〜たとたん（に）

JLPT N2

たとたんに

the moment ~; no sooner had ~

接続 · How it attaches

動詞た形＋とたん（に）

Nuance

The second event is immediate, unexpected, and outside the speaker's control. It cannot be followed by an intention, a request or a command — 開けたとたん、入ってください is wrong.

Example sentences

ドアを開けたとたん、猫が飛び出した。
The moment I opened the door, the cat shot out.

Not to be confused with

〜かと思うと／〜かと思ったら — Both are "the moment", both refuse intentions. 〜たとたん is one surprise; 〜かと思うと often contrasts two states flipping back and forth.
〜か〜ないかのうちに — 〜か〜ないかのうちに is tighter still: the first action has not even finished.