bluuebunny/crossref_metadata_2025 huggingface dataset:
Spectroscopic properties and potential energy surfaces of In2
Authors: Balasubramanian K. , Li Junqing
Published: July 2002

Complete active space MCSCF (CASSCF) followed by first-order configuration interaction (FOCI) and relativistic configuration interaction (RCI) calculations are carried out on 29 electronic states of In2. Spectroscopic properties of 19 electronic states below 34 000 cm−1 are calculated. The ground state of In2 is found to be 3Πu(0−u). The experimentally observed electronic absorption spectra of In2 are reassigned to A(3Πg)←X(3Πu) (13 000 cm−1) and B(3Πg)←X(3Πu) (27 700 cm−1) transitions. The observed emission spectrum in the King furnace is assigned to the 3Σ−u–3Σ−g system. Spectroscopic properties and potential energy surfaces of 15 more electronic states are reported which are yet to be observed. The 0+g electronic states of In2 exhibit considerable spin–orbit contaminations and avoided crossings resulting in double minima in the 0+g and 0+g(III) surfaces. The 3Σ+u(II) curve also undergoes an avoided crossing. The Mulliken population analyses of the electronic states of In2 are reported.
Multiphoton ionization of SiH3 and SiD3 radicals: Electronic spectra, vibrational analyses of the ground and Rydberg states, and ionization potentials
Authors: Johnson Russell D. , Tsai Bilin P. , Hudgens Jeffrey W.
Published: July 2002

The electronic spectra of silyl radicals, SiH3 and SiD3, were observed between 310 and 430 nm (46 000–64 000 cm−1) by resonance enhanced multiphoton ionization (REMPI) mass spectroscopy. The spectra were generated through a 2+1 REMPI mechanism. Two Rydberg series originating from planar, D3h point group states were observed. One series, of quantum defect δ=1.45(2), is comprised of the Ẽ 2A″2

(4p), J̃ 2A″2(5p), and M̃ 2A″2(6p) Rydberg states which have origins at ν0–0 =48 438, 56 929, and 60 341 cm−1 in SiH3 and at ν0–0 =48 391, 56 874, and 60 267 cm−1 in SiD3. In SiD3 the

P̃ 2A″2(7p) Rydberg origin was observed at ν0–0 =62 002 cm−1. The H̃, K̃, and Ñ states observed in the SiD3 spectrum comprise the second Rydberg series, δ=2.09, and were tentatively assigned as ns 2A′1 Rydberg states (n=5, 6, 7). The K̃ and Ñ origins were observed at ν0–0 =58 417 and 61 005 cm−1. A fit of the Rydberg formula to the np 2A″2(n≥5) origins found the adiabatic ionization potential of the SiH3 and SiD3 radicals to be IPa=8.135(+5,−2) eV and IPa=8.128(1) eV, respectively. Detailed vibrational analyses of these Rydberg states are presented. Analysis showed that in the Ẽ 2A″2 (4p) state of the SiH3 radical ω2 (a″2 symmetric bend)=796(7) and 2ω4 /2(e′ degenerate bend)=870(5) cm−1 and that in SiD3 radical ω′1 (a1 SiH3 symmetric stretch)=1576(3), ω′2 =589(3), and 2ω4 /2=635(6) cm−1. The REMPI spectra exhibited ν″2 hot bands from vibrational levels as high as Ev =2073 cm−1 in the X̃ 2A1state. Modeling calculations, which fit the numerous ν″2 hot bands, predicted barriers to inversion of Binv

=1935 cm−1 and Binv =1925 cm−1 for SiH3 and SiD3 X̃ 2A1 radicals, respectively.
Diffusion effects of hydrogen bond fluctuations. I. The long time regime of the translational and rotational diffusion of water
Authors: Bertolini Davide , Cassettari Mario , Ferrario Mauro , Grigolini Paolo , Salvetti Giuseppe , Tani Alessandro
Published: July 2002

A theoretical scheme to evaluate the long-time effects of hydrogen bond fluctuations on water dynamics is carefully reexamined in the light of recent experimental results of quasielastic neutron scattering (QENS). A molecular dynamics computer simulation is also carried out on a fairly large (343 TIP4P molecules) sample and for a total time of ≈100 ps. at three temperatures. The length of the runs has allowed to obtain good results for a number of properties. Also the temperature dependence of the static dielectric constant agrees with the experimental data. Further, some crucial predictions of our theoretical model have been carefully checked. The prediction on the deviation from Fick’s law is found to be in a satisfactory accord with the results of our computer simulation. The reasons for the less satisfactory agreement with the low-temperature results of QENS are discussed.
N2 excitations below 15 eV by the multireference coupled-cluster method
Authors: Ben-Shlomo Sigalit Berkovic , Kaldor Uzi
Published: July 2002

The multireference coupled-cluster method with singles and doubles is applied to the vertical excitations of N2 at R=2.074 bohr. Both valence- and Rydberg-type excitations are calculated, using an appropriate basis and rather large model spaces. Thirty-five excitations below 15 eV are reported and classified. Sixteen energies could be compared with experiment, and the average error was 0.13 eV. Good ionization potentials are also obtained.
Total reaction cross sections of electronic state-specified transition metal cations: V++C2H6, C3H8, and C2H4 at 0.2 eV
Authors: Sanders Lary , Hanton Scott D. , Weisshaar James C.
Published: July 2002

