bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamics of resonant third-order optical nonlinearity in perylene tetracarboxylic dianhydride studied by monitoring first- and second-order diffractions in subpicosecond degenerate four-wave mixing
Authors: Samoc Marek , Prasad Paras N.
Published: July 2002

We present a novel approach using simultaneous monitoring of temporal behavior of the usual phase conjugate signal and the second-order diffraction produced in a degenerate four-wave mixing experiment to obtain information about dynamics of resonant third-order nonlinear optical processes. The second-order diffraction is interpreted as arising from the presence of the appropriate Fourier component of the excited state grating. The higher Fourier components are expected to be generated in the presence of such excited state processes as bimolecular decay, two-photon absorption, saturation of absorption and diffusion of excitation. Third-order nonlinear optical properties of an organic dye: perylene tetracarboxylic dianhydride were studied by this approach using subpicosecond degenerate four-wave mixing at a wavelength of 602 nm. The dye is found to exhibit a strong resonant nonlinear effect with an effective χ(3) of ≊2×10−10 esu. The decay of the phase conjugate signal is power dependent and can be theoretically simulated by using a combination of monomolecular and bimolecular decay laws. The temporal behavior of the phase conjugate signal and the second-order diffraction are investigated at different intensities. The observed characteristics are well simulated by using a dominant bimolecular decay mechanism at higher excitation density.
Dielectric spectroscopy up to 20 GHz of LiCl/H2O solutions
Authors: Wei Yan-Zhen , Sridhar S.
Published: July 2002

Detailed measurements of the complex dielectric function ε=ε′−iε″ as functions of frequency between 45 MHz and 20 GHz and concentration c are presented and analyzed for LiCl/H2O solutions. In this frequency range, the dielectric response is due to free water molecules and the response of the bound water occurs at lower frequencies and is not observed. From the data, several parameters—the static and high frequency limiting values ε0 and ε∞, the dc conductivity σdc, the dielectric relaxation time τd, and the hydration number nhyd—are extracted. The dissociation is observed to be incomplete suggesting the presence of undissociated ion pairs. A dissociation parameter is inferred from the σdc data. A model which views the liquid as composed of free water, hydrated single ions, and hydrated ion pairs is used to analyze the ε0 data. With inclusion of the dissociation parameter, the model describes the data reasonably well for c≤5.1 M. At higher concentrations, a unique relaxation time is not observed indicating glasslike behavior.
Two-photon spectroscopy of the 3d Rydberg states of O2: 1Φg and 3Φg states
Authors: Park Haiyoon , Li Leping , Chupka William A. , Lefebvre-Brion Helene
Published: July 2002

The 3dδ 1Φg and 3Φg Rydberg states of O2 have been observed by (2+1) resonant enhanced multiphoton ionization from the metastable a 1Δg state and the resulting spectra rotationally analyzed. In contrast to all other observed Rydberg states of O2, the 3Φ2, 3Φ3, and 3Φ4 states show no sign of perturbation or predissociation by valence states. The 1Φ3 state is apparently weakly predissociated. The energy level structure of those 3d Rydberg states which have been reliably assigned is compared with theoretical calculations. The reasons for the failure to detect Πg states in these experiments are discussed.
A variational method for the calculation of vibrational energy levels of triatomic molecules using a Hamiltonian in hyperspherical coordinates
Authors: Carter Stuart , Meyer Wilfried
Published: July 2002

A full variational procedure is presented which is particularly suited to the calculation of vibrational (J=0) energy levels of triatomic potentials with large amplitude motions and also allows for high permutational symmetries. Starting from simple basis functions reflecting the structure of the kinetic energy operator, an optimized basis set is derived by a step-wise contraction scheme. The method is applied to H+3 and Na+3 (D3h) and to model potentials for H2O+ (C2v) and HLiH− (D∞h). The results for H+3 are shown to be superior to all previous calculations, in particular for those energy levels that lie above 20 000 cm−1. A new ab initio potential is presented for Na+3, for which converged energy levels are calculated up to 3500 cm−1. For H2O+ and HLiH−, the calculated energy levels agree with those obtained from a variational procedure in internal valence coordinates.
The structure of sodium silicate glass
Authors: Huang Chengde , Cormack A. N.
Published: July 2002

Atomistic scale computer simulations can provide a more detailed understanding of the structure of glasses. Our results on sodium silicate glasses show that sodium ions are associated with nonbridging oxygens and that the sodium ions and nonbridging oxygens tend to aggregate to form silica rich regions and alkali rich regions. Interatomic distances of our simulated glasses are in good agreement with experimental results. O–Si–O bond angles are centered around 109° with narrow distributions. Si–O–Si bond angles are broadly distributed from 120° to 180° with an average of about 153°.
Rotational features in the fluorescence excitation spectrum of O(1D2) from vacuum ultraviolet laser photodissociation of O2
Authors: Gibson S. T. , Lewis B. R. , Baldwin K. G. H. , Carver J. H.
Published: July 2002

