H. O. Lutz

Giant resonances in silver-cluster photofragmentation

Abstract The collinear ion-beam depletion technique is used to measure absolute photofragmentation cross sections of small silver-cluster ions in the range from 2.3 to 5.7 eV. Giant resonances are found which can be interpreted in terms of a collective electron oscillation. The optical spectra of spherical Ag + 9 and Ag + 21 show a large blue-shift of the resonance energy relative to the respective experimental surface value. For non-spherical clusters, a splitting of the giant resonance is found, in qualitative accordance with jellium calculations.

Photoelectron spectroscopy of silver and palladium cluster anions : electron delocalization versus localization

Photoelectrons (PE) from jet-cooled mass-identified silver and palladium cluster anions (number of atoms, n⩽ 21) were detached by UV laser light and energy-analysed in a time-of-flight (TOF) electron spectrometer of the magnetic-bottle type. For palladium PE threshold energies smoothly increase with n; for Ag, they show clear evidence of shell effects as well as an ‘even–odd oscillation’. The PE energy spectra are strongly structured, the structures being attributed to transitions involving the neutral ground states as well as contributions of low-lying excited neutral states. For silver the results can, in part, be qualitatively understood in terms of a delocalized electron Fermi-gas within the ellipsoidal deformed cluster. This picture fails for the more localized d electrons of palladium. For a thorough interpretation more elaborate calculations are necessary. The first results are available for alkali-metal clusters and will be compared to the silver and copper data.

Fundamental processes in energetic atomic collisions

This book starts with a brief review of the formal scattering theory by Massey. The chapters that follow survey a coherent selection of interesting topics in energetic ion-atom collisions. The last third of the book contains papers on coherence and correlation in atomic collisions and on new aspects in the study of atomic collisions. The papers on coherence and correlation report progress toward complete experiments where scattering amplitudes and phases are determined in coincidence experiments. The papers on new aspects cover the use of polarized particles in atomic experiments.

The pulsed arc cluster ion source (PACIS)

A new cluster source, the “PACIS”, has recently been developed [1, 2]: a pulsed high-current arc is fired between two electrically isolated electrodes. In a stream of carrier gas, the nascent metal plasma cools down, thus undergoes significant aggregation. After supersonic expansion into high vacuum, the resulting cluster ions will be subject to investigation. Here, we present the current state of the source development and further technical details. The integral intensity is estimated by film thickness measurements and yields a deposition rate of, e.g., up to 2 Å per shot for lead. About 10% of the emitted material turns out to be charged. Time-of-flight measurements show similar cluster ion mass spectra as they are known from laser vaporization. Already single shot mass spectra display completen-series which can be followed ton ≈ 40. For heavy clusters, a significant velocity slip is estimated from the beam velocities.

Pure metal and metal-doped rare-gas clusters grown in a pulsed ARC cluster ion source

Abstract The performance of a new pulsed arc cluster ion source (PACIS) is demonstrated for pure metal cluster ions and anions as well as mixed metal-rare-gas clusters. Pure metal clusters like Wn+ exhibit intensity distributions similar to those from laser vaporization sources. The ion current of the PACIS is comparable to that of a laser vaporization source. For metal-doped argon cluster ions, a single aluminium atom serves as the central ion which is surrounded by argon atoms arranged in an icosahedron. These measurements therefore strongly support the close packing model for argon clusters.

Photoemission from tin and lead cluster anions

Photoelectrons from mass-identified jet-cooled tin and lead cluster anions (Snn−, Pbn−) are detached by ultraviolet laser light (hν=3.68 eV). The photoelectron energy spectra give the detachment energies of ground state cluster anions (electron affinities) as well as excitation energies of neutral clusters in the geometry of the anions. The energy spectra for Snn− are dominated by flat thresholds with ann-dependence similar to that of other group IV clusters. In contrast, for Pbn− we find pronounced narrow lines close to threshold, generally followed by a 0.3–1.4 eV gap which indicates closed-shell behaviour of Pbn− for nearly alln.

Photoelectron spectroscopy of jet-cooled aluminium cluster anions

Aluminium cluster anions (Aln−) are produced by laser vaporization without additional ionization and cooled by supersonic expansion. Photoelectrons from mass-identified anion bunches (n=2...25) are detached by laser light (hv=3.68 eV) and undergo energy analysis in a magnetic bottle-type time-of-flight spectrometer. The measurements provide information about the electronic excitation energies from ionic ground states to neutral states of the clusters. In contrast to bulk aluminium these cluster photoelectron spectra partially have well-resolved bands which originate from low-lying excited bands. For small clusters, especially the aluminium dimer and trimer, quantum-chemical calculations will be compared to the measurements. The electron affinity size dependence of larger clusters shows conclusive evidence for “shell” effects.

Ultraviolet photodetachment spectroscopy on jet-cooled metal-cluster anions

Metal-cluster anions have been produced by laser vaporization and cooled in a supersonic expansion, without converting neutral clusters into anions via a secondary ionizing agent. After mass-selection, photoelectrons were detached (hν= 3.68 eV) from defined anion bunches and analysed in a magnetic bottle-type time-of-flight electron spectrometer. The resulting photoelectron distributions from Al–n, Ni–n, Ag–n and Sn–n, n < 22, are compared; all spectra display a dramatic dependence on the cluster size and differ substantially from the respective bulk spectra. The measurements provide information about low-lying excited electronic structures which are missing in the solid metal. Photoelectron thresholds as estimates of electron affinities in part follow a simple electrostatic model.

Sputtered metal cluster ions: Unimolecular decomposition and collision induced fragmentation

Cluster ions are produced by ion bombardment of thick metal targets and mass selected in a Wien filter. The unimolecular decomposition of Aln+, Cun+, Mon+, Wn+, and Pbn+ is investigated under UHV conditions. The time evolution of the decay allows a glimpse into the cluster formation/fragmentation process. Highly excited metal cluster ions decompose mainly by evaporating single neutral atoms with rates reaching 100%. The collision induced fragmentation (CIF) of stable mass selected metal cluster ions in a low pressure Ar and O2 gas target will be compared to the unimolecular decay.

Alkaline earth metals (Ca, Sr) attached to liquid helium droplets: Inside or out?

Superfluid helium droplets HeN at 0.4 K are doped with single alkaline earth atoms (Ca, Sr). The absorption spectra for excitation of the lowest singlet transitions display broad, blue-shifted peaks if compared to the atomic lines. The shift increases with droplet size increasing from N≈1000 up to N≈10000 but converges to only about one third of the bulk value.