bluuebunny/crossref_metadata_2025 huggingface dataset:
Towards an analytical three-body potential of Ar2Cl−
Authors: Burcl R. , Cybulski S. M. , Szcze̢śniak M. M. , Chal/asiński G.
Published: July 2002

The nonadditive interactions in the Ar2Cl− cluster are studied using the supermolecular Mo/ller–Plesset perturbation theory in conjunction with the perturbation theory of intermolecular forces. The three-body effect is rigorously dissected into fundamental nonadditive components; the exchange, the induction, and the dispersion. All three terms are important in describing nonadditivity in this cluster. The exchange nonadditivity is further divided into two terms; ES3, the electrostatic interaction between the Cl− and Ar2 subunits due to the appearance of an ‘‘exchange-quadrupole’’ moment on Ar2 [Cooper and Hutson, J. Chem. Phys. 98, 5337 (1993)], and the exchange counterpart of ES3, the X3 term. ES3 term is modeled analytically using the charge-quadrupole electrostatic term. The induction nonadditivity is approximated as an interaction of induced dipoles on two Ar atoms. The three-body dispersion term is fitted to the dipole–dipole–dipole plus dipole–dipole–quadrupole expressions. The sum of all three analytical terms reproduces well the ab initio three-body potential in the region of the trimer minimum and beyond. Possible improvements in the analytical representation are also discussed.
Random sequential adsorption: Long time dynamics
Authors: Baram Asher , Fixman Marshall
Published: July 2002

Analytical expressions are derived for the long time behavior of the coverage density ρ(t) in lattice models of irreversible deposition. For deposition processes on the square lattice with exclusion of neighboring particles out to the nearest or next nearest neighbor shell, accurate estimates of ρ(t) are obtained for all times t.
Coherent translation and libration as seen by a shear viscosity of a classical hard sphere fluid
Authors: Tang Shuang , Evans Glenn T.
Published: July 2002

Thermodynamic transport properties, such as a shear viscosity, can provide evidence of coherent phenomena. In a dilute gas, the coherence takes the form of free translation interrupted by collisions and this effect is reflected in the wavevector dependence of the shear viscosity. In a dense fluid, near the solidification transition or in a glass, the viscosity begins to diverge as a consequence of highly correlated recollisions. In this work, a generalized Langevin formulation of the kinetic theory of hard spheres is used to calculate the shear viscosity in these two limiting regimes. In the collision dominant (i.e., caging) regime, the divergence of the shear viscosity can be compensated by the vanishing self diffusion coefficient. The Dη product is well behaved, illustrates aspects of Stokes–Einstein behavior, in rough accord with the hydrodynamic predictions (obtained using slip boundary conditions) and suggests a Stokes–Einstein breakdown in accord with experiments on glassy ortho-terphenyl liquids.
Transition state dynamics of chemical reactions in clusters: A six-dimensional study of Ar(ClHCl)
Authors: McCoy Anne B.
Published: July 2002

Cluster effects on transition state dynamics are investigated through a time-dependent calculation of photodetachment spectrum of Ar(ClHCl)−. This system is studied by propagating a three-dimensional grid representation of the wave function in the H atom coordinates, coupled to three one-dimensional semiclassical Gaussians in the three lowest frequency normal coordinates of Ar(ClHCl)−. Over the 75 fs propagation times considered here, the motions of the three heavy atoms are very small. Therefore, we expect that the coupled quantum/semiclassical propagation will be in quantitative agreement with a full quantum treatment of this system. The couplings between the quantum and semiclassical degrees of freedom and between the three semiclassical degrees of freedom are introduced through the time-dependent self-consistent field (TDSCF) approximation. A computational bottleneck in applying the TDSCF approximation to such systems is that propagation of each of the modes requires the evaluation of the average of the potential over the wave functions in the three remaining modes. A solution to this problem for H transfer systems is presented and discussed. Comparisons between the dynamics of Ar(ClHCl) and ClHCl indicate that the presence of the Ar atom produces a hole in the wave function that is otherwise cylindrically symmetric. Possible observable experimental consequences of the introduction of the Ar atom to this system are discussed.
A field theory of random heteropolymers near solid surfaces: Analysis of interfacial organization and adsorption–desorption phase diagram
Authors: Gutman Lorin , Chakraborty Arup K.
Published: July 2002

Understanding the interfacial organization of heteropolymers near solid surfaces is an issue of fundamental interest that is relevant for many technological and biological applications. In this paper we address several questions pertaining to the surface-induced ordering and the adsorption–desorption phase behavior of a dilute solution of two-letter random heteropolymers interacting with a solid surface. Our analysis is based on a statistical field theoretic formulation of the propagator for the problem of interest. We employ the replica trick to alleviate analytical difficulties which arise when considering averaging over the sequence distribution of the A and the B units. In order to highlight the effects of the surface, we consider the situation wherein the intersegment interactions are of the excluded volume type while the segment–surface interactions of the A and B segments are arbitrarily different. Within the replica symmetric solution, we show that proper coarse-graining of the interaction potentials leads to exact analytical expressions for the self-consistent propagator of the heteropolymer at theta conditions and for the case where excluded volume interactions prevail. One of our interesting findings is that heteropolymers undergo an adsorption–desorption transition in the vicinity of a surface that interacts with the different types of segments in arbitrarily different ways. This is consistent with our previous numerical findings for much more restricted circumstances.

