bluuebunny/crossref_metadata_2025 huggingface dataset:
Theory of photon echoes and hole burning in low temperature glasses: How good are the standard approximations?
Authors: Geva Eitan , Skinner J. L.
Published: July 2002

The theory of optical photon echo and hole burning spectroscopies in low temperature glasses is discussed within the framework of the tunneling two-level system and stochastic sudden jump models. Exact results for the relevant theoretical quantities involve certain averages over the distributions of the two-level system energies and relaxation rates. The standard approximations for these averages are critically examined, for experimentally realistic parameters, via comparison to numerically exact calculations. The general conclusion is that the standard approximations are often used under conditions where they are not expected to be quantitatively accurate.
Electrically induced phase inversion in urethane-modified polypropylene glycol/dimethylsiloxane blend
Authors: Kimura Hiroshi , Aikawa Koichiro , Masubuchi Yuichi , Takimoto Jun-ichi , Koyama Kiyohito , Uemura Takashi
Published: July 2002

Electric-field-induced phase inversion in immiscible liquid blends of urethane-modified polypropylene glycol (UPPG)/dimethylsiloxane (DMS) was discovered. When the fraction of DMS is in the range of 23–26 wt. %, UPPG forms the particulate domains and DMS forms the continuous phase under no field. However, the application of an electric field induces connection among UPPG domains, and results in phase inversion; UPPG forms the continuous phase and DMS forms the particulate domains. This finding gives great possibilities of controlling the phase structure of blends using an electric field, and thus controlling their physical properties.
Penning ionization of the CO2 molecule by Ne* (3 3P2,0) metastable atom
Authors: Lescop B. , Ben Arfa M. , Cherid M. , Le Coz G. , Sinou G. , Fanjoux G. , Le Nadan A. , Tuffin F.
Published: July 2002

Penning ionization of the CO2 molecule by Ne* (3 3P2,0) metastable atoms is studied by electron spectroscopy. The recorded electron energy spectrum is dominated by an important vibrational progression that is decomposed over the nν1 and nν1+2ν3 vibrational series up to n=5. In addition, we observe a low-energy broad hump extending from 0.5 to 1.5 eV. The presence of this new feature is attributed to a non-van der Waals interaction between the colliding particles. Making the hypothesis that an ion–pair state channel opens during the collision, we are in a position to interpret the entire set of experimental results. We believe that the opening of this channel is responsible for the production of the CO2 molecule in an autoionized Tanaka–Ogawa Rydberg state.
Vibrationally resolved photoelectron spectroscopy of silicon cluster anions Sin− (n=3–7)
Authors: Xu Cangshan , Taylor Travis R. , Burton Gordon R. , Neumark Daniel M.
Published: July 2002

Photoelectron spectra of Sin− (n=3–7) have been measured at several photodetachment energies. The anions were created using a pulsed discharge source, resulting in considerably colder clusters than in earlier work. As a result, vibrationally resolved spectra were obtained for larger clusters and more electronic states than in previous studies of these species, leading to more accurate electron affinities, term energies, and vibrational frequencies for the ground and excited electronic states of the neutral clusters. The assignments of excited states were aided by ab initio calculations and photoelectron angular distributions.
Vibrational excitation of H2O and HOD molecules produced by reactions of OH and OD with cyclo-C6H12, n-C4H10, neo-C5H12, HCl, DCl and NH3 as studied by infrared chemiluminescence
Authors: Butkovskaya N. I. , Setser D. W.
Published: July 2002

The room-temperature reactions of OH(OD) radicals with cyclo-C6H12, n-C4H10, and neo-C5H12 have been investigated by observing the infrared chemiluminescence from the H2O(HOD) molecules generated in a fast-flow reactor. These hydrocarbon molecules are representative for abstraction from secondary and primary C–H bonds. The total vibrational energy released to H2O(HOD) was in the range of 〈fv〉=0.55–0.65. The majority (80%–85%) of the vibrational energy is in the stretching modes and the main energy release is to the local mode associated with the new OH bond. The dynamics associated with the energy disposal to H2O(HOD) resemble the H+L−H dynamics for the analogous reactions of F atoms. The data from H2O and HOD are complementary because of the different collisional coupling between the energy levels of the ν1, ν2, and ν3 modes; however, no specific isotope effect was found for the energy disposal to H2O versus HOD for reactions with the hydrocarbon molecules. In contrast, a very unusual isotope effect was found between the OH+HCl and OD+HCl pairs. The latter reaction gave the expected stretching mode excitation of HOD; however, the OH reaction gave H2O molecules with virtually no vibrational energy. This anomalous situation is partly associated with an inverse secondary kinetic-isotope effect, but the main isotope effect is on the dynamics of the energy disposal process itself.
Comparison of theoretical methods for resonant vibration–vibration energy transfer in liquids
Authors: Dodaro Frank A. , Herman Michael F.
Published: July 2002