We describe a crossed beam experiment which measures total cross sections for reaction of electronic state-specified V+ with small hydrocarbons at well-defined collision energy E=0.2 eV. The V+ state distribution created at each ionizing wavelength is directly measured by angle-integrated photoelectron spectroscopy (preceding paper). Reactant and product ions are collected and analyzed by pulsed time-of-flight mass spectrometry following a reaction time of 6 μs. Tests of the performance of the apparatus are described in detail. Our experiment defines the reactant V+ electronic state distribution and the collision energy much more precisely than previous work. For all three hydrocarbons C2H6, C3H8, and C2H4, H2 elimination products dominate at 0.2 eV. We observe a dramatic dependence of cross section on the V+ electronic term. The second excited term 3d34s(3F) is more reactive than either lower energy quintet term 3d4(5D) or 3d34s(5F) by a factor of ≥270, 80, and ≥6 for the C2H6, C3H8, and C2H4 reactions, respectively. The 3d34s(3F) reaction cross sections at 0.2 eV are 20±11 Å2, 37±19 Å2, and 2.7±1.6 Å2, respectively, compared with Langevin cross sections of ∼80 Å2. For the C2H6 and C3H8 reactions, cross sections are independent of initial spin–orbit level J within the 3F term to the limits of our accuracy. Comparison with earlier work by Armentrout and co-workers shows that electronic excitation to d3s(3F) is far more effective at promoting H2 elimination than addition of the same total kinetic energy to reactants. Electron spin is clearly a key determinant of V+ reactivity with small hydrocarbons. We suggest that triplet V+ reacts much more efficiently than quintet V+ because of its ability to conserve total electron spin along paths to insertion in a C–H bond of the hydrocarbon.
Conformational instability of the lowest triplet state of the benzene nucleus. I. The unsubstituted molecule
Authors: Buma W. J. , van der Waals J. H. , van Hemert M. C.
Published: July 2002

Experiments on benzene have established that its lowest triplet state (3B1u) is conformationally unstable owing to vibronic coupling with the next higher state (3E1u). This instability was found to be critically dependent on the influence of a crystal field. An analogous vibronic coupling is to be expected in the singlet manifold, but here no direct evidence is available for a conformational instability. The distortion behavior of benzene is of importance for the interpretation of its photophysical and photochemical properties. We have therefore determined the potential-energy surfaces of the 1,3B1u and 1,3E1u states along the two-dimensional distortion coordinate S8(ρ,φ) using ab initio multireference single and double excitation-configuration-interaction calculations. The results show that for both B1u states the hexagonal conformation is unstable and lies 800 cm−1 above a wide, virtually cylindrical trough. A calculation of the vibrational spacing in the 3B1u state yields good agreement with the experimentally observed frequency. The calculation of intensities in the absorption and emission spectrum for this state qualitatively agrees with the experiment. An estimate is made of the interaction of the excited molecule with neighboring molecules in a crystal, which indicates that the crystal-field induced energy variations in the trough should be of the order of 10 cm−1. Combination of our calculations with experimental data shows that the vibronic coupling in the B1u states of benzene should not be looked upon as a static coupling in which the molecule is permanently distorted to one conformation but as a dynamic one in which the molecule makes excursions over the entire potential-energy surface.
Photodissociation of acetylene: Determination of D00 (HCC–H) by photofragment imaging
Authors: Baldwin David P. , Buntine Mark A. , Chandler David W.
Published: July 2002

Acetylene cooled in a He supersonic expansion is photodissociated by excitation in the 201–216 nm region of the Ã 1Au −X̃ 1∑+g transition. Subsequent ionization of the H-atom fragments by 2+1 (243 nm) REMPI, and mass-selected ion imaging allows analysis of the velocity distribution of H-atoms from the HCCH hν→ C2H+H process. Measurement of the maximum velocity for H atoms from this channel produced by photodissociation of acetylene through the Ã 1Au −X̃ 1∑+g  V70K10, 110V40K10, 210V50K10 and V50K10 vibronic transitions gives a value for D00 (HCC–H) of 131±1 kcal/mol. Other channels producing hydrogen atoms (including HC2 hν→ C2+H and HCCHhν→ HCCH+ hν→ C2H++H) are detected at all photon fluxes used. These multiphoton channels produce hydrogen atoms with higher translational energy and therefore obscure measurement of the maximum velocity of H atoms produced by single-photon dissociation of acetylene. Reduction of photon flux by more than two orders of magnitude to ∼5×106 J/cm2 gives a background, multiphoton, H-atom intensity of ≤7% of the peak primary dissociation intensity. Because this multiphoton background limits the detectability of fast H atoms from single-photon dissociation of acetylene, the dissociation energy reported here is an upper limit. Calculations of potential rovibronic excitation of the C2H fragment are discussed.
On shape asymmetry of Gaussian molecules
Authors: Wei Gaoyuan , Eichinger B. E.
Published: July 2002

A method of calculating average moments, to an arbitrary order m, of the principal components of the gyration tensor of a Gaussian molecule imbedded in any k-dimensional space is presented. These average moments are then used in generalized shape parameters Am of degree m≤k, which measure the asymmetry of the shapes of Gaussian molecules. Simple formulas for A2, A3, and the average moments up to third order are given. Explicit expressions for A2 and A3 for linear and circular chains, regular stars with infinitely long arms, double rings of a large number of beads, and combs with many side chains are obtained, and the shape characteristics of these molecules are discussed. It is found that Gaussian molecules are, on the average, prolate rather than oblate even in an infinite dimensional space, with the exception of regular stars with densely radiated long arms which exhibit perfect symmetry. The problem of analytic characterizations of shape asymmetry of Gaussian molecules or non-self-avoiding random walks of any kind is thus solved in complete generality.
Effective diameter of molecules and liquid-gas critical point
Authors: Misawa M.
Published: July 2002

