bluuebunny/crossref_metadata_2025 huggingface dataset:
Crossed-beam reaction of carbon atoms with hydrocarbon molecules. II. Chemical dynamics of n-C4H3 formation from reaction of C(3P
j) with methylacetylene, CH3CCH (X1A1)
Authors: Kaiser R. I. , Stranges D. , Lee Y. T. , Suits A. G.
Published: July 2002

The reaction between ground-state carbon atoms, C(3Pj), and methylacetylene, CH3CCH (X1A1), was studied at average collision energies of 20.4 and 33.2 kJ mol−1 using the crossed molecular beams technique. Product angular distributions and time-of-flight spectra of C4H3 at m/e=51 were recorded. Forward-convolution fitting of the data yields weakly polarized center-of-mass angular flux distributions isotropic at lower, but forward scattered with respect to the carbon beam at a higher collision energy. The translational energy flux distributions peak at 30–60 kJ mol−1 and show an average fractional translational energy release of 22%–30%. The maximum energy release as well as the angular distributions are consistent with the formation of the n-C4H3 radical in its electronic ground state. Reaction dynamics inferred from these distributions indicate that the carbon atom attacks the π-orbitals of the methylacetylene molecule via a loose, reactant like transition state located at the centrifugal barrier. The initially formed triplet 1-methylpropendiylidene complex rotates in a plane almost perpendicular to the total angular momentum vector around the B\C-axes and undergoes [2,3]-hydrogen migration to triplet 1-methylpropargylene. Within 1–2 ps, the complex decomposes via C–H bond cleavage to n-C4H3 and atomic hydrogen. The exit transition state is found to be tight and located at least 30–60 kJ mol−1 above the products. The explicit identification of the n-C4H3 radical under single collision conditions represents a further example of a carbon–hydrogen exchange in reactions of ground state carbon atoms with unsaturated hydrocarbons. This channel opens a versatile pathway to synthesize extremely reactive hydrocarbon radicals relevant to combustion processes as well as interstellar chemistry.
Kinetics and dynamics of the initial adsorption of nitric oxide on Ir(111)
Authors: Davis J. E. , Karseboom S. G. , Nolan P. D. , Mullins C. B.
Published: July 2002

The interaction of nitric oxide (NO) with an Ir(111) surface has been studied with supersonic molecular beam techniques and electron energy loss spectroscopy. Initial adsorption probability S0, measurements as a function of incident kinetic energy Ei, surface temperature Ts, and angle of incidence θi reveal that separate mechanisms govern adsorption at low and high kinetic energy. This distinction is reflected in measurements of the initial molecular adsorption probability where a decrease in the value of S0 with increasing Ts (between 77 and 300 K) is observed at low kinetic energy (Ei<0.45 eV), but no surface temperature dependence is detected at high kinetic energy in this temperature range. We present a model describing both the molecular and dissociative chemisorption of NO on Ir(111). At low kinetic energy, NO adsorbs initially as a physically adsorbed species. From this state, desorption to the gas phase or conversion to a molecularly chemisorbed state on the surface are competing processes which depend on surface temperature. The molecularly chemisorbed state is the precursor to dissociation for elevated surface temperatures. At high kinetic energy, NO adsorption occurs directly into the molecularly chemisorbed well, with the probability of trapping as a physically adsorbed species near zero and with undetectable direct dissociation. Indeed, after exposure of the Ir(111) surface at 77 K to a high kinetic energy (1.3 eV) beam, surface vibrational spectroscopy measurements show only features attributable to molecularly chemisorbed NO. The success of this model in describing our measurements is demonstrated by the separate calculation from low and high kinetic energy data of rate constants corresponding to forward and reverse conversion from the molecularly chemisorbed well. Additionally, we discuss attempts to promote dissociation on the surface with vibrational energy and with a combination of translational and surface thermal energy.
Solid–fluid phase equilibrium for single component and binary Lennard-Jones systems: A cell theory approach
Authors: Cottin X. , Monson P. A.
Published: July 2002

We consider the application of the cell theory to single component and binary Lennard-Jones solids. We calculate solid phase properties and solid–fluid equilibrium using the cell theory for the solid phase and an equation of state for the fluid phase. In the single component case the thermodynamic properties as well as the solid–fluid phase diagram predicted by the theory are in quite good agreement with Monte Carlo simulation results. The introduction of correlations between the motions of nearest neighbor particles into the cell theory in a fashion suggested by Barker significantly improves the agreement. For binary Lennard-Jones 12-6 mixtures the predictions of the theory are compared with experimental data for mixtures forming substitutionally disordered solid solutions involving argon, krypton and methane. The theory correctly predicts the form of the phase diagram but the quantitative predictions are quite sensitive to the choice of potential parameters. The shape of the phase diagram is similar to that for a hard sphere mixture with the same diameter ratio.
The short-time intramolecular dynamics of solutes in liquids. I. An instantaneous-normal-mode theory for friction
Authors: Goodyear Grant , Stratt Richard M.
Published: July 2002

