Direct Detection of the Ultrafast Response of Charges and Molecules in the Photoinduced Neutral-to-Ionic Transition of the Organic Tetrathiafulvalene-p-Chloranil Solid

Ultrafast dynamics of charge and molecular degrees of freedom in a photoinduced neutral-ionic transition of an organic solid, tetrathiafulvalene-$p$-chloranil, were directly detected for the first time by transient reflectivity measurements using 15 fs laser pulses. The results reveal that ionic domains are photogenerated in the neutral lattice via collective charge-transfer processes within 20 fs, and subsequently molecular deformations and bendings occur, giving rise to large changes in the molecular ionicity and charge redistributions in molecules, respectively. We show that such couplings between charge and molecular degrees of freedom play important roles in photoinduced transitions of organic molecular compounds.