Ultrafast reaction dynamics of polyatomic molecules probed by ionization

This work focuses on the ultrafast reaction dynamics of medium-sized organic molecules involving conical intersections and the investigation of the ionization mechanism of such molecules using intense femtosecond-laser pulses. In particular, the dynamics of the photoinitiated unimolecular electrocyclic ring-opening reactions of 1,3,5-cyclooctatriene and 7-dehydrocholesterol are examined in a fs pump-probe experiment in the gas phase using photoionization in combination with time-of-flight mass spectrometry for detection. Particular care is taken in the experiment to establish well-defined conditions and, especially, to shed light on the probe step. This is exemplified by the similar-sized molecule toluene, exploring several fundamental aspects of its ionization and fragmentation behavior. Both 1,3,5-cyclooctatriene and 7-dehydrocholesterol are excited by one 266-nm photon from the electronic ground state (1A_1) into the spectroscopically bright 1B_2 state, proceed subsequently through a symmetry-allowed conical intersection to the dark 2A_1 state, and return vibrationally excited to the electronic ground state involving a symmetry-forbidden conical intersection. Concerning the 1,3,5-cyclooctatriene photoreaction, there are strong arguments for the participation of two different reaction channels (involving a three-center and a four-center substructure) which lead to three products. Studying the ring-opening reaction of 7-dehydrocholesterol, which is a substituted cyclic polyene, answers the crucial question to what extent an ultrafast reaction may be slowed down by the influence of a steric factor. (orig.)Available from TIB Hannover: RN 5339(257) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman