Photoinduced phase transition in tetrathiafulvalene-$p$-chloranil observed in femtosecond reflection spectroscopy

Photoinduced transitions from ionic $\left(I\right)$ to neutral $\left(N\right)$ and neutral $\left(N\right)$ to ionic $\left(I\right)$ phases in an organic charge transfer (CT) complex, tetrathiafulvalene-$p$-chloranil (TTF-CA), were investigated by femtosecond pump-probe reflection spectroscopy. Transient reflectivity changes of the intramolecular transition band of TTF sensitive to the degree of CT between a donor molecule of TTF and an acceptor molecule of CA are measured as a function of excitation energy, excitation density, and temperature. By adopting the multilayer model for the analysis of the obtained transient reflectivity spectra, we have derived the time characteristics of amounts and spatial distributions of photoinduced $N\left(I\right)$ states in the $I\left(N\right)$ phase. The results reveal that the $I$ to $N\left(IN\right)$ transition induced by the resonant excitation of the CT band at $4\mathrm{K}$ is composed of three processes; (1) formation of a confined one-dimensional (1D) $N$ domain, that is, a sequence of $D^0{A}^0$ pairs, just after the photoexcitation, (2) multiplication of the 1D $N$ domains to the semimacroscopic $N$ states up to $20\mathrm{ps}$ within the absorption depth of the excitation light, and (3) proceeding of the $IN$ transition along the direction normal to the sample surface. At $77\mathrm{K}$ near the $NI$ transition temperature $(T_c=81\mathrm{K})$, the size of the 1D $N$ domain initially produced is enlarged and its multiplication process is strongly enhanced. When the excitation energy is increased, the initial photoproduct is changed from the confined 1D $N$ domain to the positively and negatively charged $N$ states. The spatial size of the latter is considerably larger than that of the former, indicating that the introduction of charge carriers makes the neighboring $I$ state strongly unstable. The dynamics of the photoinduced $N$ to $I\left(NI\right)$ transition has also been investigated. The 1D $I$ domains are initially produced by lights, however, they decay within $20\mathrm{ps}$ even if the density of the $I$ domains is increased. The results demonstrate that there is a clear difference of the dynamics between the photoinduced $IN$ and $NI$ transitions. In these photoinduced transitions, three kinds of coherent oscillations with the period of $\sim 0.6$, $\sim 50$, and $\sim 85\mathrm{ps}$ have been detected on the photoinduced reflectivity changes, which are reasonably assigned to the dynamical dimeric displacements of molecules associated with the spin-Peierls instability, the shock wave driven by the sudden volume change due to the photoinduced transitions, and the oscillation of the $NI$ domain boundary. On the basis of the results, dynamical aspects of the photoinduced $IN$ and $NI$ transitions have been discussed in detail.

Figure 1

Structures of TTF $(\text{donor}:D)$ and CA $(\text{acceptor}:A)$ molecules (a), schematic illustrations of neutral $\left(D^0{A}^0\right)$ chains (b) and ionic $\left(D^{+}{A}^{-}\right)$ chains (c) along the a axis, and the energy level structures of the neutral state (d) and ionic state (e) in TTF-CA. In the ionic phase, the $D^{+}{A}^{-}$ chains are dimerized. The dimers are shown by the underlines in (c) and by the ovals in (e).

Figure 2

(a) Polarized reflectivity spectra for the electric field of light $\mathbf{E}\Vert \mathbf{a}$ and $\mathbf{E}\perp \mathbf{a}$ of TTF-CA crystal on (001) surface at 4 K (solid lines: $R_I$) and at $90\mathrm{K}$ (broken lines: $R_N$). (b) Spectra of the imaginary part of dielectric constant ${ϵ}_2$ and excitation profiles of the photoconductivity at $77\mathrm{K}$ (solid line) and $90\mathrm{K}$ (broken line) for $\mathbf{E}\Vert \mathbf{a}$ and the applied electric field $\mathbf{F}\Vert \mathbf{a}$.

