Article 19 Vienna Convention on Diplomatic Relations

1.If the post of head of the mission is vacant, or if the head of the mission is unable to perform his functions a chargé d’affaires ad interim shall act provisionally as head of the mission. The name of the chargé d’affaires ad interim shall be notified, either by the head of the mission or, in case he is
unable to do so, by the Ministry for Foreign Affairs of the sending State to the Ministry for Foreign Affairs of the receiving State or such other ministry as may be agreed.

2.In cases where no member of the diplomatic staff of the mission is present in the receiving State, a member of the administrative and technical staff may, with the consent of the receiving State, be designated by the sending State to be in charge of the current administrative affairs of the mission.

See also: Article 18 Vienna Convention on Diplomatic Relations