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...