We report ultrafast x-ray scattering experiments of the quasi-1D charge density wave (CDW) material $({\mathrm{TaSe}}_4{)}_2\mathrm{I}$ following ultrafast infrared photoexcitation. From the time-dependent diffraction signal at the CDW sidebands we identify a 0.11 THz amplitude mode derived primarily from a transverse acoustic mode of the high-symmetry structure. From our measurements we determine that this mode interacts with the valence charge indirectly through another collective mode, and that the CDW system in $({\mathrm{TaSe}}_4{)}_2\mathrm{I}$ has a composite nature supporting multiple dynamically active structural degrees of freedom.

• Received 23 December 2022
• Revised 12 June 2023
• Accepted 17 July 2023

DOI:https://doi.org/10.1103/PhysRevLett.131.076901