Br) state. By pumping the 447 nm transition with a pulsed dye laser while simultaneously monitoring gain on the HgBr (B 2∑+1/2 →X 2∑+1/2 ) band at 502 nm, the absorption cross section for the blue band was measured to be (3.0±1.5)×10−17 cm2 and the saturation intensity for the transition at 447 nm was determined to be 710±50 kW cm−2.
Calculation of free energies of Lennard-Jones crystals via molecular dynamics
Authors: Deymier Pierre , Kalonji Gretchen
Published: July 2002

The effectiveness of the determinant method and of energy distribution methods for the calculation of free energies with molecular dynamics sampling in the isothermal–isobaric ensemble has been demonstrated. The accuracy of the methods employed has been characterized. The quasiharmonic and anharmonic free energies of two-dimensional and three-dimensional crystals have been estimated over a wide range of temperatures.
The adiabatic switching of semiclassical wave functions
Authors: Skodje Rex T. , Borondo F.
Published: July 2002

The behavior of semiclassical wave functions during adiabatic deformations of the Hamiltonian is studied. A practical technique based on Gaussian wave packet dynamics is presented which allows a semiclassical eigenstate for one Hamiltonian to be adiabatically switched into an eigenstate for a different Hamiltonian. The validity of this technique is established by studying the properties of the Hamilton–Jacobi equation during adiabatic processes. The method is shown to yield good results for several one- and two-dimensional model problems. It is shown that converged semiclassical eigenstates can be obtained even if the trajectories have become chaotic by the end of the switching process.
Some ter-molecular association processes in collisions of CH+3 and its deuterated analogs with H2, HD, and D2
Authors: Bates David R.
Published: July 2002

The ter-molecular association processes involving CH+3 or a deuterated analog as the ionic reactant, H2, HD, or D2 as the neutral reactant and CH+5, CH4D+, CH3D+2, CH2D+3, CHD+4, and CH+5 as the product are investigated theoretically. Going from process to process along the sequence there are increases in the energy density of vibrational states, changes in the symmetry numbers of the reactants and of the products and changes also in the number of distinguishable isomeric products. Furthermore in the low temperature region H2 is markedly less efficient than HD or D2 as an association partner. Taking these effects into account it is found that the ter-molecular association coefficients at 80 K rise irregularly in going along the sequence from CH+5 to CH2D+3 (by a total factor of about 27) and then fall irregularly in continuing on to CD+5 (by a further total factor of 3.7). This is in apparent serious conflict with laboratory measurements in ambient helium which show much less variation along the sequence (with the values of the two factors just specified only 5.6 and 1.6, respectively). It is shown that the pressure of the ambient helium (0.4 Torr) was much too high for the third order kinetics that were implicitly assumed to prevail, even for the slowest process (CH+5 association) the ambient helium pressure would have had to be at least around ten times smaller to avoid the measured rate coefficient at 80 K being around 20% or more less than its low ambient gas density limit. The necessary corrections are made. They bring about excellent agreement between theory and experiment. The study yields a value for the CH+5 vibrational density of states.
Lie algebraic approach to quantization of nonseparable systems with internal nonlinear resonance
Authors: Farrelly David
Published: July 2002

Nonlinear resonances play an important role in a wide variety of molecular systems. Since they can lead to dramatic changes in the topology of the classical trajectories, resonances may give rise to problems in the application of semiclassical quantization schemes. In particular, the presence of resonances complicates the determination of the most appropriate action-angle variables in which to effect quantization. The best set of action-angle variables should ideally be determined by the physical system rather than by convenience (as is often the case), and, in fact, an unphysical choice of actions may give rise to unphysical singularities in the quantization procedure. In this article a new perspective is presented to the problem of defining physical action-angle variables for nonseparable coupled harmonic oscillator systems displaying internal nonlinear resonance. For the sake of illustration, the major emphasis is directed to a system exhibiting Fermi resonance. The choice of action-angle variables is based on a direct consideration of the Lie symmetries associated with the zero order (i.e., uncoupled) system, which are then used to quantize the full problem. In conjunction with the appropriate semiclassical quantization rules the method provides excellent agreement with accurate quantum results for the Fermi resonant system studied, even when the dynamics is chaotic. In addition, the treatment provides considerable insight into the application of classical perturbation theory to semiclassical quantization, and provides a consistent framework for the treatment of resonant systems. Although presented in the context of classical perturbation theory, the choice of good action variables is a central issue in most semiclassical methods (e.g., adiabatic switching), and the usefulness of the approach to these methods is briefly discussed.
Energy-donor phosphorescence and energy transfer by exchange interaction in a rigid matrix
Authors: Hara Atsuko , Gondo Yasuhiko
Published: July 2002

