Ultrafast Dynamics of Highly Excited trans-Stilbene: A Different Twist

Using femtosecond time-resolved resonant two-photon ionization coupled with Photoelectron spectroscopy, we have explored the structural dynamics of a previously uncharted, highly excited state of trans-stilbene: The molecule responds to excitation to the S-5 surface by twisting its phenylgroups about the carbon carbon bonds. The motion of the coherent wave packet is reflected in the time-dependent spectrum, yielding an oscillatory frequency of 16.7 cm(-1). This structural oscillation competes with a transition to a lower surface with a time constant of 280 (+/- 30) fs. The dynamical motions are intriguingly different from those that have previously been observed in lower excited states, revealing a structural dynamics that strongly depends on the electronic surface.