bluuebunny/crossref_metadata_2025 huggingface dataset:
Vibrational energy transfer among adsorbate modes: Picosecond dynamics on stepped H/Si(111)
Authors: Kuhnke K. , Morin M. , Jakob P. , Levinos N. J. , Chabal Y. J. , Harris A. L.
Published: July 2002

Direct measurements of interadsorbate vibrational energy flow among Si–H stretching modes on hydrogen-terminated, stepped vicinal H/Si(111) surfaces are made. A two-color picosecond infrared method is used in which one vibrational mode is pumped by a resonant infrared pulse and other vibrational modes are probed by vibrationally resonant sum frequency generation to observe energy transfer. The surfaces are prepared by chemical etching in HF solutions and have monohydride-terminated (111)-(1×1) terraces, and average terrace widths of approximately five atoms. Two types of surfaces, differing in having either monohydride- or dihydride-terminated steps, are examined. The results on both surfaces confirm that interadsorbate energy transfer competes efficiently with energy relaxation to the substrate. On the dihydride-stepped surface, the energy flow is analyzed to give a relatively complete kinetic model of the energy equilibration pathways. The model confirms that the fast relaxing dihydride-terminated steps (60–120 ps lifetime) drain a large fraction (∼2/3) of the terrace Si–H mode energy (the terrace mode intrinsic lifetime is fit to be ∼1.4 ns). The model is consistent with terrace–step energy transfer by dipole–dipole coupling between Si–H oscillators. On the monohydride-stepped surface, the experimental results suggest even stronger terrace–step coupling, but the monohydride step lifetime is long (≳500 ps) and does not drain the terrace mode energy. The coupling of the monohydride steps to the terraces by dipole interactions is in fact calculated to be strong enough so that the step and terrace modes mix, and detailed kinetic analysis of the monohydride-stepped surface is therefore ambiguous because of strong spectral interactions of the modes.
Alignment-orientation conversion by quadratic Zeeman effect: Analysis and observation for Te2
Authors: Klincare I. P. , Tamanis M. Ya. , Stolyarov A. V. , Auzinsh M. P. , Ferber R. S.
Published: July 2002

This paper reports the observation of the molecular fluorescence circularity under irradiation with linear polarized light. The phenomenon arises as a result of partial transformation from alignment of the ensemble of molecular angular momenta into orientation due to quadratic correction to Zeeman effect. Circularity rate up to 0.05 at magnetic field 0.4 T was registered in B 3Σu−(1u−)→X 3Σg−(1g−) fluorescence of 130Te2 molecules at angle π/4 with respect to Ê vector of linear polarized exciting light. Quadratic magnetic energy terms are associated with magnetic field induced ΔJ=±1 e/f mixing between 1u−∼1u+ and 1u−∼0u+ states. Circularity data fitting shows that the electronic part of Landé factor caused by 1u−∼0u+ interaction is equal to G± ≂gl〈0u‖Ja±‖1u〉+(gs−gl) 〈0u‖S±‖1u〉≂2.72.
Reactions of cobalt clusters with water and ammonia: Implications for cluster structure
Authors: Parks E. K. , Klots T. D. , Winter B. J. , Riley S. J.
Published: July 2002

Reactions of cobalt clusters in the 6 to 150 atom size range with water and ammonia are used to probe cluster geometrical structure. Clusters saturated with ammonia are found in several isomeric forms in much of this size region, including a structure based on icosahedral packing for clusters having 50 to 120 atoms. The variations with cluster size of the binding energy for a single water molecule provide evidence for icosahedral structure of the bare cobalt clusters in the same size range seen for the ammoniated clusters. However, both probes suggest that in many cases the bare clusters also have multiple isomers. The evidence for icosahedral structure in the bare and ammoniated clusters disappears above ∼120 atoms and is clearly not seen at the third icosahedral shell closing at 147 atoms. Comparison with earlier studies of hydrogenated cobalt clusters and with bare and hydrogenated nickel clusters is made. Reasons for the failure to form the closed-shell 147 atom icosahedral cluster are discussed.
Photoelectron spectroscopy of the gas-phase S
N2 reaction intermediates I−⋅CH3I and I−⋅CD3I: Distortion of the CH3I at the ‘‘ion–dipole’’ complex
Authors: Cyr Donna M. , Scarton M. Georgina , Johnson Mark A.
Published: July 2002

The I−⋅CH3I and I−⋅CD3I intermediates in the gas-phase SN2 identity reaction, I−+CH3I→CH3I+I−, are isolated, cooled in a free jet, and characterized using negative ion photoelectron spectroscopy. The spectra appear similar to bare I− but shifted to lower electron kinetic energy by the known binding energy of the complex. Vibrational fine structure in each spin–orbit subband is dominated by a simple vibrational progression in the ν3 mode of CH3I (nominally the C–I stretch). Franck–Condon analysis indicates that this bond is elongated in the complex by about 3% relative to bare CH3I with observable distortions in the C–H stretch and H–C–X bond angle (the umbrella mode). We discuss these observations in the context of the charge transfer (e.g., valence bond) model for the SN2 potential energy surface.
The kinetic-molecular model for condensation flux magnitudes applied to binary mixtures
Authors: Bauer S. H. , Wilcox C. F.
Published: July 2002

