Enhancing Infrared Laser Dissociation of Molecules with the Electromagnetic Vacuum

Abstract

Controlling bond breaking is a long-standing goal in molecular physics. Infrared nanocavities are currently being developed for reaching exotic coupling regimes of cavity QED with a few molecules, but it is not well understood how chemical reactions would proceed in such systems. To address this, we study infrared laser photodissociation of a single CS$_{2}$ molecule with a stretching mode that strongly interacts with a resonant infrared vacuum, subject to a strong laser field that either resonantly drives the molecule at its fundamental vibration frequency or injects photons at the cavity resonance. We show that the intensities required for photodissociation are significantly lower inside the cavity than in free space, with a strong dependence on the type of driving condition. By directly injecting photons into the cavity, the molecule dissociates with two orders of magnitude less laser energy than by directly driving the vibrational mode. This photodissociation enhancement is a purely quantum mechanical effect that cannot be captured semi-classically. The intracavity ladder climbing dynamics is substantially modified relative to free space due to vacuum-induced admixing a large number of vibrational quantum numbers and the cavity field acting as a surrogate molecular mode that strongly interacts with the dissociative vibrational motion. Our work provides fundamental mechanistic understanding of chemical dynamics that can be used for designing new types of nanophotonics experiments that probe single-molecule chemistry.

Figures (8)

• Figure 1: Driven molecule-cavity coupled system. (a) CS$_{2}$ molecule in free space driven by an infrared laser. Multi-photon transitions between vibrational levels lead to dissociation. (b) CS$_{2}$ in a strongly confined infrared electromagnetic vacuum generated by metallic nanostructures. Multi-photon infrared absorption occurs between polariton levels. In the molecule-driving case, the laser drives a nanotip to excite the molecule, and in the cavity-driving scenario, the cw laser injects photons directly on the near field of the coupled system.
• Figure 2: Ladder-climbing dynamics. Mean vibrational energy $\langle\hat{H}_{\mathrm{M}}\rangle$ as a function of time for free space, mean field, and intracavity scenarios. For intracavity cases, the light-matter coupling strength is $\lambda_{g}=0.02$. Molecule-driving scenarios correspond to the driving energy $E_{\mathrm{D}}\sim0.055$ aJ and for cavity-driving $E_{\mathrm{D}}\sim0.0025$ aJ. Energy in units of fundamental vibrational frequency $\omega_{10}$.
• Figure 3: Dissociation probability maps. (a) $P_{\mathrm{diss}}$ as a function of driving energy and coupling strength $\lambda_g$ for the molecule-driving scenario at $t=1.5$ ps; (b) same as in panel (a) for the cavity-driving scenario. Dissociaiton threshold contours for $P_{\mathrm{diss}}^{\rm (th)}=10^{-3}$ are shown (black solid lines). Energy in attojoules (aJ). For reference $0.01$ aJ $\equiv 23$ THz.
• Figure 4: Time-dependent dissociation probability. Photodissociation probability for different driving energies $E_{\mathrm{D}}$ in aJ for (a) the molecule-driving and (b) the cavity-driving scenarios. Dashed lines for the corresponding color of laser intensity represent the free-space scenario. In both panels $\lambda_g=0.025$. The shaded region corresponds to the numerical error threshold.
• Figure 5: ab initio potential energy curve (gray) and electric dipole moment function (blue) of the ground electronic state $\tilde{\rm X}^{1}\Sigma_{\rm g}^{+}$ for CS stretching mode of CS$_{2}$ molecule as a function of bond distance coordinate $q$ respect to equilibrium $q_{\mathrm{e}}$. $D_0$ corresponds to the dissociation energy.