bluuebunny/crossref_metadata_2025 huggingface dataset:
Exciton transitions in quasi-one-dimensional crystals. 9-Cyanoanthracene
Authors: Brillante A. , Philpott M. R. , Schlosser D. W.
Published: February 2003

The polarized reflection spectra of 9-cyanoanthracene crystal at 2 °K taken at near normal angles of incidence are reported for a wavelength range corresponding to the first singlet electronic transition. The dielectric funtion is calculated by means of the Kramers–Kronig transformation and the positions and oscillator strengths of the main exciton transitions are located. The factor group splitting in reflection is nearly 80 cm−1, but in the derived absorption spectrum ε2(ω) the splitting is much larger, being approximately 200 cm−1. One component of the dielectric function is negative in the region of the 0–0 transition and the calculation of the theoretical attenuated total reflection spectrum suggests that at 2 °K the (100) crystal surface should support an exciton surface polariton. The structure of the exciton band has been calculated for three orthogonal directions in the first Brillouin zone using the point dipole approximation. All four branches show strong negative dispersion in the kc direction, implying that the bottom of the exciton band is not a k=0 level. This is one reason why the crystal shows no ’’monomer’’ fluorescence. The qualitative validity of the point dipole model has been established by calculating the nondipolar part of the exciton transfer interaction for near neighbors. It is found that the nondipolar parts are 25% or less of the individual intermolecular exciton transfer interactions.
Tunable diode laser measurements of line strengths in the ν4 band of 12CH4
Authors: Fox K. , Reisfeld M. J. , McDowell R. S.
Published: February 2003

Absolute intensities have been measured, using a diode laser spectrometer, for several vibration–rotation lines in the ν4 fundamental of methane. The data have been analyzed, with the inclusion of Coriolis interactions, to obtain a transition moment of (0.0508±0.0015) ×10−18 esu cm. This work appears to resolve some earlier difficulties in interpreting measurements and calculations of intensities in ν4 of CH4.
Application of a dynamical S matrix method to the three-dimensional H+H2 exchange reaction
Authors: Clary D. C. , Nesbet R. K.
Published: February 2003

A quantum dynamical S matrix formalism which enables population distributions to be computed as a function of a reaction coordinate u is described and applied to the three-dimensional H+H2 exchange reaction. Quantum dynamical R matrix methods and programs of Stechel, Walker, and Light have been adapted for this purpose. The method has been applied to examine the suitability of the simple surprisal formula [pj∝pj0exp(λkj)] describing rotational product state population distributions {pj}. Previous results on the semiempirical Porter–Kaplus (PK) potential energy surface for total angular momentum J=0 showed that the computed quantum dynamical population distributions can be fitted accurately by the surprisal formula for all values of u. The microcanonical prior distribution function, p0j∝kj(2j+1), for final product states was found to be appropriate, even though the statistical justification of this function fails because of angular momentum conservation. In the present work this surprisal theory study has been extended and a comparison has been made between the population distributions computed using the PK potential and the potential of Truhlar and Horowitz derived from the ab initio CI study of Siegbahn and Liu (THSL). Although the surprisal formula is accurate for all values of u for the PK potential and for small values of u for the THSL potential, it does not give such a good fit for larger values of u for the THSL potential. This result is linked directly with the fact that for the THSL potential the H3 conformation of minimum potential energy is nonlinear for relatively large values of u. These results strongly suggest that a justification for the successes of surprisal theory must result from a detailed examination of the collision dynamics, rather than from statistical considerations. The dynamical S matrix method is also used to explain an approximate reactive flux rule for the H+H2 reaction: the sum of unnormalized reaction probabilities into product states with even rotational quantum numbers is approximately equal to the sum of reaction probabilities into odd product states.
Excitation operators and the extended random phase approximation: II. Applications
Authors: Lasaga A. C. , Karplus M.
Published: February 2003

The random phase approximation (RPA) and its extensions are tested by applying them to the π electron system of a series of linear polyenes in the PPP approximation, for which exact solutions are available. The importance of electron correlation is varied by altering the form of the electronic repulsion matrix elements in the PPP model. Emphasis is placed on a comparison between recently suggested refinements in the RPA and the more standard RPA scheme. The elements of the modifications of the RPA are illustrated by the simple example of ethylene. Calculations for longer polyenes (butadiene, hexatriene, and octatetraene) demonstrate that certain improvements in the RPA are essential if reliable results are to be obtained for highly correlated excited states. The results provide support for use of the extended RPA method in molecular excited state calculations.
The photodissociation of cadmium iodide
Authors: Kawasaki M. , Lee S. J. , Bersohn R.
Published: February 2003

A molecular beam of CdI2 molecules was photodissociated by a dye laser at 278 and 300 nm and also by a filtered Hg–Xe lamp. The time of flight distributions obtained with the polarized laser light are anisotropic, as is the distribution obtained with the lamp. The overall conclusion is that the first absorption band of CdI2 is a superposition of a parallel band dissociating to I* (2P1/2 atoms and CdI(X 2Σ) molecules and a perpendicular band dissociating to I(2P3/2) and CdI(X 2Σ). In spite of this complexity an effort is made to decompose the translational energy distribution into one for each exit channel, with the result that the available energy (aside from that appearing as electronic energy of I*) is distributed ∼40%–50% as kinetic energy of fragments.
Multiphoton ionization and two-photon fluorescence excitation spectroscopy of triethylenediamine
Authors: Parker David H. , Avouris Phaedon
Published: February 2003

