  We studied the two-level spin resonance in a new perspective. Using the
Floquet theory, the periodic interaction Hamiltonians were transfromed into a
time-independent interaction. Using the Brillouin-Wigner perturbation method, a
degenerated subspace is constructed, where the effective Hamiltonian is given
in a perturbation expansion. In this framework, we found that the upper
triangular element $\langle \alpha | H^1 | \beta \rangle$, determines whether
the resonance happens. The generalized Rabi frequency and the Bloch-Siegert
shift were solved straightforwardly as the first order and the second order
solution, proving the benefit of the developed method.