Figure 3

(a) Transient differential reflection spectra $(\Delta R∕R)$ in the intramolecular transition region measured with $\mathbf{E}\perp \mathbf{a}$ at $4\mathrm{K}$ for various time delays. The energy of the pump light $(\mathbf{E}\Vert \mathbf{a})$ is set to be $0.65\mathrm{eV}$ (the resonant CT excitation). Excitation density is $1.2\times {10}^{16}\text{photons}∕{\mathrm{cm}}^2$. The solid line shows the differential spectrum $[R_N\left(90\mathrm{K}\right)-R_I\left(4\mathrm{K}\right)]∕R_I\left(4\mathrm{K}\right)$, in which $R_N\left(90\mathrm{K}\right)$ and $R_I\left(4\mathrm{K}\right)$ are the reflectivity spectra of the $N$ phase at $90\mathrm{K}$ and the $I$ phase at $4\mathrm{K}$, respectively. (b) The differential reflection spectra $(\Delta R∕R)$ calculated by using the multilayer model. The used parameters ${\beta }^{-1}$ (nm) and $x_0$ (%) are listed in the figure.

Figure 4

Time evolutions of the transient reflectivity changes $(-\Delta R∕R)$ measured at $2.25\mathrm{eV}$ for $\mathbf{E}\perp \mathbf{a}$; (a) 4 and (b) $77\mathrm{K}$. The energy of the pump light $(\mathbf{E}\Vert \mathbf{a})$ is set to be $0.65\mathrm{eV}$. The right side panels show the dynamics in the short time region.

Figure 5

The photoinduced changes in reflectivity $(-\Delta R∕R)$ at $2.25\mathrm{eV}$ for $\mathbf{E}\perp \mathbf{a}$ as a function of the excitation density $N_{\mathrm{ex}}$; the closed circles: $T=4\mathrm{K}$ and $t_d=2\mathrm{ps}$; the closed squares: $T=4\mathrm{K}$ and $t_d=500\mathrm{ps}$; the open circles: $T=77\mathrm{K}$ and $t_d=2\mathrm{ps}$; the open squares: $T=77\mathrm{K}$ and $t_d=500\mathrm{ps}$. The energy of the pump light $(\mathbf{E}\Vert \mathbf{a})$ is set to be $0.65\mathrm{eV}$. In the inset, only the squares are plotted.

Figure 6

(a) and (b) are schematic illustrations of photoinduced formations of a 1D $N$ domain in the $I$ phase and a 1D $I$ domain in the $N$ phase, respectively. (c) and (d) show charged-solitons and charged domains in the $I$ phase, respectively.

Figure 7

Schematic illustrations of the photoinduced behaviors observed in the $I$ phase; (a) the weak excitation at $77\mathrm{K}$, (b) the weak excitation at $4\mathrm{K}$, and (c) the strong excitation at $4\mathrm{K}$.

Figure 8

The photoinduced changes of reflectivity $(-\Delta R∕R)$ at $2.25\mathrm{eV}$ $(\mathbf{E}\perp \mathbf{a})$ as a function of the absorbed photon density $N_{\mathrm{abs}}$ for various excitations $(\mathbf{E}\Vert \mathbf{a})$ ranging from $0.65\text{to}1.55\mathrm{eV}$. The measurements were performed at $4\mathrm{K}$. The broken lines give the linear relation between $-\Delta R∕R$ and $N_{\mathrm{abs}}$. The open arrows show the critical density at which the saturation of $-\Delta R∕R$ occurs.

Figure 9

Time evolutions of $-\Delta R∕R$ at $2.25\mathrm{eV}$ $(\mathbf{E}\perp \mathbf{a})$ for various excitations $(\mathbf{E}\Vert \mathbf{a})$ ranging from $0.65\text{to}1.55\mathrm{eV}$ at $4\mathrm{K}$. The excitation density is set to be very low, in which the dynamics is independent of the excitation density (see text).

Figure 10

The amplitudes of the slow decay components relative to the total amplitudes of the signal $[B∕(A+B)]$ as a function of the excitation energy (the solid squares) obtained from the results in Fig. 9. The solid line is the ${ϵ}_2$ spectrum for $\mathbf{E}\Vert \mathbf{a}$ and the broken line is the excitation profile of the photoconductivity for $\mathbf{E}\Vert \mathbf{a}$ and $\mathbf{F}\Vert \mathbf{a}$ in the $I$ phase at $77\mathrm{K}$. The open squares show the generation efficiency of the charged $N$ domains evaluated by assuming that the size of a charged $N$ domain is $20D^0{A}^0$ pairs.