The multiphoton ionization (MPI) and two-photon fluorescence excitation (TPFE) spectra of triethylenediamine (DABCO) are obtained and analyzed. A low-lying state (0–0 at 35 795 cm−1) is observed as a two-photon resonance in the four photon ionization spectrum and also in the TPFE spectrum. Circular vs linear laser polarization studies give an A′1 symmetry for this state. The state results from a 3s (+) ←n (+) orbital promotion. The vibrational progressions and sequences observed are also analyzed. The lifetime of the fluorescence resulting from two-photon 1A′1←1A′1 excitation compares well with the lifetime observed upon one-photon ?←? excitation,suggesting that the 1A′1 state is the emitting state. This 1A′1← 1A′1 one photon forbidden, two-photon allowed, transition is most probably responsible for the very weak long wavelength absorption reported in previous conventional spectroscopic studies. The MPI spectra and laser polarization characteristics of the strong (0–0 at 39 825 cm−1) transition of DABCO are also investigated. The transition is found to be both one- and two-photon allowed and is assigned as 1E″[3px,y  (+)]←1A1[n (+)].
Vibrational energy disposal by reaction of Xe(6s, 3P2) metastable atoms with chlorine containing molecules
Authors: Tamagake K. , Kolts J. H. , Setser D. W.
Published: February 2003

The bound–free XeCl spectra observed from the reactions of Xe(6s, 3P2) atoms with Cl2, PCl3, CCl4, and COCl2 have been compared to computer simulated spectra in order to assign the XeCl* vibrational energy distributions from these reactions. The 〈fV〉 values for the initial state distributions are between 0.6–0.8 for Cl2 and CCl4 and 0.2–0.3 for COCl2. The results for PCl3 are unusual in that the distribution for the XeCl (B) and XeCl (C) states apparently differ, with the 〈fV values being ∼0.47 and 0.64, respectively. This selection of reactions represents most types of XeCl spectra that have been observed and the vibrational energy disposal to XeCl by other Xe( 3P2) + RCl reactions can be estimated by comparison with these results. Model potentials, as well as the ab initio potentials of Hay, were used to represent the XeCl states for the simulation. Simulations were done for both the XeCl  (B,1/2–X,1/2) and for the XeCl  (C,3/2–A,3.2) transitions. The XeCl  (B,1/2–X,1/2) transition is the more useful for assigning vibrational distributions because it is less sensitive to the shape of the lower potential. The dependence of the derived vibrational distributions upon the XeCl potentials and the variation of the transition moment with internuclear separation is discussed. The vibrational energy disposal for Xe  ( 3P2)+Cl2 also is compared to that for Xe  ( 3P2)+F2 and Kr  ( 3P2)+F2.
Direct measurements of the Gibbs free energy of OH using a cw tunable laser
Authors: Killinger D. K. , Wang Charles C.
Published: February 2003

This paper describes an absorption measurement for determining the Gibbs free energy of OH generated in a mixture of water and oxygen vapor. These measurements afford a direct verification of the accuracy of thermochemical data of H2O at high temperatures and pressures. Our results indicate that values for the heat capacity of H2O obtained through numerical computations are correct within our experimental uncertainty of 0.15 cal/mole °K.
Laser induced fluorescence, energy transfer and dissociation of Cs2
Authors: Katô Hajime , Yoshihara Keitaro
Published: February 2003

Laser induced fluorescences of Cs2 excited by the Ar+ laser lines have been studied. A series of lines with vibrational progression and the successive diffuse bands in the region down to 16 200 cm−1 were found to be polarized and identified as the resonance fluorescence. The diffuse fluctuation band with a maximum of intensity at 16 400 cm−1 consists of several bands at regular interval and it was independent on the line of excitation. By applying the theory on the role of kinetic energy in the Franck–Condon principle proposed by Mulliken, the band was identified as a transition from the internuclear distance of maximum kinetic energy in the vibrational level of the excited state E 1πu. All the other fluorescence bands of low frequency were observed to be depolarized completely, and the intensities increased when a foreign gas was added. These bands were identified as the fluorescence from the states which were populated by collision-induced transitions from the E state. Atomic fluorescence 6p 2P3/2→6s 2S1/2 is interpreted as the production of the excited atom Cs 6p 2P3/2 by predissociation of Cs2 E 1πu through interaction with, most probably, Cs2 B 1πu.
Limiting slope of the generalized oscillator strength vs momentum transfer curve
Authors: Huo Winifred M.
Published: February 2003

Although at the limit of zero momentum transfer, the generalized oscillator strength is equal to the Born value [E. N. Lassettre, A. Skerbele, and M. A. Dillon, J. Chem. Phys. 50, 1829 (1969)], it is found that the limiting slopes (df/dK)K=0 and (df/dK2)K=0 do not obey the Born approximation. For dipole allowed transitions (df/dK)K=0 is nonzero but finite and (df/dK2)K=0 becomes infinite at all finite incident energies. Also, the non-Born correction to f0 deduced from zero angle data is just as important as the next higher multiple term in the Born approximation. Thus in the analytic fit of experimental data it is imperative to include odd power terms in K even when the Born approximation is apparently obeyed if the purpose is to deduce the limiting slope as well as the intercept. However, if only the intercept is needed and the measured f’s shows no apparent energy dependence, it is still possible to extrapolate to f0 using a series even in K. The relevance of the present result to a number of recent electron impact experiments is discussed.
A determination of adsite symmetry on surfaces via electron energy loss spectroscopy: Coadsorption of CO and NO on Ru(001)
Authors: Thiel P. A. , Weinberg W. H. , Yates J. T.
Published: February 2003

