Cluster emission under femtosecond laser ablation of silicon

Rich populations of clusters have been observed after femtosecond laser ablation of bulk silicon in vacuum. Size and velocity distributions of the clusters as well as their charge states have been analyzed by reflectron time-of-flight mass spectrometry. An efficient emission of both neutral silicon clusters Sin (up to n = 6) and their cations Sin+ (up to n = 10) has been observed. The clusters are formed even at very low laser fluences, below ablation threshold, and their relative yield increases with fluence. We show the dependencies of the cluster yield as well as the expansion dynamics on both laser wavelength and laser fluence. The mechanisms of the cluster formation are discussed

Synthesis and Laser Processing of ZnO Nanocrystalline Thin Films

We present the results of experiments on synthesis of ZnO nanoclusters by reactive pulsed laser deposition (PLD). The nanoclusters were formed and crystallized in the gas phase and deposited on SiO2 substrates. The nanostructured films were characterized by conventional photoluminescence (PL). The PL spectra consist of a narrow UV excitonic band and a broad visible band related to defects in the film. The film preparation conditions such as the substrate temperature, ambient gas nature and pressure, were optimized in order to increase the intensity of excitonic emission and prevent the formation of defects. A post-growth annealing by UV laser radiation improved the optical quality of the deposited films. The photoluminescence intensity was found to be dependent significantly on the laser fluence and on the number of shots per site. The nature of the defects responsible for the observed luminescence in a visible range is discussed

Laser ablation synthesis of zinc oxide clusters: a new family of fullerenes?

Positively charged zinc oxide clusters ZnnOm (up to n = 16, m <= n) of various stoichiometry were synthesized in the gas phase by excimer ArF laser ablation of a ZnO target and investigated using time-of-flight mass spectrometry. Depending on ablation conditions, either metal rich or stoichiometric clusters dominate in the mass spectrum. When the irradiated target surface is fairly fresh, the most abundant clusters are metal rich with Zn(n+1)On and Zn(n+3)On being the major series. The stoichiometric clusters are observed with an etched ablated surface. The magic numbers at n = 9, 11 and 15 in mass spectra of (ZnO)n clusters indicate that the clusters have hollow spheroid structures related to fullerenes. A local abundance minimum at n = 13 provides an additional evidence for the presence in the ablation plume of fullerene-like (ZnO)n clusters

Ablation de ZnO par laser UV (193 nm) : Nano-agregats en phase gazeuse

The condensation of zinc oxide nano clusters produced in gas phase has been evidenced in the ablation of a ZnO solid target by a pulsed ArF laser. We compare the spatio-temporal evolution of the shape of ZnO ablated plasma plume, both in vacuum and in controlled gas environments (Helium and/or Oxygen) from monitoring with CCD camera and from spectroscopic studies. The expansion of the plasma plume and the growth of the nanoclusters depend strongly upon the collisions between the ablated particles and surrounding gas molecules, as well as on the chemical reactions in the case of oxygen. The spectra of the observed plasma emittion are mainly due to atomic transitions in the neutral Zn. We have also observed the photoluminescence of the nanoclusters suspended in the gas and their decomposition in the beam of ArF laser.Comment: In Frenc

Strong reduction of the off-momentum halo in crystal assisted collimation of the SPS beam

A study of crystal assisted collimation has been continued at the CERN SPS for different energies of stored beams using 120 GeV/. c and 270 GeV/. c protons and Pb ions with 270 GeV/. c per charge. A bent silicon crystal used as a primary collimator deflected halo particles using channeling and directing them into the tungsten absorber. A strong correlation of the beam losses in the crystal and off-momentum halo intensity measured in the first high dispersion (HD) area downstream was observed. In channeling conditions, the beam loss rate induced by inelastic interactions of particles with nuclei is significantly reduced in comparison with the non-oriented crystal. A maximal reduction of beam losses in the crystal larger than 20 was observed with 270 GeV/. c protons. The off-momentum halo intensity measured in the HD area was also strongly reduced in channeling conditions. The reduction coefficient was larger than 7 for the case of Pb ions. A strong loss reduction was also detected in regions of the SPS ring far from the collimation area. It was shown by simulations that the miscut angle between the crystal surface and its crystallographic planes doubled the beam losses in the aligned crystal.peer-reviewe

Comparative results on collimation of the SPS beam of protons and Pb ions with bent crystals

