bluuebunny/crossref_metadata_2025 huggingface dataset:
Why does the Aitken extrapolation often help to attain convergence in self-consistent field calculations?
Authors: Cioslowski J.
Published: July 2002

The Aitken (three-point) extrapolation is one of the most popular convergence accelerators in the SCF calculations. The conditions that guarantee the Aitken extrapolation to bring about an unconditional convergence in the SCF process are examined. Classification of the SCF divergences is presented and it is shown that the extrapolation can be expected to work properly only in the case of oscillatory divergence.
Autoionization from the lone-pair orbitals of molecules containing iodine
Authors: Carlson Thomas A. , Gerard Patrick , Pullen B. P. , Grimm Frederick A.
Published: July 2002

Autoionization to the lowest spin component of the ground state of the ion has been studied for the following alkyl iodides in the gas phase: CH3I, C2H5I, and nC3H7I. Measurements have been made using the constant ionic state (CIS) method for photon energies between the ionization potentials arising from the spin–orbit split lone-pair orbitals of iodine. To carry out the measurements, angle resolved photoelectron spectrometry has been coupled with a variable photon source extracted from synchrotron radiation. A series of broad resonances together with some sharp resonances are seen for each study and the results for each molecule in the series show strong similarity to one another, illustrating the localized atom-like character of the lone-pair orbital. There are strong differences found in the extent of the background, which is attributed to the higher density of states for the longer carbon chains. Most measurements were taken for the ground state vibrational level, but results are also presented for ionization to the ν2 excited vibrational state of CH3I+. New resonances, not seen for the ground vibrational state, are found and their origin discussed. CIS spectra on the I2 molecule are reported, which are in strong contrast to the alkyl iodides. Possible reasons for the differences are presented.
Quantum theory and collisional propensity rules for rotationally inelastic collisions between polyatomic molecules (NH3 and CO2) and an uncorrugated surface
Authors: Corey Gregory C. , Alexander Millard H.
Published: July 2002

We present the general quantum theory of collisions of a symmetric top molecule with an uncorrugated surface. The similarities between the description of collisions of a molecule with a structureless atom and a flat surface allow us to exploit earlier gas-phase results. We then derive several collisional propensity rules: (1) In experiments in which both inversion states in the initial J,K doublets of para-NH3 are equally populated, both inversion states of all collisionally excited levels must also be equally populated. If, however, the initial inversion level can be state selected, then unequal populations will be observed in collisionally excited inversion doublets. (2) For transitions from the J=0 level of ortho-NH3 into rotational levels of the K=3 stack, a strong propensity will exist toward conservation of the inversion symmetry for transitions into levels with J′ odd, but toward a change in the inversion symmetry for transitions into levels with J′ even. (3) If the odd terms in the angular expansion of the potential dominate, then for transitions out of rotational levels with J>0 in the K=0 stack of ortho-NH3 into rotational levels of the K=3 stack, a strong propensity will exist toward population of the upper level of the inversion doublet if the initial state has even J, and toward population of the lower level if the initial state has odd J. Using the similarities between the wave functions of a symmetric top and those of a linear polyatomic molecule with degenerate bending modes, we derived several propensity rules for the specific case of collisions of CO2 (0000) with an uncorrugated surface. In collisions which excite the low-lying (0110) bending vibration, if the initial rotational quantum number is small, then we predict that the probability of transition into a final state with J′ odd will be much larger than for transition into a final state with J′ even.
The configurational contribution to the segmental orientation in network chains subject to perturbation by the excluded volume effect
Authors: Bahar Ivet , Mattice Wayne L.
Published: July 2002

The influence of the excluded volume effect on the configurational contribution to the segmental orientation in network chains is evaluated using rotational isomeric state theory. The chains used are rotational isomeric state models for polyethylene, poly(vinyl bromide), and poly(vinyl chloride), as well as some simplistic models for polyethylene. The long-range interactions may be attractive or repulsive. Chain expansion in response to the excluded volume effect is accompanied by an increase in orientational entropy (or disorder), which is apparent from the decrease in the absolute magnitude of the orientation function. Conversely, any tendency for orientation in a preferred direction is enhanced by chain contraction.
Structural determination of solvated copper species in formamide solution by EXAFS
Authors: Sano Mitsuru , Maruo Tetsuya , Yamatera Hideo
Published: July 2002

The EXAFS studies on CuBr2 –LiBr solutions in formamide showed that the existing species remained to be [Cu(formamide)4 ]2+ with the Cu–O distance of 1.97 Å even when the bromide concentration was increased. From the analysis of two other peaks in the EXAFS Fourier transform, the structure of the solvated species has been proposed.
Excited state relaxation in bichromophoric rotors: Time-resolved fluorescence of 1,3-di(N-carbazolyl) propane: A three-state analysis
Authors: Buchberger Eva M. , Mollay Bernhard , Weixelbaumer Wolf-Dieter , Kauffmann Harald F. , Klöpffer Walter
Published: July 2002

