bluuebunny/crossref_metadata_2025 huggingface dataset:
Morphology-dependent stimulated Raman scattering imaging. II. Experimental studies of solvent structure in the diffuse electric double layer
Authors: Zhang Jian-Xiang , Moortgat Philip A. , Aker Pamela M.
Published: July 2002

Experimental confirmation that morphology-dependent stimulated Raman scattering can be used as an optical imaging technique is given. The water structure variations present at a charged water/air interface have been mapped. The measured structure variations track precisely that given by the Gouy–Chapman theory. We present, to our knowledge, the first experimental investigations of water solvent response in the diffuse part of the electric double layer. Imaging experiments on droplets charged to both positive and negative values have enabled us to determine the neutral water/air interface potential.
Influence of hydrodynamic interactions on the ballistic deposition of colloidal particles on solid surfaces
Authors: Pagonabarraga I. , Wojtaszczyk P. , Rubí J. M. , Senger B. , Voegel J.-C. , Schaaf P.
Published: July 2002

The ballistic deposition of particles by taking hydrodynamic interactions (HI) into account has been studied by means of computer simulations. The radial distribution function of the assembly of particles deposited on a plane has been determined as a function of the coverage and compared to experimental data. It appears that the introduction of HI in the model when compared to the ballistic model (BM) predictions leads to a better agreement between experiment and simulation in particular for the radial distribution function. HI also modify the value of the first non-vanishing term (B3) in the expansion of the available surface function, Φ, in the coverage. One can estimate the ratio BBHM3/BBM3∼0.5, where BBHM3 (resp. BBM3) corresponds to simulations in which HI have been (resp. have not been) taken into account. The introduction of HI, however, leads to small changes in Φ. Finally, we conclude that, as far as average global quantities are concerned, the BM without HI constitutes a good approximation. It is only for the detailed analysis of the structure of the layer of deposited particles that HI play a significant quantitative role.
Vibrational relaxation of a polyatomic solute in a polyatomic supercritical fluid near the critical point
Authors: Urdahl R. S. , Rector K. D. , Myers D. J. , Davis P. H. , Fayer M. D.
Published: July 2002

Vibrational lifetimes and absorption spectra of the asymmetric CO stretching mode (∼1990 cm−1) of W(CO)6 in supercritical CO2 are reported as functions of solvent density and temperature. Close to the critical temperature, the observables are density independent over a twofold range of density. Possible explanations are discussed for this unique behavior.
The torsional spectrum of CH3CD3
Authors: Moazzen-Ahmadi N. , Ozier I. , McKellar A. R. W. , Zerbetto Francesco
Published: July 2002

The torsional spectrum of gaseous CH3CD3 has been measured between 230 and 280 cm−1 under relatively large pressure-path length conditions. The observations were made using a modified Bomem spectrometer at a resolution of 0.016 cm−1 and an absorption path of 20 m. The gas temperature and pressure were 190 K and 82 Torr, respectively. The prominent features of the spectrum are the P, Q, and R branches of the torsional fundamental (v6=1←0). The P and R branches consist of broad peaks from 0.3 to 0.5 cm−1 wide showing partially resolved rotational structure. A frequency analysis combining the present far-infrared frequencies with molecular beam, microwave, and mm-wave measurements from previously reported experiments yielded an improved set of torsion–rotation parameters. An intensity analysis was carried out to obtain the torsional dipole moments. The torsional dipole components μT⊥ and (μT∥−μT⊥) were determined to be 14.0(1.5) and −39.6(4.0) μD, respectively. The conditions under which there is agreement between the experimental values and those calculated from theoretical expressions are discussed.
The HNCO heat of formation and the N–H and C–N bond enthalpies from initial state selected photodissociation
Authors: Brown Steven S. , Berghout H. Laine , Crim F. Fleming
Published: July 2002

We measure upper limits for the bond enthalpies of the N–H and C–N bonds in HNCO by observation of photodissociation appearance thresholds for the NCO (X2Π) and NH (a1Δ) fragments from initially selected HNCO vibrational states. The upper limit of the dissociation energy of the H–N bond is D0(H–NCO)≤109.6±0.4 kcal/mol and that of the N–C bond is D0 (HN–CO)≤122.1±0.3 kcal/mol. Observation of unrelaxed fragment quantum state distributions at fixed energies supports the bond enthalpy measurement. The two appearance thresholds, together with known heats of formation of NH, NCO, H, and CO, provide two independent methods of calculating the HNCO heat of formation. Both methods give a value of ΔHf00 (HNCO)≥−27.7±1.1 kcal/mol. The consistency of the two methods for calculating ΔHf00 (HNCO) suggests that the actual bond enthalpies for the N–H and C–N bonds are close to the upper limits from the measurement.
Dipole–quadrupole and dipole–octopole polarizability for CH4 and CF4
Authors: Maroulis George
Published: July 2002

We have relied on accurate finite-field self-consistent field and coupled-cluster calculations to obtain for the dipole–quadrupole and dipole–octopole polarizability of CH4 the values A=9.01 e2a30Eh−1 and E=−18.9 e2a40Eh−1. For CF4 our best values combine finite-field self-consistent field and complete fourth-order perturbation theory calculations and they are A=12.4 e2a30Eh−1 and E=−27.8 e2a40Eh−1.
Atomic stress isobaric scaling for systems subjected to holonomic constraints
Authors: Melchionna S. , Ciccotti G.
Published: July 2002

