  One of the main goals of chemical dynamics is the creation of molecular beams
composed of a single (vibrational, rotational, and magnetic) quantum state of
choice. In this paper we show that it is possible to achieve {\it complete}
quantum state selectivity by producing resistance to electromagnetically
induced deflection (EID) and that the state to be selected can be "dialed in"
at will. We illustrate the method by showing in detail how to purify thermal
beams of the LiRb and IF molecules to yield molecular beams composed of a
variety of pre-chosen single internal quantum states and/or superpositions of
such states. We expect that this method will be implemented in all subsequent
explorations of the fundamentals of chemical reactions and their control, and
the use of cold molecules as a vehicle for studying some of the most profound
issues of quantum dynamics.
