  We investigate ultrafast vibronic dynamics triggered by intense femtosecond
infrared pulses in small molecules. Our study is based on numerical simulations
performed with 2D model molecules, and analyzed in the perspective of the
renown Lochfrass and Bond-Softening models. We give a new interpretation of the
observed nuclear wave packet dynamics, with a focus on the phase of the bond
oscillations. Our simulations also reveal intricate features in the
field-induced nuclear motion that are not accounted for by existing models. Our
analyses assign these features to strong dynamic correlations between the
active electron and the nuclei, which significantly depend on the carrier
envelope phase of the pulse, even for relatively ``long'' pulses, which should
make them experimentally observable.
