bluuebunny/crossref_metadata_2025 huggingface dataset:
Total scattering, surface ionization, and photoionization of a beam of H3 metastable molecules
Authors: Garvey James F. , Kuppermann Aron
Published: July 2002

In a previous paper we reported a technique for generating an intense hyperthermal beam of hydrogen atoms and metastable H3 molecules. From the flight time of the H3 species between the source and detector we estimated that its lifetime exceeds 40 μs and that it must therefore be in the 2 p 2A″2 excited Rydberg state. In this paper we report experiments utilizing this novel source of H3 molecules. Beam-gas attenuation measurements indicate that the H3–Ar cross section is roughly ten times larger than the H–Ar cross section for translational energies in the 1 to 10 eV range. This observation is consistent with the assignment of the H3 to that excited state, which has a much larger effective radius than a ground state hydrogen atom. The temperature dependence of the surface ionization of H3 by heated tungsten and platinum filaments is used to obtain effective ionizational potentials of this species. These potentials suggest that upon interaction with a metal surface, the metastable state decays to the repulsive 2 p 2E′ state which then surface ionizes to produce H+3. The production H+3 and H+ when the H3 beam is irradiated with UV light from a high pressure mercury lamp was also observed and is attributed to the relatively low ionization potential (∼3.7 eV) of the 2 p 2A′′2 metastable state of H3.
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o calculations of potential energy curves and transition moments of 1Σ+g and 1Σ+u states of N2
Authors: Ermler Walter C. , Clark Joseph P. , Mulliken Robert S.
Published: July 2002

The valence, Rydberg, and ion pair characteristics of 1Σ+u states of N2 are studied in ab initio calculations involving selectively optimized basis sets of Slater-type functions and configuration mixing. Potential energy curves, as well as transition dipole moments and second moments of charge as a function of internuclear separation, are reported for 1Σ+g and 1Σ+u states. Transition dipole moments are in good agreement with experiment as are spectroscopic constants for the observed states. Analysis of the second moment and transition moment functions for the lowest 1Σ+u state shows clearly a transformation from atom pair to valence to ion pair to nearly pure Rydberg character as a function of internuclear separation between the separated atoms and the united atom limits.
The effect of orbital alignment on the forward and reverse electronic energy transfer Ca(4s5p 1P1)+M⇄Ca(4s5p 3P
J)+M with rare gases
Authors: Bussert Wolfgang , Neuschäfer Dieter , Leone Stephen R.
Published: July 2002

Effects of orbital alignment on the relative cross sections for electronic energy transfer are determined for the near resonant transfer between Ca(4s5p 1P1) and Ca(4s5p 3PJ) states with rare gas collision partners. The experiments are carried out by pulsed laser excitation in a crossed beam. The results for the forward direction, 1P to 3P, formulated in terms of the ratio of the maximum to minimum transfer probability are: 3He 1.61±0.05; He 1.60±0.03; Ne 1.55±0.10; Ar 1.52±0.21; for Kr, transfer occurs, but no preference is distinguishable within 1±0.2; Xe 1.44±0.06. The results for He, Ne, and Ar indicate a clear preference in the transfer for the initially prepared molecular Π state. For Xe the molecular Σ state is dominant. The energy transfer is also carried out in the reverse direction, 3P1 to 1P, for He and Xe, obtaining 1.65±0.10 and 1.94±0.22, respectively. Analysis of the state preparation suggests that the reverse direction favors the asymptotic molecular Σ state for He and the molecular Π state for Xe. These alignment results provide a first experimental determination of the dominant electronic states involved in a collisional energy transfer process.
State-resolved differential cross sections for the reaction D+H2→HD+H
Authors: Buntin Steven A. , Giese Clayton F. , Gentry W. Ronald
Published: July 2002

Differential cross sections for the reaction D+H2→HD+H were measured with a novel crossed molecular beam technique, in which a pulse of energetic D atoms is formed by 193 nm photolysis of D2S and crossed with a pulse of H2 at a variable intersection angle. Speed distributions of the product HD, measured by time-of-flight to a mass spectrometer detector, show clearly resolved peaks for v=0 and v=1 at center-of-mass scattering angles near 180°, for an initial relative kinetic energy of 0.95 eV. Analysis of the data by computer simulation of the experiment yields a v=0/v=1 population ratio of 5.1, and average rotational energies of 0.26 eV in V=0 and 0.11 eV in v=1.
Predicted vibration–rotation levels of H2He+ and its isotopomers
Authors: Tennyson Jonathan , Miller Steven
Published: July 2002

Variational bound state nuclear motion calculations are performed on a fitted [Joseph and Sathyamurthy, J. Chem. Phys. 86, 704 (1987)] ab initio surface for HeH+2. The linear 4HeH+2 molecule is predicted to have an He–H+2 stretch fundamental of 732 cm−1 with the first J=0 excited bend at 960 cm−1. This isotopomer has no bound excited states of the H+2 vibrational mode. Calculations with J=1 and J=2 predict a rotational constant of 4.0 cm−1 for this system. Results are also presented for all the isotopomers containing D and 3He. The spectrum of this molecule is a good candidate for laboratory or astrophysical observation.
The dissociation dynamics of energy selected ion–dipole complexes. I. The cyclopropane ion–water complex [c-C3H+6–OH2]
Authors: Shao Jian-Dong , Baer Tomas , Morrow J. C. , Fraser-Monteiro Maria L.
Published: July 2002

