たとたんに

たとたんに expresses that something happens immediately at the very moment another event occurs. It emphasizes a sudden, almost immediate transition from one action/state to another, with little to no time between them. It can feel dramatic or surprising and often contrasts with more neutral connectors like すぐに. Commonly used after a verb in the past plain form (V-た) or after a noun (Nの) to describe what happens right after that action or state.