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

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, hnuCT decreases slowly from 0.66 to 0.55 eV, perhaps the lowest energy observed for a charge-transfer band. Below this transition, hnuCT 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 TC, the degree of charge transfer rises to a value near 0.70. The Journal of Chemical Physics is copyrighted by The American Institute of Physics.