bluuebunny/crossref_metadata_2025 huggingface dataset:
Nuclear magnetic relaxation of spin 5/2 and spin 7/2 nuclei including the effects of chemical exchange
Authors: Bull T. E. , Forsén Sture , Turner David L.
Published: February 2003

An equation describing the multiexponential longitudinal and transverse relaxation of quadrupolar nuclei, including the effects of chemical exchange in the limit of fast exchange, is derived. It is difficult to produce analytical solutions for nuclei with spins greater than 3/2, but the equation is in a form convenient for numerical solution, and such solutions are presented for both the longitudinal and transverse relaxation of nuclei with spins 5/2 and 7/2 in the absence of chemical exchange, and for the case of two-site exchange where one site is in the extreme narrowing limit. Longitudinal relaxation may be described approximately by a single exponential in all cases. However, the transverse relaxation generally must be described as the sum of three and four exponentials for the spin 5/2 and spin 7/2 cases, respectively, the amplitudes of which vary with the correlation time in the absence of exchange, and also with the ratio of the lifetimes of the nucleus in each state when two-site exchange is considered.
On dynamic scaling theories of individual polymers in solution
Authors: Adler Ronald S. , Freed Karl F.
Published: February 2003

Dynamical scaling relations are derived for the diffusion coefficient, viscosity, and the dynamic structure factor for polymer chains at infinite dilution. The derivation begins with the full diffusion equation with unaveraged hydrodynamic interaction for the dynamical motion of a continuous chain in solution. The scaling relations emerge from asymptotic dimensional analysis for very long chains. The dependence on all dimensional variables is retained, and previous scaling results are recovered. The use of interdimensional scaling enables the derivation of the dynamical exponents which are found to involve the static ones in the fashion assumed by de Gennes.
The relation between the nonlinear Debye–Hückel theory and the hypernetted chain approximation
Authors: Henderson Douglas , Bluma) Lesser
Published: February 2003

It is shown that the nonlinear Poisson–Boltzmann or Debye–Hückel theory can be obtained from the hypernetted chain approximation if the non-Coulomb part of the direct correlation in an ionic fluid is neglected.(AIP)
Laser Stark spectroscopy of the ν4 fundamental band of CH2NH at 6.1 μm
Authors: Allegrinia) M. , Johns J. W. C. , McKellar A. R. W.
Published: February 2003

The ν4 fundamental band of CH2NH has been studied by the technique of intracavity CO laser Stark spectroscopy. The short-lived CH2NH was produced in a flow system by the pyrolysis of CH3NH2 at ∼1000°C. The ν4 band origin was determined to be 1638.2976±0.0015 cm−1, and accurate values for the excited state rotational constants were found. The ground state dipole moment components were determined to be μa=1.3396 D and μb=1.4461 D, for a total dipole moment of μ=1.9712 D. Very similar dipole moments were found for the v4=1 excited state: μa=1.3445 D; μb=1.4412 D; and μ=1.9710 D.
Concentration and isotope effects in the stability of agarose gelsa)
Authors: Vento G. , Palmab) M. U. , Indovina P.
Published: February 2003

Agarose–water gelling systems are of interest as models for extended supramolecular and molecular structures of wide biological significance. Precise photometric recording techniques as recently revived are here used for a study of the influence of polymer concentration and deuterium isotope substitution on molecular and supramolecular order. These techniques are suitable for studying both types of order in one and the same sample. Experiments at different polysaccharide concentrations allowed evaluation, under limiting assumptions, of the heat of reaction of the polymer cross-linking processes. Substitution of H2O with D2O improved the stability of molecular structures as well as recognition and adhesion among ordered molecules, responsible for the growth of supramolecular order. The potential factors governing stabilization of both types of order are discussed. A sole role of a greater strength of deuterium bonding is not inferred from our data. A role of isotopic alterations of the entropy term due to interactions between solvent and the polymer nonpolar groups appears probable. Closely similar conclusions were reached in a recent study of polynucleotide systems.
Vibrational energy transfer in ethylene and ethylene–rare-gas mixtures
Authors: Häger J. , Krieger W. , Rüegg T. , Walther H.
Published: February 2003

Vibrational energy transfer in ethylene and ethylene–rare-gas mixtures has been investigated. The ν11 (CH2 stretching) mode of ethylene has been excited by the pulses of an optical parametric oscillator with a wavelength of 3.3 μm. Fluorescence from the ν7 (CH2 wagging) mode at 10.5 μm has been detected. The resulting ν7 activation rate for pure ethylene is (267±27) ms−1 Torr−1. The ν7 activation rates of mixtures with rare gases are as follows: for He (13.8±2.7) ms−1 Torr−1, for Ne (16.2±4.9) ms−1 Torr−1, for Ar (32.8±2.6) ms−1 Torr−1, for Kr (34.7±12.1) ms−1 Torr−1, and for Xe (41.3±6.2) ms−1 Torr−1. The V–T/R relaxation rate of ν7 determined in our experiments is in good agreement with former measurements. Preliminary results on the fluorescence from the ν12 (CH2 scissors) mode at 6.9 μm show a ν12 deactivation rate of about 120 ms−1 Torr−1. Possible relaxation mechanisms are discussed.
Anisotropic mobility of water in hydrated layered chalcogenides A+x(H2O)y[TaS2]x− and A+x(H2O)y [NbS2]x− as studied by NMR
Authors: Röder U. , Müller-Warmuth W. , Schöllhorn R.
Published: February 2003

