bluuebunny/crossref_metadata_2025 huggingface dataset:
Deductions about the structure of phase III from thermodynamic measurements on solid isotopic methanes
Authors: White M. A. , Morrison J. A.
Published: February 2003

In a continuing study of the solid isotopic methanes, the heat capacity of solid CHD3 has been measured in the range 0.15<T<3 K. Some structure is found in a Schottky anomaly in the region of the measurements and it is related to the composition of nuclear spin symmetry species in the solid. There is no evidence of spin conversion. The entropy of CHD3 has been calculated as a function of temperature from the heat capacity and other data, and is used to make deductions about quantum disorder in the solid. Combined results for CH3D, CH2D2 and CHD3 lead to the conclusion that the structure of phase III of solid methane is quantum disordered and that it must contain at least three types of sublattice. A model consisting of two sublattices with tetrahedral molecular fields and one with symmetry lower than tetrahedral, accounts for the experimental observations satisfactorily.
The 2 1A
g–1 1B
u energy gap in the polyenes: An extended configuration interaction study
Authors: Tavan Paul , Schulten Klaus
Published: February 2003

For a correct account of the ordering of excited states in the polyenes, in particular the low-lying 2 1Ag and 1 1Bu states, single as well as double excited configurations must be included in a CI expansion. However, for longer π systems such expansion shows, in contrast to spectroscopic observations, a divergence of excitation energies and a reversal of the 2 1Ag and 1 1Bu state ordering. We have therefore extended the CI expansion to include all triple and quadruple excitations for the polyene series CnHn+2’ n=4,6,8,10,12 and n=4,6,8,10, respectively. In our calculations we employed a PPP model Hamiltonian. The extended CI expansions correct the faults of previous treatments and predict an increase of the 2 1Ag–11Bu energy gap with increasing polyene length in agreement with recent spectroscopic observations. The effect of higher excitations is mainly due to triple excitations involving three simultaneous spin flips of ground state electrons.
Local mode overtone intensities of C–H stretching modes in alkanes and methyl substituted benzenes
Authors: Burberry Mitchell S. , Morrell James A. , Albrecht A. C. , Swofford Robert L.
Published: February 2003

A thermo-optical spectrometer is used to resolve absorption peaks due to nonequivalent C–H local mode vibrations and combination bands in several molecules containing aromatic, primary, and secondary hydrogens. Although the bandwidths vary, a constant absorption cross section per C–H is observed for the transitions to the v=3,4,5, and 6 overtone levels. This result stands in contrast to the behavior of the fundamental transitions, where C–H stretching intensities are seen to vary by as much as a factor of four between aromatic and aliphatic C–H bonds.
On the picosecond kinetics and photostability of indigo and 6,6′-dimethoxyindigo
Authors: Kobayashi T. , Rentzepis P. M.
Published: February 2003

Kinetics of the ground state repopulation and the decay lifetime of the lowest excited singlet state of indigo were measured by the use of picosecond spectroscopy. The photostability of indigo and its ring-substituted homologues can be reconciled in view of the ultrafast internal conversion from the lowest excited singlet state to the ground state enhanced by the proton transfer in the excited state.
Optical probe studies of relaxation processes in viscous liquids
Authors: Barkatt A. , Angell C. A.
Published: February 2003

The question of relaxation time distributions in viscous liquids is examined by observing time-dependent behavior of optically active elements of the liquid structure, both intrinsic and otherwise, in response to perturbations from equilibrium states. As a suitable medium for this study the polyalcohol sorbitol C6H8(OH)6 is adopted. Equilibrium distributions of intact and broken hydrogen bonds in the pure sorbitol have been determined using near IR spectra, and the loss of, and re-establishment of, this equilibrium, during down and up temperature ramping processes, has been monitored and analyzed. The hydrogen-bond equilibrium is established on essentially the same time scale and with essentially the same distribution of relaxation times as for the enthalpy. (The latter has been monitored by differential scanning calorimetry). Analysis shows that the kinetics of the bond-breaking process are well accounted for by a relaxation function, near equilibrium, of the form φe(t) =e−[(t/τ0)β], with β=0.55±0.03. As extrinsic probes, two carbocyanine dyestuffs have been introduced in low concentrations into the sorbitol, and the equilibrium distribution of monomer and dimer species has been determined as a function of temperature. The loss and recovery of the equilibrium has also been monitored during down and up temperature ramping experiments. It is found that these equilibria are lost and recovered at temperatures far above that at which solvent structure equilibrium is established on the same time scale, showing that equilibration times for the monomer–dimer equilibrium are many orders of magnitude longer than for solvent structure. In this case the probe equilibrium is established with a single relaxation time, which depends on concentration, but which is not consistent with second order reaction kinetics. Finally, the distribution of Co2+ ion probes between octahedral Co(−OH)62+ and CoCl42− sites in sorbitol + pyridinium chloride solutions has been studied at equilibrium and then monitored during temperature ramping processes. In this case the probe ion octahedral–tetrahedral equilibrium is established on the same overall time scale as the enthalpy and the hydrogen bond equilibrium, but the ’’spectrum of times’’ is distinctly narrower, i.e., the β parameter in the relaxation function is much closer to unity. This is consistent with previous observations for local structure processes in a purely ionic solvent. All observations are consistent with the idea that the spectrum of relaxation times which is observed for a given relaxation process depends on the volume of the system subspace which must be explored to relax most of the imposed stress.
Simple bond rupture reactions in multiphoton dissociation of molecules
Authors: Sudbo/ Aa. S. , Schulz P. A. , Grant E. R. , Shen Y. R. , Lee Y. T.
Published: February 2003

