bluuebunny/crossref_metadata_2025 huggingface dataset:
The reactions of iron clusters with water
Authors: Weiller B. H. , Bechthold P. S. , Parks E. K. , Pobo L. G. , Riley S. J.
Published: July 2002

The reactions of neutral iron clusters Fe7–27 with water are studied in a laser-vaporization cluster source coupled to a continuous-flow reactor. Reaction products are detected via laser ionization and time-of-flight mass spectrometry. The reactions of room-temperature clusters with H2O show adsorbate decomposition and hydrogen desorption, as do the reactions with D2O at elevated temperatures. The room-temperature reaction with D2O appears not to involve any decomposition, and is at equilibrium under the conditions of these experiments. The dependence of reaction extent on D2O pressure yields equilibrium constants for the addition of the first and second D2O molecules. The analysis is complicated by the presence of two-photon ionization processes that are treated quantitatively with a rate-equation model. This treatment also yields estimates for cluster photoabsorption cross sections, which are found to be approximately linear in cluster size, having a magnitude of 2.3×10−17 cm2 per iron atom. From the derived equilibrium constants and estimated adsorption entropies, approximate D2O–cluster binding energies are determined. They range from 0.42 to 0.59 eV, and their dependence on cluster size shows a remarkable similarity to the dependence of the rate constants for reaction of iron clusters with H2. The implications of this similarity are discussed.
Structure and bonding in the B2H5 radical and cation
Authors: Ruščić B. , Schwarz M. , Berkowitz J.
Published: July 2002

The B2 H5 radical has been generated by the F+B2 H6 reaction, and studied by photoionization mass spectrometry. The photoion yield curve for B2H+5 (B2 H5 ) is extremely weak at the adiabatic threshold (∼6.945 eV), at least three orders of magnitude weaker than at its maximum (∼9.67 eV). This observation provides support for recent ab initio calculations, which predict a singly bridged B2 H5 and a triply bridged B2H+5 as ground states. Evidence is presented for the coexistence of a doubly bridged B2 H5 isomer, ∼3 kcal/mol higher in energy. From the appearance potential of B2H+3 (B2 H5 ), a B2 H5 –H bond energy of ≲102.7 kcal/mol is obtained. From the photoion yield curve of B2H+3 (B2 H5 ), at least one, and perhaps two, excited states of B2H+5 can be inferred. The lower one also branches into parent B2 H5, an apparent violation of QET.
Quantum chaos of Ar3: Statistics of eigenvalues
Authors: Leitner David M. , Berry R. Stephen , Whitnell Robert M.
Published: July 2002

The successive diagonalization–truncation method is applied to the calculation of the vibrational eigenvalues of the Ar trimer bound by pairwise Lennard-Jones potentials. The statistics of the eigenvalues reveal strongly chaotic behavior of the cluster, consistent with the classical dynamics studies. Moreover, the zero-point energy is higher than the highest energy at which regular dynamics were found classically, indicating that for all energies physically accessible to the cluster, the dynamics are chaotic.
Multireference and relativistic effects in NiH
Authors: Marian Christel M. , Blomberg Margareta R. A. , Siegbahn Per E. M.
Published: July 2002

Large multireference CI calculations have been performed for the ground state of NiH. The effects of relativity were investigated using both a variational (no pair) theory and perturbation theory. The largest CI calculations included up to 29 reference states and were performed at the contracted CI level. The calculated and experimental results (within parentheses) are for Re 2.76 a0 (2.76 a0), for ωe 1997 cm−1 (2003 cm−1) and for the dipole moment μ 2.32 D (2.4±0.1 D). The effects of relativity are −0.03 a0, +60 cm−1 and −0.3 D, respectively. The effects of going from a reference selection threshold of 0.05 to a selection threshold of 0.02 and a different set of CASSCF orbitals was before applying Davidson’s correction +0.07 a0 for Re and about +30 cm−1 for ωe, and after applying this correction +0.06 a0 and −80 cm−1, respectively. The most accurate results for the dipole moment was obtained using the multireference ACPF method.
Rotational population and alignment distributions for inelastic scattering and trapping/desorption of NO on Pt(111)
Authors: Jacobs D. C. , Kolasinski K. W. , Shane S. F. , Zare R. N.
Published: July 2002

Rotationally resolved experiments on the NO/Pt(111) system explore the mechanisms of inelastic scattering and trapping/desorption. The rotational dynamics associated with these two regimes are markedly different. A neat supersonic NO beam is scattered at normal incidence from a Pt(111) crystal at 375–475 K. The non-Boltzmann rotational population distribution of the scattered species exhibits considerable rotational excitation beyond the energy available from the incident beam. Thus, a surface vibration to rotational energy transfer mechanism must be operative. The accompanying rotational alignment data reveal that highly excited rotational states exhibit predominantly ‘‘cartwheel’’ motion. In contrast, rotationally excited molecules that desorb from a 553 K Pt(111) surface show a preference for ‘‘helicopter’’ motion. The opposite preferences for rotational alignment in the two dynamical regimes provide insight into the anisotropy of molecule–surface interactions.
Measurement of the viscoelastic properties of silver metal liquid-like films by laser light scattering
Authors: Yogev D. , Kuo C. H. , Neuman R. D. , Efrima S.
Published: July 2002

