Optical Properties of Guanine from 2 to 82 eV

Direct reflectance measurements have been carried out on thin solid guanine films over a range of photon energies extending from 2 to 82 eV. Over this range these measurements, along with a Kramers-Kronig analysis, yield the complex refractive index n, the complex dielectric function e, and the energy loss function - Im(1/ε̃) . The structure observed in the dielectric functions is associated with π → π* transitions for energies less than ∼9 eV and with σ → σ* transitions at higher energies. A pronounced peak in the energy loss function near 25 eV is explained in terms of the collective behavior of the loosely bound electrons. This peak lies at a significantly higher energy than does a corresponding peak calculated from the directly measured energy loss spectrum. Possible reasons for this discrepancy are discussed.