The problem of resonant vibration–vibration (V–V) energy transfer in liquids is explored within a simple model in order to compare two calculational methods. Two bromine diatomics are constrained to move between two fixed argon solvent atoms in one dimension. The time-dependent probability for the transfer of a vibrational excitation between the bromine diatomics is computed semiclassically. The results of single-trajectory molecular dynamics methods are compared with those of multiple-trajectory surface hopping methods. It is found that the two methods give similar results, indicating that the simpler single-trajectory method adequately describes the resonant V to V transfer process. The proximity of the nearly degenerate potential energy surfaces leads to a phase coherence time that exceeds the time required for the transition probability to saturate for this model. As a result, the transition probability remains a nonlinear function of time, and this precludes the extraction of a rate constant from the slope of the resonant V–V transition probability curve for this simple one-dimensional model.
Simulation of phase equilibria for polymer–supercritical solvent mixtures
Authors: Gromov Dmitry G. , de Pablo Juan J. , Luna-Bárcenas Gabriel , Sanchez Isaac C. , Johnston Keith P.
Published: July 2002

Phase equilibria for mixtures of a polymer and a supercritical solvent are investigated by means of expanded Gibbs ensemble simulations. Both lower and upper critical solution temperature (LCST and UCST) phenomena are observed for such systems. A closed-loop phase diagram is observed for systems with no specific interactions. The results of our simulations for Lennard-Jones polymer–solvent mixtures are in qualitative agreement with experimental data for polymers in supercritical solvents.
A low-field paramagnetic nuclear spin relaxation theory
Authors: Westlund Per-Olof
Published: July 2002

A low-field theory of paramagnetically enhanced proton spin-lattice relaxation is developed resulting in closed analytical expressions. The theory describes paramagnetically enhanced water proton spin-lattice relaxation in complexes of transition metal ions with electron spin quantum number S=1. In the low-field regime the electron spin system is dominated by a static or permanent zero-field splitting interaction which is much larger than the Zeeman interaction. The electron spin is thus quantized in the molecular fixed principal frame of the static ZFS-interaction rather than in the laboratory fixed frame. In the molecular fixed frame the zero-field splitting Hamiltonian is characterized by an axial parameter (D) and a rhombic parameter (E). It is shown how the relative magnitude of D and E strongly influence the magnitude of the paramagnetically enhanced proton spin lattice relaxation rate. In describing electron spin relaxation in the molecular fixed frame, we consider a transient ZFS-interaction and a reorientation modulated Zeeman interaction as the two main relaxation mechanisms. Then, using Redfield theory we derive expressions for the relevant electron spin relaxation rates in terms of spectral densities. In contrast to the Zeeman or high field regime, the electron spin system in the low field limit does not have a permanent magnetic moment. The lack of this magnetic moment implies that electron spin-lattice relaxation processes do not influence the paramagnetically enhanced proton spin-lattice relaxation rates. Instead three spin–spin relaxation rates determine the enhanced proton spin-lattice relaxation profile. We express the electron spin–spin relaxation rates in terms of spectral densities determined by the mean square value of the transient zero-field splitting interaction, its characteristic correlation time, and the reorientational correlation time of rank 1.
Reactivity of niobium clusters with nitrogen and deuterium
Authors: Bérces A. , Hackett P. A. , Lian Li , Mitchell S. A. , Rayner D. M.
Published: July 2002

Absolute rate coefficients are reported for reactions of Nbn clusters (n=2–20) with D2 and N2 at 280, 300, and 370 K. Most clusters are highly reactive but there are conspicuous exceptions at n=8, 10, and 16 for both D2 and N2. The origin of this trend in reactivity with cluster size and the reason why D2 and N2 show similar trends are discussed. Density functional theory (DFT) electronic structure calculations have been used to investigate the details of the reactions for the smallest clusters Nb2 and Nb8 with H2 and N2. The steric and electronic requirements for dissociation of H2 and N2 are described in terms of frontier orbital interactions. The main conclusion from the DFT calculations is that complete dissociation of H2 or N2 requires charge transfer by transit of an avoided crossing between neutral and ionic potentials. This idea is extended to larger clusters by using a simple charge transfer model that predicts an inverse correlation between reactivity and an appropriately defined effective ionization potential. Such a correlation is observed and indicates that the effective ionization potential is the dominant influence on reactivity.
Temperature dependence of the folding rate in a simple protein model: Search for a “glass” transition
Authors: Gutin A. , Sali A. , Abkevich V. , Karplus M. , Shakhnovich E. I.
Published: July 2002

Monte Carlo simulation of model proteins on a cubic lattice are used to study the thermodynamics and kinetics of protein folding over a wide range of temperatures. Both random sequences and sequences designed to have a pronounced minimum of energy are examined. There is no indication in the kinetics of a “glass” transition at low temperature, i.e., below the temperature of the equilibrium folding transition, the kinetics of folding is described by the Arrhenius law at all temperatures that were examined. The folding kinetics is single-exponential in the whole range of studied temperatures for random sequences. The general implications of the temperature dependence of the folding rate are discussed and related to certain properties of the energy spectrum. The results obtained in the simulations are in qualitative disagreement with the conclusions of a theoretical analysis of protein folding kinetics based on certain kinetics assumptions introduced in the Random Energy Model. The origins of the discrepancies are analyzed and a simple phenomenological theory is presented to describe the temperature dependence of the folding time for random sequences.
Generalized Flory equations of state for copolymers modeled as square-well chain fluids
Authors: Gulati Harpreet S. , Hall Carol K.
Published: July 2002