The rheological properties of silver metal liquid-like films, trapped between two immiscible liquid phases, were investigated by measuring laser light scattering from the interfacial capillary waves using optical heterodyne mixing spectroscopy. All the films studied were found to have small elasticities in the range 0.27 to 1.0 dyn/cm. These values are consistent with a colloidal model of the structure of the films with diffusional processes possibly short circuiting the interfacial stresses. It also indicates a gaseous state, consistent with the charge of the surfactants and the silver colloidal particles, and the strong effect of perfluorosurfactants.
Exchange striction in the Mn2 molecule
Authors: Cheeseman M. , Van Zee R. J. , Weltner W.
Published: July 2002

Electron spin resonance evidence is presented for exchange striction in the antiferromagnetic Mn2 molecule, that is, lengthening of the interatomic distance in the spin states as S increases, 0≤S≤5.
Second-order corrections to transport coefficients of binary gaseous mixtures: N2 with He, Ne, and Ar
Authors: Gianturco F. A. , Venanzi M.
Published: July 2002

The usual kinetic theory of dilute gaseous mixtures is employed, via the infinite-order sudden (IOS) decoupling scheme for the dynamics, to obtain generalized collision integrals and transport properties for molecular mixtures containing N2 and various rare gases (He, Ne, and Ar). The relevant interaction potentials originate from a previous study [F. A. Gianturco, M. Venanzi, and A. S. Dickinson, Mol. Phys. 65, 563 (1988)] of the above systems in which a multiproperty analysis of their quality and reliability was attempted by combining scattering data, when available, with diffusion coefficients and interaction viscosity data to select the most accurate of the potential energy surfaces (PES) at hand. The extension of the calculations to second-order effects allows us to test the importance of such corrections on diffusion and viscosity coefficients, to follow their dependence on the mole fraction of the mixtures, to obtain thermal diffusion and thermal conductivity results and to futher assess the feasibility of multiproperty tests of simple atom–diatom interactions via the calculation of a broader range of transport coefficients.
Multiplex coherent anti-Stokes Raman spectroscopy study of infrared-multiphoton-excited OCS
Authors: Chen Kuei-Hsien , Lü Cheng-Zai , Avilés Luis A. , Mazur Eric , Bloembergen Nicolaas , Shultz Mary J.
Published: July 2002

The vibrational energy distribution following ν2 overtone excitation of OCS by a pulsed CO2 laser is studied by monitoring the coherent anti-stokes Raman spectrum of the ν1 mode. Because of the slow energy transfer from the pumped mode to other modes, and because the anharmonicity of the ν2 mode is small, OCS is an ideal system for studying the interaction of an intense infrared laser field with a single, nearly harmonic, oscillator. From the spectra the cross anharmonicities of the ν1 mode are determined to be x12=−6.0 cm−1 and x13=−2.7 cm−1, respectively. The time dependence of the spectra provides information on V–V energy transfer rates. In particular, the measurements put a lower limit of kν2→ν2=1 μs−1 Torr−1 on the vibrational relaxation rate within ν2 mode. At high excitation, the temperature of the ν2 mode rises up to 2000 K, and hot bands are observed up to the n=4 level. This fourth overtone peak is split because of either a Fermi resonance or vibrational angular momentum splitting.
Calculation of double differential cross sections for the interaction of electrons with a water molecule, clusters of water molecules, and liquid water
Authors: Long Keith A. , Paretzke Herwig G. , Müller-Plathe Florian , Diercksen Geerd H. F.
Published: July 2002

In this paper double differential cross sections are calculated for the interaction of electrons with a water molecule, a cluster of water molecules, and thereby in a certain approximation for liquid water. It is intended to use the data generated, in a Monte Carlo track structure code, so that the data needs to be comprehensive and accurate especially where the cross sections are largest. The method employed uses only the electron density which is calculated by means of LCAO methods using extended basis sets of atomic wave functions to approximate the total wave function. The response of the system is then calculated via an integral formulation of the density functional theory based on a path integral representation of the one particle Green’s function. This leads in the first approximation to the local density approximation in a modified form which can now be derived rigorously. The DDCS’s can be expressed in terms of the linear response of the molecular system to a moving charge.
Formation and stability of a complex of positronium with nitrobenzene from the study of the magnetic field effects
Authors: Billard I. , Abbé J. Ch. , Duplâtre G.
Published: July 2002

The magnetic quenching of the triplet state of positronium (o-Ps) has been studied in pure n-hexane at 294 K, by applying an external magnetic field. The measured annihilation parameters are characteristic of a normal second-order Zeeman effect and betray some slight expansion of the Ps wave function as compared to in vacuo. In the presence of nitrobenzene, which reacts with Ps to form a complex, very strong effects are observed, confirming previous experiments. The lifetime of the o-Ps (m=0) substate undergoes a drastic decrease, even at very low magnetic field strengths. A new approach to this unexpected phenomenon is proposed, involving the reversible formation of a weak complex between Ps and the solute. Various quantitative treatments are attempted to fit the data. As it seems, the data can only be described if one supposes that there is no spin reorientation during the reaction. In all cases, the derived fitting parameters are characteristic of the complex, therefore denoting that the field effect does not intrinsically depend on solute concentration.
Development of the flowing afterglow/Langmuir probe technique for studying the neutral products of dissociative recombination using spectroscopic techniques: OH production in the HCO+2+e reaction
Authors: Adams Nigel G. , Herd Charles R. , Smith David
Published: July 2002

