264 
place, it is said to be, before the change occurs, in a 
state of pre-dissociation [12]; a similar definition applies 
to pre-tonization. 
Symbols are assigned to the various states of parti- 
cles; the details of the electronic configuration may be 
specified; for example, by (1s)?(2s)?(2p)* for the lowest 
levels of the O atom [16, p. 45], but these symbols will 
not be explained here. For this configuration there are 
three main energy levels associated with the term sym- 
bols. 3P (ground term) and Dp, 1So, the two latter being 
metastable terms (§ 12). The ground term itself has three 
closely spaced levels, *Ps, *P1, and *Po, the first being 
the lowest. Often the suffixes in these term symbols are 
omitted. 
For atomic sodium the ground state has the symbol 
28, and the first excited state has two closely spaced 
levels 2P? and 2P;:. The symbols for the states of mole- 
cules are, as might be expected, more complicated than 
those for atoms. For example, the ground state and 
the next succeeding energy states for neutral molecular 
nitrogen have the symbols X!2,, A*2u, and Bll, [12]. 
5. Energy Units. The cgs unit of energy is the erg, 
and a larger unit is the joule (1 joule = 10’ ergs). 
Another unit much used by chemists is the calorie (or 
gram-calorie—the terms have identical meanings), 
which is the energy required to raise the temperature of 
1 ce of water at 15C by 1C (1 calorie = 4.185 joules). 
Chemists often quote energies in kilocalories (or large 
calories); 1 kilocalorie = 1000 calories. 
Physicists often use another unit called the electron 
volt (ev). An energy of V ev is equal to that acquired 
by an electron in traversing a fall of potential of V 
volts; this is eV joules, if e (the electronic charge) is 
measured in coulombs: 
4.802 X 10 electrostatic cgs units 
1.602 X 10 electromagnetic units 
1.602 X 10-* coulombs. 
Hence, as a volt is 10° electromagnetic units, 
1 electron volt = 1.602 X 10-¥ ergs. 
Hence also for a change of energy of V ev per particle, 
the change expressed in gram-calories per mole (V 
particles) is 1.602 * 10-" NV/4.185 X 10’, so that 
1 ev per particle is equivalent to 23,053 calories per 
mole. 
Yet another form of energy-reckoning by physicists, 
the wave number, is explained in the next section in 
connection with the quantum relation. 
6. Light Absorption and Emission; the Quantum 
Relation. An atomic or molecular particle may gain 
energy of amount W ergs by absorption of light of fre- 
quency »v (the frequency is the number of light-vibra- 
tions per second), or it may emit the energy W as light 
of this frequency; in either case W and » are connected 
by the quantum relation 
W = bh, 
where h denotes Planck’s constant: 
h = 6.624 X 10-*’ erg sec, 
according to the latest estimate by R. T. Birge [5]. 
€ 
e= 
€ 
THE UPPER ATMOSPHERE 
This relation can also be expressed in other forms, in 
terms of the wave length X of the light, or in terms of its 
wave number n, which is the number of waves per 
centimetre: so that if \ is expressed in centimetres 
(denoted by \em), 
m = 1/em ; 
vy and Nem are connected by the relation 
Nem’ = 6, 
where c denotes the speed of light, which zn vacuo is 
= 2.998 X 10! cm sec. 
Hence the relation W = hy can be expressed aS \anW = 
he or W = hen; energies and energy differences can 
therefore be expressed in terms of wave numbers, where 
1.986 X 10 erg. 
Hence an energy expressed as V ev has a wave number 
n given by 
n = 8068V, or V = 1.2395 X 10-4n. 
Wave lengths are generally expressed in angstrom 
units (A) or im microns (x): 
1A = 10cm, 1 » = 10*cm = 104A. 
Thus A expressed in A is 10° Aan. The relation AW = 
he, when W is expressed in electron volts V and \ in 
angstrom units, takes the form 
AV = 12395 
1 wave number = hc ergs = 
(A in A). 
Similarly if W is expressed in calories per mole, 
AW = 2.858 X 10° (Aim A). 
7. Spectra Associated with Atomic and Molecular 
Transitions. When an atomic or molecular particle 
undergoes a transition from a state of higher energy to 
one of lower energy, one way in which it can dispose of 
the energy difference W thus released is by radiation 
in light of frequency » = W/h. The spectrum of the 
light emitted by a gas whose particles are undergoing 
one or more such transitions is called an emission 
spectrum. 
If, however, the particles are undergoing transitions 
from lower to higher energy levels, one way in which 
the necessary energy W may be acquired is by absorp- 
tion of light, which must be of frequency W/h. The 
spectrum of the beam of light will be darkened at this 
frequency, because of the absorption and the conse- 
quent reduction of the transmitted intensity. This dark- 
ening, at as many frequencies as are concerned in the 
transitions taking place in the gas, gives what is called 
an absorption spectrum. 
The spectrum of sunlight shows absorption of two 
kinds: one (the Fraunhofer spectrum) due to absorption 
by gases of the sun’s own atmosphere, and another due 
to absorption in the terrestrial atmosphere (§18). 
From our knowledge of spectra for various sub- 
stances, either studied experimentally in the labora- 
tory, or in simple cases obtained from theoretical calcu- 
lation of the energy levels of particles, it is often (though 
as yet not always) possible to infer the nature of the 
