Scattering of helium atoms from adsorbates: Van Der Waals potentials and scattering formalism

Gumhalter, B.; Liu, W.-K.

doi:10.1016/0039-6028(84)90587-9

Cited by 28 publications

(4 citation statements)

References 36 publications

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“…Eqs. (4), (5), and (6)]. Third, as a result of nonvanishing resonance occupation, the transitions from the occupied part of the resonance to higher adsorbate levels also may take place [ Fig.…”

Section: -(A) + I8) 2 ]mentioning

confidence: 98%

“…5 They comprise a direct interaction appearing also in the gas phase and the surface-mediated image and interference terms which, as well as the direct one, behave as E = -C/r 6 for large r. The strength C of each of these potentials in the case of He-adsorbed-CO interaction is determined by the constants 5 …”

Section: Implications Of Fractional Occupation Of Adsorbate Resonancementioning

confidence: 99%

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Implications of Fractional Occupation of Adsorbate Resonances on van der Waals Interactions at Surfaces

Gumhalter¹,

Wandelt²

1986

Phys. Rev. Lett.

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Partial occupation n a of a resonant adsorbate valence state due to chemisorption leads to n a -dependent phase space for adsorbate dipole electronic transitions. This fundamentally influences the dynamic polarizability and related physics of an adsorption complex. We outline a unified interpretation of such different observations as the induced dipole moment of adsorbed rare-gas atoms, the cross section of He scattering from adsorbed CO, and the change in effective deexcitation energy of metastable atoms in Penning spectroscopy of adsorbates, based on the n a dependence of van der Waals forces at surfaces.PACS numbers: 73.20.HbThe concept of adsorbate-induced resonances has greatly augmented the understanding of the electronic structure of chemisorption systems. 1,2 The electronic occupation of such an adsorbate resonance depends on its position with respect to the substrate Fermi level. In the case when the resonance intersects with the Fermi level, then, in general, fractional or noninteger occupation n a of the localized resonant state | a> may be expected. Its occupied and unoccupied states can then be considered as a filled and empty part of a localized band, respectively. Such static fractional electronic occupation is often used as a criterion for the estimate of the ionic (heteropolar) versus neutral (homopolar) character of the chemisorption bond considered. Large charge transfer from (to) a localized adsorbate resonance signifies cationic (anionic) chemisorption, both being the case of ionic chemical bonding. On the other hand, small charge transfer between the adsorbate resonance and the substrate valence band of states is a characteristic of covalent or homopolar chemisorption. 1,2 In addition to this chemical aspect of the problem, the existence of a partially occupied resonance will also affect the dynamic adsorbate electronic properties, namely, all the real and virtual electronic transitions within the adsorbate and the entire adsorption complex. This, in turn, will greatly influence the physical properties of the adsorbate, such as the van der Waals (vdW) forces acting between the adsorbate and the substrate, 3,4 between the adsorbed particles themselves, and between adsorbate and gas-phase particles, 5 " 8 as well as all the electronic relaxation processes within the adsorbate. 9 Both the vdW forces and the relaxation properties play an important role in surface spectroscopies 5 " 12 involving particle collisions 5 " 8,10 ' 11 or the interaction of adsorbates with radiation. 8 " 12 In this Letter we describe a unified view of the physical origin of these various, apparently quite different surface phenomena and processes, which is based on a correlation between the fractional occupation of the resonance and the thereby modified dynamic polarizability of adsorbates.Electronic transitions in adsorption systems in their ground states are subject to usual restrictions emerging from the Pauli exclusion principle. The initial states (from which the transitions take place) have to be occupied and the ...

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Section: -(A) + I8) 2 ]mentioning

confidence: 98%

Section: Implications Of Fractional Occupation Of Adsorbate Resonancementioning

confidence: 99%

Implications of Fractional Occupation of Adsorbate Resonances on van der Waals Interactions at Surfaces

Gumhalter¹,

Wandelt²

1986

Phys. Rev. Lett.

