bluuebunny/crossref_metadata_2025 huggingface dataset:
Vibrational relaxation of HCl(v = 1) by CO, NO, and O2 and of HBr(v = 1) by CO
Authors: Adel F. Al , Doyennette L. , Margottin-Maclou M. , Henry L.
Published: February 2003

The far-from-resonance transfers and the de-excitation processes in HCl–M (M = CO, NO, O2) and HBr–CO systems have been studied by measuring the fluorescence intensity decay rate after laser exciting one then the other of the collision partners. As for the HX–CO (X = Cl or Br) systems, we have thus determined from 300 to 800 K, the rate constant kHX–CO for the energy transfer between the v = 1 vibrational levels of the HX and CO molecules. The rate constant kCOHCl for the V–R, T de-excitation of HCl(v = 1) by CO, as well as limiting values for the other de-excitation rates kCOHBr and kHXCO have also been obtained; these last rates are found to be much smaller then the respective transfer rates kHBr–CO and kCO–HX. As for the systems HCl–NO and HCl–O2, only the sums xHCl–M of the transfer rates kHCl–M and of the de-excitation rates kMHCl have been measured in the same temperature range. The rate constants have been calculated from a semiclassical theory, in which the perturbing potential is a sum of two atom–atom Morse potentials. This theory fails probably because the attractive multipolar interactions are not taken into account.
Ground-state features for Heisenberg models
Authors: Klein D. J.
Published: February 2003

Isotropic Heisenberg models with nearest-neighbor interactions between spins located at the vertices of a bipartite (or alternant) graph are considered. Six theorems helping characterize the ground state are given. Building upon the theorems of Lieb and Mattis concerning the ’’nodelessness’’ of the wave functions and their spin symmetry, we establish related results concerning certain expectation values and ’’point group’’ symmetries. Applications especially concerning valence-bond descriptions of the π-electron systems of organic molecules, are briefly indicated.
Theoretical studies on the benzene molecule. II. Criticism of the ring current model
Authors: Lazzeretti P. , Rossi E. , Zanasi R.
Published: February 2003

The pattern of the electron currents, induced in the benzene molecule by uniform magnetic fields, has been visualized by plotting modulus and direction of the quantum mechanical current density vector evaluated within the coupled Hartree–Fock approximation assuming a basis set of 198 CGTOs. The electron motion is characterized by axial and toroidal vortices and the paramagnetic σ circulation overcomes the diamagnetic π ring current. Intense localized circulations occur about carbon nuclei and carbon–carbon bonds, which give rise to important deshielding effects upon protons. The delocalized currents are most intense in the environment of carbons; delocalized currents are also present, and no actual ’’superconductivity’’ of π electrons can be accounted for. The reported results document that London’s ring current is a rough oversimplification, affected by some unnecessary and unphysical hypotheses.
A SCF-Xα-SW molecular-orbital study of a possible reaction path for Ziegler–Natta catalysis
Authors: Balazs A. C. , Johnson K. H.
Published: February 2003

SCF-Xα-SW molecular-orbital calculations for the coordination complex [CH3TiCl4 (C2H4)]n = 0,−2, which is postulated to be an intermediate in Ziegler–Natta polymerization reactions, demonstrate that the energy gap between the lower lying Ti-alkyl orbital and the higher Ti (3dyz)-olefin (π*) orbital is sufficiently large (0.9 eV for n = 0 and 1.5 eV for n = −2) to make the Cossee mechanism for the propagation step seem unlikely. A series of calculations for CH3TiCl4(C2H4), where the alkyl and olefin groups are moved closer together in a sequence of concerted steps, suggests that this migration is a plausible mechanism for the polymerization reaction.
Vibrational state of the chemisorbed molecule on metal surfaces: Role of electron-hole pair excitation
Authors: Ueba Hiromu
Published: February 2003

A theory is presented to study the vibrational state of a molecule chemisorbed on a metal surface. The dynamical interaction between localized vibrational state and continuum of electron-hole pair excitations of the coupled molecule–metal system is found to be a dominant channel of vibrational energy dissipation of chemisorbed molecules, where the charge fluctuation in the molecular electronic states during a vibration plays an important role. The vibrational line shapes thus calculated are characterized by both large red shift in the frequency and very broad width (short lifetime) compared with those of free molecules, thereby showing a fairly good agreement with what was experimentally observed in conventional vibrational spectroscopy of chemisorbed molecules on transition metal surfaces.
Efficient calculation of high-order semiclassical scattering phase shifts
Authors: Pajunen Petri , Le Roy Robert J.
Published: February 2003

A quadrature procedure for the efficient computation of high-order semiclassical (or JWKB) scattering phase shifts for any smooth spherical potential is described and tested. In most situations this method yields phase shifts of ’’quantum mechanical accuracy’’ for almost the same computational effort associated with calculation of the familiar first-order JWKB phase shifts. t t t
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o calculation of infrared intensities for the linear isoelectronic series HCN, HNC, CO, HCO+, and HOC+
Authors: Rogers Jerry D. , Hillman John J.
Published: February 2003

