F. Caridi

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...

Protons and ion acceleration from thick targets at 1010 W/cm2 laser pulse intensity

Proton ion acceleration via laser-generated plasma is investigated at relatively low laser pulse intensity, on the order of 10 10 W/cm 2 . Time-of-flight technique is employed to measure the ion energy and the relative yield. An ion collector and an ion energy analyzer are used with this aim and to distinguish the number of charge states of the produced ions. The kinetic energy and the emission yield are measured through a consolidated theory, which assumes that the ion emission follows the Coulomb-Boltzmann-Shifted function. The proton stream is generated by thin and thick hydrogenated targets and it is dependent on the free electron states, which increase the laser absorption coefficient and the ion acceleration. The maximum proton energy, of about 200 eV, and the maximum proton amount can be obtained with thick metallic hydrogenated materials, such as the titanium hydrate TiH 2 .

Investigations on low temperature laser-generated plasmas

A nanosecond pulsed Nd-Yag laser, operating at an intensity of about 10 9 W/cm 2 , was employed to irradiate different metallic solid targets (Al, Cu, Ta, W, and Au) in vacuum. The measured ablation yield increases with the direct current (dc) electrical conductivity of the irradiated target. The produced plasma was characterized in terms of thermal and Coulomb interaction evaluating the ion temperature and the ion acceleration voltage developed in the non-equilibrium plasma core. The particles emission produced along the normal to the target surface was investigated measuring the neutral and the ion energy distributions and fitting the experimental data with the “Coulomb-Boltzmann-shifted” function. Results indicate that the mean energy of the distributions and the equivalent ion acceleration voltage of the non-equilibrium plasma increase with the free electron density of the irradiated element.

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.

Carbon nanocrystals produced by pulsed laser ablation of carbon

Plasma laser ablation experiments were performed irradiating glassy-carbon targets placed in vacuum through a pulsed Nd:YAG laser operating at the second harmonic (532 nm), 9 ns pulse width and 109 W/cm2 density power. Thin films of ablated carbon were deposited on silicon oxide substrates placed at different distances and angles with respect to the target. The analysis of the deposited material was carried out by using surface profiler, scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) and Raman spectroscopy. Results show the evidence of carbon nanocrystals and nanostructures with dimension of the order of 100 nm deposited on the substrates together with a large amount of amorphous phase. The spectroscopic investigations and the SEM images indicate the formation of nanodiamond seeds as a nucleation process induced on the substrate surface. Nanostructures were investigated as a function of the laser intensity and angle distribution. Experimental results were compared with the literature data coming from nanodiamonds growth with different techniques. Experiments performed at Instituto Nazionale di Fisica Nucleare-Laboratori Nazionali del Sud (INFN-LNS) of Catania (Italy) and data analysis conducted at Dipartimento di Fisica and DFMTA of the Università of Messina (Italy), CNR-ITIS of Messina and ST-Microelectronics of Catania will be presented and discussed.

Nickel plasma produced by 532-nm and 1064-nm pulsed laser ablation

A comparison between laser ablation of nickel in vacuum by using 532-and 1064-nm Nd:YAG (Yttrium Aluminium Garnet) laser wavelengths, with an intensity of 5 × 109 W/cm2, is reported. Nanosecond pulsed ablation produces high nonisotropic emission of neutrals and ionic species. For 532-nm laser irradiation, mass quadrupole spectrometry, coupled to electrostatic ion deflection and time-of-flight measurements, allows estimation of the energy distributions of the emitted species from plasma. For 1064-nm laser ablation, a cylindrical electrostatic ion analyzer permits one to measure the yield and the charge state of the emitted ions and reconstruct the ion energy and charge state distributions. Neutrals show typical Boltzmann-like distributions, while ions show Coulomb-Boltzmann-shifted distributions depending on their charge state. Surface profiles of the ablated craters permitted study of the ablation threshold and yields of nickel in vacuum versus the laser fluence. The plasma temperature was evaluated using experimental data. Special regard is given to the ion acceleration process occurring inside the plasma due to the high electrical field generated at nonequilibrium plasma conditions and the angular distribution of the emitted species.

Evidence of plasmon resonances of nickel particles deposited by pulsed laser ablation

The optical spectra of some metals show pronounced resonance lines caused by collective excitations of conduction electrons. These excitations are known as particle plasmons, Mie plasmons, or surface plasmons. Their spectral properties have attracted a lot of interest, both for fundamental reasons and in a view of applications. Scope of the work is the growth of nanometric metal particles (Ni) and the study of its optical properties by spectroscopic ellipsometry. Ni particles are obtained by implanting SiO2 with pulsed laser ablation followed by heat treatment in inert atmosphere (N2). An analysis of the ellipsometric spectra for samples with different implantation times and energy is presented. Generally, the synthesis of such structures is performed using ion implantation techniques or chemical reaction methods, while here we propose pulsed laser ablation for the generation of these particles and annealing procedures for their activation. The experimental measurements were performed at IRST (Istituto per la Ricerca Scientifica e Tecnologica) of Fondazione Bruno Kessler in Trento and at the Physics Department of University of Messina.

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.

Measurements of gas desorption from polyethylene-UHMWPE irradiated by 5 MeV electrons

Gas desorption in vacuum from electron irradiated ultra high molecular weight polyethylene (PE) is measured with a high sensible mass quadrupole spectrometer. Measurements are performed in thick PE irradiated with 5 MeV electron beams at doses of the order of tens of kGy. The irradiation modifies the PE molecules producing dehydrogenation, emission of different C x –H y groups, C-enrichment and carbon cross-linking processes. Results indicate that the radiation damage depends on the dose and that a significant change of chemical and physical polymer properties is reached for a critical dose of 18 kGy.