The transient fluorescence profiles of 1,3-di(N-carbazolyl)-propane, DCP, were reinvestigated as a function of temperature in toluene as solvent. Typically three-exponential patterns, both for the low-energy, red edge fluorescence of the excimer FE(t) and, in part, for the monomer fluorescence FM(t) were observed in the moderate temperature range −15≤t/°C≤55, whereas at temperatures t/ °C>55 profiles were found to be approximately biexponential, within the limitations of time resolution. On the premises given in Sec. IV of this work, data were analyzed in terms of a discrete three-state model which assumes two monomeric conformers (X1=tt, X2=tg±) and a single excimer-forming conformation (X3=g∓g±) interconverting in an open, linear scheme. Starting from a generalized treatment of n-particle interaction, the analytical δ-pulse solutions to the fluorescence evolutions X1(t), X2(t), and X3(t) were formulated in terms of 18 amplitudes Aij(k) (i, j=1,2,3) and 3 eigenvalues τj =−1/Tj ( j=1,2,3) for two different, initial boundaries (k=1,2). For reasonable choices of fluorescence rate constants, the simulated parameters proved useful (a) to recover satisfactorily the experimental subnanosecond (T1) and nanosecond time constants (T2,T3), (b) to rationalize the biexponential rise of excimer fluorescence at moderate temperatures, and (c) to explain the pseudo-Birks behavior in the high-temperature regime. Results from both experiments and computation allow to specify the time scales of rotating carbazole chromophores, and they strongly indicate that the rapid conformational equilibrium hypothesis is not valid in DCP. The limitations of the minimal model have been addressed and the potential problem encountered in analyzing the data by a discrete set of multiexponentials has been discussed.
A theory of the interionic structure of graphite intercalation synthetic metals: Variations with respect to interactions and state
Authors: Chen Zhuo-Min , Karim Omar A. , Pettitt B. Montgomery
Published: July 2002

A statistical mechanical theory is developed and applied to study the structural effects that the thermodynamic state of alkali ions have on graphite intercalation compounds. The system considered is that of second stage Rb–graphite. Two-dimensional diffraction patterns are computed and compared with experimental measurements. Sensitivity to model parameters are considered. A low order density functional expansion is found to adequately describe the interionic structure of the system modeled as a two-dimensional one component plasma in an anisotropic external field.
On the molecular theory of aqueous electrolyte solutions. III. A comparison between Born–Oppenheimer and McMillan–Mayer levels of description
Authors: Kusalik P. G. , Patey G. N.
Published: July 2002

The utility of the McMillan–Mayer (MM) theory (applied at the pairwise additive level) for aqueous electrolyte solutions is examined by comparing with results previously obtained for a Born–Oppenheimer (BO) model. The structural properties and mean ionic activity coefficients calculated using both theoretical descriptions are compared for KCl, NaCl, and tetraalkylammonium bromide solutions. It is found that the success of the MM theory strongly depends upon the particular solution considered.
Trigonal sites of Eu2+ in single crystals of Cs2NaYCl6
Authors: Mondragón M. A. , Gleason R. J. , Muñoz P. E. , Boldú J. L.
Published: July 2002

Electron paramagnetic resonance measurements are reported for Eu2+ ions occupying trigonal sites in Cs2NaYCl6 single crystals. Spin–Hamiltonian parameters are reported at room temperature. A tentative model for the location of the impurity in the lattice is discussed.
Excitation of emission lines of atomic nitrogen ions by electron-impact dissociative ionization of nitrogen molecules
Authors: Rall David L. A. , Sharpton Francis A. , Lin Chun C.
Published: July 2002

Absolute optical emission cross sections have been measured for transitions of the N+ ion in the wavelength range 3800–7000 Å originating from 33 terms of the N+(2s22pnl) or N+(2s2p2nl) configurations with n from 3 to 6 produced by electron-impact ionization and dissociation of the N2 molecule. The incident electron energy ranges from threshold to 450 eV. The excitation functions show a broad maximum at about 190 eV. The mechanisms of formation of the excited atomic nitrogen ions near the threshold energy are discussed.
Optical bistability in dye molecules
Authors: Speiser Shammai , Chisena Frank L.
Published: July 2002

Optical bistability has been observed in highly concentrated substituted fluorescein dye solution and in thin (∼1 μm) doped polymeric films. For fluorescein, at concentrations larger than 10−5 mol/ℓ dye dimers are formed. The dimer–monomer equilibrium constant is 105 ℓ/mol so that most of the dye species are in the dimer form. At 480 nm the dimer absorption cross section is 10−18 cm2/molecule, while that for the dye monomer molecule is 7.6×10−17 cm2/molecule. Upon laser excitation dimers dissociate to form monomers thus providing a highly nonlinear laser induced absorption. The high nonlinear absorption coefficient can be utilized for optically bistable response of the dye system. Optical bistability was observed by placing dye solutions or dye thin films inside a Fabry–Perot resonator and exciting it with 480 nm dye laser pulses of 10 ns duration. The effect is more pronounced in 10−4 mol/ ℓ fluorescein than in 10−6 mol/ℓ fluorescein in which dimer formation is not that efficient. In disodium fluorescein, eosin Y and erythrosin B no significant dimer formation is observed even at 10−3 mol/ℓ dye concentration. The observed bistability both in solution and in thin films can be explained in terms of recent models for optical bistability in nonlinearly absorbing molecular system.
State selected ion–molecule reactions by a coincidence technique. XV. Hydrogen atom abstraction as an electron jump followed by proton transfer in the ND+3 (v)+NH3 and NH+3 (v)+ND3 reactions
Authors: Tomoda Shinji , Suzuki Shinzo , Koyano Inosuke
Published: July 2002