The decay of the energy-donor phosphorescence has been observed using a N2 laser on the benzophenone–naphthalene and p-methoxybenzaldehyde–naphthalene systems, and the results have been examined in the light of the theory developed by Inokuti and Hirayama for energy transfer by exchange interaction. Substantial discrepancies have been found between the theory and the observed phosphorescence decay curves, in particular, for the short time region after excitation. We have introduced the concept of the nonvanishing closest donor–acceptor intermolecular distance into the framework of the theory. Over the whole range of time, the experimental data have been found to agree with the theory thus amended.
Quenching of OH(A 2Σ+, v′=0) by NH3 from 250 to 1400 K
Authors: Jeffries Jay B. , Copeland Richard A. , Crosley David R.
Published: July 2002

The rate constant for collisional removal of the v′=0 level of the A 2Σ+ excited state of the OH radical by NH3 is measured over the temperature range 250–1400 K. The thermally averaged cross section σQ near room temperature is roughly twice its value at 900 K of ∼40 Å2. Between 850 and 1400 K, no variation of σQ with temperature is observed. This temperature dependence is consistent with a quenching mechanism dominated by long range attractive forces.
Kinetics of condensation in supersonic expansion (Ar)
Authors: Bauer S. H. , Chiu N-S. , Wilcox C. F.
Published: July 2002

The temperature and density in an overexpanded jet (Ar) were mapped by the method of characteristics. The kinetics of condensation in the supersaturated gas along the central streamline was analyzed on the basis of four models, subject to previously developed thermodynamic relations which must apply during the homogeneous condensation of any supersaturated vapor. Here we report on detailed dynamic analyses of the rates of cluster growth. Each association: An−1+A1⇄An was represented either by a single stabilization step, or by a sequence of two energy removing collisions, for which kinetic parameters based on Lennard-Jones (LJ) potentials were estimated. Coupled differential rate equations (up to 192) were solved, both for cases when the heat of condensation was neglected, and when it was partially or fully distributed throughout the fluid. The distribution of cluster sizes declines exponentially, according to N(n)=A exp(−an)+B exp(−bn), in agreement with generally observed distributions for monatomic species. For source pressures of 3 and 10 atm the condensation process becomes kinetically controlled within a distance downstream somewhat less than twice the aperture diameter. Our calculations indicated that after about three aperture lengths ‘‘freezing in’’ takes place if the heat of condensation is neglected. When the heat of condensation is distributed (partitioned in one of several plausible ways) freezing in is delayed beyond four aperture diameters. A modest fraction of the nascent clusters retain some of the heat of condensation and remain at a higher temperature than the ambient gas.
State-to-state differential cross sections from semiclassical energy conserving trajectory calculations: H+2(v)+H2(0)→H2(v″)+H+2(v′)
Authors: Cole S. Keith , DePristo Andrew E.
Published: July 2002

Classical state-to-state differential cross sections are presented for the charge exchange reaction H+2 (v=0,2,5,8)+H2 (v=0) to select product channels at two collision energies, E(c. m.) =8 eV and E(c. m.) =16 eV. A numerical method is presented for the extraction of scattering angles for each state-to-state transition from the average values provided by the semiclassical energy conserving trajectory (SCECT) calculation. The differential cross sections are computed in terms of these values and the results are applied to the analysis of experimental data.
Measurement of the rotational spectrum of the water cation (H2O+) by laser magnetic resonance
Authors: Strahan S. E. , Mueller R. P. , Saykally R. J.
Published: July 2002

Nine rotational transitions of v=0 X̃ 2B1 H2O+ have been measured by laser magnetic resonance. Optical combination differences have been combined with the LMR data in a weighted least-squares analysis using a Watson S-reduced rotational-fine structure Hamiltonian. Twenty-four molecular constants were simultaneously determined, including three hyperfine parameters and the axial components of both the anisotropic and rotational g tensors. The anisotropic g tensors are compared with their calculated values using the Curl relationship. An r0 structure was determined and is in good agreement with several ab initio results.
Circular dichroism in photoelectron angular distributions as a probe of atomic and molecular alignment
Authors: Dubs Richard L. , Dixit S. N. , McKoy V.
Published: July 2002

In this paper we show that circular dichroism in photoelectron angular distributions (CDAD) can be used to probe atomic and molecular alignment in the gas phase. Careful choice of photon (left or right circularly polarized) propagation and photoelectron collection directions breaks the cylindrical symmetry of the target, giving rise to dichroic effects. CDAD exists in the electric dipole approximation. We illustrate the sensitivity of CDAD to alignment by considering photoionization of the A 2Σ+ state of NO. Most of the cases of alignment we consider are created by multiphoton absorption while the others, more general, might be created in fragmentation, desorption, etc. The alignment created by n-photon absorption quickly reaches a classical limit which is reflected in the CDAD spectrum. Finally, we show that CDAD is also a sensitive probe of gas phase atomic state alignment by considering photoionization of the 7P3/2 state of cesium created by single photon absorption from the ground state.
Nuclear spin relaxation studies of the spin-rotation interaction of 13C in CO in various buffer gases
Authors: Jameson Cynthia J. , Jameson A. Keith , Buchi Kathleen
Published: July 2002