Seventh-order anti-Stokes Raman-shifted ultraviolet laser radiation is used to dissociate O2 in the 175–177 nm region of the Schumann–Runge band system, B 3Σ−u–X 3Σ−g. A cross section for the production of O(1D2) is deduced from the 762 nm fluorescence of O2(b 1Σ+g), a collisional de-excitation product of O(1D2) and O2(X 3Σ−g). Step structure observed in the spectrum is attributed to rotational thresholds for absorption from X 3Σ−g to energies above the B 3Σ−u dissociation limit. The threshold energies define a limiting rotationless dissociation energy of 57 136.4±0.9 cm−1. Shape resonances, quasibound by the rotational barrier of B 3Σ−u, are observed for the first time in O2. A theoretical calculation of the cross section is in agreement with the measured cross section when the near-dissociation outer limb of the B 3Σ−u potential has an R−5 long range form. The shape of the adopted potential is consistent with an avoided crossing with another 3Σ−u state near R=4.6 Å.
Equation of state of fluid helium to very high pressure
Authors: Kortbeek Peter J. , Schouten Jan A.
Published: July 2002

Experimental speed-of-sound data up to 1 GPa and the variational perturbation theory of Ross were used to determine new parameters of the ‘‘exponential-six’’ interatomic effective pair potential of fluid helium. The potential was used in self-consistent integral equation calculations, based on a synthesis of the hypernetted chain and the mean spherical approximation, covering the fluid range of helium from 100 to 1000 K and up to the melting line (or up to 45 GPa). The calculated equation of state data in this range are in excellent agreement with all available experimental results. The calculated data from 1 GPa upward were correlated by a 16-coefficient modified Benedict–Webb–Rubin equation of state.
Homogeneous linewidths of Rhodamine 6G at room temperature from cavity-enhanced spontaneous emission rates
Authors: Barnes M. D. , Whitten W. B. , Arnold S. , Ramsey J. M.
Published: July 2002

Fluorescence lifetimes of Rhodamine 6G in levitated micron-sized droplets have been measured using a time-correlated photon counting technique. The coupling of emission into spherical cavity modes of the droplet results in significant emission rate enhancements which allow estimation of the homogeneous linewidth at room temperature.
Ultraviolet absorption cross sections for N2H4 vapor between 191–291 nm and H(2S) quantum yield in 248 nm photodissociation at 296 K
Authors: Vaghjiani Ghanshyam L.
Published: July 2002

The absolute UV absorption cross section of hydrazine vapor at 253.65 nm was determined to be, σ253.65=(2.859±0.172)×10−20 cm2 molec−1 (where the error is 1 sigma, precision plus systematic) at 296±2 K. The UV absorbance in a column of slowly flowing N2H4/He gas mixture was monitored in an absorption cell, and the eluting hydrazine collected and quantified by titration with standard KIO3 solution in HCl acid. The relative UV absorbance spectrum of N2H4 vapor was recorded from 191–291 nm using a diode-array spectrometer and normalized to absolute absorption cross sections using the σ253.65 value. The primary quantum yield of H(2S) formation in 248.3 nm photodissociation of N2H4 was measured to be 0.85±0.15 (where the error is 1 sigma, precision plus systematic) at 296±2 K relative to photolysis of CH3SH under similar experimental conditions. The observed spectrum is essentially continuous and the high H(2S) yield is consistent with unit dissociation to the lowest Ã 1A singlet excited state. Kinetics studies also yield a value for the rate coefficient, k1(296 K)=(1.48±0.22)×10−13 cm3 molec−1 s−1 for the reaction H+N2H4→products; (k1), where the error is 1 sigma, precision plus systematic.
Rotational echo 14N/13C/1H triple resonance solid-state nuclear magnetic resonance: A probe of 13C–14N internuclear distances
Authors: Grey Clare P. , Veeman Wiebren S. , Vega Alexander J.
Published: July 2002

A novel triple resonance magic angle spinning (MAS) NMR experiment is reported that can be used to probe carbon–nitrogen interatomic distances in polycrystalline and amorphous solids, without the need for 15N isotopic labeling. Theory is presented for an S=1 nucleus under conditions of MAS and a spin-locking radio frequency (r.f.) field. An off-resonance r.f. field is only effective in periods during the rotor cycle where it is of the same order of magnitude as the first order quadrupolar splitting Q. This occurs when Q changes sign (the zero crossing) and passages between the S=1 Zeeman levels result. Numerical calculations are used to determine the conditions for adiabatic passages. An adiabatic passage results in the greatest change in the density matrix, inverting the Zeeman magnetization and creating quadrupolar order; faster passages, caused by faster MAS or a larger quadrupole coupling constant (e2qQ/h), result in the loss of some Zeeman magnetization to non-spin-locked coherences. Applying an off-resonance 14N spin-locking field, during the evolution period of a 13C spin-echo experiment, alters the evolution of the dipolar coupled spin and leads to a loss of 13C intensity at the echo. Calculations of the dephasing of 13C magnetization, caused by adiabatic 14N passages, are in good agreement with experimental results obtained at slow spinning speeds for a sample of glycine, and an estimate of the dipolar coupling between the nitrogen and directly bonded carbon can be made. Faster 14N passages result in less 13C dephasing. Despite the large value for e2qQ/h expected for the amide 14N nucleus in the polymer polyamide-6, significant dephasing is still observed for carbon atoms that are more than 3.7 Å away from the nitrogen in the polymer chain. Methods for calculating the 13C dephasing under conditions of fast 14N passages are considered.
Vibrational distributions of NH (a 1Δ) from the photolysis of HN3 from 220–290 nm
Authors: Hawley Michael , Baronavski A. P. , Nelson H. H.
Published: July 2002

