bluuebunny/crossref_metadata_2025 huggingface dataset:
Dependence of metal cluster reaction kinetics on charge state. I. Reaction of neutral (Nbx) and ionic (Nb+x, Nb−x) niobium clusters with D2
Authors: Zakin M. R. , Brickman R. O. , Cox D. M. , Kaldor A.
Published: July 2002

The effect of charge state on niobium cluster chemisorption kinetics is explored via measurement of the relative rates of D2 activation by Nb−x, Nbx, and Nb+x containing up to 28 atoms. The presence of the + or − charge is found to have only a minor effect on rate for the majority of the clusters, with the reactivity of the ions being generally within a factor of 2.5 of the corresponding neutrals. The excess charge does, however, have a profund influence on reaction rate for a number of clusters in the 7≤x≤16 size range, which may be indicative of the importance of cluster electronic structure in the chemisorption process. Kinetic data for Nb9, Nb12, and Nb+12 are found to deviate significantly from the expected pseudo-first-order behavior, suggesting the existence of structural isomers for these species. The anomalous behavior for Nb9 and Nb12 was not observed in previous neutral Nbx chemisorption studies. The maximum uptake of D2 by niobium clusters is found to be essentially independent of charge state, but varies strongly with x, consistent with the presence of a high barrier to D–D bond activation for certain of the clusters.
Photodissociation of size-selected K+n clusters
Authors: Bréchignac C. , Cahuzac Ph. , Roux J.-Ph.
Published: July 2002

Photofragmentation studies are described for size-selected ionized potassium-atom clusters, generated in a supersonic expansion and analyzed with a tandem time-of-flight system. In the range n=3 to 41 the relative ion intensities in photofragmentation spectra reflect the relative stabilities of the ion products. Moreover the photon-energy dependence of the fragmentation patterns suggests that photodissociation proceeds rather via a fast sequential evaporation of neutral monomers than via a fission process. A value can be deduced for the mean binding energy per atom which is increasing with the cluster size.
RHF energy band shapes for metallic chains: Dependence on the summation of exchange contributions. An illustration on the linear chain of hydrogen atoms
Authors: Delhalle J. , Delvaux M. H. , Fripiat J. G. , André J. M. , Calais J. L.
Published: July 2002

Minimal basis set (STO-3G) calculations on the metallic infinite chain of hydrogen atoms, (-H-)x, performed within the same computational framework as currently used for more realistic model systems, are reported to illustrate the dependence of the RHF energy bands on the summation of exchange contributions. The numerical results show the gradual decay of the density of states at the Fermi level as the number of terms in the exchange lattice sums is increased and rather convincingly verify the predictions of previous formal analyses.
Monte Carlo simulations of structural properties and infrared spectra of SF6–(Ar)n clusters
Authors: Eichenauer Dieter , Le Roy Robert J.
Published: July 2002

A method for predicting the perturbed infrared spectrum of a solvated chromophore such as SF6 in a mixed van der Waals cluster is developed and applied to a realistic model for SF6–(AR)n clusters with n=1–100. The dominant contribution to the band shift arises from the interaction of an induced dipole on the perturber with the oscillating instantaneous dipole of the vibrationally distorted SF6. Spectra generated by Monte Carlo averaging these frequency shifts over a canonical distribution of cluster configurations and over a plausible assumed cluster size distribution are in near-quantitative agreement with experiment. For the smaller clusters (n≲18), features of the simulations point to the presence of two distinct classes of cluster structure, one in which the chromophore is wrapped in a unimolecular layer of atoms, and a second in which the solvent atoms have multilayered structures. However, the question of whether the infrared spectra can provide evidence for the presence of the kind of phase coexistence identified in simulations for pure Ar clusters is not yet settled.
Concentration fluctuation of stiff polymers. I. Static structure factor
Authors: Shimada Takayuki , Doi Masao , Okano Koji
Published: July 2002

The static structure factor g(k) of the concentrated solution of stiff polymers is calculated based on the generalized random phase approximation (RPA) which accounts for the nematic interaction (i.e., the interaction causing the nematic phase of stiff polymers) between the segments. The structure factor is obtained in the form of power series expansion with respect to k. The result is: (i) the correlation length ξ is independent of the nematic interaction, and is given by the conventional RPA; (ii) the sign of the coefficient of k4 changes from positive to negative as the nematic interaction is increased; and (iii) the structure factor of rigid rod, which can be calculated exactly, shows no anomaly below the critical concentration c*, but suddenly shows anomaly above c*.
A dynamical theory of unimolecular ionic dissociation reactions in polar solvents
Authors: Zichi Dominic A. , Hynes James T.
Published: July 2002

A dynamical theory for the rates of unimolecular dissociations in polar solvents is constructed. Two classes of dissociation reactions, with dipolar and ionic transition states, are considered, and the theory is illustrated for a generalized continuum model water solvent. The rate of charge variation along the reaction coordinate is found to play a central role. Deviations from equilibrium solvation transition state theory predictions are found and discussed. Two nonequilibrium solvation regimes—nonadiabatic solvation and polarization caging—occur, and their appearance is connected to whether the solvated transition state has a reactant-like or product-like charge distribution.
The ionization potentials of NH2: The multiconfigurational spin-tensor electron propagator method (MCSTEP) applied to a polyatomic open-shell radical
Authors: Graham Richard L. , Golab Joseph T. , Yeager Danny L.
Published: July 2002

