bluuebunny/crossref_metadata_2025 huggingface dataset:
Study of phenol–water complexes using frequency- and time-resolved pump–probe photoionization
Authors: Lipert Robert J. , Colson Steven D.
Published: July 2002

Two-color multiphoton ionization has been used to study phenol–water complexes containing one phenol and one, two, and three water molecules. Photoionization efficiency spectra show that phenol–(H2O) possesses a sharp ionization threshold, giving an adiabatic ionization potential of 63 980 cm−1 (1300 cm−1 lower than previously reported). This is followed by a vibrational progression in the ion of about 240 cm−1 that has been assigned to the hydrogen bond stretching motion. Spectra obtained with the ionization wavelength set so as to avoid fragmentation of the complexes have resulted in a reassignment of the S1 state spectra of phenol–(H2O)2 and phenol–(H2O)3. Spectra taken with the ionization laser delayed by 300 ns reveal that both phenol–(H2O)2 and phenol–(H2O)3 lose two water molecules after intersystem crossing from the vibrationless level of the S1 state. However, it is known that the triplet species obtained from phenol–(H2O)3 is much longer lived than that obtained from phenol–(H2O)2. This large difference in the triplet lifetimes provides clues as to the nature of the bonding in the parent complexes.
Extended Hückel theory for ionic molecules and solids: An application to alkali halides
Authors: Wang Yansen , Nordlander Peter , Tolk Norman H.
Published: July 2002

We present a simple method to incorporate electrostatic forces into the extended Hückel scheme, thereby extending its range of application to ionic systems. A series of applications to alkali halide molecules and clusters shows the importance of including the Madelung energy in such calculations. The amount of charge transfer, the position of the valence levels, and the band gap are found to depend on the inclusion of the electrostatic potential. The calculated bulk and surface electronic structure compares well with experimental findings.
On the choice of orbital bases for configuration interaction
Authors: Tarantelli F. , Cederbaum L. S.
Published: July 2002

We study the CI energy dependence on one-particle orbital bases obtained as SCF solutions of a general parametrized Fock operator which reduces to the Hartree–Fock mean field as a special case. In the case of single excitation CI, exact analytical expressions are derived for the curvature of the CI energy surface at the Hartree–Fock point. Numerical applications to N2 and H2O show that the convergence rate of the CI expansion can be drastically increased by an appropriate and largely system independent choice of the mean field parameters. In all configuration spaces, except single plus double excitation CI, the Hartree–Fock basis is found to be the worst orbital basis for use in CI, suggesting in general that the common-practice procedure of optimizing within a given physical model as the first step to go beyond the model itself can be a very inefficient strategy.
Dynamics of electron impact dissociative excitation of XeF2: Production of XeF (B, v′, N′)
Authors: Spence David , Dillon Michael A.
Published: July 2002

By using high-resolution electron beam techniques, we demonstrate that production of XeF(B) by electron impact dissociative excitation of XeF2 occurs via the XeF2 1Σ+u excited state. Furthermore, measurements of the fluorescence yield from the transitions XeF(B,v′, N′→X, v″, N″) in the range 325–360 nm, combined with published Franck–Condon factors for these transitions, show that dissociative excitation of XeF2 produces XeF(B) that is vibrationally hot but rotationally cool. Relative vibrational populations of XeF(B, v′, N′) are derived.
Time-resolved resonance Raman study of the spectroscopy and kinetics of the Cl−2 radical anion in aqueous solution
Authors: Hynes A. J. , Wine P. H.
Published: July 2002

The resonance Raman spectrum of the chlorine molecular anion Cl−2 has been obtained using pulsed laser photolysis as a radical production method. The fundamental and nine overtone transitions are observed. Analysis of the vibrational progression gives the vibrational constants ωe =277.0±0.3 cm−1 and ωeχe =1.83±0.05 cm−1; these constants give an upper limit dissociation energy of 1.3±0.1 eV (all errors are 1σ). Time-resolved resonance Raman spectroscopy has been employed to study the kinetics of Cl−2 disproportionation. A disproportionation rate coefficient of (11±7)×109 M−1 s−1 is obtained where the reported error includes 1σ precision and estimates of systematic errors. A resonance Raman detection limit for Cl−2 of 5×10−6 M has been obtained.
State-selective detection of CO using a tunable ArF excimer laser
Authors: Meijer Gerard , Wodtke Alec M. , Voges Heiner , Schlüter Harald , Andresen Peter
Published: July 2002

CO molecules in their electronic and vibrational ground state were state selectively detected using a tunable ArF excimer laser. Rotational state selectivity is due to the one-photon resonance of the spin-forbidden a 3Π, v′=2←X 1∑+,v″=0 transition. Subsequent absorption of two additional ArF laser photons yields, via a dissociative continuum, electronically excited C(3 1P) atoms, whose fluorescence is detected.
Argon metastable–cadmium dihalide energy transfer collisions in a flowing afterglow
Authors: Bohler Christopher L. , Schearer Laird D.
Published: July 2002

The energy dependence of dissociating collisions between argon metastable atoms and cadmium dihalide molecules have been studied in a flowing afterglow apparatus. The fluorescence spectra obtained in the range of 3000–7600 Å which result from the Ar(3P2)+CdX2 interactions indicate a dominant dissociative excitation production mechanism. The emission spectra are used to narrow the uncertainty in the currently accepted values for the dissociation energy of the CdX2 molecules. The Wigner spin rule (conservation of total electronic spin) was verified for these processes as shown by the dominance of final state triplet production as compared to the virtual absence of singlet spin state production.
Raman pump/probe measurements of state-to-state energy transfer rates in the v=1 state of H2
Authors: Farrow R. L. , Chandler D. W.
Published: July 2002