A molecular beam study was carried out of the infrared multiphoton dissociation of seven molecules dissociating by a simple bond rupture. Translational energy distributions for the dissociation products were obtained, and the results can be described very well by a statistical (RRKM) theory of unimolecular reactions. A general discussion of the main features of the multiphoton dissociation process is given.
Vibrational relaxation of matrix-isolated HCl and DCl
Authors: Wiesenfeld Jay M. , Moore C. Bradley
Published: February 2003

Using infrared laser-excited vibrational fluorescence, lifetimes have been measured for HCl (v=1,2) in Ar as a function of concentration and temperature, and for dilute samples of DCl (v=2) in Ar as a function of temperature. For dilute samples at 9 K, the nonradiative decay rates (s−1) are (8.1±0.7) ×102 and (3.8±0.4) ×103 for HCl (v=1,2), and (1.2±0.2) ×102 for DCl (v=2), respectively (the DCl decay may be partially radiative). These rates increase slightly (less than a factor of 2) between 9 and 21 K. The results are consistent with a relaxation process in which the rate-determining step is intramolecular V→R transfer. No fluorescence signal is observed upon excitation of the overtone of the HCl dimer in Ar implying that relaxation of the dimer is faster than 0.3 μs. In concentrated samples the relaxation rate for HCl (v=1) (but not v=2) increases, due to energy-migration aided V→V transfer to dimers, which are an energy sink. It is argued that this system belongs to the ’’fast migration’’ regime. In N2 and O2 matrices, there is no fluorescence following excitation of HCl (v=2), presumably due to rapid V→V transfer to the host. Spectroscopic measurements of overtone absorption transitions are reported. A measurement of the integrated absorption coefficient for HCl in Ar indicates that the transition dipole moment differs less than 9% from the gas phase value.
Semiclassical study of reactive scattering in a laser field: F+H2+h/ω (1.06 μm) system
Authors: Yuan Jian-Min , George Thomas F.
Published: February 2003

Semiclassical calculations of collinear F+H2(v=0) reactive and inelastic scattering in an Nd-glass laser field of various intensities have been carried out. The decoupling approximation developed for the Miller–George theory has been used for nonadiabatic transitions between the electronic-field surfaces. Special attention is paid to the choice of a proper coordinate system for applying the decoupling approximation. The total reaction probability and population ratio of the v=3 over v=2 vibrational state of HF increase suddenly as the field intensity increases beyond 10 TW/cm2. (It is suggested that for a three-dimensional system the threshold field intensity would be smaller.) Results also show that in a laser field the H2 molecule can be vibrationally excited while the F atom is electronically excited at the same time.
Three body electron–ion recombination in molecular gases
Authors: Warman John M. , Sennhauser Eric S. , Armstrong David A.
Published: February 2003

The rate constant for the recombination of electrons with positive ions, α (cm3 s−1), has been measured at 292 K as a function of pressure in gaseous H2O, NH3, (CH3)2CO, CH3C1, CO2, and neo-C5H12. In all cases α is found to increase with increasing gas density, N (molecules cm−3), and over the density ranges studied can be described by a combination of a two and three body process, i.e., α=α2+α3N. In the same order of the gases as given above, the values found for 106 α2 are 4.1, 5.6, 6.4, 7.1, 3.9, and 7.0 cm3 s−1 and for 1025 α3 are 270, 69, 55, 26, 5.8, and 2.5 cm6 s−1. The values of α3 are discussed in the light of existing theory together with the known characteristics of electron neutral collisions in the gases studied.
Theory of phase transitions in solid methanes. XII. Orientational order of molecules in phase III
Authors: Maki Kazuo , Kataoka Yosuke , Yamamoto Tsunenobu
Published: February 2003