A b initio infrared intensities and dipole moment derivatives expressed in atomic polar tensor form are calculated using the 4–31 and 6–31G** basis sets for the isoelectronic HCN, HNC, CO, HCO+, and HOC+ series of molecules. The calculated atomic polar tensors are analyzed in terms of the charge–charge flux-overlap model, which is found to be useful in explaining some of the trends observed in the dipole moment derivatives for this series of molecules. A detailed examination of the dipole moment derivatives for the structural isomers indicates some of the ways in which experimental atomic polar tensors for one isomer should be modified to predict infrared intensities for the other isomer. The absolute intensities calculated for the HCO+ and HOC+ ions are believed to be accurate to within a factor of 2 and thus should be useful in astrophysical applications.
Photodissociation processes in the HCl molecule
Authors: van Dishoeck Ewine F. , van Hemert Marc C. , Dalgarno A.
Published: February 2003

Various ab initio methods have been employed for the study of photodissociation processes in the HCl molecule. Potential curves for selected singlet and triplet states and dipole transition moments between singlet states have been calculated. The transition moments vary significantly with internuclear distance for all states studied. The lifetime of the B 1Σ+ state is predicted to be 3 ns. The calculations show that photodissociation of HCl occurs by absorption into the repulsive A 1Π state and by absorption into the bound C 1Π state, followed by predissociation. The theoretical photodissociation cross sections for the A 1Π state and oscillator strengths for the C 1Π state are in good agreement with experimental data. The contributions from other excited states are investigated. The photodissociation rate of HCl in diffuse interstellar clouds is computed.
Anisotropy of the Na2 velocity distribution induced by a temperature gradient across a Knudsen cell
Authors: Woerdman J. P.
Published: February 2003

By studying Doppler profiles of Na2 fluorescence excitation spectra we find that the Na2 velocity distribution in a Knudsen cell is anisotropic when a temperature gradient is applied. The anisotropy is explained within the framework of a theoretical model for the surface catalyzed equilibrium 2Na↔Na2.
Energy diffusion-controlled reactions in solution
Authors: Grote Richard F. , Hynes James T.
Published: February 2003

The energy diffusion-controlled limit for reactions in solution is discussed for anharmonic oscillator models of isomerization and dissociation-recombination. Energy diffusion is described by an equation due to Zwanzig. The vibrational energy diffusion coefficient in this equation is related to the spectrum of solvent forces on the oscillator probed at its unperturbed frequencies. The energy diffusion-controlled rate constants k are calculated. For small barrier heights, k is severely depressed by inefficient, adiabatic regime vibrational energy transfer. For large barrier heights, the important energy flow region lies in the more efficient nonadiabatic vibrational energy transfer regime, and k is much larger. The relevance of our results for solution reactions is discussed.
Line shape analysis for absorption spectra of matrix isolated gold atoms
Authors: Weinert C. M. , Forstmann F. , Abe H. , Grinter R. , Kolb D. M.
Published: February 2003

A moments analysis of the 6s→6p absorption bands for Au in Ar, Kr, and Xe has been performed. Within a simple configuration coordinate model the temperature dependence of the half-width can be well described by assuming a strong linear coupling of the electronic transition to one single vibrational mode with an effective frequency. However, this model does not explain the temperature dependence of the first moment and the red tail of the bands as revealed by the third moment. Therefore, we included a quadratic coupling to a second mode. This mode is shown to have a negative force constant in the excited state, indicating an unstable state. The temperature dependence of the S1/2→P3/2 transition is described assuming a dynamical Jahn–Teller effect. The Jahn–Teller energies and the frequencies of the coupled modes are evaluated for the E⊗eg model.
Experimental and theoretical studies of resonance Raman scattering: Temperature effects in β-carotene
Authors: Ho Z. Z. , Hanson R. C. , Lin S. H.
Published: February 2003

The Raman excitation profile of β-carotene in isopentane solution has been measured at 118 and 298 K using stilbene dye laser excitation as well as argon laser lines. This data as well as absorption data have been analyzed by using a model which includes the conventional multimode and band shape function. With this model, the parameters of normal coordinate displacement and damping constant, and the effect of temperature and band shape function on the width and peak position can be explored. Using the same set of parameters, we have successfully fit the absorption spectra and the resonance Raman excitation profile of three fundamental bands at both low temperature and room temperature. With our approach the damping factor is found to be on the order of 50 cm−1 for β-carotene.
Shpol’skii effect and vibronic spectra of the phthalocyanines
Authors: Huang T.-H. , Rieckhoff K. E. , Voigt E. M.
Published: February 2003

Free base Mg, ClAl, Zn, and Cu phthalocyanines (Pc’s) are successfully incorporated in Shpol’skii matrices, although they are practically insoluble in the open chain normal alkanes which are the conventional Shpol’skii solvents. In analogy with the porphyrins (P’s) the doublet spectra of the 0–0 transitions are observed. They are attributed to removal of the degeneracy of the first excited states in metal Pc’s, and to the existence of two tautomeric states in H2Pc. Relaxation between the doublet components, separated by several tens of cm−1, are slow at the experimental temperature, 4.2 K. Their relative fluorescence intensity is found for the first time to vary with the excitation wavelength. This is attributed to a strong vibronic mixing between the doublet components. The high resolution fluorescence-excitation and fluorescence spectra of the four investigated Pc’s incorporated in the Shpol’skii matrices are structurally very rich. They are characterized by weak electron–phonon and weak vibronic interactions in total accord with our previous findings with PtPc. While the electronic transition a1u(π)→eg(π*), accounts for most of the observed intensity, the numerous vibronic transitions, through weak, are laden with information. Qualitative analysis of the vibronic spectra helps to unravel the mechanism of vibronic coupling as well as to assign the vibronic bands unique to H2Pc and ClAlPc. There is strong indication from the electronic spectrum that the corresponding electronic state in ClAlPc is strongly perturbed by the presence of Cl. It is also found that the vibronic bands in the fluorescence-excitation spectra deviate markedly from their images in the fluorescence spectra in three aspects: relative intensity; extra vibronic activity; and band halfwidth. The causes of these differences are discussed.
Low energy electron collision phenomena in HgBr2
Authors: Nighan William L. , Hinchen John J. , Wiegand Walter J.
Published: February 2003

