  Topological Dirac nodal-line semimetals host topologically nontrivial
electronic structure with nodal-line crossings around the Fermi level, which
could affect the photocarrier dynamics and lead to novel relaxation mechanisms.
Herein, by using time- and angle-resolved photoemission spectroscopy, we reveal
the previously-inaccessible linear dispersions of the bulk conduction bands
above the Fermi level in a Dirac nodal-line semimetal PtSn$_4$, as well as the
momentum and temporal evolution of the gapless nodal lines. A surprisingly
ultrafast relaxation dynamics within a few hundred femtoseconds is revealed for
photoexcited carriers in the nodal line. Theoretical calculations suggest that
such ultrafast carrier relaxation is attributed to the multichannel scatterings
among the complex metallic bands of PtSn$_4$ via electron-phonon coupling. In
addition, a unique dynamic relaxation mechanism contributed by the highly
anisotropic Dirac nodal-line electronic structure is also identified. Our work
provides a comprehensive understanding of the ultrafast carrier dynamics in a
Dirac nodal-line semimetal.
