During the last few years, the Floquet matrix approach to the theory of intense-field multiphoton dissociation processes has undergone a revival of interest which is due to the emergence of new powerful algorithms within the complex quasivibrational energy formalism. The method presented here shows that the Bloch wave operator theory is, in the framework of this theory, an efficient tool to select the active space which participates principally in the dissociation process. An illustrative numerical example reveals that the one dimensional active space model, used mainly to reproduce cw laser experimental results, fails when the laser is tuned to bound state-bound state transitions.