The kinetic-molecular model (KKM) for calculating the rates of condensation of supersaturated unary vapors has been extended to ideal and nonideal binary vapors by treating such mixtures as pseudovapors with properties assigned according to a linear combination of the properties for the pure components in the vapor phase. The single empirical KMM parameter, the hot-cluster stabilization parameter (SP) was found to vary with vapor composition, with deviations from linearity that could be correlated with cluster compactness; indeed, SP provides a measure of this structural feature.
Vibration–rotation variational calculations: Precise results on HCN up to 25 000 cm−1
Authors: Carter Stuart , Mills Ian M. , Handy Nicholas C.
Published: July 2002

Variation calculations of the vibration–rotation energy levels of many isotopomers of HCN are reported, for J=0, 1, and 2, extending up to approximately 8 quanta of each of the stretching vibrations and 14 quanta of the bending mode. The force field, which is represented as a polynomial expansion in Morse coordinates for the bond stretches and even powers of the angle bend, has been refined by least squares to fit simultaneously all observed data on the Σ and Π state vibrational energies, and the Σ state rotational constants, for both HCN and DCN. The observed vibrational energies are fitted to roughly ±0.5 cm−1, and the rotational constants to roughly ±0.0001 cm−1. The force field has been used to predict the vibration rotation spectra of many isotopomers of HCN up to 25 000 cm−1. The results are consistent with the axis-switching assignments of some weak overtone bands reported recently by Jonas, Yang, and Wodtke, and they also fit and provide the assignment for recent observations by Romanini and Lehmann of very weak absorption bands above 20 000 cm−1.
Singlet-to-triplet energy transfer via 1Π1/3Σ+1 curve crossings in group 2 and 12 metal–atom/rare-gas systems
Authors: Bililign Solomon , Gutowski Maciej , Simons Jack , Breckenridge W. H.
Published: July 2002

Of the excited 1Π1 electronic states of van der Waals complexes of Mg, Zn, and Cd atoms (M) with rare-gas (RG) atoms, only the Zn⋅Xe (1Π1) and Cd⋅Xe (1Π1) states predissociate to form the lower-lying triplet states, Zn(4s4p 3P2) and Cd(4s4p 3P2), respectively. It has been postulated that such predissociations occur by means of potential curve crossings between bound 1Π1 levels and repulsive M⋅RG (3Σ1+) states. Since the M⋅RG (1Π1) states become more bound as the RG atom becomes more polarizable, from Ne through Xe, and the M⋅RG(3∑+) states should become more repulsive as the RG atom becomes larger in the same order, the likely reason that only the Zn⋅Xe and Cd⋅Xe 1Π1 states predissociate is that they are the only states which have 1Π1/3Σ1+ curve crossings below the energies which are accessed spectroscopically. We have carried out ab initio electronic structure calculations using various basis sets, and at various levels of correlation, to examine the repulsive 3Σ1+ potential curves of Zn⋅Ar, Zn⋅Kr, Zn⋅Xe, Mg⋅Ar, and Mg⋅Xe. These calculations support the general mechanism proposed, and show that the likely reason the Mg⋅Xe(1Π1) state does not predissociate is because the 1Π1/3Σ1+ curve crossing lies slightly above the energy region probed experimentally. It was necessary to utilize very good quality basis sets and high levels of correlated calculations to obtain agreement with experimental observations. In all cases, there was a regular decrease in the repulsive character of the M⋅RG(3Σ+) states as the basis quality and level of correlation was increased.
Effect of molecular elongation on the dipolar free energy
Authors: Müller Andreas , Winkelmann Jochen , Fischer Johann
Published: July 2002

The dipolar contribution to the Helmholtz energy FD has been determined for dipolar two-center Lennard-Jones molecules of increasing elongation at one corresponding state point in the liquid. The calculation is based on the dipole–dipole energy obtained from simulations and subsequent integration according to the λ-coupling technique. It results, that at fixed values of the reduced dipole moment μ*2=μ2/(εσ3) with ε and σ being the Lennard-Jones site–site interaction parameters, the dipolar free energy FD is nearly constant as function of the molecular elongation. Keeping the effective dipole moment μ̃2=μ2(ρp/Tp) constant with Tp and ρp being the critical temperature and density obtained from WCA-type perturbation theory, FD increases somewhat with the elongation.
Lattice cluster theory for the packing of rods on a lattice: Extension to treat anisotropic orientational distributions
Authors: Huston Shawn E. , Nemirovsky Adolfo M. , Freed Karl F.
Published: July 2002

The lattice cluster theory for the free energy of a set of mutually avoiding rigid rod polymers is extended to treat anisotropic orientational distributions. The theory permits the systematic evaluation of corrections to the isotropic Flory mean field approximation for arbitrary rod orientational distributions, with the Flory theory being the zeroth order isotropic limit of the full theory. The corrections to the zeroth order mean field entropy are represented as a cluster expansion and may be evaluated as a series expansion in the polymer volume fraction φ. We compute all corrections through order φ3 that survive in the thermodynamic limit for the general anisotropic case, along with new fourth order results, which also extend the isotropic limit theory. The anisotropic rod lattice cluster theory represents an improvement over the DiMarzio theory for the packing entropy of rod polymers. This improvement first emerges at fourth order in φ and arises in the lattice cluster theory from inclusion of correlations between four rods lying along distinct lattice directions, four-rod correlations that are absent in DiMarzio’s theory.
Exponentially generated configuration interaction (EGCI) method applied to high-spin multiplicity
Authors: Ehara Masahiro , Nakatsuji Hiroshi
Published: July 2002

