Ultrafast laser excitation and rotational de-excitation of cis-stilbene

A realistic dynamical simulation is reported for ultrafast laser excitation and rotational de-excitation of cis-stilbene. Upon irradiation by a laser pulse with a FWHM of 100 fs, the molecule first rotates around its vinyl bond to about 90°, where an avoided crossing leads to electronic de-excitation. The molecule then immediately twists back to about −60° within only about 80 fs, avoiding isomerization. The other principal dihedral angles, within the phenyl rings, lag behind in time, so there is significant large strain between the phenyl rings and vinyl bond.