New experiments on crystal assisted collimation have been carried out at the CERN SPS with stored beams of 120 GeV/. c protons and Pb ions. Bent silicon crystals of 2 mm long with about 170 μrad bend angle and a small residual torsion were used as primary collimators. In channeling conditions, the beam loss rate induced by inelastic interactions of particles with the crystal nuclei is minimal. The loss reduction was about 6 for protons and about 3 for Pb ions. Lower reduction value for Pb ions can be explained by their considerably larger ionization losses in the crystal. In one of the crystals, the measured fraction of the Pb ion beam halo deflected in channeling conditions was 74%, a value very close to that for protons. The intensity of the off-momentum halo leaking out from the collimation station was measured in the first high dispersion area downstream. The particle population in the shadow of the secondary collimator-absorber was considerably smaller in channeling conditions than for amorphous orientations of the crystal. The corresponding reduction was in the range of 2-5 for both protons and Pb ions.peer-reviewe

Observation of parametric X-rays produced by 400 GeV/c protons in bent crystals

Spectral maxima of parametric X-ray radiation (PXR) produced by 400 GeV/c protons in bent silicon crystals aligned with the beam have been observed in an experiment at the H8 external beam of the CERN SPS. The total yield of PXR photons was about 10-6 per proton. Agreement between calculations and the experimental data shows that the PXR kinematic theory is valid for bent crystals with sufficiently small curvature as used in the experiment. The intensity of PXR emitted from halo protons in a bent crystal used as a primary collimator in a circular accelerator may be considered as a possible tool to control its crystal structure, which is slowly damaged because of irradiation. The intensity distribution of PXR peaks depends on the crystal thickness intersected by the beam, which changes for different orientations of a crystal collimator. This dependence may be used to control crystal collimator alignment by analyzing PXR spectra produced by halo protons.peer-reviewe

Dynamics of femtosecond laser-induced cluster emission from silicon

International audienceUltrashort laser pulses have opened unique opportunities for studyingmechanisms of laser-matter interactions by analyzing particles emittedfrom the irradiated surfaces. A number of recent experiments indicatethat efficient emission of clusters can occur under certain conditions offs-laser ablation/desorption of semiconductors. Analysis of cluster emissioncould provide a considerable insight into the complex interplay ofthermal and ultrafast, non-thermal processes in the laser-irradiated targets.Here we report time-resolved pump-probe measurements of the yield ofsingly-charged positive cluster ions (n = 2-11) from a silicon surface andtheoretical modeling of the laser-induced excitation of the Si target forthe experimental regimes. A 80-fs Ti:sapphire laser pulse was split intotwo subthreshold pulses with a variable time delay and cluster emissionwas monitored after each pair of pulses. When temporal separation betweenthe pulses is around few hundred femtoseconds, the emission isfound to be stronger than that with a single laser pulse of the same totalenergy. We demonstrate also that there is significant incubation in siliconin respect of cluster yield which is maximized after 30-40 pulses appliedto the same spot. Temporal and spatial behavior of electron and ion temperaturesin the Si target as well as surface charge density were simulatedas a function of pump-probe delay. Both modeling results and observationsimply a new mechanism of cluster emission involved. We suggestthat the first pulse induces the lattice destabilization and surfacestructural changes while the second pulse, applied to the unstable surface,acts as a trigger resulting in efficient emission of clusters. The roleof surface defects and premelting in cluster formation process will bediscussed

Production of zinc oxide nanoclusters by pulsed laser ablation

We present the experimenal results on the formation of zinc oxide nanoclusters during the ablation of sintered ZnO targets by a pulsed ArF laser both in vacuum and in the controlled gas atmosphere (helium and/or oxygen). Even in vacuum, the small clusters have been detected by time-of-flight mass-spectrometry. The presence of the surrounding gas favors the synthesis and growth of the clusters. The initial cluster ions play a role of condensation centers for the further cluster growth up to sizes of several nanometers. The evolution of the laser-induced plasma plume has been monitored by iCCD imaging and by time resolved optical spectroscopy. The radiation emitted by plasma is due mainly to atomic transition of neutral Zn. In the case of ablation in oxygen, the dynamics of plume expansion is strongly influenced by chemical reactions between the ablated particles and surrounding gas molecules. Under laser excitation, we have observed the photoluminescence (PL) of the gassuspended clusters. An intense PL band centred at the photon energy of 3.27 eV has been attributed to the radiative recombination of free excitons in crystalline ZnO at the room temperature. This result indicates that the nanoclusters are already cooled and crystallized in the gas suspension