Nitrogen emission spectrum as a measure of electric field strength in low-temperature gas discharges
Date
2016-12-22
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Abstract
Gaaslahenduses toimuvate protsesside uurimisel on sageli teadmata tegelik elektrivälja tugevus mingis kindlas lahenduse piirkonnas. Põhjuseks on tavaliselt gaaslahenduses osalevate laengukandjate poolt tekitatud ruumlaengu oluline mõju või ka väljatugevuse kiire muutumine.
Osutub, et tegelikku elektrivälja tugevust saab määrata ka optiliselt – kasutades vastavast lahenduse piirkonnast mõõdetud kiirgusspektri intensiivsuse jaotust. Optilise diagnostikameetodi suur eelis on, et mõõtmine ise ei mõjuta elektrivälja lahenduses. Optiline meetod põhineb asjaolul, et molekule ergastavate elektronide energia sõltub otseselt elektrivälja tugevusest antud piirkonnas. Kui uuritavatele spektrijoontele vastavate üleminekute ergastusenergiad üksteisest palju erinevad, siis ongi spektrijoonte intensiivsuste suhe sõltuvuses elektrivälja tugevusest.
Lämmastiku puhul on diagnostikaks sobivaimad lämmastiku molekuli suure intensiivsusega teise positiivse süsteemi 0–0 ja 2–5 üleminekutele vastavad spektrijooned lainepikkustega 337.1 nm ning 394.3 nm, ja lämmastiku molekulaarse iooni esimese negatiivse süsteemi 0–0 üleminekule vastav spektrijoon lainepikkustel 391.4 nm.
Toodud üleminekute intensiivsuste suhte sõltuvust elektrivälja tugevusest saab leida ka teoreetiliselt, samas on senised erinevate autorite tehtud arvutused andnud üksteisest oluliselt lahknevaid tulemusi. Täpse sõltuvuse teoreetiline määramine pole ka kerge ülesanne, sest see sõltub elektronide energia jaotusfunktsioonist, kasutatavate nivoode ergastusristlõigetest ja veel teistest teguritest, mille täpne väärtus pole alati teada. Selleks, et vastavaid spektraalseid mõõtmisi saaks kasutada elektrivälja tugevuse määramisel, on vaja mõõta kiirgusspektri intensiivsuse jaotus tuntud elektriväljade korral.
Käesoleva doktoritöö peamine tööülesanne oligi toodud üleminekute intensiivsuste suhete mõõtmine erinevatel taandatud elektrivälja tugevustel ja erinevatel rõhkudel õhus. Teades eelpool nimetatud sõltuvusi, on võimalik määrata elektrivälja tugevus erinevatel rõhkudel gaaslahendusplasmas ainult lämmastiku molekulaarse iooni ja neutraalse lämmastiku molekuli kiirgusspektrite alusel. Saadud sõltuvusi saab kasutada ka teiste autorite poolt tehtud teoreetiliste arvutuste õigsuse kontrolliks.
Optical emission spectrum of molecular nitrogen is an important tool for investigating electric discharge phenomena in ambient atmosphere, and it is also often used for diagnostics of low-temperature gas discharge plasmas. The study is focused on a development of a spectroscopic method for determination of electric field strength in low-temperature gas discharge plasmas in atmospheric air. Measuring electric field strength by using probes or other contact methods is possible only in limited cases. In particular, the intensity ratio of spectral bands with bandhead wavelengths at 391.4 nm and 337.1 nm (or 394.3 nm), corresponding to FNS 0–0 and SPS 0–0 (or 2–5) transitions, is often used for electric field strength determination in nitrogen-containing gas discharges. The excitation threshold energies for the upper states of these bands differ considerably and, therefore, the ratio of emission intensities is sensitive to comparatively small changes in the mean electron energy. This dependence can be used for electric field strength estimation in low-temperature gas discharges, if the excitation of nitrogen molecules from the ground state by electron impact is the dominant process. Despite a rather wide use of the above-mentioned intensity ratio for estimating electric field strength in gas discharge plasmas, the relation between the intensity ratio, electric field strength and gas number density was estimated by earlier authors only theoretically. Moreover, theoretical intensity ratios of different authors differ considerably. According to our knowledge, no experimental verification of these theoretical curves had yet been made. The aim of the present work was to experimentally determine the relationship between radiation intensity and electric field strength in air discharges for the above-mentioned transitions, and clear up physical processes that are significant for development of this relationship.
Optical emission spectrum of molecular nitrogen is an important tool for investigating electric discharge phenomena in ambient atmosphere, and it is also often used for diagnostics of low-temperature gas discharge plasmas. The study is focused on a development of a spectroscopic method for determination of electric field strength in low-temperature gas discharge plasmas in atmospheric air. Measuring electric field strength by using probes or other contact methods is possible only in limited cases. In particular, the intensity ratio of spectral bands with bandhead wavelengths at 391.4 nm and 337.1 nm (or 394.3 nm), corresponding to FNS 0–0 and SPS 0–0 (or 2–5) transitions, is often used for electric field strength determination in nitrogen-containing gas discharges. The excitation threshold energies for the upper states of these bands differ considerably and, therefore, the ratio of emission intensities is sensitive to comparatively small changes in the mean electron energy. This dependence can be used for electric field strength estimation in low-temperature gas discharges, if the excitation of nitrogen molecules from the ground state by electron impact is the dominant process. Despite a rather wide use of the above-mentioned intensity ratio for estimating electric field strength in gas discharge plasmas, the relation between the intensity ratio, electric field strength and gas number density was estimated by earlier authors only theoretically. Moreover, theoretical intensity ratios of different authors differ considerably. According to our knowledge, no experimental verification of these theoretical curves had yet been made. The aim of the present work was to experimentally determine the relationship between radiation intensity and electric field strength in air discharges for the above-mentioned transitions, and clear up physical processes that are significant for development of this relationship.
Description
Väitekirja elektrooniline versioon ei sisalda publikatsioone.
Keywords
gaaslahendused, elektrivälja tugevus, lämmastikuühendid, kiirgusspektrid, gas discharges, electric field strength, nitrogen compounds, emission spectra