SFB 450 II-III: Femtosecond Stimulated Raman Spectroscopy (FSRS) of photoreactions
in the condensed phase with conical intersection (TP B1)
in the condensed phase with conical intersection (TP B1)
Molecular reactions are examined which lead from an electronically excited state through a conical intersection (CI) to isomers in the electronically ground state. Which nuclear coordinates span the CI? How do vibrational modes change upon crossing? The last question will be answered for three typical photochemical reactions with the help of femtosecond-stimulated Raman spectroscopy, by exploiting resonances in the transient electronic spectra.
Sub-project 1: Study of (A) cis-trans isomerisation of 2,2'-Pyridocyanine, (B) cis-trans isomerisation of Indenylidene-Dihydropyridine, (C) ring opening of Benzo-Spiropyran. Femtosecond-stimulated Raman Spectroscopy will be used for this purpose. In the electronic ground state S0 the isomerisation proceeds along two paths: over a biradicaloid or over a zwitterionic transition state. In between a conical intersection with the S1 state should exist. Zwitterionic contributions to S1 are stabilized by polar solvation; as a result the CI is shifted and a global S1 minimum is formed. Here population can be accumulated and examined by transient Raman spectroscopy.
Sub-project 2: Perturbation-theoretical description of FSRS during a photoreaction, especially with regard to resonances for the ps Raman pulse. High-frequency modes are treated as depending on the reaction coordinate. For the three pulses (actinic, Raman, and chirped white-light) typically 3000 Liouville pathways must be considered. The corresponding n-fold time integrals are partly determined analytically so that spectra may be simulated fast and globally. Distributions of values for model parameters are sought with genetic algorithms.
Sub-project 3: Experimental study of the solvent effect on low-frequency modes of Anthracene and Perylene, by transient absorption spectroscopy with supercontinuum pulses 250-800 nm. The solvent affects asymetric modes which leave the molecular plane. Transient absorption spectra are being directly calculated in another project (V. May).
Financer
DFG: Sonderforschungsbereich - Beteiligungen
Duration of project
Start date: 07/2001
End date: 06/2010