The effects of the vibrational excitation of the ν2 mode of ND+3 (v) and NH+3 (v) on the three channels of their reaction with NH3 and ND3 , respectively, are investigated up to v=12 in the center-of-mass kinetic energy range from 0.9 to 4.5 eV by use of the TESICO technique. The ratio (γ) of the hydrogen/deuterium atom abstraction cross section over the competing deuteron/proton transfer cross section has a maximum systematically at a vibrational level (vmax ) slightly higher than that giving maximum Franck–Condon factor (vFC ) for the neutralization of ND+3 (v)/NH+3 (v). A new reaction model based on a nonadiabatic transition theory and the potential energy surface calculated for the ammonia dimer cation is proposed to explain the experimental results. The hydrogen/deuterium atom abstraction reaction is interpreted as a near resonant electron jump at larger intermolecular separation followed by proton/deuteron transfer which proceeds on the ‘‘electron (charge)–transferred’’ potential surface.
Transient vibrational spectroscopy by flash photolysis stimulated emission pumping: 3ν2 of singlet methylene
Authors: Xie Wei , Ritter Alan , Harkin Carmel , Kasturi Kamu , Dai Hai-Lung
Published: July 2002

A vibration–rotation spectroscopic method based on stimulated emission pumping has been developed for transient molecules generated by flash photolysis. This method may be generally applicable to radicals with electronic excited states accessible by visible or UV laser pulses. This first demonstration on the bending v2 =3 level of ã 1A1 CH2 generated a vibrational term value of 3950.45(7) cm−1 and the rotational constants A=25.665(40), B=11.470(7), and C=6.7140(61) cm−1. These constants indicate that the 3ν2 level is well below the bending-potential barrier. Perturbations of several rotational levels by the triplet state are identified. In addition, and most interestingly, abnormally intense ΔKa =3 transitions were observed in the ã←b̃ 2163 band.
Helium hydride emission spectra at 550 and 640 nm
Authors: Brooks R. L. , Hunt J. L.
Published: July 2002

Emission spectra of three isotopomers of helium hydride (4HeH, 3HeH, and 3HeD) in the visible spectral region have been acquired using proton-beam irradiation of dense helium gas (150 Torr) at 4.2 K in the presence of some solid hydrogen or deuterium. Besides the previously reported D 2Σ+→A 2Σ+ transition, near 550 nm, a second transition near 640 nm, identified as the D 2Σ+→B 2Π, has been acquired and analyzed. The spectroscopic constants for both transitions have been obtained and compared to the theoretical results based on the latest published potential curves. Further insight into the mechanism for forming HeH will be presented, which indicates that the formation process is sensitive to the hydrogen vapor pressure above the solid.
Studies of chemical reactivity in the condensed phase. IV. Density dependent molecular dynamics simulations of vibrational relaxation in simple liquids
Authors: Brown J. K. , Harris C. B. , Tully John C.
Published: July 2002

Molecular dynamics simulations of the photodissociation/recombination process for iodine in liquid xenon at several densities are reported in this paper. These simulations were performed to aid in the understanding and interpretation of recent picosecond experimental investigations on model chemical reaction systems. From these calculations, it was found that geminate recombination occurs primarily within a few picoseconds at all densities considered. This is in agreement with previous molecular dynamics simulations with significantly smaller systems, and with the current interpretation of experimental results. Simulated iodine ground electronic state vibrational relaxation times range from about 1 ns at the lowest density to approximately 250 ps at the highest density reported here. In addition, the functional form of the decay of the average iodine vibrational energy was observed to be nearly independent of density. This result is discussed in terms of simple gas phase isolated binary collision models. Various force correlation functions projected onto the iodine vibrational coordinate were also examined, and indicate that the iodine molecule significantly perturbs the local solvent environment. These force correlation functions may be helpful when assessing the usefulness of liquid phase theories of vibrational relaxation of highly excited molecules. Finally, the simulation results on iodine vibrational relaxation are compared with the available experimental data. These comparisons indicate that the molecular dynamics calculations overestimate the rate of vibrational relaxation over the lower third of the iodine ground electronic state potential surface, and that the efficiency of V–TR transfer, relative to V–V transfer, may have been underestimated. The sensitivity of these results to several system parameters are analyzed.
Collision induced dissociation of H+2 and D+2 with H2 using a surface hopping trajectory method
Authors: Eaker Charles W. , Schatz George C.
Published: July 2002

The collision induced dissociation (CID) and charge transfer (CT) cross sections have been determined for H+2 and D+2 colliding with H2 using a surface hopping trajectory method. Approximately 40 000 trajectories have been analyzed for collisions at 4.0, 6.0, and 8.0 eV (center of mass) and for H+2 (D+2 ) in vibrational states from 0 to 10. Our results are consistent with the recent experiments of Guyon, Baer, Cole, and Govers [Chem. Phys. 119, 145 (1988)]. However we have come to a different understanding of the mechanism for dissociation. We find that there are two pathways for CID: (1) formation of a H+3 intermediate followed by dissociation and (2) direct dissociation of a H+4 transition state via vibrational excitation. The H+3 intermediate pathway predominates at low collisional and low H+2 (D+2 ) vibrational energies.
Thermochemistry of the plutonium fluorides from electron impact studies
Authors: Kleinschmidt P. D.
Published: July 2002

