bluuebunny/crossref_metadata_2025 huggingface dataset:
Electromechanical properties of thin strip piezoelectric vibrators at high frequency
Authors: Ritter Timothy , Shung K. Kirk , Cao Wenwu , Shrout Thomas R.
Published: July 2002

A method was developed and used to determine the electromechanical properties of high frequency (>20 MHz) piezoelectric strip vibrators. A nonlinear regression technique was employed to fit the impedance magnitude and phase as predicted by Mason’s model to measured values. Results from experimental measurements on 30 MHz array elements supported by an attenuative backing indicated degraded performance when compared to values predicted from the electromechanical properties measured at low frequency. This degradation may be attributed to damage incurred during fabrication and grain size effects, with a fine grain sized material providing superior relative performance. This technique may be used in the evaluation and comparison of different fabrication processes and materials for high frequency medical imaging arrays.
The photoelectron spectrum of ArXe obtained using resonantly enhanced multiphoton ionization
Authors: Pratt S. T. , Dehmer P. M. , Dehmer J. L.
Published: July 2002

The photoelectron spectrum of the heteronuclear rare gas dimer ArXe was determined using resonantly enhanced multiphoton ionization-photoelectron spectroscopy (REMPI-PES). Three photoelectron peaks are observed corresponding to the production of ArXe+ in the A2Σ+1/2 B2Π3/2, and B2Π1/2 states. The kinetic energies of these peaks are used to determine lower limits for the dissociation energies of the ionic states.
Rotational spectra and structure of the ammonia–water complex
Authors: Herbine P. , Dyke T. R.
Published: July 2002

Microwave and radio frequency spectra for NH3⋅H2O and deuterated analogs have been observed by molecular beam electric resonance spectroscopy. Rotational constant, Stark effect, and nitrogen quadrupole coupling interaction data were obtained. This complex is found to have a linear, hydrogen bonded structure with water as the proton donor. The NH3 monomer symmetry axis was found to have a vibrationally averaged displacement of 23.1° from the N...O axis. No evidence for transfer of a proton from water to the ammonia was observed.
Investigation of the K(2S, 2P)–Hg interaction at collision energies between 0.5 and 6 eV
Authors: Lackschewitz U. , Maier J. , Pauly H.
Published: July 2002

Measurements of total differential cross sections for the scattering of laser-excited K(4 2P3/2) atoms by ground state mercury atoms have been performed in a crossed beam experiment at center-of-mass energies between 0.5 and 6 eV. Characteristic features of the cross section are two well resolved rainbow structures which we can assign to the ∑ and the Π branch of the excited state potential in the scope of the elastic approximation. At energies above 2.1 eV we observe an additional structure in the cross section at large angles which shows the same dependence on the polarization as the amplitude of the ∑ rainbow. This is attributed to a shoulder in the repulsive part of the B ∑ potential. The simultaneously measured differential cross sections for the scattering of K(2S) atoms by mercury have been used to determine the repulsive portion of the ground state potential by inversion, making use of the precisely known well of the potential for this system.
Kinetic energy dependence of Al++O2→AlO++O
Authors: Weber M. E. , Elkind J. L. , Armentrout P. B.
Published: July 2002

The endothermic reaction of Al+ with O2 is studied using a guided ion-beam apparatus. The reaction cross section is measured as a function of kinetic energy from 0 to 20 eV. The threshold energy for the reaction, E0, is determined from an empirical model to be 3.64±0.04 eV. Phase space calculations of the cross section performed with E0 as the only adjustable parameter yield a similar result, E0=3.60±0.02 eV, and predict the absolute magnitude of the reaction cross section within experimental error. We conservatively quote the threshold energy as 3.62±0.12 eV which is somewhat higher than the thermodynamic threshold calculated using literature thermochemistry, 3.40±0.16 eV. This may indicate that a slight barrier to the reaction exists, or that AlO+ is produced in an excited state, or that the literature thermochemistry needs revision. Our results suggest D00 (AlO+)≥1.50±0.12 eV and I.P.(AlO)≤9.75±0.13 eV.
On the correlation between electric polarizabilities and the ionization potential of atoms
Authors: Fricke B.
Published: July 2002

It is found that the electric dipole polarizabilities of neutral atoms correlate very strongly with their first ionization potential within the groups of elements with the same angular momenta of the outermost electrons. As the latter values are known very accurately, this allows a very good (<30%) prediction of various atomic polarizabilities.
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o relativistic effective potentials with spin-orbit operators. II. K through Kr
Authors: Hurley M. M. , Pacios Luis Fernandez , Christiansen P. A. , Ross R. B. , Ermler W. C.
Published: July 2002

A refined version of the ‘‘shape consistent’’ effective potential procedure of Christiansen, Lee, and Pitzer was used to compute averaged relativistic effective potentials (AREP) and spin-orbit operators for the atoms K through Kr. Particular attention was given to the partitioning of the core and valence space, and where appropriate more than one set of potentials is provided. These are tabulated in analytic form. Gaussian basis sets with expansion coefficients for the lowest energy state of each atom are given. The reliability of the transition metal AREPs was determined by comparing computed atomic excitation energies with accurate all-electron relativistic values. In all cases the maximum error was found to be less than 0.1 eV. The reliability of the spin-orbit operators was also considered.
Rotational spectrum and structure of CF3H–NH3
Authors: Fraser G. T. , Lovas F. J. , Suenram R. D. , Nelson D. D. , Klemperer W.
Published: July 2002