The effective diameters of molecules σcp of some molecular liquids, i.e., liquids halogens, carbon tetrachloride and benzene, have been estimated by a critical packing fraction of ηcp = 0.130 44 predicted for a van der Waals fluid. It turns out that the estimated values of σcp are in good agreement with the effective diameters σsf estimated independently to explain the structure factor of the liquids. It seems that both the liquid structure and the critical point are determined by a common factor, i. e., a common effective diameter of molecules, and that the simple relation of ηcp≂0.13 holds fairly well even in the molecular liquids.
Infrared absorption spectroscopy of the CO2–Ar complex in the 2376 cm−1 combination band region: The intermolecular bend
Authors: Sharpe S. W. , Reifschneider D. , Wittig C. , Beaudet R. A.
Published: July 2002

The rovibrational absorption spectrum of CO2–Ar has been recorded in the 2376 cm−1 region by using a pulsed, slit nozzle expansion and tunable IR diode laser. The spectrum is ascribed to the 410510 combination band, where ν4 corresponds to the asymmetric stretch of the CO2 moiety and ν5 is the intermolecular bend. This combination band is ∼102 times less intense and 27.82 cm−1 higher in energy than the 410 fundamental. Effective upper-state rotational constants were determined for this nearly prolate rotor, and the ν4+ν5 moments of inertia show a large inertial defect, Δ=9.48 amu Å2. In planar molecules, this indicates large vibrational amplitude, and reflects inadequacy of the rotational Hamiltonian. From the moments of inertia derived using a Watson Hamiltonian, geometric properties of ν4+ν5 can be calculated. The average O–C–Ar angle is 76.9°, while the center-of-mass separation between CO2 and Ar is 3.60 Å. These values differ from those of the ground state (83.1° and 3.51 Å) and ν4=1 (83.0° and 3.50 Å). Results are compared with previous work, which used different models and approximations to obtain intermolecular frequencies. In addition, inertial defects were calculated for the ground and vibrationally excited states using a simple normal mode model developed by Herschbach and Laurie, and Oka and Morino. With harmonic force fields, it accounts for most of the inertial defect in the ground and excited states, and yields reasonable R and θ values. However, we conclude that the good agreement between experimental and calculated inertial defects may be fortuitous, since several positive and negative contributions cancel.
Structural differences and phase separation in alkali silicate glasses
Authors: Huang Chengde , Cormack A. N.
Published: July 2002

Reasonable and significant structural differences between potassium silicate glass and sodium silicate glass can be revealed by molecular dynamics computer simulations.The differences in their immiscibility behavior reflect these structural differences. The simulation results are consistent with the experimental observations of many researchers that the variation in the number of nonbridging oxygens per silicon decreases in the order Li>Na>K. The distribution of the silicon species with various numbers of nonbridging oxygens is a reflection of the degree of homogeneity in the distribution of the modifier cation and nonbridging oxygen clusters. Nanoscale inhomogeneity seems to be an inherent nature of alkali silicate glasses.
Vibration–rotation spectrum of the acetylene-nitrous oxide van der Waals complex in the 3 micron region
Authors: Hu T. A. , Sun Ling Hong , Muenter J. S.
Published: July 2002

Infrared laser absorption spectroscopy has been carried out on a molecular beam containing the HCCH-N2O van der Waals complex. Analysis of the vibration-rotation spectrum gives ν=3281.5437(2) cm−1, A″=9387(1) MHz, A′=9382(1) MHz, B″=2829.6(3) MHz, B′=2827.2(3) MHz, C″=2166.6(2) MHz, and C′=2163.7(2). These rotational constants describe a planar complex with parallel monomer axes making right angles with the vector connecting the monomer centers of mass. The monomer center of mass separation is 3.307 Å. This geometry is very similar to that of HCCH-CO2. This similarity is also present in the comparison of model potential functions calculated for HCCH-N2O and HCCH-CO2.
Variational transition state theory for reactions in condensed phases
Authors: Pollak Eli
Published: July 2002

A generalization of the Kramers–Grote–Hynes theory for reaction rates in the spatial diffusion limit is derived for a general class of Hamiltonians. Previous restrictions to harmonic baths and bilinear system bath couplings are removed. The key ingredient is the systematic use of variational transition state theory (VTST) to identify the optimal dividing surface. A pair of collective modes are defined as a linear combination of all system and bath modes. A free energy surface is defined in the two degree of freedom collective mode phase space. The VTST estimate for the rate of reaction on this surface is shown to be an upper bound to the exact rate. The optimal definition of the collective modes is obtained by minimizing the rate. The resulting rate expression is formally identical to the Kramers–Grote–Hynes theory. However, the minimization procedure leads to a new definition of the time dependent friction. In consistence with transition state theory, this time dependent friction is constructed from equilibrium properties of the composite system and does not call for any dynamical computations. The friction parameters are determined from equilibrium centroid averages of partial derivatives of the full potential at the barrier of the potential of mean force. This removes previous ambiguities as to the definition of time dependent friction in condensed matter systems. A procedure is presented for finding collective modes along which the friction exerted by the bath is minimized. This result may be of substantial interest in the study of complex dynamical systems in biology, chemistry, and physics.
Solvent and pressure-induced perturbations of the vibrational potential surface of acetonitrile
Authors: Ben-Amotz Dor , Lee Meng-Rong , Cho Seung Y. , List Donald J.
Published: July 2002