The dissociation dynamics of energy selected n-propanol ions have been investigated by photoelectron photoion coincidence (PEPICO) and laser photodissociation. In spite of the low activation energy, the dissociation rates for the loss of H2O were found to be slow (<106 s−1) near the dissociation limit, but they increase rapidly with ion internal energy. The photodissociation experiment indicates that the n-propanol ion structure is maintained up to the dissociation limit. However, the reaction rate and the kinetic energy release show that H2O loss proceeds via an isomerized C3H8O+ structure. Ab initio calculations suggest that the isomerized structure is the ion–dipole complex formed by the cyclopropane ion and water. The dissociation rate is calculated with the statistical theory (RRKM/QET) by using vibrational frequencies determined from the ab initio calculation. Although the ion–dipole complex potential well is deeper than that of propanol, it does not account for the slow dissociation rate. The discrepancy of more than four orders of magnitude is attributed to the extremely anharmonic interaction potential associated with the ion–dipole forces.
Production and properties of singly, doubly, and triply charged N2 clusters
Authors: Scheier P. , Stamatovic A. , Märk T. D.
Published: July 2002

Clusters of N2 molecules formed in a supersonic nozzle expansion have been studied by electron impact ionization mass spectrometry. Mass resolved spectra (with n up to 190) show three homologous series, i.e., consisting of (N2)+n, (N2)nN+, and [(N2)n N]2+ ions. The distribution of (N2)+n ions shows distinct magic number effects. Moreover, there is evidence for the existence of (N2)2+n and (N2)3+n ions. The critical appearance sizes of observed multiply charged cluster ions are (N2)2+99, (N2)99N2+, and (N2)3+215. These results are compared with theoretical predictions. Moreover, a study of the electron energy dependence of singly and doubly charged N2 cluster ions close to threshold (appearance energies) gives new insight into the ionization mechanism.
Fragmentation of NH3 dimers by electron impact ionization
Authors: Buck U. , Meyer H. , Nelson D. , Fraser G. , Klemperer W.
Published: July 2002

The fragmentation of ammonia dimers by electron impact ionization is investigated by preselecting the neutral species using scattering and spectroscopic methods. In the first experiment, the measured angular and velocity dependence of the ammonia cluster scattered from a helium beam is used to separate the dimer from the other contributions. In the second experiment, the rotationally resolved spectrum obtained by microwave electric resonance technique is used to label the dimer. The ratio of the ion intensities m=18 (NH+4) to m=17 (NH+3) amu is 1.3 and 1.8, respectively. In the case of scattering, the result is independent of the initial state distribution of the dimer. The remaining difference is attributed to the different ion sources. This remarkable result is explained in terms of simple models based on the structural change from the neutral to the ionized configuration and the fragmentation pattern of the monomer.
Intramolecular vibrations of small water clusters
Authors: Honegger Evi , Leutwyler Samuel
Published: July 2002

The structures, binding energies, and vibrational frequencies of fully optimized water clusters (H2O)n, n=1– 4, were computed with ab initio molecular orbital theory at the SCF level using different basis sets. The SCF procedure allows satisfactory predictions for these properties compared with experimental results. For quantitative predictions of binding energies and geometrical parameters, a basis set including polarization functions is needed. With respect to intramolecular vibrational frequencies and frequency shifts, however, the split valence basis set 4-31G leads in all cases to the best rationalization of the available experimental data. Analysis of intramolecular force constants, frequencies, and eigenvectors for n=2 to 4 shows that (i) a transition from highly localized (n=2) to highly delocalized (n=4) vibrational modes takes place; (ii) the delocalized O–H vibrations of cyclic (H2O)n clusters (n≥3) can be described as longitudinal/transverse optical phonons; (iii) internal force constants for the hydrogen bonding O–H stretch, k(O–Hb), decrease strongly with size, leading to a decrease in the νbridge O–H frequencies; a similar decrease is found for the intramolecular bending force constant k(Θ); (iv) force constants for the free O–H stretching motions k(O–Hf) stay nearly constant, as do the corresponding vibrational frequencies; (v) intermolecular stretch–stretch couplings increase substantially with n and dominate over intramolecular stretch–stretch couplings for n≥3; similarly, intermolecular bend–libration couplings are very important.
Investigation of the electronic and geometric structure of small Li anionic clusters with quantum chemical CI procedure
Authors: Boustani Ihsan , Koutecký Jaroslav
Published: July 2002

Geometric structure of Li−n (n=2–9) clusters is determined with the analytical gradient minimization procedure of the Hartree–Fock energy using a relatively small AO basis set. The energies of the Li−n clusters for the SCF optimized geometries are calculated with the multireference diexcited configuration interaction method. The geometries of Li−n (n<5) differ appreciately from those of neutral as well as of cationic Li clusters. Linear geometries of Li− trimer and Li− tetramer can be ascribed to the electrostatic repulsion. The atomization energy of Li−n clusters as function of cluster nuclearity shows a generally increasing tendency. The calculated electron affinities of Lin clusters exhibit features which are in details analogous to the features of the adiabatic electron affinities determined experimentally for Cun clusters.
Photoelectron spectroscopy of (CO2)−n clusters with 2≤n≤13: Cluster size dependence of the core molecular ion
Authors: DeLuca Michael J. , Niu Baohua , Johnson Mark A.
Published: July 2002

