Biomolecules with several flexible sites offer a rich venue for studying single-conformation spectroscopy and the laser-induced conformational isomerization dynamics. By cooling the molecules in a supersonic free jet, the populations are collected into the zero-point levels of a few of the lowest energy minima on the potential energy surface. Double resonance spectroscopies such as UV-UV hole burning and fluorescence-dip infrared spectroscopy (FDIRS) provide the ultraviolet and infrared spectra of single conformations of these molecules free from interference from one another. Building off this spectroscopic foundation, we employ either infrared excitation or stimulated emission pumping (SEP) to prepare excited vibrational levels of single conformations with well-defined amounts of internal energy. By carrying out the excitation early in a supersonic expansion, cooling collisions are used to re-cool the vibrationally excited molecules back to their zero-point levels. Isomerization quantum yields are obtained for a series of flexible tryptophan derivatives following selective excitation of single conformations. Initial results using SEP excitation will be presented. These show great promise for measuring rate-determining barrier heights for isomerization between pairs of minima
Department of Chemistry, Purdue University, West Lafayette (USA)