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  • 1
    Electronic Resource
    Electronic Resource
    Cluster sputtering: complete fragmentation of large ammonia clusters by photons and electrons (1999)
    Springer
    Applied physics 69 (1999), S. S209 
    Details
    ISSN: 1432-0630
    Keywords: PACS: 36.40.Qv
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract. The interaction of large ammonia clusters in the size range from n=20 to 2000 with photons and electrons are investigated in a reflectron time-of-flight mass spectrometer. The clusters are generated in adiabatic expansions through conical nozzles. The clusters are detected by (1+1) resonance-enhanced multiphoton ionization via the a state at 193 nm or electron impact ionization. By varying the power density of the laser between 20 and 1000 kW  cm-2 and the electron energy between 50 and 1000 eV, the clusters were largely fragmented. The operating mechanisms were identified as those responsible for the electronic sputtering of solid matter. The yields, however, were orders of magnitude larger than those for the solid. This result is a consequence of the finite dimensions of the cluster which cannot accommodate the energy.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s003399900390
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  • 2
    Electronic Resource
    Electronic Resource
    Two- and three-body forces in the interaction of He atoms with Xe overlayers adsorbed on (0001) graphite (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 6477-6493 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In order to address the problem of three-body interactions in gas–surface scattering, we considered the collision of a He atom with the (0001) surface of graphite coated by a monolayer of Xe. To eliminate the uncertainties connected with errors in the two-body He–Xe interaction, we determined the latter by crossed-beam differential collision cross-section measurements performed at two energies (67.2 and 22.35 meV). These scattering data together with room-temperature bulk diffusion data are then fitted with a Hartree–Fock–dispersion–type function to yield an interaction potential that explains most of the properties of this system within the experimental errors and represents an improvement on previously published He–Xe potentials. Helium diffraction measurements are then carried out from the Xe overlayer and the dependence of the specular intensity from the angle of incidence is carefully determined. Further, a He–surface potential is constructed by adding together the following terms: (1) the He–Xe pairwise sum, (2) the long-range He–(0001)C interaction, (3) the three-body contribution generated by the Axilrod–Teller–Muto term, (4) the so-called surface-mediated three-body interaction He–Xe–(0001)C first considered by A. D. McLachlan [Mol. Phys. 7, 381 (1964)], and finally (5) a small correction which is meant to take into account the nonstationary nature of the surface. Using this potential, well-converged close-coupling scattering calculations are carried out, and their results compared with the data. In general, good agreement is obtained. The agreement can, however, be improved by (a) an increase of about 30% in the contribution of three-body forces, (b) the lowering of the He–graphite long-range attraction coefficient by about 15%, or (c) a reduction of the two-body interaction well depth of 1.6% (the experimental error) together with any combination of the factors under (a) and (b) reduced by an adequate amount. Elimination of the contribution of the graphite surface by studying Xe multilayers is hindered by the uncertainties in the "thermal correction'' [point (5) above] which, due to the multilayer increased "softness,'' becomes an appreciable source of uncertainty.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457364
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  • 3
    Electronic Resource
    Electronic Resource
    Rotationally inelastic scattering of NH3 with H2: Molecular-beam experiments and quantum calculations (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 6419-6432 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In crossed molecular-beam experiments, three type of cross sections have been measured: Total differential cross sections with well-resolved diffraction oscillations for oD2–NH3 at E=95.9 and 111.3 meV, differential energy-loss spectra for ND3–oD2 at E=118.3 meV which cover the center-of-mass (c.m.) angular range from 85° to 170° and are obtained by time-of-flight (TOF) analysis, and state-to-state integral cross sections for oNH3–H2 and pNH3–H2 at E=75 meV for many final rotational states which are detected by resonance enhanced multiphoton ionization. These data which are mainly sensitive to the anisotropy of the potential energy surface are well reproduced by quantum calculations in the coupled-states approximation. The potential is constructed by combining large basis-set self-consistent-field (SCF) calculations with damped long-range dispersion coefficients. The two free parameters of the damping function are fitted to a restricted set of configuration interaction (CI) calculations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458958
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  • 4
    Electronic Resource
    Electronic Resource
    Infrared photodissociation of size selected internally excited ethylene clusters (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 6276-6283 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Ethylene clusters (C2H4)n are generated in a supersonic expansion with He and size selected by scattering from a helium beam. The clusters are dissociated upon absorption of a photon from a pulsed CO2 laser by exciting the ν7 mode of the monomer. During the collision about 30 meV of internal energy is transferred to the cluster so that the laser photons interact with internally hot clusters. The frequency and fluence dependences of the photodissociation cross sections are measured for (C2H4)n with n=2,3,4,5, and 6. Nearly all spectra exhibit structure which is most pronounced for the dimer. The overall width (FWHM) decreases from 31.2 cm−1 for the dimer to 12.2 cm−1 for the hexamer, while the maximum position is nearly the same (951.6 cm−1). The structure of the dimer spectrum is attributed to hot bands. The large linewidth corresponds to a short lifetime in the ps range.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453456
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  • 5
    Electronic Resource
    Electronic Resource