Electron energy loss spectroscopy has been used to study the vibrational energies of NO and CO coadsorbed on the Ru(001) surface. Adsorption of CO competes directly with adsorption of one of the two molecular states of NO. This is interpreted in terms of the geometry of the adsorption sites for CO and NO and is consistent with models proposed previously for adsorption of CO and NO separately on the Ru(001) surface.
Gradient techniques for open-shell restricted Hartree–Fock and multiconfiguration self-consistent-field methods
Authors: Goddard John D. , Handy Nicholas C. , Schaefer Henry F.
Published: February 2003

Formulas for the analytical first derivatives of the potential energy have been derived for five specific cases of complexity beyond that of the closed-shell single-determinant self-consistent-field (SCF) approximation. The first four cases encompass most open-shell systems of chemical interest, namely, the conventional restricted SCF wave functions for doublets, triplets, open-shell singlets, and quartets. The final case considered is the two-configuration SCF (TCSCF) wave function required to describe singlet diradicals and orbital symmetry forbidden reaction pathways. An explicit example of the latter procedure is provided by the least motion insertion of CH(2Π) into H2  (1Σ+g) to yield the methyl radical. This doublet (S=1/2) TCSCF wave function is fairly typical of those required to theoretically explore problems in the physical chemistry of organic free radicals. Geometry optimization for the equilibrium structures of radical cations is illustrated by gradient calculations on H2CO+.
Electrical behavior of Se and Te to pressures of about 500 kbar
Authors: Bundy F. P. , Dunn K. J.
Published: February 2003

Using sintered diamond tipped carbide piston apparatus, and techniques of temperature-cycling the specimens while under pressure, the electrical resistance behavior of various kinds of Se specimens and of Te specimens has been explored as a function of pressure up to about 500 kbar. In general, earlier reported behavior to about 160 kbar was confirmed. New information includes: (i) Any type of initial Se goes to a stable metallic form above about 250 kbar. (ii) The ’’130 kbar metallic phase’’ of Se is probably the same as the ’’250 kbar metallic phase’’. (iii) The resistivity of metallic Se is very pressure sensitive, much more so than metallic Te. (iv) The activation energy of electrical conduction vs pressure results supports the earlier proposal that above 140 kbar Se goes into a high pressure semiconducting phase, which at about 250 kbar transforms to a metallic phase.
Laser magnetic resonance measurement of the 2 3P2–2 3P1 splitting in atomic oxygen
Authors: Saykally Richard J. , Evenson Kenneth M.
Published: February 2003

The J=2→1 fine-structure splitting in the ground state of atomic oxygen has been measured by laser magnetic resonance employing an intracavity dc glow discharge to generate the atom. The splitting, observed with the 63.1 μm laser line of 13CH3OH pumped by a CO2 laser, is 158.30298 (7) cm−1.
A high-density fluid-perturbation theory based on an inverse 12th-power hard-sphere reference system
Authors: Ross Marvin
Published: February 2003

A variational theory is developed that is accurate at normal liquid densities and densities up to 4 times that of the argon triple point. This theory uses the inverse 12th-power potential as a reference system. The properties of this reference system are expressed in terms of hard-sphere packing fractions by using a modified form of hard-space variational theory. As a result of this ’’bootstrapping,’’ a variational procedure may be followed that employs the inverse 12th-power system as a reference but uses the hard-sphere packing fraction as the scaling parameter with which to minimize the Helmholtz free energy.
Optical study of phase transformation hysteresis in p-di-iodobenzene
Authors: Soltzberg L. J. , Bobrowski Sophie J. , Parziale Patti A. , Armstrong Ellen C. , Cohn Vicki E.
Published: February 2003

The polymorphic phase transformation in p-di-iodobenzene, as followed by optical path difference measurements, shows a substantial temperature hysteresis. This hysteresis—on the order of 20 °C between the forward and reverse transformations—is surprising in light of previous descriptions of this particular phase transformation. Taken together, the various characteristics of the transformation in this material are more consistent with a nucleations and growth process than with a soft-mode transformation mechanism.
Electronic energy transfer in impurity bands of mixed organic solids
Authors: Klafter Joseph , Jortner Joshua
Published: February 2003

In this paper we consider some implications of the Anderson transition (AT) for the termination of electronic energy transfer (EET) in an impurity band of a low-temperature, isotopically mixed organic solid. The critical impurity concentration ? for the occurrence of the AT was found to be compatible with the experimental data for triplet EET. The mechanism of EET in extended states was described in terms of a strong scattering, random phase model, which leads to a minimum diffusion coefficient Dm?10−4−10−5 cm2 sec−1. Energy transfer from the impurity band to an energy sink (ETFIBTES) from extended states was described in terms of diffusion-controlled kinetics, providing a proper interpretation for the onset of EET in singlet impurity bands, which corresponds to a kinetic threshold, and the critical concentration for EET in triplet impurity bands, which marks the AT. Finally, we considered ETFIBTES from localized states, when the localization length is comparable to the spacing between the energy sinks, proposing a novel method for the determination of the localization length in disordered organic solids.
Integrated intensities in hydrogen cyanide
Authors: Kim Kwan , King W. T.
Published: February 2003

