METASTABLE (23S) HELIUM ATOM SCATTERING FROM MOLECULAR ADSORBATES ON Pd AND Cu SURFACES

Fouquet, Peter; Witte, Gregor

doi:10.1142/s0218625x99000123

Cited by 5 publications

(3 citation statements)

References 19 publications

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“…It is about 6 times larger than for the clean Cu(100) surface and remains constant for multilayer films. Very broad, cosine shaped angular distributions of the scattered He ‫ء‬ atoms are observed with a FWHM of about 40 ± in agreement with previous measurements for clean and absorbate covered surfaces [12,16]. The total He ‫ء‬ survival probability was determined by angular integration of the measured He ‫ء‬ signal and is found to be proportional to the He ‫ء‬ signal close to the specular condition [12].…”

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confidence: 88%

Observation of Metallization Transition of 2D Alkali Metal Films

Fouquet

Witte

1999

Phys. Rev. Lett.

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The deexcitation rate of metastable 3 S He ‫ء‬ atoms was measured during the growth of Na, K, and Cs films on a Cu(100) surface. As a consequence of the reduced work function, which is caused by adsorption of small amounts of alkali metals, the He ‫ء‬ deexcitation mainly takes place via Auger deexcitation. Since the incident He ‫ء‬ atoms interact only with the vacuum side tails of the electronic states of the surface atoms, the deexcitation rate for this mechanism reflects the electronic surface density of states. At coverages of about 0.4 monolayer a dramatic increase in the deexcitation rate was obtained, which is attributed to the subsequent filling of the alkali metal valence band and thus the onset of metallization. 71.20.Dg, 71.30. + h, 73.20.At The adsorption of alkali metal (AM) atoms on surfaces has been studied extensively since Langmuir discovered a large enhancement of electron emissivity for AM covered metal surfaces [1][2][3][4]. Although AM atoms are considered as prototypical chemisorption systems because of their electronic simplicity, they reveal a rather complex behavior. Because of their different electron affinity compared to metal substrates a significant fraction of the AM valence s electron is displaced towards the metal surface and creates a dipole moment. With increasing coverage these dipoles become depolarized and the AM atoms begin to form a metallic film. Since then this classical picture, originally proposed by Gurney [5], has been refined by realistic quantum theoretical calculations [4,6,7]. The metallic character of a complete AM monolayer on metal substrates is well established [2], whereas the metallization transition itself is difficult to observe experimentally.The direct observation of occupied AM s-valence states by ultraviolet photoelectron spectroscopy, the standard tool to study the electronic structure of adsorbate systems, is hampered by the low density of states (DOS) of the s band which is superimposed on the high density of states of the substrate at the Fermi level and additionally by the small photoemission cross section of s states [8]. Woratschek et al. utilized the unique surface sensitivity of metastable deexcitation spectroscopy (MDS) [9] to overcome this difficulty and measured the occupation of the 4s level of K atoms adsorbed on Cu(110) [8,10]. However, their interpretation was questioned in a later MDS study by Hemmen and Conrad who demonstrated that the observed electron emission near the Fermi edge is mainly caused by an autodetachment process of the impinging metastable He ‫ء‬ atoms [11].Here we introduce a new approach by combining the high sensitivity of ground state He atom scattering (HAS) for measuring the AM coverage and the extreme surface sensitivity of the metastable atom deexcitation rate to detect modifications of the electronic surface structure. We report on measurements of the survival probability of He ‫ء‬ atoms scattered from a Cu(100) surface as a function of the Na, K, and Cs coverage. At relative K and Cs coverages of u 0.13 and ...

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confidence: 88%

Observation of Metallization Transition of 2D Alkali Metal Films

Fouquet

Witte

1999

Phys. Rev. Lett.

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“…The same characteristic has also been observed for a number of different surfaces including various insulators (LiF, MgO and NiO), metals (Cu, Pd) or molecular adlayers (CO, NO/Pd and CO, alkanes/Cu) [20,21]. The total He * survival probability was determined by integration of the He * angular distribution which yielded typical values between 8 × 10 −6 and 8 × 10 −7 [20]. These values were found to be approximately proportional to the He * signal at specular position.…”

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confidence: 72%

“…In contrast to HAS the scattered He * atoms reveal, however, only very broad angular distributions without any diffraction features. The same characteristic has also been observed for a number of different surfaces including various insulators (LiF, MgO and NiO), metals (Cu, Pd) or molecular adlayers (CO, NO/Pd and CO, alkanes/Cu) [20,21]. The total He * survival probability was determined by integration of the He * angular distribution which yielded typical values between 8 × 10 −6 and 8 × 10 −7 [20].…”

Section: Methodsmentioning

confidence: 53%

Probing metallization transitions in two-dimensional alkali metal films by metastable He-atom scattering

Witte¹

2004

J. Phys.: Condens. Matter

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The survival probability of metastable He* atoms scattered from low work function materials depends sensitively on the electronic surface density of states which was utilized to characterize the degree of metallicity of ultrathin alkali metal (AM) films. By combining metastable He-atom scattering (MHAS) with He-atom scattering (HAS) to monitor the film growth, the coverage dependence of the metallization was studied. In this paper the metallization transition obtained for various AM (Na, K, Rb, Cs) and Ba films grown on Cu(100) and GaAs(100) surfaces are compared. It is shown that the onset of metallization observed for K, Cs and Ba on Cu(100) is in close agreement with a critical coverage estimated from a simple 2D Herzfeld model. Moreover, MHAS was also used to investigate the de-metallization of ultrathin AM films upon adsorption of O2 or CO.

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Effects of adsorbate-induced states on the metastable He atom scattering from water- and benzene-adsorbed Cu(100)

Kurahashi

Yamauchi

2005

Surface Science

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METASTABLE (2³S) HELIUM ATOM SCATTERING FROM MOLECULAR ADSORBATES ON Pd AND Cu SURFACES

Cited by 5 publications

References 19 publications

Observation of Metallization Transition of 2D Alkali Metal Films

Observation of Metallization Transition of 2D Alkali Metal Films

Probing metallization transitions in two-dimensional alkali metal films by metastable He-atom scattering

Effects of adsorbate-induced states on the metastable He atom scattering from water- and benzene-adsorbed Cu(100)

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