Wideline 1H NMR studies have been carried out in various hydrated layered chalcogenides of the type A+x(H2O)y [TaS2]x− at temperatures from 148 to 300 K, where A=Na, K, NH4, Cs, and Ca. Some measurements have been made at 77, 363, 398, and 425 K, respectively, and a sample with Nb instead of Ta has also been examined. Depending upon the temperature range and upon the particular compound, three different types of spectra have been observed. The rigid lattice H2O spectra are characterized by intra- and intermolecular dipole–dipole interactions. At elevated temperatures, the spectrum equals rather perfectly the theoretical shape of the proton spectrum for an isolated two-spin system. This suggests the occurance of a range of activated ordered mobility which is explained by a two-dimensional diffusion of water inside the interlayer space. For compounds with bilayers rather than monolayers of water, rotation of H2O about its C2 axis is also activated. This is supported by measurements of the spin–lattice relaxation time T1. Finally, an additional narrow line, which appears at high temperature, is attributed to a subphase of water undergoing random motion.
Pressure dependence of some 6p→6s emissions in vacuum ultraviolet irradiated Xe
Authors: Margani A. , Xuan C. Nguyen , Di Stefano G. , Lenzi M.
Published: February 2003

The pressure dependence of some near infrared emissions, mostly due to the production of the 6p[1/2]0 and 6p[3/2]2 Xe levels by processes related to vacuum-ultraviolet irradiation, has been investigated. Xe gas has been irradiated at twelve selected wavelengths in the range from 127.0±0.2 to 112.0±0.2 nm and the pressure effect on the absorption and the near infrared emissions compared. The results are discussed in terms of molecular dissociation and predissociation of excited Xe2 Rydberg states and of radiative coupling between atomic excited levels.
An ion cyclotron resonance study of reactions of ions with hydrogen atomsa)
Authors: Karpasb) Z. , Anicich V. , Huntress W. T.
Published: February 2003

Reactions of H2+, HeH+, and CO2+ ions with hydrogen atoms, and the reactions of D2+, CO2+, CO+, N2+ and HCN+ with deuterium atoms, were studied using ion cyclotron resonance techniques. These reactions proceed predominantly via a charge transfer mechanism. The rate constants measured are: 6.4, 9.1, 1.1, 5.0, 0.84, 0.90, 1.2, and 0.37×10−10 cm3/sec, respectively. Hydrocarbon ions of the types CHn+ and C2Hn+, where n=2–4, do not react with H or D atoms.
Photodissociation of Br2 in the visible continuum
Authors: Lindeman T. G. , Wiesenfelda) J. R.
Published: February 2003

The photodissociation of Br2 in the region 434–511 nm has been studied using the technique of laser induced photodissociation spectroscopy. Following photolysis of Br2 with a tunable dye laser, production of bromine atoms in the electronically excited, 42P1/2, and ground, 42P3/2, states was monitored by observing the attenuation of the atomic resonance emission line, 52P3/2→42P3/2, at 148.9 nm. Analysis of the absorption profiles yielded the relative contribution to the visible continuum of the B 3Π0+u←X 1Σg+ transition. Estimates of the absolute absorptivity for the transitions 1Πlu, B 3Π0 +u, A 3Πlu←X 1Σg+ were obtained by application of the reflection principle. These results are in excellent agreement with previous direct measurements but suggest that models of the Br2 absorption continuum which neglect the A 3Πlu←X 1Σg+ transition are likely to be in error.
Anisotropic hyperfine interactions of rare-gas nuclei near trapped hydrogen atomsa)
Authors: Morton J. R. , Preston K. F. , Strachb) S. J. , Adrian F. J. , Jette A. N.
Published: February 2003

The EPR spectra of hydrogen atoms trapped in xenon and krypton at 10 K have been reinvestigated. Analysis of the anisotropic 129Xe (enriched), and 83Kr hyperfine structure shows the H atoms are trapped in the octahedrally symmetric interstitial site (6 nearest neighbors) of the Kr and Xe matrices. For H in Kr, a simple model which considers only overlap of H and Kr orbitals in purely covalent Kr⋅⋅⋅H ’’molecules’’ accounts very well for the observed Kr hyperfine structure and the electronic g factor. To account for the corresponding magnetic constants of H in Xe, however, it is necessary to consider, along with overlap effects, the effects of transfer of electronic charge from the nearest neighbor Xe atoms to the interstitially trapped H atom, as described by admixture of a very small Xe+⋅⋅⋅H− ionic component into the covalent Xe⋅⋅⋅H ’’molecule’’. This charge transfer may also be a factor in the surprisingly large differences in the g factors and 129Xe hyperfine constants of H and D atoms trapped in Xe.
A theoretical study on the potential surfaces of the lower electronic states of HCO
Authors: Tanaka Kiyoshi , Davidson Ernest R.
Published: February 2003

The results of ab initio potential-energy surface calculations are presented for the lowest four doublet states of HCO. A conical Jahn–Teller intersection was found between the lowest two 2A′ surfaces. The equilibrium geometries of the lowest two quartet states were also calculated. Discussion is primarily focused on the assignment of the hydrocarbon flame bands. The A bands of the hydrocarbon flame bands are assigned as a ?2A′→?2A′ transition which concurs with Dixon’s speculation. As for the ? state which is the origin of the B bands, the most probable state is the bent second 2A″ state which is described by the configuration (6a′)2(7a′)2(1a″)1.
Angular distributions, energy disposal, and branching studied by photoelectron–photoion coincidence spectroscopy: O2+, NO+, ICl+, IBr+, and I2+ fragmentation
Authors: Eland J. H. D.
Published: February 2003

