Cyclohexadiene ring opening observed with 13 fs resolution: coherent oscillations confirm the reaction path
Abstract
The third harmonic (270 nm, 11 fs), produced in a short argon cell from Ti-sapphire laser pulses (810 nm, 12 fs), was used to excite C twist. The latter motion—after some excursion—also erects and stretches the CH2–CH2 bond, so that Woodward–Hoffmann interactions are activated after this delay (in L2). On leaving L2 (the 1B minimum) around the lower cone of the 1B/2A conical intersection, the wave packet is rapidly accelerated along an antisymmetric coordinate, which breaks the C2 symmetry of the molecule and eventually leads in a ballistic path to (and through) the last (2A/1A) conical intersection. The ring opening begins already on the 1B surface; near the 2A minimum it is already far advanced, but is only completed on the ground-state surface.