Ab initio multistate second-order perturbation theory (MS−CASPT2) calculations are used to map the reaction path for the ultrafast photochemical electrocyclic ring-opening of cyclohexa-1,3-diene (CHD). This path is characterized by evolution along a complex reaction coordinate extending over two barrierless excited state potential energy surfaces and ultimately leading to deactivation through a S1/S0 conical intersection. The observed excited-state dynamics involve three sequential phases with lifetimes (traveling times) of 10, 43, and 77 fs, respectively. In this work we associate each phase to the evolution of the CHD molecular structure along a different mode. In particular, we show that (a) the decay of CHD from its spectroscopic (1B2) state to a lower lying dark (2A1) excited state involves motion along a highly curved coordinate corresponding to a mixture of σ bond expansion and symmetry breaking skeletal bending, (b) the evolution on the 2A1 (S1) state and the final 2A1→1A1 (i.e., S1→S0) decay involve a large amplitude displacement along the same asymmetric bending mode which ultimately leads to a S1/S0 conical intersection, and (c) the application of a novel strategy for mapping the multidimensional S1/S0 intersection space indicates that the ultrashort 77 fs lifetime of the 2A1 excited state is due to the existence of an extensive set of S1/S0 conical intersection points spanning the low-lying part of the 2A1 energy surface. Points (a) and (b) are validated by discussing the results of previously reported and new femtosecond time-resolved spectroscopic data on CHD and on the two dialkyl derivatives α-terpinene and α-phellandrene. An interpretation in terms of driving forces is also given.

We found five consecutive processes with time constants 21, 15, 30, 47, and 3300 fs in Fe(CO)5 after excitation at 267 nm in the gas phase. The first four represent a continuous pathway of the molecule from the Franck−Condon region down to the lowest singlet state (S0) of Fe(CO)4 through a chain of Jahn−Teller-induced conical intersections. The motion before dissociation initially involves more than one of the equatorial ligands, but then eliminates only one CO. The product Fe(CO)4 is initially generated in its first excited singlet state S1, then it relaxes to S0 in 47 fs via a triply degenerate conical intersection at tetrahedral geometry. The pathway for this process involves pseudorotation of the ligands. The fifth step is assigned to thermal elimination of a second CO. Intersystem crossing to the triplet ground states of Fe(CO)4 and Fe(CO)3 takes more than 500 ps.

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.