The Further Extension Of The Ultra-violet Spectrum And The Progression With Atomic Number Of The Spectra Of Light Elements

VOL,. 7, 1921 PHYSICS: R. A. MILLIKA N 289 THE FURTHER EXTENSION OF THE ULTRA-VIOLET SPECTRUM AND THE PROGRESSION WITH ATOMIC NUMBER OF THE SPECTRA OF LIGHT ELEMENTS By R. A. MILLIKAN RYERSON PHYsicAL LABORATORY, UNIVIRSITY OF CHICAGO Read before the Academy April 26, 1920 The chief purpose of this investigation, outlined and begun in 19161 and briefly reported upon last year,2 has been to explore in the extreme ultra-violet the radiations which can be emitted by the second ring or shell of electrons in the atoms of atomic number from 2 to 13 (helium to aluminium).3 The results obtained to date may be very briefly summarized as follows :3 1. The ultra-violet spectrum has been photographed and its wavelengths determined down to X = 136.6 Angstroms in the case of aluminium and down to X = 149.5 Angstroms in the case of copper. There is thus a gap represented by a factor of but 10 between the shortest measured ultra-violet waves and the longest X-rays measured by the method of crystal-spectrometry which stops at 13.3 Angstroms. Fortunately, however, in the ultra-violet region, which has been already opened up and explored, the most interesting and the most important of the hitherto inaccessible radiations are found -thus 2. The La lines-of Al, Mg, and Na have been photographed and located at 144.3 A, 232.2 A, and 372.2 A, respectively. These wave-lengths are. all fairly accurately on the Mosely line connecting La frequencies andP atomic number (see fig. 1). It has thus been definitely proved that the L. series continues with its main characteristics unchanged throughout the whole range of atomic numbers from Ur (92) to Ne (io). The linear progression thus revealed clear down to neon could be very roughly inferred from the beautiful measurements of Hjalmar,4 which gave the Ka and K, lines of the elements down to sodium. For the Kossel relation between the frequencies of the K and L series, namely, K,8 - Ka = La, although known to be quite inexact, was presumably sufficiently near to the truth to enable the order of magnitude of the La wave-lengths to be predicted. Kossel has already made this use of Hjalmar's data.5 His values of La computed for Al, Mg and Na in this way are actually about 20% too low. 3. It has been found that the aluminium atom (atomic number I3) when excited by these condensed sparks in vacuo emits no radiations whatever of wave-length between I44.3 A and about 1200 A where its M spectrum, that due to its three outer electrons, begins and extends with considerable complexity into the visible. This shows that optical spectra are quite like 290 PHYSICS: R. A. MILLIKAN PROC. N. A. S. X-ray spectra in that large gaps occur between the frequencies due to the electrons in successive rings or shells. If we could neglect the influence of the negative electrons upon one another we could compute the relative diameters of these shells, for they would be inversely proportional to the limiting frequencies, i.e., in this case in about the ratio 1 to 9; but such computations are of little value save as the roughest sort of indices. The chief lines below 2000 A, due to the three outer electrons of the Al atom have the wave-lengths 1379.7 A, 1384.5 A, 1605.9 A, 1612.0 A, 1671.0 A, 1854.7 A and 1862.7 A. Magnesium shows a behavior quite like that of aluminium in that we find a complete blank between its La line at 232.2 A and the lines due to its two outer electrons whose radiations begin on our plates on the short wave-length side at about 1700 A. The chief radiations below 2000 A, arising from these two outer electrons of the magnesium atom, have the wave-lengths 1735.2 A, 1737.9 A, 1751.0 A, and 1753. 7 A. Also, sodium, quite consistently with the foregoing, is found to emit no radiations whatever between its L lines, the longest of which is at 376.5 A and the lines due to its singl