  We use optical spectroscopy to investigate the excitations responsible for
the structure in the optical self-energy of thin epitaxial films of
La1.83Sr0.17CuO4. Using Eliashberg formalism to invert the optical spectra we
extract the electron-boson spectral function and find that at low temperature
it has a two component structure closely matching the spin excitation spectrum
recently measured by magnetic neutron scattering. We contrast the temperature
evolution of the spectral density and the two-peak behavior in La2-xSrxCuO4
with another high temperature superconductor Bi2Sr2CaCu2O8+d. The bosonic
spectral functions of the two materials account for the low Tc of LSCO as
compared to Bi-2212.
