Spectrochemical analysis of trace contaminants in helium (helium–fluorine) pulsed discharge plasmas

Abstract

Time-resolved spontaneous emission and absorption spectra in the 115–860 nm spectral range were studied for high pressure He and He–F₂ pulsed discharge plasmas, the aim being the spectroscopic identification and on-line monitoring of transient and stable gaseous contaminants accumulated in an actual F₂ laser gas mixture. Various radicals (CN, CH, C₂, OH, SiF and CF₂), excited atoms (O, C, Si, N and H), molecules (CO, O₂ and N₂) and molecular ions (N₂⁺ and CO⁺) were identified as contaminants. The accumulation and self-transformation kinetics due to plasmo-chemical reactions between these species were investigated. It is suggested that carbon, oxygen and carbon monoxide species are desorbed as primary impurities from the metal electrodes during the sputtering of the electrodes from cathode (anode) hot spots. Secondary impurities such as transient radicals and more complex compounds are formed by plasmo-chemical reactions between primary and other trace impurities (H₂, N₂ and O₂) and fluorine. These reactions might be strongly catalytically activated on the surface of pure nickel particles, sputtered from the electrodes. The sensitivity of the spontaneous emission methods was estimated to be better than 1 ppm for the detection of stable CO and N₂ impurities in the He discharge plasma. Under static conditions, N₂ and O₂ impurities accumulate at a rate of about 0.1 mbar d^–1 in He and He–F₂ gas mixtures due to continuous outgassing of the laser chamber materials. Under F₂ laser running conditions, a marked increase in the rate of accumulation of O₂ contaminants (≈ 0.1 mbar per 5 × 10⁵ shots) was observed. Oxygen is the main contaminant responsible for F₂ laser output energy degradation because of the intracavity absorption at 157.6 nm in the oxygen dissociation absorption band. The strong absorption in the far VUV range (130–115 nm) observed in an aged He–F₂ gas mixture was attributed to stable fluorine-containing compounds (tentatively identified as COF₂ or HF molecules). In addition to the well known molecular fluorine emission band at 157 nm caused by the transition from the ionically bonded D′³Π_2g state, two spontaneous emission bands (structural at 255 nm and continuous at 280 nm) were observed in the discharge of an He–F₂ gas mixture. These new bands were attributed to transitions between covalently bonded excited electronic states of the fluorine molecule.