Photoelectron spectra of the negatively charged clusters of CO2 are recorded with 3.49 eV photon energy and appear as bell-shaped, unresolved vibrational envelopes similar to that observed for the monomer ion. The maxima of the photoelectron spectra, found by fitting the envelopes to Gaussian profiles, correspond to the vertical electron detachment energies (VDE) of the clusters. These VDE values, when combined with the previously measured value for CO−2, display sharp discontinuities at cluster sizes n=2 and n=6. The magnitudes of these shifts are on the order of 1 eV and are in near quantitative agreement with the calculated difference in VDE between the monomer anion and the D2d form of the dimer anion. We infer from this agreement that the dimer ion is the core of clusters 2≤n≤5 while the monomer ion forms the core for n≥7. The hexamer is special in that both forms are evident in the photoelectron spectra. These structural changes are not manifested as ‘‘magic numbers’’ in the parent spectra, which have been previously observed at n=4, 7, 10, and 14.
Intermultiplet and angular momentum transfers of excited sodium atoms in collisions with molecules. I. Experiment
Authors: Astruc J. P. , Desfrancois C. , Barbe R. , Schermann J. P.
Published: July 2002

The deexcitation processes of moderately excited sodium atoms have been studied in a pulsed crossed-beam experiment. The time resolution of the sodium beam fluorescence allows the simultaneous observation of intermultiplet and angular momentum transfers. The absolute calibration procedure is described and results are compared with those obtained with different experimental techniques. Partial quenching cross sections of the Na(4P) and Na(4D) states by He, Ar, H2, N2, O2, N2O, and SF6 are reported.
Configurational density of states and entropy of alloys: A Monte Carlo approach on body-centered cubic and face-centered cubic lattices
Authors: Bichara C. , Gaspard J.-P. , Mathieu J.-C.
Published: July 2002

The method recently developed to calculate the configurational density of states (or combinatory factor) on the regular solutions alloy model, from Monte Carlo simulation is applied to binary systems having bcc and fcc structures. This method enables one to calculate all the thermodynamic quantities of mixing of the model, including entropy. The results clearly show that the quasichemical approximation, although correct at high temperatures, does not yield a correct temperature dependence of the thermodynamic properties. In bcc alloys with ordering tendency, the Monte Carlo calculations yield configurational entropy curves that, up to now, have been interpreted in terms of the ‘‘associated model.’’ We show that an exact statistical treatment of the regular solutions model is sufficient to reproduce the strong local order observed at low temperatures. The densities of states themselves are studied; They are Gaussian near the center of the distribution, satisfying the central limit theorem. In the tails of the distribution, the density of states decreases less rapidly with E ; in a certain region of energies, the density of states is roughly exponential, giving rise to a phase transition behavior. At the far extreme of the tails, the density of states decreases faster with energy.
Intermolecular potential for thermal H2O–He collisions
Authors: Palma Amedeo , Green Sheldon , DeFrees D. J. , McLean A. D.
Published: July 2002

Theoretical potentials for rotational excitation of H2O by He were constructed via several methods, all of which start with a large basis set SCF interaction. The semiempirical Hartree–Fock with damped dispersion (HFD) model adds a damped long-range attraction with parameters adjusted to fit experimental total differential cross sections. Purely ab initio potentials add correlation energies obtained via perturbtion theory (MP2 and MP4) or a variational method (ICF1). Scattering calculations were performed on all surfaces to compare with available beam scattering and pressure broadening data and to assess sensitivity of state-to-state rates to uncertainties in the potential. From comparison with the limited experimental data, the ICF1 surface appears to be marginally better than the MP4 surface. Thermal rates calculated from this surface should be accurate to better than 50%, at least for the larger, more important rates.
Photochemistry of adsorbed molecules. III. Photodissociation and photodesorption of CH3Br adsorbed on LiF(001)
Authors: Harrison I. , Polanyi J. C. , Young P. A.
Published: July 2002

Submonolayers of CH3Br physisorbed on a LiF(001) surface were irradiated by pulsed ultraviolet (UV) in ultrahigh vacuum in a study of surface aligned photochemistry (SAP). Translational energy and angular distributions were obtained for both photofragments and photodesorbed molecules by angularly resolved time of flight to a mass spectrometer. Single-photon adsorbate photolysis (PDIS) led to photofragment distributions, CH3 and Br, which differed from gas-phase photolysis. Photodesorption (PDES) of CH3Br was nonthermal and arose from UV absorption by the LiF crystal. The dynamics for these photoprocesses were found to vary in an informative way with the coverage and the phase of the adsorbed layer.
Research of valence character HCl− states through nearly diabatic CI calculations
Authors: Rajzmann M. , Spiegelmann F. , Malrieu J. P.
Published: July 2002

A b initio CI calculations have been performed for the HCl− system, using moderately large basis sets. For large interactomic distances, the potential curves of the three bound states (two 2∑+ and one 2Π) compare favorably with those previously computed with very large basis sets. The extension of a recently published procedure yields nearly diabatic potential curves for the unbound region, and enables to discriminate between valence-like resonant states, of essentially repulsive character and diffuse states, the potential curves of which parallel one of the neutral molecule curves; the former are nearly basis-set independent while the latter are sensitive to the basis. Estimations of the widths are also given. For the HCl equilibrium distance, the first 2∑+ ‘‘resonant state’’ is located at 2.3 eV above the v=0 level of HCl, with a large width of 2 eV; the second 2∑+ and 2Π resonant states are located at 9.4 and 7.1 eV, respectively, with widths of 2.5 and 1.2 eV.
High precision dipole moments in Ã 1A2 formaldehyde determined via Stark quantum beat spectroscopy
Authors: Vaccaro P. H. , Zabludoff A. , Carrera-Patiño M. E. , Kinsey J. L. , Field R. W.
Published: July 2002