We have measured the NH(a 1Δ) vibrational distributions resulting from photolysis of HN3 at wavelengths ranging from 220 to 290 nm. The relative yields of v=1 and v=2 both show initial onsets far above the thermodynamic limit for their production and rise much faster than would be expected for a statistical distribution. The present results are compared to earlier results at 266 and 248 nm. Two possible explanations for our observations are proposed.
Semiclassical evaluation of nonadiabatic rates in condensed phases
Authors: Neria Eyal , Nitzan Abraham
Published: July 2002

A procedure for calculating nonadiabatic transition rates in the semiclassical limit is implemented and tested for models relevant for condensed phase processes. The method is based on evaluating the golden rule rate expression using a quantum description for the electronic subsystem and a semiclassical propagation for the nuclear degrees of freedom, similar to Heller’s calculation of absorption and Raman spectra. In condensed phase processes, the short lifetimes of the relevant correlation functions make it possible to implement the procedure within the frozen Gaussian method. Furthermore, because of the large density of states involved, which implies fast dephasing, incoherent superpositions of frozen Gaussian trajectories may be used for the evaluation of the rate. The method is tested using two simple exactly soluble models. One of them, consisting of two coupled electronic potential surfaces, harmonic and linear, is also used for testing and comparing a recently proposed algorithm by Tully. The other, the well-known displaced harmonic potentials model, is a prototype of many condensed phase processes. Finally, the method is applied for calculating the nonadiabatic radiationless relaxation of the solvated electron from its first excited state to the fully solvated ground state.
Temperature dependence of the reactions of small cobalt clusters with deuterium
Authors: Ho J. , Zhu L. , Parks E. K. , Riley S. J.
Published: July 2002

The kinetics of the reactions of cobalt clusters (Con, n=9–21) with deuterium are studied over the temperature range 133–373 K. The measured rate constants for the dissociative addition of the first D2 molecule to Con, and their temperature dependences, are found to be strongly dependent on cluster size. Co10–13 and Co15–17 are relatively reactive, with their reaction rates essentially independent of temperature, suggesting no significant reaction barrier for deuterium chemisorption. The reaction rate of Co20 is nearly constant above 213 K, but increases at lower temperature. For Co9 and Co14, the rate constants increase both at high and low temperatures. An increase at higher temperatures suggests the presence of activation barriers, while increases at low temperatures are interpreted as due to an increase in the initial trapping of D2 onto the clusters’ surfaces. For Co18, Co19, and Co21, evidence for more than one isomer at low temperature is found. The experimental results are discussed in terms of cluster geometrical structure and the detailed reaction mechanism.
On the vibrational temperature of metal cluster beams: A time-resolved thermionic emission study
Authors: Collings Bruce A. , Amrein Andreas H. , Rayner David M. , Hackett Peter A.
Published: July 2002

Delayed ionization rates for small niobium clusters are measured as a function of the cluster size, the laser wavelength, the number of photons absorbed, and the initial internal energy content of the cluster. It is shown, for the first time, that vibrational excitation of the clusters modulates their rate of delayed ionization. An analysis of the rate of ionization in terms of the total energy content of the cluster establishes unequivocally that delayed ionization is a statistically determined, unimolecular, activated process. It is shown that the rate of delayed ionization can be used to gauge the initial vibrational energy content of the cluster. Quantitative analysis of this effect establishes, for the first time, a thermometry for metal cluster beams. Thermal rate parameters, activation energies, and ‘‘Arrhenius factors’’ are presented for delayed ionization of a series of niobium clusters Nbn, n=5, 6, 7, 8, 9, 11, 12, and 13. The activation energies are lower than the corresponding ionization potentials by ∼1 eV. The ‘‘Arrhenius factors’’ are all in the range 1011 s−1. These parameters highlight the differences between delayed ionization and photoionization of clusters and draw attention to the need for an adequate theory of unimolecular processes in clusters taking into account both nonadiabatic effects and the important effects arising from the fluxionality of the cluster at high internal energies.
Compressibility equation for fluids in random microporous matrices
Authors: Ford David M. , Glandt Eduardo D.
Published: July 2002

Madden and Glandt have previously introduced a formalism for describing the structure of fluids in disordered microporous media [J. Stat. Phys. 51, 537 (1988)]. Structure in the system is described by pair correlation functions analogous to those used in equilibrium liquid-state theory. The applicability of the traditional ways of calculating thermodynamic properties from these functions has been somewhat in question. In this note, we introduce a new form of the compressibility equation for a fluid in a quenched matrix. When this approach is applied to a model hard-sphere system, the results compare very favorably with those from Monte Carlo simulations.
Electron-energy-loss spectroscopy of the low-lying triplet states of styrene
Authors: Swiderek P. , Fraser M.-J. , Michaud M. , Sanche L.
Published: July 2002

Low-energy electron-energy-loss spectra of styrene deposited on a thin film of solid argon are measured at a temperature of 15 K. The spectra show vibrationally resolved bands in the region of the lowest valence transitions thus allowing to locate the 0–0 transition to the lowest triplet state at 2.69 eV. The second triplet state of styrene is detected for the first time with a 0–0 transition at 3.98 eV. Semiempirical calculations are performed to characterize the bands observed in the spectrum considering the nomenclature of Platt. They suggest that the lowest triplet state has the same spacial wave function as the second singlet state and is closely related to 3La benzene. The second triplet state which has most likely Ba character cannot directly be related to a specific singlet state because the Ba and Bb states are found to mix strongly in the singlet manifold whereas among the triplets they do not.
Analytic energy gradients for the two-determinant coupled cluster method with application to singlet excited states of butadiene and ozone
Authors: Szalay Péter G. , Bartlett Rodney J.
Published: July 2002

