Puppis) to ionized helium was the first established example of the displacement rule, and constituted one of the earliest triumphs of the Bohr theory.[11] The detection and resolution of the alternate components of that series, which fall very near to the Balmer lines of hydrogen in the spectra of the hottest stars, and the consequent derivation of the Rydberg constant for helium,[12] represents an astrophysical contribution to pure physics which is of the highest importance.

IONIZATION AND EXCITATION

The ionization potential of an atom is the energy in volts that is required in order to remove the outermost electron to infinity. The excitation potential corresponding to any particular spectral series is the energy in volts that must be imparted to the atom in the normal state in order that there may be an electron in a suitable electron orbit for the absorption or emission of that series. Several different excitation potentials are usually associated with one atom. The ionization potential and the excitation potentials are collectively termed the critical potentials.

From the astrophysical point of view, ionization and excitation potentials are important as forming the basic data for the Saha theory of thermal ionization, with which the greater part of this work is concerned. A list of the ionization potentials hitherto determined is therefore reproduced in the following table. The first two columns contain the values obtained by the physical and spectroscopic methods, respectively. The third column contains “astrophysical estimates,” which are inserted here to make the table more complete. The derivation of the astrophysical values will be discussed[13] in [Chapter XI]. Physical values result from the direct application of electrical potentials to the element in question, and spectroscopic values are derived from the values of the optical terms. (See [Appendix].)

[TABLE III]

Atomic
Number 		Element Ionization potential Reference
PhysicalSpectroscopicAstrophysical
1 H14.4, 13.313.54 [1], [2], [3]
2 He25.424.47 [5], [4]
He+54.354.18 [3], [5]
3 Li 5.37 [3]
Li+40 [6]
4 Be 9.6 [7]
5 B 8.3 [7]
B+ 19.0 [7]
6 C+ 24.3 [8]
C++ 45 [9], [12]
7 N16.9 [10]
N+24 [9], [12]
N++45 [9], [12]
8 O15.513.56 [3], [11]
O+32 [9], [12]
O++5045 [12]
10 Ne16.7 [13]
11 Na5.135.12 [14], [3]
Na+30-35 [15]
12 Mg7.757.61 [16], [3]
Mg+ 14.97 [3]
13 Al 5.96 [3]
Al+ 18.18 [17]
Al++ 28.32 [17]
14 Si 10.68.5 [18], [19]
Si+ 16.27 [18]
Si++ 31.66 [18]
Si+++ 44.958.5 [18]
15 P13.3, 10.3 [20], [21]
P++ 29.8 [7]
P+++ 45.3 [7]
16 S12.210.31 [20], [12]
S+ 20 [9], [12]
S++ 32 [9], [12]
S+++ 46.8 [7]
17 Cl8.2 [13]
18 A15.1 [22]
A+33, 34, 41.5 [23], [22], [24]
19 K4.14.32 [14], [3]
K+20-23 [14]
20 Ca6.09 [3]
Ca+11.82 [3]
21 Sc6-9 [25]
Sc+12.5 [19]
22 Ti6.5 [26]
Ti+12.5 [19]
23 V6-9 [25]
24 Cr6.7 [3]
25 Mn7.41 [35]
26 Fe5.9, 8.157.5 [28], [29], [19]
Fe13 [19]
27 Co6-9 [25]
28 Ni6-9 [25]
29 Cu7.69 [3]
30 Zn9.35 [3]
Zn+19.59 [7]
31 Ga5.97 [3]
33 As11.5 [30]
34 Se12-13, 11.7 [31], [32]
35 Br1.00 [13]
36 Kr14.5 [33]
37 Rb4.14.16 [34], [3]
38 Sr5.67 [3]
Sr+10.98 [3]
42 Mo7.1, 7.35 [35], [36]
47 Ag7.54 [3]
48 Cd8.95 [3]
Cd+18.48 [7]
49 In5.75 [37]
51 Sb8.5 ± 1.0 [26]
53 I10.1, 8.0 [38], [39]
56 Ba5.19 [3]
Ba+9.96 [3]
80 Hg10.4 [40]
81 Tl6.94 [41]
82 Pb7.937.38 [42]
83 Bi8.0 [30]
Bi+14.0 [30]

[1] Horton and Davies, Proc. Roy. Soc., 97A, 1, 1920.

[2] Mohler and Foote, J. Op. Soc. Am., 4, 49, 1920.

[3] A. Fowler, Report on Series in Line Spectra, 1922.