The flowing afterglow/Langmuir probe (FALP) technique has been extended to enable the neutral products of electron–ion dissociative recombination in thermalized afterglows to be identified by spectroscopic methods. Absolute number densities of H atoms in the afterglow have been determined using vacuum ultraviolet (VUV) absorption at the Lα wavelength. By exploiting the reaction H+NO2→OH+NO, all of the H atoms can be incorporated into OH molecules and thus observation of the intensity of laser induced fluorescence (LIF) If, obtained by exciting the (1,0) band of OH(A 2Σ←X 2Π), allows a calibration to be obtained of If against the known number density of OH X 2Π(ν″=0) in the afterglow. Following this procedure, a recombining HCO+2 /electron afterglow was probed for production of ground state OH X 2Π(ν″=0) using LIF and it was established that OH(ν″=0) resulted from 17% of the recombining ground state HCO+2 ions. It was also established that a further 17% of the recombinations resulted in OH(ν″>0), i.e., that, in total, (34±6)% of the HCO+2 ions recombine to produce OH X 2Π radicals, either directly or via the electronically excited A 2Σ state. Details of the calibration procedure for H and OH number densities, of the ion chemistry involved in the production of the HCO+2 afterglow plasmas and of the checks carried out to establish that the fluorescence observed was from OH produced in the recombination reaction are presented. During these experiments, the rate coefficient at 300 K for the H+NO2 reaction was determined to be 1.3×10−10 cm3  s−1 from observations of the H-atom decay as a function of NO2 number density in the afterglow (in good agreement with previous determinations). Also the rate coefficient for the quenching reaction of OH(ν″>0) with NO to produce OH(ν″=0) was determined to be 6×10−11 cm3  s−1.
Anomaly-free Kohn variational principle
Authors: Robicheaux F.
Published: July 2002

We show how the Kohn variational principle for the K matrix is free from anomalies when complex short-range basis functions are used. Using only the real part of the Kv matrix increases the accuracy of the calculation and guarantees the unitarity of the resulting S matrix.
Photoionization thresholds of rare gas clusters
Authors: Ganteför G. , Bröker G. , Holub-Krappe E. , Ding A.
Published: July 2002

Photoionization efficiency curves of rare gas clusters generated by supersonic expansion of pure argon, krypton, and xenon gas have been measured as a function of photon energy using light from the Berlin Electron Synchrotron Facility (BESSY). Assuming that the relative shapes of the efficiency curves are independent of cluster size, ionization potentials for each cluster can be extracted from the data by using the known absolute ionization thresholds of the dimer or trimer ions. The data agree with the model that ionic clusters are composed of an ionic substructure surrounded by shells of atoms. In particular, it has been confirmed that Ar+3 is the central substructure in Ar+n for n<15±1. In addition we have estimated the ratio of the covalent binding energy to the charge-induced dipole binding energy for the ions. The larger dipole-induced binding energy in the case of xenon may explain the differences between the mass spectra of Xe and Ar clusters.
Wave packet dynamics of van der Waals molecules: Fragmentation of NeCl2 with three degrees of freedom
Authors: Gray Stephen K. , Wozny Christopher E.
Published: July 2002

We study the time evolution of wave packets describing the fragmentation of NeCl2 complexes with three degrees of freedom (total angular momentum J=0). The method of calculation involves a combination of the Fourier method to treat the dissociative degree of freedom (van der Waals stretching coordinate) and a basis set of expansion to treat the intramolecular degrees of freedom (vibration and rotation of the diatomic fragment). Resonance energies, lifetimes, and product distributions can be extracted from the evolving wave packets. Results are presented for van der Waals complexes prepared initially with zero-point energy in the van der Waals modes and several different Cl2 vibrational excitations. Two different model potential energy surfaces are considered. The fragmentation mechanism is further explored by inspection and analysis of the wave function at various time intervals.
Wave packet study of a linear collision problem
Authors: Bringer Andreas , Harris John
Published: July 2002

Using real-time wave packet propagation we consider the scattering properties of a linear collision system consisting of a harmonic oscillator coupled by truncated Morse potentials to two particles. This system is viewed as a model for the collision of a projectile D with a three particle target ABC with interactions such that the possible final states comprise backscattering of the projectile with the target intact, the ‘‘exchange’’ reaction ABC+D→A+BCD and the ‘‘fragmentation’’ reaction ABC+D→A+BC+D. For the types of interaction considered, the exchange reaction was found to proceed primarily via inelastic resonant processes and to be the dominant event for incident energies below the oscillator energy. Fragmentation occurred via nonresonant processes and dominated at higher energy. Other features of the scattering are discussed and compared with simple quantum theory and the description of the system given by classical mechanics.
Spectral hole-burning and holography. I. Transmission and holographic detection of spectral holes
Authors: Meixner Alfred J. , Renn Alois , Wild Urs P.
Published: July 2002

Holographic detection is compared with the more common transmission method. The signal shapes of the optical holes determined by both techniques are studied theoretically and experimentally as a function of the burning fluence. From a comparison of the transmission signals and the holographic signals, the homogeneous linewidth can be determined. Experimental data was obtained from free base chlorin (1,2-dihydroporphyrin) in a polyvinylbutyral film at a temperature of 1.7 K. From these measurements a homogeneous linewidth of 350 MHz was determined. Limits of sensitivity of the holographic detection method are discussed.
Collisional energy transfer from highly vibrationally excited triatomic molecules
Authors: Hynes Robert G. , Sceats Mark G.
Published: July 2002

