You are correct Oliver Semmes did command DIANA and was captured by Union forces following a mortal blow to DIANA.
DIANA had been covering withdrawal.

Semmes was subsequently taken to NOLA and thence to Fort Monroe. Loaded aboard Maple Leaf with Capt Fuller and others, he was one of the leaders of a bloodless escape.

See Escape from the Maple Leaf by Jerry Witt.

Respectfully,

Oliver Semmes