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  • 1
    Electronic Resource
    Electronic Resource
    Surface lattice dynamics and inelastic He scattering in Cu(111) (1992)
    Springer
    Il nuovo cimento della Società Italiana di Fisica 14 (1992), S. 627-641 
    Details
    ISSN: 0392-6737
    Keywords: Surface dynamics and interface vibrations
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary We present a calculation of the Cu(111) surface dynamics in the framework of the multipole model. The electronic degrees of freedom include dipole and quadrupole deformabilities of the conduction electron density, the multipole expansion points being located at the midpoints between nearest-neighbour ions. The model accounts for the anomalous longitudinal resonance by an increase of dipolar deformability at the surface. Moreover the model explains in a straight-forward way the intense He scattering from the longitudinal resonance via the dipolar and quadrupolar modulations of the surface electron density. The surface dipolar contribution also explains the intense electron scattering from the optical surface resonance localized on the second layer.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02462350
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  • 2
    Electronic Resource
    Electronic Resource
    Mass spectra and time-of-flight distributions of helium cluster beams (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 6875-6889 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Liquid helium clusters are produced by expanding gaseous 4 He into a vacuum from a cold source with temperatures between 5 and 20 K at stagnation pressures from P0 =8 to 20 bar and are studied by time-of-flight (TOF) and mass spectrometry. At low temperatures, T0 〈12 K, the mass spectra show several anomalies which can be attributed to pick-up of residual gases. At T0 〈10K, there is evidence for a very intense peak at m=16 amu which is attributed to He+4 . Depending on the temperatures, the TOF spectra reveal ions with three different velocities. These TOF observations are analyzed using isentropic lines in the known phase diagram of 4 He, which take into account deviations from ideal gas behavior. Three qualitatively different expansion regimes are identified: (I) the expansion proceeds through a region on the high temperature side of the critical point, (II) the expansion passes through or near the critical point, and (III) the expansion passes through a region on the low temperature side of the critical point. The mass spectra, peak velocities and speed ratios, when analyzed with the aid of the phase diagram, indicate that (a) two of the TOF peaks are due to clusters, (b) the fastest cluster peak is due to clusters formed by condensation of gas phase atoms, and (c) the slowest cluster peak is due to either separation into two phases (regime II) or disintegration of a liquid phase (regime III). Measured conversions of initial enthalpy into free jet kinetic energy suggest that the cluster temperature undergoes a sharp drop to a very low temperature approaching 0 K at T0 〈6.5 K where the expansion isentrope intersects the liquid–vapor line upstream from the source orifice.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458275
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  • 3
    Electronic Resource
    Electronic Resource
    A molecular beam study of the potential anisotropy of laser vibrationally excited Li2(v=0,20) scattered from Kr (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 287-294 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A beam of highly vibrationally excited (v≤20) Li2 molecules has been generated via Franck–Condon pumping using a cw ring dye laser. The linearly polarized light from a second cw dye laser is used to determine the polarization of the remaining unscattered molecules of the beam after it has passed through a scattering cell filled with krypton gas. The dependence of the state selective total integral cross section on orientation is obtained from the anisotropy of the angular momentum distribution of the beam molecules described by 1+a2P2(cos θ), where θ is the angle between the angular momentum vector and the velocity vector. The measured polarization increases from a2=−0.019±0.007 for Li2(v=0)–Kr collisions to a2=−0.074±0.016 for Li2(v=20)–Kr collisions. The measurements are interpreted in a vibrationally adiabatic approximation in terms of the potential anisotropy parameter q2 for an assumed simple anisotropic potential model V=±(Cs/Rs) [1+q2⋅P2(cos γ)], where γ is the angle between bond distance r and distance between center of mass R. For the v=20 vibrationally excited molecules we obtain q2=1.38±0.31 compared to q2=0.47±0.15 for the v=0 molecules. This large increase in anisotropy with vibrational excitation is compared with that obtained from a previously proposed model for Li2(v)–He [J. Chem. Phys. 85, 7062 (1986)].