The Generalized Flory–Dimer (GFD) equation of state has been extended to fluids containing copolymers modeled as heteronuclear chains of freely-jointed tangent spheres that interact via a site–site square-well potential. Compressibility factors are obtained for block, alternating and random copolymer fluids. The GFD theory for square-well copolymers requires expressions for the insertion factors of SW monomers in SW monomer mixtures, SW dimers in SW dimer mixtures, and SW heteronuclear dumbbells in SW heteronuclear dumbbell fluids. These insertion factors are obtained using recently-derived perturbation-theory-based equations of state. The effects of variations in composition, segment size ratios, and well-depth ratios on the compressibility factor are studied. The predictions of the Generalized Flory–Dimer theory are compared to compressibility factors obtained from discontinuous canonical molecular dynamics simulation. The Generalized Flory–Dimer theory accurately predicts the compressibility factors of square-well copolymer fluids for a variety of cases including those in which the size ratio and well-depth ratio of the two components are different.
Particle distribution of a one-dimensional imperfect annihilation reaction in the gas phase
Authors: Sheu Wen-Shyan , Chen Hsing-Yin
Published: July 2002

A one-dimensional (1D) imperfect annihilation reaction in the gas phase is studied via computer simulation. Particular attention is paid to the effects of the spatial distribution of particles on the reaction kinetics. Compared with classical kinetics, the imperfect reaction shows anomalous kinetics by the slowdown of the reaction rate and the anomaly is alleviated when the reaction probability upon contact is lower. The classical rate law is approached when the reaction probability approaches zero. The inspection of the spatial structure of particles shows that the slowdown of the reaction rate is caused by two major factors; the aggregation of the same velocity-species and the segregation of the different velocity-species. When the reaction probability is reduced, these two effects are weakened, and consequently the classical rate law is approached. Other features of the spatial distribution of particles and their effects on the kinetics are also discussed.
Phase equilibria of the modified Buckingham exponential-6 potential from Hamiltonian scaling grand canonical Monte Carlo
Authors: Errington Jeffrey R. , Panagiotopoulos Athanassios Z.
Published: July 2002

A novel method, Hamiltonian scaling grand canonical Monte Carlo, has been used to determine the phase behavior of 12 variations of the modified Buckingham exponential-6 potential. Hamiltonian scaling grand canonical Monte Carlo enables the determination of thermodynamic properties of several related potential models from a single set of simulations. The main advantage of the method is that by appropriate selection of the chemical potential and temperature for each Hamiltonian, the densities that are sampled are relevant for multiple Hamiltonians, thus increasing the simulation efficiency. The method was combined with mixed-field finite-size scaling to determine the critical parameters and phase coexistence curves of several modified Buckingham exponential-6 potentials to a high level of accuracy. In particular, the critical parameters of the models have been calculated to within 0.3% for the critical temperature and 1% for the critical density and pressure. A new potential for methane based on the Buckingham exponential-6 model is presented. The vapor pressure curve of methane from the Buckingham exponential-6 potential model is much more accurate than is possible with the Lennard-Jones potential model.
Anomalously slow intramolecular vibrational redistribution in the acetylene X̃ 1Σg+ state above 10 000 cm−1 of internal energy
Authors: Jacobson Matthew P. , O’Brien Jonathan P. , Field Robert W.
Published: July 2002

We have identified, in dispersed fluorescence spectra of acetylene Ã 1Au→X̃ 1Σg+ emission, a series of bright states between 10 000 and 15 000 cm−1 of internal energy which display anomalously slow intramolecular vibrational redistribution. That is, these bright states display virtually no fractionation at internal energies at which the majority of other observed bright states are fractionated over several hundred cm−1 in a complicated fashion. The anomalous bright states are distinguished from the other nearly isoenergetic bright states only by the way in which the vibrational excitation is distributed among the CC stretch and trans bend modes; specifically, the anomalous bright states have relatively low excitation in the trans bend mode (v4⩽8), with the remainder of the vibrational excitation in the CC stretch mode (v2⩽6). A refinement of the acetylene global effective Hamiltonian permits detailed insight into the mechanism of the anomalously slow intramolecular vibrational redistribution, and reveals that the relatively simple fractionation patterns of these bright states can be adequately described in terms of a system of 4 interacting zero-order states which are coupled by Darling-Dennison bending resonances, vibrational l-resonance, and an anharmonic “3,245” resonance. The refined effective Hamiltonian also permits the assignment of similar, minimally perturbed bright states up to at least 17 500 cm−1 of internal energy.
Do bond functions help for the calculation of accurate bond energies?
Authors: Bauschlicher Charles W. , Partridge Harry
Published: July 2002

The bond energies of eight chemically bound diatomics are computed using several basis sets with and without bond functions (BF). The bond energies obtained using the aug-cc-pVnZ+BF basis sets (with a correction for basis set superposition error, BSSE) tend to be slightly smaller that the results obtained using the aug-cc-pV(n+1)Z basis sets, but slightly larger than the BSSE corrected aug-cc-pV(n+1)Z results. The aug-cc-pVDZ+BF and aug-cc-pVTZ+BF basis sets yield reasonable estimates of bond energies, but, in most cases, these results cannot be considered highly accurate. Extrapolation of the results obtained with basis sets including bond functions appears to be inferior to the results obtained by extrapolation using atom-centered basis sets. Therefore bond functions do not appear to offer a path for obtaining highly accurate results for chemically bound systems at a lower computational cost than atom centered basis sets.
Reactive and nonreactive scattering of Na(3S) and (3P) from SF6 and SiF4
Authors: Düren R. , Färber M. , Weiß C.
Published: July 2002