Recently, the coupled cluster method has been generalized to a two-determinant reference (TD-CC method) applicable to open-shell singlet states. In this paper, we report the formulas and an efficient algorithm to calculate analytic energy derivatives for this new method. The implementation has been carried out in the ACES II many-body ab initio program system. The new method was first applied to the 1Bu state of butadiene and several excited states of ozone. We report geometries and one-electron properties for those states.
Quasirelativistic ab initio model potential calculations on the group IV hydrides (XH2, XH4; X=Si,Ge,Sn,Pb) and oxides (XO; X=Ge,Sn,Pb)
Authors: Barandiarán Zoila , Seijo Luis
Published: July 2002

In this paper we present the results of a systematic comparison between the values of the bond lengths, bond angles, and vibrational frequencies of group IV dihydrides and tetrahydrides (XH2, XH4, X=Si,Ge,Sn,Pb), and monoxides (XO, X=Ge,Sn,Pb), as well as the XH4→XH2+H2 reaction energies, calculated with the spin-free (Cowan–Griffin based) quasirelativistic ab initio core model potential method (AIMP) and the all-electron Dirac–Hartree–Fock method (DHF), using basis sets of similar quality in their valence part. The deviations between the AIMP and DHF results on the absolute values of the properties and on the sizes of the relativistic effects are of the order expected for an effective core potential method and follow the expected tendency of importance of the spin–orbit effects. The quality of the AIMP results is shown to be consistent going down the group IV of the Periodic Table.
Vapor–liquid and liquid–liquid phase equilibria of mixtures containing square-well molecules by Gibbs ensemble Monte Carlo simulation
Authors: Green Daron G. , Jackson George , de Miguel Enrique , Rull Luis F.
Published: July 2002

Gibbs ensemble Monte Carlo simulations are undertaken in order to determine the vapor–liquid and liquid–liquid phase equilibria for mixtures containing square-well particles. Two types of binary systems are examined, namely, a mixture of hard spheres and square wells, and a symmetrical mixture of square wells in which the unlike interaction is purely repulsive, i.e., hard sphere like. The latter system exhibits liquid–liquid immiscibility as well as the usual vapor–liquid coexistence. Intermolecular potential ranges which are intermediate (λ=1.5) and moderately long (λ=2) are examined in order to determine the effect of range on the phase behavior. The coexistence data are also analyzed using a Wegner expansion; the differences in densities and compositions of the two coexisting phases can both be used as the order parameter. This approach enables a description of the phase equilibria over the entire fluid range. In the case of the vapor–liquid coexistence exhibited by the mixture of hard spheres and square-wells, the leading amplitude term and a universal critical exponent are sufficient to describe the coexistence curve. The same was found for pure square-well systems in an earlier paper. However, extended scaling has to be used in order to describe the liquid–liquid coexistence curves exhibited by the symmetrical square-well mixture; the first and second Wegner correction to scaling terms are used together with a universal value of the critical exponent. In contrast to what is found for the pure component systems, an increase in the range of the potential has little effect on the shapes of the coexistence curves of the mixtures, although the phase equilibria are, of course, shifted to higher temperatures. Other authors have reported a substantial system size dependence for the liquid–liquid phase equilibria of the symmetrical square-well mixture. We do not find a significant size dependence for this system, and feel that the lack of density fluctuations in the latter study are the principal cause of the effect.
Symmetry-adapted perturbation theory calculation of the He–HF intermolecular potential energy surface
Authors: Moszynski Robert , Wormer Paul E. S. , Jeziorski Bogumil , van der Avoird Ad
Published: July 2002

Symmetry-adapted perturbation theory has been applied to compute the HeHF intermolecular potential energy surface for three internuclear distances in the HF subunit. The interaction energy is found to be dominated by the first-order exchange contribution and by the dispersion energy (including the intramonomer correlation effects). However, smaller corrections as the electrostatics, induction, and second-order exchange are found to be nonnegligible, and the final shape of the potential results from a delicate balance of attractive and repulsive contributions due to the four fundamental intermolecular interactions: electrostatics, exchange, induction, and dispersion. For a broad range of He–HF configurations the theoretical potential agrees very well with the empirical potential of Lovejoy and Nesbitt [C. M. Lovejoy and D. J. Nesbitt, J. Chem. Phys. 93, 5387 (1990)], which was adjusted to reproduce the near-infrared spectrum of the complex. Our potential has a global minimum of εm=−39.68 cm−1 for the linear He–HF geometry at Rm=6.16 bohr, and a secondary minimum of εm=−36.13 cm−1 for the linear He–FH geometry at Rm=5.59 bohr. These values are in very good agreement with the corresponding empirical results: εm=−39.20 cm−1 and Rm=6.17 bohr for the global minimum, and εm=−35.12 cm−1 and Rm=5.67 bohr for the secondary minimum.
Structure of water in the liquid and supercritical states by rapid x-ray diffractometry using an imaging plate detector
Authors: Yamanaka Koji , Yamaguchi Toshio , Wakita Hisanobu
Published: July 2002

