N. Spinelli

Generation of silicon nanoparticles via femtosecond laser ablation in vacuum

We demonstrate that femtosecond laser ablation of silicon targets in vacuum is a viable route to the generation and deposition of nanoparticles with radii of ≈5–10 nm. The nanoparticles dynamics during expansion has been analyzed through their structureless continuum optical emission, while atoms and ions, also present in the plume, have been identified by their characteristic emission lines. Atomic force microscopy analysis of the material deposited at room temperature has allowed the characterization of the nanoparticles size distribution. Taking into account the emissivity of small particles we show that the continuum emission is a blackbody-like radiation from the nanoparticles. Our results suggest that nanoclusters are generated as a result of relaxation processes of the extreme material state reached by the irradiated target surface, in agreement with recently published theoretical studies.

Double-peak distribution of electron and ion emission profile during femtosecond laser ablation of metals

Abstract Femtosecond laser ablation of metals with a Ti:sapphire laser system has been investigated by a time-of-flight mass spectroscopy technique. Ion mass spectra show a double-peak distribution, evidencing the presence of a high-energy component (up to few keV), even at moderate laser intensities (10 12 –10 13 xa0Wxa0cm −2 ). Two different ablation regimes were identified for the less energetic component, and explained in the framework of the two-temperature modeling of ultrashort laser pulse–solid target interactions. Ambipolar diffusion has been identified as the probable mechanism giving rise to the observed high-energy plasma plume component.

Lidar observations of the stratospheric aerosol layer over southern Italy in the period 1991–1995

Lidar measurements of the stratospheric aerosol layer have been carried out in Napoli (40°50 N-14°10 E) and Potenza (40°36N-15°44 E) during the period 1991-1995, covering the history of the aerosol cloud produced by the eruption of Mount Pinatubo (June 1991, Philippines). Measurements are expressed in terms of aerosol backscattering coefficient β A (z), aerosol integrated backscattering IB and aerosol optical thickness τ A at λ = 351 nm and 355 nm ; β A (z) and τ A are determined from a single-wavelength elastic return. IB, τ A , and β max , the peak aerosol backscattering, reached their maximum value in December 1991, displaying a subsequent decay with e-folding times of 237 ± 25, 250 ± 111, and 257 ± 33 days, respectively. R max , the peak scattering ratio, is characterized by a decay time of 235 ± 13 days. Measurements of the extinction-to-backscattering ratio, α A (z)/β A (z), and of the column parameter, τ A /IB, allowed us to retrieve aerosol dimensional characteristics. The time evolution of the height, z c , of the aerosol cloud center of mass was also determined. Downward gravitational settling of stratospheric aerosols with time suggests aerosol particles fall within the size range 0.1-0.3 μm. An abrupt change in IB and β maX is observed approximately 1000 days after the eruption as a result of the winter-summer transition and the tropospheric removal of the lower portion of the stratospheric aerosol layer. Changes in the values of IB, τ A /IB, and z c suggest that this transition is characterized by a change in the aerosol mean radius from 0.3 to 0.1 μm.

Two wavelength lidar analysis of stratospheric aerosol size distribution

The parameters of a single-mode lognormal size distribution for stratospheric aerosol are obtained from two-wavelength lidar measurements. The method is based on the measurement of the aerosol optical thickness, τA(λ), and integrated backscattering coefficient, BI(λ), at 351 and 580 nm. The aerosol particle median radius, rm, and distribution width, σ, are obtained by comparing experimental and computed values for τA(351)BI(351), τA(580)BI(580) andBI(580)BI(351). The method has been applied to three nights of measurement (27 April 1992, 31 May 1993 and 7 March 1994) in the aftermath of the Mount Pinatubo eruption. The time lag between the eruption and the measurements suggests that the first measurement is representative of a fresh volcanic aerosol, while the second and third measurements are representative of an old and depleted aerosol. The measurements performed on 27 April 1992, 31 May 1993 and 7 March 1994 lead, respectively, to rm = 0.31 ± 0.05 μm and σ = 1.3 ± 0.1, rm = 0.16 ± 0.04 μm and σ = 1.9 ± 0.1, rm = 0.21 ± 0.06 μm and σ = 1.8 ± 0.1. The aerosol mass content and number density for the three nights of measurement have also been determined.

