Electronically Excited States of Molecules

Research project P 14442 Electronically Excited States of Molecules Hans LISCHKA 09.10.2000 Ultrafast (femtosecond) laser pulses are used to get a better understanding of the elementary steps of chemical reactions. Built on the work by Zewail, the field of Femtochemistry has been created in which gas phase reactions, atomic and molecular cluster and surface preocesses are investigated. A close interplay between theory and experiment is required for the understanding of the details of the dynamics. In this project we are interested in the theoretical treatment of primary photochemical processes. For this purpose on needs accurate potential energy surfaces. The regions around these crossing points are crucial for the progress of the reaction since they act as funnels through which the internal conversion to the lower state (usually the ground state) takes place. It is our aim to develop accurate quantum chemichal methodology for the calculation of energy surface for excited states and for the efficient determination of conical intersections. This work is based on the analytic MR-CI gradient feature of our COLUMBUS program systems. Furthermore, in cooperation with the special research project ADLIS-P9 (Femtochemistry-Theory), dynamics calculations will be carried out. On the basis of this methodology high-level calculations of benchmark quality will be performed on outstanding problems. One is the photoisomerization of ethylene, for which so far contradictory mechanisms have been proposed by means of theoretical calculations. Another one is acetylene for which recurrences and damping of wavepackets will be investigated. Acetylene has also a very photodissociation dynamics where several excited-state surfaces are involved. In addition to these series of investigations, excited states of selected, smaller molecules with biological relevance (like formamide and alanine) will be studied. Here we want to investigate in particular protonation process and the stability of various tautomeric forms in excited states. Intramolecular proton transfer processes like in malonaldehyde will be studied as well.