In order to elucidate the structure of methane solids in phase III, orientational orderings of molecules are studied on the basis of a model potential consisting of a crystalline field (one-body potential), octopole–octopole type, octopole–hexadecapole type, and hexadecapole–hexadecapole type potentials, which are obtained through a multipole expansion of the sum of interatomic Lennard-Jones type potentials. It is assumed that carbon atoms are fixed at sites of a fcc lattice, and that the orientational order has the period not larger than twice the lattice constant of the carbon fcc lattice. In the framework of classical molecular field approximation, we solve consistency equations for molecular fields, and examine stabilities of various solutions which branch off from phase I (disordered phase) or from phase II (partially ordered phase). To this end, Landau’s theorem on the phase transition of second order is extended such that it allows us to determine possible space groups for the lower temperature phase which are related to a given irreducible representation of the space group of the higher temperature phase. Two parameters are introduced to adjust the strengths of the multipolar interactions employed. Solutions with high branching temperatures are studied in detail within a range of parameters which yields the correct structure (Fm3c) for phase II. Referring to the extinction rules found by neutron diffraction experiments on CD4, we conclude that the structure of phase III is described by a solution with the tetragonal space group P42/mbc containing 16 molecules per unit cell (a subgroup of Fm3c). Inclusion of the octopole–hexadecapole type potentials is found to be most essential for stabilizing this structure relative to phase II. Changes in site symmetries at the transition II → III are O→D2, D2d →S4, and Cs. Thus the O-molecules in phase II also become ordered in phase III. However, in comparison with strong molecular fields at S4 and Cs sites, molecules at D2 sites are subjected to a much weaker molecular field. Approximating this multisite structure by a two-site model in tetrahedral fields, we analyze the observed heat capacity anomalies of CH3D and CH2D2 in phase III, and predict their tunneling level structures. Infrared and Raman spectra are also discussed. It is also found that a solution, C2/c containing four molecules per unit cell, becomes most stable at 0 K if the values of the hexadecapolar parameters are reduced and the crystalline field is strengthened.
Low-lying excited electronic levels of Cs2UO2Cl4
Authors: Leung A. F. , Tsang K. K.
Published: February 2003

The new electronic energy level which is at 311 cm−1 above the fluorescent doublet has been confirmed by fluorescence decay measurement. The fluorescent doublet is shown to belong to the same uranyl ion rather than two fluorescent singlets associated with two different uranyl ions. Two fluorescence decay rates are observed, and they are attributed to the intrinsic relaxation of the crystal and the relaxation of impurities or defects. The low-lying energy levels of the impurities or defects are described. In addition, theoretical calculations on the temperature dependence of the fluorescence decay rates can be improved by including a temperature dependent contribution from multiphonon processes.
Lattice relaxation model of the n=1 exciton emission in solid Ne
Authors: Kunsch P. L. , Coletti F.
Published: February 2003

The emission of pure solid Ne is mostly due to atomically trapped n=1 excitons. Using the ab initio repulsive potentials of the molecular excited states and the long-range vander Waals interactions the isotropic cavity formed around an isolated Ne* atom is calculated. The lattice relaxation is extended out to the 16th shell. The static and dynamical shifts obtained in this localized description are found to be in good agreement with experimental data.
Practical application of extended valence bond diatomic calculations to the method of diatomics-in-molecules for NeHe+2
Authors: Kendrick J. , Kuntz P. J.
Published: February 2003

Potential energy curves for the lowest two 2Σ states and the lowest 2Π state of NeHe+ were calculated using the valence bond method with minimal and extended basis sets. It is shown how this ab initio information, together with information on the He2, He+2, and NeHe molecules obtained from other sources, can be incorporated into a minimal basis diatomics-in-molecules (DIM) description of NeHe+2. Particular emphasis is given to the incorporation of the extended ab initio wave functions into the DIM procedure. Two sets of semiempirical DIM potential energy surfaces for NeHe+2 were computed using the minimal and extended valence bond diatomic wave functions and energies as input. These surfaces are compared and discussed with regard to other electronic structure calculations on NeHe+2. The lower potential surfaces for NeHe+2 are particularly relevant to the collision-induced dissociation of He+2 by Ne to form Ne+ ions.
Vibrational distribution of CO and NO excited by electronic-to-vibrational energy transfer collisions with Hg(6 3P1 and 6 3P)
Authors: Horiguchi Hiroyuki , Tsuchiya Soji
Published: February 2003

The initial vibrational distributions of CO and NO excited by electronic-to-vibrational (E–V) energy transfer collisions with Hg1* (Hg in the 6 3P1 state) and Hg0* (Hg in the 6 3P0 state) have been determined by the measurements of the IR emission spectra of CO and NO utilizing a modulation technique. The Hg resonant radiation to excite Hg to Hg1* in Ar(N2) –CO or –NO mixtures was modulated in its intensity with a frequency in the range of 120 Hz to several kHz. Observation of the resulting in-phase and quadrature components of the IR emission made it possible to separate the contribution of the initial vibrational excitation to each level from that of the vibrational relaxation. In Hg*1–CO collisions, Hg1*→Hg*0 transition occurs with simultaneous excitation of CO to the v=1 level. The fraction of the electronic energy of Hg* converted to the vibrational energy of CO or NO are 27%, 32%, and 31% for Hg*0–CO, Hg*1– and Hg*0–NO collisions, respectively. The vibrational distributions resemble the Poisson type in agreement with those estimated on the basis of the impulsive ’’half-collision’’ model. However, in Hg*0–CO collisions, the model may not reproduce strictly the observed distribution, in which more populations in levels of v=4, 5, and 6 and less in those of v =1, 2, 8, and 9 are found. Contrary, in Hg*1– or Hg0*–NO collisions, the model results in a distribution which accords almost with the observed one, though the populations are found in levels of v?12, which exceed the highest level that can be achieved in the impulsive limit. All of the observed distributions deviate significiantly from the statistical prior distribution giving nonlinear surprisal plots.
Mössbauer analysis of Fe2O3–MoO3 supported on SiO2 and Al2O3
Authors: Carbucicchio M.
Published: February 2003

