M. Neeb

Spatio-temporal coherence of free electron laser pulses in the soft x-ray regime

The temporal coherence properties of soft x-ray free electron laser pulses at FLASH are measured at 23.9 nm by interfering two time-delayed partial beams directly on a CCD camera. The partial beams are obtained by wave front beam splitting in an autocorrelator operating at photon energies from h nu = 30 to 200 eV. At zero delay a visibility of (0.63+/- 0.04) is measured. The delay of one partial beam reveals a coherence time of 6 fs at 23.9 nm. The visibility further displays a non-monotonic decay, which can be rationalized by the presence of multiple pulse structure.

A high-resolution N 1s photoionization study of the molecule in the near-threshold region

The angle-resolved N 1s photoelectron spectrum of has been measured with high resolution in the threshold region. An analysis of the vibrational fine structure yields a vibrational energy of and a lifetime broadening of for the core-ionized molecule. As in the case of C 1s ionization of CO, the vibrational fine structure changes considerably with photon energy in the region of both the shape resonance and the double excitations. Both the vibrationally resolved partial photoionization cross sections and asymmetry parameters have been determined in this photon energy range. The calculated cross sections in the literature are in reasonable agreement with experiment. In addition, we have measured the cross sections and asymmetry parameters of the most intense direct and conjugate shake-up satellites. The behaviour of the satellites is found to differ significantly from that of the corresponding C 1s satellites of CO.

La@C60: A metallic endohedral fullerene

We have produced an endohedrally doped fullerene that shows a metal-like density of states at the Fermi level. Individual La@C60 clusters deposited onto graphite exhibit a zero band gap as observed by scanning tunneling spectroscopy on single clusters at room temperature. Moreover, we find that an isolated La@C60 cluster on graphite shows a reversible opening of a band gap at a transition temperature of ∼28 K. The transition is associated with a freezing of the vibrational motion of the La atom inside the fullerene cage. The metallic behavior of La@C60 is attributed to the presence of a dynamical dipole in the single cluster.

Size-dependent hot-electron dynamics in small Pdn−-clusters

Using time-resolved photoelectron spectroscopy we show that electron relaxation processes via inelastic electron–electron scattering are efficient energy dissipation channels not only in bulk metals but also in extremely small transition metal clusters. The photoelectron spectra of optically excited Pd3−, Pd4−, and Pd7− reveal effective electron relaxation times of less than 100 fs. Moreover the relaxation times vary with cluster size. In comparison to simple metal clusters the bulklike inelastic scattering rates in open d-shell transition metal clusters are attributed to the larger valence electron level density. An energy transfer to the vibrational degrees of freedom occurs within 10 ps.

Vibrational excitation in C 1s and O 1s photoionization of CO

The C 1s and O 1s photoelectron spectra of the CO molecule have been measured with high resolution. The vibrational spacing of the state is significantly lower than in the state and in the molecular ground state. Compared to the ground-state equilibrium C - O distance of 1.128 A, we extract from the analysis of the Franck - Condon factors bond lengths of 1.079(2) A and 1.167(4) A for the C 1s and O 1s core-ionized states, respectively.

Vibrationally resolved decay spectra of CO at the C and O K-edges: experiment and theory

The resonant Auger spectrum of CO has been measured at both the C and O K-edges. At the C resonance the decay spectrum was recorded selectively at the energies of the = 0, 1 and 2 vibrational states. Vibrational fine structure was not only resolved on the participator but also on the spectator lines. For the O resonance the photon energy bandwidth was sufficiently low that different, vibrationally distinct regions of the absorption profile could be selected. Ab initio calculations and their detailed analysis are presented for both the C and O decay spectra. At the C resonance the calculation explicitly accounts for the vibrational fine structure including vibrational-lifetime interference effects. The appearance of the spectator part of the spectrum is shown to be strongly influenced by the existence of unbound states and diabatic interactions. The nodal structure of the vibrational wavefunction of the core-excited state is reflected in the decay spectrum.

An experimental setup for nondestructive deposition of size-selected clusters

An experimental setup for the deposition of mass-selected clusters using a laser vaporization source and a magnetic field mass selector is presented. Nondestructive deposition and a coverage of 1% of a monolayer within 5 h are achieved for mass-selected metallofullerene clusters as demonstrated for deposited Ce@C60 on highly oriented pyrolytic graphite.

Inner-shell photoionization spectroscopy on deposited metal clusters using soft x-ray synchrotron radiation: An experimental setup

Exploration of mass-selected clusters by soft x-ray synchrotron radiation is well suited to receive element specific information on clusters in contact with a support and to systematically follow the evolution of size-dependent electronic and geometrical properties from the smallest clusters toward the bulk. Here we describe an experimental setup, which combines cluster synthesis, mass selection, soft landing, ultrahigh vacuum transfer, and photoionization experiments such as x-ray photoelectron spectroscopy, x-ray absorption, and Auger electron spectroscopy. First spectroscopic results and experimental conditions are briefly discussed for Cu(19) deposited onto the natural oxide layer of a Si-wafer surface.

One-electron versus multielectron effects in the near-threshold C 1s photoionization of acetylene

The C 1s partial photoionization cross section and asymmetry parameter β of the C2H2 molecule have been determined with high-energy resolution between threshold and 360 eV. The C 1s shake-up satellite spectrum is richly structured; the cross section of several satellites increases strongly near threshold, which indicates that there is a conjugate contribution to their intensity. It is shown that the large enhancement in the total photoabsorption cross section between 305 and 320 eV, previously attributed to a σ* shape resonance, is largely due to this photon energy dependence of the shake-up transitions.

On the correct identification of shape resonances in NEXAFS

Using core level photoemission data from free molecules, it is shown that the putative shape resonances in NEXAFS can be identified unequivocally only by measuring the main line, or single hole, cross-section. This has important consequences for the characterisation of adsorbed molecules using NEXAFS where the technique is used to determine orientation and even intramolecular bondlengths.