In Fig. , at the initial time, t = 0, the system is initialized with the zero momentum in the R direction and is in the ground state along the q direction, thus the velocity field is very small everywhere. Afterwards, the photon-nuclear density (depicted in all panels as black contour lines) starts mainly to move towards the right in the R direction. However, since we are showing the velocity field in the figure, we are able to clearly observed all along the dynamics, at all snapshots reported in Fig. , that the field changes direction often along q, and while one portion of the density moves away from the Franck-Condon region towards the right in R, the density rapidly oscillates "up and down" along the q direction. As the portion of the density moving towards the right in R approaches the avoided crossing, it gains velocity, and the nuclear component begins to be relevant, as indicated at t = 500 a.t.u. onwards. However, the oscillations in the q direction persist, and the arrows keep changing direction.