Mössbauer analysis has been carried out for Fe2O3–MoO3 supported on SiO2 and Al2O3 with different surface areas. It has been found that the supports are not inert towards the active species. Both Fe and Mo, either substituting the cations of the supports or interstitially locating, enter the silica and the alumina. In the case of alumina, Al2 (MoO4)3 containing iron can also grow. However, these compound form only at the support surface: supported Fe (III) –molybdate can grow only on this film.
Application of Redfield theory to optical dephasing and line shape of electronic transitions in molecular mixed crystals
Authors: de Bree Philippus , Wiersma Douwe A.
Published: February 2003

Redfield relaxation theory is used to study optical dephasing in molecular mixed crystals where pseudolocalized phonons play a dominant role. As a model an effective four-level system is defined which consists of the ground and excited electronic state of the guest impurity and one pseudolocalized phonon in each of these states. It is shown that coherence transfer between the purely electronic and the phonon-promoted transition is due to interference between the scattering amplitudes in the ground and excited state and only occurs when the transitions are near resonant. When the scattering amplitudes in the ground and excited state are identical the optical Redfield equations reduce to the modified Bloch equations and coherence transfer may appreciably contribute to optical dephasing. It further proves essential for coherence exchange to occur that next to electron–phonon coupling the phonon anharmonicity is taken into account. The stochastic and correlation function theories of line shape, using the same model system, have also been examined and we conclude that in these theories exchange of coherence is not properly taken into account. Next to the effect of pseudolocalized phonons the effect of band phonons on optical dephasing is also examined and within our model both effects are additive.
Rotational excitation of symmetric top molecules by collisions with atoms. II. Infinite order sudden approximation
Authors: Green Sheldon
Published: February 2003

The infinite order sudden approximation is extended to rotational excitation of symmetric tops by collisions with atoms. The formalism is developed first for ’’primitive’’ or ’’one-ended’’ tops. The proper parity adapted linear combinations which describe real rotors are then considered. Modifications needed for asymmetric rigid rotors are noted briefly. For all of these cases generalized spectroscopic relaxation cross sections are discussed; these include degeneracy averaged state-to-state integral cross sections and pressure broadening cross sections as special cases. In the IOS formalism these cross sections factor into dynamical terms which contain all the system dependent collision information and spectroscopic coefficients which contain the dependence on rotational levels and angular momentum coupling. This factorization is similar to that obtained for linear rotors, but differs insofar as the dynamical factors cannot all be expressed in terms of single state-to-state cross sections and insofar as the generalized cross sections are no longer necessarily real quantities. IOS calculations for NH3–He and H2CO–He have been performed to compare with earlier, more accurate calculations. Both of these systems provide fairly severe tests since they have rather widely spaced energy levels with only a few levels accessible at thermal energies whereas the IOS approximation should be best for the opposite case of many closely spaced levels. Nevertheless, the IOS approximation is found to provide a reasonably accurate description for these systems. Finally, implications of the IOS factorization for the interpretation of microwave double resonance experiments for these systems is considered.
Vibrational spectra and carbon–hydrogen stretching mode assignments for a series of n-alkyl carboxylic acids
Authors: Hill Ian R. , Levin Ira W.
Published: February 2003

The Raman and infrared spectra for the C4, C8, C12, and C16 carboxylic acid series were obtained at liquid nitrogen temperatures, in the 2900 cm−1 spectral region and were compared to spectra for selected branched and straight chain alkane systems. For hexadecanoic acid-d3 the Fermi resonance interaction between the CD3 symmetric stretching mode and the overtone level of the CD3 asymmetric deformation vibration was examined as a function of physical state. In this example of a negative Fermi resonance perturbation, in which the lower frequency overtone component is more intense than the higher frequency fundamental, the coupling increases as the system passes from either the matrix isolated species or solution phase to the polycrystalline state. The spectra of the carboxylic acid series, as well as the spectra of 2,2,4-trimethyl pentane and n-octane, suggest that in the 2900 cm−1 region an analogous Fermi resonance coupling exists between the CH3 symmetric stretching vibration at ∼2938 cm−1 and the 2871 cm−1 overtone transition of the CH3 asymmetric deformation. Unfortunately, the 2930–2940 cm−1 region for the solid systems is further complicated by transitions attributed to crystal interactions. Although the Fermi interaction for the methyl group has been recognized in the simpler halomethane system, for example, it has not been previously discussed in the more general context of significantly larger branched and straight chain molecules. Comparisons of the carboxylic acid spectra in the 2900 cm−1 region have also led to assignments for the α-CH2 (adjacent to the carbonyl group) and the ω-CH2 (adjacent to the methyl group) C–H stretching modes.
The potential energy curves of ArC+
Authors: Hillier I. H. , Guest M. F. , Ding A. , Karlau J. , Weise J.
Published: February 2003

Potential energy curves for different states of the molecule ArC+ have been determined by experimental and theoretical methods. Elastic scattering cross sections obtained from the scattering of C+(2P) and C+(4P) on Ar were used to determine the bound ground state X 2Π with the use of an inversion method, and to discuss qualitatively higher excited states. CI calculations performed for several doublet and quartet states dissociating into C+(2P)+Ar(1S), C+(4P)+Ar(1S), and C(3P) +Ar+(2P) are in good agreement with the experimental findings. The nature of the bonding is discussed and spectroscopic constants for the bound potential curves are given.
Vibronic spectra of the benzene crystal at 4.2 K using two-photon fluorescence excitation
Authors: Hochstrasser R. M. , Klimcak C. M. , Meredith G. R.
Published: February 2003

