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LIBS@TITO

Laser Induced Breakdown Spectroscopy - LIBS

Antonio Santagata  - antonio.santagata@ism.cnr.it

LIBS-FTIR/FemtoLAB

Tito Scalo (Pz)
 
LIBS exploits the property of the plasma emission spectrum induced during the laser ablation process. This emission spectrum, in fact, is characterized by the radiative de-excitation process involving all the electronically excited species (ions and neutral atoms) coming from the ablated material. The stoichiometric ratios of these species are characteristic of the composition of the sample under examination. The electronic excitation-de-excitation process of the plasma’s atoms is induced by continuous impacts that occur among them and the high density electrons (1017-1020 cm-3) affecting the plasma’s excitation temperature which, over time (100ns-10μs), ranges from several thousands to few thousands Kelvin. Both the excitation temperature and the electronic density of the plasma are parameters varying over time and represent the main quantities to be determined for defining the most suitable operating conditions to be used. As a matter of fact, together with the need of having an optically thin and homogeneous plasma, it is possible, through the monitoring of both the plasma’s excitation temperature and electron density, to determine the experimental conditions to be employed for fulfilling the local thermodynamic equilibrium (LTE) requirement making possible to associate the emission spectra of LIBS with the composition of the sample under analysis. The technique, however it is pretty simple, exhibits operational problems which must be verified on a case-by-case basis through the use of spectrographs having detectors with high temporal resolution (10ns -1μs). Through the use of fs laser pulsed, good stratigraphic analyses can even be performed although, as a consequence of the lack of laser-plasma interaction, the Limit of Detection (LOD) is at least one order of magnitude worse than that achievable by using ns laser pulses. These, actually, support the process of electronic excitation-de-excitation of the ablated species by the occurrence of the laser-plasma interaction process.
 
 

TECHNICAL SPECIFICATIONS

  • Spectra Physics Ti:Sa “fs” Laser
    Spitfire Pro - Regenerative Amplifier (120 fs; 1kHz; 4 mJ @ 800 nm; SH: 1.5 mJ @ 400 nm)
  • Quanta System Nd:YAG “ns” Laser
    Prototype (7 ns; 10 Hz; 100 mJ @ 532 nm)
  • Time resolved spectroscopy and imaging
    • Andor iStar “Inductively Charge Couple Device – ICCD” camera (t ≥ 2 ns; Spectral range = 250-900 nm, Pixeldim = 13 μm x 13 μm)
    • Monochromator ARC SpectraPro 300i (Spectral range = 200-1000 nm; ʎ/Δʎ = 10000)

AVAILABLE TECHNIQUES

  • Characterization of laser induced plasmas features and temporal evolution of them
    • acquisition of the emission spectra  
    • determination of the electronic density by Stark broadening  
    • outline the operating conditions that fulfill the LTE conditions  
    • definition of the excitation temperatures
       
  • Analysis of the samples under study
    • building-up of calibration lines if are available several standards having a matrix comparable with that of the sample to be analyzed (quantitative analysis)
    • use of other procedures (e.g. inverse calibration free) for semi-quantitative analyses . 
       
  • The LIBS activities are carried out in collaboration with the Physical-Chemistry Laser Laboratory of the University of Basilicata which expands the offer by the availability of other experimental equipments and techniques for materials’ characterization (e.g. μ-Raman, XPS, XRD).
 

SAMPLES

  • It is suitable for any solid, preferably flat, for which it is available at least a standard with known composition and having same matrix of the sample under investigation.

  • Sample lateral size 5 mm x5 mm (minimum) - 30 mm x 30 mm (maximum).

USED FOR

  • analysis of metal alloys

  • analysis of archaeological artifacts

  • stratigraphic analysis

  • compositional analysis of solids in the (x, y) plane with spatial resolution up to a few  μm

  • fingerprint spectra of polymeric compounds

 
 

