  Small polaron formation limits the mobility and lifetimes of photoexcited
carriers in metal oxides. As the ligand field strength increases, the carrier
mobility decreases, but the effect on the photoexcited small polaron formation
is still unknown. Extreme ultraviolet transient absorption spectroscopy is
employed to measure small polaron formation rates and probabilities in goethite
({\alpha}-FeOOH) crystalline nanorods at pump photon energies from 2.2 to 3.1
eV. The measured polaron formation time increases with excitation photon energy
from 70 {\pm} 10 fs at 2.2 eV to 350 {\pm} 30 fs at 2.6 eV, whereas the polaron
formation probability (85 {\pm} 10%) remains constant. By comparison to
hematite ({\alpha}-Fe2O3), an oxide analog, the role of ligand composition and
metal center density in small polaron formation time is discussed. This work
suggests that incorporating small changes in ligands and crystal structure
could enable the control of photoexcited small polaron formation in metal
oxides.
