COMMUTATION RULES AND EIGENVALUES OF SPIN AND ORBITAL ANGULAR-MOMENTUM OF RADIATION-FIELDS

We investigate the separation of the total angular momentum J of the e lectromagnetic field into a 'spin' part S and an 'orbital' part L. We show that both 'spin' and 'orbital' angular momentum are observables. However, the transversality of the radiation field affects the commuta tion relations for the associated quantum operators. This implies that neither S nor L are angular momentum operators. Moreover their eigenv alues are not discrete. We construct field modes such that each mode e xcitation (photon) is in a simultaneous eigenstate of S(z) and L(z). W e consider the interaction of such a photon with an atom and the resul ting effect on the internal and external part of the atomic angular mo mentum.