A. Borrielli

Particle emission from tantalum plasma produced by 532nm laser pulse ablation

A study of visible laser ablation of tantalum in vacuum by using 3ns Nd:YAG laser radiation at high pulse energy is reported. Nanosecond pulsed ablation, at an intensity on the order of 109W∕cm2, produces high nonisotropic emission of neutrals and ionic species. Mass quadrupole spectrometry, coupled to electrostatic ion deflection, allows estimation of the energy distributions of the emitted species within the plume as a function of the incident laser energy. Neutrals show typical Boltzmann distributions while ions show Coulomb-Boltzmann-shifted distributions depending on their charge state. Surface profiles of the craters and microscopy investigations permitted to study the ablation threshold, ablation yields, and deposition rates of thin films on silicon substrates. The multicomponent structure of the plume emission is rationalized in terms of charge state, ion and neutral equivalent temperatures, and plasma density. A special regard is given to the ion acceleration process occurring inside the plasma d...

Silver plasma by pulsed laser ablation

A Nd : Yag laser was employed to irradiate thick silver targets in vacuum. The ion emission from the Ag plasma was detected in situ using a ring ion collector and an electrostatic ion energy analyzer which permitted measurement of the ion kinetic energy-to-charge state ratio. The total ion yield, the ion threshold fluence, the ion energy distributions and the mean temperature for the different ion charge states of the non-equilibrium plasma were investigated. The visible radiation emission spectrum, the light emission threshold, the electronic temperature and density were also investigated by using optical spectroscopy. Ex situ surface profile measurements, performed on the generated craters after the ablation process, permitted the evaluation of the ablation threshold fluence and performance of a comparison with a semi-empirical model.

The effect of high-Z dopant on laser-driven acceleration of a thin plastic target

Acceleration of a thin (10 or 20μm) plastic foil by 120J, 0.438μm, 0.3ns laser pulse of intensity up to 1015W∕cm2 has been investigated. It is shown that the introducing a high-Z dopant to the foil causes an increase in the ablating plasma density, velocity, and collimation which, in turn, results in a remarkably higher kinetic energy and energy fluence of the flyer foil.

Polyethylene film welding induced by a 532 nm pulsed laser

A Nd:YAG laser operating at 532 nm wavelength, 3 ns pulse duration, 150 mJ pulse energy and 10 Hz repetition rate is employed to irradiate in air polyethylene-based polymers and to induce thin film welding. Polyethylene thin films of different densities, with and without enclosed carbon nanotubes, have been irradiated at different exposition times. Measurements of static tensile, shear stress and morphology permitted us to investigate the polymer modifications and welding induced by the laser irradiation. The results demonstrated that the absorption of the laser light increases in the carbon nanotubes containing polyethylene sheets, that a strong emission of nanoparticles occurs from the nanostructured polymer surfaces and that the irradiated polymer becomes less resistant to mechanical stresses after laser irradiation. Besides the coupling of different polyethylene films, it is possible to induce film welding if the laser parameters are chosen opportunely. Different typologies of welding have been investigated by varying the material and the laser irradiation time. The film welding shows a mechanical resistance depending on the features of the material.

Laser ablation threshold of cultural heritage metals

In this work we determined experimentally the threshold fluence of the most common metals found in cultural heritage, e.g. copper, silver and their alloys. We carried out the ablation process in air at atmospheric pressure with 8 ns pulsed Nd:YAG and 23 ns pulsed KrF lasers, at 532 and 248 nm, respectively. We irradiated every target by a fixed number of laser shots (repetition rate of 1 Hz) at several laser fluence values. Then, the resulting craters were characterized by a stylus surface profiler in order to obtain the dependence of ablation rate on laser fluence F. Here, we defined the ablation raate as ablated matter thickness for single laser pulse, x. Therefore, we identified the ablation threshold fluence, Fth, as the fluence value below which no ablation process would occur.

