The use of second-order vibronic Herzberg-Teller approach including static and vibronic spin-orbit coupling in the dynamic part of the adiabatic electronic wavefunctions as basis set, makes possible the derivation of a uniform model for theoretical studies of intra- molecular radiative and non-radiative processes of large molecules as well. The model provides the vibronic transition moments in a consistent manner and in form of generalized terms taking the spin classification into account. As an essential feature second-order perturbation theory is included.  In this paper exclusively the vibronic transition moments for radiative spin-allowed and spin-forbidden processes are presented. The analogous moments of non-radiative processes will be published separately (Part II).

First numerical results concerning the effect of first-order and second-order vibronic Herzberg-Teller approximations on the intensity distribution of the S_i->S_o-fluorescence of phenanthrene are discussed.