The integrated infrared intensities of five of the fundamental modes in HCN and DCN were measured by the Wilson–Wells–Penner–Weber method. The results are 53.3±0.3 and 57.7±1.1 km mol−1 for the Σ+ mode at 3312 K and the Π mode at 712 K, respectively, in HCN, and 27.2±0.4, 2.9±0.1, and 15.3±0.3 km mol−1 for the Σ+ modes at 2629 and 1921 K, and the Π mode at 569 K, respectively, in DCN. The dipole moment derivatives for the Σ+ modes obtained from these measurements and those obtained from recently reported ab initio calculations, which include valence shell configuration interaction, are found to be in essentially exact agreement with one another.
GTO basis sets for In, Sn, Sb, Te, I, and Xe
Authors: Huzinaga S.
Published: February 2003

Hartree-Fock-Roothan calculations are reported for atoms In through Xe, using Gaussian-type orbital GTO functions. (AIP).
Optical oscillator strengths for Ni
i
i
Authors: Ganas P. S.
Published: February 2003

The calculations for the optical oscillator strengths for the transitions 2s22p(2P1/2) → 2s2ns(2S1/2) and 2s22p(2P1/2) → 2s2nd(2D3/2) of N III are reported. (AIP)
Dipole coupling effects of nonchromophoric groups in molecules on frequencies, dipole strengths, and rotational strengths of chromophoric groups
Authors: Applequist Jon
Published: February 2003

Sum rules for the dipole and rotational strengths of the normal modes are derived. The calculations are used to interpret the observed frequency shifts, dipole strength and rotational strength, in chromophores whose nonchromophoric environment varies from one molecule to another. (AIP)
Semiclassical calculations of vibrational energy levels for nonseparable systems using the Birkhoff–Gustavson normal form
Authors: Swimm R. T. , Delos J. B.
Published: February 2003

We present a semiclassical method of calculating vibrational energy levels for a system of nonseparable coupled oscillators. For a Hamiltonian written as a power series in which the leading terms are given by a sum of one-dimensional harmonic oscillator Hamiltonians, the method involves transforming the original classical Hamiltonian via a succession of canonical transformations into a normal form which is a power series originally defined by Birkhoff and later generalized by Gustavson. Two cases are distinguished. If the harmonic oscillator frequencies in the unperturbed Hamiltonian are incommensurable, then the normal form is a power series whose terms are products of one-dimensional harmonic oscillator Hamiltonians; if the frequencies in the unperturbed Hamiltonian are commensurable, then additional terms which cannot be written as products of one-dimensional harmonic oscillator Hamiltonians enter into the normal form. Once the normal form is obtained, semiclassical quantization of action variables is straightforward. The incommensurable case yields a formula for the energy spectrum which is a power series in the quantum numbers. The commensurable case is more complicated, and yields a form from which energy levels may be obtained individually by numerical calculation and quantization of a one-dimensional phase integral. Nonseparable two-dimensional examples are treated for each case. The results obtained for both cases show excellent agreement with quantum mechanical calculations. The quantum calculations indicate that all of the energy levels fall into a regular pattern.
A diatomics-in-molecules (DIM) formalism for the reaction AH+ +A′→A+A′H+: A ground state potential energy surface for collinear He2H+
Authors: Meier Paul F. , Hayes Edward F. , Ellison Frank O.
Published: February 2003

A valence-bond diatomics-in-molecules (DIM) formalism is applied to the study of proton rare-gas reactions of the type AH++A′→A +A′H+. The method is used to generate potential energy surfaces for the He2H+ system. The DIM potential surface for this system is in good quantitative agreement with an LCAO–SCF–MO surface for the collinear arrangement, He–H–He+.
Effect of pressure on bonding in black phosphorus
Authors: Cartz L. , Srinivasa S. R. , Riedner R. J. , Jorgensen J. D. , Worlton T. G.
Published: February 2003

The bond lengths and bond angles of orthorhombic black phosphorus have been determined as a function of hydrostatic pressures to 26.6(5) kbar using time-of-flight neutron powder diffraction. We show that the markedly anisotropic compression reported previously results from a large pressure-induced shortening of the van der Waals bonds separating layers of atoms combined with a shear motion within the layers. Covalently bonded chains of atoms along the a direction remain very rigid. The average effective linear compressibility for van der Waals bonds is 1.48(9) ×10−3 kbar−1 while the average effective linear compressibility for covalent bonds is an order of magnitude smaller, 2.6(8) ×10−4 kbar−1.
Molecular beam measurements of inelastic cross sections for transitions between defined rotational states (j,m) of CsF in collisions with He, Ne, Ar, Kr, CH4, CF4, SF6, C2H6, N2, CO, CO2, N2O, CH3Cl, CH3Br, CF3H, and CF3Br
Authors: Borkenhagen U. , Malthan H. , Toennies J. Peter
Published: February 2003