The atom–atom encounter model developed in the accompanying paper [M. G. Sceats, J. Chem. Phys. 91, 0000 (1989)] is applied to the collisional deactivation of highly vibrationally excited triatomic molecules CS2 and SO2 by the monatomic colliders He, Ne, Ar, Kr, and Xe at 300 K. The molecular inputs are a crude normal mode analysis, vibrational frequencies and effective anharmonicities, while the collisional inputs are parameters of the atom–atom potentials. The results for CS2 are compared with the simulations of Bruehl and Schatz and the experiments of Dove, Hippler, and Troe, while those for SO2 are compared with the simulations of Schranz and Troe and the experimental results of Heymann, Hippler, and Troe. Excellent agreement is found with experiment, and the superlinear energy dependence of the average energy transfer is attributed to anharmonicity of the triatomic molecule.
Determination of tunneling rates in bound systems using the complex coordinate method
Authors: Moiseyev Nimrod , Lipkin Nurit , Farrelly David , Atabek Osman , Lefebvre Roland
Published: July 2002

Up to now tunneling rates in bound systems have been obtained primarily by semiclassical or wave packet calculations. A new accurate quantum time-independent method is presented. Those irregular eigenfunctions of bound systems which diverge asymptotically, but upon complex scaling of coordinates X→X exp(iΘ) become square integrable functions and are associated with complex eigenvalues are found to describe barrier penetration processes. The imaginary part of each of the complex eigenvalues of the complex scaled Hamiltonian contains the tunneling decay rate provided that the Balslev–Combes rotation angle is large enough. The appearance of a critical value Θc as the rotational angle Θ is varied, at which a sharp transition from a real energy spectrum of the bound system to a complex eigenvalue spectrum is an indication of an exponential decay through the potential barrier. Tunneling in multiple barrier problems is important in several areas of physics and chemistry, including isomerization reactions, Josephson junction superconductors, electron tunneling from a 1D metallic lattice under the influence of a uniform electric field (field emission), and tunneling in the EF 1Σg state of molecular hydrogen. Several representative numerical examples are presented.
Thermodynamic properties of a polyelectrolyte solution containing a mixture of counterions differing in charge
Authors: Škerjanc Jože , Dolar Davorin
Published: July 2002

From the solution of the Poisson–Boltzmann equation for the cell model of a polyelectrolyte solution with mixtures of counterions of different charge, the electrostatic internal and free energies, the degree of binding of counterions, the osmotic coefficient, and the heat of dilution are calculated. These thermodynamic properties are presented as functions of valences, and mole fractions of the counterions, and the possibility of their application for interpretation of the scarce experimental data is discussed.
On energy estimators in path integral Monte Carlo simulations:  Dependence of accuracy on algorithm
Authors: Cao Jianshu , Berne Bruce J.
Published: July 2002

Two energy estimators, the Barker estimator and the Berne virial estimator, commonly used in path integral simulations of quantum systems are compared with respect to statistical accuracy. It is found that the accuracy of these estimators is strongly affected by the algorithm used. Four common algorithms are considered here:  (1) the pure primitive algorithm, (2) the primitive algorithm augmented by whole chain moves, (3) the normal-mode algorithm, and (4) the staging algorithm. The error of the mean of the Barker energy estimator is found to grow as (P)1/2, where P is the number of discretization points of the quantum paths (or the number of chain particles in the isomorphic classical chain), for all of the algorithms above. The error of the mean of the Berne virial energy estimator is independent of P for algorithms 2, 3, and 4, and increases as (P)1/2 for algorithm 1. It is concluded that the virial estimator is far more accurate than the Barker estimator for algorithms 2, 3, and 4, and is at least as accurate for algorithm 1. Because the error analysis depends strongly on the temporal correlations in the sequence of values of the energy estimator generated during Monte Carlo or molecular-dynamics simulations, we review the general question of error analysis in simulations.
Nonradiative Rydberg↔valence relaxation of NO trapped in Ar, Kr, and Xe matrices
Authors: Chergui M. , Schwentner N.
Published: July 2002

Electronic and vibrational relaxation of NO in Ar, Kr and Xe matrices has been studied using excitation spectra and time and energy resolved emission spectra. In addition to the vibrationally relaxed emissions from a 4Π(v=0), B 2Π(v=0) and A 2Σ+(v=0) in Ar and Kr matrices, B(v=5 and 7) emissions are observed in Ar matrices. In Xe matrices, only Rydberg A 2Σ+(v=0) fluorescence is observed. Nonradiative Rydberg–valence transitions are observed in all matrices and valence–Rydberg transitions only in Ar matrices. The intensity ratios IA/IB/Ia are ∼3/9/88 in Ar, ∼9/3/88 in Kr, and ∼2–5/0/0 in Xe matrices. The quantum efficiency for total luminescence being near unity in Ar and Kr matrices. The branching ratios for intramolecular relaxation between Rydberg and valence states are described in terms of a model which combines the intramolecular Franck–Condon factors with the spectroscopically determined phonon Franck–Condon factors and solvent enhanced spin–orbit matrix elements. The latter increase from Ar to Xe matrices. This increase is rationalized in terms of a semiempirical model for the heavy-atom effect on spin–orbit mixing. Weak Rydberg–Rydberg relaxation by a Δv=2 step in Ar and a Δv=1 step in Kr is also observed and interpreted in terms of a resonant Förster–Dexter-type energy transfer. Finally in Xenon matrices, a strongly nonresonant energy transfer from the n=l exciton of solid xenon to the A(v=0) Rydberg level is observed.
Formation of water clusters in a free molecular jet of binary mixtures
Authors: Inoue Takayoshi , Kotake Susumu
Published: July 2002

