Dijagnostika i efekti ekraniranja lasersko-indukovane plazme

Abstract

Fundamentalna istraživanja interakcija laserskog zračenja i materije su naročito intenzivirana pojavom impulsnih lasera, čije je vreme trajanja svakom decenijom postajalo sve kraće. Impulsi Nd:YAG lasera, nanosekundnog trajanja, su pored brojnih primena, naročito pogodni za kreiranje plazme materijala, čije se karakteristike mogu ispitivati metodama optičke emisone spektroskopije. U ovoj disertaciji su eksperimentalno ispitivane karakteristike laserski-indukovane plazme stvorene ablacijom bakra i indijuma u zaostaloj atmosferi vazduha, vodoniku, helijumu i argonu. Primenom metoda emisione spektroskopije dijagnostikovani su plazmeni parametri; kao što su elektronska koncentracija i ekscitaciona temperatura. Na osnovu ovih podataka je sagledana dinamika prostorno-vremenskog razvoja plazme. Određeni su prostorno-vremenski intervali, u funkciji gustine snage laserskog zračenja, vrste materijala mete, vrste i pritiska ambijentalnog gasa, u kojima je optimalno posmatrati oblik i intenzitet spektralnih linija sa ciljem nalaženja atomskih parametara. Posebna pažnja je posvećena sagledavanju efekata samoapsorpcije i uslova njene minimalizacije, pa i korekciji samoapsorpcijom deformisanih profila. U cilju određivanja lokalnih vrednosti parametara u laserski-indukovanoj plazmi, korišćena je metoda zasnovana na inverznoj Abel-ovoj transformaciji. Rezonantne spektralne linije su, zbog svojih karakteristika, bile od posebnog interesa. Širenje rezonantnih spektralnih linija atoma bakra (Cu I 324.7 nm i 327.4 nm) je istraživano u spektru laserski-indukovane plazme u zaostaloj atmosferi vazduha na pritisku od 8 Pa. Na optimalnim uslovima su određeni Stark-ovi parametri ovih liniija i upoređeni sa teorijskim vrednostima. Uloga doprinosa komponenata hiperfine strukture i izotopskog efekta, rezultantnom obliku ovih spektralnih linija, je prvi put uzeta u razmatranje. Pokazano je da hiperfina struktura i izotopski efekat dodatno proširuju profile rezonantnih Cu I linija, što je značajno pri korišćenju ovih linija u dijagnostičke svrhe. Istraživanja u laserski-indukovanoj plazmi indijuma su sprovedena u zaostaloj atmosferi vazduha, vodoniku i argonu. Utvrđeno je da je ambijent argona, zbog manje izraženih prostornih i vremenskih gradijenata, izuzetno pogodan za određivanje temodinamičkih parametara plazme. Pomenuti ambijent je omogućio utvrđivanje uslova koji podrazumevaju minimalne efekte samoapsorpcije. Pod ovim uslovima su, po prvi put, eksperimentalno određeni parametri Stark-ovog širenja rezonantnih In I (410.2 nm i 451.1 nm) i jedne In II (294.1 nm) linije i upoređeni sa postojećim teorijskim vrednostima. Utvrđena je saglasnost sa dosada jedinim postojećim eksperimentalnim podacima koji se tiču pomeraja rezonantnih linija. Pokazano je da hiperfina struktura i izotopski efekat dodatno proširuju profile rezonantnih In I linija, naročito na 410.2 nm. Utvrđeno je da se doprinos hiperfine strukture na elektronskim koncentracijama iznad 1022 m-3 može zanemariti, što je značajno pri korišćenju ovih linija u dijagnostičke svrhe. Pored uzoraka metala ispitivane su karakteristike laserski-indukovane plazme hidratnih jedinjenja (CuSO4·5H2O i CaSO4·2H2O) u vidu presovanih tableta. Ovakav vid uzoraka je imao za cilj pouzdano određivanje elektronske koncentracije korišćenjem karakteristika profila vodonikovih linija iz Balmer-ove serije, za koje postoje veoma pouzdani teorijski proračuni. Pored pomenutih dijagnostičkih pogodnosti, efekat samoapsorpcije spektralnih linija je manje izražen, zbog manje koncentracije isptitivanog elementa u stvorenoj LIBS plazmi. Utvrđeno je da su hidratna jedinjenja veoma pogodna za pouzdano dijagnostikovanje parametara plazme, te je ovakav pristup predložen i za određivanje Stark-ovih parametara. U tezi su predstavljeni i rezultati istraživanja akustičkih efekata koji nastaju sa formiranjem laserski-indukovane plazme. Ispitivani su akustički odzivi proboja u vazduhu, korišćenjem različitih metala kao mete (aluminijum, bakar i zlato). Pored detaljne statističke analize opto-akustičkih signala, izvršena je vremenski integralna i vremenskirazložena analiza njihovih dominantnih učestnosti. Utvrđeno je da se ovim pristupom mogu dobiti informacije o anizotropiji i dinamici plazme, te ukazano na mogućnost korelisanja akustičkog odziva sa količinom ablirane mase. Pokazana je različitost akustičkog odziva u zavisnosti od korišćenog metala mete i sugerisane mogućnosti potencijalnih primena. Ispitivan je prag, kao i okolnosti pojavljivanja, efekata ekraniranja laserskiindukovane plazme u zavisnosti od gustine snage i talasne dužine laserskog zračenja. Utvrđeno je da pojava ekraniranja zavisi od kompleksne sprege različitih procesa, koji rezultuju apsorpcijom dela laserskog-impulsa u plazmi. Određena je vrednost praga značajnije apsorpcije za bakarni uzorak u redukovanim pristiscima helijuma, preciznim utvrđivanjem razlika mase mete pre i posle ablacije. Dobijena vrednost praga je potvrđena i metodama emisione spektroskopije na osnovu ponašanja intenziteta linija. Postignuti rezultati u ovoj tezi predstavljaju originalan doprinos proširenju postojećih rezultata u oblastima vezanim za interakciju laserskog zračenja sa materijom, kao i interakciju laserskog zračenja sa plazmom. Ovaj doprinos se ogleda i na polju atomske emisione spektroskopije, utvrđivanja atomskih parametara, ispitivanju oblika spektralnih linija, kao i prostorno-vremenske evolucije laserski-indukovane plazme. Pomenuti rezultati su i od značaja za predstojeća sveobuhvatna modeliranja u domenu kompleksnih procesa pre, tokom i posle formiranja ovakve vrste plazme.

