Electronic Spectra of Molecular Crystals

Abstract

Before any experimental data on molecular crystals had become available, Frenkel [^1–4] had formulated a theory of electronic energy levels of weakly interacting systems. He began with the tight binding approximation; the wave functions and energy levels of each molecule are unchanged from the free state; and, upon formation of the crystal, each state is shifted and split into a band of closely spaced levels. The electronic spectra of organic crystals, such as benzene and anthracene, show the features expected from Frenkel’s theory, namely, the energy levels appear to be almost those of the free molecules, but band shifts and splittings are observed. Other examples of weakly interacting systems are the d-shell states of MnF₂ [^5–7] or Cr₂O₃ [⁸] and f-shell states of PrC1₃, etc. “Molecules” such as MnO₄ ⁻ in KMnO₄ or CO₃ ²⁻ in CaCO₃ are weakly interacting in their lower electronic levels. In addition, molecules and molecular ions are also weakly interacting in their vibrational levels, and a Frenkel-type theory applies to these.