The arc spectrum of silicon has been reinvestigated from 1976 to 12 270 Å by using plane and concave grating spectrographs and Fabry–Perot interferometers. Electrodeless discharge lamps containing silicon–halogen compounds, and a silicon arc operated in argon at reduced pressure, were used to obtain measurements on 395 wavelengths, 192 of them interferometric. The total number of newly observed lines is 179, and the number of new classifications 174. Forty-two new even levels were established, principally from the 3s2 3p np (n = 6, 7) and 3s2 3p nf (n = 5, 6, 7) configurations. With the aid of vacuum-ultraviolet data provided by A. G. Shenstone, 38 new odd levels were found. The interferometric and grating investigation has led to the calculation of 92 provisional Ritz standards between 1580 and 2000 Å with uncertainties of less than 0.002 Å. The new wavelength data have also made possible the identification of 76 previously unidentified solar lines as due to Si i. Nineteen other solar lines have been identified as possibly attributable to Si i. A limit of 65 747.5±0.6 cm−1 has been derived for the Si ii level from the nd and nf level series. This corresponds to an ionization potential of 8.15116±0.00008 V.
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