We have measured state-resolved rotational energy transfer rates of H2 (v=1) in collisions with H2 (v=0) at 295 K, using optical pump/probe techniques. A single rotational level was populated using stimulated Raman pumping, and the degree of collision-induced population transfer was monitored using either coherent anti-Stokes Raman spectroscopy (CARS) or resonance-enhanced multiphoton ionization spectroscopy (REMPI). The CARS and REMPI results are in good agreement; averages of the two sets of rates are 180±27×106 s−1 amagat−1 for j=0→2 and 50±8×106 s−1 amagat−1 for j=1→3. We also observed transfer of vibrational excitation from paraspin v=1 states to ortho v=0 states due to v–v exchange, measuring a rate of 1.9±0.8×106 s−1 amagat−1 for this process.
Quadrupole interaction of 181Ta in anhydrous HfCl4
Authors: El-Kateb S. , Martin P. W.
Published: July 2002

Nuclear quadrupole interaction parameters have been determined at 293 and 77 K at the site of 181Ta nuclei in anhydrous HfCl4 by the method of time-differential perturbed angular correlations. Measurements performed on the 133–482 keV γ–γ cascade in 181Ta following the β decay of 181Hf indicate that HfCl4 does not possess cubic symmetry as claimed by some researchers.
The absorption spectrum of the PuF6 photodissociation product in the visible wavelength region
Authors: Kim K. C. , Campbell G. M.
Published: July 2002

We report the first spectroscopic observation of the transient photodissociation product following the 248 nm (KrF excimer laser) photolysis of PuF6 and present its low resolution absorption spectrum in the visible. Although this photoproduct was postulated as the primary reaction intermediate for some time, up to now it has not been detected in the gas phase because of its extreme reactivity and instability.
Tunable far infrared laser spectroscopy of van der Waals bonds: Extended measurements on the lowest Σ bend of ArHCl
Authors: Busarow Kerry L. , Blake Geoffrey A. , Laughlin K. B. , Cohen R. C. , Lee Y. T. , Saykally R. J.
Published: July 2002

A tunable far infrared laser system has been used to measure the vibration–rotation spectrum of the lowest Σ bending state of ArHCl near 24 cm−1 in a cw planar jet operating with a terminal jet temperature near 3 K. Over 60 transitions have been observed for both 35Cl and 37Cl isotopes with resolution of the quadrupole hyperfine structure. An improved set of molecular parameters was determined, including B, D, H, and eqQ for both upper and lower states. Very narrow linewidths (approximately 300 kHz) resulting in high resolution and sensitivity make this technique a powerful new method for the detailed investigation of intermolecular forces.
Equation of state of athermal lattice chains in three dimensions
Authors: Hertanto Agung , Dickman Ronald
Published: July 2002

Monte Carlo simulations of athermal chains on a simple cubic lattice are used to determine the equation of state over the full range of densities for chains of 5, 10, 20, and 40 segments. In the short chain/low density simulations, the pressure is computed from the test-chain insertion probability. For n=20 (at high density) and n=40, we use the recently devised repulsive wall method. Our results indicate that Bawendi and Freed’s correction to mean-field theory represents a significant improvement over Flory–Huggins theory in the low density regime. Various aspects of scaling behavior and polymer–wall interactions are examined.
Oscillatory transients in a non-Markovian random walk
Authors: LaViolette Randall A.
Published: July 2002

The probability distribution of a homogeneous non-Markovian random walk on the positive integers for which each step depends upon all prior states is computed and compared with its Markovian analog. Whereas the dispersion, amplitude, and entropy of the Markovian walk each increase monotonically for all times, this behavior is attained only for long times by the non-Markovian walk. The approach to monotonic time dependence is marked by decaying oscillations in each of these attributes. The unusual behavior of this walk may be related to the kinetics of the crystallization of supercooled droplets.
A systematic approach to transition moment calculations
Authors: Blomberg M. R. A. , Liu B.
Published: July 2002

Extensive configuration interaction (CI) and multiconfiguration self-consistent-field (MCSCF) calculations on the A–X transitions of CH and CH+ are reported, which show how the transition dipole matrix element between two electronic states of a molecule can be determined accurately and systematically by a sequence of ab initio calculations. Two different computational approaches are investigated: one uses different orbitals for the two states involved and the other uses the same orbitals for both states. Both methods are shown to converge rapidly when the active orbital space is increased in such a way that all orbitals of similar importance are included at the same time. It is also shown that natural orbital occupation numbers from second-order CI calculations using a valence active orbital space provide an useful guide to the relative importance of natural orbitals; all orbitals of comparable occupation numbers must be included together.
Chemisorption structure of c(2×2)S/Fe(001) determined by angle-resolved photoemission extended fine structure
Authors: Zhang X. S. , Terminello L. J. , Kim S. , Huang Z. Q. , Schach von Wittenau A. E. , Shirley D. A.
Published: July 2002