The multiconfigurational spin-tensor electron propagator method (MCSTEP) gives accurate ionization potentials (IPs) and electron affinities (EAs) for both closed-shell and open-shell molecules, including for highly correlated systems. Both principal and lower-lying shakeup IPs can be accurately obtained and straightforwardly characterized using MCSTEP. To further test this new technique, we have applied the MCSTEP approach to the open-shell, polyatomic radical NH2. We report and characterize vertical IPs 0–20 eV, including several ionizations that have not, as yet, been observed experimentally. IPs to both singlet and triplet states of NH+2 are accurately calculated using the same MCSCF reference state. We predict the presence of previously undetected, observable vertical PES IPs to states of 3A2, 1A1, and 1A2 symmetries at 16.86, 18.00, and 18.26 eV, respectively. Also, we calculate adiabatic IPs by a modified application of the procedure. By examining the two lowest adiabatic IPs with both MCSTEP and Δmultireference singles+doubles CI at geometries established by Pople’s research group and with the basis set used by that group augmented by hydrogen d polarization functions we conclude that their MP4 procedures overestimate the IPs with their best basis set by ∼0.2–0.3 eV.
Photodissociation of semiconductor positive cluster ions
Authors: Zhang Q.-L. , Liu Y. , Curl R. F. , Tittel F. K. , Smalley R. E.
Published: July 2002

Laser photofragmentation of Si, Ge, and GaAs positive cluster ions prepared by laser vaporization and supersonic beam expansion has been investigated using tandem time-of-flight mass spectrometry. Si clusters up to size 80, Ge clusters to size 40, and GaAs clusters up to a total of 31 atoms were studied. Si+n and Ge+n for n=12–26 give daughter ions of about half their original size. For both Si and Ge, this apparent positive ion fissioning appears to go over with increasing n to neutral loss of seven and ten, but for Si+n the range of n values where this is observed is rather small. At low fluences, the larger Ge+n clusters up to the maximum size observed (50) sequentially lose Ge10 (and in some cases with lower intensity Ge7). Larger Si+n clusters (n>30) always fragment primarily to produce positive ion clusters in the 6–11 size range with a subsidiary channel of loss of a single Si atom. At high laser fluences, Ge+n also fragments to produce primarily positive ion clusters in the 6–11 size range with an intensity pattern essentially identical to Si+n at similar fluences. GaxAs+y clusters lose one or more atoms in what is probably a sequential process with positive ion clusters in which the total number of atoms, x+y, is odd being more prominent.
The kinetics and thermodynamics of the reaction H+NH3⇄NH2+H2 by the flash photolysis–shock tube technique: Determination of the equilibrium constant, the rate constant for the back reaction, and the enthalpy of formation of the amidogen radical
Authors: Sutherland J. W. , Michael J. V.
Published: July 2002

Equilibrium constants K1 for the reaction H+NH3⇄NH2+H2 were measured over the temperature range 900 to 1600 K using a flash photolysis–shock tube apparatus. The experimental values of K1 ranged from 1.0 at 900 K to 2.2 at 1620 K with an estimated experimental error of about ±10%. The value obtained from the third law analysis for the enthalpy of formation of the amidogen radical, ΔH0f298 (NH2), is 45.3 kcal/mol (ΔH0f0 =46.0). The corresponding value for the bond dissociation energy, D0(NH2–H), is 107 kcal/mol. These values are in good agreement with the data tabulated in the revised JANAF tables (1982), with those derived from new measurements of the photoionization threshold for the amidogen radical, and with those from independent measurements of the rate constants of the forward and back reactions. The Arrhenius rate expression derived for reaction (−1), NH2+H2→NH3+H, is k−1(T)=5.38×10−11 exp(−6492/T) cm3 molecule−1 s−1 for the temperature range 900–1620 K. The estimated error in k−1(T) is about ±25%.
Analysis of fluorescence line shapes and Raman excitation profiles of tetradesmethyl-β-carotene
Authors: Sue James , Mukamel Shaul
Published: July 2002

The fluorescence lineshapes and Raman excitation profiles of tetradesmethyl-β-carotene in isopentane at 190 and 230 K are analyzed and related to the solvation dynamics and vibrational relaxation. We find a solvation time scale of 125 fs at 190 K, 111 fs at 230 K, and a vibrational relaxation time scale of 253 fs at both temperatures. The magnitude of the fluctuations of the electronic energy gap due to interactions with the solvent and with molecular low frequency modes is 423 cm−1 at 190 K and 480 cm−1 at 230 K.
Quantum Monte Carlo method with the model potential
Authors: Yoshida Takashi , Iguchi Kaoru
Published: July 2002

Quantum Monte Carlo (QMC) method, which is a stochastic procedure to solve the Schrödinger equation by diffusing random walk of particles, has been combined with the model potential (MP) to calculate efficiently the electronic states of many-electron atoms. This scheme has been applied to compute the valence energies of atoms such as Mg, Ca, Sr, and their cations, and ionization potentials of these atoms are evaluated. Results are compared with experimental values and they are found to be in excellent agreement.
Elastic helium scattering studies of ordered overlayers of Ar, Kr, and Xe physisorbed on Ag(111)
Authors: Gibson K. D. , Cerjan C. , Light J. C. , Sibener S. J.
Published: July 2002