Fundamental investigations concerning laser-matter interactions have been extensively conducted ever since the invention of the pulsed laser source. Consequently, the laser pulse durations have had a constant trend of reduction in a last couple of decades. Among many other applications, Nd:YAG nanosecond pulses are frequently used for the purpose of laser-induced breakdown spectroscopy (LIBS). This thesis mainly covers the various aspects of laser induced plasma of copper and indium, created in the residual atmosphere ambient, hydrogen, helium and argon buffer gas. By using methodologies of optical emission spectroscopy, the essential thermodynamic parameters were deduced; electron density and excitation temperature. These data served as a basis for constructing a more general picture about plasma dynamics and its spatiotemporal evolution. The spatiotemporal intervals, strongly dependent on irradiance, sample structure and ambient conditions, were optimally determined with the aim of investigating spectral line shapes and corresponding atomic parameters. Special care was taken in order to detect, minimize or correct the undesirable effects of self-absorption. In order to infer local values of the mentioned plasma parameters, a method based on an inverse Abel transform, was employed. The resonance spectral lines were of a particular interest, mainly because of their unique characteristics. Spectral line broadening of the resonance copper atom lines (Cu I 324.7 nm and 327.4 nm) was investigated in the laser-induced spectrum under reduced atmospheric pressure of 8 Pa. After finding the optimal set of experimental parameters, the Stark broadening coefficients were deduced and compared to the existing theoretical values. The contributions of the hyperfine structure components and isotope effect to the resonance lines shape formation were, for the first time, taken into consideration. It is shown that the hyperfine structure and isotope effect additionally broad resonance line profiles, which is of particular interest when using these lines in various diagnostic purposes. Investigations of laser-induced indium plasma were performed under different ambient environments; reduced pressures of air, hydrogen and argon. Due to less pronounced spatiotemporal gradients and effects of self-absorption, the argon buffer gas was chosen as the most adequate environment for determination of thermodynamic plasma parameters. Under those conditions, the Stark coefficients of two resonance In I (410.2 nm and 451.1 nm) spectral lines and one ionic In II (294.1 nm) line were also determined and compared to the existing theoretical values. In terms of the estimated Stark shift directions, agreement was found with the results of one experimental work, dealing with indium resonance lines. It is shown that the hyperfine structure and isotope effect additionally broad resonance line profiles, especially in the case of 410.2 nm line. This remark is of particular interest when the electron densities exceed 1022 m-3, where HFS contributions can be safely neglected. Beside metallic samples, the characteristics of the laser-induced plasma of hydrogen containing samples (CuSO4·5H2O and CaSO4·2H2O) in the form of a pressed pellets, were also studied. This kind of matrix was chosen in order to increase reliability of electron number density determination using Stark broadened hydrogen Balmer lines, extensively investigated, both theoretically and experimentally. The effects of self-absorption are also less pronounced, due to smaller concentrations of investigated element in the sample. It is concluded that these compounds are suitable for the reliable diagnosis of plasma parameters and therefore, recommended for determination of the Stark broadening coefficients, as well. This thesis also contains results of the investigations of the laser-induced plasma acoustical response, inevitably present during the plasma formation process. The acoustical response was monitored in case of an air breakdown and also for plasmas of various metals (aluminum, copper and gold). Beside detailed statistical analysis of these opto-acoustic signals, the time-integrated and time-resolved analysis of dominant frequency components, were also performed. It is determined that this approach could be used for obtaining information about plasma anisotropy and dynamics. The possibility of correlating the plasma acoustical response with the atomised fraction of the ablated mass was also considered. The found diversities of the acoustical response depending on the used sample have imposed suggestions for the potential applications. The threshold for the onset of the significant plasma screening effects was investigated depending on the laser irradiance and wavelength. It is well known that the screening process is a result of complex interplay of various processes, favoring conditions for the absorption of the trailing part of the laser pulse. The threshold value is obtained in the case of copper sample under reduced pressures of helium atmosphere, using precise estimation of mass before and after ablation. This threshold value was additionally verified using standard emission spectroscopy probing methods, based on the spectral line behavior. The results shown in this thesis present an authentic contribution and extension of existing results in the scientific fields related to both laser-material and laser-plasma interactions. Besides, there is a degree of novelty in terms of applying new approaches concerning atomic emission spectroscopy, spectral line shape investigations, atomic parameter determination and spatiotemporal evolution of the laser-induced plasma. These results should serve as a basis for the upcoming comprehensive modeling of the complex processes before, during and after plasma formation.