Cross sections for mercuric bromide (HgBr2) dissociative attachment and ionization have been measured using an electron beam experiment. The dominant products of these reactions, as identified by mass analysis, are Br− and HgBr+2. A complementary electron swarm experiment was used to determine the ion production coefficients in a variety of gas mixtures containing HgBr2. The measured ion production coefficients were found to be in excellent agreement with the attachment and ionization coefficients computed using the measured cross sections. Additionally, analysis of the variations in the measured HgBr2 ion production coefficients along with measured electron drift velocity variations is found to be consistent with the interpretation that vibrational excitation of HgBr2 is dominated by a resonance in the 3–5 eV electron energy range for which dissociative attachment is observed to occur.
Sulfur-33 isotope enrichment by isotopically selective, two-step, laser photodissociation of OCS
Authors: Zittel P. F. , Darnton L. A.
Published: February 2003

SO2 significantly enriched in the 33S isotope has been produced by isotopically selective, two-step, laser photodissociation of OCS with chemical scavenging of S atoms by O2 and NO2. A 33S enrichment factor of 2.0 has been achieved in samples containing only OCS and O2, while the addition of small amounts of NO2 to the samples has led to enrichment factors as large as 3.6. A large fractional conversion of OCS to product is obtained with a moderate number of laser pulses, and the resulting mixture of reactants and enriched products is stable with respect to chemical reaction and isotope exchange. The dependence of enrichment and conversion on OCS, O2, and NO2 pressures is reported and discussed on the basis of energy transfer and reaction mechanisms.
Stability of the ammonium and methylammonium radicals from neutralized ion-beam spectroscopy
Authors: Gellene Gregory I. , Cleary David A. , Porter Richard F.
Published: February 2003

The stability of the ammonium radical (NH4) was determined from measurements of the kinetic energy released in its fragmentation products following formation in a fast electron capture process: NH4++Na → NH4*+Na+. Scattering profiles for heavy (NH3) and light (H) dissociation products were obtained from beam measurements with 5–16 keV NH4+ ions. The existence of a predissociative barrier in the radical is inferred from edge structure and scattering continua in H atom profiles. The radical is bound with respect to a potential minimum but all of the isotopic species NH4, NH3D, NH2D2, and NHD3 undergo rapid loss of H atoms and have ground states lying above their dissociation limits. The radical ND4 has unusual stability with its ground state lying close to or below its dissociation limit. Dissociative lifetimes for stable and unstable states of ND4 differ by at least two orders of magnitude. The possible significance of these observations on the interpretation of optical transitions involving the ground states of NH4 and ND4 are discussed. From these measurements the ionization potential of 4.73±0.06 eV for NH4 is calculated. The stabilities of CH3NH3 and CH3ND3 radicals and their dissociative pathways have also been investigated.
Hot band rotational relaxation time in carbonyl sulfide by transient infrared–microwave double resonance
Authors: Leap John W. , Coleman Paul D.
Published: February 2003

Rotational relaxation time (T) has been measured for pure carbonyl sulfide (16O 12C 32S) in a highly vibrationally and rotationally excited state (031f0, J = 24). A pulsed CO2 laser [P9(24)] line was used to excite the OCS gas: (011f0, J = 25) → (031f0, J = 24). The transient population difference between l-doublet states (031f0, J = 24) and (031e0, J = 24), was monitored with a microwave (7.4 GHz) probe signal. For pressures 5–40 Pa, individual transient infrared–microwave signals were analyzed via a least squares fit. From the gain pulse shape, it is determined that population decay time (T1) and polarization decay time (T2) were equal within a factor of 2. From the pressure dependence of T, it is concluded that (pT) = 3.64±0.5 μs Pa = 27.3±4 ns Torr.
Vibrational relaxation of highly excited diatomics. III. HF(v = 5, 6, 7)+H2, D2, N2, HF, CO2, N2O, CH4, and C2H6
Authors: Dzelzkalns L. S. , Kaufman F.
Published: February 2003

Vibrational relaxation rate constants kQv are reported for HF (v = 5, 6, 7) collisions with eight quencher molecules Q = H2, D2, N2, HF, CO2, N2O, CH4, and C2H6, at 298 K under conditions of rotational equilibrium, using the fast flow infrared chemiluminescence technique. The rates are faster than had been reported by some investigators, the energy transfer probability PQv ranging from PH25 = 3.2×10−3 to PHF7 = 1.45. The rate constants rise with increasing v, i.e., n = 2.0 to 8.4 for different Q in the vn correlation. For all Q except H2 and HF, V-V transfer is suggested, but the Lambert–Salter plots have different slopes for different Q. The data are compared with published measurements and with theory, the latter especially for Q = HF.
Millimeter and submillimeter spectrum of NO+
Authors: Bowman Wayne C. , Herbst Eric , De Lucia Frank C.
Published: February 2003