We explicitly analyze the influence of the fluctuations in the sequence distribution on the conformational organization of the adsorbed chains and obtain the scaling behavior of properties of interest in the vicinity of the adsorption–desorption threshold with respect to the disorder strength and other polymer interaction parameters. Specifically, invoking the Ehrenfest theorem we find that the adsorption–desorption transition at theta conditions is a second-order phase transition while in the case where excluded volume interaction prevails the transition becomes first order. We also obtain exact analytical expressions for the adsorption–desorption threshold. The threshold exhibits quite an unusual dependence on the strength of the disorder. Finally, we compute the point of onset of repulsive forces between plates that confine a random copolymer solution as a function of chain sequence distribution. We suggest specific experiments employing the surface force apparatus that could directly test our predictions.
CIONO−2: A strongly bound negative ion
Authors: Doren Jane M. Van , Viggiano A. A. , Morris Robert A. , Miller Thomas M.
Published: July 2002

The gas phase reaction of NO2− with ClONO2 has been investigated at 300 K using a selected ion flow tube. Reaction occurs efficiently, k=1.5 (±40%)×10−9 cm3 s−1. The major ionic products are NO3− (∼90%) and ClONO2− (∼10%). The cluster ion NO2− (ClONO2) is formed in <1% of the reactive collisions under our experimental conditions. The observation of ClONO2−, arising from charge transfer from NO2−, indicates that the electron affinity of ClONO2 is ≥2.1 eV. We have determined the apparent second-order clustering rate coefficient for NO3−+ClONO2 to be 3.0 (±40%)×10−11 cm3 s−1 at 300 K in 0.5 Torr He.
Detection of CH in an oxyacetylene flame using two-color resonant four-wave mixing technique
Authors: Hung Wen-Ching , Huang Min-Lon , Lee Yu-Chang , Lee Yuan-Pern
Published: July 2002

CH free radicals in an oxyacetylene flame at atmospheric pressure were detected by means of the two-color resonant four-wave mixing technique. Ground-state grating schemes with ω1=ω2 (pump) and ω3 (probe)=ω4 (signal) were used; ω1(=ω2) and ω3 are in resonance with distinct rovibronic transitions of B 2Σ−−X 2Π and A 2Δ−X 2Π of CH, respectively. Varying the relative polarization of the pump and probe beams significantly affects the ratio of signal to noise. Two schemes with cross polarization pairs (ω1⊥ω2) and (ω3⊥ω4) produced results best suited to detect trace species in the luminous, dense medium; the relative J dependence observed for these schemes agrees with theoretical predictions. The double-resonance feature of this technique, advantageous to identify congested lines, is also demonstrated. When an A–X transition line is employed as a probe, previously unobserved predissociative levels of B 2Σ− are detected; the R(12) line of B←X (1,0) transion has a halfwidth greater than 10 cm−1, corresponding to a lifetime of 0.5 ps.
Temperature dependence of nitrogen dissociation on metal surfaces
Authors: Katz Gil , Kosloff Ronnie
Published: July 2002

The effect of crystal temperature on the dissociation dynamics of nitrogen on a catalytic metal surface is studied. The framework is a nonadiabatic mechanism where the nitrogen crosses from the physisorption potential energy surface to a dissociative chemisorption potential. Within this framework the quantum dynamics is solved in three degrees of freedom including surface vibrational excitation. In general, surface vibrations promote the dissociation. However, if the nonadiabatic coupling potential is peaked at a restricted geometry, exciting the surface vibrations can hinder dissociation. This effect is maximized for the N2/Fe mass ratio which leads to a negative temperature effect on the dissociation. For higher surface metal masses this effect disappears (N2/Ru) and even reverses to a positive temperature effect for the N2/Re mass ratio.
Potential energy surfaces and dynamics in core-ionized and core-excited states
Authors: Schulte H. D. , Cederbaum L. S.
Published: July 2002

Electronic properties of core states are discussed with respect to symmetry and localization. Particular attention is paid to the potential energy surfaces and to the construction of the Hamiltonian governing the nuclear dynamics in the manifold of interacting core states. For a system with N equivalent sites on which electrons (holes) may localize, the total multidimensional Hamiltonian is reduced to N equivalent Hamiltonians of low dimension and distorted symmetry. The various contributions to these Hamiltonians due to core and valence electrons and their interaction are discussed. The analytical results are compared with the results of ab initio computations on C3H+3 and B3N3H6 which serve as examples throughout.
Kinetics of proteinlike models: The energy landscape factors that determine folding
Authors: Betancourt Marcos R. , Onuchic José Nelson
Published: July 2002

The factors responsible for fast folding and stable 2D lattice proteins are studied by Monte Carlo kinetic simulations and by full enumeration. For weak effective coupling between residues ( i.e., high temperatures) the folding time is mainly controlled by the free energy gradient. When this coupling is strong, entrapment in local minima plays a major role. The energy gap separating the native state from the remaining conformations shows some correlation to the folding and unfolding times for a variety of sequences. These concepts are related to the ratio Tf/Tg proposed by Bryngelson and Wolynes. One source of local minima are strong, non-native (hindering) contacts. An exclusive increase in hindering contacts shows a strong correlation between the energy gap and the folding/unfolding times. Other local minima results from several native contacts that, if formed prematurely, interfere with the formation of others. To study these factors, we use a semispecific model characterized by attractive interactions between similar residues. A procedure to design stabler sequences with smoother landscapes is discussed. By comparing this model to the hydrophobic HP model, we show how higher potential heterogeneity can be used to improve folding.
Photoinduced electron transfer from π-conjugated polymers onto Buckminsterfullerene, fulleroids, and methanofullerenes
Authors: Janssen René A. J. , Hummelen Jan C. , Lee Kwanghee , Pakbaz Khashayar , Sariciftci N. Serdar , Heeger Alan J. , Wudl Fred
Published: July 2002

