OPTICAL SPECTRA EXCITED IN HIGH PRESSURE NOBLE GASES BY ALPHA IMPACT

Abstract The atomic spectra observed in the pure noble gases are interpreted through the dominant processes of direct excitation by alpha and electron impact, trapping of vacuum ultraviolet resonance radiation, and excitation transfer effects resulting from two-body collisions with ground state atoms. The pressure dependence of helium line intensities may be explained with effective depletion cross sections that are in the order of 10−15 cm2. The absolute intensities emitted from helium are in approximate agreement with cross sections calculated from the Born approximation if allowance is made for these collision effects. The results indicate that the light emitted from atomic transitions in all of the noble gases in the range between 2400 and 5500 A represents a negligible fraction of the total excitation. No evidence was found for a recombination spectrum in any of the gases. The weak lines observed in pure helium and neon were identified as atomic transitions which are normally present in the discharge spectrum at low pressures. Two weak, unidentified lines of equal intensity were found in pure argon at 3083 ± 3 A and 3094 ± 3 A. In pure krypton and xenon, broad diatomic molecular bands were found to dominate the spectrum. The integrated photon counting rates for pure xenon, krypton, argon, neon, and helium were in the ratio 240:90:3:3:1 at 350 mm Hg. Nitrogen impurity bands dominated the spectra in most helium, neon, and argon samples used, and the sensitivity to nitrogen was in that order. First negative bands of N2+ were encountered in helium and neon, and the second positive bands of N2 were found in argon. It was concluded that the following nitrogen excitation mechanisms were dominant: charge exchange with the molecular ion, He2+, in the case of helium; collision with a high excited state, probably the 43P2 or 43P1, in the case of neon; collision with the 3P2 metastable level in the case of argon. The 0,0 N2+ band dominated the helium spectrum for less than one part N2 in 108 at 350 mm Hg. The results in NeN2 mixtures place an upper limit on the available energy from thermalized Ne2+ ions of 18.75 ev. The relative C3πu populations observed in the AN2 mixtures were approximately those which would be expected if the Franck-Condon principle were obeyed in the collision excitation from the nitrogen ground state. Only the second positive bands were observed in high pressure reagent grade nitrogen and the intensities were weaker than those found in the AN2 mixtures. Comparisons are given between calculated and measured relative transition probabilities for most of the nitrogen bands observed. The relation between this work, average energy per ion pair measurements, and gas scintillation counters is discussed.