The high resolution technique of Stark quantum beat spectroscopy is used to examine the electric dipole moment function for the first excited singlet state (Ã 1A2) of formaldehyde-h2 and formaldehyde-d2. The high precision of these measurements (i.e., better than 5 parts in 104) enables detailed determination of a-axis dipole moment components (μa ) for individual J=2 rovibronic levels in the ν4 out-of-plane bending mode. In the case of 21,1 rotational levels, we find μa (40)=1.4784(7) D and μa (41)=1.4678(4) D for H2CO. For D2CO the measured 21,1 dipole moments are μa (40)=1.4698(6) D, μa (41)=1.4693(3) D, and μa (43) =1.4786(7) D. The state-specific variations in μa revealed by this study reflect the structural influences exerted by the pervasive S1∼S0 nonadiabatic interactions and the pyramidally distorted equilibrium configuration which characterize the Ã state of formaldehyde. The origin and experimental manifestation of the out-of-plane dipole moment component (μc ) in nonrigid Ã 1A2 formaldehyde is also discussed.
Unified semiclassical dynamics for molecular resonance spectra
Authors: Xiao Lin , Kellman Michael E.
Published: July 2002

A method is presented to depict the intramolecular dynamics of resonantly coupled vibrations, starting from the experimental overtone and combination spectrum. The nonlinear least-squares fit of the spectrum is used to obtain a semiclassical phase space Hamiltonian via the Heisenberg correspondence principle. This integrable Hamiltonian, corresponding to quasiperiodic motion, is used to generate a classical trajectory in phase space for each energy level in a resonance polyad. Polyad phase space profiles are shown to have complete mutual consistency starting from a fit in either the local or normal representation. It is argued that the best way to depict the phase space profile is on a spherical surface called the polyad phase sphere. Represented in this way, the local and normal mode phase spaces are seen to be a single entity, manifestly equivalent by a 90° rotation. The phase space trajectories can be converted into a coordinate space representation. This gives an easily visualized picture of the semiclassical intramolecular dynamics corresponding to each energy level. The polyad phase spheres from the fits of the experimental stretching spectra of H2O, O3 and SO2 are displayed. H2O and O3 are seen to be molecules with a local to normal modes transition, while SO2 is seen to be very near the pure normal modes limit. The experimentally determined phase space dynamics of H2O seen on the phase sphere are compared with the dynamics determined by Lawton and Child from trajectory calculations on the Sorbie–Murrell potential surface. The coordinate space trajectories corresponding to the phase spheres are compared with wave functions from the fit of the spectrum.
Electronic transitions from the A 2Πu
i(v=3) level of N+2 induced by inelastic collisions with helium atoms
Authors: Katayama Daniel H. , Dentamaro Anthony V.
Published: July 2002

An optical–optical double-resonance technique utilizing two pulsed lasers is used to study collision-induced electronic transitions from the N+2  A 2Πui(v=3) level by helium. Collisional deactivation paths are determined by this technique and found to be between this level and the X 2Σ+g (v=7 and 6) levels. The same propensity for ΔJ≊0 occurs for both of these paths in spite of an electronic energy gap size of approximately 0 cm−1 between the A(v=3) and X(v=7) levels and a large gap size of about 1950 cm−1 between the A(v=3) and X(v=6) levels. The electronic quenching rate from A(v=3) to X(v=7) is found to be only about three times larger than that to the X(v=6) level. We use this branching ratio in an electronic relaxation model to determine the collisional quenching rates between the A(v=3) and X(v=7 and 6) levels. These state specific rates are determined by fitting the model to observed radiative decay curves from the A(v=3) level obtained at various helium pressures. There is excellent agreement between the analytical and observed decay curves. The relatively efficient nature of the collision-induced electronic transition over the large energy gap is somewhat surprising in view of the fact that the nitrogen ions and helium atoms must remove most of this energy as translational kinetic energy. We have also revised our previous rate constants from the A 2Πui(v=4) level for 14N+2 and 15N+2.
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o configuration-interaction study of the photoelectron spectra of small sodium cluster anions
Authors: Bonačić-Koutecký V. , Fantucci P. , Koutecký J.
Published: July 2002

We find that the ab initio direct configuration-interaction calculations on Na−2–5 and Na2–5 account for the observed patterns of photoelectron spectra, reproduce the observed excitation energies in a semiquantitative way and permit an assignment of cluster geometries.
Rigorous bounds to molecular electron repulsion and electrostatic potential integrals
Authors: Gadre Shridhar R. , Kulkarni Sudhir A. , Pathak Rajeev K.
Published: July 2002

Novel rigorous upper and lower bounds, at primitive level, to general electron-repulsion integrals (ERIs) involving Gaussian basis sets have been derived and interconnections with the earlier works in the literature are brought out. New optimal strategies for a preemptive elimination of insignificant ERIs at atom and contraction levels are discussed and tested, resulting in a significant reduction in CPU time. Similar analysis is carried out for the computation of the molecular electrostatic potential for the first time in the literature, leading to a marked savings in computer time.
Argon, krypton, and xenon excimer luminescence: From the dilute gas to the condensed phase
Authors: Morikawa E. , Reininger R. , Gürtler P. , Saile V. , Laporte P.
Published: July 2002

