ABSTRACT
The quantum mechanical treatment of atom describes a single atom. For the atom in one quantum mechanical state, the observed value is an expectation value which is an average over a large number of measurements of the same physical variable in the same state. The physical explanation of the appearance of Bennett hole is the lower population difference caused by the transition to the upper state of a particular velocity group of atoms that resonate with the radiation frequency. It may be noted that the non-diagonal matrix elements of density matrix play an important role in the transition. The vector model of the density matrix equations illustrates the atomic transitions in the two level atoms in the presence of a near resonant electromagnetic field in a very elegant manner. The description of atom field interaction by a vector model is only analogous to the case of spin-1/2 system in nuclear magnetic resonance.