The structure of water in the liquid and supercritical states has been investigated with a newly developed rapid liquid and amorphous x-ray diffractometer using an imaging plate area detector. This new method has enabled us to reduce the measuring time to only one hour for each sample, which is less by a factor of about 100 than the time usually needed with a conventional θ–θ type diffractometer, and thus to measure x-ray scatterings of water at high temperatures and pressures, including supercritical state. In this study the temperature range of 300–649 K with pressures of 0.1–98.1 MPa was covered (Tc=647.3 K, Pc=22.12 MPa, ρc=0.322 g/cm3 for water). Densities of sample water were kept constant at 1.0, 0.95, 0.9, 0.8, and 0.7 g/cm3 by controlling temperature and pressure. The radial distribution functions (RDFs) have shown that the peaks for the second and further neighbors interactions disappear over 416 K and 0.95 g/cm3, showing the breakdown of local tetrahedral icelike structure in water. The analysis of the first peak of the RDFs has revealed that with increasing temperature the coordination number of the first neighbor interaction around 2.9 Å decreases from 3.1 at 300 K and 1.0 g/cm3 to 1.6 at 649 K and 0.7 g/cm3, whereas the interaction around 3.4 Å increases from 1.3 to 2.3 at the corresponding temperatures, resulting in a constant coordination number of around four in the first shell under the nearly constant densities. These findings are discussed with the recent results of computer simulation, neutron scattering, and Raman spectroscopic studies on water at high temperatures and pressures.
The H2–Ne interaction
Authors: Faubel M. , Gianturco F. A. , Ragnetti F. , Rusin L. Y. , Sondermann F. , Tappe U. , Toennies J. P.
Published: July 2002

New measurements of the elastic and rotationally inelastic differential cross sections for the Ne–D2, Ne–H2 system are compared with exact and approximate quantum calculations. The three most recent high quality, semiempirical interaction potentials available in the literature for the Ne–H2 system yield consistent theoretical scattering cross sections for Ne–H2 and for Ne–D2. They also agree with previous and with present inelastic cross section measurements for D2. However, the theory underestimates by 30% the newly measured rotational excitation in Ne–H2 collisions discussed here. We therefore propose a new potential with a modified repulsive barrier that succeeds in describing both Ne–D2 and Ne–H2 rotationally inelastic scattering experiments for j=0→j′=2 within an accuracy of a few percent.
The anharmonic force field and equilibrium molecular structure of ketene
Authors: East Allan L. L. , Allen Wesley D. , Klippenstein Stephen J.
Published: July 2002

A comprehensive anharmonic vibrational analysis of isotopic ketenes has been performed on the basis of a complete ab initio quartic force field constructed by means of second-order Mo/ller–Plesset perturbation theory (MP2) and the coupled-cluster singles and doubles (CCSD) approach, augmented for structural optimizations by a contribution for connected triple excitations [CCSD(T)]. The atomic-orbital basis sets of the study entailed C,O(10s6p/5s4p) and H(6s/4s) spaces multiply polarized in the valence region to give QZ(2d,2p) and QZ(2d1f,2p1d) sets. An iterative anharmonic vibrational refinement of a limited set of quadratic scaling parameters on 27 fundamentals of H2CCO, HDCCO, D2CCO, and H2C13CO generates a final quartic force field which reproduces the empirical νi data with an average absolute error of only 1.1 cm−1. This force field yields a complete and self-consistent set of Coriolis (ζij), vibrational anharmonic (χij), vibration–rotation interaction (αi), and quartic and sextic centrifugal distortion constants, providing a critical assessment of the assorted spectroscopic constants determined over many years and also facilitating future computations of vibrational state densities for detailed tests of unimolecular dissociation theories.

The harmonic frequencies ascertained for H2CCO (in cm−1), with associated anharmonicities in parentheses, are ω1(a1)=3202.2(−129.2), ω2(a1)=2197.2(−44.4), ω3(a1)=1415.2(−25.9), ω4(a1)=1146.0(−29.7), ω5(b1)=581.9(+7.1), ω6(b1)=502.6(+26.3), ω7(b2)=3308.2(−141.3), ω8(b2)=996.0(−17.9), and ω9(b2)=433.6(+5.0). The large positive anharmonicity for the ν6(b1) C=C=O bending mode, which is principally a Coriolis effect, warrants continued investigation. Explicit first-order treatments of the strong Fermi interactions within the (ν4,2ν5,ν5+ν6,2ν6) manifold reveal resonance shifts for ν4(H2CCO, HDCCO, D2CCO) of (−12.1, −10.0, +12.2) cm−1, in order. The experimental assignments for this Fermi tetrad are confirmed to be problematic. From high-precision empirical rotational constants of six isotopomers and the theoretical anharmonic force field, the equilibrium structure of ketene is derived: re(C=O)=1.160 30(29) Å, re(C=C)=1.312 12(30) Å, re(C–H)=1.075 76(7) Å, and θe(H–C–H)=121.781(12)°. A natural bond orbital (NBO) analysis shows that the unusually large methylene angle is attributable to extensive in-plane π delocalization.
Reactive scattering of O2 with the Si(111) surface: Resonance enhanced multiphoton ionization of SiO
Authors: Nakamura Kazutaka G. , Kitajima Masahiro
Published: July 2002

