Ultrafast electronic relaxation of s-trans-butadiene

By exciting 1,3-butadiene in the gas phase at 200 nm and probing it by nonr esonant multiphoton ionization, we found an internal-conversion time of abo ut 110 fs, within which one can distinguish five consecutive phases and tim e constants. The first two times (10 and 44 fs) are assigned to two phases on the initially excited 1B(u) surface and departure from it to the 'dark' 2A(g) state. The latter is then left in a remarkably short time (18 fs). Ob viously the wave packet is very early (already in the 1B(u) state) accelera ted towards the 2A(g)/1A(g) conical intersection (CI). There follow two mor e 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). (C) 2001 Elsevier Science B.V. All rights reserved.