Laser-generated plasma investigation by electrostatic quadrupole analyzer

A study of different metallic targets ablation, in vacuum, by using a 3 ns Nd:YAG laser radiation, 532 nm wavelength, is reported. Laser pulse with a high intensity generates a plasma at the target surface, with high non-isotropic emission of neutral and ion species, mainly emitted along the normal to the target surface. Mass quadrupole spectrometry, associated to the electrostatic ion deflection, allows an estimation of the emitted charge states energy distributions, within the plasma plume, for a fixed incident laser energy. Neutrals show Boltzmann-like distributions while ions show Coulomb–Boltzmann-shifted distributions. Time-of-flight measurements were also performed by using an ion collector consisting of a collimated Faraday cup placed along the normal to the target surface. The plasma is investigated in terms of velocity, kinetic energy, ion charge state and temperature of the ejected particles. A special regard is given to the parameters that influence the plasma properties, such as the evaporation latent heat, the electrical conductivity and the electron density of the ablated elements, to the plasma temperature and density and to the evaluation of the electric field producing the ion acceleration inside the plasma.

Isotopic ratio measurements with laser ablation coupled to mass quadrupole spectrometry

Physical analyses by laser ablation coupled to a mass quadrupole spectrometry technique on different metallic samples (standards of calibration, minerals and old bronze coins) have been undertaken. Laser ablation induces controllable surface layer removal in high vacuum. Contemporary mass spectrometry analyzes mass elements, mass compounds and isotopic relative content. A comparison between the experimental isotopic ratios and the expected values from the natural abundance is presented. The differences are discussed in terms of the techniques limitations and of the fluctuations due to geographic areas of the materials provenience. Measurements of stable isotopic ratios in copper, lead and tin have been performed in old Egyptian coins. Results confirm that the minerals contained in the coins come from different geographical locations and mines. Results are presented and discussed from the point of view of the physical technique, geological areas and archaeological aspects of peculiar investigated samples.

LAMQS and XRF analyses of ancient Egyptian bronze coins

A set of Egyptian bronze coins, dating back to the sixth or seventh century AD, has been studied by different experimental techniques in order to compare their composition and surface morphology, the process of coinage and, possibly, to also identify the place of production. The measurements have been performed by laser ablation with mass quadrupole spectrometry and energy dispersed X-ray fluorescence. Both analyses are non-invasive and can be safely used according to the integrity requirements of the analyzed pieces. Owing to the poor number of available samples, this work, more than to solve a numismatic question, has been carried out in order to test the validity of the above experimental techniques in view of further analyses on the same coins, based on better quality statistics. The preliminary results, presented in this paper, indicate significant differences in the chemistry of the coins’ patina, i.e. composition and isotopic species content. This seems to support, in agreement with the archaeological expectations, the hypothesis of the existence of a local mint in Antinoopolis, never before considered in Egyptian numismatics.

Ion energy enhancement in laser-generated plasma of metallic-doped polymers

Laser-generated plasma in vacuum are obtained ablating hydrogenated polymers at the Physics Department of Messina University and Prague Asterix Laser System laboratory of Prague. In the first case, a 3-ns, 532-nm Nd:Yag laser at 5×109 W/cm2 intensity was employed. In the second case, a 300-ps, 438-nm iodine laser at 5×1014 W/cm2 intensity was employed. Different ion collectors are used in time-of-flight configuration to monitor ‘on line’ the ejected ions from the plasma at different angles with respect to the normal direction to the target surface. Measurements demonstrated that the mean ion velocity, directed orthogonally to the target, increases for ablation of polymers doped with metallic elements (Br, Cu, Au and W) with respect to that obtained with no-doped polymers. The possible mechanism explaining the results can be found in the different electron density of the plasma, due to the higher number of electrons coming from the metallic doping elements. This charge enhancement increases the equivalent ion voltage acceleration, i.e. the electric field generated in the non-equilibrium plasma placed in front of the ablated target surface.