Raman-scattering studies at both ambient pressures and in a high-pressure diamond-anvil cell are used to measure gas-to-liquid vibrational frequency shifts of three normal modes of acetonitrile, CH3CN (ν1, CH stretch; ν2, CN stretch; and ν4, CC stretch) dissolved in various solvents (methylenechloride, chloroform, carbontetrachloride, toluene, pyridine, acetone, and methanol). The results are compared with calculated repulsive and attractive solvation force-induced perturbations of polyatomic vibrational potential surfaces. Repulsive solvation forces are modeled using recently developed analytical ‘‘hard-fluid’’ expressions for heteronuclear two-cavity distribution functions in hard-sphere fluids, while attractive forces are assumed to contribute a van der Waals(linearly density-dependent) mean field. Results for the CN and CC stretches of acetonitrile compare favorably with theoretical predictions, while the CH stretch appears to experience a nonlinearly density-dependent attractive frequency shift at high densities. Empirical attractive frequency-shift parameters, derived from gas-to-liquid shifts at 1 atm, agree reasonably well with those predicted using a simple dispersive and dipolar solvation force expression. Attractive solvation forces are found to correlate well with solvent polarizability (and solute bond polarizability derivatives). Dipolar solvation forces only appear to contribute significantly to the CN stretch.
An examination of substituent effects on the reaction of OH radicals with HXCO (where X=H, F, and Cl)
Authors: Francisco J. S.
Published: July 2002

In the present work, ab initio molecular orbital theory have been used in the study of the reaction of OH radicals with HXCO (where X=H, F, and Cl). Equilibrium geometries and transition state structures have been fully optimized at the unrestricted Hartree–Fock (UHF)/6-31G*, unrestricted Mo/ller–Plesset (UMP2)/6-31G* and UMP2/6-311G** levels of theory. Activation energy barriers and heats of reaction have been estimated using fourth-order Mo/ller–Plesset perturbation theory with spin projection including single, double, triple, and quadruple excitations with extended basis sets. Transition state theory treatment of the kinetics of these reactions is performed and is used to estimate the rate coefficient at 299.3 K. The results for the OH+H2CO reaction show reasonable agreement with experiment.
Fluid distributions in random media: Arbitrary matrices
Authors: Madden William G.
Published: July 2002

The graphical theory of Madden and Glandt [J. Stat. Phys. 51, 537 (1988)] for a fluid adsorbed into a quenched medium has been extended to situations in which the distribution of the immobile species has an arbitrary form, not necessarily arising from a thermal quench. The working equations of Madden and Glandt are shown to be applicable to this general case and the approximations common in the theory of equilibrium mixtures are appropriate in this application as well. Extensions to mixtures are considered and the connection with the graphical theory of small molecules is discussed.
Theoretical study of the organosulfur systems CSHn (n=0–4) and CSHn+ (n=0–5): Dissociation energies, ionization energies, and enthalpies of formationa)
Authors: Curtiss Larry A. , Nobes Ross H. , Pople John A. , Radom Leo
Published: July 2002

The Gaussian-2 (G2) theoretical procedure, based on ab initio molecular orbital theory, is used to calculate the energies of the CSHn (n=0–4) hydrides and CSHn+ (n=0–5) cations. The G2 adiabatic ionization energies of CH3SH, CH2SH, CH3S, CH2S, HCS, and CS as well as the proton affinity of CH3SH are all in satisfactory agreement with experimental values. G2 enthalpies of formation of the neutrals and cations have also been calculated and are used to help assess the reliability of the experimental values, which in some cases cover a rather wide range. The calculated enthalpy of formation of CH3S at 0 K is 31.6 kcal/mol. This supports the value of 31.44 kcal/mol reported in a recent kinetics study over values of 34.2–35.5 kcal/mol obtained in recent photofragmentation studies. Theoretical S–H and C–H bond dissociation energies for CH3SH are 86.0 and 95.0 kcal/mol, respectively. G2 theory predicts the CH2SH+ cation to lie 33.1 kcal/mol lower in energy than the CH3S+ cation, in good agreement with the value of 33.5±2 kcal/mol obtained in a recent photoionization study, but differing significantly from results of previous experimental work that gave a much larger energy difference. The G2 enthalpies of formation at 0 K of CH2S, CH2S+, HCS, and HCS+ are calculated to be 28.7, 245.0, 70.8, and 241.7 kcal/mol, respectively.
Photo-induced electron transfer in covalently linked donor–acceptor assemblies in liquid crystals. Time-resolved electron paramagnetic resonance
Authors: Hasharoni Kobi , Levanon Haim , von Gersdorff Jörg , Kurreck Harry , Möbius Klaus
Published: July 2002

Intramolecular electron transfer (ET) in cis and trans isomers of a covalently linked porphyrin–cyclohexylene–quinone, oriented in liquid crystals (LCs), is monitored by time-domain electron paramagnetic resonance (EPR) spectroscopy over an extended range of temperature, i.e., 210<T<320 K. The spectra of both isomers exhibit different line shape behavior and temperature dependence, as compared to those found in isotropic solutions (ethanol). Whereas in ethanol the range of detection is quite narrow, i.e., 130<T<160 K, the one in the LC environment includes also the nematic fluid phase, thus allowing one to monitor the ET products in this phase. The difference in spectra between the two environments is analyzed in terms of the magnetization projection on the LCs director, L. The different spectra of the two isomers are interpreted in terms of their different molecular geometry. In the case of the trans isomer, both triplet- and probably singlet-initiated ET routes can concurrently be detected, and the free energy of the charge-separated state is estimated from the spectral dynamics.
Tight-binding approximation for semi-infinite solids. Application of a transform method and of delta function normalization
Authors: Marcus R. A.
Published: July 2002

