bluuebunny/crossref_metadata_2025 huggingface dataset:
The Cu+F2 chemiluminescent reaction revisited. II. Kinetic studies
Authors: Baltayan P. , Hartmann F. , Hikmet I. , Sadeghi N.
Published: July 2002

Optical pumping, with a copper vapor laser, has been used to state select ground 2S1/2 and metastable 2D5/2 and 2D3/2 copper atoms in a fast flow, low pressure reactor combined with a hollow cathode sputtering source. The absolute densities of these atoms (around 2×1011 and 109 atoms cm−3 for 2S and 2D, respectively) were measured by resonant absorption and laser induced fluorescence techniques. Their isolated reactions with F2 have been studied in detail at 300 K. The total reaction cross section for Cu(2S) is almost four times larger than for Cu*(2D) metastable atoms. The strong chemiluminescent emission from electronically excited CuF* formed is attributed to the reaction of metastable Cu*(2D) atoms, the Cu(2S) reaction leading directly to ground state CuF(X 1Σ+) molecules. Spectral analysis of the chemiluminescence shows a highly inverted vibrational distribution in all energetically accessible excited states: a 3Σ+, b 3Π, B 1Σ+, C 1Π, and D 3Δ1 of CuF with <fv≳≊0.7. Branching ratios for formation of the molecules in these states have been determined for reaction of copper atoms in each of the three electronic states. From the highest vibrational levels populated by reaction of Cu*(2D5/2) with F2, vmax=21 and 26 for C 1Π and b 3Π states, respectively, one can deduce a more precise value of 33 560±240 cm−1 for the dissociation energy of the ground state CuF(X 1Σ+). The strong propensity observed for formation of CuF*(a,b,B,C,D) by Cu*(2D,3d94s2)+F2 and CuF(X 1Σ+) by Cu(2S,3d104s)+F2 reactions can be explained by conservation of the copper atom ionic core throughout the course of the reaction.
Steric effects and cones of reaction in gas-phase reactions
Authors: Esposito Marc , Evans Glenn T.
Published: July 2002

Information on steric effects is most easily seen through the dependence of the molecule-fixed reaction cross section on the mutual orientation of the reactants. However, the observed cross sections in oriented molecule molecular beam reactions are laboratory-fixed quantities and depend on the orientation of the ‘‘diatom’’ with respect to the relative velocity of the atom–diatom collision pair. In this work, the laboratory-fixed cross section has been determined using the venerable line of centers model for two idealized potentials which describe the dependence of the threshold energy on reactant orientation. For the painted-sphere and the linear-barrier model (here, a painted sphere with a linear dependence on the cosine of the angle of attack), the molecule-fixed cross section is nearly trivial whereas the laboratory-fixed cross section is not. In the painted-sphere model, the orientation-dependent laboratory-fixed cross section was calculated analytically and exactly and is accurately approximated by a three-term Legendre expansion. For the painted sphere with the linear barrier, we have calculated the critical angle at which the laboratory-fixed cross section vanishes. This critical angle is the laboratory-fixed cone of reaction and is in contrast with the molecule-fixed, true steric cone. At high energies, the laboratory-fixed cross section for the linear-barrier potential was represented as an energy-corrected painted-sphere model. In the energy-corrected painted-sphere approximation, the anisotropy moments of the laboratory-fixed cross section illustrate the properties of the barrier and the cone of reaction.
Spectroscopic studies of jet-cooled NiAu and PtCu
Authors: Spain Eileen M. , Morse Michael D.
Published: July 2002

Spectroscopic investigations of NiAu and PtCu have revealed that both molecules possess 2Δ5/2 ground electronic states, and are in this respect analogous to the isovalent molecule NiCu. The ground-state bond lengths (r0) have been measured as 2.351±0.001 Å and 2.335±0.001 Å for NiAu and PtCu, respectively. Ionization potentials have been bracketed as well, giving IP(NiAu)=8.33±0.38 eV and IP(PtCu)=8.26±0.07 eV. A reanalysis of previous high-temperature Knudsen effusion mass spectrometric data provides D00(NiAu)=2.52±0.17 eV. The implications of these results for the electronic structure and chemical bonding of NiAu and PtCu are discussed, and comparison is made to the other diatomic metals of the nickel and copper groups.
General method for simplifying chemical networks while preserving overall stoichiometry in reduced mechanisms
Authors: Clarke Bruce L.
Published: July 2002

A method is given for reducing the number of intermediates in reaction networks without altering the stoichiometric constraints. When a set of intermediates is to be eliminated, the reactions involving the intermediates must be replaced with the overall reactions of the extreme currents of the subnetwork whose only intermediates are the species to be eliminated. This method is automated easily for use in stability analysis and dynamical simulations.
Thermodynamics and gelation of dimerizing adhesive spheres
Authors: Weist Annemarie Ott , Glandt Eduardo D.
Published: July 2002

Wertheim’s dual density formalism is applied to study the thermodynamics and gelation behavior of dimerizing adhesive spheres. Both the thermodynamic results (critical points and site–site correlation functions) and connectivity results (gelation threshold and site–site connectedness functions) are determined for mixtures of dumbbells and spheres as a function of the fraction x1 of spheres forming dumbbells, the bond length, and the degree of adhesiveness.
Collective diffusion of Brownian particles with square well interaction
Authors: Vogel J. , Felderhof B. U.
Published: July 2002