Nuclear spin-lattice relaxation times T1 have been measured for 13C in 13C16O in pure CO gas and in CO in Ar, Kr, Xe, N2, O2, CO2, HCl, CH4, and SF6 gases as a function of temperature. The relaxation is completely dominated by the spin-rotation mechanism so that empirical values of the cross sections for rotational angular momentum transfer σJ are obtained as a function of temperature.
Isotropic Raman study of ultrarapid proton-transfer reactions in aqueous mixtures
Authors: Bratos S. , Tarjus G. , Viot P.
Published: July 2002

A theory is presented to study the exchange broadening of isotropic Raman bands due to ultrarapid proton-transfer reactions. It represents a generalization of standard theories of Raman band profiles of nonreactive liquids. The variables describing the reaction are assumed to represent a dichotomic Markovian process. The spectral behavior of various AH/H2O mixtures is studied as a function of the exchange rate and the interplay of various band shaping mechanisms is discussed in detail. Finally, the potentialities of the Raman spectroscopy as a tool to measure the rate constant are critically assessed.
Molecular dynamics of nonequilibrium infrequent events: Laser-induced desorption from surfaces
Authors: Lim Carmay , Tully John C.
Published: July 2002

An obvious shortcoming of classical trajectory simulations of chemical reactions is their limitation to short time scale processes. While procedures to extend simulations to long time scales have been developed for reactions occurring under equilibrium conditions, there are no analogous techniques for nonequilibrium situations. We propose such a method which, although not rigorous, is simple, practical, and appears to give results of useful accuracy. We illustrate the method by simulating infrared laser-induced desorption of an atom from a solid surface. At vanishingly low laser powers, desorption rate constants agree with those calculated by standard equilibrium infrequent event methods. They also agree at high laser powers with the values obtained from direct simulation in the fast time scale limit. At intermediate powers they are in accord with the results of a master-equation treatment described in the Appendix. We have examined the dependence of laser-induced desorption on the laser frequency and intensity and on the surface temperature. Enhanced desorption is observed at relatively low intensities, and a nonthermal selective desorption mechanism is indicated.
Nitrogen dioxide fluorescence from N2O5 photolysis
Authors: Oh Daniel , Sisk Wade , Young Anthony , Johnston Harold
Published: July 2002

The products of ultraviolet photolysis of N2O5 are NO3 and a wavelength dependent mixture of NO2, NO*2, and NO+O, where NO*2 represents one or more excited electronic states of nitrogen dioxide. This NO*2 emits the visible fluorescence spectrum of nitrogen dioxide (photolysis induced fluorescence, PIF), and this spectrum was compared with monochromatically excited NO2 fluorescence spectra (laser induced fluorescence, LIF). In a series of experiments, dispersed PIF and LIF spectra were measured where reactant pressure was 200 mTorr, delay time was 30 ns, and observation time was 600 ns. According to results obtained by Sugimoto and co-workers, under these conditions the continuous spectrum, which reflects the overall internal energy of NO*2, had been little modified by collision, although there was degradation of fine structure. The continuous LIF spectra were fit to an empirical function, and the PIF spectra were shown to be well represented by a linear combination of these mono-energetic excitation spectra. The coefficients of this linear combination plus other considerations were interpreted to give the almost nascent internal energy distribution of the electronically excited nitrogen dioxide molecules produced by N2O5 photolysis. This non-Boltzmann internal energy distribution indicates that electronically excited nitrogen dioxide is produced in the 2B1 state when N2O5 is photolyzed.
A comparative study of the fluorescence lifetimes of 9-cyanoanthracene in a bulb and supersonic free jet
Authors: Hirayama Satoshi
Published: July 2002

The fluorescence decays of 9-cyanoanthracene (9CNA) in a supersonic free jet have been studied for a large number of vibrational bands including the electronic band origin. The fluorescence lifetime τf varied from vibrational level to level but decreased from 28.0±0.2 at the 0–0 band origin to 4.2±0.1 ns at the excess energy of 2045 cm−1. In a bulb, however, τf of 9CNA was no more than 3.2 ns and independent of both temperature (433–553 K) and excess energy (∼4800 cm−1). We attempted to interpret the very short and temperature invariant τf of 9CNA in the bulb in terms of the vibrational level specific τf found in the supersonic jet. No simple way to rationalize the findings in the bulb was found but the results suggested an important role of optically nonactive vibrational modes in determining the radiationless transition. The nonexponential decay with the lifetimes of 3.9±0.4 and 27.3±0.2 ns found for the partially cooled state of 9CNA, which was created by seeding 9CNA in helium at the pressure of 200 Torr, can be taken as the evidence supporting this view.
Extraction of alignment parameters from circular dichroic photoelectron angular distribution (CDAD) measurements
Authors: Dubs Richard L. , Dixit S. N. , McKoy V.
Published: July 2002

In a previous paper, we showed that circular dichroism in photoelectron angular distributions (CDAD) can be used to probe alignment in gas phase atoms and linear molecules. Often this alignment is parametrized through the moments of alignment A(2), A(4), etc., which are commonly extracted from fluorescence polarization measurements. In this paper we show how these can be simply extracted from CDAD spectra. This technique can be used in principle to extract the moments to any order.
Second-quantized molecular time scale generalized Langevin equation theory: Coupled oscillator model
Authors: McDowell H. Keith
Published: July 2002

A second-quantized, coupled oscillator model is presented which explicitly displays the structure of a second-quantized MTGLE theory. The Adelman ansatz [J. Chem Phys. 75, 5837 (1981)] for a quantum MTGLE response function is shown to generate the correct response function for the model. This result paves the way for the development of a general second-quantized MTGLE theory.
Simplified statistical adiabatic channel model calculations of the reaction H+CH3→CH4 on a
b i
n
i
t
i
o potential energy surfaces
Authors: Cobos C. J.
Published: July 2002

The simplified version of the statistical adiabatic channel model has been applied to calculate limiting high pressure rate coefficients for the title recombination reaction between 200–2000 K. The energy pattern of the channel threshold energies was analyzed in terms of recently reported ab initio potential energy surfaces. The calculated rate coefficients are in very good agreement with quasiclassical trajectory, variational transition state theory, and canonical variational transition state theory calculations performed on the same surfaces. An analysis with a factorized formalism allows one to study separately the role that the motion on the reaction coordinate and the degrees of freedom orthogonal to them, play on the rate coefficients. A recent experimental rate coefficient lies between the values calculated with the ab initio surfaces constructed with data of Duchovic, Hase, and Schlegel–Brown and Truhlar, and with similar information from Hirst. In addition, translational energy dependent thermal averaged cross sections were computed and compared with quasiclassical trajectory results. Based on an experimental limiting low pressure rate coefficient, the average energy transferred per each He–CH4 collision was calculated using the low pressure range unimolecular rate theory with explicit account of the potential energy surface features. Confrontation with direct measurement of collisional energy transfer shows that the abovementioned surfaces also give the best agreement between the experimental and calculated values.
Resonance fluorescence of a two-level atom near a rough metal surface
Authors: Huang Xi-Yi , Lee Ki-Tung , George Thomas F.
Published: July 2002

Surface-dressed optical Bloch equations are solved for a two-level atom near or adsorbed on a rough surface, which is modeled as a hemispheroid protrusion on a perfectly conducting surface. Effects of the laser bandwidth are included by means of a phase-diffusion model. The presence of the surface roughness generates a dephasing broadening mechanism on the adatomic resonance fluorescence spectrum, which can be strongly enhanced by plasmon resonances depending on the shape of the hemispheroid. The dependence of the electrodynamics of the adatom on the adatom-hemispheroid distance is also evaluated.
Optical spectra in polydiacetylene crystals substituted with fluorobenzenes
Authors: Tokura Yoshinori , Koda Takao , Itsubo Akira , Miyabayashi Mitsutaka , Okuhara Kunio , Ueda Akio
Published: July 2002

Optical properties of new polydiacetylene (PDA) crystals substituted with fluorobenzene derivatives have been investigated by reflectance, Raman, and electroreflectance (ER) measurements. The results indicate that these fluorobenzene-substituted PDAs have an intermediate optical character between those of the acetylene-type and butatriene-type PDA chains. One of these PDA crystals, DFMDP, has been found to show an exceptionally high crystalline quality, which has enabled us to make a detailed spectroscopic study. ER studies on this DFMDP crystal have revealed the charge–transfer(CT)-type exciton lying at ∼2.7 eV on the higher-energy side of the lowest 1Bu exciton at ∼2.2 eV. On the basis of experimental results, it is conjectured that two-photon resonance with the CT exciton plays a major role in the third-order nonlinear optical process in PDA chains.
Infrared spectra and structure of thin films of solid oxygen: A metastable phase
Authors: Jones L. H. , Agnew S. F. , Swanson B. I. , Ekberg S. A.
Published: July 2002