It is shown that some levels of the A2Πu states of I2+, IBr+, and ICl+ are stable toward dissociation, and ought to decay by emission. The B2Σg+ state of I2+ is fully dissociated to ground state I++I, while the B2Σg+ states of IBr+ and ICl (probably) yield excited decay products. The c3Π and B1Π states of NO+ dissociate to ground state N++O fragments, while the c4Σg− and 2Πu states of O2+ near 24 eV dissociate to excited levels of O++O. The origin of anomalous time-of-flight peak shapes is discussed, and several examples are attributed to anisotropic fragment angular distributions.
Dynamics of the spin-1 Ising mean field model
Authors: Batten George L. , Lemberg Howard L.
Published: February 2003

A spin-1 Ising system with dipolar and quadrupolar interactions is placed in contact with a phonon heat bath. Using the projection operator formalism of Zwanzig and Nakajima, a master equation on the time scale of one microscopic event is derived from the Liouville equation. The transition rates thus obtained depend upon the detailed dynamics of the system, bath, and interactions between the two. Coarse-graining removes these details and yields a phenomenological transition rate similar to the form proposed by Langer. The kinetic equations which describe the most and least likely temporal evolution of the system are then derived. Finally, the time dependence of the two order parameters which characterize the system is examined numerically.
Detection of spurious avoided crossings of potential energy curves
Authors: Carrington Tucker
Published: February 2003

An operator H (λ) may have eigenvalues which cross as a function of the parameter λ. When this operator is represented in terms of its action on the vectors of a certain representation space, the eigenvalues of the representation generally have an avoided crossing. This paper discusses and illustrates the fact that the properties of such a spurious avoided crossing, and the ease with which it may be detected, depend sensitively on the relationship between the representation space and the space spanned by the exact eigenvectors one is trying to represent.
Solution of the coupled equations of inelastic atom–molecule scattering for a single initial state
Authors: Thomas Lowell D.
Published: February 2003

A variation-iteration method is proposed for solving the coupled, second-order differential equations, which arise in atom–molecule inelastic scattering theory, directly for a single, linearly independent vector of functions with the proper S-matrix boundary conditions. As a demonstration of the potential usefulness of this method a set of 210 coupled equations, nearly twice the number which can be handled with standard methods, for He–CO rotationally inelastic collisions is solved and compared to the Monte Carlo classical trajectory results.
Theory of twisting and bending of chain macromolecules; analysis of the fluorescence depolarization of DNA
Authors: Barkley Mary D. , Zimm Bruno H.
Published: February 2003

An elastic model of semiflexible chain macromolecules is developed in order to treat internal rotatory Brownian motion in the DNA helix. Dynamical equations for torsion and bending of the chain are generated, using results from classical elasticity and hydrodynamic theories. The rotational diffusion equation in normal coordinates is derived, and the initial-boundary value problem solved for the conditions of a nanosecond fluorescence depolarization experiment. The resulting time distribution function of the angular orientation of a fluorescent probe, embedded in a chain at thermal equilibrium, is used to compute the emission anisotropy. The predicted decay law is unusual, with exponentials in ∼t due to twisting and in ∼t1/4 due to bending. Comparison with published data for ethidium–DNA complex reveals that the decay of the anisotropy arises primarily from twisting of the DNA helix, with a small contribution from bending. By fitting theory and experiment, the torsional rigidity C of DNA may be obtained.
Random coupling model for multiphoton photofragmentation of large molecules
Authors: Schek Israel , Jortner Joshua
Published: February 2003

In this paper we present a quantum mechanical theory of multiphoton photodissociation of large, collision-free, molecules, which rests on the notion that the radiative coupling terms between adjacent sets of congested bound molecular states in the quasicontinuum exhibit a wide variation both in terms of magnitude and of sign. Invoking the rotating-wave approximation, neglecting spontaneous infrared decay, and assuming that near-resonant radiative coupling prevails, the equations of motion were solved within the framework of the random radiative coupling model for the radiative interactions in the quasicontinuum. In the low energy range (range I) the equations of motion for the amplitudes are determined by the effective Hamiltonian formalism, while in the quasicontinuum (range II) the populations are governed by kinetic equations for sequential reversible decay. All the features of coherent excitation are preserved for range I, while in range II intramolecular erosion of phase coherence effects prevails. This model provides a set of reasonable predictions.
Calculation of rotational constants by asymptotic expansion
Authors: Chakraborty B. P. , Grieneisen H. P. , Francke R. E. , Gallas J. A. C.
Published: February 2003

An equation has been derived for the calculation of rotational constants, Bv, using an asymptotic expansion for Morse oscillators. Bv values calculated by the new equation have been compared with those calculated numerically by other methods, and those obtained experimentally.
Laser photoelectron, photodetachment, and photodestruction spectra of O−3
Authors: Novick Stewart E. , Engelking Paul C. , Jones Patrick L. , Futrell Jean H. , Lineberger) W. C.
Published: February 2003