The formation process of water clusters is investigated experimentally with a supersonic free molecular jet of water vapor mixtures of Ar, Xe, N2, CO2 and CO, especially noticing the effect of the solute molecules on the cluster formation. The terminal concentrations of clusters formed through a supersonic molecular expansion are measured with a mass spectrometer by changing the mole fraction of water vapor at the source. It is observed that, in the range of relatively small H2 O mole fraction of binary mixtures at the source, the formation process of water clusters is controlled mainly by the thermodynamical effects of the expansion. On the other hand, at larger H2 O mole fractions, the molecular dynamical behavior of mixture molecules must be taken into account for understanding the formation process of water clusters. In both cases, the binary species play an important role in the formation process of water clusters.
The crystal azimuth dependence of the angular distribution of the desorption flux of carbon dioxide produced on palladium (110) surfaces
Authors: Matsushima Tatsuo
Published: July 2002

The reaction of CO(a) and O(a) on Pd(110) was studied with LEED and angle-resolved thermal desorption. Heating the coadlayer produces five peaks in the CO2 formation in the range of 160 to 460 K. The angular distribution of each is different. The distribution becomes sharper as the density of the coadlayer is increased, and it shows the dependence on the crystal azimuth. The correlation between the anisotropic angular distribution and the structure around the reaction site is discussed.
Translational and internal state distributions of NO produced in the 193 nm explosive vaporization of cryogenic NO films: Rotationally cold, translationally fast NO molecules
Authors: Cousins Lisa M. , Levis Robert J. , Leone Stephen R.
Published: July 2002

We report the translational, rotational, and spin-orbit state distributions of fast NO molecules which are generated by the 193 nm laser vaporization of 30 K multilayer NO films. Rotational distributions in v=0 are obtained by laser multiphoton ionization for five different velocities ranging from 900 to 2200 m s−1, corresponding to translational energies ET=0.14 to 0.71 eV. In every case, the average molecular rotational energy is more than 10 times smaller than the component of translational energy normal to the surface. Average rotational energies 〈ER〉 range from 0.009±0.002 to 0.024±0.006 eV (with corresponding best fit temperatures, TRot =105 to 220 K). For the molecules with ET=0.14 and 0.22 eV, the spin–orbit population ratios are typically comparable with TRot. For higher translational energies, the typical spin–orbit ratios are larger than expected from TRot and increase to a value F2/F1 of 1.1±0.50.4 (statistical ratio) for NO molecules with ET =0.71 eV. In some cases, the rotational distributions have a non-Boltzmann, high J component. Preliminary investigations for molecules with ET=0.56 eV indicate that the vibrational distribution v=1/v=0 is 3±1% (T≊785 K). The rotations in v=1 are also cold (〈ER〉≊0.01 eV, TRot ≊130 K). The mechanism that causes the ejection of translationally fast, rotationally cold NO molecules is considered in terms of either a collisional cooling process following desorption or rotationally constrained desorption dynamics.
The production kinetics for rare gas alkali ions
Authors: Schumann M. , Langhoff H.
Published: July 2002

In a recent paper it was shown that rare gas ions form bound molecular states which decay by emission of radiation in the VUV. They are potential candidates as storage media for VUV lasers. In the present investigation the ions Kr+K, Kr+Cs, Xe+K, and Xe+Cs are produced by electron beam excitation of suitable gas mixtures with He as buffer gas. Studies of the reaction kinetics were performed for Kr+K. The rate constant for the reaction Kr++K+He was determined to be 2.8×10−30 cm3 s−1. Kr+K is strongly quenched by plasma electrons and by reaction with Kr. The fluorescence yield at intense excitation was determined to be about 5×10−2 VUV quanta per He+ ion.
Oscillations in methylamine decomposition on Pt, Rh, and Ir: Experiments and models
Authors: Cordonier G. A. , Schüth F. , Schmidt L. D.
Published: July 2002

Oscillations with large amplitudes (>500 K), high frequencies (>8 Hz), and good reproducibility are observed in methylamine decomposition on electrically heated Pt, Rh, and Ir wires at pressures of 0.25–8.0 Torr and temperatures between 900 and 1500 K. The major reaction involves formation of HCN which is endothermic by 37 kcal/mol, so that the reaction strongly cools the wire, which counteracts resistive heating. A photodiode array was used to monitor spatial and temporal oscillations with resolutions of 1 mm and 30 Hz, respectively. On Pt, the frequency increased with increasing pressure and with decreasing wire diameter, while amplitude was a weak function of these parameters. On Ir, oscillations were more complex with regions of the wire often oscillating independently and more variability between experiments. However, the overall behavior resembles that on Pt. On Rh, frequencies were much lower (<0.03 Hz), oscillations occurred over a smaller temperature range and halves of the wire frequently oscillated out of phase with each other. Oscillatory behavior of methylamine decomposition on Pt was modeled by a simple lumped model which gives semiquantitative agreement with observations. The model assumes a simple unimolecular reaction with CH3NH2 adsorption blocked at low temperatures by formation of a strongly adsorbed CN which is known to be very stable on Pt (Ed ≂50 kcal/mol). Calculated frequencies, pressure dependencies, amplitudes, and temperature range of oscillations agree quite well with experiments using reasonable values for adsorption and reaction rate parameters, and wire geometry and properties.
Rotational populations in OD formed in the reaction O(1D)+D2 investigated by infrared rotational absorption spectroscopy
Authors: Sears Trevor J. , Hall G. E. , McAndrew J. J. F.
Published: July 2002