We describe experiments that measured the angle resolved intensity of He (Ei=18 and 66 meV) elastically scattering from the surfaces of rare gas overlayers physisorbed on Ag(111). These studies were done on a layer-by-layer basis for 1, 2, 3, and ∼25 ordered overlayers of Ar, Kr, and Xe. Two types of experiments are described. The first is diffraction, where the scattered He intensity was measured as a function of the detector angle, with the incident polar and azimuthal angles held constant. In the second type of experiment, selective adsorption, we measured the specular intensity as a function of incident angle. The purpose of these experiments was to examine the He–surface potential, to assess the relative contributions that various He–rare gas pair potentials, nonadditive multibody terms, and He–substrate interactions make to the systems studied. The experiments are compared with the results of accurate close-coupling calculations, in order to quantitatively perform these assessments. The comparisons between the selective adsorption data and scattering calculations demonstrate the extreme sensitivity that such measurements have to the He–surface potential. In particular, observable changes in the calculated selective adsorption spectra appear when different He–rare gas potentials are tested, or when various nonadditive terms are included in the potential. The results suggest that further refinements in the He-heavy–rare gas pair potentials may be in order.
One-electron diatomics in momentum space. IV. Floating single-center expansion
Authors: Koga Toshikatsu , Yamamoto Yoshiaki , Matsuhashi Toshiyuki
Published: July 2002

The floating of the expansion center is incorporated into the nonvariational single-center expansion method developed previously based on the momentum-space treatment of the one-electron diatomic Schrödinger equation. An application to the H+2 ground state shows a considerable improvement for the case of a few basis functions.
Infrared diode laser kinetic spectroscopy of the CCD radical ν3 band
Authors: Kanamori Hideto , Hirota Eizi
Published: July 2002

Infrared diode laser kinetic spectroscopy was applied to the photodecomposition of the deuterated acetylene at 193 nm to observe the ν3 (C=C stretching) band of the CCD radical. The band origin which was derived from the observed spectrum is 1743.1785 (3) cm−1, in agreement with the result of a previous matrix study. As in the case of CCH, the spin–rotation interaction constant was found to change by as much as 31% upon ν3 excitation, presumably because of the interaction with the lowest excited electronic state. Many lines with lifetimes shorter than those of the ν3 band were observed, but remained to be assigned.
Infrared double resonance of SiH4 with a tunable diode laser: Two-photon absorptions and rotational relaxation times
Authors: Millot G. , Hetzler J. , Foy B. , Steinfeld J. I.
Published: July 2002

Time-resolved infrared double resonance experiments have been carried out on silane. A pulsed CO2 laser is used to pump dyad←ground state transitions, and triad←dyad transitions are probed with a tunable diode laser. Two-photon (triad←ground state) signals are observed with the CO2 10P(20), 10P(22), and 10P(28) pump lines. Rotational relaxation rates have been measured for E, F2, and A2 symmetry components of the v4=1, J=13 level of silane in collisions with silane, argon, and methane. The relaxation efficiencies follow the order σrot (F2)>σrot (A2)>σrot (E), which parallels the behavior of pressure-broadening coefficients for infrared absorption lines of methane.
Molecular dynamics calculation of free energy
Authors: Lutsko J. F. , Wolf D. , Yip S.
Published: July 2002

The results of a systematic study of a recently proposed method by Frenkel and Ladd for calculating free energies via molecular dynamics are reported. Internal measures of the error, the effect of varying parameters, and comparison of the relative computational efficiency of the method compared to other methods is considered. In particular, agreement with the quasiharmonic method is shown for temperatures up to 75% of melting.
van der Waals bond stretch and bend frequencies in the molecules tetrazine–X (X=Xe, Kr, and Ar)
Authors: Weber Peter M. , Buontempo Joseph T. , Novak Frank , Rice Stuart A.
Published: July 2002

Fluorescence excitation and dispersed fluorescence spectra of the clusters s-tetrazine–X (X=Xe, Kr, Ar) have been studied to determine the van der Waals bond stretch and bend frequencies. In the case of tetrazine–Xe a vibrational progression spanning v=0 to 8 in the bend vibrations was detected. The harmonic frequencies of the van der Waals bond stretch and bond bend vibrations are found to be very similar for the T–Xe, T–Kr, and T–Ar clusters. Spectra of the symmetric trimer xenon–tetrazine–xenon show progressions in the van der Waals bond stretch vibration up to v″=5. The frequencies of the stretch vibration in tetrazine–xenon and the symetric stretch vibration in xenon–tetrazine–xenon support the validity of pairwise additivity of the van der Waals potentials. The excitation spectra also indicate the prescence of other, more complicated, clusters of tetrazine with xenon.
Broad spectral features in the stimulated emission pumping spectrum of acetylene
Authors: Pique J. P. , Engel Y. M. , Levine R. D. , Chen Y. , Field R. W. , Kinsey J. L.
Published: July 2002

The vibrational spectrumof acetylene is reported. This is the stimulat emission pumping spectrum obtained by stimulated downward transition from an upper electronic state to the ground electronic surface.Broad band and high resolution spectra were recorded at 25840−27250 wave numbers.(AIP)
Molecular dynamics studies of complexing in binary molten salts with polarizable anions: MAX4
Authors: Saboungi Marie-Louise , Rahman A. , Halley J. W. , Blander M.
Published: July 2002

Anion polarizations have been introduced in a molecular dynamics simulation of a complexing ionic liquid MAX4 similar to tetrachloroaluminates. The influence of the polarizability on the structure of the melt has been deduced from the radial distribution functions, the numbers of nearest and next nearest neighbor pairs, the distribution of coordination numbers, and the angular distribution of various triplets. Surprisingly, A2X6 molecular species, similar to those postulated to exist in acid haloaluminate melts, were detected in our simulated ‘‘neutral’’ melt. The stability of other species (e.g., A2X−7 and A3X−10) that were present in the simulated MAX4 melt with nonpolarizable X anions decreased when polarization was introduced.
Vibrational spectrum and potential energy surface of the CH chromophore in CHD3
Authors: Lewerenz Marius , Quack Martin
Published: July 2002

