Femtosecond dynamics of hydrogen migration and internal conversion in cycloheptatriene and derivatives studied by intense-field dissociative ionization

In the photochemical [1,7]-sigmatropic H shift in 1,3,5-cycloheptatriene (C HT) and its 7-methyl derivative, we can distinguish three consecutive phase s in the excited states, all of them having time constants < 100 fs. The ul trafast rates prove a continuous pathway from the initially excited 1 A " s tate via the dark state 2 A' to the ground-state surface via two easily acc essible conical intersections. We assign the first two times (10 and 60 fs in CHT) to travelling through two regions of 1 A " and the third (70 fs in CHT) to the departure from 2 A' and repopulation of the ground state. Perde uteration of 7-methyl-CHT only lengthens the times for region 2 and 3 (by a factor of 1.5). So, the direction of motion changes after region 1 (Franck -Condon region), only then involving the hydrogen. We suggest that the geom etries of both conical intersections are displaced from that of the ground state along two coordinates: one which is Franck-Condon active (a' CC stret ch + torsion) and one (a ") which shifts an H of the CH2 group.