A. R. B. de Castro

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.

High resolution photoemission study of CdSe and CdSe/ZnS core-shell nanocrystals

Colloidally prepared CdSe and CdSe/ZnS core-shell nanocrystals passivated with trioctylphosphine/trioctylphosphine oxide and hexadecylamine have been studied by photoelectron spectroscopy with tuneable synchrotron radiation. High-resolution spectra of the Se 3d level in CdSe nanocrystals indicate the bonding of organic ligands not only to surface Cd but also to surface Se atoms. The investigation of the CdSe/ZnS core-shell nanocrystals allows us to determine the average thickness of the ZnS shell and to study the interface between the two semiconductor nanomaterials. The photoemission spectra indicate a rather well ordered interface. No evidence for interfacial bonds other than Cd–S and Se–Zn is found.

The effect of nanocrystal surface structure on the luminescence properties: photoemission study of HF-etched InP nanocrystals.

InP nanocrystals with narrow size distribution and mean particle diameter tunable from approximately 2 up to approximately 7 nm were synthesized via the dehalosilylation reaction between InCl3 and tris(trimethylsilyl)phosphine. Specific capping of the nanocrystal surface with a shell of organic ligands protects the nanocrystals from oxidation and provides solubility of the particles in various organic solvents. InP nanocrystals with enhanced photoluminescence (PL) efficiency were obtained from the initial nanocrystals by photoassisted etching of the nanocrystal surface with HF. The resulting PL quantum efficiency of InP nanocrystals dispersed in n-butanol is about three orders of magnitude higher when compared to the nonetched InP samples and approaches approximately 40% at room temperature. High-resolution photoelectron spectroscopy with the use of synchrotron radiation was applied to reveal the changes of the nanocrystal surface responsible for the dramatic improvement of the PL efficiency. The analysis of high-resolution P 2p core-level spectra confirmed significant changes of the nanocrystal surface structure induced by the postpreparative treatments and allowed us to propose the description of the etching mechanism. In the nonetched InP nanocrystals, some surface P atoms generate energy states located inside the band gap which provide nonradiative recombination pathways. Photoassisted treatment of InP nanocrystals with HF results in selective removal of these phosphorous atoms from the nanocrystal surface. The reconstructed surface of the etched InP nanocrystals is terminated mainly with In atoms and is efficiently passivated with tri-n-octylphosphine oxide ligands.

Vacuum ultraviolet beam lines at Laboratório Nacional de Luz Síncrotron, the Brazilian synchrotron source

A TGM beamline for the spectral range 12–310 eV is being built at Laboratorio Nacional de Luz Sincrotron (LNLS). This beam line consists of safety devices, a condensing mirror, a toroidal grating monochromator, and a refocusing mirror. This beam line has been assembled up to the exit slits. A spherical grating monochromator beam line has been designed for the spectral range 200–1000 eV. The performance is limited by the surface slope error in the gratings. If the rms figure error is 2 μrad, the expected resolving power is better than 3000.

Shell explosion and core expansion of xenon clusters irradiated with intense femtosecond soft x-ray pulses

The disintegration mechanisms for xenon clusters in intense femtosecond soft x-ray pulses from the FLASH free electron laser are investigated. The clusters are irradiated at a wavelength of ? = 13.7 nm (h? = 90.5 eV) and power densities of 5 ? 1014 W cm?2. During the 10 fs pulse the Xe clusters are transformed into a highly excited, multiply charged nanoplasma. Simulating the ion kinetic energies in an electrostatic model suggests that highly charged ions explode off the surface due to Coulomb repulsion while the inner core expands in a hydrodynamic expansion. The current results yield evidence for efficient ionization of the clusters in addition to direct multistep photoemission.

Charge recombination in soft x-ray laser produced nanoplasmas

The ionization and charge separation processes of nanoplasmas created by resonant excitation of atomic clusters in intense soft x-ray pulses have been investigated. Through irradiation with femtosecond pulses from the FLASH free electron laser (FEL) at ? = 13.7 nm and power densities exceeding 1014 W cm?2 the clusters are highly ionized with transient atomic charge states up to 9+. Variation of the cluster composition from pristine to doped and core?shell systems allows tracking of the spatial origin and charge states of the fragments yielding insight into the nanoplasma dynamics. The data give evidence for efficient charge redistribution processes leading to a Coulomb explosion of the cluster outer part and recombination of the nanoplasma core. The experiments show qualitatively different processes for (soft) x-ray produced nanoplasmas from the optical (IR) strong-field regime where the clusters disintegrate completely in a Coulomb explosion.

Coherent second-harmonic generation by counterpropagating surface plasmons

We have observed second-harmonic generation by counterpropagating surface-plasmon waves. The output is in the form of a well-collimated beam along the surface normal. The results are in excellent agreement with theoretical prediction.

Modelling dynamics of samples exposed to free-electron-laser radiation with Boltzmann equations

Abstract. We apply Boltzmann equations for modelling the radiation damage in samples irradiated by photons from free electron lasers (FELs). We test this method in a study case of a spherically symmetric xenon cluster irradiated with VUV FEL photons. Qualitative agreement between the model predictions and experimental data is found. The results obtained demonstrate the potential of the Boltzmann method for describing the complex and non-equilibrium dynamics of samples exposed to FEL radiation.

Observation of excitonic satellites in the photoelectron spectra of Ne and Ar clusters

We have recorded synchrotron radiation excited photoelectron spectra of free Ne and Ar cluster beams in the valence and inner valence region. Varying the cluster sizes from a few up to some hundred atoms, the development of inelastic energy losses of the outgoing photoelectrons is clearly visible. The first few energy loss features can be related to creation of excitons and interband transitions within the cluster.

Single particle scattering from hot electrons in GaAs

Abstract Two photon-absorption in GaAs from a pulsed Nd : Yag laser induced non-equilibrium distributions of electrons and optical phonons in the bulk of the material. The Nd : Yag laser radiation itself is used as a probe to search for population changes of optical phonons and carriers by means of Raman scattering. This allowed comparison of electron and phonon temperatures at different excitation levels. Electrons reached a temperature of 560° K for a lattice temperature of 300° K.