bluuebunny/crossref_metadata_2025 huggingface dataset:
Infrared profile of NH3 trapped in argon matrix
Authors: Girardet C. , Lafhlifi A.
Published: July 2002

A detailed theoretical analysis of the spectral profile of the ν2 infrared band of NH3 trapped in an argon crystal is performed. The coupled external (rotation + lattice and molecule center of mass translation) and internal (vibration + inversion) motions of the doped crystal are studied after decomposing the total Hamiltonian into renormalized parts connected to pseudooptical states and to pseudobath states. The calculation of the optical states (vibration-inversion-rotation or inversion-rotation states) allows the bar infrared and far infrared spectra to be built and fairly well compared to experimental data. This model can quantitatively explain the narrowing of the ν2 inversion doublet, the splitting of the more intense R(0) branch into two peaks and the high frequency structures connected to the splittings of the R(1) branch by both the inversion and the hindered rotation processes. The infrared profile is then determined by using a conventional cumulant expansion method. The calculated linewidths and additional lineshifts are in close agreement with recent high resolution experimental spectra.
Effect of residual interactions on polymer properties near the theta point
Authors: Cherayil Binny J. , Douglas Jack F. , Freed Karl F.
Published: July 2002

Perturbation calculations are performed for 〈R2〉, 〈S2〉, 〈RH〉, A2, and A3 for polymers near the theta point using a continuous curve representation of the three-parameter theory of Yamakawa. Both dimensional regularization and cut-off schemes are employed, but only the latter method is shown to be in accord with observed polymer properties, demonstrating the necessity of retaining a cut-off (or its equivalent) when ternary interactions are incorporated into the three dimensional model. An effective binary interaction parameter is defined to vanish along with the second virial coefficient under theta conditions. Renormalization group calculations are then combined with the perturbation expansions to extend the predictions for these polymer properties to larger ternary interactions while still remaining in the vicinity of the theta point where the effective binary interaction is small. A comparison of calculated values of A3 and the ratios 〈S2〉1/2/RH and 〈R2〉/〈S2〉 with data from experiment and computer simulations provides a self-consistent estimate of 𝒪(10−3) for the magnitude of the three body interaction term. The ternary effect is found to be small but not negligible for flexible linear polymers. Ternary interactions are, however, expected to be more substantial for branched polymers. The cut-off theory of theta point polymers accounts satisfactorily for the shift in the theta temperature observed, but previously unexplained, in numerical studies of lattice polymers. Our calculations also reconcile many contradictory conclusions of earlier calculations on theta point polymer properties.
A stochastic model of protein conformational dynamics and electronic–conformational coupling in biological energy transduction
Authors: Cartling Bo
Published: July 2002

The principles of biological energy transduction are discussed by means of a mathematical model of a donor–acceptor system of electron transfer enzymes in which electronic and conformational states are coupled. The internal nuclear motion of the enzymes is considered to be composed of transitions between local potential energy wells, which define conformational states, and vibrations within these. The conformational transitions are treated as a stochastic process of the diffusion type on a conformational potential energy surface. Dissipative processes are avoided by restricting electron transfer with respect to conformational states and molecular mechanisms of such electron gating are discussed. Different types of transient kinetics, determined by the relative rates of electronic and conformational transitions, are demonstrated in terms of probability density functions, which describe the probability for the system to be in different electronic and conformational states as a function of time. The experimental basis for the concepts and mechanisms introduced is discussed and further experiments are proposed. The applicability of the mathematical model to other systems is indicated.
On reptation in polymer melts
Authors: Scher Harvey , Shlesinger Michael F.
Published: July 2002

A rederivation of the basic scaling laws for polymer reptation in a condensed phase yield directly for the coefficient of self-diffusion. Dr∼M−2, where M is proportional to the polymer molecular weight, as the primary relationship, independent of the polymer chain statistics. The derivation is based on a first passage time calculation for a kink to reach the end of a chain. The standard reptation time Tr is shown to be a first passage time for a chain to leave a sphere in which it is initially enclosed with the result Tr ∼M−3. The scaling law for a relaxation time TR, defined for a multichain model, is derived, with the result TR ∼M3+δ, with δ∼0.5, consistent with Dr∼M−2.
Two-photon resonance REMPI detection of the formyl radical
Authors: Tjossem Paul J. H. , Goodwin Peter M. , Cool Terrill A.
Published: July 2002

The resonance enhanced multiple photon ionization (REMPI) spectrum of HCO has been observed for dye laser wavelengths from 373 to 427 nm. Long progressions in the bending vibration have been observed for excitation of two electronic states, tentatively identified as the 3p 2Σ+ and 3p 2Π Rydberg states. Estimates are obtained for the rotational constants, vibrational frequencies, anharmonicity constants, and electronic term values for the two states. Additional strong REMPI signals were obtained in the 418–426 nm region, tentatively assigned to the 3s 2Σ+ Rydberg state. A single pulse detection limit for HCO of 1013 cm−3 was measured which suggests the feasibility of the REMPI detection of HCO at the part-per-million level in atmospheric pressure hydrocarbon flames.
Non-Franck–Condon 2σ−1u vibrational distributions in N+2. An interchannel-coupled shape resonance observed by dispersed fluorescence
Authors: Poliakoff E. D. , Ho Ming-Hang , Leroi G. E. , White M. G.
Published: July 2002