We present near-steady-state photoinduced absorption (PIA), photoluminescence, and light-induced electron spin resonance (LESR) studies on photoinduced electron transfer reactions from poly(bis-2,5-epi-cholestanoxy-1,4-phenylene vinylene) (BeCHA-PPV) as a donor to Buckminsterfullerene (C60) and a number of functionalized fullerenes as acceptors. We show that functionalized fullerenes with a solubilizing side chain linked to C60 in a [5,6] (fulleroid) or [6,6] (methanofullerene) fashion, are efficient electron acceptors towards BeCHA-PPV, as evidenced from changes in the PIA, the efficient quenching of the photoluminescence, and the LESR spectra of BeCHA-PPV cation and fullerene anion radicals. The increased miscibility of the functionalized fullerenes with the conjugated polymer as compared to C60, can be important for the preparation of future electrooptic devices, based on the photoinduced charge separation.
Resonances in the chaotic regime. Interpretation in terms of periodic orbits
Authors: Marston C. Clay
Published: July 2002

A method using classical mechanics to identify exact resonance eigenstates of a reactive scattering system is presented. Semiclassical approximate eigenstates are first constructed from classical trajectories of a defining topology. The semiclassical wave functions then serve as initial states for an exact quantum time-dependent wave packet propagation. The quality of the initial semiclassical approximation is readily apparent from the extent to which the spectrum of the resulting dynamics is dominated by a single peak. The systematic construction of the resonant eigenstate spectrum in association with a progression through defining classical topologies is emphasized, and the predictive implications of the method for approximating essential resonant eigenstate nodal structure and energies are discussed.
An evaluation of correlation-consistent basis sets in calculating the structure and energetics of (H3Si)2O, H3SiOH, and H3SiO−
Authors: Nicholas John B. , Feyereisen Martin
Published: July 2002

We studied the structure of disiloxane (H3Si–O–SiH3), silanol (H3Si–OH), and the silanol anion (H3Si–O−) with ab initio molecular orbital theory and the correlation consistent polarized basis sets of Dunning and co-workers. We present results for the correlation consistent polarized valence double zeta (cc-pVDZ), triple zeta (cc-pVTZ), and quadruple zeta [cc-pVQZ(-g)] basis sets. Optimized geometries and energies are given at both the restricted Hartree–Fock (RHF) level and with the inclusion of electron correlation by second order Mo/ller–Plesset perturbation theory (MP2). The correlation consistent basis sets provide a systematic expansion of the orbital basis set, with each set of additional functions adding a similar contribution to the correlation energy. We find that the calculated molecular properties show exponential convergence with increasing basis set size. These calculations answer long-standing questions regarding the structure and barrier to linearization of disiloxane. Results at the highest level of theory [MP2/cc-pVQZ(-g)] for disiloxane gave a Si–O–Si bond angle of 147.0°, a Si–O bond length of 1.641 Å, and a barrier to linearization of 0.4 kcal/mol. All of these values are in excellent agreement with experimental results. Similar calculations for silanol gave a Si–O bond length of 1.655 Å, an O–H bond of 0.955 Å, and a Si–O–H angle of 117.9°. The MP2/cc-pVQZ(-g) deprotonation energy (ΔE0) for silanol, calculated as the energy difference between silanol and its anion, is −366.6 kcal/mol. The effects of electron correlation at the MP4 level are studied in all three molecules using the cc-pVDZ and cc-pVTZ basis sets.
Solid friction in gel electrophoresis
Authors: Burlatsky S. F. , Deutch John M.
Published: July 2002

We study the influence of solid frictional forces acting on polymer chains moving in a random environment. We show that the total reduction in the chain tension resulting from the small friction between a polymer and fixed entanglement points is a steep nonlinear function of the number of entanglements (exponential for stretched chains). Therefore, solid friction can drastically change the dynamics and lead to trapping of long chains with a large number of entanglements. We present explicit results for the decrease of the chain tension in the presence of solid friction forces, for the limiting tension values, and for trapping thresholds for charged chains in an external field. The trapping threshold increases with the decrease of the field strength and/or application of pulsed field sequences as compared to static high fields. Our theoretical results on trapping thresholds are in good agreement with experimental data on DNA electrophoresis. Our model also predicts new nonlinear dependencies for the velocity of charged chains that are dragged through the gel by external forces. We present explicit dependencies of the velocity on charge, external force and polymer length for charged chains in external fields and for chains dragged by external forces that are applied only to chain ends. These dependencies are different in large and small force (field) limits, which correspond to stretched and harmonic chains. The strong mobility on length dependence which results from solid friction forces can serve to separate long linear charged polymers of different molecular weight.
A linear chain hybrid quantum/classical model for the photodissociation and recombination of I2(A) in rare gas matrices
Authors: Liu Li , Guo Hua
Published: July 2002

