～ Past tense + 途端[とたん]＋に
expresses what occurs immediately after an action.
it can only be used for an action that is involitional; you cannot have control over it.
窓を開けたとたんに、猫が跳んでいた。 Immediately after I opened the window, the cat jumped out.
映画を観たとたんに、眠くなりました。 Immediately after watching the film, I became sleepy.
布団に入った途端に先輩から電話がかかってきた。 Immediately after I got into the futon, I received a phone call from senpai.
Differences with similar points
Just finished doing (...) すぐ: Immediately after, not soon after. から: とたん does not imply a causality.