Legion of Superheroes Vol. 2#281

The legendary Steve Ditko's brief run on the Legion included this battle with Molecule Master, who is defeated by the combined forces of the Legionaires in a parallel-earth Smallville. Unfortunately, his disintegration hits with the force of several atomic bombs, leaving the heroic teenagers stunned and at the mercy of a suspicious miliary, who takes them prisoner.
Phantom Girl manages to carry the unconsious Superboy and Dawnstar to safety.
Source: blog