The rovibrational spectrum of trideutero-methane has been measured at resolutions mostly close to the Doppler limit on an interferometric Fourier transform spectrometer from the lowest fundamental vibration to high overtones of the CH stretching vibration (wave numbers from 900 to 12 000 cm−1). The CH chromophore spectrum is fully assigned and interpreted by means of the tridiagonal Fermi resonance Hamiltonian for the coupled CH stretching and bending vibrations. The Hamiltonian predicts and also fits the visible spectrum up to 19 000 cm−1 measured by Scherer et al., Perry et al., and Campargue et al. The effective tridiagonal Hamiltonian is derived ab initio by means of MRD-CI and full CI calculations of the potential surface of methane, a variational vibrational calculation in a normal coordinate subspace of the coupled CH stretching and bending motions and an approximate similarity transformation to tridiagonal form. Fits of the experimental results by the tridiagonal and the variational Hamiltonian lead to equivalent spectroscopic constants. A careful experimental estimate of the main Fermi resonance coupling constant gives k′sbb ≂(30±15) cm−1 in agreement with the best current ab initio result (31 cm−1). The ab initio potential in polar normal coordinates agrees with the potential derived from spectroscopic data covering an energy range of about 220 kJ mol−1 (more than half the dissociation energy). Good predictions are obtained for the parameters of the effective Hamiltonian, the spectral patterns, intensity distributions, and rotational constants in the Fermi resonance polyads. Three alternative interpretations of the parameters of the effective Hamiltonian are investigated and rejected on the basis of the available experimental and ab initio data. The results and conclusions are discussed in relation to intramolecular vibrational redistribution on the subpicosecond time scale and the recombination–dissociation kinetics of methane.
State-selective studies of T→R, V energy transfer: The H+CO system
Authors: Chawla G. K. , McBane G. C. , Houston P. L. , Schatz G. C.
Published: July 2002

Collisional energy transfer from H atoms to CO(v=0, J≂2) has been studied at a collision energy of 1.58±0.07 eV by photolyzing H2S at 222 nm in a nozzle expansion with CO and probing the CO(v″, J″) levels using tunable VUV laser-induced fluorescence. The ratio CO(v″=1)/CO(v″=0) is found to be 0.1±0.008. The rotational distribution of CO(v″=0) peaks at J″≤11 and decays gradually; population is still observed at J″≥45. The rotational distribution of CO(v″=1) is broad and peaks near J″=20. The experimental results are compared to quasiclassical trajectory calculations performed both on the H+CO surface of Bowman, Bittman, and Harding (BBH) and on the surface of Murrell and Rodriguez (MR). The experimental rotational distributions, particularly those for CO(v″=1), show that the BBH surface is a better model than the MR surface. The most significant difference between the two surfaces appears to be that for energetically accessible regions of configuration space the derivative of the potential with respect to the CO distance is appreciable only in the HCO valley for the BBH surface, but is large for all H atom approaches in the MR potential. Because the H–CO geometry is bent in this valley, vibrational excitation on the BBH surface is accompanied by appreciable rotational excitation, as observed experimentally.
Spin dependent force model of molecular liquids: Theory and simple applications
Authors: Haymet A. D. J. , Kramer Marc R. , Marshall Christopher
Published: July 2002

We introduce a spin dependent force model for molecular liquids, in which the force between two atoms depends both on the interatomic separation distance and internal, quantum degrees of freedom. In this paper we introduce an additional approximation and treat the internal degrees of freedom semiclassically. We explore some simple properties of this model applied to hydrogen atoms and molecules. Using input from quantum chemistry calculations for the electronic potential energy surfaces, we calculate an effective molecule–molecule potential energy and the dynamics of two and three atom systems. We also perform molecular dynamic simulations which propagate both position and internal ‘‘spin’’ degrees of freedom.
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o study of the electronic structure and conduction properties of mislinked heterocyclic polymers
Authors: Bakhshi A. K. , Ray N. K.
Published: July 2002

Results of ab initio band structure calculations of completely mislinked (α–β linked) polypyrrole, polythiophene, and polyfuran are reported. Comparison of the important electronic properties such as ionization potential, electron affinity, and band gap of these mislinked polymers with those of the normally linked (α–α′ linked) polymer chains indicates that the introduction of mislinkages in the normally linked polymer chains makes them more insulating and less dopantphilic. Both the ionization potential and the electron affinity values of the mislinked polymer chains are found to be sensitive to both the nature of the heteroatom and substitution at the backbone. The effect of the distributions (both periodic and random) of the mislinkages on the conduction properties of normally linked heterocyclic polymers is also investigated using negative factor counting method in its tight binding approximation. A random distribution of mislinkages, in general, has been found to produce polymer chains which are less insulating and have better prospects for forming conductive materials on doping with both electron acceptors and electron donors than the corresponding periodic distribution of mislinkages. The possible effects of the distribution of mislinkages on the conduction properties of copolymers of the heterocyclic compounds are also discussed.
Two methods of computing molecular dipole and quadrupole derivatives
Authors: Lazzeretti P. , Zanasi R. , Fowler P. W.
Published: July 2002

Polarized basis sets are used to compute dipole and quadrupole derivatives of the hydrides LiH, CH4, NH3, H2O, and HF. Analytic calculation of derivatives is compared with calculation via the dipole and quadrupole electric shielding tensors. With these basis sets, violation of the Hellmann–Feynman theorem is only about 0.01 a.u. in dipole derivatives and 0.02 a.u. in quadrupole derivatives.
Constrained diffusion dynamics in the hard-square lattice gas at high density
Authors: Ertel W. , Froböse K. , Jäckle J.
Published: July 2002