We show that in the molecular dynamics simulation of systems where the molecules are subjected to holonomic constraints the isobaric–isothermal (NPT) techniques can be applied by scaling in space the atomic coordinates rather than the molecular center of mass, as has been customary until now. The correct constrained distribution function is obtained by applying the original Andersen transformation or equivalently the Hoover-type NPT equations of motion. The latter set of equations are shown to be derived from an underlying Lagrangian. The atomic scaling is preferred to the molecular one when simulating large molecules since molecular metastabilities are more easily overcome. However, also for systems composed of small molecules, the approach can be interesting since in this way cell rotations are automatically avoided.
The structure and stability of BnH+ clusters
Authors: Ricca Alessandra , Bauschlicher, Jr. Charles W.
Published: July 2002

The geometries of the BnH+ clusters for n=1–13 have been optimized at the B3LYP level of theory. Excluding B2H+, the ground states of all clusters are low-spin coupled. The structures appear to be related to the bare Bn+ clusters in two ways: (1) an H atom is added to the Bn+ cluster or (2) a BH is added to the Bn−1+ cluster. The Bn+–H binding energies are computed at the B3LYP level of theory, and calibrated using the CCSD(T) level of theory. The computed results tend to fall between the experimental best estimates and the experimental lower bounds. The Bn+–H binding energies show an inverse correspondence with the Bn−1+ –B values, while the Bn−1+–B and Bn−1+ –BH results tend to parallel each other.
Theoretical study of barrier height to linearity of bent triatomic molecules
Authors: Lee Jae Shin
Published: July 2002

A systematic ab initio investigation has been carried out to examine the effects of a basis set and correlation method on barrier height to linearity of bent triatomic CH2 and H2O in their ground electronic states. The theoretical models employed varied from Hartree–Fock with minimal STO-3G to highly correlated quadratic configuration interaction method QCISD(T) with Dunning’s correlation-consistent aug-cc-pVQZ basis set. It is shown that the barrier height to linearity is very sensitive to the choice of the model and it is necessary to employ a proper theoretical model with large enough basis set to be assured of the convergence of the calculated barrier height. We found that calculated barrier height to linearity tends to be higher than experimentally determined barrier height in most cases, which is caused by the slower convergence of energy at linear saddle point geometry than at bent equilibrium geometry. The effect of frozen core approximation in correlated calculations is shown to always increase the barrier height to linearity slightly (less than 3% of total amount) from the value with the full activation of orbitals, regardless of the basis set or correlation method used in the study.
Theoretical study of the prewetting transition in polymer blends
Authors: Pereira Gerald G.
Published: July 2002

Prewetting transitions in polymer blends near a hard surface which favors one of the phases in the blend are studied by both mean-field and Monte Carlo methods. The mean-field results predict for systems that have a first-order wetting transition at a bulk density of (ρ∞)W, there exists first-order prewetting transitions for (ρ∞)W⩽ρ∞<(ρ∞)PW. For ρ∞>(ρ∞)PW there exist second-order transitions so that (ρ∞)PW may be identified as the prewetting critical point. Monte Carlo simulations of the bond fluctuation model on a simple cubic lattice between two hard walls H lattice spacings apart are performed and qualitative agreement is found with the mean-field predictions.
Adiabatic theory of laser-induced vibrational predesorption of physisorbed molecules: Application to a CO/NaCl system
Authors: Ohtsuki Y. , Kato T. , Fujimura Y. , Lin S. H.
Published: July 2002

Laser-induced vibrational predesorption of molecules physisorbed on insulating substrates is theoretically investigated based on the Markoff master equation. The system vibrations, which consist of intramolecular vibrations of an admolecule and admolecule-surface vibrations, are divided by the adiabatic approximation, whereby the predesorption is represented by the nonadiabatic transitions from the bound states to the desorption continuum. By using the projection operator in the double(Liouville)-space representation, the bound-continuum couplings due to the nonadiabatic and the optical interactions are explicitly included in the master equation. The adiabatic theory is applied to CO physisorbed on a NaCl(100) surface, in which CO stretching and CO-surface vibration are chosen as the system vibrations. This two-dimensional model with a shallow Morse potential for the CO-surface potential gives a desorption rate of ∼10−4 s−1, which agrees with the experimentally measured rate by Chang and Ewing [Chem. Phys. 139, 55 (1989) and Phys. Rev. Lett. 65, 2125 (1990)]. The rate equations explicitly derived from the master equation are used to analyze the desorption dynamics. It is shown that predesorption is considerably enhanced by the incoherent phonon-assisted predesorption mechanism, i.e., by thermal excitation of the CO-surface stretching in the manifold of the excited CO stretching. Excitation by a single laser is extended to the two-laser excitation scheme to accelerate the predesorption.
Fluctuations and relative Boltzmann entropy
Authors: Eu Byung Chan
Published: July 2002

It is shown in this paper that the variables conjugate to extensive macroscopic variables (or their molecular expressions) fluctuate from the thermodynamic intensive variables appearing in the extended Gibbs relation for the calortropy and that the relative Boltzmann entropy introduced in an earlier work [J. Chem. Phys. 102, 7169 (1995)] is intimately related to the fluctuations of the conjugate variables mentioned. We examine the nature of the fluctuations, and their implications and meanings for the relative Boltzmann entropy in kinetic theory. Based on this analysis, we find that even if the fluctuations vanish, the time derivative of the relative Boltzmann entropy depends on the path of evolution of irreversible processes and thus makes the time derivative of the Boltzmann entropy path-dependent, that is, a non-exact differential, in the thermodynamic space. The evolution equations for fluctuations are also presented which provide the solution of the Boltzmann equation beyond the level of description by the nonequilibrium canonical form for the distribution function. It is shown that, when the fluctuations are treated statistically, there follow thermodynamic uncertainty (thermodynamic complementarity) relations between thermodynamic conjugate variables in the case of nonequilibrium.
A Monte Carlo simulation study of the temperature dependence for the conformation distribution of 1,2-dimethoxyethane in water
Authors: Engkvist Ola , Karlström Gunnar
Published: July 2002

