ABSTRACT
In a typical time-resolved diffraction experiment, a laser pump pulse creates a time-dependent wave packet on an excited electronic potential energy surface. The probe is either an ultrafast electron bunch or an X-ray pulse, and to resolve structural changes, the electron or X-ray wavelength must be smaller than the length scale of interest. The diffraction pattern is analyzed at different pump–probe delays to determine changes in molecule structure. This chapter considers a non-time-resolved experiment looking at diffraction from aligned molecules. It utilizes a genetic algorithm to construct a measured diffraction for perfectly aligned molecules, based on the measured one for imperfectly aligned molecules. The chapter provides a series of ultrafast electron diffraction measurements following excited-state dynamics in several aromatic molecules: pyridine, picoline, and lutidine. These experiments follow radiationless decay from the excited state and are motivated by the fact that ultrafast electron diffraction can provide information from dark states not normally accessible by spectroscopic means.