The infrared absorption spectra of thin films of solid oxygen prepared by vapor deposition is examined in detail. It is shown that at deposition temperatures between 8 and 11 K the film consists of a mixture of α-O2 and a metastable phase we call m-O2. The spectrum of CO in an O2 matrix is shown to be a good marker for the presence of m-O2 and is informative concerning the structure of this metastable phase. Studies of mixtures of 18O2 and 16O2 indicate that the structure of m-O2 is complex with four nonequivalent sites for O2 molecules and at least eight molecules per unit cell. The isotopic studies suggest the presence of weakly bound dimers as well as tetramers or higher multimers in m-O2. It is shown that the presence of a few percent impurities such as nitrogen or argon allow the formation of and increase the stability of m-O2 at higher temperatures. However, with or without impurities, at deposition temperatures below 8 K less m-O2 is formed, and deposition at 5 K leads to no m-O2 but only poorly crystallized α-O2. It is concluded that m-O2 is formed by epitaxial growth from a 2D structure similar to that observed by others for 1–4 monolayers of O2 on grafoil. The spectrum of annealed α-O2 shows phonon combinations with the fundamental O–O stretch indicative of a phonon density of states from 30–90 cm−1.
High resolution IR laser spectroscopy of van der Waals complexes in slit supersonic jets: Observation and analysis of ν1, ν1+ν2, and ν1+2ν3 in ArHF
Authors: Lovejoy Christopher M. , Schuder Michael D. , Nesbitt David J.
Published: July 2002

IR spectra of jet cooled ArHF are obtained via direct absorption of a high resolution tunable difference frequency laser in a 2.54 cm path length, slit supersonic pulsed expansion at <10 K. Detection limits of 2×109 molecules/cm3/quantum state permit observation of the high frequency ν1 fundamental stretch (1000) ← (0000), the ν1+ν2 van der Waals bend plus stretch combination band (1110) ← (0000), as well as transitions to the (1002) triply vibrationally excited state that are weakly allowed via Coriolis interactions with the Π+ component of the (1110) manifold. The ground state (0000) molecular constants are in excellent agreement with previous microwave data. From the changes in rotational and centrifugal distortion constants, the vibrationally averaged van der Waals well depth is estimated to increase (+15%) with ν1 excitation, but decrease dramatically (−42%) upon subsequent excitation of the l=1 ν2 bend. L-doubling in the ν1+ν2 (1110) perpendicular bending state is large and negative [−69.8(18) MHz] and indicates the presence of a near resonant Coriolis coupled

vibration of Σ+ symmetry at lower energy. A second, localized Coriolis perturbation is observed in the (1110) state and assigned to the near resonant (1002) Σ+ fundamental plus van der Waals stretch overtone at higher energy. Analysis of this Coriolis interaction indicates that coupling can be significant even for a three quantum change in vibration. However, a perturbative, small amplitude oscillator model predicts Coriolis matrix elements only 18% of the observed values, suggesting that large amplitude, bend–stretch interactions can strongly enhance Coriolis coupling. The decrease in the B rotational constant and the vibrationally averaged well depth upon ν2 excitation confirms the strong coupling between van der Waals stretch and bend coordinates. The slit expansion geometry quenches perpendicular velocity distributions and therefore offers intrinsically sub-Doppler resolution in an unskimmed molecular beam. Residual linewidths in the ArHF spectra are all below the apparatus resolution limit of ±25 MHz, which translates into a lower limit for the predissociation lifetime of 3 ns, i.e., in excess of 2×106 ν1 vibrational periods.
First cumulant for chains with constraints
Authors: Akcasu A. Ziya , Hammouda Boualem , Stockmayer Walter H. , Tanaka Genzo
Published: July 2002

The first cumulant Ω(q) of the dynamic structure factor is calculated for freely jointed chains by applying the Fixman–Kovac formulation of chain dynamics with constraints. It is shown that the large q limit of Ω(q) for a chain of N bonds is proportional to a factor (2N+3)/3(N+1) representing the fraction of the unconstrained degrees of freedom of the chain. The relevance of this result to spin-echo neutron scattering experiments is pointed out. As an alternative approach, an expression for Ω(q) is obtained with a formalism due to Titulaer and Deutch.
Infinite-dilution activity coefficients of quadrupolar Lennard-Jones mixtures from computer simulation
Authors: Shing K. S.
Published: July 2002

The calculation of excess Gibbs’ function and activity coefficients from simulation requires the precise evaluation of chemical potentials. Such high precision is usually not attainable using the conventional test-particle simulation methods. In this paper a special implementation of Widom’s potential distribution theorem is developed and shown to give activity coefficients with high precision. Results for some model mixtures are presented. Comparison with results obtained via conventional simulation methods is made.
The effect of incident spin-orbit state on the chemiluminescent reactions of Ba(3D
J) with several oxidants
Authors: Campbell Mark L. , Dagdigian Paul J.
Published: July 2002

The dependence of the chemiluminescence channels on incident spin-orbit state has been determined by utilizing optical pumping state selection for the reactions of metastable Ba(6s5d 3DJ) with Cl2, Br2, CCl4, N2O, and NO2. The cross sections for the neighboring metastable 1D level were related to those for the 3DJ multiplet by optical pumping on an intercombination line. For the halogen-containing reagents, the spin-orbit dependence of the reactivity was substantial for BaX B–X emission and was in the order J=3>J=2>J=1. The differences in reactivity were somewhat less for the C–X chemiluminescence channel in the Cl2 and Br2 reactions. Essentially no spin-orbit dependence was observed in the N2O and NO2 reactions.
Barriers in the excited 2 1A
g state for the c
i
s,t
r
a
n
s isomerization of c
i
s,c
i
s-octatetraene: General features of the excited state potential
Authors: Kohler Bryan E. , Mitra Pradip , West Paul
Published: July 2002

