Quantum interference effects in degenerate systems. Spontaneous and   stimulated radiation
  We study the effect of quantum interference on the structure and properties of spontaneous and stimulated transitions in a degenerate V-type three-level atom with an arbitrary total momentum of each state. Explicit expressions for the factors in the terms of the relaxation operator and stimulated transition operator with account of quantum interference effects are obtained. It has been demonstrated that the condition for the dipole transition moments to be parallel is insufficient and for conventional atoms the interference cross terms are zeros for both operators. The conditions when quantum interference influences the properties of the relaxation operators have been analysed. For the stimulated transitions this condition is shown to be anisotropy of the photon field interacting with the atoms. This case is minutely studied for the D-line of alkali metals. 
A. A. Panteleev, Vl. K. Roerich