Behavior of charge-transfer absorption upon passing through the neutral-ionic phase transition

Abstract

The charge‐transfer band is determined from reflectance measurements on single crystals of TTF‐chloranil from 300 to 45 K, passing through the neutral‐ionic phase transition at 84 K. As the temperature is decreased from 300 K toward the transition, hνCT decreases slowly from 0.66 to 0.55 eV, perhaps the lowest energy observed for a charge‐transfer band. Below this transition, hνCT increases slightly and a second band appears at higher energy. From the measured oscillator strength, the temperature dependence of the degree of charge transfer has been determined. It is found to increase from ∼0.20 to ∼0.30 as the temperature is lowered from 300 to 100 K. Below T C , the degree of charge transfer rises to a value near 0.70.