Reservoir Response Described by Modified Analytical Treatments in Cavity Coupled Systems*

Vibrational cavity polaritons are produced by strongly coupling an optical mode to a molecular vibration. This results in the formation of two new modes termed polaritons. The upper and lower polariton are separated from one another by Rabi Splitting Ω. These strongly coupled systems show promise as a means of modifying chemical reactivity. Reaction modification is often signified by differing reaction product ratios or changes in the reaction rate. However, the mechanisms by which modifications occur remain unclear.  Vibrations uncoupled to the optical mode are denoted dark states and form a reservoir manifold. The reservoir dominates the density of states for vibrational cavity polaritons. Despite this, experimental studies do not typically assign spectral features to the reservoir response.  

Ultrafast infrared spectroscopic techniques are particularly well equipped to study vibrational cavity polaritons. Changes in vibrational energy relaxation (VER) are one potential indicator of modified reaction dynamics via vibrational strong coupling (VSC). Herein we use pump probe and two-dimensional infrared (2D IR) spectroscopy to describe both the polariton and reservoir response of sodium nitroprusside and W(CO)₆ inside microcavities. We couple our experimental data with an adapted analytical expression for cavity transmission that incorporates inhomogeneity. This modified form replicates 2D spectra at a range of delays by assuming all excitation begins in the reservoir. Our approach successfully describes both quickly dephasing (< 3 ps) polaritonic signatures in addition to the reservoir-dominated response at late delays. 

Lastly, we extend this model to weakly coupled systems that posess a larger degree of inhomogeneity (such as NaSCN in MeOH) to highlight the role of other optical phenomena to consider when calculating spectral signatures associated with VSC.