For the first time, the evolution of luminescence from rare gases was studied as a function of number density. Synchrotron radiation served as a light source for selective and pulsed excitation of the samples. The excitation spectra confirm previous results on perturbed Rydberg states and exciton appearance in dense media. In time-resolved emission spectra the peak energies and widths of the luminescence bands were followed. The energy separation between the fast and slow components is found to be density independent. A model proposed by Cheshnovsky et al. [Chem. Phys. Lett. 15, 475 (1972)] accounts for the change in peak width with temperature. Both lifetimes decrease with increasing density. The data extrapolate to 3.3±0.1 ns (Ar); 3.4±0.1 ns, 270±5 ns (Kr); 4.5±0.1 ns, 100±5 ns (Xe) for the low density limit. For the solid at the triple point, we obtain 1.3±0.1 ns, 82±5 ns (Kr) and 1.1±0.1 ns, 18.5±0.5 ns (Xe). Theories on density dependence of lifetimes give only a qualitative description of the experimental results.
Photoionization and photoelectron spectroscopy of O2 with coherent vacuum ultraviolet radiation
Authors: Tonkyn Russell G. , Winniczek Jerry W. , White Michael G.
Published: July 2002

The photoionization of O2 near its ionization limit has been studied with coherent vacuum ultraviolet radiation produced by third harmonic generation in free jet expansions of the rare gases. High resolution (∼2 cm−1) photoionization spectra were obtained in the ionization threshold region from 103 to 98 nm which includes three vibrational levels of the H 3Πu (3sσ) Rydberg state. The H, v=0 photoionization spectrum was assigned by simulating the H 3Πu←X 3∑+g Rydberg excitation, yielding spectroscopic constants as well as the overall autoionization lifetime. The v=1 and 2 levels have distinctly different rotational band contours which reflect perturbations with bound and dissociative levels of nearby ‘‘dark’’ states. The photoionization dynamics were probed further through measurements of photoelectron angular distributions for the v+=0 and 1 vibrational levels of O+2. In addition to strong variations in the asymmetry parameter (β) across the H state autoionization resonances, spectrally narrow variations in β were found in the surrounding continuum. These latter results suggest the presence of weak resonance features imbedded in the background continuum which nonetheless strongly influence the photoelectron ejection dynamics.
Dynamics of excess electron migration, solvation, and spectra in polar molecular clusters
Authors: Barnett R. N. , Landman Uzi , Nitzan Abraham
Published: July 2002

The dynamics of excess electron localization, migration, and solvation in water and ammonia clusters, and the time-resolved spectroscopic consequences of these processes, are investigated via computer simulations. In these simulations, the solvent evolves classically and the electron propagates in the ground state. The coupling between the polar molecular cluster and the electron is evaluated via the quantum expectation value of the electron–molecule interaction potential. Starting from an electron attached to a cold molecular cluster in a diffuse weakly bound surface state, temporal stages of the electron solvation and migration processes, leading to the formation of an internally solvated state, and the associated variations in the excitation spectra are described. The migration of the excess electron during the penetration is characterized by a nonhopping, polaronlike mechanism.
π* and σ* molecular orbitals of condensed films of chlorobenzenes and hexafluorobenzene observed by inverse photoemission
Authors: Dudde R. , Reihl B. , Otto A.
Published: July 2002

The electron affinity levels of condensed films of benzene, chlorobenzene, 1.4-dichlorobenzene, s-trichlorobenzene, and hexafluorobenzene on Cu(111) have been determined by isochromat inverse photoemission spectroscopy. In addition to the π* orbitals of the benzene ring, low-lying σ* orbitals are observed in the chlorine and fluorine-substituted molecules. Comparison to electron transmission spectra allows the assignment of the unoccupied molecular orbitals and the determination of the relaxation energies in going from the gas phase to the solid state. These relaxation energies are found to decrease in the series of benzene to trichlorobenzene.
Motion of a contact line on a heterogeneous surface
Authors: Joanny J. F. , Robbins Mark O.
Published: July 2002

Using the lubrication approximation, we calculate the dynamics of a contact line on a periodic heterogeneous plate which is advanced at constant force F or constant velocity u. For a constant force, motion starts when F exceeds a threshold Fc which is simply related to the static advancing (receding) contact angle. If the heterogeneity is smooth, the force increases with velocity as F−Fc∝u2. Alternating patches of constant wettability produce a linear relation. For constant velocity experiments, we identify weak and strong pinning regimes. In the weak pinning regime, the threshold force is zero and the force–velocity relation approaches that of a uniform surface. In the strong pinning regime, the threshold F0 is finite and approaches Fc as the strength of the heterogeneity increases. For smooth heterogeneity, F−F0∝u2/3, while alternating patches produce a linear response. The relevance of these results to experimental surfaces with random heterogeneity is discussed.
Excited-state proton transfer in gas-phase clusters: 2-Naphthol⋅(NH3)n
Authors: Droz Thierry , Knochenmuss Richard , Leutwyler Samuel
Published: July 2002