We study collective diffusion in a semidilute suspension of Brownian particles interacting with a square well potential. Under neglect of hydrodynamic interactions, we derive an explicit expression for the wave number- and frequency-dependent diffusion coefficient. At zero frequency, the diffusion coefficient exhibits a resonance structure which varies strongly with the square well parameters.
A trimer vibronic coupling model for triptycene: The Jahn–Teller and Barnett effects
Authors: Riley Mark J. , Furlan Alan , Güdel Hans U. , Leutwyler Samuel
Published: July 2002

The first singlet electronic excited state of triptycene, as measured by resonant two-photon ionization in a supersonically cooled beam, has been found to be a textbook example of the E′⊗e′ Jahn–Teller effect. Here it is shown that this E′⊗e′ vibronic coupling can be profitably viewed as a subset of a (A′1⊕E′)⊗(a′2⊕e′) vibronic coupling scheme which results from a simple trimer model. The enlarged coupling scheme has a simple physical interpretation where the wagging coordinates of the benzene subunits are strongly coupled to their excimer formation. The previously obtained parameters, in which there is a large reduction between the ground and excited electronic state frequencies of the lowest frequency e′ mode as well as an unusually large second-order vibronic coupling constant, are shown to arise naturally from a trimer viewpoint. Features of the spectra have been found which are attributed to the involvement of an a′2 vibration which couples through nonzero momentum rather than coordinate matrix elements. A coupling of this type has been termed the ‘‘molecular Barnett effect’’ and has been predicted to appear in Jahn–Teller systems when certain conditions are fulfilled. This effect has not been previously observed in molecular spectroscopy. It indicates that crude adiabatic basis functions are inadequate to describe the electronic states of the present system.
The dynamical Renner–Teller effect. III. Rovibronic energy transfer pathways in the NCO(X̃ 2Π)+He system
Authors: Macdonald R. Glen , Liu Kopin
Published: July 2002

As a linear triatomic radical in an electronic state with Λ≠0 bends, the vibrationally excited levels are split as a result of vibronic interactions. This is the well-known Renner–Teller effect. Reported here is the first study of the collision-induced transitions between these Renner–Teller components. It is found that the pathways for these processes are highly selective and their efficiencies, in some cases, can compete with rotationally inelastic collisions. A simple, intuitive argument is presented to underscore the physical mechanisms for this finding. Preliminary, more rigorous theoretical analysis confirms the essential ideas of the proposed interpretations.
Rotational spectrum, structure, and chlorine nuclear quadrupole coupling constants of the van der Waals complex Ar–Cl2
Authors: Xu Yunjie , Jäger Wolfgang , Ozier Irving , Gerry M. C. L.
Published: July 2002

The pure rotational spectrum of the van der Waals complex Ar–Cl2 has been observed between 5 and 14 GHz using a Balle–Flygare type pulsed molecular beam microwave Fourier transform spectrometer. Ten a-type rotational transitions of Ar–35Cl2, as well as five a-type rotational transitions of the mixed isotopomer Ar–35Cl37Cl, have been assigned. The rotational constants and quartic centrifugal distortion constants have been determined. Unlike its isovalent linear isomer Ar–ClF, Ar–Cl2 has been found to be a T-shaped complex. This confirms the result from electronic spectroscopy and is in accord with the atom–atom additive model. The distance from the Ar nucleus to the center of mass of the Cl2 subunit has been calculated to be 3.7190 Å for Ar–35Cl2 and 3.7184 Å for Ar–35Cl37Cl, respectively. The van der Waals stretch and bend force constants, as well as their corresponding harmonic vibrational frequencies, have been derived and compared with the theoretically predicted values. The nuclear hyperfine splittings due to the two chlorine nuclei have been resolved and the coupling constants have been accurately determined. For 35Cl these are (in MHz): χaa=54.8180(16), χbb=−110.7131(19), χcc=55.8951(19) in Ar–35Cl2; and χaa=54.8407(27), χbb=−110.706(20), χcc=55.866(20) in Ar–35Cl37Cl. For 37Cl the values are χaa=43.2111(29), χbb=−87.267(12), χcc=44.056(12) in Ar–35Cl37Cl. A value has been obtained for the nuclear quadrupole coupling constant in free diatomic chlorine.
Nonlinear counterion screening in colloidal suspensions
Authors: Löwen Hartmut , Hansen Jean-Pierre , Madden Paul A.
Published: July 2002

A new ‘‘ab initio’’ method is presented which is designed to simulate highly asymmetric systems of charged particles such as micellar solutions and charge-stabilized colloidal suspensions. The hybrid description considers the macroion degrees of freedom explicitly, while the microscopic counterions are treated within the framework of density functional theory. The counterion density profile is treated as a dynamical variable which is coupled to the macroion positions; the corresponding equation of motions are derived from a Lagrangian which contains a fictitious kinetic energy term associated with the inhomogeneous counterion density, with a fictitious mass chosen so that the counterions stay as close as possible to the surface of lowest free energy (adiabatic condition). The discontinuous behavior of the counterion density profile at the macroion surfaces is suppressed by the use of a classical pseudopotential scheme without spoiling the rapid variation of the counterion density profile outside the macroion cores. The ab initio method is implemented in Molecular and Brownian Dynamics simulations of concentrated colloidal suspensions, and the results are compared to the predictions of much simpler simulations based on the pairwise additive effective Derjaguin–Landau–Verwey–Overbeek (DLVO) potential between macroions. The density profiles calculated from the DLVO model differ considerably from the predictions of the ab initio simulations, but the macroion pair structures are in reasonable agreement. Recent ‘‘improvements’’ of the standard DLVO theory are found to overestimate or underestimate the pair structure considerably. The density functional formalism may be used to derive systematic many-body corrections to the effective DLVO pair potential. The extension of the ab initio method to treat colloidal suspensions in the presence of added salt is briefly sketched.
Renner–Teller and spin–orbit interactions in SiH2
Authors: Duxbury Geoffrey , Alijah Alexander , Trieling Reuben R.
Published: July 2002