It is sometimes useful to be able to think of the energy relaxation of a solute dissolved in a liquid as being caused by some sort of solvent-inspired friction. This intuitive association can, in fact, be made literal and quantitative in classical mechanics by casting the dynamics into a solute-centered equation of motion, a generalized Langevin equation, in which the dissipative character of the solvent is embodied in a (generally time delayed) friction force. An exact prescription is available for finding this friction, but the process is formal and the connection with microscopic degrees of freedom is rather indirect. An alternate approach due to Zwanzig, which portrays the solvent as a harmonic bath, makes explicit use of a set of solvent coordinates, but these coordinates have no immediate relationship with any of the real solvent degrees of freedom. We show here that by taking a short-time perspective on solute relaxation we can derive a generalized Langevin equation, and hence a friction kernel, which is both exact (at least at short times) and has a completely transparent connection with solvent motion at the molecular level. We find, in particular, that under these conditions the instantaneous normal modes of the solution fill the role of the Zwanzig harmonic oscillators precisely, meaning that one can analyze friction in molecular terms by appealing to the explicitly microscopic definitions of the instantaneous modes. One of the implications of this perspective is that fluctuations of the solvent are automatically divided into configuration- to-configuration fluctuations and dynamics resulting from a given liquid configuration. It is the latter, instantaneous, friction that we shall want to decompose into molecular ingredients in subsequent papers. However, even here we note that it is the character of this instantaneous friction that leads to the fluctuating force on a solute having slightly, but measurably, non-Gaussian statistics. Our basic approach to liquid-state friction and a number of results are illustrated for the special case of the vibrational relaxation of a diatomic molecule in an atomic liquid.
Collinear (He,HD+) and (He,DH+) collisions: Transition state resonances and dynamics by time-dependent quantal wave packet approach
Authors: Mahapatra S. , Sathyamurthy N.
Published: July 2002

We examine the possibility of existence of dynamical resonances in collinear (He,HD+) and (He,DH+) collisions by analyzing their respective transition state spectra on the McLaughlin-Thompson-Joseph-Sathyamurthy potential energy surface (PES). The spectra are computed by Fourier transforming the temporal autocorrelation function, C(t), of the initial wave packet. Some of the well resolved resonances are analyzed by computing their eigenfunctions and lifetimes. The vibrational state (v) - selected energy resolved reaction probabilities (PRv(E)) for collinear (He,HD+) and (He,DH+) collisions are also calculated on the same PES by computing the reactive flux in the product channel. The PRv(E) values for HeHD+ show a characteristic staircase-like structure that can be related to threshold resonances. The PRv(E) values for HeDH+ on the other hand, are highly oscillatory, in keeping with the densely packed transition state spectrum.
A position dependent friction model for solution reactions in the high friction regime: Proton transfer in triosephosphate isomerase (TIM)
Authors: Neria Eyal , Karplus Martin
Published: July 2002

A position dependent friction model based on Grote–Hynes theory is developed to describe activated rate processes in the high friction regime. The model is employed to determine the transmission coefficient, which corrects the transition state theory rate constant for recrossing of the transition state. A simple expression is derived for the transmission coefficient in the limit of a slow response of the thermal bath. The model is applied to the initial proton transfer step in the reaction catalyzed by triosephosphate isomerase, for which the standard Grote–Hynes theory was found to be inappropriate. The predictions of the position dependent friction model are in a good agreement with results of detailed molecular dynamics simulations. The method used to determine the transmission coefficient should be generally applicable to reactions that are strongly coupled to a slow thermal bath.
Classical studies of shock wave-induced desorption for model adsorbates
Authors: Dzegilenko Fedor N. , Uzer Turgay , Herbst Eric
Published: July 2002

The classical trajectory approach has been used to study the shock wave-induced desorption of a model triatomic linear adsorbate from a variety of model surfaces to which it is weakly adsorbed. The branching fraction among intact adsorbate desorption, fragmentation, and desorption with fragmentation has been analyzed as a function of shock wave energy, Debye frequency of the lattice, and the frequency and dissociation energy of the weak terminal interadsorbate bond. In general, fragmentation (with or without desorption) was observed to dominate over intact desorption. The results are explained partially in terms of resonances between low frequency adsorbate modes and the Debye frequency of the lattice. In addition, no evidence was found for a bottleneck in intact desorption which would allow the desorbing molecule to remain internally cold.
Molecular quadrupole moment function of ammonia
Authors: Piecuch P. , Špirko V. , Paldus J.
Published: July 2002

The zz component of the molecular quadrupole moment Θ of ammonia is computed at the restricted Hartree–Fock and coupled cluster theory with singles and doubles (CCSD) levels of approximation as a function of the symmetric stretching and inversion motions, considering a wide range of values for the corresponding vibrational coordinates (z is the threefold symmetry axis). Using these results and the nonrigid invertor rovibrational wave functions, effective molecular quadrupole moments are obtained for the low lying roinversional states of 14NH3. The predicted values exhibit a sizable and nonmonotonic dependence on the corresponding inversional (v2) and rotational (J,k) quantum numbers. The calculated effective quadrupole moments for the vibrational (inversional) ground states (v2=0±) are found to be about 10% smaller in absolute value than the two available experimental results [−2.42±0.04 a.u.; J. G. C. M. van Duijneveldt-van de Rijdt and F. B. van Duijneveldt, J. Mol. Struct. (THEOCHEM) 89, 185 (1982) and −2.45±0.3 a.u.; S. G. Kukolich, Chem. Phys. Lett. 5, 401 (1970); 12, 216 (1971) (E)]. Our CCSD value of the quadrupole moment pertaining to the equilibrium geometry is in a very close agreement with the best theoretical values that are available in the literature. This fact and the high quality of the roinversional wave functions that were employed lead us to believe that our theoretical predictions may be more accurate than their experimental counterparts. In particular, the difference between the theoretical and experimental values casts doubt on the reliability of the error bars (±0.04 a.u.) in one of the experimental results quoted above.
Quasiperiodicity in a detailed model of the peroxidase–oxidase reaction
Authors: Bronnikova T. V. , Schaffer W. M. , Olsen Lars F.
Published: July 2002

