What happens when light energy excites electrons in photosystem II?

The process that occurs is photolysis. Photolysis splits apart water molecules to make electrons and hydrogen ions. The hydrogen ion is used to reduce NaDP (later on). Oxygen is produced as a byproduct.

This is the equation 2H₂O⇒4e⁻ + 4H⁺ The electrons here replace the lost ones in the chlorophyll.