Reactive scattering of a pulse O2 beam with a Si(111) surface at the substrate temperature of 1250±50 K has been studied with the resonance enhanced multiphoton ionization (REMPI) mass spectroscopy. Only the SiO molecule has been observed as a reaction product. REMPI spectra of the desorbing SiO molecule shows its cooler rotational temperature compared with its vibrational temperature and the surface temperature.
Classical phase space theory for product state distributions with application to the v–j vector correlation
Authors: Klippenstein Stephen J. , Cline Joseph I.
Published: July 2002

A classical phase space theory procedure for estimating product state distributions for ‘‘barrierless’’ dissociations is described. The distributions are determined in terms of an average over the available phase space on a fixed dividing surface of the function describing the distribution of interest. The use of a weighting function corresponding to the contribution of each randomly sampled phase space point to the number of available states, as in related state counting algorithms, provides an efficient route to the determination of any particular product state distribution or correlation. The coordinates employed in this sampling are the Euler angles, describing the orientation in space of each of the fragments and of the line of centers connecting the two fragments, and their conjugate momenta. Sample applications focus on the determination of the vector correlation between the angular momentum of one fragment and the relative velocity of the fragments for the dissociations of 2-chloro-2-nitrosopropane (CNP), NCCN, and CF3NO. Comparisons with the corresponding experimental measurements for the CNP and NCCN dissociations provide further evidence for the presence of important dynamical effects in these dissociations. The calculations for CF3NO are presented with the hope of motivating an experimental verification of the predicted strong v–j correlation for the CF3 fragment in combination with the weak correlation for the NO fragment. Overall, these phase space theory results are found to provide a useful reference for the interpretation of experimental results.
On finite-size effects in computer simulations using the Ewald potential
Authors: Figueirido Francisco , Del Buono Gabriela S. , Levy Ronald M.
Published: July 2002

We discuss the origin and relevance for computer simulations of a strong finite-size effect that appears when using the Ewald summation formula. It can be understood as arising from a volume-dependent shift of the potential in a finite, periodic box relative to the infinite volume limit. This shift is due to the fact that the ‘‘zero of energy’’ for a periodic system cannot be defined by letting the interacting particles be separated by an infinite distance; the correct definition corresponds to setting its k=0 Fourier mode to zero. The implications of this effect for computer simulations are discussed.
Rotational diffusion measurements of suspended colloidal particles using two-dimensional exchange nuclear magnetic resonance
Authors: Barrall G. A. , Schmidt-Rohr K. , Lee Y. K. , Landfester K. , Zimmermann H. , Chingas G. C. , Pines A.
Published: July 2002

We present here an experimental and theoretical study of the application of two-dimensional exchange nuclear magnetic resonance spectroscopy (NMR) to the investigation of the rotational diffusion of colloidal particles. The theoretical discussion includes the nature of the NMR frequency time-correlation function where the NMR interaction is represented by the chemical shift anisotropy (CSA). Time-correlation functions for the isotropic rotational diffusion of a suspension of colloidal particles containing single and multiple sites are derived in addition to time-correlation functions for the rotational diffusion of a suspension of symmetric top particles containing an isotropic distribution of a single CSA interaction. Simulations of two-dimensional exchange spectra for particles undergoing isotropic rotational diffusion are presented. We performed two-dimensional exchange NMR experiments on a colloidal suspension of spherical poly(methyl methacrylate) (PMMA) particles which were synthesized with a 20% enrichment in 13C at the carbonyl site. Rotational diffusion time-correlation functions determined from the experimental exchange spectra are consistent with the composition of the colloidal suspension. Detailed explanations of the syntheses of the enriched methyl 13C-(carbonyl)-methacrylate monomer and the small quantities of 20% enriched 13C-(carbonyl)-poly(methyl methacrylate) microspheres used for this study are presented.
A theory for adiabatic electron transfer processes across the semiconductor/electrolyte interface
Authors: Boroda Yuri G. , Voth Gregory A.
Published: July 2002

A theory for adiabatic electron transfer between a semiconductor electrode and an electron acceptor species in a polar electrolyte is developed by extending the Anderson–Newns model as adapted for metal/electrolyte interfaces. It is shown that perturbations of the electrode electronic spectrum can lead to qualitative changes in Marcus free energy curves and thus in the electron transfer kinetics. In particular, the adiabatic solvent free energy curve may exhibit a triple well structure even when the linear response model is used to describe the solvent polarization fluctuations.
Equilibrium constants for noble-gas fullerene compounds
Authors: Jiménez-Vázquez H. A. , Cross R. J.
Published: July 2002

We have calculated equilibrium constants for the reaction of noble gas atoms becoming trapped inside C60 molecules. Two models of the potential were used: a Lennard-Jones (12,6) potential between the noble gas atom and each carbon atom and the MM3 molecular mechanics program. We assume that the vibration of the noble gas atom is uncoupled to the cage vibrations and calculate the corresponding partition function classically. The results for the two potentials agree for helium and neon but disagree strongly for krypton and xenon, with the MM3 results being much smaller than the Lennard-Jones results.
Spatial extent of the singlet and triplet excitons in transition metal-containing poly-ynes
Authors: Beljonne D. , Wittmann H. F. , Köhler A. , Graham S. , Younus M. , Lewis J. , Raithby P. R. , Khan M. S. , Friend R. H. , Brédas J. L.
Published: July 2002