Quasiperiodicity in models of the peroxidase–oxidase reaction has previously been reported in ‘‘abstract’’ or phenomenological models which sacrifice chemical realism for tractability. In the present paper, we discuss how such behavior can arise in a detailed model (BFSO) of the reaction which has previously been shown to be consistent with experimental findings. We distinguish two types of quasiperiodic behavior. Regions of what we here refer to as ‘‘primary’’ quasiperiodicity are delimited by supercritical secondary Hopf bifurcations at one end of the relevant range of parameter values and by heteroclinic transitions at the other. Regions of so-called ‘‘secondary quasiperiodicity’’ are delimited by supercritical Hopf bifurcations at both ends of the parameter range. The existence of a quasiperiodic route to chaos in a modified version of BFSO is also described. This paper emphasizes the experimental circumstances under which quasiperiodic dynamics may be detected in the lab and offers specific prescriptions for its observation.
Polyelectrolyte hydrogel instabilities in ionic solutions
Authors: English Anthony E. , Tanaka Toyoichi , Edelman Elazer R.
Published: July 2002

The phase behavior of polyelectrolyte hydrogels has been examined as a function of relative charge composition, bath salt concentration, and solvent quality. Nonlinear swelling instabilities of 2-hydroxyethyl methacrylate (HEMA) and methacrylic acid (MAAc) copolymer hydrogels manifested themselves as discontinuous first order swelling transitions as a function of bath salt concentration. A modified Flory–Huggins model was used to describe the regions of instability when bath salt concentration and solvent quality are considered as control variables. The role of ion dissociation equilibrium in the change from local or smooth transitions to nonlocal or discontinuous swelling transitions is illustrated within the framework of our model.
Detection of DCl by multiphoton ionization and determination of DCl and HCl internal state distributions
Authors: Dagdigian Paul J. , Varley David F. , Liyanage Rohana , Gordon Robert J. , Field Robert W.
Published: July 2002

A study of the 2+1 resonantly enhanced multiphoton ionization (REMPI) spectrum of DCl is reported. Transition energies for excitation of the F 1Δ–X 1Σ+ (0,0) and (1,0) bands, as well as the V 1Σ+–X 1Σ+(v′,0) bands, for v′=15–19, are presented. The derived molecular constants for the F–X (0,0) and the V–X bands agree well with those previously obtained from analysis of the one-photon VUV absorption spectrum [A. E. Douglas and F. R. Greening, Can. J. Phys. 57, 1650 (1979)]. The ion signals for excitation through various rotational lines in the E–X (0,0) and F–X (0,0) and (1,0) bands are compared with theoretical two-photon line strengths. Extensive power- and J′-dependent ion fragmentation is observed for the former band. No fragmentation is observed in the F–X bands; however, the ion signal strengths are found to vary strongly with J′. This variation of REMPI signal strengths vs J′ was shown to be due to an indirect predissociation, as in HCl. Tables of experimental line strength factors for the F–X (0,0) and (1,0) bands of HCl and DCl are reported. Finally, the relative REMPI detection sensitivities for HCl and DCl, through their respective F–X (0,0) R(1) lines, are compared.
Frequency-selective heteronuclear dephasing by dipole couplings in spinning and static solids
Authors: Bennett Andrew E. , Rienstra Chad M. , Lansbury Peter T. , Griffin Robert G.
Published: July 2002

A compensated pulse sequence for the spectrally selective reintroduction of heteronuclear dipole–dipole interactions (frequency-selective dipolar recoupling) into solid state magic angle spinning (MAS) nuclear magnetic resonance (NMR) experiments is described and shown to provide frequency-selective dipolar dephasing in weakly coupled spin systems. The experimental dipolar spin evolution is interpreted via analytical and numerical calculations, which include a simple model for the observed losses of spin coherence in the multiple pulse experiments. In the peptide glycylglycine, the selective dipolar evolution of two spins is observed while the influence of larger internuclear couplings is suppressed. This approach is aimed at obtaining several quantitative internuclear distances independently in dipolar ‘‘recoupling’’ MAS experiments by reducing multiple spin effects in the observed dipolar evolution. Similar frequency-selective dephasing experiments are also introduced for static solids, where an efficient application to measuring relative tensor orientations in powdered samples is demonstrated.
Calculation of the higher-order frequency-dependent polarizabilities using the frequency-dependent moment method
Authors: Nomura Yasushi , Miura Shouhei , Fukunaga Migifumi , Narita Susumu , Shibuya Tai-lichi
Published: July 2002

The frequency-dependent moment (FDM) method proposed by Iwata [Chem. Phys. Lett. 102, 544 (1983)] has been applied to the calculation of the second- and third-order frequency-dependent polarizabilities. It is demonstrated that the FDM ab initio calculations on the HF molecule give the same numerical results as the sum-over-state (SOS) calculations. It is important that the FDM requires only the ground-state eigensolution of the CI matrix while the SOS requires all the eigensolutions. Calculations were also made on H2O and CH4. The FDM method becomes very effective when the CI matrix is so large that its direct diagonalization becomes formidable.
Magnetic shielding surface in molecules. Neutron as a probe in the hypothetical magnetic resonance spectroscopy
Authors: Wolinski Krzysztof
Published: July 2002

