Optical study of structural phase transition in organic charge-transfer crystals K- and Rb-tetracyanoquinodimethane

Polarized reflectivity spectra have been measured on K- and Rb-tetracyanoquinodimethane single crystals over a wide temperature range covering respective structural phase-transition temperatures. From an analysis of the intramolecular vibration spectra and charge-transfer exciton bands, thermal variations of dimeric molecular displacements at the phase transition are investigated. The temperature-dependent intensity and width of the dimerization-induced intramolecular $a_g$ vibration bands can provide microscopic information, which is complementary to the x-ray data, on the structure change at the phase transition.