A transform method for treating semi-infinite solids in the tight-binding (TB) approximation is introduced. The difference equations for the TB orbital coefficients are converted, thereby, to convenient algebraic equations. For this purpose, a Dirac delta function normalization for the wave function is also introduced, instead of the usual box one. Single and coupled bands are treated, and the methods are applied elsewhere to electron transfer problems at interfaces.
The accuracy of second order perturbation theory for multiply excited vibrational energy levels and partition functions for a symmetric top molecular ion
Authors: Zhang Q. , Day Paul N. , Truhlar Donald G.
Published: July 2002

The vibrational energy levels and partition functions of the nonrotating H3+ molecular ion have been calculated by using second order perturbation theory, including constant, linear, and quadratic terms in the vibrational quantum numbers. Energy levels have been assigned to A′1, A′2, and E′ symmetry species up to 29 244 cm−1, and perturbation theory energy levels have been compared with the results of accurate quantum calculations. The root-mean-square error in 141 energy levels is 4.0 cm−1, as compared to 11.7 cm−1 in the harmonic approximation. Furthermore, perturbation-theory partition functions have errors of 7.4% or less over the factor-of-20 temperature range from 200 to 4000 K. The effect of the constant term in perturbation theory is also discussed; it improves the vibrational partition functions by ∼4% at 200 K.
The microscopic dynamics of freezing in supercooled colloidal fluids
Authors: Grier David G. , Murray Cherry A.
Published: July 2002

Using time-resolved digital video microscopy, we have tracked the reemergence of order in charge-stabilized colloidal crystals which have been shear melted into isotropic fluids. Crystallization is heterogeneously nucleated by the smooth walls of the sample container. This process is analogous to the solidification of conventional materials during casting or liquid phase epitaxy. The nonequilibrium freezing transition proceeds through the gradual formation of a layered fluid near the repulsive wall, subsequent evolution of local order within the first fluid layer, and ultimately rapid crystallization. After nucleation and initial growth, crystallites are observed to fracture, perhaps due to shear stresses imposed by neighboring crystallites. Microscopic measurements of the nonequilibrium self-diffusion coefficient are consistent with the dynamical freezing criterion recently proposed for systems in equilibrium by Löwen et al. [Phys. Rev. Lett. 70, 1557 (1993)].
Photodissociation spectroscopy of Mg+–rare gas complexes
Authors: Pilgrim J. S. , Yeh C. S. , Berry K. R. , Duncan M. A.
Published: July 2002

Weakly bound complexes of the form Mg+–RG (RG=Ar, Kr, Xe) are prepared in a pulsed nozzle/laser vaporization cluster source and studied with mass-selected photodissociation spectroscopy. The chromophore giving rise to the molecular spectra in these complexes is the 2P←2S Mg+ atomic resonance line. A 2Σ+ ground state and 2Σ+ and 2Π excited states are derived from this atomic transition. Vibrationally resolved spectra are measured for each of these complexes in the A 2Π←X 2Σ+ electronic transition. These systems are redshifted from the atomic resonance line, indicating that each complex is more strongly bound in its excited 2Π state than it is in the ground state. Extended vibrational progressions allow determination of the respective vibrational constants: Mg+–Ar, ωe′ = 272 cm−1; Mg+–Kr, ωe′ = 258 cm−1; Mg+–Xe, ωe′ = 258 cm−1. Extrapolation of the excited state vibrational progressions, and combination with the known atomic asymptotes and spectral shifts, leads to determination of the respective dissociation energies: Mg+–Ar, D0″= 1281 cm−1 (3.66 kcal/mol; 0.159 eV); Mg+–Kr, D0″ = 1923 cm−1 (5.50 kcal/mol; 0.238 eV); Mg+–Xe, D0″ = 4182 cm−1 (11.96 kcal/mol; 0.519 eV). The spin–orbit splitting in the 2Π1/2,3/2 state for all complexes is larger than expected by comparison to the Mg+ atomic value. This larger splitting in the complexes, which is attributed to configuration mixing with states on the rare gas atoms, increases for the series Ar, Kr, Xe, and decreases linearly for higher vibrational states of each complex.
Site-specific properties and dynamical dipole coupling of CO molecules adsorbed on a vicinal Cu(100) surface
Authors: Borguet Eric , Dai Hai-Lung
Published: July 2002

The dynamic and spectroscopic behavior of CO adsorption on a stepped Cu(100) surface is investigated using transient IR diode laser-reflection absorption spectroscopy. Disproportionate intensity behavior, defying Beer–Lambert’s law, is observed which makes it impossible to use spectral intensity for determining either the total or site-specific concentrations. A theoretical model, based on the Persson–Ryberg treatment of mixed isotope studies of CO at fixed coverage, is used here, with modifications added to allow for coverage dependence to account for dynamic dipole coupling between CO molecules and simulate the IR absorption spectra. This enables the spectral intensities and positions to be analyzed and the extraction of previously unattainable information on site-specific molecular spectroscopic parameters and concentrations on this CO/Cu(100) system. The CO stretching frequencies indicate that the Cu–CO bond is formed by transfer of the CO antibonding 5σ electron to copper and that the binding energy of CO at terrace sites decreases with increasing coverage. The model calculation shows that, as a result of dynamical dipole coupling, a 7% step-CO concentration, with a vibrational polarizability of 0.2 Å3, causes a 3 times larger IR absorption peak than the remaining 93% of CO at terrace sites. CO adsorption on this Cu(100) surface was found to be repulsive correlated with the order parameter determined as n=3/2. Concentrations determined from the dynamical coupling calculation show that CO occupies step and on-top terrace sites at all coverages at 90 K, with the more tightly bound step sites saturated at lower coverage. A simple model is devised to describe the equilibrium between the step and terrace CO populations and provide an estimate of the dynamical parameters governing CO motion between the step and terrace sites.
Photoinduced electron transfer and geminate recombination on a micelle surface: Analytical theory and Monte Carlo simulations
Authors: Weidemaier Kristin , Fayer M. D.
Published: July 2002

