Characteristic Energy Losses by Slow Electrons in Organic Molecular Thin Films at 77°K

Abstract

Characteristic energy losses by slow electron impact have been measured for ⪖10² Å films of several aromatic compounds as well as alkanes, alkenes, alcohols, acetone and ice at 77°K. Resonances are clearly indicated by the electron current I transmitted by the dielectric as a function of the modulated thermionic emitter voltage V, displayed as dI/dV vs V. Energy losses are measured directly by retarding potential analysis of backscattered electrons. Measurements at several incident electron energies provide the V‐dependence of the cross section. The method is particularly useful for measuring small losses (∼1 eV). There is no potential barrier for injecting electrons into the dielectrics examined, and the evidence indicates that they have positive bulk electron affinities. All systems show resonances at 0.5–1 eV which are attributed to transient negative ions. Acetone, n‐hexane and ice were examined in some detail for 0–4 eV losses by retarding potential analysis. The results indicate that electrons are also inelastically scattered by higher transient negative ion states. All compounds examined show resonances in the 3–4 eV range, which may be due to low lying triplet states. The alkanes also have considerable oscillator strength in the 4–7 eV range, approximating that in the 7–10 eV range.