The ionization potentials of the molecules PuF5, PuF4, and PuF3 have been measured using electron impact ionization. The values are, respectively, 14.5±0.1, 11.8±0.4, and 8.6±0.5 eV. The parent ion PuF+6 was not detected. From the ionization potentials and the fragment ion appearance potentials the bond dissociation energies of the molecules F5Pu–F, F4Pu–F, F3Pu–F, and F2Pu–F have been calculated to be 53±2, 108±12, 113±5, and 141±16 kcal/mol, respectively. Upon comparison with data available on the plutonium fluorides the bond energies of F3Pu–F and F2Pu–F are accurate to within the stated uncertainty. The values of the bond energies of F5Pu–F and F4Pu–F disagree with the reported enthalpy of formation of PuF6(s). Analysis of available data indicates that the enthalpy of formation of PuF6(s) is most likely −484±10 kcal/mol.
Evidence for increased chain stiffness of polydiacetylene in solution
Authors: Aime J. P. , Bargain F. , Fave J. L. , Rawiso M. , Schott M.
Published: July 2002

Evidence has been found of increased chain stiffness of isolated polydiacetylene molecules in solution, associated to an increased side-group interaction, and in at least one case, to an increased and more temperature dependent electronic conjugation length: Bromoform solutions of poly-3BCMU (3B) and poly-4BCMU (4B) were studied by visible and IR absorption spectroscopy and by small angle neutron scattering. The polymers are more rigid than in other solvents: for 3B, b>600 Å at 45 °C and for 4B, b=420±40 Å at 25 °C. Hydrogen bonding between neighboring side groups is observed, decreasing upon increasing temperature. For 3B, the visible absorption spectrum is slightly red shifted, the more so the lower the temperature, indicating an influence of chain stiffness on electronic conjugation length.
Observation of highly vibrationally excited O2 resulting from the reaction of O(1D2) with O3
Authors: Cleveland Cheryl B. , Wiesenfeld John R.
Published: July 2002

High resolution LIF studies of O2(X 3Σ−g) following the reaction of O(1D2) with O3 have been achieved using the (0,ν″)B 3Σ−u←X 3Σ−g Schumann–Runge transitions. Signals arising from ν″=14–16 and 18–22 have been observed and an approximate population ratio of P(18)/P(16)=2.2 obtained. The temporal profiles at various ozone concentrations firmly establish that the detected O†2 results frm the subject reaction, not from initial photolysis of O3.
Steric effects in molecular adsorption
Authors: Tenner M. G. , Kuipers E. W. , Kleyn A. W. , Stolte S.
Published: July 2002

The results of a molecular beam surface scattering experiment with preferentially orientated NO on Ag[111] are given. In adsorption molecules oriented with the O end towards the surface have a larger trapping probability.
CH3O(Ã–X̃) fluorescence from photodissociation of dimethyl ether
Authors: Suto Masako , Ye Chao , Lee L. C.
Published: July 2002

Photoabsorption and fluorescence cross sections of dimethyl ether, (CH3)2 O, were measured in the 110–195 nm region using synchrotron radiation as a light source. Fluorescence was observed in the 120–155 nm region with a maximum cross section of 5.5×10−19 cm2 at 137 nm. The fluorescence spectra produced at 147 and 157.5 nm were dispersed and identified to be the CH3O (Ã–X̃) transition. The radiative lifetime of CH3O*(Ã) is 1.8±0.3 μs and the quenching rate constant of CH3O(Ã) by dimethyl ether is (5.7±0.4)×10−10 cm3 /s.
Resonance enhanced multiphoton ionization spectra of the GeF and GeCl radicals from 400 to 500 nm
Authors: Johnson Russell D. , Tsai Bilin P. , Hudgens Jeffrey W.
Published: July 2002

The resonance enhanced multiphoton ionization spectra from 400 to 500 nm of the GeF and GeCl radicals are reported. In GeF bands involving transitions to the A 2Σ+, C′ 2Π, D′ 2Π, E 2Σ+, and G 2Δ states were observed. The bands involving transitions to the valence A 2Σ+ state were accounted for by 1+2 and 1+1+1 resonance enhanced multiphoton ionization (REMPI) mechanisms. Bands associated with the other Rydberg states were generated by a 2+1 REMPI mechanism. The spectrum of the GeCl radical arose from 2+1 REMPI excitation through the C′ 2Πr (5pπ) and C 2Πr (4pπ) Rydberg states. The C′ 2Πr state spin–orbit coupling constant was determined by direct measurement of C′ 2Π1/2←←X 2Π1/2 transitions and ‘‘one photon forbidden’’ C′ 2Π3/2←←X 2Π1/2 transitions. A least-squares fit determined the spectroscopic constants of the C′ 2Πr 5pπ Rydberg state of GeCl: Te=42 190±5 cm−1, Ae=81.0±3.5 cm−1, ωe=509.2±5.3 cm−1, and ωexe=3.3±1.1 cm−1.
Geometric structure of diacetylene cation in the X̃ 2Πg and Ã 2Πu electronic states
Authors: Lecoultre Jacques , Maier John P. , Rösslein Matthias
Published: July 2002

The complete rs structure of the diacetylene cation in the X̃ 2Πg and Ã 2Πu electronic states is reported. To this end the rotationally resolved origin bands in the Ã 2Πu–X̃ 2Πg laser excitation spectra of H–13C 3/4 C–C 3/4 C–H+ and H–C 3/4 13C–13C 3/4 C–H+ have been measured and analyzed. From the determined rotational constants as well as those known for H–C 3/4 C–C 3/4 C–H+ and D–C 3/4 C–C 3/4 C–D+, all the bond lengths in the two electronic states have been inferred.
Resonances in molecular photoionization. IV. Theory of one-color and two-color near-threshold photoionization of molecules
Authors: Domcke W. , Sobolewski A. L. , Lin S. H.
Published: July 2002

