Münster 1999 – wissenschaftliches Programm

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O: Oberflächenphysik

O 36: Poster (II)

O 36.26: Poster

Donnerstag, 25. März 1999, 20:00–22:30, Zelt

Mg Clusters on MgO surfaces: Characterization with MIES, UPS(HeI) and TPD — •P. Stracke¹, W. Maus-Friedrichs¹, V. Kempter¹, G. Fanjoux², J. G"unster³, St. Krischok¹, J. Stultz³, and D.W. Goodman³ — ¹Physikalisches Institut der TU Clausthal, Leibnizstra"se 4, D-38678 Clausthal-Zellerfeld — ²Facult’e des Sciences et Techniques Universit’e de Bretagne, 6 avenue V. Le Gorgeu, B. P. 809, 29285 Brest Cedex, France — ³Department of Chemistry, Texas A&M University, P. O. Box 30012, College Station, TX 77842-3012, USA

MgO films (2nm) thick were grown on Mo and W substrates while metastable impact electron (MIES) and UPS(HeI) spectra were collected in situ.Apart from the valence band emission no additional spectral features have been detected with electron spectroscopies. After exposing the oxide surface to Mg (substrate temperature between 100 and 300K) an additional peak not seen in UPS, located within the band gap, shows up in MIES. This band, located at ∼2eV above the top of the valence band with a FWHM of ∼1eV at the lowest exposures, can be detected in MIES until its intensity falls below a level of 10⁻³ of that from the valence band. The topological structure of the MgO surface partially covered by Mg was also studied by STM. This additional emission is attributed to the formation of small, nonmetallic Mg clusters. The energetic position of the cluster emission closely matches that of the expected ionization of surface F_s⁺/F_s centers. It is proposed that in the initial phase of the Mg exposure, F-center-type defects are produced close to extended defects, such as steps, corners, etc. These Mg-induced defects appear to play an important role as nucleation sites for cluster formation.