Absolute inelastic cross sections for rotational quantum state to state transitions have been measured in small angle grazing collisions of CsF molecules in collisions with He, Ne, Ar, Kr, Xe, CH4, CF4, SF6, C2H6, N2, CO, CO2, N2O, CH3Cl, CH3Br, CF3H, CH3Cl, CF3Br. For the rare gas collision partners the following Δj=1, Δm=0 transitions were measured: (j,m) = (1,0) → (2,0), (2,0) → (1,0), (2,0) → (3,0), (3,0) → (2,0) as well as the Δj=2, Δm=0 transition (3,0) → (1,0). In addition the Δj=1, Δm=1 transitions (1,1) → (2,0) and (2,0) → (3,1) and the pure Δm=1 transitions (1,1) → (1,0) and (3,0) → (3,1) were measured. In each case the signs of m and Δm are undetermined. For the other scattering partners only the Δj=1, Δm=0 and Δj=2, Δm=0 transitions were measured. In the collision region the quantization axis of the CsF molecules is defined by a weak electric field (300 V/cm) directed perpendicular to the direction of the velocity selected CsF and the target nozzle beam and thus perpendicular to the relative velocity vector. The apparatus and the techniques used to determine the absolute cross sections are given and the formulas used to derive the various types of cross sections from the pressure dependences are derived. Because of the favorable geometry and nearly monoenergetic conditions it was possible to calculate by a Monte Carlo method a transmission function for each rare gas and transition studied. The tabulated transmission functions make it possible to compare theoretical predicted differential cross sections directly with the reported values. An approximate analytic expression is also given for the (2,0) → (3,0) transition which can be used for all scattering targets. This approximate transmission function is then used to obtain absolute cross sections in the center of mass system for all systems. These cross sections are correlated with the predictions of a Born-approximation theory for the various predominant potential terms. The correlation shows that for the first 8 partners the induced dipole–quadrupole induction term is predominant. For the molecules C2H6, N2, CO, CO2 and N2O an electrostatic dipole–quadrupole potential is dominant, while for the polar symmetric top molecules the dipole–dipole potential is producing the transitions. The results suggest that the method can be used to determine quadrupole moments.
Partial photoionization cross sections of atomic iodine; irreducible tensor analysis of intensities
Authors: Berkowitz J. , Goodman G. L.
Published: February 2003

The He i photoelectron spectrum of atomic iodine has been obtained from a thermal source. The relative intensities are compared with recent data on Cl and Br. The major difference is an inversion in the relative intensities of 3P0 and 1S0 between Cl and I. An intermediate coupling angular momentum treatment, combined with spectroscopic mixing parameters, reproduces this inversion quantitatively, and, in addition, adequately describes the entire intensity pattern for all three halogens. The strong implication is that angular factors and mixing parameters are the dominant features in determining relative intensities, and radial integrals and ’’anisotropic electron–ion interactions’’ are much less important.
Further developments and applications of the Green’s function method of sensitivity analysis in chemical kinetics
Authors: Dougherty Eugene P. , Hwang Jenn-Tai , Rabitz Herschel
Published: February 2003

A numerical procedure is presented for implementing the Green’s function method of sensitivity analysis in chemical kinetics. The procedure is applied to three sets of chemical reactions: the Chapman mechanism for ozone kinetics, a mechanism for methane combustion and a model for formaldehyde oxidation in the presence of carbon monoxide. Whenever possible, comparisons with alternative methods of sensitivity analysis are made. It is shown that carefully analyzed sensitivity profiles can be used in conjunction with experiments and/or models to obtain useful information about chemical kinetic behavior. By using methods from multivariable calculus an entire family of sensitivity coefficients may be derived from the elementary sensitivities obtained by solving differential equations. Each elementary or derived sensitivity coefficient has a unique physical interpretation in terms of an experiment or modeling calculation. A simple nonlinear interpolation formula is suggested for easily estimating higher-order sensitivity information. Finally the overall computational efficacy of the Green’s function method of sensitivity analysis is assessed.
Inelastic scattering of Cl− ions in collision with H2 from 4 to 11 eV
Authors: Cheung J. T. , Datz S.
Published: February 2003

We have utilized the time-of-flight (TOF) technique to measure the energy loss of Cl− ions backscattered from H2 in the relative energy region of 4 to 11 eV. For relative energy less than 7 eV, this collision yields vibrational excitation of the H2 molecule and the amount of energy transferred is similar to that in the isoelectronic analog, i.e., K+–H2. In the case of K+, the results were in good agreement with an exact classical calculation involving only the particle masses, the K+–H2 repulsion, and the H2 potential. At higher relative energy (Erel≳7 eV), however, the process is much more inelastic with a minimum energy loss of greater than 4 eV, corresponding to dissociation of the H2 molecules. The threshold for dissociation observed here is more commensurate with that predicted for a strong [H2Cl]− surface than with simple T–V transfer kinetics.
Critical point analysis of instabilities in Hamiltonian systems: Classical mechanics of stochastic intramolecular energy transfer
Authors: Cerjan Charles , Reinhardt William P.
Published: February 2003

A critical point analysis of the nonlinear dynamics of the difference between two neighboring classical trajectories is used to develop a simple criterion for the occurrence of stochastic instabilities in conservative classical Hamiltonian systems. The technique is based on the assumption that a necessary (but not sufficient) condition for existence of macroscopic stochastic regions in phase space follows from exponentiation of such neighboring trajectories, and that the critical (or stationary) points of the difference flow controls the global behavior. The method is successfully applied to the one-dimensional cubic and Morse oscillators, and to the two-dimensional (four degrees of freedom) Henon–Heiles, Barbanis, and anti-Henon–Heiles problems. The relationship to the earlier Brumer–Duff–Toda (BDT) method, which often makes incorrect predictions of instability, is established by noting that the BDT analysis, by linearizing the difference trajectory dynamics, ignores all but one of the flow controlling critical points.
Elliptic phase boundaries between smectic and nematic phases
Authors: Klug D. D. , Whalley E.
Published: February 2003