Monte Carlo simulations of 1,2-dimethoxyethane in water were performed at two temperatures; 298 and 398 K. Analysis of the simulations shows that the anti–anti–anti conformer of 1,2-dimethoxyethane is the only of the most populated conformers that increases its probability with increasing temperature. This behavior suggests that clouding of polyethyleneoxide-water solutions is induced by conformational changes. In both simulations the amount of the anti–anti-gauche conformer is surprisingly large, even though convergence problems of the simulation at 298 K occurred. A high amount of the anti–anti-gauche conformer of 1,2-dimethoxyethane in water solution have not been reported before.
O2 transient trapping-desorption at the Ag(111) surface
Authors: Raukema Age , Butler David A. , Kleyn Aart W.
Published: July 2002

Molecular beam scattering experiments of O2 from Ag(111) carried out at a surface temperature of 150 K, which is below the desorption temperature for the molecular chemisorption state, show three different scattering paths: physisorption followed by desorption, direct-inelastic scattering and transient trapping-desorption. The transient trapping-desorption process is attributed to transient adsorption of the molecule in a metastable O2δ− state at the surface. The translational desorption energy of the transiently trapped molecules is far above thermal, strongly dependent on the surface temperature and independent of the translational energy and angle of the incident oxygen molecule. A strongly peaked intensity distribution around the surface normal is observed for the desorption. The transient trapping probability shows a sharp increase above a threshold energy and a subsequent decrease with increasing incidence energy. It is accompanied by a strong broadening in the angular direct-inelastically scattered flux distribution. The possible origin of the metastable O2δ− state will be discussed.
Toward the theory of the electron affinity of large dielectric clusters: Quantum size correction
Authors: Iakubov I. T. , Pogosov V. V.
Published: July 2002

The consecutive procedure for the calculation of a size dependence of an electron affinity to a large dielectric cluster of radius R is presented. The perturbation theory over the small parameters r̄/R and L/r̄ (L is the scattering length in the condensed medium, r̄ is the interatomic distance) is proposed for an electron binding energy in the frame of effective medium approach, using the theory of state of light quantum particle in a cellular medium. Estimations are fulfilled for liquid xenon clusters.
Potential energy surface and wave packet calculations on the Li+HF→LiF+H reaction
Authors: Aguado Alfredo , Paniagua Miguel , Lara Manuel , Roncero Octavio
Published: July 2002

In this work an analytic fit of previous ab initio points [Aguado, Suárez, and Paniagua, Chem. Phys. 201, 107 (1995)] on the potential energy surface of the LiFH system is presented and the reaction dynamics is studied using a time-dependent treatment based on local coordinates. Three-dimensional wave packet calculations performed for zero total angular momentum indicate that the reactivity for the HF reactant in its ground vibrational state is quite low, in contradiction with previous dynamical calculations using different potential energy surfaces. The differences with previous potential energy surfaces are further analyzed using a bidimensional approach. Finally, the effect of the initial vibrational excitation of the HF reactant on the reactivity is studied using the bidimensional approach.
Inner-shell excitation of monocyanoethylene, trans-dicyanoethylene, and allylcyanide by electron energy loss spectroscopy
Authors: Hubin-Franskin M.-J. , Aouni H. , Duflot D. , Motte-Tollet F. , Hannay C. , Ferreira L. F. , Tourillon G.
Published: July 2002

Inner-shell excitation spectra of gaseous monocyanoethylene, trans-dicyanoethylene, and allylcyanide have been measured at C1s and N1s edges using 2 keV electron collisions in quasi-dipolar excitation conditions and the electron energy loss spectroscopy. The energy resolution has allowed the observation of transitions from different carbon sites. Ab initio calculations have been made to help the assignment of the experimental features. The spectra below the core-electron ionization limit have been interpreted in terms of transitions to the lowest-energy valence unoccupied molecular orbitals shown to be of π* type in each molecule. The effects of the conjugation between the multiple bonds on the spectra and on the splitting of the π* type molecular orbitals have been discussed. Strong electronic relaxation effects have been evidenced.
Spectroscopy of mass-selected rhodium dimers in argon matrices
Authors: Wang Huaiming , Haouari Hanae , Craig R. , Liu Yifei , Lombardi John R. , Lindsay D. M.
Published: July 2002

The absorption (scattering depletion) spectrum and Raman spectra for Rh2 in an argon matrix prepared by the mass selected ion deposition technique have been obtained. The absorption spectrum in the visible region shows a single broad transition centered near 495 nm Resonance Raman spectra obtained by exciting with eight visible lines (457.9–514.5 nm) of an argon ion laser give a single, sharp progression with up to four Stokes transitions. These data give ωe=283.9 (18) cm−1 with ωexe=1.83 (33) cm−1, leading to an experimental dissociation energy of 1.4±0.3 eV. The Raman results are discussed in the context of previous theoretical predictions for the dimer.
Statistics of wormlike chains. II. Phase transition of polymer liquid crystals and its mixture with low molecular weight liquid crystals
Authors: Zhang W. X. , Zhao S. R. , Sun C. P.
Published: July 2002