An accurate chemisorption geometry was determined for the c(2x2)S/Fe(001) system using the angle-resolved photoemission extended fine structure (ARPEFS) method, with S(1s) photoelectron peak intensities observed along [001] and [011]. Multiple-scattering spherical-wave analysis confirmed the LEED-derived fourfold hollow site geometry, and yielded perpendicular distances for S of 1.09(2) Å above the first layer and 2.50(2) Å above the second layer atom directly below S. The S–Fe nearest-neighbor bond length is 2.30(1) Å and the M–S–M bond angle is 123(1)°. The Fe1 –Fe2 interlayer distance is contracted to 1.40(2) Å and the Fe2 –Fe3 distance expanded to 1.46(3) Å, relative to the bulk value of 1.43 Å. The results are compared with similar systems and partially explained on chemical grounds. The derived structure agrees with the results of a self-consistent field (SCF) Xα spherical wave (SW) calculation.
Molecular dynamics simulation of liquid and gaseous hydrogen selenide: Cross correlation functions
Authors: Evans M. W. , Lie G. C. , Clementi E.
Published: July 2002

The effect of the phase change from compressed gas to liquid in hydrogen selenide is explored in terms of cross correlation functions. There are considerable differences in amplitude and time dependence of the cross correlation functions which are recorded at several state points from the liquid just below the boiling point to the compressed gas. Results are obtained for 108 and 500 molecules in order to investigate the effect of sample size on cross correlation functions in the laboratory frame and in the frame (1,2,3) defined by the molecular moments of inertia.
Fine structure of triplet exciton polarons in polydiacetylene molecules
Authors: Kollmar C. , Rühle W. , Frick J. , Sixl H. , Schütz J. U. v.
Published: July 2002

Triplet states on conjugated polydiacetylene chains which are created by UV excitation are examined experimentally using ODMR spectroscopy. The observed fine structure shows that the triplet state can be ascribed to the conjugated chain rather than to the side groups and that it is localized. This leads to the suggestion of an exciton polaron. In the theoretical part the wave function of the exciton polaron is calculated using the configuration model in analogy to the description of pz radical electrons on carbene chain ends of reactive short-chain intermediates. The total fine structure tensor is obtained by summing up the fine structure tensors of the individual configurations weighted by their probability densities. The transfer integral t of the configuration model is fitted with respect to good agreement between experimentally observed and calculated fine structure parameters. Finally, comparison between the experimentally observed ESR linewidth and the calculated hyperfine structure splitting shows that the linewidth is motionally narrowed leading to the conclusion that the exciton polaron is mobile.
Configuration of a polyelectrolyte chain in the Coulombic unscreened limit
Authors: Alessandrini J. L. , Vila J.
Published: July 2002

The mean field solution of the configuration of a polyelectrolyte chain in the limit of unscreened electrostatic interactions is revisited. It is found that the Gaussian statistics behavior is recovered for weak electrostatic interactions at fixed chain size. The end to end distance is shown to crossover smoothly to that of the rod-like configuration for strong electrostatic interactions. The adsorption of a polyelectrolyte on a charged surface is briefly discussed in both limits.
High temperature chemistry of the gaseous ruthenium fluorides
Authors: Hildenbrand D. L. , Lau K. H.
Published: July 2002

Mass spectrometry was used to identify and to thermochemically characterize the gaseous fluorides RuF4 , RuF3 , RuF2 , and RuF generated by fluorination of Ru powder in a platinum effusion cell. Reaction enthalpies and bond dissociation energies were derived from equilibrium measurements involving these species, yielding the following results in kJ mol−1 : D°(F3 Ru–F)=361; D°(F2 Ru–F)=339; D°(FRu–F)=464; and D°(Ru–F)=402. The results show that the +4 oxidation state dominates the high temperature chemistry of the Ru–F system. Results are compared with the properties of the neighboring Mo–F system.
Molecular dynamics simulations on the hydration of fluoroalcohols
Authors: Kinugawa Kenichi , Nakanishi Koichiro
Published: July 2002

Molecular dynamics (MD) calculations have been carried out for aqueous solutions of isopropyl alcohol (IPA) and its fluorinated compounds, 1,1,1-trifluoro-2-propanol (TFIPA) and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIPA). The simulated systems were canonical ensembles containing 216 molecules in each, one of which was alcohol and the temperature was set to 298.15 K. The MCY (Matsuoka–Clementi–Yoshimine) potential was used for water–water interaction, whereas new potential functions were determined for alcohol–water interactions, on the basis of ab initio molecular orbital calculations on more than 1100 different dimeric configurations for each alcohol–water pair. The static properties of solvent water in the vicinity of each functional group of solute were obtained from MD calculations. It is found that the promotion of water structure and the increase of hydrogen bond between water molecules occurs not only near the fluoroalkyl group but also even near hydroxyl group of fluoroalcohols. Furthermore, the alcohol–water interaction is stronger for fluoroalcohols than for aliphatic alcohols, owing to the electronegativity and the electron withdrawing effect of fluorine atoms. The enthalpies of hydration for fluoroalcohols include the contributions from these features of both water–water and alcohol–water interactions. This is not the case for the hydration of aliphatic alcohols, and is the reason for the lack of regular change of enthalpies of hydration with the degree of the substitution of CH3 by the CF3 group. The hydration of IPA is similar to that of other aliphatic alcohols; hydrophobic hydration near the apolar group and the energetic unstabilization of water near the hydroxyl group are observed.
Matrix infrared study of NH−2 produced by electron impact on NH3
Authors: Suzer Sefik , Andrews Lester
Published: July 2002