Figure 11

Time evolutions of $-\Delta R∕R$ at $2.25\mathrm{eV}$ $(\mathbf{E}\perp \mathbf{a})$ in the time regions of 0–6 (a), 0–18 (b), and $0–500\mathrm{ps}$ (c) in the $I$ phase at $77\mathrm{K}$. The pump energy is $0.65\mathrm{eV}$ $(\mathbf{E}\Vert \mathbf{a})$ and the pump density $N_{\mathrm{ex}}$ is $0.3\times {10}^{16}\text{photons}∕{\mathrm{cm}}^2$. The arrows indicate the coherent oscillation.

Figure 12

(a) Initial response of the reflectivity change $(-\Delta R∕R)$ shown in Fig. 11a. (b) The oscillatory component obtained by subtracting background rise and decay from the time profile in (a) (the solid circles). The solid line shows a profile of a damped oscillator (see text).

Figure 13

(a) Initial response of the reflectivity change $(-\Delta R∕R)$ at $2.25\mathrm{eV}$ $(\mathbf{E}\perp \mathbf{a})$ measured at $4\mathrm{K}$. The pump energy is $0.65\mathrm{eV}$ $(\mathbf{E}\Vert \mathbf{a})$ and the pump density $N_{\mathrm{ex}}$ is $1.5\times {10}^{15}\text{photons}∕{\mathrm{cm}}^2$. (b) The oscillatory component obtained by subtracting background rise and decay from the time profile in (a) (the solid circles). The solid line shows a profile of a damped oscillator (see text). (c) The open circles show the probe energy dependence of the initial amplitudes of the oscillation indicated by the open arrow in (b). The solid line is the differential spectrum $[R_N\left(90\mathrm{K}\right)-R_N\left(300\mathrm{K}\right)]∕R_I\left(4\mathrm{K}\right)$.

Figure 14

Oscillatory components of the reflectivity change $(-\Delta R∕R)$ for various probe energies $(\mathbf{E}\perp \mathbf{a})$ at $77\mathrm{K}$. The pump energy is $0.65\mathrm{eV}$ $(\mathbf{E}\Vert \mathbf{a})$ and the excitation density is $1.2\times {10}^{16}\text{photons}∕{\mathrm{cm}}^2$.

Figure 15

(a) Transient differential reflection spectra $(\Delta R∕R)$ in the intramolecular transition region measured with $\mathbf{E}\perp \mathbf{a}$ at $90\mathrm{K}$ for various time delays. The energy of the pump light $(\mathbf{E}\Vert \mathbf{a})$ is set to be $0.65\mathrm{eV}$ (the resonant CT excitation). Excitation density is $0.77\times {10}^{16}\text{photons}∕{\mathrm{cm}}^2$. The solid line shows the differential spectrum, $[R_I\left(4\mathrm{K}\right)-R_N\left(90\mathrm{K}\right)]∕R_N\left(90\mathrm{K}\right)$. (b) The differential reflection spectra $(\Delta R∕R)$ calculated by using the multilayer model. The used parameters ${\beta }^{-1}$ (nm) and $x_0$ (%) are listed in the figure. The broken line is the differential spectrum, $[R_N\left(300\mathrm{K}\right)-R_N\left(90\mathrm{K}\right)]∕R_N\left(90\mathrm{K}\right)$.

Figure 16

Time characteristics of the photoinduced reflectivity changes $\Delta R∕R$ at $2.25\mathrm{eV}$ $(\mathbf{E}\perp \mathbf{a})$ at $90\mathrm{K}$ for several typical excitation densities; (a) short time region; (b) long time region. The energy of the pump light $(\mathbf{E}\Vert \mathbf{a})$ is set to be $0.65\mathrm{eV}$.