Double differential cross sections of sodium in both the 3S ground state and the 3P electronically excited state scattered from SF6 and SiF4 have been measured at center of mass collision energies of 0.6 and 2.0 eV for SF6 and 1.0 eV for SiF4. For the experiments with Na(3P), a two frequency excitation scheme is applied, by which an excitation efficiency (determined by scattering experiments) of 31% is obtained. In the SF6 experiments, one finds a substantial transfer of the collision energy into the vibrational and rotational degrees of freedom of SF6. A transfer of electronic energy of Na(3P) into SF6 is not observed in nonreactive collisions. For the NaF formed in the reaction, an angular distribution originating from a long-lived complex, superimposed by a forward peak caused by a direct reaction mechanism is found. Especially the ground state measurements show an angular-dependent velocity distribution of the product. Compared to forward and backward scattering in sideways collisions, a lower velocity is found. From Na(3S) to Na(3P), the general characteristic does not change. The gain of reactivity with the excitation is decreasing with the collision energy. For the experiments with Na(3P) and SiF4, a quenching channel is found. The energy transferred to SiF4 is in good agreement with the prior distribution, calculated in a simplified RRHO model. The statistical behavior and the shape of the angular distribution of the intensity suggest the interpretation by the formation of a long-lived collisional complex.
Transfer matrix method for enumeration and generation of compact self-avoiding walks. II. Cubic lattice
Authors: Kloczkowski A. , Jernigan R. L.
Published: July 2002

The transfer matrix method, developed earlier for the 2D square lattice has been generalized to enumerate and generate self-avoiding walks (chains with two ends) and self-avoiding circuits (no ends) in 3D on the cubic lattice. The method has been applied to Hamiltonian paths and circuits within simple geometries, i.e., parallelepipeds of size l×m×n for varying integer numbers l, m, n. The generalization of the method to irregular shapes has also been discussed. We also discussed an extension of this new method to permit random sampling of the conformational space.
Binding energies of palladium carbonyl cluster anions: Collision-induced dissociation of Pd3(CO)n− (n=0–6)
Authors: Spasov Vassil A. , Ervin Kent M.
Published: July 2002

The bond dissociation energies of palladium trimer anion, Pd3−, and its carbonyls, Pd3(CO)n− (n=1–6), are measured in the gas phase by the energy-resolved collision-induced dissociation method. The values obtained are D0(Pd2−−Pd)=2.26±0.36 eV for the bare cluster and D0(Pd3(CO)n−1−−CO)=1.78±0.32 eV, 1.74±0.22 eV, 1.47±0.22 eV, 1.13±0.15 eV, 1.11±0.15 eV, and 1.14±0.17 eV for n=1–6, respectively, for the carbonyls. The results show a general decrease of the bond energy with an increasing number of carbonyls, with two relatively stable structures, Pd3(CO)2− and Pd3(CO)6−. A symmetric Pd3(CO)2− structure with two three-fold bridged carbonyls is postulated.
Microwave spectra of the Ne–N2 Van der Waals complex: Experiment and theory
Authors: Jäger W. , Xu Y. , Armstrong G. , Gerry M. C. L. , Naumkin F. Y. , Wang F. , McCourt F. R. W.
Published: July 2002

High-resolution microwave spectra of the ground state Ne–1420N2, Ne–1520N2, Ne–1422N2, and Ne–1522N2 Van der Waals complexes, involving rotational levels up to J=4, are reported. Interpretation and assignment of the observed transitions were made by combining results of measurements and theoretical predictions of the MW line positions in terms of available empirical potential energy surfaces and of a new high-level ab initio potential energy surface. The deviations of the calculated MW spectra from those observed experimentally are more uniform for the ab initio potential surface than they are for the empirical potential surfaces, allowing for reduction of the deviations to within 0.07% for all isotopomers by a single-parameter scaling of the ab initio potential energy surface. The scaled Ne–N2 interaction potential was used to predict the MW line positions for the transitions J′-J″=3-2, 4-3 for all species. A simple procedure is proposed to improve the ab initio results for atom–diatom systems on the basis of atom–atom interaction components.
Fifth-order coherent light scattering: Extension of the Kramers–Heisenberg expression for light scattering and two-dimensional measurement of vibrational dynamics
Authors: Cho Minhaeng
Published: July 2002

A theoretical description of the fifth-order coherent light scattering is presented. By calculating the corresponding radiative transition rate, it is found that the intensity of the fifth-order scattering light is proportional to the squares of the number density as well as to the thickness of the optical sample. The fifth-order coherent light scattering expression is a two-dimensional extension of the Kramers–Heisenberg expression for light scattering. The intensity of each peak in the two-dimensional spectrum is determined by the product of three Raman transition matrix elements. A detailed discussion on the relative intensities of the diagonal and off-diagonal peaks is presented.
Helicity of orientation parameters of photofragments in fluorescence-imaging experiments
Authors: Chen Kuo-mei , Pei Cheng-chih
Published: July 2002

A detection scheme in fluorescence-imaging experiments is presented for determining helicity of orientation parameters of photofragments prepared by circularly polarized photolysis lasers. In a framework of density matrix theory, explicit fluorescence intensity formulas are derived for detection of right- and left-circularly polarized fluorescence photons in various transition sequences. A pattern recognition from fluorescence images of photofragments in a state of definite helicity with respect to their recoil directions has been established, where differences between left- and right-circularly polarized fluorescence image patterns should be taken. Information on the coherence among various magnetic sublevels of an angular momentum state can be obtained by monitoring fluorescence images in a ΔJ=−1 transition (P branch in absorption).
193 nm photodynamics of NO in rare gas matrices: Fluorescence, thermoluminescence, and photodissociation
Authors: Eloranta Jussi , Vaskonen Kari , Häkkänen Heikki , Kiljunen Toni , Kunttu Henrik
Published: July 2002