It is proposed to analyze the magnetic shielding tensor as a continuous function of space point coordinates in atoms and molecules. Such a three-dimensional magnetic shielding surface should be very helpful for better understanding of nuclear magnetic shieldings, which is not a trivial problem. The magnetic shielding surface provides more information about molecular electronic structure than nuclear magnetic shieldings alone. A connection between magnetic shielding at an arbitrary point and nuclear magnetic shielding can be made by “performing” a hypothetical NMR experiment using a neutron as a virtual probe, instead of nuclei with nonzero magnetic moments. The theory is illustrated by the numerical results of the magnetic shielding surface calculations for atoms and small linear molecules.
Photodissociation dynamics of the chloromethanes at the Lyman-α wavelength (121.6 nm)
Authors: Brownsword R. A. , Hillenkamp M. , Laurent T. , Vatsa R. K. , Volpp H.-R. , Wolfrum J.
Published: July 2002

The gas-phase dissociation dynamics of CH3Cl, CH2Cl2, and CHCl3 after photoexcitation at the Lyman-α wavelength (121.6 nm) were studied under collision-free conditions at room temperature. Narrow-band tunable Lyman-α laser radiation (λLα≈121.6 nm) was generated by resonant third-order sum-difference frequency conversion of pulsed-dye-laser radiation and used both to photodissociate the parent molecules and to detect the nascent H atom products via (2p2P←1s2S) laser induced fluorescence. Absolute H atom quantum yields ΦH(CH3Cl)=(0.53±0.05), ΦH(CH2Cl2)=(0.28±0.03), and ΦH(CHCl3)=(0.23±0.03) were determined employing a photolytic calibration method where the Lyman-α photolysis of H2O was used as a reference source of well-defined H atom concentrations. H atom Doppler profiles were measured for all chlorinated methanes. In the case of CH3Cl the line shapes of the profiles indicate a pronounced bimodal translational energy distribution suggesting the presence of two H atom formation mechanisms leading to a markedly different H atom translational energy release. The observed “slow” component of the H atom translational energy distribution corresponds to an average kinetic energy of (55±5) kJ/mol, while the “fast” component leads to an average kinetic energy of (320±17) kJ/mol. The relative branching ratio between the “fast” and the “slow” H atom channel was determined to be (0.71±0.15). For CH2Cl2 and CHCl3 no bimodal translational energy distributions were observed. Here the translational energy distributions could each be well described by a single Maxwell–Boltzmann distribution, corresponding to an average translational energy of (81±9) kJ/mol and (75±4) kJ/mol, respectively.
Hydration and Raman scattering studies of levitated microparticles: Ba(NO3)2, Sr(NO3)2, and Ca(NO3)2
Authors: Tang I. N. , Fung K. H.
Published: July 2002

The phase transformation and hydration of inorganic salt particles composed of alkaline earth metal nitrates, Ba(NO3)2, Sr(NO3)2, and Ca(NO3)2, are investigated in a quadrupole cell, in which a levitated single microparticle is in dynamic equilibrium with water vapor under controlled humidity conditions. Laser Raman and Mie scattering techniques are used to probe the chemical and physical states of the microparticle before and after phase transformation. Because of the high degree of supersaturation that only a suspended solution droplet can attain before solidifying, metastable states not predicted from bulk solution thermodynamics often result. Thus it is found that, except for Ba(NO3)2 particles which form the stable anhydrous crystalline state upon efflorescence, Sr(NO3)2 and Ca(NO3)2 droplets solidify to a metastable amorphous state that contains residual water persisting even in high vacuum. Raman spectra of the amorphous particles reveal that the nitrate ions form contact-ion pairs with the bivalent cations and that the residual water molecules possess very little, if any, hydrogen bond characteristics. The deliquescence properties of the amorphous particles, which are found to be different from those expected of the bulk crystalline states, are also reported.
New assignments in the UV spectroscopy of the small benzene–argonn clusters: The effects of a structure-selective vibrational predissociation
Authors: Mons M. , Courty A. , Schmidt M. , Le Calvé J. , Piuzzi F. , Dimicoli I.
Published: July 2002

In this paper, new experimental results using UV spectroscopy of the small benzene–Arn clusters are presented. We have found evidence for the vibrational predissociation of the S1 state of some of these species on a nanosecond time scale and we propose a new assignment for the UV spectral features of this system. This assignment is consistent with other experimental data and it accounts additionally for the previously reported spectral anomalies, in particular the ionization potential measurements of these species. The two-color R2PI spectra performed can thus be considered as nanosecond pump–probe experiments that allow us to estimate the relaxation rate of the S1state of these clusters. The vibrational relaxation is found to be strongly size and structure dependent: The two isomers [(1/1) and (2/0)] of the n=2 species exhibit different lifetimes differing by at least one order of magnitude. The size dependence of the relaxation process within the one-sided isomer series appears also paradoxical since the relaxation rates of the (3/0) and (4/0) species are found to be smaller than that of the (2/0). These properties are discussed in relationship with the symmetry of these species.
Scattering of fast N2 from Pd(111): A classical trajectory study
Authors: Schlathölter Thomas , Schlathölter Thorsten , Vicanek Martin , Heiland Werner
Published: July 2002