The rotational spectrum of CF3H–NH3 has been obtained using a pulsed nozzle Fourier transform microwave spectrometer. A symmetric top spectrum is observed that is consistent with free internal rotation of the NH3 subunit against the CF3H subunit. Rotational transitions have been measured for both the ground and first excited internal rotor state of the complex. The spectroscopic constants which have been obtained include: B0=1996.903(2) MHz, DJ =3.46(12) kHz, and eQqN =−3.186(8) MHz. From the quadrupole coupling constant of the nitrogen nucleus, eQqN, the bending amplitude of the NH3 unit is determined to be 22.57(10)°. The hydrogen bond length is 2.314(5) Å and the weak bond stretching force constant is 0.066(2) mdyn/Å. The bond length and stretching force constant for CF3H–NH3 are similar in value to those determined for HCCH–NH3 (2.33 Å and 0.070 mdyn/Å, respectively).
Rotationally inelastic scattering and potential calculation for NH3+He
Authors: Meyer H. , Buck U. , Schinke R. , Diercksen G. H. F.
Published: July 2002

In a crossed molecular beam experiment, time-of-flight distributions of NH3 molecules scattered from He have been measured as a function of the deflection angle at E=97.7 meV. The spectra, which cover a center-of-mass angular range from 97° to 168° show appreciable rotational energy transfer in the backward direction. These data, which are sensitive to the repulsive anisotropy of the interaction potential, are used together with the diffraction oscillations of total differential cross section to determine a new rigid rotor potential surface for NH3–He. The potential is constructed by combining large basis set self-consistent-field (SCF) calculations with damped long range dispersion coefficients. The two free parameters of the damping function are determined by fitting the measured data to calculations using the quantal coupled states approximation. The potential is in reasonable agreement with calculations where correlation effects are included and reproduces the measured second virial coefficients.
Relaxed crystalline and icosahedral molecular clusters: CO2
Authors: van de Waal B. W.
Published: July 2002

To obtain structural models for gas-phase molecular clusters of CO2, multishell cubo-octahedral fcc crystal fragments, consisting of up to five concentric shells (561 molecules) were allowed to relax, assuming -12-6-1 pairwise additive nonbonded atom–atom interactions. The resulting icosahedral deformation is most pronounced in the outer shells, whereas the inner shells remain nearly crystalline; the deformation decreases with growing cluster size. The relaxed crystalline clusters (except the one-shell cluster) do not correspond to global minima in the potential-energy surface, however. More favorable structures exist, consisting of nested, nearly undistorted icosahedra, analogous to the Mackay arrangements of equal spheres, suggesting that such arrangements are not only energetically favorable for aggregates of spherical inert-gas atoms, but also for assemblies of nonspherical molecules, with highly anisotropic interactions, including electrostatic. The relative energy difference between relaxed crystalline and icosahedral clusters of the same size ranges from 2.4% (two shells) to 0.8% (five shells). The available experimental evidence suggests bulk-crystalline structure in the N=100 range.
A test particle model potential for formamide and molecular dynamics simulations of the liquid
Authors: Sagarik Kritsana P. , Ahlrichs Reinhart
Published: July 2002

A test particle model (T-model) potential is derived for interacting rigid formamide (HCONH2) molecules and applied in MD simulations of the liquid phase. The most important features of the dimers predicted by the T-model potential are checked by large basis set SCF calculations. The most stable dimer is the cyclic arrangement with an interaction energy of −10.91 kcal/mol at the SCF level and −11.72 kcal/mol for the T model. However, the dominating molecular arrangements in the liquid phase are found to be linear-like structures. The computed thermodynamic, dynamic, and structural properties of the liquid are compared with available theoretical and experimental data.
Monte Carlo studies of the melt–vacuum interface of a lattice polymer
Authors: Madden William G.
Published: July 2002

A film of polymer on a five way cubic lattice is adsorbed on a smooth solid surface with one face exposed to the vacuum. The properties of the film are examined using a Monte Carlo algorithm which employs a combination of reptative and pseudokinetic ‘‘movements.’’ The film which forms exhibits three distinct layers, with two interfacial regions sandwiching a central region. The polymer in that central region is shown to have characteristics of the bulk melt. The density of beads, the density of ends, and the density of the centers of mass of individual chains are all calculated for a number of temperatures. The densities of beads and ends decrease monotonically across the melt–vacuum interface, but the center-of-mass density shows pronounced peak structure near both interfaces. Analysis of the conformations of the polymer in the neighborhood of the interfaces shows the chains to be asymmetrically flattened at both surfaces. The structural features near the melt–vacuum interface are not found to scale in an obvious way with the molecular weight.
Tunneling in the double-minimum E
F 1Σ+g state of molecular hydrogen
Authors: Senn P. , Dressler K.
Published: July 2002

Motivated by the observations of tunneling resonances in the rotation–vibration structures of the EF 1Σ+g double-minimum states of H2 and D2 we calculate the resonant tunneling frequencies of these states with a semiclassical formula, using accurate ab initio potential functions with appropriate adiabatic and rotational corrections. Good agreement is obtained with the corresponding two-state interaction energies which describe these tunneling resonances and which are evaluated by fitting either the experimental term values or the accurate quantum mechanical energy eigenvalues of the double-minimum oscillator.
Clarification of the electronic asymmetry of Λ doublets in 3Π electronic states of diatomic molecules
Authors: Pouilly Brigitte , Dagdigian Paul J. , Alexander Millard H.
Published: July 2002