CASE STUDIES

Experimental remarks

The emission spectrum of the plasmas generated by pulsed lasers shows a temporal evolution formed, initially, by Bremsstrahlung and ion-electron recombination continuum. This emission quenches rapidly over time, allowing the signals of electronically excited species to emerge after the first 0.1-1 μs. The features of the excited species emission are related to the composition of the sample under examination. However, the various signal intensities can be linked to the concentrations of the single species only if the following conditions: LTE, homogeneity and "thin" plasma, are fulfilled. These requirements can be determined for each type of sample by varying the delay and acquisition times of the emission’s spectra. It follows that the LIBS technique depends strongly on the matrix of the sample to be analyzed and on the need of having detector with high time resolutions so that fast detectors such as ICCD camera are employed.ossono essere correlate alle concentrazioni delle singole specie solo riuscendo a soddisfare le condizioni di LTE, omogeneità e plasma “sottile” che devono essere verificate per ogni tipologia di campione variando i tempi di ritardo e di acquisizione degli spettri di emissione. Queste condizioni rendono la tecnica dipendente dalla matrice del campione in analisi e dalla risoluzione temporale del rivelatore per cui è necessario l’uso di rivelatori veloci (e.g. ICCD) che hanno risoluzioni del ns.

See: David W., Hahn et al. Appl. Spect. 64, 335A (2010)  
DOI: 10.1366/000370210793561691

 
 
 

Compositional and stratigraphic analyses

After having determined the most suitable acquisition time intervals (delay and gate) of the emitting plasma spectrum, it is possible to carry out compositional analyses of the materials either through the known method of Calibration Free and its following upgrades or, if standards with a matrix comparable to that of the material under analysis are available, by building-up calibration graphs. The analyses processing time can be limited to few seconds per each single sampling point up to only few minutes for semi-quantitative compositional analyses. Due to both the processing speed which can be well exploited for comparative analyses and the lack of sample preparation, LIBS is widely used in archeometry for dating and classifying archaeological artifacts. Using fs laser pulses stratigraphic analyses with good spatial resolution (500 nm / pulse) can be performed. These pulses are even extremely advantageous for limiting both the damage of the surface of the analyzed material and the involvement of thermal effects which can lead to the occurrence, within the sample, of elemental interdiffusion interfering phenomena..

See: Angela, De Bonis et al., Appl. Surf. Sci. 302, 275 (2014)   
DOI: 10.1016/j.apsusc.2013.10.127

 
 

Identification of the species in Polypropylene (PP) materials deriving from automotive production wastes and end-of-life vehicles (ELVs)

A qualitative elemental analysis of LIBS spectra conducted on virgin plastic materials, production wastes and end of life vehicles (ELVs) deriving from the automotive industry, allowed us to distinguish the species in different samples characterizing the polymer matrix and the mineral fillers commonly enriching polymers. From the acquired LIBS spectra it was possible to verify the presence of the main elements, such as: Magnesium (Mg) and Silicon (Si). In addition to these, we have also been able to ascertain the presence of species such as: Aluminum (Al) and Sodium (Na) related mostly to the impurities that are commonly found in natural talc. .
As far as the polymer matrix is concerned, the characteristic LIBS signals have been traced: the Swan bands (Carbon dimers in the excited state) around 470 and 510 nm, the CN bands (generated by the interaction of the laser beam with the matrix plastic in air) and those of Carbon around 250 nm.  
Once the typical talc signals found in all the analyzed samples were excluded, the comparison of the spectrum of a model isotactic PP with those of the virgin raw materials (PP bumpers, PP defroster, PP dashboard), production wastes (PP waste) and ELV samples (ELV1 PP, ELV2 PP), has easily highlighted the presence of species deriving from impurities (especially in the waste material and in PP ELV) and /or from additives both of an organic and inorganic nature that typically characterize the formulation of almost all the plastic products on the market. In this regard, we can highlight the characteristic fingerprint of Titanium (Ti) clearly identified within some samples analyzed..

Contact: Ambra Guarnaccio - ambra.guarnaccio@ism.cnr.it

 
 

TCSPC@TITO

Time-Correlated Single Photon Counting (TCSPC) Spectroscopy

Ambra Guarnaccio  - ambra.guarnaccio@ism.cnr.it

FemtoLAB/ChemLAB

Tito Scalo (Pz)
 
The TCSPC technique allows measuring the fluorescence emission after the molecule excitation with a proper excitation wavelength,(ʎex) (generated by high-frequency pulsed laser radiation) enabling to determine the fluorescence lifetime by the fitting of the resulting decay curve. This technique allows to measure the fluorescence (radiative) emission, ʎem) after the excitation of a sample with a pulsed laser radiation. With periodic excitation, e.g., from a high frequency laser source, it is possible to extend the data collection over multiple cycles of excitation and emission. One can then accept the sparseness of the collected photons and reconstruct the fluorescence decay profile from the multitude of single-photon events collected over many cycles. The reference for the timing is the corresponding excitation pulse triggered by a fast photodiode collection.
 

