We performed polarized reflection and transmission measurements on the layered conducting oxide PdCoO₂ thin films.

For the ab-plane, an optical peak near   Ω=750 cm^-1 drives  

the scattering rate γ^*(ω) and effective mass m^*(ω)  of the Drude carrier to increase and decrease respectively for ω > Ω. 

For the c-axis,

a longitudinal optical phonon (LO) is present at Ω. as evidenced by a peak in the loss function Im[-1/ε_c(ω)]. 

Further polarized measurements in different light propagation (q) and electric field (E) configurations indicate that 

the Peak at Ω results from an electron-phonon coupling of the ab-plane carrier with the c-LO phonon, which leads to the frequency-dependent γ^*(ω) and m^*(ω). 

This unusual interaction was previously reported in high-temperature superconductors (HTSC) between a non-Drude, mid-infrared band and a c-LO. On the contrary, 

it is the Drude carrier that couples in PdCoO₂.  The coupling between the ab-plane Drude carrier and c-LO suggests that the c-LO phonon may play a significant role in the characteristic ab-plane electronic properties of PdCoO_2, including the ultra-high dc-conductivity, phonon-drag, and hydrodynamic electron transport.