Ultrafast electronic relaxation of s-trans-butadiene

By exciting 1,3-butadiene in the gas phase at 200 nm and probing it by nonresonant multiphoton ionization, we found an internal-conversion time of about 110 fs, within which one can distinguish five consecutive phases and time constants. The first two times (10 and 44 fs) are assigned to two phases on the initially excited 1Bu surface and departure from it to the `darkʹ 2Ag state. The latter is then left in a remarkably short time (18 fs). Obviously the wave packet is very early (already in the 1Bu state) accelerated towards the 2Ag/1Ag conical intersection (CI). There follow two more short processes (both 18 fs) on the ground-state surface and then single-bond isomerization, taking 270 fs at the given excess energy (6.2 eV).