METALLIC REFLECTION IN MOLECULAR CRYSTALS: THEORY OF THE DAMPING

Abstract

Metallic reflection occurs in molecular crystals when a stopping band, associated with a strong molecular electronic transition, emerges. There are interesting variations in reflectivity over the band, and we report here a calculation of the reflection bandshape for two specific cases -- 1,5-bis-(dimethylamino)pentamethinium perchlorate (BDP) and tetracyanoquinodimethane (TCNQ). Variations in reflectivity are described by a frequency dependent damping term $\Gamma (\omega)$ in the expression for the polarizability. For the case of one molecule per unit cell (BDP), the observed reflection bandshape can be matched using a theoretical expression $\Gamma (\omega)=(\gamma^{2}/N)\Sigma_{k} 1/(\omega-\omega_{k}-\Delta)$ where $\gamma$ is the single molecule vibrational relaxation constant and $\Delta$ is the frequency of the active vibration. For the more complicated case of two molecules per unit cell (TCNQ) a similar theoretical expression predicts a reflection spectrum much richer in structure than in the case of one molecule per unit cell, which moreover, is in substantial agreement with the observed spectrum of TCNQ.