  Precise shaping of coherent electron sources allows the controlled creation
of wavepackets into a one dimensional (1D) quantum conductor. Periodic trains
of Lorentzian pulses have been shown to induce minimal excitations without
creating additional electron-hole pairs in a single non-interacting 1D electron
channel. The presence of electron-electron (e-e) interactions dramatically
affects the non-equilibrium dynamics of a 1D system. Here, we consider the
intrinsic spectral properties of a helical liquid, with a pair of
counterpropagating interacting channels, in the presence of time-dependent
Lorentzian voltage pulses. We show that peculiar asymmetries in the behavior of
the spectral function are induced by interactions, depending on the sign of the
injected charges. Moreover, we discuss the robustness of the concept of minimal
excitations in the presence of interactions, where the link with excess noise
is no more straightforward. Finally, we propose a scanning tunneling microscope
setup to spectroscopically access and probe the non-equilibrium behavior
induced by the voltage drive and e-e interactions. This allows a diagnosis of
fractional charges in a correlated quantum spin Hall liquid in the presence of
time-dependent drives.
