We have studied the reflectance of the recently discovered superconductor ${\mathrm{LaO}}_{0.9}{\mathrm{F}}_{0.1}\mathrm{FeAs}$ in a wide energy range from the far infrared to the visible regime. We report on the observation of infrared active phonons, the plasma edge, and possible interband transitions. On the basis of this data and the reported in-plane penetration depth ${\lambda }_L(0)=254\mathrm{nm}$ [H. Luetkens et al., Phys. Rev. Lett. 101, 097009 (2008)] a disorder sensitive relatively small value of the total electron-boson coupling constant ${\lambda }_{\mathrm{tot}}={\lambda }_{e\mathrm{\text{−}}\mathrm{ph}}+{\lambda }_{e\mathrm{\text{−}}\mathrm{sp}}\sim 0.6\pm 0.35$ can be estimated adopting an effective single-band picture.

• Received 9 May 2008

DOI:https://doi.org/10.1103/PhysRevLett.101.257004