Experimental data on SiH2 have been reanalyzed to show the existence of a large number of strong, but erratic rotational perturbations which are correlated with anomalous fluorescence lifetimes. These results have been interpreted using a model for the combined effects of Renner–Teller coupling between the two singlet states and spin–orbit coupling with the lowest triplet. In this model, the vibrational and K-type rotational levels of the X̃ 1A1 and the Ã 1B1 states of SiH2 have been fitted by least squares to give a pair of Born–Oppenheimer curves for the combining electronic states. These have been used to calculate the interaction matrix elements of the spin–orbit coupling with the ã 3B1 state. Large interactions between the 1B1 and the 3B1 states are calculated even though the direct interaction of the states is forbidden. Thus we can rationalize the existence of both the strong rotational perturbations and the unusual radiative lifetimes. Lack of knowledge of the exact location of the triplet state, and of the rovibronic structure associated with this state, precludes a detailed fit of the type recently made for CH2. We have also calculated the rotational constants for a range of vibronic states of SiH2. Analogous rotational constants and band origins of SiD2 have also been calculated using the potential curves derived from the fit to SiH2. The agreement between the calculated origins and those measured experimentally is very satisfactory.
Measurement of cross relaxation between two selected nuclei by synchronous nutation of magnetization in nuclear magnetic resonance
Authors: Burghardt Irene , Konrat Robert , Boulat Benoit , Vincent Sébastien J. F. , Bodenhausen Geoffrey
Published: July 2002

A novel technique is described that allows one to measure cross-relaxation rates (Overhauser effects) between two selected nuclei in high-resolution NMR. The two chosen sites are irradiated simultaneously with the sidebands of an amplitude-modulated radio-frequency field, so that their magnetization vectors are forced to undergo a simultaneous motion, which is referred to as ‘‘synchronous nutation.’’ From the time-dependence observed for different initial conditions, one may derive cross-relaxation rates, and hence determine internuclear distances. The scalar interactions between the selected spins and other spins belonging to the same coupling network are effectively decoupled. Furthermore, cross relaxation to other spins in the environment does not affect the transient response of the selected spins, which are therefore in effect isolated from their environment in terms of dipolar interactions. The method is particularly suitable to study cases where normal Overhauser effects are perturbed by spin-diffusion effects due to the presence of further spins. The technique is applied to the protein bovine pancreatic trypsin inhibitor.
Correlation energy of the free-electron pair in the second-order perturbation theory
Authors: Cinal M. , Olszewski S. , Kwiatkowski T.
Published: July 2002

A standard Rayleigh–Schrödinger perturbation theory is applied to a free-electron pair enclosed in a spherical potential box having infinite walls and to an itinerant free-electron pair at its lowest energy. The second-order perturbation energy for the standing-like wave functions is found to be a large, but finite negative number independent of the box dimensions. A similar energy calculated for the itinerant free-electron pair at its lowest energy level has a much smaller absolute value; also the third-order perturbation energy for the same itinerant pair is found to have a convergent value. The convergent results obtained for the second-order and third-order perturbation energies of the free-electron pair revise the view that these energies are the divergent quantities.
Collisional energy transfer in Na(4p–3d)–He,H2 collisions
Authors: Kleiber P. D. , Wong T. H. , Bililign Solomon
Published: July 2002

We have investigated the direct collisional energy transfer process Na*(4p)+M→Na*(3d)+M, where M=He,H2 under gas cell conditions. We have measured the temporal profiles of the Na(3d–3p) sensitized fluorescence as a function of quenching gas pressure and fit the profiles to a two-state rate equation model to obtain the quenching rate coefficients from the Na*(4p) state. The total energy transfer rate coefficient out of the 4p state for He is small [(0.5±0.2)×10−10 cm3/s]. The total quenching rate coefficient out of the 4p state is much larger for H2[(3.9±0.5)×10−10 cm3/s]. Evidence suggests that the energy transfer rate coefficient for the 4p–3d process is ∼2.0×10−10 cm3/s with the remainder of the 4p quenching being predominantly reactive. We also compare the far-red wing absorption line shapes for the NaHe and NaH2 systems.
Influence of methyl group deuteration on the rate of intramolecular vibrational energy relaxation
Authors: Gambogi Joan E. , L’Esperance Robert P. , Lehmann Kevin K. , Pate Brooks H. , Scoles Giacinto
Published: July 2002

