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.

Figure 1

(a) Molecular structures of TTF-CA. (b) Reflectivity ($R$) spectra in the $N$ phase ($R_N$) and $I$ phase ($R_I$) and a differential $R$ spectrum [$(R_I-R_N)/R_N$]. Spectral profiles of pump and probe pulses are also shown. (c) Time characteristic of photoinduced $R$ changes ($\Delta R/R$). Gray shades show a cross-correlation profile of pump and probe pulse. Solid lines in the inset are simulated time characteristics.

Figure 2

(a) Oscillatory component $\Delta R_{\mathrm{OSC}}/R$. Solid lines show fitting functions. (b) Oscillations of $\rho$ by dimeric molecular displacements. (c) IM $a_g$ modes. (d) Oscillations of $\rho$ by IM $a_g$ modes.

Figure 3

(a) Left panel: Fourier transform of the oscillatory component (circles) and the fitting function (solid lines). Right panel: Wavelet transform of the oscillatory component. Broken lines show peaks in Fourier spectra. (b) Time dependence of oscillation frequencies of IM $a_g$ modes. Broken and solid lines show time profiles calculated using Eq. (2) and Eqs. (3, 4), respectively. (c) Bending modes.

Figure 4

Photoinduced $NI$ transition dynamics.