Smectic–nematic phase boundaries that resemble arcs of ellipses can be analyzed by a quadratic expansion in temperature and pressure of the change ΔG of Gibbs function. If the change of entropy or volume is known, ΔG and its first two derivatives can be determined. Data on 8OCB and mixtures of 8OCB and 6OCB and of CB9A and CB7A fit an ellipse well, which suggests that the boundary is an ellipse. The smectic phase is therefore enclosed, which appears to be unique in single-component systems and to disagree with simple mean-field theory.
Magnetic field effect on the hyperfine-induced electron spin motion in radicals undergoing diamagnetic–paramagnetic exchange
Authors: Knapp Ernst-Walter , Schulten Klaus
Published: February 2003

A semiclassical description of the hyperfine-induced spin motion of radical pairs suggested recently [K. Schulten and P. G. Wolynes, J. Chem. Phys. 68, 3292 (1978)] has been generalized to arbitrary magnetic field situations. Analytical expressions for the elements of the electron spin correlation tensor <S(o) S (t) ≳ averaged over all nuclear spin configurations are derived and applied to calculate the time evolution of the electron spin state of radical pairs initially prepared in a singlet state. The treatment includes the possibility that the radicals undergo a diamagnetic–paramagnetic exchange reaction. The effect of such exchange on the magnetic field dependence of the radical pair (triplet) recombination is predicted.
Liquid Cl2 dynamics studied by depolarized Raman and Rayleigh scattering
Authors: Topalian J. H. , Maguire J. F. , McTague J. P.
Published: February 2003

Polarized and depolarized Raman and Rayleigh spectra of liquid chlorine have been measured along the coexistence curve from 184 to 238 K. Results are compared with predictions of current simple models for rotational diffusion and vibrational dephasing in liquids. In accord with recent investigations for polyatomic species, it has been found that the temperature dependence of the single particle reorientational time τsp is proportional to the viscosity/temperature ratio, η/T. Despite evidence for specific angle-dependent valencelike forces between Cl2 molecules, comparison of the observed τsp with that predicted by the hydrodynamic ’’slip’’ and ’’stick’’ models shows clearly that the former provides much better agreement with experiment. The temperature dependence of the vibrational dephasing time is controlled by motional narrowing. A number of caveats associated with the extraction of reliable information concerning collective reorientational motions from depolarized Rayleigh scattering are briefly discussed.
Orientations and reorientational dynamics of methanol enclathrated in β-quinol (hydroquinone) from dielectric and 1H, 2H, and 13C NMR studies
Authors: Ripmeester J. A. , Hawkins R. E. , Davidson D. W.
Published: February 2003

Dielectric measurements of methanol–β-quinol clathrate between 5 and 300 K show two widely separated relaxation regions associated with relaxation of the components of the methanol dipole moment parallel and perpendicular to the c axis of the crystal. Both are sensitive to the methanol content of the cages. For high methanol content, both components tend to become ordered below the transition at 66 K. NMR studies over a wide range of temperature were made of the H line shape in CH3OD–C6D6O2, D quadrupole coupling constant (DQCC) in CD3OH–C6H6O2 and spin–lattice relaxation of H in CH3OH–C6H6O2 and D in CD3OH–C6H6O2. A 13C chemical shift anisotropy of σ∥-σ⊥=20.6 ppm for CH3OH in a single crystal was obtained at 302 K. The DQCC in CH3OD was 109 kHz at 302 K. The results are consistent with a model of six preferred orientations of methanol, three related by 120° rotation about the c axis and three generated approximately by inversion of the first three through the cage center. In the preferred orientations, the COH plane is parallel to c and the angle of 32° between the CO bond and c below 100 K increases to ∼40° at 300 K.
A merged molecular beam study of the endoergic associative ionization reaction N(2D)+O(3P) →NO++e−
Authors: Ringer Geoffrey , Gentry W. Ronald
Published: February 2003

We have measured the absolute total cross section as a function of kinetic energy for the endoergic association ionization reaction N+O→NO++e−. Analysis of data obtained with various mixtures of N atom electronic states enables us to determine that the dominant mechanism is the reaction of the N(2D) metastable excited state with the O(3P) ground state. The reaction cross section shows an abrupt onset at the expected threshold energy of 0.38 eV, and has a maximum of about 0.6 Å2 at 10.5 eV. The data in the threshold region yield thermal rate coefficients larger than those estimated previously by Lin and Teare. A theoretical interpretation of these data is given by S. E. Nielsen and J. S. Dahler in a companion paper immediately following this one.
Endoergic chemi-ionization in N–O collisions
Authors: Nielsen Svend Erik , Dahler John S.
Published: February 2003

