It has been pointed out in another communication* that some considerations connected with the investigation of ionisation potentials for the different elements made it necessary to work out their series spectra as far down in the ultra-violet as it was practicable to go. The results of some studies with quartz and fluorite spectrographs have already been published, and in the present communication an account is given of some preliminary observations made with a vacuum-grating spectrograph. This work, as well as that in other directions, has had to be discontinued for the present through the exigencies of the war situation/and is therefore incomplete. The results obtained so far, however, are interesting in that they show that it is quite feasible to investigate, with comparative ease, spectra including wave-lengths as short as λ = 584 Å. U. II. The Vacuum-grating Spectrograph . The vacuum-grating spectrograph was designed by and obtained from the Adam Hilger Company. A sectional plan and sectional elevation of this instrument is shown in fig. 1. Sketches of the grating mounting are shown in fig. 2, and others of the slits, slit taps, auto-collimating eyepiece and film carrying taps are sketched in fig. 3.
The investigation described here is a continuation of an earlier one carried out by two of the authors in which certain lines in the first spark spectrum of indium were studied and the conclusion drawn that the nuclear moment of that atom was probably ½ h /2π. During the time that elapsed between the completion of this work and its publication there appeared a paper by Jackson describing an examination of certain lines in the arc spectrum which led to an I value of unity. Two of the lines λ4511, λ4102 observed by him had appeared on our plates and been measured by us, the structure recorded in the two cases being quite different. In proceeding with the research it was therefore decided first to revise the work on these lines and then to direct our attention to lines in the ultra-violet region of the arc spectrum. The discharge tube used was very similar to Jackson’s. It was made from quartz and consisted of two wide portions 8 cm. in length and 2·5 cm. in diameter joined by a narrower one 4 cm. in length and 0·8 cm. in diameter. The electrodes were made of sheet copper wrapped around the wide parts of the tube and were coupled inductively to an oscillating circuit with a frequency range of 10,000 to 14,000 cycles. Two U. V. 861 General Electric screen grid radio frequency power amplifier tubes operating in parallel were used. A small amount of indium trichloride (Hilger’s H. S. brand) was introduced into the tube, which was then evacuated. It was usually necessary to heat the tube slightly by means of a bunsen to start the discharge, but once this was done no further heating was necessary.
The spectra of the light of the sun reflected from the major planets—Jupiter, Saturn, Uranus and Neptune—were photographed by Slipher in 1909. These spectra showed a general similarity in that there were a number of absorption bands superimposed on the ordinary solar spectrum. The intensity and width of these absorptions varied from planet to planet, increasing in general from Jupiter to Neptune in the order quoted. Of the chemical identity of the bands little is known. Some—C and F, fig. 4, for example—can be attributed to absorption by atomic hydrogen in the atmospheres of the planets; others might be due to water-vapour, though other water-vapour bands do not appear. The outstanding unidentified bands which are common to the spectra of the four planets are (see fig. 4, Plate 10):— ( a ) At λ = 5430 Å.—A rather weak band in the spectra of Jupiter and Saturn, but very strong in those of Uranus and Neptune. ( b ) At λ = 6190 Å.—This is the mid-point of a conspicuous and dense band appearing in the spectra of all the four planets, broadening from a width of some 50 Å in that of Jupiter to some 200 Å in that of Neptune. Although quite strong in the spectrum of Jupiter, it showed no tendency to become resolved in the high dispersion plates taken of the spectrum of this planet. ( c ) A strong double band at λ = 7200 to 7260 Å recorded in the spectra of Saturn and Jupiter, and probably just as strong in those of Uranus and Neptune, but not recorded because of the insensitiveness of the plates in this region.
In a paper by Frank and Hertz in the ‘Physikalische Zeitschrift,’ these investigators have shown that the minimum energy required to ionise an atom of mercury is that acquired by an electron in passing through a fall of potential of 4·9 volts. These writers have also shown in a later communication that when heated mercury vapour is traversed by electrons possessing energy slightly above this amount the vapour is stimulated to the emission of the single spectral line λ = 2536·72 Å. U. This result constitutes a new and most interesting application of the quantum theory, for it will be seen that in the relation V e = hv , where h = 6·6 x 10 -27 erg sec. 4·9 volts is the potential fall which corresponds to the frequency v of the line λ = 2636·72 Å. U. If the relation just pointed out be applicable generally to all the elements it follows that if the vapour of an element can be shown to be capable of exhibiting a single-line spectrum, the frequency of this single spectral line may be used to deduce the minimum amount of energy required to ionise the atoms of that element. With the object of establishing such a generalisation, if possible, some experiments were recently made by the writers, and it has been found that the vapours of cadmium and zinc as well as that of mercury can be stimulated to the emission of single-line spectra when traversed by electrons possessing the requisite amount of energy. With cadmium vapour the wave-length of the line constituting this single-line spectrum is λ = 3260·17 Å. U., while that of the single-line spectrum of zinc vapour is λ = 3075·99 Å. U. By the quantum theory it follows then that the minimum ionising potential for cadmium and zinc vapours are respectively 3·74 volts and 3·96 volts.
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