The two-photon excitation spectra of benzene crystals have been determined in the range 530 to 400 nm. The spectra can be related to approximately C2h symmetry benzene molecules. The E-type bands are all split by the crystal field as is already known for the one-photon absorption, the Raman, and the infrared spectra. The study has allowed the determination of the two-photon tensors for the eight ungerade vibrational fundamentals ν11, ν14–ν19 in the crystal axis system. The oriented gas transformation provides in this case an approximate diagonalization of the crystal axis based tensors. The two-photon excitation spectra are rich in two particle effects.
Time-resolved spectroscopy on the excited electronic states of NO2 in the neighborhood of 454.6 nm
Authors: Imasaka Totaro , Ogawa Teiichiro , Ishibashi Nobuhiko
Published: February 2003

The time-resolved excitation spectrum and the time-resolved fluorescence spectrum were measured by using a pulsed, tunable dye laser (Δt=4 ns, Δλ=3.5 pm) and a pulse-gated photon counter (Δt=4 ns). The three kinds (one 2B1 and two 2B2) of the excited electronic states of NO2 were resolved in the 454.3–454.8 nm region. Sharp Q and R rotational lines of the transition to the (0, 8, 0) vibrational level of the 2B1 excited electronic state appeared in the time-resolved excitation spectrum monitored through the ν2 band fluorescence. Meanwhile sharp lines and a continuum band were observed in both excitation spectra monitored through the ν1 and ν2 band fluorescence. These sharp lines could be assigned to the transition to the (1, 10, 0) vibrational level of the 2B2 state. The continuum band seems to be dominantly composed of many rotational lines to the perturbed 2B2 states.
A theory of dipole-forbidden vibrational excitation of molecules adsorbed on metal surfaces in the Born approximation
Authors: Thomas G. E. , Weinberg W. H.
Published: February 2003

The probability of vibrational excitation by slow electrons of a periodic array of molecules adsorbed on a metal surface is calculated in the Born approximation. By including both dipole and quadrupole terms, a mechanism is derived for the diffuse scattering of electrons by dipole-forbidden transitions. A comparison of this scattering probability with recent measurements reported for scattering from the β1 phase of H on W (100) serves to illustrate the magnitude of this effect in terms of the dipole and quadrupole derivatives. Dipole-forbidden scattering by this mechanism is expected to be two to three orders of magnitude weaker than dipole scattering in the specular direction, but may well make a significant contribution when considering scattering out of the specular direction.
Ultrasonic study of critical behavior in ND4Cl
Authors: Zahradnik C. , Garland C. W.
Published: February 2003

The adiabatic elastic constants c11 and c44 and the [100] longitudinal attenuation have been measured in single crystal ND4Cl, with emphasis on the region of the order–disorder transition. Unlike NH4Cl, the transition is almost continuous at 1 atm. Not only the first-order instability but also the variation of c11 in the disordered phase is very sensitive to deuteration. In contrast, the variation of c11 in the ordered phase is relatively insensitive to deuteration and can be well represented by a Pippard relation. The overall pattern of changes in velocity and attenuation on deuteration is very similar to those observed in NH4Cl on going from 1 atm to 1500 bar. The dynamical behavior corresponds to the conventional theory of critical slowing down, which is consistent with the prescence of a multicritical point in ND4Cl.
Analytical potentials from a
b i
n
i
t
i
o computations for the interaction between biomolecules. V. Formyl-triglycyl amide and water
Authors: Ragazzi M. , Ferro D. R. , Clementi E.
Published: February 2003

The interaction energy of formyl-triglycyl amide (FTGA) with water has been analyzed following the techniques described in the previous papers of this series, where the interaction of water with the naturally occurring amino acids (and some of their zwitter ions) and with the four bases of DNA were examined. FTGA has been chosen in order to obtain a realistic atomic interaction potential of water with the backbone of proteins. Since the peptide linkage (CO–NH) and the terminal formyl group are formed by atoms that are in a different electronic environment than those previously analyzed in this series of papers, new ’’classes’’ of atoms have been added to our pairwise potential library. In the computation of the wavefunction and energy of the FTGA–water complex, we have considered two different conformations of FTGA and a total of 158 positions and orientations of the water molecule. As previously done, the computed SCF–LCAO–MO interaction energies have been fitted with a set of pair potentials of very simple analytical form. The water positions corresponding to energy minima are analyzed with the help of contour maps, where isoenergetic curves are displayed.
Statistical mechanics of helical wormlike chains. VIII. Angular correlation functions
Authors: Yamakawa Hiromi , Shimada Jiro
Published: February 2003

