The ultrafast optical response of two nodal-line semimetals, ZrSiS and ZrSiSe, was studied in the near-infrared using transient reflectivity. The two materials exhibit similar responses, characterized by two features, well resolved in time and energy. The first transient feature decays after a few hundred femtoseconds, while the second lasts for nanoseconds. Using Drude-Lorentz fits of the materials equilibrium reflectance, we show that the fast response is well-represented by a decrease of the Drude plasma frequency, and the second feature by an increase of the Drude scattering rate. This directly connects the transient data to a physical picture in which carriers, after being excited away from the Fermi energy, return to that vicinity within a few hundred femtoseconds by sharing their excess energy with the phonon bath, resulting in a hot lattice that relaxes only through slow diffusion processes (ns). The emerging picture reveals that the sudden change of the density of carriers at the Fermi level instantaneously modifies the transport properties of the materials on a timescale not compatible with electron phonon thermalization and is largely driven by the reduced density of states at the nodal line.
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