A relatively comprehensive theoretical description of one-color and two-color photoionization of molecules by strong laser fields is developed. The molecular system is modeled by a number of discrete electronic configurations and a number of electronic ionization continua in a diabatic representation, allowing for intramolecular coupling of the discrete states and the continua. The vibrational degrees of freedom are included in compact operator notation without invoking the Born–Oppenheimer approximation. The relevant radiative dipole couplings are treated nonperturbatively on an equal footing with the intramolecular couplings. The important Coulomb threshold effects (accumulating Rydberg series at the electronic–vibrational ionization thresholds) are included via a threshold expansion of the complex level-shift operators representing intramolecular and radiative couplings. The weak-field, long-time ionization rate (golden-rule formula) is rederived from the general theory both in the one-color and the two-color case. In the two-color case, strong-field effects caused by either one of the two lasers are briefly discussed. We derive a simplified multichannel-quantum-defect model from the general two-color formalism which nicely reproduces a number of characteristic features of vibronic autoionization recently observed in several polyatomic molecules.
Theoretical study of the valence photoelectron spectrum of ozone: An analysis of correlation effects and configuration interaction (CI) model spaces
Authors: Decleva P. , De Alti G. , Lisini A.
Published: July 2002

Minimal basis set CI calculations with different configuration spaces including full CI are compared for the ground and ionized states of O3. A double inversion with respect to the Koopmans Theorem (KT) ordering is found, and the inadequacy of approaches based on a single configuration reference is pointed out. The strong correlation effects appear to be well described in terms of fluctuations within a quasidegenerate set comprising the three π orbitals, giving a picture of O3 as a quasi-open-shell system. Extended 2h–1p schemes based on the quasidegenerate model give good reproduction of the FCI results. Extension to the whole valence region gives indication of a further shattering of the intensity over the simple 2h–1p results. An assignment for the complete valence spectrum is proposed.
Statistical mechanics of the single occupancy system. II
Authors: Jhung Kwang Hwa Chung , Jhung Kyu Soo
Published: July 2002

The formal exact solutions of the cluster variation method applied to the single occupancy system have been obtained. The reduced distribution functions and free energy expression turn out to be exactly the same as the Born–Green–Yvon (BGY) solutions, thus implying the cluster variation method and BGY approach to the single occupancy system are equivalent.
Dynamics of a physisorbed dimer
Authors: Adams John E.
Published: July 2002

We report an application of the semiclassical spectral intensity method to the calculation of the absorption spectrum of a simple molecular dimer, (HCl)2, for both the free and physisorbed species. The calculated spectra show a clear indication of the presence of two distinct kinds of hydrogen atoms in this system, one which is involved in a hydrogen bond and one which is unaffected by the dimerization. We also find direct evidence that vibrational energy transfer plays an important role in the dynamics of this system and markedly affects the observed structure of the absorption spectrum.
Deduction of low density limit to rate coefficient for tertiary association of ions and polar molecules from measurements
Authors: Bates David R.
Published: July 2002

The low density limit k0 to the rate coefficient for tertiary association of ions and polar molecules is related through a double integral to an entity z1 or z2 that involves only the temperature, the reduced mass of the colliding pair, the polarizability and dipole moment of the neutral, and k(exp)[M], where k(exp) is the measured rate coefficient at ambient gas density [M]. The results are parametrized so that k0 may easily be found from laboratory data (irrespective of whether or not third order kinetics prevail).
Structural relaxation mechanisms in liquid Eugenol. A depolarized light scattering study
Authors: Bezot P. , Hesse-Bezot C. , Roynard D. , Jeanneaux F.
Published: July 2002

A depolarized light scattering study of liquid Eugenol, over a large temperature range including the supercooled region, is proposed. Comparisons with shear mechanical impedance measurements, obtained at lower frequencies, lead to more precise information on the viscoelastic parameters in the supercooled region. The structural relaxation process measurements by means of the photon correlation technique are compared to the dielectric and mechanical measurements. Molecular mechanisms are proposed.
Microwave spectrum, structure, mercury-201 quadrupole coupling constant, and dipole moment of methylmercury cyanide
Authors: Rego Christopher A. , Cox A. Peter
Published: July 2002

The microwave spectrum of CH3HgCN has been measured for the ground vibrational state and the lowest bending mode ν10. The rotational constants of 25 isotopic species have been measured and used to calculate the following substitution structure: Hg–C (methyl)=2.0563(1), Hg–C(cyanide)=2.0369(2), C–N=1.1570(2), C–H=1.098(10) Å, and HCH angle=109.1(3)°. The average zero-point structure has also been estimated. The nuclear quadrupole coupling constant of 201Hg (I=3/2) has been determined as eQq=−1313.8(13) MHz, along the axis of the symmetric top for the vibrational ground state. The dipole moment has been determined from Stark effect measurements on the l-type doublets of V10=1 as 4.69(4) D. The l-type doublet constant was measured as q10=2.794(11) MHz.
Thermal-energy chemiluminescent reaction of N(2D) with OCS: Nascent vibrational and rotational distributions of the NS(B 2Π) product
Authors: Obase Hiroshi , Nagano Ichiro , Tsuji Masaharu , Nishimura Yukio
Published: July 2002