The reflection symmetry of the spatial part of the electronic wave function for 3Π diatomic molecular states is examined carefully for the individual Λ doublet levels by means of an approach presented earlier [M. H. Alexander and P. J. Dagdigian, J. Chem. Phys. 80, 4325 (1984)]. The results are: For a 3Π molecule in Hund’s case (a) the electronic wave function in the Ω=1 (F2) e levels will be antisymmetric and, in the the f levels, symmetric with respect to reflection of the spatial coordinates of the electrons in the plane of rotation of the molecule. The electronic wave functions in the F1 and F3 levels will not have a defined plane of symmetry. By contrast, in the Hund’s case (b) high J limit, the electronic wave function in the F1e, F2 f, and F3e levels will be antisymmetric and, in the F1 f, F2e, and F3 f levels, symmetric with respect to reflection. Thus, the symmetry of the wave functions in the F2Λ-doublet levels reverses with the passage from case (a) to case (b). In the case (b) limit, the main branch P and R lines of a 3Π–3Σ− transition will probe antisymmetric levels, irrespective of the Fi level of the 3Π state, while the main branch Q lines will probe symmetric levels. This will be reversed for a 3Π–3Σ+ transition. At low J, in the case (a) limit, in a 3Π–3Σ− transition the P2i and R2i lines with i odd and the Q2i lines with i even will probe antisymmetric levels, while the P2i and R2i lines with i even and the Q2i lines with i odd will probe symmetric levels; the other 18 rotational branches probe levels with no well-defined reflection symmetry.
Stationary points on the potential energy surfaces of (C2H2)2, (C2H2)3, and (C2H4)2
Authors: Alberts Ian L. , Rowlands Timothy W. , Handy Nicholas C.
Published: July 2002

Minima on the potential energy surfaces of (C2H2)2, (C2H2)3, and (C2H4)2 have been located by ab initio methods, using the Mo/ller–Plesset second-order (MP2) procedure, with a DZP basis set. For (C2H2)2, a T-shaped structure is predicted and for (C2H2)3, a C3h structure. For (C2H4)2 there are two candidates, a T-shaped structure and a staggered-parallel structure, and it is difficult to distinguish between them. Other stationary points have been located on the surfaces, but they are all found to be transition states, by the method of analytic second derivatives. Existing experimental data is insufficient to decide unequivocally as to the geometry of the minima. These calculations appear to resolve these questions and there is no contradiction with the data.
A potential energy surface for the Li+HCl reaction
Authors: Palmieri Paolo , Garcia Ernesto , Laganá Antonio
Published: July 2002

The potential energy surface for the Li+HCl reaction has been calculated by performing UHF/CI ab initio computations for a large number of molecular geometries. To make the surface suitable for dynamical studies, the analytical interpolation of the CI points has been performed using a multi-body expansion of the potential and bond order variables with constraints on diatom asymptotic and transition state energies. To test the quality of the potential surface an investigation of its dynamical properties has been carried out using quasiclassical trajectories. Calculated scattering quantities have been compared to experiment.
The B 1Πu potential energy curve and dissociation energy of 39K2
Authors: Heinze Johannes , Engelke Friedrich
Published: July 2002

The 39K2 B 1Πu potential energy curve has been determined using laser spectroscopic techniques and quantum mechanical calculations. The dissociation energy is 2407.6±0.5 cm−1 (0.2985±0.0001 eV) including a potential barrier of 298±10 cm−1 (0.037±0.0013 eV) found with its maximum at 8.08±0.05 Å (15.3±0.1 bohr). The long-range behavior matches smoothly onto the form predicted from dispersion forces. The dissociation energy of the ground state X 1Σ+g, obtained by a long-range extrapolation of the vibrational separations, is De =4444±10 cm−1 (0.5506±0.0013 eV), in agreement with recent theoretical prediction.
Microwave and infrared studies of acetylene dimer in a T-shaped configuration
Authors: Prichard Diana G. , Nandi R. N. , Muenter J. S.
Published: July 2002

Radio frequency, microwave, and infrared spectra have been obtained from molecular beams of acetylene dimer. The geometry of the isomer studied here is T shaped, with the centers of mass of the monomers separated by 4.38 Å. The equilibrium structure appears not to have C2v symmetry and the vibrationally averaged structure has the ‘‘top’’ of the T rotated about its center of mass by 27°. The electric dipole moment of this complex is 0.28 D.
Photochemistry of hydrated electron clusters (H2O)−n (15≤n≤40) at 1064 nm: Size dependent competition between photofragmentation and photodetachment
Authors: Posey Lynmarie A. , Johnson Mark A.
Published: July 2002

Photofragmentation is found to be surprisingly efficient when hydrated electron clusters (H2O)−n, 15≤n≤40, are excited at 1064 nm (1.165 eV). The decay probability into ionic channels rises sharply from zero in the size range 15≤n≤20 before leveling off at a value of 0.56±0.10. The propensity of smaller clusters to detach an electron rather than fragment is correlated with the peculiar shape of the cluster ion distribution obtained by dissociative attachment of low energy electrons onto neutral water clusters, where the ionic clusters are only observed in abundance for n≥11.
Optical and magnetic properties of a nonconjugated conducting polymer
Authors: Thakur M. , Elman B. S.
Published: July 2002