A general self-consistent field (SCF) for the mixture of polymer and low molecular weight (LMW) molecules has been derived by variation principle. Considering a Maier–Saupe type of interaction, the analytical expressions of the SCF for polymer liquid crystals (PLCs) and the mixture of PLCs and LMW liquid crystals are obtained, from which the phase behaviors of PLCs as well as the mixture are studied. The theoretical results are in agreement with experimental results by adjusting a parameter.
Dynamics of poly(oxyethylene) melts: Comparison of 13C nuclear magnetic resonance spin-lattice relaxation and dielectric relaxation as determined from simulations and experiments
Authors: Smith Grant D. , Yoon Do Y. , Wade Charles G. , O’Leary Daniel , Chen Angela , Jaffe Richard L.
Published: July 2002

Molecular dynamics simulations of poly(oxyethylene) (POE) melts have been performed using a previously derived quantum-chemistry-based force field. Local chain dynamics for POE melts in the form of 13C nuclear magnetic resonance (NMR) spin-lattice relaxation times (T1) and NOE values and dielectric relaxation behavior have been determined from molecular dynamics simulations and compared with experimental measurements. 13C NMR T1 and NOE values for the methyl, α, and interior carbons from molecular dynamics simulations are in good agreement with experimental values over a wide range of temperatures. Similar agreement is seen for the dielectric relaxation strength and maximum dielectric loss frequency as a function of temperature. Non-Arrhenius behavior is seen for the correlation times of the P2CH orientational autocorrelation function (OACF), the molecular dipole moment OACF, and the torsional autocorrelation function (ACF). Very close correspondence between the torsional ACF and the molecular dipole moment OACF indicates that dielectric relaxation in POE melts is due almost exclusively to local conformational motions.
The gas-phase acidity of H3PO4
Authors: Morris Robert A. , Knighton W. B. , Viggiano A. A. , Hoffman Brian C. , Schaefer III Henry F.
Published: July 2002

The gas-phase acidity (free energy of deprotonation, ΔGacid0) of H3PO4, orthophosphoric acid, was determined both experimentally and theoretically for the first time. Ion-molecule bracketing experiments yielded the result 1372 kJ mol−1>ΔGacid0(H3PO4)>1331 kJ mol−1. The most reliable theoretical result, ΔGacid0H(H3PO4)=1343 kJ mol−1, is in agreement with experiment and was obtained using Brueckner methods with large basis sets.
Adsorption and decomposition of formic acid on MgO(001) surface as investigated by temperature programmed desorption and sum-frequency generation spectroscopy: Recurrence induced defect sites
Authors: Yamamoto Hiroyoshi , Watanabe Nobuyuki , Wada Akihide , Domen Kazunari , Hirose Chiaki
Published: July 2002

The change of the MgO(001) surface during the adsorption and thermal desorption of formic acid was investigated by temperature programmed desorption (TPD) and infrared-visible sum-frequency generation SFG spectroscopy. The TPD results revealed that the feature of desorption signals changed as the adsorption/thermal desorption process was repeated, despite no significant change of the low energy electron diffraction (LEED) pattern. The surface formate, produced by dissociative adsorption of formic acid, decomposed at 560 K and 700 K, giving H2O+CO and H2+CO2 as primary and minor products, respectively, but the broad desorption peak of H2O started to appear at 300 K. The 560 K peak appeared from an early stage, while the 700 K peak gained intensity as the experiment was repeated and the density of defects increased. The adsorption site for the 700 K peak was ascribed to the catalytic reaction of magnesia powder. The SFG spectra gave three resonance peaks of CH stretching bands at 2850 cm−1, 2870 cm−1, and 2920 cm−1 with a different dependence on the dose time, history of surface, and polarization of incident visible beam. The former two bands were assigned to the formates with bridged configuration, and the third band to the formate with unidentate configuration. The polarization dependence suggested that the CH bonds of bridged formates stood perpendicular to the surface, and that of the unidentate formate tilted approximately 30° from the surface normal. A vibration nonresonant background signal was present on the SFG signal, showing characteristic dependence on the polarization of incident light beams, as well as the coverage and history of the surface, and was interpreted as originating from the defect sites on the surface.
Kinetics of concurrent desorption and diffusion into the solid: D/Zr(0001)
Authors: Kovar M. , Griffiths K. , Kasza R. V. , Shapter J. G. , Norton P. R. , Zhdanov V. P.
Published: July 2002

Rapid adsorbate diffusion into the solid is known to suppress the desorption yield measured in a thermal desorption experiment. We show that this suppression can be controlled (at least partly) by pulsed-laser heating at rates in excess of 1010 K/s. As an example, we analyze the D/Zr system. In this case, deuterium adsorbed on a surface rapidly diffuses into the bulk of Zr with increasing temperature, and the deuterium desorption probability measured with conventional heating rates (β⩽100 K/s) is as low as ≈10−4 for polycrystalline Zr foils (deuterium desorption is not observed at all from single-crystal Zr from which dissolved H/D has been removed). Heating the Zr(0001) surface by pulsed-laser thermal excitation with β≃1011 K/s is demonstrated to result in the increase of the deuterium desorption probability up to approximately 0.01. To interpret this observation, general equations for describing associative desorption accompanied by adsorbate diffusion into the solid are simplified by employing the specifics of the temperature-programmed kinetic regimes with a linear increase of temperature. The desorption yield calculated without any adjustable parameters is in good agreement with the experimental results.
Distributed approximating functional approach to the Fokker–Planck equation: Eigenfunction expansion
Authors: Zhang D. S. , Wei G. W. , Kouri D. J. , Hoffman D. K.
Published: July 2002