Kinetic-energy distributions of charged fragments from CO2 dissociative ionization

The electron impact ionization of CO2 molecules has been studied in order to determine the dissociative paths leading to charged fragments. The electron impact energy (EIE) ranged in the interval 40-130 eV. Time-of-flight spectroscopy was used to obtain the kinetic-energy distribution (KED) extending down to thermal kinetic-energy fragments. We have evaluated the contribution of indirect processes leading to charged fragments through CO2 autoionization. The threshold for C2+ production from CO2 has been measured and a possible production channel is analysed.

EARLINET coordinated lidar observations of Saharan dust events on continental scale

EARLINET, the European Aerosol Research Lidar Network, is the best tool to investigate the horizontal and vertical transport of aerosols over Europe. Within the network, particular attention is devoted to Saharan dust events monitoring. An alert system has been established in order to perform devoted measurements in case of intrusions of desert particles on European continent. Starting from data collected within EARLINET since May 2000, a first statistical analysis of the aerosol vertical distribution on European scale during Saharan dust outbreaks, has been performed. These results highlights the fundamental role that EARLINET can have for the study of impact of Saharan dust on European scale. The current 5-year EU project EARLINET-ASOS, started in March 2006, will enhance the operation of the network through the improvement of the instruments and of the temporal coverage, and of the data analysis procedures.

Pulsed laser ablation of borocarbide targets probed by time-of-flight mass spectrometry

Abstract Excimer laser ablation of superconductive borocarbide material (YNi2B2C) in typical conditions for the deposition of superconductive thin films has been investigated using time-of-flight mass spectrometry. The mass spectra show the presence of all the target elemental ionized atoms as well as diatomics. The ablation yield of the metal ions is a strongly increasing function of the laser fluence, while the contrary is true for non-metal ions. The dependence of non-metal light mass diatomic ions on laser fluence indicates the presence of aggregation processes as the laser fluence is increased. Moreover, evidence of aggregation processes involving metallic ions at high laser fluence is also obtained by the mass spectra. An interesting aspect of our results is the observation of an ion spatial distribution characterized by the presence of the lighter species at the plume edges, while the heavier ones are concentrated at the plume center.

Optical spectroscopy diagnostics and thin film deposition of laser ablated rare earth–Ni2B2C plasma plumes

We describe optical spectroscopy diagnostics of UV laser ablated rare earth–Ni2–B2–C superconducting targets. In the light of this characterization, we have optimized the parameters for high-quality thin film deposition of borocarbide compounds. Our measurements have evidenced relevant differences in the flow velocity as well as in the spatial divergence of different plasma components. This is related to the influence of the different masses of the atoms of the multicomponent target during plume expansion. These results have led us to identify the target-to-substrate distance and substrate temperature as the critical parameters for the deposition of high-quality thin films, as clearly evidenced by residual resistivity ratio measurements. 2002 Elsevier Science B.V. All rights reserved.

Analysis of charged fragments emitted during excimer laser ablation of YNi2B2C borocarbide targets by time-of-flight mass spectrometry

Abstract Excimer laser ablation of superconductive borocarbide material (YNi 2 B 2 C) in typical condition for the deposition of superconductive thin film has been studied by using time-of-flight mass spectrometry. The mass spectra show the presence of diatomic ions together with single ions of all the elemental species. While the yield of the single elemental metallic ions increases with the laser fluence, the non-metallic ions yield decreases. Such decrease is well correlated with the observation of light diatomics (B 2 + , C 2 + , or BC + ) produced by aggregation processes in the plume. Moreover, the presence of aggregation/fragmentation processes in the plume has been also evidenced by using the covariance mapping technique.

Development of a tunable IR lidar system

Abstract A differential absorption lidar system (DIAL) based on a continuously tunable optical parametric amplifier (OPA) pumped by a Ndxa0:xa0YAG laser (200xa0mJ at λ=355xa0nm) operating at a maximum pulse repetition rate of 100xa0Hz has been developed. The system provides continuously tunable coherent radiation in the Visible–near IR range (0.4–2.5xa0μm), allowing to perform DIAL measurements in a spectral region where most of atmospheric constituents and pollutants display absorption lines. The spectral width of the OPA system is line-narrowed by using a master oscillator dye laser as seeder, achieving a linewidth of 0.04xa0cm−1 (FWHM), a spectral purity larger than 99% and a frequency stability better than 1xa0pmxa0h−1, with an output energy in the IR of 1–10xa0mJ. The OPA system was used to perform DIAL measurements in the lower troposphere. Preliminary results in terms of water vapor content and aerosol backscattering profiles are presented and discussed.