Pure rotational absorption spectra of the molecular ion NO+ is reported by means of millimeter and submillimeter microwave spectroscopy. (AIP)
Reactions of OH−⋅H2O with NO2
Authors: Paulson John F. , Dale Fred
Published: February 2003

Cross sections for reactions of OH−⋅H2O with NO2 are reported for interaction energies from 0.2 to 6 eV. Reactions producing the ions HNO3− and NO2−⋅H2O were observed, in addition to charge transfer and collisional dissociation. These reactions were not observable at an interaction energy of 0.04 eV. Based upon these results, upper and lower bounds for the electron affinity of HNO3 are 0.39⩽E.A. (HNO3)⩽0.73 eV and for the bond dissociation energy of NO2−–H2O are 0.1⩽D(NO2−–H2O) 0.6 eV. It is suggested that the strong energy dependence of the cross sections for the reactions forming HNO−3 and NO2−⋅H2O is the result of dissociation of these ions before detection.
A simple quantal model for collision-induced dissociation: An Airy basis calculation
Authors: Hunt P. M. , Sridharan S.
Published: February 2003

New matrix elements for the Airy continuum basis are employed to find quantum mechanical dissociation probabilities for the the forced Morse oscillator. The calculations performed illustrate the ease with which the continuously infinite Airy basis can be manipulated, and they illustrate the transition from vibrational enhancement to vibrational inhibition of diatomic breakup. The forced Morse oscillator model thus reproduces the behavior of more complicated collinear collision-induced dissociation systems.
On the electronic structure of exciplexes with aliphatic amines II
Authors: Van der Auweraer M. , Swinnen A. M. , De Schryver F. C.
Published: February 2003

In this contribution a theoretical model for the electronic structure of an exciplex with n-type donors is developed. The model is applied to the intramolecular exciplexes in which a parasubstituted phenyl acts as acceptor. This allows one to determine values for the emission maximum and the fluorescence rate constant which approach the experimental observed values of these quantities within the experimental error.
Statistical mechanics of electrolytes and polyelectrolytes. I. Symmetric electrolyte solutions
Authors: Ramanathan G. V. , Woodbury C. P.
Published: February 2003

Simple ionic solutions are studied using the BBGKY hierarchy. An iterative estimation technique is developed which enables one to derive various well-known approximations. It is shown that in dilute symmetric electrolytes the Poisson–Boltzmann and the hypernetted chain equations are valid to a higher degree of accuracy than in asymmetric electrolytes and plasmas. The DHX, EXP, and the PY approximations are derived from the HNC equation. A perturbative correction to the EXP approximation is obtained which gives contact values of the correlation functions and excess internal energies close to those obtained by numerical solutions to the HNC equation for 1-1 electrolytes over a wide range of molar concentrations and for 2-2 electrolytes of relatively high concentration.
Intramolecular dynamics and nonlinear mechanics of model OCS
Authors: Carter Duane , Brumer Paul
Published: February 2003

The dynamics of vibrationally and rotationally excited planar OCS is analyzed within the framework of nonlinear classical mechanics. Results include critical energies for the onset of irregularity, an analysis of energy relaxation from excited zeroth order modes relative to the K-entropy prediction, and aspects of the OCS mechanical spectra. In addition, results relevant to the possibility of OCS multiphoton dissociation are presented, such as the range of energies over which the system is classically resonant with an external field and the nature of the C–S stretch to OCS bend coupling. Finally, a useful method for mapping tori onto triangle plots is presented.
On the structure of binary hard-sphere fluid mixtures
Authors: Kok W. C. , Tang S. M.
Published: February 2003

The oscillatory behavior of the partial pair correlation functions is governed by poles of Sij(z) in the complex z plane. The hard-sphere mixture model is considered, first, for the case of equal sphere diameters where it is found that the poles and hence the partial pair correlation functions are independent of alloy composition; and second, for widely differing sphere diameters when the structure becomes dependent on alloy composition. Experimental evidence for both cases is discussed.
Crystalline features in AsF5-doped polyacetylene
Authors: Riekel C. , Hässlin H. W. , Menke K. , Roth S.
Published: February 2003

Neutron diffraction studies are reported on cis and trans-polyacetylene films in both doped and undoped conditions. (AIP)
Fast decay of the visible band electron in e-irradiated crystalline ice at low temperature: The isotope effects and the role of a mobile proton in the decay
Authors: Kawabata Keisuke , Nagata Yoshio , Okabe Shigeru , Kimura Novio , Tsumori Kunihiko , Kawanishi Masaharu , Buxton George V. , Salmon G. Arthur
Published: February 2003

The trapped electron which absorbs in the visible region in crystalline ice e−vis has been studied by pulse radiolysis in the low temperature range 6 to 77 K using 2 μs pulses, and above 77 K using pulses of 40 ps to 6 ns width. The half-life of e−vis in ice around 77 K is unusually short, 8 ns in H2O ice and 120 ns in D2O ice. The decay of e−vis in ice is found to fit Hummel’s empirical equation for the decay by geminate ion recombination in a spur. Several other pieces of evidence indicate that the decay occurs in a spur. Electron tunneling from e−vis to the OD radical does not occur in D2O ice. It is concluded that a proton (H3O+) or a deuteron (D3O+) produced in a spur by the irradiation migrates through ice to react with immobile e−vis in the same spur, and that the half-life of e−vis is determined by the mobility of the proton or deuteron. The Arrhenius plot of the half-life in the range 100 to 6 K is nonlinear and shows an activation energy of 20 meV at higher temperatures and much smaller values at temperatures below 50 K (1 meV below 15 K). The causes of the unusual Arrhenius plot and the isotope effect on the proton mobility are discussed.
The use of dipolar hyperfine couplings in studies of radiation effects
Authors: Budzinski Edwin E. , Freund Harold G. , Potienko George , Box Harold C.
Published: February 2003