We report evidence for a molecular interchannel-coupled shape resonance in N2 photoionization, observable in the vibrational branching ratios for the N+2(B 2Σ+u) state. The vibrational branching ratios for the N+2(B 2Σ+u) state are determined from vibrationally resolved dispersed fluorescence measurements on the N+2(B 2Σ+u→X 2Σ+g) band system. The results demonstrate that the partial cross section for the N+2(B 2Σ+u,v′=1) level is selectively enhanced in the photon energy range 28 <hνexc<30 eV. A shape resonance occurs in this range for the 3σg→εσu channel, and the observed enhancement is attributed to coupling between this channel and the 2σu→εσg channel. Vibrational branching ratio curves for the excited state ion also show a pronounced peak at hνexc=22 eV, which shifts for the curves that sample the v′=1 and v′=2 levels of the ion, suggesting a few interpretations. The utility of coupling monochromatized fluorescence detection with synchrotron radiation excitation for extracting relative partial photoionization cross sections is demonstrated for the first time.
Coupled channel calculation of resonances in H+CO
Authors: Romanowski Hubert , Lee Ki-Tung , Bowman Joel M. , Harding Lawrence B.
Published: July 2002

Coupled channel calculations were performed for the H+CO system, using an ab initio potential energy surface, and treating CO as a rigid rotor. Resonances for zero total angular momentum are found and analyzed. An uncoupled adiabatic stabilization analysis is able to assign most of these resonances to zero-order HCO states embedded in the continuum.
On the numerical evaluation of the rate of first passage over a barrier
Authors: Lovett Ronald
Published: July 2002

In 1940 Kramers used an extended Fokker–Planck equation to describe the dynamics of a thermally activated passage over a barrier. This classic analysis of how rapidly an initial, equilibrium-like reactant distribution develops product-like components has received much attention. In a recent characterization of nucleation dynamics it was noted that, although the dynamics of the spontaneous formation of a critical nucleus can be described with a Fokker–Planck equation, scattering boundary conditions are required. The present analysis shows that direct numerical integration of the Fokker–Planck equation exhibits the numerical instabilities associated with scattering problems. An alternate invariant embedding formulation is given which leads to a numerically stable problem for the distribution function. A simple numerical realization of the invariant embedded formulation is presented and numerical rates of nucleation of liquid drops out of a supersaturated vapor are generated. The numerical work shows that the sensitivity to the boundary conditions and detailed form of the barrier potential increases as the barrier height increases.
Accuracy of the energy-corrected sudden (ECS) scaling procedure for rotational excitation of CO by collisions with Ar
Authors: Green Sheldon , Cochrane David L. , Truhlar Donald G.
Published: July 2002

We examine the question, given a set of state-to-state rate constants k( j=0 → j′‖T) for collision-induced rotational transitions in a diatomic molecule, where j and j′ are initial and final rotational quantum numbers and T is the translational temperature, can one use scaling analyses to predict a full set of j → j′ rate constants? To answer this we consider a rigid rotator model of CO in a bath of Ar at 500 K, and we calculate accurate quasiclassical k( j → j′‖T) for j=0, 10, and 20 and j′=0–29. These are used to test the energy sudden (ES) and energy-corrected sudden (ECS) scaling procedures. Both procedures are used to predict the j=10 and j=20 rate constants from the j=0 values. The ES procedure, which has no adjustable parameters, overestimates the rates out of excited states by a factor of about 1.5 with a rms error of about 60%. The ECS procedure, in contrast, when the one parameter bc (a critical impact parameter) is about 1.75–2.0 Å, yields excellent excited-state rates on the average and has a rms error of less than 20%. The value of bc can be estimated by a weighted average impact parameter leading to inelastic collisions from a j=0 initial state.
Rotational resolution of the NeCl2 B ← X 9–1 transition
Authors: Evard Dwight D. , Thommen Fritz , Janda Kenneth C.
Published: July 2002

High resolution (∼0.001 cm−1) laser induced fluorescence spectra of the (9–1) rovibronic band of the B3Π0u+ ← X 1Σg+ system of Cl2 and the associated van der Waals molecule NeCl2 have been obtained in a free jet expansion of Ne, Cl2, and He. The spectrum of NeCl2 associated with the (9–1) transition of the free Cl2 molecule was observed 5.46 cm−1 to the blue of the band origin of uncomplexed Cl2. This study confirms the previously reported observation of a metastable vibrationally excited ground electronic state of NeCl2. Although the energy in the Cl2 stretching vibration is an order of magnitude larger than the Ne–Cl2 dissociation energy, no dissociation is observed on the microsecond time scale. This study also significantly refines the structural parameters obtained from the previous low resolution work. Rotational resolution was achieved and a detailed rotational assignment was made which allowed the Ne to Cl2 center of mass distance to be determined. This distance is 3.565±0.035 Å in the ground state, and 3.536±0.036 Å in the excited state. The structure of the NeCl2 van der Waals molecule is T-shaped, with the dipole transition moment lying along the Cl2 bond axis, giving a ‘‘b-type’’ asymmetric top spectrum. The vibrational predissociation lifetime in the B(v′=9) state was found to be 258±42 ps on the basis of homogeneous broadening.
A modification of the image dipole selection rules for surface Raman scattering
Authors: Hallmark Vickie M. , Campion Alan
Published: July 2002

We have modified the selection rules for surface Raman scattering derived from consideration of the real dipole-image dipole system for a molecule adsorbed at the surface of a metal, to bring the predictions into accord with the exact electromagnetic theory.
An improvement of the Li+HF PES based on a 3D quasiclassical trajectory test
Authors: Alvariño J. M. , Hernandez M. L. , Garcia E. , Laganá A.
Published: July 2002

