You can find the Polyatomic Ion Particle only in the Fortress Region in The First Descendant. Moreover, they can only be obtained as a drop for eliminating boss monsters.

Polyatomic Ion Particles can only be found in the Fortress Region. To get them, you need to participate in missions and kill enemies with yellow health bars (aka boss monsters).

To get Polyatomic Ion Particles in The First Descendant, you can kill any mission monster on Fortress. Mission monsters are the powerful yellow bar enemies that can spawn in during missions.