Ultrafast spectroscopy of an azide in the vibrational strong coupling regime

Vibrational strong coupling (VSC) promises systematic modification of chemical reactivity. There are now reports of reaction rate modulation, altered product branching ratios, bulk structural modification, etc. Ultrafast studies of vibrational polaritons might reveal the elusive mechanisms behind VSC-facilitated chemistry. Until now these studies have been applied to only a limited number chemical species, mainly W(CO)₆.

We apply femtosecond mid-infrared pump-probe spectroscopy to a typical organic compound, diphenylphosphoryl azide (DPPA), known to have a rather large Rabi splitting. The N₃ asymmetric stretch mode around 4.6 μm is coupled to a cavity mode, with the cavity made of two planar gold mirrors. We compare the in-cavity pump-probe spectrum to the out-of-cavity transient spectrum to investigate the early time dynamics of the upper and lower polariton states. Also, we will discuss the influence of the metallic surface, where localized surface plasmons and phonons may affect the response of the vibrational polaritons.

Understanding the ultrafast dynamics of a variety of chemical species is crucial for future research on how a chemical bond responds under VSC. This research opens the way to studying modified click chemistry under VSC.