The distributed approximating functional method is applied to the solution of the Fokker–Planck equations. The present approach is limited to the standard eigenfunction expansion method. Three typical examples, a Lorentz Fokker–Planck equation, a bistable diffusion model and a Henon–Heiles two-dimensional anharmonic resonating system, are considered in the present numerical testing. All results are in excellent agreement with those of established methods in the field. It is found that the distributed approximating functional method yields the accuracy of a spectral method but with a local method’s simplicity and flexibility for the eigenvalue problems arising from the Fokker–Planck equations.
Crossed beam reaction of atomic carbon C(3Pj) with hydrogen sulfide, H2S(X1A1): Observation of the thioformyl radical, HCS(X2A′)
Authors: Kaiser R. I. , Sun W. , Suits A. G.
Published: July 2002

One of the simplest organosulfur reactions, that between ground state carbon atoms, C(3Pj), and hydrogen sulfide, H2S(X1A1), was studied at an average collision energy of 21.0 kJ mol−1 using the crossed molecular beams technique. The product angular distribution and time-of-flight spectra of m/e=45 (HC32S) were monitored. Forward-convolution fitting of our data yields an almost isotropic center-of-mass angular flux-distribution, whereas the center-of-mass translational energy flux distribution peaks at about 50 kJ mol−1, indicating a tight exit transition state from the decomposing thiohydroxycarbene HCSH complex to the reaction products. The high energy cut-off of the translational energy flux distribution is consistent with the formation of the thioformyl radical HCS in its X2A electronic ground state. The first experimental verification of an existing thiohydroxycarbene intermediate and the rigorous assignment of the HCS radical product under single collision conditions explicitly suggest inclusion of the title reaction in chemical reaction networks of molecular clouds TMC-1 and OMC-1, the outflow of the carbon star IRC+10216, Shoemaker/Levy 9 impact-induced nonequilibrium sulfur chemistry in the Jovian atmosphere, as well as combustion of sulfur containing coal.
Integral equation theory of solutions of rigid polyelectrolytes
Authors: Shew Chwen-Yang , Yethiraj Arun
Published: July 2002

The properties of dilute and semidilute solutions of rigid polyelectrolytes are investigated using integral equation theory. The theory predicts liquidlike structure on length scales of the order of the length of the molecules in dilute solution. This structure appears at concentrations much lower than the overlap threshold concentration, and disappears when the concentration is sufficiently high. Liquidlike order reappears at higher concentrations on a lengthscale of the order of the thickness of the rods. The predictions of the theory for the static structure factor in tobacco mosaic virus solutions are in good agreement with light scattering experiments in both dilute and semidilute solutions. The theory predicts that kmax∼ρν, where kmax is the position of the maximum in the structure factor and ρ is the concentration, with ν≈1/3 and 1/2 in the dilute and semidilute regimes, respectively. These values are consistent with experimental results. Predictions are also presented for rigid starlike polymers.
Spectroscopic characterization of the Δ3(4d), Π3(4d), Σ+3(4d), and Π3(5p) Rydberg states of the MgAr van der Waals molecule
Authors: Massick Steven , Breckenridge W. H.
Published: July 2002

The Mg(3s4dσ 3DJ)⋅Ar(3Σ+), Mg(3s4dπ 3DJ)⋅Ar(3Π), Mg(3s4dδ 3DJ)⋅Ar(3Δ), and Mg(3s5pπ 3PJ)⋅Ar(3Π) Rydberg states have been characterized via Resonance Enhanced Two-Photon Ionization (R2PI) spectroscopy of transitions from the long-lived Mg(3s3pπ 3PJ)⋅Ar(3Π0+,0−) metastable states of the MgAr van der Waals molecule. The 4dπ, 4dδ, and 5pπ states are all strongly bound (D0=1230±50, 1200±40, 1270±50 cm−1, respectively). These bond energies are very similar to that of the ground-state MgAr+ “core’’ ion (D0=1240±40 cm−1), indicating very effective penetration of transversely aligned, diffuse Rydberg electron clouds on the Mg atom by the Ar atom, even for the low-lying n=4,5 Rydberg states. The Mg(3s4dσ 3DJ) ⋅Ar(3Σ+) state was substantially less bound, D0=800±40 cm−1, showing there is still some residual Mg(4dσ)/Ar(3pσ)2 electron–electron repulsion preventing penetration of the axially aligned Mg(4dσ) Rydberg electron cloud by the Ar atom. Successful computer simulations of the rotational structure of several of the vibrational transitions to the 4dΔ and 4dΣ states, assuming Hund’s case “b’’ upper-state character, resulted in R0′ values of 2.80±0.04 Å and 2.90±0.05 Å, respectively (compared to R0=2.82±0.01 Å determined previously by others for the MgAr+ ion). Because the 4dπ and 5pπ states have similarly large bond strengths as well as similar asymptotic Mg(4d 3DJ) and Mg(5d 3PJ) atomic energies, they have “mixed’’ 4dπ/5pπ character.
A simplified approach to the interpretation of nuclear spin correlations in entangled polymeric liquids
Authors: Ball R. C. , Callaghan P. T. , Samulski E. T.
Published: July 2002

We derive closed-form expressions for a sine correlation function β(2τ,τ) which is particularly sensitive to modulations of nuclear dipolar (and quadrupolar) interactions caused by slow molecular reorientations. This function is placed in the context of traditional nuclear magnetic resonance (NMR) experiments—the Free Induction Decay R(t) and the solid echo amplitude E(τ) and families of expressions are evaluated for (i) motion governed by an exponential correlation function and (ii) macromolecular motion dominated by reptation. We introduce a simple correlation function for reptation that depends on only two parameters, M2¯, the preaveraged dipolar interaction strength and τd, the tube disengagement time. From proton NMR measurements of β in poly(dimethyl siloxane) melts we estimate M2¯=1.3×106 s−2 and τd=0.019 s at 300 K, in reasonable agreement with a calculated value for M2¯ and an independent NMR estimate of the terminal relaxation time.
Millimeter-wave spectroscopy of the iron carbonyl radical (FeCO)
Authors: Tanaka Keiichi , Shirasaka Mitsuaki , Tanaka Takehiko
Published: July 2002

