  We perform a joint measurement of terahertz waves and high-order harmonics
generated from noble atoms driven by a fundamental laser pulse and its second
harmonic. By correlating their dependence on the phase-delay of the two pulses,
we determine the generation of THz waves in tens of attoseconds precision.
Compared with simulations and models, we find that the laser-assisted
soft-collision of the electron wave packet with the atomic core plays a key
role. It is demonstrated that the rescattering process, being indispensable in
HHG processes, dominant THz wave generation as well but in a more elaborate
way. The new finding might be helpful for the full characterization of the
rescattering dynamics.