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“…Besides the direct gas phase interaction, there appear two additional contributions to VdW interaction due t o the presence of the polarizable metallic (substrate) surface. These substrate mediated contributions were calculated in [3] for the case of interaction between an atom (He) and an anisotropic adsorbate (CO). The total VdW interaction is thus a sum of three contributions: the direct one, the iinage one, and the interference one.…”

Section: Surface Polarization Effectmentioning

confidence: 99%

Physisorption and Chemisorption Effects on Atom—Adsorbate van der Waals Potentials. Application to He and He* Scattering from Adsorbed CO

Lovrić

Gumhalter

1987

Physica Status Solidi (b)

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The dynamical polarizability of an atomic or molecular species may undergo significant changes upon physical and chemical adsorption onto a metallic substrate. The physical effect derives from the presence of the polarizable substrate surface which gives rise to a modification of all the polarization induced or van der Wads potentials acting between the adsorbate and the nearby atoms or molecules in the gas phase. The chemical effect, viz. chemisorption, may give rise to the broadening of adsorbate valence orbitals into resonances which will be occupied up to the substrate Fermi level. Provided the electronic density of the localized resonance states a t the Fermi level is nonvanishing, chemisorption will open a new channel of intra-adsorbate dynamic polarizability. Both effects (i.e. the physical and the chemical one) will cnhance the total van der Waals potentials encountered in the thermal energy atom scattering and Penning spectroscopy of adsorbates. Calculations are made of these effects on the adsorbate-gas phase atom van der Waals potentials and their influence on the thermal energy atom scattering (TEAS) cross sections and the effective Penning deexcitatioii energies discussed for the case of He and He* collisions with adsorbed CO.Die dynamische Polarisierbarkeit einer atomaren oder molekularen Species kann sich nach physikalischer oder chemischer Adsorption suf ein metdlisches Substrat in bedeutendem MaSe verandern. Der physikalische Effekt entwickelt sich ails der Anwesenheit der polarisierbaren Substratoberflache, die AnlaD zu einer Modifikation aller induzierten Polarisationen oder der van der WaalsPotentiale liefert, die zwischen dem Adsorbat und den benachbarten Atomen oder Molekulen in der Gasphase wirken. Der chemische Effekt, z. B. Chemisorption, kann AnlaB geben zu einer Verbreiterung der Adsorbat-Valenzorbitale in Resonanzen, die bis zur Substrat-Fermikante besetzt werden. Vorausgesetzt, dal3 die Elektronendichte der lokalisierten Resonanzzustande am Ferminiveau nicht verschwindet, offnet die Chemisorption einen neuen Kana1 der dynamischen Intra-Adsorbat-Polarisierbarkeit. Beide Effekte (d. h. der physikalische und der chemische) erhohen die gesamten van der Waals-Potentiale, die an der thermischen Atomstreuung und Penning-Spektroskopie von Adsorbaten beteiligt sind. Es wird iiber die Berechnung dieser Effekte auf die van der Waals-Potentiale der Adsorbat-Gasphasenatome berichtet nnd ihr EinfluB auf die Wirkungsquerschnitte der thermischen Atomstreuung (TEAS) und die effektiven Penning-Abregungs-Energien fur den Fall von He-und He*-StoRen mit adsorbiertem CO diskutiert.

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“…Some efforts have also been directed to the dependence on the scattering parameters, such as incidence energy and angle. These cross section studies have attracted significant theoretical interest, [7][8][9][10][11][12][13][14] concentrating almost exclusively on the dependence on the scattering parameters. Relatively little attention has been paid to the inverse question of the information content of the cross section.…”

Section: Introductionmentioning

confidence: 99%

Optical theorem and the inversion of cross section data for atom scattering from defects on surfaces

Hamburger

Gerber

1995

The Journal of Chemical Physics

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The information content and properties of the cross section for atom scattering from a defect on a flat surface are investigated. Using the Sudden approximation, a simple expression is obtained that relates the cross section to the underlying atom/defect interaction potential. An approximate inversion formula is given, that determines the shape function of the defect from the scattering data. Another inversion formula approximately determines the potential due to a weak corrugation in the case of substitutional disorder.An Optical Theorem, derived in the framework of the Sudden approximation, plays a central role in deriving the equations that conveniently relate the interaction potential to the cross section. Also essential for the result is the equivalence of the operational definition for the cross section for scattering by a defect, given by Poelsema and Comsa, and the formal definition from quantum scattering theory. This equivalence is established here. The inversion result is applied to determine the shape function of an Ag atom on Pt (111) from scattering data.

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Scattering of helium atoms from adsorbates: Van Der Waals potentials and scattering formalism

Cited by 28 publications

References 36 publications

Implications of Fractional Occupation of Adsorbate Resonances on van der Waals Interactions at Surfaces

Implications of Fractional Occupation of Adsorbate Resonances on van der Waals Interactions at Surfaces

Physisorption and Chemisorption Effects on Atom—Adsorbate van der Waals Potentials. Application to He and He* Scattering from Adsorbed CO

Optical theorem and the inversion of cross section data for atom scattering from defects on surfaces

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