A quasiclassical trajectory test of some potential energy surfaces designed for the Li+HF reaction is reported. A comparison of scattering quantities with experimental data has allowed the selection of a surface sufficiently accurate for reproducing reactive cross sections and detailed experimental data.
Direct vibrational spectroscopic observations of mixed long range and short range adsorbate interactions: The K+CO interaction on Ni(111)
Authors: Uram K. J. , Ng Lily , Folman M. , Yates J. T.
Published: July 2002

IR reflection-absorption spectroscopy has been employed to study K–CO interactions on Ni(111) at high resolution. It has been found that a mixture of long range and short range interactions are discernible. An ensemble of as many as ∼25 CO molecule/K is affected by the long range K⋅ ⋅ ⋅CO interaction. In addition, Kδ+x(CO)δ−y complexes produced by short range electrostatic effects are observed, yielding characteristic C–O stretching frequencies.
Reactions of O(3P) with saturated hydrocarbons: Vibrationally adiabatic distorted wave calculations of product rotational distributions for two triatomic model reactions
Authors: Clary D. C. , Connor J. N. L. , Southall W. J. E.
Published: July 2002

Vibrationally adiabatic distorted wave (VADW) calculations of product rotational distributions have been performed for the heavy+light–heavy atom reaction O(3P)+HR → OH(v′, j′)+R, where for v′=0, RH=neopentane=C(CH3)4, and for v′=1, RH=isobutane=(CH3)3CH. Extended London–Eyring–Polanyi–Sato potential energy surfaces are used, with R treated as a structureless particle. It is shown that plotting the rotational distributions against the rotational energy of OH (experimental and theoretical) is a better procedure than just using the rotational quantum number. Good agreement is obtained with experimental rotational distributions and with the results of quasiclassical trajectory calculations.
Measurement of the rotational spectra of OH+ and OD+ by laser magnetic resonance
Authors: Gruebele M. H. W. , Müller R. P. , Saykally R. J.
Published: July 2002

Far IR rotational transitions between the four lowest rotational levels in the X 3Σ− vibronic ground states of OH+ and OD+ have been observed by laser magnetic resonance spectroscopy. Ground state molecular constants, including the three g factors, have been determined and employed in the calculation of a Born–Oppenheimer equilibrium geometry. The centrifugal distortion of the spin-rotation interaction is found to have a significant effect on the determination of other molecular constants. Hyperfine splittings have been resolved and analyzed for OH+, but could not be observed in OD+ spectra with a 6 MHz collision-broadened linewidth.
On the distribution of included angles for the Rouse chain in strong steady shear flow
Authors: Öttinger Hans Christian
Published: July 2002

The distributions of included angles for Rouse chains in steady shear flow are calculated exactly in the limit of large shear rate. Brownian dynamics simulations confirm the exact results.
Rotational state distributions of H2 and CO following the photofragmentation of formaldehyde
Authors: Schinke Reinhard
Published: July 2002

The rotational state distributions of H2 and CO following the photo-fragmentation of formaldehyde at wavelengths of about ∼29 500 cm−1 are analyzed using a simple model. The ingredients of the model are an ab initio potential energy surface in the excit channel and the known geometry of the H2CO→H2+CO transition state. Both the ortho H2 and the CO distributions agree very well with recent measurements. While the CO rotational distribution is directly determined by the excit channel dynamics (i.e., the potential energy surface) the H2 distribution is exclusively determined by the Franck–Condon principle (i.e., the transition state geometry). CO and H2 rotations are almost completely uncorrelated. All results are easily understandable in terms of the calculated potential energy surface.
The laser induced fluorescence spectrum of the HeCl2 van der Waals molecule
Authors: Cline Joseph I. , Evard Dwight D. , Thommen Fritz , Janda Kenneth C.
Published: July 2002

Laser-induced fluorescence excitation spectra of free-jet expansions of He and Cl2 have been recorded. Spectroscopic features observed ∼3.5 cm−1 to the blue of the free 35Cl2 vibronic origins of the 7–0 through the 12–0 bands of the B 3Π0+u–X 1Σ+g system were attributed to the He35Cl2 molecule. The observed spectra were well simulated by a T-shaped, rigid, asymmetric top model. The spectra were observed to display homogeneous broadening dependent on the vibrational band excited. Predissociation lifetimes were calculated and structural parameters determined by fitting the observed contours. The distance from the He atom to the Cl2 center of mass is 3.8±0.4 and 3.9±0.4 Å for the ground and excited states, respectively.
Microemulsions in the three-phase regime
Authors: Kleinert H.
Published: July 2002

Using a simple Landau free energy we determine the shape of the three-phase regime of a ternary mixture of water, oil, and tenside (microemulsion) in the space of concentrations and temperature x, y, τ. The boundaries at x=0 (equal water and oil) and y=1 (fixed tenside) are in excellent agreement with recent data of Kahlweit et al. and Shinoda et al. We also find satisfactory agreement for the temperature behavior of fluctuation phenomena such as the intensity of scattered light and the interfacial tensions.
LPZ spectral analysis using linear prediction and the z transform
Authors: Tang J. , Norris J. R.
Published: July 2002

A new method of spectral analysis that combines linear prediction and z-transform, the LPZ is described. LPZ is compared with the fast Fourier transform FFT method of data analysis. The superior frequency resolution and sensitivity and the noise reduction is described when LPZ is used in spectral analysis. (AIP)
The calculated electronic excitation spectra of Si2C and Si3
Authors: Sabin John R. , Oddershede Jens , Diercksen Geerd H. F. , Grüner Norbert E.
Published: July 2002