193 nm excited time gated emission spectra of a NO monomer isolated in Ar, Kr, and Xe matrices are presented. In the Ar matrix a 4Π→X 2Π, B 2Π→X 2Π, and A 2Σ→X 2Π band systems are completely separable. In solid Kr, both B 2Π→X 2Π and A 2Σ→X 2Π appear promptly from the laser pulse, and in the Xe matrix only Rydberg A 2Σ→X 2Π fluorescence is observed. Prolonged photolysis at 193 nm yields electron paramagnetic resonance signals attributed to isolated S4 nitrogen atoms. This is the first observation of condensed phase photodissociation of NO. Annealing of the extensively irradiated Ar matrix produces strong a 4Π→X 2Π and B 2Π→X 2Π thermoluminescence emissions due to N(4S)+O(3P) recombination. In the Kr matrix thermoluminescence is entirely due to a 4Π→X 2Π transition. No thermoluminescence is observed in Xe. Thermoluminescence is ascribed to short-range trapping of N and O fragments, and well separated atoms do not have significant contribution to recombination.
Binary system of network-forming fluid: Study of phase stability through an analytical solution of the Percus–Yevick equation
Authors: Duda Yurko
Published: July 2002

Phase equilibria of a binary mixture of equal-sized network forming fluid (hard sphere diameters D[a]=D[b]) with associative forces between like species and hard sphere repulsion between unlike species are determined using an analytical solution of the associative Percus–Yevick integral equation. The theory shows how occurrence of coexistence lines correlates with the interparticle potential parameters, density and composition of the system. The phase behavior of the system with varying degrees and symmetry of association is studied. Namely, immiscibility curves for the mixture of chains and network-forming fluid are built and discussed.
Simulation of self-associating polymer systems. II. Rheological properties
Authors: Khalatur P. G. , Khokhlov A. R. , Mologin D. A.
Published: July 2002

Using nonequilibrium molecular dynamics (NEMD), self-associating polymer systems composed of flexible linear telechelic chains with strongly attracting end-groups are studied under the influence of external shearing forces. Interconnection between structural and rheological properties of the steady-state system is discussed in detail, including the following aspects: shear-induced intra- and intermolecular structural changes, shear-induced gelation, shear thickening and thinning, thixotropic effect. We find that a critical factor in the shear behavior of self-associating polymers is the space distribution (topology) of chain “bridges” connecting micellar aggregates. Depending on the topology of the bridges, the system may exhibit shear thickening or shear thinning. Furthermore, the applied external force may cause transitions from continuous (space-filling) associative network, which spans the whole space, to fibrous textures—threadlike objects which align along the shear field. These structural reorganizations lead to dramatic changes in the shear dependent steady-state viscosity.
Shattering of SiMe3+ during surface-induced dissociation
Authors: Schultz David G. , Hanley Luke
Published: July 2002

We provide experimental evidence that upon hyperthermal impact of Si(CD3)3+ ions with an organic surface, a portion of the ions undergo dissociation while still in contact with the surface. We use a tandem configuration of quadrupole mass spectrometers along with an energy analyzer to measure the kinetic energy distributions of the fragments that form as a result of the surface scattering of 25 eV Si(CD3)3+. These distributions are different for scattering from a clean Au(111) surface versus scattering from an organic surface composed of a self-assembled monolayer of hexanethiolate on Au(111). Parent and fragment ions recoil from the clean Au(111) surface with the same velocity, as is expected for fragmentation away from the surface. However, the same scattering products recoil from the organic surface with different velocities but similar energies, suggesting that the fragmentation dynamics are modified by surface interactions. We perform molecular dynamics simulations which predict residence times of ∼210 fs at the organic surface and ∼20 fs at the Au surface. The simulations also predict that 13% and 31% of the ions fragment within 1.1 ps of surface impact at the organic and Au surfaces, respectively. Thus, the experimental observation of dissociation at only the organic surface results from its longer ion-surface interaction time. The fragmentation time scale predicted by Rice–Ramsperger-Kassel–Marcus calculations is yet longer, suggesting that at least a portion of the surface-induced dissociation of Si(CD3)3+ may occur via a nonstatistical mechanism. Our interpretation draws heavily from an analogous “shattering” mechanism previously proposed for cluster-surface scattering [E. Hendell, U. Even, T. Raz, and R. D. Levine, Phys. Rev. Lett. 75, 2670 (1995)].
Optical line shapes of dynamically disordered ring aggregates
Authors: Bakalis Lisette D. , Coca Mircea , Knoester Jasper
Published: July 2002