The photodissociation and subsequent recombination of I2(A) in low temperature Ar or Kr matrices are investigated theoretically in a one-dimensional model. The dynamics of the I2 molecule, which is embedded in a linear chain of rare gas atoms, is explicitly treated with time-dependent quantum mechanics, while the motion of the bath atoms is described by classical trajectories. The quantum subsystem is self-consistently coupled with the classical bath via time-dependent mean fields. Our calculations show substantial energy transfer from the dissociating iodine to the solvent, and subsequent energy propagation in the solvent. The results indicate that coherence in the recombined I2 wave packet is not completely destroyed, in agreement with experiment and previous simulations. The temperature and wavelength dependence of the dynamics are also investigated.
Cross section of the transverse relaxation of NH3 colliding with spherical perturbers
Authors: Odashima Hitoshi , Ikeda Hirohiko , Shimizu Tadao
Published: July 2002

The cross sections of the transverse relaxation of NH3 colliding with various spherical perturbers are measured and examined as a function of molecular parameters, such as ionization energy, polarizability, and mass. The observed cross sections show good correlation with these molecular parameters. An empirical formula in the unit of Å2, σ=25.5 (α2M)1/6, is obtained, where σ is the cross section, α is the polarizability of the perturber in the unit of Å3, and M is the reduced mass of the colliding molecules in the unit of g/mol. This expression can be used to evaluate the cross sections of collisions between NH3 and other spherical perturbers.
Femtosecond studies of the iodine–mesitylene charge-transfer complex
Authors: Pullen Stuart , Walker Larry A. , Sension Roseanne J.
Published: July 2002

Femtosecond laser studies have been performed to investigate the initial photodissociation reactions of I2–mesitylene charge transfer complexes. Photodissociation occurs along both the I2–mesitylene ‘‘bond’’ and the I–I bond with a branching ratio of 2:3 for the two reaction coordinates. Following excitation at 400 nm, geminate recombination occurs along both reaction coordinates. The reformed I2–mesitylene complexes are formed vibrationally hot and relax on a time scale of 13 ps. The I–mesitylene spectrum is fully developed within 500 fs of the pump pulse. Approximately 40% of the I–mesitylene complexes undergo geminate recombination on a time scale of 14 ps. Most of the remaining complexes recombine with their original partners on a time scale of 400 ps. The initial anisotropy of the photoproduct absorption is 0.09±0.02. This low anisotropy is a direct result of the geometry of the complex and nature of the electronic transition rather than indicative of ultrafast motion toward an asymmetric transition state preceding dissociation.
On the structure and stability of geometrical isomers of N3F
Authors: Chaban Galina , Yarkony David R. , Gordon Mark S.
Published: July 2002

The potential energy surfaces for the N3F molecule have been studied using multiconfigurational wave functions. Two new isomers were found, one on the singlet (1A′) and one on the triplet (3A″) surface. Both isomers have a three-membered cyclic structure and Cs symmetry. The singlet cyclic isomer is endoergic relative to the open fluorine azide by 15–17 kcal/mol. Its kinetic stability is close to the stability of the open isomer: the barrier separating the cyclic isomer from the dissociation products N2(X 1Σ+g)+NF(a 1Δ) is about 13–17 kcal/mol and is lower than the barrier to isomerization. The triplet cyclic isomer is much higher in energy (about 70 kcal/mol), with a barrier to dissociation to N2(X 1Σ+g)+NF(X 3Σ−) on the order of 15 kcal/mol. Crossings of the 1A′ and the 3A″ surfaces may allow the cyclic singlet isomer to predissociate to the ground state products, N2(X 1Σ+g)+NF(X 3Σ−). It is shown, however, that the singlet–triplet surface of intersection lies ‘behind’ the barrier to singlet decomposition, so that spin-forbidden predissociation will not preclude detection of cyclic N3F.
Shock wave studies of the reactions HO+H2O2→H2O+HO2 and HO+HO2→H2O+O2 between 930 and 1680 K
Authors: Hippler H. , Neunaber H. , Troe J.
Published: July 2002

Monitoring resonance absorption at the Q1(4) line near 308.4 nm, HO concentration-time profiles were recorded during the thermal decomposition of H2O2 in shock waves between 930 and 1680 K. The results were analyzed with respect to the reactions HO+H2O2→H2O+HO2 (2) and HO+HO2→H2O+O2 (3). Above 800 K, reaction (2) shows a strong increase of the rate constant with temperature. Over the range 240≤T≤1700 K the apparent rate constant is represented as k2=[1.7⋅1018⋅exp(−14 800 K/T)+2.0⋅1012 exp (−215 K/T)] cm3 mol−1 s−1. The rate constant k3 decreases from a value of 2.0⋅1013 at 1100 K to a minimum value of 1.1⋅1013 at 1250 K, and rises again to a value of 4.5⋅1013 cm3 mol−1 s−1 at 1600 K. The anomalous temperature dependences of k2 and k3 suggest mechanisms involving intermediate complex formation.
Binding energies of carbazole⋅S van der Waals complexes (S=N2, CO, and CH4)
Authors: Bürgi Thomas , Droz Thierry , Leutwyler Samuel
Published: July 2002