Electron impact of dilute Ar/NH3 samples during condensation at 12 K produced six new infrared bands: 3552, 3217, 3152, 3133, 1523, and 1496 cm−1. The 3552 cm−1 band has been assigned to OH−, the 3133 cm−1 band to NH, and the 3217 and 1496 cm−1 bands to NH2 radical by earlier workers. The 3152 and 1523 cm−1 bands exhibit proper 15N shifts to be assigned to the NH−2 anion isolated in solid argon.
Comment on: The use of semiclassical approximations in problems involving double-minimum potentials
Authors: Senn P.
Published: July 2002

Errors arising through the use of semiclassical approximations for the energy levels of a one-dimensional double-minimum oscillator model of the hydrogen bond in chromous acid (CrOOH) may be sufficiently large to modify the conclusions drawn by Lawrence and Robertson recently.
Ion-induced chlorination of titanium leading to enhanced etching
Authors: O’Brien W. L. , Rhodin T. N. , Rathbun L. C.
Published: July 2002

The ion-induced chemical etching of titanium with chlorine has been studied. Quartz crystal microbalance studies show that the ion beam etch rate of Ti is enhanced upon addition of molecular chlorine, whereas molecular chlorine does not etch Ti in the absence of ion stimulation. This is very similar to the etching behavior of silicon in the presence of argon stimulation and chlorine gas. The etching of titanium is compared to a generalized version of the ion-assisted chemical etching model first proposed by Winters and Coburn. In this model the ion beam either enhances or induces one of the following chemical etching steps: initial adsorption, product formation, or product removal. The ion beam effect on product formation was determined by x-ray photoemission spectroscopy after sample etching. Ion beam effects on product removal were studied by measuring product distributions using modulated ion beam and time-of-flight techniques. It is found that the energetic ions induce formation of a chemically altered surface containing TiClx compounds. It is the ion-induced formation of this altered surface which leads to enhanced etching. Discussion in terms of the general model provides a comparison of the ion-assisted chemical etching mechanisms of titanium to silicon.
Spatial distribution of ions around rod-like polyelectrolytes
Authors: Einevoll G. , Hemmer P. C.
Published: July 2002

The nonlinear Poisson–Boltzmann equation for the spatial distribution of counterions and coions around thin cylindrical polyelectrolytes is studied. We give a perturbative treatment of the distribution functions when a small amount of salt is added. Activity coefficients for ions of different valencies are computed explicitly to dominant order in the salt concentration.In excess salt, and with equal valencies, the distribution functions are determined numerically for selected polyion charge densities. For line charge densities beyond a critical value these distributions are universal functions.
Hard-sphere mixtures near a hard wall
Authors: Tan Ziming , Marconi Umberto Marini Bettolo , van Swol Frank , Gubbins Keith E.
Published: July 2002

We report a study of hard-sphere mixtures of different sizes near a hard wall using both the Monte Carlo method and density functional theory. The theory is based on a semiempirical free-energy functional for an inhomogeneous hard-sphere mixture and is similar to that developed by Tarazona for pure hard-sphere fluids. Comparison between the theoretical results and the simulations for the density profiles of both species and the mole fraction profile shows that the present theory is capable of describing the structure of hard-sphere mixtures against a hard wall up to a size ratio, R≡σ2/σ1, of about 3. For R values greater than 3 the theory gives some discrepancies for densities very close to the wall.
The structure and dissociation dynamics of the Ne2Cl2 van der Waals complex
Authors: Hair Sally R. , Cline Joseph I. , Bieler Craig R. , Janda Kenneth C.
Published: July 2002

The structure and dynamics of Ne2Cl2 and Ne3Cl2 are studied by laser pump–probe spectroscopy. Analysis of a rotationally resolved B←X excitation band shows that Ne2Cl2 has a distorted tetrahedral structure with a Ne–Ne bond length of 3.23 Å and Ne2 center of mass to Cl2 center of mass distance of 3.12 Å. This structure is very close to that predicted by summing the atom–atom interactions. Excitation spectral shifts suggest a Ne3Cl2 structure with the neon atoms encircling the Cl2 bond axis. The total van der Waals binding energy of Ne2Cl2 is found to be between 145.6 and 148.6 cm−1, which is 20 cm−1 greater than 2*D0(Ne–Cl2)+D0(Ne2). For Cl2 stretching levels below υ′=10, transfer of one Cl2 vibrational quantum to the van der Waals vibrational modes is sufficient to dissociate both neon atoms from the complex. This indicates that the two neon atoms need not dissociate via independent, impulsive ‘‘half-collisions’’ which would require two Cl2 vibrational quanta. Observation of a NeCl2 dissociation fragment, however, indicates that such a sequential mechanism competes with the direct dissociation. Cl2 fragment rotational state population distributions for different initial vibrational levels are characterized using a simple rotational surprisal analysis. Comparison of these surprisal plots to those of the NeCl2 dissociation shows that as the size of the complex increases, so does the degree of statistical redistribution during the reaction. Even for Ne2Cl2, however, the extent of product rotational excitation is only weakly dependent upon the amount of energy available to the products and is always less than predicted by a statistical distribution between the translational and rotational product degrees of freedom.
Infrared–infrared double resonance investigation of the N=2 and N=3 stretching manifolds of silane and the local mode model
Authors: Chevalier M. , De Martino A.
Published: July 2002