Fixed frequency laser photoelectron spectrometry and variable frequency laser photodetachment and photodestruction spectroscopy of the ozonide ion, O−3, have been accomplished. The electron affinity of ozone is measured to be EA(O3) =2.1028(25) eV, in good agreement with previous measurements of less accuracy. Progressions in the spectra are analyzed to yield the symmetric stretching frequency and the bending frequency of the ozonide ion to be 982(30) and 550(50) cm−1, respectively. While no evidence is found for a long lived excited electronic state of O−3, an excited electronic state of neutral ozone is found roughly 0.7–1.1 eV above the ground state. Models for the dissociation of O3− are examined to explain why the photoelectron and photodetachment spectra fail to show a strong progression in the symmetric bending vibrational mode. Attempts to measure the electron affinity of CO−3 were unsuccessful. Limits placed by this attempt and our EA(O3) value are invoked in a discussion of some recent disagreements in the literature on the thermochemistry of CO−3 and O−3.
Electron–molecule scattering in momentum space
Authors: Ritchie Burke
Published: February 2003

We examine the Fourier transform of the Schrödinger equation for electron–molecule scattering, treated as potential scattering from a multicenter distribution of charged fixed in space. When the angle ϑ between R↘,the internuclear vector of a diatomic target, and q↘, the momentum transfer, is held fixed during the collision, then the directions of incidence and scattering are fixed relative to R↘. The process is then described as having a dynamical dependence on the magnitude of q↘, q, from which the scattering angle is determined, and a parametric dependence on q↘′s direction relative to R↘. This approximation is used routinely at high energies in the calculation of the Born amplitude. Fixed–nuclei coordinate–space studies suggest that this approximation can be extended to low energies, provided the amplitude is taken from the solution of the integral equation of momentum space rather than from its inhomogeneity, proportional to the Born amplitude. To obtain this solution, we use the above constraint in the inhomogeneity, and we adopt a similar constraint in the evaluation of the kernel of the integral term. That is, we constrain R↘ to be in the same direction relative to q↘′, a virtual momentum transfer belonging to the kernel, as it is to q↘. In this approximation, the process is described as having a dynamical dependence on two magnitudes, q and q′, and a parametric dependence on the single angle ϑR. This causes the equations for the ’’radial’’ or k space partial waves to be diagonal for each value of the orientation angle ϑR. Calculations are performed for the e, H2 scattering in the static approximation, and cross sections averaged over ϑR are shown to be in good agreement with cross sections calculated by use of coupled spherical and coupled spheroidal partial wave theories. The angular distribution in the static approximation is also calculated at an incident energy close to 7 eV, where exchange is relatively unimportant. This result is in reasonably good agreement with that of R matrix theory in the static–exchange approximation. The extension of the theory to treat exchange is formulated and discussed. Also its extension to treat more complicated molecular targets is discussed.
Optical absorption spectra of localized electrons generated at 1.6 K in polar matrices: Evidence for presolvated electrons
Authors: Dolivo G. , Kevan Larry
Published: February 2003

Glassy samples of ethanol, methanol +5 mol % water, 10 M and 18 M NaOH in H2O, 5 M K2CO3 and 7 M NaClO4 in D2O were x irradiated at 1.6 K. After irradiation the optical absorption spectra of the localized electrons generated were recorded at 1.6 K. Red shifts relative to spectra after irradiation at 4.2 K were observed in all matrices indicating stabilization of some presolvated electrons characterized by a nonequilibrium first solvation shell. Particularly in deuterated matrices, where observation further into the infrared is possible, a long infrared tail assigned to presolvated electrons is detected at 1.6 K; these infrared tails are not stabilized at 4.2 K. Analysis of the spectral shifts after irradiation at 77, 4.2, and 1.6 K indicates that an increasingly larger population of presolvated electrons is stabilized as the temperature is lowered and that a parallelism for electron solvation exists in all the matrices studied including ethanol. In the methanol and aqueous matrices, electron solvation is much more efficient than in ethanol, and the predominant population of localized electrons is largely solvated even at 1.6 K. The spectral shifts in 18 M NaOH glasses are also consistently interpreted in terms of presolvated electron stabilization and do not require postulation of (Na+–e−) pair stabilization as suggested in earlier work at 77 K.
Hydrodynamic theory for vibrational dephasing in liquids
Authors: Oxtoby David W.
Published: February 2003

A hydrodynamic model is presented for the dephasing of molecular vibrations in liquids; the effects of vibrational anharmonicity are included. Agreement with coherent picosecond pulse experiments and isotropic Raman line shape data is good for lower viscosity liquids, and the qualitative temperature and density dependence of the dephasing rate is predicted correctly. The final result shows a surprising formal similarity to the isolated binary collision model developed previously.
EPR studies of paramagnetic rhodium centers in LiH and LiD
Authors: Abell George C. , Bowman Robert C.
Published: February 2003