Molecular nitrogen is well known for its chemical inactivity. Experimental results for grazing incidence N2 scattering from Pd(111) surfaces in the keV range also reveal negligible influences of electronical processes on molecular fragmentation. Therefore, we carry out an appropriate classical treatment of this system. The N2–Pd(111) interaction is mediated by an analytical six-dimensional potential energy surface, based on ab initio density-functional-theory calculations and empirical data. The molecule-surface interaction seems to be strongly influenced by the azimuthal direction of incidence as well as the molecular axis orientation. Particularly, the fragmentation is found to be mainly due to vibrational excitation for highly indexed azimuthal directions, whereas for incidence along lowly indexed directions rotational excitation is more important.
Guided ion beam studies of the reactions of Fen+ (n=2–18) with O2: Iron cluster oxide and dioxide bond energies
Authors: Griffin James B. , Armentrout P. B.
Published: July 2002

The kinetic energy dependence of the reactions of Fen+ (n=2–18) with O2 are studied in a guided ion beam mass spectrometer. A variety of FemO2+, FemO+, and Fem+, where m⩽n, product ions are observed, with the dioxide cluster ions dominating the products for all larger reactant cluster ions. Reaction efficiencies are near unity for all but the smallest clusters. The energy dependence of the products is analyzed in several different ways to determine thermochemistry for both the first and second oxygen atom binding to iron cluster ions. The trends in this thermochemistry are discussed and compared to bulk phase values.
A model study of quantum dot polarizability calculations using time-dependent density functional methods
Authors: Bandy Terese , Haug Kenneth
Published: July 2002

We compare two time-dependent methods (time-dependent Hartree and time-dependent density functional methods) with a time-independent density functional method for the calculation of the frequency dependent polarizability and resulting absorption spectrum of two interacting quantum confined particles (quantum dots). The system is examined within the dipole approximation and the methods are evaluated in terms of the optical absorption spectrum. The spectral noise generated by time-dependent methods is a sensitive measure of the degree of broken correlation between the quantum degrees of freedom and the time-dependent density functional method may help to quantify the efficacy of correlation-exchange potentials that are used in density functional models. With respect to the quantum confinement issue, we find that increasing the interaction energy between nearest neighbor quantum dot sites represented in our model tends to shift absorption intensity to higher energy transitions.
Photoinduced charge-transfer dissociation in van der Waals complexes. V. Na⋯XCH3 (X=F, Cl, and Br)
Authors: Chang X. Y. , Ehlich R. , Hudson A. J. , Polanyi J. C. , Wang J.-X.
Published: July 2002

van der Waals complexes Na⋯XCH3 (X=F, Cl, and Br) have been generated by crossing a beam of sodium with the expansion region of a supersonic jet of the appropriate halide, seeded with a rare gas. The identity of these complexes was determined by photoionization time-of-flight mass spectrometry. The primary route for photodepletion of these complexes is thought to be the excitation of the Na chromophore followed by a charge-transfer dissociation: Na⋯XCH3+hν→[Na*⋯XCH3]‡→[Na+⋯XCH3−]‡→NaX+CH3. Measurement of the photo-depletion cross section as a function of the excitation wavelength provided an approach to the study of these harpooning reactions starting in selected transition states. The action spectra for the three complexes consisted of up to four broad peaks. An assignment, made by ab initio calculations, was based on the electronically excited states of Na* perturbed by the halide molecule in the complex. Peaks, ranging from the red (∼700 nm) to the blue (∼400 nm), were assigned to a superposition of Na* states 3 2Px,y, and to successively higher excited states 3 2Pz, 4 2S, and 3 2D. The transition probabilities computed for the various Na⋯XCH3→[Na*⋯XCH3]‡ transitions were generally in qualitative accord with experiment. Vibrational progressions of the covalent excited states 3 2Px,y,z were observed and analyzed.
Rotationally resolved energy-dispersive photoelectron spectroscopy of H2O: Photoionization of the C̃(0,0,0) state at 355 nm
Authors: Glab W. L. , Glynn P. T. , Dehmer P. M. , Dehmer J. L. , Wang Kwanghsi , McKoy B. V.
Published: July 2002

Measured and calculated rotationally resolved photoelectron spectra for photoionization of low rotational levels of the C̃ 1B1 Rydberg state of water are reported. This is the first example of rotationally resolved photoionization spectra beyond the special cases of H2, high-J levels, and threshold spectra. These spectra reveal very nonatomiclike behavior and, surprisingly, the influence of multiple Cooper minima in the photoelectron matrix elements.
Unimolecular dissociation dynamics of highly vibrationally excited DCO(X̃ 2A). I. Investigation of dissociative resonance states by stimulated emission pumping spectroscopy
Authors: Stöck C. , Li Xiaonong , Keller H.-M. , Schinke R. , Temps F.
Published: July 2002

