〜次第

到着(とうちゃく)次第(しだい)、ご連絡(れんらく)いたします。

Tōchaku shidai, gorenraku itashimasu.

I will contact you as soon as I arrive.

次第 is as soon as, and cannot be followed by a past form

準備(じゅんび)ができ次第(しだい)、始(はじ)めましょう。

Junbi ga deki shidai, hajimemashō.

Let us start as soon as we are ready.

It attaches to the ます stem, not the dictionary form

メールの締(し)めくくりによく使(つか)われます。

Mēru no shimekukuri ni yoku tsukawaremasu.

It is common at the close of an email.

分かり次第、ご報告します is the set closing of a business email