The self-diffusion of particles in the two-dimensional square lattice gas with nearest-neighbor exclusion is investigated. At high concentration the diffusion is severely hindered by kinetic constraints. The resulting pattern of cooperativity is analyzed and found to be parallel to that observed in the two-spin facilitated kinetic Ising model of Fredrickson, Andersen, and Brawer. It is argued that a blocking transition does not exist in the thermodynamic limit. The argument, which is based on the calculation of the percolation probability in the ‘‘rectangular-cluster percolation problem,’’ is confirmed by Monte Carlo calculations of the mean-square displacement.
The Hg–H2 system: Potential energy surfaces of the low-lying states, reactivity of the 6s6p states of Hg on H2, classical dynamic study of Hg(3P1)+H2 half collision
Authors: Bernier A. , Millie P.
Published: July 2002

Potential energy surfaces for HgH2 have been calculated using a nonempirical relativistic effective core )potential incorporating configuration interaction by means of the CIPSI algorithm (configuration interaction by perturbation with multiconfigurational zeroth-order wave function selected by iterative process). Core-valence polarization and correlation energy are included via a perturbative treatment. Spin–orbit coupling is introduced through an effective Hamiltonian. These theoretical results are used to discuss the experimental ones for the Hg(3P1)+H2→HgH(X 2Σ+)+H reaction. The different behaviors of the two van der Waals complexes are explained. Dynamical studies and classical trajectories calculations confirm that the dominant pathway is a direct dissociation on the first excited surface and is responsible for a highly peaked rotational distribution of the HgH product, in agreement with experiment.
Cluster expansion of the wave function. Valence excitations and ionizations of pyridine
Authors: Kitao O. , Nakatsuji H.
Published: July 2002

The SAC (symmetry-adapted cluster) and SAC-CI theories are applied to the calculations of the valence excitations and the ionizations of pyridine. The active space consists of 85 MOs (35π and 50 σ MOs). For the valence π→π* and n→π* excited states, S1(1 1B2), S2(1 1A1), S3(2 1B2), Sn(1 1B1), T1(1 3A1), and Tn(1 3B1), the calculated excitation energies are higher than the experimental values by within ∼0.7 eV. Although the S4(2 1A1) state is experimentally considered not to be split from S3, the present calculation predicted it to be higher than S3 by 0.31 eV. The average discrepancy from the experimental data is 0.55 eV without including S4. The transition energies and some other properties are predicted for the valence π→π* excited states, T2(1 3B2), T3(2 3A1), T4(2 3B2) and for the n→π* excited states, S′n(1 1A2) and Tn(1 3A2), for which the experimental values are unavailable so far. The following two observations seem to be interesting. (i) The valence π→π* excited states of pyridine are understood from those of benzene, reported previously, as a result of perturbation, an alteration of one C–H group to an isoelectronic nitrogen atom. The typical V states, S3 and S4, show large σ-reorganization effect, which is detected as the change in the transition energy and some one electron properties. (ii) For the relative positions between the π→π* and n→π* excitations, our results are as follows. The lowest singlet excited state is n→π*[Sn(1 1B2)]. The lowest triplet state is

π→π*[T1(1 3A1)] and n→π*[Tn(1 3B1)] exists higher than T1 by 0.15 eV. Experimentally, the lowest singlet excitation is confirmed to be n→π*, but two controversial assignments exist for the lowest triplet state: π→π* or n→π*. For the ionization potential, the experimental spectra is reproduced to within ∼0.6 eV. The nature of the first three peaks are calculated as nππ, which is supported by some recent experimental studies. The similarity and difference in the satellite peaks of benzene and pyridine are pointed out.
The negative ion states of sulfur hexafluoride
Authors: Chen Edward C. M. , Shuie Lih-Ren , Desai D’sa Ela , Batten C. F. , Wentworth W. E.
Published: July 2002

The reaction of SF6 with thermal electrons has been studied in a Ni-63 atmospheric pressure ionization source for a quadrupole mass spectrometer (API/MS). The major ions that are observed are the parent negative ion (SF−6) and the parent minus a fluorine atom (SF−5). The ratio of [SF−5]/[SF−6] is highly temperature dependent above 500 K. The dissociation energy of the ground state negative ion into SF−5 and F has been determined to be 1.35±0.1 eV. This gives values of 3.8±0.15 eV for the electron affinity of SF5 and 1.15±0.15 eV for the electron affinity of SF6. The negative ion states of sulfur hexafluoride have been described by ‘‘pseudo-two-dimensional’’ Morse potentials calculated using experimental data.
Dynamical and nondynamical correlation effects in a
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o chemisorption cluster model calculations. Ground and low lying states of H on Cu(100) and Ag(100)
Authors: Illas F. , Rubio J. , Ricart J. M.
Published: July 2002

The nondynamical correlation effects for the interaction between an adsorbate and a surface cluster model can be recovered by means of a simple and accurate method. A projection localization procedure permits to distinguish between cluster and adsorbate MOs and between lone pairs or bonds. Valence virtual MOs are obtained by an inexpensive method. Once the valence space is defined CASCI calculations account for these correlation effects, the results being in good agreement with CASSCF calculations. The dynamical correlation effects are taken into account by using a perturbative approach in which the CAS wave function is taken as the zeroth order one. Results, thus obtained, are also in agreement with accurate CIPSI calculations. The overall procedure is illustrated by means of calculations on the ground and low lying electronic states of the on top chemisorption of atomic hydrogen on the Cu5 and Ag5 cluster models simulating the (100) metal surface. The main effects of nondynamical correlation in the electronic ground state are a slight increase on the internuclear equilibrium distance of the adsorbate to the surface model and a considerable contribution to the binding energy (≊20% of the total binding energy). The dynamical effects tend to cancel the effect on the equilibrium distance and left almost unchanged the binding energy. Since these effects are quite small valence CASCI can lead to an accurate enough description of the chemisorption bonds. These general trends also hold for the excited states studied here.
The electronic spectrum of the GeH3 radical
Authors: Johnson Russell D. , Tsai Bilin P. , Hudgens Jeffrey W.
Published: July 2002

