DEOAY OF ATOMIO POLARIZATION MOMENTS II.I.Dyakonov, V.I.Perel A.F.Ioffe Physico-Technical Institute of the USSR Academy of Sciences, Leningrad, USSR The state of a free atom is described by the velocity of its motion, its internal energy and angular momentum. All these quantities may change in the act of a collision. For a long time the object of the collision theory was almost en- tirely confined to studying the changes in the velocity and energy of an atom. However, in connection with the ideas and methods of optical pumping, radiofrequency and laser spectro- scopy a new field of the collision theory is intensely deve- loping. It is concerned with p~ocesses in which the angular momentum of an atom changes. Description of Relaxation in Terms of Polarization Moments The main question was, what precisely are the characteris- tics of a collision which determine the changes in the polari- zation properties of an ensemble of atoms. Adequate means of description of these properties are provided by the density matrix f'mm'. In the simplest case, when atoms on a single energy level with the value of the total angular momentum j are studied, the subscripts m and m' correspond to different projections of the angular momentum on the quantization axis. In each collision the density matrix changes by the value of 6f'mm' which depends on the parameters of the collision and is linearly relaved to the density matrix before the col- lision. Accordingly, the variation of the density matrix in time due to collis~ons may be described by the equation -I ,j' I 2.- r m1 "'. 0 ( ) ~ - - jot ",' t,.. /It' 1 ".6 - f •• ~(",/ The right-hand side of Eq. (1) represents obviously the quantity Af'mm' summed over all collisions occurring during a-unit of time. Eq. (1) serves as a starting point for most of the work in the field considered, although, strictly speaking, it is not correct, since it does not take into