We demonstrate that chemical reactions leading to the formation of AlO radicals in plasmas produced by ablation of aluminum or Ti-sapphire with ultraviolet nanosecond laser pulses can be predicted by the model of local thermodynamic equilibrium. Therefore, emission spectra recorded with an echelle spectrometer and a gated detector were compared to the spectral radiance computed for uniform and nonuniform equilibrium plasmas. The calculations are based on analytical solutions of the radiation transfer equation. The simulations show that the plasmas produced in argon background gas are almost uniform, whereas temperature and density gradients are evidenced in air. Furthermore, chemical reactions exclusively occur in the cold plume periphery for ablation in air. The formation of AlO is negligible in argon as the plasma temperature is too large in the time interval of interest up to several microseconds. Finally, the validity of local thermodynamic equilibrium is shown to depend on time, space, and on the elemental composition. The presented conclusions are of interest for material analysis via laser-induced breakdown spectroscopy and for laser materials processing.

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 cm 2 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 g, and the fractional ionization were measured. The expelled particle flux distribution was measured by an optical transmission analysis of a Cu deposited film on a glass substrate. The plasma flow was detected along its propagation axis, between 6 and 40 cm far from the target. The ablation process expelled particles with an initial velocity of 34 km/s, while the maximum ion concentration was 1 s after the laser pulse. The plasma created propagates with a mean velocity of about 20 km/s. During the propagation, the longitudinal plasma dimension changed from 2.8 cm, near the target, to 31 cm at the maximum cup distance analyzed. At lowest distances, the cup signal wave forms presented a plateau due to the high dense plasma undergone to the space charge regime governed by the Child-Langmuir law.

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 × 108 W/cm2. 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 μg 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.

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The emission of multiply charged Cuq+ ions from a plasma produced by 308 nm excimer laser is analyzed with respect to the distance from the irradiated target. The critical zone, outside which the charge states of ions of the expanding plasma are frozen, was determined to be approximately 20 cm from the target. This value was estimated using a charge-freezing criterion expressed by a distance dependence of the total charge carried by the ions Q∝L−2, which describes the dilution of plasma by its expansion into a vacuum without collisional recombination processes.

We review the current status of metallic photocathodes based on thin films prepared by pulsed laser deposition (PLD) and we explore ways to improve the performance of these devices. PLD seems to be a very efficient and suitable technique for producing adherent and uniform thin films. Time-resolved mass spectrometric investigations definitively suggest that the deposition of high-purity metallic thin films should be carried out in ultrahigh vacuum systems and after a deep and careful laser cleaning of the target surface. Moreover, the laser cleaning of the target surface is highly recommended not only to remove the first contaminated layers but also to improve the quality of the vacuum by reducing the partial pressure of reactive chemical species as H 2 O, H 2 , and O 2 molecules. The challenge to realize high-purity Mg and Y thin films is very interesting for the photocathode R&D due to the good photoemission properties of these metals. Photocathodes based on Mg and Y thin films have been characterized by scanning electron microscopy and x-ray diffraction techniques to derive the morphological and structural features, respectively. They were also tested in a photodiode cell to deduce the photoelectron properties. The quantum efficiency of such photocathodes was systematically improved by in situ laser cleaning treatments of the surface in order to remove the contaminated layers reaching, in this way, the quantum efficiency of the corresponding bulk materials.

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 Cu 2+ 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.

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