The magnetic and optical properties of a nonconjugated conducting polymer (1,4 cis-polyisoprene) at different doping levels of iodine are discussed in this report. The EPR and static magnetic susceptibility measurements show that cation radical (spin 1/2) species are generated as a consequence of doping. The concentration of the cation radicals increases approximately linearly with the dopant concentration until saturation. The magnetic susceptibility has a Curie-like temperature dependence as anticipated. The optical measurement of doped samples show two absorption peaks—one at ∼4.27 eV and the other at ∼3.3 eV. The first peak is attributed to the excitation of the cation radicals (comparable to ethylene ions) and the second one is interpreted as due to charge transfer between localized states. These results are fully consistent with the electrical properties and the structure of the system.
The electron affinity of oxygen: A systematic configuration interaction approach
Authors: Feller David , Davidson Ernest R.
Published: July 2002

A sequence of configuration interaction (CI) wave functions, constructed so as to systematically approach the complete basis set, full CI limit, is used to argue that the only alternatives for improving the accuracy of electron affinity calculations are: (1) recovery of a sufficient fraction of the correlation energy of both anion and neutral so that the remaining error in the energy difference is acceptably small, or (2) methodological bias in favor of the more difficult to describe anion. Extended Gaussian basis sets, of the type recently employed in atomic hyperfine spin calculations are capable of recovering 95%–96% of the total O (3P) correlation energy. With much greater difficulty this basis can also recover an equivalent fraction of the O− (2 P) correlation energy. Nevertheless, the calculated electron affinity(1.31 eV) still underestimates the experimental value of 1.46 eV by 10%. Estimates based on multireference second order pertubation theory suggest that another 0.05 eV (EA=1.36 eV) is available from our present basis set, but cannot be variationally recovered due to hardware and software limitations. These results are in accord with the expectation that ΔE will not converge more rapidly than either E(O) or E(O−).
Infrared spectrum of sodium hydride
Authors: Maki Arthur G. , Olson Wm. Bruce
Published: July 2002

The infrared spectrum of gaseous NaH from 886 to 1245 cm−1 has been measured with a resolution of 0.015 cm−1 at temperatures between 670 to 720 °C. The v=1←0, 2←1, and 3←2 transitions have been observed and combined with rotational transitions measured by others to obtain appropriate rovibrational constants and Dunham potential constants. The Herman–Wallis intensity effect has been measured in order to estimate a transition moment of 0.31±0.05 D for the v=1←0 transition.
Dissociative charge transfer reactions of Ar+, Ne+, and He+ with CF4 from thermal to 50 eV
Authors: Fisher Ellen R. , Weber M. E. , Armentrout P. B.
Published: July 2002

Guided ion-beam techniques are used to measure the cross sections for reaction of CF4 with Ar+, Ne+, and He+ from thermal to 50 eV. Dissociative charge transfer followed by successive loss of F atoms are the major processes observed. Only CF+x (x=1–3) products are observed in the reactions of Ar+ and Ne+. With He+, in addition to the CF+x products, both C+ and F+ are seen at high kinetic energies. Reaction rates for these reactions are also given and compared with previous measurements. It is found that the energy dependence of the cross sections can be understood by considering the energies needed to access specific electronic states of the CF+4 ion.
Exciton migration in a ‘‘pseudo’’-one-dimensional crystal: Luminescence dynamics of doped CsMnBr3
Authors: McPherson Gary L. , Waguespack Yan Yin , Vanoy Timothy C. , Rodriguez Waldo J.
Published: July 2002

The exciton migration in CsMnBr3 (CMB), a linear chain salt which acts as a ‘‘pseudo’’-one-dimensional antiferromagnet, has been probed by examining the time-resolved luminescence of doped crystals. A number of metal ions such as Cr2+, Cr3+, Fe2+, Cu2+, Mo3+, Er3+ were observed to behave as exciton traps, while Cd2+ and Mg2+ act as exciton scatterers (antitraps). Samples doped with traps and traps along with scatterers, as well as ‘‘dilute’’ CsMnx[Mg2+ or Cd2+ ]1−xBr3 crystals were investigated. Analysis of the data indicates that exciton migration between chains is significant with a room temperature hopping frequency of 107–108 s−1 and a thermal barrier of 500 cm−1. The barrier for intrachain migration appears to be somewhat smaller, between 350 and 400 cm−1. Only at temperatures below 60 K does the exciton migration seem to be strictly confined to one dimension.
Accurate a
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o spectroscopic and thermodynamic properties for the SiC molecule
Authors: Martin J. M. L. , Francois J. P. , Gijbels R.
Published: July 2002

The spectroscopic properties of the three lowest-lying states X 3Π, A 3∑−, and a 1∑+ of SiC have been computed accurately using augmented coupled cluster methods and different basis sets. Partition functions have been set up, accounting for anharmonicity, rotation–vibration coupling, centrifugal distortion, and isotope effects. Thermodynamic properties are tabulated in JANAF style from 100 to 6000 K. A critical analysis of the effects of the various contributions is made, as well as an error analysis.
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o calculation of the OH (X 2Π, A 2Σ+)+Ar potential energy surfaces and quantum scattering studies of rotational energy transfer in the OH (A 2Σ+) state
Authors: Esposti Alessandra Degli , Werner Hans-Joachim
Published: July 2002

