70 
QUANTUM THEORY 
In a star the ionisation is so intense that few nuclei are able to retain emi 
more than 10 electrons. Our interest is therefore confined to the K and abs 
L structure. With regard to the L electrons it must be recalled that there 
are two kinds of 2 -quantum orbits according as the subsidiary number n' diff 
is 2 or 1 . We accordingly distinguish the 2 2 or circular orbits and the seq 
2 1 or elliptic orbits. The full complement consists of 2 electrons in acti 
elliptic and 6 in circular orbits*. The K orbits are necessarily circular low 
(li orbits). as 1 
To remove, say, a K electron from an atom a certain amount of work the 
W K is needed, and TF^can be deduced from experiment. For if radiation spo: 
of frequency v is passing through the material the quantum hv will have orb: 
sufficient energy to remove the electron provided that hv > W K . If the For 
removal is effected the quantum of radiation is absorbed, any surplus a re 
energy being communicated to the freed electron as kinetic energy. But 
if hv < W K the removal cannot be effected and the radiation passes through sho 
unabsorbed—so far as this absorption-mechanism is concerned. The leve 
critical frequency v K = W K jh is marked in the spectrum by a sharp inte 
absorption edge; so that by spectral measurement v K and W K can be found. in £ 
Except for the lightest elements v K and v L are in the X ray region. The elec 
vacancy left at the K level by such an ionisation may be filled by capture con* 
of a free electron with a corresponding emission of radiation. But in to L 
laboratory conditions free electrons are scarce, and usually before there is anc< 
time for such a capture an electron falls in from an upper level and fills and 
the vacancy. This will usually be succeeded by further falls from higher qua 
levels until the vacancy is at the uppermost level where it remains until is e; 
filled by a capture. If the first fall is from the L level the difference of 
energy of the atom before and after the fall is W K — W L and this must exei 
be radiated as a quantum of frequency (W K — W L )/h. The lines corre- uud 
sponding to the various possible falls constitute the (emission) X ray The 
spectrum of the element. Measurements of these lines constitute additional The 
material for determining the energy-constants W K , W L , etc. f lon 
The apparent dissymmetry between absorption and emission of X rays first 
is a consequence of the artificial production of X rays at low temperature c °ui 
in terrestrial experiments. Inside the stars there is thermodynamical l e - ] 
equilibrium; ionisations and captures, falls and ascents, occur with equal we < 
frequency. There is line absorption as well as line emission, and continuous °fi ai 
mot: 
* [The rule given by Pauli is that every possible orbit must have a different _ 
quantum specification. In addition to the quantum numbers n, n' there are two 
others (imperfectly represented in the Bohr model) of which one has only two com 
possible values (usually called \ and - |) and the other takes integral or zero values its c 
from -(w'-l) to +(«/ - 1). This gives 4n' - 2 combinations. Accordingly the foun 
number of different ( n, n') orbits is now believed to be 4 n' - 2 ihstead of the number 
n' + 1 given by the Bohr model. Evidently the change should be carried through into ^ 
the formulae for ionisation, but it has come too late to adopt in this book.] plat]