A detailed theoretical analysis of photoinduced electron transfer and geminate recombination on the surface of a spherical micelle is presented. An exact point-particle analytical theory is first developed for one donor and N competing acceptors in random fixed positions on the micelle surface. The method is applicable to any restricted geometry system. Starting with a neutral donor and acceptors, the time dependent probability of having an excited neutral donor and the time dependent probability of having ions are calculated for various numbers of acceptors and various forward and back electron-transfer parameters. The theoretical results are compared to Monte Carlo simulations of the problem, and the exact agreement obtained demonstrates that the ensemble averages are properly performed. Comparison is also made to a previously reported approximate analytical theory. The analytical theory and the Monte Carlo simulations are then extended to include the effects of donor–acceptor and acceptor–acceptor excluded volume. Although donor–acceptor excluded volume may be included exactly, inclusion of acceptor–acceptor excluded volume renders the problem intractable. An approximate method of handling acceptor–acceptor excluded volume by utilizing the pair correlation function for the system is presented and compared to Monte Carlo simulations of the full problem. An approximate technique is suggested for generating the pair correlation function for curved disks on the surface of a sphere.
Quantum dynamics study of the reaction HD+OH→H+DOH, D+HOH
Authors: Zhang Dong H. , Zhang John Z. H. , Zhang Yici , Wang Dunyou , Zhang Qinggang
Published: July 2002

Accurate time-dependent (TD) quantum wavepacket calculations are reported for the combustion reaction HD+OH. Due to the lack of symmetry, the HD+OH reaction has roughly twice the number of channels of the corresponding H2+OH reaction and produces two distinguishable products–HOH and HOD. In order to make the TD calculation possible on workstations with limited memories, we employed a normalized quadrature scheme in the wavepacket propagation by the split-operator propagator. The normalized quadrature scheme eliminates the need to store large matrices during the wavepacket propagation while preserving the unitarity of the split-operator propagator and producing numerically stable results. This approach made TD dynamics calculations possible on small-memory workstations for the title reaction and for other polyatomic reactions. Reaction probabilities, cross sections, rate constants, and reaction branching ratios are reported in this paper for the title reaction. The observed strong dependence of the reaction probabilities on the reactive HD rotation and the relative weak dependence on the nonreactive OH rotation are explained in terms of a steric effect. The isotope effect in the branching ratio is examined and physical explanation is given for the observed branching ratio at low and high kinetic energies.
The potentials of mean force of sodium chloride and sodium dimethylphosphate in water: An application of adaptive umbrella sampling
Authors: Friedman Richard A. , Mezei Mihaly
Published: July 2002

The potentials of mean force between sodium and chloride ions and between sodium and dimethylphosphate ions in aqueous solution are calculated by the probability ratio method using adaptive umbrella sampling Monte Carlo with a variety of simulation setups. The potential of mean force of sodium chloride is found to have only one minimum in contradiction to earlier molecular dynamics results with the same intermolecular potentials, but different boundary conditions. Our result in the region of contradiction is supported by large system size and free energy perturbation calculations. It is established that the difference is due to the respective boundary condition schemes employed. This result is to be regarded as a consequence of the potential function scheme employed and not necessarily as a statement about actual aqueous sodium chloride. A similar dependence of the existence of the second minimum on boundary conditions is observed for the sodium-dimethylphosphate PMF.
Martensitic transition in a confined colloidal suspension
Authors: Weiss Jessica A. , Oxtoby David W. , Grier David G. , Murray Cherry A.
Published: July 2002

We describe a nonequilibrium phase transition in a geometrically confined charge-stabilized colloidal suspension. The equilibrium configuration for the spheres in this system is a two-layer square lattice, equivalent to two layers of a body-centered cubic (bcc) crystal. After electrophoretic shear melting, the suspension rapidly crystallizes to a buckled single-layer triangular lattice. This well-ordered solid phase subsequently undergoes a martensitic phase transition back to the two-layer square lattice. We use high resolution digital video microscopy to follow the emergence and evolution of order during these phase transitions.
Electrokinetic properties of colloids of variable charge. I. Electrophoretic and electro-optic characterization
Authors: Bellini Tommaso , Degiorgio Vittorio , Mantegazza Francesco , Marsan Franco Ajmone , Scarnecchia Carmela
Published: July 2002