EPR spectra have been obtained for two independent paramagnetic Rh centers in UV-irradiated single crystals of LiH and LiD. The spin Hamiltonians for both species indicate S=1/2 and are assigned to Rh2+ ions with low-spin 4d7 electron configurations. Analysis of the anisotropic g-factor for the tetragonal center shows the unpaired electron is the Rh 4dx2−y2 orbital. The unpaired electron occupies the 4d3z 2−r 2 orbital in the orthorhombic center. The g factor, hyperfine constants, and protium superhyperfine constants for the Rh centers in LiH and LiD are compared with similar parameters for Rh2+ ions in several halide and oxide crystals. Although the isotropic protium hyperfine values indicate significant ‖1s≳−‖4d≳ orbital interactions, the total covalency in the Rh–H(D) bonds is rather small since the hydrogen ‖2p≳ orbitals cannot participate. The displacements of H ligands in the Rh centers have been determined from the direct-dipolar contributions to the superhyperfine tensor.
Comments on the magnitude and sign of the quasifree electron energy in the rare-gas solids as inferred from ESR studies of Frenkel impurity states
Authors: Edwards Peter P.
Published: February 2003

We point out that the magnetic properties of tightly-bound H atom impurities in dense, nonpolar solids are sensitive not only to local contributions to an effective potential, but also to electron– (bulk) medium interactions which dictate the nature of the ground state of an excess electron in these materials.
Spin densitites in triplet benzophenone
Authors: Chan I. Y. , Mushlin R. A.
Published: February 2003

The lowest photoexcited triplet state of benzophenone in single crystals of 4,4′-dibromodiphenyl ether has been studied by optically detected ENDOR. The hyperfine coupling tensors for eight protons and carbonyl–13C have been determined by analyzing the angular dependence of the ENDOR spectra. A point–dipole model is presented to account for the anisotropic hyperfine tensor elements based solely on spin densitites and molecular geometry. A set of spin densities and a ring–twist angle have been found which, together, best reproduce the observed dipole interaction tensors as calculated from our model. Data from zero-field ODMR experiments on 17O–benzophenone performed by S. Yamauchi and D. W. Pratt at the University of Pittsburgh enabled us to remove the correlation between n and π spin densities on oxygen. A final fit of all data shows the n electron to be localized on oxygen and the π electron to be delocalized onto the rings.
Fluorocarbons as oxygen carriers. I. An NMR study of oxygen solutions in hexafluorobenzene
Authors: Delpuecha) Jean-Jacques , Hamza M’hamed A. , Serratrice Guy , Stébé Marie-José
Published: February 2003

Relaxation times of 13C and 19F (or 1H) nuclei of hexafluorobenzene (or benzene) solutions of oxygen at 25° C are measured at several frequencies using variable concentrations of oxygen (up to 0.09 M under 6 atm). The influence of paramagnetic oxygen on neighboring solvent nuclei can be explained by dipolar electronic spin–nuclear spin interactions modulated by translation diffusion in the case of C6H6 solutions only. The results for C6F6 solutions suggest a discontinuous diffusion through the C6F6 liquid ’’lattice’’ with a short residence time (≲10−12 sec) in privileged positions of the lattice.
Raman spectrum of phase III of solid CH4 in the lattice and intramolecular regions
Authors: Medina F. D. , Daniels W. B.
Published: February 2003

Two samples of solid CH4 were grown under high pressures. The pressures of these samples at 10 K place them in phases III and IV, respectively. The Raman spectra of these samples in the lattice and intramolecular regions have been observed. These spectra provide no evidence of a III–IV phase transition. The lattice lines are assigned to translational or librational modes on the basis of their observed mode Grüneisen parameters. The translational modes are then assigned on the basis of a three-site structure recently proposed for phase III. The intramolecular bands of CH4 in phase III contain broad and sharp lines. The broad lines are assigned to lattice sidebands based on the volume dependence of their frequencies. The sharp lines are then assigned to the pure vibrational modes of three types of molecules in phase III.
On the Raman spectrum of argon dimersa)
Authors: Frenkel D. , McTague J. P.
Published: February 2003

The pressure broadening of the rotational Raman transitions of argon dimers is calculated, using the semiclassical method. Simple models to compute the line broadening of noble gas dimers are put forward. The predictions of these simple models are compared with the results of the full semiclassical calculation. The optimal experimental conditions to resolve the Ar2 Raman spectrum are discussed.
Extended accuracy for analytic model potentials for diatomic molecules
Authors: Huffaker J. N.
Published: February 2003

Formulas have recently been published which permit the determination of very accurate perturbed-Morse-oscillator (PMO) potentials from spectroscopic data. Many of the formulas were presented without a mathematically rigorous derivation. The present work contains such derivations, based on the use of Lagrange series techniques. The derivations are also modified to provide equivalent formulas for two other analytic model potentials, the Dunham and Simons–Parr–Finlan potentials. In addition, general relationships between parameters of the three models are presented, in terms of which one can directly obtain parameters of one model from those of another; the formulas are applied to determine parameters for n?20 for the other two models from published PMO parameters for CO. The suitability and appropriateness of the various models are discussed, especially in relation to problems of convergence and errors due to truncation and roundoff.
Application of field theoretical methods to the calculation of spectroscopic band shapes of molecules in strongly interacting solvents. IV. Microscopic interpretation of the extended rotational diffusion model
Authors: Paul R.
Published: February 2003

Methods of field theory have been used to derive exact formal expressions for the frequency Fourier transforms of spectroscopic memory functions. In particular these functions for free- and environment-interacting rotors have been obtained. The nature of the parameter τJ that appears in the extended diffusion models has been identified as being the imaginary part of the self energy. All theoretical methods based upon the memory function techniques must use approximations to the exact formal memory functions derived in this paper. In those regions in which hydrodynamic treatments are no longer valid, the parameter τJ must appear as a frequency dependent quantity; we have demonstrated this frequency dependence. Finally by using the finite temperature Green’s function method we have derived two expressions for τJ, one of which is of second order in the solute–solvent interaction potential and the other is a far more exact infinite order result. Both of these results show temperature dependence.
Elastic constants and diamagnetic anisotropy of p-disubstituted phenylcyclohexanes (PCH)
Authors: Schad Hp. , Baur G. , Meier G.
Published: February 2003