The exponentially generated configuration interaction (EGCI) method is extended to high-spin multiplicity. This method, in which excited, ionized, and electron attached states are calculated on the basis of the correlated wave function of the ground state, is applicable to quasidegenerate states of various spin multiplicity and various number of excitation processes. The accuracy and efficiency of the EGCI method is examined for the high-spin quasidegenerate states with multiple excitation nature. Test calculations are performed for several high-spin states of C2, C2+, N2, N2+, and m-phenylenebis(methylene) (m-PBM) molecules. The EGCI method reproduces well the full-CI results in total energy, spectroscopic constants, and spin density distributions. The EGCI method is confirmed to be accurate and useful not only for investigating the energetics involved in the study of dynamics and reactions but also for studying spin density distributions of various spin multiplet states.
Molecular dynamics calculation of the transport coefficients of liquid benzene+cyclohexane mixtures using six-center Lennard-Jones potentials
Authors: Schaink H. M. , Luo H. , Hoheisel C.
Published: July 2002

The shear viscosity, heat conductivity, and mutual diffusion coefficient of liquid benzene+cyclohexane mixtures are calculated by molecular dynamics using six-center Lennard-Jones potentials. The calculated transport coefficients in are fairly good agreement with experimental results, the deviations being ∼10%. An attempt has been made to calculate the Soret coefficient. However, the cross correlation function of the mass and the heat currents could not be obtained reliably for times larger than 0.27 ps, so it was impossible to compute the Soret coefficient accurately. It appears that the sign and our rough approximation of the Soret coefficient are in agreement with experiment.
Stimulated Raman adiabatic passage with pulsed lasers: High resolution ion dip spectroscopy of polyatomic molecules
Authors: Sussmann R. , Neuhauser R. , Neusser H. J.
Published: July 2002

The interaction of two narrow band (Δν<100 MHz) UV light pulses of different intensity with a molecular three-level system is investigated experimentally. The laser frequencies are tuned to an up (pump) and a down (dump) transition sharing a common excited rovibronic S1 level whose population is probed by a transition to the ionization continuum and ion detection. The time sequence of the two pulses results either in a lambda type coherent stimulated Raman adiabatic passage or a stimulated emission pumping (SEP) process. When the first case is realized by a 6.4 ns delay of the low intensity pump laser from the high intensity dump laser pulse, a fourfold increase of the depth of the ion dips compared to the SEP experiment is observed. This is in line with numerical calculations of the level populations using a density matrix formalism including coherent effects. Rotationally resolved ion dip spectra of the 62 state of benzene are presented and demonstrate the high sensitivity of the coherent excitation process of this work for ion dip spectroscopy.
Gd2: The highest spin diatomic molecule?
Authors: Van Zee R. J. , Li S. , Weltner W.
Published: July 2002

Ferromagnetic exchange coupling between the 4f7 inner shells in the Gd2 molecule leads to a 19Σ ground state. Electron-spin-resonance evidence is presented. Analogies to other exchange-coupled diatomics and to larger Gd clusters are considered.
Collisional and electric field effects in the delayed pulsed field ionization zero-kinetic-energy photoelectron spectrum of argon
Authors: Merkt F.
Published: July 2002

The pulsed field ionization (PFI) zero-kinetic-energy (ZEKE) photoelectron spectrum of argon has been recorded in the region of the transition from the ground neutral state (1S0) to the first two ionization limits corresponding to the two spin–orbit levels (2P3/2 and 2P1/2) of the ground state of the ion. The high-n Rydberg states (85<n<200) belonging to the series converging to the upper spin–orbit state have a lifetime which is more than 50 times longer than expected for the optically accessible ns′ and nd′ series. A series of experiments with pulsed and continuous electric fields of different magnitude shows that the nature and the lifetimes of the high-n Rydberg states probed by ZEKE spectroscopy depend critically on the experimental conditions, in particular on electric field and collisional effects. New experimental results are presented which contribute to a better understanding of the mechanisms which lead to the formation of the unexpectedly long-lived states which are observed in ZEKE spectroscopy.
Detection of OH+ in its a 1Δ state by far infrared laser magnetic resonance
Authors: Varberg Thomas D. , Evenson Kenneth M. , Brown John M.
Published: July 2002

The spectrum associated with the J=3←2 transition of OH+ in the a 1Δ(v=0) state has been observed by far infrared laser magnetic resonance spectroscopy. A new microwave discharge source enabled the detection of this spectrum, which is the first observation of the rotational spectrum of an ion in a metastable state. Assignment and least-squares fitting of the observed transitions have determined the following molecular constants: B0=494.420 388 (22) GHz, the proton hyperfine parameter a=74.84 (32) MHz, gL′ = 1.000 915 (15), and gr=−0.001 815 (18), with the 1σ uncertainties of the last digits in parentheses. The relationship of these parameters to the geometric and electronic structure of OH+ is discussed.
Symmetry considerations in molecular photoionization: Fixed molecule photoelectron angular distributions in C3v molecules as observed in photoelectron–photoion coincidence experiments
Authors: Reid Katharine L. , Powis Ivan
Published: July 2002