The germyl (GeH3) radical has been observed by resonance enhanced multiphoton ionization (REMPI) spectroscopy in the region of 370–430 nm. The spectrum arises from two-photon resonances with the 5p 2A″2 (D3h) Rydberg state that possesses an origin at 419.1 nm (ν0–0 =47 705 cm−1). A vibrational progression of 756 cm−1 was assigned to the ‘‘umbrella’’ mode, ν2. The observed X̃ 2A1 (C3v) v″2=2 to v″2=0 vibrational interval is 663 cm−1 which leads to an estimated barrier to inversion of 1530 cm−1.
A quantum mechanical treatment of vibrational mixing in ethylene dimer and rare gas–ethylene complexes
Authors: Hair Sally R. , Beswick J. A. , Janda Kenneth C.
Published: July 2002

An approximate description of the van der Waals vibrations of ethylene dimer and rare gas–ethylene complexes is presented. Using a model atom–atom potential, this calculation examines the mixing of the ethylene ν7 vibration with a set of background vibrational levels, consisting of combinations of the van der Waals vibrations and the ethylene ν10 vibration. The ethylene dimer exhibits extensive vibrational mixing, while the rare gas–ethylene molecules do not. For the ethylene dimer, calculated line strengths produce a complex spectrum of vibrational lines that span a 10 cm−1 region, in agreement with the experimental spectrum. This result suggests new explanations for the coexistence of broad and narrow lines in the ethylene dimer ν7 dissociation spectrum.
Electron impact ionization cross sections of SiF2
Authors: Shul Randy J. , Hayes Todd R. , Wetzel Robert C. , Baiocchi Frank A. , Freund Robert S.
Published: July 2002

Absolute cross sections are measured for electron impact ionization and dissociative ionization of SiF2 from threshold to 200 eV. A fast (3 keV) neutral beam of SiF2 is formed by charge transfer neutralization of SiF+2 with Xe; it is primarily in the ground electronic state with about 10% in the metastable first excited electronic state (ã 3B1). The absolute cross section for ionization of the ground state by 70 eV electrons to the parent SiF+2 is 1.38±0.18 Å2. Formation of SiF+ is the major process with a cross section at 70 eV of 2.32±0.30 Å2. The cross section at 70 eV for formation of the Si fragment ion is 0.48±0.08 Å2. Ion pair production contributes a significant fraction of the positively charged fragment ions.
Reliable diffusion quantum Monte Carlo
Authors: DePasquale Michael F. , Rothstein Stuart M. , Vrbik Jan
Published: July 2002

By introducing perturbations of O(τ) type (where τ is the time step used in the simulation) to our diffusion quantum Monte Carlo algorithm we obtain a simulated energy which is reasonably constant over a wide range of τ values. Reliable estimation of the fixed-node energy (τ=0 intercept) results, as extrapolation becomes more robust and a radical change in small τ behavior becomes less likely. We apply our techniques to the problem of estimating the ground-state energy of LiH and H2O.
Quasibound levels and shape resonances of 39K2(B 1Πu) crossed laser-molecular beam studies and analytical interpretation
Authors: Heinze J. , Kowalczyk P. , Engelke F.
Published: July 2002

Quasibound levels and shape resonances in the (B 1Πu −X 1Σ+g) band system of 39K2 have been recorded by crossed laser-molecular beam techniques. Using optical–optical double resonance, individual rovibrational levels (v″=15–18, J″=3–25) of the K2 state are prepared by Franck–Condon pumping (FCP) in a supersonic nozzle beam. Excitation into quasibound levels below and above the (B 1Πu) state barrier is detected as molecular and atomic (K4 2P3/2→4 2S1/2 only) fluorescence. The resonance transition frequencies and shapes are measured and the results are used (a) to determine the scattering resonance energies, widths, and lifetimes; (b) to compare them with values obtained by a ‘‘maximum internal amplitude’’ approach [R. J. LeRoy and R. B. Bernstein, J. Chem. Phys. 54, 5114 (1971)]; and (c) to check the agreement with exact calculations of the B state potential using the ‘‘inverted perturbation approach (IPA).’’ The bound and quasibound part of the B 1Πu state including the locus (R=8.08±0.05 Å) of the barrier maximum (298±8 cm−1 above the adiabatic dissociation limit) is found in excellent agreement with previous results. The shape resonances are not highly sensitive to the long-range interatomic forces, here the repulsive dipole–dipole resonance interaction.
Quenching of C2H emission produced by vacuum ultraviolet photolysis of acetylene
Authors: Sander R. K. , Tiee J. J. , Quick C. R. , Romero R. J. , Estler R.
Published: July 2002

Excited C2 H* is produced by vacuum ultraviolet photolysis of acetylene using a frequency tripled laser. Time-dependent emission is measured from 400–940 nm. The use of a coherent photolysis source produces an excitation spectrum in which the rotational band contour is resolved. The absorption spectrum of C2 H2 taken in the same apparatus closely resembles the excitation spectrum indicating a homogeneous predissociation. Time-dependent quenching of the C2 H* emission by Xe, Kr, Ar, He, N2, H2, D2, and C2 H2 is measured. The rapid quenching rates and lack of strong dependence on atomic weight suggest a spin-allowed process is involved in this channel of C2 H2 photolysis. Quench rates are compared with several theoretical models.
The anharmonic potential function of methylene fluoride. SCF a
b i
n
i
t
i
o computations of the cubic force field and analysis of vibration–rotation interaction constants
Authors: Gaw Jeffrey F. , Handy Nicholas C. , Palmieri Paolo , Esposti Alessandra Degli
Published: July 2002

