F. Träger

Analysis of fractional order thermal desorption

Sodium on a LiF(100) single crystal surface has been studied by thermal desorption. With increasing coverage the spectra exhibit a pronounced shift to higher desorption temperatures. We find that the results can be explained by fractional order desorption kinetics where the order of desorption is x=0.79±0.08. In addition, activation energies have been extracted. They vary between 0.55 and 0.8 eV for coverages ranging from 5×1013 to 1017 atoms/cm2. An analysis of fractional order thermal desorption is presented and the desorption energies are discussed. The results are interpreted in the framework of a microscopic model where sodium atoms desorb from the edges of Nan-clusters formed by adatom diffusion on the LiF insulator surface.

Observation of the electric hexadecapole moment of free Na atoms in a forward scattering experiment

Abstract We report the observation of an electric hexadecapole moment in the ground state of free sodium atoms. The ΔmF = 4 coherence, which corresponds to this moment, is created by the light field of a cw dye laser. The forward scattered light was used for detection, because of the higher sensitivity compared to lateral fluorescence.

Metal particles on surfaces-desorption, optical spectra, and laser-induced size manipulation

Desorption induced by electronic excitation with laser light is discussed. Emphasis is placed on nonthermal desorption where surface plasma excitation in small particles precedes the rupture of the surface chemical bond. A scenario for the mechanism underlying such a process is proposed. In this context, calculations of the electronic spectra of small sodium particles are presented and the influence of different multipole orders of the collective electron oscillation, of different shapes of the clusters and of the substrate are outlined. Furthermore, manipulation of the size distribution of metal particles on supports is described as an application of the effect. This allows the preparation of very special surfaces with novel physical and chemical properties. Methods to characterize such adsorbate-substrate combinations, especially by use of the optical spectra of the particles, are also discussed. Finally, prospects for future experiments in this field are outlined.

Nuclear charge distribution of eight Ca-nuclei by laser spectroscopy

The isotope shift in the Ca-intercombination line withλ=6,573 Å was measured for all isotopes between40Ca and48Ca with the only exception of 47Ca. The combination of the results with muonic x-ray data yields highly accurate values for the changes of the mean square nuclear charge radii. In addition, the nuclear quadrupole moments of the three isotopes41Ca,43Ca and45Ca have been determined from the hyperfine structure splitting of the 4s 4p3P1 state.

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.

Strong narrowing of the Na forward scattering signals due to the interaction with an intense dye laser field

Abstract A forward scattering experiment on the D 1 -resonance line of Na emitted by an intense dye-laser is reported. The resulting signals show strong narrowing which is attributed to the hexadecapole moment created in the Na ground state by laser radiation.

Ablation of metal particles by surface plasmon excitation with laser light

Abstract Small sodium particles supported on an insulating LiF crystal are illuminated with visible light of an Ar+ or Kr+ laser. As a result the emission of neutral metal atoms from the particle surface is observed. the strongly resonant character of this emission, centered at about 490 nm, indicates that a surface plasmon excitation precedes the ejection. Time-of-flight measurements show large mean velocities of the emitted atoms. Continued laser illumination of the metal particles leads to an ablation of typically 20% of the initial coverage. The process is explained by a nonthermal mechanism.

Nuclear electric quadrupole moment of41Ca by laser-RF spectroscopy

AbstractThe nuclear electric quadrupole moment of41Ca, which contains a single neutron outside closed proton and neutron shells (N=Z=20) has been determined. For this purpose the hyperfine structure (hfs) of the metastable 4s4p3P1 state was measured by a highly sensitive technique of combined radiofrequency laser saturation spectroscopy. Because of the small size of the quadrupole interaction constantB corrections for second order effects had to be included in the analysis. Based on corrected hfs constants for the 4s4p3P1 state of the isotopes41, 43, 45Ca and on a recent ab initio calculation of 〈r−3〉 integrals [1] one obtains in a consistent way:

Isotope shift of calcium isotopes withA=40, 42, 43, 44, 46 and 48 by laser spectroscopy

\begin{array}{*{20}c} {Q({}^{41}Ca) = - 80( 8)mb} \\ {Q({}^{43}Ca) = - 49( 5)mb[1]} \\ {Q({}^{45}Ca) = + 46(14)mb.} \\ \end{array}