Cyclohexadiene ring opening observed with 13 fs resolution: coherent oscillations confirm the reaction path.

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 1,3-cyclohexadiene to its lowest pipi* state (1B). Probing was done by transient ionization by the 810 nm pulses, measuring the yields of the parent and a fragment ion. As previously found with 10 times longer pulses, the molecule leaves in two steps (time constants tau(1), tau(2)) from the spectroscopic (1B) to a dark (2A) state and from there (within tau(3)) to the ground-state surface. In addition to slightly improved values for tau(1)-tau(3), we found in all three locations (L(1)-L(3)) on the potentials coherent oscillations, which can be assigned to vibrations. They are stimulated by slopes (driving forces) of the potentials, and the vibrational coordinates indicate the slope directions. From them we can infer the path following the initial excitation: the molecule is first not only accelerated towards CC stretching in the pi system but also along a symmetric C[double bond, length as m-dash]C twist. The latter motion-after some excursion-also erects and stretches the CH(2)-CH(2) bond, so that Woodward-Hoffmann interactions are activated after this delay (in L(2)). On leaving L(2) (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 C(2) 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.