Diode laser transient absorption/gain spectroscopy is used to monitor time-dependent populations of high rotational levels in OD (v=0) produced in the reaction of O(1D)+D2. Pure rotational transitions on species with large dipole moments offer good sensitivity, full state resolution and μs time resolution in the present apparatus. Measured nascent populations of OD in the four highest rotational levels thermodynamically accessible in this reaction are in reasonable agreement with the reported results of earlier laser-induced-fluorescence measurements, in which corrections for transition moments and predissociation introduce increasing uncertainties at high rotational levels. The relaxation kinetics of the highest rotational levels are not hopelessly complex, and evidence is presented for strong, but not complete propensity for conservation of Λ doublet symmetry during rotational relaxation.
Binding energy and electronic properties in antimony clusters: Comparison with bismuth clusters
Authors: Rayane D. , Melinon P. , Tribollet B. , Cabaud B. , Hoareau A. , Broyer M.
Published: July 2002

Antimony clusters are produced by the gas aggregation method. They are found to be built from Sb4 units. The Sb+4n clusters are demonstrated to relax their excess energy by evaporation of a tetramer and the binding energy of Sb4 in the cluster is measured to be about 1.3 eV, well below the binding energy of a Sb atom in the bulk (≊2.75 eV). The ionization potentials of Sb4n clusters are studied as a function of the size and the observed evolution confirms the nonmetallic character of their structure. These results are compared with those obtained for bismuth clusters which exhibit a very different behavior. Finally the possible metastable character of Sb4n structure is discussed and illustrated by experiments performed as a function of inert gas temperature in the clusters source.
Relaxation of spin S=3/2 in the doubly rotating tilted frame
Authors: van der Maarel J. R. C.
Published: July 2002

For spin S=3/2 the time evolution of the density operator under spin–locking is described. It is shown that outside the extreme narrowing limit triple-quantum coherences are excited. An experiment is proposed to monitor these coherences. The corresponding signal evolves according to a simple relaxation function which is suitable for fitting.
Gaussian functions in Hylleraas-configuration interaction calculations. V. An accurate a
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o H+3 potential-energy surface
Authors: Frye D. , Preiskorn A. , Lie G. C. , Clementi E.
Published: July 2002

The near-equilibrium potential-energy surface of the 1A′1 ground state of H+3 has been calculated at 69 different points with the Hylleraas-configuration interaction method using 13s3p1d Cartesian Gaussian basis functions. This new surface is found to be substantially lower in absolute energy than all previous surface calculations. The equilibrium energy of the H+3 molecule has also been calculated with a larger 13s5p3d basis set. The minimum energy was found to be E=−1.343 827 9 hartrees at an internuclear distance of R=1.6500 bohrs in the equilateral triangle configuration. This energy is significantly (>70 cm−1) lower than the previous best published variational calculation and is outside and below the error bars of the latest quantum Monte Carlo calculation. In addition, a medium-sized basis set of 13s4p2d orbitals was used to predict that the equilibrium separation is R=1.6499 bohrs.
The origin of small and large molecule behavior in the vibrational relaxation of highly excited molecules
Authors: Gordon Robert J.
Published: July 2002

An explanation is proposed for the qualitatively different types of behavior that have been reported for the vibrational relaxation of highly excited diatomic and polyatomic molecules. It is argued that all of the diatomic molecules that have been studied in bulk relax adiabatically at room temperature. In contrast, large polyatomic molecules have low frequency modes which act at ‘‘doorway’’ modes for the rest of the molecules, producing an impulsive relaxation mechanism. The theoretical work of Nesbitt and Hynes showed that impulsive collisions result in an exponential decay of the average vibrational energy of a Morse oscillator, whereas adiabatic collisions produce nonexponential power law behavior. We propose that this result explains a large body of data for the vibrational relaxation of small and large molecules.
The influence of the fluid–wall interaction potential on the structure of a simple fluid in a narrow slit
Authors: Sarman Sten
Published: July 2002

The structure of a Lennard-Jones fluid in a narrow planar slit is studied for different kinds of wall–fluid interaction potentials by applying anisotropic integral equation theories. Density profiles and in some cases the force between the walls as a function of the slit width are calculated. It is found that when the state of the fluid in the slit is close to the liquid–vapor coexistence line, there must exist a distinct attractive minimum in the wall potential in order to induce an oscillatory density profile in the slit. Then, the force as a function of the slit width is also oscillatory. The repulsive part of the wall potential has little influence on the oscillatory behavior. When the wall potential consists of a very soft repulsive part close to the wall followed by a diffuse attractive minimum further out, the layer structure disappears completely. However, when the pressure of the fluid is raised above the vapor pressure, the oscillatory structure slowly reappears. These results are utilized to discuss certain experimental surface force measurements where solid, molecularly smooth walls have been modified by adsorption of amphiphiles. It is suggested that the main reason why the oscillatory forces do not appear in such systems is that the attractive part of the wall potential becomes diffuse and not that the repulsive wall becomes soft.
Freezing, melting, spinodals, and clusters
Authors: Wales David J. , Berry R. Stephen
Published: July 2002

