The dynamics of diimide (N2H2) under excitation of femto-second laser pulses is investigated using nonadiabatic quantum molecular dynamics (NA-QMD). Distinct fragmentation channels are observed in particular by varying the laser frequency. It is found that the dynamics on the first excited state surface S-1 is dominated by cis-trans isomerization with a characteristic time scale of about tau approximate to 25 fs almost independent of the nuclear initial conditions. The long time behaviour of the electronic dynamics is classified into three categories: (1) persistence on the ground state surface, (2) persistence on the first excited state surface and (3) nonadiabatic switching between both surfaces. (c) 2006 Published by Elsevier B.V.