The results of second order polarization propagator calculations of the valence excitation energies and oscillator strengths of Si2C and Si3 are reported and compared to the previously published spectrum of SiC2. The character and properties of the excitations are explored and suggestions are made as to excitations which may be amendable to experimental study. A previously observed optical transition in Si clusters is assigned to an intense line in the Si3 spectrum at about 46 000 cm−1.
Infrared diode laser kinetic spectroscopy of the CCO radical in the X̃ 3Σ− state generated by the excimer laser photolysis of carbon suboxide
Authors: Yamada Chikashi , Kanamori Hideto , Horiguchi Hiroyuki , Tsuchiya Soji , Hirota Eizi
Published: July 2002

The ν1 band of the CCO radical in the X̃ 3Σ− ground electronic state has been observed in the gas phase by diode laser kinetic spectroscopy. The CCO radical was generated by the 193 or 248 nm excimer laser photolysis of carbon suboxide. By fixing ground state parameters to the microwave values, the band origin and the vibrational changes of the rotational (αB=B0−B1) and spin–spin interaction (αλ=λ1−λ0) constants have been determined to be 1970.864 34(95), 0.003 075 4(85), and 0.008 3(12) in cm−1 with 2.5 standard errors in parentheses.
Geminate recombination and relaxation of molecular iodine
Authors: Abul-Haj N. Alan , Kelley David F.
Published: July 2002

Picosecond absorption kinetics of I2 in several room temperature solvents have been obtained. Probe wavelengths of 500, 580, and 640 nm were used following excitation with 30 ps pulses of 532, 630, or 683 nm light. The results indicate that population of the electronically excited A(3π1u) and A′(3π2u) states occurs rapidly (<30 ps) following dissociation. Analysis of transient absorption intensities indicates that repopulation of the ground electronic state requires about 30–50 ps. We propose that the difference in population times may be understood in terms of trapping in weakly bound electronic states (perhaps 3π0−u) which relax to populate the ground electronic state, or in terms of slow atom recombination into the ground state. The results show that for I2 in CCl4, X state repopulation and vibrational relaxation occur on roughly comparable time scales. The results also show that the recombination dynamics are determined primarily by the nature of dissociative state, not the recoil energy. Vibrational relaxation rates through the ground state mainfold were interpreted in terms of vibration to translation energy transfer as given by the Schwartz, Slawsky, Herzfeld theory. This was found to work quite well for weakly interacting solvents such as CFCl3, C2Cl3F3, and CCl4. This simple theory breaks down for more strongly interacting solvents such as CH2Cl2 and CHCl3.
Macrotransport processes in the presence of bulk and surface chemical reactions
Authors: Pagitsas Michael , Nadim Ali , Brenner Howard
Published: July 2002

The theory of macrotransport processes is extended to include systems that undergo first-order irreversible bulk and/or surface chemical reactions in confined geometries. A convective-diffusion type kinetic equation incorporating both bulk and surface reaction terms is assumed to describe the microtransport of a reactive tracer particle in a multidimensional phase space. The independent positional and configurational variables appearing therein are decomposed into local (fast) and global (slow) variables, the distinction between them being based upon the existence of a small dimensionless parameter ε appearing in a scaled form of the kinetic equation. Elimination of the fast variables via a systematic perturbation scheme in ε results in a kinetic equation that governs the macrotransport of the reactive tracer. Expressions are obtained for the three physical-space phenomenological coefficients appearing therein, namely the mean velocity vector Ū*, dispersivity dyadic D̄*, and reaction velocity constant K̄*, in terms of quadratures of the seven local-space coordinate-dependent microtransport phenomenological functions appearing in the original kinetic equation. These macrocoefficients are given both Eulerian and Lagrangian interpretations.
State-resolved study of collisional energy transfer between A 2Π v=7 and X 2Σ+ v=11 rotational levels of CN
Authors: Furio Nick , Ali Ashraf , Dagdigian Paul J.
Published: July 2002

Collisional transfer from the A 2Π state of CN has been studied with initial and final state resolution by an optical–optical double resonance technique. Specific rotational levels in the v=7 vibrational manifold of the A state of CN in a flow of several Torr of argon are prepared by pulsed laser excitation in the A–X (7,2) band. After a short time delay, a second laser probes the populations of quantum levels in this vibrational manifold and in the nearly isoenergetic v=11 manifold of the X 2Σ+ state by fluorescence excitation in the overlapped B–A (8,7) and B–X (8,11) bands. The interelectronic A→X transfer rate is found to be comparable to that for purely rotational collisional transitions within the A state for all incident levels studied, regardless of whether or not they possess significant X state character, because of isolated molecule non-Born–Oppenheimer mixing. Reflecting the near homonuclear character of the CN–Ar interaction potentials, the final X state populations exhibited a significant even–odd alternation as a function of the final rotational angular momentum quantum number. These populations could be adequately fit by the sudden scaling relationship for Π→Σ cross sections derived by Alexander and Corey [J. Chem. Phys. 84, 100 (1986)].
A local HNC/HNC approximation for the 2:2-RPM electrolytes at a charged wall
Authors: D’Aguanno B. , Nielaba P. , Alts T. , Forstmann F.
Published: July 2002