The symmetry properties of fixed molecule photoelectron angular distributions in various polarization geometries are discussed, and it is shown how these place constraints on photoionization dynamics. We present model calculations of such photoelectron angular distributions for a generic C3v molecule, for limiting choices of photoionization dynamics. Finally, we show how these results help us to interpret the results of photoelectron–photoion coincidence (PEPICO) experiments.
A generalized direct inversion in the iterative subspace approach for generalized valence bond wave functions
Authors: Muller Richard P. , Langlois Jean-Marc , Ringnalda Murco N. , Friesner Richard A. , Goddard William A.
Published: July 2002

We present a greatly improved method for converging generalized valence bond (GVB) self-consistent wave functions. This method starts with the direct inversion in the interative subspace (DIIS) ideas of Pulay. Previously implemented DIIS methods were limited to special cases: closed-shell Hartree–Fock (HF), restricted open-shell HF, or a single pair GVB wave function. Here we extend this method to general wave functions including arbitrary numbers of closed-shell, restricted open-shell, and GVB orbitals (including second-order orbital mixing terms). The efficacy of GVB-DIIS is illustrated by applying it to several cases (including GVB wave functions with up to ten pairs) and comparing with other standard methods.
Aggregation in grafted polymers with attractive end groups
Authors: Gersappe Dilip , Fasolka Michael , Balazs Anna C. , Jacobson Solomon H.
Published: July 2002

We used Monte Carlo computer simulations to determine the properties of grafted polymers that contain attractive functional groups (‘‘stickers’’) on the free ends of the chains. By varying the number of stickers and the grafting density, we isolated conditions that drive the chains to aggregate into clusters. We also observed that the attraction between the stickers effectively ‘‘pulls’’ the chain ends from the bulk of the brush and localizes these sites to the top of the grafted layer. The results provide guidelines for tailoring polymer brushes that display enhanced adhesive properties.
Interference effects in the photodissociation of FNO
Authors: Cotting René , Huber J. Robert , Engel Volker
Published: July 2002

Two-dimensional wave packet calculations on the photodissociation of FNO→F+NO in the S1 state were carried out to analyze the asymmetric resonance line shapes (Fano profiles) in the absorption spectrum in terms of a continuum, a resonance, and an interference contribution. Vibrationally resolved partial cross sections were calculated which predict a strong variation of the vibrational state distribution in the NO fragment as a function of the energy in the vicinity of a resonance. This effect is due to a substantial interference contribution to the vibronic absorption cross section. The results were compared to recent experimental findings by Reisler and co-workers and to the Fano model applied to predissociation. Further, we simulated the excitation with a semi-infinite laser pulse to extract resonance energies from the time behavior of the calculated S1←S0 transition rates.
Energetics of C+16 to C+36 photodissociation
Authors: Bouyer R. , Roussel F. , Monchicourt P. , Perdrix M. , Pradel P.
Published: July 2002

Photofragmentation of mass-selected C+16 to C+36 clusters is investigated at two laser wavelengths: 532 and 355 nm. The depletion dependence on laser fluence leads to the number of absorbed photons, together with the photoabsorption cross section. From the knowledge of the internal energy deposited in the clusters before the fragmentation step, the dissociation energies are deduced. Clusters C+16 to C+29 are found to be less and less stable, whereas above C+30 an increased stability is observed.
The electronic and geometrical structure of aluminum fluoride anions AlF−n, n=1–4, and electron affinity of their neutral parents
Authors: Gutsev Gennady , Leś Andrzej , Adamowicz Ludwik
Published: July 2002

The electronic and geometrical structure of AlF−, AlF−2, AlF−3, and AlF−4 as well as their neutral parents are determined at the unrestricted Hartree–Fock (UHF) and second-order unrestricted Mo/ller–Plesset (UMP2) levels of theory. The results of the calculations are used for estimating the adiabatic electron affinity (E.A.) of the neutrals and fragmentation energies of both the neutrals and anions. All these characteristics were also recomputed using the UMP2/6-31+G* geometry at the fourth-order UMP4 level of theory. According to the results of the single-, double, triple-, and quadruple-excitation fourth-order Mo/ller–Plesset (MP4SDTQ) calculations, the AlF molecule in the ground state has no positive E.A., AlF2 possesses a rather high E.A. value of 1.90 eV, and AlF3 has a smaller E.A. of 0.93 eV, whereas the last member in the series AlF4 possesses an extremely high E.A. of 7.96 eV. Such a large value is related to high stability of the anion towards fragmentation opposite to the neutral AlF4 whose dissociation energy is computed to be only 4–5 kcal/mol. It has been predicted that all the anions are rather stable towards detachment of a fluorine anion, and AlF−4 is the most stable in the series. It is shown the AlF molecule is able to attach an additional electron at large interatomic distances and form a stable anion. This indicates a possibility of an increasing tendency to form resonances when the molecule is excited into a higher vibrational state. Our calculations also show that AlF in the lowest triplet state may attach an additional electron to form a metastable anion in the quartet state. This anion is predicted to be stable towards both detachment of the extra electron and dissociation. A similar increasing tendency to formation of resonance states with an additional electron with vibrational excitation is also predicted for AlF2. Also in this case, at some stretched geometries, the anion in the lowest triplet state becomes more stable than the neutral system. However, relaxation of its geometry leads to its decay to the neutral in the doublet state plus a free electron.
The electronic transition dipole moment of the B+u→X+g transition in iodine
Authors: Lamrini M. , Bacis R. , Cerny D. , Churassy S. , Crozet P. , Ross A. J.
Published: July 2002