The elastic and diamagnetic properties of some p-disubstituted phenylcyclohexanes (PCH) are investigated by studying the elastic deformation of a nematic layer under the influence of electric and magnetic fields. The elastic constants kii (i=1, 2, 3) and the anisotropy Δχ of the diamagnetic susceptibility are obtained from capacitance and optical measurements on nematic layers of various geometrical configurations with respect to the direction of the applied fields.
Aerosol formation on the flash photolysis of SO2/gas mixturesa)
Authors: Fogel Laurence D. , Sutherland James W.
Published: February 2003

A long-lived transient absorption observed on the flash photolysis of SO2/gas mixtures at λ?190 nm has been identified as resulting from light scattering by H2SO4 aerosols. No detectable signals were monitored on photolysis at λ?270 nm, indicating that the aerosol precursors originated from the promotion of SO2 into its second singlet level and into its dissociation continuum. The SO3 that was formed was hydrated immediately to yield H2SO4 vapor in a highly supersaturated state and heteromolecular homogeneous nucleation to produce H2SO4 aerosols ensued. This nucleation was quenched rapidly as the acid vapor was consumed by further nucleation, by condensation, and by vapor diffusion to the cell walls. A model was formulated in which the condensations of the H2SO4 and the H2O vapors on the growing droplets were considered kinetically negligible and the particles grew by coagulation; simultaneously, they were lost by tranquil gravitational settling and by diffusion to the cell walls. Computer simulations demonstrated that the observed time dependence of the absorbance data (measured at a fixed wavelength) could be accounted for by this scheme. The effects of temperature, pressure, and wavelength (of the analyzing light) were also described satisfactorily by this model.
Statistical thermodynamics of short chain molecular liquids: n-Nonane
Authors: Jaina) R. K. , Simha Robert
Published: February 2003

The hole theory previously discussed primarily in connection with amorphous high polymers and polymer melts is considered for systems with finite chainlengths. It is shown that the reduced equation of state approaches the limiting expressions for infinite chainlength only slowly. However, isotherms and isobars are superimposable over the whole range of molecular weights, except at the upper range of temperatures. Application of the theory to the detailed experimental information provided by Grindley and Lind on n-nonane over a range of 120 deg and up to 8 kbar indicates a generally good agreement at atmospheric as well as elevated pressure. Only at moderate pressures and elevated temperatures appear numerically small but systematic deviations. These may be ascribed to a slow variation in the number of effectively external degrees of freedom, and hence of the scaling temperature with increasing temperature. Increasing pressures compensate for this effect. These discrepancies are also reflected in the behavior of the isothermal derivatives of the entropy and energy. The theory, however, predicts correctly the explicit dependence of these functions on temperature as well as volume.
Photoelectron spectroscopy of phthalocyanine vapors
Authors: Berkowitz J.
Published: February 2003

The 21.2 eV photoelectron spectra of metal-free phthalocyanine and some metal-containing phthalocyanines MPc (M=Mg,Fe,Co,Ni,Cu, and Zn) have been obtained for the gaseous molecules. Comparison of these spectra reveals that the uppermost occupied orbitals are ringlike, and not metal-3d-like, in all cases. Identification of some features in the 1487.6 eV photoemission spectra of thin films of phthalocyanines with similar features in the gas phase spectra enable one to establish an absolute energy scale for the thin film work, and to locate the ionization energies of the 3d-like orbitals. The experimental results and inferences are compared with recent ab initio calculations, and indicate that the Xα local density method is a promising one for describing the electronic structure of these large molecules and the generically related biological systems, chlorophyll and hemoglobin. In some spectra, phthalonitrile was identified as an impurity, and verified by obtaining the 21.2 and 40.8 eV photoelectron spectra of this molecule.
Microwave optical double resonance spectrum of NH2. V. Hyperfine structure of the u state
Authors: Lowe R. S. , Kasper J. V. V. , Hills G. W. , Dillenschneider W. , Curl R. F.
Published: February 2003

The hyperfine structure of the u state of NH2, a rovibronic level in the ? 2B1 manifold [tentatively assigned as 220, J=3/2, ? 2B1(0,13,0)] has been obtained from an analysis of the strong microwave optical double resonance (MODR) signals which occur between the u state and the J=1/2 and 3/2 spin components of 110 in ?2 A1Π (0,10,0). The hyperfine assignment was verified with a new technique that we have chosen to call microwave modulated saturation (MMS) spectroscopy. The similarity of the hyperfine splittings in u due to the H nuclear spin with those of the 220, J=3/2 rotational level of the ? 2B1(0,0,0) state supports the assignment of u to a highly excited vibrational state of ? 2B1. The hyperfine splittings due to the 14N nucleus were found to be about 1 MHz. These splittings are considerably smaller than those of 220 X 2B1(0,0,0) (which are about 6–10 MHz). This difference has been attributed to the fact that the hyperfine coupling constants of (0,13,0) must be averaged over a large amplitude bending motion and to the presence of borrowed ? 2A1 character in the u state.
A molecular dynamics and Monte Carlo study of solvent effects on the conformational equilibrium of n-butane in CCl4a),b)
Authors: Rebertus D. W. , Berne Bruce J. , Chandler David
Published: February 2003