ENDOR measurements at V band (70 GHz) provided data on protons weakly coupled to the unpaired electron of the alkoxy radical in x-irradiated ribitol. The couplings are chiefly dipolar in character. A total of eight hyperfine coupling tensors for weakly coupled protons were determined. The prospects for utilizing weak proton hyperfine couplings as an aid in identifying free radical radiation products is discussed.
A semiclassical wave packet model for the investigation of elastic and inelastic gas–surface scattering
Authors: Agrawal P. M. , Raff L. M.
Published: February 2003

A semiclassical wave packet model that permits simultaneous study of surface diffraction, Debye–Waller effects, and inelastic energy transfer processes at the gas–surface interface is presented. The incident atomic beam is represented by a quantum mechanical wave packet whose momentum-space probability density corresponds to that employed in the molecular-beam experiments being simulated. A semiclassical forced-oscillator-type approximation couples the surface motion to that of the wave packet through a time-varying potential obtained from a solution of Hamilton’s equations of motion for the lattice. Explicit integration procedures are used to evolve the wave packet through this time-varying field. The average final kinetic energy of the beam, the angular distributions and diffraction patterns, Debye–Waller broadening effects, and the surface phonon spectrum are obtained from the scattered wave packet and its Fourier transform. The model has been applied to the case of an atomic hydrogen beam having a square distribution of incident momenta impinging upon a simple two-dimensional lattice. The results yield diffraction patterns that correlate with the known grating of the lattice. The average final kinetic energy of the beam is found to vary linearly with incident energy and with surface temperature in accord with the results of recent molecular beam experiments, and a surface phonon spectrum that exhibits one-, two-, and three-phonon processes is obtained.
The initial value representation (IVR) for three-dimensional scattering problems
Authors: Augustin Stuart D.
Published: February 2003

In this paper, we describe a new family of local integral representations for S-matrix elements. Although the historical origins of the IVR are in stationary phase semiclassical mechanics, we will show that it can be valid when the stationary phase approximation is inappropriate. Quantum mechanical corrections to the original form of the IVR are included by transforming to a representation in terms of conventional coordinates. For a simple collinear model problem, we demonstrate that this new conventional coordinate IVR is in exact agreement with the known quantum mechanical results. In the case of atom-rigid rotor scattering, the IVR can be cast as a one-dimensional integral analogous to the infinite order sudden (IOS) result. Calculations of rotational inelasticity for the He–H2 system are presented. The IVR cross sections are significantly better than the IOS values at low collision energies without requiring a substantial increase in computational effort.
Fluorescence spectra of the NaI molecule
Authors: Ragone Anthony S. , Levy Donald H. , Berry R. Stephen
Published: February 2003

The fluorescence excitation spectrum and fluorescence emission spectra of sodium iodide vapor are reported for the first time. The emission spectrum contains a v\ = 0 progression and v\ = 1 progression of a transition of the NaI molecule, as well as many other bands due to other species. The spectrum is taken from a seeded hypersonic nozzle beam excited by a tunable laser. A vibrational analysis is given, part of the upper state potential is constructed, and the results are related to photodissociation studies.
Infrared double resonance of SF6 with a tunable diode laser. III. Observations of 2ν3←ν3 transitions and collisional relaxation in v3=1
Authors: Dubs M. , Harradine D. , Schweitzer E. , Steinfeld J. I. , Patterson C.
Published: February 2003

The detailed spectroscopy and transition dipole moments of the 2ν3←ν3 transition in SF6 have been investigated by high-resolution infrared double-resonance spectroscopy. The calculated transition frequencies are found to be reliable to within ±0.003 cm−1; in addition, we find that both the l=0←1 and ν3=l=2←1 components of the 2ν3←ν3 transition carry comparable oscillator strength, and that mixing of R states becomes appreciable in 2ν3. The net relaxation rates into depleted levels of the ground vibrational state, and out of laser-populated levels of ν3, are approximately equal to each other (pτ≃35 ns Torr for both). There appear to be no restrictions on relaxation among fine-structure levels in the ground vibrational state; however, initial relaxation out of specified J levels in ν3 appears to involve a limited set of final states which may possibly belong to a vibrational level other than ν3.
Radiationless decay of 1,3,5,7-octatetraene
Authors: Ackerman John Richard , Kohler Bryan E. , Huppert D. , Rentzepis P. M.
Published: February 2003

The decay kinetics of the 2 1Ag state of the linear polyene 1,3,5,7-octatetraene in hydrocarbon solutions have been measured by nanosecond and picosecond techniques from 10 to 320 K. The observed decrease in emission lifetime with increasing temperature is well described in terms of a temperature activated intramolecular radiationless decay process proceeding over a ∼4 kcal Arrhenius barrier. This barrier is not significantly affected by intermolecular interactions such as sample viscosity or phase and may be related to rotation about essential single bonds in the excited state.
Electrodynamics of an ion near the surface of a conducting dielectric
Authors: Kayser R. F. , Hubbard J. B.
Published: February 2003