NS(B 2Π–X 2Π) chemiluminescence has been observed from the thermal-energy reaction between N(2D) atoms and OCS molecules under single collision conditions. The vibrational populations of NS(B) decrease monotonically with increasing vibrational quantum numbers from v′=0 to 4, while the rotational temperature is constant at about 1000 K for all vibrational levels. The observed vibrational distribution is more excited than the statistically predicted ones, suggesting that the reaction proceeds by a direct mode.
Rheology of homogeneous block copolymers near the microphase separation transition
Authors: Fredrickson Glenn H. , Helfand Eugene
Published: July 2002

Mode coupling and fluctuation effects on the low frequency linear viscoelastic properties of homogeneous diblock copolymers are investigated. Attention is restricted to temperatures near, but not below the microphase separation transition (MST), where a homogeneous block copolymer melt first transforms to a spatially periodic, compositionally inhomogeneous morphology. We find that long-wavelength transverse momentum fluctuations couple only weakly to compositional order parameter fluctuations; the latter are predominantly on the scale of the radius of gyration. Crossover expressions are obtained for the renormalized transport and viscometric coefficients. The fluctuation contributions to these various coefficients are smaller than originally predicted at the MST, a consequence of refined calculations showing the MST to be a weakly first order phase transition.
Charge separation efficiencies in artificial photosynthetic systems: Application to molecularly based electronic devices
Authors: Kundu Kalyan , Phillips Philip
Published: July 2002

We present exact calculations of the quantum efficiency in a general kinetic model for charge separation in artificial photosynthetic systems. The kinetic models we employ consist of a light absorber covalently linked to a linear chain of N sites. The electron is initially excited at the light absorber and subsequently migrates along the molecular chain by virtue of a random distribution of hopping rates. Decay processes are included at each site which simulate the recombination of the electron with the hole at the light absorber. An exact expression is obtained for the quantum yield for this general model of photosynthetic charge separation. From this expression we show that (1) redox biases play a sensitive role in determining the quantum yield of electrons, (2) because the e−-hole recombination rates fall off exponentially with distance, e-hole recombination beyond the nearest neighbor to the light absorber is negligible, and (3) the distribution of electron hopping rates may cause the quantum yield to decay faster than 1/N even when e−-hole recombination at multiple sites is included. The implications of these results on the experimental design of molecular electronic devices is discussed. Further, we show how the expression for the quantum yield can be used to construct the exact transit time, the moments of the transit time, and the exact expressions for the site probabilities for an arbitrarily disordered one-dimensional system when the particle is placed initially at the origin.
One-dimensional energy transfer in GdCl3
Authors: Mahiou R. , Jacquier B. , Madej C.
Published: July 2002

We report the observation of the intrinsic fluorescence of anydrous gadolinium trichloride GdCl3 when laser light is absorbed into the first excited state 6P7/2. With the aid of a frequency doubled pulsed dye laser, we are able to analyze carefully the excitation and emission spectra as well as the decay times over a range of temperatures including that at which the material undergoes a ferromagnetic transition at 2.2 K. At 4.4 K and above, experiments at low excitation density (N0<1014 excited ions/cm3) show that the intrinsic and the impurity induced trap fluorescence dynamics are well described in terms of a fast diffusion and trapping model. When the excitation density is increased anti-Stokes fluorescences appear efficiently; they are assigned to the 6D9/2, 6I7/2→8S7/2 transitions. High excitation density effect on the intrinsic decay as well as the decay of the anti-Stokes fluorescence are nicely reproduced by a theoretical model involving mainly an exciton–exciton annihilation process. Moreover, an excited state absorption mechanism contributes notably to the anti-Stokes fluorescence intensity at t=0 with increasing the excitation density. In the ferromagnetic phase of GdCl3, at 1.5 K, the one-dimensional nature of the exciton diffusion (8S7/2↔6P7/2) is confirmed by the ‘‘incoherent’’ exciton decay analysis using an appropriate model recently developed by Cibert et al. Finally a diffusion length spreading over 2500 visited Gd3+ normal sites is evaluated, knowing the hopping time as a function of the nearest neighbor interactions (nn) in the crude approximation of an ideal two level system in a linear chain of atoms.
Spectroscopic characterization of mesogen order in discotic liquid crystal
Authors: Yang X. , Waldman D. A. , Hsu S. L. , Nitzsche S. A. , Thakur R. , Collard D. M. , Lillya C. P. , Stidham H. D.
Published: July 2002

Infrared and Raman spectra of the discotic liquid crystal molecules benzene–hexa-hexanoate (BH6), benzene–hexa-heptanoate (BH7), benzene–hexa-octanoate (BH8), and benzene–hexa-nananoate (BH9), and of mixtures of BH7 with BH8, are reported as a function of temperature. The 1615 cm−1 infrared band is strong in the Raman spectrum of BH7 and is taken as a characteristic of and diagnostic for central core disorder in all these molecules and mixtures. The aliphatic side chains are shown to disorder at much lower temperatures than the central core, and order in the central core is shown to remain throughout the liquid crystalline region.
Pressure and temperature dependences of the ‘‘binary’’ ion–molecule reaction N+3+H2O→H2NO++N2
Authors: Matsuoka Shingo , Nakamura Hirone
Published: July 2002

