Coherent chemistry with THz pulses: Ultrafast field-driven isomerization of LiNC

The ability to coherently rearrange structures at the atomic scale is among the grand challenges of physical science. Some of the primary obstacles are nonadiabatic increases in energy, such as intramolecular vibrational relaxation (IVR) and electronic excitations. Motivated by recent advances in strong terahertz (THz) pulse generation, we investigate the potential of THz to circumvent these obstacles. Employing TDDFT-Ehrenfest simulations, we discover that strong THz pulses can drive isomerization of the LiNC molecule over barriers greater than 0.2 eV with very low ionization rates and, in the best case, less than 3 meV of residual excess energy. This work points to new possibilities in predictively manipulating chemical bonds in molecules and materials.

Figure 1

(a,b) Zero-field potential $\tilde{V}(\theta ,r_0)$ and relaxed radial coordinate $r_0\left(\theta \right)$. (c,d) Force constants of Eq. (1). (e) Reduced coordinates describing lithium cyanide with a rigid CN bond. Coordinates $r$ and $\theta$ are defined relative to the center of mass (COM) of the CN group. A laboratory reference frame $(\widehat{\mathbf{x}},\widehat{\mathbf{y}})$ and the molecule reference frame $(\widehat{\mathbf{r}},\widehat{\mathbf{theta}})$ are also defined. Electric fields are applied in the $(x,y)$ plane.

Figure 2

TDDFT-Ehrenfest dynamics simulation of a successful field-driven isomerization of lithium cyanide to its triangular conformation. (a) Applied electric field pulse, with $t_{\mathrm{fin}}=250$ fs, derived from Eq. (B5) with the constraints of Eqs. (B7)–(B9). (b) Ionized charge. (c) Model trajectories (black) versus TDDFT-Ehrenfest trajectories of $\theta \left(t\right)$ (green) and $\varphi \left(t\right)$ (blue), along with snapshots taken at 0, 100, 200, 400, and 600 fs. (d) Model trajectory (black) versus TDDFT-Ehrenfest trajectory (red) of $r\left(t\right)$. (e) Ionic temperature, $T_{\mathrm{ion}}=\left(2e_{\mathrm{kin}}\right)/\left(3k_{\mathrm{B}}\right)$, based on the model trajectory (black) and TDDFT-Ehrenfest trajectory (red).

Figure 3

Left side: LiNC to triangle conformation isomerizations. Right side: LiNC to linear LiCN isomerizations. (i) Final total energy $e_{\mathrm{tot}}$ (blue diamonds), total energy less ion kinetic energy $e_{\mathrm{tot}}-e_{\mathrm{kin}}$ (blue $\times \mathrm{s})$, and ionized charge (red) for different pulse durations. The dotted blue line shows the energy of the transition barrier between the initial (linear LiNC) and target (isomer) states, and the dashed blue line shows the energy of the isomer state. Energies are on the left (blue) axis, and charges are on the right (red) axis. (ii) Maximum field amplitude (black, left axis) and highest-frequency spectral component (green, right axis) for different pulse durations. Green dashed lines show the target bending mode $(\theta$ bend) and the radial stretch mode $(r$ stretch).

Figure 4

Final total energy and ionized charge for different initial molecular orientations under the isomerization pulse shown in Fig. 2. Orientations within the region outlined in red isomerize to the triangular conformation with insufficient residual energy to surmount the 0.051-eV barrier. Sampled points are marked by black dots, and the surface is interpolated by a two-dimensional cubic spline.

Figure 5

Computed vibrational frequencies and TDDFT electronic excitation spectra for the three stable isomers, along with spectral power densities of isomerization pulses. Pulse 1 is the 250-fs isomerization pulse shown in Fig. 2, and pulse 2 is the 700-fs pulse targeting the LiNC to linear LiCN isomerization. The bandwidth of these pulses overlaps primarily with the 3–6 THz molecular bending mode and exhibits little overlap with higher-frequency vibrations. The plotted electronic spectra in this figure have been truncated below the first excitation peak to remove Fourier transform artifacts.

Figure 6

TDDFT Ehrenfest dynamics compared with classical model prediction for the isomerizing electric field pulse of Fig. 2 with different electronic time step $\left(\delta t\right)$ and ionic time-scaling $\left(f_{\mathrm{ITS}}\right)$ values. The model trajectory (black) is the intended path based on the prescription of Eq. (B5), which should not be expected to coincide with the converged TDDFT-Ehrenfest path. The simulated trajectory with the smallest attempted time step (0.001 fs) is different from the model due to inherent differences between the model and the TDDFT simulations (see Discussion section). Larger time steps of $\delta t=0.003$ fs and $\delta t=0.006$ fs, as well $\delta t=0.003$ fs with $f_{\mathrm{ITS}}=5$, all yield indistinguishable trajectories. Curves are offset for clarity by 2 deg in $\theta$, 1 deg in $\varphi$, and 0.01 $\AA$ in $r$. Ionic time-scaling error begins to appear for larger $f_{\mathrm{ITS}}$ values, as evidenced by the trajectory with $f_{\mathrm{ITS}}=40$ and $\delta t=0.003$ fs (magenta).