The pure rotational spectrum of the iron-carbonyl radical FeCO produced by a dc discharge of iron pentacarbonyl Fe(CO)5 was observed in the millimeter-wave (MMW) region. Ten rotational transitions, split into triplets by the electron spin–spin interaction, were assigned to the electronic ground state of FeCO, definitely confirming that the FeCO radical has 3Σ− symmetry. Molecular constants in the ground state were derived from an analysis of the MMW spectrum combined with the rotational lines in the lowest spin component Ω=0 observed by Fourier transform microwave (FTMW) spectroscopy. The rotational and centrifugal distortion constants were determined as B0=4363.883 42(40) MHz and D0=1.217 99(84) kHz. The spin–spin coupling constant and its centrifugal distortion correction are λ0=684.470(51) GHz and λD=−258.08(17) kHz, respectively, and the spin–rotation coupling constant is γ0=−1127.80(35) MHz, where the figures in parentheses are standard errors to be attached to the last digit.
Electronic structure of vanadium cluster anions as studied by photoelectron spectroscopy
Authors: Iseda Masako , Nishio Tetsuya , Han Sang Yun , Yoshida Hiroyuki , Terasaki Akira , Kondow Tamotsu
Published: July 2002

Photoelectron spectra of vanadium cluster anions, Vn− (3⩽n⩽100), were measured at a photon energy of 3.49 eV (355 nm) by using a magnetic-bottle-type photoelectron spectrometer. The electronic density-of-state profiles and the photoelectron spectra of V4− and V3− were calculated by the spin-polarized DV (discrete variational)-Xα method for several plausible geometrical structures. The most plausible structure was determined so that the calculated photoelectron spectrum based on this structure well reproduces the observed one. The calculation showed that V4− has a square planar geometrical structure and a charge distribution of D4h symmetry. On the other hand, V3− was found to possess an equilateral triangle geometrical structure, but C2v symmetry for the charge distribution. The electronic states in the vicinity of the Fermi energy were found to consist of 3d atomic orbitals. The population differences between the minority and the majority spins per atom turned out to be 4.6 and 1.7 for V4− and V3−, respectively. The result implies that the d-electron spins are coupled ferromagnetically in these cluster anions. In addition, the size-dependence of the electron affinity of Vn was explained by a spherical conducting droplet model in the n≳9 range.
Solvation and reorganization energies in polarizable molecular and continuum solvents
Authors: Bader Joel S. , Cortis Christian M. , Berne B. J.
Published: July 2002

The solvation free energy difference, ΔG, and reorganization energy, λ, of the electronic transition between the ground and first excited state of formaldehyde are investigated as a function of the solvent electronic polarizability in aqueous solution. Solvent shifts are difficult to measure experimentally for formaldehyde due to oligomer formation; shifts for acetone, which have been measured experimentally, are used instead for comparison with computational results. Predictions of the Poisson–Boltzmann equation of dielectric continuum theory with molecular shaped cavities and charges on atomic sites calculated from ab initio quantum chemistry are compared with direct molecular dynamics simulations using the fluctuating charge model of polarizable water. The explicit molecule simulations agree with the acetone experimental results, but the continuum dielectric calculations do not agree with explicit solvent or with experiment when the default model cavity is used for both the ground and excited state molecule. Several different algorithms are used to define the size of the molecular cavity in the ground and excited states, but we are unable to find a single set of atomic radii that describe adequately all the data. Quantitative calculations from a continuum model might therefore require charge-dependent solute cavity radii.
Condensed phase electron-energy-loss spectroscopy of the low-lying triplet states of acetylene
Authors: Swiderek P. , Michaud M. , Sanche L.
Published: July 2002

Low-energy electron-energy-loss spectra of solid acetylene were recorded in the range of the low-lying valence excited triplet states at a temperature of 17 K. The spectra show that the first band, which was previously assigned to a single transition, actually contains several overlapping singlet–triplet transitions. The transition Σg+1→3Au starts with an extrapolated lowest vibronic transition at 4.89 eV and is dominated by the bending vibrational mode with frequency 86 meV (694 cm−1). Below this band the spectra show a short progression with vibrational spacings of 100 meV (806 cm−1) starting at 4.62 eV and a weak band with irregular vibrational spacings close to the energy-loss threshold. Both are assigned to the transition to the lowest triplet potential energy surface, which correlates to the linear Σu+3 state and the two bent B2 and Bu isomer states.
Pump–probe spectroscopy of photoinduced charge transfer reactions in solution
Authors: Yang Dah-Yen , Sheu Sheh-Yi
Published: July 2002

A theory of pump–probe spectroscopy of photoinduced charge separation processes in solution is developed. The pump–probe pathway is described by pump with excitation pulse which generates a nonstationary initial state in the first excited state. This initial state evolves and transfers to a second excited state (charge transfer state) nonadiabatically, and the system is probed by a second probe pulse. Doorway and window function approximations are used to separate these processes. Solution is treated as a continuum (Debye) model with one longitudinal relaxation time. Hence, after Wigner transformation of the time evolution of density matrix in Liouville space, the solvation dynamics of nonadiabatic charge separation processes obeys the semiclassical Zusman equation. Using the eigenfunction expansion method, we solve the bare absorption spectrum numerically. An electronic coherence motion is observed in the overdamped exothermic invert regime.
Doppler-free two-photon absorption spectroscopy of the A1Au←X1Ag transition of trans-glyoxal
Authors: Katô Hajime , Oonishi Takuya , Nishizawa Kiyoshi , Kasahara Shunji , Baba Masaaki
Published: July 2002