A simple theory for melting, freezing, and phase coexistence is presented with emphasis on the relation between small clusters and spinodal behavior. This theory, coupled with the results of computer simulations, suggests a reinterpretation of spinodals and first-order phase changes. We find sufficient conditions for a system of mutually attracting quasiparticles to exhibit the characteristic ‘‘S-bend’’ found, e.g., for subcritical isotherms of a van der Waals fluid, and discuss how the theory may be extended explicitly to finite clusters.
A closed form solution for the internal dynamics of polymer chains. I. Bonds with independent rotational potentials
Authors: Kloczkowski Andrzej , Mark James E. , Bahar Ivet , Erman Burak
Published: July 2002

An analytical solution to the master equation governing the conformational dynamics of linear polymer chains is formulated. Symmetric chains with N bonds subject to independent rotational potentials are considered. The eigenvalues of the transition rate matrix, which characterize the frequencies of the various relaxation modes, and the corresponding eigenvectors and eigenrows are obtained in closed form. A simple recurrence equation permits one to express the eigenvalues of the N-bond motion in terms of the nonzero eigenvalues associated with the isomeric transitions of single bonds. This leads to a clear understanding of the increase in conformational mobility with N.
Vibrational energy transfers in ozone from infrared double-resonance measurements
Authors: Ménard-Bourcin F. , Doyennette L. , Ménard J.
Published: July 2002

Vibrational energy transfer processes in pure ozone have been studied by means of a time-resolved infrared double-resonance technique using two CO2 lasers. The intermode transfer between the Coriolis-coupled ν1 and ν3 modes has been observed for the first time and its rate constant measured. The near-resonant vibrational energy transfer populating the ν2+ν3 state has been also directly observed, as well as the vibrational deexcitation of the ν1 and ν3 states including the transfer process to the ν2 mode and the direct V-T,R processes. From a kinetic analysis of the vibrational relaxation, a numerical model was formulated allowing by comparison to the experiments to determine the rate coefficients for the processes. A rate coefficient of 2.4×106 s−1 Torr−1 was obtained for the intermode transfer between the Coriolis coupled ν1 and ν3 modes. Also a rate coefficient of 7×105 s−1 Torr−1 was derived for the process populating the ν2+ν3 state. The rate coefficient corresponding to the intermode transfer from ν1 and ν3 to ν2 was found to be (2800±500) s−1 Torr−1, a value clearly smaller than that obtained in previous studies. Finally the direct deexcitation from the ν1 and ν3 states, not taken into account in previous works, was found to be rather efficient, its rate coefficient being not negligible (∼600 s−1 Torr−1) as compared to the deexcitation coefficient from the ν2 state which was found equal to (2250±300) s−1 Torr−1.
Branching ratios and rate constants for reactions of 16O− and 18O− with N2O and 14N15N16O
Authors: Morris Robert A. , Viggiano A. A. , Paulson John F.
Published: July 2002

Branching ratios for the NO− isotopic products from the gas-phase reactions of 16O− and of 18O− with 14N15N16O have been determined at 143 and 298 K using a variable temperature-selected ion flow drift tube (VT-SIFDT) instrument. The reaction of 16O− yields the products 14N16O− and 15N16O− in approximately equal abundance at both temperatures. The reaction of 18O− produces the four possible NO− isotopes, with the branching ratio being dependent on temperature. For the latter reaction the rate constant for the O− isotope exchange process has been determined at 143 and 298 K. Rate constants for the reaction of O− with N2O (unlabeled reagents) have been measured as a function of ion-neutral average center-of-mass kinetic energy (〈KEc.m.〉) at several temperatures. The temperature dependence of the rate constant is expressed as T−0.5. The energy dependences at different temperatures fall on a single curve and agree well with a previous energy dependence study at 300 K.
Simplified antisymmetrized product of geminals wave function for extended systems. II. Localized geminals
Authors: Karadakov P. , Castan̄o O. , Calais J.-L.
Published: July 2002

The suitability of a simple antisymmetrized product of localized geminals (APLG) wave function for the description of ground-state correlation effects for extended systems is studied on the example of cyclic polyenes (annulenes) with 4ν+2 carbon atoms (ν=1,2, ...) within the π-electronic approximation. It is demonstrated that the suggested APLG approach accounts for a significant part of the ground-state correlation energy irrespective of the size of the investigated system, but strongly favors bond-alternating structures.
Spectroscopy of the AlAr van der Waals complex: Rotationally resolved B 2Σ+←X 2Π1/2 electronic transitions
Authors: McQuaid Michael J. , Gole James L. , Heaven Michael C.
Published: July 2002

Rotationally resolved spectra were recorded for six bands of the AlAr B 2Σ+←X 2Π1/2 transition. Vibrational and rotational constants derived from these spectra were used to determine the upper and lower state potential energy curves. The accuracy of these potentials was assessed through calculations of the spectroscopic constants and Franck–Condon factors. Dissociation energies of D′e=440+35−8 cm−1 and D′e=180+40−10 cm−1 were obtained for the B and X states, respectively. The interaction between X 2Π1/2 and the low-lying A 2Σ+ state has been characterized by analysis of the ground-state lambda doublet splitting.
Resonant two-photon ionization spectra of the external vibrational modes of the chlorobenzene-, phenol-, and toluene-rare gas (Ne, Ar, Kr, Xe) van der Waals complexes
Authors: Mons Michel , Le Calvé Jacques , Piuzzi François , Dimicoli Iliana
Published: July 2002