Mass-selective ground-state vibronic spectra of molecular van der Waals complexes carbazole⋅S, S=N2, CO, and CH4, were measured by stimulated emission pumping followed by resonant two-photon ionization of the vibrationally hot complexes. S0-state vibrational modes were accessed from ≊200 cm−1 up to the ground-state dissociation limit D0(S0) of the van der Waals bond. Above D0, efficient vibrational predissociation of the complexes occurs, allowing accurate determination of the van der Waals dissociation energies as 627.2±7.9 cm−1 for N2, 716.5±29.8 cm−1 for CO, and 668.6±15.1 cm−1 for CH4. In the S1 excited state, the van der Waals binding energies increase to 678.5±8.0, 879.2±29.9, and 753.8±15.2 cm−1, respectively. The relative increases upon electronic excitation are about 8% and 13% for N2 and CH4, similar to the analogous rare gases Ar and Kr. For CO, the relative increase of van der Waals binding energy is 23%. The differences are primarily due to electrostatic interactions.
Effects of isotopic substitution on Eley–Rideal reactions and adsorbate-mediated trapping
Authors: Jackson Bret , Persson Mats
Published: July 2002

The dynamics of a direct reaction between a gas phase H atom and an H atom adsorbed on a Cu surface are explored. This exothermic Eley–Rideal reaction is studied by implementing both 3D quantum and quasiclassical scattering calculations. The metal surface is assumed to be flat. The effects of substituting either or both of the H atoms with D are examined in detail for five different model potential energy surfaces. The reaction cross sections, and the translational, rotational, and vibrational state distributions of the product molecules are computed. A process is considered in which the incident atom transfers enough of its kinetic energy normal to the surface into its motion parallel to the surface and into the adsorbed particle to become trapped. These trapped ‘‘hot’’ atoms can go on to react with other adsorbed atoms, giving ‘‘hot’’ products, as in the Eley–Rideal reaction. We examine the dependence of this mechanism on isotopic substitution and incident energy, in an effort to determine how one might distinguish between the two processes.
Dielectric and viscoelastic normal-mode relaxation in entangled, polydisperse cis-polyisoprene melts
Authors: Fodor Jeffrey S. , Huljak John R. , Hill Davide A.
Published: July 2002

We investigate the effects of polydispersity on dielectric and viscoelastic normal-mode relaxation of cis-polyisoprene melts above entanglement. Measured dielectric and mechanical spectra and relaxation moduli of polydisperse samples are compared to the predictions of models combining either simple reptation, double reptation or des Cloizeaux’ time-dependent-diffusion single-chain autocorrelation functions (ACF) to linear or nonlinear (Tsenoglou) mixing rules. The blending rules are tested by employing experimental data, thereby eliminating uncertainties associated with the experimental molecular weight distribution (MWD) and choice of specific forms for the ACFs. Predictions from the various models based on the MWD from size exclusion chromatography are compared to the experimental data as well. We find that dielectric spectra are satisfactorily captured by a linear mixing rule, but quantitative description of the rheology requires the use of the nonlinear (Tsenoglou) mixing rule. The single-chain ACF of reptation with a 3.7 power-law dependence of the relaxation time on molecular weight accurately predicts both dielectric and viscoelastic spectra. The double reptation ACF, while being inadequate in the dielectric case, gives viscoelastic predictions that are nearly identical to those obtained with the simple reptation ACF. The des Cloizeaux ACF fails in the dielectric case and is less satisfactory than simple or double reptation in the viscoelastic case. This limited evidence indicates that the ACFs for end-to-end vector reorientation (dielectric relaxation) and entanglement ‘‘dissolution’’ (viscoelastic relaxation), although both originating in chain reptation, reflect different mechanisms.
Influence of the solvent size on the behavior of model-polymers in solution
Authors: Komorowski Joachim , Bruns Wolfgang
Published: July 2002

Two-dimensional chains with C=4 up to C=20 segments are investigated by hard disks molecular dynamics simulations. Solvent particles are explicitly taken into account. In addition, the ratio S/σC of the diameters of the solvent molecules S and the chain segments C is varied between 0.5≤σS/σC≤40.0. Several equilibrium properties show that the polymer dimensions strongly depend on the solvent size. The data are compared to results obtained for a single chain generated by a Monte Carlo simulation. Studies of the mean square (m.sq.) end-to-end distance 2〉, the m.sq. radius of gyration 2〉 and some distribution functions indicate that the polymer coil becomes more compact in the presence of solvent molecules. Irrespective of C the most coiled configurations are found for systems with S/σC=1. For very large S/σC all quantities considered correctly approach the values found for isolated chains.
Conductivity versus spin-lattice relaxation: Contrasting behavior in a correlated disordered structure
Authors: Maass Philipp , Meyer Martin
Published: July 2002

We investigate by Monte Carlo simulations the frequency dependent conductivity σ(ω,T) and the spin lattice relaxation rate 1/T1(ω,T) due to the hopping motion of noninteracting particles in an uncorrelated and correlated disordered energy landscape. The conductivity shows a pronounced dispersion at low temperatures in both the correlated and the uncorrelated case, reminiscent of the non-Debye behavior observed in many disordered materials. In contrast, 1/T1(ω,T) exhibits the typical non-BPP form for uncorrelated energy disorder, but obeys an ordinary BPP-type behavior in the presence of finite correlations. Our results show that in general a simple relation between σ(ω,T) and 1/T1(ω,T) does not exist and that it might be difficult to account for the experimental findings, if interactions between the mobile particles are neglected.
Backbone ordering in amphiphile monolayers
Authors: Schofield Jeremy , Rice Stuart A.
Published: July 2002