We present a joint experimental and theoretical investigation of the electronic excitations in transition metal-containing phenylene ethynylenes. The influence of the metal on the nature of the lowest singlet and triplet excited states is characterized. We find that π conjugation occurs through the metal sites, which deeply modifies the optical properties of the conjugated chains. We also analyze the chain-length dependence of the singlet–singlet, S0→S1, singlet–triplet, S0→T1, and triplet–triplet, T1→Tn, transition energies; both experimental data and theoretical results indicate that the lowest triplet exciton, T1, is strongly localized on a single phenylene ring while the S1 and Tn states extend over a few repeating units. Finally, we estimate the geometric relaxation phenomena occurring in the lowest excited states and perform a Huang–Rhys analysis of the triplet emission spectrum in model systems.
Photodissociation of binary metal metallocarbohedrenes
Authors: May B. D. , Kooi S. E. , Toleno B. J. , Castleman A. W.
Published: July 2002

The 532 nm photodissociation of the three singly substituted, binary metal containing Metallocarbohedrenes, Ti7MC12+ (M=Y, Zr, or Nb), is investigated. The photofragments are identified through energy analysis of the fragment ions in a reflectron time-of-flight mass spectrometer, a new technique in photodissociation studies that results in a more accurate identification of the photofragment mass than what would normally be achievable through mass analysis using time-of-flight determinations. The findings reveal that the dominant mechanism proceeds through the loss of neutral Ti atoms for all clusters studied.
Unimolecular dissociation of trivalent metal cluster ions: The size evolution of metallic bonding
Authors: Cottancin E. , Pellarin M. , Lermé J. , Baguenard B. , Palpant B. , Vialle J. L. , Broyer M.
Published: July 2002

The unimolecular decomposition of size selected cluster cations of trivalent metals (Aln+, Gan+, and Inn+), induced by high fluence laser ionization, has been investigated in the n=7 to n=85, 55, and 75 size ranges, respectively. This method is applied for the first time to photoexcited trivalent clusters generated in an evaporative ensemble and the experimental data cover a size range that was not explored in previous pioneering experiments on their dynamics. Small clusters dissociate through the loss of a neutral or a charged atom whereas clusters larger than a well defined critical size merely dissociate through the first channel. In the framework of the RRK statistical theory, the measured evaporation rates provide some information about the size evolution of the cluster dissociation energies and their ionization potentials in the low size range. The competition between the ion and the atom evaporation is found to be consistent with the size evolution of the ionization potentials independently measured by direct photoionization. The agreement between theory and experiment is discussed in relation to cluster structure, especially in the case of gallium.
Geometric and electronic structures of silicon–sodium binary clusters. I. Ionization energy of SinNam
Authors: Kishi Reiko , Iwata Suehiro , Nakajima Atsushi , Kaya Koji
Published: July 2002

Sodium doped silicon clusters (SinNam; 1⩽n⩽14, 1⩽m⩽5) produced by two types of laser vaporization were studied. The adsorption of Na atoms on the Sin clusters leads the substantial lowering of the ionization energy, Ei, of SinNam clusters. Their reactivity toward NO molecules was measured with a fast flow reactor, and the anticorrelation between the Eis and the reactivity was clearly observed; species having low Ei exhibit high reactivity and vice versa. Moreover, the clear parallelism between the Eis of SinNa and the EAs of Sin is found. This is consistent with the fact that the structure of SinNa clusters keeps the frame of the corresponding Sin cluster unchanged and that the electronic structure of SinNa is similar to that of the corresponding negative ion Sin−. In addition to the experimental studies, the geometries, adsorption energies, and vertical ionization energies of SinNa (n=1–7) were investigated with ab initio MO calculations including electron correlation; The Mo/ller–Plesset perturbation theory was used and the configuration interaction (CI) calculation was carried out, particularly for a diatomic molecule, SiNa.
Behavior of tethered polyelectrolytes in poor solvents
Authors: Zhulina Ekaterina , Singh Chandralekha , Balazs Anna C.
Published: July 2002

Using scaling theory and a two-dimensional self-consistent field calculation (SCF), we determine the morphology and phase behavior of charged polymers tethered by one end onto a flat substrate, and immersed in a poor solvent. For degrees of ionization, α, below a critical value, α*, the effects of the poor solvent dominate and the tethered chains collapse into pinned micelles on the planar surface. For values of α>α*, the chains extend to form a laterally uniform layer. This collapsed-to-stretched transformation constitutes a first-order phase transition. The specific routes to this transition are dependent on the relative solvent quality, and are described via the scaling theory. The structure of the layers above and below α* can be visualized through the polymer density profiles, which are obtained from the SCF calculations. The SCF calculations also show agreement with predictions from the scaling analysis.
Temperature and pressure dependence of the addition reactions of HO to NO and to NO2. IV. Saturated laser-induced fluorescence measurements up to 1400 bar
Authors: Fulle D. , Hamann H. F. , Hippler H. , Troe J.
Published: July 2002