A local HNC/HNC approximation is proposed and applied to 2:2 electrolytes near a charged hard wall. The results are compared with Monte Carlo data and other theories such as HNC/HNC, HNC/MSA, BGY, and MPB5. A remarkable improvement of the density profiles and of the diffuse layer potential is obtained. We show that the HNC closure for the ion–ion correlations, when used in the local scheme, always gives better results than those obtained from the MSA closure. The necessary algorithm for solving the local HNC/HNC equations is also presented.
What do kinetic energy anisotropies tell us about chemical bonding? II. First-row A2, AO, and AF diatomics
Authors: Thakkar Ajit J. , Sharma B. S. , Koga Toshikatsu
Published: July 2002

The kinetic energy anisotropy is examined for 18 different diatomic molecules of the type A2, AO, and AF with A=Li, Be, B, C, N, O, and F. The dimensionless measure α=(Tz−Tx)/T, where Tz and Tx, respectively, are the bond-parallel and bond-perpendicular components of the total electronic kinetic energy T, is confirmed to be useful for interpretative studies. It is shown that the orbital values of α in free atoms depend only on the angular momentum quantum numbers, and can be expressed in terms of a Condon–Shortley coefficient. Comparison of α values for individual molecular orbitals with their free atom counterparts enables one to classify molecular orbitals in terms of their parent atomic orbitals. In favorable cases, α values can also be used to estimate the extent and nature of reorganization of each orbital upon bond formation. The total molecular value of α correlates roughly with the difference between the numbers of pσ and pπ electrons.
The approximate quantum mechanical calculation of reactive scattering cross sections: The fixed angle reactor model (FARM)
Authors: Jansen op de Haar B. M. D. D. , Balint-Kurti G. G.
Published: July 2002

An approximate three-dimensional quantum mechanical method for the calculation of atom–diatom reactive scattering cross sections is described and tested by application to the H+H2→H2+H problem. The method, known as the fixed angle reactor model (FARM), combines reactive quantum mechanical calculations performed at fixed angles to yield differential and integral cross sections. It encompasses the best features of the bending corrected rotating linear model (BCRLM) and the reactive infinite order sudden (RIOS) methods, while correcting shortcomings of both. The FARM method is shown to yield reliable reactive cross sections at both low energies in the threshold region and at high energies.
Electronic-structure evolution upon thermal treatment of polyacrylonitrile: A theoretical investigation
Authors: Brédas J. L. , Salaneck W. R.
Published: July 2002

We present valence effective Hamiltonian nonempirical band-structure calculations on polyacrylonitrile. We first consider the compound in its pristine state and discuss the results of calculations on an helical conformation as well as on a linear conformation. We then investigate the electronic properties of various geometric structures that have been proposed in the literature as appearing upon thermal treatment up to 300 °C. These structures namely correspond to a single conjugated structure (due to ring closure along the nitrile groups) and, at higher pyrolysis temperature, to a doubly conjugated ladder structure (polypyridinopyridine). In order to facilitate the discussion of the electronic-structure evolution in pyrolized polyacrylonitrile, we also present band-structure calculations on related compounds, such as polyethylene, polymethineimine, and polyacene. From the band stuctures, we derive the theoretical photoemission spectra of the studied polymers. The comparison with available spectra from photoelectron spectroscopy is very good and allows a detailed interpretation of the evolution of those spectra upon pyrolysis up to 300 °C.
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o CI study of the electronic spectrum of the allyl radical (CH2–CH–CH2)
Authors: Ha Tae-Kyu , Baumann H. , Oth J. F. M.
Published: July 2002

Configuration interaction (CI) studies of the ground, electronically excited and ionized states (cations) are reported for the allyl radical. The calculated excitation energies and ionization potentials are compared with previous and recent experimental values obtained from one-photon or multiphoton absorption spectroscopy and from photoelectron spectroscopy. The excited states considered are the 2B1, 2B2, and 2A2 states arising from dipole allowed transitions from the 2A2 ground state, as well as the dipole forbidden 2A1 and the quartet 4A2 state. Seven lower lying cations including the first ionized state, 1A1, are also calculated.
Spin-orbit state-selected reactions of Kr+(2P3/2 and 2P1/2) with H2, D2, and HD from thermal energies to 20 eV c.m.
Authors: Ervin Kent M. , Armentrout P. B.
Published: July 2002

Spin-orbit state-selected reactions of Kr+(2PJ), J=3/2 and J=1/2, with isotopic molecular hydrogen (H2, D2, and HD) to form KrH+ and KrD+ are investigated using guided ion beam techniques. Reaction cross sections for each spin-orbit state are measured as a function of the relative translational energy of the reactants from near thermal energies up to 20 eV c.m. At low energies, the cross sections for reaction with H2 and D2 are each about 2.6 times larger for the 2P3/2 ground spin-orbit state of krypton ion than for the 2P1/2 excited state. For the HD reaction, the 2P3/2 reactivity is enhanced by a factor of about 4.2. A higher-energy process, which exhibits an apparent activation energy of about 1 eV, is present only for the 2P3/2 spin-orbit state. The Kr+(2P3/2) spin-orbit state exhibits a strong intermolecular isotope effect, showing the unusual dependence σ(HD)>σ(H2)>σ(D2) for the total reaction cross sections at low energies. Cross sections for the Kr+(2P1/2) state show a much smaller dependence on the hydrogen isotope. The intramolecular isotope effect for the individual KrH+ and KrD+ channels in the reaction with HD shows several reversals over the energy range studied. These results are discussed in terms of the potential energy surfaces of the krypton–hydrogen system.
Rotational distributions of molecular photoions following resonant excitation
Authors: Poliakoff E. D. , Chan Jeffrey C. K. , White M. G.
Published: July 2002

