The short life and times of vibrational polaritons

There is a lot of interest in vibrational polaritons because they appear to modify chemistry.1 Following the initial studies of infrared spectra of strong coupling to polymer carbonyls and metal carbonyls dissolved in solution,2,3,4 the first transient infrared absorption studies were reported for W(CO)6 in hexane which were aimed at identifying modified vibrational dynamics,5 especially for vibrationally excited polaritons, that might account for cavity modified reactions. However, clear evidence of modified vibrational dynamics for polaritons, or even convincing demonstrations of clear spectral signatures of vibrational polaritons, remains elusive. Although aspects of the signal were initially attributed to excited state polaritons, the observed transient response is dominated by polariton contraction and enhanced excited state absorption, all phenomena described by linear optical response of the bare molecules. Two dimensional infrared (2D IR) of vibration polaritons from strong coupling to W(CO)6 showed that exciting reservoir, non-polariton states can create a response that includes polariton contraction and enhanced excited state absorption.6 Analysis of 2DIR measurements of nitroprusside anion in methanol involved subtracting the reservoir, non- polariton contributions to the signal and indicted that some of the remaining response is from excited polaritons.7 This analysis was challenged by the Kubarych group who provided insight that some early time response could be due to inhomogeneities in the molecular absorption band.8,9 In order to explain this early time response, a microscopic theory that expanded on a transfer matrix model by including excitation of the reservoir band was developed for the spectroscopy of cavity- coupled molecules that includes band inhomogeneity and can predict 2DIR spectra from molecular polaritons. The theory provides a unified picture for a global understanding of recent spectroscopic experiments on molecular polaritons. There is still the question of whether vibrationally excited polaritons exist and if so, how to reveal them.

[1] Hutchison et al., Angew. Chem., Int. Ed. 51, 1592 (2012).

[2] Long and Simpkins, ACS Photonics 2, 130, 2015.

[3] Shalabney et al., Nat. Comm. 6, 5981 (2015).

[4] Long et al. et al., ACS Photonics 2, 1460 (2015).

[5] Dunkelberger et al., Nat. Comm. 7, 13504 (2016).

[6] Xiang et al., PNAS 201722063 (2017).

[7] Grafton et al., Nature Comm. 12, 214 (2021).

[8] Simpkins et al., J. Phys. Chem. Lett. 14, 983 (2023).

[9] Duan et al., J. Phys. Chem. Lett. 12, 11406 (2021).