Space and time resolved spectroscopic diagnostics of laser induced plasma on solid target in air at atmospheric preassure

[M. Cvejić]

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Fizika plazme, laseri, elektrotehnika - Spektroskopija plazme / Physics of plasma, lasers, electrical engineering - Plasma spectroscopy

LIBS ili LIPS (Laser Induced Plasma Spectroscopy) - spektroskopija laserski indukovane plazme je relativno nova tehnika za analizu materijala, čiji je razvoj uslovljen tehnološkim napretkom na polju lasera, spektrometara i detektora. Tehnika omogućava multi-elementarnu analizu svih vrsta materijala u realnom vremenu, u raznim sredinama transparentnim za lasersko zračenje, bez zahteva za pripremom uzorka. To je čini pogodnom za mnoge primene, poput kontrole kvaliteta, zaštite čovekove okoline, recikliranje materijala i analize u nedostupnim sredinama. Kvantitativna merenja pomoću LIBS-a, međutim, i dalje pokazuju nižu granicu detekcije i generalno slabiju tačnost u odnosu na druge analitičke metode, iako je postignut značajan napredak u pouzdanosti i preciznosti LIBS merenja za različite tipove materijala. Razlog je nedovoljno poznavanje složene interakcije laserskog zračenja sa ispitivanim uzorkom, koja zavisi od karakteristika lasera (talasna dužina, vreme trajanja impulsa, energije) i svojstva materijala ispitivanog uzorka, kao i nedovoljna proučenost, prostorno i vremenski zavisne interakcije plazme i čestica. Kako su kvantitativna merenja zasnovana na povezivanju (integralnog) intenziteta spektralne linije sa gustinom svih emitujućih čestica u plazmi, i preko toga sa koncentracijom elemenata u uzorku, razumevanje fizičkih procesa i njihove povezanosti u laserski indukovanoj plazmi od suštinskog je značaja za analitičke primene. Najveći deo informacija iz LIBS-a dobija se analizom zračenja plazme. Emisioni spektar je diskretan i sastoji se od atomskih, jonskih i molekulskih linija elemenata prisutnih u plazmi (metala, oksida, stena, legura, itd.). čijim ispitivanjem se može izvršiti detaljna analiza i dijagnostika plazme. Međutim, nehomogenost i vremenske promene oblika LIBS plazme, praćene promenom parametara plazme i emisijom materijala nastalog ablacijom u plazmenom oblaku, znatno otežavaju primenu spektroskopske dijagnostike. Ovaj rad obuhvata analizu tehnika, brze fotografije i optičke emisione spektroskopije, potrebnih za prostornu i vremenski razloženu dijagnostiku laserski indukovane plazme na čvrstoj meti u vazduhu na atmosferskom pritisku. Tehnikom brze fotografije određeno je optimalno vreme posmatranja plazme za merenje oblika i intenziteta spektralnih linija prisutnih u plazmi, kako bi se izbegle teškoće vezane za nehomogenost i vremensku promenljivost LIBS plazme. Pored toga uzeto je u obzir dejstvo samoapsorpcije na oblik i intenzitet spektralnih linija. Zatim je primenjena procedura Abel-ove inverzije za određivanje radijalne raspodele intenziteta linija korišćenih za merenje novih Stark-ovih parametara pre svega ne-vodoničnih linija koje su dominantne u ispitivanoj plazmi. U radu je izvršeno i nezavisno određivanje elektronske gustine i temperature što predstavlja uslov potreban za precizno određivanje Stark-ovih parametara. Dobijeni eksperimentalni rezultati predstavljaju značajan doprinos za dijagnostiku i modelovanje laserski indukovanih plazmi a time i bolje razumevanje složenih fizičkih procesa, što vodi do ostvarivanje uslova za još širu primenu LIBS-a u različitim oblastima.

LIBS (Laser Induced Breakdown Spectroscopy) or LIPS (Laser Induced Plasma Spectrosropy) is a relatively new technique for material analysis, recently developed due to advances in laser technology, spectrometers and detectors. This technique enables a multi-element real time analysis of all kinds of materials without previous sample preparation, making it useful for various applications, like quality control, environmental protection, material recycling and analysis in unavailable environments. Even with advances in reliability and precision of LIBS measurements of different kinds of materials, quantitative measurements that are based on the connection between integral intensity of the spectral line and density of emitting particles in plasma, and via that with concentration of elements in sample, are still challenging for most of the materials and suffer from lower accuracy and detection limits compared to other analytical techniques. The main reasons for that is the complex nature of the lasersample interaction process which is dependent on several laser parameters (wavelength, pulse duration, energy) and on the characteristics of material under investigation, and space and time dependent plasma-particle interactions. Thus, understanding the physical processes and their interactions in laser induced plasma is essential for analytical applications. Most of the information obtained from LIBS measurements come from plasma radiation. Emission spectrum is discreet and includes atomic, ionic and molecular lines of all the elements present in the plasma (metals, oxides, alloys, etc.). Analysis of these spectral lines shapes is a part of analysis and diagnostics of plasma. Difficulties in applying spectral diagnostics in LIBS plasma originate from plasma inhomogeneity, changes in a plasma shape that occur over time followed by changes in plasma parameters and emission of materials due to ablation in the plasma cloud. This work includes the analysis of techniques, fast photography and optical emission spectroscopy, needed for space and time resolved diagnostics of laser induced plasma on a solid target in air at atmospheric pressure. Using the technique of fast photography, optimal time of plasma spectral observation is determined needed for recording line shapes of elements which are present in plasma. Influence of selfabsorption on spectral line shapes is taken into account during analysis. Abel inversion procedure is applied to determine radial intensity distributions of spectral lines used for measurements of new Stark broadening parameters of, in the first place, non-hydrogenic spectral lines. These lines are predominant in this kind of plasma. Independent determination of electron density and temperature is done which is necessary condition for measurements of Stark broadening parameters. Obtained experimental results represent significant a contribution to diagnostics and modeling of laser induced plasmas and to better understanding of complex physical processes. This leads to achieving a broader application of LIBS technique in different areas. Key words: laser induced plasma, electron number density, optical emission spectroscopy, Stark broadening, forbidden transitions. Scientific field: physics of plasma, lasers, electrical engineering Specific topic: plasma spectroscopy UDC number:

laserski indukovana plazma, gustina elektrona u plazmi, optička emisiona spektroskopija, Stark-ovo širenje, zabranjeni prelazi

laser induced plasma, electron number density, optical emission spectroscopy, Stark broadening, forbidden transitions

621.3(043.3)

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