The lifetime of the excited 2 1Ag state of cis,cis-octatetraene (the eight carbon oligomer of cis-polyacetylene) in a n-octane host crystal has been measured as a function of temperature in the region 8<T<116 K. The lifetime is relatively long at 8 K (130 ns), drops to a lower value at approximately 30 K, and then begins to decrease rapidly with increasing temperature in the neighborhood of 100 K. The highly resolved optical spectra show unambiguously that at 30 K the decrease in lifetime is associated with thermal isomerization to cis,trans-octatetrane in the excited 2 1Ag state. These data are combined with other spectroscopic and kinetic data to obtain estimates for the energies of all three double bond isomers in ground and excited states and the activation energies for double bond isomerization in the excited state.
Studies of mass diffusion of camphorquinone in linear and crosslinked polystyrene by the laser induced holographic relaxation technique
Authors: Zhang J. , Wang C. H. , Chen Z.-X.
Published: July 2002

Laser induced holographic grating relaxation studies of camphorquinone (CQ) diffusing in liner polystyrene (PS) (with both narrow and wide molecular weight distribution) and crosslinked PS (with bis-phenol-A-dimethacrylate as the crosslinking agent) are carried out. Diffusion coefficients of CQ in the polymer hosts are obtained from the decay rate of the intensity diffracted from the laser induced holographic grating. The diffusion coefficients (D) of CQ in polydispersed PS samples are compared with that in nearly monodispersed sample. Diffusion coefficients are found to be equal in both types of polymer samples over the whole temperature range studied, thereby indicating that at high molecular weight the molecular weight distribution has little effect on the diffusion coefficient of CQ. The diffusion coefficient of CQ in crosslinked PS is found to decrease with increasing concentration of the crosslinking agent. It decreases approximately 30 times as the concentration of the crosslinking agent increases to 20%. The relationship between the concentration of the crosslinking agent and polymer free volume is discussed. The diffusion coefficients of CQ in linear and crosslinked PS are also measured as a function of temperature. It is found that the temperature dependence of both D and D/T can be described by a Williams–Landel–Ferry (WLF) equation. However, the WLF constants C1 and C2 differ in both fits. The C2 constant obtained from the D/T vs T fit for linear PS sample agrees with that extracted from the viscoelastic measurement.
Fluids with internal degrees of freedom. I. Extended thermodynamics approach
Authors: Rodríguez R. F. , García-Colín L. S. , del Castillo L. F.
Published: July 2002

Extended irreversible thermodynamics (EIT) is used to derive a complete set of time evolution equations for the state variables describing a general polyatomic fluid. The internal degrees of freedom of the fluid are represented by suitable nonconserved variables. In particular, these time evolution equations are shown to resemble relaxation-type equations. The sign of the unknown coefficients included in such equations is undetermined. However, when a comparison is made with the results obtained from the moment solution method of a kinetic equation for a dilute polyatomic fluid, several of the general features of the phenomenological results are clarified. Indeed, the predictions of kinetic theory fully agree with the basic equations of EIT. This provides a mesoscopic foundation for the theory. The sign of the unknown coefficients appearing in the relaxation-type equations for the nonconserved variables is such that the corresponding relaxation times are positive definite. Therefore, such equations are indeed relaxation equations. Other features of the phenomenological theory are also discussed on the light of the kinetic theoretical results. Further, the usefulness of the theory will be addressed in connection with a subsequent paper where the results here obtained will be used to compute the Rayleigh–Brillouin spectrum for a polyatomic fluid.
Completely asymmetric nuclear quadrupole effects on electron spin echo modulation patterns
Authors: Romanelli M. , Kevan L.
Published: July 2002

Electron spin echo modulation patterns as modified by the presence of a completely asymmetric nuclear qudrupole interaction are evaluated by using a second order perturbation approach. The resulting modulated patterns are analyzed for S=1/2, I=1, and I=5/2 spin systems at different electron–nuclear distances and at different values of the asymmetry parameter. Comparison with the corresponding simulated patterns in the case of axially asymmetric nuclear quadrupole interaction shows remarkable differences both in the period and amplitude of the nuclear modulation.
Rotational potential functions of ammonium ions in crystalline fields of various symmetries. I
Authors: Smith David
Published: July 2002

