Control of molecular dissociation by spatially inhomogeneous near fields

Tailored intense laser fields can be used to steer electron and nuclear dynamics in molecules and control chemical reactions. Here we demonstrate that inhomogeneous nanoscopic near fields provide an additional mechanism for controlling light-induced chemical reactions. This is investigated by quantum-dynamical calculations on dissociation of ${{\mathrm{H}}_2}^{+}$ in few-cycle near fields with a controlled carrier-envelope phase. Directional dissociation asymmetry analysis reveals that the spatial asymmetry of the field is critical in controlling and in modifying field-induced dissociation pathways such as zero-photon dissociation, bond softening, and above-threshold dissociation. The degree of impact strongly depends on the maximum length scale of coherent control in a dissociation channel. The results pave the way towards coherent control of molecular reactions in a wide range of molecules in inhomogeneous near fields.

Figure 1

(a) Schematic showing the geometry for the interaction of ${{\mathrm{H}}_2}^{+}$ with an enhanced near field at a nanotip. The incident laser field is a few-cycle pulse with a controlled carrier-envelope phase. (b) Spatial variation of field enhancement calculation of the field asymmetry parameter $y$. (c) Diabatic potential energy curves and dissociation channels for ${{\mathrm{H}}_2}^{+}$.

Figure 2

(a) Instantaneous wave-packet distributions at different times. The range of the horizontal axis $z$ is from $-10.0$ to +10.0 a.u. and the range of the vertical axis $R$ is from 4.0 to 15.0 a.u. Horizontally dashed lines at $R_d=10\text{a.u.}$ indicate the position of the detector (see the text). (b) Gray lines and the right axis show temporal profiles of the field $E\left(t\right)$ for $y=0$. Blue (dark gray) lines and the left axis show variation of the dissociation asymmetry $A\left(t\right)$ for $y=0$. (c) Same as in (b) but for $y=0.2$. The inset shows the time dependence of the asymmetry contrast $C\left(t\right)$, as defined in the legend, near the asymptotic limit.

Figure 3

Variation of the dissociation asymmetry $A$ as a function of KER for (a) $y=0$ and (b) $y=0.2$ and for two different CEP values. The asymmetry contrast $C$ is indicated for two KER points.

Figure 4

Asymmetry maps $A(E,\varphi )$ for dissociation of ${{\mathrm{H}}_2}^{+}$ for (a) $y=0$ and (b) $y=0.2$. (c)–(f) The CEP variation of integrated asymmetry for different KER ranges as indicated in (a) and (b) for $y=0$ (dashed lines) and $y=0.2$ (solid lines).

Figure 5

(a) The CEP variation of the integrated asymmetry for different field asymmetries $y$. The arrows indicate the systematic up-shift of $A$ (see the text). (b) Variation of contrast $C$ between the maximum and minimum $A$ for different KER ranges and as a function of $y$. (c) and (d) Results of the semiclassical dynamics of different dissociation channels for the case of $y=0$ and $y=0.2$. (c) Time variation of the dynamical distance $R\left(t\right)$ for $\varphi =0$. The top left inset shows a closeup of the time variation of $R\left(t\right)$ for the BS and ATD channels. The bottom right inset shows the ratio between $R\left(t\right)$ for $y=0$ and $y=0.2$. (d) Trajectory of dissociation of ${{\mathrm{H}}_2}^{+}$. The black circles indicate moments of photon transitions.