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455523
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  • 4
    Electronic Resource
    Electronic Resource
    Interaction potential for one-phonon inelastic He–Cu(111) and He–Ag(111) scattering (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 3693-3710 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The interaction potential for the elastic and inelastic scattering of low-energy He atoms from the Cu(111) and the Ag (111) crystal surface is derived from the sum of semi ab initio pair potentials modified by a damping function which takes into account the smoothing effect of the conduction electrons at large parallel momentum transfer. By adjusting only two free parameters of the potential model a very good fit to published experimental diffraction intensities and bound state levels is obtained. Without any further adjustment of the potential new experimental data of one-phonon inelastic reflection coefficients can be reproduced within the experimental error by calculations based on the distorted wave Born approximation. This new potential model has made it possible to study the dependence of the inelastic intensities on potential parameters and different experimental conditions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.451972
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  • 5
    Electronic Resource
    Electronic Resource
    Helium atom scattering from CO adsorbed at steps on Ni(001) (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 1435-1441 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Diffraction of helium atoms from randomly distributed step edges on the otherwise smooth (001) surface of Ni has been observed. Measurements for different azimuths near the 〈100〉 direction indicate that the step edges are lined up perpendicular to the 〈100〉 direction. The effect of CO adsorption on both the amplitude of the diffraction oscillations and also on specular scattering has been studied. The in-plane diffraction oscillations from the steps and the specular peak intensity show a similar attenuation with CO adsorption. This is interpreted in terms of a simple model for the effect of adsorption on the step diffraction oscillations. With increasing CO adsorption, the step edge diffraction patterns are also found to be slightly shifted suggesting a continuous change in step edge shape with adsorption.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458102
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  • 6
    Electronic Resource
    Electronic Resource
    Erratum: Molecular beam translational spectroscopy of physisorption bound states of molecules on metal surfaces. I. HD on Cu(111) and Au(111) single crystal surfaces [J. Chem. Phys. 85, 2249 (1986)] (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 6678-6678 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.454762
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  • 7
    Electronic Resource
    Electronic Resource
    The exchange energy of H+2 calculated from polarization perturbation theory (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 7266-7277 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The Rayleigh–Schrödinger (polarization) perturbation theory without symmetrization is used to calculate the exchange energy of H+2 through the surface integral of Holstein–Herring. It is mathematically proven that the exchange energy series so obtained is exact in the same sense as Herring's result is exact. It is shown that the contributions to the leading term of the exchange energy series from all orders of polarized wave functions can be calculated exactly. Furthermore, it is explicitly demonstrated that the sum of these contributions converges to the exact value. The rate of convergence is relatively fast. With the first four orders of wave function, virtually 100% of the asymptotic exchange energy is recovered. With the present theory, terms other than the leading one can also be calculated systematically. This is demonstrated by the calculation of the first three terms of the exchange energy series from the first- and second-order wave functions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460211
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  • 8
    Electronic Resource
    Electronic Resource
    Observation of vibrationally resolved charge transfer in H++H2 at ECM=20 eV (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 2685-2694 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The doubly differential cross sections for both the scattered protons and H atoms have been measured at ELAB=30 eV (ECM=20 eV) from θLAB=0° to 12° (θCM=0° to 18°) for the reactions H++H2(v=0)→H++H2(vf) and →H+H+2(vf) . The energy resolution is sufficient to resolve final vibrational states in both channels. The comparison of both the angular and energy loss distributions for the two product channels provides the first clear experimental evidence of a two-step charge transfer mechanism: Vibrational excitation on the lower H++H2 surface is followed by charge transfer in the outgoing collision for only those H2 molecules which are excited vibrationally high enough (vf≥4) to overcome the endoergic barrier (ΔE=1.83 eV). The final vibrational distributions of H+2 appear to be very similar to those of H2 for vf≥4 indicating that for the angular range observed the charge transfer probability is the same for all vibrational states with vf≥4. The comparison with classical trajectory surface hopping (TSH) calculations points to some disagreement which probably can be attributed to the potential surface used.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453107
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  • 9
    Electronic Resource
    Electronic Resource
    Vibrational relaxation of hydrogen fluoride by HF dimers in a laser excited nozzle beam (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 85 (1986), S. 2653-2663 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An HF nozzle beam and a cw HF laser beam in a coaxial and antiparallel arrangement facilitated exciting HF molecules vibrationally in v=1,J up to the saturation limit. A beam consisting exclusively of monomeric HF was generated by heating the beam source to 600 K and was confirmed using a mass spectrometer. Energy-flux measurements made using a bolometer indicate that the vibrational energy distribution initiated by the laser is conserved, confirming that V–T energy transfer to monomers is slow. The maximum measured flux of HF(v=1,J) molecules was 1.5×1017 molecules s−1 sr−1 at 670 mbar stagnation pressure. Beams consisting of a mixture of monomeric and polymeric HF were generated using appropriate combinations of source temperature and pressure. In the presence of dimers, the excited monomers relax rapidly by a fast V–V transfer and subsequent predissociation of the metastable dimers. The loss of beam energy due to energy transport by the departing dimer fragments was detected bolometrically. The laser-induced depletion of dimers was determined over a wide source pressure range at T0=300 K using a mass spectrometer. From a kinetic model of these dimer-depletion data, the product of cross section and transfer probability for V–V energy transfer from HF (v=1) monomers to dimers was deduced to be 14 A(ring)2. The terminal-dimer-concentration equation for HF nozzle beams was refined. An analysis of the energy fluxes measured by the bolometer with no laser excitation of the beam revealed that, for an HF nozzle beam, the flow from the source to the detector is, in general, nonisenthalpic due to the onset of dimerization. Mach-number focusing enriches the dimer mole fractions detected by the bolometer. An analysis of the excess enthalpy due to the presence of polymers in the beam results in a new method for the experimental determination of the dissociation energy of HF dimers, determined to be 0.36 eV.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.451075
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  • 10
    Electronic Resource
    Electronic Resource
    High resolution time-of-flight spectra for reactive scattering of F+D2 (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 4676-4677 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460599
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