Dynamical Jahn-Teller effects on the generation of electronic ring currents by circularly polarized light

The generation of electronic ring currents in ring-shaped molecules by photoexcitation with circularly polarized laser light is considered in the presence of vibronic coupling effects. $(E\times e)$ Jahn-Teller distortions, unavoidable by symmetry in the $\left(E\right)$ subset of electronic states supporting the ring current, mix the clockwise and anticlockwise circulation directions of the electrons and can suppress the maximum achievable current by at least one order of magnitude, already for moderate vibronic coupling strengths, as compared to the Born-Oppenheimer limit of fixed atomic positions. The circulation direction of the electrons is found to depend on the spectral region of the $(E\times e)$ Hamiltonian. This fact results in the surprising effect that the same polarization direction of the laser pulse can trigger either clockwise or anticlockwise electronic dynamics depending on the wavelength of the photons. These findings are illustrated in a model of the triazine molecule.

Figure 1

Ring current $C_{j_q}$ values in $\hslash /m_e$ units for the eigenstates of the $q=-1$ block as a function of the coupling strength $\kappa /\omega$. The dotted-dashed curve in orange represents the current of the most optically bright eigenstate while the blue curve indicates the largest ring current achievable at a specific coupling strength.

Figure 2

(a) Absorption spectrum ($\propto |{\mu }_{0_0{j}_q}{|}^2$) for the 2D sym-triazine $(E\times e)$ JT Hamiltonian (orange, left $y$ scale) and eigenstate ring currents $[{\mathcal{C}}_{j_q}/(\hslash /m_e)]$ (blue, middle $y$ scale) as a function of the eigenstates' energy superimposed with the spectral representation of the short ($\tau =30$ fs, shaded in green) and long ($\tau =250$ fs, shaded in red) RCPs resonant with the lower band of the absorption spectrum. (b) Same as (a) but pulses are resonant with the spectral region above the CI (marked $\times$ on the energy axis). (c), (d) Time-dependent ring currents $[\mathcal{C}\left(t\right)/(\hslash /m_e)/P_{\text{ex}}\left(t_f\right)]$ generated by the pulses shown in (a) and (b), respectively. The time-dependent ring currents are normalized by the total excitation probability $P_{\text{ex}}\left(t_f\right)$ after the pulse (cf. main text). The inset in (a) shows the geometry of sym-triazine and the horizontal red line in (a) and (b) is to guide the eye.