Ultrafast Dynamics of 1,3-Cyclohexadiene in Highly Excited States

The ultrafast dynamics of 1,3-cyclohexadiene has been investigated via structurally sensitive Rydberg electron binding energies and shown to differ upon excitation to the 1B state and the 3p Rydberg state. Excitation of the molecule with 4.63？eV photons into the ultrashort-lived 1B state yields the well-known ring opening to 1,3,5-hexatriene, while a 5.99？eV photon lifts the molecule directly into the 3p-Rydberg state. Excitation to 3p does not induce ring opening. In both experiments, time-dependent shifts of the Rydberg electron binding energy reflect the structural dynamics of the molecular core. Structural distortions associated with 3p-excitation cause a dynamical shift in the - and -binding energies by 10 and 26？meV/ps, respectively, whereas after excitation into 1B, more severe structural transformations along the ring-opening coordinate produce shifts at a rate of 40 to 60？meV/ps. The experiment validates photoionization-photoelectron spectroscopy via Rydberg states as a powerful technique to observe structural dynamics of polyatomic molecules. 1. Introduction The photochemical ring-opening reaction of 1,3-cyclohexadiene (CHD) is widely studied due to its important role as a prototype for important reactions in chemistry as well as in biological systems [1, 2]. Upon excitation to the 1B valence state, the reaction path carries the molecule within about 140？fs along the ring-opening coordinate through the 1B/2A and 2A/1A conical intersections back to the ground state [3–7]. But even while these curve-crossing transitions are now well understood, numerous questions remain. Of particular interest are the geometric changes occurring in the molecule after excitation into the 1B-valence state and as the molecule travels through the electronic states, as such structural motions have eluded direct experimental observation. As has been pointed out, [5, 6, 8, 9] the wave packet remains quite narrow during the reaction, implying that “structure” is a well-defined quantity even though it rapidly evolves. Moreover, it is unknown if higher-lying electronic states follow ring-opening pathways similar to that following the 1B excitation, or if other unique mechanisms emerge. Because the dynamics is determined by the rapid motions of wave packets on intricately sculptured surfaces [8], a dependence on the excitation energy is to be expected. The observation of structural dynamics, that is, structural changes during chemical reactions, is one of the grand challenges, not only in the investigation of the CHD ring-opening reaction, but for the field of chemical