The vibrational level structure and unimolecular dissociation dynamics of highly vibrationally excited X̃ 2A DCO were investigated using the method of stimulated emission pumping spectroscopy (SEP). Single vibration-rotation states were probed with excitation energies up to E(X̃)=18 200 cm−1, ≈12 700 cm−1 above the asymptotic D-CO dissociation limit. The vibrational level structure of the molecule was found to be determined by distinctive polyads arising from a 1:1:2 resonance between the CD stretching, CO stretching, and DCO bending vibrations. Anharmonic coupling mechanisms give rise to considerable level mixings, especially regarding the CD and CO stretching motion. Thus, only a minority of vibrational states can be unambiguously assigned. The spectral line shape profiles of ≈100 highly excited “resonance states” in the continuum above the D-CO dissociation limit were measured at high resolution. The profiles are homogeneously broadened. The unimolecular decay rates, obtained from the observed line widths, were observed to fluctuate by more than two orders of magnitude in a strikingly state specific manner. The decay rates on average increase with increasing vibrational excitation energy. The state resolved experimental data are compared to predictions of the microcanonical specific unimolecular rate coefficients calculated from different statistical models. Serious problems were encountered considering the calculation of the density of states of the molecules in the continuum region of the potential energy surface regarding the contribution of the disappearing oscillator. Despite tentative corrections, the calculated rate coefficients were to too high by one to two orders of magnitude. Overall, the unimolecular dynamics of DCO appears to conform to an intermediate case between the strictly vibrationally “mode specific” and the “statistical” limits.
Two-dimensional photoelectron spectroscopy of acetylene: Rydberg-valence interaction between the (3σg)−1(3pσu)1 and (3σg)−1(3σu)1 states
Authors: Hattori Hideo , Hikosaka Yasumasa , Hikida Takumi , Mitsuke Koichiro
Published: July 2002

Two-dimensional photoelectron spectroscopy is performed for studying autoionization of acetylene in the Franck–Condon gap between the X 2Πu and A 2Ag states of C2H2+. The photoelectron spectrum in the photon energy range from 12.8 to 13.6 eV shows exclusive vibrational excitation of the symmetric C–H stretching mode ν1 of C2H2+(X 2Πu), which results from autoionization of the valence state (3σg)−1(3σu)1. Vibrational frequencies with anharmonicities of the ν1 and ν2 (the symmetric C–C stretch) modes are determined by a least-squares fit of the ionization energies of the observed peaks to a second order expansion. At the photon energy of 14.120 eV, autoionization of the Rydberg state (3σg)−1(3pπu)1 leads to a complicated photoelectron spectrum where probably the trans-bending mode ν4 of C2H2+(X 2Πu) as well as ν1 is excited, reflecting a substantial geometrical change during autoionization. Furthermore, a similar excitation of the ν4 mode is observed at ∼13.8 eV. An excellent agreement in positions of the vibrational levels between the spectra at 13.821 and 14.120 eV suggests the presence of the Rydberg state (3σg)−1(3pσu)1 at ∼13.8 eV which has not been identified previously in the photoabsorption or photoionization cross section curves. The constant-ionic-state spectra for the ν1=0–4 levels of C2H2+(X 2Πu) show two spectral features: (a) a weak shoulder (v1=0) or a small maximum (ν1=1–4) at 13.8 eV and (b) two groups of peaks in the range of 14.0–14.4 eV. The ratio of the integrated intensity of the 13.8 eV maximum to that of the two groups differs from level to level. This observation is interpreted in terms of a strong interaction between the Rydberg (3σg)−1(3pσu)1 and valence (3σg)−1(3σu)1 states.
First-order intermolecular diatomics-in-molecule potentials. Potential energy surfaces, spectra, and fragmentation dynamics of the Ne⋯Cl2 complex
Authors: Buchachenko A. A. , Stepanov N. F.
Published: July 2002

First-order perturbative approximations to the diatomics-in-molecule (DIM) approach are implemented for studying interactions between the neon atom and chlorine molecule in the X 1Σg+(0+) and B 3Πu(0+) states. Intermolecular DIM perturbation theory (IDIM PT1) [J. Chem. Phys. 104, 9913 (1996)], which accounts for the atomic component of spin-orbit interaction, is compared to the anisotropic model by Naumkin and Knowles [J. Chem. Phys. 103, 3392 (1995)] which is proven to be a first-order approximation to the nonrelativistic DIM approach. An importance of the spin-orbit effects for the ground-state potential energy surface (PES) is demonstrated. Semiempirical PESs are used in the accurate quantum calculations on the vibrationally averaged geometry, B←X vibronic spectra, and vibrational predissociation dynamics of the Ne⋯Cl2 van der Waals complex. The IDIM PT1 model is shown to provide good agreement with available experimental data. The effects of interaction potential topology on the spectroscopic and dynamic properties of the complex and the relation of DIM-based PESs to the results of ab initio calculations are discussed.
Acoustic wave enhancement of the catalytic oxidation of carbon monoxide over Pt{110}
Authors: Kelling S. , Mitrelias T. , Matsumoto Y. , Ostanin V. P. , King D. A.
Published: July 2002

Clear evidence for the nonthermal acoustic wave enhancement of a catalytic reaction over a single crystal metal surface under controlled conditions is provided in this Letter. Data demonstrating a fourfold increase in the rate of CO oxidation over a thin Pt{110} catalyst bonded onto the surface of a piezoelectric interdigital transducer is discussed here together with a possible mechanism for this effect.
Ab initio simulation of molecular Auger spectra: Nuclear dynamics effects in the spectra of carbonyl sulfide
Authors: Minelli D. , Tarantelli F. , Sgamellotti A. , Cederbaum L. S.
Published: July 2002

A theoretical ab initio simulation of the carbon and oxygen KLL and sulfur LMM Auger spectra of carbonyl sulfide is presented and discussed. The underlying vertical double ionization spectrum is computed using a Green’s function method; the Auger intensity distributions are estimated via a two-hole population analysis of the eigenvectors and the main effects of nuclear vibrational motion on the energy position and broadening of the Auger bands are taken into account. The simulation yields accurate spectra, revealing the important role played by the nuclear dynamics effects at the origin of the very different shapes of the three spectra. These effects are particularly striking in the sulfur LMM spectrum, which is additionally characterized by evident spin–orbit coupling in the decaying state.
Dinitrosyl intermediate for N2O formation from reaction of NO on Mo(110)
Authors: Queeney K. T. , Friend C. M.
Published: July 2002