We demonstrate that the photoelectron energy mediates the rotational energy distribution of N+2 ions created by photoionization, and conversely, that rotational energy determinations probe resonant excitation in molecular photoionization. Experimentally, this is accomplished by monitoring the dispersed fluorescence from N+2 (B 2Σ+u) photoions to determine their rotational energy distribution. These results demonstrate that while dipole selection rules constrain the total angular momentum of the electron–ion complex, the partitioning of angular momentum between the photoelectron and photoion depends on the photoejection dynamics. Implications for photoionization and electron impact ionizatin studies are discussed.
BaI product state distribution from the reaction Ba+CF3I
Authors: Johnson M. A. , Allison J. , Zare R. N.
Published: July 2002

Under single-collision beam–gas scattering conditions the Ba+CF3I→Bal+CF3 reaction has been studied using laser induced fluorescence to detect the BaI X 2∑+ product. The resulting BaI C 2Π–X 2∑+ excitation spectrum has a complex appearance owing to the similarity of the upper and lower state rotational constants, causing a reversal in band shading, and owing to a predissociation in the upper state, causing a break off in the fluorescence of high vibrational levels in each spin-orbit subband. From the predissociation onset an upper bound of 78.5±0.5 kcal/mol is placed on the BaI bond energy. The vibrational population distribution is shown to be bell shaped, peaking near v″=50, and accounts for most of the available energy. The average energy appearing in BaI rotation decreases with increasing BaI vibrational excitation. The BaI rotational distribution has a width comparable to that of the vibrational distribution.
Temperature dependence of 77Se, 125Te, and 19F shielding and M-induced 19F isotope shifts in MF6 molecules
Authors: Jameson Cynthia J. , Jameson A. Keith , Oppusunggu D.
Published: July 2002

The high resolution NMR spectra of 77Se, 125Te, and 19F nuclei in SF6, SeF6, TeF6, and WF6 molecules are measured as functions of density and temperature in the pure liquid and in the dilute gas. The 19F chemical shifts between the isotopomers of these molecules are observed in all but WF6, and the m′MF6–mMF6 shifts exhibit a strict proportionality to the mass factor (m′–m)/m′.
Thermochemical properties of the rare earth monoxides
Authors: Dulick M. , Murad Edmond , Barrow R. F.
Published: July 2002

Crystal field theory has been used to identify and locate the energy levels of low-lying (below 10 000 cm−1) states of the rare earth monoxides. These states were then used to calculate the thermal functions, which were, in turn, used to derive accurate dissociation energies of the rare earth monoxides. These newly derived dissociation energies are discussed in terms of a model related to the promotion energies for 4f n+15d0→4f n5d1.
Exact pair distribution function and structure factor for a one-dimensional hard rod mixture
Authors: Foidl Ch.
Published: July 2002

A one-dimensional system of N hard rods with arbitary diameters and masses is considered. For this system the pair distribution function is calculated exactly. The result is independent of the masses but depends on the specific sequence of diameters that is considered. The pair distribution function is then specialized to a mixture of Nd particles with diameter d and ND particles with diameter D, respectively, and averaged over all possible arrangements of the particles. From this the structure factor S(κ) for finite N and it’s thermodynamic limit are obtained. It is shown how in this limit a finite peak tends to the singular contribution δ(κ). Finally, the limit S(κ→0) is calculated and compared with a formula following from general considerations for three-dimensional systems.
Collision cross sections for ammonia in its ground vibrational manifold
Authors: Das A. , Townes C. H.
Published: July 2002

Rates for inelastic collision-induced transitions between rotation–inversion states are measured for the NH3 molecule colliding with He atoms and with N2 molecules. An infrared–microwave double resonance method is used to determine differences within pairs of collisional transition rates connecting different inversion pairs in the ground (ν2=0) vibrational manifold of NH3. The measured rates are analyzed for parity, and J and k selection rules. The results are consistent with Oka’s saturation experiments on NH3, but in some respects inconsistent with theoretical calculations of collisional transition rates.
Reaction dynamics for O(3P)+HD. V. Reduced dimensionality quantum and quasiclassical reaction probabilities and rate constants with an adiabatic incorporation of the bending motion
Authors: Bowman Joel M. , Wagner Albert F.
Published: July 2002

Reduced dimensionality exact quantum and quasiclassical reaction probabilities and rate constants are presented for the O(3P)+HD(ν=0,1)→OH+D and OD+H reactions. The effective potential energy surface used is the ab initio collinear MODPOLCI surface with the ground state adiabatic bending energy added in, as in the previous paper in this series. The rate constants are compared with the recent experiments of Gordon et al.
Collisional time-correlation functions in the semiclassical limit. III. Application to vibrational–rotational energy transfer in collisions of Li+ with N2
Authors: Vilallonga Eduardo , Micha David A.
Published: July 2002

We apply a previously developed treatment of energy tranfer based on collisional time-correlation functions (TCFs) to Li+ scattered from N2 at hyperthermal velocities. Double differential cross sections are calculated using a previously available, realistic potential energy surface for this system. After sucessfully testing the TCF results against independent calculations, we present an in-depth study of vibrational–rotational energy transfer over a wide range of scattering angles (θ=10–175 deg) and collision energies (E=4–17 eV). We also carry out a comprehensive comparison with time-of-flight experiments and explain the observed bimodal structure of energy loss spectra. This has led to a modification of the available translation–vibration coupling potential. The calculations are based on the conditions of slow rotational motion and low vibrational excitation, and involve classical trajectories for relative motions generated at each possible orientation of N2. Cross sections were obtained from interpolations of Tables of more than 60 000 trajectories. Vibrational and rotational motions were described quantum mechanically within a previous treatment based on operator algebras.
Full CI studies of the collinear transition state for the reaction F+H2→HF+H
Authors: Bauschlicher Charles W. , Taylor Peter R.
Published: July 2002