We report on a computer simulation study of the gauche–trans conformational equilibrium of n-butane in liquid carbon tetrachloride solvent. The study is made possible by implementing an exact statistical mechanical theorem which relates the full intramolecular distribution function for a butane molecule to that of a hypothetical species which does not possess a large potential barrier separating the trans and gauche states. In addition to determining the trans–gauche equilibrium constant, the potential of mean torsion, that is, the reversible work required to alter the conformation is determined as a function of the dihedral angle. Recent theoretical work is compared with these computer experiments, and while qualitative agreement is found, the approximate theory overestimates the solvent effect. Finally, the change in solvent structure in response to a conformational change in the solute is determined.
Proton dynamics and surface heterogeneity of silica gel with adsorbed benzene below one monolayer coveragea)
Authors: Peterson E. M. , O’Reilly D. E. , Tsangb) T.
Published: February 2003

Proton and deuteron NMR relaxation times of C6H6, C6D6, and mixtures of these molecules have been measured on a superpure silica gel (SPSG) and a sample of a Matheson silica gel (MSG) both dehydrated at 600° C and rotational (intramolecular) and translational (intermolecular) correlation times have been computed from the relaxation time data at a statistical coverage ϑ=0.6. Three kinds of adsorption sites have been observed: (1) A sites, which are probably oxygen vacancies on the surface, (2) B sites which are assigned to paired hydroxyl groups on the surface, and finally (3) C sites which comprise 80% of the occupied surface and are primarily isolated hydroxyl groups. Rotational and translational motions are highly correlated for the A and B site molecules. The mean number of molecules clustered at the A and B sites are inferred from the intermolecular second moments associated with each of these sites. The surface density of the A sites is 1.1×1012 cm−2 for SPSG and 3.1×1012 cm−2 for MSG.
On the heat of formation of NH
Authors: Piper Lawrence G.
Published: February 2003

Experiments giving information on the heat of formation of NH have been reanalyzed. It has been found that studies on the dissociative excitation or ionizaton of NH-containing species give upper limits to ΔH°f0(NH) substantially lower than the JANAF value of 3.90 eV. The most reliable of these experiments give an upper limit of 3.63±0.10 eV in agreement with recent spectroscopic studies. The experiments given most weight in the JANAF compilation have been reviewed. All have been found to be more compatible with a lower heat of formation of NH than 3.9 eV. It is recommended that a new value of 3.65±0.10 eV be adopted for ΔH°f0(NH).
On the rotational energy sudden approximation in vibrationally inelastic collisions
Authors: Secrest Don , Lin C. S.
Published: February 2003

In the rotational energy sudden approximation, certain relations exist among the differential and integral cross sections. These relations are expanded to encompass the vibrational degree of freedom. The extent to which these relations hold for exact cross sections gives an indication of the minimum error one might expect when the energy sudden approximation is used. An evaluation is made of the extent to which these relations are satisfied for He–H2 collisions. The He–H2 system is one, for which the conditions under which the energy sudden approximation is valid, are poorly satisfied. Thus, these calculations represent a worst case example for the energy sudden approximation and exhibit the greatest error one is likely to encounter in the use of the approximation. One may expect improvement in the accuracy of these relations whenever the validity conditions are better satisfied. Several features of the cross sections which the approximation cannot duplicate are pointed out.
Pressure broadening of the dipole and Raman lines of CO2 by He and Ar. Temperature dependence
Authors: Pack Russell T
Published: February 2003

Pressure broadening of CO2 lines by He and Ar is studied using a priori (electron gas plus van der Waals) potential energy surfaces and infinite-order sudden approximation dynamics. The linewidths of both dipole line (from vibration–rotation transitions) and Raman lines (from pure rotational transitions) are calculated as functions of the temperature over the range 200 to 800 °K. The dependence on uncertainties in the potential energy surfaces is also explored. Experimental results are available only at room temperature; agreement of the calculated results with them is good for small rotational quantum numbers j and deviates about as expected for large j. Accurate experimental studies of the temperature dependence of pressure broadening are suggested.
Jahn–Teller, pseudo Jahn–Teller coupling, and circular dichroism spectra of (E+A) e systemsa),b)
Authors: Zgierskic) M. Z. , Pawlikowski M.
Published: February 2003

The vibronic coupling problem in a (E+A)e system of molecular levels is analyzed. Both Jahn–Teller and pseudo Jahn–Teller interactions are taken into account. The Jahn–Teller interaction is assumed to be linear and/or quadratic, whereas the pseudo Jahn–Teller interaction is taken linear in the doubly degenerate nuclear coordinate. Systems of vibronic levels are discussed for different sets of vibronic coupling parameters and energy gaps, and corresponding chiro−optical spectra are presented.
Ultraviolet absorption by diatomic rare gas ions in E-beam excited plasmas
Authors: West John B. , Long William H.
Published: February 2003