Experiments were carried out using a time-resolved atmospheric pressure ionization mass-spectrometer (TRAPI) in N2–H2O (∼1 ppm) system at temperatures from 233 to 543 K and at pressures from 167 to 760 Torr. The title reaction showed temperature and pressure dependences which were explained by the following scheme: N+3+H2O⇄(N+3⋅H2O)* (forward and backward rate constants ka and kb; (N+3⋅H2O)*→H2NO++N2 (forward rate constant kp; (N+3⋅H2O)* +N2→H2NO++2N2 (forward rate constant ka. Assuming that kd is equal to the collision rate constant of 7.1×10−10 cm3 s−1, the individual rate constants were determined as ka =2.8×10−9 cm3 s−1 (302 K), kb =17T3.6 s−1 where T is temperature in K, and kp =2.0×109 s−1 (302 K). The product H2NO+ ion changed by successive reactions with H2O into H2NO+⋅H2O and subsequently to H3O+.
Interchain effect of 13C nuclear magnetic resonance chemical shift and electronic structure of polyoxymethylene chains in the solid state
Authors: Kurosu Hiromichi , Yamanobe Takeshi , Ando Isao
Published: July 2002

The tight-binding MO calculations of the 13C NMR chemical shifts and electronic structure for seven polyoxymethylene chains having trigonal and orthorhombic forms were carried out within the CNDO/2 sum-over-state framework, taking into account the interchain interactions. The calculation explains reasonably the experimental data. Based on these results, the interchain interactions were discussed.
Vibrational relaxation rates via equilibrium simulation of fluids: Thermal softening
Authors: Visscher P. B. , Holian B. L.
Published: July 2002

In this paper we report the results of one-dimensional simulations of vibrational relaxation in a dense classical molecular fluid. The novel feature of the simulations is that we collect statistics on the energy transfer from vibrational to translational modes in individual collisions, instead of just monitoring the total vibrational energy of the system. This gives much more information on the microscopic processes involved, and in particular reveals that under a wide range of conditions the vibrational relaxation rate is dominated by an effect which we refer to as ‘‘thermal softening’’ of the vibration frequency. This effect is very important in defining and calculating the vibrational relaxation rate.
On the use of the Gibbs free energy in macroionic solutions
Authors: Woodward C. E.
Published: July 2002

We investigate a recent theory for macroionic dispersions due to Sogami and Ise, which advocates the use of the Gibbs free energy in place of the usual Helmholtz description of the counterion thermodynamics. Employing the Poisson–Boltzmann approximation, their model predicts an effective electrostatic interaction between macroions which is attractive for spheres, in contradiction to the much used DLVO potential. By applying their analysis to a planar geometry of charged walls with only counterions we make explicit the consequences of their assumption that the thermodynamic potentials are first-order homogeneous functions of the extensive parameters in the system. This is no longer the case of the nonuniform systems inherent in the calculation of effective potentials.
Theoretical study of B2H+5, B2H+6, and B2H6
Authors: Curtiss Larry A. , Pople John A.
Published: July 2002

A theoretical study is presented of B2H+5,B2H+6, and B2H6 at a high level of theory, including correlation energy beyond fourth-order perturbation theory and large basis sets. The B2H+5 ion is predicted to have a triply hydrogen-bridged structure, with a calculated appearance potential in agreement with a recently measured value by photoionization. The most stable B2H+6 ion found corresponds to a very weakly bound complex between B2H+5 and H, likely to exist only in a metastable state.
Raman spectra of selected transuranium trihalides in the solid state
Authors: Wilmarth W. R. , Begun G. M. , Haire R. G. , Peterson J. R.
Published: July 2002

Raman spectral data have been obtained from a number of transuranium trihalides in the solid state. The Raman spectra of these actinide compounds are reported and compared to the published Raman spectra of isostructural compounds. Tentative symmetry assignments have been made for the observed Raman-active lattice vibrations based on nuclear site symmetry analysis of their respective crystal structures and comparisons to the symmetry assignments made for isostructural lanthanide compounds. The Raman spectral data obtained in this study represent a partial data base for the use of Raman spectroscopy for identifying the crystal structures exhibited by these and isostructural compounds.
The collapse transition for two-dimensional linear and ring polymers
Authors: Bishop Marvin
Published: July 2002

The collapse transition for two-dimensional linear and ring polymers is investigated by Brownian dynamics. It is found that the mean-square radius of gyration 〈S2〉 displays a different power law for strong and weak attractive forces between chain units N. For rings 〈S2〉∼N1.5 (weak forces) and N1.0 (strong forces), whereas for linear chains 〈S2〉∼(N−1)1.5 (weak forces) and (N−1)1.0 (strong forces). Moreover, it is shown that the transition between these two regimes is sharper for linear chains.
Evidence for final state interactions in the vibrational predissociation of ICl–Ne complexes
Authors: Drobits Janet C. , Lester Marsha I.
Published: July 2002

The nascent ICl rotational state distribution is examined following vibrational predissociation of ICl–Ne A state complexes from a range of initial vibrational levels, vA. The energy released to fragments after the loss of one quantum of ICl vibrational excitation varies dramatically over the initial vA levels investigated. The ICl rotational product distributions are shown to be strongly dependent on the energy available to fragments. The efficiency of energy transfer into ICl rotations is similar for vibrational predissociation of ICl–Ne complexes prepared in vA=11, 14, and 19. The energy dependence of the peaks in the ICl rotational product distributions is consistent with an impulsive final interaction between ICl and Ne, analogous to a rotationally inelastic half-collision.
Determination of population, alignment, and orientation using laser induced fluorescence with unresolved emission. II
Authors: Waldeck J. R. , Kummel A. C. , Sitz G. O. , Zare R. N.
Published: July 2002