Figure 17

(a) Initial response of the reflectivity change $(\Delta R∕R)$ at $2.25\mathrm{eV}$ $(\mathbf{E}\perp \mathbf{a})$ at $90\mathrm{K}$. The pump energy is $0.65\mathrm{eV}$ $(\mathbf{E}\Vert \mathbf{a})$ and the pump density $N_{\mathrm{ex}}$ is $0.9\times {10}^{16}\text{photons}∕{\mathrm{cm}}^2$. (b) The oscillating component obtained by subtracting background rise and decay from the time profile in (a) (the solid circles). The solid line shows a profile of a damped oscillator (see text). (c) The open circles show the probe energy dependence of the initial amplitudes of the oscillation indicated by the open arrow in (b). The solid line is the differential spectrum $[R_I\left(4\mathrm{K}\right)-R_I\left(77\mathrm{K}\right)]∕R_N\left(90\mathrm{K}\right)$.

Figure 18

The excitation density $N_{\mathrm{ex}}$ dependence of the $\Delta R∕R$ signals at $t_d=0.3$ and $500\mathrm{ps}$ ($90\mathrm{K}$, $0.65\mathrm{eV}$ excitation with $\mathbf{E}\Vert \mathbf{a}$). The broken line shows the linear relation between $\Delta R∕R$ and $N_{\mathrm{ex}}$.

Figure 19

(a) Schematic illustration of the difference of the absorption depths between the pump light $\left(l_p\right)$ and the probe light $\left(l_r\right)$. (b) Schematic illustration of the multilayer model. The sample is assumed to be composed of $N$ layers with the complex refractive index ${\tilde{n}}_l$. The $z$ axis is normal to the sample surface. (c) The definitions of electric and magnetic fields of the incident light $(E_{1x},H_{1y})$ and the transmitted light $(E_{2x},H_{2y})$, and the angle of refraction $\left({\theta }_1\right)$ associated with the first layer. $d$ is the thickness of the layer.

Figure 20

Time characteristics of $-\Delta R∕R$ at $2.25\mathrm{eV}$ $(\mathbf{E}\perp \mathbf{a})$ at $4\mathrm{K}$ corresponding to the result of Fig. 3a, and time characteristics of the total amounts of the $N$ state $\left(N_{\text{total}}\right)$, the characteristic length for the distribution of the $N$ state $\left({\beta }^{-1}\right)$, and the ratio of the $N$ state at the sample surface $\left(x_0\right)$, which were obtained by the simulation based upon the multilayer model presented in Fig. 3b.

Figure 21

(a) The time dependence of the ratio $x\left(z\right)$ of the $N$ states in the photoinduced $I$ to $N$ transition. $z$ is the distance from the sample surface. The white and gray arrows indicate the changes of the $N$ states for $t_d<20\mathrm{ps}$ and $t_d>20\mathrm{ps}$, respectively. (b) Schematic illustration of the multiplication process in the photoinduced $I$ to $N$ transition. Shade shows the coexistence of the $N$ and $I$ state and black region shows the $N$ state.

Figure 22

Schematic illustration of the observed dynamics in the photoinduced $NI$ transition (see text).

Figure 23

(a) Quantum energy of the oscillation $(\Delta E=\hslash \omega )$ observed in Fig. 14 as a function of $2k$; the closed circles: the low-frequency oscillation; the open circles: the high-frequency oscillation. $k$ is the wave number of the probe light. (b) Amplitudes of the high-frequency oscillation (the open circles) and the low-frequency oscillation (the closed circles) as a function of the probe energy. The broken line is the $\Delta R∕R$ spectrum at $t_d=500\mathrm{ps}$.

Figure 24

Schematic illustrations of the shock wave and the dispersion of the acoustic phonon. The shock wave acts as a broadband source of acoustic phonons. The probe light having the wave number $k$ is modulated by the coherent acoustic phonon with $2k$. $\Delta E$ as a function of $2k$ gives the dispersion of the acoustic phonon.

Figure 25

(a) Fourier power spectra of the oscillatory components observed in Fig. 14 at various temperatures. (b) The ratio between the amplitudes of low- $\left(I_l\right)$ and high-frequency peak $\left(I_h\right)$.

Figure 26

Schematic illustration of the observed dynamics in the photoinduced $NI$ transition (see text).