The harmonic and the cubic force fields of CH2F2 have been evaluated ab initio from the SCF energy expression by analytic derivative methods. The computed cubic force constants were used as starting values in a least squares analysis of the experimental vibration–rotation constants of CH2F2 and CD2F2. A simple scaling procedure of the ab initio cubic force constants provides a complete cubic force field for the molecule and the best fit with the experimental data.
Third order a
b i
n
i
t
i
o calculations of the f↔f transition amplitudes for ions across the lanthanide series
Authors: Smentek-Mielczarek Lidia , Andes Hess B.
Published: July 2002

The behavior of various contributions to the f↔f transition amplitude defined up to the third order in perturbation theory is analyzed. Main attention is paid to the third-order contributions which are due to electron correlation inside the rare earth ion and arise from the static and dynamic models. The discussion is based on the numerical results of exact ab initio calculations performed for Pr+3, Nd+3, Eu+3, Gd+3, Tb+3, and Tm+3 ions.
The infrared spectrum of D2HF
Authors: Lovejoy Christopher M. , Nelson David D. , Nesbitt David J.
Published: July 2002

Ultrasensitive infrared laser absorption spectroscopy in a slit supersonic expansion is used to obtain the spectrum of the HF stretching fundamental of D2HF. Both a Π←Π band due to para-D2HF and a ∑←∑ band due to ortho-D2HF are observed, in contrast to the H2HF spectrum which consists of the Π←Π band alone. Analysis of the spectrum indicates that the D2HF Π states are more strongly bound than the ∑ states. Doublet splittings in the Π←Π band are analyzed to determine barriers to internal rotation of D2 within the complex. The vibrationa1 predissociation rate of D2HF is approximately 25 times faster than that of H2HF, suggesting the opening of a channel which results in vibrational excitation of the D2 fragment.
Energy disposal in the photofragmentation of W(CO)6: Experimental observations and physical models
Authors: Holland John P. , Rosenfeld Robert N.
Published: July 2002

Energy disposal to the CO product formed upon the 351 nm photodissociation of W(CO)6 has been monitored using the method of time-resolved infrared laser absorption spectroscopy. The nascent CO product can be characterized by effective vibrational, rotational, and translational temperatures; Tv=1080±60 K, T0r(v=0)=560±50 K, and T0t(v=0 J=10) =1550±200 K. These results are considered in light of various models for energy disposal in the photofragmentation reaction. Vibrational energy disposal is consistent with a modified version of phase space theory termed ‘‘early’’ phase space theory, EPST. Rotational and translational energy release is not consistent with phase space theory or its variants, e.g., EPST and the separate statistical ensembles model, but appears in qualitative accord with an impulsive model. We propose that, in general, vibrational energy release occurs early in the exit channel for the reaction, relative to rotational and translational energy release.
Surface self-diffusion of hydrogen on a model potential: Quantum aspects and correlated jumps
Authors: Wahnström Göran
Published: July 2002

We have included the proper quantum effects into the theory for thermal rate constants between any two sites of a multisite system by Voter and Doll [J. Chem. Phys. 82, 80 (1985)]. The thermal rate constants are expressed in terms of quantum flux correlation functions and we apply the formalism to surface self-diffusion. Using a potential energy function that previously has been used in order to describe hydrogen adsorbed on a copper surface, we determine the quantum corrections to the classical transition state theory, both those associated with bound degrees of freedom and those associated with tunneling and nonclassical reflexion. Our results agree quite well with previous calculations. To evaluate the temperature and time propagators necessary for the quantum dynamics we use a fast Fourier transform technique and we demonstrate that it is feasible to study the probability of correlated jumps in the case of surface diffusion. By solving for the three-dimensional and classical motion for the hydrogen atom it is shown that it is crucial to go beyond a one-dimensional treatment in order to study the possibility of correlated jumps. In this preliminary study we have neglected the motion of the substrate degrees of freedom, but the main aim with the presented approach is to be able to treat quantitatively the multidimensional case.
Electronic and vibrational relaxation in Rydberg and valence states of NO in Ne matrices
Authors: Chergui M. , Schriever R. , Schwentner N.
Published: July 2002

New time and energy resolved data on vibrationally relaxed and unrelaxed emissions from the valence a 4 Π(v=0), B 2 Π(v=0,3,4,5), and the Rydberg A 2 Σ+ (v=0,1,2) states of NO in Ne matrices are reported. Rydberg ↔ valence and valence ↔ valence nonradiative transitions are identified. The Rydberg → valence transitions are seen to occur after lattice relaxation accommodating the Rydberg orbital. The branching ratios for intramolecular relaxation and the measured lifetimes are described in terms of a model which combines the intramolecular spin–orbit matrix elements and Franck–Condon factors with the spectroscopically determined phonon Franck–Condon factors. For the levels B 2 Π(v=5,6), a Förster–Dexter-type energy transfer between NO molecules is also invoked in the description of the relaxation cascade.
The lowest excited states of copper porphyrins
Authors: Asano Motoko , Kaizu Youkoh , Kobayashi Hiroshi
Published: July 2002