A semiclassical theory of endoergic chemi-ionization is developed and applied to the ionizing events that occur when ground state oxygen atoms collide with nitrogen atoms in the ground and first excited states. The approach we use is an adaptation and extension of earlier theories due to Bardsley, Nakamura and Miller. For energies in excess of the associative ionization (AI) threshold, EAI≳O, but less than the Penning ionization (PI) threshold, EPI≳EAI, only AI can occur.However, when E≳EPI chemi-ionization may result in the formation of a stable diatomic ion (AI), neutral and ionized atomic fragments (PI), or of a metastable diatomic rotational resonance (DI, delayed ionization). Our theory relates the experimental AI and PI cross sections to the cross sections for these three basic events. The heavy-particle motions are treated classically in terms of adiabatic potential energy functions. However, localized nonadiabatic transitions also are taken into account by using the Landau–Zener approximation. It is assumed that the Franck–Condon principle applies to the electronic transitions between the states of the neutral reactants and the continuum states of the ionic products. Numerical estimates are presented for the AI cross sections of the two reactions N(4S)+O(3P) →NO+(Σ+)  + e and N(2D)+O(3P) →NO+(Σ+)  + e, for which the threshold energies are 2.76 eV and 0.38 eV, respectively. These calculations are limited to collision energies below 10 eV. They are based on potential energy curves due to Michels and on estimates of the ionization widths that Bardsley and Lee have extracted from spectroscopic information. The theoretical predictions compare well with the results of Ringer and Gentry’s merged beam experiments which have been reported in the preceding paper.
Laser induced vibrational excitations in a diatomic molecule during molecular collisions
Authors: Mohan Man , Chand Puran
Published: February 2003

Vibrational excitation of a diatomic molecule undergoing collision in the presence of a laser beam is considered. The average vibrational excitation transition probabilities are calculated and compared with earlier results obtained in the absence of the laser field.
Configuration interaction calculations for two-electron atoms in a spherical box
Authors: Ludeña Eduardo V. , Gregori Marianne
Published: February 2003

We report calculations of the ground state (1S) correlation energy for the two electron systems He, Li+, and Be2+ when they are compressed in a spherical box with infinite potential barriers. The calculations were performed using a 41 term configuration interaction expansion generated by a 6s4p4d orbital basis. It is found that although there is an increase in the correlation energy when the sphere radius becomes smaller, this increase is extremely small with respect to the change in total energy. This fact is discussed within the context of the z−1 expansion for the energy. We also discuss the similarity between the compression process induced on an atom by increasing its nuclear charge and by the process employed in this work.
Local order and vibrational coupling in solutions of polar molecules
Authors: Mirone Paolo , Fini Giancarlo
Published: February 2003

The separation between the isotropic and the anisotropic component of some Raman bands of polar molecules is studied as a function of concentration in different solvents. It is found that it depends linearly on the ratio between the volume fraction and the static dielectric constant of the solution, and becomes zero at a finite concentration. It is suggested that this concentration threshold is related to the relaxation time of the vibrational energy. An explanation is proposed for the observed dependence on the dielectric constant.
Depolarized light scattering from dilute polymer solutions
Authors: Evans Glenn T.
Published: February 2003

Depolarized light scattering spectra of chain molecules are analyzed in terms of the contributions of the bond polarizabilities of skeletal and nonskeletal bonds and dipole-induced–dipole interactions between the chain atoms. It is found that substituents lying perpendicular to the chain contour contribute to the high wave vector part of the molecular polarizability. These high k modes are precisely the same as those detected in dielectric and spin relaxation measurements and account for the broad, molecular weight independent background observed in depolarized Rayleigh scattering from polystyrenes. The influence of dipole-induced–dipole interactions on the spectrum is also characterized. Inclusion of these effects gives rise to a weighted sum over several low frequency Rouse–Zimm modes, with the major emphasis coming from the lowest mode.
Analysis of experimental total differential scattering data of nonspherical molecules with anisotropic potentials. Application to Ar–CO2
Authors: Rotzoll G. , Lübbert A.
Published: February 2003

The analysis of total (unresolved in velocity) laboratory differential cross sections with the infinite order sudden approximation is investigated in the case of significant velocity changes. Well position anisotropies of the potential, which in the center of mass system do not affect the rainbow structure, do so in the laboratory system. The rainbow structure is damped and shifted. In averaging over experimental conditions, it is necessary to include the rotational temperature of the molecule beam. Ar–CO2 total differential scattering data are compared with calculations based on published potentials. From these comparisons we infer that the well depth anisotropy is weak. Central field calculations show that this type of analysis does not yield the spherically symmetric part of the full anisotropic potential, but a potential with a well depth approximately equal to the well depth averaged over the cosine of the body-fixed angle of orientation.
Vibrational spectroscopy of molecular constituents of one-dimensional organic conductors. Tetrathiofulvalene (TTF), TTF+, and (TTF+)2 dimer
Authors: Bozio Renato , Zanon Ines , Girlando Alberto , Pecile Cesare
Published: February 2003

An extensive vibrational assignment of TTF and TTF-d4 is achieved, improving the previously reported one through the use of polarized infrared spectra of single crystals of the monoclinic form. Infrared spectra of the monoclinic and triclinic forms are compared and the different crystal field effects discussed. Powder Raman and infrared spectra of (TTF)Br1.0 and (TTF-d4)Br1.0, Raman depolarization ratios and infrared spectra of (TTF)ClO4 and (TTF-d4)ClO4 solutions are reported. The assignment of the ag, b1u, b2u, and b3u fundamental modes of (TTF)+ and (TTF-d4)+ radicals allows the identification of most of the relevant frequency shifts following the ionization of the TTF structure. The possible use of the ionization shifts for the study of the electronic charge distribution in the conducting TTF systems is considered. The parallel investigation of the concentration effects on the visible and infrared absorption spectra of TTF+ in solution let us to identify anomalous infrared absorptions associated with the formation of (TTF+)2 dimer. They are attributable to an intensity borrowing from the charge transfer band due to the electron–molecular vibration (e–mv) interaction and their identification opens the way to an experimental evaluation of the e–mv coupling constants of TTF.
Measurement of vibrational energy exchange rates in the H2/HCl molecular system
Authors: Rosen D. , Coughlin J. , Taylor R. L.
Published: February 2003