We report on our IR–IR double resonance experiment study of the excited stretching state of silane involving two or three vibrational quanta, and on the interpretation of our results by using the local mode model. The DR spectra were recorded by means of two separately tunable IR beams, one of which was used to pump selected lines inside the υ3 or 2υ3 IR bands, and the other (the probe) to observe transitions from the pumped vibration–rotation states. We characterized 7 out of the 14 vibrational states belonging to these manifolds for the first time, and the A1+F2 diad localized previously at 6362 cm−1 has been studied with much higher accuracy than before. Only the two highest states of the N=3 manifold are still unknown. The vibrational energies are very well reproduced by the local mode model (≂1 cm−1 rms deviation), and the harmonic interbond couplings have been determined more precisely than before. The rotational structures of the excited states are also well accounted for by Champion’s theory of vibration–rotation coupling in Td, adapted to the description of stretching vibrations in terms of local modes. More high resolution (0.01 cm−1 or better) data are needed however to fully assess this point.
Collisional activation of cyclobutene by hexafluorobenzene: A chemical probe for highly energetic collisions in reactive systems
Authors: Morgulis Julie M. , Sapers Steven S. , Steel Colin , Oref I.
Published: July 2002

A new technique for studying reactive collisions is reported. A vibrationally hot donor molecule excites a cold reactant molecule via a single supercollision, transporting it above its threshold energy for reaction. In addition, this technique provides a means by which the functional form of transition probabilites can be found experimentally. As a specific example, vibrationally excited (110 kcal mol−1) electronic ground state hexafluorobenzene, produced by UV excitation followed by internal conversion, is the donor of vibrational energy in the collisional excitation of two acceptors, cyclobutene and cyclopropane. For cyclobutene φsens, the quantum yield of 1,3-butadiene formation in terms of light absorbed by the donor, increased markedly as the total pressure decreased. At the lowest pressures employed, 20 mTorr, φsens=(9.1±0.6)×10−4. The data are discussed in terms of highly energetic (strong) collisions which transfer enough energy to the cyclobutene to bring molecules above the critical energy for reaction (E0 =32.4 kcal mol−1). In the case of cyclopropane (E0 =65 kcal mol−1) no sensitized isomerization to propene was detected and φsens=(0±0.15)×10−4.
A semiclassical treatment of rotationally electronically inelastic scattering of NO from Ag(111)
Authors: Rice Betsy M. , Garrett Bruce C. , Swaminathan P. K. , Alexander Millard H.
Published: July 2002

The direct rotationally electronically inelastic scattering of NO from a rigid, uncorrugated Ag(111) surface is studied using the semiclassical self-consistent eikonal method (SCEM). Final rotational state distributions, summed over spin–orbit and Λ-doublet levels, are in good agreement with the exact quantum calculations of Smedley, Corey, and Alexander [J. Chem. Phys. 87, 3218 (1987)]. In addition to reproducing the rotational rainbows at low and high values of the final rotational quantum number J′, the SCEM calculation reproduces fine structure dependence of the final rotational distributions which is sensitive to quantum interference effects. Besides providing a quantitative alternative to fully quantum close coupling, the semiclassical method gives new insight into the dynamics of the collision process. For a translational energy of 6700 cm−1, population of states higher than J′=42.5 is dynamically limited, even though final rotational states up to J′=61.5 are energetically accessible. Similar dynamical constraints are observed for translational energies from 3200 to 10 700 cm−1. The dynamical constraints do not exist at Etot =2500 cm−1, resulting in an overestimation of the rotational excitation by the SCEM calculation. Translational-to-rotational energy transfer has a nonlinear dependence on initial translational energy over an energy range of 2500–10 700 cm−1. Additionally, the location of the high J′ rotational rainbow has a weak dependence on initial translational energy within this energy range. With increasing translational energy, the rotational distribution shifts to higher J′ while the high J′ rotational rainbow shifts to slightly lower J′. Also, the highest rotational state with significant population is only weakly dependent on the initial translational energy. By contrast, at a translational energy of 6700 cm−1, translational-to-rotational energy transfer is strongly dependent on initial rotational energy.
Hydrodynamic interactions and concentration dependence of orientational relaxation times of ions in dilute solution
Authors: Ibuki K. , Nakahara M.
Published: July 2002

We derived a theoretical expression for the limiting concentration dependence of ionic rotational friction coefficients using the approximations of point force and point torque and a form of hydrodynamic interaction between a pair of ions, the central ion rotating and the neighbor one translating or rotating. The pairwise hydrodynamic interactions were calculated by the reflection method and summed over all the surrounding ions in solution using the ionic distribution function due to the Debye–Hückel theory. Ionic rotational friction coefficients are predicted to increase linearly with concentration in dilute enough solution; the slope is determined by the translation rather than rotation of the countercation. The limiting slope increases with an increase in the surface charge density of the countercation if the charge effect on the cation friction coefficient is taken into account. The continuum theory has been applied to aqueous MNO3 solutions (M=Li, Na, K, Cs) at 25 °C for comparison with experiment; the theoretical predictions are in qualitative agreement with experiment.
Influence of the ac Stark effect on multiphoton transitions in molecules
Authors: Meerts W. Leo , Ozier Irving , Hougen Jon T.
Published: July 2002

