たとたん / たとたんに

たとたん means "the moment": right after one thing happened, something else happened, usually something sudden or unexpected. It follows the た-form of a verb, and に after it is optional. In kanji it is とたん途端.

The verb before とたん is always the た-form, never the dictionary form: ×開けるとたん. The second half reports something that actually happened, so it is nearly always in the past too.

Not for what you chose to do

The second half is something that happened to you or around you, outside your control: the weather, a feeling, a sudden realisation. Your own deliberate next step, a request or a plan doesn't fit; for those, use たらすぐ.

• × 家に帰ったとたん、テレビをつけた → 家に帰ったらすぐ、テレビをつけた
• × 駅に着いたとたん、電話してください → 駅に着いたらすぐ、電話してください

Someone else's sudden reaction is fine, because from where the speaker stands it is unexpected.

たとたん or や否や

やいなや否や also means "no sooner had… than". It follows the dictionary form, and it is literary: you meet it in novels and news writing. たとたん is the one for conversation.