Measurements of the rate of vibrational energy transfer occuring in H2/HCl molecular system at temperatures 800–2000° K are reported. (AIP)
EPR and saturation transfer EPR studies on glyceraldehyde 3-phosphate dehydrogenase
Authors: Beth A. H. , Wilder R. , Wilkerson L. S. , Perkins R. C. , Meriwether B. P. , Dalton L. R. , Park C. R. , Park J. H.
Published: February 2003

Electron paramagnetic resonance (EPR) and saturation transfer–EPR (ST–EPR) techniques were employed to investigate the hydrodynamic properties of glyceraldehyde 3-phosphate dehydrogenase (GAPDH). Both apo- and holoenzyme were spin-labeled at the active site cysteine-149 residue with N- (1-oxyl-2,2,6,6-tetramethyl-4-piperidinyl) - maleimide. The apo- and holoenzymes were observed to have the same hydrodynamic structure and the spectroscopic results were consistent with these complexes behaving as spheres with hydrated radii of 41 Å. The environment of the paramagnetic electron was significantly more polar in the spin-labeled holoenzyme than in the spin-labeled apoenzyme, suggesting that either ionic residues are positioned closer to the active site in the holoenzyme or that ionic segments of coenzyme nicotinamide adenine dinucleotide (NAD+) itself may interact with the paramagnetic electron of the maleimide spin label. The dependence of the phase quadrature second harmonic absorption ST-EPR signal upon microwave and Zeeman modulation field intensities was examined with particular attention directed to effects of the variation of these fields over samples of finite dimensions. Such considerations are crucial when comparing data obtained employing different microwave resonators or different sample geometries within the same resonator. The problem of accurately establishing phase nulls for phase quadrature measurements was examined as was the problem of estimating phase resolution. Thus, the present work represents both a specific application and a generalization of the ST-EPR methodology proposed by Thomas, Dalton, and Hyde [J. Chem. Phys. 65, 3006 (1976)].
Positronium formation in aqueous micellar solutions of sodium dodecylsulfate
Authors: Handel Edward D. , Ache Hans J.
Published: February 2003

Positron annihilation lifetime measurements on aqueous solutions of sodium dodecylsulfate (NaLS) were performed over a wide range of temperatures and surfactant concentrations. The sensitivity of the positronium formation process toward changes in physical properties accompanying micellization is demonstrated. The critical micelle concentration (CMC) is associated with a sharp decrease in positronium formation. The CMC of 8.4±0.5 mM at 20 °C in this system is in agreement with literature values. At higher temperatures the CMC is shifted to slightly higher surfactant concentrations (10.4±0.7 mM at 90 °C). The formation probability for thermal positronium as expressed by I2, the intensity of the long-lived component in the lifetime spectra, gradually decreases above the CMC with increasing surfactant concentration reaching a limiting value. A model is suggested involving the reactions of ’’hot’’ positrons or positronium atoms with micellar aggregates leading to the observed reduction of thermalized positronium atoms.
Resonant states at threshold observed in electron photodetachment cross sections of polyatomic negative ions
Authors: Jackson Robert L. , Zimmerman Albert H. , Brauman John I.
Published: February 2003

We have measured high resolution electron photodetachment cross sections for substituted acetophenone enolate anions using an ion cyclotron resonance spectrometer as an ion detector and a cw dye laser as a light source. In each case we find a series of narrow resonances (half-width∼1 nm) beginning at threshold which do not correlate with states appearing in limited basis set MO calculations. These resonances are attributed to the vibrational levels of a Rydberg-like electronic state supported by the dipole of the product neutral. We also report the observation of higher energy resonances in the low resolution cross sections. These resonances do correlate with states appearing in limited basis set MO calculations and we assign them as π–π* states of the anion. From the high resolution data, we report electron affinities for the following substituted acetophenone enolate radicals: m-fluoro: 2.218+0.005−0.010eV, p-fluoro: 2.176+0.005−0.010eV, p-H: 2.057+0.005−0.010eV, p-C(CH3)3: 2.032+0.005−0.010eV, m-methyl: 2.030+0.005−0.010eV.
Generation of N3 in the thermal decomposition of NaN3
Authors: Piper L. G. , Krech R. H. , Taylor R. L.
Published: February 2003

Gaseous azide radicals at concentrations up to 1013 molecules cm−3 appear to have been generated in a flow system in the thermal decomposition of solid NaN3 at temperatures between 150 and 350 °C. Between 150 and 250 °C the activation energy for the decomposition is 0.80±0.15 eV. The addition of small concentrations of oxygen atoms to the reactor excites NO γ-band chemiluminescence from the reaction O+N3k4→NO (A 2Σ+)+N2. The γ-band emission may be used as a sensitive tracer for N3 in kinetic measurements; k4 was determined to be (10±4) ×10−12 cm3 molecule−1 s−1. Large concentrations of O atoms quantitatively convert the N3 to NO, and the resulting O/NO afterglow has been used to determine absolute concentrations of the N3 radicals. Several other chemiluminescent emissions are observed, most notably the nitrogen first-positive emission excited in azide-radical recombination.