The electronic transition dipole moment of the B0+u→X0+g transition in molecular iodine has been determined in the range 2.633≤Rcentroid≤6.035 Å. We have measured a decrease of ‖μ̄e‖2 of about 4 orders of magnitude, perhaps the largest variation of ‖μ̄e‖2 yet observed in a molecule.
An ab initio cluster model study of the magnetic coupling in KNiF3
Authors: Casanovas Jordi , Illas Francesc
Published: July 2002

Cluster models of increasing complexity have been used to model magnetic interactions in KNiF3. These clusters contain two or four magnetic centers plus the bridge F− anions and different representations of the remaining of the crystal. The magnetic coupling constant has been obtained by computing ab initio wave functions for different spin states. These wave functions explicitly include internal and external correlation effects. Several sets of Gaussian functions have been tested and many sets of molecular orbitals have been considered in order to study the physical origin of magnetism in KNiF3. The calculated magnetic coupling constant differs from model to model but shows a fairly good convergence to the experimental result. The use of different cluster models permits to separate the magnetic coupling constant in several contributions. These are the delocalization of magnetic orbitals, the external correlation, and the collective effects normally hidden in the two body operator of the Heisenberg Hamiltonian.
Resonances in the photodissociation of HCl in the Ar–HCl van der Waals complex: How prominent are they?
Authors: Schröder Thomas , Schinke Reinhard , Mandziuk Margaret , Bačić Zlatko
Published: July 2002

We investigate the cage effect in the ultraviolet (UV) photodissociation of the Ar...HCl van der Waals complex, especially the possibility of resonance structures caused by trapping of the hydrogen atom between its heavy partners as recently highlighted by Garcia-Vela and Gerber [J. Chem. Phys. 98, 427 (1993)]. The dynamics is described by solving the time-dependent Schrödinger equation employing the standard Jacobi coordinates which are routinely used for triatomic systems. Due to the large size of the required grid, exact three-dimensional (3D) wave packet calculations are extremely time consuming and could be followed up to 20 fs only. This time is sufficient for calculating the absorption spectrum, but too short for determining the final kinetic energy distributions of the fragment atoms. Therefore, the photodissociation dynamics is mainly treated in a vibrationally sudden approximation, in which the dynamical calculations are performed for a range of fixed ArCl bond distances, and the results averaged over this bond length. 3D classical trajectory calculations show that the energy transfer out of the dissociative HCl mode is very weak (∼5% of the total energy), supporting the application of the sudden approximation. In this approximation, both the absorption spectrum and the kinetic energy distribution associated with the dissociating HCl motion exhibit very weak diffuse structures (resonances) which, following the work of Garcia-Vela and Gerber, can be assigned to the transient vibrational motion of hydrogen between Ar and Cl. However, in our calculations these structures are much less pronounced than in the work of Garcia-Vela and Gerber. The very small amplitudes of the resonance features indicate that trapping in the dissociation of HCl in Ar...HCl is marginal, and much less important than suggested by the previous studies of Garcia-Vela et al. Furthermore, in contrast to the work reported by Garcia-Vela et al., we do not find any evidence for the narrow, irregular features superimposed on the resonance structures.
The nonlinear optical spectrum of polydiacetylene in the near infrared
Authors: Gass P. A. , Abram I. , Raj R. , Schott M.
Published: July 2002

The spectrum of the modulus and the imaginary part of the third order nonlinear optical susceptibility χ(3) of polydiacetylene was studied over the range 750–1000 nm by a combination of two-photon absorption and degenerate four wave mixing measurements. A peak in the two-photon absorption spectrum near 880 nm was found to be due to induced absorption, probably caused by direct optical excitation of the long-lived triplet exciton. No clear evidence was observed for resonances due to two-photon accessible Ag symmetry states in the spectral range studied.
Laser double resonance study of collisional removal of O2(A 3Σu+,υ=7) with O2
Authors: Copeland Richard A.
Published: July 2002

In a two laser experiment, O2 molecules are excited to the υ=7 vibrational level in the A 3Σ+u electronic state by pulsed ultraviolet light. The temporal evolution of the excited O2 is monitored via resonantly enhanced ionization through the C 3Πg Rydberg state. A rate constant for removal of υ=7 state of (2.8±0.6)×10−11 cm3 s−1 by O2 is measured.
Study of the van der Zwan–Hynes model for dipole isomerization reaction rates from the viewpoint of critical phenomena
Authors: Singh Surjit , Robinson G. Wilse
Published: July 2002

We study the van der Zwan–Hynes model of nonequilibrium solvation dynamics for reactions in polar solvents. In this model those authors showed the equivalence of a multidimensional reaction coordinate picture to a simple one-dimensional description. They identified several distinct regimes and successfully described these in molecular terms. They found that in the regime of long solvent response times, the reaction rate had singular aspects which could be described in terms of simple power laws. There are two sets of exponents depending on whether the solvent is underdamped or overdamped. In this paper, we show that this behavior can be readily understood by mapping this problem onto critical phenomena. We find that the two sets of exponents correspond to two types of critical behaviors. There is also a regime when both types of critical behaviors are present and one can see the crossover effects between the two by varying the parameters. Studying the resulting crossover scaling function gives new insights into this problem and reveals its rich behavior.
Variational solutions for the thermal and real time propagator using the McLachlan variational principle
Authors: Messina Michael , Garrett Bruce C. , Schenter Gregory K.
Published: July 2002