We investigate the electrodynamic phenomena associated with an ion moving near the flat surface of a conducting dielectric solid, where the medium containing the ion has an arbitrary dielectric constant, and the solid has a surface conductivity which is independent of its bulk conductivity. For a fixed ratio of surface to bulk conductance, we show that the frictional drag on the ion is dominated by surface dissipation if the ion is sufficiently close to the surface, while bulk (i.e., ohmic) dissipation is the dominant drag mechanism at large distances. The energy dissipation turns out to be independent of the dielectric constants of the two media, given that dielectric dispersion effects are neglected.
On the mobility of an ion near a conducting boundary
Authors: Rhee Bum Ku , Schaich W. L.
Published: February 2003

The motion of a charge near a conductor gives rise to a dissipative force that may be described by a friction parameter. A formula for this force is derived which allows for the charge to be moving in a local dielectric medium above a conductor that possesses a nonlocal response. An alternate theory by Mazo is criticized.
A kinetic theory for polymer melts. IV. Rheological properties for shear flows
Authors: Bird R. Byron , Saab H. H. , Curtiss C. F.
Published: February 2003

Curtiss and Bird recently developed a kinetic theory for undiluted polymers, modeling the fluid as a collection of interacting Kramers freely jointed bead–rod chains. The theory yielded a constitutive equation in which the stress tensor is given as the sum of two integrals over the strain history. Here, the constitutive equation is used to calculate the steady-state shear flow rheological properties (shear-rate dependent viscosity and normal-stress functions) and also unsteady-state shear flow responses (shear and normal stress growth at the inception of shear flow, shear and normal stress relaxation after the cessation of shear flow, and the stress response in small-amplitude oscillatory motion). The rheological functions for an earlier slip-link network theory by Doi and Edwards are also obtained.
A kinetic theory for polymer melts. V. Experimental comparisons for shear-flow rheological properties
Authors: Saab H. H. , Bird R. Byron , Curtiss C. F.
Published: February 2003

The steady-state and unsteady-state rheological properties calculated from the Curtiss–Bird kinetic theory are compared with experimental data on polymer melts and concentrated polymer solutions. It is found that the experimental curves for viscosity and primary normal stress differences vs shear rate for some liquids have very nearly the same shapes as the theoretical curves with the link tension coefficient ε in the range 0.3 to 0.5. For sudden startup of steady shear flow at t=0, the theory correctly predicts the observed stress overshoot for both shear and normal stresses; the theory predicts a sudden jump in the shear stress at t=0, this apparently unrealistic behavior being related to the stiffness of the links in the chain model. The complex viscosity in small-amplitude oscillatory motion does not agree with the experimental data in the high-frequency range, but at low frequencies the empirical Cox–Merz rule is followed approximately.
Measurement of the rate of bimolecular electron spin relaxation between pairs of reactive radicals using time-resolved electron spin-echo spectroscopy
Authors: Syage J. A. , Lawler R. G. , Trifunac A. D.
Published: February 2003

The time-resolved electron spin echo (ESE) method has been employed to measure the bimolecular contribution to electron spin relaxation in the acetate radical ⋅CH2CO−2, formed by pulse radiolysis. Time-resolved ESE signals from solutions containing up to 4×10−4 M initial radical concentrations were analyzed using Bloch equations modified to include the effects of strong Heisenberg exchange, chemical reaction, and CIDEP. It is found that the electron exchange rate is ∼3 times faster than the chemical reaction rate, contrary to a simple model involving only statistical factors which predicts equal rates for the two processes. It is also found that the CIDEP enhancement factor is independent of initial radical concentration over the ten fold range of concentrations studied. (AIP)
Stochastically gated diffusion-influenced reactions
Authors: Szabo Attila , Shoup David , Northrup Scott H. , McCammon J. Andrew
Published: February 2003

The theory of diffusion-influenced reactions is extended to cases where the reactivity of the species fluctuates in time (e.g., the accessibility of a binding site of a protein is modulated by a gate). The opening and closing of the gate is assumed to be a stationary Markov process [i.e., it is described by the kinetic scheme (open) a⇄b (closed)]. When the reaction is described by suitable boundary conditions, by solving the appropriate reaction-diffusion equations, it is shown that the stochastically gated association rate constant (kSG) is given by k−1SG=k−1∞ + [a−1 b(a+b)κ̂u(a+b)]−1, where κ̂u(s) is the Laplace transform of the time-dependent rate constant of the ungated problem and k∞ is the corresponding steady-state rate constant. The limits when the relaxation time for gate fluctuations is larger or smaller than the characteristic time for diffusion are considered. The relation to previous work is discussed. The theory is applied to three models: (i) a gated sphere, (ii) a gated disk on an infinite plane (e.g., a channel in a membrane), and (iii) a gated localized axially symmetric reactive site on the surface of a spherical macromolecule.
The adiabatic semiclassical perturbation theory for vibrationally inelastic scattering. III. Morse oscillators
Authors: Vasan V. S. , Cross R. J.
Published: February 2003