TECHNICAL SPECIFICATIONS

  • Available laser sources:
    • Spectra Physics, Spitfire Pro, Ti:Sa Laser: ʎ 800 nm (tunabile con sistema OPA nel range 290-2500 nm); 120 fs; 4 mJ; repetition rate up to 1 KHz;
    • Tsunami: ʎ 800 nm (400 nm by SHG using a BBO crystal); 90 fs, 10 nJ, 80 MHz;
  • Electronic for timing: PicoQuant Pico Harp 300;
  • Fast photodiode: TDA 200;
  • Monochromator: Princeton Instruments ACTON SP2150;
  • Photo-multiplier tube: PicoQuant PMA-C 192-N-M (< (< 180 ps (FWHM))

AVAILABLE TECHNIQUES

  • Fluorescence decay (Int. vs Time);
  • Fluorescence anisotropy and in polarization control;
  • Fluorescence 3D maps (time vs ʎem vs Int.) of fluorescence decays over a wide wavelength range of excitation (ʎex 290-2500 nm) and emission (ʎem 230-920 nm).
 

SAMPLE

  • Preferred: diluted solutions(10-4-10-6 M) in organic solvents;

  • Thin films

 

USE FOR

  • Organic/Inorganic Semiconductors;

  • Thin films/coatings;

  • Nanoparticles.

 
 

Case Studies

COUMARIN 500 (C500) fluorescence decay

The study case of C500 has been used in order to evaluate the performance of our experimental setup using a Tsunami (80MHz, ʎex 400 nm BBO SHG). The fit analysis has highlighted that a mono-exponential decay model is useful to describe the radiative fluorescence decay (τ=5.04 ns)registered by exciting a 10-5 M of C500 in EtOH, solution by a 400 nm ultra fast laser source as reported in literature for the same dye system under analysis.

See: Sanjucta, Nad et al., J. Phys. Chem. A, 107, 501 (2003)
DOI: 10.1021/jp021141l

 
 
 

FLUORESCENCE 3D MAP of a short oligothiophene (DTBT) molecule

The experimental setup described for the C500 study case has been used to investigate the fluorescence lifetime related to a solution (10-4 M/CH2Cl2 of a short chain oligothiophene compound useful for organic photovoltaic applications (i.e. the DTBT, 1,3-di(2-thienyl)-2-benzothiophene). The evaluation of the fluorescence lifetime has been the starting point for a much wider comprehension of radiative and non-radiative processes occurring within the excited DTBT systems (by fs laser excitation source) in solution. Indeed, the fluorescence "competes" with non-radiative decay processes studied by FTAS (Femtosecond Transient Absorption Spectroscopy) measurements.

 
 

Time-Correlated Single-Photon Counting (TCSPC)

Time-correlated single-photon counting (TCSPC) is a common technique to measure photoluminescence decays in the time domain. In principle, single-photon events are detected and their time of arrival is correlated to the laser pulse, which was used for excitation of the sample. Exciting the sample by using a pulsed laser source with a high repetition rate, this process can be repeated many times so that a photon distribution over time is built up. Photons emitted by the sample are detected with a high-gain photo-multiplier and the time concerning the excitation pulse is measured. By counting many events a histogram of the photon distribution in time is built up.

TCSPC@RM TCSPC@TITO

Time-Resolved Absorption/Reflectivity in VUV/HHG

The high harmonic generation (HHG) source under development in the EFSL laboratory makes use of strongly non-linear processes in noble gases to generate high harmonics (up to 25th harmonic in Argon) by a three step process involving ionisation, electron acceleration and recombination. In the HHG set-up in EFSL laboratory the VUV light is monochromatised (probe) and combined with light from the OPA (pump) to perform UV-Vis/EUV pump-probe experiments with resolution of 100 fs.

HHG

Laser Induced Breakdown Spectroscopy (LIBS)

Laser Induced Breakdown Spectroscopy (LIBS) is a material analysis technique exploiting the composition determination of the plasma induced during the laser ablation process. Due to the versatility of the ablation process, which allows the “vaporization” in the form of plasma (plasma plume) of any solid material, no preparation of the sample under analysis is required. Thanks to the size of the spot of the incident laser (10s-100s μm) it is possible to perform compositional analyses having good spatial resolution both lateral (x, y) and vertical (z: stratigraphic analysis).

LIBS@ns LIBS@fs

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