We have performed full CI calculations on the collinear transition state for the reaction F+H2→HF+H. The full CI results are compared with those obtained from single reference and multireference CI calculations and from single reference CPF calculations. In general, only those methods which attempt to account for the effects of higher excitations, such as CPF or CI plus the Davidson correction, yield a transition state location and barrier height in good agreement with the full CI. In an extended basis, the effect of higher excitations is estimated to lower the barrier by as much as 1.5 kcal/mol; such an effect would essentially eliminate the present discrepancy between theory and experiment.
An investigation of the hydrogen-bonded dimer H3N⋅⋅⋅HBr by pulsed-nozzle, Fourier-transform microwave spectroscopy of ammonium bromide vapor
Authors: Howard N. W. , Legon A. C.
Published: July 2002

The ground-state rotational spectrum of a dimer of ammonia and hydrogen bromide has been detected by using the technique of Fourier-transform microwave spectroscopy in a Fabry–Perot cavity to examine a supersonically expanded gas pulse composed of ammonium bromide vapor entrained in argon. The spectroscopic constants B0, DJ, DJK, χ(14N), χ(79Br), and χ(81Br) have been determined (where appropriate) for the four symmetric-top type isotopic species (14NH3, H79Br), (14NH3, H81Br), (15NH3, H79Br), and (15NH3, H81Br) and for the first of these the values are as follows: B0=3226.862(1) MHz, DJ=9.0(2) kHz, DJK=142.2(6) kHz, χ(14N)=−3.183(8) MHz, and χ(79Br)=361.245(6) MHz. The spectroscopic constants have been interpreted in terms of a hydrogen-bonded dimer of C3v symmetry, having r(N⋅⋅⋅Br)=3.255 Å and the hydrogen-bond stretching force constant kσ=13.4 N m−1. A detailed analysis has demonstrated that χ(79Br) is consistent with a model of the dimer in which only a small electrical rearrangement in the HBr subunit occurs on dimer formation (as opposed to proton transfer to NH3) and that this can be viewed as the transfer of ∼0.le from H into the 4pz orbital of Br.
Mechanism of CO bond scission at alkali promoter sites—IRAS study of the system CO/K/Ni(111)
Authors: Ng L. , Uram K. J. , Xu Z. , Jones P. L. , Yates J. T.
Published: July 2002

IRAS (infrared reflection absorption spectroscopy) has been employed to study the isotopic mixing reaction: 13C16O(a)+12C18O(a)→13C18O(a)+12C16O(a) on Ni(111) with preadsorbed K adatoms. Under high temperature conditions (T>450 K) where the isotopically mixed CO is being desorbed, it has been shown that the isotopic exchange reaction cannot be detected by IRAS on the surface among CO molecules strongly interacting with K adatoms. This result implies that dissociation of CO occurs at K-promoted Ni sites followed by surface diffusion of C(a) and O(a) away from the K promoter site. Statistical recombination of C(a)+O(a) occurs with concomitant CO desorption on Ni(111) sites some distance from K centers. The empty K-promoted sites may be refilled by surface diffusion of CO from outside. Thus, alkali metal promotion of C–O bond scission may act via a ‘‘feeder-site’’ mechanism connecting promoter atoms to external surface sites via surface diffusion of atomic C and O species.
Infrared spectroscopy and vibrational predissociation of C2H2–HF
Authors: Huang Z. S. , Miller R. E.
Published: July 2002

The optothermal detection method has been used to obtain a rotationally resolved infrared spectrum of the ν1 band of C2H2–HF. This spectrum is characteristic of a parallel band of a slightly asymmetric rotor and has been completely assigned. A fit to the observed spectrum gives excited vibrational state rotational constants A1=1.130 20±0.0009 cm−1, B1=0.161 13±0.0002 cm−1, and C1=0.141 05±0.0002 cm−1. The fundamental vibrational frequency has also been accurately determined to be 3794.3646±0.004 cm−1. Intensity alternations resulting from the nuclear spin statistics clearly show that the molecule has a C2 axis, thus confirming the T-shaped geometry. The individual rotational lines are observed to be substantially broader (200 MHz FWHM) than the instrumental resolution (15 MHz FWHM). From these linewidths, the lifetime of the vibrationally excited state has been determined to be 0.8 ns. Stark spectra have also been obtained for the R(0) and P(1) transitions of the complex from which it has been possible to determine the dipole moment in the vibrationally excited state ( μ1=2.559 D±0.003 D).
Comments on a paper by Straub, Borkovec, and Berne
Authors: Zwanzig Robert
Published: July 2002

The paper referred to in the title, in J. Chem. Phys. 84, 1788 (1986), presented results of computer simulations of non-Markovian barrier crossing dynamics, and compared these results with various theoretical predictions. One comment made here is that the observed limited applicability of the Grote–Hynes theory can be explained by singular perturbation theory. The other comment concerns the importance of a second constant of the motion when the non-Markovian friction has a very long relaxation time.
Quantum dynamics of the three-dimensional Li+HF reaction: The bending corrected rotating nonlinear model
Authors: Miller Donald L. , Wyatt Robert E.
Published: July 2002

