The rotational spectrum of a highly vibrationally mixed quantum state. II.The eigenstate-resolved spectroscopy analog to dynamic nuclear magnetic resonance spectroscopy

The description of the rotational spectrum of a single molecular eigenstate in an energy region where conformational isomerization can occur is presen ted. The conformational isomerization rate can be determined from an analys is of the overall line shape of the spectrum. The isomerization dynamics ar e investigated through a time-domain analysis. It is shown that the nonstat ionary state produced by coherent microwave excitation has a well-defined c onformational structure. However, the individual molecular eigenstates are a mixture of rovibrational states associated with the two conformations. Th e structural localization decays to the eigenstate equilibrium value at a r ate defined as the isomerization rate. It is also shown that the line shape of the spectrum coalesces as the isomerization rate exceeds the difference between the characteristic rotational frequencies of the two conformers. T he line shape profile of the eigenstate-resolved spectrum is well described by the Bloch equations modified for chemical exchange. (C) 1999 American I nstitute of Physics. [S0021-9606(99)00204-4].