We study the absorption line shape caused by Frenkel excitons in one-dimensional ring-shaped molecular aggregates, such as circular light-harvesting systems, subjected to dynamic disorder with a finite correlation time. We focus on dichotomic noise and show that for arbitrary orientations of the molecular transition dipoles relative to the ring, the absorption spectrum may be calculated exactly by solving two very similar sets of 2N coupled linear equations of motion, where N is the number of molecules in the ring. These sets are a factor of N smaller than in methods developed previously, which allows us to evaluate the exact line shapes for N up to 12 and study size dependence of the spectra. Previous exact calculations were limited to hexamers (N=6). Moreover, in contrast to earlier work, we take into account long-range dipolar transfer interactions between all molecules in the ring. We find that the dipole orientation and the long-range interactions strongly affect the dependence of the spectrum on the ring size. This holds true particularly for the exchange narrowing of the linewidths in the fast-fluctuation regime.
The O(1D) yield from O2 photodissociation near H Lyman-α (121.6 nm)
Authors: Lacoursière J. , Meyer S. A. , Faris G. W. , Slanger T. G. , Lewis B. R. , Gibson S. T.
Published: July 2002

The solar H Lyman-α line is, through O2 photodissociation, an important source of O(1D) production throughout the mesosphere and lower thermosphere. To ascertain the energy balance in this altitude region, it is necessary to know the O(1D) yield across the solar H Lyman-α feature, since H Lyman-α absorption by O2 at ∼80 km accounts for a substantial fraction of the solar radiation absorbed in the mesosphere. An earlier laboratory study had provided a value of 0.44±0.05 for the O(1D) yield at the center of the solar H Lyman-α line, where the profile shows a minimum in intensity due to strong self-reversal of the line. Using tunable laser radiation, we have determined the O(1D) yield from O2 photodissociation across the entire H Lyman-α profile from 121.2 to 121.9 nm, at a spectral resolution of 0.0015 nm (1 cm−1). The results reveal a strongly wavelength-dependent window in the O(1D) yield, the origins of which are explained using calculations based on a coupled-channel Schrödinger-equations model of the O2 photodissociation. The calculations, which show significant isotopic dependence near H Lyman-α, predict that the depth of the quantum-yield window will increase significantly as the temperature is lowered.
The rotational spectrum of a highly vibrationally mixed quantum state. II. The eigenstate-resolved spectroscopy analog to dynamic nuclear magnetic resonance spectroscopy
Authors: Pate Brooks H.
Published: July 2002

The description of the rotational spectrum of a single molecular eigenstate in an energy region where conformational isomerization can occur is presented. The conformational isomerization rate can be determined from an analysis of the overall line shape of the spectrum. The isomerization dynamics are investigated through a time-domain analysis. It is shown that the nonstationary state produced by coherent microwave excitation has a well-defined conformational structure. However, the individual molecular eigenstates are a mixture of rovibrational states associated with the two conformations. The structural localization decays to the eigenstate equilibrium value at a rate defined as the isomerization rate. It is also shown that the line shape of the spectrum coalesces as the isomerization rate exceeds the difference between the characteristic rotational frequencies of the two conformers. The line shape profile of the eigenstate-resolved spectrum is well described by the Bloch equations modified for chemical exchange.
Long-range coefficients for the low-lying electronic states of BeLi and Be2
Authors: Bégué Didier , Mérawa Mohammadou , Rérat Michel , Pouchan Claude
Published: July 2002

Using the frequency-dependent dipole polarizabilities calculated for the five lowest states 2 1S, 2 3P, 2 1P, 3 3S and 3 1S of Be and for the 2 2S and 2 2P states of Li, from our time-dependent gauge invariant method, estimates of C6 were obtained for all the first molecular states dissociations: 2 1S+2 1S, 2 1S+2 3P, 2 1S+2 1P and 2 3P+2 3P of Be–Be and 2 2S+2 1S, 2 2P+2 1S and 2 2P+2 3P of Li–Be. Some higher interactions between these states were also estimated: 2 1S+3 1S, 3 3S+2 3P, 2 1P+2 1P, 3 1S+2 1P, 3 3S+3 3S, 2 1P+2 3P, and 3 1S+3 1S for the homonuclear system, 2 2P+2 1P and 2 2S+2 3P for the heteronuclear one. Experimentally, only the (X 1Σg+) ground state, and the two first (A 1Πu and B 1Σu+) states, which dissociate in 2 1S+2 1S and 2 1S+2 1P channels respectively, are known for Be2. The long-range coefficients reported for almost all of the low-lying states are new.
Modeling of oscillatory dynamics of a simple enzyme-diffusion system with hysteresis. The case of lumped permeabilities
Authors: Zou Xiaoqin , Siegel Ronald A.
Published: July 2002

We study the oscillatory dynamics of an enzyme-diffusion system with product inhibition of substrate flux. A lumped model is considered in which the diffusion medium is a thin membrane whose permeabilities to substrate and product undergo discrete transitions in response to product concentration, with hysteresis. Assuming piecewise constant permeabilities and linear reaction kinetics, we derive analytical criteria for single steady state, double steady state, and oscillatory behavior as a function of system parameters. System behavior is pulsatile in the oscillatory regime. Analytical equations for pulse widths and interpulse intervals are presented, along with numerical calculations that confirm the validity of these equations. This system is similar but not completely analogous to chemical oscillators based on bistability/feedback mechanisms.
Production of ArN+ ions in the reactions Ar++N2 and N2++Ar
Authors: Tosi Paolo , Correale Raffaele , Lu Wenyun , Bassi Davide
Published: July 2002