A new approximation to the propagator is presented. The approximation as applied to the thermal propagator (coordinate space density matrix) is obtained by using an analog of the McLachlan variational principle for the solution of the Bloch equation. The approximation as applied to the real time propagator is obtained by using the McLachlan variational principle for the solution of the time-dependent Schrödinger equation. The approximate coordinate space density matrix has the same functional form of the high temperature limit of the density matrix, while the approximate real time propagator has the same functional form as the short time propagator. We present numerical results for the thermal propagator for several test systems and compare these results to previous work of Zhang, Levy, and Freisner [Chem. Phys. Lett. 144, 236 (1988)], Mak and Andersen [J. Chem. Phys. 92, 2953 (1990)], and Cao and Berne [J. Chem. Phys. 92, 7531 (1990)]. We also present numerical results for the approximate real time propagator for several test systems and compare to the exact results and results obtained by Gaussian wave packet propagation.
Near infrared 3ν2 overtone band of H+3
Authors: Ventrudo Brian F. , Cassidy Daniel T. , Guo Zhao Yu , Joo Sangwoo , Lee Steven S. , Oka Takeshi
Published: July 2002

Near infrared spectrum of the 3ν2 overtone band (ν2=3←0) of H+3 has been observed at 1.4 μm. The spectrum is weaker than the ν2 fundamental band by a factor of ∼250. High sensitivity plasma spectroscopy using velocity modulation and unidirectional multiple passing has enabled us to observe 15 rovibrational transitions. Short-external-cavity InGaAsP diodes were used as tunable near infrared radiation sources. The narrow tuning range and the availability of diodes limited the observation to a fraction of observable transitions. Nevertheless, the observed results provide information on the rovibrational energy of the 3ν2(l=1) state which may be used to further improve the variational calculations by Miller and Tennyson.
Constant centrifugal potential approximation for atom–diatom chemical reaction dynamics
Authors: Moribayashi Kengo , Takada Shoji , Nakamura Hiroki
Published: July 2002

The constant centrifugal potential approximation (CCPA) is generalized so as to be applicable to the reactions of rotationally excited reactants. The accurate calculations of reaction probabilities are required only for J≤‖Ωi‖MAX(≤ji), where J is the total angular momentum quantum number, ji represents the initial rotational state of a reactant diatomic molecule, Ωi is the z component of J in the body-fixed frame in the initial arrangement and ‖Ωi‖MAX is the maximum of such ‖Ωi‖’s that give significant contributions to the reaction. The method is applied to the D+H2(vi=0,ji) reaction with use of the hyperspherical coordinates, and is proved to be useful by comparing the results with those, of the authors’ own accurate calculations. The reaction mechanisms are clarified with respect to the dependence on Ωi and ji.
Quantum kinetic equations incorporating the Fano collision operator: The generalized Hess method of describing line shapes
Authors: Monchick Louis
Published: July 2002

A Laplace-transformed quantum kinetic equation, quadratic in the singlet density matrix, is derived for gas mixtures in which, embedded as the collision term, the Fano relaxation tetradic allows for off-energy-shell scattering, i.e., incomplete collisions. A sufficient condition for the derivation is a stosszahl ansatz which is weaker at low frequencies than the one usually employed to derive Botlzmann-type equations. At high frequencies or, conversely, short times, it seems rather more stringent. The generalized Hess method, which is a quantum version of the Bhatnagar–Gross–Krook approximation, is used to solve it approximately, yielding a solution that describes the main features of collision broadening and Dicke narrowing. The relaxation tetradics that appear in the generalized Hess method, replace the collision term and are expressed in terms of collision integrals that are defined for finite concentration of optically active molecules. This means that self and resonant broadening and quenching are also included to some degree. The scattering operators in these collision integrals are expanded in partial waves—assuming that gas is composed of diatomic molecules—and recombined in the total angular momentum representation. Extensions to other representations seem straightforward. The reduction to the standard ‘‘impact approximation’’ or Shafer–Gordon theory is indicated as well as the symmetry effects of nuclear spin.
Double resonance infrared–visible photofragment yield spectroscopy of NO2: Interferences among overlapping quasibound levels
Authors: Reid S. A. , Reisler H.
Published: July 2002

We examine fluctuations in the unimolecular decomposition of state-selected NO2 using double resonance infrared (IR)–visible and visible–IR photofragment yield (PHOFRY) spectroscopy. Spectra for specific NO(v,J,Λ,Ω) states at excess energies (E°)=2000–2525 cm−1 exhibit marked differences in the shapes, positions, and widths of the resonance structures. We interpret this effect as arising from interferences among coherently excited overlapping quasibound levels. Comparisons of IR–visible PHOFRY spectra with one-photon spectra in the same energy region and IR–visible spectra at lower E° are used to assess the importance of thermal averaging (i.e., incoherent excitations) and to examine the transition from mild to more severe level overlap. The experimental results are compared qualitatively with results from sample calculations to shed further light on the interplay between dynamics and statistics in the decomposition and the role of overlapping quasibound levels.
Smectic-A to bilayer evolution in concentrated surfactant solutions: The role of spontaneous curvature
Authors: Granek Rony , Gelbart William M. , Bohbot Yardena , Ben-Shaul Avinoam
Published: July 2002