We have modified our previous adiabatic theory for vibrationally inelastic scattering [J. Chem. Phys. 71, 1426 (1979)]. A perturbation calculation is done using a partially adiabatic basis set. The result is exponentiated to give the full scattering matrix. Several improvements were made in the theory for collinear scattering from harmonic oscillators. The accuracy is much better than the previous version, and the computation time is less. The theory has been adapted for the Morse oscillator, and the accuracy is as good or better than for the harmonic oscillator except for the system He+HBr. The poor accuracy in the case of He+HBr seems to be due to the large perturbation involved as well as the mass combinations of the various atoms involved in the collision.
Quasi-two-dimensional motion and proton exchange in electrolyte layers of hydrated chalcogenides
Authors: Röder U. , Müller-Warmuth W. , Spiess H. W. , Schöllhorn R.
Published: February 2003

Molecular motion and proton exchange has been studied by 1H NMR in various hydrated layered chalcogenides of the type Ax+(H2O)y{MS2}x−, where A is an alkalimetal and M is Nb or Ta. Information about the anisotropy of the translational motion was deduced from temperature and frequency dependent measurements of the 1H spin-lattice relaxation rate. In the monolayer hydrates, the motion of the water molecules is essentially two-dimensional in character; significant contributions from three-dimensional motion have to be taken into account, however, for the bilayers. Proton exchange has been shown to exist even in the monolayers. It causes characteristic powder line shapes observed at elevated temperatures, which are analyzed quantitatively. At lower temperatures, values for the exchange rate were obtained from 1H spin alignment measurements. The data are in accord with the activation energy for the autoprotolysis of water.
Computer simulation of liquid anisotropy. III. Dispersion of the induced birefringence with a strong alternating field
Authors: Evans M. W.
Published: February 2003

Molecular dynamics simulation is used to compute rise transients induced by a strong alternating field of force. The dispersion of the induced anisotropy produces oscillatory rise transients whose features reflect in detail the underlying molecular dynamics on a picosecond time scale.
Transport theory of binary mixture with one trace component of disparate mass
Authors: Sung Wokyung , Stell G.
Published: February 2003

We present a simple method for calculating transport coefficients for binary hard-sphere mixtures with one trace component of extreme mass disparity (Lorentz and Rayleigh mixtures). The results that we obtain by using the Chapman–Enskog method are exact on the Enskog-theory level. In such mixtures we can expand the solute–solvent collision integral in powers of the mass ratio and thereby reduce it to a differential form, from which we can in principle calculate directly the solute–particle velocity moments of various orders relevant to the fluxes instead of determining its distribution function. Here we mainly concern ourselves with determination of the transport coefficients in the Lorentz and Rayleigh limits, including the solvent contribution for the case of shear viscosity, bulk viscosity, and thermal conductivity. We give a brief analysis of these quantities in both limits and compare with the results of the lowest Sonine-polynomial approximation.
Infrared spectrum and potential constants of silicon tetrafluoride
Authors: McDowell Robin S. , Reisfeld Martin J. , Patterson Chris W. , Krohn Burton J. , Vasquez Mariena C. , Laguna Glenn A.
Published: February 2003

Doppler-limited tunable diode laser spectra of ν4 of 28SiF4 have been analyzed and the spectroscopic constants determined. In contrast to most earlier low-resolution studies, the Coriolis constant ζ4, when combined with ζ3 as obtained from previous laser spectroscopy, yields a zeta sum that is within 5% of the expected harmonic value of 1/2. The band origins of 12 overtones and combinations have been obtained from Fourier-transform spectra (0.04 cm−1 resolution), resulting in estimates of the anharmonicity constants and harmonic frequencies. From the Coriolis constants and the isotope shifts in ν3 we have redetermined the general quadratic force field of SiF4.
The microwave spectrum and molecular structure of CO2–HCl
Authors: Altman Robert S. , Marshall Mark D. , Klemperer William
Published: February 2003

Radio frequency and microwave molecular beam electric resonance spectra have been obtained for three isotopic species of the hydrogen-bonded weakly bound complex CO2–HCl. The following spectroscopic constants have been determined: This complex most likely has a linear equilibrium geometry with an O–H bond length of 2.14 Å. A large induced dipole moment is found in this complex. A comparison of structural properties for this and other related weakly bound molecules is made.
Raman intensities of overtones and combination bands of C2H2, C2HD, and C2D2
Authors: Domingo C. , Escribano R. , Murphy W. F. , Montero S.
Published: February 2003

Measurements of absolute gas phase vibrational Raman scattering cross sections of binary overtone and combination bands of C2H2,C2HD, and C2D2 are reported. Calculated values for these cross sections, which take into account the contribution of the anharmonic force field and the electro-optical anharmonicity, are also presented. Within limitations due to approximations in the theory and uncertainties in the parameters, the comparison between the experimental cross sections and the calculated ones is quite encouraging.
Energy transfer between Eu3+ ions in LiNbO3, CaWO4, and EuxY1−xP5O14 crystals
Authors: Tyminski Jacek K. , Lawson Christopher M. , Powell Richard C.
Published: February 2003

Narrow-band laser excitation is used to selectively pump Eu3+ ions in different types of crystal field sites in four host crystals LiNbO3, CaWO4, EuP5O14, and Eu0.01Y0.99P5O14. Energy transfer between ions in nonequivalent sites is characterized using time-resolved spectroscopy techniques and phenomenological rate parameter models. In all four cases, the energy transfer is found to occur by a single-step process through a weak electric dipole–dipole interaction. However, the different spatial distributions of the Eu3+ ions in these hosts produce differences in the energy transfer characteristics.
Surface selection rules
Authors: Moskovits M.
Published: February 2003