The potential functions are given for the hindered rotation of ammonium ions, when the site symmetries of the ammonium ions are Oh, Td, D2d, D2h, C3v, C3, S4, C4v, C4h, C2v, C2, Cs, and C1. The procedure is outlined for computing the hindered rotational energy levels of an ammonium ion in crystalline fields of Cs or C1 symmetry. If the barrier to rotation is high, equations are given for the computation of the librational frequencies of ammonium ions in crystalline fields of Oh, Td, D2d, and C3v symmetry.
On understanding the relationship between structure in the potential surface and observables in classical dynamics: A functional sensitivity analysis approach
Authors: Judson Richard S. , Rabitz Herschel
Published: July 2002

The relationship between structure in the potential surface and classical mechanical observables is examined by means of functional sensitivity analysis. Functional sensitivities provide maps of the potential surface, highlighting those regions that play the greatest role in determining the behavior of observables. A set of differential equations for the sensitivities of the trajectory components are derived. These are then solved using a Green’s function method. It is found that the sensitivities become singular at the trajectory turning points with the singularities going as η−3/2, with η being the distance from the nearest turning point. The sensitivities are zero outside of the energetically and dynamically allowed region of phase space. A second set of equations is derived from which the sensitivities of observables can be directly calculated. An adjoint Green’s function technique is employed, providing an efficient method for numerically calculating these quantities. Sensitivity maps are presented for a simple collinear atom–diatom inelastic scattering problem and for two Henon–Heiles type Hamiltonians modeling

intramolecular processes. It is found that the positions of the trajectory caustics in the bound state problem determine regions of the highest potential surface sensitivities. In the scattering problem (which is impulsive, so that ‘‘sticky’’ collisions did not occur), the positions of the turning points of the individual trajectory components determine the regions of high sensitivity. In both cases, these lines of singularities are superimposed on a rich background structure. Most interesting is the appearance of classical interference effects. The interference features in the sensitivity maps occur most noticeably where two or more lines of turning points cross. The important practical motivation for calculating the sensitivities derives from the fact that the potential is a function, implying that any direct attempt to understand how local potential regions affect the behavior of the observables by repeatedly and systematically altering the potential will be prohibitively expensive. The functional sensitivity method enables one to perform this analysis at a fraction of the computational labor required for the direct method.
Dimension dependence of correlation energies in two-electron atoms
Authors: Loeser J. G. , Herschbach D. R.
Published: July 2002

Correlation energies (CEs) for two-electron atom ground states have been computed as a function of the dimensionality of space D. The classical limit D→∞ and hyperquantum limit D→1 are qualitatively different and especially easy to solve. In hydrogenic units, the CE for any two-electron atom is found to be roughly 35% smaller than the real-world value in the D→∞ limit, and about 70% larger in the D→1 limit. Between the limits the CE varies almost linearly in 1/D. Accurate approximations to real CEs may therefore be obtained by linear interpolation or extrapolation from the much more easily evaluated dimensional limits. We give two explicit procedures, each of which yields CEs accurate to about 1%; this is comparable to the best available configuration interaction calculations. Steps toward the generalization of these procedures to larger atoms are also discussed.
Phase-integral and numerical calculations of matrix elements and Franck–Condon factors for Morse oscillators
Authors: Karlsson Finn , Jȩdrzejek Czeslaw
Published: July 2002

Matrix elements and Franck–Condon factors for Morse potentials are calculated both by means of the Fröman–Fröman phase-integral method and by means of numerical integration of the Schrödinger equation with the Numerov method. The results are compared and are found to be in excellent agreement for the cases when the overlap between the classically allowed regions of the states is not too small. In some cases the accuracy of our phase-integral results is better than what can be achieved by numerical methods. The phase-integral method works better than any quasiclassical method in most cases investigated, in particular for the cases when the displacement of the Morse oscillators is not too large.
The potential energy barrier of the Na2 B 1Πu state
Authors: Chawla Gunjit K. , Vedder H. Joachim , Field Robert W.
Published: July 2002

The technique of modulated gain spectroscopy has provided detailed information about the shape of the Na2 B 1Πu state potential barrier to dissociation. By measuring the rotation–vibration energies (to ±0.006 cm−1) of all (v′=27–33) quasibound vibrational levels and the rotation-dependent tunneling rates (as obtained from linewidth measurements) of the highest quasibound vibrational level, v′=33, we have been able to characterize this barrier. Our studies show that the barrier height is U(rmax,J=0)=375.2±3.9 cm−1, relative to the center of gravity of the Na(3s)+Na(3p) atomic limit, and the barrier maximum is located at rmax=6.85±0.02 Å.
On the molecular theory of diffusion and heat conduction in multicomponent solutions
Authors: Altenberger Andrzej R. , Dahler John S. , Tirrell Matthew
Published: July 2002

