A few atoms and ions :
Precision measurements on unstable nuclei produced in an accelerator, Quantum Information Processing using atoms

Small sized clusters :
Metallic and semiconductor clusters, Approach to bulk properties, Basic for functional materials

Large sized clusters :
etc, n = 1000 - 5000
Used to understand the Van der Waals complexes, solvent interactions in gas phase

cold atoms or ions :
Cold atom physics, Bose Einstein Condensation etc

Gas/liquid phase :
Spectroscopic analysis, trace analysis, pollution detection, chemical and biochemical interactions