A multiphoton mechanism for molecular beam transitions is presented which relies on a large first-order ac Stark effect to modulate the energy separation of the initial and final states of the multiphoton transition, but which does not require the presence of any intermediate level(s). The theoretical formalism uses ideas from the laser multiphoton literature for a two-level system interacting with a monochromatic electromagnetic radiation field, together with a close analog of the rotating wave approximation. The diagonal matrix elements of the Hamiltonian operator corresponding to the large ac Stark effect are removed by a mathematical substitution which in effect transforms appropriate differences of these diagonal elements into transition moments involving higher harmonics of the frequency of the monochromatic radiation field. The electric field strength of the true monochromatic radiation field is ‘‘distributed’’ among the higher harmonics of the effective field according to an expression involving Bessel functions. Because these Bessel functions are bounded, there exists for a given time t of exposure to the radiation, a threshold for the magnitude of the transition dipole matrix element coupling the two levels: Below this threshold, the transition probability in a traditional one-photon molecular beam electric resonance experiment cannot be made unity simply by increasing the amplitude of the radiation field. In fact, if the coupling matrix element is small enough, the molecular beam electric resonance signal cannot be detected within exposure time t. The algebraic formalism described above is checked by computer solution of an initial value problem involving four real coupled linear differential equations. It is then used to explain the multiphoton transitions previously observed in molecular beam electric resonance studies on the two symmetric top molecules OPF3 and CH3 CF3, where the number of photons involved in a given transition varies from 1–40. Application of the analysis to other experiments is briefly discussed.
Theoretical study of anharmonicity in the vibrational spectra of the cyclopropenyl cation and its deuterated analogs
Authors: Xie Yaoming , Boggs James E.
Published: July 2002

Completely theoretical calculations for the structure, anharmonic force field up to partial sixth order, and vibrational frequencies of C3H+3, C3H2D+, C3HD+2, and C3D+3 have been carried out. The procedure included ab initio calculation of the vibrational energy surface at the 6-311 G4* + local MP4(SDQ) level and anharmonic vibrational configuration-interaction (CI) calculation using a variational expansion in a large, well-selected harmonic oscillator basis. The geometric parameters of C3H+3 are optimized as 1.3632 Å for the C–C bond length and 1.0795 Å for the C–H bond length. The fundamental vibrational frequencies of C3H+3 are 3193.5, 1622.1, 1015.5, 3148.8, 1297.5, 916.2, 756.8, and 1002.7 cm−1; those of C3H2D+, (3190.6, 3144.8), 2405.3, 1580.3, 1274.7, 913.4, 1001.2, 3146.3, 1295.2, 995.4, 717.8, 916.7 and 663.6 cm−1; those of C3HD+2, are 3164.7, 2434.0 (1541.6, 1506.8), 1276.0, 672.1, 824.3, 2361.6, 1261.3, 966.7, 762.3, 960.3, and 603.9 cm−1; and those of C3D+3 are (2536.7, 2451.8), 1487.4, 837.5, 2358.1, 1256.9, 671.9, 562.6, and 823.5 cm−1. The frequencies enclosed in parentheses are for Fermi resonance pairs, the prediction of which is handled automatically by this procedure. The theoretical vibrational frequencies agree with the experimental data available from both Raman and infrared spectra with 0.5% relative mean deviation. This agreement is as good as the experimental data allow, since the experiments were carried out in condensed phases and in the presence of counterions. The first overtone vibrational frequencies of C3H+3 are also predicted. The general applicability and present limitations of the method are discussed.
Imaging of spatial pattern formation in an oscillatory surface reaction by scanning photoemission microscopy
Authors: Rotermund H. H. , Jakubith S. , von Oertzen A. , Ertl G.
Published: July 2002

The rate of catalytic carbon monoxide oxidation on a Pt(100) single crystal surface under isothermal, low-pressure conditions exhibits for certain ranges of parameters (O2 and CO partial pressures, temperature) sustained temporal oscillations whose mechanism had been explored in previous work. Coupling between reaction and diffusion leads to spatial pattern formation as manifested by patches with different work function on the intrinsically homogeneous surface. Imaging is performed by means of the novel technique of scanning photoemission microscopy. Typically, nuclei with dimensions of a few microns, as determined by the instrumental resolution, are formed spontaneously and expand with sharp fronts and velocities of about 0.5 mm/min (at 480 K) up to sizes ≥1 mm. Waves with even more extended fronts propagating with somewhat higher velocities across the sample surface are responsible for the occurrence of large amplitude temporal oscillations of the integral reaction rate.
Surface photochemistry. II. Wavelength dependences of photoinduced dissociation, desorption, and rearrangement of O2 on Pt(111)
Authors: Zhu X.-Y. , Hatch S. R. , Campion A. , White J. M.
Published: July 2002

A Pt(111) surface saturated with molecular oxygen was irradiated by UV light from a mercury arc lamp with various cutoff filters. High-resolution electron energy-loss spectroscopy (HREELS) and temperature programmed desorption mass spectroscopy (TPD) were used to characterize the products retained on the surface. Upon UV irradiation at 95 K, chemisorbed O2 (peroxo) undergoes dissociation, desorption and rearrangement. Different wavelength dependences were observed for the three processes: dissociation was not observed at wavelengths longer than 295 nm, in agreement with gas phase photodissociation of the O–O bond in hydrogen peroxide; desorption and rearrangement became negligible at wavelengths longer than 420 nm, in agreement with the photolysis of an organometallic peroxoplatinum complex. For wavelengths between 230 and 315 nm, the average cross sections of dissociation and desorption were estimated to be 5.7×10−20 and 1.2×10−19 cm2 , respectively. The possible origins of the three processes are discussed.
Multiphoton excitation spectroscopy of the B 1Σ+ and C 1Σ+ Rydberg states of CO
Authors: Tjossem Paul J. H. , Smyth Kermit C.
Published: July 2002

