Dokument-ID Dokumenttyp Verfasser/Autoren Herausgeber Haupttitel Abstract Auflage Verlagsort Verlag Erscheinungsjahr Seitenzahl Schriftenreihe Titel Schriftenreihe Bandzahl ISBN Quelle der Hochschulschrift Konferenzname Quelle:Titel Quelle:Jahrgang Quelle:Heftnummer Quelle:Erste Seite Quelle:Letzte Seite URN DOI Abteilungen OPUS4-541 Wissenschaftlicher Artikel Götz, T.; Hoheisel, W.; Träger, F.; Vollmer, Michael Characterization of Large Supported Metal Clusters by Optical Spectroscopy Small sodium and silver particles were generated on dielectric substrates like LiF, quartz and sapphire under ultrahigh vacuum conditions. The optical transmission spectra of the clusters were measured as a function of cluster size and shape, for low and high substrate temperatures as well as for s- and p- polarization of the incident light. Excitation of dipolar surface plasmon oscillations in the directions normal and parallel to the substrate surface could be identified. Furthermore, optical spectra for Na and Ag clusters were calculated with the classical Mie theory. The measured spectra vary strongly if the experimental conditions are changed and can be exploited, for example, to characterize the particles with regard to their size and shape. In particular, the axial ratio of the spheroidal clusters could be determined. Its value is considerably different for the two investigated metals and depends on the substrate material. Furthermore, the temperature of the substrate has a pronounced influence on the shape of the particles. At low temperature of T=100 K two-dimensional island growth is predominant. The particles extend only little in the direction perpendicular to the surface and coalesce readily at small coverage of metal atoms. In contrast, the clusters are truly three-dimensional at T=300 K. At this stage, sodium particles still exhibit a rather small axial ratio whereas silver clusters appear almost spherical. Thus, measurements of the optical spectra permit direct in situ monitoring of cluster growth during the nucleation of adsorbed atoms and of temperature induced shape variations. In addition to investigations of the shape of the particles, the quadrupolar surface plasmon mode was observed for Ag clusters. 1995 In: Zeitschrift für Physik / D Vol. 33 (1995) 2, pp 133-141 10.1007/BF01437432 OPUS4-544 Wissenschaftlicher Artikel Götz, T.; Träger, F.; Vollmer, Michael Desorption of metal atoms with laser light of different polarization Laser-induced desorption of metal atoms from the surface of small metal particles has been investigated as a function of the shape of the particles and the polarization of the incident laser light. The particles were supported on LiF, quartz or sapphire substrates. In a first set of experiments, the shape of the particles was determined by recording optical transmission spectra with s- and p-polarized light incident under an angle of typically 40° with respect to the surface normal. The metal particles turn out to be oblate, the ratio of the axes perpendicular and parallel to the substrate surface being on the order of 0.5. This ratio decreases with increasing particle size. Also, the particles change shape if the temperature is raised. In further experiments, s- and p-polarized light has been used to stimulate desorption of atoms via surface plasmon excitation. It is found that the desorption rate markedly depends on the polarization of the light. This is explained by excitation of the collective electron oscillation along different axes of the non-spherical particles. 1993 3 In: Applied physics / A 57 (1993) 1, pp 101-104 101 104 10.1007/BF00331225 OPUS4-545 Wissenschaftlicher Artikel Hoheisel, W.; Vollmer, Michael; Träger, F. Desorption of metal atoms with laser light: Mechanistic studies Results on laser-induced desorption of metal atoms from small metal particles are presented. Experiments have been performed on sodium, potassium, and silver particles supported on a LiF(100) single-crystal surface under ultrahigh vacuum conditions. Measurements include the determination of the desorption rate as a function of laser wavelength, laser intensity, average particle size, and substrate temperature, the determination of the kinetic energy of the desorbed atoms, the investigation of the optical spectra of the supported metal particles, and the study of the influence of adsorbate molecules on the desorption rate. Furthermore, theoretical extinction and absorption spectra of the metal particles have been calculated with the classical electrodynamical Mie theory as a function of average particle size and excitation wavelength. Also, the radial electric field at the particle surface was computed. The results of the experiments and theoretical calculations are combined to give a consistent picture of the mechanism of metal-atom desorption by electronic excitation with laser light. A realistic surface potential from which the atoms escape and nonlocal optical effects are taken into account. The latter introduce additional absorption channels by the formation of electron-hole pairs in the surface layer of the particle which relax into antibonding states before desorption occurs. Finally, the mechanism is discussed in the light of similar phenomena observed for thin metal films. Possibilities for future work are outlined. © 1993 The American Physical Society 1993 In: Physical Review / B 48 1993) 23, 17463-17476 10.1103/PhysRevB.48.17463 OPUS4-547 Wissenschaftlicher Artikel Götz, T.; Hoheisel, W.; Träger, F.; Vollmer, Michael Interplay between collective and single electron excitations in large metal clusters 1993 In: Zeitschrift für Physik / D 26 (1993), 267-269 OPUS4-504 Wissenschaftlicher Artikel Hoheisel, W.; Vollmer, Michael; Träger, F. Laserdesorption von Metallatomen Beleuchtet man die Oberfläche eines Festkörpers mit Laserlicht, können darauf adsorbierte Atome oder Moleküle abgelöst werden. Solche Desorptionsprozesse sind in jüngster Zeit in zahlreichen Experimenten untersucht worden, wobei sich das Interesse sowohl auf die Erforschung der zugrundeliegenden Mechanismen als auch auf mögliche Anwendungen dieser Reaktionen, zum Beispiel zur gezielten Modifizierung von Oberflächen, konzentriert. Besonders interessant ist das laserinduzierte Aufbrechen von Bindungen zur Oberfläche dann, wenn es nicht einfach als thermischer Prozeß durch eine Temperaturerhöhung hervorgerufen wird, sondern direkt auf einer elektronischen Anregung beruht. Solche nichtthermischen Prozesse werden selbst für Metalle beobachtet, obwohl die Kopplung der elektronischen Anregung an das Substrat hier besonders schnell abläuft. Der zugrundeliegende Mechanismus beruht auf dem Zusammenspiel einer kollektiven Elektronenoszillation und einer lokalisierten Einzelelektronenanregung, wobei auch nichtlokale optische Effekte in der Metalloberfläche eine wichtige Rolle spielen. 1993 In: Physikalische Blätter 49 (1993) 9, 795 - 799 10.1002/phbl.19930490909