Excited-state proton transfer was studied in supersonically cooled neutral acid-base clusters of 2-naphthol⋅(NH3)n with n=1-10, using size-selective two-color resonant two-photon ionization (R2PI) and fluorescence emission techniques. The smaller clusters (n≤3) show vibrationally resolved spectra and do not exhibit excited-state proton transfer. Two rotamers (cis and trans) were observed for each cluster size; these exhibit very different electronic-vibrational couplings to the intermolecular modes, reflecting the orientation of the (NH3)n cluster relative to the electronic transition dipole moment of the aromatic chromophore. Excited-state proton transfer occurs for n≥4, as evidenced by (a) a large spectral red shift (≊8000 cm−1) of the fluorescence emission band; (b) large increase in the emission bandwidth; (c) similarity of the fluorescence band position and width to that of 2-naphtholate anion in neutral or basic aqueous solution. In absorption, the S1←S0 bands are substantially broadened for clusters with n≥4, but are not shifted to the red, as is the case for the analogous 1-naphthol⋅(NH3)n clusters. The cluster-size dependence of the proton transfer reaction is mainly controlled by the cluster proton affinity.
A turbidity study on colloidal silica particles in concentrated suspensions using the polydisperse adhesive hard sphere model
Authors: Penders M. H. G. M. , Vrij A.
Published: July 2002

A new type of analysis of turbidimetric measurements is proposed for concentrated sterically stabilized colloidal dispersions. It is applied to dispersions of silica spheres, coated with stearylalcohol in aromatic solvents up to high volume fractions of 0.30–0.40. Baxter’s polydisperse adhesive hard sphere model is used to explain the results. Influence of diameter, stickiness, and polydispersity in diameter on turbidity will be discussed. Model calculations fit the experimental curves very well up to high volume fractions and from the data it can be concluded that there exists a small attraction between the particles.
Pseudospectral Hartree–Fock theory: Applications and algorithmic improvements
Authors: Ringnalda Murco N. , Belhadj Mahfoud , Friesner Richard A.
Published: July 2002

Several additions to the pseudospectral Hartree–Fock theory are described, including a localized least-squares procedure, various numerical cutoff algorithms, and calculation of all integrals in the diatomic frame. This pseudospectral method is tested on 23 molecules, ranging in size from two to twenty atoms (200 6-31G** basis functions). A direct comparison of accuracy and computational efficiency is made with the conventional electronic structure programs gamess, gradscf, gaussian 86, and gaussian 88. The pseudospectral code is shown to be up to nine times faster than any of the above programs for the molecules tested here; moreover, this timing advantage increases with molecular size, suggesting that ab initio calculations may soon be possible on large systems not accessible by the Roothaan–Hall procedure.
Measurement of the lifetime of metastable triatomic hydrogen
Authors: Bordas C. , Cosby P. C. , Helm H.
Published: July 2002

We measured the survival probability of the metastable (N=0, K=0) level of the 2p2A″2 of H3 as a function of time elapsed from formation of the molecule by probing its population with photoionization. We find lifetimes τ=640+300−100 ns for the ground vibrational state and τ=740+300−100 ns for the symmetric stretch-excited level. Equally short lifetimes are obtained from an analysis of the photoinduced bleaching of the spontaneous dissociation signal of metastable H3. These lifetimes are about 2 orders of magnitude shorter than those expected on the basis of the allowed radiative transition 2p2A″2→2s2A′2. We attribute the faster decay channel to weakly allowed radiative transitions between the metastable state and the degenerate mode-excited repulsive ground state of H3, as well as to predissociation of the metastable levels by the repulsive ground state of H3 induced by spin–orbit coupling.
High resolution and Doppler-limited laser investigation of atomic oxygen production in O2–noble gases radio-frequency discharges
Authors: Sasso A. , Schisano M. I. , Tino G. M. , Inguscio M.
Published: July 2002

The result of a Doppler-limited and sub-Doppler optical laser spectroscopic investigation of atomic oxygen is presented. Atomic oxygen is produced in a O2–noble gas radio-frequency discharge of moderate power. The analysis of the atoms produced is performed by monitoring both fluorescence and the optogalvanic signal for oxygen optical transitions between highly excited levels. Doppler broadening measurements illustrate a nonequilibrium thermal distribution selective on the spin value and on the gas used to sustain the discharge. A simple model based on energy transfer processes is proposed to explain the observed phenomena.
The Cs(7P)+H2→CsH+H reaction. I. Angular scattering measurements by Doppler analysis
Authors: L’Hermite J.-M. , Rahmat G. , Vetter R.
Published: July 2002

The Cs(7P)+H2→CsH (X 1Σ+) (v″=0,J″)+H reaction has been studied in a crossed-beam experiment, with electronic excitation of Cs atoms and laser-induced fluorescence detection of CsH products. The Doppler technique has been applied to determine the center of mass angular scattering probability of CsH products, for all rotational levels between 1 and 14 (in v″=0) and for various values of the collision energy between 0.03 and 0.13 eV. We have used two complementary arrangements of the analysis laser beam: With a ‘‘parallel’’ arrangement (the laser beam is along the collision axis), the shape analysis of fluorescence profiles leads directly to the angular scattering probability but experimental constraints limit the range of measurements; with a ‘‘perpendicular’’ arrangement (the laser beam is perpendicular to the collision plane), fluorescence profiles are symmetrical and ‘‘forward’’ contributions cannot be separated from ‘‘backward’’ ones. However, extensive shape analyses have been performed with this perpendicular arrangement, since the direction of scattering was determined earlier by recording several profiles with the parallel arrangement. For the title reaction, it is shown unambiguously that, in the center of mass, CsH products scatter preferentially at small angle, in the forward direction with respect to the Cs initial velocity. The angular scattering probability varies with the collision energy but is independent of the rotational level of the product molecule (thus of its recoil velocity). These results are compatible with the harpooning mechanism which was suggested for the reaction.
Photoabsorption of negative cluster ions near the electron detachment threshold: A study of the (O2)−n system
Authors: DeLuca Michael J. , Han Chau-Chung , Johnson Mark A.
Published: July 2002

