Transmission of 0–15 eV monoenergetic electrons through aliphatic and alicyclic hydrocarbon films

Abstract

High-resolution electron transmission spectra and their second energy derivatives were measured in thin (∼100 Å) organic films deposited on polycrystalline and single crystal metal substrates. For all compounds investigated (i.e., benzene, pentene, hexane, 1-hexane, trans-2, trans-4-hexadiene, 1,3,5-hexatriene, norbornene, norbornadiene, 1,3-cycloheptadiene, and 1,3,5-cycloheptatriene), these spectra contained broad maxima observable in dc curves and much sharper and less intense features, that were usually only visible in the doubly differentiated spectra (DDTS). The broad maxima were interpreted to result from a convolution of inelastically scattered electron currents that increased the total transmitted current, mainly due to a change in their reflection coefficient at the film-vacuum interface. The sharper features were more difficult to interpret. In hexatriene, norbormadiene, and heptatriene, they could be correlated with gas phase electronic transitions. As previously observed in C6-alicyclic hydrocarbons, this correlation could be achieved by shifting the energy scale of the DDTS relative to the gas phase electronic energy levels. This phenomenon is expected to result from either pronounced structure in the ’’conduction band’’ density of states or the production of electron–exciton complexes. Using a modified Wigner–Seitz model and variational electron wave functions we have shown that electron–exciton complex formation can explain the 2.2 and 0.8 eV shift in energy scale in solid benzene and cycloheptatriene, respectively. In other films of the present series, sharp features could not be correlated to gas phase energy levels and we suggest that they may result from interference effects.