Ab Initio Calculations of Strongly Coupled Molecule-Plasmon Systems

When coherent energy exchange between an electromagnetic field mode (such as a plasmon mode) and quantum emitters is faster than the decay and decoherence of either constituent, the system enters into the strong coupling regime. The system excitations then become light-matter hybrid states which display Vacuum Rabi Splitting, i.e., an energy splitting caused solely by the vacuum EM field. Strong coupling in plasmonic systems is been extensively studied due to their promising applications. One particularly interesting realization of strong coupling uses organic molecules as the quantum emitters, allowing Rabi splittings of up to 1 eV. Pioneering experimental studies show that even chemical structure and chemical reactions could be modified and controlled using strong coupling.

In most theoretical descriptions of strong coupling, quantum emitters are modelled as two-level systems, however, organic molecules possess a complex internal structure due to nuclear motion. This internal structure is often described phenomenologically through Lindblad decay and dephasing terms in a master equation description, which can not describe chemical modifications and dynamics. Additionally, it is not a priori obvious if the underlying approximations are valid in the strong coupling regime. In this contribution, we investigate an ab initio model in which both electronic and nuclear motion in the molecule are taken into account fully, allowing the description of chemical dynamics within a single unified framework. This is achieved using a model molecule with only one electronic and one nuclear degree of freedom.

We study the applicability of different approximations to obtain observables such as the absorption spectrum by comparing to the full ab initio results. In particular, we will discuss how the introduction of a new timescale due to strong coupling affects the validity of the Born-Oppenheimer approximation, which is typically used in molecular physics to separate the slow vibrational and fast electronic degrees of freedom. As for large enough Rabi splittings, the energy exchange with the EM field takes place on an intermediate time scale between vibrational and electronic motion, it could potentially form a ”bridge” between them and affect the validity of the Born-Oppenheimer approximation.

javier.galego@uam.es