We have studied the production of ArN+ starting from the two symmetric charge-state reactants Ar++N2  and N2++Ar. For both reactions we measured the total cross sections as a function of the relative energy, in the energy range from 5 to 45 eV. Estimates of the reaction thresholds and symmetry considerations suggest that the reaction Ar++N2 produces ArN+ essentially in its first excited state A 3Π, while the reaction N2++Ar produces ground state ArN+ ions. This fact explains the discrepancies between different estimates of the dissociation energy of ArN+ obtained in previous studies.
Measurement of C6H6 densities in a diamond deposition system
Authors: Goyette A. N. , Anderson L. W. , Peck J. R. , Lawler J. E.
Published: July 2002

We report the absolute column densities of gas phase C6H6 and upper limits on absolute C4H2 and C2H3 concentrations in a hot filament diamond chemical vapor deposition (CVD) reactor. The steady state density of C6H6 is monitored as a function of C2H2 and H2 input fractions. The C6H6 column density is a quadratic function of input C2H2 fraction at low C2H2 fractions. Above a critical C2H2 input fraction, the C6H6 column density becomes independent of C2H2 input fraction. The column density of gas phase C6H6 is relatively insensitive to the input H2 fraction. The weak relationship between C6H6 and H2 input fraction is in contradiction to the conclusion drawn from chemical kinetics modeling of diamond CVD that H2 strongly suppresses the formation of aromatic species under diamond CVD conditions. Moreover, comparison of C6H6 column densities with previously measured CH3 column densities under similar conditions indicate that a comparable fraction of gas phase carbon atoms in the steady state are incorporated in C6H6 as are incorporated in the growth species CH3.
Collective dynamics of random polyampholytes
Authors: Müller-Nedebock Kristian K. , Vilgis Thomas A.
Published: July 2002

We consider the Langevin dynamics of a semi-dilute system of chains which are random polyampholytes of average monomer charge q and with fluctuations in this charge of size Q−1 and with freely floating counter-ions in the surrounding. We cast the dynamics into the functional integral formalism and average over the quenched charge distribution in order to compute the dynamic structure factor and the effective collective potential matrix. The results are given for small charge fluctuations. In the limit of finite q we then find that the scattering approaches the limit of polyelectrolyte solutions.
Crossed beam reaction of the cyanogen radical, CN(X 2Σ+), with acetylene, C2H2(X 1Σg+): Observation of cyanoacetylene, HCCCN(X 1Σ+)
Authors: Huang L. C. L. , Lee Y. T. , Kaiser R. I.
Published: July 2002

The chemical dynamics to cyanoacetylene, HCCCN(X 1Σ+), formation via the neutral–neutral reaction of cyanogen, CN(X 2Σ+), with acetylene, C2H2(X 1Σg+), is investigated in a crossed molecular beams experiment at a collision energy of 21.1 kJ mol−1. The laboratory angular distribution and time-of-flight spectra of the HCCCN product are recorded at m/e=51 and 50. Forward-convolution fitting of our data reveals that the reaction dynamics are governed by an initial attack of the CN radical to the π electron density of the acetylene molecule to form a HCCHCN collision complex on the A′2 surface. The four heavy atoms are rotating in plane almost perpendicular to the total angular momentum vector J around the C axis of the complex which undergoes C–H bond rupture through a tight transition state to HCCCN and H. The H atom is emitted almost perpendicular to the HCCCN axis to yield a nearly “sideways” peaking of T(θ). The explicit identification of the cyanoacetylene reaction product represents a solid background for the title reaction to be included with more confidence in reaction networks modeling the chemistry in dark, molecular clouds, outflow of dying carbon stars, hot molecular cores, as well as the atmosphere of hydrocarbon rich planets and satellites such as the Saturnian moon Titan.
Decay of metastable states: Mean relaxation time formulation
Authors: Drozdov Alexander N. , Brey J. Javier
Published: July 2002

The mean relaxation time formalism introduced by Nadler and Schulten [J. Chem. Phys. 82, 151 (1985)] in their generalized moment expansion method is extended to a general diffusion process in arbitrary dimensions. The utility of the approach is demonstrated by calculating analytically the rate of noise-induced transitions in a bistable system with an isolated transition point. The rate formula obtained summarizes in a uniform manner much of what had been done before in this field. Limitations of its validity are discussed and a perturbation procedure to systematically improve it is proposed. The validity of our theoretical predictions for the rate is confirmed by comparing with exact numerical results.
Pump-dump control and the related transient absorption spectroscopies
Authors: Shen Zhenwen , Yan YiJing , Cheng Jixin , Shuang Feng , Zhao Yi , He Guozhong
Published: July 2002

We combine theories of optimal pump-dump control and the related transient probe absorption spectroscopy in order to elucidate the relation between these two optical processes and the possibility of experimental realization. In the weak response regime, we identify the globally optimal pair of pump-dump control fields, and further propose a second-order difference detection scheme to monitor the wave packets dynamics that is jointly controlled by both the pump and dump fields. The globally optimal solution serves also as the initial input for the iterative search for the optimal control fields in the strong response regime. We use a model I2 molecule to demonstrate numerically the pump-dump control and the detection of a highly vibrationally excited wave packet focusing dynamics on the ground X surface in both the weak and strong response regimes. The I2B surface serves as the intermediate to assist the pump-dump control and the optical detection processes. Demonstrated in the strong response regime are the optimal pair of pump-dump molecular-π pulses that invert nearly total population onto the predefined target region within a half period of vibration motion.
Structure of dipolar liquids near charged solid surfaces: A nonlinear theory based on a density functional approach and Monte Carlo simulations
Authors: Das D. , Senapati S. , Chandra A.
Published: July 2002