The potential energy surfaces of OH+Ar, which correlate asymptotically with OH(X 2Π)+Ar(1S) and OH(A 2Σ+)+Ar(1S), have been calculated using the coupled electron pair approximation (CEPA) and a very large basis set. The OH–Ar van der Waals complex is found to be bound by about 100 cm−1 in the electronic ground state. In agreement with several recent experimental studies the first excited state is found to be much more stable. The A state potential energy surface has two minima at collinear geometries which correspond to isomeric OH–Ar and Ar–OH structures. The dissociation energies De are calculated to be 1100 and 1000 cm−1, respectively; both forms are separated by a barrier of about 1000 cm−1. The equilibrium distances for OH–Ar and Ar–OH are calculated to be 2.9 and 2.2 Å, respectively, relative to the center of mass of OH. In order to investigate the nature of the strong binding in the A state, we have calculated accurate dipole and quadrupole moments as well as dipole and quadrupole polarizabilities for the X and A states of the OH radical and for the Ar atom. These data are used to estimate the contributions of induction and dispersion forces to the long-range OH–Ar potential. The calculated potential energy surfaces have been fitted to an analytical function and used in quantum scattering calculations for collision induced rotational energy transfer in the A state of OH. From the integral cross sections rate constants have been evaluated as a function of the temperature. The theoretical rate constants are considerably larger than the corresponding experimental values of Lengel and Crosley [J. Chem. Phys. 67, 2085 (1977)], but in good agreement with recent measurements of Jörg, Meier, and Kohse-Höinghaus [J. Chem. Phys. (submitted)]. Our potential energy surface has also been used to calculate the bound rovibrational levels of the OH–Ar complex.
Reactions of fourth-period metal ions (Ca+−Zn+) with O2: Metal-oxide ion bond energies
Authors: Fisher Ellen R. , Elkind J. L. , Clemmer D. E. , Georgiadis R. , Loh S. K. , Aristov N. , Sunderlin L. S. , Armentrout P. B.
Published: July 2002

Reactions of Ca+, Zn+ and all first-row atomic transition metal ions with O2 are studied using guided ion beam techniques. While reactions of the ground states of Sc+, Ti+, and V+ are exothermic, the remaining metal ions react with O2 in endothermic processes. Analyses of these endothermic reactions provide new determinations of the M+–O bond energies for these eight elements. Source conditions are varied such that the contributions of excited states of the metal ions can be explicitly considered for Mn+, Co+, Ni+, and Cu+. Results (in eV) at 0 K are D0(Ca+–O)= 3.57±0.05, D0(Cr+–O)=3.72±0.12, D0(Mn+–O)=2.95±0.13, D0(Fe+–O)=3.53±0.06 (reported previously), D0(Co+–O)=3.32±0.06, D0(Ni+–O) =2.74±0.07, D0(Cu+–O)=1.62±0.15, and D0(Zn+–O)=1.65±0.12. These values along with literature data for neutral metal oxide bond energies and ionization energies are critically evaluated. Periodic trends in the ionic metal oxide bond energies are compared with those of the neutral metal oxides and those of other related molecules.
Kinetic nucleation theory: A new expression for the rate of homogeneous nucleation from an ideal supersaturated vapor
Authors: Girshick Steven L. , Chiu Chia-Pin
Published: July 2002

The ‘‘kinetic theory’’ of homogeneous nucleation developed by Katz and Wiedersich is extended to derive a new expression for the rate of nucleation from an ideal supersaturated vapor. Compared to the classical expression for the nucleation rate, the new expression has a slightly different dependence on supersaturation, and a substantially different dependence on temperature. A comparison of the new expression with experimental data on nucleation rates of several organic liquids indicates that in some but not all cases the new expression gives much closer agreement with the data than does the classical expression. Discrepancies between the theory and the data are ascribed mainly to the physical assumptions of the theory presented, which are the same as in the classical theory—particularly, that the physical properties of microscopic clusters are the same as those of the bulk liquid.
The C9 cluster: Structure and infrared frequencies
Authors: Heath J. R. , Saykally R. J.
Published: July 2002

The high resolution infrared spectrum of the C9 cluster has been measured in direct absorption by infrared diode laser spectroscopy of a pulsed supersonic carbon cluster jet. Fifty-one rovibrational transitions have been assigned to the ν6 (σu ) antisymmetric stretch fundamental of the 1Σ+9 linear ground state of C9. The measured rotational constant [429.30(50) MHz] is in good agreement with ab initio calculations and indicates an effective bond length of 1.278 68(75) Å, consistent with cumulenic bonding in this cluster. Several perturbations are observed in the upper state, and the upper- and lower-state centrifugal distortion constants are observed to be anomolously large, evidencing a high degree of Coriolis mixing of the normal modes.
Angle-energy distributions of Penning ions in crossed molecular beams. I. Evidence for discrete nonadiabaticity in the He*(2 1S)+H,D→He+H+,D++e− reaction
Authors: Khan A. , Siddiqui H. R. , Siska P. E.
Published: July 2002

Relative doubly differential cross sections for the Penning ionization of H and D by He*(2 1S) are reported at a mean collision energy of 9.2 kcal/mol for H and 7.9 for D in an experiment employing crossed supersonic beams and a rotatable mass spectrometer detector. The products are scattered sharply forward in the center-of-mass frame, with release of an additional 3.5 kcal/mol into translational energy at the peak of the recoil distribution. The energy release is significantly lower than predicted on the basis of quantum dynamics on a single adiabatic ab initio complex potential, or of the experimental low-energy Penning electron spectrum (PIES). Discrete nonadiabatic behavior is suggested and a model is developed based on modifications of ab initio potentials, Penning ionization atom–atom theory on a single complex potential, and an adiabatic-velocity modification of Landau–Zener curve-crossing theory (which may be more generally useful). The dynamical model compares well with exact two-state complex potential calculations for the magnitude and energy dependence of the total ionization cross section, and improves the emulation of both the low-energy PIES data and our recoil distributions. Significant modifications to the ab initio potentials include a deepening of the well in the He*(2 1S)+H adiabatic potential by 2.0 to 10.2±1 kcal/mol, a quadrupling of the magnitude of the resonance width, and a reduction in the adiabatic splitting at the avoided crossing between 2 1S and 2 3P potentials. Our low-energy calculations suggest that available data on the associative ionization (HeH+) branching fraction is inconsistent with the PIES data.
Microscopic shifts of size-assigned p-cresol/H2O-cluster spectra
Authors: Pohl M. , Schmitt M. , Kleinermanns K.
Published: July 2002

