So what is the chlorine not sublimation but atomization

Chlorine (Cl₂) exists as a diatomic molecule in its standard state, which is a gas at room temperature and pressure.

• Sublimation is the transition from solid to gas. Since chlorine is already a gas in its standard state, sublimation is not the relevant process here.
• Atomization is the process of forming one mole of gaseous atoms from a substance in its standard state. For chlorine, this means converting gaseous Cl₂(g) into gaseous chlorine atoms (Cl(g)).

The process is:
Cl₂(g) → 2Cl(g)

The energy change for this reaction is the enthalpy of atomization for chlorine.