Doppler-free two-photon absorption spectra of the A1Au←X1Ag(π*−n) transition of trans-glyoxal have been measured by means of two-photon absorption spectroscopy with counter-propagating light beams of identical photons within an external cavity. The relative energies of transition lines are measured with accuracy better than 0.0001 cm−1. Rotational lines are fully resolved, and 1809 lines of the A1Au(v7=1)←X1Ag(v=0) transition are assigned for J=0−75,K=0−17. Rotational constants of the A1Au(v7=1) and X1Ag(v=0) states are determined by a least-squares fitting of eigenvalues of the A-reduced rotational Hamiltonian to energies of the assigned lines. Energy shifts, intensity anomalies, and line splittings are observed for several lines. When an external magnetic field is applied, remarkable changes are observed for these lines. They are identified as originating from perturbations between the A1Au and a3Au states, which become appreciable when perturbing levels are close in energy. Splittings into three lines are observed for strongly perturbed levels of I=1, and these splittings are identified as the hyperfine splitting caused by mixing of the a3Au state.
On the heats of formation of trifluoromethyl radical CF3 and its cation CF3+
Authors: Asher Robert L. , Ruscic Branko
Published: July 2002

The CF+ and CF3+ fragment ion yield curves from C2F4 have been remeasured by photoionization mass spectrometry. Fits with appropriate model curves yield the appearance potentials AP0(CF3+C2F4)=13.721±0.005 eV and AP0(CF+/C2F4)=13.777±0.005 eV and an accurate difference in ionization potentials, IP(CF)−IP(CF3)=0.055±0.003 eV. With the existing photoelectron value IP(CF)=9.11±0.01 eV, this produces IP(CF3)=9.055±0.011 eV. The CF3+ fragments from CF3Cl, CF3Br, and CF3I have also been remeasured, and their ion yield curves fitted with model functions. The experimentally derived AP0(CF3+/CF3Cl)<12.867±0.008 eV has been found to be only an upper limit. The Analogous CF3+CF3+ fragment yield curves from CF3Br and CF3I produce AP0(CF3+/CF3Br)=12.095±0.005 eV and AP0(CF3+/CF3I)=11.384±0.005 eV, leading to D0(CF3−Br)=70.1±0.3 kcal/mol (70.8±0.3 kcal/mol at 298 K) and D0(CF3−I)=53.7±0.3 kcal/mol (54.3±0.3 kcal/mol at 298 K). Based on tabulated values for ΔHf∘(CF3Br) and ΔHf∘(CF3I), which appear to be inconsistent by ∼1 kcal/mol, a compromise value of ΔH298 f∘(CF3)=−111.4±0.9 kcal/mol (−110.7±0.9 kcal/mol at 0 K) is selected, resulting in ΔH298f∘(CF3+)=97.4±0.9 kcal/mol (98.1±0.9 kcal/mol at 0 K). Additionally, IP(CF4)≡AP0(CF3+/CF4)=14.67±0.04 eV can be inferred. From data on C2F4, ΔH298f∘(CF)=62.5±1.1 kcal/mol (61.7±1.1 kcal/mol at 0 K) can be deduced. Many earlier literature values for appearance potentials of CF3+ from CF3X, leading to very low ΔHf∘(CF3+) and/or IP(CF3) values, are demonstrated to be in error.
A coupled-cluster ab initio study of triplet C3H2 and the neutral–neutral reaction to interstellar C3H
Authors: Ochsenfeld Christian , Kaiser Ralf I. , Lee Yuan T. , Suits Arthur G. , Head-Gordon Martin
Published: July 2002

For the initially formed C3H2 collision complexes of molecular beam experiments ab initio calculations are presented. Resolving energetics and properties of these intermediates is essential for the understanding of the reaction of C(3P) with C2H2 to form interstellar cyclic and linear isomers of C3H. Computed reaction energies agree with results from molecular beam experiments. The combination of crossed molecular beam experiments and ab initio calculations allows us to identify two reaction channels for the carbon–hydrogen exchange and to explain astronomical observations of a higher c-C3H to 1-C3H ratio in dark clouds as compared to hotter envelopes of carbon stars.
Electron spin echo detection of the microwave-induced recombination of transient radical pairs produced in photochemical reactions
Authors: Astashkin Andrei V. , Sakaguchi Yoshio
Published: July 2002

The effects of the geminate recombination induced by the pulsed microwave irradiation on the electron paramagnetic resonance spectra of radicals generated in the photoinitiated hydrogen abstraction reaction of 2-methyl-1,4-naphthoquinone in a micellar sodium dodecylsulphate solution have been studied using an inversion recovery method of pulsed electron paramagnetic resonance. It was found that the stimulated nuclear polarization resulting from the microwave-induced recombination of the geminate radical pairs at one (or a limited group) of nuclear spin configurations affects the polarization and intensity of the electron paramagnetic resonance spectra of escaped radicals via the electron-nuclear cross-relaxation mechanism. The long lifetimes of the radical pairs in a micellar system facilitate the observation of the effects. Similar effects, though of much smaller scale, were found to be detectable in certain conditions also in homogeneous solutions.
Spin dynamics calculations of electron and nuclear spin relaxation times in paramagnetic solutions
Authors: Abernathy Shawn M. , Sharp Robert R.
Published: July 2002