In this paper we describe a lattice density functional theory for the rotator-herringbone phase transition in a liquid supported monolayer system of long chain acids or alcohols. It is assumed that both phases have long-range translational order and that the rotator phase is characterized by a uniform distribution of backbone orientation. We find that orientational interactions among the backbone planes are sufficient to drive a second order transition, and we characterize the phase of the system by a parameter associated with the width of the peaks in a distribution which describes the orientations of the backbones. We also briefly discuss generalizations of the theory to include lattice vibrations and distortions.
Competing predissociation and preionization in the photoabsorption of H2 above the ionization threshold
Authors: Stephens J. A. , Greene Chris H.
Published: July 2002

We present a straightforward multichannel quantum defect theory (MQDT) procedure to calculate the broadening of preionization resonances due to competing predissociation in the ionization continuum of molecular hydrogen. We discretize the dissociative continuum by forcing the ionic vibrational basis functions to vanish at a boundary R0, after which we average the differential oscillator strength over a range R0+dR. The relevant averaging range dR depends on the density of discrete vibrational levels in the vicinity of selected preionization resonances. This approach yields directly the full spectral linewidth including the effect of predissociation. We apply the method to regions of the photoabsorption spectrum of para-H2 (v″=0, J″=0) between the photon wavelengths 782.0 and 786.0 Å. This spectral region exhibits extensive resonance structures due to preionization and predissociation. We obtain semiquantitative agreement with linewidths and branching ratios for ionization and dissociation obtained from experiment, and the MQDT/R-matrix method of Jungen [Phys. Rev. Lett. 53, 2394 (1984)].
Random matrix treatment of intramolecular vibrational redistribution. II. Coriolis interactions in 1-butyne and ethanol
Authors: Go Jungsug , Perry David S.
Published: July 2002

A random matrix methodology has been applied to simulate the molecular eigenstate resolved infrared spectra of the 1-butyne ν16 band and the ethanol ν14 band. In these methyl C–H stretch bands, each rotational transition is fragmented into a clump of molecular eigenstates. The frequencies and intensities of these discrete features carry information about the rate and mechanism of the intramolecular vibrational redistribution (IVR) which would follow the coherent excitation of the zero-order state. The simulations include anharmonic and Coriolis x-, y-, and z-type interactions. These interactions mix the bright state with the bath and also mix the bath states with each other. Since the vibrational identities of the bath states are assumed to be sufficiently mixed, the vibrational parts of the coupling matrix elements are treated stochastically following the development in Paper I of this series [J. Chem. Phys. 98, 6665 (1993)]. The rotational parts of the matrix elements are treated dynamically based on the known rotational quantum number dependence of the Coriolis effect. A stochastic treatment cannot expect to reproduce the detailed line positions and intensities of the experimental spectra, therefore three measures of IVR are used as the basis for comparison of the simulation with experiment.

The measures are the dilution factor φd, the interaction width Δε, and the effective level density ρeffc. In the presence of multiple coupling mechanisms (near the best fit to the ethanol ν14 band), the correlations between φd and Δε and the bright-bath Coriolis coupling mechanisms follow the expected trends. It was also found that ρeffc is sensitive to the x, y Coriolis coupling among the bath states. The results were not sensitive to the z-type Coriolis coupling among the bath states in the region of the ethanol simulation, but ρeffc was sensitive to it in the simulation of the 1-butyne ν16 band. Best-fit coupling parameters were obtained for both simulated bands. The rms bright-bath z-type Coriolis coupling was found to be 0.028±0.005 cm−1 which is about three times the value obtained from a naive approach which neglects the interaction of the multiple coupling mechanisms. A direct count vibrational level density, ρvib, provided good agreement with the experiments when a full treatment of the torsional modes was included and a 20% enhancement of the density from neglected diagonal anharmonicities was added. A method of quantifying the conservation of the rotational quantum number, K, is provided by the inequalities, ρvib≤ρeffc≤(2J+1)ρvib. For 1-butyne, ρeffc is closer to ρvib than for ethanol indicating that K is more nearly conserved. While this work treats only anharmonic and Coriolis coupling, the random matrix formalism provides the ability to treat a wide variety of coupling schemes.
Measurement of the beam intensity in a laser desorption jet-cooling mass spectrometer
Authors: Boogaarts Maarten G. H. , Meijer Gerard
Published: July 2002

In a laser desorption jet-cooling molecular beam spectrometer the concentration of translationally and internally cooled laser desorbed organic molecules that can be achieved is experimentally determined. Sensitive direct absorption detection of laser desorbed jet-cooled diphenylamine (DPA) via cavity ring down (CRD) spectroscopy on the S1←S0 transition around 308 nm is used to measure the line-integrated absolute absorption of the pulse of laser desorbed DPA molecules. The absolute cross section for the various vibrational bands of the electronic transition that is used, is determined in a separate two-color ionization experiment. It is concluded that the optimum beam intensity that is obtained with laser desorption is comparable to the beam intensity that is obtained in the same spectrometer by conventional seeding of the desired species at a partial pressure of 10−4.
ESR observation of the H⋅⋅⋅H, H⋅⋅⋅D, and D⋅⋅⋅D spin-pair radicals in rare gas matrices
Authors: Knight Lon B. , Rice William E. , Moore Louie , Davidson Ernest R.
Published: July 2002

