L. Torrisi

Angular distribution of ejected atoms from Nd:YAG laser irradiating metals

A Nd:YAG pulsed laser is employed to irradiate different metals in vacuum at the ECLISSE facility of the Laboratorio Nazionale del Sud, Catania, INFN. Laser pulse energy, 9 ns in width, ranges between 100 and 900 mJ. The ejection of atoms by means of laser irradiation is studied in terms of angular distribution, laser etching yield and film thickness deposited on a substrate. Light elements (Ni, Cu) show an angular distribution that is larger than heavy ones (W, Pb). A theoretical approach, applied to fit experimental data, indicates that the distribution depends on the high power of cosu200aθ and that the flow velocity of ejected atom ranges between 27u200a000 and 88u200a000 m/s and the kinetic energy of ejected species ranges between 0.7 and 4.4 keV.

Ion and neutral emission from pulsed laser irradiation of metals

Abstract An Nd:YAG pulsed laser at 1064 nm wavelength with 9 ns pulse width and a maximum pulse energy of 900 mJ is focused on different metallic targets (Al, Ti, Ni, Cu, Nb, Sn, Ta, W, Au and Pb) placed in vacuum. The interaction produces a high etching for a pulse energy higher than a threshold value typical of each metal. Near the threshold a strong neutral emission takes place; at high pulse energy a stronger ionic emission occurs. The experimental thresholds of the ion emission are very similar to the threshold of the neutral emission. The atomic neutral emission is monitored by a mass quadrupole spectrometer and by the vapor thin film deposition technique. The ionic emission is detected through ion collectors (IC) using Faraday cups and time-of-flight measurements. The energy thresholds, the emission yields, the angular distribution, the fractional ionization, the kinetics and characteristics of the plasma production and the ion charge state are presented and discussed.

Angular distribution of ions emitted from Nd:YAG laser-produced plasma

Angular distribution of ion currents emitted from laser-produced plasmas are reported for a Nd:YAG laser with intensities lower than 1×1010u200aW/cm2. This distributions are strongly peaked along the normal to the target surface for Cu, Sn, Ta, W, Au, and Pb ion streams, independent of the incidence angle of the irradiated target. For Al, Ni, and Nb the main axis tends to decline to about −10°. The comparison of fits of Gaussian function and cosP(α−α0)+y0 to the experimental data verified the formal equivalency of both the functions. Fitted values of the FWHM and of the exponent P are compared for all the elements used. The angular distribution of mean ion velocity 〈v〉 and ion kinetic energy 〈E〉 are presented.

Production of low energy, high intensity metal ion beams by means of a laser ion source

The ECLISSE (ECR coupled to Laser Ion Source for charge State Enhancement) project started in 1999 with the aim to obtain an intense beam of highly charged ions (pulsed mode) by means of the coupling between a laser ion source (LIS) and an electron cyclotron resonance (ECR) ion source. The major points to be investigated appeared to be the coupling efficiency between the ion beam produced by the LIS and the ECR plasma, as well as the possibility to enhance the available charge state by an ECRIS with respect to the standard methods which are used to produce ion beams from solid samples (e.g., evaporation, sputtering). The calculations have confirmed that this concept may be effective, provided that the ion energy from the LIS is lower than a few hundred eV. The main features of the calculations will be shown, along with the results obtained in the off-line test facility at laser power densities below 1011u2009W/cm2.

Gold ions produced by 1064 nm pulsed laser irradiation

Abstract A Nd:Yag laser, 9 ns pulse width and 900 mJ maximum energy, is employed to irradiate gold targets in vacuum. Using a laser fluence higher than 15 J/cm 2 , the irradiation produces a plasma formation in front of the target with a plume directed along the normal to the laser spot surface. On line measurements have been obtained with a quadrupole mass spectrometer, with a time of flight technique and with an energy-to-charge ratio ion analyzer. Results demonstrate that the threshold fluence for neutral and ion emission is practically the same. Moreover, ions at different charge state are produced with an energy spectrum from hundreds of eV up to some keV.