The adsorption and subsequent reaction of nitric oxide (NO) on Mo(110) has been studied by temperature programmed reaction, electron energy loss, and infrared reflectance absorbance spectroscopies. The predominant reaction pathway for a saturated NO overlayer is dissociation to atomic nitrogen and oxygen; in fact, for low NO coverages, dissociation is the only reaction and largely takes place below 300 K. At NO coverages above 65% of θsat, evolution of N2O, N2, and NO is also observed at low temperature. Temperature programmed reaction of isotopically mixed overlayers demonstrates that N2O formation occurs via reaction of two intact NO molecules, suggestive of a dimeric surface intermediate. Electron energy loss and infrared spectroscopies identify three ν(NO) features which are assigned to three distinct species; the frequencies of the ν(NO) peaks in the infrared spectrum of a saturated 14NO overlayer at 100 K are 1860, 1821, and 1720 cm−1. The 1860 and 1720 cm−1 features are assigned to monomeric NO. Based on the infrared spectrum of a mixed overlayer of 14NO and 15NO, and on comparison to reference spectra of NO adsorbed on MoO3, the 1821 cm−1 peak is identified as νs(NO) of a surface dinitrosyl complex, i.e., two NO molecules bound to one Mo site. The weakly bound NO with the stretch frequency of 1860 cm−1 is associated with NO desorption, while the dinitrosyl leads to low-temperature N2 and N2O formation and some dissociation.
Energies and derivative couplings in the vicinity of a conical intersection. II. CH2(2 3A″,3 3A″) and H2S(1 1A″,2 1A″), unexpected results in an ostensibly standard case
Authors: Matsunaga Nikita , Yarkony David R.
Published: July 2002

The 2 3A′′−3 3A′′ and the 1 1A′′−2 1A′′ seams of conical intersection in CH2 and H2S, respectively, are considered. The nuclear coordinate dependence of the seam of conical intersection, the energy of the lower adiabatic potential energy surface along closed loops containing the conical intersection, and the nonremovable part of the derivative coupling in the region contained within the closed loops are studied. The energetics and derivative couplings in the vicinity of the conical intersections are analyzed in terms of the characteristic parameters of a conical intersection, determined at the configuration interaction level using analytic gradient techniques. The characteristic parameters are found to predict, in a qualitative manner, the energetics at moderate distances from the conical intersection. Loops containing the conical intersection that exhibit and do not exhibit the geometric phase effect are considered. An unusual trifurcation of the C2v seam of conical intersection in CH2 into a C2v branch and 2Cs branches is discovered. For H2S, over the range of nuclear coordinates considered, only points of conical intersection with C2v symmetry were found.
Reactant-product decoupling approach to half-scattering problems: Photodissociation of H2O in three dimensions
Authors: Wang Dunyou , Zhu Wei , Zhang John Z. H. , Kouri Donald J.
Published: July 2002

In this paper, we present the RPD (reactant-product decoupling) approach to the calculation of final-state distribution in photodissociation of H2O in three-dimensional space. Although the RPD approach was recently developed for bimolecular state-to-state reactive scattering calculations, its application to photodissociation dynamics is very attractive. Specifically in photodissociation, the interaction (reactant) component wavefunction ψr (which in the present case of photodissociation is replaced by the interaction component ψint) is nonzero only in the strong interaction region, which greatly simplifies the numerical calculation for ψint in comparison to that for ψr in a full bimolecular reactive scattering calculation. In the following report, the time-dependent implementation of the RPD approach to the photodissociation of H2O in three dimensions is given and the calculated rovibrational state distributions of the OH fragment are presented.
Semiclassical initial value theory for dissociation dynamics
Authors: Campolieti G. , Brumer Paul
Published: July 2002

A time-dependent initial value semiclassical propagator is developed and applied to dissociation dynamics. Numerically implementable formulas are given for computing detailed dissociation dynamics and photofragmentation matrix elements. The method is applied to the study of two- and three-dimensional HOH/HOD photodissociation in the Ã state. In the two-dimensional case, results obtained by a grid-based numerical integration method using relatively few classical trajectories show very good agreement with known quantum results. The three-dimensional study uses a stationary-phase Monte Carlo approach to computing dissociation cross sections. In this case a comparison with exact quantum calculations shows only qualitative agreement.
Light scattering properties of paramagnetic particles
Authors: Du Y.-B. , Tong P.
Published: July 2002

We present an experimental study of light scattering properties of paramagnetic particles. To account for the magnetic dipole radiation and the Brownian motion of the particles in a thermal equilibrium solution, we calculate the scattering intensity and its auto-correlation function g(t). Experimentally, we examine the scattering properties of the paramagnetic particles and compare the results with those from isotropic and anisotropic dielectric particles. The experiment verifies the calculation and reveals that the magnetic dipole radiation of the paramagnetic particles is unusually large and equals to approximately 1/3 of the electric dipole radiation of the particles. Dynamic light scattering measurements show that the measured g(t) for the depolarized scattering is strongly influenced by the size distribution of the particles. This is because the large paramagnetic particles tend to have more magnetite content and hence weigh more in the depolarized scattering. With a simple sedimentation method, we are able to separate the particles of different sizes and obtain relatively monodispersed scattering samples. These samples give the expected translational and rotational diffusion constants of the particles.
Comparison of the Smoluchowski approach with modern alternative approaches to diffusion-influenced fluorescence quenching: The effect of intense excitation pulses
Authors: Naumann Wolfgang , Szabo Attila
Published: July 2002

