Spectrum and the energy levels of neutral argon, Ar i

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Abstract
The spectrum of Ar i has been reobserved with grating spectrographs in large parts of the photographic range. The energy levels have been calculated taking into consideration interferometric and grating observations made since the publication of Atomic Energy Levels (1949) and by use of the dispersion formula for air adopted by the International Astronomical Union. Four ground-level combinations between 1066 and 876 Å have been carefully measured on spectrograms taken with the 10.7-m vacuum spectrograph at the National Bureau of Standards; two in the second and two in the fourth order. They give for the lowest excited level, 3p5 (2P1(1/2))4s[1(1/2)]2 1s5 in Paschen’s notation), the value 93143.76 ± 0.05 cm−1 relative to the ground level zero. By use of this level, the 3p5nl level system as a whole has been accurately located relative to the ground level. From the levels, 34 lines in the vacuum ultraviolet between 1066 and 792 Å have been calculated according to the combination principle. Between 894 and 806.8 Å, these should be accurate to ± 0.0004 Å or better, whereas for the 1066- and 1048-Å lines and several lines below 806.8 Å, the limits of error are somewhat larger. From interferometrically determined values of levels of types n s (n > 5) and n d (n > 3) accurate combinations with those of type 4p in the 10068–4768-Å range have been calculated. The (3p5 2P1(1/2)) ionization energy, referred to the ground state of Ar ii, is 127109.80 ± 0.10 cm−1.

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