The adaptation of the helical wormlike chain to a given real chain, or its rotational isomeric state model on the monomer-unit level, is studied with angular correlation functions between two localized coordinate systems affixed to the chain or between two monomer units as functions of their separation along the chain, as defined as the expansion coefficients of the Green’s functions. Since these correlation functions for the latter model depend on the orientation of a localized system affixed to the monomer unit with respect to that unit, they are first formulated in the Flory system and then transformed to those in a different system appropriate for a comparison with the helical wormlike chain. This transformation is determined to preserve certain symmetry properties of the helical wormlike chain in the correlation functions for the rotational isomeric state model. Numerical computations are carried out for polymethylene, polyoxymethylene, poly(dimethylsiloxane), tactic polystyrene, and tactic poly(methylmethacrylate) chains. The analysis shows that the decay of the torsional correlation in the helical wormlike chain is slow, while the higher order correlation functions for the rotational isomeric state model decay slowly. The imaginary parts of the correlation functions are related to the asymmetry of the chain, and their symmetry properties for the two models do not completely agree with each other in the case of the asymmetric chains. However, the imaginary parts are of minor importance since their magnitudes are small compared to the real parts; in particular, they vanish for the symmetric chains. As a whole, there is rather good agreement between the two models. The analysis provides also a general procedure of determining the orientation of the localized system of the helical wormlike chain with respect to the monomer unit, and therefore also to, for instance, the principal axes of the polarizability tensor of the latter.
Time-dependent birefringence, linear dichroism, and optical rotation resulting from rigid-body rotational diffusion
Authors: Wegene William A. , Dowben F Robert M. , Koester Vaughn J.
Published: February 2003

Expressions are derived for the time-dependent relaxation behavior of a monodisperse suspension of arbitrarily shaped rigid bodies, after initial alignment by an externally applied field. We show that five exponential terms are necessary for a complete description of birefringence, linear dichroism, and optical rotation decay phenomena in a force-free rotational diffusion process. The explicit form for the multiplicative coefficients of the exponential relaxation terms are presented; they are expressed in terms of the optical anisotropy tensor and a tensor characteristic of the initial alignment conditions. Symmetry constraints that involve special relationships between the optical anisotropy tensor, the alignment tensor, and the diffusion tensor or that involve the initial orientational distribution conditions, are shown to lead to a reduction in the number of required exponential relaxation terms. We concern ourselves mainly with alignment by means of an electric field in a Kerr cell, but other alignment techniques are treated in a general formalism with application to hydrodynamic flow fields. We also present an alternate formulation of the birefringence decay which provides physical insight as regards the sign and the monotonic or nonmonotonic behavior of the time-dependent relaxation process.
EPR powder pattern analysis for Mn2+ in cubic crystalline fields
Authors: Rubio O. J. , Muñoz P. E. , Boldú O. J. , Chen Y. , Abraham M. M.
Published: February 2003

The EPR powder pattern analysis for a d5 spin system in a cubic crystalline field is discussed. From the study of the EPR absorption curves for the powder, it has been established that if divalent manganese occupies sites of cubic symmetry in single crystal MgO, CaO, and SrO, it also occupies cubic sites in the powder. The EPR spectra of Mn2+ in MgO, CaO, and SrO single crystals were first observed along different magnetic field orientations and compared to the subsequent powder spectra obtained after the same samples were ground to a fine powder. For each host, a one to one correspondence between each allowed EPR transition in the 〈100〉, 〈110〉, 〈111〉 single crystal spectra and the derivative peaks of the powder spectrum was found, leading to the conclusion that the powder spectrum is due solely to the contributions from these principal directions. Spin-Hamiltonian parameters g, a, and A obtained from EPR measurements of the powder spectrum agree with those obtained from the single crystal, and both sets of parameters are in good agreement with those already reported for the single crystal samples. Forbidden hyperfine lines ‖ΔMS‖=1, ΔmI=±1 were also observed in all the powder spectra. Contrary to some previous published works, it was not necessary to use any axial crystal-field term to explain the forbidden lines.
The structural phase change in s-triazine: Reconciliation of x-ray diffraction and NQR measurements
Authors: Rae A. I. M.
Published: February 2003

Previous x-ray diffraction studies of the low-temperature phase of s-triazine identified the order parameter with a shear strain of the unit cell and interpreted the data in terms of a weakly first-order transition. Independent measurements of the nuclear quadrupole resonance spectrum showed that the 14N lines split in the low temperature phase; this splitting was taken to be proportional to the order parameter and its variation with temperature was described in terms of that associated with a second-order phase change, with a fluctuation contribution in the region of the transition. The present work shows that the first order nature of the transition is a necessary consequence of the symmetry of the system and the temperature dependence of the NQR frequencies can be explained if they are expressed as a sum of a term proportional to the order parameter together with a relatively large quadratic term whose magnitude can be qualitatively understood by a consideration of the environment of the nitrogen atoms.
Vibrational relaxation of D2O(010)
Authors: Kim Y. H. , Shin H. K.
Published: February 2003

The vibrational relaxation of D2O(010) is calculated for temperatures ranging from 200−400°K. The calculation is based on a model that the vibrational energy hv is transferred to both the free rotational and hindered rotational motions of subunit molecules in a weekly bound complex D2O...D2O.(AIP)
Exchangeable proton hyperfine couplings in free radical radiation products
Authors: Box Harold C. , Budzinski Edwin E.
Published: February 2003