Two-photon absorption spectra of the CO 3s B 1∑+←←X 1∑+ (v′,v″)=(0,0), (1,1), (2,2), (1,0), and 3pσ C 1∑+←←X 1∑+ (0,0) transitions have been observed in a room temperature cell and in an atmospheric pressure hydrocarbon flame using fluorescence (B 1∑+→A 1Π) and ionization detection. The ratio of the transition strengths for the B 1∑+←←X 1∑+ (0,0) Q-branch head is found to exceed 200:1 for excitation with two identical linearly polarized photons compared with two circularly polarized photons. Weak O and S branches are observed for the first time for the B 1∑+←←X 1∑+ (0,0) transition and show the expected 2:3 polarization ratio. These measured polarization ratios are well described by two-photon absorption theory for ∑←←∑ transitions. In contrast, an earlier study by another group obtained a value of 0.7:1 for the Q-branch polarization ratio which did not fit the predictions of two-photon absorption theory.
Quantum dynamical calculations for the vibrational predissociation of the He–ICl complex: Product rotational distribution
Authors: Waterland Robert L. , Lester Marsha I. , Halberstadt Nadine
Published: July 2002

Three-dimensional quantum close-coupling calculations are presented for the vibrational predissociation of He–ICl B state complexes containing two quanta of ICl vibrational excitation. The dynamics are evaluated for the lowest quasibound van der Waals levels of He–ICl with total angular momentum J=0 and 1. The vibrational predissociation lifetime and final ICl B(v=1, j ) rotational distribution are calculated using the golden rule approximation. The calculated ICl product rotational distributions are broadly bimodal with maxima at j=7 and 15, as experimentally observed. The computed rotational distributions exhibit pronounced oscillations, which are expected to be suppressed when averaged over the initial angular momentum distribution sampled in the experiment. The theoretical analysis points to the dominant role of final-state interactions in determining the rotational distribution of the ICl fragments. The zero-point bending motion of the He–ICl complex and the coupling between the initial and final vibrational states make only small contributions to the final ICl rotational-state distributions. The extensive rotational excitation of the ICl product is primarily due to the anisotropic intermolecular interaction between the separating ICl and He fragments.
Slow spinodal decomposition in binary liquid mixtures of polymers. III. Scaling analyses of later-stage unmixing
Authors: Izumitani Tatsuo , Takenaka Mikihito , Hashimoto Takeji
Published: July 2002

Later-stage unmixing process of a near critical mixture of polybutadiene (PB)/poly(styrene- r-butadiene)(SBR) were examined at real time t and in situ at several temperatures T by time-resolved light scattering method. The magnitude of scattering vector qm(t,T) at which the intensity becomes maximum and the maximum intensity Im(t,T) were analyzed in order to characterize the coarsening processes of the later-stage spinodal decomposition. The variations of Im and qm with t at different T’s were found to fall onto master curves on the reduced plots, thus assuring the scaling postulate that the data obtained at different t and T for given mixtures are properly scaled with the temperature-dependent characteristic wave number qm(0,T) and characteristic time tc(T).
The angular distribution of photoelectrons ejected from the hydrated electron cluster (H2O)−18
Authors: Campagnola Paul J. , Posey Lynmarie A. , Johnson Mark A.
Published: July 2002

The angular distribution of the photoelectrons ejected from the hydrated electron cluster (H2 O)−18 was measured to determine the spatial character of the orbital in which the excess electron resides. The asymmetry parameter β was determined to be 0.92±0.1, consistent with a roughly spherical orbital, similar to that of the hydrated electron in solution.
Dynamics of phase separation in a binary polymer blend of critical composition
Authors: Chakrabarti Amitabha , Toral Raul , Gunton James D. , Muthukumar M.
Published: July 2002

We report results of a detailed numerical study of the phase separation process in a three dimensional model of polymer blends. The model system considered by us is described by Flory–Huggins–de Gennes free-energy functional. For a critical quench, we numerically integrate the corresponding time evolution equation for the conserved order parameter based on the above free-energy functional. We study the time dependence of the characteristic domain size as well as the pair correlation function and the structure factor for several quench temperatures. The results indicate that the growth law for the characteristic domain size is given by a modified Lifshitz–Slyozov law and the growth law exponent is independent of the quench temperature. We also find that both the pair correlation function and the structure factors show dynamical scaling at late times.
Electronic states and potential energy surfaces of H2Te, H2Po, and their positive ions
Authors: Sumathi K. , Balasubramanian K.
Published: July 2002

Geometries, bond energies, ionization potentials, dipole moments, other one-electron properties, and potential energy surfaces of six valence electronic states of H2Te and H2Po species are obtained using the relativistic complete active space multiconfiguration self-consistent field (CASSCF) followed by full second-order configuration interaction (SOCI) and relativistic configuration interaction (RCI) calculations including spin–orbit coupling. In addition, Rydberg states of H2Te and H2Se are studied to interpret the experimental spectra.

