The energy loss spectra of 25 keV electrons passing through thin condensed films of the nucleic acid bases guanine and cytosine have been measured over the range of energy losses from 0 to 500 eV. Transitions from π and σ molecular orbital levels and atomic K levels to bound excited states of the molecules as well as ionizations were observed. Cross sections for these energy loss processes and also cross sections for damage of the molecules by the electron beam are given. By describing the energy loss spectra in terms of the dielectric theory, the optical constants for these materials are calculated and used in interpretation of the energy loss mechanisms.

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