Hyperfine couplings are reported from ENOR measurement on singles crystals of L-histidine HCl and DL-serine. The crystals were x-irradiated at 4.2°K and the free rachcal species studied. The exchangeables proton couplings in c−+NH3 are deduced from ENOR measurements.(AIP)
Reply to comments on the theory of dipolar fluids
Authors: Chan Derek Y. C. , Mitchell D. John , Ninham Barry W. , Pailthorpe Bernard A.
Published: February 2003

The dipole−dipole potential for a hard sphere fluid is discussed. A reply to Ramshaw’s comments on a previous paper by the authors is given.(AIP)
Comment on ’’Observation of saturation in three-body ion–neutral association reactions’’ by D. K. Bohme, D. B. Dunkin, F. C. Fehsenfeld, and E. E. Ferguson
Authors: Fehsenfeld F. C. , Ferguson E. E.
Published: February 2003

The association reactions of N2+ with N2 and O2+ with O2 as reported by Bohme et al are interpreted by taking into account the temperature dependence of the rate constants for reactions of He(235) (career gas in ref. 1) with N2 and O2. (AIP)
Measurement of the rate coefficients for the bimolecular and termolecular de-excitation reactions of He(2 3S) with selected atomic and molecular species
Authors: Collins C. B. , Lee F. W.
Published: February 2003

This work reports the measurement of second and third order excitation transfer reactions of metastable 2 3S helium atoms with 15 reactants selected because of their widely varying values of polarizability and dipole moment. In this study He(2 3S) destruction frequencies have been experimentally determined from measurements of the optical absorption of the 3889 Å, 2 3S →33P, helium transition in high pressure afterglows of mixed gases excited by intense electron beam discharges. Data have been obtained as functions of helium pressure over the range 800–2000 Torr and as functions of the partial pressure of reactant from 14 to 200 mTorr. From this data pressure dependent rate coefficients have been extracted and subsequently resolved into contributions from second order, bimolecular, and third order, termolecular components. The bimolecular components have been found to agree with tabulated values appearing in the literature. The sensitivity of the method has been sufficient to detect termolecular components as small as 0.3×10−30 cm6 sec−1 and values were found to range from 0.9×10−30 cm6 sec−1 for Kr to 15×10−30 cm6 sec−1 for CCl3F. A classical approximation to the three-body capture rate has been derived which explains these values in therms of the rates at which third body encounters change glacing collisions into inwardly spiraling orbits. The data reported here are in agreement with this model and indicate that the reaction probabilities are generally approximated by values of 0.25, 0.55, and 1.0 for monatomic, diatomic, and polyatomic reactants, respectively. The sizes of these termolecular rates suggests their general importance in excitation transfer reactions in plasmas at atmospheric pressures.
Multiple time scale stochastic formulation for collision problems with more than one degree of freedom
Authors: Augustin Stuart D. , Rabitz Herschel
Published: February 2003

Individual molecular collisions have been described by nonequilibrium statistical mechanics in previous work. The present paper deals mainly with refinements and extensions of the theory for systems with more than one internal degree of freedom. For example, it is shown how quantum mechanics for one internal mode can be combined with equations for the other kinds of motion. A multiple time scale stochastic formulation, which allows each degree of freedom its own ’’natural’’ time scale is also described. It is shown, by an application to vibration-rotation inelasticity in the He4–para-H2 system, that this method gives results in good agreement with full quantum calculations and experimental measurements. A computationally simple technique for restoring microscopic reversibility to time-dependent quantum calculations that employ the classical path approximation is also described.
Molecular dynamics of polymeric systems
Authors: Bishop Marvin , Kalos M. H. , Frisch H. L.
Published: February 2003

Molecular dynamics calculations have been performed on polymeric systems containing a single chain immersed in a solvent. All atoms interact via the repulsive (shifted) Lennard-Jones potential. In addition, nearest neighbors along chains are linked by a quasiharmonic potential which permits limited pair extensions. Chains of length five and ten have been studied. Equilibrium properties such as the virial, various pair correlations, the end-to-end distance, the radius of gyration, and the polymer asphericity were computed. Where comparisons with alternative models were possible, the agreement was good. Chain dynamics has been monitored by examining the relaxation of the velocity autocorrelation function, the end-to-end distance, and the radius of gyration. The five unit chain appears to exhibit elastic resistance to bending modes of deformation.
Two-photon excited fluorescence spectroscopy
Authors: Chen Kuo-mei , Yeung Edward S.
Published: February 2003

The intensities of two-photon excited fluorescence (TPEF) experiments are derived by the density matrix formalism for a symmetric top molecule. Although we only consider systems under collision-free conditions and unpolarized ground state, this treatment can be extended to incorporate collisional relaxation processes and polarized ground state. The two-photon excitation and the fluorescence detection operators are resolved into their irreducible tensor components. For an identical-frequency two-photon absorption, the two-photon excitation operator can be decomposed into a scalar and a quadrupole part. The scalar operator only prepares the population of the excited state density matrix ρe. The quadrupole operator can prepare ρe into monopole through hexadecapole polarizations. In addition to these two sources, a mixed term which combines the scalar and the quadrupole operator can propare ρe into a quadrupole polarization for the Q rotational branch of ΔK=0 two-photon transitions for linearly polarized light. In two-photon absorption experiments which only detect the population of ρe, the mixed term cannot be probed. Since the fluorescence detection operator detects the population, orientation and alignment of ρe, the fluorescence intensities in irreducible tensor operator representation can be put into a condensed form that clearly describes all the polarization and angular momentum dependences.
Trapped electrons in irradiated single crystals of polyhydroxy compounds
Authors: Box Harold C. , Budzinski Edwin E. , Freund Harold G. , Potter William R.
Published: February 2003