The potential energy surfaces of two electronic states of H2Te+ and H2Po+ are obtained. The ground states of both H2Te and H2Po are found to be of X 1A1(A1) symmetry with bent (C2v) equilibrium geometries of H2Te:re=1.668 Å, θe=91.2°; and H2Po:re=1.835 Å and θe=90.9°. The ground states of H2Te+ and H2Po+ are X 2B1 with H2Te+:re=1.676 Å, θe=90.7° and H2Po+:re=1.828 Å and θe=88°. The De (HTe–H) and De (HPo–H) including spin–orbit effects are calculated as 63.2 and 39.4 kcal/mol, respectively. The X 2B1(E)−A 2A1(E) energy separations of H2Te+ and H2Po+ ions are calculated as 66.6 and 76 kcal/mol, respectively. The adiabatic IPs of H2Te and H2Po are calculated as 8.47 and 7.79 eV, respectively. In addition CASSCF/SOCI/RCI calculations are also carried out on the X 2Π3/2 and 2Π1/2 states of TeH and PoH diatomics. The X 2Π3/2–2Π1/2 energy separations of TeH and PoH are computed as 3710 and 9920 cm−1, respectively. Spin–orbit effects are thus found to be very significant for PoH and H2Po. All excited states of H2Te and H2Po are above 3.7 and 3.1 eV, respectively. Properties and energy separations of H2Te and H2Po are compared with the lighter group (VI) H2Ch species.
Continuous time simulation of transient polymer network models
Authors: Biller Peter , Petruccione Francesco
Published: July 2002

A continuous time simulation algorithm for polymer melts is presented. The method is introduced via an explicit application to transient polymer network models, but it may be applied to a much larger class of models. The central quantity of the simulation is the lifetime of a strand. This can be calculated according to a proper distribution, which may depend on the orientation and length of the generated new strand. As the equation of motion of the strand can be integrated analytically during the lifetime of the strand, the procedure works much more efficiently than a previous simulation algorithm for transient network models. The material functions can be calculated as ensemble averages. The new method is shown to work for time-dependent flows. A simplified algorithm for steady flows is also given. Some predictions are made for a specific configuration-dependent transient network model.
Heavy atoms and tunneling in the X̃ state of tropolone
Authors: Redington Richard L.
Published: July 2002

Large (6.9 to 16.3 cm−1 ) tunneling splittings are uniquely observed for the ν27 (OD stretch), ν31 (carbonyl stretch), and ν34 (C=C–C stretch) fundamentals of tropolone-OH and tropolone-OD in the X̃ 1A1 (ground) electronic state. These same three modes are predicted by the molecular geometry to interact strongly with tunneling because the dominant vibrational and tunneling displacements involve the same atoms. The heavy atom tunneling displacements (≊0.07 Å) are small enough to plausibly consider heavy atom tunneling phenomena—especially in appropriate excited vibrational states—and the tunneling splittings appear consistent with behavior expected at zero order for adiabatic reaction surface theory with a 2D reaction surface defined by C=O/C–O and C=C–C heavy atom coordinates. This model attributes tunneling in the X̃ state of tropolone to heavy atom motion followed adiabatically by H atom motion rather than the reverse. Energy balance equations are used to obtain estimates for the vibrational state-specific tunneling barrier heights of tropolone-OH (13.7 and 9.3 kcal/mol for the zero-point and ν27 states) and tropolone-OD (14.3 and 11.0 kcal/mol for the zero-point and ν27 states).
Zero field nuclear magnetic resonance in high field
Authors: Tycko Robert
Published: July 2002

We present a complete description of techniques for obtaining nuclear magnetic resonance (NMR) spectra of solids that have the appearance of zero field spectra but are obtained entirely in high field. With these techniques, we gain the resolution and simplicity of zero field NMR while preserving the full sensitivity and isotopic selectivity of high field NMR. Combinations of rapid sample rotation and the synchronous application of sequences of resonant radio frequency pulses are used to average nuclear spin couplings in solids from their usual orientation-dependent forms in high field to scalar, zero field forms. We describe the theoretical basis for the techniques, the derivation of specific combinations of sample rotations and pulse sequences that produce the desired scalar average couplings, and the experimental implementation of the techniques. We use exact dynamical simulations to assess the effects of experimental nonidealities on the spectra and present new sequences designed to be less sensitive to the dominant nonidealities. Experimental proton NMR spectra of selectively deuterated, polycrystalline benzene samples are presented. The spectra show sharp lines with splittings that depend only on internuclear distances as in zero field spectra, rather than the inhomogeneously broadened ‘‘powder pattern’’ lines of traditional high field NMR spectra. Applications of the techniques to structural studies of polycrystalline and noncrystalline solids are discussed.
Evidence for coupling between hydrodynamic interactions and excluded volume effects in polymer coils
Authors: Ellis A. R. , Schaller J. K. , McKiernan M. L. , Selser J. C.
Published: July 2002

In a side-by-side comparison, the q dependence of the collective internal decay constant ∼(ΔΓ)∼qγ of linear polystyrene in good and theta solvents was determined using photon correlation spectroscopy. The results q2.9 and q3.9 for the theta and good solvents, respectively, correspond to relatively strong hydrodynamic interactions (nondraining) when excluded volume effects are minimal for the compact coil at theta and to relatively weak hydrodynamic interactions (partial draining) when excluded volume effects are manifest in the swollen coil in good solvent.