Giulia Lorenzetti

Effect of laser parameters on plasma shielding in single and double pulse configurations during the ablation of an aluminium target

The process of nanosecond-laser ablation in air is a complex phenomenon, where the drilling process is a plasma mediated mechanism strongly affected by the shielding of the trailing part of the laser pulse. The laser pulse parameters as well as the target and ambient gas properties affect the dynamics of the process, modifying the importance and the initiation times of plasma shielding and laser-supported wave mechanisms. In this paper, the laser?target coupling of a Nd?:?YAG laser pulse in air at 1.06??m impinging on an aluminium target was investigated by spectroscopic analysis of plasma emission and by inspection of craters with video-confocal microscopy. The effects of laser pulse width and energy were investigated both in the single pulse configuration and in the orthogonal pre-pulse double pulse configuration. While in the single pulse case, a strong plasma shielding is observed at irradiances higher than (7?9) ? 108?W?cm?2, which is thought to coincide with the onset of laser-supported wave mechanism, such a change of regime is not observed in the double pulse case. A physical interpretation of the data is proposed.

Recovery of archaeological wall paintings using novel multispectral imaging approaches

New approaches in the application of multispectral imaging to the recovery of archeological wall paintings are presented, based on statistical techniques and on a novel method of image treatment (Chromatic Derivative Imaging – ChromaDI) which offers a way of embedding information coming from four spectral bands into a standard RGB image. The methods are applied to some wall paintings from the Tomb of the Monkey, an Etruscan tomb in the necropolis of Poggio Renzo, near the city of Chiusi (Siena), Italy, dated around 480-470 BC. It is shown that the techniques described are able to highlight and enhance a number of details that cannot be perceived in either any of the original channel images or any single processed output channel.

Micro-Laser-Induced Breakdown Spectroscopy (Micro-LIBS) Study on Ancient Roman Mortars

The laser-induced breakdown spectroscopy (LIBS) technique was used for analyzing the composition of an ancient Roman mortar (5th century A.D.), exploiting an experimental setup which allows the determination of the compositions of binder and aggregate in few minutes, without the need for sample treatment. Four thousand LIBS spectra were acquired from an area of 10 mm2, with a 50 µm lateral resolution. The elements of interest in the mortar sample (H, C, O, Na, Mg, Al, Si, K, Ca, Ti, Mn, Fe) were detected and mapped. The collected data graphically shown as compositional images were interpreted using different statistical approaches for the determination of the chemical composition of the binder and aggregate fraction. The methods of false color imaging, blind separation, and self-organizing maps were applied and their results are discussed in this paper. In particular, the method based on the use of self-organizing maps gives well interpretable results in very short times, without any reduction in the dimensionality of the system.

Extracting Time-Resolved Information from Time-Integrated Laser-Induced Breakdown Spectra

Laser-induced breakdown spectroscopy (LIBS) data are characterized by a strong dependence on the acquisition time after the onset of the laser plasma. However, time-resolved broadband spectrometers are expensive and often not suitable for being used in portable LIBS instruments. In this paper we will show how the analysis of a series of LIBS spectra, taken at different delays after the laser pulse, allows the recovery of time-resolved spectral information. The comparison of such spectra is presented for the analysis of an aluminium alloy. The plasma parameters (electron temperature and number density) are evaluated, starting from the time-integrated and time-resolved spectra, respectively. The results are compared and discussed.

Multi-technique study of a ceramic archaeological artifact and its content

In this paper we report the results of a study performed with different physical and chemical methods on a ceramic vase originally attributed to the I century CE. The joint use of infrared spectroscopic analysis and laser techniques, as well as pyrolysis-gas chromatography coupled with mass spectrometry and thermoluminescence, allowed us to characterize the vase material and its content. The chemical data were combined with morphological and stylistic examinations of the object and helped in defining its actual geographical and chronological pertinence.

X-Ray Fluorescence Analysis of XII–XIV Century Italian Gold Coins

An extensive analytical study has been performed on a large number of gold coins (Norman-Swabian Augustale and Tari, Grosso of Lucca, Florin of Florence) minted in Italy from the end of XII century to XIV century. The X-ray fluorescence (XRF) technique was used for verifying the composition of the coins. XRF is a nondestructive technique particularly suited for in situ quantitative analysis of gold and minor elements in the precious alloy. The Florins turned out to have a gold content very close to 24 carats (pure gold) although in a couple of cases we observed relatively high concentrations of iron (around 2%) or lead (around 1%). The Grosso of Lucca has a similar composition, with a measured gold content around 97% due to a higher silver percentage (about 2%), with respect to the average Florin. The Augustali analyzed showed, on average, a gold content around 89%. The average gold content of the Tari analysed is around 72%, with a relatively large variability. The analysis revealed the use of native gold for the coinage of the Florins, excluding the possibility of recycling gold coming from other sources. On the other hand, the variability observed in the compositions of the Tari and Augustali could suggest the reuse of Islamic and North African gold. The study could shed some light on the sudden diffusion of gold coins in Italy around the first half of XIII century, allowing hypotheses on the provenience of the gold used for a coinage that dominated the economic trades from then on.

High-resolution three-dimensional compositional imaging by double-pulse laser-induced breakdown spectroscopy

In this paper we present a new instrument specifically realized for high-resolution three-dimensional compositional analysis and mapping of materials. The instrument is based on the coupling of a Double-Pulse Laser-Induced Breakdown Spectroscopy (LIBS) instrument with an optical microscope. The compositional mapping of the samples is obtained by scanning the laser beam across the surface of the sample, while the in depth analysis is performed by sending multiple laser pulses on the same point. Depths of analysis of several tens of microns can be obtained. The instrument presented has definite advantages with respect to Laser Ablation-ICP Mass Spectrometry in many applications related to material analysis, biomedicine and environmental diagnostics. An application to the diagnostics of industrial ceramics is presented, demonstrating the feasibility of Double-Pulse LIBS Imaging and its advantages with respect to conventional single-pulse LIBS imaging.

Applications of LIBS to the Analysis of Metals

In this chapter we will discuss the basic theoretical principles of the laser ablation process on metal alloys, and will present successful applications of the LIBS technique to the analysis of such materials (aluminium alloys, iron based alloys, copper based alloy, precious alloys, and others). A special emphasis will be given to analytical applications of LIBS performed in line and/or in situ.

Novel polystyrene-based nanocomposites by phosphorene dispersion

Polystyrene-based phosphorene nanocomposites were prepared by a solvent blending procedure allowing the embedding of black phosphorus (BP) nanoflakes in the polymer matrix. Raman spectroscopy, X-ray Diffraction and TEM microscopy were employed to characterize the structural and morphological characteristics of the achieved hybrids, with the aim of evaluating the dispersion level of black phosphorus layers. TGA, DSC analysis as well as thermal oxidation and photo-degradation techniques were employed to investigate the thermal- and the photo-stability of the samples. The collected results showed evidence of better thermal and photostability of both the polymer matrix and dispersed layered phosphorus, suggesting really interesting polymer-nanofiller synergic effects ascribable to the presence and the good dispersion of the 2D-nanomaterial.

Double and Multiple Pulse LIBS Techniques

This chapter will introduce the physical principles underlying double and multiple pulse LIBS analysis and the main applications of this technique for stand-off analysis of hazardous materials, for environmental analysis, for the analysis of biological materials and for cultural heritage and archaeological studies. The available commercial instrumentation for double pulse LIBS analysis will also be presented and discussed.