The first experimental observation of the fundamentally important spin-pair radicals, H...H, H...D, and D...D is reported. ESR (electron spin resonance) studies of rare gas matrices near 4 K containing extremely high concentrations of H and D atoms revealed the presence of these spin exchange-coupled molecules whose ESR spectra are analyzed in terms of a 3Σ electronic ground state. The observed D tensor (zero-field splitting) for a specific isotopic spin pair was surprisingly constant in all four rare gases but showed large changes among the three isotopic species. As expected, the hyperfine splitting (Aiso) for the spin-pair was observed to be one-half that of the free H or D atom.
Scattering angular distributions in collisionally activated dissociation of some high mass ions: Analysis of mass-analyzed ion kinetic energy peak shapes
Authors: Lee Young Jin , Kim Myung Soo
Published: July 2002

A method has been developed to determine scattering angular distribution in collisionally activated dissociation (CAD) of high mass ions by analyzing translational energy spectra recorded by mass-analyzed ion kinetic energy spectrometry. Scattering angular distributions have been obtained for CAD of C6F5I+̇, Cr(CH3COCHCOCH3)+̇3, and Cs5I+4 with He. Large energy losses observed in the translational energy spectra are attributed to the elastic energy transfer to the collision gas accompanying collisional deflection. Deflections of the parent ions get larger as the translational energy for relative motion in the center-of-mass coordinate system decreases. This has been explained by the reactive line-of-centers model. Either vibrational excitation via momentum transfer or vibronic excitation via nonadiabatic interaction is compatible with the experimental data. On the other hand, electronic excitation via the Franck–Condon process is not a feasible mechanism to explain the excitation process in CAD of high mass ions at keV translational energy.
Secondary ion mass spectrometry without secondary ion emission. Recombinative scattering of hyperthermal Cs+ ions from a Si(111) surface adsorbed with water
Authors: Yang M. C. , Lee H. W. , Kang H.
Published: July 2002

Collision of hyperthermal Cs+ ion beams with a Si(111) surface partially covered with water gives rise to emission of CsX+ cluster ions (X is a surface atom or molecule) even when the monomer X+ ions are not produced. The yield for atomic and cluster ion emission is examined as a function of Cs+ collision energy, based on which, we propose that CsX+ species are formed by recombination of the scattered, low kinetic energy Cs+ ions and the gaseous neutral species emanating from a surface. It is also demonstrated that under this condition the secondary neutral flux contains a large fraction of molecular units.
Microcalorimetric investigation of the upper phase boundary of water-in-oil microemulsions
Authors: Vollmer D. , Ganz P.
Published: July 2002

Experimental data on the thermodynamics of phase separation of an ionic microemulsions [sodium bis(2-ethylhexyl)sulfosuccinate-water-isooctane] are presented. They are obtained by differential scanning microcalorimetric measurements across the upper phase boundary. Sample compositions are chosen in such way that the transition resembles a liquid-gas-like phase transition. Increasing attractive interaction between the droplets is assumed to cause an instability against phase separation into a system with two droplet phases of different droplet density, but equal droplet size on the average. Our data yield first thermodynamic information on the dynamics related to this phase transition. For finite scan speed we observe two peaks in the specific heat. The first peak arises at the phase separation temperature and it is immediately followed by another one. The enthalpy related to the second peak depends quadratically on droplet size and linearly on droplet concentration. We propose that internal degrees of freedom due to hopping possibilities of the surfactant molecules in the contact region of touching droplets form its origin. In the limit of zero scan speed both peaks merge. Together they give rise to the latent heat of the first order phase transition.
Observation of spin-correlated radical pairs via two-dimensional electron paramagnetic resonance nutation spectroscopy
Authors: Hanaishi Ryuji , Ohba Yasunori , Akiyama Kimio , Yamauchi Seigo , Iwaizumi Masamoto
Published: July 2002

Two-dimensional (2D) Fourier-transform (FT) electron paramagnetic resonance (EPR) nutation experiments were done in order to verify anomalous flip-angle dependence of a FT–EPR spectrum of a photochemically generated spin-correlated radical pair (SCRP). It was demonstrated by both the experiment and theory that the signals of SCRP have additionally higher and lower nutation frequencies than that of a free radical. This spectroscopy provides a direct spectroscopic evidence of spin-correlation within SCRP, and enables one to partially separate SCRP signals from free radical signals in the 2D EPR spectrum.
Autoionizing Rydberg and zero electron kinetic energy states in Ar
Authors: Muhlpfordt A. , Even U.
Published: July 2002

We present spectroscopic data for the autoionizing np Rydberg manifold of Ar atoms together with the lifetimes of its Zeke states (Rydbergs of very high n’s studied by time resolved ZEKE spectroscopy). The lower n Rydberg states have finite widths, indicating a fast autoionization process that diminishes with increasing n (as n3). This trend cannot be used to predict the behavior of the high Zeke states whose lifetimes are almost 2 orders of magnitude longer than the lifetimes extrapolated from the low n values.
The dipole moment of 18-crown-6: Molecular dynamics study of the structure and dynamics of the macrocycle in vacuo and in cyclohexane
Authors: Leuwerink F. T. H. , Briels W. J.
Published: July 2002