We present an experimental study of the electrokinetic properties of a model colloidal system in which the number of ionizable sites per particle is controlled by exploiting the competitive adsorption between a ionic and a nonionic amphiphile. The adsorption of the nonionic amphiphile is monitored by light scattering. By measuring the electrophoretic mobility, we obtain the zeta-potential ζ of the particles. We find that ζ is independent from the ionic strength and strongly dependent on the surface density of ionizable sites. This indicates that in our system we can separately control zeta potential and ionic strength. In the investigated range of ionic strength and particle concentration, the Kerr constant of the dispersion is found to be proportional to the particle concentration, and does not contain any significant collective effect.
The torsional potential of perfluoro n-alkanes: A density functional study
Authors: Röthlisberger Ursula , Laasonen Kari , Klein Michael L. , Sprik Michiel
Published: July 2002

The structural, vibrational, and conformational properties of small perfluoro n-alkanes CnF2n (n≤5) have been studied with different density functional models. Our calculations show that the relative conformational energies are severely underestimated within the local density approximation (LDA). The inclusion of gradient corrections (GC), on the other hand, leads to results in close agreement with experimental values, e.g., the barrier to internal rotation in C2F6 is calculated to be 2.9 kcal/mol and 3.8 kcal/mol at the LDA and GC level, respectively, whereas corresponding experimental values range from 3.7–3.9 kcal/mol. A calculation of the torsional potential about the central C–C bond in C4F10 gives two degenerate chiral minimum energy configurations (t+ and t−) shifted away from the usual trans position at zero dihedral angle by ∼±12°. These two minima are separated by a barrier of ∼0.3 kcal/mol. At least four local minima were determined on the torsional potential energy surface. Two enantiomeric gauche minima (g+ and g−) at φ=±125° are ∼1.0 kcal/mol higher in energy than the t configuration. Two further minima (g′+ and g′−) with a relative energy of 1.9 kcal/mol are located at φ=±83°. The role of nonbonded interactions in determining the conformational energy landscape is discussed in some detail.
Cumulative reaction probability via transition state wave packets
Authors: Zhang Dong H. , Light John C.
Published: July 2002

A new time-dependent approach to the cumulative reaction probability, N(E), has been developed based on the famous formulation given by Miller and co-workers [J. Chem. Phys. 79, 4889 (1983)], N(E)=[(2π)2/2] tr[δ(E−H)Fδ(E−H)F]. Taking advantage of the fact that the flux operator has only two nonzero eigenvalues, we evaluate the trace efficiently in a direct product basis of the first flux operator eigenstates and the Hamiltonian eigenstates on the dividing surface (internal states). Because the microcanonical density operator, δ(E−H), will eliminate contributions to N(E) from an internal state with the energy much higher than the total energy E, we can minimize the number of internal states required by choosing a dividing surface with the lowest density of internal states. If the dividing surface is located in an asymptotic region, one just needs to include all the open channels, i.e., with internal energy lower than the total energy. Utilizing the Fourier transform for δ(E−H), we can obtain the information for all the energies desired by propagating these wave packets once. Thus the present approach will be much more efficient than the initial state selected wave packet (ISSWP) approach to N(E) for systems with many rotation degrees of freedom because the density of states in asymptotic region for such systems is much higher than that in the transition state region. With the present method one can also calculate the cumulative reaction probability from an initial state (or to a final state) by locating the second flux operator in the corresponding asymptotic region. This provides an alternative to the ISSWP approach which may be more efficient if the reaction probabilities from a large number of initial states are desired. The method is applied to the 3D H + H2 (even rotation) reaction for J=0 by locating the first dividing surface in the transition state region. The demonstration also shows an aspect less than ideal; the contribution to N(E) from a wave packet may be slightly larger than 1 or slightly smaller than 0, making it improper to interpret the contribution as a probability.
Photofragmentation spectrum of the Sr+Ar complex
Authors: Lüder Ch. , Velegrakis M.
Published: July 2002

The photofragmentation spectrum of the weakly bound Sr+Ar is measured over the 418–448 nm wavelength region. Two vibrational progressions are observed and are attributed to transitions from the X2 Σ1/2 electronic ground state to vibrational levels of the excited A 2Π1/2 and A 2Π3/2 states. Isotope-resolved measurements of several of the observed transitions are performed to obtain the absolute vibrational numbering. From these, the spectroscopic constants of the involved states are deduced. We obtain ωe′=120.8 cm−1, ωeχe′=1.67 cm−1, and D0′=2303±232 cm−1 for the A 2Π1/2 state and ωe′=122 cm−1, ωeχe′=1.6 cm−1, and D0′=2575±256 cm−1 for the A 2Π3/2 state. For the ground X 2Σ1/2 state the observed hot bands yield ΔG1/2″=48 cm−1. Furthermore a value of 803±244 cm−1 is estimated for D0″. Finally the potential constants are compared with pseudopotential calculations from literature and the trends of binding energies for group II cation–argon complexes are discussed.
Microscopic simulation of chemical bistability in homogeneous systems
Authors: Baras F. , Mansour M. Malek , Pearson J. E.
Published: July 2002

Microscopic simulation is used to clarify the status of stochastic theories of homogeneous chemical systems operating in the multiple steady state region. The results demonstrate the failure of the Langevin approach, but show excellent agreement with the master equation formulation.
Theory of size-dependent resonance Raman intensities in InP nanocrystals
Authors: Shiang J. J. , Wolters R. H. , Heath J. R.
Published: July 2002

The resonance Raman spectrum of InP nanocrystals is characterized by features ascribable to both longitudinal (LO) and transverse (TO) optical modes. The intensity ratio of these modes exhibits a strong size dependence. To calculate the size dependence of the LO and TO Raman cross sections, we combine existing models of Raman scattering, the size dependence of electronic and vibrational structure, and electron vibration coupling in solids. For nanocrystals with a radius >10 Å, both the LO and TO coupling strengths increase with increasing radius. This, together with an experimentally observed increase in the electronic dephasing rate with decreasing size, allows us to account for the observed ratio of LO/TO Raman intensities.
Absorption line shapes and solvation dynamics of CH3I in supercritical Ar
Authors: Egorov S. A. , Stephens M. D. , Skinner J. L.
Published: July 2002