We study the two-dimensional (2-D) structural and thermodynamic changes in smectic-A/lamellar phases of self-assembling surfactant systems, in which the rim associated with a bilayer edge has a preferred curvature. This property was not considered in previous studies of 2-D aggregation, where an infinite bilayer emerges already at very low concentrations. A lattice Hamiltonian is used to describe the bending energy of the rim: An occupied lattice site corresponds to a minimum, disklike, micelle, and a bending energy penalty is associated with corners and straight edges depending on the value of the spontaneous curvature. When the spontaneous radius of curvature of the rim is small and the bending modulus is large, we find that the ‘‘condensation’’ transition—i.e., the ‘‘collapse’’ of the finite aggregates into a continuous bilayer—is postponed to high concentrations. At low concentrations the bending energy leads to an effective repulsive interaction between the aggregates, which in turn can result in ordered (modulated) structures for not too large ratios of thermal energy to bending energy (which is the expected situation in most systems of interest). Our model should be applicable to the systems of decylammonium chloride and cesium perflourooctanoate studied by Boden and co-workers (NMR and conductivity measurements) and Zasadzinski and co-workers (freeze fracture), where monodisperse micellar disks are observed to layer in stacked planes. In the latter system a 2-D order of disk-shaped aggregates appears within the smectic-A layers, which is also consistent with our theory. Experimental studies of the structural evolution under further condensation of the system are not yet available.
Kinetic theory for a mobile adsorbed gas
Authors: Koch Donald L.
Published: July 2002

The equations of motion for a classical adsorbed gas on a single crystal surface, in which the kinetic energy of the adsorbate is large compared with the potential barrier between sites, are derived. If the corrugation of the surface is sufficiently small, an isolated adsorbate will translate in a ballistic motion parallel to the surface for a distance that is much larger than its diameter before transferring a significant fraction of its parallel momentum to the lattice. A kinetic theory that includes effects of adsorbate–adsorbate collisions as well as the adsorbate–lattice coupling is derived. If the only role of the substrate was to confine the adsorbed species to a thin layer, then the equations of motion would be those for a two-dimensional gas. However, the transfer of momentum and energy between the adsorbate and the lattice affects the form of the transport equations and is essential to obtain Fick’s law for diffusion due to adsorbate concentration gradients.
Microphase separation in thin films of the symmetric diblock-copolymer melt
Authors: Kikuchi M. , Binder K.
Published: July 2002

By means of Monte Carlo simulations of a lattice model, microphase separation transition of symmetric diblock-copolymer melts confined in the thin-film geometry between parallel hard walls is studied. We impose a surface field which acts repulsively only on one of the two species, to stabilize lamellar order parallel to the surfaces. However, interplay between two characteristic lengths, that is, the natural thickness of the lamellar l and the thickness of the film D, causes complicated behavior. In case that the two lengths are compatible with each other, clear lamellar order parallel to the surfaces is observed at temperatures lower than the bulk transition temperature, as expected. On the other hand, tilted or deformed lamellar structure, or even coexistence of lamellae in different orientations are found in cases of strong conflict. In both cases, lamellae are fully established. Even at temperature higher than the bulk transition temperature, weak order is induced by the surface field, and a gradual transition between such surface-induced order and bulklike order is observed. Film thickness and temperature dependence of the ordered structure is discussed, as well as a density reduction near the walls and in the interfaces between the segregated regions.
The nonadiabatic photodissociation of BrNO at 355 nm
Authors: Cao Jianying , Loock Hans-Peter , Qian Charles X. W.
Published: July 2002

Nozzle cooled BrNO molecules were dissociated with a polarized laser at 355 nm. The NO fragments were then state-selectively probed using a second tunable laser. Resonance-enhanced multiphoton ionization and fragment time-of-flight techniques were combined to obtain scalar and vector properties of the NO fragment. For each of the NO states probed, the Br*/Br ratio was determined. From these data, a comprehensive picture of the dissociation process was obtained. The excitation at 355 nm has a transition dipole moment parallel to the Br–N bond (β≊2.0). The NO fragment was observed with high rotational excitation (J≊50) and nonstatistical spin–orbit and Λ-doublet state distributions.
Comparative energy derivative analyses of the HBO–BOH and AlOH–HAlO potential energy hypersurfaces
Authors: Yamaguchi Yukio , DeLeeuw Bradley J. , Vacek George , Richards Claude A. , Schaefer Henry F.
Published: July 2002

A study of first and second derivatives of the orbital, electronic, nuclear, and total energies for closed-shell self-consistent-field (SCF) wave functions has been applied to the ground state HBO–BOH and AlOH–HAlO potential energy hypersurfaces. At the stationary points, these energy derivative quantities are uniquely transformed from the Cartesian to the normal coordinate system. Using the four equilibrium and four transition state structures on the two potential energy hypersurfaces, it is demonstrated that the energy derivative method may be used as a powerful quantitative model in understanding and interpreting various chemical phenomena including structures and reactivities. Specifically, the inversion (bending through linear geometry) motions for the bent BOH and AlOH molecules are found to be electronically favorable processes. The isomerization reaction between the linear HBO and bent BOH is seen to be electronically unfavorable, whereas the corresponding reaction between the bent AlOH and linear HAlO is observed to be an electronically favorable process.
Polymers in quenched random environment: Weak replica symmetry breaking
Authors: Haronska P. , Vilgis T. A.
Published: July 2002