A nonlinear theory for the calculation of density and polarization of dipolar solvents near charged surfaces is developed by using a density functional approach. The theory is based on a weighted density approximation for the isotropic part of the density and the anisotropic (or dipolar) part is calculated by using a perturbative approach. The theory, however, retains the full nonlinear dependence on the surface charge density or the external field. Explicit numerical results are obtained for different values of the external field. It is found that the number density of the solvent near the surface increases with increasing surface charge density indicating the presence of electrostriction. The polarization is found to increase nonlinearly with the external field and exhibits the presence of dielectric saturation at high field strength. The predictions of the present nonlinear theory are compared with the results of Monte Carlo simulations and a good agreement is found for both the density and polarization near charged surfaces. The present simulations clearly reveal a nonlinear behavior of dipolar molecules in presence of the charged surfaces. Also, the nonlinear effects are found to be especially important for interfacial molecules, in agreement with the predictions of the analytical theory presented here.
Ultrafast relaxation processes of excitons to the relaxed excited state in one-dimensional dimethylglyoxime platinum complexes driven by intramolecular vibration
Authors: Iwai Shinichiro , Kamata Toshihide , Murata Shigeo , Fukaya Toshio , Kodaira Testuya , Mizukami Fujio , Tachiya M. , Yamamoto Kaoru , Ohta Toshiaki
Published: July 2002

Optical response due to the photo excited carrier or exciton dynamics in a one-dimensional (1D) dimethylglyoxime Pt complex [Pt(dmg)2] has been investigated by femtosecond pump–probe spectroscopy. Measurements were made at several excitation energies between the exciton state and the free electron hole pair state. Induced absorption was observed in the low-energy side of the exciton band at any excitation energy. The spectral shape of the induced absorption changed with time between 0.1 and 0.3 ps after instantaneous rise up. These results can be explained by the formation and thermalization processes of the relaxed excited state on the adiabatic potential surface. The unthermalized relaxed excited state is formed in a very short time (<50 fs). From the Raman spectrum, the frequency of the intermolecular stretching mode along the chain was estimated to be 80 cm−1 (T∼400 fs). These facts suggest that, in contrast to other 1D systems such as polydiacetylenes (PDA) and halogen bridged mixed valence metal complexes (MX), the formation of the relaxed excited state is caused not by the stretching vibration along the 1D chain. We propose the possibility that the triggering of the primary relaxation is caused by a high frequency (>600 cm−1) intramolecular mode which is not in the direction of the 1D chain.
Correcting for solvent–solvent electrostatic cutoffs considerably improves the ion-pair potential of mean force
Authors: Resat Haluk
Published: July 2002

A recently developed algorithm based on the continuum treatment of the solvent molecules beyond the electrostatic cutoff sphere is applied to the potential of mean force results between sodium and chloride ions to study the effects of the solute–solvent and solvent–solvent cutoff errors. The results show that although the solute–solvent correction improves the thermodynamic results slightly, physically realistic results are obtained only when the solvent–solvent correction is applied. This further supports past findings that proper treatment of solvent–solvent interactions is as important as that of the solute interactions, and should not be ignored.
Infrared spectra of cyclic-O6+ and trans-O6+ in solid neon and argon
Authors: Zhou Mingfei , Hacaloglu Jale , Andrews Lester
Published: July 2002

Charged transient species in the oxygen system have been trapped in solid argon and neon using electron impact, Townsend discharge, and laser-ablation methods. The previously identified O3−, O4−, and O4+ species are observed in these experiments. Absorptions at 1435.0 and 1429.5 cm−1 in solid neon are characterized as cyclic-O6+ and trans-O6+, respectively, on the basis of annealing behavior, isotopic substitution, multiplet structure in mixed O216+18O2 experiments, and density functional calculations. Cyclic-O6+ is observed at 1416.1 cm−1 in solid argon, a smaller displacement than found for cyclic-O4+ in solid argon.
Intermolecular interaction in the CH3+–He ionic complex revealed by ab initio calculations and infrared photodissociation spectroscopy
Authors: Olkhov Rouslan V. , Nizkorodov Sergey A. , Dopfer Otto
Published: July 2002

The infrared photodissociation spectrum of the degenerate asymmetric CH stretch (ν3) vibration of the CH3+–He ionic complex has been recorded. The rotational structure and vibrational frequency of the observed transition are consistent with a π-bonded C3v cluster geometry where the He ligand is attached to the 2pz orbital of the central C atom of CH3+. The intermolecular bond in the ground vibrational state is characterized by an averaged intermolecular separation of Rcm=2.18 Å. The origin of the ν3 vibration of the complex is slightly blue shifted (7 cm−1) compared to the monomer frequency, indicating that vibrational excitation is accompanied by a slight destabilization of the intermolecular bond. Ab initio calculations at the MP2/aug-cc-pVTZ# level of theory confirm that the π-bonded configuration corresponds to the global minimum structure of the complex (De=707 cm−1, Re=1.834 Å). The calculated intermolecular potential energy surface of this “disk-and-ball” ionic complex reveals substantial angular-radial couplings in the region of the global minimum, which account for the large discrepancy between vibrationally averaged and calculated equilibrium intermolecular separations, Rcm−Re=0.35 Å.