We have performed very long simulations of the 18-crown-6 molecule in the gas phase and in cyclohexane. For the isolated molecule we have used two different sets of charges. For all simulations the average dipole moment was in moderate agreement with experiment. Therefore we have examined in some detail the theoretical models used for the interpretation of the experiments. We propose a new formula, based on the Kirkwood equation, to calculate the molecular dipole moment from the experimental dielectric constants. With previously published experimental data, we have calculated a dipole moment that is somewhat larger than the originally reported value. We conclude that the charges that have been used in all potential models up to now may, at best, be treated as effective charges and that polarization is expected to be important. We made an extensive investigation of the structure of 18-crown-6 during the simulations. It was observed that conformational statistics was almost the same in vacuo and in the apolar cyclohexane. The structure of the crown ether is found to fluctuate around the centrosymmetric Ci conformation. A comparison is made with previously published statistical mechanical studies. We also examined the average shape of 18-crown-6 by looking at the mass distribution within the molecule. Again it was found that the crown ether, on average, displays an elliptical shape, consistent with the other results. Further, it was found that many of the samples show the same structural features, although they do not exhibit the same conformation. Finally, the dynamics of the different systems was investigated. As expected, it has been found that the solvent slows down the dynamics of the crown ether molecule.
Search of the liquid–vapor coexistence of the two-dimensional restricted primitive model
Authors: Manificat G. , Caillol J.-M.
Published: July 2002

We report Monte Carlo simulations of a two-dimensional classical Coulomb system with a short range attractive interaction between charges which can be varied independently from the electrostatic interaction. We show that, as this attractive interaction is weakened, the critical temperature of the liquid–vapor transition decreases and the density of the gas, in equilibrium with the liquid, also decreases. But the situation of a pure soft core Coulomb system was not reached.
Photoinduced charge transfer reactions at surfaces: CF3I on Ag(111)
Authors: Sun Z.-J. , Schwaner A. L. , White J. M.
Published: July 2002

The surface photochemistry of submonolayer to multilayer amounts of CF3I, adsorbed on Ag(111) at 95 K, has been studied using 248 and 193 nm pulsed laser excitation. For low doses, there is some thermally activated dissociation, limited to 30% of the first monolayer, to form adsorbed CF3 and I. The remaining CF3I adsorbs molecularly. Neither CF3 nor I is photoactive, but adsorbed CF3I is photodissociated, by C–I bond cleavage, at both 248 and 193 nm. A fraction of the resulting CF3 and I desorbs during photolysis; the remainder is retained as chemisorbed CF3 and I. The former processes were probed using time-of-flight and Fourier transform mass spectrometry. The retained products were detected by post-irradiation temperature programmed desorption and Auger electron spectroscopy. The photochemistry varied with wavelength and coverage. Regarding the mechanism, for both 193 and 248 nm, there is good evidence that both submonolayer and multilayer CF3I molecules absorb photons and dissociate into CF3 and I, i.e., direct photodissociation. There is evidence, based on time-of-flight distributions of CF3 photofragments, that I(2p1/2), electronically excited I, is produced at both wavelengths, while ground state atomic iodine is produced only at 248 nm. At both 193 and 248 nm, and for coverages up to three monolayers, there is also evidence for a charge transfer process involving hot electrons produced by photon absorption in Ag(111), i.e., substrate mediated photodissociation. These hot carriers attach to CF3I, and the resulting anion dissociates into CF3 and I−. The latter is detected by Fourier transform mass spectrometry and the former as a low-velocity component in time-of-flight mass spectrometry. At 193, but not 248 nm, there is evidence for a second, substrate independent, charge transfer process also leading to CF3 and I−. For coverages exceeding ten monolayers, approximately 80% of the reaction was through this channel, the remaining 20% occurring through the direct photodissociation channel. To account for this second charge transfer channel, photoinduced intermolecular charge transfer is proposed.
Pattern recognition by polyelectrolytes
Authors: Muthukumar M.
Published: July 2002

The statistical mechanical problem of recognition of different patterns imprinted on a surface by polyelectrolyte chains is studied. The necessary condition for the binding of a polyelectrolyte in a salt solution to an oppositely charged pattern is derived in terms of the charge densities of the polymer and the pattern, Debye length, characteristic size of the pattern, and the molecular weight of the polymer. The key features of the criterion for the binding are supported by Monte Carlo simulations. We find the kinetics of the recognition process to take place in two distinct stages: a fast complexation without registry and a very slow occurrence of registry. We show that the distance between different trajectories in the second stage diverges with time due to the untimely minimization of temporal free energy.
G2 ab initio calculations of the enthalpy of formation of hydrazoic acid, methyl azide, ethyl azide, methyl amine, and ethyl amine
Authors: Rogers Donald W. , McLafferty Frank J.
Published: July 2002

G2 calculations on the title compounds yield total energies, E0=−164.560 33, −203.776 24, −243.004 79, −95.666 91, and −134.894 58 hartrees and ΔHf298=70.4, 71.0, 64.5, −5.9, and −12.4 kcal mol−1, respectively. Agreement with literature values for hydrazoic acid and both amines is good. We discuss the calculation for methyl and ethyl azide in relation to our recent experimental results from hydrogen calorimetry of a group of higher organic azides. While a substantial improvement over the existing literature value, the calculation yields an estimate for ΔHhyd298=−76.9 kcal mol−1, that is, 2.2 kcal mol−1 less exothermic than the lower limit of our estimated experimental uncertainty.