The recombination reactions HO+NO+M⇒HONO+M(1) and HO+NO2+M⇒HNO3+M(2) have been investigated over an extended pressure (1–1000 bar) and temperature (250–400 K) range. HO radicals were generated by laser flash photolysis of suitable precursors and their decays were monitored by saturated laser-induced fluorescence (SLIF) under pseudo-first-order conditions. The measured rate constants were analyzed by constructing falloff curves which provide the high pressure limiting rate constants k∞. In the given temperature range, these rate constants are k1,∞=(3.3±0.5)×10−11×(T/300 K)−(0.3±0.3) and k2,∞=(7.5±2.2)×10−11 cm3 molecule−1 s−1.
Fourier transform millimeter-wave spectroscopy of the HCS radical in the A′2 ground electronic state
Authors: Habara Hideta , Yamamoto Satoshi , Ochsenfeld Christian , Head-Gordon Martin , Kaiser Ralf I. , Lee Yuan T.
Published: July 2002

The 101–000 rotational transition of the HCS radical in the X̃ A′2 ground electronic state has been observed with a Fourier transform millimeter-wave spectrometer in combination with a pulsed discharge nozzle. The radical is produced by discharging a mixture of CH4 and H2S diluted in Ar. Six fine and hyperfine components are detected, and the effective rotational constant, spin–rotation interaction constant, and hyperfine interaction constants are determined accurately. The Fermi contact term of the hydrogen nucleus is found to be much smaller than that for the isovalent radical, HCO, indicating that the HCS radical is more close to a linear structure than the HCO radical.
Delayed ionization in transition metal–carbon clusters: Further evidence for the role of thermionic emission
Authors: Kooi S. E. , Castleman A. W.
Published: July 2002

The delayed ionization behavior of several single and binary transition metal–carbon cluster systems, in which metallocarbohedrenes (Met-Cars) are produced, is reported. The single metal Met-Cars, M8C12, where M=Ti, Nb, or Zr, and the binary metal Met-Cars, TixMyC12, where M=Zr, Nb, or Ta and x+y=8, are investigated at various selected photoionization wavelengths (266, 355, 532, 590, and 620 nm) and over a range of fluences (20 to 120 mJ/cm2). In addition, a variable temperature experiment is performed in order to estimate the initial internal energy of the clusters after expansion from a laser vaporization source. Delayed ionization, which is observed at all wavelengths studied, is well described by a statistical model based on an adaptation of the Richardson–Dushman relation. The new findings reported herein provide evidence that all of phase space is sampled, supporting the proposal that the observed delayed ionization is thermionic in character. The temporal dependence of the ionization is found to correlate with the ionization potentials of two systems whose values are available. The ionization potential trends of the mixed metal Met-Cars can by predicted using the thermionic emission model.
Electric and magnetic properties of fullerenes
Authors: Jonsson Dan , Norman Patrick , Ruud Kenneth , Ågren Hans , Helgaker Trygve
Published: July 2002

We present fully analytical ab initio calculations of the electric polarizability, the second hyperpolarizability, and the magnetizability of the fullerenes C70 and C84 at the self-consistent field level of theory using large basis sets and—in the case of the magnetizability—London atomic orbitals in order to obtain gauge-origin independent results. These calculations are the first ab initio studies of such properties for C70 and C84, and all results are expected to be of near Hartree–Fock limit quality. By comparison with similar results reported earlier for C60, valuable insight into the electronic structure of the fullerenes is obtained.
Efficient memory equation algorithm for reduced dynamics in spin-boson models
Authors: Golosov Andrei A. , Friesner Richard A. , Pechukas Philip
Published: July 2002

The dynamics of a one-dimensional quantum system coupled to a harmonic bath can be expressed through Feynman’s path integral expression for the reduced density matrix. In this expression the influence of the environment is seen in correlations between positions of the system that are nonlocal in time. Makri and Makarov [J. Chem. Phys. 102, 4600 (1995)] showed that for many practical problems correlations over only a few time steps, Δkmax, need to be taken into account, which led to an efficient iterative scheme. However, this algorithm scales as the size of the system to the power of 2(Δkmax+1), which restricts the size of the system that can be studied with this method. In this work we present an efficient algorithm which scales linearly with Δkmax. In our method the reduced density matrix is written as a convolution of its past values with an integral equation kernel. The calculation of that kernel is based on a perturbative expansion of the discretized quasiadiabatic path integral expression for the reduced density matrix. The expansion ignores certain types of correlations.
Phase transition and decomposition of 90% hydrogen peroxide at high pressures
Authors: Cynn Hyunchae , Yoo Choong-Shik , Sheffield Stephen A.
Published: July 2002

Physical and chemical changes of 90 wt% hydrogen peroxide have been investigated to pressures of 12 GPa by using a diamond-anvil cell, synchrotron x-ray diffraction, and Raman spectroscopy. Hydrogen peroxide freezes at 1.5 GPa and ambient temperature into a tetragonal structure (P41212, Z=4, denoted as H2O2-I. This is the same transition that has previously been reported in this material at 253 K. The unit cell parameters at 6.3 GPa are a=3.759 Å, c=7.397 Å, and V=15.74 cm3/mol, representing 21% compression from that at ambient pressure. H2O2-I has been found to transform into a high pressure phase H2O2-II at 7.5 GPa, and it decomposes into water and oxygen at the onset of melting, which may be incongruent. In contrast to water, hydrogen peroxide exhibits a relatively simple polymorphism and a positive initial slope of the melting curve at high pressures.