In order to better understand the propensity of negative cluster ion systems to photodissociate when excited near their photodetachment thresholds, we present a detailed photochemical study of the oxygen cluster negative ions, (O2)−n. Irradiation of the 3≤n≤6 parent ions at or slightly below their photodetachment thresholds in the near ir (1064 nm) is found to result in significant photofragmentation, even though the dimer is effectively photostable at this wavelength. The cross sections for n≥3 photofragmentation are approximately constant over the higher clusters and are about a factor of 3 larger than the O−2 photodetachment cross section. These observations suggest that photoabsorption of the higher clusters may result from a charge-transfer process between the O−4 core ion and the ‘‘solvent’’ O2 ligands.
The rates of HCl loss from energy-selected ethylchloride ions: A case of tunneling through an H-atom transfer barrier
Authors: Booze Jon A. , Weitzel Karl-Michael , Baer Tomas
Published: July 2002

The dissociation rates of energy-selected ethylchloride and deuterated ethylchloride ions were measured as a function of the parent-ion internal energy by the method of photoelectron photoion coincidence. Previously performed ab initio calculations indicated that the rate-determining step for this reaction is an H-atom transfer from the β carbon to the Cl atom via a substantial energy barrier of 92 kJ/mol (referenced to the zero-point energy). The ion internal energy range in which the experimental rates varied between 105 and 107 s−1 was found to lie well below the calculated barrier for H-atom transfer. The rates were modeled with the RRKM statistical theory which includes a tunneling step through an Eckart potential. The vibrational frequencies of both the normal and deuterated ethylchloride ions were determined by ab initio molecular-orbital methods. The theory accounted very well for the absolute rates including the strong deuterium isotope effect. The measured kinetic-energy release distribution appears nonstatistical. This indicates that the ion–dipole complex, which lies in between the transition state and the C2H+4+HCl products, is ineffective in randomizing the potential energy of the reverse activation barrier.
‘‘Magic number’’ carbon clusters: Ionization potentials and selective reactivity
Authors: Zimmerman Jeffrey A. , Eyler John R. , Bach Stephan B. H. , McElvany Stephen W.
Published: July 2002

The ionization potentials (IPs) of several large carbon clusters Cn (n≥48), including the enhanced abundance (‘‘magic number’’) clusters C50, C60, and C70, have been determined by Fourier transform ion cyclotron resonance (FTICR) mass spectrometric charge transfer bracketing experiments. The IPs of C50, C60, and C70 were bracketed by the same two charge transfer compounds, leading to a common value of 7.61±0.11 eV. The IPs of even numbered clusters adjacent to these magic number clusters were found to be lower by as much as 0.5 eV and all clusters between C50 and C200 were determined to have IPs greater than 6.20 eV. The reaction rates of C+60 and C+70 with metallocenes were anomalously slow in comparison to the other large carbon cluster ions. IP and reactivity results suggest that C50, C60, and C70 may indeed have different or more stable structures than neighboring clusters, which supports the hypothesis of closed-shell, spherical species. The implications of these results for the mechanism of C+n formation by direct laser vaporization are also discussed.
Positions, lifetimes, and partial widths of metastable quasienergy states by solving the time-dependent complex-scaled Schrödinger equation
Authors: Ben-Tal Nir , Moiseyev Nimrod , Leforestier Claude , Kosloff Ronnie
Published: July 2002

The method presented here is based on the solution of the time-dependent complex-scaled Schrödinger equation to provide the complex-scaled evolution operator after one optical cycle. This method is mainly suitable in the study of multiphoton ionization or dissociation under the influence of high intense fields. An illustrative numerical example is given.
Infrared spectroscopy of HX (X=Br,Cl) adsorbed on LiF(001): Alignment and orientation
Authors: Blass P. M. , Jackson R. C. , Polanyi J. C. , Weiss H.
Published: July 2002

The infrared spectra of HBr and HCl on LiF(001) single crystal surfaces were measured as a function of coverage at temperatures ≤83 K using Fourier-transform infrared (FTIR) spectroscopy. For each hydrogen halide three different spectral features could be distinguished. At low coverages broad absorptions centered at 2265±20 cm−1 (HBr) and at 2515±20 cm−1 (HCl) were observed. These absorptions were attributed to molecules hydrogen-bonded to F− anions of the surface, the angle between the molecular axis and the surface being 21±5° for HBr and 19±5° for HCl as determined from experiments employing polarized infrared radiation. Hydrogen bonding was evidenced by: (i) redshifts with respect to the gas phase (∼300 cm−1), (ii) broad infrared absorption (FWHM: 265±25 cm−1 for HBr, 295±15 cm−1 for HCl), and (iii) enhancement of the infrared absorption intensity compared to the gas phase by more than one order of magnitude for both HBr and HCl. With increasing coverage a second absorption was observed before the first one saturated (HBr:ν=2461±5 cm−1, FWHM=75±10 cm−1; HCl: ν=2763±5 cm−1, FWHM=80±10 cm−1). This absorption was attributed to molecules adsorbed in a second layer. The smaller redshift and spectral width for the second layer were consistent with weaker hydrogen bonding, probably to the halogen of molecules adsorbed in the first layer. Further increase in coverage resulted in the appearance of the well-known doublet absorptions due to formation of solid. Coadsorption of HBr and HCl, as well as experiments under adsorption–desorption equilibrium conditions, confirmed that the first and second layers could coexist. The isotherms could best be understood on the assumption of a repulsive interaction within the first layer.
Formation of negative cluster ions from (CO2)m in collision with high-Rydberg atoms
Authors: Misaizu Fuminori , Mitsuke Koichiro , Kondow Tamotsu , Kuchitsu Kozo
Published: July 2002