p-cresol and its complexes with H2O and CH3OH were cooled in a pulsed supersonic free jet and studied by resonant multiphoton ionization with time-of-flight mass analysis. Detailed mass and concentration analysis allowed an unambiguous assignment of cluster size. The electronic origins of p-cresol (H2O)1,2,3 show irregular red- and blueshifts with change of cluster size, which is referred to the bivalent role of p-cresol as proton donor and acceptor. Ab initio and semiempirical quantum chemical calculations support this interpretation and show the spectral shifts to be essentially due to the inductive effect of the solvent molecules Y exerted on X in X-H...Y. While the vibronic bands of p-cresol (H2O)2 are quite broad, those of p-cresol (H2O)3 are sharp again. The ab initio calculations show that this may be attributed to the quite rigid ‘‘open cyclic’’ structure of p-cresol (H2O)3. Our experimental and theoretical investigations show a completely analogous behavior of phenol (H2O)1,2,3 clusters
Cluster ion dip spectroscopy of hydrogen bonded phenol(H2O)n clusters, n=0–4
Authors: Stanley R. J. , Castleman A. W.
Published: July 2002

Employing a laser based time-of-flight mass spectrometer system, ion dip spectra for phenol (Ph), Ph(H2O)1, Ph(H2O)3, and Ph(H2O)4 were obtained in the range of 500–1300 cm−1 from a variety of initially pumped states. Dramatic enhancement of the signal-to-noise ratio of the cluster ion dip spectra relative to that of the bare phenol is attributable to the increase in the excited state singlet lifetime of the hydrated phenol chromophore. Several dips in the Ph(H2O)1 spectrum exceed the ‘‘saturation’’ limit of 50%, indicating that significant relaxation of the downpumped ground state is occurring via low frequency vibrational modes of the H2O solvent ‘‘bath.’’ Excitation of the hydrogen bond stretch (σ 10=156 cm−1 ) in the S1 state of the Ph(H2O)1 cluster reveals that the ground state (S0) hydrogen bond stretch, σ 01, is 151(±1) cm−1, a mode which appears to be built off of phenol fundamental and combination bands. A second intermolecular band is also evident at 141(±2) cm−1. There is no evidence of an analogous wag mode when pumping the S1 bend (β10), suggesting that the intermolecular modes in S1 are highly coupled. Attempts to obtain ion dip spectra for Ph(H2O)2 went unrewarded, presumably due to the anomalously short S1 lifetime of the Ph(H2O)2 cluster. Spectra for Ph(H2O)3 and Ph(H2O)4 were obtained which show prominent phenol bands, with low frequency (∼10 cm−1 ) progressions built off of these bands. The intermolecular hydrogen bond stretch for Ph(H2O)3 and Ph(H2O)4 in the ground state are 189(±1) cm−1 and 185(±1) cm−1, respectively.
The small shear rate response of electrorheological suspensions. II. Extension beyond the point–dipole limit
Authors: Klingenberg D. J. , van Swol Frank , Zukoski C. F.
Published: July 2002

Electrorheological (ER) suspensions display dramatic changes in their rheological properties when subjected to large electric fields. In an accompanying paper, a molecular dynamics-like simulation technique was developed to investigate sheared ER suspensions. The results of that study predicted a dynamic yield stress that saturates at large concentrations of the disperse phase. Experimental results are presented here which confirm the predicted saturation of the yield stress. The experimentally determined yield stresses are larger than the values predicted by the simulations in the point–dipole limit. This discrepancy is accounted for by reconciling approximations made in the simulation model. Incorporation of multipole and multibody contributions to the electrostatic interaction between spheres through a perturbation analysis suggests that accurate predictions of yield stresses can be made. However, we find that the ER response is sensitive not only to the material dielectric properties, but also the details of the pair interactions between particles at vanishing separations.
Group V trimers and their positive ions: The electronic structure and potential energy surfaces
Authors: Balasubramanian K. , Sumathi K. , Dai Dingguo
Published: July 2002

Complete active space multiconfiguration self-consistent field (CAS-MCSCF) followed by multireference configuration interactions (MRCI) calculations are made on the electronic states of P3, As3, Sb3, Bi3 and all their positive ions. All group V trimers have Jahn-Teller distorted 2A2 ground states arising from the 2E″(D3h) state while the positive ions have 1A′1(D3h) equilateral-triangular ground states. For the neutral species, the 2B1 Jahn-Teller component was found to be nearly degenerate with the 2A2 component. Extensive studies on As3, Sb3, and Bi3 reveal the existence of 4A2, 4E′, and 2E′ (D3h) excited states which also undergo Jahn-Teller distortion. The binding energies and the ionization potentials (IP) of all group V trimers were computed and compared with known experimental data on some of these species. The IPs and the binding energies per atom of group V trimers were found to be significantly smaller than dimers thus exhibiting odd–even alternation in these properties. Our computations explain the dramatically different photofragmentation patterns for antimony and bismuth clusters observed by Geusic et al.
Observation of eliminative cationic polymerization within van der Waals clusters
Authors: El-Shall M. Samy , Schriver K. E.
Published: July 2002

