The dynamics of very high molecular Rydberg states, with special attention to the extreme long time behavior, is discussed using a quantum mechanical multichannel scattering theoretic formulation. Detailed computational results are provided for an electron revolving about a dipolar core in the presence of an external de field. Two distinct effects are highlighted, trapping and dilution where the former is an origin of long time stability when there are very many bound states but much fewer states that are directly coupled to the continuum. Both trapping and dilution act to elongate the intermediate time decay of the high Rydberg states. The extent of dilution can be varied by changing the magnitude of the external de electrical field. The formalism and specific results are discussed also towards the implications to other types of unimolecular processes. In particular it is argued that the study of molecular Rydberg states does suggest a possible route to mode selective chemistry.