Negative ions of CO2 clusters (CO2)m produced by electron transfer from highly excited Rydberg atoms were investigated by mass spectrometry. The relative intensities of (CO2)−n (n≥3) (size distribution) showed a characteristic structure with magic numbers at 9, 14, and 16. The dependence of the intensities of (CO2)−n (n=7–20) on the stagnation pressure of the cluster beam source was measured. The threshold pressure at which a cluster anion of a given size started to appear increased irregularly with the size. These observations suggest that evaporation of CO2 molecules occurs by dissipation of the excess energy after electron attachment and that the number of evaporating molecules m–n depends on the size of the resulting ion. The evaporating molecules are fewer at n=9, 14, and 16 than those at adjacent sizes. The cluster ions of these sizes are shown to have higher dissociation energies than other ions, and the magic numbers observed in the mass spectrum are ascribed to the difference in the stability of the ions.
Controlling the internal energy content of size-selected cluster ions: An experimental comparison of the metastable decay rate and photofragmentation methods of quantifying the internal excitation of (H2O)−n
Authors: Campagnola Paul J. , Posey Lynmarie A. , Johnson Mark A.
Published: July 2002

The metastable decay rates of (H2O)−n clusters are found to be strongly dependent on source conditions and are correlated to the distribution of photofragments. We relate these variations to changes in the internal energy content of the clusters and analyze the effects to provide two independent determinations of the internal energy variation. The two methods are found to be in surprisingly good agreement. Measurements on larger clusters are carried out using photofragmentation, and changes on the order of 0.30 eV are easily affected by changing the pressure of argon backing the expansion. An interesting result of this work is that the cooling efficiency increases with cluster size.
The wave packet motion and intramolecular vibrational redistribution in CHX3 molecules under infrared multiphoton excitation
Authors: Marquardt Roberto , Quack Martin
Published: July 2002

We report results from quantum dynamical simulations of ultrafast vibrational redistribution processes in the CH chromophore of CHX3 molecules (CHD3, CHF3) during and after infrared-multiphoton excitation. The vibrational Hamiltonian is based on results from high resolution spectroscopy and ab initio calculations of the potential hypersurfaces for these molecules. The quantum dynamical calculations involve accurate solutions of the time dependent quantum equations of motion by means of both Floquet and quasiresonant approximations. We find mode selective redistribution between the CH stretching and bending modes on a time scale of 50 to 100 fs. Other modes participate only on much longer time scales (>1 ps), as was shown previously by analysis of the spectra. For the real, strongly anharmonic systems (k′sbb≂30 to 100 cm−1 ), the redistribution is nonclassical with fast spreading to a quasimicrocanonical distribution, which is particularly pronounced if a narrow range of energies (for example, the N=6 polyad) is initially excited. The effect can be interpreted as an intrinsic quantum statistical behavior induced by anharmonicity. In comparison, a weakly anharmonic hypothetical model system (ksbb≤2 cm−1) leads to quasiclassical motion of the wave packet with quasiperiodic exchange between stretching and bending motions. We present an approximate analytical investigation of the Fermi modes underlying the dynamics which provides a semiquantitative understanding of the Fermi-resonance spectra. On the basis of these results, we discuss possibilities of mode selective reaction control in unimolecular processes with laser excitation and some aspects of intramolecular ‘‘chaos.’’
Fragmentation of the hexafluorobenzene dication formed by photoabsorption in the 30–120 eV energy range
Authors: Ibrahim K. , Lablanquie P. , Hubin-Franskin M.-J. , Delwiche J. , Furlan M. , Nenner I. , Hagan D. , Eland J. H. D.
Published: July 2002

The dissociation pattern of the doubly charged C6F2+6 ion produced by single photon ionization has been investigated in the 27–120 eV energy range using time-of-flight (TOF) mass spectrometry, double photoion–photoion coincidence (PIPICO), and triple photoelectron–photoion–photoion coincidence (PEPIPICO) spectroscopies. A ‘‘metastable’’ C6F2+6 ion is observed with an appearance energy of 27.3±0.4 eV dissociating slowly to the lowest thermodynamical limit leading to the ion pair products CF++C5F+5 (appearance energy of 30.7±0.5 eV). At higher energies, several other charge separation reactions appear as well as covalent channels in which neutral fragments are ejected from C6F2+6. Additional pairs of correlated ions are formed by secondary fragmentations. In the threshold region, double ionization of the molecule is probably followed by structural rearrangements to distinct C6F2+6 isomers, each dissociating to specific dissociation channels. Partial cross sections of selected ion pairs measured in the 30–120 eV photon energy region exhibit strong resonances suggesting that a rather large part of double ionization is an indirect phenomenon.