We report the first observation of eliminative cationic polymerization within van der Waals (vdW) clusters following electron impact ionization at pressures of 10−8 Torr. The elimination reactions of C2H3Cl+ within the clusters terminate after three successive steps, each involving elimination of HCl or Cl. The results provide a mechanism for the early stages of gas phase cationic polymerization of vinyl chloride and demonstrate the feasibility of using vdW clusters as a means of studying gas phase cationic polymerization.
Study of the low-lying electronic states of Si2 and Si−2 using negative ion photodetachment techniques
Authors: Kitsopoulos T. N. , Chick C. J. , Zhao Y. , Neumark D. M.
Published: July 2002

The low-lying electronic states of Si−2 and Si2 were studied using both photoelectron spectroscopy and threshold photodetachment spectroscopy of Si−2 . Our measurements show that the ground state of Si2 is the X 3Σ−g state and that the X 3Σ−g–D 3Πu splitting is 0.083±0.010 eV. Additional spectroscopic constants for the X 3Σ−g, D 3Πu, a 1Δg, b 1Πu and c 1Σ+g states of Si2 were also determined. For Si−2 , the first two electronic states were identified as: 2Πu (Te =0, re =2.207±0.005 Å, and ν=533±5 cm−1) and 2Σ+g (Te =0.025±0.010 eV, re =2.116±0.005 Å, and ν=528±10 cm−1). The electron affinity for Si2 was found to be 2.176±0.002 eV. Our results provide definitive orderings and splittings for the low-lying electronic states in both Si2 and Si−2 .
Friction coefficient of polymer networks of gels
Authors: Tokita Masayuki , Tanaka Toyoichi
Published: July 2002

The friction coefficient between the polymer network of poly(acrylamide) gel and water is measured as a function of the temperature, the polymer concentration of the gel, and the concentration of the crosslinker by a newly designed apparatus. The friction decreases in proportion to the viscosity of water as the temperature is increased, but the ratio of the friction coefficient and the fluid viscosity is constant in the temperature range from 0 to 60 °C. The concentration dependence of the friction coefficient is well described by a power law relationship f∼φν with the exponent of ν=1.5, which coincides with the prediction of the scaling theory. The friction decreases as the crosslinker concentration increases. The decrease, contradictory to our intuition, is presumably a result of the spatial structural inhomogeneity induced by the critical density fluctuations of the polymer network due to the small solubility of the crosslinker in water. In such an inhomogeneous network water passes through the regions with less network density avoiding the denser regions. This indicates that the effective pore size of a network is determined by the correlation length of the network inhomogeneities rather than by the distance between the neighboring two chemical crosslinkings.
Electronic structure of group IV tetramers (Si4–Pb4)
Authors: Dai Dingguo , Balasubramanian K.
Published: July 2002

Geometries and energy separations of ten electronic states of Ge4, Sn4, and Pb4 are computed and compared with Si4. We employ complete active space multiconfiguration self-consistent-field followed by multireference singles + doubles configuration interaction calculations. The ground states of Ge4–Pb4 are found to be 1Ag states with equilibrium geometries of rhombus similar to Si4. Relativistic effects are found to be significant for Pb4. Atomization/dissociation energies of Ge4–Pb4 are computed and compared with Si4. Pb4 was found to be considerably less bound compared to other tetramers. The energy separations of the excited electronic states were found to decrease, especially the splitting between the rhombus and tetrahedral structures, as one goes down the group.
Weighted-density-functional theory calculation of elastic constants for face-centered-cubic and body-centered-cubic hard-sphere crystals
Authors: Laird Brian B.
Published: July 2002

The isothermal elastic constants for the face-centered-cubic (fcc) and body-centered-cubic (bcc) hard-sphere crystal are calculated for a range of densities using the modified weighted-density functional of Denton and Ashcroft [Phys. Rev. A 39, 4701 (1989)]. The fcc elastic constants are shown to be in excellent agreement with the computer simulation data and to represent a significant improvement over the predictions of other density-functional methods. The bcc crystal is predicted correctly to be unstable to shear, in agreement with simulation. This fact supports the conclusion that the bcc hard-sphere solid, even though mechanically unstable, is well described by such methods.
The grating decomposition method: A new approach for understanding polarization-selective transient grating experiments. II. Applications
Authors: Fourkas John T. , Trebino Rick , Fayer M. D.
Published: July 2002

We apply the theory developed in Paper I to two transient grating problems that present difficulties in interpretation and/or calculation. The first application is general, and illustrates the ability of the grating decomposition method (GDM) to facilitate calculations and to provide intuition and insight in complex orientational grating experiments: we apply the GDM to nuclear optical Kerr effect (OKE) polarization gratings. We show that the circularly polarized component gratings of the polarization-grating decomposition do not contribute to the signal, and that the OKE polarization grating can therefore be viewed as the sum of two gratings with orthogonal net molecular alignments. We also use the GDM and this system to explain why polarization gratings can rotate the polarization of the probe beam. The second example is a detailed application of the GDM to an experiment in which the data cannot be fully interpreted using standard diagrammatic perturbation methods: picosecond transient gratings on the D lines of gas-phase sodium atoms. We use the GDM and effective two-interaction matrix elements to greatly simplify this problem. We show why, in atmospheric-pressure experiments, Na intensity-grating decays are dominated by excited-state quenching, whereas Na polarization-grating decays are not. We show that the polarization-grating decays are dominated by Na diffusion and are influenced by scattering among the ground-state magnetic sublevels, but are unaffected by excited-state decay. We further show why the envelopes of polarization decays do not match the corresponding intensity-grating decays at large fringe spacings in low-pressure Na cells.
The grating decomposition method: A new approach for understanding polarization-selective transient grating experiments. I. Theory
Authors: Fourkas John T. , Trebino Rick , Fayer M. D.
Published: July 2002