Using resonant two-photon ionization and time-of-flight mass spectrometry techniques, original spectra of the external vibrational modes of ten van der Waals (vdW) complexes are presented. The complexes are formed in a pulsed supersonic expansion between a rare gas atom (Ne, Ar, Kr, Xe) and a monosubstituted benzene derivative chlorobenzene, phenol, or toluene. For each complex, the red shift of the S1←S0 000 energy due to complexation, and the vdW stretching and bending frequencies are determined. In some cases, the bending mode anharmonicity and Fermi resonances could be analyzed from the extended progressions observed for the bending vibration. The diatomic model is used to estimate the stretching force constant. Intensity, mass, and dissymmetry effects induced by the X substituent on the benzene ring are analyzed. In particular, in these Cs symmetry complexes, every bending A′ level is observed, not just the even bending levels found in C6v or C2v complexes. Finally, some correlations are shown from the comparison of the present results with the few available data of the literature.
Elastic constants of the metastable orientationally disordered phase Ia of CCl4 by Brillouin spectroscopy
Authors: Zuk J. , Kiefte H. , Clouter M. J.
Published: July 2002

The technique of Brillouin spectroscopy has been used to determine the adiabatic elastic constants, including their temperature dependence, and the elasto-optic coefficient ratios of single crystals of the metastable orientationally disordered phase Ia of CCl4. The values of the elastic constants at 244.1 K are C11=35.7, C12=31.6, and C44=11.1 (in units of kbar) and the elasto-optic coefficient ratios are P12/P11=0.85 and P44/P11=0.22. The elastic constants results suggest that there is strong rotation–translation coupling in the k=0 acoustic wave vector regime and that this coupling is likely responsible for the metastability of the fcc phase Ia with respect to the rhombohedral phase Ib.
Pressure dependence of zero-field splittings in organic triplets. II. Carbonyls
Authors: Chan I. Y. , Qian X. Q.
Published: July 2002

We have conducted optically detected magnetic resonance (ODMR) experiments at pressure up to 40 kbar for neat biactyl (BA), neat benzil (BZ), and acetophenone (AP) doped in dibromobenzene (DBB). The pressure dependences of their zero-field splitting (ZFS) parameters D and E are reported. For BA and BZ systems, the ‖D‖ value decreases greatly with increasing pressure. This behavior is in contrast with that of benzophenone (BP), whose ‖D‖ value increases sigmoidally 13% over the same pressure range. These results may be rationalized in a qualitative theory based on pressure modulation of the spin-orbit coupling (SOC) contribution to the ZFS. ln aromatic ketones, lattice compression modifies the twist angle of the phenyl ring(s) relative to the carbonyl frame, thus changing the energy of the 3ππ* state relative to that of the 3nπ* state. This variation of the energy denominator in a second order perturbation enhances the SOC contribution to the ZFS. In comparison, the increase of spin–spin (SS) dipolar interaction by isotropic compression is relatively unimportant. Consistent with this picture, the very small 3ππ*–3nπ* energy gap produces an enormous pressure sensitivity of D and E in AP/DBB. The behavior of the ZFS in this case may be interpreted as a consequence of pressure tuning of the 3ππ* state through an anticrossing region. In addition, a new set of high frequency ODMR signals appears under pressure. This is attributed to a new site of AP having the 3nπ* as the phosphorescent triplet state. The pressure dependence of ZFS for benzil shows complicated fine structure. This is a testimony to the flexible nature of benzil in both the dihedral angle of the dicarbonyl fragment and the phenyl twist angle.
Mode-specific infrared photodissociation of nitric oxide dimers: High-resolution infrared spectroscopy of (14NO)2 and (15NO)2
Authors: Matsumoto Yoshiyasu , Ohshima Yasuhiro , Takami Michio
Published: July 2002

The high-resolution infrared absorption spectra of the symmetric (ν1) and the antisymmetric NO stretching (ν4) bands of nitric oxide dimer (NO)2 have been measured for 14NO and 15NO in supersonic free jets. The ν1 and ν4 bands exhibit a dramatic difference in linewidth: approximately 200 MHz [full width at half-maximum (FWHM)] for the ν1 band and approximately 5 GHz (FWHM) for the ν4 band. The predissociation lifetimes deduced from the linewidths are in excellent agreement with those reported in the recent time-resolved measurement for 14NO [Casassa et al., J. Chem. Phys. 89, 1966 (1988)]. There is no systematic dependence of the linewidth on the rotational states of (NO)2. Isotope substitution does not influence the linewidths significantly. However, the ν4 band structure of (15NO)2 is very different from that of (14NO)2, a difference that may be explained by a perturbation from a low-lying singlet vibronic state. All of the experimental results obtained to date may be accounted for if it is assumed that the predissociation of (NO)2 is enhanced by an electronically nonadiabatic transition to a repulsive triplet surface. Vibrational potential coupling between the NO stretching and intermolecular modes, particularly an in-plane NO bending mode, appears to play a key role in the mode specificity.