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15 May 1998 Probing of ultrafast photoinduced isomerization and dissociation reactions by intense-field dissociative ionization
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Ionization by the electric field of intense laser radiation offers some advantages to study the dynamics of ultrafast chemical reactions in pump-probe experiments: any electronically excited state is expected to give an enhanced ion signal independent of its electronical and vibrational nature, depending mainly on its ionization potential; ionization is followed by extensive fragmentation which providers additional information on transient intermediate states and/or species. We applied this approach to study the femtosecond dynamics of a number of photoinduced isomerization and dissociation reactions in the gas phase including electrocyclic ring opening of 1,3-cyclohexadiene, pericyclic hydrogen migration of 1,3,5-cycloheptatriene, UV photodissociation of metal hexacarbonyls. Monitoring the pump-probe delay kinetics of the parent and various fragment ions, we are able to follow the molecule from state to state all along the primary photochemical reaction path which is completed within 200 fs. The common feature of these different chemical reactions is the ultrafast passage from S1 to S0 within a time in the range of 40 to 80 fs. Such ultrafast processes can only take place, if there is a continuous pathway between the two potential surfaces. The observations are therefore a strong support for the S1/S0 conical intersections predicted by quantum chemistry.
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Werner Fuss, Karl Ludwig Kompa, T. Schikarski, W. E. Schmid, and Sergei A. Trushin "Probing of ultrafast photoinduced isomerization and dissociation reactions by intense-field dissociative ionization", Proc. SPIE 3271, Laser Techniques for State-Selected and State-to-State Chemistry IV, (15 May 1998);

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