Relaxation processes of excited copper porphyrins were studied with relevance to the structure of the substates of the lowest excited states. Lifetimes of luminescence at room temperature were determined as 17, 29, 69, and 105 ns for T(EtO)PPCu [T(EtO) PP: 5, 10, 15, 20-tetra (p-ethoxyphenyl)porphin], TPPCu(TPP: 5, 10, 15, 20-tetraphenylporphin), TFPPCu[TFPP: 5, 10, 15, 20-tetra(pentafluorophenyl)porphin], and OEPCu(OEP: 2, 3, 7, 8, 12, 13, 17, 18-octaethylporphin) in toluene, respectively. Emission intensities and lifetimes of OEPCu and TFPPCu measured as a function of temperature show a variation ascribed to a Boltzmann distribution between the lowest trip–doublet and –quartet with an energy gap of 300–400 cm−1. The anomalous temperature dependence for TPPCu and T(EtO)PPCu is explained by a larger energy gap and larger vibronic distortions in the excited state. The difference in behavior is attributed to the orbital nature of the triplet: ‖2,4[b31(a1e)]〉 for OEPCu and TFPPCu but ‖2,4[b31(a2e)]〉 for TPPCu and T(EtO)PPCu. The assumption of a low energy charge transfer state is not necessary for our analysis.
Intracavity, laser absorption spectroscopy of Ar2: Aπu5p and Aσu5p Rydberg states
Authors: Shannon D. C. , Eden J. G.
Published: July 2002

Inter-Rydberg transitions of Ar2 in the green (∼460–530 nm) have been studied by intracavity, laser absorption spectroscopy. All five Hund’s case (c) members of the Aπu5p manifold, predicted by the guidelines established by Mulliken [J. Chem. Phys. 52, 5170 (1970)] for Xe2 and later adapted by Chang and Setser [J. Chem. Phys. 69, 3885 (1978)] for Ar2, have been observed. The molecular transitions originate from Aσg4s(1u,0−u) and terminate on the A2 Σ+ 1/2  u ion core, 3Πg on the A2 Σ+ 1/2  u ion co-derived (5p Rydberg electron 0−g, 0+g, 2g, 1g, and 1g. Transitions involving three of these are strongly red degraded [revealing that Re4s(1u, 0−u) <Re5p(0−g) ≲Re(0+g)<Re(1g upper)] whereas the bands associated with the 2g and lowest 1g levels are not. Analysis of newly observed hot bands (Δv≡v′−v″<0) yields the following vibrational constants for ω″e =297.2±1.0 cm−1, ω″e x″e =3.3±0.4 cm−1, and ω″e y″e =0.08±0.05 cm−1. Several clear vibrational sequences are observed and tentative assignments for the Aπu5p separated atom limits are reported. The 0−g, 0+g, (2g, 1g), and 1g (upper) states apparently correlate (in the separated atom limit) with Ar(1S0)+Ar 5p[ 1/2 ]1, [ (5)/(2)

]2, [ (5)/(2) ]3, and [ (3)/(2) ]1, respectively, which is consistent with the known 4p, 5p→4s oscillator strengths, the Ar 3p5 5p fine structure splittings, and the measured energy defects between the Aπu5p substrates. Rather than being degenerate at small R, the 0−g and 0+g states of the πu5p manifold are split by 247 cm−1 near Re. Bandheads arising from the Aσu5p(0−g)←Aσg4s (1u, 0−u) transition have also been observed. Calculations show that the Aσu5p(0−g) state dissociation energy (D0≂1.0 eV) is noticeably smaller than that for the Aπu5p levels (D0=1.26±0.01 eV).
Numerical multiconfiguration self-consistent field study of the total (electronic and nuclear) parallel polarizability and hyperpolarizability for the OH, OH+, OH−
Authors: Adamowicz Ludwik
Published: July 2002

Vibrational contributions to the molecular polarizability and hyperpolarizabilities are calculated for OH, OH+, and OH− for an electric field directed along the bond axis. It was found that vibrational averaging of the polarizability works reasonably well for OH and OH−, but fails for OH+ , giving a result 16% to low. For the hyperpolarizabilities, the vibrational averaging strongly underestimates the vibronic contribution, whose size can exceed the electronic contribution, and therefore disqualifies this procedure for evaluation of these types of higher-order molecular properties.
Studies in binary nucleation: The dibutylphthalate/dioctylphthalate system
Authors: Okuyama Kikuo , Kousaka Yasuo , Kreidenweis Sonia , Flagan Richard C. , Seinfeld John H.
Published: July 2002

A continuous-flow mixing apparatus has been developed for the study of binary nucleation. This apparatus has been used to investigate the nucleation of mixed dibutylphthalate/dioctylphthalate vapors, and the interaction of the two vapors in particle formation has been demonstrated. A model that considers competition between nucleation and condensation processes during particle formation is applied in the analysis of the experimental results, allowing comparison of the magnitudes of the theoretical and actual nucleation rates in both the single-component and mixed vapor systems.
Dynamics of endoergic substitution reactions. I. Br+chlorinated aromatic compounds
Authors: Robinson Gary N. , Continetti Robert E. , Lee Yuan T.
Published: July 2002

The endoergic substitution reactions Br+R–Cl→Cl+R–Br (R=o-, m-, and p-CH3C6H4, C6H5, C6F5; ΔH°≂15 kcal/mol) have been studied using the crossed molecular beams method in the collision energy (Ec ) range 20–35 kcal/mol. The CH3C6H4Br and C6F5Br products were found to be mostly forward scattered with respect to the incident Br beam indicating that the lifetimes of the Br–R–Cl collision complexes are short compared to their rotational periods. The product translational energy distributions and excitation functions for these reactions are well reproduced by statistical calculations that assume that only a few vibrational modes in the collision complexes participate in intramolecular energy redistribution prior to Cl elimination. Ring substituents are found to affect both the extent of energy redistribution in the complexes and the probability of Br addition. For example, no substitution product was observed with m-CH3C6H4Cl or C6H5Cl. The relative magnitudes of the cross sections are explained in terms of possible features of the potential energy surfaces along their reaction coordinates.