  We show that, when a spatially localised electric pulse is applied at the
edge of a quantum spin Hall system, electron wavepackets of the helical states
can be photoexcited by purely intra-branch electrical transitions, without
invoking the bulk states or the magnetic Zeeman coupling. In particular, as
long as the electric pulse remains applied, the photoexcited densities lose
their character of right- and left-movers, whereas after the ending of the
pulse they propagate in opposite directions without dispersion, i.e.
maintaining their space profile unaltered. Notably we find that, while the
momentum distribution of the photoexcited wave packets depends on the
temperature $T$ and the chemical potential $\mu$ of the initial equilibrium
state and displays a non-linear behavior on the amplitude of the applied pulse,
in the mesoscopic regime the space profile of the wave packets is independent
of $T$ and $\mu$. Instead, it depends purely on the applied electric pulse, in
a linear manner, as a signature of the chiral anomaly characterising massless
Dirac electrons. We also discuss how the photoexcited wave packets can be
tailored with the electric pulse parameters, for both low and finite
frequencies.
