  We analyze existing optical data in the superconducting state of LiFeAs at $T
=$ 4 K, to recover its electron-boson spectral density. A maximum entropy
technique is employed to extract the spectral density $I^2\chi(\omega)$ from
the optical scattering rate. Care is taken to properly account for elastic
impurity scattering which can importantly affect the optics in an $s$-wave
superconductor, but does not eliminate the boson structure. We find a robust
peak in $I^2\chi(\omega)$ centered about $\Omega_R \cong$ 8.0 meV or 5.3 $k_B
T_c$ (with $T_c =$ 17.6 K). Its position in energy agrees well with a similar
structure seen in scanning tunneling spectroscopy (STS). There is also a peak
in the inelastic neutron scattering (INS) data at this same energy. This peak
is found to persist in the normal state at $T =$ 23 K. There is evidence that
the superconducting gap is anisotropic as was also found in low temperature
angular resolved photoemission (ARPES) data.