The modification of the absorption, emission, and Raman scattering intensities of molecules near metal surfaces is discussed with emphasis on the last. These modifications are usually referred to as surface selection rules. We show that in Raman scattering, the ‘‘rule,’’ which states that only those modes will be active which belong to the irreducible representation to which α′zz also belongs, is only approximately obeyed. The circumstances under which modes which are represented by nondiagonal derived polarizibility tensors are ‘‘surface-active’’ are considered, as well as the geometrical strategies which should be followed in studying the Raman spectra of molecules adsorbed on flat, metal surfaces.
Spectral narrowing and infrared laser fragmentation of jet-cooled UO2(hfaa)2 TMP and UO2(hfaa)2 THF: Volatile uranyl compounds
Authors: Dietz T. G. , Duncan M. A. , Smalley R. E. , Cox D. M. , Horsley J. A. , Kaldor A.
Published: February 2003

The temperature dependence of the CO2 laser-induced decomposition spectral linewidths and yields for jet-cooled UO2(hfaa)2 TMP and UO2(hfaa)2 THF molecules has been studied using laser-induced fluorescence detection and photoionization time-of-flight mass spectrometry on a pulsed supersonic beam of uranyl molecules seeded into a helium carrier gas. The two uranyl species, although chemically very similar, show a remarkable difference in the temperature dependence of the linewidth. For UO2(hfaa)2 TMP, the linewidth decreases upon cooling by a factor of ∼15, while the UO2(hfaa)2 THF linewidth decreases by only a factor of ∼2.6 over the same temperature range. The differences in the linewidth behavior are reflected in the yield curves for the laser-induced reaction for the cooled molecules. These results for UO2(hfaa)2 TMP, to our knowledge, represent the first measurements of the temperature dependence of a T2 line broadening process for an isolated gas phase molecule. Detection of beam molecules as well as molecular decomposition fragments by multiphoton ionization is found to be ∼103 times more efficient than detection by resonant two-color photoionization.
Optical spectroscopy of Mn2+ ions in solid solution and Suzuki phase in NaCl single crystals
Authors: Rodríguez F. , Moreno M. , Jaque F. , López F. J.
Published: February 2003

The optical spectroscopy of Mn2+ ions both in dipolar form and forming Suzuki-phase precipitates has been studied in NaCl by photostimulated luminescence at room and liquid nitrogen temperatures. An analysis of the positions of the crystal field transitions shows that the 10 Dq value is slightly higher (about 7%) in the Suzuki than in the dipolar phase while the covalency is the same in both cases. Such analysis allows us to estimate the value fσ−fπ=4.8% from the optical data which is in agreement with the EPR data for [MnCl6]4− clusters. Moreover, an intense band peaking at 231 nm which disappears when the dipolar phase is formed has been ascertained to be associated with the Suzuki phase. This band has been assigned to the double excitation 6A1(S) +6A1(S) → 4T1(G)+[4A1(G), 4E(G)]; that is consistent with the existence of Mn2+ ions in close positions in the Suzuki phase.
Vibrational optical activity calculations using infrared and Raman atomic polar tensors
Authors: Freedman Teresa B. , Nafie Laurence A.
Published: February 2003

Expressions for vibrational circular dichroism (VCD) and Raman optical activity are formulated using infrared and analogous Raman atomic polar tensors. Calculations using finite electric field perturbation with CNDO wave functions are presented for the VCD of CH-stretching vibrations in L-alanine and (+)-(3R)-methylcyclohexanone. These compare favorably with experiments and previous models.
Classical solvent dynamics and electron transfer. II. Molecular aspects
Authors: Calef Daniel F. , Wolynes Peter G.
Published: February 2003

Charge transfer reactions in polar solvents are accompanied by reorganization of the solvent. In many cases, the dynamics of this reorganization determines the rate for charge transfer. In this paper the dynamics of this process is investigated with special emphasis on the molecular nature of the solvent. The reaction coordinate is identified and an equation of motion for this coordinate is derived. The equation describes the diffusion of the reaction coordinate in a bistable potential, which can be treated using Kramers’ theory. The form of the bistable potential is discussed in detail, with special emphasis on the distinction between this effective potential and a nonequilibrium free energy. As an example the resulting expressions for the rate of electron transfer between the charged and uncharged states of a zwitterion are calculated using a simple, approximate model for the fluid structure. The difference between these results and those from the continuum dielectric treatment of the solvent are discussed.
Interfacial hydrodynamics: A microscopic approach
Authors: Baus M. , Tejero C. F.
Published: February 2003

Linearized hydrodynamic equations for a nonuniform anisotropic fluid are obtained from the exact Mori–Zwanzig equations for the conserved densities. In the particular case of a two-phase system with a planar equilibrium interface, these equations can be reduced to the ordinary hydrodynamic equations inside each bulk phase and to surface hydrodynamic equations for the interfacial layer. Surface transport coefficients and surface thermodynamic parameters are hereby obtained as Gibbs surface excess values. All the known phenomenological equations can be recovered by suitable approximations. Various correction terms to the phenomenological results, including Laplace’s formula, are found.