K. von Haeften

Multiple ionization of atom clusters by intense soft X-rays from a free-electron laser

Intense radiation from lasers has opened up many new areas of research in physics and chemistry, and has revolutionized optical technology. So far, most work in the field of nonlinear processes has been restricted to infrared, visible and ultraviolet light, although progress in the development of X-ray lasers has been made recently. With the advent of a free-electron laser in the soft-X-ray regime below 100 nm wavelength, a new light source is now available for experiments with intense, short-wavelength radiation that could be used to obtain deeper insights into the structure of matter. Other free-electron sources with even shorter wavelengths are planned for the future. Here we present initial results from a study of the interaction of soft X-ray radiation, generated by a free-electron laser, with Xe atoms and clusters. We find that, whereas Xe atoms become only singly ionized by the absorption of single photons, absorption in clusters is strongly enhanced. On average, each atom in large clusters absorbs up to 400 eV, corresponding to 30 photons. We suggest that the clusters are heated up and electrons are emitted after acquiring sufficient energy. The clusters finally disintegrate completely by Coulomb explosion.

A time resolved VUV fluorescence study of hydrogen clusters: evidence of a liquid phase

Abstract The mobility of Xe atoms captured from a cross jet by hydrogen clusters is investigated with time resolved VUV fluorescence excitation spectroscopy. A band blue shifted by 0.5 eV with respect to the 5p→6s transition of free Xe atoms is observed. It is attributed to Xe atoms profoundly embedded in hydrogen clusters and surrounded by several layers of hydrogen. The complete absence of features related to surface sites gives evidence that after the pick up process the xenon atoms penetrate deep into the interior of the hydrogen clusters. This indicates that hydrogen clusters show a liquid-like behavior similar to helium clusters, though their estimated temperature (∼6 K) is much below the triple point of hydrogen (14 K).

Dissociation and suppressed ionization of H2O molecules embedded in He clusters: The role of the cluster as a cage.

Electronic structure and energy transfer in H2O doped HeN clusters (N≈104) is studied with photoexcitation in the spectral range of 140–40 nm (9–30 eV). The reaction dynamics is investigated by fluorescence of neutral OH* and H* and ionic H2O+* fragments. The rotational temperature of embedded water molecules has been estimated from the 124 nm line shape (3pa1 C 1B1←1b1 X 1A1 transition). Two different temperatures (T1⩽5 K and T2≈30 K) have been found. We propose that the lower temperature (T1) is due to completely thermalized water molecules trapped inside helium clusters, while the warmer molecules (T2) are formed if they are first captured by helium clusters but then leave the clusters again. Predissociation of H2O with excitation below the ionization limit (λexc>100 nm) is found to be unaffected by the cluster environment. On the other hand, the ionization (λexc<100 nm) seems to be suppressed inside helium clusters in favor of the fragmentation into neutral products.

Evolution of the charge localization process in xenon cluster ions: From tetramer to dimer cores as a function of cluster size

The charge localization process in XeN+ cluster ions (N=40–20 000) is investigated with fluorescence spectroscopy methods. New discrete and continuous luminescence bands in the visible and near infrared spectral range are observed and are assigned to radiative transitions of ionic dimers, trimers and tetramers inside Xe clusters. The bands are related to the 5p5 2P1/2→5p5 2P3/2 transition of electronically excited atomic Xe ions and the 6p→6s transitions of electronically excited Xe neutrals. The dependence of the size of the ionic centers on the Xe cluster size is discussed. In large clusters discrete lines are due to embedded dimer emission and they are identified as 2(1/2)u→1(3/2)g transitions between different vibrational levels. Line positions are blue-shifted by 30 (±1) meV with respect to free molecular dimer ions. The energy shift is due to the interaction of the surrounding neutral Xe cluster atoms with the embedded ionic Xe molecules.

Energy relaxation and quenching processes of doped rare-gas clusters with a shell-like geometric structure

Energy relaxation processes of photo-excited Kr50¯ clusters covered with a shell of Ar atoms (up to 40), which are embedded inside large Ne7500¯ clusters are investigated with energy resolved fluorescence spectroscopy. In the energy range of the characteristic Ne cluster absorption (16.5–18 eV) a strong energy transfer to the embedded Kr cluster is observed, which results in the desorption of electronically excited Kr* atoms. Kr* atoms move through the Ne cluster, desorb and emit visible and near-infrared light in the vacuum (5p→5s). By coating the Kr clusters with Ar atoms, the Kr lines disappear and 4p→4s transitions of Ar* become dominant. Additionally, new emission bands occur, which are assigned to transitions of perturbed atomic Kr 5p-states inside Ne clusters. Due to the interaction of electronically excited Kr* atoms with neutral Ar atoms in the surrounding shell, several excited Kr states namely 5p [1/2]0 and 5p [3/2]2 decay nonradiatively. This is in agreement with the well-known “energy-gap law...

Probing collective excitations in helium nanodroplets: Observation of phonon wings in the infrared spectrum of methane

The authors have recorded the nu(3) infrared spectrum of methane in helium nanodroplets using our cw infrared optical parametric oscillator. In a previous paper, Nauta and Miller [Chem. Phys. Lett. 350, 225 (2001)] reported the observation of the monomer rovibrational transitions of methane in helium nanodroplets. Here, they report the observation of additional absorption bands in the frequency range between 2990 and 3070 cm(-1) blueshifted compared to the monomer transitions. They attribute these absorption features to phonon wings of individual rovibrational transitions, i.e., the simultaneous excitation of collective excitation modes of the quantum fluid and the rovibrational excitation of the methane monomer in the helium nanodroplet.

Electronic structure and excited state dynamics of clusters: What can we learn from experiments with synchrotron radiation?

Abstract The investigation of the electronic structure and dynamics of clusters is a topic of current interest. Spectroscopy with vacuum ultraviolet (VUV) photons has two aspects: (i) systems with a large band gap can be studied and (ii) excitation of inner shell electrons allows us to obtain element-specific information. This paper gives an overview of recent developments in the field of cluster research with VUV radiation. Two topics will be discussed in detail: (i) Electronic excitation and relaxation dynamics in helium clusters studied with fluorescence spectroscopy. It turned out that desorption of electronically excited atoms and molecules is an important relaxation process. (ii) Inner shell photoionisation of NaCl clusters produced with a new pick-up cluster source. The near edge absorption structure (XANES) at the Cl 2p edge contains information on the geometric structure of the clusters.