Absorption has been measured at 295 and 320 nm in electron-beam excited mixtures of argon, krypton, and fluorine. The absorbing species are identified as the diatomic rare gas ions Ar+2, ArKr+, and Kr+2. By using kinetic models to calculate ion number densities in the absence of fluorine, absorption cross sections are determined for these species. At 295 nm, they are 5.8×10−17 cm2 for Ar+2, 1×10−17 cm2 for ArKr+, and 2.6×10−17 cm2 for Kr+2. At 320 nm, the cross sections are 5.4×10−17 cm2, 2.5×10−17 cm2, and 3.1×10−17 cm2, respectively. Errors of ±18% are estimated for these values. Absorption in mixtures containing fluorine is higher, and the presence of additional absorbing species is indicated. The most likely candidates are the triatomic excimers Ar2F, Kr2F, and possibly ArKrF.
The vibrational infrared spectrum of the group IV transition metal nitride gaseous molecules
Authors: DeVore T. C. , Gallaher T. N.
Published: February 2003

The infrared spectrum of TiN, ZrN, and HfN has been observed in absorption by using a microwave discharge through MCl4/N2 mixtures as the molecular source. The molecular constants determined for each molecule are as follows: The agreement between the values of the rotational constants reported previously for TiN and ZrN establishes that each molecule has a 2Σ ground state. The appearance of the infrared spectrum indicates that HfN also has a 2Σ ground state.
Quantum corrections to the equation of state of a fluid with a triangular-well potential
Authors: Sinha A. K. , Sinha S. K.
Published: February 2003

Using the ’’modified’’ WK expansion, we have extended the method of Kemmer and Jancovici to cover more general hardcore plus attractive tail potenials and given explicit expressions for the first two quantum corrections to the free energy. This enable us to derive the general expressions for the second and third virial coefficients of the equation of state. The theory is applied to evaluate the first two quantum corrections to the third virial coefficient of a fluid with the triangular-well potential. The result is reported in the ascending power of (λ/d).
Detailed dynamics of collinear F+H2 trajectories
Authors: Hutchinson John S. , Wyatt Robert E.
Published: February 2003

Details of classical trajectory dynamics are analyzed to develop the relationships between classical and quantum results, and between classical and statistical results. A definition of the vibrational energy in the transition region is introduced which allows comparisons to earlier quantum results. Trajectory ensembles are seen to model the quantum wavefunction well in the early stages of the reaction. The failures of trajectory studies are due to wells in the local vibrational levels of the transition states. Action-angle plots and Poincaré surfaces of section display the stochastic behavior of some trajectory sets. This behavior is shown to be predicted by a new quantity, the stochastic impulse integral, which relates trajectory dynamics to the applicability of statistical theories.
A stochastic solvent model for electron exchange reactions
Authors: Efrima Shlomo , Bixon Mordechai
Published: February 2003

Outer sphere electron transfer reactions are investigated in the framework of a semiclassical theory with a density matrix representation. The coupling of the electron exchange system to the polar solvent is represented as a stochastic Gaussian process. Expressions for the transition probability in the nonadiabatic case are derived in terms of the correlation function for the fluctuating coupling. Some models for the correlation functions are discussed together with explicit results for the transition probabilities.
Charge transfer to a solvent. II. Luminescence studies of tryptophan in aqueous solvent at 300 and 77 K
Authors: Truong Thu Ba
Published: February 2003

The phosphorescence of tryptophan, viewed at λa=440 nm, can be observed several seconds after irradiating the solid solutions of tryptophan in neutral or acidic 4.5M CaCl2 ices or in pure ices at 77 K with visible light of energy 2.3 and 2.9 eV (546 and 435 nm), respectively. Based on our results of optical absorption and luminescence studies, we interpret these results in term of a charge transfer (CT) exciplex between the tryptophan and the solvent matrix. A possible decrease of ∼5.0 eV in the ionization energy of tryptophan in these media at 77 K is discussed.
Charge transfer to the solvent state. III. Luminescence of tryptophan and indole in aqueous alkaline solvent at 300 and 77 K
Authors: Truong Thu Ba
Published: February 2003

In strong alkaline ices (?12M OH−), the emission of 2.8 eV of indole and tryptophan can be induced by monophotonic light of 2.85 eV. Following previous observations, we interpret this emission is due to (a) the direct phosphorescence of the solute–solvent exciplex, (b) the phosphorescence of the excited chromophore which is photodissociated from the exciplex during irradiation, and (c) the spontaneous recombination luminescence due to back reaction (chromophore cation +e−). The red end of the solute–solvent charge transfer absorption band is thus associated with prephotoionization and its tail end energy is approximately equivalent to the ionization energy Ip of the exciplex. The Ip for the solute–solvent exciplex and for the solvated electron in different aqueous matrices are linearly dependent. This leads to our deductions that (a) the solute–solvent exciplex is a ’’solvated anion’’, (b) the solvent electron affinity (E.A.) plays an important role in photoionization processes in solid solutions (increasing the solvent E.A. would cause an increase in the solute Ip), and (c) at least in the solid phase, photoionization seems to proceed from an excited ’’solvated solute’’ or a CTTS state.
Ionization potentials and electron affinites for a spacially nondegenerate doublet state in a second-order electron propagator approach
Authors: Albertsen Preben , Jo/rgensen Poul
Published: February 2003

The identification of poles in approximate electron propagator calculations on open shell systems has been analyzed. The electron propagator consistent through second order in electronic repulsion has been derived for a 2Σ reference state. Calculations are performed on Li, Na, and BeH and the accuracy of the result is comparable to the one obtained for closed shell systems.