Electronic strong coupling of gas-phase molecular iodine

Molecular polaritons, hybrid light–matter states formed from the strong coupling of molecular transitions and discrete photonic modes, are a compelling platform for optical control of chemical reactivity. Despite the origins of the field of polaritonics in atomic gases, strong coupling of molecular gases remains underexplored. The pristine, solvent-free gas-phase environment may prove ideal for gaining mechanistic understanding of molecular behavior under strong light–matter coupling. In this work, we achieve electronic strong coupling of the B–X, ν1 = 0 → 32, J = 53 → 52 and B–X, ν1 = 0 → 34, J = 103 → 102 rovibronic transitions of gas-phase iodine (I2) lying near 532.2 nm. We access a range of coupling strengths and detuning conditions with fine control over molecular number density and cavity length stabilization. This effort represents the first demonstration of electronic polaritons in a molecular gas and opens a new platform for polariton photochemistry and photophysics.