The high resolution spectra of the fundamental and first overtone of the acetylenic C–H stretch in tert.-butylacetylene-d9 and (trimethylsilyl)acetylene-d9 have been measured using optothermal detection of a collimated molecular beam. IVR lifetimes determined from the homogeneously broadened lineshapes are compared to those of their undeuterated analogues. It is found that for both molecules, at both levels of excitation, deuterating the methyl rotors results in an increased rate of IVR. The results indicate that the previously suggested methyl rotor effect, as an enhancer for IVR, plays a secondary role to increasing the number of low order resonances to which the C–H stretch can couple. Although the torsional modes are important for the molecules to exhibit statistical case IVR and contribute to the filled-in homogeneous lineshapes, the rate of energy relaxation seems to be dominated by the number of low order resonances.
Bleaching of hydrogen centers in solid xenon by thermally activated tunneling
Authors: Wittl F. , Eberlein J. , Epple Th. , Dechant M. , Creuzburg Martin
Published: July 2002

The thermal bleaching of the characteristic UV-absorption bands in doped solid xenon has been studied. These bands are attributed to quasistable caged hydrogen atoms produced by photolysis of the dopant in the xenon lattice. Hydrogen atoms can recreate their parent molecules via thermally activated tunneling. A semiclassical description of this process is given considering the potential of hydrogen in a rigid cage. Its vibrational energy is equal to that determined spectroscopically and explains quantitatively the temperature dependence of the bleaching rate. Deviations from the experimental results for the deuterium atom give evidence for the influence of a dynamic lattice.
Classical versus quantum mechanical desorption kinetics in molecule/surface scattering: The NO/diamond case
Authors: Vach Holger , Châtelet Marc
Published: July 2002

A model is presented that is based on the tunneling effect to explain the scattering behavior observed during the interaction of NO molecules with a diamond (111) surface. To account for the previously reported surface residence times that are unusually long, but temperature independent, the temporary formation of a metastable surface–molecule complex is assumed. The need for a quantum mechanical picture is discussed in comparison to alternative classical models.
Comment on: Surface modes on polymer solutions by surface light-scattering techniques
Authors: Dorshow Richard B. , Turkevich Leonid A.
Published: July 2002

We critically examine this paper’s claim for a surface elastic mode on concentrated polymer solutions. Their polymer system does not exhibit bulk viscoelasticity, making unlikely a surface elastic mode. A capillary mode analysis of their surface laser light scattering data yields the measured rheology. We conclude that a surface elastic mode has not been observed.
A beam experiment on excimer formation in collisions of Kr*(3P), Kr*(3P2), and Xe* atoms with Br-containing molecules
Authors: Vredenbregt E. J. D. , van Besouw M. P. M. , Vrakking M. J. J. , Mietus M. J. , Gerrits C. E. P. , Beijerinck H. C. W.
Published: July 2002

The reactions of metastable Kr* and Xe* atoms with several Br-containing molecules are studied with a beam-gas experimental apparatus. For Kr*, state selection of the metastable atom beam is employed to investigate the influence of the initial fine-structure state Kr*(3P0) and Kr*(3P2) on the reaction. Trial-and-error simulation of the observed emission spectra results in modified potential energy curves for the X, A(3/2), B, and C states of the KrBr and XeBr excimer products and corresponding transition moments. The propensity for conservation of the Kr+(2P1/2) ion core in the reactions of Kr*(3P0) is observed to be between 36% and 51%, depending on the target, while for the Kr+(2P3/2) core the propensity is close to 100%. This is in general agreement with the results of Sadeghi, Cheaib, and Setser [J. Chem. Phys. 90, 219 (1989)] for Ar*. The reactive cross section is appreciably smaller for Kr*(3P0) than for Kr*(3P2). For several reagents, the analysis leads to a preference for formation of KrBr and XeBr in the C state, different from results of flowing afterglow experiments. This points to incomplete correction for collisional relaxation and for overlap of B→X and C→A(3/2) emission in previous work. For most reagents, the vibrational distributions are analogous for both XeBr and KrBr in both the C and B states. For XeBr(B), the results are generally in agreement with the work of Tamagake, Kolts, and Setser [J. Chem. Phys. 74, 4286 (1981)].
Extreme covalence in the Tc–Cl bond from polarized neutron diffraction
Authors: Figgis B. N. , Reynolds P. A. , Cable J. W.
Published: July 2002

A polarized neutron diffraction experiment on a salt containing the 4d1 TcNCl4− ion (S=1/2) yielded the flipping ratios of six reflections confined to the hk0 plane. Using nuclear structure factors, FN, extrapolated from the 123 and 295 K x-ray structures, magnetic structure factors, FM, were obtained from the flipping ratios. The values of FM were fitted to a model of spin populations in the Tc-4d and the Cl-3(sp) orbitals. It was found that 46(5)% of the spin density is located on the Cl atoms, corresponding to exceptionally high covalence in the Tc–Cl bonding. The amount is distinctly higher than found indirectly by ESR measurements, but is to some extent supported by a local density functional DV–Xα calculation. Because the data is limited to the one projection, it is not possible to separate the Tc and N spin density contributions.
Reaction of O2+(X 2Πg) with H2 , D2 , and HD: Guided ion beam studies, MO correlations, and statistical theory calculations
Authors: Weber M. E. , Dalleska N. F. , Tjelta B. L. , Fisher E. R. , Armentrout P. B.
Published: July 2002

