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Ionization Potential of Helium.
J. Franck. Bemerkung über Anregungs- und Ionisierungsspannung des Heliums, Ztschr. f. Ph, XI, 155, 1922.
Despite the large number of experimental and theoretical works devoted to determining the excitation and ionization potential of helium, the question of the arrangement of the lines computed from these data into serial schemes—i.e. the question of the possible orbits of helium—has until now remained controversial. The scheme proposed by Franck and Knipping, given in Table 1, met with objections from Davies, Horton, Kemball, and others.
TABLE 1 ¹)
| Measured | Serial designation | Remarks | Computed wavelength |
|---|---|---|---|
| 20.45 volts | 1S — 2S | Voltage of transformation of one-quantum parahelium into two-quantum metastable orthohelium. | No radiation. |
| 21.25 ” | 1S — 2S | Transition forbidden by the selection principle, consequently corresponding, under normal conditions, to a very weak line. | 585 Å |
| 21.9 ” | 1S — 2P | Absorption series of normal helium. | 569 Å |
| 23.6 ” | 1S—3P | Absorption series of normal helium. | 523 Å |
| 25.3 ” | 1S | Series limit. | 493 Å |
¹) In all serial designations I have used Sommerfeld’s designations, in which the fractional numbers standing before the symbols S are increased, being completed to the nearest whole number (addition of 0.5).
In all the above-mentioned works the methods of electron impacts were used. The recently published work of Lyman (Nature, 26 Aug. 1922), who succeeded in photographing the extreme ultraviolet part of the helium spectrum, puts an end to all these disputes. Lyman was able to overcome the difficulty connected with the absorption of the photographed lines in the atmosphere of the gas itself by dividing his apparatus with a partition having a very narrow slit and, with the aid of a continuously operating powerful pump, maintaining a pressure difference in both halves of the apparatus, so that one part of it served as the discharge tube and the other as the vacuum spectrograph. Lyman obtained 5 lines in the extreme ultraviolet and arranged them in a scheme according to Table II.
As comparison with Table I shows, this scheme can be brought into complete agreement with the scheme of Franck and Knipping if a systematic correction of \(-0.7\) volts is introduced into the data of these observers. This correction is explained by the existence of two modifications of helium (orthohelium, to which in serial schemes the lowercase letters \(s, p, d\ldots\) correspond, and parhelium, with the designations \(S, P\ldots\)), the difference between the two quantum orbits \(2S — 2s\) being equal to \(0.8\) volts and introducing a systematic error into all the values of the potential noted by F. and K. (the accuracy of their measurements is \(0.25\) v.).
TABLE II¹).
| Lines observed by Lyman | Series designations | Calculated excitation voltage | Excitation voltage measured by Franck and Knipping, reduced by 0.7 v. | Remarks |
|---|---|---|---|---|
| — | \(1S — 2S\) | 19.77 volts | 19.75 | Transformation voltage, to which no radiation corresponds. |
| 600.5 A° | \(1S — 2S\) | 20.55 ” | 20.55 ” | Forbidden by the selection principle and therefore a very weak line. |
| 584.4 A° | \(1S — 2P\) | 21.12 ” | 21.2 ” | First member of the absorption series, and therefore a very bright line. |
| 537.1 A° | \(1S — 3P\) | 22.97 ” | 22.3 ” | — |
| 522.3 A° | \(1S — 4P\) | 23.62 ” | — | — |
| 515.7 A° | \(1S — 5P\) | 23.92 ” | — | — |
| (502. A°) calculated. | \(1S\) | 24.5 ” | 24.6 | Ionization voltage. |
Thus, it should be regarded as finally established that the value of the ionization potential for helium is \(24.6\) volts; this is the value that should be used for correcting all results obtained by the electron-impact method, where mixtures of the gas under investigation with He as the standard gas were studied.
Tr. Landsberg.