For low intensity excitation pulses, the fluorescence intensity in the presence of quenchers is obtained, within the framework of the Smoluchowski approach, by convoluting the pulse profile with the survival probability of an excited fluorophore initially surrounded by an equilibrium distribution of quenchers. This conventional approach is generalized to handle excitation pulses of arbitrary intensity. The resulting expression is exact in the limit that the fluorophore is static and the quenchers diffuse independently. Modern alternative approaches to this problem are based either explicitely or implicitely on truncating the reduced many-particle distribution function hierarchy by means of a superposition approximation. For a delta-function excitation pulse all approaches yield the identical result. For arbitrary pulses, the modern approaches predict different, albeit numerically similar, results. This difference, however, does not constitute an improvement over the Smoluchowski approach. Rather, it is a reflexion of an additional approximation that is made in the modern approaches.
A study of the predissociation of NaK molecules in the 6 1Σ+ state by optical–optical double resonance spectroscopy
Authors: Jabbour Z. J. , Huennekens J.
Published: July 2002

Predissociation of a high-lying Σ+1 state of NaK is studied using the optical–optical double resonance technique. A single-mode ring dye laser is set to a particular 2(A)1Σ+(v,J)←1(X)1Σ+(v′′,J′′) transition. Another single-mode laser (Ti–sapphire) is then used to excite the molecule from the 2(A)1Σ+(v,J) level, to rovibrational levels of a higher predissociating electronic state, which we identify as 6 1Σ+. The predissociation is monitored by the atomic potassium emission on the 3 2D3/2→4 2P1/2 transition at 1.17 μm, while bound state radiative processes are monitored by total violet fluorescence from the upper state to the various rovibrational levels of the ground 1(X)1Σ+ state. By scanning the Ti–sapphire laser, different rovibrational levels of the 6 1Σ+ state can be excited. The vibrational levels probed range from v=13 to 20 with rotational states ranging from 9 to 99. The bound state energy level positions are measured from the center frequencies of lines recorded with the Ti–sapphire laser excitation scans. The 6 1Σ+ state is then described by the following molecular constants which are calculated from the experimental values of the level energies: Te=25 560.373 cm−1, ωe=89.179 26 cm−1, ωexe=0.730 691 cm−1, Be=0.067 327 0 cm−1, αe=0.000 675 35 cm−1, De=−3.298 31×10−8  cm−1, βel=1.518 17×10−8  cm−1. The potential well depth is De=4416.0 cm−1, if we assume the most likely asymptotic limit of Na(3 2S1/2)+K(5 2P1/2). The equilibrium separation is Re=4.158 Å. We also report measured and calculated intensities (Franck–Condon factors) for the 6 1Σ+→1(X)1Σ+ violet band. The absolute predissociation rates of 6 1Σ+ levels are directly measured from the linewidths recorded on the Ti–sapphire laser excitation scans. We measure predissociation rates ranging up to 9.4×109 s−1. The dependence of the absolute predissociation rates on rovibrational quantum numbers is studied with an attempt to predict the shape of the repulsive potential curve causing the predissociation, its crossing point with the bound state, and the type of perturbative interaction leading to the predissociation. The state causing the predissociation is determined from correlation diagrams to be the continuum of either the 3 3Π, the 3 1Π, or the 5 3Σ+ state with Na(3S)+K(3D) dissociation limit. We measure the collisional broadening rate coefficients of some 6 1Σ+←2(A)1Σ+ lines due to both argon and potassium perturbers, and obtain the average values, kbrAr=(1.1±0.2)×10−8 cm3 s−1 and kbrK=(1.1±0.6)×10−8 cm3 s−1. Velocity-changing collisions and collisional excitation transfer between individual rotational levels of the 2(A)1Σ+ state are also investigated.
Impurity rotations in quantum versus classical solids: O2 in solid hydrogens
Authors: Li Z. , Apkarian V. A.
Published: July 2002

Molecular dynamics simulations based on pseudopotentials are used to characterize the difference between impurity rotations in classical versus quantum solids. The method is first applied to the pure solids and demonstrated to faithfully reproduce static and dynamical properties, in the form of pair distributions and phonon density of states of solid H2(D2). Then the rotations of molecular oxygen in the ground X(3Σg−) and electronically excited state A(3Δu) is investigated. Where the substitutional impurity is small, O2(X), in the classical solid, the cavity remains nearly spherical and the molecule undergoes rotation-translation coupled motion. In contrast, in the quantum solid, the lattice locally distorts around the impurity and forces librations with occasional reorientational hops as rotation-distortion coupled motion. These effects are amplified in the excited O2(A) state, in which due to the larger molecular bond length, the angular anisotropy of the guest–host interaction is larger. Now, in the classical solid a small cage distortion forces the molecule into large amplitude librations. The molecule, however, reorients occasionally, when the lattice fluctuations lead to a nearly spherical cage geometry. In the quantum host, O2(A) becomes a strict librator, due to a large and permanent deformation of the soft cage. The results are used to rationalize experimental observations.