Nonlinear spectroscopy and chemical reactivity of anharmonic vibrational polaritons in the ultrastrong coupling regime

Light-matter interaction of molecular vibrations with confined infrared fields is a powerful resource for controlling the spectroscopy and chemical reactivity of molecular materials [1]. Experiments show that the properties of vibrational polariton states can strongly depend on the detailed internal structure of the coupled molecules [2]. We introduce fully-quantum theory for the coupling of a Morse oscillator with an infrared cavity, taking into account both transition and permanent molecular dipole moments [3], and show that at the onset of ultrastrong coupling, polar molecular modes can spontaneously dissociate when placed inside a cavity [4]. The same theory has been successfully used to describe ultrafast polariton-to-polariton transitions in two- dimensional infrared cavity spectroscopy [5]. Our work thus offers mechanistic insights on the spectroscopy and chemistry hybrid vibration-cavity states.