In this paper and the following Paper (II) we introduce a new method of viewing transient holographic grating experiments in which the gratings are formed by laser beams of orthogonal linear or circular polarizations (or one of each). In this paper, we show that the two traditional methods of modeling these gratings, electric-field pictures and diagrammatic perturbation theory, may be augmented. We demonstrate that any grating can be decomposed into component intensity gratings that are related to the polarizations in its electric-field picture. Each of these component gratings may be analyzed separately (with or without diagrammatic perturbation theory), facilitating the incorporation of secondary effects (such as transport and heat deposition) into the grating calculation. The grating decomposition method (GDM) illuminates spatial structure that is not evident in standard perturbative calculations; it also provides a physical description that makes qualitative insights more readily obtainable, while at the same time making the electric-field approach rigorous and quantitative. Furthermore, the GDM reduces the complexity of many diagrammatic perturbation theory calculations. We also introduce effective two-interaction matrix elements (ETIMEs), which can be used to greatly simplify perturbative grating calculations. We show that ETIMEs, when considered in conjunction with the symmetry properties of the third-order susceptibility (χ(3)), can often be used to prove that some of the component gratings in a decomposition do not contribute to the signal and therefore need not be considered. In II, we apply this theory to two grating problems.
Statistical modeling of ion–molecule electrostatic capture
Authors: Smith Sean C. , Troe J.
Published: July 2002

A statistical model is presented which enables rapid and accurate calculation of capture rate coefficients for the interaction of ions with neutrals possessing any combination of dipole, quadrupole, and isotropic or anisotropic induced-dipole moments. Rate coefficients at very low temperatures are calculated via the state-resolved statistical adiabatic channel model (SACM). At higher temperatures, a classical state-counting technique is utilized which may be derived either from microcanonical variational transition state theory or from the high-temperature limit of the quantized SACM approach. The predicted rate coefficients are generally accurate to within 10% in comparison with classical trajectory calculations.
A density functional investigation on the structure and stability of the SiFn and SiF−n series, n=1–5
Authors: Gutsev G. L.
Published: July 2002

The electronic and geometrical structure of the ground and low lying excited states of the SiFn and SiFn− series (n=1–5) are calculated within a local spin density approximation (LSDA) augmented with the nonlocal gradient corrections to the exchange functional. Energies of fragmentation through different decay channels, evaluated for both series, are in good accord with the experimental data and results of the post-Hartree–Fock (HF) calculations. The adiabatic electron affinity (EAad) of all the title neutrals is estimated for the first time. The SiF4− anion is found to be stable towards dissociation and its neutral precursor possesses the near-zero EAad. The SiF5− anion is highly stable towards dissociation although stability of its neutral precursor, SiF5, is close to zero threshold. The intriguing peculiarity of the anions consists in nearly the same energy for the fluorine anion detachment that means an approximate equality of the affinity to F− for all the neutral silicon fluorides considered.
Analysis of long range dispersion and exchange interactions between two K atoms
Authors: Zemke Warren T. , Tsai Chin-Chun , Stwalley William C.
Published: July 2002

This paper critically surveys the best available spectroscopic data for the two lowest electronic states (X 1Σ+g and a 3Σ+u) of K2. Since both states are known up to dissociation, they can be used to determine Coulomb and exchange contributions to the intermediate and long range interaction potentials. The multipolar expansion representation of the Coulomb (dispersion) energy at long range (−ΣnCnR−n) and the exponential representation of the exchange energy (Ae−αR) as well as a variety of theoretical calculations are compared with these empirical results. Finally, dissociation energy values are discussed and improved dissociation energies for the X 1Σ+g (De=4449.1±1.0 cm−1) and the a 3Σ+u state (De=252.9±1.1 cm−1) proposed.
Equation of motion coupled cluster method for electron attachment
Authors: Nooijen Marcel , Bartlett Rodney J.
Published: July 2002

The electron attachment equation of motion coupled cluster (EA-EOMCC) method is derived which enables determination of the various bound states of an (N+1)-electron system and the corresponding energy eigenvalues relative to the energy of an N-electron CCSD reference state. Detailed working equations for the EA-EOMCC method are derived using diagrammatic techniques for both closed-shell and open-shell CCSD reference states based upon a single determinant. The EA-EOMCC method is applied to a variety of different problems, the main purpose being to establish its prospects and limitations. The results from EA-EOMCC calculations are compared to other EOMCC approaches, starting from different reference states, as well as other theoretical methods and experimental values, where available. We have investigated electron affinities for a wide selection of both closed-shell and open-shell systems. Excitation spectra of atoms and molecules with an odd number of electrons are obtained, taking the closed-shell ground state of the ion as a reference in the EA-EOMCC calculation. Finally we consider excitation spectra of some closed-shell systems, and find in particular that the electron attachment approach is capable of yielding accurate triplet excitation energies in an efficient way.