Absolute cross sections are measured for the reactions of O2+(X 2Πg) with H2 , D2 , and HD from thermal energies to over 4 eV. The OH++OH, HO2++H, O++H2O, and H2O++O product channels (and the corresponding isotopic analogs) are observed, although H2++O2 is not. While the first three products appear at their thermodynamic thresholds, formation of H2O++O, the least endothermic channel, exhibits a barrier to reaction. In the HD system, the DO2+ product ion is strongly favored over the HO2+ product. Results for internally excited O2+ reactants, probably the a 4Πu state, are also presented. Analysis of the excitation functions, molecular orbital arguments, and statistical kinetic theories are used to understand the mechanisms and dynamics of this reaction. It is shown that the inefficiency of the O+ product channel is due to spin and symmetry constraints. The other three product channels proceed through a long-lived intermediate, but formation of this intermediate from reactants requires surmounting a barrier measured to be 1.1±0.1 eV. The intramolecular isotope effects are shown to be due to statistical and dynamic effects.
Correlations and free energies in restricted primitive model descriptions of electrolytes
Authors: Hummer Gerhard , Soumpasis Dikeos Mario
Published: July 2002

The pair and triplet correlations of a restricted primitive model (RPM) description of aqueous NaCl solutions have been computed in the concentration regime 0.5–4 M via Monte Carlo simulations, using both the method of configuration statistics and the particle insertion (chemical potential) method. Comparison of the triplet data to Kirkwood’s superposition approximation and the Abe and convolution approximations shows that deviations occur only for distances close to contact. The entropy computed via the entropy expansion including terms up to third order and the insertion method is dominated essentially by packing effects. The pair correlations and thermodynamic properties derived from the simulations were compared to the hypernetted chain (HNC) approximation which was found to be very good corroborating the conclusions drawn from earlier RPM studies.
Formation of protonated ammonia cluster ions: Two-color two-photon ionization study
Authors: Misaizu Fuminori , Houston Paul L. , Nishi Nobuyuki , Shinohara Hisanori , Kondow Tamotsu , Kinoshita Minoru
Published: July 2002

The mechanism of the resonance-enhanced two-photon ionization (RE2PI) and the dissociation of ammonia clusters has been investigated by using the two-color ionization technique and the reflectron time-of-flight mass spectrometer. The clusters are firstly excited to the intermediate state corresponding to the Ã 1A2 state of an ammonia molecule. The lifetime of the intermediate species is found to be <10 ns for the production of NH4+ and (NH3)2+, and longer than 1 μs for (NH3)nNH4+ (1≤n≤6). This result implies that NH4+ is produced by the ionization of the dimer followed by the dissociation of NH2, whereas the higher protonated cluster ions are produced by the second photon ionization of the (NH3)nNH4 radicals produced by the dissociation of the parent clusters in the intermediate state. The appearance potentials of (NH3)2+ and (NH3)nNH4+ (0≤n≤4) are also determined by two-color RE2PI technique. Those values are discussed in connection with the two possible mechanisms for the observed ion production, which confirms the scheme obtained from the lifetime measurement.
The spectral representation technique for an active-electron-only molecular orbital calculation applied to the case of nonspherical frozen charge
Authors: Katsuki Shinichi
Published: July 2002

The new approach, in which the interactions between the nonspherical dormant electrons and the active electrons are replaced with a kind of effective potential operator Veff, is introduced, taking a functional group COOH as a working example of the nonspherical dormant electron charges. To construct Veff, the use of the spectral representation technique is made. The validity of the new idea is tested by calculating the electronic structures of molecules H3C–COOH, H3C–CH2COOH, and H3C–CH2CH2COOH as a function of C–C bond length with use of Veff, and by comparing the results with those obtained by the all-electron calculations. The present approach is shown to be promising. The possibility of saving computation is also discussed.
Spectroscopy of SH (A–X) transition in Ar and Kr matrices: The caging of predissociation
Authors: Zoval J. , Imre D. , Apkarian V. A.
Published: July 2002

The A(2Σ+)–X(2Π) transition of SH isolated in Ar and Kr matrices is studied by laser induced fluorescence spectroscopy. The (0,0), (1,0), and (2,0) transitions are observed in excitation. These lines are structureless and homogeneously broadened (200–300 cm−1). The (0,0), (0,1), and (0,2) emission bands, are strongly Stokes shifted and further broadened (≥1000 cm−1). The spectra can be successfully interpreted in analogy with OH–Rg pair interactions. The SH(A)–Rg potential is deeply bound (∼1000 cm−1 in Ar and ∼2000 cm−1 in Kr) with nearly 1 Å compression along this coordinate. Franck–Condon analysis indicates that the equilibrium bond length of SH(A) is 0.055 Å longer than in the free radical. The A state, which is strongly predissociated in the free radical, relaxes radiatively in the matrix: τ=425 and 770 ns in Ar and Kr, respectively. Predissociation is treated as a two-dimensional tunneling process between two electronic surfaces. Poor Franck–Condon factors along the solvent coordinate, due to differential solvation of electronic states, leads to complete arrest of electronic predissociation.
A quantum molecular-dynamics study of proton-transfer reactions along asymmetrical H bonds in solution
Authors: Azzouz H. , Borgis D.
Published: July 2002

