Bismuth oxyiodide (BiOI) is a promising material for photocatalysis with intriguing optical and structural properties. We demonstrate that excitation by a femtosecond laser pulse creates coherent phonons inducing a time-variant oscillating modulation of the optical density. The two underlying frequencies originate from vibrations along the stacking direction of oppositely charged layers in BiOI. This is consistent with a subpicosecond charge separation driven by a built-in dipolar field. This partially screens the field, launching coherent phonons. Our results demonstrate the presence of an electric field and show its role in efficient charge separation that is crucial for photocatalytic applications of BiOI.
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