Spin dynamics (SD) methods have been developed to compute NMR paramagnetic relaxation enhancements (NMR-PRE) produced by solutes with electron spin S⩾1 in solution. The spin dynamics algorithms, which are implemented as the computer program SpinDyn.f, are similar in spirit to molecular dynamics calculations in statistical mechanics, except that the spin motion is propagated numerically in time using quantum mechanical equations of motion of the spin operators, rather than Newtonian equations of motion of the molecular degrees of freedom as in MD simulations. SD simulations as implemented in SpinDyn.f provide accurate, flexible, and rapid calculations of NMR-PRE phenomena with few of the assumptions or limitations of previous analytical theories. The program calculates inter- and intramolecular NMR-PRE phenomena for both integer and half-integer spin systems processing under arbitrary Zeeman and zfs Hamiltonians in the presence of Brownian reorientation. Isotropic Brownian reorientation is simulated by means of a finite-step algorithm with adjustable step size. Simulations computed by SpinDyn.f have been used in a systematic study aimed at better understanding the influence of Brownian reorientation on the NMR-PRE of an S=1 ion in a non-Zeeman-limit physical situation. Conditions required for the validity of zfs-limit analytical theory are given. SpinDyn.f has also been used to assess quantitatively the effects of molecular reorientation on a prior analysis of NMR-PRE data for the model S=2 complex ion [tris-(acetylacetonato)manganese(III)] in acetone solution; this system was found to be well described by zfs-limit analytical theory.
Laser velocity modulation spectroscopy of the Δ(3d4s)–X33Φ(3d2) visible system of TiCl+ and characterization of the spin–orbit structure
Authors: Focsa C. , Dufour C. , Pinchemel B. , Hadj Bachir I. , Huet T. R.
Published: July 2002

A single mode cw dye laser excitation along with velocity modulation detection was used to record between 17 100 and 18 600 cm−1 the absorption spectrum of the Δ(3d4s)–X33Φ(3d2) visible system of TiCl+ produced in an ac glow discharge with a gas mixture of He/TiCl4. The rotational structure of the (0,0) and (1,0) vibrational bands has been observed and fully analyzed for the main isotopomer Ti35Cl+ as well as for Ti37Cl+. Beside the confirmation of the nature of the two electronic states, the observation of the forbidden Δ23–3Φ2 and Δ33–3Φ3 intercombination transitions allowed the first experimental determination of the spin–orbit constants and a detailed characterization of both the Δ3 and Φ3 states through a matricial fitting of the data. Molecular parameters have been derived.
Vibrations, coverage, and lateral order of atomic nitrogen and formation of NH3 on Ru(101̄0)
Authors: Dietrich H. , Jacobi K. , Ertl G.
Published: July 2002

The dissociative chemisorption of nitrogen on the Ru(101̄0) surface has been studied using high-resolution electron energy loss spectroscopy (HREELS), thermal desorption spectroscopy (TDS) and low-energy electron diffraction (LEED). To prepare a surface covered by atomic nitrogen we have used ionization-gauge assisted adsorption. A saturation coverage of θN=0.6 is achieved of which about 30% is in the subsurface region. At saturation coverage a ( 2 1−1 1) LEED pattern is observed. The ν∥(Ru–N) mode at 41 meV and the ν⊥(Ru–N) mode at 60 meV are identified. Upon exposing the nitrogen covered surface to hydrogen at 300 K we have observed the formation of NH3 which is characterized by its symmetric bending mode δs(NH3) at 149 meV. At 400 K, NH3 could not be detected. The reaction intermediate NH is stable up to 450 K and has been identified by its vibrational losses ν(Ru–NH) at 86 meV, and ν(N–H) at 408 meV. The TD spectra of mass 14 show three desorption states of nitrogen, Nα at 740 K (from subsurface N), Nβ shifting from 690 to 640 K with increasing coverage, and Nγ at 550 K. The activation energy for desorption via the Nβ state is 120±10 kJ/mol. The TD spectra of mass two showed three desorption states at 450, 550, and 650 K due to the decomposition of NHx.
Distribution functions for H2 nuclear magnetic resonance band shapes for polymerized surfactant molecules forming triply periodic surfaces
Authors: Góźdź Wojciech T. , Hol/yst Robert
Published: July 2002

We present theoretical predictions of the distribution functions for H2 NMR bandshape for polymerized surfactant monolayers in triply periodic surfaces formed in ternary mixtures. We have calculated the distribution function for many triply-periodic structures of different topology, geometry, and symmetry. We have investigated applicability and usefulness of this new experimental technique to study the microstructures formed by surfactant molecules. The results presented in this paper can help experimentalists in better interpretation and analysis of nuclear magnetic resonance (NMR) bandshape experiments.
Probing solute–solvent electrostatic interactions: Rotational diffusion studies of 9,10-disubstituted anthracenes
Authors: Hartman R. S. , Konitsky W. M. , Waldeck D. H. , Chang Y. J. , Castner E. W.
Published: July 2002

The rotational diffusion of 9,10-disubstituted anthracenes is studied in various solvents. By systematic variation of the anthracene functionalities the influence of the electrostatic properties of the solute on the solute–solvent frictional coupling is investigated. These studies explore the range of validity of continuum based dielectric friction models for describing the frictional coupling. It is found that a continuum model of the solvent with an extended charge distribution for the solute is adequate in unassociated solvents, but does not adequately describe the friction in the associated solvent, benzyl alcohol.