A molecular-dynamics study of a model for AH−B⇄A−−H+B reactions in liquid chloromethane is presented. The parameters of the model are fitted to those of typical OH–N proton-transfer complexes. The rate constant is computed at a quantum level for complexes of various H-bond strength and A-B equilibrium distance. The influence of the properties of the complex on the proton-transfer mechanism is outlined. Also the static and dynamical role of the solvent, the tunneling contribution to the rate, and the associated kinetic isotope effect are discussed. The rate calculations are based on two independent methods. First a curve-crossing, transition-state rate formula which, although related to standard charge-transfer theories, presents some original features and allows the determination of the rate at very low computational cost is developed. The curve-crossing results are compared to those of a path-integral, quantum transition-state calculation. The overall agreement between the two methods is satisfactory, although there is a discrepancy in the adiabatic reaction regime; a rigorous estimation of the transmission coefficients would be needed then. Finally, it is shown that zero-point energy and parabolic barrier tunneling factors added to the classical transition-state-theory rate constant are unable to describe properly the quantum effects in the present case.
The ridge of the specific opacity surface in heavy heavy–light chemical reactions
Authors: Alvariño José María , Cuadrado Luis , Laganà Antonio
Published: July 2002

An approximate one-to-one correspondence between the value of the product vibrational state and the location of the interval of impact parameter open to reaction has been found for direct Mg+HF→MgF+H reactive collisions. This gives rise to a ridge in the specific opacity function surface.
Thermodynamic properties of hard-chain molecules
Authors: Bokis Costas P. , Cui Yuping , Donohue Marc D.
Published: July 2002

The density and molecular size dependence of the equation of state for molecules that consist of freely jointed, tangent hard spheres is investigated. It is shown that the properties of hard-chain molecules can be accurately described using the properties of spherical molecules and a function c which varies approximately linearly with the reduced density. Monte Carlo and molecular dynamics simulation data for hard chains were used to develop the expression for c. The new equation of state is simple in form and more accurate for the calculation of compressibility factors for hard-chain molecules than either Wertheim’s first-order thermodynamic perturbation theory or the generalized Flory dimer theory (GFD). Further, this new equation reduces to the correct second virial coefficient limit as the density approaches zero.
The NaZn excimer: Blue–green band
Authors: Azinović Davorka , Li Xinghua , Milošević Slobodan , Pichler Goran , van Hemert Marc C. , Düren Rudolf
Published: July 2002

Electronically excited NaZn excimers have been produced in a photochemical reaction of Na2(C 1Πu) with Zn(4 1S0). The Na–Zn vapor mixture was prepared by introducing a controlled stream of sodium vapor into zinc vapor generated in a heat-pipe oven. A new band (bound–free transition) was observed in the region from 450 to 487 nm. Potential curves of the eight lowest electronic states of NaZn were calculated. On the basis of the calculations, the observed emission is interpreted as resulting from the 2 2Π–X 2Σ+ transition. The conditions favorable for the observation of the photochemical reaction are discussed.
A surface reaction with atoms: Hydrogenation of sp- and sp2-hybridized carbon by thermal H(D) atoms
Authors: Biener J. , Schubert U. A. , Schenk A. , Winter B. , Lutterloh C. , Küppers J.
Published: July 2002

Hydrogen containing carbon (C:H) films have been prepared by ion beam deposition of 160 eV ethane ions on a carrier consisting of a Pt(111) single crystal surface covered with a monolayer of graphite. Several monolayers thick films deposited at 350 K contain hydrogen bound to sp, sp2, and sp3 hybridized carbon. Vibrational spectroscopy reveals that thermal D(H) atoms directed to the C:H film surface hydrogenate unsaturated CH groups at the surface and transform sp and sp2 hybridized carbon to sp3. The observed reaction mechanism explains the microscopic processes in chemical erosion of graphite by hydrogen atoms which is a crucial reaction step in the low pressure synthesis of diamondlike carbon.
The correspondence between classical nonlinear resonances and quantum mechanical Fermi resonances
Authors: Roberts Floyd L. , Jaffé Charles
Published: July 2002

The correspondence between classical nonlinear resonances and quantum mechanical Fermi resonances is discussed. The principle result is the recognition that the classical resonant behavior does not contribute to the diagonal matrix elements, but rather contributes to the off-diagonal matrix elements. As a direct consequence, it is not necessary for the classical dynamics corresponding to the quantum mechanical states involved in a Fermi resonance to exhibit resonant behavior. Instead, it is the classical dynamics associated with the matrix element which connects the quantum mechanical states involved in the Fermi resonance which must exhibit resonant behavior. These results are illustrated numerically using a very simple model of two kinetically coupled Morse oscillators.
Measurement of the cross relaxation rate for a polymer dispersed liquid crystal system
Authors: Cross Collin W. , Fung B. M.
Published: July 2002

Proton cross relaxation between liquid crystal and polymer in a polymer dispersed liquid crystal (PDLC) system has been studied by the use of magic angle spinning and polarization transfer. The cross relaxation rates have been determined as functions of temperature, irradiation power, and off-resonance frequency setting. The data at each temperature are treated by a new set of relaxation equations which are not limited by the assumption of complete saturation of the polymer protons. The cross relaxation rates for the aromatic and aliphatic protons are similar over the temperature range studied, indicating that the liquid crystal molecules anchor parallel to the polymer surface. Before the glass transition, the cross relaxation rates increase with temperature due to a reduction in the contact time. After the glass transition, at about 37 °C, the cross relaxation rates decrease rapidly with temperature due to an increase in the motions of the polymer molecules which cause a reduction in the efficiency of spin energy exchange.
Potential energy function for CH3+CH3⇄C2H6: Attributes of the minimum energy path
Authors: Robertson S. H. , Wardlaw D. M. , Hirst D. M.
Published: July 2002