    Electron bombardment fragmentation of size selected NH3 clusters (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 4250-4255 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Ammonia clusters that are generated in a supersonic expansion with He are size selected by a scattering process with He atoms. By measurements of angular dependent mass spectra and time-of-flight distributions at all fragment masses, the complete fragmentation pattern for electron impact ionization of clusters from the dimer to the pentamer are obtained for electron energies of 70 eV. The dimer fragmentation which is dominated by the NH+4 ion (0.58) can be explained completely by fast ion–molecule reactions of the monomer fragments with the partner molecule. For the larger clusters up to n=11 the NH+4 ion is still the largest fragment channel. The other protonated ions NH+4(NH3)x start to contribute to the fragmentation with increasing cluster size but never exceed the contribution at mass 18 amu.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457783
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  • 6
    Electronic Resource
    Electronic Resource
    Improved potential energy surface for He–CO2 (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 4671-4679 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The anisotropic potential energy surface of He–CO2 is determined by the simultaneous analysis of newly measured high resolution total differential cross sections, differential energy loss spectra, new low temperature second virial coefficients, new diffusion, and viscosity data. The calculations are carried out in the infinite-order-sudden approximation. The repulsive anisotropy of the potential is determined from the rotationally inelastic cross sections and the quenching of the diffraction oscillations, while the absolute scale is fixed by the position of these oscillations. The second virial coefficient data are essentially sensitive to the general features of the spherical effective potential well. The transport data are then correctly predicted by this potential surface which differs both in the anisotropy and the spherical part from the recently derived multiproperty fit potential for this system.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455687
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  • 7
    Electronic Resource
    Electronic Resource
    Fragmentation of NH3 dimers by electron impact ionization (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 3028-3031 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453945
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  • 8
    Electronic Resource
    Electronic Resource
    Erratum: Two- and three-body forces in the interaction of He atoms with Xe overlayers adsorbed on (0001) graphite [J. Chem. Phys. 91, 6477 (1989)] (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4492-4492 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459724
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  • 9
    Electronic Resource
    Electronic Resource
    Infrared photodissociation of size-selected methanol clusters (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 6017-6029 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Size-selective infrared photodissociation spectra of (CH3OH)n clusters from n=2 to n=9 were measured near the absorption band of the C–O stretching mode of the monomer at 1033.5 cm−1 . The experiments were carried out in a molecular-beam apparatus in which the clusters are generated in a supersonic expansion and afterwards size selected in a scattering experiment with helium atoms. The internally excited clusters are dissociated by the radiation of a cw CO2 laser in a new antiparallel arrangement of the scattered cluster beam and laser beam. The observed spectra vary from the dimer, for which a double-peak structure appears, to the single-peak spectra of the trimer, tetramer, and pentamer which are continuously shifted to higher frequencies. A special transition is seen from the pentamer to the hexamer, for which again a double-peak structure is observed which continues to larger clusters. Applying an intermolecular model potential, a correlation between the observed spectra and the cluster configuration of minimum energy is derived. The line shifts of the dimer to the red and blue are caused by the nonequivalent position of the donor and acceptor in the hydrogen bond. The next three larger clusters are nearly planar rings, while from the hexamer onwards, only distorted rings and similar isomeric structures appear. These calculations together with the evaluation of the integrated dissociation cross section show that only internally excited dimers and trimers can be dissociated with one or two CO2 laser photons, respectively.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458373
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  • 10
    Electronic Resource
    Electronic Resource
    Multiple collision rotational rainbows: Theory and experiment for Xe–CO2 (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 82 (1985), S. 202-216 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In a crossed molecular beam experiment differential energy loss spectra have been measured for Xe+CO2 collisions at energies of 0.2, 0.58, 1.0, and 1.6 eV. Nearly the complete angular range from 40° to 180° in the center-of-mass system was covered. At large deflection angles and small energy transfers (Δ E/E(approximately-equal-to)0.1) the spectra exhibit a large intensity peak which cannot be explained by usual rotational rainbow theory. Quantum and classical calculations in the centrifugal sudden approximation demonstrate that this effect is a multiple collision rotational rainbow. In the first collision the kinetic energy is nearly completely transferred to rotational energy of the CO2 molecule. Since the heavy Xe atom leaves the interaction region very slowly, a second collision occurs and the rotational motion is deaccelerated. The classical excitation function J (γi), which relates the final angular momentum with the orientation angle of the molecule, has three extrema, two of which give rise to the multiple collision rotational rainbow. Various test calculations show that the effect depends strongly on the reduced mass, on the anisotropy and, in contrast to the normal rotational rainbow, also on the slope of the repulsive part of the interaction potential. Exact three-dimensional classical trajectory calculations at E=1.0 eV based on a realistic model potential agreed satisfactorily with the experimental results.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.448792
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