We consider the electronic spectroscopy of dilute CH3I in supercritical Ar fluid. Absorption line shapes for the B←X transition of CH3I have been measured previously in low-density argon, which yielded results for the CH3I/Ar pair potentials. Using these potentials, Kalbfleisch et al. [J. Chem. Phys. 105, 7034 (1996)] have performed molecular dynamics simulations to calculate the absorption line shapes at higher densities, and also the solvation correlation function. We compare the results of several analytic theories to the simulated line shapes and solvation correlation functions.
Spin-polarized H/D atoms and radiation chemistry in amorphous silica
Authors: Shkrob I. A. , Trifunac A. D.
Published: July 2002

Time-resolved pulsed electron paramagnetic resonance (EPR) was used to study spin-polarized mobile H and D atoms in radiolysis of silica glass containing ∼1200 ppm of OH or OD groups. The EPR spectra of the H/D atoms indicate the occurrence of CIDEP in reactions of H/D atoms with radiolytically induced metastable spin centers. The EPR kinetics observed at 180–530 K exhibit fast growth and slow decay. The slow component (Ea=15–18 kJ/mol) is due to diffusion-controlled scavenging of H and D atoms by defects. For D atoms, this scavenging is 1.1–1.5 times slower than for H atoms; the formation rate of spin polarization is the same for H and D atoms. Below Tc=320±20 K, the decay kinetics can be fit by [H]t=[H]0 exp[−(kst)β], with β≈T/Tc; above Tc, β≈1. This behavior is indicative of dispersive diffusive transport of the H/D atoms through the glass network. The fast component with Ea=6–8 kJ/mol is due to rapid decay of a mobile spin partner of the H/D atoms, which we assume to be a polaron, in reactions with OH groups. It is argued that the large difference in the EPR signals from H and D atoms, relative to the concentration of the OH/OD defects (approximately 4–8 times), must be due to the substantial isotope effect on radiolytic yield of the H/D atoms (α∼1.5±0.2). The most likely reaction that can yield such an isotope effect is recombination of metastable protons with electrons. Our result seems to indicate that the latter reaction is the main route to the H atoms in room-temperature silica. At low temperatures, the H atoms are formed mainly due to dissociation of the OH defects following their reaction with triplet excitons, and α≈1.
Computer simulation studies of anisotropic systems. XXVI. Liquid crystal dimers: A generic model
Authors: Luckhurst G. R. , Romano S.
Published: July 2002

A generic model has been proposed with which to understand the unusual properties of liquid crystal dimers, in particular their behavior at the nematic–isotropic transition. In the model the many conformational states available to real dimers are replaced by just two, a linear and a bent conformer which are allowed to interconvert. The nematic behavior of the model has been explored with the aid of the molecular field approximation and found to be in good qualitative agreement with that of real dimers, thus providing a valuable aid to our understanding of them. To see to what extent the use of the molecular field approximation influences the predictions we have undertaken a Monte Carlo computer simulation study of the generic model. To facilitate the simulations the particles have been confined to the sites of a simple cubic lattice with the anisotropic interactions being restricted to nearest neighbors and having the form −(ε/4)P2(cos β), where β is the angle between the anisometric groups. The other advantage of this model is that for the linear conformer alone it is identical to the Lebwohl–Lasher model for nematics which has been studied in depth and so allows the influence of the presence of the interconverting bent conformer on the liquid crystal behavior to be readily discerned. We have also extended the discrete generic model, with just two interconverting conformers, to a continuous model in which the torsional angle defining the conformational state varies continuously from 180° (linear) to 0° (bent); this makes the model far more general and more analogous to real dimers. A wide range of properties have been determined from the computer simulations; these include the second rank orientational order parameter for the anisometric groups, the singlet orientational distribution function for these, the second rank short range orientational order parameter, the internal order parameters and the distribution function for the torsional angle. Armed with this wealth of information we are able to make a searching test of the molecular field theories for the two models as well as commenting on the behavior of real liquid crystal dimers.
Electronic structure and optical properties of electroluminescent spiro-type molecules
Authors: Johansson N. , dos Santos D. A. , Guo S. , Cornil J. , Fahlman M. , Salbeck J. , Schenk H. , Arwin H. , Brédas J. L. , Salanek W. R.
Published: July 2002

The electronic structure and optical properties of the propeller-shaped spiro molecules, 2,2′-bis(5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl)-9,9′-spirobifluorene, and 2,2′,7,7′-tetrakis (biphenyl-4-yl)-9,9-spirobifluorene, have been studied by photoelectron spectroscopy, Raman spectroscopy, and spectroscopic ellipsometry. The experimental spectra are analyzed with the help of results from quantum-chemical calculations. The excellent agreement between experiment and theory allows for a detailed characterization of the frontier electronic structure and lowest-energy optical transitions in these novel compounds. It follows that most of the electronic and optical properties of the these spiro-molecules can be deduced by consideration of a single branch; in this case a sexi-phenyl and PBD branch. In particular, although the branches in the spiro molecules are perpendicular to each other, there is considerable interaction at the spiro-center between the π-electron systems of the two branches, indicating, among other things, that chemistry involving either branch is dependent on the other branch.