TABLE OF CONTENTS Lasing on the Nd ions was produced in a very wide range of active media (crystals and glass) where the majority of these media operates in the four-level system. The absorption and luminescence spectra of the neodymium ions Nd 3+ are determined by three electrons of the 4 f-shell which is sufficiently screened by 5 s and 5 p-shells. The energy of interaction with 4 f-electrons is only 100 cm -1. This energy is lower in comparison with the energy of spin-orbital interaction. The weak crystalline field does not fracture the bond between the orbital and spin moments of the electrons and the Stark splitting of the energy levels of the ions is smaller in comparison with the splitting of its fine structure. The total angular moment J is retained; (2 J + 1) of energy sublevels correspond to different projections of the moment on the direction of the electric field.
The position of the energy levels of the Nd ions is determined by the Coulomb interaction of the electrons with the nucleus of with each other (Fig. 2.1). As a result of the spin-orbital interaction of the electrons, the energy levels of the ions are split into sublevels (multiplets) with a spectral range ~10 3 cm -1, having the same moments L and S, but differing in the mutual orientation of the moments, i.e. in the total moment