The reaction Li+HF→LiF+H is studied using three-dimensional quantum reactive scattering theory. The rotational and bending motions are treated using the bending corrected rotating nonlinear model (BCRNM) developed in the body of this paper. Reaction probabilities and total reaction cross sections are calculated for energies up to 1 eV. Reaction cross sections are compared with experimental results at 0.38 and 0.63 eV. Results are in excellent agreement with experiment at the lower energy. We calculate a total reaction cross section σ of 0.72 Å2 vs the measured value of 0.80 Å2. At the higher energy, the BCRNM overestimates the cross section: We calculate σ=2.46 Å2, while the experimental value is σ=0.97 Å2.
Experimental tests of reaction rate theory: Mu+H2 and Mu+D2
Authors: Reid Ivan D. , Garner David M. , Lee Lap. Y. , Senba Masayoshi , Arseneau Donald J. , Fleming Donald G.
Published: July 2002

Bimolecular rate constants for the thermal chemical reactions of muonium (Mu) with hydrogen and deuterium—Mu+H2→MuH+H and Mu+D2→MuD+D—over the temperature range 473–843 K are reported. The Arrhenius parameters and 1σ uncertainties for the H2 reaction are log A (cm3 molecule−1 s−1)=−9.605±0.074 and Ea =13.29±0.22 kcal mol−1, while for D2 the values are −9.67±0.12 and 14.73±0.40, respectively. These results are significantly more precise than those reported earlier by Garner et al. For the Mu reaction with H2 our results are in excellent agreement with the 3D quantum mechanical calculations of Schatz on the Liu–Siegbahn–Truhlar–Horowitz potential surface, but the data for both reactions compare less favorably with variational transition-state theory, particularly at the lower temperatures.
Rotational dynamics of electronically excited aniline in solution from picosecond fluorescence anisotropies
Authors: Myers Anne B. , Pereira Marco A. , Holt Patrick L. , Hochstrasser Robin M.
Published: July 2002

Ultraviolet fluorescence upconversion gating has been used to measure the fluorescence anisotropy of aniline in four hydrocarbon and alcoholic solvents. The deconvoluted anisotropy decay time of 1.2 ps in isopentane is only about a factor of 2 longer than calculated for free rotation of collision-free molecules, and increasing the solvent viscosity by a factor of 15 by going from isopentane to hexadecane increases the rotational relaxation time by only another factor of 2. These results imply that the orientational motion of electronically excited aniline in hydrocarbons may be significantly nondiffusive and that unhindered inertial rotation through large angles may be possible. The experimental anisotropy decays do, however, differ significantly from those calculated for completely unhindered free rotation. The anisotropy decays much more slowly in methanol and sec-butanol than in the hydrocarbons, presumably due to dipole–dipole and/or hydrogen bonding interactions in the hydroxylic solvents. The rotational relaxation of N,N-dimethylaniline is slower than that of aniline in hydrocarbon solvents but faster in methanol, perhaps reflecting reduced hydrogen bonding. Fluorescence anisotropy decays of 4-cyclohexylaniline and 2,4,6-tri-tert-butylaniline have also been examined in order to estimate the true initial rotationless anisotropy [r(0)] for aniline.
Adiabatic energies and radial couplings of the 3Σ+u states of H2
Authors: Borondo F. , Martín F. , Yáñez M.
Published: July 2002

We present a calculation of the potential energy curves of the 3Σ+u states of the H2 molecule in the region R=1–30 a.u. The features of the correlation diagram are discussed, and a new series of avoided crossings in the region of R<5 a.u. is predicted. We calculate and analyze the radial couplings which are of interest in the study of the excitation process H(1s)+H(1s)→H(2s,2p)+H(1s). Finally, we introduce a simple analytical model which accounts for the 2 3Σ+u−3 3Σ+u interaction.
Coupled torsional and bending motions in s-c
i
s methyl vinyl ether
Authors: Meyer Rolf , Ha Tae-Kyu , Oldani Markus , Caminati Walther
Published: July 2002

New microwave measurements on s-cis methyl vinyl ether and a study of the interactions among skeletal and methyl torsions and COC bending are reported. Using pulsed microwave Fourier transform spectroscopy, the small methyl torsional A–E splitting (0.16 MHz) in the vibrational ground state has been resolved for the first time. Large splittings are observed in the first excited states of skeletal torsion and COC bending. Ab initio results on the torsional coupling, allowing for structural relaxation, are reported and used to specify, in part, a flexible model for the torsional and bending motions. The spectroscopic properties of this three-dimensional model sytem, as estimated from the results for the one-dimensional and two-dimensional subsystems, explain the relevant experimental data. The gearing type torsional interaction predicted ab initio is confirmed by this treatment. The adjusted potential function as well as the structural relaxations upon torsion suggest repulsive interaction between the methyl hydrogen atoms and the methylenic hydrogen atom next to the methyl group.
Hydrates in supersaturated binary sulfuric acid–water vapor: A reexamination
Authors: Jaecker-Voirol A. , Mirabel P. , Reiss H.
Published: July 2002

The influence of hydrates formation on the energetics of nucleation has been reexamined. The distribution Nh of hydrates containing one acid molecule and h water molecules has been calculated as a function of the relative humidity and acid vapor concentration. A new formula giving the reversible work required to form a droplet has been derived. Hydrate formation can exert an appreciable effect on the energetics of nucleation in the H2SO4–H2O system and must be included in any theory of nucleation rate.