The intermolecular trapping of electrons has been observed in single crystals of dulcitol and L(+) arabinose x-irradiated at 4.2 °K. Attribution of a major component of the ESR absorption to trapped electrons is based upon the character of the hyperfine pattern, which arises from multiple anisotropic hyperfine interactions with exchangeable protons, and on the g value of the absorption, which is always less than the free spin value. The removal of the trapped electron absorption upon irradiation with visible light has also been demonstrated. In these experiments all of the electrons are trapped in identical sites. This circumstance provides some important advantages in the study of the factors affecting the stabilization of charge in an environment of polarizable molecules.
Theory of chemically-induced dynamic spin polarization. IV. Low-field effects
Authors: Zientara Gary P. , Freed Jack H.
Published: February 2003

The Pedersen–Freed theory of chemically-induced dynamic spin polarization is extended to low field cases where the T±→S transitions must be included. The solutions of the stochastic-Liouville equation are again obtained by numerical finite-difference (FD) methods, but order of magnitude reductions in the size of the matrices to be inverted are realized by introduction of a simple modification of the FD expressions. The simplest case of a single nuclear spin of I=1/2 on one radical is treated in detail. The results show unequivocally the significant dependence of both CIDNP and CIDEP signals on both the magnitude and the range of the exchange interaction. Also, significant polarizations which should be observable are predicted for both types of experiment under appropriate low field conditions. All the computed results are consistent with a simple qualitative model involving two primary regions in r space: in the outer region the hyperfine terms induce T±→S transitions and J (r) =0, while in the inner region J (r) modulates the effectiveness of these transitions. Consideration is also given to magnetic-field effects on chemical reactivity as well as T±→S contributions in high field but viscous CIDEP.
Molecular theory of translational diffusion: Microscopic generalization of the normal velocity boundary condition
Authors: Hynes James T. , Kapral Raymond , Weinberg Michael
Published: February 2003

A simple molecular theory is presented for the diffusion constant D for a test hard sphere translating in a hard sphere solvent. It is argued that there is a breakdown of the applicability of hydrodynamics in the neighborhood of the test particle due to collisional effects. It is shown that, as a consequence, the traditional hydrodynamic boundary condition (BC) on the particle–solvent normal relative velocity is incorrect for molecular motion. An approximate replacement for this BC is constructed from collisional considerations. With this new BC and the usual hydrodynamic equations, D is found to have two additive contributions. The first is the microscopic, collisional Enskog diffusion constant; the second is of the hydrodynamic Stokes–Einstein form. It is shown how the standard hydrodynamic Stokes–Einstein relation for D can hold numerically to a good approximation despite the dominance of (or significant contribution to) the motion by microscopic collisional contributions. Observed trends of D with size and mass ratios which contradict the analytic Stokes–Einstein relation are reproduced. The predicted D values are compared with available results of renormalized kinetic theory and Boltzmann-level kinetic theory. High density deficiencies of the new BC are discussed.
Charge transfer reactions of ground N+(3P) and excited N+(1D) state ions with neutral molecules
Authors: Moran T. F. , Wilcox J. B.
Published: February 2003

Total charge transfer cross sections have been measured for the reactions of 1–3 keV N+(3P) and N+(1D) ions with Ar, H2, N2, O2, CO, NO, and CO2. Time-of-flight techniques have been used to measure fast neutral N products from charge transfer reactions of N+ ions which have been produced by electron impact ionization of nitrogen. Charge transfer reactions have been examined as a function of the electron energy used to produce the reactant N+ ions and reaction cross sections determined for both ground N+(3P) and N+(1D) state ions. Total charge transfer cross sections involving 1–3 keV N+(3P) and N+(1D) ions are similar in magnitude as a result of similar energy defects for reactions of both ground and excited state ions.
Quantum mechanics of electronic–rotational energy transfer in F(2P)+H2 collisions
Authors: Wyatt Robert E. , Walker Robert B.
Published: February 2003

Theoretical study is made of electronic–rotational energy transfer in F(2P)+H2 three-dimensional collisions, with electronic matrix elements from DIM theory. The quantum close-coupled equations are integrated via the R-matrix propagation method. Inelastic quenching probabilities are emphasized, with and without simulated open reaction channels. Interweaving patterns in the transition probability for even and odd nuclear parity vs J (total angular momentum quantum number) are analyzed in terms of avoided crossing structure in the electrotational energy correlation diagrams. Localized regions where electronic quenching is dominant are identified in the correlation diagrams, and are confirmed in separate calculations which neglect interchannel mixing in local regions of the atom–molecule separation. Open reaction channels are found to have little influence on the quenching probabilities in these low energy (Etr<0.15 eV) calculations.