The region of the potential energy surface for the title reaction in the vicinity of its minimum energy path has been predicted from the analysis of ab initio electronic energy calculations. The ab initio procedure employs a 6–31G** basis set and a configuration interaction calculation which uses the orbitals obtained in a generalized valence bond calculation. Calculated equilibrium properties of ethane and of isolated methyl radical are compared to existing theoretical and experimental results. The reaction coordinate is represented by the carbon–carbon interatomic distance. The following attributes are reported as a function of this distance and fit to functional forms which smoothly interpolate between reactant and product values of each attribute: the minimum energy path potential, the minimum energy path geometry, normal mode frequencies for vibrational motion orthogonal to the reaction coordinate, a torsional potential, and a fundamental anharmonic frequency for local mode, out-of-plane CH3 bending (umbrella motion). The best representation is provided by a three-parameter modified Morse function for the minimum energy path potential and a two-parameter hyperbolic tangent switching function for all other attributes. A poorer but simpler representation, which may be satisfactory for selected applications, is provided by a standard Morse function and a one-parameter exponential switching function. Previous applications of the exponential switching function to estimate the reaction coordinate dependence of the frequencies and geometry of this system have assumed the same value of the range parameter α for each property and have taken α to be less than or equal to the ‘‘standard’’ value of 1.0 Å−1. Based on the present analysis this is incorrect: The α values depend on the property and range from ∼1.2 to ∼1.8 Å−1.
Molecule–corrugated surface collisions: Converged close coupling wave packet and quasiclassical trajectory calculations for N2 scattering from corrugated lattices
Authors: Bowen H. F. , Kouri Donald J. , Mowrey Richard C. , Yinnon A. Tamar , Gerber R. Benny
Published: July 2002

The close coupling wave packet (CCWP) and quasiclassical trajectory methods are used to study rotationally inelastic scattering of N2 from static, corrugated surfaces. The collision energy in these calculations ranges from 10 to 100 meV; 18 711 quantum states are included in the highest energy calculations to ensure convergence. The scattered molecules are analyzed with respect to the polarization of the final angular momentum vector and the amount of energy transferred into rotational motion and translational motion parallel to the surface. Comparisons of quantum and quasiclassical results show that quantum effects are important even with the relatively large mass of N2 and the high scattering energies used and can be seen even after summing over many final quantum states. A test of a factorization relation derived from the coordinate-representation sudden (CRS) approximation gives qualitative agreement with the exact quantum results.
Rotational structure in the near-infrared absorption spectrum of ozone
Authors: Anderson Stuart M. , Hupalo Peter , Mauersberger Konrad
Published: July 2002

Ozone absorption spectra near 1 μm exhibit fine structure indicative of rotational subbands, suggesting that the upper electronic state is metastable. Preliminary analysis supports a previous assignment to the 3A2←1A1 transition. A binding energy of ∼0.1 eV is inferred from the breakoff in the observed structure.
Structural defects in self-assembled organic monolayers via combined atomic beam and x-ray diffraction
Authors: Camillone N. , Chidsey C. E. D. , Eisenberger P. , Fenter P. , Li J. , Liang K. S. , Liu G.-Y. , Scoles G.
Published: July 2002

We present the results of a combined He atom and x-ray diffraction study of CH3(CH2)n−1SH monolayers self assembled on Au(111) surfaces. By combining these two complementary probes, we have characterized both the surface and the interior structure of the monolayers. In both cases, we find the same structure containing four molecules per unit mesh. However, we demonstrate that there are significant differences in both the diffraction linewidths and the dependence of the linewidth upon chain length for these two techniques.
A study of solvent effects on hyperpolarizabilities: The reaction field model applied to acetonitrile
Authors: Willetts Andrew , Rice Julia E.
Published: July 2002

The reaction field method has been applied in the calculation of the nonlinear optical properties of acetonitrile (CH3CN) in the liquid phase. Both self-consistent field (SCF) and second order perturbation theory (MP2) methods are used to calculate the dipole moment, polarizability, first and second static hyperpolarizabilities. Based on previous gas-phase studies which stress the importance of electron correlation in the calculation of hyperpolarizabilities, the MP2 results should be more reliable than the corresponding SCF results. The choice of a cavity radius is of considerable importance. For the two choices made, one based on the liquid density and the other on van der Waals’ parameters, the total second hyperpolarizability changed by as much as a factor of 3 and these calculated values bracket the two differing experimental results. The form of the reaction field factor suggests that the radius dependence should be less for acetonitrile in a solvent of reduced dielectric constant. Although this is true, as demonstrated by calculations involving acetonitrile dissolved in chloroform, significant dependence is still observed. The effect of using an ellipsoidal cavity as opposed to a spherical one is also examined. In this case the hyperpolarizability is less sensitive to the change in cavity parameters from those corresponding to the liquid density to those based on van der Waals’ radii. The hyperpolarizability determined is intermediate between the two values obtained with the spherical cavity and closer to one of the experimental values. Further work using more sophisticated solvation models is required in order to establish whether the reaction field model, with an appropriate choice of cavity parameters, can be used to investigate the hyperpolarizabilities of molecules in solution reliably.
Relativistic effects on the electronic structure and chemical bonding of UF6
Authors: Onoe Jun , Takeuchi Kazuo , Nakamatsu Hirohide , Mukoyama Takeshi , Sekine Rika , Kim Bong-Il , Adachi Hirohiko
Published: July 2002

