V. Nassisi

Comparison of nanosecond laser ablation at 1064 and 308 nm wavelength

To study the solid Cu ablation in vacuum, two different laser sources operating at 1064 and 308 nm wavelength are employed at similar values of laser fluences. The infrared laser is a Q-switched Nd:Yag having 9 ns pulse width (INFN-LNS, Catania), while the ultraviolet one is a XeCl excimer having 20 ns pulse width (INFN-LEA, Lecce). Both experiments produced a narrow angular distribution of the ejected material along the normal to the target surface. The ablation showed a threshold laser power density, of about 7 and 3 J/cm 2 at 1064 and 308 nm, respectively, below which the ablation effect was negligible. The laser interaction produces a plasma at the target surface, which expands very fast in the vacuum chamber. Time-of-flight (TOF) measurements of the ion emission indicated an average ion velocity of the order of 4:7 � 10 4 and 2:3 � 10 4 m/s for the infrared and ultraviolet radiation, respectively. We also estimated approximately the corresponding temperature of the plasma from which ions originated, i.e. about 10 6 and 10 5 K for IR and UV wavelength, respectively. A discussion of the analysis of the ablation mechanism is presented. At the used laser power densities the produced Cu ions showed ionisation states between 1þ and 5þ in both cases. # 2003 Elsevier Science B.V. All rights reserved.

A study of the parameters of particles ejected from a laser plasma

We report on the results concerning the characteristics and the behavior of expanding plasma generated by a Laser Ion Source ~LIS!. The LIS technique is an efficient means in producing of multi-charged ions utilizing pulsed laser beams. In order to extract Cu ions, in this experiment an XeCl excimer UV laser was employed, providing a power density on the target surface up to 5 3 10 8 W0cm 2 . Two typologies of diagnostic systems were developed in order to detect the plasma current and the ion energy. The time-of-flight ~TOF! measurements were performed exploiting either a Faraday cup or an Ion Energy Analyzer ~IEA!. This latter allowed getting quantitative information about the relative ion abundances, their kinetic energy and their charge state. To study the plasma characteristics we measured the total etched material per pulse at 70 mJ. It was 0.235 mg and the overall degree of ionization, 16%. The angular distribution of the ablated material was monitored by optical transmission analysis of the deposited film as a function of the angle with respect to the normal to the target surface. Applying a high voltage to an extraction gap a multi-charged ion beam was obtained; different peaks could be distinguished in the TOF spectrum, resulting from the separation of ions of hydrogen, adsorbed compounds in the target and copper.

Laser-induced plasmas from the ablation of metallic targets: The problem of the onset temperature, and insights on the expansion dynamics

Laser-induced plasmas are transient systems rapidly aging in few nanoseconds of evolution. Time-of-flight spectrometry allowed studying initial plasma characteristics based on frozen translational degrees of freedom, hence overcoming intrinsic limitations of optical spectroscopy. Experimental ion velocity distributions were reconstructed as developed during the longitudinal plasma expansion. The obtained onset plasma temperatures are in the range of ∼18–45eV depending on the ablated metals. Also the ion angular spreads were found to be a function of ablated metal, e.g., the narrowest for Fe, the broadest for Al, due to different collisional coupling in the plasma population.

Characterization of a nonequilibrium XeCl laser-plasma by a movable Faraday cup

In this work the experimental results of a nonequilibrium laser-plasma induced by an ultraviolet 308 nm excimer laser are reported. All measurements were performed fixing the laser energy at 70 mJ. It was concentrated on a 0.0099 cm2 spot by a convergent focal lens of 15 cm focal length. The utilized target was a 99.99% pure Cu disk. An 8 cm in diameter movable Faraday cup was developed in order to detect the plasma flow pulse at different positions along a drift tube. Analyzing the time-of-flight pulse under different cup bias voltage, we were able to distinguish the electron pulse, the suprathermal ions, and the thermal evolution of the plasma. In addition, by applying a breakdown voltage as polarizing cup voltage, we characterized the duration of the neutral component. To determine the system particle production efficiency, the total etched material per pulse, 0.235 μg, and the fractional ionization were measured. The expelled particle flux distribution was measured by an optical transmission analysis ...