The case of polymers in strong quenched disorder is investigated. The disorder is modeled by a quenched random potential which is sampled by a flexible polymer chain. It is shown that the problem, originally formulated in terms of Edwards type path integrals, can be transformed into an effective field theory. Localized phases of the polymer, where the size of the chain is independent of its contour length, i.e. R2∝(1/Δ)2/(4−d), can be found by a weak breaking of the replica symmetry. Δ is a measure for the disorder. The same formalism predicts at d=4 an essential singularity R2 ∝exp(2π2/Δ) for vanishing disorder Δ→0.
Hyperfine interaction of nd10(n+1)s1 impurities in zinc chalcogenides
Authors: Bucher Manfred
Published: July 2002

The measured hyperfine shifts of ten nd10(n+1)s1 impurities in zinc chalcogenide hosts are found to scale with an orbital-moments ratio of both the host anions and the impurity ions. The scaling relation is employed to predict hyperfine shifts of the remaining eight impurity-host systems.
Cross sections for molecular aggregation with positional and orientational restrictions
Authors: Ziabicki Andrzej , Jarecki Leszek
Published: July 2002

Effects of molecular symmetry and intermolecular interactions on the cross section for association–dissociation reactions are discussed. The cross section controls thermodynamic equilibrium, kinetics of aggregation, and sensitivity to external orienting fields. Example calculations indicate variation of the cross section for aggregation by several orders of magnitude and substantial changes in crystallization temperature of asymmetric rigid molecules.
Determining the density of states and partition function for polyatomic molecules
Authors: Kusnezov Dimitri
Published: July 2002

Absolute thermodynamic properties can be extracted for complex molecules which are described algebraically. By exploiting some group theoretical properties of the Hamiltonians, which often include large anharmonicities, the partition function, and properly normalized density of states can be found. When the first few energies of the system are known, one can extend the results accurately to zero temperature. Examples are given for diatomic molecules as well as the polyatomic molecule C2H2.
Quantum stochastic approach for molecule/surface scattering. II. Adsorption resonances of He atoms on Xe overlayers
Authors: Bittner Eric R. , Light John C.
Published: July 2002

He scattering from Xe monolayers adsorbed onto Ag substrates is studied using a quantum stochastic method. Both the He atom and the surface and bulk modes are treated quantum mechanically using a mean-field partitioning of the quantum Liouville equation. The resulting equations of motion permit the evaluation of the reduced density matrix of the He atom subsystem by averaging over a series of ‘‘quantum trajectories.’’ The final reduced density matrix of the atomic subsystem, when projected onto the asymptotic states, provides an accurate estimate of energy transfer processes between the atom and the surface. Our calculations indicate that inelastic energy transfer is enhanced near adsorption resonances due primarily to the increased lifetime of the atom near the surfaces as evidenced by marked deviations from the Debye–Waller thermal attenuation rates.
Fluctuations in absorption spectra and final product state distributions following photodissociation processes
Authors: von Dirke Michael , Heumann Bernd , Kühl Klaus , Schröder Thomas , Schinke Reinhard
Published: July 2002

We present a quantum mechanical wave packet study for the unimolecular dissociation of a triatomic molecule into an atom and a diatom. The 3D potential energy surface used in the dynamics calculations is that of the B̃ state of water corresponding to the second absorption band. Both OH stretching coordinates and the bending angle are included. What is not taken into account is the strong nonadiabatic coupling to the lower-lying Ã and X̃ states which in reality drastically shortens the lifetime in the B̃ state. For this reason the present study is not a realistic account of the dissociation dynamics of water in the 122 nm band. It is, however, a representational investigation of a unimolecular reaction evolving on a realistic potential energy surface without barrier. The main focus is the resonance structure of the absorption spectrum and the final rotational state distributions of the OH fragment. The total absorption spectrum as well as the partial dissociation cross sections for individual rotational states of OH show drastic fluctuations caused by overlapping resonances. The widths of the individual resonances increase, on average, with the excess energy which has the consequence that the cross sections become gradually smoother. Although the low-energy part of the spectrum is rather irregular, it shows ‘‘clumps’’ of resonances with an uniform spacing of ∼0.1 eV. They are discussed in the context of IVR and a particular unstable periodic orbit. In accordance with the fluctuations in the partial dissociation cross sections as functions of the excess energy the final rotational state distributions show pronounced, randomlike fluctuations which are extremely sensitive on the energy.

The average is given by the statistical limit (PST), in which all levels are populated with equal probability. With increasing excess energy the distributions more and more exhibit dynamical features which are reminiscent of direct dissociation like rainbows and associated interferences. Classical trajectories for small excess energies are chaotic, as tested by means of the rotational excitation function, but become gradually more regular with increasing energy. Our wave packet calculations hence demonstrate how the transition from the chaotic to the regular regime shows up in a fully quantum mechanical treatment. The results of the present investigation are in qualitative accord with recent measurements for the unimolecular dissociation of NO2.