Surface-Sensitive Mapping of Anisotropic Phonon Cascades in T$_{d}$-WTe$_{2}$
Understanding how energy flows from photoexcited carriers into the lattice is essential for describing nonequilibrium phenomena in low-symmetry quantum materials. Here, we use ultrafast low-energy electron diffraction and diffuse scattering to probe momentum-resolved phonon dynamics at the surface of T$_d$-WTe$_2$, a strongly anisotropic semimetal. Following optical excitation, the Debye--Waller suppression of Bragg peaks exhibits a biexponential increase of the mean-squared atomic displacement, indicating sequential lattice relaxation. Analysis of the diffuse background reveals a preferential intensity build-up parallel to the tungsten-chain axis in the material, attributed to anisotropic electron--phonon coupling during electronic cooling which precedes anharmonic phonon--phonon scattering and subsequent thermalization across the surface Brillouin zone. The results identify a hierarchical relaxation pathway in which energy is first deposited into selected finite-momentum phonons before spreading through the broader lattice bath. Our work highlights the importance of momentum-resolved diffuse scattering for disentangling electron--phonon and phonon--phonon relaxation in anisotropic topological semimetals.