PHOTOCHEMICAL PROCESSES IN THE UPPER ATMOSPHERE 265 
emitting or absorbing particles responsible for given 
emission or absorption spectra, and also, from the 
intensity of the spectrum, to infer how many transitions 
of any such kind are taking place, per unit cross section 
of the beam of light per second. (See §§18, 19.) 
8. Line, Band, and Continuous Spectra. Atomic par- 
ticles have definite configurations and energy levels, 
and definite energy differences and associated frequen- 
cies, corresponding to definite nes in their emission or 
absorption spectrum. Their spectra are therefore called 
line spectra. 
Molecular particles have far more energy levels and 
energy differences and associated frequencies than 
atomic particles have, because of their additional energy 
of vibration and/or rotation. Any “‘line” in their spec- 
trum, associated with a change purely of electron con- 
figuration, may be accompanied by many lines of rather 
different frequencies, corresponding to the same elec- 
tronic change accompanied by any one of many possible 
changes of vibratory or rotational energy. In spectra 
of moderate dispersion some of these lines may be so 
crowded together as to appear like a continuous band; 
hence the name band spectra for molecular spectra. 
A definite minimum energy (see § 4) is required to 
ionize or dissociate a particle. Corresponding minimum 
frequencies are associated with these processes when 
they are induced by light absorption; but light of any 
greater frequency may also induce the process, the 
excess energy going into the kinetic energy of separa- 
tion of the resulting two particles. Hence the absorption 
spectrum can be continuous on the high frequency (or 
short-wave) side of the frequency corresponding to the 
ionization or dissociation potential. Similarly for the 
emission spectrum resulting from the recombination (with 
radiation) of ions and electrons or combining particles; 
for example, in the combinations 
O- + 0+ + 02, or O + O2 > Os, 
the energy released may exceed the ionization or disso- 
ciation energy, because of the kinetic energy of ap- 
proach of the combining particles. Such continuous 
spectra are an indication of the occurrence of ionization 
or dissociation, or of their converse, recombination. 
Certain bands in molecular spectra indicate pre-disso- 
Ciation or pre-ionization (§4) leading to dissociation or 
ionization with certain probabilities. 
9. Atomic and Molecular Spectra: Different Ranges 
of Wave Length. The range of the visible spectrum 
extends from about 4000 A (violet) to 7600 A (red); for 
dX < 4000 A the spectrum is called ultraviolet, and for 
\ > 7600 A, infrared. The energy differences between 
the lower electronic levels of atoms and molecules are 
often as much as 1 to 10 ev, corresponding to light of 
wave lengths from 12395 A to 1239 A, extending from 
the infrared to the ultraviolet. The energy differences 
between different vibrational states of a molecule in the 
same electronic configuration are of order 0.1 ev to 
1 ev, corresponding to wave lengths from 12395 A to 
123950 A (or from 1.2 » to 12 w) in the “near” infrared. 
Purely rotational transitions involve energy differences 
about one-tenth as great, corresponding to wave lengths 
from 12 u to 120 uw in the “far” infrared. Light of a single 
wave length is called monochromatic. 
Numerous bands are named after their discoverers or 
interpreters, working either in the laboratory or with 
natural light in emission or absorption. Examples are 
the Hartley and Huggins bands of ozone, extending re- 
spectively from about 2000 to 3200 A and from 3200 to 
3000 A, and the Chappuis band in the visible region 
(maximum absorption, in air, at about 6100 A); ozone 
also has infrared bands. Oxygen (Q2) has Schumann- 
Runge and Herzberg bands with ranges from about 1750 
to 1930 A and 3100 to 3800 A, respectively, and others 
im the near infrared. Nitrogen (N») has the “first posi- 
tive” system (about 6000 to 6500 A) in the red, as well 
as the Vegard-Kaplan system (3100 to 4500 A), and 
ionized nitrogen (V2) has the “first negative” system 
(3800 to 4700 A). Hydroxyl (OH) has strong infrared 
(Meimel) bands (A > 7200 A). The ranges of wave length 
here specified are somewhat rough and depend upon 
conditions which may be different in the laboratory 
from those in “natural” emission or absorption in the 
atmosphere. 
10. Absorption Coefficients. When light of frequency 
v passes through a gas which contains particles that can 
absorb it, it is weakened proportionately to its own 
intensity and to the number 7 of the absorbing particles 
per cubic centimetre. This is expressed by 
dl = —k,nIdl, 
where dJ is the reduction of the intensity J in travers- 
ing a path length dl; hence /, has the same dimensions 
as 1/ndl, which is length squared or area. The factor 
k, is called the atomic (or molecular) absorption coeffi- 
cient, or alternatively the absorption cross section. In 
the case of some processes in which light is absorbed 
k, can be calculated theoretically; for example [80], for 
the ionization of neutral atomic oxygen by radiation 
whose quanta have energy between 13.55 and 16.86 ev, 
k, is of the order 3 X 10—8 em? . In other cases ky must 
be determined experimentally by observing the de- 
crease of intensity in passing through a known amount 
of the gas. 
If m = mo, where m is the number of molecules per 
cubic centimetre of gas at normal temperature and 
pressure, and a = k,n, the absorption in a gas of uni- 
form density with this value of n is given by dI = 
—aldl, where a is constant. This leads to the relation 
T=, emo = J) 10-04848e0 
Here a is called the absorption coefficient; 0.4343a is 
sometimes called the decimal absorption coefficient and 
denoted by ai, the suffix bearing reference to the 
replacement of e by 10 in the formula above. For ozone 
at about \ = 2500 A, aio is of order 100, and it has 
the same order of magnitude for molecular oxygen at 
about 1500 A. 
The value of k, or a depends greatly on the wave 
length. For example, for ozone, aj sinks to 10~* for 
somewhat greater than 3000 A. 
11. Monochromatic Absorption in an Exponential At- 
mosphere. The term exponential atmosphere is used to 
