時間・時点N2
〜たとたん（に）
the moment... / as soon as...
Structure
V-た形 + とたん（に）
Explanation
Indicates that the moment the first action ends, the second happens immediately, emphasizing a very short interval. Attached to the ta-form of verbs; the main clause is often something unexpected or instantaneous.
Examples
家に着いたとたん、雨が降り出した。
The moment I got home, it started raining.
立ち上がったとたんに、めまいがした。
The moment I stood up, I felt dizzy.
Note
Similar to 〜か〜ないかのうちに but more colloquial. The main clause cannot express volition or commands.
Related Grammar
〜か〜ないかのうちに〜やいなや