A statistical mechanical analysis is presented of diffusion and heat conduction in multicomponent systems. Explicit molecular expressions are derived for the generalized wave vector and frequency dependent Onsager kinetic coefficients and transport coefficients appropriate to various reference frames and specific to several different sets of independent thermodynamic forces. Relationships are established among the coefficients referred to different flow reference frames and to the lab frame coefficients as well. It is shown that the reciprocal relations are fulfilled only for certain very specific sets of independent thermodynamic forces.
Excitation transport in solution: A quantitative comparison between GAF theory and time-resolved fluorescence profiles
Authors: Hart David E. , Anfinrud Philip A. , Struve Walter S.
Published: July 2002

Time-correlated single photon counting was used to monitor fluorescence concentration depolarization for DODCI in glycerol. For DODCI concentrations below ∼10−3 M, the present experiments have minimized self-absorption and excitation trapping artifacts to the extent where they contribute negligibly to the observed differences between experimental fluorescence profiles and profiles computed from the Gochanour–Andersen–Fayer three-body theory for excitation transport in solution. The three-body theory accurately describes fluorescence depolarization at the lower dye concentrations. At higher concentrations, the measured decay in Gs(t), the probability that the excitation resides on the laser-excited molecule, is perceptibly slower than predicted by the three-body theory. This deviation may arise from nonrandom dye distributions in solution, rather than from errors in the three-body theory. The experimental decay is equally well described at all concentrations by an earlier analytic theory which was developed by Huber, Hamilton, and Barnett.
Convergence of ℒ2 methods for scattering problems
Authors: Staszewska Grażyna , Truhlar Donald G.
Published: July 2002

We compare 35 different methods for calculating reactance matrix elements from ℒ2 basis sets. By systematically classifying the methods, we are able to draw conclusions about several lines of approach. For example, the explicit subtraction of unscattered waves or the least-squares minimization of the iterative correction do not lead to significant improvements in accuracy. However, expansions of the amplitude density are systematically more rapidly convergent than expansions of the wave function. The most efficient methods are variational methods based on expanding the amplitude density, but the method of moments for the amplitude density may also be useful since it leads to reasonable accuracy with smaller programming effort.
Inelastic electron tunneling spectra of 1- and 2-naphthols
Authors: Ratinen H. , Kiviharju M.
Published: July 2002

Inelastic electron tunneling spectra of 1- and 2-naphthols were measured. Doping from liquid phase and through sublimation was applied. The latter method gave better spectra. The results were compared with corresponding IR and Raman spectra.
Molecular dynamics study of a supercooled soft-sphere fluid
Authors: Amar Jacques G. , Mountain Raymond D.
Published: July 2002

Results obtained from equilibrium molecular dynamics simulations (performed on a 1000-particle system) for a soft-sphere fluid in the supercooled region are presented. The hydrodynamic length lt corresponding to the range of dynamical correlations in the transverse current is found to increase rapidly as the amount of supercooling increases. The shear viscosity and bulk viscosity are also found to increase rapidly in the supercooled region, due to the increasing contribution of the tails of the corresponding correlation functions. While the bond-orientational order parameters Q6 and Q8 do not increase significantly, the lifetimes of the corresponding time correlation functions also increase rapidly in the supercooled region.
The liquid–vapor interface of simple polar fluids. II. Computer simulation
Authors: Eggebrecht J. , Thompson S. M. , Gubbins K. E.
Published: July 2002

The molecular dynamics method is used to study the liquid–vapor interface of the Stockmayer fluid. We compare our results for the singlet density-orientation distribution function and surface tension with the predictions of integral equation and perturbation theory. We find that at low temperature and high moment conditions, dipolar orientations in the interfacial region parallel to the surface are more probable. The pressure tensor, mean squared force, and mean squared torque are also determined as functions of position with respect to the interface. Directional anisotropy in the mobility of particles in the vicinity of the surface is found by decomposing the mean squared displacements into components parallel and perpendicular to the interfacial plane.
Relevance of a two-variable Oregonator to stable and unstable steady states and limit cycles, to thresholds of excitability, and to Hopf vs SNIPER bifurcations
Authors: Bar-Eli Kedma , Noyes Richard M.
Published: July 2002

A stiffly-coupled Oregonator model based on the two independent variables Y and Z has been examined in detail with the use of the stoichiometric factor as a single disposable parameter. The trajectories and periods of the stable limit cycles can be generated with unanticipated accuracy from simple linear differential equations. Transitions between stable limit cycles and stable steady states takes place by means of subcritical Hopf bifurcations. Unstable limit cycles and thresholds of excitability can be recovered by integrating the equations of motion backward in time; such procedures cannot be applied so easily for models based on more than two independent variables. We have examined claims of experimental evidence for saddle-node infinite period (SNIPER) bifurcations and have concluded that all currently available evidence is equivocal. Until unambiguous criteria can be established for identifying SNIPER bifurcations in real systems, and until chemical mechanisms have been proposed which generate such bifurcations, observations should be interpreted by existing models based on Hopf bifurcations. Experimental behaviors are listed which can and cannot be simulated in terms of two independent composition variables.