Fractional ionization in plasmas produced by pulsed laser ablation

A pulsed infrared laser (Q-switched Nd:Yag) is employed to irradiate different metal targets having atomic number from Z =13 up to Z =82. The high laser fluence deposited on the metals, of the order of 100 v J/cm, produces high ablation yield and a plasma generation at the target surface. The emitted species are neutral and ionized atoms. Both components have been investigated in terms of yield emission, time-of-flight measurements and angular distribution. Results indicate that the main emission occurs mainly along the normal to the target surface, that the etching, at high fluence, is of the order of 10 v atoms/ pulse, that the atoms velocities are of the order of 10 v m/s, that the maximum ion energies are of the order of keV. During the laser irradiation, expanding and non-equilibrium plasma is produced in front of the target. The plasma has a fraction ionization depending by the metal species and generally within 10% and 80%. The plasmatemperature, at high fluence, can be theoretically calculated and reaches about 10 v K. The fractional ionization of the plasma, experimentally measured, has been investigated as a function of the laser fluence and of the energy binding of the target molecules. The ion emission yield is presented and discussed in view of the possibility to realize a laser ion source for ion accelerators.

Non-equilibrium plasma production by pulsed laser ablation of gold

A gold target has been irradiated with a Q-switched Nd:Yag laser having 1064u200anm wavelength, 9u200ans pulse width, 900u200amJ maximum pulse energy and a maximum power density of the order of 1010u200aW/cm2. The laser–target interaction produces a strong gold etching with production of a plasma in front of the target. The plasma contains neutrals and ions having a high charge state. Time-of-flight (TOF) measurements are presented for the analysis of the ion production and ion velocity. A cylindrical electrostatic deflection ion analyzer permits measurement of the yield of the emitted ions, their charge state and their ion energy distribution. Measurements indicate that the ion charge state reaches 6+ and 10+ at a laser fluence of 100u200aJ/cm2 and 160u200aJ/cm2, respectively. The maximum ion energy reaches about 2u200akeV and 8u200akeV at these low and high laser fluences, respectively. Experimental ion energy distributions are given as a function of the ion charge state. Obtained results indicate that electrical fields, produced in the plume, along the normal to the plane of the target surface, exist in the unstable plasma. The electrical fields induce ion acceleration away from the target with a final velocity dependent on the ion charge state. The ion velocity distributions follow a “shifted Maxwellian distribution”, which the authors have corrected for the Coulomb interactions occurring inside the plasma.

Radiation damage in polyvinyltoluene (PVT)

Abstract Polyvinyltoluene (PVT) is an organic polymer which is luminescent when it is irradiated by ionizing radiation. The luminescence yield is proportional to the radiation stopping power and to the absorbed dose. High stopping power radiations, such as ion beams, induce radiation damage in PVT with the production of Cue5f8H bond break, dehydrogenation and free radical formation, as investigated by the mass quadrupole spectrometry. The damage reduces the luminescence yield significantly. A reduction of about 50% is obtained with 300xa0keV proton and argon ions, having 300xa0keV energy, at a dose of about 10 14 and 10 13 xa0ions/cm 2 , respectively.

Radiation effects induced by pulsed laser ablation in tantalum surfaces

Abstract An investigation about the pulsed laser irradiation of tantalum surfaces is presented. The employed laser is a Q-switched Nd:Yag, 1064xa0nm wavelength, with 9xa0ns pulse width and a maximum power density of 1010xa0W/cm2. The tantalum shows a high-energy threshold for the ablation, a low etching rate depending on the laser fluence, and a narrow angular emission produced along the normal to the target surface. The irradiation produces an expanding plasma in front of the target. The ion emission is investigated by time-of-flight measurements. The plasma is characterized in terms of average ion velocity and kinetic energy, ion temperature, fractional ionization and ion charge state. A special regard is given to the ion energy distribution and to the charge state distribution, which are presented and discussed.

Luminescence degrading in polyvinyltoluene by ion beam irradiations

Abstract Polyvinyltoluene is an organic polymer based on a structure of benzenic rings bonded to methylic and vinylic groups. This molecular structure has peculiar luminescent properties in the visible and near ultraviolet wavelength range. The fluorescence induced by photons is investigated as a function of the polymer irradiation with keV–MeV ion beams. Because the luminescence yield is proportional to the radiation energy released by the incident ions to the polymer, these organic materials are employed as scintillators to detect energetic particle in nuclear physics. A significant damage induced by radiation at high stopping power reduces the luminescence yield and degrades the polymer properties. The radiation damage increases with the stopping power of the incident particles. A luminescence yield reduction of about 90% is obtained with an absorbed ion dose of the order of 1014/cm2.