Time-of-flight profile of multiply-charged ion currents produced by a pulse laser

The ion emission from a Cu-plasma produced by a 308 nm excimer laser is analysed by time-of-flight spectroscopy and by deconvolution of measured ion currents. The ion current signals recorded by an ion collector outside the critical zone, where the charge-states of ions of the expanding plasma are frozen, have been deconvoluted by the use of the Kelly and Dreyfus equation. The meaningful recovered currents recorded for Cu+ and Cu2+ ions have been compared with the current signals reconstructed from the ion energy analyser spectra. A velocity distribution and the abundance of the above ions are presented. A time-resolved average charge-state of ions is also determined. The application of the law Q ∝ l−2, based on the dilution of the total charge, Q, carried out by ions at long distances, l, from the target, is shown to be fundamental for a characterization of the laser-produced plasma.

Improved lasing performance of KrCl excimer laser

A compact, small active volume (70 cc) discharge pumped KrCl laser with UV preionization is described. The results obtained show that KrCl compares well with the performance obtainable from the well established XeCl and KrF lasers. Considerably improved laser output energy density (≂2.5 J/1) is achieved at very high discharge power loadings (≂24 MW/cc), with a good power conversion efficiency (0.8%) and a long gas static lifetime. The output laser beam is reproducible and uniform with a cross section of 2×1 cm2.

Study of particle acceleration of Cu plasma

The experimental results of particle acceleration by plasma generated using a XeCl laser are described. The laser ion source developed is able to accelerate specific particles and to overcome the plasma effects which occur specially during the application of the accelerating voltage. In order to successfully execute this experiment, plasma expansion was highly necessary before the accelerating voltage application. For this goal an almost hermetic expanding chamber with a hole at its end, used as extraction electrode, was made. In this way arcs were eliminated and specific particles propagate in the drift tube. Time-of-flight and current intensity measurements of the ion beam have been done. The output signal, measured at 147 cm from the target, resulted modulated on ion mass-to-charge ratio and its maximum current was 220u2002μA at 18 kV accelerating voltage. Under the same accelerating value the bunch charge was estimated to be 4.2 nC.

Significant role of the recombination effects for a laser ion source

This study is devoted to characterizing a free expansion laser plasma produced by an excimer laser with respect to recombination phenomena. The plasma was ejected from a Cu target after the laser pulse. The diagnostic system was a Faraday cup located at different distances from the target in order to collect the ions carried by the plasma. Ion recombination effects were clearly observed at distances close to the target, due to the high density of the laser-produced plasma at the beginning of its expansion. The recombination range and critical distance were experimentally estimated measuring the ion charge variation on the distance from the target. Beyond it, the charge loss due to the recombination effects was negligible and the freezing of the charge states set in. In this region the plasma dilution, which was due to its free expansion into the vacuum, occurred, and the collected ion charge, Q, followed the law Q ∝ L−2 as a function of the distance from the target, L. The assessment of recombination processes is very important for the optimization of a laser ion source.

A suitable plane transmission line at 900MHz rf fields for E. coli DNA studies

Utilizing a suitable irradiating transmission line chamber, the effects of 900MHz radio frequency fields on DNA mutability and repair in Escherichia coli strains were investigated. The transmission line was very versatile and able to easily apply different field values. In this experiment the maximum electric and magnetic fields were 66V∕m and 260nT, respectively, in the absence of cell plates. Slight decrease in spontaneous mutability to erythromycin resistance was demonstrated in mismatch-repair proficient bacteria exposed to the radio frequency electromagnetic field (RF-EMF) during their growth on solid medium. The anti-mutagenic effect of the RF-EMF was much more impressive at the level of a hypermutagenic cytosine repeat, whose stability is strongly dependent on the activity of the mismatch repair system. In contrast, in mismatch repair-defective background RF-EMF neither affected the general DNA mutability nor the stability of the cytosine repeat, suggesting that the anti-mutagenic effect of the 900...

Characteristics of target polarization by laser ablation

Experimental results are obtained concerning the target polarization, which aptly characterizes the laser ablation. The charge separation in the laser-produced plasma, structure of the ion front, and the current of fast electrons expanding into the vacuum chamber ahead of ions are of crucial importance for the interpretation of multi-peak structure of target currents appearing much later than the laser pulse. Of particular interest is the correlation between the partial maxima in the time-resolved target current and the square root of mass number of ionized species. The late-time negative charging of targets provides evidence for production of very slow ions by ionization of neutrals ablated at the target crater by radiation from plasma produced by 23 ns excimer krypton fluoride laser.