We have studied the relativistic effects in the electronic structure and chemical bonding for the ground state of UF6, using the relativistic and nonrelativistic discrete-variational Xα molecular orbital calculations. It is found that two relativistic effects appear in the valence levels; the energy level splitting and upward shift of energies of the molecular orbitals. From the Mulliken population analysis of the valence levels, it is shown that the level splitting is due to mixing of the uranium atomic orbitals with a strong spin–orbit interaction, such as U6p, and the upward shift due to the increase in the screening of the nuclear charge and charge redistribution. The strength of U–F bonding remarkably increases for the relativistic case, because the changes in the radial distributions due to the relativistic effects induce both the decrease in the antibonding interactions and the increase in the bonding ones.
Photodecomposition of acetyl chloride on the excited singlet state surface
Authors: Sumathi R. , Chandra A. K.
Published: July 2002

We investigate the plausible mechanism of fast decomposition of acetyl chloride upon 1nπ* (CO) excitation through the selective C–Cl bond-fission on the lowest excited singlet state surface using ab initio quantum chemical methods. Effects of zero point energy corrections and of electron correlation have been considered. The pathway involves dissociation, via a φClφ*CO and a φClφ*C–Cl configurations where φ’s stand for wave functions of σ-molecular orbitals.
Rotational energy transfer in highly vibrationally excited HCN
Authors: Wu J. , Huang R. , Gong M. , Saury A. , Carrasquillo M. E.
Published: July 2002

The state-resolved collisional self-relaxation of HCN at a vibrational energy content of 10 000 cm−1 is probed directly by combining direct overtone vibration excitation, to prepare energized molecules in the (0,00,3) level, with a laser induced fluorescence monitor of the population evolution from different rotational states. Pure rotational energy transfer dominates the collision dynamics while vibrational relaxation results from only a small fraction of the inelastic events. The depopulation of single j levels proceeds with high efficiency. It is characterized by rates up to 14 times faster than the Lennard-Jones gas kinetic rate conforming to a j dependent distribution which peaks near the Boltzman population maximum and decreases to higher and lower angular momentum values. Approximately 70% of the collisional population removal from the j=4 level proceeds via the ΔJ=±1 channel and 28% proceeds via the ΔJ=±2 direct population transfer step. The results support a long range dipole–dipole mechanism for the energy transfer. This work also investigated various empirical scaling relations and determined that a two parameter fitting law based on the momentum gap or a three parameter modified scaling expression based on the energy gap successfully models the rotational relaxation.
Resonance enhanced multiphoton ionization spectroscopy of the NF molecule: 1Σ+ and 1Δ 3p Rydberg states
Authors: Dyke J. M. , Keszthelyi T. , Lee E. P. F. , Low R. J. , Richter R.
Published: July 2002

Two Rydberg states of the NF molecule, the 2 1Δ and 2 1Σ+ states, have been investigated by multiphoton ionization (MPI) spectroscopy. These states are observed via the two-photon resonance enhancements they provide in the multiphoton ionization spectra of NF a 1Δ and NF b 1Σ+. Both states are assigned on the basis of experimental evidence as Rydberg states with the dominant configuration ...1π45σ22π1 (3pπ)1. Ab initio calculations performed at the multireference determinant configuration interaction (MRDCI) level showed that these states were of mixed Rydberg-valence character with the Rydberg character [... 1π45σ22π1 (3pπ)1] dominating at short bond lengths (<1.35 Å) and the valence character (... 1π35σ22π3) becoming more important at longer bond lengths (≳1.35 Å). These calculations also proved useful in providing a mechanism to account for the rotational predissociation observed in the experimental spectra.
Calculation of hyperfine coupling constants of the ground state X 3Σ− of NH and B2
Authors: Fernández Berta , Jo/rgensen Poul , McCullough E. A. , Simons Jack
Published: July 2002

Following a systematic examination of basis set and electron correlation effects, accurate hyperfine coupling constants have been determined for the X 3Σ− states of NH and B2 using the multiconfiguration self-consistent-field (MCSCF) restricted–unrestricted (RU) response function approach. These species were chosen for study because their unpaired electrons reside in π orbitals; so at the single configuration self-consistent-field (SCF) approach, they display zero hyperfine coupling constants. The approach advocated here has been tested successfully on σ-radical species with unpaired electrons occupying σ orbitals; this work represents the extension to π-radical species which are expected to be more difficult cases. In designing the atomic orbital basis sets, effects of uncontraction of the orbitals (to permit maximal flexibility especially in describing electron density near nuclei) and of addition of diffuse and tight functions were taken into account. Our final bases give hyperfine coupling constants that agree with numerical Hartree–Fock (HF) and with numerical complete active space valence (CASV) MCSCF results, which indicates that our basis sets are accurate enough to be used in further studies that treat electron correlation more accurately. For dealing with electron correlation in a manner that, based on our past experience, could provide the requisite over all accuracy in the final coupling constants, the CASV configuration spaces were systematically extended to larger CAS (complete active space) spaces using natural orbital occupation numbers to determine which orbitals to include in active spaces for each symmetry. Our final results compare favorably with the available experimental data. The results show that the hyperfine coupling constant for B in B2 and N in NH results from a near cancellation of large and opposite signed core and valence contributions.