When an anisotropic ensemble of isolated atoms or molecules absorbs radiation from a polarized laser beam and the resulting fluorescence is collected independent of wavelength (1+1 unresolved LIF) with optional polarization analysis, the moments of the spatial distribution of the angular momentum J of the initial state in the ensemble may be determined. Previously, work by A. C. Kummel, G. O. Sitz, and R. N. Zare [J. Chem. Phys. 88, 7357 (1988)] correctly derived what possible moments can be found when cylindrical symmetry is present. This work shows that several additional alignment moments, not mentioned in KSZ, can be found in ensembles having noncylindrical symmetry.
Methyl rotation, vibration, and alignment from a multiphoton ionization study of the 266 nm photodissociation of methyl iodide
Authors: Ogorzalek Loo R. , Haerri H.-P. , Hall G. E. , Houston P. L.
Published: July 2002

The photodissociation dynamics of CH3I and CD3I have been examined by using multiphoton ionization to probe the CH3, CD3, I(≡5 2P3/2) and I*(≡I 5 2P1/2) photoproducts. The parent compounds were cooled in a supersonic expansion, collimated into a molecular beam, and dissociated at 266 nm. For the CD3I dissociation, the ratio of CD3(v=0)/(v=2) was estimated to be about 1.1, with multiple determinations ranging from 0.47–2.1. The quantum number v here denotes the nascent excitation of the ν2 ‘‘umbrella’’ mode. Measurements of the CD3(v=1) and (v=3) vibronic bands indicated that the (v=1)/(v=3) ratio is greater than unity, with some measurements suggesting values as large as 10. A value for the CH3(v=0)/(v=2) ratio from dissociation of CH3I could not be estimated, although it was clearly larger than that for CD3. The CH3(v=0) and CD3(v=0) products from this dissociation are fit by 120±30 K and 105±30 K rotational distributions, respectively. The dissociation mechanism produces alignment in the molecular frame such that there is a strong preference for K=0 (rotation perpendicular to the top axis). Assuming that the relative velocity vector lies along the CH3 C3 axis, the velocity and rotation vectors tend to be perpendicular. It is likely that K=0 molecular frame alignment is produced in photodissociation through both the I and I* channels.
Diffusion influenced reactions at short times: Breakdown of the Debye–Smoluchowski description
Authors: Eads Daniel D. , Periasamy N. , Fleming Graham R.
Published: July 2002

Fluorescence quenching studies were used to explore the validity of the Debye–Smoluchowski equation on the 0.5–200 ps time scale, using fluorescence upconversion. Complementary studies extending to 3.5 ns were carried out using time correlated single photon counting. We find, for the system rhodamine B quenched by ferrocyanide, that the fluorescence decay (i. e., reaction rate) is much faster at short times than implied by fits to the lower time resolution data. Possible reasons for the breakdown of the Debye–Smoluchowski equation at short times are briefly discussed.
Multiplet-specific shape resonant features in 3σg photoionization of O2
Authors: Braunstein M. , McKoy V. , Smith Maile E.
Published: July 2002

We report multiplet-specific photoionization cross sections and photoelectron angular distributions for the 3σg orbital of O2 leading to the b 4Σ−g and B 2Σ−g ion states obtained using Hartree–Fock photoelectron orbitals. These cross sections show significant nonstatistical behavior at low photoelectron energies which arises from the sensitivity of the kσu shape resonance to the exchange potentials of these two molecular ions. Specifically, the oscillator strength associated with the shape resonance in the B 2Σ−g cross section is shifted to lower energy compared to that of the b 4Σ−g cross section. This shift gives rise to a quartet to doublet cross section ratio of more than 15:1 near threshold. These effects are difficult to assess in the measured cross sections due to the presence of strong autoionization features. Significant multiplet dependence is also seen in the calculated photoelectron angular distributions.
Exchange dynamics of 23Na and structural incommensuration in NaMo4O6: Dynamic line shape for the second order central transition of quadrupolar nuclei
Authors: Chu P. J. , Gerstein B. C.
Published: July 2002

Sodium nuclei in NaMo4O exhibit motion as evidenced by the change in asymmetry parameter describing the central transition of 23Na with variable temperature. The nonaxially symmetric electric field gradient observed in the slow motion limit implies that the preferred occupation of the sodium nucleus is not at the center of the tetragonal oxygen cage. At room temperature the sodium jumps randomly within the cage in such a manner as to experience an axially symmetric electric field gradient on average. Dynamic NMR line shape simulations invoking second order broadening of the central transition were performed based on models of four site exchange, and all-site random exchange. Both models yield results indistinguishable from experiment, with an activation energy Ea of 1.95 k cal mol−1. An anomolous upfield shoulder in the central transition is related to the existence of a structural incommensuration. A lower transition temperature Tc=140 K is evidenced by jump discontinuities in both the temperature dependence of T1 and of the quadrupolar coupling constant e2qQ. Using a plane wave approximation to describe the structural incommensuration, the appearance of, and the gradual downfield shift of the upfield shoulder can be satisfactorily fit to the data. From the temperature dependence of the quasicontinuous frequency distribution associated with the incommensurate structure, the upper transition temperature Ti is estimated to be 520 K.
Analytical evaluation of one- and two-center Coulomb and two-center hybrid integrals for Slater-type orbitals. II
Authors: Guseinov I. I.
Published: July 2002

General formulas are derived for the two-center Coulomb and hybrid integrals for real and complex STOs with respect to the common molecular coordinate system. The angular and radial parts of these integrals are expressed through the spherical harmonics and the auxiliary functions ∼(Gq−NN′), respectively.