This book is devoted to the investigation of a rather prevalent process
in nature: interaction of atoms with electromagnetic radiation. Primary
attention is given to the low and intermediate photon energy region,
from tens to hundreds of electron-volts. It is in this region that the
probability of photon absorption and photoionization is largest. Data in
this energy region are very interesting and useful in astrophysics and
plasma physics, solid-state physics and quantum electronics, and in a
number of other branches of science and technical applications. Formulae
for hydrogen atom photoionization are given in almost all textbooks on
quantum mechanics. Together with the limited amounts of experimental
data available up to the beginning of the sixties, the formulae gave an
impression of the completeness of the study of photoionization, of the
absolute clarity of the mechanism of the process, and of the possibility
of calculating rather easily its probability using the formulae.